Enhanced Photocatalytic Activity of Nitrogen-Doped Titania by

Jul 2, 2009 - N-doped TiO2 deposited with Au (Au/N−TiO2) was successfully prepared via hydrolysis of titanium sulfate and ammonia followed with a Au...
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J. Phys. Chem. C 2009, 113, 14689–14695

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Enhanced Photocatalytic Activity of Nitrogen-Doped Titania by Deposited with Gold Yongmei Wu, Haibei Liu, Jinlong Zhang,* and Feng Chen Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China ReceiVed: May 13, 2009; ReVised Manuscript ReceiVed: June 17, 2009

N-doped TiO2 deposited with Au (Au/N-TiO2) was successfully prepared via hydrolysis of titanium sulfate and ammonia followed with a Au deposition-precipitation method. The resulting samples were characterized by X-ray diffraction (XRD), atomic absorption flame emission spectroscopy (AAS), UV-vis absorbance spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 physical adsorption, high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra. The photocatalytic activities of the samples were evaluated for degradation of methyl orange (MO) under visible light and UV light irradiation. It was found that the Au/N-TiO2 samples presented much higher photocatalytic activity than N-doped TiO2 under both UV and visible light irradiation. Diffuse reflectance UV-vis spectra showed an extension of light absorption into the visible region for Au/N-TiO2, and PL analysis of the sample indicated that the electron-hole recombination has been effectively inhibited after Au particles deposition. XPS analysis displayed Au existing as Au0 on the surface of the N-TiO2 photocatalyst and nitrogen species in the form of N-Ti-O and Ti-O-N. The excellent photoactivities of Au/N-TiO2 compared with N-doped TiO2 could be explained by its appropriate Au particle sizes and cooperation effect between N species and metallic Au particles. 1. Introduction Titanium dioxide, commonly regarded as one of the most active and stable photocatalysts, has long been investigated for environmental applications.1-3 However, its bandgap is too large (Eg ) 3.20 eV) to be excited only by ultraviolet light with a wavelength of no longer than 387.5 nm. The high rate of electron-hole recombination often results in a low quantum yield and poor efficiency of photocatalytic reactions. Many studies have been carried out to improve the photocatalytic activity by reducing the recombination reaction through deposition of noble metals such as Au, Ag, Pt, and Pd on the surface of TiO2, and it was confirmed that the deposition of noble metal on TiO2 nanoparticles was an essential factor for maximizing the efficiency of photocatalytic reactions.4-10 The noble metal, which acts as a sink for photoinduced charge carriers, promotes interfacial charge-transfer processes. Great success has been reached in the preparation of gold nanoparticles over TiO2 for photocatalytic application; however, these studies were primarily concerned with Au deposition on undoped TiO2 or P25 titania for photocatalytic applications.11-18 There has been little research focusing on Au deposition on the surface of modified TiO2.19-22 Vicente et al. have reported that Au deposited on the TiO2-Al2O3 sample by deposition-precipitation using urea is more active than that of the Au/TiO2 and Au/P25 for methyl tert-butyl ether (MTBE) decomposition.19 Our previous research on Au deposition on the Fe-doped TiO2 has shown that the Au/ Fe-TiO2 photocatalyst exhibited excellent visible light and UV light activity, and the synergistic effects of Fe3+ and Au was responsible for improving the photocatalytic activity.20 Li et.al synthesized a novel biomorphic N-doped TiO2 templated with cotton and further assembled Au nanoparticles within the nanopores, but they have not applied this material to the catalytic reaction.21 Sanz et al. reported that Au preadsorption on rutile * To whom correspondence should be addressed. Fax: +86-21-64252062. E-mail: [email protected].

