Photocatalytic, structural and optical properties of mixed anion solid solutions Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey

Abstract

Nine members of two contiguous solid solutions, Ba₃Sc_2−xIn_xO₅Cu₂S₂ and Ba₃In₂O₅Cu₂S_2−ySe_y (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 ų for Ba₃Sc₂O₅Cu₂S₂ increasing to 481.0 ų for Ba₃In₂O₅Cu₂S₂ and finally to 499.0 ų 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 Ba₃In₂O₅Cu₂Se_1.5S_0.5. The most active sample was found to be Ba₃Sc₂O₅Cu₂S₂, the material with the largest band gap.