Synthesis and Characterization of Nitrogen-Doped TiO2

Apr 21, 2007 - Synthesis and characterization of nitrogen-doped TiO 2 coatings on reduced graphene oxide for enhancing the visible light photocatalyti...
16 downloads 8 Views 200KB Size
Download PDF
6976

J. Phys. Chem. C 2007, 111, 6976-6982

Synthesis and Characterization of Nitrogen-Doped TiO2 Nanophotocatalyst with High Visible Light Activity Ye Cong,† Jinlong Zhang,*,† Feng Chen,† and Masakazu Anpo‡ 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, and Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan ReceiVed: December 11, 2006; In Final Form: March 14, 2007

Nitrogen-doped TiO2 nanocatalysts with a homogeneous anatase structure were successfully synthesized through a microemulsion-hydrothermal method by using some organic compounds such as triethylamine, urea, thiourea, and hydrazine hydrate. Analysis by Raman and X-ray photoemission spectroscopy indicated that nitrogen was doped effectively and most nitrogen dopants might be present in the chemical environment of Ti-O-N and O-Ti-N. A shift of the absorption edge to a lower energy and a stronger absorption in the visible light region were observed. The results of photodegradation or the organic pollutant rhodamine B in the visible light irradiation (λ > 420 nm) suggested that the TiO2 photocatalysts after nitrogen doping were greatly improved compared with the undoped TiO2 photocatalysts and Degussa P-25; especially the nitrogen-doped TiO2 using triathylamine as the nitrogen source showed the highest photocatalytic activity, which also showed a higher efficiency for photodecomposition of 2,4-dichlorophenol. The nitrogen doping concentration had an optimal value, and accordingly, the photocatalyst showed the highest photocatalytic activity. This suggested that nitrogen doping has important effects on the improvement of photocatalytic activity: on one hand, nitrogen doping could narrow the band gap of titania to extend the adsorption of catalyst to the visible light region; on the other hand, nitrogen doping could inhibit the recombination of the photoinduced electron and thereafter increase the efficiency of the photocurrent carrier.

1. Introduction Titanium dioxide, as a cheap, nontoxic, and highly efficient photocatalyst, has been extensively applied for degradation of organic pollutants, for air purification, as a deodorant, for sterilization, and as a demister.1-3 However, because of the wide band gap of titanium dioxide, only a small UV fraction of solar light (3-5%) can be utilized. Therefore, the most important and challenging issue is to develop efficient visible light sensitive photocatalysts by the modification of titanium dioxide. Impurity doping is one of the typical approaches to extend the spectral response of the titanium dioxide to the visible light region. Some metal elements such as Fe, Cr, Co, Mo, and V have been employed to tune the electronic structure and enhance the photocatalytic activity of the titanium dioxide.4-7 However, metal doping can result in thermal instability and increasing carrier trapping, which may decrease the photocatalytic efficiency. Furthermore, the preparation of transition-metal-doped TiO2 requires more expensive ion-implantation facilities.8,9 Recently, many efforts have been made to modify titanium dioxide with nonmetals, such as B, C, N, S, and F, to efficiently extend the photoresponse from the UV to the visible light region.10-15 Furthermore, some theoretical calculations have also been performed to suggest that anion doping of TiO2 has considerable effects on the band gap alteration.12,16 Since the first report by Sato, considerable research has been done with respect to the preparation, characterization, photocatalytic * To whom correspondence should be addressed. Phone and fax: +8621-64252062. E-mail: [email protected]. † East China University of Science and Technology. ‡ Osaka Prefecture University.

