Solar-assisted water splitting is an environmentally friendly method to obtain storable hydrogen fuel. To reduce the overpotential of the Oxygen Evolution Reaction (OER), which is the bottleneck of the overall process, an efficient photoanode is required. Building, a high-throughput device with low-cost and stable material and using easy and highly reproducible techniques are the main challenges to covering the gap between research and industry.

In this respect, Silicon (Si)  is a promising candidate material due to its excellent optoelectronic properties, cost-effectiveness, and potential for large-scale production. However, its low catalytic properties and instability in alkaline solutions hinder its application as a photoanode. To overcome these limits, a strategy is offered by coupling Silicon with a protective layer of Transition Metal Oxides (TMOs), which are cost-effective, abundant, and highly catalytic toward the OER. In such a system a high charge transfer and low charge recombination at the interface of the two solid materials is mandatory to boost the efficiency of the OER. To this respect, a passivating thin interlayer, i.e. AlO_x/Au or TiO₂, can be introduced or a buried junction can be adopted, though increasing significantly the complexity of the system [1],[2].

As a representative case study, we focused on a system of nanostructures of Co₃O₄/SiO_x/Si prepared by magnetron sputtering deposition of Co and thermal annealing[3].

Here, we engineered the electric field at the solid-solid interface by reducing SiO_x defects through a pre-deposition annealing of Si at high temperatures in a vacuum. In a related way, a significant improvement in the onset of the photocurrent is observed and 500 mV of photovoltage at 10 mA/cm² is achieved. The same results were found in several similar photoanodes, showing a high reproducibility. Furthermore, a stable photocurrent in 1M KOH under working conditions has been measured for more than 70 h[4].

Si/SiO_x interface was investigated by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). Current-voltage and Impedance spectroscopy in the solid state were used to study the electrical properties of the Co₃O₄/SiO_x/Si heterojunction, elucidating the charge transfer process. A full set of electrochemical measurements (LSV, CV, EIS) were employed to investigate the photo-electrocatalytic properties. Morphological and structural characterization by GIXRD and SEM complete the analyses.

The experimental results show that pre-deposition annealing of Si is mandatory to reduce defects, and the charge transfer at the solid-solid junction is pivotal for an efficient solar-to-energy conversion system. Similar results were also obtained by a system of NiO/SiO_x/Si corroborating that this approach can be used as a general strategy to optimize the solid-solid interface of Si-TMOs photoanodes.

Considering the feasibility of the fabrication process using the versatile and highly scalable technique of sputtering deposition this work paves the way for enhancing the efficiency of Si-TMO photoanodes and potentially accelerates the transition to renewable hydrogen fuel production.