Abstract
Photocatalysts and methods of using photocatalysts for producing hydrogen and oxygen from water are disclosed. The photocatalysts include an iodide modified photoactive material having an electrically conductive material attached to the iodide ions.
Claims
1. A photocatalyst comprising: a) a photoactive material comprising titanium dioxide and iodide ions attached to the surface of the titanium dioxide; and b) an electrically conductive material attached to the iodide ions, wherein the electrically conductive material includes a metal cation.
2. The photocatalyst of claim 1, wherein the metal is gold.
3. The photocatalyst of claim 2, wherein the gold is gold cations and ionic bonds are formed between the iodide ions and gold cations.
4. The photocatalyst of claim 3, comprising less than 1 wt. % of gold.
5. The photocatalyst of claim 4, wherein the gold is in the form of particles having an average particle size of ≦1 nm to 10 nm.
6. The photocatalyst of claim 1, wherein the titanium dioxide comprises anatase, rutile, brookite or a mixture thereof.
7. The photocatalyst of claim 6, wherein the titanium dioxide comprises single phase anatase.
8. The photocatalyst of claim 6, wherein the titanium dioxide comprises anatase and rutile, and the ratio of anatase to rutile ranges from 4:1 to 5:1.
9. (canceled)
10. The photocatalyst of claim 1, wherein the metal is platinum.
11. The photocatalyst of claim 1, wherein the metal is gold, ruthenium, rhenium, rhodium, palladium, silver, copper, osmium, iridium, platinum, or combinations thereof.
12. The photocatalyst of claim 11, wherein the metal is gold, palladium, or a combination thereof.
13. The photocatalyst of claim 1, wherein the electrically conductive material has an average particle size of 1 nm to 10 nm.
14-17. (canceled)
18. The photocatalyst of claim 1, wherein the photocatalyst is uncalcined.
19-20. (canceled)
21. A method for producing hydrogen gas and oxygen gas from water, the method comprising obtaining a water-splitting photocatalyst of claim 1 and subjecting the composition to the light source for a sufficient period of time to produce hydrogen gas from the water.
22-24. (canceled)
25. A method of making the photocatalyst of claim 1, the method comprising: a) obtaining an iodide modified titanium dioxide having iodide ions attached to the surface of the titanium dioxide; and b) treating the iodide modified titanium dioxide with a metal salt solution comprising a metal salt solubilized in a solvent to form metal cations attached to the iodide ions to obtain the photocatalyst.
26. The method of claim 25, wherein the iodide treated titanium dioxide is suspended in the metal salt solution.
27. The method of claim 26, wherein a particle size of the metal cation is proportional to the amount of time the titanium dioxide is suspended in the metal salt solution and wherein the amount of time the titanium dioxide is suspended in the metal salt solution is 1 to 5 minutes.
28. The method of claim 25, wherein the iodide treated titanium dioxide from step a) is obtained by treating titanium dioxide with an iodide solution comprising an iodide solubilized in a second solvent to form iodide ions attached to the surface of the titanium dioxide.
29. The method of claim 28, wherein the titanium dioxide is suspended in the iodide solution for 1 to 48 hours.
30. (canceled)
31. The method of claim 25, wherein the produced photocatalyst is not subjected to a calcination treatment.
32. (canceled)
33. (canceled)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a non-limiting schematic of the photocatalyst of the present invention.
[0030] FIG. 2A is a non-limiting schematic of a water-splitting system employing the photocatalyst of the present invention.
[0031] FIG. 2B is a non-limiting schematic of a water-splitting pathway for production of hydrogen and oxygen using the photocatalyst of the present invention.
[0032] FIG. 2C is a non-limiting schematic of another water-splitting pathway for production of hydrogen and oxygen using the photocatalyst of the present invention.
[0033] FIG. 3 are spectra of UV-Vis absorption of photocatalysts of the invention.
[0034] FIG. 4 is a graph of hydrogen production versus time that the iodide treated TiO.sub.2 (anatase) substrates were doped with gold cations.
[0035] FIG. 5 is a graph of hydrogen production rate in (μmol/g.sub.catalyst/min) versus time the iodide treated TiO.sub.2 substrate was suspended in the HAuCl.sub.4 solution.
