EP2260123A1 - Procédés électrochimiques de fabrication de nanostructures - Google Patents

Procédés électrochimiques de fabrication de nanostructures

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Publication number
EP2260123A1
EP2260123A1 EP09714589A EP09714589A EP2260123A1 EP 2260123 A1 EP2260123 A1 EP 2260123A1 EP 09714589 A EP09714589 A EP 09714589A EP 09714589 A EP09714589 A EP 09714589A EP 2260123 A1 EP2260123 A1 EP 2260123A1
Authority
EP
European Patent Office
Prior art keywords
metal
nanostructures
electrolyte
anode
cathode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP09714589A
Other languages
German (de)
English (en)
Inventor
Shrisudersan Jayaraman
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Corning Inc
Original Assignee
Corning Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US12/038,847 external-priority patent/US8101059B2/en
Priority claimed from US12/363,162 external-priority patent/US20100193363A1/en
Application filed by Corning Inc filed Critical Corning Inc
Publication of EP2260123A1 publication Critical patent/EP2260123A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/26—Anodisation of refractory metals or alloys based thereon
    • 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
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Definitions

  • Embodiments of the invention relate to methods of making nanostructures and more particularly to electrochemical methods of making nanostructures.
  • Metal oxides, metals, mixed metals, metal alloys, metal alloy oxides, and metal hydroxides are material systems explored, in part, due to these systems having several practical and industrial applications.
  • Metal oxides for example, titanium (IV) oxide (titania) , are used in a wide range of applications such as in paints, cosmetics, catalysis, and bio-implants.
  • Nanomaterials possess unique properties that are not observed in the bulk material, for example, the optical, mechanical, biochemical and catalytic properties of particles are closely related to the size of the particles. In addition to very high surface area-to-volume ratios, nanomaterials exhibit quantum-mechanical effects which can enable applications that are otherwise impossible using the bulk material.
  • One of the challenges with nanotechnology is the manufacture of nanomaterials in an economically viable process. As a result, only a very few nanotechnology based applications have been commercialized, although a wide spectrum of nanotechnology based applications have been demonstrated on a laboratory scale. SP09-011
  • Titania for example, is a material system where nanotechnology based applications have been demonstrated on a laboratory scale and where the nanomaterials could be used in a wide range of practical applications. Titania nanomaterials can be used, for example, in photovoltaic applications such as dye-sensitized solar cells, metal-semiconductor Junction Schottky Diode solar cells, and doped-Ti0 2 nanomaterials based solar cells. Titania nanomaterials can be used in photocatalysis, photo-degradation of various organic pollutants, for example, Rhodamine B, Chloroform, Acid Orange II, Phenol, Salicylic Acid, and Chlorophenols .
  • organic pollutants for example, Rhodamine B, Chloroform, Acid Orange II, Phenol, Salicylic Acid, and Chlorophenols .
  • titania nanomaterials are useful in hydrogenation reactions, for example, hydrogenation of propyne (CH 3 CCH) , photocatalytic water splitting.
  • titania nanoparticles can be used in electrochromic devices such as electrochromic windows and displays, in hydrogen storage, in sensing applications, for example, humidity sensing and gas sensing such as in hydrogen, oxygen, carbon monoxide, methanol, and ethanol sensors.
  • Titania nanomaterials can be used in lithium batteries as insertion electrodes.
  • nanomaterials such as titania nanostructures, for example, sol-gel, micelle and inverse micelle, sol, hydrothermal, solvothermal, direct oxidation, chemical vapor deposition, physical vapor deposition, electrodeposition, sonochemical, microwave, organic templated synthesis, aerogel, and TiC> 2 nanosheets, for example, through delaminated layer synthesis from protonic titanate.
  • a colloidal suspension or sol is formed from precursors, typically inorganic metal salts or metal-organic compounds, for example, metal alkoxides through hydrolysis and polymerization reactions. Loss of SP09-011 solvent and complete polymerization leads to the transition into a sol-gel phase which is then converted into a dense ceramic through further drying and heat treatment.
