Langmuir 2004, 20, 9441-9444
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Direct Observation of the One-Electron Reduction of Methyl Viologen Mediated by the CO2 Radical Anion during TiO2 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 565-0047, Japan Received July 27, 2004. In Final Form: September 10, 2004 The one-electron reduction of methyl viologen (MV2+) mediated by the carbon dioxide radical anion (CO2•-) during photocatalytic reactions in a colloidal TiO2 aqueous solution (pH 2) has been investigated by time-resolved absorption spectroscopy. The formation of MV•+ generated from the one-electron reduction reaction with CO2•-, which is generated from the one-electron oxidation reactions with the photogenerated holes (h+), was directly observed. The spectral features of the photogenerated charge carriers and the kinetic analysis of the formation process of MV•+ revealed that the CO2•-, desorbed from the surface, reacts with MV2+ via a homogeneous electron-transfer process in the bulk solution.
Introduction When TiO2 is illuminated with UV light, electron (e-) and hole (h+) pairs are generated, and they reduce and oxidize adsorbates on the surface, respectively. These reactions are known as photocatalysis,1 and much research has been conducted on these reactions from the viewpoint of applications for water and air purification,2-5 antibacterial agents,6,7 and self-cleaning surfaces of various substrates coated with TiO2.8 A valence band h+ (hVB+) has a powerful oxidation strength, while the ability of a conduction band (CB) e(eCB-) to drive useful reductions is limited by the fact that most inorganic and organic substrates are reduced at potentials more negative than the CB edge. Hence, with the exception of some nitro- and halogenated compounds of environmental significance, the cathodic process is restricted to the reduction of dissolved oxygen. Recent studies have revealed that the initial oxidation of some organic additives such as a formate (HCO2-) by hVB+ or hydroxyl radicals (OH•) can generate the CO2 radical anion (CO2•-) with a strong reducing power, and CO2•- then easily reduces other substrates.9-11 To date, a number of studies have been conducted on the CO2•-mediated one-electron reduction of substrates such as metal cations. In the case of metal cations, an induced * To whom correspondence should be addressed. Tel: +81-6-6879-8495. Fax: +81-6-6879-8499. E-mail: majima@ sanken.osaka-u.ac.jp. (1) (a) Honda, K.; Fujishima, A. Nature 1972, 238, 37. (b) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (4) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: London, 1993. (5) Mills, A.; Hunte, S. L. J. Photochem. Photobiol., A 1997, 108, 1. (6) Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1997, 106, 51. (7) Wei, C.; Lin, Y.; Zainai, Z.; Williams, N. E.; Zhu, K.; Kruzic, A. P.; Smith, R. L.; Rajeshwar, K. Environ. Sci. Technol. 1994, 28, 934. (8) Heller, A. Acc. Chem. Res. 1995, 28, 503. (9) (a) Ming, Y.; Chenthamarakshan, C. R.; Rajeshwar, K. J. Photochem. Photobiol., A 2002, 147, 199. (b) Somasundaram, S.; Ming, Y.; Chenthamarakshan, C. R.; Schelly, Z. A.; Rajeshwar, K. J. Phys. Chem. B 2004, 108, 4784. (10) Perissinotti, L. L.; Brusa, M. A.; Grela, M. A. Langmuir 2001, 17, 8422. (11) Nguyen, V. N. H.; Amal, R.; Beydoun, D. Chem. Eng. Sci. 2003, 58, 4429.
