Single-Layered Biosynthesized Copper Oxide (CuO) Nanocoatings as Solar-Selective Absorber

Abstract

Herein, spectrally selective single-layered CuO nanocoatings were successfully demonstrated via green synthesis and deposited on stainless steel (SS) substrates using a spin coater at 700, 800, 900, and 1000 rpm. The morphological, structural, and compositional analyses of the obtained nanocoatings were studied using SEM, XRD, EDX, and Raman spectroscopy. The SEM images show nanorod-like structure surfaces with dense surface morphology. The XRD patterns confirmed the presence of peaks indexed to a monoclinic structural phase of CuO. The EDX spectra clearly revealed the presence of Cu and O elements, and XPS spectra showed peaks of Cu2p and O1s core levels, which are typical characteristics of Cu (II) and O(II), respectively, in CuO. The Raman spectra showed peaks at 305, 344, and 642 cm⁻¹ attributed to Raman active (A_g+2B_g) modes for Cu-O stretching. Rutherford backscattering spectrometry (RBS) determined the content of the elements and the changes in the thicknesses of the coatings with the rotational speed (RS) of the spin coater. The elemental content of Cu and O atoms were, respectively, 54 and 46%. The thicknesses were calculated to be 1.406 × 10¹⁸ atoms/cm² (296.3 nm), 1.286 × 10¹⁸ atoms/cm² (271.0 nm), 1.138 × 10¹⁸ atoms/cm² (239.8 nm), and 0.985 × 10¹⁵ atoms/cm² (207.5 nm) at 700, 800, 900 and 1000 rpm, respectively. The optical properties of the CuO nanocoatings were characterized using UV–Vis–NIR and FTIR spectrometers; its vital solar selectivity parameters of solar absorptance (α) and emissivity (ε) were evaluated in the ranges of 0.3–2.5 and 2.5–20 µm wavelengths, respectively. The obtained coatings exhibited solar parameters (α = 0.90, and ε = 0.31) associated with 700 rpm due to an intrinsic and interference-induced absorption as well as higher attenuation of light.

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 (α ≥ 0.90) at solar spectrum wavelengths (300 ≤ λ ≤ 2500 nm), retain low emissivity (ε ≤ 0.10) at infrared (IR) wavelengths (λ ≥ 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 Fe₂O₃ [15], NiO [16], Cr₂O₃ [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. G. Toquer et al. [23] reported that a CuO nanofilm was deposited onto a silicon wafer via electrophoretic deposition (EPD), and achieved a maximum solar absorptance (α) of 0.97 and thermal emissivity (ε) of 0.14. S. Kim et al. [24] fabricated CuO nanostructured as SSA on a Cu substrate by dipping it into an alkali solution of NaOH, NaClO₂, and water at 95 °C, varying the reaction time, and reported an optimum solar absorbance (α) of 95% and a low thermal emissivity (ε) of 11%. Barshilia et al. [25] fabricated CuO thin films on silicon (Si) substrates using reactive sputtering at high temperatures and reported high absorptance (α = 0.95) and emissivity (ε = 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.

2. Experimental Section

2.1. Materials and Chemicals

Methanol (CH₃OH, 99.8%), isopropanol (H₃CCHOHCH₃, 96%), trichloroethylene (C₂H₂Cl₂, 99.5%), copper nitrate trihydrate (Cu(NO₃)₂.3H₂O, 99.99%), stainless steel (SS) (1.5 × 1.5 cm × 1 mm) substrates, electronic balance, ultrasonic bath, spin coater, vacuum oven, and an air furnace were used from iThemba Labs, MRD, Cape Town, South Africa.

2.2. Plant Extraction

The cactus pear was freshly collected from eastern Tigray, Ethiopia. Initially, the cactus pear was washed several times using tap water and then deionized water to remove dust particles and contaminants present on the surface of the plant material. The cactus pear was cut into small pieces followed by drying at 60 °C in an oven for 12 h and then ground using an electric blender. Typically, the required amount of cactus pear powder (10 g) was added to 200 mL of deionized water and heated in the mixture solution at 80 °C for an hour under constant magnetic stirring. The solution cooled down to room temperature, filtered using Whatman (No. 1) filter paper, and then kept in a refrigerator at 5 °C for further use.

