Fabrication of [email protected] composite nanofibers by electrospinning and autocatalytic electroless plating techniques

Results in Physics 12 (2019) 853–858

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Fabrication of Ni@SiC composite nanofibers by electrospinning and autocatalytic electroless plating techniques Nan Wu

⁎,1

T

, Su Ju1, Yingde Wang , Dingding Chen ⁎

College of Aerospace Science and Technology, National University of Defense Technology, Changsha 410073, China

ARTICLE INFO

ABSTRACT

Keywords: Nanocomposites Electrospinning Silicon carbide Fiber technology Ceramics

In this work, nickel-coated silicon carbide (Ni@SiC) nanofibers were successfully fabricated via electrospinning and autocatalytic electroless plating techniques. The in situ formed nickel oxide (NiO) seeds on the surface of SiC nanofibers were applied to catalyze the plating reaction instead of expensive palladium. The quality of nickel layer was determined by the reaction temperature, deposition time and NiO content in the fibers. After electroless plating for 2 h at 70 °C, the thickness of nickel layer was 60 nm and the average diameter of composite nanofibers was 610 nm. The volume density of the obtained Ni@SiC fibrous membrane was measured to be 1.3 g cm−3. Such composite fibrous membranes with uniform nickel coating and interconnected pore structure possess potential applications as catalysts, supercapacitor and shielding materials.

Introduction

ceramic matrix when pyrolyzing the polymers together with metal organic precursors at high temperature [11,12]. Furthermore, the metal oxides have been successfully applied to initiate the plating process without the existence of Pd2+ [13]. Therefore, the generated metal and metal oxides may also act as the seeding catalysts replacing noble metals for electroless nickel plating apart from antibacterial and pollutant degradation properties. In this work, SiC nanofibers functionalized by the in situ formed NiO activator were successfully synthesized through electrospinning and the following thermal treatments at 1100 °C in argon. Uniform nickel layer was then deposited on the SiC nanofibers via an electroless plating approach without pre-treatments with Sn4+ and Pd2+ solution. The influence of NiO contents, reaction temperature and growth time on the plating results was investigated.

Recently, introducing magnetic nickel layers onto ceramic fibers has been widely investigated for the fabrication of functional materials, such as flexible electrodes [1], heterogeneous catalysts [2], thermal barrier coatings [3] and electromagnetic interference shielding materials [4]. Up to now, various coating methods have been established, such as chemical vapor deposition, physical vapor deposition, sol-gel, electroplating and electroless plating, etc [5]. Particularly, electroless plating is a promising technique due to its advantages including uniform deposition, scale up and environmental friendless [6]. In general, sensitizing with Sn4+ and activating with Pd2+ on the surface of fibers is necessary to induce the reduction reaction of nickel ions [7]. Although some research groups have presented different procedures by replacing noble Pd2+ with Ag+ to reduce the overall cost, the complicated double-steps process and consumption of noble metals (Pd or Ag) still need to be optimized. Silicon carbide (SiC) fibers synthesized by electrospinning and polymer derived ceramics techniques, are gaining increasing attention as hydrophobicity and electromagnetic wave absorption materials [8,9]. The excellent stability, high porosity, large specific surface area and interconnected pore structure of the three-dimensional SiC fibrous membrane further make them potential candidates as catalyst supports and smart filters in harsh environments [10]. Previous studies have shown the possibility of in situ synthesis of metal or metal oxide in

Experimental Preparation of NiO-doped SiC nanofibers In a typical procedure, 60 mg of polystyrene, 20 mg of sodium dodecyl sulfate, 180 mg of polycarbosilane (PCS) and nickel acetylacetonate (Ni(acac)2) with different concentration (0.5, 1.0, 1.5 and 2.0 wt%) were dissolved in the mixed solvent of xylene and dimethylformamide. The obtained solution was electrospun by a Drum Electrospinning Unit (Kato Tech Inc., Japan) at 0.8 mL h−1 with an

Corresponding authors. E-mail addresses: [email protected] (N. Wu), [email protected] (Y. Wang). 1 N. Wu and S. Ju contributed equally to this work. ⁎

https://doi.org/10.1016/j.rinp.2018.12.051 Received 8 December 2018; Accepted 11 December 2018 Available online 17 December 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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N. Wu et al.

