Synthesizability and Properties of Carbon Nitride Semiconductors for Solar Energy Conversion

Metastable semiconductors in many chemical systems have been discovered that have desirable photoelectrochemical 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. Recent results will be presented that demonstrate flux-mediated synthetic routes to metastable Sn(II)- and Cu(I)-containing titanates, niobates and stannates. These semiconductors possess some of the smallest known visible-light band gaps that also maintain suitable band energies for the photocatalytic reduction and oxidation of water. 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, with a kinetic stabilization that can be controlled via solid solutions that inhibit phase segregation. Results will be presented on their photoelectrochemical properties for the production of molecular hydrogen and molecular oxygen as a function of their chemical compositions and crystalline structures.