2.3. Green Synthesis of Copper Oxide (CuO) Nanoparticles

The cactus pear extracts were used as a chelating and stabilizing agent for the green synthesis of CuO nanoparticles. In a typical procedure, 100 mL of cactus extract was mixed with 200 mL of 5 mM Cu(NO₃)₂·3H₂O aqueous solution and was allowed the reaction mixture heated at 100 °C for 7–8 h under vigorous stirring until a brownish black color precipitate was formed. The obtained product was centrifuged at 4000 rpm for 30 min three times over the successive addition of deionized water and thoroughly washed the obtained precipitates using deionized water to remove remained impurities and then dried at 100 °C for an hour. Finally, the dried sample was calcinated at 420 °C in a furnace for 2 h and stored at room temperature in a tight container for further use.

2.4. Thin Film Deposition

The green synthesized CuO NPs were deposited on the SS substrate using a spin coater. Before the deposition, the SS substrates were cleaned by sonicating while dipped in acetone, isopropanol, trichloroethylene, and distilled water for 15 min each and then dried under nitrogen flow. The suspension solution was prepared by adding 0.25 g of CuO in 1.5 mL of deionized water and stirring for 1 h. In the spin-coating procedure, a vacuumed spin coater (WS400-6NPP-Lite) was used to deposit the prepared CuO suspension solution on SS substrates, and then 50 μL of suspension was spin-coated at 700, 800, 900, and 1000 rpm for 15 s. Subsequently, the deposited CuO coatings were dried at 80 °C in a vacuum oven for 2 h. Figure 1 illustrates the extraction, biosynthesis, and deposition of CuO coating for harvesting sunlight.

2.5. Characterizations

The crystal structure, surface morphology, roughness, and elemental composition of CuO nanocoatings were characterized using X-ray diffraction (XRD, Bruker with 1.5406 $\dot{\mathrm{A}}$ wavelength radiations), Scanning electron microscopy (SEM, Leo-Steros Scan 440), Atomic force microscope (AFM, Veeco Nasoscope IV Multi-Mode), and Energy dispersive spectroscopy (EDS), respectively. The Raman spectroscopy (Jobin T64000) was applied to analyze the existing phases and lattice vibrations within the coatings. Rutherford backscattering spectrometry (RBS) characterization was carried out to determine the thickness and elemental compositions of the coatings using 2 MeV He⁺ ion with the detector at a backscattering angle of 165°. The Spectral reflectance properties of the coatings was analyzed from diffuse reflectance spectra measurements in 0.3–2.5 μm, and 2.5–25 μm the wavelength ranges using UV−Vis−NIR (Varian, DRA-2500), and FTIR (Buker Tensor 27) spectrophotometers, respectively. The solar absorptance (α) is defined by the weighted integration of spectral reflectance with AM 1.5 in the solar spectrum of 0.3–2.5 µm wavelength range, while the emissivity (ε) is evaluated by the weighted integration of the reflectance with the perfect Planck’s black body radiations. Both the absorptance and emittance (α/ε) of the coatings were calculated from the diffuse reflectance spectra using Equations (1) and (2), respectively [29,30,31,32,33].

$$\alpha =\frac{{{\displaystyle \int }}_{0.3\mu m}^{2.5\mu m}\left(1-R\left(\lambda \right)\right)·I_{\mathrm{sol}}\left(\lambda \right)d\left(\lambda \right)}{{{\displaystyle \int }}_{0.3\mu m}^{2.5\mu m}{I}_{\mathrm{sol}}\left(\lambda \right)d\lambda }$$

(1)

where λ is the wavelength, I_sol is incident solar power, and R is the measured reflectance.

$$\epsilon =\frac{{{\displaystyle \int }}_{2.5\mu m}^{20\mu m}\left(1-R\left(\lambda \right)\right)·I_b\left(\lambda ,T\right)d\left(\lambda \right)}{{{\displaystyle \int }}_{2.5\mu m}^{20\mu m}{I}_b\left(\lambda ,T\right)d\lambda }$$

(2)

where I_b (λ,Τ) is emitted radiative power by the blackbody at λ and T.

