J. Phys. Chem. C 2007, 111, 11339-11346
11339
Can OH Radicals Diffuse from the UV-Irradiated Photocatalytic TiO2 Surfaces? Laser-Induced-Fluorescence Study Yoshinori Murakami,* Kenji Endo, Ikki Ohta, Atsuko Y. Nosaka, and Yoshio Nosaka* Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka, 940-2188, Japan ReceiVed: March 20, 2007; In Final Form: May 18, 2007
Diffusion of OH radicals from UV-irradiated TiO2 surface to the gas phase was successfully detected using a laser-induced-fluorescence technique for various types of TiO2 powders. The diffusion time of OH radicals was found to vary with the types of TiO2 powders and to be affected by the heat treatments of these powders, depending on the treatment temperatures. The diffusion mechanism was discussed based on the characteristic OH-LIF intensities for individual TiO2 powders and the observations of OD-LIF after the exposure of D2O vapors over the TiO2 powders. The quantum yield of OH radicals diffused from the TiO2 surface was estimated to be about 5 × 10-5 by comparing the OH-LIF intensities produced by the 266-nm photolysis of HNO3.
Introduction TiO2 photocatalysts have been widely studied because of their unique photochemical characteristics for potential applications to solar-to-chemical conversion (water splitting reactions)1-3 and environmental cleaning (photodecomposition of dirt or harmful pollutants).4-7 However, the initial process in the photocatalytic reactions has not been fully understood so far. Electrons and holes generated after the band gap excitation can reduce and oxidize the organic and inorganic compounds on the TiO2 surface, respectively. Furthermore, under ambient conditions, active oxygen species such as hydroxyl radicals (OH),8-10 superoxide radicals (O2•-),11-13 hydrogen peroxide (H2O2),14-16 and singlet oxygen17-19 are also formed at the TiO2 surface as a consequence of the primary photocatalytic processes. However, the relative roles of the active oxygens for degradation of chemical species are still not well understood. Especially the roles of OH radicals for the photocatalytic degradation processes have been under debate for a long time. The pioneering work of detecting OH radicals in the photocatalytic reactions of TiO2 was performed by Jaeger and Bard.8 They detected OH radicals using the spin-trapping technique and concluded that OH radicals were produced by the photocatalytic oxidation of water. Anpo et al.9 also detected the ESR signals of OH radicals, but because of the similarity of the ESR spectra of trapped holes20 and also because of the short lifetimes of OH radicals during the photocatalytic reactions, it is still not clear whether OH radicals were produced in the photocatalytic reactions of TiO2. Recently, Kamat et al.21 investigated the relative roles of active oxygen for the photocatalytic oxidation of arsenite and concluded that the reaction of As(III) with OH radical was important based on the observation of the transient absorption of adsorbed hydroxyl radicals. On the other hand, Ishibashi et al.10 suggested that the quantum yield of OH radicals was estimated to be 7 × 10-5 in aqueous solution using a terephthalic acid as a fluorescence probe and suggested that direct oxidation by the photogenerated holes played the major role in the degradation process of chemical species. Nosaka et al. showed that the TiO2 photocatalytic reaction of a sterically hindered cyclic amine to * To whom correspondence should be addressed. E-mail: murakami@ chem.nagaokaut.ac.jp (Y.M.).