  • Typical synthesis of titanium oxide nanostructures using the sol-gel method includes adding titanium alkoxide (e.g. titanium tetraisopropoxide) precursor to a base such as tetramethyl ammonium hydroxide at 2°C in alcoholic solvents.
  • hydrothermal synthesis is performed in an autoclave or high pressure reactor with Teflon® liners under controlled temperature and pressure with the reactions occurring in aqueous solutions.
  • a variation of this method is the solvothermal method wherein organic solvents are used instead of an aqueous environment.
  • Typical synthesis of titanium oxide nanowires involves reacting titanium chloride with an acid or inorganic salt at from 50 0 C to 150 0 C in an autoclave for 12 hours. This is followed by washing powders of nanomaterial in DI water and ethanol and drying at 60 0 C for several hours.
  • Some of the other conventional hydrothermal methods for making titania nanoparticles are hydrothermal reaction of titanium butoxide (in isopropanol) with water (water: Ti ratio of 150:1) at 7O 0 C for 1 hour followed by filtration and heat 67
  • titania nanowires are deposited using an anodic alumina membrane (AAM) as template.
  • AAM anodic alumina membrane
  • the substrate is subsequently heated to 500 0 C for 4 hours followed by removal of the AAM template.
  • a prerequisite for this method is the availability of a template that can be removed without leaving any residue using a moderate removal process. Otherwise, regular electrodeposition yields bulk sized particles. Additionally, handling of corrosive electrolyte like titanium chloride in an industrial process can be challenging.
  • synthesis of titania nanotubes involves applying a voltage of from 10 volts to 20 volts for from 10 minutes to 30 minutes between two titanium plates in a 0.5% hydrogen fluoride (HF) solution.
  • HF hydrogen fluoride
  • Methods of making nanostructures address one or more of the above-mentioned disadvantages of conventional methods of making nanostructures, for example, titania nanostructures, and provide one or more of the following advantages: increased compositional and size control with reduced capital and/or manufacturing costs and, since the nanostructures can be grown directly on substrates, the nanostructures possess an inherently high electrical conductivity. Inherently high electrical conductivity is particularly useful in photovoltaic and photocatalytic applications and can lead to materials and systems with improved architecture .
  • One embodiment is a method of making nanostructures.
  • the method comprises providing an electrolytic cell, which comprises an anode and a cathode disposed in an electrolyte comprising a hydroxide, wherein the anode or the cathode comprise a surface exposed to the electrolyte; and applying an electrical potential to the electrolytic cell for a period of time sufficient to obtain nanostructures on the surface of the anode or the cathode exposed to the electrolyte.
  • Another embodiment is a method of making titania nanostructures .
  • the method comprises providing an SP09-011 electrolytic cell, which comprises an anode and cathode disposed in an electrolyte, wherein the anode or the cathode comprise a titanium surface exposed to the electrolyte; and applying an electrical potential to the electrolytic cell for a period of time sufficient to obtain titania nanostructures on the titanium surface of the anode or the cathode exposed to the electrolyte.
  • Figure 1 is an electrolytic cell used in a method according to one embodiment.
  • Figure 2a and Figure 2b show the cyclic voltammetry of a Ti substrate.
  • SP09-011 [0026]
  • Figure 3a, Figure 3b, Figure 3c, Figure 3d are SEM micrographs of titania nanostructures made according to one embodiment .
  • Figure 4a, Figure 4b, Figure 4c, Figure 4d are SEM micrographs of Ti electrodes.
  • Figure 5a, Figure 5b are SEM micrographs of titania nanostructures made according to one embodiment.
  • Figure 6a, Figure 6b are SEM micrographs of titania nanostructures made according to one embodiment.
  • Figure 7a, Figure 7b are cross-sectional SEM micrographs of the embodiment shown in Figure 5a.
  • Figure 8a, Figure 8b, Figure 8c, Figure 8d are a series of SEM micrographs at increasing magnifications of the embodiment shown in Figure 6a.
