Fundamental Transport Mechanisms and Advancements of Graphene

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Review

Fundamental transport mechanisms and advancements of graphene oxide membranes for molecular separation Tieshan Yang, Han Lin, Kian Ping Loh, and Baohua Jia Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03820 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Chemistry of Materials

Fundamental transport mechanisms and advancements of graphene oxide membranes for molecular separation Tieshan Yang,a Han Lin,a Kian Ping Loh,b,c Baohua Jia*a a. Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia. b. Department of Chemistry, National University of Singapore, Singapore 117543, Singapore. c. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore. ABSTRACT: Graphene shows excellent mechanical, electrical and optical properties. Its unique structure and attributes have made it and its derivatives (e.g., graphene oxide (GO) and reduced graphene oxide (rGO)) promising candidates in molecular separation. Excellent molecular separation performance, including permeability and selectivity, has been demonstrated by GO membranes recently. In this review, the mechanisms, i.e., in-plane pores (defects), interlayer spacing and functional groups assisted molecular separation, of selective molecular transport through GO membranes are summarized. In addition, preparation and interlayer spacing tuning strategies of GO membranes are provided. This timely review aims to present the latest advancements in both the theoretical and experimental demonstration of GO membranes for molecular separation. Moreover, current challenges and emerging perspectives of GO membranes for separation are also discussed.

1. Introduction Membrane separation has emerged as a highly demanded technology for seawater desalination and gas separation, and has profound environmental impacts during the past years.1 A perfect membrane should be permeable and thin enough to maximize flux and reduce energy consumption, mechanically robust to prevent fracture, and have defined pores to increase molecular selectivity in an effective manner.2-10 There exist two types of conventional membranes: polymeric membranes are commonly used membranes on the market with excellent salt rejection. However, it is limited by the slow diffusive water transport through the polymer is slow.1, 11 In addition, there are several practical disadvantages of the polymeric membranes, including the large pore size distribution, the poor thermal and chemical stability, the poor mechanical strength, and the polymeric membranes are easy to foul and require pretreatment. On the other hand, inorganic membranes, like zeolite, show separation performances. However, it is challenging to scale up these inorganic membranes in a cost-effective way.9 New membrane materials are thus urgently needed to enhance molecular separation with high permeability, excellent rejection and fouling resistance. In this regard, the synthesis and processing of two-dimensional (2D) materials, including graphene and its derivatives (e.g. graphene oxide (GO), reduced graphene oxide (rGO)) offer an exciting opportunity for developing novel membranes possessing outstanding separation properties owing to its atomic-thickness, nearly frictionless surface, high tensile strength, which enable a lower transport resistance and higher permeate flux for practical applications.1, 12-16

Graphene membranes with defined in-plane pores are highly efficient for molecular separation. In theory, nanoporous monolayer graphene membranes are able to achieve salt rejection of 100% because the only pathway for ions and water molecules is the subnanometer pores.3, 17-27 However, precisely controlling pore sizes with a narrow size distribution and high density on a large-scale graphene film are challenging. 13, 23, 28, 29 Various methods have been explored recently, such as focused electron beam (FEB) irradiation,30, 31 ultraviolet-induced oxidative etching,32 ion bombardment followed by chemical etching,18 oxygen plasma etching3 and focused ion beam (FIB) irradiation33 for generating in-plane nanopores in graphene. Despite advances in how to produce pores in graphene, it is challenging to achieve high density pores and narrow sized distribution with low cost for targeted applications.1, 13, 23, 28, 34 In addition, the high density of pores in graphene membranes may compromise the mechanical strength. Additionally, it is challenging to fabricate single-crystal, continuous largearea graphene, and the defect-free transfer of graphene is non-trivial.28 Unlike hydrophobic graphene, GO, which is the oxidized form of graphene, possesses rich oxygen-containing functional groups, such as epoxy, hydroxyl, carbonyl and carboxyl groups on the basal planes and edges of the sheet, and is thus hydrophilic.35-38 These oxygen-containing functional groups provide electrostatic repulsion between GO sheets and widen the interlayer spacing, which serves as a water permeation channel in addition to any pores on the basal plane or grain boundaries between GO flakes. Moreover, these oxygen-containing functional groups enable the high hydrophilicity of GO, which can be used to design the microstructures and properties of GO membranes.12, 22 Additionally, GO can be produced in a large scale by cost-effective

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methods.39 The solution synthesis method offers the possibility of surface functionalization, thereby imparting GO membranes with diverse functions. Detailed studies of the permeability and selectivity of GO membranes have been carried out recently, which provides in-depth understanding of the molecular transport mechanism. This timely review not only provides a detailed update on the state-of-the-art GO membranes for molecular separation, like gas separation and water desalination, but also reveals the fundamental transport mechanisms of GO membranes, as shown in Figure 1. Molecular separation with GO membranes can occur either through in-plane pores (pore size rejection), interlay spacing (layer spacing

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rejection) or interactions with the oxygen-containing functional groups (charge rejection) mechanisms. In particular, we highlight strategies to tune the interlayer spacing in the GO membrane and achieve long-term membrane stability through thermal, chemical and photo reduction and functionalization. Insights on the correlations between structures and properties of GO membranes in the context of molecular separation are presented. Particularly, we highlight that various synergistic effects are responsible for the overall separation performance. Through summarizing the outstanding challenges and future perspectives in the field, we aim at stimulating discussions and ideas accelerating the development of stacked GO membranes towards diverse applications.

Figure 1. (a) Schematic illustration of molecules or ions passing through 2D stacked GO and fundamental transport mechanisms of GO membranes: (b) in-plane pore size (defects), (c) interactions (charge effect) with the oxygen-containing functional groups and (d) interlayer spacing. 2. Graphene oxide membranes 2.1 Structure of GO nanosheets GO is a monolayer-thin graphene decorated with epoxy, hydroxyl, carbonyl and carboxyl groups on the basal planes and edges.39 Lerf-Klinowski model is the most recognized structural model, as shown in Figure 2.35 Both the Young’s modulus and intrinsic strength decrease as the coverage of the oxidized regions increase.35, 36, 40 Due to the stacked structures and the oxidized regions of GO that keep graphene planes apart, the non-oxidized regions create nearly frictionless channels for small molecules across GO laminates.41 2.2 Properties of GO membrane GO shows many unique properties due to these oxygen-containing functional groups.4, 36, 38, 39, 42-45 On one hand, GO can be dispersed in aqueous medium, which

makes GO compatible with many membrane-processing methods. On the other hand, GO sheets are negatively charged when they are dispersed in water. Additionally, carbon atoms bonded to oxygen atoms in the form of epoxy, hydroxyl, carbonyl and carboxyl groups tend to form amorphous regions due to distortions from the sp3 C-O bonds.46-48 Structural defects are formed in the basal plane of the GO sheets, which can be used for molecular transport. Moreover, these oxygen-containing functional groups are beneficial for various surface modifications to develop functionalized GO-based membranes with largely enhanced performances and anti-biofouling applications. Comparing to porous graphene membranes, where in-plane pores provide channels for molecular transport,17 the 2D nanochannels between GO nanosheets provide another way for molecular separation.41


