ZnO/NiO heterostructures with enhanced photocatalytic activity obtained by ultrasonic spraying of a NiO shell onto ZnO nanorods

Degradation of organic pollutants such as methylene blue (MB) from water resources is currently of particular interest. Employment of a heterojunction device with optimized layer properties and proper interface engineering can enhance the photocatalytic performance by taking advantage of efficient charge separation. In this work, we develop an efficient photocatalytic system for the MB degradation based on ZnO nanorod (ZnONR)/NiO core-shell heterostructure with an optimized chemical and electronic structure for achieving record MB degradation efficiency of ∼70 %. ZnONR were grown by hydrothermal technique, whereas homogeneous crystalline NiO thin films were prepared by a robust and easy for up-scaling method of ultrasonic spray pyrolysis (USP). The optimum preparation conditions of photocatalytically efficient ZnONR/NiO heterostructures imply NiO film deposition from two USP cycles at 500 °C followed by air annealing heterostructures at 600 °C. The photocatalytic performance of ZnONR/NiO core-shell structure was investigated in comparison to counterpart layers and ZnO/NiO bilayer system. Chemical composition and band alignment at the ZnONR/NiO interface were investigated by X-ray photoelectron spectroscopy, Kelvin probe and photoelectron yield spectroscopy. Current transport studies indicated the presence of built-in electric field at the n-ZnO/p-NiO heterointerface responsible for the enhanced photocatalytic activity and based on this the degradation mechanism of MB is discussed.