Template-Assisted Synthesis of Nickel Sulfide Nanowires: Tuning the

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Template-assisted synthesis of nickel sulfide nanowires: tuning the compositions for supercapacitors with improved electrochemical stability Xiaoxian Zang, Ziyang Dai, Jun Yang, Yi-Zhou Zhang, Wei Huang, and Xiao-Chen Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08409 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Applied Materials & Interfaces

Template-assisted synthesis of nickel sulfide nanowires: tuning the

compositions

for

supercapacitors

with

improved

electrochemical stability Xiaoxian Zang, Ziyang Dai, Jun Yang, Yizhou Zhang, Wei Huang *, Xiaochen Dong * Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China E-mail: [email protected]; [email protected] Abstract Ni nanowires were firstly synthesized via a chemical method without surfactants or a magnetic field. A series of nickel sulfide nanowires (Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS) have been successfully prepared by a controlled sacrificial template route based on the conductive Ni nanowire template. Electrochemical characterizations indicate that Ni3S2-NiS nanowires present superior redox reactivity with a high specific capacitance of 1077.3 F g-1 at 5 A g-1. Besides, its specific capacitance can maintain about 76.3% after 10,000 cycles at 20 A g-1. On the contrary, the nickel-preserving sulfide nanowires (Ni3S2-Ni, Ni3S2-NiS-Ni) deliver enhanced cycling stability, as 100% of the initial specific capacitance of Ni3S2-Ni is retained after 10,000 cycles. The outstanding electrochemical stability can be attributed to the interaction between nickel sulfides and the conductive nickel nanowires. Keywords: supercapacitors; nickel sulfide; nanowires; composition; electrochemical stability

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Introduction Transition metal sulfides, such as FeS, CoS, and ZnS, have gained intensive interest as semiconductor functional materials owning to their excellent properties and wide applications in energy storage and conversion, catalysts, and sensors.1-5 In particular, nickel sulfides with various compositions and phases (Ni3S2, α-NiS, β-NiS, NiS2) have received considerable attention due to their promising applications in practical devices.6-7 Since the first report of NiS as an electrode for supercapacitors by Hou et al., numerous researches have been conducted to probe the electrochemical performance of nickel sulfides.8-10 It is noted that the poor electrical conductivity of nickel sulfides, like other sulfides and oxides, seriously inhibits the fast electron transport required for electrochemical stability at high current density and thereby hinders their practical application in energy storage devices.11 To overcome this drawback, considerable efforts have been devoted recently to explore the effective strategies, including fabrication of novel nanostructures (e.g. nanotubes,12 nanosheets,13-14 and hollow nanostructures15-16) and hybridization with conductive materials (e.g. carbon nanotube,17 graphene18-19). Among these methods, a novel and effective approach emerges to grow the active materials in situ on metal nanowires, utilizing the metallic nanowires as a template. For example, Zhou et al. used nickel nanochains, prepared by polyvinyl pyrrolidone (PVP) modification method, as sacrificial templates to fabricate Ni/Ni3S2 peapod nanochains and nickel sulfide hollow chains.20 In another work, Liu and co-workers synthesized Ni nanowires under a magnetic field in order to produce a free-standing Ni@NiO membrane electrode.21 The unique one dimensional, high aspect ratio morphology of metallic


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nanowires provides a large specific surface area and also shortens the diffusion pathways for ions. What’s more, the intimate connection between the active materials and metals results in a better electrochemical performance. The template approach based on metallic nanowires is highly promising and can be extended to many materials systems. However, detailed studies on the specific influence of procedures on various electrochemical performances, such as capacitance, rate capacity, and cycle stability, are lacking and strongly needed. In this report, Ni nanowires, prepared by a chemical reduction method without using any surfactants or a magnetic field, were used as conductive template. A series of nickel sulfide nanowires (Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS) were grown in situ on the metallic nanowires. For the Ni-preserving nickel sulfide nanowires (Ni3S2-Ni and Ni3S2-NiS-Ni), the active materials (Ni3S2 or NiS) were generated and wrapped on the Ni nanowires with strong interfacial interaction, which enabled a good electrical contact facilitating ion/electron transfer. As a result, Ni3S2-NiS presented the highest capacity primarily owing to the redox reactions of the nickel sulfides. On the other hand, the nickel-preserving nickel sulfide nanowires simultaneously deliver a high-rate capacity and much better cycling stability owing to the conductive nickel template, albeit showing a relatively low capacitance. The results demonstrate the dilemma in satisfying the different aspects of electrochemical performance using the template approach of synthesizing composite materials. Experimental section Synthesis of Ni nanowires