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Graphene oxide

Oxygen-containing functional groups

Epoxy

Hydroxyl

Carbonyl

Carboxyl

Figure 2. Schematics of the molecular structure of graphene oxide. 3. Mechanisms of selective molecular transport Different from molecular separation using graphene membrane, which mainly depends on the in-plane pore sizes selective mechanism, molecular separation with GO membranes can through in-plane pores (defects), interlayer spacing or interactions with the oxygen-containing functional groups, as shown in Figure 1. Practically, the synergistic effects may enhance the separation performance.1, 14 3.1 In-plane pores for molecular separation In-plane pores (defects) provide a mechanism for molecular sieving, which could be as diffusion ways for molecules passing through the GO membranes, as shown in Figure 1b. Through ultraviolet-induced oxidative etching, bilayer graphene can generate with a pore size of ≈3.4 Å, which showed a CO2 (kinetic diameter: 3.3 Å) / CH4 (3.8 Å) selectivity of 6000.32 If membrane has inplane pores, which are smaller than the mean free path but larger than the kinetic diameters of gas molecules, the membrane will show free molecular effusion behaviors. Theoretical studies 17, 49, 50 showed through introducing pores in graphene sheets with subnanometer sizes, the membranes could provide water permeance two orders of magnitude higher than that of the pristine GO membrane. So, it is promising to improve the separation performance of GO membrane by introducing in-plane pores. In ultrathin GO membranes (thickness: 1.8 nm) for H2 separation, the dominant transport channel is via structural defects between GO sheets, since the laminar structure has not been formed at an ultrathin thickness. Effusive flow for separating gas mixtures through an in-plane pores membrane is by the Knudsen transport as 𝑄 = ∆𝑃⁄√2𝜋𝑚𝑘𝐵 𝑇 , which is related to the molecular weight.51 In the equation, Q is the gas permeance, P is the pressure, m is the molecular weight, kB is the Boltzmann constant, and T is the temperature.1 3.2 Interlayer spacing for molecular separation In addition to in-plane pores, interlayer spacing between two graphene lattice can be precisely modulated

and well used for molecular separation, as shown in Figure 1d.41 The membranes with micrometer-thickness allow unimpeded permeation of water, but impermeable to helium. GO have two different regions: pristine and oxidized. The pristine regions form a network of capillaries, connecting with the oxidized regions. The oxidized regions interact with water and the capillary-like pressure drives the transport of water, resulting in ultrafast water permeation through GO membrane.41 GO membranes block all solutes with a hydrated radius > 4.5 Å in the fully hydrated state, not depending on the charge of ions, while ionic species with hydrated radius < 4.5 Å passes through the GO nanochannels.52, 53 Thus, interlayer spacing of GO membranes for molecular separation is mainly based on size-selective mechanisms. 3.3 Functional groups for molecular separation Apart from in-plane pores and interlayer spacing, functional groups of GO are also crucial for molecular separations, since they can have interactions with molecules, as shown in Figure 1c.54 When rGO membrane (interlayer spacing: ≈3.5 Å) is used for gas separation, the permeation of CO2 (3.3 Å) is 12 times higher than that of H2 (2.9 Å). This result indicates the effect of the oxygencontaining functional groups, which interact with the CO bonds of CO2, leading to CO2 adsorption and diffusion. The adsorption affects CO2 transport, depending on the structures of GO membranes. Additionally, the oxygencontaining functional groups would interact with H2O molecules to form hydrogen bonds, affecting water transport.1, 14 The functional groups can control the transport of ions, since the negatively charged functional groups on GO could electrostatically interact with the ions. The permeation of Na+ with various anions is in the trend of NaHCO3 < NaHSO4 < NaOH via a GO membrane (thickness < 10 μm) with an interlayer spacing of ≈8.3 Å.54 When NaHCO3 solution is permeated, chemical reactions between the HCO3– and the carboxyl groups generate CO2, which suppresses the permeation. For NaHSO4 solution, the H+ prohibits the ionization of the functional groups, which decreases the interlayer spacing, and thus lowing the


3


permeation. Hydroxide ions interact with hydroxyl and carboxylate groups, which enlarges the interlayer spacing because of electrostatic repulsion, and thus promoting the permeation of Na+ and OH−.54 In summary, molecular transport through GO membranes may be dominated by the synergetic effects of the above mentioned factors (e.g. in-plane pores, interlayer spacing and functional groups). The structure defects and functional groups of GO membranes offer not only nanochannels, but interact with molecules or ions. Therefore, there are several methods to utilize these nanochannels for molecular separation, which will be discussed in the following parts. 4. Preparation and property tuning of GO membranes GO nanosheets can be mass-produced in an effective method by chemical oxidization of graphite. The oxygencontaining functional groups modify the surface properties (e.g. the charge properties and hydrophilicity) of GO. In contrast to pristine graphene, graphene oxide is hydrophilic because of the attachment of the oxygen-containing functional groups. As a result, GO flakes can be dissolved in water, which enable the low-cost solutionbased membrane synthesis technique. In addition, it is feasible to modify the GO membrane to tune the chemical properties because of the oxygen-containing functional groups.55 These unique features enable GO an attractive platform for building well-defined multifunctional nanostructures. GO membranes worked in separation can be classified into two kinds: membranes with inplane pores and laminar structure. In the following sections, we summarize the strategies on the pore generation, laminar structure creation, and the control of the interlayer spacing and the functional groups of the GO membranes. 4.1 Membranes with in-plane pores Kim et al. used one method to prepare GO membranes: touching the surface of the polymeric substrate to the air-liquid interface of GO solution, followed by spin-coating (as Method one in Figure 3a).1, 51 Multilayer GO membranes were prepared and these GO membranes included in-plane pores generated by the edges of non-interlocked GO sheets, thus gases may diffuse through such in-plane nanopores.28 In addition, thermal reduction of GO to form rGO can also induce in-plane pores.51, 56 The oxygen-containing functional groups are removed with carbon atoms in thermal reduction of GO, which leads to the release of CO and CO2 gases and generates in-plane pores.51 About 0.3 nm pores in graphene were created through ion and electron bombardment.57 Using KOH activation of microwave-exfoliated GO, a porous graphene sheet with pore size smaller than 5 nm was prepared.58 O2 etching was also used to introduce pores at high temperature in the pristine GO sheets.59 H2O2 was also used to open pores on rGO sheets.60 These porous GO sheets were easily damaged, and it is difficult to assemble lamellar membrane through filtration.61

