Effects and Importance of Surface States

Effects and Importance of Surface States. In the usual model (10, 19) of charge transfer across semiconductor-electrolyte interfaces, it is assumed that this process occurs isoenergetically. 

This model would thus predict that for a series of redox couples within the band gap, the redox couple having the most positive redox potential would be kinetically favored for photoxidation reactions, while for photoreduction the couple with most negative redox potential would be kinetically favoured. 

However, experimental results with Ti02, SrTi03, CdS, GaP, 

and Si (20-26) show that the inverse is generally true. Redox couples with the most thermodynamically favorable redox potential, and hence with the poorest energy overlap with the semiconductor bands, are usually the favoured reaction. These results are generally explained by invoking the existence of surface states for the semiconductor that lie within the band gap. These surface states form an energy band such that photoexcited holes or electrons are first trapped by the surface states, and then transferred isoenergetically to the electrolyte. This mechanism permits the most thermodynamically favoured redox reaction to be also kinetically favorable. 

Another important role for surface states has been proposed to explain how semiconductor band edges can become unpinned at the semiconductor-electrolyte interface. This important effect is discussed in detail below in the next section. 

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