Characterization and Electrochemical Investigation of Heterogeneous Sb-Cu Coatings

Abstract

In recent years, Cu-Sb-based alloys have been considered promising materials for various green applications; for instance, they can serve as catalysts for CO₂ reduction, anodic material for metal-ion batteries, and a light-absorbing layer in solar cell. Our research is focused on the electrodeposition of antimony-copper alloy layers with copper content between 18–30 wt.%. The layers were observed by scanning, atomic force and transmission electron spectroscopies. Their phase structure, roughness, and hardness were also determined. The electrochemical properties were investigated by using electrochemical impedance spectroscopy and potentiodynamic polarization tests in 0.5 M sulfuric acid. Decreasing the copper content leads to an increase in chemical and morphological heterogeneity and an increase in the roughness from 1.74 to 2.94 μm due to the separation of antimony crystal clusters protruding above the surface but does not significantly affect the hardness and electrochemical behavior of the layers. Copper is recorded only in the form of the Cu₂Sb phase. The corrosion current density and potentials of the layers are close to those of pure antimony (about 20 µA cm⁻² and about −30 mV vs. Ag/AgCl). The highly porous structure obtained after anodic polarization seems suitable for catalytic purposes.

1. Introduction

The Cu-Sb-based materials are successfully entering the realms of the green and renewable energy technologies. They are effective as catalysts for electrochemical CO₂ reduction, achieving a good CO selectivity of 85%–90% [1,2,3,4,5,6], as well as enhancing the photoelectrocatalytic performance of BiVO₄ for photoelectrochemical splitting water technology [7]. A Cu₂Sb layer was tested as a sensor layer catalyzing the oxidation of antibiotic drugs [8]. Antimony-copper alloys are used as a precursor for the fabrication of ternary Cu-Sb-S and Cu-Sb-Se semiconductor layers for electronics and solar cells [9,10,11] and as additives to lead-free solders [12]. In the last ten years, they have been intensively investigated as an anode material for metal-ion batteries [13,14,15,16,17], as well as a cathode for liquid-metal batteries [18].

Different engineering approaches are used to achieve a high surface area and increasing porosity for Sb-Cu-based materials developed for catalytic purposes and for the intercalation of metal ions (Li⁺, Na⁺, and K⁺). In this regard, the main approaches are (i) the sintering of pure Sb and Cu nanoparticles to a highly porous structure [4,16]; (ii) the potentiostatic electrodeposition of a Zn-Sb alloy and subsequent dissolution of the zinc to formation of meso- and micropores [15]; the (iii) growth of Cu-Sb nanowires on a nickel supporting layer, using a nanoporous template [14]; and (iv) hierarchical porous Sb film on a three-dimensional (3D) Cu substrate [15].

In metallurgically obtained alloys, the solubility of antimony in the copper phase is 7.8 at.% at 645 °C and decreases with the lowering of the temperature [19,20]. In a Cu-Sb bimetallic system, the content of copper is higher than that of antimony in all intermetallic phases. At a content level of more than 32 at.%, the antimony separates into a single phase [19], and the copper is entirely in the form of a Cu₂Sb phase. The addition of copper to antimony lowers the melting temperature, reduces the cost, and extends the cycle life of the anode in a metal-ionic battery, thus stabilizing the structure [18]. Additionally, various Cu-Sb alloys and intermetallic compounds are synthesized via the microwave-assisted solvothermal method [4], thermal evaporation of Cu and Sb metallic stacks [9], hydrothermally from CuCl₂ and SbCl₃ [8], ionic-liquids-based synthesis of Cu₂Sb [21], co-reduction of Cu²⁺ and Sb³⁺ using a NaBH₄ solution in an ice bath [6], and combination of electroless deposition and laser ablation of solids in liquids to synthesize nanocomposite thin films [22]. Compared to the methods listed above, electrochemical deposition is a cheap and simple method that allows for precise control of the composition and morphology of the copper–antimony layers. A wide range of structures, such as thin films [3,23,24], dendrites [10,25,26], nanosized particles [7,27], and nanowires [14], have been obtained via electrodeposition.