  • One embodiment is a method of making nanostructures.
  • the method comprises providing an electrolytic cell 100, as shown in Figure 1, which comprises an anode 10 and a cathode 12 disposed in an electrolyte 14 comprising a hydroxide, wherein the anode or the cathode comprise a surface 17 exposed to the electrolyte; and applying an electrical potential to the electrolytic cell for a period of time sufficient to obtain nanostructures on the surface of the anode or the cathode exposed to the electrolyte.
  • the anode and the cathode each comprise a surface 16 exposed to the electrolyte as shown in Figure 1.
  • the anode and SP09-011 the cathode each comprise at least two surfaces exposed to the electrolyte.
  • the nanostructures can be obtained for instance on the surface of an anode exposed to the electrolyte, on the surface of a cathode exposed to the electrolyte, or on the surface of both an anode and a cathode exposed to the electrolyte .
  • Reference to "a surface” or “the surface” of an anode or a cathode therefore includes one or several surfaces of the anode or the cathode, or both the anode and the cathode, when either is exposed to the electrolyte or having nanostructures obtained thereon.
  • the surface of the anode or the cathode exposed to the electrolyte comprises a metal oxide, a mixed metal oxide, a metal, a mixed metal, a metal alloy, a metal alloy oxide, or combinations thereof.
  • the nanostructures in one embodiment, comprise a metal oxide, a mixed metal oxide, a metal, a mixed metal, a metal alloy, a metal alloy oxide, a metal hydroxide, or combinations thereof.
  • the metal oxide can comprise, for example, titanium oxide, molybdenum oxide, zinc oxide, cobalt oxide, or some other metal oxide.
  • the nanostructures when the nanostructures comprise a mixed metal or a mixed metal oxide, the nanostructures can comprise a mixture of two or more metals or metal oxides.
  • Several combinations of nanostructures can be obtained after electrochemical processing such as, when a surface exposed to the electrolyte comprises a metal, a mixed metal, and/ or a metal alloy, then the metal or metals could be converted to an oxide or hydroxide or could remain a metal. For instance, all of the metals, one or more of the metals, or none of the metals could be converted to an oxide or SP09-011 hydroxide, or any combination thereof. Conversion of the metal (s) to an oxide or a hydroxide can be dependent upon the specific starting material, for example, dependent upon the material' s electrochemical behavior when exposed to the electrolyte .
  • a surface exposed to the electrolyte comprises a metal oxide, a mixed metal oxide, or a metal alloy oxide.
  • Conversion of the metal oxides to a metal or a hydroxide can be dependent upon the specific starting material, for example, dependent upon the material's electrochemical behavior when exposed to the electrolyte.
  • the metal oxides can, in some embodiments, remain oxides but the stoichiometry may change.
  • the composition of the nanostructures when a surface comprises C0 3 O4, after electrochemical processing, the composition of the nanostructures can remain C0 3 O 4 or can be converted to CoO or can be converted to Co or a combination thereof.
  • the composition of the nanostructures when a surface comprises CoO, after electrochemical processing the composition of the nanostructures can remain CoO or can be converted to C0 3 O4 or can be converted to Co or combinations thereof.
  • the electrolyte further comprises one or more additives.
  • the additives are selected from boric acid, phosphoric acid, carbonic acid, sodium sulfate, potassium sulfate, sodium sulfite, potassium sulfite, sodium sulfide, potassium sulfide, sodium phosphate, potassium phosphate, sodium nitrate, potassium nitrate, sodium nitrite, potassium nitrite, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, a sodium 67
  • SP09-011 halide a potassium halide
  • surfactant can be ionic, nonionic, biological, or combinations thereof.
  • Exemplary ionic surfactants are (1) anionic (based on sulfate, sulfonate or carboxylate anions) , for example, perfluorooctanoate (PFOA or PFO), perfluorooctanesulfonate (PFOS) , sodium dodecyl sulfate (SDS) , ammonium lauryl sulfate, and other alkyl sulfate salts, sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES) , alkyl benzene sulfonate, soaps, and fatty acid salts; (2) cationic (based on quaternary ammonium cations), for example, cetyl trimethylammonium bromide (CTAB) (a.k.a.
