Synthesizability and Properties of Metastable Semiconductors for Solar Energy Conversion

Abstract

Metastable semiconductors in many chemical systems have been discovered that have desirable semiconductor properties for driving fuel-producing redox reactions from water and sunlight, such as strong visible light absorption, optimal band-edge energies, extreme defect tolerance, and enhanced carrier mobilities and charge separation. These properties are intimately related to their metastable nature, i.e., their crystalline structures and compositions lead to thermodynamic instabilities. New results will be presented that demonstrate flux-mediated synthetic routes to new metastable n-type Sn(II)-titanates and p-type Cu(I)-niobates. These semiconductors possess some the smallest known visible-light band gaps that also maintain suitable conduction and valence band energies for the photocatalytic reduction and oxidation of water to molecular hydrogen and oxygen, respectively. Their electronic structures also show advantageous features for solar energy conversion, such as small bandgaps, high band dispersion and defect tolerance. These characteristics are intimately related to their metastability and kinetic stabilization, which can be achieved via solid solutions that inhibit phase segregation. Thus, the solid-solution semiconductors Cu5(Ta1-x Nb x )11O30 and (Ba1-x Sn x )(Zr1-yTi y )O3 have been prepared using flux-mediated synthesis methods. 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 chemical compositions.