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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 10 5121ndash5128 2010

Three-Dimensional CuO Nanobundles Consisted ofNanorods Hydrothermal Synthesis Characterization

and Formation Mechanism

Huiyu Chen1 Dong-Wook Shin2 Jong-Hak Lee2 Sung-Min Park2Kee-Won Kwon3 and Ji-Beom Yoo12lowast

1School of Advanced Materials Science and Engineering (BK21) Sungkyunkwan University Suwon 440-746 Republic of Korea2SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746 Republic of Korea3Department of Semiconductor Systems Engineering Sungkyunkwan University Suwon 440-746 Republic of Korea

Novel monoclinic CuO nanobundles 08ndash1 m in size were synthesized at 130 C in the presenceof sodium dodecyl benzenesulfonate (SDBS) by a simple hydrothermal method Each nanobundlewas comprised of many nanorods with one ends growing together to form a center and another endsradiating laterally from this center The length and the diameter of these assembled nanorods are inthe range of 200ndash300 nm and about 20ndash30 nm respectively HRTEM and SAED results indicatedthat the CuO nanorods grow along the [010] direction An investigation of the hydrothermal processrevealed that the reaction time temperature and surfactant play important roles in the formationof the resultant CuO nanostructures Isolated CuO nanorods were obtained when the temperaturewas increased to 190 C and CuO microflowers composed of many nanosheets were producedat 130 C when cetyltrimethylammonium bromide (CTAB) was employed instead of SDBS Thepossible mechanism for the formation of these CuO nanostructures was discussed simply on thebasis of the experimental results

Keywords CuO Nanostructures Hydrothermal Synthesis

1 INTRODUCTION

Over the past few decades the synthesis and investiga-tion of low-dimensional (0D 1D and 2D) nanomateri-als have attracted particular attention in view of theirwide range of potential applications12 The fabrication ofthree-dimensional (3D) complex architectures consistingof low-dimensional nanostructures is of great interest tomaterial scientists and chemists because this type of mate-rial not only shows unique properties but also gives insightinto the construction of micro- and nano-scale devices3

A variety of 3D nanomaterials have been investigatedextensively via a soft template-assisted approach Forexample Guo et al synthesized dendritic silver crystals ina cetyltrimethylammonium bromide (CTAB) and sodiumdodecyl benzenesulfonate (SDBS) mixed surfactant solu-tion at room temperature and found that the concentra-tion of CTABSBDS and molar ratio have a significantinfluence on the final silver shape4 Uniform cobaltmicrospheres composed of ordered nanoplatelets with a

lowastAuthor to whom correspondence should be addressed

thickness of approximately 20 nm were obtained in thepresence of SDBS5 Flowerlike bismuth tungstate struc-tures which were assembled by nanosheets consisting ofnumerous square nanoplatelets were fabricated by varyingthe amount of poly(vinylpyrrolidone) (PVP) surfactant6

Different driving mechanisms including surface tensioncapillary effects and magnetic forces were proposed forthe formation of 3D complex nanostructures7ndash9 Among allcomplex 3D architectures there are few reports on bundle-like nanostructures10114142 because of the extremelynovel shape Hong et al proposed that a planar networkstructure of gadolinium oxalate served as a molecular tem-plate to fabricate Gd(OH)3 nanobundles10 A decrease insurface energy was proposed to explain the formation ofsodium tungstate nanobundles with the driving force forthe assembly of nanorods originating from lateral capillaryforces11 All these bundle-like nanostructures were consti-tuted directly by parallel nanorods connecting togetherAs an important p-type semiconductor cupric oxide

(CuO) has drawn much attention in recent years onaccount of its promising applications It forms the basisof several high-Tc superconductors and materials with

