Synthesis, Characterization, and Effects of Morphology on the Magnetic Application Base Properties of Pure Nickel Oxide (NiO) and Cobalt-Doped Nickel Oxide/Nickel Hydroxide (CoxNi1-xO/Ni(OH)2) Nanocomposites ()
1. Introduction
The nanoparticle size effect in technological evolution has manifested advantages in physical, chemical, and biological application base properties. Nickel oxide is a perovskite material whose size effect and morphology of nanoparticles have attracted much attention due to the strong correlation between properties, size and morphology. The nickel oxide nanoparticle is a p-type semiconductors with a band gap ranging from 3.1 to 4.0 eV, which has exhibited increased chemical stability, electrocatalytic properties, super conductance and electron transfer capability. Nano particle morphology is at the origin of potential application base properties exploited in several engineering and technology domains, such as photovoltaic [1] , Solar cells [2] , Catalysis [3] , Magnetic [4] , Electric [5] , sensing [6] and energy storage [7] applications. The general properties and particle morphology of nickel oxide NPs can be modified by the introduction of impurity atoms or ions by doping. Several wet chemical methods have been successfully employed for the synthesis of nanoparticles, but the oxalate route has singled itself out by showing control over the nanoparticle (NP) morphology. The oxalate route has the metal oxalate as a single molecular precursor with carboxylate groups acting as chelating ligands between metal atoms and the resultant highly regular 3D structures generally called metal-organic frameworks (MOFs). The oxalate route takes advantage of the fact that the precursor thermally decomposed under mild thermal conditions producing CO2/CO gas mixtures and either pure metal and/or metal oxide depending on the environmental conditions (oxidizing; air or oxygen), neutral (nitrogen, helium) or reducing (hydrogen) [8] . The Carambola fruit juice method is a modified oxalate method which uses naturally occurring dissolved oxalate in the fruit sap. This natural juice rich in oxalate mitigates the solubility problems of commercial oxalates. The juice is ecofriendly, cheap and contains a consequent renewable source of oxalate. Many research efforts have been devoted to thermal decomposition studies of nickel oxalate. In a previous study, Nguimezong et al. [9] and in conformity with other literature demonstrated that the thermal decomposition of nickel oxalate dihydrate proceeded via two main stages; dehydration and decomposition but the characteristics of these stages are greatly influenced by many factors, such as the heating rate, and mass of the sample [10] and, the temperature of preparation and catalysts [11] . The thermal decomposition of nickel oxide in air has been broadly studied [12] [13] [14] , and in all these studies, the decomposition stage was observed to be exothermic with the final product found to be essentially NiO under oxidizing conditions (air, O2). In addition, decomposition in air occurs at a lower temperature. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are the most available techniques used for studying the process of dehydration/decomposition of metal oxalates. These thermal analyses have been coupled with mass spectrometry (MS), gas-liquid chromatography, and Fourier transform infrared spectroscopy to monitor evolving gases. Nickel oxide has been co-doped with various transition metals to modify its morphology and stability. The effects of Co, Cu, and Zn dopants on the optical, morphological, electrochemical, and electrochromic properties of NiO thin films prepared via sol-gel spin coating have been reported, and the NiO thin films were composed of nanoparticles and exhibited increased surface coverage compared with the undoped sample [15] . By incorporating Co, Cu, and Zn dopants, the Co-doped NiO sample demonstrated excellent long-term electrochemical stability, retaining a capacity of ~88% after 2000 cycles, representing a notable improvement compared with the values of ~70%, ~56%, and ~56% recorded for undoped, Cu-, and Zn-doped NiO, respectively [15] . Copper-doped nickel oxide has been evaluated for hybrid nano lubricants as a compressor oil, and the results showed that doping increased the coefficient of performance (COP) value by 18.45% [16] . The effect of cobalt doping on the electrochemical properties of sprayed nickel oxide thin films was reported in the literature [17] , and Co doping of nickel oxide led to a significant enhancement in the electrochemical properties of the NiO electrode material, which is a suitable alternate electrode material for supercapacitor applications. Highly porous Co-doped NiO nanorods with a high specific surface area and abundant nanopores have been synthesized by the oxalate route, and the products showed a significant increase in catalytic activity for the oxygen evolution reaction (OER), which was attributed to a combination of a highly porous NiO nanoarchitecture and the effect of the Co dopant [18] . In an investigation on the suitability of Co-doped NiOx nanoparticles as HTMs, nanoparticles containing different concentrations (0 - 5 mol%) of Co ions were synthesized, and they demonstrated a double effect of Co doping. On the one hand, Co doping arises from surface recombination while on the other hand, Co doping influences the properties of the perovskite grown on top of the Co-NiOx substrates, improving the morphology as a consequence of grain size enlargement and reducing bulk recombination [19] . We think that impregnating NiO with OH may reveal even more interesting morphological changes which will infer on the magnetic properties of the new materials. In this study, nano sized nickel oxide/hydroxide composites were obtained from the thermal decomposition of a single molecular precursor synthesized by a modified oxalate route using Carambola fruit juice as a precipitating agent. The compositional and morphological variations were modified by introducing cobalt as an impurity ion at different w/w % fractions of cobalt (0, 0.1, 0.3, 0.5, 3, 4.0 and 5.0) into the microstructure of the nickel oxide NPs.
