MULTI-LAYERED WATER-SPLITTING PHOTOCATALYST HAVING A PLASMONIC METAL LAYER WITH OPTIMIZED PLASMONIC EFFECTS

Abstract

Photocatalysts and methods of using the same for producing hydrogen and oxygen from water are disclosed. The photocatalysts include a photoactive layer having a thickness of 10 nanometers (nm) to 1000 nm and a plasmonic metal layer having a thickness of 2 nm to 20 nm and having surface plasmon resonance properties in response to ultra-violet and/or visible light, wherein the plasmonic metal layer is positioned proximal to the photoactive layer.

Claims

1. A photocatalyst comprising: a substrate; a photoactive layer having a thickness of 10 nanometers (nm) to 1000 nm; and a plasmonic metal layer having a thickness of 2 nm to 20 nm and having surface plasmon resonance properties in response to ultra-violet and/or visible light, wherein the plasmonic metal layer is coated on the substrate and the photoactive layer is coated on the plasmonic metal layer.

2. The photocatalyst of claim 1, wherein the plasmonic metal layer has a thickness of 4 nm to 12 nm, preferably 6 nm to 10 nm, more preferably from 7 nm to 9 nm, or most preferably about 8 nm.

3. The photocatalyst of claim 1, wherein the plasmonic metal layer is a discontinuous layer having a plurality of noncontiguous regions each having a thickness of less than 10 nm or a continuous layer having a thickness of at least 10 nm.

4. The photocatalyst of claim 3, wherein the combined surface area of the plurality of noncontiguous regions is up to 30% of the surface area of the photoactive layer.

5. The photocatalyst of claim 1, wherein the plasmonic metal layer is gold, silver, copper, or an alloy thereof.

6. The photocatalyst of claim 1, wherein the plasmonic metal layer is gold.

7. The photocatalyst of claim 1, wherein the thickness of the photoactive layer is 100 nm to 500 nm, preferably, 200 nm to 400 nm, or more.

8. The photocatalyst of claim 1, wherein the photoactive layer is a titanium dioxide layer, a zinc oxide layer, or a cadmium sulfide layer, or a layer having any combination of titanium dioxide, zinc oxide, and/or cadmium sulfide.

9. The photocatalyst of claim 8, wherein the photoactive layer is a titanium dioxide layer having anatase, rutile, brookite, or a mixture thereof, preferably anatase or a mixed-phase comprising anatase and rutile.

10. The photocatalyst of claim 9, wherein the ratio of anatase to rutile is 1.5:1 to 10:1.

11. The photocatalyst of claim 1, wherein the photoactive layer is impregnated with a metal that is less than 5, 4, 3, 2, 1, 0.5 or 0.1 wt. % of the total weight of the photoactive layer selected from palladium, silver, platinum, gold, rhodium, ruthenium, rhenium, iridium, nickel, or copper, or any combinations or oxides or alloys thereof.

12. The photocatalyst of claim 1, wherein the plasmonic metal layer is in direct contact with the photoactive layer.

13. The photocatalyst of claim 1, wherein at least one interlayer is positioned between the plasmonic metal layer and the photoactive layer.

14. The photocatalyst of claim 13, wherein the interlayer is a metal oxide layer, preferably a SiO.sub.2 layer.

15. The photocatalyst of claim 1, wherein the photocatalyst is capable of catalyzing the photocatalytic electrolysis of water.

16. An aqueous composition comprising the photocatalyst of claim 1.

17. A water-splitting system for generating hydrogen from water, the system comprising a reaction vessel comprising water and any one of the photocatalysts of claim 1.

18. A method for enhancing the electric field produced at an interface between a photoactive layer having a thickness of 10 nanometers (nm) to 1000 nm and a plasmonic metal layer having a thickness of 2 nm to 20 nm and having surface plasmon resonance properties in response to ultra-violet and/or visible light, the method comprising coating the plasmonic layer on a substrate, and subsequently coating the plasmonic metal layer on the photoactive layer.

