SEMICONDUCTOR/M1/CD XM1-XS BASED PHOTOCATALYST FOR EFFICIENT HYDROGEN GENERATION

Abstract

Embodiments of the invention are directed to Z-scheme photocatalyst for efficient hydrogen generation from water. The Z-scheme photocatalyst can include a hybrid metal that includes a semiconductor material/M1/Cd.sub.xM.sub.1xS material. M1 can be transition metal and M can Zn, Fe, Cu, Sn, Mo, Ag, Pb and Ni.

Claims

1-20. (canceled)

21. A photo electrochemical (PEC) thin film comprising: metal nanoparticles positioned between a layer of a Cd.sub.xM.sub.1xS semiconductor material, where x is 0.7 to 0.9 and M is Zn, Fe, Cu, Sn, Mo, Ag, Pb or Ni, or combinations thereof and a layer of a metal oxide semiconductor material, wherein the metal nanoparticles are Au, Pd, Au/Pd, or Pd/Ag nanoparticles and the metal oxide is TiO.sub.2, SrTiO.sub.3, WO.sub.3, or BiVO.sub.4.

22. The PEC thin film of claim 21, wherein M is Zn or Ni.

23. A PEC thin film, comprising TiO.sub.2@Ag/Pd@Cd.sub.xM.sub.1xS where x is 0.7 to 0.9.

24. The PEC thin film of claim 21, wherein the photocatalyst is TiO.sub.2@Pt@Cd.sub.xNi.sub.1xS where x is 0.7 to 0.9.

25. The PEC thin film of claim 21, wherein the layer of the Cd.sub.xM.sub.1xS semiconductor material has a thickness of 100 nm to 5000 nm.

26. The PEC thin film of claim 21, wherein the layer of Cd.sub.xM.sub.1xS semiconductor material is deposited on a conducting support.

27. A photocatalytic reactor comprising a reactor having an inlet for feeding water or aqueous solution to a reactor chamber, the reaction chamber comprising: (i) a photo electrochemical (PEC) assembly comprising a PEC thin film of claim 21 deposited on a conducting support and a hydrogen co-catalyst deposited on a second portion of the conductive support material; (ii) a H.sub.2 gas product outlet; (iii) O.sub.2 gas product outlet; and (iv) ion exchange membrane.

28. The reactor of claim 27 wherein the reactor chamber is transparent to visible light.

29. The reactor of claim 27, wherein the hydrogen co-catalyst is a metal alloy, such as Mo/Ni in a weight ratio of 10:1 to 1:10.

30. The reactor of claim 27, wherein the conductive support is stainless steel, molybdenum, titanium, tungsten, tantalum, or an alloy thereof.

31. The reactor of any one of claims 27, wherein the metal oxide semiconductor material further comprises an oxygen co-catalyst thin film on the surface of the metal oxide material.

32. The reactor of claim 31, wherein oxygen co-catalyst thin film is a metal oxide having the general formula of AO.sub.y or B.sub.zN.sub.1zO.sub.y, where A and B are metals, and z is <1 and y is a value that balances the valence of the oxide.

33. The reactor of claim 31, wherein A or B is one or more of Pt, Pd, Au, Ag, Ir, Ru, Rh, Mo, Ni, Ce, Cu, Co, Fe, W and Sn; and combinations thereof.

34. The reactor of any one of claims 31, wherein the oxygen co-catalyst thin film comprises a promoter element or metal.

35. A method of producing hydrogen comprising irradiating the photo electrochemical (PEC) thin film in the reactor of claim 21 with light in the presence of water.

36. A method of producing hydrogen comprising irradiating the photo electrochemical (PEC) thin film in the reactor of claim 22 with light in the presence of water.

37. The PEC thin film of claim 22, wherein the layer of the Cd.sub.xM.sub.1xS semiconductor material has a thickness of 100 nm to 5000 nm.

38. The PEC thin film of claim 22, wherein the layer of Cd.sub.xM.sub.1xS semiconductor material is deposited on a conducting support.

39. The PEC thin film of claim 24, wherein the layer of Cd.sub.xM.sub.1xS semiconductor material is deposited on a conducting support.

40. The reactor of claim 29, wherein the conductive support is stainless steel, molybdenum, titanium, tungsten, tantalum, or an alloy thereof.

