Fabrication and characterization of TiO2 deposited black electroless Ni-P solar absorber

Applied Surface Science 496 (2019) 143632

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Fabrication and characterization of TiO2 deposited black electroless Ni-P solar absorber

T

Mohammad Amin Razmjoo Khollari, Mohammad Ghorbani , Abdollah Afshar ⁎

Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran

ARTICLE INFO

ABSTRACT

Keywords: Electroless Ni-P Blackening Solar absorber Titania Antireflection coating Corrosion

Preparing a selective, efficient, and low-cost solar absorber is one of the main challenges in solar to thermal energy conversion. In this paper, black electroless NieP (ENi-P) solar absorber has been fabricated, and the effect of nanoporous TiO2 antireflection layer (ARL) on its optical and corrosion properties has been investigated. The optimum black coating was obtained by blackening in 9 M nitric acid solution at 50 °C for 40 s, in which a solar absorptance of 99.3% was achieved. Deposition of TiO2 ARL increased the solar absorptance of coating to 99.7% and addition of 0.8 g Pluronic F127 (F127) as pore former, further increased this value to 99.9% and solar-to-heat efficiency of the coating from 78.1 to 78.7%. F127 added coatings exhibited elongated and irregularly shaped pores with dimensions of a few tens of nanometers. Also, deposition of TiO2 ARL decreased the corrosion current density (icorr) of black ENi-P coating in 3.5 wt% NaCl solution from 20 to 4 μA/cm2. The results of this work indicate that the TiO2 deposited black ENi-P coating can be a suitable choice for black coating applications.

1. Introduction The growth of population and industries has led to an increase in energy consumption and demand [1]. Fossil fuels supply a major part of our energy demand, but these sources of energy have two major problems: first, they are not renewable, and second, they are environmentally hazardous. These problems have forced researchers to find a suitable alternative to fossil fuels. Geothermal, wind, nuclear, and solar energy are some of the possible candidates. Although these resources are considered green and renewable, their main problem is low efficiency and high cost of energy production [2,3]. Being abundant and accessible, solar energy can be a promising alternative to fossil fuels. In 2004, the world's primary energy production in one day was only 0.0012% of the delivered solar energy to the earth, which indicates the solar energy potential for providing the bulk of our energy [4]. High-efficiency solar absorbers are used to convert photons energy to heat [5,6]. The most important feature of solar absorbers is “selective absorption”, which means they should exhibit high solar absorptance (αsol > 0.95) in UV–Vis-NIR spectrum (0.3–2.5 μm) and low thermal emittance (εthe < 0.1) in IR region (2.5–25 μm). Therefore, the solar absorber efficiency can be calculated according to the η = α-ε or η = α/ ε ratio, and the higher the ratio, the performance will be better [7,8].

⁎

Black coatings have wide usage in photo-thermal convertors, absorbing materials, optical instruments, sensors and decorative coatings [9–11] Up to now, black coatings have been generally fabricated by electrodeposition or vapor phase deposition methods. However, a high-efficiency black coating can be obtained by chemical and electrochemical etching or anodic oxidation of ENi-P coating in acidic electrolytes [12]. Good corrosion and wear resistance, excellent hardness, and uniformity of the nickel-phosphorus (NieP) coating have made the ENi-P coatings a well-known candidate in various industries [13]. The blackening process involves the formation of nickel oxides (NiO and Ni2O3) and surface cavities, both of which are responsible for absorbing the incident light [9,14]. A portion of incident light is reflected when it propagates across a boundary between two media with different refractive indices. Deposition of an ARL on solar absorber surface can reduce optical losses and therefore, lead to an increase in solar absorptance. For best performance, ARL should have a proper refractive index to increase the solar absorptance and, at the same time, should be thin enough so as not to increase the substrate emittance [15]. The optical properties of ARL produced by the sol-gel method can be controlled in the presence of a “pore former” agent. Removal of this polymeric material during calcination can result in the formation of pores in the coating. Miao et al. have reported that the addition of F127 in the sol of TiO2 and SiO2

Corresponding author. E-mail addresses: [email protected] (M.A.R. Khollari), [email protected] (M. Ghorbani), [email protected] (A. Afshar).

https://doi.org/10.1016/j.apsusc.2019.143632 Received 24 May 2019; Received in revised form 14 July 2019; Accepted 9 August 2019 Available online 15 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 496 (2019) 143632