19. The method of claim 18, wherein the plasmonic metal layer has a thickness of 4 nm to 12 nm, preferably 6 nm to 10 nm, more preferably from 7 nm to 9 nm, or most preferably about 8 nm.

20. The method of claim 18, wherein the plasmonic metal layer is a discontinuous layer having a plurality of noncontiguous regions each having a thickness of less than 10 nm or a continuous layer having a thickness of at least 10 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIGS. 1A-D: (A) Schematic of a cross-sectional view of a photocatalyst of the present invention where a photoactive layer is in direct contact with a plasmonic metal layer; (B) Schematic of a cross-sectional view of a photocatalyst of the present invention where a photoactive layer is in indirect contact with a plasmonic layer (e.g., a third intermediate/interlayer is positioned between the photoactive layer and the plasmonic metal layer); (C) a top view of a continuous plasmonic metal layer supported by a substrate; and (D) a top view of a discontinuous plasmonic metal layer having a plurality of noncontiguous regions supported by a substrate.

[0034] FIGS. 2A-B: Schematic of a water splitting system of the present invention where the photoactive catalyst is (A) coated on the surface of the reaction container's walls and (B) coated on a substrate that is placed inside the reaction container.

[0035] FIG. 3: Schematic of an organic photovoltaic cell incorporating a photocatalyst of the present invention.

[0036] FIG. 4: Scanning electron microscope (SEM) image of Au plasmonic metal layers thermally evaporated on glass substrates. Layers having a thickness below 10 nm are discontinuous layers. Layers having a thickness above 10 nm are continuous layers.

[0037] FIGS. 5A-B: (A) Optical absorption as function of wavelength of Au plasmonic layers with different thicknesses; and (B) % R as function of wavelength of Au plasmonic layers with different thicknesses.

[0038] FIGS. 6A-B: (A) Hydrogen production of TiO.sub.2 photocatalyst as function of Au plasmonic metal layer thickness (reaction conditions include Quartz reactor, Xenon lamp with UV flux (300-380 nm) about 5 mW/cm.sup.2, 30 mL H.sub.2O with 5 vol. % glycerol); (B) Hydrogen production of TiO.sub.2 photocatalyst as function of Au plasmonic metal layer thickness under UV and visible light radiation.

[0039] FIGS. 7A-B: (A) Optical absorption of non-plasmonic platinum layer; and (B) Hydrogen production of TiO.sub.2 photocatalysts as function of plasmonic Au layer and non-plasmonic Pt layer thicknesses (reaction conditions include Quartz reactor, Xenon lamp with UV flux (300-380 nm)5 mW/cm.sup.2, 30 mL H.sub.2O with 5 vol. % glycerol).

[0040] FIGS. 8A-B: Optical simulations (Finite Difference Time Domain (FDTD)) of TiO.sub.2 on Au plasmonic films by using commercial software, COMSOL Multiphyisics version 4.4. COMSOL use finite element method (FEM) to solve Maxwell's equations for specific electromagnetic wave condition and gives electrical field intensity (|E|.sup.2) as an output. The incident electromagnetic field was assumed to be 1 V/m, with wavelength of incident electromagnetic field set to be at 500 nm and polarized in y-direction.

[0041] FIG. 9: (A) EF enhancement at interface of Au plasmonic metal layer and TiO.sub.2 photoactive layer as function of Au layer thickness; and (B) Hydrogen production from TiO.sub.2 photocatalysts on Au plasmonic metal layers (Circles equals rates under experimental conditions. Square equals rates normalized to the EF enhancement obtained from the optical simulations).

