**2. Highly conductive graphene-alumina-based hybrid monoliths for electrical, thermal, and mechanical applications**

Low strength and brittle attributes are the main properties of ceramics. The most widely used structural ceramic is alumina, due to its good thermal conductivity and the shape stability [7]. Alumina has a wide range of applications, some of the fields include dental implants, high speed cutting tools, chemical insulators, electrical insulators, and wear resistant coatings. Scientists have observed that mechanical properties of alumina may be improved using carbon nanotubes, for example, fracture toughness (by 94%), flexural strength (6.4%), and hardness (by 13%), respectively. On the addition of graphene platelets, about 40% enhancement in the fracture toughness of the ball milled alumina/zircon/graphene have been noticed. In another research study, the alumina-rGO core shell nanocomposites were fabricated using the method known as the sol-gel method, and it was found through this study that the BET surface area of the rGO is essential to enhance the surface charge properties of the hybrids. In another study, alumina graphene composite films were reported with a low optical gap of about 1.53 eV. Alumina-rGO nano-composites obtained via deposition during the process showed a unique morphology of aluminum nanoparticles with low prosperity and BET surface area of 242.4 m2 g−1.

**75**

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

Moreover, scientists have found that in a microwave preparation of alumina-rGO composites, the grain size of the alumina matrix was reduced from 475 to 180 nm, which was obtained from the conventional sintering process, leading to an increase in the Young's modulus from 148 to 180 GPa. Scientists have found that using solvothermal-hot press processing route, highly conductive alumina-rGO hybrids may be obtained, which consist of Al2O3 nanorods and rGO, respectively [7, 8, 10]. The same solvothermal method was used to form hybrids from cross-linked Al2O3 nanorods and reduced graphite oxide (rGO) platelets. Then after hot pressing, the hybrid monoliths were obtained, which were utilized for the systematical study of improved physical properties of hybrids. Using the same method, it is noticed that with the 3 h-calcinated hybrid, the Al2O3-rGO monoliths show enhanced electri-

strength (90% increase), thermal conductivity (80% increase), and a much higher dielectric constant (12 times) than the bare Al2O3. The highest values of electrical

rGO hybrid which is calcinated for about 1 h. It was noticed that the functional groups that contain oxygen on GO were useful for the adsorption of aluminum isopropoxide, leading to the dispersion of rGO and the Al2O3, which were obtained during the solvothermal process by the hydrolysis of the aluminum isoropoxide [7]. The improvement in the mechanical properties was caused due to the elongated Al2O3 nanorods, which was indicated by the study of aspect ratio of the nanorods. Graphene platelets, functional groups present, and their surface properties are driving forces for enhancement in the physical properties of alumina-rGO hybrids.

**2.1 Preparations of highly conductive graphene-alumina-based hybrid** 

In the past, alumina rGO hybrids have been prepared using sol gel, molecular level mixing, and powder coating methods, but scientists have tried some conventional preparation methods followed by high temperature treatments [7, 11]. Such methods have shown great enhancement in physical properties of hybrids. Here, we discuss one of such advanced methods, that is, the preparation of Al2O3-rGO hybrids using solvothermal-hot press processing route. Al2O3-rGO was prepared by the mixing of GO and with cyclohexane and the aluminum isopropoxide (C9H21AlO3), which was followed by the solvothermal process. The procedure involves 0.1 g of GO being first dispersed in 35 mL of cyclohexane, then 3.5 mL of aluminum isoproperoxide (C9H21AlO3) being added dropwise. The mixture is then stirred continuously at room temperature at the rate of 1000 rpm for several days until the GO powder is dispersed homogeneously but the color of the suspension changes with time. Then the products are separated by centrifugation, and the products were then washed several times with cyclohexane. The solid sample obtained are denoted as Al[O]x/GO. Then it was sent for the hydrothermal treatment. For this, it was dispersed in 50 mL cyclohexane and then transferred to the 100 mL Teflon-lined stainless steel autoclave, after which the reaction was carried out for about 6 h at about 453 K, and the resultant sample was again centrifuged and dried at about 303 K, which is denoted as Al[O]x/rGO. It was then calcined at 723 K for about 1–3 h to form Al2O3/rGO hybrids. The condition of calcination is limited supply of air. The calcination process was controlled by using the quartz tubular furnace with open ends that will allow the calcination to occur in the limited supply of air; also, the furnace was heated to the desired temperature of about 723 K for the calcination times for 1–3 h. In the initial stage, the temperature was increased by using heating rate of about 15°C min−1. As a result, the free standing

