Surfactant-free synthesis of novel copper oxide (CuO) nanowire–cobalt oxide (Co3O4) nanoparticle heterostructures and their morphological control

Abstract

A simple and surfactant-free synthesis of novel heterostructures comprising of copper oxide (CuO) nanowires uniformly decorated with cobalt oxide (Co3O4) nanoparticles was demonstrated by combining thermal growth and wet-coating method. The heterostructures were synthesized by thermally decomposing cobalt salt (cobalt nitrate) into Co3O4 nanoparticles onto vapor–solid (VS)-grown CuO nanowires. X-ray diffraction (XRD) and high resolution transmission electron microscopy (TEM) confirmed the presence of CuO and Co3O4 phases as well as a narrow size distribution of Co3O4 nanoparticles (average diameter ~7.0 ± 1.5 nm) on CuO nanowires (average diameter of nanowire tips ~67.9 ± 18.6 nm). Unique interfacial lattice relationship was observed for (111) Co3O4 nanoparticles on (200) CuO nanowire surface resulting in hemispherical shape of the former. For the first time, further systematic studies were performed to understand the influence of various parameters (cobalt salt concentration and annealing temperature, atmosphere, and time) on the morphological evolution of Co3O4 nanoparticles on CuO nanowires. Interestingly, by varying these parameters, it was possible to grow Co3O4 in different shapes (spherical, triangular, rectangular, cubical, and hexagonal nanoparticles) and forms (shells and nanorods). It was observed that all these parameters play a critical role in influencing the surface migration, nucleation, and growth of Co3O4 nanoparticles on CuO nanowires and this assisted in understanding the involved growth mechanisms. Finally, UV–vis–NIR spectroscopy and band gap energies for these heterostructures were evaluated that showed higher photocatalytic degradation efficiency for Rhodamine B under low-power visible-light illumination.