Enhancing Solar Absorption with Double-Layered Nickel Coatings and WS₂ Nanoparticles on Copper Substrates

Abstract

This study focused on the development and characterization of multi-layered nickel coatings doped with WS₂ nanoparticles and electrodeposited on copper substrates. To enhance the solar collector’s performance by improving the solar radiation conversion into heat, two distinct undercoatings were evaluated, along with the incorporation of WS₂ nanoparticles in the black nickel layer. X-ray diffraction (XRD) analysis revealed that the bright and dull nickel undercoatings consisted of metallic nickel, whereas the black coatings comprised amorphous nickel oxide, inferred to be Ni₂O₃ based on energy-dispersive X-ray spectroscopy (EDS) analysis. Scanning electron microscopy (SEM) analysis of the undercoatings and black nickel morphology showed a compact surface with a relatively homogenous microstructure composed of polyhedric grains, which was free of visible cracks or pinholes. The undercoating influenced the brightness, the reflectivity and the reflectance of the black nickel films, with the dull undercoated sample showing the most promising results, with a total absorbance of 0.94. The incorporation of WS₂ nanoparticles induced the formation of cracks and increased the porosity of the black nickel film. With an appropriate content of WS₂ nanoparticles and the use of a dull undercoat, these drawbacks can be avoided. Concerning the integration of WS₂ nanoparticles, a minor decrease in the brightness of the black films and a subsequent increase in the total absorbance ultimately led to an enhancement of the conversion of solar energy into thermal energy.

1. Introduction

The Sun, which is the primary source of energy for the Earth system, provides an average of about 340 watts of energy per square meter of the Earth’s surface, accounting for both day and night as well as seasonal variations.

This energy reaches the Earth mainly as visible light, but it also includes infrared and ultraviolet radiation and other wavelengths of the electromagnetic spectrum.

As sunlight reaches the Earth, approximately 30% of it is reflected back to space, with clouds and the atmosphere contributing to this reflection (22.6%), along with bright surfaces like snow- and ice-covered areas (6.7%). The remaining sunlight is absorbed by the Earth system, with 22.7% absorbed by the atmosphere and 48% absorbed by the Earth’s surface [1].

Despite the attenuation, the total amount of solar energy available on the surface of the Earth, ≈163 watts per square meter, remains substantial. However, due to its low density and intermittency, it must be collected and stored efficiently [2].

There are two primary methods for utilizing solar energy for energy production: solar–electric conversion, which involves converting solar energy directly into electrical energy using photovoltaic solar cells, and solar–thermal conversion, which involves converting solar energy into thermal energy using solar collectors [3].

In solar–thermal systems, solar irradiation is absorbed by the collector as heat, which is then transferred to a working fluid such as air, water, or oil. This heat can be utilized to provide domestic hot water and heating or stored in a thermal energy storage tank for future use. Consequently, solar collectors must have excellent optical performance, absorbing as much heat as possible [2].

Due to metals’ inherent low thermal emittance, selective solar absorber coatings are commonly applied to metallic substrates known for their good corrosion resistance and high thermal conductivity [4,5].

These selective coatings can be applied by several techniques, including electroplating, sol-gel, and chemical vapor deposition [5]. Electrodeposition is considered a low-temperature, robust, and cost-effective method that is easily scalable. This technique can produce porosity-free finished products without the need for subsequent consolidation processing. Additionally, electrodeposition requires low initial capital investment and supports high production rates with minimal limitations on shape and size [6,7].

The most common materials used for collector plates include copper, aluminum, and stainless steel [8]. Among these, copper stands out as an ideal substrate due to its superior thermal conductivity, stability, and high reflectance/low emittance. Moreover, copper’s suitability for electrodeposition makes it an attractive choice, eliminating the need for the complex pre-treatment steps required for electrodeposition onto other metals like stainless steel or aluminum [4,7,9].

Typical examples of electrodeposited selective coatings are black chrome and black nickel, both of which exhibit excellent selective optical properties. Black chrome coatings have been preferred, despite the requirement for high current densities (up to 20 A/dm²) during the electrodeposition process [10,11].

