Carbon and Nitrogen Co-doped TiO2 with Enhanced Visible-Light

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Ind. Eng. Chem. Res. 2007, 46, 2741-2746

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MATERIALS AND INTERFACES Carbon and Nitrogen Co-doped TiO2 with Enhanced Visible-Light Photocatalytic Activity Daimei Chen, Zhongyi Jiang, Jiaqing Geng, Qun Wang, and Dong Yang* Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin, 300072, People’s Republic of China

To utilize visible light more efficiently in photocatalytic reactions, carbon-doped TiO2 (C-TiO2), nitrogendoped TiO2 (N-TiO2), and carbon and nitrogen co-doped TiO2 (C-N-TiO2) nanoparticles with different nitrogen and carbon contents were prepared by a sol-gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. XRD results showed that the doping of C and N atoms could suppress the crystal growth of TiO2, and the effect of C doping was more pronounced than that of N doping. XPS, UV-vis spectroscopy, and lattice parameter analysis revealed that N atoms could incorporate into the lattice of anatase through substituting the sites of oxygen atoms, while most C atoms could form a mixed layer of deposited active carbon and complex carbonate species at the surface of TiO2 nanoparticles. The photocatalytic activities of obtained C-TiO2, N-TiO2, and C-N-TiO2 samples with different C and N contents were evaluated by methylene blue degradation under visible light irradiation. It was found that C-N-TiO2 nanomaterials exhibited the highest photocatalytic activity, which could be assigned to the synergistic effect of doped C and N atoms. Introduction Titanium dioxide (TiO2) is the most widely used photocatalyst for the decomposition of various organic pollutants because of its cheapness and nontoxicity, in addition to its high activity, optical properties, and chemical stability. The band gap energy of TiO2 (3.0-3.2 eV) requires UV light irradiation, and thus only a small portion of the solar spectrum is absorbed in the UV region (λ < 380 nm). Hence, much effort1 has been devoted to preparing a TiO2 photocatalyst that is capable of efficient utilization of the visible light constituting the main part of the solar spectrum. Until now, several strategies including the doping of TiO2 with transition metals,2-4 the synthesis of reduced forms with TiO2-x structure by the plasma treatment (i.e., formation of oxygen vacancy),5,6 anchoring organic dyes onto the surface of TiO2,7,8 and the doping of TiO2 with anionic nonmetals have been investigated. Among them, nonmetal dopants, such as C,9,10 N,11-13 S,14,15 P,16 and halogen atoms,17-19 may be more appropriate for the extension of photocatalytic activity of TiO2 into the visible region than other methods because their impurity states are near the valence band edge, but they do not act as charge carriers, and their role as recombination centers might be minimized compared to metal cation doping.20 Among all nonmetal-doped TiO2 materials, nitrogen- and carbon-doped TiO2 nanomaterials have been found to exhibit superior photocatalytic activity under visible light irradiation. Until now, much effort has been focused on the research of nitrogen-doped TiO2 (N-TiO2), because the doping of N atoms can effectively narrow the band gap of TiO2. However, different conclusions concerning the state of doped nitrogen in the * To whom correspondence should be addressed. Tel.: +86-2227892143. Fax: +86-22-27892143. E-mail: [email protected].

N-TiO2 lattice and the mechanism of band gap narrowing have been derived. For example, Asahi et al.21 proposed that substitutional-type doping using N was effective for the band gap narrowing of TiO2 due to the mixing of N 2p with O 2p states in the valence band based on spin-restricted local density approximation calculations on anatase. Another viewpoint22,23 confirmed that interstitial-type doping of N atoms was related to the photothreshold energy decrease, which induced localized N 2p states within the band gap just above the top of the valence band, facilitating the production of oxygen vacancies. As for the investigation of TiO2 doping with carbon, similar to the N doping, there was also a debate that the doped type of carbon is substitutional20,24,25 or interstitial,13,26-28 which was accounted for by the crystal form and synthesis method of TiO2. In addition, some reports9,10 suggested that large band gap narrowing could also originate from residual carbon-containing species with complex composition, through incomplete removal of organic compounds in the calcination procedure. More recently, the simultaneous doping of two kinds of atoms into TiO2 has attracted considerable interest, since it can result in a higher photocatalytic activity and peculiar characteristics compared with single element doping into TiO2. For example, Li et al.29,30 found that N-F co-doped TiO2 nanomaterials had a higher visible-light photocatalytic activity than TiO2 doped solely with nitrogen or fluorine. This can be ascribed to a synergistic effect of two elements codoping, in which the doped N atoms improved the visible light absorption and the doped F atoms led to the enhancement of surface acidity and the adsorption of reactant. Cong et al.31 synthesized C-N-TiO2 nanoparticles by a microemulsion-hydrothermal process and researched their structure and photocatalytic activity simply. However, they did not deeply discuss the reason for the increase of photocatalytic activity; moreover, their preparation method

