Surfactant-assisted solvothermal synthesis of pure nickel submicron spheres with microwave-absorbing properties
© The Author(s). 2016
Received: 22 June 2016
Accepted: 21 July 2016
Published: 29 July 2016
Pure metallic nickel submicron spheres (Ni-SSs), flower-like nickel nanoflakes, and hollow micrometer-sized nickel spheres/tubes were controllably synthesized by a facile and efficient one-step solvothermal method with no reducing agent. The characteristics of these nickel nanostructures include morphology, structure, and purification. Possible synthesis mechanisms were discussed in detail. The resultant Ni-SSs had a wide diameter distribution of 200~800 nm through the aggregation of small nickel nanocrystals. The ferromagnetic behaviors of Ni-SSs investigated at room temperature showed high coercivity values. Furthermore, the microwave absorption properties of magnetic Ni-SSs were studied in the frequency range of 0.5–18.0 GHz. The minimum reflection loss reached −17.9 dB at 17.8 GHz with a thin absorption thickness of 1.2 mm, suggesting that the submicron spherical structures could exhibit excellent microwave absorption properties. More importantly, this one-pot synthesize route provides a universal and convenient way for preparation of larger scale pure Ni-SSs, showing excellent microwave absorption properties.
With the rapid development of nanoscience and nanotechnology, magnetic nanostructured materials have attracted significant interest because of their interesting optical, electrical, and catalytic properties, as well as response and manipulability under magnetic fields [1–3]. As a result of these characteristics, their potential applications have been proposed in several fields such as optoelectronics, magnetics, catalysis, biologic engineering, information storage, and photovoltaic technology [4–6]. In particular, extensive research has been focused on magnetic metallic nanoparticles such as nickel (Ni), cobalt, and iron which represent a group of promising nanomaterials to ascertain their usefulness in several areas such as high density magnetic recording media, chemical and photochemical catalysis, ferrofluids, and medical diagnostics because of their special structures and distinctive magnetic and physical properties [7–9].
As an important class of ferromagnetic transition metal, Ni nanoparticles are emerging and displaying many characteristics such as high magnetism, high surface area, large surface energy, excellent chemical stability, low melting point, resource-rich, and low cost [10–12]. They are widely used in several important technological fields such as magnetic materials catalysts, magnetic fluids, microwave devices, and high-sensitive gas sensors [13–15]. Specifically, metal Ni nanoparticles as an important conducting and magnetic-anisotropy material have attracted wide significant interest because of their intriguing electronic and magnetic properties under magnetic fields [16, 17]. Over the past several years, Ni nanomaterials provide the possibility of a good candidate for microwave absorbers due to their proper combination of dielectric and magnetic loss leading to wave impedance matching. It is commonly known that the microwave absorption properties of the electromagnetic wave absorbers depend on the complex permittivity and permeability . Nevertheless, it is relatively difficult to achieve impedance match conditions under many conditions owing to unilateral dielectric loss or magnetic loss . For example, some studies have illustrated that an array of Ni nanowires has a negative magnetic permeability as well as a negative permittivity in the resonance frequency . In fact, the attenuated permeability and weak magnetocrystalline anisotropy may limit its applications at higher frequencies due to the dimension decrease and the repression of skin effect . Furthermore, this phenomenon obviously shows that the physical and chemical performances of Ni nanomaterials strongly depend on the makeup, structure, size, nanoscale morphology, and polydispersity [22, 23] which are the key factors for its further application including the electromagnetic wave absorption characteristic and intensity. However, the control of the Ni/Ni-based nanomaterials fabrication is sensitive to the preparation methods.
In recent years, multiple methods are adopted for the fabrication of Ni nanomaterials, which include sputtering, solution glow discharge process, thermal decomposition, pulsed laser ablation, and metal evaporation condensation [24–26]. To date, Ni nanomaterials have been synthesized by using different controllable synthesis routes for various applications with different purities, sizes, structures, and shapes, such as nanowires, nanoparticles, nanotubes, nanosheets, hollow microspheres, and microstructures [27–29]. Although the abovementioned Ni-based nanomaterials have been made, to our knowledge, there are only few reports on the synthesis of pure Ni nanoparticles. However, the successful routes for fabrication of the desired uniform architectures are usually complicated, which always involve the use of various surfactants and still give unstable assemblies with the small nanocrystal . In addition, the metallic Ni nanoparticles are easily oxidized in the presence of nonmagnetic components . In spite of their high electrocatalytic activity, these nanomaterials often have certain disadvantages including poor stability and high cost . Herein, many efforts have been directed toward synthesizing metal Ni nanoparticles with a rapid, simple, economical, well-reproducible, and green way.