Control of the layer composition is most often performed by varying the copper ion concentration in the electrolyte [7,24]. There is a wide variety of electrolytes in the literature, in which SbCl₃ [3,7,24,27,28] and Sb₂O₃ [14,26] are most often used as a source of Sb³⁺. The copper (II) ions are added in the electrolyte in the form of CuSO₄ [24], Cu(NO₃)₂ [3,14,26], and CuCl₂ [7,27]. The type of substrate also affects the processes of electrochemical co-deposition of copper-antimony [29]. To obtain Cu-Sb alloys, a combination of electrodeposition with thermal fusion was also used. In this case, an antimony layer is initially deposited onto a copper substrate or successive alternation of electrodeposited Cu and Sb layers, followed by the heat-treating of films [10,13]. However, the electrodeposition of copper–antimony alloys is rarely reported; only a few of the cases reported are for alloys with more than 50 at.% antimony and in the form of nanoparticles [7]. It is interesting to note that various nanostructured Sb/Sb₂O₃ composites in the form of a flower-like Sb deposits and oxygen-rich bottom layer were obtained from the antimony electrolyte [30,31,32].

The Cu-Sb alloys obtained through the use of different methods are well characterized in terms of composition, structure, and physical properties, with respect to their future applications. In this regard, the electrochemical study of Cu-Sb materials is oriented towards testing their electrochemical activity and stability in the expected operational environment, such as organic solutions of alkali metal ions during the development of Me-ion battery anodes, CO₂-saturated carbonate solutions when studying the electroreduction of CO₂, Na₂SO₄ electrolyte for determining the photoelectrochemical activity, etc. The anodic behavior of antimony-containing alloys in sulfuric acid environments has been studied in relation to their use as a positive grid in lead-acid batteries and the formation of antimony oxide with various applications (electro- and photocatalysts, thin capacitive layers, and electrochromic displays) [33,34,35]. In terms of corrosion resistance, it has been found that alloying antimony with other elements, such as copper and tin, leads to a deterioration in corrosion resistance. For example, Gulevskii et al. showed a 10-times-higher corrosion resistance than pure antimony in environments containing chloride ions, such as 5% NaCl and 10% HCl [36]. Ninomiya et al. studied dissolution and passivation of Cu-5 wt.% Sb anodes under galvanostatic anodic polarization in aqueous H₂SO₄-CuSO₄ electrolytes in connection with the electrorefining of Cu [20]. They found that the anodic dissolution of Cu generated an Sb-rich compound layer on the anode surface, accompanying the formation of pits. However, the electrochemical behavior of antimony–copper alloys in sulfuric acid environments has not been fully investigated.

In this study, the galvanostatic electrodeposition of antimony-rich Sb-Cu layers with chemical and morphological heterogeneity was performed. The influence of the composition of the coatings on their structure, hardness, roughness, and electrochemical behavior in 0.5 M sulfuric acid was investigated. The electrochemical results were compared with those for pure antimony and copper metals.

2. Materials and Methods

An electrolyte containing 0.016 mol/L CuSO₄, 0.12 mol/L K(SbO)C₄H₄O₆, 0.28 mol/L methanesulfonic acid (CH₄O₃S), and 0.64 mol/L tartaric acid (C₄H₆O₆) was used for the electrodeposition of the antimony-copper alloy coatings. Tartaric acid was used as an additive to increase the solubility of the antimony salt. We used methanesulfonic acid as an alternative to sulfuric acid. In addition to high conductivity, this acid is biodegradable and safer to use [37]. Moreover, the overvoltage of copper deposition in methanesulfonate electrolyte is significantly lower, and copper coatings have finer crystallites compared to coatings deposited from sulfate electrolyte [38]. Electroplating was performed at a cathodic current density of 0.4–0.7 A dm⁻² in a two-electrode electrochemical cell on a platinum or a brass cathode and an inert platinum anode. No stirring was applied. The layers were obtained at 15 C cm⁻² quantity of electricity, at which a thickness of about 10 µm was achieved. The X-ray fluorescence (XRF) analysis was used to determine the average composition and thickness of the resulting layers. It was performed on a Fischerscope X-RAY XDAL.

