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

2.1 Synthesis procedure

There are different methods to synthesize Co₃O₄ nanoparticles, which, among others, are chemical coprecipitation, mechanochemical reaction, thermal decomposition and oxidation route, and long-time calcining method. Manigandan et al. [11] used thermal decomposition method for the synthesis of Co₃O₄ nanoparticles by dispersing 0.01 M cobalt chloride in 500 mL distilled water and 10% of glycerol. The suspension was stirred for 20 min by a magnetic stirrer at a temperature of 50°C; after that the dissolved ammonium hydroxide solution (50 mL) was added slowly to control the agglomeration. The obtained cobalt hydroxide is calcined in air for 3 h at a temperature of 450°C yielding the Co₃O₄ nanoparticles. The TEM image of the synthesized Co₃O₄ nanoparticles is shown in Figure 1. Salavati-Niasari et al. [13] prepared Co₃O₄ nanoparticles from a solid organometallic molecular precursor of N,N′-bis(salicylaldehyde)-1,2-phenylenediamino cobalt(II); Co(salophen) estimated the magnetic behavior of the Co₃O₄ nanoparticles. In another study, Salavati-Niasari et al. [14] used thermal deposition method for the preparation of Co₃O₄ nanoparticles by using benzene dicarboxylate complexes, especially phthalate ones, as precursors and characterized using Fourier transform infrared and X-ray photoelectron spectroscopy and observed temperature-dependent magnetization curve in zero-field-cooled, where Co₃O₄ nanoparticles exhibit weak ferromagnetism. Alrehaily et al. [15] synthesized Co₃O₄ nanoparticles by gamma irradiation of 0.2–0.3 mM of CoSO₄ solutions. Syam Sundar et al. [18] used chemical coprecipitation method for the synthesis of Co₃O₄ nanoparticles, and they estimated their thermal properties at different particle volume concentrations and temperatures.

2.2 Thermal properties

Co₃O₄ nanofluids have a potential use in several mechanical engineering applications; in particular, as replacement of low thermal conductivity fluids such as water and ethylene glycol, as a consequence, the thermal properties of nanofluids are of great interest. Mariano et al. [12] prepared Co₃O₄ nanofluids by dispersing cobalt(II, III) oxide nanopowder in ethylene glycol and determined experimentally their thermal conductivity and viscosity. The thermal conductivity and viscosity of Co₃O₄/EG nanofluids are shown in Figure 2a and b; it is noticed that the increase of particle volume concentration ϕyields increased values of thermal conductivity and viscosity. They observed thermal conductivity enhancement for 5.7% volume concentration of Co₃O₄/EG nanofluids is 27% at temperature of 323.15 K (Figure 2a); similarly, the viscosity enhancement for 5.7% volume concentration of Co₃O₄/EG nanofluids is 40% at a temperature of 303.15 K (Figure 2b).

2.3 Toxicity

Knowledge about the toxicity of Co₃O₄ nanoparticles is very important considering their eventual use in medical applications. Cavallo et al. [16] studied the cytotoxicity of Co₃O₄ nanoparticles in human alveolar (A549) and bronchial (BEAS-2B) cells exposed to1−40μg/mL. In A549 cells, they found no cytotoxicity; however, BEAS-2B cells presented viability reduction at 40μg/mLand early membrane damage at 1, 5, and40μg/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.

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 CoCl₂·6H₂O 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 2θposition 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⁻¹ is the aromatic C=C group, the peak at 1727 cm⁻¹ is the C=O vibration of carboxylic group, and the peaks at 1045 cm⁻¹, 1220 cm⁻¹, and 1410 cm⁻¹ 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⁻¹ and 671 cm⁻¹ 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⁻¹ reflect C=C, C=O, and C▬O▬C groups related to GO and the peaks at 584 and 671 cm⁻¹ 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 2 M KOH aqueous electrolyte solution, and the measured specific capacitance was 472F/gat 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

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

4.1 Synthesis procedure

The ND-Co₃O₄ nanoparticles were synthesized by Syam Sundar et al. [19] using in situ and the chemical coprecipitation method. The synthesis route and TEM results are shown in Figure 11. The synthesis route contains dispersion of 0.5 g of ND particles and 0.5 g (0.003 M) of CoCl₂·6H₂O in 100 mL, adds 0.379 (0.01 M) g of NaBH₄ gradually, and observes the formation of light black color precipitate. The XRD, VSM, and XPS results of ND-Co₃O₄ nanocomposite are reported in Figure 10.

