s 0884291413002380 a

Address alla)e-mail: wb)e-mail: kjDOI: 10.15

Solvothermal synthesis of shape-controlled manganese oxidematerials and their electrochemical capacitive performances

Wen-Yin Ko,a) Lung-Jing Chen, Yu-Hung Chen, and Kuan-Jiuh Linb)

Department of Chemistry, National Chung-Hsing University, 40227 Taichung, Taiwan

(Received 15 May 2013; accepted 9 August 2013)

We present a simple and quick procedure for the one-pot synthesis of manganese oxides undera basic solvothermal condition in the presence of cationic surfactants acting as the template ina 2-butanol/water solution. Three-dimensional spinel-type MnO2 microspheres composed of smallnanoparticles have been fabricated for the first time using our method. Their correspondingelectrochemical performances in the applications of supercapacitor electrodes exhibit a goodspecific capacitance (SC) value of ;190 F/g at 0.5 A/g and excellent SC retention and Coulombicefficiency of ;100% and ;95% after 1000 charge/discharge cycles at 1 A/g, respectively.This suggests its potential applications in energy storage devices. Further, we demonstrate that thissolvothermal technique enables the morphological tuning of manganese oxides in various formssuch as schists, rods, fibers, and nanoparticles. This work describes a rapid and low-cost techniqueto fabricate novel architectures of manganese oxides having the desired crystal phase, which willhighly benefit various supercapacitor applications.

I. INTRODUCTION

Over the past few decades, the synthesis of inorganicnanomaterials with desired sizes and shapes has been in-tensively pursued due to their unique electronic, magnetic,optical, and catalytic properties with respect to multiplefundamental and practical technological applications.1–7

By means of further investigations on crystal growth kinet-ics and shape evolution, nanomaterials with different archi-tectures have been successfully fabricated in the solutionphase.8,9 By means of the cooperative self-assembly of in-organic species and surfactants, where the surfactant surfaceaggregates as templates, the surfactant-assisted strategy isknown to be a general and efficient method for the prep-aration of uniform inorganic nanomaterials with controllablesizes and shapes. Recently, various nano- and microstruc-tures of metal oxides (particularly those of MnO2, SiO2, andV2O5) in the form of spheres, porous shapes, layers, tubes,wires, and rods have been successfully fabricated by usingthis approach.10–14

Manganese oxide (MnO2)—an important functionalmetaloxide—and its derivative compounds offer the followingadvantages: possibility of numerous attainable oxidationstates ofMn, cost-effectiveness, variety of electrochemicalbehaviors, and environmental compatibility; because ofthese advantages, they have been the subject of intenseinvestigations for a wide range of applications such ascatalysts, water purification, water splitting, molecular

correspondence to these [email protected]@dragon.nchu.edu.tw57/jmr.2013.238

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absorption, and battery materials.15–17 In addition, theyalso provide reversible Faradaic (redox) reaction, appro-priate electrode potential window, and high theoreticalcapacitance; therefore, they have been considered foruse as a promising electrode material in electrochemicalsupercapacitors for yielding high power density, excel-lent reversibility, and long cycle.18,19 More recently,manganese oxides with various crystal types, morphol-ogies, and sizes have been systematically investigatedwith great enthusiasm for their unique properties thatpromote their performances in applications involvingenergy storage (e.g., supercapacitors).16,20,21 Among them,MnO2 structures with a spinel-type crystal phase ofthree-dimensional (3D) interconnected tunnels havebeen recently fabricated to obtain better capacitanceperformances in aqueous solutions as they can offer alarger number of electrolyte transport paths for electrontransfer and proton/cation diffusion, thereby allowingenhanced charge transport efficiency through the electro-des during the charge/discharge process and leading tobetter supercapacitive performances.22 In addition, as weknow, hierarchical microspheres assembled from smallnanostructured particles have been reported to exhibitdistinctive properties—different from those of the constit-uent nanoparticles—with various potential applications23;therefore, the creation of such a shape of MnO2 is ofgreat significance. Several types of morphologies ofMnO2 materials have been previously reported includ-ing nanotubes, nanorods, nanowires, nanosheets, andnanoparticles10,21,24–26; however, to the best of ourknowledge, spinel-type MnO2 microspheres (MS-MnO2)constructed from small nanoparticles have not been

