Room Temperature Synthesis Routes to the 2D Nanoplates and 1D

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Room Temperature Synthesis Routes to the 2D Nanoplates and 1D Nanowires/Nanorods of Manganese Oxides with Highly Stable Pseudocapacitance Behaviors Da-Young Sung,† In Young Kim,† Tae Woo Kim, Min-Sun Song, and Seong-Ju Hwang* Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Korea

bS Supporting Information ABSTRACT: The 2D nanoplates of δ-MnO2 and the 1D nanowires/nanorods of R-MnO2 can be synthesized at room temperature via one-pot oxidation reaction of commercially available divalent manganese compounds. Treating the MnO or MnCO3 precursor with persulfate ions for 1
2 days yields layered δ-MnO2 2D nanoplates, whereas the same oxidation reaction for the MnSO4 precursor produces γ-MnO2-structured 3D urchins. As the reaction time is extended for ∼14
21 days, not only δ-MnO2 nanoplates but also γ-MnO2 urchins are changed to wellseparated 1D nanostructured R-MnO2 materials with controllable diameters. According to N2 adsorption
desorption isotherm measurements and Mn K-edge X-ray absorption spectroscopy, all the obtained manganate nanostructures show expanded surface areas of ∼50
120 m2 g
1 and the mixed oxidation state of Mn3þ/Mn4þ, respectively. All the present nanostructured manganese oxides exhibit pseudocapacitance behaviors with large specific capacitance and excellent capacitance retention, highlighting their promising functionality as a supercapacitor electrode. Among the materials under investigation, the δ-MnO2 2D nanoplates show the largest specific capacitance (∼180
210 F g
1). The present finding clearly demonstrates that the room-temperature oxidation reaction of the MnO or MnCO3 precursor can provide a facile soft-chemical route to 2D δ-MnO2 nanoplates and 1D R-MnO2 nanowires/nanorods with highly stable pseudocapacitance behaviors.

’ INTRODUCTION Over the past decades, low-dimensional nanostructured manganese oxides attract intense research interest because of their promising functionalities as electrodes for supercapacitors and lithium secondary batteries, gas adsorbents, catalysts, and so on.1
8 Many synthetic methods such as vapor deposition, template synthesis, and hydrothermal synthesis are developed for these nanostructured materials.9
11 For the practical application, it is of high importance to economically synthesize these materials in a large quantity. The solution-based synthesis is quite advantageous in that a large amount of nanostructured manganese oxides can be obtained in a single batch. A great deal of studies are devoted to the solution-based synthesis of 1D nanostructured manganates and their hierarchical assemblies having diverse crystal structure of R-MnO2, β-MnO2, γ-MnO2, δ-MnO2, and spinel.12
23 Most solution-based synthetic methods contain a hydrothermal treatment step at elevated temperature of >100 °C. There are only a limited number of examples for the room-temperature synthesis of nanostructured manganese oxides. In one instance, 1D nanostructured manganese oxide can be synthesized via the rolling process of layered manganate precursors at room temperature.16,24 This synthesis however is a two-step process, i.e., the synthesis of precursors and the formation of nanostructures, and shows poor yield. Thus, this method is not suitable for mass production. Alternatively, via solution-based reaction of precursor r 2011 American Chemical Society

MnSO4, the R-MnO2 and γ-MnO2 materials can be prepared at room temperature in the form of aggregated 3D urchin morphology, not in the form of well-separated nanostructure morphology.25,26 To take advantage of the unique functionalities of nanostructured manganese oxides originating from their limited crystal dimension, the isolated 1D and 2D nanostructures have better morphology than the hierarchically assembled 3D urchin.27 Of particular importance is that the porous assembly of exfoliated manganate 2D nanosheets shows promising electrode performance for supercapacitors.28 However, this 2D material is synthesized by a multistep exfoliation
reassembling process, which is not beneficial in terms of cost and efforts. Thus, it is highly requested to develop a room-temperature route to 2D nanostructured MnO2 materials as well as to 1D nanostructured manganese oxides with nonaggregated morphology. Our group investigates the effect of Mn valence in the solid state precursor on the formation of nanostructured manganese oxides upon hydrothermal treatment, clearly demonstrating the importance of precursor solubility in the formation of nanostructured manganese oxide.29 This finding gives an impetus to adopt divalent manganese compounds of MnO, MnCO3, and MnSO4 with considerable Received: March 3, 2011 Revised: June 3, 2011 Published: June 06, 2011 13171

