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2024/07/19

Press Release

Positive and Negative Impacts of Interfacial Hydrogen Bonds on Photocatalytic Hydrogen Evolution
-Identifying the reaction conditions that maximize catalytic activity in photocatalytic hydrogen evolution under water vapor atmospheres-

 

    Photocatalytic hydrogen evolution from water is a key technology for achieving sustainable hydrogen production. However, the direct impact of the microscopic structure of interfacial water molecules on photocatalytic reactivity remains unexplored. In this study, the crucial roles of interfacial hydrogen bond structure and dynamics, as well as the optimal interfacial water environment for promoting H2 evolution were uncovered. These findings provide molecular-level insights that can guide the design of interfacial water conditions to enhance photocatalytic performance.


    Hydrogen production via photocatalytic water splitting is a sustainable solution for next-generation energy by utilizing light energy at room temperature. However, the design of innovative photocatalysts remains a challenge due to a limited molecular-level understanding of interfacial water molecules and their hydrogen bond networks. Unveiling the physicochemical properties of these interfacial water molecules is critical to optimizing photocatalytic efficiency and achieving breakthroughs in sustainable hydrogen production.

    Researchers (Zhongqiu Lin et al.) led by Toshiki Sugimoto, Associate Professor at Institute for Molecular Science / The Graduate University for Advanced Studies, SOKENDAI, have comprehensively investigated the impact of interfacial H-bond networks using various TiO2 photocatalysts and uncovered a crucial role of interfacial H-bond structure/dynamics and optimal interfacial water environment for H2 evolution. They controlled the thickness of adsorbed water from sub-monolayer to multilayers by precisely adjusting water vapor pressure. With this approach, they succeeded in directly demonstrating the correlation between H2 formation rate and the microscopic structure of H-bond networks using real-time mass spectrometry and infrared absorption spectroscopy (Figure 1). Regardless of the crystalline structure of the TiO2 photocatalyst (brookite, anatase, or a mixture of anatase and rutile), they observed a linear increase in H2 formation rate with water adsorption up to three layers (Figure 1a), indicating that reactive water molecules are present not only in the first adsorbed layer but also in several overlying layers. However, the H2 formation rate turned to decrease dramatically when more than three layers of water covered the TiO2 surface (Figure 1a). In this situation, infrared spectra clearly indicated two distinct types of adsorbed water on the TiO2 surface: interfacial water and liquid-like water (Figure 1b). Due to many-body interactions among adsorbed water molecules, the liquid-like water adsorbed in more than three layers led to strengthening of interfacial H-bond (Figure 1c), which hinder interfacial proton-coupled hole transfer and drastically decreased the H2 formation rate. Based on these microscopic insights, their study suggests that depositing three water layers in a water vapor environment is optimal for photocatalytic hydrogen evolution (Figure 2). 

    Photocatalysis has been extensively studied for over half a century, predominantly in aqueous solution environments. In this context, this study represents a potential paradigm shift, demonstrating the effectiveness of water vapor environments compared to traditional liquid-phase reaction systems. These findings open new avenues for the molecular-level design and engineering of interfacial water toward the development of more innovative photocatalytic systems for next-generation renewable energy production.


Figure 1: Impact of water adsorption on the H2 formation rate and interfacial H-bonds for the brookite TiO2 photocatalyst (TIO19PB).
(a) Change in the rate of hydrogen formation through photocatalytic water splitting when the number of water layers (amount of adsorbed water molecules) is systematically changed under various water vapor pressures. (b) The dependency of the O−H stretching band area on the number of water layers (black: total spectrum area, green: spectrum area of interfacial water component, blue: spectrum area of liquid-like water component). The spectrum of the interfacial water component, which shows a spectral shape clearly different from that of bulk liquid water, is nearly saturated in area at two molecular layers, while the liquid-like water component, which shows a spectral shape almost the same as that of bulk liquid water, increases when water molecules are adsorbed in layers thicker than three. (c) Change in the O-H vibration spectrum of the interfacial water component induced by the adsorption of more than three molecular layers of water.
(Credit: Zhongqiu LIN, Toshiki Sugimoto, License: CC BY)


Figure 2: Summary of positive and negative impacts of interfacial hydrogen bonds on photocatalytic hydrogen evolution.
Schematic of changes in the reaction activity associated with variation of water layer thickness on the photocatalyst surface. The H2 formation rate increases with increasing water layers up to three layers. When more than three water layers cover the surface, the liquid-like water overlayers strengthen/harden the interfacial H-bond networks. The hardened interfacial H-bond networks hinder the interfacial proton-coupled hole transfer, resulting in a dramatic decrease in the H2 formation rate.
(Credit: Zhongqiu LIN, Toshiki Sugimoto, License: CC BY)

 

Paper Information:

Authors: Zhongqiu Lin, Hikaru Saito, Hiromasa Sato, and Toshiki Sugimoto
Journal Name: Journal of the American Chemical Society
Journal Title: “Positive and Negative Impacts of Interfacial Hydrogen Bonds on Photocatalytic Hydrogen Evolution”
DOI: 10.1021/jacs.4c04271

 

Financial Support:

・JSPS KAKENHI
Grant-in-Aid for Scientific Research (A), 22H00296
Grant-in-Aid for JSPS Fellow, 23KJ1003
・JST-CREST, JPMJCR22L2
・JST-FOREST, JPMJFR221U
・Joint Research by the National Institutes of Natural Sciences (NINS), 01112104
・Demonstration Project of Innovative Catalyst Technology for Decarbonization through Regional Resource Recycling, the Ministry of the Environment, Government of Japan
 

Contact Person:

Name: Toshiki Sugimoto
TEL: +81-564-55-7280
E-mail: toshiki-sugimoto_at_ims.ac.jp (Please replace the "_at_" with @)