Tokyo Institute of Technology scientists have shown that an oxyfluoride is capable of visible light-driven photocatalysis, a process of converting solar to fuel energy using visible-light-absorbing semiconductor materials. 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.

Experiments reveal the strong visible-light absorption of Pb2Ti2O5.4F1.2.  The inset (on the right) shows a photograph of Pb2Ti2O5.4F1.2, 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. Image Credit: Tokyo Institute of Technology. Click image for the largest view.

Now, such efforts have taken a surprising turn, with the discovery of a new photocatalytic material called a pyrochlore [1] 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, the difference in energy of an electron in the valence band and the conduction band that indicates the conductivity of a material, 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 [2] 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.”

Specifically, it is the strong interaction between certain orbitals [3] (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.

Terms Explanation:

[1] 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.

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

[3] Orbitals: The regions where electrons can be calculated to be present within atoms.

These “newly discovered” posts are the most interesting. Something brand new is found, built, works. Here is one. Sure the yield is barely there, its the first try and it works. Plus the market in artificial photosynthesis doesn’t have a huge built in inertia to work over. No market for disruption here.

These points are fuel for intensifying effort. There is a long way to go to get to a market ready product. But it will happen. It will be worth it.


2 Comments so far

  1. Benjamin Cole on June 5, 2018 7:14 AM

    These points are fuel for intensifying effort. There is a long way to go to get to a market ready product. But it will happen. It will be worth it.–Brian W.

    I dunno. It seems like every time I watch these pie-in-the-sky energy solutions, I just get crapped on, and really hard.

    I mean, toilet-time over and over again.

    You never feel the same way?

  2. Roy Long on June 7, 2018 8:37 PM


    I know what you feel like. Most of these promises are still in the lab and a long way off from being commercial.

    But take a look at HyperSolar and their progress.
    They’ve got a nano-chip sized solar cell splitting water for grid sized power hydrogen production.

    Roy Long.

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