Effects of Capping Agents on Shape, Stability, and Oxygen Evolution Reaction Activity of Copper Nanoparticles

Here, five different capping agents’ polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and oleylamine (OAm) are used to form copper nanoparticles (Cu-NPs), along with their electrocatalytic oxygen evolution reaction (OER) properties. The produced Cu-NPs’ mono-dispersity and their interactions with PEG, CTAB, PVA, PVP, and OAm were examined. The mean particle size determined by TEM images for the uncapped Cu-NPs as generated, PVA-capped, PVP-capped, CTAB-capped, PEG-capped, and OAm-capped samples, respectively, were 2.71 nm, 2.22 nm, 3.10 nm, 3.31 nm, 1.49 nm, and 1.71 nm. Greater than prepared catalysts, commercial Pt/C has a 3.7 nm size. It’s interesting to note that the CTAB-capped Cu-NPs considerably exhibit the strongest OER activity, with a very low overpotential of 222 mV at 10 mA cm−2, lower than many previously reported and the high electrocatalytic activity for OER of the commercial RuO2 catalysts. However, an expedited stress experiment shows that the CTAB-capped NPs have greater structural stability during electrochemical cycling. Their strong capping tendency and particle shape are the key causes of this. The approach for creating Cu nanocatalysts described in this paper has several potential uses for the creation of bimetallic catalysts.

Oxygen evolution reaction (OER) is a sluggish four-electrontransfer process, and since it requires a significantly larger overpotential to drive it, it loses energy at a much faster pace. OER is consequently believed to be the bottleneck in water electrolyzers. [1][2][3] Additionally, metal-air batteries, regenerated fuel cells, and carbon dioxide electrolyzers utilize the oxygen evolution process in anodes. 4 Therefore, for these electrochemical energy devices, the development of efficient OER electro-catalysts is essential. 5 Precious metal oxide catalysts, such as RuO 2 and IrO 2 , are considered the industry standard for OER because of their high reactivity and stability. [6][7][8] Since their limited reserves and high-cost limit their large-scale application, research has been done to identify their substitutes, which are Earth-abundant, affordable, and high activity. [9][10][11] Recently, transition metal oxides have drawn a lot of interest and demonstrated good OER performance, including those formed of Mn, Fe, Co, and Ni. [12][13][14] Among these transition metal oxides that have been studied, Cubased catalysts have the greatest alternative to replace all such precious metals in the upcoming years because of their affordability, extraordinarily large specific surface area, superior electrical properties, sufficient stability in harsh conditions, yield great, incredible compositional characteristics, higher reliability against electron transfer, high surface to volume ratio, as well as a good number of surfactant molecules. [15][16][17][18] How these catalysts operate in response circumstances is yet not well understood. When assessing the efficacy of such catalysts, it is important to consider how stable they are under reactional conditions, especially when there is a chance that the presence of gases will cause segregation, [19][20][21] which will alter the surface's composition and structure and possibly cause the catalyst to lose its catalytic activity. [22][23][24][25] Capping agents are essential because they serve as stabilizers during colloidal production, preventing nanoparticle aggregation and coagulation and limiting nanoparticle growth. [26][27][28][29] The capping ligands stabilize the contact between the nanoparticles and the preparation medium. [30][31][32] Certain structural features of nanoparticles are caused by the capping on their surface. The stabilizing compounds are essential for altering the action. [33][34][35] The steric effects of capping agents placed on the surface of nanoparticles are what produce these changing physical and chemical characteristics. [36][37][38][39] In this study, five different capping agents were used to form copper nanoparticles (Cu-NPs), along with their electrocatalytic OER properties. The CTAB-capped Cu-NPs considerably exhibit the strongest OER activity, with a very low overpotential of 222 mV at 10 mA cm −2 , lower than many previously reported, and the high electrocatalytic activity for OER of the commercial RuO 2 catalysts. The approach for creating Cu nanocatalysts described in this paper has several potential uses for the creation of bimetallic catalysts.
