ARTICLE IN PRESS

0022-0248/$ - se

doi:10.1016/j.jc

�Corresponding, Central So

+86 731 887961

E-mail addr

Please cite thi

Journal of Crystal Growth ] (]]]]) ]]]–]]]

www.elsevier.com/locate/jcrysgro

Facile synthesis of a-MnO2 nanorods and theirelectrochemical performances

Hong-en Wanga, Dong Qiana,b,�, Zhouguang Lua, Yujie Lia, Ruijing Chenga, Wei Zhanga

aCollege of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR ChinabState Key Laboratory of Powder Metallurgy, Changsha 410083, PR China

Received 30 November 2006; received in revised form 9 January 2007; accepted 28 January 2007

Communicated by T.F. Kuech

Abstract

Single-crystalline a-MnO2 nanorods have been successfully prepared via in situ hydrothermal decomposition of single KMnO4 under

different acidic conditions. The as-synthesized products were characterized with X-ray powder diffraction (XRD), scanning electron

microscope (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), high-resolution transmission

electron microscope (HRTEM), and Fourier transformation infrared spectroscopy (FT-IR). The effects of various experimental

conditions on the phase structures and morphology of the final products were investigated. Formation mechanism for the MnO2

nanorods was discussed on the basis of the experimental results. The results show that the usage and kinds of acids used in the

experiments can influence the phase of the final products considerably. Phase-pure a-MnO2 nanorods can be obtained in a wide

temperature range from 100 to 150 1C. Electrochemical studies of the MnO2 materials prepared at 150 1C for different hydrothermal

times indicate that the MnO2 materials prepared for 5 and 8 h show fine capacitive behaviors and high specific capacitances of 83.9 and

79.3F/g, respectively.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.07.Bc

Keywords: A1. Crystal morphology; A1. One-dimensional structure; A2. Hydrothermal crystal growth; B1. Manganese oxide; B1. Nanomaterial

1. Introduction

In recent years, one-dimensional (1-D) nanomaterials,including nanowires, nanorods, and nanotubes, havereceived increasing interest for their superior optical,electrical, catalytic and magnetic properties, based on theirlow dimensionality and quantum confinement effect, whichcan be widely exploited as fundamental building blocks fornanoscience and nanodevices [1]. Therefore, much atten-tion has been paid to the fabrication of various 1-Dnanomaterials with alterable morphology, crystallographicforms and distinctive performances.

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.01.034

ing author. College of Chemistry and Chemical Engineer-

uth University, Changsha 410083, PR China. Tel./fax:

6.

ess: qiandong6@vip.sina.com (D. Qian).

s article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

MnO2 octahedral molecular sieves (OMS) materials mayexist in many kinds of polymorphs, such as a, b, g, e andd-type, according to different linking means of their basic[MnO6] octahedron units, which have gained increasingattention because of their unique structural characteristicsand extensive applications. For instance, MnO2 can beemployed as redox catalysts and molecular/ion sieves [2,3].In particular, MnO2 have been intensively investigated aspromising electrode materials in primary/secondary bat-teries and electrochemical capacitors due to their excellentelectrochemical performance, low-cost, nonpoisonous nat-ure, environmental friendliness and convenient preparation[4–7]. It has been documented that the electrochemicalproperties of MnO2 strongly depend upon the parameterssuch as particle morphology, phase structure and bulkdensity [8]. 1-D nanostructures are so far the smalleststructures for the efficient electron transport, which might

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESSH.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]2

provide the possibility of detecting the theoretical operat-ing limits of a Li-ion battery [9]. Therefore, it is ofsignificance to develop controllable synthesis approachesfor 1-D MnO2 nanomaterials and to explore their potentialapplications. Great efforts have been made on thefabrication of 1-D MnO2 nanostructures with variouspolymorphs, such as a-MnO2 nanorods/nanowires [10–12],b-MnO2 nanorods/nanotubes [13,14], g-MnO2 nanowires/nanotubes [15–17] and d-MnO2 nanofibers [18]. Amongthese reports, a common approach for the synthesis ofsingle-crystalline a-MnO2 nanorods was based on thehydrothermal reaction of MnSO4 and KMnO4, whichwas proposed by Wang and Li [10]. However, some minordifferences in the morphology of the final products havealso been observed when some specific reaction conditionswere slightly altered [19]. Therefore, the controlled synth-esis of 1-D a-MnO2 nanostructures with favorablemorphology, phase structure, crystallinity, and highreproducibility remains a challenge.

