**2. Efficient fabrication of single layer graphene oxide nanosheets**

### **2.1. Rheologically driven exfoliation of graphite oxide**

Conventional sonicators vigorously destroy the structure of GO, which results in producing small-sized GO nanosheets due to acoustical wave agitation in solution. An alternative way to minimize the destructive effect of exfoliation of graphite oxide is to use a homogenizer to apply a shear force in the solution (**Figure 2(A)**). The average lateral size of sonicated GO (SGO) nanosheets (a few square micrometers) was smaller than that of the homogenized GO (HGO) nanosheets (a few hundred square micrometers) in the optical images in **Figure 2(B)** and **(C)**. The SGO nanosheets exhibited some agglomerated GO on the silicon substrate due to the small size distribution of the sheets. To confirm the exfoliation effects of HGO and SGO sheets, we carried out homogeneous dispersion of GO sheets in aqueous solution without using small size graphite powder (70 μm). The rheologically derived or sonicated exfoliation and dispersion of GO sheets was accomplished (**Figure 1(d)**) in an aqueous NaOH solution at pH 10 for 1 h. After diluting it using dimethylformamide (DMF), the RGO solution was prepared by chemical reduction with hydrazine for preparing transparent conducting films. The enhanced sheet resistance of the reduced HGO (HRGO) thin film was found to be 2.2 kΩ/sq. at 80% transmittance. The effective exfoliation method has great potential for application for high performance GO-based flexible electrodes.

#### **2.2. Extremely efficient liquid exfoliation of graphite oxide using unusual acoustic cavitation**

properties. As shown in **Figure 1**, GO nanosheets are typically produced by oxidizing graphite using strong acids and oxidants, followed by exfoliation in aqueous solutions [5–8]. It should be noted that, the characteristics of GO and RGO nanosheets critically depend on the oxidation states of graphite oxide and its exfoliation. Moreover, for real-life applications, the dispersion stability of RGO inks or pastes is a prerequisite. The dispersion of high-quality chemically exfoliated graphene (CEG) or RGO in polar solvents, which contain few oxygen functional groups and defects, has been impossible due to the hydrophobic nature of graphene without post treatment or addition of dispersant molecules. The stability of RGO dispersion is one of the crucial factors for preserving their unique properties such as electrical conductivity and mechanical strength. Therefore, this chapter describes some of the research on CEG nanosheets conducted over the past 8 years that addresses these and other challenges, with an emphasis on our own efforts. We began with the efficient fabrication method of single layer GO nanosheets from graphite, and then described the stable dispersion of RGO in solutions. Furthermore, we described the applications of GOs as p-type dopants, conductors and interfacial modifiers. We concluded with some discussion of future directions and the remaining challenges in chemically exfoli-

**Figure 1.** Production schematics of chemically exfoliated graphene from graphite. Here, GIC stands for graphite

**2. Efficient fabrication of single layer graphene oxide nanosheets**

Conventional sonicators vigorously destroy the structure of GO, which results in producing small-sized GO nanosheets due to acoustical wave agitation in solution. An alternative way

**2.1. Rheologically driven exfoliation of graphite oxide**

ated graphene technologies.

intercalation compound and LIB for lithium ion batteries.

128 Graphene Oxide - Applications and Opportunities

Acoustic cavitation, also called sonication, has been used to fabricate two-dimensional (2-D) nanosheets via exfoliation of bulk-layered crystal materials in solution to fabricate fascinating materials such as graphene, transition metal dichalcogenides, and transition metal oxides. The

**Figure 2.** Fabrication of GO and RGO nanosheets by using different exfoliation methods. (A) Exfoliation of graphite oxide by sonication (S) or homogenization (H). (B) and (C) optical images of GO samples prepared by sonication (B) and homogenization (C) deposited on a 300-nm-thick SiO2 substrate (inset: AFM images of the GO sheets). (D) Fabrication of transparent conducting films (TCFs) with RGO nanosheets by the contact printing of filtrated RGO films. Here PDMS is polydimethylsiloxane, PET is polyethylene terephthalate, and RGO-TCFs are reduced graphene oxide transparent conductive films. The scale bars in (B) and (C) are 10 μm, and those in the respective insets are 2 μm [9].

amplitude at the 2.5 cm probe depth. Moreover, bubbling due to the liquid surface instability under acoustic oscillation is also helpful for the efficient exfoliation of graphite oxide. Bubbling by aeration at the liquid surface is also helpful for the dispersion of nanomaterials because bubbling can produce a greater shearing effect on the particles in suspension under an acoustic flow with lower energy. **Figure 3(b)** and **(c)** shows the scanning electron microscopy (SEM) images of exfoliated GO nanosheets under different cavitation conditions. The maximum size of GO was dramatically increased by adjusting the probe depth from 2.5 to 0.5 cm. At a probe depth of 2.5 cm, the lateral size of GO was less than 5 μm even at 10% amplitude after 10 min of sonication (**Figure 3(b)**) because of breakage in the stretched C–C or C–O–C bonds [10, 11] due to the high energetic physical phenomena of microjets and shock waves [12]. However, at a probe depth of 0.5 cm, GO nanosheets with a maximum 30 μm size were produced even at the high output power setting (amplitude 100%) by reducing the detrimental effect of the high

