**3. Highly conductive graphene-silica-based hybrid monoliths for dielectric applications**

Improved physical properties may be achieved for O2 *Si* -rGO monoliths using calcination followed by hot press processing. If adsorption of ethyl silicate( CHO ) 10 20 4 *Si* is required as if the group on graphene oxide contains oxygen, then it is beneficial for adsorption because it helps in uniform dispersion of rGO within the O2 *Si* matrix, which can be obtained during the hydrothermal reaction by hydrolysis of ethyl silicate [8, 16]. If O2 *Si* spheres in hybrids become more crystalline, then good physical properties in the hybrid can be obtained. Crystallinity in the O2 *Si* spheres can be enhanced by increasing calcination temperature and further hot press processing at 750°C. Graphene is a material having good physical properties. As experimentally proved by scientists worldwide, the thermal, electrical, and mechanical properties of polymers, metals, and ceramics may be improved using graphene. Graphite oxide-derived graphene has tunable surface functionalization and the potential for large scale production, so it can be used to enhance the physical properties of hybrids. For the decomposition of ammonium perchlorate, *Co*3 4 O can be used as a catalyst [8, 17]. To increase the catalytic effect of *Co*3 4 O , rGO can be used. Basically, rGO helps in uniform deposition of *Co*3 4 O in inorganic hybrids. Silica has good functionalized ability and is very stable, so it can be used as an additive in numerous applications. In biomedical, polymer, and ceramics engineering, silica is used for various purposes. rGO can be

used to enhance the physical properties of silica. Epoxy- O2 *Si* -rGO hybrids have enhanced thermal conductivity (0.452 W 1 1 m K− − ), storage modulus (3.56), and dielectric constant (77.23). In another study, rGO has been used to improve the gas sensing performance, which was obtained by an electrostatic self- assembly approach. If dispersion, corrosion resistance, and barrier properties of hybrids are required to be enhanced, then the presence of rGO is essential, and this was found when nanocomposites of silica-graphene oxide were fabricated using in situ gel method. O2 *Si* -graphene hybrids are better gas sensors as compared to rGO-based sensors [18, 19]. Toward 50 ppm *NH*<sup>3</sup> for 850 s, the gas sensing response of rGObased sensors is 1.5% and that of O2 *Si* -graphene-based sensors is 31.5%, respectively. It has been shown that O2 *Si* -rGO composites having enhanced BET surface area (676 2 1 m g<sup>−</sup> ) can be obtained by one-step hydrothermal method. On addition of ultrathin graphene, O2 *Si* -polyvinyldiene fluoride having high dielectric constant (72.94) and low dielectric loss (0.059) is obtained. Composites of epoxy-silicagraphene oxide have enhanced tensile strength, which leads to an increase in fracture toughness and Young's modulus [8]. The physical properties of O2 *Si* -rGO such as dielectric, electrical, mechanical, and thermal properties need to be improved further. So, in this chapter, we are discussing various physical properties of silica-rGO hybrids for dielectric applications.

#### **Figure 4.**

*SEM images of (a) SiO2-rGO-6.75% (sample b) and (b) SiO2-rGO-10.80% (sample c) fabricated at calcination temperature of 800 K for 1-h. (c) TEM images of the same SiO2-rGO-10.80% (sample c) at lower magnification and (d) at higher magnifications.*

**81**

*Surface Science of Graphene-Based Monoliths and Their Electrical, Mechanical, and Energy…*