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

giant magnetoresistance12 and is also used as a sensorcatalyst and solar cells13ndash15 Thus far many effortshave been directed towards the preparation of CuOnanostructures to enhance its performance in currentlyexisting applications Various low dimensional CuO nano-structures such as nanowires16 nanorods17ndash22 nanotubes17

nanoleaves2324 nanoplatelets25ndash29 nanosheets3031

nanobelts and nanoribbons162032ndash34 have been obtainedsuccessfully via a solution-based approach because thismethod has been considered to be one of the mostpromising synthetic routes with high efficiency low costand large-scale production Meanwhile some other CuOarchitectures including nanodendrites35 dandelions36 hol-low microspheres37 and flower-like structures3839 havealso been fabricated using wet chemical methods How-ever it is still a challenge to find a convenient route forpreparing nanorod- or nanosheet-based CuO 3D complexnanostructures with high yield and uniform size In thisstudy we report a simple hydrothermal method for syn-thesizing CuO nanobundles and isolated nanorods withthe assistance of surfactant sodium dodecyl benzenesul-fonate (SDBS) Up to now this is the first report aboutCuO nanobundles composed of many nanorods with oneends growing together to form a center and another endsradiating from this center rather than aggregative parallelnanorods or nanoplates in the previous reports4142 CuOmicroflowers consisted of many nanosheets were obtainedwhen cetyltrimethylammonium bromide (CTAB) wasemployed instead of SDBS

2 EXPERIMENTAL DETAILS

In a typical synthesis 17 g CuCl2middot2H2O (99 puritySigma-Aldrich) and 2 g dodecyl benzenesulfonate (SDBSSigma-Aldrich) were dissolved in 250 mL water with vig-orous stirring for 30 min which was followed by the addi-tion of 20 mL of NaOH (5 M in distilled water) solutionIn the whole mixed solution system the concentrationsof copper salt and SDBS were 0037 M and 0021 Mrespectively The above mixture was then transferred intoa 375 mL Teflon-lined stainless steel autoclave and main-tained at 130 C for 24 h in an electric oven After thereaction the autoclave was allowed to cool naturally toambient temperature and the precipitate was collected bycentrifugation rinsed several times with distilled water andabsolute ethanol and dried at 45 C in air for 8 h Othercontrolled experiments were carried out by changing thereaction time and temperature respectively The surfac-tant CTAB was used instead of SDBS for the synthesisof CuO microflowers while the other synthetic parame-ters and procedures were the same as those of the typicalreactionThe crystal structures of the obtained samples were

characterized by X-ray powder diffraction (XRD) usinga Bruker D8 focus diffractometer with Cu K radiation

( = 015406 nm) Raman spectroscopy was performedusing a RM1000-Invia (Renishaw) spectrometer from 200to 700 cmminus1 at room temperature The 514 nm line of thelaser was used as the excitation source with the capabilityof supplying 20 mW Scanning electron microscopy (SEM)images were taken by a JEOL JSM6700F field emis-sion scanning electron microscope Transmission electronmicroscopy (TEM and HRTEM) images and the corre-sponding selected area electron diffraction (SAED) pat-terns were taken with a JEOL JEM2100F transmissionelectron microscope performed at an accelerating voltageof 200 kV

3 RESULTS AND DISCUSSION

The morphology and microstructure of the CuO productsobtained in the typical synthesis were characterized byFESEM and TEM Figure 1(a) shows a low-magnificationFESEM image of the CuO products It can be clearlyobserved that the samples consist mainly of uniformbundle-like nanostructures (nanobundles) in large quanti-ties with an individual size ranging from 08 to 1 mThe magnified SEM image (Fig 1(b)) further confirmedthe 3D bundle-like morphology and revealed that eachnanobundle consisted of many nanorods with one endsgrowing together to form a center and another ends later-ally radiating from it The length of these assembled rods

(a)

1 microm

500 nm

(b)

Fig 1 (a) Low-magnification and (b) high-magnification FESEMimages of the CuO nanobundles prepared at 130 C for 24 h with thesurfactant SDBS