TiO2 (110) surfaces significantly increased the reachable amount of N implanted in the oxide and a synergism between implanted N and deposited Au atoms lead to a higher stabilization of N species as well as a stronger adsorption of Au atoms and the Au/TiNxO2-y system was found to be active for the thermal dissociation of water and the production of H2 through the watergas shift reaction.22 Tian et. al synthesized Au particles loading on the N-doped TiO2 which was prepared by precipitedhydrothemal method and the Au/N-TiO2 photocatalyst exhibited higher visible activity than N-doped TiO2 and Au/TiO2.23 Different from their preparation method, herein we prepare N-doped TiO2 through the hydrolysis of titanium sulfate by ammonia followed with urea. Gold was deposited on the surface of N-doped TiO2 by the deposition-precipitation method with urea, since it has been reported that with this method highly dispersed gold particles were obtained.24 The characterization of the photocatalyst was made by means of nitrogen adsorption, XRD, UV-vis absorption spectra, and HRTEM. The photocatalytic properties were evaluated in the liquid phase decomposition of methyl orange under both visible and UV light irradiation. On the best of our knowledge, only few researches havebeenpreviouslyreportedfortheAu/N-TiO2 photocatalysts,21-24 and here we have demonstrated the excellent photoacticvities of Au/N-TiO2 under both UV light and visible light compared with N-doped TiO2, and the synergistic effect between N species and metallic Au particles. 2. Experimental Sections 2.1. Preparation of Photocatalysts. Nitrogen-doped titania (N-TiO2) was prepared by the hydrolysis of titanium sulfate by ammonia followed dealing with urea. A 25% ammonia solution was added dropwise to aqueous titanium sulfate until the pH value of this mixture was 7.0. The white precipitate was obtained immediately. After aging at room temperature for 24 h, the precipitate was filtered and washed by distilled water until SO42ions were removed, and then dried at 100 °C for 8 h and ground

10.1021/jp904465d CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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to obtain amorphous titanium hydroxide. Four grams of urea was added to 50 mL of water containing 3 g titanium hydroxide. After stirring vigorously for 6 h, water was removed in vacuum at 60 °C, and the residue was dried again in an oven at 100 °C for 6 h. Then the resulting powder was calcined in the muffle finance under air at 400 °C for 2 h, with a heating rate of 2 °C/min. Au particles were deposited on the surface of N-TiO2 by using the deposition-precipitation method with urea. One gram of N-TiO2 was added to 100 mL of aqueous solution containing HAuCl4 and urea (0.42 M). The suspension, thermostatted at 80 °C, was vigorously stirred for 4 h. Decomposition of urea leads to a gradual rise in pH. These sequences of preparation in solution were performed in the dark, since light is known to decompose the gold precursors and to reduce them. The solids gathered after centrifugation were washed with water four times, and then dried under vacuum at 100 °C for 2 h in the dark, calcined at 350 °C under a flow of industrial air (100 mL · min-1) with a heating rate of 2 °C/min, and then maintained for 4 h. The N-doped TiO2 was also handled under flow air. Au weight loading of the sample is expressed in grams of Au per grams of sample: wt%Au ) [mAu/(mAu + mN-TiO2)] × 100. The nominal content of Au in the samples was 1.0, 3.0, 4.0, and 5.0 wt %, designated as N-TiO2, 1.0Au/N-TiO2, 3.0Au/ N-TiO2, 4.0Au/N-TiO2, 5.0Au/N-TiO2. 2.2. Characterization of Photocatalysts. XRD analysis of the prepared photocatalysts was carried out at room temperature with a Rigaku D/max 2550 VB/PC apparatus using Cu KR radiation (λ ) 1.5406 Å) and a graphite monchromator, operated at 40 kV and 30 mA. Diffraction patterns were recorded in the angular range of 10° ∼ 80° with a stepwidth of 0.02° s-1. The surface morphologies and particle sizes were observed by transmission electron microscopy (TEM, JEM-2011), using an accelerating voltage of 200 kV. The actual content of Au deposited on N-TiO2 was determined by atomic absorption flame emission spectroscopy (Shimadzu AA-6400F). UV-vis absorption spectra were obtained using a scan UV-vis spectrophotometer (Varian Cary 500) equipped with an integrating sphere assembly, while BaSO4 was used as a reference. To investigate the chemical states of the photocatalysts, X-ray photoelectron spectroscopy (XPS) was recorded with a Perkin-Elmer PHI 5000C ESCA System with Al KR radiation operated at 250 W. The shift of binding energy due to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard. The textural properties of the samples were determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). All of the samples were degassed at 473 K for 2 h before the measurement. The recombination of electron-hole in the samples was studied by the photoluminescence (PL) emission spectra, which was measured on a luminescence spectrometry (Cary Eclips) at room temperature under the excitation light at 280 nm. The conditions are fixed as far as possible in order to compare the photoluminescence intensity directly. 2.3. Photocatalytic Activities Test. The photocatalytic activity of each sample was evaluated in terms of the degradation of methyl orange (MO). MO was selected as a model pollutant because it is a common contaminant in industrial wastewater and has good resistance to light degradation. The photocatalyst (0.08 g) was added into a 100 mL quartz photoreactor containing 80 mL of a 20 mg L-1 MO solution. The mixture was sonicated for 10 min and stirred for 30 min in the dark in order to reach the adsorption-desorption equilibrium. A 1000 W tungsten halogen lamp equipped with a UV cutoff filters (λ > 420 nm) was used as a visible light source (the average light intensity was 60 mW · cm-2.) and a 300 W high-pressure Hg lamp for

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Figure 1. Absorption spectrum of methyl orange.

