RAO RICHUAN (SG)
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| CLAIMS claimed is: A metal oxide nanomaterial having photocatalytic properties and having H2- plasma-induced surface defects that do not reduce the oxidation state of the metal oxide, wherein the metal oxide is Ti02, ZnO, WO3, Sn03 or ln203. The nanomaterial of Claim 1 wherein the metal oxide is Ti02 and wherein the nanomaterial has H2-plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti02. The nanomaterial of Claim 2, wherein the H2-plasma-induced surface defects include structural disorder. The nanomaterial of Claim 3, wherein the black Ti02 nanomaterial absorbs visible and IR light. The chemical composition of Claim 4, wherein the Ti02 nanomaterial is a nanoparticle, a nanotube or a nanowire. A method of making a metal oxide nanomaterial having photocatalytic properties, the method comprising: contacting a metal oxide nanomaterial starting material with hydrogen plasma at a temperature, pressure and hydrogen plasma flow rate for a period of time sufficient to produce H2-plasma induced surface defects on the metal oxide but not reduce the oxidation state of the metal oxide, wherein the metal oxide nanomaterial has photocatalytic properties and is selected from Ti02, ZnO, WO3, Sn03 or ln203. The method of Claim 6, wherein the metal oxide nanomaterial starting material is white Ti02 nanomaterial. 8. The method of Claim 6, wherein the metal oxide nanomaterial having photocatalytic properties is black Ti02 nanomaterial. 9. The method of Claim 6, wherein the pressure is a pressure lower than atmospheric pressure. 10. The method of Claim 9, wherein the Ti02 nanomaterial having photocatalytic properties is black Ti02 nanomaterial having H2-plasma- induced surface defects that impart a black color to the nanoparticles but do not reduce the oxidation state of Ti02. 11. The method of Claim 10, wherein the H2-plasma-induced surface defects include structural disorder. 12. The method of Claim 1 1, wherein the Ti02 nanomaterial is a nanoparticle, a nanotube, or a nanowire. 13. A method for decomposing an organic compound in water, the method comprising: (a) contacting an aqueous solution comprising an organic compound with a metal oxide nanomaterial having photocatalytic properties, wherein the metal oxide is Ti02, ZnO, W03, Sn03 or ln203, to form a mixture; and (b) exposing the mixture of step (b) to a visible light source, the visible light source optionally including an ultraviolet filter, for an amount of time sufficient to decompose the organic compound. 14. The method of Claim 13, wherein the metal oxide nanomaterial having photocatalytic properties is a Ti02 nanomaterial having H2-plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti02. 15. The method of Claim 14, wherein the H -plasma-induced surface defects include structural disorder. 16. The method of Claim 14, wherein the Ti02 nanomaterial having photocatalytic properties absorbs visible and IR light. 17. The method of Claim 16, wherein the Ti02 nanomaterial having photocatalytic properties is a photocatalyst for decomposition of an organic compound. 18. The method of Claim 17, wherein the photocatalyst is reusable in the process of decomposition of an organic compound. 19. A method for splitting water to form H2 gas, the method comprising: (a) contacting an aqueous solution comprising methanol and a metal oxide nanomaterial having photocatalytic properties, wherein the metal oxide nanomaterial is Ti02, ZnO, W03, Sn03 or ln203; with a platinum species to form a mixture; and (b) irraditating the mixture of step (a) with a visible light source to form H2 gas. 20. The method of Claim 19, wherein the metal oxide nanomaterial having photocatalytic properties is a Ti02 nanomaterial having H2-plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of TiO?. 21. The method of Claim 20, wherein the H2-plasma-induced surface defects include structural disorder. 22. The method of Claim 21, wherein the Ti02 nanomaterial having photocatalytic properties absorbs visible and IR light. |
DECONTAMINATION IN WATER
RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No.
61/568,246, filed on December 8, 2011. The entire teachings of the above application are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The pollution of groundwater by hazardous organic compounds, such as industrial dyes, gasoline and other petroleum-derived hydrocarbons, is becoming a global problem [1,2]. In the past, conventional biological and physical treatment methods, such as adsorption, ultrafiltration and coagulation, have been widely utilized techniques to remove organic pollutants from various waters and
wastewaters. However, the decontamination of many emerging anthropogenic organic pollutants, particularly those with high toxicity at very low concentrations, requires novel techniques to chemically transform the pollutants into non-hazardous compounds [2].
Recently, efforts have been put forth to examine the decomposition of organic pollutants in water by utilizing photocatalysis fueled by solar energy. In particular, semiconductors have been studied as catalysts. The effectiveness of solar light-driven photocatalytic processes is dictated to a great extent by the
semiconductor's capability of absorbing visible and infrared light, as well as its ability to suppress rapid recombination of photo-generated electrons and holes.
