First-principles studies of water splitting at semiconductor and oxide surfaces Poul Georg Moses and Chris G. Van de Walle , Materials Department, University of California, Santa Barbara A photoelectrochemical cell consists of a semiconductor photoelectrode which absorbs light in the solar spectrum with a valence-band maximum (VBM) and conduction-band minimum (CBM) that straddle the redox potential of water and hydrogen. We have investigated InGaN alloys using first-principles electronic-structure calculations and have determined the band gap and band positions as a function of In content. We combine calculations of VBM positions with calculations of the onset potential for water splitting and find that wurtzite nitrides and oxides are well suited for water splitting. Band positions as a function of In content Activity Volcano for water splitting Onset potential VBM positions and band gaps for InGaN Onset potentials and VBM positions