nanofibers were fabricated by electrospinning. After stabilization and pyrolysis process, NiO-doped SiC nanofibers with an average diameter of 510 nm were prepared. The obtained fibrous membrane was black due to the generated free carbon from the decomposition of PCS [14]. After electroless plating, nickel nanoparticles were uniformly deposited onto the NiO-doped SiC nanofibers and the membrane turned to be gray. It is worth mentioning that no nickel coating was observed in the gap among the nanofibers. The average diameter of nickel-coated SiC (Ni@SiC) composite fibers increased to 610 nm. XRD pattern in Fig. 2a shows three weak diffraction peaks at 35.7°, 60.0° and 71.7°, attributing to (1 1 1), (2 2 0) and (3 1 1) plane of β-SiC (JCPDS, No 29-1129) in SiC-1.5 [15]. However, no other peaks, being related to the active components, are found in SiC-1.5. After electroless plating, three obvious diffraction peaks at 44.5°, 52.0° and 76.5°, corresponding to (1 1 1), (2 0 0) and (2 2 0) planes of nickel (JCPDS, No 040850) [16], respectively, were observed to prove the successful coating of nickel. As can be seen in Fig. 2b, no nickel layer could be found on the pure SiC nanofibers after the same electroless plating treatments. This reveals that activator must exist on the surface of SiC −1.5, catalyzing the reduction reaction. TEM and XPS were used to further explore the detailed components of the activator. As shown in Fig. 3a, SiC-1.5 exhibited a rough surface. Fig. 3b is the selected area electron diffraction (SAED) pattern for SiC1.5. Three diffraction rings corresponded to the (1 1 1), (2 2 0) and (3 1 1) crystal planes of β-SiC [17]. Apart from the characterized lattice distance for β-SiC in the high-resolution TEM (HRTEM) image (Fig. 3c) [18], (1 1 1) plane of NiO with an interplanar spacing of 0.247 nm and free carbon could also be detected on the surface of SiC-1.5 [19]. Meanwhile, two main peaks at 853.4 and 856.5 eV, corresponding to NiO phase [20,21], were detected in high-resolution Ni 2p spectrum of SiC-1.5 (Fig. 3d). According to the above results, we believed that NiO phase on the surface of SiC-1.5 worked as the seeds for electroless nickel plating. In situ formed NiO seeds deriving from the decomposition of Ni(acac)2 contributed to catalyzing the growth of continuous nickel layer in the plating bath. The influence of NiO contents in SiC fibers on the surface morphology and nickel yield of Ni@SiC was investigated. As shown in Fig. 4a, no nickel phase was observed when the nickel content was 0.5 wt% (SiC-0.5), suggesting the active NiO seeds were not enough to catalyze the reduction reaction. Uniform nickel coating with many aggregations could be found when the nickel content was increased to 1.0 wt% (Fig. 4b). Perfect nickel layer could be obtained when the nickel content increased up to 1.5 wt% or above (Fig. 4c and d). XRD pattern in Fig. 4e also displays the absence of peaks for nickel in Ni@

Table 1 Typical composition of nickel electroless plating bath. No

Component

Amount

1 2 3 4 Plating temperature Time for plating

Nickel sulfate Trisodium citrate dihydrate NaOH (N2H4)·H2O

0.02 M 30 g L−1 pH = 11∼12 0.1 mL 60, 70, 80 °C 0.5∼3 h

applied voltage of 12 kV. Then, the as-spun nanofibers were stabilized in air at 210 °C for 2 h. Finally, NiO-doped SiC nanofibers (denoted as SiC-x, x represents the nickel acetylacetonate weight percentage in the solution) were obtained by pyrolyzing the stabilized nanofibers at 1100 °C under argon atmosphere. Electroless nickel plating The typical composition of nickel electroless plating bath was summarized in Table 1. Nickel sulfate was the source of nickel and NaOH was used to keep the pH between 11 and 12. N2H4 was used as reductant. The plating temperature was controlled at 60–80 °C. The fibrous membrane with a size of 1 × 2 cm was dipped into 5 mL of plating bath for different time to obtain the biggest nickel yield. After the plating process, the nickel-coated silicon carbide nanofibers membrane (Ni@SiC) was carefully removed from the plating bath and rinsed with deionized water. The resulting Ni@SiC membrane was dried in an oven at 80 °C for 12 h. Characterizations The morphology and surface microstructure of the nanofibers were observed using a scanning electronic microscopy (SEM, Hitachi S4800). X-ray diffraction (XRD) patterns were collected using a Rigaku TTR III diffractometer (Cu-Kα radiation, λ = 1.5406 Å) working at 40 kV and 200 mA. The inner microstructure was detected by transmission electron microscopy (TEM, Titan G2 60–300). The surface chemical environment was explored using X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, Al-Kα). Results and discussion The fabrication process of Ni@SiC was illustrated in Fig. 1. As-spun

Fig. 1. Schematic illustration of the fabrication process of Ni@SiC and the corresponding SEM images.

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Fig. 2. (a) XRD patterns of SiC-1.5 and [email protected]. (b) SEM image of pure SiC nanofibers without the addition of Ni(acac)2 after eletroless plating.