**3.1 Preparation of highly conductive graphene-silica based hybrid monoliths for** 

In brief preparation using solvothermal-hot press processing route [8], GO is mixed with cylcohexane and ethylsilicate ( CHO ) 10 20 4 *Si* , followed by hydrothermal reaction to form O2 *Si* -rGO hybrids. For the preparation, 0.1 g of GO is dispersed in 50 mL cyclohexane, followed by addition of 5 mL of ethylsilicate dropwise. Then GO powder is homogeneously dispersed by stirring the mixture at the speed of 1500 rpm for several days at room temperature. The products are then separated by using centrifugation. The products are then washed with cyclohexane for several times. The obtained solid samples are represented as (O)*<sup>x</sup> Si* /GO. From suspension of 0.1, 0.2 and 0.3 g GO, (O)*<sup>x</sup> Si* /GO powders were obtained, respectively. The solid samples obtained are then dispersed in 75 mL cyclohexane, and then for hydrothermal reaction, the mixture is transferred to a 100 mL Teflon-lined stainless-steel autoclave. The reaction is then carried out at 420 K for 4 h, and the sample obtained is then centrifuged and dried at 303 K. The sample obtained is denoted as (O)*<sup>x</sup> Si* rGO. Then calcination of (O)*<sup>x</sup> Si* -rGO is carried out at 800 K for 1 h. To study the physical properties, using the same method, hybrids of O2 *Si* -rGO consisting of 1.55, 6.75, and 10.82 wt% rGO are obtained. 1.55, 6.75, and 10.82% rGO is referred to as sample-a, sample-b, and sample-c, respectively. Without adding GO by using the same procedure, pure O2 *Si* (referred to as sample-d) is obtained. The calcination temperature is then set as 500, 600, 700, and 800 K for a processing time of 1 h

to study the effect of calcination temperature on crystallinity.

**hybrid monoliths for dielectric applications**

sphere size changes at various temperatures [9, 22].

matrix, as shown in **Figure 4**.

applications [8, 12, 23].

silica [8, 24].

**3.2 Surface and physical properties of highly conductive graphene-silica-based** 

Researchers have developed a hydrothermal-hot press processing technique, a simple and efficient method that can improve the thermal, electrical, dielectric, and mechanical properties of the hybrid [8, 20]. By a hydrothermal reaction, GO is dispersed in cyclohexane and ethylsilicate to produce hybrids composed of rGO and silica monoliths [20, 21]. The SEM morphology of hybrids has shown sphere-particle-like morphology with thin layers of rGO, which act as a support for elongated

SEM images of SiO2-rGO-1.55% (sample a) at various temperatures are shown in **Figure 5**. At all temperatures, hybrids have shown sphere-like morphology, but

The solvothermal-hot press processing method shows the best reported electrical conductivity (0.143 <sup>1</sup> Sm<sup>−</sup> ), thermal conductivity (1.612 W 1 1 m K− − ), and higher dielectric constants for O2 *Si* -rGO monoliths. Thus, due to enhanced physical properties of the nano hybrids, it can be applied as electrolytes, catalysts, conductive and electrochemically active materials, and dielectrics for high-temperature

**Table 2** have shown BET surface area and mesoporous volume % analysis for the hybrids. From the table, it is confirmed that BET surface area has been increased with more rGO in the hybrids, while mesoporous volume % increased with more

The dielectric properties of the SiO2-rGO hybrids and bare SiO2 were measured using an LCR meter as shown in **Figure 6**. The dielectric properties of the hybrids were measured at a frequency of 1 kHz. For SiO2, its dielectric constant is found

*DOI: http://dx.doi.org/10.5772/intechopen.93318*

**dielectric applications**

*Surface Science of Graphene-Based Monoliths and Their Electrical, Mechanical, and Energy… DOI: http://dx.doi.org/10.5772/intechopen.93318*

