Journal of Solid State Chemistry 179(2006)1757–1761
A simple method of fabricating large-area a -MnO 2
nanowires and nanorods
Yi Liu a,b ,Meng Zhang a ,Junhao Zhang a ,Yitai Qian a,Ã
a
Hefei National Laboratory for Physical Sciences at Microscale,Department of Chemistry,University of Science &Technology of China,
Hefei 230026,PR China
b
Department of Chemistry,Zaozhuang University,Zaozhuang 277100,PR China
Received 20December 2005;received in revised form 14February 2006;accepted 17February 2006
Available online 6March 2006
Abstract
a -MnO 2nanowires or nanorods have been selectively synthesized via the hydrothermal method in nitric acid condition.The a -MnO 2nanowires hold with average diameter of 50nm and lengths ranging between 10and 40m m,using MnSO 4ÁH 2O as manganese source;meanwhile,a -MnO 2bifurcate nan
orods with average diameter of 100nm were obtained by adopting MnCO 3as starting material.The morphology of a -MnO 2bifurcate nanorods is the first one to be reported in this paper.X-ray powder diffraction (XRD),field scanning electron microscopy (FESEM),transmission electron microscopy (TEM),selected area electron diffraction (SAED)and high-resolution transmission electron microscopy (HRTEM)were used to characterize the products.Experimental results indicate that the concentrated nitric acid plays a crucial role in the phase purity and morphologies of the products.The possible formation mechanism of a -MnO 2nanowires and nanorods has been discussed.r 2006Elsevier Inc.All rights reserved.
Keywords:a -MnO 2;Nanowires and nanorods;Hydrothermal reaction;X-ray powder diffraction (XRD);Field scanning electron microscopy (FESEM);Transmission electron microscope (TEM);Selected area electron diffraction (SAED);High-resolution transmission electron microscope (HRTEM)
1.Introduction
In the past few years,controlling the shape of nanostructures at the mesoscopic level is one of challenging issues presently faced by material scientists [1].Nanowires and nanorods,which are one-di
mensional (1-D)objects,have stimulated great interest among synthetic material operators due to their peculiar properties and potential application [2–9].Several techniques for the preparation of nanowires or nanorods have been reported,such as the solid–vapor process [2],laser ablation [3],arc discharge [4],electrochemical techniques [5],virus-templating [6],exfo-liating method [7,8],and hydrothermal method [9].As a popular inorganic-function material,manganese dioxide and derivative compound have attracted special attention and been widely used not only as catalysts,molecular sieves
[10,11],but also as promising candidate materials for cathodes in lithium ion batteries [12–15].Generally speak-ing,a -and g -MnO 2can be converted by electrochemical Li +intercalation into cubic spinel,Li 1Àx Mn 2O 4,which has channels through which Li +can move [13,14].Recently,many efforts have been focused on preparing manganese oxide 1-D nanostructures,and their synthesis methods are generally based on the redox reactions of MnO 4Àand/or
Mn 2+
reaction to a book or an article
[16–23].For example:Y.D.Li et al.[20,21]reported a selected-control low-temperature hydrothermal method of synthesizing 1-D MnO 2nanostructure through the
oxidation of Mn 2+by S 2O 82À,MnO 4À
or ClO
Àwithout any existence of catalysts or templates;Z.Q.Li et al.[22]provided a simple room-temperature solution-based cata-lytic route to fabricate a novel hierarchical structure of a -MnO 2core-shell spheres with spherically aligned nanor-ods on a large scale.The previous experimental results indicated that a -MnO 2tended to form in acidic conditions,the pH of solution had crucial effect on the formation of 1-D nanostructural a -MnO 2[23].The influence of the
www.elsevier/locate/jssc
0022-4596/$-see front matter r 2006Elsevier Inc.All rights reserved.doi:10.1016/j.jssc.2006.02.028
ÃCorresponding author.Fax:+865513607402.
E-mail addresses:liuyi67@mail.ustc.edu (Y.Liu),ytqian@ustc.edu (Y.Qian).
anion on growth of the products had been investigated by Kijima et al.[19],and their results showed that a -MnO 2could be prepared in concentrated H 2SO 4rather than HCl or HNO 3.Thus far,the synthesis of a -MnO 2nanowires or nanorods has seldom been reported under concentrated nitric acidic conditions.
