Great Enhancement of Photocatalytic Activity of Nitrogen-Doped

Jul 1, 2006 - Beijing 100080, P.R. China; College of Physics, Nankai UniVersity, Tianjin ... nonmetal ion doping,9-12 generation of an oxygen vacancy,...
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J. Phys. Chem. B 2006, 110, 14391-14397

14391

Great Enhancement of Photocatalytic Activity of Nitrogen-Doped Titania by Coupling with Tungsten Oxide Bifen Gao,†,‡ Ying Ma,*,† Yaan Cao,§ Wensheng Yang,| and Jiannian Yao*,†,⊥ Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China; College of Physics, Nankai UniVersity, Tianjin 300071, P.R. China; Chemistry College, Jilin UniVersity, Changchun 130023, P.R. China; and Graduate School of Chinese Academy of Sciences, Beijing 100039, P.R. China ReceiVed: April 21, 2006; In Final Form: June 2, 2006

The TiO2-N-x%WO3 composite photocatalysts were prepared by introducing WO3 into nitrogen-doped TiO2. The composite catalysts present much higher photocatalytic activity than TiO2 and nitrogen-doped TiO2 under both ultraviolet and visible light irradiation. Diffuse reflectance UV-vis spectra, XPS analysis, and IR spectra show that the coordinated nitrogen species (or N-metal-O linkages) may contribute to the visible light photocatalytic activity. WO3 coupling increases the active nitrogen species and thus enhances the visible light activity of the composite photocatalysts. The superior activity of TiO2-N-x%WO3 composite photocatalysts upon UV light irradiation can be rationalized in terms of efficient charge separation and high adsorption affinity of WO3.

1. Introduction TiO2 photocatalysis has attracted extensive attention as a promising technique for the degradation of inorganic and organic pollutants in water and air due to the high oxidative power, photostability, and nontoxicity of TiO2.1-4 Generally, TiO2 is only sensitive to UV light due to its large band gap (3.2 eV).5,6 In recent years, great efforts have been made to develop TiO2based photocatalysts sensitive to visible light in order to make use of solar energy more efficiently in practical applications. Some strategies, such as surface modification,7,8 metal or nonmetal ion doping,9-12 generation of an oxygen vacancy,13-15 combination with other semiconductors,16,17 and so forth, have been widely adopted to prepare TiO2 photocatalysts sensitive to visible light. Among these strategies, doping TiO2 with nonmetal elements has been considered one of the most promising ways to develop TiO2 photocatalysts sensitive to visible light.11,12,18 Especially, TiO2 doped with nitrogen has been known to be one of the best visible light photocatalysts up till now.19,20 WO3 coupling has been widely used to improve the photoelectrochemical and photocatalytic performance of TiO2 since WO3 is more acidic than TiO2 and can serve as an electronaccepting species.21-25 A beneficial influence of the addition of WO3 on TiO2 has been observed for some photocatalytic processes, such as the photooxidation of 2-propanol21 and butyl acetate22 and the photodegradation of 1,4-dichlorobenzene23 in aqueous media. Here we report a simple method to improve the photocatalytic activity of nitrogen-doped TiO2 by WO3 coupling. The experimental results show that the composite photocatalysts present higher photocatalytic activity under both visible and UV light irradiation than the TiO2 photocatalyst * Corresponding authors. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. § College of Physics, Nankai University. | Chemistry College, Jilin University. ⊥ Tel: 86-10-82616517. Fax: 86-10-82616517. E-mail: [email protected].

doped only by nitrogen. In addition to serving as an efficient electron acceptor, we found that WO3 apparently increased the nitrogen species sensitive to visible light. 2. Experimental Section 2.1. Catalyst Preparation. Nitrogen-doped titania (TiO2N) was prepared by the hydrolysis of titanium tetrachloride by ammonia. A 25% ammonia solution was added dropwise to aqueous titanium tetrachloride until the pH of this mixture was 5.5. A white precipitate was observed immediately. After aging at room temperature for 24 h, the precipitate was filtered, dried at 70 °C, and then calcined at 400 °C for 4 h. Pure TiO2 was prepared by the same procedure but replacing ammonia by sodium hydroxide. Na+ ions were removed by washing with Milli-Q water, and the complete elimination of Na+ ions was confirmed by XPS measurement. The tungsten oxide-loaded catalysts (TiO2-N-x%WO3, where x represents the nominal molar ratio of WO3 to TiO2) were obtained by dispersing TiO2-N powders in dilute ammonia solution containing different amounts of tungstic acid. The obtained suspension was dried at 70 °C to evaporate the solvent, and the residual powder was annealed at 450 °C for half an hour. 2.2. Characterization. The diffuse reflectance UV-vis absorption spectra were collected on a UV-vis spectrometer (UV3010, Shimadzu). BaSO4 was used as the reflectance standard in the experiment. Infrared transmission spectra were recorded for KBr disks containing the powder sample with an FTIR spectrometer (Bruker Tensor 27). Each spectrum was the result of co-adding 16 scans collected at a 4-cm-1 resolution. Raman spectra were taken on a Renishaw-2000 Raman spectrometer at a resolution of 2 cm-1 by using the 514.5-nm line of an Ar ion laser as the excitation source. XPS measurements were carried out with an SECA Lab 220i-XL spectrometer by using an unmonochromated Al KR (1486.6-eV) X-ray source. All the spectra were calibrated to the binding energy of the adventitious C1s peak at 284.6 eV. The BET surface areas of the samples were determined by nitrogen adsorption-desorption