DETAILED DESCRIPTION OF THE INVENTION
[0036] While hydrogen-based energy from water has been proposed by many as a solution to the current problems associated with carbon-based energy (e.g., limited amounts and fossil fuel emissions), the currently available technologies are expensive, inefficient, and/or unstable. The present application provides a solution to these issues. The solution is predicated on the use of a photocatalyst having highly dispersed nanoparticles or sub-nanoparticles of electrically conductive materials on the surface of a photoactive material that has been treated with iodide ions. This combination of electrically conductive materials and a photoactive material results in the efficient production of hydrogen and oxygen in a water-splitting reaction by catalyzing hydrogen atom recombination to form H.sub.2 and reducing electron hole recombination events.
[0037] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.
A. Photoactive Catalysts
[0038] The photoactive material includes any semiconductor material able to be excited by light in a range from 360-600 nanometers. In a preferred embodiment, the photoactive material is titanium dioxide. Titanium dioxide can be in the form of three phases, the anatase phase, the rutile phase, and the brookite phase. Anatase and rutile phases have a tetragonal crystal system, whereas the brookite phase has an orthorhombic crystal system. While anatase and rutile both have a tetragonal crystal system consisting of TiO.sub.6 octahedra, their phases differ in that anatase octahedras are arranged such that four edges of the octahedras are shared, while in rutile, two edges of the octahedras are shared. These different crystal structures result in different density of states (DOS) may account for the different efficiencies observed for transfer of charge carriers (electrons) in the rutile and anatase phases and the different physical properties of the catalyst. For example, anatase is more efficient than rutile in the charge transfer, but is not as durable as rutile. Each of the different phases can be purchased from various manufactures and supplies (e.g., titanium (IV) oxide anatase nano powder and titanium (IV) oxide rutile nano powder in a variety of sizes and shapes can be obtained from Sigma-Aldrich® Co. LLC (St. Louis, Mo., USA) and from Alfa Aesar GmbH & Co KG, A Johnson Matthey Company (Germany)); all phases of titanium dioxide from L.E.B. Enterprises, Inc. (Hollywood, Fla. USA)). They can also be synthesized using known sol-gel methods (See, for example, Chen et al., Chem. Rev. 2010 Vol. 110, pp. 6503-6570, the contents of which are incorporated herein by reference).
[0039] In one aspect of the invention, mixed phase titanium dioxide anatase and rutile may be a transformation product obtained from heat-treating single phase titanium dioxide anatase at selected temperatures. Heat-treating the single phase titanium dioxide anatase nanoparticle produces small particles of rutile on top of anatase particles, thus maximizing the interface between both phases and at the same time allowing for a large number of adsorbates (water and ethanol) to be in contact with both phases, due to the initial small particle size. Single phase TiO.sub.2 anatase nanoparticles that are transformed into mixed phase TiO.sub.2 nanoparticles have a surface area of about 45 to 80 m.sup.2/g, or 50 m.sup.2/g to 70 m.sup.2/g, or preferably about 50 m.sup.2/g. The particle size of these single phase TiO.sub.2 anatase nanoparticles is less than 95 nanometers, less than 50 nm, less than 20, or preferably between 10 and 25 nm. Heat treating conditions can be varied based on the TiO.sub.2 anatase particle size and/or method of heating (See, for example, Hanaor et al. in Review of the anatase to rutile phase transformation, J. Material Science, 2011, Vol. 46, pp. 855-874), and are sufficient to transform single phase titanium dioxide to mixed phase titanium dioxide anatase and rutile. Other methods of making mixed phase titanium dioxide materials include flame pyrolysis of TiCl.sub.4, solvothermal/hydrothermal methods, chemical vapor deposition, and physical vapor deposition methods. Using a ratio of anatase to rutile of 1.5:1 or greater can substantially increase the photocatalytic activity of the semiconductor material. The mixed phase TiO.sub.2 nanoparticles of the present invention can have a ratio of anatase and rutile phase ranges from 1.5:1 to 10:1, from 6:1 to 5:1, from 5:1 to 4:1, or from 2:1. As explained above, it is believed that this ratio allows for the efficient transfer of charge carriers (electrons) from the rutile phase to the anatase phase, where said charge carries in the anatase phase have an increased chance of being transferred to the metal conducting materials rather than undergoing an electron-hole recombination event.
[0040] The electrically conductive material can be a metal or metal alloy. Non-limiting embodiments of metals include gold, ruthenium, rhenium, rhodium, palladium, silver, copper, osmium, iridium, platinum, or any combination thereof. In some instances, the electrically conductive material is a plasmon resonance material. Non-limiting embodiments of plasmon resonance materials include silver, gold, copper, and palladium.
