Synthesis and Structure of Nanocrystalline TiO2 with Lower Band Gap

Department of Chemical Engineering, Indian Institute of Science, Bangalore-560012, India. Received September 24, 2003. In Final Form: January 21, 2004...
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Synthesis and Structure of Nanocrystalline TiO2 with Lower Band Gap Showing High Photocatalytic Activity K. Nagaveni,† M. S. Hegde,*,† N. Ravishankar,‡ G. N. Subbanna,‡ and Giridhar Madras§ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India, Materials Research Centre, Indian Institute of Science, Bangalore-560012, India, and Department of Chemical Engineering, Indian Institute of Science, Bangalore-560012, India Received September 24, 2003. In Final Form: January 21, 2004 Nanocrystalline TiO2 was synthesized by the solution combustion method using titanyl nitrate and various fuels such as glycine, hexamethylenetetramine, and oxalyldihydrazide. These catalysts are active under visible light, have optical absorption wavelengths below 600 nm, and show superior photocatalytic activity for the degradation of methylene blue and phenol under UV and solar conditions compared to commercial TiO2, Degussa P-25. The higher photocatalytic activity is attributed to the structure of the catalyst. Various studies such as X-ray diffraction, Raman spectroscopy, Brunauer-Emmett-Teller surface area, thermogravimetric-differential thermal analysis, FT-IR spectroscopy, NMR, UV-vis spectroscopy, and surface acidity measurements were conducted. It was concluded that the primary factor for the enhanced activity of combustion-synthesized catalyst is a larger amount of surface hydroxyl groups and a lowered band gap. The lower band gap can be attributed to the carbon inclusion into the TiO2 giving TiO2-2xCx •• VO 2.

Introduction The presence of several organic pollutants in industrial wastewater results in a serious environmental problem. Photocatalytic oxidation using semiconductors is one of the advanced oxidation processes for the quick removal of such organic pollutants. Although several semiconductors such as ZnO, Fe2O3, WO3, and CdS have been used, TiO2 has been the photocatalyst of choice due to its photostability, nontoxicity, low cost, redox efficiency, and availability.1-5 In general, it is found that the rutile form of TiO2 is less photoactive than anatase.3,5,6 Since the activity of amorphous titania is negligible, an anatase powder with a high degree of crystallinity with high surface area is important to improve the photocatalytic activity.7 In most of the studies, commercially available titania powder, Degussa P-25, was employed. The problem with the P-25 powder is that the TiO2 disperses in water forming a white colloidal solution and it is difficult to separate from the reaction mixture for reuse. Filtration is energy intensive, and therefore, it would be desirable to have a powder that settles easily and can be reused. Several attempts have been made to improve the performance of TiO2 as a photocatalyst under UV illumination and to extend its absorption and conversion capacity into the visible portion of the solar spectrum.8-14 * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +91-80-3601310. † Solid State and Structural Chemistry Unit. ‡ Materials Research Centre. § Department of Chemical Engineering. (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (2) Linsebrigler, A. L.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (3) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (4) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (5) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (6) Brown, J. D.; Williamson, L. d.; Nozik, A. J. J. Phys. Chem. 1985, 89, 3076. (7) Ohtani, B.; Ogawa, Y.; Nishimoto, S.-i. J. Phys. Chem. B 1997, 101, 3746.

