Keywords: Photocatalyst; titanium(iv) oxide; visible-light response photocatalyst; transition-metal ion; switching redox sites

Transcription

1 Elsevier Editorial System(tm) for Applied Catalysis A Manuscript Draft Manuscript Number: Title: Switching Redox Site of Photocatalytic Reaction on Titanium(IV) Oxide Particles Modified with Transition Metal Ion Controlled by Irradiation Wavelength Article Type: Research Paper Keywords: Photocatalyst; titanium(iv) oxide; visible-light response photocatalyst; transition-metal ion; switching redox sites Corresponding Author: Dr. Teruhisa Ohno, Corresponding Author's Institution: First Author: Naoya Murakami, Ph.D. Order of Authors: Naoya Murakami, Ph.D.; Tetsuo Chiyoya; Toshiki Tsubota; Teruhisa Ohno

2 * List of Potential Reviewers List of Potential Reviewers 1. Name: Hiroshi Kominami, Associate professor Organization: Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University 2. Name: Detlef W. Bahnemann, Professor Organization: Institute for Technical Chemistry, University Hanover 3. Sigeru Ikeda, Associate professor Organization: Research Center for Solar Energy Chemistry, Osaka University Address, FAX and of the corresponding author: Name: Teruhisa Ohno Organization: Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology Adress: 1-1 Sensuicho, Tobata, Kitakyushu , Japan address: TEL&FAX: We appreciate your kind handling of this manuscript for publication. Sincerely yours, Naoya Murakami

3 * Graphical Abstract Graphical abstract The redox site of photocatalytic reaction on titanium(iv) oxide particles modified with a transition metal ion was switched by the wavelength of photoirradiation, i.e., reduction took place over the metal ion under ultraviolet irradiation, while oxidation proceeded over the metal ion under visible-light irradiation

4 * Manuscript Click here to view linked References Switching Redox Site of Photocatalytic Reaction on Titanium(IV) Oxide Particles Modified with Transition Metal Ion Controlled by Irradiation Wavelength Naoya Murakami, Tetsuo Chiyoya, Toshiki Tsubota and Teruhisa Ohno* Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu , Japan Abstract Titanium(IV) oxide (TiO 2 ) particles were modified with several kinds of transition-metal ions (iron(iii), copper(ii), nickel(ii) and chromium(iii) ions). Photocatalytic activity for acetaldehyde decomposition over metal-ion modified TiO 2 showed higher photocatalytic activity than bare TiO 2 under ultraviolet (UV) as well as visible-light irradiation. The mechanism of photocatalytic reaction over metal-ion modified TiO 2 was investigated by double-beam photoacoustic (DB-PA) spectroscopy, which enables observation of electron migration in the TiO 2 particles. DB-PA measurements suggested that metal ions on TiO 2 surface acted differently depending on the wavelength of photoirradiation, i.e., as electron acceptors under UV irradiation and as electron injectors under visible-light irradiation. Keywords: Photocatalyst, titanium(iv) oxide, visible-light response photocatalyst, transition-metal ion, switching redox sites * Corresponding author. FAX: address: tohno@che.kyutech.ac.jp (T. Ohno)

5 1. Introduction Titanium(IV) oxide (TiO 2 ) is one of the most attractive materials for a semiconductor photocatalyst with advantages of photostability, oxidation ability and availability [1]. TiO 2 photocatalysts required ultraviolet (UV) irradiation, the amount of which corresponds to only 3% of total solar energy, for generation of photoexcited electrons and positive holes. However, a large proportion of them recombine with each other without being utilized in chemical reactions. Thus, it is important to prevent recombination in order to make efficient use of the photoexcitation. One conventional method for preventing recombination is utilization of noble metal catalysts, e.g., platinum and rhodium, which are capable of collecting a large number of electrons and enhancing reduction activity, though the use of noble metals as co-catalysts is a problem from the viewpoints of cost and shortage. Recently, we have succeeded in enhancement of photocatalytic activity of sulfur-doped TiO 2 (S-TiO 2 ) by adsorption of iron(iii) ion (Fe 3+ ) as an electron acceptor [2,3]. Transition-metal ions with more positive redox-potential than the conduction band of TiO 2, such as Fe 3+, can act as electron acceptors, resulting in an increase of reduction rate. Such electron acceptors are expected to have a larger influence on rutile TiO 2 than anatase TiO 2 because reduction activity of rutile TiO 2 is thought to be inferior to that of anatase TiO 2 due to less negative potential of the conduction band. On the other hand, some studies have indicated that rutile TiO 2 shows superior activity to that of anatase TiO 2 for some oxidative reactions [4,5]. Therefore, the photocatalytic activity of rutile TiO 2 has greater potential than that of anatase TiO 2 to be enhanced with the aid of electron acceptors because it is difficult to enhance oxidation activity of TiO 2 photocatalysts with a co-catalyst. Furthermore, some metal ions are also capable of working as sensitizers for visible light by formation of a metal complex, such as a ruthenium complex, which is a well-known sensitizer for dye-sensitized solar cells [6]. However, development of visible-light response photocatalyst by modification of sensitizer on TiO 2 has been failed because of poor stability under photoirradiation. Recently, Kisch et al. reported a new type of visible-light response photocatalyst developed by - 2 -

