**3. Dispersion of reduced graphene oxide**

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%

and LRGO films prepared by sonication with probe depths of 0.5 and 2.5 cm [13].

hybridized region during probe sonication. (g) Real THz conductivity of the GO, SRGO,

the defect site or from the sp3

130 Graphene Oxide - Applications and Opportunities

**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

#### **3.1. TiO2 precursor-assisted dispersion of reduced graphene oxide in solution**

The problems associated with the aggregation of the RGO sheets in organic solvents were addressed by introducing noncovalent interactions among the sp2 carbons of the RGO sheets and the TiO2 precursor sol, as shown in **Figure 4(a)**. Titanium dioxide is also a promising charge screening candidate because it can interact electrostatically with oxygen moieties causing charge trapping [14, 15]. The TiO2 precursor sol was prepared from a titanium isopropoxide (TIP)/acac stabilizer (1/5 molar ratio) solution, which was added to the GO solution. In order to determine the minimum amount of used TiO<sup>2</sup> precursor for RGO dispersion, the varying amount of TiO2 precursor sols were added into the GO solution prior to hydrazine reduction. The weight ratio between GO and TIP in the precursor TiO<sup>2</sup> sol was varied between 0 and 1.5. Just a 0.1 weight ratio was required to stabilize the RGO solution in dimethylformamide (DMF) after hydrazine reduction. This stable RGO/TiO<sup>2</sup> precursor sol 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, which can decrease their electrical properties. However, wrinkle-free RGO/TiO<sup>2</sup> hybrid multilayer films can be built up on SiO<sup>2</sup> by automatic spray-coating. The electrical transport characteristics of the RGO and RGO/TiO<sup>2</sup> hybrid films were investigated by preparing 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> layer into the RGO multilayer film, despite the amorphous insulating characteristics of TiO<sup>2</sup> . This was due to the hole-doping effect caused by increasing the TiO<sup>2</sup> amount between the RGO nanosheets, which was demonstrated by observing a significant blue shift of the G peaks in Raman spectra.

Here, the electrostatic binding enthalpy of cations to a π system (−ΔH = 19.2 kcal/mol) was higher than that that of water (−ΔH = 17.9 kcal/mol). As the aging time is optimized, GO can be formed due to the cation interacting GO (CIGO) as shown in **Figure 5(e)**. Interestingly, the dispersion stability of noninteracting GO and CIGO were similar in aqueous solutions because of the presence of oxygen functional groups. However, the significant differences occurred after chemical reduction, which is described in **Figure 5(c)** and **(e)**. These

**Figure 5.** (a) Structure of graphite oxide, (b) as-exfoliated GO in NaOH solution, (c) highly interacting cations with oxygen functional groups. (a, b, d, e) procedure for obtaining the cation–π interacting GO. (d) Intermediate state of GO by mild deoxygenation aging in NaOH solution (e) decoration of cations on the partially reduced GO surface via a cation-π interaction. (f) FE-SEM image of CIGO on a silicon wafer (inset: AFM images of CIGO sheets). The scale bars in

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the reduction process. Following the hydrazine reduction, the CIGO formed a dispersion of cation–π interacting RGO (CIRGO), whereas, the noninteracting GO aggregated in aqueous solution, as shown in **Figure 5(c)**. Moreover, the atomic force microscopy (AFM) image of the

For real-life applications of RGO nanosheets, alcohol-based formulations of graphene are sometimes needed for graphene processing if the use of harsh organic solvents is not possible. Alcoholic solvents are not good for the dispersion of RGO in solutions due to their solubility parameters. Therefore, for the stable dispersion of RGO in alcoholic solvents, dispersant molecules should be added before the chemical reduction of GO in solutions. Recently, it has been reported that hexamethyldisilazane (HMDS) is a good candidate for the dispersion of RGO in alcohol because HMDS can be easily hydrolyzed into trimethylsilanol and ammonia in the presence of water. Furthermore, for the reduction of GO in solutions, hydrazine can be *in-situ* synthesized in a GO suspension by mimicking a typical reaction cycle involving GO (using alternative ketone molecules as catalysts) and ammonia and hydrogen peroxide as reagents.

single-layered CIGO in **Figure 5(f)** confirmed its 2 μm size and 0.9 nm thickness.

**3.3. Spontaneous reduction and dispersion of graphene oxide nanosheets with** 

*in-situ* **synthesized hydrazine assisted by hexamethyldisilazane**

carbon did not desorb from the CIGO after

results show that the cations with interacting sp2

(f) and the inset are 2 μm [17].

**Figure 4.** (a) Proposed mechanism for the dispersion of RGO sheets by the TiO2 precursor sol via a hydrophobic interaction. (b) Dispersion stability of RGO solution in DMF after chemical reduction with hydrazine monohydrate; gradual increase of absorbance of GO solution at 550 nm and vial images shows the stable dispersion of RGO dispersion. (c) UV–Vis absorption spectra of RGO/TiO<sup>2</sup> hybrid multilayer films; the linear increase of absorption intensity shows the regular deposition of films by spraying. (d) C1s XPS spectra of GO, RGO reduced by hydrazine vapor (H-RGO), RGO/ TIP:acac (TiO<sup>2</sup> precursor), and RGO/TiO<sup>2</sup> hybrid film thermally treated at 200°C [16].