Figure 2. XRD patterns of different samples: (a) N-TiO2; (b) 1.0Au/ N-TiO2; (c) 3.0Au/N-TiO2; (d) 4.0Au/TiO2; and (e) 5.0Au/N-TiO2.

which the strongest emission wavelength is 365 nm was used as a UV light source (the average light intensity was about 1230 µW/cm2). The lamp was cooled with flowing water in a quartz cylindrical jacket around the lamp, and ambient temperature was maintained during the photocatalytic reaction. At the given time intervals, the analytical samples were taken from the mixture and immediately centrifuged, then filtered through a 0.22 µm Millipore filter to remove the photocatalysts. The concentration of the filtrate was analyzed by checking the absorbance at 464 nm with a UV-vis spectrophotometer (Varian Cary 100). The absorption spectrum of MO and molecular structure are shown in Figure 1. 3. Results and Discussion 3.1. XRD and TEM Analysis. The XRD patterns of the studied samples are present in Figure 2. With the gold loading ranging from 1.0 to 4.0 (wt%), only the diffraction peaks of anatase TiO2 [JCPDS no. 21-1272, space group: I41/amd(141)] can be observed and no reflections due to gold or gold oxide were detected, suggesting either the low gold content below the detection limit or that well-dispersion of the gold particles on the surface of N-TiO2. When gold loading is up to 5.0%, a very weak peak at around 44.4° showed up, which was attributed to the (200) peak of metallic gold. From the (101) peak of anatase TiO2, the average size of crystallite was calculated using the Scherrer equation:

D)

Kλ βcos θ

Photocatalysis of N-Doped Titania with Au Deposition TABLE 1: Textural Property of Au/N-TiO2 Photocatalysts, and the Gold Content and Average Size of Gold Particles pore pore specific nominal Au crystallite surface diameter volume Au content loading (wt%) size (nm)a area (m2/g) (nm) (cm3/g) (wt%)b 0 1.0 3.0 4.0 5.0

13 13 13 13 13

88.5 85.8 88.2 72.3 75.8

4.5 4.4 4.4 4.8 4.5

10.2 10.1 10.1 10.1 10.1

a Determined by XRD using Scherrer equation. atomic absorption flame emission spectroscopy.