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Figure 1. Powder XRD patterns of the oxidized derivatives of the precursors (a) MnO, (b) MnCO3, and (c) MnSO4 with various reaction times of 1
21 days. In (a), the asterisk symbols denote the Bragg reflections of the precursor MnO.

solubility as precursors for the development of a room-temperature synthesis route to 1D and 2D nanostructured manganates. Also, attempts are made to control the crystal structure, structural dimensionality, and crystal morphology of the nanostructured manganese oxides by changing the precursor composition and reaction time. The resulting manganese oxides with various crystal structures/shapes can provide a useful opportunity to elucidate a relationship between the crystal structure/morphology and electrode activity of nanostructured manganese oxides. In this study, the 2D nanoplates of δ-MnO2 and the 1D nanowires/nanorods of R-MnO2 are synthesized at room temperature by treating commercially available divalent manganese compounds with persulfate ions, together with the hierarchically assembled 3D urchins of γ-MnO2. The crystal structure, crystal morphology, and electronic structure of the obtained manganese oxides are characterized with a combination of diffraction, microscopy, and spectroscopy. The capacitance properties of the obtained nanostructured manganese oxides are investigated to test their applicability as a supercapacitor electrode.

’ EXPERIMENTAL SECTION Preparation. The nanostructured manganese oxides were prepared by persulfate treatments of the commercially available divalent manganese compounds of MnO, MnCO3, and MnSO4 at room temperature. For comparison, the same reactions were carried out for the higher valent manganese oxides of Mn3O4, Mn2O3, and MnO2. All of the precursors were purchased from Sigma-Aldrich Co. and used without further purification. The precursors were reacted with 0.5 M aqueous solution of (NH4)2S2O8 at 298 K for 1
21 days in conventional glassware. After the oxidation reaction, the obtained powders were washed thoroughly with distilled water and dried at 50 °C in air. Characterization. The effects of persulfate treatment on the crystal structures of the precursor manganese compounds were characterized by powder X-ray diffraction, or XRD (λ = 1.5418 Å, 298 K), measurements. The morphology and chemical composition of the nanostructured manganese oxides were probed by field emission-scanning electron microscopy/energy-dispersive X-ray spectrometry, or FE-SEM/EDS (Jeol JSM-6700F). The crystal dimension and local crystal structure of the manganese oxides were examined by high resolution-transmission electron microscopy/selected area electron diffraction, or HR-TEM/ SAED (Jeol JEM-2100F microscope, 200 kV), measurement. The water content and thermal behavior of the manganese oxides were examined with thermogravimetric analysis, or TGA. To

determine the surface area and pore structure of the nanostructured manganese oxides, N2 adsorption
desorption isotherms were measured volumetrically at 77 K after degassing of the samples at 150 °C for 2 h under vacuum. X-ray absorption spectroscopy, or XAS, data at the Mn K-edge were collected from the thin layer of powder samples deposited on transparent adhesive tape in a transmission mode at the beamline 7C at the Pohang Accelerator Laboratory, or PAL (Pohang, Korea), operated at 2.5 GeV and 180 mA. The measurements were carried out at room temperature with a Si(111) single-crystal monochromator. All the spectra were calibrated by measuring the spectrum of Mn metal foil. The data analysis for the experimental spectra was performed by the standard procedure reported previously.17 Electrochemical Measurement. The capacitance behaviors of the nanostructured manganese oxides were investigated by measuring cyclic voltammetry, or CV, and galvanostatic charge
discharge cycling data. All the electrochemical measurements were carried out with a conventional three-electrode cell using a potentiostat/galvanostat (WonA Tech). The working electrode materials were prepared by mixing the active material (manganese oxides), acetylene black, and polyvinylidenefluoride, or PVDF, in a mass ratio of 75:20:5. The mixtures were stirred for 1 h in N-methylpyrrolidone to mix the components homogeneously. Each of the resulting slurries was then pressed on a stainless steel substrate, and the electrodes were dried in a vacuum oven at 80 °C for 30 min. A platinum mesh was adopted as the counter electrode, and a saturated calomel electrode, or SCE, was used as the reference electrode. Aqueous solutions (1 M) of Li2SO4 and Na2SO4 were employed as electrolyte. CV experiment was performed from 0 to 1 V at a scan rate of 20 mV s
1. The galvanostatic charge
discharge cycling was done at a constant current density of 0.5 mA cm
2 to determine the specific capacitance of the present materials.

