PHOTOCATALYST

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U. S. Provisional Patent Application No. 62/537,552 filed July 27, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns methods of preparing nanocomposite heteroj unction photocatalysts. In particular, the method includes electrostatically coupling two semiconductors and heat-treating the electrostatically coupled material to form the nanocomposite heteroj unction. The nanocomposite heteroj unction can have photocatalytic activity.

B. Description of Related Art

[0003] T1O2 is commonly used as a photocatalyst due to its availability, durability, long- term stability, and nontoxicity. However, its wide bandgap and its tendency to undergo fast charge carriers recombination can lead to a low quantum efficiency. In order to obtain a commercially feasible solar to fuel efficiency (about 10% conversion) system, T1O2 has been coupled with narrow band gap semiconductors such as, g-C ₃ N ₄ , CdS, M0S2, and ImS ₃ as photosensitizers. However, identifying a stable narrow bandgap semiconductor with a suitable band alignment with T1O2 is a challenge. Among these semiconductors, graphitic carbon nitride (g-C ₃ N ₄ ) is highly attractive because it can be excited with visible light. It possesses a two-dimensional structure and has excellent chemical stability. However, g-C ₃ N ₄ suffers from rapid recombination of photo-generated electron-hole pairs resulting in low photocatalytic activity. Further, only a few materials can be coupled with g-C ₃ N ₄ to form a heterostructure. In order to form an efficient visible light excited g-C ₃ N ₄ based hetero-structure, the potential materials should fulfill the following two requirements:

(1) An appropriate band structure, which is beneficial to create a coupling hybridization with C ₃ N ₄ ; the difference in chemical potential between the semiconductors and the g-C ₃ N ₄ should generate a band bending at the junction interface. Thus, the generated band bending induces a built-in field, which impels the photo-generated electrons and holes to transfer in opposite directions, resulting in a spatially efficient separation of the electrons and holes pairs on different sides of the heterojunction; and

(2) The crystal structure in the junction domain of the heterostructure is also important in strengthening the quantum efficiency of the photocatalyst. A difference in lattice spacing between two semiconductors could lead to lattice mismatch of the interface forming defects, which will capture the photo-generated charge carriers and thus inhibit their diffusion to the surface.

[0004] T1O2 fulfills most of the requirements in terms of band structure and crystal facets. A stable interface can be formed between the plane (22-40) of g-C ₃ N4 and the plane (110) of anatase T1O2, both of which have similar d-spacing of 0.35 nm and the same orientation. Therefore, T1O2 can be used as a coupling candidate for fabricating C3N4 base heterojunctions. For instance, Chai et al. (Physical Chemistry Chemical Physics, 2012, 14(48), 16745-16752) prepared a g-C ₃ N4-Pt-Ti02 (P25 T1O2) composite having a mass ratio of 70:30 that showed maximum H2 generation rate under visible light with stable photocurrent. The activity reported was 1.5 times higher than that of the bare g-C ₃ N4. Yang et al. (Physical Chemistry Chemical Physics, 2015, 17(27), 17887-17893) reported a C ₃ N4-sensitized T1O2 nanotube array-based photoanode, which was designed and fabricated via in situ growth of C ₃ N4 on the surface of T1O2 nanotubes. It showed a stable and significantly improved hydrogen generation rate under visible light irradiation; 7 times higher than that of C ₃ N4. In another example, Dai et al. (Applied Catalysis B: Environmental, 2014, 156-157, 331-340) describes coupling of surface- fluorinated T1O2 nanosheet onto g-C ₃ N4 nanosheet using a hydrothermal method of mixing HF, T1O2 and g-C ₃ N4 together and heating under autogenous pressure at 180 °C. These fabrication methods, however, suffer from self-aggregation of both C ₃ N4 and T1O2, which leads to three separate phases of T1O2, C ₃ N4, and Ti02@C ₃ N4. These phases can interfere with the direct crystal contact between the two materials and ultimately reduce the catalytic activity of the material.

SUMMARY OF THE INVENTION

[0005] A solution to some of the problems associated with making nanocomposite heterojunctions has been discovered. The solution is premised on electrostatically coupling two semiconductors and heat-treating the electrostatically coupled semiconductors to form a nanocomposite heterojunction. The electrostatically coupling can be achieved through surface functionalization of each semiconductor with a positive and negatively charged ligand, respectively. By way of example, a semiconductor (X) nanomaterial can be functionalized with a negatively charged ligand (L ^" ) to form a XL ^" nanomaterial. A different semiconductor nanomaterial (Y) can be functionalized with a positively charged ligand (L ⁺ ) to form a YL ⁺ nanomaterial. XL ^" and YL ⁺ can be combined under conditions sufficient to form a XL ^" XL ⁺ nanomaterial, which is electrostatically coupled. The XL ^" YL ⁺ nanomaterial can be heat treated to remove the ligands and form a XY nanocomposite heteroj unction material, where X is uniformly dispersed on the Y surface. A metal particle can be dispersed on the surface of the XY nanocomposite. The XY heteroj unction can have photocatalytic properties. Notably, H2 generation rate from water using a Pd on Ti02/g-C ₃ N ₄ photocatalytic hydrogen generation heteroj unction prepared by the processes of the present invention is 14 times and 8 higher than that of as prepared g-C ₃ N ₄ and 0.3% Pd/C ₃ N ₄ , respectively.

[0006] In one aspect of the invention, a method of preparing a nanocomposite heteroj unction is described. The method can include combining a first semiconductor nanomaterial (X) having its surface functionalized with a negatively charged ligand (L ^" ) with a second semiconductor nanomaterial (Y) having its surface functionalized with a positively charged ligand (L ⁺ ) under conditions suitable to electrostatically couple XL ^" with YL ⁺ to form an electrostatically coupled XL ^" YL ⁺ material. The process can further include heat-treating the electrostatically coupled XL ^" YL ⁺ material to form a XY heteroj unction nanocomposite. Heat-treating can include heating the XL ^" YL ⁺ material to a temperature of 300 °C to 450 °C, 325 °C to 375 °C, or about 350 °C in the presence of air at 0.1 MPa to 2 MPa pressure. The XY nanocomposite can include 5 to 35 wt.% of X, preferably 10 wt.% to 30 wt.% X, or more preferably 20 wt.% to 25 wt.% of X, and/or 60 to 95 wt.% Y, preferably 70 wt.% to 90 wt.% Y, or more preferably 75 to 80 wt.% Y.