activity, and mechanism of nitrogen-doped TiO2 in visible light.17-19 The success in nitrogen doping and increasing the photocatalytic activity of TiO2 in the visible light region provides good opportunities for extensive applications such as oxidation of CO, ethanol, gaseous 2-propanol, acetaldehyde, and NOx and the decomposition of dyes such as methylene blue. The reason for the improvement of photocatalytic activity is usually attributed to the decrease of the band gap, which is due to either mixing the nitrogen 2p states with O 2p states on the top of the valence band12 or the creation of a N-induced midgap level20 as well as some impurity species (NOx, NHx).21,22 Concerning the chemical states of N1s in the X-ray photoemission spectra (XPS), the assignment of XPS peak is still under debate with no consensus in the literature. In general, the peak of N1s in the XPS spectra mostly lies in the range of 396-404 eV, and there are different spectral features with different preparation methods and conditions. At present, N-doped TiO2 has been prepared by various methods such as mechanochemical reaction, sputtering, ion implantation, chemical vapor deposition, spray pyrolysis, sol-gel, and oxidation of TiN. Most of the above methods need a higher temperature or complicated and expensive equipment; therefore, it is promising work to develop a simple and lower temperature method to extend the application of the nitrogen-doped TiO2 photocatalyst. In this study, nitrogen-doped nanocrystalline titanium dioxides were successfully synthesized by a wet method, i.e., a microemulsion-hydrothermal process, which did not need a higher calcination temperature and consequently averted the agglomeration and sintering of the TiO2 particles. Some organic compounds were used as nitrogen sources such as thiethylamine, urea, thiourea, and hydrazine hydrate. Using the photodecom-

10.1021/jp0685030 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/21/2007

Synthesis of Nitrogen-Doped TiO2 Nanophotocatalyst

J. Phys. Chem. C, Vol. 111, No. 19, 2007 6977

position of rhodamine B under visible light irradiation as a model, a considerable improvement of the photocatalytic activity was found for all the nitrogen-doped TiO2 powders in comparison to Degussa P-25 and the undoped TiO2 prepared under the same conditions. By comparing the photocatalytic activities of the nitrogen-doped TiO2 with different nitrogen sources, triethylamine was chosen as the optimum, the characterizations of the structure, the chemical states, and the doping concentration effect of which were subsequently investigated in detail. To further investigate the photocatalytic activity, 2,4-dichlorophenol was also chosen as another probe organic. Furthermore, the reasons attributed to the improvement of the photocatalytic activity are also discussed generally. 2. Experimental Section 2.1. Preparation of the Catalysts. In the microemulsion system, Triton X-100 was used as the surfactant, 1-hexanol as the cosurfactant, cyclohexane as the continuous oil phase, and tetrabutyl titanate dissolved in nitric acid (5 mol/L) as the aqueous phase. Then the aqueous phase was slowly added to the oil phase and the resulting solution stirred until it became transparent. Some organic nitrogen compounds were employed for the nitrogen-doped source, such as triethylamine, urea, thiourea, and hydrazine hydrate. The mole ratio of nitrogen source to tetrabutyl titanate (i.e., N/Ti) varied from 0 to 0.5, 1, 2, and 4. With continuous stirring, the nitrogen source was dropped into the above microemulsion solution. After being continuously stirred for 6 h, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and then heated under 120 °C for 13 h. After the autoclave was cooled to room temperature, the precipitate at the bottom of the autoclave was obtained and washed with ethanol and deionized water several times to remove the oil, surfactant, and cosurfactant. The product was kept in an infrared drying oven to dry overnight, and yellow powders were obtained. The obtained photocatalysts are denoted as N-Ti-x, where x represents the mole ratio of nitrogen source to tetrabutyl titanate (i. e. N/Ti). No further treatment was needed. 2.2. Materials Characteriazation. X-ray diffraction (XRD) patterns of all samples were collected in the range 20-80° (2θ) using a Rigaku D/MAX 2550 diffractometer (Cu KR radiation, λ ) 1.5406 Å), operated at 40 kV and 100 mA. The crystallite size was estimated by applying the Scherrer equation to the full width at half-maximum (fwhm) of the (101) peak of anatase and the (110) peak of rutile, with R-silicon (99.9999%) as a standard for the instrumental line broading. The Raman spectra were recorded on an inVia + Reflex spectrometer equipped with an optical microscope at room temperature. For excitation, the 514.5 nm line from an Ar+ ion laser (Spectra Physics) was focused, with an analyzing spot of about 1 µm, on the sample under the microscope. The power of the incident beam on the sample was 1 mW. The time of acquisition was varied according to the intensity of the Raman scattering. The instrument employed for XPS studies was a Perkin-Elmer PHI 5000C ESCA system with Al KR radiation operated at 250 W. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 285.0 eV as an internal standard. The elemental analysis was carried out on a VARIO EI III, the measuring range of which was within 0.3%. The UV-vis absorbance spectra were obtained for the dry-pressed disk samples using a Scan UV-vis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly, using BaSO4 as the reflectance sample. The spectra were recorded at room temperature in air within the range 200-800 nm. The photo-