’ RESULTS AND DISCUSSION Powder XRD Analysis and Phase Transition Behavior. Figure 1 represents the powder XRD patterns of the oxidized derivatives of the precursor manganese compounds. The persulfate treatment of the precursor MnO for 1 day initiates the formation of the layered δ-MnO2 phase, but there still remains a large amount of the precursor MnO phase. An extension of reaction time to 2 days leads to the formation of δ-MnO2 structure as a main phase. After 3 days, the γ-MnO2 phase starts to appear as a secondary phase. As the reaction time is extended to 14
21 days, both the δ-MnO2 and γ-MnO2 phases are 13172

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Figure 2. FE-SEM images of the oxidized derivatives of the precursors (a) MnO, (b) MnCO3, and (c) MnSO4 with the reaction times of 1
21 days.

changed to the tunnel-structured R-MnO2 phase, suggesting a higher stability of the latter phase in the given condition. A similar gradual phase transformation of precursor f δ-MnO2 f γMnO2 f R-MnO2 occurs for the precursor MnCO3 (see Figure 1b). In comparison with the precursor MnO, the formation of the δ-MnO2 phase is more rapid with the precursor MnCO3, which would be related to the higher solubility of the manganese carbonate.30 As plotted in Figure 1c, the persulfate treatment of the precursor MnSO4 for 1 day yields an X-ray amorphous phase, instead of the layered δ-MnO2 phase. An elongated reaction for 3 days results in the formation of the γ-MnO2 phase as a main phase. As the reaction is extended to 4 days, the R-MnO2 phase starts to be formed. The formation of the R-MnO2 phase is observed earlier for the MnSO4 precursor than for the MnO and MnCO3 precursors. Like the cases of the other precursors, the extended reaction for 14
21 days produces the R-MnO2-structured material as a main phase. Conversely, the higher Mn valent precursors of Mn3O4, Mn2O3, and MnO2 with the Mn oxidation state of >þ2 do not experience any structural change upon the persulfate treatment (see Supporting Information), indicating the important role of the Mn oxidation state in the phase transformation of the precursors. FE-SEM and HR-TEM Analyses. The effects of oxidation reactions on the crystal morphology of the resulting manganese oxides are examined with FE-SEM and HR-TEM. As illustrated in the FE-SEM images of Figure 2, the hydrothermal treatment of MnO and MnCO3 precursors for 1
2 days leads to the formation of 2D plate-type crystallites with ∼5
10 nm in thickness and ∼300
500 nm in lateral size. The observed morphology is fairly consistent with the XRD results showing the formation of the layered δ-MnO2 structure (Figure 1). As the reaction time is extended, 1D morphology starts to appear (Figure 2). On the basis of the XRD results in Figure 1, the observed new morphology can be attributed to the newly formed γ-MnO2 phase. After the reaction for 21 days, the well-separated 1D nanowires with a

higher aspect ratio can be observed. The powder XRD analysis of the corresponding sample allows us to assign this highly anisotropic morphology as the R-MnO2 structure having a strong preference to the anisotropic 1D crystal growth along the direction of 2  2 tunnels. In the case of the precursor MnSO4, 2D plate-type morphology is not observed during the entire stage of the oxidation reaction, which agrees well with the XRD results showing no formation of the layered δ-MnO2 phase. Instead of the 2D nanoplate, the oxidation reaction for 1 day produces aggregated secondary particles composed of irregular polyhedral crystals. After the reaction for 3 days, the 3D urchin-like structure starts to be formed. As the reaction is preceded up to 21 days, the 1D nanostructures with nonaggregated morphology appear in the corresponding FE-SEM images. On the basis of the powder XRD analysis (Figure 1), the observed 3D and 1D nanostructured morphologies can be assigned as the γ-MnO2 and R-MnO2 phases, respectively. In comparison with the highly anisotropic R-MnO2 nanowires synthesized with the precursors MnO and MnCO3, the homologue prepared with the MnSO4 precursor exhibits a nanorodlike morphology with a thicker diameter of ∼70 nm and a lower aspect ratio of ∼7. The change of precursors makes it possible to control the diameter and aspect ratio of the resulting 1D nanostructured materials. Figure 3 represents the HR-TEM images and SAED patterns of the obtained nanostructured manganese oxides. The δ-MnO2structured material obtained from the precursor MnO shows 2D nanoplate morphology, which is in good agreement with the FESEM results (Figure 2). Due to the disordered stacking of several very thin nanoplates, a ring-type SAED pattern is observed for this material. As shown in Figure 3b, the urchin-type 3D superstructured sphere appears for the γ-MnO2-structured material prepared from the MnSO4 precursor, together with a ring SAED pattern that reflects its hierarchical morphology. In Figures 3c and 3d, the 1D nanostructure-type crystal shapes are observed in 13173