In this paper, a facile synthesis approach for thefabrication of single-crystalline a-MnO2 nanorods via in

situ hydrothermal decomposition of single KMnO4 undervarious acidic conditions has been proposed. The effects ofvarious experimental conditions on the morphology andphase structures were investigated. The formation mechan-ism of such nanorods was studied and discussed. Electro-chemical properties of the as-prepared nanorods aselectrodes in supercapacitors were also investigated withgalvanostatic charge/discharge tests and cyclic voltamme-try (CV) studies.

2. Experimental section

2.1. Synthesis

All the chemical reagents were analytically pure and usedwithout further purification. The hydrothermal reactionswere performed in a 36-mL Telfon-lined stainless steelautoclave under autogenous pressure. In a typical proce-dure, 2mmol KMnO4 and 1mL concentrated H2SO4 wereadded into 23mL distilled water under stirring to form asolution, which was then loaded into the autoclave, sealed,and heated in an oven at 150 1C for 12 h. After the reactionwas finished, the autoclave was taken out and coolednaturally to room temperature. The resulting brown-blackprecipitates were filtered, washed with distilled water forseveral times to remove excess ions, and finally dried at100 1C in air overnight.

2.2. Characterization

Phase structure and purity of the as-prepared productswere characterized by X-ray powder diffraction (XRD)taken on a Rigaku-D-Max rA 12 kW diffractometer withCu Ka radiation (l ¼ 1.54056 A) at an operation voltageand current of 40 kV and 300mA, respectively. Fouriertransformation infrared (FT-IR) spectrum was recorded on

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

a US Nicolet-5DX Fourier-IR spectrometer with KBr ascompressed slices. Scanning electron micrograph (SEM)was taken on a JEOL JSM-6360LV scanning electronmicroscope. Morphology and particle size of the productswere further observed with a Hitachi H-800 transmissionelectron microscope (TEM) and JEOL-3010 high-resolu-tion transmission electron microscope (HRTEM), operatedat an acceleration voltage of 200 kV. Chemical compositionof the as-obtained materials was analyzed by inductivelycoupled plasma emission spectrograph (ICP-ES), and theaverage oxidation state (AOS) of Mn in the manganeseoxides was determined by chemical titration with sodiumthiosulfate described in the literature [20]. Brunauer–Emmett–Teller (BET) surface area of the as-synthesizedmaterials was measured by nitrogen adsorption–desorptionmeasurements carried on a Micromeritics ASAP 2010apparatus at 77K. Prior to adsorption tests, the sampleswere outgassed under vacuum for 24 h at 150 1C.

2.3. Electrochemical measurement

The electrodes used for the evaluation of the electro-chemical properties were prepared by mixing the as-prepared MnO2 samples, acetylene black and PTFE latexwith a mass ratio of 85:10:5 to form a slurry. The slurrywas dried in a vacuum oven at 120 1C, and then filled into anickel mesh with an apparent area of 1� 1 cm2, which wasthen roll pressed to about 0.5mm thick and dried againovernight. The capacitor was assembled with the MnO2-positive electrode and MnO2-negative electrode separatedby a polypropylene separator. The masses of the positiveand the negative active materials are kept the same.Galvanostatic charge/discharge tests of the MnO2

capacitors were performed using a Land battery measure-ment system in a three-electrode system, where the MnO2

electrodes were used as the positive electrode and negativeelectrode, saturated calomel electrode (SCE) as thereference electrode. All the work was performed between0 and 1.4V at a specific current of 200mA/g in a 6M KOHelectrolyte at room temperature. Discharge-specific capa-citances of the capacitors were calculated according to theformula C ¼ q/mDV (where q is the charge delivered, DV isthe difference of discharge voltage, and m is the total massof active materials of the two electrodes). CV measure-ments were carried on a CHI-660 electrochemical work-station with a three-electrode configuration, in whichMnO2 electrode was used as the working electrode, Ptwire as the counter, and SCE as the reference. CV testswere done between 0 and 1.0V in a 6M KOH electrolyte atdifferent scan rates of 5, 10, and 20mV/s, respectively.

