Facile green synthesis of zinc oxide nanoparticles using Thymus vulgaris extract, characterization, and mechanism of chromium photocatalytic reduction

Abstract

Zinc oxide nanoparticles (ZnO NPs) were synthesized via a green and simple method using extract of Thymus vulgaris (TV). The green synthesized zinc oxide nanoparticles (GZnO NPs) were characterized by field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), Ultra violet-visible diffuse reflectance spectra (UV–vis-DRS), and Fourier Transform Infra-Red )FTIR( spectrophotometer.The results of various analyses confirmed the formation of GZnO NPs. UV–vis spectrum exhibited a blue shift absorption peak at 368 nm and the band gap energy was calculated 3.4 eV. FTIR study suggested that polyphenols and other bio-chemicals were responsible for reduction and stabilization of synthesized ZnO NPs. FE-SEM and TEM images showed particles with spherical shape and less than 40 nm diameter in size. In the photocatalytic reduction study, effect of some important variables such as catalyst dosage, methanol hole scavenger, contact time, and Cr(VI) concentration was investigated. To study photocatalytic activity of GZnO NPs, the adsorption control (AP) and photolysis (PP) experiments were conducted in same condition of photocatalytic process (PCP). Synergistic effect of PCP showed the high activity of GZnO NPs as catalyst for Cr(VI) reduction in the photocatalytic reactor (about 83% at 15 min, 0.5 mg l−1 of Cr(VI) and 0.015 g l−1 of GZnO NPs concentration). EDS analysis confirmed the presence of chromium on GZnO after adsorption control and photocatalytic processes. Kinetic constant for Cr(VI) reduction by PCP and PP obtained 0.154 and 0.008, which confirmed the increasing removal efficiency of chromium in the present of GZnO in photo-reactor as catalyst under UV light.