Enhanced photocatalytic activity of chemically deposited ZnO nanowires using doping and annealing strategies for water remediation

Water pollution represents one of the most challenging ecological threats humankind faces nowadays, resulting in the fast growing development of the heterogeneous photocatalysis using ZnO nanowires. A typical approach to improve the photocatalytic activity consists in achieving their extrinsic doping with group-III elements, but the reasons accounting for the resulting, modified photocatalytic processes are multifactorial and still under debate. In this work, we investigate the effect of the Al and Ga doping of ZnO nanowires grown by chemical bath deposition and of the thermal annealing under oxygen atmosphere on their photocatalytic activity and establish the dependence of the photocatalytic processes on their structural morphology, dimensions, dopant-induced defects, surface properties, and optical absorption and emission. We reveal that the photocatalytic processes strongly depend on the nature of dopants and are systematically enhanced after thermal annealing. The photocatalytic activity of annealed, Al-doped ZnO nanowires is found to be more efficient, through the direct action of holes besides the efficient action of radicals in the degradation process of organic dyes. These findings show the significance of decoupling the intricate contributions to the photocatalytic activity of ZnO nanowires when the extrinsic doping is used and of thoroughly selecting the nature of dopants.