Unrevealing the effect of transparent fluorine-doped tin oxide (FTO) substrate and irradiance configuration to unmask the activity of FTO-BiVO4 heterojunction

Three FTO-BiVO4 photoelectrodes are fabricated modifying the BiVO4 thickness, and systematically evaluating the influence of FTO substrate on the optical, electrical properties, and photoelectrochemical performance of BiVO4 semiconductor. The catalysts are characterized using two light sources with back-side or front-side irradiations, to investigate the impacts of different energy sources and configuration illumination on the FTO-BiVO4 photoactivity. This analysis reveals the existence of an additional charge transfer resistance increasing with thickness film subjected to front-side illumination, while the resistance remarkably diminishes when this interface is directly irradiated under back-side illumination. The highest photocurrent is achieved with the LED lamp under back-side illumination, condition selected to compare the degradation of 20 mg L-1 ciprofloxacin (CIP) in 0.05 M NaCl through electrocatalysis, photocatalysis, and photoelectrocatalysis using front-side or backside illuminations. In these evaluations, modified FTO contributes to the photogeneration of reactive chlorine species, whence it cannot be considered as a simple substrate. Back-side illumination presents a higher CIP elimination in comparison with front-irradiation. A schematic energy band diagram relying on Tauc and MottSchottky plots, and incorporating FTO as a photoactive semiconductor is established to rationalize the formation of oxidant species in the system. A degradation mechanism is established based on HPLC measurements of the different treatment methods.