3. Results and discussion

3.1. Characterization of the a-MnO2 products

Fig. 1 depicts the XRD pattern and FT-IR spectrum ofthe resultant products. In Fig. 1a, all peaks in the XRD

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESS

Fig. 1. (a) XRD pattern and (b) FT-IR spectrum of the as-prepared

a-MnO2 products.

Fig. 2. SEM image of the as-prepared a-MnO2 products.

Fig. 3. (a) and (b) TEM images and SAED pattern (inset image in (b)) and

(c) HRTEM images of the as-obtained a-MnO2 products.

H.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] 3

pattern can be readily indexed to the pure tetragonal phaseof a-MnO2 (space group I4/m(87), JCPDS 44-0141) withlattice constant a ¼ 9.823 A and c ¼ 2.856 A. No peaks forother types are observed, indicating that the as-preparedproducts are of single-phase a-MnO2 materials. The sharpdiffraction peaks reveal that the products are perfectlycrystallized. The average crystallite size is estimated to be33.6 nm, according to the (2 1 1) reflection using theScherrer formula. In addition, it is noteworthy that theintensity ratio of (1 1 0) peak and (2 1 1) peak is 51.0%,much stronger than the standard value of PDF card(37.0%), suggesting that the MnO2 products might have apreferential growth direction along the [0 0 1]. Fig. 1bshows the FT-IR spectrum of the resulting samples. It isfound that it is relatively simple, which is ascribed to theirhighly structural symmetry. The bands appearing ataround 3416.6 and 1642.3 cm�1 correspond to the O–H

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

vibrating mode of traces of absorbed water. The bandslocated at ca. 709.9, 528.6, and 470.3 cm�1 that are below750 cm�1 can be ascribed to the Mn–O vibrations of[MnO6] octahedra in a-MnO2 nanorods. The FT-IRanalysis presented here is consistent with the resultsreported by Yang et al. [11].Fig. 2 shows the SEM image of the resulting a-MnO2

samples. It is clear that all the samples display fine fibrousmorphology. These a-MnO2 nanorods have diameters of30–80 nm and lengths up to several micrometers.Fig. 3 shows the TEM and HRTEM micrographs of the

as-prepared a-MnO2 products. In Fig. 3a, all the samplesare constructed of nanorods with diameters 30–70 nm andlengths up to 2.6 mm. Fig. 3b is a representative TEMimage of a single nanorod with uniform diameter. Asindexed in the SAED pattern taken from this rod (theinset image in Fig. 3b), a series of dots arraying in a singleline and perpendicular to the rod can be seen clearly,implying that it is a single-crystalline rod. The HRTEMimage in Fig. 3c demonstrates that the a-MnO2 nanorodhas uniform lattice fringes, indicating that it is astructurally uniform single crystal, which is in agreementwith the SAED pattern. The d-spacing is calculated to be0.69 nm, corresponding to the (1 1 0) plane of a-MnO2

(JCPDS 44-0141).The production yield of MnO2 is above 95% based on

Mn in the current experiments. The molar ratio of K/Mn inthe MnO2 products is ca. 0.15, determined by the ICP-ESanalysis. The AOS of Mn in the products is 3.84 according

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESSH.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]4

to the chemical titration analysis. BET surface area of theMnO2 products is 44.4m2/g, which is ca. 1.9 times of thedata reported in the literature [21]. However, the BETsurface area presented here is relatively low compared tothe result reported by Wei and co-workers [7]. Therelatively low surface area may further account for thatthe as-obtained product is well crystalline, which isconsistent with the previous XRD results.