10.1021/ie061491k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007

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also had some disadvantages including low yield, use of an organic solvent, high cost, etc., which retarded the practical application of TiO2. Therefore, it is necessary to find a facile way to prepare C-N co-doped TiO2 nanomaterials and elucidate the mechanism of the synergistic effect of carbon and nitrogen codoping. In this study, C-doped, N-doped, and C-N co-doped TiO2 nanoparticles were prepared by a simple sol-gel method, and their intrinsic characteristics were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. The photocatalytic activity of as-synthesized TiO2 samples were evaluated by the degradation rate of methylene blue (MB) under visible light irradiation, and the synergistic effect of C and N co-doping was discussed. By making use of the codoping of nitrogen and carbon in TiO2, we expect that the resultant TiO2 nanomaterials will have high visible-light photocatalytic activity and can be used in practice. Experimental Section Preparation of Photocatalysts. Titanium tetra-n-butyl oxide (TTB), urea, and tetrabutylammonium hydroxide (TBAH) were used as the precursors of TiO2, nitrogen, and carbon, respectively. All chemicals used in the experiment were of analytical reagent grade. C-N-TiO2 nanoparticles were prepared by the following procedure: 0.02 mol of TTB was added dropwise to 50 mL of mixed aqueous solution of urea and TBAH under vigorous stirring at room temperature. Subsequently, the resulting solution was stirred in a closed beaker at room temperature for 24 h to further hydrolyze TTB, and a sol solution containing monodisperse TiO2 nanoparticles was obtained. The sol solution was dried at 100 °C for 8 h in air to vaporize water and alcohol to obtain xerogel, and then calcined at high temperature in air for 1 h to make TiO2 crystallize. The preparations of C-TiO2 and N-TiO2 are the same as that of C-N-TiO2 nanoparticles except for the difference of the doped substance. A series of C-TiO2, N-TiO2, and C-N-TiO2 were prepared by changing the weight of urea and the volume of 25% TBAH solution. Here, TiO2 samples doped with different inorganic elements were denoted as Cx-Ny-TiO2, Cx-TiO2, and Ny-TiO2, where “x” and “y” represent the volume of 25% TBAH solution (x ) 1, 1.5, 3) and the weight of urea (y ) 1, 3, 6, 8), respectively. In this work, x cannot exceed 3; otherwise TiO2 xerogel cannot form even when the sol solution is stirred for several days because TBAH can suppress the hydrolysis of TTB. For comparison, undoped TiO2 nanoparticles were also prepared with the same procedure. Characterization. The particle morphology was observed with a JEM-100CX II transmission electron microscope (TEM). The Brunauer-Emmett-Teller (BET) surface areas (SBET) of the powder samples were determined by nitrogen adsorptiondesorption isotherm measurements at 77 K on a CHEMBET3000 nitrogen adsorption apparatus. The powder X-ray diffraction (XRD) patterns were obtained with a Philips X’Pertpro diffractometer using Co KR radiation with an accelerating voltage of 40 kV and current of 40 mA, which was used to determine the crystallite size and identity of TiO2 samples. The surface property of TiO2 samples was characterized by X-ray photoelectron spectroscopy (XPS) in a Perkin-Elmer PHI 1600 ESCA system with a monochromatic Mg KR source and a charge neutralizer. The UV-vis spectra of TiO2 samples in the 200-1100 nm range were recorded using a Perkin-Elmer Lambda35 UV-vis spectrophotometer equipped with an integrating sphere, and BaSO4 was used as the reference.

Figure 1. XRD patterns of (a) undoped TiO2, (b) C3-TiO2, (c) N3-TiO2, and (d) C3-N3-TiO2 calcined at (A) 400 and (B) 500 °C, respectively.

