Green synthesis of nanoparticles (NPs) is a revolutionary approach in terms of cost-effectiveness as well as minimization of environmental pollution by using perilousness-free bio-sources (different parts of plant, microorganism, plant-based waste). The different phytochemicals present in bio-sources play the role of potential reducing, stabilizing and capping agent during the synthesis of nanoparticles. An impetus in the rise of zinc oxide (ZnO) semiconductor has been perceived over the past decade as advanced nanomaterial to ameliorate the photocatalytic characteristics. Specifically, the use of ZnO is regarded as quite promising, not only because of the choice of green material but also owing to their alluring characteristics like high surface area to volume ratio, varying morphology etc. This review article represents several literature reports revealing the photocatalytic activities of green synthesized ZnO, prepared by using different green sources in degrading various organic pollutants which are present in wastewater. The method of derivation of phytochemicals during green synthesis approach of ZnO formation and the proposed mechanism of ZnO nanoparticles formation by interaction of those phytochemicals has been discussed thoroughly in this critical review. Numerous scientific investigations have been performed which elucidate on the controlling factors of ZnO (band gap, surface defects, surface morphology) and influence the photocatalytic activity. A summarization of such important reports has been synopsised in this article. The optimized parameters influencing the effective green synthesis of ZnO nanoparticles e.g. pH, temperature, metal precursor concentration and bio extract concentration have been concisely discussed with some published reports. Finally, a conclusion and future prospectus of the pioneering research arena are emphasized. Graphical Abstract

Present study demonstrates an intensive experimental work based on the tin oxide (SnO₂) nanoparticle synthesization which was successfully carried out by a simple conventional precipitation method followed by calcination at 700 °C. The synthesized nanoparticles were characterized by X-ray powder diffraction (XRD), UV–vis spectroscopy, FTIR (Fourier Transformed Infrared Spectroscopy), Field emission scanning electron microscopy (FESEM) and EDAX. The XRD pattern proves that tetragonal rutile structure SnO₂ nanoparticles were formed. The crystallite particle size calculation from Scherer's equation revealed the average size of 28.5 nm. The absorption spectrum of SnO₂ nanoparticles showed absorption band at about 290 nm and the band gap energy (E_g) from Tauc plot was obtained at 3.8 eV. The photocatalytic degradation of pharmaceutical compound, 4-aminopyridine (5 ppm) using synthesized SnO₂ nanoparticle, was assessed. The effect of variable catalyst dosage, pH and irradiation sources, were studied. The optimum catalyst dosage and pH were found to be 1.5 gm/L and 6.5 respectively. The degradation efficiency of water contaminant 4-aminopyridine under UV light and solar light irradiation for 120 mins were found to be 97% and 11%, respectively. The reusability of the catalyst was checked and has been found stable after three photocatalytic runs.