Nanophase titanium dioxide (Ti0 2 ), which has a large surface area with which to facilitate fast rates of surface reactions, is a semiconductor that is widely used as a photocatalyst for solar-driven applications such as water-splitting to produce H 2 and decontamination of water that contains organic pollutants [2-7]. As a photocatalyst, however, Ti0 2 absorbs only ultraviolet (UV) light, due to its wide band gap, which limits the catalytic utility of Ti0 2 in visible and IR light. A number of efforts have been put forth to effectively narrow the band gap, which should give the material photocatalytic properties under visible light. Metallic and non-metallic dopants have been used to add donor or acceptor states in the bandgap, however the visible and IR light absorption of such materials has not seen sufficient
improvement. More significant improvements in the visible and IR light absorption of these nanoparticles have stemmed from engineering defects into the nanoparticle surface [7,8], however, these methods exhibit the disadvantage of changing the chemical stoichiometry of the nanomaterial.
There is a need to develop methods to improve the visible and IR-light absorbance of semiconductor nanomaterials such as Ti0 2 in order to efficiently employ such materials in solar-driven applications such as the decontamination of water.
SUMMARY OF THE INVENTION
The present invention is directed to a metal oxide nanomaterial having photocatalytic properties and having H 2 -plasma-induced surface defects that do not reduce the oxidation state of the metal oxide, wherein the metal oxide is Ti0 2 , ZnO, W0 3 , Sn0 3 or ln 2 0 3 . The present invention is further directed to Ti0 2
nanomaterials that have H 2 -plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti0 2 . The H 2 -plasma-induced surface defects include structural disorder. The black Ti0 2 nanomaterial of the present invention absorbs visible and IR light, and can be a nanoparticle, a nanotube or a nanowire.
The present invention is further directed to a method of making a metal oxide nanomaterial having photocatalytic properties, the method comprises contacting a metal oxide nanomaterial starting material with hydrogen plasma at a temperature, pressure and hydrogen plasma flow rate for a period of time sufficient to produce H 2 -plasma induced surface defects on the metal oxide but not reduce the oxidation state of the metal oxide, wherein the metal oxide nanomaterial has photocatalytic properties and is selected from Ti0 2 , ZnO, W0 3 , Sn0 3 or ln 2 0 3 . The present invention is also directed to a method of making Ti0 2 nanomaterials having photocatalytic properties, wherein the metal oxide
nanomaterial starting material is white Ti0 2 nanomaterial and further wherein the metal oxide nanomaterial having photocatalytic properties is black Ti0 2
nanomaterial. The method of making a metal oxide nanomaterial having photocatalytic properties is carried out at a pressure lower than atmospheric pressure. The present invention further describes a method of making a Ti0 2 nanomaterial having photocatalytic properties having H 2 -plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti0 2 . The H 2 -plasma-induced surface defects include structural disorder, and the black Ti0 2 nanomaterial is a nanoparticle, a nanotube, or a nanowire.
The present invention further describes a method for decomposing an organic compound in water, the method comprises (a) contacting an aqueous solution comprising an organic compound with a metal oxide nanomaterial having photocatalytic properties, wherein the metal oxide is Ti0 2 , ZnO, W0 3 , Sn0 3 or ln 2 0 3 , to form a mixture; and (b) exposing the mixture of step (b) to a visible light source, the visible light source optionally including an ultraviolet filter, for an amount of time sufficient to decompose the organic compound.
The present invention further describes a method for decomposing an organic compound in water, wherein the metal oxide nanomaterial having photocatalytic properties is a Ti0 2 nanomaterial having H 2 -plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti0 2 . The H 2 -plasma-induced surface defects include structural disorder. The present invention further describes that the Ti0 2 nanomaterial having photocatalytic properties absorbs visible and IR light, and that the Ti0 2
nanomaterial is a photocatalyst for decomposition of an organic compound, and further that the photocatalyst is reusable in the process of decomposition of an organic compound.
The present invention is further directed to a method for splitting water to form H 2 gas, the method comprises (a) contacting an aqueous solution comprising methanol and a metal oxide nanomaterial having photocatalytic properties, wherein the metal oxide nanomaterial is Ti0 2 , ZnO, W0 3 , Sn0 3 or ln 2 0 3 ; with a platinum species to form a mixture; and (b) irraditating the mixture of step (a) with a visible light source to form H 2 gas.
The present invention describes that the metal oxide nanomaterial having photocatalytic properties utilized in a method of splitting water to form H 2 gas is a Ti0 2 nanomaterial having H 2 -plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of Ti0 2 . The H 2 -plasma-induced surface defects include structural disorder. Further, the Ti0 2 nanomaterial having photocatalytic properties absorbs visible and IR light.
The black Ti0 2 nanomaterials described herein exhibit enhanced visible and IR light absorption capabilities. The materials comprise surface defects that generate mid-band gap states, and are particularly advantageous over previously described materials in that the surface defects are structural rather than chemical; that is, the Ti0 2 does not contain oxygen defects or reducted titanium species, such as Ti 3+ . The methods of creating the particles described herein use H 2 -plasma treatment of anatase Ti0 2 nanoparticles under low pressure to form black Ti0 2 nanoparticles with enhanced visible light photocatalytic properties for efficient decomposition of organic wastes in water. Methods described herein are highly adaptable to an industrial scale, unlike high pressure hydrogenation reactions. The black Ti0 2 nanoparticles produced via H 2 -plasma treatment can be used as an efficient photocatalyst for visible light decontamination of organic wastes in water, and further may be used in visible light photocatalytic water splitting to produce H 2 .
