Single-crystal α-MnO2 nanorods: synthesis and electrochemical properties

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Nanotechnology 18 (2007) 115616 (5pp) doi:10.1088/0957-4484/18/11/115616

Single-crystal α-MnO2 nanorods:synthesis and electrochemical propertiesHongen Wang1,2,3, Zhouguang Lu1, Dong Qian1,2,3, Yujie Li1 andWei Zhang1

1 College of Chemistry and Chemical Engineering, Central South University,Changsha 410083, People’s Republic of China2 State Key Laboratory of Powder Metallurgy, Changsha 410083, People’s Republic of China

E-mail: hong enwang@yahoo.com.cn and qiandong6@yahoo.com.cn

Received 13 November 2006, in final form 12 January 2007Published 14 February 2007Online at stacks.iop.org/Nano/18/115616

AbstractSingle-crystal α-MnO2 nanorods were prepared by hydrothermal reaction ofsingle KMnO4 under acidic conditions. The nanorods have a diameter of30–70 nm and a length up to 2 μm. The formation mechanism for theα-MnO2 nanorods was investigated. Electrochemical properties of the MnO2nanomaterials prepared for different hydrothermal times were characterizedby galvanostatic charge/discharge tests and cyclic voltammetry (CV) studies.The results indicate that the MnO2 nanorods prepared for 5 and 8 h show finecapacitive behaviour with high specific capacitances of 71.1 and 70.9 F g−1,respectively.

1. Introduction

In recent years, one-dimensional (1D) nanostructures, includ-ing nanowires, nanorods and nanotubes, have attracted increas-ing attention due to their superior optical, electrical, catalyticand magnetic properties, which can be extensively explored asfundamental building blocks for nanoscale science and tech-nology [1]. Therefore, much effort has been devoted to thefabrication of various 1D nanomaterials with alterable mor-phology, phase and distinctive performances.

MnO2 has many kinds of polymorphs, such as α, β ,γ and δ-type according to different linkage ways of itsbasic MnO6 octahedral units. MnO2 has wide applicationssuch as redox catalysts [2], molecular/ion sieves [3] andelectrode materials in batteries or capacitors [4–7]. Recently,increasing interest has been focused on the fabrication of1D MnO2 nanostructures. Various 1D MnO2 nanomaterials,such as α-MnO2 nanorods/nanowires [8–10], β-MnO2

nanorods/nanotubes [11, 12], γ -MnO2 nanowires/nanotubes[13–15] and δ-MnO2 nanofibres [16] have been successfullyprepared by oxidation of Mn2+ or reduction of KMnO4.In addition, single-crystal MnO2 nanowires of α, β andγ -type have also been conveniently prepared by simplehydrothermal treatment of commercial γ -MnO2 particles inwater or ammonia solution [17, 18]. Further, it is found

3 Authors to whom any correspondence should be addressed.

that these 1D MnO2 nanomaterials show finer electrochemicalproperties than their bulk counterparts, which can be adoptedas promising electrode materials in Zn–MnO2 batteries, Li–MnO2 cells and electrochemical capacitors, etc [7, 9, 14, 19].

In this paper, we report the preparation of single-crystalα-MnO2 nanorods by direct hydrothermal decomposition ofsingle KMnO4 under acidic conditions involving no templatesor surfactants. The formation mechanism of such nanorodswas studied by tracing their growth process at different reactionstages. Electrochemical properties of the as-prepared nanorodsas electrode materials in electrochemical capacitors werealso investigated with galvanostatic charge/discharge tests andcyclic voltammetry (CV) studies. The results show that theseMnO2 nanorods display ideal capacitive behaviour, in whichthe MnO2 nanomaterials prepared for 5 and 8 h have highspecific capacitances of 71.1 and 70.9 F g−1, respectively.

2. Experimental details

All the chemical reagents were analytical grade and usedwithout further purification. All the hydrothermal reactionswere performed in a 36 ml Teflon-lined stainless steelautoclave under autogenous pressure. In a typical synthesis,2 mmol KMnO4 and 1 ml concentrated H2SO4 were added to23 ml distilled water under stirring to form a solution, whichwas then loaded into the autoclave, sealed and heated in an

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Nanotechnology 18 (2007) 115616 H Wang et al

oven at 150 ◦C for 12 h. After the reaction was complete,the autoclave was taken out and naturally cooled to roomtemperature. The resulting brown-black precipitates werefiltered off, washed with distilled water several times to removeexcess ions, and finally dried at 120 ◦C in air overnight.

