Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel

Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel

Solar Energy Materials & Solar Cells 146 (2016) 99–106 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepa...

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Solar Energy Materials & Solar Cells 146 (2016) 99–106

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel Xingli Wang, Xiaofeng Wu, Long Yuan, Cuiping Zhou, Yanxiang Wang, Keke Huang, Shouhua Feng State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 2 July 2015 Received in revised form 23 October 2015 Accepted 29 November 2015

Currently, solar absorber plays a key role among all components of the concentrating solar power (CSP) system, which is considered as an important way for solar energy utilization. Due to the outstanding features of surface texture, nano-structured materials have been extensively utilized for solar energy harvesting and conversion, but most of them were synthesized by expensive or complicated techniques. Herein, for the first time, solar selective absorbers with uniquely foamed nanostructure, are grown in situ by facile hydrothermal method on stainless steel, without using any porogen or template. The selective absorber films, which comprised a large number of nanoparticle agglomerates and nanopores, have a vital role on the solar selective absorptance owing to the increased optical path as well as enhanced sunlight trapping via foamed nanostructure. They show an excellent solar thermal performance with the solar absorptance and the thermal emittance of 0.92 and 0.12, respectively. Besides, the solar absorber films also exhibit considerable solar thermal performance and benign thermal stability at high temperature. Due to the relieving internal stresses from foamed nanostructure, there is no obvious cracking within the entire structure even after heat treatment. Consequently, the hydrothermal method used in the present investigation happens to be a novel, pollution-free, low-cost, and suitable for simultaneous mass production for large-size absorber films. Moreover, the pre-treatment of substrate and posttreatment of films are also very convenient and environmental, without any complex procedure. Accordingly, the resultant films with foamed nanostructure will have a good prospect as a new-family of solar selective absorbers, which may have a tremendous potential for industrial production in the future. & 2015 Published by Elsevier B.V.

Keywords: Foamed nanostructure Solar selective absorber Hydrothermal method Stainless steel Self-assembled Industrial production

1. Introduction Solar absorber plays a key role among all components of the CSP system, it absorbs solar radiation and then converts into heat. As for high-performance solar absorber, the key requirement is its spectral selectivity, namely a strong absorptance in visible-nearinfrared region coupled with low emission in the infrared (IR) region [1,2]. Surface texturing is a common technique to obtain spectral selectivity by the optical trapping of solar energy [3]. In recent years, the application of nano-structured materials for solar energy harvesting and conversion has been gaining momentum [4–7]. Two and three dimensional (2D and 3D) periodic nano- or micro-structured surfaces were proposed to be alternatives for better spectral control [8–10]. The porous microstructures [11,12], tandem structures [13–16], needle-like structure [17] and branched nanowire forest [18] have been shown great superiority [19] owing to the increased optical path [5,11]. H. Sai et al. prepared 2D E-mail address: [email protected] (S. Feng). http://dx.doi.org/10.1016/j.solmat.2015.11.040 0927-0248/& 2015 Published by Elsevier B.V.

surface with submicron holes on tungsten substrates by fast atom beam etching [20]. Due to the standing wave resonance between the electromagnetic fields and the standing wave mode in the holes, this surface obtained the absorptivity of 0.82 and the emittance of 0.057 at 400 K. A. Lasagni et al. reported the surface structure on copper and stainless steel by laser interference metallurgy [21]. The resultant structure performed a desired spectral selectivity with the optimal absorptivity and emittance of 0.498 and 0.093 at 300 K, respectively. T. Kim et al. synthesized the tandem-structured layers of CuO nanowires and Co3O4 nanoparticles, which exhibited superior solar absorbing properties (figure of merit values is c.a. 0.90), the adhesion between the absorber and the substrate needed to be further improved yet [13]. So far, although lots of high-performance solar absorbers have been reported through plating, etching, magnetron sputtering and vapor deposition etc. However, many of them are often weakly adhesive, expensive, or multi-step, even need complicated equipment or high energy consumption in preparation. In previous reports [22–24], toxic chemical polishing agents such as acetone, isopropyl alcohol etc. were also always utilized in the pre-

