Synthesis, physical and photoelectrochemical characterizations of Sr0.5Nb3O8·1.7H2O: application to the Rhodamine B oxidation under solar light

Abstract

The layered niobate Sr0.5Nb3O8·1.7H2O is synthesized by soft chemistry in aqueous electrolyte via Sr2+ → H+ exchange between strontium nitrate and niobic acid HNb3O8·H2O. The material is identified by X-ray diffraction using Rietveld refinement, thermal analysis (TG/DSC) and optical measurements. The semiconducting and photoelectrochemical properties are investigated for the first time. The band gap of Sr0.5Nb3O8·1.7H2O is evaluated at 3.67 eV, and the transition is directly allowed due to the charge transfer O2−: 2p → Nb5+: 4d. The thermal variation of the electrical conductivity shows that 4d electrons are localized and the data are fitted by a small-polaron hopping model: σ = σo exp {− 0.13 eV/kT}. The capacitance measurement done in the ionic electrolyte (Na2SO4, 10−2 M) indicates n-type semiconductivity with a flat band potential of − 0.09 VSCE. The conduction band, made up of Nb5+: 4d orbital, is located at − 0.22 VSCE. As application, Rhodamine B (RhB) is oxidized by photocatalysis on Sr0.5Nb3O8·1.7H2O through O2· radicals; 56% of the initial concentration (10 mg L−1) is eliminated after 3 h under solar light (90 mW cm−2), and the Rh B oxidation follows a first-order kinetic with a rate constant of 0.246 h−1.