J. Phys. Chem. C 2007, 111, 693-699
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Photoluminescence Characteristics of TiO2 and Their Relationship to the Photoassisted Reaction of Water/Methanol Mixture Jianying Shi, Jun Chen, Zhaochi Feng, Tao Chen, Yuxiang Lian, Xiuli Wang, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed: September 4, 2006; In Final Form: October 25, 2006
This work investigated the photoluminescence characteristics of TiO2 and discussed the relationship between the photoluminescence features of TiO2 and the photoassisted reaction of water/methanol mixture. It is found that anatase TiO2 displays a visible luminescence band centered at about 505 nm and rutile TiO2 mainly shows a near-infrared luminescence band centered at about 835 nm, which are respectively ascribed to the oxygen vacancies in anatase TiO2 and the intrinsic defects in rutile TiO2. The visible luminescence band is easily quenched by the Pt deposited on the surface of TiO2, while the near-infrared luminescence band is hardly influenced by the deposited Pt. It is suggested that the excited electrons trapped in the oxygen vacancies of anatase are facilely transferred to Pt to contribute to the photoassisted reaction, but the electrons trapped in the intrinsic defects of rutile are not.
1. Introduction Photoluminescence is a nondestructive and high-sensitivity technique widely used to investigate the photophysical and photochemical properties of solid semiconductors, particularly in the photocatalysis field.1-7 Photoluminescence techniques can supply information such as surface oxygen vacancies and defects as well as the separation and recombination of photoinduced charge carriers. The application of photoluminescence as a technique for the characterization of solid surface in relation to adsorption, catalysis, and photocatalysis has been reviewed.8 TiO2 is one of the most widely used photocatalysts because of its exceptional optical and electronic properties, strong oxidizing power, nontoxicity, chemical stability, and low cost.9 Anatase and rutile are the two main crystalline phases structures of TiO2 with band energies at 3.210 and 3.0 eV,11 respectively. It is well-known that the photoactivity of TiO2 heavily depends on its crystal structure12 and surface properties,3,13,14 etc. While the crystal structures15-18 and the surface properties3,5,6,19 of TiO2 also have essential correlation with the luminescence features of TiO2. Therefore, investigation of TiO2 by photoluminescence spectroscopy can supply information about the surface sites of TiO2 and their contributions to the photoreaction performance of TiO2.1,4,7,19 As TiO2 is an indirect wide-gap semiconductor, the band edge luminescence is difficult to observe.20 It has been reported that the main features of TiO2 are the broad and structureless visible or near-infrared luminescence bands. However, the relationship of the photoluminescence features with the crystal structures of TiO2 is ambiguous, and the correlation between the photoluminescence of TiO2 and its photoassisted reaction performance has not been well investigated. In present work, the photoluminescence features of TiO2 are studied in the processes of phase transformation of TiO2 from anatase to rutile, and the origins of the luminescence bands are also studied by in situ photoluminescence spectroscopy. On the basis of the influence of deposited Pt on the photoluminescence features and photoactivities, the relationship between photoluminescence properties of TiO2 and photoactivities of H2
production from the photoassisted reaction of water/methanol mixture is discussed. 2. Experimental Section 2.1. Sample Preparation. TiO2 was prepared by hydrolysis of Ti(OC4H9)4. All chemicals were purchased as guaranteed reagents and used without further purification, and the water was double-distilled and deionized. Tetrabutyl titanate was mixed with ethanol to form an alkoxide solution. A solution of water and ethanol was slowly added dropwise to the alkoxide solution with continuous stirring. After hydrolysis for 24 h with vigorous stirring, the precipitate was filtered and washed three times with water and ethanol, and then was kept at 120 °C overnight. The resultant powder [mostly Ti(OH)4] was calcined at different temperatures in air for 2 h to get TiO2 sample. The