EP2630086A1

EP2630086A1 - Photocatalytic material

Info

Publication number: EP2630086A1
Application number: EP11833926.6A
Authority: EP
Inventors: Ming-Chung Wu; András SÁPI; Mika Huuhtanen; Géza TÓTH; Akos Kukovecz; Zoltan Konya; Riitta Keiski; Jyri-Pekka Mikkola; Krisztián KORDÁS; Imre Kiricsi
Original assignee: University of Szeged
Current assignee: University of Szeged
Priority date: 2010-10-21
Filing date: 2011-10-21
Publication date: 2013-08-28
Also published as: EP2630086A4; WO2012052624A1; FI20106086A0

Abstract

The present invention provides a photocatalyst material comprising nitrogen-doped Ti0₂ nanofibers decorated with nanoparticles of high work function material or with p-type semiconductors. The present invention also provides a method for producing said photocatalyst material and applications related to said material.

Description

Photocatalytic material

Field of the invention The present invention relates to a photocatalyst material comprising N-doped Ti0₂ nanofibers. The present invention also relates to a method for producing said photocatalyst material and methods for using said material.

Background of the invention

Photocatalytic processes can be used to address several aspects of modern renewable energy production, organic contaminant removal from air and industrial waste water as well as anti-microbial surface treatment - just to mention a few. Amongst others, titanium dioxide (Ti0₂) is probably the most promising photocata- lyst being environmentally friendly, with low cost, good photocatalytic activity and excellent photostability as has been demonstrated in electrochemical photolysis of water to produce hydrogen, to oxidize organic compounds, or to destroy cancer cells. Ti0₂ is a wide-band-gap semiconducting material with three kinds of natural crystalline phases such as rutile, brookite and anatase, from which the latter one is exhibiting the best photocatalytic activity. Doping as well as decoration of Ti0₂ with metals and metal oxides such as Pt, Au, Pd, PdO, Ni and Ag, and also with carbon nanotubes will enhance the photocatalytic properties and may be exploited in antimicrobial coatings, photocatalytic hydrogen generation from alcohol, and in removal of xylene from air. Partial substitution of O in the lattice with N, or C leads to a shift of the absorption edge towards visible wavelengths thus enhancing further the photocatalytic efficiency.

Improvements of hydrothermal synthesis of various types of titanate nanofibers and nanotubes synthesized from Ti0₂ powders in alkaline solutions may open new possibilities for large scale and simple production of Ti0₂ nanofibers by simply annealing the obtained titanate nanofibers in air. In a number of practical applications, nanofibers can perform much better activity than the corresponding nano- particles. In electrical devices applying nanoparticles (e.g. for interconnects, electrodes, field-effect transistor channels, gas sensing layers) it is vital to have a per- colated electrical network of the particles, which is much easier to achieve with elongated 1 -dimensional particles than with ordinary 0-dimensional nanoparticles. On the other hand, bundling of nanofibers can also contribute to some mechanical robustness of tangled networks and also results in better entanglement with other nanofibers when forming macroscopic films.

Wu et al. (Synthesis and electrochemical study of novel Pt-decorated Ti nano- wires, Electrochemistry Communications 1 1 (2009), 736-739) disclose the preparation of free-standing Ti nanowires and Pt-decorated Ti nanowires via a one-step environment-friendly and template-free process. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) spectroscopy were employed to characterize the synthesized nanowires, showing that highly dense Ti nanowires with diameters of similar to 45 nm were directly grown on the Ti substrate and that the Pt nanopar- ticles coated onto the surface of Ti nanowires were uniform and ultrafine (similar to 3 nm). The active surface area of the Pt-decorated Ti nanowires is over 265 times larger than that of a polycrystalline Pt electrode and that the Pt-decorated Ti na- nowires exhibit much higher activity for electrochemical oxidation of methanol compared to the polycrystalline Pt electrode.

Lin et al. (Langmuir 2008, 24 9907-9915) study the effect of calcination temperature on the structure of a Pt/Ti0₂ (B) nanofiber and its photocatalytic activity in ge- nerating H₂. Platinized Ti0₂ (B) nanofiber was prepared and the activity thereof in generating H₂ gas from neat ethanol in photocatalytic dehydrogenation reaction was investigated.

There is still need for more efficient catalytic materials as well as production me- thods thereof. Many of the production methods known in the art are not suitable for industrial-scale production. Expensive precursors are needed (sol-gel reactions) if large amounts are to be synthesized. The prior art processes are usually for low volume production only (sputtering) or not very practical (reactive ball milling). Summary of the invention

It was surprisingly found out in the present invention that when combining N-doped Ti0₂ nanofibers with specific coating or decoration with specific materials it was possible easily and in large scale to obtain photocatalytic materials with superior characteristics and catalytic activities. The present invention provides a photocatalyst material comprising N-doped Ti0₂ nanofibers decorated with nanoparticles of high work function material or p-type semiconductors. The present invention also provides a method for producing said photocatalyst material, said method comprising

(1 ) synthesizing the titanate nanofibers by hydrothermal methods from Ti0₂ particles

(2) cation exchanging of titanates by soaking/washing in acids to form hydrogen titanate nanofibers

(3) annealing hydrogen titanates in ammonia or other nitrogen-containing base to obtain N-doped titanium dioxide nanofibers, and

(4) impregnating said titanium dioxide nanofibers with inorganic or organic precursor of high work function material or p-type semiconductor followed by decomposi- tion of the metal/semiconductor-compounds to metal/semiconductor. The present invention also provides a photocatalytic material obtained with said method.

Furthermore, the invention points to applications in which the catalyst materials are used as prepared or applied in the form of composites with polymers (both synthetic and biopolymers) or inorganic materials.

The present invention also provides methods for removing contaminants from surfaces, air or water, and methods for disinfecting thereof. Further, the present invention provides methods for generating H₂ fuel gas from water or alcohol/water mixture. The present invention is characterized in the claims.

It is an advantage of the present invention that the obtained materials are efficient photocatalytic materials. For example the materials have enhanced visible-light activity. It is another advantage of the present invention that the materials are easy to produce even in industrial quantities with the methods of the invention. High ammonium concentrations during the preparation are avoided. A specific advantage of applying nanofiber-based catalysts is the versatility of post processing such materials because with the elongated nanoparticles it is simple to form large area films, coatings, and porous membranes as well as composites with polymers, all which are practical in several chemical processes due to easily accessible catalytic surfaces even when the photocatalyst is applied in a porous host of a composite matrix.

Brief description of the figures

Figure 1 shows EFTEM images of Pt and Pd nanoparticles supported on Ti0₂ nanoparticles (reference samples). The average particle sizes are 1 .8±0.3 nm and 3.5±1 .2 nm, respectively.

Figure 2 shows (a) 25 mg Pd-Ti0₂ NFs and 25 mg cellulose immobilized on a filter paper by drop casting from aqueous suspension followed by drying. The diameter of the catalyst papers is -12 cm. (b) Two catalyst films wrapped around a plastic frame and (c) placed in the tube reactor. The 6 light sources are outside of the reactor volume, (d) Concentration of evolved H₂ gas under UV-B exposure (in 400 ml/min N₂ carrier flow).

Figure 3 shows (a) XRD patterns of sodium hydrogen titanate nanofibers cal- cined in air at 400, 550, 600, 700 and 850^QC for 2 hours (heating rate of 5 ^QC/min). (b) Sodium hydrogen titanate nanofibers calcined for 2 h (heating rate of 5 ^QC/min) and for 12 h (heating rates of 1 and 5 ^QC/min).

Figure 4 shows TEM images of (a) Ti0₂ nanofibers containing sodium impuri- ties (Ti0₂-NFs), (b) 1 .0 wt% Pt-decorated, and (c) 1 .0 wt% Pd-decorated Ti0₂- NFs. Insets in panels (b) and (c) show the corresponding Pt and Pd metal nano- particle size distributions, respectively.

