Using a hematite photocatalyst, a team led by researchers from Kobe University has succeeded in producing both hydrogen gas and hydrogen peroxide at the same time from sunlight and water. An open-access paper on the work is published in Nature Communications.

Hydrogen has gained attention as one of the possible next generation energy sources. Ideally, photocatalysts could use sunlight and water to produce hydrogen, however it is necessary to achieve a conversion rate of 10% to enable such a system to be adopted industrially. It has been pointed out that even if this efficiency is achieved, the cost of hydrogen will not reach the desired value.

To overcome these issues, there is strong demand for the development of a competitive next-generation solar water-splitting system with high added value that can produce other useful chemicals at the same time as hydrogen.

In previous research, Associate Professor TACHIKAWA Takashi and colleagues at Kobe developed “mesocrystal technology”, which involves precisely aligning nanoparticles in photocatalysts to control the flow of electrons and their holes. Recently, they have succeeded in increasing the light energy conversion efficiency by applying this technology to hematite (α-Fe₂O₃), an iron oxide that in addition to being safe, inexpensive and stable (pH > 3), can absorb a wide range of visible light (approx. under 600nm).

Up until now, hematite has not been applied to the production of hydrogen peroxide. In this new study, the researchers discovered that by modifying the surface of the hematite with a composite oxide of tin and titanium ions it was possible to produce both hydrogen and hydrogen peroxide in a highly efficient and selective manner.

Associate Professor Tachikawa and colleagues found that by preparing electrodes with mesocrystals doped with two different metal ions (tin and titanium) and sintering it, it was possible to produce hydrogen peroxide as well as hydrogen safely, cheaply and stably. Hydrogen peroxide is used for many purposes including disinfecting, bleaching and soil improvement.

The research group’s next aim is to implement this technology. While continuing to improve the high efficiency of the developed photocatalyst electrode, they will try to assemble the cells into a compact module as a step towards societal implementation. They also plan to develop this mesocrystal technology with various materials and reaction systems.

This was a joint research project with Nagoya University’s Institute of Materials and Systems for Sustainability (Professor MUTO Shunsuke) and the Japan Synchrotron Radiation Research Institute (JASRI) (Chief Researcher OHARA Koji and Researcher INA Toshiaki).

Mesocrystal technology. The main problem that causes a conversion rate decline in photocatalytic reactions is that the electrons and holes produced by light recombine before they can react with the molecules (in this case, water). Tachikawa et al. created 3D structures of hematite mesocrystals with highly oriented nanoparticles via solvothermal synthesis. Furthermore, they were able to develop mesocrystal photoelectrodes for water splitting by coating and sintering the mesocrystals on the conductive glass substrate.

Mesocrystal photocatalyst for hydrogen and hydrogen peroxide production. A hematite mesocrystal is a superstructure of particles, each around 20 nanometers in size. The mesocrystals were doped with Sn²⁺ and Ti⁴⁺ which were thermally induced to diffuse, segregating to form a composite oxide (SnTiO_x) layer. The tin (Sn) on the uppermost layer is oxidized and becomes tin oxide (SnO₂).

Formation of a co-catalyst for producing hydrogen oxide via dopant segregation. Normally, photocatalytic water-splitting using hematite results in oxygen being produced from the oxidation of the water. Doping this hematite with tin ions (Sn²⁺) and titanium ions (Ti⁴⁺) and then sintering it at 700°C causes segregation of the tin and titanium dopants, leading to the formation of a composite oxide (SnTiO_x) co-catalyst with high selectivity for hydrogen peroxide production. This structural change was revealed by performing synchrotron-based X-ray total scattering measurements using beamlines BL01B1 and BLO4B2 at the SPring-8 facility, and by using a high-resolution electron microscope incorporating electron energy loss spectroscopy.

Photocatalyst formation and performance. The water-splitting reaction was promoted when voltage was applied to the photocatalyst electrode illuminated by artificial sunlight. The researchers investigated the photoelectric current density and the Faradiac efficiency which indicate the hydrogen production efficiency and the hydrogen peroxide selectivity, respectively.

It was revealed that there were positive and negative effects on hydrogen and hydrogen peroxide production if the photocatalyst was doped with only one of the metal ions. On the other hand, hematite doped with both Sn²⁺ and Ti⁴⁺ could produce hydrogen and hydrogen peroxide at the same time in a highly efficient and highly selective manner. In addition, first principle calculations suggested that the SnTiO_x co-catalyst on the hematite consisted of SnO₂/SnTiO₃ layers of a few nanometers in thickness.

Resources

• Zhang, Z., Tsuchimochi, T., Ina, T. et al. (2022) “Binary dopant segregation enables hematite-based heterostructures for highly efficient solar H₂O₂ synthesis.” Nat Commun 13, 1499 doi: 10.1038/s41467-022-28944-y