WO2011085731A1

WO2011085731A1 - Materials for photoelectrocatalytic hydrogen production

Info

Publication number: WO2011085731A1
Application number: PCT/DK2011/050008
Authority: WO
Inventors: Billie Abrams; Peter Vesborg; Ib Chorkendorff; Yidong Hou; Su-il In; Konrad Herbst
Original assignee: Danmarks Tekniske Universitet
Priority date: 2010-01-14
Filing date: 2011-01-14
Publication date: 2011-07-21

Abstract

The invention relates to the direct conversion of solar energy to chemical fuels, e.g. hydrogen, utilizing a combination of a visible and/or infrared light absorber with a non-platinum (Pt) hydrogen evolution reaction (HER) catalyst deposited on its surface. The invention discloses a hydrogen evolution reaction (HER) material comprising a molecular cluster core of the formula L_xN_yM_z, where L is selected from molybdenum (Mo), tungsten (W), tantalum (Ta), rhenium (Re), iron (Fe), nickel (Ni), niobium (Nb), chromium (Cr) and cobalt (Co); N is selected from oxygen (O), nitrogen (N), sulphur (S), selenium (Se) and tellurium (Te); M is selected from copper (Cu), cobalt (Co), manganese (Mn), or any other transition metal; x is an integer selected from 2, 3, 4 and 5; y is an integer selected from 1, 2, 3, 4 and 5; z is an integer selected from 0, 1, 2, 3, 4 and 5. The invention further discloses a system comprising said HER material and a photoabsorptive positive type (p-type) semiconductor with a low band gap, where said HER material is positioned on the surface of said photoabsorptive semiconductor, thereby enhancing the hydrogen production.

Description

Materials for photoelectrocatalytic hydrogen production

The invention relates to the direct conversion of solar energy to chemical fuels, e.g. hydrogen, utilizing a combination of a solar energy absorber with a non-platinum (Pt) hydrogen evolution reaction (HER) catalyst deposited on its surface.

Background

The consequences of the ongoing global warming has increased the focus on exploiting alternative energy sources in order to reduce the use of fossil fuels such as oil or coal as the main fuel sources. It is thus desirable to mimic Nature's utilization of the solar energy by directly converting solar energy to chemical fuels such as hydrogen gas (H₂ gas).

One of the main requirements when creating artificial systems for hydrogen production is that materials involved are cost-effective materials capable of absorbing sun light, i.e. ultra violet (UV), visible light and near infra red (IR) light efficiently.

The production of hydrogen gas using electrolysis is based on splitting of wa- ter molecules by a two-fold process, where first water is split into oxygen gas (O₂ gas) and protons (H⁺) and secondly the protons are reduced to hydrogen gas (H₂ gas) in a hydrogen evolution reaction (HER):

_{2H 0} solarenergy _? ^ + ₊ ^ solar energy , HER _{? 2fJ} ^ ₊ ^

The process of splitting water requires photons with a Gibbs free energy of AG = 1 .23eV (1008nm). However, due to various loss mechanisms this value should be closer to about 2eV (620nm) for practical application. Thus, photons from the high energy part of the visible spectrum extending to the ultra violet (UV) part can be used for this reaction, if only one photon is used for each transferred electron. In order to fully take advantage of the entire solar spectrum, it would be advantageous to split the reaction up and also utilize the low energy part of the visible spectrum and the IR part, thereby making it a two-photon process. Focusing on the HER, the requirements for materials are among other things that they are stable, do not corrode and are cost-effective. It is further advantageous if the hydrogen evolving photocatalyst can absorb visible and/or IR light, thereby utilizing the low energy part of the solar spectrum. Oxides have been proposed as light absorbing materials for HER. However, oxides generally have too large band gaps and thus are only able to access the UV portion of the solar spectrum, ultimately limiting their efficiency. Some oxides, such as e.g. copper oxide (Cu₂O), have shown to be able to work as HER using visible light. However, these oxides are neither active nor stable.

Silicon (Si) is an interesting candidate as a light absorbing material for HER, as it has a low band gap enabling harvesting of the visible and IR light from the sun. Si is additionally very abundant and thus inexpensive. However, Si alone has a low HER activity, and modification of the material is therefore needed.

Examples of hydrogen evolving photocatalyst, where platinum-based (Pt- based) structures are employed as electrocatalyst on Si, have been demonstrated. However, Pt is an expensive and rare material and the cost of such systems is far too high for large scale production. Hydrogen gas produced by Pt-based photoelectrocatalytic systems will thus not be a commercially viable and competitive fuel when compared to traditional fuels such as oil and coal.

As alternatives to Pt-based structures employed as electrocatalysts on Si, electrocatalysts based on palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium (Ir) or alloys thereof have been suggested. However, as all of these structures are based on rare materials, they too are expensive to produce and the cost of such systems is far too high for large scale production and thus not commercially competitive compared to traditional fuels such as oil and coal.

