WO2013006427A1 - Procédés et systèmes utiles pour le stockage d'énergie solaire - Google Patents

Procédés et systèmes utiles pour le stockage d'énergie solaire Download PDF

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Publication number
WO2013006427A1
WO2013006427A1 PCT/US2012/044899 US2012044899W WO2013006427A1 WO 2013006427 A1 WO2013006427 A1 WO 2013006427A1 US 2012044899 W US2012044899 W US 2012044899W WO 2013006427 A1 WO2013006427 A1 WO 2013006427A1
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product
nanostructure
reactant
cases
catalytic
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PCT/US2012/044899
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English (en)
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Steven Y. Reece
Tom MADDEN
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Sun Catalytix Corporation
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Publication of WO2013006427A1 publication Critical patent/WO2013006427A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/50—Processes
    • C25B1/55—Photoelectrolysis
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/10—Process efficiency
    • Y02P20/133—Renewable energy sources, e.g. sunlight
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W10/00—Technologies for wastewater treatment
    • Y02W10/30—Wastewater or sewage treatment systems using renewable energies
    • Y02W10/37—Wastewater or sewage treatment systems using renewable energies using solar energy

Definitions

  • the present disclosure generally relates to methods and systems for solar energy storage and/or electricity generation.
  • the methods and/or systems comprise nanostructures.
  • the present disclosure provides methods, the methods comprising exposing, to electromagnetic radiation, a reactant in contact with a photocatalytic material so as to effect a catalytic redox reaction that produces at least a first product and a second product in different phases.
  • the present disclosure provides methods, the methods comprising exposing a fluid containing particles to sunlight to cause a redox reaction promoted by the particles which produces at least a first product and a second product at an efficiency of at least 1% over the course of at least 24 hours.
  • systems suitably including a vessel comprising a reactant in contact with a photocatalytic material, the photocatalytic material being capable of, when exposed to electromagnetic radiation, effecting a redox reaction that produces from the reactant at least a first product and a second product; and a device capable of promoting a reaction between at least the first product and the second product so as to produce electricity.
  • the components include a vessel comprising a reactant and a photocatalytic material, the photocatalytic material being adapted to, when exposed to electromagnetic radiation, effect a redox reaction on the reactant that produces at least a first product and a second product in different phases.
  • Figures 1A-1C illustrate a non-limiting embodiment of a nanostructure of the present disclosure and use of the nanostructure for photocatalytic water splitting.
  • Figures 2A-2J illustrate non-limiting examples of nanostructures of the present disclosure.
  • Figures 3A-3D illustrate a non-limiting example of a method of the formation of a nanostructure comprising association of a first catalytic material and a second catalytic material with a first and a second region of a bulk, respectively, thereby forming a nanostructure, followed by use of the nanostructure for photocatalytic water splitting.
  • Figures 4A-4E illustrate the formation of a catalytic material on a bulk material to form a nanostructure, according to some embodiments.
  • Figure 5 depicts a dispersed photocatalyst in a plastic reactor
  • Figure 6 depicts a tandem configuration for a two-compartment reactor design
  • Figure 7 depicts a UF membrane cartridge embodiment for a two- compartment reactor design
  • Figure 8 depicts the use of conductive electrodes as redox mediators for a two-compartment reactor design.
  • Figure 9 depicts a schematic of a photocatalytic core-shell particle that comprises a light-absorbing core material that is unstable under water splitting conditions
  • Figure 10 depicts a schematic of an iron oxide particle for use in
  • Figure 11 depicts a schematic of a Tas s particle decorated with both HEC and OEC material.
  • Figure 12 depicts a non-limiting example of a system of the present disclosure
  • Figure 13 illustrates hydrogen production in a system according to the present disclosure
  • Figure 14 illustrates hydrogen production in an alternative system according to the present disclosure.
  • the present disclosure generally relates to methods and systems for solar energy storage and/or electricity generation.
  • the methods and/or systems comprise nanostructures.
  • the subject matter of the present disclosure involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.
  • the present disclosure provides systems and/or methods for storage of energy. These systems and methods address the limitations of current technologies, which limitations include (but are not limited to) the high cost associated with producing and/or installing the materials/systems, poor and/or limited efficiency (e.g., due to recombination of electron-hole pairs), and the difficultly and/or inability to store energy and/or products that are generated and/or are to be recombined at a later time point.
  • the methods and/or systems of the present disclosure make use of photocatalysis and/or photoelectro lysis promoted by a photoactive material.
  • Photocatalysis e.g., involving an oxidation and/or reduction reaction
  • the methods/systems allow for the conversion of a reactant (e.g., HBr) into a first product (e.g., Br 2 ) and a second product (e.g., H 2 ).
  • the reactant is suitably in a fluid state, although fluid form is not a requirement.
  • energy can be stored, via a reactive pathway involving the systems/methods of the disclosure, in the form of the first product and the second product.
  • the first product and the second product may in turn be recombined at a later time point to product electricity and/or reform the reactant.
  • This approach gives rise to an illumination-rechargeable power system, which operates by storing photocatalytically-evolved energy until the user requires such energy, which energy is in turn evolved by reforming the reactant.
  • a photoactive material promotes an oxidation-reduction reaction (which may, in some cases, be termed a "redox reaction”) upon exposure to electromagnetic radiation.
  • oxidation-reduction reaction means a chemical reaction between two species involving the transfer of at least one electron from one species to the other species.
  • the oxidation portion of the reaction involves the loss of at least one electron by one of the species, and the reduction portion involves the addition of at least one electron to the other species.
  • the redox reaction may produce a first product and a second product, wherein the first product and the second product may be recombined at a later time point to reform the reactant, and thereby produce electricity
  • HBr is a non-limiting, exemplary reactant used herein for exemplary purposes only, as the present disclosure is not necessarily limited to systems and methods that use HBr.
  • a system comprises a container for containing a fluid.
  • the fluid contains a photoactive material (e.g., in the form of particles), wherein, upon exposure to electromagnetic radiation, the particles are capable of promoting a redox reaction to produce at least a first product and a second product.
  • the system may also comprise a device capable of promoting a reaction between at least the first product and the second product to produce electricity and optionally reform the reactant.
  • Figure 12 One non-limiting example of a system of the present disclosure and/or a system which may be used in connection with a method of the present disclosure is shown in Figure 12. It should be understood that the materials, components, and arrangements depicted in and described in connection with Figure 12 are by way of example only, and those of ordinary skill in the art will know of alternative components, materials, and arrangements which would be suitable for use in connection with this exemplary system.
  • Container 2 contains fluid 4, which fluid comprises a reactant (e.g., HBr, in this example) and at least one photoactive material.
  • the photoactive material comprises plurality of nanostructures 8 dispersed in fluid 4.
  • at least a portion of container 2 is transparent or substantially transparent such that fluid 4 can be exposed to electromagnetic radiation (e.g., from sun 3).
  • a least top edge 6 of container 2 is transparent.
  • Container 2 is shown inclined at an angle to the vertical, which inclination may aid in the separation of the products produced during the photocatalysis/photoelectro lysis of the reactant, as described herein.
  • Photoactive material 8 is suitably capable of promoting photocatalysis (e.g., photoelectrolysis) of the reactant, upon exposure to electromagnetic radiation (e.g. sun 3), to form a first product and a second product.
  • photoactive material 8 may be matter comprised of a semi-conductor material, in which a bandgap exists by virtue of the chemical structure (see, e.g., United States Application 13/452,258, "Nanostructures, Systems, And Methods For Photocatalysis," filed on April 20, 2012, incorporated herein by reference in its entirety for all purposes). Products may be collected and stored and/or optionally combined, either concomitantly or at a later point in time, to reform the reactant and electricity.
  • Br 2 and H2 are the first and second products, respectively, and are formed from the photocatalysis and/or photoelectrolysis of the reactant HBr promoted by photoactive material 8 upon exposure to electromagnetic radiation.
  • the first product e.g., Br 2
  • the second product H 2
  • Fluid 4 may optionally comprise a complexing agent.
  • Br 2 forms a fluid which spontaneously separates from fluid 4 and second product H 2 and collects in the bottom most portion 10 of container 2, which is optionally connected to outlet 12.
  • the second product, H 2 is formed as a gas that spontaneously separates from fluid 4 and first product Br 2 and collects in the upper most portion 14 of container 2, which is optionally connected to outlet 16.
  • Outlets 12 and 16 may be each connected to a storage container (e.g., storage containers 18 and 20) and/or to a device (e.g., flow battery stack 22) that is capable of promoting a reaction between the products to form electricity and optionally reform the reactant.
  • the products produced in the photocatalysis/photoelectrolysis reaction may be stored for any suitable period of time. For example, during times when container 2 is exposed to electromagnetic radiation such that the photocatalysis/photoelectrolysis reaction is occurring, the products formed may then be stored. In some cases, the products are formed and stored during the day and/or during times when the container is exposed to sunlight.
  • the products may be recombined at a later time (or immediately following formation, if desired) using a device (e.g., flow battery stack 22) capable of promoting the reformation of the reactant (e.g., HBr) and the production of electricity.
  • a device e.g., flow battery stack 22
  • the electricity may be stored and/or may be used to power other components and/or systems (e.g., to power household appliances, or even to power one or more system components, such as pumps).
  • Reformed reactant e.g., HBr
  • the system depicted in Figure 12 is a regenerative system which is capable of being operated solely using electromagnetic radiation (e.g., the sun) as a power source.
  • the fluid 4 may be converted to the first and second products by sun exposure during the daytime.
  • the fluid may be reformed by the products at nighttime so as to product electricity during nighttime hours when sunlight is not available.
  • a user might use solar cells to power a facility during the day and reform a reactant (that has been photocatalytically processed to form the two products during the daytime) to product electricity that powers the facility at nighttime.
  • a user may operate a system that acts to provide power to a facility essentially continuously (or on-demand) throughout the day and night.
  • a system of the disclosure comprises an at least partially transparent container comprising a photoactive material (e.g., nanostructures), wherein the photoactive material is capable of promoting a redox reaction of at least one reactant to produce at least a first product and a second product of different phases which can separate spontaneously from the fluid and each other; and a device capable of promoting a reaction between at least the first product and the second product to thereby reform the reactant.
  • a system may optionally comprise other components which aid in and/or are required to operate the system.
  • tubing e.g., between container 2 in Figure 12 and storage containers, flow battery stack, etc.
  • membrane(s) e.g., between container 2 in Figure 12 and storage containers, flow battery stack, etc.
  • air inlet(s) and/or outlet(s) e.g., power and/or light management system(s)
  • compressor(s) e.g., 26, Figure 12
  • pump(s) e.g., 28, Figure 12
  • containers e.g., 18, 20, and 24 in Figure 12).
  • the systems and/or methods described herein involve the formation of a first product and a second product from a reactant contained in a fluid, wherein the first product and the second product can separate spontaneously from the fluid and/or each other. That is, the first product is formed in a phase, wherein the first product separates (e.g., via phase separation) from the second product and/or the fluid in which the reactant was provided. Similarly, the second product is suitably formed as a phase such that the second product separates (e.g., phase separates) from the first product and/or the fluid in which the reaction was provided.
  • suitable redox chemistries which will form products having suitable phases.
  • At least one product is a gas.
  • the gas may spontaneously separate from the fluid.
  • at least one product is a liquid, wherein the liquid is substantially immiscible or immiscible with the fluid in which the reactant is provided.
  • a liquid product has a density which is greater than the density of the fluid in which the reactant is provided. Thus, the liquid product may pool or otherwise collect as a separate phase from the fluid in which the reactant is provided.
  • at least one product is a solid, wherein the solid is able to be separated from the fluid in which the reactant is provided (e.g., wherein the solid is dense such that it accumulates in a portion of the container in which the fluid comprising the reactant is held).
  • the first product is a gas and the second fluid is a liquid which is immiscible or substantially immiscible with the fluid containing the reactant.
  • the fluid containing the reactant may optionally comprise one or more complexing agents which aid in the phase separation of a product.
  • an optional complexing may be employed which can limit the bromine vapor pressure and/or which aids in the formation of a phase separated fluid.
  • complexing agents for use with Br 2 include N-ethyl-N-methyl-morpholinium bromide (MEM), N-ethyl-N-methyl-pyrrolidinium bromide (MEP), and tetra-butyl ammonium bromide (TBA).
  • the complexing agent may be an compound which is commonly used/present in flow batteries. Other complexing agents may be used, and a user may also add a precipitating agent, a ligand, a chelating agent, or other like material so as to separate one product from another, or even to separate a product from the reactant.
  • Table 1 provides non-limiting examples of reactants and reactions which may be employed in a method/system of the present disclosure. The first product and the second product may be stored and/or recombined at some later time point to reform the reactant and/or to produce electricity, as described herein.
  • Table 2 provides non-limiting examples of particular combinations of oxidation/reduction reactions. As would be understood by those of ordinary skill in the art, the potentials provided in Tables 1 and 2 are standard state potentials and may vary depending on conditions, including, but not limited to changes in temperature, pressure, pH, and other conditions.
  • the formation of a first and a second product that separate from each other (e.g., spontaneously) and/or the fluid in which the reactant is provided may be advantageous, as the products can be easily collected and optionally stored.
  • spontaneous separation may reduce and/or limit the reduction of efficiency of the system which can be caused be recombination of the redox products (e.g., to reform HBr as opposed to the formation of H 2 and Br 2 ).
  • the systems and methods may be configured so as to increase the rate and/or ease of separation of the first product and the second product.
  • the system/method may make use of gravitational forces to aid in the separation of a solid or a liquid product having a density greater than the fluid in which the reactant is contained.
  • the container for the fluid which contains the reactant may be designed such that the container narrows at the bottom edge such that a fluid product is encouraged towards a small portion of the container.
  • the container comprising the fluid containing a reactant may be situated at an angle and/or may have a sloped bottom edge such that gravitational forces aid in the collection of the liquid product at a bottom edge/corner/point of the container (e.g., see Figure 12).
  • the bottom edge/corner/point of the container may comprise an output for the liquid product.
  • the container may be designed such that the container narrows at the top edge such that a gaseous product is encouraged towards a small portion of the container.
  • the container comprising the fluid containing a reactant may be situated at an angle and/or may have a sloped top edge such that the gaseous product (e.g., having a density greater than air) is drawn to an upper edge/corner/point of the container (e.g., see Figure 12).
  • the upper edge/corner/point of the container may comprise an output for the gaseous product.
  • the container comprising the fluid containing a reactant may be any suitable shape (e.g., square, rectangular, circle, ellipsoid, triangle, trapezoid, etc.) and may be formed of any suitable materials (e.g., polymer, glass, metal). Generally, at least one portion of the container is transparent or substantially transparent so as to allow exposure of the fluid contained in the container to electromagnetic radiation.
  • a photoactive material may be associated with at least one catalytic material.
  • the catalytic material may include a low-cost, earth-abundant material, or multiple materials of this type (and/or other materials), and/or the methods for forming the catalytic material may be carried out with ease.
  • the photoactive material is provided as a plurality of nanostructures (e.g., nanoparticles, nanocylinders, and the like) contained in the fluid (e.g., also containing the reactant), optionally associated with at least one catalytic material.
  • nanostructures and compositions which may be used in connection with a system and/or method of the present disclosure as a photoactive material are described in detail in U.S. Provisional Patent Application No. 61/478,364, filed April 22, 2011, entitled
  • Nanostructures, Systems, And Methods For Photocatalysis incorporated herein by reference in its entirety for all purposes. It should be understood that a nanostructure need not be perfectly spherical or regular in shape, as nanostructures may be oblong or irregular in form.
  • the disclosed systems and methods may operate with a disperse population of nanostructures, i.e., nanostructures that differ from one another in terms of size, shape, composition, or any of the foregoing.
  • a nanostructure may suitably comprise a semiconductor material, in which a bandgap exists by virtue of the chemical structure.
  • This bandgap may result from an intrinsic semiconductor chemical structure (e.g., Ti0 2 , CdS) or a doped semiconductor yielding a p-n junction (e.g. n or p-type silicon), as further described in United States patent application 61/478,364, and also in United States patent application 13/452,258, "Nanostructures, Systems, And Methods For Photocatalysis,” filed on April 20, 2012, both of which applications are incorporated herein in their entireties for all purposes.
  • an intrinsic semiconductor chemical structure e.g., Ti0 2 , CdS
  • a doped semiconductor yielding a p-n junction (e.g. n or p-type silicon)
  • a method comprises exposing a fluid (e.g., comprising a reactant) to electromagnetic radiation, wherein the exposure causes a catalytic redox reaction which produces at least a first product and a second product.
  • the first product and the second product may have different phases, wherein the first product and the second product separate spontaneously from the fluid and each other, as described herein.
  • a method comprises exposing a fluid containing particles (e.g., nanostructures, as described herein) to electromagnetic radiation (e.g., sunlight), thereby causing a redox reaction promoted and/or catalyzed by the particles which produces at least a first product and a second product, as described herein.
  • electromagnetic radiation e.g., sunlight
  • the performance of a system may be analyzed by determining the quantum efficiency.
  • quantum efficiency is given its ordinary meaning in the art and refers to a measure of the efficiency of the system for utilizing photons of a given energy to catalyze a given reaction.
  • quantum efficiency may be determined from measuring the monochromatic light power density and the rate of the photocatalyzed chemical reaction.
  • the quantum efficiency of a system is greater than about 1%, about 2%, about 5%, about 10%, about 20%, about 25%, about 30%, about 40%, about 50%, about 75%, about 100%.
  • the performance of the system may be analyzed by determining the efficiency of the system for utilizing AM 1.5 simulated sunlight to product a given product.
  • the first product and/or second product are produced at an efficiency of at least 1%, at least about 2%, at least about 3%, at least about 4%, at least about 5%, at least about 6%, at least about 7%, at least about 8%, at least about 9%, at least about 10%, at least about 15%, at least about 20% over the course of at least 24 hour.
  • the storage system may comprise a container, and optionally, a compressor (e.g., for storage of gaseous products).
  • Electromagnetic radiation may be provided by any suitable source.
  • electromagnetic radiation may be provided by sunlight and/or an artificial light source.
  • the electromagnetic radiation is provided by sunlight.
  • light may be provided by sunlight at certain times of operation of a system (e.g., during daytime, on sunny days, etc.) and artificial light may be used at other times of operation of the system (e.g., during nighttime, on cloudy days, etc.).
  • Non-limiting examples of artificial light sources include a lamp (mercury-arc lamp, a xenon-arc lamp, a quartz tungsten filament lamp, etc.), a laser (e.g., argon ion), and/or a solar simulator.
  • the spectra of the artificial light source may be substantially similar or substantially different than the spectra of natural sunlight.
  • the light provided may be infrared (wavelengths between about 1 mm and about 750 nm), visible (wavelengths between about 380 nm and about 750 nm), and/or ultraviolet (wavelengths between about 10 nm and about 380 nm).
  • the electromagnetic radiation may be provided at a specific wavelength, or specific ranges of wavelengths, for example, through use of a monochromatic light source or through the use of filters.
  • the power of the electromagnetic radiation may also be varied.
  • the light source provided may have a power of at least about 100 W, at least about 200 W, at least about 300 W, at least about 500 W, at least about 1000 W, or greater. The formation and properties of the composition are described herein.
  • a system may include a light management system and/or solar concentrator, which are capable of focusing electromagnetic radiation and/or solar energy.
  • light management systems or solar concentrators may receive electromagnetic radiation and/or solar energy over a first surface area and direct the received radiation to a second, smaller, surface area.
  • Light management systems and solar concentrators will be known to those of ordinary skill in the art and may comprise, for example, mirrors, magnifying lenses, parabolic mirrors, and/or Fresnel lenses for focusing incoming light and/or solar energy.
  • the light management system or solar collector may collect and act as a waveguide to deliver light to an area or surface of the system, for example, a surface associated with the catalytic material, to a nanostructure, or to other locations.
  • Ambient conditions define the temperature and pressure relating to the system and/or method.
  • ambient conditions may be defined by a temperature of about 25°C and a pressure of about 1.0 atmosphere (e.g., 1 atm, 14 psi).
  • the systems and/or methods as described herein may proceed at temperatures above ambient temperature.
  • a system and/or method may be operated at temperatures greater than about 30 °C, greater than about 40 °C, greater than about 50 °C, greater than about 60 °C, greater than about 70 °C, greater than about 80 °C, greater than about 90 °C, greater than about 100 °C, greater than about 120 °C, greater than about 150 °C, greater than about 200 °C, or greater.
  • Efficiencies can be increased, in some instances, at temperatures higher than ambient.
  • systems and/or methods as described herein may proceed at temperatures below ambient temperature, although the disclosed systems and methods may of course operate at ambient conditions.
  • the systems and methods as described herein may be carried out at elevated pressures.
  • elevated pressures include at least about 1.5 atm, at least about 2 atm, at least about 3 atm, at least about 5 atm, at least about 10 atm, at least about 20 atm, at least about 50 atm, at least about 100 atm, at least about 200 atm, or greater.
  • the pressure is between about 1 atm and about 200 atm, between about 1 atm and about 100 atm, between about 10 atm and about 100 atm, between about 50 atm and about 200 atm, or between about 100 atm and about 200 atm.
  • An electrolyte as known to those of ordinary skill in the art is any substance containing free ions that is capable of functioning as an ionically conductive medium.
  • the fluid containing the reactant and/or photoactive material may comprisean electrolyte.
  • the electrolyte may be a liquid, a gel, and/or solid.
  • the electrolyte may also comprise methanol, ethanol, sulfuric acid, methanesulfonic acid, nitric acid, mixtures of HQ, organic acids like acetic acid, and the like.
  • the electrolyte comprises mixtures of solvents, such as water, organic solvents, amines and the like.
  • the pH of the electrolyte may be about neutral.
  • the pH of the electrolyte may be between about 5.5 and about 8.5, between about 6.0 and about 8.0, about 6.5 about 7.5, and/or the pH is about 7.0. In a particular case, the pH is about 7.0. In other cases, the pH of the electrolyte is about neutral or acidic. In these cases, the pH may range from about 0 to about 8, about 1 to about 8, about 2 to about 8, about 3 to about 8, about 4 to about 8, about 5 to about 8, about 0 to about 7.5, about 1 to about 7.5, about 2 to about 7.5, about 3 to about 7.5, about 4 to about 7.5, about 5 to about 7.5.
  • the pH may be between about 6 and about 10, about 6 and about 11, about 7 and about 14, about 2 and about 12, and the like. In a specific embodiment, the pH is between about 6 and about 8, between about 5.5 and about 8.5, between about 5.5 and about 9.5, between about 5 and about 9, between about 3 and about 11, between about 4 and about 10, or any other combination thereof.
  • Water may be provided to the systems and/or for the methods provided herein, using any suitable source.
  • the water is provided from a substantially pure water source (e.g., distilled water, deionized water, chemical grade water, etc.).
  • the water may be bottled water.
  • the water is provided from a natural and/or impure water source (e.g., tap water, lake water, river water, ocean water, rain water, lake water, pond water, sea water, potable water, brackish water, industrial process and/or waste water, etc.).
  • the water is not purified prior to use (e.g., before being provided to the system).
  • photocatalysis refers to a process of the acceleration of a photoreaction in the presence of a photocatalyst.
  • the methods and/or systems may solely use solar energy (e.g., sunlight) as the power source.
  • Electriclysis refers to the use of an electric current to drive an otherwise non-spontaneous chemical reaction.