DETAILED DESCRIPTION OF THE INVENTION

[0042] 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 discovery that plasmonic metal layers having a certain thickness can dramatically enhance hydrogen production rates from a water-splitting reaction. Without wishing to be bound by theory, it is believed that when a plasmonic metal layer having a thickness of 2 nm to 20 nm, preferably 4 nm to 12 nm, more preferably 6 nm to 10 nm, most preferably from 7 nm to 9 nm or about 8 nm, is positioned proximal to a photoactive layer, the electric field produced by the plasmonic metal layer, when subjected to UV and/or visible light, increases the charge carrier life time of the electrons and holes produced in the photoactive layer. This leads to an increase in hydrogen production through reduction of hydrogen ions rather than an electron-hole recombination event. As illustrated in non-limiting embodiments in the Examples, a critical range of thickness for the plasmonic metal layer has been identified to achieve this increase in hydrogen production. In a most preferred embodiment, the photoactive layer is a TiO.sub.2 layer and the plasmonic metal layer is a gold layer, with the highest hydrogen production being obtained with a gold plasmonic layer having a thickness of 7 nm to 9 nm, with the peak production being about a thickness of 8 nm.

[0043] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Photoactive Catalysts

[0044] Referring to FIGS. 1(A)-(D), multi-layered photoactive catalysts 10 of the present invention are illustrated through non-limiting schematics. By way of example, the photoactive catalysts 10 can include a photoactive layer 12 that is coated directly onto a plasmonic metal layer 13 (FIG. 1(A). Alternatively, a third layer (also referred to as intermediate layer or interlayer) 15 can be positioned between the photoactive layer 12 and plasmonic metal layer 13 (FIG. 1(B)). Although not shown, fourth, fifth, sixth, or more layers can also be positioned between the photoactive layer 12 and plasmonic metal layer 13. When an intermediate layer 15 is present, the thickness of this layer 15 and the materials of the layer can be selected to ensure that the electric field produced by the plasmonic layer 13 still exerts its effects on the photoactive layer 12. In preferred instances, the distance between the photoactive layer 12 and plasmonic metal layer is 0 nm (i.e., direct contact) or within 20 nm or less. The photoactive catalysts can be supported by a support 14. In a preferred instance, the plasmonic metal layer 13 is positioned closer to the support 14 then the photoactive layer 12. When the support 14 is transparent, then light (h) can contact the photoactive layer 12 and the plasmonic layer 13 in either direction as illustrated in FIGS. 1(A) and (B). When the support material 14 is opaque or reflective, then the light (h) typically contacts the photoactive layer 12 first and then the plasmonic metal layer 13. The light (h) can be ultraviolet light (280 nm to 400 nm) or visible light (400 nm to 700 nm). In preferred instances, a combination of ultraviolet light and visible light can be used to maximize electron/hole formations and H.sub.2 and O.sub.2 production from the water splitting reaction.

[0045] One of the discoveries of the present invention is that the LSPR or plasmonic resonance effect of the plasmonic metal layer 13 can be optimized by modifying or tuning the thickness of this layer 13. As illustrated in non-limiting aspects in the Examples, a thickness range of 2 nm to 20 nm leads to an optimization in the LSPR. It was further discovered that a thickness of 10 nm and greater leads to a continuous layer 13. A non-limiting illustration of the continuous layer 13 is provided in FIG. 1(C), which is a top view of the layer 13. In FIG. 1(C), although portions of the substrate 14 can be seen through gaps or regions in which the plasmon metal layer 13 is not present, it is continuously connected around these gaps or regions. Although not illustrated, the continuous layer 13 can be made where no such gaps or regions exist. When the thickness of the plasmonic metal layer is less than 10 nm, then the layer can exhibit a discontinuous layer morphology, which is illustrated in FIG. 1(D), a top view of layer 13. In FIG. 1(D), the discontinuous plasmonic metal layer 13 is represented by a plurality of noncontiguous regions 13 each having a thickness of less than 10 nm.

[0046] Still further, the photactive layer 13 can be impregnated with or coated with additional materials that further enhance the efficiency of the water-splitting reaction and ultimate production of H.sub.2 and/or O.sub.2. By way of example, the photoactive layer 13 can be impregnated with metals or oxides or alloys thereof or can be coated with metal nanostructures or oxides or alloys thereof. Non-limiting examples of such metals include palladium, silver, platinum, gold, rhodium, ruthenium, rhenium, iridium, nickel, or copper, or any combinations or oxides or alloys thereof.