Description of the invention

Disclosed herein is a hydrogen evolution reaction (HER) material for photo- electrocatalytic hydrogen production, said HER material comprising a molecular cluster core of the formula L_xNyM_z, where L is selected from molybde- num (Mo), tungsten (W), tantalum (Ta), rhenium (Re), iron (Fe), nickel (Ni), niobium (Nb), chromium (Cr) and cobalt (Co); N is selected from oxygen (O), nitrogen (N), sulphur (S), selenium (Se) and tellurium (Te) ; M is selected from copper (Cu), cobalt (Co), manganese (Mn), or any other transition metal ; x is an integer selected from 2, 3, 4 and 5; y is an integer selected from 1 , 2, 3, 4 and 5; z is an integer selected from 0, 1 , 2, 3, 4 and 5. Further, said HER material is to be positioned on the surface of a photoabsorptive semiconductor having a low band gap, whereby said HER material is enhancing the hydrogen production. By the term a low band gap is meant a material with a band gap, which enables harvesting of the visible and I R light from the sun. Normally a band gap of 2.0eV or less is adequate for this. In combination with the molecular cluster core, the HER material also normally contains hydrophobic ligands, such as e.g. Cp-ligands (cyclopentadienyl ligands or methylcyclopentadienyl ligands). Depending on which ligands are used, the HER process can be enhanced to different degrees. In order to be able to attach the HER material to the photoabsorptive positive type (p-type) semiconductor, the ligands need to be hydrophobic as opposed to hydrophilic. Counter anions, such as e.g. p- toluenesulfonate, sulfate, or hexafluorophosphate, are also normally present in combination with the HER material. Which of these anions that are present, is a matter of preparation. By the above is obtained a photoelectrocatalytic material for hydrogen production with direct utilization of visible/infra red (IR) solar energy to produce hydrogen through water splitting. The material is inexpensive and abundant, thereby making it cost-effective and, in combination with an oxygen evolution reaction site, a promising competitor to fossil fuels. Further, the system is stable over long time periods during activity measurements, and it does not corrode. In one or more embodiments, said molecular cluster core of said HER material has an incomplete cubane cluster core structure, which is advantageous, as the structure is known to be active.

In one or more embodiments, said molecular cluster core of said HER mate- rial is of the formula Mo_xS_y. This is a non-Pt based, i.e. non-precious materials-based system, and is as such inexpensive and viable. In one or more embodiments, said molecular cluster core of said HER material is of the formula Mo₃S₄. In one or more embodiments, said molecular cluster core of said HER material has a cubane cluster core structure, which is advantageous, as the structure is known to be active.

In one or more embodiments, said molecular cluster core of said HER mate- rial is of the formula Mo_xSyMz, where M = Cu, Co, Mn, or any other transition metal. This is a non-Pt based, i.e. non-precious materials-based system, and is as such inexpensive and viable. In one or more embodiments, said molecular cluster core of said HER material is Mo₃S₄Cu. In one or more embodiments, said HER material further comprises x hydrophobic ligands, where x is said integer selected from 2, 3, 4 and 5. It is thereby obtained that the HER material can be attached to e.g. a Si surface. Using a corresponding cubane cluster with hydrophilic ligands would make this nearly impossible.

In one or more embodiments, said x hydrophobic ligands are cyclopentadi- enyl ligands or methylcyclopentadienyl ligands. More specifically, in one or more embodiments, said molecular cluster core of said HER material is of the formula Mo₃S₄ and said hydrophobic ligands are methylcyclopentadienyl ligands. In other embodiments, said molecular cluster core of said HER material is of the formula Mo₃S₄Cu and said hydrophobic ligands are methylcy- clopentadienyl ligands.

Disclosed herein is also a system for hydrogen production comprising a hydrogen evolution reaction (HER) material comprising a molecular cluster core of the formula L_xNyM_z, where L is selected from molybdenum (Mo), tungsten (W), tantalum (Ta), rhenium (Re), iron (Fe), nickel (Ni), niobium (Nb), chro- mium (Cr) and cobalt (Co); N is selected from oxygen (O), nitrogen (N), sulphur (S), selenium (Se) and tellurium (Te) ; M is selected from copper (Cu), cobalt (Co), manganese (Mn), or any other transition metal ; x is an integer selected from 2, 3, 4 and 5; y is an integer selected from 1 , 2, 3, 4 and 5; z is an integer selected from 0, 1 , 2, 3, 4 and 5; and a photoabsorptive positive type (p-type) semiconductor with a low band gap, where said HER material is positioned on the surface of said photoabsorptive semiconductor, thereby enhancing the hydrogen production.

By the term a low band gap is meant a material with a band gap, which en- ables harvesting of the visible and I R light from the sun. Normally a band gap of 2.0eV or less is adequate for this. In combination with the molecular cluster core, the HER material also normally contains hydrophobic ligands, such as e.g. Cp-ligands (cyclopentadienyl ligands or methylcyclopentadienyl ligands). Depending on which hydrophobic ligands are used, the HER proc- ess can be enhanced to different degrees. In order to be able to attach the HER material to the photoabsorptive positive type (p-type) semiconductor, the ligands need to be hydrophobic as opposed to hydrophilic. Counter anions such as e.g. p-toluenesulfonate, sulfate, or hexafluorophosphate are also normally present in combination with the HER material. Which of these anions that are present, is a matter of preparation.