Materials characterization.-The field emission scanning electron microscopy (FESEM, Hitachi S-4800). Transmission electron microscopy (TEM, JEM-2100F) was used to clarify the microstructure of the electrocatalysts. X-ray powder diffraction (XRD, D8 Advance X-ray diffractometer) measurements were used to determine the electrocatalysts' crystal structures. The materials' X-ray photoelectron spectroscopy (XPS) was examined using the ESCALAB-250Xi. The specific surface area and pore size distribution of the samples were measured by the Quanta chrome gas adsorption analyzer (USA).
Electrochemical measurement.-The electrocatalytic measurements were carried out with the CHI760E and 1.0 M KOH as the electrolyte in a conventional three-electrode system, in which the catalysts as prepared were used directly as the working electrode, carbon rod, and Hg/HgO electrodes were used as the counter and reference electrodes, respectively. According to the following equation: E corrected = E RHE -iR, the reference was calibrated against and converted to the reversible hydrogen electrode (RHE): E RHE = E HgHgO + 0.059 pH + 0.098. z E-mail: anand@snnu.edu.cn

Results and Discussion
With the use of various capping agents and copper nitrate as a metal precursor, a variety of Cu nanocatalysts have been produced (Table I). By using EDX, the chemical compositions of these produced catalysts were identified. The EDX results were quite consistent with theoretical calculations ( Table I). The composition analysis results, on the other hand, suggested that the Cu concentration in the produced nanocatalysts was extremely near the feeding ratio. Due to oxidized Cu species being harder to reduce than nonoxidized species, the levels of Cu in the produced nanocatalysts were lower than the feeding ratio. 3 The pure uncapped Cu NPs were likewise made using the same procedure as the capped Cu nanocatalysts under the present system. 6 The produced catalysts' line scan elemental profiles and mapping for oxygen (green) and Cu (yellow) contents show that the catalysts have a high concentration of Cu (Fig. 1). Cu nanocatalysts' significant oxygen signal in the EDX spectrum shows that the metals are phase-separated and include a lot of Cu oxides (Fig. 1). Figure 1's depiction of copper oxides and the EDX analysis's significant oxygen signal are both indicative of the existence of copper oxides. 11 Pure uncapped Cu NPs were created similarly to how Cu capped NPs were created to further investigate this outcome (Fig. 2). The pure Cu samples' EDX examination revealed that they also contain a significant quantity of oxygen, demonstrating the propensity of Cu to oxidize (Fig. 1a). Strong oxygen signals in both the capped and uncapped Cu nanocatalysts' EDX spectra indicate that the metals are phase-separated, with areas that are rich in Cu oxides.
Broad peaks of Cu, CuO, and Cu 2 O structures form the majority of the XRD pattern, which denotes an amorphous structure (Fig. 3). Based on the XRD data, computations are shown in Table II. The (111) diffraction peak appears to have split slightly upon examination of the XRD spectra, indicating the coexistence of another fcc phase. The two distinct diffraction peaks at 2 = 43.52°and 50.71°, which correspond to the (111) and (200) crystal planes, respectively, Additionally, NPs sizes were calculated from XRD data and included in Table II. The nanocatalysts' XRD patterns show one significant diffraction peak that corresponds to the (111) planes. (Fig. 3) Most Pt-Cu samples exhibit an extra thin peak or shoulder close to the main diffraction peak, suggesting the existence of unalloyed Cu or a particular Cu-rich alloy. There aren't any other peaks that may be a second phase for any contaminants. This indicates that, at least at the level of X-ray detection, our copper nanoparticles are stable against oxidation. The capping layer that is present at the particles' surface is most likely responsible for their resistance to oxidation. Cu ions could coordinately interact with N or O atoms found in capping agents during particle production, resulting in a layer of these capping agents being absorbed on top of the formed copper particles.  As shown in Table III, pore volume and size estimation of the samples were necessary for the specific surface area study. Various stoichiometric compositions of catalysts were tested for their specific surface area. Comparing the catalyst's surface area to other produced materials, the findings of CTAB-capped Cu NPs (136.63 m 2 g −1 ) revealed the largest surface area (Table III, Fig. 4). Due to the high stoichiometric composition of Cu-O, it is possible that the specific surface area of other samples has decreased. Additionally, the pore volume and size of the produced catalyst samples exhibit the same fluctuation patterns as the corresponding specific surface areas. Stoichiometric compositions can vary, which is what is responsible for this behavior. The surface area of NPs was shown to be impacted by concentrations. Agglomeration and nucleation may be to blame for this phenomenon. Most of the pores' sizes fell within the 2-50 nm range, and their mesoporous-range pore diameters. Based on these findings, it appeared that mesoporous materials made up all the samples. The surface area per unit volume affects every aspect of a nanomaterial's behavior, including its physical, chemical, and biological characteristics. Given that there are a lot of reacting sites on the surface of the materials, their huge surface area may speed up reactions.
