Synthesis of hierarchical flower-like nitrogen functionalized graphene oxide‑zinc oxide photocatalyst for the sonophotocatalytic degradation of tetracycline antibiotic

Abstract

In this study, a nitrogen-functionalized graphene oxide‑zinc oxide (NGZ) photocatalyst, with varying GO contents, were synthesized using an in-situ hydrothermal method. X-ray diffraction results demonstrate improvements in the crystallinity of ZnO nanoflowers with increase in GO contents with the simultaneous increase in crystallite size from 18.59 to 21.30 nm. Raman spectroscopy revealed slight reduction in ID/IG ratio from 1.01 to 0.97 to reflect reduced structural disorderness. Ultraviolet-diffuse reflectance spectroscopy revealed trivial but inconsistent changes in the band gap within the range 3.19 to 3.25 eV. The band gap values were used to estimate the locations of valence and conduction bands in NGZ photocatalysts. Time-resolved photoluminescence and photocurrent measurements provide evidences for the improved lifetimes and enhance separation of charge carriers. Scanning and transmission electron microscopies established the formation of ZnO nanoflowers. Sonophotocatalytic tests conducted using ZnO and various NGZ catalysts revealed increase in the removal efficiency of tetracycline (TC) from 21 % to 65 % as GO content was increased from 0 % to 0.5 %, while further increase in GO contents to 1 % and 2 % have negative influence on the removal efficiency. Under optimized reaction conditions of time (180 min), catalysts (50 mg), and TC concentration (30 mg/L), an optimal removal of 96 % was recorded with 0.5%NGZ. The kinetics of the process, when fitted using first order kinetics, demonstrated 4.5 folds increase in the rate constant value of 0.5%NGZ, as compared to ZnO. The study demonstrated the crucial role of hydroxyl radicals in the sonophotocatalytic removal of TC through scavenging tests.