Figure 5 shows XRD patterns of the Ti0₂-NFs and metal decorated Ti0₂-NFs catalyst samples.

Figure 6 shows (a) a linearized kinetic plot for the degradation of methyl orange using different types of Ti0₂-based catalyst materials, (b) an UV-Vis absorbance spectra of methyl orange (initial concentration of 10.0 mg/l) as a function of illumi- nation time when applying Pd-Ti0₂-NF catalyst (10.0 mg dispersed in 10 ml solution).

Figure 7 shows the mechanism for photocatalytic generation of hydrogen from ethanol aqueous solution over Pt/Ti0₂ and Pd/Ti0₂ catalyst as a consequence of optical electron-hole pair generation, and efficient charge separation by the Schottky interface between metal nanoparticles of large work function and n-type semiconducting Ti0₂. Protons forming by the reaction between holes and water as well as organic compounds are reduced by electrons injected previously to the metal nanoparticles.

Figure 8 shows photographs of Ti0₂-based catalyst/cellulose composite films showing the degradation of an organic dye deposited on the surface from a color ink. Each membrane (diameter of -38 mm) is filtered from an aqueous dispersion of 70 mg cellulose and 60 mg Ti0₂ NPs or Ti0₂-NFs based catalyst (Pd or Pt content is 0.6 mg). Figure 9 shows scanning electron micrographs of cellulose fiber surfaces in (a) cellulose/Pt-Ti0₂ NPs and (b) cellulose/Pt-Ti0₂NFs catalyst composite. Insets show lower magnification electron micrographs of the composite films, (c) Photograph taken from a bent catalyst membrane. Figure 10 shows (a) hydrogen formation over parent and metal loaded (1 w%) catalyst materials (0.2 g each) using ethanol: water (molar ratio 1 :3) mixture with UV-B irradiation (6 lamps), and (b) the effect of illumination power on hydrogen formation (using 0, 2, 4, 6, 4, 2, and 0 lamps) for Pt/Ti0₂-NF and Pd/Ti0₂-NF catalysts.

Figure 1 1 shows TEM images of (a) pristine Ti0₂ NF, (b) N-Ti0₂(A) NF, and (c) N-Ti0₂(B) NF. Panels in the bottom right corners show high-magnification images of the lattice with the corresponding fast Fourier transformed pattern of each sample.

Figure 12 shows TEM images of (a) N-Ti0₂(A)-Pd NF, (b) N-Ti0₂(A)-Pt NF, (c) N-Ti0₂(B)-Pd NF, and (d) N-TiC½(B)-Pt NF. In each case metal loading is -1 .0 wt %. Insets show histograms of the corresponding metal nanoparticle size distributions.

Figure 13 shows hydrogen evolution from ethanol/water mixture (molar ratio 1 :3) over parent and noble metal loaded (1 .0 wt %) catalyst materials (100 mg each) under (a) UV-A (total UV power on the reactor ~1 .54 W, A(lmax) -365 nm) and (b) UV-B (total UV power on the reactor -1 .46 W, A(lmax) -312 nm) irradiation. N₂ gas was bubbled through the reactor at a flow rate of 400 mL/min, serving also as a purging gas for the evolving gaseous products. Figure 14 shows normalized absorption spectra of the Ti0₂ based materials used in the photocatalytic experiments.

Detailed description of the invention

The present invention is described in more detail in Wu et al. (ACS Nano, Vol 5, No 6, 5025-5030, 201 1 ) which is incorporated herein by reference. The document describes the synthesis of N-doped Ti0₂ nanofibers and high photocatalytic efficiency in generating hydrogen from ethanol-water mixtures under UV-A and UV-B irradiation. Titanate nanofibers synthesized by hydrothermal method are annealed in air and/or ammonia to achieve N-doped anatase fibers. Depending on the synthesis route, either interstitial N atoms or new N-Ti bonds appear in the lattice, resulting in slight lattice expansion as shown by XPS and HR-TEM analysis, respectively. These nanofibers were then used as support for Pd and Pt nanoparticles deposited with wet impregnation followed by calcination and reduction. In the hydrogen generation tests, the N-doped samples were clearly outperforming their undoped counterparts, showing remarkable efficiency not only under UV-B but also with UV-A illumination. When 100 mg of catalyst (N-doped Ti0₂ nanofiber decorated with Pt nanoparticles) was applied to 1 I of water-ethanol mixture, the H₂ evolution rates were as high as 700 μιτιοΙ/h (UV-A) and 2250 pmol/h (UV-B) corresponding to photo energy conversion percentages of -3.6 and -12.3%, respectively.

In the present invention a photocatalyst (or photocatalytic) material comprising N- doped Ti0₂ nanofibers is decorated or coated with nanoparticles of high work function material or with nanoparticles of semiconductor material having less n- type semiconducting (i.e. more electron acceptor or more hole donor) character. The term "decoration" as used herein refers to non-continuous coating i.e. the nanoparticles may be at a certain distance from each other on the nanofibers "deco- rating" the fiber. The term "coating" may be understood to refer to similar coverage, as full 100% coverage is not required in the applications provided herein. Both terms may be used interchangeably, as well as the sometimes used term "deposited". The amount of metal or semiconducting nanoparticles decorating the nanowires is typically relatively low, such as between 0.5-5 w%. The average par- tide sizes are typically between 0.5-20 nanometers.

In one embodiment the Ti0₂ nanofibers contain alkali metal impurities. Generally, one method of making Ti0₂ nanofibers is a three-step process: 1 ) hydrothermal synthesis of sodium-titanate nanofibers from Ti0₂ nanoparticles, 2) washing of sodium-titanate nanofibers with HCI to replace sodium ions with protons and get hydrogen-titanate nanofibers, and 3) annealing of the hydrogen-titanate nanofibers at elevated temperatures to transform those to Ti0₂ nanofibers. If step 2 is not made long enough, there are still some sodium ions left in the titanate lattice i.e. in step two there may be sodium-hydrogen titanate. The alkali metal impurities are not limited to sodium, but also other alkali metals may be present, depending on the alkaline used in the first step, such as lithium or potassium. The presence of Na⁺ or other alkali metal ions on the surface may influence water molecule adsorp- tion and lead to pH change at the catalyst/aqueous solution interface, thus changing electrochemical potentials in water splitting reactions, and can also be an additional source of charge carriers enhancing or inhibiting photochemical excitation effect. The Ti0₂ nanofibers are N-doped. "N-doping" as used herein refers to partial substitution (typically between 0.1 -2.0 atom-% or about 1 N per 100-200 O) of lattice oxygen with nitrogen (Ti0₂-_xN_x). The doping introduces visible-light activity from UV to the Ti0₂ and such doped material generally absorbs the light at less than 500 nm, such as at the range of 390-600 nm. One effect of the N-doping is intro- duction of visible-light activity to the photocatalytic material. Another effect is the enhancement in UV-A light activity in addition to UV-B light activity. The N-doping enhances the overall catalytic activity of the material.

The high work function material refers to any suitable material having the electron work function of 4 eV or more. In one embodiment the high work function material is a platinum group metal, such as platinum or palladium. In other embodiments the high work function material may be for example a various form of carbon, such as a carbon nanotube, graphene, amorphous carbon or fullerene, or conductive polymer such as poly-thiophene, poly-acetylene, poly-pyrrole, or metal such as Ag, Au, Pt, Pd, Rh, or Ir, or other conductor forming Schottky junctions with TiO₂ or with N-doped TiO₂. Semiconducting materials can be for example Co₃O₄, CuO, NiO, Cr₂O₃, Cu₂S/CdS, Cu₂O, ll-VI semiconductors (ZnO, ZnS, ZnSe, CdS, CdSe, CdTe, HgS, HgSe, HgTe) as well as Si, Ge or other compound semiconductors which make p-n junctions with TiO₂ and with N-doped TiO₂.

