Effects of Trifluoroacetic Acid Modification on the Surface

DTA/TGA of the samples was carried out. Figure 1 shows. (18) Paz, Y.; Heller, A. J. Mater. Res. 1997, 12, 2759-2766. (19) Paz, Y.; Luo, Z.; Rabenberg,...
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Langmuir 2003, 19, 3889-3896

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Effects of Trifluoroacetic Acid Modification on the Surface Microstructures and Photocatalytic Activity of Mesoporous TiO2 Thin Films Jimmy C. Yu ,*,† Wingkei Ho,† Jiaguo Yu,†,‡ S. K. Hark,§ and Kwansai Iu§ Department of Chemistry and Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China, and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received March 26, 2002. In Final Form: January 21, 2003 Enhancement of the photocatalytic activity of sol-gel-derived TiO2 thin films by a simple treatment with trifluoroacetic acid (TFA) was investigated. The thin films were characterized with N2 adsorption, Fourier transform infrared spectroscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, X-ray diffraction, atomic force microscopy, and differential thermal analysis/ thermogravimetry analysis. The photocatalytic activity of the thin films was evaluated by the photocatalytic decomposition of acetone in air. The results show that TFA is chemisorbed on the surface of the TiO2 films as a trifluoroacetate complex. The photocatalytic activity of modified TiO2 thin films is higher than that of unmodified TiO2 thin films, and the modified film treated at 250 °C shows the highest activity. This is ascribed to the fact that the TFA complex bound on the surface of TiO2 acts as an electron scavenger and, thus, reduces the recombination of photogenerated electrons and holes. The enhancement is only temporary, however, as the TFA eventually decomposes under the strong oxidizing environment of photocatalysis. Reaction schemes are proposed to explain the interaction between TFA and TiO2. This detailed study may shed some light on the future development of more durable enhanced photocatalysts.

Introduction The application of photocatalysis for pollution treatment has attracted considerable attention in recent years.1-8 Titanium dioxide is the most widely used photocatalyst because of its exceptional optical and electronic properties, strong oxidizing power, nontoxicity, chemical stability, and low cost.5 Conventional powder photocatalysts, however, have a serious limitationsthe need for post-treatment separation in a slurry system. This can be overcome by immobilizing the TiO2 particles as thin films on a solid substrate. These materials would have new industrial applications, including as antibacterial and self-cleaning glass and ceramic tiles.5 The efficiency of TiO2 photocatalysis must be further improved, however, for this technology to be commercialized on a large scale. Many methods have been developed for enhancing the efficiency of TiO2 thin films. These include doping with fluoride and metal ions,5,9,10 coupling the films with metal * Corresponding author. Telephone: (852) 2609-6268. Fax: (852) 2603-5057. E-mail: [email protected]. † Department of Chemistry, The Chinese University of Hong Kong. ‡ Wuhan University of Technology. § Department of Physics, The Chinese University of Hong Kong. (1) Somorjai, G. A. Chem. Rev. 1996, 96, 1223-1225. (2) Watanabe, T.; Nakajima, A.; Wang, R.; Minabe, M.; Koizumi, S.; Fujishima, A.; Hashimoto, K. Thin Solid Films 1999, 351, 260-263. (3) Hidaka, H.; Zhao, J.; Pelizzeti, E.; Serpone, N. J. Phys. Chem. 1996, 96, 2226-2230. (4) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier Science: New York, 1993. (5) Hoffmann, M. S.; Martin, T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96. (6) Ovenstone, J. J. Mater. Sci. 2001, 36, 1325-1329. (7) Fox, M. A.; Duby, M. T. Chem. Rev. 1993, 93, 341-357. (8) Sopyan, I.; Murasawa, S.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1994, 723-726. (9) Hattori, A.; Yamamoto, M.; Tada, H.; Ito, S. Chem. Lett. 1998, 707-708. (10) Hattori, A.; Tada, H. J. Sol.-Gel Sci. Technol. 2001, 22, 47-52.

