Regeneration of an efficient, solar active hierarchical ZnO flower photocatalyst for repeatable usage: controlled desorption of poisoned species from active catalytic sites

Abstract

To date, reusable properties of nano photocatalysts (PCs) have been investigated for five to ten consecutive cycles with a mild decrease in photocatalytic performance. Systematic investigation on the decrease in photocatalytic performance and regeneration of the nanocatalyst after repeatable usage have rarely been reported. Hierarchical forms of self-assembled zinc oxide (ZnO) with a flower-like structure have been successfully synthesized by an aqueous solution based precipitation and show visible photocatalytic degradation efficiency of 98% against methylene blue (MB) dye in 120 min under natural solar irradiation and visible irradiation. Reusable catalytic properties were studied for 50 consecutive cycles and the degradation efficiencies obtained were around 85.9, 61.4, 47.3, 31.6 and 21.3% in 120 min of photo irradiation for 10, 20, 30, 40 and 50 consecutive repeatable cycles, respectively. On increasing the number of cycles, the photocatalytic efficiency of the ZnO flowers decreased due to adsorption of hydrocarbon molecules on the active sites of the catalyst, a phenomenon termed as catalytic poisoning. Furthermore, we show that the photocatalytic performance of ZnO hierarchical flowers could be completely reverted by a simple chemical treatment for further repeatable usage by removing the hydrocarbon groups on the active sites of the surface of the nanocatalysts. Our results provide insights into the effective visible photocatalytic nature and controlled process which removes the poisoning effect and regenerates the catalytic properties.