## 3 Results and Discussion
### Photoelectrochemical Characterization  
The NiO photoelectrodes functionalized with both **P1** dye and the Pt HEC, according to the procedures previously discussed, were tested by means of linear sweep voltammetry (LSV) and chronoamperometry (CA) under chopped light irradiation (calibrated at 1 sun) in N\({}_{2}\)-purged 0.1 M acetate buffer at pH 4. The associated maximum photocurrent density, viz. the maximum difference between the measured current density in the presence of light subtracted by that in the absence of light, was then extracted as a figure of merit for comparison purposes (see the estimate for an exemplar in Figure S13). The experimental conditions were fixed because at a more acidic pH, rapid discoloration of the electrode was observed, ascribed to substantial desorption of the **P1** dye from the NiO surface, while at  
Figure 3: SEM micrographs (**A,D**), corresponding EDS elemental maps (**B,E**), and AFM images (**C,F**) of sensitized NiO electrodes of type A after Pt deposition using Method 1 at 20 s deposition time (**A–C**) and Method 5 (**D–F**). a higher pH, the resulting photocurrents were considerably smaller than those measured here, thus preventing a reliable evaluation of the different deposition methodologies. Furthermore, since we aimed for a fast screening of the deposition methodology, the CA measurements were limited to a time window of 5 min. Figure 4 shows the relevant LSV curves recorded for the sensitized and platinized NiO photoelectrodes of type A. As can be clearly seen, all platinization procedures, regardless of their nature, yielded active photocathodes with measured photocurrent densities featuring onset potentials between 0.4 and 0.7 V vs. SCE and values well above those of sensitized NiO without Pt. This experimental evidence can be taken as a clear indication that all functionalized electrodes can effectively promote electron transfer processes to the electrolyte mediated by the Pt metal on the surface. In particular, the observed improved photocurrents can be ascribed to the occurrence of photoinduced electron transfer processes involving hole injection from the excited state of the **P1** dye (**\"P1**) into the valence band (VB) of NiO, followed by electron transfer from the reduced form of the dye (**P1\({}^{-}\)**) to the Pt HEC that mediates proton reduction to dihydrogen (Figure 1A). More importantly, the measured photocurrent densities appreciably depend on the procedure employed for the attachment of the catalytic metal. Figure 4: LSV of sensitized electrodes: (**A**) NiO-A **P1** **P1**-**1**, (**B**) NiO-A **P1** **Pt-2**, and (**C**) NiO-A **P1** **Pt-3-5** in N2-purged 0.1 M acetate buffer at pH 4 under chopped irradiation (in every graph, a comparison is made with the blank non-platinized electrode, grey trace); (**D**) photoaction spectra of NiO-A **P1** **Pt-1** at 20 s deposition time, NiO-A **P1** **Pt-5**, and the blank non-platinized electrode. For electrodes prepared using Method 1 (hereafter named NiO-A \(\mid\)**P1**\(\mid\)Pt-**1**; see Figure 4A for two relevant exemplars and Figure S14 for the whole series), the maximum photocurrent densities achieved were also affected by the deposition time: \(-15\)\(\mu\)A\(\cdot\)cm\({}^{-2}\) at 10 s, \(-25\)\(\mu\)A\(\cdot\)cm\({}^{-2}\) at 20 s, \(-15\)\(\mu\)A\(\cdot\)cm\({}^{-2}\) at 30 s, \(-18\)\(\mu\)A\(\cdot\)cm\({}^{-2}\) at 50 s, and \(-13\)\(\mu\)A\(\cdot\)cm\({}^{-2}\) at 100 s. Interestingly, the largest values were attained at short to intermediate deposition times (i.e., 20 s), most likely resulting as the best balance in terms of catalyst loading. A similar effect was already observed in our previous study using CdTe/CdS quantum dots as sensitizers [16]. A large catalyst loading for NiO-A \(\mid\)**P1**\(\mid\)Pt-**1** electrodes at longer deposition times (see, e.g., 100 s in Figure 4A) was further confirmed by the noticeable increase in cathodic dark current, likely ascribable to direct electrocatalysis by Pt.