oxide,11 modifying the substrates,12,13 and treating the films with acid.14,15 Liao et al. reported the adsorption and photoreactions of acetic acid on TiO2 powders.16 The chemisorption of trifluoroacetic acid (TFA), an acid with strong electron-withdrawing groups, has also been reported.17 However, the substrate used in that particular study was tin(IV) oxide gel. Because the electronwithdrawing CF3 groups are good electron scavengers, they may be able to reduce the recombination rate of photogenerated electrons and holes of TiO2 thin films. To the best of our knowledge, systematic studies on the surface modification of TiO2 films by adsorbing a layer of TFA have never been reported. The aim of this work is to examine the effects of TFA treatment on the surface microstructures and photocatalytic activity of mesoporous TiO2 thin films and to elucidate the underlying reaction mechanism. The TiO2 films were coated by a sol-gel method on soda lime (SL) glass and then modified by dipping the films in a TFA aqueous solution. The modified and unmodified TiO2 thin films were characterized with N2 adsorption, Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and differential thermal analysis (DTA)/thermogravimetry analysis (TGA). The photocata(11) Song, K. Y.; Park, M. K.; Kwon, Y. T.; Lee, H. W.; Chung, W. J.; Lee, W. I. Chem. Mater. 2001, 12, 2349-2355. (12) Watanabe, T.; Fukayama, S.; Miyauchi, M.; Fujishima, A.; Hashimoto, K. J. Sol.-Gel Sci. Technol. 2000, 19, 71-76. (13) Hattori, A.; Shimoda, K.; Tada, H.; Ito, S. Langmuir 1999, 15, 5422-5425. (14) Yu, J. G.; Zhao, X. J. Mater. Res. Bull. 2001, 36, 97-107. (15) Yu, J. C.; Yu, J. G.; Zhao, J. C.; Appl. Catal., B 2002, 36, 31-43. (16) Liao, L. F.; Lien, C. F.; Lin, J. L. Phys. Chem. Chem. Phys. 2001, 3, 3831-3837. (17) Harrison, P. G.; Guest, A. J. Chem. Soc., Faraday Trans. 1991, 87, 1929-1934.