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

is in the range of 400ndash500 nm and the diameter is about20ndash30 nmFigures 2(a and b) shows typical TEM images that are

in accordance with the FESEM observations As can beseen a single bundle possessed many nanorods with oneends radiating from a central point which was formed byanother ends growing together The structures and shapeswere still preserved even after the CuO nanobundles weredispersed in ethanol by ultrasonic vibration for 30 minbefore being deposited on a carbon-coated copper gridfor the TEM observations This suggests that the cen-ter of the nanobundle was produced as an integral partwhen one end of the nanorods grew together rather thanloosely aggregated The novelty of this nanostructure isthat the entire CuO nanobundle exhibits lateral radiationfrom this center The full size of a single bundle wasapproximately 1 m and the typical length as well asthe diameter of the nanorods is in the range of 350ndash500 nm and close to 20ndash30 nm respectively Figure 2(c)is a corresponding SAED pattern taken from an individualnanorod of CuO nanobundles The SAED pattern can beindexed to be the [001] zone axis of a monoclinic phaseCuO nanorod indicating that it is single crystalline Incontrast SAED of the entire CuO nanobundle produceda complex polycrystalline pattern (Fig 2(d)) The EDrings were not continuous but were composed of discretespots suggesting a preferential orientation of the collective

(a)

(e)

(f) (g)

(b) (c)

(d)

100 nm100 nm

Fig 2 TEM images of (a) CuO nanobundles with a panoramic view and (b) high-magnification view of half of a CuO nanobundle (c) SAED patternof a single assembled nanorod circled in (b) (d) SAED pattern of the whole CuO nanobundle circled in (a) (e) HRTEM image of a single assemblednanorod circled in (b) (f) XRD pattern and (g) Raman spectra of the CuO nanobundles

assembled nanorods constituting the CuO nanobundlesHRTEM image taken from the tip of a single CuO nanorodis shown in Figure 2(e) which reveals that the nanorod isa single crystal in nature The distance between the adja-cent fringes was examined to be 0270 nm correspondingto that of the (110) planes of CuO Based on SAED andHRTEM analyses it can be demonstrated that the CuOnanorods grow along the [010] direction The direction isthe same with the previous reports of CuO nanoleaves andnanoplatelets2326

The structure and crystal phase of the resultant CuOnanobundles were investigated by XRD pattern as shownin Figure 2(f) All the observed peaks were indexed tothe monoclinic phase CuO (JCPDS 05-0661 a= 4684 Aringb = 3425 Aring c = 5129 Aring and = 9947 space groupC2c No other characteristic peaks for impurities suchas Cu(OH)2 or Cu2O were detected indicating the highpurity of CuO nanobundles obtained via our current syn-thetic route The XRD results are well consistent with theabove HRTEM and SAED analyses The samples of CuOnanobundles were further examined by Raman spectraCuO belongs to the c62h space group with two moleculesper primitive cell There are 12 zone-center optical phononmodes including six infrared active modes (3Au + 3Bu)three acoustic modes (Au+2Bu) and three Raman activemodes (Ag + 2Bg) Figure 2(g) shows the Raman spectraof the as-synthesized CuO nanobundles It can be seen that

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

there are three Raman peaks at 273 320 and 608 cmminus1

corresponding to the Ag Bg and Bg modes of CuO nano-structures respectively The results are in good agree-ment with the reported values19 and demonstrate that CuOnanobundles are single crystal with a monoclinic structureIn order to investigate the growth mechanism of the

as-prepared CuO nanobundles systematic time-dependentexperiments were carried out at 130 C Figures 3(andashd)shows the SEM images of CuO samples synthesized at130 C with a reaction time of 100 min 2 10 and18 h illustrating the morphological evolution of the CuOnanobundles A large number of flake-like CuO wereobtained when the hydrothermal reaction proceeded for100 min and the typical SEM image was shown inFigure 3(a) The surface of some CuO nanoflakes was

