Article https://doi.org/10.1038/s41467-023-39533-y Nanoconfinement enabled non-covalently decoratedMXenemembranes for ion-sieving Yuan Kang1, Ting Hu1, Yuqi Wang 2, Kaiqiang He1, Zhuyuan Wang3, Yvonne Hora1, Wang Zhao1, Rongming Xu4, Yu Chen 5, Zongli Xie 6, Huanting Wang 1, Qinfen Gu 7 & Xiwang Zhang 1,3 Covalent modification is commonly used to tune the channel size and func- tionality of 2D membranes. However, common synthesis strategies used to produce such modifications are known to disrupt the structure of the mem- branes. Herein, we report less intrusive yet equally effective non-covalent modifications on Ti3C2Tx MXene membranes by a solvent treatment, where the channels are robustly decorated by protic solvents via hydrogen bond network. The densely functionalized (-O, -F, -OH) Ti3C2Tx channel allows multiple hydrogen bond establishment and its sub-1-nm size induces a nano- confinement effect to greatly strengthen these interactions by maintaining solvent-MXene distance and solvent orientation. In sub-1-nm ion sieving and separation, as-decorated membranes exhibit stable ion rejection, and proton- cation (H+/Mn+) selectivity that is up to 50 times and 30 times, respectively, higher than that of pristinemembranes. It demonstrates the feasibility of non- covalent methods as a broad modification alternative for nanochannels inte- grated in energy-, resource- and environment-related applications. Membranes based on rolled, perforated, or stacked 2D materials pos- sess channels in 1-nm and sub-1-nm scales and present relevance and significance across disciplines1–3. With channel dimension approaching the size of most hydrated ions (<1 nm), gases (<0.5 nm), and solvent molecules (<2 nm), 2Dmembranes allow controllablemass separation, a core step to drive critical technologies in resource recovery, envir- onmental remediation, energy conversion, and storage. Particularly in the stack form, the membranes have regularly aligned neighboring layers so that interlayer channels with narrow size distribution can be formed to induce high selectivity between physiochemically similar species. Accompanying the ongoingmaterial exploration on graphene derivatives, metal nitrides/carbides (MXene), metal-organic frame- works (MOFs), covalent organic frameworks (COF), a few such chan- nels have been composed with primary width between 0.3 and 2 nm, and functioned well in several separation processes4–7. To extend their use for numerous other separating purposes and in various media (e.g., dry, wet, or solvated), these pristine channels always need customized tailoring on their size and/or surface chem- istry to satisfy each specific requirement. A typical example is aquatic- used graphene oxide (GO) and MXene channels that show effective widths of ~0.6 to 0.8 nm1,6. While this intrinsic size meshes out polya- tomic ions andorganic pollutants over 1 nm inwastewater purification, it needs an additional reduction to below 0.6 nm to enable monoa- tomic ion sieving as in desalination, and proton isolation as in redox batteries3. The majority of such modifications are achieved via cova- lent crosslinking or decoration, where guest species are chemically bonded toboth or one side of the channels so thatdesired channel size and functionalities can be obtained1,4. While providing reliable mod- ification efficacy, covalent methods are discouraged from fulfilling their decoration potential largely due to the emergence of Received: 22 November 2022 Accepted: 15 June 2023 Check for updates 1Department of Chemical and Biological Engineering, Monash University, Clayton, VIC 3800, Australia. 2School of Materials Science and Engineering, Zhejiang University, 310058 Zhejiang, China. 3UQ Dow Centre for Sustainable Engineering Innovation, School of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia. 4School of the Environment, Nanjing University, 210023Nanjing, China. 5MonashCentre for ElectronMicroscopy, Monash University, Clayton, VIC 3800, Australia. 6CSIRO Manufacturing, Private Bag 10, Clayton South 3169, Australia. 7Australian Synchrotron, ANSTO, Clayton, VIC 3168, Australia. e-mail: qinfeng@ansto.gov.au; xiwang.zhang@uq.edu.au Nature Communications | (2023) 14:4075 1 12 34 56 78 9 0 () :,; 12 34 56 78 9 0 () :,; http://orcid.org/0000-0001-7722-393X http://orcid.org/0000-0001-7722-393X http://orcid.org/0000-0001-7722-393X http://orcid.org/0000-0001-7722-393X http://orcid.org/0000-0001-7722-393X http://orcid.org/0000-0001-7882-1077 http://orcid.org/0000-0001-7882-1077 http://orcid.org/0000-0001-7882-1077 http://orcid.org/0000-0001-7882-1077 http://orcid.org/0000-0001-7882-1077 http://orcid.org/0000-0002-4610-0758 http://orcid.org/0000-0002-4610-0758 http://orcid.org/0000-0002-4610-0758 http://orcid.org/0000-0002-4610-0758 http://orcid.org/0000-0002-4610-0758 http://orcid.org/0000-0002-9887-5555 http://orcid.org/0000-0002-9887-5555 http://orcid.org/0000-0002-9887-5555 http://orcid.org/0000-0002-9887-5555 http://orcid.org/0000-0002-9887-5555 