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Formation of the Dimer Radical Cation of Aromatic Sulfide on the TiO2 Surface during Photocatalytic Reactions Takashi Tachikawa, Sachiko Tojo, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Received February 11, 2004 The formation of the dimer radical cation (D•+) of 4-(methylthio)benzoic acid on a TiO2 surface is demonstrated using the time-resolved diffuse reflectance technique. The observed time-resolved diffuse reflectance spectral shape significantly depends on the substrate concentrations. The substrate concentration dependences of the initial transient signal intensity (%abs.t)0) and the amount of adsorbates (nad) clearly suggest that the formation of D•+ is attributable to the high local substrate concentration on the TiO2 surface. The electronic influence of substituents on the formation of D•+ is also discussed.
Photocatalytic decomposition of organic pollutants with the aid of TiO2 particles or layers is a promising process for the purification and sterilization of environmental aqueous media.1-7 It is well-known that the absorption of a photon with an energy greater than the band gap energy results in the formation of a conduction band electron (ecb-) and a valence band hole (hvb+). These photogenerated carriers migrate to the particle surface and participate in redox processes at the surface. For colored pollutants, the degradation can also be initiated using visible light. From the dye in the excited state, an electron is injected into the conduction band of TiO2 and is captured by the surfaceadsorbed O2 to produce O2•-, and the dye radical cation subsequently decomposes from the reactions with O2 or O2•-. Organic radical cations are important intermediates in photochemical electron-transfer reactions8-10 and have received much attention regarding their structures and reactivities on/in heterogeneous materials such as zeolite, silica, and organic-inorganic hybrid materials.6b,9,10 It is * To whom correspondence should be addressed. Tel.: +81-66879-8495. Fax: +81-6-6879-8499. E-mail address: majima@ sanken.osaka-u.ac.jp. (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (2) Mills, A.; Hunte, S. L. J. Photochem. Photobiol., A 1997, 108, 1. (3) Ollis, D. F., Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: London, 1993. (4) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemannt, D. W. Chem. Rev. 1995, 95, 69. (b) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (5) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (6) (a) Kamat, P. V. Chem. Rev. 1993, 93, 267. (b) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (7) (a) Nakaoka, Y.; Nosaka, Y. J. Photochem. Photobiol., A 1997, 110, 299. (b) Nosaka, Y.; Kishimoto, M.; Nishio, J. J. Phys. Chem. B 1998, 102, 10279. (8) (a) Fox, M. A., Chanon, M., Eds. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988; Parts A-D. (b) Balzani, V., Ed. Electron Transfer in Chemistry; Wiley-VCH: New York, 2001; Vol. 2. (c) Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401. (9) (a) Scaiano, J. C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783. (b) Garcia, H.; Roth, H. D. Chem. Rev. 2002, 102, 3947. (c) Mao, Y.; Thomas, J. K. Langmuir 1992, 8, 2501. (d) Zhang, G.; Thomas, J. K. J. Phys. Chem. B 2003, 107, 7254. (e) Morkin, T. L.; Turro, N. J.; Kleinman, M. H.; Brindle, C. S.; Kramer, W. H.; Gould, I. R. J. Am. Chem. Soc. 2003, 125, 14917. (10) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668.
