The synthesis of nanosized or mesosized crystalline metal oxides is increasingly gaining research interest because of their adsorptive properties, large surface area, surface defects and fast diffusivities [1]. They exhibit novel material properties that are very different from those of their bulk counterparts due to their small sizes. It has been established that metal and metal oxides nanoparticles are capable of increasing chemical reaction activities due to the high ratio of surface atoms with free valences to the cluster of total atoms [2].

Cobalt (Co)-based nanomaterials have attracted numerous research interest because of their good electro-activity and low cost [2–4]. Cobalt is a transition metal and is capable of exhibiting variable oxidation states (Co2+, Co3+, Co4+). This multivalent state of cobalt provides it the ability to be present in various spin states in its oxide forms: Low, high as well as intermediate spin [5]. Co3O4 exists as a regular spinel structure with Co3+ in the octahedral sites and Co2+ in the tetrahedral site. This is attributable to the stability of the Co3+ (low spin d6) in the octahedral crystal field. The presence of octahedral and tetrahedral sites in Co3O4 results in two-sublattices in the crystal, hence in its bulk form, Co3O4 is anti-ferromagnetic with a Neel temperature of 33K [6,7]. However, unlike the bulk form, Co3O4 particles exhibit a weak ferromagnetic behaviour. This is consistent with early predictions by Neel who suggested that at the nanoscale the reduced coordination of surface spins will result in a change in magnetic order making materials that are anti-ferromagnetic in their bulk form to become weakly ferromagnetic or superparamagnetic [6]. The effects of quantum confinement and surface effects and variable oxidation state of cobalt have made Co3O4 nanoparticles find immense applications in areas such as catalysis, intercalation compounds for energy storage in Li-ion batteries, gas sensors, electrochemical devices, pigments, high-temperature solar absorbers, magnetic materials among others [8,9].

Previous research work on Co3O4 focused on the synthesis of novel morphologies of the oxide and their applications. Some notable physical and chemical synthetic routes have been used in the synthesis of nanocrystalline Co3O4. Sol–gel synthesis, spray pyrolysis, coprecipitation, reduction, electrodeposition, chemical vapour deposition, thermal decomposition, solvothermal, microemulsion, pulsed laser deposition, sputtering, among others have been reported [8,10–15]. Though, these methods have shown varying degrees of successes in producing Co3O4 nanoparticles with good properties and some control over the obtained morphology, there are some drawbacks such as high cost, and dependence of particle growth kinetics on parameters such as pH, temperature, adsorption of surfactants [16]. Furthermore, some of the surfactants and stabilizers applied in the synthesis procedure tend to be hazardous with carcinogenic and cytotoxic effects making the synthesized nanomaterials unsuitable for clinical and biomedical applications. Consequently, current research interest is drifting towards green synthesis involving the use of environmentally friendly biological materials in the synthesis of nanomaterials (biosynthesis). Besides producing environmentally friendly nanostructures, biosynthetic routes provide control over the obtained morphology of particles by serving as templates for the hierarchical assembly of obtained nanostructures into complex micro, meso and macro entities [16]. Biosynthesis typically employs the use of bacteria, yeast, viruses, plants, fungus, actinomycetesetc. for the production of nanomaterials. Among these biomaterials, the use of plants provides the unique advantages of simplicity, scalability for production on a large scale. It is much faster since it does not require the complex multi-step processes of isolation, culture preparation, and culture maintenance typical in microbe-based synthesis [16]. Biosynthetic routes based on plants typically employ the phenolic, alkaloids, proteins, sugars, and terpenoids present in plants as stabilizers and reducing agents for the synthesis of the desired nanostructures. Initially, these chemicals aid the conversion of the metal precursor from their high oxidation states to zerovalent states, followed by nucleation. Growth occurs thereafter to yield nanoparticles which form aggregates based on the intermolecular interactions between the nanoparticles and the plant extracts to yield the most stable nanoparticles’ morphologies [16]. The pH, concentration (of both the metal precursors and plant extracts), temperature, reaction time are among the factors that control particle shape, sizeand morphology. Some notable research efforts involving the use of plant extracts in nanoparticles synthesis have been recorded [5,17].

