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. 2022 Nov 26;7(48):43958–43964. doi: 10.1021/acsomega.2c05281

Sulfur-Mediated Synthesis of Spherical Nickel Nanoparticles in a Chemical Vapor Reactor

Yong-Su Jo †,‡, Mansurbek Abdullaev §, Gwang-Hwa Jin †,‡, Bhabani Sankar Swain †, Byongpil Lee ∥, Hyung-Sub Eom ∥, Seok-Hong Min ⊥, Seok Ki Kim #,*, Young Keun Kim ‡,*, Seung-Min Yang †,*
PMCID: PMC9730454  PMID: 36506157

Abstract

graphic file with name ao2c05281_0009.jpg

In this study, we investigated the effect of the sulfur content in the NiCl2 precursor on the shape of nickel nanoparticles (Ni-NPs) prepared by chemical vapor synthesis. We obtained spherical Ni-NPs when using anhydrous NiCl2 mixed with NiSO4 or Na2SO4 with a molar ratio of 0.002 as precursors without changing any other process parameters whereas faceted Ni-NPs when using only anhydrous NiCl2 as a precursor. First-principles calculations supported experimental results, which showed that NiSO4-mixed NiCl2 and Na2SO4-mixed NiCl2 precursors favored the growth of spherical NPs.

1. Introduction

Tuning the shape and size of nanoparticles is technologically important because macroscopic properties of NPs depend on their shape and size.1−8 Enhancement of magnetic properties or catalytic activity by changing the shape of NPs has been reported.9,10 Ni-NPs are most often used as electrode materials for multilayer ceramic capacitors (MLCCs). If the packing density of Ni-NPs is low when they are printed on the ceramic layer of MLCCs, then the porosity of electrodes will be high, and it may result in short circuits. If cubic NPs are ideally well-arranged, a higher packing density than spherical NPs can be achieved, but this is difficult due to their poor flowability.11 Therefore, spherical NPs with a good flowability are preferred over cubic for electrode materials.

Several synthetic methods such as wet chemical synthesis12 and vapor-phase synthesis13−17 have been used for the preparation of Ni-NPs. For MLCC applications, highly crystalline Ni-NPs prepared by vapor-phase synthesis (VPS) such as chemical vapor synthesis (CVS)13−15 and physical vapor synthesis (PVS) are preferred. Therefore, highly crystalline Ni-NPs are preferably synthesized via high-temperature processes such as chemical vapor synthesis (CVS)13−15 and physical vapor synthesis (PVS).17,18 CVS possesses an advantage over PVS in that it is easy to synthesize a narrow particle size range of NPs. The commonly employed approach is to synthesize Ni-NPs via thermal reduction of NiCl2 in a reducing atmosphere, and the commonly observed shape of NPs is cubes or truncated cubes. This occurs because the equilibrium crystal shape of Ni nanocrystals favors the (100) crystal plane by selective adsorption of NiCl2 on (100) in the CVS process.14,19,20 During particle formation, NiCl2 vapor is adsorbed on the surface of the Ni-NPs and forms a cubic shape by lowering the surface energy of a specific surface. Therefore, a rapid reaction must be induced to prevent the formation of cubic structures.14 A fast reaction and short residence time of NPs in CVS can be used to obtain spherical nickel. However, the process window is narrow, which inevitably acts as a constraint during the scale-up. Therefore, we attempted to synthesize spherical particles by adding the nonpreferential-adsorption type of impurity to the surface of Ni, which could destabilize the nanocrystal facet and strategically tune the shape of the Ni-NPs.21,22

It is necessary to manipulate selective impurities that can alter the adsorption properties such as the surface energy and surface anisotropy of Ni. We chose sulfur among the various elements that are adsorbed nonpreferentially on the Ni surface and cause a sintering delay effect that can eliminate the defects in MLCCs.23,24 H2S is a suitable candidate for the easy adsorption of S to Ni.25,26 However, H2S is toxic and dangerous for direct use in CVS. To overcome this problem, a suitable chemical that can locally produce H2S in a CVS reactor would be highly beneficial for controlling the shape of the Ni-NPs. We chose NiSO4 and Na2SO4 as additives to realize our strategy to synthesize spherical Ni-NPs.