6 modification of platinum(iv) chloride surface complex [7-10]. The photocatalyst has attracted much attention because of simple preparation without any impurity doping, which causes formation of defects in a semiconductor and reduces photocatalytic activity under UV irradiation. In this study, TiO 2 particles were modified with various inexpensive transition-metal ions, including iron(iii) ion (Fe 3+ ), copper(ii) ion (Cu 2+ ), nickel(ii) ion (Ni 2+ ) and chromium(iii) ion (Cr 3+ ), by the impregnation method. Photocatalytic reaction and electron migration on TiO 2 modified with metal ions under UV and visible-light irradiation were investigated. 2. Experimental 2.1. Materials and instruments TiO 2 (MT-600B) particles having a rutile phase were obtained from TAYCA. The relative surface area of MT-600B was 27 m 2 g -1. Acetaldehyde (CH 3 CHO) was obtained from Aldrich, and other chemicals were obtained from commercial sources as guaranteed reagents and were used without further purification. The crystal structures of TiO 2 particles were determined from X-ray diffraction patterns measured with an X-ray diffractometer (JEOL, JDX3500) with a Cu target K -ray ( = Å). The relative surface areas of the particles were determined by using a surface area analyzer (Micromeritics, FlowSorb II 2300). The diffuse reflection (DR) spectra were measured using a Shimadzu UV-2500PC spectrophotometer equipped with an integrating sphere unit (ISR-240A). X-ray photoelectron spectra (XPS) of the TiO 2 particles were measured using a JEOL JPS90SX photoelectron spectrometer with an Mg K source ( ev). The shift of binding energy due to relative surface charging was corrected using the C 1s level at ev as an internal standard. 2.2 Sample preparation An aqueous suspension composed of rutile TiO 2 particles and an aqueous solution of iron(iii) chloride (FeCl 3 ), iron(iii) nitrate enneahydrate (Fe(NO 3 ) 3 9H 2 O), copper(ii) chloride dehydrate (CuCl 2 2H 2 O), nickel(ii) chloride hexahydrate (NiCl 2 6H 2 O), chromium(iii) chloride hexahydrate - 3 -

7 (CrCl 3 6H 2 O), the amount of which corresponds to 0.1 wt% modification of metal, was stirred for 3 h under an aerated condition at ca. 80 o C. The residue was washed with deionized water several times until ph of the filtrate was neutralized. The particles were dried under reduced pressure at 60 C for 12 h. Then metal-ion-modified TiO 2 (M n+ /TiO 2 ) was obtained. 2.3 Photocatalytic decomposition of acetaldehyde The photocatalytic activity of a M n+ /TiO 2 sample was evaluated by photocatalytic decomposition of acetaldehyde. One hundred mg of TiO 2 particles was spread on a glass dish, and the glass dish was placed in a Tedlar bag (As one) with a volume of 125 cm 3. Five hundred ppm of gaseous acetaldehyde was injected into the Tedlar bag, and the photoirradiation was carried out at room temperature after acetaldehyde had reached an adsorption equilibrium. A 500-W xenon lamp (Ushio, SX-UI501XQ) was used as a light source and the wavelength of photoirradiation was controlled by UV-35 and L-42 filter (Asahi Techno Glass Co.). Concentrations of acetaldehyde and carbon dioxide (CO 2 ) were estimated by gas chromatography (Shimadzu, GC-8A, FID detector) with an FEG-20 M 20% Celite 545 packed glass column and by gas chromatography (Shimadzu, GC-9A, FID detector) with a TCP 20% Uniport R packed column and methanizer (GL Sciences, MT-221), respectively. 2.4 Double-beam photoacoustic spectroscopic measurement A gas-exchangeable photoacoustic (PA) cell equipped with two valves for gas flow was used, and a TiO 2 sample was placed in the cell. The atmosphere was controlled by a flow of artificial air or N 2 containing ethanol vapor (air + EtOH, N 2 + EtOH), and the measurements were conducted after shutting off the valves, i.e., in a closed system at room temperature. A light-emitting diode (LED) emitting light at ca. 625 nm (Luxeon LXHL-ND98) was used as a probe light, and the output intensity was modulated by a digital function generator (NF DF1905) at 80 Hz. In addition to the modulated light, a UV-LED (Nichia NCCU033, emitting light at ca. 365 nm, 2.5 mw cm -2 ) and a blue-led (Luxeon LXHL-NB98, emitting light at ca. 470 nm, 8.1 mw cm -2 ) were also used as - 4 -