Figure 1. Photocatalytic decomposition of rhodamine B by N-TiO2 doped with different nitrogen sources and under various mole ratios of N to Ti.

luminescence (PL) spectra were measured with a fluorospectrophotometer (Cary Eclipse) by using the 280 nm line of a Xe lamp as the excitation source at room temperature. 2.3. Photocatalytic Activity Measurement. The photocatalytic activity was evaluated by measuring the decomposition of the distilled water solution of rhodamine B (with a concentration of 20 mg/L) and 2,4-dichlorophenol (100 mg/L). A 1000 W halogen lamp was used as the light source of the homemade photoreactor, surrounded with a water circulation facility at the outer wall through a quartz jacket. The temperature of the photocatalytic reaction was kept below 40 °C. The shortwavelength components (λ < 420 nm) of the light were cut off using a glass optical filter. The distance between the lamp and the center of the quartz tube was 10 cm. For a typical photocatalytic experiment, a total of 0.05 g of catalyst powders was added to 50 mL of the above rhodamine B solution or 2,4dichlorophenol solution in the quartz tube. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium. The above suspensions were kept under constant air-equilibrated conditions before and during the irradiation. At given time intervals, about 4 mL aliquots were sampled, centrifuged, and filtered through a 0.22 µm membrane filter to remove the remaining particles. The filtrates were analyzed by recording variations in the absorption in UV-vis spectra of rhodamine B or 2,4-dichlorophenol using a Cary 100 ultravioletvisible spectrometer. According to the standard curve of concentration and absorption, the value of ∆C/C0 was calculated and indicated the decomposition efficiency. 3. Results and Discussion Varying the mole ratio of nitrogen source to tetrabutyl titanate (i.e., N/Ti) from 0 to 0.5, 1, 2, and 4, the photocatalytic activities of decomposition of rhodamine B suspension using different nitrogen sources were investigated (shown in Figure 1). It is found that the optimal mole ratio of N to Ti for triethylamine, urea, thiourea, and hydrazine hydrate is 2, 1, 0.5, and 0.5, respectively. When the mole ratio of Ti to N is higher than the optimal value, the photocatalytic activities decrease rapidly, whereas the photocatalytic activity of nitrogen-doped titanium dioxide with triethylamine is higher than those of the other nitrogen-source-doped titanium dioxides in all mole ratios of Ti to N. Figure 2 shows the photocatalytic activities of Degussa P-25 and undoped and doped TiO2 with different nitrogen sources in the optimal mole ratio of N to Ti. Compared to that of the

6978 J. Phys. Chem. C, Vol. 111, No. 19, 2007

Cong et al.

Figure 2. Decomposition rate of Degussa P-25, undoped TiO2, and TiO2 doped with different nitrogen sources at the optimal N/Ti ratio after visible light irradiation for 1 h.

Figure 3. X-ray diffraction pattern of N-doped TiO2 using triethylamine as the nitrogen source with different N/Ti ratios: (a) 0, (b) 1, (c) 2, (d) 4.

