## 3 Results and Discussion
### Photoelectrochemical Characterization
NiO-A \(\mid\)**P1**\(\mid\)Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)Pt-**5**) were further characterized to gain additional insight into the photoelectrochemical per formances. We first checked if the observed photocurrent enhancements were assignable to hydrogen generation by determining the Faradaic efficiency (FE) using a generator-collector method adapted from the literature [20]. Hydrogen was produced with FEs of 38 (\(\pm\)2)% and 60 (\(\pm\)5)% for NiO-A \(\mid\)**P1**\(\mid\)**Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)**Pt-**5**, respectively (Figures S18 and S19), which substantially surmount the negligible FE measured in the absence of the Pt HEC (3 (\(\pm\)2)%, Figure S20). These data clearly confirm that the improvement in terms of photocurrent densities can be unambiguously assigned to profitable charge transfer to the aqueous electrolyte involving proton reduction to dihydrogen mediated by the Pt HEC, as previously inferred. The photoaction spectra of both NiO-A \(\mid\)**P1**\(\mid\)**Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)**Pt-**5** were then determined and compared with that of the sensitized NiO electrode without Pt HEC (Figure 4D). For all samples tested, the photoaction spectrum closely resembled the absorption spectrum of the **P1** dye [17], thus confirming that in all cases, the photoelectrochemical response is triggered by light absorption by the molecular chromophore. More interestingly, higher efficiencies were measured in the platinized electrodes, with maximum incident photon-to-current conversion efficiency (IPCE) values of 0.15% and 0.18% for NiO-A \(\mid\)**P1**\(\mid\)**Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)**Pt-**5**, respectively. Taking Equation (2) and considering a light-harvesting efficiency (LHE) of \(\sim\)0.8 at the absorption maximum--as possibly estimated according to Equations (1) and (3) considering the surface concentration of **P1** dye previously calculated (\(\sim\)11.7 mmol-cm\({}^{-2}\)) and the molar extinction coefficient at the absorption maximum (\(\epsilon\) = 58,000 M\({}^{-1}\)cm\({}^{-1}\)) [17]--these data correspond to absorbed photon-to-current conversion efficiency (APCE) values of 0.187% and 0.225% for NiO-A \(\mid\)**P1**\(\mid\)**Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)**Pt-**5**, respectively. According to Equation (2) and the almost unitary yield of hole injection (\(\Phi_{inj}\)) by the **P1** dye into NiO (\(\sim\)97%) [30], charge collection efficiencies (\(\eta_{cc}\)) of 0.19% and 0.23% can be estimated for NiO-A \(\mid\)**P1**\(\mid\)**Pt-**1** at 20 s and NiO-A \(\mid\)**P1**\(\mid\)**Pt-**5**, respectively. \[\text{APCE}=\text{IPCE}(\lambda)/\text{LHE}(\lambda)=\Phi_{inj}\cdot\eta_{cc} \tag{2}\]  
\[\text{LHE}(\lambda)=1-10^{-\text{A}(\lambda)} \tag{3}\]  
The same platinization procedures used for electrodes of type A were employed for the attachment of the Pt HEC onto NiO electrodes of type B sensitized with the **P1** dye (hereafter named NiO-B \(\mid\)**P1**\(\mid\)**Pt-**n**, where n = 1-5) in order to evaluate the potential general applicability of our deposition methods. Figure 5 shows the relevant LSV curves obtained with such electrodes under chopped irradiation in 0.1 M acetate buffer at pH 4 (the corresponding CA experiments at an applied bias of \(-\)0.2 V vs. SCE are reported in the Supplementary Materials, Figures S21-S23). Similar to type A NiO electrodes, different deposition times were considered for Methods 1 and 2 (Figures S24 and S25), and the best performances were obtained at deposition times of 20 s (Figure 5A).