10.1021/la025775v CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003

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lytic activity of the films was evaluated by the photocatalytic decomposition of acetone in air. Experimental Section Preparation. All chemicals were purchased from Aldrich and were used as received. Millipore water was used in all experiments. Titanium tetraisopropoxide (TTIP) was used as a titanium source. A total of 0.1 mol of TTIP was dissolved in 3.2 mol of absolute ethanol under vigorous stirring. A total of 0.1 mol of acetylacetone and 0.1 mol of water were added to the alcohol solution. A total of 20 g of the triblock copolymer HO(CH2CH2O)20[CH2CH(CH3)O]70(CH2CH2O)20H (average molecular weight ca. 4400, designated P123) was added to the above solution. The mixture was stirred for 1 h to dissolve the P123. The beaker was sealed and stored at room temperature for 1 h to complete the hydrolysis process. The chemical composition of the resulting TiO2 sol was 1:1:32:1 TTIP/acetylacetone/EtOH/H2O. SL glass (83 × 98 × 2 mm) was used as the substrate for dip coating. To prevent the thermal diffusion of the sodium ions from the glass to TiO2, a SiO2 layer was precoated on the SL glass by the solgel method (hereafter, the substrate will be referred to as SiO2SL glass).18-20 The TiO2 thin films deposited on the SiO2-SL glass were prepared from the above TiO2 sol solution by the dipcoating method in an ambient atmosphere. The withdrawal speed was 4 mm s-1. The thin films were dried at 100 °C for 60 min, then heat-treated in air at a heating rate of 3 °C min-1 to 500 °C, and kept at that temperature for 1 h to decompose the triblock copolymer. The thickness of the films was adjusted by repeating the cycles from withdrawal to heat treatment. The surface modification of the films was conducted by dipping the asprepared TiO2 films in an aqueous 0.25 M TFA solution at room temperature. After 1 h, the films were withdrawn from the TFA solution and then heat-treated at different temperatures for 1 h in air. Powder samples were also prepared from the same TiO2 sol. The sol was dried at 100 °C in air to obtain a gel, then heattreated in air at a heating rate of 3 °C min-1, and kept at 500°C for 1 h. The solid was ground to powder with an agate mortar. The powder was then mixed with a small amount of a 0.25 M TFA solution and dried in air at room temperature. The dried samples would undergo further heat treatment at different temperatures. Characterization. The thermal-decomposition behavior of TFA adsorbed on the surface of TiO2 is monitored using a DTA/ TGA instrument (model TG-DTA 92-16, Setarem, Caluire, France). The XPS measurements were performed on a PHI Quantum 2000 XPS System with a monochromatic Al KR source and charge neutralizer; all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface-adventitious carbon. The film thickness was measured using a surface profiler (Alphastep 500, Tencor Instrument Inc., USA). The surface roughness and morphologies of TiO2 thin films were evaluated by AFM (NanoScope 3a, Digital Instruments Inc., Santa Barbara, CA). The XRD patterns obtained on an X-ray diffractometer (Philips MPD 18801) using Cu KR radiation at a scan rate of 2θ ) 0.05° s-1 were used to determine the phase constitution and crystallite size. The accelerating voltage and applied current were 35 kV and 20 mA, respectively. The crystallite size was calculated by the X-ray line-width analysis using the Scherrer formula. The PL emission spectra of the samples were measured with the following procedure. Each sample was dry-pressed into a 10mm-diameter round disk containing about 200 mg of mass. The sample disks were illuminated with a 10-mW, 325-nm He-Cd laser at an ambient temperature. Then, the PL from the samples was collected and focused into a spectrometer (Spex 1702) and detected by a photomultiplier tube (PMT; Hamamatsu R943). Finally, the signal from the PMT was sent into a lock-in amplifier before being recorded by a computer. IR spectra were obtained by a FTIR spectrometer (Magna-IR 560, Nicolet). The Brunauer(18) Paz, Y.; Heller, A. J. Mater. Res. 1997, 12, 2759-2766. (19) Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. J. Mater. Res. 1995, 10, 2842-2848. (20) Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1997, 106, 51-56.

Figure 1. DTA/TGA curves of the TiO2 powders after being suspended in an aqueous TFA solution and then dried at room temperature for 24 h. Emmett-Teller (BET) surface area (SBET) and pore parameters of the powder samples were determined by nitrogen adsorptiondesorption isotherm measurements at 77 K on a nitrogen adsorption apparatus (Micromeritics ASAP 2000). All the samples measured were degassed at 180 °C before the actual measurements. Pore size distributions were calculated from the desorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method using the Halsey equation. The UV-vis spectra of the films were obtained using a UV-vis spectrophotometer (Cary 100 Scan Spectrophotometers, Varian). Photocatalytic Activity. Acetone (CH3COCH3) is a common chemical that is used extensively in a variety of industrial and domestic applications. For example, acetone is frequently used as a solvent in the printing industry and in analytical laboratories; it is also a major constituent of many common household chemicals.21 Therefore, we chose it as a model contaminant chemical. The photocatalytic oxidation of acetone is based on the following reaction:21,22

CH3COCH3 + 4O2 f 3CO2 + 3H2O

(1)

The photocatalytic activity experiments on the TiO2 thin films for the oxidation of acetone in air were performed at an ambient temperature using a 7000-mL reactor. The detailed experimental process was described in our previous papers.15,22 During the photocatalytic reaction, a near 3:1 ratio of carbon dioxide products to acetone destroyed was observed,22,23 and the acetone concentration decreased steadily with increasing UV illumination time. Each reaction was followed for 60 min. The photocatalytic oxidation of acetone is a pseudo-first-order reaction, and its kinetics may be expressed as ln C/C0 ) -kt, where k is the apparent reaction rate constant.24-27 C0 and C are the initial and reaction concentrations of acetone, respectively.