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Table 1 Chemical composition of 1050 Al alloy (wt%). Si

Fe

Cu

Mg

Mn

Zn

Ti

Al

0.1

0.35

0.01

0.008

0.03

0.007

0.005

99.49

Table 2 Chemical composition of Zincate bath. Bath constituents

Quantity (g/L)

ZnO NaOH NaNO3 KNaC4H4O6.4H2O

4 120 1 50

Table 3 Chemical composition of NieP electroless bath. Bath constituents NiSO4.6H2O H2NaO2P·H2O CH3COONa CH3COOH Pb(C2H3O2)2 Temp. (°C) pH Time (h)

Fig. 1. XRD patterns of a) ENi-P coating, b) black ENi-P coating and c) TiO2 deposited black ENi-P coating after calcination at 400 °C.

Quantity (g/L) 28 20 12 10 0.001 85 ± 1 4.7 ± 0.1 2

increases the porosity and transmittance of the coating [16]. Boström et al. investigated the effect of Al2O3 and SiO2 ARLs on the optical performance of Nickel-Alumina solar absorber and have found that deposition of ARL improves solar absorption. The optimum sample exhibited an increase in solar absorptance from 0.79 to 0.93 [15]. Similarly, Lira-Cantu et al. studied the effect of sol-gel derived SiO2 ARL on electrochemically deposited black Nickel solar absorber and their results showed an increase in solar absorptance from 0.91 to 0.92 [17]. In this study, fabrication of black ENi-P coating by chemical blackening in Nitric acid has been investigated, and its optical and corrosion properties before and after deposition of TiO2 sol-gel derived ARL have been examined. Moreover, the effect of F127 addition on the optical properties of coating has been investigated.

Fig. 2. FTIR spectrum of black ENi-P coating before and after deposition of TiO2 antireflection layer and calcination at 400 °C.

into the mixture and stirred for 15 min (Solution 1). At the same time, 0.25 ml nitric acid was added to 15 ml ethanol and after 15 min stirring, different amounts of F127 (0, 0.4, 0.8 and 2 g) was added to the mixture and the solution stirred for another 15 min (Solution 2). Finally, Solution 2 was gradually added to Solution 1, stirred for 1 h and then aged for 1 day [18]. TiO2 layer was deposited on the black ENi-P coating surface by the dip-coating method using a withdraw rate of 1 mm/s and immersion time of 1 min. The dip-coated samples were first dried at 80 °C and then calcined at 400 °C for 1 h.

2. Experimental procedures 2.1. Material and method Commercial 1050 Al alloy samples (50 × 30 × 1.5 mm3) were used as substrate. The chemical composition of the alloy is given in Table 1. Before ENi-P plating, all the samples were grounded with SiC sandpapers to a grit of 2000. Degreasing of the samples was carried out in a 10 wt% NaOH solution at 40 °C for 1 min. After rinsing with distilled water, samples were immersed in the Zincate bath for 10 s, the composition of which is given in Table 2. In the following, samples were soaked in a 10 Vol% Nitric acid solution and after rinsing with distilled water, again immersed in the zincate bath for 10 s (two-step zincating). Afterwards, the pretreated samples were immersed in ENi-P bath. The composition and the condition of the plating bath are presented in Table 3. Blackening of the as-plated samples was performed by immersing them in 9 M nitric acid at 40–50 °C for 30–50 s, and the samples were rinsed quickly with distilled water and dried afterward. The TiO2 ARL was prepared by sol-gel method with different amounts of F127 as pore former agent. At first, 0.25 ml Acetylacetone (AcAc) was added to 10 ml ethanol and the solution stirred for 15 min. Then 0.75 ml Titanium Tetraisopropoxide (TTIP) was added dropwise

2.2. Characterization A Philips X'Pert PRO MPD X-ray diffractometer was used for structure evaluation of the prepared coatings with a Cu-target (λ = 1.542 Å) at grazing mode in the 2θ range from 20 to 80° and incident angle of 1°. FTIR spectrum of the coatings was recorded on a Perkin Elmer RXI FTIR spectrometer (USA), using KBr pellets. The surface morphology, cross-section, and chemical composition of the coatings were investigated using a MIRA3 TESCAN-XMU FESEM equipped with EDX system. Bio-AFM (Ara research, Iran) in non-contact mode was employed for further study of the morphology of the coatings. Total reflectance of samples in the UV–Vis-NIR range was measured by a Perkin-Elmer Lambda 950 spectrophotometer equipped with a 60 mm diameter integrating sphere with 8° angle of incidence. The IR 2