### **3.1 Preparation of highly conductive graphene-silica based hybrid monoliths for dielectric applications**

In brief preparation using solvothermal-hot press processing route [8], GO is mixed with cylcohexane and ethylsilicate ( CHO ) 10 20 4 *Si* , followed by hydrothermal reaction to form O2 *Si* -rGO hybrids. For the preparation, 0.1 g of GO is dispersed in 50 mL cyclohexane, followed by addition of 5 mL of ethylsilicate dropwise. Then GO powder is homogeneously dispersed by stirring the mixture at the speed of 1500 rpm for several days at room temperature. The products are then separated by using centrifugation. The products are then washed with cyclohexane for several times. The obtained solid samples are represented as (O)*<sup>x</sup> Si* /GO. From suspension of 0.1, 0.2 and 0.3 g GO, (O)*<sup>x</sup> Si* /GO powders were obtained, respectively. The solid samples obtained are then dispersed in 75 mL cyclohexane, and then for hydrothermal reaction, the mixture is transferred to a 100 mL Teflon-lined stainless-steel autoclave. The reaction is then carried out at 420 K for 4 h, and the sample obtained is then centrifuged and dried at 303 K. The sample obtained is denoted as (O)*<sup>x</sup> Si* rGO. Then calcination of (O)*<sup>x</sup> Si* -rGO is carried out at 800 K for 1 h. To study the physical properties, using the same method, hybrids of O2 *Si* -rGO consisting of 1.55, 6.75, and 10.82 wt% rGO are obtained. 1.55, 6.75, and 10.82% rGO is referred to as sample-a, sample-b, and sample-c, respectively. Without adding GO by using the same procedure, pure O2 *Si* (referred to as sample-d) is obtained. The calcination temperature is then set as 500, 600, 700, and 800 K for a processing time of 1 h to study the effect of calcination temperature on crystallinity.

### **3.2 Surface and physical properties of highly conductive graphene-silica-based hybrid monoliths for dielectric applications**

Researchers have developed a hydrothermal-hot press processing technique, a simple and efficient method that can improve the thermal, electrical, dielectric, and mechanical properties of the hybrid [8, 20]. By a hydrothermal reaction, GO is dispersed in cyclohexane and ethylsilicate to produce hybrids composed of rGO and silica monoliths [20, 21]. The SEM morphology of hybrids has shown sphere-particle-like morphology with thin layers of rGO, which act as a support for elongated matrix, as shown in **Figure 4**.

SEM images of SiO2-rGO-1.55% (sample a) at various temperatures are shown in **Figure 5**. At all temperatures, hybrids have shown sphere-like morphology, but sphere size changes at various temperatures [9, 22].

The solvothermal-hot press processing method shows the best reported electrical conductivity (0.143 <sup>1</sup> Sm<sup>−</sup> ), thermal conductivity (1.612 W 1 1 m K− − ), and higher dielectric constants for O2 *Si* -rGO monoliths. Thus, due to enhanced physical properties of the nano hybrids, it can be applied as electrolytes, catalysts, conductive and electrochemically active materials, and dielectrics for high-temperature applications [8, 12, 23].

**Table 2** have shown BET surface area and mesoporous volume % analysis for the hybrids. From the table, it is confirmed that BET surface area has been increased with more rGO in the hybrids, while mesoporous volume % increased with more silica [8, 24].

The dielectric properties of the SiO2-rGO hybrids and bare SiO2 were measured using an LCR meter as shown in **Figure 6**. The dielectric properties of the hybrids were measured at a frequency of 1 kHz. For SiO2, its dielectric constant is found

*21st Century Surface Science - a Handbook*

of silica-rGO hybrids for dielectric applications.

used to enhance the physical properties of silica. Epoxy- O2 *Si* -rGO hybrids have enhanced thermal conductivity (0.452 W 1 1 m K− − ), storage modulus (3.56), and dielectric constant (77.23). In another study, rGO has been used to improve the gas sensing performance, which was obtained by an electrostatic self- assembly approach. If dispersion, corrosion resistance, and barrier properties of hybrids are required to be enhanced, then the presence of rGO is essential, and this was found when nanocomposites of silica-graphene oxide were fabricated using in situ gel method. O2 *Si* -graphene hybrids are better gas sensors as compared to rGO-based sensors [18, 19]. Toward 50 ppm *NH*<sup>3</sup> for 850 s, the gas sensing response of rGObased sensors is 1.5% and that of O2 *Si* -graphene-based sensors is 31.5%, respectively. It has been shown that O2 *Si* -rGO composites having enhanced BET surface area (676 2 1 m g<sup>−</sup> ) can be obtained by one-step hydrothermal method. On addition of ultrathin graphene, O2 *Si* -polyvinyldiene fluoride having high dielectric constant (72.94) and low dielectric loss (0.059) is obtained. Composites of epoxy-silicagraphene oxide have enhanced tensile strength, which leads to an increase in fracture toughness and Young's modulus [8]. The physical properties of O2 *Si* -rGO such as dielectric, electrical, mechanical, and thermal properties need to be improved further. So, in this chapter, we are discussing various physical properties