  • electrolysis may involve a change in redox state of at least one species and/or formation and/or breaking of at least one chemical bond, by the application of an electric current.
  • Electrolysis of water may involve splitting water into oxygen gas and hydrogen gas, or oxygen gas and another hydrogen- containing species, or hydrogen gas and another oxygen-containing species, or a combination thereof.
  • Photoelectrolysis utilizes electromagnetic radiation to drive an otherwise non- spontaneous chemical reaction. That is, photoelectrolysis is the conversion of light into a current, and then the division of a molecule using that current.
  • the present disclosure provides methods. These methods include exposing, to electromagnetic radiation, a reactant in contact with a photocatalytic material so as to effect a catalytic redox reaction that produces at least a first product and a second product in different phases.
  • the reactant may have the photocatalytic material dispersed within. This may be in the form of a suspension, colloid, mixture, and the like, where particles (e.g., nanoparticles) of the photocatalytic material are disposed within the fluid.
  • the photocatalytic material may also be present on the walls of a container within which the reactant is disposed.
  • the photocatalytic material may also be disposed on baffles, stirrers, projections, or other structures that contact the reactant, as there is no requirement that the photocatalytic material be in particulate form.
  • Photocatalytic material that is in nanostructure form e.g., a body having a cross-sectional dimension in the range of from about 1 nm to about 100 nm
  • the reactant need not be perfectly pure, as the fluid may include one or more impurities.
  • impurities may be non-participative in the reaction.
  • An impurity may be dirt, dust, sand, or other material.
  • the methods may be performed such that first product and the second product separate from each other. This may be effected in embodiments when the first and second products are in different phases from one another.
  • the first product may be a solid, and the second product may be in a liquid state.
  • the first product may be in a gas state, while the second product is in a solid state.
  • the first and second products may also be immiscible liquids.
  • at least one of the first and second product may separate from the reactant. This facilitates recovery of the product or products that have separated from the reactant.
  • the first product can be any of a solid, liquid or gas
  • the second product may be any of a solid, liquid, or gas.
  • reactant can refer to a fluid that is or comprises a material that undergoes a photocatalytic process to produce the first and second products.
  • HBr is considered an especially suitable reactant, but the present disclosure is not limited to HBr.
  • suitable reactants include, for example, ZnCl, sodium iodide sulfate, vanadium sulfate, nickel cerium hydroxide, water, vanadium quinone, and the like.
  • the reactant may comprise metal (e.g., an alkalai metal, such as sodium), halogen (e.g., CI, I, F, Br), or even both of these.
  • the reactant suitably comprises material (such as a reactant) capable of being photochemically processed to firm at least two products, with the products in turn suitably being formed in separate phases or being immiscible with one another.
  • the disclosed methods further include, in some embodiments, separating the first product and second product. This may be effected by gravity, decanting, a membrane, condensation, a chemical reaction, or any combination thereof.
  • separating the first product and second product This may be effected by gravity, decanting, a membrane, condensation, a chemical reaction, or any combination thereof.
  • Figure 12 shows a vessel from which gas product (H 2 ) and liquid product (B3 ⁇ 4) are removed.
  • gas product (H 2 ) and liquid product (B3 ⁇ 4) are removed.
  • the illustration of the system using HBr is exemplary only and does not act to limit the present disclosure to HBr only.
  • the electromagnetic radiation applied in he disclosed methods may be natural, such as sunlight.
  • the electromagnetic radiation may also be synthetic, such as incandescent light or fluorescent light.
  • the electromagnetic radiation need not be visible, as infrared radiation or even ultraviolet light may also be used in the disclosed methods.
  • the disclosed methods also include, as described elsewhere herein, reforming the reactant from the first product and second products so as to effect generation of electricity.
  • This reforming may be effected by an electrochemical cell, such as a fuel cell (e.g., a proton exchange membrane fuel cell or similar device) or flow battery.
  • a fuel cell e.g., a proton exchange membrane fuel cell or similar device
  • flow battery e.g., a flow battery
  • photocatalytic materials may be used, including those set forth in United States Patent Application 13/452,258, "Nanostructures, Systems, And Methods For Photocatalysis," filed on April 20, 2012, incorporated herein in its entirety for any and all purposes.
  • Photocatalytic materials that comprise a semiconductor are considered especially suitable for the disclosed methods, including those described in United States patent application 13/452,258, “Nanostructures, Systems, And Methods For Photocatalysis," filed on April 20, 2012; United States patent application 12/576,066; United States patent application 12/486,694; and United States patent application 12/870,530, all of which are incorporated herein by reference in their entireties for all purposes.
  • One method for selecting photocatalytic materials and reactants is as follows.
  • the user may select a photocatalytic material (e.g., a semiconductor), in which the potential of the conduction band is more negative than the reduction potential of the reducing half-redox reaction (of the reactant), and in which the valence band potential is more positive than the reduction potential of the oxidizing half-redox reaction.
  • a photocatalytic material e.g., a semiconductor
  • the potential of the conduction band is more negative than the reduction potential of the reducing half-redox reaction (of the reactant)
  • the valence band potential is more positive than the reduction potential of the oxidizing half-redox reaction.
  • the photocatalytic material would also suitably possess a potential for the valence band that is more positive than the reduction potential of Br 2 ;
  • a semiconductor having these properties is titanium dioxide (Ti0 2 ), which has a conduction band potential of -0.05 V vs. NHE (i.e., more negative than the reduction potential of H + ) and a valence band potential of 3.15 V vs. NHE (i.e., more positive than the reduction potential of Br 2 ).
  • One semiconductor with these properties is silicon (Si), which has a conduction band potential of -0.65 V vs. NHE and a valence band potential of 0.55 V vs. NHE.
  • the present disclosure also provides methods. These methods suitably include exposing a fluid in contact with a photocatalytic material to electromagnetic radiation (e.g., sunlight) so as to effect a redox reaction that produces at least a first product and a second product at an efficiency of at least 1% over the course of at least 24 hours.
  • electromagnetic radiation e.g., sunlight
  • the efficiency of 1% means that at least 1% of the incident irradiance (e.g., solar radiation, 1 kW/m 2 peak flux) is converted into stored chemical energy (rate of production of chemicals per unit area x Voltage stored in the chemical reaction (delta E) > 0.01 x lkW / m 2 ).
  • incident irradiance e.g., solar radiation, 1 kW/m 2 peak flux
  • stored chemical energy rate of production of chemicals per unit area x Voltage stored in the chemical reaction (delta E) > 0.01 x lkW / m 2 ).
  • the present disclosure also provides systems. These systems suitable include a vessel (e.g., a tank, barrel, or other container) that contains a reactant in contact with a photocatalytic material, the photocatalytic material being capable of, when exposed to electromagnetic radiation, effecting a redox reaction that produces from the reactant at least a first product and a second product; and a device capable of promoting a reaction between at least the first product and the second product so as to produce electricity.
  • a vessel e.g., a tank, barrel, or other container
  • a photocatalytic material being capable of, when exposed to electromagnetic radiation, effecting a redox reaction that produces from the reactant at least a first product and a second product
  • a device capable of promoting a reaction between at least the first product and the second product so as to produce electricity.
  • Suitable reactants are described elsewhere herein, as are suitable photocatalytic materials.
  • the reaction between the first product and the second product may comprise reforming the reactant.
  • the use may then re-expose the reactant to electromagnetic radiation, which exposure in turn produces photocatalytic products from the reactant, which product may in turn be separated and then used to again reform the reactant and generate electricity.
  • the products may be stored until needed. In this way, a user may operate the disclosed systems in a cyclic manner.
  • the system may even, depending on the user's needs and the scale of the system, be operated in a batch manner or even semi-continuously or continuously such that reactant that has been photocatalytically processed into products for future reformation is replaced by fresh reactant that is itself then been photocatalytically processed into products for future reformation.
  • the reactant and photocatalytic material may comprise a suspension of the material within the fluid. It is not necessary that the photocatalytic material be in suspension within a fluid, as the system may include baffles, filters, porous monoliths, posts, and the like of photocatalytic material that may contact the fluid.
  • the vessel may be in fluid communication with a device capable of separating the first product and second product from one another.
  • a device capable of separating the first product and second product from one another.
  • a device may be a funnel, a filter, a condenser, and the like.
  • the system may, as described above, include a device capable of promoting a reaction between at least the first product and the second product so as to produce electricity.
  • a device capable of promoting a reaction between at least the first product and the second product so as to produce electricity.
  • Such devices may be electrochemical cells, fuel cells (e.g., membrane fuel cells), flow batteries, and the like, as describe elsewhere herein.
  • the first and second products may be immiscible with one another; the first and second product may also reside in different phases, as described elsewhere herein.
  • the present disclosure also provides components.
  • the components suitably include
  • a vessel comprising a reactant and a photocatalytic material, the photocatalytic material being adapted to, when exposed to electromagnetic radiation, effect a redox reaction on the reactant that produces at least a first product and a second product in different phases.
  • the photocatalytic material may be in nanostructure form.
  • the optimal process conditions will depend on the needs of the user.
  • the photocatalytic material particularly in embodiments where the material is dispersed within a fluid, is suitably present in the range of from about 0.01 - 10 mg photocatalyst/mL of fluid.
  • the reactant may be present in a variety of ranges.
  • a reactant e.g., HBr
  • the intensity of electromagnetic radiation may also vary depending on environmental conditions and also on the user's needs; irradiance (e.g., solar) may in some embodiments be in the range of from 400 - 1500 kW/m 2
  • a system may produce a range of electricity, depending on the size and configuration of the system. For example, systems according to the present disclosure may product from about 1 W to about 1 MW of electricity, or from about 10 W to about 100 kW of electricity, or even from about 100 W to about 1 kW of electricity.
  • the solar panel is clear plastic container 2 which contains a suspension of colloidal semiconductor particles 8 for solar illumination.
  • the particles may be provided to container 2 from storage container 24 comprising an HBr solution.
  • illumination of the particles in the suspension results in electron-hole pair formation within the particle.
  • the electron-hole pair can reacts with the HBr aqueous phase to generate gaseous hydrogen and liquid bromine.
  • Optional complexing agents used in flow batteries using bromine such as N-ethyl-N-methyl-morpholinium bromide (MEM), N-ethyl-N-methyl-pyrrolidinium bromide (MEP), or tetra-butyl ammonium bromide (TBA) may be used to limit the bromine vapor pressure (e.g., in so far as blocking of sunlight does not occur) and/or generate a fluid which is substantially immiscible with the aqueous HBr solution.
  • MEM N-ethyl-N-methyl-morpholinium bromide
  • MEP N-ethyl-N-methyl-pyrrolidinium bromide
  • TAA tetra-butyl ammonium bromide
  • the semiconductor particles can be designed so that the bandgap of the particle results in 1) efficient absorption of the solar spectrum and/or 2) the generation of a) electrons with sufficient reducing potential for hydrogen evolution and b) holes of sufficient oxidizing potential for bromine evolution.
  • Kinetics may also be enhanced by the disposition of hydrogen-evolving and oxygen-evolving catalytic materials associated with the particle (e.g., on the surface).
  • the housing/container may be designed such that a gravitational separation of the gaseous hydrogen and the immiscible bromine phase occurs. Effecting a rapid separation is important in minimizing direct contact of the hydrogen and bromine which could result in self-discharge.
  • the hydrogen may be compressed via external compression to appropriate pressures for storage in tanks.
  • the immiscible bromine phase can be collected and stored in a separate tank.
  • Vessels and containers used in the disclosed systems and methods may comprise glass, polymer, or other suitable materials.
  • the colloidal suspension can be repeatedly circulated until adequate charging of the solutions occurs.
  • the "charged" photochemical reaction products can be fed to a cell or even a cell-stack structure; the reaction products may be stored (e.g., in storage vessels) until needed.
  • This stack may comprise a plurality of cells through which the suspension may be circulated to serve as the electrolyte.
  • hydrogen gas and the bromine phase can be introduced to the stack at the anode and cathode, respectively, to facilitate discharge in the form of DC electricity. Circulation may be effected by a pump, gravity, or other methods.
  • Figure 2 illustrates hydrogen produced in a dispersion of 5mg Pt-CdS illuminated in 3mL 0.24M Na 2 S and 0.35 M Na 2 S0 3 versus M 0 of AM 1.5 filtered light from a 150 W Xe arc lamp.
  • Figure 3 illustrates hydrogen production at various concentrations of Pt-CdS in 3mL 0.24M Na 2 S and 0.35 M Na 2 S0 3 under AM 1.5 filtered illumination.
  • the present disclosure generally relates to nanostructures and related compositions, methods of making and using the nanostructures, and related systems. Many aspects and embodiments of the disclosure involve nanometer-scale articles and systems, but larger articles and systems are provided as well.
  • the subject matter of the present disclosure involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.
  • nanostructures are provided for a variety of applications.
  • the nanostructures may be used for photocatalytic reactions.
  • Photocatalysis e.g., involving an oxidation and/or reduction reaction
  • the nanostructures may be used for photocatalytic reactions.
  • Photocatalysis e.g., involving an oxidation and/or reduction reaction
  • nanostructures allow for the conversion of water to hydrogen gas and/or oxygen gas using at least one nanostructure and without an external power and/or energy source.
  • energy can be stored, via a reactive pathway involving nanostructures of the disclosure, in the form of oxygen gas and hydrogen gas. It should be understood that a user may expose one or even a plurality of nanostructures according to any part of the present disclosure to
  • a user may expose one or more nanostructures according to any part of the present disclosure so as to catalyze a reduction reaction.
  • photochemical of water splitting is discussed in many embodiments described herein, this is by no means limiting, and other photochemical reactions may be carried out using the nanostructures, compositions, methods, and systems of the present disclosure, as described herein. These reactions include, but are not limited to, the conversion of nitrogen to ammonia, the conversion of hydrohalic acids to hydrogen and halide (e.g. 2HBr ⁇ 3 ⁇ 4 + !3 ⁇ 4), the conversion of methanol to hydrogen and an oxidized form of carbon, and the conversion of carbon dioxide to carbon monoxide and/or the conversion of and/or formation of any number hydrocarbon fuels (e.g., methane, ethane, etc.).
  • Nanostructures of the disclosure can comprise at least one catalytic material.
  • the catalytic material may include a low-cost, earth-abundant material, or multiple materials of this type (and/or other materials), and the methods for forming the catalytic material may be carried out with ease.
  • many of the methods and systems may be operated under ambient conditions (e.g., temperature, pressure) and at about neutral pH, with good efficiency.
  • water for photocatalysis may be supplied from an impure water source. Accordingly, because the systems can be operated under mild conditions, the system components may be greatly simplified as compared to systems which are not operated under mild conditions.
  • a photocatalytic system for water splitting may comprising a plurality of nanostructures suspended in impure water in a plastic bag, and the electrolysis may occur upon exposure of the nanostructures to sunlight. Further benefits of the nanostructures, compositions, methods, and systems are described herein.
  • nanostructure refers to structures which are nanoscopic.
  • nanostructure-scale refers to structures which are nanoscopic.
  • nanoscale-scale refers to structures which are nanoscopic.
  • nanoscale generally refers to elements or articles having widths or diameters of less than about 1 micron.
  • the specified widths can be a smallest width (i.e. a width as specified where, at that location, the article can have a larger width in a different dimension), or a largest width (i.e. where, at that location, the article has a width that is no wider than as specified, but can have a length that is greater).
  • nanostructures include nanowires, nanorods, nanotubes, nanoparticles, etc., as describe herein.
  • a nanostructure may comprise one or more materials and/or components.
  • nanostructures of the present disclosure operate according to the following mechanism.
  • charge separation may occur between the first region and the second region, wherein an excess of electrons are present in the first region and an excess of holes are present in the second region.
  • a nanostructure comprising a first region defining primarily or exclusively a first semiconductor material and a second region defining primarily or exclusively a second semiconductor material
  • electrons may be excited from the valence band to the conduction band in the first region (e.g., comprising an n-type semiconductor), thereby creating holes in the valence band and free electrons in the conduction band.
  • the excited electron and corresponding electron-hole may separate spatially within the n-type semiconductor material from the point of generation.
  • the electrons produced at the first region may be transported (e.g., via the nanostructure material) to the second region of the nanostructure (e.g., comprising a p-type semiconductor).
  • first and second regions are both semiconductors are considered especially suitable.
  • the first and second regions need not be the same semiconductor.
  • the separated electron-holes may be transported to a nanostructure- electrolyte interface at each region where they may be used to carry out an electrochemical reaction, e.g., a redox reaction.
  • the holes/electrons can react with a water molecule, resulting in the formation of oxygen gas and/or hydrogen ions.
  • the nanostructure may include a first region that catalyzes the formation of 02 from water.
  • the nanostructure may also include a second region that catalyzes the formation of one or more of H2, a reduced form of N2, or even a reduced form of C02.
  • Suitable reduced forms of N2 include NH3, NH4+ ions, N2H4, urea, and the like.
  • Suitable reduced forms of C02 include CO, formic acid, formate, formaldehyde, hydrocarbons, and even alcohols. Some such materials include methane, ethylene, ethanol, methanol, and the like.
  • the holes/electrons can react with a halide ion (e.g., Br-; alternatively in the form HBr), resulting in the formation of halogen (e.g., B3 ⁇ 4; or H2 and Br2).
  • Nanostructures of the disclosure are composed of a bulk composition or material, and a surface-presenting catalytic material (which can be the same or different than the underlying bulk material).
  • a surface-presenting catalytic material which can be the same or different than the underlying bulk material.
  • another material hereafter referred to as a shell material, may cover at least a portion of the surface of the bulk composition and may, in some cases, partially or fully separate the bulk material from the surface-presenting catalytic material.
  • the bulk material in most embodiments, is a composition including at least one type of semiconductor material.
  • the nanostructure comprises a semiconductor material with dopants which are primarily n-type in nature in a first region and dopants which are primarily p-type in nature in a second region.
  • the nanostructure is a nanorod
  • one region (e.g., end) of the nanorod can exhibit more n-type dopants with a second region (e.g., the other end) exhibiting more p-type dopants.
  • “Bulk” in this context means a majority of the mass of the nanostructure including, or excluding, a small region near the surface of the nanostructure.
  • the bulk can define at least 60%, 70%, 80%, 90%, or greater the mass of the nanostructure, typically the central portion of the nanostructure extending outwardly toward the surface.
  • the makeup of the bulk composition of the nanostructure defines a non-uniform semiconductor characteristic.
  • the nanostructure includes two relatively discrete sections, one comprising a primarily n-type semiconductor material and the other comprising a primarily p-type semiconductor material, with a relatively abrupt transition between those two semiconductor materials.
  • a nanorod can be made up of essentially uniform n-type semiconductor material along a portion of its length and then, relatively abruptly, the material can shift to p-type semiconductor material and remain relatively uniformly p-type semiconductor material for the remainder of its length.
  • a transition between any n-type or p-type semiconductor material is not abrupt and the structure defines a first section of relatively uniform n-type semiconductor material, a second section of relatively p-type semiconductor material, and an intermediate section involving different material or a transition, gradual, or otherwise, between the n-type and p-type semiconductor material sections.
  • various sections of nanostructure bulk can exist, with different characteristics of any number of them, so long as, overall, the nanostructure exhibits primarily n-type dopants at one section and primarily p-type dopants at a different section.
  • the nanostructure bulk is primarily (but not exclusively) p-type at one end, primarily (but not exclusively) n- type at another end, with a transition between those ends from n-type to p-type where the transition can represent a mixture of the two semiconductor types in changing ratio along the pathway from the first end to the second end.
  • the transition can be linear (i.e., essentially consistently gradual) or can be non-linear as would be understood by those of ordinary skill in the art.
  • the disclosure contemplates any number of other arrangements of n-type and p-type material within the bulk of the nanostructure that provide a result suitable for use in the disclosure.
  • the bulk material may be silicon with a dopant or dopants (e.g. germanium), the concentration of which varies from one end or region of the nanostructure to another end or region of the nanostructure.
  • the changes in concentration may be abrupt, such as in a multi-junction Si nanorod, between neighboring regions of the nanostructure.
  • the changes in concentration may be abrupt, such as in a multi-junction Si nanorod, between neighboring regions of the nanostructure.
  • concentration of the dopant may vary smoothly or in a more linear fashion from one region of the nanostructure to another region.
  • compositions suitable for use as a bulk material see FIGS. 2A-2E and additional configurations described herein.
  • any number of materials can substitute from n-type and/or p-type semiconductor material so long as the bulk (in this set of embodiments in which bulk composition varies) can be used to establish a nanostructure particle allowing different photocatalytic reactions to occur at different sections of the nanostructure.
  • the first region may comprise a first type of semiconductor material and the second region may comprise a second type of semiconductor material, wherein the first type of semiconductor material and the second type of semiconductor materials do not differ only by the dopant (e.g., the materials are not simply a semiconductor material doped with a first dopant to produce the n-type material and the same or essentially similar semiconductor material doped with a second dopant to produce a p-type material).
  • the first type of semiconductor material may be CdSe and the second type of semiconductor material may be CdS.
  • a nanorod can have an essentially uniform bulk with one end (e.g., a first region) of the nanorod having a surface-presenting catalytic material that is present in less quantity, or essentially absent from the other end (e.g., a second region) of the nanorod.
  • one end (e.g., a first region) of the nanorod can carry a first surface-presenting catalytic material and the other end (e.g., a second region) of the nanorod can carry a second, different, surface-presenting catalytic material.
  • the entire nanorod can include, uniformly or nonuniformly, a surface-presenting catalytic material that differs in composition across the nanostructure, e.g., the nanostructure might include a first material and a second material (and, optionally, additional materials) with the first material present predominantly at a first section (e.g., a first region) of the nanostructure surface and a second material present predominantly at a second section (e.g., a second region) of the
  • the nanostructure surface with a mixture of the first and second materials present at a third section of the nanostructure surface can exist essentially throughout the surface of the nanostructure but in different ratios such that the first material is more prevalent at one section or region (e.g., one end) of the nanostructure and the second material is more prevalent at another section or region (e.g., the opposite end) of the nanostructure.
  • the first and second materials can be catalytic materials that catalyze different oxidation and/or reduction reactions, respectively, and/or can be materials which can attract and/or bind, in later synthetic steps and/or within a photocatalytic reaction itself, catalytic materials.
  • the first and/or second materials can be both provided
  • the bulk of the nanostructure can include one section or region (e.g., one end) that is predominantly an n-type semiconductor material and another section or region (e.g., another end) that is predominantly a p-type semiconductor material, and one or both of the n-type and/or p-type sections can carry a surface-presenting catalytic material that is different from the majority of the n-type and/or p- type bulk.
  • Surface-presenting catalytic materials are described herein, and in some cases, may comprise photosensitizing agent and/or a catalytic material
  • an optional shell material can be provided between the bulk of the nanostructure and any surface-presenting catalytic material.
  • the shell can be selected to provide any of a variety of functionalities, as will be understood by those of ordinary skill in the art upon reading the description of this disclosure, which may be advantageous in carrying out the disclosure.
  • the shell can be made of material selected to inhibit corrosion of a bulk semiconductor material of the nanostructure. A variety of such materials can be used for the shell, as those of ordinary skill in the art would understand.
  • the shell may be a semiconductor material that is transparent to visible light, including, but not limited to,Ti0 2 , SrTi0 3 , ZnO, indium tin oxide (ITO), fluorine tin oxide (FTO), Ce0 2 , S1O2,
  • the shell may be a polymer that is transparent to visible light, such as PTFE.