Measurement of Photocatalytic Activity. The photocatalytic activity under visible light irradiation of Cx-TiO2, Ny-TiO2, and Cx-Ny-TiO2 nanoparticles was evaluated on the basis of the degradation rate of methylene blue. A 100 mL volume of methylene blue (1.8 × 10-5 M) aqueous solution containing 0.125 g of photocatalyst powder was mixed in a 250 mL beaker. Prior to photocatalytic reaction, the suspension was allowed to reach adsorption equilibrium with the photocatalyst in darkness. The irradiation was performed with a 150 W xenon arc lamp that was installed in a light-condensing lamp housing, and a 420 nm cutoff filter (420 nm > λ > 700 nm; Schott Glass) was placed in front of the reaction vessel to obtain visible light. The reaction solution was bubbled with air at a rate around 0.5 dm3/ min continuously. At a defined time interval, the concentration of methylene blue in the photocatalytic reaction was analyzed using the UV-vis spectrophotometer at 664 nm. For comparison, the same experiment was also done in the presence of pure TiO2 under the same conditions. Results and Discussion Crystal Structure. XRD was carried out to investigate the crystal identity of TiO2 samples and the effect of carbon and nitrogen doping on the crystal structure of TiO2. Figure 1 shows the XRD patterns of undoped TiO2, C3-TiO2, N3-TiO2, and C3-N3-TiO2 nanoparticles calcined at 400 °C (Figure 1A) and 500 °C (Figure 1B), respectively. From these patterns, it is found that all samples are of the anatase phase, and C3-TiO2, N3TiO2, and C3-N3-TiO2 samples have a clear peak broadening due to the smaller crystalline size of TiO2 nanoparticles versus that of undoped TiO2. The crystallite sizes of eight samples calculated with the Scherrer formula are listed in Table 1, suggesting that the doping of nitrogen and carbon atoms in TiO2 can remarkably suppress the crystal growth; the effect of carbon doping is more pronounced than that of nitrogen doping. Based on Bragg’s law (2d sin θ ) λ) and a formula for a tetragonal system,

h2 + k2 l2 l ) + 2 2 d a2 c the lattice parameters of eight TiO2 samples were obtained and are listed in Table 1. The lattice parameter of N3-TiO2 nanoparticles in the c-axis is a little larger than that of undoped TiO2 sample, indicating a lattice expansion along the c-axis due to the incorporation of nitrogen atoms. This result hints that nitrogen atoms may substitute the sites of oxygen atoms in the lattice of anatase, which causes the increase of the unit cell. When organic carbon is doped into TiO2 nanoparticles, the lattice parameters change little. Considering that the TBA+ ion,

Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2743 Table 1. Summary of Physicochemical Properties of TiO2 Samples Calcined at 400 and 500 °C sample

calcination temperature (°C)

undoped TiO2 C3-TiO2 N3-TiO2 C3-N3-TiO2 undoped TiO2 C3-TiO2 N3-TiO2 C3-N3-TiO2

400 400 400 400 500 500 500 500

Lattice Parameter (Å) a c 3.7910 3.7923 3.7902 3.7989 3.7875 3.7867 3.7840 3.7726

