Two-Dimensional-Materials-Based Membranes E-Book
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Two-Dimensional-Materials-Based Membranes
An authoritative and up to date discussion of two-dimensional materials and membranes
In Two-Dimensional-Materials-Based Membranes: Preparation, Characterization, and Applications, a team of distinguished chemical engineers delivers a comprehensive exploration of the latest advances in design principles, synthesis approaches, and applications of two-dimensional (2D) materials—like graphene, metal-organic frameworks (MOFs), 2D layered double hydroxides, and MXene—and highlights the significance and development of these membranes. In the book, the authors discuss the use of membranes to achieve high-efficiency separation and to address the challenges posed in the field.
The book also discusses potential challenges and benefits in the future development of advanced 2D nanostructures, as well as their impending implementation in applications in the fields of energy, sustainability, catalysis, electronics, and biotechnology.
Readers will also find:
- A thorough introduction to fabrication methods for 2D-materials-based membranes, including the synthesis of nanosheets, membrane structures, and fabrication methods
- Descriptions of three types of 2D-materials-based membranes: single-layer membranes, laminar membranes and mixed-matrix membranes
- Comprehensive discussions of 2D-materials-based membranes for water and ions separation, solvent-water separation and gas separation
- Explorations of transport mechanism of 2D-materials-based membranes for molecular separations
Perfect for membrane scientists, inorganic chemists, and materials scientists, Two-Dimensional-Materials-Based Membranes will also earn a place in the libraries of chemical and process engineers in industrial environments.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Introduction
References
2 Fabrication Methods for 2D Membranes
2.1 Introduction
2.2 Synthesis of Nanosheets
2.3 Membrane Structures and Fabrication Methods
2.4 Summary and Outlook
References
3 Nanoporous Single‐Layer Graphene Membranes for Gas Separation
3.1 Introduction
3.2 Gas‐Separation Potential of N‐SLG Membranes
3.3 Engineering Gas‐Selective Vacancy Defects
3.4 Fabrication of Large‐Area N‐SLG Membranes
3.5 Summary and Outlook
References
4 Graphene‐Based Membranes for Water Separation
4.1 Introduction
4.2 Water Transport Mechanisms in Graphene‐Based Membranes
4.3 Graphene‐based Membrane Water Separation Applications
4.4 Conclusions and Perspectives
References
5 Graphene‐Based Membranes for Ions Separation
5.1 Introduction
5.2 Single‐Layer Graphene
5.3 Graphene Oxide Membranes
5.4 Summary and Perspective
References
6 Graphene‐Based Membranes for Pervaporation
6.1 Introduction
6.2 Mass‐Transport Mechanism
6.3 Progresses in GO Membranes for Pervaporation
6.4 Summary and Perspective
References
7 Two‐Dimensional‐Materials Membranes for Gas Separations
7.1 Introduction
7.2 2D‐Materials Membranes
7.3 Preparation of 2D Nanosheets
7.4 Preparation of 2D‐Materials Membranes
7.5 Gas Separations
7.6 Conclusions and Perspectives
References
8 Layered Double Hydroxide Membranes for Versatile Separation Applications
8.1 Introduction on LDHs and LDH‐Based Membranes
8.2 Strategy for LDH‐Based Membrane Preparation
8.3 Research Progress on LDH‐Based Membranes
8.4 Summary and Outlook
References
9 MXene: A Novel Two‐Dimensional Membrane Material for Molecular Separation
9.1 Introduction
9.2 Synthesis and Processing
9.3 MXene‐Based Membranes for Molecular Separation
9.4 Conclusions and Perspective
References
10 2D‐Materials Mixed‐Matrix Membranes
10.1 Introduction
10.2 Two‐Dimensional Materials as Dispersed Phase of MMMs
10.3 Two‐Dimensional Material as Continuous Phase of MMMs
10.4 Conclusion and Outlook
References
11 Transport Mechanism of 2D Membranes
11.1 Introduction
11.2 Fundamentals of Mass Transport Through Membranes
11.3 Nanofluidic Transport Through Confined Dimensions
11.4 Unique Mass‐Transport Properties in 2D Membranes: Structural Aspects
11.5 Summary and Outlook
References
12 Conclusions and Perspectives
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Water separation performance of a few representative graphene‐base...
Chapter 7
Table 7.1 H
2
/CO
2
separation performance of 2D nanosheet‐based membranes.
Table 7.2 H
2
/N
2
separation performance of 2D nanosheet‐based membranes.
Table 7.3 H
2
/CH
4
separation performance of 2D nanosheet‐based membranes.
Table 7.4 CO
2
/N
2
separation performance of 2D nanosheet‐based membranes.
Table 7.5 CO
2
/CH
4
separation performance of 2D nanosheet‐based membranes.
Table 7.6
P
‐xylene/
o
‐xylene, He/H
2
, He/N
2
,
n
‐butane/isobutane, H
2
/C
3
H
6
, CO
2
/...
List of Illustrations
Chapter 2
Figure 2.1 Schematic representation of the sonication exfoliation process of...
Figure 2.2 Schematic illustration of the two‐step chemical ion‐intercalation...
Figure 2.3 Synthesis of GO nanosheets by water electrolytic oxidation. (a) S...
Figure 2.4 Schematic illustration of the synthesis process of MXenes from MA...
Figure 2.5 GO membranes fabricated by vacuum suction method. (a) Digital pic...
Figure 2.6 (a) Schematic of a pressurized ultrafiltration system. Source: Ta...
Figure 2.7 Fabrication and characterization of the Fe
3
O
4
@GO membrane. (a) Sc...
Figure 2.8 (a) Schematic of the external force‐driven assembly method for fa...
Figure 2.9 Fabrication, characterization, and ion permeation tests of GO mem...
Figure 2.10 (a) Schematic of stabilizing a GO membrane through interlaminar ...
Figure 2.11 (a) Schematic of the assembly of GO nanosheets in polymeric envi...
Figure 2.12 AFM images for GO sheets with lateral sizes of (a) 100–200 nm, (...
Figure 2.13 Illustration of the synthesis process of ultrathin ZIF‐8/GO memb...
Chapter 3
Figure 3.1 (a) Lattice structure of graphene showing the unit cell composed ...
Figure 3.2 (a) Gas transport across nanopores in SLG where a schematic shows...
Figure 3.3 (a) All‐hydrogen passivated pore‐10 in SLG with an electron‐densi...
Figure 3.4 (a) A 2D nanoporous network for H
2
and He sieving. (b) Calculated...
Figure 3.5 (a) Calculated adsorption energies and energy barriers of H
2
and ...
Figure 3.6 Impact of pore geometry on gas transport. (a) Various pore geomet...
Figure 3.7 (a) A hydrogen‐terminated 6‐Å‐sized pore covered with a monolayer...
Figure 3.8 High‐performance postcombustion capture membranes based on N‐SLG ...
Figure 3.9 The concept of graphene pore masking to block the direct gas‐phas...
Figure 3.10 Illustration of complexities in selective gas transport from a p...
Figure 3.11 (a) HRTEM image of an intrinsic vacancy defect in as‐synthesized...
Figure 3.12 An example of the bottom‐up synthesis of highly porous N‐SLG. (a...
Figure 3.13 (a) Schematic of pore‐6 where zigzag pore edge carbon atoms are ...