In one embodiment the photocatalyst material is a composite with synthetic polymer, biopolymer or inorganic material. The synthetic polymer may be a step- growth polymer such as polyamide, polyacetate, polyester, nylon, bakelite, or a chain-growth polymer such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, neoprene or synthetic rubber. The biopolymer may be a polysaccharide such as celluloses and derivatives thereof, hemicellulose, starch, glycogen or chi- tin, or polypeptide, such as wool, silk, or other polymer such as lignin or polyiso- prene. The inorganic material may be selected from ceramic-type materials, clays, cements, silicates, and alumino-silicates balanced with several type of alkali, earth and transition metal cations, transition and earth metal oxides, boron oxides and alloys thereof, mica, feldspar, carbides and nitrides of silicon and boron, various forms of alumina and hetero-atomic polymers. The cement may be hydraulic ce- ment, such as Portland cement or Portland-based cement, pozzolana cement, gypsum cement, high alumina cement, slag cement, silica cement, kiln dust or mixtures thereof. Hetero-atomic polymers may be selected from polyborazylenes; polysiloxanes, polysilazanes, polyphosphazenes, polythiazyls and polysulfides or perfluorated, chlorinated polymers with or without oxygen containing functional groups.

Depending on the application (such as air filters, self-cleaning surfaces, waste water cleaning or H₂ gas generation in batch, tube, slurry, centrifugal or film reactors) of the photocatalytic material of the invention it may be for example in the form of a dispersion, a membrane, a film or a coating.

In one embodiment the Ti0₂ nanofiber is a particle having at least one dimension less than 100 nm, such as a nanofiber having the length in the range of 0.2-5 μιτι and diameter in the range of 10-100 nm. In one embodiment the Ti0₂ nanofiber is a particle having the length of about 1 μιτι and the diameter of about 50 nm. In one embodiment the Ti0₂ is in the form of anatase.

The present invention also provides a method for producing a photocatalyst material comprising N-doped Ti0₂ nanofibers coated with nanoparticles of high work function material or with p-type semiconductor as described above, said method comprising

(1 ) synthesizing the titanate nanofibers by means of hydrothermal methods from Ti0₂ particles including flakes or films

(2) cation exchanging of titanates by soaking/washing in acids to form hydrogen- titatane nanofibers (3) annealing hydrogen-titanates in ammonia or other nitrogen-containing base to obtain N-doped titanium dioxide nanofibers, and

(4) impregnating said titanium dioxide nanofibers with inorganic or organic precursor of high work function material, or with inorganic or organic precursor of p-type semiconductor followed by decomposition of the metal/semiconductor-compounds (precursors) to metal/semiconductor. This may be done at elevated temperatures (such as about 300^QC) followed by reduction in H₂ to pure metal/semiconductor (such as PdO and/or at about 500^QC). The impregnation may be wet impregnation. The decomposition of the metal precursor may be carried out in air. As described previously, the nanofibers may contain alkali metal impurities. Preferably the method is carried out at the substantial absence of separate catalyst metal.

The required amount of ammonia or other nitrogen-containing base is very low, generally less than 5%, such as about 2. In one embodiment the amount of am- monia or other nitrogen-containing base is in the range of 1 -50%, preferably 1 - 10%, e.g. 1 -5%.

The high work function material or p-type semiconducting material may be any of the materials described herein.

In one embodiment the titanate nanofibers are synthesized in alkaline solutions at 120-175^QC.

The inorganic or organic precursor of high work function material may be nitrate, chloride, acetylacetonate, acetate or hydrochloroacidic form of the mentioned metals. Precursors of the semiconducting materials may be organic complexes, hydrides. In case of oxide and sulfide semiconductors, the precursors can be metal salts/complexes that are reacted in oxidative environment or in the presence of sulfides.

In one embodiment in step (3) the hydrogen titanates and/or alkali metal hydrogen titanate are annealed in air to obtain titanium dioxide (Ti0₂, anatase) nanofibers.

In another embodiment in step (3) the hydrogen titanates are annealed in air to obtain titanium dioxide (Ti0₂, anatase) nanofibers prior annealing in ammonia to obtain nitrogen-doped titanium oxide (N-Ti0₂). In one embodiment the photocatalytic material of the invention is obtained by any of the methods described herein.

In still another embodiment the N-doped titanium dioxide nanofibers are further dispersed with liquids or cellulose fibers in liquids, the dispersion is applied to a surface and dried to obtain a photocatalyst material membrane, film or coating for instance by painting, lamination, spraying, spinning.

Upon absorption of photons by the semiconductor, electrons are excited from the valence to the conduction band. In the case of metal (or semiconductor) decorated Ti0₂-based catalysts, the photogenerated electrons will inject to the decorating metal (or semiconductor) and get trapped there if decorating metals have high enough work function to form good rectifying Schottky contacts. If decorated with semiconductors, diode-like junction should form to have similar rectifying effect. Once the electrons and holes are efficiently generated and separated, those can take part in chemical reduction and oxidation, respectively. For instance, when a contaminant is adsorbed on the catalyst, photogenerated holes in the semiconductor can initiate oxidative decomposition of the molecules (breaking chemical bonds, partially or completely oxidizing parts of the molecules).

The photocatalytic materials described herein may be used in applications wherein organic or inorganic molecules are photodegraded. Such molecules can be considered as contaminants and may include for example organic contaminants such as polychlorinated biphenyls (PCBs), microorganisms, and pesticides, fungicides, and insecticides.

Further non-limiting examples of organic contaminants include non-aromatic chlorinated hydrocarbons (chloromethane; tetrachloroethylene; dichloromethane; dich- loroethane; trichloromethane; 1 ,1 -dibromoethane; carbon tetrachloride; 1 ,2- dibromoethane; dibromomethane; ethylene dibromide; tribromomethane; monoch- loroacetic acid; trichloroethylene; and dichloroacetic acid); aromatic chlorinated hydrocarbons (PCBs; 1 ,2,4-trichlorobenzene; dioxins; 3,3-dichlorobiphenyl; chlo- robenzene; 2,4,5-trichlorophenoxyacetic acid; and 4,4'-DDT); aromatic and non- aromatic hydrocarbons (benzene; alkanes; toluene; alkenes; xylene; alkynes; and ethyl benzene); surfactants (sodium dodecylbenzene sulphonate); organic acids (salicylic acid; butyric acid; acetic acid; propionic acid; formic acid; octanoic acid; oxalic acid; and pyruvic acid); phenols (phenol; 4-chlorophenol; 1 -chlorophenol; 3,4-dichlorophenol; 2-chlorophenol; pentachlorophenol; and 2,4,5-trichlorophenol); nitrogen containing organics (2,4-dinitrotoluene; benzonitrile; nitrobenzene; nitriles; aniline; caprolactam; proteins; and cyanide); inorganics (lead ions; chromium ions; mercury ions; and copper ions); insecticides (chlorpyrifos - phosphorylated pyridine; endosulfan - chlorinated cyclodiene; and permethrin - pyrethriod); herbicides (pronamide amide); and fungicides (dichloran - chlorinated nitroaniline; triadimefor - triazole).

One embodiment of the present invention provides a method for removing organic contaminant from surfaces, air or water comprising contacting said surface, air or water with the photocatalyst material of the invention to degrade the contaminants. The organic compounds may be for example grease, oil, toxic moieties, typically on the surfaces, volatile organic compounds in the air or alcohols, phenolic compounds, hydrocarbons or fatty-acids, esters in water. The water may be any suitable water or aqueous liquid, such as waste water, industrial water, domestic water, or the like. "Removing" refers to any degree of reducing the amount of said contaminants by degrading the contaminant molecules. Generally the degradation is photodegradation.

Another embodiment of the present invention provides a method for disinfecting surfaces, air or water comprising contacting the air or the water with the photocatalyst material of the invention to kill microorganisms present on the surface, such as bacteria, yeast, fungi, viruses and the like.