adsorption by the addition of HCO2- was observed,9 indicating that the interfacial adsorption of substrates on the TiO2 surface plays an important role in the reaction mechanisms. However, no direct observation of the oneelectron reduction process of substrates by CO2•- in the bulk solution and/or on the surface of the TiO2 has been reported; thus, a definite mechanism still seems to remain in dispute. The transient absorption technique is a powerful tool for the investigation of mechanisms involving photochemical reactions. Recently, we have successfully investigated the one-electron oxidation and subsequent reactions of several aromatic compounds in a TiO2 colloidal aqueous solution or adsorbed on the surface of a TiO2 powder by time-resolved absorption and diffuse reflectance spectroscopies.12 In the present study, we investigated the one-electron reduction of methyl viologen (MV2+) mediated by CO2•during the TiO2 photocatalytic reactions using the transient absorption measurement. MV2+ is known for its diverse applications, such as a herbicidal and toxicological agent13 and an electron-accepting agent in photochemical and photoelectrochemical devices.14 We directly observed the one-electron reduction of MV2+ by CO2•- and discuss the reaction mechanisms in detail. Experimental Section Colloidal aqueous solutions of TiO2 were prepared by the controlled hydrolysis of TiCl4 at 2 °C. In a typical preparation, 7.58 g of fresh TiCl4 (Wako) was slowly added dropwise over 1 h to 1 dm-3 of Milli-Q water (2-4 °C) with vigorous stirring. The 0.3 dm-3 TiO2 colloidal solutions were subsequently dialyzed at (12) (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. (c) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. J. Phys. Chem. B 2004, 108, 5859. (d) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Langmuir 2004, 20, 4327. (e) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Tetrahedron Lett. 2004, 45, 3753. (f) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Chem. Phys. Lett. 2004, 392, 50. (g) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. J. Phys. Chem. B 2004, 108, 11054. (13) (a) Bus, J. S.; Aust, S. D.; Gibson, J. E. Biochem. Biophys. Res. Commun. 1974, 58, 749. (b) Bird, C. L.; Kuhn, A. T. Chem. Sci. Rev. 1981, 10, 49. (14) (a) Derwent, J. R. J. Chem. Soc., Chem. Commun. 1980, 805. (b) Crutchley, R. J.; Lever, A. B. P. J. Am. Chem. Soc. 1980, 102, 7128. (c) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. Nano Lett. 2004, 4, 1231.
10.1021/la048100w CCC: $27.50 © 2004 American Chemical Society Published on Web 09/28/2004
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Figure 1. Steady-state diffuse reflectance spectra observed for the TiO2 powders slurried in Ar-saturated water (pH 7) containing MV2+ (0.2 mM) and in the absence (a) and presence (b) of HCO2Na (10 mM) under UV irradiation (366 nm, 150 W, 2 min); a′ and b′ are the photographs of powders a and b, respectively. 4 °C (Visking-tube presoaked for 1 week in approximately 2.5 dm-3 of Milli-Q water replaced several times per day) resulting in a pH of 2.0 for the colloidal solution ([Cl-] ) 4.5 × 10-6 M). Dynamic light scattering (Horiba, LB-550) indicated that the mean particle size of the material was 16 nm. The methyl viologen dichloride hydrate (MV2+) (Tokyo Kasei) and sodium formate (HCO2Na) (Nakarai Tesque) were used without further purification as the source of ions. Time-resolved absorption measurements were performed using the third harmonic generation (355 nm, 5 ns full width at halfmaximum (fwhm)) from a Q-switched Nd:YAG laser (Continuum, Surelite II-10) for the excitation operated with a temporal control by a delay generator (Stanford Research Systems, DG535). The analyzing light from a pulsed 450 W Xe-arc lamp (Ushio, UXL451-0) was collected by a focusing lens and directed through a grating monochromator (Nikon, G250) to a silicon avalanche photodiode detector (Hamamatsu Photonics, S5343). The analyzing lamp, sample, monochromator, and a silicon avalanche photodiode detector all lie on the same axis with the excitation beam incident at 90° to the axis. The transient signals were recorded with a digitizer (Tektronix, TDS 580D). All measurements were carried out at room temperature. The concentration of absorbed photons was determined from Nph/Vir, where Nph is the number of absorbed photons of a molecule in the irradiated volume (Vir) and was estimated from the absorbance at 355 nm and the irradiation energy which was directly measured using a power meter. Zeta potentials were determined using a Zetasizer Nano from Sysmex Co. at room temperature.
