Materials synthesis, characterization and DFT calculations of the visible-light-active perovskite-like barium bismuthate Ba1.264(4)Bi1.971(4)O4 photocatalyst

A perovskite-like barium bismuthate of the BaBi2O4 class, Ba1.264(4)Bi1.971(4)O4, has been prepared by solid-state synthesis and subsequently characterized by a number of experimental techniques (XPS, DRS, SEM, EDX and Raman spectroscopy), as well as by a DFT computational approach using the GGA Perdew-Burke-Ernzerhof (PBE) density functional to determine the energy band structure. XRD peaks were indexed to a rhombohedral cell (R3m) with parameters close to Ba0.156Bi0.844O1.422 (i.e., to Ba0.439Bi2.374O4), which upon Rietveld refinement gave Ba1.264(4)Bi1.971(4)O4. The Bi-O bond lengths determined from this refinement (1.86 and 2.31 angstrom) accorded with the bond lengths estimated from Raman spectra (1.97 and 2.26 angstrom). DFT calculations revealed the bismuthate to display two bandgaps that correspond to lower-energy indirect (2.28 eV) and to higher-energy direct (2.36 eV) electronic transitions in good agreement with the experimental bandgaps of 2.26 eV and 2.43 eV, respectively, from Tauc plots of DRS spectra. Relative to the indirect bandgap energy of 2.26 eV, the energies of the valence band and of the conduction band were, respectively, +1.14 eV and -1.12 eV. The photoactivity of Ba1.264(4)Bi1.971(4)O4 was examined toward the photoreduction of the greenhouse gas CO2 in aqueous media photoelectrochemically yielding alcohols and alkanes, while in the gas phase in an infrared cell reactor the products were carbon monoxide and alkanes.