Producing hydrogen fuel through solar-driven photoelectrochemical (PEC) water splitting represents an attractive approach for the development of sustainable energy. 2D semiconducting Transition Metal dichalcogenide (TMDC) nanosheets are attractive materials for solar energy conversion and have been applied as photocathodes for the Hydrogen Evolution Reaction (HER). To improve the performances of the photocathodes, optimized TMDC film microstructure was proposed involving a self-assembly at a liquid-liquid interface. In addition, in combination with a hexyl-tri-silane surfactant treatment, molecularly thin 2D WSe2 nanosheets films were coated by a Pt-Cu catalyst. In this process, the increase of photocurrent resulting from the surfactant treatment was attributed to edge site passivation of the 2D nanosheets drastically decreasing the charge carrier recombination site density. Indeed, for a large scale development of these photocathodes, replacement of Pt-Cu by a precious metal-free catalyst is highly desirable. In this talk, we will present our recent results concerning Mo thio complexes as catalysts anchored on WSe2 nanosheets for efficient water splitting under visible light irradiation. We used WSe2 films prepared by a simple film forming process, i.e. drop casting, and an aqueous environmentally friendly process for the catalyst preparation. Active molecular Mo catalysts including (Mo2S12)2- were deposited via a selection process from a solution allowing the deposition of the most interactive Mo complexes towards the WSe2 nanosheets thus improving their anchorage and durability. Our ultrathin films showed a current density up to -2 mA/cm² at 0 V vs NHE without any passivation additive such as HTS surfactant. The respective contributions namely in electro-catalysis and site passivation observed experimentally for the various molecular Mo complexes involved were supported by DFT calculations of i) Adsorption energies of the Mo thio complexes on the WSe2 substrates and ii) Free energies changes for the hydrogen Evolution Reaction on the various adsorbed molecular Mo complexes. These results will greatly help to better design molecular photo-catalysts for photo-electrodes displaying high surface recombination site density.