Reactive species specific RhB assisted collective photocatalytic degradation of tetracycline antibiotics with triple-layer Aurivillius perovskites

Abstract

Transition-metal incorporated triple-layer titano-niobate perovskites of the Aurivillius family, Bi_2.5Sr_1.5Nb₂Ti_0.5M_0.5O₁₂ (M = Cr, Mn, Fe), synthesized by conventional solid state reactions are reported. A strategy for collective photocatalytic removal of coloured and colourless organic pollutants with the compounds is devised. The triple-layer Aurivillius perovskites show significant shifts in absorption edges from UV toward the visible region upon transition metal incorporation in the parent Bi₂Sr₂Nb₂TiO₁₂. The Cr-compound displays the highest activity which correlates with higher adsorption, longer charge carrier lifetime, smaller charge transfer resistance (R_CT) and higher photocurrent density. Tetracycline (TC) antibiotics, a colorless pollutant, which remained unaffected when present alone, are degraded by more than 80% in the presence of the visible-light absorber Rhodamine B (RhB) dye in acidic aqueous solution. Combined adsorption, scavenger tests and mechanistic investigations reveal RhB degradation to be hole dominated while TC degradation is superoxide radical specific. Relative positioning of the HOMO–LUMO levels of the pollutants and the band edges of the perovskite semiconductors facilitates RhB-sensitized TC degradation and is responsible for reactive species specificity. The Aurivillius perovskites show excellent post catalytic chemical and structural stability and maintain activity over multiple cycles of degradation without any regeneration step between the cycles. The approach will enable the use of an existing colored co-pollutant or a deliberately added natural dye as a sensitizer for collective degradation of many colorless pollutants, including antibiotics from contaminated waters using visible light.