Structural, optical and mechanical characterisations of nanostructured copper cobalt oxide coatings synthesised via Sol-gel method for solar selective absorber

The search for clean renewable energy sources to fulfil global energy needs, incorporating environmentally-friendly technologies, is currently unabated. Solar thermal collectors are technologies that harness unlimited solar radiation then convert it into usable heat for numerous industries or domestic needs. The solar selective absorber thin film coating is the key component in determining the efficiency of a solar thermal collector. Many challenges still exist in terms of the fabrication of high quality selective absorber material, in order to meet the criteria of better cost-effectiveness and environmentally-friendly characteristics especially in the context of flat-plate absorbers. In this study, novel copper cobalt oxide (CuxCoyOz) thin film coatings on highly reflective aluminium substrate were synthesised via a facile, environmentally friendly and cost-effective sol-gel dip-coating method. The structural, surface morphology and composition, optical properties, mechanical properties and thermal durability were characterised using a wide range of complementary techniques, namely, XRD, FESEM/SEM, EDX, AFM, XPS, synchrotron radiation XPS and NEXAFS, UV-Vis-NIR, FTIR, nanoindentation and FEM modelling, as well as an accelerated thermal durability test. The copper cobalt oxide thin films showed a nano-sized grain-like morphology forming a porous surface structure with distinctively high solar absorptance compared to the manganese- and nickel- cobalt oxide coatings. XRD results demonstrated a relatively weak crystallinity of copper cobalt coating through the annealing temperature of 500 °C, the XPS and EDX analyses corroborated the existence of Cu-O and Co-O bonding structures within different copper cobalt oxide composition ratios. The optimised solar absorptance value of 83.4% was achieved from the copper cobalt oxide thin films synthesised using 0.25 M of copper acetate and 0.25 M cobalt chloride precursors ([Cu]/[Co]=1) with the withdrawal rate of 120 mm/min by four dip-drying cycles at annealing temperature of 500°C. Higher absorptance value could be accomplished by a thin film with [Cu]/[Co] of 0.5, however, its reflectance spectra curve was less satisfactory in terms of a good selectivity curve profile. The difference in [Cu]/[Co] ratios in the synthesis process has a direct influence on the degree of porosity of surface morphology which slightly alters the surface compositions, electronic structure and local coordination of the coatings. The mechanical properties analysis such as the elastic modulus and hardness via a nanoindentation test revealed that the coatings exhibit much higher wear resistance compared to the aluminium substrate especially for [Cu]/[Co] = 1.0. Finite element modelling (FEM) indicated that, under spherical loading conditions, the higher stress and the plastic deformation were predominantly concentrated within the coating layer, with marginal effect on the substrate. The high absorptance value (i.e. without an anti-reflective layer) accompanied by the high wear resistance of copper cobalt oxide made it a very promising candidate for solar selective absorbers application. Higher annealing temperatures treatment of up to 650 °C improved the crystalline structure of copper cobalt oxide, but it relatively did not change the surface compositions and bonding structures. The absorptance of coatings slightly increased with the annealing temperature up to 550 °C and then decreased from 550 °C to 650 °C due to the increase of scattering from larger crystallite. Even though the elastic modulus and the hardness improved, the wear resistance was slightly decreased as temperature was increased. To minimize the reflectance of absorber material and protect it from any degradation due to external factors, a silica anti-reflection (AR) layer was fabricated on top of the copper cobalt oxide coatings. The AR layer evidently changed the reflectance spectra which cause the increase of the absorptance value in the UV-Visible-near infrared (UV-Vis-NIR) area and unfortunately also increase the emittance value due to the strong phonon absorption by the silica in the range from mid to far infrared. The optimum absorptance and emittance values were 84.96 and 5.6 %, respectively. The accelerated thermal durability test revealed that the degradation of the copper cobalt oxide with a silica AR layer was more governed by the temperature regime fluctuations compared to the change in exposure time, indicating that the coating is applicable for uses in the low temperature range solar collectors such as for domestic solar water heater (≤ 150 oC). The sol-gel dip-coating synthesised copper cobalt oxide thin film coatings present high absorptance in UV-Vis-NIR range and low emittance (or high reflectance) in the mid – far infrared range with good mechanical properties. All these attributes render the coatings promising as a solar selective absorber for applications in the solar energy industry. However, further research may require the development of an appropriate anti-reflective layer to maximise absorptance and to minimise emission and then achieve a high selectivity of coatings stack.