The Effects of Sintering on the Photocatalytic Activity of N-Doped TiO2

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The Effects of Sintering on the Photocatalytic Activity of N-Doped TiO2 Nanoparticles Yixin Zhao, Xiaofeng Qiu, and Clemens Burda* Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106 ReceiVed October 23, 2007. ReVised Manuscript ReceiVed January 21, 2008

N-Doped titanium oxide nanoparticles (NPs) were synthesized through the hydrolysis of N-substituted titanium isopropoxide precursors and postsynthesis treated by sintering at different temperatures in a nitrogen atmosphere and in air. X-ray diffraction (XRD) results revealed that the presence of oxygen during the sintering also affects the crystallization of the N-doped TiO2 NPs. The air and N2 sintering produced different nitrogen concentrations and visible-light absorbances. The N-doped TiO2 NPs sintered at 200–250 °C in air for 1/2 h produced the best photocatalytic nanoparticles for visible-light decomposition of methylene blue in water. On the other hand, sintering in N2 atmosphere at any temperature provided photocatalysts with about half-the activity for visible-light decomposition of methylene blue compared to the air-sintered NPs. The zeta potential of the N-doped TiO2 NPs solution used in the photocatalytic tests shows that higher sintering temperatures create more negative charges on the NP surface and that the sintering in air creates more negative charges compared to the treatment in N2, due to the carboxylation of organic surface residues. Computational and ATR-FTIR results indicate that the doping of the nitrogen into the matrix occurs as a complexation between the Ti central metal ion and the N atom and not from a covalent Ti-N bond.

Introduction N-doped TiO2 nanoparticles (NPs) received a great deal of attention as promising photocatalysts for their high visiblelight activity, low fabrication cost and controllable synthesis.1–4 The biggest advantage for N-doped TiO2 NPs is their lower excitation energy compared to pure TiO2 NPs, which not only allows the absorbance of the UV portion of solar light, which covers about 5% of the solar spectrum, but also the visible portion of the solar spectrum, which covers >50% of the solar energy. While there is an ongoing research whether the doped TiO2 NPs enhance the visible-light absorbance due to a narrowed band gap or predominately due to introduced impurity sensitization or simply due to oxygen vacancies,5,6 the enhanced visible-light photocatalytic activity of N-doped TiO2 NPs attracts currently major scientific attention. Much of the work is focused on modifying the N-doped NPs by increasing the nitrogen concentration through different nitrogen sources and synthesis routes because of the nitrogen concentration dependent photocatalytic performance of the nitrogen-doped TiO2.7,8 There are experimental and computational studies on exploring the * Corresponding author. E-mail: [email protected].

(1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) Burda, C.; Lou, Y. B.; Chen, X. B.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049. (3) Chen, X.; Lou, Y.; Samia, A. C. S.; Burda, C.; Gole, J. L. AdV. Funct. Mater. 2005, 15, 41. (4) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen, X. J. Phys. Chem. B 2004, 108, 1230. (5) Kuznetsov, V. N.; Serpone, N. J. Phys.Chem. B 2006, 110, 25203. (6) Serpone, N. J. Phys. Chem. B 2006, 110, 24287. (7) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384.

