A review on two-dimensional metal oxide and metal hydroxide nanosheets for modification of polymeric membranes

Journal Pre-proof A review on two-dimensional metal oxide and metal hydroxide nanosheets for modification of polymeric membranes Mahdie Safarpour, Samira Arefi-Oskoui, Alireza Khataee

PII:

S1226-086X(19)30580-5

DOI:

https://doi.org/10.1016/j.jiec.2019.11.002

Reference:

JIEC 4844

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

2 July 2019

Revised Date:

27 October 2019

Accepted Date:

1 November 2019

Please cite this article as: Safarpour M, Arefi-Oskoui S, Khataee A, A review on two-dimensional metal oxide and metal hydroxide nanosheets for modification of polymeric membranes, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.002

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A review on two-dimensional metal oxide and metal hydroxide nanosheets for modification of polymeric membranes

Mahdie Safarpour a, Samira Arefi-Oskoui b, Alireza Khataee b,c,*

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Department of Chemistry, Faculty of Basic Science, Azarbaijan Shahid Madani University,

83714-161 Tabriz, Iran Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department

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b

of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran

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Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam

Department of Environmental Engineering, Gebze Technical University, 41400 Gebze,

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c

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Turkey

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* Corresponding author: Email: [email protected]

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[email protected]

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Graphical abstract

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Abstract

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Recently, ultrathin two-dimensional (2D) materials, known as nanosheets, have attracted great interest owning to their ultimate structural and desirable physical, chemical, thermal, electrical,

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and optical characteristics. Various potential applications of 2D nanomaterials have been reported in electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells,

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sensors, photocatalysis, and other environmental processes. Modification of polymeric membranes using nanosheets is increasingly reported in the literature. The incorporation of

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inorganic nanostructured additives, such as metal oxide and metal hydroxide nanosheets, can impart desired characteristics to the surface and matrix of the membrane and improves its

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filtration performance. This review summarizes the preparation methods of the metal oxide and metal hydroxide nanosheets, with a special emphasis on the recent advances in this field. The structural and physicochemical properties of the 2D nanosheets have been discussed, and after that, the applications of these nanosheets for modification of polymeric membranes are presented with a focus on the membranes used for environmental applications. Finally, the

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existing challenges and future perspectives for synthesis and membrane-related applications of 2D metal oxide and metal hydroxide nanosheets are discussed.

Abbreviations and symbols Term Zero-dimensional One-dimensional Two-dimensional Three-dimensional

AMOST

Aqueous miscible organic solvent treatment

CMC DMF

Carboxyl methyl cellulose dimethylformamide

DS

Dodecyl sulfate

FESEM LDH LSH MMMs MOF NF TBA TBAOH TEOA TFC TFN TMA TMAOH PAN pDA PEI PES PIP PMAA PPO PSf PVDF RATRP SBMA

Field Emission Scanning Electron Microscopy

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Layered double hydroxide Layered single hydroxide

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Mixed matrix membranes Metal organic framework Nanofiltration Tetrabutylammonium Tetrabutylammonium hydroxide Triethanolamine Thin film composite Thin film nanocomposite Tetramethylammonium Tetramethylammonium hydroxide Polyacrylonitrile Polydopamine Polyethyleneimine Polyethersulfone Piperazine Poly(methacrylic acid) Polyphenylene pxide Polysulfone Polyvinylidene fluoride Reverse atom transfer radical polymerization Sulfobetaine methacrylate

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Abbreviation/ symbol 0D 1D 2D 3D

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SEM TEM TMC UF ZHT

Scanning electron microscope Transmission electron microscopy Trimesoyl chloride Ultrafiltration Szwitterion-hydrotalcite

Keywords: Nanosheet; Metal oxide; Metal hydroxide; Polymeric membrane; Membrane

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modification; Mixed matrix membrane.

Content

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1. Introduction

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2. Preparation methods of 2D metal oxide and metal hydroxide nanosheets 2.1. Preparation of 2D metal oxides

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2.2. Preparation of 2D metal hydroxides

3. Application of metal oxide and metal hydroxide nanosheets in modification of polymeric

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membranes

3.1. Metal oxide nanosheets

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3.2. Metal hydroxide nanosheets 3.3. Functionalized metal oxide and metal hydroxide nanosheets 3.4. Nanocomposites of metal oxide and metal hydroxide nanosheets

4. Existing challenges and future perspectives 5. Conclusion

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1. Introduction

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During recent decades, along with the rapid advances in nanoscience and technology, a wide

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variety of low-dimensional nanostructures has been explored because of their potential applications in different areas. The previous classification of nanomaterials included threedimensional (3D) nanostructures, quasi-one-dimensional (1D) nanowires and nanotubes, and

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“zero-dimensional” (0D) nanoparticles and quantum dots [1]. But, the discovery of graphene introduced the two-dimensional (2D) materials which have two dimensions beyond the

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nanometric range, now generally known as nanosheets [2]. Nanosheets have essentially

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molecular thickness with large lateral dimensions. Graphene, as the most known 2D material, contains a honeycomb lattice of carbon atoms in a monolayer sheet that may have lateral sizes up to millimeters in monocrystalline form. 2D materials have very high surface-to-volume ratios compared with their corresponding 3D analogues, and show unique shape-dependent characteristics owing to their certain geometries [3]. To date, different types of 2D materials have been prepared and reported in the literature, including graphene and graphene oxide,

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layered dichalcogenides, layered oxides, layered double hydroxides (LDHs), MXenes (2D transition-metal carbide and/or nitrides), layered zeolites, 2D polymers, and 2D metal-organic frameworks (MOFs) [4]. Polymeric membranes are currently the most widely used membranes in the industry due to their straightforward fabrication methods, high flexibility, small footprints required for installation and relatively low costs [5]. However, the polymeric membranes, especially membranes used for water and wastewater treatment, suffer from some practical problems such

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as high fouling tendency, low mechanical strength, and trade-off between permeability and selectivity [6, 7]. So, the development of membranes with improved structural and practical characteristics is of great importance and interest for researchers. The incorporation of

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inorganic nanomaterials into the polymeric membrane’s matrix is one of the most reported and promising solutions to remove or reduce the weaknesses of these membranes.