### **3.2. Dispersion of reduced graphene oxide nanosheets by monovalent cation-π interaction**

The cation-π interaction on crystallized RGO, which has fewer defects and oxygen functional groups, can be enhanced the dispersion stability in various solvents due to Coulombic repulsion between the cations on the in-plane of graphene. (Jeong et al. [17]) **Figure 5** shows the stable dispersion of RGO by monovalent cation–π interactions. The interactions did not directly occur on the basal plane of GO because the cations usually interact with the oxygen functional groups of highly oxidized GO (as described in **Figure 5(a)**–**(c)**). In order to effectively activate the cation interaction on π stage, the sp<sup>2</sup> carbon state on the basal plane is exposed through the reduction process (**Figure 5(a)**, **(b)**, **(d)**, and **(e)**). Therefore, mild reduction and aging processes are necessary to increase the six-membered sp2 carbon states as described in **Figure 5(d)**. Chemically Exfoliated Graphene Nanosheets for Flexible Electrode Applications http://dx.doi.org/10.5772/intechopen.77284 133

**Figure 5.** (a) Structure of graphite oxide, (b) as-exfoliated GO in NaOH solution, (c) highly interacting cations with oxygen functional groups. (a, b, d, e) procedure for obtaining the cation–π interacting GO. (d) Intermediate state of GO by mild deoxygenation aging in NaOH solution (e) decoration of cations on the partially reduced GO surface via a cation-π interaction. (f) FE-SEM image of CIGO on a silicon wafer (inset: AFM images of CIGO sheets). The scale bars in (f) and the inset are 2 μm [17].

Here, the electrostatic binding enthalpy of cations to a π system (−ΔH = 19.2 kcal/mol) was higher than that that of water (−ΔH = 17.9 kcal/mol). As the aging time is optimized, GO can be formed due to the cation interacting GO (CIGO) as shown in **Figure 5(e)**. Interestingly, the dispersion stability of noninteracting GO and CIGO were similar in aqueous solutions because of the presence of oxygen functional groups. However, the significant differences occurred after chemical reduction, which is described in **Figure 5(c)** and **(e)**. These results show that the cations with interacting sp2 carbon did not desorb from the CIGO after the reduction process. Following the hydrazine reduction, the CIGO formed a dispersion of cation–π interacting RGO (CIRGO), whereas, the noninteracting GO aggregated in aqueous solution, as shown in **Figure 5(c)**. Moreover, the atomic force microscopy (AFM) image of the single-layered CIGO in **Figure 5(f)** confirmed its 2 μm size and 0.9 nm thickness.

#### **3.3. Spontaneous reduction and dispersion of graphene oxide nanosheets with**  *in-situ* **synthesized hydrazine assisted by hexamethyldisilazane**

**3.2. Dispersion of reduced graphene oxide nanosheets by monovalent cation-π** 

**Figure 4.** (a) Proposed mechanism for the dispersion of RGO sheets by the TiO2

The cation-π interaction on crystallized RGO, which has fewer defects and oxygen functional groups, can be enhanced the dispersion stability in various solvents due to Coulombic repulsion between the cations on the in-plane of graphene. (Jeong et al. [17]) **Figure 5** shows the stable dispersion of RGO by monovalent cation–π interactions. The interactions did not directly occur on the basal plane of GO because the cations usually interact with the oxygen functional groups of highly oxidized GO (as described in **Figure 5(a)**–**(c)**). In order to effectively activate

interaction. (b) Dispersion stability of RGO solution in DMF after chemical reduction with hydrazine monohydrate; gradual increase of absorbance of GO solution at 550 nm and vial images shows the stable dispersion of RGO dispersion.

regular deposition of films by spraying. (d) C1s XPS spectra of GO, RGO reduced by hydrazine vapor (H-RGO), RGO/

hybrid film thermally treated at 200°C [16].

the reduction process (**Figure 5(a)**, **(b)**, **(d)**, and **(e)**). Therefore, mild reduction and aging pro-

carbon state on the basal plane is exposed through

hybrid multilayer films; the linear increase of absorption intensity shows the

carbon states as described in **Figure 5(d)**.

precursor sol via a hydrophobic

**interaction**

TIP:acac (TiO<sup>2</sup>

the cation interaction on π stage, the sp<sup>2</sup>

(c) UV–Vis absorption spectra of RGO/TiO<sup>2</sup>

132 Graphene Oxide - Applications and Opportunities

precursor), and RGO/TiO<sup>2</sup>

cesses are necessary to increase the six-membered sp2

For real-life applications of RGO nanosheets, alcohol-based formulations of graphene are sometimes needed for graphene processing if the use of harsh organic solvents is not possible. Alcoholic solvents are not good for the dispersion of RGO in solutions due to their solubility parameters. Therefore, for the stable dispersion of RGO in alcoholic solvents, dispersant molecules should be added before the chemical reduction of GO in solutions. Recently, it has been reported that hexamethyldisilazane (HMDS) is a good candidate for the dispersion of RGO in alcohol because HMDS can be easily hydrolyzed into trimethylsilanol and ammonia in the presence of water. Furthermore, for the reduction of GO in solutions, hydrazine can be *in-situ* synthesized in a GO suspension by mimicking a typical reaction cycle involving GO (using alternative ketone molecules as catalysts) and ammonia and hydrogen peroxide as reagents. Thus, HMDS can be used as a source of ammonia molecules for synthesizing hydrazine and dispersing RGO (**Figure 6**). The step-wise heating of the solution at 50 and 100°C is required to utilize keton groups in GO for in-situ synthesis of hydrazine molecules at high temperature for reduction of GO.