http://orcid.org/0000-0001-9209-4208 http://orcid.org/0000-0001-9209-4208 http://orcid.org/0000-0001-9209-4208 http://orcid.org/0000-0001-9209-4208 http://orcid.org/0000-0001-9209-4208 http://orcid.org/0000-0002-4319-527X http://orcid.org/0000-0002-4319-527X http://orcid.org/0000-0002-4319-527X http://orcid.org/0000-0002-4319-527X http://orcid.org/0000-0002-4319-527X http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-39533-y&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-39533-y&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-39533-y&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-39533-y&domain=pdf mailto:qinfeng@ansto.gov.au mailto:xiwang.zhang@uq.edu.au performance-compromising non-idealities to membrane structure in the reaction process. These include unwanted defects, corrugations, and interrupted channel alignment caused by harsh reaction condi- tions (e.g., high temperature and strong acid/base) and volatile by- products8–10. Equally interesting, but much less explored, is to modify nano- channels in non-covalent routes. Known as a collective of van der Waals forces, electrostatic interactions, hydrogen bonds, and π–π interaction, non-covalent interactions universally form within a 2 nm threshold intermolecular distance, promising easy membrane mod- ifications without major structural compromise. Nevertheless, their low strength (−0.4 eV) and exponential weakening beyond the threshold distance present a potential instability problem for non- covalent modifications, making them less a favor than covalent ones (−1.0 eV) in wide conventional channels. Recent research on nanofluid, however, proves otherwise by discovering a nanoconfinement effect where non-covalent affinity can be boosted by one order ofmagnitude in constrained geometry11. With most 2Dmembranes possessing 1-nm channels, such a phenomenon can be utilized to enable non-covalent decorations. Considering its stable 1-nm channel size12, we herein take 2D Ti3C2Tx (Tx represents –O, –OH, and –F) MXene as a platform and propose an almost non-intrusive, effective non-covalent decoration strategy by facile solvent treatment. When streaming into MXene channels, organic molecules, particularly protic ethanol, can attach themselves onto the functional groups ofMXene surface via hydrogen bond (H-bond) network. Owing to the sub-1-nm space and the estab- lishment of multiple H-bonds, the confined ethanol is able to set up closer contactwithMXene channels than in bulk solution to generate a much stronger and more stable interaction (−1.29 eV). The as- decorated nanochannels exhibit improved ion sieving ability, proton- ion selectivity, and long-term stability, providing a new, applicable category of modification methods for 1-nm and sub-1-nm channels. Results and discussion MXene membrane preparation and characterizations Highly crystallized, monolayered Ti3C2Tx nanosheets (Supplementary Fig. 1) with low oxidation degree and small lateral size distribution (Supplementary Fig. 2) were etched from parent MAX phase Ti3AlC2 13,14. When self-assembled into laminar membranes, the rear- ranged nanosheets illustrated a defect-free surface and layer-like cross-section structure, indicating the formation of internal 2D inter- layer channels (Supplementary Fig. 3). By a simple solvent immersion method, these untreated MXene membranes (Untreated-M) turned into ethanol-treated membranes (EtOH-M, Supplementary Fig. 4). In accordance with previous studies, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) analysis revealed the mem- brane chemical composition, comprising –O, –OH and –F groups linked to Ti and C atoms across the basal plane and around the edge of nanosheets (Fig. 1a, b, and Supplementary Fig. 5)15,16. These char- acteristic groups were also detected in Fourier-transform infrared spectroscopy (FTIR), but with a clear red shift at some peaks after ethanol treatment (Fig. 1c). Two major peaks assigned to –OH (3460 cm−1), and Ti–O (626 cm−1) moved to lower wavenumber of 3410 and 606 cm−1 17–19. This was largely because of the hydrogen bond formed between them and the –OH from the inserted ethanol, which causes the electron cloud density change and elongation of involving bonds. Consequently, the strength of the bonds will be decreased to show a lower stretching vibration frequency in FTIR20–22. Also, two joint peaks emerging at 2920 and 2850 cm−1, which are normally attributed to –CH3/–CH2–, also prove the successful insertion of ethanol mole- cules into MXene membranes. Further X-ray diffraction (XRD, Fig. 1d and Supplementary Fig. 6) characterizationon themembrane showed two-fold implications on its channel interiors. The hydrated interlayer channel size remained the same at around 16.5 Å regardless of the ethanol treatment, suggesting likelymodifications via decoration (MX-EtOH) rather than crosslinking Fig. 1 | Physicochemical characterizations on Ti3C2Tx MXene membranes. a, b XPS elemental analysis of Ti (Ti 2p) and C (C 1s) in MXene. c FTIR spectra of Untreated-Mand EtOH-M. Thehighlighted regions indicate FTIRpeakchanges after EtOH treatment. d XRD patterns of Untreated-M and EtOH-M. Inset, corresponding d-spacing is calculated from the patterns. e Schematic of ethanol decoration in Ti3C2Tx channels. The error bars in this figure represent the standard deviations of three parallel tests. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 2 (MX-EtOH-MX), because the latter usually results in smaller interlayer distances by XRD23. More importantly, an almost unchanged peak full width at half maximum (FWHM) was recorded before (0.73°) and after (0.76°) solvent insertion24. It proves that unlike inmost covalent cases, non-covalent membrane modifications can be achieved without dis- rupting the channel order, as depicted in Fig. 1e. Ion sieving and separating performance of EtOH-M The ion sieving performance of both untreated and EtOH-Mwas tested and compared towards a few common ions to evaluate the decoration efficiency (Fig. 2a). For a 300-nm-thick untreated membrane, the permeation rate of K+, Na+, Li+, Ca2+, and Mg2+ reached 377.5, 367.5, 333.7, 298.3 and 272.5mmol h−1 m−2, respectively, similar to previous studies (Fig. 2b)25,26. The relatively fast ion transport is reasonable considering the effective channel size of 7.7 Å (16.5−8.8 Å, the thick- ness of a single Ti3C2Tx nanosheet) 12. Once decoratedwith ethanol, it is expected that part of the channel is occupied to narrow down the effective channel width and obstruct the passage of ions. Accordingly, membrane ion rejection improved by 15 to 60 times, allowing only 25.4, 15.8, 14.3, 4.9, and 5.5mmol h−1 m−2 rate for corresponding ions. Such improved ion sieving performance is comparable to that of previously reported covalently and physically modified 2D membrane channels, suggesting similar decoration efficacy via non-covalent methods (Supplemental Table 1). Considering that non-covalent interactions including hydrogen bonding are generally low in strength, long-term and cyclic tests were then conducted using Na+ solution to examine the stability of the channel decoration strategy. In a 2-week-long continuous test, the permeation rate of Na+ in Untreated-M showed an obvious upward trend after the 4th day, implying the gradual Ti3C2Tx oxidation that compromised its ion- rejecting ability25. In opposition, the rate remained rather stable for EtOH-M at around 15.0mmol h−1 m−2 until the 17th day (Fig. 2c). Such contrast not only proved the sufficient affinity of decorated ethanol to the channel but also highlighted its bonus role to prevent Ti3C2Tx membranes from degrading via a “molecular shielding” effect (Sup- plementary Fig. 7)27. In cyclic experiments using Na+ solution, EtOH-M went through washing and drying (or drying only) after each testing session, to mimic the real membrane operating situations (Fig. 2d). Despite aminor rise between the 1st and the 2nd cycle, the permeation rate ofNa+ is stable during operationmode. It is alsoworthmentioning that to increase solvent treatment time and membrane thickness had limited influenceonmembrane ion sieving performance, implying that the intake of ethanol into the channel would soon reach a capacity (Supplementary Fig. 8). This is understandable considering the stiff configuration of ethanol that only allows limited rotation and vibra- tion. After a fewmolecules are anchored, the diffusion of the following ethanol will be impeded as they cannot shrink or squash like hydrated ions to pass the already obstructed channel26,28. Besides, to demonstrate the decoration effect on channel selec- tivity,we then testedmembranes’ ability to separate protons (H+, 2.8Å) from the above salts (>6.6Å, K+) before and after solvent treatment. Figure 2e shows that proton transport was also retarded in the EtOH- anchored MXene channels, albeit by a much smaller extent compared to other salt transport. While the permeation rate of the proton was reduced from 625.4 to 146.9mmol h−1 m−2 in EtOH-M, its selectivity against K+, Na+, Li+, Ca2+, andMg2+ increased from around 2.0–5.7, 12.5, 12.0, 28.7, 27.0 respectively. The enhanced selectivity was rationalized by the Arrhenius plot of temperature against ion permeation rate (Fig. 2f). Although both ions experienced higher energy barrier in EtOH-M than in Untreated-M, the activation energy increase for Na+ (from 20.8 to 39.1 kJmol−1) was obviously more pronounced than that for H+ (11.3–23.1 kJmol−1). This implies that the decorated EtOH mole- cules created a substantially larger barrier for Na+ to dehydrate while they only moderately interfered with the “hopping” (Grotthuss-like Fig. 2 | Ion sieving performance of Untreated-M and EtOH-M. a Ion permeation through MXene membranes in a U-shape device. b The permeation rate of K+, Na+, Li+, Ca2+ and Mg2+ (0.2M) through Untreated-M and EtOH-M. c Long-term Na+ permeation rate through Untreated-M and EtOH-M. Inset, accumulated permeated Na+ along with testing time. d Na+ permeation rate in cyclic tests. e H+/Mn+ (K+, Na+, Li+, Ca2+, and Mg2+) selectivity in Untreated-M, EtOH-M, and a commercial Nafion membrane. Inset, H+ permeation rate in these membranes. f Arrhenius plots of temperature against H+ and Na+ permeation rate for Untreated-M and EtOH-M. The error bars in this figure represent the standard deviations of three parallel tests. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 3 transport) of H+. Such high proton permeance and proton-cation selectivity of EtOH-M far exceed those of current Nafion membranes (proton permeation rate: 25.1mmol h−1 m−2; Selectivity: <2.2), and show its potentials as proton-selective membranes for electrolysis and flow batteries. Solvent-dependent non-covalent decoration efficacy To further verify the non-covalent (H-bond) nature of the proposed solvent decoration strategy, we then prepared and compared Ti3C2Tx membranes treated by non-polar cyclohexane (Cy-M), and polar yet aprotic acetone (Ace-M) besides aprotic EtOH-M. While having a similar size to ethanol (4.4 Å), cyclohexane (4.7 Å) and acetone (4.6 Å) are chemically different, which supposedly leads to varying inherent capabilities of establishing an H-bond network. This hypothesis was first supported by FTIR results in Fig. 3a. Unlike EtOH-Mwhich showed multiple peak redshifts, Cy-M displayed an almost identical spectrum to that ofUntreated-M, andAce-Mspectrumonly recorded a –OHshift from 3460 to 3382 cm−1. Meanwhile, while ethanol generated two obvious characteristic C–H (–CH3 or –CH2–) peaks between 3000 and 2800 cm−1, acetone and cyclohexane could only generate much weaker ones, implying their lower or even no adsorption into MXene channels. This difference can bewell explained by theH-bond forming mechanism: hydrogen (H) directly connected to a highly electro- negative atom (X, donor, usually N, O, F, and S) is approached by another alike atom with lone-pair electrons (Y, acceptor)29. As a non- polar hydrocarbon solvent, Cyclohexane is inept at H-bond formation (Fig. 3b). With a polar carbonyl yet no active proton, acetone can barely form H-bond with –OH on MXene (Ace =O ··· H–O–MX). As for ethanol, its hydroxyl enables its versatile binding with MXene via –O (Et–O–H ··· O–MX), –F (Et–O–H ··· F–MX), and especially –OH (Et–O–H ··· OH–MXand Et–HO ··· H–O–MX). HigherH-bond forming probability will translate into larger decoration coverage in MXene channels, and ultimately superior ion-sieving and proton-separating performance (Supplementary Fig. 9). Correspondingly, EtOH-Mwas found to have a 23-time Na+ rejection enhancement relative to Untreated-M, but Cy-M and Ace-M saw only 1.2-times and minor 2.6-times increase, respectively (Fig. 3c). At the same time, EtOH-M and Ace-M showed an H+/Na+ selectivity of around 12 and 4, respectively, while Untreated-M and Cy-M could only separate H+ and Na+ by a factor of 2. To quantitatively analyze the decoration difference, we then employed density-functional theory (DFT) to study each solvent- functionality pair. This was achieved by calculating the binding energy of each pair in a simplified “1-on-1” mode where a single solvent molecule is allowed to stabilize on one MXene functional group for all three Cy-M, Ace-M, and EtOH-M samples (Fig. 3d and Supplementary Table 2). In good agreement with experimental findings, the DFT results revealed two obvious characteristics. (1) all the pairs involving ethanol demonstrated negative binding energy (Eabs < 0), which again proves the tendencyof ethanol to be adsorbed ontoMXene via various surface groups. Comparatively, acetone and cyclohexane could only set up this spontaneous link via Ace-OH (−0.216 eV) and Cy-OH (−0.063 eV) pairs, implying their lower chance todecorate the channel; (2) When paired with any surface groups (–F, –O and –OH) from MXene nanosheets, ethanol always generated stronger combination than the other two. Especially in hydroxyl-containing pairs, EtOH-OH showed the highest Eabs of −0.479 eV, being 2 and 7 times as much as that of Ace-OH and Cy-OH, respectively. In addition, the dynamic and thermodynamic stabilities of the EtOH decoratedMXene structure are Fig. 3 | Solvent-dependent non-covalent channel decoration efficacy and ion sieving performance. a FTIR spectra of Untreated-M, Cy-M, Ace-M, and EtOH-M. The highlighted regions indicate FTIR peak changes after solvent treatment. b Schematic of the possible hydrogen bonding between MXene nanosheets and cyclohexane, acetone, and ethanol. c Na+ sieving performance of various MXene membranes. d DFT calculation of binding energy between MXene functionalities (–O, –F, –OH) and different solvents on “1-on-1”mode. The error bars in this figure represent the standard deviations of three parallel tests. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 4 further verified by ab-initio molecular dynamics (AIMD) calculations. After a dynamic simulation of 10 ps with a time step of 1 fs, the EtOH-M structure has neither significant deformation nor EtOH molecule dis- association, which reflects its thermodynamic stability. Such higher solvent-dependent combination chance and strength not only eluci- dates the non-covalent origin of our decoration method but also advises that protic functional groups like –OH, whether from guest materials or host channels, is critical to non-covalently decorating MXene nanochannels in a dense and intense fashion. In addition, we also treated Ti3C2Tx membranes with a few more solvents including methanol (MeOH), Acetaldehyde (MeCHO), and n-Hexane (Hex), which respectively, resembled EtOH, Ace, and Cy in terms of chemical properties. Similar ion-rejecting and selective performance trends were obtained, which demonstrated the reproducibility and uni- versality of our method (Supplemental Fig. 10). Nanoconfinement-enabled stability and ion-sieving ability Although the “1-on-1” DFT calculations corroborate that appropriate solvents can absorb ontoMXene channels via H-bond, the durability of these absorption-based decorations remaine a major concern. This is because even for the strongest EtOH–OH, its binding strength of −0.479 eV is still below the chemical absorption range between −0.622 and −1.036 eV (60–100 kJmol−1), a bar generally acknowledged for stable and irreversible adsorption. To rationalize the long-term experimental stability observed for EtOH-M, we first reviewed the real-world surface chemistry of MXene nanosheets. Unlike GO and MoS2 nanosheets bearing sparse functional groups, MXene surface is almost fully functionalized due to its unique progressive etching pre- paration method30–32. The massive amount of surface groups means that any solvent near MXene is highly likely to interact with more than one surface group. Therefore, we further calculated the binding energy of ethanol with two adjacent groups in an adjusted “1-between- 2” mode (Fig. 4a). Apparently, the transition of any “1-on-1” combination to corresponding “1-between-2” configuration would increase binding strength by 2–35 times (Supplementary Table 3). In particular, the incorporation of an extra group into the EtOH–OH could lead to chemically stable HO–EtOH–OH (−1.290 eV), F–EtOH–OH (−0.990 eV) andO–EtOH–OH (−0.890 eV) configurations, which was estimated to decorate around 27% of an MXene nano- channel (Supplementary Fig. 11). However, further enlarging the con- figuration into a “1-between-3” mode oppositely yielded decreased solvent-MXene binding energy. We attributed the looser combination to the deviating ethanol position from all three –OH in the “1-between- 3” system, in which the prolonged intermolecular distance greatly weakened theoverall hydrogenbondnetwork (Supplementary Fig. 12). Considering the high dependence of the H-bond on distance, we then investigated the effect of nanoconfinement on its formation and continuation by adjusting the ethanol–MXene distance (dO–H) in DFT calculations. The results found ethanol most energetically stable when 1.3 Å away from –OH on MXene, and that the binding energy expect- edly decreased with increased dO–H (Fig. 4b). More importantly, the dO–H-eV plot displayed two different changing trends in the adjusted distance from 1.3 to 4.8Å. Below 3.3 Å, Eads changed rapidly when ethanol approached or deviated from theMXene surface, showing the stronger attraction imposed by H-bond within this range. Beyond 3.3 Å, however, Eabs would only change slightly in response to altered dO–H. The cut-off point of 3.3 Å well reflects the short-range (<3.5 Å) feature of the H-bond and thus implies the critical role of nano- confined space to intensify otherwise much weaker non-covalent interactions23. To better mimic the real-world aquatic operating situation, we also studied the interplay among MXene, ethanol, and water in the confined channel. It was revealed that the binding energy between water and the anchored ethanol molecule was only −0.1 eV, substantially lower than that of MXene–ethanol pairs (Fig. 4a). The weaker water–ethanol interaction can be attributed to the relatively fixed orientation of ethanol due to the nanoconfinement in theMXene aa Untreated-M (0 ns) Untreated-M (100 ns) Ace-M (100 ns) EtOH-M (100 ns) c b 2. 3 �m es 18 � m es d Fig. 4 | DFT and MD simulation on channel stability and ion sieving ability. aDFT calculation of binding energy between ethanol andMXene functional groups in “one-between-two”mode.bThe effect of ethanol–MXenedistanceon its binding strength. c Schematic and d results of Na+ permeation through the simulated channel of Untreated-M, Ace-M, and EtOH-M over the course of 100ns. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 5 channels. While its –OH tends to face toward the MXene surface to maintain an energetically favorable position, this position prevents full contact between water and the polar –OH of ethanol. Therefore, local H2O-OHmiscibility via H-bond is greatly reduced compared to that in the bulk phase, thus ensuring the durability of the non-covalent dec- oration in solutions. After clarifying the stability issue,we thenprepared the channel of Untreated-M, Ace-M, and EtOH-M in molecular dynamics (MD) simu- lations and studied monovalent ion transport through them (Fig. 4c and Supplementary Fig. 13). The preparation of solvent-treated chan- nels followed the same process in the lab, and interestingly, stabilized ethanol-treated channels were found having 12.5 times higher dec- oration coverage than acetone-treated ones. (Supplementary Fig. 14). This agreed well with our DFT- and XPS-based analyzing results, pro- viding amutual validation of both our simulation and characterization methodologies. As shown in Fig. 4d and Supplementary Fig. 15, monovalent ions could free-pass untreated channels and rapidly accumulate at the channel exit. By contrast, channels stuffed with solvents greatly retarded their permeation depending on the decora- tion coverage. In particular, the permeation rate of simulated Na+ was reduced by a respective factor of 2.3 in Ace-M and 18 in EtOH-M, in excellent accordance with experimental results. The discrepant ion sieving ability among different channels also displayed a positive correlation with channel decoration degree, which suggests an ion sieving mechanism mainly based on size exclusion. In summary, 2D membranes possessing selective 1-nm or sub-1- nm channels can potentially extend their separating capability into numerous application settings with appropriate modifications. The easy and universal establishment of non-covalent interactions without radical reactions offers an efficient modifying route that largely maintains membrane integrity and channel alignment. As a proof-of- concept, we demonstrate the possibility of effective and stable non- covalent decoration to Ti3C2TxMXene channel by building upmultiple hydrogen bond networks between protic solvents and channel surface groups such as–O, –F, and –OH.More importantly, the angstrom-scale channel width induces the nanoconfinement effect to regulate the distance and orientation of the solvent to channel walls, rendering the generally weak H-bond sufficiently robust to yield improved ion siev- ing and separating performances. This study proves the feasibility of non-covalent modifications in nanoconfined space, which is promis- ingly extendable to other appropriately-sized nanochannels of various forms (e.g., porous and mixed matrix) and materials (GO, MoS2, MOF, COF, etc.). It also indicates that stable preferential settling of one species by non-covalent interactions can boost its selectivity against others in a separation process, providing a new perspective to understanding transporting phenomena in 1-nm and sub-1-nm chan- nels.We believe that thesemethodologies and underlying theories can be applied to develop and modulate membranes used for a broader range of applications including organic solvent nanofiltrations, solvent separations, gas separations, and beyond. Methods Preparation of MXene materials MAX parent phase Ti3AlC2 (400 mesh, >99.0%) was purchased from Kaixi Tech, China. Lithium fluoride (LiF, BioUltra, >99.0%) was pur- chased from Sigma-Aldrich while hydrochloride acid (HCl, 36%) was obtained from RCI Labs. All chemicals were used without further purification. The etching process was conducted in the same fashion and under similar conditions as described in previous studies but with slightly more concentrated HCl (7M)26. In detail, LiF (1 g) was first mixed into HCl (20ml) in a 150ml Teflon container under magnetic stirring at 300 rpm for 5min. MAX (1 g) powder was then slowly added into the mixture over 5min to avoid overheating. The temperature of the container was maintained at 35 °C for the next 24 h for sufficient etching. Upon the finishing of the etching, the etched product was washed via a “centrifugation-dispersion” method using deionized water. Normally the centrifugation was carried out at RCF of 550×g (Rotor 12181, Sigma 2-16P) until the pH of the supernatant reaches around 6, and the sediment was re-dispersed mildly sonicated for 10min to further delaminate few-layeredMXene stacks. The sonicated product was finally centrifuged for another 1 h at an RCF of 550×g to remove most of the unexfoliated Ti3AlC2. The concentration of the as- preparedMXene solution was determined based on a UV-vis spectrum “absorbance–concentration” plot (Shimadzu UV-2600 UV–Visible spectrometer, wavenumber from 900 to 200). Generally, after careful collection,MXene dispersion (around 0.7mg/ml) could be obtained. A chemically inert gas such as Ar was then pumped into the dispersion before storing it at a low temperature (lower than 4 °C). MXene membranes preparation To fabricate laminar membranes, the calculated amount of MXene would be filtrated on aNylon substrate (47mmdiameter, 0.2 µmpore size) with vacuumassistance. For instance, to prepare a 300-nm thick membrane, a dispersion containing 1.12mg of MXene would be extracted and diluted with 200ml of deionized water. As-prepared membranes were dried at room temperature for 24 h before further