Over the years, black chromium coatings were produced using chemical baths containing hexavalent chromium (Cr(VI)). However, since 2013, the use of Cr(VI) baths has been limited and discouraged due to their toxicity and carcinogenic effects on both human health and the environment. As a result, less toxic trivalent chromium has been considered as a potential replacement for hexavalent chromium baths [12,13].

This technology is already in commercial use for decorative coatings. However, achieving thicker coatings is more challenging, and maintaining the bath is more demanding compared to hexavalent chromium. Additionally, trivalent chromium baths are more sensitive to contamination by foreign ions and less tolerant of impurities from substrates or long-term process carry-over [12,13,14].

Electrodeposited nickel coatings have been considered as potential replacements for hard chromium, offering several advantages. Black nickel, for instance, can be electrodeposited at lower current densities (around 0.1 A/dm²), and nickel electrolytes have superior dispersion properties compared to chrome ones. This is particularly important for mass production of large and irregularly shaped units. However, many black nickel coatings degrade quickly, especially those electrodeposited from sulfur baths, which exhibit poor corrosion resistance in aggressive environments [10,11,14].

This drawback can be mitigated by using chloride-based chemical baths, by the application of an undercoat of dull or bright nickel, and by embedding nanoparticles on the black nickel coatings [4,7,9,10,13].

Owing to the rapid advancements of nanotechnology, various nanoparticles, such as Fe₂O₃, Al₂O₃, TiO₂, SiO₂, ZrO₂, WC, WS₂, and MoS₂, have been introduced into coatings. Nanoparticle-embedded composite coatings can improve the properties of, or introduce new functionalities to, metallic coatings such as nickel, enhancing their performance and durability [15].

In recent years, tungsten disulfide (WS₂), a two-dimensional layered material with lubricating properties, has mainly been used as a solid lubricant in applications to improve the antifriction properties of composite coatings. It was already reported that the incorporation of WS₂ nanoparticles on an Ni–Co alloy coating reduced the friction coefficient by 3/2 and the corrosion rate by about half [16]. However, the impact of WS₂ nanoparticles as a dispersed phase material on the selective optical properties of black nickel coatings remains unexplored.

Given the context outlined above, this study aims to develop and characterize black nickel coatings, with a particular emphasis on the effects of a bright/dull nickel undercoat and the incorporation of WS₂ nanoparticles on the selective optical properties of the black nickel films.

2. Materials and Methods

2.1. Electrodeposition Procedure

Rectangular pieces of copper with an area of 6 cm² and a thickness of 1 mm were used as the substrate.

The electrodeposition was performed in a polyethylene container, using the substrate as the cathode (with an electrodeposition area of 4 cm², delimited with Kapton tape), and two rectangular pieces of nickel (99.99%, Testbourne Ltd., Basingstoke, UK) as the anodes, as depicted in Figure 1.

The current density was controlled by the EA-PSI 9360-15 (Elektro-Automatik, Viersen, Germany) power supply and a 2831E digital multimeter (B&K Precision, Yorba Linda, CA, USA), which was installed as an ammeter.

Immediately before the electrodeposition process, in order to effectively remove the protective oxide layer from the surface of the copper and prevent adhesion issues, the rectangular pieces were immersed in a 25% v/v aqueous solution of HCl (25% v/v, Panreac, Barcelona, Spain) for 60 min.

For the deposition of the bright and dull nickel undercoats, an electrolyte containing NiCl₂·6H₂O (98%, Thermo Scientific, Waltham, MA, USA), NaCl (≥99%, Sigma-Aldrich, St. Louis, MO, USA) and H₃BO₃ (≥99.8%, Fisher Chemical, Pittsburgh, PA, USA) was used.

The process occurred at room temperature, with a stirring speed of 200 rpm, performed by a cylindrical PTFE-coated steel magnetic follower with an 8 mm diameter and a 30 mm length.

The pH of the bath, which was not adjusted, remained in the range of 4.83–4.93, and the current density applied was 3.75 mA/cm² for 15 (bright nickel undercoating—sample Bu) and 45 min (dull nickel undercoating—sample Du).

Using the bright and dull nickel-coated copper substrate, the black nickel layer was electrodeposited, also at room temperature and with the same bath (excluding the H₃BO₃) and stirring conditions, resulting in the samples BuBN and DuBN.

