Photocatalytic, structural and optical properties of mixed anion solid solutions Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey†
Nine members of two contiguous solid solutions, Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey (x, y = 0, 0.5, 1, 1.5 and 2), were synthesised at temperatures between 800 °C and 900 °C by stoichiometric combination of binary precursors. Their structures were determined by Rietveld refinement of X-ray powder diffraction data and found to adopt the SmNi3Ge3 structure with I4/mmm symmetry. Approximate Vegard law relationships were found within each solution between the lattice parameters and composition, with an observed cell volume of 466.4 Å3 for Ba3Sc2O5Cu2S2 increasing to 481.0 Å3 for Ba3In2O5Cu2S2 and finally to 499.0 Å3 for Ba3In2O5Cu2Se2. In the first solid solution, this volume increase is driven by the replacement of scandium by the larger indium ion, generating increased strain in the copper chalcogenide layer. In the second solution the substitution into the structure of the larger selenium drives further volume expansion, while relieving the strain in the copper chalcogenide layer. Band gaps were estimated from reflectance spectroscopy and these were determined to be 3.3 eV, 1.8 eV, and 1.3 eV for the three end members Ba3Sc2O5Cu2S2, Ba3In2O5Cu2S2, and Ba3Sc2In2O5Cu2Se2, respectively. For the intermediate compositions a linear relationship between band gap size and composition was observed, driven in the first solution by the introduction of the more electronegative indium lowering the conduction band minimum and in the second solution by the substitution of the electropositive selenium raising the valance band maximum. Photocatalytic activity was observed in all samples under solar simulated light, based on a dye degradation test, with the exception of Ba3In2O5Cu2Se1.5S0.5. The most active sample was found to be Ba3Sc2O5Cu2S2, the material with the largest band gap.