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treatment of substrates. Thus, it is very important to introduce a convenient and environment-friendly strategy, that the pretreatment is facile and nontoxic and the solar selective absorbers are low-cost, firmly-adhesive and easy for large-scale preparation. Stainless steel is widely used in solar heat collector for solar power plant. It has high thermal conductivities, the excellent corrosion resistance to working fluids, and contains transition metal elements such as Fe, Cr, Ni [23,25,26], whose corresponding oxides are common used for solar energy absorption material. Besides, the smooth surface of stainless steel plate generally has low emissivity. However, the poor solar absorption performance makes it unsuitable to be used as solar selective absorbers directly. Hydrothermal method is an excellent strategy to prepare various materials with many novel structure and morphology due to its high temperature and high pressure [27]. Under normal temperature and alkaline condition, many kinds of stainless steel can be seldomly corroded. Hydrothermal condition may accelerate their corrosion processes [28], and obtain special nanostructure oxide surface. Herein, the Fe–Cr–Ni–Mo stainless steel was firstly utilized in NaOH solution by hydrothermal method to prepare the solar selective absorbers. These as-prepared foamed nanostructure absorbers, are grown in situ on stainless steel without the use of any porogen. They are constructed on the stainless steel surface firmly, and are composed of nanopores and nanoparticle aggregations. The as-prepared absorbers display a solar absorptance of 0.92 and a thermal emittance of 0.12 at 300 K. Thus, these films with self-assembled foamed nanostructure will be very promising for integration into the CSP devices.

2. Experimental details 2.1. Film preparation Herein, Fe–Cr–Ni–Mo stainless steels were purchased from baoshan iron& steel, NaOH (AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. Deionized water was used in all procedures of experiments. As for film preparation, the stainless steel sheets were wiped with alcohol swabs and were not subjected to either chemical or mechanical polishing at first. Then they were placed into Teflonlined autoclaves along with 15 M sodium hydroxide solution and the mixture was subjected to hydrothermal treatment at 200 °C for 24 h. Finally, the films with foamed nanostructure could be obtained after ultrasound treatment, washing and drying. 2.2. Characterization method X-ray diffraction (XRD) data were collected using a Rigaku D/ Max 2500 V/PC X-ray diffractometer with monochromated Cu Kα radiation (λ ¼0.15418 nm) to identify the crystal structures of the films at a scanning rate of 6 ° min  1 in the 2θ range of 20–70°. The EDX and morphologies of the films were examined using field emission scanning electron microscopy (SEM) on a Helios NanoLab 600I from FEI Company. The cross sections of the selective absorber films were obtained by focused ion beam (FIB), and the measured micro area was protected by deposited platinum before being etched by gallium ion. EDX results for each film was measured for 6 times, and the average intensity was counted. Reflectance in the wavelength interval 0.3–2.5 mm was measured by a Perkin-Elmer Lambda 950 UV/vis/NIR double beam spectrophotometer equipped with an integrating sphere. Over the infrared range 2.5–20 mm, a Fourier transform infrared reflectance (FTIR) spectrometer TENSOR 27 (Bruker Optics) was used, which was equipped with an integrating sphere coated with gold.

Fig. 1. XRD patterns of stainless steel substrate (black), as-prepared film (red) and the film heated in air for 24 h at 450 °C (blue). The as-prepared film is prepared by hydrothermal reaction for 24 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. XRD results The diffraction profile of the selective absorber films (Fig. 1) have been successfully indexed to the Fe3O4 phase (JCPDS: 653107) [29], similar to spinel structure. In XRD patterns, the peaks (at 2θ ¼43.48°, 50.54°) represent stainless steel. The characteristic peaks (2θ ¼30.18°, 35.58°, 53.64°, 57.0°, 62.70°), marked with their indices ((220), (311), (422), (511), (440)), are coincided with spinel structure as Fe3O4. Peaks of the as-prepared film have a small shift due to the doping of other elements (Cr, Ni) into the crystal lattice. This was in agreement with the energy dispersive spectra (EDS) and mapping analysis. We also note that the characteristic peaks of as-prepared film are broadened as a result of the nano-sized agglomerations on the film surface. After heating in air for 24 h at 450 °C, characteristic spinel peaks become narrower and sharper, owing to the sintering of the nano-agglomerates leading to larger particles and strong crystallization. 3.2. Morphology of the films Surface and cross-sectional SEM images of the as-prepared film and the heat-treated (24 h @ 450 °C) film are shown in Fig. 2. The surface of the as-prepared film appears a typical foamed nanostructure, with a large amount of nanopores distributed in the surface uniformly (Fig. 2a1), it also can be seen that the supports of the foamed nanostructure are assembled by nanoparticles with the size of 30–40 nm from the enlarged view (Fig. 2a2). The crosssectional image (Fig. 2a3) clearly shows a well-defined foamed nanostructure, the thickness of the layer is 2.32 um (The calculated thickness of the sample is based on the measured length under 52° tilted, the film surface is impregnated by some platinum as a protecting layer). The number and volume of the pores on film surface are reduced after 450 °C treatment for 24 h (Fig. 2b1), mainly due to the obvious aggregation of nanoparticle agglomerations as shown in Fig. 2b2. The roughness root mean square (RMS) of the untreated and 24 h @ 450 °C treated films are 4.077 nm and 9.317 nm, respectively. The cross section confirmed that the pores are reduced in the number and volume, and the foamed nanostructure is weakened (Fig. 2b3). There is no sharp interface between the in-situ grown foamed layer and the stainless steel substrate, implying a well bonding even after high

