The magnetic nanoparticles (NPs) are offering a wide range of applications in medical and engineering fields. Among all the magnetic nanoparticles, cobalt oxide (Co3O4) nanoparticles and its composite-based nanoparticles are attracting more interest from researchers because of its unique mechanical, thermal, and magnetic properties. The aim of this book is to bring together a number of recent contributions regarding the cobalt oxide-based composite nanoparticles from several researchers all over the world. The latest research results, innovations, and methodologies are reported in the book in order to support the discussion and to circulate ideas and knowledge about the applications of these materials in medical and engineering applications. This chapter presents the methodology for the synthesis and characterization and applications of cobalt oxide-based composite nanoparticles. The detailed analysis related to toxicity of these nanocomposite materials is also included in this book chapter.
- cobalt oxide
- thermal properties
- electrical properties and toxicity
Magnetic nanoparticles (NPs) are attracting increased researchers’ interest due to their potential wide use in many engineering and medical applications. The most commonly used magnetic materials are Fe2O3, Fe3O4, Ni, Co, CoO, and Co3O4. However, among all these magnetic materials, cobalt oxide (Co3O4) nanoparticles are being preferred due to their good magnetic properties. Pure cobalt is not stable at room temperature as it can be converted to oxides like CoO, Co2O3, and Co3O4; Co3O4 is the most stable phase. It is a P-type semiconductor, and it has high Young’s modulus, which varies between 116 and 160 GPa. Co3O4 exhibits a normal spinel crystal structure with occupation of tetrahedral sites by Co2+ and octahedral sites by Co3+. Its magnetic moment arises due to Co2+ ions largely because of spins, with a small contribution from spin-orbit coupling. Co3O4 has excellent properties such as gas-sensing, catalytic, and electrochemical properties, and it has been studied widely for applications in solid-state sensors, electrochromic devices, and heterogeneous catalysts as well as lithium batteries and also medical applications [1, 2, 3, 4, 5]. There are several methods to synthesize the Co3O4 nanoparticles, which include the Co3O4 nanowires , the surfactant-templated approach for fabricating Co3O4 nanoboxes , the mechanochemical reaction method for the synthesis of Co3O4 nanoparticles , the thermal decomposition and oxidation route for the growth of Co3O4 nanorods , and the Co3O4 nanowalls .
Some of the reports dealing with the synthesis of Co3O4 nanoparticles and their potential use are succinctly reviewed below. Manigandan et al.  used the thermal decomposition method. Mariano et al.  synthesized Co3O4 nanoparticles and prepared ethylene glycol-based nanofluids. Salavati-Niasari et al.  prepared Co3O4 nanoparticles from solid organometallic molecular precursors. Salavati-Niasari et al.  used another method by considering benzene dicarboxylate complexes, in particular phthalate ones, as precursors. Alrehaily et al.  synthesized Co3O4 nanoparticles by gamma irradiation. All the above researchers synthesized the Co3O4 nanoparticles for engineering applications. Cavallo et al.  studied the cytotoxicity of Co3O4 nanoparticles in human alveolar (A549) and bronchial (BEAS-2B) cells. Alarifi et al.  investigated the toxicity of Co3O4 nanoparticles in HepG2 cells. Based on these studies, pure Co3O4 nanoparticles are toxic.
Cobalt-based compounds also offer interesting advantages in various applications; typical cobalt-based compounds are grapheme oxide/cobalt oxide, nanodiamond-cobalt oxide, zeolite Y/cobalt oxide, and carbon nanotubes/cobalt oxide. Syam Sundar et al.  synthesized GO/Co3O4 hybrid nanoparticles and studied their thermal properties. Syam Sundar et al.  also synthesized ND-Co3O4 nanoparticles and investigated their thermal properties and toxicity. Shi et al.  prepared different concentrations of Co3O4/GO, studied their catalyst activity, and observed the highest catalytic activity when the Co3O4 mass loading was about 50% in the catalyst. Xiang et al.  synthesized rGO/Co3O4, which was used as the pseudocapacitor electrode in the 2 M KOH aqueous electrolyte solution.