TABLE 1: Some Structural Characteristics of Pure TiO2 and N-TiO2 sample

N/Ti

crystallite size (nm)

d spacing (Å)

a b c d

0 1 2 4

9.80 8.83 8.64 8.40

3.52 3.51 3.52 3.51

undoped TiO2, the phtotocatalytic activities of nitrogen-doped TiO2 with organic nitrogen sources are improved greatly and much higher than that of Degussa P-25. Especially the nitrogendoped TiO2 using triethylamine as the nitrogen source shows the highest photocatalytic decomposition efficiency. Thereafter, taking the nitrogen-doped TiO2 with triethylamine (denoted as N-TiO2) as an example, we further investigate the crystal phase, characters, and chemical states of the doped nitrogen and illustrate the probable reasons for the improvement in decomposition of rhodamine B and 2,4-dichlorophenol. In Figure 3, the XRD patterns of N-TiO2 samples in different mole ratios of N to Ti are provided, using triethylamine as the nitrogen source. The anatase phase has been retained without phase change after nitrogen doping. It can be seen that the N-TiO2 samples show peak broadening compared to the undoped TiO2. The crystallite sizes of the samples were calculated using the Debye-Scherrer equation (eq 1),23 and the

D ) Kλ/(β cos θ)

(1)

values are given in Table 1 along with other physical characteristics, where D is the average crystallite size in angstroms,

Figure 4. Raman spectra of N-doped TiO2 with different N/Ti ratios: (a) 0, (b) 1, (c) 2, (d) 4.

K is a constant which is taken as 0.89 here, λ is the wavelength of the X-ray radiation (Cu KR, 0.15406 nm), β is the corrected band broadening (fwhm) after subtraction of equipment broadening, and θ is the diffraction angle. Some structural characteristics of pure TiO2 and N-TiO2 are shown in Table 1. It is suggested that the particle size decreased after nitrogen doping; however, the increase of the doping concentration had less effect on the structure and size. In addition, there is no change in the “d” space values, which indicates that N has been introduced into the lattice without changing the average unit cell dimension.24 Raman spectroscopy is a powerful technique for the investigation of various phases of titanium dioxide. This technique is capable of elucidating the photocatalyst structural complexity as peaks from each material are clearly separated in frequency, and therefore, the phases are easily distinguishable. The modes A1g (519 cm-1), B1g (399 and 519 cm-1), and Eg (144, 197, and 639 cm-1) are Raman-active, and thus, six fundamental transitions are expected in the Raman spectrum of anatase.25 The four modes A1g (612 cm-1), B1g (143 cm-1), B2g (826 cm-1), and Eg (447 cm-1) are Raman-active modes of rutile.26 Figure 4 shows the Raman spectra of the nitrogen-doped titanium dioxides with different mole ratios of N to Ti. The observed peaks at 151, 404, 513, and 634 cm-1 can be attributed to the characteristics of the anatase phase, which indicates that the anatase is the predominant phase structure. From a measurement of the maximum of the low-frequency Raman band, it is possible to determine the nanoparticle size since the particle size can cause large shifts in the location of the scattered Raman peaks and their widths, namely, the quantum size confinement effect.27-29 Consequently, comparing the lowest frequency peak at 151 cm-1 of the different samples, it can be evidently seen that the intensities of this peak are dramatically increased and its widths are broadened after nitrogen doping. This indicates that the crystallinity is enhanced and the particle size is decreased, which is consistent with the results of the XRD measurement. To investigate the effect of nitrogen doping on the structure of the titanium dioxide, the Raman spectrum of N-TiO2-2 was fitted, and the result is shown in Figure 5. As can be seen, the peaks at 404, 513, and 634 cm-1 with a solid line are the vibration peaks of anatase. In the fitting curves, three peaks at about 320, 560 and 700 cm-1 with a dotted line can be observed distinctly, which are attributed to the vibration of Ti-N. According to previous reports, in the Raman spectra of the layers, the bands at 200, 330, and 550 cm-1 are the first-order

Synthesis of Nitrogen-Doped TiO2 Nanophotocatalyst

J. Phys. Chem. C, Vol. 111, No. 19, 2007 6979

Figure 5. Raman spectrum and fitting curves for N-TiO2 when the mole ratio of Ti to N was 2. The solid line corresponds to anatase features, and the dotted line corresponds to TiO2-xNx scattering of nonstoichiometric titanium nitride.

