Preparation of high surface area Cu-Au bimetallic nanostructured materials by co-electrodeposition in a deep eutectic solvent

Our sustainable future requires ﬁnding new, affordable and green routes to prepare nanostructured ma- terials used in renewable energy conversion. In this work we present an electrodeposition method in a deep eutectic solvent (DES) to prepare bimetallic high surface area nanostructures of Cu and Au with tunable structure and composition. The metal electrodeposition performed in choline chloride within a urea deep eutectic solvent allows us to tailor the size, morphology and elemental composition of the deposits. We combine electrochemical methods with scanning electron microscopy (SEM), X-ray pho-toelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) to characterize the elec- trodeposited nanostructured materials. We assess the increase of the electroactive surface area through the analysis of the lead underpotential deposition (UPD) on the prepared ﬁlms. Integrated Pb UPD charge values of ca. 160 0–40 0 0 μ C/cm 2 for the prepared Cu-Au ﬁlms have been calculated, suggesting a 5– 14 fold increase of the active surface area compared to ﬂat surfaces of polycrystalline Cu or Au. Our work reports a versatile and environmentally friendly route for the electrodeposition of Cu-Au bimetallic nanostructures in a DES. The combination of a tailored morphology and composition with the high ac- tive surface area of the nanostructured materials show that electrodeposition in DES is promising for the development of multimetallic electrocatalysts.


Introduction
As society continues to move towards a carbon-free economy, there is an increase in the urgent demand to produce more efficient nanotechnologies for clean energy conversion [ 1 , 2 ]. During the past decade, the scientific community has dedicated substantial efforts to design novel nanocatalysts for a variety of applications such as energy conversion and storage, as well as sustainable electrosynthesis processes, necessary in the green energy transition [3][4][5][6] . In this regard, developing new methods and strategies for controlled preparation of tailored nanostructured materials, in an environmentally friendly manner and without compromising its competitiveness, is crucial to decarbonize the industry [ 1 , 2 ].
Chemical synthesis of nanoparticles usually employs colloidal routes in which different surfactant agents or templates are neces-sary to guarantee crystallinity of the nanoparticles [7] . The methods require to remove the surfactant agent, which poisons the catalyst surface and affects its catalytic performance [ 8 , 9 ]. Physical methods, such as sputtering from metallic-targets and in ultra-high vacuum conditions, allow for the controlled preparation of multimetallic nanoparticulate thin films, although at the expense of high energy consumption [10] . In recent years, electrodeposition of metallic nanostructures in green organic solvents has been proposed as a sustainable alternative for the synthesis of new materials [11][12][13] . Electrodeposition is a versatile and relatively inexpensive technique for tuning the deposition of different metallic alloys and metal oxides [14] . Tailored morphology and composition is obtained by simply fine-tuning certain parameters, such as applied potential conditions and composition of the electrodeposition bath, which contains the electrolyte and the metal precursors [12] . Traditionally, electrodeposition was performed in aqueous solutions and in the presence of additives, to facilitate the reduction of metals with very negative reduction potentials, while avoiding the interference of the solvent co-reduction [ 15 , 16 ]. This is the case of e.g. highly toxic cyanide bath solution for the deposition of Au-Cu compounds [17] . Alternatively, deep eutectic solvents (DES) have been suggested as green solvents for the electrodeposition of nanostructures [ 18 , 19 ]. Formed by the eutectic mixture of a quaternary ammonium salt and a neutral proton donor molecule, they provide wide potential limits, enough conductivity and usually do not require the use of additives [19] . The single electrodeposition of the majority of metals has been successfully conducted in DES [ 12 , 20-24 ] and, in recent years, DES have also been used for the electrodeposition of a few bimetallic compounds [25][26][27][28][29] . However, electrodeposition of noble bi-or multimetallic nanostructures in DES is a topic still in its infancy.
