A team led by Professor Kazuhiko Maeda and graduate student Haruka Yamamoto at the Institute of Science Tokyo has developed a dye-sensitized photocatalyst that absorbs long-wavelength visible light up to about 800 nanometers. The study, published in ACS Catalysis on December 5, 2025, reports up to a twofold increase in solar-to-hydrogen conversion efficiency compared with conventional systems. This performance gain indicates that the new material converts a larger fraction of incident photons into hydrogen under illumination conditions.
Dye-sensitized photocatalysts combine a light-absorbing dye molecule with a catalytic material. In these systems, the dye acts as an antenna that captures visible light and transfers the excitation energy or charge to the catalyst surface, where hydrogen evolution reactions occur. The choice of metal complex in the dye strongly influences which wavelengths are absorbed and how effectively the system drives charge transfer.
"Dye-sensitized photocatalysts typically use ruthenium complexes as the photosensitizing dyes. However, ruthenium-based complexes typically absorb only shorter visible wavelengths up to 600 nm," explains Maeda.
To extend absorption into longer wavelengths, the team replaced the ruthenium metal center in the complex with osmium. This substitution broadened the absorption profile, enabling the photocatalyst to use light with wavelengths beyond 600 nanometers and harvest a larger portion of the solar spectrum. The osmium-containing dye generates additional excited electrons that participate in hydrogen evolution, which contributes to the reported twofold efficiency increase.
The improvement is linked to the heavy-atom effect of osmium, which enhances singlet - triplet excitation in the metal complex. This low-energy electronic transition allows absorption of long-wavelength visible photons that ruthenium dyes do not effectively capture. By exploiting this effect, the new photocatalyst accesses a spectral region that is abundant in natural sunlight but previously underused in many dye-sensitized systems.
"In our efforts to extend the range of light absorption, osmium proved to be a key element in accessing wavelengths that ruthenium complexes could not use, leading to a 2-fold increase in hydrogen production efficiency," says Maeda.
The osmium-based system shows improved performance even under weak or diffuse sunlight, indicating operation under real-world outdoor conditions. This behavior is important for technologies such as artificial photosynthesis and solar-energy conversion materials, which must function under variable irradiance and atmospheric scattering. Enhanced utilization of long-wavelength light could help stabilize hydrogen output across different weather and seasonal conditions.
The researchers note that further optimization of the metal complexes and photocatalyst architecture remains an active area of work. Their current results establish a design framework for next-generation dye-sensitized photocatalysts that exploit heavy-metal effects and singlet - triplet transitions to extend light absorption. This approach could support broader deployment of solar-driven hydrogen production and related sustainable energy systems.
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