Figure 3.14 (a) TEM image of a nanopore drilled on multilayer graphene by an...
Figure 3.15 (a) An ensemble of nanopores drilled on a suspended bilayer grap...
Figure 3.16 A graphene etching strategy based on nucleation by ion bombardme...
Figure 3.17 (a) Raman spectroscopy data from SLG treated with O
2
plasma for ...
Figure 3.18 Etching of graphene by O
2
.
Atomic force microscopy
(
AFM
) images ...
Figure 3.19 Reaction mechanism of CO
2
with the zigzag edge of graphene that ...
Figure 3.20 (a) Optical microscope image of cracks developing in partially o...
Figure 3.21 (a) HRTEM images of nanopores incorporated in graphene after O
3
...
Figure 3.22 (a) Schematic of the MGR setup for etching graphene in milliseco...
Figure 3.23 (a) Stacking of Cu foils to improve packing density of foils ins...
Figure 3.24 (a) Schematic of the NPC‐film‐based mechanical reinforcement of ...
Chapter 4
Figure 4.1 Schematic of the geometry of a typical 2D graphene‐based membrane...
Figure 4.2 Effects of internal geometry on water transport in graphene‐based...
Figure 4.3 Effect of surface chemistry on water transport in graphene‐based ...
Figure 4.4 Effects of solution chemistry and pressure on the structure and w...
Figure 4.5 Effects of guest materials in GO membranes stabilization. (a) Cro...
Figure 4.6 Temperature‐gated mass transport through GO–PNIPAM membranes. (a)...
Figure 4.7 pH‐gated mass transport through GO–PEI membranes. (a) Water trans...
Figure 4.8 Design of development of graphene‐based membranes for nanofiltrat...
Figure 4.9 Design of development of graphene‐based membranes for forward osm...
Chapter 5
Figure 5.1 (a) Graphene nanopores terminated with hydrogen atoms and –OH gro...
Figure 5.2 (a) Schematic illustration of the single‐layer graphene suspended...
Figure 5.3 Energy costs and optimized atomic structures of migrations of (a)...
Figure 5.4 (a) Illustration of the desalination mechanisms of GO membranes u...
Figure 5.5 (a) Illustration of the proposed nanostructures of the GO membran...
Figure 5.6 (a) Illustration of the
d
‐spacing of graphene, dry GO, and fully ...
Figure 5.7 (a) Illustration of the K
+
cross‐linked GO membrane. Other ca...
Figure 5.8 (a) Illustration of the formation of surface wrinkle from initial...
Figure 5.9 (a) Illustration of the surface charged GO membranes by coating p...
Figure 5.10 (a) Illustration of the photo‐induced ion transport device. (b) ...
Chapter 6
Figure 6.1 (a) The
xy
‐plane number density map of the water molecules inside...
Figure 6.2 (a) Schematic diagram of the brine desalination process in GO mem...
Figure 6.3 Summary of the structural manipulation strategies of GO membranes...
Figure 6.4 (a) Schematic diagrams of composite GO membranes fabricated throu...
Figure 6.5 (a) Schematic of the self‐assembly of TU–GOF membrane. (b) Pervap...
Figure 6.6 The representative cross‐linkers in GO membranes for pervaporatio...
Figure 6.7 Proposed model of the bond locations between metal ions and GO na...
Figure 6.8 Schematic diagram of the molecular transport in GO membrane with ...
Figure 6.9 Schematic illustration of (a) the zwitterionic functionalization ...
Figure 6.10 Proposed chemical interactions in GO/CS (a) and rGO/CS (b). Phot...
Figure 6.11 (a) Schematic representation of the GO layer and GO–PVA layers. ...
Figure 6.12 (a) Cartoons for demonstration of the mechanism for the pervapor...
Figure 6.13 (a) Molecular transport in the PAA–SPB/GO membrane. (b) Influenc...
Figure 6.14 (a) Schematic of the water‐organic separation process using the ...
Chapter 7
Figure 7.1 Different types of promising 2D‐materials membranes for gas separ...
Figure 7.2 (a) Proposed structure for a single MFI nanosheet. Source: Choi e...
Figure 7.3 (a) Low‐magnification and (b) high‐magnification SEM images of th...
Figure 7.4 (a) Single‐layer graphene structure.
Figure 7.5 (a) Single‐layer GO and rGO structures. Source: Toda et al. [66]/...
Figure 7.6 (a) Four‐layered stacking diagram of Zn
2
(Bim)
3
precursors along t...
Figure 7.7 (a) Scheme of the iCON membrane with staggered packing pattern. (...
Figure 7.8 (a) Structure of the PTI layer (g‐C
3
N
4
). (b) SEM image of the PTI...
Figure 7.9 (a) Structures of the MXene Ti
3
C
2
T
x
[102]/with permission of John ...
Figure 7.10 (a) Scheme of the h‐BN structure. Source: Lu et al. [29]/with pe...
Figure 7.11 Different synthesis types of 2D nanosheets.
Figure 7.12 (a) Scheme of the preparation of lithium intercalated MoS
2
dispe...
Figure 7.13 (a) Top: four‐layered stacking diagram of Zn
2
(Bim)
3
precursors a...
Figure 7.14 Synthesizing 2D nanosheets by a bottom‐up strategy. (a) Modulate...
Figure 7.15 (a) Scheme of vacuum filtration method. Source: Wang et al. [113...
Figure 7.16 (a) Scheme of the spin‐coating technique Source: Wang et al. [11...
Figure 7.17 Scheme of the Layer‐by‐Layer assembly of TpEBr@TpPa‐SO
3
Na iCON m...
Figure 7.18 (a) Scheme of the highly oriented Zn
2
(bIm)
4
nanosheet membranes ...
Chapter 8
Figure 8.1 Crystal structure of Mg
2
Al–Cl LDHs, [Mg
2
Al(OH)
6
]Cl·2H
2
O. Green ba...
Figure 8.2 SEM images of (a, b) top and (c) the cross‐section views of a vac...
Figure 8.3 Schematic illustration of the concept of selective molecular siev...
Figure 8.4 (Left panel) XRD patterns of (a) starting
NO
3
−
‐intercalated...
Figure 8.5 Permeances of the single gases (H
2
, CO
2
, N
2
, and CH
4
) of the NiAl...
Figure 8.6 (a) Illustration of the preparation of the ZIF‐8–ZnAl–NO
3
composi...
Figure 8.7 Schematic illustration of the preparation of LDH membranes with i...
Figure 8.8 Reaction pathway for the formation of the PEIE–MgAl–CO
3
complex....
Figure 8.9 Schematic illustration of CoAl–Fe‐cage LDH composite NF membrane ...
Figure 8.10 Schematic illustration of
in situ
solvothermal growth of ZIF‐8 m...
Figure 8.11 (a) Schematic illustration of the engineering of vertically alig...
Figure 8.12 Schematic illustration of surface modification of commercial tex...
Figure 8.13 Schematic illustration of the function of TFC FO membranes based...
Figure 8.14 Schematic illustration of the fabrication of ZIF‐8 membrane by p...
Figure 8.15 Schematic illustration of the preparation of ZHT via (a) anion‐e...
Figure 8.16 Comparison of the traditional solvent–non‐solvent phase inversio...
Figure 8.17 (a) Schematic showing the
in situ
growth of Co(OH)
2
nanosheets o...
