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Influence of Thermal Treatment on the Adsorption of Oxygen and Photocatalytic Activity of TiO2 Jimmy C. Yu,*,† Jun Lin,† D. Lo,‡ and S. K. Lam‡ Departments of Chemistry, and Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received March 2, 2000. In Final Form: June 16, 2000 Thermal treatment in air has a marked influence on the adsorption of oxygen and the photocatalytic activities of TiO2 (P25) for the oxidation of acetone in air. The photoactivity of TiO2 increases with the increase of thermal treatment temperature until 400 °C, above which more rutile is formed. On the basis of the photocatalytic performance of TiO2 thermally treated in a vacuum and the results of polycrystalline X-ray diffraction analysis, BET surface area, adsorption oxygen measurements and ESR spectra, the increase in photoactivity can be attributed to a catalyst surface with more adsorbed oxygen. This would generate more superoxide anion radicals under UV irradiation, resulting in a better separation of photoexcited electrons and holes.
Introduction Among the various semiconductor oxides used in photocatalysis, TiO2 has been proven to be the benchmark semiconductor for effective decomposition of volatile organic pollutants in air or in water.1,2 This is because the anatase TiO2 has suitable thermodynamic positions of the valence and conduction bands in addition to its stability with respect to photocorrosion and chemical corrosion.3,4 Photoexcitation of the semiconductor irradiated by UV light with energy which matches the band gap energy yields electron-hole pairs. The separation of the pairs and their transfer to chemical substrates at the semiconductor surface becomes very important for the initiation of photocatalytic reaction and enhancement in reaction rate. Therefore, to compete with the electron-hole recombination, the electron acceptor or donor should be confined to the semiconductor surface. Recently, several methods have been used to separate the electrons and holes or to control the electron transfer. These include the mixture of metal oxide with TiO2,5-7 substitution of Ti in anatase and rutile TiO2 lattice with some transition metals,8,9 and electrostatic association of electron acceptor at the semiconductor surface.10 In the field of semiconductor photocatalysis, it is widely recognized that superoxide anions and hydroxyl radicals • OH produced from scavenging the electron-hole pair by * Corresponding author. E-mail:
[email protected]. Fax: (852)2603-5057. † Department of Chemistry. ‡ Department of Physics. (1) Serpone, N., Pelizzetti, E., Eds.; Photocatalysis Fundamentals and Applications; John Wiley & Sons: New York, 1989. (2) Ollis, D. F., Al-Ekabi, H., Eds.; Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (4) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (5) Fu, X.; Clark, L. A.; Yang, Q.; Anderson, M. A. Environ. Sci. Technol. 1995, 30, 647. (6) Anderson, C.; Bard, A. J. J. Phys. Chem. 1995, 99, 9882. (7) Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawa, Y.J. Phys. Chem. 1989, 90, 1633. (8) Yu, J. C.; Lin, J.; Kwok, R. W. M. J. Phys. Chem. B 1998, 102, 5094. (9) Lin, J.; Yu, J. C.; Lo, D.; Lam, S, K. J. Catal. 1999, 183, 368. (10) Frank, A. J.; Willner, I.; Goren, Z.; Degani, Y. J. Am. Chem. Soc. 1987, 109, 3568.
adsorbed molecular oxygen and water act as active reagents for the mineralization of organic pollutants.11 The hydroxyl radicals attack the C-H bonds of organic compounds to lead to complete mineralization. The effective separation of electron-hole pairs, or the enhanced degradation of organic pollutants can be realized by the increase in the concentration of electron donors such as surface hydroxyl groups and by the enhanced reduction of adsorbed molecular oxygen. The increase in the concentration of surface hydroxyl groups may be achieved by the addition of foreign metal oxides into TiO2.5-7 The use of solid solution Ti1-xSnxO2 and size-quantized semiconductor TiO2 can also enhance reduction of surface molecular oxygen.9,12-15 We found in this work that the photocatalytic activity of TiO2 was increased after thermal treatment in air. Similar observations were also reported by other researchers, but the explanations were not consistent.16-20 Some people thought that thermal treatment could somehow improve the crystallinity of TiO2, which resulted in an increase in photocatalytic activity.19 Sclafai et al. demonstrated that photocatalytic activity of TiO2 was dependent on its preparation methods.21 Rusu and Yates reported that thermal pretreatment on the rutile TiO2(110) surface in a vacuum produced anion vacancy defect sites which lead to enhanced photooxidation.22 Funda(11) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (12) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5546. (13) Hoffman, A. J.; Yee, H.; Mills, G.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5540. (14) Hoffman, A. J.; Carraway, E. R.; Hoffmann, M. R. Environ. Sci. Technol. 1994, 28, 776. (15) Nosaka, Y.; Ohta, N.; Miyama, H. J. Phys. Chem. 1990, 94, 3752. (16) Watanabe, T.; Kitamura, A.; Kojima, E., Nakayama, C. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993; p 747. (17) Tanaka, K.; Hisanaga, T.; Rivera, A. P. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993; p 169. (18) Weng, Y.; Wang, F.; Lin, L.; Xie, R. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993; p 713. (19) Porter, J. F.; Li, Y. G.; Chan, C. K. J. Mater. Sci. 1999, 34, 1. (20) Sato, S.; Kadowaki, T.; Yamaguti, K. J. Phys. Chem. 1984, 88, 2930. (21) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (22) Rusu, C. N.; Yates, J. T. Langmuir 1997, 13, 4311.