b

0.79 2.20 3.35 4.39

Determined by

where β is the full width half-maximum (fwhm) of the 2θ peak, K is a shape of factor of the particles (it equals to 0.89), and θ and λ are the incident of angle and the wavelength of the X-rays, respectively. The crystallite size of all of the samples is shown in Table 1. As shown in Table 1, the crystallite size of Au/ N-TiO2 samples is similar to that of the corresponding matrix. It also imply that the Au loading does not have much influence on the crystallize size. TEM and HRTEM images (inset) for typical 1.0Au/N-TiO2 and 4.0Au/N-TiO2 photocatalysts are shown in Figure 3. The micrographs of these two photocatalysts showed that the gold particles are well dispersed on the surface of N-TiO2. HRTEM confirms gold particles and anatase N-TiO2 are highly crystallized. The mean particle diameter of gold for 1.0Au/N-TiO2 is about 2.6 nm and for 4.0Au/N-TiO2 is about 2.8 nm. In the case of 4.0Au/N-TiO2 sample, the major fraction of gold particles is in the range of 1.5-2.5 nm, and few are above 5.0 nm. Chang et al. have reported the limitation of XRD for reduced gold crystallite corresponding to a particle size smaller than 5 nm.25 The failure in observing gold phases on the XRD pattern of the 4.0Au/N-TiO2 sample can be explained due to its smaller particle size and well dispersion on the surface of N-TiO2. It also showed that the deposition-precipitation method by urea allows the smaller gold particle size. 3.2. Nitrogen Adsorption Analysis and Atomic Absorption Flame Emission Spectroscopy (AAS) Studies. Figure 4 depicts the N2 adsorption-desorption isotherms and the BJH pore size distribution of the 4.0Au/N-TiO2 sample. The isotherms are of type IV, characteristic for mesoporous materials according to the Brunaner, Deming, Deming, and Teller (BDDT) classification.26 The pore size distribution of 4.0Au/N-TiO2 obtained with BJH method is quite narrow and mainly at 5 nm. The other samples have similar N2 adsorption-desorption isotherms and the BJH pore size distributions to 4.0Au/N-TiO2 sample (not shown) and their textural properties of the N-TiO2 and Au/N-TiO2samples are listed in Table 1. The support N-TiO2 has a specific surface area of 88.5 m2/g. The surface areas of 1.0Au/TiO2, 3.0Au/N-TiO2, 4.0Au/N-TiO2, 5.0Au/N-TiO2 samples are 85.8, 88.2, 72.8, 75.3 m2/g, respectively, which are very close to N-TiO2. The surface area is not greatly affected by the lower Au loading. The slight decrease in the BET surface area of 4.0Au/N-TiO2 and 5.0Au/N-TiO2 is probably due to the blockage of some of the TiO2 pores by higher Au loading. Additionally, the differences in the pore sizes and volumes of the N-TiO2 with different gold loading are insignificant. It can be concluded that the deposition of gold could not change the texture properties of N-doped TiO2. Similar results have been reported on Au/P25 photocatlyst using the deposition-precipitation method with NaOH by Lambert’s group.13

J. Phys. Chem. C, Vol. 113, No. 33, 2009 14691 The actual content of gold in different samples is determined by AAS, the results are listed in Table 1. It is shown that the actual content of gold measured by AAS is slightly lower than that of the nominal value, which is due to the low adsorption forces between AuCl3(OH)- ions and TiO2 surface, part of gold species may be washed out by water. 3.3. UV-vis Absorbance Spectra. Figure 5 shows the UV-vis absorbance spectra of samples with different gold loading. It can be seen that the N-TiO2 sample presents a significant absorption in the visible region between 400 and 550 nm, which was the typical absorption feature of nitrogen doped TiO2 and was caused by the excitation of electrons from localized N doping level in the band gap.27 In the case of Au/ N-TiO2 samples, additional peak shoulder at around 560 nm was observed, which was attributed to the absorption of the gold surface plasmon. This absorption is due to collective oscillation of free conduction band electrons of the gold particles in response to optical excitation.12 When the content of gold increased, the intensity of the plasmon absorbance band increase which is affected by the gold particle size, shape, the content of gold, and the surrounding environment,28,29 3.4. XPS Analysis. The XPS spectra of N-TiO2 and 4.0Au/ N-TiO2 are shown in Figure 6. Figure 6(a) displays the XPS Ti 2p spectra of these two samples. Two peaks for the Ti 2p spectra were observed at 458.8 and 464.4 eV, assigned to Ti 2p3/2 and Ti 2p1/2, in good agreement with the presence of Ti(IV). With respect to 4.0Au/ N-TiO2 sample, the binding energy of Ti 2p3/2 and 2p1/2 are at around 458.4 and 464.0 eV, respectively, positive shift 0.4 eV in comparison with N-TiO2, suggesting that there may be interaction between N-TiO2 and Au particles. Figure 6(b) shows the XPS of O1s spectra of N-TiO2 and 4.0Au/N-TiO2. The O1s binding energies of all the samples were located at a little higher value than 530.0 eV, which was assigned to bulk oxide (O2-) in the TiO2 lattice. Figure 6(c) displays the XPS Au 4f spectra of 4.0Au/N-TiO2 samples. Binding energy values found for Au 4f7/2 and Au 4f5/2 levels are 82.9 and 86.6 eV, respectively, suggesting that Au species exist in their metallic state.30,31 No oxidized gold species was detected. Zanella et al.28 reported oxidation gold can be completely reduced in metallic gold when the Au/TiO2 catalyst prepared by deposition-precipitation with urea was calcinated at a temperature above 473 K under air flow. According to the XPS handbooks and previous reports,32,33 binding energy values of 4f7/2 and 4f5/2 for metallic Au were centered at 84.0 and 87.7 eV, respectively. The shift of Au 4f peaks of Au/N-TiO2 toward lower binding energies indicates strong interaction between Au particles and the N-TiO2 substrate. As we know, Au and TiO2 have different Fermi level positions; the former is higher than the latter. When the two materials are connected electrically, electron migration from the semiconductor to the metal occurs until the two Fermi levels are aligned. Hence, the surface of the metal acquires an excess negative charge in the case of Au cluster supported on N-TiO2. Figure 6(d) shows the XPS spectra for the N1s region of N-TiO2 and 4.0Au/N-TiO2 samples and their fitting curves. A broad peak extending from 394 to 404 eV is observed for both of N-TiO2 and 4.0Au/N-TiO2 samples, which is in the range (396-404 eV) observed by several other researchers and is typical of nitrogen-doped titanium dioxide.27,34-36 After fitting of the curve, two peaks are obtained at 399.0-399.7 eV and 400.3-401.0 eV, respectively. In many cases, the peak at about 399 eV is attributed to the anionic N- in O-Ti-N linkages, inconsistent with some literature proposals. The origin of another peak above 400 eV is still under debate and subject to much