By the above is obtained a system for hydrogen production with direct utilization of visible solar energy to produce hydrogen through water splitting. The system is based on inexpensive and abundant materials, thereby making it cost-effective and, in combination with an oxygen evolution reaction site, a promising competitor to fossil fuels. Further, the system is typically stable over long time periods during activity measurements, and it does not corrode. In one or more embodiments said HER material is deposited on said surface of said photoabsorptive semiconductor by the method of drop casting, which is a simple, quick and cheap method yielding controllable distribution of the HER material. In one or more embodiments said HER material is deposited on said surface of said photoabsorptive semiconductor by the method of electrophoretic deposition, whereby the concentration and the distribution of the HER material is controllable. In one or more embodiments, said photoabsorptive semiconductor is silicon (Si), which is an effective absorber of visible and IR-light. In combination with an oxygen evolution reaction side, which utilizes the ultra violet and the high- energy part of the visible light from the sun, the entire solar spectrum is utilized. In one or more embodiments, said Si has a nanostructure or microstructure with a non-planar structure. This gives a significant enhancement of the hydrogen evolution efficiency by enhancing the surface area and making the charge carrier collection and transfer more efficient as compared to planar surfaces. In one or more embodiments, said non-planar Si structure is Si pillars.

Disclosed herein is further a method for hydrogen production, wherein said method comprises the step of illuminating a photoelectrocatalytic cell com- prising a system according to the above with sun light. In one embodiment, visible and/or infra red light from the sun light is used for illuminating said photoelectrocatalytic cell, whereby direct utilization of visible solar energy to produce hydrogen through water splitting is obtained. The method is based on inexpensive and abundant materials, thereby making it cost-effective and, in combination with an oxygen evolution reaction site, a promising competitor to the now used fossil fuels.

Disclosed herein is also the use of a system according to the above for hydrogen production, whereby direct utilization of visible and infra red solar en- ergy to produce hydrogen through water splitting is obtained. As the system described above is based on inexpensive and abundant materials, the use of it for hydrogen production produces a cost-effective and, in combination with an oxygen evolution reaction site, a promising competitor to fossil fuels. Brief description of the drawings

Figure 1 illustrates a cell in which the photoelectrochemical measurements are performed.

Figure 2a illustrates the principle behind an embodiment of the photoelectro- catalytic system according to the invention. Figure 2b illustrates the principle behind an embodiment wherein the photo- e!ectrocatalystic system is embedded in a 'chemical solar cell'.

Figures 3a-c illustrate scanning electron microscope (SEM) images of an embodiment of the Si pillars according to the invention.

Figure 4 illustrates an embodiment of the molecular cluster core HER material. Figure 5 illustrates the concentration dependence of the HER catalyst shown in figure 4 deposited on planar Si.

Figure 6 illustrates the HER photocurrent for different Si samples with and without the HER catalyst shown in figure 4.

Figure 7 illustrates the incident photon to current efficiency (IPCE) for different combinations of Si structures with and without the HER catalyst shown in figure 4. Figure 8a and b illustrate another embodiment of the molecular cluster core HER material and the cluster core HER material with ligands, respectively.

Figure 9 illustrates the HER photocurrent for different Si samples with and without the HER catalyst shown in figure 8.

Figure 10 illustrates the incident photon to current efficiency (IPCE) for different combinations of Si structures with and without the HER catalyst shown in figure 8. Figure 1 1 illustrates the dry etch procedure for processing of Si pillars. Description of preferred embodiments

Figure 1 illustrates a pyrex glass cell 100 wherein the photoelectrochemical measurements are normally performed. The pyrex glass cell 100 comprises two compartments 102, 104; one for the working electrode 106 and one for the counter electrode 108, separated by a glass frit 1 10. In the working electrode compartment 102, a flat surface is created in order to let light pass with minimal scattering. The working electrode compartment 102 is connected to a third compartment 1 12, which houses the reference electrode 1 14, Hg/HgS0₄. The counter electrode 108 for all experiments is a platinum (Pt) mesh. The electrolyte for all experiments is HCI0 . The counter electrode compartment 104 is bubbled with hydrogen 1 16, thereby making it a hydrogen electrode. This additionally creates a lower overall cell potential, since the Pt counter electrode 108 only needs to oxidize hydrogen, not water. The working electrode compartment 102 is bubbled with Argon. For all experi- ments the Hg/HgS0₄ potential is calibrated against a RHE (reversible hydrogen electrode). Alternatively, the measurements can be done in other electrolytes, e.g. H₂S0₄, with other reference electrodes and another cell configuration. In a preferred embodiment of the invention, the working electrode 106 is made from silicon (Si). The Si can have different nano-structural designs, such as e.g. a planar surface, a pillared or pyramidal surface. The preparation of a planar single crystal (100) positive type (p-type) Si electrode can be done by a process, wherein each Si sample is cleaned in an ethanol/Millipore water mixture and subsequently sonicated to remove any possible particulate contamination. Alternatively, the Si samples can be cleaned in a Piranha solution of concentrated H₂S0₄ and 30% H₂0₂ in a 3:1 mixture.

