(Invited) Mixed-Metal Oxides: Tuning Their Structures and Properties for Uses in the Capture and Conversion of Solar Energy

Abstract

The conversion of solar energy to chemical fuels, e.g., in the renewable production of hydrogen, has attracted intense research interest as both a practical and environmentally responsible way to meet our growing energy needs. Semiconducting p-type and n-type films can facilitate solar-driven reduction and oxidation reactions of water to produce molecular hydrogen and oxygen, respectively. Our research efforts have focused on promising new mixed-metal oxide systems (i.e., M = Cu(I) or Sn(II) with M’ = Nb(V), Ta(V), or Ti(IV)) that exhibit bandgap sizes spanning a wide range of visible-light energies, e.g. for CuNbO3, Sn2TiO4, Cu5Ta11O30, as well as for related solid solutions. These mixed-metal oxides can be prepared as p-type or n-type polycrystalline films. Electrochemical impedance spectroscopy shows that their conduction-band energies are located at suitable energies for driving fuel-producing reduction reactions at their surfaces, while their bandgap sizes range from ~2.8 eV to ~1.5 eV as a function of chemical composition and structure. For example, the n-type Sn2TiO4 phase was synthesized using flux methods and found to have one of the smallest visible-light bandgap sizes known that also maintains suitable conduction and valence band energies for driving photocatalytic water-splitting reactions. Further, as a suspended powder it is active for both oxygen and hydrogen production in aqueous solutions. Electronic structure calculations have also been used to probe the nature and type of the bandgap transitions. For the Cu(I)-niobates and Cu(I)-tantalates, the relationship of their reactivity to surface nanostructuring of the polycrystalline films has been determined and found to be responsible for the significant enhancements of their cathodic photocurrents under irradiation. Photocatalytic activities for hydrogen and oxygen production have been investigated in the form of suspended powders in solution, as well as in the form of p-type and n-type photoelectrode films. These results reveal new routes to tuning the band energies and bandgap sizes of mixed-metal oxides for the efficient use of sunlight for fuel producing redox reactions.