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Typically, NiCl2·6H2O (0.5 mmol) was dissolved in ethylene glycol (EG, 100 ml) by ultrasonic treatment for 10 min to form an aquamarine blue solution. Then, the solution was heated at 70 °C in a water bath. During the heating process, hydrazine monohydrate liquid (4 mL, 80%, v/v) and sodium hydroxide aqueous solution (8 mL, 2.5 M) were successively injected into the mixture with magnetic stirring for 30 min. Afterward, the collected black magnetic precipitate was washed with ethyl alcohol and deionized (DI) water for several times, followed by drying in the oven at 80 ℃for 6 h. Synthesis of Ni3S2-Ni nanowires The as-prepared Ni nanowires (10 mg) and Na2S (40 mg) were dispersed in DI water (20 mL) by ultrasonic treatment for 10 min. Then, the mixture was placed in a Teflon-lined stainless steel autoclave and heated in an oven at 80 °C for 12 h. Finally, the collected product was washed with ethyl alcohol and DI water for several times, followed by drying in the oven at 80 °C for 6 h. Synthesis of Ni3S2-NiS-Ni nanowires The synthesis procedure of Ni3S2-NiS-Ni nanowires was similar as the preparation of Ni3S2-Ni nanowires, except that the solvent was replace by EG and the sulfuration reaction was conducted at 140 °C. Synthesis of Ni3S2-NiS nanowires The synthesis procedure of Ni3S2-NiS nanowires was similar as the preparation of Ni3S2-Ni nanowires, except that the solvent was replace by EG and the sulfuration reaction was conducted at 180 °C.


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Characterization The crystallographic structure, composition and morphology of the products were characterized by a range of methods including X-ray diffraction (XRD, Bruker D8 Advance), field emission scanning electron microscope (FESEM, Hitachi, S-4800, Japan), and transmission electron microscopy (TEM, Hitachi, HT7700), respectively. High-resolution TEM (HRTEM) and selected area electron diffraction (SAED) investigation were carried out by a JEOL JEM-2010 instrument. X-ray Photoelectron Spectroscopy (XPS) spectra of the products were recorded by an ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, USA) with Al (Kα) radiation. Electrochemical measurements Electrochemical characterizations were investigated in a conventional three-electrode system with a working electrode, a reference electrode (Hg/HgO electrode), and a counter electrode (platinum wire). To fabricate the working electrode, the slurry containing active materials, conductive agent (acetylene black) and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 was coated onto the nickel foam. The mass of the active material loaded on nickel foam substrate was ~4.0 mg cm-2. A 3.0 M KOH aqueous solution was served as the electrolyte. Electrochemical measurements including cyclic

voltammetry

(CV),

galvanostatic

charge-discharge

tests

(GCD),

and

electrochemical impedance spectroscopy (EIS) (0.1-100 000 Hz) were operated on a CHI 660D electrochemical workstation (CH instrument Inc, China). Results and Discussion As illustrated in Scheme 1, a two-step procedure has been carried out to fabricate nickel



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sulfide nanowires. In the first step, Ni nanowires were synthesized as a conductive template by the reduction of Ni2+. Then, the nickel nanowires were reacted with S2- ions to obtain three kinds of sulfide nanowires with different amounts of Ni in component. Fig. 1a presents the XRD pattern of the as-prepared Ni nanowires. Three distinct diffraction peaks at 2θ = 44.5°, 51.8°, and 76.4° can be well indexed to metallic Ni (JCPDS no. 04-0850).22-23 No additional peaks can be detected as impurities, indicating the high purity of the Ni template. SEM images of the obtained Ni nanowires were observed to possess a length of 1-3 µm, as shown in Fig. 1b. The Ni nanowires with rough surfaces can be characterized in the magnified SEM image (inset in Fig. 1b). TEM image (Fig. 1c) reveals that the average diameter of the Ni nanowires is ~60 nm. In addition, the corresponding SAED pattern (Fig. 1d) exhibits a distinguishable ring-like feature with the diffraction rings from the inner to the outer corresponding to (111), (200) and (220) lattice planes of Ni in sequence. The above results demonstrate that the Ni nanowires with appropriate structure and morphology were successfully synthesized as a conductive and sacrificial template. Based on the Ni nanowire sacrificial template, three samples (Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS) with different sulfide compositions were synthesized. The evolution of the components of the samples was identified by means of XRD analysis (Fig. 2a). The XRD pattern of Ni3S2-Ni shows the main peaks matched well for Ni3S2 (JCPDS no. 44-1418).24 It should be noticed that the asterisk peaks assigned to the preserved Ni phase (JCPDS no. 04-0850) are also clearly present in the patterns of both Ni3S2-Ni and Ni3S2-NiS-Ni. As illustrated in the XRD pattern of Ni3S2-NiS, the peaks marked with