Choi et al.10 conducted a systematic study of 21 metal ions doping in quantum-sized TiO2, but no appreciable change in the band-gap energy of TiO2 was observed. Doping of anions has also been investigated. Khan et al.11 reported a chemically modified, carbon-substituted TiO2 absorbing light at wavelengths below 535 nm and having a lower band gap. Asahi et al.12 reported that TiO2-xNx shows a shift in its optical absorption and photodegradation of methylene blue and gaseous acetaldehyde in the visible region of λ < 500 nm. Nanosize titania powders have been synthesized by various methods such as the aerosol process,15,16 the solgel method,17,18 inert gas condensation,19 and the hydrothermal process.20,21 The aerosol process yields high-purity products and does not involve multiple steps, but the high temperature employed in the aerosol process leads to aggregation of the particles. On the other hand, the solgel process employs costly chemicals and involves multiple steps. In the case of the inert gas condensation process, gas condensation is carried out completely under ultrahigh (8) Soria, J.; Conesa, J. C.; Augugliaro, V.; Schiarello, M.; Sclafani, A. J. Phys. Chem. 1991, 95, 274. (9) Lin, J.; Yu, J. C.; Lo, D.; Lam, S. K. J. Catal. 1999, 183, 368. (10) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (11) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (12) Asahi, R.; Morikawa, T.; Ohwaki, T.; AoKi, K.; Taga, Y. Science 2001, 293, 269. (13) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808. (14) Hattori, A.; Shimoda, K.; Tada, H.; Ito, S. Langmuir 1999, 15, 5422. (15) Tewillinger, C. D.; Chiang, Y. M. Nanostruct. Mater. 1993, 2, 37. (16) Akhtar, M. K.; Pratsinis, S. E.; Mastrangelo, S. V. R. J. Am. Ceram. Soc. 1992, 75, 3408. (17) Schneider, M.; Baiker, A. J. Mater. Chem. 1992, 2, 587. (18) Campbell, L. K.; Na, B. K.; Ko, E. I. Chem. Mater. 1992, 4 (6), 1329. (19) Rubio, J.; Oteo, J. L.; Villegas, M.; Duran, P. J. Mater. Sci. 1997, 32, 643. (20) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G. Chem. Mater. 1997, 9 (10), 2197. (21) Stathatos, E.; Lianos, P.; Del Monte, F.; Levy, D.; Tsiourvas, D. Langmuir 1997, 13, 4296.

10.1021/la035777v CCC: $27.50 © 2004 American Chemical Society Published on Web 02/19/2004

Synthesis and Structure of Nanocrystalline TiO2

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vacuum conditions and the cost of production is high. The hydrothermal processing has shown great promise, producing particles at a relatively lower temperature at around 300 °C. However, even these particles require calcination at a higher temperature of 550-600 °C to reach their photocatalytic potential. The solution combustion method has been found to give highly crystalline fine particles/large surface area oxide materials such as alumina, ceria, titania, and zirconia.22,23 The flame pyrolysis of Ti metal gives carbon-substituted TiO2-xCx with lower band gap energy.11 Therefore, we considered it worthwhile to investigate whether the solution combustion method with fuels such as glycine, hexamethylenetetramine (HMT), and oxalyldihydrazide will give such chemically modified TiO2 with lower band gap. Herein we report the synthesis of high surface area, nanocrystalline lower band gap anatase TiO2 by the solution combustion method and show that carbide ion substituted TiO2 is a highly efficient photocatalyst both in UV and solar conditions for various reactions. The activity is much higher than that of commercial TiO2, Degussa P-25. Experimental Section Synthesis. Nanosize titania was obtained by the combustion of aqueous solutions containing stoichiometric amounts of TiO(NO3)2 and fuels such as glycine (Merck), hexamethylenetetramine (S.D. Fine Chemicals, India), and oxalyldihydrazide (ODH). The controlled hydrolysis of titanium isopropoxide under icecold (4 °C) conditions with vigorous stirring gives white precipitate of TiO(OH)2. The precipitate was washed several times in distilled water and then dissolved in nitric acid to get a clear, transparent titanyl nitrate [TiO(NO3)2] solution. This solution was used as the precursor for the synthesis of TiO2. Further details of the preparation have been described elsewhere.23 The product was yellow in color when glycine was used as a fuel [hereafter called TiO2(G)], and it was clear white with HMT [TiO2(H)] and pale yellow with ODH [TiO2(O)]. As-synthesized TiO2 is used in photochemical reactions. Characterization of the Combustion Product. The X-ray diffraction (XRD) patterns of titania were recorded on a Siemens D-5005 diffractometer using Cu KR radiation in the 2θ range from 5 to 100° with a scanning rate of 1°/min. The crystallinity was analyzed by Raman spectroscopy (Bruker RFS 100/S). Transmission electron microscopy (TEM) of powders was carried out using a JEOL JEM-200CX transmission electron microscope operated at 200 kV. The surface area of the titania was determined with a standard Brunauer-Emmett-Teller (BET) apparatus (NOVA-1000, Quantachrome). X-ray photoelectron spectra of these materials were recorded with ESCA-3 Mark II spectrometer (VG Scientific Ltd., U.K.) using Al KR radiation (1486.6 eV). Temperature-programmed desorption studies of as-prepared TiO2 were carried out, and the desorbed species were analyzed with quadrupole mass spectrometer QXK300 (VG Scientific). The assynthesized TiO2 was subjected to thermogravimetric-differential thermal analysis (TGA-DTA) (Perkin-Elmer-Pyris Diamond) to determine the adsorbed water. FT-IR studies were carried out in the 400-4000 cm-1 frequency range in the transmission mode (Perkin-Elmer, FTIR-Spectrum-1000). UVvis absorption spectra of TiO2 powders were obtained using a UV-vis spectrophotometer (GBC Cintra 40, Australia) between 270 and 800 nm. Each sample was dry-pressed into a 10 mm diameter round disk containing about 150 mg of mass. Absorption spectra were referenced to BaSO4. The acidity of the titania was determined by temperature-programmed desorption (TPD) of ammonia using a thermal conductivity detector. Catalytic Studies. The photocatalytic activity of combustionsynthesized TiO2 was evaluated by measuring degradation rates of methylene blue (MB) and phenol in solar and UV conditions. (22) Patil, K. C.; Amar Sekar, M. M. Int. J. Self-Propag. High-Temp. Synth. 1994, 3, 181. (23) Sivalingam, G.; Nagaveni, K.; Hegde, M. S.; Madras, G. Appl. Catal., B 2003, 45, 23.