In this study, we carefully analyzed the impurity content of the initial NiCl2 precursor to strategically modify the sulfur concentration of NiCl2 in the precursor to be used. Toward this end, we prepared NiSO4-mixed NiCl2 and Na2SO4-mixed NiCl2 precursors via spray-drying and used them to synthesize spherical Ni-NPs. We observed that the Na2SO4-mixed NiCl2 precursor offered better results than NiSO4-mixed NiCl2 in terms of particle size distribution and the agglomeration ratio.

2. Methods

2.1. Synthesis of Ni-NPs Using the NiCl2 Precursor

A vertical hot-wall CVS reactor was used to prepare Ni-NPs with a fixed heating zone temperature of 1000 °C.15 A schematic of the CVS reactor used in our experiments is shown in Figure S1. Nickel chloride hexahydrate (NiCl2·6H2O, 97%, Research Institute of Industrial Science & Technology) was used as a precursor for the synthesis of Ni-NPs. The composition of the initial NiCl2 precursor is listed in Table S1. Anhydrous NiCl2 was prepared by spray-drying (B-290, Buchi) and vacuum-drying NiCl2·6H2O. The prepared NiCl2 precursor was injected into the CVS reactor using a powder feeder (ROVO, Fine Technics) with N2 (99.9%) carrier gas at a flow rate of 34.09 standard liters per minute (SLM) (sample 1). H2 reducing gas was flown separately into the reactor at a flow rate of 1 SLM into the particle formation zone, as shown in the schematic of the CVS reactor (Figure S1).

2.2. Synthesis of Ni-NPs Using NiCl2 with S Additives

We used nickel sulfate hexahydrate (NiSO4·6H2O, Sigma Aldrich, 98%) and sodium sulfate (Na2SO4, Sigma Aldrich, 99%) as additives for the synthesis of Ni-NPs. We prepared anhydrous NiSO4-mixed NiCl2 and Na2SO4-mixed NiCl2 precursors by spray-drying and vacuum-drying. The details of the process parameters, such as the molar ratio, feed rate, and flow rate, for the synthesis of Ni-NPs are summarized in Table 1. We collected Ni-NPs from a powder collector connected to the reactor outlet and characterized them using the methods described in the following section.

Table 1. Process Parameters.

    molar ratio
  
sample number feeding rate of the precursor (g/h) MNa2SO4/MNiCl2 MNiSO4/MNiCl2 total flow rate (LPM)
1 80.00     35.09
2 80.00   0.002 35.09
3 80.00 0.002   35.09
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2.3. Characterization

The shape and size of the as-synthesized Ni-NPs were analyzed by field-emission scanning electron microscopy (FESEM, Quanta 250 FEG, FEI) after Au-Pd thin-film coating by sputtering. Additionally, more detailed shape and elemental mappings were analyzed using a field-emission high-resolution transmission electron microscope (HRTEM, JEM F200, TFEG). The geometric standard deviation (GSD) and the count median diameter (CMD) of Ni-NPs were determined via image analysis considering 200 particles. The agglomeration ratio was calculated by dividing the number of primary particles constituting the agglomerate by the sum of the primary particles constituting the agglomerate and nonagglomerating particles, which was obtained through image analysis of more than 500 particles. The details of the calculation methods are described in eqs 1–3 in the SI. The phase and crystallinity of the Ni-NPs were analyzed using X-ray diffraction (XRD, Empyrean, PANalytical) with Cu Kα radiation in the 2θ/θ scan mode with a step size of 0.013°. Elemental analysis was performed using X-ray fluorescence (XRF; M4 TORNADO, Bruker) with an excitation energy of 0–10 keV.

2.4. Computational Methods

All density functional theory (DFT)27 calculations were performed using the Vienna ab initio simulation package (VASP).28,29 The electronic structure was described using the Perdew (Burke)–Ernzerhof exchange–correlation function within a generalized gradient approximation and projector-augmented wave pseudopotentials with a cutoff energy of 400 eV.29−31 The conjugate gradient method was implemented to optimize the geometry, and the criteria for the convergence of energy and force were set at 10–6 eV and 0.01 eV/Å, respectively.

