By synchronizing periodic excitations of photocatalysts using a Michelson interferometer in operando FT-IR spectroscopy, a team of researchers led by Toshiki Sugimoto successfully detected and characterized the reactive electron species involved in photocatalytic hydrogen production. This study challenges the conventional view, revealing that it is not the freely available electrons within metal cocatalysts that play a role in photocatalysis; rather, it is the electrons that are trapped on the edges of these cocatalysts that make a direct contribution.
Since Honda and Fujishima first discovered photochemical hydrogen production in 1972, the field of heterogeneous photocatalysis has received considerable attention and continues to be a dynamic area of research. Gaining insights into the reactive electron species and the active sites involved in photocatalytic reduction is essential for the development of new catalysts that can enhance hydrogen generation as a sustainable energy source.
Despite its critical significance, achieving a detailed understanding of the microscopic processes in photocatalysis has proven challenging. This difficulty arises from the weak spectroscopic signals generated by photoexcited reactive electron species being easily masked by the strong background signals from thermally excited nonreactive electrons, especially under prolonged photon irradiation. This background interference often results from the temperature increase of catalyst samples during real photocatalytic reactions.
A research team (Dr. Hiromasa Sato and Prof. Toshiki Sugimoto) from the Institute for Molecular Science at The Graduate University for Advanced Studies, SOKENDAI, managed to significantly reduce the signals from thermally excited electrons, thereby allowing the observation of reactive photogenerated electrons involved in photocatalytic hydrogen production. This breakthrough was accomplished through a novel method that synchronizes millisecond periodic excitations of photocatalysts with FT-IR spectroscopy via a Michelson interferometer.
This technique was successfully demonstrated on metal-loaded oxide photocatalysts during steam methane reforming and water splitting reactions. Traditionally, it has been assumed that these loaded metal cocatalysts act as electron sinks and serve as active sites for reduction reactions. However, the researchers discovered that the free electrons within the metal cocatalysts do not directly participate in photocatalytic reduction. Instead, electrons that are weakly trapped in the in-gap states of the oxides are primarily responsible for enhancing the hydrogen production rate when metal cocatalysts are present. The abundance of electrons in these in-gap states, particularly those induced by metal at the semiconductor surface, strongly correlates with reaction efficiency, indicating that these metal-induced semiconductor surface states at the edges of the metal cocatalysts are crucial for photocatalytic hydrogen evolution.
These new microscopic discoveries challenge the previously held beliefs regarding the function of metal cocatalysts in photocatalysis and lay the groundwork for the strategic design of metal/oxide interfaces that could serve as effective platforms for nonthermal hydrogen production. Moreover, the innovative operando infrared spectroscopy approach has broad applicability across numerous catalytic systems and materials that operate under the influence of light and/or external electric fields. Consequently, this novel technique has significant potential to reveal hidden factors that could boost catalyst efficacy, fostering advancements in environmentally friendly energy technologies for a sustainable future.
