Gas transport through two-dimensional nanoslits

Journal Pre-proof Gas transport through two-dimensional nanoslits Wen Ying, Amin Khan, Xinsheng Peng PII:

S2588-8420(20)30003-1

DOI:

https://doi.org/10.1016/j.mtnano.2020.100074

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MTNANO 100074

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Materials Today Nano

Received Date: 13 September 2019 Revised Date:

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Accepted Date: 5 February 2020

Please cite this article as: Ying W., Khan A. & Peng X., Gas transport through two-dimensional nanoslits, Materials Today Nano, https://doi.org/10.1016/j.mtnano.2020.100074. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Gas transport through two-dimensional nanoslits Wen Ying,a Amin Khan,a Xinsheng Penga* a

State Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China. The corresponding author: [email protected]

1

Abstract Two-dimensional (2D) material nanosheets have been widely investigated for gas separation due to their atomic thick layered structures. For 2D nanosheet, there mainly are two transport paths for the gas: (i) the pores in the nanosheet plane, (ii) the nanoslits constructed by layer-stacked nanosheets. The in-plane pores separate the gases by size-exclusion or functional groups on the pores. The 2D nanoslits in layer-stacked nanosheets have a much longer path than the in-plane pores and have more complex transport mechanisms. Herein, we have summarized the construction of 2D nanoslits and mechanisms for gas transport. A newly developed composite membrane based on the 2D framework is also highlighted. The transport mechanisms in a variety of 2D materials are also introduced. Keywords: Two-dimensional materials, two-dimensional nanoslits, gas transport, separation membrane

1. Introduction Gas separation is widely established in industries, such as to capture greenhouse gas and purify the gas.1 Membrane separation is blooming for its energy efficiency and pro-environment, compared with those traditional strategies, like adsorption, solvent absorption, cryogenic and thermal desalination.2-9 A qualified membrane for gas separation should be thin and selective to enable separation efficiency, as well as mechanically robust. Among the available commercial membranes, polymeric membranes dominate the current market due to their cost-effective nature and easy processability.10-12 However, the intrinsic drawbacks of poor chemical and thermal stabilities limit their application. There exists a trade-off between permeability and selectivity in case of polymeric membranes which is called Robeson Upper bound.13 Much work has been done to break the above limitation.14-16 Utilizing two-dimensional (2D) materials to construct membranes is one of the promising methods. The study of 2D materials started with the discovery of graphene.17 2D material 2

nanosheets with unique atomic thickness and micrometer lateral dimensions are rapidly emerging as desirable building blocks for the design of ultrathin separation membranes,18-21 which can maximize the flux. However, a perfect graphene nanosheet is impermeable to gases as small as helium, because the electron density of its aromatic rings is substantial enough to repel atoms and molecules.22 Thereby, much attention has been given to create the pores effectively. Theoretical studies showed that the designed N-functionalized pores, with the size of 3.3 Å, and all-H pores, with the size of 2.5 Å, had H2/CH4 selectivity with the order of 108 and 1023, respectively,23 whereas the obtained experimental selectivity was much lower than the theoretical selectivity. Therefore, a significant obstacle, in this case, was to create pores precisely and uniformly, except for the preparation of defect-free nanosheet. Many technologies, such as oxidative etching,24 electron beam irradiation,25 ion-beam bombarding,26 plasma,27,28 and template method29 were used to create pores in graphene nanosheet. While, some intrinsic porous materials, such as metal-organic framework (MOF), were exfoliated into 2D MOF nanosheets.30-33 Besides, layer-stacked membranes can also be prepared by using 2D materials for gas separation. A widely used material for these stacked membranes is the oxidative derivative of graphene i.e. graphene oxide (GO).34-36 Apart from this, layered transition-metal dichalcogenides (TMDs),37,38 2D transition-metal carbide and/or nitrides (MXenes),39,40 2D MOFs30-33 and Layered double hydroxides (LDHs)41,42 have also opened a new era of membranes. The above 2D materials are readily assembled into layered stacks, which act as 2D separation nanoslits. In this review, we have focused on the preparation of 2D nanoslits and the respective mechanisms of gas transport through 2D nanoslits: size sieving, Knudsen diffusion, solution-diffusion and facilitated diffusion. Moreover, a composite membrane based on 2D materials is also introduced, which exhibited excellent separation performance.

2. Construction of 2D nanoslits The prepared 2D nanosheets can be assembled into a layer-stacked membrane with a well-defined interlayer distance which forms regular 2D nanoslits for gas transport. Different from the fabrication of porous nanosheet, which can be prepared by chemical 3

Fig. 1. Proposed conceptual interlayer nanostructures of GO membranes prepared by slow and fast deposition rates. When prepared at slow deposition rate, oxygen-containing groups on adjacent SLGO flakes prefer to self-assemble with each other to form thermodynamically favored interlayer structure. In contrast, at fast deposition rate oxygen-containing groups may 45

arrange in a more random fashion. Reprinted with permission.