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Figure 3. HR-TEM/SAED data of (a) δ-MnO2 nanoplates, (b) γ-MnO2 urchins, (c) R-MnO2 nanowires, and (d) R-MnO2 nanorods.

the HR-TEM image of the R-MnO2-structured materials. The R-MnO2 nanorods prepared from the MnSO4 precursor show a diameter of ∼70
80 nm, which is thicker than the diameter of the R-MnO2 nanowires prepared from the MnO precursor (∼10
20 nm). These 1D nanostructured materials commonly display characteristic SAED patterns of the R-MnO2-structure, which is well-consistent with the XRD results. According to the diffraction and electron microscopy results presented here, the MnO and MnCO3 precursors are transformed into the layered δ-MnO2 nanoplates at the initial stage of the nanostructure formation. The following phase transition from δ-MnO2 nanoplates to 1D nanostructures of γ-MnO2 seems to agree with the rolling mechanism.23,24 Conversely, the final phase transition of γ-MnO2 f R-MnO2 cannot occur via a simple rearrangement of Mn ions in the presynthesized γ-MnO2 material since the formation of the R-MnO2 phase with larger 2  2 pores requires the incorporation of additional large spacer cations (i.e., H3Oþ, NH4þ, etc.) for the pore stabilization.31,32 Thus, this γ-MnO2 f R-MnO2 transition is supposed to occur via the redissolution of presynthesized γ-MnO2 urchins and the subsequent recrystallization of R-MnO2 nanowires. In contrast to the precursors MnO and MnCO3, the oxidation reaction of the precursor MnSO4 forms 3D nanostructured γ-MnO2 and/or 1D nanostructured R-MnO2 without the formation of the 2D δ-MnO2 phase, suggesting the different formation mechanism of these 1D nanostructures via the direct crystallization from the solvated MnOx clusters. Among the precursors under investigation, MnSO4 has the highest solubility and tends to be easily dissolved to form the MnOx species in a high concentration. This facilitates the direct crystal growth of the 1D nanostructure and the enlargement of the diameter of 1D nanostructure. For this reason, the previous experiments using precursor MnSO4 cannot produce the 2D nanoplates of the δ-MnO2 phase.25,26 Mn K-Edge XANES Analysis. The effects of persulfate treatment on the local crystal structure and electronic configuration of the precursor manganese compounds are investigated with Mn K-edge X-ray absorption near-edge structure, or XANES, spectroscopy. Figure 4 represents the Mn K-edge XANES spectra of the precursor manganese compounds and their oxidized derivatives for 2
21 days, compared with several reference spectra. Regardless of reaction time, all of the present oxidized derivatives show a higher edge energy than the precursor compounds, underscoring the oxidation of Mn ions under the given condition. In the pre-edge region of 6535
6545 eV, all the present materials display weak