  • CTAB cetyl trimethylammonium bromide
  • CPC cetylpyridinium chloride
  • POEA polyethoxylated tallow amine
  • BAC benzalkonium chloride
  • BZT benzethonium chloride
  • zwitterionic (amphoteric) for example, dodecyl betaine, cocamidopropyl betaine, and coco ampho glycinate.
  • Exemplary nonionic surfactants are alkyl poly (ethylene oxide), alkylphenol poly (ethylene oxide), copolymers of poly (ethylene oxide) and poly (propylene oxide) (commercially called Poloxamers or Poloxamines) , alkyl polyglucosides (including: octyl glucoside, decyl maltoside) , fatty alcohols (including: cetyl alcohol, oleyl alcohol), cocamide MEA, cocamide DEA, and polysorbates (including: Tween 20, Tween 80, dodecyl dimethylamine oxide) .
  • Exemplary biological surfactants are micellular-forming surfactants or surfactants that form micelles in solution, for example, DNA, vesicles, and combinations thereof.
  • the nanostructures can become ordered, for example, similar to self-assembly. 01067
  • the nanostructures further comprise a borate, a phosphate, a carbonate, a boride, a phosphide, a carbide, an intercalated alkali metal, an intercalated alkali earth metal, an intercalated hydrogen, a sulfide, a nitride, or combinations thereof.
  • the composition of the nanostructures can be dependent on the selection of the additive or additives incorporated in the electrolyte.
  • titania nanostructures can be caused to further comprise sodium or potassium in the nanostructure matrix by adding sodium sulfate or potassium sulfate in the electrolyte prior to electrochemical processing
  • cadmium nanostructures can be caused to further comprise cadmium sulfide by adding sodium sulfite, sodium sulfide, sodium sulfate, or combinations thereof in the electrolyte prior to electrochemical processing.
  • the potential can be applied via a power supply 18, for example, a direct current (DC) power supply which can supply a constant voltage or a bipotentiostat, for example, which can supply a cyclic voltage.
  • a power supply 18 for example, a direct current (DC) power supply which can supply a constant voltage or a bipotentiostat, for example, which can supply a cyclic voltage.
  • the potential is not limited to a cyclic voltage, for example, any potential program can be used according to the method.
  • a triangular wave, a pulsed wave, a sine wave, a staircase potential, or a saw-tooth wave are exemplary potential programs. Other applicable potential programs could be used such as other potential programs known by those skilled in the art.
  • the potential is greater than 0.0 volts. In another embodiment, the potential is 0.5 volts or more.
  • the potential is 5 volts or less, for example, in the range of from 0.6 volts to 5.0 volts.
  • the potential according to one embodiment, is applied continuously for 1 minute or more.
  • the potential according to another embodiment is applied for 24 hours or less.
  • the potential in some embodiments, is applied SP09-011 continuously for from 30 minutes to 24 hours, for example, for 4 hours to 18 hours .
  • the electrolyte is a solution comprising sodium hydroxide, potassium hydroxide, or combinations thereof.
  • the solution in some embodiments, can be at a concentration of from 1 molar to 10 molar, for example, at a concentration of from 3 molar to 8 molar, for example, 5 molar.
  • the anode and cathode independently comprise a material selected from a uniform metal, a metal layer, a metal foil, a metal alloy, multiple metal layers, a mixed metal layer, multiple mixed metal layers and combinations thereof.
  • the layer (s) can be, in some embodiments, a thick film, a thin film, a mesh, a patterned layer where the metal (s) is/are present in strips, discrete areas, a spot, spots, or combinations thereof.
  • An example of a mixed metal layer is a co-deposited alloy.
  • the pattern comprises the same material.
  • the pattern comprises any number of dissimilar materials, for example, a strip of metal could be next to a spot of mixed metal, which is next to a square of metal alloy. The strip, spot, and square could be touching or could be spaced apart from each other.