S m−1), thermal conductivity (2.53 Wm−1 K−1), dielectric

), and Young's modulus (3.7 GPa) are determined for the alumina-

S m−1), mechanical tensile

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

cal conductivity (changes from 5.1 × 10−10 to 6.7 × 101

conductivity (8.2 × 101

constant (104

**monoliths**

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

Moreover, scientists have found that in a microwave preparation of alumina-rGO composites, the grain size of the alumina matrix was reduced from 475 to 180 nm, which was obtained from the conventional sintering process, leading to an increase in the Young's modulus from 148 to 180 GPa. Scientists have found that using solvothermal-hot press processing route, highly conductive alumina-rGO hybrids may be obtained, which consist of Al2O3 nanorods and rGO, respectively [7, 8, 10]. The same solvothermal method was used to form hybrids from cross-linked Al2O3 nanorods and reduced graphite oxide (rGO) platelets. Then after hot pressing, the hybrid monoliths were obtained, which were utilized for the systematical study of improved physical properties of hybrids. Using the same method, it is noticed that with the 3 h-calcinated hybrid, the Al2O3-rGO monoliths show enhanced electrical conductivity (changes from 5.1 × 10−10 to 6.7 × 101 S m−1), mechanical tensile strength (90% increase), thermal conductivity (80% increase), and a much higher dielectric constant (12 times) than the bare Al2O3. The highest values of electrical conductivity (8.2 × 101 S m−1), thermal conductivity (2.53 Wm−1 K−1), dielectric constant (104 ), and Young's modulus (3.7 GPa) are determined for the aluminarGO hybrid which is calcinated for about 1 h. It was noticed that the functional groups that contain oxygen on GO were useful for the adsorption of aluminum isopropoxide, leading to the dispersion of rGO and the Al2O3, which were obtained during the solvothermal process by the hydrolysis of the aluminum isoropoxide [7]. The improvement in the mechanical properties was caused due to the elongated Al2O3 nanorods, which was indicated by the study of aspect ratio of the nanorods. Graphene platelets, functional groups present, and their surface properties are driving forces for enhancement in the physical properties of alumina-rGO hybrids.

### **2.1 Preparations of highly conductive graphene-alumina-based hybrid monoliths**

In the past, alumina rGO hybrids have been prepared using sol gel, molecular level mixing, and powder coating methods, but scientists have tried some conventional preparation methods followed by high temperature treatments [7, 11]. Such methods have shown great enhancement in physical properties of hybrids. Here, we discuss one of such advanced methods, that is, the preparation of Al2O3-rGO hybrids using solvothermal-hot press processing route. Al2O3-rGO was prepared by the mixing of GO and with cyclohexane and the aluminum isopropoxide (C9H21AlO3), which was followed by the solvothermal process. The procedure involves 0.1 g of GO being first dispersed in 35 mL of cyclohexane, then 3.5 mL of aluminum isoproperoxide (C9H21AlO3) being added dropwise. The mixture is then stirred continuously at room temperature at the rate of 1000 rpm for several days until the GO powder is dispersed homogeneously but the color of the suspension changes with time. Then the products are separated by centrifugation, and the products were then washed several times with cyclohexane. The solid sample obtained are denoted as Al[O]x/GO. Then it was sent for the hydrothermal treatment. For this, it was dispersed in 50 mL cyclohexane and then transferred to the 100 mL Teflon-lined stainless steel autoclave, after which the reaction was carried out for about 6 h at about 453 K, and the resultant sample was again centrifuged and dried at about 303 K, which is denoted as Al[O]x/rGO. It was then calcined at 723 K for about 1–3 h to form Al2O3/rGO hybrids. The condition of calcination is limited supply of air. The calcination process was controlled by using the quartz tubular furnace with open ends that will allow the calcination to occur in the limited supply of air; also, the furnace was heated to the desired temperature of about 723 K for the calcination times for 1–3 h. In the initial stage, the temperature was increased by using heating rate of about 15°C min−1. As a result, the free standing