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings.
FIG. 1 is a schematic showing the visible light photocatalytic
decontamination of organic pollutants in water using the interface engineered Ti0 2 nanoparticles in the present invention. These Ti0 2 nanoparticles have with mid- band gap states that participate in the excitation of electrons.
FIG. 2 is a schematic drawing showing the H 2 -plasma treatment of white anatase Ti0 2 nanoparticles (NPs) to form the black TiO? NPs.
FIG. 3 shows the UV-Vis absorption spectrum for white, black, grey and yellow Ti0 2 nanoparticles.
FIG. 4 shows X-ray diffraction (XRD) spectra of untreated (white) Ti0 2 nanoparticles and H 2 -plasma-treated (black) Ti0 2 nanoparticles of the present invention.
FIG. 5 shows Raman spectra of untreated (white) TiO? nanoparticles and H 2 - plasma-treated (black) Ti0 2 nanoparticles of the present invention.
FIGS. 6 A and 6B shows X-ray photoelectron spectroscopy (XPS) spectra of untreated (white) Ti0 2 nanoparticles and H 2 -plasma-treated (black) Ti0 2
nanoparticles of the present invention.
FIG. 7 contains digital pictures demonstrating the photocatalytic
degeneration of methylene blue (5 mg/L) after simulated sun light irradiation for (a) 0 min, (b) 10 min and (c) 20 min. Vial (1) contains black Ti0 2 nanoparticles of the present invention, vial (2) contains white Ti0 2 nanoparticles, and vial (3) contains no Ti0 2 nanoparticles. At (a) t = 0 min, each vial contains a visually-identical blue solution. At (b) t = 10 min, vial (1) exhibits a mostly gray solution, with a subtle blue tint. Vial (2) is a blue solution, though the blue is less vivid than at t = 0 min. Vial (3) remains the vivid blue color it was at t = 0 min. At (c) t = 20 min, vial (1) is completely gray in color, vial (2) is blue-gray, and vial (3) is a vivid blue color. In FIG. 7, the mass of the black Ti0 2 nanoparticles and white Ti0 2 nanoparticles used for vials (1) and (2), respectively, is 0.2 mg, the solution volume is 5 mL, and the concentration of methylene blue is 5 mg/L.
FIG. 8A shows the photocatalytic degradation of methylene blue with black Ti0 2 nanoparticles of the present invention under simulated sun light without any filter.
FIG. 8B shows absorption spectra of methylene blue solution (5 mg/L) using black Ti0 2 under simulated sun light with different exposure durations. FIG. 9A shows photocatalytic degradation of methylene blue for white (square) and black (upward triangle) Ti0 2 nanoparticles of the present invention. The red-colored lines are obtained under irradiation with an optical filter to filter out incident light with wavelengths shorter than 400 nm. The mass of the black Ti0 2 nanoparticles is 10 mg, the solution volume is 30 mL, and the concentration of methylene blue is 5 mg/L.
FIG. 9B shows cycling tests of solar-driven photocatalytic degradation of methylene blue for black Ti0 2 nanoparticles. The mass of the black Ti0 2 nanoparticles is 1 mg, the solution volume is 10 mL, and the concentration of methylene blue is 5 mg/L.
FIG. 10A shows the photocatalytic degradation of Rhodamine B by black Ti0 2 nanoparticles under simulated sunlight. FIG. 10B shows photocatalytic degradation of Rhodamine B by black (square) and white (circle) Ti0 2 nanoparticles under simulated sunlight. The mass of the black Ti0 2 nanoparticles is 0.2 mg, the solution volume is 10 mL, and the concentration of Rhodamine B is 5 mg/L.
FIG. 1 1A shows the photocatalytic degradation of phenol (15 mg/L) by black Ti0 2 under simulated sunlight. FIG. 1 IB shows photocatalytic degradation of phenol by black (square) and white (circle) Ti0 2 under simulated sunlight.
FIG. 12 shows an Ultraviolet Photoelectric Spectroscopy (UPS) spectrum of the valence band of the white Ti0 2 nanoparticles and the black Ti0 2 nanoparticles.
FIGs. 13A and 13B show Brunauer-Emmett-Teller (BET) data for the white Ti0 2 nanoparticles and the black Ti0 2 nanoparticles. Specifically, BET measures the specific surface area of a composition to examine physical adsorption of gases onto a solid surface.
FIG. 14 is a graph demonstrating generation of H 2 gas from water splitting catalyzed by black Ti0 2 nanoparticles under solar light irradiation.
DETAILED DESCRIPTION OF THE INVENTION
A description of example embodiments of the invention follows.