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Figure 1. Schematic diagram of luminescence experimental setup.
mental studies were carried out in the 70’s on the amount of water chemisorbed and oxygen photoadsorbed on TiO2 thermally treated at different temperatures.23-25 Sato et al. investigated the effect of lattice oxygen on the photocatalytic activity of titanium dioxide calcined at different temperatures by using the oxygen isotope exchange (OIE) method.20 They also found that the photocatalytic activity is enhanced after calcination at about 500 °C and suggested that the calcination at high temperature released lattice oxygen from TiO2 resulting in an enhancement in photooxidation reaction rate. The purpose of our study was to investigate the relationship between oxygen adsorption and photocatalytic activity of thermally treated TiO2. We measured the photocatalytic activity of TiO2 thermally treated at different temperatures in air and in a vacuum. By combination of the results of polycrystalline X-ray diffraction, ESR (electron spin resonance) spectra, adsorption oxygen, and BET surface area measurements, a new explanation for this improvement in photocatalytic activity is offered. Experimental Section Thermal Treatment of TiO2 (P25). TiO2 powder sample was kindly provided by Degussa. The TiO2 sample was divided into two batches. One batch was thermally treated in air at 200, 300, 400, and 500 °C, respectively for 3 h and then cooled to room temperature in air. The other batch was degassed at 10-4 Torr for 20 min in glass tubes and then sealed and thermally treated at 200, 300, 400, and 500 °C, respectively, for 3 h before cooling to room temperature in a vacuum. Measurement of Photocatalytic Activity. The photocatalytic activity measurements on the TiO2 thermally treated at different temperatures for the oxidation of acetone were carried out at room temperature by using a 7000-mL reactor. The photocatalysts for the experiment were prepared by coating an aqueous suspension of the TiO2 onto three dishes with diameters of 6.0 cm. The weight of the photocatalyst used for each experiment was kept at about 0.27 g. The dishes containing the photocatalyst were pretreated in an oven at 100 °C for 2 h and then cooled to room temperature before use. After the dishes coated with the photocatalysts were placed in the reactor, a small amount of acetone was injected into the reactor with a syringe. The reactor was connected to a pump and a dryer containing CaCl2 for adjusting starting concentration of acetone and controlling the initial humidity in the reactor. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was performed with a photoacoustic IR multigas monitor (INNOVA Air Tech Instruments model 1312). The (23) Munuera, G.; Stone, F. S. Discuss. Faraday Soc. 1971, 52, 205. (24) Boonstra, A. H.; Mutsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1694. (25) Boonstra, A. H.; Mutsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1940.
