Facile One-Pot Synthesis of Nickel Nanoparticles by Hydrothermal Method

The one-pot synthesis process has emerged as an economical synthesis method without the involvement of purification or formation of intermediate compounds. Therefore, nickel nanoparticles were selectively synthesized by a simple hydrothermal method using nickel(II) chloride hexahydrate and borane–ammonia complex as a precursor and reducing agent, respectively. The morphology and crystal growth were observed by controlling the precursor concentration ratio of Ni:AB from 1:0.1 to 1:4 under various temperatures ranging from 80 to 140 degrees. In addition, we observed that the crystal growth rate under the influence of NaCl and KCl resulted in spherical Ni particles with size distributions controlled in the range of 297.65 nm to 1082.15 nm and 358.6 nm to 605 nm, respectively.

Methods applied to chemical synthesis (mentioned above) use techniques such as ball milling, sol-gel, immersion, spray drying, solvothermal, and hydrothermal methods, respectively [11,[22][23][24][25][26][27]. However, in the case of wet synthesis, after forming the primary intermediate compound with a reducing agent such as NaOH, KOH, ammonia, or hydrazine, an additional reduction process of two or more steps or a thermal annealing treatment in a hydrogen atmosphere after the formation of the dry powder is required [24,26,27]. In addition, since there are difficulties in fine particle refinement with impurity control associated with these two or more synthesis steps, it is necessary to develop a synthesis method that can overcome them. Recently, borane-ammonia complex (AB) has mainly been used as a hydrogen storage medium, and has attracted attention as a reducing agent capable of synthesizing various metal nanoparticles such as Au, Ag, Cu, Pd, and Ir [28][29][30]. In this study, a process that can be synthesized in one pot without an additional reduction or heat treatment was studied using the hydrothermal synthesis method using nickel(II) chloride hexahydrate as a precursor and a borane-ammonia complex reducing agent with NaCl and KCl as effective additives to the growth of spherical Ni nanoparticles.
The shape and particle size of the nanoparticle powders were measured using a field emission scanning electron microscope (FE-SEM, Hitachi, Su-5000) and field emission transmission electron microscope (FE-TEM, FEI Technai G2 200 kV). The crystal structure of the nanoparticle powder was analyzed with a powder X-ray diffractometer (XRD, X-ray Diffractometer system, Rigaku, Miniflex 600) using a Cu-Kα radiation light source (λ = 0.15056 nm). The chemical composition change and surface bonding state of the nanoparticles were determined using X-ray photoelectron spectroscopy (XPS, X-ray photoelectron Spectroscopy, Thermo Scientific, Sigma Probe) with a photon energy of 1486.6 eV (Al Kα). Figure 1 shows the SEM images of Ni nanoparticle powder synthesized at 80 • C for 12 h using nickel(II) chloride hexahydrate and AB as precursors.