Description

DESCRIPTION OF THE DRAWINGS

[0027] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0028] FIGS. 1A and 1B depict schematics of synthesis of two Z-Scheme catalysts of the present invention. FIG. 1A depicts the schematic of the synthesis of TiO.sub.2/M1/Cd.sub.xZn.sub.1xS. FIG. 1B depicts the schematic of the synthesis of TiO.sub.2/M1/Cd.sub.xNi.sub.1xS.

[0029] FIG. 2 depicts a schematic of a total water-splitting system that includes (1) hydrogen production catalyst, (2) conductive support, (3) PEC thin film based Z-scheme catalyst of the present invention, and (4) oxygen production catalyst.

[0030] FIGS. 3A and 3B depict X-ray diffraction (XRD) patterns of two Z-Scheme photocatalysts of the present invention. FIG. 3A is an XRD pattern of TiO.sub.2/AgPt/Cd.sub.0.8Zn.sub.0.2S based systems. FIG. 3B is an XRD pattern of TiO.sub.2/M1/Cd(Ni)S based systems.

[0031] FIGS. 4A and 4B depict ultra-violet (UV) visible (vis) spectra of two Z-Scheme catalysts of the present invention. FIG. 4A depicts UV-vis absorption spectra of Cd.sub.0.8Zn.sub.0.2S based materials. FIG. 4B depicts UV-vis absorption spectra of TiO.sub.2/M/Cd(Ni)S based systems.

[0032] FIGS. 5A-5C depict graphs of photocatalytic hydrogen generation of photocatalysts of the present invention. FIG. 5A depicts hydrogen production versus time for Cd.sub.0.8Zn.sub.0.2S based photo-catalysts. FIG. 5B depicts hydrogen production versus time for Cd(Ni)S based catalysts. FIG. 5C depicts comparison of the hydrogen production rate of various photo-catalysts of the present invention and comparative catalysts of PdAg/TiO.sub.2 and 1 wt. % Au/TiO.

DESCRIPTION

[0033] A solution to at least some of the problems associated with light harvesting associated with photocatalytic systems has been discovered. The solution is premised on an integrated photocatalyst that shows a redox potential scheme corresponding to the Z-scheme, the total potential difference of which is sufficient to permit cleavage of water into hydrogen and oxygen when the catalyst is irradiated with light (e.g., sun light) that includes a wavelength of at least 420 nm, 430 nm, 440 nm, 450 nm, and up to 700 nm. An integrated photocatalyst described herein can be in the form of a plate, a film, or a tube. In certain aspects, the integrated photocatalyst is a photo electrochemical (PEC) thin Film. A water-splitting PEC thin film described herein can include metal nanoparticles positioned between a first semiconductor and a second semiconductor to form a Z-scheme for total water-splitting.

[0034] Semiconductor materials can include: elements from Column 4 of the Periodic Table; materials including elements from Column 3 and Column 5 of the Periodic Table; materials including elements from Columns 2 and 4 of the Periodic Table; materials including elements from Columns 1 and 7 of the Periodic table; materials including elements from Columns 4 and 6 of the Periodic Table; materials including elements from Columns 5 and 7 of the Periodic Table; and/or materials including elements from Columns 2 and 5 of the Periodic Table. Other materials with semiconductive properties can include layered semiconductors, metallic alloys, miscellaneous oxides, some organic materials, and some magnetic materials.

[0035] First semiconductorIn certain aspects, the first semiconductor can include cadmium-based materials having a band gap that from 1.7 to 2.8 eV, or 2.0 to 2.5, or 2.1 to 2.3 eV, or any range or value there between. The first semiconductor can be a Cd.sub.xM.sub.1xS material having a band gap from and including 1.7 to 2.8 eV, where x is less than one, from 0.01 to 0.99, from 0.7 to 0.9, or any value at least, equal to, or between any two of 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.70, 0.75, 0.8, 0.85, 0.9, 0.95, and 0.99, but less than 1. In certain aspects, a first semiconductor can be a Cd.sub.xZn.sub.1xS (2.4 eV) semiconductor, where x is less than one, from 0.01 to 0.99, from 0.7 to 0.9, or any value at least, equal to, or between any two of 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.70, 0.75, 0.8, 0.85, 0.9, 0.95, and 0.99, but less than 1. In some instances, a Cd.sub.xNi.sub.1xS (2.0 eV) semiconductor, where x is less than one, from 0.01 to 0.99, from 0.7 to 0.9, or any value at least, equal to, or between any two of 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.70, 0.75, 0.8, 0.85, 0.9, 0.95, and 0.99, but less than 1.

