## 3 Results and Discussion
### Spectroelectrochemical Measurements  
Spectra of black-CuO/Pd and black-CuO electrodes recorded in 0.1 M KOH solution in normal and heavy water at fixed \(E\) values are shown in Figures 4 and 5, respectively. From the viewpoint of HER, the main difference between Pd and Cu electrodes was that on the palladium surface, this process was reversible and took place at \(E=0\) V (RHE), while in the case of copper, the hydrogen evolution reaction was irreversible and proceeded at an overvoltage of about 0.5 V [12]; therefore, a more negative \(E\) range was chosen for the investigations. It is evident from the comparison of Figures 4 and 5 that the intensity of spectra in the case of the Pd coated black-CuO electrode was about 10 times less. This, most likely, should be attributed to blocking of the black-CuO surface by Pd, rather than a decrease in electrode surface area,because the evaporated \(\sim\)200 nm thick Pd layer could not significantly reduce the true surface area of the electrode, as we found in earlier SERS studies [21]. Figure 4: Dependence of the SERS spectra from the black-CuO/Pd electrode upon the potential in 0.1 M KOH solution in water H\({}_{2}\)O (**a**) and heavy water D\({}_{2}\)O (**b**). Measurement conditions: laser excitation 785 nm; laser power at the sample 30 mW; integration time 300 s. The experiment was carried out under N\({}_{2}\) flow. The same electrode was used in both experiments. The spectral positions of the characteristic peaks are shown on vertical markers; red colour markers denote CuO related bands. The main peak at 565 cm\({}^{-1}\) in (a) corresponds to 70.1 meV energy or 6.76 kJ mol\({}^{-1}\) (see the upper horizontal axis). The inset in (a) shows electric dipole formation (a charge pair) at the interface in the applied E-field of light. The polarisibility of such a dipole is small, and the reduced mass \(\mu\) is determined by the electron mass; the same Raman lines were observed in H\({}_{2}\)O and D\({}_{2}\)O. Line markers are shown as eye-guides; the \(\sim\)1070 cm\({}^{-1}\) band was observed also on Pt and could be related to (H\({}_{3}\)O\({}^{+}\))\({}_{\rm ad}\)[26; 27; 28; 29]. Figure 3: Cyclic voltammograms of: (**a**) black-CuO and black-CuO/Pd; (**b**) metallic Cu and Pd electrodes in 0.1 M KOH solution, potential scan rate 50 mV s\({}^{-1}\). The positive \(E\) limit of the CV of metallic Cu in (b) is at 1.6 V, just prior to the beginning of the oxygen evolution reaction; the full range is not shown here for the sake of easier comparison with the curves in (**a**). (**c**) CVs within the \(E\) range \(-\)0.5-1.85 V (RHE). All spectra in Figures 4 and 5 can be conditionally divided into several regions. With respect to wavenumbers, one can make a distinction between the range at 100-800 cm\({}^{-1}\), where the most intensive bands were observed, and the other one at 800-1800 cm\({}^{-1}\) with less intensive spectral features. From the viewpoint of electrode potential values, there was a clear distinction between the spectra recorded within 0.7-0.3 V, i.e., in the oxide region, and those obtained at \(E<0.3\) V, i.e., in the region of oxide reduction and hydrogen evolution reactions. The most prominent bands, which appeared in all the spectra at 100-300 cm\({}^{-1}\) and 400 and 600 cm\({}^{-1}\) at the potentials just below 0.3 V and intensified as the electrode potential turned more negative, should be assigned to cuprous oxide Cu\({}_{2}\)O, which was produced by means of the electrochemical reduction of CuO, as reflected by the cathodic reduction peak within 0.15-0.4 V (Figure 3a). At more negative potentials, the reduction of Cu\({}_{2}\)O to Cu proceeded, and this was the reason why the intensity of the bands indicated decreased significantly (see the spectra at \(E<0.1\) V in Figure 5a). The assignments of the CuO and Cu\({}_{2}\)O bands of the nanotextured surface was done following the earlier study of electrodes prepared by the same method [30], as well as literature data [31; 32; 33; 34; 35; 36] and are summarized in Table 1; see Appendix A.