Effect of Zinc Doping on Structural, Optical, Magnetic, and Catalytic Behavior of Co3O4 Nanoparticles Synthesized by Microwave-Assisted Combustion Method

The microwave-assisted combustion process (MCP) was adapted to prepared Zinc doped Co3O4 spinel nanoparticles. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM) techniques were used to investigate the structural, optical, morphological, magnetic, and catalytic properties. The cubic spinel structure was obtained without impurities in the X-ray diffraction (XRD) patterns of undoped Co3O4 and Zn2+ doped Co3O4 (x = 0.1 and 0.3) respectively. However, as the Zn2+ concentration increased, at x = 0.5, a new hexagonal phase appeared in addition to the cubic phase, with mean crystallite size of the cubic spinel structure extending from 48.6 to 25.5 nm. It is found that Zn2+ doping in Co3O4 matrix can induce a negative shift in the flat-band potential (VFB) and increases the isoelectric point. The Co–O stretching mode of the cubic spinel Co3O4 structure is responsible for the occurrence of FT-IR bands at about 662 and 573 cm−1. Kubelka–Munk (K–M) method is utilized to deduce the direct band gap and decline in the band gap values (1.87–1.72 eV) observed with rise in Zn2+ content. TG–DTA analysis confirms the weight loss and exothermic transitions. Scanning and transmission electron microscopy were used to study the morphology and the images depicted with intragranular pores, fused grains with different grain boundaries and homogeneous distributions. The transition from paramagnetic to super-paramagnetic behavior was most likely caused by the exchange of Zn and Co ions, as well as the phase composition of ZnO (hexagonal phase) and Co3O4 (cubic phase). The as-fabricated Zn2+ doped Co3O4 nanoparticles were evaluated for the catalytic activity tests carried out in a batch reactor operating under atmospheric conditions. The high doping concentration (about, x = 0.5) sample exhibited excellent catalytic activity and it exhibited better conversion efficiency and selectivity of 97.3% and 95.3%, respectively.