Following a water rinse, the blank Si sample is dipped in a 1 % HF solution in order to etch away the native oxide and hydrogen-terminate the surface. After this step, an ohmic contact to the unpolished backside of the Si wafer is made by using a gallium indium (Ga-ln) eutectic mixture in the liquid form, which is scratched into the backside of the Si. The Ga-ln solution is deposited onto the copper wire coil that is used to make connections through a glass tube making a sealed and enclosed system. The Cu wire is glued to the Ga-ln mixture on the backside of the Si with silver paint. After these components have dried, Hysol 1 C epoxy is used to seal the glass tube to the other components and backside of the sample. It is also used to insulate and protect all the contact components from the electrolyte. The epoxy is cured at approximately 80°C. It is important not to get the epoxy on the front side of the sample where eventually the HER material will be deposited.

High surface area silicon samples as shown in figures 3a-c can be prepared by an advanced dry etch process, which is explained in detail in the description of figure 1 1 . As such, there are no metallic catalysts present at the tips of the pillars as in other processing methods such as vapor-liquid-solid processing (VLSP) of pillars.

Figure 2a illustrates the principle behind one embodiment of the photoelec- trocatalytic system 200 according to the invention, wherein a HER material 202, e.g. with the molecular cluster core Mo₃S₄ or Mo₃S₄Cu, is deposited on the surface of a Si pillar 204. Solar energy 206 is exciting the Si pillar 204, whereby an electron is promoted from the valence to the conduction band creating an electron-hole pair. The HER material utilizes the electrons to reduce the protons thereby generating hydrogen gas as shown in the figure.

In combination with the molecular cluster core, the HER material 202 also normally contains hydrophobic ligands, such as e.g. Cp-ligands (cyclopenta- dienyl ligands or methylcyclopentadienyl ligands). Depending on which ligands are used, the HER process can be enhanced to different degrees. Counter anions such as e.g. p-toluenesulfonate, sulfate, or hexafluorophos- phate are also normally present in combination with the HER material 202. Which of these anions that are present, is a matter of preparation. The anions can very easily be exchanged by an ion-exchange process. However, it is difficult and time-consuming to exchange the ligands at the metals in the cluster core.

Figure 2b illustrates the principle behind an embodiment wherein the photo- eiectrocataSystic system is embedded in a 'chemical solar cell'. The pillared structure ensures an effective harvesting of the photons and an effective photocurrent collection (by virtue of the length available for absorption com- bined with short, radial, minority carrier transport distances) while also exposing a large surface area for the catalytic reactions. In the 'chemical solar cell' the blue part 208 of the solar spectrum is absorbed by an 'oxidizing' photoanode 210 - possibly with a oxygen evolution catalyst 212 - where water is oxidized to release oxygen and protons.

The red part of the solar spectrum 214, unabsorbed by the large band-gap photoanode 210, passes through to be absorbed by the 'reducing' photo- cathode 204 where protons are reduced to evolve hydrogen. In this embodiment of the 'reducing' photocathode according to this invention, a HER mate- rial 202 - e.g. with the molecular cluster core Mo₃S₄ or Mo₃S₄Cu with hydrophobic ligands, e.g. cyclopentadienyl ligands or methylcyclopentadienyl ligands - is deposited on the surface of a Si pillar 204. The two-photon tandem approach allows access to a larger part of the solar spectrum than single-photon water splitting. The two photoanodes are separated by a mem- brane 216.

Since Si has a band gap of -1 .1 eV, low energy illumination such as red light or even infrared light from the sun can be used to excite the charge carriers. Subsequently, the minority carriers, i.e. the electrons, generated from this excitation process can be transferred to the HER catalyst enhancing, or rather catalyzing, the reduction of protons to hydrogen. An example of the blank Si pillars 204 is shown in scanning electron microscope (SEM) images in figures 3a-c. The pillars are normally of the p-type. Figure 3c is a zoom of figure 3b, which again is a zoom of figure 3a showing the Si pillars 204 and the geometrical structure in which they are arranged. The embodiment of the Si pillars 204 shown in the figures has a diameter of 3 μιτι and a length of 40-50 μιτι, which creates a much shorter distance between the charge carrier generation site and the surface of the pillars as compared to planar Si. Hence, there is a larger probability for the charge car- riers to reach a HER material deposited onto the surface of the Si pillars as compared to planar Si.

