Submolecular Insights into Interfacial Water by Hydrogen-Sensitive Scanning Probe Microscopy

Abstract

Conspectus Water–solid interfaces have attracted extensive attention because of their crucial roles in a wide range of chemical and physical processes, such as ice nucleation and growth, dissolution, corrosion, heterogeneous catalysis, and electrochemistry. To understand these processes, enormous efforts have been made to obtain a molecular-level understanding of the structure and dynamics of water on various solid surfaces. By the use of scanning probe microscopy (SPM), many remarkable structures of H-bonding networks have been directly visualized, significantly advancing our understanding of the delicate competition between water–water and water–solid interactions. Moreover, the detailed dynamics of water molecules, such as diffusion, clustering, dissociation, and intermolecular and intramolecular proton transfer, have been investigated in a well-controlled manner by tip manipulation. However, resolving the submolecular structure of surface water has remained a great challenge for a long time because of the small size and light mass of protons. Discerning the position of hydrogen in water is not only crucial for the accurate determination of the structure of H-bonding networks but also indispensable in probing the proton transfer dynamics and the quantum nature of protons. In this Account, we focus on the recent advances in the H-sensitive SPM technique and its applications in probing the structures, dynamics, and nuclear quantum effects (NQEs) of surface water and ion hydrates at the submolecular level. First, we introduce the development of high-resolution scanning tunneling microscopy/spectroscopy (STM/S) and qPlus-based atomic force microscopy (qPlus-AFM), which allow access to the degrees of freedom of protons in both real and energy space. qPlus-AFM even allows imaging of interfacial water in a weakly perturbative manner by measuring the high-order electrostatic force between the CO-terminated tip and the polar water molecule, which enables the subtle difference of OH directionality to be discerned. Next we showcase the applications of H-sensitive STM/AFM in addressing several key issues related to water–solid interfaces. The surface wetting behavior and the H-bonding structure of low-dimensional ice on various hydrophilic and hydrophobic solid surfaces are characterized at the atomic scale. Then we discuss the quantitative assessment of NQEs of surface water, including proton tunneling and quantum delocalization. Moreover, the weakly perturbative and H-sensitive SPM technique can be also extended to investigations of water–ion interactions on solid surfaces, revealing the effect of hydration structure on the interfacial ion transport. Finally, we provide an outlook on the further directions and challenges for SPM studies of water–solid interfaces.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy

“…24,25 Its signals are proportional to the square of the vibration's second-order nonlinear susceptibility ( ) eff (2) , which vanishes when a material has inversion symmetry under the electric dipole approximation and becomes nonzero when inversion symmetry is broken. The signals from the interfacial water at a charged surface usually involve two contributions (eq 2): one from the intrinsic second-order nonlinear susceptibility ( Res (2) ) and the other from the (3) contribution that originates from symmetry breaking due to the presence of the surface potential (Φ 0 ). 26,27 + + +…”

“…When a surface of condensed matter contacts an aqueous solution, it often generates an electric surface charge, forming an electrical double layer (EDL). , Such interfacial phenomena are ubiquitous in nature and play critical roles in a wide range of chemical, physical, environmental, and biological processes. − Insights into the water structure and dynamics of the EDL are essential for understanding these various processes on the macroscopic scales. In comparison to the bulk water structure and dynamics, the behavior of interfacial water at the EDL is much less understood.…”

Concerted versus stepwise mechanisms of cyclic proton transfer: Experiments, simulations, and current challenges

“…These NQEs can be reflected by many phenomena, e.g., quantum delocalization of H, quantum tunneling, and zero-point energy (ZPE) effects. [15][16][17][18][19] Proton transfer along H-bonds are a class of processes that possess rich quantum behavior. While it is fair to say that in processes involving a single PT, the microscopic details have been relatively well-understood, [20][21][22][23][24][25][26] when multiple protons are involved, however, the underlying mechanisms become more complicated and atomic-scale understanding is still lacking.…”

Self-flooding behaviors on the fuel cell catalyst surface: an in situ mechanism investigation

“…4,28 Visualization of the water distribution is required for comprehensive investigation of the flooding behaviors at the catalyst layer. [29][30][31][32] In the last few decades, both numerical simulations and experimental observations have been conducted, including computational fluid dynamics (CFD) simulations, [33][34][35][36] neutron imaging, 29,[37][38][39] nuclear magnetic resonance (NMR) imaging, [40][41][42][43] electron microscopy, [44][45][46][47] and X-ray techniques. [48][49][50] However, the above-mentioned techniques usually stop at the GDL rather than the catalyst layer, which could be attributed to the complex parameters required for the simulation, including the temperature, ion concentration, and pressure, and the insufficient spatial and temporal resolutions for the characterization of the threephase interfaces based on the multiple-layered structure of FC devices.…”