The course of electrochemical reactions is fundamentally governed by the atomic structure at the interface between an electronic conductor, the electrode, and an ionic conductor, the electrolyte. Since the time of Helmholtz, consensus has been developing that this interface is composed of a layer of charge on the electrode surface and a layer of ionic charge with opposite sign in the adjacent electrolyte. Such a scenario, known as the electric double layer, became recognized as the most critical factor controlling the performance of many electrocatalytic reactions, for example, hydrogen evolution/oxidation reaction, oxygen reduction/evolution reaction and CO2/N2 reduction reaction. A long-standing puzzle in the physics and chemistry of electric double layer is how water molecules in this interfacial region behave as a function of electrode potential. One of the ideal platforms to study the in-situ atomic structure of interfacial water is on the atomically flat and well-defined surface of single crystal. So far, enormous effort has been invested in the direct detection of interfacial water on single crystal surface, using advanced surface characterization techniques such as surface-enhanced infrared absorption spectroscopy, sum-frequency generation spectroscopy and X-ray spectroscopy. Nevertheless, these techniques often suffer from poor sensitivity to the subtle changes in the signal of water molecules, rendering their potential-dependent structure hard to unravel.
Surface-enhanced Raman spectroscopy (SERS) is emerging in recent years as a powerful tool for probing interfacial water, because it can effectively mitigate the interference from bulk water and capture the information of vibration and bending of the interfacial water molecules. However, early studies using SERS are restricted to polycrystalline electrodes, relying on the “borrowing” strategy in which the inner core significantly enhances Raman signals. Based on SERS, a novel technique named SHINERS (shell-isolated nanoparticle-enhanced Raman spectroscopy) has been devised, which can yield in-situ Raman spectra of interfacial water on single crystal surfaces (Fig. 1a). Through the combinatorial use of SHINERS and ab initio molecular dynamics (AIMD) simulations, the atomic-level understanding on the issue of potential-dependent rearrangement of interfacial water has been advanced to an unprecedented degree.
In the application of SHINERS, the single crystal surface of metal is prepared in the form of a half bead via hydrogen flaming and mechanical polishing. The quality of the metal surface can be characterized by electrochemical cyclic voltammetry. The shell-isolated nanoparticles (SHINs) are synthesized by coating the Au core with SiO2 shell, where the core is prepared by reducing the HAuCl4 with sodium citrate and the shell is formed from sodium silicate. The size of both the Au core and SiO2 shell are controllable, and pyridine adsorption test will be carried out to evaluate the pinhole. The enhancement of SHINs on different substrates can be estimated with pyridine employed as the Raman probe molecule.
The SHINs are then assembled on the single crystal surface for in-situ electrochemical Raman study of interfacial water. The Raman cell is vertically fixed (Fig.1b) for the removal of bubbles so that the detection will not be disturbed by gaseous products. An ultrathin liquid layer (50~100 μm) is constructed over the single crystal surface to diminish the signal from bulk solution. A layer of Au@SiO2 SHINs are dispersedly assembled on the electrode surface, which serves as an amplifier of the Raman signal through the effect of electromagnetic fields at the junctions between the SHINs and the substrate. Signals at fairly positive potential will be taken as reference and subtracted so as to guarantee that bulk electrolyte has little contribution to the final Raman spectra. AIMD simulations are employed for the construction of orientational configurations of interfacial water as a function of electrode potential, which establishes the atomic-level model correlated with the SHINERS results.
In our previous work, Au single crystal was taken as a model system to study the dynamic structural variation of interfacial water at bias potentials2. The frequency of the O-H stretching mode of interfacial H2O was obtained by SHINERS. Combined with AIMD results, the orientation transitions and changes of the hydrogen-bond network formed by interfacial water was investigated. Afterwards, the structure and dissociation of interfacial water on Pd single crystal surface was studied3. The important role of M.H2O in improving charge transfer efficiency and enhancing electrocatalytic performance was studied thoroughly. The cations in electric double layer were confirmed as “co-catalyst”, which continuously deliver water to the interface and transfer the hydroxyl radical to the bulk electrolyte during water dissociation.
The combined investigation via SHINERS and AIMD simulation is a fascinating approach to probe the microscopic structure of interfacial water during electrocatalytic reactions. We expect that further in-depth understanding of mechanisms taking place at electric double layer will be achieved with this protocol, and the relationship between interfacial water and reaction intermediates on a wide range of electrocatalysts could be tentatively explored in the future.
For detailed procedure and researches of the SHINERS and AIMD study of interfacial water on single crystal surfaces, please read our paper in Nature Protocols. “In situ electrochemical Raman spectroscopy and ab initio molecular dynamics study of interfacial water on a single crystal surface”.
Figure. In situ electrochemical Raman spectroscopy and ab initio molecular dynamics study of interfacial water on a single crystal surface. (a) Procedures for the study of interfacial water on single crystal surface using SHINERS and AIMD. (b) Vertically fixed homemade Teflon Raman cell, with laser focused on single crystal surface. (c) Statistical analysis of interfacial water via atomic-level simulations.
1 Li, J. F. et al. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392-395. (2010).
2 Li, C. Y. et al. In situ probing electrified interfacial water structures at atomically flat surfaces. Nat. Mater. 18, 697-701 (2019).
3 Wang, Y. H. et al. In situ Raman spectroscopy reveals the structure and dissociation of interfacial water. Nature 600, 81-85 (2021).
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