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Recently, mesopores smaller than 5 nm could be generated through re-oxidizing the pristine GO sheets via KMnO4 by Ying et al..61 The oxidant species could easily attack the defective domains and remaining functional groups on the carbon skeleton, and pores can be generated in GO sheets.61 After sonication, these porous GO sheets could be easily assembled into GO membranes via filtration.61 4.2 Membranes with laminar structure In additional to the in-plane pores, GO can be assembled into laminar structures with a defined interlayer spacing. In this part, we will discuss several ways for the generation of stacked layers.14 GO can be assembled into stacked structures by vacuum filtration,20, 52, 62-64 drop-casting,54 spin-coating,41, 51 layer-by-layer (LBL) deposition65 and other ways. Vacuum filtration is used to make GO membranes on substrates and the thickness of the membrane can be controlled by the amount of GO in the solution. Additionally, other functional materials, like molecules and nanoparticles can be incorporated into GO if they are compatible.66 Drop-casting is used to fabricate freestanding GO membranes, but it is not suitable in the composite system, since the interfacial adhesion between the GO and the substrate is not enough.13 Spin-coating can arrange GO nanosheets in a fast process. The LBL method is scalable and cost-effective for providing a stabilizing force by covalent bonding, electrostatic interaction, or both effects during layer deposition,66 which enables precise control of the GO membrane thickness at the molecular level.66 If the interlayer spacing is well manipulated, it will provide additional pathways for molecules. Therefore, precise tuning of the interlayer spacing becomes a basic method to achieve high-performance GO laminar membranes. In order to get a stacked structure, Kim et al. prepared few-layer GO membranes via a spin-casting method by directly dropping a constant volume of GO solution onto substrate (as Method two in Figure 3a).51, 67 The spin casting generates face-to-face attractive capillary forces, which overcomes the repulsive edge-to-edge GO sheet interaction, and thus the dense stacking occurs. Abraham et al. used epoxy physical confinement to tune the interlayer spacing from ∼9.8 to 6.4 Å of GO membrane, which provided a sieving size smaller than the diameters of hydrated ions.68 In order to reduce the spacing and to keep this spacing against swelling when immersed in aqueous solution, Chen et al. showed cationic control of the interlayer spacing of GO membranes with ångström precision using cationic ions.69 LBL assembly of GO is suitable to produce a laminar structure with a tuned spacing by modifying the GO materials through chemical methods, controlling the thickness by changing the cycles of deposition, and designing interlayer interactions by varying the thickness of the introduced species.13, 66, 70 The GO sheets were cross-linked by 1, 3, 5-benzenetricarbonyl trichloride (TMC) by Hu et al. via LBL method, as shown in Figure 3b.65 The interlayer spacing between GO layers is ~ 1 nm. The LBL


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method not regulated the interlayer spacing, but also enhanced the GO stability, which could be well used for molecular separation for a long time. Enlarged GO spacing of 1-2 nm can be obtained by using large chemical groups between GO nanosheets, which will make GO membranes suitable for water purification.66 If nanoparticles or nanowires with even larger size are used as spacers, GO membranes with interlayer spacing of more than 2 nm can be created for biomedical applications that require precise separation of large biomolecules.66 In order to make GO membranes with tunable pore sizes and interlayer spacing at a relatively large range, nanoparticles or nanowires can be intercalated between GO nanosheets. As shown in Figure 3c, intercalating positively charged copper hydroxide nanostrands into the negatively charged GO layers, followed by partially reducing GO and then dissolving the copper hydroxide nanostrands, Huang et al. fabricated a nanostrand-channelled GO membrane. The GO membranes possess numerous nanochannels with diameters of 3-5 nm,63 which is suitable for separation to get optimized selectivity and permeability. Recently, Shen et al. proposed an external force driven assembly way to generate sub-nanometer channels for fast transporting.71 In Figure 3d-i, the “outer” external forces include centrifugal, compressive and shear forces; the “inner” ones are molecular interactions that are inside the laminar. Centrifugal rotation provided external forces are along x, y axes and vacuum-suction supplied external forces are along z axis.71 The negative effect of repulsive interactions can be suppressed in x, y and z directions. As a result, the stacking of GO nanosheets begins to be ordered, resulting in the quasi-ordered laminar structure. The synergic function external forces facilitated the highly ordered laminar structure with an interlayer spacing of 4.2 Å (Figures 3d-ii), which is promising for transport applications.71 4.3 Functional group tuning In additional to the GO membranes with in-plane pores and laminar structures, the functional groups of GO are also important for molecular separations, since they can have interactions with molecules. The oxygen-containing functional groups of GO would interact with water molecules to form hydrogen bonds, hindering fast water transport through the GO membrane.1 It is crucial to functionalize GO membranes by tuning oxygen-contain-

ing functional groups in order to be well used for molecular separation to get a higher permeability and selectivity. To tune the functional groups of GO membranes, various methods have been reported. Reduction method is effective to precisely control the functional groups. For example, chemical reduction (e.g. hydroiodic or ascorbic acids) is very gentler and it can get rGO with few structure defects.72 Thermal reduction can be used to control the functional groups by changing the heating temperature.56, 73 Photo reduction can be used to finely tune the functional groups by changing the laser power and scanning speed.74-76 Microwave can also be used to completely reduce GO to tune the functional groups.77 5. GO membranes stability The instability of GO membranes in water is a big problem as separation membranes because they will disintegrate.28 Therefore, it is necessary to stabilize GO membranes by tuning oxygen-containing functional groups in order to be used for a long period.78-81 To improve stability of GO membranes, various methods have been reported: (1) chemical crosslinking by ions (Na +, K+, Mg2+, Ca2+ and Li+),69, 82 multifunctional small molecules (isophorone diisocyanate,83 1,3,5-benzenetricarbonyl trichloride (TMC),65 diamine,78 dicarboxylic acids, polyols,84 tannic acid 85 and etc.) and functional polymers (poly(Nisopropyl acrylamide),86, 87 polydopamine 88 and polyethylenimine89). (2) Increasing π–π interaction by additives such as porphyrin and partially rGO sheets. Both methods can improve the adhesion between GO sheets. To facilitate molecular separation, interlayer spacing can be tuned and stability can also be improved by chemical or photo reduction to remove the functional groups of GO.90 (1) For chemical reduction (e.g. hydroiodic or ascorbic acids), recent studies have shown that hydroiodic acid (HI) or ascorbic acid as a reducing agent is much gentler, and it could obtain rGO with high quality and leave fewer structural defects and little deformation.91 It is challenging to maintain the structural integrity of GO nanosheets and the entire laminates in chemical reduction of GO membranes.28 (2) In addition to chemical reduction, photo reduction will be a promising and useful strategy to precisely control the oxygencontaining functional groups to obtain a well-defined layered structure and hydrophobic property of GO membranes.34, 92-94 Therefore, reducing them can improve stability via enhancing the π-π interactions between GO nanosheets.90


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(a)

Page 6 of 23

(b)

(c)

(d) (i)

(ii)

Figure 3. (a) Schematic illustration of two different GO coating methods. Reproduced with permission.51 Copyright 2013, American Association for the Advancement of Science. (b) Schematic illustration of a LBL procedure to synthesize the GO membrane. Reproduced with permission.64 Copyright 2013, American Chemical Society. (c) Illustration of the fabrication process of nanostrand-channeled GO membrane. A multi-step process consisting of formation of a dispersion of positively charged copper hydroxide nanostrands (CHNs) and negatively charged GO sheets on a porous support, followed by hydrazine reduction, and finally CHN removal. Reproduced with permission. 63 Copyright 2013, American Chemical Society. (d) Design and construction of 2D channels. (i) 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 channel unit. (ii) Hypothetical evolution of surface and cross section of GO-assembled 2D channels from intrinsic force induced disordered structure (left) to ordered laminar structures (right) driven by introduced synergistic external forces. Reproduced with permission.71 Copyright 2015, American Chemical Society.