While monometallic nanostructures with variable shape and sizes have been extensively investigated for different electrocatalytic reactions [30][31][32] , new bimetallic and multimetallic nanostructures continue to emerge [33][34][35] . When two or more metals are combined, the aim is to enhance the catalytic performance towards desirable compounds in different electrocatalytic reactions, by tuning the electronics of the multimetallic surface [ 4 , 36 ]. Among other bimetallic catalysts, the Cu-Au system presents interesting properties for a variety of reactions, including CO conversion [37] , electrochemical and photoelectrochemical hydrogen evolution [ 38 , 39 ], design of non-enzymatic biosensors [40] , and electroreduction of CO 2 into fuels and CO [41][42][43][44] . Developing clean approaches to selectively tailor morphology, shape and size of multimetallic materials is, thereby, relevant in order to design efficient catalysts for different applications [ 3 , 4 , 45 , 46 ].
In addition to the improved catalytic properties when combining two or more metals, nanostructured substrates usually display enhanced electrocatalytic activities (per mass unity of catalyst) compared to flat or bulk electrodes [ 47 , 48 ]. This is, among other reasons, because of their high electrochemically active surface area (ECSA), which accounts for the total number of reaction sites versus geometric area of the surface catalyst [49][50][51][52] . In this work, we present an alternative methodology to prepare bimetallic structures of Cu and Au with high extended surface area, via metal co-electrodeposition in choline chloride plus urea deep eutectic solvent. A glassy carbon electrode was selected as deposition substrate due to its low activity towards the reduction of the DES, thus providing a wide potential window of around 4 V [ 53 , 54 ]. Cyclic voltammograms (CVs) and chronoamperometric curves were recorded to clarify the co-deposition mechanism and to establish the best potential conditions for the preparation of the bimetallic films. Ex-situ field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to assess the morphology, composition and distribution of Cu and Au in the nanostructured deposits. The co-electrodeposition of Cu and Au was conducted using two baths with different concentrations of Cu and Au salt precursors, to tailor the structure, composition and morphology of the bimetallic nanostructures. Finally, we analyzed the ECSA by means of lead underpotential deposition (Pb UPD) on the prepared films. Pb UPD on metallic surfaces is a surface process sensitive to the structure and area of the catalyst which allows for a more reliable quantification of the electroactive surface area [55] .

Experimental section
Choline chloride (ChCl, Across organics, 99%) and urea (Sigma-Aldrich, 99%) were purchased at the highest available purity. The DES was prepared by mixing both choline chloride and urea salts in a 1:2 molar ratio and under constant stirring at 40 °C. Dehydrated AuCl 3 and CuCl 2 salts were similarly purchased (Sigma-Aldrich, 99%). To prepare the different employed bath solutions in this work, specific amounts of AuCl 3 and CuCl 2 salts were dissolved in the DES under magnetic stirring, vacuum and heating conditions ( T < 60 °C) overnight. This protocol was used to minimize the amount of water in DES and allow for optimal deposition of the metals [23] . Four bath solutions were prepared and tested in this work: i) 0.1 M CuCl 2 + DES solution; ii) 0.1 M AuCl 3 + DES solution; iii) a 0.05 M CuCl 2 : 0.05 M AuCl 3 + DES solution, i.e., Cu and Au 1:1 molar ratio solution; iv) a 0.075 M CuCl 2 : 0.025 M AuCl 3 + DES solution, i.e., Cu and Au 3:1 molar ratio solution. Ultra-pure perchloric acid, potassium perchlorate (Merck, Supra-pur®, 99.995 % purity) and lead (II) perchlorate hydrate (Sigma-Aldrich Merck, 99.995% of purity) were purchased to prepare a 2 mM Pb(ClO 4 ) 2 + 0.1 M KClO 4 + 1 mM HClO 4 employed for the Pb underpotential deposition.
The working electrodes were: a glassy carbon (GC) disk in which the geometric surface area was 0.1963 cm 2 ; a polycrystalline Cu rod with a geometric surface area of 0.2826 cm 2 and a polycrystalline Au bead electrode (0.183 cm 2 ) prepared following the Clavilier's method [56] . For the pre-treatment of the electrodes, the glassy carbon was polished until mirror finish using waterbased α-alumina powder of 0.3-0.05 μm coarseness (Struers). After polishing the glassy carbon rod, the electrode was sonicated in milli-Q (18.2 M cm, TOC < 5 ppm) water for 1 min, and dried with a nitrogen stream before the electrodeposition. The Cu(poly) rod electrode was polished with α-alumina (0.3 μm) and, then, electropolished in a H 3 PO 4 (analytical degree, 85% volume/volume) plus H 2 SO 4 (analytical degree, 95% volume/volume) plus water solution with a volume ratio of 10:3:1, respectively. The gold bead electrode was pre-treated by flame-annealing and cooled down in air. The electrochemical measurements were carried out with the Cu and GC surfaces in the meniscus configuration while the Au bead was immersed into the solution.
