Fabrication and Morphological Control of Ni-Based Nanowires by Self-Assembled Solution Synthesis
2019 Volume 60 Issue 10 Pages 2188-2194
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2019 Volume 60 Issue 10 Pages 2188-2194
In this study, we investigated the formation of Ni-based nanowires (NWs) and precise control of their surface morphology via solution synthesis. Ni2+ ions were reduced to metallic Ni with hydrazine, and Ni nanoparticles were formed in ethylene glycol solution. The Ni nanoparticles then self-assembled along one direction to form NWs. The surface morphology of the Ni NWs depended on the hydrazine concentration. Needle-like protrusions were formed on the surfaces of the Ni NWs at high hydrazine concentrations, whereas Ni NWs with smooth surfaces were obtained at low hydrazine concentrations. The inclusion of NaBH4 as a proportion of the reducing agent led to the formation of NWs containing a Ni–B alloy. The catalytic activity of the NWs containing a Ni–B alloy in H2 generation from NaBH4 solution was higher than that of the Ni NWs.
Nanowires (NWs), which have high aspect ratios, have received significant attention because of their unique properties, which originate from their one-dimensional structure. The one-dimensional structure, which allows spatial expansion from the nanoscale to the microscale along the long axis, unlike the case for nanoparticles or nanosheets, makes NWs promising materials for a number of applications, e.g., in electrical,1–3) optical,4) and chemical- and bio-sensing devices.5,6) Many techniques for the fabrication of NWs have been developed, e.g., template-based growth,7) vapor–liquid–solid growth,8,9) and lithographic methods.10) Metallic, semiconductor, and polymer NWs have been fabricated via these methods. These techniques enable the fabrication of patterned and oriented NWs, but such processes are generally expensive because they need templates such as porous materials and complicated processes in a vacuum. Solution synthesis is a comparatively simple and low-cost process, therefore methods for solution synthesis of nanomaterials have been developed and widely used to fabricate various nanomaterials. One-dimensional nanomaterials can also be fabricated by anisotropic growth in a solution by using capping agents,11,12) but the resulting materials generally have relatively low aspect ratios and do not have satisfactory one-dimensional properties.
Recently, solution synthesis of one-dimensional magnetic metals, e.g., Ni and Co, under application of a magnetic field has been reported.13,14) In this process, the nanoparticles that are formed by the reduction of Ni2+ or Co2+ ions in the solution are self-assembled one-dimensionally because of the magnetic interactions between the nanoparticles and form one-dimensional nanobeads or NWs. This enables the production of metallic NWs with a high aspect ratio by a simple and cost-effective process. Because solution synthesis is easy to scale up and suitable for mass production, the Ni NWs fabricated by this method are expected to be useful in practical applications, e.g., in flexible electrodes for Li batteries,14) catalysts,15) and antibacterial applications.16)
The nanostructure and morphology of NWs are important factors in determining the properties of NWs. In addition to the magnetic field that is applied to the solution during NW formation, the reducing agent used to reduce the Ni2+ ions in the solution plays an important role in determining the nanostructure and morphology of the NWs. Hydrazine and sodium or potassium borohydride (NaBH4 or KBH4) are strong reducing agents and are frequently used in the solution synthesis of metallic nanoparticles via reduction of metallic ions in solution. Hydrazine has been used in Ni NW synthesis, but its effects on the nanostructure and morphology of the NWs are still unclear. It has been reported that Ni–B alloy nanoparticles are formed when NaBH4 or KBH4 is used as the reducing agent.17–19) Nickel–boron (Ni–B) alloys show high catalytic activities, therefore NWs of such alloys could provide easy-to-use catalysts.
In this study, we investigated Ni-based NW formation via solution synthesis in terms of (i) the effects of hydrazine concentration on the nanostructure and morphology of the NWs and (ii) the effects of use of a combination of NaBH4 and hydrazine as reducing agents on the phase obtained. We found that the surface morphology of the Ni NWs varied depending on the hydrazine concentration. Needle-like protrusions were formed on the surfaces of the Ni NWs produced with high concentrations of hydrazine, whereas those produced with low hydrazine concentrations had smooth surfaces. We obtained NWs containing a Ni–B alloy by using NaBH4 as a proportion of the reducing agent. The NWs containing a Ni–B alloy showed good catalytic activity in H2 generation from NaBH4 solution.
