Controllable preparation of Ni nanoparticles for catalysis of coiled carbon fibers growth
© Jian et al.; licensee Springer. 2014
Received: 6 January 2014
Accepted: 20 July 2014
Published: 29 July 2014
The mass preparation of high-purity coiled carbon fibers (CCFs) remains challenging due to the high complexity and low controllability of reaction. In this work, a controllable growth of Ni particles was fulfilled by liquid phase reduction of nickel sulfate with hydrazine hydrate. The impacts of the reaction temperature, NaOH concentration, and reaction time on the particle size and purity were investigated. The as-deposited Ni particles were characterized by scanning electron microscopy and X-ray diffraction. In addition, these Ni particles were also applied in preparing high-purity CCFs both on graphite and ceramic substrates. The diameter of the as-grown carbon microcoil was about 500 nm, and the related growth mechanism was discussed.
Coiled carbon materials exhibit a variety of unique characteristics, such as super-elasticity , wide band absorption of electromagnetic waves , and hydrogen adsorption . In particular, researchers have focused on the preparation [4–9], characterization [10, 11], and growth mechanism [12, 13] of the coiled carbon materials because these helical materials are currently not commercially available and they possess great potential applications [14–18]. At present, artificial coiled structures at the mesoscale usually have simple helical geometries of one-dimensional helical fibers depending on the growth condition such as temperature, flow rate, and carbon source.
It was reported that several coiled carbon fibers (CCFs) can be obtained using appropriate catalyst on some substrate or with the help of electric and magnetic field. For example, Chen and Motojima prepared the carbon microcoils by the Ni-catalytic pyrolysis of acetylene containing a small amount of thiophene . Three-dimensional (3D) spring-like carbon nanocoils were obtained in high purity by the catalytic pyrolysis of acetylene at 750°C to 790°C using a Fe-based catalyst, and the nanocoils have a tubular shape of diameter of about 10 to 20 nm . Besides, the carbon nanocoils having coil diameters of 50 to 450 nm can be obtained by applying a magnetic field in the reaction zone or using sputtered thin films of Au and Au/Ni as catalysts .
In fact, Ni catalyst plays a significant role in control of the helical structure during the growth of carbon coils . Though several methods of preparing nickel particles, such as hydrothermal reduction technique , electrodeposition , sol-gel process , and microwave irradiation method  have been reported, the agglomeration of the particles should be prevented or else this would result to the nonuniformity of the as-prepared Ni particles. One of the crucial factors to obtain high-purity CCFs is the controllable synthesis of catalyst nanoparticles. Since Ni grain is one of the most typical catalysts for carbon microcoil (CMC), it is necessary to synthesize uniform Ni particles with designed sizes and to study the effects on the preparation and growth mechanism of the Ni particles. In this study, we prepare Ni nanoparticles by reduction of nickel sulfate with hydrazine hydrate employing the surfactant polyvinylpyrrolidone (PVP) to prevent agglomeration of particles. The as-prepared Ni particles were also used for the growth of CCFs.
Nickel sulfate (NiSO4 · 6H2O, analytical reagent (AR)), PVP (K30, AR, average molecular weight 40,000), sodium hydroxide (NaOH, AR) and hydrazine hydrated (N2H4 · H2O, AR) were purchased from Chengdu Jinshan Chemical Reagent Limited Company, Chengdu, China. Acetylene (C2H2, 99.9%), nitrogen (N2, 99.999%), and hydrogen (H2, 99.99%) were purchased from Chengdu Liuhe Chemical Industry, Chengdu, China. All reagents were used without any further purification.
Preparation of Ni nanoparticles
Two kinds of solution were firstly prepared. Solution A was formed by adding NaOH solution (0.8 to 1.5 M) in 20 ml hydrazine hydrated (6 M) with pH ranging from 10 to 14. Solution B was formed by dissolving 5.256 g of nickel sulfate (NiSO4 · 6H2O) in distilled water, which contained 1 g of PVP polymer as dispersant. Solution A was added to a beaker with a capacity of 100 ml and was magnetically stirred for 15 min at 60°C ~ 80°C. Then, slowly dropwise, adding solution B into A, it was stirred continuously for 45 min. The black precipitates were separated from the mother liquor by magnetic separation and washed repeatedly with distilled water and acetone until the pH was 7. The grey-black powder was finally dried in vacuum at 25°C.