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Fig. 2. Surface and cross-sectional SEM images of the foamed nanostructure films: (a1–a3) the as-prepared film. (b1–b3) the heated film ( 24 h at 450 °C).

temperature treatment. In addition, the relationship between the thickness of the foamed layer and the reaction time were also studied. At the beginning, the layer thickness increases drastically. The thickness of the film is 284 nm and 1.58 μm for the reaction time of 1 and 3 h, respectively. With the extension of the reaction time to 10 h or more, the thickness increases slowly. This phenomenon can be easily explained by the reaction mechanism, with the reaction proceeding, the alkaline solution getting difficult to react with the substrate as the foamed layer becomes thicker. Fig. 3a–f. shows the morphology of the films with the treatment conditions of 2d @ 450 °C, 3 d @ 450 °C, 5d @ 450 °C, 7d @450 °C, 1d @ 250 °C and 1d @ 500 °C, respectively. It is found that, the diameter of the nanoparticle agglomeration is not significantly increased with the extension of heat treatment time (Fig. 3a–d). But some particle agglomerations gradually reunite, resulting in a decrease in the number of pores per unit area, simultaneously their surface roughness increased. The film also was heat treated for 1d at different temperatures (Fig. 3e–f). After treated at 250 °C, the surface particle agglomerations of film only have a little growth and the surface still has a lot of pores (Fig. 3e), compared with the as-prepared film. After 500 °C heat treatment, the particle agglomerations on the film surface gather obviously, their diameters become larger, leading to a drop in the number of pores per unit area and an increasing of the surface roughness (Fig. 3 f), which is unfavorable to the solar selective absorptance.

3.3. EDS results The elemental concentration and cross-sectional distribution for as-prepared and heat treated films (450 °C, 24 h) are analyzed by EDS (Fig. 4). In Fig. 4b, c, e, f, h, i, k and l, the intensity of brightness represent the relative element concentrations of tested micro-area for the as-prepared (Fig. 4a) and heat treated films (Fig. 4g), namely the brighter of the color, the higher of the relative element concentration or proportion. The element ratios of O, Fe, Cr and Ni for the as-prepared selective absorber film are 26.2%, 43.3%, 9.7% and 20.7% (Fig. 4d), respectively. On account that the electrode potential (relate to the activity of the metal reaction) of the Cr and Fe in stainless steel is lower than that of Ni, especially under high temperature and high pressure, therefore the former is more likely to be corroded, therefore the Cr and Fe concentrations at the surface are lower than that in the interior, while the distribution of Ni and O gradually changes in the opposite direction (Fig. 4b, c, e and f). Based on these results we propose that, from the surface to the interior of film, spinel ceramic phases show compositional inhomogeneity, namely the proportion of spinel ceramic phases at the surface is higher than that at the bottom, which leads to a gradual changing for refractive index and permittivity, as a function of depth from the surface to the base of the film. Those are in favor of solar absorptance [25,30]. While for heat-treated films of 450 °C, the compositional gradient

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Fig. 3. Surface SEM images of the films heated by different temperature and time in air: (a–d) 450 °C, 48–168 h. (e–f) 250–500 °C, 24 h.

