Water-Mediated Ion Selectivity in 2D MXene Channels

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Water behavior differs significantly under Ångström-scale confinement compared to bulk water.
• Understanding confined water's role in selective ion transport is crucial for nanofluidic systems.
• Classical models for ion transport are inadequate in subnanometer channels, necessitating advanced experimental control.

Purpose of the Study:

• To investigate how 2D Ti3C2Tx MXene nanosheets with controlled properties influence ion transport.
• To elucidate the mechanisms of water reorganization and ion-specific interactions under ultranarrow confinement.
• To establish design principles for advanced nanofluidic devices.

Main Methods:

• Utilized 2D Ti3C2Tx MXene nanosheets with tunable interlayer spacing (0.9-5.0 Å), surface terminations, and electrode potentials.
• Performed experimental measurements including ion permeation, spatial secondary-ion mass spectrometry, and Fourier transform infrared spectroscopy.
• Conducted molecular dynamics simulations to complement experimental findings.

Main Results:

• Ultrananoconfinement reorganizes water structure, creates ion-specific dehydration penalties, and modifies ion-MXene interactions.
• Achieved a two-orders-of-magnitude increase in Li+ permeation using horizontally aligned Ti3C2Tx channels compared to vertically aligned membranes.
• Demonstrated electrochemical surface charge modulation for fine-tuning ion selectivity.

Conclusions:

• Confined water, surface chemistry, and energetics create a transport regime beyond classical diffusion.
• These findings provide a framework for designing artificial ion channels and high-performance membranes.
• The study offers pathways for improved ion separation, water desalination, and sustainable water treatment.