3. Self-assembly of micro- and nanostructures

Due to their amphiphilic character GO sheets are valuable building blocks for preparing a variety of carbon-based nano- and microsized nanostructures by a self-assembly process. Because GO sheets are few nanometers in size, their self-assembly hierarchy proceeds to develop 1D, 2D, and3D nano- and microsized materials. It thus enables to use templates for directing the self-assembly of GO sheets into complex structures with the specific shape and morphology for a given application.

It is noteworthy that the GO sheets'self-assembly was observed to occur at interfaces such as solidliquid and air-liquid [12], and hence some hybrid metal- or metal oxide-GO nanocomposites comprise GO-coated inorganic nanoparticles.

Regarding GO self-assembled micro- and nanostructures, the presence of functional groups on the GO sheet surface promotes the assembly of nanoscale GO sheets into macroscopic 2D structures (films or fibers) and 3D bulk graphene by GO solution filtration or hydrothermal treatment. The forces that drive the self-assembly process are quite like those forces that participate in the self-assembly process of colloidal nanocrystals. Potential applications for these emergent structures are flexible fiber-type actuators, robots, motors, photovoltaic cells, and supercapacitors.

#### 3.1. Metal-rGO hybrid nanocomposites

#### 3.1.1. Broad classification of nanocomposites

Graphene and derivative materials are widely used to develop novel nanocomposites when combined with polymers and/or nanoparticles (semiconductors, metals, or metal oxides) [72]. These materials display superior physicochemical properties than those of their individual components and are currently essayed for water remediation, sensing, catalysis, photovoltaic films, materials reinforcement, and biomedical applications.

At present, a number of nanocomposites have been prepared by diverse methods and with specific physicochemical properties for biomedical [73], energy conversion, environmental and electrochemical storage [12], and miscellaneous [9] applications, as reported in recent review articles.

Among the variety of chemical and physical synthesis methods reported in the literature, we just include representative examples of the four classes of nanocomposites as described below.

According to their final morphology, rGO hybrid nanocomposites are broadly classified as supported, encapsulated, incorporated, and multilayered composites [7, 13]. Schematic representation of nanocomposites is presented in Figure 4.

#### 3.1.2. Processing methods

The primary nanostructure GO sheets and nanoparticles (metal, metal oxide, or semiconductor) to develop nanocomposite materials are mainly processed by chemical methods.

The supported-rGO nanoparticles can be prepared by either direct synthesis of inorganic nanoparticle in the rGO dispersion or by mixing of previously prepared rGO and nanoparticles colloids. In the first approach, precursors are first dissolved in a convenient solvent, and then poured into the rGO dispersion. For preparing rGO-metal or rGO-metal oxide nanocomposites, the preferred method consists in adding the metal precursors (chlorides, nitrates, etc.) and a reducing agent (vitamin C, citric acid, L-lauric acid, etc.) into a previously pristine GO colloid. The whole processing can be performed using a range of synthesis systems such as microwave oven, hydrothermal, electrodeposition, sonication, and so on.

Figure 4. Kinds of metal-rGO nanocomposites. (a) Supported rGO surface is decorated by metal nanoparticles. (b) Encapsulated nanocomposites, few or multilayers of rGO are wrapping individual or clusters of MNP. (c) Incorporated layers of rGO are intercalated by metal layers. (d) rGO sheets are present in a metal matrix.

For instance, Kim et al. [74] used ascorbic acid to simultaneously reduce GO, Pd, Pt, Au, and Ag. An aqueous GO dispersion was kept at 100C, then metal precursor and ascorbic acid solution were sequentially added. The final product consisted of rGO-supported nanosized noble metals. They used the rGO/Pd nanocomposite as catalytic material for Suzuki coupling reaction and observed that the nanocomposite catalytic activity was almost fully restored after five cycles.

3.1. Metal-rGO hybrid nanocomposites 3.1.1. Broad classification of nanocomposites

140 Graphene Materials - Structure, Properties and Modifications

articles.

3.1.2. Processing methods

films, materials reinforcement, and biomedical applications.

sentation of nanocomposites is presented in Figure 4.

oven, hydrothermal, electrodeposition, sonication, and so on.