8 simultaneous continuous irradiation for photoexcitation of M n+ /TiO 2. The PA signal acquired by a condenser microphone buried in the cell was amplified and monitored by a digital lock-in amplifier (NF LI5640). Detailed setups of double-beam photoacoustic (DB-PA) spectroscopic measurements have been reported [11]. 3. Results and Discussion 3.1 Photoabsorption properties of M n+ /TiO 2 sample The color of M n+ /TiO 2 samples was changed by modification with metal ion and depended on the kind and amount of metal ion adsorbed on TiO 2. Figure 1a shows DR spectra of bare TiO 2 and M n+ /TiO 2 samples. The absorption edge of M n+ /TiO 2 samples shifted to a longer wavelength region of nm, but the range of red shift depended on the kind of metal ion (Cr 3+ > Fe 3+ ~ Ni 2+ > Cu 2+ at 420 nm). Figure 1b shows DR spectra of a mixture of TiO 2 and metal oxide fine particles, the amount of which corresponds to 0.1 wt% of metal atom. Similar DR spectra were observed between M n+ /TiO 2 and TiO 2 particles physically mixed with NiO and CuO fine particles, while rather large differences were observed in the DR spectra for TiO 2 particles physically mixed with Fe 2 O 3 and Cr 2 O 3 fine particles. This suggests that state of Fe 3+ and Cr 3+ on the TiO 2 surface was very different from the bulk state of metal oxide. 3.2 Photocatalytic reaction of M n+ /TiO 2 sample under UV-vis irradiation Figure 2 shows the time course of CO 2 evolution of acetaldehyde decomposition under UV-vis irradiation. M n+ /TiO 2 showed a higher photocatalytic activity than that of the bare sample. As suggested by our previous studies [2,3], a metal ion possibly prevents an excited electron from recombining with a positive hole by acting as an electron acceptor. Moreover, the activity depended on the kinds of metal ion (Fe 3+ > Cu 2+ > Ni 2+ > Cr 3+ ), and the order of photocatalytic activity coincided with the order of redox potential (Fe 3+ /Fe 2+ = V, Cu 2+ /Cu + = V, Ni 2+ /Ni = V, Cr 3+ /Cr 2+ = V vs. SHE). This result suggests that a metal ion with a more positive redox potential worked as an effective acceptor for enhancement of photocatalytic - 5 -

