Investigation of the Thermal Stability of a Solar Absorber Processed through a Hydrothermal Technique

: In this work, we study the thermal stability of a hydrothermally treated stainless steel (SS) selective solar absorber by annealing in air in a temperature range between 300 ◦ C and 700 ◦ C for a soaking time of 2 h. Thermal stability testing in the presence of air is critical if the vacuum is breached. Therefore, the SS was characterized by X-ray diffraction (XRD), mechanical, and optical techniques. The XRD analysis shows that the grain size of the as-treated absorber is 67 nm, whereas those of the annealed absorbers were found to be in the range between 66 and 38 nm. The phase of the as-treated and annealed SS was further identiﬁed by XRD as Fe 2 O 3 . The EDS result shows that the elemental components of the SS were C, Cr, Fe, and O. The strain ( ε ) and stress ( σ ) calculated for the as-treated absorber are 1.2 × 10 − 1 and − 2.9 GPa, whereas the annealed absorbers are found in the range of 4.4 × 10 − 1 to 5.2 × 10 − 1 and − 121.6 to − 103.2 GPa, respectively, at 300–700 ◦ C. The as-treated SS absorbers exhibit a good spectra selectivity of 0.938/0.431 = 2.176, which compares with 0.941/0.403 = 2.335 after being annealed at 300 ◦ C and 0.884/0.179 = 4.939 after being annealed at 700 ◦ C. These results indicate a small improvement in absorptivity (0.941) and emissivity (0.403) after annealing at 300 ◦ C, followed by a signiﬁcant decrease after annealing at 700 ◦ C. The obtained analysis conﬁrms that the annealed SS absorber exhibits excellent selectivity and is suitable to withstand any thermal condition ( ≤ 700 ◦ C) in air. Thus, using a cost-effective approach as demonstrated in this study, the as-treated and annealed SS absorber could be used for photo-thermal conversion applications.


Introduction
As a result of the global awareness of greenhouse gas emissions, air pollution, and energy security issues, many governments and researchers around the world have been motivated to search for alternative energies that must be environmentally friendly, clean, affordable, and sustainable [1]. Seeking an ecofriendly, cost-effective, and feasible alternative to overcome the energy crisis is one of the most significant challenges humanity Solar 2022, 2 416 methods suffer from a lack of adhesion between the film and the substrate, and they exhibit high emission, which has a significant negative effect on the absorber's performance [20,21].
Therefore, the hydrothermal method may be a better alternative because it has several advantages that overcome some of the above-mentioned challenges. These include low process cost, simplicity, pollution-free operation, and ease of application on a large scale [3]. As a result, the method was used to treat the surface of the stainless steel (SS) in this study in order to enhance its solar absorption property and to investigate the effect of annealing temperatures on the structural and optical properties of the treated surface. Indeed, due to its superior qualities compared to other metallic systems, SS is widely used in a wide variety of applications [22]. Stainless steel (SS) is often used in the fabrication of solar absorbers and can be rarely corroded under normal temperature and alkaline conditions. It has been reported that the corrosion process of the SS can be accelerated by hydrothermal conditions, leading to a special nano/microstructure oxide surface. The hydrothermal condition is very sensitive to film preparation parameters, such as time and temperature, which are very useful for obtaining the desired morphology and exploring an improved new optical phase of the material. For this reason, the hydrothermal heat treatment has been demonstrated to be a feasible technique to prepare solar selective absorbers for a photo-thermal conversion application [3].
Thermal stability of selective solar absorber coatings is critical, as the absorber degrades over time at operating temperatures when exposed to vacuum or air, reducing the life of the absorber and eventually resulting in failure [23][24][25]. Thermal stability testing in the presence of air is critical if the vacuum is breached [26]. Herein, we report the thermal stability of stainless steel (SS) solar absorbers that are hydrothermally treated and annealed in the temperature range between 300 and 700 • C.