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There are several review papers on the modification of polymeric membranes using different

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2D materials [8] such as graphene-based materials [9-12], zeolites [13-15] and MOFs [16-19]. However, we couldn’t find any review on the mixed matrix membranes modified with metal oxide and metal hydroxide nanosheets in the available sources. So, this review paper focuses

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on the modification of polymeric membranes using metal oxide and metal hydroxide nanosheets. For this purpose, the preparation methods of 2D metal oxide and metal hydroxide

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nanosheets will be described first and will be followed by introducing the physicochemical

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properties of these materials. After that, the application of metal oxide and metal hydroxide nanosheets in the modification of polymeric membranes will be reviewed by the classification of different types of these nanosheets. Finally, existing challenges in this field and future perspectives will be mentioned and concluded.

2. Preparation methods of 2D metal oxide and metal hydroxide nanosheets

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2D structures can be prepared by both bottom-up and top-down techniques. In the bottom-up strategy, nanosheets are synthesized by controlled assembly of atomic or molecular precursors from the solution or gas phase [20]. In most of the bottom-up techniques, a template is utilized to conduct the formation of a proper 2D configuration. This method has specific requirements and challenges to control the anisotropic growth of sheets with atomic thickness [21]. In the top-down strategy, a 3D layered material is first prepared, and then, it is delaminated or exfoliated into 2D nanosheets or platelets [20]. As an advantage of the top-down technique, the

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initial 3D layered compounds are often synthesized at high temperatures where the kinetics are fast and thermodynamic equilibrium conditions can be obtained. So, highly crystalline exfoliated 2D metal oxide and metal hydroxide nanosheets, with low structural defects will be

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prepared, like to equilibrated bulk materials and unlike the structures fabricated by most bottom-up methods [22]. In most of the metal oxide/ hydroxide systems, relatively strong ionic

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interactions are established between the crystalline nanosheet layers which usually require a

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chemical interaction to delaminate such ionic crystals into its 2D constituents and counter ions [23].

In this paper, special focus will be on the metal oxide and metal hydroxide nanosheets prepared

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by top-down approaches, although some examples of nanosheets prepared by bottom-up

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methods will also be mentioned.

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2.1. Preparation of 2D metal oxides A literature review shows that the top-down approach is the preferred method for cost-effective and scalable preparation of 2D nanosheets [24-26]. Different techniques have been reported for the synthesis of 2D nanosheets, i.e. mechanical force-assisted exfoliation and soft chemical exfoliation. During mechanical force-assisted exfoliation, a physical force is applied by solution-phase sonication to delaminate the layers of the bulk crystals by breaking the weak

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van der Waals connections between layers [27]. The applied force cannot affect the strong inplane covalent interactions of each layer. The mechanical exfoliation has been utilized for preparation of 2D metal oxides such as V2O5 [28] and MnO2 [29]. But, in most of the layered metal oxides, the interlayer interactions are strong due to the negatively charged host layers, and the solution-phase sonication cannot be effectively used for delamination. Therefore, the exfoliation of such compounds needs a proper chemical force to overcome the strong electrostatic interaction. Different 2D metal oxide structures have been prepared by chemical

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exfoliation method. A typical soft chemical exfoliation process includes four steps schematically illustrated in Fig. 1. First, the layered metal oxide parent is synthesized by usually a solid state reaction and then, an ion exchange process performs by protonation of the

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initial compound. After that, intercalation occurs by usually an organoaluminum compound,

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the monolayer metal oxide [21].

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known as osmotic swelling step. Finally, a solution-phase exfoliation is performed to obtain

Liu et al. [30] reported the synthesis of K-birnessite nanobelts and delamination of the protonated form into MnO2 nanosheets. They synthesized the parent K-birnessite nanobelts by

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hydrothermal method in a concentrated KOH solution and then, treated the K-birnessite nanobelt with a weakly acidic (NH4)2S2O8 aqueous solution for protonation of the initial

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compound. The intercalation step was promoted by tetrabutylammonium hydroxide (TBAOH)

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and tetramethylammonium hydroxide (TMAOH) aqueous solutions. Final exfoliation into MnO2 nanosheets was conducted by gentle shaking of the swollen structures at 80 rpm for 2 days. The single sheet MnO2 obtained with a morphology like its parent nanobelts in long-axis direction and lateral sizes of micrometer order. They found that the relative dose of MnCl2 and KMnO4 is a crucial synthetic parameter that affects the size and purity of the product. When Mn2+/MnO4− > 2, the products were contaminated by hausmannite particles, the amount of

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which increased as the Mn2+/MnO4− ratio increased. The hausmannite impurity formed due to the excess of Mn2+. Omomo et al. [31] also prepared the unilamellar 2D crystallites of MnO2 by swelling and exfoliation of layered protonic manganese oxide (H0.13MnO2·0.7H2O) in a TBAOH solution. Their results showed that at low amounts of TBAOH, normal intercalation occurs on the layered MnO2, making an intercalated phase with a height of 1.25 nm. At extra amounts of TBAOH, MnO2 undergoes the osmotic swelling, providing a wide intersheet space of 3.5-7 nm. At intermediate TBAOH concentrations, both intercalation and osmotic swelling

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occurs and creates a sample with broad X-ray diffraction profile. In other research, the intercalation/deintercalation behavior and interlayer distances of the 2D layered MnO2 was investigated in the presence of various metal ions [32].