[0047] 1. Materials Used

[0048] The photoactive layer 12 can be made from any type of photoactive material that is capable of producing excited elections in response to ultraviolet and/or visible light. In preferred embodiments, the photoactive material can include titanium dioxide, zinc oxide, or cadmium sulfide, or any combinations thereof. In particular instances, 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 resulting in different density of states 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).

[0049] The plasmonic metal layer 13 can be made from any type of material that includes LSPR or plasmonic resonance effects when exposed to ultraviolet and/or visible light. In preferred instances, the material can be metal selected from gold, silver, copper, or an alloy thereof.

[0050] The intermediate layer 15 can be made from any type of material. Preferably, the material would be of a kind that enhances the efficiency of the water-splitting reaction and ultimate production of H.sub.2 and/or O.sub.2. In one non-limiting aspect, the intermediate layer 15 can be silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2O.sub.3), or alkaline earth metal oxides such magnesium oxide (MgO), calcium oxide (CaO), or the like. The thickness of this interlayer can be up to 20 nm, preferably 6 nm or most preferably 2 nm.

[0051] The substrate 14 can be any type of material that is capable of supporting the photoactive layer 12, the plasmonic layer 13, and/or any intermediate layers 15. Non-limiting examples of materials that can be used for the substrate include glass, quartz, polymers such as polyethylene, PET, PEN, polyimide, polyamide, polyamidoimide, polycarbonate (e.g., Lexan, which is a polycarbonate resin offered by SABIC Innovative Plastics), liquid crystal polymers, cyclic olefin polymers, silicon, metal, etc. The substrate 14 can be any surface of an article of manufacture (e.g., the walls of a container, the walls of a reactor such as a water-splitting reactor, etc.).

[0052] 2. Process of Making the Photocatalysts

[0053] Non-limiting examples for making photocatalysts are disclosed in the Examples of the present specification. Generally, the following steps can be used to manufacture catalysts of the present invention.

[0054] The plasmonic metal layer 13 can be coated onto the substrate 14 with processes known to those having ordinary skill in the art. Non-limiting examples include thermal evaporation, sputtering, atomic layer deposition, or e-beam evaporation. In some preferred aspects, the substrate surface can be first cleaned, for example, by ultra-sonication in acetone, ethanol, and/or deionized (DI) water. Subsequently, the plasmonic metal layer 13 can be deposited by thermal evaporation in a vacuum chamber. The deposition can be done at room temperature with a constant deposition rate of 0.1 A/s to 0.5 A/s, preferably about 0.2 A/s. Subsequently, the photoactive layer 12, or the intermediate layer 15 if one is desired, can be coated onto the surface of the plasmonic metal layer 13 with processes known to those having ordinary skill in the art. If an intermediate layer 15 is first deposited onto the plasmonic metal layer 13, then the same type of coating process used for the intermediate layer 15 can be used to apply the photoactive layer 12 to the intermediate layer 15. Non-limiting processes include drop casting, dip coating, spin coating, blade coating, or spray coating. The thickness of the photoactive layer 12 and/or the intermediate layer 15 can be modified or tuned as desired by modifying the amount of materials used and/or the timing of the coating process. In preferred instances, the thickness of the photoactive layer 12 can be 10 nm to 1000 nm, more preferably 100 nm to 500 nm, still more preferably, 200 nm to 400 nm, or most preferably 250 nm to 350 nm. If used, the thickness of the intermediate layer 15 can be up to 10 nm.