The shape and particle size of the nanoparticle powders were measured using a field emission scanning electron microscope (FE-SEM, Hitachi, Su-5000) and field emission transmission electron microscope (FE-TEM, FEI Technai G2 200 kV). The crystal structure of the nanoparticle powder was analyzed with a powder X-ray diffractometer (XRD, Xray Diffractometer system, Rigaku, Miniflex 600) using a Cu-Kα radiation light source (λ = 0.15056 nm). The chemical composition change and surface bonding state of the nanoparticles were determined using X-ray photoelectron spectroscopy (XPS, X-ray photoelectron Spectroscopy, Thermo Scientific, Sigma Probe) with a photon energy of 1486.6 eV (Al Kα). Figure 1 shows the SEM images of Ni nanoparticle powder synthesized at 80 °C for 12 h using nickel(II) chloride hexahydrate and AB as precursors. Scheme 1 shows that the size and shape of Ni particles change depending on the temperature and the presence or absence of additives when the Ni:AB value is 1:1. In addition, we experimented with different concentrations for each temperature. Figure 1 shows SEM images of Ni particles synthesized for 12 h at 80 °C When the Ni:AB precursor concentrations were 1:0.1, 1:0.5, 1:1, 1:2, and 1:4, the average size of the aggregated particles was determined in Figure 1a as ~300 nm, in (b), it was observed to be ~200 nm, (c) ~500 nm, (d) ~300 nm, (e) ~250 nm, respectively. As the concentration of AB increases, Ni Scheme 1 shows that the size and shape of Ni particles change depending on the temperature and the presence or absence of additives when the Ni:AB value is 1:1. In addition, we experimented with different concentrations for each temperature. Figure 1 shows SEM images of Ni particles synthesized for 12 h at 80 • C When the Ni:AB precursor concentrations were 1:0.1, 1:0.5, 1:1, 1:2, and 1:4, the average size of the aggregated particles was determined in Figure 1a as~300 nm, in (b), it was observed to be~200 nm, (c)~500 nm, (d)~300 nm, (e)~250 nm, respectively. As the concentration of AB increases, Ni in the form of an agglomerated amorphous film is observed ( Figure 1e). EDX data shows that there are no impurity components without Ni or O ( Figure S1). in the form of an agglomerated amorphous film is observed (Figure 1e). EDX data shows that there are no impurity components without Ni or O ( Figure S1).    Figure 3a shows that when the ratio is 1:0.1, the particle size is agglomerated, with a distribution of 100 nm to 400 nm, and Figure 3b shows that the mulberry-like particles were distributed with an average size of ~200 nm. Figures 3c-e show that when the Ni:AB precursor ratios are 1:1, 1:2, and 1:4, the average particle size is ~3 μm, ~5 μm, and ~2.5 μm, respectively. Since the decomposition of AB proceeds at temperatures around 110 °C , it is considered that as the concentration of AB increases, the reducing power of the surface decreases due to dehydrogenation [32].    Figure 3a shows that when the ratio is 1:0.1, the particle size is agglomerated, with a distribution of 100 nm to 400 nm, and Figure 3b shows that the mulberry-like particles were distributed with an average size of ~200 nm. Figures 3c-e show that when the Ni:AB precursor ratios are 1:1, 1:2, and 1:4, the average particle size is ~3 μm, ~5 μm, and ~2.5 μm, respectively. Since the decomposition of AB proceeds at temperatures around 110 °C , it is considered that as the concentration of AB increases, the reducing power of the surface decreases due to dehydrogenation [32]. EDX data show there are no impurity components without Ni or O (Figure   Figure 3a shows that when the ratio is 1:0.1, the particle size is agglomerated, with a distribution of 100 nm to 400 nm, and Figure 3b shows that the mulberry-like particles were distributed with an average size of 200 nm. Figure 3c-e show that when the Ni:AB precursor ratios are 1:1, 1:2, and 1:4, the average particle size is~3 µm,~5 µm, and~2.5 µm, respectively. Since the decomposition of AB proceeds at temperatures around 110 • C, it is considered