Chapter 9
Figure 9.1 (a) Schematic diagram of the clay LiF/HCl method (Route 1) and th...
Figure 9.2 (a) Schematic of the effect of Al
3+
intercalation on d‐space;...
Figure 9.3 (a) Schematic diagrams of polymer‐based membrane filled with MXen...
Figure 9.4 (a) Pervaporation performance of MXene/CS MMMs With different MXe...
Figure 9.5 (a) Schematic diagram of MXene membrane; SEM images of MXene memb...
Figure 9.6 (a) Schematic diagrams of MXene–GO membrane on separation mechani...
Figure 9.7 (a) Schematic diagrams of hydrophilic MXene membrane for efficien...
Figure 9.8 (a) Cross‐sectional TEM image of MXene membrane with 2D channels;...
Chapter 10
Figure 10.1 Schematic illustration of MMMs with 2D materials as dispersed ph...
Figure 10.2 (a) Schematic illustration of mechanism for selective water perm...
Figure 10.3 Schematic diagram of (a) the synthesis process of PEG−PEI−GO and...
Figure 10.4 Schematic illustration of CO
2
transmission in Pebax/GO–DA–Zn
2+
...
Figure 10.5 Schematic illustration of (a) the synthesis process of Ag+@UGO l...
Figure 10.6 Schematic illustration of CO
2
transmission in Pebax/CuMOF membra...
Figure 10.7 Schematic illustration of the molecule transport through the PES...
Figure 10.8 (a) TpPa‐1 synthesized by Schiff base aldehyde−amine condensatio...
Figure 10.9 (a) Schematic illustration of COF‐42/PDMS membrane for separatin...
Figure 10.10 (a) Schematic illustration of channel‐mediated facilitated tran...
Figure 10.11 (a) Schematic illustration of structure and facilitated transpo...
Figure 10.12 Structural illustration of GO and PDA‐g‐GO membranes with diffe...
Figure 10.13 Schematic illustration of the assembly process of GO–PAMAM/PTFE...
Figure 10.14 Schematic illustrations for (a) the preparation of the cross‐li...
Figure 10.15 The preparation of GO–PVAm‐silica membranes.
Figure 10.16 (a) Schematic illustration of the fabrication of the 2D Zn
2
(Bim...
Figure 10.17 Mixed‐dimensional assembly of TpTG
Cl
and CNFs. (a) A schematic ...
Figure 10.18 (a) Strategy to fabricate polyCOF membranes via a three‐compone...
Chapter 11
Figure 11.1 Schematic illustrations of (a) mass transport through membranes ...
Figure 11.2 Schematic representation of diffusion mechanisms in porous membr...
Figure 11.3 Mass transport through a nonporous membrane based on solution–di...
Figure 11.4 Schematic illustrations of (a, b) the distribution of the co‐ion...
Figure 11.5 Representative upper‐bound relations in polymer membranes for di...
Figure 11.6 Transitions in mass‐transport properties from bulk state to nano...
Figure 11.7 Classification of confinement architectures and the correspondin...
Figure 11.8 (a) Three cases of slip flow through a stationary surface with t...
Figure 11.9 Transport model and mechanism of gas transport through porous mo...
Figure 11.10 Schematic illustration of electric double layer (EDL) in the so...
Figure 11.11 Effects of surface charge in charge distribution, electric pote...
Figure 11.12 Typical confinement effects in mass transport behaviors. (a) Na...
Figure 11.13 Typical structures of 2D material‐based membranes. Here, NATM s...
Figure 11.14 Gas transport properties of graphene‐based NATM coated onto por...
Figure 11.15 (a) Schematic illustration of ion‐transport properties through ...
Figure 11.16 (a) Preparation methods of 2D laminar membranes. (b) Schematic ...
Figure 11.17 (a) Schematic illustration of gas transport pathway and (b) the...
Figure 11.18 (a–c) Fundamental water and ion‐transport properties of freesta...
Figure 11.19 Schematic illustration of (a) resistance model approach assumin...
Figure 11.20 (a) Schematic illustration of the definition of the directions ...
Guide
Cover Page
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Two‐Dimensional‐Materials‐Based Membranes
Preparation, Characterization, and Applications
Edited by Gongping Liu and Wanqin Jin
Editors
Prof. Gongping Liu
Nanjing Tech University
College of Chemical Engineering
30 Puzhu Road (S)
211816 Nanjing
China
Prof. Wanqin Jin
Nanjing Tech University
College of Chemical Engineering
30 Puzhu Road (S)
211816 Nanjing
China
Cover Image: © LAGUNA DESIGN/Getty Images
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Preface
Synthetic membranes have been used in various processes, from water treatment, gas purification to energy conversion. The conventional membrane processes, including microfiltration, ultrafiltration, and reverse osmosis, have been commercialized and widely used in food, chemical, and medical industries. However, it remains challenging to realize the precise separation of the molecules/ions with subnanometer sizes, such as saline water, organic solvent solution, and gas mixtures. As we know, there is a general trade‐off between permeability and selectivity for polymeric membranes. Although inorganic membranes could achieve both high permeability and selectivity, the high cost of membrane fabrication restricts their development. The key lies in the design and fabrication of high‐performance (high permeability, selectivity, and stability) membranes for efficient molecular separation.
Two‐dimensional (2D) materials (e.g. nanosheets of graphene family materials, zeolites, metal–organic frameworks, or layered double hydroxides) have become excellent nanobuilding blocks to develop next‐generation separation membranes featuring unique nanopores and/or nanochannels. Recently, 2D materials‐based membranes have been appearing as a very hot topic both in theoretical and experimental studies and have attracted a huge surge of interest. Important new results demonstrated that the fascinating 2D‐material membranes showed extraordinary permeation properties, opening the door to ultrafast and highly selective membranes for water purification, desalination, gas separation, and bioseparation. Up to now, books lacked an instructive view on the important contemporary topic “2D materials membranes.” There is an urgent need for a book that brings alive the new results, and current significances and challenges concerning 2D‐material membranes. In this book, we aim to give comprehensive information on design, fabrication, and application of 2D‐material membranes that are used for molecular separation.
Various membrane structures based on different 2D materials are involved in the book, including porous nanosheet membranes, 2D laminar membranes, and mixed‐matrix membranes (MMMs) with 2D materials serving as nanofillers. Fabrication strategies, physicochemical properties, morphologies, host–guest interactions, transport characteristics, and mechanisms of 2D‐material membranes will be thoroughly discussed based on advanced characterization techniques and theoretical calculations. Meanwhile, the book contains several typical applications of 2D‐material membranes in fields, such as water desalination, ion separation, solvent dehydration, and gas separation. In addition, milestones in the large‐scale fabrication of 2D‐material membranes for wide implementation are briefly introduced. Finally, we conclude this book with an overview of remaining challenges and new opportunities that have opened for 2D‐material membranes in molecular separation.
At last, we would like to express our appreciation and gratitude to the chapter authors: Dr. Kumar Varoon Agrawal from École Polytechnique Fédérale de Lausanne (EPFL), Dr. Xiwang Zhang from Monash University, Dr. Hongwei Zhu from Tsinghua University, Dr. Dan Zhao from National University of Singapore, Dr. Yi Liu from Dalian University of Technology, Dr. Fusheng Pan from Tianjin University, and Ho Bum Park from Hanyang University.
