Photocatalytic Water Oxidation at the GaN (10(1)over-bar0)-Water Interface

Domen has observed that the GaN/ZnO semiconductor alloy serves, in the presence of a sacrificial electron scavenger, as a photocatalyst for solar water oxidation, producing H+ and O-2 at the aqueous/semiconductor interface. With a suitable cocatalyst, the same solar photoexcitation process also generates H-2 from H+. The active sites, mechanisms, and reaction intermediates are not known. This paper describes atomistic modeling and proposes a sequence of intermediate steps for the water oxidation process at a pure GaN/water interface. Pure GaN is known to be photocatalytically active but only in the UV region, because the semiconductor band gap is 3.4 eV, outside the visible region of the spectrum. However, it serves as an appropriate model system in the absence of more detailed information. A fiat (10 (1) over bar0) nonpolar surface is chosen to model an active site. Ab initio molecular dynamics simulations examine the fully solvated aqueous interface at ambient temperature. An appropriate cluster model, that includes a polarizable continuum in addition to explicit solvent water molecules, is cut out from snapshots of these AIMD simulations for additional DFT-based calculations of the water oxidation mechanism. The reaction intermediates follow a sequence of four proton-coupled electron transfers. Four UV photons are consumed to generate the four photoholes which drive the oxidation, producing 4H(+) + O-2 from 2H(2)O. Calculated standard free energies show that the photogenerated holes in GaN have sufficient energy to drive the overall water oxidation reaction. Implications for the operation of GaN/ZnO alloy photocatalysts, which absorb in the visible wavelength range, are presented. The calculated potentials show a remarkable parallelism to the known potentials for the sequential one-electron oxidation of water in homogeneous aqueous solution, suggesting that the proposed sequence may apply more generally than for the specific GaN (10 (1) over bar0) surface catalyst.