Materials Research Society 2013 107

W-Y. Ko et al.: Shape-controlled manganese oxide materials

investigated. In this paper, we report a simple and quickprocedure for the one-pot synthesis of manganese oxidesunder a basic solvothermal condition in the presence of acationic surfactant acting as a template in a 2-butanol/watersolution for the direct synthesis of spinel-type MS-MnO2

comprising small MnO2 nanoparticles. The galvanostaticcharge/discharge and cyclic voltammetry (CV) measure-ments in 1-MNa2SO4 aqueous electrolytes demonstrate thatthe MS-MnO2 display an improved specific capacitance(SC) of ;190 F/g at 0.5 A/g. Interestingly, a superior SCretention of ;100% and a stable Coulombic efficiency of;95% after 1000 cycles can be obtained, yielding excellentlong-term charge/discharge cycling stability. In addition, bymeans of the solvothermal method, various morphologiesof manganese oxides including schists, rods, fibers, andnanoparticles can be easily obtained by changing the ratioof 2-butanol and water and the nature of the precursors aswell as cation surfactants. Therefore, the proposed methodcan provide a simplistic technique to fabricate MnO2

having novel architectures.

II. EXPERIMENTAL SECTION

A. Materials

Cetyltrimethyl ammonium bromide (CTAB), manganesesulfate (MnSO4�H2O), and (6)2-butanol used in the studywere purchased from Sigma-Aldrich. Tetraethyl ammoniumbromide (TEA; 98%) and potassium permanganate(KMnO4) were obtained from Alfa Aesar (UK) andSHOWA (Japan), respectively. All these reagents wereused in the experiments without any further purification.

B. Preparation of MS-MnO2

In a typical experiment, the MnO2 precursor of theTEAMnO4 powder, which was presynthesized by using amodifiedBrockmethod by the cation-exchange process thatinvolved the replacement of a hard potassium cation of thepermanganate precursor with a soft organic TEA cation,27

was dissolved in 2 mL of deionized water to yield ahomogeneous solution; then, the solution was added to10 mL of 2-butanol under stirring, where 2-butanol actedas a solvent as well as a reducing agent and TEA served asa template. The mixed solution was subsequently sealedin a Teflon-lined autoclave and heated to 120 °C at therate of 5 °C/min and maintained at this temperaturefor 3 h. This aging process accompanied an increasein the pH, from the neutral pH of the initial sols up to pHof 11–12. After the solvothermal process, the precip-itates (brownish-black color) were collected by centri-fugation and washed in sequence with deionized waterto remove the possible unreacted material. The finalproducts of the MnO2 materials, denoted as MS-MnO2,were dried in vacuum desiccators at the ambienttemperature.

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C. Characterization

Powder x-ray diffraction patterns were recorded by ananalytical X’Pert Pro MRD equipped with Cu Ka as thex-ray source (k 5 0.15418 nm). An operational voltageand current of 45 kV and 40 mA, respectively, were usedand the data were collected stepwise in the 2h range of10–80° with a scan step of 0.033°/s. A Zeiss Ultra Plusfield emission scanning electron microscope (FE-SEM;Germany) was used to examine the morphology and sizeof the as-prepared MS-MnO2. A JEM 2010 high-resolu-tion transmission electron microscope (HRTEM; Japan)operated at 200 kV was used to examine the morphologyand lattice structure of the MS-MnO2. The samples usedin the HRTEM analysis were prepared by placing a dropof the MS-MnO2 aqueous suspension onto a copper gridcovered with a carbon film and evaporating the drop in air.Thermal gravimetric analysis (TGA) curves were obtainedusing a Perkin-Elmer TGA-7lab instrument. In a typicalanalysis, approximately 10 mg of the as-synthesizedMS-MnO2 sample was analyzed in a platinum pan at aheating rate of 5 °C/min under nitrogen atmosphere with aflow rate of 150 mL/min.

