Thickness dependent morphological, structural and optical properties of SS/CuO nanocoatings as selective solar absorber

In this paper, selective solar absorber (SSA) of copper oxide (CuO) nanocoatings on stainless steel (SS) substrate are conveniently prepared by sputtering and chemical oxidation route for solar thermal energy harvesting. The copper (Cu) thin films were deposited on SS substrate by using RF sputtering for different thicknesses (i.e 300, 400, 500 and 600 nm), and then oxidized to form CuO nanocoatings. The AFM, SEM, EDS, XRD, Raman spectroscopy, UV-Vis-NIR, and FTIR techniques were employed to characterize the surface morphology, microstructure, and optical properties of the CuO nanocoatings. The morphological, structural, and optical properties are greatly influenced by the thickness of the coatings. Both SEM and AFM shows the presence of nanoflake as well as nanosheet-like structures, respectively. The crystallinity, lattice vibration and purity of the CuO nano coatings are confirmed by XRD, Raman and EDS spectra, respectively. The optical properties the obtained nanocoatings are measured from spectral reflectance of UV-Vis-NIR as well as FTIR data's and the absorption edge slightly shifts towards higher wavelength with a coating thickness, this enables to increase the solar absorptance (alpha), but at the same time emittance (epsilon) also increased. High solar absorptance (alpha) of 97.4% and emissivity (epsilon) of 40.8% is associated to the thickest coating of 600 nm. The optical band gap of the coatings is determined from diffuse reflectance by using Kubelka-Munk (K-M) model and the values are ranging between 1.27 and 1.62 eV.