The XRD, VSM, and prepared nanofluids are shown in Figure 12a–d. From the XRD patterns (Figure 12a), the 2θposition for the ND nanoparticles for the plane (111) is 43.73^°; similarly, the 2θposition for the Co₃O₄ nanoparticles for the plane (311) is 36.81^o, and the ND-Co₃O₄ nanocomposite nanoparticles contains both the ND and Co₃O₄ nanoparticles planes. The weight percentage of ND and Co₃O₄ present in the ND-Co₃O₄ nanocomposite was measured from vibrating sample magnetometer (Cryogenic, UK) instrument (Figure 12b). The total saturation magnetization of Co₃O₄ nanoparticles is 14.3 emu/g, whereas the total saturation magnetization of ND-Co₃O₄ nanocomposite is 4.7 emu/g. It is decreased because of the presence of nonmagnetic material of ND. The coercivity results of ND-Co₃O₄ and of pure Co₃O₄ nanoparticles are 334 and 490 Oe, respectively (Figure 12c). Based on the total sum rule, it is observed that there is 67% of ND and 33% of Co₃O₄ present in the ND-Co₃O₄ nanocomposite nanoparticles. The prepared ND-Co₃O₄ nanofluid’s samples are shown in Figure 12d, and the final ND-Co₃O₄ nanocomposite particles showing magnetic behavior are observed.

The surface composition of ND-Co₃O₄ nanocomposite particles was measured using X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 13a–c. The Co 2p spectra has two main peaks at binding energies (BEs) of 780.7 and 796.3 eV, which can be related to Co 2p_3/2 and Co 2p_1/2 spin-orbit lines, respectively (Figure 13a). The determination of oxidation state of the each and every component is very important, and also it is very difficult. The shape of the satellites and the energy gap between the satellites are the key parameters used to discriminate between different oxidation states of Co. For instance, the presence of a pronounced satellite like that founded in the present sample at 785.7 eV can be ascribed to CoO. For the case of Co₃O₄ compounds, the satellite is generally detected at BEs higher than 10 eV with respect to the main peak. It is observed from the XPS analysis that the Co₃O₄ particles are covered by a thin layer of CoO.

The O 1s core level is presented in Figure 13b for the cobalt nanoparticles (bottom) and for the nanocomposite (upper). The first component, centered at a BE = 529.5 eV (gray), is ascribed to oxygen atoms in the cobalt particles, while the others are related to different oxygen species. In particular, the components at 531.6 eV (red) and 533.7 eV (green) are ascribed to OH⁻ and C▬O/O=C▬O, respectively. Moreover, in the case of the nanocomposite, the C 1s core level (Figure 13c) shows interesting features. Four components were needed for fitting this peak, appearing at BEs of 284.6 eV (gray), 286.5 eV (red), 287.8 eV (green), and 289.6 eV (blue), which can be ascribed to C▬C, C▬OH or C▬O▬C, C=O, and O=C▬O, respectively. Thus, XPS indicates that the cobalt particles are integrated with the treated nanodiamonds.

4.2 Thermal properties

The water- and ethylene glycol-based ND-Co₃O₄ nanofluid’s thermal conductivity and viscosity were measured by Syam Sundar et al. [19], and the data is shown in Figure 14a and b at different particle weight concentrations and temperatures. The water-based ND-Co₃O₄ nanofluid’s samples are shown in Figure 14b, and particle size distribution is shown in Figure 14c. They observed thermal conductivity enhancement of 2 and 6% for 0.05 wt.% of water-based ND-Co₃O₄ nanofluid and the thermal conductivity enhancement of 8.7 and 15.7% for 0.15 wt.% of water-based ND-Co₃O₄ nanofluid at temperatures of 20 and 60°C, respectively, compared with water data (Figure 14a). They also observed thermal conductivity enhancement of 1.16 and 3.97% for 0.05 wt.% of EG-based ND-Co₃O₄ nanofluid and the thermal conductivity enhancement of 4.68 and 8.71% for 0.15 wt.% of EG-based ND-Co₃O₄ nanofluid at temperatures of 20 and 60°C, respectively, compared with EG data (Figure 14d).