3.2. Influence of experimental parameters on crystallinity

and morphology of the final products

It was reported that the reaction temperatures, pH valueand kinds of anions may all play important roles indetermining the crystal structures and morphologies of thefinal MnO2 products [21–24]. In this experiment, the effectsof reaction conditions, such as hydrothermal temperatures,amount of the acid used and kinds of acids, on thestructure and morphology of the final products werestudied in detail.

Fig. 4a shows the XRD spectra of the products obtainedin sulfuric solutions at different temperatures for 12 h. It isevident that the products prepared at 100, 120, and 150 1C,respectively, are all of pure a-MnO2. Besides, the diffrac-tion peaks become narrower and sharper along with theincrease of reaction temperatures, indicating that the

Fig. 4. XRD spectra of the MnO2 products obtained under various experime

temperatures (100, 120, and 150 1C, respectively); (b) hydrothermally prepared

(0.1, 1, and 2mL, respectively); (c) hydrothermally prepared at 150 1C for 12 h

respectively); (d) hydrothermally prepared at 150 1C for different times (1.5, 5

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

products possess higher crystallinity at higher hydrother-mal temperatures. For comparison, if this hydrothermalreaction was carried out at 80 1C under atmosphericpressure for 12 h, only a few black precipitates wereobtained as a result. Therefore, the a-MnO2 products canbe appropriately obtained in a temperature range from 100to 150 1C in the current experiments.Fig. 4b demonstrates the XRD patterns of the samples

derived from the addition of different amounts of sulfuricacid. It is found that single-phase a-MnO2 materials can beharvested with the addition of 1 or 2mL acid, respectively.However, a potassium manganese oxide hydrateK0.27MnO2 � 0.54H2O (space group R-3m(166), JCPDS52-0556) with rhombohedral structure was obtained whenthe amount of sulfuric acid used was 0.1mL. Hence, theamount of acid added does play a crucial role in the phaseof the resulting products.The effects of the kinds of acids on the final products

were also studied in our experiments. Three kinds of acids,namely, H2SO4, HNO3, and H3PO4, were used, respec-tively. The XRD results are shown in Fig. 4c, accordingly.From Fig. 4c, the diffraction peaks of the productsobtained using sulfuric acid and nitric acid can all bereadily indexed to pure a-MnO2 phase, and no peaks forother manganese oxides are detected. However, in contrast,the XRD pattern of samples synthesized with H3PO4

ntal conditions: (a) hydrothermally prepared for 12 h at different reaction

at 150 1C for 12 h with the addition of different volumes of sulfuric acid

with the addition of different kinds of acids (H2SO4, HNO3, and H3PO4,

, and 8 h, respectively).

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESSH.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] 5

exhibits obviously mixed phase structures. Apart from thecharacteristic diffraction peaks of a-MnO2, some diffrac-tion peaks for monoclinic MnPO4 �H2O (space group C2/c(15), JCPDS 44-0071) can also be observed clearly.Therefore, H2SO4 or HNO3 is believed to be necessary toobtain pure phase a-MnO2 products in the currentexperiment, which may oxidize the low-valence manganeseoxides species to Mn4+.

Fig. 5 shows the microscopic morphologies of theresulting products under various synthetic conditions.Fig. 5a and b displays the TEM and SAED patternof the samples obtained in H2SO4 at 100 1C for 12 h. InFig. 5a, the as-obtained samples are mainly composed ofnanorods (20–50 nm in diameter and 300–800 nm inlength). Besides, some nanorods had stacked together toform some agglomerates. Fig. 5b snatches a photo of anindividual nanorod with particle diameter of ca. 45 nm andlength of 700 nm. The SAED (inset in Fig. 5b) taken fromthis rod reveals it is a single crystal. In addition, it is foundthat the nanorod is unstable under electron irradiation, inwhich an amorphous layer (see the arrow) was formedunder electron beams for a long time observation in theTEM test. Fig. 5c depicts the micrograph of the productsobtained in HNO3. It can be seen that the product showsrod-like nanostructure similar to that of Fig. 5a. Fig. 5dshows a panoramic view of the samples obtained in H3PO4

solution. Different from the products derived from H2SO4

Fig. 5. TEM micrographs and SAED pattern of the products prepared

under various experimental conditions: (a) and (b) in H2SO4 at 100 1C for

12 h; (c) and (d) in HNO3 and H3PO4 at 100 1C for 12 h, respectively.