(a)

(c) (d)

(e)(f)

(b)

Fig 3 SEM images of CuO products prepared with SDBS at 130 C with a hydrothermal reaction time of (a) 100 min (b) 2 h (c) 10 h and (d) 18 h(e) SEM image of the sample prepared without SDBS for 24 h and (f) XRD pattern of products prepared with SDBS at 130 C for various time

rough and some flakes connected to each other At theinitial stage the orthorhombic Cu(OH)2 precursor precip-itated as small nanorods or nanoflakes due to the connec-tion of (010) planes through H-bonds1840 In the followinghydrothermal period Cu(OH)2 lost H2O molecules bybreaking the interplanar H-bonds resulting in the forma-tion of monoclinic flake-like CuO nanostructures It shouldbe noted that 100 min was the appropriate time to com-plete the reaction as investigated by the XRD patternshown in Figure 3(f) Some blue precursors coexisted withthe black CuO products when the reaction time was lessthan 100 min which suggested the transformation fromthe Cu(OH)2 precursor to the final resultant CuO wasnot fully complete When the reaction was prolonged for2 h the edges of most CuO nanoflakes partially split into

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

many irregular short nanostrips due to the splitting pro-cess By careful observation some tips of the short stripswere curved while the center part of the flakes was stillintegrated (Fig 3(b)) This process is similar to the forma-tion of CuO nanoribbons and nanorings33 With the assis-tance of SDBS in the low temperature region the CuOnanoflakes split easily into small nanoribbons possiblybecause the intense Brownian movement of the surfac-tant molecules might destroy the integrity of CuO crys-tal facets As the reaction time was increased further thepartially split parts of these CuO nanoflakes might growthicker and larger to produce rod-like structures As theSEM image shown in Figure 3(c) many irregular rod-likestructures with a variety of lengths were produced whenthe hydrothermal reaction time was 10 h One ends ofthese nanorods were connected together in the central partof flakes which was preserved well as those in the pre-vious stage However the nanorods were not straight andthe diameter varied according to the position It was obvi-ous that the CuO rod-like structures were formed in situfrom the split parts of nanostrips and the various lengthsmight result from the different extent of partially split-ting When the hydrothermal treatment was progressedfor 18 h the CuO nanobundles were formed to someextent (Fig 3(d)) Compared to the CuO products obtainedfrom 24 h hydrothermal treatment process the nanobun-dles produced at this stage were not perfect Howevermany straight nanorods existed with one ends connectingtogether to form a whole as center of the bundle Thewhole center was the one preserved from the nanoflake atthe initial reaction stage With reaction time prolongingthe CuO nanobundles will continue to grow into perfect

(a)

(e) (d)

(b)

(c)

Fig 4 Schematic diagram of a possible growth mechanism for CuO nanobundles The entire procedure includes five main steps (a) blue Cu(OH)2precursor precipitates as very small nanoflakes due to the H-bonds in its (010) planes (b) Irregular CuO nanoflakes with a rough surface form bythe dehydration of Cu(OH)2 molecules (C) The edges of CuO nanoflakes split partially as a result of an interaction between SDBS and the CuOcrystal surface while the central area is still integral (d) Irregular nanorods are formed in situ at the previous partially split positions and (e) Straightnanorods and perfect CuO nanobundles are formed finally

ones as what are displayed in Figures 1(a and b) In con-trast the sample prepared at 130 C for 24 h without sur-factant of SDBS only included some small nanoplateletsand some layered CuO nanostructures due to the directdecomposition of Cu(OH)2 precursor (Fig 3(e))A possible growth mechanism for the formation of

bundle-like CuO nanostructures is proposed based on theabove controlled experimental results The growth of CuOnanobundles in our case consisted of a five-step process asschemed in Figure 4(1) The Cu(OH)2 precursor precipitates as very smallnanoflakes due to the H-bonds existing and connecting its(010) planes (step a)1840