The electrochemical experiments were performed in a threeelectrode cell configuration using a Bio-Logic potentiostat. The metal electrodeposition in DES was performed in a thermostated small-volume (25mL) electrode cell. Copper wires for single deposition of copper, and gold wires for the deposition of gold and Cu-Au bimetallics were used as both quasi-reference and counter electrodes. Potential values in the electrodeposition process, were referred to the Ag|AgCl reference electrode scale. The temperature was kept at 70 °C in all experiments carried out in DES solutions. We have selected this temperature to enhance the deposition rates and ensure that solvent co-reduction affecting the bimetallic film deposition does not happen [23] . For the voltammetric analysis in aqueous solution and at room temperature, a classical standard three-electrode cell was employed. A saturated calomel electrode (SCE) from Crison was used as a reference electrode and placed in a Lugging capillary. The counter electrode was a gold wire in case the deposits were composed by gold or gold-copper mixtures, and a copper wire was used for the analysis of the pure copper deposits. For the preparation of the nanostructured Cu-Au deposits, chronoamperometry value was performed for several minutes, after reaching the desired amount of circulated charge. The circulated charge was carefully monitored by using Biologic software.
The morphology of the samples was analyzed using the highresolution Zeiss Gemini 500 field emission scanning electron microscope at Haldor Topsøe S/A. An Inlens and SE2 detector were used at low voltage (2 keV) to take high resolution images with the FE-SEM. Higher voltages (15 kev) were then used when collecting energy dispersive X-ray (EDS) data. The EDS was performed using a Thermo Scientific UltraDry silicon drift detector and processed using Pathfinder Software. All XPS measurements were conducted by a Theta Probe instrument (Thermo Scientific) using an Al anode X-ray source (K α line = 1,486.6 eV). The XPS chamber's base pressure was between 8.0 ×10 −9 and 5.0 ×10 −10 mbar. The X-ray beam size was 400 μm and the pass energy utilized was 100 eV. Before the ex-situ morphological characterization, the samples were cleaned with milli-Q water at near 90 °C, in order to remove the remaining DES on the surface.

Single Au deposition and Cu deposition
Before studying the co-deposition of gold and copper, we assessed the gold and copper single electrodeposition on glassy carbon. The formation of chlorocomplexes of Au(III)-Clx and Cu(II)-Clx allows for dissolution of the metallic salt precursors. [57] Cyclic voltammograms (CVs) as well as chronoamperometric curves at different applied potentials, were recorded for either Cu and Au single electrodeposition in DES, aiming to establish the optimal potential range at which each single metal deposits. The CVs as well as the recorded chronoamperometric transients appears in Fig. S1 of the supporting information (S.I.), with our results being in line with previous reports [ 23 , 24 ]. The S.I. also contains the detailed description of the experimental voltammetric and chronoamperometric curves. The morphological aspect of the prepared Au and Cu single deposits in DES was assessed by using FE-SEM. Fig. 1 A shows the FE-SEM images of the gold deposit performed at a moderate overpotential of -0.45 V vs Ag|AgCl and after reaching a charge of -25 mC. Flower-shaped gold nanoparticles (NPs) with a diameter of ca. 300 nm are formed. This morphology is likely induced by the particle agglomeration promoted by a high surface diffusion on the GC substrate. All gold NPs display a similar shape and size. Fig. 1 B shows the FE-SEM image of a less covered sample prepared at -10 mC of circulated charge. The gold nanoparticles have the same flower-shape but show a lower degree of agglomeration and more pristine areas of exposed GC surface [23] . These results are in line with studies carried out by Wei et al. [24] , at 300 s of deposition time and low Au coverage on the glassy carbon substrate. EDS analysis (Fig. S2B) confirmed the presence of gold. Fig. 1 C shows the FE-SEM images of the copper deposit performed at a moderate overpotential of -1.1 V vs Ag|AgCl and after circulating a charge of -25 mC. Under these conditions, nanostructured copper plates with a triangle shape are formed, showing that the particular composition of the DES induces a specific  morphology on the Cu deposits. A less covered deposit of -10 mC prepared at -1.1 V vs Ag|AgCl ( Fig. 1 D) displays rounded copper nanoclusters ranging between 300 and 500 nm of diameter [58] . These rounded nanoparticles are likely the precursors to form the triangular plated morphology in Fig. 1 C at longer deposition times. In addition, no formation of dendrites, under these conditions, is observed at lower magnification ( Fig. S3A and B). The presence of pure copper was confirmed by EDS analysis (Fig. S3C).

