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Photocatalysis—a chemical reaction driven by light in the presence of a photocatalyst—is poised to play a key role in next-generation technologies, including hydrogen production through water splitting, carbon dioxide reduction, and environmental purification by utilizing sunlight. Thanks to these promising applications, photocatalysis is attracting attention as a major tool for building sustainable cities and societies.

However, photocatalysis involves a series of interconnected processes—light absorption, carrier excitation and transport, and surface redox reactions—that proceed in a continuous manner. This makes it challenging to identify the rate-limiting step of the overall reaction. As a result, optimizing photocatalytic reactions for maximum efficiency remains a significant challenge.

In a recent breakthrough, researchers from the Graduate School of Advanced Science and Technology at the Japan Advanced Institute of Science and Technology, Japan, led by Research Assistant Professor Yohei Cho and Professor Toshiaki Taniike, have introduced a novel methodology to pinpoint the bottleneck metrics and thus determine rate-limited regimes in photocatalysis. Their findings have been in the Journal of Materials Chemistry A.

"In this study, we categorized photocatalytic reactions into two key processes: charge supply, which refers to the supply of excited carriers to the surface, and charge transfer, which involves redox (oxidation–reduction) reactions. Since surface reactions are more sensitive to temperature changes, we introduced the Onset Intensity for Temperature Dependence (OITD) as a crucial metric. It marks the point at which the reaction rate begins to respond to temperature, allowing us to clearly distinguish which of the two processes is rate-limiting," explains Dr. Cho.

The researchers measured photocatalytic reaction rates under varying temperatures and light intensities to identify the OITD and determine whether the reaction was limited by charge supply and charge transfer. Using the decomposition of methylene blue as a model reaction, they studied titanium dioxide (TiO₂) and zinc oxide (ZnO) as representative photocatalysts.

TiO₂ exhibited temperature dependence only at high light intensity, suggesting that the material is relatively more constrained by charge supply. In contrast, ZnO showed temperature sensitivity even at lower light intensity, suggesting that its performance is relatively more limited by surface reactions. These findings reveal distinct rate-limiting behaviors for different materials.

Furthermore, the study highlighted that enhancing surface accessibility through nanoparticle formation plays a more important role in improving charge supply than increasing crystallinity. This insight offers a concrete design principle for optimizing photocatalytic material design.

Dr. Cho says, "Our diagnostic method supports the rational design of photocatalysts for solar-driven hydrogen production, carbon dioxide reduction, and environmental remediation. It enables rapid screening of materials and informs targeted optimization strategies—such as co-catalyst loading or nanostructuring—for efficient and practical solar energy utilization technologies.

"Ultimately, this can accelerate the development of sustainable energy and environmental technologies, potentially contributing to carbon neutrality and cleaner water and air."

In conclusion, OITD offers a straightforward yet powerful diagnostic for identifying whether a photocatalytic reaction is limited by charge supply or surface charge transfer, paving the way for smarter catalyst design and improved reaction efficiency.