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6. Separation performance by GO membranes Recent investigations have showed GO membranes demonstrate potential in gas, ions and molecules separation due to their prominent structural properties

through in-plane pores, interlayer spacing or functional groups-assisted separation mechanisms. We summarize the latest advancements on GO membranes for molecular separation in the following sections, as shown in Table 1.

Table 1. Molecular separation performance of GO membranes Applications

GO

mem-

branes

Thick-

Post-treat-

Interlayer

Membrane

ness

ment

spacing

classifica-

preparation

Flux

Rejection

Mechanisms

Author (year)

tion

method Gas

separa-

Spin-coating

tion

~

3-7

N/A

N/A

N/A

nm

H2 Permeability: ~ 5 × 10

5

barrer Spin-casting

~

3-7

N/A

N/A

N/A

nm

CO2

Permea-

bility:

H2/CO2 se-

in-plane

lectivity:

pores

~30

fects)

(2013)51

N/A

interlayer

Kim

spacing

al.

~8500

Kim (de-

1.8 nm

N/A

N/A

N/A

tration

al.

et

(2013)51

barrer Vacuum fil-

et

Separation se-

N/A

in-plane

lectivities:

pores

H2/CO2 3400;

fects)

Li et al. (de-

(2013)64

H2/N2 900 Vacuum-

N/A

N/A

~ 4.2 Å

N/A

spin

cation

Vacuum fil-

H2/CO2 se-

interlayer

Shen

ity:

lectivity:

spacing

al.

840-1200

barrer

technique Water purifi-

H2 Permeabil-

~ 5 μm

N/A

~ 13 ± 1 Å

RO

Na+, 3−

tration

(2016)71

29-33

K+, AsO4

~ 10

mol h−1 m−2

et

glycerol,

interlayer

Joshi

sucrose

spacing

al.

was

et

(2014)52

en-

tirely blocked Vacuum fil-

~

tration

μm

100

Epoxy physically

~ 6.4-9.8 Å

RO

H2O ~ 6 L h−1

NaCl 97%

m−2 bar−1

con-

interlayer

Abraham

spacing

et

al.

(2017)

68

finement Layer-by-

~ 14 nm

Cross-

~ 1 nm

NF

by

80-276 L m-2 h-

MB

46-

1

66%;

R93-

bar-1

interlayer

Hu et al.

spacing

(2013)65

EB 83 ±

interlayer

Huang et

1%

spacing

al.

layer assem-

linked

bly

1,3,5-ben-

WT

zenetricar-

95%

bonyl

tri-

chloride (TMC) Vacuum filtration

~ 2030 ± 40nm

Cross-

~ 3-5 nm

linked

by

NF

695 ± 20 L m-2 -1

h bar

(2013)63

nanoparticles,

-1

then

hydrazine treatment


7


Vacuum fil-

750 nm

tration

Drop-casting

Vacuum filtration

Cationic

~ 11.4 ± 0.2

~ nm

100

0.1 L m-2 h-1

Å

control

~ 10 μm

NF

Page 8 of 23 Na+ < 10−2 −2

mol

m

interlayer

Chen

spacing

al.

h−1

N/A

8.2 Å

Hydriodic

3.5 Å

NF

FO

et

(2017)69

Na+ permeated

CuSO4,

Functional

Sun et al.

quickly

MnSO4,

groups inter-

(2013)54

CdSO4

actions

57.0 L m −2 h −1

acid (HI) re-

NaCl: 1.3 gm

−2

h

−1

interlayer

Liu et al.

spacing

(2015)72

duction

NF (nanofiltration), RO (reverse osmosis), FO (forward osmosis), 1barrer = 10 −10 cm3 (STP) cm cm−2 s−1 cmHg−1 at standard temperature and pressure (STP), Methylene blue (MB), Rhodamine-WT (R-WT), Evans Blue (EB). 6.1 Gas separation Theoretical investigations of porous graphene membranes for gas separation via molecular dynamics (MD) simulations have been performed more widely than experiment,11 while transport via GO membranes has been less investigated by simulation methods because the structure and chemical properties of GO are complicated.11 The first experiment of GO membrane for molecular separation was reported by Nair et al. in 2012, which showed the freestanding GO membranes (thickness: ~1 μm) in dry state blocking liquids (e.g. ethanol, alcohol), vapors and gas molecules (helium), except water at hydrated GO membranes.48 This was an endeavor to utilize a GO membrane for filtration. Figure 4 shows the schematic for water pass through GO laminates. GO sheets have two different regions: pristine and oxidized. The pristine regions form a network of capillaries, connecting with the oxidized regions. The oxidized regions interact with water and the capillary-like pressure drives the transport of water, resulting in ultrafast water permeation through GO membrane.1, 41

Figure 4. Schematic view of permeation through the GO laminates. L is the average lateral length of the GO sheets, d is the interlayer spacing. Typical L/d is ~1000. Zoomed area: Model for graphene capillaries within the GO films. When the pristine-graphene capillaries are wide open, monolayer water can move through. Under a low humidity, the capillaries become narrower, and there is not

enough van der Waals distance to graphene walls to accommodate a water molecule. Reproduced with permission.41 Copyright 2013, American Association for the Advancement of Science. 6.1.1 In-plane pores dominated gas separation Subsequent to the pioneering work conducted by Nair et al., several works have been conducted in the following years.51, 52, 54, 62-64, 68, 69, 73, 81, 91, 95-111 The GO membranes prepared via spin-coating (Method one in Figure 3a) by Kim et al. showed gas permeation explained by Knudsen transport, in which gas permeance decreases according to the molecular weight, except CO2 (Figure 5a).51 During the test, CO2 permeance decreased rapidly with time and reached constant, whereas other gas permeance kept a constant value. The GO membrane included in-plane nanopores. Thus, gas may diffuse through such in-plane pores.11 Moreover, lots of carboxylic acid groups distributed at GO edges offer a preferential site for CO2 adsorption in the presence of water, enhancing the selectivity of gas pairs (e.g. CO2/CH4, CO2/H2 and CO2/N2).112 They also used thermal reduction treatment to generate porous structures of ultrathin rGO membranes for gas separation. The intrinsic defects in rGO can also work as nanopores, making rGO membrane for separations. The H2/CO2 selectivity at 140°C was as high as 40, one of the highest values among other reported membranes.51 In another work by Li et al., the H2 separation using 1.8 nm GO membranes onto anodic aluminum oxide support by vacuum filtration method was studied.64 Selective separations of 3400 for H2/CO2 mixtures and 900 for H2/N2 mixtures were observed, respectively,113 which were one or two orders of magnitude higher than those of the current membranes. The permeation pathway is through the GO structural defects, which is similar to spin-coating method used by Kim et al. to prepare the GO membranes for gas separation. H2 (2.9 Å) permeates ~300 times faster than CO2 (3.3 Å) through GO membrane (thickness: 18 nm) at 20 °C was observed (Figure 5c),113 suggesting a critical mesh size of pores for permeation may be ~ 2.9 - 3.3 Å.11, 64 Based on these results,


