## 4 Conclusions  
A systematic investigation of Pt deposition onto NiO electrodes sensitized with the standard **P1** dye has been accomplished using simple methodologies based on potentiostatic or potentiodynamic procedures either by employing or not employing an additional light bias. Photoelectrochemical characterization of the resulting electrodes showed that all platinization procedures lead to active photocathodes displaying enhanced photocurrent densities compared to the blank sample without Pt HEC. The best performances were achieved using fast deposition methods that couple both potential bias and light irradiation and were associated with a homogeneous distribution of small Pt islands on the sensitized NiO substrate. In these cases, the active participation of the **P1** dye in the deposition process possibly leads to favorable chromophore-catalyst interactions that light-driven HER catalysis may benefit from. The similar results obtained using two different NiO substrates finally suggest that the simple photoelectrodeposition methods described here can be employed as general procedures for the preparation of NiO electrodes sensitized with novel dyes--either molecular- or material-based ones--towards the realization of active photocathodes for light-driven hydrogen production. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12104955/s1](https://www.mdpi.com/article/10.3390/app12104955/s1)]([https://www.mdpi.com/article/10.3390/app12104955/s1](https://www.mdpi.com/article/10.3390/app12104955/s1)). Figure S1. LSV of sensitized NiO electrode of type A in N\({}_{2}\)-purged 0.1 M acetate buffer at pH 4 in the presence of 0.01 M [Co(NH\({}_{3}\))SCl]Cl\({}_{2}\) as an irreversible electron acceptor. Figure S2. LSV of sensitized NiO electrode of type B in N\({}_{2}\)-purged 0.1 M acetate buffer at pH 4 in the presence of 0.01 M [Co(NH\({}_{3}\))SCl]Cl\({}_{2}\) as an irreversible electron acceptor. Figure S3. Film thickness of NiO electrodes of type A and type B measured using profilometry. Figure S4. Section analyses of NiO electrodes of type A and type B measured using AFM. Figure S5. Absorption spectrum of a solution (optical pathlength of 1 cm) obtained after desorption of a sensitized NiO electrode of type A (average of three samples, surface area of 1.5 cm\({}^{2}\)) with 20 \(\mu\)L TBAOH 1 M in methanol and further dilution with methanol to a total volume of 3 mL. Figure S6. Absorption spectrum (average of three samples) of a NiO electrode of type B sensitized after overnight soaking in a 0.5 mM acetonitrile solution of the **P1** dye. Figure S7. Absorption  
Figure 5: LSV of sensitized electrodes (**A**) NiO-B \(|\)**P1**\(|\)Pt-1 and NiO-B \(|\)**P1**\(|\)Pt-2 at 20 s deposition time and (**B**) NiO-B \(|\)**P1**\(|\)Pt-3**-5 in N\({}_{2}\)-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). spectrum of a solution (optical pathlength of 1 cm) obtained after desorption of a sensitized NiO electrode of type B (average of three samples, surface area of 1.5 cm\({}^{2}\)) with 20 \(\upmu\)L TBAOH 1 M in methanol and further dilution with methanol to a total volume of 3 mL. Figure S8. Exemplars of current vs. time trace recorded during the deposition with Method 1 of sensitized NiO electrodes of type A. Figure S9. Exemplar of current vs. voltage traces recorded during the deposition with Method 3 of sensitized NiO electrodes of type A. Figure S10. Exemplar of current vs. voltage traces recorded during the deposition with Method 4 of sensitized NiO electrodes of type A. Figure S11. Exemplar of current vs. voltage traces recorded during the deposition with Method 5 of sensitized NiO electrodes of type A. Figure S12. SEM/ EDS analysis of NiO electrodes of type A prepared using Method 2 at 20 s. Figure S13.