Black Si Photocathode with a Conformal and Amorphous MoS 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) thin film on a nanostructured surface of black Si using atomic layer deposition (ALD). The a-MoS 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 has a smaller work function (4.0 eV) than that of crystalline 2H-MoS (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 and an onset potential of 0.27 V at 1 mA/cm. Furthermore, the a-MoS 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 layer and leads to a breakthrough in the architectural design of PEC cells to ensure both high performance and stability.