Professor in Chemical Engineering
Gongping Liu
Nanjing Tech University
Wanqin Jin
Nanjing, China
10 January 2022
1Introduction
Gongping Liu and Wanqin Jin
Nanjing Tech University, College of Chemical Engineering, State Key Laboratory of Materials‐Oriented Chemical Engineering, 30 Puzhu South Road, Nanjing, 211816, P. R. China
Two‐dimensional (2D) materials have been emerging stars in condensed matter physics, materials science, and chemistry since the successful exfoliation of graphene by Novoselov and Geim in 2004 [1–3]. The atomic thickness and micrometer lateral dimensions endow 2D materials with great potential in membrane separation. Synthetic membranes are used widely in many separation processes, from industrial‐scale processes, such as removing salt from seawater and separating atmospheric gases, to smaller‐scale processes in chemical synthesis and purification [4, 5]. The membranes function by forming a selective barrier between the two phases, restricting the movement of some molecules while letting others penetrate. On one hand, the membranes assembled from ultrathin nanosheets can minimize the transport resistance and maximize the mass‐transfer rate. On the other hand, the intrinsic or artificial nanopores and interlayer galleries can provide excellent molecular sieving properties [6, 7]. Hence, 2D materials, including graphene‐family, zeolites, metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), metal carbides and nitrides (MXenes), and layered double hydroxides (LDHs), have been demonstrated as excellent building blocks for high‐performance membranes [8, 9].
2D materials are either porous or nonporous based on their atomic structure. The intrinsic nanopores in zeolites and MOFs or the drilled nanopores in graphene can provide molecular/ionic transport pathway. In contrast, the nonporous nanosheets (e.g. graphene oxide and MXene) must be assembled into laminates with interlayer channels for mass transfer, which is driven by external forces, such as pressure difference, centrifugal force, and molecular interaction [10]. Hence, according to the difference in membrane structures, 2D‐material‐based membranes can be categorized into three types—(i) porous nanosheet membranes, (ii) laminar membranes, and (iii) 2D‐material‐based mixed‐matrix membranes (MMMs). The fabrication methods and unique properties of these membranes will be detailedly discussed in the following chapters. By the precise construction and manipulation of the in‐plane nanopores/slits and interlayer channels, 2D‐material‐based membranes have exhibited outstanding molecular separation properties in various membrane processes, such as ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, pervaporation, and gas separation [9].
With the rapid development of 2D‐material‐based membranes, a comprehensive summary of the breakthroughs and milestones for these membranes is urgently needed to guide newcomers and inspire innovations in this field. The following chapters summarize the recent progress of 2D‐material‐based membranes for molecular separation, focusing on membrane preparation, characterization, and application. Chapter 2 introduces the fabrication methods for 2D materials and membranes. Chapter 3 presents the development of porous graphene‐based nanosheet membranes. Chapters 4–6 describe the design and application of graphene‐based membranes for water separation, ions separation, and pervaporation, respectively. Chapter 7 shows the development of 2D‐material membranes for gas separation. Chapter 8 introduces the advance in 2D‐LDHs membranes. Chapter 9 demonstrates the design of MXene and other emerging 2D‐material membranes. Chapter 10 summarizes the milestones for 2D‐material mixed‐matrix membranes. Chapter 11 reveals the transport mechanism of 2D‐material membranes. Finally, Chapter 12 gives the conclusions and perspectives of this research topic.
References
1
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2Fabrication Methods for 2D Membranes
Long Cheng Gongping Liu and Wanqin Jin
Nanjing Tech University, College of Chemical Engineering, State Key Laboratory of Materials‐Oriented Chemical Engineering, 30 Puzhu South Road, Nanjing, 211816, P. R. China
2.1 Introduction
Two‐dimensional‐material membranes have demonstrated great potential in molecular separation. The synthesis of high‐quality nanosheets with integrated structure is a prerequisite for fabricating defect‐free two‐dimensional‐material membranes. The synthetic methods of nanosheets aimed at membrane application include mechanical‐force exfoliation, ion‐intercalation exfoliation, oxidation‐assisted exfoliation, selective‐etching method, chemical vapor deposition (CVD), hydro/solvothermal synthesis, and interfacial synthesis. All of these methods can be divided into two categories, top‐down and bottom‐up methods [1], which are discussed in detail. After the nanosheets are successfully synthesized, novel membrane fabrication methods should be proposed for tuning the in‐plane pores and constructing interlayer channels. There are three main types of 2D‐material membranes—(i) nanosheet membranes; (ii) laminar membranes; and (iii) mixed‐matrix membranes (MMMs). The first type consists of a monolayer or a few layers of 2D material with intrinsically uniformly sized pores (e.g. zeolite, metal‐organic frameworks (MOFs)) or drilled nanopores (graphene) for molecular sieving. The second type, laminar membrane, is composed of orderly stacked nanosheets with tunable interlayer channels by various assembly strategies. The size and functional groups of the interlayer channels play an important role in selective permeation of small molecules. Thirdly, the addition of 2D fillers into polymer matrix can combine the unique transport property of nanosheets and the excellent processability of a polymer, further enhancing separation performance and even mechanical strength of pristine polymer membrane. Owing to the difference in membrane structures, various fabrication methods have been proposed. In this chapter, we aim to summarize the advances in design and fabrication of nanosheets and 2D‐material membranes. We focus on the discussion of different approaches for assembling the nanosheets, and the prominent effect of tuning the nanostructures and physicochemical properties of transport channels is highlighted. Opportunities and challenges concerning the development of fabrication methods for 2D‐material membranes are discussed as well.
2.2 Synthesis of Nanosheets
The synthetic methods of 2D nanosheets can be categorized into top‐down and bottom‐up methods. The top‐down method, including mechanical‐force exfoliation, ion‐intercalation exfoliation, oxidation‐assisted exfoliation, and selective‐etching method, relies on the exfoliation of bulk crystals into 2D nanosheets. Conversely, the bottom‐up method refers to the construction of 2D frameworks from basic units. CVD, hydro/solvothermal synthesis, and interfacial synthesis are well‐established bottom‐up methods. All the aforementioned methods are introduced and discussed, respectively.
2.2.1 Top‐Down Method
2.2.1.1 Mechanical‐Force Exfoliation
The early mechanical‐force exfoliation method is the micromechanical cleavage technique [2], namely “Scotch‐tape” method. The micromechanical force can weaken the van der Waals interaction between the layers of bulk crystals, which is beneficial for the isolation of neighboring layers, and single‐ or few‐layer nanosheets can be obtained after several times of cleavage. The sizes of the as‐prepared nanosheets can reach a few to tens of micrometers, and this nondestructive technique can contribute to clean surfaces and excellent crystal quality, making them good candidates for investigating the intrinsic physical properties. Nevertheless, the low yield of this method restricts its practical application. There are more easily scalable strategies, such as sonication, shear force, and ball‐milling exfoliations. For the sonication method, the layered crystals were firstly dispersed in a proper solvent, followed by a sonication treatment (Figure 2.1) [3]. To realize efficient exfoliation, the surface energy of the chosen solvent should match that of the bulk crystal. Coleman and coworkers pioneered the liquid exfoliation of graphite into graphene in organic solvents, which is a scalable method to produce high‐quality graphene nanosheets [4]. They further extended this method for the synthesis of other 2D nanosheets, such as MoS2, WS2, and h‐BN [5]. Moreover, the addition of polymers or surfactants can tune the surface tension of the aqueous solution to match the surface energy of bulk crystals. For instance, Coleman's group demonstrated that the introduction of sodium dodecylbenzenesulfonate (SDBS) or sodium cholate (SC) into the aqueous solution can largely improve the exfoliation efficiency, contributing to high yield of graphene nanosheets [6, 7]. In addition, the control of sonication parameters, such as ultrasonic powder, sonication time, sonication temperature, and the shape of vessels, can tune the properties (e.g. concentration, lateral size, and thickness) of as‐synthesized nanosheets [8].