Materials
Six sheets of stainless steel (434-L. SAE Grade) with square shapes of size 2.00 cm × 2.00 cm, NaOH in pellet form (500 g, Semiconductor Grade, 99.99% Trace metals basis) and solvents such as ethanol (200 Proof, Anhydrous, ≥99.5%) and acetone (Laboratory Reagent, ≥99.5%) were obtained from Sigma Aldrich. Teflon-line autoclaves (with 4 cm of diameter and 6 cm of height) used for hydrothermal treatment were obtained from KIMIX Chemical & Lab Supplies (Ruco bank unit 13, Boston circle airport Industrial north, Western Cape, 7525 South Africa). Chemical products such as ethanol, acetone, and NaOH in pellet form were of analytical grade and hence used without any further purification.

Methods
The hydrothermal technique was used to treat six sets of stainless steel (SS). The treatment process of the SS is detailed elsewhere [3]. Briefly, the SS was cleaned in ethanol, followed by acetone, and finally rinsed using deionized water. The sample in the autoclave in the alkaline solution was heated in a laboratory oven for 1 h at 200 • C to modify its surface layer, thus forming a film of different structure and composition.
To determine the thermal stability of the treated SS solar absorber, five specimens with the obtained films were isothermally heated between 300 • C and 700 • C in air in a furnace (Elite Thermal Systems Limited model TSH12/50/610-2416CG) for 2 h at each selected temperature: 300, 400, 500, 600 and 700 • C. The temperature was raised from room temperature to the required temperature at a rate of 9 • C/min and then cooled at a rate of 10 • C/min after attaining the required duration. The used furnace has a proportional-integral-derivative (PID) temperature control system to improve the accuracy of the annealing process; we estimated an average of 0.1 • C in precision after performing a number of experiments using the same furnace. One film was not annealed in order to serve as the reference of the sample.

Structural and Optical Characterization
The structural composition of samples was investigated by using X-ray diffraction (Bruker AXS D8 X-ray diffractometer, Cu-Kα radiation of average wavelength~1.54 Å, operating at 40 kV and 35 mA in Bragg-Brentano geometry, Billerica, MA, USA). XRD measurements provide significant information on the width and grain size of crystallites, and phase's identification [27]. The morphology and elemental composition of all samples were studied by High Resolution-Scanning Electron Microscopy (HR-SEM: Hitachi X-650 electron microscopy unit with a resolution limit of 0.12 nm, coupled with the Energy Dispersive X-rays Spectroscopy (EDS), Chiyoda City, Japan). The optical reflectance of the samples was evaluated in the wavelength range of 0.25-2.50 µm by Cary series 5000 UV-Vis-NIR double beam spectrophotometer, while the reflectance in the thermal wavelength zone of 3.00-20.00 µm was characterized by Thermo-Nicolet 8700 Fourier transform infrared (FT-IR) spectroscopy.
Solar absorptance (α) values for the samples were calculated using [8], where R, λ, and I sol denote reflectance, wavelength, and direct normal solar irradiance, respectively, as defined by ISO standard 9845-1 (1992), with an air mass (AM) of 1.5. For opaque materials, the absorptance (α) is expressed in terms of the total reflectance (R λ ).
The thermal emittance (ε) was obtained using [28], where B (λ, T) denote radiance of a blackbody at temperature T.

