Dye-sensitized photocatalyst achieves doubled efficiency in solar-driven hydrogen generation

A research team led by Graduate Student Haruka Yamamoto and Professor Kazuhiko Maeda from the Department of Chemistry, School of Science at the Institute of Science Tokyo has successfully developed a new dye-sensitized photocatalyst that can utilize visible light wavelengths that were previously unusable. This achieves approximately double the energy conversion efficiency when generating hydrogen from solar energy. Their research was published in ACS Catalysis.

Solar light contains a wide range of wavelengths, but to utilize it for hydrogen generation, the light must first be absorbed by a photosensitizer. Dye-sensitized photocatalysts using ruthenium complexes as photosensitizers have been extensively studied and are known to exhibit high performance in water-splitting actions for hydrogen generation.

Maeda's research team previously developed a dye-sensitized photocatalyst by adsorbing ruthenium complexes that consume visible light onto HCa₂Nb₃O₁₀ oxide nanosheets, successfully achieving hydrogen generation reactions utilizing visible light. However, conventional ruthenium complexes used in dye-sensitized photocatalysts can absorb visible light only up to approximately 600 nanometers. This highlighted the need for new design guidelines to utilize longer wavelength light.

The research team has now successfully developed a new dye-sensitized photocatalyst capable of utilizing visible light in the previously unusable long wavelength region (600-800 nanometers). By replacing the central metal of the conventional ruthenium complex with the heavier osmium, the heavy atom effect becomes functionally significant, enabling singlet-triplet excitation that involves electron spin reversal - a process not prominently observed in ruthenium complexes. Leveraging this mechanism highlighted that long wavelength light of 600-800 nanometers, which could not be absorbed by conventional ruthenium complex systems, can be effectively utilized. In hydrogen generation reactions under visible light irradiation, the system showed higher apparent quantum yields than ruthenium complex systems and achieved a solar energy conversion efficiency of 0.21%, approximately double that of conventional systems.

This research demonstrates that solar energy can be converted into hydrogen more efficiently by utilizing the long wavelength components of sunlight that were previously underutilized. The findings show that long wavelength light can be harnessed by triplet excitation in osmium complexes. In the future, further optimization of complex structures through more advanced molecular design is expected to enable solar hydrogen production systems which use less expensive reducing agents with fewer resource constraints.

Among photoenergy conversion systems where metal complexes are responsible for light absorption, there are dye-sensitized solar cells in practical use already. In basic research, there are similarly photocatalysts that convert carbon dioxide to useful substances. The findings of this research may also contribute to the development of photoenergy conversion materials across these different fields.

Journal Information
Publication: ACS Catalysis
Title: Charge Transfer Dynamics in Dye-Sensitized Photocatalysts Using Metal Complex Sensitizers with Long-Wavelength Visible Light Absorption Based on Singlet-Triplet Excitation
DOI: 10.1021/acscatal.5c06687