Preparation of coiled carbon fibers
The as-prepared Ni nanoparticles were used as catalyst for CCFs and dispersed on a graphite substrate by spraying and drying the suspension of Ni particles. Then CCFs were obtained on the graphite by catalytic pyrolysis of acetylene containing a small amount of thiophene as the liquid catalytic addictives. Acetylene, hydrogen, and nitrogen were introduced into a horizontal reaction tube (quartz, 28 mm i.d.) which was heated from the outside by a tubular furnace. The flow rates of acetylene and nitrogen were fixed at 20 and 60 ml/min (sccm), respectively, and the hydrogen flow rate ranged from 100 to 140 sccm. Several kinds of CCFs grew exclusively on the upper region of the source gas steam.
The crystal structure of catalyst particles and helical carbon fibers was investigated using X-ray diffraction (XRD with Ni filter, Panalytical X’Pert PRO diffractometer, Almelo, the Netherlands). The size and morphology analyses of nickel particles and CCFs were performed using environmental scanning electron microscopy (ESEM; FEI, Quanta 200, FEI Company, Hillsboro, OR, USA) with an accelerating voltage of 20.0 kV and high-resolution transmission electron microscopy (HRTEM; JEM-2100 F, JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 200 kV.
Results and discussion
Effects on the preparation of Ni particles
To obtain controllable catalyst particles, factors, such as reaction temperature and time, pH values, and the concentration of nickel ions, should be considered. Among these factors, reaction temperature and pH value were addressed in the following discussion.
Effect of reaction temperature
Effect of NaOH concentration
When the molar concentration of NaOH in the NiSO4 solution is low, the reduction rate of nickel ion becomes slow and numerous light green clusters of Ni(OH)2 generate in the initial stage of reaction of about 15 min. Then Ni nanoparticles form gradually by the reduction of uniform clusters of Ni(OH)2 during the following 100 min. In contrast, the clusters of Ni(OH)2 become larger and the amount of the clusters decreases when the molar concentration of NaOH is higher than 1 M.
Structural characterization of Ni particles
where D is the crystallite size, k = 0.89 is a correction factor to account for particle shapes, β is the full width at half maximum (FWHM) of the most intense diffraction peak (111) plane, λ = 1.5406 Å is the wavelength of Cu target, and θ is the Bragg angle. The average crystallite sizes of the produced nickel powders are reckoned as 5.07, 4.56, and 5.70 nm when the molar concentration of NaOH is 0.8, 1.0, and 1.2 M (mol/l), respectively. It is pointed that the particle sizes calculated from the XRD pattern are considerably smaller than those determined from the SEM images. The analysis suggests that the spherical nickel particles may contain a number of ultra small crystals, which agrees with the observation of morphology.
Preparation of coiled carbon fibers and corresponding mechanism
By controlling the reaction temperature and NaOH concentration, Ni nanoparticles with designed size can be obtained by reduction of nickel sulfate with hydrazine hydrate employing the surfactant of PVP. Ni nanoparticles of about 90 nm were obtained at 70°C when the molar concentration of NaOH solution was 0.8 M. The as-prepared Ni nanoparticles of about 90 nm contain some ultra small crystals less than 50 nm, and they are effective for catalytic growth of CCFs. The diameter of coiled carbon fibers is remarkably larger than that of the Ni particle catalysts. It was proposed that the aggregation and shape changes occurred during the growth of coiled carbon fiber, and the morphology of carbon helix can be adjusted by choosing the proper substrate of Ni catalyst.
This work was financially supported by the National Natural Science Foundation of China (No. 51173148 and No. 51202228), the Special Research Fund for Doctoral Program of Higher Education (No. 20060613004), the 2011 Doctoral Innovation Funds of Southwest Jiaotong University, the Fundamental Research Funds for the Central Universities (No. 2010XS31), and the scientific research expenses Foundation (for new teachers) of University of Electronic Science and Technology of China (No. Y02002012001007).
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