X-ray Diffraction (XRD)
The X-ray diffraction profile of the as-obtained ( Figure 1a) and annealed films (Figure 1b-f) have been indexed to the Fe 2 O 3 phases (JCPDS card no. 00-039-1346) and (JCPDS card no. 00-033-0664). Figure 1 shows two different sets of peaks with different structures of the (a) as-obtained and (b-f) annealed film within the temperature range of 300-700 • C. The crystal structures of the (a) as-obtained and (b-f) film annealed at different temperatures were evaluated within the 2θ angular range of 15-90 • by X-ray Diffraction. The XRD analysis for the as-obtained and annealed films were tabulated in Tables A1-A6, respectively (See Appendix A).
The results of the analysis, such as inter-planar distance (d), crystallites grain size (φ), full width at half maximum (FWHM), lattice constant (a) and other XRD values for the samples, were evaluated and are summarized in Table 1. The results of the analysis, such as inter-planar distance (d), crystallites grain size (ϕ), full width at half maximum (FWHM), lattice constant (a) and other XRD values for the samples, were evaluated and are summarized in Table 1.
where λ~1.541 Å is the wavelength of Cu-K radiation, n = 1 and θ is the Bragg angle (half of the measured diffraction angle). As shown in Table A1 (See Appendix A) the average ratio of Dd/dbulk (where Dd = dexp − dbulk) is constantly positive, ranging from 0.002 to 0.007 for all hkl reticular plans, except for the film annealed at 400 °C, which shows a negative value (−0.006). This indicated that the peaks of the film annealed at 400 °C are subjected to small compressive strain at the various crystallographic directions, whereas the peaks  The inter-planer distances (d) were calculated for spectra using Bragg's law [29], where λ~1.541 Å is the wavelength of Cu-K radiation, n = 1 and θ is the Bragg angle (half of the measured diffraction angle). As shown in Table A1 (See Appendix A) the average ratio of Dd/d bulk (where Dd = dexp − d bulk ) is constantly positive, ranging from 0.002 to 0.007 for all hkl reticular plans, except for the film annealed at 400 • C, which shows a negative value (−0.006). This indicated that the peaks of the film annealed at 400 • C are subjected to small compressive strain at the various crystallographic directions, whereas the peaks of the film prepared and annealed at 300, 500, 600 and 700 • C are under tensile strain conditions. The average grain size of the crystallite (Ø) was evaluated using Debye-Scherrer, expressed as [30], where λ, Dθ 1/2 and θ B are the X-ray wavelength (~1.541 Å), Full Width at Half Maximum (FWHM) in radian, and Bragg's diffraction angle, respectively. The FWHM of the treated (as-obtained) film is 0.024 (rad), whereas the film annealed at different temperatures records FWHM in the range of 0.025-0.043 (rad). The average grain size of the crystallite (Ø) for the as-obtained film is 67 nm, whereas the annealed film at various temperatures records Ø in the range of 66-38 nm, as shown in Table 2. Similarly, it is observed ( Table 2) that as the annealing temperature increases, the average grain sizes decreases, whereas the FWHM tends to increase. The increment in FWHM can be attributed to island coalescence [31], whereas the reduction in Ø can be attributed to reduction in surface roughness of the film [3].
where Ø is the average grain size of the crystallite. The δ was found to be 2.2 × 10 −4 nm −2 for the as-treated film, while 2.3 × 10 −4 , 2.7 × 10 −4 , 3.9 × 10 −4 , 5.4 × 10 −4 , and 6.9 × 10 −4 nm −2 were obtained for the films annealed at 300, 400, 500, and 700 • C, respectively. It is observed that the film annealed at 700 • C exhibited a smaller value of δ (6.9 × 10 −4 nm −2 ), which implies that the film had fewer lattice defects and good crystalline qualities [37]. The lattice constants (a) were calculated using [3], where d exp hkl is the experimental inter-planar spacing obtained from Bragg's law, and h, k, and l are the Miller indices denoting the plane. The bulk lattice constant (result obtained from database) of the as-obtained film is~8.35 Å (Fe 2 O 3 , JCPDS card no. 00-039-1346), whereas the corresponding experimental average lattice constant for this film (annealed film) was estimated to be~8.45 Å, which is a little bit higher than the bulk value. The obtained bulk lattice constant of the film annealed is~5.04 Å (Fe 2 O 3 , JCPDS card no. 00-033-0664), whereas the corresponding experimental lattice constant of this film annealed at various temperatures (Appendix A Table A1) was estimated to be in the range of~7.27 to 7.66 Å. The small differences between the experimental lattice constant (a exp ) and the corresponding bulk lattice constant (a bulk ) may be due to the effects of tensile strains between the substrate and film (Fe 2 O 3 ). The strains (ε) and stress (σ) are given as, where a exp and a bulk are the calculated experimental and bulk lattice constant of the asobtained and annealed films. The strains (ε) and stress (σ) along the a-axis in the as-treated film were found to be 1.2 × 10 −1 and −2.9 GPa, whereas the annealed film at 300, 400, 500, 600, 700 • C records the values of the strain (ε) and stress (σ) at 4.4 × 10 −1 , 5.1 × 10 −1 , 4.6 × 10 −1 , 5.2 × 10 −1 , 5.2 × 10 −1 and −103.2, −119.2, −107.8, −121.6, and −120.6 GPa, respectively, using Equations (8) and (9). The stress (σ) negative sign obtained for the films annealed shows that the film is in a state of compressive stress [29]. The presence of the compressive stress can be attributed to the lattice mismatch between the bulk materials (~5.04 Å) and films annealed (in the range of~7.27 to 7.66 Å). According to the reports, the lattice mismatch between the film and the substrate results in a variety of strains of varying degrees [38].
However, examining the ε and σ in the film provides significant information on the evolution of the defect, which is critical for better understanding and optimizing the electrical and optical properties of the film. It has been reported that the band structure of a material can change with the strain field, thereby changing its optical properties [39]. Therefore, the purpose of observing both ε and σ in this study is to determine the level of cracks and other defects in the film treated (as-obtained film) and annealed in the range of 300-700 • C. Fortunately, the obtained results of ε and σ in the films are not significant enough to affect the optical performance of the films (absorber surface). Figure 2 reports the surface morphology images of the (a) as-obtained and (b-f) films annealed at 300, 400, 500, 600 and 700 • C, respectively. SEM images confirm the presence of micropores distributed across the surfaces of all films. It has been reported that micropores on the surface of a material minimize surface reflection, which in turn influences photoabsorption by trapping the incident light and subjecting it to multiple reflections [3]. Solar 2022, 2, FOR PEER REVIEW 7 7.66 Å. The small differences between the experimental lattice constant (a ) and the corresponding bulk lattice constant (a ) may be due to the effects of tensile strains between the substrate and film (Fe2O3).