X-ray powder diffraction (XRD) was taken on a Rigaku-D-Max rA 12 kW Diffractometer with Cu Kα radiation(λ = 0.154 06 nm) at an operation voltage and currentof 40 kV and 300 mA, respectively. The Fouriertransform infrared (FT-IR) spectrum was recorded on aUS Nicolet-5DX Fourier-IR spectrometer with KBr pellets.Transmission electron micrographs (TEM), selected areaelectron diffraction (SAED) and high-resolution transmissionelectron micrographs (HRTEM) were obtained from aHitachi H-800 transmission electron microscope and aJEOL-3010 high-resolution transmission electron microscope,respectively. Chemical composition of the as-obtainedmaterials was analysed by an inductively coupled plasmaemission spectrograph (ICP-ES). Average oxidation state ofMn in the final products was determined by chemical titrationwith sodium thiosulfate as described elsewhere [20].

The electrodes used for the electrochemical measurementswere prepared by mixing the as-prepared MnO2 samples,acetylene black and PTFE latex with a mass ratio of 80:10:10to form a slurry. The slurry was dried at 120 ◦C, andthen filled into a Ni mesh, which was then roll pressed anddried again. The capacitor was assembled with the MnO2

positive electrode and MnO2 negative electrode, separated by apolypropylene separator. The masses of the positive-electrodeand the negative-electrode active materials are kept the same.

Galvanostatic charge/discharge tests of the MnO2 capac-itors were performed using a Land battery test system in atwo-electrode system, in which MnO2 electrodes were used asthe positive electrode and the negative electrode. All the testswere performed between 0 and 1.0 V at a specific current of300 mA g−1 in a 0.5 M Na2SO4 electrolyte at room temper-ature. Discharge specific capacitances of the capacitors werecalculated according to the formula C = I t/m�V (where I isthe discharge current, t is the discharge time, �V is the differ-ence in discharge voltage and m is the total mass of active ma-terials of the two electrodes). Cyclic voltammetry (CV) stud-ies were carried out on a CHI-660 electrochemical workstationwith a three-electrode configuration, in which a MnO2 elec-trode was used as the working electrode, Pt wire as the counterand a saturated calomel electrode (SCE) as the reference. CVtests were done between −0.2 and 1.0 V in a 0.5 M Na2SO4

electrolyte at different scan rates of 5, 10 and 20 mV s−1, re-spectively.

3. Results and discussion

Figure 1 shows the XRD pattern of the resultant MnO2 product.All the peaks can be indexed to the pure tetragonal phaseof α-MnO2 (JCPDS No. 44-0141) with lattice constantsof a = 0.982 nm and c = 0.286 nm. No peaks forother types of manganese oxides are observed, indicating thatthe as-prepared products are phase-pure α-MnO2 nanorods.The sharp diffraction peaks reveal that the products are wellcrystalline. In addition, it is worth noting that the relativeintensity of the (110) peak (51.0%) is much stronger than that

Figure 1. Typical XRD pattern of the as-prepared MnO2 products.

of the standard JCPDS card (37.0%). Likewise, the relativeintensity of the (220) peak (7.5%) is 1.5 times of the value fromthe standard JCPDS card. However, the relative intensities ofother diffraction peaks are basically comparable to those of thestandard JCPDS card. Thus, these MnO2 nanorods might havea preferential growth along the [001] direction.

Figure 2(a) gives a panoramic TEM image of the as-obtained α-MnO2 samples. It is found that all the MnO2

samples are composed of nanorods with diameters 30–70 nmand lengths up to 2 μm. Figure 2(b) shows a representativeTEM micrograph of a single nanorod with uniform diameter.As indexed in the SAED pattern taken from this rod (insetimage of figure 2(b)), a series of dots arrayed in a single lineand perpendicular to the rod can be seen clearly, implyingthat it is a single-crystal rod. The HRTEM image of part ofan individual rod demonstrates that the nanorod has uniformlattice fringes as shown in figure 2(c). The lattice spacing iscalculated to be 0.69 nm (figure 2(d)), which corresponds tothe (110) plane of α-MnO2 (JCPDS No. 44-0141).