*21st Century Surface Science - a Handbook*

cal conductivity (106

and technological applications.

is considered as an outstanding candidate for enhancing the structural, electrical, mechanical, and thermal properties of materials (for example, metals, ceramics, and polymers) [7, 8]. In hybrid nanostructures, the physical property enhancement may be possible due to excellent physical properties of the graphene. Excellent physical properties included higher thermal conductivity (5000 Wm−1 K−1), electri-

for enhancement in the physical properties of hybrids. Among the various types of graphene materials, graphite oxide-derived graphene plays an important role in increasing the physical properties of hybrids because of its surface functionalization and its ability of large-scale production at any level. Even a tiny amount of graphene in hybrids (either polymers or ceramics or metals) may alter its physical properties to a great extent. In case of graphene, the compatible structural properties and how it makes bond with various types of nanostructures are reasons for improved properties in the end product (hybrids or composites). For example, reduced graphite oxide (rGO)-polystyrene composites with a low threshold content of 0.1 (volume %) rGO have shown greatly improved electrical conductivity (approx. 0.1 S m−1); this has been possible due to good dispersion of rGO in the polymer composite matrix. Similarly, in inorganic hybrids, rGO has been used for the deposition of Co3O4 particles for increased catalytic effects, which may have been used for the decomposition of ammonium perchlorate because of the complex properties of GO and Co3O4. In another research, rGO was used to improve the mechanical properties of the bulk silicon nitride (i.e. toughness is enhanced by up to 235%), which may be used for high-performance mechanical and structural applications [8, 9]. In short, graphene being the toughest, strongest, lightweight material may act as a wonder material for future scientific revolution in every aspect of life. Even if it is combined with polymers, metals, and ceramics, it may play a significant role in improving physical properties due to its versatile surface, morphology, chemistry, and physical properties. In this chapter, we will discuss graphene combination with various ceramics and how it has been used to improve their physical properties, and porous carbon for energy storage, respectively. This book chapter will be a significant contribution to advance studies on physical properties

**2. Highly conductive graphene-alumina-based hybrid monoliths for** 

Low strength and brittle attributes are the main properties of ceramics. The most widely used structural ceramic is alumina, due to its good thermal conductivity and the shape stability [7]. Alumina has a wide range of applications, some of the fields include dental implants, high speed cutting tools, chemical insulators, electrical insulators, and wear resistant coatings. Scientists have observed that mechanical properties of alumina may be improved using carbon nanotubes, for example, fracture toughness (by 94%), flexural strength (6.4%), and hardness (by 13%), respectively. On the addition of graphene platelets, about 40% enhancement in the fracture toughness of the ball milled alumina/zircon/graphene have been noticed. In another research study, the alumina-rGO core shell nanocomposites were fabricated using the method known as the sol-gel method, and it was found through this study that the BET surface area of the rGO is essential to enhance the surface charge properties of the hybrids. In another study, alumina graphene composite films were reported with a low optical gap of about 1.53 eV. Alumina-rGO nano-composites obtained via deposition during the process showed a unique morphology of aluminum nanoparticles with low prosperity and BET surface area of 242.4 m2

g−1.

**electrical, thermal, and mechanical applications**

S m−1), and Young's modulus (1 TPa), which are a driving force

**74**

Al2O3 nanorods were formed as a result of calcination treatment and also the GO was reduced to rGO. The physical properties were studied by obtaining the Al2O3/ rGO hybrid powder samples consisting of 16.707, 12.830, and 7.705 wt% using the same solvothermal process. The same process was then used for the preparation of the pure Al2O3 without the addition of GO. The calcination temperature was altered and was set at different temperatures for the processing time of about 1 h. For the analysis of crystallinity, it was set as 500, 600, 650, 700, 750, and 800 K and for the analysis of the effect of calcination temperature and time on the nanorods structure, the calcination time was set as 1, 2, 3, 4, and 5 h for the temperature of 723, 823, and 923 K. Hot pressing of powder samples was carried out in a vacuum furnace. The furnace was fitted with a hydraulic press which compresses the samples in a graphite pressing die. The heating temperature was made such that to increase from the room temperature at the heating rate of 10°C min−1 up to 900°C, which was then maintained constant for about 60 min. When the hybrids reached the set temperature, the pressure of about 25–30 MPa was then applied to the hybrids.

## **2.2 Improved physical properties of the highly conductive graphene-alumina based hybrid monoliths**

In the case of hybrids, higher the rGO, higher will be the enhancement in the physical properties such as electrical, thermal, dielectric, and mechanical properties. In the hybrids, the surface area has been increased, and as a result, greater will be the interfacial interaction of the rGO [7, 12]. The higher rGO platelets will improve the physical properties because it provides a large surface area for interfacial interactions at nano-level. Due to higher surface area of graphene, BET surface area has been improved in the hybrids as represented in the **Table 1**, in comparison with various fabrication methods [7]. Scientists believe that the higher mechanical strength is caused due to the elongated dimensions of nanorods in alumina-rGO hybrids. From the literature, it is found that 90% increase in tensile strength and 75% increase in compressive strength occur when the content of rGO is increased up to 7.707% in the hybrid. The addition of rGO affects the dielectric constant, and it increases by four orders of magnitude through a second percolation threshold [7, 8].

Further, the hot press processing sustains the quality of rGO in the hybrids. An increase in calcination temperature resulted in enhanced crystallinity in the Al2O3 nanorods and rGO hybrids as also shown in XRD of hybrid (**Figure 1a**). From the surface science point of view, this may cause enhancement in the diameters and lengths of the nanorods in the hybrid as shown in the **Figure 1b**. TEM images showing variations in diameters of nano-rod structures with various calcination temperatures are presented in **Figure 1c–h**. As a result, after calcination and hot-press processing, Al2O3-rGO monoliths were obtained with enhanced physical properties. Researchers have found that with very little rGO in the alumina hybrid, higher electrical conductivity (8.2 × 101 S m−1), higher dielectric constant by four orders of magnitude, and improved thermal conductivity (1.4 Wm−1 K−1) have been achieved [7]. Hot pressing at 900°C ensured the complete reduction of GO and the higher crystallinity of Al2O3, resulting in enhanced physical properties. The elongated and fine Al2O3 nanorod morphology, atomic-level layered structure, and excess surface free electrons of rGO resulted in the best reported BET surface area (408 m<sup>2</sup> g−1 in the 2 h-calcinated alumina–rGO), best thermal conductivity (2.53 Wm−1 K−1 in the 1 h-calcinated alumina—rGO), and relatively small density (0.92 g cm3 in the 1 h-calcinated alumina–rGO) and high strength (3.7 GPa in the 1 h-calcinated alumina–rGO), respectively [7].