Figure 1. Degradation profiles of methylene blue with an initial concentration of 100 ppm and a catalyst loading of 1 kg/m3 with combustion-synthesized and Degussa P-25 TiO2 under (a) solar and (b) UV conditions. The activity of this catalyst was compared with that of commercial TiO2, Degussa P-25. All the experiments were carried out using a cylindrical annular batch photoreactor. The design, the operation of the reactor, and the characterization of the lamp have been discussed in detail elsewhere.23 All degradations were performed in an open system wherein the top surface of the photoreactor was open to air. This indicates that atmospheric air provided enough oxygen for the oxidative degradation of pollutants. All the solar experiments were carried out in a cylindrical borosilicate glass reactor with i.d. of 8 cm and 400 cm3 volume. Direct sunlight was used in the present study, and the average solar intensity was 0.753 kW/m2. Samples were collected at regular intervals for subsequent analysis by a UV/visible spectrophotometer (Shimadzu, UV-2100). Chemical analysis of the filtrate was carried out in a HPLC system (Waters Inc., USA).

Results Photocatalytic Activity. It was observed that no detectable degradation of the pollutants occurs without TiO2 or irradiation (UV and solar) alone. The adsorption capacity of the combustion-synthesized titania for MB and phenol was evaluated, and the results indicate that there was no measurable amount of adsorption of MB as well as phenol over combustion-synthesized catalysts. Hence, the initial concentration was taken to be C0 in all the cases. The effects of catalyst loading and the initial concentration on photocatalytic degradation were studied, and the optimal catalyst loading of 1 kg/m3 and initial concentration of 100 ppm of dye and 0.5 mM/L phenol were employed throughout the study. Figure 1a shows the degradation profile of MB in solar irradiation with an initial concentration of 100 ppm with the 1 kg/m3 (100 mg/100 mL) catalyst loading. The complete degradation of the dye was observed in 220 min with TiO2(G), and the conversion was essentially complete at 300 min over TiO2(H). However, when the degradation was catalyzed by Degussa, an initial decrease in the concentration up to 50 ppm and no further degradation

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Figure 2. Degradation profiles of phenol with an initial concentration of 0.5 mM (48 ppm) and a catalyst loading of 1 kg/m3 with combustion-synthesized and Degussa P-25 TiO2 under (a) solar and (b) UV conditions.