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Figure 3. TEM and HRTEM images of (a) 1.0Au/N-TiO2 and (b) 4.0Au/N-TiO2.

Figure 4. N2 adsorption-desorption isotherms and the BJH pore size distribution (inset) of the 4.0Au/N-TiO2sample.

controversy. Asahi et. al27 and Kisch et. al37 attribute the N1s peaks at binding energy at 400 and 402 eV to molecularly adsorbed nitrogen species. Burda et al. suggested that it is attributed to the N atom in the environment of O-Ti-N.34 We assigned this peak to the formation of the Ti-O-N structure.38 Compared with N-TiO2, the binding energy of N1s of 4.0Au/ N-TiO2 samples are positive shifted 0.7 eV, indicating some charges from nitrogen species may be transferred to Au species on the surface of TiO2. Sanz et.al22 reported that Au preadsorption on TiO2 (110) surfaces significantly increased the reachable amount of N by direct reaction of NH3 with Au/TiO2(110), and the stabilization of the embedded N was due to an electron transfer from the Au 6s levels toward the N 2p levels. Thus, there may be strong interaction between doped N and Au

Figure 5. UV-vis absorbance spectra of all samples.

particles on the surface of TiO2. Our result is different from Sanz’s group. One possible reason is due to the different preparation of Au/N-TiO2 system. The N-doped TiO2 catalyst was prepared and then Au was deposited on the surface of N-doped TiO2, whereas Sanz et al. used Au preadsorption on TiO2 (110) surfaces and then N-doped under ammonia. So it may be reasonably considered that there may be strong interactions between Au particles and N-TiO2, which may change the electronic property of Au/N-TiO2. 3.5. Photoluminescence Emission Analysis. Photoluminescence (PL) emission is useful to disclose the efficiency of charge carrier trapping, immigration, and transfer, and to understand the fate electron-hole pairs in semiconductor particles.39 It is known that the PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity may

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Figure 6. XPS spectra of N-TiO2 and 4.0Au/N-TiO2 samples (a) Ti 2p, (b) O1s, (c) Au 4f, and (d) N1s.

Figure 7. Photoluminescence spectra of (a) N-TiO2 and (b) 4.0Au/ N-TiO2.

indicate the lower recombination rate of electron-holes under light irradiation.40 Figure 7 shows the PL spectra of N-TiO2 and 4.0Au/N-TiO2. Two peaks appearing at about 360 and 380 nm are ascribed to the emission of the band gap transition.41 It is observed that the PL intensity of Au/N-TiO2 sample decreases greatly in comparison with the N-TiO2 sample. This result indicates the recombination of charge carriers is effectively suppressed after Au deposition on the surface of the N-TiO2 sample. The presence of gold on the surface of N-TiO2 sample favors the migration of photoproduced electron to gold particles, thus improving the electron-hole separation. 3.6. Photocatalytic Activities. The photodegradation of methyl orange (MO) is employed to evaluate the photocatalytic activities of N-TiO2 and Au/N-TiO2 samples because MO is a photostable dye and cannot be photodegraded in the absence of any photocatalyst under UV light or visible light irradiation. The experimental results are displayed in Figure 8, parts (A) and (B). Under visible light irradiation, the Au/N-TiO2 samples