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Fig. 3. a) FESEM image and b) EDX spectrum of ENi-P coating, c) and d) FESEM image of black ENi-P coating (blackened at 9 M nitric acid at 50 °C for 40 s) under 5000× and 50,000× magnification, respectively.

measurements were performed on a FTIR Tensor 27 (Bruker) with a gold mirror reference. Normal solar absorptance (αsol) and normal thermal emittance (εthe) were calculated based on Eqs. (1) and (2), respectively, where Isol is the direct normal solar irradiance which is defined according to ISO standard 9845-1 (1992) for air mass of 1.5, IB is the blackbody spectral intensity at the solar absorber temperature, and R(λ) is the total reflectance at a certain wavelength [7,19]. sol

=

the

=

2.5 I ( )(1 0.2 sol 2.5 I ( 0.2 sol

25 I ( 2.5 B

)(1

25 I ( 2.5 B

ENi-P coating before and after deposition of TiO2 ARL. The XRD patterns of ENi-P coating before and after the blackening process are shown to be similar. The patterns present a single broad diffuse peak in the range of 2θ = 35 to 2θ = 55° corresponding to Ni3P (231) or (141), and a sharp peak at 2θ = 45.3° related to Ni (111) diffraction (JCPDS No. 34-0501 and JCPDS No. 87-712). It can be concluded that both coatings contain a mixture of crystalline and amorphous phases [20,21]. Also, the absence of sharp diffraction peaks of the black ENi-P coating indicates an amorphous structure [22,23]. New XRD peaks corresponding to crystalline Ni, Ni3P and TiO2 phases appear after deposition of TiO2 ARL and calcination at 400 °C. During calcination, the semi-amorphous structure of ENi-P coating transforms to the crystalline structures of F.C.C. Ni and Ni3P phases. Also, the peaks at 2θ = 25.4° and 2θ = 37.8° are evidence of the formation of Anatase TiO2 after calcination [20,21,24–26]. The FTIR spectra of the black ENi-P coating before and after deposition of TiO2 ARL are shown in Fig. 2. Black ENi-P spectrum shows a peak at 447 cm−1 corresponded to the bending vibration of Ni–O. After deposition of TiO2 layer, a peak appears at 472 cm−1 which is attributed to the stretching vibration of Ti–O [27–29]. Also, three bands at 3400, 1610 and 1390 cm−1 are observed in the spectrum caused by the stretching vibration of the hydroxyl (O–H) group of the TiO2 ARL, bending vibration of coordinated H2O as well as Ti–OH, and TieO modes, respectively [30,31].

R ( )) d )d

(1)

R ( )) d )d

(2)

The corrosion resistance of the substrate and prepared coatings were evaluated by potentiodynamic polarization measurement in 3.5% NaCl solution. All tests were performed after 30 min stabilization of samples in the testing solution by an AUTOLAB PGSTAT302N apparatus in a three-electrode cell. A platinum plate was used as the counter electrode and a saturated calomel as the reference electrode. The exposed area was 1 cm2, and polarization curves were measured from −100 to +100 mV relative to open circuit potential (OCP) at a scan rate of 1 mV/s. 3. Results and discussion

3.2. Morphology

3.1. Structural characterization

As shown in Fig. 3a, the ENi-P coating exhibits a spherical nodular structure [32,33] with globules of 1–10 μm. The deposited coating is

Fig. 1 presents the structural evaluation of ENi-P coating and black 3

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Fig. 4. a) Cross sectional FESEM image and b) EDX spectrum of black ENi-P coating (blackened at 9 M nitric acid at 50 °C for 40 s), c) cross sectional FESEM image with indication of EDX scan line across black layer and d) variation of Ni, P and O concentration along the specified scan line of figure c.