According to the survey spectrum, the synthetic sample is only made up of copper, oxygen, and carbon, with the carbon and oxygen peaks being assigned to the residual carbon in the sample and the adventitious hydrocarbon in the instrument, respectively (Fig. 5). The four peaks in the C1s spectrum are C-C=C (284.7 eV), C-C (284.9 eV), C-N-C (286.9 eV), and C=O (290.6 eV) (Fig. 5a). Two peaks at 531.9 and 533.6 eV, which correspond to the C-O and O-O bonds, respectively, may be seen in the deconvoluted O1s spectrum (Fig. 5b). The peaks of Cu + 2P3/2 and Cu 2+ 2p1/2 in the spectra of Cu2p are, respectively, 932.9 eV, 934.2 eV, 954.0 eV, and 942.4 eV. A high-spin Cu 2+ ion is associated with the peak at 962.7 eV, which is a satellite peak (Fig. 5c). Although two minor signals of Cu 2+ are seen owing to the oxidation of surface Cu atoms in the air, the Cu2p spectrum shows two significant peaks at binding energies that indicate the majority of Cu (Fig. 5). The presence of copper was verified by two peaks in the product's XPS measurement at 932.7 and 952.5 eV, respectively, which correspond to Cu2p3/2 and Cu2p1/2. It should be noted that the copper nanoparticles may have a small coating of Cu 2 O or CuO on their surface. However, the XRD also picks up on these microscopic copper oxide layers. Two   samples, reducing their exposure to air. The presented spectra demonstrated a significant degree of CTAB-capped NPs surface oxidation into CuO, validating the polymer's effective stabilizing abilities.
Using the TEM method, the morphology of the nanocatalysts was observed. The TEM micrographs of these catalysts, shown in Fig. 6, show that they are equally scattered without aggregation and have a spherical shape. As illustrated in Fig. 6, the produced nanocatalysts display well-scattered metal NPs with an estimated average diameter of ca. 2.1 nm based on statistics of 100 NPs. When prepared catalysts' TEM images are compared, it becomes clear that the preparation process significantly lessened the aggregation of the nanocatalysts. Agglomeration was reduced when capping agents were present, however, the particle dispersion varied from site to site. The mean particle size determined by the TEM images for the PVA-capped, PVP-capped, CTAB-capped, PEG-capped, and OAmcapped Cu NPs, as well as the uncapped Cu NPs generated as-is, was 2.71 nm, 2.22 nm, 3.10 nm, 3.31 nm, 1.49 nm, and 1.71 nm, respectively. Greater than prepared catalysts, commercial Pt/C has a 3.7 nm size. All the particles have a high degree of dispersion, which benefits the exploitation of the precious metal and high specific surface areas. It was assumed that the Cu atoms would be    evenly dispersed and equally distributed; however, it was demonstrated that the preparation procedure utilized in this experiment reduced particle size and enhanced particle dispersion of the catalyst. As a result of this intervention, the Cu inter-atomic distance at the surface decreases, the electrical and crystalline characteristics are altered, and the surface's electrocatalytic activity is ultimately improved. As a result, the dispersion and form parameters of catalysts are almost identical.
