full papers Nanowire Arrays
Synthesis of Free-Standing Metal Sulfide Nanoarrays via Anion Exchange Reaction and Their Electrochemical Energy Storage Application Xinhui Xia, Changrong Zhu, Jingshan Luo, Zhiyuan Zeng, Cao Guan, Chin Fan Ng, Hua Zhang, and Hong Jin Fan*
Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and corebranch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The selfsupported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.
1. Introduction In recent years, tremendous research effort has been devoted to electrochemical devices with high power density and fast recharge capability. Among all these power sources, nanostructured electrode batteries and supercapacitors are the paradigm devices as they are promising to deliver power source with both high energy and power densities. Metal oxides and conducting polymers have been widely explored Dr. X. H. Xia, C. R. Zhu, J. S. Luo, C. Guan, C. F. Ng, Prof. H. J. Fan Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore, 637371, Singapore E-mail: [email protected] Dr. X. H. Xia State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering Zhejiang University Hangzhou, 310027, P. R. China Dr. Z. Y. Zeng, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University Singapore, 639897, Singapore DOI: 10.1002/smll.201302224
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for constructing batteries and supercapacitors with increased specific capacity/capacitance and high energy densities arising from faradic redox reactions.[1–13] Metal sulfides have recently emerged as a new promising class of active materials due to their excellent redox reversibility and relatively high capacity/capacitance. Metal sulfides have been demonstrated with outstanding properties for hydrodesulfurization catalysis,[14] biological labeling,[15] medical diagnostics,[16] solar cells,[17,18] lithium ion batteries,[19–21] and supercapacitors,[22–27] oxygen reduction reaction.[28] It has been demonstrated that metal sulfides undergoes reversible redox reaction according to MSOH (MS + OH− ↔ MSOH + e−, M = Ni, Co…) in the alkaline electrolyte.[25] Take NiS and CoS for examples, their theoretical capacities are ≈294 mAh/g. Although several nanostructured metal sulfides (such as NiS, CuS, CoS2 and CoS) have been synthesized and applied for batteries and supercapacitors,[19–27,29–31] these materials are in powder form and thus need to be mixed with polymer binders, introducing supplementary and undesirable interfaces and risks negating the nanosize benefits. It is known that the electrochemical properties are determined by the kinetic features controlled by the transportation of electrons and ions into the active materials.[3] One of effective approaches to enhance the transport kinetics is to construct nanoarray electrodes with a direct mechanical and electrical contact with the current collector. This configuration allows an easy
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diffusion of ions, lower internal resistance and subsequently, realization of high-power performance. So far, there are only a few reports dedicated to metal sulfides nanoarrays directly on conductive substrates for high-rate battery/supercapacitor application. Lin and Chou reported an electro-deposited nanosheet-like CoS film via thiourea and its pseudo-capacitive performance.[32] However, their cycling test was conducted only for 1000 cycles and no full cell was demonstrated. A large variety of methods have been reported for the synthesis of metal sulfides, such as solid state reaction,[20] wet chemical precipitation,[22,33] hydrothermal method,[34] solvothermal synthesis methods,[30] and electrochemical deposition method.[35] These synthesis methods are suitable for powder materials, but have not been proven applicable to oriented metal sulfide nanoarrays on conductive substrates. In addition, these conventional methods usually require toxic H2S source and organometallic precursors. It is therefore of great interest to develop a green synthesis method for high-quality metal sulfide nanoarrays for energy storage applications. The solution-based ion exchange reaction (IER) has proven an effective and low-cost method for chemical transformation of nanomaterials.[36] The IER method circumvents the drawbacks of high temperatures, high pressure and toxic precursor sources, which significantly reduces the fabrication cost while maintaining an excellent reproducibility. The Alivisatos group pioneered the application of cation ion exchange method for the synthesis of colloidal nanoparticles such as Ag2Se, CdSe, ZnS, and CdS−Ag2S.[37–40] More recently, the similar method has also been employed to fabricate nanowire p−n junctions (CdS−Cu2S) for photovoltaics,[41] counter electrode for solar cells,[17] and photosensitizer layers for photoelectrochemical electrodes.[42,43] To date, there is little literature on anion exchange reaction for the synthesis of free-standing metal sulfide nanoarrays with adjustable compositions and nanostructures on conductive substrates, and their applications for electrochemical energy storage. In the present work, we apply the facile anion exchange method to fabricate metal sulfide nanostructured arrays (including nanowires, core-branch nanowires, and nanowalls) directly from their metal oxide or hydroxide pre-forms without using any organosulfur compound precursors or H2S source. As a result of shape-preserved reaction, the obtained sulfides retain not only the topotactical relationship in morphology but also rigid contact with the conductive carbon fibre cloth and nickel foam substrates. As a preliminary demonstration, the CoS nanowire arrays exhibit outstanding redox reactivity and highrate capability when tested as the pseudo-capacitor cathode. Our results demonstrate the effectiveness of IER to the construction of various sulfide-based 3D nanostructures for the energy applications in supercapacitor, lithium ion batteries, and solar cells.
2. Results and Discussion 2.1. Oxide-to-Sulfide Transformation via Anion Exchange Reaction The schematic in Figure 1 illustrates the generality of the IER approach which can convert various metal oxide or small 2014, 10, No. 4, 766–773
Figure 1. Schematic of the conversion of metal oxide nanoarrays to metal sulfides nanoarrays via anion exchange reactions.
hydroxide nanostructures (from 1D, 2D to 3D) to the corresponding metal sulfides while maintaining the array architecture, alignment, and physical contact with the substrates. The Co3O4 nanoarrays of both nanowire and nanowall grown on carbon fibre cloth are chosen for elucidation of the IER process. The whole carbon fibre cloth is uniformly covered by hydrothermal-synthesized Co3O4 nanowires with diameters of ≈80 nm (Figure 2a,b) and height of ≈5 μm. The Co3O4 nanowires are composed of numerous nanoparticles of ≈5 nm and show mesoporous structure ranging from 3−5 nm (Figure 2c). After IER, the obtained CoS nanowire surfaces are rough (Figure 2d,e). The thickness of the nanowire film is also around 5 μm (Figure 2e), the same as the pristine Co3O4 nanowires film. TEM result indicates that, unlike the mesoporous Co3O4, the converted CoS nanowires exhibit a denser but still polycrystalline structure (Figure 2f). The measured lattice fringe with an interplane spacing of 0.19 nm in the HRTEM image corresponds to the (102) plane of CoS phase (Figure 2f). The phase change is also verified by XRD, Energy dispersive X−ray spectroscopy (EDS) and X−ray photoelectron spectroscopy (XPS) results. Prior to ion exchange, XRD pattern confirms the spinel Co3O4 phase (JCPDS 42-1467) and its crystalline nature (Figure 3a). After the conversion, all the diffraction peaks can be indexed to the hexagonal CoS phase (JCPDS 75-0605) (Figure 3b), indicating the complete reaction via ion exchange. For the XPS results, the Co3O4 nanowire arrays show the Co 2p3/2 peak around 779.9 eV, which can be deconvoluted to two peaks corresponding to two charge states, Co2+ and Co3+ (Figure 3c). The Co3O4 nanowire arrays do not exhibit any peak in the region of S 2p (Figure 3d). After IER, the Co 2p3/2 peak shifts to a lower binding energy of 777.9 eV, characteristic of the Co−S bonding. Simultaneously, a new peak at 162.9 eV is noticed in the region of S 2p.[35,44,45] In the O1s
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After the same IER, the nanowall network architecture is well maintained, but the surface of the obtained NiS nanowalls becomes rougher than the pre-forms (Figure 4g,h). XRD results confirm the conversion from NiO (JCPDS 4-0835) to NiS (JCPDS 12-0041) (Figure 5). Similar morphologies of metal sulfide nanoarrays can also be derived when nickel foam substrates are used. Furthermore, this IER method can be extended to fabricate CoS/NiS core-shell nanowire array when the pre-form of Co3O4/NiO core-shell nanowire array is utilized (Figure 6). The crystal phases prior to and after IER are verified by XRD (Figure S4). More interestingly, we also obtained Co9S8 nanotube arrays from basic cobalt carbonate precursor through the same IER procedure (see electron micrographs in Figure 7 and XRD patterns in Figure S5). Note that, while the nanowire outline is preserved, the nanotube walls are polycrystalline. All in all, the above demonstrations serve as strong testament that the IER method is powerful and versatile for constructing high-quality metal sulfide nanoarrays on arbitrary substrates.