[0007] X and Y can be two different semiconductors. By way of example, the heteroj unction nanocomposite can include an oxygen containing semiconductor (e.g., W0 ₃ , B1VO4, T1O2, ZnO, or, SrTiCb, or combinations thereof) and a non-oxygen containing semiconductor (e.g., graphitic carbon nitride (g-C ₃ N ₄ ), CdS, CdZnS, CdNiS, InN, GaN, InGaN, GaAs, InAs, InGaAs, InP, GaP, or InGaP, or combinations thereof). In a preferred embodiment, X is T1O2 and Y is g-C ₃ N ₄ . [0008] In some embodiments, L ^" is a negatively charged bi-functional organic ligand, and L ⁺ is a positively charged bi-functional ligand or a positively charged hydrogen atom (H ⁺ ). The negatively charged bi-functional organic ligand can have the general structure of: _R ^--CH ₂ R ₂ CH— _R ^ where: Ri is a heteroatom-containing group, preferably -SH, -Si(OCH3)3, -Si(OC2H ₅ )3 and attaches to X; R2 is an aliphatic group or an aromatic group, preferably a group having 1 to 10 carbon atoms; and R3 is a negatively charged functional group, preferably, -SO3 ^" , -CO2 ^" and PO3 ^2" and attaches to L ⁺ . In some embodiments, the negatively charged bi-functional organic ligand can have the structure of:

where R ₄ , R5, and R ₆ are each independently an alkyl group having 1 to 5 carbon atoms, preferably -CH3 or -CH2CH3. In some embodiments, L ⁺ can have the general structure of: where: R7 is a heteroatom-containing group, preferably -SH, -Si(OCH3)3, -Si(OC2H ₅ )3, and attaches to Y; and Rs and R9 are each independently a hydrogen atom, an aliphatic group, or an aromatic group, preferably a hydrogen atom. The ammonium can attach to L ^" . In a preferred aspect, the electrostatically coupled XL ^" YL ⁺ material is T1O2- (CH30)3SiCH2CH ₂ CH ₂ S03-C3N4H ⁺ .

[0009] The method can further include depositing a metal particle (e.g., palladium (Pd), platinum (Pt), gold (Au), or alloys thereof, preferably Pd) on the surface of the XY nanocomposite to form a metal/XY nanocomposite. Deposition of the metal can include suspending the XY nanocomposite and the metal particle in a solvent, preferably water and ethanol, exposing the suspension to visible light in an aqueous alcohol solution to attach the metal particle to the XY nanocomposite to form a metal/XY nanocomposite, and collecting the metal/XY nanocomposite. The metal/XY nanocomposite can include 1 wt.% to 2 wt.%, 0.2 wt.%) to 1 wt.%), or 0.3 to 0.5 wt.%> of the metal particle. In a preferred embodiment, the metal/XY nanocomposite is a Pd/Ti02/g-C3N ₄ nanocomposite. [0010] In another aspect of the invention, a photocatalytic hydrogen generating heteroj unction is described. The photocatalytic hydrogen generating heteroj unction can include: a metal oxide (X), non-oxide (Y) nanocomposite having a X:Y weight ratio of 1 : 19, where X can be uniformly dispersed on the Y surface; and 0.1 wt.% to 0.5 wt.% of a metal particle can be dispersed on the surface of the XY nanocomposite. In some embodiments, X can be TiCh and Y can be g-C ₃ N ₄ and the metal particle can be Pd. The particle size of Pd particle can be from 1 to 8 nm, 2 to 7 nm, or 3 to 5 nm. The particle size of the T1O2 can be 5 to 500 nm. The particle size of g-C ₃ N ₄ can be 10 nm to 5000 nm.

[0011] In yet another aspect of the invention, a method of producing H2 from water is described. The method can include obtaining an aqueous solution comprising the photocatalyst of the present invention, a sacrificial agent, and water, and subjecting the mixture to a light source for a sufficient period of time to produce hydrogen (H2) from the water.

[0012] In the context of the present invention 20 embodiments are described. Embodiment 1 is a method for preparing a nanocomposite heteroj unction, the method comprising: (a)combining a first semiconductor nanomaterial (X) having its surface functionalized with a negatively charged ligand (L ^" ) with a second semiconductor nanomaterial (Y) having its surface functionalized with a positively charged ligand (L ⁺ ) under conditions suitable to electrostatically couple XL ^" with YL ⁺ to form an electrostatically coupled XL ^" YL ⁺ material; and (b) heat-treating the electrostatically coupled XL ^" YL ⁺ material to form a XY heteroj unction nanocomposite. Embodiment 2 is the method of embodiment 1, wherein X and Y comprise an oxygen containing semiconductor and a non-oxygen containing semiconductor, respectively. Embodiment 3 is the method of embodiment 2, wherein the oxygen containing semiconductor is W0 ₃ , BiV0 ₄ , T1O2, ZnO, or SrTiCb, or combinations thereof, and the non- oxygen containing semiconductor is graphitic carbon nitride (g-C ₃ N ₄ ), CdS, CdZnS, CdNiS, InN, GaN, InGaN, GaAs, InAs, InGaAs, InP, GaP, or InGaP, or combinations thereof. Embodiment 4 is the method of embodiment 3, wherein X is T1O2 and Y is g-C ₃ N ₄ . Embodiment 5 is the method of any one of embodiments 1 to 4, further comprising depositing a metal particle on the surface of the XY nanocomposite to form a metal/XY nanocomposite. Embodiment 6 is the method of embodiment 5, wherein the deposition comprises: suspending the XY nanocomposite and the metal particle in a solvent, preferably water and ethanol; exposing the suspension to visible light in an aqueous alcohol solution to attach the metal particle to the XY nanocomposite to form a metal/XY nanocomposite; and collecting the metal/XY nanocomposite. Embodiment 7 is the method of any one of embodiments 5 to 6, wherein the metal particle comprises palladium (Pd), platinum (Pt), or gold (Au), or alloys thereof, preferably Pd. Embodiment 8 is the method of any one of embodiments 5 to 7, wherein metal/XY nanocomposite comprises 0.1 wt.% to 2 wt.%, 0.2 wt.% to 1 wt.%, or 0.3 wt.% to 0.5 wt.%) of the metal particle. Embodiment 9 is the method of any one of embodiments 5 to 8, wherein the metal/XY nanocomposite is a Pd/Ti02/g-C3N4 nanocomposite. Embodiment 10 is the method of any one of embodiments 1 to 9, wherein L- is a negatively charged bi- functional organic ligand, and L+ is a positively charged bi-functional ligand or a proton (H+). Embodiment 1 1 is the method of embodiment 10, wherein the negatively charged bi-functional organic ligand has the general structure of:

where: Ri is a heteroatom-containing group, preferably -SH, -Si(OCH3)3, -Si(OC2H ₅ )3 and attaches to X; R2 is an aliphatic group or an aromatic group, preferably a group having 1 to 10 carbon atoms; and R3 is a negatively charged functional group, preferably, -SO3 ^" , -CO2 ^" and PO3 ^2" and attaches to L ⁺ . Embodiment 12 is the method of embodiment 11, wherein the negatively charged bi-functional organic ligand has the structure of:

where R ₄ , R5, and R ₆ are each independently an alkyl group having 1 to 5 carbon atoms, preferably -CH3 or -CH2CH3. Embodiment 13 is the method of any one of embodiments 10 to 12, wherein positively charged bi-functional ligand has the general structure of: where: R7 is a heteroatom-containing group, preferably -SH, -Si(OCH3)3, -Si(OC2H ₅ )3, and attaches to Y; and Rs and R9 are each independently a hydrogen atom, an aliphatic group, or an aromatic group, preferably a hydrogen atom. Embodiment 14 is the method of

embodiment 1, wherein electrostatically coupled XL-YL+ material is T1O2- (CH30)3SiCH2CH ₂ CH ₂ S03-C3N4H+. Embodiment 15 is the method of any one of embodiments 1 to 14, wherein the heat treating in step (c) comprises heating the electrostatically coupled XL ^" YL ⁺ material to a temperature of 300 °C to 450 °C, 325 °C to 375 °C, or about 350 °C in the presence of air at 0.1 MPa to 2 MPa pressure. Embodiment 16 is the method of any one of embodiments 1 to 15, wherein the XY nanocomposite comprises 5 to 35 wt.% of X, preferably 10 wt.% to 30 wt.% X, or more preferably 20 wt.% to 25 wt.% of X, and/or 60 to 95 wt.% Y, preferably 70 wt.% to 90 wt.% Y, or more preferably 75 to 80 wt.% Y.

[0013] Embodiment 17 is a photocatalytic hydrogen generating heteroj unction comprising: (a) a metal oxide (X), non-oxide (Y) nanocomposite having a X: Y weight ratio of 1 :4 to 1 :25, preferably 1 : 19, wherein the X is uniformly dispersed on the Y surface; and (b) 0.1 wt.% to 0.5 wt.%) of a metal particle dispersed on the surface of the XY nanocomposite. Embodiment 18 is the photocatalytic hydrogen generating heteroj unction of embodiment 17, wherein: X is WO3, B1VO4, T1O2, ZnO, or SrTiCb, or combinations thereof, preferably T1O2; Y is g-C ₃ N ₄ , CdS, CdZnS, CdNiS, InN, GaN, InGaN, GaAs, InAs, InGaAs, InP, GaP, or InGaP, preferably g-C ₃ N ₄ ; and the metal particle is a transition metal, preferably palladium (Pd), platinum (Pt), or gold (Au), or alloys thereof, more preferably Pd. Embodiment 19 is the photocatalytic hydrogen generation heteroj unction of any one of embodiments 17 to 18, wherein X is T1O2 and Y is g-C ₃ N ₄ and the metal particle is Pd, and wherein the particle size of Pd particle is from 1 to 8 nm, 2 to 7 nm, 3 to 5 nm, the particle size of the T1O2 is 5 to 500 nm, the particle size of g-C ₃ N ₄ is 10 nm to 5000 nm. [0014] Embodiment 20 is a method of producing hydrogen (H2) from water, the method comprising: obtaining an aqueous solution comprising the photocatalyst of any one of embodiments 17 to 19, a sacrificial agent, and water; and subjecting the mixture to a light source for a sufficient period of time to produce hydrogen (H2) from the water.

[0015] The following includes definitions of various terms and phrases used throughout this specification.

[0016] An aliphatic group is an acyclic or cyclic, saturated or unsaturated carbon group, excluding aromatic compounds. A linear aliphatic group does not include tertiary or quaternary carbons. Non-limiting examples of aliphatic group substituents include halogen, hydroxyl, alkyoxy, haloalkyl, haloalkoxy, carboxylic acid, ester, amine, amide, nitrile, acyl, thiol and thioether. A branched aliphatic group includes at least one tertiary and/or quaternary carbon. Branched aliphatic group substituents include, but are not limited to, alkyl, halogen, hydroxyl, alkyoxy, haloalkyl, haloalkoxy, carboxylic acid, ester, amine, amide, nitrile, acyl, thiol and thioether. A cyclic aliphatic group is includes at least one ring in its structure. Polycyclic aliphatic groups may include fused, e.g., decalin, and/or spiro, e.g., spiro[5.5]undecane, polycyclic groups. Non-limiting examples of cyclic aliphatic group substituents include alkyl, halogen, hydroxyl, alkyoxy, haloalkyl, haloalkoxy, carboxylic acid, ester, amine, amide, nitrile, acyl, thiol and thioether. [0017] An alkyl group is linear or branched, substituted or unsubstituted, saturated hydrocarbon (CnH2n+i). Non-limiting examples of alkyl group substituents include halogen, hydroxyl, alkyoxy, haloalkyl, haloalkoxy, carboxylic acid, ester, amine, amide, nitrile, acyl, thiol and thioether.

[0018] An aryl group or aromatic group is a substituted or unsubstituted, mono- or polycyclic hydrocarbon with alternating single and double bonds within each ring structure. Non-limiting examples of aryl group substituents include alkyl, halogen, hydroxyl, alkyoxy, haloalkyl, haloalkoxy, carboxylic acid, ester, amine, amide, nitrile, acyl, thiol and thioether.

[0019] "Nanostructure" or "nanomaterial" refer to an object or material in which at least one dimension of the object or material is equal to or less than 1000 nm (e.g., one dimension is 1 to 1000 nm in size). In a particular aspect, the nanostructure includes at least two dimensions that are equal to or less than 1000 nm (e.g., a first dimension is 1 to 1000 nm in size and a second dimension is 1 to 1000 nm in size). In another aspect, the nanostructure includes three dimensions that are equal to or less than 1000 nm (e.g., a first dimension is 1 to 1000 nm in size, a second dimension is 1 to 1000 nm in size, and a third dimension is 1 to 1000 nm in size). The shape of the nanostructure can be of a wire, a particle (e.g., having a substantially spherical shape), a rod, a tetrapod, a hyper-branched structure, a tube, a cube, or mixtures thereof. "Nanoparticles" include particles having an average diameter size of 1 to 1000 nanometers.

[0020] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0021] The terms "wt.%," "vol.%," or "mol.%" refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component. [0022] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0023] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

[0024] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0025] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0026] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include"), or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0027] The methods and heteroj unction nanocomposites of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non-limiting aspect, a basic and novel characteristic of the photocatalytic hydrogen generating heteroj unctions of the present invention are their abilities to catalyze production of H ₂ from water.

[0028] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings. [0030] FIG. 1 is a schematic of a method to produce a heteroj unction nanocomposite of the present invention.

[0031] FIG. 2 is electrostatically coupled Ti02-(CH ₂ 0)3SiCH2CH2CH ₂ S03 ^" /H ⁺ g-C3N4, of the present invention.

[0032] FIG. 3 is a schematic of a method to produce a heteroj unction nanocomposite of the present invention having metal deposited on its surface.

