CHARACTERIZATIONS

Some AFM and SEM results from a prepared GO hybrid material indicate that the average flake size was about 2 pm. The average thickness results of the GO flake was ~1 nm as presented in Figure 4.5a. The average size was

Schematic depiction of the formation of

FIGURE 4.3 Schematic depiction of the formation of: (a) magnetic chitosan; and (b) MCGO. Reprinted with permission from Lulu Fan et al. [26]. Copyright (2012) Elsevier.

Synthesis of MCGO and their application for adsorption of methylene blue (MB) with the use of an external magnetic field. Reprinted with permission from Lulu Fan et.al [26]. Copyright (2012) Elsevier

FIGURE 4.4 Synthesis of MCGO and their application for adsorption of methylene blue (MB) with the use of an external magnetic field. Reprinted with permission from Lulu Fan et.al [26]. Copyright (2012) Elsevier.

AFM image of (a) GO; and (b) GONH Reprinted with permission from Surech Kumar et al. [22]. Copyright (2014) American Chemical Society

FIGURE 4.5 AFM image of (a) GO; and (b) GONH Reprinted with permission from Surech Kumar et al. [22]. Copyright (2014) American Chemical Society.

~6 nm as presented in Figure 4.5b, and refers to the NP grown on the surface of GO as measured by AFM. Furthermore, XRD patterns of GO, were confirmed using Cu Ka radiation (X= 1,542 A). Figure 4.6a presents the GO-XRD pattern with a diffraction peak at scattering angle 20=9.4°, which was attributed

XRD pattern of GO

FIGURE 4.6 XRD pattern of GO: (a) NP; (b) GONH; and (c) and typical FESEM image of GONH (inset of (c)). Reprinted with permission from Surech Kumar et al. [22], Copyright (2014) American Chemical Society.

to (001) plane with an interlayer separation ~9.5 A. Figure 4.6b presents the XRD pattern from the NP [22].

The Debye-Scherrer equation was used to calculate the average particle size, which seems to be ~ 11 nrn (corresponding to (311) line). Figure 4.6c displays the XRD pattern of the GONH. revealing both the NPs and the GO flakes' diffraction peaks. It shows that during the experimental preparation of the nanohybrids, because of the partial reduction of the graphene oxide, the consequent decrease in the peak due to (001) reflection plane. The diffraction peaks corresponded to - Fe202 and - Mn02. on the surface of the NPs were absent in the XRD pattern, which confirms the formation of NPs and GONH. XRD analysis pictures displayed that the average size of the nanoparticles was ~7.5 nm.

The peak of GO, in the XRD pattern of the nanohybrids, was very much reduced due to the fact, that the NP grown on the epiphany of graphene oxide averts its restacking. Furthermore, the fact that the size of the NPs was decreased could be attributed to the reason that on the side of the NPs, growth was blocked in the case of in situ growth onto the graphene surface. Figure 4.6c presents the typical SEM surface micrograph of the synthesized nanohybrid. The coverage of the NPs on graphene was confirmed to be uniform for different samples [22].

Figure 4.7a presents the FTIR spectrum of GO, where the characteristic absorption peaks were observed at 1236, 1046, 1415, 1620 and 1729 cm'1, which can be attributed to the epoxy C-O stretching vibrations, alkoxy C-O stretching, O-H deformation, and due to adsorbed water molecules, C=C in-plane stretching vibrations or C=0 stretching, respectively.

All of these measurements, clearly confirm the formation of GO and the appearance into the graphene skeleton of oxygenated functionalities, which have been used to the experimental process for the growth of magnetic NPs. Furthermore, the pH of the mixture affects the charge to the - OH and - COOH groups. Figure 4.7b displays the absorption peaks of NP at 577 and 490 cm'1, due to manganese ferrite (metal О stretching vibrations).

FTIR measurements confirm the formation of MnFe204 NPs. Figure 4.7c presents the FTIR spectrum of the GONH and presents the characteristic peaks of both NP and GO. These results confirm the successful preparation of the nanohybrids [22].

