Metastable semiconductors within many chemical systems have been demonstrated to efficiently drive water oxidation and/or reduction under visible-light irradiation as suspended powders or thin films, respectively. These include for mixed-metal oxides (e.g., MM’Ox; M = Ti(IV), Nb(V) or Ta(V) cation; M’ = Ag(I), Cu(I), Sn(II), or Pb(II)) which have diverse structure types and compositions. For example, metastable n-type Sn(II)-titanates and p-type Cu(I)-niobates have been synthesized by flux methods and exhibit some of the smallest known visible-light band gaps that also maintain suitable conduction and valence band energies for driving the photocatalytic water-splitting reactions. Their electronic structures also show advantageous features for solar energy conversion, such as a high band dispersion and defect tolerance. These characteristics are intimately related to their metastability and necessitate synthetic routes to their kinetic stabilization, such as by using solid solutions that can inhibit phase segregation. Current results will be presented on the syntheses, structures and photoelectrochemical properties of the solid solutions Cu5(Ta1-xNbx)11O30 and (Ba1-xSnx)(Zr1-yTiy)O3 with band gaps that span the visible light energies from ~1.9 to 3.1 eV. Their band gaps, band edge energies and photocatalytic activities for the production of molecular hydrogen and molecular oxygen are found to be a sensitive function of their Ta-to-Nb or Zr-to-Ti and Ba-to-Sn molar ratios.

Metastable semiconductors: Synthesizabiliy and properties for solar energy conversion

ACS Fall 2021

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