commercial anatase (99%) and rutile (99.99%) TiO2 were purchased from Acros Organics and Alfa Aesar, respectively. The samples obtained by calcining commercial anatase at 900 °C for 6 h and 1200 °C for 24 h in air were denoted as A-900 °C and A-1200 °C, respectively. For Pt/TiO2 samples, 0.3 wt % Pt was photodeposited on TiO2 in a mixture of platinizing solution (H2PtCl6 in methanol solution) irradiated by a 300-W Xe lamp for 6 h. The special Brunauer-Emmett-Teller (BET) (nitrogen) surface areas of TiO2 calcined at 500, 650, and 900 °C are 40.1, 4.0, and 2.6 m2‚g-1, respectively. The particle sizes of TiO2 prepared at 500 °C and the commercial anatase are about 15 and 100 nm deduced from the TEM images, respectively. (XRD results of commercial TiO2 and TEM images of TiO2 calcined at 500 °C and of commercial anatase can be obtained in the Supporting Information.) 2.2. Spectroscopy Measurement. UV-Raman spectra were measured at room temperature on a Jobin-Yvon T64000 triplestage spectrograph with spectral resolution of 2 cm-1. The photoluminescence spectra were measured in a home-built laserinduced luminescence spectrograph,21 which is a very convenient design for in situ luminescence studies under working conditions. The photoluminescence signal was collected with an ellipsoidal collecting mirror and focused onto a 320 mm
10.1021/jp065744z CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006
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Figure 1. Photoluminescence spectra of TiO2 calcined at different temperatures.
monochromator (Jobin-Yvon Triax 320) by passing through a filter with cutoff wavelength below 360 nm. A charge-coupled device (Jobin-Yvon Spectrum One CCD 3000) was mounted at the focal plane in the exit of the monochromator to detect the luminescence signal. Prior to the experiments, the wavelength calibration of this setup was carried out with a mercury lamp. A continuous-wave 325-nm He-Cd laser was used as the exciting source for the measurement of Raman and photoluminescence spectra. The photoluminescence and Raman spectra were measured for the powder samples in air at room temperature. In the in situ experiments, the samples were pressed into a thin disk and fixed in a homemade quartz cell.21a The details of the in situ treatment processes are described as follow: first, heating samples at 520 °C in O2 for 1 h; second, annealing samples at 520 °C in H2 for 2 h; third, exposing samples to water vapor at room temperature for 30 min (water vapor was introduced with high-purity N2 passing through a H2O saturator); then the adsorbed water on the sample was degassed at 100, 300, and 500 °C step by step in N2. The notation “T-A-t” is used to denote the treatment conditions in the in situ experiment, where T denotes the thermal treatment temperature, A is the thermal treatment atmosphere, and t is the thermal treatment time. All the in situ spectra were measured after the sample was cooled down to room temperature in the treatment atmosphere and purged by high-purity N2 for 30 min, except the spectra of the samples exposed to water, which were measured in water vapor at room temperature. 2.3. Photoactivity Measurement. Photoassisted reaction was performed in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. The light source was a 300-W Xe lamp. TiO2 powder (0.2 g) was suspended in 200 mL of solution containing 160 mL of H2O and 40 mL of CH3OH. For Pt-loaded TiO2 catalyst, 0.3 wt % Pt was photodeposited on the TiO2 from H2PtCl6 solution under irradiation. The amount of H2 produced was analyzed by on-line gas chromatography. 3. Results 3.1. Photoluminescence and UV-Raman Characteristics of TiO2. Figure 1 shows the photoluminescence spectra of TiO2 calcined at different temperatures. The sample calcined at 500 °C displays a visible luminescence band centered at 505 nm. With increasing calcination temperature, a near-infrared luminescence band centered at 835 nm appears, while simultaneously, the visible luminescence band centered at 505 nm is weakened. Further increasing calcination temperature, the near-infrared emission become stronger while the visible emission is quenched to a large degree. The sample calcined at 900 °C displays the
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Figure 2. UV-Raman spectra of TiO2 calcined at different temperatures.
