Solid State Nanostructured Metal Oxides as Photocatalysts and Their Application in Pollutant Degradation: A Review

Abstract

Most dyes used in various industries are toxic and carcinogenic, thus posing a serious hazard to humans as well as to the marine ecosystem. Therefore, the impact of dyes released into the environment has been studied extensively in the last few years. Heterogeneous photocatalysis has proved to be an efficient tool for degrading both atmospheric and aquatic organic contaminants. It uses the sunlight in the presence of a semiconductor photocatalyst to accelerate the remediation of environmental contaminants and the destruction of highly toxic molecules. To date, photocatalysis has been considered one of the most appealing options for wastewater treatment due to its great potential and high efficiency by using sunlight to remove organic pollutants and harmful bacteria with the aid of a solid photocatalyst. Among the photocatalysts currently used, nanostructured metal oxide semiconductors have been among the most effective. This review paper presents an overview of the recent research improvements on the degradation of dyes by using nanostructured metal oxide semiconductors obtained by a solid-state method. Metal oxides obtained by this method exhibited better photocatalytic efficiency than nanostructured metal oxides obtained using other solution methods in several cases. The present review discusses examples of various nanostructured transition metal oxides—such as TiO2, Fe2O3, NiO, ReO3, IrO2, Rh2O3, Rh/RhO2, and the actinide ThO2—used as photocatalysts on methylene blue. It was found that photocatalytic efficiency depends not only on the bandgap of the metal oxide but also on its morphology. Porous nanostructured metal oxides tend to present higher photocatalytic efficiency than metal oxides with a similar band gap.