FTIR spectra of

FIGURE 4.7 FTIR spectra of: (a) GO; (b) NP; (c) GONH. Reprinted with permission from Surech Kumar et al. [22]. Copyright (2014) American Chemical Society.

Some interesting microscopic images can also be a tool for the examination of the surface of nanohybrid graphenes. Figure 4.8 depicts the low and high magnification images of GO, Fe304/rG0, Ppy/rGO and Ppy-Fe304/rG0. As presented in Figure 4.8a, the prepared GO was sheetlike in shape morphology, with a slick surface and single layer structure with wrinkled edges. The formation of loosely packed Fe304 nanoparticles onto rGO sheets, as displayed in Figure 4.8b, was achieved while a big amount of granular particles attach on the rGO surface after the addition of Fe3+. The diameter of the spherical Fe304 nanoparticles was 50-80 nm. Figure 4.8c presents the rough surface morphology of the Ppy decorated in the rGO sheets. In Figure 4.8d the ternary hybrids are observed [24].

PREPARATION METHODS OF GO MEMBRANES

Based on stable aqueous dispersity, as well on as the high aspect ratio structure of GO, GO membranes can be easily fabricated via different methods such as the filtration-assisted method, casting/coating-assembly method, and layer-by-layer (LbL) assembly method. Additionally, the evaporation-assisted method, templating method, shear-induced alignment method, and hybrid method are also applied to prepare GO membranes. The different preparation methods for GO membranes are detailed in Table 4.1

SEM images of (a) GO; (b) Fe^O^rGO; (c) rGO/Ppy; and

FIGURE 4.8 SEM images of (a) GO; (b) Fe^O^rGO; (c) rGO/Ppy; and (d) Ppy-Fe304 /rGO composites. The inset is the corresponding lower magnification images. Reprinted with permission from Hou Wang et al. [24]. Copyright (2015) Elsevier.

Methods for the preparation of GO membranes

TABLE 4.1

Method

Description

Note

Filtration-

assisted

Vacuum filtration Pressure filtration

Good nanoscale control over the membrane thickness; laminar structure of GO membranes is dictated by the filtration force; highly scalable

Casting/

coating-

based

Spinning-casting

/coating

Drop-casting

Dip-coating

Spray-coating

Doctor blade-casting

Nonuniform deposition of GO nanosheets; poor control over the membrane thickness; producing highly continuous GO membranes; highly scalable

LbL

assembly

Layer-by-layer

assembly

Easily control of the GO layer number, packing, and thickness

Others

I lybrid approach

Evaporation- assembled method Tcmplating method Langmuir-Blodgett (LB) assembly Shear-alignment method

Easily control of the GO assembly, industrial-scalability, rapid throughput.

Scale-up, easily control of the membrane thickness and size

Producing highly uniform, close-packed monolayered GO membrane

Scale-up, industrial-scalability, producing large-area GO membrane, rapid throughput

Filtration-Assisted Method

The filtration-assisted method, including vacuum filtration and pressure-assisted filtration, is a widely used approach to prepare GO membranes at present, especially for the free-standing GO membranes. Dikin et al. [27] fabricated a freestanding GO membrane by vacuum filtration, in which GO nanosheets were bonded together in a near-parallel manner. They reported that the physicochemical property of GO nanosheets did not change during the preparation process. Tsou et al. [28] investigated the influence of GO membrane structure prepared via three distinct self-assembly methods (pressure-, vacuum-, evaporation-assisted technique) on membrane separation performance. Results showed that the GO membrane obtained via pressure-assisted technique exhibited exceptional PV performance and superior operating stability at a high temperature (70°C) due to its dense packing and highly ordered laminate structure. In another study, a highly ordered GO/mPAN (modified polyacrylonitrile) composite membrane was prepared via pressure-assisted self-assembly (PASA) technique [29]. The resultant GO/mPAN composite membrane exhibited excellent PV performance for an isopropyl alcohol (IPA)/water mixture. They reported that the membrane thickness could be readily adjusted by changing the concentration and volume of GO solutions. From the aforementioned discussion, we can conclude that filtration-assisted method allows reasonable and easy control over the membrane thickness and microstructure, and is a potential route for large-scale preparation of GO membrane.

