3. Graphene oxide (GO)/cobalt oxide (Co3O4) nanoparticles

### 3.1 Synthesis procedure

The GO/Co3O4 composite nanoparticles were synthesized and used for various applications. Liang et al. [22] synthesized Co3O4/N-doped graphene hybrid nanoparticles as catalyst for oxygen reduction. They prepared GO sheet using the modified Hummers method; after the Co3O4/rmGO hybrid was synthesized using a Co(OAc)2 aqueous solution dispersed in GO/ethanol at room temperature and stirred for 10 h at a temperature of 80°C, then NH4OH was added to the solution. They used the GO/Co3O4 composite nanoparticles for catalytic activity. Syam Sundar et al. [18] synthesized the GO/Co3O4 hybrid nanoparticles using the chemical coprecipitation method. Their procedure involved first the preparation of GO sheets using modified Hummers method, after that, 0.2 g of GO-COOH nanosheet dispersed in 100 mL of distilled water and added the solution to 0.4 g of CoCl26H2O dispersed in 20 mL of distilled water and added 0.2932 g of NaBH4, which was accompanied by the formation of a black precipitate. They prepared GO/Co3O4 hybrid nanofluids and observed higher values of thermal conductivity and viscosity when particle concentration and temperature increase. The synthesis method and TEM results are shown in Figure 6. The XRD, FTIR, and VSM results are presented in Figure 7a–c. The XRD patterns of GO/Co3O4 nanoparticles contain

> both the peaks of Co3O4 and GO. The 2θ position of GO/Co3O4 nanoparticles is 11.68, 19.2, 31.2, 36.9, 44.8, 55.43, 59.4, and 65.2° which can be indexed as (002) plane for GO and (111), (220), (311), (400), (511), and (440) planes for Co3O4 nanoparticles (Figure 7a). The IR spectra of GO nanoparticle (Figure 7c) show the GO peak at 1622 cm<sup>1</sup> is the aromatic C=C group, the peak at 1727 cm<sup>1</sup> is the C=O

> The Cobalt Oxide-Based Composite Nanomaterial Synthesis and Its Biomedical and Engineering…

DOI: http://dx.doi.org/10.5772/intechopen.88272

GO/Co3O4 nanoparticles: (a) XRD patterns, (b) FTIR spectra, and (c) M-H curves [18].

1410 cm<sup>1</sup> are contributed to C▬O▬C vibration of epoxy or alkoxy group. The IR spectra of Co3O4 nanoparticle show the strong band peak at 584 cm<sup>1</sup> and 671 cm<sup>1</sup> is due to the Co▬O vibration of Co3O4 nanoparticles, which shows that Co2+ has been oxidized into Co3O4 nanoparticles. The IR spectra of GO/Co3O4 nanoparticles show the peaks at 1622, 1727, 1045, 1220, and 1410 cm<sup>1</sup> reflect C=C, C=O, and C▬O▬C groups related to GO and the peaks at 584 and 671 cm<sup>1</sup> reflect the Co▬O groups related to Co3O4 nanoparticles. The total saturation magnetization of GO/ Co3O4 (Figure 7c) is 4.67 emu/g and pure Co3O4 nanoparticles is 14.23 emu/g. The weight percentages of GO and Co3O4 nanoparticles present in the GO/Co3O4 nanocomposite particles were 67 and 33%, respectively. Shi et al. [20] prepared different concentrations of Co3O4/GO such as 20% Co3O4/GO, 30% Co3O4/GO, 50% Co3O4/GO, 70% Co3O4/GO, 90% Co3O4/GO, 95% Co3O4/GO, and pure Co3O4, and they studied their catalyst activity; the highest catalytic activity was observed when the Co3O4 loading was about 50% in the catalyst. Xiang et al. [21] synthesized 20-nm-sized Co3O4 nanoparticles, which were in situ grown on the chemically reduced graphene oxide (rGO) sheets to form an rGO/Co3O4 composite during hydrothermal processing; they were used as the pseudocapacitor electrode in the

, 1220 cm<sup>1</sup>

, and

vibration of carboxylic group, and the peaks at 1045 cm<sup>1</sup>

Figure 7.

37

Figure 6. GO/Co3O4 nanoparticles: (a) synthesis method and (b) TEM image [18].