that as the concentration of AB increases, the reducing power of the surface decreases due to dehydrogenation [32]. EDX data show there are no impurity components without Ni or O ( Figure S3). We performed FE-TEM measurements on a representative 1:0.2 sample. The low-magnification TEM image shows individual Ni nanoparticles with an average size of 200 nm ( Figure S4a). Figure S4b shows the lattice-resolved image of Ni nanoparticles. A d-spacing of 2.0Å is observed, which is the (111) plane of a face-centered cubic Ni structure (JCPDS No. 87-0712, a = 0.3523 nm). S3). We performed FE-TEM measurements on a representative 1:0.2 sample. The lowmagnification TEM image shows individual Ni nanoparticles with an average size of 200 nm ( Figure S4a). Figure S4b shows the lattice-resolved image of Ni nanoparticles. A dspacing of 2.0Å is observed, which is the (111) plane of a face-centered cubic Ni structure (JCPDS No. 87-0712, a = 0.3523 nm).       Figure S4a). Figure S4b shows the lattice-resolved image of Ni nanoparticles. A dspacing of 2.0Å is observed, which is the (111) plane of a face-centered cubic Ni structure (JCPDS No. 87-0712, a = 0.3523 nm).  Figure 4 shows an SEM image of Ni particle powder obtained at 140 °C . For all Ni:AB ratios, spherical particles were not observed. It was confirmed that the crystals grew in the form of agglomerated two-dimensional sheets, having a thickness of 50 nm, which was observed when the concentration of the AB precursor was 1:4. EDX data show there are no impurity components without Ni or O ( Figure S5).    It is believed that crystal growth is limited due to the activation of the AB complex [31]. dominant peak at 44.5° of the Ni (111) at a Ni:AB concentration range of 1:1~1:4 and other peaks were weakly observed. It is believed that crystal growth is limited due to the activation of the AB complex [31].  Figure 6 shows the XPS fine spectra of Ni, confirming the surface chemical bonding state of nanoparticle powders synthesized at various temperatures and concentrations. For the 80 °C sample, the main peaks of Ni 2p3/2 and Ni 2p1/2 were observed at 855.2 eV and 872.2 eV, respectively, which means that the state of the oxidation number considered as Ni(OH)2 exists on the surface [33][34][35] (Figure 6a). In addition, as the AB concentration increased, red shifts were observed from 0.2 eV to 0.4 eV. It was confirmed that the Ni 0 state existed at 852.6 eV when the Ni:AB precursor concentration ratio was 1:1 and 1:2. In Figure 6b, it is shown that 100 °C samples reveal peaks of Ni 2p3/2 and Ni 2p1/2 at 855.7 eV and 873.4 eV, respectively. The entire range of the Ni:AB precursor concentration ratio from 1:0.1 to 1:4 contains the Ni 0 state at 852.4 eV. In the case of the 120 °C samples, in  Figure 6 shows the XPS fine spectra of Ni, confirming the surface chemical bonding state of nanoparticle powders synthesized at various temperatures and concentrations. For the 80 • C sample, the main peaks of Ni 2p 3/2 and Ni 2p 1/2 were observed at 855.2 eV and 872.2 eV, respectively, which means that the state of the oxidation number considered as Ni(OH) 2 exists on the surface [33][34][35] (Figure 6a). In addition, as the AB concentration increased, red shifts were observed from 0.2 eV to 0.4 eV. It was confirmed that the Ni 0 state existed at 852.6 eV when the Ni:AB precursor concentration ratio was 1:1 and 1:2. In Figure 6b, it is shown that 100 • C samples reveal peaks of Ni 2p 3/2 and Ni 2p 1/2 at 855.7 eV and 873.4 eV, respectively. The entire range of the Ni:AB precursor concentration ratio from 1:0.1 to 1:4 contains the Ni 0 state at 852.4 eV. In the case of the 120 • C samples, in Figure 6c, the main peaks of Ni 2p 3/2 and Ni 2p 1/2 were observed at 855.1 eV and 872.7 eV, respectively. In particular, when the ratio of the Ni:AB precursor is 1:2, it can be seen that the full width at half maximum is the largest. It was considered that the proportion of exposed Ni(OH) 2 or NiOOH increases as the surface area increases, as shown in Figure 3.

