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we present an approach of using GO?MnO2 nanocomposites as electrode material for supercapaci-best of our knowledge,few studies on the

of GO?MnO2nanocomposites have been

far.The electrochemical properties of as-nanocomposites with different mass ratios investigated,together with their individual com-(nano-MnO2and GO)and bulk MnO2for com-

Moreover,the possible formation mechanism

nanocomposites was derived.Most impor-

experiments suggest that the application of

solvent system plays a signi?cant role in an-needle-like MnO2onto GO sheets and that the

these crystals in turn is of great bene?t to fur-exfoliating GO.

AND DISCUSSION

XRD patterns and Raman spectra of GO and (chemically synthesized GO?MnO2nanocompos-the feeding ratio of MnO2/GO is3/1;the

MnO2is calculated by MnCl2according to the

2KMnO4?3MnCl2?2H2O?5MnO2?2KCl

presented in Figure1.The spectral features nanocomposites with other feeding ratios are simi-displayed in Figure1a,the most intensive peak of

around2??10.2°corresponds to the(001)re-

the interlayer spacing(0.87nm)was much

that of pristine graphite(0.34nm)due to introduction of oxygen-containing functional

the graphite sheets.34The diffraction peaks as-synthesized CMG3are similar to those of a nan-

phase of?-MnO2(JCPDS44-0141,a?

?2.8630?),where the(001)re?ection peak

GO has almost disappeared.Our results cor-

with the previous studies that the diffraction become weakened or even disappear whenever stacks of GO are exfoliated.35Moreover,

seldom differences between GO and CMG3

Raman spectra in the range of1200?1800 (Figure1b),indicating that GO was not reduced applied conditions and existed as a compo-CMG3.This conclusion is also supported by the XRD,Raman,and FTIR spectroscopies of GO and GOI(GO treated with isopropyl alcohol,

ods)in Figure1S(Supporting Information).

safe for us to derive that GO with its destroyed structure and?-MnO2in a tetragonal phase

the prepared nanocomposites.

The heterostructure of these nanocomposites be veri?ed by morphological analyses.Figure veals that GO sheets are multilayers with diameters a few micrometers,while nano-MnO2(Figure needle-like morphology with typical diameters

20?50nm and lengths of200?500nm.Figure

is the representative TEM images of as-synthesized nanocomposites(take CMG3for example).

XRD patterns(b)Raman spectra of GO and CMG3.

Figure2.(a,b)TEM images of GO and nano-MnO2;(c,d)

bright-?eld and dark-?eld images of CMG3;(e,f)the HRTEM

images of a MnO2nanoneedle.

Figure3.TEM images of CMG3samples collected at differ-

ent time intervals:(a)10s,(b)1min,(c)3min,(d)5min,(e) 10min,(f)30min.

clearly seen from Figure2c that the GO sheets have

been exfoliated and decorated randomly with the MnO2 nanoneedles;consistent with the XRD observations.In-terestingly,no obvious difference in the morphology of MnO2in Figure2b and c suggests a similar growth pro-cess for the crystals on and out of the GO supports.

Comparing the bright-?eld and dark-?eld TEM images

in Figure2c,d,the single layered graphene oxide sheet can be clearly distinguished from the background.Re-

markably,we?nd that some MnO2nanoneedles are

brighter than others which seem to be enveloped by a Figure4.(a)UV?vis spectra of CMG3samples collected at differ-ent time intervals;(b)plot of UV?vis peak position of CMG3 samples as a function of reaction time.

Figure5.(a)Raman spectra of CMG3samples collected at different time intervals;(b)plot of C/(D?G)peak area ratio of CMG3samples as a function of reaction time.

crystals are the most energetic ones,37and that

H2O molecules coordinate with MnO6octahedron

units more easily than isopropyl alcohol due to the polarity,steric hindrance,electrostatic effect,etc.45 Therefore,H2O molecules will interact with the O at-

oms of MnO6units in the(001)direction in priority.

With the addition of a smaller amount of water

(H2O/isopropyl alcohol?5:50mL),thermodynami-

cally only O atoms along the(001)direction would

bind with H2O molecules,whereas the ones in other directions would coordinate with isopropyl alcohol.

It should be noted that the coordination of H2O and isopropyl alcohol with the O atoms of MnO6has a discrep-

ant effect on the packing of the crystal growing species.