give the corresponding nitroxide radical proceeds via trapped holes but not via OH radical.22 Cermenati et al.23 also suggested the important roles of O2•- for the degradation of quinoline from the product analysis of the TiO2 photocatalytic reactions. Thus, to clarify the relative roles of OH radicals in TiO2 photocatalysts, the direct detection of OH radicals produced by the photocatalytic TiO2 surface is now very important. Recently, Tatsuma et al. found for the first time that a dye24 and an organic film25 were oxygenated and decomposed even when these substances were separated by a small gap (50 µm2.2 mm). Subsequent studies on such remote oxidation processes of TiO2 photocatalysts were also performed and they found that silicon,26,27 diamond,28 silicon carbide,28 soot,29,30 poly(vinyl chloride),31 and noble metals including copper and silver26 can be oxidized by these remote oxidation processes. Since the electrons and holes cannot diffuse out of the TiO2 surface, it was supposed that OH radicals, which were known as the strongest oxidant among the active oxygen species mentioned above, diffused from the TiO2 surface and oxidized the organic and inorganic compounds separated from TiO2 surface. Majima et al.32,33 performed a single-molecule fluorescence imaging of the remote TiO2 photocatalytic oxidation and concluded that OH radicals were the active oxygen diffused from the TiO2 surface, although singlet oxygen could also possibly bleach the fluorescent dye. In this study, we present the first direct observation of the OH radicals diffused from the TiO2 surface using the laserinduced-fluorescence (LIF) technique. The LIF technique has already been applied by many of researchers to monitoring OH radicals in the gas phase,34 and recently this technique has also been applied to monitoring the diffusion of OH radicals to the gas phase from the catalyst35-37 as well as from the liquid surface.38,39 With this highly sensitive and selective LIF detection system, attempts to monitor the diffusion of the OH radicals from the photocatalytic TiO2 surface were performed and the roles of OH radicals on the TiO2 photocatalytic reactions were investigated. Experimental Section Since the experimental setup has already been described elsewhere,40 a brief description is given here. The reaction cell
10.1021/jp0722049 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
11340 J. Phys. Chem. C, Vol. 111, No. 30, 2007 is composed of quartz glass tubes connected with a rotary pump to evacuate the air in the reaction cell. Just below the center of the cell, a stainless steel holder containing about 1 g of the TiO2 powders was placed for investigating the photocatalytic reaction. After evacuation of the air in the cell, the water vapor was flowed at a typical pressure of 0.5 Torr since it was recognized that the water vapor was very important for forming the surface hydroxyl groups on the TiO2 surface. The third harmonic of a Nd:YAG laser (Continuum Minilite II) was irradiated to the TiO2 powders with a typical energy of 3 mJ/pulse at 355 nm with a repetition rate of 10 Hz. Before each experiment the leakage of the reaction cell was kept as low as about 10-3 Torr (1 Torr ) 133.3 Pa) per second when the TiO2 powders were absent from the reaction cell. To see the influence of the gases such as water vapor and oxygen on the formation of OH radicals from the photocatalytic TiO2 powders, the gas lines for flowing these gases were also connected to the reaction cell. The total pressure and partial pressure in the reaction cell were measured with a capacitance manometer (MKS Baratron, 622A11TAE). Various kinds of titanium dioxide (TiO2) powders were obtained from Ishihara Sangyo (ST-01), Japan Aerosil (P25), Showa Taitanium (F4, F6), and Tayca (AMT-100, AMT-600, MT500B). Helium gas (Nippon Sanso, 99.99999%) was used without further purification, and water (Milli-Q) and heavy water (D2O, 99.9%, Merck) were used after degassing by multiple freeze-pump-thaw cycles. The OH radicals diffused from the TiO2 surface were detected by monitoring the laser-induced fluorescence on the A-X transition. The OH radicals were excited by the A2Σ+(V′)1) r X2Π(V)0) transition around 282 nm, and the fluorescence of the A2Σ+(V′)1) f X2Π (V′′)1) transition around 310 nm was collected on a photomultiplier tube (Hamamatsu, R106UH) through two quartz glass lenses and a monochromator (JovinYvon, H10-UVA, fwhm ) 10 nm). The probe laser, whose diameter was within 2 mm, was generated by frequency doubling in the KDP crystal of the dye laser (Spectron, SL4000B) pumped by a Nd:YAG laser (Spectron, SL803) and was irradiated perpendicular to the 355-nm laser light about 5 mm above the surface of the TiO2 powders. The delay times between the pump and probe laser were controlled by a pulse generator (Stanford Research, DG535) and monitored by an oscilloscope (Lecroy, LT262). The signals were accumulated by boxcar integrators (Stanford Research, SR250) and transferred to a computer. Results and Discussion Identification of OH Radicals Diffused from the TiO2 Surface. Figure 1 shows an example of the laser-inducedfluorescence (LIF) spectrum of OH radicals observed in the 355-nm laser irradiation of the ST-01 TiO2 powders under the flow of the 0.5 Torr water vapor at a delay time of around 40 µs between the 355-nm laser and the probe laser lights. As shown in Figure 1, the rotational transitions in the 1-0 band of the A-X transition was resolved and assigned according to Dieke and Crosswhite.41 When the wavelength for exciting the surface of the TiO2 photocatalysts was changed to 532 nm, no OH-LIF signals were observed. Also, when the powders were changed from the TiO2 powders to the nonphotocatalytic powders such as MgO, BaSO4, and SiO2, no OH-LIF appeared even after the laser excitation at 355 nm. From these results, it was concluded that the electrons and holes generated by the band gap excitation of the TiO2 photocatalysis played important roles in the formation of OH radicals in the gas phase from the TiO2 surface.