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In a comparative study, the using of tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) cations were investigated in the osmotic swelling and exfoliation behaviors of a

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lepidocrocite-type titanate (H1.07Ti1.73O4·H2O) [33]. The results of this research showed that

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for both types of cations, crystallites of a few stacks were observed at the first step of the process in the zone where exfoliation predominated. But, when using TBA+, a remarkable portion of unilamellar nanosheets was found, while with TMA+, unilamellar nanosheets were

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rarely observed. However, at high reaction times, unilamellar nanosheets were finally achieved as an ultimate product for both TBA+ and TMA+. Other different metal oxide nanosheets such

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as MoO2 [34], CoO [35, 36], TaO3 [37], cesium tungstate (Cs6+xW11O36) [38], Ti2Nb2O9 [39],

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KCa2Nb3O10 [40, 41], titanium niobate and titanium tantalate (HTiNbO5, HTi2NbO7, and HTiTaO5) [42], K0.8Ti1.73Li0.27O4 [43], and SrNb2O6F− [44] were prepared by chemical exfoliation technique. It is worth mentioning that Kim et al. [45] performed research on the direct exfoliation and dispersion of 2D materials in pure water via controlling the temperature without using any chemicals or surfactants. Their results showed that the preparation of the 2D materials at high

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temperatures improves their dispersion stability in water. Also, they observed that the overall yield for materials exfoliated at low temperature (30 °C) was lower than those sonicated at higher temperatures. Since cavitation is the key factor for dispersion of the 2D materials in water, the particle dissolution yield can be increased by heating the water more and lowering the pressure, which improves the level of cavitation. The exfoliation and dispersion of the 2D materials in the pure water enable their cost-effective and large-scale applications, and facilitate

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research on the water-based experiments.

2.2. Preparation of 2D metal hydroxides

Some of the layered single hydroxide (LSH) compounds can be prepared by chemical or

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mechanical exfoliation of naturally layered lattices of metal elements networked in the layers with strong hydrogen bonding. Some others can be obtained from the insufficiency of

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hydroxide ions, creating a charged network which can be compensated by intercalation of

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counter anions [20, 46]. Different layered hydroxide compounds such as zinc hydroxide [47, 48], different lanthanide hydroxides [49-52], iron (oxy) hydroxides [53], nickel hydroxide [54, 55] and cobalt hydroxide [56] have been prepared by this method. Zhu et al. [57] successfully

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synthesized ultra-thin (∼4 nm thick) (Y0.95Eu0.05)2(OH)5NO3·xH2O (Y/Eu LRH) nanoplates in one step hydrothermal processing of mixed nitrate solution in the presence of TBAOH template.

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Layered double hydroxides (LDHs) include an inner layer of cationic metal atoms (M2+ and

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M3+) sandwiched between octahedral hydroxide layers and containing charge-balancing anions (An−) in the interlayer gallery (Fig. 2) [46, 58]. In a typical LDH, M2+ cation may be Mg2+, Fe2+, Co2+, Ni2+, Zn2+, etc.; M3+ cation can be Al3+, Fe3+, Co3+, etc.; and An− anion can be CO32−, Cl−, NO3−, ClO4−, etc. [59-61]. LDHs have attracted increasing research interest in recent years owning to their promising applications in environmental, energy-related and biomedical technologies. Among various methods reported for the synthesis of LDHs, the co-precipitation

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is the most common and preferred method. In this method, the pH of aqueous solution containing metal cations is usually elevated to around 8 ≤ pH ≤ 10 using NaOH or Na2CO3 to precipitate the metal hydroxides from solution. After precipitation of the hydroxides, nonlayered structures such as amorphous networks or “sand-rose” morphology may be formed; so, a nucleation agent can be utilized to promote the formation of a crystalline layered network [20]. The as-synthesized LDHs have a high layer charge density which is bonded strongly to anionic moieties, as well as by hydrogen bonds between hydroxyl slabs, interlayer water

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molecules and anions [20]. Therefore, more effective exfoliation techniques are required to prepare the individual nanosheets of LDHs from parent compounds [62, 63]. Ma et al. [62] and Yu et al. [64] have reviewed the exfoliation methods of LDHs into single layers with ultimate 2D

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anisotropy and nanometer thickness. The recently reported exfoliation methods of LDHs have been

summarized in Table 1. In most of these methods, the interlayer distance of LDH is increased

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by intercalating an organic modifier such as an anionic surfactant, amino acid, or lactate into

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the LDH gallery, and leads to a reduction in the layer-to-layer interactions. The weakened interlayer force can be broken in an appropriate solvent and the LDH can then be exfoliated

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into well-defined single-layer nanosheets.

Li et al. [65] introduced direct delamination of Mg-Al-NO3 LDH in formamide solvent without

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intercalation of organic compounds. Moreover, other researchers have used direct delamination

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due to its simplicity and cost-effectiveness [62]. It can be concluded from literature review that among different intercalation anions, e.g. Cl−, NO3−, ClO4−, the best yield, and exfoliation performance were achieved using nitrate anion. So, the combination of nitrate-LDH and formamide solvent under ultrasonication may be optimum for the formation of well-defined LDH nanosheets. Since the delamination of layered compounds is a time-consuming procedure, the development of rapid and time-efficient techniques for industrial-scale

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production of 2D materials is of great importance. Yuan et al. [66] reported an in-situ ion intercalation-assisted exfoliation driven by an acid-base reaction to provide a fast method for the preparation of titanate nanosheets. Their results showed that direct exfoliation of protonated layered titanate occurred within seconds after mixing of the reactants, without a swelling route.