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

and HNO3 solutions, the resultant samples are composedof both nanorods and nanoagglomerates (see the arrow),which can be ascribed to their different phase structurespresented in the previous XRD result.

3.3. Possible formation mechanism of the a-MnO2 nanorods

In the present synthesis of MnO2 nanorods, the sourcematerials were merely KMnO4, no templates or surfactantswere involved through the whole experimental section. Toclarify the formation mechanism of such a-MnO2 nanor-ods, the growth processes of the products have been tracedcarefully by analyzing a series of samples obtained atdifferent growth stages. That is, after each reaction reachedthe set time, the autoclave was taken out of the oven andallowed to cool to room temperature naturally. Fig. 4ddepicts the XRD patterns of the samples taken fromdifferent reaction stages: (a) 1.5 h, (b) 5 h, and (c) 8 h. Themain phase of the products obtained in a short hydro-thermal dwell time of 1.5 h is tetragonal a-MnO2, and thecoexisting minor phase is Mn7O13 � 5H2O (JCPDS 23-1239)with orthorhombic structure. When the hydrothermal timeis prolonged to 5 h, the products are found to be composedof pure a-MnO2. In addition, the diffraction peaks becomesharper along with the prolongation of hydrothermal dwelltime, indicating the crystallinity of the products increaseswhen the reaction time is prolonged. The TEM micro-graphs and SAED patterns of the products hydrothermallysynthesized in 1.5 h are shown in Fig. 6a–c. It can be seenthat the products are mainly composed of flower-likenanostructures and some aggregates of nanorods. Inaddition, very few individual nanorods are also detected.The SAED pattern (inset image in Fig. 6c) taken from anindividual nanorod demonstrates that it is a single crystal.When the hydrothermal reaction was prolonged to 5 h, theproducts are all composed of nanorods, and no flower-likeaggregates are detected, as shown in Fig. 6d and e.However, it is worth noting that some nanorods havecongregated and stacked together to form some agglom-erates. Fig. 6f shows the TEM image of the productsobtained after hydrothermal reaction for 8 h. It is evidentthat the products show fine rod-like morphology with theprolongation of hydrothermal treatment.On the basis of the experimental results, the formation

process of the nanorods may be outlined as follows: (1)KMnO4 is not stable in an acidic solution under ahydrothermal environment, which will decompose intoMnO2 and O2 quickly. (2) The produced MnO2 colloidstend to form agglomerates firstly due to their highconcentration and surface energies. Meantime, minorcolloids might also grow into nanorods because of their1-D growth habits under the specific reaction condition. (3)As the reaction proceeded, the reaction rate was sloweddown with the decrease of the reactants. After the KMnO4

was consumed completely, the whole reaction system wouldthus tend to be a thermodynamically stable environment.Then, a subsequent dissolution–recrystallization process

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESS

Fig. 6. TEM images and SAED patterns of the products prepared at different hydrothermal reaction times: (a–c) 1.5 h; (d, e) 5 h; (f) 8 h.

H.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]6

should be involved, in which the amorphous components inthe agglomerates would dissolve again and grow intonanorods. This process should obey the well-known‘‘Ostwald ripening process’’, in which the larger nanorodsgrow at the cost of the small ones because of their energydifferences.