Copyright 2017 American

Chemical Society.

vapor deposition, preparation of 2D nanoslits always starts with the suspension of 2D nanosheets. 2.1. Filtration The 2D nanosheets have been assembled to consistent stacks during the flow of suspension under pressure difference. The different interactions between the 2D nanosheets, such as hydrogen-bond, van der Waals force and electrostatic, lead to the layered structure and form uniform interlayer distance.43 The thickness of resulted membrane depends on the concentration and volume of suspension, ranging from nanometers to micrometers.35,44 Furthermore, the recent research showed that the deposition rate had an influence on the 2D nanoslits structure.45 At slow deposition rate, the oxidic area of graphene matched with oxidic area, and pristine area matched with pristine area. But at fast deposition rate, the oxidic area and pristine area of graphene were mismatched (Fig. 1). Filtration is a facile way to construct functionalized 2D nanoslits, by just mixing 2D nanosheets with additional functionalized molecules or nanoparticles, as long as they are incorporable.46,47 The structure of 2D nanoslits can greatly expand in this way. 2.2. Coating Coating is another widely used method to prepare layer-stacked membranes. The 2D membrane is formed by dropping the suspension of 2D nanosheets on the surface of a 4

substrate. The additional suspension is removed from the surface to obtain the final 2D

Fig. 2. Schematic illustration of two different GO coating methods (method one and method 50

two). Reprinted with permission.

Copyright 2013 the American Association for the

Advancement of Science.

membrane. Different coating methods such as drop-casting, dip-coating, spin-coating, and spray-coating have been widely adopted.34,48-50 Freestanding GO membrane with interlayer spacing of 0.82 nm have been prepared by drop-casting,49 and during the preparation process, GO suspension was drop-casted on a paper and dried at room temperature. Spin-coating or spray-coating34 were also used to prepare the GO membranes with thickness of 0.1 to 10 µm and interlayer spacing of 1 nm. The prepared GO membrane could withstand a pressure difference up to 100 mbar, with the impermeability even for helium. GO/ceramic membrane can be simply prepared by dip-coating,48 with an automatic capillary infiltration process. A study was performed to compare the results of two available coating methods to prepare GO membranes.50 In the first method, the support substrate contacted the air-water interface of GO suspension and after that the spin-coating was performed. In the second method, the GO suspension was dropped on the support substrate, accompanied by spin-casting. The above methods resulted into different degrees of nanosheets interlocking (Fig. 2). In the first method, 5

electrostatic and hydrophilic interaction between GO nanosheets dominated the GO deposition. As GO nanosheets edges carried negative charge, the edge-to-edge interaction between GO nanosheets was repulsive and thus resulted into the island-like assembly of GO nanosheets on the support substrate. In the second method, the contact between support substrate and GO suspension was accompanied by spin-casting. The face-to-face attractive capillary force, created by spin-coating, overcame the repulsion of edge-to-edge interaction, and thus led to a denser GO stacking. In a nutshell, nanoslits constructed by coating is a comprehensive result of intrinsic edge-to-edge interaction of 2D nanosheets and face-to-face capillary force during coating. The thickness of resulted membrane depends on the concentration or volume of 2D suspension and the number of coating iterations. 2.3. Langmuir-Blodgett assembly Langmuir-Blodgett is a method in which the 2D monolayer from the surface of a liquid is deposited onto a solid substrate by immersing the solid substrate into the liquid. The edge-to-edge and face-to-face interaction are two fundamental interactions between 2D nanosheets. The edge-to-edge electrostatic repulsion of 2D nanosheets prevents the overlap of 2D nanosheets and ensures the stabilization of monolayer without any stabilizing agent. The face-to-face interaction leads to irreversible stack of single layers with different sizes, and thus forms the double layers.51 The 2D material monolayer can be transferred to the solid substrate like glass or quartz to construct 2D membrane, and the number of layers can be easily tuned by repeating the process. This is the most precise method to control the 2D membrane thickness. And it is also easy to modify the structure of 2D nanoslits by functional molecules. For example, 1,3,5-benzenetricarbonyl trichloride (TMC) could cross-link GO nanosheets by assembling one nanosheet on top of another one (Fig. 3).52 By soaking substrate into GO suspension and TMC solution in turn, TMC functionalized GO nanoslits were formed with the interlayer spacing of around 1 nm.

6

Fig. 3. Schematic illustration of (a) a step-by-step procedure to synthesize the GO membrane, (b) the mechanism of reaction between polydopamine and TMC, and (c) the mechanism of 52

reaction between GO and TMC. Reprinted with permission.

Copyright 2013 American

Chemical Society.