Scanning Electron Microscopy (SEM) Analysis
The strains (ε) and stress (σ) are given as, ε = a − a a (8) σ = −2.33 × 10 a − a a (9) where a and a are the calculated experimental and bulk lattice constant of the asobtained and annealed films.
However, examining the ε and σ in the film provides significant information on the evolution of the defect, which is critical for better understanding and optimizing the electrical and optical properties of the film. It has been reported that the band structure of a material can change with the strain field, thereby changing its optical properties [39]. Therefore, the purpose of observing both ε and σ in this study is to determine the level of cracks and other defects in the film treated (as-obtained film) and annealed in the range of 300-700 °C. Fortunately, the obtained results of ε and σ in the films are not significant enough to affect the optical performance of the films (absorber surface). Figure 2 reports the surface morphology images of the (a) as-obtained and (b-f) films annealed at 300, 400, 500, 600 and 700 °C, respectively. SEM images confirm the presence of micropores distributed across the surfaces of all films. It has been reported that micropores on the surface of a material minimize surface reflection, which in turn influences photoabsorption by trapping the incident light and subjecting it to multiple reflections [3]. After the digitization of various HR-SEM images, the diameter of the average size of the pores was found to be 0.47 ± 0.06 μm for the as-treated film, while 0.53 ± 0.07, 0.49 ± 0.11, 0.52 ± 0.10, 0.59 ± 0.11, and 0.69 ± 0.11 μm were obtained for the annealed films at 300, 400, 500, and 700 °C, respectively. Table 2 shows that as the annealing temperature increases, the size of the pores on the film increases. However, films annealed at 400 and 500 °C (sample c & d) are not in trend, and this could be the effect of different Teflon-line autoclaves used for film treatment via the hydrothermal technique.