  • the same material can be layered on top of each other.
  • different materials can be layered on top of each other, for example, one metal on top of an alloy, on top of a mixed metal, with several combinations envisioned.
  • the metal film can be, for example, a thin film or a thick film of Ti metal.
  • the thin film can be, for example, from a few nanometers in thickness to a few microns in thickness.
  • the thick film can be, for example, from tens of SP09-011 microns in thickness to several hundreds of microns in thickness.
  • the electrical conductivity of the Ti surface can facilitate electron transfer at the solid-liquid interface and the electrical connection given to the Ti portion of the substrate.
  • the substrate can comprise a flat surface or can comprise a non-flat surface.
  • the substrate can be a flexible substrate or a substrate with a deformable surface.
  • the material is disposed on a conductive support, a non-conductive support, or combinations thereof.
  • the conductive support comprises a material selected from a metal, a metal alloy, nickel, stainless steel, indium tin oxide (ITO) , copper, and combinations thereof.
  • ITO indium tin oxide
  • the non-conductive support comprises a material selected from a polymer, plastic, glass, and combinations thereof.
  • the anode and the cathode can comprise a material selected from titanium metal, titanium foil, titanium film disposed on a conductive support, titanium film disposed on a non-conductive support, and combinations thereof.
  • the conductive support in some embodiments, comprises a material selected from ITO, copper, and combinations thereof.
  • the conductive support in some embodiments, is any conductive metallic substrate.
  • the non-conductive support in some embodiments, comprises a material selected from a polymer, plastic, glass, and combinations thereof.
  • the method can further comprise cleaning the substrates prior to contacting the electrolyte.
  • the method can be used at ambient conditions, for example, room temperature and atmospheric SP09-011 pressure and can utilize low voltage and current, thus, lower energy.
  • the method further comprises heating the electrolyte to a temperature of from 15 degrees Celsius to 80 degrees Celsius, for example, from 30 degrees Celsius to 80 degrees Celsius, for example, from 30 degrees Celsius to 60 degrees Celsius. Heating the electrolyte can be realized by a number of heating methods known in the art, for example, a hot plate placed under the electrolytic cell. The temperature can be adjusted depending on desired nanostructures and materials used.
  • the method further comprises agitating the electrolyte.
  • agitation methods known in the art can be used to agitate the electrolyte, for example, a magnetic stirring bar placed in the electrolyte with a stirrer placed under the electrolytic cell. Mechanical stirring or ultrasonic agitation, for example, can also be used.
  • the method further comprises cleaning the anode and the cathode after obtaining the nanostructures.
  • the cleaning in some embodiments, comprises acid washing.
  • the acid can be selected from hydrochloric, sulfuric, nitric, and combinations thereof.
  • Another embodiment is a method of making titania nanostructures. The method comprises providing an electrolytic cell, which comprises an anode and cathode disposed in an electrolyte, wherein the anode or the cathode comprise a titanium surface exposed to the electrolyte; and applying an electrical potential to the electrolytic cell for a period of time sufficient to obtain titania nanostructures on the titanium surface of the anode or the cathode exposed to the electrolyte.
  • the anode and cathode each comprise a titanium surface exposed to the electrolyte.
  • titanium substrates available from Alfa Aesar were cut and cleaned by being sonicated in 1:1:1 mixture of acetone, iso-propanol, and water for 15 minutes. The titanium substrates were then rinsed in deionized (DI) water and further sonicated in DI water for 15 minutes. The titanium substrates were dried under a stream of nitrogen.
  • DI deionized
  • the electrolyte was prepared using certified ACS sodium hydroxide and certified ACS potassium hydroxide, both available from Alfa Aesar, in DI water.
  • Electrolytic cells for example, electrochemical cells of different sizes (1.5" x 1" x 1" and 3" x 1.5" x 3.5" internal dimensions) were made using Teflon. Teflon was chosen since Teflon is stable in basic environment as opposed to glass or metal vessels that can be susceptible to etching and/or corrosion effects. Other materials that are resistive to a basic pH can be used to build the electrochemical cells.
