ZnO/CeO2/carbon xerogel composites with direct Z-scheme heterojunctions: Enhancing photocatalytic remediation of 4-chlorophenol under visible light

Abstract

This paper aims to create visible light driven ternary photocatalysts using zinc oxide (ZnO), cerium (IV) oxide (CeO2), and carbon xerogel (CX) as constituent materials. The use of CeO2 is based on the creation of direct-Z-scheme heterojunctions with the ZnO and the consequent diminishing of charge recombination, whereas the carbon xerogel inclusion is predicted to minimize bandgap energy, decrease electron-hole recombination, and boost specific surface area. Furthermore, the choice of the black-wattle tannin as a carbonaceous precursor was targeted at the development of an environmentally friendly and affordable composite. The existence of the hexagonal phase of zinc oxide and cubic structure of the cerium (IV) oxide in the ternary material was confirmed by X-ray diffractometry and X-ray photoelectron spectroscopy, with the latter also suggesting chemical bonding between the ZnO and the CX due to the creation of zinc oxycarbide complexes. The inclusion of the carbon xerogel provokes a significant modification in the morphology of the ternary material, resulting in an increased surface area and smaller particle aggregates. The CX/ZnO–CeO2 ternary composite obtains the highest photocatalytic efficiency among all the materials studied, degrading 100% of 4-chlorophenol under simulated sunlight and 68% under visible radiation, after 5 h. The increased photocatalytic activity can be attributed to the formation of direct Z-scheme heterojunctions between the semiconductors, higher visible light response, and higher specific surface area, as evidenced by the results obtained by active radical scavenging, chronoamperometry, diffuse reflectance spectroscopy, and N2 adsorption–desorption isotherms.