correlation between the nitrogen doping level and the electronic structure in order to understand the origin of the enhanced visible-light photocatalytic activity.9–13 Several studies show that higher nitrogen doping levels can narrow the band gap by lowering the conduction band or increasing the valence band,10–13 Recent investigations have demonstrated that doped TiO2 can contribute enhanced magnetic properties to TiO2 nanoparticles, which was shown to introduce enhanced catalytic activities.14 However, there are no studies on the postsynthesis treatment like sintering, which optimizes the visible-light absorbance and photocatalytic activity by modifying the surface chemistry, nitrogen concentration, and crystal and electronic structure. Several methods have been reported to synthesize visiblelight sensitive N-doped TiO2.15,16 The sol–gel method is by far the most often used method to prepare N-doped TiO2.4,8,12,17–23 Using organic reagents for N-doping in this synthesis leads initially to organic residues on the NPs. It is (8) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (9) Yang, K. S.; Dai, Y.; Huang, B. B.; Han, S. H. J. Phys. Chem. B 2006, 110, 24011. (10) Di Valentin, C.; Pacchioni, G.; Selloni, A. Phys. ReV. B 2004, 70, 085116. (11) Dai, K. Y. Y.; Huang, B. B. J. Phys. Chem. C 2007, 111, 12086. (12) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666. (13) Qiu, X.; Zhao, Y.; Burda, C. AdV. Mater. 2007, 19, 3995. (14) Li, Q. K.; Wang, B.; Zheng, Y.; Wang, Q.; Wang, H. Phys. Status Solidi 2007, 1 (5), 217–219. (15) Chen, X. B.; Samuel, S.; Mao, Chem. ReV. 2007, 107, 2891. (16) Qiu, X.; Burda, C. Chem. Phys. 2007, 339, 1. (17) Matsumoto, T.; Iyi, N.; Kaneko, Y.; Kitamura, K.; Ishihara, S.; Takasu, Y.; Murakami, Y. Catal. Today 2007, 120, 226. (18) Sano, T.; Negishi, N.; Koike, K.; Takeuchi, K.; Matsuzawa, S. J. Mater. Chem. 2004, 14, 380.

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a well-accepted fact, but much less documented and still needs to be studied in more detail, that the postsynthesis sintering treatment of the N-doped TiO2 NPs plays a very important role in the preparation of N-doped TiO2 NPs and in the enhancement of their visible-light photocatalytic activity.24–34 Belver et al. recently reported a study on the evolution of gaseous effluents during sintering under conditions with and without O2 by using mass spectroscopy and IR spectroscopy to monitor the effluent changes during the sintering.24,25 Toward this end, the current work is focusing on the investigation of the effect of sintering treatment on surface functional groups, crystallinity, visible-light absorbance and photocatalytic activity of N-doped TiO2 NPs. This study aims to understand the chemical fine-tuning and further optimization of the visible-light photocatalytic activity of N-doped TiO2 NPs through their surface chemistry. Experimental Section Synthesis of N-Doped TiO2 NPs. N-doped TiO2 NPs are synthesized according to a previously published procedure.16 Specifically, a mixture of 4 mL of Ti(OPr)4 (Aldrich 97%), 20 mL id ethylenediamine, and 150 mL of 1-hexanol solution was refluxed until the solution became amber. This precursor solution was cooled to room temperature and 50 mL of CH3CO2H was added to neutralize the excess of ethylenediamine. The following hydrolysis process was achieved by adding 50 mL of distilled water dropwise into the solution under vigorous stirring. The resulting yellowish precipitate was centrifuged and washed subsequently with distilled water and ethanol. Finally, the N-doped TiO2 NPs were vaccuumdried for 24 h. Characterization. The N-doped TiO2 NP powder was analyzed by thermogravimetric analysis (TGA) with a Q500 TGA Instrument using a 10 °C/min ramp up to 800 °C, then sintered for 30 min in air and in N2 atmosphere at a steady flow rate of 200 mL/min in a quartz tube oven at 150, 200, 250, 300, 350, and 400 °C. The sintering temperatures were selected according to the obtained TGA curve. The infrared analyses were carried out on the sintered products by using a Thermo Nexus 870 FTIR spectrometer with an attenuated total reflection (ATR) accessory. The crystal structure (19) Drygas, M.; Czosnek, C.; Paine, R. T.; Janik, J. F. Chem. Mater. 2006, 18, 3122. (20) Ihara, T.; Miyoshi, M.; Iriyama, Y.; Matsumoto, O.; Sugihara, S. Appl. Catal., B 2003, 42, 403. (21) Liu, Y.; Chen, X.; Li, J.; Burda, C. Chemosphere 2005, 61, 11. (22) Valentin, C. D.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. J. Phys. Chem. B 2005, 109, 11414. (23) Wang, Z.; Cai, W.; Hong, X.; Zhao, X.; Xu, F.; Cai, C. Appl. Catal., B 2005, 57, 223. (24) Belver, C.; Bellod, R.; Fuerte, A.; Fernandez-Garcia, M. Appl. Catal., B 2006, 65, 301. (25) Belver, C.; Bellod, R.; Stewart, S. J.; Requejo, F. G.; Fernandez-Garcia, M. Appl. Catal., B 2006, 65, 309. (26) Huang, D. G.; Liao, S. J.; Liu, J. M.; Dang, Z.; Petrik, L. J. Photochem. Photobiol. A 2006, 184 (3), 282. (27) Irokawa, Y.; Morikawa, T.; Aoki, K.; Kosaka, S.; Ohwaki, T.; Taga, Y. Phys. Chem. Chem. Phys. 2006, 8, 1116. (28) Joung, S. K.; Amemiya, T.; Murabayashi, M.; Itoh, K. Appl. Catal., A 2006, 312, 20. (29) Liu, M.; Fang, Q.; He, G.; Li, L.; Zhu, L. Q.; Li, G. H.; Zhang, L. D. App. Phys. Lett. 2006, 88, 19. (30) Pan, C. C.; Wu, J. C. S. Mater. Chem. Phys. 2006, 100, 102. (31) Shi, Q.; Yang, D.; Jiang, Z. Y.; Li, J. J. Mol. Catal. B: Enzym. 2006, 43, 44. (32) Yin, S.; Aita, Y.; Komatsu, M.; Sato, T. J. Eur. Ceram. Soc. 2006, 26, 2735. (33) Yu, J. G.; Zhou, M. H.; Cheng, B.; Zhao, X. J. J. Mol. Catal. A: Chem. 2006, 246, 176. (34) Simakov, S. A.; Tsur, Y. J. Nanopart. Res. 2007, 9, 403.