Here we report a novel,large-area synthesis method for obtaining nanowires and nanorods with uniform sizes.The a -MnO 2nanowires have average diameter of 50nm and lengths of 10–40m m,using MnSO 4ÁH 2O as manganese source;meanwhile,a -MnO 2bifurcate nanorods with average diameter of 100nm were obtained by adopting MnCO 3as starting material.In our presentation,we choose concentrated nitric acid as acid source to tune the pH of the system.Our experiments show that pure-phase a -MnO 2can be readily obtained in a wide range of nitric acid concentrations.This result may be a useful comple-mentarity to previous experimental results that a -MnO 2could be only produced in H 2SO 4surroundings.2.Experimental procedure
All the reagents of analytical grade were purchased from Shanghai Chemical Reagent Company and used without further purification.In a typical procedure,1mmol MnSO 4ÁH 2O or MnCO 3and 2mmol KClO 3powders were successively put into a beaker with 15mL concen-trated nitric acid,the solution was magnetically stirred for 20min at 801C to form brown colloid.The slurry solution was transferred into a 50mL stainless-steel autoclave with a Teflon-liner,the beaker was washed with 25–30mL distilled water,and washing solution was put into above-mentioned Teflon-liner.The autoclave was sealed and maintained at 1201C for 12h,then air cooled to room temperature.The brown products were filtered off,washed several times with distilled water and absolute ethanol,and then dried in vacuum at 801C for 1h.
The X-ray powder diffraction (XRD)pattern of the as-prepared samples was determined using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite
monochromatized Cu K a radiation (l ¼1:541874A)
in the 2y ranging from 101to 701.The morphology and size of the final products were determined by field scanning electron microscopy (FESEM)images,taken with JEOL-6700F scanning electronic microanalyzer.Transmission electron microscope (TEM)image and selected area electron
diffraction (SAED)pattern,which were characterized by Hitachi H-800TEM with a tungsten filament and an accelerating voltage of 200kV.High-resolution transmis-sion electron microscope (HRTEM)image was recorded on a JEOL 2010microscope.The samples used for TEM and HRTEM characterization were dispersed in absolute ethanol and were ultrasonicated before observation.3.Results and discussion
The synthesis of a -MnO 2nanowires and nanorods is based on the hydrothermal method in a strong acidic (nitric acid)circumstance.The experimental results by using nitric acid as acidification agent,different manganese sources,and KClO 3as the oxidizer are summarized in Table 1.Under our experimental conditions,the different size and morphological products can be obtained by varying the c
oncentration of nitric acid.
From this table we can see that only under concentrated nitric acid condition pure a -MnO 2can be obtained.The volume of concentrated nitric acid can be in the range of 3–20mL.The yields and morphology change greatly when different amounts of nitric acid were introduced.We found that the most optimal conditions of obtaining uniform a -MnO 2nanowires were fixed on 15mL concentrated nitric acid and reaction temperature of 1201C.Moreover,when different Mn compounds were selected as starting materi-als,the size and morphologies can be changed greatly,as shown in the lines 1and 4of Table 1.The result of experiments clearly indicates concentrated nitric acid plays a crucial role in the formation of a -MnO 2with 1-D structure.