The pH of the bath remained in the range of 6.40–6.50 and the current density, applied for 6 min, was 2.50 mA/cm².

Afterwards, 1.3 and 2.7 g/L of tungsten disulfide (WS₂) nanoparticles (>99.99%, Nanografi, Ankara, Turkey), 35–75 nm in diameter, was added to the black nickel electrolyte, with the electrodeposition being performed under the same conditions, over a dull nickel layer, since it was the most promising undercoating, leading to the samples DuBN_1.3 and DuBN_2.7, respectively. However, to prevent the sedimentation of the nanoparticles, stirring was performed using a cylindrical PTFE-coated steel magnetic follower with an 8 mm diameter and a 50 mm length.

Table 1. Electrolytes and experimental conditions.

Reagents and Parameters	Bright Nickel	Dull Nickel	Black Nickel	Black Nickel + WS2 NPs
NiCl·6H2O	75 g/L	75 g/L	75 g/L	75 g/L	75 g/L
NaCl	30 g/L	30 g/L	30 g/L	30 g/L	30 g/L
H3BO3	27 g/L	27 g/L	----------	----------	----------
WS2 NPs	----------	----------	----------	1.3 g/L	2.7 g/L
T (°C)	Room temperature
ω (rot/min)	200
pH	4.83–4.93		6.40–6.50	3.74–3.84	3.10–3.20
J (mA/cm2)	3.75		2.50
t (min)	20	45	6

displays the composition of the electrolytes and the experimental parameters.

2.2. Sample Characterization

To analyze the structure of the coatings deposited on the steel substrates, the samples were characterized by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer (PANalytical, Almelo, the Netherlands) with Cu Kα (λ = 1.54060 Å) radiation, operating at 45 kV and 40 mA, and with an incidence angle of 2°. The measurements were performed in parallel beam geometry, with a 2θ range set from 30° to 100°, a step size of 0.04°, and an exposure time of 3 s per step.

The 3D digital microscope RX-100 (Hirox, Tokyo, Japan) was used to examine the prepared samples and to estimate the full thickness of the coatings (bright, dull and black nickel) deposited.

The surface and the cross-section of the samples was characterized by scanning electron microscopy (SEM) using an SU3800 microscope from Hitachi (Tokyo, Japan) and a Zeiss-Merlin microscope (Zeiss, Oberkochen, Germany), operating in secondary electrons mode. The analysis of the chemical composition of the films was performed by energy dispersive X-ray spectroscopy (EDS), using Bruker Nano (Berlin, Germany) equipment, with an accelerating voltage of 10 kV.

The reflectivity and color of the coatings were measured using a GretagMacbeth Color-Eye^® XTH spectrophotometer (X-Rite, Grand Rapids, MI, USA). This equipment measures the specular and diffuse reflection components, in a wavelength range between 360 nm and 750 nm, and the color coordinates in the CIE-L*a*b* color space, which is the most commonly used approach for human color perception.

The reflectance spectra of the coatings were obtained using a UV-3600 UV–Vis–NIR spectrophotometer (Shimadzu Co., Kyoto, Japan) in the UV–Vis–NIR wavelength range of 200–2000 nm.

To evaluate the performance of the DuBN, DuBN_1.3 and DuBN_2.7 samples, they were placed in contact with a container filled with 58.0 mL of deionized water. The surface of the samples was illuminated with a lamp (useful luminous flux of 910 lm and spectral power distribution in the range of 250 nm to 800 nm) simulating the solar spectrum and the temperature of the water was measured.

3. Results and Discussion

3.1. Double-Layered Nickel Coatings

The surface profiles of the undercoatings were examined using a 3D digital microscope. Figure 2a displays a 3D image of the dull nickel, sample Du, with the corresponding surface profile shown in Figure 2b. From the surface profiles, the measured thickness of the samples Bu and Du were (1.1 ± 0.2) and (3.0 ± 0.3) μm, respectively. According to these results, the undercoatings have a deposition rate between 3.3 and 4.0 μm/h.