scatterings of nonstoichiometric titanium nitride,30,31 and the common component at 680-700 cm-1 is ascribed to secondorder scatterings of nonstoichiometric titanium nitride.31 In our Raman spectra, the bands at 330 and 550 cm-1 slightly shift compared with the results of Gyorgy,30 and the band at 200 cm-1 cannot be clearly observed, which might be because the peak is not high enough and is hidden under the peak at 151 cm-1. The first-order Raman spectrum is forbidden for a 1:1 stoichiometry, perfect TiN crystal. The low-frequency scattering around 200 and 300 cm-1 is caused by acoustical phonons, while the high-frequency scattering around 550 cm-1 is due to optical phonons. In general, the vibrational characteristic band of Ti-N can be clearly found in the Raman spectra, which indicates that nitrogen assuredly substitutes for some oxygen atoms in the titania lattice. To investigate the chemical states and the concentration of nitrogen atoms incorporated into the TiO2 photocatalyst, the N1s binding energy was measured by XPS. Up to now, the assignment of the XPS peak of N1s has still been under debate, and controversial hypotheses have been provided. In many cases, the peak at about 396 eV is attributed to substitutional nitrogen doping.12,32 In recent literature, the feature was found to be completely absent, while peaks at higher binding energies (399404 eV) were detected.33-35 Furthermore, in some other cases, both features have been observed.12,18,19 Figure 6 shows the XPS spectra for the N1s region of N-doped TiO2 with different N/Ti ratios and its fitting curves. It can be seen that there is a broad peak from 397 to 403 eV, which is in the range (396-404 eV) observed by several other researchers and is typical of nitrogen-doped titanium dioxide.12,20,22,33-35 After fitting of the curve, two peaks are obtained at 399.2 eV (peak 1) and 401.2 eV (peak 2). We attribute peak 1 to the anionic N- in O-Ti-N linkages, which is consistent with the present characteristics of other literature.24,35,36 The binding energy of this peak is higher than that of TiN appearing at E397.5 eV, which may be because nitrogen doping into the TiO2 lattice reduces the electron density on the nitrogen due to the high electronegativity of oxygen. Thus, changes in the nitrogen environment can produce significant differences in the nitrogen 1s XPS spectral region. This is also further supported by the results of XPS spectra for the Ti 2p region (shown in Figure 7A). The Ti2p3/2 core levels of the pure TiO2 and N-TiO2 appear at 459.05 and 458.25 eV, respectively. The binding energy of Ti2p3/2 after nitrogen doping decreases and suggests different electronic interactions of Ti with anions, which causes partial electron transformation from

Figure 6. Fitting XPS spectra for the N1s region of N-doped TiO2 with different mole ratios of N to Ti: (A) 1, (B) 2, (C) 4.

the N to the Ti and an increase of the electron density on Ti because of the lower electronegativity of nitrogen compared to oxygen.24,35,36 This further testifies that nitrogen incorporates into the lattice and substitutes for oxygen. The other peak at higher binding energy is controversial, and it is hard to identify its origin from the N1s XPS spectra alone. Therefore, we combined the N1s and O1s core levels to analyze. As shown in Figure 7B, an additional peak for N-TiO2 appears at about 532 eV and previously was attributed to the presence of Ti-O-N bonds.37 It is a feature that was first observed by Saha et al.38 and was most recently characterized by Gyorgy et al.39 They assigned this feature to the formation of oxidized Ti-N, which led to the Ti-O-N structure. It also coincides with the fact that the presence of oxidized nitrogen such as Ti-O-N should appear at higher binding energy (above 400 eV). From the above observations it can be concluded that the chemical states of the nitrogen doped into TiO2 may be various and coexist in the form of N-Ti-O and Ti-O-N. According to the XPS spectra of the N1s region, the total nitrogen percentage and fitting peak nitrogen percentage in the TiO2 were calculated (shown in Table 2). From the results, the theoretical N content and the calculated value from the XPS are different. First, in the process of preparation, not all of the nitrogen source could be incorporated into the titania photocatalyst. Second, the calculated value from the XPS is not the whole value of nitrogen doped in the titania since XPS is a measurement technique mainly for surface characterization. Therefore, it is only part of the nitrogen doped into the lattice of titania. Compared with the results of elemental analysis, the calculated results are slightly higher, which is probably due to the fact that XPS is a measurement technique mainly for surface characterization; moreover, there is error in the calculated nitrogen value in the fitting and calculation process.

