Introduction

Nickel oxide has attracted remarkable attention owing to its applications in various areas, such as batteries, gas sensors, catalysis, and energy devices [1,2,3]. Various methods, such as electrospinning [4], and sonochemical [5] and thermal evaporation [6] methods, have been applied to synthesize NiO. However, the electrospinning technique is associated with toxic residual solvents [7], whereas the sonochemical method has low efficiency [8], and thermal evaporation suffers from low material utilization and high energy consumption [9]. The hydrothermal method is a simple and low-cost technique that facilitates crystal growth owing to the high vapor pressures in the synthesis process [10]. Furthermore, the morphology and size of the products of the hydrothermal process can be controlled using surfactants as different surfactants can induce selectivity in the direction of particle growth [11].

The morphology of NiO plays an essential role in its performance [12]. Therefore, several studies have focused on synthesizing NiO nanomaterials with different morphologies, such as nanoparticles (0D) [4], nanosheets (2D) [13], nanorods (1D) [14], and flower-like structures (3D) [15], to enhance the surface area of the particles. Among these, flower-like NiO, which has a porous structure, exhibits superior physicochemical features compared with low-dimensional morphologies [16] because of its complicated structure, which improves the surface to volume ratio and prevents stacking [15, 17].

Although various studies related to the hydrothermal synthesis of NiO with a flower-like structure have been reported, the factors that affect the hydrothermal process, particularly the solvent, are not understood in detail. In this study, we report a hydrothermal method to enhance the uniformity and surface area of NiO nano-flower-like particles. The effects of the molar ratio of the reactants, solvent, and surfactant on the structure and morphology of the as-prepared NiO are discussed. Additionally, a mechanism for the particle growth is proposed.

Experimental

Preparation of Samples

Nickel(II) nitrate hexahydrate (Ni(NO3)2∙6H2O), urea (CO(NH2)2) (Sigma-Aldrich), and cetrimonium bromide (CTAB; C19H42BrN) obtained from Sigma-Aldrich were used as the reactants and surfactants (Figs. 1, 2). Ethanol (Samchun) and deionized water (DI) were used as solvents.

First, Ni(NO3)2∙6H2O and urea (and CTAB) were dissolved in 80 mL solvent. After stirring for 30 min, the mixed solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 150 °C for 2 h. The green product was collected via vacuum filtration and dried overnight at 80 °C in an oven. Finally, the flower-like NiO was obtained by calcinating the green powder at 400 °C for 4 h. The experimental details are listed in Table 1.

Table 1 Experimental details of the synthesis of flower-like NiO
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Fig. 1
figure 1

Schematic of the synthesis of NiO via the hydrothermal method

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Characterization

The morphologies of the precursors and NiO were investigated via scanning electron microscopy (SEM, FEI QUANTA 400). The structure of NiO was examined via X-ray diffraction (XRD, Bruker D8 ADVANCE) using Cu K\(\alpha\) radiation in the 2θ range of 10°–80° and transmission electron microscopy (XPS, Thermo Fisher Scientific K-Alpha).

Results and Discussion

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 [18,19,20].

$${\text{CO}}\left( {{\text{NH}}} \right)_{{\text{2}}} + {\text{ 2H}}_{{\text{2}}} {\text{O}} \to {\text{2NH}}_{{\text{3}}} + {\text{ CO}}_{{\text{2}}}$$
(1)
$${\text{NH}}_{{\text{3}}} + {\text{ H}}_{{\text{2}}} {\text{O}} \to {\text{NH}}_{{\text{4}}} ^{ + } + {\text{ OH}}^{{-}}$$
(2)
$${\text{Ni}}^{{{\text{2}} + }} + {\text{ 2OH}}^{{-}} \to {\text{Ni}}\left( {{\text{OH}}} \right)_{{\text{2}}}$$
(3)
$${\text{Ni}}\left( {{\text{OH}}} \right)_{{\text{2}}} \to {\text{NiO }} + {\text{ H}}_{{\text{2}}} {\text{O}}$$
(4)

Particle Morphology

Fig. 2
figure 2

SEM images of Ni(OH)2-the precursors of a, b NiO-1; c, d)NiO-2; e, f NiO-3; g, h NiO-4; i, j NiO-5; and k, l NiO-6

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Effect of Molar Ratio

Figure 2a–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

Figure 2c, 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. 2c, the precursor obtained using the solvent H2O did not possess a clear flower-like structure. As shown in Fig. 2i, 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].

$$\frac{1}{r}=A+\frac{B}{\epsilon },$$
(5)

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. (5), 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
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Fig. 3
figure 3

a Chemical structure of CTAB; b Schematic of the formation of the precursor in the presence of CTAB

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Effect of Surfactant

Figure 2g, h, k, and l show the differences among the morphologies of the precursors obtained using the cationic surfactant CTAB. As shown in Fig. 2k 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 Fig. 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

Fig. 4
figure 4

a SEM images, and b EDS elemental mapping and EDS spectrum of NiO-6

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Figure 4a 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. 4b. The elemental mapping and spectrum confirmed the presence of Ni and O. The weight% of each element shown in the inset table reveals the high Ni content of the final product.

Crystal Phase

Fig. 5
figure 5

XRD patterns of selected NiO samples

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The XRD patterns of the NiO-2, NiO-4, and NiO-6 samples are shown in Fig. 5. All observed peaks correspond to cubic-structure NiO (JCPDF-01-071-1179). Compared with the peaks of NiO-2, those of NiO-4 and NiO-6 were significantly sharper and higher intensity, thereby suggesting better crystallization [26].

Specific Surface area

Fig. 6
figure 6

Nitrogen adsorption–desorption isotherm of NiO-6

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Table 3 Parameters of various NiO materials
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Table 3 compares the BET specific capacity of NiO-6 (Fig. 6) with those of previously reported NiO materials. Owing to its larger specific capacity, NiO-6 is expected to have more active adsorption and reaction sites.

XPS Analysis

Fig. 7
figure 7

a XPS survey spectrum, b high-resolution Ni 2p XPS spectrum, and c high-resolution O 1s XPS spectrum of the NiO-6 sample

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Figure 7a shows the XPS spectrum of the NiO-6 sample. Evidently, Ni, O, and C were observed in the sample. The XPS peak at 285 eV corresponded to the C 1s core level of adventitious hydrocarbon. The high-resolution Ni 2p XPS spectrum (Fig. 7b) revealed peaks at 853.8 eV and 855.8 eV associated with Ni 2p3/2 and peaks at 871.2 eV and 873.2 eV associated with Ni 2p1/2 [30]. The additional peaks at 861.1 eV and 879.1 eV were assigned as Ni 2p3/2 and Ni 2p1/2 satellites, respectively. Figure 7c shows the O 1s peaks at 529.4 eV and 531.1 eV, which were attributed to NiO and hydroxyl groups on the NiO surface, respectively [31].

Conclusion

In summary, flower-like NiO was successfully synthesized via a urea-based hydrothermal reaction in the presence of the surfactant CTAB as a support for crystallization. The results indicated that the reactant ratio, solvent, and surfactant have a significant impact on the particle size, uniformity, and crystal structure of the final products. The reaction was optimized at a molar ratio of 1:2 for nickel ions and urea. The addition of ethanol to the solvent slowed the transfer of ions and led to slower reaction and smaller particles. The presence of CTAB controls particle growth during the hydrothermal process. NiO has a high BET-specific surface area of 62.7 m2 g–1; thus, it has potential for application in electrodes, catalysis, and adsorption.