tests, treatments, and characterizations. The solvent-modified membranes were prepared via a facile “immersion-evaporation” method, where these pristinemembranes were immersed in solvents (5ml) including ethanol (anhydrous, >99.5% Sigma-Aldrich), acetone (ACS reagent, >99.5% Sigma-Aldrich), cyclohexane (anhydrous, 99.5% Sigma-Aldrich), methanol (HPLC, >99.9% Sigma-Aldrich), acetaldehyde (ACS reagent, >99.5% Sigma-Aldrich), n-hexane (HPLC, >97% Sigma-Aldrich) for 2 h. After that, the solvent-treated mem- branewas placed in a 50 °Coven to let any residual organicmolecules evaporate. All untreated and solvent-treated membranes were fur- ther dried at room temperature for 12 h before further testing or characterizations. Characterizations The morphology and size distribution of Ti3C2Tx nanosheets and membraneswas characterized by scanning electronmicroscope (SEM) on a Nova NanoSEM 450 microscope (FEI, USA), operated with an accelerating voltage of 5 kV, 2.0 spot size. All samples were sputtered with iridium before SEM image taking. More detailed morphology and crystallinity information of singleMXene nanosheets were obtained by transmission electron microscopy (TEM) under conventional imaging and diffraction mode, respectively, on an FEI Tecnai G2 T20 micro- scope operated at an accelerating voltage of 200 kV. The EDX data were collectedusing an FEI TecnaiG2 F20 FEGTEMoperating at 200 kV with a Bruker EDX detector. The height profile was acquired by atomic force microscopy (AFM) using a tapping mode on Bruker Dimension Icon Microscope (Bruker, Billerica, MA, USA). To analyze the crystal- lographic structure of bulk Ti3AlC2 and restacked Ti3C2Txmembranes, XRD was conducted on a Bruker D2 Phaser Diffractometer with a Cu- Kα radiation source (15Ma and 40 kV) and the data was analyzed by coupling software DIFFRAC. The examination of MXene surface che- mical properties including atomic ratio, atomic chemical states, and functional groups was performed by XPS on a Thermo Scientific Nexsa Surface Analysis System equipped with a hemispherical analyzer and the data were processed either automatically (element analysis) or manually (peak fitting) on a supporting software Avantage. The inci- dent radiation was monochromatic Al Kα X-rays (1486.6 eV) at 72W (6mA and 12 kV, 400 × 800μm). To determine the inner chemical properties of our samples, depth profile elemental analysis was added, which was realized by etching away top MXene layers for hundreds of seconds. Meanwhile, a qualitative and semi-quantitative probing of MXene functional groups was carried out by FTIR on a Perkin Elmer Spectrum Two FTIR Spectrometer with the widest spectral range from 4000 to 400 cm−1. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 6 Ion-sieving performance test The ion-sieving ability of MXene-based membranes was evaluated by an ion-diffusion experiment. It was carried out using a Teflon-based U- shape apparatus. In a typical permeation test, 15ml of 0.2M XCl (X for H+, K+, Na+, Li+. Mg2+ and Ca2+) solution and DI water were simulta- neously injected into different sides as feed and permeate solution, respectively. Magnetic stirring at 350 rpm was used to minimize pos- sible concentration polarization. To figure out the permeation rate of ions through different membranes, the conductivity of the permeates was measured with a LabCHEM Conductivity-pH meter at room tem- perature. The relation between measure conductivity and permeate salt concentration can be expressed as: C = κ Λm ð1Þ whereC is the to-be-calculatedpermeate concentrationwhile κ andΛm respectively represent measured conductivity and molar conductivity of the used salt solution. The obtained C can thereby be converted to ion permeation rate (Ji, mmol h−1 m−2) based on: Ji = VC At ð2Þ DFT calculations Calculations are performed by density functional theory (DFT) based on the projected augmented wave (PAW) in the Vienna ab initio simulation package 5.4 (VASP)33,34. The Perdew–Burke–Ernzerhof (PBE) form of generalized gradient approximation (GGA) is used as the exchange-correlation function of energy6. The DFT-D3 method is used to describe the weak interlayer van der Waals effect6. The cutoff energy of the planewave is set at 550 eV, and the convergence criteria for energy and force are set at 10−5 eV and 0.005 eV/Å, respectively. The 3 × 3 × 1 K-point grids in the Brillouin zone are used for structural optimization and electronic structure calculations. Simplified mem- brane structures are constructed using 2 layers of surface groups (–O, –OH, –F) decorated MXene supercells (30 × 26 × 55 Å3) with an initial interlayer distance of 16.5 Å derived from XRD experiment results. A 25 Å vacuum layer is set to avoid the interaction between the repeated unit cell normal to 2D layers. Over 60 types of conformation of EtOH to MXene have been tested to find out the optimized structure. To simulate the real application in membrane structure, “1-on-1”, “1-between-2”, and “1-between-3” configurations of