J. Phys. Chem. C 2008, 112, 6211
Reply to “Comment on ‘Mechanisms for Photooxidation Reactions of Water and Organic Compounds on Carbon-doped Titanium Dioxide, as Studied by Photocurrent Measurements’ ” Haimei Liu,†,‡ Akihito Imanishi,†,‡ and Yoshihiro Nakato*,‡,§ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka, 560-8531, Japan, The Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, Ibaraki, Osaka, 567-0047, Japan, and CREST, JST, Kawaguchi, Saitama 332-0012, Japan ReceiVed: January 7, 2008 Many thanks to L. H. Zhu for his interest in and comments on our recent work1 about mechanisms of photooxidation reactions of water and organic compounds on C-doped TiO2. We also thank J. Phys. Chem. C for giving us an opportunity to express our opinions on the comments. We agree with the comment of Zhu that the mechanisms of photooxidation reactions on particulate TiO2 film electrodes are affected by many factors and are complex. However, we want to emphasize that our conclusions are obtained from comparative studies based on well-controlled experiments under a variety of conditions. Marked differences in photocurrent behavior are observed by changes in electrodes (non-doped, N-doped, and C-doped TiO2, and TaON), wavelengths of irradiated light (UV and visible), and electrolyte compositions (the addition of a reductant such as methanol, formic acid, iso-propanol, and acetone to 0.5 M Na2SO4 in the presence and the absence of oxygen),1,2 but all of the experimental results are explained systematically by our mechanisms. This gives strong support to them. Unfortunately, Zhu’s criticisms of our mechanisms are based on a simple general kinetic equation for the photocurrent. He then claims that it is possible to explain our experimental results by factors other than our mechanisms with neither referring to detailed analyses of actual reactions nor proposing any alternative systematic mechanism. For example, he points out that the large increase in the photocurrent by the addition of methanol under UV-light irradiation does not necessarily give support to * Author to whom correspondence should be addressed. E-mail: nakato@ chem.es.osaka-u.ac.jp. † Division of Chemistry, Graduate School of Engineering Science, Osaka University. ‡ CREST. § The Institute of Scientific and Industrial Research (ISIR), Osaka University.
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the direct reaction mechanism because the result can be explained by the indirect reaction mechanism if a high rate constant for the current doubling term is assumed. However, this argument is wrong because the observed photocurrent enhancement by the addition of methanol is about 5.6 times, much larger than twice, which is the maximum enhancement expected from the current doubling mechanism. Zhu also points out that the observed small photocurrent under the visible-light irradiation in the presence of methanol does not necessarily support the indirect reaction mechanism because the result is explained by the direct reaction mechanism if a low rate constant for the current doubling term is assumed. This argument is also unreasonable because the assumption of the direct reaction mechanism should give a much higher photocurrent (>2 µA cm-2) than the observed one (about 0.3 µA cm-2) even without the current doubling, as is actually realized in the case of the addition of formic acid (Figure 7 of ref 1). It is to be noted also that the rate constant for the current doubling term is independent of the direct or indirect mechanism; that is, it should be the same for both the mechanisms. Zhu also criticizes our mechanism for the enhancement of the photocurrent by the addition of oxygen under the visiblelight irradiation in the presence of methanol. He claims why the same mechanism is not applied to the photocurrent under the UV-light irradiation. It is important to note that the interfacial oxidation reaction under the UV-light irradiation proceeds efficiently, yielding a high photocurrent, while that under the visible-light irradiation proceeds only inefficiently. How the effect of the addition of oxygen appears thus depends on such a difference in reaction kinetics, even though the reaction of O2 and •CH2OH radical occurs similarly in both cases. Namely, the photocurrent enhancement by the reaction of O2 and •CH2OH radical under the UV-light irradiation is not significant because the interfacial oxidation reaction is originally efficient. The photocurrent enhancement is rather overwhelmed by the photocurrent decrease because of the trapping of a high density of conduction-band electrons produced by efficient interfacial oxidation reaction by added O2. It is natural that our mechanisms should be regarded as plausible ones, not final. However, it is also a fact that our mechanisms offer a systematic explanation to all of the varied experimental results and are given strong support. Further detailed studies will be necessary to elaborate our mechanisms and to reach a definite conclusion. References and Notes (1) Liu, H. M.; Imanish, A.; Nakato, Y. J. Phys. Chem. C 2007, 111, 8603. (2) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617.
10.1021/jp8001299 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