dense, uniform, and almost free of defects (no obvious holes, cracks, etc. are detectable). Based on the EDX spectrum of Fig. 3b, the coating contains 6.5 wt% P and 93.5 wt% Ni and no impurities are observed in it. To achieve the lowest reflectance after the blackening process, the phosphorous content of the coating should be between 5 and 7 wt%. At a higher P content, the blackening process will become more difficult, while at a lower content, selective etching will not occur [34]. The morphology of the optimum black ENi-P coating is shown in Fig. 3c and d. The micrographs show a “crater” structure which consists of a dense array of conical cavities perpendicular to the surface with a diameter range from hundreds of nanometers to several micrometers [14,23,35]. This diameter range can result in enhanced light absorption in the UV–Vis-NIR region due to the fact that the cavities preferably trap irradiations with a wavelength close to their dimension [9,23]. Fig. 4a shows the cross sectional FESEM image of the optimum black ENi-P coating. The thin and lighter layer at the outermost edges of the coating is the black layer, which is formed during the blackening process [36]. The cross-section of cavities is conical and their depth is about a few micrometers. The EDX spectrum of the black layer surface is shown in Fig. 4b. Comparing Figs. 4b and 3b shows that after the blackening process, the Ni content is reduced from 93.5 to 83.6 wt%, and the P and O content of the coating are increased from 6.5 to 10.5 wt % and from 0 to 5.9 wt%, respectively. The decrease in Ni content and increase in P content are valid evidence of selective etching of Ni. Also, the presence of O element in EDX spectrum proves the formation of oxide phases during the blackening process [9,37]. Based on Fig. 4c, the thickness of the black layer is about 0.3 μm. The EDX line scan (Fig. 4d)

confirms the decrease of Ni content across the black layer. The Ni content reduction accompanied with O content increase gives a proof of the Ni oxides formation during the blackening process. The O to Ni ratio in black layer varies between 1 and 1.2, which indicates the black layer consists of a mixture of NiO (O/Ni = 1) and Ni2O3 (O/Ni = 1.33). These oxide phases, besides the surface cavities, are mainly responsible for solar absorption [9,14,23]. The surface morphology and cross section of the TiO2 ARL deposited black ENi-P coating are illustrated in Fig. 5a and b. The walls of the cavities and the area between them are uniformly covered with the TiO2 layer, and no crack and defect are observed in the film. This uniform TiO2 ARL can reduce the unwanted light reflection from the black surface which can result in an increase in the solar absorptance. Based on Fig. 5b, the thickness of the TiO2 ARL is about 90 nm. The Ni, P, Ti, and O distribution of Fig. 5a are depicted in Fig. 5c to f, respectively. According to Fig. 5e and f, the TiO2 ARL has uniformly covered the surface of black ENi-P coating. AFM analysis was employed to study the morphology of the coatings in more detail. The two and three-dimensional AFM images of the ENi-P coating are shown in Fig. 6a and b. According to the images, the surface topography of the ENi-P coating is composed of nearly hemispherical grains with a diameter range of 1–10 μm and an average surface roughness of 608 nm. In addition, Fig. 6c and d represent the two and three-dimensional AFM images of TiO2 deposited black ENi-P coating with 0.8 g F127. Selective etching of Ni element during the blackening process has led to a “crater” structure with a large number of conical cavities. The height difference between the lowest and highest surface 4

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Fig. 5. a) Top view and b) cross sectional FESEM image of TiO2 deposited black ENi-P coating and corresponding EDX elemental mapping of c) Ni, d) P, e) Ti and f) O elements of figure a.

features is about 3 μm, indicating the cavities depth is in the range of a few micrometers. Fig. 7 shows the effect of F127 as a pore former agent. No pores are observable in the TiO2 layer prepared from the solution without F127 (inset of Fig. 7a). However, after addition of 0.8 g F127, the porosity of the coating is increased, and elongated and irregularly shaped pores with a dimension of a few tens of nanometers have been obtained (inset of Fig. 7b). The nano-scale pores are formed due to the removal of F127 after calcination at 400 °C. These pores can give rise to a desirable refractive index of TiO2 ARL and improvement in solar absorption [38–40].

Fig. 8 shows the effect of F127 on the total reflectance of TiO2 deposited black ENi-P coating in UV–Vis-NIR region. In order to compare, the reflectance spectra of ENi-P coating and TiO2 deposited ENi-P coating with 0.8 g F127 are shown in the figure. Table 5 exhibits the calculated solar absorptance and thermal emittance of the coatings. According to Fig. 8, it is clear that the deposition of TiO2 layer results in a broadband antireflective effect and a decrease in the total reflectance of the black ENi-P coating (an improvement of the solar absorptance of the black ENi-P coating from 99.3% to 99.7%). This can be explained by elimination of unwanted light reflection and therefore increase in solar absorption [17,43,44]. As can be seen, the reduction is more severe in the ultraviolet and visible region. Moreover, the addition of F127 up to 0.8 g decreases the total reflectance of the black coating. The TiO2 deposited black ENi-P coating with 0.8 g F127 shows a solar absorptance of 99.9% and a minimum reflection of 0.001 at 630 nm. However, in a higher amount of F127, the solar absorptance of the coating reduces. For a non-absorbing substance with a refractive index of ns, the refractive index of ARL (nc) should satisfy the following condition (for absorbing surfaces, the equation is more complicated):

