Fabrication and chararcterization of spectrally selective solar absorber copper oxide (CuO) nanocoatings for photothermal application

Abstract

Solar-to-thermal energy is considered to be the most direct way of converting solar radiation into usable forms of energy for a wide range of applications, including seawater desalination, heating water, photocatalysis, space heating and cooling, thermophotovoltaics etc. Spectrally solar selective absorber (SSSA) surfaces are the major components in photothermal energy conversions and ideally exhibits a high solar absorptance (α ≥ 0.90) in the wavelength rage (300 ≤ λ ≤ 2500nm) and low emissivity (ε ≤ 0.10) in the IR wavelength range (λ > 2500nm). Copper oxide (CuO, tenorite) is a transitional metal oxide from two elements copper (Cu, ([Ar]4s1 3d10)), and oxygen (O, [He]2s2 2p4 ). The Cu ions are coordinated by four oxygen ions in a monoclinic phase of CuO crystals. Basically, CuO is a p-type semiconductor due to Cu vacancies, and interstitial oxygen within the structure, and it has narrow band gap values of 1.2-1.9 eV that allow it to have a high solar absorptivity in the solar region. In this investigation, spectrally selective single-layered CuO and Ag@CuO nanocermet coatings deposited on stainless steel (SS) substrate are introduced. The SS has been widely used as a substrate for various range applications due to its thermal and chemical stability, environmental friendliness, and good optical properties. CuO and its plasmonic nanocermet coatings were successfully demonstrated using facile and reproducible green synthesis, electrodeposition, and sputtering methods aimed at high absorptance(α), and low emissivity (ε) values for solar-to-thermal conversion application. In green synthesis, spectrally selective single-layered CuO nanocoatings and Ag@CuO nanocermet coatings were synthesized from copper nitrate trihydrate (Cu(NO3)2.3H2O) and silver nitrate (AgNO3) salt precursors using plant extract (cactus pear) as stabilizing and reducing agent and then deposited on SS substrates using spin coater at 700, 800, 900, and 1000 rpm. In electrodeposition, the Cu thin films were reduced on the electrode or SS substrate surface from Cu(NO3)2.3H2O electrolyte at 15, 20 and 25 min deposition time at room temperature and then annealed in a furnace, results in the growth of nanostructured CuO. Conversely, the Cu films were deposited using RF sputtering on SS substrate at different thicknesses and then oxidized in alkaline solution at room temperature. The morphological, structural, compositional, chemical states and thickness of the coatings were analysed using scanning electron microscopy (SEM), Atomic microscopy (AFM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), X-ray photometer spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). The SEM images confirmed the growth of CuO nanorods, nanowalls, and nanoplates (NPs) from green synthesis, electrodeposition, and sputtering methods, respectively. The Ag@CuO nanocermet coatings also showed a better dispersibility of white plasmonic Ag NPs in the nanorods of CuO matrix. The XRD patterns revealed a well-crystalline nature of the monoclinic phase of CuO, and face centered cubic of Ag metal, the incorporated Ag NPs did not affect the monoclinic phase of CuO. The EDS clearly confirms compositional purity of the coatings. The grain size, surface roughness and crystalline size of the coatings depend on the thickness of the coatings, and were found to increase with coating thickness. The content of the elements in the coatings and the thicknesses of the coatings were determined by RBS. The thickness of the coatings is calculated to be 1416×1015 atoms/cm2 (298.2 nm), 1296×1015 atoms/cm2 (272.8 nm), 1153×1015 atoms/cm2 (242.7 nm) and 998×1015 atoms/cm2 (210.2 nm) at 700, 800, 900, and 1000 rpm, respectively. Raman spectra showed peaks attributed to Raman active (Ag+2Bg) modes which are characteristics of Cu-O stretching vibrations and XPS spectra revealed peaks of Cu2p, O1s, and Ag3d core levels; These peaks are typical characteristics of Cu (II), O(II) and Ag(I), respectively. The optical properties of CuO nanocoatings, and Ag@CuO nanocermet coatings was characterized as spectrally selective absorbers using UV-Vis-NIR, and IR spectrometers. The vital solar selectivity parameters of solar absorptivity (α) and emittance (ε) were evaluated, respectively from UV-Vis-NIR and IR spectral reflectance in a wavelength range of 300- 2500, and 2500-20000 nm. The optimized coatings exhibit a solar absorptance (α = 0.93, 0.92 and 0.97), and thermal emissivity of (ε = 0.23, 0.28, and 0.40) from green synthesis, electrodeposition, and sputtering methods, respectively. The incorporated Ag NPs improved the intrinsic absorption and reflectivity properties of green synthesized CuO nanocoatings from (α/ε = 0.90/0.31) to (α/ε = 0.93/0.23) at 700 rpm. This is due to the concentrated free electrons which contribute a plasma resonance frequency and its particle sizes are comparable to or smaller than the wavelength of incident light. The optical bandgap energy (Eg) of CuO coatings was estimated from reflectance spectra using Kubelka-Munk (K-M) function and found in the range of 1.65-1.27 eV. The lower band gap values are attributed to higher solar absorption above the band gap energy. Hence, the CuO nanocoatings and Ag@CuO nanocermet coatings are capable of a potential candidate(s) for SSSA surfaces in solar to thermal energy conversion systems.