of MB was observed. Even though TiO2(H) and TiO2(O) show complete degradation of MB, the initial rate of degradation was less compared to the rate of degradation with TiO2(G). The initial rates of the reaction are determined by conducting a series of concentration versus time experiments at different initial concentrations, and each run is extrapolated back to the initial conditions.24 The initial rates of degradation of MB are 0.153, 0.027, 0.024, and 0.018 µmol L-1 s-1 for TiO2(G), TiO2(H), TiO2(O), and Degussa P-25 TiO2, respectively. Experiments were also carried out under UV with the same initial concentration and catalyst loading over TiO2(G) and compared with results for Degussa P25. The results are shown in Figure 1b. Disappearance of MB was complete in 50 min over TiO2(G), while the concentration of the dye remained at 4.5 ppm with Degussa P25 even at 120 min. The initial rate of degradation of dye was 0.427 and 0.271 µmol L-1 s-1 with TiO2(G) and Degussa P-25 TiO2, respectively. For higher initial concentrations (200 ppm), complete degradation of MB was observed in 65 min under UV when catalyzed by TiO2(G). However, the concentration of MB shows only an initial decrease up to 100 ppm and then there is no further degradation of the dye when catalyzed by Degussa P25, as seen from the inset of the figure. We also evaluated the photodecomposition of phenol with the combustion-synthesized TiO2 in solar and UV conditions. Figure 2a shows the degradation profile of phenol with an initial concentration of 0.5 mM/L (48 ppm) and a catalyst loading of 1 kg/m3 in solar conditions. The phenol completely degrades when catalyzed by TiO2(G), and the photoactivity of TiO2(H) was comparable with that of TiO2(G), while the concentration shows saturation at 30 ppm when catalyzed by Degussa P25. The TiO2(O) also shows degradation of phenol, but the initial degrada(24) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley & Sons: New York, 1995; p 70.

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tion rate is less compared to the initial degradation rate of phenol by TiO2(G) and TiO2(H). The initial rates of degradation of phenol over different catalysts are in the following order: TiO2(G) > TiO2(H) > TiO2(O) > Degussa P-25 TiO2. The activity of combustion-synthesized titania was higher in UV conditions, as seen from Figure 2b. The complete degradation of phenol was observed at 120 min with combustion-synthesized TiO2(G), while the concentration of phenol shows saturation around 38 ppm with Degussa P-25. The initial rates of degradation of phenol in UV with combustion-synthesized TiO2 and Degussa P-25 are 0.023 and 0.012 µmol L-1 s-1, respectively, which is about 2 times higher than that of Degussa P-25 TiO2. HPLC analysis of the solution from phenol degradation with combustion-synthesized TiO2 showed no intermediates such as catechol (CC) or hydroquinone (HQ) during the reaction, and the concentration of phenol continuously decreased with time. However, when the degradation of phenol was conducted with Degussa P-25, two major peaks for CC and HQ were detected and the concentration of HQ and CC increased with the decrease in phenol concentration. It appears that secondary hydroxylation and ring-opening reactions for phenol are extremely fast with the combustion-synthesized catalyst. Most of the phenolic species including CC and HQ are also water pollutants. Since no significant amounts of these species are formed when phenol is degraded by combustionsynthesized TiO2, this reduces the degree of pollution in the water bodies compared to the commercial material Degussa P-25. These results demonstrate the higher photocatalytic activity of combustion-synthesized TiO2 under UV and solar exposure compared to the photocatalytic activity of Degussa P-25 under identical experimental conditions. In addition to higher photocatalytic activity, the combustion-synthesized titania is easier to separate from water. Degussa P25 TiO2 forms a milky white turbid suspension in aqueous media. Though the combustion-synthesized TiO2 crystallites are 6-10 nm in size, they do not form a turbid suspension. Further, this catalyst settles faster and it is easier to separate from the reaction mixture by centrifugation. The high catalytic activity of combustion-synthesized TiO2 must be attributed to its structure, and therefore we have examined the structure in detail. Structural Analysis of the Catalyst XRD Studies. XRD patterns of TiO2(G), TiO2(H), TiO2(O), and commercial TiO2, Degussa P25, are shown in Figure 3. The pattern can be indexed to TiO2 in the anatase phase only. The rutile and brookite phases of TiO2 were not observed. The crystallite size is determined from each 101 peak in the XRD pattern using the Sherrer formula, and the sizes are 6, 7, 9, and 32 nm for TiO2(G, H, O) and Degussa P25 TiO2, respectively. Rietveld refinements of the diffraction profiles of combustion-synthesized titania were carried out using the Fullprof-98 program, and a good agreement between calculated and observed patterns was observed (see the Supporting Information, Figure S1). The background of the X-ray pattern (Figure 3 and Figure S1) is flat, indicating that combustion-synthesized TiO2 is crystalline. For TiO2(G), the RBragg, RF, and RP values are 4.67, 3.36, and 9.32%, respectively. The lattice parameters for TiO2(G) are a ) 3.7865 (5) Å and c ) 9.509 (1) Å. The total oxygen in all the TiO2 samples was 2. Refinements of TiO2(H) and TiO2(O) also showed a good agreement with the calculated pattern, and no significant variation in the lattice parameters and oxygen content is observed.