  • a shell of titanium dioxide that covers portions of the nanostructure bulk i.e., which separates portions of the semiconductor bulk from a surrounding medium and/or from a surface-presenting catalytic material, can be selected.
  • the shell coats portions or all of the nanostructure bulk.
  • a semiconductor bulk can be presented (e.g., comprising a predominantly n-type semiconductor material in a first region or section and a predominantly p-type semiconductor material is a second region or section) and coated with a shell of titanium dioxide or other material selected to prevent corrosion of the semiconductor materials.
  • Surface-presenting catalytic material can exist upon one or more sections of the shell, as described above with respect to the arrangement to surface-presenting catalytic materials on sections of the bulk material.
  • the shell may be selected to be of a composition and thickness so as to be sufficiently transparent to electromagnetic radiation at a wavelength that activates catalytic activity of the bulk nanostructure material, and of a conductivity that allows electron and/or hole transport between the bulk
  • the shell is selected, both in material composition and thickness, to be transparent in a direction orthogonal to the surface to electric magnetic radiation at a wavelength promoting photocatalysis in an amount of at least 50%, 60%, 70%, 80%, or 90% transmittance.
  • FIGS. 2F-2J Non-limiting examples of suitable arrangements for a nanostructure of the present disclosure are shown in FIGS. 2F-2J.
  • each of the bulk materials 100 in FIGS. 2F-2J may comprise a single or any number of combinations of materials, for example, those described throughout the specification, included those depicted in FIGS. 2A-2E.
  • a shell material is optional in each case, and may or may not completely or evenly coat the bulk of the nanostructure. Further, where surface-presenting catalytic materials are illustrated, they can but need not define a relatively even coating on the bulk and/or on the shell.
  • FIG. 2F is an illustration of one embodiment, including a nanostructure comprising bulk material 100 and shell material 102.
  • FIG. 2G is an illustration of an embodiment including a nanostructure comprising bulk material 100, shell material 102, and first type of catalytic material 104.
  • FIG. 2G is a non-limiting illustration of a nanostructure comprising bulk material 100, shell material 102, first type of catalytic material 104, and second type of catalytic material 106.
  • FIG. 2H is a non-limiting illustration of a nanostructure comprising bulk material 100, shell material 102, first type of catalytic material 104, second type of catalytic material 106, and first type of photosensitizing agent 108 provide between first type of catalytic material 104 and shell material 102.
  • FIG. 2 J is a non-limiting illustration of a nanostructure comprising bulk material 100, shell material 102, first type of catalytic material 104, second type of catalytic material 106, first type of photosensitizing agent 108 provided between first type of catalytic material 104 and shell material 102, and second type of photosensitizing agent 1 10 provided between second type of catalytic material 106 and shell material 102.
  • FIGS. 2F-2J are in no way limiting, and other suitable arrangements will be understood by those of ordinary skill of the art.
  • another suitable arrangement includes the nanostructure depicted in FIG. 2J, without the inclusion of shell 102.
  • FIGS. 2F-2J are depicted as blunt, this is by no means limiting, and similar arrangements may be provided where at least some or all of the boundaries are blended or gradual.
  • the bulk material can be essentially homogenous or can include separate materials and/or sections, a shell can be used but need not be, and the nanostructure can include one or more surface-presenting catalytic materials different from the bulk of the nanostructure.
  • surface-presenting catalytic materials they need not coat the nanostructure in a way that would separate the bulk of the nanostructure from a surrounding medium, but can be provided in an island-like manner, a pattern, or otherwise partially (or fully) cover the bulk nanostructure.
  • the element composition of a nanostructure may be assessed using electron microscopy (e.g., transmission electron microscopy) in combination with energy dispersive x-ray spectroscopy.
  • electron microscopy e.g., transmission electron microscopy
  • a nanostructure as described herein may be used for photocatalysis.
  • photocatalysis refers to a process of the acceleration of a photoreaction in the presence of a photocatalyst.
  • a nanostructure as described herein may be used for the photoelectrolysis of water (or other materials), and in some cases, covert light to electrical energy. In some cases, the
  • Electrolysis refers to the use of an electric current to drive an otherwise non-spontaneous chemical reaction.
  • electrolysis may involve a change in redox state of at least one species and/or formation and/or breaking of at least one chemical bond, by the application of an electric current.
  • Electrolysis of water can involve splitting water into oxygen gas and hydrogen gas, or oxygen gas and another hydrogen-containing species, or hydrogen gas and another oxygen-containing species, or a combination.
  • Photoelectrolysis utilizes electromagnetic radiation to drive an otherwise non-spontaneous chemical reaction. That is, photoelectrolysis is the conversion of light into a current, and then the division of a molecule using that current.
  • oxidation-reduction reaction means a chemical reaction between two species involving the transfer of at least one electron from one species to the other species. This type of reaction is also referred to as a "redox reaction.”
  • the oxidation portion of the reaction involves the loss of at least one electron by one of the species, and the reduction portion involves the addition of at least one electron to the other species.
  • FIG. 1A illustrates nanostructure (e.g., nanorod) 31 comprising first region 30 and second region 32.
  • the nanostructure comprising a bulk material having first region 30 comprising an n-type semiconductor material and second region 32 comprising a p-type semiconductor material.
  • holes propagate to the n-type semiconductor material, as indicated by arrow 36 and electrons propagate to the p-type semiconductor material, as indicated by arrow 38, as shown in FIG. IB.
  • the holes in the n- type semiconductor material may be used for an oxidation reaction, for example, the production of oxygen gas from water, as indicated by arrow 40 in FIG. 1C, and the electrons in the p-type semiconductor material may be used for a reduction reaction, for example, the production of hydrogen gas from water, as indicated by arrow 44 in FIG. 1C.
  • Other suitable reduction reactions include the formation of H2, the reduction of C02, the reduction of N2, and the like.
  • a bulk material may comprise two or more regions having different compositions.
  • regions differing in composition may comprise different materials or elements, phases or crystallographic orientations or other structural differences so as to provide different behavior, or they may comprise the same materials or elements, but at different ratios or concentrations.
  • Each region may be of any size or shape within the bulk material.
  • FIG. 2A illustrates bulk material (e.g., nanorod) 1 1 having first region 10 and second region 12 of approximately equal lengths and sizes, wherein the junction between the first and second region is abrupt.
  • FIG. 2B illustrates bulk material (e.g., nanorod) 15 having a first region 14 and second region 16 having different sizes and lengths, wherein the junction between the first region and the second region is abrupt.
  • FIG. 2C illustrates bulk material (e.g., nanorod) 17 having a gradual change between first region 18 and second region 20.
  • a bulk material may comprise more than two regions.
  • the first region and the second region may be separated by a third region, wherein the third region comprises a material (e.g., a conductive material) which allows for electrical communication between the first region and the second region.
  • a material e.g., a conductive material
  • bulk material (e.g., nanorod) 25 comprises first region 22, second region 24, and third region 26.
  • the bulk material may comprise a plurality of regions (e.g., in the case of a multi-junction semiconductor substrate).
  • bulk material (e.g., nanorod) 26 comprises a multi- junction layout (e.g., multi-junction or triple-junction silicon), where areas 27 comprise an n- type semiconductor material, areas 28 comprise a i-type semiconductor material, and areas 29 comprise a p-type semiconductor material.
  • the bulk material comprises a semiconductor material. In some embodiments, the bulk material comprises a single or substantially uniform semiconductor material. In some embodiments, the bulk material comprise more than one type of semiconductor material. For example, in some cases, the first region of a bulk material comprises a first type of semiconductor material and the second region of a bulk material comprises a second type of semiconductor material. In some cases, the first type of semiconductor material and the second type of semiconductor material comprise similar types of semiconductor materials, but the first region comprises the semiconductor material which has been n-doped and the second region comprises the semiconductor material which has been p-doped.
  • the first type of semiconductor material and the second type of semiconductor materials are different (e.g., are not an n-type doped and a p-typed doped version of a single semiconductor material).
  • the materials may be selected such that a junction (e.g., a heterojunction or a p/n-junction, as described herein) is formed between the first region and the second region.
  • junctions may form between two or more different semiconductor materials of a bulk material.
  • the junction is a p/n-junction, wherein the bulk material is provided comprising a first region and a second region, wherein the first region comprises an n-type semiconductor material and the second region comprises a p-type semiconductor material. That is, the bulk material comprises a semiconductor material, wherein a first portion of the semiconductor material is doped with an n-type element and a second portion of the semiconductor material is doped with a p-type element material.
  • the nanostructure comprises two different semiconductor materials, and a heterojunction is formed.
  • junctions are p/p junctions, n/n junctions, p/i junctions (where i refers to an intrinsic semiconductor), n/i junctions, i/i junctions, and the like.
  • bulk material having more than one junction between two regions having different compositions are also contemplated.
  • a nanostructure may have 2, 3, 4, or more overlap regions.
  • the number of periods and the repeat spacing may be constant or varied during growth.
  • the bulk semiconductor material comprises a multiple junctions, for example a triple junction.
  • the bulk material comprises a triple junction silicon nanostructure (e.g., nanorod). Multi-junction materials will be known to those of ordinary skill in the art.
  • the regions of the bulk material may be distinct from each other with minimal cross-contamination, or the composition of the bulk material may vary gradually from one region to the next.
  • the regions may be both longitudinally arranged relative to each other, or radially arranged (e.g., as in a core/shell arrangement) on the bulk material.
  • the junction between two differing regions may be "atomically-abrupt," where there is a sharp transition at the atomic scale between two adjacent regions that differ in composition.
  • the junction between two differing regions may be more gradual.
  • the "overlap region" between the adjacent regions can comprise a few nanometers wide, for example, less than about 10 nm, less than about 20 nm, less than about 40 nm, less than about 50 nm, less than about 100 nm, or less than about 500 nm.
  • the overlap region between a first region having a composition and a second region having a composition different from the first region can be defined as the distance between where the composition of the overlap region ranges between about 10 vol% and about 90 vol% of the composition of the first region, with the remainder having a complementary amount of the composition of the second region.
  • Non-limiting examples of semiconductor materials include Ti0 2 , WO 3 , SrTi0 3 , Ti0 2 -Si, BaTi0 3 , LaCr0 3 -Ti0 2 , LaCr0 3 -Ru0 2 , Ti0 2 -In 2 0 3 , GaAs, GaP, p-GaAs/n- GaAs/pGa 0 .
  • the semiconductor material may be provided in any suitable morphology or arrangement.
  • the semiconductor material is a metal oxide and/or metal hydroxide. In some cases, the semiconductor material is not silicon oxide and/or hydroxide. In some embodiments, the semiconductor material is an iron oxide and/or iron hydroxide.
  • a semiconductor material may be doped.
  • doping refers to the process of introducing impurities into a pure semiconductor to change its electrical and/or physical properties.
  • each region of the bulk material e.g., nanorod
  • a dopant may be present in an essentially undoped amount such that about 80%, about 90%, about 95%, or nearly 100% of the charge carriers within the bulk material (e.g., nanorod) do not arise from dopant. In some cases, there are no dopants present in a region of a bulk material (e.g., nanorod) at a detectable level. If a dopant is present, the dopant may be, for example, a solid solution of various elemental semiconductors.
  • a dopant is present in an amount of or in an amount of at least about 1 atom%, about 2 atom%, about 3 atom%, about 4 atom%, about 5 atom%, about 10 atom%, about 20 atom%, about 30 atom%.
  • a semiconductor material may be doped with an n-type element, where the n-type element is capable of providing extra conduction electrons to the host material.
  • a semiconductor material may be doped with a p-type element, wherein the p-type element is capable of increasing the number of free charge carriers in the semiconductor material.
  • T1O2 may be doped with Y, V, Mo, Cr, Cu, Al, Ta, B, Ru, Mn, Fe, Li, Nb, In, Pb, Ge, C, N, S, Si, etc.
  • the dopant or the semiconductor may include mixtures of Group IV elements, for example, a mixture of silicon and carbon, or a mixture of silicon and germanium.
  • the dopant or the semiconductor may include a mixture of a Group III and a Group V element, for example, BN, BP, BAs, AIN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, or InSb. Mixtures of these may also be used, for example, a mixture of BN/BP/BAs, or BN/A1P.
  • the dopants may include alloys of Group III and Group V elements.
  • the alloys may include a mixture of AlGaN, GaPAs, InP As, GalnN, AlGalnN, GalnAsP, or the like.
  • the dopants may also include a mixture of Group II and Group VI
  • the semiconductor may include ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, or the like. Alloys or mixtures of these dopants are also possible, for example, (ZnCd)Se, or Zn(SSe), or the like. Additionally, alloys of different groups of semiconductors may also be possible, for example, a combination of a Group II-Group VI and a Group Ill-Group V semiconductor, for example, (GaAs) x (ZnS)i- x .
  • dopants may include combinations of Group IV and Group VI elements, such as GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, or PbTe.
  • Other semiconductor mixtures may include a combination of a Group I and a Group VII, such as CuF, CuCl, CuBr, Cul, AgF, AgCl, AgBr, Agl, or the like.
  • a p-type dopant may be selected from Group III, and an n-type dopant may be selected from Group V, for example.
  • a p-type dopant may be selected from the group consisting of B, Al and In, and an n-type dopant may be selected from the group consisting of P, As and Sb.
  • a p-type dopant may be selected from Group II, including Mg, Zn, Cd and Hg, or Group IV, including C and Si.
  • An n-type dopant may be selected from the group consisting of Si, Ge, Sn, S, Se and Te. It will be understood that the disclosure is not limited to these dopants, but may include other elements, alloys, or materials as well.
  • a bulk material (or nanostructure) of the present disclosure comprises iron oxide (e.g., Fe 2 0s).
  • the iron oxide may be alpha-Fe 2 0 3 , also known as hematite.
  • the iron oxide may comprise a first region and a second region.
  • a nanostructure may comprise a first region comprising an n-type iron oxide and a second region comprising a p-type iron oxide.
  • the first region may comprise iron oxide doped with Ta, Zr, In, Sb, or Si, etc.
  • the second region may comprise iron oxide doped with Mg, Ca, Ti, Mn, Co, Ni, Sn, or Zn, etc.
  • a semiconductor material may be selected to have an appropriate band gap.
  • the semiconductor may be unstable.
  • the band gap may be selected such that the material(s) advantageously absorb(s) a sufficient fraction of solar energy.
  • the semiconductor materials may be selected such that the band gap is between about 1.0 eV and about 3.0 eV, between about 1.0 eV and about 2.5 eV, between about 1.5 eV, and about 2.5 eV, between about 1.7 eV and about 2.3 eV, between about 1.8 eV and about 2.2 eV, between about 1.2 and about 1.8 eV, between about 1.4 and about 1.8 eV, between about 1.5 and about 1.7 eV, or is about 2.0 eV. In some cases, the bad gap is less than about 2.0 eV.
  • the band gap of a material is the energy difference between the top of the valence band and the bottom of the conduction band. If a photon has energy greater than or equal to the band gap of the material, then electrons can form in the conduction band and holes can form in the valence band, related by the following Equation 1 :
  • h Planck's constant
  • v the frequency of the photon
  • e' is an electron
  • h * is an electron hole.
  • an electric field or bias e.g., provided through doping of the semiconductor material and/or through the application of an external voltage
  • the electron and the hole may recombine.
  • many embodiments of the present disclosure avoid this challenging problem, as described herein, for example, through use of catalytic materials.
  • the use of catalytic material(s) may reduce the number of holes and electrons that recombine prior to participation in a reaction.
  • Equation 2 the process that takes place at a first region is shown in Equation 2.
  • Equation 2 may take place at the first region (e.g., comprising an n-type semiconductor material). This process produces oxygen gas which may be released, stored, and/or used in various systems/methods.
  • the electrons and the hydrogen ions may combine at the second region (e.g., comprising a p-type semiconductor material) to form hydrogen gas, as shown in Equation 3.
  • Equation 4 The overall reaction that takes place is shown in Equation 4.
  • Half-reactions of interest e.g., reactions which may be carried out using the methods and systems described herein
  • HE normal hydrogen electrode
  • the semiconc uctor materials comprised in the bulk materia may be selected, at least in part, according to the energy levels of the conduction and valence bands.
  • the energy level of valence band of the first region is selected to be less positive than the energy level of the valence band of the second region; the semiconductor material comprising the first region may be selected such that its valence band potential is more positive than the reduction potential for oxidation half- reaction; the energy level of the conduction band of the first region may be selected to be more negative than the energy level of the conduction band of the second region; and/or the semiconductor material comprising the second region may be selected such that its conduction band potential is more negative than the reduction potential for the desired reduction half-reaction at the given pH (e.g., the evolution of hydrogen from water, the evolution of carbon monoxide and/or hydrocarbons from carbon dioxide, the evolution of ammonia from nitrogen).
  • the given pH e.g., the evolution of hydrogen from water, the evolution of carbon monoxide and/or hydrocarbons from carbon dioxide
  • a semiconductor material may be transparent, substantially transparent, substantially opaque, or opaque.
  • a semiconductor material may be a solid, semi-solid, semi- porous, or porous.
  • a bulk material e.g., for a nanostructure
  • a nanostructure bulk material e.g., a nanorod
  • a nanostructure bulk material may be grown in sequential fashion with the appropriate p-type dopant followed by the appropriate n-type dopant, thereby forming a nanostructure with a p/n-junction within the nanostructure.
  • a nanostructure bulk material (e.g., nanorod) may be produced using techniques that allow for direct and controlled growth of the nanostructure.
  • Techniques for forming nanostructure bulk material include, but are not limited to, direct fabrication of nanostructure bulk materials comprising heterojunctions during synthesis, or doping of nanostructure bulk materials via post-synthesis techniques (e.g., annealing of dopants from contacts or solution-processing techniques).
  • a p/n-junction may be formed during synthesis by changing the dopant at any point during the growth of the nanostructure bulk material, whereas a heterojunction may be formed during synthesis by changing the materials being employed for the growth of the nanostructure bulk material.
  • a method of preparing a bulk material may utilize metal-catalyzed CVD techniques ("chemical vapor deposition").
  • CVD synthetic procedures useful for preparing individual wires directly on surfaces and in bulk form are generally known, and can readily be carried out by those of ordinary skill in the art.
  • Nanowires or nanorods may also be grown through laser catalytic growth.
  • nanowires or nanorods with uniform size (diameter) distribution can be produced, where the diameter of the wires or rods is determined by the size of the catalytic clusters.
  • nanowires or nanorods with different lengths can be grown.
  • a bulk material (e.g., for a nanostructure) may be formed using catalytic chemical vapor deposition ("C-CVD").
  • C-CVD catalytic chemical vapor deposition
  • reactant molecules are formed from the vapor phase.
  • the nanostructure bulk material may be doped by introducing the doping element into the vapor phase reactant.
  • the doping concentration may be controlled by controlling the relative amount of the doping compound introduced in the composite target.
  • the final doping concentration or ratios are not necessarily the same as the vapor-phase concentration or ratios. By controlling growth conditions, such as temperature, pressure or the like, nanostructures having the same doping concentration may be produced.
  • the ratio of gas reactant may be varied (e.g. from about 1 ppm to about 10%, from about 10 ppm to about 20%, from about 100 ppm to about 50%, or the like), and/or the types of gas reactants used may be altered during growth of the nanostructure bulk material (e.g. to form a second, differing, semiconductor material, or to change from an n-type dopant to a p-type dopant, or vice versa).
  • a bulk material (e.g., for a nanostructure) may be formed using laser catalytic growth ("LCG").
  • LCG laser catalytic growth
  • reactants and/or dopants are controllably introduced during vapor phase growth of nanostructure bulk materials.
  • Laser vaporization of a composite target composed of a desired material and a catalytic material may create a hot, dense vapor. The vapor may condense into liquid nanostructure bulk materials through collision with a buffer gas.
  • Growth may begin when the liquid nanostructure bulk materials become supersaturated with the desired phase and can continue as long as reactant is available. Growth may terminate when the nanostructure bulk material passes out of the hot reaction zone and/or when the temperature is decreased.
  • the nanostructure bulk material wire may be further subjected to different semiconductor reagents during growth.
  • vapor phase semiconductor reactants required for nanostructure bulk material growth may be produced by laser ablation of solid targets, vapor-phase molecular species, or the like.
  • the addition of the one or more of the first-provided reactants may be stopped during growth, and then a one or more second-provided reactants may be introduced.
  • the catalytic clusters or the vapor phase reactants may be produced by any suitable technique.
  • laser ablation techniques may be used to generate catalytic clusters or vapor phase reactant that may be used during LCG.
  • Other techniques may also be contemplated, such as thermal evaporation techniques.
  • the laser ablation technique may generate liquid nanoclusters that may subsequently define the size and direct the growth direction of the nanoscopic wires.
  • the diameters of the resulting nanostructure bulk materials may be determined by the size of the catalyst cluster, which in turn may be determined using routine experiments that vary the growth conditions, such as background pressure, temperature, flow rate of reactants, and the like. For example, lower pressure generally produces nanowires or nanorods with smaller diameters. Further diameter control may be achieved by using uniform diameter catalytic clusters.
  • a bulk material (e.g., for a nanostructure) may be formed using a nanostructure-forming catalyst (e.g., different from the catalytic materials described herein which may be associated with a nanostructure).
  • a nanostructure-forming catalyst can be, for example, gold or a gold-containing material in certain embodiments.
  • a wide range of other nanostructure-forming catalysts may also be contemplated for forming a nanostructure bulk material, for example, a transition metal such as silver, copper, zinc, cadmium, iron, nickel, cobalt, and the like.
  • any metal able to form an alloy with the desired semiconductor material but does not form a more stable compound than with the elements of the desired semiconductor material may be used as the catalyst.
  • the buffer gas may be any inert gas, for example, 2 or a noble gas such as argon. In some embodiments, a mixture of H 2 and a buffer gas may be used to reduce undesired oxidation by residual oxygen gas.
  • a reactive gas used during the synthesis of the nanostructure bulk material may also be introduced when desired, for example, ammonia for semiconductors containing nitrogen, such as gallium nitride.
  • Nanostructure bulk materials may also be flexibly doped by introducing one or more dopants into the composite target. The doping concentration may be controlled by controlling the relative amount of doping element, for example, between 0 and about 10% or about 20%, introduced in the composite target.
  • nanostructure bulk materials of any of a variety of materials may be grown directly from vapor phase through a vapor-solid process.
  • nanostructure bulk materials may also be produced by deposition on the edge of surface steps, or other types of patterned surfaces.
  • nanostructure bulk materials may be grown by vapor deposition in or on any generally elongated template.
  • the porous membrane may be porous silicon, anodic alumina, a diblock copolymer, or any other similar structure.
  • the natural fiber may be DNA molecules, protein molecules carbon nanotubes, any other elongated structures.
  • the source materials may be a solution or a vapor.
  • the template may also include be column micelles formed by surfactant.
  • a nanostructure bulk material may be formed using supercritical fluid phase inclusion techniques (see, for example, Crowley et al., Chem. Mater., 2003, 14, 3518).
  • a metal oxides and/or metal hydroxides can be formed by the oxidation and/or hydrolysis of a metal-containing compound.
  • a doped iron oxide can be formed by the oxidation and/or hydrolysis of an iron-containing compound in the presence of a dopant.
  • a semiconductor material may be doped after formation (e.g., of the nanostructure).