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blue dots correspond to the NiS phase (JCPDS no. 12-0041)25 and the diffraction peaks of Ni completely disappear, indicating metallic Ni is entirely converted to Ni-S phase at an elevated temperature (180 °C). The chemical states of the nickel sulfide nanowires were further characterized by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum (Fig. S1) confirms that Ni and S elements both present in the products. In the Ni 2p spectrum (Fig. 2b), the binding energies of Ni 2p1/2 and Ni 2p3/2 peaks at around 874 eV and 856 eV are the characteristic of Ni+ and Ni2+ in Ni3S2. 26-28 Consistent with the result of XRD spectrum, the distinct 2p3/2 peaks in the Ni 2p line of Ni3S2-Ni appear at 852.0 eV, indicating the presence of Ni0 species.29 Besides, the 2p3/2 peaks in the Ni 2p line of Ni3S2-NiS at 853.1 eV can be assigned to divalent Ni species.30 According to the XPS analysis, the mass ratio of Ni : S in each sample is listed in Table S1. SEM (Fig. S2) and TEM images (Fig. 3a, 3c, and 3e) further confirm that the nanowire structures of the products are well-preserved except a slight increase of their diameters. In addition, thin nanosheets are spontaneously wrapped on the surface of the nanowires; as a consequence, the products are endowed with heterostructure and large specific surface area. The phase transformation of the three samples, as illustrated by HRTEM images in Fig. 3b, 3d, and 3f, is in accordance with the analysis by XRD. According to the above characterizations, the formation mechanism of the nickel sulfide nanowires might be described as follows. The rough surface of Ni nanowires with defects compared to the unexposed part may provide more reactive sites.31 Therefore, the Ni atoms at the surface of the nanowires might react with S2- ions firstly. Thin nanosheets



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are formed on the surface of Ni nanowires, which is consistent with the structure and morphology of Ni3S2-Ni. As the reaction proceeds, the anion exchange reaction takes place gradually in the Ni nanowires from outer to inner to form Ni3S2-NiS-Ni and Ni3S2-NiS. As a result, the obtained heterostructure of the products might promote electron transfer behavior during the electrochemical charge/discharge process. The electrochemical performances of the as-prepared nickel sulfide nanowires were evaluated in 3 M KOH aqueous solution. Fig. 4a shows the typical CV curves of the Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS measured at a scan rate of 20 mV s-1. It can be observed that all of the CV curves present a pair of well-defined redox peaks, showing a typical pseudo capacitive behavior.32 Moreover, the integral area of Ni3S2-NiS is larger than those of Ni3S2-Ni, and Ni3S2-NiS-Ni, indicating the Ni3S2-NiS possesses higher specific capacitance. For comparison, the CV curve of pure Ni foam is also measured (Fig. S3). Its inappreciable integrated area, attributed to the unavoidable oxidation of Ni, indicates that the current collector almost makes no contribution to the electrochemical capacitance.33 Furthermore, the CV curves of the three electrodes at different scan rates ranging from 2.0 to 20 mV s-1 are shown in Fig. S4a, S4c, and S4e. The peak currents present a corresponding increase with the scan rate raised, which indicates a desired capacitive behavior of supercapacitors. Fig. 4b presents the GCD curves of the Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS measured at a current density of 5 A g-1. The performances of the electrodes have a positive correlation with their contents of electrochemical active ingredients (including Ni3S2 and NiS). The specific capacitances evaluated from GCD curves at current densities of 5, 10,


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15, 20, 30 A g-1 (Fig. S4b, 4d, 4f) are plotted in Fig. 4c and the detailed data are listed in Table S2. As expected, Ni3S2-NiS achieves the highest specific capacitance (1077.3 F g-1) at a current density of 5 A g-1. It also should be noticed that the specific capacitances of Ni3S2-Ni represent only a slight decline when increasing the current density from 5 to 30 A g-1. Although Ni3S2-Ni and Ni3S2-NiS-Ni present relative lower specific capacitances compared with Ni3S2-NiS, their higher rate capabilities confirm that Ni nanowires play a vital role to enhance the electronic transmission in the electrochemical process. Fig. 4d shows the Nyquist plots of the Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS electrodes. The equivalent series resistance (ESR) values of the three electrodes, derived from the point crossing with the horizontal axis, are 0.45, 0.40, and 0.44 Ω, respectively.34 The charge-transfer resistance (Rct) is small enough, since no obvious semicircles appearance can be observed.35 Moreover, the linear part in the low frequency range related to the diffusion limitation demonstrates a representative capacitive behavior.36 The cycling performances of Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS at a current density of 20 A g-1 were also measured, as shown in Fig. 5a. It can be observed that the specific capacitance of Ni3S2-NiS decreased slightly to ~76.3% of its initial values after 10,000 cycles. However, there is no decrease of the capacitance for Ni3S2-Ni electrode under the same test. According to the TEM images presented in Fig. S5, it can be found that the morphology of Ni3S2-Ni nanowires can be well maintained after 10000 cycles. The better long-term cycling stability can be attributed to the interaction between nickel sulfides and nickel nanowires, as well as the improved electrical conductivity due to the relatively larger amount of nickel in component.