This book chapter emphasizes on the various synthesis methods for cobalt oxide and engineering and medical applications of this material. In addition, synthesis, characterization, and engineering and medical applications of cobalt oxide-based composite materials are also reviewed.
2. Cobalt oxide (Co3O4) nanoparticles
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.  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.  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.  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.  synthesized Co3O4 nanoparticles by gamma irradiation of 0.2–0.3 mM of CoSO4 solutions. Syam Sundar et al.  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.  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 yields 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).
Knowledge about the toxicity of Co3O4 nanoparticles is very important considering their eventual use in medical applications. Cavallo et al.  studied the cytotoxicity of Co3O4 nanoparticles in human alveolar (A549) and bronchial (BEAS-2B) cells exposed to . In A549 cells, they found no cytotoxicity; however, BEAS-2B cells presented viability reduction at and early membrane damage at 1, 5, and . The results related to toxicity study are presented in Figures 3 and 4. Alarifi et al.  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. 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.  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.  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 2position 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.  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.  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 at a scan rate of 2 .
3.2 Thermal properties
The water- and ethylene glycol-based GO/Co3O4 nanocomposite nanofluid’s thermal properties were measured by Syam Sundar et al. , 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.  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 with an increase in the current density from 0.5 to 2.0 , respectively. The long-term stability of the rGO/Co3O4 electrode was observed at the current density of 2.0 . 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 after 1000 cycles (Figure 10d), which demonstrated its high cycling stability. Lai et al.  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 (first cycle efficiency of 77%), and 910 of capacity is retained after 100 cycles. Seventy percent of Co3O4/tRG-O delivers a reversible capacity of 750 , and the irreversible capacity loss during the first cycle is 700 . 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. Nanodiamond (ND)-cobalt oxide (Co3O4) nanoparticles
4.1 Synthesis procedure
The ND-Co3O4 nanoparticles were synthesized by Syam Sundar et al.  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 2position for the ND nanoparticles for the plane (111) is 43.73°; similarly, the 2position for the Co3O4 nanoparticles for the plane (311) is 36.81o, 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 2p3/2 and Co 2p1/2 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. , 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.  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. Zeolite Y/cobalt oxide (Co3O4) nanoparticles
5.1 Synthesis procedure
The zeolite Y/cobalt oxide (Co3O4) nanoparticles were synthesized by Davar et al. , 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. Carbon nanotubes/cobalt oxide (Co3O4) nanoparticles
6.1 Synthesis procedure
The f-SWCNT/Co3O4 nanocomposite was prepared by Abdolmaleki et al.  using the electrostatic coprecipitation route. They noted that the specific capacitance of f-SWCNT/Co3O4 is high with a value of , while that of the Co3O4 nanoparticles is only (Figure 17). Zarnegar et al.  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.
The cobalt oxide-based composite nanoparticles and its engineering and medical applications were discussed in this book chapter. The pure Co3O4 nanoparticles reveal the presence of toxicity, whereas cobalt compounds are nontoxic. In this book chapter, the synthesis procedure for cobalt and cobalt-based compounds such as GO/Co3O4, ND-Co3O4, zeolite Y/Co3O4, and MWCNT-Co3O4. All the synthesized cobalt-based compound materials present magnetic properties, which can be benefit for the thermal applications, electrical applications, and medical applications. Cobalt oxide-based compounds have improved synergistic properties as compared to pure cobalt oxide nanoparticles.
The author (LSS) acknowledges the Foundation for Science and Technology (FCT, Portugal) for the financial support received through the grant SFRH/BPD/100003/2014. The author (ACMS) acknowledges the 2017 Visiting Scientist Fellowship awarded to him under the Chinese Academy of Sciences President’s International Fellowship Initiative. TEMA/DEM researchers also acknowledge the FCT grant UID/EMS/00481/2019-FCT, the infrastructures support CENTRO-01-0145-FEDER-022083-Centro Portugal Regional Operational Programme (Centro2020), and Project 33912-AAC no. 03/SI/2017, under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund.