The effects of the substrate and graphene on the interfacial structures of DMSO revealed by heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and molecular dynamics analysis

Understanding molecular structures at electrode/electrolyte interfaces is essential for optimizing high-performance electrochemical reactions. Here, we investigated the interfacial structures of dimethyl sulfoxide (DMSO), a widely used aprotic solvent, at the air/DMSO, SiO2/DMSO, and air/graphene/DMSO interfaces by heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and molecular dynamics (MD) simulation. HD-SFG spectroscopy is an interface-selective nonlinear spectroscopic technique capable of determining the absolute orientation of interfacial molecules. To enable HD-SFG measurements at the buried SiO2/DMSO interface, where precise optical phase control is challenging, an octadecyltrichlorosilane self-assembled monolayer was introduced on the SiO2 substrate as a phase reference for accurate determination of the interfacial complex susceptibility. The obtained spectra revealed that the hydrogen atoms of the methyl group preferentially orient toward the air side (CH3-up configuration) at the air/DMSO interface, whereas they orient toward the bulk DMSO side (CH3-down configuration) at the SiO2/DMSO interface. Furthermore, introducing graphene, an optically transparent two-dimensional carbon material relevant to practical carbon electrodes, shifts the most probable configuration at the interface: the S=O bond points toward the liquid phase, while the methyl groups orient nearly perpendicular to the surface normal. MD simulations with polarizable models well reproduced the spectral band shapes and signs, revealing the physical meaning of the signs of imaginary amplitudes in terms of orientation and local solvation effects. These results provide molecular-level insights into how the substrate and graphene affect the interfacial structures and demonstrate HD-SFG spectroscopy, combined with polarizable MD simulations, as a powerful approach for elucidating molecular-level structures at buried electrochemical interfaces for energy storage systems.