An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoO_x, a NiO_x/NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoO_x/NiO_x/NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoO_x/NiO_x/NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm⁻² at 1.23 V vs. RHE for OER, as compared to n-Si/CoO_x, even significantly surpassing the counterpart n-Si/CoO_x/NiO_x/FeOOH and n-Si/CoO_x/NiO_x/NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoO_x/NiO_x/NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.