Furthermore, the cross-sections of the BuBN and DuBN samples were also analyzed. Figure 2c presents the cross-sectional SEM image of the sample DuBN, showing that the black film can be estimated to be around 0.7 μm.

The XRD patterns of the substrate, undercoatings and double-layered samples are shown in Figure 3. In the substrate spectrum, only peaks corresponding to cubic copper were observed, consistent with the ICDD 00-004-0836 [17]. For the undercoatings and double-layered samples, besides the peaks from the substrate, all the remaining peaks can be assigned to cubic nickel (ICDD 00-004-0850 [17]). This implies that the black coating exclusively consists of amorphous nickel compounds, which is in agreement with previous investigations [4,7,18].

The structural analysis performed is consistent with the bright/dull nickel and black nickel electrodeposition processes.

The generally accepted mechanism for the electrodeposition of metallic nickel comprises two sequential one-electron charge transfers and the participation of an anion, X⁻, with the formation of an adsorbed complex.

Since the anions present in the electrochemical bath are OH⁻ and Cl⁻, previous studies show that the anion X⁻ is usually Cl⁻.

This mechanism can be described by the following equations [4,19,20,21]:

Ni²⁺ + X⁻ → NiX⁺

(1)

NiX⁺ + e⁻ → Ni X_ads

(2)

Ni X_ads + e⁻ → Ni + X

(3)

Conversely, black nickel electrodeposition involves multiple reduction processes, along with oxidative processes, resulting in a complex composition that can contain α-Ni(OH)₂, NiOOH, Ni₂O₃, NiO, water and metallic Ni, depending on different factors, such as the surface pH, reagent concentrations and temperature [4,7].

With increased magnification, Figure 4a,b show that the undercoatings exhibit a compact surface with a relatively homogeneous microstructure composed of polyhedral grains, with the increase in the deposition time contributing to a visible increase in the grain size.

The black nickel film surfaces, as shown in Figure 4c,d, are also compact but consist of smaller grains with less defined polyhedral shapes, especially in the BuBN sample, which aligns with the nature of the undercoatings’ morphology and their impact on the formation of the black layer.

Additionally, Figure 4c,d depict the atomic concentration of nickel and oxygen on the black nickel coating, with a ratio of approximately 2:3 for Ni:O, suggesting the formation of Ni₂O₃.

Figure 5a,b show the reflectivity, total and diffuse, as a function of the wavelength for the undercoatings and for the black nickel films.

As expected, the bright nickel has higher total reflectivity than the dull nickel, which increases, in both cases, with the increase in the wavelength. In the case of the black films, the total reflectivity is almost wavelength independent. Nonetheless, the slightly higher values for the BuBN coating show that the undercoating’s influence on the surface morphology of the black layers impacts the reflectivity. Regarding the diffuse reflectivity, the increase in this component suggests a rougher and less uniform coating, meaning that the black films have a smoother surface than the undercoatings, as was already observed in the surface micrographs in Figure 4.

The CIE L*a*b* color coordinates of the undercoatings and double-layered nickel coatings were measured via reflectivity and the results are shown in Figure 5c,d.

When comparing the two undercoatings and the two double-layered nickel coatings, the a* and b* coordinates show small variations, with all four samples positioned in the same semi-quadrant.

The brightness, L*, ranges from 0, which corresponds to absolute black, to 100, which corresponds to absolute white. To ensure low reflectance and improve the absorbance in the solar spectrum wavelength, black surfaces are essential. Since both double-layered samples present L* values below 40, their surfaces are considered visually dark [22,23,24,25]. The positive influence of the undercoating can be seen, as the L* value for the dull sample (DuBN) is closer to absolute black, which might be beneficial for its solar absorbance characteristics.

Figure 6 presents the measured reflectance in the UV–visible–NIR region of the double-layered nickel coatings, with the total solar absorptance, α, being displayed in the inset.

The total solar absorptance of the coatings was determined according to Equation (4) from the overlap of the absorbance spectrum of the samples with the solar spectrum [4,26].