Fig. 3. (a) TEM image, (b) SAED pattern and (c) HRTEM image of SiC-1.5. (d) High-resolution XPS spectrum of Ni 2p on the surface of SiC-1.5.

SiC-0.5. To calculate the yield of nickel after plating for 3 h at 70 °C, the membrane mass before and after electroless plating was measured and the nickel yield was calculated through dividing the total nickel content in the plating solution by the plated nickel on Ni@SiC. The nickel yield of [email protected] and [email protected] is 5.8% and 66.3% (Fig. 4f), respectively. With the nickel content in SiC fibers increased to 1.5 wt% and 2.0 wt%, the nickel yield increased to 87.3% and 88.5%, respectively. Therefore, we choose SiC-1.5 as the substrate for the following experiments.

As well known that reaction temperature and growth time are two key parameters to determine the deposition rate and the amount of metals during electroless plating [22]. No obvious metal particles could be found in Fig. 5a when the temperature is 60 °C, suggesting the plating bath was not active at low temperature. Moreover, uniform nickel distributed throughout the surface of fibers when the temperature increased to 70 °C (Fig. 5b). With the temperature increasing to 80 °C, the nickel aggregations grown bigger (Fig. 5c) due to the bath unsteady. Furthermore, the nickel yield increased to 88.1% (Fig. 5d)

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Fig. 4. SEM images of (a) [email protected], (b) [email protected], (c) [email protected], (d) [email protected]. (e) XRD patterns of Ni@SiC-x; (f) Nickel yield of Ni@SiC-x after electroless plating for 3 h at 70 °C.

compared to the sample at 70 °C. Considering about the coating morphology and the excessive energy consumption, 70 °C was selected as the optimized plating temperature. As displaying in Fig. 6a, SiC nanofibers were not fully covered by nickel nanoparticles in the initial 0.5 h and the nickel yield was only 6.7%. With extending the reaction time to 1.0 h, the isolated nickel clusters jointed together to construct a continuous metal layer (Fig. 6b). After electroless plating for 2.0 h, the nickel yield reached to 86.8% and the weight ratio of nickel in the composite fiber was 85.1% (Fig. 6d and f). When further extending the time to 3.0 h, the nickel yield only increased from 86.8% to 87.3%. Therefore, 2.0 h was enough to finish the plating process. As can be seen in Fig. 7a, the uniform nickel products could only be observed on the surface of SiC nanofibers, suggesting that NiO play key roles in the nickel growth process. The area density and volume density of the hybrid [email protected] nanofibers after electroless plating at 70 °C for 2.0 h were measured to be 5.1 mg cm−2 and 1.3 g cm−3, respectively.

The enlarged SEM image in Fig. 7b further demonstrates that nickel cluster is constructed of abundantly small particles with size of 25 nm. The disordered stack of nanoparticles leads to the formation of many pores among the metal layers. The thickness of nickel coating is 60 nm (Fig. 7c). Furthermore, the interconnected Ni@SiC fibers without any breakages constructed into a three-dimensional open architecture, which could facilitate mass transportation inside the membrane. Conclusions In conclusion, this work presented a facile and low-cost method to prepare nickel-coated SiC composite nanofibers. In situ formed NiO on the surface of SiC nanofibers acted as activator to catalyze the nickel growth. A uniformly continuous nickel layer with a thickness of 60 nm was obtained at 70 °C after deposition for 2.0 h, when the nickel content inside the fiber was 1.5 wt%. The area density and volume density of Ni@SiC membrane is 5.1 mg cm−2 and 1.3 g cm−3, respectively. The

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Fig. 5. SEM images of [email protected] after electroless plating for 3 h at different temperature: (a) 60 °C; (b) 70 °C; (c) 80 °C; (d) Nickel yield of [email protected] after electroless plating for 3 h at different temperature.

Fig. 6. SEM images of Ni@SiC after electroless plating at 70 °C for different time (a) 0.5 h; (b) 1.0 h; (c) 1.5 h; (d) 2.0 h; (e) 3.0 h; (f) Nickel yield of [email protected] after electroless plating at 70 °C for different time.

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Fig. 7. (a) Low-resolution and (b, c) high-resolution SEM images of [email protected] after electroless plating at 70 °C for 2.0 h.

interconnected Ni@SiC membrane with high porosity exhibits great potential in purification, supercapacitor, high-temperature shielding materials and catalysis fields. Simultaneously, this method can be extended to other metal-ceramic composites requiring metal layers.

[9] [10]

Acknowledgements [11]

The work was supported by the National Natural Science Foundation of China (51773226, 51503223). Thanks to Jungsu Choi in University of British Columbia for the help in nickel plating experiments.

[12] [13]

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