## 3 Results and Discussion
### Spectroelectrochemical Measurements
The reason why the bands attributable to Cu\({}_{2}\)O were less prominent in the spectra recorded in the solution of 0.1 M KOH in heavy water was, most likely, related to the fact that these spectra were collected after the experiments in normal water, during which the majority of the CuO phase on the black-CuO substrate was reduced. Figure 5: Dependence of the SERS spectra from the black-CuO 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 spectral positions of the characteristic peaks are shown with vertical markers; red colour markers denote CuO related bands. The presence of the Pd layer on the black-CuO surface gave rise to the occurrence of distinct bands at 1328, 1410, and 1569 cm\({}^{-1}\) observed in SERS spectra measured in the \(E\) range between 0.3 V and 0 V (RHE) (Figure 4). These bands correspond to the energy of adsorptive interaction between the particle and electrode surface (see the inset in Figure 4a); in the case of reversible HER, they could be assigned to the adsorption of ionic intermediates (H\({}_{2}\))\({}^{+}\)\({}_{\text{ad}}\), (H\({}_{2}\))\({}^{+}\)\({}_{\text{ab}}\) (H\({}_{3}\)O\({}^{+}\))\({}_{\text{ad}}\) on the Pd surface with an energy of 15-21 kJ mol\({}^{-1}\). Such energy corresponds to the Langmuir-type adsorption process. Moreover, the above indicated values correspond to the energy of H\({}_{2}^{+}\) and H\({}_{3}\)O\({}^{+}\) adsorption on the Pt electrode, i.e., 12-26 kJ mol\({}^{-1}\) as determined from electrochemical data [15]. The dipole nature (Raman inactive) of an ion-electron pair separated through the interface Pd-electrolyte was expected to have a weak Raman signature. Hence, the use of a nano-rough electrode was essential for detection of inelastic light scattering via SERS. The exact band assignment requires ab initio density functional theory (DFT) calculations [20], which are currently not available. The smaller mass of H\({}_{2}^{+}\) ion should have the larger energy (wavenumber) signature as compared with the \"electron-H\({}_{3}\)O\({}^{+}\)\" ion pair (see the discussion of the isotopic effect below). It is interesting to note that in the studies [26; 27; 28; 29], devoted to IR spectroscopic investigations of HER/HOR processes on the Pt electrode in 0.5 M H\({}_{2}\)SO\({}_{4}\), it was presumed that adsorption of (H\({}_{3}\)O\({}^{+}\))\({}_{\text{ad}}\) was reflected by the band at 1100 cm\({}^{-1}\), whereas the band within 2080-2095 cm\({}^{-1}\) was assigned to the discharging particle, which was H\({}_{\text{OPD}}\), the overpotential deposited H atom. In terms of the energy of interaction between the particle and the surface, this would correspond to 14-26 kJ mol\({}^{-1}\). Recently, a lack of understanding of HER at the molecular level and the role of the interface had been articulated as the next focus area in HER research [37]. Direct comparison of the Raman spectra of the CuO electrode with and without Pd are replotted in Appendix A, Figures 15 and 16 for H\({}_{2}\)O and D\({}_{2}\)O, respectively, at the potential range of 0.3-0 V preceding to HER. A Pd deposition resulted in the suppression of the most prominent Raman bands related to CuO; however, there was almost no effect at the spectral window of 1700-1300 cm\({}^{-1}\). Especially for the HER at 0 V in H\({}_{2}\)O, there was almost no decrease in Raman intensity.