3. Application of metal oxide and metal hydroxide nanosheets in modification of polymeric membranes

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Nowadays, polymeric membranes are the most widely used membranes in various applications due to their simple fabrication methods, tunable morphology, higher flexibility, easier installation, and relatively low costs compared to ceramic membranes [67]. However, the

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widespread utilization of the polymeric membranes is still restricted by some challenges including high hydrophobicity, low mechanical stability, trade-off between permeability and

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selectivity, and high fouling tendency. So, researches are ongoing to development of

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membranes with high antifouling ability, both improved permeability, and selectivity, as well as proper mechanical strength for energy-efficient and cost-effective operation. Mixed matrix membranes (MMMs), as a combination of polymeric membranes and inorganic materials, have

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been introduced as a promising type of advanced membranes to overcome the mentioned challenges of the polymeric membranes [68]. Among various inorganic materials, the 2D

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nanostructures have attracted considerable attention for the fabrication of MMMs due to their

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special properties such as high surface area (endowed by large lateral size and atomic thickness), and great chemical stability [69]. In the following sections, improvement of the polymeric membranes using 2D metal oxide and metal hydroxides is discussed.

3.1. Metal oxide nanosheets

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Although many metal oxides with non-layered morphology have been used for modification of polymeric membranes, there are few examples of using metal oxide with layered morphology for polymeric membranes modification. Park et al. [84] fabricated mixed matrix membranes containing MgO nanosheets and AgBF4 in a comb copolymer matrix for separation of olefin/paraffin mixtures. They synthesized 3D mesoporous MgO particles consisting of 2D nanosheets, with approximate size and thickness of 2-3 µm and 20-30 nm, respectively, by a facile non-hydrothermal method using MgCl2 as precursor and calcination at 400 °C. Results

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showed that MgO nanosheets acted as the olefin carrier and led to specific interaction with silver to increase the activity of silver. MgO nanosheets also led to enhance diffusivity due to its mesoporous nature, representing the dual-functionality of the nanosheets. The layered tin

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phosphate hydrate [Sn(HPO4)2·nH2O (SnP)] was used in the form of nanosheet and particle for preparation of organic/inorganic composite membranes of sulfonated poly(ether sulfone) [85].

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The addition of SnP improved the chemical and thermal stabilities of the membranes. Results

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showed that the effect of SnP nanosheets on the structural stabilization and proton-conducting properties of the membranes was more significant compared to SnP particles. This observation related to the networks of hydrogen bonds formed at the interface of polymer and SnP in the

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composite membranes, which was more efficient in the membranes containing larger interfacial area of SnP nanosheets.

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Stacked nanosheet membranes are another important type of separation membranes using 2D

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nanosheets. These membranes are fabricated by assembling nanosheets into ultrathin layers consist of nanochannels between the stacked sheets that allow water permeation while rejecting unwanted solutes. In a most recently published study, Nakagawa et al. [86] fabricated a stacked membrane by vacuum filtration of niobate nanosheets on the cellulose nitrate support. They prepared the niobate nanosheets by hydrothermal (HT-NbO) and exfoliation methods (EXNbO). In the hydrothermal synthesis, triethanolamine (TEOA) was used as an adsorbing-

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chelating ligand for stabilizing niobate against rapid hydrolysis in an alkaline solution. Conventional exfoliation was performed in the presence of tetra(n-butylammonium) hydroxide (TBAOH) and TBA cations were intercalated into the interlayers of the bulkier niobate. Their results showed that the membrane prepared by both types of nanosheets had a dense structure and proper structural resistance in water due to chemical interactions between sheets. However, EX-NbO membranes showed higher water permeance and lower rejection of polyethylene glycol, Na2SO4, and organic dye compared with HT-NbO membranes. Better performance of

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the EX-NbO compared with HT-NbO was related to the formation of large nanochannels in the EX-NbO. Fig. 3 shows the schematic illustration of the different pore structures created between stacked nanosheets in the HT-NbO and EX-NbO membranes. Since the formed

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channel size depends on the number of layers and interlayers, the channel size of EX-NbO membranes was considered to be larger than that of HT-NbO membranes which was confirmed

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by cross-sectional SEM images of the membranes. Liu et al. [69] also prepared an ultrafiltration

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membrane by layer-by-layer stacking of 2D kaoline nanosheets with a cationic polyacrylamide cross-linking agent, grafted on a porous cellulose acetate support. The fabricated stacked kaolin membrane showed a flux of ~ 4000 L.m-2.h-1.bar-1, several times higher than that of a

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commercial polyethersulfone ultrafiltration membrane. Despite a higher permeability, the kaolin membrane exhibited a lower fouling rate compared with commercial membranes,

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mainly due to its extremely hydrophilic nature.

3.2. Metal hydroxide nanosheets Table 2 summarizes some of the polymeric membranes modified with metal hydroxide nanosheets. Mg(OH)2 nanoplatelets were used for antifouling enhancement of PVDF microfiltration membranes [87]. Protein and E. coli adsorption investigations showed that the added Mg(OH)2 nanoplatelets prevented the formation of biofilm on the membrane surface

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resulting in the higher antifouling ability of the modified membranes. The presence of -OH groups in the modified membrane was responsible for the hydrophilicity increase and antifouling improvement. LDHs, as the most known form of metal hydroxides, have highly uniform and adjustable interlayer spacing which create a great opportunity to construct 2D channels in the membrane structure. LDHs are often used in nanoparticle form and sometimes in nanosheet form for modification of polymeric membranes. Nicotera et al. [88] prepared composite polymeric membranes based on Mg-Al LDH platelets for H2/air-fed fuel cells. They

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incorporated LDH into Nafion polymer for the fabrication of a hybrid membrane utilizable at high-temperature fuel cells. The results showed that both water absorption and diffusion improved in the modified membranes with considerable mechanical and thermal properties and

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high proton conductivity. The test of LDHs synthesized with different intercalating anions showed that the best proton conductivity obtained using LDH-NO3- composite membrane,

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whereas, very low conductivity was observed in the case of LDH-CO32- based membrane due

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to the instability of this anion in the electrochemical environment. Anion-exchange composite membranes were also fabricated using quaternized polysulfone and exfoliated Mg-Al LDH [89]. Fig. 4 shows the TEM image of the composite membrane containing 10% of Mg-Al LDH.

