Advanced nanostructured photocatalysts based on reduced graphene oxide-TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye

Abstract

Reduced graphene oxide–TiO2 composites (GOT) were prepared by liquid phase deposition followed by post-thermal reduction at different temperatures. The composite materials were systematically evaluated as photocatalysts for the degradation of an important pharmaceutical water pollutant, diphenhydramine (DP), and an azo-dye, methyl orange (MO), under both near-UV/Vis and visible light irradiation as a function of the graphene oxide (GO) content. A marked compositional dependence of the photocatalytic activity was evidenced for DP and MO pollutants degradation and mineralization under both UV/Vis and visible light. Especially under visible light, optimum photocatalytic performance was obtained for the composites treated at 200 °C comprising 3.3–4.0 wt.% GO, exceeding that of the benchmark P25 (Evonik) catalyst. According to scanning electron microscopy, Raman spectroscopy, and porosimetry analysis data, this was attributed to the optimal assembly and interfacial coupling between the reduced GO sheets and TiO2 nanoparticles. Almost total degradation and significant mineralization of DP and MO pollutants (in less than 60 min) was achieved under near-UV/Vis irradiation for the optimum GOT composites. However, higher GO content and calcination temperatures (350 °C) led to detrimental effects due to the GO excess and the disruption of the GO–TiO2 binding. Photocatalytic experiments employing sacrificial hole and radical scavenging agents revealed that photogenerated holes are the primary active species in DP degradation for both bare TiO2 and GOT under UV/Vis irradiation, while an enhanced contribution of radical mediated DP oxidation was evidenced under visible light. These results combined with the distinct quenching of the GO photoluminescence under visible and NIR laser excitation, indicate that reduced GO acts either as electron acceptor or electron donor (sensitizer) of TiO2 under UV and visible light, respectively. Fine-tuning of the reduced GO–TiO2 interface is concluded as a very promising route to alleviate electron–hole recombination and circumvent the inherently poor light harvesting ability of TiO2 in the visible range.