Zhao et al. of the products was examined with a Scintag X-1 Advanced X-ray powder diffractometer (XRD, 2.4 °/min, Cu KR radiation). The optical properties of the products were determined with a Varian Cary Bio50 UV–vis spectrometer with a Barrelino remote diffuse reflection probe (reference material: MgO). The nitrogen doping level was determined by X-ray photoelectron spectroscopy (XPS, PHI 5600 XPS System). The zeta potential was measured by electrophoretic light scattering (ELS) using a NICOMP 370 ZLS Zeta potential and particle size analyzer. Photodegradation of Methylene Blue. The photocatalytic activity of the N-doped TiO2 NPs was determined by measuring the decomposition of methylene blue (MB) under the irradiation with visible light (>400 nm). A 150 W high-pressure Xenon arc lamp was used as light source. The concentration of methylene blue (MB) was determined by measuring the absorbance of the MB solution in a Varian Cary Bio50 UV–vis spectrometer. For comparison, a photocatalytic reaction was carried out using commercial titania (Degussa P25) NPs. Computations. The calculations were performed with the Becke’s three parameter hybrid method with the Lee, Yang, and Parr (B3LYP) gradient corrected correlation functional and the standard 6–31 g(d) basis sets. The geometries were fully optimized and then followed by frequency calculations. All the calculations were performed with Gaussian 03W.