a noteworthy issue, therefore, to clarify the interfacial electron-transfer reaction dynamics and the subsequent reactivity of the transient intermediates at the heterogeneous surface for the novel development of functional nanoscale architectures as well as efficient photocatalytic systems. Recently, we studied the one-electron oxidation of several aromatic compounds adsorbed on the surface of a TiO2 powder by time-resolved diffuse reflectance (TDR) spectroscopy and concluded that the -OH group plays an important role in the adsorption on the surface of TiO2 and the efficiency of the one-electron oxidation of the substrates.11 In the present letter, we investigated the one-electron oxidation of 4-(methylthio)benzoic acid (MTBA) and 4-(methylthio)phenol (MTP) adsorbed on the surface of a TiO2 powder (P25, Japan Aerosil) slurried in acetonitrile by TDR spectroscopy. The TDR method is a powerful tool for the investigations of photocatalysis under various conditions.11-15 We report the formation of a new transient species on the TiO2 surface, that is, a dimer radical cation (D•+) between a neutral substrate (S) and the radical cation of S (S•+) during the photocatalytic one-electron oxidation reactions. To our knowledge, no experimental evidence has been reported for the formation of D•+ on the TiO2 surface. Figure 1a shows the TDR spectra obtained at 40 ns after a 430-nm laser flash (1.5 mJ pulse-1, 5 ns full width at half-maximum, fwhm) of the charge-transfer complexes between MTBA (concentrations of MTBA, [MTBA] ) 1, 2, and 10 mM, M ≡ mol dm-3) and the TiO2 powder slurried in Ar-saturated acetonitrile at room temperature. The TDR measurements were performed using the third harmonic generation (355 nm, 5 ns fwhm) from a Qswitched Nd:YAG laser (Continuum, Surelite II-10) for the excitation operated with a temporal control by a delay generator (Stanford Research Systems, DG535).11 The (11) (a) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Chem. Phys. Lett. 2003, 382, 618. (b) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Langmuir 2004, 20, 2753. (12) Kessler, R. W.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1 1981, 77, 309. (13) (a) Draper, R. B.; Fox, M. A. J. Phys. Chem. 1990, 94, 4628. (b) Draper, R. B.; Fox, M. A. Langmuir 1990, 6, 1396. (14) Colombo, D. P., Jr.; Bowman, R. M. J. Phys. Chem. 1996, 100, 18445. (15) Furube, A.; Asahi, T.; Masuhara, H.; Yamashita, H.; Anpo, M. Res. Chem. Intermed. 2001, 27, 177.
10.1021/la0496439 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004
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Figure 1. Transient diffuse reflectance spectra obtained at 40 ns after a laser flash during the 430-nm laser photolysis of TiO2 powder in the presence of MTBA (1, 2, and 10 mM) in Arsaturated acetonitrile at room temperature (a). MTBA concentration dependences of the initial signal intensities (%abs.t)0) at 545 (solid triangles) and 730 nm (open triangles) and the amount of adsorbates on the TiO2 surface (nad; open circles) for MTBA in air at room temperature (b). Solid lines are visual guides.
TDR spectra with two peaks at around 545 and 730 nm were clearly observed immediately after the laser flash photolysis as shown in Figure 1a. The absorption band with the peak at 545 nm observed for 1 mM MTBA was very similar to that of MTBA•+ already reported.16 In the present systems, MTBA•+ was generated by the electron injection from MTBA to TiO2 as shown by reaction 1,
[TiO2‚‚‚MTBA]CT + hν f ecb- + MTBA•+
(1)
On the other hand, in the longer wavelength region, an absorption band at 730 nm was observed and the signal intensity significantly increased with increasing [MTBA], compared with those obtained at 545 nm, as shown in Figure 1b. To assign this new absorption band, we examined every conceivable influence on the spectral shapes and time traces. Gra¨tzel et al. reported a transient absorption band with a peak at around 600 nm immediately after the laser flash photolysis of TiO2 and concluded that this absorption band was attributable to trapped electrons.17 In the present systems, the signal intensities due to the trapped electrons at 600 nm were hardly influenced by the addition of methanol and triethylamine as hole scavengers. On the other hand, the signal intensities of the absorptions at 545 and 700 nm clearly decreased with the addition of methanol as shown in Figure 2b. In addition, a similar spectral shape was also observed for the O2-saturated sample (oxygen molecule is an electron scavenger), although the signal intensities slightly increased. Thus, we suggest that the absorption band observed at around 730 nm is not attributable to trapped electrons. Recently, Sawaki et al. investigated the one-electron oxidation processes of aromatic sulfides in acetonitrile by the laser flash photolysis method and concluded that dimer radical cations (D•+) absorbing at around 460-500 nm are the σ-type complex of the sulfur-sulfur three-electron bond and that the radical cations absorbing at around (16) Gawandi, V. B.; Mohan, H.; Mittal, J. P. Res. Chem. Intermed. 2003, 29, 51. (17) (a) Rothenberger, G.; Moser, J.; Gra¨tzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. Soc. 1985, 107, 8054. (b) Serpone, N.; Lawless, D.; Khairutdinov, R.; Pelizzetti, E. J. Phys. Chem. 1995, 99, 16655.
