Revealing Water Structure at Neutral and Charged Graphene/Water Interfaces through Quantum Simulations of Sum Frequency Generation Spectra

The structure and dynamics of water at charged graphene interfaces fundamentally influence molecular responses to electric fields, with implications for applications in energy storage, catalysis, and surface chemistry. Leveraging the realism of the MB-pol data-driven many-body potential and advanced path-integral quantum dynamics, we analyze the vibrational sum-frequency generation (vSFG) spectrum of graphene/water interfaces under varying surface charges. Our quantum simulations reveal a distinctive dangling OH peak in the vSFG spectrum at neutral graphene, consistent with recent experimental findings yet markedly different from earlier studies. As the graphene surface becomes positively charged, interfacial water molecules reorient, decreasing the intensity of the dangling OH peak as the OH groups turn away from the graphene. In contrast, water molecules orient their OH bonds toward negatively charged graphene, leading to a prominent dangling OH peak in the corresponding vSFG spectrum. This charge-induced reorganization generates a diverse range of hydrogen-bonding topologies at the interface, driven by variations in the underlying electrostatic interactions. Re- markably, these structural changes extend into deeper water layers, creating an unequal distribution of molecules with OH bonds pointing toward and away from the graphene sheet. This imbalance amplifies bulk spectral features, underscoring the complexity of many-body interactions that shape the molecular structure of water at charged graphene interfaces.