Results and Discussion Figure 1 shows the steady-state diffuse reflectance spectra observed for the TiO2 powder (20 g dm-3) (Japan Aerosil, P25) slurried in water (pH 7) containing MV2+ (0.2 mM) in the absence (a) and presence (b) of HCO2Na (10 mM) under UV irradiation (366 nm, 150 W, 2 min). A significant blue color change in the TiO2 powder with the addition of HCO2Na was observed as shown in photographs a′ and b′. The absorption band with a peak at about 600 nm is attributable to MV•+,15 suggesting that the reduction of MV2+ was significantly enhanced by the addition of HCO2Na. (15) (a) Farrington, J. A.; Ebert, M.; Land, E. J.; Fletcher, K. Biochim. Biophys. Acta 1973, 314, 372. (b) Krasna, A. I. Photochem. Photobiol. 1980, 31, 75. (c) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617. (d) Meisel, D.; Mulac, W. A.; Matheson, M. S. J. Phys. Chem. 1981, 85, 179. (e) Das, T. N.; Ghanty, T. K.; Pal, H. J. Phys. Chem. A 2003, 107, 5998.
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Figure 2. Transient absorption spectra observed at 0.08 µs after the laser flash during the 355-nm laser photolysis (25 mJ pulse-1) of the Ar-saturated TiO2 colloidal aqueous solution (pH 2) in the absence (solid circles) and presence (solid triangles) of HCO2Na (10 mM). The broken lines indicate the transient absorption spectra of e- observed during the 355-nm laser flash photolysis of the TiO2 powder (Ishiahra Sangyo, ST-01) slurried in methanol, in which the ∆O.D. values were normalized at 827 nm for each subtraction. The open circles and triangles indicate the net transient absorption spectra due to trapped h+ calculated by subtracting the transient absorption spectra of e- (broken lines) from the observed spectra (solid symbols), respectively.
As a first step, to confirm the generation of CO2•-, we examined the scavenging of h+ by HCO2- adsorbed on the TiO2 surface.16 Figure 2 shows the transient absorption spectra observed at 0.08 µs after the laser flash during the 355-nm laser photolysis (25 mJ pulse-1) of an Ar-saturated TiO2 colloidal aqueous solution (pH 2) in the absence (solid circles) and presence (solid triangles) of HCO2Na (10 mM). To separate the absorption band of h+ from that of e-, we observed the transient absorption spectra during the 355-nm laser flash photolysis of the TiO2 powder (Ishihara Sangyo, ST-01) slurried in methanol.17 In the presence of an h+ scavenger such as methanol, the absorption band attributed to e- in the TiO2 particles, which increases with the increasing wavelength in the present wavelength range, was observed. The absorption band represented by the broken lines in Figure 2 is quite consistent with (16) (a) Formate is generally considered to bond to surfaces in three different ways: (i) bridging bidendate, (ii) bidendate, and (iii) monodendate. Recently, Rotzinger et al. studied the adsorption of formate and acetate from aqueous solutions onto the TiO2 rutile (110) surface by attenuated total reflection FTIR spectroscopy.16b They suggested that formate and acetate adsorb via the bridging bidentate structure. On the other hand, in recent infrared work on formate adsorbed on anatase powders, an asymmetric formate was reported in which one of the formate oxygens forms a strong bond to a surface Ti atom, whereas the other bonds more weakly to the surface.16c In comparison to the rutile surface, such a structure may be more favorable because Ti-Ti distances are 2.9616d and 3.785 Å16e for rutile (110) and anatase (101), respectively. (b) Rotzinger, F. P.; Kesselman-Truttmann, J. M.; Hug, S. J.; Shklover, V.; Gra¨tzel, M. J. Phys. Chem. B 2004, 108, 5004. (c) Popova, G. Y.; Andrushkevich, T. V.; Chesalov, Y. A.; Stoyanov, E. S. Kinet. Catal. 2000, 41, 805. (d) Baur, W. H. Acta Crystallogr. 1961, 14, 209. (e) Cromer, D. T.; Herrington, K. J. Am. Chem. Soc. 1955, 77, 4708. (17) (a) To obtain the spectrum of e- in the present colloidal TiO2 solution, methanol and several sugars were used as h+ scavengers. However, we found that the particle size of the TiO2 changed with the addition of such scavengers and the efficiency of the h+ scavenging was not very high due to the competitive adsorption of water on the TiO2 surface. Therefore, we obtained the spectrum of e- from the laser flash photolysis of a TiO2 powder slurried in neat methanol. (b) Shkrob, I. A.; Sauer, M. C., Jr. J. Phys. Chem. B 2004, 108, 12497. (c) Shkrob, I. A.; Sauer, M. C., Jr.; Gosztola, D. J. Phys. Chem. B 2004, 108, 12512.