[0033] FIG. 4 is a schematic of a system to produce H2 from water using the photocatalytic heteroj unction nanocomposite of the present invention.

[0034] FIG. 5 is a schematic of a mechanism for charge transfer between g-C3N ₄ and T1O2 through the heteroj unction interface under visible light irradiation. [0035] FIGS. 6A and 6B show (6A) X-ray diffraction (XRD) patterns of 0.3 wt.% Pd/g- C3N4 and Pd/Ti02/g-C3N ₄ composites with different mass ratios of T1O2 and g-C3N ₄ and (6B) 002 peak of FIG. 6 A

[0036] FIGS. 7A and 7B show (7 A) ultra violet-visible diffuse reflectance absorption spectra (DRS) of the patterns of 0.3 wt.% Pd/g-C3N ₄ and Pd/Ti02/g-C3N ₄ composites with different mass ratios of T1O2 and g-C3N ₄ and (7B) bandgap values.

[0037] FIGS. 8A-8I show high resolution transmission electron microscopy (FIR-TEM) images of 0.3 wt.% Pd/0.5 wt.% T1O2/ 95 wt.% g-C ₃ N ₄ : (8A) low magnification; (8B) high magnification; (8C) lattice fingers of Pd; (8D) lattice spacing of T1O2; (8E) FFT diffraction pattern of T1O2; and (8F) FFT diffraction pattern of Pd; (8G) transmission electron microscopy (TEM) images of the areas (circled) that performed EDX analysis, (8H) EDX results of the circled areas in (8G); and (81) STEM images.

[0038] FIGS. 9A through 9E show the X-ray photonelectron spectra (XPS) of 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C ₃ N ₄ : (9A) Cls; (9B) Nls; (9C) Ti 2p; (9D) Ols; and (9E) Pd 3d; (9F) S [0039] FIG. 10 shows a scanning electron microscope (SEM) image of 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C ₃ N4 with a scale of 100 micrometers.

[0040] FIG. 11 shows photocurrent responses of C3N4 (bottom line), Pd/ C ₃ N4 (first line from bottom), 0.1% Pd/5% Ti0 ₂ /C ₃ N ₄ (second line from bottom line), 0.1% Pd/10% Ti0 ₂ /C ₃ N4 (third line from bottom line and crosses over second and fourth line), 0.1% Pd/30% Ti0 ₂ /C ₃ N ₄ (fourth line from bottom), and 0.1% Pd/20% Ti0 ₂ /C ₃ N ₄ (top line) under Xe lamp irradiation with 400 nm cutoff filter;

[0041] FIG. 12 shows photoluminescence (PL) spectra of water (bottom line), 0.1% Pd/20% Ti0 ₂ /C ₃ N ₄ (second line from bottom), 20% Ti0 ₂ /C ₃ N ₄ (third line from bottom), and C ₃ N ₄ (top line).

[0042] FIG. 13 shows photocatalytic H ₂ evolution rates of g-C ₃ N4, Pd-g-C ₃ N4, and Pd/Ti0 ₂ /g-C ₃ N4 composites with different mass ratios: 20 mg catalyst, 30 mL 10 vol. % triethanol amine aqueous solution, 300 W Xe-lamp with 23% intensity (45 mW/cm ² ) for the visible-light (λ > 420 nm) irradiation. [0043] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0044] A discovery has been made that provides a solution to making heteroj unction nanocomposites. The solution is premised on electrostatically coupling two different semiconductors and heat-treating the electrostatically coupled semiconductors to form a heteroj unction nanocomposite. In some aspects, a metal particle can be dispersed on the surface the heteroj unction nanocomposite. The resulting heteroj unction nanocomposite has a first semiconductor material dispersed on the surface of the second semiconductor material. The heteroj unction nanocomposites of the present invention are capable of producing H ₂ from water.

[0045] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the FIGS. 1 to 5. The systems and methods of described in FIGS. 1 to 5 can also include various equipment that is not shown and is known to one of skill in the art of chemical processing. For example, some controllers, piping, computers, valves, pumps, heaters, thermocouples, and/or pressure indicators may not be shown. A. Method of Preparing a Heteroj unction Nanocomposite

[0046] FIG. 1 depicts a schematic of the method of the present invention to form a heteroj unction nanocomposite. In a first step of method 10, functionalized semiconductor 12 (e.g., XL ^" ) can be combined with functionalized second semiconductor 14 (e.g., YL ⁺ > to form an electrostatically coupled nanomaterial. Functionalized semiconductors 12 and 14 can be made using known synthetic methods, or as exemplified in the Examples and described in the Materials Section. Combining can include incrementally adding a solution of the functionalized 14 nanomaterial to a solution of the functionalized nanomaterial 12 and agitating the solution at 20 to 30 °C, or about 25 °C, for a period of time (e.g. 8 to 15 hours, or about 12 hours). The solutions can include the charged semiconductor and a polar solvent such as water or aqueous alcohol (e.g., methanol or ethanol). After agitating, the solvent can be removed to isolate the electrostatically coupled (XL ^{" +} LY) nanomaterial 16. In some embodiments, the electrostatically coupled semiconductors can be Ti02-(CH20)3SiCH2CH2CH2S03 ^" /H ⁺ g-C3N4, which is shown in FIG. 2. Solvent removal can be performed by evaporative methods and/or filtration methods (e.g., filtration, centrifugation, or the like). The isolated nanomaterial can be dried at 70 °C to 100 °C, or 75 °C to 90 °C, or 80 °C to 85 °C until deemed dry (e.g., 2 to 5 hours). In step 2, the mixture can be heated to remove the ligands (L ^" and L ⁺ ) and form the heteroj unction nanocomposite 18. Heat-treating can include heating the heteroj unction nanomaterial 16 at 300 °C to 450 °C, 325 °C to 375 °C, or a temperature greater than, equal to, or between at least two of 300 °C, 310 °C, 320 °C, 325 °C, 330 °C, 335 °C, 340 °C, 345 °C, 350 °C, 355 °C, 360 °C, 365 °C, 370 °C, 375 °C, or about 350 °C in the presence of air at 0.1 MPa to 2 MPa pressure.