Still another embodiment of the present invention provides a method for generat- ing H₂ fuel gas from starting material, such as water, alcohol, alcohol/water mixture, or other organic compounds such as hydrocarbons or oxygen containing organic molecules, comprising contacting said starting material with the photocatalyst material of the invention to degrade said starting material to release H₂ gas. In principle, any hydrogen-containing molecule may be a potential source.

Light is applied in the catalytic reactions described herein, such as visible or UV light i.e. the methods described herein are carried out in the presence of light. In one embodiment UV light is used, for example at the wavelength in the range of 300-400 nm, such as UV-A or UV-B light (about 365 nm and 312 nm respective- ly). Examples

In the following examples, anatase nanofibers (TiO₂-NFs) are synthesized from acid treated sodium titanate nanofibers by calcination at 600^QC and then used as support for Pt and Pd nanoparticles deposited by wet impregnation. Enhanced photocatalytic activity of the metal decorated TiO₂ nanofibers with or without alkali metal impurities has been demonstrated by (i) decomposing organic dyes in water, (ii) degrading organic stains on the surface of flexible freestanding composite catalyst films and by (iii) generating hydrogen from ethanol. The presented results are compared to corresponding catalyst materials based on commercial TiO₂ nanopar- ticle powders. Furthermore, examples of nitrogen-doped TiO₂ nanofibers for generating hydrogen from ethanol are demonstrated.

Catalyst synthesis

Hydrogen titanate nanofibers and/or sodium hydrogen titanate nanofibers were prepared by suspending 20-30 g anatase in 1000 ml of 10-15 M NaOH aqueous solution followed by thermal treatment in a Teflon-lined autoclave at 120-175^QC for 24-72 h applying 10-60 rpm revolving around its short axis. The product was then filtered and washed with deionized water and 0.1 M HCI aqueous solution then washed again with deionized water to reach pH ~7 and the absence of chloride, and finally dried in air at 70^QC. In order to obtain TiO₂ anatase nanofibers (TiO₂- NFs), the hydrogen titanate nanofibers were calcined at 600^QC at a heating rate of 1 ^QC/min for 12 h. Anchoring of Pt and Pd nanoparticles on TiO₂-NFs was carried out by wet impregnation. In a typical process, 20.4 mg platinum (II) acetylaceto- nate (Aldrich, 99.99%) or 29.2 mg palladium (II) acetyl aceton ate Aldrich, 99%) is dissolved in 200 ml acetone and mixed with 1 .0 g of TiO₂-NFs by ultrasonic agitation for 3 hours and following stirring for 6 hours. After evaporating the solvents at ~80^QC, the samples were calcined in air at 300^QC for 2 hours, and then reduced in 15% H₂ (in Ar buffer) flow at 500^QC for 4 hours to obtain the product, i.e. Pt-TiC½- NFs and Pd-TiO₂-NFs with ~1 w% metal load, respectively. Metal decorated reference samples with a commercial TiO₂ support (referred as Pt-TiO₂ and Pd-TiO₂) were made (Fig. 1 ) using the same routine as applied for the nanofibers. Nitrogen doping

N-doped TiO₂ nanofibers were synthesized using different calcination methods of the titanate nanowires. One example is a direct doping by annealing in ammonia gas (e.g. at 600^QC in N₂ buffer). Another route is, when the titanate is first annealed to obtain titania and then subsequently annealed in ammonia gas (e.g. at 600^QC in N₂ buffer). Annealing in the presence of amines or other nitrogen- containing bases may be also applied for N-doping.

Catalyst film fabrication and photocatalytic decomposition of organic stains on the films

Different types of freestanding flexible catalyst films were made by dispersing 70.0 mg cellulose fibers (Sigma-Aldrich, C6288) and 60.0 mg of each Ti0₂-based materials (all together 6 types of samples) in 20 ml deionized water by ultrasonic agitation for 30 min followed by subsequent stirring for 30 min. The dispersion was then filtered through polycarbonate membranes of 0.2 μιη pore size, dried at room temperature for 12 hours, and finally detached from the filter. To mimic decomposition of organic stains on the freestanding flexible catalyst membrane, a highlighting pen (Stabilo Flash, Art. No. 555/24) was used to deposit an organic dye. Each sample was exposed to UV light (Philips, HPR 125 W) for 48 hours and the fading of the written pattern was recorded with a digital camera. Photocatalytic degradation of methyl orange

The photocatalytic activity of Ti0₂-NFs, Pt-Ti0₂-NFs, Pd-Ti0₂-NFs and the reference Ti0₂, Pt-Ti0₂, Pd-Ti0₂ commercial Ti0₂-based (Degussa P25) photocatalyst materials were tested in the decoloration of methyl orange widely used as a model reaction in photocatalysis. In a typical experiment, 10.0 mg of catalyst was sonicated for 2 min in 10 ml methyl orange (Reanal) aqueous solution having concentration of 10 mg/l. The suspension was irradiated with UV light (Mercury-vapor lamp, 80 W) under vigorous stirring at ambient conditions. After a centrifuging process (for 15 min at 3200 rpm) the UV-Vis spectrum of remained methyl orange and its derivatives in the supernatant was recorded (Hitachi U-2001 , UV-Vis spectrophotometer) in the 200-700 nm wavelength range. The methyl orange concentration was calculated from the absorbance at λ = 464 nm extrapolated to a previously plotted calibration curve. Hydrogen generation from ethanol-water mixture

Photocatalytic hydrogen generation tests were carried out using 1 :3 molar ratio mixture of ethanol and water (1 I) in which 0.2 g of Ti0₂-based catalyst was sus- pended before each experiments. The temperature of the mixture was kept near the room temperature. For light source 6 pieces of UVB lamps (Sankyo Denki, G15T8E, /max at Λ-313 nm, Puv~3 W) were placed in a hexagonal arrangement around the reactor. In order to avoid sedimentation of the catalyst powders the reaction liquid was circulated by pumping. In addition, N₂ gas was bubbled through the reactor with 400 ml/min flow rate, serving also as a purging gas for the evolving gaseous products. The outlet of the reactor was connected to cold traps and to a molecular sieve (for condensing and removal the vapours of water and ethanol), and then to a hydrogen analyzer (General Electric, XMTC-6C-1 1 ). The illumination time was 60 minutes in each experiment. In addition, hydrogen production rates over Pt and Pd decorated Ti0₂-NFs catalysts were tested with different irradiation powers. Experiments using immobilized catalyst were also implemented. Each catalyst powder (25 mg) was mixed with 25 mg cellulose, then suspended in water and finally deposited on the surface of a filter 5 paper (~12 cm in diameter) by drop casting. The dried catalyst coated paper sheets were then folded to cylinders and fixed in the tube reactor to perform similar experiments as described above (Fig. 2).

Structural characterization of the catalysts

The microstructure of composite films and diameter/structure of individual nanofi- bers and the decorating metal nanoparticles were studied by field-emission scanning electron microscopy (FESEM, Jeol JSM-6300F), by transmission electron microscopy combined with electron diffraction (EFTEM, LEO 912 OMEGA, 120 kV) as well as by X-ray diffraction (XRD, Siemens D5000 and Philips PW 1380, both using Cu Ka radiation).

Results and Discussion In order to prepare the catalytically active anatase TiO₂ nanofibers with sodium impurities from the sodium hydrogen titanate nanofibers, a quick screening for finding the appropriate calcination temperature has been carried out. The samples were heated in air at 400, 550, 600, 700 and 850^QC for 2 hours then analyzed by X-ray diffraction as shown in Figure 1 (a). Calcination at temperatures up to 600^QC results in the formation of anatase phase only. The intensity of reflections increases with the temperature indicating better ordering of the lattice, however at temperatures above 600^QC formation of the catalytically inactive rutile-phase starts as shown by the appearance of (1 10) reflection at -27.2- [PDF #77-0445]. To optim- ize the calcination conditions further - i.e. to improve crystallization of anatase phase without forming rutile the calcination was tested at 600^QC for 1 2 h using two different heating rates (1 and 5^QC/min). The appearance of reflections from low intensity higher index planes in the diffraction patterns clearly demonstrates the effect of elongated calcination on the formation of highly crystalline anatase (Figure 1 (b)). All the peaks can be perfectly indexed as the body-centered tetragonal lattice structure [JCPDS No. 89-4921 ] of Ti0₂, with lattice constants a = b = 3.78 A and c = 9.50 A. Though the effect of heating rate seems to have only minor effect on crystallization (slightly better crystal formation when lower heating rate is ap- plied), we decided to use the slower heating throughout this work for synthesizing the Ti0₂ nanofibers with sodium impurities serving as starting materials of our pho- tocatalysts (Figure 3).