6980 J. Phys. Chem. C, Vol. 111, No. 19, 2007

Cong et al.

Figure 7. XPS spectra of pure TiO2 and N-TiO2: (A) Ti2p and (B) O1s core levels.

TABLE 2: Nitrogen Doping Percentage of Different Samplesa Nc (%) Ne (%)

N-TiO2-1 (A)

N-TiO2-2 (B)

N-TiO2-4 (C)

1.48 0.95

1.88 1.21

0.98 0.63

a

Nc is the nitrogen doping percentage calculated according to the curve fitting of the XPS spectra for the N1s region. Ne is the nitrogen percentage of the elemental analysis results.

Figure 9. Photoluminescence emission spectra for N-TiO2 powders measured at room temperature, excitation wavelength 280 nm.

Figure 8. Diffuse reflectance spectra of different samples.

The diffuse reflectance spectra of Degussa P-25, pure TiO2, and nitrogen-doped TiO2 with different doping concentrations are shown in Figure 8. The samples Degussa P-25, undoped TiO2, TiO2-N-1, TiO2-N-2, and TiO2-N-4 show the band gap absorption onset at 400, 412, 425, 447, and 451 nm, corresponding to energy band gaps of 3.04, 2.95, 2.86, 2.72, and 2.70 eV, respectively. Consequently, the optical absorption edges of the doped samples shift to the lower energy region compared to those of the undoped sample, and the absorptions after nitrogen doping are drastic and stronger in the range of wavelengths from 400 to 600 nm. Furthermore, the absorption strengthens with increasing doping concentration. It is expected that nitrogen doping contributed to the red shift because of the narrowing of the band gap. PL emission spectra have been widely used to investigate the efficiency of charge carrier trapping, migration, and transfer and to understand the fate of electron-hole pairs in semiconductor particles since PL emission results from the recombination of free carriers.19,40,41 In general, it is difficult to observe the photoluminescence phenomenon at room temperature for bulk TiO2, even in the monocrystal state, due to its indirect transition nature. On the contrary, some nanometer-sized TiO2 particles and mesoporous structured TiO2 powders have been reported to exhibit room-temperature photoluminescence.19,40

Figure 9 shows the PL spectra of the undoped and nitrogendoped TiO2 with different mole ratios of Ti to N. Three peaks appear at about 387, 487, and 525 nm in two main regions (340-420 and 470-550 nm). The former is ascribed to the emission of the band gap transition,40 while the latter two (inset of Figure 9) are emission signals originating from the chargetransfer transition of an oxygen vacancy trapped electron.19,41 Because the PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity of the doped sample indicates a lower recombination rate.42 As shown in Figure 9, the doping of nitrogen into titania catalyst leads to the efficient quenching of the photoluminescence with different efficiencies. Consequently, more electrons can transfer from the valance band to the conduction band and enhance the charge separation efficiency of the photoinduced electron and hole. The effective quenching of the photoluminescence can be attributed to the two following pathways: the electron is trapped by the oxygen vacancy, while the hole is trapped by the doped nitrogen (eqs 2 and 3). On the other hand, the excited electron can transfer from the valance band to the new levels that exist upper of the conduction band introduced by nitrogen doping, which can also decrease the photoluminescence intensity.

Ov0 + eCB- f Ov-

(2)

N- + hVB+ f N0

(3)

To specify the effect of the doping concentration on the photocatalytic activities, the decomposition of rhodamine B (20 mg/L) and 2,4-dichlorophenol (100 mg/L) in the visible light irradiation of N-TiO2 in different mole ratios of N to Ti was carried out and compared with that of undoped TiO2 (shown in

Synthesis of Nitrogen-Doped TiO2 Nanophotocatalyst

J. Phys. Chem. C, Vol. 111, No. 19, 2007 6981

Figure 10. Decomposition rate of rhodamine B for 1 h (A) and 2,4-dichlorophenol for 5 h (B) on undoped TiO2 and TiO2 doped with different N/Ti ratios under visible light irradiation.