  • a nanostructure bulk material having a substantially homogeneous composition is first synthesized, then is doped post-synthetically with various dopants. Such doping may occur throughout the entire nanostructure bulk material, or in one or more portions of the nanostructure bulk material, for example, in a nanowire or nanorod having multiple regions differing in composition.
  • a nanostructure comprises at least one surface- presenting catalytic material which is compositional different than the bulk material (e.g., a semiconductor material).
  • a first catalytic material is associated with a first region of a bulk and a second catalytic material is associated with a second region of the bulk, wherein the first type and the second type of catalytic materials are compositionally different than the bulk material(s).
  • Association of at least one surface-presenting catalytic material with the first and/or second region of a bulk material may aid in the photocatalytic reaction.
  • the association of at least one surface-presenting catalytic material with the bulk material may increase the efficiency of the photocatalysis.
  • a challenge associated with the use of semiconductor materials is that the charge separation of the photogenerated electrons and holes may not be long-lived. That is, the electrons may diffuse back into the semiconductor to recombine with a hole within a time frame faster than what is needed for the electrochemical reaction to proceed.
  • association of at least one catalytic material may increase the efficiency of the photocatalysis by allowing for rapid reaction between the photoexcited electron and/or electron-hole with the bulk.
  • a nanostructure of the present disclosure comprises a bulk material (e.g. comprising one or more semiconductor material) and at least one surface- presenting catalytic material (e.g., which is compositionally different than the bulk material).
  • a first type of surface-presenting catalytic material is associated with a first region of the bulk material and a second type of surface-presenting catalytic material is associated with a second region of the bulk material.
  • the surface-presenting catalytic material is a catalytic material.
  • Many species of the class of catalytic material provided by the disclosure are made of readily-available, low-cost material, and are simple to make, and non-limiting examples are described herein.
  • a "catalytic material” as used herein, means a material that is involved in and increases the rate of a chemical reaction , but is largely unconsumed by the reaction itself, and may participate in multiple chemical transformations.
  • a catalytic material may also be referred to as a catalyst and/or a catalyst composition.
  • a catalytic material is generally not simply a bulk semiconductor material.
  • a catalytic material might involve a metal center which undergoes a change from one oxidation state to another during the catalytic process.
  • the catalytic material might involve metal ionic species which bind to one or more oxygen atoms from water and release the oxygen atoms as dioxygen (i.e., (3 ⁇ 4).
  • catalytic material is given its ordinary meaning in the field in connection with this disclosure.
  • a catalytic material of the disclosure that may be consumed in slight quantities during some uses and may be, in many embodiments, regenerated to its original chemical state.
  • a catalytic material upon oxidation catalyzed an oxidation reaction
  • a catalytic material upon reduction catalyzes a reduction reaction.
  • methods are provided for forming a nanostructure comprising a bulk material (e.g., comprising a first region and a second region), and at least one surface-presenting catalytic material associated with at least the first region and/or the second region.
  • the nanostructure comprising at least one surface-presenting catalytic material may be formed by exposing a bulk material (e.g.
  • the metal ionic species and anionic species may associate with the bulk and form a composition (e.g., a catalytic material) associated with the bulk, thereby forming a nanostructure.
  • a first catalytic material associates with a first region of a bulk and a second catalytic material associates with a second region of the bulk substantially simultaneously.
  • substantially simultaneously when used in connection with the formation of a first catalytic material and the second catalytic material means that the catalytic materials associate with the bulk during the same period, but does not necessarily mean that the catalytic materials form in the same quantity or quality.
  • a greater amount of the first catalytic material may associate with the first region of the bulk as compared to the amount of second catalytic material which associates with the second region of the semiconductor material, a lesser amount of the first catalytic material may associate with the first region of the bulk as compared to the amount of second catalytic material which associates with the second region of the bulk.
  • a bulk may comprise a first region and a second region (e.g., a first material and a second material)
  • the catalytic material may not necessarily be associated with all portions of the first region and the second region.
  • the first catalytic material may associate with only a portion of the first region.
  • the catalytic material may associate with essentially the entire first region.
  • the association occurs at the surface of the material.
  • the catalytic material may form in the pores of the material.
  • catalytic materials which associate with a semiconductor material upon exposure of the semiconductor material to appropriate catalyst-forming conditions to electromagnetic radiation.
  • the disclosure provides methods for forming one or more catalytic material on a semiconductor substrate, wherein the semiconductor substrate is in the form of a nanostructure.
  • a semiconductor substrate e.g., a bulk material
  • a semiconductor substrate comprising a first region comprising an n-type semiconductor material and a second region comprising a p-type semiconductor material.
  • the separation of holes and electrons may occur in the n-type and p-type semiconductor materials.
  • the holes in the n-type material may be used for the formation of a first catalytic material and the electrons in the p-type material may be used for the formation of the second catalytic material.
  • the catalytic materials may aid in an oxidation and/or reduction reaction of an electrochemical reaction.
  • surface-presenting catalytic materials may form as follows.
  • a semiconductor substrate e.g., a bulk material
  • a solution comprising at least one metal ionic species and at least one anionic species.
  • the metal ionic species may be oxidized from an oxidation state of (n) to an oxidation state greater than (n), and a catalytic material comprising the metal ionic species in an oxidation state greater than (n) may associated with the first region.
  • the first catalytic material may addition comprise anionic species.
  • the metal ionic species may be reduced from an oxidation state of (n) to an oxidation state less than (n) (e.g., 0), and a catalytic material comprising the metal ionic species in an oxidation state less than (n) may then associate with the second region.
  • a similar protocol can be carried out but, instead of driving the formation of the nanostructures' catalytic regions with electromagnetic radiation (or, in addition to the use of electromagnetic radiation), a chemical reaction and/or a change in the chemical and/or physical environment can be used to drive formation. E.g., a change in pH, temperature, and/or the like of an environment to which (e.g. a solution in which) the nanoparticles are exposed, addition of reaction-driving constituents, etc.
  • Those of ordinary skill in the art will be able to select reactants and/or conditions to drive such reactions with aid of the present disclosure.
  • FIG. 3 A non-limiting example of a method of forming a first and/or second catalytic material associated with a semiconductor substrate (e.g., a bulk material) comprising a first region and a second region is illustrated in FIG. 3.
  • Semiconductor substrate 51 comprising first region 50 and second region 52 is provided, wherein first region 50 comprises an n-type semiconductor material and second region 52 comprises a p-type semiconductor material.
  • the semiconductor substrate is immersed in a solution comprising metal ionic species (grey circles) having an oxidation state of (n) and anionic species (white circles).
  • holes propagate to the n-type material, as indicated by arrow 56 and electrons propagate to the p- type material, as indicated by arrow 58, as illustrated in FIG. 3B.
  • the holes in the n-type material may aid in the formation of and/or association of first catalytic material 60 comprising the metal ionic species (dashed circles) in an oxidation state greater than (n) and the anionic species (white circles) with first region 50 and the electrons in the p-type material may aid in the formation of and/or association of second catalytic material 62 comprising metal ionic species (dotted circles) having an oxidation state less than (n) to with second region 52, as illustrated in FIG. 3C.
  • the first catalytic material in combination with the holes present in the n-type material
  • the second catalytic material in combination with the electrons present in the p-type material
  • a reduction reaction for example, the production of hydrogen gas from water, as indicated by arrow 66 in FIG. 3D.
  • a catalytic material "associated with" a semiconductor substrate e.g., a bulk material
  • a metal ionic species and/or anionic species which can define a catalytic material of the disclosure.
  • the anionic species and the metal ionic species may interact with each other prior to, simultaneously to, and/or after the association of the species with the semiconductor substrate, and result in a catalytic material with a high degree of solid content resident on, or otherwise immobilized with respect to, the semiconductor substrate.
  • the catalytic material can be solid including various degrees of electrolyte or solution (e.g., the material can be hydrated with various amounts of water), and/or other species, fillers, or the like, but a unifying feature among such catalytic material associated with semiconductor substrate is that they can be observed, visually or through other techniques described more fully below, as largely resident on or immobilized with respect to the semiconductor substrate, either in electrolyte solution or after removal of the semiconductor substrate from solution.
  • the catalytic material may associate with a semiconductor substrate via formation of a bond, such as an ionic bond, a covalent bond (e.g., carbon- carbon, carbon-oxygen, oxygen-silicon, sulfur-sulfur, phosphorus-nitrogen, carbon-nitrogen, metal-oxygen, or other covalent bonds), a hydrogen bond (e.g., between hydroxyl, amine, carboxyl, thiol, and/or similar functional groups), a dative bond (e.g., complexation or chelation between metal ions and monodentate or multidentate ligands), Van der Waals interactions, and the like.
  • a bond such as an ionic bond, a covalent bond (e.g., carbon- carbon, carbon-oxygen, oxygen-silicon, sulfur-sulfur, phosphorus-nitrogen, carbon-nitrogen, metal-oxygen, or other covalent bonds), a hydrogen bond (e.g., between hydroxyl, amine, carboxyl, thiol,
  • the interaction between a metal ionic species and an anionic species may comprise an ionic interaction, wherein the metal ionic species is directly bound to other species and the anionic species is a counterion not directly bound to the metal ionic species.
  • an anionic species and a metal ionic species form an ionic bond and the complex formed is a salt.
  • a catalytic material associated with a bulk may be most often arranged with respect to the bulk so that it is in sufficient electrical communication with the bulk to carry out purposes of the disclosure as described herein.
  • Electrical communication is given its ordinary meaning as would be understood by those of ordinary skill in the art whereby electrons can flow between the bulk and the catalytic material in a facile enough manner for the bulk to operate as described herein. That is, charge may be transferred between the bulk and the catalytic material (e.g., the metal ionic species and/or anionic species present in the catalytic material).
  • the composition is in direct contact with the bulk.
  • a material may be present between the composition and the bulk (e.g., a shell as described herein, an insulator, a conducting material, etc.).
  • a surface-presenting catalytic material may be in "direct electrical communication" with the bulk.
  • Direct electrical communication is given its ordinary meaning as defined above with respect to electrical communication, but in this instance, the bulk and the surface-presenting catalytic material are in direct contact with one another (e.g., as opposed to through a secondary material, through use of circuitry, etc.).
  • the bulk and the surface-presenting catalytic material may be integrally connected.
  • integrally connected when referring to two or more objects, means objects that do not become separated from each other during the course of normal use, e.g., separation requires at least the use of tools, and/or by causing damage to at least one of the components, for example, by breaking, peeling, dissolving, etc.
  • a surface-presenting catalytic material may be considered to be in direct electrical communication with a bulk material during operation of a nanostructure even in instances where a portion of the surface- presenting catalytic material may dissociate from the photoactive composition when taking part in a dynamic equilibrium.
  • a catalytic material on a bulk material e.g., comprising a semiconductor material
  • the catalytic material comprising metal ionic species in an oxidation state greater than the oxidation state in solution may proceed according to the following example.
  • the bulk e.g., a plurality of nanorods each comprising at least one semiconductor material
  • M metal ionic species
  • anionic species e.g., A "y ).
  • the bulk (e.g., the plurality of nanorods) may be exposed to electromagnetic radiation, and metal ionic species near to the bulk material (or nanostructure) may be oxidized to an oxidation state of (n+x) (e.g., M ⁇ n+x) ).
  • the oxidized metal ionic species may interact with an anionic species near the bulk material (or nanostructure) nanostructure to form a substantially insoluble complex, thereby forming a catalytic material.
  • the catalytic material may be in electrical communication with the bulk material.
  • FIG. 4A shows a single metal ionic species 140 with an oxidation state of (n) in solution 142.
  • Metal ionic species 144 may be near bulk (e.g., nanorod) 146, as depicted in FIG. 4B.
  • metal ionic species may be oxidized to an oxidized metal ionic species 148 with an oxidation state of (n+x) and (x) electrons 150 may be transferred to bulk 152 or to another species near or associated with the metal ionic species and/or the bulk.
  • FIG. 4D depicts a single anionic species 154 nearing oxidized metal ionic species 156.
  • anionic species 158 and oxidized metal ionic species 160 may associate with bulk 162 to form a catalytic material.
  • the oxidized metal ionic species and the anionic species may interact and form a complex (e.g., a salt) before associating with the bulk.
  • the metal ionic species and anionic species may associate with each other prior to oxidation of the metal ionic species.
  • the oxidized metal ionic species and/or anionic species may associate directly with the bulk and/or with another species already associated with the bulk.
  • the metal ionic species and/or anionic species may associate with the bulk (either directly, or via formation of a complex) to form the catalytic material (e.g., a composition associated with the bulk).
  • a catalytic material on a semiconductor substrate e.g., a nanorod comprising a least one semiconductor material
  • the catalytic material comprising metal ionic species in an oxidation state less than the oxidation state in solution may proceed according to the following example.
  • a semiconductor substrate may be immersed in a solution comprising metal ionic species (M) with an oxidation state of (n) (e.g., M n ).
  • M oxidation state of (n) e.g., M n .
  • the semiconductor substrate may be exposed to electromagnetic radiation, and metal ionic species near to the semiconductor substrate may be reduced to an oxidation state of (n-w) (e.g., M ⁇ n w) ).
  • the reduced metal ionic species may thereby form a catalytic material associated with the semiconductor substrate.
  • the catalytic material may be in electrical communication with the semiconductor substrate.
  • the catalytic material may comprise species in addition to the reduced metal species.
  • first catalytic material and a second catalytic material comprising the same metal species in different oxidation states
  • first catalytic material and the second catalytic material can comprise different metal species.
  • a first catalytic material may comprise cobalt in an oxidation state of (II), (III) and/or (IV)
  • the second catalytic material may comprise nickel in an oxidation state of (0). Formation of two catalytic material comprises different metal species may be
  • the first catalytic material may comprise the first metal species (e.g., Co(III) and/or Co(IV)) in an oxidation state greater than the oxidation state of the first metal species in solution and the second catalytic material may comprise the second type of metal species (e.g., Ni(0)) in an oxidation state less than the oxidation state of the second metal species in solution.
  • the first catalytic material may comprise a first metal species and the second catalytic material may comprise the first metal species and a second metal species.
  • the first catalytic material may comprise Ni (e.g., in an oxidation state of (II) and/or (III)) and the second catalytic material may comprise Ni (e.g., in an oxidation state of (0)) and molybdenum (e.g., in an oxidation state of (0)).
  • the metal ionic species is chosen and presented in an oxidation state such that both lower and higher oxidation states are available.
  • Co(II) may be provided, wherein cobalt has access to lower oxidation states (e.g., Co(0)) and higher oxidation states (e.g., Co(III), Co(IV)).
  • the metal ionic species is selected such that it capable of forming a first catalytic material comprising the anionic species according to the methods, guidelines, and parameters described elsewhere (e.g., see U.S. Provisional Patent Application Serial No. 61/103,898, filed October 8, 2008, entitled "Catalyst Compositions and Photoanodes for Photosynthesis Replication and Other
  • the metal ionic species (M n ) and the anionic species (A "y ) may be selected such that they exhibit the following properties in connection with forming a catalytic material wherein the oxidation state of the metal ionic species is greater than the oxidation state of the metal ionic species in solution.
  • the solubility of a material comprising anionic species and oxidized metal ionic species may influence the association of the metal ionic species and/or anionic species with the current collector. For example, if a material formed by (c) number of anionic species and (b) number of oxidized metal ionic species is substantially insoluble in the solution, the material may be influenced to associate with the current collector. This non-limiting example may be expressed according to Equation 9:
  • M (n+x) is the oxidized metal ionic species
  • a "y is the anionic species
  • ⁇ [M] b [A] c ⁇ n+x) ⁇ c(y)) is at least a portion of catalytic material formed, where b and c are the number of metal ionic species and anionic species, respectively. It should be understood, however, that the catalytic material does not necessarily consist essentially of a material defined by the formula ⁇ [M] b [A] c ⁇ (n+x y) , as, in most cases, additional components can be present in the catalytic material (e.g., a second type of anionic species).
  • a metal ionic species will also have access to oxidation states less than the oxidation states of the metal ionic species in solution.
  • the selection of the metal ionic species may also take into consideration the ease of reduction of the metal ionic species to a lower oxidation state of that in solution, for example, such that a second catalytic material forms on a second portion of the semiconductor substrate.
  • the reduction potential necessary to reduce the metal ionic species in an oxidation state of (n) (e.g., the oxidation state in solution) to an oxidation state of (0) is within the range of the electron energy available via exposure of the semiconductor substrate to electromagnetic radiation.
  • Metal ionic species useful as one portion of a catalytic material of the disclosure may be any metal ion selected according to the guidelines described herein.
  • the metal ionic species have access to oxidation states of at least (n) and (n+x).
  • the metal ionic species have access to oxidation states of (n), (n+1) and (n+2).
  • the value, (n) may be any whole number, and includes, but is not limited to, 0, 1, 2, 3, 4, 5, 6, 7, 8, and the like.
  • (n) is not zero.
  • (n) is 1, 2, 3 or 4.
  • the value, (x), may be any whole number and includes, but is not limited to 0, 1, 2, 3, 4, and the like.
  • (x) is 1, 2, or 3.
  • metal ionic species include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Rh, Ru, Ag, Cd, Pt, Pd, Ir, Hf, Ta, W, Re, Os, Hg, Pb, Au and the like.
  • the metal ionic species may be a lanthanide or actinide (e.g., Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, Pa, U, etc.).
  • the metal ionic species comprises cobalt ions, which may be provided as a catalytic material in the form of Co(II), Co(III) or the like. In some embodiments, the metal ionic species is not Mn.
  • the metal ionic species may be provided (e.g., to the solution) as a metal compound, wherein the metal compound comprises metal ionic species and counter anions.
  • the metal compound may be an oxide, a nitrate, a hydroxide, a carbonate, a phosphite, a phosphate, a sulphite, a sulphate, a triflate, and the like.
  • An anionic species selected for use as a catalytic material of the disclosure may be any anionic species that is able to interact with the metal ionic species as described herein and to meet threshold catalytic requirements as described.
  • the anionic compound may be able to accept and/or donate hydrogen ions, for example, H 2 PO 4 " or HPO 4 " 2 .
  • Non-limiting examples of anionic species include forms of phosphate (H3PO 4 or HPO 4 "2 , H 2 P0 4 “2 or PO4 3 ), forms of sulphate (H 2 S0 4 or HS0 4 " , S0 4 “2 ), forms of carbonate (H 2 C0 3 or HCO3 “ , CO3 “2 ), forms of arsenate (H 3 As0 4 or HAs0 4 "2 , H 2 As0 4 “2 or As0 4 “3 ), forms of phosphite (H3PO3 or HPO3 "1 , H 2 P0 3 “2 or P0 3 “3 ), forms of sulphite (H 2 S0 3 or HS0 3 “ , SO3 “2 ), forms of silicate, forms of borate (e.g., H3BO3, ⁇ 2 ⁇ 3 ⁇ , HBO3 “2 , etc.), halides (F “ , CI “ , Br “ , T), nitrate, n
  • the anionic species may be a form of phosphonate.
  • a phosphonate is a compound comprising the structure PO(OR 1 )(OR 2 )(R 3 ) wherein R 1 , R 2 , and R 3 can be the same or different and are H, an alkyl, an alkenyl, an alkynyl, a heteroalkyl, a heteroalkenyl, a heteroalkynyl, an aryl, or a heteroaryl, all optionally substituted, or are optionally absent (e.g., such that the compound is an anion, dianion, etc.).
  • R 1 , R 2 , and R 3 can be the same or different and are H, alkyl, or aryl, all optionally substituted.
  • a non-limiting example of a phosphonate is a form of PO(OH) 2 R 1 (e.g., PO ⁇ OHXR 1 ) " , P0 3 (R 1 ) "2 ), wherein R 1 is as defined above (e.g., alkyl such as methyl, ethyl, propyl, etc.; aryl such as phenol, etc.).
  • the phosphonate may be a form of methyl phosphonate (PO(OH) 2 Me), or phenyl phosphonate (PO(OH) 2 Ph).
  • phosphorus-containing anionic species include forms of phosphinites (e.g., PCOR ⁇ R 2 ⁇ ) and phosphonites (e.g., P OR ⁇ OR ⁇ R 3 ) wherein R 1 , R 2 , and R 3 are as described above.
  • the anionic species may comprise one any form of the following compounds: R ⁇ O ⁇ OR 2 )), SCXOR ⁇ OR 2 ), CCXOR ⁇ OR 2 ), PCXOR ⁇ OR 2 ), AsO(OR 1 )(OR 2 )(R 3 ), wherein R 1 , R 2 , and R 3 are as described above.
  • substituents may be chosen to tune the properties of the catalytic material and reactions associated with the catalytic material.
  • the substituent may be selected to alter the solubility constant of a composition comprising the anionic species and the metal ionic species.
  • the anionic species may be provided as a compound comprising the anionic species and a counter cation.
  • the counter cation may be any cationic species, for example, a metal ion (e.g., K + , Na + , Li + , Mg +2 , Ca +2 , Sr +2 ), R (e.g., NH 4 + ) , H + , and the like.
  • the compound employed may be K 2 HP0 4.
  • a catalytic material of the disclosure may comprise more than one type of metal ionic species and/or anionic species (e.g., at least about 2 types, at least about 3 types, at least about 4 types, at least about 5 types, or more, of metal ionic species and/or anionic species).
  • more than one type of metal ionic species and/or anionic species may be provided to the solution in which the bulk material is immersed.
  • the catalytic material may comprise more than one type of metal ionic species and/or anionic species.
  • a first type of metal ionic species e.g., Co(II)
  • second type of metal ionic species e.g., Ni(II)
  • the catalytic material comprises the first type of metal ionic species and the second type of metal ionic species (e.g., Co(II) and Ni(II)).
  • a first and second type of metal ionic species are used together, each can be selected from among metal ionic species described as suitable for use herein.
  • both the first and second species need not both be catalytically active, or if both are catalytically active they need not be active to the same level or degree.
  • the ratio of the first type of metal ionic and/or anionic species to the second type of metal ionic and/or anionic species may be varied and may be about 1 : 1, about 1 :2, about 1 :3, about 1 :4, about 1 :5, about 1 :6, about 1 :7, about 1 :8, about 1 :9, about 1 : 10, about 1 :20, or greater.
  • the second type of species may be present in a very small amount and serve as a dopant to, for example, to improve the conductivity or other properties of the material.
  • the ratio of the first type of species to the second type of metal ionic species may be about 1 :0.1, about 1 :0.005, about 1 :0.001, about 1 :0.0005, etc.
  • a catalytic material comprising more than one metal ionic species and/or anionic species may be formed by first forming a catalytic material comprising a first type of metal ionic species and a first type of anionic species, followed by exposing the nanostructure to a solution comprising a second type of metal ionic species and/or second type of anionic species and exposing the nanostructure to electromagnetic radiation. This may cause the second type of metal ionic species and/or second type of anionic species to be comprised in the catalytic material.
  • the catalytic material may be formed by exposing a bulk material nanostructure to a solution comprising the components (e.g., first and second type of metal ionic species, and anionic species) and applying a voltage to the nanostructure, thereby forming a catalytic material comprising the components.
  • a solution comprising the components (e.g., first and second type of metal ionic species, and anionic species) and applying a voltage to the nanostructure, thereby forming a catalytic material comprising the components.
  • the catalytic material may be formed by exposing a bulk material or nanostructure to a solution comprising the components (e.g., first and second type of metal ionic species, and anionic species) and an appropriate reducing (e.g., LiBH 4 , LiAlH 3 , lithium triethylborohydride, Zn, Na, Li, etc.) or oxidizing agent (e.g., (3 ⁇ 4, 3 ⁇ 4(3 ⁇ 4, hypochlorite salts, Ce +3 salts, etc.) in solution.