Figure 2.1 Schematic representation of the sonication exfoliation process of graphite in the absence (top right) and presence (bottom right) of surfactant molecules.
Source: Ciesielski and Samori [3]/with permission from Royal Society of Chemistry.
Another mechanical force for efficient exfoliation is the shear force, which can be generated from the high‐shear rates or the ball‐milling impact. Coleman and coworkers employed a high‐shear rotor–stator mixer to provide high‐shear rates in liquid, facilitating the exfoliation of bulk graphite [9]. They successfully produced graphene nanosheets in 1‐methyl‐2‐pyrrolidinone (NMP) solvent in this shear‐force‐assisted method. Besides the rotor–stator mixer, a commercially available kitchen blender was proved to generate sufficient shear rates for fabrication of various nanosheets, including graphene and MoS2 [10, 11], which implies the potential of industrial rotating blade stirred‐tank reactors in large‐scale graphene production. Ball‐milling method was also employed as the shear‐force‐assisted exfoliation. The rotating speed should be carefully controlled to avoid the in‐plane defects caused by shock stress and ensure the dominant shear stress. Chen and coworkers demonstrated the exfoliation of graphite in N,N‐dimethylformamide (DMF) by the ball‐milling method [12]. After high‐speed centrifugation, they obtained single‐ and few‐layer graphene nanosheets with a thickness around 0.8–1.8 nm. The monolayer fraction could be further increased by optimizing the milling conditions (e.g. the diameters of milling balls, the milling time, and the concentration of graphite). Yang and coworkers proposed a soft‐physical strategy combining the ball‐milling and sonication processes to exfoliate layered Zn2(bim)4 (bim = benzimidazole) crystals [13]. The mixture of methanol and propanol is proved to be the most appropriate for the exfoliation—small methanol molecules can enter the interlayer galleries to promote the exfoliation, and propanol helps to stabilize the exfoliated nanosheets by adsorbing on the nanosheets with its hydrophobic alkane tails. Therefore, the selection of proper solvents is critical for successful exfoliation, which is universally applicable to almost all exfoliation methods. In general, the mechanical force exfoliation can contribute to high yield and massive production of nanosheets, but the sonication or ball milling may cause some defects on the nanosheets, affecting the separation performance of assembled membranes.
2.2.1.2 Ion‐Intercalation Exfoliation
The ion‐intercalation is accomplished by intercalating cation ions (e.g. Li+, Na+, and K+) into the interlayer galleries of bulk crystals, which would weaken the var der Waals interaction between neighboring layers and expand the interlayer space for easier cleavage. In 1980s, Morrison and coworkers employed n‐butyl lithium, as the intercalator, to synthesize MoS2 nanosheets [14]. The bulk MoS2 was first soaked in hexane containing n‐butyl lithium for at least two days to intercalate Li into the MoS2. Then the obtained Li‐intercalated compound was transferred into water, followed by sonication to assist in the exfoliation. The reaction between intercalated Li and water continuously generated hydrogen gas and further facilitated the exfoliation process. In addition, Loh and coworkers proposed a two‐step chemical ion‐intercalation method to synthesize large‐sized nanosheets (Figure 2.2) [15]. N2H4 was first intercalated into the bulk crystals and could expand the volume of the crystals after its decomposition. After that, the product was secondly intercalated by metal naphthalenide (metal = Li, Na, K) to form intercalated compounds. In this way, the yield of MoS2 reached 90%, and the lateral size was up to 400 μm2. It is worth pointing out that organometallic compounds used in this method are highly explosive and very sensitive to moisture and oxygen, so the experiment needs to be operated in a glovebox with extreme caution. Bearing this in mind, Zheng and coworkers demonstrated that some safer inorganic salts, including NaCl and CuCl2, can serve as intercalators for efficient exfoliation [16, 17]. The graphite was dispersed in saturated NaCl or CuCl2 solution, which was heated at 100 °C to evaporate the water and obtain the cation‐intercalated graphite. The intercalated compound was further sonicated in organic solvent (e.g. DMF, ethanol, and NMP) to obtain graphene nanosheets. Moreover, they extended this method for the synthesis of other nanosheets, including MoS2, MoSe2, WS2, and WSe2[17], demonstrating the universality of the safer ion‐intercalation exfoliation.
Figure 2.2 Schematic illustration of the two‐step chemical ion‐intercalation method for the preparation of MoS2 nanosheets. (a) Bulk MoS2 was pre‐exfoliated by the decomposition products of N2H4. (b) Pre‐exfoliated MoS2 was secondly intercalated by metal naphthalenide, followed by exfoliation in water. Photographs of (c) bulk single‐crystal MoS2, (d) pre‐exfoliated MoS2, and (e) Na‐exfoliated single‐layer MoS2 dispersion in water.
Source: Zheng et al. [15]/with permission from Springer Nature.
2.2.1.3 Oxidation‐Assisted Exfoliation
Oxidation‐assisted exfoliation is a well‐established method for the synthesis of graphene oxide (GO). This method was firstly proposed by Brodie in 1859, and further developed by Staudenmaier and Hummers [18], forming the three primary synthetic routes. The exfoliation is based on the oxidation of graphite into graphite oxide by strong oxidizing agents. The generated oxygen‐containing functional groups can greatly expand the interlayer space of layered graphite and weaken the van der Waals interaction between neighboring nanosheets. After the oxidation treatment, the subsequent sonication can further transform the graphite oxide into single‐layer GO nanosheets. Both Brodie and Staudenmaier methods use potassium chlorate and nitric acid as the oxidizing agent, and the most commonly used Hummers' method employed the potassium permanganate and concentrated sulfuric acid to ensure sufficient oxidation. The advantage of this method lies in the high yield of single‐layer GO nanosheets. However, the oxidation process can cause severe explosions at elevated temperatures, and a huge amount of wastewater containing acid and metal ions can be produced and lead to environmental pollution. Therefore, a greener and safer exfoliation method based on electrochemical oxidation process was proposed for the synthesis of GO nanosheets. Ren and coworkers reported a two‐step electrochemical method to fabricate GO nanosheets with a high level of oxygen content (Figure 2.3) [19]. In the first intercalation step, the commercial graphite paper serving as the anode was immersed in 98 wt% H2SO4 at a low voltage of 1.6 V, resulting in the blue graphite intercalation compound. The second step (rapid oxidation process) required a suitable concentration of H2SO4, normally in the range of 40–60 wt%, to achieve highly oxidized GO with C/O ratio of 1.5–1.8, surpassing that of classic chemical oxidation routes (around 2.2) [20]. In addition, the as‐synthesized GO nanosheets exhibited good dispersibility and nanoscale thickness, which are similar to those synthesized by Hummers' method. They attributed the extensive graphite oxidation to the abundant radical intermediates (e.g. *OH, *O, and *OOH) from the electrolytic water splitting. In this way, the oxidation of graphene lattice was only within a few seconds, which is much faster than the Hummers' method. The electrochemical oxidation method combined the advantages of safety, environmental friendliness, ultrafast synthesis, and easiness to scale up, which paves the way for industrial production and application of GO sheets at a low cost.
