## 3 Results and Discussion
### Atomic Force Microscopy (AFM)  
The AFM images for a bulk copper sheet before and after electrochemical dissolution in choline chloride-based IL are presented in Figure 10. It can be seen that metallic copper undergoes pitting under several conditions and electropolishing under the others, noticing that on the native (unpolished) surface, machining marks and scratches can clearly be seen. Over the anodic sweep, it can be seen that the average surface roughness value, Ra, of the electrode prior to anodic polarisation was 0.75 \(\upmu\)m, while the value after polarisation was 3.75 \(\upmu\)m. When the electrode was rotated at 3000 rpm, the 0.63 \(\upmu\)m was gained. If the liquid was stirred (by rotating the electrode at 3000 rpm) then no film was produced at the electrode's surface and the solution became green.  
Galvanostatic etching for short times resulted in pitting taking place, leading to an uneven surface, whereas increasing the etch time led to a visibly brighter surface with less microscopic roughness. The machining marks were removed by the electro polishing process. Similarly, the same pattern was observed for the electropolishing of stainless steel in choline chloride-based IL [36]. In commercial electropolishing electrolytes, it is well-known that levelling only really takes place once the electrolyte is saturated with metal ions. Electropolishing the metallic copper samples with 0.81 M CuCl\({}_{2}\) added to the solution caused two different morphologies on the electrode's surface. It is worth noting that at low etch times, pitting was more observable, whereas longer timescales gave more even surface finishes. It is worth mentioning the hydrogen evolution occurs at the cathode during the anodic dissolution of metallic copper in choline chloride-based IL at 20\({}^{\circ}\)C. On the one hand, a negligible gas evolution at low current density (\(<\)20 mA\(\cdot\)cm\({}^{-2}\)) is observed in the form of bubble. On the other hand, at higher current density the evolution is remarkable in the addition of water at any amount. This can be related to either the electrolysis of a trace of water or EG [36]. The hydrogen gas evolution resulted in a lowering of the current efficiency of the electrochemical dissolution process, which is undesirable. DESs are generally insensitive to moisture compared to ILs, but the impact of water on the electrochemical process is still effective [37].  
Figure 9: A photograph of a sample of metallic copper sheet after electrochemical dissolution in choline chloride-based IL after being held at +0.18 V for 10 min at 20 \({}^{\circ}\)C, unpolished (u) and polished (p).  
On the one hand, there is an alteration in morphology of metallic copper surface from dark to surface brightness, as shown in Figure 10. On the other side, roughness is revealed in Figure 10; the copper sheet after dissolution made the subjects rougher, but the fewer crevices are considered the key feature of the copper sheet. So as to make a decision on whether electropolishing occurred or not, herein at least, the brightness accomplished had to be sufficient. Moreover, this is desirable from the electropolishing perspective that can be linked to the kind of electrolyte because the electropolishing of copper robustly depends on the nature of the electrolyte.  
It is also impressive to notice that the nature of interfacial region governs the nature of electrochemical-polishing which is different to large extent from that of aqueous counterparts. It is well-reported that electrochemical polishing occurs at the interface region between the electrode and the electrolyte where dissolved ions diffuse from the electrode's surface into bulk electrolyte [38; 39].