Murakami et al.
Figure 1. OH A-X (1,0) LIF excitation spectrum produced by 355-nm laser irradiation of TiO2 (ST-01) powders. Pressure of H2O ) 0.5 Torr; photolysis-probe delay ) 40 µs; distance surface-probe beam ) 5 mm.
Figure 2. Time-resolved OH-LIF intensities after 355-nm laser irradiation of TiO2 (ST-01) powders with different surface-probe distances of (a) 5, (b) 6, (c) 7, and (d) 8 mm. Pressure of H2O ) 0.5 Torr.
The time profile for the OH-LIF intensity at one of the rotational lines formed by the irradiation of 355-nm laser light to the ST-01 TiO2 powders was obtained by varying the delay times between the 355-nm and the probe laser lights and is shown in Figure 2. The OH-LIF intensity was decreased with increasing delay time between the two lasers. Figure 2 also shows the time profiles of OH-LIF intensity with different distances between the probe laser beam and the TiO2 surface. At longer distances, each time profile of LIF intensity showed a maximum at the earliest time and later a relatively slow decay. When the distance between the two lasers increased, the time of the maximum intensity was shifted to the longer delay time and the intensity was also decreased. The same trend was already observed in the gas-liquid interfacial reaction of O(3P) atoms with hydrocarbons,38,39 where the sharp onset of the OH-LIF signals was correlated with the round trip flight time of O(3P) to the surface and the return of the OH radicals. In our case the OH radicals formed at the TiO2 surface after the 355-nm laser irradiation diffused to the observation area and therefore some induction period is required to obtain the OH-LIF signal. Assuming the cross section between the water molecules and OH radicals is 1 nm2, the mean free path under the 0.5 Torr of water vapor at room temperature is about 5 × 10-2 mm. Therefore, more than 100 collisions have to be suffered until arriving at the observation area when the distance between the probe laser beam and the TiO2 was 5 mm. Because of such numerous collisions the number of OH radicals arriving at the observation area may decrease with increasing distances. This
OH-LIF on TiO2 Powders
Figure 3. Time-resolved OH-LIF intensities after 355-nm laser irradiation of TiO2 (P25) powders with different environments: b, vacuum; O, 0.5 Torr of He; 2, 0.5 Torr of H2O; 4, 0.5 Torr of O2.
is consistent with the experimental observation that the OHLIF intensities decreased with the increasing distance between the probe laser beam and the TiO2 surface as shown in Figure 2. The increased activity of TiO2 photocatalysts by adding water vapor has been reported by several researchers in relation to the formation of hydroxyl radicals by the photooxidation of water on the TiO2 surface.42,43 On the other hand, the acceleration of the photodegradation of chemical species by adding molecular oxygen has also been reported.24 However, the roles of water vapor and oxygen in the formation of active oxygen species have not been clearly understood. Therefore, to find out the relative roles of water vapor and oxygen gas in the formation of OH radicals from the TiO2 surface, the influence of the surrounding gases on the OH-LIF intensities was investigated. For comparison the time profile of the OH-LIF intensity under the conditions without any gases and also under the conditions with a flow of about 0.5 Torr of He gas was also investigated. Figure 3 shows the time profiles of OH-LIF intensity from the P25 TiO2 surface at various surrounding gases. The OH-LIF signal was found even in the evacuated conditions (normally under total pressure of less than 0.2 Torr mainly because of the vapor pressure of the adsorbed water on the TiO2 powders). Since the surface hydroxyl groups survived on the TiO2 surface even under evacuation by the rotary pump, it is not surprising that the OH-LIF signal appeared without the flow of any gases. However, the intensities of the OH-LIF signal decreased during the 355-nm laser irradiations in the absence of water vapor flow. Since under the flow of water vapor in the reactor the OH-LIF intensities were almost constant, the water vapor supplies the surface hydroxyl groups which are the source of the OH radicals diffused from the photocatalytic TiO2 surface. The time profile of the OH-LIF intensity in He gas is also shown in Figure 3. The time profile obtained under the flow of He was similar to those obtained under the rough evacuation, but showed a slight shift of the maximum time of the OH-LIF intensities. It is known that the fluorescence is quenched by the collisions of the surrounding gases and the quenching efficiencies of the fluorescence are different with these gases because of the different interactions of these gases with the excited state of the fluorescent state.44 The quenching cross sections of the fluorescence of OH(A2Σ+ r X2Π) transition have already been determined to be σ(He) ) 4 × 10-5 nm2, σ(O2) ) 0.18 nm2, and σ(H2O) ) 0.8 nm2, respectively.45 Therefore, little difference in the time profiles with and without He is explained by the small quenching cross section of He gas. On the other hand, the introduction of water and oxygen on the TiO2 surface induced the large shift of the