Bimetallic Cu-Au nanostructures
After analyzing the single deposition of Cu and Au on GC, we performed the co-deposition of both metals from a 0.05 M AuCl 3 : 0.05 M CuCl 2 + DES solution (molar ratio between Cu and Au of 1:1) at 70 °C. Fig. 2 A shows the CVs of the Cu and Au codeposition at different cathodic potential limits. Fig. 2 A , curve a) shows the shortest potential window between -0.55 V and 1.05 V vs Ag|AgCl. In this potential window, a reduction current starting at -0.35 V vs. Ag|AgCl is observed until peak II is reached, which corresponds to gold electrodeposition. The counter-oxidation peak (peak II') overlaps with the characteristic feature related with the first Cu 2 + to Cu 1 + electron transfer (peaks I-I') [59] . The absence of the characteristic oxidation peak of metallic Cu corroborates, as expected, that gold deposition occurs prior to the onset of Cu deposition in the Cu and Au 1:1 molar ratio solution. Enlarging the cathodic potential limit progressively from -0.55 V to -1.05 V vs Ag|AgCl (curves b), c) and d) in Fig. 2 A) causes the appearance of a third reduction peak (peak III) most likely involving both Cu and Au co-electrodeposition. Consequently, a broad Cu 0 to Cu 1 + oxidation peak between -0.1 V and -0.3 V vs Ag|AgCl has emerged (peak III'), while the oxidation peak of gold increases in intensity (peak II'). The intensity of the gold oxidation peak is considerably higher than the metallic Cu oxidation peak, suggesting that, for the Cu and Au 1:1 solution and selected potential range, the Au presence in the co-deposition process is higher. Chronoamperometric transients were recorded at different applied potentials ( Fig. 2 B). The j-t transients only display a single maximum current peak rather than multiple peaks, and the current curves practically overlaps ( Fig. 2 B) at longer times, which is an indication that Cu and Au follow a nucleation and growth mechanism as well.
The morphological analysis of a Cu-Au bimetallic nanostructured electrode from the 1:1 molar ratio bath solution was assessed by FE-SEM. Fig. 3 A shows the FE-SEM image of a Cu-Au sample prepared at -0.72 V vs Ag|AgCl and under stationary conditions. Rounded nanoparticles were formed with a diameter below 300 nm and with plates emerging from the surface. Additionally, the nanoparticles are homogenously distributed across the entire surface ( Fig. 3 A, left panel). Cu-Au bimetallic samples were also prepared at slightly lower applied potential (at -0.69 V vs Ag|AgCl), but under stirring conditions (200 rpm) in order to increase the mass diffusion. Fig. 3 B shows the morphology of the new Cu-Au sample, which displays slightly bigger rounded nanoparticles than in Fig. 3 A. The rounded nanoparticles are also homogeneously distributed and cover the entire surface ( Fig. 3 B, left panel). These rounded nanoparticles display similar morphology to those prepared with no stirring, showing small plates on top of the NPs, which indicates that stirring conditions have little effect on the morphological aspect. This generated morphology is clearly different than the morphology of either pure deposited Au or Cu in DES ( Fig. 1 ). These results suggest that tuning the applied potential allows both Cu and Au to grow forming the bimetallic structure. The EDS colour maps of Cu (purple) and Au (red) elements in Fig. 3 C, verifies the presence of Cu and Au in the sample. The EDS analysis of the abundance of the elements in the sample ( Fig.  S4A and B) indicates that the mass relation between Au and Cu is approximately 2:1, respectively. However, EDS results are only semi-quantitative.