The Si pillars 204 shown in figures 3a-c are just one possible example of a diameter-length configuration, and multiple other diameter-length configura- tions, which can show to be more effective, can also be utilized. Generally, a larger amount of HER material 202 can be deposited on the surface if the length of the pillars 204 is increased or if the density of Si pillars 204 is increased, e.g. by using thinner Si pillars 204. The HER efficiency will, as a consequence, depend on the length of the pillars 204, the length-diameter ratio of the pillars 204 and the pattern in which they are arranged on a surface.

The pattern 300 of the Si pillars displayed in figures 3a-c with empty circles 302, 304 placed in between the Si pillars is arbitrarily chosen. A surface with- out the empty circles, with fewer or more circles, or with empty spaces in the shape of squares, triangles or any other type of structure could has been used as an alternative.

Besides round Si pillars as shown in figures 3a-c, different geometrical struc- tures of the Si can be used. These include flat surfaces, hollow tubes, circular dots, oval tubes, triangular shaped tubes, and the like. Also, instead of Si, alternative semiconducting materials with a low band gap of 2.0 eV or less can be used. Some examples of such materials are germanium (Ge), indium phosphide (InP), indium arsenide (InAs), gallium arsenide (GaAs) and semiconducting compounds with tin (Sn), which are all known to be able to harvest the visible and the I R light from sun.

The HER material 202 can in general be described as material with a molecular cluster core with a structure of the type L_xNyM_z, where L = molybde- num (Mo), tungsten (W), tantalum (Ta), rhenium (Re), iron (Fe), nickel (Ni), niobium (Nb), chromium (Cr) and cobalt (Co); N = oxygen (O), nitrogen (N), sulphur (S), selenium (Se) and tellurium (Te) ; M = copper (Cu), cobalt (Co), manganese (Mn), or any other transition metal ; and x is an integer between 2-5, y is integers between 0-5, and z is an integer between 0-5. In combina- tion with the molecular cluster core, the HER material 202 also normally contains hydrophobic ligands (not shown in this figure), such as e.g. Cp-ligands (cyclopentadienyl ligands or methylcyclopentadienyl ligands). Depending on which ligands are used, the HER process can be enhanced to different degrees. Counter anions such as e.g. p-toluenesulfonate, sulfate, or hexafluorophosphate are also normally present in combination with the HER material 202. Which of these anions that are present, is a matter of preparation. The anions can very easily be exchanged by an ion-exchange process. However, it is difficult and time-consuming to exchange the ligands at the metals in the cluster.

An example 400 of a molecular core structure of the HER material Mo₃S₄Cu 202 is shown in figure 4, wherein the molybdenum (Mo) atoms are denoted 402, the sulphur (S) atoms 404 and the copper (Cu) atom 406. Normally, the HER material 202 has a cubane cluster core structure e.g. deposited on the semiconducting material, e.g. the Si pillars. The HER material 202 can be electrophoretically deposited onto the pillared Si surface. In one embodiment of the invention, the HER material 202 is electrophoretically deposited onto the pillared Si surface by pressing a silicon electrode against an O-ring sealed opening in an electrochemical cell, where- in 0.25 cm² is exposed to the electrolyte. A counter electrode is used, wherein the cross section of the counter electrode is parallel to that of the silicon electrode. The distance between the two electrodes is kept at around 4 mm. A certain amount, e.g. 0.30 ml, of a HER material solution in a dichloro- methane/methanol solvent mixture, typically a 1 :1 volume mixture, is trans- ferred to the electrochemical cell. The silicon electrode and a graphite electrode are connected to the negative and positive terminals of the power supply, respectively, where after a DC of 10 V is applied between the two electrodes for 5 min. The HER material 202 can also be deposited onto the Si surface in a number of different ways such as e.g. spin coating, spray coating, drop casting, dip coating, electrochemical deposition, electroless deposition, gas phase deposition, chemical vapour deposition and pulse injection. Figure 5 illustrates the concentration dependence of the HER material of figure 4 with the molecular core structure Mo₃S Cu, which has been electro- phorectically deposited on planar p-type Si 500. The concentration of Mo₃S₄Cu is 0 μΜ, 1 μΜ, 5 μΜ, 10 μΜ and 20 μΜ in the curves 502, 504, 506, 508 and 510, respectively. The ligands in this embodiment are methylcy- clopentadienyl.

As can be seen from figure 5, the optimum concentration of Mo₃S₄Cu on planar Si is about 10 μΜ. Figure 6 illustrates the HER photocurrent enhancement 600 of planar Si 602, planar Si with 10 μΜ of Mo₃S₄Cu 604, Si pillars 606 as shown in the SEM images in figures 3a-c and Si pillars with 10 μΜ of Mo₃S₄Cu 608 after illuminating the samples with light having wavelengths above 645 nm obtained from a xenon lamp. The ligands in this embodiment are methylcyclopentadi- enyl.

By using the type of blank Si pillars 204 shown in figures 3a-c, a surface area enhancement of a factor of 6 is observed compared to blank planar Si, where the comparison is made between the curves 606 and 602. However, the photocatalytic HER activity is enhanced by a factor of 50 when changing from blank planar Si 602 to blank Si pillars 606.

