Water adsorption and dissociation on the surface of bismuth vanadate causes the localization of excess electrons into small polaron states at vanadium sites, represented by the yellow and blue clouds. Image: HZB/J. Am. Chem. Soc. 2022.Every green leaf is able to convert solar energy into chemical energy, storing it as chemical compounds. However, an important sub-process of photosynthesis can already be imitated technically, in the form of solar hydrogen production – sunlight generates a current in a so-called photoelectrode that is used for splitting water molecules. This produces hydrogen, a versatile fuel that can be used to store solar energy in chemical form and release it when needed.
At the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany, many teams are working on this vision. The focus of their research is on producing efficient photoelectrodes, which are semiconductors that remain stable in aqueous solutions and are highly active. Not only do photoelectrodes convert sunlight into electrical current, but they can also act as catalysts to accelerate the splitting of water. Among the best candidate materials for inexpensive and efficient photoelectrodes is bismuth vanadate (BiVO4).
"Basically, we know that just by immersing bismuth vanadate in the aqueous solution the chemical composition of the surface changes," says David Starr at the HZB Institute for Solar Fuels.
"Although there are a great many studies on BiVO4, it has not been clear until now exactly what implications this has on the surface electronic properties once they come into contact with the water molecules," adds Marco Favaro, also at the HZB Institute for Solar Fuels. In this study, the researchers have now addressed this question, reporting their findings in a paper in the Journal of the American Chemical Society,
They used resonant ambient-pressure photoemission spectroscopy at the Advanced Light Source at Lawrence Berkeley National Laboratory to study single crystals of BiVO4 doped with molybdenum when exposed to water vapor. A team led by Giulia Galli at the University of Chicago then performed density functional theory calculations to help interpret the data, with the aim of untangling the contributions of individual elements and electron orbitals to the electronic states.
“In situ resonant photoemission has allowed us to understand how the electronic properties of our BiVO4 crystals changed upon water adsorption,” says Favaro. The combination of measurements and calculations showed that due to excess charge, generated by either doping or defects on certain surfaces of the crystal, so-called polarons may form. These are negatively charged localized states where water molecules can easily attach and then dissociate. The hydroxyl groups formed via water dissociation help to stabilize further polaron formation.
"The excess electrons are localized as polarons at VO4 units on the surface," explains Starr.
"What we can't yet assess for sure is what role the polarons play in charge transfer. Whether they promote it and thus increase efficiency or, on the contrary, are an obstacle, we still need to figure that out.".
These results provide valuable insights into processes that modify the surface chemical composition and electronic structure of BiVO4, and might foster the knowledge-based design of better photoelectrodes for green hydrogen production.
This story is adapted from material from Helmholtz-Zentrum Berlin für Materialien und Energie, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.