Ni-based NWs were fabricated via solution synthesis with nickel(II) acetate (99%, Wako Pure Chemical Industries, Ltd., Japan) as the Ni source and hydrazine monohydrate (Wako Pure Chemical Industries, Ltd., Japan) and/or NaBH4 (99%, Sigma-Aldrich, USA) as the reducing agent. When hydrazine monohydrate was used as the reducing agent, nickel(II) acetate (12.4 mg, 0.05 mmol) was dissolved in ethylene glycol (4 mL; 99%, Wako Pure Chemical Industries, Ltd., Japan) in a vial. Hydrazine monohydrate (15, 30, 75, and 150 µL; 0.31, 0.62, 1.55, and 3.1 mmol, respectively) was added to the solution. The vials were placed on a hot plate and heated to 130°C in air at a rate of 10°C/min. A hot plate with and without a magnetic stirrer was used for the synthesis with or without application of a magnetic field. The magnetic flux density applied in the synthesis with a magnetic field was 20 mT in the horizontal direction against the reaction vessel. In all cases, the solutions were not stirred during heating. The temperature was kept at 130°C for 30 min and then the solution was cooled to room temperature in air. The formed NWs were separated by filtration, washed several times with ethanol, and dried in air.
2.2 Synthesis of Ni–B NWsNi–B NWs were synthesized using both NaBH4 and hydrazine as the reducing agent. Nickel(II) acetate (12.4 mg, 0.05 mmol) was dissolved in ethylene glycol (4 mL). NaBH4 (10 mg, 0.26 mmol) was added and the solution was stirred at room temperature for 2 h. Stirring was stopped after 2 h. Hydrazine monohydrate (150 µL, 3.1 mmol) was added and the solution was heated to 130°C. The temperature was maintained for 30 min, with application of a magnetic field (20 mT) by using a hot plate with a magnetic stirrer. The solution was then cooled to room temperature in air. The formed NWs were separated by filtration, washed several times with ethanol, and dried in air.
The NWs were examined by field-emission scanning electron microscopy (FE-SEM; JEOL-6335F, Japan). X-ray diffraction (XRD) was performed using a diffractometer (Rigaku, RINT-2500, Japan), with Cu Kα radiation.
2.3 Evaluation of catalytic activity of NWsThe catalytic properties of the obtained Ni-based NWs in H2 generation from NaBH4 aqueous solution were evaluated. NaBH4 was dissolved in water at a 1 mM concentration and the Ni-based NWs (5 mg) were dispersed in the NaBH4 aqueous solution. The H2 gas generated was collected and quantified from the downward displacement of water by using a graduated cylinder. The catalytic activity of Pt nanoparticles (Pt black, <20 µm, Sigma-Aldrich, USA) was also evaluated by the same procedures for comparison.
When hydrazine was added to the nickel(II) acetate solution, the color of the solution immediately changed from light green to blue because of formation of a Ni ion–hydrazine complex.15) With increasing temperature, the solution gradually became opaque and dark purple. At 130°C, the products were entangled and floated in the solution, and the solution became transparent and colorless. This indicates that the Ni ions in the solution were completely consumed in the reaction. Figure 1(a) shows the XRD patterns of the sample synthesized using 15 µL of hydrazine without application of a magnetic field and the sample synthesized under a magnetic field (Fig. 1(b)). In both cases, all the diffraction peaks were identified as those of face-centered cubic Ni (fcc-Ni). The SEM images in Fig. 2 show that Ni NWs of diameter 250–450 nm were formed. NWs with bead-like structures, in which individual particles were connected to other particles continuously, were clearly observed, even in the case of the NWs formed in the absence of a magnetic field (Fig. 2(a) and (b)). These results clearly indicate that the Ni NWs self-assembled from Ni nanoparticles even in the reaction without a magnetic field, although previous reports described Ni NW formation with application of a magnetic field.14,15) However, the NWs synthesized without a magnetic field had more bends and kinks (Fig. 2(a) and (b)) than those obtained under application of a magnetic field (Fig. 2(c) and (d)). This clearly indicates that a magnetic field assists nanoparticle self-assembly along one direction. This is because the magnetic domains in the Ni nanoparticles align along the magnetic field direction, as previously reported.14,15) This method enables the simultaneous synthesis of a large number of Ni NWs, as shown in Fig. 3.
XRD patterns of Ni NWs synthesized without (a) and with (b) a magnetic field.
SEM micrographs of Ni NWs synthesized by reduction of Ni ions with hydrazine with (a, b) and without (c, d) a magnetic field.
SEM micrographs of Ni NWs at magnifications of (a) ×350, (b) ×1000, (c) ×2500, and (d) ×50 000.