D. Electrochemical measurements

All the electrochemical measurements were carried outin a 1-M Na2SO4 aqueous electrolyte solution at roomtemperature by using the 672A electrochemical system(CH Instruments, Austin, TX) with a three-electrode cellthat was composed of a MS-MnO2-based electrode as theworking electrode, a Pt film as the counter electrode,and an Ag/AgCl electrode as the reference electrode.The working electrodes were fabricated using a mixtureof 70 wt% MS-MnO2 powder with 25 wt% carbon blackand 5 wt% poly(tetrafluoroethylene) (PTFE) binder; sub-sequently, a small amount of alcohol was added into themixture to form a clay-like paste. The paste was thenincorporated in nickel foam (1� 1 cm), dried in a vacuumoven at room temperature, and followed by a compressiontreatment under a pressure of 6000 kg/cm2 for 5 min toensure that the active material completely adhered to thenickel foam. The CV measurements were performedbetween 0 and 0.9 V (versus Ag/AgCl) for scan rates inthe range of 2–100 mV/s. The charge/discharge propertieswere examined galvanostatically in the potential range of0–0.9 V (versus Ag/AgCl) at current densities of 0.5 and1 A/g. The SC of the fabricated electrodes was calculatedaccording to the relation C (F/g) 5 IDt/DVm, where Idenotes the constant discharge current, Dt is the dischargetime,m is the mass of the active materials (g), andDV is thetotal potential difference (V).

III. RESULTS AND DISCUSSION

Figures 1(a)–1(e) show the FE-SEM images of thetypical samples of the MnO2 material prepared using our

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FIG. 1. FE-SEM images of the as-synthesized MnO2 materials under solvothermal conditions and heating at 120 °C for (a) 1 h, (b) 3 h, (c) 6 h,(d) 12 h, and (e) 48 h.


approach, revealing the morphological evolution withthe reaction time. A series of impressive microsphereswere observed at the reaction times in the range of 3–6 h[Figs. 1(b)–1(c)]; when the reaction time was above 12 h, aneedle-like morphology of manganese oxides was gener-ated [Fig. 1(d)]. Notably, the population of such shapesincreased with an increase in the reaction time, whereasthe population of the microspheres decreased concomi-tantly. After 48 h, nearly all the initial microspheres wereconverted into such needle-like structures, as shown inFig. 1(e). These observations clearly show that the MnO2

composites with a needle-like architecture evolve from theinitial microspheres. From the above observations, we canshow that the optimal condition for the production of attrac-tive MS-MnO2 can be obtained under a solvothermal condi-tion at a heating temperature of 120 °C and a gel time of 3 h.

A large number of microspheres can be clearly seen inthe FE-SEM image shown in Figs. 2(a) and 2(b), with the

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size distribution ranging from 0.5 to 4.5 lm [Fig. 2(c)].The corresponding TGA profile, shown in Fig. 3(a), in-dicated that the thermal decomposition curve of MS-MnO2

involved three weight-loss steps: the first weight loss of4.9% occurred below 145 °C, which was due to the evap-oration of physically adsorbed water molecules inMS-MnO2, followed by a second weight loss of 34.4%between 145 and 230 °C, which was caused by the releaseof the TEA1 species according to the Hofmann degrada-tion reaction; in the third step, a weight loss of 4.3% in thetemperature range of 230–500 °C was demonstrated,which was associated with the combustion of remainingTEA and the phase transformation of MnO2 to Mn2O3 andMn3O4.

22,28 Importantly, a weight loss of 43.6% aftercalcination of MS-MnO2 was observed, and the weightloss of sample at temperatures 145–230 °C was significantlygreater than that at any other heating temperature, whichsuggests that the TEA-based template played a significant

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FIG. 2. (a, b) FE-SEM image and (b) size distribution of the as-synthesized MS-MnO2 using the solvothermal method and heating at 120 °C for 3 h.