stronger near-infrared luminescence band together with the negligible visible luminescence band. It should be mentioned that the fringes in the near-infrared (NIR) region of the spectrum originate from the etaloning effect of back-illuminated CCD. Raman spectra can sensitively identify the anatase and rutile phases of TiO2 on the basis of their characteristic Raman bands. The anatase phase shows major Raman bands at 144, 197, 399, 515, 519 (superimposed with the 515 cm-1 band), and 639 cm-1, which are attributed to the six Raman-active modes of anatase phase with the symmetries of Eg, Eg, B1g, A1g, B1g, and Eg, respectively.22 The Raman bands of rutile phase appear at 143, 235, 447, and 612 cm-1, which are ascribed to the B1g, twophonon scattering, Eg, and A1g modes of rutile phase, respectively.23 Figure 2 displays the UV-Raman spectra of TiO2 calcined at different temperatures. The sample calcined at 500 °C shows the five characteristic Raman bands at 143, 195, 395, 515, and 638 cm-1, which means this sample is pure anatase phase. When the sample is calcined at 600 °C, the Raman bands at 445 and 612 cm-1 are observed besides the Raman bands related to anatase phase; that is, part of anatase phase has been transformed into rutile phase at 600 °C. With the increase of calcination temperature, the characteristic Raman bands of rutile phase predominate in the spectra and the Raman bands of anatase phase are diminished. The sample calcined at 900 °C is completely transformed into rutile phase according to the characteristic Raman bands of rutile. By comparison of the photoluminescence spectra with the UV-Raman spectra of TiO2 (Figures 1 and 2), it can be seen that the position of luminescence bands is related to the crystalline structure of TiO2. The sample calcined at 500 °C is pure anatase phase and displays the visible luminescence band centered at 505 nm. When the calcination temperature is elevated to 600 °C, the rutile phase begins to be detected by Raman spectroscopy; correspondingly, the near-infrared luminescence band appears in the photoluminescence spectra. With the increase of the calcination temperature, anatase phase is gradually transformed into rutile phase, while the visible luminescence band is quenched and the near-infrared luminescence band prevails. When anatase phase completely transforms into rutile phase, the visible luminescence band nearly disappears and the near-infrared luminescence band predominates in the photoluminescence spectra. Therefore, it is deduced that the visible luminescence band located at 505 nm is related to the anatase structure and the near-infrared luminescence band centered at 835 nm is associated with the rutile structure. In order to clarify the relationship between the crystal structure and the photoluminescence characteristics, the spectra of commercial TiO2 with pure anatase or rutile phase are
Photoluminescence Characteristics of TiO2
Figure 3. UV-Raman spectra of commercial TiO2: (a) anatase, (b) anatase calcined at 900 °C for 6 h, (c) anatase calcined at 1200 °C for 24 h, and (d) rutile.
Figure 4. Photoluminescence spectra of commercial TiO2: (a) anatase, (b) anatase calcined at 900 °C for 6 h, (c) anatase calcined at 1200 °C for 24 h, and (d) rutile.
Figure 5. In situ photoluminescence spectra of TiO2 calcined at 500 °C: (a) after being calcined at 520 °C for 1 h in O2 and purged by N2; (b) after being calcined at 520 °C for 2 h in H2 and purged by N2; (c) after exposure to water vapor for 30 min; (d) after water vapor was purged by N2; and followed by thermal desorption of water in N2 for 20 min at (e) 100 °C, (f) 300 °C, and (g) 500 °C.