[0036] Second semiconductorA second semiconductor can be a semiconducting material with band gap range different than the first semiconductor. By way of example, the second semiconductor can have a band gap from 2.4-3.2 eV. Non-limiting examples of semiconductor materials include carbon nitride materials, TiO.sub.2, SrTiO.sub.3, metal doped-SrTiO.sub.3, metal doped-WO.sub.3, WO.sub.3, metal doped-BiVO.sub.4 or BiVO.sub.4). In certain aspects, the second semiconductor is C.sub.3N.sub.4 or a metal oxide such as TiO.sub.2, SrTiO.sub.3 and BiVO.sub.4, or combinations thereof. In a preferred embodiment, the metal oxide is TiO.sub.2.

[0037] Metal nanostructuresCertain aspects of the invention are directed to the lowering the amount of platinum (Pt) used or replacement of platinum nanostructures with lower cost nanostructures and utilizing a plasmonic effect to enhance hydrogen generation rate and improve the stability of the catalyst. The amount of Pt loading in the Z-scheme catalysts can be from 0.05 wt. % to 1 wt. %, or 0.1 wt. % to 0.8 wt. %, or at least, equal to, or between any two of 0.05 wt. %, 0.1 wt. %, 0.15 wt. %, 0.2 wt. %, 0.3 wt. %, 0.4 wt. %, 0.5 wt. %, 0.6 wt. %, 0.7 wt. %, 0.8 wt. %, 0.9 wt. %, and 1 wt. %. In some embodiments, the nanostructures are nanoparticles. The present invention provides the advantage of using less expensive metals (Ag, Pd, Cu, and Ni) as well as bimetallic systems (Au/Ni, Ag/Ni, Au/Pd, Ag/Pd, Au/Cu and Ag/Cu). The preparation of the Z-scheme catalyst can be done using known catalysts preparation methods. In a first step, the metal nanostructures can be deposited on the second semiconductor material to form a M1/second semiconductor material. By way of example, a metal precursor solution can be added to an alcoholic suspension of the first semiconductor particles. A reducing action can be added to the solution and the solution agitated until the metal precursor material forms a zero valent metal. The resulting particles can be isolated (e.g., centrifugation) and dried to give a M1@ second semiconductor material. The M1@ second semiconductor material can be dispersed in an alcoholic solution and heated to an appropriate temperature (e.g., 55 to 65 C.). A metal precursor of the CdMS series (first semiconductor material) can be added to the heated alcoholic dispersion of the M1@ second semiconductor material. A cadmium metal precursor can be an alcoholic solution, and a reducing agent (e.g., sodium sulfide) can be added to the metal precursor/M1@ second semiconductor dispersion. The solution can be agitated for a period of time and the resulting second semiconductor@M1@CdMS material can be isolated, washed with an aqueous methanol solution, and the then dried at 50 to 75 C. to yield the final second semiconductor/M1@Cd.sub.xM.sub.1xS material. In some embodiments, the CdMS material is formed and then added to the M1/second semiconductor material. Non-limiting schematic of the preparation of the TiO.sub.2/M1/Cd.sub.xNi.sub.1xS (M1=Au, Ag, Ni, Cu, Au/Ni, Ag/Ni, Au/Pd, Ag/Pd, Au/Cu and Ag/Cu) and TiO.sub.2/M1/Cd.sub.xNi.sub.1xS are shown in FIGS. 1A and 1B, respectively. As shown in FIG. 1A, the semiconductor/M1 and CdMS material are layered. As shown in FIG. 1B, the CdMS forms a shell over M1 which is on the surface of the first semiconductor material.