Results and Discussion 1. Adsorption of TFA on the Surface of TiO2. 1.1. DTA/TGA Measurements. To study the adsorption behavior and thermal stability of TFA on the surface of TiO2, DTA/TGA of the samples was carried out. Figure 1 shows (21) Zorn, M. E.; Tompkins, D. T.; Zeltner, W. A.; Anderson, M. A. Appl. Catal., B 1999, 23, 1-8. (22) Lin, J.; Yu, J. C.; Lo, D.; Lam, S. K. J. Catal. 1999, 183, 368372. (23) Yu, J. C.; Lin, J.; Lo, D.; Lam, S. K. Langmuir 2000, 16, 73047308. (24) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735-758. (25) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C2000, 1, 1-26. (26) Fernandez, A.; Lassaletta, G.; Jimenez, V. M.; Justo, A.; Gonzalez-Elipe, A. R.; Herrmann, J. M.; Tahiri, H.; Ait-Ichou, Y. Appl. Catal., B 1995, 7, 49-63. (27) Yu, J. G.; Zhao, X. J. Mater. Res. Bull. 2000, 35, 1293-1301.

TiO2 Thin Films

Figure 2. XPS survey spectra for the surface of the unmodified (a) and modified (b) TiO2 films.

the DTA/TGA curves of TFA-modified TiO2 powders after drying at room temperature for 24 h. The DTA curve shows a broad endothermic peak below 100 °C. This is due to the desorption of the physically adsorbed water and TFA. A large and sharp exothermal peak appears at 292 °C, corresponding to the thermal decomposition of TFA. The TGA curve in Figure 1 can be divided into three stages. The first stage is from room temperature to 245 °C. A mass loss of about 0.83% is observed, which can be attributed to the evaporation of the physically adsorbed water and TFA. The largest weight loss (1.77%) is in the temperature range from 245 to 325 °C. This could be a result of the thermal decomposition of the chemically adsorbed TFA on the surface of TiO2. In the third stage from 325 to 700 °C, the mass loss is about 0.41%. This can be ascribed to the combustion of the organic residues and vaporization of the chemisorbed water. On the basis of the above results, all subsequent samples were heated to only 250 °C to avoid thermal decomposition of the adsorbed TFA. 1.2. XPS Studies. The XPS survey spectra of both the unmodified and the modified TiO2 films are shown in Figure 2a,b, respectively. The unmodified TiO2 film contains only Ti, O, and C, with sharp photoelectron peaks appearing at binding energies of 458 (Ti 2p), 531 (O 1s), and 285 (C 1s) eV. The carbon peak is attributed to the residual carbon from the precursor solution and adventitious hydrocarbon from the XPS instrument itself.28 An additional peak at 688 eV (F 1s) is found in the spectrum of the modified film. The high-resolution XPS spectrum of the F 1s region (Figure 3) consists of a single peak corresponding to the CF3 groups of the chemically adsorbed TFA. The highresolution XPS spectra of the C 1s region of the unmodified and modified samples are shown in Figure 4a,b, respectively. Deconvolution of the C 1s region of the unmodified sample shows three peaks. The major peak is related to the carbon atoms in the CsC, CdC, and CsH bonds.29,30 The two minor peaks can be accounted for by the hydroxyl (CsOH) and carbonyl (OsCdO) groups.31,32 A new peak (28) Wagner, C.; Muilnberg, G. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (29) Ng, S. C.; Fu, P.; Yu, W. L.; Chan, H. S. O.; Tan, K. L. Synth. Met. 1997, 87, 119-122. (30) Fabre, B.; Kanoufi, F.; Simonet, J. J. Electroanal. Chem. 1997, 434, 225-234. (31) Kim, T. K.; Yang, X. M.; Peters, R. D.; Sohn, B. H.; Nealey, P. F. J. Phys. Chem. B 2000, 104, 7403-7410.

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Figure 3. High-resolution XPS spectrum of the F 1s peak of the modified TiO2 film.