Figure 2.3 Synthesis of GO nanosheets by water electrolytic oxidation. (a) Schematic illustration of the fabrication process of electrochemically synthesized GO (EGO). Source: (a) Pei et al. [19]/Springer Nature/CC BY 4.0. (b) Photograph of the graphite intercalation compound paper (GICP) (blue area) after the reaction with 98 wt% H2SO4 at 1.6 V for 20 minutes. (c) Photograph of GO (yellow area) resulting from the water electrolytic oxidation of GICP in 50 wt% H2SO4 at 5 V for 30 seconds. (d) Comparison of EGO with GO prepared by Hummers' method (HGO) in water (1 mg ml−1). (e) AFM images of EGO nanosheets.
Source: Pei et al. [19]/with permission from Springer Nature.
2.2.1.4 Selective‐Etching Method
In 2011, Gogotsi and coworkers proposed the selective‐etching method for the exfoliation of bulk MAX phases into the nanosheets of transition metal carbides and/or carbonitrides (MXenes) [21]. The term MAX phases reflect the chemical composition of Mn+1AXn, where n = 1, 2, or 3 (M2AX, M3AX2, or M4AX3), “M” is an early transition metal, “A” is an A group (mostly groups 13 and 14) element, and “X” is C and/or N [22]. In the MAX phases, the M layers are nearly closed packed with the X atoms filling the octahedral sites, and the Mn+1Xn layers are interleaved with layers of A atoms. The MX bond has a mixed covalent/metallic/ionic character, while the MA bond is metallic. Compared with the weak van der Waals interactions in graphite and transition metal dichalcogenides (TMDs), the metallic bonds are too strong to be broken by the traditional mechanical forces. To solve this problem, Gogotsi and coworkers reported the selective etching of the Al from Ti3AlC2 by introducing the hydrogen fluoride (HF) (Figure 2.4) [22]. Owing to the differences in character and relative strengths of the TiAl bond compared with the TiC bond, the Al layers can be selectively etched without disrupting the TiC bond. The etched intermediate product was further sonicated to obtain Ti3C2 nanosheets. MXenes prepared by this method possess numerous terminations (e.g. O, –OH, and –F) on the surface, which are favorable for stabilizing them in solution and the assembly into membranes. It is worth pointing out that the etchant HF is highly corrosive and dangerous. Therefore, Gogotsi's group developed the mixture of fluoride salts and HCl or H2SO4 as the alternative to the corrosive HF [23], which improved the safety of the selective‐etching method. Moreover, more kinds of MXenes, such as Ti2C, Nb2C, V2C, and Nb4C3, have been successfully synthesized by this method [24], demonstrating its great potential for wider application. The limitation is that the method is not applicable to the synthesis of other nanosheets, such as graphene or TMDs.
Figure 2.4 Schematic illustration of the synthesis process of MXenes from MAX phases by selective‐etching method.
Source: Naguib et al. [22]. Reproduced with permission. Copyright 2012, American Chemical Society.
2.2.2 Bottom‐Up Method
2.2.2.1 Chemical Vapor Deposition
The first industrial application of CVD technique was dated back to 1897 when de Lodyguine reduced tungsten hexachloride with hydrogen to deposit tungsten onto the carbon filament of lamps [25]. Then this technique was further employed for the production of high‐purity materials, including Ti, Ta, Zr, and Si. Over the past decade, CVD has been considered as an efficient technique for the massive production of 2D nanosheets. Typically, the substrates are placed in a furnace, and gas/vapor precursors are cycled through the chamber to react and/or decompose on the surface of substrates. Sometimes the catalysts, in the shape of a film, are needed in the reaction process. Then the nanosheets can be obtained and further transferred to the desired substrates. Somani and coworkers pioneered the synthesis of graphene films from camphor pyrolysis on Ni substrates via the CVD method [26]. Despite the film thickness of about 30 layers, this work demonstrated the feasibility of the CVD method for the fabrication of graphene. Following this work, many efforts have been devoted to optimizing the experimental conditions, such as the precursors, substrates, catalysts, temperature, and atmospheres, which are critical to the structural features of the obtained graphene products. Pollard et al. demonstrated the CVD growth of single‐layer graphene on the Ni film evaporated on the SiO2/Si substrate [27]. It is worth noting that the Ni film served as the substrate, which supported the growing graphene and the catalyst facilitating the nucleation of precursors at the same time. In addition, Li et al. synthesized large‐area graphene film with lateral size of centimeters and single‐layer rate over 95% by CVD method on copper foil substrate [28]. Methane and hydrogen were chosen as gas sources, and the substrate was removed by the treatment with iron nitrite solution to obtain freestanding graphene films. Similarly, the CVD growth method can be applied for the fabrication of other nanosheets, including h‐BN [28, 29], MoS2[30], and so on. Despite the great potential of CVD technique in the massive production of nanosheets with high crystal quality and purity, the synthetic conditions, such as high temperatures (∼1000 °C) and inert atmosphere, lead to high production costs and restrict its industrial application.
2.2.2.2 Hydro/Solvothermal Synthesis
The hydro/solvothermal synthesis, in which water or organic solvent serves as the reaction medium in a sealed vessel at high temperature and pressure, has been a popular strategy for preparing nanosheets, such as 2D metal oxides, metal chalcogenides, and MOFs [31]. During the synthesis, the key issue lies in how to selectively block the growth of nanomaterials along one direction to form desired nanosheets. The introduction of structure‐directing agents is demonstrated to be an effective strategy. Tsapatsis and coworkers proposed the bottom‐up synthesis of MFI nanosheets via the hydrothermal growth of MFI seeds with the assistance of bis‐1,5(tripropyl ammonium) pentamethylene diiodide (dC5) [32]. The dC5 was proven to direct the growth of plate‐like MFI with the thin crystal dimension along the b axis, and the straight micropores aligned along this direction, endowing the 2.5‐unit‐cell thick (5 nm) MFI nanosheets with great potential in membrane separation. The hydro/solvothermal synthesis method is a scalable method for the high‐yield production of ultrathin nanosheets, but the product is quite sensitive to the synthesis parameters, such as the concentration of precursors, surfactant and solvent type, and reaction temperature/time, thus influencing the reproducibility of each batch [1].