2.2. Reaction Mechanism Figure 2. Transformation from Co3O4 nanowires (left column) to CoS nanowires (right column). a,b) SEM images of Co3O4 nanowire arrays on carbon fibre cloth (large-scale image in inset). c) TEM image of individual Co3O4 nanowire (magnified image in inset). d,e) The corresponding CoS nanowire arrays after ion exchange reaction with Na2S solution (largescale image and side view in insets). f) TEM image of individual CoS nanowire (HRTEM image and magnified image in insets).
spectrum (Figure 3e), the peaks at 530.3 and 531.5 eV indicate the existence of Co-O bonds, as is consistent with the Co 2p spectrum.[3,46] After the anion exchange, no Co-O bonds are observed in this region. Moreover, the EDS spectra and element mapping analysis also verified the component of the nanowires is Co and S (Supporting Information Figure S1). CoS nanowall arrays can also be prepared through the same anion exchange conversion from Co3O4 or Co(OH)2 nanowall arrays. The electrodeposited Co3O4 film has a network of interconnected nanowalls with thicknesses of ≈10 nm (Figure 4a). The Co3O4 nanowall arrays align vertically to the substrate and the walls contain mesopores of sizes ranging from 2−3 nm (Figure 4b). The anion-exchanged CoS keeps the vertical network configuration and shows a film thickness of ≈1 μm (Figure 4e,f). Compared to the Co3O4 mesoporous walls, the CoS walls exhibit a rough texture and dense structure (Figure 4f). Similar results have been obtained when the Co(OH)2 nanowall array were used as the starting material (see Figure S2). To verify the universality of the IER method in synthesizing metal sulfides, we extended the method to NiS nanowall array by using chemical-bath-deposited NiO and Ni(OH)2 nanowall arrays as the starting materials (Figure 4 and Figure S3). The NiO and Ni(OH)2 nanowall arrays also have a network structure composed of nanowalls with thickness around 10 nm (Figure 4c,d). The NiO and Ni(OH)2 nanowalls both show a smooth texture from TEM observations.
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The growth mechanism of the metal sulfides (CoS and NiS) based on IER is proposed as follows. The key parameter for ion exchange reactions is the solubility product constant (Ksp) of the material.[36] In general, materials with lower Ksp values are thermodynamically more stable that those with higher Ksp values. As a result, the latter are prone to conversion into the former in the specific solution (with the same cations or anions). In our experiments, the Ksp values for Co3O4, Co(OH)2, NiO, Ni(OH)2, CoS and NiS are about 3.1 × 10−18, 5.9 × 10−15, 1.5 × 10−17, 5.5 × 10−16, 3 × 10−26 and 1.3 × 10−25, respectively.[47,48] Since the Ksp values of metal sulfides (CoS and NiS) are drastically lower than the corresponding metal oxides/ hydroxide pre-forms, the former can be obtained by simple exchange reactions of the latter in a solution containing S2− anions. The reactions involved in our experiments can be expressed as follows.[17,31] Na2 S ↔ 2Na+ + S2− Co3 O4 + 3S
2−
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the reaction rates are much faster than in bulk materials. Consequently, complete and fully reversible reactions in nanoscale materials may occur, leading to more homogeneous molecule-like reaction kinetics. This opens a new way for composition manipulation of colloidal nanoparticles in any dimension. Our nanoarray pre-forms (nanowires or nanowalls) are composed of tiny nanocrystallites of sizes in a few nanometers. These nanoarrays possess a large surfaceto-volume ratio and surface areas (see BET measurement result in Figure S6), which is favorable for complete exchange reactions with S2− ions. As for the particular case of Co9S8 nanotube, it is probably due to the pseudo Kirkendall effect involved in the reaction of the solid Co2(OH)2CO3 nanowires with the Na2S solution.[37,49] The reaction takes place in a way that the outward diffusion of cobalt ions is faster than the inward diffusion of S2− ions, thus creating voids at the center of the nanowire and finally forming Co9S8 nanotubes. Given that the pure Co3O4 nanowires do not convert to hollow Co9S8 nanotubes but solid CoS nanowires (see above), it is hypothesized that the faster diffusion of cobalt cations in this reaction should be related to the release of CO32− and OH− ions. small 2014, 10, No. 4, 766–773
To demonstrate the potential application in electrochemical energy storage, we investigated the electrochemical properties of the as-prepared CoS nanowire arrays grown on nickel foam as cathode materials for pseudo-capacitors. Here, it is important to note that the electrochemical behavior of pseudo-capacitor is similar to that of battery (Conway proposed the concept of pseudo-capacitor in 1975 on the basis of the similarity to conventional secondary batteries).[4,50] In our case, in order to obtain real and accurate values of the energy storage and release, the capacities were calculated based on the battery equation (unit, mAh g−1), not the equation of traditional supercapacitor (capacitance, F g−1). A further detailed clarification on this point is provided in the supporting information. Figure 8a and b shows the SEM and optical images and three-electrode configuration of the quasi-vertical aligned CoS nanowires. The electrochemical behavior of the CoS nanowire arrays was investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge tests in 2 M KOH. Figure 8c shows the CV curves of the CoS nanowire arrays at different scanning rates. The redox couple corresponds to the conversion between CoS and CoSOH, simply illustrated as follows.[25,29]
CoS + OH− ↔ CoSOH + e−
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The CV curves at different cycles also show that the electrochemical reaction is quasi-reversible (Figure S7), meaning that the electrochemical energy can be reversibly stored and released in the CoS nanowire arrays. The specific capacity values of the CoS nanowire arrays are calculated from the galvanostatic discharge curves at various current densities (Figure 8d,e). The values range from 129 mAh g−1 at 2 A g−1 to 102 mAh g−1 at 40 A g−1, meaning that 79 % of the capacity is retained when the rate changes from 2 to 40 A g−1. It is noteworthy that the Ni foam has a small contribution to the capacity with prolonged cycling arising from the oxidation of Ni. So in our work, the contribution of the nickel foam has been subtracted. Furthermore, the asprepared CoS nanowire arrays show a good cycle life with 91% capacity retention (117 mAh g−1) after 3000 cycles at 2 A g−1 (Figure 8f). The CoS nanowire array electrode also shows a high and stable coulombic efficiency of ∼98% during charge–discharge cycles. The overall nanowire architecture is well preserved after 3000 cycles, which implies that the array structure could effectively alleviate the structural
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Figure 6. Transformation from a,b) Co3O4/NiO core/shell array to c,d) CoS/NiS metal sulfide array.