Figure 10). The results indicate that the photocatalytic activity is greatly improved by nitrogen doping. As can be seen from Figure 10A, when the mole ratio of N to Ti is 2, the decomposition rate is the highest. Accordingly, it can be obviously observed from Table 2 that the nitrogen percentage doped into the TiO2 is the highest at the same time. When the addition N/Ti value of triethylamine is higher than 4, the actual doping percentage is decreased instead. Consequently, we consider that the nitrogen doping concentration has an optimal value. Considering the DRS, XPS, and PL results, the local structures of the titanium dioxide species are altered by nitrogen doping, which not only improves the absorption of titania in the visible light region but also decreases the recombination of the photoinduced electron and hole. Therefore, it is reasonable that the photocatalytic activities of nitrogen-doped titania are improved accordingly. In addition, when the N/Ti ratio is 2, the photocatalyst has the highest photocatalytic activity because of its stronger absorption, the highest actual nitrogen doping percentage, and the lowest photoluminescence efficiency. Because rhodamine B can absorb visible light (λ > 420 nm), it can be degraded under visible light irradiation via two competitive processes: a photocatalytic process and a selfphotosensitized process. To elucidate that the higher photocatalytic activity of our photocatalyst after nitrogen doping is not mainly via the self-photosensitized process, we also degrade the solution of 2,4-dichlorophenol (100 mg/L), which does not absorb visible light. The results are shown in Figure 10B. As can be seen, the photocatalytic activities of TiO2 after nitrogen doping are improved compared with those of pure TiO2. Furthermore, when the N/Ti ratio is 2, the photocatalyst has the highest photocatalytic activity, which is consistent with the results of the decomposition of rhodamine B. The decomposition rate of 2,4-dichlorophenol (100 mg/L) is lower than that of rhodamine B, which may be because of the different absorptions of goal pollutants on the surface of the photocatalyst, the different surface oxygen concentrations, the different concentrations and structures of the goal pollutants, and so on. All the above results suggest that the nitrogen doping can effectively improve the photocatalytic activity in the visible light region. However, which chemical state (N-Ti-O or/and TiO-N) has the decisive effect on the photocatalytic activity needs to be further investigated. 4. Conclusions Nitrogen-doped TiO2 nanophotocatalyst with a homogeneous anatase phase and small particle size was prepared by the microemulsion-hydrothermal process using some organic compounds as the nitrogen source at lower temperature. N-TiO2

using triethylamine as the nitrogen source showed the highest photocatalytic activity for the decomposition of rhodamine B in the visible light region. The results of Raman spectra and XPS spectra indicated that the chemical environments of doping nitrogen were N-Ti-O and Ti-O-N. A stronger optical absorption and higher photocatalytic activity were observed. Furthermore, on the basis of the actual doping concentration calculated according to the XPS spectra and elemental analysis, the photocatalytic activity of the photocatalysts was more greatly improved with an increase of the actual nitrogen doping concentration. The nitrogen doping had predominant effects on the improvement of the photocatalytic activity: on one hand, it could narrow the band gap of titania to extend its absorption to the visible light region; on the other hand, it could increase the separation efficiency of the photoinduced electron and hole. Acknowledgment. This work has been supported by the Program for New Century Excellent Talents in University (Grant NCET-04-0414), Shanghai Nanotechnology Promotion Centre (Grant 0552nm019), National Nature Science Foundation of China (Grant 20577009), and Ministry of Science and Technology of China (Grants 2006AA06Z379 and 2006DFA52710). References and Notes (1) Linsebigler, A. L.; Lu, G.; Yates, Y. T., Jr. Chem. ReV. 1995, 95, 735. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2001, 1, 1. (4) Zhu, J.; Chen, F.; Zhang, J.; Anpo, M. J. Photochem. Photobiochem., A 2006, 18, 511. (5) Zhu, J.; Deng, Z.; Chen, F.; Zhang, J.; Chen, H.; Anpo, M.; Huang, J.; Zhang, L. Appl. Catal., B 2006, 62, 329. (6) Zhu, J.; Zheng, W.; He, B.; Zhang, J.; Anpo, M. J. Mol. Catal. A 2004, 216, 35. (7) Di Paola, A.; Marci, G.; Palmisanok, L.; Schiavello, M.; Uosaki, K.; Ikeda, S.; Ohtani, B. J. Phys. Chem. B 2002, 106, 637. (8) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707. (9) Wang, Y.; Cheng, H.; Hao, Y.; Ma, J.; Li, W.; Cai, S. Thin Solid Films 1999, 349, 120. (10) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. J. Am. Chem. Soc. 2004, 126, 4782. (11) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (12) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (13) Sato, S.; Nakamura, R.; Abe, S. Appl. Catal., A 2005, 284, 131. (14) Yu, J. C.; Ho, W.; Yu, J.; Yip, H.; Wong, P. K.; Zhao, J. EnViron. Sci. Technol. 2005, 39, 1175. (15) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (16) Umebayshi, T.; Yamaki, T.; Yanamoto, S.; Miyashita, A.; Tanaka, S.; Sumita, T. J. Appl. Phys. 2003, 93, 5156.

