Scientists in Japan have shown that an oxyfluoride is capable of visible light-driven photocatalysis—i.e., converting solar energy to fuel energy using visible-light-absorbing semiconductor materials. The finding opens new doors for designing materials for artificial photosynthesis and solar energy research.

Pb₂Ti₂O_5.4F_1.2. The inset (on the right) shows a photograph of Pb₂Ti₂O_5.4F_1.2, which is capable of absorbing visible light. This ability is thought to be due to strong interaction between Pb and O, which is enabled by the short Pb-O bond in the pyrochlore lattice.

Over the last decade, research has intensified to develop efficient 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 the new Pb₂Ti₂O_5.4F_1.2 material.

Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visible-light-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N^3–, S^2–) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine.

Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride Pb₂Ti₂O_5.4F_1.2 that has a small band gap (ca. 2.4 eV). With suitable modification of Pb₂Ti₂O_5.4F_1.2 by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, Pb₂Ti₂O_5.4F_1.2 worked as a stable photocatalyst for visible-light-driven H₂ evolution and CO₂ reduction.

—Kuriki et al.

Kazuhiko Maeda of Tokyo Institute of Technology (Tokyo Tech), Kengo Oka of Chuo University and collaborators in Japan have succeeded in demonstrating that Pb₂Ti₂O_5.4F_1.2 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 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.

The oxyfluoride belongs to a group of compounds that had until now been largely overlooked due to the highest electronegativity 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 Pb₂Ti₂O_5.4F_1.2 lies in the unique features specific to the pyrochlore-type structure.”

Namely, it is the strong interaction between certain orbitals (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 Pb₂Ti₂O_5.4F_1.2 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.

Resources

• Ryo Kuriki, Tom Ichibha, Kenta Hongo, Daling Lu, Ryo Maezono, Hiroshi Kageyama, Osamu Ishitani, Kengo Oka, and Kazuhiko Maeda (2018) “A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light Hydrogen Evolution and Carbon Dioxide Reduction” Journal of the American Chemical Society doi: 10.1021/jacs.8b02822