## 3 Result and Discussion
### Optical Properties  
The spectral reflectance of green synthesized CuO nanocoatings was analyzed using UV-Vis-NIR spectrometer in the wavelength range of 300-2500 nm and combined with FTIR covering the 2500-20,000 nm wavelength range. The solar parameters performance of both the solar absorptivity and emissivity (\(\alpha\), \(\varepsilon\)) of the coatings were deduced from their diffuse reflectance spectra using Equations (1) and (2), respectively. Figure 10a,b depicts spectral reflectance characteristics of CuO nanocoating absorbers in the solar and IR regions as a function of RS. It exhibits relatively low reflectance spectra in the 300-2500 nm wavelength range, while higher reflectance spectra in the wavelength range of 2500-20,000 nm indicate good characteristics of spectrally selective coatings were obtained. The spectra reflectivity of CuO nanocoatings in the visible wavelength (300-800 nm) region is minimal (below 2%), while above the UV-Visible (or from 800-2500 nm), a gradual transition from low to high reflectance is observed. In the IR region (\(\lambda\geq 2500\) nm), the reflectivity spectra of the coatings are above 40%, and increase with the increasing RS of the spin coating [55], as revealed in Figure 10a,b. At 700 rpm a lower reflectance value is observed both in solar as well as in the IR spectrum and achieved the broader absorption or highest solar absorptance (\(\alpha\)) of 0.90 and emissivity (\(\varepsilon\)) of 0.31 as shown in Figure 10a,b. When the RS increases to 800 and 900 rpm, the reflectivity spectra of the coatings are raised, and the solar absorptance (\(\alpha\)) value is gradually decreased to 0.88 and 0.85 as the same time, the emissivity is decreased to 0.28 and 0.23, respectively. Further increasing in RS to 1000 rpm, a higher reflectance is observed; thus, both (\(\alpha\), \(\varepsilon\)) are smoothly decreased to 0.82 and 0.19, respectively, as revealed in Figure 10a,b.  
Figure 9: Raman spectra of CuO (**a**) 700 rpm, (**b**) 800 rpm, (**c**) 900 rpm, and (**d**) 1000 rpm.  
It is well known that at a lower RS of the spin coater, the coatings tended to be slightly thicker with rough surfaces [13] (also observed from AFM and RBS); this increases the solar absorption due to an intrinsic and interference-induced absorption of CuO in the solar spectrum. In the IR region, it is caused to have high infrared absorption as well as higher attenuation of light. Consequently, the surface reflection of the coatings is lowered, which results in the rise of the emissivity (\(\varepsilon\)) of the coatings [47, 57, 58]. Moreover, the appropriate surface roughness can determine the optical property of absorber surfaces. When the dimension of surface roughness is longer than the wavelength (\(\lambda\)) of the incident light, the electromagnetic wave (EW) gets more trapped energy which contributes to better solar absorptivity in the solar region. On the contrary, when the wavelength (\(\lambda\)) of the sunlight is longer than the surface roughness, the coating surface exhibits mirror-like properties, and most of IR light would be reflected in the IR wavelengths so a lower emissivity (\(\varepsilon\)) could be achieved [36, 43, 59].