10.1021/jp0624606 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/01/2006

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Figure 1. Diffuse reflectance UV-vis spectra of TiO2, TiO2-N, and the composite samples. The TiO2-N-NH3 sample was prepared by the same process as TiO2-N-3%WO3 but without tungstic acid.

isotherm measurement at 77 K (Micromeritics Automatic Surface Area Analyzer Gemini 2360, Shimadzu). The samples were degassed at 180 °C prior to actual measurements. The XRD patterns were acquired on a Rigaku D/max 2500 X-ray diffraction spectrometer (Cu KR, λ ) 1.54056 Å) at a scan rate of 0.02° 2θ S-1. The average crystallite size was calculated according to the Scherrer formula (D ) k λ/B cos θ). 2.3. Photoluminescence Measurement. The photoluminescence spectra of the samples were measured with the following procedure. Each sample was dry-pressed into a 10-mm-diameter round disk containing about 200 mg of mass. The sample disks were illuminated with a 10-mW, 325-nm He-Cd laser at an ambient temperature. Then, the PL from the samples was collected and focused into a spectrometer (Spex 1702) and detected by a photomultiplier tube (PMT; Hamamatsu R943). Finally, the signal from the PMT was sent into a lock-in amplifier before being recorded by a computer. 2.4. Evaluation of Photocatalytic Activity. The photocatalytic decomposition of 4-chlorophenol was performed with 1and 5-mg amounts of catalysts suspended in a 40-mL aqueous 4-chlorophenol solution (5 × 10-5 mol L-1) under UV light and visible light irradiation, respectively. A sunlamp (Philips HPA 400/30S, Belgium) was employed for the UV light photocatalytic reaction, and a 400-nm cutoff filter was employed to remove ultraviolet light for the visible light reaction. Prior to photocatalytic reactions, the suspension was magnetically stirred in the dark, and the concentration of 4-chlorophenol was monitored. The 4-chlorophenol concentration did not change after stirring for 30 min, which indicates that 30 min is enough to reach the adsorption equilibrium of 4-chlorophenol. Therefore, all of the suspensions were stirred for 30 min in the dark to ensure adsorption equilibrium before illumination. Oxygen gas was continuously bubbled through the 4-chlorophenol solution at a flux of 5 mL min-1. Variations in the concentration of 4-chlorophenol under illumination were monitored by a UVvis spectrometer with the help of 4-aminoantipyrine as the chromogenic reagent. Each photocatalytic experiment was repeated four times with the photocatalysts, which were prepared independently by the same procedure. Errors corresponding to standard deviation were calculated from these four photocatalytic experiments. 3. Results and Discussion 3.1. UV-Vis Spectra. Figure 1 gives the diffuse reflectance UV-vis absorption spectra of pure TiO2, TiO2-N, and the

Figure 2. (A) Raman spectra of TiO2, TiO2-N, and various WO3loaded samples in the region of 100-900 cm-1; (B) Raman spectra of the TiO2-N-x%WO3 samples: (a) x ) 3, (b) x ) 5, (c) x ) 10, in the region of 700-1100 cm-1.