**77**

**Sample type** Solvothermal-hot

γ-Al

O2 3 (1 h

γ-Al

O2 3-rGO

γ-Al

O2 3-rGO

γ-Al

O2 3-rGO

γ-Al

O2 3 (1 h

γ-Al

O2 3-rGO

γ-Al

O2 3-rGO

γ-Al

O2 calcination time)

3-rGO (1 h

(2 h calcination

time)

(3 h calcination

time)

calcination

time)

(1 h calcination

time)

(2 h calcination

time)

(3 h calcination

time)

calcination

time)

280 Al

O2 3

243

Al

O2 3 286.62

Al

O2 3

N/A

361

408 Al

O2 3-rGO

327

Al

O2 3-rGO

119.71

Al

O2 3-rGO

242.4

379

2.75 Al

O2 3

2.40

Al

O2 3 2.816

Al

O2 3

N/A

1.61

1.37 Al

O2 3-rGO

1.65

Al

O2 3-rGO

0.003

Al

O2 3-rGO

N/A

0.92

press processing

method

Meso-porous

Al

O2 3-rGO Core-shell flakes

Al

O2 3-rGO In situ deposition

Al

**Table 1.**

*BET surface area and density comparison for* γ*-Al*

*O2*

*3-rGO (1, 2 and 3 h calcination time) and pure* γ*-Al*

*O2*

*3 (1 h calcination time), compared with various fabrication methods [7].*

O2 3-rGO

**BET surface area (m2 g−1)**

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

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

**Bulk density (g/cm3**

**)**


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

*21st Century Surface Science - a Handbook*

the hybrids.

**based hybrid monoliths**

percolation threshold [7, 8].

electrical conductivity (8.2 × 101

alumina–rGO), respectively [7].

Al2O3 nanorods were formed as a result of calcination treatment and also the GO was reduced to rGO. The physical properties were studied by obtaining the Al2O3/ rGO hybrid powder samples consisting of 16.707, 12.830, and 7.705 wt% using the same solvothermal process. The same process was then used for the preparation of the pure Al2O3 without the addition of GO. The calcination temperature was altered and was set at different temperatures for the processing time of about 1 h. For the analysis of crystallinity, it was set as 500, 600, 650, 700, 750, and 800 K and for the analysis of the effect of calcination temperature and time on the nanorods structure, the calcination time was set as 1, 2, 3, 4, and 5 h for the temperature of 723, 823, and 923 K. Hot pressing of powder samples was carried out in a vacuum furnace. The furnace was fitted with a hydraulic press which compresses the samples in a graphite pressing die. The heating temperature was made such that to increase from the room temperature at the heating rate of 10°C min−1 up to 900°C, which was then maintained constant for about 60 min. When the hybrids reached the set temperature, the pressure of about 25–30 MPa was then applied to

**2.2 Improved physical properties of the highly conductive graphene-alumina** 

In the case of hybrids, higher the rGO, higher will be the enhancement in the physical properties such as electrical, thermal, dielectric, and mechanical properties. In the hybrids, the surface area has been increased, and as a result, greater will be the interfacial interaction of the rGO [7, 12]. The higher rGO platelets will improve the physical properties because it provides a large surface area for interfacial interactions at nano-level. Due to higher surface area of graphene, BET surface area has been improved in the hybrids as represented in the **Table 1**, in comparison with various fabrication methods [7]. Scientists believe that the higher mechanical strength is caused due to the elongated dimensions of nanorods in alumina-rGO hybrids. From the literature, it is found that 90% increase in tensile strength and 75% increase in compressive strength occur when the content of rGO is increased up to 7.707% in the hybrid. The addition of rGO affects the dielectric constant, and it increases by four orders of magnitude through a second

Further, the hot press processing sustains the quality of rGO in the hybrids. An increase in calcination temperature resulted in enhanced crystallinity in the Al2O3 nanorods and rGO hybrids as also shown in XRD of hybrid (**Figure 1a**). From the surface science point of view, this may cause enhancement in the diameters and lengths of the nanorods in the hybrid as shown in the **Figure 1b**. TEM images showing variations in diameters of nano-rod structures with various calcination temperatures are presented in **Figure 1c–h**. As a result, after calcination and hot-press processing, Al2O3-rGO monoliths were obtained with enhanced physical properties. Researchers have found that with very little rGO in the alumina hybrid, higher