To reflect the effect of adsorbates on particle shape formation, we performed calculations by introducing adsorbates with different surface coverages onto two low-Miller-index planes: Ni(100) and Ni(111). These facets were represented by slabs composed of four-layered (2 × 4) and (4 × 4) surface unit cells. The bottom two layers of the slab were fixed at their bulk-optimized positions, and the top two layers, including any adsorbates, were allowed to relax. A Monkhorst–Pack (4 × 4 × 1) k-point mesh grid was implemented for all facets to integrate the Brillouin zone. For all modeled surface facets, the vacuum thickness along the z-direction was set at 15 Å to avoid unintended interactions between the slabs.

Since the surface densities were different in the aforementioned facets, the surface coverage of an adsorbate on a facet was calculated based on its surface area, which was normalized to that of the most closely packed Ni(111). This normalization enabled consistent surface coverage among all facets, regardless of the number of actual adsorption sites.

The adsorption energy was calculated as follows:

2.4. 1

where Esystem is the total energy of the optimized system, Eslab is the total energy of the bare Ni slab, and Eadsorbate is the total energy of the isolated adsorbate.

The corresponding surface energy at a certain coverage was obtained according to eq 2:

2.4. 2

where Eref is the reference energy of the unit composition obtained from the bulk calculation, Eads is the absolute sum of the adsorption energies of the intermediates at a given coverage, n is the number of unit compositions on the surface, and A is the surface area.

3. Results and Discussion

Figure 1a,b shows the FESEM image and the particle size histogram of Ni-NPs prepared by CVS using an anhydrous NiCl2 precursor (sample 1). The faceted crystals including cubes and polyhedrons were observed in sample 1. The GSD and CMD of sample 1 were 1.35 and 243 nm, respectively. Notably, no agglomeration of particles occurred. Figure 1c,d shows the FESEM image and the particle size histogram of Ni-NPs prepared by CVS using the NiSO4-mixed NiCl2 precursor (sample 2). The GSD and CMD of sample 2 were 1.25 and 132 nm, respectively. The Ni-NPs in sample 2 were spherical with a smooth surface, but a few of them appeared to agglomerate. The calculated agglomeration ratio of sample 2 was 13.50%. Figure 1e,f shows the FESEM image and the particle size histogram of Ni-NPs prepared by CVS using the Na2SO4-mixed NiCl2 precursor (sample 3). The GSD and CMD were 1.23 and 108 nm, respectively. Only spherical Ni-NPs were observed in sample 3, and the calculated agglomeration ratio of NPs in sample 3 was 4.34%. The calculated GSD, CMD, and agglomeration ratio data for samples 1–3 are summarized in Table 2, and low-magnification scanning electron micrographs of samples 1–3 are shown in Figure S2 in the SI.

Figure 1.

Figure 1

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FESEM images and particle size histograms of Ni-NPs synthesized in the CVS reactor using different precursors; (a,b) NiCl2, (c,d) NiSO4/NiCl2, and (e,f) Na2SO4/NiCl2.

Table 2. CMD, GSD, and Agglomeration Ratio of Ni-NPs Summarized for Ni-NPs Synthesized Using Different Precursors; Sample 1: NiCl2, Sample 2: MNa2SO4/MNiCl2 = 0.002, Sample 3: MNiSO4/MNiCl2 = 0.002.

sample number CMD (nm) GSD agglomeration ratio (%)
1 243 1.35  
2 132 1.25 13.50
3 108 1.23 4.34
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Figure 2 panels (a and b), (d and e), and (g and f) show the high-resolution transmission electron microscopy (HRTEM) images of samples 1, 2, and 3, respectively. While only spherical Ni-NPs were observed in samples 2 and 3, various faceted crystals including cubes and polyhedrons were observed in sample 1. Figure 2 panels (c), (f), and (i) show selected-area electron diffraction (SAED) patterns of samples 1, 2, and 3, respectively. All patterns match face-centered cubic (FCC) Ni regardless of the particle shape.

Figure 2.

Figure 2

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HRTEM images and SAED patterns of Ni-NPs synthesized in the CVS reactor using different precursors; (a–c) NiCl2, (d–f) NiSO4-mixed NiCl2, and (g–i) Na2SO4-mixed NiCl2.