Results and Discussion Powder X-ray Diffractometry. The crystal structure of a photocatalyst is an important property for the catalytic activity of the NPs. The crystal structures of the synthesized N-doped TiO2 NPs were studied by X-ray powder diffraction (XRD). It is found that N-doped TiO2 NPs are amorphous when sintered at temperatures below 400 °C whether in N2 atmosphere or air. Figure 1 shows the XRD patterns of N-doped TiO2 NPs sintered at different temperatures in N2 atmosphere and air. The XRD patterns of N-doped TiO2 sintered at 400 °C in air correspond to the anatase TiO2. The 105 and 211 peaks of the N-doped TiO2 NPs sintered at 400 °C in N2 atmosphere overlap to form one double peak. All peaks of N-doped TiO2 NPs sintered at 400 °C in N2 atmosphere are weaker and broader than the ones of the NPs sintered at 400 °C in air, which shows that oxygen affects the crystallization of the N-doped TiO2 NPs. The crystallization of the N-doped TiO2 NPs is faster when sintered in air than in N2 atmosphere. Diffuse Reflectance Spectroscopy. The optical absorbance and reflectance was used to study the capability to photosensitize the TiO2 NPs.13,15 The absorbance shift of the N-doped TiO2 NPs can be observed from the reflectance spectra of undoped and N-doped TiO2 NPs in Figure 2. The absorbance region of these N-doped TiO2 NPs was found to be broader with the increase of the sintering temperature whether in air or N2. The absorbance region of the samples sintered in N2 atmosphere shifted more gradually than those sintered in air. The color of the samples sintered above 250 °C became dark yellow, which may result from the carbonization of the organic residues because of high temperature sintering, and the reflectance spectra show lower reflectance from the samples. X-ray Photoelectron Spectroscopy. The nitrogen concentrations are thought to significantly affect the electronic structure, which then affects the visible-light absorbance and

Effects of Sintering on Photocatalytic ActiVity

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Figure 1. (A) XRDs for N-doped TiO2 NPs sintered at different sintering temperatures for 30 min in N2 atmosphere, (B) XRDs for N-doped TiO2 NPs sintered at different sintering temperatures for half an hour in air.

Figure 2. (A) Diffuse reflectance spectra of undoped and N-doped TiO2 NPs sintered at different temperatures in N2, (B) diffuse reflectance spectra of undoped and N-doped TiO2 NPs sintered at different temperatures in air.

Figure 3. (A) XPS for N-doped TiO2 NPs sintered in N2; (B) XPS for N-doped TiO2 NPs sintered in air. The insets display the enlarged C 1s and N 1s regions of the full-scale XPS spectra.

photocatalytic activity.1,3,13,33–36 The change of the nitrogen content in the N-doped TiO2 NPs with sintering temperature and atmosphere was investigated by core-level X-ray photoelectron spectroscopy (XPS). XPS spectra of N-doped TiO2 NPs sintered at different temperatures in N2 atmosphere and air is shown in Figure 3. All of these spectra showed typical TiO2 features and a clear N 1s binding energy around 400 (35) Chen, X.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (36) Chen, X. B.; Lou, Y. B.; Dayal, S.; Qiu, X. F.; Krolicki, R.; Burda, C.; Zhao, C. F.; Becker, J. J. Nanosci. Nanotechnol. 2005, 5, 1408.

eV, which indicates that N is successfully incorporated into the titanium oxides. 2–4,14 When the N-doped TiO2 NPs were sintered in N2 atmosphere, the C1s peak at 284.6 eV was found to be broader and had a shoulder at higher binding energy for temperatures below 200 °C, which shows that there are some organic residues besides the carbon tape used in the XPS measurement. The C1s peak becomes sharper above 300 °C, which shows that there is less or even no organic residues other than the carbon tape used. However, when the N-doped TiO2 NPs are sintered in air, the C1s peak

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Figure 4. Photodecomposition of methylene blue catalyzed by pure TiO2 and N-doped TiO2 NPs sintered at different temperatures in air and N2 for 30 min, irradiation of visible light (>400 nm) for up to 160 min. (A) N-doped TiO2 NPs sintered in N2 atmosphere, (B) N-doped TiO2 NPs sintered in air.