2. Materials and Methods
2.1. Materials
Cobalt (II) chloride and nickel (II) chloride were obtained from Sigma Aldrich with 97% purity, while the fruit juice for precipitation was obtained from Ripe Carambola fruits. The juice was prepared as reported in the literature [9] [20] and kept in a freezer until further use.
2.2. Methods
2.2.1. Synthesis Protocol
Cox-Ni1−xO NPs were synthesized by a co-precipitation procedure (x = 0, 0.1, 0.3, 0.5, 3, 5, 40.0 and 50.0). One hundred milliliters of the juice extract were poured into a 250 mL round bottom flask and immersed into a water bath maintained at 80˚C. The appropriate metal ion solutions [(1 − x) + x] 100 mL were added slowly into the juice while stirring for 2 hrs. The mixtures were allowed to cool to room temperature and the precipitates obtained, filtered and washed three times with distilled water followed by ethanol and then dried in a glass oven at 80˚C for 2 h. The dried samples (Precursors) obtained were preserved for analysis.
2.2.2. Characterization Techniques
FTIR spectra of the samples were recorded on a PerkinElmer Spectrum Two Universal Attenuated Total Reflectance Fourier Transform Infrared (UATR-FTIR) spectrometer in the range of 390 - 4200 cm−1. Thermogravimetric analysis (TGA) was obtained using a Pyris 6 PerkinElmer TGA 4000 thermal analyzer. The thermal studies to determine the right annealing temperature were conducted between 30˚C and 900˚C in air at a flow rate of 20 mL/min and a temperature ramp of 10˚C/min. To identify the structure and phase purity of the prepared sample, powder X-ray diffraction (XRD) analysis was performed on a Philips PW-1710 diffractometer with CuKα radiation (λ = 1∙540598 Å) at room temperature. The surface morphology of the prepared sample was determined by scanning electron microscopy (SEM, Philips XL-20) and low magnification TEM, while the magnetization measurement (M-H characteristics) of the sample was recorded using a vibrating sample magnetometer (VSM) where the magnetic field could be varied up to 1∙8 Tesla.
3. Results and Discussion
3.1. Characterization of the Samples
3.1.1. Thermogravimetric/Differential Thermogravimetric Analysis (TGA/DTG)
Thermal studies by TGA/DTG analysis revealed the decomposition profile of the precursors and suggested the decomposition temperature of the samples. Figure 1(a) shows the TGA/DTG profiles of the precursors as consisting of two stages, notably dehydration and decomposition, while the EDS reveals that nickel was the metal ion present in the final decomposition product. The dehydration stage occurring at 108˚C ± 2˚C is accompanied by a mass loss of 17.4% ± 0.4% relative to 19.718% theoretically expected. The difference of 2.318% is attributed to the existence of dehydrated and hydrated nickel oxalate [21] [22] or adsorbed water molecules in the pores of the dehydrated nickel oxalate crystallites [23] . The decomposition stage which occurred at 380˚C ± 8˚C and accompanied by a mass loss of 29.27%, which compares well with the theoretically calculated value (29.372%). However, the difference in water of hydration now liberated at the decomposition stage revealed a net mass of 2.42% attributed to some molecules of hydroxide Ni(OH)2 in the pores of the nickel oxide crystallites formed or just adsorbed on the nickel oxide surface by chemisorption. This observation was confirmed by the EDS of the decomposed samples integrated in Figure 1(a). However, annealing at a higher temperature (600˚C), leads to the decomposition of the relatively stable hydroxide intermediate to pure NiO nanoparticles. The relatively stable intermediate also decomposed when annealing at 400˚C is prolonged to 6hrs. These results suggested that the total decomposition of the reaction intermediate required prolonged heating (calcinations for a longer period). It is interesting to note that the samples exhibited the same thermal decomposition profile, as shown in Figure 1(a) and Figure 1(b). The observation is attributed to the closeness of the chemistry of nickel oxide to that of cobalt oxide (CoO and Co3O4), as observed from their heat of formations −57.3 ± 0.1 and −57.1 ± 0.3 kcal/mol. for NiO and CoO, respectively (Boyle B. J., 1954) [24] . It is by this advantage that the nanocomposite Ni1−xCoxO/Ni(OH)2 is obtained by decomposing the precursor at 400˚C for 3 hrs.
