Optimization of hydrothermal synthesis of nickel oxide with ower-like structure

Herein, nano-ower NiO is successfully fabricated via a simple hydrothermal process using urea and nickel(II) nitrate as reactants, followed by a calcination reaction. Final products with different morphologies are obtained by varying the molar ratio of the reactants, varying the solvent, and using a surfactant. The results reveal that the NiO particles obtained using a molar ratio of 1:2 (Ni: urea) in a mixture of water and ethanol as the solvent and in the presence of cetyl trimethyl ammonium bromide (CTAB) exhibit the best uniformity and an excellent BET speci�c surface area of 62.97 m 2 g –1 . The increase in uniformity and decrease in particle size can be attributed to the ethanol in the solvent, which slows ion diffusion in the solution and CTAB, thereby controlling the growth of particles.


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 e ciency [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 ower-like structures (3D) [15], to enhance the surface area of the particles.Among these, ower-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 ower-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-ower-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.
First, Ni(NO 3 ) 2 •6H 2 O and urea (and CTAB) were dissolved in 80 mL solvent.After stirring for 30 min, the mixed solution was transferred into a 100 mL Te on-lined stainless-steel autoclave and heated at 150°C for 2 h.The green product was collected via vacuum ltration and dried overnight at 80°C in an oven.
Finally, the ower-like NiO was obtained by calcinating the green powder at 400°C for 4 h.The experimental details are listed in Table 1.

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 radiation in the 2θ range of 10°-80° and transmission electron microscopy (XPS, Thermo Fisher Scienti c K-Alpha).

Results and discussion
The ower-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),  2+ 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 nonuniformity.shown in Fig. 2(i), the precursor obtained using ethanol had a ake-like shape rather than a ower-like structure.The precursor obtained using the H 2 O/ethanol solvent possessed a ower-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].

Effect of the solvent
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 nal morphology of the product.A highdielectric-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 ( ower-like structures) [23].Therefore, the product obtained in water alone exhibited a lowerdimensional morphology, in which the ower-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]: where is the total cohesion energy of the solvent; and , , and are the dispersive polar and hydrogen-bonding solubility parameters, respectively.A smaller 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-ake structure for the solvent of ethanol.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 akes 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 ower-like product with a smaller size.
Figure 4(a) shows the morphology of NiO-6.Evidently, after calcination, the ower-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 con rmed the presence of Ni and O.The weight percentage of each element shown in the inset table reveals the high Ni content of the nal product.
3.2.Crystal phase 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 signi cantly sharper and higher intensity, thereby suggesting better crystallization [26].Table 3 compares the BET speci c capacity of NiO-6 (Fig. 6) with those of previously reported NiO materials.Owing to its larger speci c capacity, NiO-6 is expected to have more active adsorption and reaction sites.

Conclusion
In summary, ower-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 signi cant impact on the particle size, uniformity, and crystal structure of the nal 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

Figures 2 (
Figures2 (c), (d), and (g)-(j) show the SEM images of the precursors obtained using different solvents.Evidently, the solvent signi cantly affected the assembly and packing of the particles.As shown in Fig.2(c), the precursor obtained using the solvent H 2 O did not possess a clear ower-like structure.As

Figure 7 (
Figure 7(a) 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. 7(b)) revealed peaks at 853.8 eV and 855.8 eV associated with Ni 2p 3/2 and peaks at 871.2 eV and 873.2 eV associated with Ni 2p 1/2 [30].The additional peaks at 861.1 eV and 879.1 eV were assigned as Ni 2p 3/2 and Ni 2p 1/2 satellites, respectively.Figure 7(c) 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].

Figures Figure 1 2 SEM
Figures