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The uniform dispersion and plate-like morphology of the LDH nanosheets with a thickness and lateral sizes of about 8-10 nm and 60-120 nm are observed in the TEM image. The

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nanocomposite membrane containing 5% of LDH showed lower water uptake, higher ionic

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conductivity and improved tensile strength compared to the unmodified membrane. In order to reduce the aggregation of LDH in the membrane matrix, Liu et al. [90] used exfoliated Mg-Al LDHs as the nanofiller for the fabrication of positively charged polyethersulfone (PES) ultrafiltration and nanofiltration membranes. Their results showed that Mg-Al hydrotalcite successfully exfoliated in DMAc and positively charged prepared membranes exhibited improved hydrophilicity, water flux, and rejections.

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A composite loose nanofiltration (NF) membrane was developed by chelating-assisted in-situ growth of Co-Ni LDHs for dye/salt fractionation of textile wastewater [91]. The fabrication of LDHs/polymer hybrid NF membrane (see Fig. 5) started by hydrolyzing the porous polyacrylonitrile membrane using NaOH aqueous solution to create a negatively charged surface. After that, the positively charged polyethyleneimine (PEI) with high chelating ability

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to metal ions was deposited on the pretreated membrane surface. Then, Co2+ ions were immobilized on the modified membrane by soaking the membranes at CoCl2 solution to provide a Co2+ source for the in-situ growth of Co-Ni LDHs. Final composite membranes were

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obtained by immersing the Co2+/PEI membrane into the freshly mixed solution of NiCl2 for different reaction times. It was believed that the ridge-and-valley surface structure of the

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LDHs/polymer hybrid membrane increases the steric hindrance effect and prevents the passage

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of macromolecules, but it can easily transport the salts ions and water molecules resulting in fractionation of dye/salt solutions. The optimized membrane showed a permeability of 198.6 L/(m2 h MPa), high rejection of organic dyes (methyl blue 97.9% and acid fuchsin 97.5%) and

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low salt rejection (less than 3%).

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In a recently published study, Cui et al. [92] prepared a grass-like structured composite

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membrane by nanoarray growth of NiCo-LDH onto the polydopamine (pDA) modified PVDF for oil-water emulsion separation (Fig 6). They controlled the surface morphology of the composite membrane by the hydrothermal time of LDH synthesis. The obtained modified membranes showed superhydrophilicity/underwater superoleophobicity for the separation of both surfactant-free and surfactant-stabilized oil-water emulsions by capillary effect. Also, the composite membranes displayed supreme cyclic performance for long-term operation due to

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the improved antifouling properties and low adhesion of oil molecules on the micro-nano grasslike structured LDH layers. Ma et al. [93] also grafted poly(methacrylic acid) (PMAA) on the PVDF membrane via plasma inducing graft copolymerization as anchor sites for binding heterogeneously tailored LDH nanosheet. The nanosheets were grafted with alternately arranged sodium 1-dodecanesulfonate (SDS) and 3-aminopropyltriethoxysilane (APTS) chains during preparation. So, the surface-tailored LDH nanosheets could be bound to the PMAA grafted membrane with a simple dip-coating operation and cross-linking between the amine

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groups of APTS and carboxyl groups of PMAA. Finally, a membrane with heterogeneous and infiltration selective (due to steric exclusion) surface was created due to the presence of long hydrophobic SDS and short hydrophilic APTS chains on the nanosheets. The formation of such

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hydrophilic moieties on the membrane surface provides superhydrophilic but oleophobic

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characteristics.

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LDHs were also used for modification of thin film composite (TFC) membranes which consist of a dense ultrathin selective layer on top of a microporous support [4, 94]. Tajuddin et al. [95] used Cu-Al LDH as a nanofiller in the TFC nanofiltration membranes fabricated by interfacial

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polymerization. The obtained thin film nanocomposite (TFN) membranes showed a relatively smooth surface and a less nodular structure as well as enhanced surface hydrophilicity

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compared with the bare TFC membrane. They related the smoother surface of the modified

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membranes to the lower diffusion rate of piperazine (PIP) and trimesoyl chloride (TMC) monomers during the interfacial polymerization process in the presence of Cu-Al LDH nanosheets which leads to lower surface roughness. Moreover, the incorporation of Cu-Al LDH improved salt rejection (Na2SO4 (96.8%), MgCl2 (95.6%), MgSO4 (95.4%), and NaCl (60.8%)) and antifouling ability of the TFN membranes. Lu et al. [96] fabricated TFC forward osmosis membranes containing Mg-Al LDH nanoparticles embedded in the polysulfone (PSf) support.