3. Result and Discussion

3.1. Possible Reaction Mechanism of Green Synthesis Cuo Nanoparticles

The phytochemicals present in plant materials are rich in secondary metabolites with different functional groups, such as -C-O-H, –C–O–, -C-N-C-, –C=C-, –C=O– etc., which have a significant role in the formation, nucleation, accumulation, and stabilization of the nanomaterials. Different plant parts, such as roots, stems, leaves, seeds, fruits, peel, or flowers, can be used in the biosynthesis of nanoparticles (NPs). The cactus pear, commonly known as Prickly Pear belongs to Angiosperm Cactaceae family. The cactus pear (Opuntia ficus indica) is the source of several phytochemicals like flavonoids, saponins, phenylpropanoids, alkaloids, Rutin, essential oils, and steroids. In biosynthesis, the plant extracts are mixed with metal precursor salts at ordinary temperature, which is easy, environmentally friendly, cost-effective, and non-toxic [17,27,34].

In this reaction mechanism, we hypothesized the organic molecules present in plant extracts with poly-hydroxyl functional groups as a phytochemical agent for CuO NPs formation as displayed in Figure 2. In this biosynthesis of CuO NPs, the phytochemicals in the cactus extracts activate the copper (Cu) metal in the copper nitrate-trihydrate (Cu(NO₃)₂·3H₂O) into Cu²⁺ ion in a mixture solution, leading to the formation of complex structures with phytochemicals of the plant extracts or [Phyto-Cu-Phyto]²⁺ complex structures. The formed complex structure is converted into Cu(OH)₂ and byproduct, and then to CuO after being dried in a vacuum oven temperature of 100 °C. Finally, the obtained amorphous CuO stabilized and capped into CuO nanopowders by annealing at 420 °C [17,27,34,35].

3.2. Surface Morphology

Surface morphology is one of the crucial factors that influence the performance of solar selective absorber (SSA) surface coatings [36]. The morphology of single-layered green synthesized CuO nanocoatings was characterized using SEM and AFM. Figure 3a–d shows SEM images of CuO nanocoatings on SS substrate deposited using a spin coater at 700, 800, 900, and 1000 rpm, respectively. From the SEM images, nanorod-like structures are observed. The surface structures are densely covered and compacted at lower RS of 700 and 800 rpm, as shown in Figure 3a,b. When the RS increase to 900, and 1000 rpm, a nanorod-like surfaces with lower density is observed, as shown in Figure 3c,d. The particle size of CuO nanocoatings are found to be 57.6, 51.2, 43.1, and 36.7 nm at 700, 800, 900, and 1000 rpm, respectively, these results confirm that the thickness of the coating decrease with an increase in RS of the spin coating [37,38].

Figure 4a–d depicts the 3D AFM images of green synthesized CuO nanocoatings deposited by spin-coating on SS substrate at RS of 700, 800, 900, and 1000 rpm, respectively. The AFM images confirm the presence of vertically aligned nanorod-like structures on the CuO nanocoating surfaces. From Figure 4a,b at RS of 700 and 800 rpm, growth of vertically aligned nanorod-like structures with relatively uniform distribution and better coverage of nanorod surfaces is observed, and these are characterized by higher surface roughness of 28.3 and 24.5 nm, respectively. From Figure 4c,d, a further increase in the RS to 900 and 1000 rpm complicates the uniform distribution of the particles, and nanorod-like surfaces are reduced as well as appeared smooth surfaces with a surface roughness of 21.2 and 17.6 nm, respectively. As the RS increases the surface homogeneity of vertically aligned nanorod-like surfaces and grain size are reduced, indicating that the crystallization quality and thickness of the coatings are decreased. Moreover, the surface roughness of CuO is gradually decreased with an increase in RS due to the lowering of the coating thickness and surface uniformity of the coatings [39,40], as shown in Figure 4a–d. It should be noted that surface roughness has a key role in the absorber surface, as the incident light can be trapped efficiently in the absorber surfaces. When the incident light encounters the absorber surface of the coatings, some of them are absorbed, and some are reflected. The suitable surface roughness creates optical traps between nanosized particles, which can increase the light scattering of short wavelengths and reflection of long wavelengths [40,41]. Moreover, the distribution of peaks in the surface structure traps more light in the inner structure of the coatings by multireflection; consequently, the intrinsic absorption of CuO nanocoatings can be enhanced, which contributes to better solar absorption properties [42,43].

