New Approach to a Rotating Ring Disk Electrode

Abstract

The mechanism of the oxidation of water by photogenerated holes in an n-type semiconductor was elucidated by using a rotating ring disk electrode (RRDE) with n-TiO2 as a disk. The heterogeneous electrode process was found to be hydrox3~] radical (OH*) formation. Hydrogen peroxides were produced in solution by the homogeneous second-order chemical reaction of hydroxyl radicals. The rate constant k2 for OH* + OH* k2 H~O2 was found to be 1.5 • 1011 M 1 s-1 (1 tool 1 s-l) on the basis of a model that is developed herein. The catalytic rate parameter of electrochemically generated radicals with redox species such as Fe z § could also be determined using this model. The rate constant kl for the catalytic reaction Of Fe(CN)~ 4-with H202 and OH* was found to be 180 M -1 s -I. It was confirmed in some detail that the following Haber-Weiss cycle was operative at an illuminated TiO2/aqueous electrolyte system 2Fe 2+ + 3H202 ~ 2Fe 3+ + 2 OH-+ 02 + 2H20The rotating ring disk electrode (RRDE) is a powerful device for investigating electrode kinetics (1-5). Several attempts have been made to apply RRDE methods to investigate electrode kinetics at a semiconductor electrode (6-12). At a metal electrode, the electrode reaction can be controlled by the electrode potential. A useful potential range where no unfavorable reactions occur can be chosen with ease. For example, oxygen or hydrogen evolution in an aqueous electrolyte can be avoided between 0 and 1.23V at pH = 0. On the contrary, in a large bandgap n-type semiconductor electrode, the energy level of valence bandedge where photogenerated holes appear is generally far below the redox energy levels in the electrolyte. An external bias cannot alter this situation because it only alters the band bending of the semiconductor. It is likely that holes react with unfavorable redox species. Even the oxidation of solvent such as water occurs. It is well known that hydrogen peroxides are formed at an illuminated TiOJaqueous electrolyte system (8, 13). The hydrogen peroxide shows a catalytic reaction with redox species added in solution such as H202 + 2A--> 2A § + 2 OH-where A and A + denote redox species. This complicates the interpretation of the RRDE data. It is also well known that photogenerated holes also react with the semiconductor causing photocorrosion. Especially, TiO2 may suffer from anodic dissolution when it is illuminated in the presence of the sulfate ion (14-16). Photocorrosion influences the reproducibility of the experimental data.Needless to say, the semiconductor has a forbidden energy gap, in which surface states exist. Surface or interface states are believed to determine the electrode process (17, 18). However, a detailed mechanism of the charge transfer process has not been clarified.The objective of the present study is to elucidate the mechanism of oxidation of water by photogenerated holes in a TiQ electrode. An analytical method for interpreting the complicated metal-semiconductor ring-disk data has been proposed.It is shown that the complicated RRDE data can...

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