ZnO Nanostructures by Hot Water Treatment for Photocatalytic Bacterial Disinfection

Photocatalysis has received significant interest as an alternative method for microbial disinfection in water. Photocatalyst materials absorb light, produce electron–hole pairs, and generate reactive oxygen species (ROS), which cause damage to the cell walls and decomposes cellular materials of bacteria. A photocatalytic water disinfection method is an environmentally friendly approach and more effective compared to traditional methods such as chlorination, ozonation and direct UV – light disinfection. It is also well known that photocatalysts in the form of nanostructures have high surface area and enhanced light absorption capabilities, and thereby increased reactivity. Among various metal oxide photocatalysts, zinc oxide (ZnO) has attracted a great deal of attention due to its wide bandgap, remarkable optoelectronic properties, and non-toxicity. However, most of the ZnO nanostructure synthesis methods use toxic chemicals, are expensive, complicated, and non-scalable. Besides that, ZnO nano – photocatalysts are often synthesized in powder form, which are suspended in water. Although suspended nanostructures have high surface area, their recovery and reuse are challenging, and may cause secondary pollution. Hence, there is a need for synthesizing robust nanostructured photocatalysts that are immobilized on supports, in a scalable, cost – effective and environmentally friendly way, for water disinfection applications. Here we introduce a simple and chemical- free nanostructure synthesis method using a hot water treatment (HWT) to produce immobilized metal oxide nanostructures. With this method, ZnO nanostructures can be grown on Zn plates, just by the immersion of Zn plates in hot deionized (DI) water typically at temperatures ranging from 75°C to 95°C. In this work, we immersed Zn plates in DI water at 75°C for different time durations, which resulted in the formation of ZnO nanostructures of different lengths and diameters. Morphology and crystal structure of the ZnO nanostructures were characterized using scanning electron microscopy and X–ray diffraction. We then tested the antimicrobial efficacy of these novel nanostructured surfaces and coupled this surface treatment with UV radiation. It was found that the nanostructured ZnO plates had a statistically significant antimicrobial effect, and increased kill when coupled with UV radiation. This is believed to be due to the increased formation of ROS due to the photocatalytic ZnO nanostructures.