## 3 Results and Discussion  
### Voltammetric Characterization  
Figure 3a shows cyclic voltammograms of the black-CuO and black-CuO/Pd electrodes recorded in 0.1 M KOH solution within the \(E\) range between 0 and 1.0 V. The shape of both curves was essentially the same, and the nature of the processes reflected by the voltammetric peaks can be easier understood when compared with the responses of the pure metallic Pd and Cu electrodes presented in Figure 3b. The anodic part of the cycles (\(i>0\)) in Figure 3a display two peaks at 0.5-0.7 V and 0.75-1.0 V, the position of which corresponds to the thermodynamic potential ranges of Cu\({}_{2}\)O and CuO formation, respectively [10]. Such ascription of the peaks is supported by the fact that their positions coincide with those of anodic peaks observed in the voltammogram of the pure Cu electrode in Figure 3b. In the cathodic range of the CVs (\(i<0\)), two peaks were observed as well (Figure 3a). The first minor one at \(E=0.7\) V was situated in the range where reduction of CuO to Cu\({}_{2}\)O was thermodynamically possible; however, the amount of charge was very low. This peak could be ascribed to the reversible reduction of oxide species formed on the electrode surface in the preceding positive-going scan, whereas the main mass of nanostructured CuO, formed by means of chemical oxidation of the Cu surface, was most likely reduced to Cu\({}_{2}\)O in the range of the second cathodic peak within 0.4-0.15 V. The latter peak was shifted towards more negative \(E\) values as compared with the first cathodic peak corresponding to transition CuO/Cu\({}_{2}\)O in the case of the pure Cu electrode in Figure 3b. This means that reduction proceeded at an overpotential, which could be associated with the thickness of the oxide layer on the black-CuO electrode (it is noteworthy that current density values in Figure 3a are tenfold compared to those in Figure 3b). Reduction of Cu\({}_{2}\)O to metallic Cu on the black-CuO electrode proceeded at \(E<0.1\) V (Figure 3a). In the case of the Cu electrode, the sharp peak of Cu\({}_{2}\)O reduction was between 0.2 V and 0 V (Figure 3b). The comparison of the CVs of the black-CuO and Cu electrodes within a wide range of potentials, i.e., \(-0.5\) V-1.85 V, encompassing the whole \(E\) window between O\({}_{2}\) and H\({}_{2}\) evolution reactions, is shown in Figure 3c. One can see from the voltammogram of black-CuO in Figure 3c that complete reduction of Cu\({}_{2}\)O was achieved at potentials as negative as \(-0.3\) V. The voltammetric response of Pd in alkaline medium shown in Figure 3b helped to identify the ranges of potential where one could expect to observe the influence of palladium in the CV of the black-CuO/Pd electrode. The increase in cathodic current at \(E<0.3\) V on the Pd electrode corresponded to HER, whereas the anodic peak within 0.3-0.5 V was related to H\({}_{2}\) oxidation and desorption processes. One can observe higher cathodic and anodic currents in the \(E\) ranges described above on black-CuO/Pd electrode as compared to the Pd-free black-CuO sample (see Figure 3a); however, the assignment of these differences to the influence of Pd was ambiguous. Most likely, the effect of Pd on black-CuO could not be discerned in cyclic voltammograms, because the Pd layer deposited by magnetron sputtering was very thin compared to the nanostructured CuO formed by the chemical oxidation of the Cu substrate. The influence of Pd on the SERS response of black-CuO samples was significantly more obvious as described further.