[0041] Referring to FIG. 1, a schematic of a surface of a photocatalyst of the present invention is depicted. Photocatalyst 10 can have iodide ions 12 between a photoactive material (for example, titanium dioxide particle) 14 and electrically conductive material 16. The photocatalyst 10 can be prepared from a photoactive material, an iodide ion source, and an electrically conductive material (e.g., metal) source. The number of iodide ions associated with the electrically conductive particles balances the valence of the electrically conductive particle. In a preferred embodiment, the electrically conductive particle 16 represent an Au.sup.+3 cation associated with three iodide ions 12. In other embodiments, Au.sup.+1, Au.sup.+2, or a different electrically conductive material having a valence of +1 or +2, can be associated with one iodide ion 12 or two of the iodide ions 12, respectively. Without wishing to be bound by theory, it is believed that the iodide ions inhibit agglomeration of the metal particles and, thus allow smaller particles of the metal to be dispersed on the surface of the catalysts while leaving sufficient surface area for the catalysis of water-splitting. In the embodiment shown, the photoactive material 14 has a generally circular cross-section. The photoactive material 14 can additionally be of any shape compatible with function in the photocatalyst 10 of the present invention, including but not limited to spherical, rod-shaped, irregularly shaped, or combinations thereof. The photoactive material 14 can also be, as non-limiting examples, a bulk material, a particulate material, or a flat sheet. The photoactive material 14 can be of any microstructure or larger size suitable for use in the photocatalyst system 10. In some embodiments, the photoactive material 14 are microstructures, meaning that they have at least one dimension measuring between 0.1 and 100 μm and no dimensions measuring 0.1 μm or less. Attachment of the electrically conductive material and the iodide ions to the photoactive material can be accomplished, for example, by contacting an iodide modified photoactive material with metal ions using the methods described in the Examples section and throughout this Specification. A non-limiting embodiment of a method that can be used to make the photocatalyst 10 of the present invention includes formation of an aqueous solution of metal iodide (for example, potassium iodide) and adding photoactive material to the solution (for example, adding titanium dioxide particles) to form a suspension. The suspension of photoactive material and metal iodide can be stirred for a desired amount of time (for example, 0.5, 1, 2, 10, 15, 20, or 24 hours) or until sufficient iodide ions are deposited on the surface of the photoactive material or in the interstitial spaces of the photoactive material's crystal lattice. The iodide modified photoactive material can be separated from the aqueous metal iodide solution using known techniques such as filtration, vacuum filtration, centrifugation or the like. The iodide modified photoactive material can be added to an aqueous solution of a salt of the electrically conductive material (for example, aqueous solution of HAuCl.sub.4) for a desired amount of time to deposit the desired amount of the electrically conductive material on the iodide modified photoactive material. In non-limiting embodiments, the photoactive material is contacted with the