  • reducing e.g., LiBH 4 , LiAlH 3 , lithium triethylborohydride, Zn, Na, Li, etc.
  • oxidizing agent e.g., (3 ⁇ 4, 3 ⁇ 4(3 ⁇ 4, hypochlorite salts, Ce +3 salts, etc.
  • first type of anionic species and a second type of anionic species may be provided to the solution and/or otherwise used in combination in a catalytic material of the disclosure.
  • first and second catalytically active anionic species they can be selected from among anionic species described as suitable for use herein.
  • the first type of anionic species is hydroxide and/or oxide ions
  • the second type of anionic species is not hydroxide and/or oxide ions.
  • the species might not be provided to the solution but instead, may be present in the water or solution the species is provided in and/or may be formed during a reaction (e.g., between the first type of anionic species and the metal ionic species).
  • the composition e.g., catalytic material
  • anionic species comprising phosphorus (e.g., HPO 4 "2 ).
  • the composition may additionally comprise cationic species (e.g., K + ).
  • An anionic species comprising phosphorus may be any molecule that comprises phosphorus and is associated with a negative charge.
  • anionic species comprising phosphorus include H 3 PO 4 , H 2 P0 4 ⁇ , HPO4 "2 , PO4 3 , H3PO3, H 2 P0 3 " , HPO3 "2 , PO3 "3 , 'PC OH) ⁇ R 1 P0 2 (0H) " , R 1 P0 3 "2 , or the like, wherein R 1 is H, an alkyl, an alkenyl, an alkynyl, a heteroalkyl, a heteroalkenyl, a heteroalkynyl, an aryl, or a heteroaryl, all optionally substituted.
  • metal ionic species and anionic species can serve as selective screening tests for identification of particular metal ionic and anionic species useful for particular applications.
  • Those of ordinary skill in the art can, through simple bench-top testing, reference to scientific literature, simple diffractive instrumentation, simple electrochemical testing, and the like, select metal ionic and anionic species based upon the present disclosure, without undue experimentation.
  • a catalytic material (e.g., associated with a semiconductor substrate) is a material that is capable of catalyzes the production of 3 ⁇ 4 from water in the presence of (3 ⁇ 4.
  • the catalytic material is capable of catalyzing the production of hydrogen gas at a current density that is at least about 10 times, at least about 50 times, at least about 100 times, at least about 200 times, at least about 500 times, at least about 1000 times, at least about 2000 times, at least about 5000 times, or at least about 10000 times greater than the current density at the catalytic material for the reduction of (3 ⁇ 4 at overpotentials less negative than about -400 mV for the production of 3 ⁇ 4 from water.
  • the above may be determined for a catalytic material by using comparing the current density for the production of 3 ⁇ 4 using a system comprising the catalytic material, in the absence and presence of oxygen gas, under substantially similar conditions.
  • such a catalytic material may be associated with a semiconductor substrate by reduction of a metal ionic species near to the semiconductor substrate (e.g., M n ) to an oxidation state of (n-w) (e.g., M n , or in some cases, M°).
  • a catalytic material comprises a plurality of metal elements (e.g., two, three, four, five, or more, metal elements).
  • the catalytic material may be an alloy.
  • alloy is given its ordinary meaning in the art and refers to a composition comprising two or more metals or metalloids that have physical properties different than those of any of the metals by themselves.
  • Alloys may be binary, ternary, quaternary, etc., depending on the number of metals or metalloids present in the mixture.
  • An alloy may be single phase solid solutions, stoichiometric compounds or consist of two or more phases where each phase may be a solid solution or stoichiometric compound. The alloy may or might not have the same composition throughout.
  • the catalytic material comprises nickel.
  • compositions include binary alloys (e.g., NiMo, NiFe, NiSn, NiS, NiZn, NiP, NiW, NiCu, NiCo, NiAl, CoP, CoMo, NiTi, etc.), ternary alloys (e.g., NiMoX where X is a metal such as Fe, Cu, Zn, Co, W, Cr, Cd, V, Ti, or the like, NiCoP, NiFeP, NiFeZn, NiCoZn, NiCuFe, NiCuMo, LaNiSi, etc.), or quaternary alloys (e.g., NiCoMnAl, etc.).
  • binary alloys e.g., NiMo, NiFe, NiSn, NiS, NiZn, NiP, NiW, NiCu, NiCo, NiAl, CoP, CoMo, NiTi, etc.
  • ternary alloys e.g., NiMoX where X is a metal
  • each of the metals or metalloids in the composition may be present in an atomic percent between 0.001 and 99.999%, such that the total atomic percent of the metal, metalloids, and/or other elements or compounds present totals about 100%.
  • the amount of each of the metal or metalloid component of the composition may be varied in the composition. This may be accomplished using techniques known to those of ordinary skill in the art, for example, by providing varying amounts of each of the starting material prior to forming the composition.
  • the catalytic material comprises or consists essentially of nickel and molybdenum.
  • the physical structure of the catalytic materials described herein may vary.
  • the catalytic material may be a coating (e.g., film) and/or particles associated with at least a portion of the bulk material or nanostructure (e.g., surface and/or pores).
  • the catalytic material might not form a coating associated with the bulk material or nanostructure.
  • a pattern in some cases can form spontaneously upon deposition of catalytic material onto the bulk material or nanostructure.
  • the thickness of catalytic material may be of substantially the same throughout the material. In other cases, the thickness of the catalytic material may vary throughout the material (e.g., a film does not necessarily have uniform thickness). In some cases, the average thickness of the catalytic material may be less than about less than about 100 nm, less than about 50 nm, less than about 25 nm, less than about 15 nm, less than about 15 nm, less than about 10 nm, less than about 5 nm, less than about 2 nm, less than about 1 nm, less than about 0.5 nm, less than about 0.1 nm, or less.
  • the performance of a photocatalyst may be analyzed by determining the quantum efficiency.
  • quantum efficiency is given its ordinary meaning in the art and refers to a measure of the efficiency of the photocatalyst for utilizing photons of a given energy to catalyze a given reaction. .
  • quantum efficiency may be determined from measuring the monochromatic light power density and the rate of the photocatalyzed chemical reaction.
  • the quantum efficiency of a photocatalyst of the present disclosure is greater than about 1%, about 2%, about 5%, about 10%, about 20%, about 25%, about 30%, about 40%, about 50%, about 75%, about 100%.
  • a photocatalyst as described herein is capable of producing at least about 1 umol (micromole), at least about 5 umol, at least about 10 umol, at least about 20 umol, at least about 50 umol, at least about 100 umol, at least about 200 umol, at least about 500 umol, at least about 1000 umol oxygen and/or hydrogen, or more, per cm 2 of incident simulated sunlight (i.e., AM 1.5 illumination) respectively, per hour.
  • any of the nanostructures described herein may be incorporated into devices known to those of ordinary skill in the art.
  • the device may be a solar energy conversion device, wherein the device converts solar energy (e.g., sunlight) via a chemical or electrical reaction.
  • solar energy e.g., sunlight
  • the solar energy may be used to drive a reduction and/or oxidation reaction (e.g., as described herein).
  • a device comprises a plurality of nanostructures in a solution.
  • the solution in which the nanostructures are immersed may be formed from any suitable material.
  • the solution may be a liquid and may comprise water.
  • the solution may comprise a material for reaction (e.g., material in which a redox reaction is to take), for example, a hydrocarbon, HX, etc.).
  • the solution may contain a gas (e.g., N 2 ), wherein the solution may be saturated or substantially saturated with the gas.
  • the solution may consist of or consist essentially of water, i.e.
  • the solution may be essentially pure water or an aqueous solution that behaves essentially identical to pure water, in each case, with the minimum electrical conductivity necessary for an electrochemical system to function.
  • the solution may be selected such that the metal ionic species and the anionic species used for forming catalytic materials are substantially soluble.
  • the solution when the nanostructure is to be used in a system immediately after formation, the solution may be selected such that it comprises water (or other fuel) to be oxidized by a system and/or method as described herein.
  • the solution may comprise water (e.g., provided from a water source).
  • the solution may be contained within a container which is substantially transparent to visible light (e.g., such that the nanostructure may be exposed to electromagnetic radiation through the container).
  • the metal ionic species and the anionic species may be provided to a solution (e.g. for forming a nanostructure and/or for operation of a system comprising nanostructure) by substantially dissolving compounds comprising the metal ionic species and the anionic species. In some instances, this may comprise substantially dissolving a metal compound comprising the metal ionic species and anionic compound comprising the anionic species. In other instances, a single compound may be dissolved that comprises both the metal ionic species and the anionic species.
  • the metal compound and/or the anionic compound may be of any composition, such as a solid, a liquid, a gas, a gel, a crystalline material, and the like.
  • the dissolution of the metal compound and anionic compound may be facilitated by agitation of the solution (e.g., stirring) and/or heating of the solution. In some cases, the solution may be sonicated.
  • the metal species and/or anionic species may be provided in an amount such that the concentration of the metal ionic species and/or anionic species is at least about 0.1 mM, at least about 0.5 mM, at least about 1 mM, at least about 10 mM, at least about 0.1 M, at least about 0.5 M, at least about 1 M, at least about 2 M, at least about 5M, and the like.
  • the concentration of the anionic species may be greater than the concentration of the metal ionic species, so as to facilitate the formation of the catalytic material, as described herein.
  • the concentration of the anionic species may be about 2 times greater, about 5 times greater, about 10 times greater, about 25 times greater, about 50 times greater, about 100 times greater, about 500 times greater, about 1000 times greater, and the like, of the concentration of the metal ionic species. In some instances, the concentration of the metal ionic species will be greater than the concentration of the anionic species.
  • the pH of a solution may be about neutral. That is, the pH of the solution may be between about 6.0 and about 8.0, between about 6.5 and about 7.5, and/or the pH is about 7.0. In other cases, the pH of the solution is about neutral or acidic.
  • the pH may be between about 0 and about 8, between about 1 and about 8, between about 2 and about 8, between about 3 and about 8, between about 4 and about 8, between about 5 and about 8, between about 0 and about 7.5, between about 1 and about 7.5, between about 2 and about 7.5, between about 3 and about 7.5, between about 4 and about 7.5, or between about 5 and about 7.5.
  • the pH may be between about 6 and about 10, between about 6 and about 11, between about 7 and about 14, between about 2 and about 12, and the like.
  • the pH of the solution may be about neutral and/or basic, for example, between about 7 and about 14, between about 8 and about 14, between about 8 and about 13, between about 10 and about 14, greater than 14, or the like.
  • the pH of the solution may be selected such that the anionic species and the metal ionic species are in the desired state.
  • some anionic species may be affected by a change in pH level, for example, phosphate. If the solution is basic (greater than about pH 12), the majority of the phosphate is the form P0 4 ⁇ 3 . If the solution is approximately neutral, the phosphate is in approximately equal amounts of the form HP0 4 "2 and the form H 2 P0 4 ⁇ ⁇ If the solution is slightly acidic (less than about pH 6), the phosphate is mostly in the form H 2 P0 4 " .
  • the pH level may also affect the solubility constant for the anionic species and the metal ionic species.
  • a nanostructure in addition to the bulk material, at least one catalytic material, and/or at least one shell, may comprise a photosensitizing agent.
  • the photosensitizing agent is present between a catalytic material and the bulk and/or a shell material associated with the bulk.
  • the photosensitizing agent may or may not be presented symmetrically or evenly on the nanostructure.
  • the photosensitizing agent may form on only a portion of the bulk material (or shell material associated with the bulk material).
  • a nanostructure may comprise more than one type of photosensitizing agent.
  • a bulk material may comprise a first region and a second region, wherein the first region is associated with a first photosensitizing agent and/or a first surface-presenting catalytic material and the second region is associated with a second photosensitizing agent and/or a second surface-presenting catalytic material. See, for example, FIGS 2I-2J for non-limiting arrangements.
  • the photosensitizing agent may comprise functional groups which aid in the association of a catalytic material with the photosensitizing agent.
  • a ruthenium dye is associated with (e.g. provided upon) a bulk material (e.g., comprising a semiconductor material), wherein the dye contains a chelate that is capable of associating with a ruthenium oxide catalytic material.
  • the incorporation of at least one photosensitizing agent may increase the conversion efficiency of a reduction and/or oxidation reaction.
  • electromagnetic radiation absorbed by a dye causes dye molecules to be transferred from a ground-state (Dye) to an excited state (Dye*) (e.g., see Equation 5).
  • the excited state dyes may transfer electrons to the bulk (e.g., comprising a semiconductor material), resulting in the formation of a higher oxidation state dye (Dye + ) and a reduced nanostructure (e ) (e.g., see Equation 6).
  • the oxidized dye molecules may react with water, thereby resulting in the formation of oxygen (e.g., see Equation 7).
  • the electrons may be transferred from a first region of the nanostructure to a second region of the nanostructure where they may react with protons to produce hydrogen gas (e.g., see Equation 8).
  • the photosensitizing agent is formed on a surface of the bulk or shell associated with a bulk.
  • the photosensitizing agent may have a single, a narrow range (e.g., less than about 100 nm range), a plurality, and/or a wide range (e.g., greater than about 100 nm range) of light absorption peaks.
  • the absorption may occur at a wavelength(s) between about 300 nm and about 1000 nm.
  • the photosensitizing agent may comprise a metal complex dye, an organic dye, quantum dots, etc.
  • Quantum dots will be known to those of ordinary skill in the art and may comprise ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, B12S 3 , HgS, HgSe, HgTe, MgTe, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlAs, A1P, AlSb, A1S, and the like, or combinations thereof (e.g., CdTe/CdSe(core/shell), CdSe/ZnTe(core/shell)). Quantum dots may allow for improved stability as compared to some metal or organic dyes, tailoring of the band gap of the quantum dots (e.g., by size quantification), and/or tailoring of the optical absorption of the quantum dots.
  • the photosensitizing agent can be a polyoxometalate (POM), i.e., typically a polyatomic ion, usually an anion, including three or more transition metal oxyanions linked by shared oxygen atoms, where the metal atoms are usually Group 5 of Group 6 transition metals in high oxidation states.
  • POMs are a class of inorganic metal- oxygen clusters. They generally comprise a polyhedral cage structure or framework bearing at least one negative charge which may be balanced by cations that are external to the cage.
  • the framework of a polyoxometalate generally comprises a plurality of metal atoms, which can be the same or different, bonded to oxygen atoms.
  • the POM may also contain centrally located heteroatom(s) surrounded by the cage framework.
  • classes of POMs which will be known to those of ordinary skill in the art include Keggin-type POMs (e.g., [XM12O40D, Dawson-type POMs (e.g., [X 2 Mi 8 062] n" ), Lindqvist-type POMs (e.g., [M 6 0i 9 ] n ⁇ ), and Anderson-type POMs (e.g., [XM 6 0 2 4] n ) where X is a heteroatom, n is the charge of the compound, M is a metal (e.g., Mo, W, V, Nb, Ta, Co, Zn, etc., or combinations thereof), and O is oxygen.
  • the photosensitizing agent can be a metal complex dye and may comprise a metal such as ruthenium, platinum, or any other suitable metal and an organic component (e.g., a ligand) such as biquinoline, bipyridyl, phenanthroline, thiocyanic acid or derivatives thereof.
  • an organic dye may comprise an organic component such as a porphyrin-based system. The organic dyes may or might not comprise at least one metal (e.g., Zn, Mg, etc.).
  • the sensitizing agent may comprise a composition of the formula ML x (L') y (SCN) z where M is a metal (e.g., Ru), L and L' may be the same or different and are polypyridyl ligands (e.g., 4,4"-(C0 2 H)-2,2"-bipyridine), and x, y, and z can be the same or different and are any whole number 0, 1, 2, 3, etc.
  • M is a metal (e.g., Ru)
  • L and L' may be the same or different and are polypyridyl ligands (e.g., 4,4"-(C0 2 H)-2,2"-bipyridine)
  • x, y, and z can be the same or different and are any whole number 0, 1, 2, 3, etc.
  • the photosensitizing agent comprises a porphyrin-based system, for example:
  • R 1 , R 2 , R 3 , and R 4 can be the same or different and are H, an alkyl, an alkenyl, an alkynyl, a heteroalkyl (e.g., CF 2 CF 2 CF 3 ), a heteroalkenyl, a heteroalkynyl, an aryl, or a heteroaryl, all optionally substituted, or are optionally absent (e.g., such that the compound is an anion, dianion, etc.).
  • additional carbons on the porphyrin may be optionally substituted.
  • the porphyrin may be an anion, dianion, etc.
  • the porphyrin-based system may comprise a metal ion (e.g., such that the porphyrin is an anion or a dianion, etc., and the metal ion is coordinated in the center of the porphyrin by the nitrogen atoms).
  • a metal ion e.g., such that the porphyrin is an anion or a dianion, etc., and the metal ion is coordinated in the center of the porphyrin by the nitrogen atoms.
  • metals include Ru, Rh, Fe, Co, Mg, Al, Ag, Au, Zn, Sn, etc., as known to those of ordinary skill in the art.
  • X is a halide (e.g., F, CI, Br, I), etc.
  • porphyrins include, but are not limited to:
  • Additional suitable photosensitizing agents may include, for example, dyes that include functional groups, such as carboxyl and/or hydroxyl groups that can chelate to the nanoparticles, e.g., to Ti(IV) sites on a T1O2 surface.
  • suitable dyes include, but are not limited to, anthocyanins, phthalocyanines, merocyanines, cyanines, squarates, eosins, and metal-containing dyes.
  • a metal-containing dye may be a polypyridyl complex of ruthenium(II) (e.g., cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'- dicarboxylato)-ruthenium(II), tris(isothiocyanato)-ruthenium(II)-2,2':6',2"-terpyridine-4,4',4"- tricarboxylic acid, cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium, cis-bis(isocyanato)(2,2'-bipyridyl-4,4'dicarboxylato)ruthenium (II), and tris(2,2'-bipyridyl-4,4'-dicarbox
  • the nanostructures and/or photocatalysts of the present disclosure may be used for photocatalytic reactions other than water splitting. Based on the teaching described herein, those of ordinary skill in the art would be able to select suitable nanostructure materials and/or related catalytic materials which could be used for other photocatalytic reactions.
  • Non-limiting examples of photocatalytic reactions which may be carried out using a nanostructure of the present disclosure include formation of X2 and H 2 from HX, wherein X is a halide (e.g., F, CI, Br, I, etc.); formation of CO2 and H 2 from organic compounds (e.g., CH 4 , CH 3 OH, CHOH, EtOH, n-propanol, glycerol, isopropyl alcohol, CH2CH2, etc.); and formation of O2 and hydrocarbon from CO2 + water; formation of ammonia and O2 from 2 and water.
  • X is a halide
  • CO2 and H 2 from organic compounds
  • organic compounds e.g., CH 4 , CH 3 OH, CHOH, EtOH, n-propanol, glycerol, isopropyl alcohol, CH2CH2, etc.
  • Electromagnetic radiation may be provided by any suitable source.
  • electromagnetic radiation may be provided by sunlight and/or an artificial light source.
  • the electromagnetic radiation is provided by sunlight.
  • light may be provided by sunlight at certain times of operation of a system (e.g., during daytime, on sunny days, etc.) and artificial light may be used at other times of operation of the system (e.g., during nighttime, on cloudy days, etc.).
  • artificial light sources include a lamp (mercury-arc lamp, a xenon-arc lamp, a quartz tungsten filament lamp, etc.), a laser (e.g., argon ion), and/or a solar simulator.
  • the spectra of the artificial light source may be substantially similar or substantially different than the spectra of natural sunlight.
  • the light provided may be infrared (wavelengths between about 1 mm and about 750 nm), visible (wavelengths between about 380 nm and about 750 nm), and/or ultraviolet (wavelengths between about 10 nm and about 380 nm).
  • the electromagnetic radiation may be provided at a specific wavelength, or specific ranges of wavelengths, for example, through use of a monochromatic light source or through the use of filters.
  • the power of the electromagnetic radiation may also be varied.
  • the light source provided may have a power of at least about 100 W, at least about 200 W, at least about 300 W, at least about 500 W, at least about 1000 W, or greater. The formation and properties of the composition are described herein.
  • a system may comprise a light management system and/or solar concentrator, which are capable of focusing electromagnetic radiation and/or solar energy.
  • light management systems or solar concentrators may receive
  • the light management system or solar collector may collect and waveguide the light to an area or surface of the system, for example, a surface associated with the catalytic material, a nanostructure, etc.
  • the systems and methods as described herein, in some cases, may proceed at about ambient conditions.
  • Ambient conditions define the temperature and pressure relating to the system and/or method.
  • ambient conditions may be defined by a temperature of about 25°C and a pressure of about 1.0 atmosphere (e.g., 1 atm, 14 psi).
  • the conditions may be essentially ambient.
  • essentially ambient temperature ranges include between about 0 °C and about 40 °C, between about 5 °C and about 35 °C, between about 10 °C and about 30 °C, between about 15 °C and about 25 °C, at about 20 °C, at about 25 °C, and the like.
  • Non-limiting examples of essentially ambient pressure ranges include between about 0.5 atm and about 1.5 atm, between about 0.7 atm and about 1.3 atm, between about 0.8 and about 1.2 atm, between about 0.9 atm and about 1.1 atm, and the like. In a particular case, the pressure may be about 1.0 atm.
  • Ambient or essentially ambient conditions can be used in conjunction with any of the systems, compositions, catalytic materials, and/or methods described herein, in conjunction with any conditions (for example, conditions of pH, etc.).
  • the systems and/or methods as described herein may proceed at temperatures above ambient temperature.
  • a system and/or method may be operated at temperatures greater than about 30 °C, greater than about 40 °C, greater than about 50 °C, greater than about 60 °C, greater than about 70 °C, greater than about 80 °C, greater than about 90 °C, greater than about 100 °C, greater than about 120 °C, greater than about 150 °C, greater than about 200 °C, or greater.
  • Efficiencies can be increased, in some instances, at temperatures higher than ambient.
  • the temperature of the system may be selected such that the water provided and/or formed is in a gaseous state (e.g., at temperatures greater than about 100 °C).
  • systems and/or methods as described herein may proceed at temperatures below ambient temperature.
  • a system and/or method may be operated at temperatures less than about 20 °C, less than about 10 °C, less than about 0 °C, less than about -10 °C, less than about -20 °C, less than about -30 °C, less than about -40 °C, less than about -50 °C, less than about -60 °C, less than about -70 °C or the like.
  • the temperature of the system and/or method may be affected by an external temperature source (e.g., a heating and/or cooling coil, infrared light, refrigeration, etc.).
  • the temperature of the system and/or method may be affected by internal processes, for example, exothermic and/or endothermic reactions, etc.
  • the system and/or method may be operated at approximately the same temperature throughout the use of the system and/or method.
  • the temperature may be changed at least once and/or gradually during the use of the system and/or method.
  • the temperature of the system may be elevated during times when the system is used in conjugation with sunlight or other radiative power sources.
  • the systems and methods as described herein may be carried out at elevated pressures.
  • elevated pressures include at least about 1.5 atm, at least about 2 atm, at least about 3 atm, at least about 5 atm, at least about 10 atm, at least about 20 atm, at least about 50 atm, at least about 100 atm, at least about 200 atm, or greater.