9 activity. With longer photoirradiation, CO 2 evolution for most of the sample showed a saturation tendency before reaching 1000 ppm CO 2 due to accumulation of intermediate species that were not easily decomposed, e.g., acetic acid [3]. On the other hand, 1000 ppm of CO 2 evolution over Fe 3+ /TiO 2 suggests that Fe 3+ /TiO 2 has a higher oxidation activity than that of other M n+ /TiO 2. After photoirradiation for 20 h, CO 2 evolution over Fe 3+ /TiO 2 was ca. 650 ppm larger than that over the bare sample. This suggests that 8.5 mol of exited electrons was trapped by Fe 3+ adsorbed on the TiO 2 surface, assuming that total decomposition of 1 mol acetaldehyde requires 10 number of hole reaction (eq. 1). CH 3 CHO + 3H 2 O + 10h + 2CO H + (1) Thus, the turnover number of Fe 3+ was more than ca. 5 because the amount of Fe 3+ adsorbed on TiO 2 was calculated to be 1.7 mol. This means that Fe 2+ ions generated as a result of the reaction between Fe 3+ and photoexcited electrons can be recovered to Fe 3+ ions by reacting with oxygen species (OS) [2,3]. Therefore, the recovery process is also important for enhancement of photocatalytic activity in a long reaction. 3.3 DB-PA spectroscopic detection of electron transfer to metal ion The electron migration on TiO 2 particles under UV irradiation was observed using DB-PA spectroscopy, which enables observation of electron behavior by detection of trivalent titanium (Ti 3+ ) species on TiO 2 particles [11]. PA measurements were carried out under an aerated condition in order to estimate reactivity of photoexcited electrons in M n+ /TiO 2 particles. Figure 3 shows the time-course curves of PA signals of bare TiO 2 and M n+ /TiO 2 under UV irradiation in the presence of air + EtOH. PA intensity of all samples was increased under UV irradiation because Ti 3+ species were generated by accumulation of electrons on TiO 2, as a counter part of hole consumption by ethanol (eq. 2) [11]. In longer UV irradiation, PA intensity approached an equilibrium value due to the balance between reduction of Ti 4+ by excited electrons and oxidation of Ti 3+ by atmospheric oxygen. The equilibrium values of PA intensity of the M n+ /TiO 2 sample were largely reduced compared to the bare sample. This result suggested that metal ions on TiO 2-6 -

10 consumed some of the photoexcited electrons on TiO 2 by acting as an electron acceptor (eq. 3) without electron accumulation on TiO 2 (eq. 2). Ti 4+ + e - Ti 3+ (rate k 2 ) (2) M n+ + e - M (n 1)+ (rate k 3 ) (3) M (n 1)+ + OS M n+ + OS - (rate k 4 ) (4) However, a difference in the PA signal was hardly observed among the different kinds of metal ions in PA measurements (Fig. 3), though a relatively large difference was observed in photocatalytic activity evaluations (Fig. 2). One possible reason for this is that the rate of electron transfer to the acceptor was faster the rate of reoxidation of the metal ion (k 3 > k 4 ). An electron spin resonance study suggested that Fe 2+ can be quickly recovered to Fe 3+ by atmospheric oxygen under an aerated condition (k 3 < k 4 ) [2]. However, the PA measurements in the present study were carried out in the presence of ethanol, which is a well-known electron donor, and electron accumulation in the presence of ethanol was much faster than that in the presence of acetaldehyde. Therefore, dependence on the kind of metal ion was hardly observed because a large proportion of the metal ions were exhausted as electron acceptors in the present PA conditions due to the reoxidation process of a metal ion (eq. 4). 3.4 Photocatalytic reaction of M n+ /TiO 2 sample under visible-light irradiation Figure 4 shows the time course curves of CO 2 evolution of acetaldehyde decomposition on bare TiO 2 and M n+ /TiO 2 under visible-light irradiation. M n+ /TiO 2 showed a higher photocatalytic activity even under visible-light irradiation than that of bare sample, and their activity depended on the kind of metal ion. The concentration of CO 2 evolution over Fe 3+ /TiO 2 monotonically increased with photoirradiation time and reached ca. 490 ppm with 20-h photoirradiation, while the bare sample showed a saturation tendency of CO 2 evolution at ca. 180 ppm. As mentioned in section 3.1, the DR spectra suggested that M n+ /TiO 2 absorbed visible light in the wavelength of nm. This result suggested that only metal ions adsorbed on the TiO 2 surface were photoexcited - 7 -