*SEM images of (a) SiO2-rGO-6.75% (sample b) and (b) SiO2-rGO-10.80% (sample c) fabricated at calcination temperature of 800 K for 1-h. (c) TEM images of the same SiO2-rGO-10.80% (sample c) at lower* 

**80**

**Figure 4.**

*magnification and (d) at higher magnifications.*

#### **Figure 5.**

*SEM images of SiO2-rGO-1.55% (sample a) at a calcination temperature of (a) 500 K, (b) 600 K, (c) 700 K, and (d) 800 K, respectively.*


#### **Table 2.**

*BET surface area, mesoporous volume % of SiO2 (sample d), SiO2-rGO-1.55% (sample a), SiO2-rGO-6.75% (sample b), and SiO2-rGO-10.82% (sample c).*

to be around 3.79, which is closer to that of pure silica. For sample a, the dielectric constant significantly increased by a value of 497, which indicates the presence and proximity of a first percolation threshold.

The enhanced dielectric constant (up to order of 105 and 107) was determined for samples b and c, which is much higher compared to that for sample d. Formation of conductive pathways is one of the main reasons for an increase in the overall dielectric constants. In sample c, significant leakage current leads to higher dielectric loss (300). By further increasing the rGO, the dielectric constant increased by seven orders of magnitude, indicating the presence of a second percolation

**83**

area of about 3134 m2

**Figure 6.**

*hybrid.*

of this hierarchy carbon, i.e. 1.11 cm3

*Surface Science of Graphene-Based Monoliths and Their Electrical, Mechanical, and Energy…*

threshold, which is achieved through the higher value of dielectric constant. Similarly, the dielectric loss indicates very similar behavior in the real part of the dielectric constant as shown in the inset of **Figure 6**. Scientists have experimentally proved that a small amount of rGO in hybrids can enhance dielectric properties to a great extent. The existence of a double percolation threshold in SiO2 and the rGO hybrids can be significant for applied applications because it can be used to enhance the dielectric permittivity (up to 107) with the addition of a small percentage of rGO in the hybrids. Silica-rGO hybrids may be used as dielectric materials for high-

*Dielectric constant as a function of % rGO in the hybrid; inset is dielectric loss as a function of % rGO in the* 

temperature applications due to better dielectric properties [7, 8, 23, 25].

A process called one-step carbonization-activation which is used to transform frozen tofu, mainly a source of carbon (C) and nitrogen (N), into a co-doped porous carbon having N (0.6–6.7 wt%) and O (3.6–9.5 wt %) and bearing a specific

of micropores with a regular pore size appropriation somewhere in the range of 0.8–4 nm [9, 26]. When used as electrodes in supercapacitors, this porous carbon shows a specific capacitance of 243 F g−1 with sulfuric acid used as electrolyte and retains 93% of its initial capacitance after 10,000 cycles. In 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), a specific resistance of 170 F g−1 and a reliable rate capability can be observed using above prepared carbon which also provides an energy density of 72 W h kg−1 (calculated at an average power density

g−1. Mesopores and micropores constitute a high volume

g−1 consists of mesopores and 0.71 cm3

g−1

**4. Porous carbon for superior energy storage**

*DOI: http://dx.doi.org/10.5772/intechopen.93318*

*Surface Science of Graphene-Based Monoliths and Their Electrical, Mechanical, and Energy… DOI: http://dx.doi.org/10.5772/intechopen.93318*

**Figure 6.**

*21st Century Surface Science - a Handbook*

**82**

**Figure 5.**

SiO2 rGO-1.55

SiO2 rGO-6.75

SiO2 rGO-10.8%

**Table 2.**

*and (d) 800 K, respectively.*

**Sample type BET surface area (m2**

 **g−1)**

**Total volume (cm3 g−1)**

to be around 3.79, which is closer to that of pure silica. For sample a, the dielectric constant significantly increased by a value of 497, which indicates the presence and

*BET surface area, mesoporous volume % of SiO2 (sample d), SiO2-rGO-1.55% (sample a), SiO2-rGO-6.75%* 

*SEM images of SiO2-rGO-1.55% (sample a) at a calcination temperature of (a) 500 K, (b) 600 K, (c) 700 K,* 

Pure SiO2 333.07 0.3821 0.3459 0.0362 90.52

**Mesoporous volume (cm3**

 **g−1)**

611.21 0.4580 0.3694 0.0886 80.65

677.53 0.5521 0.3571 0.1950 64.68

712.01 0.6812 0.3891 0.2921 57.11

**Microporous volume (cm3**

 **g−1)**

**Mesoporous volume (%)**

The enhanced dielectric constant (up to order of 105 and 107) was determined for samples b and c, which is much higher compared to that for sample d. Formation of conductive pathways is one of the main reasons for an increase in the overall dielectric constants. In sample c, significant leakage current leads to higher dielectric loss (300). By further increasing the rGO, the dielectric constant increased by seven orders of magnitude, indicating the presence of a second percolation

proximity of a first percolation threshold.

*(sample b), and SiO2-rGO-10.82% (sample c).*

*Dielectric constant as a function of % rGO in the hybrid; inset is dielectric loss as a function of % rGO in the hybrid.*

threshold, which is achieved through the higher value of dielectric constant. Similarly, the dielectric loss indicates very similar behavior in the real part of the dielectric constant as shown in the inset of **Figure 6**. Scientists have experimentally proved that a small amount of rGO in hybrids can enhance dielectric properties to a great extent. The existence of a double percolation threshold in SiO2 and the rGO hybrids can be significant for applied applications because it can be used to enhance the dielectric permittivity (up to 107) with the addition of a small percentage of rGO in the hybrids. Silica-rGO hybrids may be used as dielectric materials for hightemperature applications due to better dielectric properties [7, 8, 23, 25].

#### **4. Porous carbon for superior energy storage**

A process called one-step carbonization-activation which is used to transform frozen tofu, mainly a source of carbon (C) and nitrogen (N), into a co-doped porous carbon having N (0.6–6.7 wt%) and O (3.6–9.5 wt %) and bearing a specific area of about 3134 m2 g−1. Mesopores and micropores constitute a high volume of this hierarchy carbon, i.e. 1.11 cm3 g−1 consists of mesopores and 0.71 cm3 g−1 of micropores with a regular pore size appropriation somewhere in the range of 0.8–4 nm [9, 26]. When used as electrodes in supercapacitors, this porous carbon shows a specific capacitance of 243 F g−1 with sulfuric acid used as electrolyte and retains 93% of its initial capacitance after 10,000 cycles. In 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), a specific resistance of 170 F g−1 and a reliable rate capability can be observed using above prepared carbon which also provides an energy density of 72 W h kg−1 (calculated at an average power density

of 889 W kg−1). A total of 25 light emitting diodes (LEDs) which are connected in parallel fashion may be empowered immediately for more than 2 min in the wake of being charged for 25 s, using supercapacitors comprising of porous carbon, at a current density of 10 A g−1. What's more, the porous carbon displays a high reversible charge capacity of 2120 mA h g−1 in the first cycle (estimated at 0.1 A g−1) or 1035 mA h g−1 after 300 cycles (estimated at 1 A g−1), when used as an anode for Li-ion batteries [9, 26, 27].