  • the pressure is between about 1 atm and about 200 atm, between about 1 atm and about 100 atm, between about 10 atm and about 100 atm, between about 50 atm and about 200 atm, or between about 100 atm and about 200 atm.
  • Water may be provided to the systems, nanostructures, and/or for the methods provided herein, using any suitable source.
  • the water is provided from a substantially pure water source (e.g., distilled water, deionized water, chemical grade water, etc.).
  • the water may be bottled water.
  • the water is provided from a natural and/or impure water source (e.g., tap water, lake water, river water, ocean water, rain water, lake water, pond water, sea water, potable water, brackish water, industrial process and/or waste water, etc.).
  • the water is not purified prior to use (e.g., before being provided to the system/nanostructures for electrolysis).
  • an electrolyte as known to those of ordinary skill in the art is any substance containing free ions that is capable of functioning as an ionically conductive medium.
  • an electrolyte may comprise water, which may act as the water source.
  • the electrolyte may be a liquid, a gel, and/or solid.
  • the electrolyte may also comprise methanol, ethanol, sulfuric acid, methanesulfonic acid, nitric acid, mixtures of HQ, organic acids like acetic acid, etc.
  • the electrolyte comprises mixtures of solvents, such as water, organic solvents, amines and the like.
  • the pH of the electrolyte may be about neutral.
  • the pH of the electrolyte may be between about 5.5 and about 8.5, between about 6.0 and about 8.0, about 6.5 about 7.5, and/or the pH is about 7.0. In a particular case, the pH is about 7.0. In other cases, the pH of the electrolyte is about neutral or acidic. In these cases, the pH may range from about 0 to about 8, about 1 to about 8, about 2 to about 8, about 3 to about 8, about 4 to about 8, about 5 to about 8, about 0 to about 7.5, about 1 to about 7.5, about 2 to about 7.5, about 3 to about 7.5, about 4 to about 7.5, about 5 to about 7.5.
  • the pH may be between about 6 and about 10, about 6 and about 11, about 7 and about 14, about 2 and about 12, and the like. In a specific embodiment, the pH is between about 6 and about 8, between about 5.5 and about 8.5, between about 5.5 and about 9.5, between about 5 and about 9, between about 3 and about 11, between about 4 and about 10, or any other combination thereof.
  • the electrolyte when the electrolyte is a solid, the electrolyte may comprise a solid polymer electrolyte.
  • the solid polymer electrolyte may serve as a solid electrolyte that conducts protons and separate the gases produces and or utilized in the electrochemical cell.
  • Non-limiting examples of a solid polymer electrolyte are polyethylene oxide, polyacrylonitrile, and commercially available NAFION.
  • Non-limiting embodiments of non-liquid electrolytes include electrolytes formed by using a lithium salt and an ion-conductive polymer such as polyethylene oxide or polypropylene oxide; gel polymer electrolytes formed by using a non-ionic conductive polymer such as poly(vinyl chloride), polyacrylonitrile, polymethyl methacrylate, poly(vinylidene fluoride), poly(vinyl) sulfone, or combinations thereof.
  • the system may comprise an ion exchange membrane
  • anion exchange membranes and/or cation exchange membranes i.e. ones with anion and/or cation exchangeable ions
  • anionic exchange membranes include poly(ethylene-co-tetrafluoroethylene), poly(hexafluoropropylene-co- tetrafluoroethylene), poly(epichlorhydrin-ally glycidyl ether), poly(ether imide),
  • poly(ethersulfone) cardo poly(2,6-dimethyl-l,4-phenylene oxide), polysulfone, or polyethersulfone, associated with a plurality of cationic species (e.g., quaternary ammonium groups, phosphonium groups, etc.).
  • cationic species e.g., quaternary ammonium groups, phosphonium groups, etc.
  • the electrolyte may be used to selectively transport one or more ionic species.
  • the electrolyte(s) are at least one of oxygen ion conducting membranes, proton conductors, carbonate (CO 3 "2 ) conductors, OH " conductors, and/or mixtures thereof.
  • the electrolyte(s) are at least one of cubic fluorite structures, doped cubic fluorites, proton-exchange polymers, proton-exchange ceramics, and mixtures thereof.
  • oxygen-ion conducting oxides that may be used as the
  • electrolyte(s) include doped ceria compounds such as gadolinium-doped ceria (Gdi_ x Ce x 02-d) or samarium-doped ceria (Smi_ x Cex02-d), doped zirconia compounds such as yttrium-doped zirconia (Yi_ x Zr x 02-d) or scandium-doped zirconia (Sci_ x Zr x 02-d), perovskite materials such as Lai_ x Sr x Gai_ y Mg y 0 3 _ d , yttria-stabilized bismuth oxide, and/or mixtures thereof.
  • doped ceria compounds such as gadolinium-doped ceria (Gdi_ x Ce x 02-d) or samarium-doped ceria (Smi_ x Cex02-d
  • doped zirconia compounds such
  • proton conducting oxides that may be used as electrolyte(s) include, but are not limited to, undoped and yttrium-doped BaZr03_d, BaCe03_d, and SrCe03_d as well as Lai_ x Sr x b03_d.
  • the electrolyte may comprise additives.
  • the additive may be an anionic species (e.g., as comprised in the catalytic material associated with a bulk material).
  • the electrolyte may comprise the at least one anionic species.
  • the electrolyte can comprise an anionic species which is different from the at least one anionic species comprised in the catalytic material.
  • the catalytic material may comprise phosphate anions and the electrolyte may comprise borate anions.
  • the electrolyte may comprise counter cations (e.g., when the anionic species is added as a complex, a salt, etc.).
  • the anionic species may be good proton-accepting species.
  • the additive may be a good proton-accepting species which is not anionic (e.g., is a neutral base).
  • Non-limiting examples of good proton- accepting species which are neutral include pyridine, imidazole, and the like.
  • a composition comprising a plurality of nanostructures of the disclosure.
  • the plurality of nanostructure comprise individual nanostructures.
  • individual nanostructure means a nanostructure free or essentially free of contact with another nanostructure (but not excluding contact of a type that may be desired between individual nanostructures, e.g., as in a crossbar array).
  • an "individual” or a "free-standing” article may, at some point in its life, not be attached to another article, for example, with another nanostructure, or the free-standing article may be in solution. This is in contrast to nanotubes produced primarily by laser vaporization techniques that produce materials formed as ropes.
  • nanostructures of the present disclosure may be of any size or shape.
  • a nanostructure is a nanowire or a nanorod.
  • the term "nanorod” refers to a nanoparticle having a longest dimension of at most 200 nm, and having an aspect ratio of from 3 : 1 to 20: 1.
  • the nanorod has a maximum length of about 200 nm, or about 150 nm, or about 100 nm, or about 50 nm, or about 25 nm, or about 10 nm, or about 5 nm, or about 1 nm.
  • nanowire means a nanofiber having a longest dimension greater than about 200 nm, about 300 nm, about 400 nm, about 500 nm, about 600 nm, about 700 nm, about 800 nm, about 900 nm, about 1 um, or greater.
  • the nanowire has at least one cross-sectional dimension ranging from 0.5 nm to 200 nm.
  • the cross-section of the elongated semiconductor may have any arbitrary shape, including, but not limited to, circular, square, rectangular, tubular, or elliptical, and may a regular or an irregular shape.
  • the wires or rods may have an aspect ratio (length to thickness) of at least about 2: 1, or greater than about 5: 1, or greater than about 10: 1, or greater than about 100: 1, or greater than about 500: 1, or greater than about 1000: 1, or greater.
  • the nanostructure size may be determined using techniques known to those of ordinary skill in the art, for example, standard microscopy techniques, including transmission electron microscopy (TEM) or dynamic light scattering (DLS).
  • standard microscopy techniques including transmission electron microscopy (TEM) or dynamic light scattering (DLS).
  • a nanostructure is a nanotube.
  • a “nanotube” is generally nanoscopic wire that is hollow, or that has a hollowed-out core, including those nanotubes known to those of ordinary skill in the art.
  • the dimensions of nanotubes can be similar to those described above for nano wires.
  • a nanostructure is a nanoparticle.
  • Nanoparticle generally refers to a particle having a maximum cross-sectional dimension of no more than 1 micron. Nanoparticles are generally spherical in shape, though other shapes are also possible. As used herein, the term “particle size” refers to the diameter of a particle, such as a substantially spherical particle, as determined by microscopy. In the event that a particle of the disclosure is not absolutely spherical, then size is determined by approximating the shape of the particle in the form of a sphere.
  • a composition comprising a plurality of individual nanostructures may be formed by providing a solution of a plurality of nanostructures in solution. Nanostructures in solution may form a suspension wherein the nanostructure are free or essentially free of contact with other nanostructures. In some cases, the nanostructures in solution may be agitated (e.g., stirring, shaking) to prevent the nanostructures from settling in the solution. In some cases, however, the size of the nanostructures may be selected such that the nanostructures generally remain suspended in solution (e.g., do not settle) based upon solution mechanics and/or properties. In some cases, the nanostructures have a maximum dimension of less than 200 nm, thereby promoting the nanostructures to remain suspended in solution. In some cases, the nanostructures are nanorods having an average length of less than about 200 nm, less than about 150 nm, less than about 100 nm, or less.
  • a nanostructure is capable of carrying out photochemical water splitting.
  • the method may comprise exposing a plurality of nanostructures (e.g., as described herein, and in some embodiments, comprising a first region and a second distinct region) to electromagnetic radiation.
  • an oxidation reaction may occur at the first region (e.g., n-type doped region of a nanostructure) and a reduction reaction may occur at the second region (e.g., p-type doped region of a nanostructure).
  • the nanostructures may be separated in the nanostructure, and the electrons and holes may be used to carry out a photochemical reaction (e.g., H 2 0 to 3 ⁇ 4 and/or (3 ⁇ 4; 2 and H2O to NH 3 and O2; HX to H 2 and X 2 , etc.).
  • a photochemical reaction e.g., H 2 0 to 3 ⁇ 4 and/or (3 ⁇ 4; 2 and H2O to NH 3 and O2; HX to H 2 and X 2 , etc.
  • the nanostructures comprise metal oxides.
  • the photochemical reaction is water splitting, wherein oxygen and hydrogen gases are produced from water.
  • the photochemical reaction is water splitting, wherein oxygen and hydrogen gases are produced from water.
  • nanostructure comprises a heterojunction (e.g., between two differing semiconductor materials with appropriate band gaps, and/or a p/n-junction).
  • a heterojunction e.g., between two differing semiconductor materials with appropriate band gaps, and/or a p/n-junction.
  • electrons which are excited in the nanostructure upon exposure to electromagnetic radiation can travel to the p-type region of the nanostructure, where they can be used to reduce protons to hydrogen.
  • formed electron holes can travel to the n- type region of the nanostructure, where they can be used to oxidize water to oxygen.
  • an “elongated” article e.g. a semiconductor or a section thereof
  • a "width" of an article is the distance of a straight line from a point on a perimeter of the article, through the center of the article, to another point on the perimeter of the article.
  • a "width” or a "cross-sectional dimension" at a point along a longitudinal axis of an article is the distance along a straight line that passes through the center of a cross-section of the article at that point and connects two points on the perimeter of the cross-section.
  • the "cross-section" at a point along the longitudinal axis of the article is a plane at that point that crosses the article and is orthogonal to the longitudinal axis of the article.
  • the "longitudinal axis" of an article is the axis along the largest dimension of the article.
  • a “longitudinal section” of an article is a portion of the article along the longitudinal axis of the article that can have any length greater than zero and less than or equal to the length of the article.
  • the "length" of an elongated article is a distance along the longitudinal axis from end to end of the article.
  • a "cylindrical" article is an article having an exterior shaped like a cylinder, but does not define or reflect any properties regarding the interior of the article.
  • a cylindrical article may have a solid interior or may have a hollowed- out interior.
  • a cross-section of a cylindrical article appears to be circular or approximately circular, but other cross-sectional shapes are also possible, such as a hexagonal shape.
  • the cross-section may have any arbitrary shape, including, but not limited to, square, rectangular, or elliptical. Regular and irregular shapes are also included.
  • the term "Group,” with reference to the Periodic Table, is given its usual definition as understood by those of ordinary skill in the art.
  • the Group II elements include Mg and Ca, as well as the Group II transition elements, such as Zn, Cd, and Hg.
  • the Group III elements include B, Al, Ga, In and Tl;
  • the Group IV elements include C, Si, Ge, Sn, and Pb;
  • the Group V elements include N, P, As, Sb and Bi;
  • the Group VI elements include O, S, Se, Te and Po.
  • aliphatic includes both saturated and unsaturated, straight chain (i.e., unbranched) or branched aliphatic hydrocarbons, which are optionally substituted with one or more functional groups, as defined below.
  • aliphatic is intended herein to include, but is not limited to, alkyl, alkenyl, alkynyl moieties.
  • Illustrative aliphatic groups thus include, but are not limited to, for example, methyl, ethyl, n-propyl, isopropyl, allyl, n-butyl, sec- butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, tert-pentyl, n-hexyl, sec-hexyl, moieties and the like, which again, may bear one or more substituents, as previously defined.
  • alkyl is given its ordinary meaning in the art and may include saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups.
  • An analogous convention applies to other generic terms such as “alkenyl,” “alkynyl,” and the like.
  • alkyl,” “alkenyl,” “alkynyl,” and the like encompass both substituted and unsubstituted groups.
  • a straight chain or branched chain alkyl may have 30 or fewer carbon atoms in its backbone, and, in some cases, 20 or fewer. In some
  • a straight chain or branched chain alkyl has 12 or fewer carbon atoms in its backbone (e.g., C1-C12 for straight chain, C3-C12 for branched chain), has 6 or fewer, or has 4 or fewer.
  • cycloalkyls have from 3-10 carbon atoms in their ring structure or from 5, 6 or 7 carbons in the ring structure. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, tert-butyl, cyclobutyl, hexyl, cyclochexyl, and the like.
  • the alkyl group might not be cyclic.
  • non-cyclic alkyl include, but are not limited to, methyl, ethyl, propyl, isopropyl, n-butyl, tert- butyl, n- pentyl, neopentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, and dodecyl.
  • alkenyl and alkynyl refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but that contain at least one double or triple bond respectively.
  • Alkenyl groups include, but are not limited to, for example, ethenyl, propenyl, butenyl, l-methyl-2-buten-l-yl, and the like.
  • Non-limiting examples of alkynyl groups include ethynyl, 2- propynyl (propargyl), 1-propynyl, and the like.
  • heteroalkenyl and “heteroalkynyl” refer to unsaturated aliphatic groups analogous in length and possible substitution to the heteroalkyls described above, but that contain at least one double or triple bond respectively.
  • halogen or halide designates -F, -CI, -Br, or -I.
  • aryl refers to aromatic carbocyclic groups, optionally substituted, having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings in which at least one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring may have a conjugated Pi electron system, while other, adjoining rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, and/or heterocycyls.
  • the aryl group may be optionally substituted, as described herein.
  • Carbocyclic aryl groups refer to aryl groups wherein the ring atoms on the aromatic ring are carbon atoms.
  • Carbocyclic aryl groups include monocyclic carbocyclic aryl groups and polycyclic or fused compounds (e.g., two or more adjacent ring atoms are common to two adjoining rings) such as naphthyl group.
  • Non-limiting examples of aryl groups include phenyl, naphthyl, tetrahydronaphthyl, indanyl, indenyl and the like.
  • heteroaryl refers to aryl groups comprising at least one heteroatom as a ring atom, such as a heterocycle.
  • heteroaryl groups include pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, isoquinolinyl, and the like.
  • aryl and heteroaryl moieties may be attached via an aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, alkyl or heteroalkyl moiety and thus also include -(aliphatic)aryl, -(heteroaliphatic)aryl, - (aliphatic)heteroaryl, -(heteroaliphatic)heteroaryl, -(alkyl)aryl, -(heteroalkyl)aryl, - (heteroalkyl)aryl, and -(heteroalkyl)-heteroaryl moieties.
  • aryl or heteroaryl and “aryl, heteroaryl, (aliphatic)aryl, -(heteroaliphatic)aryl, - (aliphatic)heteroaryl, -(heteroaliphatic)heteroaryl, -(alkyl)aryl, -(heteroalkyl)aryl, - (heteroalkyl)aryl, and -(heteroalkyl)heteroaryl” are interchangeable.
  • substituted is contemplated to include all permissible substituents of organic compounds, “permissible” being in the context of the chemical rules of valence known to those of ordinary skill in the art. It will be understood that “substituted” also includes that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. In some cases, “substituted” may generally refer to replacement of a hydrogen with a substituent as described herein.
  • substituted does not encompass replacement and/or alteration of a key functional group by which a molecule is identified, e.g., such that the "substituted” functional group becomes, through substitution, a different functional group.
  • a "substituted phenyl group” must still comprise the phenyl moiety and can not be modified by substitution, in this definition, to become, e.g., a pyridine ring.
  • the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds.
  • Illustrative substituents include, for example, those described herein.
  • the permissible substituents can be one or more and the same or different for appropriate organic compounds.
  • the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valencies of the heteroatoms.
  • substituents include, but are not limited to, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, - CF 3 , -CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halide
  • the substituents may be selected from F, CI, Br, I, -OH, -N0 2 , -CN, -NCO, -CF 3 , -CH 2 CF 3 , -CHCI2, -CH 2 OR x , - CH 2 CH 2 OR x , -CH 2 N(R X ) 2 , -CH 2 S0 2 CH 3 , -C(0)R x , -C0 2 (R x ), -CON(R x ) 2 , -OC(0)R x , - C(0)OC(0)R x , -OC0 2 R x , -OCON(R x ) 2 , -N(R X ) 2 , -S(0) 2 R x , -OC0 2 R x , -NR x (CO)R x , - NR x (CO)N(R x ) 2 , wherein each occurrence of R x independently includes,
  • a user may employ a single reactor compartment, container, or chamber.
  • the systems may include a photocatalyst adapted to catalytically evolve H 2 and 0 2 from water under illumination; a container capable of containing H 2 , and a separator system adapted to separate H 2 from a mixture comprising at least H 2 and 0 2 .
  • Such reactors use water as a substrate for H 2 and 0 2 production.
  • the water need not be perfectly pure, as the water may include salt, minerals, or other impurities.
  • Sea water may be used, as well as lake, stream, river, and rain water.
  • Purified water may be used, but purified water is not a requirement.
  • the water may even be waste water, and may include a supporting electrolyte (e.g. salts of hydroxide, chloride, sulfate, phosphate, borate, nitrate, and mixtures thereof).
  • the water may be liquid or gaseous form.
  • the systems also suitably include a photocatalyst material.
  • photocatalyst is suitably adapted to photocatalyze the evolution of 0 2 and H 2 from water.
  • the photocatalyst is suitably disposed within the water container. Suitable photocatalysts are described herein, as well as in United States application 61/478,364 and in international application PCT/US2009/005521, both of which applications are incorporated herein by reference in their entireties for any and all purposes.
  • Photocatalysts suitably include a semiconductor, a molecule, a polymer, a composite material, a protein or other biomolecule, or a biological organism (e.g. bacteria or algae).
  • Photocatalysts may be of various shapes and sizes (e.g. rods, spheres, cubes, tetrapods, or irregular shapes and from 0.1 nm - 1 cm or larger in size).
  • suitable photocatalysts may operate according to the following mechanism.
  • a semiconductor particle may be in contact with water. Particles sizes may vary from less than lnm to about lnm -10 nm, 5 nm-50 nm, 20 nm - 200 nm, 100 nm - 1 micro-m, and larger than 1 micro-m.
  • electrons are produced in the semiconductor conduction band and electron-holes are produced in the semiconductor valence band. Electrons may then react with water to produce H 2 and the corresponding electron-holes may react with water to produce (3 ⁇ 4.
  • Typical semiconductor materials include metal oxides (e.g.
  • III-V semiconductor materials e.g. GaAs, GaP, AlGaAs, InGaAs, InGaP, GaAsP, AlInAs
  • II- VI semiconductor materials e.g., CdS, CdSe, CdTe, ZnTe, CdZnTe
  • silicon as well as doped versions thereof.
  • a nanostructure comprising a first region and a second region is exposed to light
  • charge separation may occur between the first region and the second region, wherein an excess of electrons are present in the first region and an excess of holes are present in the second region.
  • a nanostructure comprising a first region defining primarily or exclusively a first semiconductor material and a second region defining primarily or exclusively a second semiconductor material
  • electrons may be excited from the valence band to the conduction band in the first region (e.g., comprising an n-type semiconductor), thereby creating holes in the valence band and free electrons in the conduction band.
  • the excited electron and corresponding electron-hole may separate spatially within the n-type semiconductor material from the point of generation.
  • the electrons produced at the first region may be transported (e.g., via the nanostructure material) to the second region of the nanostructure (e.g., comprising a p-type semiconductor).
  • the separated electron-holes may be transported to a nanostructure-electrolyte interface at each region where they may be used to carry out an electrochemical reaction, e.g., a redox reaction.
  • the holes/electrons can react with a water molecule, resulting in the formation of oxygen gas and/or hydrogen ions.
  • the holes/electrons can react with a halide ion (e.g., Br-; alternatively in the form HBr), resulting in the formation of halogen (e.g., B3 ⁇ 4 or 3 ⁇ 4 and Br 2 ).
  • a halide ion e.g., Br-; alternatively in the form HBr
  • halogen e.g., B3 ⁇ 4 or 3 ⁇ 4 and Br 2 .
  • the nanostructure comprises a semiconductor material with dopants which are primarily n-type in nature in a first region and dopants which are primarily p-type in nature in a second region.
  • one region (e.g., end) of the nanorod can exhibit more n-type dopants with a second region (e.g., the other end) exhibiting more p-type dopants.
  • a second region e.g., the other end
  • suitable photocatalysts may be found in United States application 61/478,364 and in international application PCT/US2009/005521.
  • the semiconductor particle may be in contact with a co-catalyst for the hydrogen evolution reaction (HER) and/or the oxygen evolution reaction (OER).
  • Co-catalysts may consist of one or more islands deposited on the surface of the semiconductor particle or may consists of uniform shells which coat the particle.
  • Typical HER catalysts include metals and metal alloys, such as Pt, Pd, Ni and Ni-based alloys (e.g. NiMo, NiZn, NiMoCd, NiMoZn, etc), Co and Co-based alloys (e.g.
  • OER catalysts include metal oxides, such as oxides of Ir (e.g. 3 ⁇ 43 ⁇ 4), oxides of Ru (e.g. Ru0 2 ), oxides of Ni (e.g. NiO, NiO(OH)), oxides of Co (e.g. Co 3 0 4 , CoO(OH)), mixed metal oxides (e.g. NiFe), and various perovskites and pyrochlores.
  • Other OER catalysts may include mixed metal-anionic species (e.g.
  • Co-phosphate, Co-borate, and Ni-borate as described in the original MIT case
  • molecular catalysts including coordination compounds and clusters of Ru and Ir
  • enzymes and other biomolecules. Exemplary embodiments are described in United States application 61/478,364 and in international application
  • the photocatalyst may be dispersed in water (Fig 1).
  • the dispersion may be in the form of a colloidal suspension in which the particles do not settle out over a time (e.g. 1 week).
  • the dispersion may be a suspension of photocatalyst particles that readily settle from solution.
  • the dispersion may or may not be mixed. Mixing may be achieved through convection, pumping, or stirring of the fluid.