decoration mole- cules are configured. In order to determine the configurationwith the lowest energy, the binding energies of the decorated membrane structures are evaluated by the following equation: Eb = Etotal � EMembrane � Edecoration ð3Þ where Etotal, EMembrane, andEdecoration represent the total energies of the decorated membrane structures, a double-layered MXene membrane, and an isolated decoration molecule (EtOH, cyclohexane, acetone), respectively. Amore negative value indicates stronger binding energy. The optimized interlayer spacing of 16.4 Å is larger than the sumof the radii of O, OH, and F groups ofMXene, indicating that there is a typical van derWaals interaction between theMXene layers of the membrane sample. Furthermore, the effect of nanoconfinement on its formation by adjusting the ethanol-MXene distance is investigated based on the Climbing Image Nudged Elastic Band (CINEB) method to illustrate the energy profile for EtOH migration path. MD simulations To investigate the ion transport properties, MD simulations with three modelswere carriedout: surface-modifiedMXMs (MXenechannelswith a d-spacing of 16.5 Å) equilibrated in water, in ethanol, and in acetone, respectively26. The MXM repeat unit is originally from the crystal structure of Ti3C2F2, the functional -F groups are randomly replaced by -O- and -OH reaching a ratio of –O-/-F/-OH=0.47/0.38/0.1435. The box size in the solvent equilibrium process is 5.54 nm × 5.33 nm × 3.28 nm. The systems were filled with water/ethanol/acetone molecules through the solvationprocess. Thewatermoleculesweredescribedby the SPC/E model36. The universal force field UFF forcefield with QEq charge is assigned to other molecules through Openbabel and OBGMX codes19,37–40. Systems were subjected to multi-step steepest-descent energy minimization followed by CG energy minimization26. After that, 100nsNVT equilibrium simulationswere performed. TheMXene atoms were frozen in the simulations since theMXene nanosheets were rather rigid. In NVT simulations, the time step is 2.0 fs, and bonds to hydrogen atoms were maintained with the LINCS algorithm41. And a constant simulation temperature of 298.15 K was maintained by the V-rescale thermostat42. The rcoulomb and rvdwwere set as 12 Å. The electrostatic interactions were evaluated using the particle mesh Ewald algorithm. For Na+/Li+/K+ ion transport behaviors, the simulation box is enlarged into 12 nm×5.33 nm×3.28nm with an additional feed region at the left side of the MXene sheet. Salt ions were randomly placed in the feed chamber with a concentration of 0.42M. 100ns NVT production simulationswereperformedafter energyminimization. Thepermeation rate was calculated as the number of cations permeating into the membrane divided by the simulation time. All MD simulations in this work were performed using the GROMACS 2019.643. 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Author contributions Y.K. and X.Z. raised the conceptualization and designed the research framework, and they also finished themanuscript writing joined byH.W. Y.K. conducted the experimentswith the assistance of Y.W. and K.H. and R.X. Y.H., Z.W., W.Z., and Y.C. conducted most characterizations while these parts were also supported by Z.X. T.H. finished Molecular Dynamics simulation and Q.G. covered all the DFT calculation section. All the authors take part in the discussion and reviewing of the project towards its completion. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-023-39533-y. Correspondence and requests for materials should be addressed to Qinfen Gu or Xiwang Zhang. Peer review informationNatureCommunications thanksSiQin, Abdul P. Rasheed, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available. Reprints and permissions information is available at http://www.nature.com/reprints Publisher’s note Springer Nature remains neutral with regard to jur- isdictional claims in published maps and institutional affiliations. Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 8 https://doi.org/10.1038/s41467-023-39533-y http://www.nature.com/reprints Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. 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To view a copy of this licence, visit http://creativecommons.org/ licenses/by/4.0/. © The Author(s) 2023 Article https://doi.org/10.1038/s41467-023-39533-y Nature Communications | (2023) 14:4075 9 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Nanoconfinement enabled non-covalently decorated MXene membranes for ion-sieving Results and discussion MXene membrane preparation and characterizations Ion sieving and separating performance of EtOH-M Solvent-dependent non-covalent decoration efficacy Nanoconfinement-enabled stability and ion-sieving ability Methods Preparation of MXene materials MXene membranes preparation Characterizations Ion-sieving performance test DFT calculations MD simulations Data availability References Acknowledgements Author contributions Competing interests Additional information