The electrochemical behavior of the layers was investigated via electrochemical impedance spectroscopy (EIS) and the potentiodynamic polarization method. The EIS tests were performed after 10 min of open-circuit potential (OCP) stabilization in the corrosion environment at a potential amplitude of ±10 mV in the frequency interval from 0.1 to 10⁵ Hz. Polarization tests were started immediately after receiving the EIS data at a potential scan rate of 1 mV s⁻¹ in the anodic direction, from −0.25 V vs. OCP to 1.0 V vs. Ag/AgCl.

The electrochemical studies of the Sb-Cu layers were carried out in 0.5 M sulfuric acid at room temperature, without stirring. The results were compared with those obtained under identical conditions for antimony (99.999% polycrystalline metallic Sb) and copper (99.5%) electrodes. A three-electrode electrochemical cell with a 1 cm² area working electrode, a platinum mesh counter electrode, and a silver chloride reference electrode (Ag/AgCl/3.0 M KCl with potential vs. standard hydrogen electrode of E_Ag/AgCl = 0.210 V_SHE) were used. All potential values in this study are presented versus a Ag/AgCl reference electrode. All solutions were prepared from analytical-grade reagents and monodistilled water. The electrochemical equipment consisted of Autolab potentiostat/galvanostat PGSTAT302N with an FRA32M module and controlled by NOVA 2.1.4 software (Metrohm, Herisau, Switzerland).

The surface of the alloy layers before and after the corrosion tests was observed via scanning electron microscope (SEM) Tescan LYRA (TESCAN, Brno, Czech Republic) equipped with an energy-dispersive X-ray (EDX) analyzer Bruker (Bruker AXS GmbH, Karlsruhe, Germany), and atomic force microscope (AFM) Veeco-Nanoscope III (Veeco Instruments Inc., NY, USA). The microhardness HV was measured at a set load of 20 gf, a time to reach the load of 10 s, and a hold time under load of 10 s (MicroDuromat microhardness tester Reichert-Jung, Leica Microsystems AG, Wetzlar, Germany). The mean roughness depth (R_Z) and the arithmetical mean roughness (R_a) were measured with a Perthen profilometer (Mahr, Goettingen, Germany) on 4 samples with layered spiral structures (PPVS) in 5 sections of the total measurement length of the coating.

The crystal structure of the Sb-Cu layers was investigated using an XRD PANanalytical Empyrean Pixel 3D multichannel detector (Malvern Panalytical, Malvern, UK) at Cu Kα. The diffractograms were obtained in the interval of 10–100° 2θ, with an accuracy of 0.01°. Laser ablation of a SbCu18 sample was performed in aqueous medium, with a power of 2 kW and frequency of 15 pps, using an ND: YAG pulsed laser, Continuum Surellite. Nanoparticles were separated from the obtained suspension and observed using transmission electron microscopy (TEM HR STEM JEOL JEM 2100, equipped with a CCD camera GATAN (JEOL Ltd., Tokyo, Japan).

3. Results

3.1. Electrodeposition of Sb-Cu Layers

The potential-time dependencies in the galvanostatic deposition of alloy coatings on a platinum substrate are presented in Figure 1. The galvanostatic dependencies are characterized by an initial sharp decrease in cathodic polarization. It can be seen from Figure 1 (inset) that two minima are observed in the first 10 s of electrodeposition. The higher the applied current density, the more pronounced and closer the minima are. The steady state is reached after about 20 s. The course of potential–time dependence is presented schematically in Figure 1b. It can be assumed that the copper, as a more noble metal, is deposited first on the Pt surface (Figure 1b, stage 1—nucleation of Cu nuclei on Pt).

The initiation of the copper deposition process can also be related to the high overpotential of antimony deposition on a foreign substrate [39]. Gradual coating of the surface with copper lowers the deposition overpotential (Figure 1b, stage 2—growth of a thin Cu layer). After the platinum surface is completely coated, the copper deposition is realized at the potential of stage 3. Since the concentration of copper ions is very low (0.016M Cu²⁺), they are depleted near the electrode, once again increasing the cathodic polarization (Figure 1b, stage 4—depletion of Cu²⁺ ions) until conditions for antimony co-deposition are created (Figure 1b, stage 5—start of Sb co-deposition). The process shows a tendency to become stationary after less than 1 min (Figure 1a,b, stage 6—steady-state alloy deposition). Increasing the cathodic current density did not significantly affect the steady-state value of the alloy deposition potential, and after 15 min, the difference is below 15 mV.

