The flower-like NiO particles were synthesized via a hydrothermal route using urea as a precipitant. The chemical process used to form NiO, i.e., the chemical reactions related to the decomposition of urea (1), generation of OH– (2), precipitation of Ni(OH)2, and decomposition of Ni(OH)2 to form NiO at high temperature (3–4), can be described as follows [18–20].
3.1. Particle Morphology
Effect of molar ratio
Figures 2 (a)–(f) show SEM images of the precursors prepared with different ratios of Ni2+ and urea. The precursors exhibited a decreased particle size with increasing urea concentration. Generally, particle formation involves two processes: nucleation and crystal growth [19]. The number of nuclei is closely related to the concentration of urea, which is the source of OH–. As the amount of urea is increased, nucleation tends to increase, thus leading to a decrease in particle size. However, when the concentration of urea is extremely high, nuclei are formed too quickly, thus resulting in agglomeration and non-uniformity.
Effect of the solvent
Figures 2 (c), (d), and (g)–(j) show the SEM images of the precursors obtained using different solvents. Evidently, the solvent significantly affected the assembly and packing of the particles. As shown in Fig. 2(c), the precursor obtained using the solvent H2O did not possess a clear flower-like structure. As shown in Fig. 2(i), the precursor obtained using ethanol had a flake-like shape rather than a flower-like structure. The precursor obtained using the H2O/ethanol solvent possessed a flower-like architecture with no agglomeration, higher uniformity, and smaller particle size compared with the precursors obtained using the two other solvents. Note that crystallization is affected by the dielectric constant of the solvent and the dispersion of the particles in the solvent. The relationship between particle size and dielectric constant can be described by the following formula [21].
$$\frac{1}{r}=A+\frac{B}{\epsilon },$$
1
where r is the radius of the particles; A and B are constants; and ε is the dielectric constant of the solvent. Based on Eq. (1), an increase in the dielectric constant leads to a larger particle size. The dielectric constant of ethanol is 24.55 (at 25°C) [22], which is lower than that of water (78.4 at 25°C) [22]. Therefore, the greater the proportion of ethanol in the solvent, the smaller the particle size will be. Moreover, the dielectric constant of the solvent affects the final morphology of the product. A high-dielectric-constant solvent can weaken the interactions between ions, thus resulting in 1D linear crystal growth. In contrast, a low dielectric constant facilitates ion diffusion, thus enabling 2D or 3D crystal growth (flower-like structures) [23]. Therefore, the product obtained in water alone exhibited a lower-dimensional morphology, in which the flower-like architecture was not clear.
Particle dispersion in solvents is predicted by the Hansen solubility parameters (HSP) (Table 2), which describe the liquid cohesion property as [24]:
$${\delta }_{\text{t}\text{o}\text{t}\text{a}\text{l}}^{2}={\delta }_{\text{d}}^{2}+{\delta }_{\text{p}}^{2}+{\delta }_{\text{h}}^{2},$$
where \({\delta }_{\text{t}\text{o}\text{t}\text{a}\text{l}}\) is the total cohesion energy of the solvent; and \({\delta }_{\text{d}}\), \({\delta }_{\text{p}}\), and \({\delta }_{\text{h}}\) are the dispersive polar and hydrogen-bonding solubility parameters, respectively. A smaller \({\delta }_{\text{t}\text{o}\text{t}\text{a}\text{l}}\) value is expected to result in stronger dispersibility [24]. Good dispersibility is necessary to obtain uniform particles. However, too high dispersibility prevent the assembly of particle, resulting in nano-flake structure for the solvent of ethanol.
Table 2
Hansen solubility parameters of the solvents
Solvent
|
\({\delta }_{\text{d}}\) (MPa0.5)
|
\({\delta }_{\text{p}}\) (MPa0.5)
|
\({\delta }_{\text{h}}\) (MPa0.5)
|
\({\delta }_{\text{t}\text{o}\text{t}\text{a}\text{l}}\) (MPa0.5)
|
Water [25]
|
15.6
|
16.0
|
42.3
|
47.8
|
Water: Ethanol
|
15.7
|
12.4
|
30.8
|
36.7
|
Ethanol [25]
|
15.8
|
8.8
|
19.4
|
26.5
|
Effect of surfactant
Figures 2 (g),(h), (k), and (l) show the differences among the morphologies of the precursors obtained using the cationic surfactant CTAB. As shown in Figures 2 (k) and (l), the particle size of the precursor obtained using CTAB was smaller than that of the precursor synthesized without surfactant. Moreover, the assembly of the flakes was more rigid owing to the effect of CTAB. The crystallization mechanism in the presence of CTAB is shown in Figure 3. Initially, the precursor was formed in an amorphous state. During the hydrothermal synthesis reaction, CTAB particles covered the outside of the amorphous precursor and prevented its agglomeration during crystallization, thereby resulting in a flower-like product with a smaller size.
Figure 4(a) shows the morphology of NiO-6. Evidently, after calcination, the flower-like structure was maintained, with an average diameter of 3 µm.
The chemical composition of NiO-6 was analyzed using EDS, as shown in Fig. 4(b). The elemental mapping and spectrum confirmed the presence of Ni and O. The weight percentage of each element shown in the inset table reveals the high Ni content of the final product.