the carbon precursor, can be surrounded by water molecules due to ion-dipole interaction, they hardly diffusion into the TiO2 bulk. Thus, it is inferred that carbon atoms are difficult to weave into the TiO2 lattice, and most likely exist on the surface of TiO2 nanoparticles and form a layer composed of complex carbonate species, resulting in the suppression of TiO2 crystal growth. Figure 2 represents the typical TEM images of TiO2 samples doped with different elements calcined at 500 °C for 1 h. It can be observed that the aggregation of C3-TiO2 and C3-N3-TiO2 is a little more serious than that of undoped TiO2 and N3TiO2, suggesting that the doped carbon species can locate at the surface of TiO2 nanoparticles and act a role of linker between the aggregated nanoparticles. Correspondingly, the BET surface areas of TiO2 samples containing carbon element, C3-TiO2 (34.8 m2/g) and C3-N3-TiO2 (39.5 m2/g), are smaller than that of TiO2 samples without carbon element, undoped TiO2 (48.5 m2/g) and N3-TiO2 (49.5 m2/g). However, phase separation in doped TiO2 samples is not observed, indicating that they all possess a homogeneous structure. XPS Studies. Figure 3 illustrates the XPS survey spectra of N3-TiO2 and C3-N3-TiO2, respectively, which show that both N3-TiO2 and C3-N3-TiO2 samples contain C, O, Ti, and N elements. Undoped TiO2 and C3-TiO2 only have C, O, and Ti elements: the atomic composition of C, O, and Ti elements for C3-TiO2 is 53.6, 34.7, and 11.7 atom %, respectively, and for undoped TiO2 it is 48.3, 38.4, and 13.3 atom %, respectively. According to XPS element analysis, N atomic composition in C3-N3-TiO2 is 1.5%, about 2 times that in N3-TiO2 sample, indicating that TBAH can apparently facilitate nitrogen uptake. It is inferred that TBAH can suppress the hydrolysis of TiO2 colloidal solution, and thus smaller TiO2 nanoparticles form, which makes the nitrogen uptake much easier due to larger lattice strain and lattice distortion.32-34 To further investigate the states of the doped N atoms, high-resolution and curve fitting of N 1s XPS spectra of two samples were recorded and are shown in Figure 4. A pair of N 1s features is observed: one is at around 399.9 eV, which is assigned to chemisorbed N2 molecules on the surface of TiO2 nanoparticles; the other is at 396.0 eV, which can be attributed to substitutionally bound N-. The two-feature XPS spectrum is comparable to that presented by Asahi et al.21 and Diwald et al.,35 confirming that nitrogen atoms undoubtedly incorporate into TiO2, and may substitute the sites of oxygen atoms. Similarly, C 1s XPS spectra of undoped TiO2, N3-TiO2, C3TiO2, and C3-N3-TiO2 powders calcined at 500 °C were recorded and analyzed, and are shown in Figure 5. It can be found that undoped TiO2 and N3-TiO2 only share a peak at 284.4 eV, which can be ascribed to adventitious hydrocarbon from the XPS instrument itself. As for C3-TiO2 and C3-N3TiO2, they have the same composition of peaks, and include three other peaks at 282.5, 286.5, and 288.4 eV. The first peak arises from active carbon of TBAH residual on the surface of

9.4449 9.4483 9.5130 9.5259 9.4745 9.4793 9.5244 9.5477

crystalline size (nm)

degradation of MB after 7 h (%)

band gap (eV)

12.6 8.7 9.3 9.0 22.2 12.9 20.3 12.7

26 70 61 90 21 51 45 69

3.15 2.97 3.10 2.95 3.22 3.05 3.17 3.01

TiO2, and the other two suggest the existence of C-O and Cd O, respectively, which indicates the formation of carbonated species.36 A peak around 281 eV resulting from Ti-C bond37 was not observed, so carbon does not substitute oxygen atom in the lattice of anatase. Considering that TiO2 powders containing carbon element present only the anatase phase in their XRD spectra, and their lattice parameters hardly change compared with those of undoped TiO2, it can be further speculated that the doped carbon can form a layer on the surface of TiO2 nanoparticles with a complex mixture of active carbon and carbonate species. This layer can retard the crystal growth and increase the absorption ability of TiO2 for visible light like organic dyes as the photosensitizer. UV-Vis Absorption Spectra. To study the optical response of doped TiO2 nanoparticles, their UV-vis absorption spectra were measured. As shown in Figure 6, a noticeable shift of absorption edge to the visible light region was observed for the C3-TiO2, N3-TiO2, and C3-N3-TiO2 samples in comparison with undoped TiO2. The band gap of all samples calcined at 400 and 500 °C are listed in Table 1, indicating that both carbon and nitrogen doping can evidently narrow the band gap of TiO2. Meanwhile, C3-N3-TiO2 sample has the strongest absorption for visible light, followed by C3-TiO2 and N3-TiO2, which may be assigned to the synergistic effect of carbon and nitrogen codoping. Additionally, TiO2 samples calcined at 400 °C (Figure 6A) have much stronger absorption in the visible light region than the sample calcined at 500 °C (Figure 6B). The reason may come from the fact that the amount of N and C doping in TiO2 decreases with the increase of calcination temperature, as in an earlier report.38 Through carefully observing the UV-vis absorption spectrum of N3-TiO2 nanoparticles in Figure 6A, one can find that it has a new absorption band around 420 nm as a shoulder peak, indicating the formation of an impurity energy level with the band gap. It is estimated that the induced effect of nitrogen doping allowing sub-band-gap excitation is due to an isolated N 2p state rather than to mixed states of N 2p and O 2p, in good agreement with a few early reports.22,23 Photocatalytic Activity. The photocatalytic activity of TiO2 samples was evaluated by measuring the decomposition rate of methylene blue under visible light irradiation. Figure 7 shows the degradation of methylene blue (MB) without any solid and in the presence of undoped TiO2, C3-TiO2, N3-TiO2, and C3N3-TiO2 after 7 h under visible light irradiation. As shown in Figure 7A, the direct visible light illumination without any solid can lead to only about 5% decomposition of MB molecules within 7 h, which can be omitted. Under the same reaction condition, the photodegradation efficiency of pure TiO2 nanoparticles calcined at 400 °C is about 26% (Figure 7B), which can be accounted for by the photosensitized capability of MB molecules. C3-TiO2, N3-TiO2, and C3-N3-TiO2 all have higher photocatalytic activity than undoped TiO2; moreover, the photocatalytic activity of C3-N3-TiO2 is higher than that of