Figure 3a–c shows EDS results of sample 1, where 99.99 wt % Ni and 0.01 wt % S were detected. Note that we did not add additional S to the precursor used in sample 1, but trace amounts of S were present as shown in Table S1. Figure 3d–f shows EDS results of sample 2, where 99.87 wt % Ni and 0.13 wt % S were detected. Figure 3g–j shows EDS results of sample 3, where 99.91 wt % Ni and 0.09 wt % S were detected. Na was added to the precursor used in sample 3, but it was difficult to quantify because it was present in small amounts.

Figure 3.

Figure 3

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EDS mapping of Ni-NPs synthesized in the CVS reactor using different precursors; (a–c) NiCl2, (d–f) NiSO4-mixed NiCl2, and (g–j) Na2SO4-mixed NiCl2.

Comparing the microstructures of Ni-NPs prepared by CVS using the three different precursors, it was observed that both NiSO4-mixed and Na2SO4-mixed NiCl2 precursors were more favorable for preparing spherical Ni-NPs than the pure NiCl2 precursor. This confirmed that the formation of faceted Ni-NPs could be prevented by the intentional addition of S to the precursor. In addition, the agglomeration rate, which was 13.50% with NiSO4-mixed NiCl2, was reduced to 4.34% with Na2SO4-mixed NiCl2. This will be discussed later in this section.

Figure 4 shows the XRD patterns of samples 1–3. The peaks appeared at 2θ = 44.5, 51.8, 76.4, 92.9, and 98.4° for the (111), (200), (220), (131), and (222) planes, respectively, of FCC Ni (JCPDS 04-0850). No peaks of residues or byproducts of the chemical reactions in the CVS were observed. The chemical reactions and their feasibility are explicitly discussed later in this section (eqs 1–9).

Figure 4.

Figure 4

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XRD patterns of Ni-NPs synthesized by CVS using different precursors.

Figure 5 shows the XRF results for samples 1–3. The XRF results of sample 1 showed the presence of 99.60 wt % Ni and 0.40 wt % Cl. The Cl found in sample 1 might be a nonreacted NiCl2 precursor that precipitated on the surface of the Ni-NPs.14 In samples 2 and 3, Ni was expected along with Cl and S, as we used metal sulfate additives in our precursors. Because the intensities of S and Cl are relatively very small compared to that of Ni, a magnified view of the XRF data in the range of 2.0–3.5 keV is presented in the inset of Figure 5. We observed 98.65 wt % Ni, 1.33 wt % Cl, and 0.03 wt % S in sample 2 and 98.27 wt % Ni, 1.72 wt % Cl, and 0.01 wt % S in sample 3. The chemical compositions of samples 1–3 are summarized in Table 3.

Figure 5.

Figure 5

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XRF patterns of Ni-NPs synthesized by CVS using different precursors. (Inset) XRF data of S, Cl, and Na in the spectral region of 2.0–3.5 keV.

Table 3. Chemical Composition (wt %) of Samples 1–3.

  chemical composition
sample number Ni Cl S Na
1 99.60 0.40    
2 98.65 1.33 0.03  
3 98.27 1.72 0.01 0
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Results indicated that metal sulfate as an additive in the NiCl2 precursor enables the synthesis of spherical Ni-NPs in the CVS process. Moreover, Na2SO4 was found to be more beneficial than NiSO4 in terms of its low agglomeration ratio. This is because NaCl, a byproduct of Na2SO4, inhibits agglomeration between particles owing to the previously reported in-flight coating.15 However, we did not observe the presence of NaCl in the XRD and XRF results of sample 3. We assumed that NaCl was not observable because it is a very small and light molecule. Therefore, we investigated the effect of excessive Na2SO4 addition on the shape of Ni-NPs. The microstructure of Ni-NPs synthesized using a Na2SO4-mixed NiCl2 precursor with a molar ratio of 0.02 is shown in Figure S3a,b. The XRD pattern of Ni-NPs synthesized using the Na2SO4-mixed NiCl2 precursor with a molar ratio of 0.02 shows the NiCl2 and NaCl phases in the sample (Figure S4). This byproduct was also confirmed by elemental analysis using XRF (Figure S5). As a result, we confirmed the formation of NaCl as a Na2SO4 byproduct, and we can conclude that a Na2SO4-mixed NiCl2 precursor with a molar ratio of 0.002 provides an optimal amount of S for synthesizing spherical Ni-NPs.