Figure 4. Transformation from metal oxide nanowall arrays (left column) to metal sulfides (right column). a,b) SEM and TEM images of Co3O4 nanowall array on carbon fiber cloth (large-scale image in inset). e,f) The corresponding CoS nanowall array after ion exchange reaction with Na2S solution (large-scale image and side view in insets). c,d) SEM and TEM images of NiO nanowall array on carbon fiber cloth (large-scale image in inset). g,h) The corresponding NiS nanowall array (large-scale image and side view in insets).
Figure 7. Transformation from Co2(OH)2CO3 nanowire arrays a,b) to Co9S8 nanotube arrays c,d). A pseudo Kirkendall effect might involve during the anion exchange reaction, accounting for the formation of the hollow core.
damage caused by volume expansion during the cycling process (Figure S8). In short, these results reveal the high specific capacity, good rate capability and cycle life of the CoS nanowire arrays that are potentially useful for highperformance batteries.
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The electrochemical properties are compared between CoS nanowire arrays and conventional CoS powder (see Figure S9 for detailed electrochemical characterization). First, the self-supported CoS nanowire arrays have a narrower oxidation-reduction potential separation than the powder counterpart. As shown in Figure S9b, the nanowire arrays exhibit an oxidation peak at 0.48 V and a reduction peak at 0.31 V, whereas the CoS powder exhibits lower reduction peak potential (0.24 V) and higher oxidation peak potential (0.53 V). Second, the peak currents of the nanowire arrays are much higher than the CoS powder. From the galvanostatic charge–discharge curves (Figure S9c), one can also see the nanowire arrays present 40 50 60 70 lower charge voltage plateau and higher 2θ (degree) discharge voltage plateau compared to the powder electrode. This indicates that
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Figure 8. Electrochemical properties of CoS nanowire arrays on nickel foam. a) SEM image with large-scale view and optical image in inset. b) Schematics of the three-electrode electrochemical cell. c) CV curves at different scanning rates. d) Charge–discharge curves at different current densities (unit: A g−1). e) Specific capacities at different current densities. f) Cycling performance at 2 A g−1.
the CoS nanowire arrays has weaker polarization and better electrochemical activity. Finally, the CoS nanowire arrays show higher specific capacity and better high-rate capability (Figure S9d). It can be concluded that the free-standing CoS nanowire arrays have a superior electrochemical activity electrode compared to the powder counterpart. To prove the potential application of these metal sulfide nanoarrays for electrochemical energy storage, full pseudocapacitor prototype devices with the CoS nanowire arrays as the cathode and activated carbon as the anode are assembled and tested (see Figure S10 for detailed electrochemical characterization and chemical reaction details at both electrodes). In our case, the capacity of anode is much larger than the cathode to ensure the maximum performance of the cathode (see methods). The reactions involved in our full devices based on CoS nanowire arrays are given as follows.[25] Cathode : CoS + OH − e− ↔ CoSOH
3. Conclusion We have demonstrated that the ion exchange reaction is an effective and powerful method that allows a shape-preserved transformation from metal oxide or hydroxide nanostructure arrays to the corresponding metal sulfides. As examples, CoS and NiS arrays of nanowires, core-shell nanowires, nanowalls, and nanotubes have been achieved directly on conductive substrates. The occurrence of anion exchange reactions depends on their Ksp values and nanoscale morphology. The as-obtained CoS nanowire arrays show outstanding electrochemical properties with a high capacity, good cycle life and a high-rate capability. This method could be extended to the fabrication of other metal sulfide nanostructure arrays or thin films for solar cells, photo- or electrocatalysis, and electrochemical energy storage applications.
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Anode : C + K + e ↔ K + //C (8) (K+//C represents the K+ is absorbed on the surface of activated carbon) Overall : CoS + OH− + C + K+ ↔ CoSOH + K+ //C small 2014, 10, No. 4, 766–773
The cathode exhibits a specific capacity of 131 mAh g−1 at the working current of 50 mA (Note: 50 mA corresponds to 2 A g−1 or 6.8 C based on the total CoS mass of 25 mg) and 121 mAh g−1 at a very high working current of 250 mA (10 A g−1 or 34 C). While the low-rate capacity is comparable to that of LiFePO4/C composites (≈135 mA h−1 at 5 C),[51–53] the high-rate capacity is much higher than that of LiFePO4/C composites (≈80 mAh g−1 at 20 C),[51–53] as well as that of LiFePO4/ conducting polymers (Polypyrrole and Polyaniline) composites (≈64 mAh g−1 at 10 C).[54] It is also noteworthy that the working voltage of the metal sulfides arrays in this study is much lower than LiFePO4 as our assembled pseudo-capacitors are in an aqueous system, unlike the LiFeFO4/C in a non-aqueous system. Cycle life is an important parameter for pseudo-capacitor application. Impressively, the assembled pseudo-capacitor exhibits fairly good cycling stability. After 16 000 cycles at 2 A g−1 (charge–discharge current of 50 mA), the pseudo-capacitor still delivers a specific capacity of 103 mAh g−1 with a retention of 79% (Figure S10d). The tandem devices (three pseudo-capacitors in series) can easily power the green LEDs with a working voltage of 3.2 V. These results demonstrate the functionality of metal sulfide nanoarrays potentially for pseudo-capacitors with both high specific capacity and rate capability.