The as-made anatase nanofibers with sodium impurities are having length of up to a few micrometers and diameter of -50 nm (Figure 4). Decoration of anatase Ti0₂ nanofibers were obtained by wet impregnation with metal-acetylacetonates followed by decomposition in air at 300^QC and reduction in hydrogen at 500^QC. The weak and broadened XRD reflections found at 2Θ ~40^Q for both Pt and Pd-based catalyst are assigned to metallic Pd(1 1 1 ) and Pt(1 1 1 ) and suggest very small size nanoparticles (Figure 5). Based on TEM analysis the Pt and Pd nanoparticles are homogeneously distributed on the surface of Ti0₂ nanofibers and show uniform size distribution with average particle diameters of 2.3 ± 0.5 nm and 3.8 ± 1 .1 nm, respectively. The photocatalytic activities of the synthesized Ti0₂-based materials (pristine or intact Ti0₂-NFs, Ti0₂ powder, and their Pd and Pt decorated derivatives) were tested by UV light-induced photodegradation of methyl orange in aqueous solutions. UV-Vis spectra of methyl orange as function of UV light irradiation time were recorded (Figure 6), and from the absorbance measured at λ = 464 nm the corres- ponding dye concentration could be calculated using a calibration curve measured previously. Ti0₂ catalyzed photodegradation of different dyes follows well the Langmuir-Hinshelwood kinetics, which can be simplified to an apparent first-order kinetics at lower initial dye concentrations mathematically described as \n(c₀/c)=kt, where c is the concentration of the dye at time t, c₀ is the initial concentration and k is the apparent reaction rate constant. Plotting the logarithm of the reciprocal of the measured dye concentration as the function of time, linear slopes were obtained for each catalyst studied in good agreement with the Langmuir- Hinshelwood model. Anatase Ti0₂ nanofibers with sodium impurities show almost the same activity as the commercial Ti0₂ and its Pt and Pd sensitized derivatives. Despite the fact that the enhancement with noble metals on Ti0₂ was negligible (reaction rate constants were -0.08 min^"1 for each of those catalysts), the effect of metal addition to Ti0₂ nanofibers on the photocatalytic activity was found consi- derable. The apparent rate constants with Pt-Ti0₂-NFs and Pd-Ti0₂-NFs were measured to be -0.12 min^"1 and 0.16 min^"1 , respectively.

Table 1 Apparent reaction rate constants for the photodegradation of methyl orange over different Ti0₂ based catalysts

In earlier works, metals such as Au, Pt and Pd attached on Ti0₂ nanoparticles have been confirmed to help achieving better activity for the catalyst due to the rectifying Schottky barrier forming at the interface between metal nanoparticles and Ti0₂. Considering the electron affinity of Ti0₂ {χ = 3.9 eV) and the work functions of Pt (<pPt = 5.7 eV) and Pd (<pPd = 5.1 eV) the barrier heights are 1 .8 eV and 1 .2 eV, respectively. Photogeneration of electrons and holes takes place by the absorption of high energy photons followed by charge separation. Since the electrons at the higher energy levels in the conduction band of Ti0₂ can pass through the interface (while electron injection from metal to semiconductor is limited by the Schottky barrier) a much more efficient separation of charges with lower recombination rates is expected than that in pristine Ti0₂. As a result, the negatively charged metal nanoparticles contribute to a possible chemical reduction of molecules adsorbed on the metal nanoparticles; meanwhile in the proximity of the met- al nanoparticles, the depleted (i.e. positively charged) Ti0₂ surface enhances oxidative processes (see Figure 7). The lower Schottky barrier at the contact of Ti0₂ and Pd (compared to Pt) may explain the better photocatalytic activity of Pd- based catalyst in our experiments regardless whether nanoparticles or nanofibers are the support. Using photocatalysts to degrade organic compounds on surfaces has raised attention in the recent years due to applications as coatings towards self-cleaning and antimicrobial surfaces. In this example cellulose nanofibers and Ti0₂ based catalysts were adapted to prepare composite films that enable easy handling and use of the metal-decorated Ti0₂ catalysts. Using the catalyst composite films the degradation of organic dyes deposited on the surface of thin catalyst membranes as shown in Figure 8 was demonstrated. A letter "A" is written on each cellulose- catalyst composite film, then the membranes were exposed to UV light (Philips, HPR 125 W) for 48 hours. The composite membranes consisting of only cellulose and Ti0₂ or Ti0₂-NFs show only minor change in the intensity of the color pattern, whereas, the other films in which also Pd and Pt nanoparticles are present degrade the deposited ink considerably. In the case of the cellulose/Pd-Ti0₂-NFs and cellulose/Pt-Ti0₂-NFs composites, the original stain hardly can be seen, i.e. fading of the dye is practically complete.

In case of each composite, the cellulose microfibers are uniformly coated with the catalyst materials as shown in Figure 9. The composites are flexible and can be folded until a curvature radius of ~5 mm (when the membrane buckles and then breaks). The composites with Ti0₂ nanoparticles are significantly softer than those made of the nanofibers. The difference in stiffness is unexpected since the typical dimensions of the cellulose fibers (length hundreds of micrometers and diameter between 10 and 25 μιτι) are about 3 orders of magnitude larger than those for the Ti0₂-based nanofibers (length of a few micrometers, and diameter between 20 and 60 nm), which rules out a conventional mechanical tangling. However, the presence of Ti0₂ with the different shapes in the different types of catalyst materials can influence mechanical friction and sticking of adjacent cellulose fibers coated with the nanoparticles, thus affecting flexibility.

In Figure 10 (a) hydrogen production over 1 wt-% Pd and Pt decorated Ti0₂ and Ti0₂-NF catalysts using UV-B irradiation is presented. The formation was found to be the highest over Pd/Ti0₂. Pd and Pt decorated Ti0₂-NF catalysts showed a bit lower hydrogen production than respective Ti0₂ catalysts. Pd was observed to be more active material than Pt. The hydrogen formed over was found to be around 1500 μιτιοΙ/h for Pd/Ti0₂, 950 pmol/h for Pd/Ti0₂-NF, 750 pmol/h for Pt/Ti0₂, and 450 μιτιοΙ/h for Pt/Ti0₂-NF. The hydrogen formation over parent Ti0₂ and Ti0₂-NF was detected to be very low (<30 μιτιοΙ/h for both) The effect of illumination power on Pd and Pt loaded Ti0₂-NF catalysts is shown in Figure 10 (b). It can clearly be seen that the increase in illumination power, i.e. more light, increase the formation of hydrogen. The lamps were on by pairs, i.e. using 2, 4, and, 6 lamps. The curves indicate that the change in illumination power has immediate effect to hydrogen formation rate. However, the improvement in hydrogen production was found to be smaller as more lights are lighted on. Thus, it can be assumed that formation of H₂ will be stabilized when certain illumination power is reached. Among the synthesized N-doped Ti0₂ photocatalyst materials, hydrogen generation rates from ethanol-water mixtures up to 0.700 mmol/h and 2.250 mmol/h could be achieved when using 0.1 g catalyst (Pt decorated) under UV-A and UV-B irradiation, respectively. The corresponding photo-efficiency values are -3.6 and -12.3%, respectively.