measured and the samples obtained by calcining commercial anatase at 900 and 1200 °C are also measured for comparison. The Raman and photoluminescence spectra of commercial TiO2 are displayed in Figures 3 and 4, respectively. It can be seen that the two commercial TiO2 samples are in pure anatase and rutile phases (Figure 3a,d). Anatase TiO2 displays a visible luminescence band centered at 505 nm (Figure 4a) while rutile TiO2 shows a near-infrared luminescence band centered at 835 nm (Figure 4d). The A-900 °C sample, obtained by calcination of anatase at 900 °C for 6 h in air, is still in pure anatase phase (Figure 3b) and its luminescence band is still located at 505 nm (Figure 4b). When anatase is calcined at 1200 °C for 24 h
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Figure 6. (A) In situ photoluminescence spectra of TiO2 calcined at 600 °C: (a) after being calcined at 520 °C for 1 h in O2 and purged by N2; (b) after being calcined at 520 °C for 2 h in H2 and purged by N2; (c) after exposure to water vapor for 30 min; (d) after water vapor was purged by N2; and followed by thermal desorption of water in N2 for 20 min at (e) 100 °C, (f) 300 °C, and (g) 500 °C. (B) Enlarged segment of panel A from 700 to 1000 nm.
Figure 7. In situ photoluminescence spectra of TiO2 calcined at 900 °C: (a) after being calcined at 520 °C for 1 h in O2 and purged by N2; (b) after being calcined at 520 °C for 2 h in H2 and purged by N2; (c) after exposure to water vapor for 30 min; (d) after water vapor was purged by N2; and followed by thermal desorption of water in N2 for 20 min at (e) 100 °C, (f) 300 °C, and (g) 500 °C.
in air, the A-1200 °C sample completely transforms into rutile phase (Figure 3c) and the corresponding luminescence band shifts to the near-infrared region centered at 835 nm (Figure 4c). The results obtained from commercial TiO2 are consistent with those obtained from the prepared TiO2, which further indicates that the visible luminescence band located at about 505 nm and the near-infrared luminescence band centered at about 835 nm are respectively related to anatase and rutile structures. It should be mentioned that the evident difference of phase transformation temperatures between prepared TiO2 and the commercial TiO2 is mainly caused by the obvious difference in their particle sizes. Gribb and Banfield24 had demonstrated that the increase in favorable nucleation sites is a likely cause of increased transformation rate at small crystallite sizes. 3.2. In Situ Photoluminescence Characteristics of TiO2. The photoluminescence process is closely related to the surface stoichiometry and the kinds of surface states, which could usually be changed by an annealing process.25 Sekiya et al.26 reported that the defect states of TiO2 can be controlled by heat treatments under oxidation or reduction atmosphere. In the in situ experiments, pure oxygen and hydrogen are used as the oxidation and reduction atmospheres, respectively, and nitrogen is used as the inert atmosphere to purge the sample. The samples calcined at 500, 650, and 900 °C are chosen in the in situ experiment for their distinct luminescence characteristics.
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Figure 8. Photoluminescence spectra of pure TiO2 and Pt/TiO2 powders calcined at (A) 500 °C, (B) 650 °C, and (C) 900 °C.
Figure 5 gives the in situ luminescence features of TiO2 sample calcined at 500 °C. After the sample is heated in O2 at 520 °C for 1 h and cooled down to room temperature, the relatively weak visible luminescence band centered at 505 nm is observed (spectrum a). A following thermal treatment in H2 at 520 °C for 2 h leads to increased intensity of this band by a factor of 6 (spectrum b). After exposure of the partially reduced sample to water vapor for 30 min, the visible-luminescence intensity is further increased (spectrum c). When the water vapor was purged by high-purity N2 (spectrum d), the intensity of this band slightly declined. The photoluminescence spectra with the water desorption at 100, 300, and 500 °C for 20 min in N2 step by step are shown in spectra e-g, respectively. It can be seen that the desorption of water at 100 °C has no obvious impact on the intensity of visible emission centered at 505 nm (spectrum e). But the intensity of this band declines dramatically