3.4. Electrochemical properties of the as-prepared MnO2

nanorods

Electrochemical properties of the MnO2 samples pre-pared in H2SO4 solution at 150 1C for 1.5, 5, 8, and 12 hwere investigated using galvanostatic charge/discharge andCV tests, respectively. Fig. 7a shows the charge/dischargecurves of the MnO2 capacitors in 6M KOH electrolytes.All the capacitors were charged to 1.4V and discharged to0V with charge/discharge-specific current of 200mA/g(based on the mass of a single electrode active material). Itis seen that all the charge/discharge profiles display finecapacitive behaviors. In addition, the charge duration anddischarge duration for the samples prepared at 150 1C for 5and 8 h are longer than that of the samples prepared for 1.5and 12 h, indicating the charge/discharge capacity of theformers should be higher than that of the latter. Thedischarge-specific capacitances of the MnO2 materialsprepared at 1.5, 5, 8, and 12 h are 74.8, 83.9, 79.3, and64.6 F/g, respectively.

Cyclic voltammograms (CV) recorded at different scanrates in the 6M KOH electrolyte for MnO2 samplesprepared at 150 1C for 8 h are shown in Fig. 7b. It is

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

observed that the CV curve at a scan rate of 5mV/s exhibitsan ideal capacitive behavior with fine rectangular shape. Inaddition, the CV curves deviate from the rectangularshape to some extent with the increase of scan rates.Two mechanisms have been proposed to account for thecharge storage in MnO2-based capacitors. One is basedon the concept of the intercalation of proton or alkalimetal cations such as Na+ in the electrode duringreduction process and deintercalation on oxidationprocess [25]. The other is based on the adsorption ofcations in the electrolyte on the electrode [26]. In thecurrent CV tests, the redox reaction of MnO2 electrodein the KOH electrolyte is generally recognized as the so-called proton–electron mechanism due to the bigger radiusof K+. The increase of the scan rate will have aconsiderable effect on the diffusion of protons into theMnO2 matrix, which would influence the capacitivebehavior accordingly.Fig. 8 shows the cycling stability of the MnO2 electrodes

prepared at 150 1C for different hydrothermal times over800 cycles. It is seen that the samples prepared at 150 1C for5 and 8 h have high specific capacitances, compared to thesamples obtained for 1.5 and 12 h, respectively. Thevariation of specific capacitance for MnO2 productsprepared from different times may be ascribed to theintegration of their specific structural characteristics, suchas crystallinity, morphology, K+ existed in the cavity andspecific surface area. In addition, the specific capacitance ofthe products prepared for 1.5 h is unstable, which might bedue to their various morphologies.

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESS

Fig. 8. Specific capacitances of MnO2 products prepared at 150 1C for 1.5,

5, 8, and 12 h over the first 800 cycles, respectively.

0 300 600 900 1200 1500 1800 2100 2400

0.0

0.3

0.6

0.9

1.2

1.5

5 h

8 h

12 h

1.5 h

Voltage (

V v

s.

SC

E)

Time (s)

0.0 0.2 0.4 0.6 0.8 1.0

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

Voltage (V vs. SCE)

Cu

rre

nt (A

)

20 mV/s

10 mV/s

5 mV/s

b

Fig. 7. (a) Typical charge/discharge cycling of the MnO2 products

prepared at 150 1C for 1.5, 5, 8, and 12 h, respectively, at a specific

current of 200mA/g; (b) CV curves of MnO2 materials prepared for 8 h at

5, 10, and 20mV/s, respectively.

H.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] 7

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

4. Conclusions

In conclusion, single-crystalline a-MnO2 nanorodshave been successfully synthesized via a facile hydrother-mal approach involving no templates and surfactants.The kinds and usage of acids play important roles inthe phase structures of the final products. Single-phasea-MnO2 products can be obtained in a wide temperaturerange from 100 to 150 1C in current experiments. Electro-chemical studies indicate that the electrodes made ofMnO2 materials prepared for different hydrothermal timesshow fine capacitive behaviors. The MnO2 prepared for5 and 8 h have high specific capacitances of 83.9 and 79.3F/g, respectively, which are higher than that preparedfor 1.5 and 12 h, respectively. This simple syntheticapproach may provide a convenient route for the fabrica-tion of a-MnO2 nanorods with alterable diameter andlength and other cation-doped MnO2 1-D nanomaterialson a large scale. Moreover, these nanorods may findpotential applications in catalysts, batteries, capacitors,and so on.

