Anomalous Freezing of Low Dimensional Water Confined in Graphene Nanowrinkles

1. Problem Statement

Water exhibits unique physical properties when confined at the nanoscale and frequently deviates significantly from its bulk behavior (e.g., changes in melting temperature, density, and phase transitions). While previous studies have investigated water in carbon nanotubes or between flat graphene layers, the behavior of water confined in heterogeneous, corrugated environments (specifically between a silicon dioxide substrate and a graphene monolayer with nanofolds) remains poorly understood.

Key challenges addressed in this study include:

• Detection Limits: The amount of water confined within nanofolds is too small for conventional characterization techniques such as X-ray or neutron scattering.
• Substrate Complexity: Most theoretical models assume flat substrates, whereas real graphene on silicon dioxide is corrugated and features folds, leading to varying confinement thicknesses.
• Monitoring Phase Transitions: There is a lack of in-situ, real-time methods to track the phase transitions (melting/freezing) of such tiny amounts of confined water without destroying the sample.

2. Methodology

The researchers pursued a multimodal approach combining advanced sample preparation, cryogenic spectroscopy, and extensive molecular dynamics simulations.

A. Sample Preparation and Water Confinement

• Technique: A novel nitrocellulose-assisted transfer method was developed. Graphene grown on copper via chemical vapor deposition (CVD) was transferred onto a SiO₂/Si substrate using a nitrocellulose polymer as a temporary carrier.
• Mechanism: The transfer process takes place in an aqueous environment, trapping water molecules between the graphene monolayer and the silicon dioxide substrate.
• Result: This creates a stable system where water is confined within a network of regular nanofolds (lateral dimension ~4 nm, height ~4 nm) and flat regions. The graphene acts as both a confinement barrier and a sensor.

B. Experimental Characterization

• Cryogenic Raman Spectroscopy: Raman maps (20 × 20 µm²) were acquired over a temperature range of 10 K to 370 K.
• Spectral Deconvolution: Due to the limited spatial resolution of the microscope relative to the fold size, the Raman spectra were deconvoluted into two components:
  1. Flat Regions: Graphene adhered to the substrate (high doping, low strain).
  2. Folded Regions: Curved graphene detached from the substrate (lower doping, high strain).
• Monitored Parameters: The study monitored the frequencies and intensities of the G-band and 2D-band to calculate changes in the strain and doping levels of the graphene, which serve as indicators for the state of the underlying water.

C. Computational Simulations

• Classical MD: Used to model system geometry and water behavior.
• Path Integral Molecular Dynamics (PIMD): Indispensable for capturing nuclear quantum effects, which are crucial for an accurate description of water structure and phase transitions at low temperatures.
• Models:
  • Water: TIP4P/ICE (classical) and q-TIP4P/F (quantum mechanical).
  • Graphene: AIREBO potential.
  • Substrate: Silicon dioxide modeled with force fields according to Emami et al.
• System Size: The simulations encompassed over 90,000 atoms to replicate the experimental fold geometry (~40 Å height/width).

3. Main Contributions

1. Novel Transfer Method: A robust method was demonstrated to permanently confine water molecules within graphene nanofolds via nitrocellulose-assisted transfer, thereby creating a stable "nanoreactor."
2. Graphene as an In-Situ Sensor: It was established that the Raman spectral response of the encapsulating graphene (specifically shifts in strain and doping) can serve as a highly sensitive, non-invasive probe for the phase state of confined water.
3. Quantum Simulations of Confined Water: PIMD simulations produced results that successfully reproduced experimental anomalies and proved that nuclear quantum effects are necessary to explain the observed behavior of confined water.
4. Extension of the Phase Diagram: The anomalous melting behavior of water in a realistic, corrugated confinement geometry was mapped, distinguishing it from idealized flat or cylindrical confinements.

4. Main Results

• Anomalous Melting Temperatures:
  • Surface Premelting: Simulations and experimental data suggest that the confined ice begins to premelt at the interface at approximately 200 K.
  • Complete Melting: The melting process is completed at approximately 240 K, which is about 30–40 K lower than the melting point of bulk water (273 K) and the melting point of the TIP4P/ICE model for bulk (~270 K).
• Spectroscopic Signatures:
  • Doping: A distinct "kink" behavior in the doping level of the flat graphene regions was observed at ~200 K, correlating with the onset of water reorientation and increased dynamics.
  • Strain: The strain in the folded graphene regions showed a steep drop upon cooling to ~200 K. This corresponds to the volume change and structural reorganization of water during the transition from an ordered ice-like state to a disordered liquid-like state.
• Water Orientation Dynamics:
  • At low temperatures, water molecules prefer a "flat" orientation parallel to the graphene surface.
  • As temperature rises above 200 K, the distribution of orientations becomes blurred, indicating increased rotational freedom and dynamics.
  • The transition is more abrupt in the folded regions (ice surface premelting) than in the flat regions (disordered 2D layer).
• Validation: The PIMD-simulated temperature dependence of the C-C bond length in graphene perfectly matched the experimentally derived strain data, confirming that the observed spectral shifts are driven by phase changes in the water rather than intrinsic thermal expansion of the graphene.

5. Significance

• Access to the "No Man's Land": This study provides a pathway to investigate the "No Man's Land" of the water phase diagram (supercooled water) at the nanoscale, an area that has been experimentally difficult to access hitherto.
• New Sensing Paradigm: It demonstrates that atomically thin materials like graphene can act as intrinsic spectroscopic sensors for confined liquids, thereby bypassing the need for bulky external probes that cannot resolve nanoscale features.
• Fundamental Understanding: The results underscore the critical interplay between confinement geometry (folds vs. flat), substrate interactions, and the thermodynamic properties of water. They suggest that confinement in realistic, corrugated environments leads to significantly depressed melting points compared to idealized models.
• Applications: This approach opens doors for investigating chemical reactions, ion transport, and phase transitions in nanoscale confined spaces, with potential applications in nanofluidics, energy storage, and understanding biological water dynamics.