Thermodynamic Oxidation
and Reduction Potentials of
Photocatalytic Semiconductors in Aqueous Solution
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Abstract
An approach is introduced to calculate the thermodynamic
oxidation
and reduction potentials of semiconductors in aqueous solution. By
combining a newly developed ab initio calculation method for compound
formation energy and band alignment with electrochemistry experimental
data, this approach can be used to predict the stability of almost
any compound semiconductor in aqueous solution. Thirty photocatalytic
semiconductors have been studied, and a graph (a simplified Pourbaix
diagram) showing their valence/conduction band edges and oxidation/reduction
potentials relative to the water redox potentials is produced. On
the basis of this graph, the thermodynamic stabilities and trends
against the oxidative and reductive photocorrosion for compound semiconductors
are analyzed, which shows the following: (i) some metal oxides can
be resistant against the oxidation by the photogenerated holes when
used as the n-type photoanodes; (ii) all the nonoxide semiconductors
are susceptible to oxidation, but they are resistant to the reduction
by the photogenerated electrons and thus can be used as the p-type
photocathodes if protected from the oxidation; (iii) doping or alloying
the metal oxide with less electronegative anions can decrease the
band gap but also degrade the stability against oxidation