disappears leading to a reduction in the solar energy absorptance efficiency. The element ratios of O, Fe, Cr and Ni for the heat treated film are 55.9%, 35.1%, 4.3% and 4.7% by EDS, respectively (Fig. 4j). Oxygen concentration is higher than that of as-prepared film and approaching to that proportion in spinel. Combining with the strong XRD peaks of spinel and uniform elements distributions within the entire heat-treated films (Fig. 4h, i, k and l), it is clear that the whole film has become a spinel ceramic phase. 3.4. XPS analysis The valence states of the transition metals in the as-prepared film surface are determined by XPS spectra (Fig. 5). After deconvolution, the Fe 2p3/2 peak has been resolved into two peaks corresponding to the B.E. positions of 710.7 eV and 713.0 eV, which are related to Fe2 þ 2p3/2 and Fe3 þ 2p3/2 (Fig. 5a), respectively [31,32]. The high-resolution Cr 2p spectrum can be divided into two peaks of Cr 2p3/2 (576.5 eV) and Cr 2p1/2 (586.5 eV) (Fig. 5b). The presence of Cr 2p3/2 leads evidence to the existence of Cr(III) [33]. The Ni 2p3/2 peak values are 855.5 and 861.4 eV (Fig. 5c), which may be attributed to NiO and Ni2O3 [34–36]. The presence of mixed valent transition metals, especially Fe(III), Fe(II), Cr(III) and Ni(II) and Ni(III), are responsible for the solar absorptance in the selective absorber films [5,25,37,38].

3.5. Solar absorptance and thermal emittance results The reflection spectra for the as-prepared and 24 h @ 450 °C treated films are shown in Fig. 6, which are measured at room temperature in UV–vis–NIR (300–2500 nm) and infrared (2.5– 20 mm), respectively. According to the formula (1) [39], solar absorptance can be calculated by reflectance spectroscopy (300– 2500 nm), referring to the ISO standard 9845-1 (1992) and air mass (AM) 1.5. According to the formula (2) [39,40], thermal emittance can be calculated by reflectance spectroscopy (2.5– 20 mm). The as-prepared films receive excellent absorptance and thermal emittance (calculated to be 0.922 and 0.120, respectively), and realize effective absorptance in the solar spectral region (300– 2500 nm) and effective inhibiting the black body emittance of 300 K (2.5–20 mm). After heating in air for 24 h at 450 °C, the absorptance and thermal emittance of the film turn out to be 0.834 and 0.230, separately (Fig. 6).

α¼

ε¼

R 2:5  0:3

    1  R λ P sun λ dλ   R 2:5 0:3 P sun λ dλ

    R 20  2:5 1  R λ P b λ dλ R 20   2:5 P b λ dλ

ð1Þ

ð2Þ

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Fig. 4. Elemental mappings and EDS of as-prepared film (a–f) and heat treated film (24 h at 450 °C) (g–l).

Fig. 5. XPS spectra of the foamed nanostructure film: (a) Fe 2p; (b) Cr 2p; (c) Ni 2p; (d) O 1s.

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Fig. 6. Reflectance spectra of the as-prepared film (black), heat treated film (24 h at 450 °C) (red), 300 K blackbody radiation under AM 1.5 solar illumination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 8. Reflectance spectra of films heated for different time at 450 °C, 48 h (black), 72 h (red), 120 h (blue) and 168 h (pink). 300 K blackbody radiation under AM 1.5 solar illumination. The inset is the enlarged reflectance spectra in the wavelength of 0.3–2.5 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Reflectance spectra of films heated at different temperature for 24 h. 250 °C (black), 300 °C (red), 350 °C (blue), 400 °C (green) and 500 °C (pink). 300 K blackbody radiation under AM 1.5 solar illumination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For further estimate the performance stability of the asprepared films, we have treated the films at 250 °C, 300 °C, 350 °C, 400 °C and 500 °C for 24 h, their reflection spectra showed in Fig. 7. The films acquire solar absorptance of about 0.903, 0.894, 0.857, 0.837 and 0.826, the thermal emittance of about 0.145, 0.161, 0.173, 0.192 and 0.321, respectively. While the temperature of heat treatment is below 450 °C, the film can maintain considerable solar thermal performance. In case the temperature increases to 500 °C, the nanoparticle agglomerations of the foamed nanostructure surface aggregate obviously and the number of nanopores per unit area decreases, reducing the solar absorptance of the film. Meanwhile the aggregation of particle agglomerations brings about an increasing of film surface roughness, resulting in a rise of thermal emittance [41]. In addition, we also studied the thermal stability of as-prepared film at 450 °C for 48 h, 72 h, 120 h and 168 h, their reflection spectra showed in Fig. 8. They achieve solar absorptance of about 0.830, 0.821, 0.814 and 0.809, and the thermal emittance of about 0.234, 0.293, 0.334 and 0.391, respectively. As shown in SEM images in Fig. 3, it was found that the diameters of the particle agglomerations are not significantly increased, from 48 h to 168 h