(2) Flake-like CuO nanostructures with some rough sur-faces form through both the decomposition of Cu(OH)2molecules and the breakage of H-bonds (step b)(3) The edges of the CuO nanoflakes split partially withthe assistance of the surfactant SDBS while the centralpart is still integrated (step c)(4) Irregular CuO nanorods with different lengths areformed in situ at the previous partially split positions Therods are connected together with one end at the centralpart of CuO nanoflakes (step d)(5) The irregular CuO nanorods transform gradually intostraight ones and perfect bundle-like CuO nanostructuresare produced (step e) finally

It is obvious that SDBS is very crucial during the entireCuO growth process In the beginning SDBS interactsdirectly with the CuO facets to produce partially split CuOnanoflakes and subsequently it acts as a soft template forthe growth of CuO nanorods and the formation of the finalbundles

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Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

giant magnetoresistance12 and is also used as a sensorcatalyst and solar cells13ndash15 Thus far many effortshave been directed towards the preparation of CuOnanostructures to enhance its performance in currentlyexisting applications Various low dimensional CuO nano-structures such as nanowires16 nanorods17ndash22 nanotubes17

nanoleaves2324 nanoplatelets25ndash29 nanosheets3031

nanobelts and nanoribbons162032ndash34 have been obtainedsuccessfully via a solution-based approach because thismethod has been considered to be one of the mostpromising synthetic routes with high efficiency low costand large-scale production Meanwhile some other CuOarchitectures including nanodendrites35 dandelions36 hol-low microspheres37 and flower-like structures3839 havealso been fabricated using wet chemical methods How-ever it is still a challenge to find a convenient route forpreparing nanorod- or nanosheet-based CuO 3D complexnanostructures with high yield and uniform size In thisstudy we report a simple hydrothermal method for syn-thesizing CuO nanobundles and isolated nanorods withthe assistance of surfactant sodium dodecyl benzenesul-fonate (SDBS) Up to now this is the first report aboutCuO nanobundles composed of many nanorods with oneends growing together to form a center and another endsradiating from this center rather than aggregative parallelnanorods or nanoplates in the previous reports4142 CuOmicroflowers consisted of many nanosheets were obtainedwhen cetyltrimethylammonium bromide (CTAB) wasemployed instead of SDBS

2 EXPERIMENTAL DETAILS

In a typical synthesis 17 g CuCl2middot2H2O (99 puritySigma-Aldrich) and 2 g dodecyl benzenesulfonate (SDBSSigma-Aldrich) were dissolved in 250 mL water with vig-orous stirring for 30 min which was followed by the addi-tion of 20 mL of NaOH (5 M in distilled water) solutionIn the whole mixed solution system the concentrationsof copper salt and SDBS were 0037 M and 0021 Mrespectively The above mixture was then transferred intoa 375 mL Teflon-lined stainless steel autoclave and main-tained at 130 C for 24 h in an electric oven After thereaction the autoclave was allowed to cool naturally toambient temperature and the precipitate was collected bycentrifugation rinsed several times with distilled water andabsolute ethanol and dried at 45 C in air for 8 h Othercontrolled experiments were carried out by changing thereaction time and temperature respectively The surfac-tant CTAB was used instead of SDBS for the synthesisof CuO microflowers while the other synthetic parame-ters and procedures were the same as those of the typicalreactionThe crystal structures of the obtained samples were

characterized by X-ray powder diffraction (XRD) usinga Bruker D8 focus diffractometer with Cu K radiation

( = 015406 nm) Raman spectroscopy was performedusing a RM1000-Invia (Renishaw) spectrometer from 200to 700 cmminus1 at room temperature The 514 nm line of thelaser was used as the excitation source with the capabilityof supplying 20 mW Scanning electron microscopy (SEM)images were taken by a JEOL JSM6700F field emis-sion scanning electron microscope Transmission electronmicroscopy (TEM and HRTEM) images and the corre-sponding selected area electron diffraction (SAED) pat-terns were taken with a JEOL JEM2100F transmissionelectron microscope performed at an accelerating voltageof 200 kV