The intermolecular hydrogen bonds formed among the

H2O coordinated species make the species favorable to

form highly ordered aggregates;whereas a shortage of

such strong intermolecular interactions in isopropyl alco-

hol coordinated species leads the crystal stacking to be disordered.As a result,the overall system had only one preferable crystal growth direction,for the case with less

water(5mL)employed,yielding MnO2with a1D nanostructure.

Signi?cantly,the manner of adding KMnO4solution

exerts an in?uence on the formation and stacking of

the nuclei in the crystal growth process.A rapid intro-

duction of KMnO4solution causes the formation of a

large amount of growing species in a short time.

Dissolution?crystallization and oriented attachment

are two simultaneous processes in crystal growth,37and kinetically,the center of the primary nanorod has a

higher growing speed compared with the other direc-

tions,thus needle-like products are produced in the

end.

Moreover,as indicated by the results of UV?vis measurements,the exfoliation of GO sheets and the

growth of crystals are two simultaneous processes.

Thus,the growth of in situ formed crystals may also in

turn contribute to the destruction of regular layered GO

and the formation of exfoliated graphene

Furthermore,as reported by previous studies,

can also react with carbonaceous materials,

CNTs,active carbon,etc,forming manganese

carbon composites.46?48Thus controlled ex-periments using only GO and KMnO4as the precursors similar procedure were conducted.But few MnO2 nanoneedles can be observed(Figure2S,Supporting Information).On the other hand,nano-MnO2synthe-chemically in the absence of GO via a similar pro-

shows a needle-like morphology(Figure2b).So it is speculated that the formation of needle-like

may be attributable to the direct reaction between MnCl2·4H2O and KMnO4in the water?isopropyl

hol system as reported by our previous study.40 studies on the growth mechanism and the effects other reaction parameters(such as solvents,manga-nese precursors,concentrations,reaction time,

are still in progress.

To explore the potential applications of as-synthesized nanocomposites,the samples were

cated into supercapacitor electrodes and characterized with cyclic voltammograms(CVs)and galvanostatic

1.The formation mechanism for GO?MnO2nanocomposites.

charge/discharge measurements.CV response of as-synthesized samples carried out at a scan rate of

5mV·s?1in the potential range of0?1V using1M Na2SO4aqueous electrolyte solution is shown in Figure 6.The rectangular and symmetric CV curves of nano-MnO2indicate the ideal pseudocapacitive nature of the fabricated electrode,while the lack of symmetry in CMG0.5,CMG3,and CMG15is probably due to combined double-layer and pseudocapacitive contribution to the total capacitance.49Interestingly,with the feeding ra-tio of MnO2/GO increasing from0.5to15,CV plots of CMG ratio became more close to that of the nano-MnO2 (Figure6a),which can be attributed to the incremental pseudocapacitive contribution to the overall capacitance.

Moreover,the average areas of CV curves of these samples were calculated.Since C s is proportional to the average areas of CVs,50the results shown in Fig-ure6c indicate that C s of CMG ratio increases with the ratio of MnO/GO from0.5to10.Nevertheless,no

Of note is that C s of the nano-MnO2is much higher than that of its bulk counterpart,probably owing to the shape-and size-dependent properties appearing on the nanoscale.As reported by Devaraj et al.,28two factors are thought to make contribu-tions to the high C s:(1)higher speci?c area of nano-scale MnO2over the bulk one and(2)good align-ment of nanoneedles that can provide well-ordered tunnels,convenient for insertion/extraction of alkali cations into/from MnO2.

To get more information about the potential of as-synthesized GO?MnO2nanocomposites as electrode materials for supercapacitors,galvanostatic charge/dis-charge measurements were carried out in1M Na2SO4 between0and1V at a current density of200mA·g?1. As illustrated in Figure7a,during the charging and dis-charging steps,the charge curve of CMG15is almost symmetric to its corresponding discharge counterpart with a slight curvature,indicating the pseudocapacitive contribution along with the double layer contribution.

Figure6.CVs of(a)CMG0.5,CMG3,CMG15;(b)bulk MnO2,GO,CMG15,and nano-MnO2at5mV·s?1in1M Na2SO4solution;(c)plot of av-erage area of CV curves as a function of different ratios of CMG ratio.

Figure7.(a)Galvanostatic charge/discharge curves of CMG15,nano-MnO2,GO,and bulk MnO2at200mA·g?1;(b)galvanostatic charge/discharge curves of CMG15at150,200,500,and1000mA·g?1;(c)cycle life of CMG15and nano-MnO2at200mA·g?1in1M Na2SO4solution.