3.1.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis
The FTIR profiles for the pure and Co-doped nickel oxalate (precursors) are presented in Figure 2. The principal FTIR characteristic absorption bands of samples x = 0.0%, 0.1%, 0.3%, 3.0% and 5.0% and their assignments are summarized in Table 1. The profiles of the samples in the series of precursors and decomposed products are similar within each series but differ between the series, indicating that new bonding modes existed in the products. However, within each series, slight shifts were revealed as the doping fractions increased. The shift
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Figure 1. (a) Decomposition profiles (TGA/DTG) of the as-synthesized precursors x = 0.0%, and EDS, showing the metal ions present in decomposed samples; (b) decomposition profiles (TGA/DTG) of the as-synthesized precursors (x = 0.1%, 0.3%, 3.0%, and 5.0%).
is attributed to the presence of cobalt ions in the microstructure of NiO, and the insignificance in variability between the profiles after doping could be attributed to the closeness of the bonding properties between nickel and cobalt. In the decomposed products and for all doping fractions, two absorption bands were observed at 542 cm−1 and 358 cm−1 corresponding to the metal-oxygen bonds (Ni-O and Co-O, respectively), which were in agreement with the expected results. The absence of the free carbonyl [ν(C=O)] vibration expected at approximately 1700 cm−1 is an indication that the carboxylate group is completely involved in the coordination. The bonding mode of oxalate is determined by calculating the separation between the bands, Δν (= νCOOasym − νCOOsym), which presented Δν between 140 cm−1 and 200 cm−1 suggesting that the bonding mode of the oxalate ions with the metal atoms is the bridged bidentate mode [25] . Our results showed that Δν was 150 cm−1, corresponding to the bridged bidentate, which indicated.
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Table 1. FTIR absorption bands of samples x = 0.0%, 0.1%, 0.3%, 3.0%, and 50.0%, and their assignments.
υ: stretching (as: asymmetric; s: symmetric); δ: deformation in the plane; π: deformation outside the plane and ρ: wagging.
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Figure 2. (a) FTIR spectra of the as-synthesized precursors; (b) FTIR spectra of the decomposed products.
3.1.3. Phase Identification
The powder X-ray diffractograms (PXRD) for the calcined products of the samples are shown in Figure 3(a) (sample calcined at 400˚C for 6 hrs.) and Figure 3(b) (samples calcined at 400˚C for 4 hrs.). Diffraction peaks identified from Figure 3(a) are at 2θ values of 37.70˚, 43.54˚, 61.84˚, and 74.65˚ indexed to the (111), (200), (220) and (311) crystal planes, which match with [JCPDS card No: 47-1049] of nickel oxide nanofibers. However, in Figure 3(b), in addition to the peaks observed in Figure 3(a), there is another peak at 2θ values of 51.64 indexed to the (112) crystal plane which matches with [JCPDS card no. 14-0481] of Ni(OH)2. There were no other impurity peaks suggesting that the samples calcined at 400˚C for 3 hrs are made up of a mixed phase of NiO and Ni(OH)2.
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Figure 3. (a) X-ray/EDS of the as-synthesized composites (x = 0) calcined at 400˚C for 6 hrs; (b) PXRD of the as-synthesized composites (x = 0.0%, 0.1%, 0.3%, 0.5%, 3.0% and 5.0%) calcined at 400˚C for 3 hrs.