[0047] In some embodiments, a metal particle (M) can be deposited on the surface of the heteroj unction nanocomposite 18 to form metal heteroj unction nanocomposite. FIG. 3 depicts a schematic of the method 30 of the present invention to form a metal heteroj unction nanocomposite, M/XY. Step 1 and step 2 of method 30 are the same as in FIG. 1. In step 3, the heteroj unction nanocomposite 18 and metal particle 32 can be suspended in a solvent (e.g., aqueous ethanol). In step 4, the suspension can be exposed to visible light (400 to 650 nm wavelength) for a time sufficient (e.g., 1 to 10 hours, 2 to 8 hours or about 4 hours) to attach metal particle 32 to the heteroj unction nanocomposite 18 and form a metal/XY nanocomposite 34. A non-limiting source of visible light is a Xenon lamp (Asahi spectra MAX-303) with a total flux of 42.5 mW cm ^"2 (350-650 nm) (UV - 3.0 mW cm ^"2 )). Without wishing to be bound by theory, it is believed that performing the metal deposition under visible light allows the metal to be deposited on the sites where the electrons migrate and consequently improve the reactivity of the catalyst (e.g., hydrogen production with the metal being the hydrogen co- catalyst). In step 5, the M/XY nanocomposite 34 can be isolated from the solvent using liquid/solid separation methods (e.g., filtration, centrifugation, and the like) and dried at 70 °C to 100 °C, or 75 °C to 90 °C, or 80 °C to 85 °C (e.g., for 2 to 10 hours).

B. Photocatalytic Heteroj unction Nanocomposite

[0048] The photocatalytic hydrogen generating heteroj unction (XY) of the present invention can include a metal oxide (e.g., X) and a non-oxide (e.g., Y) nanocomposite. The nanocomposite can have a X:Y weight ratio of 1 :25 to 1 :4, 1 : 19 to 1 :4, 1 : 19 to 1 :9, or 1 :9 to 1 :4, or any ranges there between. In a preferred aspect, the metal oxide (e.g., TiCh) to non- oxide (e.g., g-C ₃ N ₄ ) ratio is about 1 : 19. The metal oxide (e.g., X, oxygen-containing semiconductor) can be uniformly dispersed on the non-oxide (e.g., Y, non-oxygen containing semiconductor) surface. By way of example, at least 80%, at least 85%, or at least 90% of the oxygen-containing semiconductor can be dispersed on the surface of the non- oxygen containing semiconductor. The photocatalytic heteroj unction nanocomposite can also include at least 0.1 wt.% up to 0.5 wt.%, 0.2 wt.% to 0. 4 wt.%, or 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%), 0.5 wt.%) or any range or value there between of a metal particle dispersed on the surface of the XY nanocomposite. The particle size of the metal particle can range from 1 nm to 8 nm, 2 nm to 7 nm, 3 nm to 5 nm, or 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm or any range or value there between. The particle size of the metal oxide can be 5 nm to 500 nm, 10 nm to 400 nm, 50 nm to 300 nm, 100 nm to 200 nm, or greater than, equal to, or between any two of: 5, 50, 100, 200, 300, 400, 500. The particle size of the g-C ₃ N4 can be 10 nm to 5000 nm, 100 nm to 2500 nm, 500 nm to 1000 nm or greater than, equal to, or between any two of: 10, 100, 200, 300, 400, 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 45000, 5000. In a preferred embodiment, the metal/heterojunction nanocomposite can be Pd/Ti02/g-C ₃ N4 with the particle size of Pd particle can be from 1 to 8 nm, 2 to 7 nm, 3 to 5 nm, the particle size of the T1O2 is 5 to 500 nm, the particle size of g-C ₃ N ₄ is 10 nm to 5000 nm.

C. Materials

[0049] Materials used for the production of functionalized semiconductors and metal deposition can be obtained from commercial sources. Non-limiting examples of commercial source of such materials is Sigma-Aldrich® (U.S.A.) and Huntsman chemicals (U.S.A.). 1. Semiconductors

[0050] The photocatalytic heteroj unction nanocomposite can include two semiconductors that are different (e.g., X and Y can be different materials in FIG. 1). In some embodiments, the photocatalytic heteroj unction nanocomposite can include an oxygen containing semiconductor (e.g., X in FIG. 1) and a non-oxygen containing semiconductor (e.g., Y in FIG. 1). Non-limiting examples of oxygen containing semiconductors include WO3, BiV0 ₄ , T1O2, ZnO, or SrTiCb, or combinations thereof. Non-limiting examples of non-oxygen containing semiconductors include g-C ₃ N ₄ , CdS, CdZnS, CdNiS, InN, GaN, InGaN, GaAs, InAs, InGaAs, InP, GaP, or InGaP, or combinations thereof. The photocatalytic heteroj unction nanocomposite can include from 5 wt.% to 35 wt.% 10 wt.% to 30 wt.%, 20 wt.% to 25 wt.%, or greater than, equal to, or between any two of 5, 10, 15 20, 25, 30, and 35 wt.% of the oxygen- containing semiconductor (e.g., X in FIG. 1) with the balance being substantially or completely the non-oxygen containing semiconductor (e.g., Y), based on the total weight of the heteroj unction nanocomposite. In some embodiments, the heteroj unction nanocomposite can include 60 wt.% to 95 wt.%, 70 wt.% to 90 wt.%, 75 wt.% to 80 wt.% or greater than, equal to, or between any two of: 5, 10, 15 20, 25, 30, 35 wt.% of the non-oxygen containing semiconductor (e.g., Y in FIG. 1), with the balance being substantially or completely the metal oxide semiconductor (e.g., X), based on the total weight of the heterojunction nanocomposite. In a preferred embodiment, the oxygen-containing semiconductor is T1O2, and the non-oxygen containing semiconductor is g-C ₃ N ₄ .

2. Ligands

[0051] The surface of the semiconductors of the semiconductors can be functionalized with a negatively charged ligand (L ^" ) or a positively charged ligand (e.g., L ⁺ ). The ligands can include two functional groups, where one of the functional groups is capable of attaching (covalently bonding) to the semiconductor surface and the other functional group bears a charge.

[0052] The bifunctional negatively charged ligand can have the general structure of:

Ri can be a heteroatom-containing group that attaches to the semiconductor surface. Non- limiting examples of Ri include a sulfides (SH) or a siloxane (e.g. -Si(OCH ₃ ) ₃ , -Si(OC2H ₅ )3). R2 can be an aliphatic group or an aromatic group. R2 can be a group having 1 to 10 carbon atoms (e.g., methyl, ethyl, propyl, aryl, and the like). R3 can be a negatively charged functional group that electrostatically attaches to the paired semiconductor through a positively charged ligand (L ⁺ ). Non-limiting examples of R3 can include a sulfoxide (e.g., -SO3 ^" ) a carbonate (- CO2 ^" ), or a phosphite (ΡΟ3 ^2" )· In a preferred embodiment, the negatively charged ligand can be -(CH30)3SiCH2CH2CH2S03 ^" and the oxygen-containing semiconductor can be T1O2

[0053] The positively charged ligand can be a proton (H ⁺ ) or a bifunctional positively charged ligand (e.g., a functionalized ammonium compound). In some embodiments, the positively charged ligand can have the general structure of:

R7 can be a heteroatom-containing group that attaches to the surface of the semiconductor (e.g. Y surface). Non-limiting examples of R7 include sulfides (SH) or siloxane (e.g. -Si(OCH3)3, - Si(OC2H ₅ )3). R ₈ and R9 can each independently be a hydrogen atom, an aliphatic group, or an aromatic group. Non-limiting examples of R ₈ and R9 are methyl groups, ethyl groups, phenyl groups, substituted phenyl groups. In a preferred embodiment, R ₈ and R9 are both H. The charged nitrogen (ammonium ion) can electrostatically couple with the negatively charged ligand attached to the paired semiconductor (e.g., XL ^" ). In preferred instance, the positively charged ligand (L ⁺ > is H ⁺ , and the functionalized non-oxygen containing semiconductor (e.g. YL ⁺ ) is protonated g-C3N ₄ , as shown below.