3.3. Structural Properties

The structural analysis of CuO nanocoatings was evaluated from XRD spectra. Figure 5a–d shows the XRD patterns of CuO nanocoatings spin-coated on SS substrates at RS of 700, 800, 900, and 1000 rpm. The crystallographic orientations originated at 2θ values of 32.6°, 35.5°, 38.6°, 48.8°, 53.5°, and 58.2°, respectively, corresponding to (110), (11$\overline{1}$), (111), (20$\overline{2}$), (020), and (202) planes of the monoclinic phase structure of CuO, and it is well-matched with the previous literature (according to the JCPDS card No. 80-1917) [44,45]. For all the obtained nanocoatings, characteristic of diffraction patterns at 2θ values of 43.58° and 50.72° attributed to (111), and (200) planes of face centered cubic (fcc) of Chromium Iron Nickel (Cr0.19 Fe0.7Ni0.11) (SS) substrate are observed (according to the JCPDS card No. 033–0397). The average crystalline size (D) of the coatings was calculated from Scherrer’s formula, D = 0.9 λ/βcosθ, where λ is the wavelength, β is the full width at half maximum (FWHM) of the peak at the diffracting angle θ by determining the width of (111) Bragg reflection [46] and found, respectively to be 76, 57, 44, and 36 nm at RS of 700, 800, 900, and 1000 rpm. The results are in agreement with the trend observed by SEM. As shown in Figure 5a–d, increasing RS results decrease in the intensity of the coatings, which confirms that the thickness, particle size, and crystallite size of the deposited coatings are decreased [39,47].

The elemental composition of the green synthesized CuO nanocoatings deposited on SS substrates was analyzed by using the EDX technique. EDX of typical CuO nanocoatings with nanorod-like structures (as observed in SEM) are shown in Figure 6a–d. The EDX spectra reveal the presence of strong peaks attributing to Cu and O in the spectrum confirming the purity of CuO nanocoatings. The EDX spectra also show weak peaks of Fe and Cr originating from the SS substrate.

The surface electronic states and elemental composition of CuO nanocoating on SS substrate were analyzed via X-ray photometry spectroscopy (XPS) and the results displayed in Figure 7 are associated with 700 rpm. The wider scan spectra of CuO nanocoating confirm the presence of Cu, O, C, Ca, N, and Na elements. The high resolution of XPS spectra of Cu2p shows two peaks of Cu 2p_1/2 and Cu 2p_3/2 at 953.15 and 933.72 eV binding energy, respectively, atypical characteristic peaks of Cu (II) for CuO. The binding energy of Cu2p_1/2 and Cu2p_3/2 are separated by 20 eV, essentially identical binding energies of Cu2p of Cu(II). The satellite peak at 944.13 eV confirms the presence of paramagnetic chemical states of Cu²⁺. From XPS spectra a peak with low intensity centered at 932.62 eV binding energy is associated to Cu¹⁺. As demonstrated in Figure 7, the corresponding XPS spectra O in CuO reveals two-peaks, indicating that there are two-oxygen states. The peak at 529.30 eV binding energy corresponding to the metal-oxygen (M-O) lattice confirms the formation of CuO [48,49,50]. Among them, the C element might derive from impurities on the surface of samples.