FIG. 3. (a) TGA data and (b) XRD pattern of the as-synthesized MS-MnO2 using the solvothermal method and heating at 120 °C for 3 h.

FIG 4. (a) TEM image of the MS-MnO2 fabricated using the solvothermal process and heated at 120 °C for 3 h. (b) HRTEM image of the fringeregion of an individual MS-MnO2 sample, indicating that MS-MnO2 was constructed from sphere-like nanoparticles surrounded by surfactants.(c) HRTEM image of an individual sphere-like nanoparticle of MS-MnO2 taken along the [011] axis and its corresponding Fourier-transformeddiffraction pattern (inset), which can be indexed as the spinel-type structure of MnO2.


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role in the formation of the spinel MnO2 microspheres.To understand the crystalline structures of the as-preparedMS-MnO2 material, its corresponding XRD measurementwas carried out, as shown in Fig. 3(b). All the peaks can beindexed as pure spinel-type MnO2 (JCPDS no. 44-0992),where it can be shown that the three main peaks at2h 5 18.73°, 36.14°, and 65.09° can be correspondingly

FIG. 5. Schematic illustration of the solvothermal formation of the as-prepared spinel-type MS-MnO2.

FIG. 6. FE-SEM images of the synthesized MnO2 materials through the devexperimental conditions and changing the ratio of 2-butanol and water, nature


indexed to the (111), (311), and (440) reflections of thespinel-type MnO2; this indicates that the as-prepared prod-ucts are phase-pure spinel-type MnO2 microspheres.To further examine the configuration of these microstruc-tures, the HRTEM images were recorded. From Fig. 4(b), itis revealed that the MS-MnO2 material is composed ofnumerous spherical nanoparticles with an average diameterof 5 nm; interestingly, it is distinctly visible that thesenanoparticles are surrounded by a TEA-based template.The corresponding clear lattice fringes and fast-Fourier-transformed diffraction pattern data of the individualsmall spherical nanoparticles of MS-MnO2, as shown inFig. 4(c), indicate their efficient crystallization; addition-ally, two clear lattice fringes with basal distances of 2.02 Åand 1.86 Å can be observed, which are consistent withthe (400) and (331) planes of the spinel-phase MnO2,respectively. These results indicate that the spinel-typeMS-MnO2 microstructures are, in fact, built from thesesmall nanoparticles that are spontaneously assembledthrough the assistance of the TEA-based template,finally forming a 3D network of spherical microstruc-tures (Fig. 5). This is the first time that the spinel-typeMnO2 with the shape of a microscopic sphere has beenfabricated; further, we believe that it can offer additionalelectrolyte-transport paths for electron transfer andproton/cation diffusion due to its 3D interconnected

eloped solvothermal method and heating at 120 °C for 3 h under differentof the precursors, and cation surfactants. See also SI, Table 1.

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tunnel crystal phase and unique microspherical archi-tecture constructed from small nanoparticles. This architec-ture permits enhanced charge-transport efficiency throughthe electrodes during the charge/discharge process and leadsto better supercapacitive performance. The considerableimpact of the morphology on the electrochemical propertiesof MnO2 materials has been pointed out29; therefore, mucheffort has been expended in the controllable synthesis ofMnO2 materials with desired and uniform structures forenergy storage applications. In this paper, we also employedthe proposed one-pot solvothermal method for the successfulsynthesis of MnO2 with different architectures, includingnanoparticles, fibers, rods, and schists, by changing the ratioof 2-butanol and water and the nature of the precursors aswell as the cation surfactants, as shown in Fig. 6.