Figure 4. High-resolution XPS spectra of the C 1s region of the unmodified (a) and modified (b) TiO2 films.

shows up in the modified film (Figure 4b), which must come from the carbon in the CF3 groups of TFA. The O 1s region of both the unmodified and the modified samples can be fitted by three peaks, as shown in Figure 5. The main peak is attributed to the contribution of Tis O in TiO2.15 The others are related to the hydroxyl groups from the chemisorbed water and oxygen in the carbonoxygen bonds (CsO and CdO) from the residual carbon or TFA.15,30,33 Table 1 lists the results of curve fitting of the XPS spectra of the F 1s, C 1s, and O 1s regions for the unmodified and modified samples. The unmistakable CsF and CF3 signals and much higher carbonyl (OsCdO) content in the modified film confirm that the TFA is chemisorbed on the TiO2 surface. We believe the interactions between the carbonyl groups of TFA and the Ti atom in TiO2 results in the formation of a trifluoroacetate complex. If TFA were physisorbed on TiO2, it would be easily desorbed under the ultrahigh-vacuum condition of the XPS system, and the carbon-fluorine and carbonyl signals would not have been detected. To evaluate the integrity of the chemisorbed TFA on the TiO2 thin films during photocatalysis, the highresolution spectra of the C 1s region of the modified (32) Fusalba, F.; Belanger, D. J. Phys. Chem. B 1999, 103, 90449054.

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Table 2. Results of Curve Fitting of the High-Resolution Spectra of the C 1s Regions for the Modified TiO2a Samples after UV Irradiation CsC, CdC, CsH

b

CdOH

OsCdO

CF3

UV illumination time

Eb (eV)

rib (%)

Eb (eV)

rib (%)

Eb (eV)

rib (%)

Eb (eV)

rib (%)

0h 1h 3h 5h 7h 10 h

284.9 284.8 284.8 284.9 284.8 284.7

45.2 56.2 62.8 56.2 57.8 57.0

286.3 286.2 286.3 286.3 286.3 286.3

17.0 16.4 15.8 18.4 29.6 35.7

289.4 289.4 289.3 288.9 288.9 288.8

19.5 11.2 10.2 16.2 12.6 7.3

292.8 292.7 292.8 292.8

18.3 16.2 11.2 9.2

a The TiO thin films deposited on the SiO -SL glass were dipped in an aqueous 0.25 M TFA solution and then heat-treated at 250 °C. 2 2 ri represents the ratio of Ai/∑Ai (Ai is the area of each peak).

Figure 5. High-resolution XPS spectra of the O 1s region of the unmodified (a) and modified (b) TiO2 films. Table 1. Results of Curve Fitting of the High-Resolution Spectra of the F 1s, C 1s, and O 1s Regions for the Unmodified and Modified TiO2a Samples without TFA modification F 1s C 1s C 1s C 1s C 1s O 1s O 1s O 1s

CsF CsC, CdC, CsH CsOH OsCdO CF3 TisO OHCsO, CdO

Eb (eV)

rib (%)

284.8 286.3 288.9

73.7 20.2 6.1

529.7 531.8 532.7

81.1 13.5 5.4

Figure 6. High-resolution XPS spectra of the C 1s region of the modified TiO2 films after UV irradiation for different time periods.

with TFA modification Eb (eV)

rib (%)

688.3 284.9 286.3 289.0 292.9 529.8 531.7 532.8

100 44.3 15.5 19.6 20.6 63.6 13.7 22.7

a The TiO thin films deposited on the SiO -SL glass were dipped 2 2 in an aqueous 0.25 M TFA solution and then heat-treated at 250 °C. b ri represents the ratio Ai/∑Ai (Ai is the area of each peak).

samples were examined after the samples were irradiated by UV light for specific periods. Figure 6a shows that the C 1s region of the sample without UV irradiation is composed of four peaks at 284.9, 286.3, 289.4, and 292.8 eV. These are attributed to the CsC, CdC, and CsH; CsOH; OsCdO; and CF3 groups, respectively. From Figure 6b-f, the intensity of the CsF peak from CF3 decreases steadily with an increase in UV illumination time. Moreover, the binding energy of the OsCdO peak shifts slightly from 289.4 to 288.9 eV after 5 h of illumination. This may be caused by the cleavage of the carbonyl group of the adsorbed TFA. Table 2 shows the results of curve fitting of the C 1s regions for the modified TiO2 samples after UV irradiation. It can be seen that with increasing UV illumination time the amount of CF3 groups decreases gradually, indicating a photocatalytic decomposition of chemisorbed TFA.