Depositing 10 μΜ of the HER material with the molecular core structure Mo₃S₄Cu onto the Si pillars further increases the efficiency by a factor of 4 compared to the activity of the blank Si pillars, where the comparison is made between the curves 608 and 606. Hence, in total there is an activity enhancement of 200 when using Si pillars with Mo₃S Cu compared to the efficiency of the planar Si surface, where the comparison is made between the curves 608 and 602. All the enhancement factors are for a photon to electron efficiency at 0.1 V bias. However, even at zero V. i.e., no applied over poten- tial, it can bee seen from figure 6 that there is a significant current per area of about 1 .5mA/cm².

The significant enhancement in activity observed upon comparing planar blank Si 602 to Si pillars with Mo₃S₄Cu deposited on 608 does not seem to be solely due to an enhancement in the surface area of Si, since this only corresponds to a factor of 15. In other words, the surface area enhancement factor of 15 cannot alone explain the activity being increased by a factor of 200. One explanation is that there is an enhanced charge carrier capture, i.e. a lower recombination loss and a lower reflection loss. In other words, the charge carriers in the planar Si compared to the Si pillars must travel too far before they can reach the surface and be transferred to a HER catalyst. During that distance they can be scattered or recombined. Thus, factors such as charge transfer and transport are significantly affected by the change in Si geometry.

Note that the enhancement in activity shown in figure 6 is for one configuration where Mo₃S₄Cu has been electrophoretically deposited onto the pillared Si surface at a coverage that approximately corresponds to less than one monolayer. Other enhancement factors will be observed if different sizes of Si pillars are used, e.g. when changing the diameter and the length of the pillars, or if an entirely different structure forming a non-planar Si surface is used. Figure 7 illustrates the incident photon to current efficiency (IPCE) 700 at -0.1 V vs. RHE for planar blank Si (702), planar Si with Mo₃S₄Cu (704), blank Si pillars (706), and Si pillars with Mo₃S₄Cu (708). The ligands in this embodiment are methylcyclopentadienyl. At -0.1 V vs. RHE, the maximum IPCE obtained is about 4.5 %. However, this can be improved through focused op- timizations of e.g. the deposition procedures of the HER material and the structure of the underlying Si.

A different example of a molecular core structure of the HER material 800 is the incomplete cubane-like structure Mo₃S₄ shown in figure 8a and b, wherein the molybdenum (Mo) atoms are denoted 802 and the sulphur (S) atoms 804. The position of three hydrophobic methylcyclopentadienyl ligands (Cp ligands) 806 attached to the Mo atoms is also shown in figure 8b.

The HER material 800 is in one embodiment deposited onto the pillared Si surface by drop casting. By choosing a hydrophobic ligand such as methylcyclopentadienyl as shown in figure 8b, the HER cubane-like structure be- comes insoluble in water but soluble in polar organic solvents, which is a necessity for obtaining an efficient drop-casting of the material onto the planar Si and the Si pillars. Various amounts of Mo₃S₄ cluster were drop-casted onto the planar Si and the Si pillars and a concentration of 2 nmol was found to be optimal. X-ray photoelectron spectroscopy (XPS) has shown that the Mo₃S₄ clusters are deposited on the Si surface in a concentration of 2.6 x 1 0¹³ Mo₃S₄ clusters/cm². This implies that most of the deposited clusters are lost into the electrolyte. Since pulling the sample out of the electrolyte in inert gas does not change the activity, 2.6 x 1 0¹³ Mo₃S₄ clusters/cm² is also the area density of the Mo₃S₄ clusters during measurements.

Figure 9 illustrates the HER photocurrent enhancement 900 of planar Si 902, planar Si with 2 nM of Mo₃S₄ 904, Si pillars 906 and Si pillars with 2 nM of Mo₃S₄ 908 after illuminating the samples with light having wavelengths above 645 nm obtained from a xenon lamp. The ligands in this embodiment are me- thylcyclopentadienyl as shown in figure 8b. The HER material has been deposited on the Si surfaces by us of the drop casting method.

In the dark all electrodes 902, 904, 906, 908 show negligible current. Under illumination, the blank planar Si 902 has an onset 91 0 of photocurrent at URHE = -0.4 V. However, upon depositing the Mo₃S₄ clusters onto the Si planar surface, a significant enhancement in the photoactivity is observed, where the onset 91 2 is now shifted to URHE = +0.1 5 V due to improved catalysis. This results in a hydrogen evolution current density of 8 mA/cm² at the reversible potential, U RHE = 0 V marked with the dotted line 91 4. This corresponds to an IPCE of 47%. Considering that only 2.6 x 1 0¹³ Mo₃S₄ clusters/cm² is present during operation, this rate of hydrogen evolution at the reversible potential corresponds to a turnover frequency (TOF) of 960 sec^"1 for the Mo₃S₄ modified planar Si surface. The blank Si pillars 906 are considerably improved relative to planar Si. The limiting current density of the pillar electrode 906 is below 16 mA/cm² at an over potential larger than -1 V, corresponding to an IPCE of 93%. This is 33% higher than that of the planar Si 902. Depositing 2 nmol Mo₃S₄ cluster onto the Si pillar (908) displayed a hydrogen evolution current density of 9.4 mA/cm² at the reversible potential, U_RHE = 0 V, corresponding to an IPCE of 55%. Bubbles dislodging from the photocathode could clearly be observed under these conditions.

