Solar Energy: Mixed Anion Compounds with ‘Fluorine’ Works as New Photocatalytic Material

Nobel photocatalyst composed of abundant elements overturns conventional views

photocatalysts
Figure 1. Experiments reveal the strong visible-light absorption of Pb2Ti2O5.4F1.2 The inset shows an image of Pb2Ti2O5.4F1.2, shown to be capable of absorbing visible light of a wavelength of around 500 nm. This ability is thought to be due to the bonding structure around the Pb cation within the pyrochlore lattice, shown on the right.

Scientists in Japan have shown that an oxyfluoride is capable of visible light-driven photocatalysis1. The finding opens new doors for designing materials for artificial photosynthesis and solar energy research.

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 pyrochlore2 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 gap3 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 electronegativity4 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 orbitals5 (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. (See Figure 1.)

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.

Source : Tokyo Institute of Technology