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11341 maximum time and the decrease of the OH-LIF signals. The large shift of the maximum time in OH-LIF intensities corresponds to the effect on the time for diffusion of OH radicals to the observation area, and the diffusion coefficient is proportional to the product of the mean free path of the gas.46 Thus it is consistent that the maximum times of OH-LIF intensities were in the order of He < H2O < O2. However, the OH-LIF intensity in 0.5 Torr of H2O vapor was stronger than that in 0.5 Torr of O2 gas, which contradicts the fact that the quenching cross section of water is much higher than that of oxygen. These results show that water not only quenches the OH-LIF intensity, but also enhances the OH formation from the photocatalytic reactions of TiO2 surface. To confirm further the roles of water vapor on the OH formation from TiO2 surface, heavy water (D2O) vapor was flowed to the reaction cell and attempts to detect the OD-LIF signals were performed. As shown in Figure 4a, the OD-LIF signals corresponding to the OD(A2Σ+(V′)1) f X2Π(V′′)0)) transition38 appeared soon after the exposure of D2O vapor on the TiO2 surface. Similar to the OH-LIF signals, the time profile of the OD-LIF intensity was investigated by varying the delay time between the 355-nm and the probe laser lights. The results are given in Figure 4b. The time profile of the OD-LIF intensity showed a maximum around 100 µs, which was similar to the time profile of the OH-LIF intensity under the exposure of H2O vapor on the TiO2 (P25) surface, which was already shown in Figure 3. Thus, the mechanism of the diffusion of OD radicals from TiO2 surface is similar to that of OH radicals. Figure 5 is the time history of the OD-LIF intensity at one of the rotational lines of OH-LIF and OD-LIF before and after the exposure of D2O vapor. In Figure 5 the time when the D2O vapor was flowed in the reaction cell was set to zero. The intensity of the ODLIF signals became maximum about 20 s after the exposure of D2O vapors, as shown in Figure 5. If the physically adsorbed D2O moecules and their prompt dissociation played a key role in the OD formation from the photocatalytic TiO2 surface, the appearance of the OD-LIF signals just after the exposure of D2O must occur, which is not the case in the present work. Thus such an initial dead time for the OD-LIF appearance strongly suggests that the chemisorbed water, that is, the surface hydroxyl groups, plays a key role in this OD-LIF appearance. The dissociation of water on the rutile TiO2(110) surface has recently been investigated by using scanning tunneling microscopy, and it is concluded that the oxygen vacancies acted as the active site for water dissociation and then the two hydroxyl groups were formed after the reaction of the water molecule with the bridging-oxygen vacancies.47-49 Hence when the D2O molecules were introduced to the TiO2 surface, they reacted with TiO2 surface to form the surface OD groups, which might act as the precursor of OD radicals diffused to the gas phase from the TiO2 surface. Comparison of the Time-Resolved Intensities of the OHLIF Signals for Various TiO2 Powders. To confirm further the roles of the surface OH groups on the diffusion of OH radicals from the photocatalytic TiO2 powders, the time-resolved intensities of the OH-LIF signals for various kinds of commercially available TiO2 powders have been investigated. Figure 6 shows typical examples of the time profiles of OHLIF intensity for the ST-01 and P25 TiO2 powders without and with 0.5 Torr of H2O gas. The time profiles for ST-01 in this figure show little differences between without and with 0.5 Torr of H2O gas, but the time profile for P25 shows a maximum about 100 µs after the 355-nm laser excitation when 0.5 Torr of H2O gas was added. The time profiles of the OH-LIF
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Figure 4. (a) OD A-X (1,0) LIF excitation spectrum produced by 355-nm laser irradiation of TiO2 (P25) powders. p(D2O) ) 0.5 Torr; photolysisprobe delay ) 60 µs; distance surface-probe beam ) 5 mm. (b) Time-resolved OD-LIF intensities after 355-nm laser irradiation of TiO2 (P25) powders under flow of D2O vapors. The surface-probe distance was 5 mm, and the pressure of D2O was 0.5 Torr.