using 2-ureido-4[1*H*]pyrimidinone (UHP) moieties to provide QHB motifs (**Figure 7(a)**). QHB arrays are much stronger than triple hydrogen bond arrays and are easily accessible synthetically. **Figure 7(b)** shows the well-dispersed RGO paste in DMF illustrating the striking synergy effect of QHB moieties into graphene nanosheets on the fabrication of dispersant-free RGO pastes. This unique paste can be used in electrochemical and printed electrodes and

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**Figure 7.** (a) Synthetic scheme for fabrication of graphene nanosheets functionalized with 2-ureido-4[*1H*]pyrimidinone moieties via a sequential reaction with toluene diisocyanate (TDI) and 2-amino-4-hydroxy-6-methyl-pyrimidine (AHMP)

to form. (b) Photographs of well-dispersed RGO paste and printed electrode on the plastic substrate [19].

could be formed into flexible buckypaper.

### **3.4. Dispersant-free dispersion of reduced graphene oxide by supramolecular chemistry**

Highly concentrated colloidal suspensions of graphene nanosheets are of great interest for a variety of applications ranging from flexible electronics and conducting fibers to electrochemical electrodes for energy harvesting or storage devices. Unfortunately, many additives such as organic surfactants and polymeric dispersants should be added to prepare highly concentrated graphene pastes. These organic dispersant molecules can give detrimental effects on their electrical or thermal properties because graphene nanosheets can be separated by insulating organic materials if it is removed. Quadruple hydrogen bond (QHB) networks can overcome these issues for fabricating printable, spinnable, and chemically compatible conducting pastes containing high quality graphene nanosheets in organic solvents without the need for additional dispersion agents. Motivated by the self-assembly of donor-donor-acceptoracceptor (DDAA) arrays of hydrogen bonding sites, GO nanosheets were functionalized

**Figure 6.** Roles of hexamethyldisilazane (HMDS): (i) ammonia source for the GO-assisted production of hydrazine upon the addition of hydrogen peroxide and (ii) RGO dispersion agent in ethanol, via hydrophobic interactions [18].

using 2-ureido-4[1*H*]pyrimidinone (UHP) moieties to provide QHB motifs (**Figure 7(a)**). QHB arrays are much stronger than triple hydrogen bond arrays and are easily accessible synthetically. **Figure 7(b)** shows the well-dispersed RGO paste in DMF illustrating the striking synergy effect of QHB moieties into graphene nanosheets on the fabrication of dispersant-free RGO pastes. This unique paste can be used in electrochemical and printed electrodes and could be formed into flexible buckypaper.

Thus, HMDS can be used as a source of ammonia molecules for synthesizing hydrazine and dispersing RGO (**Figure 6**). The step-wise heating of the solution at 50 and 100°C is required to utilize keton groups in GO for in-situ synthesis of hydrazine molecules at high temperature

Highly concentrated colloidal suspensions of graphene nanosheets are of great interest for a variety of applications ranging from flexible electronics and conducting fibers to electrochemical electrodes for energy harvesting or storage devices. Unfortunately, many additives such as organic surfactants and polymeric dispersants should be added to prepare highly concentrated graphene pastes. These organic dispersant molecules can give detrimental effects on their electrical or thermal properties because graphene nanosheets can be separated by insulating organic materials if it is removed. Quadruple hydrogen bond (QHB) networks can overcome these issues for fabricating printable, spinnable, and chemically compatible conducting pastes containing high quality graphene nanosheets in organic solvents without the need for additional dispersion agents. Motivated by the self-assembly of donor-donor-acceptoracceptor (DDAA) arrays of hydrogen bonding sites, GO nanosheets were functionalized

**Figure 6.** Roles of hexamethyldisilazane (HMDS): (i) ammonia source for the GO-assisted production of hydrazine upon the addition of hydrogen peroxide and (ii) RGO dispersion agent in ethanol, via hydrophobic interactions [18].

**3.4. Dispersant-free dispersion of reduced graphene oxide by supramolecular** 

for reduction of GO.

134 Graphene Oxide - Applications and Opportunities

**chemistry**

**Figure 7.** (a) Synthetic scheme for fabrication of graphene nanosheets functionalized with 2-ureido-4[*1H*]pyrimidinone moieties via a sequential reaction with toluene diisocyanate (TDI) and 2-amino-4-hydroxy-6-methyl-pyrimidine (AHMP) to form. (b) Photographs of well-dispersed RGO paste and printed electrode on the plastic substrate [19].
