The Role of Morphology and Phase Engineering in Molybdenum Sulfide for Robust Photoelectrochemical Hydrogen Evolution on III-V Photoelectrodes.

Solar-driven hydrogen production via photoelectrochemical (PEC) water splitting offers a direct route to decarbonized fuels without grid dependence. Although III-V semiconductors exhibit exceptional optoelectronic properties, their reliance on platinum group cocatalysts limits scalability. Here, we report a platinum group metal-free (PGM-free) hybrid photocathode integrating an epitaxially grown GaAs/Si:p junction with mixed-phase MoS2 nanostructures for efficient hydrogen evolution in acidic media. Through systematic control of morphology, crystallinity, and phase composition, we demonstrate that 1T'-rich MoS2 nanoflowers outperform exfoliated nanosheets, bulk 2H-MoS2, and amorphous powder MoSx. The nanoflower architecture delivers higher saturation photocurrent densities and a positive shift in onset potential, attributed to enhanced charge transport, catalytic kinetics, and light penetration, while simultaneously improving stability. Notably, the hierarchical nanoflower morphology and 1T' phase synergistically enhance photocurrent performance. The optimized GaAs/Si:p photocathodes modified with MoS2 nanoflowers exhibit stable operation with a Faradaic efficiency of ~97% for hydrogen evolution. These findings establish morphology-phase synergy as a critical design principle for developing PGM-free PEC systems, paving the way for cost-effective solar fuel production.