Results and Discussions
Regarding the 140 • C samples in Figure 6d, the entire main peaks of Ni 2p 3/2 and Ni 2p 1/2 were observed at 855.7 eV and 873.5 eV, respectively, and distinct Ni 0 peaks at 851.9 eV were observed. Figure 6c, the main peaks of Ni 2p3/2 and Ni 2p1/2 were observed at 855.1 eV and 872.7 eV, respectively. In particular, when the ratio of the Ni:AB precursor is 1:2, it can be seen that the full width at half maximum is the largest. It was considered that the proportion of exposed Ni(OH)2 or NiOOH increases as the surface area increases, as shown in Figure 3. Regarding the 140 °C samples in Figure 6d, the entire main peaks of Ni 2p3/2 and Ni 2p1/2 were observed at 855.7 eV and 873.5 eV, respectively, and distinct Ni 0 peaks at 851.9 eV were observed.  Figure 7 shows SEM images of particle growth rate changes with the addition of NaCl and KCl. The formation of spherical Ni nanoparticles was confirmed when the Ni:NaCl precursor was added in a ratio of 1:1. The average particle size distribution was 455.75 nm, 659 nm, 1082.15 nm, 626.4 nm, and 297.65 nm when the Ni:NaCl ratio was 1:0.1, 1:0.5, 1:1, 1:5, and 1:10, respectively, as shown in Figure 8a. The crystal growth reaction in response to the addition of extra Na and Cl ion concentrations was confirmed.  7 shows SEM images of particle growth rate changes with the addition of NaCl and KCl. The formation of spherical Ni nanoparticles was confirmed when the Ni:NaCl precursor was added in a ratio of 1:1. The average particle size distribution was 455.75 nm, 659 nm, 1082.15 nm, 626.4 nm, and 297.65 nm when the Ni:NaCl ratio was 1:0.1, 1:0.5, 1:1, 1:5, and 1:10, respectively, as shown in Figure 8a. The crystal growth reaction in response to the addition of extra Na and Cl ion concentrations was confirmed.
In the case of KCl addition, when the ratio of Ni:KCl precursor was increased to 1:0.1, 1:0.5, 1:1, 1:5, and 1:10, the average particle size was measured to 536.85 nm, 572.8 nm, 605 nm, 430.65 nm, and 358.6 nm, respectively. The overall particle size is smaller than when NaCl additive is used. It is considered that not only the Cl ion radius but also the K Materials 2023, 16, 76 7 of 10 ion radius (r = 152 pm) are larger than that of Na ions (r = 116 pm), which was matched well, with crystal growth more effectively suppressed [36][37][38][39].  In the case of KCl addition, when the ratio of Ni:KCl precursor was increased to 1:0.1, 1:0.5, 1:1, 1:5, and 1:10, the average particle size was measured to 536.85 nm, 572.8 nm, 605 nm, 430.65 nm, and 358.6 nm, respectively. The overall particle size is smaller than when NaCl additive is used. It is considered that not only the Cl ion radius but also the K ion radius (r = 152 pm) are larger than that of Na ions (r = 116 pm), which was matched well, with crystal growth more effectively suppressed [36][37][38][39].

Conclusions
We studied one-pot synthesis of Ni nanoparticles using a nickel(II) chloride hexahydrate precursor and a borane-ammonia complex (AB) reducing agent using a hydrothermal synthesis method. The ratio of Ni and AB was adjusted to 1:0.1~1:4, and as a result of synthesis for 12 h at a temperature of 80 °C to 140 °C, it was confirmed that single-crystal Ni particles were formed by XRD. In addition, it was confirmed that the size distributions of spherical Ni particles were controlled in the ranges of 297.65 nm to 1082.15 nm and 358.6 nm to 605 nm by NaCl and KCl additives, respectively. We suggest that this ecofriendly one-pot synthesis method be applied to control the particle size and grain size of other transition metal nanoparticle powders to improve their chemical properties.

Conclusions
We studied one-pot synthesis of Ni nanoparticles using a nickel(II) chloride hexahydrate precursor and a borane-ammonia complex (AB) reducing agent using a hydrothermal synthesis method. The ratio of Ni and AB was adjusted to 1:0.1~1:4, and as a result of synthesis for 12 h at a temperature of 80 • C to 140 • C, it was confirmed that single-crystal Ni particles were formed by XRD. In addition, it was confirmed that the size distributions of spherical Ni particles were controlled in the ranges of 297.65 nm to 1082.15 nm and 358.6 nm to 605 nm by NaCl and KCl additives, respectively. We suggest that this ecofriendly one-pot synthesis method be applied to control the particle size and grain size of other transition metal nanoparticle powders to improve their chemical properties.