acetone vapor was allowed to reach an adsorption equilibrium with the photocatalyst in the reactor prior to an experiment. The initial concentration of acetone after the adsorption equilibrium was 400 ppm, which remained constant until a 15-W 365-nm UV lamp (Cole-Parmer Instrument Co.) in the reactor was turned on. The intensity of UV light striking the catalyst was 540 ( 10 µW/cm2. The initial concentration of water vapor was 1.20 ( 0.01 vol %, and the initial temperature was 25 ( 1 °C. During the photocatalytic reaction, a near 3:1 ratio of carbon dioxide produced to acetone destroyed was observed in the monitor, and the acetone concentration decreased linearly with increase in UV irradiation time. Each reaction was followed for 30 min. The degradation rate constant was used to express the photocatalytic activity of the TiO2. Characterization of Thermally Treated TiO2 Bulk and Surface. All TiO2 samples used for characterization are under the actual conditions used in the photocatalytic experiments. The polycrystalline X-ray diffraction patterns of TiO2 thermally treated at different temperatures in air and in a vacuum were recorded with a Philips MPD 18801 diffractometer using Cu KR radiation, and single-crystal silicon was used as the standard to determine instrument peak broadening. The BET surface areas of the TiO2 thermally treated at different temperatures were determined by using nitrogen adsorption data at 77 K obtained with a Micromeritics ASAP2000 accelerated surface area and porosimetry system. Measurements of Oxygen Adsorbed on Thermally Treated TiO2. A sketch of our experiment setup for oxygen measurement is shown in Figure 1. There is an oxygen sensor in the stainless steel test chamber. This oxygen sensor is made of dye-doped sol-gel silica. After the sensor is excited by 5 ns (fwhm) pulses of the second harmonic of a Nd:YAG laser into a triplet state, it will emit phosphorescence which is collected by a fiber bundle and send to a monochromator equipped with a ICCD intensified charge couple device. If in the presence of a quencher in the chamber, such as oxygen, the emitted phosphorescence becomes weak as compared to that in a vacuum, and the higher the oxygen concentration, the weaker the emitted phosphorescence. These luminescence-quenching systems are based on the following processes:
M + hv f *M
photon absorption
*M f M + hv *M f M + ∆ *M + Q f M + Q
(1)
luminescence
(2)
nonradiative decay
(3)
dynamic quenching
(4)
The whole process and principle were described in greater detail in our previous paper.26 A standard curve of the phosphorescence vs oxygen pressure was measured by varying the pressure of oxygen in the chamber. A TiO2 sample was put into a quartz tube as shown in Figure 1 and degassed at 5 × 10-6 Torr (26) Lam, S. K.; Namdas, E.; Lo, D. J. Photochem. Photobiol. A: Chem. 1998, 118, 25.
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Figure 2. Effect of thermal treatment on photocatalytic activity.
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Figure 3. Polycrystalline X-ray diffraction patterns of TiO2 thermally treated at different temperatures in air. “A” represents the most intense peak of anatase phase; “R” indicates the most intense peak of the rutile phase.
for 10 min before measurement and heating. After the TiO2 sample was heated at 400 °C for 1.5 h, the thermally desorbed gas was released into chamber for measurement. ESR Measurements. ESR spectra under continuous UV irradiation were recorded on an electron spin resonance spectrometer (JEOL JES-TE100) equipped with an X-band gun oscillator and a rectangular resonant cavity. The sample powder was placed into quartz cell (30 cm length, 4 mm diameter). UV irradiation was carried out with a superhigh-pressure Hg lamp (500 W) equipped with a band-pass filter of 350 nm.
Results Photocatalytic Activity for the Oxidation of Acetone in Air. Figure 2 illustrates the rate constant for the photocatalytic degradation reaction of acetone over the TiO2 thermally treated at different temperatures in air and in a vacuum, respectively. The reproducibility as estimated from triplicate measurements is (15%. The TiO2 sample thermally treated at 100 °C in air is considered to be the same as that without thermal treatment since each sample was required to be pretreated at 100 °C for 2 h before use. According to Figure 2, the photocatalytic activity of TiO2 thermally treated in air increases with increase in thermal treatment temperature and begins to drop as thermal treatment temperature reaches 500 °C. Interestingly, the samples thermally treated in a vacuum at 200-400 °C show a much smaller increase in photocatalytic activity, and the one treated at 500 °C in a vacuum exhibits the lowest activity of all. Charaterization Results. On the basis of polycrystalline X-ray diffraction patterns of the TiO2 thermally treated at different temperatures in air and in a vacuum (as shown in Figures 3 and 4), there is no difference in phase constitution among those thermally treated at and below 400 °C. However, the samples thermally treated at 500 °C, in particular, TiO2 thermally treated in a vacuum, exhibit an increase in rutile content, indicating that a phase transformation takes place at this temperature. The crystal size was determined from the broadening of corresponding X-ray spectral peaks by Scherrer’s formula:27
D ) Kλ/(βc - βs) cos θ Here D is the crystal size, λ is the wavelength of the X-ray radiation, and βc and βs are the line widths at halfmaximum height of the sample and standard, respectively. The crystal sizes of the TiO2 thermally treated at different (27) Cullity, B. D. Elements of X-ray Diffraction Addition-Wesley: Reading, WA, 1978.