Graphene and derivative materials are widely used to develop novel nanocomposites when combined with polymers and/or nanoparticles (semiconductors, metals, or metal oxides) [72]. These materials display superior physicochemical properties than those of their individual components and are currently essayed for water remediation, sensing, catalysis, photovoltaic

At present, a number of nanocomposites have been prepared by diverse methods and with specific physicochemical properties for biomedical [73], energy conversion, environmental and electrochemical storage [12], and miscellaneous [9] applications, as reported in recent review

Among the variety of chemical and physical synthesis methods reported in the literature, we just include representative examples of the four classes of nanocomposites as described below. According to their final morphology, rGO hybrid nanocomposites are broadly classified as supported, encapsulated, incorporated, and multilayered composites [7, 13]. Schematic repre-

The primary nanostructure GO sheets and nanoparticles (metal, metal oxide, or semiconduc-

The supported-rGO nanoparticles can be prepared by either direct synthesis of inorganic nanoparticle in the rGO dispersion or by mixing of previously prepared rGO and nanoparticles colloids. In the first approach, precursors are first dissolved in a convenient solvent, and then poured into the rGO dispersion. For preparing rGO-metal or rGO-metal oxide nanocomposites, the preferred method consists in adding the metal precursors (chlorides, nitrates, etc.) and a reducing agent (vitamin C, citric acid, L-lauric acid, etc.) into a previously pristine GO colloid. The whole processing can be performed using a range of synthesis systems such as microwave

Figure 4. Kinds of metal-rGO nanocomposites. (a) Supported rGO surface is decorated by metal nanoparticles. (b) Encapsulated nanocomposites, few or multilayers of rGO are wrapping individual or clusters of MNP. (c) Incorporated

layers of rGO are intercalated by metal layers. (d) rGO sheets are present in a metal matrix.

tor) to develop nanocomposite materials are mainly processed by chemical methods.

An interesting version of supported-rGO nanoparticles approach is one in which rGO wraps the nanoparticle. In some cases, rGO sheets conformally enwrap the nanoparticle developing a hermetically sealed multilayer coating. The resultant composite comprises GO sheets decorated with a number of nanoparticle/rGO structures. These core/shell nanostructures are of current interest, since they are protected against oxidation or degradation. Chemical methods in solution and chemical vapor deposition have successfully been used to prepare such nanocomposites. They are used as lithium storage electrodes, high performance anodes, and biomedical applications.

Other interesting approach to obtain coated-rGO nanoparticles is the aerosol encapsulation technique reported by Chen et al. [75], to coat citric acid-stabilized Ag nanoparticles. These workers used an ultrasonic system to generate an aerosol composed of GO and Ag nanostructures, which was transported into a furnace at 600C by using N2 as the carrier gas. After draying, the individual drops transformed into a sample composed of Ag/rGO microstructures. This composite could be of interest for applications in tissue engineering, magnetic resonance imaging, X-ray computed tomography, and bioimaging contrast agent.

The layer-by-layer method is a thin film deposition technique in which alternating layers are successively deposited, and a film with a multilayered structure is obtained. Techniques such as immersion, spray coating, spin coating, and electrochemical are suitable ones to deposit multilayered nanocomposite films with large interfacial area. These kinds of nanocomposites are ideal for energy storage and generation.

Jang et al. [76] reported rGO/maghemite multilayered nanocomposite preparation. The GO was exfoliated by thermal expansion in vacuum at 200C and then heated up to 300C for 5 h. The powder of exfoliated GO, iron acetylacetonate, and oleic acid were mixed together and mechanically ground with a pestle and mortar. After heating at 600C for 3 h, they obtained intercalated rGO-maghemite nanocomposite, and studied its performance as an anode material of Li ion batteries. In contrast with individually tested rGO and iron oxide samples, the nanocomposite displayed enhanced cycling stability and rate performance.

Incorporated nanocomposites have a low GO content (less than 1% vol) and are usually prepared by the ball-milling technique and a postsintering process. In this case, the properties to be exploited are mechanical and electrical properties, since they are applied in structural elements and implants.

Recently, the preparation of rGO incorporated in Al, Ni, Mg, and Cu matrices was reported [77]. It is expected that rGO sheets replace carbon nanotubes as a reinforcement material because rGO can be produced at large scale and at a lower cost. Zhang and Zhan [78] reported rGO-reinforced copper by ball milling and spark plasma sintering. They found that the presence of 0.1–1% vol rGO greatly enhances its mechanical properties (yield and tensile strengths) compared to those of pure Cu.

#### 3.2. GO thin films and membrane

Currently, the GO self-assembled micro- and nanostructures are being essayed as semiconductor in thin film transistors, transparent electrode of solar cells, active material for chemical sensing, etc. These applications require paper-like and thin film of self-assembled GO nanosheets.

There are plenty of reports on the GO self-assembly into 2D and micro- and nanostructures [79, 80]. Shao et al. [14] did an exhaustive description of the mechanistic aspects of the GO selfassembly at diverse interfaces. Herein, we present the more recent findings on 2D microstructures, including thin films.

Until now several self-assembly mechanisms to form GO thin films have been proposed. The GO thin films formation frequently occurs at liquid-air-type interfaces by evaporation and Langmuir-Blodgett assembly [81, 82]. The techniques employed are simple in almost all cases and also let the assembly on the suspension surface, for instance, dip-coating, drop-casting, spin-coating, and spray-coating for mentioning some of them.