  • the system may include catalyst disposed on a surface of the container or compartment that contacts the water.
  • the system may also include a catalyst support (e.g., a porous medium) on which the photocatalyst is deposited. Such embodiments allow the user to increase the contact between the photocatalyst and the water.
  • the reactors may also suitably include an apparatus, material, or system to collect gas (e.g., hydrogen) evolved within the system.
  • the collector may be configured to contain gas at a given purity and, if desired, at a given pressure. Ranges for purity may be greater than 50%, greater than 70%, greater than 90%, greater than 95%, greater than 99%, and the like. Ranges for pressure include greater than 0.5 atm, of or about 1 atm, greater than 1 atm, greater than 10 atm, greater than 100 atm, greater than 1000 atm, and the like.
  • the system may also include a gas purification apparatus.
  • the apparatus may be used to separate H and 0 2 .
  • the H 2 and 0 2 are present in a gaseous mixture.
  • the H 2 and 0 2 are to have an initial and final molar ratio, corresponding to the ratio of moles of H 2 to 0 2 before and after being processed by the gas purification apparatus.
  • Initial ratios of H 2 :0 2 may be greater than 100: 1, greater than 50: 1, greater than 10: 1, or or about 2: 1, or less than 1 : 10, less than 1 :50, or less than 1 : 100.
  • the H 2 and 0 2 may be dissolved in solution or may consist of bubbles in solution (e.g. during the generation of H 2 and 0 2 from water).
  • the gas purification apparatus may contain an electrochemical device.
  • This device may, in some embodiments, be a proton-exchange membrane (PEM) fuel cell that is run in hydrogen pump mode to effect a selective transport of H 2 across the PEM membrane.
  • the anode electrode in the cell may be comprised of supported platinum catalyst and a hydrocarbon ionomer (non-PFSA) ionomer.
  • PFSA hydrocarbon ionomer
  • Such ionomers include aromatic structures with sulfonic acid functionalities, such as sulfonated polysulfones and the like. These ionomers have the characteristic of low oxygen solubility, which may aid in avoiding a combustible mixture being formed near the catalyst during the hydrogen pump.
  • a gas diffusion medium may be impregnated with an oxygen-absorbing material (e.g cauliflower
  • cells comprised of porous water transport bipolar plates (e.g. as found in United States Patent 6,723,461) may also be used to effectively bathe the electrode in water. This has the dual benefit of imparting high conductivity through high hydration, and quenching thermal energy should hydrogen-oxygen combustion occur over the catalyst due to both the high enthalpy of vaporization for water, and the local dilution of the combustible mixture that would result.
  • the gas purification apparatus may also include a gas absorbing material.
  • the gas absorbing material may have a higher affinity for oxygen in the presence of hydrogen (deemed an "oxygen absorbing material") or a higher affinity for hydrogen in the presence of oxygen (deemed an "hydrogen absorbing material”).
  • oxygen absorbing materials include those used in the separation of oxygen from air, such as cyano- complexes of cobalt, cobalt sal en complexes, and suitable polymer-based membranes.
  • Other oxygen absorbing materials include globins (e.g. hemoglobin, myoglobin), oxygen binding proteins and molecules, and fluorinated polymers (fluorocarbons).
  • Exemplary hydrogen absorbing materials include metals (e.g.
  • the gas absorbing material may be loaded and depleted of the desired gas through changes in temperature and/or pressure. Such pressure- and temperature-swing processes are well known to those of ordinary skill in the art.
  • the gas purification apparatus may separate hydrogen and oxygen through condensation. Because oxygen has a higher boiling point (90.15 K) than hydrogen (20.28 K), thus oxygen may be preferentially condensed as a liquid in the presence of hydrogen, as is known to those of ordinary skill in the art.
  • the condenser may, in some embodiments, be powered by a fuel cell that itself operates using hydrogen, oxygen, or both that is evolved from the photocatalysis of water.
  • the gas collection apparatus may comprise a containment vessel for the water and the photocatalyst. At least a portion of this containment vessel is suitably transparent to at least a portion of the solar radiation spectrum.
  • Containment vessels may comprise plastic, glass, metal, or combinations thereof or a host of other materials.
  • the containment vessel is a plastic bag or a plastic container.
  • the containment vessel may comprise a natural and/or man-made pond, lake, or body of water, which has been covered with a material to prevent mixing of the evolved gases with the atmosphere.
  • the container is suitably impermeable to hydrogen, oxygen, or both.
  • the container may be made of such an impermeable material.
  • the container may be made of a first material and then be coated (inside or outside) with a quantity of impermeable material so as to confer impermeability on the container.
  • the container may be rigid or flexible.
  • a system may include a plurality of bladders (e.g., formed in rollable plastic) that are in fluid communication with a gas collector or gas purifier. In this way, a user may have 2, 5, 10, or more hydrogen-producing vessels that feed to a gas purifier or separator that collects the hydrogen.
  • a condenser or other separator may be disposed within or connected to the container. Alternatively, the separator may be in fluid communication with the container.
  • the container may have an outlet that places gas evolved from water within the container into fluid communication with a condenser or other separator system.
  • the separator system may include a material that preferentially retains (e.g., adsorbs or absorbs) hydrogen or oxygen.
  • the user may deploy one or more containers across a body of water (e.g., ocean, lake, river).
  • the containers may contain photocatalyst and be operated to evolve H2 and 02 from sea water.
  • the containers may comprise separator systems to separate H2 from 02, or, alternatively, may be in fluid connection with a separator system that separates gas evolved from two or more containers.
  • Containers may also be deployed to collect rainwater, dew, or other condensation. The system may then be operated to evolve H2 or 02 from the gas mixture evolved from the water.
  • the evolved gas is used to operate a fuel cell so as to produce electricity.
  • the electricity may be used to power a device, or, alternatively, the electricity may be stored for later use.
  • the evolved gases may also be combusted to provide energy in a combustion engine or other device.
  • a separator system may comprises a membrane that preferentially admits hydrogen or oxygen.
  • the separator may also include an electrochemical device that, under application of power, is capable of transporting H2 across a membrane, or any combination thereof.
  • the disclosed systems may be configured or operated such that headspace above the water substrate may be flooded with hydrogen so as to reduce the relative concentration of oxygen present. This may reduce the risk of combustion with the hydrogen and oxygen.
  • the user may modulate the gas content within a container or compartment such that the ratio of H2 to 02 is below about 5:95 or above about 95:5.
  • the reactor systems include two compartments or containers. As described elsewhere herein, the reactor suitably uses water as a substrate for H 2 and O2 production.
  • the systems suitably include a HER photocatalyst (absorbs light and catalyzes HER reaction).
  • the systems also suitably include an OER photocatalyst (absorbs light and catalyzes OER reaction).
  • the HER and OER photocatalysts suitably evolve H2 and 02, respectively, from water when illuminated.
  • the systems also suitably include a gas collection apparatus (suitable for collection hydrogen, oxygen, or both), a redox mediator, and an ion conductor.
  • the HER photocatalyst is suitably disposed within one compartment, and OER photocatalyst is suitably disposed within another compartment, which are separated by a porous separator, membrane, salt bridge, or other mechanism to separate the H 2 and O2 while allowing for the conduction of ions.
  • the water substrate may be water that is purified or unpurified, as described elsewhere herein.
  • the HER and OER photocatalysts suitably absorb light and may comprise a semiconductor, a molecule, a polymer, a composite material, a protein or other
  • the HER and OER photocatalysts are comprised of semiconductors selected such that the conduction and valence bands are of sufficient energy to catalyze the HER and OER reaction from water, respectively.
  • Typical HER semiconductor photocatalyst materials include metal oxides (e.g. CuO, ⁇ 3 ⁇ 40) III-V semiconductor materials (e.g. GaAs, GaP, AlGaAs, InGaAs, InGaP, GaAsP, AlInAs) , II-VI semiconductor materials (e.g., CdS, CdSe, CdTe, ZnTe, CdZnTe), silicon, and doped versions thereof.
  • Typical OER photocatalyst materials include metal oxides (e.g.
  • III-V semiconductor materials e.g. GaAs, GaP, AlGaAs, InGaAs, InGaP, GaAsP, AlInAs
  • II-VI semiconductor materials e.g., CdS, CdSe, CdTe, ZnTe, CdZnTe.
  • photocatalysts may comprised materials that are connected in series via an ohmic contact to generate the required voltage to catalyze the HER and OER.
  • Photocatalysts may be of various shapes and sizes (e.g. rods, spheres, cubes, tetrapods, or irregular shapes and from 0.1 nm - 1 cm or larger in size).
  • the photocatalysts are dispersed in water.
  • the dispersion may be in the form of a colloidal suspension in which the particles do not settle out over a reasonable period of time (e.g. 1 week).
  • the dispersion may be a suspension of photocatalyst particles that readily settle from solution. In this case, the dispersion may or may not be mixed. Mixing may be achieved through convection, pumping, or stirring of the fluid.
  • other components may be of various shapes and sizes (e.g. rods, spheres, cubes, tetrapods, or irregular shapes and from 0.1 nm - 1 cm or larger in size).
  • the photocatalysts are dispersed in water.
  • the dispersion may be in the form of
  • the catalyst is present on a surface of a reactor compartment, or even on a catalyst support (e.g., porous material) that is present inside the compartment.
  • a catalyst support e.g., porous material
  • These two-compartment systems may also include a gas collection apparatus.
  • the apparatus serves to afford at least hydrogen (as obtained from the interaction of light, water, and the HER and OER photocatalysts) of given purity and within a containment vessel at a given pressure.
  • Ranges for purity may be greater than 50%, greater than 70%, greater than 90%, greater than 95%, greater than 99%, and the like.
  • Ranges for pressure include greater than 0.5 atm, of or about 1 atm, greater than 1 atm, greater than 10 atm, greater than 100 atm, greater than 1000 atm, and the like.
  • the gas collection apparatus serves to define at least two compartments.
  • at least one compartment comprises water and the HER photocatalyst (termed the "hydrogen compartment") and at least one other compartment comprises water and the OER photocatalyst (termed the "oxygen compartment”).
  • the hydrogen and oxygen compartments may be selected so as to increase the overall efficiency of the photochemical conversion of sunlight and water to hydrogen and oxygen.
  • the oxygen compartment may be arranged such that it is between the incident solar irradiation and the hydrogen compartment.
  • OER photocatalysts may be selected such that the oxygen compartment absorbs higher energy photons of the solar spectrum (e.g. purple or blue light) and HER photocatalysts may be selected such that the hydrogen compartment absorbs lower energy photons of the solar spectrum (e.g. orange or red light).
  • both the hydrogen and oxygen compartment may be irradiated with the same spectrum (e.g. the full spectrum) of incident solar irradiation.
  • the hydrogen and oxygen compartment may be arranged such that they are both between the incident solar spectrum and the earth (e.g. next to each other).
  • the systems may have multiple containers, i.e., two or more containers for hydrogen evolution, two or more containers for oxygen evolution, or both.
  • the "tandem" configuration may include two, three, or even more containers arranged in a stack, so as to reduce the footprint of the device and increase the amount of gas evolved per square foot of device footprint.
  • the containers are suitably transparent or at least configured such that at least a portion of water and catalyst disposed within the containers may be exposed to illumination from exterior to the container.
  • the systems may also include one or more redox mediators.
  • the redox mediator suitably serves to transport electrons from the OER photocatalyst to the HER photocatalyst and may consist of one or a variety of substance.
  • the redox mediator may be an atom, a molecule, a protein or other biomolecule, a solid state material.
  • a non-exhaustive list of redox mediators includes carbon, nitrogen, sulfur, boron, a halogen (fluoride, chloride, bromide, iodide), tin, uranium, a molecule, a dye, biomolecules (e.g., ferrodoxin, azurin, plastocyanine, cytochrome c, NADP + /NADPH, NAD NADH), transition metal ions(e.g., iron, vanadium, manganese, cobalt, chromium, copper, nickel, palladium, platinum, ruthenium, osmium, rhodium, iridium, rhenium), and/or coordination complexes (e.g.
  • the redox mediators are dispersed in solution with the HER and OER photocatalyst.
  • electrons upon illumination of the system, electrons are transferred from the OER photocatalyst to a redox mediator or group of redox mediators.
  • the "reduced" redox mediator or group of mediators is transported from the oxygen compartment to the hydrogen compartment, where electrons are then transferred from the mediator or group of mediators to the HER photocatalyst.
  • the mediator or group may be transported through a membrane or separator which separates the hydrogen and oxygen compartments.
  • the membrane or separator may be a porous separator, an ion exchange membrane, or other material as known to those skilled in the art.
  • Various geometries for the membrane or separator are available with respect to the hydrogen and oxygen compartments.
  • the membrane or separator may comprise a portion or all of the interface between the hydrogen and oxygen compartments.
  • the membrane or separator may consist of a ultrafiltration (UF) membrane cartridge ( Figure 7).
  • UF ultrafiltration
  • the UF cartridge provides a size selective separation method and may be used to pass water and other ions while preventing mixing of the larger photocatalyst particles.
  • UF cartridges are commercially available (e.g. hollow fiber and cross flow cartridges) and may be readily known by those skilled in the art.
  • a pump, piston, or other device may be used to transport or otherwise circulate the redox mediator.
  • the redox mediator may comprise conductive electrodes which are immersed in the hydrogen and oxygen compartments and which are connected by wires ( Figure 8).
  • the redox mediation may be a conductive material, such as a metal.
  • electrons are transferred from the OER photocatalyst to the electrode in the oxygen compartment, through the wire, to the electrode immersed in the hydrogen compartment, and to the HER photocatalyst.
  • the electrodes may line a portion or all of the inner walls of the hydrogen and/or oxygen compartments or they may be extend from the walls of the compartments into the hydrogen and/or oxygen compartments.
  • An ion conductor serves to transport ions between the hydrogen and oxygen compartment (e.g. the transport of protons or hydronium from the oxygen compartment to the hydrogen compartment, the transport of hydroxide from the hydrogen compartment to the oxygen compartment, etc) and complete the circuit for photocatalytic water splitting.
  • an ion conductor places two containers into ionic communication with one another.
  • the ion conductor may be a membrane or separator which separates the hydrogen and oxygen compartments.
  • the membrane or separator may be a porous separator, an ion exchange membrane, or other material as known to those skilled in the art.
  • Various geometries for the membrane or separator are available with respect to the hydrogen and oxygen compartments.
  • the membrane or separator may comprise a portion or all of the interface between the hydrogen and oxygen compartments. In other embodiments, the membrane or separator represents a portion of the interface between the compartments.
  • the ion conductor may preferentially conduct hydronium, protons, hydroxide, sodium, potassium, ammonium, phosphate, borate, or any combination thereof.
  • the present disclosure also provides methods or producing hydrogen, oxygen, or both. These methods suitably include contacting water to a photocatalytic material adapted to catalytically evolve 3 ⁇ 4 and (3 ⁇ 4 from water under illumination. The methods also include illuminating the water and photocatalytic material (e.g., with sunlight) so as to catalytically evolve a mixture of 3 ⁇ 4 and (3 ⁇ 4 from the water. The user may also separate 3 ⁇ 4 from the mixture of 3 ⁇ 4 and (3 ⁇ 4, separate (3 ⁇ 4 from the mixture of 3 ⁇ 4 and (3 ⁇ 4, or both.
  • H 2 and/or (3 ⁇ 4 separation may be effected by a separator system.
  • separator system include condensers, materials that preferentially retain or admit 3 ⁇ 4 or (3 ⁇ 4.
  • the separator may also, as described elsewhere herein include a membrane.
  • the separator may also include an electrochemical device that, under application of power, is capable of transporting 3 ⁇ 4 across a membrane. Separation may be effected by pressure-swing or temperature-swing processes.
  • Additional production methods include, in a first chamber, contacting water and a first photocatalyst adapted to evolve 3 ⁇ 4 from water under illumination; in a second chamber, contacting water and a second photocatalyst adapted to evolve (3 ⁇ 4 from water under illumination; illuminating at least a portion of the first chamber (e.g., with sunlight) so as to effect catalytic production of 3 ⁇ 4; and illuminating at least a portion of the second chamber (e.g., with sunlight) so as to effect catalytic production of 0 2 .
  • the methods further include comprising effecting transport of electrons from the second chamber to the first chamber.
  • the methods also include effecting the transport of ions from the first chamber to the second chamber, from the second chamber to the first chamber, or both.
  • the user may collect at least a portion of the H 2 evolved in the first chamber, at least a portion of the 0 2 evolved in the second chamber, or any combination thereof. Methods of collecting (and separating) gases are described elsewhere herein in detail.
  • Electron transport may be effected by a conductive material, such as a metal.
  • Ion transport may be effected by a material that transports anions, cations, or both.
  • Porous separators known to those of ordinary skill in the art may bused for this purpose, as well as an ion exchange membrane, such as NafionTM.
  • the user may recover at least a portion of the evolved H 2 , at least a portion of the evolved O2, or both.
  • Ions conducted during the disclosed processes include protons, hydronium, hydroxide, alkali metal ions (e.g. Li+, Na+, K+), alkaline earth metal ions (e.g., Mg2+, Ca2+, Sr2+), halide ions (e.g. F-, C1-, Br-, I-), oxyanions of nitrogen (e.g., nitrate, nitrite), oxyanions of sulfur (e.g. sulphate), oxyanions of halogens (e.g. chlorate, chlorite, perchlorate, bromate, perbromate, iodate, periodate), oxyanions of boron (e.g. borate), oxyanions of phosphorus (e.g. phosphate), and the like.
  • alkali metal ions e.g. Li+, Na+, K+
  • alkaline earth metal ions e.g., Mg2+, Ca2+, Sr2
  • the disclosed materials include a light-absorbing semiconductor materials present in particulate form. These semiconductor materials may, in some embodiments, have a bandgap equal to or smaller than about 2.6 eV. Such materials may absorb much of the visible light in the solar spectrum, and attractive solar-to-hydrogen conversion efficiencies are thus obtained.
  • the particles catalytically convert sunlight and water to H2 and 02 with a solar-to-hydrogen efficiency of more than about 0.1% over a period of at least about 100 hours.
  • the energetic position of the conduction and valence band edges of the materials are less than about 0 V vs. NHE (normal hydrogen electrode) and greater than about 1.23 V vs. NHE at the standard state, respectively.
  • photogenerated electrons have sufficient potential to participate in hydrogen evolution and photogenerated holes in the valence band have sufficient potential to realize 02 evolution.
  • simultaneous H2 and 02 evolution may be effected.
  • the disclosed materials may perform H2 and 02 evolution without the presence of sacrificial electron donors/acceptors.
  • the disclosed materials may include a Hydrogen Evolution Co-catalyst (HEC) and/or an Oxygen Evolution Co-catalyst (OEC) so as to further effect H2 and 02 evolution.
  • HEC Hydrogen Evolution Co-catalyst
  • OEC Oxygen Evolution Co-catalyst
  • light-absorbing materials may be surmounted by a shell. This shell may serve to prevent anodic/cathodic
  • Bodies formed of the disclosed light-absorbing materials may be made sufficiently small (e.g., less than 5 nm in cross-sectional dimension) so as to prevent recombination during carrier transport from the point of generation towards the
  • cross-sectional dimension is meant thickness, diameter, or even width, as the bodies may be present as, e.g., particles, flakes, platelets, sheets, and the like.
  • the disclosed materials may be in a core-shell structure; core-shell nanoparticles are considered especially suitable.
  • the structure comprises a core that is surmounted by a shell of an insulating material. Bodies having this configuration suitably have at least one cross-sectional dimension of less than about 0.5 mm.
  • a platelet or flake configuration may also be used, where a central layer (which need not be speherical in shape) may be encased in a shell material.
  • the shell suitably encases the entirety of the core, although total coverage is not a requirement.
  • a core formed from a light- absorbing but unstable material may be coated with a shell of an insulating material that protects the core against corrosion.
  • a light-absorbing particle e.g. Cd 2+ dissolution from CdS
  • the core materials are suitably photoactive, being capable of evolving electrons and electron holes in response to illumination.
  • Some existing catalytic materials entail protection of unstable materials by encapsulation in conductive shells, such as a gold or gold-Si02 shell.
  • a conductive coating leads to increased recombination, as the conductive coating may act as a pathway for shunt currents, resulting in increased electron-hole recombination and concomitant lower conversion efficiencies.
  • a conductive shell frustrates the creation of materials that have two catalytic sites at different potentials.
  • the disclosed materials operate by way of tunneling transport, and the disclosed coatings do not exhibit the above-mentioned problems of conductive shells.
  • Coatings having a thickness in the range of from about 0.5 nm to about 10 nm are considered especially suitable.
  • a non-exhaustive listing of core materials includes:
  • Chalcogenides such as MoS 2 , WS 2 , MoSe 2 , FeS 2 , CdS, CdSe, and CdTe, etc.
  • III-V semiconductor particles such as InP, GaAs, GalnP, InAs, etc.
  • Crystalline and/or amorphous silicon, germanium, and their alloys are Crystalline and/or amorphous silicon, germanium, and their alloys.
  • any of the forgoing materials may also be doped for use in the disclosed materials.
  • Suitable dopants include Ag + , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Zn 2+ , Ca 2+ , Cu 2+ , In 3+ , Ga 3+ , Si 4+ , Ge 4+ , Ti 4+ , Pt 4+ , V 5+ , and Nb 5+ .
  • Other transition metal cations and combinations of any of the foregoing may be used.
  • These materials can be synthesized by precipitation techniques, (hydro)thermal synthesis, solid state reactions, gas-phase growth techniques (e.g. VLS, Vapor-Liquid-Solid growth) or a combination of these techniques.
  • Insulating shell materials include, for example, oxide materials such as Si0 2 , Al 2 0 3 , Ti0 2 , Zr0 2 , Hf0 2 , Sn02, and the like.
  • the shell (e.g., oxide) layer may be disposed conformally on the light-absorbing particles and is suitably stable against anodic/cathodic dissolution over a wide range of pH values.
  • Film layers on particles can be deposited with thickness control by using Atomic Layer Deposition (ALD) in a fluidized bed reactor or rotary reactor configuration. ALD is considered an especially suitable deposition method.
  • a shell is suitably uniform in thickness and is also suitably non-porous so as to prevent corrosion.
  • Conformal coatings of insulating layers may also be achieved by means of St5ber, sol-gel, and/or microemulsion techniques.
  • the choice of a particular shell material may depend on the medium in which water splitting will be performed. In cases when water splitting may be performed under basic conditions (pH of greater than about 12), T1O 2 or Hf0 2 are considered especially suitable in alkaline conditions.
  • the alignment of energy levels between the light absorber and the oxide layer may also inform the choice of the material for the oxide shell.
  • the energy level of the conduction band edge of Ti0 2 is positioned lower than that of CdS; after illumination of Ti0 2 -coated CdS particles, the electron may be transferred to the lower- lying T1O 2 CB edge, thereby losing some of its potential energy.
  • This charge transfer is not possible when CdS is coated with high-bandgap oxides (e.g. S1O 2 , HfC ⁇ ); applying shells of these materials, ballistic electron tunneling is expected to take place without energy losses.
  • the present disclosure also provides iron oxide (e.g., hematite (a-Fe 2 0s) or maghemite (y-Fe203)) bodies. These bodies are suitably nanoparticles (which need not be spherical in configuration) of less than about 50 nm diameter, on which both water oxidation (O 2 evolution) and proton reduction (H 2 evolution) take place.