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structural defects within GO flakes may be the major transport pathway for gas molecules.107 From the above mentioned examples using GO membranes for gas separation, the nanocapillary networks in GO membrane were not created, since a few layers of GO flakes were stacked to form the GO membranes. Gas permeated mostly through the in-plane pores. 6.1.2 Interlayer spacing dominated gas separation However, in the spin-casting method (Method two in Figure 3a) conducted by Kim et al, highly interlocked laminar structures were formed and had extraordinary gas permeation behavior.51 The gas permeation varied in the trend of N2 < O2 < CH4 < He < H2 < CO2 at 20 °C (Figure 5b).113 These GO membranes were also more selective, but were less gas permeable than those prepared by spin-coating, indicating gases diffused selectivity between the GO interlayer spacing.51 This study represents the latest performance on GO membranes for gas transport. (a)

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Figure 5. Gas transport behaviour through GO membranes. (a) Gas permeances of GO membranes as a function of molecular weight (Spin-coating; dashed line represents the ideal Knudsen selectivity) under dry and humidified conditions. (b) Gas permeances of GO membranes as a function of kinetic diameter (Spin-casting) under dry and humidified conditions. Reproduced with Permission.51 Copyright 2013, American Association for the Advancement of Science. Single-gas permeation through GO membranes supported on porous AAO at 20°C. (c) Permeances of seven molecules through a ~18nm-thick GO membrane. (d) Permeances of H2 and He through GO membranes with different thicknesses. Reproduced with Permission.64 Copyright 2013, American Association for the Advancement of Science. Recently, Shen et al. used an external force driven assembly method to realize 2D channels with interlayer spacing of ~ 4.2 Å, as shown in Figure 2d.71 The external forces were utilized to finely control the 2D channels.

The nanochannels provide GO membrane with perfect sieving properties, which provide H2 permeability of 2-3 orders of magnitude higher and H2/CO2 selectivity of 3fold enhancement compared with polybenzimidazole and polyimide membranes.71 Moreover, the nanochannels with micrometer lateral size and subnanometer thickness may work as a promising platform for studying transport properties of molecules and ions.71 6.2 Water purification Water desalination is one method for water purification, which is the process that purifies water from seawater and it is an effective solution to water shortage.12 A membrane is necessary in the desalination process. For example, materials with uniform pores can effectively separate molecules based on their sizes by selectively adsorbing small molecules from a mixture containing molecules too large to pass through its pores. Notably, a distribution of pore sizes would limit the ability of the material to separate molecules of various diameters. To serve this purpose, an efficient membrane for water passage should be thin enough to get maximized permeance, be strong to withstand the pressure and have a narrow pore sizes distribution for perfect selectivity. MD simulations conducted by Cohen-Tanugi et al. have showed a graphene monolayer with subnanometer and chemical functionalized pores could separate salt from water effectively.28 Hydrophilic pores (28Å2) have higher water permeation but lower salt rejection than that of hydrophobic pores (23Å2).17, 28 In theory, nanoporous graphene has a water permeability of 66 L cm-2 day1 MPa-1, which is 2-3 orders of magnitude higher than commercial membranes.17 This work showed the functionalized, nanoporous graphene could be worked as a high-permeability membrane. Despite the huge potential for molecular separation using nanoporous graphene membranes, scalable production of large-area graphene films and introducing nanopores into the graphene plane remains a major challenge. In comparison, GO membranes are hydrophilic, and have been also explored for molecular separation.11 GO membranes for anomalous penetration of water was reported using the first principles calculations by Boukhvalov et al. (Figures 6a, b).114 They established models composed of water and bilayer GO, revealing the anomalous water behavior inside the GO. The water monolayer is between a perfect graphene layer (top) and an imperfect graphene layer (bottom) with a hole. In order to simulate hydrophilic edges of GO sheets, the edges in the bottom layer were modified by hydroxyl groups (Figure 6a).114 The results show that the ice structure was distorted and the height of the barriers was increased by two orders of magnitude compared with the bulk case because of the interactions of water with hydroxyl groups.114 It is much easier for the water monolayer to slide along the carbon sheet when reaching the edge than to pass through the hole.114 The migration of the second ice layer through the edge along the zigzag direction (Figure 6b) is energetically more favorable than its gliding along the first ice layer in the same


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plane.114 Since GO membranes swell in water, the enlarged interlayer spacing is useful for the formation of bilayer ice. Thus, the formation of an ice bilayer and the melting transition of the ice at the GO flake edges are responsible for the rapid transport of water molecules. Previous works have demonstrated the interlayer spacing can allow for fast water permeation, indicating the interlayer regions might not set a substantial barrier for water permeation.91 Additionally, the interlayer spacing has been found to facilitate selective permeation.91 Wei et al. also studied water permeation in GO membranes by considering the interlayer spacing, expanded channels and pores within the sheets using simulations, as shown in Figures 6c, d.99 Although nanoconfinement can enhance flow, fast water transport through pristine graphene channels is inhibited by a side-pinning effect from capillaries formed within oxidized regions, where enhanced flow is reduced by hydrogen bonds between water in the pristine and oxidized regions.99 This finding contradicts with the reported result of fast water flow confined between pristine channels in GO membranes where pristine regions offer a network of capillaries that allow frictionless flow of a water monolayer.41, 99 These findings are helpful to get a better understanding of water permeation through the porous and laminar microstructure and can help rationally design GO membranes for molecular separation. (a)

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Figure 6. Energy costs (meV/H2O) and optimized atomic structures for: (a) Ice monolayer migration over the hydroxyl-passivated GO edge. (b) Ice bilayer sliding and destruction transition over the pore edge. Reproduced with