Chemically Exfoliated Graphene Nanosheets for Flexible Electrode Applications

http://dx.doi.org/10.5772/intechopen.77284

131

 **precursor-assisted dispersion of reduced graphene oxide in solution**

The problems associated with the aggregation of the RGO sheets in organic solvents were

charge screening candidate because it can interact electrostatically with oxygen moieties

isopropoxide (TIP)/acac stabilizer (1/5 molar ratio) solution, which was added to the GO

varied between 0 and 1.5. Just a 0.1 weight ratio was required to stabilize the RGO solution

mixture can be deposited onto the large area substrate by air-spraying without postreduction process. Usually, RGO films are fabricated by deposition of GO nanosheets on the substratem, followed by thermal or chemical reduction at elevated temperatures. Moreover, direct deposition of RGO solution onto the substrate induces formation of wrinkled structures,

graphene field-effect transistors (FETs) on heavily doped Si substrates, which are commonly employed as gate electrodes. It is worth noting that the conductivity of the RGO film at the neutral charge point was maximized for GO/TIP (1/0.7 ratio) by inserting a thin TiO<sup>2</sup>

nanosheets, which was demonstrated by observing a significant blue shift of the G peaks in

to hydrazine reduction. The weight ratio between GO and TIP in the precursor TiO<sup>2</sup>

which can decrease their electrical properties. However, wrinkle-free RGO/TiO<sup>2</sup>

into the RGO multilayer film, despite the amorphous insulating characteristics of TiO<sup>2</sup>

in dimethylformamide (DMF) after hydrazine reduction. This stable RGO/TiO<sup>2</sup>

precursor sol, as shown in **Figure 4(a)**. Titanium dioxide is also a promising

carbons of the RGO sheets

precursor for RGO dis-

amount between the RGO

sol was

hybrid

layer

. This

precursor sol

precursor sol was prepared from a titanium

precursor sols were added into the GO solution prior

by automatic spray-coating. The electrical trans-

hybrid films were investigated by preparing

energy cavitation process (**Figure 3(c)**).

**3.1. TiO2**

and the TiO2

Raman spectra.

**3. Dispersion of reduced graphene oxide**

causing charge trapping [14, 15]. The TiO2

persion, the varying amount of TiO2

multilayer films can be built up on SiO<sup>2</sup>

port characteristics of the RGO and RGO/TiO<sup>2</sup>

addressed by introducing noncovalent interactions among the sp2

solution. In order to determine the minimum amount of used TiO<sup>2</sup>

was due to the hole-doping effect caused by increasing the TiO<sup>2</sup>

**Figure 3.** (a) Temperature change (*ΔT*) over time during sonication of pure water and of a GO suspension containing different initial amounts of graphite oxide. (b) and (c) FESEM images of GO nanosheets fabricated using probe sonication by dipping probe into the liquid surface by 2.5 and 0.5 cm, respectively, for 10 min. The large GO nanosheet was fabricated at 0.5 probe depth condition. (d) Shear viscosity of GO paste samples showing different rheological behavior due to their sizes. (e) Raman spectra of the chemically reduced GO nanosheets demonstrating the effect of acoustic cavitation at different probe depth on the crystalline structure of RGO (f) breakage of GO nanosheet initiated at the defect site or from the sp3 hybridized region during probe sonication. (g) Real THz conductivity of the GO, SRGO, and LRGO films prepared by sonication with probe depths of 0.5 and 2.5 cm [13].

high energetic transient acoustic cavitation; the formation, growth, and implosive collapse of bubbles at high ultrasonic intensities (10–30 W cm−2) in a liquid medium, allow to give physical effects on exfoliation of layered materials. However, the high energetic transient cavitation phenomenon can give a detrimental effect on 2D materials by generating defects on the surface, which decrease their electrical and other useful properties. Recently large (>10 μm) chemically modified graphene nanosheets have been developed from graphite oxide. These have fewer defects than those produced by other methods without requiring further separation processes and can be produced by combining ultrasonic acoustic cavitation with sufficient acoustic shearing and additional microbubbling by aeration in an extremely short time (10 min). It can be achieved by adjusting the ultrasound parameters (amplitude, time, and probe immersion depth) for the delivered power (related to temperature change (*ΔT*) **Figure 3(a)**), the acoustic flow rate, and the bubbling behavior in 200 mL water using conventional flat tip probes with a 12.7 mm diameter. In order to reduce the detrimental effect of transient cavitation, the probe tip was located at a 0.5 cm depth. Subsequently, the acoustic flow rate decreased from 0.62 to 0.47 m s−1 and then increased to 0.73 m s−1 at 100% amplitude, which was faster than the 10% amplitude at the 2.5 cm probe depth. Moreover, bubbling due to the liquid surface instability under acoustic oscillation is also helpful for the efficient exfoliation of graphite oxide. Bubbling by aeration at the liquid surface is also helpful for the dispersion of nanomaterials because bubbling can produce a greater shearing effect on the particles in suspension under an acoustic flow with lower energy. **Figure 3(b)** and **(c)** shows the scanning electron microscopy (SEM) images of exfoliated GO nanosheets under different cavitation conditions. The maximum size of GO was dramatically increased by adjusting the probe depth from 2.5 to 0.5 cm. At a probe depth of 2.5 cm, the lateral size of GO was less than 5 μm even at 10% amplitude after 10 min of sonication (**Figure 3(b)**) because of breakage in the stretched C–C or C–O–C bonds [10, 11] due to the high energetic physical phenomena of microjets and shock waves [12]. However, at a probe depth of 0.5 cm, GO nanosheets with a maximum 30 μm size were produced even at the high output power setting (amplitude 100%) by reducing the detrimental effect of the high energy cavitation process (**Figure 3(c)**).