3. Metals

[0054] Metals deposited on the heteroj unction nanocomposite (e.g., 16 in FIGS. 1 and 3) can include noble metals, transition metals, or any combinations or any alloys thereof. Noble metals include palladium (Pd), platinum (Pt), gold (Au), rhodium (Rh), ruthenium (Ru), rhenium (Re), osmium (Os), or iridium (Ir) or any combinations or alloys thereof. Transition metals include iron (silver (Ag), Fe), copper (Cu), nickel (Ni), zinc (Zn), manganese (Mn), chromium (Cr), molybdenum (Mo), tungsten (W), or tin (Sn), or any combinations or alloys thereof. In some embodiments, the nano- or micro structure includes 1, 2, 3, 4, 5, 6, or more transition metals and/or 1, 2, 3, 4 or more noble metals.

D. Process of Producing H ₂ from Water Using the Photocatalytic Heteroj unction Nanocomposite of the Present Invention [0055] Referring to FIG. 4, a non-limiting representation of a water-splitting system 40 of the present invention is provided. The system includes a plurality of the photocatalysts 34, a light source 42, and container or reaction vessel 44 that can be used to hold aqueous solutions or water 46. In some embodiments, photocatalyst 18 is used. The plurality of photocatalysts 34 can be suspended in the aqueous solution 46. Although not shown, the system 40 can also include at least one inlet for the aqueous solution/water 46 and at least one or more outlets for produced hydrogen and oxygen formed during the water-splitting reaction. Although not shown, the photocatalyst 34 can be coated onto the walls of the container 44 or can be packed in a bed (or plurality of beds), which is then immersed in the aqueous solution 46.

[0056] The container 44 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 ₂ . The light source 42 can includes either one of or both of visible and (400-600 nm) and ultraviolet light (280-400). In some embodiments, visible light can excite the non-oxygen containing photoactive semiconductor (e.g., g-C ₃ N ₄ ) to excite an electron in the valence band to the conductive band as shown in FIG. 5. Although T1O2 is not typically activated under the visible light because of the high band gap of 3.2 eV, the excited electrons (e ^" ) move through the heteroj unction interface and move to the conductive band, leaving a corresponding hole (h ⁺ ). The electrons can be extracted by the metal (e.g., Pd) nanoparticles and then captured by H ⁺ , resulting in a better charge separation. The excited electrons (e ^" ) can be used to reduce hydrogen ions to form hydrogen gas, and the holes (h ⁺ ) can be used to oxidize sacrificial agents and/or oxygen atoms. Metals can act as an electron sink to reduce protons and produce H ₂ . The hydrogen gas and the oxygen gas can then be collected and used in down-stream processes. Due to the hole scavenging properties of the cobalt oxide and conductive properties of palladium, excited electrons (e ^" ) are more likely to be used to split water before recombining with a hole (h ⁺ ) than would otherwise be the case. The system 40 does not require the use of an external bias or voltage source. Further, the efficiency of the system 40 as well as the hole scavenging properties of heteroj unction nanocomposite 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, or amino alcohols (e.g., triethanol amine) 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 46. The presence of the sacrificial agent can increase the efficiency of the system 40 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 triethanol amine, ethylene glycol, or glycerol, or a combination thereof is used.

[0057] 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 example, light energy may be provided to a photocell and from the light energy a voltage between the anode and the cathode can be produced and water molecules can be 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 34 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) or M0S2 cathodes (See, International Journal of Hydrogen Energy, February 2013, Vol. 38, issue 4, pages 1745-1757). [0058] 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

(Synthesis of Pd/Ti02/ -C3N4 of the Present Invention)

[0059] The heteroj unction nanocomposite of the present invention was made using a sulfonic acid-functionalized T1O2 electrostatically coupled with protonated g-C ₃ N ₄ using the methods described below. [0060] Sulfonic acid-functionalized T1O2 (Ti0 ₂ -Pr-S0 ₃ Na). To a solution of T1O2 (3 g, anatase, origin HOMBIKAT UV 100 produced by Huntsman Chemical (U.S.A.) with particle size around 5 nm, BET surface area 340 m ² /g) in dry ethanol (30 mL) and H3PO4 (1 mL), (3- mercaptopropyl) trimethoxysilane (3 mL) was added, and the reaction mixture was refluxed for 24 h. After this period, the mixture was filtered, washed with acetone and dried in air to obtain TiCh-Pr-SH. The resulting TiCh-Pr-SH was dispersed in H2O2 (10 wt.%) in methanol (20 mL) and stirred for 24 h at room temperature. The prepared sample was then treated with H2SO4 (IN) at ambient temperature for complete protonation, filtered, re-dispersed in water, and the pH was adjusted to about 10. The resulting dispersion was filtered, washed with H2O, then by acetone to obtain TiC -Pr-SCbNa.

[0061] Positively charged g-C ₃ N ₄ . Urea was heated at 675 °C for 4 hours with heating rate of 0.5 °C/min to produce g-C ₃ N4. The yield was around 6%. The prepared g-C ₃ N4 (1 g) was added to hydrochloric acid (HC1, 1M, 200 mL) solution and ultra-sonicated for 1 h, stirred for 4 h, centrifuged, washed with deionized water (DI) water, and then dried at 80 °C for 12 h.

[0062] Ti0 ₂ -Pr-S0 ₃ -/g-C ₃ N ₄ +. The prepared positively charged g-C ₃ N +) (95 mg) was dispersed in DI water (70 mL) and then added dropwise to the prepared TiC -Pr-SCbNa (5 mg) dispersed in DI water (30 mL). The resulting mixture was stirred at 25 °C overnight, filtered and dried at 80 °C for 3 h to form 5 wt.% Ti0 ₂ -Pr-S0 ₃ (-)/g-C ₃ N ₄ (+) of the present invention.

[0063] Ti0 ₂ /g-C ₃ N ₄ Heteroj unction Nanocomposite. Dried Ti0 ₂ -Pr-S0 ₃ (-)/g-C ₃ N ₄ (+) of the present invention was calcined at 350 °C for 4 hours to remove the ligands and produce the TiC"2/g-C ₃ N4 heteroj unction nanocomposite of the present invention.