2.2.2.3 Interfacial Synthesis
The interfacial synthesis is another bottom‐up method for the fabrication of 2D nanosheets. During the synthesis process, the monomers react at the confined interfaces to guarantee a low thickness. The diffusion rates of the monomers toward the interface should be carefully controlled, which determines the structural integrity of the as‐prepared nanosheets. Gascon and coworkers proposed a three‐layer synthesis strategy for the preparation of MOF nanosheets [33]. The system consisted of three liquid layers composed of mixtures of DMF and a suitable miscible co‐solvent in appropriate ratios, which were vertically arranged according to their different densities. The top and bottom layers contained Cu(NO3)2 and 1,4‐benzenedicarboxylic acid (BDCA), respectively, while the intermediate region acted as a highly diluted medium for the growth of MOF nanosheets. The obtained nanosheets exhibited lateral size of 0.5–4 μm and thickness in the range of 5–25 nm. Moreover, the method can be extended to a variety of MOFs by changing the metal nodes or organic linkers, demonstrating the versatility of the three‐layer synthesis strategy. Besides MOF nanosheets, covalent organic framework (COF) nanosheets can be prepared by the interfacial synthesis method as well. Payamyar et al. successfully synthesized COF nanosheets at the water/air interface [34]. They designed an amphiphilic monomer that can preorganize on the water surface and realize face‐to‐face stacking of the anthracene units. As a result, the monomers were connected via the photochemical anthracene [4+4]‐cycloaddition dimerization to construct COF nanosheets. Although the interfacial synthesis method can produce nanosheets with integrated structure, the yield is relatively low and cannot meet the demand for industrial‐scale production.
2.3 Membrane Structures and Fabrication Methods
According to the differences in membrane structures, two‐dimensional‐material membranes can be categorized into three main types, including nanosheet membranes, laminar membranes, and MMMs, and the fabrication methods for each structure differ from one another. In this section, we focus on various strategies for membrane fabrication and introduce the latest advances in different types of membranes. Importantly, the effect of constructing and tuning the in‐plane pores or interlayer channels on the membrane structure and separation performance is systematically summarized. In addition, the pros and cons of the fabrication methods are briefly compared. In the last part, some hybrid membranes beyond the mentioned types are discussed as well.
2.3.1 Two‐Dimensional‐Material Nanosheet Membranes
2.3.1.1 Zeolite Membrane
Zeolites are microporous materials based on crystalline silica with pore sizes within several angstroms to nanometers. The thickness of 2D zeolite nanosheets can be reduced as thin as the dimension of unit cells, which enables the nanosheets to be promising material for fabricating ultrathin membranes with superior separation performance. Tsapatsis and coworkers have been devoted to the fabrication of high‐quality zeolite nanosheets and molecular‐sieving membranes. In 2011, they reported the synthesis of highly crystalline MFI nanosheets via the melt‐blending method combined with sonication [35]. The obtained nanosheets were ∼3.4 nm, only 1.5‐unit‐cell thick along the b axis. Then they employed filtration method to deposit nanosheets on the porous support to form the nanosheet coating, which was oriented since the short dimension of nanosheets was perpendicular to the support surface. Nevertheless, nanometer‐sized pinhole defects existed in the deposits, which seriously affected the separation performance. The secondary solvothermal growth method was proposed to eliminate the defects, and the preferred orientation could be maintained by the introduction of appropriate structure‐directing agents. The thickness of as‐prepared ultrathin zeolite membranes can be reduced to ∼100 nm, which was 10 times thinner than typical zeolite membranes [36]. Importantly, the membrane showed excellent molecular‐sieving property and high‐separation performance for xylene isomer mixtures. Although the zeolite nanosheets synthesized in this method were of high purity, the yield was low (∼0.01% w/w nanosheets in the dispersion) and the lateral size of nanosheets was relatively small. Bearing this in mind, they further demonstrated the direct synthesis of zeolite nanosheets via hydrothermal growth of MFI seeds [32], which was previously discussed in Section 2.2.2. The as‐synthesized nanosheets combined the lateral dimension of conventional MFI crystals (1.5–2.0 μm) with nanometer‐scale thickness. The large lateral dimensions were favor for lowering the density of grain boundaries in the membranes, which would lead to nonselective transport pathways and reduce the sieving property. After gel‐free secondary growth, a dense layer with thickness ranging between 250 nm and 1 μm was formed on the porous support. The successful synthesis of high‐quality zeolite nanosheets endowed the membrane with excellent p‐xylene/o‐xylene separation performance (p‐xylene permeance: 0.56 × 10−6 mol Pa−1 m−2 s−1, separation factor: 2000 at 150 °C). The membrane throughput can be further enhanced if the membrane thickness can be reduced to 10 nm. Furthermore, more zeolite crystals with layered structures should be discovered for improving the diversity of zeolite nanosheets. The early advance in 2D zeolite membranes lays the foundation for the fast development of 2D‐material membranes.
2.3.1.2 MOF Membrane
Owing to the abundant porous structure and functional surface groups, MOFs can serve as building blocks for molecular‐sieve membranes. A large amount of MOF crystals with layered structures can be exfoliated into ultrathin nanosheets, demonstrating the great potential of 2D MOF membranes. However, it is challenging to retain the morphological and structural integrity of the synthesized nanosheets, which is a critical factor for fabricating defect‐free MOF membranes. Yang and coworkers demonstrated the fabrication of ultrathin MOF nanosheets and high‐performance molecular‐sieve membranes [13]. The Zn2(bim)4 nanosheets were exfoliated via a soft‐physical strategy, which was discussed in Section 2.2.1. Filtration method is commonly used for the preparation of nanosheet membranes. Nevertheless, they found this method could lead to the restacking of nanosheets back to ordered pristine structures, which would partially or totally block the molecular‐sieve pores. A hot‐drop coating method was proposed to realize the disordered stacking of nanosheets. In a typical preparation, a certain amount of nanosheet suspension was deposited dropwise onto the surface of the porous support, which was heated on a heating plate. As a result, the MOF nanosheets were uniformly deposited on the support to form a continuous layer. The texture of the underlying support was distinguishable, indicating an ultrathin MOF layer. In addition, the detection of aluminum signal from Al2O3 support by X‐ray photoelectron spectroscopy (XPS) proved that the membrane was only several nanometers thick. Moreover, the structure–performance relationship was investigated as well. The low‐angle hump and (002) reflection corresponded to the expanded stacking and ordered restacking of nanosheets, respectively, which would block the pathway for H2 and reduced the separation performance. The ultrathin MOF membrane, consisting of disordered nanosheets, exhibited excellent H2/CO2 separation performance with H2 permeance of up to 2700 GPU and selectivity reaching 291. Wang et al. proposed the freeze–thaw approach to exfoliate the bulk MAMS‐1 (mesh adjustable molecular sieve, Ni8(5‐bbdc)6(μ‐OH)4, 5‐bbdc = 5‐tert‐butyl‐1,3‐benzenedicarboxylate) crystals into ∼4 nm nanosheets [37]. Then the MAMS‐1 nanosheets were assembled into molecular‐sieve membranes via the hot‐drop casting method. The as‐prepared membranes possessed pore openings parallel to gas concentration gradient, which allowed high gas flux and high selectivity at the same time. In addition, they demonstrated the reversed thermos‐switchable property of the gas transport pathways. The H2/CO2 selectivity exhibited a temperature‐dependent behavior, with the maximum of 245 at 20 °C and the minimum of c. 5 at 100 °C. Based on in situ variable temperature powder X‐ray diffraction (PXRD) and molecular dynamics simulation, this phenomenon was attributed to the flexibility of the MAMS‐1 nanosheets. Hence, the MOF membrane with reversed thermo‐switchable molecular‐sieving property could have potential in the temperature‐related gas separation process.