Casting/Coating-Assisted Method

At present, many GO membranes have been developed based on casting/coating- assembly method, which includes drop-casting, dip-coating, spraying-coating /casting, and spin-coating approaches. Park et al. [25] fabricated several layered GO membranes via spin-coating method on a polyethersulfone (PES) substrate and studied their gas separation performance. They reported that high gas separation selectivity could be achieved by controlling gas flow channels through adjusting stacking manner of GO nanosheets. Robinson et al. [30] proposed that large-area and ultrathin GO membranes with excellent mechanical properties could be obtained by a modified spin-coating method. In this procedure, dry nitrogen was utilized to accelerate GO solution evaporation, which correspondingly obtained continuous GO membranes with strong interfacial adhesion force between GO nanosheets and substrate surfaces. Meanwhile, membrane thickness could be controlled on nanometer scales through varying GO concentration in solution or volume of GO suspension. Individual GO nanosheets within GO membranes fabricated by casting/coating-assembly method are strongly held together with hydrogen bonding and van der Waals force.

Layer-by-Layer Assembly Method

Recently, LbL assembly approach has been attracting great attention for the preparation of GO membranes. An interlayer stabilizing force can be conveniently introduced into laminate GO membranes by electrostatic interaction or covalent bonding through this method. Hu et al. [31] have developed a new type of water purification membrane through this approach. The negatively charged GO nanosheets were interconnected with positively charged poly (ally- larnine hydrochloride) (PAH) via electrostatic interaction and then assembled onto a porous PAN support. Results showed that the resultant GO membrane reserved a compact structure in solutions of low ionic strength and showed excellent separation performance. Typically, the membrane thickness can be easily adjusted by changing the number of LbL deposition cycles [31].

Other Methods

Apart from the aforementioned methods for the preparation of GO membrane, some novel preparation methods such as the evaporation-assisted method, tem- plating-assisted method [32], Langmuir-Blodgett assembly method [33, 34], hybrid method [55], and shear-induced alignment method [56] have also been utilized to fabricate GO membranes. Recently, facile engineering of GO membranes was realized via a hybrid approach by Guan et al. [35], in which spray-coating and solvent evaporation-induced assembly techniques were included. The team reported that the membrane structure could be finely and conveniently manipulated by adjusting the spraying times and evaporation rate. The resultant GO membranes with ordered and compact structure presented excellent gas separation performance, which exceeded the upper bound of most polymeric membranes. Specifically, this process was less time consuming and more productive compared with the filtration method. This study provided a rather facile and productive approach for large-scale preparation of defect-free GO membranes.

Chen et al. [36] fabricated large-area free-standing GO membranes via an evaporation-driven self-assembly method. They reported that the thickness and area of the membrane could be readily adjusted by controlling the evaporating time and the liquid/air interface area. This is a facile and scale-up approach for preparation of GO membranes. Akbari et al. [56] provided a rapid, scalable, and industrially adaptable method, shear-induced alignment method, to produce large-area GO-based membranes by taking advantage of the flow properties of a discotic nematic GO fluid. The resultant membranes had a large in-plane stacking order of GO sheets and showed remarkable enhancement in water permeability with comparable or better retention of small organic molecules and ions by molecular sieving and electrostatic repulsion. Meanwhile, the obtained membranes showed good stability in aqueous environments and excellent fouling resistance due to the hydrophilic groups on GO membranes. This shear-alignment processing method is conducive to bridging laboratory curiosity to industrial productivity for GO membranes.

From the previous description, it can be concluded that various methods have been developed and utilized to fabricate GO-based membranes. Specifically. it should be noted that the structure and separation performance of the resultant GO membranes significantly depend on the fabrication method and corresponding fabrication conditions. Hence, in a specific practical application, a desired GO membrane can be obtained by appropriate preparation methods and optimized fabrication conditions.

 
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