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4. Experimental Section Preparation of CoS Nanowire Arrays: The CoS nanowire arrays were prepared by the combination of hydrothermal synthesis and ion exchange reactions. Firstly, self-supported Co3O4 nanowire
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arrays were prepared by a facile hydrothermal synthesis method. The solution was prepared by dissolving Co(NO3)2 (2 mmol), NH4F (4 mmol) and CO(NH2)2 (10 mmol) in distilled water (50 mL). Then the resulting solution was transferred into Teflon-lined stainless steel autoclave liners. Various substrates (2 cm × 6 cm in sizes) such as carbon fibre cloth and nickel foam were immersed into the reaction solution. Top sides of the substrates were uniformly coated with a polytetrafluoroethylene tape to prevent the solution contamination. The liner was sealed in a stainless steel autoclave and maintained at 110 °C for 5 h, and then cooled down to room temperature. The samples were annealed at 350 °C in normal purity argon for 2 h to form Co3O4 nanowire arrays. Then, the CoS nanowire arrays were obtained by placing Co3O4 nanowire arrays in a sealed cup with a solution containing sodium sulfide (0.1 M) at 90 °C for 9 h. Finally, the CoS nanowire arrays were collected and rinsed with distilled water several times. The load weight of CoS is about 2 mg cm−2. Preparation of CoS Nanowall Arrays: The CoS nanowall arrays were fabricated through direct IER from two Co species nanowall arrays [Co(OH)2 and Co3O4] prepared via cathodic electrodeposition. The electrodeposition was performed in a standard threeelectrode glass cell at 25 °C, the carbon fibre cloth or nickel foam as the working electrode, saturated calomel electrode (SCE) as the reference electrode and a Pt foil as the counterelectrode. The Co(OH)2 nanowall array were electrodeposited from aqueous solution containing Co(NO3)2 (1 mol L−1) and NaNO3 (0.1 mol L−1) at a constant cathodic current of 2 mA cm−2 for 300 s using a Chenhua CHI660C model Electrochemical Workstation (Shanghai). The Co(OH)2 nanowall array could be converted into Co3O4 nanowall array after annealing at 250 °C in argon for 1 h. Afterwards, the Co(OH)2 and Co3O4 nanowall array were converted into CoS nanowall array under the same ion exchange procedure as above. Preparation of NiS Nanowall Arrays: The NiS nanowall arrays were fabricated through direct ion exchange from two Ni species nanowall array (Ni(OH)2 and NiO) prepared via chemical bath deposition. In a typical procedure, clean carbon fibre cloth or nickel foam (masked with polyimide tape to prevent deposition on the back sides) were placed vertically in a 250 mL pyrex beaker. Solution for chemical bath deposition (CBD) was prepared by adding aqueous ammonia (20 mL, 25−28 %) to the mixture of nickel sulfate (100 ml, 1 mol L−1) and potassium persulfate (80 mL, 0.25 mol L−1). After immersing in the CBD solution for 30 min at 25 °C, the substrates were taken out and rinsed with distilled water. The Ni(OH)2 nanowall array were obtained after annealing at 200 °C in argon for 1.5 h. The NiO nanowall arrays were formed when the annealing temperature increased to 350 °C. Afterwards, the Ni(OH)2 and NiO nanowall array were converted into NiS nanowall array under the same IER as above. The load weight of NiS is about 1 mg cm−2. Preparation of Co9S8 Nanotube Arrays: The Co9S8 nanotube arrays were converted from Co2(OH)2CO3 nanowire arrays, which were prepared by a facile hydrothermal synthesis method. The hydrothermal solution was prepared by dissolving Co(NO3)2 (2 mmol), NH4F (5 mmol) and CO(NH2)2 (10 mmol) in distilled water (50 mL). The liner was sealed in a stainless steel autoclave and maintained at 110 °C for 5 h. Finally, the Co2(OH)2CO3 nanowire arrays were converted into the Co9S8 nanotube arrays under the same ion exchange procedure as above. Preparation of CoS Powder: The CoS powder was converted from Co3O4 powder, which was prepared by a facile chemical
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precipitation method. Added NaOH (50 mL, 0.1 mol L−1) into Co(NO3)2 (25 mL, 0.05 mol L−1) to form green precipitate. Then, the precipitate was annealed at 350 °C in air to form Co3O4 powder. Finally, the Co3O4 powder was converted into the CoS powder under the same ion exchange procedure as above. Characterization of Metal Sulfide Nanoarrays: The samples were characterized by X-ray diffraction (XRD, RIGAKU D/Max-2550 with Cu Kα radiation), field emission scanning electron microscopy (FESEM, FEI SIRION), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F) and X-ray photoelectron spectroscopy (XPS, PHI 5700). The surface area of the film that scratched from the substrate was determined by BET measurements using a NOVA-1000e surface area analyzer. Electrochemical Measurements: The electrochemical measurements were carried out in a three-electrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were performed on a CHI660c electrochemical workstation (Chenhua, Shanghai). CV measurements were carried out at different scanning rates between 0−0.65 V at 25 °C, using the metal sulfide nanoarrays grown on nickel foams as the working electrode, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. The galvanostatic charge–discharge tests were conducted using a LAND battery program-control test system. The core-shell nanowire arrays electrode, together with a nickel mesh counter electrode and an Hg/HgO reference electrode were tested in a three-compartment system. Specific capacities were calculated from the galvanostatic discharge curves using the following equation:
Cspecific =
I t Q I t , = 3600 = M M 3600M
where C (mAh g−1) is specific capacity, Q is the quantity of charge, I (mA) represents the discharge current, and M (g), Δt (s) designate the mass of active materials and total discharge time, respectively. Fabrication of Pseudo-Capacitors and Electrochemical Measurements. The pseudo-capacitors were assembled based on the CoS nanowire arrays as cathode (active area ≈12.5 cm2, the total mass of active materials is ≈25 mg, equal to 2 mg cm−2) and an active carbon (AC)-based as anode (5.5 cm × 5.5 cm, total mass of ≈800 mg, the capacity of anode is much larger than the cathode to ensure the cathode performing best). The AC-based anode was fabricated by mixing active carbon (YP-1, Kuraray, Japan) with a certain proportion of carbon black (10 wt%) and binders (poly(vinyl difluoride) (PVDF) 15 wt%) to form a slurry. Then, the slurry was filled into a foam nickel substrate (1.5-mm-thick) and dried at 90 °C for 5 h. Then, the AC-based anode was rolled to a thickness of 0.5 mm. Afterwards, the cathode and anode electrodes were separated by a porous non-woven cloth separator and assembled into an full battery, in which the capacities were determined by the cathode. The capacities were determined based on the mass of CoS nanowire arrays, instead of the whole weight of electrode. The electrode containing CoS powder was fabricated as the same procedure of active carbon (AC)-based anode above. The average mass CoS powder is ≈2 mg cm−2. A series of electrochemical tests including cyclic voltammetry (CV) and galvanostatic charge–discharge measurement were performed on CHI660c electrochemical
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workstation (Chenhua, Shanghai) and Xinwei battery program-control test system.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Xinhui Xia and Changrong Zhu contributed equally to this work. This research is supported by SERC Public Sector Research Funding (Grant number 1121202012), Agency for Science, Technology, and Research (A*STAR), Singapore MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034), AcRF Tier 1 (RG 61/12), and NTU Start-Up Grant (M4080865). The research is also funded by the Singapore National Research Foundation and the publication is supported under the Campus for Research Excellence And Technological Enterprise (CREATE) programme (Nanomaterials for Energy and Water Management). Support by Singapore-France MERLION Programme (Project No. 2.04.11) is also appreciated.
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Received: July 21, 2013 Revised: September 23, 2013 Published online: November 27, 2013
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Synthesis of Free-Standing Metal Sulfide Nanoarrays via Anion Exchange Reaction and Their Electrochemical Energy Storage Application Xinhui Xia, Changrong Zhu, Jingshan Luo, Zhiyuan Zeng, Cao Guan, Chin Fan Ng, Hua Zhang, and Hong Jin Fan*
Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and corebranch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The selfsupported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.