composite samples. Only a strong absorption in the ultraviolet region attributed to the band-band transition can be observed for pure TiO2. Compared to the spectrum of pure TiO2, TiO2-N presents a significant absorption in the visible region between 400 and 550 nm, which is the typical absorption feature of nitrogen-doped TiO2.10,18,26 This visible light absorption becomes stronger upon WO3 loading on TiO2-N surface and strengthens with the increased amount of the WO3 loaded. This means TiO2-N-x%WO3 is more sensitive to visible light than TiO2 and TiO2-N. To rule out the effects of probable increased nitrogen species incorporated during tungsten oxide loading, TiO2-N was also treated by ammonia solution without tungstic acid and denoted as TiO2-N-NH3. The absorption character of TiO2-N-NH3 is almost in overlapping with that of TiO2N, suggesting that the stronger absorption of TiO2-N-x%WO3 in the visible region should be attributed to the tungsten oxide loading but not the adsorption or doping of more nitrogen species. It is reasonable that the active centers interacting with NH3‚H2O on the TiO2-N surface have been removed after calcination at 400 °C as confirmed by Kuroda et al.27 3.2. Raman and XRD Analysis. Figure 2A shows the Raman spectra of TiO2, TiO2-N, and the composite samples in the 100-900 cm-1 region. The observed characteristic bands at 144, 196, 396, 516, and 638 cm-1, assigned to the Eg, B1g, A1g, B2g, and Eg vibrational modes of TiO2, respectively, indicate the

Enhancing Photocatalytic Activity of N-Doped TiO2

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TABLE 1: Crystallite Size and Specific Surface Area of Pure TiO2, TiO2-N, and the Composite Samples sample

TiO2

TiO2-N

TiO2-N-3%WO3

TiO2-N-5%WO3

TiO2-N-10%WO3

crystallite size/nm SBET/m2 g-1

18 44.9

12 87.5

12 85.9

12 85.2

12 84.5

Figure 3. XRD patterns of TiO2, TiO2-N, and the TiO2-N-x%WO3 samples. Arrows indicate the diffraction peaks of crystalline WO3.

presence of the anatase phase in all these samples.28 However, for the TiO2-N-10%WO3 sample, besides the vibrational modes of anatase TiO2, three additional peaks were observed at 271,713, and 805 cm-1, which are characteristic of monoclinic tungsten oxide.29 No discrepancy can be found among the other samples. Figure 2B presents the Raman spectra of the TiO2N-x%WO3 samples in the region of 700-1100 cm-1. Two peaks at 795 and 965 cm-1 are observed for the TiO2-N3%WO3 sample. The peak around 795 cm-1 is attributed to the second-order feature of anatase TiO2,30 while the peak at 965 cm-1 is attributed to the symmetrical WdO stretching mode of the dispersed tungsten oxide species on the surface of TiO2N.31 It has been reported that at least 3 mol % of WO3 is needed to cover the TiO2 surface with monolayer thickness,23,32 so it is likely that TiO2-N is covered with a monolayer of WO3 for TiO2-N-3%WO3. With the increase of the WO3 loading, the peak shifts from 965 to 975 cm-1, suggesting the increased polytungstate species with an octahedral environment.33 For TiO2-N-10%WO3, in addition to the WdO stretching mode at 975 cm-1, the peak at 805 cm-1 assigned to the W-O stretching mode of monoclinic tungsten oxide clearly shows up and overwhelms the peak at 795 cm-1, as shown in curve c of Figure 2B. X-ray powder diffraction measurements (Figure 3) show results similar to those of the Raman study. Only the diffraction peaks of anatase TiO2 can be observed in most samples except for TiO2-N-10%WO3, in which a weak peak at around 23.1° also shows up, indicating the presence of a small amount of monoclinic WO3. From the (101) peak of anatase TiO2, a crystallite size of about 18 and 12 nm for pure TiO2 and TiO2-N, respectively, can be roughly estimated. The WO3 loading does not have much influence on the crystallite size. As shown in Table 1, the crystallite size of the composite catalysts is similar to that of the corresponding matrix, whereas, compared to that of TiO2-N, the specific surface areas of the TiO2-N-x%WO3 catalysts decrease gradually, from 87.5 (x ) 0) to 84.5 (x ) 10), with the increase of the WO3 loading (Table 1). 3.3. TEM Observation. TEM observation (Figure 4A) reveals that the TiO2-N-3%WO3 sample consists of agglomerates of primary particles of 10-20 nm in diameter, which is in

Figure 4. TEM (A) and HRTEM (B) images of the TiO2-N-3%WO3 sample.