The phase and purity of the products were firstly examined by XRD.Fig.1shows a typical XRD pattern of the as-synthesized samples at 1201C for 12h,all the reflection peaks can be readily indexed to body-centered tetragonal a -MnO 2phase (space group I 4/m ),with lattice
constants of a ¼9:816A,
and c ¼2:853A,which are in agreement with the standard values (JCPDS 72-1982,
a ¼9:815A;
c ¼2:847A Þ.No other phase was detecte
d in Fig.1indicating th
e high purity o
f the final products.The morphologies and structure information were further obtained from FESEM,TEM and SAED.Fig.2provides FESEM images of the as-prepared a -MnO 2single-crystal nanowires.Figs.2(a)and (b)are the low-and high-magnification FESEM images of the as-prepared a -MnO 2
Table 1
Summary of the results on the products obtained under different manganese sources,the content of concentrated nitric acid and reaction temperature for 12h,using KClO 3as the oxidizer Sample no.Manganese source Concentrated nitric acid (mL)Reaction temperature (1C)Product morphology 1MnSO 4ÁH 2O 15120a -MnO 2nanowires 2MnSO 4ÁH 2O 0120Nonexistence of MnO 23MnSO 4ÁH 2O 0180Minor b -MnO 2
4MnCO 315120Flowery a -MnO 2nanorods 5
MnCO 3
120
Nonexistence of MnO 2
Y.Liu et al./Journal of Solid State Chemistry 179(2006)1757–1761
1758
single-crystal nanowires when MnSO 4ÁH 2O served as manganese source.These images show that the products of a -MnO 2consisted of a large quantity of uniform nanowires,with diameters of 50nm and lengths up to several hundreds of micrometers.Fig.3(a)shows the TEM image of as-prepared a -MnO 2nanowires,and the TEM images further demonstrate that the obtained product has a uniform wire-like morphology.The results reveal the product of a -MnO 2was composed of nanowires.The diameters and lengths of nanowires were consistent with
(541)
(002)
(521)
(600)
(411)
(510)
(321)
(301)
(420)
(330)
(211)
(400)
(310)
(220)
(101)
(200)
(110)
i n t e n s i t y
2θ/degree
Fig.1.Typical XRD pattern of as-prepared a -MnO 2
.
Fig.2.Low-magnification FESEM image (a)and high-magnification FESEM image (b)of a -MnO 2nanowires (MnSO 4ÁH 2O as manganese
source).
Fig.3.TEM images of as-prepared single-crystal a -MnO 2nanowires (a),TEM image (b),SAED pattern (c)and HRTEM image (d)of the single a -MnO 2nanowire.
Y.Liu et al./Journal of Solid State Chemistry 179(2006)1757–1761
1759
those of FESEM results.The TEM image (Fig.3(b))of representative single nanowires and HRTEM observation for individual nanowire provide additional insight into the structure of a -MnO 2with MnSO 4ÁH 2O as manganese source.The typical SAED pattern of the single a -MnO 2nanowire is shown in the inset of Fig.3(c).Fig.3(d)is the HRTEM image taken from the single a -MnO 2nanowire,which shows the clearly resolved lattice fringes.The
separated spacings of 2.73and 3.12A
correspond to ð101Þand (310)planar of a -MnO 2,respectively.This image clearly reveals that the as-synthesized nanowire has no defect of dislocation and further substantiates that the nanowires are single crystalline,which is consistent with the SAED pattern.According to HRTEM image and SAED pattern recorded on the single a -MnO 2nanowire,the deduced growth direction of nanowire is ½101 .
If MnCO 3was introduced into the reaction system,the products are mainly composed of nanorods,as revealed by the corresponding FESEM images.Figs.4(a)and (b)are the low-and high-magnification FE
SEM images of the as-prepared a -MnO 2nanorods with MnCO 3as manganese source.The low-magnification FESEM image (Fig.4(a))reveals that the product of a -MnO 2is consisted of a large quantity of flowery nanorods with average diameter of 100nm.Fig.4(b)is the high-magnification FESEM image of the as-prepared a -MnO 2,in which we seem to observe obvious features of bifurcate rod-like structure.It is worth to note that the morphology of a -MnO 2bifurcate nanorods has never been reported previously.Comparing Figs.4(a)and (b)to Figs.2(a)and (b),it can be found that the nanowires with MnSO 4ÁH 2O as manganese source are much slenderer than the bifurcate nanorods with MnCO 3as manganese source.
Generally,pH is believed to have great impact on the crystal forms of final products [17,19,24,25].In our experiment,a series of hydrothermal synthesis were carried out in a wide range of acidity with pH value less than 7,we found that the final products to be a -MnO 2nanowires or nanorods with 1-D morphology whether MnSO 4ÁH 2O or MnCO 3as manganese source.Therefore,this method is very effective for the large-scale synthesis of a -MnO 2with 1-D nanostructures.
