Semiconductor photoelectrodes reduction processes

The conclusions from these considerations are that semiconductor photoelectrodes can be used to effect either reductions (p-type semiconductors) or oxidations (n-type semiconductors) in an uphill fashion. The extent to which reaction can be driven uphill, Ey, is no greater than Eg, but may be lower than Eg owing to surface states between Eqb and Eye or to an Inappropriate value of Ere(jox. Both Eg and Epg are properties that depend on the semiconductor bulk and surface properties. Interestingly, Ey can be independent of Ere(jox meaning that the choice of Ere(jox and the associated redox reagents can be made on the basis of factors other than theoretical efficiency, for a given semiconductor. Thus, the important reduction processes represented by the half-reactions (3)-(5) could, in principle, be effected with the same efficiency at a Fermi level pinned (or... 

A silicon electrode with a small amount of deposited noble metals such as Pt does not behave as a buried junction and the photoelectrode properties are still determined by contact of the electrolyte, not the metal. The photovoltaic response of the Pt-deposited silicon electrode is due to the incomplete coverage of the surface by Pt so that the interface energetics remains dominated by the contact of the semiconductor surface with the electrolyte. The mechanism of the hydrogen reduction processes involves first photoexcitation of electron-hole pairs followed by charging of the small Pt islands with excited electrons. The charged Pt islands react with H2O at a high rate. 

In such devices the light-absorbing semiconductor electrode immersed in an electrolyte solution comprises a photosensitive interface where thermodynamically uphill redox processes can be driven with optical energy. Depending on the nature of the photoelectrode, either a reduction or an oxidation half-reaction can be light-driven with the counterelectrode being the site of the accompanying half-reaction. N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, and... 

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