CuNb1−xTaxO3 (x ≤ 0.25) solid solutions: impact of Ta(v) substitution and Cu(i) deficiency on their structure, photocatalytic, and photoelectrochemical properties

Abstract

Solid solutions of Cu(I)-containing oxide p-type semiconductors provide key opportunities to probe the fundamental relationships between chemical compositions and crystal structures, bandgap sizes, band energies, and photoelectrochemical properties. Members of the CuNb_1−xTa_xO₃ (0 < x ≤ 0.25) solid solution have been synthesized via high temperature solid-state methods. The structure of CuNbO₃ was found to be Cu-deficient Cu_0.965NbO₃ after heating in air at 250 °C for 3 hours, i.e., under similar conditions as those used to prepare it as a polycrystalline film. Powder X-ray diffraction techniques confirmed the purity of each composition up to x ≤ 0.25 and the lattice parameters were refined as the molar ratio of Nb(V) and Ta(V) was varied (a = 9.499 to 9.506 Å, b = 8.439 to 8.451 Å, c = 6.768 to 6.781 Å and β = 90.847 to 90.694°). An increase in the amount of Ta(V) yielded a small blue shift of the bandgap size from ∼1.89 eV to ∼1.97 eV for CuNb_1−xTa_xO₃ from x = 0 to 0.25. Polycrystalline films of each member of the CuNb_1−xTa_xO₃ solid solutions produced relatively comparable p-type photocurrents of up to −0.5 mA cm⁻², while the stability of the cathodic photocurrent also remained similar with increasing Ta(V) content. Mott–Schottky analysis of CuNb_1−xTa_xO₃ showed that the conduction band edge of −1.5 (vs. SHE) provides a sufficient overpotential (∼800 mV) to drive the reduction of water to hydrogen gas at the surface. The capability of the solid solutions to drive hydrogen production was confirmed through suspended particle photocatalysis. Further characterization of the CuNb_0.91Ta_0.09O₃ composition included scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analyses. These data show that Cu(I) is oxidized to Cu(II) as CuNb_1−xTa_xO₃ is heated in air. Thus, the formation of Cu(II) rich regions at the surface, together with the Ta(V) content, are found to play important roles in the stability and magnitude of the cathodic photocurrents produced under visible-light irradiation. Importantly, these results demonstrate that solid solution compositions can be used in films for solar energy conversion, notwithstanding their inherent atomic disorder.