B. Water-Splitting System

[0055] Referring to FIGS. 2A and B, a non-limiting representation of a water-splitting system 20 of the present invention is provided. The systems each include a photocatalyst 10, a light source 21, and container or reaction vessel 22 that can be used to hold aqueous solutions or water 23. Although not shown, the system 20 can also include at least one inlet for the aqueous solution/water 23 and at least one or more outlets for produced hydrogen and oxygen formed during the water-splitting reaction. In one embodiment, the photocatalyst 10 can be coated onto the walls of the container 22 (See FIG. 2A), preferably with the plasmonic metal layer 13 contacting the container 22 wall and the photoactive layer 12 contacting the water 23. In this instance, the substrate 14 is the walls of the container 22. Alternatively, and in another embodiment, the photocatalyst can be supported by a substrate 14 and then placed into the water (See FIG. 2B). In certain instances, a plurality of supported photocatalysts 10 can be used to maximize hydrogen and oxygen production. To maximize efficiency, the substrate can be transparent, thereby allowing for light to contact both the photoactive layer 12 and the plasmonic metal layer 13 in different directions.

[0056] In either instance, the container 22 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 21 can includes either one of or both of visible and (400-600 nm) and ultraviolet light (280-400). The light can excite the photoactive layer 12 to excite an electron in the valence band 24 to the conductive band 25. The light can also excite the metal plasmon resonance layer 13 such that an electric field is generated. The excited electrons (e.sup.) leave a corresponding hole (h.sup.+) when the electrons move to the conductive band. The excited electrons (e.sup.) are used to reduce hydrogen ions to form hydrogen gas, and the holes (h.sup.+) are used to oxidize oxygen ions to oxygen gas. The hydrogen gas and the oxygen gas can then be collected and used in down-stream processes. Due to the electric field produced by the metal plasmonic layer 13, excited electrons (e.sup.) are more likely to be used to split water before recombining with a hole (h.sup.+) 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, 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 example, 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 MoPt 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).

C. Photovoltaic Application

[0057] In addition to water splitting applications, the photocatalysts of the present invention can also be used in other applications that utilize excited electrons. By way of example, the photocatalysts can be used in a photovoltaic cell. Referring to FIG. 3, this figure is a cross-sectional view of a non-limiting photovoltaic cell that incorporates the photocatalyst of the present invention. The photovoltaic cell 30 can include a transparent substrate 31, a front electrode 32, an active layer 33, and a back electrode 34 which can also act as substrate 14. The active layer 33 includes the photoactive layer 12 and plasmonic metal layer 13 of the present invention. Preferably, the photoactive layer 12 can be positioned next to the front electrode 32 and the plasmonic metal layer 13 can be positioned next to the back electrode 34. Alternatively, the photoactive layer 12 can be positioned next to the back electrode 34 and the plasmonic metal layer 13 can be positioned next to the front electrode 32. Additional materials, layers, and coatings (not shown) known to those of ordinary skill in the art can be used with photovoltaic cell 30. Generally speaking, the photovoltaic cell 30 can convert light into usable energy by: (a) photon absorption to produce excitons; (b) exciton diffusion; (c) charge transfer; and (d) charge separation and transportation to the electrodes.

[0058] The front electrode 32 can be used as a cathode or anode depending on the set-up of the circuit. It is stacked on the substrate 31. The front electrode 32 can be made of a transparent or translucent conductive material. Alternatively, the front electrode 32 can be made of opaque or reflective material. Typically, the front electrode 32 is obtained by forming a film using such a material (e.g., vacuum deposition, sputtering, ion-plating, plating, coating, etc.). Non-limiting examples of transparent or translucent conductive material include metal oxide films, metal films, and conductive polymers. Non-limiting examples of metal oxides that can be used to form a film include indium oxide, zinc oxide, tin oxide, and their complexes such as indium stannate (ITO), fluorine-doped tin oxide (FTO), and indium zinc oxide films. Non-limiting examples of metals that can be used to form a film include gold, platinum, silver, and copper. Non-limiting examples of conductive polymers include polyaniline and polythiophene. Also, the sheet resistance of the front electrode 32 is typically 10 /sq or less. Further, the front electrode 32 may be a single layer or laminated layers formed of materials each having a different work function.