Figure 4 shows SEM images of a series of Ni NWs synthesized from solutions with various hydrazine concentrations without application of a magnetic field. Although Ni NWs were formed in all cases, the surface morphologies differed significantly depending on the hydrazine concentration. At low hydrazine concentrations (Fig. 4(a) and (b)), the Ni NW surfaces were smooth and nanobeads were clearly observed. When the concentration of hydrazine increased, the surface morphology became bumpy (Fig. 4(c)) and then needle-like (Fig. 4(d)). The formation of Ni NWs with similar needle-like protrusions, referred to as “prickly NWs”15) or “urchin-like nanostructures”,20) has previously been reported. In those studies, the needle-like structures were characterized by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction; the phase consisting of the needle-like structures was identified as cubic Ni. The HRTEM images clearly showed a lattice spacing of 0.20 nm, which corresponds to the (111) plane of fcc-Ni, and indicates growth of needle-like structures along the [111] direction of fcc-Ni.15,20) In solution synthesis of nanomaterials, anisotropic crystal growth can be caused by suppression of growth along a specific direction by chemical capping of the corresponding crystal face. This type of surface capping significantly affects the nanocrystal shape. Zhu et al. reported the formation of needle-like structures on the surfaces of Ni nanoparticles or nanobeads in solution synthesis with hydrazine as a reducing agent and ethylenediamine as a shape controller and stabilizer.20) The length and density of the needle-like structures varied depending on the ethylenediamine concentration. They suggested that needle-like anisotropic growth of Ni was induced by ethylenediamine, which acted as a capping agent and shape controller. In the present study, the surface structure varied depending on the concentration of hydrazine, without addition of a capping agent such as ethylenediamine, as shown in Fig. 4. It is reasonable to conclude that hydrazine, which is a simple diamine, acts as both a reducing agent and a capping agent in the present synthesis. At low hydrazine concentrations, i.e., addition of 15 or 30 µL of hydrazine, a significant proportion of the added hydrazine was consumed in the reduction of Ni2+ ions and the amount of hydrazine that remained was insufficient to cap the surfaces of the precipitated Ni particles. Needle-like anisotropic growth therefore did not occur and the Ni NW surfaces were smooth. At a high hydrazine concentration, i.e., addition of 150 µL of hydrazine, the amount of hydrazine that remained after the reduction of Ni2+ ions was sufficient to cap the surfaces of the precipitated Ni particles. Preferential growth therefore occurred in the [111] direction of fcc-Ni because crystal growth in the other direction was prevented by capping with hydrazine, as described in previous reports. Needle-like anisotropic growth of Ni therefore occurred and needle-like protrusions were formed on the NWs.
SEM micrographs of Ni NWs formed by solution synthesis in ethylene glycol containing nickel(II) acetate and (a) 15, (b) 30, (c) 75, and (d) 150 µL of hydrazine.
The mechanism of Ni NW formation in the present synthesis is summarized in Fig. 5. First, Ni2+ ions were reduced to metallic Ni by hydrazine and small Ni nanoparticles were formed. The small nanoparticles were unstable because of their high surface energy, and therefore aggregated and formed larger nanoparticles. Next, the Ni particles self-assembled to form one-dimensional structures. In the absence of a magnetic field, the Ni nanoparticles formed assemblies by magnetic dipole–dipole interactions between the particles. However, the connections between particles were not straight because the magnetic dipole–dipole interactions were weak because of the random orientation of the magnetic domains in the Ni nanoparticles, as shown in the upper part of the scheme in Fig. 5. Under application of a magnetic field, the alignment of the formed nanoparticles was straight because the magnetic dipole moments of all the nanoparticles were aligned along the external magnetic field. The Ni NWs synthesized under a magnetic field were therefore straight (lower part of the scheme in Fig. 5). In the third step, in the presence of excess hydrazine, hydrazine was adsorbed on the NW surfaces and capped specific crystal faces. This enabled fcc-Ni growth along the [111] direction. As a result, needle-like protrusions were formed on the surfaces of the Ni NWs.
Schematic diagram of formation of Ni NWs.
Figure 6 shows SEM images of Ni nanostructures synthesized by two-step injection of NaBH4 and hydrazine as reducing agents. The images show that each particle was continuously connected, but many branching or twisting parts can be observed, unlike the case for the Ni NWs synthesized with only hydrazine as a reducing agent, even with application of a magnetic field. The connected particles are of size less than 100 nm, which is smaller than that of the Ni NWs (see Fig. 2). The XRD pattern of these structures is shown in Fig. 7. The major diffraction peaks, at 2θ = 44°, 52°, 77°, and 92°, which are attributed to fcc-Ni, are broader than those in the Ni NW pattern (see Fig. 1) because of the small crystal size. In addition, a broad peak at around 40°, which overlaps with the Ni (111) peak, can be clearly observed. On the basis of a previous report of the formation of amorphous Ni–B nanoparticles by the reaction between NaBH4 or KBH4 and Ni2+ ions,17–19) the position and broadness of this peak at ∼40° indicate the presence of amorphous Ni–B compounds.
