A review on green synthesis of ZnO nanoparticles

Nanoparticles and Nanostructured materials are playing an ever-important role in affordable healthcare, environment remediation, renewable energy, agriculture, consumer electronics, cosmetics etc. However, progress in these sectors has to be sustainable, environmentally friendly and requires sustainable synthesis process of nanoparticles and nanomaterials with net zero toxic byproducts. Therefore, green synthesis techniques are being actively pursued by researchers everywhere. When naturally occurring precursors replace industrially produced chemicals; it is always cost effective and facilitates direct as well as indirect employments to common man. Zinc oxide (ZnO) is one of the few materials which has wide spread application in all of the above sectors due to its unique physical, chemical, optical and electronic properties. In this review, various green synthesis techniques for ZnO nanoparticles used by different researchers in last 5–8 years are discussed and reviewed. In the beginning, the conventional synthesis techniques of ZnO nanoparticles are discussed briefly including ball milling, sol–gel, hydrothermal and precipitation methods. In the second part, different green synthesis techniques are discussed using various plant extracts. Particularly, the use of green tea leaf extracts in ZnO nanoparticle synthesis is discussed in detail. The factors that affect the morphology of nanomaterials are also discussed. Finally, the challenges and issues still remaining to be addressed are outlined with a conclusion. The review will be useful to researchers who want to pursue green synthesis of nanoparticles in general and ZnO in particular as beginners. It will be beneficial to biochemist, biologist, biotechnologist, environmentalist, industrialist and policy makers interested in progress towards sustainable science and technology.