Ti0₂ nanofibers decorated either with Pt or Pd nanoparticles show excellent pho- tocatalytic behavior as demonstrated in decomposing organic dyes in water, degrading organic stains on the surface of flexible freestanding cellulose/catalyst composite films and in generating hydrogen from ethanol using both suspended and immobilized catalysts. The performance of the nanofiber-based catalyst materials competes with - and in some cases outperforms - their conventional nano- particle-based counterparts. In all cases, Pd decorated Ti0₂ nanoparticles and nanofibers proved to be more efficient than their Pt-based counterparts making the Pd-based catalysts industrially relevant especially if we consider the lower cost of Pd metal (approximately 20-25% of Pt metal). Furthermore, the feasibility of cellulose/catalyst composites poses a novel way of utilizing photocatalyst materials in large-area coatings and freestanding films, all which could be produced (after some optimization of cellulose/catalyst size and amount) in extremely large quantities e.g. by roll-to-roll processes applied by paper and printing industries. Prelimi- nary experiments conducted using immobilized catalyst materials deposited on large area filter papers showed similar catalytic activity in H₂ generation as the powder samples (Figure 2).

The preparation methods and uses described above for non-doped nanofibers may also be applied to any nitrogen-doped nanofibers described herein. Nitrogen-doped anatase nanofibers decorated with noble metal nanopar- ticles for photocatalytic production of hydrogen

Wu et al. (ACS Nano, Vol 5, No 6, 5025-5030, 201 1 ) disclose the synthesis of the present nitrogen-doped Ti0₂ nanofibers (N-Ti0₂ NFs) and their metal (Pt and Pd) decorated derivatives, and demonstrate ultra-efficient H₂ generation from water- ethanol mixtures under UV-A and UV-B irradiation.

Methods

Sodium titanate (Na₂Ti_y0₂y₊i) nanofibers were synthesized through the hydrother- mal synthesis route from anatase Ti0₂ in aqueous NaOH solution (10 M) at 175^QC for 24 h using a rotating autoclave applying 120 rpm revolving around its short axis. Washing of Na₂Ti_y0_2y+i in 0.1 M HCI was applied to exchange Na⁺ ions to protons in the nanofibers. Finally, the product was washed with deionized water to reach pH~7 and finally filtered and dried in air at 70^QC.

Two kinds of nitrogen-doped Ti0₂ nanofibers, referred as N-Ti0₂(A) NF and N- Ti0₂(B) NF, were synthesized using different calcination methods. N-Ti0₂(A) NF was synthesized by the calcination of H_2-xNa_xTi_y0₂y₊i nanofibers at 600^QC in ammonia gas flow (50 ml/min, 2% NH₃ in N₂ buffer) for 15 h. N-Ti0₂(B) NF was synthesized also from H_2-xNa_xTi_y0₂y₊i nanofibers but in two subsequent calcination steps: first calcined in air at 600^QC for 12 h to form Ti0₂ anatase nanofibers, followed by a second calcination step in ammonia gas (50 ml/min, 2% NH₃ in N₂ buf- fer) at the same temperature for 3 h. The Na/Ti atomic ratios of the two products were -30.4% in N-Ti0₂(A) NF and -25.5% in N-Ti0₂(B) NF.

The as-made N-Ti0₂(A) NF and N-Ti0₂(B) NF were used as support for Pt and Pd nanoparticles deposited by wet impregnation. In a typical process, 20.4 mg of pla- tinum(ll) acetyl aceton ate (Aldrich, 99.99%) or 29.2 mg of palladium(ll) acetylace- tonate (Aldrich, 99%) was dissolved in 100 ml of acetone and mixed with 1 .0 g of N-Ti0₂ NF by ultrasonic agitation for 3 h and stirring for 6 h. After the solvents were evaporated at ~80^QC under N₂ flow, the samples were calcined in air at 300^QC for 2 h, and then reduced in 15% H₂ (in Ar buffer) flow at 500^QC for 4 h to obtain the products: N-Ti0₂(A)-Pt, N-Ti0₂(A)-Pd, N-Ti0₂(B)-Pt, and N-Ti0₂(B)-Pd each with -1 .0 wt% metal load. Pt-decorated Ti0₂ nanoparticles (Degussa P25), used as reference catalyst, were made using the same routine as with the nanofibers described above. The microstructure of diameter/structure of individual nanofibers (N-Ti0₂(A) NF and N-Ti0₂(B) NF) and the decorating metal nanoparticles (N-Ti0₂(A)-Pd NF, N- Ti0₂(A)-Pt NF, N-Ti0₂(B)-Pd NF, and N-Ti0₂(B)-Pt NF) were studied by transmis- sion electron microscopy (EFTEM, LEO 912 OMEGA, 120 kV) as well as by X-ray diffraction (XRD, Siemens D5000 and Philips PW 1380, both using Cu KR radiation). High-resolution TEM images of TiO₂ NF, N-TiO₂(A) NF, and N-TiO₂(B) NF were taken with a JEOL 2000FX (JEOL Ltd.) electron microscope operated at 200 kV (LaB₆ gun).

The ζ-potential of the catalysts (dispersed in ethanol) was determined from their electrophoretic mobility using a ζ-potential analyzer, 90Plus/BI-MAS (Brookhaven Instruments Corporation). The velocity of the particles was measured by laser Doppler velocimetry. The analysis of chemical composition and oxidation state was carried out by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, mono Al KR source, analysis area of 0.3x0.7 mm², applying charge neutralizer).

Photocatalytic H₂ generation tests were carried out using 1 :3 molar ratio mixture of ethanol and water (1 I) in which 100 mg of TiO₂-based catalyst was suspended before each experiments. The temperature of the mixture was kept near the room temperature. For light source, six pieces of UV-A lamps (Philips Actinic BL 15W/10 SLV, the wavelength of maximum emission of UV-A lamp was at ~365 nm and the power in the UV range was ~3.15 W) or six pieces of UV-B lamps (Sankyo Denki G15T8E UV-B lamps, the wavelength of maximum emission of UV-B lamp was -312 nm, and the power in UV range was 3.0 W) were placed in a hexagonal arrangement around the reactor. When the geometry of the experimental setup was taken into account, the total UV-A and UV-B powers reaching the reactor were -1 .54 and -1 .46 W, respectively. To avoid sedimentation of the catalyst powders, N₂ gas was bubbled through the reactor with a flow rate of 400 ml/min, serving also as a purging gas for the evolving gaseous products. The outlet of the reactor was connected to a cold trap and to a molecular sieve and then to a hydrogen analyzer (General Electric, XMTC-6C-1 1 ). Results and discussion

Two kinds of nitrogen-doped Ti0₂ nanofibers, referred as N-Ti0₂(A) NF and N- Ti0₂(B) NF, were synthesized using different calcination methods. N-Ti0₂(A) NF is synthesized by the calcination of H_2-xNa_xTi_y0₂y₊i nanofibers at 600^QC in ammonia gas flow (50 ml/min, 2% NH₃ in N₂ buffer) for 15 hours. N-Ti0₂(B) NF is synthesized also from H_2-xNa_xTi_y0₂y₊i nanofibers but in two subsequent calcination steps: first, calcined in air at 600^QC for 12 hours to form Ti0₂ anatase nanofibers, followed by a second calcination step in ammonia gas (50 ml/min, 2% NH₃ in N₂ buf- fer) at the same temperature for 3 hours.