6982 J. Phys. Chem. C, Vol. 111, No. 19, 2007 (17) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (18) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2588. (19) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2596. (20) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (21) Rodriguez, J. A.; Lindgren, T.; Lu, J.; Granquist, C. G.; Lindquist, S. E. J. Phys. Chem. B 2000, 104, 319. (22) Diwald, O.; Thompson, T. L.; Zubkov, T. E.; Goralski, G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 6004. (23) Lin, J.; Lin, Y.; Liu, P.; Meziani, M. J.; Allard, L. F.; Sun, Y. J. Am. Chem. Soc. 2002, 124, 11514. (24) Sathishi, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349. (25) Berger, H.; Tang, H.; Kevy, F. J. Cryst. Growth 1993, 130, 108. (26) Porto, S. P. S.; Fbeury, P. A.; Damen, T. C. Phys. ReV. 1967, 154, 522. (27) Bassi, A. L.; Cattaneo, D.; Russo, V.; Bottani, C. E.; Bartoriri, E.; Mazza, T.; Pisen, P.; Midani, P.; Ernst, F. O.; Wegner, K.; Pratsinis, S. E. J. Appl. Phys. 2005, 98, 74305. (28) Iida, Y.; Furukawa, M.; Aoki, T.; Sakai, T. Appl. Spectrosc. 1998, 52, 673.

Cong et al. (29) Bersani, D.; Lottici, P. P.; Ding, X.-Z. Appl. Phys. Lett. 1998, 72, 73. (30) Gyogy, E.; Perez del Pino, A.; Serra, P.; Morenza, J. L. Appl. Surf. Sci. 2002, 186, 130. (31) Bernard, M.; Deneuville, A.; Thomas, O.; Gergaud, P.; Standstrom, P.; Birch, J. Thin Solid Films 2002, 380, 252. (32) Irie, H.; Watanabe, Y.; Hashmoto, K. J. Phys. Chem. B 2004, 107, 5483. (33) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384. (34) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen, X. J. Phys. Chem. B 2004, 108, 1230. (35) Chen, X.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (36) Li, H.; Li, J.; Huo, Y. J. Phys. Chem. B 2006, 110, 1559. (37) Battiston, G. A.; Gerbasi, R.; Gregori, A.; Porchia, M.; Cattarin, S.; Rizzi, G. A. Thin Solid Films 2001, 371, 126. (38) Saha, N. C.; Tomkins, H. C. J. Appl. Phys. 1992, 72, 3072. (39) Gyorgy, E.; Perez del Pino, A.; Serra, P.; Morenza, J. L. Surf. Coat. Technol. 2003, 173, 265. (40) Li, F. B.; Li, X. Z. Chemosphere 2002, 48, 1103. (41) Zhang, J.; Hu, Y.; Matsuoka, M.; Yamashita, H.; Minagawa, M.; Hidaka, H.; Anpo, M. J. Phys. Chem. B 2001, 105, 8395. (42) Tang, H.; Prasad, K.; Sanjines, R.; Schmidd, P. E.; Levy, F. J. Appl. Phys. 1994, 75, 2042.