Figure 5. Comparison of OH-LIF and OD-LIF intensities before and after exposure of D2O vapors over TiO2 powders. The solid line indicates the OD-LIF signals, and the dashed line indicates the OHLIF signals. t ) 0 is the time for the initiation of the D2O flow over the TiO2 powders.
intensities for the other commercially available TiO2 powders such as F4, F6, AMT-100, AMT-600, and MT-500B were also investigated. Then, the influence of the water vapor on the OHLIF time profiles was found to be categorized in two types. We will designate “type A” for the same category as ST-01 and “type B” for the same category as P25. The difference in the feature of time profiles was specified by the relative change of the OH-LIF intensity at 180 µs by the addition of 0.5 Torr of water vapor. For type A the relative change was less than unity. That is, the OH-LIF intensity observed at 5 mm above the TiO2 surface at 180 µs after the excitation was decreased. On the other hand, type B profile is characterized with the increase of the OH-LIF intensity by the addition of 0.5 Torr of water vapor; i.e., the relative change is greater than unity as listed in Table 1. Besides the categories of the OH-LIF time profiles, the properties of the commercially available TiO2 powders such as anatase content, primary particle size, secondary particle size, BET surface areas, and amount of surface OH groups are listed in Table 1. The TiO2 powders consisting of 100% anatase are mostly categorized into type A, and the TiO2 powders containing rutile crystallite or sonsisting of 100% rutile are categorized into type B. Though AMT-600 consists of 100% anatase, the OH-LIF profile is categorized into type B such as P25 and F4. A similar classification of TiO2 powders has been reported by the present authors. In our previous study using 1H NMR spectroscopy, the chemical shift of tightly bound surface water was smaller than that of loosely bound water for AMT600, F4, and P25, while it was larger for AMT-100 and ST-01. The difference has been explained by the characteristic state of OH groups on the TiO2 surface.50 It is known that TiO2 surface has two types of hydroxyl groups, called terminal OH and
bridged OH. The time profiles of the OH-LIF intensity might be attributed to the different types of the surface hydroxyl groups, because each commercially available TiO2 powder has different ratios for the terminal and bridged hydroxyl groups. P25, F4, and F6 were prepared from TiCl4 at relatively higher temperature, while AMT-600 was prepared with heat treatment from the hydrolysis of titanium sulfonate. Since the bridged OH groups have greater thermal stability than the terminal OH groups,51 the calcinations of the TiO2 powders at various temperatures and controlling the relative ratios of the terminal and the bridged OH groups will give some insights into the relationship between the types of hydroxyl groups and the time profiles of the OH-LIF intensity. Hence, in the present work, studies of the influence of the calcined temperature on the OHLIF intensities were carried out for ST-01 and P25 TiO2 powders. Time-Resolved Intensities of the OH-LIF Signals for the Calcined TiO2 Powders. Figure 7 shows the results of the effect of calcination on OH-LIF profiles. For the ST-01 TiO2 powder calcined at 200 °C, the OH-LIF intensities were slightly increased compared to the noncalcined ST-01 TiO2 powder. When ST-01 TiO2 powder was calcined at 550 °C, the time profile of OH-LIF intensity showed a maximum about 100 µs after the 355-nm excitation, which is a typical time profile categorized into type B. Calcination of the ST-01 at higher temperatures resulted in the decrease of the OH-LIF intensities. For the P25 TiO2 powder the calcination at 200 °C also increased the OH-LIF signals and the calcinations at higher temperatures just reduced the OH-LIF intensity without a significant change in the peak position of the OH-LIF intensities. According to recent 1H NMR experiments for adsorbed water on the TiO2 surface,52 the heat treatments at 423 K (150 °C) for 2 h decrease the physisorbed water on the TiO2 surface and the calcinations below 873 K (600 °C) eliminate the terminal OH groups leaving the bridged OH groups. The treatments at a higher heat remove the bridged OH groups from the TiO2 powder. Figure 8 shows schematically the relative roles of the terminal and bridged OH groups after the heat treatments. Thus the plausible explanation for the increase in the OH-LIF intensities calcined at 200 °C for both ST-01 and P25 TiO2 powders may be the removal of the physisorbed water, because the physisorbed water may quench the OH-LIF intensities or otherwise inhibit the diffusion of the OH radicals from the TiO2 surface. The appearance of the peak for the time profile of ST01 TiO2 calcined at 550 °C is due to the removal of the terminal OH groups. That is, the OH-LIF signal arriving before 100 µs after the 355-nm laser excitation was the OH radicals diffused from the terminal OH groups. For further calcinations of ST01 above 550 °C, the OH-LIF intensity disappeared because of
OH-LIF on TiO2 Powders
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11343
Figure 6. Time-resolved OH-LIF intensities after 355-nm laser irradiation of various TiO2 powders with (b) and without (O) 0.5 Torr of H2O vapor: (a) TiO2 (ST-01), (b) TiO2 (P25).