To explain what occurs when two different additives are inserted into the CVS reactor with NiCl2 in a reducing atmosphere, all possible chemical reactions involved during the synthesis of Ni-NPs are written as follows. In the conventional reduction process, NiCl2 reacts with H2 to form Ni and HCl gases (eq 3).

3. 3

In the presence of NiSO4, it undergoes several chemical reactions, as follows (eqs 4–6).

3. 4
3. 5
3. 6

In the presence of Na2SO4, several chemical reactions occur, as follows (eqs 7 and 8).

3. 7
3. 8

Following the above reactions (eqs 4–8), NiS is produced by the following reaction (eq 9).

3. 9

The values of the change in Gibbs free energy with temperature for each reaction (eqs 3–9) are presented in Figure 6. According to the Gibbs free energy calculations, all reactions discussed have negative values of the Gibbs energy change and are spontaneous reactions.

Figure 6.

Figure 6

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Values of the Gibbs free energy change with temperature for each reaction.

We calculated the surface energy of different facets for total surface coverages up to 0.5 ML, which is sufficient to observe the distinct effects of S addition on the change in Ni surface energy. Two planes with low Miller indices, Ni(100) and Ni(111), were modeled to observe the changes in the surface energies for the various surface coverages of Ni, Cl, and S species because these two facets mostly equilibrate the Ni-NPs.32 Note that when surface energies with different structures become similar, the exposure preference for a particular surface diminishes, resulting in the formation of spherical particles. Although the surface energy of a high-index Ni plane can provide insight into the particle shape change, we adopted the most representative and plausible surfaces, Ni(100) and Ni(111), to be exposed. This is to simplify the phenomenon that different types of adsorbates vary the point where surface energies become similar. Surface energies for Ni(100) and Ni(111) were initially calculated without any adsorbates and were found to have the values γ100 = 2.08 J/m2 and γ111 = 1.91 J/m2. These results are consistent with those obtained in most previous studies.33

The surface energies of the adsorbates were calculated assuming that the adsorbate–adsorbate interactions were identical among the adsorbates. For instance, the simultaneous adsorption of Ni and Cl with a coverage of 0.0625 ML each was considered as 0.125 ML in total. Changes in the surface energy as a function of coverage are shown in Figure 7.

Figure 7.

Figure 7

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Effect of adsorbate species on the surface energy of Ni(111) and Ni(100) as a function of surface coverage (θ).

The surface energies of Ni(100) and Ni(111) decreased with the adsorbate coverage for all cases; however, the addition of S made the slope steeper. In particular, the Ni(100) surface energy was more sensitive to adsorption and crossed the Ni(111) line. In the case of S addition, the point at which Ni(111) and Ni(100) crossed appeared at a lower surface coverage, suggesting isotropic growth of the Ni surface; furthermore, the formation of spherical particles was easy in the presence of S.

The aforementioned DFT calculations indicate that S plays an important role in lowering the surface energy of the crystal facets to form spherical Ni-NPs.

4. Conclusions

Spherical Ni-NPs were prepared using NiSO4-mixed NiCl2 and Na2SO4-mixed NiCl2 precursors in a CVS reactor. The shape change from faceted to spherical Ni-NPs occurred due to the presence of S atoms, which were identified to be the major contributing factor. The results of first-principles calculations indicated that the added S atom stabilizes Ni (hkl) more than Cl. This concept may help fabricate other spherical NPs using S impurities in the CVS reactor. This study provides a new route for changing the shape and size of Ni-NPs using NiCl2 with an added S content.

Acknowledgments

This study was supported by the Korea Institute of Industrial Technology [grant number UR220003] and the National Research Foundation of Korea [grant number 2021M3H4A6A03103721 and 2202R1F1A1075082].

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c05281.

  • Additional experimental details, materials, and methods, including photographs of the experimental setup (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c05281_si_001.pdf (574KB, pdf)

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Supplementary Materials

ao2c05281_si_001.pdf (574KB, pdf)

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