Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents.

Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction.