**4. Physical properties of silica-graphene hybrids for technological applied applications**

With rGO in the SiO2-rGO hybrids, the hybrids powder shows a prominent change in the color after calcination. TGA of SiO2-rGO powder samples shows that

*Silicon Materials*

**Figure 14.**

hybrids with the addition of a small rGO % in the hybrid. From compressive and tensile stress-strain analysis, it is evident that with an increase of rGO content in the hybrid the mechanical compressive and tensile strength is increased as compared to pure alumina. This further caused more strength in alumina hybrids, i.e., higher compressive, tensile strength and higher compressive young modulus values for these hybrids (**Figures 15** and **16**). The enhanced mechanical properties of γ-Al2O3 and rGO hybrids can be attributed to covalent interaction of rGO with γ-Al2O3 and to efficient load transfers between rGO and nanorods of γ-Al2O3. Further, this is closely bound with the elongated and fine γ-Al2O3 nanorods and atomic-level rGO layers with a covalent interaction with γ-Al2O3. Young modulus of γ-Al2O3-rGO with 1-, 2- and 3-h calcination time and γ-Al2O3 with 1-h calcination time are calculated as 3.7, 3.2, 2.65 and 1.80 GPa. In this case, lower tensile and compressive strength in alumina can be due to the availability of powder instead of single crystals of alumina. Increase in calcination temperature has reduced wt.% of rGO in a hybrid. This is the reason of having more strength in hybrids with lower calcination

*Compressive and tensile strength as a function of rGO for hybrid γ-Al2O3-rGO with error bars.*

*Dielectric properties vs. % rGO in γ-Al2O3-rGO hybrid with error bar.*

**170**

**Figure 15.**

different initial GO suspensions (0.1, 0.2, and 0.3 g) have led to different concentrations of rGO in the hybrids. The TGA curves of all hybrids show a stable weight loss up to 100°C, as a result of moisture loss and between 150 and 300°C, as a removal of unreduced GO functional groups, and from 350 to 600°C, as a result of removal of all carbon related materials which are due to the decomposition of rGO, and other impurities (if any) after heating SiO2-rGO hybrids to 800°C in an air atmosphere. By keeping 1-h calcination time, with initial 0.1 g GO suspension in hybrid, the wt.% loss of 1.55 was calculated. With initial 0.2 g GO suspension in hybrid, the wt.% loss of 6.75 was calculated. With initial 0.3 g GO suspension in hybrid, the wt.% loss of 10.82 was calculated. From the wt.% loss, it is found that % rGO is determined as 1.55, 6.75 and 10.82 in the SiO2-rGO hybrids, which are obtained from 0.1, 0.2 and 0.3 g GO suspensions, respectively. All powder samples before calcination (at 800 K for 1 h) but after autoclave heating (at 420 K for 4 h) have been referred to as Si(O)x/ rGO. Lower and higher magnification SEM images for hybrid SiO2-rGO-1.55% (calcinated at 800 K for 1 h) are shown as in **Figure 17a** and **b**. From SEM images, it is clear that SiO2 have sphere morphology with location side by side along with overlapping and wide size distribution of spheres, ensuring very close contact among SiO2 spheres in the whole network. The diameters of spheres have variation in size ranging from 5 nm to 3 μm. Bare SiO2 particles (calcinated at 800 K for 1 h) have shown spherical morphology with the little rough surface as shown in the **Figure 17b** and **d**. The rough texture of SiO2 sphere is confirmed by the closely looking to **Figure 17d**. Diameters of SiO2 spheres vary in size from 1 μm to a few micrometers. TEM is carried out to verify the morphology, which is obtained in SEM. Further, TEM image of SiO2-rGO-1.55% shows that the SiO2 spheres were uniformly encapsulated by the thin layers of rGO (marked with an arrow) as in the **Figure 17e**. After hydrothermal (420 K for 4 h) and calcination (800 K for 1 h) treatments, in SiO2-rGO-1.55% hybrid, the SiO2 spheres is fully decorated with small rGO sheets.

In **Figure 17e**, the low-contrast features are small portions of the graphene layers on which SiO2 spheres are uniformly distributed. After thermal treatments, uniform distribution of SiO2 spheres and rGO sheets is the main factor for enhanced properties of the SiO2-rGO hybrid. TEM image of SiO2-rGO-1.55% is shown in the

#### **Figure 17.**

*SEM images of (a) SiO2-rGO-1.55% (b) pure SiO2 fabricated at a calcination temperature of 800 K for 1 h at lower magnification. SEM images of same (c) SiO2-rGO-1.55% (d) pure SiO2 at higher magnification. (e) TEM images of same SiO2-rGO-1.55% at lower magnifications, inset is SAED pattern for same sample and (f) TEM images of same pure SiO2 at lower magnifications.*

**173**

**Figure 18.**

*temperature of 800 K for 1 h.*

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

**Figure 17e**, from TEM image it is evident that diameters of SiO2 spheres ranges from few nanometers to few micrometers, and agreed with SEM results. For the SiO2-rGO-1.55% hybrid, the selected area electron diffraction (SAED) pattern is presented in the inset of **Figure 17e**, and it shows the hybrid is composed of SiO2 spheres and thin rGO layers. In SAED pattern, the electron diffraction rings indicate the amorphous nature of SiO2. Rough texture and the non-uniform surface of a pure SiO2 sphere are confirmed from TEM results in **Figure 17f**. Diameters of spherical SiO2 vary from 1 μm to a few micrometers as evident from TEM characterization (**Figure 17f**, and in agreement with SEM results of **Figure 17b** and **d**. For more insight into structural properties of SiO2-rGO hybrids, the fabrication of the amorphous SiO2 was confirmed from the XRD results as shown in the **Figure 18**. XRD peak of the pure SiO2 confirmed the presence of amorphous SiO2 in the sample, which is prepared from the same method and under same experimental

In the XRD spectra of all the samples with % rGO of 1.55, 6.75 and 10.82 and pure SiO2, the presence of characteristic peaks of amorphous SiO2 is evident and matched with JCPDS card no. 01-086-1561. Relatively intense and broader peaks are observed in the XRD of pure SiO2, SiO2-rGO-1.55% and SiO2-rGO-6.75% hybrids in comparison to SiO2-rGO-10.82% hybrid. In XRD pattern (**Figure 18**), the absence of XRD peak at 10.28o is the evidence of the successful reduction of GO in hybrids after hydrothermal and calcination treatments. Raman spectroscopy confirmed the presence of carbon in hybrids. Meanwhile, SiO2-rGO hybrids were tested for FTIR spectra to evaluate the reduction of GO and successful attachments of SiO2 spheres with rGO platelets, as shown in **Figure 19**. With 1.55, 6.75 and 10.82 wt.% rGO, it is

, which are

, which is due to the bending

, IR bands are mainly due

found that SiO2-rGO hybrids have shown IR band at 3417 and 1396 cm<sup>−</sup><sup>1</sup>

mainly due to the stretching and bending vibrations of ▬OH structural group and

vibration of absorbed water molecules. In SiO2-rGO hybrids, the IR bands at 1110,

vibrations, respectively. According to previous reports, the IR bands at 1110, 800

and such bands appear in IR spectra of all three SiO2-rGO hybrids. In hybrids, IR

*XRD of SiO2-rGO hybrids with 1.55, 6.75 and 10.8% rGO and pure SiO2 fabricated at a calcination* 

are assigned to the Si-O-Si asymmetric stretching and bending

are main IR characteristics bands for amorphous nature of silica,

is associated with the Si-OH wagging mode. In-between 500

, the appearance of IR bands is mainly because of assembling of SiO2

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

conditions but without the addition of GO.

stretching vibrations of absorbed water.

800 and 479 cm<sup>−</sup><sup>1</sup>

band at 380 cm<sup>−</sup><sup>1</sup>

and 479 cm<sup>−</sup><sup>1</sup>

and 780 cm<sup>−</sup><sup>1</sup>

All hybrids have shown the IR band at 1642 cm<sup>−</sup><sup>1</sup>

spheres and rGO platelets in hybrids. Below 1000 cm<sup>−</sup><sup>1</sup>

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

**Figure 17e**, from TEM image it is evident that diameters of SiO2 spheres ranges from few nanometers to few micrometers, and agreed with SEM results. For the SiO2-rGO-1.55% hybrid, the selected area electron diffraction (SAED) pattern is presented in the inset of **Figure 17e**, and it shows the hybrid is composed of SiO2 spheres and thin rGO layers. In SAED pattern, the electron diffraction rings indicate the amorphous nature of SiO2. Rough texture and the non-uniform surface of a pure SiO2 sphere are confirmed from TEM results in **Figure 17f**. Diameters of spherical SiO2 vary from 1 μm to a few micrometers as evident from TEM characterization (**Figure 17f**, and in agreement with SEM results of **Figure 17b** and **d**. For more insight into structural properties of SiO2-rGO hybrids, the fabrication of the amorphous SiO2 was confirmed from the XRD results as shown in the **Figure 18**. XRD peak of the pure SiO2 confirmed the presence of amorphous SiO2 in the sample, which is prepared from the same method and under same experimental conditions but without the addition of GO.

In the XRD spectra of all the samples with % rGO of 1.55, 6.75 and 10.82 and pure SiO2, the presence of characteristic peaks of amorphous SiO2 is evident and matched with JCPDS card no. 01-086-1561. Relatively intense and broader peaks are observed in the XRD of pure SiO2, SiO2-rGO-1.55% and SiO2-rGO-6.75% hybrids in comparison to SiO2-rGO-10.82% hybrid. In XRD pattern (**Figure 18**), the absence of XRD peak at 10.28o is the evidence of the successful reduction of GO in hybrids after hydrothermal and calcination treatments. Raman spectroscopy confirmed the presence of carbon in hybrids. Meanwhile, SiO2-rGO hybrids were tested for FTIR spectra to evaluate the reduction of GO and successful attachments of SiO2 spheres with rGO platelets, as shown in **Figure 19**. With 1.55, 6.75 and 10.82 wt.% rGO, it is found that SiO2-rGO hybrids have shown IR band at 3417 and 1396 cm<sup>−</sup><sup>1</sup> , which are mainly due to the stretching and bending vibrations of ▬OH structural group and stretching vibrations of absorbed water.

All hybrids have shown the IR band at 1642 cm<sup>−</sup><sup>1</sup> , which is due to the bending vibration of absorbed water molecules. In SiO2-rGO hybrids, the IR bands at 1110, 800 and 479 cm<sup>−</sup><sup>1</sup> are assigned to the Si-O-Si asymmetric stretching and bending vibrations, respectively. According to previous reports, the IR bands at 1110, 800 and 479 cm<sup>−</sup><sup>1</sup> are main IR characteristics bands for amorphous nature of silica, and such bands appear in IR spectra of all three SiO2-rGO hybrids. In hybrids, IR band at 380 cm<sup>−</sup><sup>1</sup> is associated with the Si-OH wagging mode. In-between 500 and 780 cm<sup>−</sup><sup>1</sup> , the appearance of IR bands is mainly because of assembling of SiO2 spheres and rGO platelets in hybrids. Below 1000 cm<sup>−</sup><sup>1</sup> , IR bands are mainly due

#### **Figure 18.**

*XRD of SiO2-rGO hybrids with 1.55, 6.75 and 10.8% rGO and pure SiO2 fabricated at a calcination temperature of 800 K for 1 h.*

*Silicon Materials*

fully decorated with small rGO sheets.

*(f) TEM images of same pure SiO2 at lower magnifications.*

different initial GO suspensions (0.1, 0.2, and 0.3 g) have led to different concentrations of rGO in the hybrids. The TGA curves of all hybrids show a stable weight loss up to 100°C, as a result of moisture loss and between 150 and 300°C, as a removal of unreduced GO functional groups, and from 350 to 600°C, as a result of removal of all carbon related materials which are due to the decomposition of rGO, and other impurities (if any) after heating SiO2-rGO hybrids to 800°C in an air atmosphere. By keeping 1-h calcination time, with initial 0.1 g GO suspension in hybrid, the wt.% loss of 1.55 was calculated. With initial 0.2 g GO suspension in hybrid, the wt.% loss of 6.75 was calculated. With initial 0.3 g GO suspension in hybrid, the wt.% loss of 10.82 was calculated. From the wt.% loss, it is found that % rGO is determined as 1.55, 6.75 and 10.82 in the SiO2-rGO hybrids, which are obtained from 0.1, 0.2 and 0.3 g GO suspensions, respectively. All powder samples before calcination (at 800 K for 1 h) but after autoclave heating (at 420 K for 4 h) have been referred to as Si(O)x/ rGO. Lower and higher magnification SEM images for hybrid SiO2-rGO-1.55% (calcinated at 800 K for 1 h) are shown as in **Figure 17a** and **b**. From SEM images, it is clear that SiO2 have sphere morphology with location side by side along with overlapping and wide size distribution of spheres, ensuring very close contact among SiO2 spheres in the whole network. The diameters of spheres have variation in size ranging from 5 nm to 3 μm. Bare SiO2 particles (calcinated at 800 K for 1 h) have shown spherical morphology with the little rough surface as shown in the **Figure 17b** and **d**. The rough texture of SiO2 sphere is confirmed by the closely looking to **Figure 17d**. Diameters of SiO2 spheres vary in size from 1 μm to a few micrometers. TEM is carried out to verify the morphology, which is obtained in SEM. Further, TEM image of SiO2-rGO-1.55% shows that the SiO2 spheres were uniformly encapsulated by the thin layers of rGO (marked with an arrow) as in the **Figure 17e**. After hydrothermal (420 K for 4 h) and calcination (800 K for 1 h) treatments, in SiO2-rGO-1.55% hybrid, the SiO2 spheres is

In **Figure 17e**, the low-contrast features are small portions of the graphene layers on which SiO2 spheres are uniformly distributed. After thermal treatments, uniform distribution of SiO2 spheres and rGO sheets is the main factor for enhanced properties of the SiO2-rGO hybrid. TEM image of SiO2-rGO-1.55% is shown in the

*SEM images of (a) SiO2-rGO-1.55% (b) pure SiO2 fabricated at a calcination temperature of 800 K for 1 h at lower magnification. SEM images of same (c) SiO2-rGO-1.55% (d) pure SiO2 at higher magnification. (e) TEM images of same SiO2-rGO-1.55% at lower magnifications, inset is SAED pattern for same sample and* 

**172**

**Figure 17.**

**Figure 19.** *FTIR of SiO2-rGO hybrids with 1.55, 6.75 and 10.8% rGO.*

to reduced oxygen and ▬OH groups, which is a further indication of a successful reduction of GO in hybrids after hydrothermal and calcination treatments. FTIR spectra of all hybrids possess less pronounced IR bands, and which are associated with the presence of amorphous silica. Amorphous silica can be distinguished from crystalline silica based on far FTIR region measurements. For more information of the chemical composition and elemental states, the XPS spectroscopy of hybrids is carried out. With 1.55, 6.75 and 10.82 wt.% rGO, XPS spectra of SiO2-rGO hybrids is presented in **Figure 20**.

In case of all the hybrids, XPS spectra show distinct peaks for C1s, O1s, O2s, Si2s, and Si2p. In case of SiO2-rGO-10.82% hybrid, the intensity ratios of the C1s peak to the Si 2 s and Si 2p peaks are measured, and intensity ratios are higher in comparison to hybrids with 6.75 and 1.55% rGO. This may be due to more rGO in hybrids, as confirmed by the TGA results. The samples were tested for Brunauer-Emmett-Teller (BET) surface areas. In hybrids, higher % of rGO helps to increase surface area compared to that of pure SiO2. With 1.55, 6.75 and 10.82% rGO, the nanohybrids of SiO2-rGO have BET surface areas of 611.21 ± 19.02, 677.53 ± 25.21, and 712.01 ± 13.21 m2 •g<sup>−</sup><sup>1</sup> , respectively. For pure SiO2, the BET surface area is 333.07 ± 21.57 m2 •g<sup>−</sup><sup>1</sup> . With 1.55, 6.75 and 10.82% rGO, the bulk densities of SiO2 rGO have values of 1.41 ± 0.07, 1.29 ± 0.19, and 0.89 ± 0.03 g cm<sup>−</sup><sup>3</sup> , respectively. For bare SiO2, the bulk density is 2.75 ± 0.12 g cm<sup>−</sup><sup>3</sup> . It is found that mesoporosity

**175**

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

is highest with mesopores volume % of 90.52, which is measured for pure SiO2. It is quite clear that minimum mesopores volume % is 57.11, which is calculated for SiO2-rGO hybrid with 10.82% rGO. With higher wt.% of SiO2, hybrids have shown an increase of mesopores volume %. Clearly, the presence of more rGO has led to higher BET surface area in hybrids. Well-aligned SiO2 spheres with ultrathin rGO platelets resulted in a higher surface area and lower density compared to pure SiO2. In hybrids, the presence of more rGO has increased the BET surface area, which is the main reason for enhanced thermal and electrical conductivity due to the availability of more surface electrons of rGO platelets. The higher value of surface areas, electrical conductivity and thermal conductivity are significant factors for applications in thermal and electrical engineering. The effect of hot pressing pressure on SiO2 and rGO hybrid was conducted. Raman spectroscopy is conducted on SiO2-rGO-6.75, which are hot pressed at 10, 20 and 30 MPa, respectively. The higher hot pressing pressure led the D peak intensity to reduce. For SiO2-rGO-6.75 hybrid at 30 MPa, D peak is almost disappeared as compared to the hot-pressing pressure of 10 and 20 MPa. It shows that higher hot pressing pressure leads to remove defects from the rGO platelets and reconstruction of sp2 carbon network in hybrid. The D/G intensity ratio has reduced with higher hot pressing pressure. Solvothermalhot press processed SiO2 and rGO hybrids have shown a significant improvement in the physical properties such as the electrical conductivity, thermal conductivity, tensile strength and the dielectric constant. In SiO2-rGO-10.8% hybrid, the electri-

is calculated. For the same sample, the thermal

is calculated, which is enhanced by more than 255%.

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

cal conductivity of 0.143 S•m<sup>−</sup><sup>1</sup>

K<sup>−</sup><sup>1</sup>

With little rGO in hybrid, dielectric constant increases by seven orders of magnitude through second percolation threshold, such values for dielectric constants are higher. In solvothermal-hot press processed SiO2-rGO, improved physical properties are due to more rGO platelets, high-temperature calcination, high-temperature hot pressing and formation of conductive pathways between rGO platelets and SiO2 spheres.

This book chapter has explained ceramics-graphene hybrids, enhanced properties and possible applications in ceramics-graphene industry. Further, by the solvothermalhot pressing method, a complete systematic study on enhanced physical properties of the hybrids has been made, which can further implement hybrids in advanced technological applications. This can lead to a significant contribution for the applications of ceramics-graphene assembly nanomaterials which can be further applied as electrolytes, catalysts, conductive, electrochemically active, and as dielectric materials

for the high-temperature applications due to enhanced physical properties.

Authors have declared no "conflict of interest."

conductivity of 1.612 Wm<sup>−</sup><sup>1</sup>

**5. Conclusions**

**Conflict of interest**

**Figure 20.** *XPS spectra of SiO2-rGO hybrids with 1.55, 6.75 and 10.8% rGO.*

## *Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

is highest with mesopores volume % of 90.52, which is measured for pure SiO2. It is quite clear that minimum mesopores volume % is 57.11, which is calculated for SiO2-rGO hybrid with 10.82% rGO. With higher wt.% of SiO2, hybrids have shown an increase of mesopores volume %. Clearly, the presence of more rGO has led to higher BET surface area in hybrids. Well-aligned SiO2 spheres with ultrathin rGO platelets resulted in a higher surface area and lower density compared to pure SiO2. In hybrids, the presence of more rGO has increased the BET surface area, which is the main reason for enhanced thermal and electrical conductivity due to the availability of more surface electrons of rGO platelets. The higher value of surface areas, electrical conductivity and thermal conductivity are significant factors for applications in thermal and electrical engineering. The effect of hot pressing pressure on SiO2 and rGO hybrid was conducted. Raman spectroscopy is conducted on SiO2-rGO-6.75, which are hot pressed at 10, 20 and 30 MPa, respectively. The higher hot pressing pressure led the D peak intensity to reduce. For SiO2-rGO-6.75 hybrid at 30 MPa, D peak is almost disappeared as compared to the hot-pressing pressure of 10 and 20 MPa. It shows that higher hot pressing pressure leads to remove defects from the rGO platelets and reconstruction of sp2 carbon network in hybrid. The D/G intensity ratio has reduced with higher hot pressing pressure. Solvothermalhot press processed SiO2 and rGO hybrids have shown a significant improvement in the physical properties such as the electrical conductivity, thermal conductivity, tensile strength and the dielectric constant. In SiO2-rGO-10.8% hybrid, the electrical conductivity of 0.143 S•m<sup>−</sup><sup>1</sup> is calculated. For the same sample, the thermal conductivity of 1.612 Wm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> is calculated, which is enhanced by more than 255%. With little rGO in hybrid, dielectric constant increases by seven orders of magnitude through second percolation threshold, such values for dielectric constants are higher. In solvothermal-hot press processed SiO2-rGO, improved physical properties are due to more rGO platelets, high-temperature calcination, high-temperature hot pressing and formation of conductive pathways between rGO platelets and SiO2 spheres.