Langmuir-Blodgett assembly leads to the formation of GO very thin films and GO single layer, so when GO is obtained, it can be dispersed in a highly volatile organic solvent in the presence of little amounts of water, and then as the solvent evaporates, the GO begins to aggregate at the interface of water-air, forming a GO monolayer. The assembled material at the interfaces can be collected later by dipping a substrate [83]. The main advantage lies in the collected GO very thin films on the substrate, and they are a source to reduce the GO into graphene sheets by either chemical or thermal treatment [19, 84].

The resulting GO films have high transmittance, high surface chemical activity, and low sheet resistance. The morphology and dispersion degree can be modified by the pH of the synthesis solution; so, the pH modulates the amphiphilic nature of GO layers as evidenced by Cote et al. [85]. The self-assembled GO films are dominated by attractive forces as van der Waals forces and π–π interaction which lead to stacking of single GO layer.

In the evaporation case, the solvent is heated to accelerate its evaporation and the agglomeration of GO sheets in the interface of water-air, any solvent with relatively low evaporation point can be used to disperse and afterward evaporate it to promote the self-assembly on the solvent surface. In this way, it is possible to obtain both thin films and membranes. Similarly, the Langmuir-Blodgett assembly and evaporation induced lead to the formation of GO membranes by staking layer-by-layer (Figure 5). Cote and Shao [85, 86] provide examples of selfassembly using the Langmuir-Blodgett and evaporation-induced mechanisms.

As started below, the GO membrane can be considered as 2D microstructures with a morphology that depends either on the attractive or repulsive interaction among individual GO layers.

#### 3.3. 1D and 2D microstructures (membranes and fibers) and 3D bulk structures

The GO sheet self-assembly can produce thin films and 2D membranes as aforementioned; however, this would occur at a liquid-air interface. If the interface is now liquid-solid type, a variety of microstructures of 2D and 3D can be obtained by self-assembly of single GO layers in the presence of a solid. The GO sheets' interactions with the solid surface involve π–π interaction, hydrogen bonding, electrostatic forces, and surface tension.

Green Routes for Graphene Oxide Reduction and Self-Assembled Graphene Oxide Micro- and Nanostructures… http://dx.doi.org/10.5772/67403 143

3.2. GO thin films and membrane

142 Graphene Materials - Structure, Properties and Modifications

tures, including thin films.

Currently, the GO self-assembled micro- and nanostructures are being essayed as semiconductor in thin film transistors, transparent electrode of solar cells, active material for chemical sensing, etc. These applications require paper-like and thin film of self-assembled GO nanosheets.

There are plenty of reports on the GO self-assembly into 2D and micro- and nanostructures [79, 80]. Shao et al. [14] did an exhaustive description of the mechanistic aspects of the GO selfassembly at diverse interfaces. Herein, we present the more recent findings on 2D microstruc-

Until now several self-assembly mechanisms to form GO thin films have been proposed. The GO thin films formation frequently occurs at liquid-air-type interfaces by evaporation and Langmuir-Blodgett assembly [81, 82]. The techniques employed are simple in almost all cases and also let the assembly on the suspension surface, for instance, dip-coating, drop-casting,

Langmuir-Blodgett assembly leads to the formation of GO very thin films and GO single layer, so when GO is obtained, it can be dispersed in a highly volatile organic solvent in the presence of little amounts of water, and then as the solvent evaporates, the GO begins to aggregate at the interface of water-air, forming a GO monolayer. The assembled material at the interfaces can be collected later by dipping a substrate [83]. The main advantage lies in the collected GO very thin films on the substrate, and they are a source to reduce the GO into graphene sheets

The resulting GO films have high transmittance, high surface chemical activity, and low sheet resistance. The morphology and dispersion degree can be modified by the pH of the synthesis solution; so, the pH modulates the amphiphilic nature of GO layers as evidenced by Cote et al. [85]. The self-assembled GO films are dominated by attractive forces as van der Waals forces

In the evaporation case, the solvent is heated to accelerate its evaporation and the agglomeration of GO sheets in the interface of water-air, any solvent with relatively low evaporation point can be used to disperse and afterward evaporate it to promote the self-assembly on the solvent surface. In this way, it is possible to obtain both thin films and membranes. Similarly, the Langmuir-Blodgett assembly and evaporation induced lead to the formation of GO membranes by staking layer-by-layer (Figure 5). Cote and Shao [85, 86] provide examples of self-

As started below, the GO membrane can be considered as 2D microstructures with a morphology that depends either on the attractive or repulsive interaction among individual GO layers.

