Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution

Catalysts for the oxygen reduction and evolution reactions are central to key renewable-energy technologies including fuel cells and water splitting. Despite tremendous effort, the development of oxygen electrode catalysts with high activity at low cost remains a great challenge. In this study, we report a generalized sol-gel method for the synthesis of various oxide nanocrystals (TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5) with appropriate transition metal dopants for an efficient electrocatalytic oxygen evolution reaction (OER). Although TiO2 and ZnO nanocrystals alone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals exhibit greatly enhanced OER activity. A remarkable finding is that Co dopant produces higher OER activity than the other doped metals. X-ray photoelectron and X-ray absorption spectroscopies revealed the highly oxidized metal ions that are responsible for the enhanced catalytic reactivity. The excellent OER activity of the Co-doped nanocrystals was explained by a synergistic effect in which the oxide matrix effectively guards the most active Co dopants at higher oxidation states by withdrawing the electrons from the metal dopants. The metal-doped NCs exhibit enhanced catalytic activity under visible light irradiation, suggesting their potential as efficient solar-driven OER photoelectrocatalysts.