[0059] The back electrode 34 can be used as a cathode or anode depending on the set-up of the circuit. This electrode 34 can be made of a transparent or translucent conductive material. Alternatively, it 34 can be made of opaque or reflective material. This electrode 34 can be stacked on the active layer 33. The material used for the back electrode 34 can be conductive. Non-limiting examples of such materials include metals, metal oxides, and conductive polymers (e.g., polyaniline, polythiophene, etc.) such as those discussed above in the context of the front electrode 32. When the front electrode 32 is formed using a material having high work function, then the back electrode 34 can be made of material having a low work function. Non-limiting examples of materials having a low work function include Li, In, Al, Ca, Mg, Sm, Tb, Yb, Zr, Na, K, Rb, Cs, Ba, and the alloys thereof. The back electrode 34 can be a single layer or laminated layers formed of materials each having a different work function. Further, it may be an alloy of one or more of the materials having a low work function and at least one selected from the group consisting of gold, silver, platinum, copper, manganese, titanium, cobalt, nickel, tungsten, and tin. Examples of the alloy include a lithium-aluminum alloy, a lithium-magnesium alloy, a lithium-indium alloy, a magnesium-silver alloy, a magnesium-indium alloy, a magnesium-aluminum alloy, an indium-silver alloy, and a calcium-aluminum alloy.

[0060] In some embodiments, the front 32 and back 34 electrodes can be further coated with hole transport or electron transport layers (not shown in FIG. 3) to increase the efficiency and prevent short circuits of the photovoltaic cell 30. The hole transport layer and the electron transport layer can be interposed between the electrode and the active layer 33. Non-limiting examples of the materials that can be used for the hole transport layer include polythiophene-based polymers such as PEDOT/PSS (poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)) and organic conductive polymers such as polyaniline and polypyrrole. As for the electron transport layer, it can function by blocking holes and transporting electrons more efficiently.

EXAMPLES

[0061] 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 and Characterization of Photocatalysts of the Present Invention

[0062] The photocatalytic materials were fabricated on glass substrates. First glass slides were cleaned by ultra-sonication in acetone, ethanol and DI water. Thin Au films were deposited on these glass slides by thermal evaporation in a vacuum chamber. The deposition was done at room temperature with a constant deposition rate of 0.2 A/s. To prepare the photocatalyst, anatase TiO.sub.2 (supplier: Hombikat) with an average particle size of about 7 nm and BET surface area of about 320 m.sup.2/g was impregnated with PdCl.sub.2 salt solution. Excess water was evaporated to dryness under constant stirring with slow heating at 80 C. The dried photocatalysts was calcined at 350 C. for 5 hours. The resulting photo-catalysts with 0.4 wt % Pd loading on anatase TiO.sub.2 had an average particle size of about 10-12 nm and BET surface area of approximately 120 m.sup.2/g. Similarly, comparative devices using non-plasmonic metal films (platinum (Pt) films) were prepared, where the Pt was deposited using Radio Frequency (RF) sputtering.

[0063] The TiO.sub.2 photocatalysts were coated on the Au films by the spin coating method. A TiO.sub.2 dispersion (1.5 wt. %) was prepared in ethanol and spun coated on the Au thin film at 500 rpm for 20 sec. The coating process was repeated 5 times and the thin films were heated at 90 C. for 20 min to remove ethanol.

[0064] UV-VIS absorbance spectra of the catalysts were collected over the wavelength range of 250-2000 nm on a Thermo Fisher Scientific spectrophotometer equipped with praying mantis diffuse reflectance accessory. Absorbance (A) and reflectance (R) of the samples were measured.

[0065] FIG. 4 shows a high resolution scanning electron microscopy (HRSEM) images of the Au plasmonic metal films with thicknesses of 2, 4, 8, 12, 16 and 20 nm deposited on glass slides. A Volmer-Weber growth is seen (islands growth) for the 2 and 4 nm Au films. See Kaiser, N. Review of the fundamentals of thin-film growth, Appl. Opt. 2002, 41, 3053-3060; Orr et al., A Model for Strain-Induced Roughening and Coherent Island Growth, EPL (Europhysics Letters) 1992, 19, 33; Seel et al., Tensile stress evolution during deposition of Volmer-Weber thin films, Journal of Applied Physics 2000, 88, 7079-7088; Zhang et al., Atomistic Processes in the Early Stages of Thin-Film Growth, Science 1997, 276, 377-383. With increasing thickness these islands composed of Au, particles start to coalesce. The average size and irregularity of the islands increase with increasing film thickness. The formation of worm-like particles is the direct evidence of aggregation of Au NPs due to touching and merging of adjacent particles. The formation of inter-links between the coalescences of Au NPs was greatly enhanced as the film continued growing, and a continuous film was eventually formed as the film thickness reached about 12 nm.

[0066] This unique island-like structure discontinuous film of noble metals leads to interesting optical properties. FIG. 5(A) shows the absorption spectra of Au films as a function of thickness. As seen in FIG. 5(A) there are three regions of absorption. Absorption due to inter-band transitions are observed at 260 and 380 nm. The localized surface plasmon resonance LSPR for 2 nm Au films is located around 580 nm and is red shifted with increasing thickness up to 8 nm Au films. From FIG. 4 Au discontinuous island regions are between 10 and 20 nm for the 2 nm thick layer and up to 30 nm for the 4 nm thick layer. This is observed in the absorption spectra in FIG. 5 where a shift from 590 nm (2 nm-thick layer) to 640 nm (8 nm-thick layer) is seen. For films thicker than 8 nm, the formation of interlinks (conductive percolation paths) between the Au islands due to their aggregation delocalize the free electrons making a Drude absorption more significant and consequently suppresses LSPR. Reflectance (% R) measurements of the Au films show the same trend as seen in FIG. 5(B).

Example 2

Photocatalytic Activity of the Photocatalysts of the Present Invention

[0067] Photocatalytic reactions were evaluated in a 190 mL volume quartz reactor. 30 mL of 5 vol % glycerol aqueous solution was used to evaluate the water splitting activity. The coated slides were inserted vertically into the reactor and the reactor was purged with N.sub.2 gas to remove any O.sub.2. The photoreactions were carried out using a Xenon lamp (Asahi spectra MAX-303) at a distance of 9 cm from the reactor with a total UV flux of 5-6 mW/cm.sup.2 in the 280-380 nm range. Product analysis was performed by gas chromatograph (GC) equipped with thermal conductivity detector (TCD) connected to Porapak Q packed column (2 m) at 45 C. and N.sub.2 was used as a carrier gas.

[0068] The H.sub.2 production rates of the photocatalysts of the present invention under UV and visible light excitation (280-650 nm) is presented in in FIG. 6. The photocatalytic activity was stable and reproducible. Pure anatase TiO.sub.2 with 0.4 wt. % Pd loading, showed H.sub.2 production rates of abut 200 molg.sup.1min.sup.1. When the photocatalysts were coated on Au plasmonic films, it showed a dramatic increase in the hydrogen production rates. With 2 nm thickness, the rates increased 2.5 times to about 550 molg.sup.1min.sup.1 and reached a maximum with a thickness of 8 nm. Further increasing the thickness of underlying Au plasmonic film led to a decrease in activity as seen for films from 12 to 20 nm thickness. The trend in H.sub.2 production was similar to the trend seen in LSPR from these Au films as discussed in FIG. 5 where for film thickness greater than 8 nm, the LSPR was suppressed due to the Drude absorption. In other words, normalization of the rates to the LSPR peak area or intensity results in no or negligible changes FIG. 6(A).