4.3 Toxicity of ND-Co₃O₄ nanoparticles

The toxicity of ND-Co₃O₄ nanoparticles was studied by Syam Sundar et al. [24] on Allium cepa, and the results are shown in Figure 15. The untreated root tip cells showed a mitotic index of 71.3 ± 2.2%. However, a dose-dependent effect on mitotic index was noted for Co₃O₄ and Co₃O₄-cND. In particular, the mitotic indices were found to be 58.07 ± 1.7, 37.8 ± 1.2, and 28.6 ± 0.8% upon exposure to 5, 10, and 20 μg/mL Co, respectively. Notably, decreases in MI were insignificant at these concentrations of cND with values of 68.3 ± 2.0, 65.7 ± 1.9, and 59.0 ± 1.7, respectively. The ameliorative effect of Co-accrued impacts is demonstrated by low (5 μg/mL) and moderate (10 μg/mL) concentration values of cND-Co₃O₄. This indicates that, if accidentally released into the environment, cND-Co₃O₄ would be safe for biotic life to a maximum concentration of 10 μg/mL. The observed Co-accrued cyto-genotoxic consequences coincide with similar earlier studies, where Co oxide nanoparticles were reported to spoil the whole cellular metabolism and stages of cell division mainly by blocking water channels through adsorption and/or by impacting genetic material by causing various types of chromosomal aberrations.

In summary, insignificant changes in MI with moderate concentration (10 μg/mL) of cND-Co₃O₄ also confirm that cND-Co₃O₄ was unable to interfere with the normal development of mitosis mainly by its incapacity to prevent cells from entering the prophase and blocking the mitotic cycle during interphase inhibiting DNA/protein synthesis. Moreover, 20 μg/mL of cND-Co₃O₄ compared to 5, 10, and 20 μg/mL of Co₃O₄ and 10 μg/mL of cND-Co₃O₄ presents insignificant and infrequent chromosome aberrations (such as stickiness, breaks, disturbed, and scattered metaphase); therefore, these results strongly support the environment-friendly nature of the cND-Co₃O₄ nanocomposite, as demonstrated by the toxicity tests conducted using Allium cepa (Figure 15A–D). For comparison purpose, similar tests are also performed for various concentrations of cobalt oxide nanoparticles (Figure 15E–G) and cND-Co₃O₄ (H–K) (Figure 15H–K).

5.1 Synthesis procedure

The zeolite Y/cobalt oxide (Co₃O₄) nanoparticles were synthesized by Davar et al. [25], and the schematic diagram is depicted in Figure 16. The Co₃O₄ nanocomposite was synthesized by an ion exchange of cobalt ions and zeolite Y in the presence of sodium hydroxide and calcination treatment; the synthesized zeolite Y/Co₃O₄ has a paramagnetic behavior at room temperature.

6.1 Synthesis procedure

The f-SWCNT/Co₃O₄ nanocomposite was prepared by Abdolmaleki et al. [26] using the electrostatic coprecipitation route. They noted that the specific capacitance of f-SWCNT/Co₃O₄ is high with a value of 343F/g, while that of the Co₃O₄ nanoparticles is only 77F/g(Figure 17). Zarnegar et al. [27] used a new sonochemical synthesis of polyhydroquinolines having as catalyst Co₃O₄-CNT nanocomposites for aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate in an ethanol medium. These nanocomposites proved to be a highly effective catalytic system, and they provide a green strategy to generate a variety of polyhydroquinolines under sonic conditions. The TEM image of Co₃O₄ nanoparticles (Figure 18A), carbon nanotubes (Figure 18B), Co₃O₄-CNT nanocomposites at microscope 100 nm range (Figure 18C), and Co₃O₄-CNT nanocomposites at microscope 40 nm range (Figure 18D) is shown in Figure 18.