1. Introduction In recent years, tremendous research effort has been devoted to electrochemical devices with high power density and fast recharge capability. Among all these power sources, nanostructured electrode batteries and supercapacitors are the paradigm devices as they are promising to deliver power source with both high energy and power densities. Metal oxides and conducting polymers have been widely explored Dr. X. H. Xia, C. R. Zhu, J. S. Luo, C. Guan, C. F. Ng, Prof. H. J. Fan Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore, 637371, Singapore E-mail: [email protected] Dr. X. H. Xia State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering Zhejiang University Hangzhou, 310027, P. R. China Dr. Z. Y. Zeng, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University Singapore, 639897, Singapore DOI: 10.1002/smll.201302224
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for constructing batteries and supercapacitors with increased specific capacity/capacitance and high energy densities arising from faradic redox reactions.[1–13] Metal sulfides have recently emerged as a new promising class of active materials due to their excellent redox reversibility and relatively high capacity/capacitance. Metal sulfides have been demonstrated with outstanding properties for hydrodesulfurization catalysis,[14] biological labeling,[15] medical diagnostics,[16] solar cells,[17,18] lithium ion batteries,[19–21] and supercapacitors,[22–27] oxygen reduction reaction.[28] It has been demonstrated that metal sulfides undergoes reversible redox reaction according to MSOH (MS + OH− ↔ MSOH + e−, M = Ni, Co…) in the alkaline electrolyte.[25] Take NiS and CoS for examples, their theoretical capacities are ≈294 mAh/g. Although several nanostructured metal sulfides (such as NiS, CuS, CoS2 and CoS) have been synthesized and applied for batteries and supercapacitors,[19–27,29–31] these materials are in powder form and thus need to be mixed with polymer binders, introducing supplementary and undesirable interfaces and risks negating the nanosize benefits. It is known that the electrochemical properties are determined by the kinetic features controlled by the transportation of electrons and ions into the active materials.[3] One of effective approaches to enhance the transport kinetics is to construct nanoarray electrodes with a direct mechanical and electrical contact with the current collector. This configuration allows an easy
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diffusion of ions, lower internal resistance and subsequently, realization of high-power performance. So far, there are only a few reports dedicated to metal sulfides nanoarrays directly on conductive substrates for high-rate battery/supercapacitor application. Lin and Chou reported an electro-deposited nanosheet-like CoS film via thiourea and its pseudo-capacitive performance.[32] However, their cycling test was conducted only for 1000 cycles and no full cell was demonstrated. A large variety of methods have been reported for the synthesis of metal sulfides, such as solid state reaction,[20] wet chemical precipitation,[22,33] hydrothermal method,[34] solvothermal synthesis methods,[30] and electrochemical deposition method.[35] These synthesis methods are suitable for powder materials, but have not been proven applicable to oriented metal sulfide nanoarrays on conductive substrates. In addition, these conventional methods usually require toxic H2S source and organometallic precursors. It is therefore of great interest to develop a green synthesis method for high-quality metal sulfide nanoarrays for energy storage applications. The solution-based ion exchange reaction (IER) has proven an effective and low-cost method for chemical transformation of nanomaterials.[36] The IER method circumvents the drawbacks of high temperatures, high pressure and toxic precursor sources, which significantly reduces the fabrication cost while maintaining an excellent reproducibility. The Alivisatos group pioneered the application of cation ion exchange method for the synthesis of colloidal nanoparticles such as Ag2Se, CdSe, ZnS, and CdS−Ag2S.[37–40] More recently, the similar method has also been employed to fabricate nanowire p−n junctions (CdS−Cu2S) for photovoltaics,[41] counter electrode for solar cells,[17] and photosensitizer layers for photoelectrochemical electrodes.[42,43] To date, there is little literature on anion exchange reaction for the synthesis of free-standing metal sulfide nanoarrays with adjustable compositions and nanostructures on conductive substrates, and their applications for electrochemical energy storage. In the present work, we apply the facile anion exchange method to fabricate metal sulfide nanostructured arrays (including nanowires, core-branch nanowires, and nanowalls) directly from their metal oxide or hydroxide pre-forms without using any organosulfur compound precursors or H2S source. As a result of shape-preserved reaction, the obtained sulfides retain not only the topotactical relationship in morphology but also rigid contact with the conductive carbon fibre cloth and nickel foam substrates. As a preliminary demonstration, the CoS nanowire arrays exhibit outstanding redox reactivity and highrate capability when tested as the pseudo-capacitor cathode. Our results demonstrate the effectiveness of IER to the construction of various sulfide-based 3D nanostructures for the energy applications in supercapacitor, lithium ion batteries, and solar cells.
2. Results and Discussion 2.1. Oxide-to-Sulfide Transformation via Anion Exchange Reaction The schematic in Figure 1 illustrates the generality of the IER approach which can convert various metal oxide or small 2014, 10, No. 4, 766–773
Figure 1. Schematic of the conversion of metal oxide nanoarrays to metal sulfides nanoarrays via anion exchange reactions.
hydroxide nanostructures (from 1D, 2D to 3D) to the corresponding metal sulfides while maintaining the array architecture, alignment, and physical contact with the substrates. The Co3O4 nanoarrays of both nanowire and nanowall grown on carbon fibre cloth are chosen for elucidation of the IER process. The whole carbon fibre cloth is uniformly covered by hydrothermal-synthesized Co3O4 nanowires with diameters of ≈80 nm (Figure 2a,b) and height of ≈5 μm. The Co3O4 nanowires are composed of numerous nanoparticles of ≈5 nm and show mesoporous structure ranging from 3−5 nm (Figure 2c). After IER, the obtained CoS nanowire surfaces are rough (Figure 2d,e). The thickness of the nanowire film is also around 5 μm (Figure 2e), the same as the pristine Co3O4 nanowires film. TEM result indicates that, unlike the mesoporous Co3O4, the converted CoS nanowires exhibit a denser but still polycrystalline structure (Figure 2f). The measured lattice fringe with an interplane spacing of 0.19 nm in the HRTEM image corresponds to the (102) plane of CoS phase (Figure 2f). The phase change is also verified by XRD, Energy dispersive X−ray spectroscopy (EDS) and X−ray photoelectron spectroscopy (XPS) results. Prior to ion exchange, XRD pattern confirms the spinel Co3O4 phase (JCPDS 42-1467) and its crystalline nature (Figure 3a). After the conversion, all the diffraction peaks can be indexed to the hexagonal CoS phase (JCPDS 75-0605) (Figure 3b), indicating the complete reaction via ion exchange. For the XPS results, the Co3O4 nanowire arrays show the Co 2p3/2 peak around 779.9 eV, which can be deconvoluted to two peaks corresponding to two charge states, Co2+ and Co3+ (Figure 3c). The Co3O4 nanowire arrays do not exhibit any peak in the region of S 2p (Figure 3d). After IER, the Co 2p3/2 peak shifts to a lower binding energy of 777.9 eV, characteristic of the Co−S bonding. Simultaneously, a new peak at 162.9 eV is noticed in the region of S 2p.[35,44,45] In the O1s
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After the same IER, the nanowall network architecture is well maintained, but the surface of the obtained NiS nanowalls becomes rougher than the pre-forms (Figure 4g,h). XRD results confirm the conversion from NiO (JCPDS 4-0835) to NiS (JCPDS 12-0041) (Figure 5). Similar morphologies of metal sulfide nanoarrays can also be derived when nickel foam substrates are used. Furthermore, this IER method can be extended to fabricate CoS/NiS core-shell nanowire array when the pre-form of Co3O4/NiO core-shell nanowire array is utilized (Figure 6). The crystal phases prior to and after IER are verified by XRD (Figure S4). More interestingly, we also obtained Co9S8 nanotube arrays from basic cobalt carbonate precursor through the same IER procedure (see electron micrographs in Figure 7 and XRD patterns in Figure S5). Note that, while the nanowire outline is preserved, the nanotube walls are polycrystalline. All in all, the above demonstrations serve as strong testament that the IER method is powerful and versatile for constructing high-quality metal sulfide nanoarrays on arbitrary substrates.