peaks P and/or P0 corresponding to the 1s f 3d transitions. Since these electronic transitions are forbidden by the dipole-selection rule under centrosymmetric octahedral geometry,33,34 the observed weak intensity of these peaks provides strong evidence for the presence of manganese ions in the octahedral geometry. Among the materials under investigation, the MnO precursor shows a somewhat higher intensity for the peak P, but it is just a result of simple overlap between this feature and the main-edge jump. In the main-edge region, there are some peaks A and B corresponding to the dipole-allowed 1s f 4p transitions.34,36 After the persulfate treatments, all these features are displaced toward the higher energy region, confirming the increase of the Mn oxidation state. The resonance peak B can provide a sensitive measure for the relative concentration of edge-sharing over corner-sharing of MnO6 octahedra; a sharp and intense peak appears for the manganese oxide whose lattice is composed of edge-shared MnO6 octahedra only.34 As illustrated in Figure 4, the layered LiMn0.9Cr0.1O2 shows a narrow peak width for the feature B, whereas a broader peak B appears for the reference β-MnO2 material. While the layered structure of δ-MnO2 and LiMn0.9Cr0.1O2 phases is composed of edge-shared MnO6 octahedra only, the R-MnO2, β-MnO2, and γ-MnO2 phases are crystallized with the interconnected networks of corner- and edge-shared MnO6 octahedra.36 In the case of the oxidized MnO (Figure 4a), a sharp peak B can be observed for the oxidized derivative for 2 days, whereas a broader feature B appears for the spectrum of the sample oxidized for 21 days. This is fairly consistent with the powder XRD results (Figure 1) revealing the formation of layered δ-MnO2 and tunnel R-MnO2 materials after 2 and 21 days, respectively. Conversely, the precursor MnCO3 subjected to the 3 day reaction exhibits a rather broad peak B (Figure 4b) because this material contains the γ-MnO2 phase as well as the layered δ-MnO2 phase (Figure 1). Also, this material displays a marked intensity decrease of the peak B, reflecting the coexistence of two phases (i.e., γ-MnO2 and δ-MnO2) that results in the destructive interference in XANES signal. Similarly to the precursor MnCO3, only a broad feature B is discernible for both the oxidized derivatives of the precursor MnSO4, which is well-consistent with their γ-MnO2 or R-MnO2 structure (see Figure 4c). TGA and Elemental Analysis. For further characterizations, four samples are selected as representatives for the crystal structures and morphologies of the obtained many nanostructured materials: (1) δ-MnO2-structured 2D nanoplates prepared from the precursor MnO after 2 days, (2) γ-MnO2-structured 3D 13174

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Figure 4. Mn K-edge XANES spectra of the precursor compounds (dot-dashed lines) and their oxidized derivatives for 2
3 days (dashed lines) and 21 days (solid lines) and the references LiMn0.9Cr0.1O2 (circles) and β-MnO2 (crosses). The panels (a), (b), and (c) represent the spectra of the samples prepared from the precursors MnO, MnCO3, and MnSO4, respectively.

Figure 5. TGA curves of (a) δ-MnO2 nanoplates, (b) γ-MnO2 urchins, (c) R-MnO2 nanowires, and (d) R-MnO2 nanorods.

urchins prepared from the MnSO4 precursor after 3 days, (3) RMnO2-structured 1D nanowires prepared from the MnO precursor after 21 days, and (4) R-MnO2-structured 1D nanorods prepared from the MnSO4 precursor after 21 days. The TGA curves of these four representative nanostructured manganese oxides are illustrated

in Figure 5. A considerable mass loss of ∼5
10% occurs commonly for all the manganese oxides in the temperature range of 100
250 °C. This mass decrease is attributable to the dehydration and dehydroxylation of manganese oxide.16,37 Among the present materials, the δ-MnO2-structured material shows the most prominent 13175

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Figure 6. N2 adsorption
desorption isotherms of (a) δ-MnO2 nanoplates, (b) γ-MnO2 urchins, (c) R-MnO2 nanowires, and (d) R-MnO2 nanorods.

decrease in mass, indicating the highest water content of this material. This observation confirms that the layered lattice of δ-MnO2 can accommodate a lager amount of water molecules in its interlayer space compared with the nonlayered lattices of R-MnO2 and γ-MnO2 phases. In the higher temperature region of >550 °C, all the materials under investigation display a mass decrease corresponding to oxygen loss caused by the reduction of tetravalent Mn4þ ions into trivalent Mn3þ ions.19,37 This observation confirms that all the materials possess a higher Mn oxidation state of >þ3. The chemical compositions of the present nanostructured materials are investigated with EDS analysis, showing the absence of a nitrogen element in all these materials (see Supporting Information). In general, the R-MnO2 or δ-MnO2 structure is stabilized by the presence of large spacer cations in the intrachannel 2  2 pore or interlayer space of each lattice.31,32 Since the EDS results show the absence of ammonium ions, we believe that these lattices contain H3Oþ cations as a spacer ion. N2 Adsorption
Desorption Isotherm Measurements. N2 adsorption
desorption isotherm measurements are carried out to determine the surface area and pore structure of the nanostructured manganese oxides (see Figure 6). There is a distinct hysteresis in the pp0
1 > 0.5 region of the isotherm of the δ-MnO2 nanoplates and R-MnO2 nanorods, indicating the presence of mesopores formed by the porous stacking of nanostructured crystals.38,39 The observed isotherms can be classified