Fig. 9. Solar absorptance, thermal emittance and selectivity factors of as-prepared and heat treated films.

of heat treatment (Fig. 3a–d). Therefore, the nanopore number of the foamed nanostructure surface does not reduce obviously, resulting in a little change of the solar absorptance. Due to the aggregation of the particle agglomerations on the film surface, the thermal emission of the film increases after a long time of heat treatment. The absorptance, emittance and selectivity factors of all samples at different temperature and time are shown in Fig. 9. The selectivity factor, which is the ratio of absorptance and emittance, varies from 7.68 to 2.57 with the treatment time and temperature increased. It represents that the foamed nanostructure films are relatively sensitive to high temperature, especially over 400 °C, such as 450 °C heat treatment for more than 48 h. The higher temperature and longer time of the treatment, the more significantly decreasing of film selectivity factor. In the next experiment, SiO2 or AlN are proposed to be added on the as-prepared film surface by spin coating or magnetron sputtering, for protecting the foamed nanostructures from being weakened while heated at high temperature in air, simultaneously reducing the reflection of the film surface.

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temperature. Finally, the hydrothermal method used in the present investigation happens to be simple, convenient, pollution-free and novel. Besides, this method is also very low-cost for it only utilize NaOH and stainless steel as starting materials. The NaOH can also be recycled after the reaction. And the stainless steel sheets do not require any chemical or mechanical polishing. Furthermore, as each Teflon-lined autoclave can simultaneously accommodate many sheets of stainless steel inside for preparation, leading to a significant decreasing for the cost of each selective absorber film. Thus, the selective absorber films with foamed nanostructure, which are prepared by hydrothermal method, have a tremendous potential for their use in the CSP for large-scale production in the future.

Fig. 10. The schematic of absorbing sunlight and emitting infrared for the foamed nanostructure film. Qrad represents radiative emittance at the surface. Qheat represents heat flux converted from sunlight.

Here, the foamed nanostructure plays a great role in defining the solar thermal properties of the selective absorber film. Firstly, sunlight can be effectively trapped in the interior spaces of the foamed nanostructure by multi-reflection within the inner surface of individual pores [19,42]. Secondly, the solar absorptance of the film is greatly influenced by the response of the absorbing nanoparticles [3]. Thirdly, the 3D foamed nanostructure is relatively an ideal air-filled medium with varying dielectric constant [5,11], resulting in the weakening of reflection from the film surface. Fourthly, the intrinsic combination of spinel oxides on the film also contribute to solar selective absorptance. The solar thermal property is superior to other spinel films prepared by spin-coating or dip-coating (with absorptance of 0.80–0.91, and thermal emittance of 0.2–0.5) [37], which could be attributed to the foamed nanostructures with excellent spectral selectivity through efficient sunlight trapping of solar energy. Our results clearly outline the importance and influence of surface morphology on the absorptance and emittance of the selective absorber films. On the one hand, the surfaces of the as-prepared films have smaller-sized nanoparticle agglomerates, resulting in more proportion of nano-voids or pores per unit micro-area of film surface, thereby leading to enhanced solar absorptance and trapping within pores (Fig. 10). Therefore the film represents lower reflectance, namely higher absorptance in the 0.3–2.5 mm (according to formula (1)) [5,11]. On the other hand, the nanoparticle agglomerations and surface roughness of the as-prepared films are so small in size (far less than IR wavelength), that the film surface is relatively flat and smooth compared with the infrared wavelength. Therefore the reflectance of the selective absorber film in the IR region is higher, namely lower for thermal emission in 2.5–20 mm.

4. Conclusions In summary, the solar selective absorbers with foamed nanostructure, for the first time are grown in situ by hydrothermal method on stainless steel, without using any porogen or template. The selective absorption films comprise a large number of nanoparticle agglomerates and nanopores, which have a vital role on the selective absorptance, leading to excellent solar thermal performance. With solar absorptance of 0.92 and thermal emittance of 0.12, these films represent a new-family of absorbers for CSP applications. As for the thermal stability, the films are unruptured pointing to the excellent adhesivity and structural integrity, and show considerable solar thermal performance at high

Acknowledgment This work was supported by the National Natural Science Foundation of China (Grants 21427802, 21131002 and 21201075) and the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP Grant 20110061130005).

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