always shows a broad peak and shoulder even at 400 °C and the C concentration decreased sharply from 36% at 300 °C to 18% at 400 °C, while the C concentration changed from 36% at 300 °C to 33% at 400 °C when the N-doped TiO2 NPs were sintered in N2 atmosphere. This indicates that organic residues were oxidized when sintered in air. It is important to note that the XPS studies have to be carried out on a carbon (C) tape and therefore the concentrations of carbon relative to other elements are always unreliable and should not be taken as quantitive data. Even pure TiO2 shows carbon peaks at 284 eV in the XPS spectrum. In general, it is found that the nitrogen content decreased with increasing sintering temperature. At 400 °C the nitrogen content of the N-doped TiO2 NPs sintered in N2 is higher than the counterpart sintered in air. This difference may come from the oxidation of the doped nitrogen by oxygen when sintered in air, which decreases the nitrogen concentration at 400 °C. Photocatalysis. The visible-light photocatalytic activity of the N-doped TiO2 NPs was tested by visible-light photodecomposition of methylene blue. Figure 4 shows the normalized optical density change of methlyene blue at 680 nm under visible light irradiation (>400 nm) photocatalyzed by N-doped TiO2 NPs sintered at different temperatures in N2 atmosphere and air as a function of time. TEM and dynamic light scattering (DLS) measurements show that the particle sizes vary from 10 to 100 nm. We have not found any systematic correlation between particle sizes and photocatalytic conversion efficiencies. The photocatalytic activity of N-doped TiO2 NPs sintered at 150, 200, and 250 °C is much higher than that when sintered in N2 atmosphere. It is found that the pure TiO2 NPs Degussa P25, which is a wellknown UV-photocatalyst, shows almost no visible-light photocatalytic activity compared to the decomposition of methylene blue solution without any catalyst at otherwise the same conditions as blank sample. All the N-doped TiO2 NPs sintered in N2 atmosphere showed higher visible-light photocatalytic performance compared to pure TiO2 NPs. On the other hand, the photocatalytic activity of the N-doped TiO2 NPs sintered in air exhibited a strong dependence on the sintering temperature as shown in Figure 5. The photocatalytic activities of these sintered N-doped TiO2 NPs were maximized after 200 °C sintering. This sintering-temperature dependent activity shows that not

Figure 5. Photocatalytic activity of N-doped TiO2 NPs sintered at different temperatures for 30 min in air and N2 atmospheres measured after 160 min of irradiation by a Xe arc lamp with > 400 nm wavelength filter under continuous stirring in an open cuvette.

only the N-dopant concentration13 is important for activity but also other factors, as discussed below. Thermogravimetric Analysis. The above results show that the properties of the N-doped TiO2 NPs change with sintering temperatures and sintering atmospheres. To understand the loss of organic residues on the surface of the N-doped TiO2 NPs under sintering, we used the thermogravimetric analysis to study the weight loss of the N-doped TiO2 NPs. The TGA results of the N-doped TiO2 NPs in Figure 6 and Table 2 show that there is less weight loss, about 20% from room temperature up to 150 °C, which is mostly due to the loss of physically adsorbed or embedded water and solvent on the surface of the N-doped TiO2 NPs.26 From 150 to 250 °C, there is about a 30% weight loss of the N-doped TiO2 NP sample. From 250 to 400 °C there is much slower weight loss, about 10% of the N-doped TiO2 NPs. Above 400 °C, there is very little, about 3–5% weight loss of the N-doped TiO2 sample in association with the crystallization of these NPs, as confirmed by XRD.25 The latter weight loss is about the same as the nitrogen content in the N-doped TiO2 NPs sintered at 400 °C in air, as shown in the XPS in Figure 3. FTIR-ATR. To study the suspected sintering effect on organic residues, one method is to study the gas effluents by MS and FTIR.24,25 Here, we studied the surface organic


Figure 6. Thermogravimetric analysis (TGA) curve of N-doped TiO2 NPs from RT to 800 °C.