Aiming for a higher Cu content in the bimetallic nanostructures, the concentration of copper in DES was increased. Fig. 4 A illustrates the CVs of Cu and Au co-deposition at different potential limits, and from a solution containing 0.075 M CuCl 2 : 0.025 M AuCl 3 in DES, i.e, with a 3:1 molar ratio of Cu and Au. The 3:1 molar ratio metallic solution shows the same voltammetric features as those that were obtained in the 1:1 molar ratio solution. As expected, peak III ´, which is related with the oxidation of the deposited Cu, is more intense due to the higher amount of copper deposited in this bath solution. Fig. 4 B illustrates the corresponding 3-D nucleation and growth chronoamperometric transients at different applied potentials. Similar to the results in Fig. 2 B, the curves present a good overlapping at longer times. The j-t transients display a profile more similar to those for single Cu electrodeposition (Fig. S1D), which suggests an important contribution of copper in the formation of the film when the Cu and Au 3:1 solution is used.
FE-SEM samples were prepared at moderate overpotential to investigate the morphology of the obtained nanostructured films under these conditions. Fig. 5 A contains the FE-SEM images of the Cu and Au co-deposition on GC from the 3:1 molar ratio solution at -0.65 V vs Ag|AgCl, -25 mC of circulated charge and under stationary conditions. The obtained co-deposited nanostructured electrode displays the formation of a network with tiny plates in which the rounded shape has completely disappeared. In Fig. 5 B, the Cu-Au bimet allic obt ained from the 3:1 bath solution was carried out at the same applied overpotential, i.e, at -0.65 V vs Ag|AgCl, but after decreasing the circulated charge to -18 mC. This sample still shows a high population of plates but in which, unlike in Fig. 5 A, clusters up to 300 nm can be differentiated. The EDS analysis confirmed the presence of both Cu and Au homogeneously distributed over the surface ( Fig. 5 C). The semi-quantitative chemical analysis obtained from the EDS shows a higher amount of Cu in any of the samples prepared from the 3:1 molar ratio solution (Fig. S5). The wt% content of Cu in the sample prepared at -25 mC (Fig. S5A) is approximately double than Au. However, the EDS analysis evidences a lower Cu mass content in the sample prepared at -18 mC (Fig. S5B). The mass content of Cu is 1.5 times higher than Au in this sample . These results clearly evidence that increasing the content of copper in the bimetallic film, causes the loss of the rounded shape morphology obtained in the 1:1 bath solution, and enhances the growth of plates.
To investigate the chemical composition and chemical states of the elements at the surface layers of the prepared deposits, XPS analysis was conducted. Fig. S6 shows the XPS data of the bimetallic deposits prepared in Figs. 3 A and 5 B. The Cu 2p and Au 4f spectra were recorded for each Cu-Au sample, which confirm the presence of both elements in the bimetallic films. Details of the XPS spectra are provided in the S.I. of this paper. Interestingly, the intensity of the Au 4f peaks are substantially low, particularly in samples prepared using the Cu and Au 3:1 bath solution in which they are almost negligible. A plausible explanation to this behavior is that Au deposition starts at a lower applied potential than Cu (Fig. S1), thereby activating the Cu electrodeposition, where its deposition on top of Au would be enhanced. The increase of Cu at the surface in relation to Au is especially accentuated at the samples prepared from the DES solution with higher content of Cu salt precursor, i.e. using the Cu and Au 3:1 solution. However, since the XPS measurements have been conducted ex-situ , it is also possible that the elements in the surface have redistributed because of the oxidation of Cu by exposure of the sample to air (see S.I).
In summary, results obtained by the ex-situ characterization analysis suggest that higher concentration of gold in the bimetallic Cu-Au film induces the formation of rounded nanoparticles. This rounded morphology is lost as the content of copper increases, which enhances the agglomeration of the particles and induces the formation of plates. The ratio between Cu and Au in the bimetallic film dramatically affects the morphological aspect of the deposits with an increase of Cu in the more superficial layers.

Electrochemical determination of the electroactive surface area
Electrochemical characterization of the Cu-Au films was conducted by recording the blank cyclic voltammograms in 0.1 M sodium bicarbonate solution ( Fig. 6 ), and in the oxide region up to 0.9V vs SCE. Fig. 6 A and 6 B shows the blank CVs of a Au bead and a Cu polycrystalline rod in bicarbonate solution. While the Au bead only displays a reduction peak at 0.35 V vs SCE, peak (i), which corresponds to the reduction of the gold oxide produced in the anodic region (i ) [60] , the copper electrode shows two prominent features in the cathodic scan. These two peaks appear centred at -0.20 V vs SCE peak (iii) and -0.64 V vs SCE peak (ii) and were ascribed to the reduction of Cu(II) to Cu(I), and Cu(I) to Cu(0) oxide species, respectively [61] . The voltammetric features of the prepared Cu-Au films, in the oxide region, are clearly affected by the relative content of Cu and Au in the film. The blank CV of the 1Cu:1Au film ( Fig. 6 C) shows two broad oxidation peaks at 0.13 V vs SCE (iv ) and at 0.78 V vs SCE (v ) in the anodic scan which we ascribed to the formation of Cu and Au oxide, respectively. The counter peaks in the reductive scan appeared at 0.19 V vs SCE, peak (v) and 0.03 V vs SCE, peak (iv). Increasing the Cu content in the 3Cu:1Au film ( Fig. 6 D) causes the appearance of two sharp reduction peaks in the cathodic region, (vi) at -0.47 V vs SCE and (vii) at -0.23 V vs SCE, showing a profile similar to the CV of pure Cu, which also supports that Cu concentrates in the more external layer, in line with the XPS results. A tiny peak at 0.005 V vs SCE (viii ) , which we did not observe on the Cu polycrystalline electrode ( Fig. 6 B), has appeared in the Cu-Au film. Our blank CVs of the prepared Cu-Au films agree with the previous ones reported by Liu et al. [43] in which the authors prepared Cu-Au films by magnetron sputtering, thereby supporting that the use of the ChCl:urea DES allows preparing clean bimetallic surfaces of Cu and Au by electrodeposition technique.
To assess the ECSA of the prepared co-deposited films, lead underpotential deposition (Pb UPD) on the nanostructured bimetallic films was evaluated. Underpotential deposition is the reversible deposition of a sub-monolayer or monolayer of a foreign element onto the substrate, at potentials more favorable than the thermodynamic reduction potential of this element. Pb UPD is, thereby, a surface process sensitive to the structure and active area of the catalyst [55] . To calculate the ECSA of the prepared films, we have integrated the charge (in micro coulombs, μC) involved in the cathodic voltammetric curve of the Pb UPD recorded on each Cu-Au sample ( Fig. 7 ) [60] . The integrated charge values were normalized by the geometric area of the glassy carbon substrate to obtain the surface charge density or surface charge area values of the nanostructured films ( μC/cm 2 ). These integrated charge values were divided by the average value between the theoretical surface charge densities ( μC/cm 2 ) of both Au and Cu polycrystalline flat surfaces, to assess the increase in effective area or roughness factor (R) of the nanostructured films. Polycrystalline Au and Cu are considered to approximate flat surfaces, i.e., their roughness factor is equal to 1 (R = 1). This means that the geometric area of the electrode correlates with its electroactive surface area in these surfaces [60] .