show higher visible light activities than that of the N-TiO2 sample, confirming that Au depositing on N-TiO2 is an effective way to improve photoactivity. The highest activity is observed at 4.0 wt % Au loading, and about 85.4% of MO was degraded after reaction for 5 h under visible light. It has been pointed out by some groups that inclusion of nonmetal ions decreases photocatalytic activity of TiO2 under UV light illumination despite enhanced light absorption in the visible region.42,43 It is interesting to note that gold deposited on N-TiO2 samples have also higher photocatalytic activities under UV light illumination. The overall trend of the activity is very similar to that under visible light irradiation, and the superior catalytic activity of 4.0Au/N-TiO2 is evident, slightly lower than that of P25, a well-known commercial photocatalyst with high UV photoactivity. When the deposited gold exceed 5 wt %, the photoactivity of 5.0/N-TiO2 sample decreases even though it has higher visible light absorption. Higher content of gold not only influences the penetration of light but also become the recombination center, and results in low photoactivity. Several factors have been reported to influence the photocatalytic activity of titania, such as crystallite size, crystallinity, surface area, surface acidity, and Au particle size. The high photocatalytic activity of 4.0Au/N-TiO2 may be attributed to the following reasons. On the one hand, it was reported that deposition of Au nanoparticles on polycrystalline TiO2 resulted in quasi-Fermi level shift. Such a shift improved charge separation in the Au-TiO2 composite and enhanced the efficiency of the interfacial charge-transfer process.44 It is generally agreed that the rate limiting step in photocatalytic reactions of this type is electron transfer from the TiO2 surface to adsorbed O2.2 Thus, the photoelectrons can be captured by gold particles and subsequently be transferred to the adsorbed O2 to yield highly oxidizing peroxy or superoxy species, leading to the

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Figure 8. Photocatalytic degradation rate of MO under (A) visible light illumination for 5 h and (B) UV light illumination for 75 min over (a) N-TiO2; (b) 1.0Au/N-TiO2; (c) 3.0Au/N-TiO2; (d) 4.0Au/ N-TiO2; (e) 5.0Au/N-TiO2; and (f) P25.

effective separation of electrons and holes which has been confirmed by PL measurement and therefore substantially enhance the rate of photocatalytic reaction. Meanwhile, it has been reported that smaller gold particles induce more negative quasi-Fermi level shifts than bigger particles with the negative shift in the quasi-Fermi level indicating a better charge separation and thus a higher reductive power for the photocatalyst.45,46 With respect to the 4.0Au/N-TiO2 sample, TEM image shows that Au particles was dispersed highly and uniformly on the surface of N-TiO2, and the mean particle size of Au is about 2.8 nm. Haruta et.al have shown that gold particles smaller than 5 nm were the most active for the oxidation of CO.47 Orlov et al. have reported the higher activity for smaller Au particles in methyl tert-butyl ether and 4-cholophenol photodegradation over Au/TiO2 supported catalysts.13,15 Our results are in good agreement with literature proposal. While an opposite depen-

Wu et al. dence, higher degradation efficiency under visible light irradiation along with large Au particle size has been reported.48 The correlation of photocatalytic activity between Au particle size and the TiO2 substrate is still under debate. On the other hand, the synergic effect between N species and Au particle are responsible for the enhancement of photoactivity. The minor nitrogen species in the form of N-Ti-O and Ti-O-N is evidenced by the weak signal of 399.0-399.7 eV and 400.3-401.0 eV and interaction between N species and metallic Au particles is confirmed by XPS. Scheme 1(a) illustrates a mechanism of degradation of MO for 4.0Au/N-TiO2 under visible light. It is generally accepted that N-doping can form a new state lie just above the valence band for the substitutional nitrogen, which could narrow the band gap of TiO2 and absorb visible light. In the presence of visible light irradiation, the excited electrons from nitrogen species is transferred to the conduct band of TiO2 and then captured by Au particles. Subsequently, the electron is transferred to oxygen adsorbed on the surface of TiO2, producing O2 · -, which is capable of degrading organic compound. In addition, Tian et al.49 found that the Au-TiO2 as a kind of visible-light sensitive photocatalyst can oxidation ethanol and methanol as well as reduce oxygen. The gold nanoparticles were photoexcited due to plasmon resonance and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO2 conduction band and simultaneous transfer of compesative electrons from a donor in the solution to the gold particle. Kowalska et al. also reported the visible light-induced oxidation of 2-propanol on gold-modified titania was initiated by excitation of gold surface plasmon by using action spectrum analyses and they speculated that an electron may be injected from Au particles into the conduction band of TiO2 and then reduce molecular oxygen adsorbed on the surface of TiO2. The resultant electron-deficient Au could oxidize organic compounds such as 2-propanol to be recovered to the original metallic state.48 Two major roles by Au particles played could be considered. The first one is to transfer electron from the TiO2 surface to adsorbed O2. Another positive effect is that photoexcited electrons of gold surface plasmon may be injected into the TiO2 conduction band, creating separated electrons and holes, which then undergo charge transfer reactions with adsorbates. Therefore, this may not only improve the visible light absorption but also reduce the chance of recombination of photoinduced electron-hole pairs, hence increasing the photocatalytic activity. Similar behavior of the Au/TiNxO2-y system was found to be active for the thermal dissociation of water and the production of H2 through the water-gas shift reaction.23 In order to check the influence of Au plasmon adsorption on photoactivity, photocatalytic degradation of MO (10 mg/L) over N-TiO2 and 4.0Au/ N-TiO2 samples using a UV cutoff filter >550 nm was carried out because MO has no absorption of visible light above