Energy-Dispersive X-ray Spectroscopy (EDS) Analysis
The elemental composition of the (a) as-obtained and (b-f) films annealed at 300, 400, 500, 600, and 700 °C were determined using EDS. All samples contain similar elemental components (i.e., C, Cr, Fe, and O) but have varying atomic percentages. The variation in the atomic percentage (%) for each sample is summarized in Table 3, while Figure 3 illustrates the EDS analysis.  After the digitization of various HR-SEM images, the diameter of the average size of the pores was found to be 0.47 ± 0.06 µm for the as-treated film, while 0.53 ± 0.07, 0.49 ± 0.11, 0.52 ± 0.10, 0.59 ± 0.11, and 0.69 ± 0.11 µm were obtained for the annealed films at 300, 400, 500, and 700 • C, respectively. Table 2 shows that as the annealing temperature increases, the size of the pores on the film increases. However, films annealed at 400 and 500 • C (sample c & d) are not in trend, and this could be the effect of different Teflon-line autoclaves used for film treatment via the hydrothermal technique.

Energy-Dispersive X-ray Spectroscopy (EDS) Analysis
The elemental composition of the (a) as-obtained and (b-f) films annealed at 300, 400, 500, 600, and 700 • C were determined using EDS. All samples contain similar elemental components (i.e., C, Cr, Fe, and O) but have varying atomic percentages. The variation in the atomic percentage (%) for each sample is summarized in Table 3, while Figure 3 illustrates the EDS analysis.   It is observed that both as-obtained and annealed films contain Carbon (C), Chromium (Cr), Iron (Fe) and Oxygen (O), but the atomic percentage (%) of these elements in the films varies. It can be observed from Table 4 that the as-obtained film (a) contains the highest atomic % of O (54.60%), whereas the film annealed at 700 °C (f) exhibits the lowest atomic % of O (48.67%). This confirms that as the annealing temperature increases, the atomic % of O decreases. Table 3 revealed that the film annealed at 600 °C (e) contains the highest atomic % of Fe (32.41%), whereas the film annealed at 300 °C (b) has the lowest It is observed that both as-obtained and annealed films contain Carbon (C), Chromium (Cr), Iron (Fe) and Oxygen (O), but the atomic percentage (%) of these elements in the films varies. It can be observed from Table 4 that the as-obtained film (a) contains the highest atomic % of O (54.60%), whereas the film annealed at 700 • C (f) exhibits the lowest atomic % of O (48.67%). This confirms that as the annealing temperature increases, the atomic % of O decreases. Table 3 revealed that the film annealed at 600 • C (e) contains the highest Solar 2022, 2 423 atomic % of Fe (32.41%), whereas the film annealed at 300 • C (b) has the lowest atomic % of Fe (24.25%). It is also observed that the atomic % of Cr in the film decreases from 9.22 to 8.67% when the film is annealed from 200 to 300 • C and later increases to 9.53% at 400 • C. Table 4. Absorptance (α) and emittance (ε) of the (a) as-obtained and (b-f) film annealed for 2 h in air at different temperature.

Sample
T However, among the elemental components of the films, Cr and Fe, which are transitional metals, play a vital role in minimizing the thermal radiation from the material. Indeed, the presence of transitional metal in a material helps to enhance its optical infrared reflectance property, which is required to achieve high selectivity for photo-thermal conversion applications. However, the element O also plays a significant role in the absorption property of a metal. Metal atoms are composed of a metallic ion's d-shell, which is partially filled. When this metallic ion reacts with oxygen, the electrons become localized, forming metallic oxide, a new material with a high absorption capacity. Transition metals' high infrared reflectance is due to the free electrons contained within their atoms, whereas their absorption is influenced by the bonded electrons. Thus, the oxidized surface of the metal exhibits a high absorption characteristic, whereas the transitional base metal exhibits a high infrared property, which aids in minimizing thermal emission loss from the absorber surface; thus, the combination of the two phases results in a good selective absorber with the required optical properties for photo-thermal conversion applications [3].