11 and induced photocatalytic decomposition of acetaldehyde. Kisch et al. proposed that their mechanism employed chlorine atom on degradation of 4-chlorophenol over TiO 2 modified with platinum(iv) chloride surface complex [7-10]. However, it was thought that our samples did not include chloride ion (Cl - ) because of complete removal of Cl - by washing with water in the preparation of M n+ /TiO 2. Complete removal of Cl - ions on TiO 2 was confirmed by XPS analyses, the results of which are not shown here. A detail discussion of the mechanism is given in the following section. 3.5 PA spectroscopic detection of electron injection from metal ion under visible-light irradiation Figure 5 shows the time-course curves of PA signals of bare TiO 2 and M n+ /TiO 2 under visible-light irradiation in the presence of N 2 + EtOH. PA measurements were carried out under a deaerated condition in order to estimate accumulation of electrons in TiO 2 particles. The PA signal of M n+ /TiO 2 increased even under photoirradiation of ca. 470 nm and was larger than that of the bare sample, while the PA signal showed no increase under photoirradiation of > 550 nm. These results suggested that metal ions adsorbed on TiO 2 absorbed visible light of ca. 470 nm and induced generation of Ti 3+ species in TiO 2. The mechanism of Ti 3+ generation can be explained as follows: (1) photoexcited metal-ion (M n+ ) injected electrons into TiO 2 and became oxidized state (M (n+1)+ ), (2) injected electrons reduced Ti 4+ to Ti 3+, (3) oxidized state of metal ions (M (n+1)+ ) oxidized ethanol to go back initial state of metal ions (M n+ ). Under an aerated condition, electrons injected into the conduction band of TiO 2 must be consumed by reaction with OS adsorbed on TiO 2 for continuous photocatalytic reaction. Although the injected electrons may also transfer back to an oxidized state of metal-ion (M (n+1)+ ) without being utilized in chemical reactions, the efficient electron transfer into OS is thought to be enhanced by metal ions located on the dark part of TiO 2 particles during irradiation, which act as electron acceptor as analogy with reaction under UV irradiation (Figure 6). The amount of injected electrons estimated from the time course curve of PA intensity showed a good relationship with photocatalytic activity as shown in Fig. 4 (Fe 3+ > Cu 2+ > Ni 2+ > - 8 -

12 Cr 3+ ), though it showed no relationship with photoabsorption properties as shown in Fig. 1 (Cr 3+ > Fe 3+ ~ Ni 2+ > Cu 2+ at 470 nm). This result suggests that the rate of electron injection may be one of most important factors for control of the photoctalytic activity of acetaldehyde decomposition. 3.6 Mechanism of photocatalytic reaction under visible-light irradiation It was thought that the metal ion on the TiO 2 surface formed a surface complex, metal oxide or hydroxide. However, we have no evidence on it because there was not enough amount of modified metal-ion on TiO 2 particles to analyze its structure. One plausible structure is a surface complex of transition metal. Some studies have suggested that TiO 2 shows photocatalytic activity under visible-light irradiation by formation of a surface complex on the surface [12]. Recently, Kisch et al. reported visible-light response photocatalysis by modification of platinum(iv) ion [12]. They suggested that a platinum(iv) chloride surface complex, such as [PtCl 4 ] 2-, was formed on the TiO 2 surface and that it adsorbed visible light of wavelength of nm. Their DR spectra of TiO 2 modified with a platinum(iv) chloride surface complex were similar to those of our M n+ /TiO 2 samples. However, the M n+ /TiO 2 samples in the present study hardly contain Cl - because of complete removal by washing with water in the preparation procedure. Actually, Fe 3+ /TiO 2 prepared from an aqueous solution of Fe(NO 3 ) 3 showed a DR spectrum similar to that of the sample prepared from an aqueous solution of FeCl 3. This result suggests that the absorption is not attributed to formation of a metal-ion complex with coordination of Cl -. If the surface state is attributed to a metal complex, plausible state of transition metal is surface complex formed by dehydration reaction between the hydroxyl group on the TiO 2 surface and an aqua complex, such as hexaaqua iron(iii) complex ([Fe(H 2 O) 6 ] 3+ ). Another plausible structure is a heterojunction semiconductor consisting of two kinds of semiconductor with different bandgap energies. For instance, excited electrons in CdSe are capable of migrating into TiO 2 by formation of a heterojunction under visible-light irradiation [13]. However, electrons in the conduction band of Fe 2 O 3 cannot migrate into that of TiO 2 due to V (vs. SHE at ph 3.8) of flat band potential of Fe 2 O 3 [14], though Fe 2 O 3 can absorb visible light of - 9 -