Figure 4. Polycrystalline X-ray diffraction patterns of TiO2 thermally treated at different temperatures in a vacuum. “A” represents the most intense peak of anatase phase; “R” indicates the most intense peak of rutile phase. Table 1. Crystal Size and Surface Area of TiO2 Thermally Treated at Different Temperatures in Air and in Vacuum sample
cryst size (nm)
surf area (m2/g)
TiO2 treated at 100 °C in air TiO2 treated at 200 °C in air TiO2 treated at 300 °C in air TiO2 treated at 400 °C in air
23.68 23.80 22.86 27.16
52.21 50.47 50.65 51.06
temperatures in air and in a vacuum have been listed in Table 1; they show little change with exceptions of samples treated at 400 and 500 °C, in which a small increase in crystal size was observed. BET measurement results for the surface area are also shown in Table 1. The surface area of TiO2 (P25) powders treated at 100 °C was found to be 52.21 m2/g, which was consistent with a value of 55 m2/g reported by Minero et al.28 From the data in Table 1, it can be concluded that the surface area remains more or less the same for samples treated in air and vacuum over the temperature range of 100-500 °C. Results of Adsorbed Oxygen Measurements. Figure 5 shows the plot of Io/I vs oxygen pressure, where Io is the intensity of phosphorescence emitted by a dye sensor in a vacuum and I represents that emitted by the dye sensor at various oxygen pressures. The phosphorescence decreases with increase in oxygen pressure and that the dye sensor becomes very sensitive to oxygen for oxygen pressures higher than 10-3 Torr. On the basis of this (28) Minero, C.; Catozzo, F.; Pelizzetti, E. Langmuir. 1992, 8, 481.
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Figure 5. Ratio of Io/I versus oxygen pressure.
Figure 6. ESR spectra of TiO2 after UV irradiation in the presence of O2 at 77 K: (I) TiO2 thermally treated at 400 °C; (II) TiO2 thermally treated at 100 °C.
calibration curve and the volume of the chamber, we estimated that the amounts of oxygen adsorbed on TiO2 thermally treated at 100 and 400 °C were 1.86 × 1014 and 2.12 × 1014 molecules/cm2, respectively. More oxygen molecules were adsorbed on the surface of TiO2 thermally treated at high temperature in air. Results of ESR Measurements. The ESR spectra of TiO2 thermally treated at 100 and 400 °C in air after UV irradiation for 20 min in the presence of O2 at 77 K are shown in Figure 6. The UV irradiation of these two samples in the presence of O2 at 77 K produced signals A and B in the detection range (282-362 mT). It is obvious that signal A generated photochemically on TiO2 thermally treated at 400 °C has higher intensity than that on TiO2 treated at 100 °C and that signal B formed on TiO2 thermally treated at 400 °C shows a lower intensity as compared to that on TiO2 treated at 100 °C. Discussion Effects of Phase Constitution, Crystal Size, and Surface Area on Photoactivity. From the above experimental results, thermal treatment in air leads to an enhancement in photocatalytic activity of TiO2 for the oxidation of acetone. Such a treatment also induces small
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changes in phase constitution, crystal size, and surface area. The results of polycrystalline X-ray diffraction show the catalysts TiO2 thermally treated at different temperatures in air have the same phase constitutions with an exception of that thermally treated at 500 °C, in which more rutile content was found (as shown in Figure 3). It is well-known that TiO2 in rutile form is less effective than that in the anatase form as a photocatalyst for the oxidation of most volatile organic compounds.4,29,30 Therefore, it is normal that the TiO2 thermally treated at 500 °C has been shown to have lower photoactivity than that thermally treated at 400 °C. The crystal sizes of the TiO2 anatase phase present in the samples are listed in Table 1. It shows a small increase in the anatase crystal size at thermal treatment above 400 °C. In general, the increase in catalyst crystal size is detrimental to catalytic activity since it results in the reduction of reactant/catalyst contact area. On the other hand, no increase in crystal size is found on the samples after thermal treatment at 200 and 300 °C, but the two samples also exhibit an enhancement in photoactivity. Thus, the increase in the crystal size of TiO2 treated at 400 °C is not responsible for the enhanced photoactivity. Table 1 also shows little change in surface area among these samples thermally treated at different temperatures. From Figure 1, Figure 4, and Table 1, similar changes in crystal size and surface area were observed after thermal treatment in a vacuum in the above temperature range. However, no marked increase in