TABLE 1: Relationships between Properties72 and Types of the OH-LIF Time Profiles
name
anatase/%
primary particle size/nm
ST-01 P-25 F-4 F-6 AMT-100 AMT-600 MT-500B
100 80 90 80 100 100 0
7 32 28 16 6 30 38
secondary particle size/nm
BET surface area/nm2 g-1
surface OH groups/wt %
change in OH-LIF with 0.5 Torr of H2O at 180 µs
3.73 1.11 0.23 0.53 0.52 0.62 2.49
320 49 56 98 260 50 38
5.2 1.8 4.9 6.2 5.1 0.5 s
0.7 2.6 1.8 1.2 0.9 1.3 2.4
the removal of the bridged OH groups. That is, the OH-LIF signal arriving 100 µs after the 355-nm laser excitation was the OH radicals diffused from the bridged OH groups. The P25 TiO2 powders, which were treated at relatively higher temperatures and therefore had some rutile phase, have more bridged OH groups than terminal OH groups, and this might be the reason the time profile of the OH-LIF intensities showed the time profile characteristic of that for the bridged OH groups. 355-nm Laser Power Dependence of the OH-LIF Signal Intensities from the TiO2 Surface and the Plausible Mechanism of OH-Radical Diffusion from the TiO2 Surface. To understand the mechanism for OH radical formation at the TiO2 surface, the power dependence was investigated for the OHLIF intensities at 40 and 180 µs after the 355-nm laser excitation. Based on the above discussions on the time profiles of the OHLIF intensity after the heat treatment of the TiO2 powder, the OH-LIF intensities at 40 and 180 µs correspond to the OH radicals diffused from the terminal and the bridged OH groups, respectively. The results are shown in Figure 9 as a log-log plot. From the slope, the power dependence of the OH-LIF
OH-LIF type type A type B type B type B type A type B type B
intensities at 40 µs was 1.82, suggesting that the diffusion of OH radicals from the terminal OH groups in TiO2 surface is a kind of two-photon process. On the other hand, the power dependence of the OH-LIF intensity at 180 µs was 0.91, suggesting that the diffusion of OH radicals from the bridged OH groups is a kind of one-photon process. Since the water has a broad continuum absorption from 145 to 186 nm,53 the two-photon absorption of H2O via the excitation to the TiO2 conduction band can occur at the wavelength of 355 nm with some probability. When the adsorbed water on the terminal site of the TiO2 surface or the terminal OH groups on the TiO2 surface absorbed two photons of the 355-nm laser light via the TiO2 conduction band, it is energetically possible that OH radicals diffuse to the gas phase from the TiO2 surface. On the other hand, the bridged OH groups could contribute to the formation and diffusion of OH radicals from the photocatalytic TiO2 surface, because the power dependence of the OH-LIF intensities at the delay time of 180 µs was nearly unity. The important role of the bridged Ti-O site on the photooxidation of water has been proposed by Nakamura et
Figure 7. Time-resolved signals of OH-LIF intensities after 355-nm laser irradiation of various TiO2 powders ((a) TiO2 (ST-01), (b) TiO2 (P25)) calcined at various temperatures. Pressure of H2O was 0.5 Torr.
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Figure 10. Dependence of OH-LIF intensities from the TiO2 (MT500B) surface on partial pressure of oxygen gas. Surface-probe distance ) 5 mm; photolysis-probe delay ) 20 µs. Pressure of H2O was 0.0 Torr.
that the adsorbed H2O2 on the TiO2 surface is believed to form OH radicals by the following reduction reaction of hydrogen peroxide based on the probe of the UV absorption of formaldehyde60 and the fluorescence probe of terephthalic acid:61
H2O2 + e- f •OH + OH-
Figure 8. Schematic figures of surface hydroxyl groups on TiO2 surface calcined at various temperatures.