  • Iron oxide bodies (e.g., particles) according to the present disclosure are modified by incorporating cation dopants (e.g. Mg 2+ , Zn 2+ , Si 4+ transition metal cations, etc.) so as (without being bound to any particular theory) to shift the conduction band edge to a position favorable for proton reduction.
  • cation dopants e.g. Mg 2+ , Zn 2+ , Si 4+ transition metal cations, etc.
  • the doped p-type iron oxide bodies are suitably of dimensions similar to or smaller than the hole diffusion length - this size enables the carriers to reach the interface and the presence of suitable dopants enables hydrogen evolution on Fe 2 0 3 , opening the prospect of efficient oxygen and hydrogen evolution on Fe203 particles.
  • the iron oxide bodies (which may be present as particles) can be coated with a thin shell consisting of an oxide material (which may be different than iron oxide) as mentioned elsewhere herein, the iron oxide bodies may be doped.
  • Possible dopants include Mg 2+ , Zn 2+ , Ca 2+ , Cu 2+ , Si 4+ , Ge 4+ , Ti 4+ , Pt 4+ , V 5+ , and Nb 5+ .
  • Other transition metal cations and combinations of any of the foregoing may be used.
  • Dispersions of particles in aqueous media can be stabilized electrostatically because protons, hydroxyl-ions, or other ions present in the dispersion (e.g. phosphate, borate, perchlorate, sulfate, nitrate, etc.) adsorb to the surface of the particle. Under these conditions, the particles are charged, and electrostatic repulsion prevents aggregation and subsequent precipitation.
  • the surface charge of particles is the result of acid-base equilibria.
  • the surface charge is zero and the dispersion may be unstable.
  • the PZC is a material property; for instance, the PZC for hematite ranges from about 5.5 to about 9.
  • Water oxidation on hematite may be performed under basic conditions (e.g., pH in the range of from about 9 to about 14), which is relatively close to the PZC of hematite. Under such conditions, agglomeration of hematite particles may occur.
  • the PZC of silica S1O2
  • S1O2 silica
  • coating hematite particles with an insulating silica shell reduces or prevents Fe203 particle agglomeration in basic (water oxidation) conditions, promoting long-term catalytic activity.
  • Bodies (e.g., particles) of the disclosed materials may also be decorated with both HEC and OEC materials, i.e., the disclosed materials may present HEC and OEC regions.
  • the above mentioned generation of H 2 and O2 using photocatalytic particles is based on materials having relatively low bandgap ( ⁇ about 2.6 eV).
  • bandgap ⁇ about 2.6 eV
  • Suitable OEC materials include metal oxides (e.g. Ru02, Ir02, and various cobalt- and nickel-based oxides), amongst others.
  • Suitable HEC materials include Pt, Rh, Ir, Pd, Ru, Ru02, NiO, and Ni, amongst others.
  • the HEC and OEC can be deposited by various methods such as photodeposition and precipitation.
  • photodeposition is that catalyst material is deposited on those surface sites where photogenerated electrons and holes accumulate spontaneously.
  • the HEC and OEC materials may be deposited on 'active sites' requiring less catalyst material than in the case where light-absorbing particles are conformally coated by HEC/OEC material.
  • HEC/OEC material may also be used to selectively deposit HEC/OEC material on oxide-coated light-absorbing particles. Adding a precursor solution (for HEC/OEC) to a dispersion of oxide-coated light-absorbing particles may result in selective precipitation of the HEC/OEC material onto the already present particles, as the surface tension at the solid-solid interface is smaller than the surface tension at the solid- solution interface, favoring heterogeneous precipitation at the particle surface over homogeneous precipitation in the solution.
  • Conventional electrodeposition is also a suitable technique for HEC/OEC deposition.
  • the present disclosure provides a catalytic body comprising a core portion comprising a material having a bandgap in the range of from about 1.23 eV to about 2.6 eV, the catalytic body being capable of catalytically converting sunlight and water to 3 ⁇ 4 and O2 with a solar-to-hydrogen efficiency of more than about 0.1% over a period of at least about 100 hours.
  • the solar-to-hydrogen efficiency n is defined as where Rm is the rate of hydrogen production (per unit of illuminated reactor area), the factor 2 corresponds to the amount of electrons required to make one 3 ⁇ 4 molecule, F is the Faraday constant,, E re d is the standard redox potential of 1.23 V for water splitting, and P so i is the power density of solar irradiation.
  • the rate of H 2 production is defined as moles of 3 ⁇ 4 per second per unit area reactor that is illuminated. This measurement thus refers to the moles of 3 ⁇ 4 that are produced in a given reactor volume, defined by the unit surface area multiplied with reactor depth.
  • the power density of solar irradiation is defined as being about 100 mW/cm 2 for AMI .5 irradiation.
  • the upper limit of the catalytic bodies' efficiency is at least partially defined by the bandgap of the light-absorbing core, determining how many photons can be absorbed, hence setting the upper value for R m -
  • catalytic bodies according to the present disclosure may also include a shell portion that surmounts at least a portion of the core portion.
  • the shell suitably has a thickness in the range of from about 0.5 nm to about 10 nm, or even in the range of from about 1 nm to about 5 nm.
  • the bandgap of the core portion is suitably measured using UV-VIS absorption spectroscopy.
  • the thickness of the shell portion may be measured using electron microscopy.
  • a catalytic body according to the present disclosure suitably has a cross- sectional dimension (defined elsewhere herein) in the range of from about 0.5 nm to about 5 mm, or even in the range of from about 10 nm to about 1 mm, or even from about 1 nm to about 10 nm.
  • a catalytic body may be characterized as being configured such that the energetic position of the conduction band edge is less than 0 V relative to NHE at the standard state, and wherein the energetic portion of the valence band is greater than about 1.23 V relative to the Normal Hydrogen Electrode (NHE) at the standard state of about 25°C and about 1 bar.
  • NHE Normal Hydrogen Electrode
  • the shell may be characterized as being essentially transparent to illumination in the range of from about 380 nm to about 700 nm, or from about 400 nm to about 650 nm, or even about 500 nm.
  • Oxides are considered especially suitable materials for use as shells.
  • Suitable oxides include Si02, A1203, Ti02, Zr02, Hf02. MgO, ZnO, NiO, Sn0 2 , Ta205, V2O5, cobalt oxide, and the like. Combinations or mixtures of oxides are also suitable.
  • the shell is suitably chemically stable in photocatalytic water splitting conditions, and the shell material is also suitably capable of being applied as a thin film, e.g., via atomic layer deposition (ALD).
  • ALD atomic layer deposition
  • the shell may be essentially free (i.e., having less than about 1%) of metal, or may even be a pure oxide that is entirely free of metal.
  • the cores may be formed from a number of materials.
  • a non-exhaustive listing of such materials includes chalcogenides, (oxy)nitrides, (oxy)sulfides, a Group III-V semiconductor (e.g. InP or GaAs), crystalline silicon, amorphous silicon, crystalline germanium, amorphous germanium, any alloy thereof, or any combination thereof.
  • Photoactive materials may be purchased commercially.
  • core particles may be synthesized by techniques known in the art such as (co-)precipitation, sol- gel methods, solid-state synthesis, and gas-phase growth techniques (e.g. chemical vapor deposition or vapor-liquid-solid growth), amongst others.
  • Suitable chalcogenides include MoS2, WS2, MoSe2, FeS2, CdS, CdSe, CdTe, PbS, and combinations thereof. CdS, CdSe, and MoSe 2 are considered especially suitable.
  • TaON, GaN, Ge3N4, Ta3N5, or combinations thereof are all considered suitable oxynitrides; TaON and TasNs are especially suitable.
  • Iron oxides including maghemite, hematite, and other iron oxide forms are also suitable.
  • Suitable (oxy)sulfide materials include Sm 2 Ti 2 0 5 S 2 , La-based oxysulfides, FeS, NiS, Ag2S, CoS, or any combination thereof.
  • Suitable group III-V semiconductors (which may be present in particle form) include InP, GaAs, GalnP, InAs, and the like, as well as combinations thereof. InP is especially suitable.
  • the disclosed bodies may also include hydrogen-evolving (HEC) and oxygen-evolving co- catalyst (OEC) materials. The HEC and OEC materials may contact the core of the body or even contact the shell of the body if such a shell is present.
  • FIG. 9 presents a schematic of a photocatalytic core-shell particle.
  • the particle comprises a light-absorbing core material that is unstable under water splitting conditions.
  • CdS cadmium sulfide
  • illumination in aqueous environment results in dissolution of Cd2+ ions from the particle.
  • the core CdS may be protected by a thin (e.g., from about 0.5 nm to about 10 nm) shell of stable insulating oxide material.
  • This dense film can effectively protect the core particle from dissolution but is sufficiently thin to allow tunneling of photogenerated charge carriers through the thin film, enabling them to participate in water splitting at the HEC and the OEC.
  • suitable shell materials include Ti02, Zr02, Si02, Hf02, amongst others.
  • Such bodies suitably include a core comprising iron oxide (e.g., a- Fe 2 0 3 , y-Fe 2 0 3 , or both), the core further comprising a cationic dopant, and the core defining a cross-sectional dimension in the range of from about 0.5 nm to about 50 nm.
  • a variety of cationic dopants may be used, e.g., Mg 2+ , Zn 2+ , Ca 2+ , Cu 2+ , Si 4+ , Ge 4+ , Ti 4+ , Pt 4+ , V 5+ , Nb 5+ .
  • the cationic dopant may be present at from about 0.01% to about 10% atomic concentrations, or from about 2% to about 8% atomic concentration. Atomic concentrations of from about 0.1% to about 5% atomic concentration are considered especially suitable.
  • the energetic position of the conduction band edge is suitably less than 0 V relative to NHE at the standard state, and wherein the energetic portion of the valence band is greater than 1.23 V relative to the Normal Hydrogen Electrode (NHE) at the standard state of 25°C and 1 bar.
  • NHE Normal Hydrogen Electrode
  • a shell portion encloses the core.
  • oxides are considered suitable shell materials; the materials is suitably chemically stable in photocatalytic water splitting conditions (over a wide pH range) and may be applied as a very thin film.
  • a stable colloidal dispersions may be achieved when, the value of the point of zero charge of the shell material differs by about 5 pH units from the pH value in which water splitting is performed. The difference need not be about 5 pH units, as the difference may be less than 5 units (e.g., 1, 2, 3, or about 4 units) or even greater than 5 units (e.g., 6, 7, 8, or even 9 units).
  • the shell is suitably essentially transparent to visible light, as described above.
  • Illustrative, non-limiting Figure 10 presents a schematic of an iron oxide particle according to the present disclosure, and suitable for use in photocatalytic water splitting.
  • Iron oxide is earth-abundant, inexpensive, and absorbs visible light, but the material also presents comparatively poor carrier mobility (causing photogenerated carriers to recombine before they reach the iron oxide/electrolyte interface) and also that the potential of electrons in the iron oxide conduction band is too low to participate in hydrogen evolution.
  • These disadvantages may be overcome by applying small (about 5 nm in diameter) p-type iron oxide particles. By reducing the size of the particles to a diameter of about 5 nm, carriers can reach the surface before recombining.
  • the conduction band edge shifts upwards, enabling photogenerated electrons to participate in hydrogen evolution.
  • any of the foregoing catalytic bodies may also comprise a first co-catalyst.
  • This first co-catalyst may be present on exterior surface of the body. It should be understood that this exterior surface of the body may be an exterior surface of the core or, depending on the embodiment, may be an exterior surface of an optional shell, if such a shell (e.g., an oxide shell) is present.
  • the first co-catalyst may be an oxygen evolving catalyst or a hydrogen evolving catalyst.
  • Suitable oxygen evolving catalysts include RuC ⁇ , IrC ⁇ , gold, palladium, platinum, silver, nickel, NiO, NiFeO, nickel-oxyhydroxide, Ni-borate OEC, Co-borate OEC, Co-phosphate OEC, C0 3 O 4 , CoOx, rhodium, RhxCr 2 -x0 3 , or any combination thereof.
  • the light absorbing material or body performs the function of H2 evolution from water.
  • one co-catalyst performs 02 evolution, and another co-catalyst effects both light absorption and is also a catalyst for H2 evolution.
  • the core material acts to produce electrons and electron holes under illumination, and a HEC co-catalyst evolves H2, and a OEC co- catalyst evolves 02.
  • the core material acts to produce electrons and electron holes under illumination as well as to evolve H2, and a OEC co-catalyst evolves 02.
  • the core material acts to produce electrons and electron holes under illumination and evolves 02, and a HEC co-catalyst evolves H2.
  • the core material may thus be a source of electrons and electron holes, a source of electrons and electron holes and H2 evolution, a source of electrons and electron holes and 02 evolution, or even a source of electrons and electron holes and H2 and 02 evolution.
  • a core material that is capable of itself evolving H2 may be supplemented with one or more HEC co-catalysts and optionally one or more OEC catalysts.
  • a core material that is capable of itself evolving 02 may be supplemented with one or more OEC co-catalysts and optionally one or more HEC catalysts.
  • Suitable hydrogen evolving catalysts include platinum, ruthenium, gold, palladium, silver, Ru0 2 , Ni, and alloys thereof, (e.g. NiFe, NiMo, NiMoCd, NiMoZn, and the like) NiO,CoOx, or any combination thereof.
  • the first co-catalyst may be an oxygen evolving catalyst, and wherein the body further comprises a hydrogen evolving catalyst.
  • the first co-catalyst may be a hydrogen evolving catalyst, and the body further comprises an oxygen evolving catalyst.
  • a catalytic body may contain from about 1 to 2 weight percent HEC and/or OEC.
  • Catalyst loading may, however, be in the range of from about 0.01 to about 10 or even about 15 weight percent, depending on the user's needs and on other operational parameters.
  • iron oxide particles may be comparatively small (e.g, about 5 nm in diameter), which small size in turn requires that only a modest amount of electrons/holes need to be converted.
  • a CdS particle may be comparatively large (e.g, 1 mm in diameter), which particles will in turn absorb more light. Because the surface area of a comparatively larger particle (relative to the volume) is small, more catalyst may be required to process all photogenerated electrons/holes.
  • Illustrative, non-limiting Figure 1 1 presents a schematic of an exemplary Tas s particle decorated with both HEC and OEC material.
  • Existing photocatalytic particles e.g., Tas s
  • charge separation is more efficient (reducing recombination)
  • reaction rates are enhanced
  • the desired half reactions outcompete undesired half reactions, such as particle dissolution.
  • the disclosed materials present a higher activity for simultaneous hydrogen and oxygen evolution.
  • the present disclosure also provides methods of evolving hydrogen and oxygen from water. These methods suitably include contacting a catalytic body as described in the present disclosure to a fluid comprising water, and illuminating the catalytic body and water so as to evolve H2, 02, or both.
  • the fluid may be ocean water, river water, rain water, or virtually any other aqueous fluid.
  • the fluid may be pure or may contain pollutants, metals, salts, and the like. It should be understood that these methods encompass the use of multiple particles according to the present disclosure, and that a user may contact water with particles that differ from one another in one or more aspects (e.g., size, material composition).
  • the user may separate the evolved H2, 02, or both.
  • This separation may be effected, for example, by a condenser or by pressure- or temperature- swing adsorption.
  • Exemplary methods of effecting separation are set forth in United States patent application 61/566,078, filed December 2, 2011, "Systems And Methods For Photocatalytic Production Of Hydrogen From Water," the entirety of which is incorporated herein by reference.
  • the present disclosure also provides methods of evolving H2 and 02 from water. These methods include contacting an iron oxide catalytic body as described elsewhere herein to a fluid comprising water; and illuminating the catalytic body and water so as to evolve H2, 02, or both.
  • the evolution of H2 and 02 is performed at a solar-to-hydrogen efficiency of greater than about 0.1% over a period of at least about 100 hours.
  • the user may separate evolved H2, evolved 02, or both; suitable separation methods are set forth in United States patent application no. 61/566,078.
  • Cationic dopant may be introduced during the nucleation of the one or more particles, during the growth of the one or more particles, or both.
  • One may introduce the cationic dopant to a particle precursor material.
  • one method for synthesizing y-Fe 2 0 3 or a-Fe 2 0 3 particles is to add a solution containing Fe 2+ and Fe 3+ ions to an alkaline medium, so as to co-precipitate Fe 3 0 4 particles (which are oxidized into y-Fe 2 0 3 or a-Fe 2 0 3 in a subsequent step).
  • Doping of the initial Fe 3 0 4 particles can be achieved by adding a certain amount of dopant ions to the Fe 2+ /Fe 3+ precursor solution.
  • the dopant may be present in the range of from about 0.1 to about 5 atomic percent.
  • a user may also introduce a cationic dopant by annealing the one or more particles in the presence of the dopant.
  • Doping may, for instance, be achieved by mixing undoped iron oxide powder with the dopant and by subsequent annealing in an oven containing a gaseous atmosphere.
  • an aqueous dispersion to which the dopant is added can be annealed under hydrothermal conditions (e.g. in an autoclave in which temperatures > 100°C can be obtained for aqueous dispersions).
  • the present disclosure also provides methods. These methods include suspending, under illumination, a catalyst body within a solution that comprises a precursor material for an O2 evolving catalyst, H2 evolving catalyst, the illumination giving rise to photogenerated electrons or holes on the surface of the catalytic body, the photogenerated electrons or holes reducing or oxidizing the precursor material for the O2 evolving catalyst, H 2 evolving catalyst, or both so as to result in deposition of O2 evolving catalyst, H 2 evolving catalyst, or both on the photo-excited particle.
  • the catalyst precursor materials e.g., hexachloroplatinic acid for Pt deposition or i( 03)2 for Ni/NiO deposition
  • an aqueous dispersion containing the photocatalytic bodies The dispersion may be illuminated by a white light source, e.g., a Xe lamp.
  • a white light source e.g., a Xe lamp.
  • the catalyst body may be a catalyst body according to the present disclosure, i.e., one that includes a core portion comprising a material having a bandgap in the range of from about 1.23 eV to about 2.6 eV, the catalytic body being capable of catalytically converting sunlight and water to H2 and 02 with a solar-to-hydrogen efficiency of more than about 0.1% over a period of at least about 100 hours.
  • the catalyst body may include a shell as described elsewhere herein, or may lack such a shell.
  • the present disclosure also provides systems. These systems suitably include a container configured to contact a catalyst body with water, the container being adapted to permit illumination of the catalyst and water with a source of illumination.
  • the catalyst body is suitably a catalyst body according to the present disclosure.
  • the body may, as described elsewhere herein, include a shell, but the inclusion of such a shell is not a requirement.
  • the catalyst body may be disposed on the surface of a support, such as a porous material.
  • the system may also include a device capable of separating H2 from 02, the device being in fluid communication with the container. Such devices include condensers, materials that preferentially adsorb hydrogen over oxygen or oxygen over hydrogen, and the like.
  • the systems may include a fluidic connection between the container and the device capable of separating H2 from 02.
  • the system may further include a fluidic connection between the device capable of separating H2 from 02 and one or more containers adapted to contain H2, 02, or both.
  • the container of the system is suitably adapted to permit illumination of the catalyst and water with sunlight.
  • a container may also include a region that is essentially transparent to sunlight.
  • Systems may also include a source of illumination configured to illuminate catalyst and water disposed within the container.
  • illumination sources may be white light sources, monochromatic light sources, and the like.
  • Xenon lamps, light-emitting diodes, and the like are suitable illumination sources.
  • the following prophetic example describes photocatalytic production of hydrogen and oxygen gases from water using a colloidal suspension of p/n-doped nanowires or nanorods.
  • the nanowires are comprised of at least one semiconductor material, such that a p/n-junction is formed within the nanowires or nanorods.
  • excited electrons travel to one end of the nanowires or nanorod and the
  • an iron oxide employed as the semiconductor is generally stable to corrosion under the conditions for water splitting.
  • the band positions and band gap of a p/n-junction of an iron oxide is appropriate for (1) efficient absorption of visible light, and (2) the production of hydrogen and oxygen gas from water (e.g., see Leygraf et al,. J. Catalysis 1982, 78, 341-351 ; Turner et al, J. Electrochem Soc. 1984, 131, 1777-1783).
  • the p-type and n-type iron oxide may be formed by doping iron oxide with Mg and Si, respectively, as described previously.
  • Iron oxide nanowires or nanorods can be prepared as described previously (e.g., see Crowley et al, Chem. Rev. 2003, 15, 3518-3522), but using further incorporation of the selected components to be doped.
  • the nanowires or nanorods may be exposed to a solution comprising selected metal ionic species and anionic species and exposed to electromagnetic radiation (e.g., sunlight).
  • Excited electrons can travel to the p-type end of the wire, where they can reduce protons to hydrogen.
  • the electron holes can travel to the n-type end of the wire, where they can oxidize water to oxygen.
  • a hydrogen evolution and oxygen evolution catalysts may be formed, substantially simultaneously, at the surface of the p:n nanowires or nanorod, respectively.
  • the nanowires or nanorods may be dissolved in a solution containing Ni 2+ ions and borate buffer electrolyte.
  • a nickel-oxide-borate catalyst can form on the n-type side of the nanowires or nanorod, which can serve as a catalyst for the oxidation of water to oxygen.
  • the Ni 2+ ions can be reduced to Ni metal at the p-type end of the nanowires or nanorod, where the Ni-metal can then serve as a catalyst for the reduction of protons to hydrogen.
  • nanoreactors are comprised of low-cost, earth abundant materials and may operate in buffered borate electrolyte at near neutral pH (pH 9.2).
  • the suspension may be prepared in low-cost containers (i.e. plastic bags) to hold the water and collect the resulting hydrogen and oxygen gases, which may be separated outside of the reactor.
  • the following prophetic example describes photocatalytic production of hydrogen and oxygen gases from water using a colloidal suspension of dye-sensitized nanoparticles.
  • Most of the photocatalyst materials explored to date are comprised of metal oxide semiconductors having bandgaps of 3 eV or larger. These materials have proven to be excellent photocatalysts for water oxidation, however their poor absorption of visible light limits the overall efficiency for solar energy conversion.
  • Photoactive dye molecules may be chemically adsorbed to the surface of wide bandgap, metal oxide particles that have been loaded with catalysts for the hydrogen evolution reaction (HER). In this scheme, excitation of the dye molecule promotes the transfer of an electron from the dye to the particle.
  • the electron resides in the semiconductor conduction band, allowing transport of the electron to the hydrogen evolution catalyst.
  • the corresponding electron hole remains on the dye, where it is later consumed by an associated catalytic material.
  • Dye sensitization of T1O2 with polypyridyl complexes of Ru 11 has been used for the photoproduction of H 2 from protons, with methanol (Kajiwara, T.; Hasimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982, 86, 4516), and later ⁇ (Abe, R.; Sayama, K.; Arakawa, H. Chem. Phys. Lett. 2003, 379, 230) serving as the electron source.
  • the semiconductor and dye molecule should be interfaced with appropriate and selective catalysts for HER and the oxygen evolution reaction (OER), respectively.
  • a reference to "A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.
  • the phrase "at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements.
  • This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified.
  • At least one of A and B can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

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Abstract

La présente invention concerne des procédés et des systèmes de stockage d'énergie solaire et de génération d'électricité. Dans certains modes de réalisation, les procédés et / ou les systèmes comportent des nanostructures.
PCT/US2012/044899 2011-07-01 2012-06-29 Procédés et systèmes utiles pour le stockage d'énergie solaire WO2013006427A1 (fr)

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