[0069] The photoreactions were also carried out under UV light only (<400 nm). LSPR is a resonance condition, which is, established when the frequency of incident photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei. By cutting of the visible light, LSPR would be considerably attenuated. As seen in FIG. 6(B), the activity under UV light only was much lower. It is to be noted that the trend of rates under UV is the same with maximum activity for 8 nm Au layers at about 380 molg.sup.1min.sup.1. This is a further indication that it is the LSPR property of the Au thin films, which helps improve the activity of the photocatalysts of the present invention, and the LSPR of Au will be active at the resonant frequency.

[0070] To further confirm the plasmon resonance response has increased the reaction rate rather than increasing the interface between Au and the photoactive catalyst, the plasmonic Au films were replaced with non-plasmonic platinum (Pt) films. Pt films were deposited with different thicknesses from 5 to 20 nm. FIG. 7(A) shows the absorbance of Pt films deposited on quartz where the absence of LSPR is noticed and only the Drude absorption seen for films with thickness above 15 nm. The TiO.sub.2 photocatalyst was coated on top of Pt similar to the Au devices. The photocatalytic activity of these materials is shown in FIG. 7(B) and conducted under identical conditions to those of the Au series. The H.sub.2 production rates showed marginal gradual increase after adding Pt thin film up to 310 molg.sup.1min.sup.1 for 20 nm Pt films. The difference in H.sub.2 production rates from Au and Pt films is highlighted in FIG. 7(B).

Example 3

Electric Field Enhancement of the Photocatalysts of the Present Invention

[0071] To identify the mechanism of how the LSPR helps enhancing the photocatalytic activity, optical simulations of TiO.sub.2 on Au films as a function of thickness was conducted using commercial software, COMSOL Multiphysics version 4.4., in RF module. COMSOL uses finite element method (FEM) to solve Maxwell's equations for the specific electromagnetic wave condition and gives electrical field intensity (|E|.sup.2) as an output. The incident electromagnetic field was taken as 1 V/m; with wavelength of incident, electromagnetic field set to be at 500 nm and polarized in the y-direction. The incident electromagnetic field was set normal to the Au films or glass substrate. Dielectric permittivity of Au was taken from Johnson-Christy report and the Au island size for 2, 4 and 8 nm Au discontinuous films was taken from the collected SEM images while continuous films were assumed for 12, 16 and 20 nm thickness. The optical simulation domain contains nanoparticles in a homogeneous medium, covered with perfectly matched layers (PMLs) at the computational boundaries to avoid any reflection in the domain. The scattering cross-section was also simulated. The results are presented in FIG. 8. The electric filed enhancement in FIG. 8 are for two representative Au films (2 and 20 nm thick) and for two different planes (YZ-plane and XY plane). The XY-plane shows the electric field enhancement in the boundary between TiO.sub.2 nanoparticles (NPs) and Au films while the YZ-plane shows the electric field enhancement along the system TiO.sub.2Au films-glass substrate. The red color in the figure represents the highest enhancement and the blue the lowest. It can be seen that the enhancement of the electric field was largely isotropic (no much changes in the enhancement in the YX plane when compared to that of the YZ plane). It can also be seen that the effect increased from the 2 nm thick layer of Au to that of 8 nm then decreased again for the 20 nm thick layer.

[0072] The data of the electric field enhancements for different Au thickness is in FIG. 9(A). With 2 nm Au film (particle size 13 nm) the enhancement is about 5 times at the surface of TiO.sub.2 particle. Increasing the Au film thickness dramatically improves the EF enhancement with up to 19 times higher EF for 8 nm films and then starts dropping for thicker films. This was observed in both XY and YZ plane as see in FIG. 9(A). XY-plane shows the electric field enhancement in the boundary between TiO.sub.2 nanoparticles (NPs) and Au films. YZ-plane shows the electric field enhancement along the system (TiO.sub.2Au films-glass substrate). Notably, a correlation has been observed between the electric field enhancement and the photocatalytic activity. Both of them show similar pattern, with 8 nm-thickness show the highest enhancement.