The present invention is directed to metal oxide nanomaterials that have photocatalytic properties, to methods of making the metal oxide nanomaterials and to methods of using the metal oxide nanomaterials to decompose organic wastes in water. In one embodiment, the metal oxides are Ti02 nanomaterials having H 2 - plasma-induced surface defects that impart a gray to black color to the nanoparticles but do not reduce the oxidation state of the Ti0 2 . Although metal oxides in addition to Ti0 2 can be made and used in accordance with the methods described herein, for purposes of discussion, the Ti0 2 embodiment will be discussed in detail below but such discussion is not intended to be limiting to the Ti0 2 .
The present invention describes methods of treating anatase Ti0 2
nanoparticles with H 2 -plasma under low pressure to form black Ti0 2 nanoparticles with enhanced visible light photocatalytic properties. The present invention further describes methods of using black Ti0 2 nanoparticles with enhanced visible light photocatalytic properties in the decomposition of organic wastes in water, under the frame of environment and water technology. The photocatalytic degradation of organic molecules for black (H 2 -plasma treated) Ti0 2 nanoparticles of the invention is demonstrated by using methylene blue under simulated sun light irradiation, and is compared to degradation results using white (untreated) Ti0 2 nanoparticles. It is found that the efficiency for photocatalytic degradation of organic molecules is about one order of magnitude higher for black Ti0 2 nanoparticles of the invention as compared with the white Ti0 2 nanoparticles. Even with the use of an optical filter to filter UV-light (wavelength <425nm), the black Ti0 2 still displays pronounced efficiency for photocatalytic degradation of methylene blue. The black Ti0 2 nanoparticles produced via H 2 -plasma treatment can be used as efficient
photocatalysts for visible light decontamination of organic wastes in water. The Ti0 2 nanoparticles described herein can be also used for visible light photocatalytic water splitting to produce H 2 .
Black T1O2 nanomaterials
The invention described herein is directed to a black Ti0 2 nanomaterial comprising a single oxidation state of Ti and further comprising H 2 -plasma-induced surface disorder. What is meant by "a single oxidation state" is that Ti0 2 is found only as the Ti(IV) oxidation state as in Ti0 2 , there are no reduced Ti species such as Ti in the nanomaterial. These novel materials exhibit high efficiency in the degradation of organic compounds in water.
In some embodiments of this invention, the black Ti0 2 nanomaterials are nanoparticles. In alternate embodiments of the invention, the black Ti0 2
nanomaterials are nanotubes, nanowires, nanoparticles, or mesoporous
nanomaterials. In some embodiments of the invention, the nanomaterials comprise black Ti0 2 nanoparticles. In alternate embodiments of the invention, the
nanomaterials comprise ZnO nanoparticles, ZnO nanotubes, ZriO nanowires, or mesoporous ZnO nanomaterials.
The black Ti0 2 nanomaterial of the present invention comprises H 2 -plasma- induced surface defects. In some embodiments of the invention, the surface defects are created by the H 2 -plasma impacting the nanoparticle or nanomaterial surface. In further embodiments of the invention, the surface defects are a layer with structural disorder. The defects created in the Ti0 2 nanoparticle or nanomaterial do not comprise oxygen defects, which are the defects manufactured in the method of Sugihara [8].
In some embodiments of the invention, the Ti0 2 nanomaterials of the present invention comprising H 2 -induced surface disorder are black. In further
embodiments, the nanomaterials are a shade of gray. It is noted that for
convenience, the nanoparticles of the invention are referred to as black nanoparticles throughout the application. The methods of making or using gray nanoparticles are analogous to the methods of making or using black nanoparticles.
In some embodiments of the invention, the black Ti0 2 nanoparticles produced by H 2 -plasma treatment absorb visible and IR-light. In further
embodiments of the invention, the black Ti0 2 nanoparticles absorb visible, IR, and UV light. The absorptivity of IR and visible light by the black Ti0 2 nanoparticles of the present invention is much higher than white Ti0 2 nanoparticles which have not been treated by H 2 -plasma. The black Ti0 2 nanoparticles of the present invention show an efficiency for photocatalytic degradation of organic molecules that is one order higher than that of untreated white Ti0 2 nanoparticles under visible light and solar light irradiation. Other Metal Oxide Nanomaterials
The invention described herein is also directed to metal oxide nanomaterials having photocatalytic properties. In some embodiments of the invention, these metal oxide nanomaterials have a black to gray color. In some embodiments of the invention, the metal oxide nanomaterials comprise a single oxidation state of the metal. The metal oxide nanomaterials exhibit visible light absorbance, and further exhibit photocatalytic properties under IR, visible, UV and full spectrum light sources.
The metal oxide nanomaterials of the present invention comprise Ti0 2 , as discussed above, and in further embodiments, the metal oxide nanomaterials comprise ZnO, W0 3 , Sn0 3 or ln 2 0 3 . In some embodiments of the invention, the nanomaterials are nanotubes, nanowires, nanoparticles, or mesoporous
nanomaterials.