residues on the N-doped TiO2 NP surface by the FTIR attenuated total reflection (ATR) technique. The functional groups of the organic compounds used in the synthesis of the N-doped TiO2 NPs are shown in Figure 7A: the 2500–3500 cm-1 region displays the C-H, N-H, and O-H stretch vibrations from the organic precursor compounds used in the synthesis. The sharp peak at 1700 cm-1 originates from the C)O group in acetic acid, the 1595 cm-1 peak is due to the bend vibration of the N-H group in ethylenediamine. The 1000–1500 cm-1 region displays the bend vibrations of the organic compounds as shown in Figure 7A. The functional groups on the surface of these N-doped TiO2 NPs, which were sintered at different temperatures for 30 min, are assigned in Figure 7B and C. One finds that the signal below 1000 cm-1 is made of the characteristic Ti-OTi lattice vibrations.3 The IR experiments confirm that there are initially organic residues adsorbed on the surface of the N-doped TiO2 NPs. The intensity of these peaks decreases significantly after sintering 30 min at 150 and 200 °C in air and N2, which resulted in the rapid weight loss in the TGA experiment. These organic group signatures start to disappear when the sample is sintered above 300 °C, whether in an air or N2 atmosphere. The 3294 cm-1 peak, which comes from ethylenediamine, decreases slower and becomes sharper when sintered at 150 °C in N2 compared to that in air, which shows that these organic residues can be removed quicker when sintered in air compared to in N2. The 1630 cm-1 signal may come from -CONH-, which also includes the characteric 1537 cm-1 signal, the characteristic -NH- bend vibration. The -NH- group in the ethylenediamine appears at 1594 cm-1, which indicates that this NH group has been doped into the lattice or attached to the N-doped TiO2 NPs, which is confirmed by the following computations (see below). The 1537 cm-1 signal is characteristic of the -CONH- group, which is a condensation product of the acetic acid and the amine, a most undesirable side product. The 1405 cm-1 peak is the characteristic bend vibration mode of OH. The 1300 cm-1 signal is characteristic of C-H, the 1178 and 1045 cm-1 peaks indicate a C-O bond in 1-hexanol and acetic acid, respectively, as shown in the Figure 7A.

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The IR intensity change of the 1537 cm-1 peak for different sintering temperatures (shown in Figure 8) indicates that the organic residues, which contain nitrogen, decrease with increasing sintering temperatures and most of the organic residues were removed when these samples were sintered at temperatures above 300 °C, which is consistent with the TGA results as shown in Figure 6 where the weight loss curve becomes almost flat above 300 °C. This result also indicates that the N indeed has been doped into the TiO2 lattice. Otherwise there would be no N peak in the XPS in the sample sintered above 300 °C. Computation. A simplified model for the N-doped TiO2 is shown in Scheme 1. The nitrogen atom replaces the oxygen position in form of an organic amine. In model A, a coordinative bond between the N and the Ti are assumed. The calculated N-H bend vibration frequency of this amine-doped TiO2 is 1673.5 cm-1. In consideration of the systematic over estimation of the IR calculation by DFT, this value is relatively close to the experimental FTIR result of 1630 cm-1 as shown in the above discussion. On the other hand, for model B, where a covalent bond between N and Ti is assumed, there is no N-H bend vibration to be found based on the calculation, which is inconsistent with the strong 1630 cm-1 peak in the ATR experiments. This consistency between the experimental and theoretical calculation for model A suggests that amines are doped into the TiO2 NPs through coordinating bonds between the N lone pairs and the TiIV centers as shown in model A and not covalently as shown in model B. Zeta Potential Measurement. Nanoparticles in solution can aggregate into larger particles, which means that the actual particle size in solution can differ from the size measured initially as powder. Therefore, it is important to know the particle size and size distribution also in solution for comparing the photocatalytic activity of different samples. While particle sizes are in the 10 to 80 nm regime and increase with sintering termperature, a more interesting variable is the zeta potential of the NP. The zeta potential of a colloid, arises from the net charge on the surface of the particle in solution. The increased zeta potential of the colloidal solutions of pure nanosized Degussa TiO2 NPs and N-doped TiO2 NPs sintered at different temperatures in N2 atmosphere and in air, are listed in Table 1. From these results, we found that the undoped Degussa TiO2 NPs has the smallest zeta potential, which means that the Degussa TiO2 NPs has the smallest number of charges on the particle surface. The zeta potential of the sintered N-doped TiO2 NPs increased with increasing sintering temperature and the zeta potential of the N-doped TiO2 NPs sintered in air is in general higher than those counterparts sintered in N2 atmosphere. Discussion N-doped TiO2 NPs can be prepared in different ways. We found that complexation of organic amines on the Ti metal center creates highly efficient precursors for N-doped TiO2 NPs. Advantages of this route are high doping levels, controllable doping concentrations, and aqueous chemistry that results in a hydrophilic surface at low cost. A disadvantage of this method is that as synthesized organic residues