3. Separation mechanism of 2D nanoslits The channel of 2D nanoslits, constructed by layer-stacked 2D nanosheets, can be considered as a pore while the interlayer spacing can be reckoned as the pore size. The difference is that the 2D nanoslits have a much longer diffusion path. Pores can be classified into micropores, mesopores and macropores, depending on the pore size.53 Thereby, several possible mechanisms have been postulated to explain the transport of molecules through 2D nanoslits. 3.1. Size-exclusion When the interlayer spacing is smaller than the kinetic diameter of a gas molecule, the gas molecule cannot go through the nanoslit because of the steric effect.54 Each atom occupies a finite space, and when atoms are too close, there is a repulsion of the overlapping electron clouds. It is an effective way to separate gases with different kinetic diameters by nanoslits whose interlayer spacing is in between. Take the Ni3AlCO3-based membrane, constructed by Caro et al.,55 as an example. Its free space for gas transport was 0.31 nm, which lied between the kinetic diameters of H2 and other gases. Thus, the membrane presented much higher H2 permeance than the permeance of other gases. 7

Fig. 4. The separation capability of the GO membrane is tunable by adjusting the nanochannel 57

size. Reprinted with permission.

Copyright 2014 the American Association for the

Advancement of Science.

3.2. Knudsen diffusion When the interlayer spacing is larger than the kinetic diameter of gas molecule but smaller than its mean free path, the collisions between the gas molecule and nanoslit wall dominate in the nanoslits, and thus gas transport follows Knudsen diffusion mechanism i.e. larger gas molecular weight leads to smaller gas permeance.56 Size-exclusion and Knudsen diffusion are mainly based on the intrinsic interlayer spacing of 2D nanoslits, and the interlayer spacing can be easily tuned by introducing specific molecules or ions, which has been summarized (Fig. 4) in the review by Mi et al.57 As most interlayer spacings of 2D nanoslits fall in between kinetic diameter and mean free path of gases, Knudsen diffusion is dominated during the pure 2D nanoslits diffusion process. In Knudsen diffusion, gas permeance is relative to gas molecular weight, which is constant, and thus there is little room for improvement. The review by Zhao et al.58 provides a deeper insight of molecule diffusion through different pore sizes. It is promising to modify 2D nanoslits to break Knudsen diffusion’s limitation, and thus new diffusion mechanisms are introduced.

8

Fig 5. Gas permeances of GO membranes as a function of molecular weight (dashed line represents the ideal Knudsen selectivity) under dry and humidified conditions. amu, atomic 50

mass unit. Reprinted with permission.

Copyright 2013 the American Association for the

Advancement of Science.

3.3. Solution-diffusion Solution-diffusion mechanism is a well-known mechanism for gas transport through polymeric membranes while gas permeance is the product of gas solubility and diffusivity in this case.59 By completely filling the 2D nanoslits with polymer or other liquids, like water50,60 or ionic liquid (IL),16,61 the diffusion mechanism will change from Knudsen diffusion to solution-diffusion mechanism. From the Kim’s work50, it was observed that the CO2 permeance of the prepared membranes got increased under humidified conditions as compared to the membranes under dry conditions (Fig 5). Under dry conditions, pore size determined the gas transport and it showed typical Knudsen diffusion. However, under humidified conditions, water was present in the GO nanoslits, due to which, solution-diffusion mechanism dominated the transport. The high CO2 solubility in the water led to the high permeance. The gas solubility and diffusivity vary with the fillers, so it is likely to select specific liquids for target gas transport. The 2D nanoslits are not only used for the framework, but its nanospace can also change the property of the confined liquid.62 9

Fig. 6. (a) Schematic illustration of the carrier-mediated transport mechanisms for a mobile carrier membrane, a semi-mobile carrier membrane and a fixed-site carrier membrane. (b) The four typical types of reversible reactions for facilitated transport membranes together with their 63

corresponding carriers and target permeants. Reprinted with permission. Copyright 2014 the Royal Society of Chemistry.

3.4. Facilitated diffusion Inspired by cell membranes which can selectivity transport target molecules or ions through carrier protein, facilitated diffusion through specific carrier has attracted much attention. It also can be applied to gas transport through 2D nanoslits by introducing some specific molecules or groups as carriers, that can react with target gas molecules. According to the carrier mobility, facilitated diffusion can be divided into three types (Fig. 6a): fixed carrier diffusion, semi-mobile carrier diffusion and mobile carrier diffusion.63 Fixed carrier diffusion can be also named as “hopping mechanism”,64 because the carrier is fixed and the target gas molecule needs to go through the nanoslits by jumping from one carrier to another carrier. In pure nanoslits, these carriers are always fixed on the nanoslit walls. While in the nanoslits filled by filler such as polymer, carriers can be fixed in the filler. Mobile carrier diffusion based on the liquid-filled nanoslits is considered as an improvement of solution-diffusion mechanism. These carriers can be dissolved into the liquid with considerable diffusivity. This diffusion process contains the reaction between target gas molecules and carriers, and the diffusion of carriers in the solvent. The 10