SCHEME 1: Proposed Mechanism for Phototocatalytic Degradation of Organic Pollutant over Au/N-TiO2 Photocatalyst (a) under Visible Light Irradiation and (b) under UV Light Irradiation

Photocatalysis of N-Doped Titania with Au Deposition 550 nm (as shown in Figure 1). The N-TiO2 sample showed low level activity (less than 10%) due to its absorption wavelength at around 450 nm. While 4.0Au/ N-TiO2 exhibited 28% degradation rate of MO under visible light irradiation (550-800 m) for 5 h, which confirm the visible light photocatalytic reaction can be initiated by excitation of gold surface plasmon. Mechanism of degradation of MO for 4.0Au/N-TiO2 under UV light is shown in Scheme 1(b). Different from mechanism under visible light, a lot of electrons and holes generated by band gap excitation of TiO2 under UV light irradiation. It should be noted that the oxygen vacancies by N-doping become the recombination center and lead to lower UV light photoactivity of N-doped TiO2. Our experimental results show that the presence of Au nanoparticles on the surface of N-TiO2 has an important role in improving the UV photocatalytic activity of N-TiO2. Au particles can capture electrons and transfer them to the adsorbed O2 on the surface of TiO2 to form superoxide anion radicals, improving the charge separation and hence inhibition of surface recombination process. In addition, research by Zanella et al.50 and our previous work20 indicated that Au nanoparticles favor deposition on the defects between the anatase crystallite produced by metal ions doped, which could work as particle pinning centers of the gold particles for hindering their diffusion as well as preventing the formation of larger gold particles. Sanz et al. also found that the most favored site for Au atoms are filling into oxygen vacancies when Au is deposited on the surface of reduced TiO2.22 It can be safely concluded that the oxygen vacancies produced by N-doping might decrease after Au nanoparticles deposition on the surface of N-TiO2, and hence inhibition of surface recombination process. 4. Conclusions A novel composite photocatalyst was successfully prepared by depositing Au on N-doped TiO2. This as-prepared photocatalyst presents higher photocatalytic activity than N-doped TiO2 under both UV light and visible light irradiation. Results of various characterization methods indicate an extension of light absorption into the visible region for Au/N-TiO2, and the electron-hole recombination has been effectively inhibited. Au existed as Au0 on the surface of the N-TiO2 photocatalyst, and the nitrogen species was in the form of N-Ti-O and Ti-O-N. Interaction between N species and metallic Au particles would not only improve the visible light absorption but also reduce the chance of recombination of photoinduced electron-hole pairs, which is responsible for the higher photocataltic activity. Acknowledgment. This work has been supported by Shanghai Nanotechnology Promotion Centre (0752 nm001), National Nature Science Foundation of China (20773039), National Basic Research Program of China (973 Program, 2007CB613306, 2004CB719500), and the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710). A part of research work was finished in Shanghai Nanotechnology Joint Lab. References and Notes (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C. 2000, 1, 1. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Chen, X. B.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (4) Paola, A. D.; Marci, G.; Palmisano, L.; Schiavello, M.; Uosaki, K.; Ikeda, S.; Ohtani, B. J. Phys. Chem. B 2002, 106, 637.

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