When light is applied to photocatalysts, electrons and holes are produced on the surface of the catalyst; hydrogen is obtained when these electrons reduce the hydrogen ions in water. However, in traditional photocatalysts the holes that are produced at the same time as the electrons mostly recombine on the surface of the catalyst and disappear, making it difficult to increase conversion efficiency.

Professor Tachikawa’s research group developed a novel perovskite strontium titanate (SrTiO₃) mesocrystal superstructure with well-defined orientation of assembled cubic nanocrystals. They deliberately created a lack of uniformity in size and arrangement of the crystals.

This new photocatalyst is able to spatially separate the electrons and electron holes to prevent them recombining. As a result, it has a more efficient conversion rate for producing hydrogen than conventional nanoparticulate photocatalysts.

The team developed a new method called “topotactic epitaxial growth” that uses the nanometer-sized spaces in mesocrystals. Topotactic epitaxy is a solid-tate reaction in which the orientations of the epitaxially grown product crystals are determined by the orientation of the initial crystals.

Synthesis of SrTiO mesocrystals by Topotactic Epitaxial Growth. Using the spaces in mesocrystals, SrTiO3 mesocrystals are synthesized from TiO2 mesocrystals using a one-step hydrothermal process. Click to enlarge.

Using this method, they synthesized strontium titanate from a compound with a different structure, titanium oxide (TiO₂), using a simple one-step hydrothermal reaction. By lengthening the reaction time, they could also grow larger particles near the surface while preserving their crystalline structure.

When they attached a co-catalyst to the synthesized mesocrystal and applied ultraviolet light in water, the reaction occurred with approximately 7% light energy conversion efficiency.

Under the same conditions, SrTiO₃ nanoparticles which had not been converted into mesocrystals reached a conversion efficiency of less than 1%, showing that the reaction efficiency increased tenfold under the mesocrystal structure.

When each particle was examined under a fluorescent microscope, the team found that the electrons produced during the reaction gathered around the larger nanocrystals.

When exposed to ultraviolet light, the electrons in this newly-developed photocatalyst move smoothly between the nanoparticles inside the mesocrystal, gather around the larger nanocrystals generated on the surface of the crystal, and efficiently reduce the hydrogen ions to create hydrogen.

Hydrogen synthesis in SrTiO3 mesocrystals. (a) The electrons produced under ultraviolet light move between the nanoparticles in the mesocrystal, gather around the comparatively large nanocrystals exposed on the surface, and reduce the hydrogen ions. (b) In the nanocrystals that aggregated with a disordered structure, most of the electrons and electron holes recombine and disappear, so the amount of hydrogen produced is low. Click to enlarge.

The discovery of this powerful photocatalyst started with the researchers’ idea to “deliberately break down the ordered structure of mesocrystals”, a concept that could be applied to other materials. The strontium titanate used this time is a cubic crystal—there is no variation in molecular adsorption or the reaction strength for each crystal plane. By regulating the size and spatial arrangement of the nanocrystals, which form the building blocks for this structure, it may be possible to greatly increase the light energy conversion efficiency of the existing system.

Using these findings, the research group plans to apply mesocrystal technology to realizing the super-efficient production of hydrogen from solar energy. The perovskite metal oxides, including strontium titanate, the target of this study, are the fundamental materials of electronic elements, so their results could be applied to a wide range of fields.

Resources

• P. Zhang, T. Ochi, M. Fujitsuka, Y. Kobori, T. Majima, T. Tachikawa (2017) “Topotactic Epitaxy of SrTiO₃ Mesocrystal Superstructures with Anisotropic Construction for Efficient Overall Water Splitting” Angew. Chem. Int. Ed. 56, 5299 doi: 10.1002/anie.201702223