general agreement with the XRD determination. From the HRTEM image (Figure 4B), the uniform lattice fringes of titania can be observed over an entire primary particle, and no WO3 clusters are found around titania. This suggests that WO3 is uniformly dispersed on the surface of TiO2-N probably with monolayer thickness.21,23 3.4. XPS and FTIR Spectroscopy. The states of nitrogen in TiO2-N and the TiO2-N-x%WO3 samples were analyzed by XPS. As shown in Figure 5, none of the catalysts shows the peak at 396-397 eV attributed to atomic nitrogen.18 A broad peak extending from 396 to 403 eV with a maximum at about 399.5 eV is observed for TiO2-N. TiO2-N-3%WO3 shows an unsymmetric N1s peak centered at 399.5 eV, and the intensity

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Figure 5. N1s XPS spectra of (a) TiO2-N, (b) TiO2-N-3%WO3, and (c) TiO2-N-10%WO3.

of the higher-binding-energy side is stronger than that of TiO2N. A weak broad feature peaking at 401.5 eV is found to be strongly skewed to a higher binding energy for the TiO2-N10%WO3 sample. The signal of the higher-binding-energy side becomes stronger and the signal around 399.5 eV becomes weaker with the increased amount of the WO3 loaded. The origin of the broad peak in the 396-403-eV region is still under debate and subject to much controversy. Asahi et al.18 and Kisch34 attributed the N1s peaks at binding energies at 400 and 402 eV to molecularly adsorbed nitrogen species. These assignments are also supported by Saha and Tomkins35 in their XPS study of the oxidation of titanium nitride films. Burda et al.36 observed an N1s core level at 401.3 eV in nitrogen-doped titania nanoparticles and suggested that it is attributed to the N atom in the environment of O-Ti-N. A symmetric peak at 398.2 eV observed by Viswanath37 and Ma38 was also assigned to the O-Ti-N linkages. Considering the weakening of the peak at 399.5 eV during the surface loading of WO3, we propose that the signal at 399.5 eV arises from adsorbed nitrogen species such as hyponitrite. 34,39 Upon WO3 loading, the N1s core level shifts to a higher binding energy, indicating the decrease of the electron density of the N atom. In other words, some N atoms become more positive after WO3 coupling. It is likely that nitrogen species coordinated to tungsten loaded to form the O-W-N linkage, similar to the O-Ti-N observed by Burda et al. To get further evidence, we also examined the W states by XPS spectra. Figure 6 presents the W4f and W4d XPS spectra of TiO2-N-3%WO3 and WO3. WO3 shows typical W4f7/2 and W4f5/2 peaks at 35.3 and 37.4 eV, respectively. Although the W4f5/2 peak and Ti3p overlap in TiO2-N3%WO3, it still can be clearly seen that the W4f7/2 peak of TiO2-N-3%WO3 shifts to a lower binding energy compared to that of WO3. To exclude the interference from Ti, W4d was also analyzed by XPS, and the results are shown in Figure 6, too. The W4d peaks of TiO2-N-3%WO3 experience the same shift as W4f peaks compared to those of WO3. The shift of W4f and W4d peaks to a lower binding energy indicates higher electron density of the W atom in the TiO2-N-3%WO3 sample. The peaks of N1s and W4f (W4d) shift in opposite directions

Figure 6. (A) W4f and (B) W4d XPS spectra of TiO2-N-3%WO3 (solid line) and WO3 (dot line).

and further suggest the linkage of N and W in TiO2-N3%WO3. FT-IR spectra of TiO2, TiO2-N, and the WO3-loaded samples in the 400-2000 cm-1 region are shown in Figure 7. All the samples show a main band at 400-800 cm-1 and a relatively strong band at 1642 cm-1. The former is attributed to Ti-O stretching and Ti-O-Ti bridging stretching modes, while the latter is attributed to the bending vibration of H2O adsorbed on the surface of catalysts.40,41 In curve b, a new band at 1385 cm-1, which was assigned to the vibrational mode of hyponitrite,34,39 is observed for the TiO2-N sample. This agrees well with the aforementioned XPS analysis. In curves c and d, when WO3 was loaded on TiO2-N, the band at 1385 cm-1 decreases its intensity and cannot be detected, whereas another band at 1401 cm-1 occurs. The occurrence of this new band at 1401 cm-1 may suggest the formation of coordinated nitrogen species,42 in accordance with the above XPS analysis. 3.5. PL Spectra. The PL emission spectra can be useful to disclose the efficiency of charge carrier trapping, immigration, and transfer and to understand the fate of electrons and holes in the semiconductor since PL emission results from the recombination of free carriers. Figure 8 shows the PL spectra of TiO2, TiO2-N, TiO2-N-3%WO3, and TiO2-N-10%WO3. Two peaks around 450 and 540 nm are observed for the pure TiO2 catalyst. Generally, the former is attributed to surface defects, such as an oxygen vacancy.43 The latter is ascribed to the charge transfer from Ti3+ to the oxygen anion in a TiO68complex.44,45 The emissions are significantly weakened in

