Synchronizing periodic excitations of photocatalysts with a Michelson interferometer on operando FT-IR spectroscopy, researchers led by Toshiki Sugimoto succeeded in observing and identifying the reactive electron species for photocatalytic hydrogen evolution. In contrast to the standard belief, this study demonstrates that not the free electrons in metal cocatalysts however the electrons trapped within the periphery of cocatalysts directly contribute to the photocatalysis.
For the reason that discovery of photoelectrochemical hydrogen evolution by Honda and Fujishima in 1972, heterogeneous photocatalysis has been intensively investigated and continues to be a hot topic in science and technology. Specifically, understanding of reactive electron species and lively response sites on photocatalytic reduction response are vital for designing and manufacturing modern catalysts with improved evolution activity of hydrogen as sustainable energy carrier.
Nonetheless, despite its fundamental importance, microscopic understanding on photocatalysis stays a highly difficult issue owing to an inherent difficulty in experimentally observing and extracting weak spectroscopic signals originating from the photoexcited reactive electron species. That is predominantly attributable to inevitable temperature increment for catalyst samples under actual photocatalytic response conditions upon continuous photon irradiation. On this case, the weak signals derived from reactive photoexcited electron species are readily overwhelmed by intense background signals originating from thermally excited nonreactive electrons.
Researchers (Dr. Hiromasa Sato and Prof. Toshiki Sugimoto) at Institute for Molecular Science / The Graduate University for Advanced Studies, SOKENDAI succeeded in significantly suppressing the signals derived from thermally excited electrons and observing the reactive photogenerated electrons contributing to the photocatalytic hydrogen evolution. Such innovation was achieved by a brand new method based on synchronization of the millisecond periodic excitations of photocatalysts with a Michelson interferometer used for FT-IR spectroscopy.
This demonstration was achieved for metal-loaded oxide photocatalysts under steam methane reforming and water splitting conditions. Even though it has long been conventionally believed that loaded metal cocatalysts function as sinks for reactive photogenerated electrons and lively sites for reduction reactions, they found that the free electron species within the metal cocatalysts were indirectly involved within the photocatalytic reduction response. Alternatively, the electrons shallowly trapped within the in-gap states of oxides contributed to enhancing the hydrogen evolution rate upon the loading of metal cocatalysts. The electron abundance within the in-gap states, especially metal-induced semiconductor surface states, was clearly correlated to the response activity, suggesting that such metal-induced semiconductor surface states formed within the periphery of the metal cocatalyst play key roles within the photocatalytic hydrogen evolution.
These microscopic insights shift a paradigm on the traditionally believed role of metal cocatalysts in photocatalysis and supply a fundamental basis for rational design of the metal/oxide complex interfaces as promising platforms for nonthermal hydrogen evolution. Moreover, the brand new approach of operando infrared spectroscopy is widely applicable to other various catalytic response systems and materials driven by the help of photons and/or external electric field/potential. Subsequently, the brand new approach would have great potential to uncover hidden key aspects to reinforce catalyst performance toward innovation of environmentally friendly energy technology for next-generation sustainable society.