Figure 7. IR spectra of the unmodified TiO2 powders (a) and modified TiO2 powders in a 0.25 M TFA solution for 1 h with further heat treatment at 25 °C (b), 250 °C (c), 300 °C (d), and 500 °C (e) for 1 h.

1.3. FTIR Spectra. Figure 7 shows the IR spectra of the unmodified and modified TiO2 powders calcined at various temperatures. The spectrum of the modified powder dried at 25 °C shows the presence of new absorption bands at 1145, 1200, 1428, 1450, 1623, and 1671 cm-1. The two bands observed at 1145 and 1200 cm-1 are assigned to the ν(CsF) stretching vibration.17 The peaks at 1428 and 1450 cm-1 as well as 1623 and 1671 cm-1 are characteristic absorptions of the symmetric and asymmetric stretching of a carbonyl group,16 respectively. At 250 °C, the intensity of all the peaks (1145, 1200, 1428, 1450, 1623, and 1671 cm-1) decreases as a result of the desorption of the

TiO2 Thin Films

Langmuir, Vol. 19, No. 9, 2003 3893 Scheme 1

Scheme 2

Figure 8. Two- and three-dimensional images of the unmodified (a,c) and modified (b,d) TiO2 thin films.

physisorbed TFA. At temperatures of 300 °C or higher, all the peaks disappear as a result of the thermal decomposition of the chemisorbed TFA. The observed bands can be interpreted in terms of the species shown in Schemes 1 and 2. When the TiO2 powders are dispersed in a TFA solution, TFA is adsorbed on the surface of TiO2 by hydrogen bonding {shown as species I in Schemes 1 and 2 with characteristic bands at 1671 [νas(CO)] and 1428 [νs(CO)] cm-1}. At the same time, some of the adsorbed TFA near the interfaces of the TiO2 films can bind at the Lewis acid sites (Ti4+) through the oxygen lone-pair electrons of the carbonyl group to form species II with characteristic bands at 1623 [νas(CO)] and 1450

[νs(CO)] cm-1.34 When heated to 250 °C, some of the physisorbed TFA would be vaporized and some might turn into chemisorbed trifluoroacetate (species III). Therefore, the intensity of the IR peaks in Figure 7 starts to drop. The trifluoroacetate complexes adsorbed on the surface of TiO2 are ionic in character but may be associated with either one or two surface Ti4+ ions. The surface complexes could exist in either bridging (Scheme 1) or chelating (Scheme 2) arrangements.34,35 We believe that fluorine, (33) Yu, J. G.; Zhao, X. J.; Du, J. C.; Chen, W. M. J. Sol.-Gel Sci. Technol. 2000, 17, 163-171. (34) Griffiths, D. M.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1977, 73, 1913-1926.