OER performances.- Figure 7a shows typical polarization curves of prepared materials, and the commercial Pt/C (20 wt%) and RuO 2 . The overpotential of the uncapped-Cu-NPs (305 mV), PVAcapped Cu NPs (300 mV), CTAB-capped Cu NPs (222 mV), PVPcapped Cu NPs (268 mV), PEG-capped Cu NPs (271 mV), OAmcapped Cu NPs (287 mV), RuO 2 (252 mV), and Pt/C (304 mV) recorded at 5 mV s −1 in 1 M KOH at a current density of 10 mA cm −2 . When compared to commercial RuO 2 , CTAB-capped Cu NPs had the lowest overpotential for delivering 10 mA cm −2 for the OER operation. Low charge transfer resistance at the electrode/ electrolyte interface helps in dramatically improved OER kinetics, as demonstrated by lower overpotential values. The behavior of the CTAB-capped Cu NPs is increased due to the electronegativity difference between nitrogen and carbon and improved spin density due to asymmetric charging. The CTAB-capped Cu NPs have excellent activity due to more active sites, and wide surface area, with a conductive carbon skeleton. The Cu NPs anchored structure has strong conductivity and can facilitate electron transport. The corresponding Tafel plots of Uncapped-Cu NPs (108 mV dec −1 ), RuO 2 (81 mV dec −1 ), and Pt/C (86 mV dec −1 ), are much higher than CTAB-capped Cu NPs (53 mV dec −1 ) (Fig. 7b). The CTAB-capped Cu NPs had a lower Tafel slope which explained why CTAB-capped Cu NPs had the superior OER kinetics. The introduction of CTAB changed the local electronic structure, provided more active sites, created a volatile electronic environment to form more valence states, and enhanced the electron transfer capability of catalysts, contributing to improved catalytic efficiency.
Since ECSA is proportional to Cdl, the electrochemical doublelayer capacitance (Cdl) reflects the variation in the electrochemical active surface area (ECSA). CTAB-capped Cu NPs have a Cdl of 0.39 mF cm −2 which is higher than RuO 2 's 0.30 mF cm −2 suggesting that the former has a higher ECSA (Fig. 8a). EIS is used to assess a catalyst's capacity for electron transport. OER on CTAB-capped Cu NPs has a substantially lower electron-transfer resistance than RuO 2 (Fig. 8b), which could contribute to its strong catalytic activity. At 10 mA cm −2 , RuO 2 had inferior stability vs CTAB-capped Cu NPs. 10,000 s show little change in the overpotential of CTAB-capped Cu NPs, demonstrating their great catalytic stability. The layer of capping agents on the particle's surface is responsible for its stability. During particle production, Cu ions can coordinately interact with N or O atoms in these capping agents, resulting in a coating of these capping agents being absorbed on top of the formed copper particles.