3 RESULTS AND DISCUSSION

The morphology and microstructure of the CuO productsobtained in the typical synthesis were characterized byFESEM and TEM Figure 1(a) shows a low-magnificationFESEM image of the CuO products It can be clearlyobserved that the samples consist mainly of uniformbundle-like nanostructures (nanobundles) in large quanti-ties with an individual size ranging from 08 to 1 mThe magnified SEM image (Fig 1(b)) further confirmedthe 3D bundle-like morphology and revealed that eachnanobundle consisted of many nanorods with one endsgrowing together to form a center and another ends later-ally radiating from it The length of these assembled rods

(a)

1 microm

500 nm

(b)

Fig 1 (a) Low-magnification and (b) high-magnification FESEMimages of the CuO nanobundles prepared at 130 C for 24 h with thesurfactant SDBS

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

is in the range of 400ndash500 nm and the diameter is about20ndash30 nmFigures 2(a and b) shows typical TEM images that are

in accordance with the FESEM observations As can beseen a single bundle possessed many nanorods with oneends radiating from a central point which was formed byanother ends growing together The structures and shapeswere still preserved even after the CuO nanobundles weredispersed in ethanol by ultrasonic vibration for 30 minbefore being deposited on a carbon-coated copper gridfor the TEM observations This suggests that the cen-ter of the nanobundle was produced as an integral partwhen one end of the nanorods grew together rather thanloosely aggregated The novelty of this nanostructure isthat the entire CuO nanobundle exhibits lateral radiationfrom this center The full size of a single bundle wasapproximately 1 m and the typical length as well asthe diameter of the nanorods is in the range of 350ndash500 nm and close to 20ndash30 nm respectively Figure 2(c)is a corresponding SAED pattern taken from an individualnanorod of CuO nanobundles The SAED pattern can beindexed to be the [001] zone axis of a monoclinic phaseCuO nanorod indicating that it is single crystalline Incontrast SAED of the entire CuO nanobundle produceda complex polycrystalline pattern (Fig 2(d)) The EDrings were not continuous but were composed of discretespots suggesting a preferential orientation of the collective

(a)

(e)

(f) (g)

(b) (c)

(d)

100 nm100 nm

Fig 2 TEM images of (a) CuO nanobundles with a panoramic view and (b) high-magnification view of half of a CuO nanobundle (c) SAED patternof a single assembled nanorod circled in (b) (d) SAED pattern of the whole CuO nanobundle circled in (a) (e) HRTEM image of a single assemblednanorod circled in (b) (f) XRD pattern and (g) Raman spectra of the CuO nanobundles

assembled nanorods constituting the CuO nanobundlesHRTEM image taken from the tip of a single CuO nanorodis shown in Figure 2(e) which reveals that the nanorod isa single crystal in nature The distance between the adja-cent fringes was examined to be 0270 nm correspondingto that of the (110) planes of CuO Based on SAED andHRTEM analyses it can be demonstrated that the CuOnanorods grow along the [010] direction The direction isthe same with the previous reports of CuO nanoleaves andnanoplatelets2326

The structure and crystal phase of the resultant CuOnanobundles were investigated by XRD pattern as shownin Figure 2(f) All the observed peaks were indexed tothe monoclinic phase CuO (JCPDS 05-0661 a= 4684 Aringb = 3425 Aring c = 5129 Aring and = 9947 space groupC2c No other characteristic peaks for impurities suchas Cu(OH)2 or Cu2O were detected indicating the highpurity of CuO nanobundles obtained via our current syn-thetic route The XRD results are well consistent with theabove HRTEM and SAED analyses The samples of CuOnanobundles were further examined by Raman spectraCuO belongs to the c62h space group with two moleculesper primitive cell There are 12 zone-center optical phononmodes including six infrared active modes (3Au + 3Bu)three acoustic modes (Au+2Bu) and three Raman activemodes (Ag + 2Bg) Figure 2(g) shows the Raman spectraof the as-synthesized CuO nanobundles It can be seen that