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permission.114 Copyright 2013, American Chemical Society. Microstructures of graphene-derived membranes. (c) The percolated water transport channel is composed of interlayer, interedge spaces, wrinkles and pores within the graphene sheets. (d) The pristine and oxidized patterns on GO (left) are modeled in a quasi-2D molecular model (center) with oxygen-containing functionalization groups on both sides (right). Reproduced with permission.99 Copyright 2014, American Chemical Society. In summary, mechanism for the molecular transport phenomenon was investigated via calculations and atomistic simulations. The above works lay the foundation for the advancement of molecular transport via GO membranes, which is important for developing functionalized GO membranes for molecular separation.107 6.2.1 Interlayer spacing dominated water desalination Many experiments have been conducted using GO membranes for molecular separation. The permeation of species with various ionic charges through micrometerthick (thickness: 5 μm), freestanding GO membranes was investigated by Joshi et al..52, 113 There was no permeation for glycerol, toluene, ethanol, benzene and dimethyl sulfoxide for many weeks. But water can permeate quickly through GO membranes.11 The permeability of various ions and organic molecules is only related to hydration radii of particles, independent on the charge of ions (Figure 7a). When immersed in ionic solutions, the interlayer spacing of the nanosheets can be swelled to ~ 9 Å. Therefore, any ionic species with hydrated radius > 4.5 Å is blocked, while ionic species with hydrated radius < 4.5 Å passes through the nanochannels.25 The above example shows GO membranes have exceptional molecular separation properties, confirming interlayer spacing plays an important role in separation. However, their applications in ion sieving are restricted by a permeation cut-off of ~ 9 Å, which is larger than the diameters of the common hydrated ions. Achieving a smaller interlayer spacing for the laminates immersed in water is challenging. Abraham et al. used epoxy physical confinement to tune interlayer spacing from ∼9.8 to 6.4 Å, smaller than the diameters of hydrated ions.68 They achieved accurate and tunable ion sieving. Permeation rates of ions decreased exponentially with decreasing sieving size but water transport was weakly affected, as shown in Figures 7b, c. This is an easy way to precisely control the interlayer spacing of GO nanosheets and well used for ion sieving. In order to enable GO membranes with largely tunable interlayer spacing, nanoparticles or nanowires with various sizes can be intercalated between GO nanosheets. A nanostrand-channelled GO membrane with numerous nanochannels with diameters of 3-5 nm was prepared by Huang et al. (Figure 3c), showing perfect separation performance of small molecules and ultrafast water permeation of 695 ± 20 L m-2 h-1 bar-1 (Figure 7d).63 Pressuredependent separation behavior is also shown (Figures


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7e, f). Water flux does not increase monotonically with increasing pressure, while the rejection of Evans Blue (EB) decreases with increasing pressure, and then increases again at a certain pressure.11 This is a method to get tunable interlayer spacing, and nanochannel crosssectional area could be changed by applied pressure in order to get optimized rejection rate and permeability. In order to investigate the factors controlling filtration in GO membranes, Huang et al. investigated the filtration properties by changing pH, salt concentration and pressure.115 In Figure 8a, flux rate through GO membranes decreases rapidly with increasing NaCl concentration. They found rejection rate of 85% for EB and a water flux of 71 L m-2 h-1 bar-1 for GO membranes, as shown in Figure 8b. They concluded filtration process

depends on type and concentration of charge near GO membrane. At low pH, the protonation of carboxyl acid and the increasing of ion concentration reduces the interlayer spacing, resulting in water flux decreasing and rejection rate increasing.115 At high pH, the ion-screening effect coming from the ion concentration increasing make GO nanochannels shrinking, resulting in flux decreasing and rejection increasing.115 The nanochannels can be changed by the pressure from the result of applied pressure on water flux and rejection rate of EB molecules (Figures 8c, d). These results show the separation performance can be tuned by PH, salt concentration and applied pressure, showing a way for optimizing the performance of GO membranes.


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Figure 7. (a) Solute size sieving through the GO membrane (1 M feed solution). Reproduced with permission. 52 Copyright 2014, American Association for the Advancement of Science. (b) Permeation rates through epoxy physically confined GO membranes with different interlayer spacing. (c) Permeation rates for K+ and Na+ depend exponentially on the interlayer spacing (left axis). Water permeation varied only linearly with interlayer spacing (blue squares, right axis). Reproduced with permission.68 Copyright 2017, Nature Publishing Group. (d) Water flux and rejection of EB molecules through nanostrand-channelled GO membrane as a function of pressure. The rejection rate refers to the first pressureloading process. (e) The change in the nanochannel cross-sectional area with respect to the applied pressure obtained by MD simulations. (f) The morphology changes of GO nanochannel in response to the applied pressure in MD simulations. Reproduced with permission.63 Copyright 2014, Nature Publishing Group.

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Figure 8. (a) The flux of NaCl solution through a GO membrane as a function of concentration. The inset shows the zeta potential of the GO dispersion as a function of NaCl concentration. (b) Water flux and rejection of EB molecules as a


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function of pH. (c) Water flux and rejection of EB molecules as a function of pressure. The rejection of EB molecules refers to the first pressure-loading process. (d) UV-vis absorption spectra of EB solutions filtered through GO membranes under pressures of 0.2 MPa and 0.5 MPa respectively during the second pressure-loading process. Reproduced with permission.115 Copyright 2013, Royal Society of Chemistry. h−1, respectively (Figure 9c).69 Whereas, GO-750 memVery recently, in order to reduce the interlayer spacbranes treated with KCl showed low Na+, Mg2+ and Ca2+ ing and keep this spacing against swelling when impermeation rates below the cation detection limits, mersed in aqueous solution, Chen et al. showed cationic which demonstrates ion sieving effect of the KCl-concontrol of the interlayer spacing of GO membranes with trolled GO sheets, showing ion rejection of > 99% relaångström precision using K+, Na+, Ca2+, Li+ or Mg2+ ions.69 tive to untreated GO membranes.69 Figure 9d demonThey soaked freestanding GO membranes in water, folstrates that a KCl-controlled thinner GO membrane (GOlowed by KCl solution. The corresponding interlayer 280, thickness: 280 nm) exhibited a higher water flux of spacings were 11.4 ± 0.2, 11.4 ± 0.1, 11.2 ± 0.2 and 11.2 0.36 L m−2 h−1, and effectively rejected Na+, with the per± 0.1 Å for KCl + NaCl, KCl + CaCl2, KCl + LiCl and KCl + meation rate reduced by a factor of ~ 150 compared with MgCl2, respectively (Figures 9a, b).69 the untreated GO-280 membrane.69 Overall, they preTo further show the performance, they fabricated GO cisely controlled the interlayer spacing in GO memmembranes for ion permeation. Untreated GO-750 branes, with a precision of less than 1 Å, and achieved (thickness: ~ 750 nm) membranes showed Na+, Mg2+ and expected ion rejection. Ca2+ permeation rates of 0.190, 0.025 and 0.019 mol m−2

