Mixed Anion Compounds with 'Fluorine' Work as New Photocatalytic Material

Now, such efforts have taken a surprising turn with the discovery of a new photocatalytic material called a pyrochlore² oxyfluoride (Pb₂Ti₂O_5.4F_1.2).

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 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 Pb₂Ti₂O_5.4F_1.2 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 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.

The inset shows an image of Pb₂Ti₂O_5.4F_1.2, 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.

Technical terms

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

²Pyrochlore：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.

³Band 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.

⁴Electronegativity: 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.

References

Ryo Kuriki^1,2, Tom Ichibha³, Kenta Hongo^4,5,6,7, Daling Lu⁸, Ryo Maezono^3,7, Hiroshi Kageyama⁹, Osamu Ishitani¹, Kengo Oka¹⁰, and Kazuhiko Maeda*¹, 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 
¹ Department of Chemistry, School of Science, Tokyo Institute of Technology
² Japan Society for the Promotion of Science
³ School of Information Science, JAIST
⁴ Research Center for Advanced Computing Infrastructure, JAIST
⁵ Center for Materials Research by Information Integration, Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science
⁶ PRESTO, Japan Science and Technology Agency
⁷ Computational Engineering Applications Unit, RIKEN
⁸ Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology
⁹ Graduate School of Engineering, Kyoto University
¹⁰ Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University

Related links

https://www.titech.ac.jp/english/news/2017/038625.html

https://www.titech.ac.jp/english/research/stories/faces15_ishitani.html

http://www.chemistry.titech.ac.jp/~ishitani/en/index_en.html

About Tokyo Institute of Technology

Tokyo Institute of Technology stands at the forefront of research and higher education as the leading university for science and technology in Japan. Tokyo Tech researchers excel in a variety of fields, such as material science, biology, computer science and physics. Founded in 1881, Tokyo Tech has grown to host 10,000 undergraduate and graduate students who become principled leaders of their fields and some of the most sought-after scientists and engineers at top companies. Embodying the Japanese philosophy of "monotsukuri," meaning technical ingenuity and innovation, the Tokyo Tech community strives to make significant contributions to society through high-impact research. www.titech.ac.jp/english/

About the Faculty of Science and Engineering, Chuo University

The Faculty of Science and Engineering at the Korakuen Campus is conveniently located in the center of Tokyo. Our 4,000 students are studying a wide range of science and engineering fields throughout our ten departments. We emphasize "research-based studies" in over 100 advanced laboratories as well as rich education of the fundamental natural sciences. In the summer of 2011, our new research building was completed with state-of-the-art facilities.

SOURCE Tokyo Institute of Technology; Chuo University