The electrodeposition process, as well as the structure of the resulting layers, is analogous to brass, which is why it was used as a substrate in all subsequent tests to characterize the layers.

3.2. Characterization of Sb-Cu Layers

The composition, microhardness, and roughness of the Sb-Cu alloy coatings depending on deposition rate are presented in Table 1. It should be emphasized that the composition in Table 1 (as determined by XRF) is averaged both over the entire electrode surface and over the depth of coatings. The thickness of all the obtained layers is about 10 µm, but as the deposition rate increases, not only their composition but also their surface morphology change. Increasing the cathode current density from 0.4 to 0.7 A dm⁻² decreases the copper content linearly by about 16% (from 30 to 18 wt.%). This trend is even more pronounced in terms of the mean roughness depth (R_Z), which increases by nearly 70% for the same range of current density. However, the arithmetic mean roughness (R_a) increases only for alloys SbCu21 and SbCu18. The microhardness is not significantly affected by the deposition rate but remains about 40 kgf/mm² higher than that measured for the brass substrate (160.95 kgf/mm²).

More complete information on the topography of the maximum roughness coating was obtained via AFM on the roughest layer. The AFM images presented in Figure 2 reveal a wavy structure of the SbCu18 coating with two step lengths. Sharper peaks are spaced at a distance of about 2–3 µm (Figure 2a). In the intermediate zones, the topology is also wavy, but with less pronounced peaks, located at a step of about 0.8 µm (Figure 2b). The AFM imaged roughness data show a maximum roughness of 1.5–2.0 μm, which is consistent with the results of the microroughness test (Table 1).

Visually, all layers are silvery in color, and as the copper content decreases, the surface becomes matte. Figure 3 presents SEM images of the top surface of Sb-Cu alloy coatings. The richest-in-copper layer, the SbCu30 layer, is bright and smooth, with no visible defects from hydrogen evolution and inclusions in the layer (Figure 3a). As the copper content decreases to 25.8 wt.% various heterogeneous areas gradually begin to form barely noticeable wavy matting (Figure 3b). Well-defined hilly structures with a tendency to spiral formations are observed on SbCu21 and SbCu18. Thus, the layer with minimum copper content demonstrates a well-noticeable relief (Figure 3d), with distinct crystalline formations in the convex areas (Figure 3e). The crystals obtained in these zones strongly resemble the layers obtained by Lin et al. via the potentiostatic electrodeposition of antimony in H₂SO₄-NH₄F-SbF₃ solutions [40].

The SbCu18 layer, which has the minimum copper content and most pronounced heterogeneity, was also observed in cross-section view (Figure 3f). The SEM image reveals a layered structure, which indicates an inhomogeneous distribution of elements in the coating depth, too.

Based on the SEM images in backscattered-electron (BSE) mode, it can be assumed that the observed inhomogeneity is the result of a different distribution of the alloying elements. An EDX point analysis was performed to determine the copper and antimony content in different points of the SbCu18 surface. Protruding fine crystals were found to have an average antimony content of over 90 wt.% (Figure 3e, point 1); meanwhile, in the adjacent smoothed concave valleys, the copper content reached 16 wt.%. Therefore, the EDX point analysis of the outer layers revealed that they are poorer in copper than the average content (Table 1). This result is also observed at an EDX cross-section of the SbCu18 layer (Figure 3f), where the composition in the middle of the layer corresponds to that presented in Table 1 (Spectrum 1). At the same time, an increase in copper intensity (red line) can be observed at the interface with the substrate.

Fine cracks are observed on all layers (Figure 3). The cracks are probably the result of strong intrinsic tensile stresses generated by the high overpotential during electrodeposition. Obtaining Sb-Cu coatings with a high antimony content requires a high overpotentials. According to the theory of island growth in electrodeposition, these operating conditions lead to small-sized metal nucleus [41], with high surface free energy at the grain boundaries [42]. The stresses generated by crystallite coalescence are maintained during subsequent film growth. However, based on the layered structure of the coatings, it can be assumed that the tensile stress is stronger and localized in the outermost antimony-rich layers. As a result, the cracks do not reach the substrate and are mainly confined to the surface. This was observed in our previous studies [43].