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Figure 9. (a) Interlayer spacing for GO membranes immersed in pure water or in various 0.25 M salt solutions. (b) Interlayer spacing of GO membranes that were soaked in KCl solution, followed by immersion in various salt solutions. (c) Na+, Ca2+ and Mg2+ permeation rates of untreated and KCl-treated GO membranes. Dashed lines indicate the detection limits of the different cations. (d) Na+ permeation rates of untreated GO membranes (71.84 ± 6.75 × 10−2 mol m−2 h−1) and KCl treated GO membranes (0.48 ± 0.07 × 10−2 mol m−2 h−1) with a thickness of ~ 280 nm. In ion permeation tests for the untreated GO membranes, the feed side included 35 mL deionized water, and the draw side included 35 mL 0.25 M target salt aqueous solution (NaCl, CaCl 2 or MgCl2); the water flux was 0.85 ± 0.09 L m−2 h−1. For KCl controlled GO membranes, the feed side included 35 mL 0.25 M KCl aqueous solution, and the draw side included 0.25 M KCl with 0.25 M target salt aqueous solution (NaCl, CaCl2 or MgCl2); the water flux was 0.36 ± 0.06 L m−2 h−1. Error bars indicate the standard deviation from three different samples. Reproduced with permission. 69 Copyright 2017, Nature Publishing Group.


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6.2.2 Functional groups dominated water desalination In order to test functional groups dominated water desalination, Sun et al. used drop-casting method to prepare micrometer-thick freestanding GO membranes with laminar structure and wrinkled surface.54 As shown in Figure 10a, Na+ could pass through the GO membranes quickly, while the permeation of heavy metal salts was much more slowly. Rhodamine B (RhB) and copper sulfate molecules are almost fully blocked (Figure 10b), indicating heavy-metal salts and organic contaminates can be removed from water via GO membranes. The interlayer spacing within a fully wetted GO membranes is 3-5 nm. Small molecules and nanoparticles could pass through these 2D nanochannels. But the permeation of heavy-metal ions (e.g. Cu2+, Cd2+ and Mn2+) is much slower, even fully blocked because of the strong interactions between the heavy-metal ions and the functional groups (Figure 10c).54 Additionally, GO membranes also show selective permeation of alkali and alkaline earth cations due to the different strength of cations-π interactions.98 It is found these cations (Cl-, K+, Mg2+, Ca2+ and Ba2+) have different velocity of the transport rate, which could facilitate the separation of specific ions. As shown in Figures 10d, e, the ion permeabilities are weakened through varying the lateral size of GO flakes from several hundred nanometers to several micrometers, indicating the nanocapillaries between the adjacent GO flakes influence the permeation of ions and water molecules. Ion permeation via GO membranes indicated that alkali and alkaline-earth cations interact with the sp2 clusters via “cation-π” interactions, while transition metallic cations interact with the sp3 C-O matrix via coordination interactions.98 Additionally, the negative charges of GO membranes can generate electrostatic repulsion towards anions and electrostatic attraction towards cations.98 The GO membrane conducted by Hu et al. via LBL deposition of GO nanosheets (Figure 3b) exhibited high water flux (80-276 L m-2 h-1 bar-1), which is about 4-10 times higher than commercial membranes.65 Water flux

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does not decrease linearly with the increasing number of GO layers (Figure 10f). However, GO membranes exhibited relatively low rejection of 6-46% of monovalent and divalent salts; whereas, they showed a medium rejection of 46-66% of Methylene blue (MB) and a high rejection of 93-95% of Rhodamine-WT (R-WT). The rejection rate of monovalent and divalent salts is low but the rejection rate for organic dyes is high (Figure 10g). The rejections for NaCl and Na2SO4 both decrease with the increasing ion strength, showing ion rejection is dominated by the charge effect (Figure 10h). The electrostatic repulsion between anions and the negatively charged GO membrane weakens with the Debye length decreasing (the ion strength increasing), leading to the degradation of the ion rejection rate. They concluded the permeation through GO membranes is largely dependent on the electrostatic repulsion between the charged membrane and ions.25 Pervaporation, a new membrane separation technology for water desalination.116-119 Chemical potential between the two sides of the membrane is the driving force for mass transfer in the pervaporation process.118 Currently, the water fluxes of all these pervaporation membranes prepared by polymer, inorganic and polymericinorganic hybrid materials are low.118 GO with laminar structure and functional surface property, can work as a promising pervaporation membrane. Liang et al. fabricated pervaporation composite membranes using a vacuum filtration assisted assembly method by depositing GO films with nanochannels on functionalized polyacrylonitrile (PAN) membranes.118 GO/PAN composite membranes show great potential for desalination, exhibiting a high water flux up to 65.1 L m-2 h-1 with high rejection about 99.8% at 90 °C.118 In a brief summary, the functional groups of GO dominated molecular separation is mainly based on the strong interactions between ions and the functional groups, thus influencing the separation performance.120 Maybe we can tune the functional groups of GO membranes in order to get the optimized ion rejection rate and water permeability.121


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Figure 10. Selective mass transport through freestanding GO membrane. (a) Permeations of various salt solutions (0.1 M). (b) UV-vis absorption spectra of the RhB solution (0.1 mg mL -1) and its corresponding source and drain solutions after 3 h transmembrane permeation. (c) Schematic diagrams of GO membrane and the interaction with different ions. Reproduced with permission.54 Copyright 2013, American Chemical Society. (d, e) Schematic views for the GO laminates composed of nanometer- and micrometer-sized GO flakes respectively and the corresponding transmembrane permeations of alkali and alkaline-earth cations. Reproduced with permission.98 Copyright 2014, American Chemical Society. GO membrane performance: (f) water flux with different numbers of GO layers, (g) rejection of salts and organic dyes with different numbers of GO layers, and (h) effect of salt concentration on the rejection by the 15-layered GO membrane. The data at 0 layer are those of the polydopamine-coated membrane. All flux and rejection tests were performed under 50 psi (0.34 MPa). The rejection tests in (g) were performed with 20 mM NaCl, 10 mM Na 2SO4, 7.5 mg L-1 MB, and 7.5 mg L-1 R-WT solutions, respectively. Reproduced with permission.65 Copyright 2013, American Chemical Society.


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7. Separation performance by rGO membranes The instability of GO membranes in water is a big issue as separation membranes in aqueous media, since they will swell and disintegrate over time. Therefore, it is necessary to stabilize GO membranes.4, 21, 29, 113, 122, 123 Through reduction methods to remove the oxygen-containing functional groups, surface properties can be controlled and stability can be enhanced. To evaluate rGO for molecular separations, Lin et al. used MD simulations to reveal the defects formation during thermal reduction (Figure 11a) in monolayer rGO membranes and separation performance.91 They established the relation between rGO synthesis parameters and defect sizes, providing a possible way to control the nanopores sizes.91, 124 Figure 11b shows structures of

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rGO after thermal reduction at 2500 K, when GO flakes with various initial epoxy and oxygen concentrations or epoxy/hydroxyl ratios are used. With the increase of oxygen and epoxy concentration, bigger nanopores has been generated in rGO since more carbon is removed.124 Then, they studied rGO desalination performance (Figure 11c). Initial oxygen concentration of 17% results in complete water blocking, independent on reduction temperature and initial epoxy concentration or epoxy/hydroxyl ratio.124 When higher initial oxygen concentration (25% and 33%) are used, high water flux and salt rejection of 99% can be achieved depending on reduction temperature and initial epoxy/hydroxyl ratio, indicating reducing GO with desired starting material may result in high filtration performance.91

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* Initial epoxy/hydroxyl ratio. The letter ‘(L)’ indicates that nanopores in rGO membranes are too large and both the water

molecules and salt ions can freely pass through the rGO membrane.