Acknowledgments

This work was supported by the opening subject ofState Key Laboratory of Powder Metallurgy (No.200506123105A). The authors also thank Dr. L.Y. Wu ofHunan University for TEM and HRTEM tests.

References

[1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin,

F. Kim, H. Yan, Adv. Mater. 15 (2003) 353.

[2] Y.S. Ding, X.F. Shen, S. Sithambaram, S. Gomez, R. Kumar,

V.M.B. Crisostomo, S.L. Suib, M. Aindow, Chem. Mater. 17 (2005)

5382.

[3] Y. Tanaka, M. Tsuji, Y. Tamaura, Phys. Chem. Chem. Phys. 2 (2000)

1473.

[4] Y.S. Horn, S.A. Hackney, C.S. Johnsom, M.M. Thackeray,

J. Electrochem. Soc. 145 (1998) 582.

[5] R.N. Reddy, R.G. Reddy, J. Power Sources 132 (2004) 315.

[6] M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184.

[7] V. Subramanian, H. Zhu, R. Vajtai, P.M. Ajayan, B. Wei, J. Phys.

Chem. B 109 (2005) 20207.

[8] S. Bach, M. Henry, N. Baffer, J. Livage, J. Solid State Chem. 88

(1990) 325.

[9] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind,

E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.

[10] X. Wang, Y.D. Li, Chem. Commun. 124 (2002) 764.

[11] R. Yang, Z. Wang, L. Dai, L. Chen, Mater. Chem. Phys. 93 (2005)

149.

[12] M. Wei, Y. Konishi, H. Zhou, H. Sugihara, H. Arakawa,

Nanotechnology 16 (2005) 245.

[13] X. Liu, S. Fu, C. Huang, Powder Technol. 154 (2005) 120.

[14] D. Zheng, S. Sun, W. Fan, H. Yu, C. Fan, G. Cao, Z. Yin, X. Song,

J. Phys. Chem. B 109 (2005) 16439.

[15] Y. Xiong, Y. Xie, Z. Li, C. Wu, Chem. Eur. J. 9 (2003) 1645.

[16] Z. Yuan, Z. Zhang, G. Du, T. Ren, B. Su, Chem. Phys. Lett. 378

(2003) 349.

[17] F. Cheng, J. Chen, X. Gou, P. Shen, Adv. Mater. 17 (2005) 2753.

[18] F.A. Al-Sagheer, M.I. Zaki, Colloids Surf. A 173 (2000) 193.

oi:10.1016/j.jcrysgro.2007.01.034

ARTICLE IN PRESSH.-en. Wang et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]8

[19] F. Cheng, J. Zhao, W. Song, C. Li, H. Ma, J. Chen, P. Shen, Inorg.

Chem. 45 (2006) 2038.

[20] Y. Shen, S.L. Suib, C.L. O’Young, J. Am. Chem. Soc. 116 (1994) 11020.

[21] N. Kijima, H. Yasuda, T. Sato, Y. Yoshimura, J. Solid State Chem.

159 (2001) 94.

[22] T. Xiao, P.R. Strutt, M. Benaissa, H. Chen, B.H. Kear, Nanostruct.

Mater. 10 (1998) 1051.

Please cite this article as: Hong-en Wang, et al., J. Crystal Growth (2007), d

[23] R.N. DeGuzman, Y. Shen, E.J. Neth, S.L. Suib, C.L. O’Young,

S. Levine, J.M. Newsam, Chem. Mater. 6 (1994) 815.

[24] X. Shen, Y. Ding, J. Liu, J. Cai, K. Laubernds, R.P. Zerger,

A. Vasiliev, M. Aindow, S.L. Suib, Adv. Mater. 17 (2005) 805.

[25] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147

(2000) 444.

[26] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 144 (1999) 220.

oi:10.1016/j.jcrysgro.2007.01.034