For testing the electrochemical performances ofthe spinel-type MS-MnO2 materials, galvanostaticcharge/discharge measurements and CV measurementswere carried out in 1-M Na2SO4 aqueous electrolytes bymeans of a conventional three-electrode system (Fig. 7).Broad redox peaks were obtained in the spinel-typeMS-MnO2 which illustrated that the intercalation/deintercalation of protons and electrolyte cations of Na1

ions into the bulk material accompanied by a Faradaic

FIG. 7. (a) Representative CV curves (third curve) of the as-prepared spaqueous solution using a three-electrode system with Ag/AgCl as the refecharge/discharge cycling tests recorded at a constant current density of 0.5 Acharge/discharge curves and (d) relationship of the SC and Coulombic efficmeasured at a current density of 1 A/g in 1-M Na2SO4 aqueous electrolyt


reaction occurred in the spinel-type MS-MnO2 materials[Fig. 7(a)]30; the Faradaic reaction can be a beneficialeffect by the combination of surface-based double-layercapacitance and volume-based “pseudocapacitance,” lead-ing to boost the energy current density of the 3D transport-pathway-based spinel-type MS-MnO2 electrode andresulting in improved capacitance performances. The SCvalues of the spinel-typeMS-MnO2were estimated throughgalvanostatic charge/discharge cycling at 0.5 A/g in 1-MNa2SO4 aqueous solution, as shown in Fig. 7(b). It can beclearly observed that the spinel-type MS-MnO2 exhibitsa good SC value (190 F/g); significantly, the obtained SCvalues are competitive with those of other reported spinel-type MnO2 materials, including the slightly truncatednanoparticles (21 F/g at 20 mV/s),31 interconnected nano-fibers (241 F/g at 5 mV/s),22 and particles with polyhedralshapes (53 F/g at 10 mA/cm2).32 The cycling stabilitycan be an area of concern for the practical applicationsof supercapacitor electrodes; therefore, an endurancetest at a constant current density of 1 A/g for 1000 cycleswas conducted for the as-synthesized spinel-type MnO2

electrode [Figs. 7(c) and 7(d)]. Almost 100% of theinitial capacitance was retained and the correspondingCoulombic efficiency remained significantly high at;95%

inel-type MS-MnO2 material at a scan rate of 5 mV/s in 1-M Na2SO4

rence electrode and Pt wire as the counter electrode. (b) Galvanostatic/g for MS-MnO2 in 1-MNa2SO4 aqueous electrolyte. (c) Galvanostaticiency with the number of cycles for the spinel-type MS-MnO2 electrodee.

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in the entire cycling process. Such results indicate that theproposed spinel-type MS-MnO2 material with 3D transporttunnels has the potential application as a supercapacitorelectrode that can be used in long charge/discharge cyclingwithout any significant microstructural modification orphase changes in the crystals of the MS-MnO2 materialand without any dissolution of the active material of theelectrode during the charge/discharge processes.

IV. CONCLUSION

In this paper, we have described the one-pot synthesisof a 3D network of spinel-type MS-MnO2 with the sizedistributions in the range of 0.5–4.5 lm; these microspheresare built from small MnO2 nanoparticles with average sizesof 5 nm assembled using the cationic surfactant as thetemplate. Such spinel-type MnO2 microspheres have beenfabricated for the first time by means of a solvothermalmethod in a 2-butanol/water solution under a basic condi-tion. CV and galvanostatic charge/discharge measurementswere carried out to characterize the fabricated MS-MnO2

material in the form of a supercapacitor electrode; its SCvalue was calculated to be ;190 F/g. Furthermore, ap-proximately 100% of the SC retention and approximately95% of the Coulombic efficiency were obtained after 1000charge/discharge cycles, exhibiting its excellent long-termstability. Significantly, the proposed solvothermal methodcan enable the morphological tuning of MnO2, which hasbeen successfully used to obtain architectures of schists,rods, fibers, and nanoparticles. Therefore, this method canpossibly offer an attractive technique toward synthesizingother metal oxides having the form of novel morphologiesthat can widen the range of its applications in biosensors,catalysis, fuel cells, and electronic nanodevices.

ACKNOWLEDGMENTS

We gratefully acknowledge the financial supportfrom National Science Council of Taiwan (Grant Nos.NSC-101-2113-M-005-014-MY3 and NSC 101-2628-M-007-006). We also appreciate Dr. Lih J. Chen atNational Tsing Hua University, Taiwan, for providingtechnical assistance in HRTEM measurements.