Manihot esculenta Crantz is a woody shrub found in South America. It has been identified as a staple food for more than 500 million people in the tropics, many of whom are very poor [18]. It is considered a major source of industrial raw material and income for rural communities in most African Countries [19,20]. However, It contains anti-nutritional cyanogenic glucosides such as Linamarin and Lotaustralin The main organ of storage of the plant is the root and it has three distinct parts: bark (Periderm), Peel (Cortex) and Parenchyma. However, the consumption of Manihot esculenta Crantz roots and leaves are restricted by the cyanide content. The root Parenchyma has a cyanide content ranging from 10 to 500mg cyanide equivalent/kg dry matter [21]. In this study, we report for the first time, the use of a natural extract of Manihtot esculenta Crantz Parenchyma as a chelating agent in the production of magnetic cobalt oxide (Co3O4) for multifunctional applications.

The X-ray diffraction pattern of the as-synthesized Co3O4 mesoparticles is shown in Fig. 1a. The XRD pattern indexed to (111), (220), (311), (222), (400), (511) (440) and (620) planes corresponding to the face centre cubic phase which matched with JCPDS No. 00-042-1467 with FD- 3m (227) space group and lattice constant, α=8.08A. No characteristic peaks of other impure phases were detected. This showed that the as-synthesized black powder is of high quality and highly crystalline. The XRD results further showed that the Co3O4 samples crystallized at a single phase but, they have different FWHM which means they have different particle sizes. The precipitated Co3O4 sample had higher FWHM than the supernatant as evaluated from the XRD peaks. This resulted in reduced particle size as seen from the calculated Scherrer's equation.

Fig. 1.

(a) X-ray diffraction pattern of the as-synthesized cobalt oxide particle annealed at 500°C; (b) FT-IR spectra of the as-synthesized cobalt oxide particles.

(0.17MB).

The crystallite size of the synthesized Co3O4 was determined using the Scherrer's formula Eq. (1) to be ca. 36nm while the lattice strain is 0.0032.

Comparison of the intensity of the XRD peaks from the two annealed samples showed that the particles obtained from the supernatant were more crystalline than those obtained from the supernatant. This was also in agreement with the result obtained from the selected area diffraction in Fig. 4a and b. The FTIR spectrum of the prepared Co3O4 samples is shown in Fig. 1b. Characteristic peaks were observed at 561 and 665cm−1. These correspond to Co–O bending mode of Co3O4[13]. There were also weak bands at 2922cm−1 and 1733cm−1. These could indicate the presence of O-H stretching vibrations from water vapour absorption during analysis.

Electron microscopy such as Scanning electron and Transmission electron microscopy (SEM and TEM) were used to evaluate the morphology of the Co3O4 mesoparticles. Fig. 2a and b shows the SEM while and Fig. 3c and d shows the TEM micrographs of the as-synthesized Co3O4 particles annealed at 500°C. The particles were found to be agglomerated and the relative particle sizes could not be measured. This is attributed to nucleation and crystal growth at temperature [23]. Yin and Alivisatos reported that semiconducting nanoparticles growth by thermal decomposition method takes place in many stages [24]. Firstly, the molecules of the precursors decompose and convert into complex intermediate reportedly called monomers. When this monomer exceeds the critical value, nucleating is reached by clustering of the monomers. This enables crystalline particles to grow around the nucleation sites.

Fig. 2.

(a) SEM images of hierarchical cobalt oxide mesostructures showing fracture free octahedron-like shape. (b) SEM images of mesostructure cobalt oxide particles showing the layer–layer arrangement of the supernatant.

(0.19MB).

The crystallinity of the as-prepared sample was improved by the annealing process. This annealing process became necessary, because, the synthesis was a facile green approach (ambient temperature). After annealing, the samples become highly crystalline as seen from the selected area electron diffraction (SAED), Fig. 3a and b. It is believed that the particle size is controlled by the reaction temperature at the initial stage of nucleation. Thus, the reaction took place at room temperature and this caused the reacting precursor to change into amorphous cobalt oxide, hence giving rise to particles with large particle size distribution and agglomeration. This observation is similar to previous report [23] where it was noted that further annealing of amorphous cobalt oxide nanoparticles with broad particle size gives rise to asymmetric particle size growth of nanocrystals with large size distribution.

Fig. 3.

