# Electrochemical Deposition of Copper on Epitaxial Graphene
## 3 Results and Discussion  
The voltammetry of epitaxial graphene (Figure 1) in the pure supporting electrolyte does not show any faradaic current in the double layer region of potentials (between \(+0.5\) and \(-0.4\) V). The larger negative potentials (below \(-0.6\) V) applied on epitaxial graphene in pure acidic electrolytes were characterized by the appearance of faradaic currents due to the appearance of a hydrogen evolution reaction. The addition of copper ions to the thousand-times excess of background electrolytes led to the increase in capacitive currents (ca. 2.5 times at 0.5 V). This manifests the strong adsorption of di-cations on epitaxial graphene [77; 78; 93]. In parallel, the redox phenomena associated with the electrodeposition and dissolution of the metal species became explicit. The voltammetry scan in the negative direction shows the appearance of an intensive cathodic process at potentials below \(-0.2\) V. Being a poor complexing ion, perchlorate disables the stabilization and appearance of Cu\({}^{1+}\)-based intermediates, which is illustrated by the absence of any redox processes at the higher potentials [94; 95]. This implies that the copper electrodeposition on epitaxial graphene proceeds via single, irreversible bi-electronic reduction. The nucleation loop observed for the cathodic process is assigned to the new phase formation [96]. Taking into account the differences in the concentration of copper ions as well as the irreversible (non-Nernstian) character of the deposition process [3], the current density of the nucleation loop reaches values comparable with copper electrodeposition on glassy carbon [3] and pyrolytic graphite [4] at the potentials shifted on ca. 400 mV in a negative direction. This overpotential illustrates the typically slower kinetics of the generalized faradaic process on an epitaxial graphene monolayer in comparison with the common bulk graphitic interfaces [97] contributed by reactive graphene edges. Figure 1: Cyclic voltammograms related to the electrodeposition and stripping of copper on the epitaxial graphene (in absence and in presence 0.1 mM Cu\({}^{2+}\), as dashed black and solid red curves, respectively; 0.1 M HClO\({}_{4}\), scan rate 20 mV\(\cdot\)s\({}^{-1}\)). The voltammetry scan in the positive direction showed the appearance of discrete steps of dissolution (stripping) of the copper deposits. The direction-dependent voltammetry response illustrates the difference between the pristine and copper-modified epitaxial graphene monolayer presented at potentials higher than 0.5 and lower than \(-\)0.5 V, respectively. The oxidation peak current (O1), located at \(-\)0.03 V. corresponds to the single-electron oxidation of metallic copper to Cu\({}^{1+}\). Further increases in the applied potential in a positive direction led to the unusual appearance of the negative current (R1) at 0.1 V. In contrast to the kinetically slow electrochemical deposition of copper on the epitaxial graphene monolayer, the aforementioned instability of Cu\({}^{1+}\) might enable an exergonic (non-faradaic) route towards the metallic copper at the surface via fast disproportionation. Here, the state of the surface is not pristine yet. As soon as the copper ions undergo the faster reduction in metallic copper in comparison with bare carbon [96], the metallic copper might act as an electrocatalyst for the reduction, in comparison with epitaxial graphene originating the appearance of the negative currents. Further increases in potential in a positive direction showed the appearance of the sharp peak current typical in complete oxidation to Cu\({}^{2+}\) ions. The scenario of the electrode processes was repeated with continuous voltammetry cycling. We performed DFT calculations in order to assay the reduction thermodynamics. The Cu\({}^{2+}\) ion was considered, when coordinated with six water molecules [98], energetically, the most favorable complex (Figure 2a).