[0038] The metal nanoparticles (M1) can be positioned/placed between a metal oxide (See, FIG. 2) and Cd.sub.xZn.sub.1xS with the purpose of constructing an effective Z-scheme photocatalyst. The ratio between the second semiconductor (e.g., metal oxide or carbon nitride) and sulfur, second semiconductor and metal nanostructure (M1), x value in Cd.sub.xM.sub.1xS, or combinations thereof can be optimized to maximize the hydrogen generation rate. The resulting material can be a film.

[0039] The Z-scheme photocatalysts of the present invention can be used in a water-splitting system that include a hydrogen co-catalyst and an oxygen co-catalyst. FIG. 2 depicts a schematic of a water-splitting catalyst system 200. System 200 can include water-splitting systems that include the Z-scheme photocatalyst of the present invention 202 in combination with a hydrogen co-catalyst 204 and an oxygen co-catalyst 206. The Z-scheme photocatalyst 202 can be a multi-layer film that is electrically and photo active (e.g., a PEC film). Z-scheme photocatalyst can include first semiconductor material 208, metal nanostructure 210, and second semiconductor material 212. First semiconductor material 208 can have a thickness of 100 nm to 5000 nm, 500 nm to 3000 nm, or 1000 nm to 2000 nm or any value or range there between. Metal nanostructure 210 can have a size of 0.5 nm to 20 nm, 1 nm to 10 nm, 2 nm to 5 nm or about 3 nm or any value or range there between. Metal oxide layer 212 have a thickness of 10 to 500 nm, 50 to 400 nm, or 100 to 300 nm or about 200 nm. In some instances, first semiconductor material 202 can have a thickness of 2000 nm, the metal nanostructure can be 3 nm, and the second semiconductor can have a thickness of 20 nm. Z-scheme photocatalyst 202 can be deposited on conducting support material 214 (e.g., a stainless steel support).

[0040] Hydrogen co-catalyst 204 can be deposited on a second portion of support material 214 opposite of Z-scheme photocatalyst 202. A thickness of hydrogen co-catalyst can be 0.01 to 50 nm, 1 to 30 nm or 5 to 15 nm, or any value or range there between, or about 10 nm. The hydrogen generation catalyst can have two metals at a ratio of 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, to 1:10. Non-limiting examples of hydrogen production co-catalysts can include Pt, Pd, Au, Ag, Ir, Ru, Rh, Mo, Ni, Ce, Cu, Co, Fe, W and Sn as well as combinations thereof (e.g., Mo:Ni catalyst in a 1:1 weight ratio). In certain aspect, the Mo/Ni hydrogen catalyst can have a Mo:Ni a ratio of 10:1 to 1:10, including all values and ranges there between.

[0041] Oxygen producing co-catalyst 206 can be deposited on a portion of the second semiconductor 212 (e.g., metal oxide or carbon nitride material). A thickness of oxygen producing co-catalyst 206 can be 0.01 to 50 nm, 1 nm to 40 nm or 10 to 30 nm, or any range or value there between, or about 30 nm. Oxygen co-catalyst 206 can be a metal oxide (AO.sub.y) or (A.sub.zB.sub.1zO.sub.y) where z<1 and y is a value sufficient to balance the valence of the metal. The metal (A) can be Pt, Pd, Au, Ag, Ir, Ru, Rh, Mo, Ni, Cu, Co, Fe, W, Sn, and combinations thereof. A non-limiting example of an oxygen co-catalyst 206 includes IrNiO.sub.3. Parts of the catalysts can be deposited, for example, on TiO.sub.2/M/Cd.sub.xZn.sub.1xS by a light deposition method. In some embodiments, the water-splitting component is a wireless total water-splitting system having a TiO.sub.2/M/Cd.sub.xZn.sub.1xS base.

[0042] An apparatus or system for the production of hydrogen from water or aqueous solutions of organic compounds by using the Z-Scheme photocatalyst of the present inventions or apparatus described herein can include one or more of (i) a light source (such as a visible light source), (ii) a reactor (optionally a transparent portion for light if the light source is external to the reactor), (iii) an inlet for feeding water or aqueous solution to the reactor, and (iv) a gas product outlet for releasing hydrogen liberated in the reaction chamber. The photocatalyst described herein can be located inside the reactor. The apparatus or system for the production of hydrogen can also include a storage chamber for collecting and storing the molecular hydrogen produced. The storage chamber can be in communication with the reaction chamber via the gas or product outlets. The storage chamber may be pressurized.

