Soft and hard X-ray ambient pressure photoelectron spectroscopy of semiconductor/electrolyte interfaces for water splitting applications
26.04.2017
| Date | 26.04.2017 |
|---|---|
| Time | 11:15 |
| Place |
|
| Speaker | David Starr |
| Area of expertise | Physics |
| Host | Dep. Physik |
| Contact | |
| Abstract | Due to their potential long term stability in aqueous solutions, ease of synthesis, and low cost of production, semiconducting metal oxide materials have recently garnered much attention for use as photoelectrodes for photoelectrochemical water splitting. Most studies have focused on binary semiconducting oxide materials. However, currently no binary oxide material has met all the criteria listed above. This has driven researchers to expand the materials database and investigate more complex multinary oxides. Among multinary oxide materials, bismuth vanadate, BiVO4, is currently viewed as one of the most promising for water splitting. The monoclinic scheelite phase of BiVO4 has a band-gap of 2.4 eV and recent reports of >5 % solar to hydrogen conversion efficiency under AM 1.5 illumination makes it the most successful multinary oxide photoanode to date. We have used ambient pressure X-ray photoelectron spectroscopy to gain a molecular-level understanding of the BiVO4/aqueous electrolyte interface. With soft X-rays (AP-XPS) water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under an electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on BiVO4(010) single crystal surfaces indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium in the surface which leads to occupied states above the valence band maximum. We have also investigated the open-circuit behaviour of the thin-film BiVO4/bulk-like potassium phosphate electrolyte interface when illuminated with a solar-simulator. Upon illumination we observe changes in the BiVO4 consistent with the production of a small amount of H+ and significant restructuring of the electrolyte near the interface. These results provide fundamental information about the general behaviour of water splitting photoelectrodes under illumination. |