Using Kirchoff’s law, spectral absorptance α(λ) can be expressed in terms of the total reflectance R(λ) for opaque materials: α(λ) = 1 − R(λ) [26], where P_sun(λ) is the normal solar spectral irradiance defined by ISO standard 9845-1 (2022) for air mass (AM) 1.5 [27].

$$\alpha ={\displaystyle \frac{{{\displaystyle \int }}_{280 \mathrm{nm}}^{2000 \mathrm{nm}}\left[1-R\left(\lambda \right)\right]P_{sun}\left(\lambda \right) d\lambda }{{{\displaystyle \int }}_{280 \mathrm{nm}}^{2000 \mathrm{nm}}{P}_{sun}\left(\lambda \right) d\lambda }}$$

(4)

For both samples, it is observed that the reflectance is lower in the UV–visible range but begins to increase in the near-infrared region. Around 1400 nm, the reflectance of BuBN starts to decrease, while that of DuBN remains stable, at least until 1800 nm, where a band can be observed in both cases.

For optimum efficiency, a solar absorber should possess the maximum possible absorptance (or minimum reflectance) in the solar spectrum while showing a minimum infrared emittance. This may be achieved by using a so-called “selective” absorber coating, with the selectivity originating from a substance that exhibits optical properties that differ significantly depending on the spectral region [28].

Considering this, along with the fact that the DuBN sample exhibits higher total solar absorptance, it demonstrates more promising properties and has been selected to proceed with the incorporation of WS₂ nanoparticles.

3.2. Double-Layered Nickel Coatings with WS₂ Nanoparticles

The cross-sections of the double-layered nickel coatings with WS₂ nanoparticles, the samples DuBN_1.3 and DuBN_2.7, were analyzed by SEM, as shown in Figure 7. The estimated thicknesses changed from (0.6 ± 0.1) μm and (0.4 ± 0.12) μm, respectively.

The introduction of the WS₂ nanoparticles into the electrochemical bath led to a decrease in the black nickel film thickness, meaning that the deposition rate decreases, namely with the increase in these nanoparticles.

The XRD patterns of the double-layered nickel coatings with WS₂ nanoparticles are shown in Figure 8. As anticipated, only peaks corresponding to cubic copper, from the substrate, and cubic nickel, from the undercoating, were observed. Concerning the absence of the peaks related to WS₂, one possible explanation can be related to the very low amount incorporated.

Figure 9 displays the surface SEM micrographs of the double-layered nickel coatings with WS₂ nanoparticles, revealing that the deposits have no obvious pinholes or pits. The DuBN_1.3 sample presents visible and open grain boundaries, however, with the grains showing the same polyhedral structure identified in the corresponding black nickel film without nanoparticles. A closer examination reveals the presence of some cracks.

The DuBN_2.7 sample exhibits a morphology closely resembling that of DuBN, but with an increased grain size, indicating a greater influence of the undercoating morphology. This is consistent with the reduced thickness of the DuBN_2.7 sample.

The thickness and surface morphology of the black layers allow us to infer that in the sample with a lower nanoparticle content, there is a higher deposition rate compared to the DuBN_2.7 sample, which results in a less dense material that may be subject to stresses, causing the observed cracks. Additionally, the slightly greater thickness of the black layer helps to better mask the surface of the dull nickel. Conversely, in the sample with a higher nanoparticle content, as previously mentioned, the deposition rate decreases. The process is slower, which allows more time for the coating to organize and densify but results in a thinner layer where the morphology of the dull nickel is less effectively obscured.

Figure 10 presents the EDS spectra of the DuBN_1.3 and DuBN_2.7 samples.

The overall analysis indicates that the films contain a minor amount of chlorine, implying its incorporation during the electrochemical deposition of black nickel. Furthermore, as expected, the incorporation of WS₂ nanoparticles increases with their higher concentration in the electrochemical bath, as seen by the higher intensity W signal (note that the sulfur signal is overlapped in the EDS spectra with the W signal).

Figure 11 presents the elemental mapping of the element W for the DuBN_1.3 and DuBN_2.7 samples. Consistent with the information obtained from the EDS spectra, it is visible that the content of WS₂ nanoparticles in the deposited films increases according to their concentration in the electrochemical bath. Additionally, the nanoparticles are evenly distributed in both cases, without visible clusters or segregations.