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Fig. 5b shows the Nyquist plots of the three electrodes after 10,000 charge-discharge cycles. After long cycling test, the ESR value of Ni3S2-NiS present a distinct increase (0.74 Ω). While the slight changes of Ni3S2-Ni and Ni3S2-NiS-Ni electrodes in ESR values during the cycling test further confirm that the enhanced electrochemical performance comes from the conductive nickel template preserved in the active material. Table S3 presents the electrochemical performances of nickel sulfides and their hybrids electrodes reported recently. As comparison, the electrodes in this work exhibit superior electrochemical stability. The achieved electrochemical performances are related to the heterostructure with the active materials (Ni3S2 or NiS) wrapped on the surfaces of Ni nanowires, which could be able to form a conductive channel for electron transport.37-39 Considering that the high capacitance mainly results from the redox reaction of active materials, the careful control over the content of metallic nickel is a viable strategy to achieve excellent electrochemical stability as well as high specific capacity. Conclusion In summary, a series of nickel sulfide nanowires (Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS) were successfully synthesized by using Ni nanowires as a template. The effect of the preserved nickel template on the electrochemical performance of the composites has been explored by the detailed electrochemical characterization. On the one hand, Ni3S2-NiS nanowires exhibit outstanding electrochemical performance with high specific capacitance (1077.3 F g-1 at 5 A g-1) and relatively poor cycle stability. On the other hand, the nickel-preserving sulfide nanowires (Ni3S2-Ni and Ni3S2-NiS-Ni) show superior electrochemical stability (100% and 87.2% capacitance retention after 10,000 cycles at


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20 A g-1, respectively). The enhanced electrochemical stability clearly sheds light on the design and synthesis of materials for various electrochemical applications. Furthermore, the compromise between the specific capacitance and cycle stability is particularly interesting for the application of the synthetic approaches of composite materials via metallic templates.

Supporting Information: XPS spectrum, weight ratio of Ni, S, C and O coming from XPS analysis, SEM images, comparison of CV curves of pure Ni foam, Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS electrodes, CV and GCD curves, TEM images of the electrodes after 10,000 charge-discharge cycles, and comparison of electrochemical performances of nickel sulfides and their composites.

Acknowledgements The project was supported by the NNSF of China (61525402, 21275076), 973 program (2014CB660808), Key University Science Research Project of Jiangsu Province (15KJA430006), Jiangsu Provincial Funds for Distinguished Young Scholars (BK20130046),

Program

for

New

Century

Excellent

Talents

in

University

(NCET-13-0853), QingLan Project, and Nantong Key Laboratory of New Materials Industrial Technology.



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Scheme 1. Illustration of the formation process of nickel sulfide nanowires.


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Fig. 1 (a) XRD pattern, (b) SEM image, (c) TEM image, and (d) SAED of Ni nanowires. The inset in Fig. 1b showed the magnified SEM image of Ni nanowires.



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Fig. 2 (a) XRD patterns and (b) XPS patterns of Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS.


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Fig. 3 TEM and corresponding HR-TEM images of (a, b) Ni3S2-Ni, (c, d) Ni3S2-NiS-Ni, and (e, f) Ni3S2-NiS.



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Fig. 4 (a) CV curves, (b) Galvanostatic charge-discharge curves, (c) Specific capacitance, and (d) Nyquist plots of Ni3S2-Ni, Ni3S2-NiS-Ni, and Ni3S2-NiS electrodes.


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Fig. 5 (a) Cycling stability of the electrodes measured at a current density of 20 A g−1. (b) Nyquist plots of the electrodes after 10,000 charge-discharge cycles at a current density of 20 A g−1.



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Table of Contents Graphic


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Template-Assisted Synthesis of Nickel Sulfide Nanowires: Tuning the

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