[0064] Pd/Ti0 ₂ /g-C ₃ N ₄ . Pd(N0 ₃ ) ₂ solution in water (0.3 mL of 1 mg (Pd)/mL solution) was added to a suspension of the prepared Ti02/g-C ₃ N4 (100 mg in 100 mL 10% TEA solution). The resulting mixture was stirred under visible light for 4 hours. The resulting mixture was filtered, washed with water, then ethanol, and dried to give the 0.1 wt.% Pd/TiC"2/g-C ₃ N4 metal deposited heteroj unction nanocomposite of the present invention.

[0065] Using the procedure above, 0.3 wt.% Pd/TiC"2/g-C ₃ N4 heteroj unction nanocomposites of the present invention were made varying amounts of T1O2 and are listed in Table 1. The amounts of T1O2 and g-C ₃ N4 are weight based on the total weight of the TiCh/g- C ₃ N4 heteroj unction nanocomposites prior to deposition of the palladium Table 1

Example 2

(Characterization of Pd/Ti0 ₂ /g-C3N ₄ Heteroj unction Nanocomposites of the Present

Invention) [0066] XRD Analysis. XRD spectra was recorded using a Bruker D8 Advance X-ray diffractometer with Cu Κα (λ= 1.5406 A) radiation over the range of 2Θ interval between 20 and 90° with a step size of 0.010° and a step time of 0.2 s/step were used. FIG. 6A shows XRD patterns of samples 1-4 and the comparative sample. Most of the characteristic peaks for both components are visible in all studied ratios (Samples 1-4). The two main characteristic peaks of interlayer packing (100) and interplannar stacking (002, FIG. 6B) of g-C ₃ N ₄ were observed at 2Θ = ~ 13° and ~27° respectively. In addition, all the other TiCh peaks were assigned as shown in FIG. 6A. Furthermore, the peak intensities of T1O2 became stronger and the diffraction peaks of the (002) plane of g-C ₃ N ₄ showed a clear shift towards higher angle with increasing of the T1O2 content in the Ti02/g-C ₃ N ₄ , which may be attributed to interaction between g-C ₃ N ₄ and T1O2. The 0.1 wt.% Pd/20 wt.% Ti0 ₂ /C ₃ N ₄ showed slightly larger shift on crystal phase (002) while the peak position of 0.1 wt.% Pd/5 wt.% Ti02/g-C ₃ N ₄ was almost same as that of 0.1 wt.% Pd/g-C ₃ N ₄ in FIG. 6B. Interestingly, these are two hybrid samples that gave the highest and the lowest H2 generation rates therefore one may conclude that the bigger shift indicates stronger interaction between two semiconductors and stronger the interaction better the charge separation which subsequently results in higher H2 generation rates as shown in FIG. 13. No peak was attributed to Pd, which was attributed to its complete dispersion as small nanoparticles.

[0067] Ultra-violet Visible (UV-Vis) Diffuse Reflectance Adsorption Spectra (DRS).

UV-vis absorbance spectra of the powdered catalysts were collected over the wavelength range of 250-700 nm on a Thermo Fisher Scientific spectrophotometer equipped with praying mantis diffuse reflectance accessory. FIG. 7 shows the DRS spectra of Samples 1-4 and the comparative sample. The absorption edge of T1O2 was not visible from the spectrum. The main absorption edge of g-C ₃ N ₄ was observed at around 440 nm and the band gap was estimated to be around 2.8 eV from the Tuac plots.

[0068] High Resolution Transmission Electron Microscopy (HR-TEM). TEM

Samples were dispersed in alcohol and a drop of the suspension was placed over a grid with holey-carbon film. The microscope used for STEM and HRTEM was a FEI Tecnai F20 (FEI, U.S.A.) operating at 200 kV. HR-TEM images of 0.3% Pd/5 wt% T1O2/C3N4 (Sample 4) were obtained. FIGS. 8A-8F show HR-TEM images of Sample 4; (8 A) low magnification; (8B) high magnification; (8C) lattice fingers of Pd; (8D) lattice spacing of T1O2; (8E) FFT diffraction pattern of T1O2; and (8F) FFT diffraction pattern of Pd. From FIG. 8A, it was determined that the particle size of Pd and T1O2 were around 5 nm and 15 nm, respectively and they were densely and uniformly dispersed on the surface of C ₃ N ₄ layers. No phase separation was observed, which indicated a uniform mixing of all three components. Lattice fingers revealed in FIG. 8C and 8D are for the d-spacing of 0.2 nm and 0.19 nm corresponding to the (200) planes of anatase T1O2 and Pd respectively. The corresponding d-spacings were calculated according to the FFT diffraction patterns from FIGS. 8E and 8F. A further important observation made for Pd/Ti02/C ₃ N ₄ (FIG. 8B) was the interparticle heterojunctions between T1O2 and C ₃ N ₄ as well as T1O2 and Pd. Although, the inter-particle heteroj unction between C ₃ N ₄ and Pd was also detected, the Pd density on T1O2 is much higher than that of C ₃ N ₄ as shown in STEM (FIG. 81). Using the SEM, a ratio of Pd density on T1O2 to Pd density to C ₃ N ₄ was determined to be 41. Moreover, nanoscale EDXs are performed at the points where the T1O2 and C ₃ N ₄ are separated (FIG. 8G). The Pd signal was only detected on T1O2 (FIG. 8H). These results clearly indicates that the electron generated by visible light flows from the conduction band of C ₃ N ₄ to that of T1O2 since the Pd was light deposited to the surface of the Ti02/C ₃ N ₄ . This is direct evidence confirming the proposed mechanism in FIG. 5. [0069] X-ray Photon Spectroscopy (XPS). The XPS spectra of the 0.1 wt.% Pd/20 wt.% Ti02/g-C ₃ N ₄ were collected by Thermo Scientific Escalab 250 XI XP spectrometer with Al Ka X-ray source (Thermo Fischer Scientific Inc., U.S.A.). The X-ray spot size was 650μπι ² . The charge compensation was carried out using standard flood gun. Before collecting XPS data, samples were etched using Ar ions for 5 min. at ion energy of 1000 eV. The data was acquired using following settings before and after etching: All the peaks were corrected with respect to the binding energy of adventitious Cls peak at 284.5 eV. All peaks were fitted using SMART background option and Lorentzian/Gaussian. FIGS. 9A through 9E show the XPS of 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C ₃ N ₄ : (9A) Cls; (9B) Nls; (9C) Ti 2p; (9D) Ols; and (9E) Pd 3d. FIG. 9F shows the XPS spectra of S 2p of T1O2. The regional spectrum of Cls in FIG. 9A displayed three peaks including the reference Cls (284.5 eV) with binding energy values of 285.9 and 287.9 eV, respectively. The peak located at 284.5 eV was assigned to the C-C and/or adventitious carbon, and the peaks at 286.8 and 287.8 eV were ascribed to the C-N-C and the C-(N) ₃ group of C3N4. The Nls high resolution spectrum in FIG. 9B can be fitted into three peaks, assigned to C-N=C at 398.4 eV, N-(C) ₃ at 399.7 eV and C-N-H at 401.0 eV. The Ti2p spectrum (FIG. 9C) shows two symmetric peaks at 458.5 and 464.2 eV, which was attribute to the binding energy of Ti 2p ₃ /2 and 2pm, correspondingly. Furthermore, the Ols spectrum (FIG. 9D) was fitted into two peaks, and the binding energy of 530.1 and 531.8 eV was ascribed to T1O2 and H2O, respectively.

