Evidence for a dual mechanism in the TiO2/CuxO photocatalyst during the degradation of sulfamethazine under solar or visible light: Critical issues

This study presents the photocatalytic degradation of sulfamethazine (SMT) on TiO2/CuxO nanotubes (NTs) by a differentiated mechanism under low intensity solar light and indoor visible light irradiation. In the presence of TiO2/CuxO nanotubes, the SMT-degradation was complete within 3 h (in acid aqueous solution). The surface of the photocatalyst used was registered by scanning and transmission electron microscopy SEM/TEM. By X-ray diffraction (XRD), the anatase and rutile phases were detected in the TiO2/CuxO(1%) materials. This photo catalyst led to the fastest SMT-degradation. By X-ray photoelectron spectroscopy (XPS) the TiO2 and CuxO species deconvoluted signals provided the evidence for the redox catalysis taking place during SMT-degradation. Cu2O was the major component in the TiO2/CuxO(1%) samples as detected by XPS. The SMT-degradation kinetics was monitored by high performance liquid chromatography (HPLC). The reactive oxidative species (ROS) generated by TiO2/CuxO surface under solar and visible light irradiation were unambiguously identified by appropriate scavengers. The band-gap of the TiO2/CuxO NTs prepared in this study is reported. The stability of the TiO2/CuxO leading to SMT-photodegradation was monitored. The interfacial charge transfer (IFCT) photo activated by solar light on the TiO2/CuxO surface is suggested to proceed via a Schottky barrier. But under visible light a mechanism involving surface plasmon resonance (SPR) mechanism is suggested to account for the observed IFCT.