Figure 3 shows the FT-IR spectrum of the resultingnanorods. The bands at around 3416.6 and 1642.3 cm−1

correspond to the O–H vibrating mode of traces of absorbedwater. The bands at about 709.9, 528.6 and 470.3 cm−1 thatare below 750 cm−1 can be attributed to the Mn–O vibrationsof MnO6 octahedra in α-MnO2 nanorods. The relatively simpleFT-IR pattern is ascribed to the highly structural symmetry ofα-MnO2 nanorods. The IR analysis here is consistent with theresults reported in the literature [10].

The production yield of MnO2 is repeatedly above 95%based on Mn in our experiments. The molar ratio of K/Mnin the MnO2 products is about 0.15 determined by ICP-ESanalysis. The average oxidation state of Mn in the productsis 3.84 according to the chemical titration analysis.

In our synthesis of MnO2 nanorods, the source materialswere merely KMnO4; no templates or surfactants were used.To investigate the formation process of α-MnO2 nanorods,a series of samples were prepared by hydrothermal reactionfor different times. Figure 4 shows the XRD patterns ofthe samples hydrothermally prepared for 1.5, 5, 8 and 12 h,respectively. Clearly, the main reflection peaks of the productsprepared for 1.5 h can be indexable to the α-MnO2 phase. Inaddition, a small amount of Mn7O13·5H2O (JCPDS No. 23-1239) can also be observed. When the hydrothermal time

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Nanotechnology 18 (2007) 115616 H Wang et al

Figure 2. ((a), (b)) TEM images of the as-prepared α-MnO2 nanorods; the inset shows the corresponding SAED pattern. ((c), (d)) HRTEMimages of the α-MnO2 nanorods.

Figure 3. FT-IR spectrum of the as-synthesized α-MnO2 nanorods.

is increased to 5 h, the products are composed of single-phase α-MnO2, and no peaks for other manganese oxidesare detected any more. Meanwhile, the diffraction peaksbecome narrower and sharper along with the prolongation ofhydrothermal time, indicating the crystallinity of the productsenhances with increased reaction time. TEM micrographs ofthe products hydrothermally synthesized at different times areshown in figure 5. Figures 5(a)–(c) show the TEM images ofthe samples hydrothermally prepared for 1.5 h. We can see thatthe products are composed of flower-like nanoarchitecturesand some aggregates of nanorods. In addition, few individualnanorods are also detected. The SAED pattern taken from an

Figure 4. XRD patterns of the manganese oxides prepared atdifferent hydrothermal times.

individual nanorod (inset image of figure 5(c)) demonstratesthat it is a single crystal, though it is not very uniform indiameter. When the hydrothermal reaction was prolongedto 5 h, the products are all composed of nanorods, and noflower-like aggregates are detected, as shown in figures 5(d)and (e). Besides, some nanorods have congregated and stackedtogether. Figure 5(f) shows the TEM image of the productsobtained after hydrothermal reaction for 8 h. It is evidentthat the products exhibit fine rodlike morphology with theprolongation of the hydrothermal treatment.

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Nanotechnology 18 (2007) 115616 H Wang et al

Figure 5. TEM images and SAED patterns of the resulting manganese oxides prepared at different hydrothermal times. ((a)–(c)) 1.5 h;((d), (e)) 5 h; (f) 8 h.

Based on the above experimental results, the formationmechanism of these nanorods is deduced as follows. Firstly,KMnO4 is generally unstable in acidic conditions, whichwould decompose into MnO2 and O2. This decompositionreaction would be accelerated considerably under a sealedhydrothermal environment, resulting in a large quantity ofMnO2 colloids. Next, the produced MnO2 colloids tendedto form some larger agglomerates due to their relativelyhigh concentration and surface energies. Meantime, minorcolloids might grow into a few small nanorods becauseof their 1D growth habits under the specific experimentalconditions. As the reaction proceeded, the reaction ratewould be slowed down due to the decrease of the reactants.After the KMnO4 was consumed completely, the wholesystem would thus tend to be thermodynamically stable.Subsequently, a dissolution–recrystallization process shouldbe involved. That is, the amorphous and poorly crystallinecomponents in the agglomerates would dissolve again andgrow into nanorods. This process obeyed the well-known‘Ostwald ripening process’, in which the larger nanorodsgrew at the cost of the small ones because of their energydifferences. The growth process of the α-MnO2 nanorodspresented here is somewhat different from those reported inthe literature [18, 21].