Characterization of Co3O4 mesoscale particles synthesized by a facile green approach using Manihot esculenta extract. Characterization of Co3O4 nanoparticles synthesized by a facile green approach using Manihot esculata extract. (a & b) SAED patterns, (c & d,) TEM images, (e & f) lattice fringes and (g & h) EDAX spectra of the as-synthesized Co3O4 mesooparticles (precipitate & supernatant) annealed @ 500°C.

(0.61MB).

The corresponding selected area electron diffraction (SAED) pattern (Fig. 4a & b) showed spot diffraction pattern indicating that the Co3O4 mesocrystals are arranged more or less in the same crystallographic direction. This is one of the characteristic for crystal formation via solvent-mediated mechanism. The SAED patterns consist of well-defined rings which could be indexed with face-centred cubic Co3O4 phase. It is important to know that no reflection of other phases was found. This is further supported by the XRD result in Fig. 1a showing the pure and single phase of Co3O4 mesoscale particles with regular lattice fringes as shown in Fig. 3e and f. This is further supported by the EDAX spectra in Fig. 3g and h, showing peaks corresponding to Co, and O2. A closer look at the octahedron crystal shown in Fig. 2a and b and the SAED pattern in Fig. 3a and b obviously shows evidence of the formation of polycrystals with well-defined diffraction spots.

Fig. 4.

SEM images of the assembled mesostructured cobalt oxide particles. a) Precipitate and b) Supernatant.

(0.26MB).

3.2Magnetic and optical properties

The magnetic hysteresis curve at 5K and 300K is presented in Fig. 5a. The magnetization of the samples was measured as a function of the magnetic field (H,T) at 5K and 300K in the applied magnetic field sweeping between ±5T. The magnetization curve looks antiferromagnetic. This antiferromagnetic behaviour exhibited by the as-synthesized Co3O4 mesoscale particles is similar to antiferromagnetic behaviour of the bulk Co3O4. These implies that an ideal bulk antiferromagnetic Co3O4 exhibits magnetic moment per particle which is equal to zero. Therefore, there could be complete compensation of sub-lattice spins [15,31]. This explains the mesoscale particles size.

Fig. 5.

(a) Saturation magnetization; (b) band gap of Co3O4 mesoparticles particles.

(0.1MB).

This implies that Co3O4 particles have a normal cubic spinel structure with an antiferromagnetic exchange between ions which occupy the tetrahedral and the octahedral sites [32,33]. Hence, it has zero net magnetization due to complete compensation of sub-lattice magnetization

The optical spectra of the mesostructured Co3O4 particles were measured on an opaque substrate Fig. 5b.

The band gap of the as-synthesized Co3O4 mesoparticles was evaluated using Tauc relation[34].

where, h: Plank's constant, v: frequency of vibration, α: absorption coefficient, Eg: bandgap, A: proportional constant, n: denote the nature of the electronic transition of the sample which is responsible for adsorption. The values are ½, 3/2, 2 or 3. In this analysis, direct allowed electronic transition was used, therefore n was ½.

The acquired diffuse reflectance spectrum was converted to Kubelka–Munk function as the sample was measured on an opaque substrate [35]. Thus, the vertical axis was converted to F(R∞) quantity, which is proportional to absorption coefficient. Furthermore, α in the Tauc equation is replaced with (R∞), the actual relation used in the experiment becomes,

It is reported that bandgap varies with crystallite sizes, usually due to O2−–Co3+and O2–Co2+ charge transfer processes [36,37]. We obtained a band gap of 2.86eV and 2.54eV which was slightly higher and lower than the reported bulk value (Eg=3.17eV and 177eV respectively). As earlier mentioned, increase in band gap can be attributed to quantum confinement and small grain size of the Co3O4 mesoparticles [14,38].

This study showed that mesostructured Co3O4 of high industrial potentials can be synthesized by a more environmentally friendly process using the extract of Manihot esculenta Crantz extract as a chelating/stabilizing agent. The as-synthesized mesostructured Co3O4 had a prism like-anchored octahedron morphology having antiferromagnetic behaviour and a band gap of 2.86eV and 2.54eV respectively. It is hoped that this facile green, more environment-friendly approach will be adopted in the synthesis of Co3O4 mesoparticles.

No conflict of interest was reported by the authors.