Effects of Cu and Ni co-doping on magnetic, photocatalytic and dielectric properties of Co₃O₄ nanoparticles

Magnetic, photocatalytic and dielectric properties of Cu²⁺ and Ni²⁺ co-doped Co₃O₄ nanoparticles (NPs) have been studied in detail. XRD analysis confirmed the single pure phase formation of Co₃O₄ NPs and co-doped (CN26, CN44 and CN62) NPs samples. The average crystallite size increases from 16.5 to 30.9 nm with co-doping due to stress and strain induced by the co-dopants. FTIR analysis revealed the formation of desired molecular bands especially at 578 cm⁻¹ (v₁) and 661 cm⁻¹ (v₂) originating from stretching vibrations caused by Co-O bonds confirming the formation of spinel oxide structure. Tauc plots from UV–Vis spectroscopy showed increased bandgap from 3.68 to 3.82 eV with co-doping due to Burstein-Moss effect. Magnetic measurements revealed the presence of weak ferromagnetism (FM) in antiferromagnetic (AFM) Co₃O₄ NPs with co-doping due to the increased FM exchange interactions caused by the incorporation of Cu²⁺ and Ni²⁺ ions. A maximum magnetization and coercivity were observed in CN44 sample (with equal percentage of dopants) which is due to increased magnetic interactions caused by co-dopants. In ZFC/FC curves of undoped Co₃O₄ NPs, a broad AFM peak is observed at 42 K with overlapped ZFC/FC corresponds to the freezing temperature of AFM spins. In co-doped samples, the weak FM behavior is evident by the splitting of ZFC/FC curves at low temperatures. The increase in magnetization with co-doping can be attributed to both Cu/Ni doping and uncompensated surface spins. In photocatalysis, the degradation rate (for methyl orange dye) increases from 84.23 to 89.39 % with co-doping which is due to increase in bandgap and decrease in electron-hole recombination. A decrease in dielectric constant was observed by co-doping which is caused by the conducting nature of both the co-dopants. Dielectric losses were increased with co-doping because of increased charge population available for polarization.