The absence of the peak at 2θ value of 45.34 in the X-ray diffractograms of the products indicated the absence of Ni metal in the decomposed sample. Although there are no metal oxide impurity peaks at low Co doping fractions, there are slight phase shifts in the XRD peak position of the crystal planes of the NiO phase towards higher angles as the doping fraction increased, which demonstrated the incorporation of Co impurity atoms in the host matrix. The substitution came with a little change in the Co/Ni atomic radius ratio (although there is a close similarity in the radius of Co(0.79 Å) and Ni(0.83 Å) atoms), which leads to the distortion in the crystallite microstructure of Nickel oxides and the eventual shifts in 2θ values. A separate evaluation of the individual X-ray diffractograms showed very little change in peak intensity, especially after 0.5% mol Co doping, indicating very little variability in the crystallite sizes. However, the initial increase in intensity for doping fractions up to 0.3% mol Co-doping indicated an increase in the number of crystallites and similar observations were reported in the literature [28] [29] [30] .
The crystalline sizes of the particles (pure and cobalt-doped NiO/Ni(OH)2 nanoparticles) were determined using Debye Scherer’s relation, Equation (1)
(1)
where D is the crystalline size, β is the full width half-maximum value of the high intensity peak, k is the shape factor (0.9), λ = wavelength of the radiation with value (λ) = 0.15406 nm, θ = Bragg’s angle and β = full width at half maximum (FWHM) peak. Line broadening due to the instrument was subtracted from the peak width before calculating the crystallite size using Equation (2):
(2)
where β means = measured full width at half maximum of peak β, equi = instrumental broadening Standard Silicon sample was used, whose β value was 0.09821 at 2θ = 29˚ with (hkl) values (200) for NiO and (112) for Ni2O3 profiles in Figure 3. The particle distancing was calculated using Bragg’s equation (Equation (3)):
(3)
For n = 1, and λ = 0.154 nm.
The crystallite sizes are found to decrease with increase in dopant concentrations and vary between 7.8 nm, 7.3 nm, and 7.1 nm for various identified diffraction peaks. These results compare with those reported in the literature [31] .
3.1.4. Morphology: Scanning Electron Microscopy (SEM)/Low Magnification Transmission Electron Microscopy (TEM)
Figure 4 shows the scanning electron micrographs of the synthesized samples. Three different morphologies are observed as the concentration of the dopant increased. The predominant morphology is the porous nanorods, which are exhibited by five different test samples (x = 0.0, 0.1, 0.2, 0.3, 0.5).
A clearer view of the nano rod morphology is observed under TEM at low magnification (Figure 4(b)), where the nano rods are seen to be rough surfaced, made up of nano grains with distinct boundaries, regularly arranged and interspaced with pores. The nano rods measured about 1 μm in length and 100 nm in diameter. The presence of pores granted hydroxides molecules easy access. The dopants ions had three possibilities of reacting with the main matrix: to replace nickel atoms, fill the pores, or be adsorbed on the rough nano rod surfaces. At lower Co doping mol% fractions, such as 0.1 mol% and 0.3 mol%, the cobalt atoms are systematically substituted for Ni atoms in the nano rod microstructure, thereby modifying the surface area and the size of the nano rods while at relatively high dopant concentrations, such as 3%, both impregnation of the pores and adsorption of the Co atoms in the NiO matrix occur. As the doping progressed to very high concentrations of dopants (Co), such as 40% and 50%,
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Figure 4. (a) SEM pictures of Ni1-xCoxO (x = 0.0%, 0.1%, 0.2%, 0.3%, and 0.5%); (b) low-magnification TEM image of a pure NiO nano rod (x = 0.0%); (c) SEM pictures of composites with high concentrations cobalt ions (x = 3.0%, 40.0%, and 50.0%).
the pores are further impregnated with the dopants leading to the disintegration of the nano rod framework into cuboidal and spherical grains, attributed to CoO and Nickel Oxide faces (Figure 4(c)). These grains agglomerated into porous aggregated clusters held together by strong attractive forces which interconnect the masses. The particle sizes observed from the TEM micrograph are in the range of 10 nm, which confirms the grain size calculation from the XRD results (between 7.0 - 7.8 nm).
