Abstract

By combining ab initio molecular dynamics simulations and many-body perturbation theory calculations of electronic energy levels, we determined the band edge positions of functionalized Si(111) surfaces in the presence of liquid water, with respect to vacuum and to water redox potentials. We considered surface terminations commonly used for Si photoelectrodes in water splitting experiments. We found that, when exposed to water, the semiconductor band edges were shifted by approximately 0.5 eV in the case of hydrophobic surfaces, irrespective of the termination. The effect of the liquid on band edge positions of hydrophilic surfaces was much more significant and determined by a complex combination of structural and electronic effects. These include structural rearrangements of the semiconductor surfaces in the presence of water, changes in the orientation of interfacial water molecules with respect to the bulk liquid, and charge transfer at the interfaces, between the solid and the liquid. Our results showed that the use of many-body perturbation theory is key to obtain results in agreement with experiments; they also showed that the use of simple computational schemes that neglect the detailed microscopic structure of the solid-liquid interface may lead to substantial errors in predicting the alignment between the solid band edges and water redox potentials.

Highlights

  • The photocatalysis of water splitting is a promising way to capture and store solar energy and is an active research field.[1]

  • The optimal band gap of the solid should be larger than 1.9 eV and smaller than 3.1 eV in order to fall within the visible range of the solar spectrum.[1,3]

  • We devised a computational strategy to compute, from firstprinciples, band edge positions of semiconductors and insulators interfaced with liquid water, with respect to vacuum and to water redox potentials

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Summary

Introduction

The photocatalysis of water splitting is a promising way to capture and store solar energy and is an active research field.[1]. A potentially more efficient PEC design is based on two semiconductor−liquid junctions: an n-type semiconductor, for the photoanode where water oxidation occurs, and a p-type one, for the photocathode where water reduction takes place.[2,4] Each photoelectrode provides part of the water splitting potential, and semiconductors with smaller band gaps that absorb a larger fraction of the visible light can be utilized to improve the overall conversion efficiency of the device For this design to work, the CBM of the photocathode must be higher than the water reduction potential H+/H2O, and the VBM of the photoanode, lower than the oxidation potential O2/H2O. Irrespective of the scheme chosen to build a PEC cell, one of the critical factors to select candidate semiconductors for the electrodes is the alignment between their band edge positions and water redox potentials

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