An XRD analysis provides more detailed information about the crystallographic structure and chemical composition of the layers. The diffractograms presented in Figure 4 reveal only reflexes of the antimony (JCPDS No. 35-0732) and the phase Cu₂Sb (JCPDS No. 65-2815). The peaks of pure antimony at 28.8, 40.1, 42.2, 49.1, 51.9, 59.6, and 68.8 2θ corresponding to crystallographic planes (012), (104), (110), (006), (202), (024), and (122) are observed [15,18,44]. Characteristic copper reflexes were not recorded. This element is represented only as the phase Cu₂Sb at 26.6°, 31.6°, 34.9°, 43.6°, and 54.9° 2Theta [27,45]. This corresponds to the metallurgically obtained copper–antimony phase diagram, according to which, at an antimony content above 33 at.% (48.55 wt.%), it forms a pure Sb phase [19,46]. The XRD results show an enrichment of the outer layers with antimony. For example, for layer SbCu18, a composition of 73 wt.% Sb phase and 27 wt.% Cu₂Sb phase is determined, corresponding to 86.2 wt.% Sb. This is, on average, about 4% Sb above the average layer composition (Table 1), which supports the conclusion from the galvanostatic curves of increased copper content at the substrate–coating interface.

From X-ray diffraction peaks, the mean crystallite sizes of Sb and Cu₂Sb phases are calculated using the Scherrer equation, and they are presented in Table 2. The Scherrer constant is taken as 0.9. The obtained results show a gradual increase in the crystallite size with an increase in the cathodic current density during electrodeposition.

Together, the results of the EDX and XRD analyses clearly show that the Cu₂Sb phase is located in the smoother concave regions of the surface, while the convex crystalline regions are composed of pure antimony phase. The recording of small amounts of copper (around and below 10 wt.%) in the convex areas is probably a result of a signal from the copper-rich underlayer.

Laser ablation was used to fabricate nanosized particles of the Sb-Cu layer for TEM analysis. During the irradiation with the high-energy laser beam, intermetallic nanosized particles were expected to separate from the surface, whose structure and composition could be determined. The nanoparticles from the suspension were observed via TEM. The obtained images clearly demonstrate the formation of individual and aggregated antimony and Cu₂Sb nanoparticles (Figure 5a). The particle size varies from a few nm to 150 nm. The high-resolution TEM revealed that the well-ordered antimony lattice fringes with spacing distances of 0.31 nm might be attributed to the (012) planes of rhombohedral Sb (Figure 5b). The distance between the adjacent lattice plane in Cu₂Sb nanoparticles was measured to be about 0.61 nm, and it is associated with the tetragonal structure [8]. Therefore, laser ablation is a suitable method to obtain a mixture of Sb and Cu₂Sb nanoparticles, which could be an attractive material for incorporation into nanocomposite layers to impart functional properties [22].

3.3. Electrochemical Investigation of Sb-Cu Layers in Sulfuric Acid Solution

The electrochemical behavior and corrosion resistance of antimony and its alloys have most often been studied in sulfuric acid solutions [20,33,34,35,47,48,49,50]. For this reason, we chose 0.5 M H₂SO₄ as an electrolyte when studying the electrochemical behavior of Sb-Cu layers. That gives us the opportunity to compare the obtained results with those of other research groups. Additionally, in these studies, we also tested reference samples of copper and antimony.

First, the time dependence of the open-circuit potential (OCP) was monitored (Figure 6). The OCP for all Sb-Cu layers starts in a narrow range of potentials, from −0.01 V to 0.04 V. After an initial decrease in the OCP of about 20 mV, the potentials show a tendency to stabilize within 5 min. The copper and antimony electrode potentials after 10 min are slightly more positive than those of the alloy layers (0.03 V vs. Ag/AgCl).