2.3.1.3 Porous Graphene Membrane
The perfect single‐layer graphene sheet is impermeable to gases as small as helium because the electron density of its aromatic rings is too substantial to allow the transport of atoms and molecules through these rings [38]. Therefore, researchers were motivated to drill holes in graphene sheets to fabricate porous graphene membranes, which hold the potential to achieve superior separation performance due to their single‐atom thinness. At first, theoretical studies on porous graphene membranes were reported. Jiang et al. designed pores functionalized with different atoms on the graphene nanosheet and investigated the gas separation performance of the porous graphene membrane by first‐principles density‐functional theory calculations [39]. Cohen‐Tanugi and Grossman reported the desalination performance of single‐layer graphene with nanometer‐scale pores based on classical molecular dynamics [40]. They found that the pore diameter of the porous nanosheet determined the salt rejection performance and the hydrophilic groups, such as hydroxyl groups, bonded to the pore edges contributed to prominently enhanced water flux. In addition, the water permeability of the porous graphene was several orders of magnitude higher than the conventional reverse osmosis membrane, which exhibited the great potential of porous graphene in water purification.
Apart from the theoretical calculations, experimental studies on drilling pores in graphene sheets were reported. The ultraviolet‐induced oxidative etching method was firstly proposed for creating pores in graphene membranes [41]. After the introduction of nanopores, the pressurized blister test combined with mechanical resonance was employed to monitor the molecular transport through nanopores. The as‐fabricated membrane exhibited a gas permeation rate comparable to that of computational modeling and molecular‐sieving property, making an important step toward the realization of macroscopic porous graphene membranes. Celebi et al. reported the physically perforated double‐layer graphene by the focused ion beam [42]. The perforated graphene membrane possessed lots of nanopores with narrowly distributed diameters ranging from <10 nm to 1 μm, endowing the membrane with orders‐of‐magnitude enhancement in gas, water, and water vapor permeances. Moreover, the O2 plasma‐etching method was employed to fabricate porous graphene membranes for water desalination [43]. The membrane with finely tuned nanopores showed fast water transport and a salt rejection rate of nearly 100%, proving the feasibility of porous graphene membrane for desalination. To create nanopores with angstrom precision at a high pore density, Zhao et al. developed synergistic defect nucleation and pore‐expansion strategy by combining O2 plasma and O3 treatment [44]. Raman spectra were employed to detect the evolution of defects and porosity of the porous graphene nanosheet. As a result, they achieved a high density (c. 2.1 × 1012 cm−2) of H2‐sieving pores and record‐high H2/CH4 and H2/C3H8 separation performance. More importantly, this pore‐etching strategy is highly scalable and will contribute to the rapid development of single‐layer graphene‐based membranes.
Although many strategies have been proposed to make pores on the graphene nanosheet, the problem of structural integrity of the atomic‐thin membrane will restrict its practical application. Yuan and coworkers reported the fabrication of large‐area graphene‐nanomesh/single‐walled carbon nanotube (GNM/SWNT) hybrid membrane with excellent mechanical strength [45]. In a typical process, single‐layer graphene was prepared on Cu foil via the CVD method. A layer of SWNT membrane consisting of interconnected SWNTs was transferred on top of the graphene, and a freestanding membrane with SWNT‐supported graphene was obtained by etching the Cu foil. Then a layer of mesoporous SiO2 film with perpendicular mesoporous channels was created on the graphene surface, serving as the porous template. O2 plasma was employed to drill pores in the graphene to form the graphene nanomesh, and the meso‐SiO2 layer was removed by hydrofluoric acid etching. As a result, the GNM/SWNT hybrid membrane was obtained, in which the mechanically strong SWNT webs formed strong π–π interaction with the supported GNM and acted as a microscopic framework to support the GNM, ensuring the structural integrity of the atomically thin GNM over macroscopic scale. Moreover, the high‐density subnanometer pores in the GNM allow the fast transport of water molecules, while blocking solute ions/molecules to enable selective separation. The as‐fabricated ultrathin GNM/SWNT hybrid membrane show high water permeance and a high rejection ratio for salt ions or organic molecules.
In all, the porous graphene membrane has demonstrated its huge potential in the process, such as water purification and gas separation, but the technical issue of creating high‐density nanopores with narrowly distributed diameter in the membrane area of m2 level remains to be solved.
2.3.2 Two‐Dimensional‐Material Laminar Membranes
2.3.2.1 Assembly Strategies of Laminates
For the laminar membrane assembled from 2D nanosheets, the selection of proper assembly strategy determines the microstructure of membranes and thus the transport behavior. For GO membranes prepared by self‐assembly, the intrinsic repulsive force between the carboxyl group of neighboring nanosheets will cause the disordered stacking of nanosheets and the generation of nonselective defects [8], which greatly influence the separation performance of laminar membranes. Hence, various driving forces, including pressure difference, shear force, centrifugal force, and electric‐field force, are employed to regulate the assembly of 2D nanosheets and reduce the interlayer defect.
To construct continuous GO laminates with few defects, vacuum filtration method was employed to surpass the repulsive force between neighboring nanosheets. Yu and coworkers fabricated ultrathin (1.8–18 nm) GO membranes on the anodic aluminum oxide (AAO) support via filtration for hydrogen purification [46]. The membrane thickness could be easily tuned by controlling the deposition amount of GO nanosheets. They observed that the reduced GO membrane with narrowed interlayer spacing showed similar gas permeance to the pristine membrane, suggesting the selective structural defects within GO flakes instead of the interlayer spacing determined the gas transport behavior. The as‐prepared GO membrane exhibited ultrahigh H2/CO2 selectivity of 3400 and H2/N2 selectivity of 900. Moreover, our group firstly proposed a vacuum suction method for depositing GO nanosheets on the ceramic hollow fiber (Figure 2.5) [47], which possesses high‐packing density and is beneficial to the scale‐up fabrication of GO membranes. Owing to the preferential water‐sorption ability and fast water diffusivity through the GO layer, the as‐fabricated GO membrane showed excellent pervaporation dehydration performance for the dimethyl carbonate (DMC)/water mixture. Similarly, based on the driving force of pressure difference, pressurized filtration method was employed to prepare the freestanding GO membrane (Figure 2.6a) [48]. The filtration pressure was proved to influence the packing density of GO nanosheets and thus the interlayer spacing of GO laminates. The freestanding GO membrane was applied in the pervaporation separation of water/ethanol mixture, but the binary‐component selectivity was much lower than the ideal water/ethanol selectivity, which was attributed to the swelling of GO laminates in aqueous solution. The strategy for stabilizing GO membranes is discussed in Section 2.3.2.2.
Besides the pressure difference, centrifugal force was employed to assist the assembly of GO laminates. Kim et al. reported the fabrication of few‐layered GO membranes via the spin–coating method [52]. They employed two different coating methods to prepare GO membranes. In method one, they contacted the surface of the support to the air–liquid interface of a GO solution, followed by spin coating. For comparison, they directly spin‐coated the GO solution on the support in method two. As a result, two GO membranes exhibited similar surface morphology but quite different gas transport behavior. The repulsive edge‐to‐edge interaction from the carboxylic groups of GO nanosheets led to a heterogeneous GO deposition in method one, while the face‐to‐face attractive capillary forces during spin coating overwhelmed the repulsive interaction in method two, contributing to the dense stacking of GO nanosheets. The GO membrane prepared by method two formed highly interlocked structure and exhibited excellent CO2/N2 separation performance. In addition, we firstly proposed to fabricate GO laminar membranes by the spray–evaporation method (Figure 2.6b) [49]