electrically conductive material for 0.1 min., 0.5 min., 1 min., 1.25 min., 1.5 min., 1.75 min., 2 min., 2.25 min., 2.5 min., 2.75 min., 3 min. 3.25 min., 3.5 min., 3.75 min., 4 min., 4.25 min., 4.5 min., 4.75 min., 5.0 min., 5.25 min., 5.5 min. 5.75 min., 6.0 min., 6.25 min., 6.5 min., 6.75 min., 7.0 min., 7.25 min., 7.5 min., 7.75 min., 8.0 min., 8.25 min., 8.5 min., 8.75 min., 9.0 min. 9.25 min., 9.75 min., 10.0 min., 20 min., 30 min., 60 min., or any range derivable therein. In non-limiting embodiments, the amount of electroconductive material deposited on the surface can be 0.0500 wt. %, 0.0525 wt. %, 0.0550 wt. %, 0.0575 wt. %, 0.0600 wt. %, 0.0625 wt. %, 0.0650 wt. %, 0.0675 wt. %, 0.0700 wt. %, 0.0725 wt. %, 0.0750 wt. %, 0.0775 wt. %, 0.0800 wt. %, 0.0825 wt. %, 0.0850 wt. %, 0.0875 wt. %, 0.0900 wt. %, 0.0925 wt. %, 0.0950 wt. %, 0.0975 wt. %, 0.1000 wt. %, 0.1250 wt. %, 0.1500 wt. %, 0.1750 wt. %, 0.2000 wt. %, 0.2250 wt. %, 0.2500 wt. %, 0.2750 wt. %, 0.3000 wt. %, 0.3250 wt. %, 0.3500 wt. %, 0.3750 wt. %, 0.4000 wt. %, 0.4250 wt. %, 0.4500 wt. %, 0.4750 wt. %, 0.5000 wt. %, 0.5250 wt. %, 0.0550 wt. %, 0.5750 wt. %, 0.6000 wt. %, 0.6250 wt. %, 0.6500 wt. %, 0.6750 wt. %, 0.7000 wt. %, 0.7250 wt. %, 0.7500 wt. %, 0.7750 wt. %, 0.8000 wt. %, 0.8250 wt. %, 0.8500 wt. %, 0.8750 wt. %, 0.9000 wt. %, 0.9250 wt. %, 0.9500 wt. %, 0.9750%, up to 1.0%, or any range derivable therein, based on the total weight of the photocatalyst, of electrically conductive material. In preferred non-limiting embodiments, the amount of electroconductive material deposited on the surface of the iodide modified photoactive material can be 0.0500 wt. %, 0.0525 wt. %, 0.0550 wt. %, 0.0575 wt. %, 0.0600 wt. %, 0.0625 wt. %, 0.0650 wt. %, 0.0675 wt. %, 0.0700 wt. %, 0.0725 wt. %, 0.0750 wt. %, 0.0775 wt. %, 0.0800 wt. %, 0.0825 wt. %, 0.0850 wt. %, 0.0875 wt. %, 0.0900 wt. %, 0.0925 wt. %, 0.0950 wt. %, 0.0975 wt. %, 0.1000 wt. %, 0.1250 wt. %, 0.1500 wt. %, 0.1750 wt. %, 0.2000 wt. %, or any range derivable therein, based on the total weight of the photocatalyst, of electrically conductive material. In other non-limiting embodiments, the photoactive material is contacted with the electrically conductive material for 0.1 min., 0.5 min., 1 min., 1.25 min., 1.5 min., 1.75 min., 2 min., 2.25 min., 2.5 min., 2.75 min., 10 min or any range derivable therein and 0.0500 wt. %, 0.0525 wt. %, 0.0550 wt. %, 0.0575 wt. %, 0.0600 wt. %, 0.0625 wt. %, 0.0650 wt. %, 0.0675 wt. %, 0.0700 wt. %, 0.0725 wt. %, 0.0750 wt. %, 0.0775 wt. %, 0.0800 wt. %, 0.0825 wt. %, 0.0850 wt. %, 0.0875 wt. %, 0.0900 wt. %, 0.0925 wt. %, 0.0950 wt. %, 0.0975 wt. %, 0.1000 wt. %, 0.1250 wt. %, 0.1500 wt. %, 0.1750 wt. %, 0.2000 wt. %, or any range derivable therein, based on the total weight of the photocatalyst, of electrically conductive material is deposited on the iodide modified photoactive material. Without wishing to be bound by theory, it is believed that the metal cation attaches to the iodide ions through ionic bonding. Adjusting the time period, controls the amount of metal cation that is available for ionic bonding. Furthermore, when gold is used as the electrically conductive material, due to the ionic bonding of the Au (III) cation (electrically conductive material) with three iodide ions (I.sup.−), the gold does not require thermal reduction to elemental gold prior to use as the Au (III) can catalyze the recombination of hydrogen atoms.