In the case of the prepared high surface area nanostructures, the roughness factor (R) provides the relation between the effective electroactive surface catalyst area (ECSA) and the geometric area (A GEO ) of the employed glassy carbon substrate [62] . on Cu single crystalline electrodes of around 300 μC/cm 2 that increased to 350 μC/cm 2 due to anion adsorption [65][66][67]. Our calculated values are in line with those obtained for single crystalline electrodes. Thereby, we have assumed that the surface charge area of a Cu-Au film with R equal to 1 must be close to 318 ± 56 μC/cm 2 , which is the average between the lead UPD on the Au bead and the Cu polycrystalline surface that we have previously determined. Fig. 7 A also shows the lead UPD of the deposited gold Au (black solid line) and Cu (red solid line) in DES. The Pb UPD of the deposited nanostructured Au has a voltammetric profile that is similar to the Pb UPD on the Au bead and displays higher current densities. The RF of the deposited Au was 2.87. The Pb UPD on the deposited Cu was also similar to the Pb UPD on a Cu polycrystalline electrode, and it displayed the characteristic cathodic peak centered at -0.30 V vs SCE, similar to the Cu polycrystalline CV. However, in the case of the Cu deposit, a small increase in the electroactive surface area ( Fig. 7 A, red line) was observed (RF about 1), a result that we attributed to the low adherence of the Cu deposit compared to Au, and the instability of Cu films with high coverages. Pb UPD on the Cu and Au co-deposits prepared from either the 1:1 molar ratio solution ( Fig. 7 B) and the 3:1 molar ratio solution ( Fig. 7 C), are compared with the Pb UPD on polycrystalline Cu and Au flat surfaces. Unlike the deposited Cu, the Cu-Au films had a good adherence. Fig. 7 B and 7C clearly show a strong increase of the current density values of the Pb UPD on the deposited Cu-Au bimetallic nanostructures, after normalizing them with the geometric area of the glassy carbon substrate. The integrated charge values for the Pb UPD on the Cu-Au deposits were 4148 μC/cm 2 for the 1Cu:1Au, and 1637 μC/cm 2 for the 3Cu:1Au film, charge values that are considerable higher than the charge values we reported for the Cu(poly) (366 ± 17 μC/cm 2 ) and the Au bead (270 ± 12 μC/cm 2 ). These results clearly reflect the increase in number of active sites per unit area in the deposited nanostructured films. The integrated charges from the Pb UPD CVs and estimated EC- SAs as well as the R for the prepared deposits, are summarized in Table 1 . The obtained roughness factors (R) range between 5 and 14. Based on our Pb UPD results, we estimated an increase of the active surface area of at least 5 to 14 times higher than the geometric surface area. We conclude that the preparation of nanostructured bimetallic Cu-Au films through metal electrodeposition in deep eutectic solvents, generates materials with high extended surface areas which ultimately will cause an increase in the electrochemical activity. Future work combining electrochemical measurements and insitu /operando characterization techniques should be conducted to obtain information on the parameters governing the nucleation and growth mechanism of these bimetallic nanostructures [ 68 ]. In particular, it would be of huge interest to get experimental insights on the first stages of bimetallic electrodeposition. At short times of deposition, different unknown events can occur before the formation of stable bimetallic nanostructures, such as the formation and dissolution of nuclei, or surface diffusion and coalescence of atomic arrangements, events that determine the electrodeposition mechanism [ 68,69 ].

Conclusions
We studied, in detail, the preparation of tailored nanostructured bimetallic catalysts of copper and gold, with high active surface area, by metal electrodeposition in a deep eutectic solvent. We have also shown that size, morphology and elemental composition of the deposits can be tailored by selectively adjusting the applied potential and bath composition. The FE-SEM and EDS analysis confirm the formation of nanostructures of either Cu, Au and bimetallic Cu-Au electrodes. Higher Au/Cu ratios in the sample generate rounded gold NPs with similar size (about 200 nm), and with small plates covering the surface. On the other hand, an increased Cu content promotes the development of a network structure with tiny plates. The ex-situ XPS analysis confirmed the presence of both Cu and Au in the deposited films, in agreement with the EDS data. Interestingly, it also showed a substantial higher concentration of Cu on the surface compared to Au. We ascribe the increase of the surface content of Cu to the co-electrodeposition mechanism of Cu and Au, although we do not discard that the oxidation of the sample at open circuit potential could also bring Cu to the surface.
The ECSA of the prepared nanostructured bimetallic films was estimated by Pb UPD. The current intensity of the Pb UPD considerable increases when the Cu-Au bimetallic nanostructures are deposited on the glassy carbon substrates. An ECSA of 5 to 14 times higher than the geometric active area was estimated by Pb UPD. Thus, we confirm the successful preparation of high surface area nanostructured bimetallic catalysts of Cu and Au, by metal electrodeposition in a deep eutectic solvent. We highlight that the electrodeposition in deep eutectic solvents is a promising research field for the future design and development of bimetallic or multimetallic high surface area catalysts with high interest in the field of electrocatalysis. Future work is necessary for rationalization of the nucleation and growth mechanism of bimetallic nanostructures electrodeposited in ionic liquids. 8