[0043] Valves may also be present, to control the flow of water or aqueous solution into the reactor via the inlet and release of gas via the outlet. Control means may also be present to adjust the light source intensity or even switch it on or off (e.g., provide access or block sunlight) as required. The reaction chamber or reactor may further comprise a waste outlet for removal of waste or by-products or unreacted water or aqueous solution, the waste outlet optionally having a valve. Still further, the hydrogen production device may comprise control means operably linked to the valves for controlling the flow of water or aqueous solution into the reaction chamber, the flow of molecular hydrogen through the outlet (and into the storage chamber if present), and/or the flow of waste or by-products or unreacted water or aqueous solution through the waste outlet.

EXAMPLES

[0044] The following examples as well as the figures are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples or figures represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

Materials

[0045] Stock solutions were prepared according to Table 1.

TABLE-US-00001 TABLE 1 MW Mass Mole Molar Chemicals (g/mol) (g) (mmol/ml) V (mL) conc. Purity Zn(CH.sub.3CO.sub.2)2H.sub.2O 220 1.76 0.08 100 0.08M 99% (MeOH) Cd(CH.sub.3CO.sub.2)2H.sub.2O 266.5 0.08 100 0.08M 99% (MeOH) Na.sub.2SxH.sub.2O 78 1.3 0.1 100 0.1M 60% (MeOH) NaBH.sub.4 38 0.152 0.04 100 0.04M 99% (MeOH) PdCl.sub.2 177.3 0.12 1.2 mg/mL 100 0.0067M 99% (H.sub.2O) HAuCl.sub.43H.sub.2O 393.8 0.394 0.01 (1.9 mg 100 0.01M 99% Au/mL) (H.sub.2O) AgNO.sub.3 169.8 0.79 1 mg Ag/mL 500 0.0093M 99% (H.sub.2O) H.sub.2PtCl.sub.66H.sub.2O 409 0.961 1.9 mg Pt/mL 200 0.012M 99.9% (H.sub.2O) Ni(CH.sub.3CO.sub.2)2H.sub.2O 220 1.76 0.08 100 0.08M 99% Cd(CH.sub.3CO.sub.2)2H.sub.2O 266.5 0.08 100 0.08M 99% Na.sub.2SxH.sub.2O 78 1.3 0.1 100 0.1M 60% NaBH.sub.4 38 0.152 0.04 100 0.04M 99% HAuCl.sub.43H.sub.2O 393.8 0.394 1.9 mg Au/mL 100 0.01M 99% AgNO.sub.3 169.8 0.79 1 mg Ag/mL 500 0.0093M 99% PdCl.sub.2 177.3 0.12 1.2 Pd mg/mL 100 0.0093 99% H.sub.2PtCl.sub.66H.sub.2O 409 0.961 1.9 Pt/mL 100 0.012M 99%

Example 1

(Synthesis of M1 on Second Semiconductor Material

[0046] Synthesis of Ag/Pd@TiO.sub.2. AgNO.sub.3 (1 mg (Ag/mL, 10 mL) was added drop-wise to a suspension of PdCl.sub.2 (1mg (Pd)/mL, 30 mL) and TiO.sub.2 (10 g, Hombikat, American Elements, U.S.A.). The suspension was stirred at 80 C. until all the solvent was evaporated and the resulting powder was crushed and calcined at 350 C. for 5 hours to give core/shell structure of Ag (0.1 wt %)/Pd (0.3 wt. %)@ TiO.sub.2 in quantitative yield.

[0047] Synthesis of Au@TiO.sub.2. HAuCl.sub.4(1.97 mg (Au)/mL, 1.7 mL) was added drop-wise to a methanolic solution of TiO.sub.2(0.33 g, 4.1 mmol) nanocrystals, followed by NaBH.sub.4 (40 mM, 5 mL) aqueous solution. The solution was stirred for 10 min. The solution color became black due to the formation of platinum on TiO.sub.2 nanoparticles. The resulting solution was centrifuged, filtered and dried in air to give core/shell structure of Au (0.5 wt. %) @ TiO.sub.2 in quantitative yield.

