Enhancement of photocatalytic activity of ZnO–SiO2 by nano-sized Ag for visible photocatalytic reduction of Hg(II)

Abstract

ZnO–SiO2 nanoparticles were synthesized by a sol-gel technique from Zn (NO3)2.6H2O and tetraethyl orthosilicate. The synthesized samples were further modified by nano-sized Ag from silver nitrate solution through photo-assisted deposition (PAD) and impregnation (Img) routes. The obtained samples were characterized by a series of techniques including X-ray diffraction, UV–vis diffuse reflectance spectroscopy, N2 adsorption, extended X-ray absorption fine structure, and transmission electron microscopy. The nano-sized Ag metal with a mean diameter (d) of ca. 2nm having a narrow size distribution was found on the PAD-Ag/ZnO–SiO2 catalyst, whereas the aggregated Ag metal with various sizes were observed on Img-Ag/ZnO–SiO2 catalyst (d = 12 nm). The surface area of the synthesized samples was decreased from 550 to 480 and 350 m2/g with ZnO–SiO2, PAD-Ag/ZnO–SiO2, and Img-Ag/ZnO–SiO2, respectively. The UV–visible diffuse reflectance spectra spectra analysis confirmed the lowest band gap of PAD-Ag/ZnO–SiO2 with a value of 2.5, compared to 2.8 and 3.2 with Img: Ag/ZnO–SiO2 and ZnO–SiO2, respectively. The photocatalytic reduction of Hg2+ to Hg0 was performed by aqueous solution containing formic acid. The obtained results revealed that the photocatalytic activity of the prepared samples increased in the order; ZnO–SiO2 < Img-Ag /ZnO–SiO2<PAD-Ag/ZnO–SiO2. The surface area decreased from 550 to 480 and 450 m2/g, while the efficiency of the photocatalytic reduction Hg (II) increased from 30 to 73 and 100%, with the ZnO–SiO2, Img-Ag/ZnO–SiO2 and PAD-Ag /ZnO–SiO2 samples, respectively.