Methods for producing black T1O 2 nanomaterials
The present invention is further directed to methods of making the black
Ti0 2 nanomaterials comprising a single oxidation state of Ti and further comprising H 2 -plasma-induced surface disorder. The methods of making black Ti0 2 nanomaterials described herein impart photocatalytic properties to the material.
Prior methods of manufacturing Ti0 2 nanoparticles with surface defects capable of improved visible light absorption have included high-pressure hydrogenation reactions [7] and treatment with H 2 -plasma [8]. Both the Chen [7] and Sugihara [8] methods yield Ti0 2 nanoparticles with surface defects, however each method also produces particles with reduced oxidation state, namely containing Ti 3+ . The presence of reduced titanium species is particularly shown in Fig. 3A of Chen [7], wherein the Ols XPS spectrum of the black Ti0 2 nanoparticles exhibits a shift. Conversely, in FIG. 12 of the present invention, the UPS spectra of the valence band of the black and white Ti0 2 nanoparticles are closely correlated to one another, and exhibit no new Fermi level of the valence band. There are several disadvantages of having a reduced form of Ti in nanomaterials, disadvantages that are overcome by the black Ti0 2 nanomaterials of the present invention. First, reduced Ti species, especially Ti(0), gives the Ti0 2 nanomaterials metallic properties, causing the band gap to disappear, as shown in valence band XPS spectrum of Chen, Fig.3C [7]. Without a bandgap, electrons that are excited by photons will quickly recombine with the holes because of the metallic properties of the nanomaterial, which decreases the photo-excitation efficiency. Secondly, the presence of reduced Ti species destabilizes the photocatalyst. The reduced Ti species is highly reactive with anions in solution, which corrodes the photocatalyst and disables the photocatalytic reaction process.
This invention presents a new approach to improve the visible light photocatalyzed decontamination of water via interface engineering of Ti0 2 nanomaterials utilizing exposure to H 2 -plasma. This method induces significant surface defects in the form of structural disorder, and therefore generates substantial mid-gap states for enhanced light absorption in visible region, as shown by the schematic in FIG. 1. Importantly, however, the black Ti0 2 nanomaterials produced by the method described herein contain no reduced Ti-species, such as Ti 3+ , and no titanium species are found in the metallic form. In the method of manufacturing black Ti0 2 nanomaterials from white Ti0 2 nanomaterials described herein, the chemical stoichiometry of the Ti0 2 material is unchanged. Further advantages over Chen include shorter reaction times, and low pressure H 2 plasma treatment, as compared to the high pressure hydrogenations of Chen, which are difficult to implement on an industrial scale. The present invention also exhibits many advantages over Sugihara. The method for generation of H 2 plasma in Sugihara utilizes a Tesla coil. Conversely, the method of H 2 plasma generation in the present invention utilizes RF frequency discharge between two plate electrodes, generating an H 2 plasma having a higher energy density than that of Sugihara. The methods disclosed herein also enable accelerated hydrogen ions directly bumping on the Ti0 2 nanomaterial surfaces with high efficiency. It is of important note that exposure of Ti0 2 nanomaterials to H 2 plasma generated by Tesla coil cannot make black Ti0 2 nanomaterials.
As depicted in FIG. 2, the black Ti0 2 nanomaterials of the invention are fabricated through H 2 -plasma treatment of white Ti0 2 nanomaterials under low pressure. A Ti0 2 nanomaterial of the present invention, having photocatalytic properties is made by introducing white Ti0 2 nanomaterial into a chamber. The white Ti0 2 nanomaterial is exposed to H 2 -plasma under reduced pressure at an elevated temperature and a H 2 -plasma flow rate. In certain embodiments of the invention, the elevated temperature is from about 500 °C to about 750 °C. In preferred embodiments of the invention, the elevated temperature is from about 650 °C to about 750 °C. In certain embodiments of the invention, the H 2 -plasma flow rate is from about 30 seem to about 100 seem. In preferred embodiments of the invention, the H 2 -plasma flow rate is from about 40 seem to about 80 seem. In certain embodiments of the invention, the reduced pressure is from about 10 3 to about 10 "4 atm. In preferred embodiments of the invention, the reduced pressure is about 8 x 10 ~4 atm. The nanomaterial is exposed to the conditions described for a period of time sufficient to generate a Ti0 2 nanomaterial of the present invention, having photocatalytic properties. In some embodiments of the invention, the period of time sufficient to generate a Ti0 2 nanomaterial is from about 2 hours to about 10 hours. In a preferred embodiment of the invention, the period of time sufficient to generate a Ti0 2 nanomaterial is from about 7 hours to about 8 hours at 110W. In an example embodiment, commercially available white anatase Ti0 2 particles (MTI Corporation, average particle diameter about 8-10 nm) is placed into the chamber of Plasma-enhanced chemical vapor deposition (PECVD). Hydrogen gas with a flow rate of 60sccm are introduced at a temperature of 720 °C, a low pressure (10 ~4 to 10 3 atm), and an RF plasma of 110W. The black powders are collected after 8 hours. In some embodiments of the invention, the plasma is generated within the PECVD chamber by RF frequency discharge between two plate electrodes.