[0048] Synthesis of Ag@TiO.sub.2. AgNO.sub.3 (1mg (Au)/mL, 4 mL) was added drop-wise to a methanolic solution of TiO.sub.2 (0.33 g, 4.1 mmol) nanocrystals, followed by NaBH4 (40 mM, 5 mL) aqueous solution. The solution was stirred for 10 min. The solution color became black due to the formation of platinum on TiO.sub.2 nanoparticles. The resulting solution was centrifuged, filtered and dried in air to give Ag (0.5 wt. %) @ TiO.sub.2 in quantitative yield.

[0049] Synthesis of Au/Pd@TiO.sub.2. A mixture of HAuCl.sub.4 (1.97 mg (Au)/mL, 0.76 mL) and PdCl.sub.2 (1.2 mg (Pd)/mL, 1.3 mL) was added drop-wise to a methanolic solution of TiO.sub.2 (0.33 g, 4.1 mmol) nanocrystals, followed by NaBH.sub.4 (40 mM, 5 mL) aqueous solution. The solution was stirred for 10 min. The resulting solution was centrifuged, filtered and dried in air to give Au (0.5 wt %)/Pd (0.5 wt %) @ TiO.sub.2 in quantitative yield.

[0050] Synthesis of Pd/TiO.sub.2. PdCl.sub.2 (1.2 mg (Pd)/mL, 0.83 mL) was added drop-wise to a methanolic suspension of TiO.sub.2 (0.33 g, 4.1 mmol) nanocrystals, followed by 5 mL of NaBH.sub.4 (40 mM) aqueous solution. The solution was stirred for 10 min. The solution color became black due to the formation of palladium on TiO.sub.2 nanoparticles. The resulting solution was centrifuged, filtered and dried in air to give Pd (0.3 wt %) @ TiO.sub.2 in quantitative yield.

[0051] Synthesis of Au@TiO.sub.2. HAuCl.sub.4 (1.97 mg (Au)/mL, 1.7 mL) was added drop-wise to a methanolic suspension of TiO.sub.2 (0.33 g, 4.1 mmol) nanocrystals, followed by 5 mL of NaBH.sub.4 (40 mM) aqueous solution. The solution was stirred for 10 min. The resulting solution was centrifuged, filtered and dried in air to give Au (1 wt %) @ TiO.sub.2 in quantitative yield.

[0052] Synthesis of Pt@TiO.sub.2. H.sub.2PtCl.sub.6 6H.sub.2O (1.9 mg (Pt)/mL, 1.8 mL) was added drop-wise to a methanolic suspension of TiO.sub.2 (0.33 g, 4.1 mmol) nanocrystals, followed by 5 mL of NaBH.sub.4 (40 mM) aqueous solution. The solution was stirred for 10 min. The solution color became black due to the formation of platinum on TiO2 nanoparticles. The resulting solution was centrifuged, filtered and dried in air to give Pt (1 wt %) @ TiO.sub.2 in quantitative yield.

Example 2

(Synthesis of Cd(Ni)S

[0053] Nickel acetate (0.1 mmol) from the stock solution (80 mM, 1.25 mL), the cadmium acetate (0.9 mmol) from the stock solution (80 mM, 11.25 mL) were mixed and stirred for 15 min at 60 C. then sodium sulfide (3 mmol) from (100 mM, 30 mL) methanolic stock solution was added dropwise. The resulting suspension was stirred for 1 hour. The precipitates were separated by centrifugation, washed with H.sub.2O/MeOH (1:1) mixture and dried at 60 C. overnight to give the final product of Cd(Ni)S.

Example 3

(Synthesis of Pd@Cd(Ni)S)

[0054] PdCl.sub.2(1.2 mg (Pd)/mL, 0.6 mL) was added drop-wise to a suspension of Cd(Ni)S (200 mg) nanoparticles in 100 mL water solution of benzyl alcohol and acetic acid mixture (2.5-2.5 v/v %). The resulting mixture was illuminated under UV (=360 nm) light with a light intensity of 5 mW/cm.sup.2 for 4 hours then filtered, washed with water and dried in air to give Pd (0.3 wt %)@ Cd(Ni)S.

Example 4

(Synthesis of TiO.SUB.2.@M@Cd(Ni)S.