REFERENCES

1. F. Kim, S. Connor, H. Song, T. Kuykendall, and P.D. Yang:Platonic gold nanocrystals. Angew. Chem. Int. Ed. 43, 3673 (2004).

2. Z.W.Chen, Z. Jiao,D.Y. Pan, Z. Li,M.H.Wu,C.H. Shek, C.M.L.Wu,and J.K.L. Lai: Recent advances in manganese oxide nanocrystals:Fabrication, characterization, and microstructure. Chem. Rev. 112,3833 (2012).

3. J.E. Millstone, W. Wei, M.R. Jones, H.J. Yoo, and C.A. Mirkin:Iodide ions control seed-mediated growth of anisotropic goldnanoparticles. Nano Lett. 8, 2526 (2008).

4. W.Y. Ko, W.H. Chen, S.D. Tzeng, S. Gwo, and K.J. Lin: Synthesisof pyramidal copper nanoparticles on gold substrate. Chem. Mater.18, 6097 (2006).


5. W.Y. Ko, W.H. Chen, C.Y. Cheng, and K.J. Lin: Architecturalgrowth of Cu nanoparticles through electrodeposition. NanoscaleRes. Lett. 4, 1481 (2009).

6. J.Z. Chen, Y.C. Yen, W.Y. Ko, C.Y. Cheng, and K.J. Lin: The roleof the fabrication of anatase-TiO2 chain-networked photoanodes.Adv. Mater. 23, 3970 (2011).

7. J.Z. Chen, W.Y. Ko, Y.C. Yen, P.H. Chen, and K.J. Lin:Hydrothermally processed TiO2 nanowire electrodes withantireflective and electrochromic properties. ACS Nano 6, 6633(2012).

8. Y.G. Sun and Y.N. Xia: Shape-controlled synthesis of gold andsilver nanoparticles. Science 298, 2176 (2002).

9. H. Lee, S.E. Habas, S. Kweskin, D. Butcher, G.A. Somorjai, andP.D. Yang: Morphological control of catalytically active platinumnanocrystals. Angew. Chem. Int. Ed. 45, 7824 (2006).

10. G.H. Qiu, H. Huang, S. Dharmarathna, E. Benbow, L. Stafford, andS.L. Suib:Hydrothermal synthesis ofmanganese oxide nanomaterialsand their catalytic and electrochemical properties. Chem. Mater. 23,3892 (2011).

11. S.L. Brock, M. Sanabria, J. Nair, S.L. Suib, and T. Ressler:Tetraalkylammonium manganese oxide gels: Preparation, struc-ture, and ion-exchange properties. J. Phys. Chem. B 105, 5404(2001).

12. N. Pinna, M. Willinger, K. Weiss, J. Urban, and R. Schlogl: Localstructure of nanoscopic materials: V2O5 nanorods and nanowires.Nano Lett. 3, 1131 (2003).

13. G.J.D. Soler-illia, C. Sanchez, B. Lebeau, and J. Patarin: Chemicalstrategies to design textured materials: From microporous andmesoporous oxides to nanonetworks and hierarchical structures.Chem. Rev. 102, 4093 (2002).

14. T.D. Nguyen and T.O. Do: Solvo-hydrothermal approach for theshape-selective synthesis of vanadium oxide nanocrystals and theircharacterization. Langmuir 25, 5322 (2009).

15. X.K. Huang, D.P. Lv, H.J. Yue, A. Attia, and Y. Yang: Controllablesynthesis of alpha- and beta-MnO(2): Cationic effect on hydrothermalcrystallization. Nanotechnology 19, 225606 (2008).

16. L.C. Zhang, Z.H. Liu, H. Lv, X.H. Tang, and K. Ooi:Shape-controllable synthesis and electrochemical propertiesof nanostructured manganese oxides. J. Phys. Chem. C 111, 8418(2007).

