Water dynamics and sum-frequency generation spectra at electrode/aqueous electrolyte interfaces

Revealing the 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 those of 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. Importantly, 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.

Heterodyne-Detected Sum-Frequency Generation Vibrational Spectroscopy Reveals Aqueous Molecular Structure at the Suspended Graphene/Water Interface

Graphene, a transparent two-dimensional conductive material, has brought extensive new perspectives and prospects to various aqueous technological systems, such as desalination membranes, chemical sensors, energy storage, and energy conversion devices. Yet, the molecular-level details of graphene in contact with aqueous electrolytes, such as water orientation and hydrogen bond structure, remain elusive or controversial. Here, we employ surface-specific heterodyne-detected sum-frequency generation (HD-SFG) vibrational spectroscopy to re-examine the water molecular structure at a freely suspended graphene/water interface. We compare the response from the air/graphene/water system to that from the air/water interface. Our results indicate that theχ y y z 2 ${{\chi }_{yyz}^{\left(2\right)}}$ spectrum recorded from the air/graphene/water system arises from the topmost 1-2 water layers in contact with the graphene, with the graphene itself not generating a significant SFG response. Compared to the air/water interface response, the presence of monolayer graphene weakly affects the interfacial water. Graphene weakly affects the dangling O-H group, lowering its frequency through its interaction with the graphene sheet, and has a very small effect on the hydrogen-bonded O-H group. Molecular dynamics simulations confirm our experimental observation. Our work provides molecular insight into the interfacial structure at a suspended graphene/water interface, relevant to various technological applications of graphene.

Neural Network-Based Sum-Frequency Generation Spectra of Pure and Acidified Water Interfaces with Air

The affinity of hydronium ions (H3O+) for the air-water interface is a crucial question in environmental chemistry. While sum-frequency generation (SFG) spectroscopy has been instrumental in indicating the preference of H3O+ for the interface, key questions persist regarding the molecular origin of the SFG spectral changes in acidified water. Here we combine nanosecond long neural network (NN) reactive simulations of pure and acidified water slabs with NN predictions of molecular dipoles and polarizabilities to calculate SFG spectra of long reactive trajectories including proton transfer events. Our simulations show that H3O+ ions cause two distinct changes in phase-resolved SFG spectra: first, a low-frequency tail due to the vibrations of H3O+ and its first hydration shell, analogous to the bulk proton continuum, and second, an enhanced hydrogen-bonded band due to the ion-induced static field polarizing molecules in deeper layers. Our calculations confirm that changes in the SFG spectra of acidic solutions are caused by hydronium ions preferentially residing at the interface.