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as Brunauer
Deming
Deming
Teller, or BDDT, type I and IV isotherms and H3-type hysteresis loop in the IUPAC classification.38,39 This classification suggests the presence of the open slit-shaped capillaries with very wide bodies and narrow short necks in these materials. In contrast, the isotherms of the other nanostructured materials of R-MnO2 nanowires and γMnO2 urchins display only a negligible adsorption of nitrogen in the low pp0
1 region and no significant hysteresis in the mid pp0
1 region. This observation indicates small populations of micropores and mesopores in these materials. The surface areas of the nanostructured manganese oxides are determined by the fitting analysis based on the Brunauer
Emmett
Teller, or BET, equation. As expected, the formation of low-dimensional nanostructures increases the surface area to ∼116 m2 g
1 for the δ-MnO2 nanoplates, ∼82 m2 g
1 for the γMnO2 urchins, ∼53 m2 g
1 for the R-MnO2 nanowires, and ∼84 m2 g
1 for the R-MnO2 nanorods. The pore sizes of all the present materials are calculated with the Barrett
Joyner
Halenda, or BJH, method (see Supporting Information). Both the δ-MnO2 nanoplates and the R-MnO2 nanorods possess the mesopores with the average diameter of ∼3.8 nm. The presence of mesopores in the former compounds is attributable to the porous stacking structure of 2D plate-shaped and thick rodshaped crystals. Conversely, no distinct peak appears in the pore size diagram of the γ-MnO2 urchins and R-MnO2 nanowires, confirming the absence of mesopores in these materials. Such an absence of mesopores is responsible for the smaller surface area of R-MnO2 nanowires than that of R-MnO2 nanorods. Electrochemical Measurements. As discussed in the Introduction section, manganese oxides can be used as electrode materials for lithium ion batteries and supercapacitors. Between the two kinds of energy storage devices, the lithium ion batteries can store electrical energy via the reversible intercalation/deintercalation of lithium ions into/from the electrode lattice, in which transition metal components of the electrode materials experience repeated reduction/oxidation reaction.40 The capability of the electrode material to reversibly accommodate Liþ ions in its crystal lattice is crucial to obtain excellent electrode performance for lithium ion batteries. Conversely, the storage of electrical energy in the supercapacitor device is achieved by the formation of an electric double layer near the electrode surface via the polarization process as well as by a partial Faradic redox reaction of electrode materials near the surface.41 Thus, the storage capacity of the supercapacitors can be enhanced by the use of nanostructured electrode materials with expanded surface area. Due to the different mechanisms of charge storage in the two energy storage devices, the lithium ion batteries show a larger energy density than the supercapacitor, and thus they are widely used as main power sources for mobile electronics, power tools, electrical vehicles, etc.41 Conversely, the supercapacitor boasts a higher powder density than the Li ion batteries, which is suitable as auxiliary power sources for electrical vehicles and uninterruptible power supply.41 Among the nanostructured manganese oxides synthesized in this study, the R-MnO2- and δ-MnO2-structured materials possess void lattice sites available for Li intercalation, while the γ-MnO2-structured materials are not suitable for the intercalation of Li ions. In addition, taking into account the large surface area of the present nanostructured manganese oxides, these materials are expected to be more applicable as electrodes for supercapacitors than for lithium ion batteries. The CV measurements are carried out for the nanostructured manganese oxides 13176

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Figure 7. CV curves up to the 1000th cycles for (a) δ-MnO2 nanoplates, (b) γ-MnO2 urchins, (c) R-MnO2 nanowires, and (d) R-MnO2 nanorods collected with the electrolyte of 1.0 M Na2SO4 solution.