In contrast to the majority of the published electrocatalysts, it is remarkable that our Cu NPs have good activity at the cutting edge. The electrocatalytic activity of CTAB-capped Cu NPs may be attributed to the electrocatalyst's optimal metal loading, NPs' tiny size, twinned effects, and efficient electron interaction. In this work, Cu NPs are abundant, which results in increased catalytic activity. 33 Cu could increase OH production, which would improve CO tolerance. 15 For the manufacture of bimetallic NPs with various capping agents and a wide range of potential applications, we think the rational design and synthesis approach of Cu NPs presented in the current study may offer a fresh concept. 37 Effect of capping agent type.-Remarkably, the capping ingredient is necessary for dispersing nanoparticles in the solution. Because of the nanoparticles' high surface energy, their aggregation and reunion produce larger particles. By including a capping agent, it is possible to prevent particle agglomeration and promote the stability of their suspension. 11 In nature, there are two separate groups on either side of the chemical structure of steric capping agents. 7 While the hydrophilic group may cling to copper particle surfaces, the hydrophobic group repels water (to be exposed to the dissolving media). Thus, to avoid copper core collisions, an organic thin coating is placed on the surface of the copper nanoparticles. This organic thin layer also controls the formation of the Cu particles and improves their ability to disseminate and resist oxidation. 39 To investigate this factor, the reaction was run both in the presence and absence of capping agents such as PEG, CTAB, PVA, PVP, and OAm. The reaction by-products were investigated using XRD, FE-SEM, EDS, and TEM techniques. Surfactants can build an absorption layer on the copper nanoparticles' surface, inhibiting the formation of aggregates by enhancing the attraction between the particles. 4 The TEM images show that the mean copper nanoparticle sizes are uniformly distributed. In this ratio, the particles are wellseparated and spherical. The copper nanoparticles are spherical in size, size distribution, and form. 6 This proves conclusively that using capping agents results in a decrease in the size of the Cu nanoparticles. The capping agent molecules are adsorbed on the surface of the Cu nanoparticles, which prevents the Cu nanoparticles from becoming too large and instead causes smaller nanoparticles to develop.
One of the most important steps in the production of metal nanoparticles is the ability to keep them physically apart from one another to prevent oxidation and agglomeration processes. Nanoparticle stabilization typically involves the employment of surfactants as protective molecules. 26 These compounds stop aggregation by sticking to the nanoparticle surfaces. Capping agents are one of these substances that have been used often 18,27 as steric stabilizers or capping agents to avoid nanoparticle aggregation. The stability of metal colloids and the size and shape of nanomaterials are both significantly influenced by the concentration of the capping agent solution. 25 In this research, capping agents serve as both a polymeric capping agent and a size controller by preventing the nuclei from aggregating through the polar groups, which are intensely adsorbed at the surface of copper particles on the surface with coordination bonds. 2 The characteristic diffraction peaks are visible in the XRD patterns for pure metallic copper nanoparticles. Further diffraction peaks for contaminants have not been able to be discovered. Additionally computed for both capped and uncapped reaction media were the usual crystallite sizes of the nano-sized copper products. 1 The decreased crystallite size of copper nanoparticles in the reaction media including capping agents is probably caused by two main causes. As reported, 23 capping agents have been demonstrated to reduce Cu 2+ to Cu + , which may encourage the development of copper nanoparticle nuclei. As such, they are considered weak reducing agents. In addition to being a surfaceactive agent, they also delayed the crystallization of the metallic copper particles.
To further investigate the impact of the capping agents on the morphology and purity of the copper products, FE-SEM (Fig. 2) and EDS techniques (Fig. 1) were applied. Metallic copper nanoparticles that were produced both with and without capping chemicals were photographed using a field-effect scanning electron microscope (FE-SEM). The results demonstrated that spherical copper nanoparticles in metallic copper nanoparticles that had not been capped were more evenly distributed than those in those that had. These conclusions are consistent with what the XRD analysis showed. Additionally, only the element Cu was seen in the EDS spectra, validating the copper products' high purity and low oxygen content. Additionally, copper nanoparticles with capping agents contain less oxygen than copper products without capping agents.

Conclusions
In this work, the impact of capping agents on the production of Cu NCs, OER activities, electrochemical stability, and structural stability was examined. To the best of our knowledge, this is the first occasion where it was possible to accomplish this. CTAB-capped Cu NPs, among the series of produced catalysts, demonstrate exceptional OER performance. The findings offer a solid foundation for the hydrothermal method for creating nanoparticles with increased electrochemical activity. This research not only provides a fundamental understanding of the origin of the high-performance catalyst activity but also provides a fresh perspective on how to design and create more affordable and effective oxygen electrocatalysts.