Figure 9. OH-LIF intensity versus 355-nm laser intensity at delay times of 40 (O) and 180 µs (b). TiO2 (ST-01) powders were used, and the pressure of H2O was 0.5 Torr. The solid lines were the least-square fits to these plots.
al.54-57 as related to the identification of the intermediates during the photooxidation reaction of water during the water splitting reactions.58,59 They detected the surface peroxo species such as TiOOH and TiOOTi using multiple internal reflection infrared absorption spectroscopy and proposed a new reaction scheme for the formation of the surface peroxo species as a nucleophilic attack of H2O molecule on a photogenerated hole at the bridged Ti-O site.
[Ti-O-Ti]s + h+ + H2O f [Ti-O‚HO-Ti]s + H+ (1) [Ti-O‚HO-Ti]s + h+ f [Ti-O-O-Ti]s + H+
(2)
Then the Ti-O-O-Ti site further reacted with water molecule to form [Ti-OOH HO-Ti]s:55
[Ti-O-O-Ti]s + H2O f [Ti-OOH HO-Ti]s + H+
(3)
The [Ti-OOH HO-Ti]s is the alternative form of the adsorbed H2O2 on the bridged Ti-O site. It has been reported
(4)
Therefore it is plausible that the observation of OH radicals diffused to the gas phase from the photocatalytic TiO2 surface were due to such reduction reactions of the surface adsorbed H2O2 molecules. Although H2O2 molecules are also formed by the reduction reactions of oxygen molecules and the subsequent reactions
O2 + e- f O2-
(5)
O2- + O2- + 2H+ f H2O2 + O2
(6)
the enhancement of the formation of the OH radicals was not clearly observed because of the quenching of the OH-LIF intensities by the oxygen molecules as shown in Figure 3. To investigate further the roles of oxygen in the formation of OH radicals from the UV-irradiated TiO2 surface, the oxygen gas was flowed with much lower partial pressures, where the quenching of the OH-LIF intensities by the oxygen molecules was not so important. For these measurements the delay time between the 355-nm and the probe lasers was fixed at 20 µs. As shown in Figure 10, the OH-LIF intensity was increased to 0.1 Torr and then decreased with increasing partial pressure of oxygen gas. If OH radicals were formed by the photochemical and ablation processes of the TiO2 surface, the OH-LIF intensity was independent of the partial pressure of oxygen gas. Therefore, the increase of the OH-LIF intensities around 0.1 Torr of oxygen may be attributed to the photocatalytic reduction of H2O2 (eq 4). However, the enhancements of the charge separation because of the adsorption of oxygen on the TiO2 surface may also accelerate the OH radical formation. Thus we have demonstrated the important roles of the adsorbed H2O2 on the diffusion of OH radicals from the UV-irradiated photocatalytic TiO2 surface. Dependence of the OH-LIF Intensities on the Excitation Wavelength. In the present work, we proposed the OH radicals were formed by the surface reduction of H2O2 formed by the photocatalytic reactions at the TiO2 surface. On the other hand, Tatsuma et al.62,63 proposed that the mechanism of the photocatalytic remote oxidation was the photolysis of H2O2 in the gas phase produced and diffused from the photocatalytic TiO2 surface. Park et al.64 found that the remote oxidation process could occur even on the surface-fluorinated TiO2, but they doubted the double excitation scheme proposed by Tatsuma et
OH-LIF on TiO2 Powders
Figure 11. Time-resolved OH-LIF intensities after 355- (O) and 266nm (b) laser irradiation of TiO2 (ST-01) powders under evacuated conditions.
Figure 12. Time-resolved OH-LIF intensities after 355-nm laser irradiation of treated TiO2 (ST-01) powders under evacuated conditions: O, nontreated TiO2 (ST-01); b, H2O2-sintered TiO2 (ST-01); 9, CH3OH-sintered TiO2 (ST-01).