semi-mobile carrier diffusion also exists in liquid-filled nanoslits, with the carrier mobility between mobile carrier and fixed carrier. There are four typical interactions between carriers and target gases, summarized in Fig. 6b. For example, some transition metal ions such as silver and copper can be regarded as olefin carriers as they can form complexes with olefins by π-bond complexation. The GO membrane whose nanoslits are fixed with silver ions can be used for ethylene/ethane separation.65,66 Facilitated diffusion is the most flexible mechanism for gas diffusion as it can persistently select specific carrier to transport gas molecules. 3.5. External force External force, such as electric field and magnetic field, can be applied on 2D nanoslits to modulate gas transport through 2D nanoslits. Theoretically, external force cannot change the mechanism of gas transport, it just enhances or blocks gas diffusion through nanoslits by acting on the liquid that filled in nanoslits or directly on gases.66 Ionic liquid (IL) consists of anions and cations, so they can be influenced by the external electric field. In this perspective, a typical work was performed in which an external electric field was applied to GO supported ionic liquid membrane (GO-SILM).67 Molecular dynamics simulations showed that the gas adsorption/desorption free energies, free volumes of IL and interaction energies between anions and cations were changed under electric field. Furthermore gas solubility and diffusivity were also improved.

4. Application of gas transport through 2D nanoslits Numerous 2D materials have been prepared after the discovery of graphene. In this section, we will focus on the 2D nanoslits that are constructed by different 2D materials and employed for gas separation. 4.1. Graphene oxide (GO) nanoslits Graphene is a well-known 2D material18 in which a perfect graphene nanosheet is impermeable to gases as small as helium. The direct preparation of laminated graphene membrane is a challenge.68 Basically, GO is the oxide derivative of graphene, and can be exfoliated from graphite oxide through strong acid and oxidant followed by sonication.20 There are oxidized and non-oxidized regions in GO nanosheets. These oxidized regions 11

Fig. 7. Design and construction of 2D channels. (a) External force driven assembly approach for fabricating 2D channels. It involves three dimensional external forces in x, y, and z axes. Enlarged schematic shows force analysis for one 2D channel unit consisting of GO nanosheets and polymer chain. Three main types of forces are included: intrinsic force, “outer” external forces (compressive force, centrifugal force and shear force) which are applied outside the 2D channel unit and “inner” external force (GO-polymer molecular interactions) which are applied inside the 2D channel unit. (b) Hypothetical evolution of surface and cross section of GO-assembled 2D channels from intrinsic force-induced disordered structure (left) to highly ordered laminar structures (right) driven by introduced 70

synergistic external forces. Reprinted with permission.

Copyright 2016 American Chemical

Society.

expand the interlayer spacing to around 0.8 nm in the layer-stacked GO nanosheets.16,69 Gas transport in majority of the GO membranes takes place through nanoslits irrespective of the work reported by Yu et al.35 for ultrathin GO membranes, in which the gases were transported through the selective pores or defects rather than the nanoslits. As 0.8 nm is larger than the kinetic diameters of common gases and smaller than their mean free paths, the mechanism will follow Knudsen diffusion. However, due to the influence of preparation technology, the structure of nanoslits shows some differences. For example, the work reported by Kim et al.50 showed two kinds of coating methods for the preparation of GO membranes, as above-mentioned in the section 2.2. The two different preparation methods showed different gas separation performance. The membrane prepared by the first method had loose nanosheets stacking and islands-like structure and it followed Knudsen diffusion mechanism as a result. However, the membrane prepared by the 12

second method had unusual high CO2 permeance, that came from the closely-packed interlocked layered structure. Many attempts have been done to improve the limitations of Knudsen diffusion. GO membrane with interlayer spacing of 0.4 nm was prepared by Xu et al.70 through a vacuum-spin technique (Fig. 7), in which the size-sieving effect played an important role. Another typical work was performed by Jiang et al.,71 in which the GO nanoslits were modified by poly(ethylene glycol) diamines (PEDGA). In this way, CO2-philic region was introduced which contributed to the high CO2/CH4 selectivity (Fig. 8). The fabricated GO membranes were possibly suitable at industrial scale due to their large area.72,73 4.2. 2D metal-organic framework (MOF) nanoslits MOFs are a class of porous materials with metal ions linked by organic ligands.74 MOFs are crystalline compounds and their regular repetition or layered structure offer the opportunity of preparing 2D MOF nanosheets. Different from graphene or GO single nanosheet, which only consists of carbon atoms, single 2D MOF layer is made of line-up units of MOF, and is usually much thicker. The intrinsic porous structure in nanosheets gives additional transport path except for the nanoslits having layer-stacked nature. Peng et al.30 firstly used exfoliated MOF nanosheets to construct gas separation membrane in 2014. 2D MOF nanosheets, Zn2(bim)4, were obtained by top-down method, in which the layered MOF crystals were treated by ball-milling and sonication. 2D MOF nanoslits were constructed by hot-drop coating, in which different temperature could form different structures, like disordered stacking, expanded stacking and restacking (Fig. 9a-c). Among these structures, disordered stacking had the best H2/CO2 selectivity, because lamellar ordering of nanosheets would block the transport path of H2 with little effect on CO2. The plenty of MOFs offer multitudinous structures of 2D MOF nanosheets. The same group used another MOF, [Zn2(bim)3(OH)(H2O)]n, to construct 2D MOF membrane by the same method i.e. hot-drop coating. By increasing coating temperature, highly interlocked, closed-packed nanosheet structure was obtained which made the interlayer space more constrictive.75 The decrease in CO2 permeance and unaffectedness of H2 permeance suggested that there were two transport paths in the 2D MOF membrane (Fig. 10a-c) i.e. 13