  • Figure 2a and Figure 2b show the cyclic voltammetry of a Ti substrate in 1 molar (M) NaOH and IM KOH solutions.
  • M 1 molar
  • IM KOH 1 molar
  • the reaction is a surface oxidation process that may be limited by the mass transfer of the hydroxyl ions towards the electrode surface.
  • the current increases to further positive values indicating the onset of further electron-transfer reaction or reactions.
  • this second electron-transfer reaction is a kinetically controlled oxidation reaction that is not affected by the concentration of hydroxyl ions in the solution (at least at concentrations >1 M) .
  • the cyclic voltammetry can be used as a guide for predictive experimentation, i.e. the potential to be applied can be chosen to influence reaction-specific changes to the surface of the anode and/or the cathode.
  • FIG. 2b shows the cyclic voltammetry of a Ti substrate in IM KOH.
  • the electrochemical behavior of Ti in KOH and the electrochemical behavior of Ti in NaOH electrolytes are different, although the pH of the two solutions is the same.
  • the Ti surface of the substrate is unaffected at potentials below 0.8 V.
  • potentials above 0.8 V a diffusion- controlled oxidation reaction up to a potential of 5 V as indicated by a single peak with positive current.
  • the cyclic voltammetry of Ti in the KOH electrolyte can be used a guide for predictive experimentation to control the surface reactions and eventually surface structure and/or composition.
  • Titanium electrodes (anode and cathode) were subjected to electrochemical control, for example, a constant potential control, in NaOH and KOH solutions.
  • Solution concentrations of 1 M, 5 M and 10 M were tested and it was found that 5 M solutions produced the desired titania nanostructures .
  • No or very little nanostructures were observed on the electrodes that were prepared in 1 M solutions, even at increased times.
  • 10 M solutions although surface roughness was observed after electrochemical control, feature sizes were several hundreds of micrometers with little evidence of nanometer sized structures.
  • Controls corresponding to each electrochemical example were prepared by immersing Ti substrates in the respective electrolyte for the respective time without any applied potential. Electrodes were also subjected to varying time (i.e. the time under electrochemical control) . For the electrodes with electrochemical control for 30 minutes and 2 hours, no nanostructures were observed both in NaOH and KOH solutions. Scanning electron microscope (SEM) micrographs of SP09-011 these electrodes (not shown) were similar to those of the controls .
  • SEM scanning electron microscope
  • Figure 3a, Figure 3b, Figure 3c, and Figure 3d are SEM micrographs of Ti substrates that were subjected to a constant potential of 5 V for 6 hours in 5 M NaOH solution.
  • Figure 3a and Figure 3c correspond to those of the anode (i.e. the surface experiences a positive potential) and
  • Figure 3b and Figure 3d correspond to those of the cathode (i.e. the surface experiences a negative potential) .
  • FIG. 3a and Figure 3b are SEM micrographs of the Ti substrates after being rinsed in DI water and dried under a nitrogen flow following electrochemical processing.
  • the titania nanostructures comprise an open (porous) network 18 connected by short, nanometer sized (width) Ti ⁇ 2 nanowires 20.
  • the "grainy" features are due, in part, to the presence of the leftover NaOH that did not wash out during DI water rinse. This was confirmed by the presence of sodium peaks in X-ray diffraction (XRD) analysis.
  • XRD X-ray diffraction
  • FIG 3c and Figure 3d are SEM micrographs of the substrates after being rinsed, acid-washed and dried following electrochemical processing.
  • the substrates were immersed in a mild acid, for example, 1 M HCl, for 30 minutes followed by rinsing in DI water.
  • Well defined titania nanostructures similar to those observed in Figure 3a and Figure 3b are present sans the graininess. This is due, in part, to the complete removal of NaOH by acid-washing.
  • the titania nanostructures comprise an open (porous) network 18 connected by short, nanometer sized (width) TiO ⁇ nanowires 20. This represents a very high surface area surface with very good electrolyte access to the entire surface through open pores .