Figure 6(a) shows the charge/discharge curves of theMnO2 capacitors in 0.5 M Na2SO4 electrolytes. All thecapacitors were charged to 1.0 V and discharged to 0 V at acharge/discharge specific current of 300 mA g−1 (based on themass of a single-electrode active material). It is seen that allthe charge/discharge curves show ideal capacitive behaviour.In addition, the charge duration and discharge duration for the

Figure 6. (a) Typical charge/discharge curves of manganese oxidesprepared at 1.5, 5, 8 and 12 h, respectively. (b) CV curves of theMnO2 nanorods prepared for 8 h at scan rates of 5, 10 and20 mV s−1, respectively.

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Nanotechnology 18 (2007) 115616 H Wang et al

samples prepared for 5 and 8 h are longer than those preparedfor 1.5 and 12 h, indicating the charge/discharge capacitancesof the former are higher than those of the latter. The dischargespecific capacitances of the MnO2 electrode materials preparedfor 1.5, 5, 8 and 12 h are 66.3, 71.1, 70.9, and 56.3 F g−1,respectively.

Cyclic voltammograms (CV) recorded at different scanrates in a 0.5 M Na2SO4 electrolyte for the MnO2 samplesprepared at 8 h are shown in figure 6(b). We can see thatthe CV curve at a scan rate of 5 mV s−1 exhibits an idealcapacitive behaviour with fine rectangular profiles. In addition,the CV curves deviate from the rectangular shape to someextent with the increase of scan rates. There have beentwo mechanisms proposed to account for the charge storagein MnO2-based capacitors. One is based on the conceptof the intercalation of proton or alkali metal cations suchas Na+ in the electrode during the reduction process anddeintercalation on the oxidation process [22]. The other isbased on the adsorption of cations in the electrolyte on theelectrode [23]. In current CV tests, the redox process shouldbe mainly accomplished by the intercalation/deintercalationof Na+ and H+ from the electrolyte into the porous MnO2

electrodes. Therefore, the increase of the scan rate will havea considerable effect on the diffusion of Na+ ions into theMnO2 matrix. That is to say, the faster scan rates will leadto the Na+ ions accessing only the outer surfaces of MnO2

electrodes instead of the interior cavities of the MnO2 matrix,which would significantly reduce the available capacitance.The specific capacitance of the MnO2 electrode according tothe CV curves is calculated to be 79.8 F g−1 at a scan rate of5 mV s−1. When the scan rates were increased to 10 mV s−1

and 20 mV s−1, the specific capacitances would be decreasedto 65 F g−1 and 51 F g−1, respectively. The differenceof specific capacitance for the MnO2 products prepared atdifferent times should be ascribed to the integration of theirspecific structural characteristics, such as morphology, particlesize, specific surface area and K+ concentration in the MnO2

matrix. In addition, the experimental parameters, such ascharge–discharge current rates and scan rates in CV, will alsohave a considerable influence on their specific capacitance.Further studies on the relationship between their structuralcharacteristics and electrochemical properties are still underway.

4. Conclusion

In conclusion, single-crystal α-MnO2 nanorods were preparedvia a facile hydrothermal approach involving no templatesand surfactants. Based on our experimental results, adissolution–recrystallization followed by an ‘Ostwald ripeningprocess’ should be responsible for the formation of nanorods.Electrochemical studies indicate that the MnO2 materials

show ideal capacitive behaviour, in which the MnO2 samplesprepared after 5 and 8 h have high specific capacitances of 71.1and 70.9 F g−1, respectively. This simple synthetic approachmay provide a convenient route for the fabrication of α-MnO2

nanorods with alterable particle size and other cation-dopedMnO2 1D nanomaterials on a large scale. Further, theseMnO2 nanorods may find potential applications in catalysis andelectrochemical power sources.

Acknowledgments

This work was supported by the opening subject of State KeyLaboratory of Powder Metallurgy (No. 200506123105A). Theauthors also thank Dr LY Wu of Hunan University for TEM andHRTEM tests.

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