Passivation Effect of Rock-Salt Nitride Materials on a Planar <i>p</i>-Si Photocathode for Solar Water Reduction

Photoelectrochemical water splitting, which uses sunlight to produce clean hydrogen fuel, is gaining traction with rising concerns about fossil fuels and pollution. Silicon (Si) photocathodes are promising for this process due to their light absorption and favorable band alignment for hydrogen production. However, limitations such as reflectance, low photovoltage, and rapid photocorrosion hinder their overall performance and stability. An interfacial insulating layer such as SiO_<i>x</i> ensures superior durability to <i>p</i>-Si photocathodes. However, the inherent heterogeneity and intrinsic defects pose challenges to effective surface passivation. This study investigates the challenges of photocorrosion and utilization of a thin titanium nitride (TiN) passivation layer deposited using direct-current magnetron sputtering to protect the native silicon oxide (SiO_<i>x</i>) layer. Additionally, a molybdenum oxynitride (Mo(O,N)_<i>x</i>) bifunctional layer is employed to enhance both the electrocatalytic activity and durability of the photocathode. The presence of oxygen and nitrogen within this cocatalyst layer modifies the surface chemistry of the <i>p</i>-Si photocathode, promoting favorable interfacial interactions with electrolyte species during PEC reactions. This modification facilitates efficient charge transfer processes and accelerates reaction kinetics, ultimately optimizing the performance and operational lifetime of the photocathode. The resulting Mo(O,N)_<i>x</i>/TiN/<i>p</i>-Si photocathode exhibited a remarkable onset potential of +0.46 V_RHE in harsh acidic conditions under simulated sunlight (AM 1.5G illumination, 100 mW·cm^-2). Notably, the photocathode demonstrated exceptional long-term stability exceeding 140 h, highlighting the combined TiN passivation and Mo(O,N)_<i>x</i> cocatalyst as a protection strategy.