Electrochemical impedance spectroscopy (EIS) was used to further electrochemically characterize the layers at the open-circuit potential. Figure 7 presents the Nyquist and Bode plots of the four alloy coatings compared to the data for copper and antimony electrodes. Nyquist plates have a form of a depressed capacitive incomplete semicircle, which is characteristic for a copper electrode in an aerated solution of 0.5 M H₂SO₄ [51]. There is no clear tendency for the variation of the diameter of the capacitive semicircle of the Nyquist plots depending on the content of the alloying elements (Figure 7a). However, it is observed that those for antimony and the layer with the greatest smoothness (SbCu30) are of similar diameter. The main difference in the course of the Nyquist plots of pure antimony and those of the alloy layers is that, for Sb-Cu surfaces, at low frequencies, their course deviates from the typical capacitive semicircle. The recorded deviation is most visible in the Nyquist plots of SbCu30 and SbCu18, and at low frequencies, the dependence remains almost horizontal on the abscissa. This could be attributed to a barrier at the metal–electrolyte interface of corrosion products that are insoluble in acidic media. The barrier layer introduces additional resistance into a system and a charge at the solid–electrolyte interface.

The Bode impedance plots for layers with a copper content of 18%–26% coincide with weak deviations being observed only in the low frequency range (when the electrochemical reaction occurs), where the impedance values for these layers are between 400 and 600 Ω at 0.1 Hz (Figure 7b). The behavior of the SbCu30 layer, characterized by the smoothest surface, is intermediate between that of the alloy layers and the reference electrodes. At low frequencies, it is closer to the response of Sb and Cu electrodes, and at high frequencies, it coincides with that of Sb-Cu layers with pronounced morphological heterogeneity.

The deviation of the phase angle to −10° in the lower frequency region demonstrates the dominating kinetic control of the reaction. The maximum negative value of the phase angle for all alloy layers and for the antimony electrode is around 20 Hz, while for the copper electrode, the maximum is shifted visibly to higher frequencies.

The best fit of the Nyquist and Bode plots was obtained using the equivalent circuit (1) presented in Figure 7d. A circuit is denoted by the circuit description code [R_s(Q_ox[R_ox(Q_dlR_ct)])], where R_s is the solution resistance, and Q_ox and R_ox are the corrosion product layer capacitance and resistance, respectively. The symbols R_ct and Q_dl are the charge transfer resistance and double-layer capacitance, respectively. Both the capacitances (of the layer of corrosion products and of the electrical double layer) are expressed by a constant phase element, since the maximum of log|Z| = f(log f) is below −80°; then, Q_ox and Q_dl are far from an ideal capacitor, and the values of n are very low (Table 3). The behavior of the antimony electrode is described by the simpler equivalent circuit (2) of Figure 7d. This equivalent scheme was used in the active dissolution of antimony [50].

The validation of the EIS data was performed with the Kramers-Kronig test, and the results are presented in the last column of Table 3.

Figure 8 presents the polarization dependencies in 0.5M H₂SO₄ of Sb-Cu alloy layers with various levels of copper content, as well as of the copper and antimony electrodes used as a reference sample. The corrosion potential (E_corr) and corrosion current density (J_corr) determined from the polarization dependencies are summarized in Table 4. The course of the polarization dependences for the alloys is similar. In the cathode region, they are similar to the copper electrode, starting with a steep region corresponding to the hydrogen reaction. At weaker cathodic polarization, the dissolved oxygen reduction reaction and the associated low-slope cathodic region dominate [51,52]. On the surface of the antimony electrode, the cathodic reaction is only associated with the hydrogen evolution.

The corrosion potential values for all tested layers are in a relatively narrow range (from −0.041 to 0.021 V) and fall between those determined for antimony and copper, respectively, −0.080 V and 0.013 V. A similar observation was made about the corrosion current densities. There is no clear tendency for the influence of copper on the values of these characteristic parameters. However, it can be noted that the E_corr for the SbCu30 layer is the most positive, the J_corr is the lowest, and the values are closest to those of the copper electrode. This is probably a result of both the highest copper content in this layer and the lowest surface roughness. The fast increase of the anodic current after the corrosion potential is due to an active dissolution of copper and SbCu30 at a slightly positive polarization (around 0.08 V). These results are consistent with the literature data [51,52].

At a low anodic polarization, three weakly expressed steps are observed on the polarization dependencies, starting at potentials of 0.054, 0.112, and 0.182 V. They are related to the transition of antimony through various oxo and hydroxo forms of Sb³⁺ with different solubilities, such as (SbO)_ads, (SbOH)_ads, and SbO⁺ and with gel-like layers formed on the antimony’s surface [33,53,54]. Despite the enrichment of the near-electrode layer with slightly soluble corrosion products and the formation of different permeable layers, the surfaces are not passive. This confirms the EIS results for the phase angle at low frequencies (Figure 8) for the absence of a passive layer and a kinetically controlled response. A more pronounced diffusion-limited process starts at a potential above 0.3 V, where the current densities for antimony and Sb-Cu alloys tend to stabilize at values around 10 mA cm⁻².