magnitude, and improved thermal conductivity (1.4 Wm−1 K−1) have been achieved [7]. Hot pressing at 900°C ensured the complete reduction of GO and the higher crystallinity of Al2O3, resulting in enhanced physical properties. The elongated and fine Al2O3 nanorod morphology, atomic-level layered structure, and excess surface free electrons of rGO resulted in the best reported BET surface area (408 m<sup>2</sup>

in the 2 h-calcinated alumina–rGO), best thermal conductivity (2.53 Wm−1 K−1 in the 1 h-calcinated alumina—rGO), and relatively small density (0.92 g cm3

the 1 h-calcinated alumina–rGO) and high strength (3.7 GPa in the 1 h-calcinated

S m−1), higher dielectric constant by four orders of

g−1

in

**76**

#### **Figure 1.**

*(a) XRD of* γ*-Al2O3-rGO hybrids taken from 500 to 800 K, (b) average diameter of nano rods (nm) as function of calcination temperature (K), and (c–h) TEM images showing variations in diameters of nano rod structures with various calcination temperatures. Units for diameters are in (nm).*

#### **Figure 2.**

*SEM images of hot pressed samples: (a)* γ*-Al2O3-rGO (1 h calcination time), (b)* γ*-Al2O3-rGO (2 h calcination time), (c)* γ*-Al2O3-rGO (3 h calcination time), and (d) pure* γ*-Al2O3 (1 h calcination time).*

Hot press processing may have an impact on the physical properties of hybrids; SEM images of hot pressed samples have shown particle-like morphology, as represented in **Figure 2**.

Moreover, well-aligned, elongated, and fine nanorod morphology of alumina is the reason for improvement in the mechanical strength [7, 13, 14]. Aspect ratio

**79**

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

studies have confirmed that alumina-rGO hybrids (1 h calcinated) have more strength compared to hybrids that are calcined at more time (2 and 3 h), as

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

calcination followed by hot press processing. If adsorption of ethyl sili-

dimensions and improved physical properties [7, 15].

Thus, nano-hybrids of alumina monoliths and rGO can be further applied as electrolytes, catalysts, and electrochemically active materials because of nanometer

*Young's modulus as a function of average aspect ratio of nano-rods in hot pressed samples* γ*-Al2O3-rGO hybrids* 

Improved physical properties may be achieved for O2 *Si* -rGO monoliths using

cate( 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

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

shown in **Figure 3**.

*with 1, 2, and 3 h calcination time.*

**Figure 3.**

**dielectric applications**

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

#### **Figure 3.**

*21st Century Surface Science - a Handbook*

**Figure 1.**

**78**

**Figure 2.**

represented in **Figure 2**.

Hot press processing may have an impact on the physical properties of hybrids;

*SEM images of hot pressed samples: (a)* γ*-Al2O3-rGO (1 h calcination time), (b)* γ*-Al2O3-rGO (2 h calcination* 

Moreover, well-aligned, elongated, and fine nanorod morphology of alumina is the reason for improvement in the mechanical strength [7, 13, 14]. Aspect ratio

SEM images of hot pressed samples have shown particle-like morphology, as

*time), (c)* γ*-Al2O3-rGO (3 h calcination time), and (d) pure* γ*-Al2O3 (1 h calcination time).*

*(a) XRD of* γ*-Al2O3-rGO hybrids taken from 500 to 800 K, (b) average diameter of nano rods (nm) as function of calcination temperature (K), and (c–h) TEM images showing variations in diameters of nano rod* 

*structures with various calcination temperatures. Units for diameters are in (nm).*

*Young's modulus as a function of average aspect ratio of nano-rods in hot pressed samples* γ*-Al2O3-rGO hybrids with 1, 2, and 3 h calcination time.*

studies have confirmed that alumina-rGO hybrids (1 h calcinated) have more strength compared to hybrids that are calcined at more time (2 and 3 h), as shown in **Figure 3**.

Thus, nano-hybrids of alumina monoliths and rGO can be further applied as electrolytes, catalysts, and electrochemically active materials because of nanometer dimensions and improved physical properties [7, 15].