B. Water-Splitting System
[0042] Referring to FIGS. 2A-C, a non-limiting representation of a water-splitting system 20 of the present invention is provided. The system includes the photocatalyst 10, a light source 22, and container 24. The photocatalyst includes electrically conductive material 16 attached (bonded) to the iodide ions 12 which are attached or dispersed on the photoactive material 14. The container 24 can be a transparent, translucent or even opaque such as those that can magnify light (e.g., opaque container having a pinhole(s)). The photocatalyst 10 can be used to split water to produce H.sub.2 and O.sub.2. The light source 22 includes visible and (400-600 nm) and ultraviolet light (360-410). The ultraviolet light excites the photoactive material 14 while the visible light excites “resonance” electrons from Au (and/or Ag) atoms (plasmonic excitation). Referring to FIGS. 2B and 2C the pathways for production of hydrogen and water are depicted. In both pathways, the excited electrons (e−) transition from their valence band 26 to their conductive band 28 thereby leaving a corresponding hole (h+). In the first pathway shown in FIG. 2B, the excited electrons (e−) can be transferred to the electrically conductive material where reduction of hydrogen ions on the surface of the metal occurs through electron transfer to form molecular hydrogen (H.sub.2). The holes (h+) are used to oxidize oxygen ions to molecular oxygen (O.sub.2). In the second pathway shown in FIG. 2C, the excited electrons (e−) can be reduced to hydrogen atoms at the surface of the photoactive material. The hydrogen atoms migrate to the metal surface where the electrically conductive material 16 can catalyze the recombination of the hydrogen atoms to molecular hydrogen (H.sub.2). The holes (h+) are used to oxidize oxygen ions to oxygen gas. In both pathways, the hydrogen gas and the oxygen gas can then be collected and used in other processes. Without wishing to be bound to theory, it is believed that both pathways can exist in a photocatalytic system, however, the catalysis pathway (FIG. 2B) is about 100 to 1000 times faster than that of electron transfer pathway (FIG. 2A). Due to the electrically conductive material 16 having small particle sizes and being highly dispersed on the surface of the photoactive material 14, excited electrons (e−) are more likely to be used to split water before recombining with a hole (h+) than would otherwise be the case. Notably, the system 20 does not require the use of an external bias or voltage source. Further, the efficiency of the system 20 allows for one to avoid or use minimal amounts of a sacrificial agent such as methanol, ethanol, propanol, butanol, isobutanol, methyl tert-butyl ether, ethylene glycol, propylene glycol, glycerol, oxalic acid, triethylamine, triethanolamine, or any combination thereof. In certain aspects, however, 0.1 to 10 w/v %, or preferably 2 to 7 w/v %, of a sacrificial agent can be included in the aqueous solution. The presence of the sacrificial agent can increase the efficiency of the system 20 by further reducing the likelihood of hole/electron recombination via oxidation of the sacrificial agent by the hole rather than recombination with the excited electron. Preferred sacrificial agents ethylene glycol, glycerol, or a combination thereof is used. In addition to being capable of catalyzing water splitting without an external bias or voltage, the photocatalysts of the present invention may be included in an anode of an electrochemical cell capable of forming oxygen and hydrogen by electrolysis of water. In a non-limiting embodiment, light energy may be provided to a photocell and from the light energy a voltage between the anode and the cathode is produced and water molecules are split to form hydrogen and oxygen. The method can be practiced such that the hydrogen production rate from water can be modified as desired by subjecting the system to various amounts of light energy or light flux. For example, the photoactive catalyst 10 can be used as the anode in a transparent container containing an aqueous solution and used in a water-splitting system. An appropriate cathode can be used such as Mo-Pt cathodes (See, International Journal of Hydrogen Energy, June 2006, Vol. 31, issue 7, pages 841-846, the contents of which are incorporated herein by reference) or MoS.sub.2 cathodes (See, International Journal of Hydrogen Energy, February 2013, Vol. 38, issue 4, pages 1745-1757, the contents of which are incorporated herein by reference).
EXAMPLES
[0043] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.
Example 1
Production of Photocatalysts of the Present Invention
[0044] I.sup.−/TiO.sub.2 Substrate.
[0045] The iodide ion modified titanium dioxide substrate (I.sup.−/TiO.sub.2) was made using a treatment method to obtain iodide ions coated on the surface of the titanium dioxide anatase phase substrate. A solution of potassium iodide (10 mM KI) was prepared by dissolving potassium iodide (KI, 350 mg) into deionized water (210 mL). The TiO.sub.2 (3.0 g) was added to the aqueous KI solution to form a suspension. The suspension was stirred for about 12 hours (overnight). The suspension was vacuum filtered and the iodide ion modified TiO.sub.2 particles were stored.
[0046] Au.sup.+3/I.sup.−/TiO.sub.2 Photocatalyst.
