CRYSTAL GROWTH & DESIGN
Shape-Controlled Synthesis of 3D Hierarchical MnO2 Nanostructures for Electrochemical Supercapacitors
2009 VOL. 9, NO. 1 528–533
Peng Yu, Xiong Zhang, Dongliang Wang, Lei Wang, and Yanwei Ma* Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ReceiVed July 30, 2008; ReVised Manuscript ReceiVed September 21, 2008
ABSTRACT: A simple hydrothermal method has been used to fabricate sea urchin shaped R-MnO2 without using any template and surfactant. When other conditions were kept the same and Al3+ exists in the solution, the product is R-MnO2 nanorods clusters; when Al3+ was substituted by Fe3+ in the solution, the phase of the products was transformed from R-MnO2 to ε-MnO2; relatively, the morphology of the product was changed from urchinlike clusters to 3D clewlike. The sea urchin shaped R-MnO2 and R-MnO2 nanorods clusters consist of radially grown single crystalline MnO2 nanorods of ca. 30-40 nm in diameter and ca. 0.87-1.2 µm in length. The 3D clewlike ε-MnO2 nanostructures are composed of nanosheets with a thickness of about 17 nm. The electrochemical properties of the prepared MnO2 materials were studied using cyclic voltammetry (CV) and ac impedance measurements in aqueous electrolyte. 3D clewlike ε-MnO2 nanostructures have a specific capacitance of 120 F g-1 and lower charge-transfer resistance. The results indicate that metal ions (Fe3+, Al3+) have great effects on the structure and morphology of MnO2.
1. Introduction Nowadays materials with three-dimensional (3D) architectures have attracted much attention because of their attractive chemical and physical properties.1-4 3D aperiodic hierarchical porous graphitic carbon material and nanotubular arrayed architecture of hydrous RuO2 have been synthesized and used for electrode materials in electrochemical capacitors (ECs) with good performances.5,6 The core-shell nanostructures and hollow spheres also show distinctive properties when used as catalysis, microcapsule reactors, and energy storage materials.7-9 Recently, ZnO hollow spheres and carbon hollow spheres have been successfully prepared.10,11 Many different methods have been reported for the synthesis of such special materials, including a hydrothermal reaction,12 thermal decomposition,13 microemulsion method,14 sol-gel process,15 electrodeposition method,16 photochemical deposition,17 and template methods.18-20 Manganese dioxides are some of the most attractive inorganic materials because of their structural flexibility and wide applications in catalysis, ion exchange, molecular adsorption, biosensors, and energy storage.21-24 MnO2 exists in several crystallographic forms, such as R-, β-, γ-, δ-, λ- and ε-type, when the basic unit [MnO6] octahedron links in different ways. 3D manganese dioxides, such as R-MnO2 core-shell microspheres, γ-MnO2 urchin microspheres, γ-MnO2 hollow microspheres and microcubes, have been fabricated.25-27 In previous work, Li et al. pointed that the hollow urchin structure R-MnO2 had not been obtained by using other reducing reagents except Cu foil; this implies that Cu foil or Cu2+ ions may have a great effect on the morphology of MnO2.28 Gao and co-workers investigated the influences of different anions on the morphology of R-MnO2 nanocrystal.29 The effects of some cations (K+, NH4+, and H+) on the hydrothermal crystallization of MnO2 were studied by Huang et al.30 However, how to control both the morphology and the crystalline structure of MnO2 by the direct reaction route is still unsolved, and a complete understanding of formation mechanisms has not been achieved. In this paper, we report a simple hydrothermal method to fabricate sea urchin shaped R-MnO2, R-MnO2 nanorods clusters, * To whom correspondence should be addressed. E-mail: ywma@ mail.iee.ac.cn.
Table 1. MnO2 with Different Crystallographic Forms Obtained under Different Reaction Conditionsa sample
K2S2O8 (mmol)
MnSO4 (mmol)
H2SO4 (mL)
Fe(NO3)3 (mmol)
Al(NO3)3 (mmol)
phase
S1 S2 S3
2 2 2
2 2 2
2 2 2
0 2 0
0 0 2
R-MnO2 ε-MnO2 R-MnO2
a
All samples were produced at 110 °C for 6 h.
Scheme 1. Illustration of the Formation of the r-MnO2 Urchinlike Structures
and 3D clewlike ε-MnO2 nanostructures without using any template and surfactant. To the best of our knowledge, the 3D clewlike ε-MnO2 nanostructures have never been observed before. The influences of different reaction conditions on the morphology and crystalline structure of final products have been discussed in detail. The electrochemical properties of such prepared MnO2 were also investigated under a three-electrode cell configuration at room temperature. The 3D clewlike ε-MnO2 nanostructures have a specific capacitance of 120 F g-1 at the scan rate of 5 mV s-1 in 1 mol L-1 Na2SO4 aqueous electrolyte solution.
2. Experimental Section 2.1. Synthesis of Manganese Dioxide 3D Hierarchical Nanostructures. Manganese dioxides with different crystal structures and morphologies were synthesized using a hydrothermal method by varying the process parameters (in the Table 1). All the chemicals used in this experiment were of analytical grade and used without further purification. In a typical procedure, MnSO4 · H2O (0.3415 g, 2 mmol), K2S2O8 (0.5434 g, 2 mmol), and 2 mL of concentrated sulfuric acid were mixed in 38 mL of deionized (DI) water and stirred with a magnetic stirrer for 10 min to form a homogeneous solution at room temperature. Then the solution was transferred to a Teflon-lined stainless steel autoclave (50 mL) of 80% capacity of the total volume and loaded into an oven
10.1021/cg800834g CCC: $40.75 2009 American Chemical Society Published on Web 12/02/2008
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Crystal Growth & Design, Vol. 9, No. 1, 2009 529 level of the active material is 0.2 mg cm-2, a slice of Pt was used as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. In these studies, all electrodes were tested in 1 mol L-1 Na2SO4 aqueous electrolyte solution, and all electrochemical experiments were carried out at room temperature. The specific capacitance was evaluated from the loop area of the charge and discharge curves of the CV plots.
3. Results and Discussion
Figure 1. XRD pattern of the S1, S2, and S3 produced at 110 °C for 6 h. preheated to 110 °C for 6 h. The autoclave was allowed to cool to room temperature naturally. The precipitates were collected, washed with DI water and absolute ethanol respectively for several times to remove impurities, and then dried at 60 °C for 8 h in air. The product was denoted as S1. In another two experiments, Fe(NO3)3 · 9H2O (0.8203 g, 2 mmol) and Al(NO3)3 · 9H2O (0.7578 g, 2 mmol) were respectively added to the above solution, the following processes were similar to the former one, and the final products were denoted as S2 and S3. 2.2. Material Characterization. XRD analyses were performed using a X′ Pert Pro system with Cu KR radiation (λ ) 1.540 60 Å) operated at 40 kV and 40 mA. The morphologies of the synthesized materials were investigated by an FEI Strata DB 235 (FIB/SEM) system. The samples were suspended in absolute ethanol and dispersed on AuPd-coated silicon chips previously mounted onto stainless-steel sample holders using two-sided carbon tape. TEM and HRTEM studies were carried out using a TECNAI F30 TEM operating at an accelerating voltage of 300 kV and equipped with an energy dispersive X-ray (EDX) analysis system. The chemical composition of the samples was determined by EDX. 2.3. Electrochemical Measurement. The cyclic voltammetry (CV, 0-0.8 V) and ac impedance (0.1 Hz-100 kHz) were operated in a three-electrode cell configuration. The working electrode was Ni foil coated with a mixture of MnO2-acetylene black-polyvinylidene difluoride (PVDF) with a weight ratio of 0.70:0.20:0.10. The loading
3.1. Structure Characterization. The XRD patterns of the obtained manganese dioxide samples S1, S2, and S3 are shown in Figure 1. Almost all the diffraction peaks in Figure 1, line S1 and S3, can be assigned to the tetragonal phase of R-MnO2 (JCPDS 44-0141, a ) 9.784, c ) 2.863 Å), indicating the high purity and crystallinity of S1 and S3. Lattice constants of S1 and S3 are calculated to be a ) 9.826, c ) 2.854 Å and a ) 9.823, c ) 2.852 Å, respectively. This implies that Al3+ ions have no influence on the structure of the final product. The XRD pattern in Figure 1, line S2, corresponds to the hexagonal ε-MnO2 with lattice constants of a ) 2.846, c ) 3.530 Å, (JCPDS 30-0820, a ) 2.80, c ) 4.45 Å). Obviously, the structure of the synthesized product would be changed when Fe3+ ions exist in the system. 3.2. Morphology of the Products. The morphologies of the products were observed using SEM and TEM. The SEM study indicates that S1 is a uniform urchin-shaped R-MnO2 with diameter of 2.5-3.5 µm, which consists of several straight and radially grown nanorods with uniform diameter around 30-40 nm (Figure 2a,d), and S2 is a uniform 3D clewlike nanosphere with diameter of 600-800 nm, which composed nanosheets (Figure 2b,e). The inset of Figure 2e is the typical morphology of the surface of S2. From it one can see that the nanosheets with the thickness of about 17 nm aggregate and constitute the nanosphere; S3 is R-MnO2 nanorods cluster, the diameter of the structure being about 1.7 µm (Figure 2c,f). All the samples are not destroyed after a long period of ultrasonic treatment, indicating the robustness of the structures. TEM images of S1, S2, and S3 are presented in Figure 3a-f. Figure 3a,b indicates that S1 has the core-shell structure. Li et al. reported that the existence of Ag+ was essential for the
Figure 2. (a) Low-magnification and (d) high-magnification SEM image of S1. (b) Low-magnification and (e) high-magnification SEM image of S2. (c) Low-magnification and (f) high-magnification SEM image of S3. The inset in part e is the SEM image of the typical surface morphology of S2. The scale bar is 200 nm, and all products were obtained at 110 °C for 6 h.
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Figure 4. EDXS patterns of the prepared MnO2 materials at 110 °C for 6 h.
Figure 3. TEM images of (a) S1 with core-shell structure at low magnification, (b) nanorods on the shell of S1, (c) S2 at low magnification, (d) S2 at high magnification (the inset is the corresponding SAED pattern), (e) S3 at low magnification, (f) high magnification of a selected area of part e, (g) the corresponding FFT pattern of a selected area of part f, and (h) HRTEM image of a selected area of part f. All products were obtained at 110 °C for 6 h.
formation of R-MnO2 core-shell urchinlike structure, because only with catalyst Ag+ could the core-shell structures be obtained.25 But in our experiment, we have not added any template, surfactant, or catalyst, This implies that a core-shell urchinlike structure can be obtained without catalyst Ag+ in solution. Figure 3c,d shows that the inners of S2 are solid, and no interstice is observed. The inset in Figure 3d is the corresponding selected area electron diffraction (SAED) pattern, which demonstrates the poor crystalline nature of S2. Figure 3e,f shows the typical TEM images of S3, which are different from that of S1. The SAED pattern in Figure 3g demonstrates that S3 is composed of single crystals; the interplanar distance
calculated from the lattice fringes of HRTEM in Figure 3h is 2.405Å, which corresponds to the distance between two (211) crystal planes. Figure 4 shows energy dispersive X-ray analysis (EDXS) patterns of the three samples. It reveals that the main elements in three samples are Mn, O, and K. C and Cu come from the carbon-coated copper grids that are used for supporting the samples. 3.3. Electrochemical Properties. The typical rectangular shapes of cyclic voltammogram (CV) curves (Figure 5a) were measured at scan rate of 5 mV s-1 in 1 M Na2SO4 solution between 0 and 0.8 V (vs SCE) for S1, S2, and S3. The specific capacitance (C) could be calculated from CV curves according to C ) It/m∆E, where I is the charge-discharge current, t is the discharge time, m is the mass of active material, and ∆E is the voltage difference. The specific capacitance of S1 is 46 F g-1, of S2 is 120 F g-1, and of S3 is 100 F g-1 at a scan rate of 5 mV s-1. The dependence of the capacitance loss on the scan rate of CV from 5 to 200 mV s-1 for S1, S2, and S3 is shown in Figure 5b. Increasing scan rate has a direct impact on the diffusion of Na+ into the MnO2 matrix. At high scan rate, the Na+ ion will only approach the outer surface of the electrode, the material in the deep pores has little contribution to pseudocapacitance, and a decrease in pseudocapacitance is expected. Variation of voltammetric cathodic current density at 0.4 V with scan rate is shown in Figure 5c. There is a linear increase in current with increasing in scan rate, confirming the capacitive behavior of nano-MnO2. The ac impedance plots of three samples at an applied potential of 0.5 V (vs SCE) in 1 M Na2SO4 solution are presented in Figure 6. The electrochemical impedance spectroscopy (EIS) of the three samples is composed of a partially overlapped semicircle and a straight sloping line. Such a pattern of the EIS can be fitted by an equivalent circuit shown in the inset of Figure 6,31 where the Rb is bulk resistance of the electrochemical system, Rct is Faradic charge-transfer resistance, and W is the Warburg impedance. For all three samples, Rb (at very high frequencies, the intercept at real part Z′) is almost the same, which means that the electrodes obtained by different materials have the same combination resistance of electrolyte, intrinsic resistance of active materials, and contact resistance at the active material/current collector interface.32 The chargetransfer resistance (the semicircle in the high frequencies range) of three samples is in the order of S2 < S3 < S1. At the lower frequencies, a straight sloping line represents the diffusive
3D Hierarchical MnO2 Nanostructures
Crystal Growth & Design, Vol. 9, No. 1, 2009 531
Figure 6. Ac impedance plots of S1, S2, and S3 at an applied potential of 0.5 V (vs SCE) in 1 M Na2SO4 solution. The inset is the equivalent circuit.
Figure 7. SEM images of the intermediates of sea urchin shaped R-MnO2 collected at different times: (a) obtained under 110 °C hydrothermal conditions for 1 h followed by fast cooling and placed in room temperature after 24 h, obtained under 110 °C hydrothermal conditions for (b) 1.5 h and (c) 2 h followed by fast cooling, and (d) obtained under 110 °C hydrothermal conditions for 6 h.
Figure 5. (a) Cyclic voltammograms of S1, S2, and S3 recorded in 1 M Na2SO4 at 5 mV s-1. (b) Dependence of the capacitance loss on the scan rate of CV from 5 to 200 mV s-1 for S1, S2, and S3. (c) Dependence of voltammetric current density on scan rate.
resistance (Warburg impedance). The precipitous slope denotes fast electric double-layer forming speed;33 the forming speed of the electric double-layer is in the order of S1 < S3 < S2. From above analysis, S2 has the best electrochemical properties among the three samples. The charging-discharging process of MnO2 in neutral Na2SO4 aqueous electrolyte is mainly governed by the insertion and/or release of Na+ from the electrolyte into the nanostructured MnO2 matrix and/or from the MnO2 matrix into the electrolyte. The reasons for the lower specific capacity of S1
may be due to its compact morphology; the diffusion of ions from the electrolyte has more resistance than for S3, and otherwise, S1 and S3 are R-MnO2, S2 is ε-MnO2, and the crystal structures play important roles in its electrochemical properties.34 3.4. Formation of MnO2 3D Structures. To investigate the formation process of the sea urchin shaped R-MnO2 structures, we collected some intermediates of them during the formation process and observed them by the SEM. As shown in Figure 7a, the intermediates obtained under 110 °C hydrothermal conditions for 1 h followed by fast cooling and placed in room temperature after 24 h are microspheres consisting of small nanoparticles. Figure 7b shows a SEM image of intermediates obtained under 110 °C hydrothermal conditions for 1.5 h followed by fast cooling; three kinds of morphologies are found, indicating that many nanorods are epitaxially grown from nanoparticles on the surface of microspheres. Upon further increasing the hydrothermal reaction time to 2 h, the microspheres transformed into sea urchin shaped R-MnO2 (shown in Figure 7c). Figure 7d shows a SEM image of the final products obtained under 110 °C hydrothermal conditions for 6 h;
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Figure 8. SEM images of the intermediates of sea urchin shaped R-MnO2 cluster collected at different times. Samples obtained under 110 °C hydrothermal conditions for (a) 0.5 h and (b, c) 1 h followed by fast cooling and placed in room temperature after 24 h, obtained under 110 °C hydrothermal conditions for (d) 1.5 h and (e) 2 h followed by fast cooling, and (f) obtained under 110 °C hydrothermal conditions for 6 h.
compared to the intermediates, the nanorods on the surface of the final urchinlike structures became thin and long. On the basis of the above evolution of the time-dependent morphology, the formation process of the R-MnO2 urchinlike structures is illustrated in Scheme 1. In the initial stage, a large number of nuclei were formed in a short time (Scheme 1a) and aggregated to microspheres (Scheme 1b) through the reaction between MnSO4 and K2S2O8 in acid solution. Then, slow crystal growth followed: nanorods were epitaxially grown along the surface of initial microspheres and formed the solid urchins (Scheme 1c); the nanorods located on the outside would serve as starting points for the subsequent crystallization process of cores, and then an interior cavity was gradually formed via a core evacuation process due to higher surface energies (Scheme 1d); finally, the core-shell R-MnO2 structures were formed. All the experimental results indicate that the mechanism for the formation of the core-shell R-MnO2 structures is the “Ostwald ripening process”,35 which can be used to explain the formation of the different structured R-MnO2.28 To further understand the influence of the Fe3+ and Al3+ on the evolution of 3D MnO2 structures, detailed time-dependent evolution of morphology was studied by the SEM. Figure 8 shows the SEM images of samples obtained at the different reaction stages when Al3+ exists in the solution. The formation process was similar to that of the core-shell R-MnO2 structures in the initial stage (Figure 8a-c), but during the process of the slow crystal growth, the microspheres were divided into many small urchinlike structures (Figure 8d,e). Further increasing the hydrothermal reaction time to 6 h led to formation of R-MnO2 nanorods clusters (Figure 8f), no core-shell R-MnO2 structures were formed, and the net result of the above process should be
Yu et al.
Figure 9. SEM images of the intermediates of 3D clewlike ε-MnO2 nanostructure collected at different times: (a-c) obtained under 110 °C hydrothermal conditions for 0.5 h followed by fast cooling and placed in room temperature after 24 h, (d,e) obtained under 110 °C hydrothermal conditions for 1 h followed by fast cooling, and (f) obtained under 110 °C hydrothermal conditions for 6 h.
the nanorods growing around one common center. Two reasons might be proposed for the effect of Al(NO3)3 in our synthesis of R-MnO2 nanorods cluster. First, the addition of Al(NO3)3 increased the chemical potential of the solution, which contributes to the growth of one-dimensional nanostructure.36,37 Second, the addition of Al(NO3)3 significantly decreased the viscosity of the solution; thus, the components of the system can move more expediently and the atoms and ions can adopt appropriate positions in the development of crystal lattices.38 A similar phenomenon occurred for the creation of β-MnO2 nanowires in Zhou’s previous work.39,40 The evolution of 3D clewlike nanosphere at different hydrothermal dwell times at 110 °C is shown in Figure 9. No product formed after 0.5 h followed by fast cooling, but the microspheres were formed when placing the solution in room temperature after 24 h (Figure 9a-c). Further increasing the hydrothermal reaction time to 1 h, the 3D clewlike nanospheres were formed (Figure 9d,e). We found that for the synthesis using Fe(NO3)3 without MnSO4 under 110 °C hydrothermal conditions for 6 h, no precipitate was obtained; the result indicated that no iron phase precipitate formed in the reaction. In our experiment, R-MnO2 was transformed to ε-MnO2 with the introduction of iron; this indicated that the added Fe3+ affected the hydrothermal growth of crystalline MnO2. Fe element may substitute for Mn element, which results in the change of structure and morphology of MnO2. The above experimental results indicate that metal ions (Al3+, 3+ Fe ) have a great effect on the morphology of product, but at present, the detailed reason is not completely understood. To further investigate the growth mechanism and the effect of metal ions (Al3+, Fe3+) in the MnO2 3D structure formation, more experimental studies are required.
3D Hierarchical MnO2 Nanostructures
4. Conclusions In summary, sea urchin shaped R-MnO2 and 3D clewlike ε-MnO2 nanostructures have been prepared through a facile hydrothermal method at a low temperature (110 °C) under different reaction conditions without using any template and surfactant. The electrochemical properties of the three samples are different; the 3D clewlike ε-MnO2 has a specific capacitance of 120 F g-1 at the scan rate of 5 mV s-1. The metal ions (Fe3+, Al3+) have great effects on the structure and morphology of MnO2. In fact, the prepared materials are also believed to have applications as catalysts, as absorbents and separation materials, and in electromagnetic and electronic devices. Acknowledgment. This work was supported by the National natural Science Foundation of China under grant Nos. 50572104 and 50777062 and the “Bairen” program of CAS.
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