UV-Vis-NIR Diffuse Reflectance Analysis
The optical reflectance of the (a) as-obtained and (b-f) films annealed for 2 h in air at different temperatures was investigated by analyzing UV-Vis-NIR diffuse reflectance data in the wavelength region of 0.25-2.50 µm, as illustrated in Figure 4. It is observed that the film annealed at 300 • C (Figure 4b) exhibits the lowest reflectance of~7%, whereas the film annealed at 700 • C exhibits the highest reflectance of about 20%. Figure 4 further shows that as the annealing temperature increases, the reflectance of the films also increases, which confirms the negative effect of the high annealing temperature on the reflectivity behavior of the film. It has been reported that the lower a surface's reflectivity is in the short wavelengths of the solar spectrum, the greater its absorptance value, whereas the opposite is true in the longer wavelengths of the thermal spectrum; i.e., the higher the reflectivity of a surface in the mid/long IR wavelength region, the lower the emissivity of a material. Indeed, the two major requirements for achieving high selectivity of a material for photo-thermal conversion application are the low reflectivity of the incident solar radiation and the high reflectivity of the thermal radiation [3].

FT-IR Diffuse Reflectance Analysis
The optical reflectance of the (a) as-obtained and (b-f) films annealed for 2 h in air at different temperatures was studied by analyzing the FT-IR diffuse reflectance data in the wavelength zone of 3.0-20.0 μm, as shown in Figure 5. It is observed that the film annealed at 700 °C (Figure 5f) exhibits the highest reflectance of ~90%, whereas the as-obtained film exhibits the lowest reflectance of about 60%. Figure 5 further shows that as the annealing temperature increases, the reflectance of the film also increases. The reflectivity reduction at IR wavelength zone causes high emissivity to the absorber material, which in turn reduces its selectivity. Hence, decrement in reflectivity in the IR region does not favor solar selective absorber material for photo-thermal conversion application.
The decrease in reflectance spectra (%R) at the IR wavelength zone of the film can be attributed to the increment in atomic % of oxygen (O), as revealed by EDS results. The atomic % of the oxygen of the film annealed at 700 °C is 48.67%, and the value keeps increasing up to 54.60% (film annealed at 0 °C) as the annealing temperatures decrease, as revealed in Table 3. Indeed, increased oxidation has been shown to result in a decrease in reflectivity in the mid/far-infrared wavelength region, causing high emissivity on a material [3]. The decrease in the reflectance spectra (%R) of the films in the UV-Vis-NIR wavelength zone can be attributed to surface oxidation induced by the NaOH used in the hydrothermal treatment, as confirmed by EDS analysis. Another possible explanation for the decrease in reflectance in this wavelength zone is the presence of micropores on the surface of the films, as revealed by SEM analysis [3].

FT-IR Diffuse Reflectance Analysis
The optical reflectance of the (a) as-obtained and (b-f) films annealed for 2 h in air at different temperatures was studied by analyzing the FT-IR diffuse reflectance data in the wavelength zone of 3.0-20.0 µm, as shown in Figure 5. It is observed that the film annealed at 700 • C (Figure 5f) exhibits the highest reflectance of~90%, whereas the as-obtained film exhibits the lowest reflectance of about 60%. Figure 5 further shows that as the annealing temperature increases, the reflectance of the film also increases. The reflectivity reduction at IR wavelength zone causes high emissivity to the absorber material, which in turn reduces its selectivity. Hence, decrement in reflectivity in the IR region does not favor solar selective absorber material for photo-thermal conversion application.
The decrease in reflectance spectra (%R) at the IR wavelength zone of the film can be attributed to the increment in atomic % of oxygen (O), as revealed by EDS results. The atomic % of the oxygen of the film annealed at 700 • C is 48.67%, and the value keeps increasing up to 54.60% (film annealed at 0 • C) as the annealing temperatures decrease, as revealed in Table 3. Indeed, increased oxidation has been shown to result in a decrease in reflectivity in the mid/far-infrared wavelength region, causing high emissivity on a material [3].