photocatalytic activity was observed. Therefore, the enhancement in photoactivity after thermal treatment in air cannot be attributed to the growth of crystal size and unstable lattice oxygen resulting from thermal treatment. Effect of Surface-Adsorbed Oxygen on Photoactivity. The results for determination of oxygen adsorption show that more oxygen molecules are adsorbed on TiO2 after thermal treatment at 400 °C in air. These oxygen molecules may be enriched on the surface by coordinating with Ti4+ ions. Surface oxygen has a significant contribution to the photocatalytic activity of TiO2. The presence of more surface oxygen can result in an effective separation of the photoexcited electron and hole since the formation of superoxide anion radical stabilizes hydroxyl radical by preventing recombination according to the following equations:1
O2 + e- f O2-
(5)
OH-+ h+ f •OH
(6)
or H2O + h+ f •OH + H+
(7)
OH + e- f OH- (inhibited)
(8)
•
As more oxygen molecules are present on the surface of TiO2 thermally treated at 400 °C, more photoexcited electrons would be scavenged by surface oxygen to form superoxides on the TiO2 treated at 400 °C than that treated at 100 °C, and its photocatalytic activity is higher than that thermally treated at 100 °C for the oxidation of acetone in air. On the other hand, the small increases in photoactivity on the samples treated in a vacuum at 200400 °C (Figure 2) can be attributed to the formation of anion vacancy defect sites.22 The enhancement is not significant because these anion vacancies occur at the (29) Sclafani, A.; Herrmann, J. M. J. Phys. Chem. 1996, 100, 13655. (30) Yu, J. C.; Lin, J.; Kwork, R. W. M. J. Photochem. Photobiol. A: Chem. 1997, 111, 199.
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rutile TiO2(110) which represents only a minor component of the P25 powders. The explanation of adsorbed oxygen correlates well with the ESR experimental results. On the basis of Figure 6, irradiation of TiO2 thermally treated at 400 °C at 77 K in the presence of oxygen produced signals A and B, and the two signals were also obtained on irradiated TiO2 thermally treated at 100 °C under the same conditions. The signal B can be identified and assigned to Ti3+ ions since it has g-tensor components identical with those of Ti3+ formed on irradiated TiO2 at 77 K.31,32 The signal A on the two irradiated samples has parameters (g1 ) 2.026, g2 ) 2.016, and g3 ) 2.004) similar to those of the superoxide radical anion O2- formed on the anatase surface.33,34 Meriaudeau et al. found that this species can be formed on thermally activated TiO2 and anhydrous anatase in the presence of O2, respectively.34,35 The assignment has been confirmed by analysis of 17O-enriched O2.24 Therefore, it is reasonable that signal A is attributed to O2-. The signal A formed on the TiO2 thermally treated at 400 °C has higher intensity than that on the TiO2 treated at 100 °C. The reason for this is due to more oxygen molecules adsorbed on TiO2 thermally treated at 400 °C. On the (31) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1985, 89, 4495. (32) Rajh, T.; Ostafin, A. E.; Micic, O. I.; Tiede, D. M.; Thurnauer, M. C. J. Phys. Chem. 1996, 100, 4538. (33) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1987, 91, 3906. (34) Meriaudeau, P.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 2. 1976, 72, 472. (35) Naccache, C.; Meriaudeau, P.; Che, M.; Tench, A. J. Trans. Faraday Soc. 1971, 67, 506.
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other hand, the fact that signal B formed on irradiated TiO2 thermally treated at 400 °C has lower intensity than that on TiO2 treated at 100 °C indicates that there are less oxygen molecules on TiO2 treated at 100 °C since adsorbed oxygen functions as a very efficient electron scavenger, and the presence of more oxygen can inhibit the formation of Ti3+. Hence, the formation of more superoxide anion radicals O2- stabilizes hydroxyl radicals by preventing combination, which accounts for the enhancement in photocatalytic activity of TiO2 thermally treated at 400 °C. Conclusions An enhancement in the photocatalytic activity of TiO2 (P25) for the oxidation of acetone in air can be achieved by a simple thermal treatment in air. This thermal treatment in air allows more oxygen molecules to be adsorbed on the surface of TiO2. These oxygen molecules act as photoexcited electron traps and inhibit the recombination of photoexcited electrons and holes. Acknowledgment. The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (RGC Ref. No. CUHK4124/98). The authors thank Dr. Ming Fang of Hong Kong University of Science and Technology and Dr. Daniel Kwong of Hong Kong Baptist University for their technical assistance. LA000309W
