Molecular and Material Engineering of Photocathodes Derivatized with Polyoxometalate-Supported {Mo3S4} HER Catalysts.

Molecular engineering of efficient HER catalysts is an attractive approach for controlling the spatial environment of specific building units selected for their intrinsic functionality required within the multistep HER process. As the {Mo3S4} core derived as various coordination complexes has been identified as one as the most promising MoSx-based HER electrocatalysts, we demonstrate that the covalent association between the {Mo3S4} core and the redox-active macrocyclic {P8W48} polyoxometalate (POM) produces a striking synergistic effect featured by high HER performance. Various experiments carried out in homogeneous conditions showed that this synergistic effect arises from the direct connection between the {Mo3S4} cluster and the toroidal {P8W48} units featured by a stoichiometry that can be tuned from two to four {Mo3S4} cores per {P8W48} unit. In addition, we report that this effect is preserved within heterogeneous photoelectrochemical devices where the {Mo3S4}-{P8W48} (thio-POM) assembly was used as cocatalyst (cocat) onto a microstructured p-type silicon. Using a drop-casting procedure to immobilize cocat onto the silicon interface led to high initial HER performance under simulated sunlight, achieving a photocurrent density of 10 mA cm-2 at +0.13 V vs RHE. Furthermore, electrostatic incorporation of the thio-POM anion cocat into a poly(3,4-ethylenedioxythiophene) (PEDOT) film is demonstrated to be efficient and straightforward to durably retain the cocat at the interface of a micropyramidal silicon (SimPy) photocathode. The thio-POM/PEDOT-modified photocathode is able to produce H2 under 1 Sun illumination at a rate of ca. 100 µmol cm-2 h-1 at 0 V vs RHE, highlighting the excellent performance of this photoelectrochemical system.

Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-Decorated Silicon Photocathodes by the Temperature and Time of Electroless Pt Deposition.

The electroless deposition of Pt nanoparticles (NPs) on hydrogen-terminated silicon (H-Si) surfaces is studied as a function of the temperature and the immersion time. It is demonstrated that isolated Pt structures can be produced at all investigated temperatures (between 22 and 75 °C) for short deposition times, typically within 1-10 min if the temperature is 45 °C or less than 5 min at 75 °C. For longer times, dendritic metal structures start to grow, ultimately leading to highly rough interconnected Pt networks. Upon increasing the temperature from 22 to 75 °C and for an immersion time of 5 min, the average size of the observed Pt NPs monotonously increases from 120 to 250 nm, and their number density calculated using scanning electron microscopy decreases from (4.5 ± 1.0) × 108 to (2.0 ± 0.5) × 108 Pt NPs cm-2. The impact of both the morphology and the distribution of the Pt NPs on the photoelectrocatalytic activity of the resulting metallized photocathodes is then analyzed. Pt deposited at 45 °C for 5 min yields photocathodes with the best electrocatalytic activity for the hydrogen evolution reaction. Under illumination at 33 mW cm-2, this optimized photoelectrode shows a fill factor of 45%, an efficiency (η) of 9.7%, and a short-circuit current density (|Jsc|) at 0 V versus a reversible hydrogen electrode of 15.5 mA cm-2.

Protected Light-Trapping Silicon by a Simple Structuring Process for Sunlight-Assisted Water Splitting.

Macroporous layers are grown onto n-type silicon by successive photoelectrochemical etching in HF-containing solution and chemical etching in KOH. This specific latter treatment gives highly antireflective properties of the Si surface. The duration of the chemical etching is optimized to render the surface as absorbent as possible, and the morphology of the as-grown layer is characterized by scanning electron microscopy. Further functionalization of such structured Si surface is carried out by atomic layer deposition of a thin conformal and homogeneous TiO2 layer that is crystallized by an annealing at 450 °C. This process allows using such surfaces as photoanodes for water oxidation. The 40 nm thick TiO2 film acts indeed as an efficient protective layer against the photocorrosion of the porous Si in KOH, enhances its wettability, and improves the light absorption of the photoelectrode. The macroporous dual-absorber TiO2/Si has a beneficial effect on water oxidation in 1 M KOH and leads to a considerable negative shift of the onset potential of ∼400 mV as well as a 50% increase in photocurrent at 1 V vs SCE.