In some embodiments of the invention, the surface defects are created by the H 2 -plasma impacting the nanoparticle or nanomaterial surface. In further embodiments of the invention, the surface defects are a layer with structural disorder. The defects created in the Ti0 2 nanoparticle or nanomaterial do not comprise oxygen defects, which are the defects manufactured in the method put forth by Sugihara.
In some embodiments of the invention, the longer the duration of white Ti0 2 nanomaterial exposure to H 2 -plasma, the more the the UV-visible light absorptivity of the nanomaterial improves (FIG. 3). In some embodiments of the invention, the nanomaterial is exposed to H 2 plasma for about 2 hours to about 10 hours. In preferred embodiments, the nanomaterial is exposed to H 2 plasma for about 7 hours to about 8 hours. Characterization and structure of the black T1O 2 nanomaterials
In some embodiments of the invention, the crystal structure of the black Ti0 2 nanoparticles is evaluated by X-ray diffractometry (XRD) and Raman spectroscopy. In some embodiments of the invention, the Ti0 2 nanoparticles are of the anatase form of Ti0 2 , the rutile form of Ti0 2 , the brookite form of Ti0 2 , or the mixed phase form of Ti0 2 . In a preferred embodiment, the Ti0 2 nanoparticles of the invention are of the anatase form of Ti0 2 . In further embodiments of the invention, the black Ti0 2 nanomaterials may be nanowires or nano wires. X-ray photoelectron spectroscopy (XPS) is carried out on the materials of the invention in order to determine characteristics of the surface of the material, including elemental composition, empirical formula, chemical state and electronic state. The black Ti0 2 nanoparticles of the present invention do not exhibit any shift in orbital energy or intensity relative to the white Ti0 2 nanoparticle starting material as examined by XPS. This data shows that the black Ti0 2 nanomaterials of the present invention do not include any reduced Ti-species, such as Ti 3+ and further do not include any titanium species in the metallic form (Ti°). In some embodiments of the invention, the surface of the Ti0 2 nanomaterial comprises surface defects. In further embodiments of the invention, the surface defects include structural disorder and H interstitials.
Photocatalytic decomposition of organic compounds
The compositions of the present invention are useful for the photocatalytic degradation of pollutant organic compounds in a water source. In some
embodiments of the invention, the water source is groundwater, industrial wastewater, municipal wastewater, hospital wastewater, or laboratory wastewater. In preferred embodiments of the invention, the water source is industrial wastewater or groundwater. When exposed to a light source, the electrons in the valence band of the compositions of the present invention are excited to a mid-gap state, or similarly, from a mid-gap state to a conduction band. The excited electrons are consumed by 0 2 dissolved in water, and this reactive oxygen species oxidizes the organic compounds into C0 2 and water. This reaction necessitates the
photoexcitation process of the photocatalyst. H 2 -plasma treatment induced mid-gap states enable the photoexcitation of the electrons by visible light enabling the decomposition of organic compounds under visible light irradiation. In some embodiments of the invention, the visible light is solar light. In other embodiments of the invention, the visible light is tungsten light or fluorescent light. The light used for the excitation of the nanomaterial of the invention comprises visible light or infrared (IR) light, and optionally comprises ultraviolet (UV) light.
The water source that is treated with the Ti0 2 nanomaterials of the present invention comprises water, organic compounds, and optionally comprises other components including but not limited to pathogens such as viruses and bacteria, non-pathogenic bacteria, organic particulate matter, soluble organic material, inorganic particulate matter, soluble inorganic material, gases, pharmaceuticals, and toxins. Many organic compounds are subject to decomposition upon treatment with a light source and a composition of the present invention. In some embodiments of the present invention, the organic compounds include methylene blue, Rhodamine B, and phenol. Other compounds for use with the invention include aniline, pesticides such as chlorobenzene, and herbicides, such as 4-chlorophenyl isocyanate.
In some embodiments of the invention, the organic compound is treated with a light source and a black Ti0 2 nanomaterial for about one minute to about 60 minutes. The length of visible light irradiation required to decompose an organic compound is a function of the concentration of the organic material, the wavelength and intensity of the light source, and the concentration of the photocatalyst in solution. In an example embodiment, a typical length of visible light irradiation is from about 10 minutes to about 20 minutes at a concentration of 5 mg/L of the organic compound.
In some embodiments of the invention, the black Ti0 2 nanoparticles produced by H 2 -plasma treatment show an efficiency for photocatalytic degradation of organic molecules that is one order higher than that of untreated white Ti0 2 nanoparticles under solar light and visible light irradiation.
