## 1 Introduction  
The generation of molecular hydrogen through photochemical water splitting currently represents a fundamental reaction in the framework of sustainable development [1; 2; 3; 4]. One of the possible approaches to target such a challenging process is represented by the dye-sensitized photoelectrochemical cell (DS-PEC). This device is ideally conceived as being comprised of two compartments, viz. a photocathode and a photoanode [5; 6]. The former is based on a p-type semiconductor functionalized with a sensitizer and a hydrogen evolving catalyst (HEC), whereas the latter is composed of an n-type semiconductor, a chromophore unit, and an oxygen evolving catalyst (OEC).  
The design of a tandem DS-PEC that incorporates both a photocathode and a photoanode necessitates the study and optimization of each single compartment. As far as the photocathode side is concerned, its structural and functional role takes inspiration from that of dye-sensitized photocathodes employed in p-type dye-sensitized solar cells (p-DSSCs) [7; 8; 9]. In this respect, nanocrystalline NiO currently represents the standard material of choice for the design of photocathodes, and its coupling with photochemical and catalytic active components, particularly of a molecular nature, has been pursued to target photoelectrochemical hydrogen evolution [10]. As to the attachment of both active components onto the NiO surface, different approaches have been attempted to date, which include, among others, co-functionalization of NiO with separated dye/HEC units [11], use of molecular dyads [12], and layer-by-layer deposition of a sensitizer and a catalyst [13]. Quite unexpectedly, within this framework, the coupling of a dye-sensitized photoelectrodewith material-based catalysts--potentially merging the tunability of the dye components with the efficiency and stability of solid-state catalysts--has rarely been considered [14; 15].  
We have recently reported on photoelectrochemical hydrogen evolution using photo-cathodes based on mesoporous NiO, CdTe/CdS quantum dots as sensitizers and either a molecular nickel complex or heterogeneous platinum as the HEC [16]. In the latter case, attachment of the Pt HEC was performed through a simple photoelectrodeposition method. Interestingly, the photoelectrochemical performance of the resulting photoelectrodes was observed to be largely affected by the deposition time and conditions. Since no systematic evaluation of Pt deposition onto sensitized NiO electrodes has been attempted so far, we decided to perform a detailed investigation on the effects of the deposition conditions on the hydrogen evolving reaction (HER) promoted at a NiO photocathode. In order to make this study as general as possible, we considered direct Pt deposition onto mesoporous NiO sensitized with the standard **P1** dye (Figure 1), a reference organic compound used in p-DSSCs and whose photochemical behavior on NiO is rather well-established [17; 18; 19]. We also employed two mesoporous NiO substrates prepared via different procedures in order to evaluate potential effects from the semiconducting support as well as the possible generalization of the methodology presented here. The results highlight that the best photoelectrochemical performances can be achieved using appreciably fast deposition methods that couple both light and potential bias.