Raman and IR spectra of water under graphene nanoconfinement at ambient and extreme pressure-temperature conditions: a first-principles study

TL;DR

This study uses first-principles simulations to analyze how graphene nanoconfinement alters the vibrational spectra and structural properties of water under various pressure-temperature conditions, revealing unique phases and spectroscopic signatures.

Contribution

It provides the first detailed theoretical Raman and IR spectra of nanoconfined water at extreme conditions, linking spectral features to structural transformations and superionic phases.

Findings

Nanoconfinement alters Raman stretching and low-frequency bands.

Spectroscopic evidence shows changes in hydrogen bond networks.

Confined water transitions into superionic phase at high pressure and temperature.

Abstract

The nanoconfinement of water can result in dramatic differences in its physical and chemical properties compared to bulk water. However, a detailed molecular-level understanding of these properties is still lacking. Vibrational spectroscopy, such as Raman and infrared, is a popular experimental tool for studying the structure and dynamics of water, and is often complemented by atomistic simulations to interpret experimental spectra, but there have been few theoretical spectroscopy studies of nanoconfined water using first-principles methods at ambient conditions, let alone under extreme pressure-temperature conditions. Here, we computed the Raman and IR spectra of water nanoconfined by graphene at ambient and extreme pressure-temperature conditions using ab intio simulations. Our results revealed alterations in the Raman stretching and low-frequency bands due to the graphene confinement. We also found spectroscopic evidence indicating that nanoconfinement considerably changes the tetrahedral hydrogen bond network, which is typically found in bulk water. Furthermore, we observed an unusual bending band in the Raman spectrum at ~10 GPa and 1000 K, which is attributed to the unique molecular structure of confined ionic water. Additionally, we found that at ~20 GPa and 1000 K, confined water transformed into a superionic fluid, making it challenging to identify the IR stretching band. Finally, we computed the ionic conductivity of confined water in the ionic and superionic phases. Our results highlight the efficacy of Raman and IR spectroscopy in studying the structure and dynamics of nanoconfined water in a large pressure-temperature range. Our predicted Raman and IR spectra can serve as a valuable guide for future experiments.