SEM micrograph of Ni–B-containing Ni NWs synthesized by reduction of Ni ions with NaBH4 and hydrazine under a magnetic field (a); (b) magnified version of (a).
XRD pattern of Ni–B-containing Ni NWs synthesized with both NaBH4 and hydrazine as reducing agents.
In this synthesis, in which NaBH4 and hydrazine are injected in stages, in the first step, some of the Ni2+ ions were reduced by and/or reacted with NaBH4 and small nanoparticles of Ni and/or a Ni–B compound were formed. The remaining Ni2+ ions were reduced by hydrazine and small nanoparticles of Ni were formed. These small Ni nanoparticles then aggregated and formed large nanoparticles incorporating small Ni–B nanoparticles. It is inferred that the Ni–B nanoparticles subsequently inhibited the aggregation of Ni nanoparticles. The Ni–B NWs were formed by connection of these particles, therefore the diameter of the Ni–B NWs were smaller than that of the Ni NWs. In addition, these Ni particles self-assembled but the connections between particles were not aligned in accordance with the external magnetic field because of the non-magnetic Ni–B compound. As a result, many branches and twists appeared in the nanostructures.
3.3 Catalytic activity of Ni-based NWsThe catalytic activities of Ni–B compounds in H2 generation from alkaline borohydride solutions are higher than those of other Ni-based catalysts.21–23) We therefore evaluated the catalytic activity of the Ni–B-containing Ni NWs and compared it with that of the pure Ni NWs. Figure 8 shows the amounts of H2 generated from NaBH4 aqueous solutions with Ni–B-containing Ni NWs, pure Ni NWs, or Pt nanoparticles as the catalyst. The results show that the rate of H2 generation with the Ni–B-containing Ni NWs was much higher than that with pure Ni NWs (see Fig. 8 and Table 1). However, the catalytic activity of the Ni–B-containing Ni NWs was lower than that of the Pt nanoparticles. The H2 generation rate with the Ni NWs was 16 mL H2/min/g of catalyst, which is comparable to that achieved with Ni powders (19.5 mL H2/min/g of catalyst).24) The H2 generation rate with the Ni–B-containing Ni NW catalyst, i.e., 262 mL H2/min/g of catalyst, was 1.6 times higher than that previously reported for a Ni–B catalyst supported on carbon black, i.e., 159 mL H2/min/g of catalyst.21) This clearly indicates that the Ni–B compounds were well exposed on the surfaces of three-dimensionally expanded mesh-like networks of Ni NWs and were in contact with the NaBH4 solution. Although the amount of H2 generated was much lower than that generated with Pt nanoparticles, Ni–B-containing Ni NWs have potential as an alternative to expensive noble-metal catalysts because they consist of less-noble metals and can be synthesized by a low-cost solution process.
H2 generation from NaBH4 solutions with Ni NWs, Ni–B-containing Ni NWs, and Pt nanoparticles as catalysts.
Ni NWs were synthesized in nickel(II) acetate-containing ethylene glycol solutions with hydrazine as the reducing agent. Ni NWs were formed by self-assembly of Ni nanoparticles, even in the absence of a magnetic field, although in previous studies of Ni NW formation, the synthesis was performed under a magnetic field. The surface morphology of the Ni NWs varied depending on the hydrazine concentration because hydrazine acted as both a reducing and a capping agent in the present synthesis. Needle-like protrusions were formed on the Ni NW surfaces at high concentrations of hydrazine because the excess hydrazine was adsorbed on the Ni NW surfaces and capped specific crystal faces. At low hydrazine concentrations, the Ni NW surfaces were smooth.
We successfully synthesized Ni NWs containing a Ni–B alloy by two-step injection of NaBH4 and hydrazine as reducing agents. Some of the Ni2+ ions were reduced by and/or reacted with NaBH4 and small nanoparticles of Ni and/or a Ni–B compound were formed in the first step. The remaining Ni2+ ions were reduced by hydrazine and small nanoparticles of Ni were formed. Ni particles incorporating small Ni–B nanoparticles self-assembled and formed three-dimensionally expanded mesh-like networks, which contained many branched and twisted parts. The NWs containing a Ni–B alloy showed good catalytic activity in H2 generation from NaBH4 solution.
The H2 generation rate with the Ni–B-containing Ni NWs was much higher than that with pure Ni NWs because the Ni–B compounds, which have high catalytic activity, were well exposed on the surfaces of three-dimensionally expanded mesh-like networks of Ni NWs, and in contact with the NaBH4 solution. We suggest that this simple and cost-effective solution synthesis of Ni nanostructures will be useful in the development of catalytic nanomaterials.
This work was supported by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (No. 16K05905) and “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).