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

there are three Raman peaks at 273 320 and 608 cmminus1

corresponding to the Ag Bg and Bg modes of CuO nano-structures respectively The results are in good agree-ment with the reported values19 and demonstrate that CuOnanobundles are single crystal with a monoclinic structureIn order to investigate the growth mechanism of the

as-prepared CuO nanobundles systematic time-dependentexperiments were carried out at 130 C Figures 3(andashd)shows the SEM images of CuO samples synthesized at130 C with a reaction time of 100 min 2 10 and18 h illustrating the morphological evolution of the CuOnanobundles A large number of flake-like CuO wereobtained when the hydrothermal reaction proceeded for100 min and the typical SEM image was shown inFigure 3(a) The surface of some CuO nanoflakes was

(a)

(c) (d)

(e)(f)

(b)

Fig 3 SEM images of CuO products prepared with SDBS at 130 C with a hydrothermal reaction time of (a) 100 min (b) 2 h (c) 10 h and (d) 18 h(e) SEM image of the sample prepared without SDBS for 24 h and (f) XRD pattern of products prepared with SDBS at 130 C for various time

rough and some flakes connected to each other At theinitial stage the orthorhombic Cu(OH)2 precursor precip-itated as small nanorods or nanoflakes due to the connec-tion of (010) planes through H-bonds1840 In the followinghydrothermal period Cu(OH)2 lost H2O molecules bybreaking the interplanar H-bonds resulting in the forma-tion of monoclinic flake-like CuO nanostructures It shouldbe noted that 100 min was the appropriate time to com-plete the reaction as investigated by the XRD patternshown in Figure 3(f) Some blue precursors coexisted withthe black CuO products when the reaction time was lessthan 100 min which suggested the transformation fromthe Cu(OH)2 precursor to the final resultant CuO wasnot fully complete When the reaction was prolonged for2 h the edges of most CuO nanoflakes partially split into

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

many irregular short nanostrips due to the splitting pro-cess By careful observation some tips of the short stripswere curved while the center part of the flakes was stillintegrated (Fig 3(b)) This process is similar to the forma-tion of CuO nanoribbons and nanorings33 With the assis-tance of SDBS in the low temperature region the CuOnanoflakes split easily into small nanoribbons possiblybecause the intense Brownian movement of the surfac-tant molecules might destroy the integrity of CuO crys-tal facets As the reaction time was increased further thepartially split parts of these CuO nanoflakes might growthicker and larger to produce rod-like structures As theSEM image shown in Figure 3(c) many irregular rod-likestructures with a variety of lengths were produced whenthe hydrothermal reaction time was 10 h One ends ofthese nanorods were connected together in the central partof flakes which was preserved well as those in the pre-vious stage However the nanorods were not straight andthe diameter varied according to the position It was obvi-ous that the CuO rod-like structures were formed in situfrom the split parts of nanostrips and the various lengthsmight result from the different extent of partially split-ting When the hydrothermal treatment was progressedfor 18 h the CuO nanobundles were formed to someextent (Fig 3(d)) Compared to the CuO products obtainedfrom 24 h hydrothermal treatment process the nanobun-dles produced at this stage were not perfect Howevermany straight nanorods existed with one ends connectingtogether to form a whole as center of the bundle Thewhole center was the one preserved from the nanoflake atthe initial reaction stage With reaction time prolongingthe CuO nanobundles will continue to grow into perfect

(a)

(e) (d)

(b)

(c)