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Table 1. Zeta Potential of the Colloidal Solutions of Degussa TiO2 and N-Doped TiO2 NPs Used in the Photocatalytic Experiments above Degussa/RT

TiO2-xNx NPs/150 °C/air




-24.00 mV TiO2-xNx NPs/RT -40.40 mV

-39.57 mV TiO2-xNx NPs /150 °C/N2 -32.84 mV

-40.42 mV TiO2-xNx NPs/250 °C/N2 -33.41 mV



Table 2. Tentative Assignment of the Effluent in the TGA in Air

weight loss compd

RT-150 °C

150–250 °C

250–400 °C

400–800 °C

∼20% water, solvent

∼30% surface organics

∼10% deeper seated organics

3–5% doped nitrogen

cover the NP surface, which have to be removed by sintering treatments. Moreover the crystallinity and impurity level can be adjusted by sintering as well. The above-mentioned aspects of possible impurities, surface carbon control and low crystallinity of N-doped titania NPs are often discussed for as-synthesized samples but were so far not systematically investigated. Clearly much more work has to be carried out on the optimization of such nanocatalysts in order to maximize their impact on catalytic, environmental, or biomedical applications. Here, we decided to study the sintering effect on N-doped TiO2 NPs, which have organic precursor molecules. The N-doped TiO2 NPs were synthesized by hydrolysis of N-substituted titanium isopropoxide precursors in alcohol solution. In the synthesis the nitrogen has been doped into the lattice and/or attached to the NP surface. However, at the same time, there is water and organic residue adsorbed on the surface and enclosed into the amorphous and porous N-doped TiO2 powder. During the sintering, the adsorbed water and organics are first released at temperatures up to 150 °C. These organics can be removed quickly with the presence of oxygen because the oxygen can decompose the adsorbed organics compared to the pure physical desorption in a N2 atmosphere. Based on the data above, the TGA curve can be interpreted as shown in Table 2. This can explain the much faster decrease of the 2500–3500 cm-1 IR signal for the organic groups of the N-doped TiO2 NPs when sintered in air compared to N2. On the other hand, the nitrogen containing organics were removed only a little slower when sintered in air compared to N2, as seen in Figure 8A and B. At the same time, the visible light refelectivit changes (Figure 8C). However, as can be seen in Figure 8, the surface changes and the changes in light absorbance are not necessarily correlated. Much rather are XPS nitrogen concentrations and reflectivity correlated with the photocatalytic activity. At high temperature >400 °C, most organic residues can be removed whether sintered in an air or N2 atmosphere. The oxygen not only decomposes the organic residues but also replaces the doped nitrogen, which finally results in a sharp decrease of nitrogen concentration for the N-doped TiO2 NPs sintered in air. On the other hand, the nitrogen concentration for the N-doped TiO2 NPs sintered in N2 atmosphere decreases slower because of the absence of oxygen. The sintering temperature and presence of oxygen plays an important part in the modification of the properties of the N-doped TiO2 NPs surface.