At a high anodic potential, approximately 0.6 V vs. Ag/AgCl, the current density for the Sb-Cu layers decreases slightly, and thus decrease is related to the formation of Sb₂O₅ in a highly acidic environment [20]. Sb₂O₅ is the thermodynamically stable form of antimony in the presence of oxygen. Under highly oxidizing conditions in very acidic solutions, Sb₂O₃ oxidizes to Sb₂O₅. The presence of Sb₂O₅ on the surface of Sb-containing steels further inhibits the anodic reactions, thus reducing the corrosion rate [55].

3.4. Sb-Cu Surface Observation after Polarization Test in Sulfuric Acid Solution

After anodic polarization to 1 V in 0.5 M H₂SO₄, the surface of the alloy layers appears to be attacked primarily in the antimony-rich regions (Figure 9). In layers SbCu21 and SbCu18, the initial surface heterogeneity remains clearly distinguishable (Figure 9b,d). At a higher magnification, however, the antimony crystals that are clearly visible before the polarization test are no longer observed. The convex areas of the Sb phase appear to be preferentially attacked to form a highly porous structure; meanwhile, in the concave areas, where the Cu₂Sb phase dominates, the surface appears to be less affected (Figure 9c). This result demonstrates that antimony dissolves preferentially and mostly at the edges of the crystals, leading to the flattening of macro-asperities and the appearance of micro- and submicrosized pores. Ninomiya et al. report that during linear sweep voltammetry measurements in the anodic polarization of a Cu-5 wt.% Sb alloy, the Cu-Sb phase remains during Cu dissolution [20]. As a result, segregated Sb-rich regions are formed on the surface, accompanying the formation of pits [20]. Comparing the cited results with our observations, it can be assumed that the Cu_xSb phases are more resistant to anodic polarization than the pure Cu or Sb metals.

However, the dissolution of antimony is not complete. The raised zones remain antimony-rich, which can be observed in the EDX map of a helical section of the SbCu18 layer (Figure 9d). Moreover, oxygen was also recorded in these areas, thus supporting the results of the electrochemical tests for the formation of various oxo and hydroxo compounds of antimony in sulfuric acid medium. The element sulfur was not detected during this analysis. We assume that the obtained nanoporous surface is covered with a thin oxide layer and would exhibit similar properties as the Sb/Sb₂O₃ composites obtained by other researchers and intended as an anode material for Na-ion batteries [56,57,58].

4. Conclusions

This study involved the electrochemical deposition of Sb-Cu coatings with copper contents from 18 to 30 wt.%. The composition of the layers was controlled by the applied cathodic current density. As the copper content decreased from 30% to 18%, an increase in the mean roughness depth from 1.7 to 2.9 μm was observed as a result of the separation of a crystal Sb phase protruding above the remaining surface. In all tested layers, the copper was entirely in the Cu₂Sb-phase form. The coatings have a layered structure with a decreasing copper content in the outer layers.

The SbCu30 coating has the most positive open-circuit potential, as well as better corrosion resistance in 0.5 M sulfuric acid, as a result of both the highest copper content and the lowest surface roughness. However, the electrochemical behavior of the Sb-Cu layers remains close to that of the antimony and copper.

In anodic polarization, as a result of selective dissolution of antimony under a layer of oxide corrosion products, Sb-Cu coatings with a highly developed porous surface were obtained. This morphology seems suitable for applications such as self-supported catalytic layers, as well as a binder-free electrode for sodium storage. Sb and Sb-Cu nanoparticles obtained by laser ablation can be used to produce functional nanocomposite materials and solder paste additives.

Author Contributions

Conceptualization, B.T. and V.K.; investigation, V.K.; data curation and writing, B.T.; funding acquisition, V.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

We acknowledge the Institute of Physical Chemistry at the Bulgarian Academy of Sciences and especially Tsvetina Dobrovolska and Georgi Avdeev for the technical support in obtaining some of the presented results.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Potential-time dependence of the galvanostatic deposition of (a) Sb-Cu alloy coatings at a range of cathodic current densities 0.4–0.7 A dm−2 on a Pt electrode; (b) schematic representation of potential-time dependence at Sb-Cu alloy deposition.