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the most electronegative element, can exert a pull on the electron density of the C atoms (species IV). Furthermore, the CF3 groups of the adsorbed trifluoroacetate complexes on the surfaces of the TiO2 films can attract the photogenerated electrons and reduce the recombination rate of the photogenerated electrons and holes. The photocatalytic activity of the films is, therefore, enhanced.5,24 2. Surface Microstructures of the TiO2 Films. 2.1. AFM Analysis. Figure 8 shows the two- and threedimensional AFM images of the unmodified (a,b) and modified (c,d) TiO2 thin films deposited on the SiO2-SL glass. The surface morphologies of both types of films show similar porous structures, but the surface roughnesses are different. The Rrms values of the unmodified and modified samples are 1.062 and 0.587 nm, respectively. Figure 8b,d also shows that the surface morphology of the unmodified TiO2 film is rougher than that of the modified one. This can be ascribed to the fact that the adsorbed trifluoroacetate complexes block the pores in the modified TiO2 thin films and, hence, reduce the surface roughness. 2.2. XRD Study. XRD was used to investigate the phase constitution of the TiO2 samples. The XRD results (not shown here) confirm that both the unmodified and the modified films contain only the anatase phase. Moreover, the diffraction patterns are virtually the same after TFA treatment. This suggests that TFA modification occurs only on the surface of the TiO2 films and does not affect the bulk composition. 2.3. Pore Structure Characterization. The BET surface areas of the mesoporous TiO2 thin films on the SiO2-SL glass could not be measured directly by the nitrogen adsorption apparatus because the amount of the thin film on the substrate was too small. Instead, we measured the BET surface areas of the powder samples prepared through the same procedure as the thin films. Figure 9a,b shows the BJH pore-size-distribution curve and nitrogen sorption isotherms (inset) of the unmodified and modified mesoporous TiO2 powders, respectively. Both samples appear to have narrow pore size distribution. The sharp decline in the desorption curve is an indication of mesoporosity. The pore structure parameters (Table 3) show that the average pore size of the modified samples is greater than that of the unmodified ones. However, the BET surface area, porosity, and pore volume of the modified TiO2 powders are smaller than those of the unmodified ones. This is due to the blockage of the small pore openings by the trifluoroacetate complexes. 3. PL Spectra. Figure 10a,b shows the PL spectra of the unmodified and modified TiO2 powders, respectively. The PL of the TiO2 powders originates from the chargetransfer transition from Ti3+ to the oxygen anion in a TiO68- complex. The large difference of about 0.9 eV between the band-gap energy (anatase, 3.2 eV) and the emission peak energy (about 2.3 eV) is due to the Stokes shift caused by the Franck-Condon effect. Because PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity of the modified sample indicates a lower recombination rate.36,37 As shown in Figure 10, the PL intensity of the modified TiO2 powder calcined at 250 °C is significantly lower than that of the unmodified one. 4. UV-Vis Spectra. Figure 11 shows the UV-vis spectra of the unmodified and modified TiO2 thin films deposited on the SiO2-SL glass. The transmittance of the unmodified films was about 96% in the visible region from 400 to 800 nm. However, a lower transmittance in the (35) Guo, Q.; Cocks, I.; Williams, E. M. J. Chem. Phys. 1997, 106, 2924-2931.

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Figure 9. Pore-size-distribution curve calculated from the adsorption branch of the nitrogen isotherm by the BJH method. Inset: the corresponding nitrogen adsorption-desorption isotherms of the unmodified (a) and modified (b) TiO2 powders. Table 3. BET Surface Areas and Pore Parameters of the Unmodified and Modified TiO2 Powders

samples modified TiO2 unmodified TiO2

pore SBETa porosityb volumec pore sized pore size (m2 g-1) (%) (mL g-1) (nm) range (nm) 29.97 69.86

13.3 24.6

0.042 0.088

5.70 5.04

2.68-7.49 2.78-7.38

a The BET surface area is calculated from the linear part of the BET plot (P/Po ) 0.05-0.3). b The porosity is estimated from the pore volume determined using the adsorption branch of the N2 isotherm curve at the single point P/Po ) 0.995. c The total pore volume is taken from the volume of N2 adsorbed at P/Po ) 0.995. d The average pore diameter is estimated using the adsorption branch of the isotherm and the BJH formula.

400-600-nm range is observed after modification. This slight decrease is attributed to the difference in film thickness and light absorption caused by TFA modification. The fast decrease below 380 nm is due to the absorption of light caused by the excitation of electrons from the valence band to the conduction band of the TiO2 films. In addition, the absorption edge of the modified TiO2 films is the same as that of the unmodified sample. This indicates that the crystal structures of the TiO2 films do not change, even after TFA modification. To estimate the change in the band-gap energy after TFA modification, the absorption coefficients (R) of the films near the absorption edge are calculated from the transmittance (T) and reflectance (R) data using the simplified relation T ) (1 - R)2e-Rd/(1 - R2e-2Rd), where

TiO2 Thin Films

Figure 10. PL spectra of the unmodified (a) and modified (b) TiO2 powders.

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Figure 12. Plots of R1/2 versus photon energy for the unmodified (a) and modified (b) TiO2 thin films deposited on the SiO2-SL glass.