  • SP09-011 is a very high surface area surface with very good electrolyte access to the entire surface through open pores.
  • the sizes of the nanowires in these networks ranged between from IOnm to 40 nm with an average around 30 nm.
  • These high-surface area structures possess an increased accessibility for liquids or gases to the entire surface area or gases which is an advantageous attribute in applications where material utilization is to be maximized (e.g. photovoltaic cells).
  • Figure 4a, Figure 4b, Figure 4c, and Figure 4d are SEM micrographs of Ti electrodes that were subjected to a constant potential of 5 V for 6 hours in 5 M KOH solution.
  • Figure 4a and Figure 4c correspond to those of the anode and
  • Figure 4b and Figure 4d correspond to those of the cathode.
  • Figure 4a and Figure 4b are SEM micrographs of the Ti substrates after being rinsed in DI water and dried under a nitrogen flow following electrochemical processing.
  • Figure 4c and Figure 4d are SEM micrographs of the substrates after being rinsed, acid-washed and dried following electrochemical processing.
  • the substrates were immersed in a mild acid, for example, 1 M HCl, for 30 minutes followed by rinsing in DI water. No to minimal discernible nanostructures were formed under these conditions.
  • Figure 4a appears to have some structure on the surface, of which disappears after acid wash, as shown in Figure 4c.
  • Figure 5a and Figure 5b are SEM micrographs of Ti substrates processed under a constant potential control of 5 V SP09-011 for 16 hours in 5 M NaOH solution.
  • Figure 5a corresponds to the anode and
  • Figure 5b corresponds to the cathode .
  • the surface exhibits webbed titania nanostructures with the connecting titania nanowires 22 having finer sizes as compared to the 6 hour electrode, shown in Figure 3a.
  • the average sizes of the titania nanowires are less than 10 nm and several titania nanowires are bundled together forming a high surface area network.
  • the titania nanostructures 24 on the counter electrode seem to have collapsed, since they are more closed than the corresponding 6 hour electrode, shown in Figure 3b, possibly due to some sort of a coalescence effect. Nevertheless, these disordered structures are still in the sub-100 nm regime.
  • Figure 6a and Figure 6b are SEM micrographs of Ti substrates processed under a constant potential control of 5 V for 16 hours in 5 M KOH solution.
  • Figure 6a corresponds to the anode
  • Figure 6b corresponds to the cathode.
  • both the anode and the cathode possess an interwoven network of titania nanostructures 26, for example, titania nanowires.
  • the titania nanowires have high surface area and good accessibility to the titania nanostructures even deep into the substrate.
  • the anode possesses uniform distribution of sub-10 nm sized titania nanowires while the cathode possesses titania nanowires that are predominantly around 30 nm.
  • An advantageous feature of the titania nanostructures is the amount of surface connectivity.
  • the titania nanowires are intricately and inseparably connected to each other to the point where it is almost impossible to identify the start and end of any given strand of titania nanowire.
  • the surface structure of the titania nanostructures can be manipulated by manipulating processing conditions such as electrolyte composition, time, electrode polarity (anode vs. cathode), electrode potential or combinations thereof.
  • Figure 7a and Figure 7b are cross-sectional SEM micrographs of the 16 hour electrode synthesized in 5 M NaOH
  • the titanium to titania interface 28 illustrates a good substrate-to-nanostructure connectivity.
  • the layer of titania nanostructures 30 across the titanium substrate 32 is fairly uniform.
  • the average thickness of the layer of nanostructures is around 500 nm.
  • the thickness can be controlled, for example, by controlling the time of electrochemical control within the optimum time range, as too little ( ⁇ 6 hours) or too high a time will not yield the desired nanostructures.
  • a 72 hour experiment Ti under potential control in KOH or NaOH caused the collapse of nanostructures; this might be due to the mechanical collapse of the nanostructures as Ti surface is continually being subjected to continuous dissolution- redeposition.
