## 3 Results and Discussion
### Preparation of the Electrodes and Characterization
\[\text{A}=\Gamma\cdot\varepsilon\cdot 1000 \tag{1}\]  
Deposition of heterogeneous Pt was performed on sensitized NiO electrodes immersed in the platinizing solution based on H\({}_{2}\)PtCl\({}_{6}\) as the metal precursor [22] using simple and fast procedures upon application of a potential bias or a combination of both potential and light biases. Five different methodologies were considered. In Methods 1 and 2, a stationary potential of \(-\)0.2 V vs. SCE was applied, either coupled or not to light irradiation, and different time intervals were considered. Under these conditions, Pt deposition implies the reduction of Pt(IV) to Pt(0), most likely occurring through the transient formation of a Pt(II) species [24]. The typical current vs. time responses obtained using Methods 1 and 2 are reported in the Supplementary Materials (Figure S8). Both feature an increase in cathodic current, ascribable to nucleation and progressive growth of Pt onto the sensitized NiO surface, until reaching a plateau associated with deposition conditions limited by diffusion of Pt(IV) to the electrode surface [25]. Interestingly, the rates to achieve such plateau currents are different for the two methods, suggesting that the nucleation and growth of Pt islands onto sensitized NiO might differ to some extent when the photoexcited dye can mediate the reduction of Pt(IV) ions as in Method 1 (i.e., under irradiation conditions after hole injection into the NiO valence band) or not (i.e., under dark conditions, Method 2). Following the suggestions in the literature [26], we also employed potentiodynamic conditions for Pt attachment onto our sensitized NiO electrodes (Methods 3-5). Three cycles of cyclic voltammetry spanning the potential range between +0.3 and \(-\)0.2 V vs. SCE at a scan rate of 0.1 V-s\({}^{-1}\) were considered, either coupled or not to light irradiation (see Section 2.2). We envisioned that the application of the negative potential enabling the reduction of Pt(IV) will promote the nucleation of Pt metal onto the surface, while the subsequent anodic scan will renew the Pt(IV) electrolyte solution close to the electrode, thus potentially favoring a cleaner deposition upon the subsequent cathodic scan. The current vs. voltage traces recorded during the platinization process of sensitized NiO electrodes of type A are reported in the Supplementary Materials (Figures S9-S11). An increase in cathodic current was observed in the presence of the light bias, possibly due to the active participation of the **P1** dye in the deposition event [14]. Interestingly, a noticeable current enhancement was also observed at negative potentials, regardless of the method used, in the second and third scans, which can be attributed to progressive, favorable growth of Pt islands following kinetically limited nucleation at the first cathodic scan [26]. Combined SEM/EDS and AFM analyses were employed to characterize exemplars of sensitized and platinized NiO electrodes. Figure 3 depicts the data for two electrodes of type A obtained via photoelectrodeposition, viz. Method 1 with a deposition time of 20 s and Method 5, whereas the results for an exemplar prepared via the dark Method 2 are reported in the Supplementary Materials (Figure S12). For all the samples tested, the observation of characteristic S and Pt emissions in the EDS maps clearly confirms the simultaneous presence of both the **P1** dye and the Pt HEC, respectively. Furthermore, both the EDS and AFM analyses suggest that regardless of the deposition method, the distribution of the Pt metal is rather homogenous, supporting preferential formation onto the sensitized NiO surface of small, well-distributed Pt islands over large metal clusters. The failure to observe, at the current size resolution, appreciably different morphological features in the case of light vs. dark conditions possibly suggests that such differences are, if at all present, intrinsically small.