3.3. Optical properties The effect of blackening process on solar absorptance and thermal emittance of the black ENi-P coatings are illustrated in Table 4. All the samples show a solar absorptance of > 95%. The high solar absorptance of the black ENi-P coating is due to the selective etching of Ni during the blackening process. It leads to the formation of surface cavities (as observed in Fig. 3c and d) that are able to trap and absorb the incident light by multiple scattering (as illustrated in Fig. 6d) [41]. Based on the results, increasing the blackening temperature enhances solar absorptance and decreases thermal emittance of the coating. Similarly, at a constant temperature, increase in blackening time can enhance solar absorptance and decrease thermal emittance. It can be explained by a higher surface cavity density, which increases the amount of light trapping [14,35]. The optimum blackening condition is obtained in 9 M nitric acid at 50 °C for 40 s (αsol = 99.3%, εthe = 21.2% and η = 78.1%). Further increase in the blackening time leads to a slight decrease in solar absorptance. Since cavities preferably absorb radiation with a wavelength close to their diameter, their dimension is a determinative factor. Hence, excessive blackening will result in undesirable cavity size distribution and reduces solar absorptance [4,42].

nc =

ns × nair

(3)

By increasing the F127 content, the porosity of ARL increases (based on Fig. 7) and hence its refractive index decreases. It seems that addition of F127 up to 0.8 g leads to an optimal refractive index (based on Eq. (3)) [43,45]. It should be mentioned that in the black ENi-P coating spectrum (Fig. 8), the absorption edge of Nickel oxide is observed in the range of 250–400 nm that is attributed to the electronic transition from the valence band to the conduction band [46]. The TiO2 deposited black ENiP coatings depict a slight redshift of the absorption edge. It can be ascribed to the lower optical bandgap of the anatase TiO2 ARL (Eg = 3.2 eV), compared to that of Nickel oxide (Eg = 3.4–4.3 eV) [47–49]. 5

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Fig. 6. AFM 2D and 3D surface micro-graphs of a,b) ENi-P coating and c,d) TiO2 deposited black ENi-P coating with 0.8 g F127, respectively.

Fig. 7. FESEM image of TiO2 deposited black ENi-P coating, a) without and b) with 0.8 g Pluronic F127. The insets in a) and b) show a magnified view of TiO2 ARL.

Based on Fig. 9, TiO2 ARL has not a significant effect on the thermal emittance of the black ENi-P coating. It can be explained by the nanometric thickness of ARL that prevents the interaction with IR radiation [15]. Based on the results, among all the samples, TiO2 deposited black ENi-P with 0.8 g F127 shows the maximum efficiency (η = 78.7%). Fig. 10 compares the reflectance spectra of the black ENi-P coating, TiO2 deposited black ENi-P coating with 0.8 g F127, and an ideal selective absorber. The ideal selective absorber has zero reflectance in

UV–Vis-NIR and unity reflectance in the IR region. It is clear that the TiO2 deposited black ENi-P coating with 0.8 g F127 shows a selective absorption and behave close to an ideal absorber. 3.4. Corrosion properties Polarization curves of Al substrate, ENi-P coating, black ENi-P coating, and TiO2 deposited black ENi-P coating with 0.8 g F127 are 6

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Table 4 Effect of blackening conditions on solar absorptance and thermal emittance of black ENi-P coating. αsol (%)

εthe (%)

αsol - εthe (%)

Blackening at 40 °C 30 s 40 s 50 s

97.4 98.3 98.9

35.2 32.3 28.3

62.2 66.0 76.6

Blackening at 50 °C 30 s 40 s 50 s

99.1 99.3 99.2

23.7 21.2 21.2

75.4 78.1 78.0

Variable

Table 5 Effect of TiO2 ARL with different amounts of Pluronic F127 on solar absorptance and thermal emittance of optimum black ENi-P coating. Sample Black ENi-P coating TiO2 deposited black TiO2 deposited black TiO2 deposited black TiO2 deposited black

