MnOx

Article pubs.acs.org/JPCC

Kirkendall Effect Induced One-Step Fabrication of Tubular Ag/MnOx Nanocomposites for Supercapacitor Application Yonghe Li,† Haoyu Fu,† Yuefei Zhang,*,† Zhenyu Wang,† and Xiaodong Li†,‡ †

Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, P. R. China Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, United States

‡

S Supporting Information *

ABSTRACT: One-dimensional (1D) tubular Ag/MnOx nanocomposites were synthesized by the solvothermal method via the Kirkendall effect between potassium permanganate (KMnO4) and Ag nanowire templates. The morphology and electrochemical performance of Ag/MnOx composites were tuned by varying the pH levels. Tubular MnOx nanosheets with ultrafine Ag nanoparticles were formed in an acidic environment (pH 0.76), whereas the Ag nanoparticles entrapped in amorphous MnO2 nanotubes were prepared in a neutral environment (pH 7.00). Based on a series of volume-dependent experiments, it was confirmed that the Kirkendall effect was involved in the formation of these morphologies. When tested as an electrode for supercapacitors, the hierarchical tubular Ag/MnOx nanosheet composites prepared in an acidic environment exhibited an optimized electrochemical performance, with specific capacitance of 180 F g−1 at current density of 0.1 A g−1, and still maintained 80% of initial capacity after 1000 cycles at current density of 1 A g−1. The proposed synthetic mechanism and the developed synthetic strategy may provide design guidelines for synthesizing other hierarchical transition metal/transition metal oxide nanocomposites.

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INTRODUCTION To meet the urgent need for environment-friendly, highefficiency energy storage and conversion devices for high-power electronic devices, backup power supplies, and electric vehicles, efforts have been made to develop batteries, fuel cells, and electrochemical capacitors (ECs) with the aim of replacing fossil fuel use.1 ECs, also called supercapacitors or ultracapacitors, have received much attention because they can simultaneously achieve high power and energy densities.2−5 Supercapacitors have shown great promise in applications such as high-power electronic devices, backup power supplies, and electric vehicles.3,6,7 Based on their energy storage mechanisms, ECs are divided into two types, namely, electrical double layer capacitors (EDLCs) and pseudocapacitors.3 EDLCs store energy by electrostatic charge separation,8−10 whereas pseudocapacitors realize their energy storage function via fast, reversible redox reactions at the surface of the electrode materials.11−13 Ruthenium dioxide (RuO2) has been the first material explored for pseudocapacitor application and attracted much attention because of its high specific capacitance, high conductivity, and cycle stability.14,15 However, the high cost and toxicity of RuO2 have limited its application as a supercapacitor electrode material. Thus, the application of other transition oxides such as MnO2,16−19 V2O5,20 Co3O4,21,22 and NiO23,24 as a supercapacitor electrode material have been explored. MnO2 is an © 2014 American Chemical Society

ideal replacement for RuO2 because of its good reversibility, high theoretical capacitance (1100 F g−1), and low cost. Because capacitance strongly depends on the conductivity of the material and surface area, considerable research has been conducted on the synthesis of nanostructured25 and porous MnO2 materials26 to improve their electrochemical performance. Moreover, combining MnO2 with other conductive components, such as carbonaceous materials,27−29 conductive polymers,30 and metals decorated,31,32 also significantly improves electrochemical performance by enhancing their electrical conductivity and charge-storage capability. Recently, Wang et al.31 fabricated ternary ordered WO3−x@Au@MnO2 nanowires on carbon cloth as supercapacitor electrode via an Au layer sputtering on WO3−x nanowires array and a following MnO2 electrodepositing. Tang et al.32 synthesized threedimensional MnO2/Pt/nickel foam hybrid electrodes prepared by double-pulse polarization and potentiostatic deposition technologies. However, one-step fabrication of such a hybrid nanostructure with high surface area combined with conductive materials remains challenging. Here we demonstrate a new design and a one-step fabrication route for 1D hierarchical tubular Ag/MnO x Received: December 12, 2013 Revised: March 5, 2014 Published: March 10, 2014 6604

dx.doi.org/10.1021/jp412187n | J. Phys. Chem. C 2014, 118, 6604−6611

The Journal of Physical Chemistry C

Article

measurements were performed in 1 M Na2SO4 aqueous electrolyte at room temperature in a three-electrode setup, which included the as-prepared working electrode, a platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical performance of the composites on an electrochemical workstation (PARSTAT2273, Ametech Inc.). CV tests were conducted within the potential window between 0.0 and 1.0 V (vs SCE) at different scan rates of 1, 5, 10, 30, 50, and 100 mV s−1. EIS measurements were performed in the frequency range from 100 kHz to 0.01 Hz at open circuit potential with ac perturbation of 10 mV. Galvanostatic charge/ discharge curves (Land, Wuhan, China) were measured at current densities of 0.1, 0.3, 0.5, 0.8, 1.0, 2.0, and 5.0 Ag−1.

