Understanding how interfacial water governs photocatalytic hydrogen evolution is essential for rational photocatalyst design. Here, we directly characterize the adsorption state and hydrogen-bonding structure of interfacial water and correlate them with hydrogen evolution activity. We reveal that relatively weaker water-TiO2 interactions and more flexible hydrogen-bond networks favor higher interfacial water reactivity. These findings provide a molecular basis for engineering water-catalyst interfaces to enhance photocatalytic performance.
Hydrogen (H2) evolution via photocatalytic water splitting is an environmentally friendly and sustainable technology for solar-to-chemical energy conversion. Although interfacial interactions are recognized as key determinants of photocatalytic performance, systematic experimental studies explicitly targeting the structure and reactivity of the water-catalyst interface remain limited. A major challenge lies in probing the molecular structure of interfacial water, especially under hydrogen-evolving conditions. Furthermore, apparent H2 evolution activity is highly sensitive not only to the photocatalyst surface area but also to the amount of interfacial water. Consequently, despite decades of research, the relationship between the molecular structure and reactivity of interfacial water remains poorly understood, and rational guidelines for photocatalyst surface design based on molecular-level understanding have yet to be established.
Dr. Zhongqiu Lin, together with Associate Professor Toshiki Sugimoto and colleagues at the Institute for Molecular Science, systematically investigated the relationship between the microscopic structure and photocatalytic reactivity of interfacial water using a series of anatase TiO2 photocatalysts with different surface characteristics. They combined infrared spectroscopy with real-time mass spectrometry under precisely controlled hydration conditions ranging from sub-monolayer to several monolayers. By normalizing the H2 formation rates by both the specific surface area and the number of adsorbed water layers, the team quantitatively distinguished the reactivity of interfacial water from effects arising simply from differences in the amount of available interfacial water across different water-TiO2 interfaces.
Based on this framework, the team systematically investigated the adsorption state of interfacial water, including adsorption strength and adsorption mode (molecular or dissociative). Contrary to the conventional view that strong water-TiO2 interactions are generally favorable for photocatalysis because they enhance photocarrier trapping, suppress charge recombination, and thereby prolong charge-carrier lifetimes, the team found that relatively weaker water-TiO2 interactions were associated with higher reactivity of interfacial water. Because interfacial water not only interacts with the TiO2 surface but also forms hydrogen-bond networks with collective structural and dynamical properties, the team next examined how the hydrogen-bonding environment influences its reactivity. They found that weaker and more flexible hydrogen-bond networks were associated with higher reactivity of interfacial water. This finding further provides molecular insight into the rate-determining initial water oxidation in photocatalytic H2 evolution, which proceeds through proton-coupled charge transfer at the water-TiO2 interface. From the perspective of Marcus theory, greater flexibility and fluctuations of the hydrogen-bond network facilitate the molecular reorganization required for this reaction, consistent with the experimentally observed higher reactivity of more flexible interfacial water. Photocatalyst design has traditionally favored hydrophilic interfaces with strong interactions between water and catalyst surfaces because of their beneficial effects on photogenerated charge carriers. This study reveals that relatively weaker water-TiO2 interactions, associated with more flexible hydrogen-bond networks, favor higher reactivity of interfacial water toward H2 evolution. These findings open new avenues for engineering photocatalyst interfaces by controlling the molecular structure and dynamics of interfacial water.