ENi-P ENi-P ENi-P ENi-P

coating coating coating coating

(0 g F127) (0.4 g F127) (0.8 g F127) (2 g F127)

αsol (%)

εthe (%)

αsol - εthe (%)

99.3 99.7 99.8 99.9 99.7

21.2 21.6 21.5 21.2 21.2

78.1 78.1 78.3 78.7 78.5

Fig. 9. Variation of Total Reflectance with wavelength of ENi-P coating before and after deposition of TiO2 ARL with 0.8 g F127 and optimum black ENi-P coating before and after deposition of TiO2 ARL with 0, 0.4, 0.8 and 2 g F127 at IR region.

Fig. 8. Variation of Total Reflectance with wavelength of ENi-P coating before and after deposition of TiO2 ARL with 0.8 g F127 and optimum black ENi-P coating before and after deposition of TiO2 ARL with 0, 0.4, 0.8 and 2 g F127 at UV–Vis-NIR region.

given in Fig. 11. Corresponding Corrosion parameters of coatings are summarized in Table 6. After blackening of the ENi-P coating, the corrosion current density (icorr) increases from 1 to 20 μA/cm2 and the corrosion potential (Ecorr) shifts from −491 to −731 mVSCE, indicating that the blackening process reduces the corrosion resistance of the ENiP coating. It can be referred to the reduction of the coating uniformity and the formation of surface cavities during the blackening process, which increase the anodic reaction rate and hence, the corrosion rate of the black ENi-P coating [34]. After deposition of TiO2 ARL, icorr decreases to 4 μA/cm2 and Ecorr shifts to −811 mVSCE. The decrease in corrosion current density implies that the TiO2 layer slows the corrosion kinetics of black ENi-P coating. The uniform TiO2 layer (as observed in Fig. 5) acts as a ceramic barrier and protects the black substrate from

Fig. 10. Solar absorption selectivity comparison of black ENi-P and TiO2 deposited black ENi-P with 0.8 g F127 coatings with ideal solar absorber.

corrosive media and by suppressing the anodic reaction, improves the corrosion resistance of the substrate [50–54]. Based on the polarization curves, corrosion resistance follows the sequence: 1050 Al substrate > ENi-P coating > TiO2 deposited black ENi-P coating > black ENi-P coating. 7

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Fig. 11. Potentiodynamic polarization curves of 1050 Al substrate, ENi-P coating, black ENi-P coating and TiO2 deposited black ENi-P coating with 0.8 g Pluronic F127 in 3.5% NaCl.

Table 6 Corrosion characteristics of Al substrate (1050 alloy), ENi-P coating, black ENiP coating and TiO2 deposited black ENi-P coating with 0.8 g F127 in 3.5% NaCl determined by Tafel extrapolation method. Sample 1050 Al ENi-P Black ENi-P coating TiO2 deposited black ENi-P coating (0.8 g F127)

βa (mV/dec)

βc (mV/dec)

icorr (μA cm−2)

Ecorr (mVAg/SCE)

114 155 115 105

−121 −71 −133 −255

0.4 1 20 4

−728 −491 −731 −811

4. Conclusion The present paper describes the fabrication process of the black ENiP solar absorber and investigates the effect of TiO2 ARL on its optical and corrosion properties. Blackening of the ENi-P coatings in Nitric acid led to selective etching of Ni, the formation of Ni oxides (NiO and Ni2O3) and surface cavities that are responsible for high solar absorptance (99.3%) of the coating. The results showed that the diameter of surface cavities ranges from hundreds of nanometers to several micrometers, which absorb the incident light in the UV–Vis-NIR spectrum. Deposition of a nanometric and uniform TiO2 ARL reduced the total reflectance of the black ENi-P coating by eliminating unwanted light reflection and led to an increase in solar absorptance (99.7%). Also, the addition of F127 up to 0.8 g to TiO2 sol optimized the refractive index and performance of ARL. The lowest solar absorptance (99.9%) and maximum efficiency (78.7%) was obtained in this case. The removal of F127 by calcination at 400 °C resulted in the formation of nano-scale pores that can reduce the refractive index of ARL. The potentiodynamic polarization measurements revealed that deposition of TiO2 ARL increases the corrosion resistance of black ENi-P coating by acting as a barrier to corrosive medium. Declaration of competing interest None. 8

Applied Surface Science 496 (2019) 143632

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