Enhancing Photocatalytic Activity of N-Doped TiO2

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TABLE 2: Photodegradation of 4-Chlorophenol under Visible Light Irradiation (λ g 400 nm) sample

4-chlorophenol degradeda (c0 -c)/c0

kb (min-1)

t1/2 (min)

specific photocatalytic activity (mol g-1 h-1)

blankc TiO2 TiO2-N TiO2-N-3 %WO3

0 0.065 ( 0.005 0.131 ( 0.007 0.291 ( 0.028

(3.66 ( 0.34) × 10-4 (1.43 ( 0.12) × 10-3 (3.52 ( 0.28) × 10-3

(1.89 ( 0.18) × 103 (4.88 ( 0.40) × 102 (1.97 ( 0.16) × 102

0 (1.24 ( 0.10) × 10-5 (2.61 ( 0.14) × 10-5 (5.68 ( 0.55) × 10-5

a After reaction for 2 h. b Apparent rate constant deduced from the linear fitting of ln(c0/c) versus reaction time. c The blank was the photolysis of 4-chlorophenol.

Figure 7. FTIR spectra of (a) TiO2, (b) TiO2-N, (c) TiO2-N3%WO3, and (d) TiO2-N-10%WO3.

Figure 8. Photoluminescence spectra of (a) TiO2, (b) TiO2-N, (c) TiO2-N-3%WO3, and (d) TiO2-N-10%WO3.

nitrogen-doped titania. Further decrease of PL intensity can be seen upon WO3 coupling with TiO2-N. The obvious lower PL intensity at 540 nm in TiO2-N and TiO2-N-x%WO3 implies that the recombination of charge carriers is effectively suppressed, which reasonably leads to a higher photocatalytic activity since the photodegradation reactions are evoked by these charge carriers. The lower PL of TiO2-N may be attributed to the efficient capture of charge carriers by the surface states related to the nitrogen species. The weakened PL of the TiO2-

N-x%WO3 catalyst may be partly due to the increased active nitrogen species as well as to the photoinduced electron transfer from TiO2-N to WO3.24,25 It is also notable that TiO2-N10%WO3 shows a higher PL intensity than TiO2-N-3%WO3, which is probably due to too many surface states serving as recombination centers. 3.6. Photocatalytic Activity. The photodegradation of 4-chlorophenol is employed to evaluate the photocatalytic activities of pure TiO2, TiO2-N, and the composite catalysts, and the experiment results are illustrated in Tables 2 and 3, respectively. For all of the samples, the ln(c0/c) values of 4-chlorophenol show a linear relationship with irradiation time, suggesting a pseudo-first-order reaction. Under visible light irradiation with λ g 400 nm (Table 2), the pure TiO2 photocatalyst shows a low photocatalytic activity, and nitrogen-doped TiO2 (TiO2N) shows a much higher visible light activity than TiO2. In contrast, the composite catalyst TiO2-N-3%WO3 presents an even higher visible light activity than TiO2-N. The photocatalytic activity, that is, the photodegraded rate and the specific photocatalytic activity, of TiO2-N-3%WO3 is about 2.2 times that of TiO2-N, confirming that coupling TiO2-N with WO3 is an effective way of preparing TiO2-based materials with a visible light photoactivity. In comparison with TiO2-N, the composite photocatalysts TiO2-N-x%WO3 show an improved visible light activity in a wide range of WO3 loading as shown in Figure 9. The highest activity is observed at 3 mol % tungsten loading, and about 73.7% of 4-chlorophenol was degraded after reaction for 6 h under visible light irradiation (λ g 400 nm) in the TiO2-N-3%WO3 suspension. In the case of UV light irradiation (Table 3), the three samples show activity in the order TiO2-N-3%WO3 > TiO2-N > TiO2. The overall trend of the activity is very similar to that under visible light irradiation, and the superior catalytic activity of TiO2-N-3%WO3 is evident. It has been proven that the monolayer coverage with WO3 on the surface of TiO2 increases the photocatalytic activity of TiO2 in the UV region due to either the efficient charge separation or high adsorption affinity.21,23 This is the reason the composite catalyst TiO2-N-3%WO3 shows higher activity than TiO2-N under UV light irradiation. However, TiO2-N3%WO3 presents only 1.3-1.4 times the photocatalytic activity of nitrogen-doped TiO2 (Table 3). In the case of visible light irradiation, the photocatalytic activity of TiO2-N-3%WO3 is much higher than that of TiO2-N, indicating that efficient charge separation and high adsorption affinity may be not be the only contribution to the increased photoactivity as in the UV region. The origin of the visible light response in nitrogen-doped titania has been extensively investigated but remains under dispute to date. Asahi et al.18 consider that this arises from band narrowing by mixing of N2p and O2p orbitals. Kisch10,34 and Gole46 attribute the visible light photocatalytic activity to the generation of a great deal of surface states that are located close to the valence band edge. If this visible light absorption stems from the surface hyponitrite detected in XPS, it will decrease

