Solar energy: Mixed anion compounds with 'fluorine' works as new photocatalytic material

Over the last decade, research has intensified to develop efficient, manmade photocatalysts that work under visible light -- an important target for renewable energy systems.

Now, such efforts have taken a surprising turn, with the discovery of a new photocatalytic material called a pyrochlore[2] oxyfluoride (Pb₂Ti₂O₅.₄F₁.₂).

Kazuhiko Maeda of Tokyo Institute of Technology (Tokyo Tech), Kengo Oka of Chuo University and collaborators in Japan have succeeded in demonstrating that Pb₂Ti₂O₅.₄F₁.₂ works as a stable photocatalyst for visible light-driven water splitting and carbon dioxide reduction, with the aid of proper surface modifications.

The new material has an unusually small band gap[3] of around 2.4 electron volts (eV), meaning that it can absorb visible light with a wavelength of around 500 nanometers (nm). In general, band gaps bigger than 3 eV are associated with inefficient utilization of sunlight, whereas those smaller than 3 eV are desirable for efficient solar energy conversion.

What's more, the oxyfluoride belongs to a group of compounds that had until now been largely overlooked due to the highest electronegativity[4] of fluorine, a property that essentially ruled them out as candidates for visible light-driven photocatalysts.

The new oxyfluoride is "an exceptional case," the researchers say in their study published in the Journal of the American Chemical Society.

Based on structural considerations and theoretical calculations, they conclude that "the origin of the visible light response in Pb₂Ti₂O₅.₄F₁.₂ lies in the unique features specific to the pyrochlore-type structure."

Namely, it is the strong interaction between certain orbitals[5] (Pb-6s and O-2p) enabled by short Pb-O bonding in the pyrochlore structure that is thought to give rise to the material's ability to absorb visible light.

One limitation is that the yield of the new photocatalyst currently remains low, at a figure of around 0.01% at 365 nm for hydrogen evolution. The research team is therefore investigating how to boost the yield by modifying Pb₂Ti₂O₅.₄F₁.₂ through refinement of methods for synthesis and surface modification.

The present study arose as a result of collaborations between institutes including Tokyo Tech, Japan Advanced Institute of Science and Technology (JAIST), the National Institute for Materials Science (NIMS), RIKEN, Kyoto University and Chuo University.

The findings are expected to lead to new directions in materials research and future development of heterogeneous photocatalysts under visible light.

Technical terms

[1] Visible light-driven photocatalysis: The process of converting solar to fuel energy using visible-light-absorbing semiconductor materials.

[2] Pyrochlore: One of crystal structures represented by a chemical formula of A₂B₂X₆X', where A and B show cations, X and X' show anions. The A and B elements are generally rare-earth or transition metal elements. The presence of two short bonds between A site ion (Pb) and X' site (O) is the characteristic of this structure.

[3] Band gap: Refers to the difference in energy of an electron in the valence band and the conduction band, which indicates the conductivity of a material.

[4] Electronegativity: A property whereby electrons are held tightly to the nucleus. Fluorine has the highest electronegativity among all elements.

5 Orbitals: The regions where electrons can be calculated to be present within atoms.