(205e) Hydrogen Evolution Activity and Water Oxidation Selectivity Tuning By Intermediate-Band Oxides

Intermediate-band oxides possess tunable electronic structures and tunable transport energy levels in their band gaps: they have initially shown to serve as chemically-stable, transparent conductive layers for stabilizing photocatalytic interfaces that otherwise corrode or passivate in water under light illumination. Recent reports about manganese-alloyed titanium oxide, (Ti,Mn)O_x, showed the incorporation of transition metal cation impurities, e.g. Mn³⁺, into an otherwise insulating TiO₂ coating. The high concentration of Mn³⁺ impurities induces a continuous intermediate band. This talk will leverage the interfacial electronic structure characterization to further demonstrate the use of this new class of interfacial layers to boost hydrogen evolution activity and tune the water oxidation selectivity.

On the one hand, this intermediate band induced electronic interactions between the coating and underlying electro-catalysts. For example, the nano-interface of ALD MnO_x and NiFe layered double hydroxides lowered the electrocatalysts Fermi level by ~0.2 eV, which is correlated with improved water-oxidation activity. Similarly, the hydrogen evolution activity of Ni-Fe sulfide bi-metallic catalysts can be boosted via continuous tuning of interfacial electronic structures by growing TiO₂ permeable coatings of atomically precise thickness. On the other hand, the tunable intermediate bands in an otherwise forbidden band gap uniquely achieved potential-dependent selectivity. So far, manipulating selectivity only concerns factors, such as molecular structures and activation barriers, because conventional metallic electro-catalysts cannot confine charge transfer to specific energy levels thereby voiding potential-dependent selectivity. We will demonstrate tunable intermediate bands invoked 100% water-oxidation selectivity to produce hydrogen peroxide, even though the thermodynamically favored pathway of O₂ evolution by an overpotential of 0.55 V. DFT calculations will elucidate the catalytic pathways and support the observed potential-dependent selectivity.