After the annealing processes, both products have different colors than the un- doped material. While the undoped material is a white powder, N-Ti0₂(A) NFs are bluish and N-Ti0₂(B) NFs appear as a pale gray color. High resolution TEM im- ages of pristine Ti0₂ NF and nitrogen-doped Ti0₂ nanofibers (Figure 1 1 ) show that the calcination processes were sufficient to form a highly crystalline anatase structure with both routes. An increased d spacing for the (101 ) crystal plane is observed only for the N-Ti0₂(A) samples, suggesting that the one-step calcination method resulted in incorporation of interstitial N atoms into the lattice, while the two-step calcination method introduced the nitrogen atoms onto substitutional locations. In N-Ti0₂(A) NFs (titanate annealed in NH₃ at 600^QC for 15 h), the change of (101 ) spacing is from 3.41 to 3.46 A; however, in N-Ti0₂(B) NFs, no considerable expansion of the lattice is visible (Figure 1 1 ). The two different high-temperature treatments in NH₃ result in nitrogen doping of the raw materials as calculated from N 1 s XPS spectra (see Table 1 in Wu et al. 201 1 ). The N 1 s component at 397.8 eV of N-Ti0₂ (A) NF can be assigned to interstitial nitrogen atoms (N/Ti ratio of 0.01 1 1 ), while the peak at 396.3 eV of N-Ti0₂ (B) NF is due to substitutional nitrogen (formed Ti-N bond, N/Ti ratio of 0.0021 ). The other components with higher binding energies (>399.0 eV) detected for each sample correspond to decomposition products of NH₃ molecules and/or other surface contamination. The difference in the N/Ti atomic ratios measured for the interstitial and substitutional nitrogen is reasonable considering the different synthesis condition. It is important to point out that a significant amount of sodium ions (Na atom concentration: 8.54 at %) was found in the samples probably due to limited ion exchange in the interior of the initial sodium titanate (Na_xTi_y0_2y+i) nanofibers. The presence of Na⁺ ions on the surface may influence water molecule adsorption and lead to pH change at the catalyst/aqueous solution interface, thus changing electrochemical potentials in water splitting reactions, and can also be an additional source of charge carriers enhancing or inhibiting photochemical excitation effect. The XPS results on the N content of the samples are consistent with the HR-TEM analysis because the considerable amount of interstitial N atoms may indeed contribute to the expansion of the lattice.

The average ζ-potentials of original Ti0₂ NF, N-Ti0₂(A) NF, and N-Ti0₂(B) NF measured on powders dispersed in ethanol are -10.14, -18.02, and -16.32 mV, respectively. The somewhat lower potential values (calculated with the Henry equ- ation from the electrophoretic mobility) for the N-doped samples compared to the original Ti0₂ NFs suggests slight accumulation of additional negative charge on the surface as a consequence of nitrogen doping.

The as-prepared N-doped Ti0₂ nanofibers were finally decorated with Pt and Pd nanoparticles (1 w% each sample) by wet impregnation with Pt- and Pd- acetyla- cetonate in acetone followed by drying, thermal decomposition in air at 300^QC for 2 hours and reduction in H₂ flow at 500^QC for 4 hours (for details see Supporting Information). The anatase crystalline structure is left intact during the metal nanopar- ticle deposition step as verified by X-ray diffraction. Furthermore, the weak and broadened reflections of the Pd (1 1 1 ) and Pt (1 1 1 ) at 2Θ-40.0⁵ and the Pt (200) at 2Θ-46.5⁵ indicate the presence of small metal particles in the samples.

Analysis of the metal decorated nanofibers with transmission electron microscopy (TEM) shows that the deposited metal nanoparticles are well dispersed on the sur- face (Figure 12). The average size of Pt nanoparticles is considerably smaller than that measured for Pd on both types of supporting surfaces (1 .4 ± 0.3 nm on N- Ti0₂(A) and 1 .9 ± 0.3 nm N-Ti0₂(B) for Pt; and 4.7 ± 1 .4 nm on N-Ti0₂(A) and 5.9 ± 1 .2 nm on N-Ti0₂(B) NF for Pd). The particle size is determined by several factors as decomposition of the noble metal source compounds, seed formation rates, and diffusion properties. While both platinum acetyl aceton ate and palladium acetylacetonate decompose around 200^QC, the activation energy of the diffusion of platinum is 1 .4-1 .5 times higher than that in the case of palladium which leads to a slower surface diffusion of platinum and results in nanoparticle formation from a smaller area around the seed.

Hydrogen production from aqueous ethanol solution with metal nanoparticle- decorated N-Ti0₂(A) NF and N-Ti0₂(B) NF catalysts shows about an order of magnitude higher rate than on the reference catalyst (same support materials without metal) under both UV-A and UV-B irradiation (Figure 13). Using UV-A irradiation, the rates of H₂ evolution were found to be around 330 μιτιοΙ/η for N- Ti0₂(A)-Pd NF, 330 μιτιοΙ/h for N-TiC½(A)-Pt NF, 250 μιτιοΙ/h for N-Ti0₂(B)-Pd NF, and 700 pmol/h for N-Ti0₂(B)-Pt NF, while only 50 pmol/h for N-Ti0₂(A) NF and 30 μιτιοΙ/h for N-Ti0₂(B) NF. When using UV-B light source - as expected - the generation rates were considerably higher: 1350 μιτιοΙ/h for N-Ti0₂(A)-Pd NF, 1550 μιηοΙ/h for N-TiC½(A)-Pt NF, 1530 pmol/h for N-Ti0₂(B)-Pd NF, and 2250 pmol/h for N-Ti0₂(B)-Pt NF, and 120 pmol/h for N-Ti0₂(A) NF and 30 pmol/h for N-Ti0₂(B) NF. As the catalyst amount was 100 mg in all of the experiment, the hydrogen production rate normalized to the total catalyst mass is 3300 μιτιοΙ/gh for N- Ti0₂(A)-Pd NF, 3300 pmol/gh for N-TiC½(A)-Pt NF, 2500 pmol/gh for N-Ti0₂(B)-Pd NF, and 7000 pmol/gh for N-Ti0₂(B)-Pt NF, while only 500 pmol/gh for N-Ti0₂(A) NF and 300 μιτιοΙ/gh for N-Ti0₂(B) NF. When using UV-B light source - as expected - the generation rates were considerably higher: 13 500 μιτιοΙ/gh for N- Ti0₂(A)-Pd NF, 15 500 pmol/gh for N-Ti0₂(A)-Pt NF, 15 300 pmol/gh for N- Ti0₂(B)-Pd NF, and 22 500 pmol/gh for N-Ti0₂(B)-Pt NF, and 1200 pmol/gh for N- Ti0₂(A) NF and 300 pmol/h for N-Ti02(B) nanofibers. The highest values of efficiency of the energy conversion from photo energy to chemical energy stored in hydrogen are 5.3 and 10.6% for UV-A and UV-B photons, respectively, calculated by photo energy conversion (%) = (output energy of hydrogen evolved/energy of incident light) x 100 To further demonstrate the high efficiency of the present N-doped and metal- decorated catalysts, commercial Ti0₂ nanoparticles (Degussa P25) decorated with 1 .0 wt% Pt nanoparticles were prepared and their photocatalytic performance was measured using the same conditions as with the nanofiber-based samples. Comparison of the absorption spectra (Figure 14) of the Ti0₂-based materials shows evidence of higher absorptivity in the visible region. As shown in Figure 13b, all of the four new catalysts clearly outperform the conventional Ti0₂-Pt as well as other metal-decorated Ti0₂-based samples synthesized by other groups (see Table S1 in Supporting Information of Wu et al. 201 1 ). The better catalyst activity under UV-B exposure is explained by the larger number of photons of high enough energy to induce electron hole pairs in the semiconductor. The reason for better photocatalytic activity for the metal-decorated nanofibers as compared to the clean support materials is due to the efficient electron hole separation, which inhibits recombination, thus improving the photoefficiency of the catalyst. Doping Ti0₂ with nitrogen results in the formation of new p-states near the valence band (similar to deep donor levels in semiconductors), enabling electron transitions with lower energies than in the undoped Ti0₂. Since the N/Ti ratio for the interstitial N atom of N-Ti0₂(A) NF is higher than that for the N-Ti bond of N-Ti0₂(B) NF, one may expect a smaller band gap (or more p-states near the valence band) and consequently a better photocatalytic activity for the catalysts with N-Ti0₂(A) NF. Experiments with the pristine N-doped support materials are in agreement with the expectations; that is, N-Ti0₂(A) NF performs better than N- Ti0₂(B) NF. The metal-decorated samples seem to perform quite similarly except N-Ti0₂(B)-Pt NF, which produces almost twice as much hydrogen as the other metal-decorated catalysts. This difference is not thoroughly understood; however, a direct electron transition from the p-states to any empty states of the Pt nanopar- ticle might give a reasonable explanation as such transitions should be more fa- vored energetically than the ones from the valence band and/or the p-states to the conduction band of the semiconductor followed by a subsequent transition to the metal. On the other hand, Pt-decorated samples have advantages over the Pd- decorated ones derived from the smaller particle size and accordingly higher dispersion value of the catalyst particles. To have insight into this aspect, the turnover frequency values were calculated (see Supporting Information of Wu et al. 201 1 ) and higher ones for Pt in three of the four cases were found. As the particle size and dispersion was not fully correlated to the product amount and use of turnover frequency has controversies in photocatalytic processes, the product rate/amount of catalyst values for comparing different methods was used, and this value has the practical advantage to lead easily to product amount/cost of catalyst calculations.

In summary, a set of novel photocatalyst materials based on N-doped TiO₂ nanofi- bers were developed and tested for hydrogen generation from ethanol-water mix- tures of 1 :3 molar ratio by applying UV-A and UV-B irradiation. Each photocatalyst was found to be highly efficient outperforming the conventional metal decorated TiO₂ nanoparticle catalyst materials. Among the synthesized photocatalysts, the highest hydrogen generation rates were obtained with N-doped TiO₂(B) nanofibers decorated with Pt nanoparticles (diameter of 1 .9 ( 0.3 nm, 1 .0 wt % Pt in the cata- lyst). When applying only 100 mg of catalyst in 1 I water-ethanol mixture - without optimizing the reactor conditions - the H₂ evolution rates were as high as 700 μιτιοΙ/η (UV-A) and 2250 μιτιοΙ/h (UV-B) corresponding to photoefficiency values of -3.6 and -12.3%, respectively.

Claims

1 . A method for producing a photocatalyst material comprising N-doped titanium dioxide (Ti0₂) nanofibers decorated with nanoparticles of high work function ma- terial or with p-type semiconductor, said method comprising

(2) cation exchanging of titanates by soaking/washing in acids to form hydrogen- titatane nanofibers (3) annealing hydrogen-titanates in ammonia or other nitrogen-containing base to obtain N-doped titanium dioxide nanofibers (N-Ti0₂), and

(4) impregnating said N-doped titanium dioxide nanofibers with inorganic or organic precursor of high work function material or p-type semiconductor followed by decomposition of the metal/semiconductor-compounds to metal/semiconductor to obtain N-doped titanium dioxide (Ti0₂) nanofibers decorated with nanoparticles of high work function material or with p-type semiconductor.

2. The method of claim 1 , characterized in that the Ti0₂ nanofibers contain alkali metal impurities.

3. The method of claim 1 or 2, characterized in that in step (3) the hydrogen- titanates are annealed in air to obtain anatase nanofibers prior to annealing in ammonia.

4. The method of any of the preceding claims, characterized in that the obtained titanium dioxide nanofibers are further dispersed with liquids or cellulose fibers in liquids, the dispersion is applied to a surface and dried to obtain a photo- catalyst material membrane, film or coating.

5. The method of any of the preceding claims, characterized in that the high work function material is a metal, such as Ag, Au, or platinum group metal, such as Pt, Pd, Rh or Ir.

6. The method of any of the claims 1 -4, characterized in that the high work function material is a form of carbon such as a carbon nanotube, graphene, amorphous carbon or fullerene.

7. The method of any of the claims 1 -4, characterized in that the high work function material is a conductive polymer such as poly-thiophene, poly-acetylene or poly-pyrrole

8. The method of any of the preceding claims, characterized in that the semi- conductor nanoparticle is selected from Co₃0₄, CuO, NiO, Cr₂0₃, Cu₂S/CdS, Cu₂0, ll-VI semiconductors (ZnO, ZnS, ZnSe, CdS, CdSe, CdTe, HgS, HgSe, HgTe), or Si, Ge or other compound semiconductors which make p-n junctions with Ti0₂ and with N-doped Ti0₂.

9. Photocatalyst material comprising N-doped Ti0₂ nanofibers decorated with nanoparticles of high work function material or p-type semiconductor.

10. The photocatalyst material of claim 9, characterized in that the Ti0₂ nanofibers contain alkali metal impurities.

1 1 . The photocatalyst material of claim 9 or 10, characterized in that the high work function material is a metal, such as Ag, Au, or platinum group metal, such as Pt, Pd, Rh or Ir.

12. The photocatalyst material of claim 9 or 10, characterized in that the high work function material is a form of carbon such as a carbon nanotube, graphene, amorphous carbon or fullerene.

13. The photocatalyst material of claim 9 or 10, characterized in that the high work function material is a conductive polymer such as poly-thiophene, poly- acetylene or poly-pyrrole

14. The photocatalyst material of any of the claims 9-13, characterized in that the semiconductor nanoparticle is selected from Co₃0₄, CuO, NiO, Cr₂O₃, Cu₂S/CdS, Cu₂O, ll-VI semiconductors (ZnO, ZnS, ZnSe, CdS, CdSe, CdTe, HgS, HgSe, HgTe), or Si, Ge or other compound semiconductors which make p-n junctions with TiO₂ and with N-doped TiO₂.

15. The photocatalyst material of any of the claims 9-14, characterized in that the Ti0₂ nanofiber is a particle having at least one dimension less than 100 nm, such as a nanofiber having the length in the range of 0.2-10 μιτι and the diameter in the range of 10-100 nm, such as the length of about 1 μιτι and the diameter of about 50 nm.

16. The photocatalyst material of any of the claims 9-15, characterized in that it is obtained with the method of any of the claims 1 -8.

17. The photocatalyst material of any of the claims 9-16, characterized in that it is a composite with synthetic polymer, biopolymer, or inorganic material.

18. The photocatalyst material of claim 17, characterized in that the synthetic polymer is a step-growth polymer such as polyamide, polyacetal, polyester, nylon, bakelite, or a chain-growth polymer such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, neoprene or synthetic rubber.

19. The photocatalyst material of claim 17, characterized in that the biopolymer is polysaccharide, such as celluloses and derivatives thereof, hemicellulose, starch, glycogen, chitin, or polypeptide, such as wool, silk, or other polymer such as lignin or polyisoprene.

20. The photocatalyst material of claim 17, characterized in that the inorganic material is selected from ceramic-type materials, clays, cements, silicates and alumino-silicates balanced with several type of alkali, earth and transition metal cations, transition and earth metal oxides, boron oxides and alloys of them, mica, feldspar, carbides and nitrides of silicon and boron and various forms of alumina, and hetero-atomic polymers selected from polyborazylenes; polysiloxanes, polysi- lazanes, polyphosphazenes, polythiazyls and polysulfides or perfluorated, chlori- nated polymers with or without oxygen-containing functional groups.

21 . The photocatalyst material of any of the claims 9-20, characterized in that it is in the form of a dispersion, a membrane, a film or a coating.

22. A method for removing organic contaminant from surfaces (such as grease, oil, toxic moieties), air (such as volatile organic compounds) or water (such as alcohols, phenolic compounds, hydrocarbons, fatty-acids, esters) comprising con- tacting said surface, air or water with the photocatalyst material of any of the claims 9-21 to degrade said contaminant.

23. A method for disinfecting surfaces, air or water comprising contacting the surface, air or the water with the photocatalyst material of any of the claims 9-21 to kill microorganisms present on the surface.

24. A method for generating H₂ fuel gas from starting material comprising contacting said starting material with the photocatalyst material of any of the claims 9- 21 to degrade said starting material to release H₂ gas.

25. The method of claim 24, characterized in that the starting material is water, alcohol, alcohol/water mixture, or other organic compound such as hydrocarbon or oxygen-containing organic molecule.