Absorptance (α) and Emittance (ε) Evaluation
Solar absorptance (α) was calculated for the films using Equation (1) and weighted by solar irradiance based on the standard air mass 1.5 solar spectrum in the wavelength range 0.25-2.50 μm, while thermal emittance (ε) was calculated using Equation (3) and measured Blackbody reflectance data [3]. The "α" and "ε" results of the films are shown in Table 4, which reveals that as the annealing temperature increases, the absorptance increases first from 0.938 (as-obtained SS) to 0.941 (annealed) at 300 °C and then continuously decreases as the temperature proceeds to 700 °C, where the film exhibits the lowest absorptance of 0.884. It is similarly observed that as the annealing temperature increases, the film emittance decreases in the range from 0.431 to 0.179, as shown in Table 4. The decrease in film absorptance is attributed to its low surface reflectivity in the UV-Vis-NIR spectrum, whereas the decrease in emittance is attributed to high surface reflectivity in the mid/far-IR wavelength zone, as illustrated in Figures 4 and 5. The error/standard deviation on these data is ±0.11. The increase in "α" is beneficial to the solar absorber's properties, whereas the increase in "ε" is detrimental to the solar absorber's surface. This is because a high emittance value results in a greater loss of energy absorbed by the surface [3].
However, Figure 6 has been plotted to show the variation of α, ε, and η of the films with an increment in annealing temperatures. Figure 6 reveals that both the absorptance (α) and emittance (ε) of the films tend to decrease as the annealing temperature increases, whereas the selectivity factor (η) enclosed in Figure 6 tends to increase. The selectivity factors (η) of the films were evaluated and found to vary from 2.176 to 4.983 ± 1.26, as illustrated in Figure 6. This increment in η implies an improvement in the selectivity of the film annealed at 700 °C.

Absorptance (α) and Emittance (ε) Evaluation
Solar absorptance (α) was calculated for the films using Equation (1) and weighted by solar irradiance based on the standard air mass 1.5 solar spectrum in the wavelength range 0.25-2.50 µm, while thermal emittance (ε) was calculated using Equation (3) and measured Blackbody reflectance data [3]. The "α" and "ε" results of the films are shown in Table 4, which reveals that as the annealing temperature increases, the absorptance increases first from 0.938 (as-obtained SS) to 0.941 (annealed) at 300 • C and then continuously decreases as the temperature proceeds to 700 • C, where the film exhibits the lowest absorptance of 0.884. It is similarly observed that as the annealing temperature increases, the film emittance decreases in the range from 0.431 to 0.179, as shown in Table 4. The decrease in film absorptance is attributed to its low surface reflectivity in the UV-Vis-NIR spectrum, whereas the decrease in emittance is attributed to high surface reflectivity in the mid/far-IR wavelength zone, as illustrated in Figures 4 and 5. The error/standard deviation on these data is ±0.11.
The increase in "α" is beneficial to the solar absorber's properties, whereas the increase in "ε" is detrimental to the solar absorber's surface. This is because a high emittance value results in a greater loss of energy absorbed by the surface [3].
However, Figure 6 has been plotted to show the variation of α, ε, and η of the films with an increment in annealing temperatures. Figure 6 reveals that both the absorptance (α) and emittance (ε) of the films tend to decrease as the annealing temperature increases, whereas the selectivity factor (η) enclosed in Figure 6 tends to increase. The selectivity factors (η) of the films were evaluated and found to vary from 2.176 to 4.983 ± 1.26, as illustrated in Figure 6. This increment in η implies an improvement in the selectivity of the film annealed at 700 • C.