2.2. Reaction Mechanism Figure 2. Transformation from Co3O4 nanowires (left column) to CoS nanowires (right column). a,b) SEM images of Co3O4 nanowire arrays on carbon fibre cloth (large-scale image in inset). c) TEM image of individual Co3O4 nanowire (magnified image in inset). d,e) The corresponding CoS nanowire arrays after ion exchange reaction with Na2S solution (largescale image and side view in insets). f) TEM image of individual CoS nanowire (HRTEM image and magnified image in insets).
spectrum (Figure 3e), the peaks at 530.3 and 531.5 eV indicate the existence of Co-O bonds, as is consistent with the Co 2p spectrum.[3,46] After the anion exchange, no Co-O bonds are observed in this region. Moreover, the EDS spectra and element mapping analysis also verified the component of the nanowires is Co and S (Supporting Information Figure S1). CoS nanowall arrays can also be prepared through the same anion exchange conversion from Co3O4 or Co(OH)2 nanowall arrays. The electrodeposited Co3O4 film has a network of interconnected nanowalls with thicknesses of ≈10 nm (Figure 4a). The Co3O4 nanowall arrays align vertically to the substrate and the walls contain mesopores of sizes ranging from 2−3 nm (Figure 4b). The anion-exchanged CoS keeps the vertical network configuration and shows a film thickness of ≈1 μm (Figure 4e,f). Compared to the Co3O4 mesoporous walls, the CoS walls exhibit a rough texture and dense structure (Figure 4f). Similar results have been obtained when the Co(OH)2 nanowall array were used as the starting material (see Figure S2). To verify the universality of the IER method in synthesizing metal sulfides, we extended the method to NiS nanowall array by using chemical-bath-deposited NiO and Ni(OH)2 nanowall arrays as the starting materials (Figure 4 and Figure S3). The NiO and Ni(OH)2 nanowall arrays also have a network structure composed of nanowalls with thickness around 10 nm (Figure 4c,d). The NiO and Ni(OH)2 nanowalls both show a smooth texture from TEM observations.
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The growth mechanism of the metal sulfides (CoS and NiS) based on IER is proposed as follows. The key parameter for ion exchange reactions is the solubility product constant (Ksp) of the material.[36] In general, materials with lower Ksp values are thermodynamically more stable that those with higher Ksp values. As a result, the latter are prone to conversion into the former in the specific solution (with the same cations or anions). In our experiments, the Ksp values for Co3O4, Co(OH)2, NiO, Ni(OH)2, CoS and NiS are about 3.1 × 10−18, 5.9 × 10−15, 1.5 × 10−17, 5.5 × 10−16, 3 × 10−26 and 1.3 × 10−25, respectively.[47,48] Since the Ksp values of metal sulfides (CoS and NiS) are drastically lower than the corresponding metal oxides/ hydroxide pre-forms, the former can be obtained by simple exchange reactions of the latter in a solution containing S2− anions. The reactions involved in our experiments can be expressed as follows.[17,31] Na2 S ↔ 2Na+ + S2− Co3 O4 + 3S
2−
(1)
+ 3H2 O → CoS + 6OH
Co(OH)2 + S2− → CoS + 2OH− NiO + S
2−
+ H2 O → NiS + 2OH
Ni(OH)2 + S2− → NiS + 2OH−
−
(2) (3)
−
(4) (5)
It should be mentioned that the ion exchange reaction in the solids are closely related to the size of the materials. It is accepted that the reaction in bulk materials is generally very slow and incomplete because of the high activation energies for the volume diffusion of atoms and ions in the solid. In contrast, the scenario is very different in the nanometer scale. Previously, Alivisatos and co-workers[40] reported that both thermodynamics and kinetics of ion exchange reactions could change with a decrease in the nanosize, and
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b Intensity (a. u.)
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530
532
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Binding Energy (eV) Figure 3. XRD patterns of (a) Co3O4 and (b) CoS nanowire arrays. XPS spectra of Co3O4 and CoS nanowires: (c) Co 2p3/2, (d) S 2p and (e) O1s.
the reaction rates are much faster than in bulk materials. Consequently, complete and fully reversible reactions in nanoscale materials may occur, leading to more homogeneous molecule-like reaction kinetics. This opens a new way for composition manipulation of colloidal nanoparticles in any dimension. Our nanoarray pre-forms (nanowires or nanowalls) are composed of tiny nanocrystallites of sizes in a few nanometers. These nanoarrays possess a large surfaceto-volume ratio and surface areas (see BET measurement result in Figure S6), which is favorable for complete exchange reactions with S2− ions. As for the particular case of Co9S8 nanotube, it is probably due to the pseudo Kirkendall effect involved in the reaction of the solid Co2(OH)2CO3 nanowires with the Na2S solution.[37,49] The reaction takes place in a way that the outward diffusion of cobalt ions is faster than the inward diffusion of S2− ions, thus creating voids at the center of the nanowire and finally forming Co9S8 nanotubes. Given that the pure Co3O4 nanowires do not convert to hollow Co9S8 nanotubes but solid CoS nanowires (see above), it is hypothesized that the faster diffusion of cobalt cations in this reaction should be related to the release of CO32− and OH− ions. small 2014, 10, No. 4, 766–773
To demonstrate the potential application in electrochemical energy storage, we investigated the electrochemical properties of the as-prepared CoS nanowire arrays grown on nickel foam as cathode materials for pseudo-capacitors. Here, it is important to note that the electrochemical behavior of pseudo-capacitor is similar to that of battery (Conway proposed the concept of pseudo-capacitor in 1975 on the basis of the similarity to conventional secondary batteries).[4,50] In our case, in order to obtain real and accurate values of the energy storage and release, the capacities were calculated based on the battery equation (unit, mAh g−1), not the equation of traditional supercapacitor (capacitance, F g−1). A further detailed clarification on this point is provided in the supporting information. Figure 8a and b shows the SEM and optical images and three-electrode configuration of the quasi-vertical aligned CoS nanowires. The electrochemical behavior of the CoS nanowire arrays was investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge tests in 2 M KOH. Figure 8c shows the CV curves of the CoS nanowire arrays at different scanning rates. The redox couple corresponds to the conversion between CoS and CoSOH, simply illustrated as follows.[25,29]
CoS + OH− ↔ CoSOH + e−
(6)
The CV curves at different cycles also show that the electrochemical reaction is quasi-reversible (Figure S7), meaning that the electrochemical energy can be reversibly stored and released in the CoS nanowire arrays. The specific capacity values of the CoS nanowire arrays are calculated from the galvanostatic discharge curves at various current densities (Figure 8d,e). The values range from 129 mAh g−1 at 2 A g−1 to 102 mAh g−1 at 40 A g−1, meaning that 79 % of the capacity is retained when the rate changes from 2 to 40 A g−1. It is noteworthy that the Ni foam has a small contribution to the capacity with prolonged cycling arising from the oxidation of Ni. So in our work, the contribution of the nickel foam has been subtracted. Furthermore, the asprepared CoS nanowire arrays show a good cycle life with 91% capacity retention (117 mAh g−1) after 3000 cycles at 2 A g−1 (Figure 8f). The CoS nanowire array electrode also shows a high and stable coulombic efficiency of ∼98% during charge–discharge cycles. The overall nanowire architecture is well preserved after 3000 cycles, which implies that the array structure could effectively alleviate the structural
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Figure 6. Transformation from a,b) Co3O4/NiO core/shell array to c,d) CoS/NiS metal sulfide array.