[0047] A solution of hydrogen chloroauric acid (HAuCl.sub.4, Sigma-Aldrich®) was obtained commercially. Iodide modified TiO.sub.2 particles (1 gram) were added to the HAuCl.sub.4 solution for each catalyst prepared and contacted for different periods of deposition time (1 min, 3, minutes, 5 minutes, 20 minutes, 30 minutes, and 60 minutes). The suspensions were sonicated for 1 to 3 minutes. At the end of the contact time period, the solution was vacuum filtered using a fine filter and then washed with excess deionized water to obtain the gold iodide modified titanium dioxide (Au.sup.+3/I.sup.−/TiO.sub.2) photocatalysts. The photocatalysts were dried at 70° C. overnight (about 12 hours). FIG. 3 are spectra of UV-Vis absorption of Au.sup.+3/I.sup.−/TiO.sub.2 of the invention made by contact times the iodide I.sup.−/TiO.sub.2 with Au.sup.+3 for 5 min., 10 min., 20 min., 30 min. and 60 min. Data line 30 is the TiO.sub.2 reference with no doping and data lines 31, 32, 33, 34, 35 are the I.sup.−/TiO.sub.2 with Au.sup.+3 for 5 min, 10 min., 20 min., 30 min. and 60 minutes respectively, photocatalysts versus wavelength. As shown by the spectra in FIG. 3, the Au.sup.+3/I.sup.−/TiO.sub.2 all of photocatalysts exhibited a strong plasmon resonance (i.e., strong Au absorption between 500 and 600 nm.) The amount of gold on the surface of each Au.sup.+3/I.sup.−/TiO.sub.2 photocatalysts was determined using X-ray photoelectron spectroscopy (XPS). The gold weight percent on the surface after 10 minutes of deposition was found to be about 0.6 wt. %, while the gold deposited on the surface after 30 minutes was 0.9 wt. %. The amount of gold in the bulk of each Au.sup.+3/I.sup.−/TiO.sub.2 was determined by elemental analysis utilizing inductively coupled plasma (ICP). The gold weight percent in the bulk for the photocatalysts was 0.5 wt. %.
Example 2
Use of the Photocatalysts of the Invention in Water-Splitting Reactions
[0048] Water-Splitting Reaction Using Example 1 Photocatalysts.
[0049] Catalytic reactions were conducted in a borosilicate (Pyrex®, Corning) glass reactor having a capacity of 100 mL. A photocatalyst prepared as described in Example 1 was added to the glass reactor in a concentration of 0.1 g/L (10 mg in 21 mL total volume). Deionized water (20 mL) and sacrificial agent (ethanol, 5 v/v % based on total water, 1 mL) were added to the reactor. The reaction mixture was irradiated with sunlight, with a light flux at the front side of the reactor of between 2 to 10 mW/cm.sup.2 at 360 nm. The mixture containing photocatalyst, water and sacrificial agent was stirred constantly under dark conditions to disperse the catalyst and sacrificial agent in the water. The reactor was then exposed to a UV light source (100 Watt UV lamp (H-144GC-100, Sylvania par 38) with a flux of about 2-5 mW/cm2 at a distance of 10 cm with the cut off filter (360 nm and above). Product analysis of the produced gas was done using a gas chromatography (Porapak™ Q (Sigma Aldrich) packed column 2 m, 45° C. (isothermal), with nitrogen as a carrier gas) with a thermal conductivity detector. FIG. 4 is a graph of hydrogen production versus time that for the iodide treated TiO.sub.2 (anatase) substrates doped with gold cations for 5 min., 20 min., 30 min. and 60 min. Data 40 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst made at 1 min. deposition time optimization (DTO), data 42 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst made at 3 min. DTO, data 44 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst made at 5 min. DTO, data 46 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst at 20 min. DTO, data 48 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst at 30 min. DTO, and 50 represents hydrogen production using a Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst at 60 min. DTO. The Au.sup.+3/I.sup.−/TiO.sub.2 photocatalyst having a 30 minute DTO had a gold content of 0.9 wt. %. FIG. 5 is a graph of hydrogen production rate in (μmol/g.sub.catalyst/min) versus time that the iodide treated TiO.sub.2 substrate was suspended in the HAuCl.sub.4 solution. Data 52 represents hydrogen production after 1 min. DTO, data 54 represents hydrogen production after 3 min. DTO, data 56 represents hydrogen production after 5 min. DTO, data 58 represent hydrogen production after 20 min. DTO, data 60 represents hydrogen production after 30 min. DTO, and data 62 represents hydrogen production after 60 min. DTO. Table 1 is a listing of the deposition time in minutes and the hydrogen production in μmol/g.sub.catalyst/min. for FIG. 5. From the data in FIGS. 4 and 5 and Table 1, it was concluded that a greater increase in hydrogen gas production was observed for catalysts that had the shorter deposition time. For example, the hydrogen production using the catalyst prepared by soaking iodide treated TiO.sub.2 substrate in the HAuCl.sub.4 solution for 1 minute is greater than the hydrogen product using the catalyst prepared by soaking the iodide treated TiO.sub.2 substrate in HAuCl.sub.4 solution for 60 minutes.
TABLE-US-00001 TABLE 1 Deposition Time Hydrogen (H.sub.2) Production (minutes) (μmol/g.sub.catalyst/min) 1 93.4 3 61.5 5 57.9 20 36.1 30 28.4 60 24.7