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Langmuir, Vol. 20, No. 22, 2004 9443
those of e- reported elsewhere.17-19 By subtracting the spectra of e- (broken lines) from the observed ones (solid symbols), it was found that a small amount of the trapped h+ (htr+) was scavenged by HCO2- adsorbed on the surface of the TiO2 particles (see open symbols). A significant increase (∼30%) in ∆O.D. at 827 nm, mainly attributed to e- in the TiO2 particles, was simultaneously observed. Using the molar absorption coefficient () of 700 M-1 cm-1 at 800 nm for e-,19 the concentrations of (1.5 ( 0.1) × 10-5 and (1.9 ( 0.1) × 10-5 M for e- were calculated at 0.08 µs after the laser flash in the absence and presence of HCO2-, respectively. HCO2has been reported to be a current doubling agent due to the more negative reduction potential (Ered) of CO2•- (Ered ) -1.90 V vs normal hydrogen electrode (NHE)) compared with that (-0.25 V vs NHE at pH 2) of the CB.20-22 Therefore, the observed difference ((4.4 ( 0.4) × 10-6 M) in the concentration of e- is mainly attributable to the scavenging of h+ by HCO2- and the electron injection from CO2•-, which is generated from the one-electron oxidation reaction with h+, to the CB of the TiO2. Using the value of 420 M-1 cm-1 at 520 nm for htr+,23 one can estimate the decrease of (2.5 ( 0.3) × 10-6 M in the concentration of htr+ due to the hole transfer from h+ to HCO2- adsorbed on the TiO2 surface. The yield of the scavenging of h+ by HCO2- of about 3.5% was also estimated from the absorbed photon concentration of 7.2 × 10-5 M. Assuming that an e- of 2.5 × 10-6 M was generated by the scavenging of h+, the concentration of the injected e- is expected to be (1.9 ( 0.3) × 10-6 M. This value is slightly lower than the decrease ((2.5 ( 0.3) × 10-6 M) in the concentration of htr+ due to the scavenging by HCO2-, suggesting that CO2•molecules of 10-7-10-6 M were desorbed from the surface. Therefore, we considered that CO2•- generated from the one-electron oxidation reaction with h+ is quickly desorbed from the surface or injects an e- into the CB of the TiO2 as given by reactions 1-3,24
HCO2-(a) + h+ f CO2•-(a) + H+
(pKa ) -0.2)25 (1)
CO2•-(a) f CO2(a) + eCB-
(2)
CO2•-(a) a CO2•-(f)
(3)
where a and f in parentheses denote the surface-bound and free species, respectively. These reactions occurred (18) (a) Yoshihara, T.; Katoh, R.; Furube, A.; Tamaki, Y.; Murai, M.; Hara, K.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2004, 108, 3817. (b) Yamakata, A.; Ishibashi, T.; Onishi, H. Chem. Phys. Lett. 2001, 333, 271. (c) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2001, 105, 7258. (d) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2002, 106, 2922. (19) Safrany, A.; Gao, R.; Rabani, J. J. Phys. Chem. B 2000, 104, 5848. (20) (a) The CB potential (ECB) of -0.25 V vs NHE is calculated from ECB ) -0.13-0.059 pH.20b (b) Duonghong, D.; Ramsden, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1982, 104, 2977. (21) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1989, 93, 409. (22) (a) Rajeshwar, K. In Electron Transfer in Chemistry; WileyVCH: New York, 2001; Vol. 4, p 279. (b) Itoh, K.; Baba, R.; Fujishima, A. Chem. Phys. Lett. 1987, 135, 521. (23) Assuming that the same number of charge carriers was generated during the 355-nm laser photolysis of the TiO2 colloidal aqueous solution, for htr+ was estimated from the value of 700 M-1 cm-1 at 800 nm for e-.19 The spectral shape was identical within experimental error in the present time period (