Figure 4. Transformation from metal oxide nanowall arrays (left column) to metal sulfides (right column). a,b) SEM and TEM images of Co3O4 nanowall array on carbon fiber cloth (large-scale image in inset). e,f) The corresponding CoS nanowall array after ion exchange reaction with Na2S solution (large-scale image and side view in insets). c,d) SEM and TEM images of NiO nanowall array on carbon fiber cloth (large-scale image in inset). g,h) The corresponding NiS nanowall array (large-scale image and side view in insets).
Figure 7. Transformation from Co2(OH)2CO3 nanowire arrays a,b) to Co9S8 nanotube arrays c,d). A pseudo Kirkendall effect might involve during the anion exchange reaction, accounting for the formation of the hollow core.
damage caused by volume expansion during the cycling process (Figure S8). In short, these results reveal the high specific capacity, good rate capability and cycle life of the CoS nanowire arrays that are potentially useful for highperformance batteries.
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The electrochemical properties are compared between CoS nanowire arrays and conventional CoS powder (see Figure S9 for detailed electrochemical characterization). First, the self-supported CoS nanowire arrays have a narrower oxidation-reduction potential separation than the powder counterpart. As shown in Figure S9b, the nanowire arrays exhibit an oxidation peak at 0.48 V and a reduction peak at 0.31 V, whereas the CoS powder exhibits lower reduction peak potential (0.24 V) and higher oxidation peak potential (0.53 V). Second, the peak currents of the nanowire arrays are much higher than the CoS powder. From the galvanostatic charge–discharge curves (Figure S9c), one can also see the nanowire arrays present 40 50 60 70 lower charge voltage plateau and higher 2θ (degree) discharge voltage plateau compared to the powder electrode. This indicates that
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Figure 8. Electrochemical properties of CoS nanowire arrays on nickel foam. a) SEM image with large-scale view and optical image in inset. b) Schematics of the three-electrode electrochemical cell. c) CV curves at different scanning rates. d) Charge–discharge curves at different current densities (unit: A g−1). e) Specific capacities at different current densities. f) Cycling performance at 2 A g−1.
the CoS nanowire arrays has weaker polarization and better electrochemical activity. Finally, the CoS nanowire arrays show higher specific capacity and better high-rate capability (Figure S9d). It can be concluded that the free-standing CoS nanowire arrays have a superior electrochemical activity electrode compared to the powder counterpart. To prove the potential application of these metal sulfide nanoarrays for electrochemical energy storage, full pseudocapacitor prototype devices with the CoS nanowire arrays as the cathode and activated carbon as the anode are assembled and tested (see Figure S10 for detailed electrochemical characterization and chemical reaction details at both electrodes). In our case, the capacity of anode is much larger than the cathode to ensure the maximum performance of the cathode (see methods). The reactions involved in our full devices based on CoS nanowire arrays are given as follows.[25] Cathode : CoS + OH − e− ↔ CoSOH
3. Conclusion We have demonstrated that the ion exchange reaction is an effective and powerful method that allows a shape-preserved transformation from metal oxide or hydroxide nanostructure arrays to the corresponding metal sulfides. As examples, CoS and NiS arrays of nanowires, core-shell nanowires, nanowalls, and nanotubes have been achieved directly on conductive substrates. The occurrence of anion exchange reactions depends on their Ksp values and nanoscale morphology. The as-obtained CoS nanowire arrays show outstanding electrochemical properties with a high capacity, good cycle life and a high-rate capability. This method could be extended to the fabrication of other metal sulfide nanostructure arrays or thin films for solar cells, photo- or electrocatalysis, and electrochemical energy storage applications.
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Anode : C + K + e ↔ K + //C (8) (K+//C represents the K+ is absorbed on the surface of activated carbon) Overall : CoS + OH− + C + K+ ↔ CoSOH + K+ //C small 2014, 10, No. 4, 766–773
The cathode exhibits a specific capacity of 131 mAh g−1 at the working current of 50 mA (Note: 50 mA corresponds to 2 A g−1 or 6.8 C based on the total CoS mass of 25 mg) and 121 mAh g−1 at a very high working current of 250 mA (10 A g−1 or 34 C). While the low-rate capacity is comparable to that of LiFePO4/C composites (≈135 mA h−1 at 5 C),[51–53] the high-rate capacity is much higher than that of LiFePO4/C composites (≈80 mAh g−1 at 20 C),[51–53] as well as that of LiFePO4/ conducting polymers (Polypyrrole and Polyaniline) composites (≈64 mAh g−1 at 10 C).[54] It is also noteworthy that the working voltage of the metal sulfides arrays in this study is much lower than LiFePO4 as our assembled pseudo-capacitors are in an aqueous system, unlike the LiFeFO4/C in a non-aqueous system. Cycle life is an important parameter for pseudo-capacitor application. Impressively, the assembled pseudo-capacitor exhibits fairly good cycling stability. After 16 000 cycles at 2 A g−1 (charge–discharge current of 50 mA), the pseudo-capacitor still delivers a specific capacity of 103 mAh g−1 with a retention of 79% (Figure S10d). The tandem devices (three pseudo-capacitors in series) can easily power the green LEDs with a working voltage of 3.2 V. These results demonstrate the functionality of metal sulfide nanoarrays potentially for pseudo-capacitors with both high specific capacity and rate capability.