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They reported an improvement in the hydrophilicity, overall and surface porosity, mechanical strength and thermal resistance of the nanocomposite membranes. It was also concluded that the incorporation of LDH nanosheets led to the formation of a layer-structure consist of one finger-like top layer and a bottom layer containing both ellipsoidal macrovoids and spongelike morphology in the cross-section of the membranes. In another noteworthy study, the effect of LDHs location on the structural and separation properties of the TFC forward osmosis membranes was investigated [97]. For this purpose, four different forward osmosis membranes

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including TFC (polyamide layer on PSf support), TFC-LDH (polyamide layer on PSf support containing LDHs), LDH@TFC (LDH deposited on polyamide layer on PSf support) and LDH@TFC-LDH (LDH deposited on polyamide layer on PSf support containing LDHs) were

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fabricated (Fig. 7). The forward osmosis performance of the prepared membranes is shown in Fig. 8. Among four types of the membranes, the LDH@TFC-LDH membrane showed the best

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chlorine resistance and antifouling properties.

Exfoliated Mg-Al-Fe LDH was used for the preparation of mixed matrix PES membranes for adsorptive removal of phosphate and fluoride from aqueous solutions [98]. In this study, a

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mixed formamide/dimethylformamide (DMF) solvent was utilized for the simultaneous exfoliation of LDHs and dissolution of PES. Since the exfoliation of LDHs was time-

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consuming in the mentioned mixed solvent (more than 24 hr), carboxyl methyl cellulose

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(CMC) was added as a stabilizer to facilitate the exfoliation and stabilization of the LDHs. This method decreased the exfoliation time of the LDHs to 2 h and increased the stability of the exfoliated sample to more than 2 days. Results showed that the well-exfoliated LDHs/PES membrane had much higher phosphate adsorption capacity and faster adsorption rate compared to un-exfoliated LDHs/PES membranes. Another adsorptive membrane was prepared by intercalation ethylenediaminete-traacetic acid (EDTA) into Mg-Al LDH and subsequent

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encapsulation into polyacrylonitrile (PAN) polymer matrix for Cu(II) removal from wastewater [99]. The obtained composite membrane had the combined advantages of LDH@PAN nanofiber membrane (high surface area, facile separation, and no aggregation and loss) and EDTA (high chelating ability).

3.3. Functionalized metal oxide or metal hydroxide nanosheets The 2D nanostructures are good candidates for attachment of different functional groups due

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to their large surface area. Different surface and/or structural properties can be provided by the functionalization of the nanosheets and incorporation of them into polymeric membranes. Zwitterionic functionalized Mg/Al hydrotalcite nanosheets were used for the fabrication of

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charged mosaic membranes with high salt permeability and high rejection of low molecular weight organics [112]. Mg/Al hydrotalcite nanosheets were first positively charged and then,

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sulfobetaine methacrylate (SBMA) grafted on the surface via surface initiated reverse atom

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transfer radical polymerization (RATRP). The resultant zwitterion-hydrotalcite (ZHT) nanosheets were added to the casting solution of the PES membranes with different concentrations. Fig. 9 shows the schematic separation mechanism of the prepared pristine and

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composite mosaic membranes. The charged composite membranes presented high dye rejection (86.7% for reactive red 49), excellent salt permeation, and high water flux (80.2 Lmh-1 at 0.4 MPa) indicating their potential application for dye/salt fractionation.

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2

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Functionalized LDH was applied for the modification of hydroxide exchange membranes used in alkali membrane fuel cells [113]. Want et al. functionalized the hexagonal LDH with Nspirocyclic ammonium (ASU-LDH) to increase surface-ionic charges and electrorheological effect of the ASU-LDH and enhance ion conductivity of the related membranes. The asobtained ASU-LDH composite was added to the polyphenylene oxide (PPO) solution for the fabrication of ion exchange membranes. The prepared ASU-LDH/PPO solution was finally

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fixed in an electric field to conduct the directional alignment of the ASU-LDH composite and formation of the aligned membrane. Fig. 10 displays schematically the alignement of ASULDH under electric field.

3.4. Nanocomposites of metal oxide and metal hydroxide nanosheets Different nanocomposites containing 2D nanomaterials have been reported for the modification of polymeric membranes. Nair and Jagadeesh Babu [114] fabricated a

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hierarchical membrane containing a thin layer of TiO2 nanosheet-graphene oxide photocatalyst for water purification. They used the nanosheet form of TiO2 to enable the stable layer formation of the thin sheet structure during deposition on the surface of the cellulose acetate

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membrane. The sheet structure of graphene oxide can also increase the stability of the coating layer and acts as a support for interconnecting TiO2 nanosheets. Fig. 11 shows the microscopic

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images of the nanosheets and the modified membrane. The results showed that at low catalyst

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loadings, the photodegradation is the main treatment mechanism of the dye, while at high catalyst concentration, the dye adsorption is more important. It should be noted that the permeate flux of the membranes dropped seriously at high catalyst loading due to the additional

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cake resistance of the catalyst coating. A similar photocatalytic membrane was prepared by

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embedding Ag-TiO2 nanosheet for water treatment under solar radiation [115].

Charradi et al. [116] incorporated sepiolite clay/LDH nanocomposite in the Nafion membrane to operate at high temperature fuel cells. They prepared the Mg-Al LDH-sepiolite hybrid by co-precipitation method and dispersed it in the polymeric matrix up to 6 wt.%. In this hybrid, the sepiolite fibers create a large external surface area for attachment of LDH particles resulting in higher water retention, better thermal properties and enhanced proton conductivity of the

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modified membranes compared with pure nafion membrane. However, the mechanical properties of the membranes slightly decreased after embedding of the nanofiller. Zhang et al. [117] synthesized a heterostructured filler by in-situ growth of ZIF-8 on the Zn-Al LDH surface (ZIF-8@LDH), and incorporated it into Pebax to fabricate mixed matrix membranes for gas separation. The resultant composite membranes showed significant enhanced CO2 separation due to the favorable affinity of ZIF-8 and numerous hydroxyl groups of LDH toward CO2 which increases the physical selectivity of CO2/CH4. In addition, the oriented distribution of

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ZIF-8 allows higher CO2 concentration around LDH moiety, facilitating the CO2 transport and thus the chemical selectivity. Moreover, the layered structure of the LDH prevents the agglomeration of ZIF-8 particles at high concentrations. Sun et al. [118] fabricated a composite

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membrane containing anionic graphene oxide and cationic Co-Al (or Mg-Al) LDH nanosheets for highly selective ion transport. Ghalamchi et al. [119] used Ag3PO4/ZnAlCu-LDH

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nanocomposite for modification of PES microfiltration membranes. They reported an

owing

to

the

improved

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improvement in the flux, wettability and antifouling properties of the modified membranes hydrophilicity

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composite

membranes.