al. because no CO2 generation from the degradation of stearic acid was observed when H2O2 and the glass plate was used as a control experiment. To clarify this point, we have performed the OH-LIF experiments by changing the wavelength for the excitation of the photocatalytic TiO2 powders from 355 to 266 nm because the absorption cross section of H2O2 was different between these wavelengths (σ(265 nm) ) 4.4 × 10-20 cm2, σ(355 nm) < 5.3 × 10-22 cm2).65 Since the absorption cross section of H2O2 for 266 nm was 2 orders magnitude larger than that for 355 nm, it was expected that the OH-LIF intensities formed by the 266-nm laser excitation of the TiO2 powders were much larger than those formed by the 355-nm laser excitation of the TiO2 powders. Figure 11 shows the comparison of the time profiles of the OH-LIF intensity for the excitation at 355-nm and 266-nm laser lights. As shown in Figure 11, the differences of the OH-LIF intensities were within a factor of 2 and no significant effects on the OH-LIF intensities due to the differences of the absorption coefficients of H2O2 between 355 and 266 nm have been observed. Thus it was suggested that the OH radicals were formed by the surface reduction of adsorbed H2O2 formed by the photocatalytic reactions of TiO2 powders. Influences of the OH-LIF Intensities on the Addition of H2O2 and CH3OH. To observe further the effects of the electrons and holes at the conduction band and valence band, respectively, the time profiles of OH-LIF intensity obtained for the ST-01 and the ST-01 sintered with H2O2 and also the ST01 sintered with methanol under the evacuated conditions (that is, no water flow) were investigated; the results are shown in Figure 12. When the ST-01 was sintered in H2O2, the OH-LIF intensity was increased compared with the pure ST-01 powders. This suggests that OH radicals were formed by the reduction
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11345 of H2O2 (eq 4), which is consistent with the mechanism of the OH radical diffusion in the gas phase from the TiO2 surface. Relatively faster decay of the OH-LIF intensities is due to the reaction of OH radicals with H2O2. On the contrary, when the ST-01 TiO2 powder was sintered with methanol, the OH-LIF signals completely disappeared. The reaction of methanol (CH3OH) with holes has been well-known and often used as the hole scavengers for investigating the half-reaction of the water splitting reaction.66 The reaction of the holes with methanol have also been directly and indirectly observed by the transient absorption of the trapped holes in TiO2 nanocrystalline films67 and time-resolved infrared absorption spectroscopy.68 The inhibition effects of the OH formation by the methanol were due to such hole-scavenging effects. Once the holes were scavenged by the methanol molecules, the oxidation of water on the TiO2 surface was inhibited, resulting in the inhibition of H2O2 formation on the TiO2 surface. Also, the decrease of the surface hydroxyl groups by the substitutions of the methoxy groups may be another explanation. Further work should be done for clarifying the effect of methanol. Quantum Yields. Finally, the quantum yield of the OH radicals diffused from the TiO2 surface was estimated. To determined the absolute concentration of OH radicals in the gas phase, the 266-nm laser photolysis of HNO3 gas was used since the quantum yield for OH formation and the absorption cross section at 266 nm were well established. To avoid the complications arising from the quenching of the OH-LIF by the water vapor, the OH-LIF intensities diffused from the TiO2 powders were measured under the evacuated conditions without any gases. Using the quantum efficiency φ ) 1 and the absorption cross section at 266 nm (σ(HNO3, λ ) 266 nm) ) 1.7 × 10-20 molecules cm-3),69,70 a typical concentration of OH radicals inside the observation area was determined to be around 5.0 × 1012 molecules/cm3. Assuming that all of the 355-nm laser light was absorbed to the TiO2 powders and that the concentration of the OH radicals diffused from the TiO2 surface was constant within the volume of 0.05 cm3 (this value is based on the product of the laser beam area 0.1 cm2 and the maximum diffusion height 0.5 cm), the quantum yield was estimated to be 5 × 10-5, which is nearly equal to the quantum yields of 2 × 10-4 (polystyrene) and 3 × 10-5 (polyethylene) by the TiO2-coated glass with a small gap of 50 µm,25 but much larger than the quantum yield of H2O2 (1.4 × 10-7 in ref 15, 1.8 × 10-7 in ref 63). This value is not high enough compared with the quantum yields of superoxide radicals (O2•-)71 and singlet oxygen (1O2),19 which were determined to be 0.4 and 0.12-0.38, respectively, but the diffused OH radicals from the TiO2 surface may also act as the important active oxygen species for the degradations of the hard-degradable chemical compounds when the other active oxygen species could not decompose these compounds. Acknowledgment. This work was supported in part by a Grant-in-Aid on Priority Areas (417) from the Ministry of Education, Culture, Science and Technology (MEXT). We thank Prof. Katsuyoshi Yamasaki, Hiroshima University, and Dr. Tatsuo Oguchi, Toyohashi University of Technology, for their helpful suggestions. We also thank Prof. Tetsuro Majima, The Insititute of Scientific and Industrial Research, Osaka University, for useful suggestions. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Sato, S.; White, M. J. Chem. Phys. Lett. 1980, 72, 83. (3) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082.
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