The influence of the reaction time on the growth of the nanowires and nanorods was investigated.The correspond-ing samples were tested by FESEM.Fig.5shows FESEM images of the as-obtained samples measured (a)after 0.5h,(b)after 3h,(c)after 6h,(d)after 12h,and other conditions kept constant
at the same time.Thereinto,Figs.5(a)–(d)are FESEM images of the products with MnSO 4ÁH 2O as manganese source.As can be seen,the reaction lasted for 0.5h;the products were composed of aggregated particles (see Fig.5(a)).When the reaction time
Fig.4.Low-magnification FESEM image (a)and high-magnification FESEM image (b)of a -MnO 2nanorods (MnCO 3as manganese
source).
Fig.5.The FESEM images of products obtained by heating in the acidic solution for various reaction times,MnSO 4ÁH 2O (a–d)as manganese source:(a)0.5h,(b)3h,(c)6h,(d)12h and MnCO 3(e–h)as manganese source:(e)0.5h,(f)3h,(g)6h,(h)12h.
Y.Liu et al./Journal of Solid State Chemistry 179(2006)1757–1761
1760
prolonged to3h,on the surfaces of these particles,lamellar structures appeared,and some of these lamellar split to tiny nanowires,indicating the beginning of the formation of a-MnO2nanowires(see Fig.5(b)).This process continued and more nanowires formed after6h(see Fig.5(c)).Until the reaction time was extended to12h,most of the products are nanowires with average diameter of50nm and lengths ranging between10and40m m,as shown in Fig.5(d).Further elongating the reaction time shows little effects on the size and phase-purity of the products. This growth process is similar to the results of C.Z.Wu et al.[26],we call this a‘‘rolling-broken-growth’’process. According to above results and previous research [20,21,27],the possible formation mechanism of a-MnO2 nanowires by adopting MnSO4ÁH2O as manganese source could be explained as follows:(1)when temperature was maintained at801C,the interaction of KClO3and manganese source with Mn2+ion happened only when concentrated nitric acid exists.In the synthetic process,a large number of the MnO2colloidal particles had been formed in concentrated nitric acid before hydrothermal operation.(2)Under hydrothermal conditions,owing to the absence of surfactants,the MnO2colloidal particles are prone to aggregate and form bigger particles.(3)The surface of aggregated big particles grows gradually into sheets of a-MnO2with lamellar structure through an elevated temperature and pressure,and then these sheets of a-MnO2will curl by extending reaction time to form a-MnO21-D nanostructres.(4)Much evidence has demonstrated that the lamellar structure had a strong tendency to form1-D nanostructures[20,27].The
structure of a-MnO2comprises a macromolecular lamellar net with octahedral[MnO6]units coordinated Mn and O atoms [20],which can give rise to formation of1-D nanostruc-tures.As the layer structure of a-MnO2is in a metastable state,these sheets of a-MnO2with lamellar structure split into nanowires.(5)Anisotropic nature of crystal growth makes thefinal products turn into a large number of uniform a-MnO2nanowires.Moreover,we found when MnCO3serves as manganese source,a similar growth procedure was observed,as shown in Figs.5(e)–(h).We believe this a-MnO21-D nanostructural formation process is universal despite different manganese sources were involved in the hydrothermal process.This observation may spread to other nanomaterials synthesis.The above mechanism is in good agreement with our experiment results.
4.Conclusion
In summary,a-MnO2nanowires and nanorods with a uniform diameter have been successfully synthesized on a large scale via a simple nitric-acid-assisted hydrothermal process at low temperature.It belongs tofirstly report that the morphology of a-MnO2bifurcate nanorods can be acquired when MnCO3serves as manganese source.The concentrated nitric acid plays a crucial role in the formation of a-MnO2nanowires and nanorods.This experimental result is different from the previous conclu-sion that the concentrated nitric acid seems to be an unfavorable condition to form a-
MnO2.This observation may be expanded to synthesize other nanomaterials. Acknowledgments
Financial support from the National Natural Science Foundation of China and the973Project of China is greatly appreciated.
References
[1]A.P.Alivisatos,Science271(1996)933.