Exfoliated

hydrotalcite/graphene oxide hybrid nanosheets were dispersed in aqueous phase solution of the

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interfacial polymerization for the fabrication of nanofiltration membranes [120]. The result showed that the incorporation of hybrid nanosheet into the polyamide layer of the NF

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membrane could remarkably increase its water flux and surface positive charge.

4. Existing challenges and future perspectives 2D nanostructures, known as nanosheets, possess versatile properties due to their unique structural characteristics that enable the preparation of advanced 2D materials and devices. 2D metal oxide and metal hydroxides have supreme mechanical stability, flexibility, high active surface and optical transparency owing to their atomic thickness and strong in-plane covalent

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bonds. Their bulk compounds mostly lack these specifications. Owing to the recent developments in the high yield and time-efficient synthesis of crystalline 2D metal oxide and metal hydroxide nanosheets, there is a promising potential for scale-up and practical applications of these materials in different fields. 2D metal oxide and metal hydroxide nanosheets are interesting candidates for designing high-performance separation membranes, either as free-standing membrane materials or in combination with polymeric membranes. The latter one, composite polymeric membranes, has attracted increasing attention in recent years

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due to the several advantages of incorporating metal oxide and metal hydroxide nanosheets including 1) simple preparation methods and low process cost; 2) uniform and tailorable thickness and gallery height; 3) large surface area with functionalizing potential; 4) good

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exfoliation capability; 5) excellent antifouling property; 6) easy scale-up and 7) remarkable stability [121]. The main reported applications of polymeric membranes modified with 2D

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metal oxide and metal hydroxide nanosheets are gas separation, ultrafiltration, nanofiltration,

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forward osmosis, membrane adsorption, oil/water separation, and fuel cells. Although worthy progress has been achieved in the gas/liquid separation processes using metal oxide and metal hydroxide nanosheets modified membranes, further researches are needed to

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commercialize these membranes and industrial use of them. One of the major challenges facing these membranes is accurate control of the interlayer spacing for specific separation. In

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addition, since the separation performance of the membranes containing 2D metal oxide and

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metal hydroxide nanosheets strongly depends on the microstructure of the nanosheets (such as preferred orientation, grain boundary defects and thickness of sheets), simple and cost-effective methods for careful control of their microstructures should yet to be developed. In the case of metal oxides, the shape and size of oxide nanosheets are often not uniform and new synthesis and fabrication techniques are needed to solve this issue. And finally, the uniform dispersion of the inorganic nanosheets in the organic matrix of a polymeric membrane is still a remaining

22

challenge. Careful dispersion of the nanosheets is needed to avoid the breaking of them into smaller fragments under too large shear forces, especially for very thin nanosheets with subnanometer thicknesses.

5. Conclusion In this review paper, the application of two-dimensional (2D) metal oxide and metal hydroxide nanosheets for modification of polymeric membranes was discussed. The structural properties

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and preparation methods of metal oxide and metal hydroxide nanosheets were briefly reported with a special focus on the top-down preparation approaches. The application of metal oxide and metal hydroxide nanosheets for modification of polymeric membranes was discussed in

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separate sections. The functionalized metal oxide and metal hydroxide nanosheets and nanocomposites containing these nanosheets have been also used for modification of polymeric

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membranes which were reviewed in this paper. The main effects of metal oxide and metal

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hydroxide nanosheets in the polymeric membranes structure can be summarized as hydrophilicity improvement, permeation increase, fouling reduction and selective transport. These advantages provide a promising perspective for the application of membranes containing

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metal oxide and metal hydroxide nanosheets in different areas from fuel cells to water and

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wastewater treatment.

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Declaration of Interest Statement We would like to confirm that there is no known conflict of interest associated with this publication.

Acknowledgements

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Authors thank the University of Tabriz and Azarbaijan Shahid Madani University for all the support. We also acknowledge the Iran National Science Foundation (INSF) for supporting

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under grant number of 97017831.

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Fig. 1. Schematic illustration of the typical soft chemical exfoliation process for the

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preparation of metal oxide nanosheets.

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ur

na

Fig. 2. Schematic model of LDH structure.

Fig. 3. Schematic diagram of nanochannels in HT-NbO and EX-NbO membranes. Reprinted from Nakagawa et al. [86] with permission from Elsevier.

33

ro of

Fig. 4. TEM image of composite membrane containing 10 wt.% Mg-Al LDH.

Jo

ur

na

lP

re

-p

Reprinted from Liu et al. [89] with permission from Elsevier.

Fig. 5. The fabrication process of loose LDHs/PEI hybrid NF membrane. Reprinted from Zhao et al. [91] with permission from Elsevier.

34

ro of

Fig. 6. Preparation scheme of grass-like structured NiCo-LDH/PVDF composite membrane.

na

lP

re

-p

Reprinted from Cui et al. [92] with permission from Elsevier.