Comparison of the blank Si surfaces 902, 906 and the Si surfaces with Mo₃S₄ cluster 904, 908 shows that the limiting current under large cathodic potential suffers slightly when Mo₃S₄ clusters are present. The reason for the decrease in limiting current could be hydrogen bubbles adhering to the surface causing the loss of effective surface area. The cluster-modified surface is more hydrophobic than the untreated surface making bubbles adhere much better.

Figure 9 also shows that the Si pillars with Mo₃S₄ cluster 908 have better activity than the planer Si with Mo₃S₄ cluster 904, although the gain is modest near the onset potential 912. More importantly, the pillar structure increases the limiting current (and IPCE) significantly. For high-quality single-crystal Si samples at low over potential, not much is gained by using Si pillars as opposed to planar Si surfaces, except for a lower reflectance loss. However, in a cheap gas-phase grown system, where the minority carrier lifetime would be much lower, pillars would be crucial for success. In effect, the Si pillars constitute a "model system" for vapour-liquid-solid grown Si pillars, and they demonstrate that solar driven hydrogen evolution can indeed be achieved with such configurations without using any Pt-group metals. Figure 10 illustrates the incident photon to current efficiency (IPCE) 1000 at 0 V vs. RHE for planar blank Si (1002), planar Si with Mo₃S₄ (1004), blank Si pillars (1006), and Si pillars with Mo₃S₄ (1008). The ligands in this embodiment are methylcyclopentadienyl as shown in figure 8b. The figure highlights the results seen in figure 9, emphasizing the significant increase in IPCE obtainable when depositing the hydrophobic Mo₃S₄ onto Si surfaces by drop casting. The maximum IPCE obtained is 55 % and is observed at 0 V vs. RHE for Si pillars with Mo₃S₄.

Figure 1 1 illustrates the dry etch procedure 1 100 for processing of the Si pillars, shown in detail in figures 3a-c. This procedure is step wise, where the first step 1 102 is a 30 sec buffered hydrogen fluoride (BHF) etch followed by, in the second step, 1 104 deposition of 2.2μΜ of the photoresist AZ-5214E. In the subsequent step 1 106, the Si is exposed to UV-light for 5s through a honeycomb pattern mask. In the next step, the Si is 1 108 firstly baked at 120°C for 120s and secondly flood exposed for 30s. In the subsequent step 1 1 10, the photoresist is developed for 80s in an aqueous solution of the developer AZ 351 . This is followed by 1 1 12, firstly another BHF etch and secondly a deep reactive-ion etching (DRIE) for 9-27min. In the final step 1 1 14, there is a resist strip in the plasma asher, thereby creating the Si pillars 1 1 16.

References

100 pyrex glass cell 100 for photoelectrochemical measurements

102 working electrode compartment

104 counter electrode compartment

106 working electrode (e.g. Si electrode)

108 counter electrode (Pt mesh)

1 10 glass frit

1 12 reference electrode compartment

1 14 reference electrode (Hg/HgS0₄)

1 16 hydrogen bubbling

200 photoelectrocatalytic system

202 HER material, e.g. Mo₃S₄Cu or Mo₃S₄

204 Si pillar

206 solar energy

208 blue part of the solar spectrum

210 photoanode

212 oxygen evolution catalyst on the photoanode 210

214 red part of the solar spectrum

216 membrane

300 pattern of the Si pillared surface

302 small empty circle without Si pillars

304 large empty circle without Si pillars

400 molecular core structure of the HER material Mo₃S₄Cu

402 molybdenum (Mo) atom

404 sulphur (S) atom

406 copper (Cu) atom

500 Mo₃S₄Cu electrophorectically deposited on planar p-type Si 502 planar Si

504 planar Si with 1 μΜ of Mo₃S₄Cu

506 planar Si with 5 μΜ of Mo₃S₄Cu

508 planar Si with 10 μΜ of Mo₃S₄Cu

510 planar Si with 20 μΜ of Mo₃S₄Cu

600 HER photocurrent enhancement

602 HER photocurrent of planar Si

604 HER photocurrent of planar Si with 10 μιη of Mo₃S₄Cu 606 HER photocurrent of Si pillars

608 HER photocurrent of Si pillars with 10 μιτι of Mo₃S₄Cu

700 incident photon to current efficiency (IPCE) at -0.1 V vs. RHE

702 IPCE for planar blank Si

704 IPCE for planar Si with Mo₃S₄Cu

706 IPCE for blank pillared Si

708 IPCE for pillared Si with Mo₃S₄Cu

800 molecular core structure of the HER material Mo₃S₄

802 molybdenum (Mo) atom

804 sulphur (S) atom

806 methylcyclopentadienyl ligand

900 HER photocurrent enhancement by drop-casting method 902 HER photocurrent of planar Si

904 HER photocurrent of planar Si with Mo₃S₄

906 HER photocurrent of Si pillars

908 HER photocurrent of Si pillars with Mo₃S₄

910 onset of the photocurrent for sample 902

912 onset of the photocurrent for samples 904 and 908

914 reversible potential 1000 incident photon to current efficiency (IPCE) at 0 V vs. RHE