The differences in photocatalytic performance of these sintered N-doped TiO2 NPs may come from the changes of the active sites on these NPs, during air sintering. While sintered in air, the number of active sites upon sintering in air increases with the removal of organic residues on the surface up to 200 °C. The active sites of the N-doped TiO2 NPs will then be passivated by oxidation, which leads to a less active catalytic surface and less photocatalytic activity when sintered above 250 °C. However, sintered in N2 atmosphere, there are less active sites formed during the removal of organic residues, which makes the N2 sintered N-doped TiO2 NPs less photocatalytically active compared to those sintered in air below 200 °C. These active sites will not be passivated in N2 at higher temperatures because of the oxygen absence, which leads to the remaining visiblelight photocatalytic activity when sintered in N2 at temperatures above 200 °C, as seen in Figure 5. The zeta-potential result shows that the sintering in air will decompose the surface organics more efficiently and create more negative charges on the N-doped TiO2 NPs compared to sintering in N2. This is likely due to the oxidation of remaining surface organics and the formation of anionic functional groups, which explains the high wetability and immeasurably small contact angles of these samples. The difference is due to the oxygen that also generates increased photocatalytic activity on the air-sintered NPs compared to the ones sintered in N2. However, the further increase of sintering temperature above 200 °C in air will then passivate the NPs, which is shown by the fact that higher-temperature sintered N-doped TiO2 NPs in air show a relatively lower photocatalytic activity compared to the lower-temperature ones (Figure 4). Surface charges help to increase the wetability of the catalyst and can have a dramatic effect on the catalytic performance, particularly for watersoluble substrates as demonstrated here. For lipophilic substrates, the situation may be different and a less charged surface can enhance the catalysts performance.37 Conclusions N-doped TiO2 NPs have been synthesized and treated by sintering. The nitrogen precursor ethylenediamine was found to incorporate nitrogen into the lattice of the NPs indicated by ATR-FTIR and computational results, which also indicated that the nitrogen is bound to Ti through a coordinating bond from the N lone pair to the Ti metal center, not through a Ti-N covalent bond. It was found that most of the organic residues can be removed by sintering above 300 °C for 30 min both in N2 atmospheres and in air. The amorphous N-doped TiO2 NPs crystallize by sintering above 400 °C. (37) Clouser, S.; Samia, A. C. S.; Navok, E.; Allred, J.; Burda, C. Top. Catal. 2007,in press,


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Figure 7. ATR spectra of (A) organic precursor compounds used in the synthesis of N-doped TiO2 NPs, (B) N-doped TiO2 NPs sintered at indicated temperatures for 30 min in N2. (C) The same samples sintered at indicated temperatures for 30 min in air.

Figure 8. (A) IR intensity of 1537 cm-1 peak for different sintering temperatures in N2 atmosphere and air. (B) IR intensity of 1630 cm-1 peak for different sintering temperatures in N2 atmosphere and air. (C) The change of reflectance at 600 nm with increasing sintering temperatures. (D) The change of the zeta potential of the NPs in solution with increasing sintering temperatures.

However, the crystallization is faster in the presence of oxygen. The nitrogen concentrations of the N-doped TiO2 NPs decrease with increasing sintering temperatures. The

nitrogen loss rate is lower at low temperatures in N2 atmosphere compared to air. The oxygen in air removes the doped nitrogen much faster when sintering above 400 °C.

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Scheme 1. Simplified Models of N-Doped TiO2; Compared are Two Forms of Possible Ti-N Binding: (A) Ti and N Form a Bond through the N Lone Pair Coordinating to the Ti Center; (B) Ti and N Form a Covalent Bond

The visible-light absorbance of the N-doped TiO2 NPs varied significantly with sintering temperatures in air while less upon sintering in N2 atmospheres. The zeta potential of the N-doped TiO2 NP solutions is higher than that of TiO2 NPs solutions, which implies that the N-doping and sintering

Zhao et al.

creates more charges on the NP surface. The zeta potential of the sintered N-doped TiO2 NPs increased with increasing sintering temperature. When sintered in air it is higher than for those NPs sintered in N2 atmosphere. In summary, sintering at different temperatures in different atmospheres modifies the surface chemistry, crystallinity, visible-light absorbance and photocatalytic activity. The synthesized N-doped TiO2 NPs sintered at mild sintering temperatures 200–250 °C for 30 min produce the best visible-light photocatalysts for the decomposition of organic pollutants in water. Acknowledgment. C.B. gratefully acknowledges financial support from the NSF (CHE-0239688). CM703043J

The Effects of Sintering on the Photocatalytic Activity of N-Doped TiO2

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