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New mesocrystal photocatalyst enhances light-driven hydrogen production

18 May 2017

A group of Japanese researchers has developed a novel photocatalyst for increased hydrogen production. The strontium titanate mesocrystal exhibits three times the efficiency for hydrogen evolution compared to conventional disordered systems in alkaline aqueous solution. The mesocrystal also exhibits a high quantum yield of 6.7% at 360 nm in overall water splitting and even good durability up to 1 day.

The discovery was made by a joint research team led by Associate Professor Takashi Tachikawa (Molecular Photoscience Research Center, Kobe University) and Professor Tetsuro Majima (Institute of Scientific and Industrial Research, Osaka University). Their findings were published in the journal Angewandte Chemie International Edition.

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.

2017_05_01_01-1
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.

2017_05_01_01-5
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

May 18, 2017 in Catalysts, Hydrogen, Hydrogen Production, Solar | Permalink | Comments (6)

Comments

It would be interesting to know on what grounds critics of the hydrogen economy have ruled out perovskite strontium titanate mesocrystals fopr the production of hydrogen, or if like me they have never heard of it either, but reckon that they can rule it out anyway along with any other method of producing hydrogen.

Well, it saves studying the subject and subsequently forming an opinion I suppose.

Whatever it is, it won't work anyway.

Really profound.

This is a lab discovery that could lead to others.
Science asks WHY this is so, if they know they can do even better.

Can they construct bi fuel cars with 2 tanks. One tank for hydrogen and one conventional tank of gas. You then mix some hydrogen and gas in an ice car. If hydrogen is made cheaply with no tax , it can lower our fuel bills, decrease gas consumption and alleviate pollution.

Hydrogen is hard to store and transport. This is the main reason for the non-appearance of the hydrogen economy.
Thus, Gorr, it would be tricky (expensive and bulky) to have two storage tanks.

@mahonj if they fit a really big hydrogen tank to a small car when the tank go empty, we can thus have a cheap helicopter because the empty hydrogen tank get lighter than air. And depending on the wind we can fly far away for no cost.

Than makes no sense, but you don't know that.

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