IMPS analysis of multistep photoelectrochemical reactions with adsorbed intermediates

11 IMPS ANALYSIS OF MULTISTEP PHOTOELECTROCHEMICAL REACTIONS WITH ADSORBED INTERMEDIATES 

The behaviour of p-InP discussed in Section 8.9 was judged to be nonideal on the basis of a simplified theory for one electron transfer reactions. However, the reduction of protons to hydrogen by photogenerated electrons must be a two step process. There are two possible mechanisms  

Both mechanisms involve a hydrogen intermediate which must be adsorbed at the surface, modifying the potential distribution across the interface and hence the band bending. This intermediate may also be responsible for surface recombination by the reaction 

The intermediate can therefore be considered as a photogenerated surface state. The formation of adsorbed intermediates is a common feature of multi-electron transfer reactions. Other examples are encountered in the photodecomposition of compound semiconductors, for example the pho-toanodic decomposition of n-CdS (cf., equation (8.9)). 

Recently Fermin et al. [31] have used IMPS to study the photo-evolution of oxygen on /i-Ti02 and have concluded that the response is consistent with a Case II type mechanism involving OH ads intermediates. The same work showed that the OHads intermediate is also involved in the photooxidation of formic acid. 

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