Synthesis and Structure of Nanocrystalline TiO2

Figure 3. X-ray diffraction patterns of (i) Degussa P-25 TiO2, (ii) TiO2(G), (iii) TiO2(H), and (iv) TiO2(O).

After photocatalytic reactions, XRD patterns of the catalysts were recorded and there was no change in the structure and the line width of the peaks. To find the stability of combustion-synthesized TiO2, samples were heated at 400 °C for 48 h. Even after heat treatment of TiO2, only the anatase phase with increased crystallite size from 6 to 15 nm was observed. The color of TiO2(G) changed from yellow to white. Raman Studies. The Raman spectrum of TiO2(G) (see the Supporting Information, Figure S2) shows that the material is well crystallized in the anatase phase without any rutile impurity. The spectrum shows a strong sharp band at 146 cm-1, three mid-intensity bands at 396, 516, and 641 cm-1, and a weak band at 200 cm-1, which correspond to the fundamental vibrational modes of anatase titania. TiO2(H) and TiO2(O) also show only the anatase phase. TEM Studies. Bright-field images recorded in the transmission electron microscope of TiO2(G), TiO2(H), and TiO2(O) are shown in Figure 4a-c. Since it was difficult to identify the separate crystallites using bright-field images of combustion-synthesized titania, dark-field imaging was also carried out and the crystallite sizes were determined from the dark-field images. The crystallite sizes are 6-8, 7-9, and 11-13 nm for TiO2(G), TiO2(H), and TiO2(O), respectively. The sizes obtained from the dark-field images are in close agreement with the XRD values. The bright-field image of Degussa P25 TiO2 is shown in Figure 4d for comparison. Clearly the sizes of

