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 (TiO 2, ZnO, Nb 2 O 5, In 2 O 3, SnO 2, and Ta 2 O 5 ) with appropriate transition metal dopants for an efficient electrocatalytic oxygen evolution reaction (OER). Although TiO 2 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.