## 1 Introduction  
Due to the energy crisis and environmental concerns, studies on renewable energy sources for clean and sustainable growth are needed. Solar energy is abundant, widespread, and green and is a sustainable solution for a series of issues, such as the energy crisis and environmental pollution caused by the use of fossil fuels [1; 2; 3; 4; 5]. Solar-to-thermal energy conversion is a prospective solar energy-harvesting technology used for various important applications [6; 7], including water heating, sterilization, seawater desalination, distillation, and thermal power generation [7; 8; 9; 10; 11].  
In solar-to-thermal conversion, spectrally selective solar absorber (SSSA) surfaces are core devices that harvest and convert incident radiation into thermal energy. The SSSA surface exhibit high absorptances (\(\alpha\geq\) 0.90) at solar spectrum wavelengths (300 \(\leq\)\(\lambda\leq\) 2500 nm),retain low emissivity (\(\epsilon\leq 0.10\)) at infrared (IR) wavelengths (\(\lambda\geq 2500\) nm) to enhance the conversion efficiency, and are thermally stable at their operational temperatures [12; 13; 14]. So far, different spectrally selective nanostructured transition metal oxide (MO) coatings, such as Fe\({}_{2}\)O\({}_{3}\)[15], NiO [16], Cr\({}_{2}\)O\({}_{3}\)[17], MoO [18], etc., have been reported to improve the conversion efficiency of absorber surfaces. Among the transition MOs, black copper oxide (CuO) is a promising candidate for harvesting incident solar energy and then converting it into thermal energy; CuO is basically a p-type semiconductor and has a monoclinic structure with lower bandgap values of 1.2-1.9 eV, which allows it to have higher solar absorptivity at solar spectrum wavelengths and a good IR mirror to suppress the reemission loss in the long-wavelength IR region [19; 20; 21; 22]. Different spectrally selective CuO coatings have been reported using different chemical and physical methods. [PERSON] et al. [23] reported that a CuO nanofilm was deposited onto a silicon wafer via electrophoretic deposition (EPD), and achieved a maximum solar absorptance (\(\alpha\)) of 0.97 and thermal emissivity (\(\epsilon\)) of 0.14. [PERSON] et al. [24] fabricated CuO nanostructured as SSA on a Cu substrate by dipping it into an alkali solution of NaOH, NaClO\({}_{2}\), and water at 95 \({}^{\circ}\)C, varying the reaction time, and reported an optimum solar absorbance (\(\alpha\)) of 95% and a low thermal emissivity (\(\epsilon\)) of 11%. [PERSON] et al. [25] fabricated CuO thin films on silicon (Si) substrates using reactive sputtering at high temperatures and reported high absorptance (\(\alpha\) = 0.95) and emissivity (\(\epsilon\) = 0.52). However, these methods are associated with the use of high temperatures and toxic chemicals, which are not ecological or safe [26].  
Recently, green synthesis of nanoparticles (NPs) has shown more advantages over conventional physical and chemical procedures due to its inevitability to develop clean, eco-friendly, and effective synthesis techniques [17; 27; 28]. To our knowledge, plant-mediated synthesis of CuO as a spectrally selective absorber surface has not been reported yet. Herein, we report spectrally selective single-layered CuO nanocoatings via green synthesis (i.e., plant extract) and spin-coated on a stainless steel (SS) substrate with high optical properties for solar-to-thermal energy application. The morphological, structural, and optical properties of the nanocoatings were characterized by using optical and non-optical characterization techniques.