17. J.H. Kim, T. Ayalasomayajula, V. Gona, and D. Choi: Fabricationand electrochemical characterization of a vertical array of MnO2

nanowires grown on silicon substrates as a cathode material forlithium rechargeable batteries. J. Power Sources 183, 366 (2008).

18. J.W. Lee, A.S. Hall, J-D. Kim, and T.E. Mallouk: A facile andtemplate-free hydrothermal synthesis ofMn3O4 nanorods on graphenesheets for supercapacitor electrodes with long cycle stability. Chem.Mater. 24, 1158 (2012).

19. W.F. Wei, X.W. Cui, W.X. Chen, and D.G. Ivey: Manganeseoxide-based materials as electrochemical supercapacitor electrodes.Chem. Soc. Rev. 40, 1697 (2011).

20. O. Ghodbane, J.L. Pascal, B. Fraisse, and F. Favier: Structural insitu study of the thermal behavior of manganese dioxide materials:Toward selected electrode materials for supercapacitors. ACS Appl.Mater. Interfaces 2, 3493 (2010).

21. J. Zhu, W. Shi, N. Xiao, X. Rui, H. Tan, X. Lu, H.H. Hng, J. Ma,and Q. Yan: Oxidation-etching preparation of MnO2 tubularnanostructures for high-performance supercapacitors. ACS Appl.Mater. Interfaces 4, 2769 (2012).

22. O. Ghodbane, J-L. Pascal, and F. Favier: Microstructural effectson charge-storage properties in MnO2-based electrochemicalsupercapacitors. ACS Appl. Mater. Interfaces 1, 1130 (2009).

23. Y.Wang, Q.S. Zhu, and L. Tao: Fabrication and growth mechanismof hierarchical porous Fe3O4 hollow sub-microspheres and theirmagnetic properties. CrystEngComm 13, 4652 (2011).

No. 1, Jan 14, 2014 113


24. H. Xia, J.K. Feng, H.L. Wang, M.O. Lai, and L. Lu: MnO2

nanotube and nanowire arrays by electrochemical deposition forsupercapacitors. J. Power Sources 195, 4410 (2010).

25. D. Portehault, S. Cassaignon, E. Baudrin, and J.P. Jolivet: Structuraland morphological control of manganese oxide nanoparticles uponsoft aqueous precipitation through MnO4

-/Mn21 reaction. J. Mater.

Chem. 19, 2407 (2009).26. K. Kai, Y. Kobayashi, Y. Yamada, K. Miyazaki, T. Abe,

Y. Uchimoto, and H. Kageyama: Electrochemical characterizationof single-layer MnO2 nanosheets as a high-capacitance pseudoca-pacitor electrode. J. Mater. Chem. 22, 14691 (2012).

27. S.L. Brock, M. Sanabria, S.L. Suib, V. Urban, P. Thiyagarajan, andD.I. Potter: Particle size control and self-assembly processes innovel colloids of nanocrystalline manganese oxide. J. Phys. Chem.B 103, 7416 (1999).

Supplementar

Supplementary materials can be viewed in this isvisiting http://journals.cambridge.org/jmr.


28. M.A. Camblor, A. Corma, and S. Valencia: Characterization ofnanocrystalline zeolite beta. Microporous Mesoporous Mater. 25,59 (1998).

29. S. Bach, M. Henry, N. Baffier, and J. Livage: Sol-gel synthesis ofmanganese oxides. J. Solid State Chem. 88, 325 (1990).

30. T. Brousse, M. Toupin, R. Dugas, L. Athouel, O. Crosnier, andD. Belanger: Crystalline MnO2 as possible alternatives to amorphouscompounds in electrochemical supercapacitors. J. Electrochem. Soc.153, A2171 (2006).

31. S. Devaraj and N. Munichandraiah: Effect of crystallographicstructure of MnO2 on its electrochemical capacitance properties.J. Phys. Chem. C 112, 4406 (2008).

32. Y. Xue, Y. Chen, M-L. Zhang, and Y-D. Yan: A new asymmetricsupercapacitor based on lambda-MnO2 and activated carbonelectrodes. Mater. Lett. 62, 3884 (2008).

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