[0055] TiO.sub.2/M (0.2 g, 2.5 mmol) nanoparticles of Example 1 were redispersed in 70 mL methanol and the temperature was raised to 60 C. In order to form Cd.sub.0.9Ni.sub.0.1S layers, the required amount of nickel acetate (0.25 mmol) from nickel acetate stock solution (80 mM, 3.1 mL) was added to the dispersion and then cadmium acetate (2.25 mmol) from (80 mM, 28.1 mL) stock solution and sodium sulfide (3 mmol) from (100 mM, 30 mL) methanolic stock solutions were added drop wise while stirring. Stirring was continued for an additional 30 min. Products were separated by centrifugation and washed with H.sub.2O/MeOH mixture and dried at 60 C. overnight to give the final products.

Example 5

(Synthesis of TiO.SUB.2.@M@Cd.SUB.0.8.Zn.SUB.0.2.S Compounds)

[0056] TiO.sub.2/M (0.2 g, 2.5 mmol) nanoparticles of Example 1 were re-dispersed in 70 mL methanol and the temperature was raised to 60 C. In order to form Cd.sub.0.8Zn.sub.0.2S layer of the particles, zinc acetate (0.5 mmol) from zinc acetate stock solution (80 mM, 6.25 mL) was added to the dispersion and then the cadmium acetate (2 mmol) from (80 mM, 25 mL) stock solution and sodium sulfide (3 mmol) from (100 mM, 30 mL) methanolic stock solutions were added drop wise simultaneously while stirring the solution. Stirring was continued for 30 min more. Products were separated by centrifugation and washed with H.sub.2O/MeOH mixture and dried at 60 C. overnight to give the final product (0.4 g, 88% in yield).

Example 6

(Characterization)

[0057] The materials were characterized by UV-vis, and X-ray Diffraction (XRD) to study the band gap, composition, and crystallinity. XRD spectra were recorded using a Bruker D8 Advance X-ray diffractometer. Cu K(=1.5406 {acute over ()} ) radiation over the range of 2 interval between 20 and 90 with a step size of 0.010 and a step time of 0.2 s/step were used.

[0058] The XRD pattern of AgPd/TiO.sub.2, AgPd/Cd.sub.0.8Zn.sub.0.2S, and [TiO.sub.2].sub.a/AgPd/Cd.sub.0.8Zn.sub.0.2S (a=1 to 4) compounds listed in Table 2 are depicted in FIG. 3A. The hybrid system shows a clear mixture of anatase TiO.sub.2 and cubic Cd.sub.0.8Zn.sub.0.2S phases in FIG. 3A. The Cd:Zn ratios were confirmed by Vergard's law. The XRD pattern of Cd(Ni)S, TiO.sub.2/M1/Cd(Ni)S compounds listed in Table 2 are depicted in FIG. 3B. The peaks located at 25.4 and 47.9 correspond to the (101) and (200) planes of the TiO.sub.2 anatase phase (JCPDS 21-1272). The diffraction feature of Cd(Ni)S appearing at 26.7 , 43.2 and 52.1 correspond to the (111), (220), (311) planes of cubic Cd(Ni)S (JCPDS 42-1411). All the characteristic peaks in FIG. 3B are broadened due to the small particles size of each components indicating the polycrystalline nature of the samples.

TABLE-US-00002 TABLE 2 Compound Pattern FIG. No. Compound Position No. 1 AgPdTiO.sub.2 Bottom pattern 4A 2 AgPd/Cd.sub.0.8Zn.sub.0.2S Above pattern of 4A No. 1 3 [TiO.sub.2].sub.4/AgPd/Cd.sub.0.8Zn.sub.0.2S Above pattern of 4A No. 2 4 [TiO.sub.2].sub.3/AgPd/Cd.sub.0.8Zn.sub.0.2S Above pattern of 4A No. 3 5 [TiO.sub.2].sub.2/AgPd/Cd.sub.0.8Zn.sub.0.2S Above pattern of 4A No. 4 6 TiO.sub.2/AgPd/Cd.sub.0.8Zn.sub.0.2S Above pattern of 4A No. 4 7 TiO.sub.2/0.3 wt. % Pd/Cd(Ni)S Bottom pattern 4B 8 TiO.sub.2/0.1 wt. % Au/Cd(Ni)S Above pattern of 4B No. 7 9 TiO.sub.2/0.1 wt. % Pt/Cd(Ni)S Above pattern of 4B No. 8 10 Cd(Ni)s Above pattern of 4B No. 9

