Enhanced ROS generation by ZnO-ammonia modified graphene oxide nanocomposites for photocatalytic degradation of trypan blue dye and 4-nitrophenol

Abstract

There has been consistent effort on manipulating surface properties of ZnO NPs to produce higher concentration of reactive oxygen species (ROS) to enhance sustained photocatalytic degradation. Here we present a very simple strategy of synthesizing nanocomposites of ZnO NPs and ammonia modified graphene oxide (ZG) as an efficient photocatalyst for enhanced ROS generation and degradation of trypan blue azo dye and 4-nirophenol. The batch of photocatalyst made of ZnO NPs and 5wt% ammonia modified graphene oxide (ZG5) has been well characterized and its stability in aqueous medium is confirmed from minimal zinc leaching. The photocatalytic degradation of the dye and 4-nitrophenol followed Langmuir-Hinshelwood kinetics model. At optimized conditions, i.e., NH3 concentration for modifying graphene oxide, photocatalyst dose (ZG5), pH of the medium and contact time, the rate of photocatalytic degradation of trypan blue (k=7.3×10−3min−1) and 4-nitrophenol (k=5.5×10−3min−1), were more than 7–10 folds higher than that of pristine ZnO NPs. The enhanced rate of photocatalytic degradation by the ZG5 nanocomposite was correlated with higher ROS generation, owing to diffusion of photoexcited electrons and holes to the surface of the nanocomposite via conducting reduced graphene oxide to favour ROS generation. The ROS scavenger studies confirmed in-situ generation of superoxide radicals, hydroxyl radicals and singlet oxygen in the reaction medium. The degradation pathway for 4-nitrophenol has been proposed via formation of 1,4-benzoquinone as revealed from HPLC studies. The GC–MS studies confirmed nearly complete mineralization of the dye and 4-nitrophenol without formation of any by-product.