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4. Experimental Section Preparation of CoS Nanowire Arrays: The CoS nanowire arrays were prepared by the combination of hydrothermal synthesis and ion exchange reactions. Firstly, self-supported Co3O4 nanowire
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arrays were prepared by a facile hydrothermal synthesis method. The solution was prepared by dissolving Co(NO3)2 (2 mmol), NH4F (4 mmol) and CO(NH2)2 (10 mmol) in distilled water (50 mL). Then the resulting solution was transferred into Teflon-lined stainless steel autoclave liners. Various substrates (2 cm × 6 cm in sizes) such as carbon fibre cloth and nickel foam were immersed into the reaction solution. Top sides of the substrates were uniformly coated with a polytetrafluoroethylene tape to prevent the solution contamination. The liner was sealed in a stainless steel autoclave and maintained at 110 °C for 5 h, and then cooled down to room temperature. The samples were annealed at 350 °C in normal purity argon for 2 h to form Co3O4 nanowire arrays. Then, the CoS nanowire arrays were obtained by placing Co3O4 nanowire arrays in a sealed cup with a solution containing sodium sulfide (0.1 M) at 90 °C for 9 h. Finally, the CoS nanowire arrays were collected and rinsed with distilled water several times. The load weight of CoS is about 2 mg cm−2. Preparation of CoS Nanowall Arrays: The CoS nanowall arrays were fabricated through direct IER from two Co species nanowall arrays [Co(OH)2 and Co3O4] prepared via cathodic electrodeposition. The electrodeposition was performed in a standard threeelectrode glass cell at 25 °C, the carbon fibre cloth or nickel foam as the working electrode, saturated calomel electrode (SCE) as the reference electrode and a Pt foil as the counterelectrode. The Co(OH)2 nanowall array were electrodeposited from aqueous solution containing Co(NO3)2 (1 mol L−1) and NaNO3 (0.1 mol L−1) at a constant cathodic current of 2 mA cm−2 for 300 s using a Chenhua CHI660C model Electrochemical Workstation (Shanghai). The Co(OH)2 nanowall array could be converted into Co3O4 nanowall array after annealing at 250 °C in argon for 1 h. Afterwards, the Co(OH)2 and Co3O4 nanowall array were converted into CoS nanowall array under the same ion exchange procedure as above. Preparation of NiS Nanowall Arrays: The NiS nanowall arrays were fabricated through direct ion exchange from two Ni species nanowall array (Ni(OH)2 and NiO) prepared via chemical bath deposition. In a typical procedure, clean carbon fibre cloth or nickel foam (masked with polyimide tape to prevent deposition on the back sides) were placed vertically in a 250 mL pyrex beaker. Solution for chemical bath deposition (CBD) was prepared by adding aqueous ammonia (20 mL, 25−28 %) to the mixture of nickel sulfate (100 ml, 1 mol L−1) and potassium persulfate (80 mL, 0.25 mol L−1). After immersing in the CBD solution for 30 min at 25 °C, the substrates were taken out and rinsed with distilled water. The Ni(OH)2 nanowall array were obtained after annealing at 200 °C in argon for 1.5 h. The NiO nanowall arrays were formed when the annealing temperature increased to 350 °C. Afterwards, the Ni(OH)2 and NiO nanowall array were converted into NiS nanowall array under the same IER as above. The load weight of NiS is about 1 mg cm−2. Preparation of Co9S8 Nanotube Arrays: The Co9S8 nanotube arrays were converted from Co2(OH)2CO3 nanowire arrays, which were prepared by a facile hydrothermal synthesis method. The hydrothermal solution was prepared by dissolving Co(NO3)2 (2 mmol), NH4F (5 mmol) and CO(NH2)2 (10 mmol) in distilled water (50 mL). The liner was sealed in a stainless steel autoclave and maintained at 110 °C for 5 h. Finally, the Co2(OH)2CO3 nanowire arrays were converted into the Co9S8 nanotube arrays under the same ion exchange procedure as above. Preparation of CoS Powder: The CoS powder was converted from Co3O4 powder, which was prepared by a facile chemical
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precipitation method. Added NaOH (50 mL, 0.1 mol L−1) into Co(NO3)2 (25 mL, 0.05 mol L−1) to form green precipitate. Then, the precipitate was annealed at 350 °C in air to form Co3O4 powder. Finally, the Co3O4 powder was converted into the CoS powder under the same ion exchange procedure as above. Characterization of Metal Sulfide Nanoarrays: The samples were characterized by X-ray diffraction (XRD, RIGAKU D/Max-2550 with Cu Kα radiation), field emission scanning electron microscopy (FESEM, FEI SIRION), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F) and X-ray photoelectron spectroscopy (XPS, PHI 5700). The surface area of the film that scratched from the substrate was determined by BET measurements using a NOVA-1000e surface area analyzer. Electrochemical Measurements: The electrochemical measurements were carried out in a three-electrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were performed on a CHI660c electrochemical workstation (Chenhua, Shanghai). CV measurements were carried out at different scanning rates between 0−0.65 V at 25 °C, using the metal sulfide nanoarrays grown on nickel foams as the working electrode, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. The galvanostatic charge–discharge tests were conducted using a LAND battery program-control test system. The core-shell nanowire arrays electrode, together with a nickel mesh counter electrode and an Hg/HgO reference electrode were tested in a three-compartment system. Specific capacities were calculated from the galvanostatic discharge curves using the following equation:
Cspecific =
I t Q I t , = 3600 = M M 3600M
where C (mAh g−1) is specific capacity, Q is the quantity of charge, I (mA) represents the discharge current, and M (g), Δt (s) designate the mass of active materials and total discharge time, respectively. Fabrication of Pseudo-Capacitors and Electrochemical Measurements. The pseudo-capacitors were assembled based on the CoS nanowire arrays as cathode (active area ≈12.5 cm2, the total mass of active materials is ≈25 mg, equal to 2 mg cm−2) and an active carbon (AC)-based as anode (5.5 cm × 5.5 cm, total mass of ≈800 mg, the capacity of anode is much larger than the cathode to ensure the cathode performing best). The AC-based anode was fabricated by mixing active carbon (YP-1, Kuraray, Japan) with a certain proportion of carbon black (10 wt%) and binders (poly(vinyl difluoride) (PVDF) 15 wt%) to form a slurry. Then, the slurry was filled into a foam nickel substrate (1.5-mm-thick) and dried at 90 °C for 5 h. Then, the AC-based anode was rolled to a thickness of 0.5 mm. Afterwards, the cathode and anode electrodes were separated by a porous non-woven cloth separator and assembled into an full battery, in which the capacities were determined by the cathode. The capacities were determined based on the mass of CoS nanowire arrays, instead of the whole weight of electrode. The electrode containing CoS powder was fabricated as the same procedure of active carbon (AC)-based anode above. The average mass CoS powder is ≈2 mg cm−2. A series of electrochemical tests including cyclic voltammetry (CV) and galvanostatic charge–discharge measurement were performed on CHI660c electrochemical
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workstation (Chenhua, Shanghai) and Xinwei battery program-control test system.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Xinhui Xia and Changrong Zhu contributed equally to this work. This research is supported by SERC Public Sector Research Funding (Grant number 1121202012), Agency for Science, Technology, and Research (A*STAR), Singapore MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034), AcRF Tier 1 (RG 61/12), and NTU Start-Up Grant (M4080865). The research is also funded by the Singapore National Research Foundation and the publication is supported under the Campus for Research Excellence And Technological Enterprise (CREATE) programme (Nanomaterials for Energy and Water Management). Support by Singapore-France MERLION Programme (Project No. 2.04.11) is also appreciated.
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Received: July 21, 2013 Revised: September 23, 2013 Published online: November 27, 2013
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