after the desorption of water at 300 °C (spectrum f). With the desorption of water at 500 °C, the intensity of this band (spectrum g) is decreased to the initial level of that treated in O2 (spectrum a). The luminescence features of TiO2 sample calcined at 650 °C are shown in Figure 6. Both the visible and the nearinfrared luminescence bands are observed after annealing of the sample at 520 °C in O2 for 1 h (spectrum a). The change trend of the visible luminescence band centered at 505 nm (Figure 6A) is similar to that shown in Figure 5. After the sample was treated in H2, the intensity of the visible luminescence band increased obviously (spectrum b). The exposure of the sample to water vapor lead to further intensity increase of this band (spectrum c). With removal of the water vapor (spectrum d) in the cell and water adsorption on the sample (spectra e-g), this band is gradually quenched and finally comes back to the level of that treated in O2 (spectrum a). Figure 6B shows enlarged spectra in the 700-1000 nm range of Figure 6A. The nearinfrared luminescence band displays high intensity after the sample was annealed in O2 (spectrum a). Treatment of the sample in H2 quenches this band (spectrum b) and it is not recovered in the adsorption and desorption processes of water (spectra c-g). Figure 7 gives the in situ luminescence features of the sample calcined at 900 °C, which mainly exhibits the near-infrared emission at 835 nm. The near-infrared luminescence band in Figure 7 displays similar changes to those shown in Figure 6B. After the sample was annealed at 520 °C in O2 for 2 h, this band presented higher intensity (spectrum a). Treating the sample in H2 led to the quenching of this emission (spectrum b), and exposure of the sample to water had no obvious influence on this band (spectra c-g). As shown in Figures 5-7, the visible and near-infrared luminescence bands exhibit obviously different characteristics, which indicate that the two luminescence bands originate from
Figure 9. Photocatalytic H2 evolutions from methanol-water solution under UV light illumination on pure TiO2 and Pt/TiO2 catalysts. Calcination temperature of TiO2 is given in parentheses. Light source, 300-W Xe lamp.
different luminescence centers. In the whole process of in situ experiments, the intensities of the two luminescence bands vary significantly but the positions of these bands remain unchanged. It is suggested that the kind of luminescence center is not changed but the amount of each center varies in the process of treatment. 3.3. Influence of Deposited Pt on Photoluminescence Properties and Photoactivities of TiO2. It is well-known that Pt deposited on the surface of TiO2 plays a vital role in the photoassisted reaction of water/methanol mixture. Figure 8 compares the photoluminescence features of pure TiO2 and Pt/ TiO2. It can be seen that the visible luminescence band of TiO2 is obviously quenched after loading of Pt on TiO2, while the near-infrared luminescence band slightly declines after deposition of Pt on TiO2. The photoactivities of pure TiO2 and Pt/ TiO2 are also compared in Figure 9. On the pure TiO2 samples calcined at 500, 650, and 900 °C, the amount of H2 production is negligible, while the evident activities of hydrogen production are detected after loading of Pt on these TiO2 samples. The order of photoactivities of TiO2 calcined at different temperature is 500 °C > 650 °C > 900 °C; that is, the photoactivity of anatase TiO2 is higher than that of rutile TiO2. 4. Discussion 4.1. Origins of Visible and Near-Infrared Luminescence Emissions. Photoluminescence is mostly a surface phenomenon, and a change in the surface environment would have a significant effect on the photoluminescence process. It was reported that treating TiO2 in hydrogen atmosphere could generate oxygen vacancies,27-29 which were accompanied by the generation of Ti3+ ions,26,30,31 while oxidizing TiO2 in oxygen atmosphere could remove oxygen vacancies and produce the stoichiometric surface.26,32 In the in situ photoluminescence
Photoluminescence Characteristics of TiO2
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Figure 10. Schematic illustration of the photoluminescence evolution with the phase transformation of TiO2 and the deexciting processes of the photoinduced electrons in anatase TiO2.
