Electrodeposition of nanostructured copper oxide (CuO) coatings as spectrally solar selective absorber: Structural, optical and electrical properties

Herein, a nanostructured CuO coatings as selective solar absorber (SSA) was fabricated through electrochemical deposition and thermal oxidation techniques. Initially, the copper (Cu) thin films were electrodeposited from copper nitrate trihydrate (Cu(NO3)2·3H2O) electrolyte at 15, 20, and 25 min on the electrode or stainless steel (SS) substrate at room temperature and then annealed in a furnace in the presence of oxygen (O2). The surface morphology, structural, and compositional analysis of the coatings were characterized using scanning electron microscopy (SEM), Atomic force microscopic (AFM), X-Ray Diffraction (XRD), and Energy-dispersive X-Ray Spectroscopy (EDX). The surface morphology confirms the presence of nanowall-like structures and exhibits grain growth as well as an increment in surface roughness with deposition time. The XRD patterns reveal well-crystalline monoclinic nature of CuO, and EDX spectra confirm high-purity phase composition of CuO coatings. Raman spectra show peaks at 305, 344, and 642 cm−1 attributed to Raman active (Ag + 2Bg) modes for characteristics of CuO lattice vibrations. A nanoprofilometer was used to measure the thickness of the coatings and found to be 42, 60, and 74 nm at 15, 20, and 25 min deposition time, respectively. The electrical properties of the prepared films were examined using Keithley 617 programmable electrometer at room temperature which showed the lowest resistivity (ρ) of 1.19x102 Ωcm at 25 min deposition time. The optical properties of the obtained CuO coatings were characterized using UV–Vis-NIR and IR spectrophotometers in the wavelength range of 0.3–2.5 μm and 2.5–20 μm, respectively. The CuO exhibits a higher solar absorptance (α) value of 0.92 and infrared emissivity (ε) of 0.28 at the deposition of 25 min. The optical band gap energy (Eg) of the obtained CuO coatings was estimated using Kubelka-Munk (K-M) model from the diffuse reflectance spectra and found in the range of 1.45–1.50 eV; the lower band gap values are attributed to higher solar absorption. Several parameters, such as crystallinity, grain growth, and stress of the coatings, may influence the optical band-gap energy (Eg).