The Cobalt Oxide-Based Composite Nanomaterial Synthesis and Its Biomedical and Engineering… DOI: http://dx.doi.org/10.5772/intechopen.88272

Figure 7. GO/Co3O4 nanoparticles: (a) XRD patterns, (b) FTIR spectra, and (c) M-H curves [18].

both the peaks of Co3O4 and GO. The 2θ position of GO/Co3O4 nanoparticles is 11.68, 19.2, 31.2, 36.9, 44.8, 55.43, 59.4, and 65.2° which can be indexed as (002) plane for GO and (111), (220), (311), (400), (511), and (440) planes for Co3O4 nanoparticles (Figure 7a). The IR spectra of GO nanoparticle (Figure 7c) show the GO peak at 1622 cm<sup>1</sup> is the aromatic C=C group, the peak at 1727 cm<sup>1</sup> is the C=O vibration of carboxylic group, and the peaks at 1045 cm<sup>1</sup> , 1220 cm<sup>1</sup> , and 1410 cm<sup>1</sup> are contributed to C▬O▬C vibration of epoxy or alkoxy group. The IR spectra of Co3O4 nanoparticle show the strong band peak at 584 cm<sup>1</sup> and 671 cm<sup>1</sup> is due to the Co▬O vibration of Co3O4 nanoparticles, which shows that Co2+ has been oxidized into Co3O4 nanoparticles. The IR spectra of GO/Co3O4 nanoparticles show the peaks at 1622, 1727, 1045, 1220, and 1410 cm<sup>1</sup> reflect C=C, C=O, and C▬O▬C groups related to GO and the peaks at 584 and 671 cm<sup>1</sup> reflect the Co▬O groups related to Co3O4 nanoparticles. The total saturation magnetization of GO/ Co3O4 (Figure 7c) is 4.67 emu/g and pure Co3O4 nanoparticles is 14.23 emu/g. The weight percentages of GO and Co3O4 nanoparticles present in the GO/Co3O4 nanocomposite particles were 67 and 33%, respectively. Shi et al. [20] prepared different concentrations of Co3O4/GO such as 20% Co3O4/GO, 30% Co3O4/GO, 50% Co3O4/GO, 70% Co3O4/GO, 90% Co3O4/GO, 95% Co3O4/GO, and pure Co3O4, and they studied their catalyst activity; the highest catalytic activity was observed when the Co3O4 loading was about 50% in the catalyst. Xiang et al. [21] synthesized 20-nm-sized Co3O4 nanoparticles, which were in situ grown on the chemically reduced graphene oxide (rGO) sheets to form an rGO/Co3O4 composite during hydrothermal processing; they were used as the pseudocapacitor electrode in the

(BEAS-2B) cells exposed to 1 40 μg=mL. In A549 cells, they found no cytotoxicity; however, BEAS-2B cells presented viability reduction at 40 μg=mL and early membrane damage at 1, 5, and 40 μg=mL. The results related to toxicity study are presented in Figures 3 and 4. Alarifi et al. [17] investigated the toxicity of Co3O4 nanoparticles in HepG2 cells and observed cytotoxicity and genotoxicity in HepG2 cells through ROS and oxidative stress, and their results are presented in Figure 5.

The GO/Co3O4 composite nanoparticles were synthesized and used for various

3. Graphene oxide (GO)/cobalt oxide (Co3O4) nanoparticles

applications. Liang et al. [22] synthesized Co3O4/N-doped graphene hybrid nanoparticles as catalyst for oxygen reduction. They prepared GO sheet using the modified Hummers method; after the Co3O4/rmGO hybrid was synthesized using a Co(OAc)2 aqueous solution dispersed in GO/ethanol at room temperature and stirred for 10 h at a temperature of 80°C, then NH4OH was added to the solution. They used the GO/Co3O4 composite nanoparticles for catalytic activity. Syam Sundar et al. [18] synthesized the GO/Co3O4 hybrid nanoparticles using the chemical coprecipitation method. Their procedure involved first the preparation of GO sheets using modified Hummers method, after that, 0.2 g of GO-COOH nanosheet

dispersed in 100 mL of distilled water and added the solution to 0.4 g of

GO/Co3O4 nanoparticles: (a) synthesis method and (b) TEM image [18].

CoCl26H2O dispersed in 20 mL of distilled water and added 0.2932 g of NaBH4, which was accompanied by the formation of a black precipitate. They prepared GO/Co3O4 hybrid nanofluids and observed higher values of thermal conductivity and viscosity when particle concentration and temperature increase. The synthesis method and TEM results are shown in Figure 6. The XRD, FTIR, and VSM results are presented in Figure 7a–c. The XRD patterns of GO/Co3O4 nanoparticles contain

3.1 Synthesis procedure

Cobalt Compounds and Applications

Figure 6.

36

2 M KOH aqueous electrolyte solution, and the measured specific capacitance was 472 F=g at a scan rate of 2 mV=s.

#### 3.2 Thermal properties

The water- and ethylene glycol-based GO/Co3O4 nanocomposite nanofluid's thermal properties were measured by Syam Sundar et al. [18], and the results are shown in Figures 8 and 9 at different volume concentrations and temperatures. It is noticed that the thermal conductivity of nanofluids increases linearly with the increase of particle volume concentrations and temperatures. Similarly, the viscosity of nanofluids increases with an increase of particle volume concentrations and decreases with an increase of temperature. The thermal conductivity of 0.05% nanofluid is enhanced by 2.82% and 8.58% at temperatures of 20 and 60°C, respectively, as compared to water. The thermal conductivity of 0.2% nanofluid is enhanced by 7.64 and 19.14% at temperatures of 20 and 60°C, respectively, as compared to water (Figure 8, left side). The viscosity of 0.05% volume concentration of nanofluid is enhanced by 1.075 times and 1.166 times; the viscosity of 0.2% volume concentration of nanofluid is enhanced by 1.49 times and 1.70 times at temperatures of 20 and 60°C compared to water (Figure 8, right side).

The thermal conductivity of 0.05% nanofluid is enhanced by 2.71 and 4.44% at temperatures of 20 and 60°C, respectively, as compared to EG. The thermal conductivity of 0.2% nanofluid is enhanced by 5.81 and 11.85% at temperatures of 20 and 60°C, respectively, as compared to EG (Figure 9, left side). The viscosity enhancement of 0.05% volume concentration of nanofluid is 1.028 times and 1.096 times; the viscosity enhancement of 0.2% volume concentration of nanofluid is 1.22 times and 1.42 times at temperatures of 20 and 60°C compared to EG (Figure 9, right side).

3.3 Electrical properties

DOI: http://dx.doi.org/10.5772/intechopen.88272

Figure 9.

39

Xiang et al. [21] measured the electrical properties of GO/Co3O4 using pseudocapacitor electrode in 2 M of KOH aqueous electrolyte solution. Figure 10 depicts the galvanostatic charge discharge curve of rGO and the rGO/Co3O4 composite electrodes between 0 and 0.85 V at different current densities. Both the samples of rGO and rGO/Co3O4 electrodes exhibited good symmetric shape with the coulomb efficiency close to 1. The rGO/Co3O4 composite electrode presented longer charge discharge time than the rGO electrode, indicating larger specific capacitance (Figure 10a and b). The specific capacitance of the rGO/Co3O4 electrode was investigated with a progressively increasing current density (Figure 10c). The specific capacitance decreases from 458 to 416 F=g with an increase in the current density from 0.5 to 2.0 A=g, respectively. The long-term stability of the rGO/Co3O4 electrode was observed at the current density of 2.0 A=g. The specific capacitance of the rGO/Co3O4 electrode increased during the first 100 cycles, which was due to an activation process in the super capacitor electrode. About 95.6% of the specific capacitance of rGO/Co3O4 electrode was retained at the current density of 2.0 A=g after 1000 cycles (Figure 10d), which demonstrated its high cycling stability. Lai et al. [23] synthesized Co3O4 nanoparticles grown on nitrogen-modified microwave-exfoliated graphite oxide (NMEG) with weight ratio controlled from 10 to 70%; due to their electrochemical performance, they are used as Li-ion battery anode, and they exhibit improved cycle stability. Seventy percent of the Co3O4/NMEG composite has an initial irreversible capacity of 230 mAh=g (first cycle efficiency of 77%), and 910 mAh=g of capacity is retained after 100 cycles. Seventy percent of Co3O4/tRG-O delivers a reversible capacity of 750 mAh=g, and the irreversible capacity loss during the first cycle is 700 mAh=g. It is noted that the composite material synthesized with Co3O4 exhibits larger specific capacitance than rGO material when they are used as electrode. The Co3O4/NMEG composite material shows higher capacitance when they are used as Li-ion battery

Ethylene glycol-based GO/Co3O4 nanofluids: thermal conductivity (left side) and viscosity (right side) [18].

The Cobalt Oxide-Based Composite Nanomaterial Synthesis and Its Biomedical and Engineering…

The Cobalt Oxide-Based Composite Nanomaterial Synthesis and Its Biomedical and Engineering… DOI: http://dx.doi.org/10.5772/intechopen.88272

Figure 9. Ethylene glycol-based GO/Co3O4 nanofluids: thermal conductivity (left side) and viscosity (right side) [18].