nanocomposites for use as supercapacitor electrodes. We determine that the morphology and electrochemical performance of the nanoscale architecture can be tuned by varying the pH level in the solution and that the Kirkendall effect is involved in the formation of the morphology of these novel composites. The proposed synthetic mechanism and the developed synthetic strategy may provide design guidelines for synthesizing other transition metal/transition metal oxide hybrid nanocomposites.

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EXPERIMENTAL SECTION All chemicals were of analytical grade and used without further purification. Ag nanowire solution (20 mg mL−1, ethanolbased) was purchased from Blue Nano, Inc., and potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4) solution were from Beijing Chemical Works. All solutions were prepared in distilled water, and the pH of the aqueous solution was measured with a pH meter (PHS-3C, INESA Scientific Instrument Co., Ltd.). Samples of Ag/MnOx nanocomposites with different morphologies were synthesized and characterized by the following procedure. Synthesis of Ag/MnOx Nanocomposites. Tubular Ag/ MnOx nanocomposites were prepared by the solvothermal method. Specifically, 20 mg of ethanol-based Ag nanowires was rinsed with distilled water and then dispersed into 10 mL of distilled water using ultrasonic vibration. The Ag nanowire solution was added dropwise to 30 mL of 0.1 M KMnO4 aqueous solution with vigorous stirring, and then the pH of the solution was adjusted to 0.76 by the addition of concentrated H2SO4. The as-prepared acidic solution and neutral solution (without H2SO4 solution) were oil-bathed at 80 °C for 90 min. The brownish as-prepared powders were carefully rinsed with distilled water and ethanol several times before being dried at 80 °C under vacuum for 12 h. To reveal the nature of the evolution from Ag nanowire to tubular Ag/MnOx composite nanostructure, we performed a series of volume-dependent experiments. Specifically, 20 mg of distilled-water-rinsed Ag nanowires was dispersed in 30 mL of distilled water, and 0.03 M KMnO4 solution was added dropwise into the Ag nanowire solution until 0.5, 1.0, 2.5, 5.0, 10.0, 20.0, and 25.0 mL of KMnO4 solution had been added. Subsequently, the solution was exposed to ultrasonic vibration at room temperature for 30 min. The as-prepared powders were carefully rinsed with distilled water and ethanol several times before being dried at 80 °C under vacuum for 12 h. Characterization of As-Prepared Ag/MnOx Samples. The crystallographic structure of the synthesized materials was determined by a powder X-diffraction system (XRD, D8 Advance) equipped with Cu Kα radiation (λ = 0.154 06 nm). The microstructure of the samples was investigated by field emission scanning electron microscopy (FESEM, JEOL JSM 6500F, operated at 20 kV), transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (EDS) instruments (TEM, FEI TECHNAI G20, operated at 200 kV), and field emission transmission electron microscopy (FETEM, JEM-2010F, operated at 200 kV). Nitrogen adsorption− desorption experiments were carried out at 77.35 K by means of an Autosorb-1 (Quantachrome Instruments) analyzer. Electrochemical Measurements. The process of fabricating working electrodes specifically involved mixing the asprepared composites, carbon black, and poly(tetrafluoroethylene) in a mass ratio of 80:15:5, respectively, and then was pressed onto nickel foam. All electrochemical

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RESULTS AND DISCUSSION Figure 1 shows a representative XRD pattern of the pristine Ag nanowires and the as-prepared Ag/MnOx composites. The

Figure 1. Typical XRD patterns of pristine Ag nanowires and the asprepared Ag/MnOx products. The inset shows the magnified XRD pattern of the sample prepared at pH 0.76.