[0070] Moreover, the binding energy values of Pd 3ds/2 and 3d ₃ /2 can be observed to be 334.4 and 339.4 eV as shown in FIG. 9E, which are very close to the standard binding energy of Pd metal, thus it was determined that Pd existed as a metallic form in the nanocomposite material. In addition, only S ⁺⁶ species of sulfur (S 2p ₃ /2 =168 eV) was detected on TiC -SCbNa (FIG. 9F) indicating all the SH from pervious step was oxidized into -S0 ₃ ^" to form negatively charges T1O2.

[0071] XPS spectra of 0.1% Pd-20% Ti0 ₂ /g-C ₃ N ₄ CPS CI, Nls are consistent with those reported for G-C ₃ N ₄ in the literature. Pd metal despite its low loading was detected (around 0.1 wt.%) and similar loading was also measured by inductively coupled plasma atomic emission spectroscopy (ICP) and shown in Table 2. Binding energy was attributed to Pd 3ds/2 and 3d ₅ /2 at 334.4 and 339.4 eV. The presence of S was clear, this was due to the ligand used to link T1O2 to g-C ₃ N ₄ : binding energy at 168 eV was attributed to S 2p ₃ /2 for TiCh-SCbNa indicating the SH from pervious step was oxidized into -S0 ₃ ^" to form negatively charges T1O2 surface, supporting the method of the present invention represented in FIGS. 1-3.

Table 2

[0072] Scanning Electron Microscopy (SEM) and Energy dispersive X-ray Spectroscopy (EDX). SEM and EDX analysis was performed at 20 kV (Quanta 200, FEI, OR) on the 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C3N ₄ material. FIG. 10 is the SEM image at 100 micrometers scale. From the SEM and EDX analysis it was determined that the Ti atoms of the Ti0 ₂ (brighter portions of SEM) were well dispersed over the g-C ₃ N ₄ material.

[0073] Photocurrent Responses. Photocurrent responses of the catalysts under Xe lamp irradiation with 400 nm cutoff filter were performed. A fast and steady photocurrent response was observed for each working electrode (FIG. 11). The highest photocurrent density was achieved on 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C ₃ N ₄ which was more than 10 times that of Pd/C ₃ N ₄ and the great enhancement in the photocurrent can be ascribed to the improved charge separation and accelerated change transfer by the selective positioning of Pd on Ti0 ₂ . [0074] Photoluminescence (PL) spectra were measured at room temperature on A10094 fluorescence spectrometer (Hamamatsu Photonics K. K., Japan) with an excitation wavelength of 320 nm and the suspension concentration was 0.06 mg/mL in deionized water. PL measurements were conducted on the best performing catalyst of 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g- C ₃ N ₄ (FIG. 12) and its precursors, in order to probe into the charge recombination process at room temperature with an excitation wavelength of 320 nm. There was a significant decrease in the PL intensity of 0.1 wt.% Pd/20 wt.% Ti0 ₂ /g-C ₃ N ₄ compared to those of 20 wt.% Ti0 ₂ /C ₃ N ₄ and pristine g-C ₃ N ₄ . The weak intensity of PL signal was attributed to a lower recombination rates of the photo-generated charge carriers also consistent with other studies over metal supported Ti0 ₂ . Therefore, one can draw the conclusion that introducing Ti0 ₂ on the surface of g-C ₃ N ₄ could effectively decrease the electron-hole recombination rates. The addition of Pd, even in such a small amount has resulted in further decreasing the PL signal which is again a further evidence of electron transfer from the CB of the hybrid semiconductor to Pd metal.

Example 3

(Production of H ₂ from H ₂ 0)

[0075] Photocatalytic General Procedure. Photocatalytic reactions were evaluated in a 137-mL-volume Pyrex glass reactor using catalyst from Samples 1-4 (25 mg) or the comparative sample, and a triethanol amine (TEA) aqueous solution (30 mL of 10% TEA). The slurry was purged with N ₂ gas to remove any 0 ₂ and subjected to constant stirring before the reaction. A Xenon lamp (Asahi spectra MAX-303) with a total flux of 42.5 mWcrrT ² (up to 650 nm) at a distance of 2 cm was used as the excitation source. Product analyses were performed by gas chromatography (GC) equipped with thermal conductivity detector (TCD) connected to Porapak Q packed column (2 m) at 45 °C and N2 was used as a carrier gas.

[0076] The photocatalytic H2 production rates over the Pd/Ti02/g-C3N4 composites containing different mass ratios (Samples 1-5) are shown in FIG. 13. Control experiments showed no appreciable H2 evolution in the absence of either photocatalyst or light irradiation. Moreover, the H2 evaluation rate of as prepared g-C ₃ N4 under visible light (400 nm cut off) was very low which was doubled when 0.3% Pd was deposited on top. The visible-light- induced photocatalytic H2 evolution rate was enhanced when the T1O2 content was increased in the Pd/Ti02/g-C ₃ N4 composite photocatalyst. Furthermore, the H2 production rate showed increasing trends, and reached a maximum rate when the T1O2 content was 20 wt.%; whereas further increasing the content of T1O2 in the composite leads to a decrease in the H2 production rate. Without wishing to be bound by theory, it is believed that the H2 production rate enhancement along with the increase of the T1O2 was due to the synergistic effect between the T1O2 and g-C ₃ N4. The synergic effect is believed to be caused by the electron transfer from the valence band of g-C ₃ N4 to the CB of T1O2 owning to the band positions as shown in FIG. 5, resulting in slower charge recombination rates. Moreover, Pd, as a H2 co-catalyst, acts as an electron drain hindering the charge recombination, which further improved H2 generation. On the other hand, the decrease in photoactivity upon increasing the T1O2 mass ratio was attributed to the fact that there were not enough active sides of g-C ₃ N4 to be exposed for photo-oxidation because of the excess surface coverage by T1O2. Furthermore, it is believed that the excess T1O2 caused aggregation of T1O2 nanoparticles, which created the bulk resistance for electron transfer. H2 generation rate of the Pd/Ti02/g-C ₃ N4 was extremely low (almost zero) under UV and UV+visible light, which meant that the electron-hole recombination was enhanced on the interface when both semiconductors were activated at the same time.