In this work, the flower-like Ni nanoflakes, hollow micrometer-sized Ni spheres/tubes, and Ni submicron spheres (Ni-SSs) were synthesized by a facile and efficient one-step solvothermal method with no reducing agent and only one surfactant used. The structure and morphology of the series of resultant Ni nanostructures were characterized. The possible formation mechanism for three Ni hierarchical architectures was proposed, which was confirmed by the controllable surfactant-assisted synthesis routes in addition of poly (ethylene glycol) (PEG) agents. Importantly, the pure Ni-SSs were obtained successfully. Furthermore, the microwave properties of the as-made pure Ni-SSs were investigated in a wide frequency range, and it was found that the pure Ni-SSs exhibit a strong and broad electromagnetic absorption peak with excellent microwave properties. More importantly, our findings inspire a universal and convenient way, which not only controls the low-cost reaction process of pure Ni metal-based nanomaterials as microwave absorbers but also presents operational simplicity for the large-scale production.
Nickel chloride hexahydrate (NiCl2·6H2O), ethylene glycol (HOCH2CH2OH, EG), anhydrous sodium acetate (CH3COONa, NaAc), and PEG (M n = 600, 2000, 6000, 10,000) were purchased from Kelong Regent Co. Ltd, Chengdu, China. All the chemicals were used without any further purification.
Synthesis of Nickel Submicron Spheres
The morphologies of the obtained samples were observed by scanning electron microscopy (SEM) (JSM, 6490LV). An energy dispersive X-ray spectrometer (EDS, EDAX GENESIS 2000XMS) associated with SEM was used to characterize the compositions of the samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were acquired using a JEOL JEM 2010F microscope working at 200 kV. The obtained samples were further characterized by X-ray powder diffractometer (XRD) (Rigaku RINT2400 with CuKα radiation). Thermal gravimetric analysis (TGA) was performed on a TA Instruments Q50. The samples (10–15 mg) were heated from ambient temperature to 500 °C at a rate of 20 °C·min−1 under nitrogen flow with a purge of 40 ml min-1. The magnetic properties were examined using a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525) at 27 °C. The microwave absorption properties were measured using a vector network analyser (Agilent 8720ET) in the frequency range of 0.5–18.0 GHz. The Ni-10000 powder samples were pulverized and mixed with paraffin with a mass ratio of 3:1 and then pressed into a cylindrically shaped compact with an outer diameter of 7.0 mm and an inner diameter of 3.0 mm.
Results and Discussion
Morphology and Structure
Then, thermal properties of the Ni-0, Ni-2000, and Ni-10000 powders were characterized using TGA under a nitrogen atmosphere. The TGA curves were shown in Fig. 5b. The total weight losses of the Ni-0, Ni-2000, and Ni-10000 powders at 500 °C were 1.1, 7.2, and 0.3 % respectively, showing excellent thermal stabilities and high purity. However, the Ni-0, Ni-2000, and Ni-10000 samples showed a two-stage decomposition process: the maximum-rate decomposition temperatures of the first stage were at 241.5, 235.8, and 231.4 °C, respectively; the maximum-rate decomposition temperatures of the second stage were observed at 303.6, 321.4, and 253.1 °C, respectively. The thermal results reveal that the first stage may be attributable to the thermal degradation of the organic components. Moreover, the second decomposition as well as increased weight losses corresponds to the decomposition reaction of the Ni which is probably related to the consequence of Ni structural crystallization .
It can be assured that the Ni-0 flower-like nanoplates and nanospheres can be obtained without any surfactant and special capping ligands, and so it is reasonable that the Ni spheres were generated in the liquid polyols phase by a duplicative oxidation of acetaldehyde previously produced by dehydration of EG. Additionally, we may think that there are two other mechanisms that may be involved in the formation of three Ni hierarchical architectures including two basic mechanisms: the Ostwald ripening process and the aggregation growth process [40, 41]. At the beginning of the nucleation process, ethylene glycol as a high-boiling-point solvent [42, 43] and a reducing agent has been effective polyol reaction medium, which may act as a bridge to resolve and deliver the free Ni2+ ions  at an elevated reaction temperature (200 °C). According to the Ostwald ripening mechanism, the free Ni cations were nucleated in the EG solution. Afterward, it is likely that the Ni cations reacted with anhydrous sodium acetate to form a relative stable structure of the metal precursor, which is analogous to sodium formate in the literature . Although anhydrous sodium acetate acts as an electrostatic stabilization agent to prevent particle agglomeration, it was found that no dark solid products were obtained in the absence of anhydrous sodium acetate. Our own experimental evidence has led us to believe that anhydrous sodium acetate plays an important role in the formation of metal precursors. In fact, in a generalized hydrothermal process, sodium borohydride or hydrazinium hydroxide as a complexing agent as well as a strong reducing agent has been often used for the metal salts to produce small metal particles both in aqueous and non-aqueous solutions [31, 37]. However, some complications can arise with the corresponding metal borides if the stoichiometry is not carefully adjusted. Therefore, in this work, the similar Ni2+ complexes can also be formed without the special complexing agent. Under appropriate solvothermal conditions, the similar Ni2+ complexes as the precursor could decompose and release free Ni2+ to form nanocrystalline Ni through the chemical reduction effect of EG. This might result from the fact that the probability of collision between Ni atoms and Ni nuclei has existed in the reduction reaction. In other words, once a few Ni nuclei were formed, the growth process would be superior to nucleation with enhanced reduction .