nanoslits constructed by nanosheets and the apertures in nanosheets. The work done by Wang et al.76 also confirmed the two transport paths (Fig. 11a-e). They exfoliated a layered MOF, MAMS-1 (Mesh Adjustable Molecular Sieve, Ni8(5-bbdc)6(µ-OH)4) into 2D nanosheets and constructed 2D membrane through hot-drop coating. The first path was the aperture in MOF nanosheet with size of 0.29 nm which was nearly perpendicular to nanosheet basal plane. The second path was parallel to nanosheet basal plane with size of 0.5 nm. In fact, the second path was just the nanoslits constructed by two parallel nanosheet basal plane. The first path had an ideal size-exclusion effect and the second path resulted in the Knudsen diffusion. However, the highly polar wall of the second path contributed by [Ni8(µ3-OH)4] clusters contained CO2-philicity, and thus had a resistance to transport for CO2.

Fig. 8. (a) Mixed gas permeation performances of the composite membranes tested in the dry state (50/50 vol%, total feed pressure 2 bar, 30 °C). The membranes were prepared from a filtrate volume of 10 mL. The text below each membrane type is the corresponding calculated interlayer nanochannel size. (b) Representation of gas molecules transport in the GO stacking membranes channels containing both CO2-philic and non-CO2-philic nanodomains. Reprinted 71

with permission. Copyright 2017 John Wiley and Sons. 14

Fig. 9. (a) Architecture of the layered MOF precursor. The ab planes are highlighted in purple to better illustrate the layered structure. (b) Powder XRD patterns of Zn2(bim)4. The top trace is the experimental pattern, whereas the bottom trace is the pattern simulated based on the single-crystal data (CCDC-675375). The asymmetric unit of Zn2(bim)4 is also presented to illustrate the coordination environment of Zn atoms. (c) Powder XRD patterns of four membranes with different separation properties. The cartoons at the left schematically illustrate the microstructural features of the nanosheet layers. The yellow and green portions correspond to the low-angle humps and the (002) peaks in the XRD patterns, respectively. All the membranes were measured for equimolar mixtures at room temperature and 1 atm. 30

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15

Fig. 10. (a) Four-layered stacking diagram of Zn2(Bim)3 precursors along the c-axis. Zn green, N orange, C gray, H white, O red. The Zn coordination polyhedra are displayed in green, the layers with benzimidazole ligands along c-axis are depicted in purple, and the others in yellow. (b) Two-layered Zn2(Bim)3 structure highlighting the AB stacking mode. (c) Illustration of the hypothesis of gas separation through porous Zn2(Bim)3nanosheets. Only Zn atoms are shown for clarity, and the light blue planes represent the nanosheets regardless of their amphiprotic natures. The solid and dashed lines represent the pathways of H2 (blue) 75

and CO2 (red). Reprinted with permissions. Copyright 2017 John Wiley and Sons.

16

Fig. 11. (a) FE-SEM image of layered MAMS-1 crystals. Scale bars, 5 µm. (b) Crystal structure of MAMS-1. The ab planes are highlighted in magenta to illustrate the layered structure. (c) Tilted vertical view of ab plane in MAMS-1 monolayer featuring PW1. (d) Amplified view of PW1 gated by two pairs of tert-butyl group highlighted in magenta. (e) View 76

along a axis of MAMS-1 monolayer featuring PW2. Reprinted with permissions.

Copyright

2017, Springer Nature.

4.3. Transition metal dichalcogenides (TMDs) nanoslits TMDs are lamellar materials that can be exfoliated by the intercalator, alkyl lithium. They are represented as MX2, where M is the transition metal element and X is the chalcogenide element such as S, Se or Te, respectively. A single layer of 2D TMDs has three atomic layers. A metal atom is in the middle, with two chalcogenide atoms located above and below. Molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are two typical TMDs, which are exfoliated into 2D nanosheets and constructed into 2D membranes.37,38 MoS2 and WS2 nanosheets are non-porous and gas transport takes place in membrane through the nanoslits constructed by layered nanosheets. Jin et al.77 were among the pioneers who firstly used MoS2 nanosheets membrane for gas separation. The MoS2 nanosheets were exfoliated from MoS2 flakes by alkyl lithium, and the membrane was simply prepared by filtration. Different from GO membrane, there were 17

two

kinds

of

interlayer

spacing,

0.62

nm

Fig. 12. (a) AFM image of single-layered MoS2 nanosheet. (b) XRD pattern of a MoS2 membrane with a thickness of 100 nm. (c) Schematic of gas permeation pathway across MoS2 membranes before and after heating at 160 °C. Afte r heating the 1T MoS2 (grey sheets) is converted to the 2H phase (black sheets), which decreases the interlayer spacing, but as the overall thickness of the membranes remains the same the decrease in d-spacing creates empty spaces or channels inside the membranes through which gas molecules can diffuse 77,78

fast. This results in the increase in gas permeability. Reprinted with permissions.