Figure 11. (a) Illumination of the rGO formation. Epoxy and hydroxyl functional groups are distributed on both sides of the GO sheet. Some atoms are removed via thermal reduction, resulting in nanopores in rGO materials with different sizes. (b) Representative defective structures of rGO after reduction at 2500 K. The epoxy/hydroxyl ratio and initial oxygen concentration of GO sheets vary along the horizontal and vertical directions, respectively. (c) Performance of rGO membranes in water desalination. Reproduced with Permission.91 Copyright 2015, Nature Publishing Group. No changes were made to the copyrighted material. https://creativecommons.org/licenses/by/4.0/ Furthermore, rGO membranes can also be used for forward osmosis (FO).72 FO is driven by the osmotic pres-

sure of the draw solutions and it costs less energy. Membrane support is crucial for FO performance, but the internal concentration polarization (ICP) inevitably occurs


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within the support, which could reduce permeability.124 Recently, Liu et al. showed a facile method to create freestanding rGO membranes (thickness: 100 nm) by HI vapor for water desalination (Figure 12), which greatly alleviated the ICP.72 The water permeability of the rGO membrane reached 57.0 L m-2 h-1 when using NaCl (2.0 M) as draw solution, as shown in Figure 12a, 72 which is about five times of the commercial cellulose triacetate

membrane. The rejection rate of NaCl was also higher than those of the GO membranes without HI treatment, which was caused by the smaller rGO nanochannels.72 Water permeability showed linear increase as the draw solution concentration (0.5-2.0 M) increased, indicating the ICP was almost eliminated. The reverse NaCl flux was measured to be 0.02 mol m-2 h-1 after 12 h operation. 72

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Figure 12. FO test of the freestanding rGO membrane. (a) Variations of water fluxes of the freestanding rGO (thickness: 100 nm) and CTA membranes as a function of salt (draw solution) concentrations. (b) Comparison of the osmotic pressure profiles for the conventional supported membrane (left) and freestanding rGO (right) membrane in FO process. (c) Variations of reverse salt fluxes of the freestanding rGO and commercial CTA membranes as a function of salt concentrations. (d) Permeation rates of Na+ and Cl- and water fluxes through rGO, GO, and CTA membranes using NaCl as feed solution and ammonia as draw solution, respectively. Reproduced with permission. 72 Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. require clear theoretical predications and experimental verifications. 8. Conclusions and outlook Deep understanding of transport mechanisms under practical conditions can give rise to rational design and Separation processes based on membrane have grown optimization of GO membranes with improved separarapidly because of many merits compared with convention performance. Furthermore, due to the mechanical tional processes. Currently, most researchers are looking strength, the fabrication of a highly permeable GO memforward to finding membranes with high flux and reabrane in a large scale is one of the greatest challenges. sonable selectivity in a cost-effective way. For potential Remaining challenges like tuning the interlayer spacing membrane candidates, materials should have high selecto improve ion permeability and enhancing mechanical tivity and permeability, excellent film formation and strength and integrity in order to achieve the desirable good producing ability. GO is intriguing material because separation performance are yet to be well resolved. it has many intrinsic advantages, such as being atomiMore works need to be conducted in order to close the cally thin, and having excellent mechanical and chemical gap between theory and experiment on water/ion stability. Separation properties through GO membranes transport. Furthermore, the performance of separation via in-plane pores, interlayer spacing and functional membranes is still much lower than the theoretical pregroups assisted transport have been extensively redictions, which may be due to an ambiguous understandported. The GO membrane represents a high-flux and ing of the structure-property relations of the complex cost-effective membrane for gas separation and water nanostructures. Advancement of theoretical models is purification. The important aspects of water desalinacrucial to provide in depth understanding of the tion processes in GO membranes reported in simulations


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transport behavior in the GO membranes clearly, accompanied by the enhanced characterization of the physical nanostructures. It is also important to balance the compromise between the flux decrease and fouling rejection to further improve the antifouling capability of GO membranes. To be able to target real-life applications, significant efforts are required to develop membranes with high mechanical strength and stable performance under real operating conditions. AUTHOR INFORMATION Corresponding Author *Baohua Jia: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT Baohua Jia acknowledges the support from the Australia Research Council through the Discovery Project scheme (DP190103186). REFERENCES 1. Liu, G.; Jin, W.; Xu, N., Graphene-based membranes. Chem. Soc. Rev. 2015, 44 (15), 5016-5030. 2. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M., Science and technology for water purification in the coming decades. Nature 2008, 452 (7185), 301-310. 3. Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M., Water desalination using nanoporous single-layer graphene. Nat. Nanotechnol. 2015, 10 (5), 459-464. 4. Huang, H.; Ying, Y.; Peng, X., Graphene oxide nanosheet: an emerging star material for novel separation membranes. J. Mater. Chem. A 2014, 2 (34), 13772-13782. 5. Sun, C.; Wen, B.; Bai, B., Recent advances in nanoporous graphene membrane for gas separation and water purification. Sci. Bull. 2015, 60 (21), 1807-1823. 6. Goh, P. S.; Ismail, A. F., Graphene-based nanomaterial: The state-of-the-art material for cutting edge desalination technology. Desalination 2015, 356, 115-128. 7. Hegab, H. M.; Zou, L., Graphenek oxide-assisted membranes: Fabrication and potential applications in desalination and water purification. J. Membrane Sci. 2015, 484, 95-106. 8. Das, R.; Vecitis, C. D.; Schulze, A.; Cao, B.; Ismail, A. F.; Lu, X.; Chen, J.; Ramakrishna, S., Recent advances in nanomaterials for water protection and monitoring. Chem. Soc. Rev. 2017, 46, 6946-7020. 9. Wang, E. N.; Karnik, R., Water desalination: Graphene cleans up water. Nat. Nanotechnol. 2012, 7 (9), 552-554. 10. Raidongia, K.; Huang, J., Nanofluidic ion transport through reconstructed layered materials. J. Am. Chem. Soc. 2012, 134 (40), 16528-16531. 11. Yoon, H. W.; Cho, Y. H.; Park, H. B., Graphenebased membranes: status and prospects. Philos. Trans. A Math. Phys. Eng. Sci. 2016, 374, 2060.

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Fundamental Transport Mechanisms and Advancements of Graphene

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