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Figure 3. XPS spectra of (A) N3-TiO2 and (B) C3-N3-TiO2 samples.

Figure 4. N 1s high-resolution XPS spectra of (A) N3-TiO2 and (B) C3N3-TiO2 samples calcined at 500 °C.

Figure 5. C 1s high-resolution XPS spectra of (A) undoped TiO2, (B) N3-TiO2, (C) C3-TiO2, and (D) C3-N3-TiO2 samples calcined at 500 °C.

Figure 2. TEM photographs of (a) undoped TiO2, (b) C3-TiO2, (c) N3TiO2, and (d) C3-N3-TiO2 calcined at 500 °C.

C3-TiO2 and N3-TiO2, indicating the existence of a synergistic effect of carbon and nitrogen codoping. Figure 8 shows the change of MB concentration with reaction time for Cx-TiO2 (Figure 8A) and Ny-TiO2 samples calcined at 500 °C (Figure 8B). From Figure 8A, it can be seen that the photocatalytic activity of Cx-TiO2 monotonically increases with the increase of x value, because the visible light absorption becomes higher with the increase of carbon content. However, for the Ny-TiO2 samples, there is an inflection point when y ) 6, before which the photocatalytic activity slightly increases and after which the photocatalytic activity starts to decrease, with

Figure 6. UV-vis absorption spectra of TiO2 samples calcined at (A) 400 and (B) 500 °C.

the increase of carbon content. It is reported39 that apart from introducing an isolated band above the valence band edge, nitrogen doping also introduces a new set of states positioned energetically in a 1.30 eV broad range just below the conduction band edge, which can act a recombination center for holes and electrons. When the y value enhances, the number of nitrogen atoms replacing the oxygen sites increase, thus causing an

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Figure 7. Degradation of methylene blue under visible light irradiation (A) without any solid and in the presence of different TiO2 samples calcined at 400 °C ((B) undoped TiO2; (C) N3-TiO2; (D) C3-TiO2; (E) C3-N3TiO2).

Figure 9. Degradation of methylene blue on (A) C3-Ny-TiO2 samples with different y values calcined at 500 °C and (B) C3-N3-TiO2 samples calcined at different temperatures.

Figure 8. Degradation of methylene blue on (A) Cx-TiO2 and (B) NyTiO2 samples with different x and y values calcined at 500 °C.

increase of oxygen vacancy and Ti3+, leading to the enhancement of photocatalytic activity. Some oxygen vacancies and Ti3+ sites become the recombination centers of photoproduced holes and electrons when the y value is enhanced continuously, so the photocatalytic activity begins to decrease. The photocatalytic activity of a series of Cx-Ny-TiO2 samples calcined at 500 °C was also measured, and the typical results of C3-Ny-TiO2 samples with different y values and C3N3-TiO2 samples calcined at different temperatures are shown in Figure 9. When fixing the y value and changing only the x value, the photocatalytic activity becomes larger gradually with the increase of carbon content, in which x ) 3 is maximum. However, it becomes a different change when only the y value is changed. As shown in Figure 9A, C3-N3-TiO2 has the highest photocatalytic activity followed by C3-N6-TiO2, C3N1-TiO2, and C3-N8-TiO2 samples, although x ) 3 and y ) 6 are the optimal C and N contents in solely carbon- and nitrogen-doped TiO2 nanoparticles, respectively. This may be why TBAH added during the preparation of Cx-Ny-TiO2 appears to facilitate nitrogen uptake, causing the nitrogen content