3.2. Magnetic Studies: Vibrating Sample Magnetometry (VSM)
Five of the samples selected from three different morphologies (Porous nano rods, nano rods with filled/filling pores, and interconnect masses) were subjected to magnetic studies to evaluate the effects of dopant concentration and/or morphology of the products on the magnetic properties. The magnetic parameters of the pure products and Co-doped nanoparticles are presented in Table 2, while Figure 5 shows the magnetic characteristic curves of the samples analyzed using a vibrating sample magnetometer at room temperature. The magnetization curve for the synthesized magnetite nanocomposites all showed very small hysteresis behaviors and exhibit relatively very low remnant magnetization (0.00105 - 0.00903 emu/g). These suggest that the synthesized nano composites display super paramagnetic behavior at room temperature. The observation is attributed to the very small particle sizes (7.0 - 7.8 nm) of the paramagnetic synthesized Nanocomposites which are smaller than the critical size of the magnetic domain size (20 nm) [27] . Furthermore, the nanocomposites also show very low saturation magnetization (Ms) values compared to bulk. The saturated magnetization values of the nanocomposites obtained were in the range (0.0183 - 0.240 emu/g) suggesting the existence of paramagnetic behavior.
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Figure 5. Room temperature hysteresis loops of the as-synthesized nanocomposites.
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Table 2. Magnetic parameters of pure, and co-doped NiO/Ni(OH)2.
The retention (the ability of the nanoparticle to retain magnetization after the applied field is removed) increases with increase in the concentration of Co without respecting the doping fraction trend. This is an indication that another parameter which does not have a direct correlation with crystallite size is responsible for the variation. However, this observation is suggesting that the modification has a link with the surface area of interaction while the surface area is itself linked to the morphology. The highest retention was recorded for the CoO/NiO sample (x = 50%). In a similar manner, the coercivity also varied with particle morphology, while the saturation magnetization increased with increasing concentration of cobalt, then decreased at x = 3.0%, and further increased for x = 50% (CoO/NiO). At the early stages of doping, x < 0.5%, the morphology (porous nano rods) presents a larger surface area of interaction due to porosity and small particle size giving rise to uncompensated spins in the magnetic nanoparticles [32] . At doping fractions of 0.3% < x < 3%, the Co ions, begin to fill the pores instead of replacing the Ni ions and as a consequence reducing the surface area of interaction and the saturation magnetization begins to decrease. At very high doping fractions, X = 50%, there is a complete breakdown of the nano rod microstructure, giving way to a continuous smooth interconnected network of spherical and cubic nano grains, which give rise to the unusual magnetic behavior resulting from the breaking of a large number of exchange bonds.
This also suggests the presence of small magnetic clusters on the surface with lattice imperfections as the translation symmetry of the system is broken down, the antiferromagnetic ordering of NiO becomes disturbed, and the uncompensated spin values increase. The squareness values (remanence ratio) for the as-prepared samples are presented in Table 1. The samples have R values in the range 1 > R > 0.5, indicating the existence of exchange coupling in the NPs except CoO/NiO with 0.5 > R, where NPs interact by magnetic interaction [33] . Particle sizes of 7.0 - 7.8 nm are far below the critical particle size of 20.0 nm [34] . The small particle sizes give rise to a high surface area, which are further favored by the morphology of the microstructure which encourages ferromagnetic behavior at room temperature due to a higher concentration of unpaired spins on the surface. The NPs in the samples form single domains with the spins aligned in the same direction, leading to uniform magnetization of the sampled material. The relatively large coercivity is attributed to the absence of domain walls, and this situation allows the NPs to freely reverse spin by rotation.
4. Conclusion
The magnetic properties manifested by the as-synthesized nanocomposites are known to improve the magnetic memory and hardness of the magnetic materials. Therefore, we can conclude that the Cobalt-doped Nickel Oxide/nickel hydroxide (CoxNi1−xO/Ni(OH)2) Nano composites synthesized for the very first time by a modified oxalate route using Carambola fruit juice as a precipitating agent, have potential applications in Magnetic memories and hardness of magnetic materials.
Data Availability
All dataset generated to support the conclusions of the study have been completely exploited and are available within the text in the form of Tables, Figures (graphs or photos) for the readers appreciation.
Ethical Approval
This article does not contain any studies with human participants or animals performed.
Authors Contributions
All authors have participated sufficiently in all the phases of the work and agree to take public responsibility for the content, including participation in the concept, design, analysis, writing and/or revision of the manuscript.