Design of Medium Band Gap Ag–Bi–Nb–O and Ag–Bi–Ta–O Semiconductors for Driving Direct Water Splitting with Visible Light

Two new metal oxide semiconductors belonging to the Ag–Bi–M–O (M = Nb, Ta) chemical systems have been synthesized as candidate compounds for driving overall water splitting with visible light on the basis of cosubstitution of Ag and Bi on the A-site position of known Ca₂M₂O₇ pyrochlores. The low-valence band edge energies of typical oxide semiconductors prevents direct water splitting in compounds with band gaps below 3.0 eV, a limitation which these compounds are designed to overcome through the incorporation of low-lying Ag 4d¹⁰ and Bi 6s² states into compounds of nominal composition “AgBiM₂O₇”. It was found that the “AgBiTa₂O₇” pyrochlores are in fact a solid solution with an approximate range of Ag_xBi_5/6Ta₂O_6.25+x/2 with 0.5 < x < 1. The structure of Ag_4/5Bi_5/6Ta₂O_6.65 was determined from the refinement of time-of-flight neutron diffraction data and was found to be a cubic pyrochlore with a = 10.52268(2) Å and a volume of 1165.143(6) ų. The closely related compound, AgBiNb₂O₇, appears to have an integer stoichiometry and to adopt an orthorhombically distorted pyrochlore-related structure with a subcell of a = 7.50102(8) Å, b = 7.44739(7) Å, c = 10.5788(1) Å, and V = 590.93(2) ų. Density functional theory-based calculations predict this distortion should result from A-site cation ordering. Fits to UV–vis diffuse reflectance data suggest that AgBiNb₂O₇ and “AgBiTa₂O₇” are both visible-light-absorbing semiconductors with the onset of strong direct absorption at 2.72 and 2.96 eV, respectively. Electronic structure calculations for an ordered AgBiNb₂O₇ structure show that the band gap reduction and the elevation of the valence band primarily result from hybridized Ag d¹⁰–O 2p orbitals that lie at higher energy than the normal O 2p states in typical pyrochlore oxides. While the minimum energy gap is direct in the band structure, the lowest energy dipole allowed optical transitions start about 0.2 eV higher in energy than the minimum energy transition and involve different bands. This suggests that the minimum electronic band gap in these materials is slightly smaller than the onset energy for strong absorption in the optical measurements. The elevated valence band energies of the niobate and tantalate compounds are experimentally confirmed by the ability of these compounds to reduce 2 H⁺ to H₂ gas when illuminated after functionalization with a Pt cocatalyst.