The GO sheet self-assembly can produce thin films and 2D membranes as aforementioned; however, this would occur at a liquid-air interface. If the interface is now liquid-solid type, a variety of microstructures of 2D and 3D can be obtained by self-assembly of single GO layers in the presence of a solid. The GO sheets' interactions with the solid surface involve π–π

assembly using the Langmuir-Blodgett and evaporation-induced mechanisms.

3.3. 1D and 2D microstructures (membranes and fibers) and 3D bulk structures

interaction, hydrogen bonding, electrostatic forces, and surface tension.

spin-coating, and spray-coating for mentioning some of them.

and π–π interaction which lead to stacking of single GO layer.

by either chemical or thermal treatment [19, 84].

Figure 5. Self-assembly process into GO thin layers at the interface of liquid-air. The assembly is assisted by the solvent evaporation.

GO self-assembly driven by electrostatic forces produce several morphologies of 2D and 3D such as thin films, membranes, and capsules [87]. In this case, the ionization of COOH groups provides a negative charge distribution at the GO sheet edge. This charge distribution can be controlled by the pH of GO dispersion. An electric field applied in the GO solution is able to drift the negatively charged GO layers toward the positive electrode (solid element) dipped in the dispersion. GO sheets are agglomerated on the electrode surface and forced to assemble by staking. An interesting effect is presented during the GO sheets drift, since the GO layers that assemble are electrochemically reduced. So the electric field can remove the oxygen-based functional groups [88] and promote the assembly only by π–π interaction.

The presence of particles or nanoparticles in a GO dispersion can also drive the self-assembly, in this case foreign particles act as agglomeration centers, which destabilize the GO dispersion. This can be considered as a GO colloid in a disperse state [83, 89]. Therefore, attractive forces originated from particles overcome the electrostatic repulsion (colloidal stabilization) and lead to agglomeration and assembly.

Three-dimensional GO structures with polyhedral-like morphology were reported in Ref. [90]. In this case, the self-assembly process was observed at microscale. GO is synthesized by Hummers' method and the material is patterned on a silicon wafer. The patterned 2D GO membranes become the building blocks. A metallic frame is deposited around the GO membranes; it drives the folding of each membrane by surface tension forces. Considering the membrane size, the attractive forces as van der Waals are not manifested at this scale; therefore, surface tension forces are dominant here. These 3D cubes can be used as microcontainer of liquids and gases. Different assembly stages are shown in Figure 6, initially any material can be patterned as isolated blocks, Figure 6(a), and then the metallic frame is deposited by a photolithographic process, Figure 6(b). The metallic frame is constituted by two different metals, which linked the assembly blocks and allow the self-folding [91].

1D GO microfibers have been obtained by self-assembly of single GO layers; however, this is an example of microstructures' self-assembly at a liquid-air-type interface. As reported by Tian et al. [92], which these 1D fibers are formed by two forces combination, π–π interaction and van der Waals attractive forces are gradually manifested in the GO dispersion. These forces drive the single GO layers agglomeration toward the GO dispersion surface and staking layer

Figure 6. Self-assembly process of 3D microcubes, schematizing different stages by self-folding.

by layer. Then, progressive accumulation of GO sheets produces these 1D GO microfibers. The fibers are annealed later to obtain rGO. The authors in this case do not provide further information about the rolling up process that give place to the 1D fibers.

#### 4. Summary

Synthesis, reduction, and advanced application of graphene oxide (GO) are fast growing research areas because there exist a great variety of preparation techniques for mass production, the chemical-based ones being the most promising. For its chemical richness, chemically obtained GO is an extraordinary product in various aspects. First, it can be obtained by means of scalable, simple, and low-cost techniques, which is important for gram- or kilogram-scale applications (e.g., rGO-metal-based composites for the lithium battery anode, rGO-based foams, water cleaning, etc.). Second, it has demonstrated to be an excellent precursor material for developing advanced materials, such as graphene, graphane when treated under hydrogen atmosphere, and Teflon-like materials when fluorinated.

This chapter presents an overview on the GO reduction by green methods, on the production methods of carbon-based structures by GO sheets self-assembly, and on preparation methods of GO-based metal nanocomposites.

The so-called green methods for GO reduction demand that both, starting chemicals and byproducts, are safe to handle and environmentally friendly. Technologies such as bioreduction, photoreduction, reduction by polymers, reduction by metals, mechanochemical reduction, and electrochemical reduction fulfill both criteria.

On the other hand, the amphiphilic character of GO sheets make them valuable as building blocks for preparing a variety of carbon-based structures produced by their self-assembly, as well as hybrid nanocomposites when combined with metal semiconductor nanoparticles. The self-assembled carbon structures and hybrid nanocomposites are currently essayed for water remediation, sensing, catalysis, photovoltaic films, materials reinforcement, and biomedical applications.
