Light-assisted redox reactions

As shown in this symposium, interest in chemical modification of electrode surfaces has been extended in many directions, including the study of light-assisted redox reactions, and the use of modified electrodes in electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (A,5). 

Unfortunately, many of the semiconductor materials which would satisfy requirement (a) and (b) are not stable and undergo light-assisted corrosion instead of driving the desired redox reaction. Several important methods have been devised to chemically protect the surfaces of such materials as Si, CdS or CdSe, and the GaAs with the result however that the redox reactions that can be light-assisted are dictated by the redox e.m.f. of the surface-attached species (6). 

Rieke, P.C. and Armstrong, N.R. (1984) Light-assisted, aqueous redox reactions at chlorogallium phthalocyanine thin-film photoconductors dependence of the photopotential on the formal potential of the redox couple and evidence for photoassisted hydrogen evolution. I. Am. Chem. Soc., 106, 47-50. 

Fig. 7.4 Schematic eneigy band diagram for an ideal photoelectrochemical cell with a singleabsorber semiconductor photoanode and a metal cathode for light assisted water splitting. The electrochemical potentials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are represented by —qE°, where represents the reduction potential for the corresponding redox couples...

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