to examine their capacitive behaviors. Figure 7 illustrates the CV data of the nanostructured δ-MnO2, γ-MnO2, and R-MnO2 materials with the electrolyte of 1.0 M Na2SO4 solution. All of the present materials show pseudorectangular-type CV curves, indicating their pseudocapacitance behaviors. After several initial cycles, only weak changes occur in the CV data of all the present manganese oxides up to the 1000th cycle, indicating their good cyclability. Similar capacitive behaviors can be observed with the electrolyte of aqueous Li2SO4 solution (see Supporting Information). Except for R-MnO2 nanorods, the larger capacitances are obtained with the aqueous Li2SO4 solution than with the Na2SO4 solution; with the electrolyte of aqueous 1.0 M Li2SO4 solution, the initial capacitance is estimated as 213 F g
1 for δ-MnO2 nanoplates, 142 F g
1 for γ-MnO2 urchins, 110 F g
1 for R-MnO2 nanowires, and 120 F g
1 for R-MnO2 nanorods, respectively. The smaller initial capacitance is obtained with the 1.0 M Na2SO4 solution for δ-MnO2 nanoplates (180 F g
1), γ-MnO2 urchins (100 F g
1), and R-MnO2 nanowires (94 F g
1). Conversely, R-MnO2 nanorods show a larger initial capacitance of 127 F g
1. The capacitive behavior of manganese oxide can occur via two different mechanisms,42 i.e., the adsorption/ desorption of lithium ions on the surface of the manganate electrode

and the intercalation/deintercalation lithium ions into/out of the manganate lattice. Since the δ-MnO2 nanoplates have the largest surface area and a 2D layered structure favorable for cation intercalation, the observed largest specific capacitance of this material is well understood in terms of both mechanisms. Although the γ-MnO2 lattice has neither a 2D interlayer space nor a 1D channel available for cation diffusion, the γ-MnO2-structured material shows a larger capacitance than the R-MnO2-structured nanowires. Considering the larger surface area of the γ-MnO2-structured material, the larger capacitance of this compound can be regarded as evidence for the important contribution of the adsorption mechanism. This conclusion is further supported by the fact that, between the two samples of the R-MnO2 phase, the 1D nanorod with larger surface area delivers larger capacitance than the 1D nanowire with smaller surface area. That most materials except the R-MnO2 nanorods show greater capacitances with the Li2SO4 electrolyte than with the Na2SO4 electrolyte reflects the additional contribution of Li intercalation to the total specific capacitance of the present manganese oxides. In the case of the R-MnO2 nanorods showing smaller capacitances with the Li2SO4 electrolyte, the contribution of the intercalation process seems to be negligible because of their thick 13177

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Figure 8. Galvanostatic charge
discharge curves of a few cycles near the 1st and the 500th cycles at a constant current density 0.5 mA cm
2 of (a) δMnO2 nanoplates, (b) γ-MnO2 urchins, (c) R-MnO2 nanowires, and (d) R-MnO2 nanorods. (e) Variations of the capacitance retention as a function of the number of cycles for δ-MnO2 nanoplates (triangles), γ-MnO2 urchins (squares), R-MnO2 nanowires (crosses), and R-MnO2 nanorods (circles).

diameter and limited 1D channel unfavorable for Li intercalation. The electrochemical activity of the present manganese oxides is also investigated by monitoring galvanostatic charge
discharge cycling (see Figure 8). All of the present materials commonly show a linear variation of potential with time, confirming their capacitive behavior.43 The linear shapes of the charge
discharge curves of these materials remain nearly unchanged after the 500th cycle, confirming their high electrochemical stability. Figure 8e represents variations of the specific capacitances of the nanostructured manganese oxides as a function of cycle number. With the electrolyte of aqueous Na2SO4 solution, there occurs no significant fading of capacitance up to the 1000th cycle for all the present materials, highlighting their excellent cyclability. Basically the observed excellent cyclability of the present materials can be attributed to the fact that the storage of electrical energy in the supercapacitor is attributed to the polarization and/ or Faradic redox process near the electrode surface, and such surface processes cause no significant modification in the crystal and electronic structures of the electrode material. Thus, the repeated cycling in the supercapacitors causes negligible damage for the electrode materials, which is mainly responsible for the observed good cyclability.41 Also, the high electrochemical stability of these materials is well correlated with the powder XRD results showing the maintenance of their original MnO2 structures after electrochemical cycling (see Supporting Information). A closer inspection for Figure 8e reveals that most of the present manganese oxides display a gradual increase of