Figure 11. UV-vis transmission spectra of the unmodified (a) and modified (b) TiO2 thin films deposited on the SiO2-SL glass.

d is the thickness of the film.36-38 The intercept of the tangent to the R1/2 versus photon energy (hv) plot is used to obtain an estimation of the band-gap energies for the TiO2 films. Plots of R1/2 versus hv are drawn for the unmodified and modified TiO2 thin films in Figure 12. The band-gap energies estimated from the intercept of the tangents to the plots are about 3.4 eV for both the unmodified and the modified TiO2 films. The extrapolated optical absorption band gaps of both the TiO2 films are found to be about 3.4 eV, larger than the band-gap energy of bulk TiO2 (3.2 eV). This can be explained by the smaller crystallite size of the TiO2 thin films prepared by the solgel method.30 As a result of the quantum size effects, the band gaps of the unmodified and modified TiO2 films become larger. Therefore, its absorption edge shows a pseudo “blue shift”.5,24,38 In addition, the estimated binding energies of the unmodified and modified TiO2 films are almost the same, indicating no chemical change in the TiO2 thin films after TFA modification. 5. Photocatalytic Activity. The photocatalytic activity of the unmodified and modified TiO2 thin films is shown (36) Tang, H.; Prasad, K.; Sanjines, R.; Schmidd, P. E.; Levy, F. J. Appl. Phys. 1994, 75, 2042-2047. (37) Tang, H.; Berger, H.; Schmidd, P. E.; Levy, F.; Burri, G. Solid State Commun. 1993, 87, 847-850. (38) Rahman, M. M.; Krishna, K. M.; Soga, T.; Jimbo, T.; Umeno, M. J. Phys. Chem. Solids 1999, 60, 201-210.

Figure 13. Photocatalytic activity of the unmodified and modified TiO2 thin films on the SiO2-SL glass at different heattreatment temperatures.

in Figure 13. The activity of the modified TiO2 films is higher than that of the unmodified ones. Furthermore, the modified film heat-treated at 250 °C shows the highest photocatalytic activity, which is 2 times higher than that of the unmodified film. However, the photocatalytic activity decreases sharply for the samples heat-treated at temperatures of 300 °C or higher. These observations are consistent with the results described in previous sections. To study the durability of the modified films, the photocatalytic activity was recorded over a 10-h period. Figure 14 shows that the photocatalytic activity of the films decreases gradually and then becomes constant after 7 h. Furthermore, this constant photodegradation rate matches that of the unmodified TiO2 films shown in Figure 13. This is clearly due to the decomposition of the chemisorbed TFA complexes under the strong oxidizing environment of photocatalysis. Conclusions 1. TFA can be easily adsorbed on the surface of TiO2 thin films via hydrogen bonding and Lewis acid-base interactions. After heat treatment at 250 °C, only the more stable chemisorbed TFA complexes would remain on the

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roacetate complexes blocks the smaller pores on the surface of TiO2. However, the phase structures and transmittance of the TiO2 thin films show no obvious changes. 3. The photocatalytic activity of the modified TiO2 thin films is greater than that of the unmodified TiO2 thin films. Furthermore, at 250 °C, the modified film shows the highest photocatalytic activity. This is due to the fact that the chemisorbed trifluoroacetate complexes can scavenge the photogenerated electrons in the conduction band of TiO2 and, thus, reduce the electron-hole recombination. 4. Although the enhancement in photocatalytic activity is only temporary, this detailed study of surface modification opens up new possibilities for future investigations of more long-lasting enhanced photocatalysts.

Figure 14. Photocatalytic activity of the modified TiO2 thin films on the SiO2-SL glass after UV irradiation for different time periods.

TiO2 surface. The chemisorbed TFA can be removed at temperatures of 300 °C or higher. 2. After TFA modification, the BET surface area decreases and the average pore size increases slightly. This is due to the fact that the chemisorption of trifluo-

Acknowledgment. This work was partially supported by a grant from the National Natural Science Foundation of China and Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. N_CUHK433/00). This work was also financially supported by the Excellent Young Teachers Program of MOE_P.R.C. and the National Natural Science Foundation of China (50272049). LA025775V