[2]Y.Wu,P.Yang,Chem.Mater.12(2000)605.
[3](a)  A.M.Morales,C.M.Lieber,Science279(1998)208;
(b)M.S.Gudiken,C.M.Lieber,J.Am.Chem.Soc.122(2000)8801.
[4](a)S.Iijima,Nature354(1991)56;
(b)T.Seeger,P.Kohler-Redlich,M.Ruhle,Adv.Mater.12(2000)
279.
[5]Y.Zhou,S.H.Yu,X.P.Cui,C.Y.Wang,Z.Y.Chen,Chem.Mater.
11(1999)545.
[6]C.Mao,D.J.Solis,B.D.Reiss,S.T.Kottmann,R.Y.Sweeney,A.
Hayhurst,G.Georgiou,B.Iverson,A.M.Belcher,Science303(2004) 213.
[7]G.H.Du,L.-M.Peng,Q.Chen,S.Zhang,W.Z.Zhou,Appl.Phys.
Lett.83(2003)1638.
[8]G.H.Du,Q.Chen,Y.Yu,S.Zhang,W.Z.Zhou,L.M.Peng,J.
Mater.Chem.14(2004)1437.
[9]G.H.Du,Q.Chen,R.C.Che,L.M.Peng,Appl.Phys.Lett.79(2001)
3702.
[10]M.M.Thackeray,Prog.Solid State Chem.25(1997)1.
[11]A.R.Armstrong,P.G.Bruce,Nature381(1996)499.
[12]B.Ammundsen,J.Paulsen,Adv.Mater.13(2001)943.
[13]Q.Feng,H.Kanoh,K.Ooi,J.Mater.Chem.9(1999)319.
[14]L.I.Hill,A.Verbaere,D.Guyomard,J.Power Sources226(2003)
119.
[15]M.M.Thackeray,J.Am.Ceram.Soc.82(1999)3347.
[16]Y.F.Shen,R.P.Zerger,S.L.Suib,L.McCurdy,D.I.Potter,C.L.
O’Young,Science260(1993)511.
[17]R.N.DeGuzman,Y.F.Shen,E.J.Neth,S.L.Suib,C.L.O’Young,S.
Levine,J.M.Newsam,Chem.Mater.6(1994)815.
[18]M.Benaissa,M.Jose-Yacaman,T.D.Xiao,P.R.Strutt,Appl.Phys.
Lett.70(1997)2120.
[19]N.Kijima,H.Yasuda,T.Sato,Y.Yoshimura,J.Solid State Chem.
159(2001)94.
[20]Y.D.Li,X.L.Li,R.R.He,J.Zhu,Z.X.Deng,J.Am.Chem.Soc.124
(2002)1411.
[21]X.Wang,Y.D.Li,Chem.Eur.J.9(2003)300.
[22](a)Z.Q.Li,Y.Ding,Y.J.Xiong,Q.Yang,Y.Xie,Chem.Commun.
(2005)918;
(b)Z.Q.Li,Y.Ding,Y.J.Xiong,Y.Xie,Cryst.Growth Des.5
(2005)1953.
[23](a)Y.Q.Gao,Z.H.Wang,J.X.Wan,G.F.Zou,Y.T.Qian,J.Cryst.
Growth279(2005)415;
(b)Y.Chen,C.Liu,F.Liu,H.M.Cheng,J.Alloy Compd.19(2005)
282.
[24]J.Luo,S.L.Suib,J.Phys.Chem.B101(1997)10403.
[25]T.D.Xiao,P.R.Strutt,M.Benaissa,H.Chen,  B.H.Kear,
Nanostruct.Mater.10(1998)1051.
[26]C.Z.Wu,Y.Xie,D.Wang,J.Yang,T.W.Li,J.Phys.Chem.B107
(2003)13583.
[27]Y.D.Li,X.L.Li,Z.X.Deng,B.C.Zhou,S.S.Fan,J.W.Wang,X.M.
Sun,Angew Chem.Int.Ed.Engl.41(2002)333.
Y.Liu et al./Journal of Solid State Chemistry179(2006)1757–17611761

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