in Cx-Ny-TiO2 to be higher than that in the corresponding NyTiO2. Therefore, the optimal point of the y value reaches ahead in Cx-Ny-TiO2 samples. Figure 9B shows the photocatalytic activity of C3-N3-TiO2 calcined at 400, 500, and 600 °C, respectively. The sample calcined at 400 °C has the highest photocatalytic activity, since a higher temperature can increase the particle size and decrease the content of C and N co-doped in the TiO2 matrix. Synergistic Effect of Carbon and Nitrogen Codoping. It is inferred that the reasons for absorption enhancement in the visible region of N-TiO2 and C-TiO2 are different in our experiment: the former arises from the doped nitrogen atoms with substitutional type; the latter arises from a complex mixture of active carbon and carbonate species at the surface of TiO2 nanoparticles, similar to Sakthivel’s report.10 It is well-known that, for effective degradation, the organic substance should be preconcentrated on the surface of the semiconductor particles to effectively utilize the photoexcitons.40 The adsorption experiment of MB on different TiO2 samples indicated that the amount of MB adsorbed on the C3-doped (68 µmol/g) and C3-N3-doped (73 µmol/g) TiO2 calcined at 400 °C is obviously higher than that of undoped (40 µmol/g) and N3-doped (47 µmol/g) TiO2 calcined at 400 °C. These results suggest that the carbonaceous species formed at the surface of TiO2 samples could adsorb more MB molecules than pure TiO2, besides the capability of exciting by visible light like organic dyes. When MB molecules are degraded, other MB molecules adsorbed on the carbonaceous layer can transfer to the residual vacancies through surface diffusion, which may be a faster process than free diffusion in solution. One possible mechanism for this photocatalytic activity enhancement of C-N-TiO2 materials includes two main ways, as shown in Scheme 1. One is that the reactive electrons (er-) from the conduction band reduce O2 to its radical anion, which can further transform to H2O2 and OH•, resulting in the oxidation of methylene blue at last. The other is that the reactive holes (hr+) oxidize MB to its radical cation either directly or through a primarily formed OH• produced by the oxidation of ubiquitous water. In the first process of interfacial electron transfer, the

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Scheme 1. Tentative Mechanism for the Degradation of Methylene Blue on the C-N-TiO2 Nanoparticlesa

a MB, P, IB, and FB represent the methylene blue, carbonaceous species, intra-band-gap state, and flat-band state, respectively.

carbonaceous species (P) formed by doped C atoms act a role of photosensitizer like organic dyes, which can be excited and inject electrons into the conduction band of TiO2; then the rate of electron transfer to oxygen absorbed on the TiO2 surface increases. On the other hand, the N atoms doped in TiO2 can also promote this electron transfer. It is reported that nitrogen doping can not only create intra-band-gap states (IB) close to the valence band edges, which induces visible light absorption at the sub-band-gap energies hV2, but also shift its position of flat-band (FB) potential (dashed line shown in Scheme 1) to a higher level than that of undoped TiO2, which increases the driving force of electron injected from carbonaceous species and accelerates the reductive process of interfacial electron transfer. Recently, Cong et al.42 also mentioned a synergistic effect of C and N codoping for the high photocatalytic activity, but it was different from ours, which may be due to the different precursor of C and the preparation method. Conclusions In summary, C-TiO2, N-TiO2, and C-N-TiO2 nanoparticles were prepared by a simple sol-gel method, and their crystal structures, optical properties, and photocatalytic properties were compared. The doped nitrogen atoms weaved into the lattice of anatase and induced new band states in the band gap of TiO2, through replacing the sites of oxygen atoms, while most of the carbon atoms could form a layer composed of a complex carbonaceous mixture at the surface of TiO2. TiO2 nanomaterials co-doped with carbon and nitrogen had higher photocatalytic activities than those doped solely with carbon or nitrogen under visible light, which is accounted for by a synergistic effect of carbon and nitrogen codoping. This work may open a new avenue to prepare TiO2 nanomaterials for efficient utilization of solar energy, and promote the progress of TiO2 photocatalysts toward commercial application. Acknowledgment This work is supported by Programme of Introducing Talents of Discipline to Universities (No. B06006) and Changjiang Scholars and Innovative Research Teams in University (PCSIRT). We thank Professor Zhong Shunhe and Professor He Fei for their kind assistance in UV-vis spectroscopy and XPS measurements, respectively. Literature Cited (1) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 4428. (2) Kisch, H.; Zang, L.; Lange, C.; Maier, W. F.; Antonnius, C.; Meissner, D. Angew. Chem., Int. Ed. 1998, 37, 3034.

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ReceiVed for reView November 21, 2006 ReVised manuscript receiVed February 12, 2007 Accepted February 24, 2007 IE061491K