experiments, the visible luminescence band is quenched after annealing of TiO2 in O2 and is enhanced after the following reduction treatment in H2. Therefore, it is concluded that the visible luminescence band originates from the oxygen vacancies associated with Ti3+ in anatase TiO2. Similar ascription of the visible luminescence band was reported by Mochizuki et al.,33 Ferna´ndez et al.,34 and Serpone et al.35 On the contrary, the near-infrared luminescence band is increased after oxidation treatment in O2 and quenched after reduction treatment in H2. It is deduced that the near-infrared luminescence centers are related to the intrinsic defects in rutile TiO2. Poznyak et al.16 also reported that this band was associated with the luminescence centers of intrinsic defects in TiO2, which were more likely from the rutile lattice compared with the anatase lattice. The nature of these intrinsic defects in rutile phase will be investigated in future work. The interaction between water and the surface of TiO2 has been studied by a variety of techniques, such as temperatureprogrammed desorption (TPD), scanning tunneling microscopy (STM), and X-ray and ultraviolet photoemission spectroscopies (XPS and UPS). Kurtz et al.36 reported that the dissociative adsorption of H2O on TiO2 surface resulted in increased of Ti3+ defect state numbers, which means an increased number of oxygen vacancies. That could explain the increase in the visible emission intensity after exposure of the surface of TiO2 to water vapor. The interaction between water and the defective TiO2 surfaces is actually involved not only in the adsorption and dissociation of water on the surface but also in the surface redox reaction where the reduced Ti3+ states are reoxidized by the adsorbed water to Ti4+.37,38 The oxygen atoms in water molecules are preferentially extracted by the substrate to fill the surface oxygen vacancies. Therefore, gradual quenching of the visible luminescence band is observed in the following
processes of thermal treatment of TiO2 at 100, 300, and 500 °C. After treatment at 500 °C, the surface of TiO2 is fully oxidized by water and accordingly a nearly perfect surface is formed; as a result, the visible-luminescence intensity decreases to the level of that annealed in O2. The interaction between water and the surface of TiO2 further indicates that the visible luminescence centers are the oxygen vacancies in TiO2. It is well-known that anatase and rutile are the two major crystal structures of TiO2 and they are commonly used in photoassisted reactions. The structures of these two crystals can be described in term of chains of TiO6 octahedra; they differ by the distortion of each octahedron and by the assembly pattern of the octahedral chains.39 The differences in the structures of rutile and anatase crystals govern the differences in their electrical and optical properties.23,40,41 It is reasonable that the crystalline structures of TiO2 determine the type of luminescence center; that is, oxygen vacancies related to visible emission prevail in anatase, while defect states related to near-infrared emission predominate in rutile. With the phase transformation of TiO2 from anatase to rutile, the luminescence centers change from oxygen vacancies to intrinsic defects, and the visible luminescence emission is correspondingly replaced by nearinfrared luminescence emission. 4.2. Relationship between Photoluminescence Features and Photoassisted Reaction Performance of TiO2. When Pt is deposited on TiO2, excited electron migration from the semiconductor TiO2 to the metal Pt occurs until the two Fermi levels are aligned.39 The Schottky barrier formed at the Pt/TiO2 interface can serve as an efficient electron trap, preventing electron-hole recombination in photocatalysis.39 For TiO2 alone, the photogenerated carriers are mainly deexcited as the luminescence is emitted through the recombination centers of different defects and its photoactivity is negligible. After