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these crystallites are in the range of 100-150 nm. The sizes obtained in TEM are significantly higher than that calculated using the Sherrer formula for Degussa P25. The electron diffraction studies indicate that combustionsynthesized titania is highly crystalline, as the pattern could be indexed to the anatase phase only (Figure 4e). The d spacing calculated from the electron diffraction pattern matches the values obtained from XRD patterns. The crystalline nature of the combustion-synthesized titania is evident from the absence of the diffuse halo normally associated with the presence of amorphous phases. Analysis of the lattice fringes of individual small crystallites in the high-resolution image of TiO2(G) (see the Supporting Information, Figure S3) showed the lattice spacing of 3.5 ( 0.1 Å, which is in good agreement with the anatase (101) lattice spacing of 3.52 Å. There is no evidence for the presence of amorphous phases in these high-resolution images, further corroborating that the combustion-synthesized titania is fully crystalline. Surface Area. The BET surface areas measured for combustion-synthesized TiO2 particles are 246, 164, and 143 m2/g for TiO2(G), TiO2(H), and TiO2(O), respectively, while the surface area of commercial TiO2, Degussa P-25, was 50 m2/g. The surface area for TiO2(G) calculated by assuming all the particles are cubic with size of 6 nm and density of 3.84 g/m2 is 260 m2/g. This value is close to the measured value of 246 m2/g. Thus, TiO2(G) is indeed a nanocrystalline, high surface area material. XPS Studies. Ti(2p) core level spectra of the TiO2(G, H, and O) and Degussa P25 were recorded (see the Supporting Information, Figure S4). The two peaks observed at 459.0 and 464.8 eV are due to Ti4+(2p3/2) and Ti4+(2p1/2), respectively. There was no Ti3+ species observed in XPS. The line width of Ti(2p3/2) of Degussa P25 TiO2 is slightly higher compared to that of the TiO2 prepared here.25 This may suggest that Degussa P25 is not as crystalline as the TiO2 prepared here. The total Ti(2p) intensity of Degussa P25 TiO2 is much higher than that of the TiO2 synthesized here. Obviously, the density of Ti in the Degussa P25 pellet is higher than that of combustion-synthesized titania, suggesting a higher crystallite size of Degussa P25 which agrees well with XRD and TEM. Figure 5a shows the oxygen core level spectra of combustion-synthesized and Degussa P25 TiO2. The O(1s) core level spectrum of TiO2(G) shows two peaks at 530.1 and 532.4 eV. The peak at 532.4 eV can be attributed to the surface hydroxyl groups or chemisorbed water molecules on the titania, while the 530.1 eV peak is due to O2- ion. TiO2(H) and TiO2(O) also show a shoulder to the main O(1s) peak at higher binding energy, but the intensities are low. Thus TiO2(G) contains more adsorbed water. The C(1s) spectra of the combustion-synthesized TiO2 show a peak at 285.5 eV which can be assigned to graphitic carbon as shown in Figure 5b. TiO2(G) has a lower binding energy peak at 284.3 eV unlike TiO2(H), TiO2(O), and Degussa P25. This lower binding energy peak can be assigned to a carbidic species. To test this, TiO2(G) was heated in a furnace for 48 h at 400 °C. The color of the TiO2(G) pellet changed from yellow to white. The low binding energy C(1s) peak vanishes upon heating TiO2(G) [see Figure 5b]. Therefore, XPS of TiO2(G) is different compared to that of TiO2(H) and TiO2(O) in terms of more adsorbed water and the presence of carbide species. To investigate any nitrogen incorporation, the N(1s) core level spectra were recorded for all the samples (see (25) Nanda, J.; Kuruvilla, B. A.; Sharma, D. D. Phys. Rev. B 1999, 59, 7473.

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Figure 4. Transmission electron micrographs of (a) TiO2(G), (b) TiO2(H), (c) TiO2(O), and (d) Degussa P25 TiO2 and (e) electron diffraction of TiO2(G).

the Supporting Information, Figure S5). In all the samples, a N(1s) peak of significant intensity is observed at 400 ( 0.5 eV which can be assigned to chemisorbed γ-N2.26 The intensity ratio of N(1s)/Ti(2p3/2) is 0.066 for TiO2(H) and TiO2(O) compared to 0.057 for Degussa P25 TiO2. The value is 0.37 in TiO2(G). Generally, N(1s) of N3- ion is observed at ∼398 ( 0.5 eV. From the N(1s) binding energy, the total nitrogen in these materials is not in the N3state and it can be assigned to -NH-like species on the surface. However, the intensity of N(1s) in TiO2(G) is 5-6 times higher. The N(1s) peak is broad, and it may contain a sufficient amount of N3- at 398.5-399 eV in addition to -NH-like species. TPD Studies. Thermal desorption studies of the combustion-synthesized titania from room temperature to 500 °C in a vacuum were carried out, and the results reveal that the principle species to leave the surface of TiO2 is water. The appearance of two water desorption peaks (see the Supporting Information, Figure S6) indicates that two types of water exist on the surface of titania. In the lower temperature region (