Exploring the Spatial Structure of Two-Dimensional Membrane Materials: A New Perspective Enabled by Low-Field NMR

Background

In recent years, the rapid growth of the new energy sector has driven a surge in demand for lithium ions (Li⁺). Extracting lithium from salt lake brines has become one of the most important sources of lithium. However, most brines contain high concentrations of magnesium ions (Mg²⁺) and relatively low concentrations of Li⁺, which poses a major challenge to lithium resource utilization. The key technical bottleneck lies in the poor selectivity during Li⁺/Mg²⁺ separation. Among cutting-edge separation methods, 2D membrane technology has demonstrated outstanding performance in Li⁺/Mg²⁺ separation. However, a fundamental trade-off still exists between permeability and selectivity. To overcome this, optimizing the membrane’s microstructure has become critical for achieving both high flux and selectivity. Two-dimensional (2D) materials—materials with thicknesses of only one or several atomic layers—have emerged as promising candidates for fabricating high-performance separation membranes.

In this application, the authors developed a high-permeability and highly stable composite membrane (ZIF-8@MLDH) by in situ deposition of functional ZIF-8 nanoparticles into nanopores formed at framework defects in a layered double hydroxide (MLDH) membrane.

Additionally, the authors innovatively utilized a low-field NMR device to characterize the microstructure of the separation membrane—a level of structural insight that cannot be obtained through conventional XRD or electron microscopy. The application of low-field NMR provides a new pathway to better understand the spatial architecture of 2D membrane materials.

Low-Field NMR for Microstructural Characterization of 2D Membrane Materials

Figure 1: Relaxation time spectrum using water and ethanol to deliver ZIF-8 nanoparticles as probes

In this study, water or ethanol was used to carry ZIF-8 nanoparticles as probe molecules to characterize the microstructure of the 2D membrane. As a separation membrane, a 2D material presents three distinct environments for the probe: First, the interlayer spaces between nanosheets, which are extremely confined and tortuous, restrict probe movement and result in short relaxation times (0.1–10 ms); second, nanosheet defect regions, where the restriction is moderate, with relaxation times typically ranging from 10 to 10³ ms; and third, open free spaces where probe molecules experience no restriction, resulting in long relaxation times (10³–10⁴ ms).

As shown in Figure 1, there was no change observed in the 0.1–10 ms range, which corresponds to interlayer regions. This confirms that the ZIF-8 nanoparticles did not deposit between the 2D nanosheets. However, a distinct peak emerged in the 10–10³ ms range, indicating that ZIF-8 nanoparticles had deposited at defect sites within the membrane framework. This experiment successfully identified the specific deposition region of the ZIF-8 nanoparticles.

Conclusion

This application demonstrates the use of low-field NMR to characterize the microstructure of separation membranes, offering a novel approach for spatial structural analysis of 2D materials.

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Reference

[1] Yahua Lu, Rongkun Zhou, Naixin Wang, et al. Engineer Nanoscale Defects into Selective Channels: MOF-Enhanced Li⁺ Separation by Porous Layered Double Hydroxide Membrane. Nano-Micro Lett, 2023, 15(9): 325–336.