Figure 12 shows the reflectivity, total and diffuse, as a function of the wavelength for the double-layered nickel coatings with WS₂ nanoparticles (and DuBN for comparison), as well as the CIE L*a*b* color coordinates.

The behavior of the DuBN_1.3 sample closely resembles that of the DuBN sample, which can be attributed to the lower content of WS₂ nanoparticles, combined with the higher thickness, when compared with the DuBN_2.7, allowing us to infer a reduced influence of the surface morphology.

Using the DuBN sample as a reference, with CIE L*a*b* color coordinates of 22.1, 0.66, and 2.38, respectively, the incorporation of WS₂ nanoparticles resulted in a slight decrease in brightness for higher NPs incorporation (DuBN_2.7 sample), showing that the incorporation of the nanoparticles can have a positive effect on the solar absorbance characteristics.

Figure 13 presents the measured reflectance in the UV–visible–NIR region of the double-layered nickel coatings with WS₂ nanoparticles (and DuBN for comparison).

For the DuBN_1.3 sample, it is observed that the reflectance is lower in the UV–visible range but begins to increase in the near-infrared region. Around 1500 nm, the reflectance stabilizes, at least until ≈1800 nm, where a band can be observed. This trend was already observed in relation to the DuBN film.

In the case of the DuBN_2.7 sample, the reflectance begins to increase in the high visible region, exhibiting a band centered at 1150 nm. This differing behavior could be attributed to a combination of factors, including the higher content of WS₂ nanoparticles, coupled with the reduced thickness. Thus, it seems that the incorporation of nanoparticles (especially for lower contents) can show improvements in the solar absorbance properties.

Previous studies, such as those conducted by Wäckelgård [9], have shown similar results when electrodepositing black nickel onto a copper substrate in a two-step procedure, reporting a total absorbance of 0.96. However, to achieve this outcome, the author had to interrupt the electrodeposition process to dry the surface in an N₂ gas flow before the second plating. Oskam et al. [4] reported a total absorbance of 0.94 for a bright/black nickel double layer electrodeposited on copper and 0.95 when the black nickel was deposited directly onto the copper substrate. However, in terms of this latter approach, delamination of the coating was pointed out.

The adoption of stainless steel as a substrate for black nickel deposition has also been explored. Gómez-Romero et al. [29] presented a total absorbance of 0.94 for a double layer of bright and black nickel electrodeposited onto an AISI316L stainless steel substrate. In this case, an intermediate step of drying with nitrogen was also required. The higher total solar absorbance reported by Oskam et al. [7], who electrodeposited a bright nickel interlayer and black nickel overlayer onto stainless steel, was 0.90. In this case, the substrates were activated by an anodic treatment followed by a cathodic treatment.

3.3. Conversion of Solar Energy into Thermal Energy

Studying the optical properties enabled the estimation of the total absorptance. However, to assess the conversion of solar energy into thermal energy, it is essential to analyze the heat transfer processes.

The results, displayed in Figure 14, show that the conversion of light into heat occurred, with the DuBN_1.3 sample exhibiting an improved performance. The slightly better absorbance of this sample results in the faster heating of the deionized water and a higher steady-state temperature.

4. Conclusions

This study investigated the development of black nickel coatings for solar collectors, focusing on the impact of a bright/dull nickel undercoat and the incorporation of WS₂ nanoparticles.

The double-layered nickel coatings were applied on a copper substrate, at room temperature, using two sequential electrodeposition processes.

The XRD analysis showed that the undercoatings are composed of cubic nickel, while the black coating exclusively consists of amorphous nickel compounds.

The findings indicated that the presence of a bright or dull nickel undercoat influences the morphology, brightness, and selective optical properties of the black nickel coatings, with the sample with a dull nickel undercoating showing higher total solar absorptance and being more promising for further enhancement.

The successful incorporation of WS₂ nanoparticles was confirmed by the EDS spectra, and the SEM-EDS mapping demonstrated their even distribution, without the presence of clusters.

The integration of WS₂ nanoparticles into the black nickel coatings reduced the thickness and uniformity of the black layer. However, the reflectance measurements indicated that, with a specific WS₂ content, an improved performance can be achieved in terms of the conversion of solar energy into thermal energy.