Synthesis and Characterization of CuO Nanoparticles Using Heat Treatment Approach: Annealing Effects on Crystallite...

Copper oxide (CuO) was synthesized through a heat treatment approach from copper-(II) nitrate trihydrate {Cu-(NO₃)₂·3H₂O} as source material and polyvinylpyrrolidone (PVP) as capping agent. The obtained powder was annealed from 300 to 500 °C (in 50 °C steps) and further characterized at each of these temperatures with a battery of techniques to understand the influence of temperature on their final structures and electrical performance. XRD patterns proved that CuO nanoparticles have high purity and crystallinity (crystallite size ∼43 nm). FESEM images showed that nanoparticles are colloidal, irregular-shaped, and rod-like structures whose size is influenced by the thermal treatment. Their stoichiometry was confirmed by EDX spectrum, and HR-TEM corroborated CuO planes. Infrared and X-ray photoelectron spectroscopy studies correlate well with CuO nanoparticles being formed and free from PVP residues when annealing to T _a > 350 °C. UV-vis spectrophotometry revealed a decrease in the energy band gap up to T _a = 400 °C, after which it increases, with electrical conductivity showing this same trend. Resistivity shows a sharp increase (maximum value at T _a = 500 °C). The fluorescent intensity decreased with annealing temperature due to the recombination of electrons and holes of CuO-free excitons, demonstrated by photoluminescence. Small amounts of Cu-(I) present on the nanoparticle surface were investigated by CO adsorption (FTIR), revealing the largest amount observed when T _a = 400 °C. The wide bandgap CuO NPs obtained can handle high temperatures and maintain environmental stability, and their synthesis by heat treatment is proposed as a potential alternative way of preparation due to its simple nature.