peaks of Ag (JCPDS 65-2871) are observable for the sample prepared at pH 7.00 with no peaks corresponding to MnOx, indicating that amorphous MnOx was produced in a neutral environment. Moreover, the intensity of the peaks at 38°, 44°, 65°, and 77° for the sample prepared at pH 7.00 is lower than that of the pristine Ag nanowire sample. The lower intensity of the Ag peaks can be attributed to the depletion of Ag due to the outward shifting of cation and the uniform coating of the amorphous MnOx at the surface of the nanowire. The Ag peaks almost disappeared when the sample was prepared in an acidic environment, and four broad peaks with low intensity at around 17°, 29°, 32.5°, 36.5°, and 66° (inset of Figure 1) are observable, which correspond to MnO2 and Mn3O4 (JCPDS 44-0141 and 24-0734). The XRD pattern of the sample prepared at pH 0.76 suggests the exhaustion of Ag and the low crystallinity of the as-prepared MnO2 and Mn3O4 nanostructure. Morphology characterizations and elemental profiles of the pristine Ag nanowires and as-prepared samples are shown in Figure 2. The pristine Ag nanowires (Figure 2a) used as the templates exhibit a diameter of ∼120 nm, and EDS profiles (Figure 2b) demonstrate the existence of single-element Ag, confirming the purity of the Ag nanowires. The TEM characterization (Figures 2c,d) and the SAED pattern (inset 6605

dx.doi.org/10.1021/jp412187n | J. Phys. Chem. C 2014, 118, 6604−6611

The Journal of Physical Chemistry C

Article

Figure 2. FESEM image, EDS spectra, and TEM images of (a−d) pristine Ag nanowires, (e−h) Ag/MnO2 nanotubes prepared at pH 7.00, and (i−l) tubular Ag/MnOx nanosheets produced at pH 0.76.

Ag/MnO2 nanotubes shows that the wall thickness of Ag/ MnOx nanotubes ranges from 25 to 35 nm, and numerous Ag nanoparticles ranging from 10 to 15 nm in diameter were homologous and entrapped at the nanotube surface (Figure 2h). The SAED pattern (inset of Figure 2h) of the pH 7.00 sample reveals the existence of an amorphous phase, and the numerous dispersing diffraction dots could be attributed to entrapped Ag nanoparticles. When the pH was acidic (pH 0.76), the 1D hierarchical Ag/MnOx nanostructure comprised

of Figure 2d) of the Ag nanowires indicate their single-crystal nature. The samples prepared at pH 7.00 (Figure 2e) exhibit 1D hollow morphology with diameter of ca. 160 nm and ca. 120 nm hollow area, duplicating the shape of pristine Ag nanowires. According to the EDS spectrum shown in Figure 2f, the lowered intensity of Ag peaks and the appearance of Mn and O peaks (atomic ratio Mn:O = 1:2) indicate the production of MnO2 and the depletion of Ag, which is well consistent with XRD results. The TEM image (Figure 2g) of 6606

dx.doi.org/10.1021/jp412187n | J. Phys. Chem. C 2014, 118, 6604−6611

The Journal of Physical Chemistry C

Article

Figure 3. TEM and HRTEM images at different magnifications of (a−c) Ag/MnO2 nanotube prepared at pH 7.00 and (d−f) tubular Ag/MnOx nanosheets prepared at pH 0.76.

Figure 4. TEM characterization of the evolution from Ag nanowires to Ag/MnO2 nanotubes prepared under different KMnO4 volume: (a) pristine Ag nanowire; (b) 0.5, (c) 1.0, (d) 2.5, (e) 5.0, (f) 10, (g) 20, and (h) 25 mL of KMnO4. (i) Schematic of the evolution process from pristine Ag nanowire to Ag/MnO2 nanotube. 6607

dx.doi.org/10.1021/jp412187n | J. Phys. Chem. C 2014, 118, 6604−6611

The Journal of Physical Chemistry C

Article

Figure 5. (a, b) CV curves of pH 0.76, pH 7.00 samples at various scan rates in 1 M Na2SO4. (c) Specific capacitance of pH 0.76 (blue) and pH 7.00 (red) samples at different scan rates derived from CV. (d, e) Galvanostatic constant-current charge/discharge performance of pH 0.76, pH 7.00 samples at various current densities. (f) Specific capacitance of pH 0.76 (blue) and pH 7.00 (red) samples at different current densities derived from the galvanostatic charge/discharge process.

nanosheets produced with diameters of ∼490 nm (Figure 2i). The EDS pattern (Figure 2j) shows that the atomic ratio of Ag decreased to