Conclusions
We have treated the surface of stainless steel to enhance its solar absorption property using a hydrothermal technique. Following the thermal treatment at different annealing temperatures, the SS absorber was characterized. The SEM images reveal micropores with decreasing grain sizes and increasing full width at half maximum (FWHM) when annealing temperature is increased, as confirmed by XRD analysis. The EDS spectrum indicates the presence of the transitional elements Cr and Fe, which were evident in the optical response. The SS absorber exhibited a lower reflectance at the UV-VIS-NIR wavelength zone, and this can be attributed to the presence of micropores, which were evident in the SEM images. The optical reflectance of the annealed films at the far-IR wavelength zone were high (60 to 90%), resulting in the obtained minimum thermal emission of the annealed SS absorbers. The major components of the as-treated and annealed film that play a significant role in enhancing the selectivity of the SS absorber are Cr, Fe, and O, as evidenced by the EDS result. Cr and Fe are transitional metals that help to enhance the optical infrared reflectance property of a metal/film, whereas element O (deposited on SS absorber through NaOH used during the hydrothermal treatment) enhances the absorption property of the SS absorber. Thus, the oxidized surface (Fe2O3) of the SS exhibits a high absorption characteristic, whereas Fe and Cr exhibit a high infrared reflectance property, which aids in minimizing thermal emission loss from the absorber surface; thus, the existence of these elements (Fe and Cr) in film results in a good selective absorber with the required optical properties for photo-thermal conversion applications.
The as-obtained SS absorbers exhibit spectra selectivity of 2.176 (0.938/0.431), whereas SS annealed at 700 °C exhibits 4.939 (0.884/0.179). These results indicate a significant improvement in the selectivity (absorptivity/emissivity) of the SS annealed at 700 °C. Hence, the annealed SS absorber could be a promising candidate for a photo-thermal conversion application.

Conclusions
We have treated the surface of stainless steel to enhance its solar absorption property using a hydrothermal technique. Following the thermal treatment at different annealing temperatures, the SS absorber was characterized. The SEM images reveal micropores with decreasing grain sizes and increasing full width at half maximum (FWHM) when annealing temperature is increased, as confirmed by XRD analysis. The EDS spectrum indicates the presence of the transitional elements Cr and Fe, which were evident in the optical response. The SS absorber exhibited a lower reflectance at the UV-VIS-NIR wavelength zone, and this can be attributed to the presence of micropores, which were evident in the SEM images. The optical reflectance of the annealed films at the far-IR wavelength zone were high (60 to 90%), resulting in the obtained minimum thermal emission of the annealed SS absorbers. The major components of the as-treated and annealed film that play a significant role in enhancing the selectivity of the SS absorber are Cr, Fe, and O, as evidenced by the EDS result. Cr and Fe are transitional metals that help to enhance the optical infrared reflectance property of a metal/film, whereas element O (deposited on SS absorber through NaOH used during the hydrothermal treatment) enhances the absorption property of the SS absorber. Thus, the oxidized surface (Fe 2 O 3 ) of the SS exhibits a high absorption characteristic, whereas Fe and Cr exhibit a high infrared reflectance property, which aids in minimizing thermal emission loss from the absorber surface; thus, the existence of these elements (Fe and Cr) in film results in a good selective absorber with the required optical properties for photo-thermal conversion applications.
The as-obtained SS absorbers exhibit spectra selectivity of 2.176 (0.938/0.431), whereas SS annealed at 700 • C exhibits 4.939 (0.884/0.179). These results indicate a significant improvement in the selectivity (absorptivity/emissivity) of the SS annealed at 700 • C. Hence, the annealed SS absorber could be a promising candidate for a photo-thermal conversion application. Acknowledgments: This work was supported by the UNESCO-UNISA Africa Chair in Nanosciences & Nanotechnology, and the National Research Foundation (NRF) of South Africa. We gratefully acknowledge the help provided by the Department of Materials Division (MRD), iThemba-LABS, South Africa, for the provision of the research lab and equipment. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Table A1. XRD values and crystallites size of the as-obtained film (Figure 1a).  Table A2. XRD values and crystallites size of the annealed film at 300 • C (Figure 1b).  Table A3. XRD values and crystallites size of the annealed film at 400 • C (Figure 1c).  Table A4. XRD values and crystallites size of the annealed film at 500 • C (Figure 1d).  Table A5. XRD values and crystallites size of the annealed film at 600 • C (Figure 1e).