3.4. Rutherford Backscattering Spectrometry

Rutherford backscattering spectrometry (RBS) was employed to study elemental compositions, concentrations, and thickness of green synthesized CuO nanocoatings. RBS is the most precise quantitative and versatile technique of ion beam analysis for elemental surface analysis and depth profiling of thin films without the need for standards [51,52,53]. Figure 8a–d shows the RBS spectra of CuO nanocoatings deposited on SS substrate at RS 700, 800, 900, and 1000 rpm, respectively, using a beam of 3.6 MeV alpha particles. The fitting of RBS spectra was simulated using SIMNRA 7.03 software to analyze the elemental composition and thickness of the coatings. For all coatings, two peaks of alpha particles backscattered from Cu and O atoms at channels 2424 and 1112, respectively, are observed as shown in Figure 8a–d. The elemental contents of Cu and O atoms are 54 and 46%, respectively; the results did not show remarkable changes in the content of the elements as the RS increased from 700 to 1000 rpm. The thickness of coatings was obtained in RBS unit (atoms/cm²) and calculated to be 1.406 × 10¹⁸ atoms/cm² (296.3 nm), 1.286 × 10¹⁸ atoms/cm² (271.0 nm), 1.138 × 10¹⁸ atoms/cm² (239.8 nm), and 0.985 × 10¹⁸ atoms/cm² (207.5 nm), respectively, at 700, 800, 900, and 1000 rpm.

3.5. Raman Spectroscopy

Raman spectroscopy is a versatile and non-destructive technique used to probe the phase and vibrational properties of materials. CuO belongs to C⁶_2h symmetry and has twelve optical phonon modes (i.e 4A_u + 5B_u +A_g + 2B_g) in the Raman spectrum; Of these only three (A_g + 2B_g) are Raman active modes [20,54]. Figure 9a–d shows the Raman spectra of green synthesized CuO nanocoatings recorded over 200–700 cm⁻¹ at room temperature using 532 nm radiation. The Raman spectrum exhibited the characteristic peaks of CuO at 305.5, 344.3, and 642.7 cm⁻¹. The first peak at 305.5 cm⁻¹ is related to the A_g mode, and the two peaks at 344,3, and 642.7 cm⁻¹ are attributed to the B_g mode [54,55]. As shown in Figure 9a–d, at lower RS of the spin coater, the peak intensity is more intense and sharper which confirms that the thickness, particle size, and crystallinity are improved [56], complementing the obtained XRD results. Xu et al. [56] reported that the Raman spectra peaks become stronger as well as sharper and shift to higher waves as the grain size increases. Moreover, the Raman spectrum shows a weak peak at 596.5 cm⁻¹, which indicates the presence of the Cu₂O phase [56]. From the Raman analysis, both CuO and Cu₂O phases coexist in the coatings and also, the XPS spectra indicats the presence of the Cu⁺ chemical state. However, the XRD spectra do not reveal the presence of the Cu₂O phase, and this confirms that Raman spectroscopy and XPS are more surface-sensitive than XRD, and provides more surface information [54].

3.6. 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 (α, ε) 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 (λ ≥ 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 (α) of 0.90 and emissivity (ε) 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 (α) 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 (α, ε) are smoothly decreased to 0.82 and 0.19, respectively, as revealed in Figure 10a,b.

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 (ε) 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 (λ) 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 (λ) 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 (ε) could be achieved [36,43,59].

4. Conclusions

Herein, spectrally selective CuO nanocoatings via a novel green synthesis method deposited by spin coating on SS substrate at different rotational speeds (RS) is presented. The influences of rotational speed (RS) of the spin coater on characteristic parameters such as surface morphology, the structural and optical properties of the coatings were studied. The nanorod-like surfaces were observed from SEM, and the XRD patterns confirmed the existence of the well-crystalline CuO single phase with a monoclinic structure. The particle size, surface roughness, and crystalline size of the coatings decrease with increasing RS of the spin coater; these confirmed that lowering of the thickness of the coating with increasing RS. RBS reveals the change in thickness of the coatings with RS of the spin coater. Raman spectra attributed to Raman active (A_g+2B_g) modes for the characteristic of CuO, and also revealed the presence of the Cu₂O phase. The optical properties of the coatings were performed using UV-Vis-NIR and FTIR spectrometers in the wavelength range of 0.3–2.5μm and 2.5–20μm, respectively as sola selective absorber surfaces. The coatings exhibited a higher solar absorptance (α = 0.90) and emissivity (ε = 0.31), and both (α,ε) are decreased, respectively from 0.90 to 0.85 and 0.31 to 0.19 as RS is increased from 700 to 1000 rpm. The thickness of the coatings is reduced with an increase RS; this strengthened the reflection spectra in the solar region and higher attenuation of light and higher intrinsic reflectance property of the coatings in the IR region.