Figure 2. Transient diffuse reflectance spectra obtained at 40, 200, and 5000 ns after a 430-nm laser flash of the chargetransfer complexes between TiO2 and MTBA ([S] ) 10 mM) in Ar-saturated acetonitrile at room temperature (a). The percent absorption values at 560 and 700 nm versus time after a 430nm laser flash of the charge-transfer complexes between TiO2 and MTBA ([S] ) 10 mM) in the absence and presence of MeOH (b).
800 nm are the π-type complex associated with two phenylthio groups.18 As shown in Figure 2a, the absorption band observed for the MTBA system is similar to those for D•+ of thioanisoles such as the p-chloro and pmethylthioanisoles. The absorption peak at around 545 nm slightly shifted toward a shorter wavelength with the increasing [MTBA], while that around 730 nm was insensitive to [MTBA]. Therefore, we conclude that the formation processes of σ-type D•+ as well as π-type D•+ are involved in the reaction dynamics of MTBA•+ on the TiO2 surface at high [MTBA] (>2 mM). As shown in Figure 1b, the initial signal intensities (%abs.t)0, solid and open triangles) significantly depended on [MTBA]. The %abs.t)0 values increased and then decreased with increasing [MTBA]. On the other hand, the amount of adsorbates (nad, open circles) increased with increasing [MTBA] in accordance with the Langmuir-type adsorption isotherms.11 From the Langmuir-type adsorption isotherms, the degree of coverage of the adsorption sites is estimated to be about 20 and >90% at [MTBA] values of 1 and 10 mM, respectively. This high coverage degree obtained at a [S] of 10 mM clearly suggests that the formation of D•+ is attributable to the high local [S] on the TiO2 surface. We summarize the reaction scheme at high [S] as follows. In the case of the 430-nm excitation, S•+ is generated from the electron injection from the surfacebound S to the conduction band in TiO2 as described above (reaction 1). S•+ then reacts with neutral S on the surface within the dead time (∼50 ns) as indicated by reaction 2,
S•+ + S a D•+
(2)
Almost the same decay rates were also observed for MTBA at 545 and 700 nm as shown in Figure 2b, suggesting that S•+ is in rapid equilibrium with D•+. This result is consistent with that reported for D•+ of other aromatic sulfides.19,2019-20 (18) Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki, Y. J. Am. Chem. Soc. 1998, 120, 12728. (19) (a) Ioele, M.; Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997, 101, 2979. (b) Engman, L.; Lind, J.; Mere´nyi, G. J. Phys. Chem. 1994, 98, 3174. (c) Mohan, H.; Asmus, K.-D. J. Phys. Chem. 1988, 92, 118.
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Figure 3. Transient diffuse reflectance spectra obtained at 40, 200, and 5000 ns after a 430-nm laser flash of the chargetransfer complexes between TiO2 and MTP ([S] ) 10 mM) in Ar-saturated acetonitrile at room temperature. Inset: percent absorption versus time after a 430-nm laser flash of the chargetransfer complexes between TiO2 and MTP ([S] ) 10 mM).
On the other hand, a different tendency was observed for MTP. Figure 3 shows the TDR spectra obtained at 40, 200, and 5000 ns after a 430-nm laser flash of the chargetransfer complexes between TiO2 and MTP ([S] ) 10 mM) in Ar-saturated acetonitrile at room temperature. The (20) (a) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S. Bull. Chem. Soc. Jpn. 1995, 68, 958. (b) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Org. Chem. 1996, 61, 7793.
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TDR spectra attributed to D•+ were not clearly observed, although a remarkable broadening of the spectral lines was observed at >600 nm. It is well-known that the formation of D•+ is very sensitive to the steric and electronic factors of substituents.18-20 MTP has a strong electrondonating -OH group, and, hence, the delocalization of the positive charge over the aromatic rings is more effective than that in MTBA. Therefore, the formation of D•+ is unfavorable and could not be clearly observed for MTP. In summary, we have successfully studied the D•+ formation of MTBA on the TiO2 surface during photocatalytic reactions. It was found that the formation processes of σ-type D•+ as well as π-type D•+ are involved in the reaction dynamics of MTBA•+ on the TiO2 surface at high [MTBA]. The present results should contribute to a proper understanding of the photochemical reaction on the TiO2 surface and the subsequent reactivity of the transient species generated on the TiO2 surface. Acknowledgment. This work has been partly supported by a Grant-in-Aid for Scientific Research on Priority Area (417), 21st Century COE Research, and others from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. LA0496439