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2015 the Royal Society of Chemistry. Copyright 2016 the Royal Society of Chemistry.

and 1.0 nm, calculated from XRD patterns (Fig. 12a and b). The larger interlayer spacing was resulted from the island-like assembly of MoS2 nanosheets, just as mentioned above in section 2.2. The interlayer spacing determined that gas transport through membrane followed Knudsen diffusion mechanism. The experimental results showed a typical Knudsen selectivity as expected. However, the larger interlayer spacing got flattened and densified by increasing pressure, which in turn stipulated larger influence on H2 or He permeance as compared to the gases with bigger molecular weight. Another work by Achari et al.78 confirmed that there were two phases on MoS2 membrane, 1T and 2H phase. Phase transition from 1T to 2H phase occurred under specific temperature, and gas separation performance of MoS2 membrane got improved (Fig. 12c). It was because that gas transport through the interlayer spacing of 1T phase was changed to through inter-bundler space of 2H phase. In another word, the phase transition resulted into the 18

rebuilding of nanoslits with different interlayer spacing.

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Fig. 13. (a) SEM image of the delaminated Mxene nanosheets on porous anodic aluminum oxide (AAO) (scale bar, 1 µm). Inset is the Tyndall scattering effect in MXene colloidal solution in water. (b) HRTEM image of the MXene nanosheet with SAED pattern in the inset (scale bar, 5 nm, inset b, 5 nm-1). (c) AFM image of the MXene nanosheet on cleaved mica. The height profile of the nanosheet corresponds to the blue dashed line (scale bar, 500 nm). Note that the adsorbed molecules, such as H2O, also contribute the detected thickness of 1.5 nm. (d) SEM image of the MXene membrane surface (scale bar, 500 nm). Inset is a photograph of a Mxene membrane. (e) Cross-sectional SEM image of the MXene membrane (scale bar, 1 µm). Inset is a tweezer bent membrane. (f) Cross-sectional TEM image of the MXene membrane with 2D channels (scale bar, 10 nm). (g) XRD patterns of the MAX (Ti3AlC2) powder and MXene (Ti3C2TX) membrane with inset of the magnified XRD pattern at low Bragg angles. (h) Illustration of the spacing between the neighboring MXene nanosheets in the membrane. (i) Structures and gas transport of H2-selective and CO2-selective MXene 39,79

nanofilms. Reprinted with permissions.

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2018, Springer Nature. 20

4.4. Mxenes nanoslits Mxenes are new 2D layered materials with the formula of Mn+1AXn, where n=1, 2, or 3, M is an early transition metal, A represents the group IIIA/IVA element and X refers to C and/or N. The typical method to obtain Mxene nanosheets is by using HF aqueous to etch A layers in MAX phase. Among variety of Mxenes, Ti3C2Tx is the most studied one, where T represents O, OH and/or F. Ding et al.79 reported a lamellar stacked Ti3C2Tx membrane with regular nanoslits for gas separation. The Ti3C2Tx was obtained by using hydrochloric acid and lithium fluoride. The nanoslits with the interlayer spacing of 1.35 nm was constructed after ordered stacking. As the monolayer thickness of Ti3C2Tx was around 1 nm, the actual free space for gas transport is 0.35 nm, which served as a gas sieve (Fig. 13a-h). The gas separation performance and molecular dynamic simulation both confirmed this mechanism. The results showed that H2 and He, with kinetic diameters smaller than the free space, could transport the nanoslits quickly, while the gases which had the kinetic diameters slightly larger than the free space transported 100 times slower. Due to the quadrupole moment of CO2 and interaction between CO2 and Ti3C2Tx, the CO2 transport got trapped and thus produced much smaller permeance. The CO2 sorption in Mxene membrane was further studied by Jin et al.39 through functionalizing Mxene nanosheets. Jin and co-workers introduced borate molecules and polyethylenimine (PEI) to crosslink the Mxene nanosheets, and to modulate the stacking and interlayer spacing. Borate molecules and PEI played a key role of CO2-philic domains to increase the CO2 solubility, which overcame the energy barrier for CO2 entering into the Mxene nanoslits, and finally resulted into facilitated transport of CO2 (Fig. 13i). 4.5. Layered double hydroxides (LDHs) nanoslits Different from the above negatively charged 2D nanosheets, LDHs are positively charged, with the formula of [M1-x2+Mx3+(OH)2][An-]x/n·zH2O, where M2+ and M3+ represent di- and tri- valent metal ions, An- is the n-valent anion and water is the interlayer water. The interlayer spacing can be modulated by the compositional metal ions and charge compensating anions from nanometer to sub-nanometer. Caro et al.55 prepared a well-intergrown Ni3AlCO3 LDH membrane for H2 separation, with the interlayer spacing of 21

0.79

Fig. 14. (a) Schematic illustration of the concept of interlayer gallery-based separation. In the figure, layered compounds with a gallery height of 0.31 nm represent NiAl–CO3 LDH membranes as mentioned below. Gas molecules with kinetic diameters of 0.29 nm and 0.38 nm represent H2 and CH4, respectively. (b) Permeances of the single gases (H2, CO2, N2 and CH4) of the NiAl-CO3 LDH membrane at ∆P = 1 bar, T = 180 °C as a function of molecular 55,80

kinetic diameters with the Wicke–Kallenbach technique. Reprinted with permissions.