13 < ca. 560 nm. Semiconductor nanoparticles of a few nanometers in size are known to show expansion of the bandgap by a quantum size effect, which leads to negative and positive shifts of the conduction and valence bands, respectively. Thus, smaller nanoparticles of Fe 2 O 3 may be capable of injecting electrons into the conduction band of TiO 2 particles. On the other hand, Fe 2 O 3 with larger particle size is presumably capable of acting as an electron acceptor like the heterojunction structure of TiO 2 and tungsten(vi) oxide [15], and enhancing reduction rate. The mechanism of electron transfer between TiO 2 and metal oxide nanoparticles loaded on TiO 2 is now under investigation. 4. Conclusion Photocatalytic activity of and electron migration in M n+ /TiO 2 samples were investigated under UV and visible-light irradiation. M n+ /TiO 2 samples showed a higher photocatalytic activity than that of the bare sample under UV irradiation. The reason for this enhancement is that the metal ion acted as an electron acceptor, the influence of which strongly depended on the kind of metal ion. Furthermore, M n+ /TiO 2 samples showed higher photocatalytic activity even under visible-light irradiation. DB-PA measurements suggested that the metal ion acted as acceptor under UV irradiation, while photoexcited metal ions injected electrons into the conduction band of TiO 2 as a counter part of oxidation by the oxidized state of metal ions under visible-light irradiation. These results suggest that the redox site of M n+ /TiO 2 was switched by the wavelength of photoirradiation, i.e., reduction took place over the metal ion under UV irradiation, while oxidation occurred over the metal ion under visible-light irradiation. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, and Technology (MEXT), Japan and Nissan Science Foundation

14 References [1] M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann, Chem. Rev. 95 (1995) [2] T. Ohno, Z. Miyamoto, K. Nishijima, H. Kanemitsu, F. Xueyuan, Appl. Catal. A 302 (2006) 62. [3] K. Nishijima, B. Ohtani, X. Yan, T. Kamai, T. Chiyoya, T. Tsubota, N. Murakami, T. Ohno, Chem. Phys. 339 (2007) 64. [4] T. Ohno, K. Sarukawa, M. Matsumura, New J. Chem., 26 (2002) [5] T. Torimoto, N. Nakamura, S. Ikeda, B. Ohtani, Phys. Chem. Chem. Phys. 4 (2002) [6] M. Gratzel, Nature 414 (2001) 338. [7] H. Kisch, L. Zang, C. Lange,W.F. Maier, C. Antonius, D. Meissner, Angew. Chem. Int. Ed. 37 (1998) [8] L. Zang, C. Lange, I. Abraham, S. Storck, W.F. Maier, H. Kisch, J. Phys. Chem. B, 102 (1998) [9] L. Zang, W. Macyk, C. Lange, W.F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 6 (2000) 379. [10] W. Macyk, H. Kisch, Chem. Eur. J. 7 (2001) [11] N. Murakami, O.O.P. Mahaney, R. Abe, T. Torimoto, B. Ohtani, J. Phys. Chem. C, 111 (2007) [12] S. Ikeda, C. Abe, T. Torimoto, B. Ohtani, J. Photochem. Photobio. A: Chem. 160 (2003) [13] I. Robel, M. Kuno, P. Kamat., J. Am. Chem. Soc., 129 (2007) [14] J.H. Kennedy and K.W. Frese Jr., J. Electrochem. Soc. 125 (1978) [15] T. Tatsuma, S. Saitoh, P. Ngaotrakanwiwat, Y. Ohko, A. Fujishima, Langmuir; 18 (2002)

15 Figures and Captions Figure 1. DR spectra normalized at 350 nm (R rel. : relative reflection) of (a) M n+ /TiO 2 and (b) a mixture of TiO 2 and metal oxide

16 Figure 2. Time course of CO 2 evolution of acetaldehyde decomposition on bare TiO 2 and M n+ /TiO 2 under UV-vis irradiation ( > 350 nm, I = 12 mw cm -2 )

17 Figure 3. Time course curves of PA signals of bare TiO 2 and M n+ /TiO 2 under UV irradiation ( ex = 365 nm, I = 2.5 mw cm -2 ) in the presence of air + EtOH

18 Figure 4. Time course of CO 2 evolution of acetaldehyde decomposition on bare TiO 2 and M n+ /TiO 2 under visible-light irradiation ( > 420 nm, I = 12 mw cm -2 )

19 Figure 5. Time course curves of PA signals of bare TiO 2 and M n+ /TiO 2 under visible-light irradiation ( ex = 470 nm, I = 8.1 mw cm -2 ) in the presence of N 2 + EtOH

20 Figure 6. Schematic image of postulated photocatalytic reaction under (a) UV and (b) visible-light irradiation

21 Graphical abstract The redox site of photocatalytic reaction on titanium(iv) oxide particles modified with a transition metal ion was switched by the wavelength of photoirradiation, i.e., reduction took place over the metal ion under ultraviolet irradiation, while oxidation proceeded over the metal ion under visible-light irradiation

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