[0059] FIG. 4A presents UV-vis diffused reflectance spectra (from bottom to top) of the solid solutions of compounds 1 (bottom spectra), 3, 4, 5, and 6 (top spectra) of Table 2. The intense absorption bands with steep edges are observed which indicates that the light absorption is due to intrinsic band gap transitions. Kubelka-Munk function versus energy of incident light are shown in the inset of FIG. 4A, the band gap position is almost the same, around 2.4 eV. FIG. 4B shows UV-vis absorbance spectra and Tauc plots for the TiO.sub.2/M/Cd(Ni)S systems 7 (bottom spectra), 9, 8 and 10 (top spectra) of Table 2. All the TiO.sub.2 containing samples show intense absorption below 400 nm because of the band-gap excitation of TiO.sub.2. Furthermore, all samples have the distinct future of Cd(Ni)S which has absorption around 560 nm. The band gap determined from the Kubelka-Munk function versus energy of incident light are shown in the insert in FIG. 4B, the band gap of Cd(Ni)S (around 2.2 eV) is slightly lower compared to CdS (2.4 eV), which is due to the formation of Cd(Ni)S solid solution. The band gap of TiO.sub.2 determined by Tauc plots was 3.2

Example 7

(Production of Hydrogen Using the Catalysts of the Present Invention)

[0060] TiO.sub.2/M1/Cd.sub.xZn.sub.1xS System. The rate of photocatalytic hydrogen generation of Z-scheme photocatalysts of the present invention was determined. The photocatalyst of the present invention (7 mg) or a comparative photocatalysts was dispersed in a water solution of benzyl alcohol and acetic acid mixture (2.5-2.5 v/v %) and irradiated with a light source at 23% light intensity (42.5 mW/cm.sup.2) of Xenon lamp and 1 cm.sup.2 of the area of irradiation.

[0061] FIG. 5A depicts the hydrogen production versus time for the TiO.sub.2/M1/Cd.sub.xZn.sub.1xS series (compounds 3-6 of Table 2). The ratio of Cd:Zn (4:1) was kept the same during this study. Factors that influenced the hydrogen generation rate for of the were determined to be the (TiO.sub.2).sub.a:S ratio and (TiO.sub.2).sub.a:M1 ratio. The Z-Scheme catalyst of TiO.sub.2/Ag (0.1 wt %)/Pd (0.3 wt %) Cd.sub.0.8Zn.sub.0.2S gave the best rate (FIG. 5A). In this system the molar ratio of metal oxide to M1 was 1:1.

[0062] TiO.sub.2/M1/Cd(Ni)S System. Hydrogen production of the TiO.sub.2/M1/Cd(Ni)S system was determined using the same procedure as for the TiO.sub.2/M1/CdZnS system. FIG. 5B depicts hydrogen production versus time for Cd(Ni)S based catalysts. FIG. 5C depicts comparison of the hydrogen production rate of the Cd(Ni)S based catalysts and comparative catalysts of PdAg/TiO.sub.2 and 1 wt. % Au/TiO. Referring to FIG. 5B, the bottom data lines are TiO.sub.2/PdAg, Cd(Ni)S, and TiO.sub.2/1% Au/Cd(Ni)S, the middle data lines are 0.3 wt. % Pd/Cd(Ni)S and TiO.sub.2/1 wt. % Pd/Cd(Ni)S, and the top data line is TiO.sub.2/1 wt. % Pt/Cd(Ni)S. The TiO.sub.2/Pt/Cd(Ni)S catalysts far exceeding those of the single-and two-component systems due to an efficient electron-hole recombination rate between the TiO.sub.2 conduction band and Cd(Ni)S valence band through Pt nanoparticles. Interestingly, the hydrogen generation rate of TiO.sub.2/1% Au/Cd(Ni)S was very low. Moreover, 0.3% Pd/Cd(Ni)S gave a better rate than that of TiO.sub.2/0.3% Pd/Cd(Ni)S. Thus, it was determined that changing the metal and its content in the TiO.sub.2/M/Cd(Ni)S catalyst series considerably affects the hydrogen generation rate under the same light intensity.