Copyright 2014 the Royal Society of Chemistry. Copyright 2013 the Royal Society of Chemistry.

nm. By subtracting the thickness of a brucite-like layer, 0.48 nm, the free space was 0.31 nm, which was smaller than most of the gas kinetic diameters. So, it was an ideal membrane for H2 separation by size exclusion, and the experimental results confirmed this mechanism (Fig. 14). However, when Ni(NO3)3 was replaced by Zn(NO3)2, the free space for gas transport in ZnAlNO3 LDH membrane turned out to be 0.41 nm, which was larger than the kinetic diameters of CH4, N2 and CO2.80 Thus, the size exclusion effect disappeared and selectivity for H2/gases declined sharply compared with Ni3AlCO3 LDH membrane. 4.6. 2D material based composite nanoslits There is an interesting phenomenon in the already mentioned Kim’s work, 50,60 that the 22

wet GO membrane has pretty good CO2 permeance in comparison with other gases, like O2, H2 or N2. It results from the presence of water in the GO nanoslits. As water has a better solubility for CO2 than other gases, the water in the nanoslits plays the role of “CO2 carrier”. If the nanoslits are full of water, the diffusion mechanism will change from Knudsen diffusion to solution-diffusion mechanism. The gas solution and diffusion will dominate the transport procedure. Inspired by this idea, GO-SILM was developed.16 IL has higher CO2 solubility and better stability than water. If the GO nanoslits are filled by IL, gas has to transport the nanoslits through the filled IL. The solubility of CO2 in IL (such as 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4]) is much higher than other gases (H2, CH4 and N2),81 and this high solubility results in extraordinary high CO2 selectivity over the later gases. In addition, the interaction between nanoslits wall and IL will in turn change the IL property, by redistributing the special position of anions and cations of IL. It leads to the change of gas solubility and diffusivity. And the anions further form a facilitated path for CO2 diffusion. So GO-SILM even shows a better CO2 selectivity performance than pure IL. Different from those existed composite membranes formed by incorporating 2D nanosheets into a polymer matrix, in which 2D nanosheets played a role of reinforcer,82,83 and the proportion of 2D nanosheets was typically low. GO-SILM was a composite membrane based on the matrix of GO membrane. GO took half of the membrane weight. Except GO nanosheets, MoS2 nanosheets,61 WS2 nanosheets84 and mica nanosheets85 have also been used to formulate corresponding supported IL membrane by incorporating IL into 2D nanoslits through capillary force. A typical construction process has been shown in Fig. 15, taking WS2 as an example. All these 2D material-IL composite membranes had excellent CO2 selective performance, and most are above Robeson upper bound (Fig. 16). Among these 2D material based composite membranes, GO-SILM showed better performance according to the literature. It is because of the oxygen functional groups in GO nanosheets which can lead to a stronger interaction with confined IL compared with MoS2, WS2 and mica. Not only IL can be used as the filler, but also those solvents with high solubility for specific gases, and with high stability can also be adopted. 23

Fig. 15. Schematic diagrams of the synthesis process of the WS2 SILM through the spin 84

coating method and selective CO2 transportation. Reprinted with permission. Copyright 2018 the Royal Society of Chemistry.

Also, applying an external electric field (EEF) on GO-SILM or MoS2 supported IL membrane can further redistribute anions and cations of IL,67,86 on the base of nanoconfinement, and thus increases the CO2 selectivity and permeance (Fig. 17).

Fig. 16. The CO2/N2 performance of GO-SILM, MoS2-SILM, WS2-SILM and Mica-SILM compared with Robeson upper bound. 24

Fig. 17. The CO2/N2 separation performance of GO-SILM and MoS2-SILM with and without EEF.

5. Summary and outlook In this work, a deeper understanding of gas transport through 2D nanoslits have been presented. Three main preparation methods: filtration, coating and Langmuir Blodgett assembly are introduced for the preparation of 2D nanoslits. Langmuir Blodgett assembly can more precisely modulate the structure of 2D nanoslits and thickness of 2D membranes but still, filtration and coating are two more conventional methods due to their process ease. The nanoslits can separate gases through size-exclusion, Knudsen diffusion, solution-diffusion and facilitated transport mechanisms. Size-exclusion is a highly efficient separation mechanism which does not have a complex chemical reaction. However, it needs complete nanosheets and regular stacking. Knudsen diffusion is conventional in pure 2D-membrane separation and usually has poor separation selectivity. The solution-diffusion mechanism mainly depends on the used solvent, like water, IL or polymer, thereby, separation performance can be modulated in a wide range through the choice of solvents. Facilitated transport uses functional groups on the nanoslits or in the 25

composite solvents as carriers, and it can transport specific gases through the interaction between gases and carriers. Solution-diffusion and facilitated transport mechanism are more flexible than size-exclusion and Knudsen diffusion mechanism, and thus have more potential in the future. Table 1. Gas separation performance of different 2D materials introduced in this work. Material

