Black Si Photocathode with a Conformal and Amorphous MoS x Catalytic Layer Grown Using Atomic Layer Deposition for Photoelectrochemical Hydrogen Evolution.

We demonstrated how the photoelectrochemical (PEC) performance was enhanced by conformal deposition of an amorphous molybdenum sulfide (a-MoS x ) thin film on a nanostructured surface of black Si using atomic layer deposition (ALD). The a-MoS x is found to predominantly consist of an octahedral structure (S-deficient metallic phase) that exhibits high electrocatalytic activity for the hydrogen evolution reaction with a Tafel slope of 41 mV/dec in an acid electrolyte. The a-MoS x has a smaller work function (4.0 eV) than that of crystalline 2H-MoS 2 (4.5 eV), which induces larger energy band bending at the p-Si surface, thereby facilitating interface charge transfer. These features enabled us to achieve an outstanding kinetic overpotential of ∼0.2 V at 10 mA/cm 2 and an onset potential of 0.27 V at 1 mA/cm 2 . Furthermore, the a-MoS x layer provides superior protection against corrosion of the Si surface, enabling long-term PEC operation of more than 50 h while maintaining 87% or more performance. This work highlights the remarkable advantages of the ALD a-MoS x layer and leads to a breakthrough in the architectural design of PEC cells to ensure both high performance and stability.