A review on biological synthesis of ZnO nanostructures and its application in photocatalysis mediated dye degradation: An overview

Zinc oxide (ZnO) has several industrial applications due to its versatile properties, which lead to its continuously increasing demand in different industrial sectors. Additionally, ZnO nanostructures possess unique photocatalytic activity, and because of this, they are being applied to degrade organic dyes through photocatalysis for wastewater treatment. Nevertheless, chemical synthesis methods to develop ZnO nanostructures have raised concerns related to environmental issues, furthermore, these methods are found to be costly and tedious. As a result, the synthesis of ZnO nanostructures using green methods is gaining popularity due to its low cost and eco-friendly mode, while avoiding the use of toxic chemicals. Green synthesis of ZnO nanostructures using different biological approaches involving plants, algae, and different microorganism-derived bioactive compounds has been well reported for diverse applications. Among different applications, ZnO nanostructures that enable photocatalysis to degrade dye have been found to be imperative for wastewater treatments. Therefore, the current review explores recent studies on green synthesis approaches to prepare ZnO nanostructures via adopting different biological methods that rely on plants, algae, and bacterial microorganisms. The properties of ZnO nanostructures, along with their green synthesis routes and feasible mechanisms, have also been discussed in this review. This review focuses on the use and efficiency of green route synthesized ZnO nanostructures as nanophotocatalysts for the degradation of organic dyes in wastewater treatment. Additionally, existing challenges in green synthesis methods and the efficiency of ZnO nanostructures to degrade organic dyes following photocatalysis has been discussed.