1002 IPCE for planar blank Si

1004 I PCE for planar Si with Mo₃S₄

1006 IPCE for blank pillared Si

1008 I PCE for pillared Si with Mo₃S₄

1 100 dry etch procedure for processing of the Si pillars

1 102 an initial etch for 30 sec in buffered hydrogen fluoride (BHF)

1 104 deposition of 2.2μΜ of the photoresist AZ-5214E

1 106 exposed to UV-light for 5s through a so called "Honeycomb dummy 1 " mask

1 108 baking at 120 °C for 120s and flood exposed for 30s

1 1 10 developing for 80s in an aqueous solution of the developer AZ 351 1 1 12 a second BHF etch and deep reactive-ion etching (DRIE) for 9- 27min.

1 1 14 a resist strip in the plasma asher

1 1 16 Si pillars

Claims

1 . A hydrogen evolution reaction (HER) material for photoelectrocatalytic hydrogen production, said HER material comprising a molecular cluster core of the formula L_xN_yM_z, where:

- L is selected from molybdenum (Mo), tungsten (W), tantalum

(Ta), rhenium (Re), iron (Fe), nickel (Ni), niobium (Nb), chromium (Cr) and cobalt (Co);

- N is selected from oxygen (O), nitrogen (N), sulphur (S), selenium (Se) and tellurium (Te);

- M is selected from copper (Cu), cobalt (Co), manganese (Mn), or any other transition metal;

- x is an integer selected from 2, 3, 4 and 5;

- y is an integer selected from 1 , 2, 3, 4 and 5;

- z is an integer selected from 0, 1 , 2, 3, 4 and 5;

where said HER material is to be positioned on the surface of a photoab- sorptive semiconductor having a low band gap, whereby said HER material is enhancing the hydrogen production.

2. A HER material according to claim 1 , wherein said molecular cluster core of said HER material has an incomplete cubane cluster core structure.

3. A HER material according to claim 2, wherein said molecular cluster core of said HER material is of the formula Mo_xS_y.

4. A HER material according to claim 3, wherein said molecular cluster core of said HER material is of the formula Mo₃S₄.

5. A HER material according to claim 1 , wherein said molecular cluster core of said HER material has a cubane cluster core structure.

6. A HER material according to claim 1 or 5, wherein said molecular cluster core of said HER material is of the formula Mo_xSyM_z, where M = Cu, Co, Mn, or any other transition metal.

7. A HER material according to claim 6, wherein said molecular cluster core of said HER material is Mo₃S₄Cu.

8. A HER material according to any of the claims 1 -7, wherein said HER material further comprises x hydrophobic ligands, where x is said integer selected from 2, 3, 4 and 5.

9. A HER material according to claim 8, wherein said x hydrophobic ligands are cyclopentadienyl ligands or methylcyclopentadienyl ligands.

1 0. A HER material according to claim 9, wherein said molecular cluster core of said HER material is of the formula Mo₃S₄ and said hydrophobic ligands are methylcyclopentadienyl ligands.

1 1 . A HER material according to claim 9, wherein said molecular cluster core of said HER material is of the formula Mo₃S₄Cu and said hydrophobic ligands are methylcyclopentadienyl ligands.

1 2. A system for hydrogen production comprising :

• a hydrogen evolution reaction (HER) material according to any of the claims 1 -1 1 ;

· a photoabsorptive positive type (p-type) semiconductor with a low band gap, where said HER material is positioned on the surface of said photoabsorptive semiconductor, thereby enhancing the hydrogen production.

13. A system according to claim 12, wherein said HER material is deposited on said surface of said photoabsorptive semiconductor by the method of drop casting.

14. A system according to claim 12, wherein said HER material is deposited on said surface of said photoabsorptive semiconductor by the method of electrophorectic deposition.

15. A system according to any of the claims 12-14, wherein said photoabsorptive semiconductor is silicon (Si).

16. A system according to claim 15, wherein said Si has a nanostructure or microstructure with a non-planar structure.

17. A system according to claim 16, wherein said non-planar Si structure is Si pillars.

18. A method for hydrogen production, wherein said method comprises the step of illuminating a photoelectrocatalytic cell comprising a system ac- cording to any of the claims 12-17 with sun light.

19. A method according to claim 18, wherein visible and/or infra red light from the sun light is used for illuminating said photoelectrocatalytic cell.

20. Use of a system according to any of the claims 12-17 for hydrogen production.