In the aggregation growth mechanism, Ni nuclei have a strong tendency to coalesce to form the initial aggregates with the interaction of PEG, serving as a surfactant and structure-directing agent and often altering the order of free energies on the metal surface . According to the minimization of interfacial energy , smaller crystalline Ni nanoparticles may begin to aggregate together in the early stages and then lead to the generation of larger aggregates with the driving force for oriented aggregation. More importantly, to decrease the high surface energy, the nanoparticles preferentially agglomerate during their formation in the liquid-phase process . Then, more uniform and larger nanostructures, such as flower-like nanoplates and hierarchical hollow structures, were gradually formed by tiny Ni nanoparticles with different PEG polymers. As the PEG molecules weight increases, the polymeric chain became longer and the entanglement became more pronounced. At the same time, hydrophilic groups in the PEG molecules adsorbed on the surface of Ni nanoparticles increased, leading to the decreasing of their aggregation; this also resulted in the disappearance of the flower-like hierarchical architectures. However, these Ni nuclei still further gathered the adjacent particles by magnetic dipole-dipole interaction and van der Waals force through self-organization in a common crystallographic orientation and joined these submicron particles at a planar interface [48, 49]. Consequently, the final polydisperse polyhedral Ni-SSs with a quasi-spherical shape and high purity can be obtained by the polyol reaction process when the long PEG-10000 polymer chains were added.
The performance of Ni-based nanomaterials
In addition, to achieve the desired uniform architectures, the methods used are usually complicated, which always involve the use of various surfactants and reducing agents. In our work, a one-step synthesis method with no reducing agent and only one common surfactant has been applied. Thus, the pure Ni-SSs obtained by our solvothermal method exhibited relatively higher performances than most of other the nanostructures, and the shape of the as-obtained Ni nanostructures could be controlled by optimizing the only surfactant we used, thanks to one-pot synthesize route in the absence of any reducing agents. To examine the magnetic properties of the samples, the Ni-10000 powder was dispersed in ethanol easily at room temperature. But the mixture remains stable for only 15 min and then become clear. As shown in Fig. 7b, these pure Ni-SSs move quickly along the magnetic field as holding the sample close to a commercial magnet, indicating that it is possible to manipulate these magnetic Ni-SSs by an external magnetic field.
Microwave Absorption Properties
where f is the frequency of the microwave, c is the velocity of light in free space, and d is the thickness of the absorber, ε r (ε r = ε′-jε′′) and μ r (μ r = μ′-jμ′′) are the measured relative complex permittivity and permeability, respectively. The sensitivity of the reflection loss minimum is dependent on the thickness of the sample, which also affects the position of the frequency as one of the crucial parameters. Therefore, to investigate the influence resulting from the thickness, the absorber thickness is termed as matching thickness with different thicknesses.
In conclusion, pure metallic nickel submicron spheres have been successfully synthesized by a facile and efficient one-step solvothermal method using different PEG agents. The as-synthesized Ni-SSs can be obtained without any further processing. The solid-cores were nearly spherical in shape with a wide diameter distribution of approximately 200~800 nm through the aggregation of small Ni nanocrystals. The pure Ni-SSs with a ferromagnetic behavior exhibited high coercivity values. Furthermore, the microwave absorption properties of the magnetic Ni-SSs researched were in the frequency range of 0.5–18.0 GHz. The minimum reflection loss reached −17.9 dB at 17.8 GHz with the thickness of 1.2 mm, suggesting excellent microwave absorption properties. Therefore, this one-pot synthesize route provides a universal and convenient way for preparations of larger scale pure Ni-SSs, which can be potentially used in industrial, commercial, and defense applications.
The work is supported by the National Natural Science Foundation of China (Grant Nos. 11104010 and 61474014).
HG completed all the experiments and wrote the manuscript. BP, HC, JY and YZ assisted with manuscript preparation. JY, BB and HL revised the manuscript. XN conceived the study, revised the manuscript, and supervised the work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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