In further embodiments of the invention, the black Ti0 2 nanomaterials described herein are used for photocatalytic water splitting to produce H 2 gas under visible light. A method of water-splitting is described by Chen [7], the teachings of which are incorporated by reference. In an embodiment of the invention, a solar light simulator is used as an excitation source, pointed at a solution of the black Ti0 2 nanoparticles of the invention, a catalytic portion of Pt, and a solvent of water and methanol mixture. In some embodiments of the invention, the Pt is Pt(0). In further embodiments of the invention, the Pt is a Pt(II) or Pt(rV) source, such as H 2 PtCl 6 . In an embodiment of the invention, an amount of Pt from about 0.5 weight % to about 1 weight % is added to a solution of about 20 mg to about 50 mg of the black Ti0 2 nanoparticles of the present invention in about 20 ml to about 50 ml of about 4: 1 water-methanol solution. The resulting mixture is kept in a sealed beaker with quartz windows, through which light is irradiated. In some embodiments of the invention, the light is generated by a solar simulator. The H 2 gas that is generated is collected into a container connected to the beaker. In some embodiments of the invention, the volume of H 2 is measured in the collection container. In alternate embodiments of the invention, the H 2 is measured by gas chromatography.
The black Ti0 2 nanomaterials described herein exhibit enhanced visible and
IR light absorption capabilities. The materials comprise surface defects that generate mid-band gap states, and are particularly advantageous over previously described materials in that the surface defects are structural rather than chemical; that is, the Ti0 2 does not contain oxygen defects or reduced titanium species, such as Ti 3+ . The method of creating the particles described herein uses H 2 -plasma treatment of anatase Ti0 2 nanoparticles under low pressure to form black Ti0 2 nanoparticles with enhanced visible light photocatalytic properties for efficient decomposition of organic wastes in water. This method is highly adaptable to an industrial scale, unlike high pressure hydrogenation reactions. The black Ti0 2 nanoparticles produced via H 2 -plasma treatment can be used as an efficient photocatalyst for visible light decontamination of organic wastes in water, and further may be used in visible light photocatalytic water splitting to produce H 2 .
DEFINITIONS
As used herein, "interface engineered" means modified at the surface and interface.
As used herein, a "mid-band gap state" is an accessible electronic state between the conduction band and the valence band of a semiconductor material.
The term "nanomaterial" can mean a nanoparticle, nanowire, nanotube, or mesoporous nanomaterial. When used in the context of a photocatalytic reaction of the decomposition of organic compounds or of water-splitting, "nanomaterial" can be used interchangeably with "photocatalyst".
As used herein, the term "surface defects" means structural defects in the surface of a material.
As used herein, "structural disorder" means the atoms are located in disordered positions and not the ordered position. For example, Ti and O atoms are located in disordered positions and not ordered positions as in a Ti0 2 crystal.
Further, some H interstitials may induce disorder.
As used herein "untreated Ti0 2 " refers to Ti0 2 that has not undergone treatment with H 2 -plasma. "Untreated Ti0 2 " may be used interchangeably with "white Ti0 2 " and "commercially available Ti0 2 ."
The term "metallic form" refers to a form of a compound that exhibits metallic properties. For example, Ti 2 0 3 is regarding herein as a metallic form of a titanium species due to its high conductivity. A metallic form of a titanium species may also include Ti(0).
An organic compound is a small molecule comprising carbon and hydrogen. Classes of organic compounds for use in the present invention include aromatic compounds, heteroaromatic compounds, aliphatic compounds, alcohols, amines, and other organic compounds that may be oxidized.
EXAMPLES The following examples are provided to illustrate one or more preferred embodiments of the invention, but are not limited embodiments thereof. Numerous variations can be made to the following examples that lie within the scope of the invention. Example 1: Production of black T1O2 nanoparticles
500 mg of commercially available white anatase Ti0 2 particles (MTI Corporation, average particle diameter about 8-10 nm) were placed into the chamber of Plasma-enhanced chemical vapor deposition (PECVD). Hydrogen gases with a flow rate of 60sccm were introduced at a temperature of 720 °C, a low pressure of 10 "4 to 10 "3 atm, and an RF plasma of 110W. The black powders were collected after 8 hours.
Example 2: Characterization and structure of black T1O2 nanoparticles
The crystal structure of the black Ti0 2 nanoparticles was evaluated by XRD as shown in FIG. 4 and Raman spectroscopy as shown in FIG. 5. X-ray
diffractometry (XRD) was performed on a Bruker AXS D8 system using Cu Ka radiation. Room temperature micro-Raman scattering analyses were carried out with a Renishaw spectrometer using Ar + (514.5nm) laser excitation source. X-ray photoelectron spectroscopy (XPS) was carried out in a homemade ultrahigh vacuum system with Omicron Twin- Anode X-ray gun. As shown in FIG. 4 and FIG. 5, the H 2 -plasma treatment does not affect the crystal structure of Ti0 2 nanoparticles and they remain in the pristine anatase phase. From XPS measurement (FIGs. 6A and 6B), we did not observe any reduced chemical state for Ti0 2 , such as Ti + . This suggests that the H 2 -plasma treatment mainly induces formation defects related gap states at the surface region of Ti0 2 nanoparticles and hence leads to the enhanced visible light absorption. Further, Brunauer-Emmett-Teller (BET) data obtained for the white Ti0 2 nanoparticles and the black Ti0 2 nanoparticles demonstrates that the surface area and pore volume of Ti0 2 nanoparticles changes only slightly after treatment with H 2 plasma (FIG. 13).