### 3.3 Electrical properties

2 M KOH aqueous electrolyte solution, and the measured specific capacitance was

The water- and ethylene glycol-based GO/Co3O4 nanocomposite nanofluid's thermal properties were measured by Syam Sundar et al. [18], and the results are shown in Figures 8 and 9 at different volume concentrations and temperatures. It is noticed that the thermal conductivity of nanofluids increases linearly with the increase of particle volume concentrations and temperatures. Similarly, the viscosity of nanofluids increases with an increase of particle volume

concentrations and decreases with an increase of temperature. The thermal conductivity of 0.05% nanofluid is enhanced by 2.82% and 8.58% at temperatures of 20 and 60°C, respectively, as compared to water. The thermal conductivity of 0.2% nanofluid is enhanced by 7.64 and 19.14% at temperatures of 20 and 60°C, respectively, as compared to water (Figure 8, left side). The viscosity of 0.05% volume concentration of nanofluid is enhanced by 1.075 times and 1.166 times; the viscosity of 0.2% volume concentration of nanofluid is enhanced by 1.49 times and 1.70 times at temperatures of 20 and 60°C compared to water

The thermal conductivity of 0.05% nanofluid is enhanced by 2.71 and 4.44% at temperatures of 20 and 60°C, respectively, as compared to EG. The thermal conductivity of 0.2% nanofluid is enhanced by 5.81 and 11.85% at temperatures of 20 and 60°C, respectively, as compared to EG (Figure 9, left side). The viscosity enhancement of 0.05% volume concentration of nanofluid is 1.028 times and 1.096 times; the viscosity enhancement of 0.2% volume concentration of nanofluid is 1.22 times and 1.42 times at temperatures of 20 and 60°C compared to EG (Figure 9,

Water-based GO/Co3O4 nanofluids: thermal conductivity (left side) and viscosity (right side) [18].

472 F=g at a scan rate of 2 mV=s.

Cobalt Compounds and Applications

3.2 Thermal properties

(Figure 8, right side).

right side).

Figure 8.

38

Xiang et al. [21] measured the electrical properties of GO/Co3O4 using pseudocapacitor electrode in 2 M of KOH aqueous electrolyte solution. Figure 10 depicts the galvanostatic charge discharge curve of rGO and the rGO/Co3O4 composite electrodes between 0 and 0.85 V at different current densities. Both the samples of rGO and rGO/Co3O4 electrodes exhibited good symmetric shape with the coulomb efficiency close to 1. The rGO/Co3O4 composite electrode presented longer charge discharge time than the rGO electrode, indicating larger specific capacitance (Figure 10a and b). The specific capacitance of the rGO/Co3O4 electrode was investigated with a progressively increasing current density (Figure 10c). The specific capacitance decreases from 458 to 416 F=g with an increase in the current density from 0.5 to 2.0 A=g, respectively. The long-term stability of the rGO/Co3O4 electrode was observed at the current density of 2.0 A=g. The specific capacitance of the rGO/Co3O4 electrode increased during the first 100 cycles, which was due to an activation process in the super capacitor electrode. About 95.6% of the specific capacitance of rGO/Co3O4 electrode was retained at the current density of 2.0 A=g after 1000 cycles (Figure 10d), which demonstrated its high cycling stability. Lai et al. [23] synthesized Co3O4 nanoparticles grown on nitrogen-modified microwave-exfoliated graphite oxide (NMEG) with weight ratio controlled from 10 to 70%; due to their electrochemical performance, they are used as Li-ion battery anode, and they exhibit improved cycle stability. Seventy percent of the Co3O4/NMEG composite has an initial irreversible capacity of 230 mAh=g (first cycle efficiency of 77%), and 910 mAh=g of capacity is retained after 100 cycles. Seventy percent of Co3O4/tRG-O delivers a reversible capacity of 750 mAh=g, and the irreversible capacity loss during the first cycle is 700 mAh=g. It is noted that the composite material synthesized with Co3O4 exhibits larger specific capacitance than rGO material when they are used as electrode. The Co3O4/NMEG composite material shows higher capacitance when they are used as Li-ion battery

Figure 10.

GO/Co3O4 nanomaterial: (a) charge-discharge curves at the current density of 0.5 A=g, (b) charge-discharge curves at different current densities (0.5, 1.0, and 2.0 A=g), (c) cycling stability at varying the current density, and (d) long-term stability at a current density of 2.0 A=g [21].

anode. So, it is understood that the composite material exhibits synergistic (superior electrical) properties compared to the single-phase nanoparticles.