Mechanism

Method

H2/CO2

CO2/H2

CO2/C

Permeance

H4

(GPU)

Ref

GO

Size-exclusion

Coating

30

-

-

H2:1000a

70

GO-PEDGA

Solution-diffusion

Filtration

-

-

69.5

CO2:175.7

71

Wet GO

Solution-diffusion

Coating

-

13

32

CO2:100

60

Size-exclusion

Coating

291

-

-

H2:2700

30

Size-exclusion

Coating

166

-

-

H2:2394

75

Size-exclusion and

Coating

268

-

-

H2:800

76

membrane Zn (bim) 2

4

[Zn (bim) (OH)(H 2

3

O)]

2

n

MAMS-1

Knudsen diffusion MoS2

Knudsen diffusion

Filtration

4.4

-

-

CO2:560

77

MoS2

Knudsen diffusion

Filtration

3.79

-

-

CO2:210

61

MoS2-SILM

Solution-diffusion

Filtration

-

14.95

43.52

CO2:47.9

61

and Coating WS2

Knudsen diffusion

Filtration

4.96

-

-

CO2:467

84

WS2-SILM

Solution-diffusion

Filtration

-

13.56

68.81

CO2:47.3

84

-

24

234

CO2:68.5

16

-

7.7

28.6

CO2:80

85

and Coating GO-SILM

Solution-diffusion

Filtration and Coating

Mica-SILM

Solution-diffusion

Filtration and Coating

Ti C T

Size-exclusion

Filtration

>160

-

-

H2:>2200a

79

Ti C T

Size-exclusion

Filtration

27

-

-

H2:1584

39

3

3

2 x

2 x

26

Ti3C2Tx-borate-PEI

Solution-diffusion

Filtration

-

1.4

15.3

CO2:350

39

Ni3AlCO3

Size-exclusion

In situ

25.5

-

-

H2:134

55

5.8

-

-

H2:120

80

-

42.8

262.4

CO2:259

67

-

34

174

CO2:195

86

growth ZnAl–NO3

Size-exclusion

In situ growth

GO-SILM-EEF

Solution-diffusion

Filtration and Coating

MoS2-SILM-EEF

Solution-diffusion

Filtration and Coating

a

Permeability, the unit is Barrer

Among all the 2D materials, graphene and graphene oxide are widely studied. The gas separation application is still limited even after the development of several 2D materials. The existed membranes mainly depend on the intrinsic nanoslits structure, which restraints the development to some extent. Table 1 summarizes the gas separation performance through different 2D nanoslits mentioned in this review. From Table 1, it can be observed that the composite membrane that infuses solvent in the nanoslits has much higher selectivity though the permeance is lower. Gas transport through nanoslits changes to solution-diffusion or facilitated transport mechanism after infusing solvent. The infused solvent dominates the gas transport whereas nanoslits also influences the property of the confined solvent. It offers more avenues to modulate gas transport. To keep the stability of the membrane and to achieve promising separation performance, the solvent should be chemically and thermally stable, should not easy to be squeezed out, and should selectively solute specific gas. IL is an ideal but not the only choice, and all the solvents fitting the above criteria can endeavor. The main drawback of this composite membrane is its low gas permeance. The questions is that how to enlarge the permeance while keeping selectivity the same. In this scenario, there are three main proposed methods which can make some progress in our opinion. Firstly, the choice of solvent should be done on the basis of high gas solubility and diffusivity. Secondly, the nanoslits can influence the solvent 27

property by the interaction between the nanoslits wall and solvent, like electrostatic interaction and hydrogen bonds. So, different 2D nanoslits structures may have different influences on solvent property. In addition, the interlayer spacing, functional groups and charge of nanosheets are also critical factors. Therefore, more 2D materials can be consumed to get better outcomes. Lastly, it is reported that the external stimuli, such as electric field, magnetic field, light, temperature or pressure, can play a vital role in the property of the nanoconfined solvent. The synergistic effect of 2D materials, solvents and external stimuli show considerable and vast room for modulation. Therefore, composite 2D materials having different functionalities should be prepared irrespective of their usage as a matrix or filler for novel outcomes.

Conflict of interest The authors have no conflict of interest.

Acknowledgements This work was supported by the National Natural Science Foundations of China (21671171,

21875212),

National

Key

Research

and

Development

Program

(2016YFA0200204), and the Zhejiang Natural Science Foundation (LD18E02001).

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33

Conflict of interest

Dear Editor, On behalf of all the authors, we declare that we have no conflict of interest for this work. We have got the corresponding copyrights for the all Figures adapted from the references. Thanks a lots. Best regards. Sincerely yours, Xinsheng Peng