Figure 2. AFM images of SbCu18 surface of an area with a side length of (a) 10 µm and (b) 1.2 µm.

Figure 3. SEM images in BSE mode of (a) SbCu30, (b) SbCu26, (c) SbCu21, (d) SbCu18 low magnification, and (e) SbCuSb18 high magnification with EDX point analysis; and in SE mode of (f) SbCu18 cross-section with EDX line analysis of Cu (red), Sb (green) and C (blue).

Figure 4. XRD patterns of Sb-Cu layers.

Figure 5. TEM image of nanosized particles obtained after laser ablation of a Sb-Cu layer: (a) aggregated Sb-Cu2Sb nanoparticles and (b) Sb nanoparticle.

Figure 6. Open-circuit potential versus time for Sb-Cu layers in 0.5 M H2SO4.

Figure 7. EIS results for Sb-Cu layers: (a) Nyquist plots, (b) Bode magnitude plot, (c) Bode phase-angle plots (same legend), and (d) equivalent circuits used to fit EIS data.

Figure 8. Potentiodynamic dependencies of Sb-Cu alloy coatings in 0.5 M H2SO4.

Figure 9. SEM images in BSE mode of Sb-Cu surfaces after polarization test: (a) SbCu26; (b) SbCu21 low magnification; (c) SbCu21 high magnification; (d) SbCu18 low magnification; and (e) SbCu18 high magnification and EDX maps of SbCu18 layer.

Table 1

Average values of copper content, roughness, and microhardness in Sb-Cu alloy layers obtained at cathodic current densities from 0.4 to 0.7 A dm⁻².

Layer Abbreviation	SbCu30	SbCu26	SbCu21	SbCu18
Cathodic current densities, A dm−2	0.4	0.5	0.6	0.7
Cu, wt.% (at.%)	29.5 (44.5)	25.8 (40.0)	20.6 (33.2)	17.9 (29.4)
Microhardness, kgf/mm2	201.3	204.56	197.21	214.28
RZ, µm	1.74	1.97	2.44	2.94
Ra, µm	0.364	0.338	0.418	0.506

Table 2

The mean crystallite size of Sb-phase and Cu₂Sb, as estimated by the Scherrer equation.

Coating	Sb Phase (nm)	Cu2Sb Phase (nm)
SbCu30	20.2	20.9
SbCu26	25.4	31.2
SbCu21	31.2	32.6
SbCu18	37.6	33.8

Table 3

Equivalent electrical circuit parameters for Sb-Cu alloy layers at OCP in 0.5 M H₂SO_4.

Sample	RS (Ω)	Rox (Ω)	Qox (×10−6 Ω−1 sn)	n ox	Rct(kΩ)	Qdl(×10−6 Ω−1 sn)	n dl	χ 2
SbCu30	4.92	790	179	0.89	1.06	876	0.62	8.93 × 10−6
SbCu26	1.63	310	359	0.94	0.299	1090	0.69	3.32 × 10−6
SbCu21	2.89	14.8	246	0.97	0.392	746	0.51	2.13 × 10−7
SbCu18	2.98	48.5	307	0.98	0.676	1140	0.50	3.69 × 10−7
Cu	4.77	26.3	24.1	0.92	1.22	195	0.49	9.21 × 10−6
Sb	5.53	7.32	124.4	0.70	1.68	72.4	0.80	3.10 × 10−4

Table 4

Corrosion potential and corrosion current density for alloy layers tested in 0.5 M H₂SO₄.

Sample	Ecorr, V vs. Ag/AgCl	Jcorr, µA cm−2
SbCu30	−0.008 ± 0.044	9.50 ± 2.2
SbCu26	−0.023 ± 0.018	25.2 ± 5.1
SbCu21	−0.012 ± 0.004	12.1 ± 1.5
SbCu18	−0.021 ± 0.011	23.2 ± 9.6
Sb	−0.080 ± 0.007	23.0 ± 2.7
Cu	0.013 ± 0.005	5.44 ± 1.0

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