698 J. Phys. Chem. C, Vol. 111, No. 2, 2007 deposition of Pt on TiO2, a large number of photogenerated electrons are transferred to Pt and readily consumed by the photoassisted reaction; as a consequence, the effective enhancement of H2 evolution rate is observed for Pt/TiO2 (Figure 9). When Pt is deposited on the surface of TiO2, the visible luminescence band is obviously quenched (Figure 8), which indicates that the excited electrons trapped in oxygen vacancies of anatase are facilely transferred to the Pt. On the contrary, the excited electrons trapped in the intrinsic defects of rutile are hardly transferred to the Pt, based on the little influence of Pt on the near-infrared luminescence band (Figure 8). Namely, Pt on the surface of TiO2 inhibits the photoinduced carrier recombination at oxygen vacancies in anatase, but has little influence on photoinduced carrier recombination at the intrinsic defects in rutile. That may be a reason why the photoactivity of anatase TiO2 is usually higher than that of rutile TiO2. In our previous work,42 it was proposed that phase transformation of TiO2 starts from the interfaces between the anatase particles in the agglomerated TiO2 particles. The scheme for photoluminescence evolution with the phase transformation of TiO2 is illustrated in Figure 10. The sample calcined at 500 °C is pure anatase phase and a number of the oxygen vacancies distribute on the surface of TiO2 particles. The unique visible luminescence band is observed in the photoluminescence spectra. With the increase of calcination temperature, the intrinsic defects in rutile appear, accompanied with the formation of rutile TiO2; meanwhile, the number of oxygen vacancies is reduced. Hence, the visible luminescence band is decreased while the near-infrared luminescence band is observed in the photoluminescence spectra. When the sample is completely transformed to rutile phase, the visible emission is quenched and the near-infrared emission predominates in the photoluminescence spectra. The deexciting processes of the photogenerated electrons in anatase TiO2 are also illustrated in Figure 10. For TiO2 alone, the photoexcited electrons in TiO2 are trapped in oxygen vacancies (process 1), then deexcited by the visible emission at these oxygen vacancy sites (process 2). After Pt is deposited on the surface of TiO2, in addition to the photoexcited electrons that are directly transferred to Pt (process 3), the excited electrons trapped in oxygen vacancies are also transferred to Pt (process 4) to contribute to the photoassisted reaction. That is, the Pt deposited on the surface of TiO2 changes the distribution of photoexcited electrons in TiO2. 5. Conclusions In this paper, the photoluminescence characteristics of TiO2 are investigated and the relationship between photoluminescence features and photoactivity of the photoassisted reaction in water/ methanol mixture is discussed. It has been found that the photoluminescence properties depend, to a considerable extent, on the crystalline structure of TiO2. Anatase TiO2 shows a visible luminescence band centered at about 505 nm, and rutile TiO2 mainly displays a near-infrared luminescence band at about 835 nm. The visible and near-infrared luminescence bands are respectively ascribed to oxygen vacancies in anatase TiO2 and intrinsic defects in rutile TiO2. On the basis of obvious quenching of the visible luminescence band and less change in the near-infrared luminescence band after Pt is deposited on the surface of TiO2, it is deduced that the excited electrons trapped in oxygen vacancies of anatase are facilely transferred to Pt to participate in the photoassisted reaction, while the electrons trapped in intrinsic defects of rutile are still deexcited by near-infrared emission. This may be a reason why the