Fig. 7. TFC forward osmosis membranes with different LDHs locations. (a) TFC, (b) TFC-

with permission from Elsevier.

Jo

ur

LDH, (c) LDH@TFC and (d) LDH@TFC-LDH membranes. Reprinted from Lu et al. [97]

35

ro of

Fig. 8. Forward osmosis performance of TFC membranes with different LDHs locations (1 M

-p

NaCl as draw solution and DI-water as the feed solution). Reprinted from Lu et al. [97] with

ur

na

lP

re

permission from Elsevier.

Fig. 9. Schematic separation mechanism of the pristine and composite mosaic membranes.

Jo

Reprinted from Wang et al. [112] with permission from Elsevier.

36

Fig. 10. Schematic of aligned ASU-LDH nanosheets under the applied-electric field.

ur

na

lP

re

-p

ro of

Reprinted from Wang et al. [113] with permission from Elsevier.

Jo

Fig. 11. a) FESEM and, b) TEM image of TiO2 nanosheets; c) FESEM image of TiO2 nanosheet-graphene oxide, and d) cross-sectional SEM image of the modified membrane. Reprinted from Nair and Jagadeesh Babu [114] with permission from Elsevier.

37

Table. 1. A number of methods used for exfoliation of LDHs. Intercalated anion

Exfoliation method Refluxing [Zn-Al-DS] LDH in butanol at 120 °C for 16 h Ultrasonication of LDH dispersed in CCl4 and toluene Stirring/heating/sonicating in alcohols Exfoliation with high shearing in acrylate monomers at 70 °C Exfoliation in nonpolar xylene solution of polyethylene Delamination in formamide (HCONH2) solvent at room temperature

Ref.

Zn-Al

Dodecyl sulfate

Mg-Al Zn-Al

Dodecyl sulfate Dodecyl sulfate

Mg-Al

Dodecyl sulfate

Zn-Al

Dodecyl sulfate

Mg-Al

Glycine

Cd-Cr Zn-Cd-Cr

Dodecyl sulfate

Delamination in formamide at room temperature

[76]

Mg-Al

Dodecyl sulfate

Synthesis using a reverse microemulsion method, introducing a traditional coprecipitation system into an oil phase of isooctane

[77]

Co-Al

Nitrate, perchlorate, acetate, lactate, dodecyl sulfate, and oleate

Agitating vigorously in formamide solvent in a mechanical shaker for 2 days

[78]

Mg2Al-Cl

-

Ni-Al

Nitrate

Co-Al

Lactate

[71] [72] [73] [74] [75]

-p

re

[79] [80] [81]

Ultrasonication in cyclohexanone

[82]

Mechanical liquid-exfoliation in formamide

[83]

Jo

ur

na

Ni-Al

lP

Mg-Al

Reverse microemulsion composed of toluene, isopropanol, and an aqueous solution as the dispersed phase Synthesis by acetone mediated aqueous miscible organic solvent treatment (AMOST) Delamination by shaking in water at ambient temperature

Nitrate, dodecyl sulfate and dodecylbenzene sulphonate Nitrate

[70]

ro of

LDH

38

Table 2. Reported polymeric membranes modified with metal hydroxide nanosheets. Membrane material Poly(vinylidene fluoride

Metal hydroxide

Application

Ref.

Phase inversion

Mg(OH)2

Microfiltration

[87]

Nafion

Solving casting followed by acid activation

Mg-Al LDH

Fuel cell

[88]

Polysulfone

Solution casting

Mg-Al LDH

Fuel cell

[89]

Solution casting

Mg-Al LDH

Fuel cell

[100]

Electrostatic spraying

Mg-Al LDH

Fuel Cell

[101]

Solution casting

Mg-Al LDH

Fuel Cell

[102]

Solution casting

Zn-Al LDH

Fuel Cell

[103]

Interfacial polymerization

Mg-Al LDH

Forward osmosis

[96, 97, 104]

Interfacial polymerization/ Phase inversion

Mg-Al LDH

Forward osmosis

[105]

Cu-Al LDH

Nanofiltration

[95, 106]

Mg-Al LDH

Removal of pharmaceutical environmental contaminants

[107]

Co-Ni LDH

Dye wastewater treatment

[91]

Interfacial polymerization

-p

Sulfonated polysulfone Polyamide/ Polysulfone (TFC) Polyamide/ polyvinyl chloride (TFC) Polyamide/ Polysulfone (TFC)

re

Nafion

lP

Sulfonated poly(ether ketone) Poly(2,6-dimethy l-1,4phenylene oxide)

ro of

Preparation method

Phase inversion

Polyacrylonitrile

Chelating-assisted in-situ growth of LDHs/polymer hybrid layer

Polyvinylidene fluoride

Phase inversion

Ni-Co LDH

Cellulose

Microwave hydrothermal processing

Zn-Al LDH

Polyvinylidene fluoride

Phase inversion

Mg-Al LDH

Ultrafiltration

[93]

Polyethersulfone

Phase inversion

Zn-Al LDH

Ultrafiltration

[109]

Polyethersulfone

Phase inversion

Mg-Al LDH

Ultrafiltration

[90]

Polyvinylidene fluoride

Phase inversion

Nano-layered Mg-Al LDH

Ultrafiltration

[59, 110, 111]

Jo

ur

na

Cellulose acetate

39

Oil-water emulsion separation Oil removal from oily water

[92] [108]

Phase inversion

Mg-Al-Fe LDH

Adsorption of phosphate and fluoride

[98]

Polyacrylonitrile

Electrospinning

Mg-Al LDH

Cu(II) adsorption

[99]

Jo

ur

na

lP

re

-p

ro of

Polyethersulfone

40