Room-temperature deposition of ZnO-graphene nanocomposite hybrid photocatalysts for improved visible-light-driven degradation of methylene blue

Abstract

ZnO-graphene nanocomposites (NCs) of ZnO and graphite microparticles were directly deposited by a one-step kinetic spray process at room temperature on a nickel foam (NF) porous substrate. The ZnO-graphene NCs/NF with various graphite contents (25, 50, and 75 wt%) were utilized as hybrid photocatalysts for methylene blue (MB) degradation. Analysis of Raman spectra revealed the improvement of various disorder-related phonons, confirming the fragmentation of microparticles into the nano-size range in all the ZnO-graphene NCs/NF hybrid photocatalysts. Also, the degree of layer separation and fragmentation of micro-sized graphite in the ZnO-graphene NCs/NF was shown to be strongly dependent on the graphite content. The surface morphology of the nanostructured ZnO/NF thin films consisted of nanorods and nanosheets. The increase of graphite content in the ZnO-graphene NCs/NF hybrid photocatalysts was accompanied by a two-step morphology transformation into graphene nanoflakes and nanoflowers. The interfacial bonding states of the ZnO-graphene NCs/NF were investigated using X-ray photoelectron spectroscopy, which indicated synergy improvement by the evolution of multiple bonded states between the ZnO and graphene species in all the hybrid photocatalysts. UV–visible optical absorbance spectra revealed a decrease in the optical bandgap of the ZnO-graphene NCs/NF with graphene species incorporation, indicating the improvement of visible light harvesting. The NCs hybrid photocatalyst with 50 wt% graphite content exhibited the lowest bandgap of 2.9 eV compared with the nanostructured ZnO/NF with 3.18 eV. The photocatalytic activity of ZnO-graphene NCs/NF was evaluated using the degradation of MB under visible light irradiation. The NCs hybrid photocatalysts with 50 wt% graphite content exhibited the highest degradation efficiency and the highest reaction kinetics rate constant compared with the other nanostructured photocatalysts.