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This study explores oxide passivation in Photoelectrochemical (PEC) electrodes featuring nanostructures. The introduction of nanostructures offers the advantage of improved light absorption, but also introduces challenges such as surface defects and hydrophobic surface properties. Oxide passivation provides a simple solution to these problems by offering a variety of material options. However, the high bandgap nature of plain oxides requires thickness optimization due to their dual role as passivation and insulating layers. Among various materials, TiO2 was selected for passivation of nanoporous silicon photocathode electrodes due to its relatively narrow bandgap and conduction band position, which closely matches the hydrogen evolution potential. In this study, we meticulously analyzed the PEC property changes induced by varying the thickness in steps of 0.5 nm, with the goal of identifying the optimal thickness for improved PEC performance. The results conclusively demonstrate that a 1.5 nm thick direct tunnelable oxide layer provides the best PEC performance. By applying a 1.5 nm thin nanolayer for passivation, a 1.6-fold improvement in photocurrent characteristics compared to Black Si was observed at -1.23 V relative to RHE. Deposition of TiO2 not only increased the photocurrent but also enhanced the photovoltage up to 110 mV, resulting in an overall positive shift in the electrode potential that is favorable for HER applications. This study highlights the importance of fine-tuning the oxide layer thickness to optimize the performance of PEC systems and highlights the potential of TiO2 passivation in overcoming limitations posed by the unique properties of nanostructured electrodes.
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