The physical properties of clays and micas can be controlled by exchanging ions in the crystal lattice. Atomically thin materials can have superior properties in a range of membrane applications, yet the ion-exchange process itself remains largely unexplored in few-layer crystals. Here we use atomic-resolution scanning transmission electron microscopy to study the dynamics of ion exchange and reveal individual ion binding sites in atomically thin and artificially restacked clays and micas. We find that the ion diffusion coefficient for the interlayer space of atomically thin samples is up to 104 times larger than in bulk crystals and approaches its value in free water. Samples where no bulk exchange is expected display fast exchange at restacked interfaces, where the exchanged ions arrange in islands with dimensions controlled by the moiré superlattice dimensions. We attribute the fast ion diffusion to enhanced interlayer expandability resulting from weaker interlayer binding forces in both atomically thin and restacked materials. This work provides atomic scale insights into ion diffusion in highly confined spaces and suggests strategies to design exfoliated clay membranes with enhanced performance.
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The work was supported by EPSRC grants EP/M010619/1 (S.J.H.), EP/P00119X/1 (R.R.-N., K.S.N.), EP/S021531/1 (S.J.H.) and EP/P009050/1 (S.J.H, D.G.H.), the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant ERC-2016-STG-EvoluTEM-715502 (S.J.H., Y.-C.Z.), the ERC Synergy Hetero2D project 319277 (K.S.N., Y.-C.Z.) and ERC contract 679689 (R.R.-N)) and The Royal Society (M.L.-H., R.G.), Graphene Flagship (K.S.N., R.G.). L.M. acknowledges the EPSRC NOWNano programme for funding. The work was supported by the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1 and EP/P025498/1 (equipment access). We thank the Diamond Light Source for access and support in use of the electron Physical Science Imaging Centre (Instrument E02 and proposal numbers MG21981, MG21597 and MG24290) that contributed to the results presented here (S.J.H.). We thank L. S. Campos from Brasil Minerios for sourcing vermiculite. Part of this work was supported by the Flemish Science Foundation (FWO-Vl) (F.M.P.) and the Sun Yat-sen University 29000-18841290 (Y.-C.Z.).
The authors declare no competing interests.
Peer review information Nature Materials thanks Benjamin Gilbert, Jiaxing Huang and Jani Kotakoski for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
a–d, Steps used in the preparation of cross-sectional samples. a, Mechanical exfoliation of mica(clay) flakes. b, Ion exchange processes for the selected flakes: electrolyte immersion; then drying in a N2 gas environment and quality inspection by optical microscopy. c, Cover the target flakes with thin graphite to avoid surface damage or contamination. d, FIB lift out of the graphite/SiOX/mica(clay) stack, followed by FIB thinning and S/TEM imaging. e–h, Steps used in the preparation of plan-view samples. e, Mechanical exfoliation of mica(clay) crystals onto a PVA/PPC coated SiOx /Si substrate, followed by dissolving PVA in water to leave a floating PPC/mica(clay) stack. f, Transfer of the PPC/mica(clay) stack by micromanipulation onto a SiNx/Si TEM grid, followed by removal of PPC in solvent to leave a clean and suspended flake. g, Ion exchange and cleaning of the mica(clay) flake on TEM grid. h, STEM and TEM imaging.
a, ADF-STEM cross-sectional image taken from a Cs exchanged 2L vermiculite sample. Scale bar, 1 nm. b, c Consecutively acquired EDS elemental line profiles along the line I to II (summed vertically for the whole ADF region shown in a). b, EDS line profile from t=0 to t=100 s with t being the EDS acquisition time, total electron fluence of ~1.6×107 e·Å−2. c, EDS line profile collected from t=100 to t=200 s with material having experienced a total electron fluence of ~3.2x107 e·Å−2. The peak values for Cs+ counts are marked by red dashed lines. The lower peak Cs concentration for the second measurement shows that Cs+ diffuses away from the region of interest during STEM imaging (Cs+ counts in b > c). A similar decrease is seen in the Mg concentration. Note that the total dose required for high-resolution STEM EDS elemental mapping is >107 e·Å-2, much larger than the total electron fluence that would amorphize the Cs-vermiculite crystals (105 e·Å-2), and hence it is not possible to perform atomic-scale chemical mapping or quantitative atomic analysis in these clay samples.
Extended Data Fig. 3 Plan-view ADF-STEM images taken from the edge of ion exchanged few-layered muscovite and biotite.
a, Muscovite (Mus), b, Biotite (Bio), both Cs-exchanged for 300 h. Cs+ ions are visible as the brightest yellow dots. The largest penetration depth of Cs+ from the edge into the interlayer space in the crystal (∆P) is indicated, yielding D6L-Mus ~10−11 µm2 s−1, and D4L-Bio ~10−11 µm2 s−1. Scale bars, 5 nm.
Extended Data Fig. 4 Exchange speed characterization of vermiculite samples with different exposure times for Cs+ ion exchange, analysed using XRD.
a, XRD taken from bulk crystals (t being the CsCl solution treatment time). b, XRD taken from the laminates made from exfoliated nanosheets. A right shift of (002) peaks can be observed with increasing exposure time due to the contraction of interlayer spacing caused by Cs+ incorporation. c, Flake thickness distribution in the laminates measured by AFM. Inset, optical image of a typical Mg-vermiculite laminate (membrane). Scale bar, 5 mm.
Extended Data Fig. 5 Comparison of the swelling behaviour for vermiculite crystals of different thicknesses in NaCl solution.
a–d, AFM results for a 2L vermiculite region (outlined by red dashed lines) and a local 1L region (outlined by black dashed lines). a, b, AFM height image taken a, in ambient air and b, in aqueous 0.1 M NaCl. c, d, Corresponding height profiles in air and liquid extracted from the positions of the lines (I, II, III, IV) for c, 2L region and d, 1L region. e–h, AFM results for a 3L vermiculite (green dashed lines) containing a local 1L region (black dashed lines). e, f, AFM height image taken e, in ambient air, and f, in aqueous 0.1 M NaCl. g, h, Corresponding height profiles extracted from the positions of the lines (V, VI, VII, VIII) for g, 3L region, and h, 1L region. i, Statistical results showing the AFM measured height (z) values for vermiculite as a function of N, in ambient air (magenta) and liquid NaCl (blue). Swelling values (∆z) are extracted as height difference in liquid and ambient, with the standard deviation extracted from at least three different measurements. j, Mean swelling values per interlayer as a function of N, calculated as ∆z x (N−1)−1 using ∆z values shown in i. Error bars represent the standard deviation (s.d.) from the mean for at least 3 different samples. Dotted line is a guide to the eye. Scale bars in a, b, e, f are all 200 nm.
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Zou, YC., Mogg, L., Clark, N. et al. Ion exchange in atomically thin clays and micas. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01072-6
Nature Materials (2021)