specific capacitances as the cycle proceeds. Such a capacitance increase is attributable to the improvement of ion transport near the surface of the electrode. This phenomenon is more prominent for the R-MnO2 nanorod sample with thicker diameter than the other materials. In comparison with the electrochemical data collected with the Na2SO4 electrolyte, those obtained with the Li2SO4 electrolyte show poorer cyclability for all the present materials (see Supporting Information). This finding indicates the detrimental effect of Li intercalation on the structural stability and cyclability of the nanostructured manganese oxides due to the accompanying inelastic deformation of the crystal lattice.

’ CONCLUSION Room-temperature synthesis routes to the δ-MnO2 2D nanoplates and R-MnO2 1D nanowires/nanorods are developed with the oxidation reaction of divalent manganese compounds. The adaptation of divalent manganese compounds as a precursor is indispensable for the formation of nanostructured manganese oxides at room temperature. The crystal structure, morphology, and dimension of the nanostructured manganese oxides are strongly dependent on the solubility of solid state precursor and reaction time; at the initial stage of reaction, less soluble MnO and MnCO3 precursors are transformed into δ-MnO2 2D nanoplates, whereas the use of a more soluble MnSO4 precursor yields γ-MnO2 3D urchins. The extension of reaction time to 14
21 days produces the well-isolated R-MnO2 1D nanostructure whose diameter depends 13178

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The Journal of Physical Chemistry C heavily on the type of the precursor. All of the synthesized nanostructured manganese oxides show pseudocapacitance behaviors with large specific capacitance and excellent cyclability. Among the present materials, the 2D nanostructured δ-MnO2 material can deliver the largest specific capacitance. Taking into account the low price of precursors adapted here, simple synthesis procedure, and low synthesis temperature, the present one-pot oxidation reaction of MnO or MnCO3 at room temperature is considered as a valuable synthetic route to highly promising 2D and 1D nanostructured electrode materials for supercapacitor application. Our current project is the development of a soft-chemical route to hybrid-type electrode materials composed of 2D nanostructured manganese oxide and 2D graphene species with further optimized electrode performance.

’ ASSOCIATED CONTENT

bS

Supporting Information. Powder XRD patterns of the oxidized derivatives of Mn3O4, Mn2O3, and MnO2. EDS data, pore size distribution curves, and CV data collected with the Li2SO4 electrolyte for the nanostructured manganese oxides and powder XRD patterns of nanostructured manganese oxide after the 1000th electrochemical cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ82-2-3277-4370. Fax: þ82-2-3277-3419. E-mail: [email protected]. Author Contributions †

These two authors contributed equally to this work.

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029065), by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2010-0027517), and by the Converging Research Center Program through the Ministry of Education, Science and Technology (2010K001087). The experiments at PAL were supported in part by MOST and POSTECH. ’ REFERENCES (1) Arico, A. S.; Bruce, P.; Tarascon, J. -M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366–377. (2) Singhal, A.; Skandan, G.; Amatucci, G.; Badway, F.; Ye, N.; Manthiram, A.; Ye, H.; Xu., J. J. J. Power Sources 2004, 129, 38–44. (3) Conway, B. E. Scientific Fundamentals and Technological Applications; Kluwer Academic: Plenum: New York, 1999. (4) Chen, X.; Li, X.; Jiang, Y.; Shi, C.; Li, X. Solid State Commun. 2005, 136, 94–96. (5) Li, W. B.; Yang, X. F.; Chen, L. F.; Wang, J. A. Catal. Today 2009, 148, 75–80. (6) Zhang, W.; Wang, H.; Yang, Z.; Wang, F. Colloids Surf. A 2007, 304, 60–66. (7) Ma, R.; Bando, Y.; Zang, L.; Sasaki, T. Adv. Mater. 2004, 16, 918. (8) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. J. Phys. Chem. B 2005, 109, 20207. (9) Wang, X.; Li, Y. Mater. Chem. Phys. 2003, 82, 419–422. (10) Ouyang, L.; Thrall, E. S.; Deshmukh, M. M.; Park, H. Adv. Mater. 2006, 18, 1437. (11) Sander, M. S.; Gao, H. J. Am. Chem. Soc. 2005, 127, 12158.

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