Example 3: Photocatalytic decomposition of methylene blue The photocatalytic degradation of methylene blue under visible light irradiation by filtering out incident light with wavelengths shorter than 425 nm was carried out with Abet technologies sun 2000 solar simulator (lOOmW/cm 2 ). In a typical experiment, 10 mg of the white or black Ti0 2 was added into an aqueous solution (30 mL) containing 5mg/L methylene blue. The mixed solution was placed in a 50ml beaker and stirred with a magnetic stirrer. The circulated water jacket was used to keep the temperature of the beaker at around 25 °C during irradiation. After a given irradiation time, the UV-visible absorption spectrum (Shimadzu UV3600) was measured to estimate the concentration of the remaining methylene blue in the solution by the integration of the absorption peak.
FIG. 7 shows the digital pictures of photocatalytic degradation of methylene blue (5mg/L) after simulated sun light irradiation using 0.2 mg black and white Ti0 2 in comparison with the condition of without any Ti0 2 nanoparticles. Clearly, black Ti0 2 nanoparticles displays much better efficiency in terms of photocatalytic decomposition of methylene blue. FIG. 8 shows the corresponding summarized results for photocatalytic decomposition of methylene blue with different concentration, and representative UV-Vis spectra during the photocatalytic decomposition of methylene blue and with different durations. FIG. 9 shows the comparison of the photocatalytic decomposition of methylene blue using black and white Ti0 2 , and with or without optical filter. It is found that the efficiency for photocatalytic degradation of organic molecules is about one order higher for black Ti0 2 as compared with the white Ti0 2 nanoparticles. In particular, by using optical filter to filter out UV-light (wavelength < 425 nm), the black Ti0 2 still displays pronounced efficiency for photocatalytic degradation of methylene blue. Further, the photocatalytic degradation of methylene blue using the black Ti0 2 of the present invention proceeds to completion with less irradiation time (FIG. 9A) than with the black Ti0 2 of Chen [7] under analogous conditions (Chen, Fig. 2A).
Example 4: Photocatalytic decomposition ofRhodamine B and phenol
The photocatalytic degradation of Rhodamine B and phenol under simulated sun light were also carried out with Abet technologies sun 2000 solar simulator. A mass of 0.2mg of the white or black Ti0 2 was added into an aqueous solution (lOmL) containing 5mg/L Rhodamine B or 15mg/L phenol. After a given irradiation time, the UV- visible absorption was recorded to estimate the
concentration of the remaining Rhodamine B or phenol in the solution (FIG. 10A and FIG. 11 A). FIGs. 10B and 1 IB show the comparison of the photocatalytic decomposition of Rhodamine B and phenol, respectively, using black and white Ti0 2 . Results show higher performance of black Ti0 2 than white Ti0 2 in
Rhodamine B and phenol degradation by solar light irradiation.
Example 5: Hydrogen production from photocatalytic water splitting 50 mg black Ti0 2 nanoparticles of the invention were dispersed in 5 ml DI water by ultrasonication and 100 μΐ Chloroplatinic Acid (H 2 PtCl ) solution
(10.5mg/ml) was added. The mixed solution was kept in UV irradiation (Philips 8W, 253.7nm) for 2 hours and dried in an oven at 90 °C for 10 hours. The dried black Ti0 2 with 1 wt Pt was then added into methanol solution (V met hanoi: V water = 1 :4). The pH value of the solution was measured by a pH meter (Fisher Scientific, accument AB15) and adjusted to pH = 7 by additions of NaOH solution. The final solution was then transferred and sealed in a beaker with a quartz window. Light irradiation generated by the solar generator (Abet technologies sun 2000 solar simulator) was then applied onto the samples through the window. H 2 gas generated from the beaker was then collected by a bottom-up water container connected to the reaction beaker by a pipe. The whole reaction was kept at room temperature and atmospheric pressure (1 atm). The H 2 gas volume was recorded according to the irradiation time to get the H 2 production rate. As seen in FIG. 14, the H 2 production rate in the water-splitting reaction catalyzed by H 2 -plasma treated black Ti0 2 nanoparticles of the present invention is 23 mmoM '-g 1 , which is two times greater than the H 2 production rate in the analogous experiment performed using the black Ti0 2 nanoparticles of Chen [7] (Chen, page 748, H 2 generation rate reported as 10 mmol'h ^g "1 ).
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5. Chen, X. et al. Chem. Rev. 107, (2007), 2891. 6. Fujishima, A. et al. Surf. Sci. Rep. 63 (2008), 515.
7. Chen, X., et al. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 331 (2011), 746-750.
8. Sugihara, S. Visible Radiation Type Photocatalyst and Production Method Thereof. US patent 6,908,881 Bl. 21 June 2005.
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.