Zinc oxide NPs (ZnO NPs) have attracted significant interest due to their versatile physico-chemical and biological characteristics that supports the diverse applications in electronics, catalysis, healthcare, agriculture and in environmental remediation. This article reports a detailed study of the different synthesis methods for ZnO NPs, namely chemical, physical and green approaches and their effects on the particle size, morphology, crystallinity and functionality. The novel Green synthesis approach that involves the use of plant extracts, microorganisms and algae has been compared with conventional methods in terms of yield, reproducibility, cost and the environmental impacts. The eco-friendly approaches such as green synthesis discourages the use of hazardous chemicals and the energy intensive processes while producing ZnO NPs with enhanced surface activity, improved biocompatibility and lower toxicity. Green synthesized ZnO NPs exihibit the improved photocatalytic efficiency (up to 90–95% for the organic dyes) and better antimicrobial efficacy when compared with their chemically synthesized counterparts. The doping with metals and non-metal plays an significant role in tuning the structural, optical, electronic and magnetic properties of ZnO nanostructures for specific applications. Dopants such as Cu, Co, Ni, Ag, N and S has effectively modified the band gap (typically~2.8 eV–3.3 eV) and the charge carrier kinetics that leads to the enhanced photocatalytic activity and antimicrobial performance. Despite the many advantages of the doped ZnO NPs there still remains the challenges related to the large scale production, reproducibility and the incomplete understanding of structure property relationships. In this article the recent advances in ZnO based nanomaterials have been summarized with future directions for their use in sustainable technologies.
Sharma et al. (Sun,) studied this question.