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TABLE 3: Photodegradation of 4-Chlorophenol under Ultraviolet Irradiation sample

4-chlorophenol degradeda (c0 - c)/c0

kb(min-1)

t1/2(min)

specific photocatalytic activity (mol g-1 h-1)

blankc TiO2 TiO2-N TiO2-N-3%WO3

0.084 ( 0.009 0.614 ( 0.026 0.647 ( 0.051 0.919 ( 0.083

(1.50 ( 0.17) × 10-3 (1.44 ( 0.21) × 10-2 (2.21 ( 0.24) × 10-2 (4.51 ( 0.51) × 10-2

462.1 ( 51.8 48.1 ( 6.8 31.8 ( 3.4 15.4 ( 1.7

(1.28 ( 0.05) × 10-3 (1.34 ( 0.10) × 10-3 (1.91 ( 0.17) × 10-3

a After reaction for 1 h. b Apparent rate constant deduced from the linear fitting of ln(c0/c) versus reaction time. c The blank was the photolysis of 4-chlorophenol.

visible light, and the monolayer coverage of WO3 promotes the separation of photogenerated carriers. Thus the composite TiO2N-x%WO3 photocatalyst presents improved photocatalytic activity compared to TiO2 and nitrogen-doped TiO2. It is expected that extended investigations on such composite TiO2N-x%WO3 catalysts will be helpful to prepare a TiO2-based photocatalyst suitable for practical applications. Acknowledgment. This work was supported by National Natural Science Foundation of China (Nos. 50221201, 90301010, 50502033, 50472035), the Chinese Academy of Sciences, and the Natural Science Foundation of Tianjin City (Grant No. 043612411). We thank Dr. Hongwei Ma (Institute of Chemistry, Chinese Academy of Sciences) for calculation of crystal parameters and Dr. Jianfeng Wang (Institute of Semiconductors, Chinese Academy of Sciences) for PL measurement. Figure 9. The photodegradation of 4-chlorophenol after visible light irradiation (λ g 400 nm) for 6 h in the suspensions of TiO2-N loaded with different content of tungsten oxide.

upon WO3 loading since such nitrogen species decrease as evidenced by XPS analysis. On the contrary, the visible light response becomes stronger with the increase of the tungsten loading and the generation of the N-W-O linkage in our experiments. So the surface hyponitrite or NHx-containing species is unlikely to be responsible for the visible light response of TiO2-N. The minor nitrogen species in the form of N-Ti-O which is evidenced by the weak signal at 401.5 eV in the N1s XPS of TiO2-N catalyst is proposed to contribute to the visible light response.36 It is well-known that WO3 retains a much higher Lewis surface acidity than TiO2 and has a higher affinity for chemical species having unpaired electrons.21,22 Therefore, it is much easier for nitrogen species to coordinate with tungsten dispersed on TiO2 surface than TiO2 itself to form N-W-O linkages, which also contribute to visible light responses. The active nitrogen species increased considerably after WO3 loading, thus the visible light activity of TiO2-N-x%WO3 catalysts is improved. When the loaded WO3 exceeds 5 mol %, the photocatalytic activity of TiO2-N-x%WO3 decreases even though the visible light absorption still increases. On one hand, increased active nitrogen species may serve as the recombination centers of photogenerated electrons and holes. On the other hand, the charge transfer of the composite catalysts to O2 or target molecules could be retarded with too much WO3 coupling.25 4. Conclusion We successfully prepared a new kind of composite photocatalysts by coupling WO3 with nitrogen-doped TiO2. The composite photocatalyst presents higher photocatalytic activity than TiO2 and nitrogen-doped TiO2 under both UV and visible light irradiation. The surface states of nitrogen due to the generation of the N-Ti-O or N-W-O linkage in the photocatalyst allow the more efficient utilization of both UV and

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