Energetics and Solvation Effects at the Photoanode/Catalyst Interface: Ohmic Contact versus Schottky Barrier

The design of optimal interfaces between photoelectrodes and catalysts is a key challenge in building photoelectrochemical cells to split water. Iridium dioxide (IrO_2) is an efficient catalyst for oxygen evolution, stable in acidic conditions, and hence a good candidate to be interfaced with photoanodes. Using first-principles quantum mechanical calculations, we investigated the structural and electronic properties of tungsten trioxide (WO3) surfaces interfaced with an IrO_2 thin film. We built a microscopic model of the interface that exhibits a formation energy lower than the surface energy of the most stable IrO_2 surface, in spite of a large lattice mismatch, and has no impurity states pinning the Fermi level. We found that, upon full coverage of WO_3 by IrO_2, the two oxides form undesirable Ohmic contacts. However, our calculations predicted that if both oxides are partially exposed to water solvent, the relative position of the absorber conduction band and the catalyst Fermi level favors charge transfer to the catalyst and hence water splitting. We propose that, for oxide photoelectrodes interfaced with IrO_2, it is advantageous to form rough interfaces with the catalyst, e.g., by depositing nanoparticles, instead of sharp interfaces with thin films.