  • Table 1 shows the summary of XRD analysis performed on the Ti electrodes synthesized in 5 M NaOH and 5 M KOH solutions for 16 hours under electrochemical control.
  • the electrodes were subjected to heat-treatment prior to XRD analysis.
  • the heat treatment comprised heating the electrodes to 500 0 C at a rate of 1O 0 C per minute and holding at 500 0 C for 1 hour.
  • the controls in both the electrolytes did not yield any oxides showing that the surface remained in the metallic state.
  • the anode (working electrode) in both cases showed the presence of metallic Ti and Rutile and Anatase phases of Ti ⁇ 2.
  • the metallic phase is the background from the Ti substrate.
  • the cathode (counter electrode) exhibited the presence of only the Rutile phase of Ti ⁇ 2 in addition to the Ti metal background from the substrate.
  • This feature could be favorably exploited to selectively synthesize TiO 2 nanostructures with a desired phase or phases.
  • the nanostructures remained intact after heat treatment. Also, one could subject these electrodes to further heat treatment to obtain the desired phases.
  • Figure 8a, Figure 8b, Figure 8c, and Figure 8d are a series of SEM micrographs of the 16 hour anode synthesized in KOH solution taken at increasing magnifications (500X, 2500X, 10,00OX and 25,000X) shown in Figure 6a.
  • This electrode was chosen for illustrative purposes only; other electrodes show similar behavior.
  • the titania nanostructures are formed uniformly across the entire surface and not merely discrete islands of nanostructures.
  • This is an advantage of using an electrochemical process where the entire surface can be manipulated uniformly. This has an important implication in terms of scalability and manufacturability of this process.
  • a bigger substrate along with a bigger electrochemical cell can be used to manufacture SP09-01 1 various quantities (few mm 2 to several m 2 ) of TiO 2 nanostructures .
  • the method comprises making the nanostructures in a batch process. In another embodiment, the method comprises making the nanostructures in a continuous process .
  • the process could be a batch process where sheets of Ti or Titanium coated substrates (for example, a Ti film on an indium tin oxide (ITO) or a copper substrate or a Ti film on a polymer substrate such as polyethylene terephthalate (PET)) can be immersed in the electrolyte (NaOH or KOH) and nanostructures created by applying an electric potential .
  • ITO indium tin oxide
  • PET polyethylene terephthalate
  • Another embodiment that could be envisioned is a continuous process wherein two Ti or Ti coated substrate rolls could be continuously fed into a tank containing NaOH or KOH while electric potential is being applied.
  • a downstream cleaning and/or rinsing step could be integrated producing rolls of TiO 2 nanostructured surfaces.
  • the reaction since the reaction is limited to the surface that is in contact with the electrolyte, excellent process control can be achieved. In both embodiments, the process can be monitored by monitoring the current as a function of time.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Electrodes For Compound Or Non-Metal Manufacture (AREA)
  • Inorganic Compounds Of Heavy Metals (AREA)
  • Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

Abstract

L'invention porte sur des procédés électrochimiques pour fabriquer des nanostructures, par exemple, des nanostructures d'oxyde de titane (TiO2). La morphologie des nanostructures peut être manipulée par commande de paramètres de réaction, par exemple, la composition de solution, la tension appliquée et le temps. Les procédés peuvent être utilisés dans des conditions ambiantes, par exemple, la température ambiante et la pression atmosphérique, et utiliser des potentiels électriques modérés. Les procédés sont adaptables avec un haut degré d'aptitude à la commande et de reproductibilité.
EP09714589A 2008-02-28 2009-02-18 Procédés électrochimiques de fabrication de nanostructures Withdrawn EP2260123A1 (fr)

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US12/038,847 US8101059B2 (en) 2008-02-28 2008-02-28 Methods of making titania nanostructures
US12/363,162 US20100193363A1 (en) 2009-01-30 2009-01-30 Electrochemical methods of making nanostructures
PCT/US2009/001067 WO2009108286A1 (fr) 2008-02-28 2009-02-18 Procédés électrochimiques de fabrication de nanostructures

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