(351h) Understanding Photoelectrochemistry on Epitaxial Oxides through Surface Electronic Structure

The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. One earth abundant storage option is water electrolysis, storing energy in the bonds of O₂ and H₂. Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces can convert sunlight to chemical fuels without external circuitry, such as “splitting” water into O₂ and H₂ upon illumination of a photoabsorber and catalyst. Tuning the location of the band edges of a semiconductor tailors the wavelength of photoabsorption and the charge transfer to adsorbed species. The efficacy of conversion depends in part on the rectifying properties of semiconductor–electrolyte junctions, as the band bending present in the semiconductor space-charge region from Fermi level equilibration drives the separation of electron–hole pairs. However, the relationship between band structure, band bending/flattening, photocatalytic activity, and surface functionality (such as hydroxylation properties and hydrophilicity) is not well understood. Improving our understanding of the semiconductor/electrolyte interface is crucial in the design of efficient photoelectrode materials for harvesting solar energy in a PEC system.

In the particular case of the semiconductor Ge, favorable conduction band alignment with the hydrogen evolution reaction (HER) suggests the potential for efficient solar-to-hydrogen conversion with visible light. However, material instability in aqueous environments and the potential for photocorrosion limits its utility in PEC applications. In order to protect the photoabsorber surface while maintaining favorable alignment of the conduction band, we epitaxially grow SrTiO₃ on top of p-Ge. We will present studies of such model oxide photoelectrodes grown by molecular beam epitaxy (MBE) on single crystal substrates that display a known crystallographic orientation, surface area, path for charge transport, and strain. Photoelectrochemical measurements on these heterostructures can establish the intrinsic activity of oxide catalysts in a way that cannot be realized with polydisperse nanoparticle systems. Insight into the band bending between the substrate and oxide overlayer, as well as at the semiconductor surface, can be obtained from X-ray photoelectron spectroscopy (XPS).¹ Measurement of XPS at ambient pressures (AP-XPS) can further elucidate the relationship between adsorbates and surface band bending.² This fundamental insight will build understanding necessary for the design of active, earth-abundant photocatalysts that can be integrated into PEC devices for efficient conversion of solar energy into chemical fuels.

References

1. S.A. Chambers, Y. Du, R.B. Comes, S.R. Spurgeon, P.V. Sushko, Applied Physics Letters, 110, 082104 (2017).

2. K.A. Stoerzinger,R. Comes,S.R. Spurgeon, S. Thevuthasan, K. Ihm,E.J. Crumlin, S.A. Chambers. J. Phys. Chem. Lett. 8, 1038 (2017).