Shi et al. photoactivity of anatase is usually higher than that of rutile. This work further demonstrates that photoluminescence is a powerful technique for characterizing semiconductor materials and investigating the mechanism of photoassisted reactions. Acknowledgment. This work was financially supported by the National Science Foundation of China (NSFC, Grants 20273069 and 20273070), the National Basic Research Program of China (Grant 2003CB214504), and the Knowledge Innovation Program of the Chinese Academy of Science (DICP K2006E2). Supporting Information Available: XRD results of commercial TiO2 and TEM images of TiO2 calcined at 500 °C and commercial anatase. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello, E. J. Phys. Chem. 1985, 89, 5017. (2) Anpo, M; Shima, T.; Kubokawa, Y. Chem. Lett. 1985, 1799. (3) Anpo, M.; Tomonari, M.; Fox, M. A. J. Phys. Chem. 1989, 93, 7300. (4) Nakajima, H.; Itoh, K.; Murabayashi, M. Chem. Lett. 2001, 304. (5) Nakajima, H.; Mori, T.; Watanabe, M. Jpn. J. Appl. Phys. 2004, 43, 3609. (6) Nakajima, H.; Mori, T.; Watanabe, M. J. Appl. Phys. 2004, 96, 925. (7) Jung, K. Y.; Park, S. B.; Anpo, M. J. Photochem. Photobiol. A: Chem. 2005, 170, 247. (8) Anpo, M.; Che, M. AdV. Catal. 2000, 44, 119. (9) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (10) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (11) Rao, M. V.; Rajeshwar, K.; Pai Verneker, V. R.; Dubow, J. J. Phys. Chem. 1980, 84, 1987. (12) (a) Tsai, S.; Cheng, S. Catal. Today 1997, 33, 227. (b) Tanaka, K.; Capule, M. F. V.; Hisanaga, T. Chem. Phys. Lett. 1991, 187, 73. (c) Nishimoto, S.; Ohtani, B.; Kajiwara, H.; Kagiya, T. J. Chem. Soc., Faraday Trans. 1 1985, 81, 61. (d) Ding, Z.; Lu, G. Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104, 4815. (e) Ohno, T.; Sarukawa, K.; Matsumura, M. J. Phys. Chem. B 2001, 105, 2417. (f) Fujihara, K.; Ohno, T.; Matsumura, M. J. Chem. Soc., Faraday Trans. 1998, 94, 3705. (13) Yu, J. C.; Lin, J.; Lo, D.; Lam, S. K. Langmuir 2000, 16, 7304. (14) Wu, N. L.; Lee, M. S.; Pon, Z. J.; Hsu, J. Z. J. Photochem. Photobiol. A: Chem. 2004, 163, 277. (15) Tang, H.; Berger, H.; Schmid, P. E.; Le´vy, F. Solid State Commun. 1994, 92, 267. (16) Poznyak, S. K.; Sviridov, V. V.; Kulak, A. I.; Samtsov, M. P. J. Electroanal. Chem. 1992, 340, 73. (17) Montoncello, F.; Carotta, M. C.; Cavicchi, B.; Ferroni, M.; Giberti, A. J. Appl. Phys. 2003, 94, 1501. (18) Nakajima, H.; Mori, T.; Shen, Q.; Toyoda, T. Chem. Phys. Lett. 2005, 409, 81. (19) Nakajima, H.; Itoh, K.; Murabayashi, M. Bull. Chem. Soc. Jpn. 2002, 75, 601. (20) Emeline, A. V.; Ryabchuk, V. K.; Serpone, N. J. Phys. Chem. B 2005, 109, 18515. (21) (a) Chen, J.; Feng, Z. C.; Ying, P. L.; Li, M. J.; Han, B.; Li, C. Phys. Chem. Chem. Phys. 2004, 6, 4473. (b) Chen, J.; Feng, Z. C.; Ying, P. L.; Li, C. J. Phys. Chem. B 2004, 108, 12669. (c) Chen, J.; Feng, Z. C.; Shi, J. Y.; Ying, P. L.; Zhang, H. D.; Li, C. Chem. Phys. Lett. 2005, 401, 104. (22) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321. (23) Chaves, A.; Katiyan, K. S.; Porto, S. P. S. Phys. ReV. B 1974, 10, 3522. (24) Gribb, A. A.; Banfield, J. F. Am. Mineral. 1997, 82, 717. (25) Zacharias, M.; Fauchet, P. M. Appl. Phys. Lett. 1997, 71, 380. (26) Sekiya, T.; Yagisawa, T.; Kamiya, N.; Mulmi, D. D.; Kurita, S.; Murakami, Y.; Kodaira, T. J. Phys. Soc. Jpn. 2004, 73, 703. (27) Cronemeyer, D. C. Phys. ReV. 1959, 113, 1222 (28) Go¨pel, W.; Rocker, G.; Feierabend, R. Phys. ReV. B 1983, 28, 3427. (29) Salvador, P.; Garcı´a Gonza´lez, M. L.; Mun˜oz, F. J. Phys. Chem. 1992, 96, 10349. (30) Qian, L.; Jin, Z. S.; Zhang, J. W.; Huang, Y. B.; Zhang, Z. J.; Du, Z. L. Appl. Phys. A 2005, 80, 1801.
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