Fig 4 Schematic diagram of a possible growth mechanism for CuO nanobundles The entire procedure includes five main steps (a) blue Cu(OH)2precursor precipitates as very small nanoflakes due to the H-bonds in its (010) planes (b) Irregular CuO nanoflakes with a rough surface form bythe dehydration of Cu(OH)2 molecules (C) The edges of CuO nanoflakes split partially as a result of an interaction between SDBS and the CuOcrystal surface while the central area is still integral (d) Irregular nanorods are formed in situ at the previous partially split positions and (e) Straightnanorods and perfect CuO nanobundles are formed finally

ones as what are displayed in Figures 1(a and b) In con-trast the sample prepared at 130 C for 24 h without sur-factant of SDBS only included some small nanoplateletsand some layered CuO nanostructures due to the directdecomposition of Cu(OH)2 precursor (Fig 3(e))A possible growth mechanism for the formation of

bundle-like CuO nanostructures is proposed based on theabove controlled experimental results The growth of CuOnanobundles in our case consisted of a five-step process asschemed in Figure 4(1) The Cu(OH)2 precursor precipitates as very smallnanoflakes due to the H-bonds existing and connecting its(010) planes (step a)1840

(2) Flake-like CuO nanostructures with some rough sur-faces form through both the decomposition of Cu(OH)2molecules and the breakage of H-bonds (step b)(3) The edges of the CuO nanoflakes split partially withthe assistance of the surfactant SDBS while the centralpart is still integrated (step c)(4) Irregular CuO nanorods with different lengths areformed in situ at the previous partially split positions Therods are connected together with one end at the centralpart of CuO nanoflakes (step d)(5) The irregular CuO nanorods transform gradually intostraight ones and perfect bundle-like CuO nanostructuresare produced (step e) finally

It is obvious that SDBS is very crucial during the entireCuO growth process In the beginning SDBS interactsdirectly with the CuO facets to produce partially split CuOnanoflakes and subsequently it acts as a soft template forthe growth of CuO nanorods and the formation of the finalbundles

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

is in the range of 400ndash500 nm and the diameter is about20ndash30 nmFigures 2(a and b) shows typical TEM images that are

in accordance with the FESEM observations As can beseen a single bundle possessed many nanorods with oneends radiating from a central point which was formed byanother ends growing together The structures and shapeswere still preserved even after the CuO nanobundles weredispersed in ethanol by ultrasonic vibration for 30 minbefore being deposited on a carbon-coated copper gridfor the TEM observations This suggests that the cen-ter of the nanobundle was produced as an integral partwhen one end of the nanorods grew together rather thanloosely aggregated The novelty of this nanostructure isthat the entire CuO nanobundle exhibits lateral radiationfrom this center The full size of a single bundle wasapproximately 1 m and the typical length as well asthe diameter of the nanorods is in the range of 350ndash500 nm and close to 20ndash30 nm respectively Figure 2(c)is a corresponding SAED pattern taken from an individualnanorod of CuO nanobundles The SAED pattern can beindexed to be the [001] zone axis of a monoclinic phaseCuO nanorod indicating that it is single crystalline Incontrast SAED of the entire CuO nanobundle produceda complex polycrystalline pattern (Fig 2(d)) The EDrings were not continuous but were composed of discretespots suggesting a preferential orientation of the collective

(a)

(e)

(f) (g)

(b) (c)

(d)

100 nm100 nm

Fig 2 TEM images of (a) CuO nanobundles with a panoramic view and (b) high-magnification view of half of a CuO nanobundle (c) SAED patternof a single assembled nanorod circled in (b) (d) SAED pattern of the whole CuO nanobundle circled in (a) (e) HRTEM image of a single assemblednanorod circled in (b) (f) XRD pattern and (g) Raman spectra of the CuO nanobundles

assembled nanorods constituting the CuO nanobundlesHRTEM image taken from the tip of a single CuO nanorodis shown in Figure 2(e) which reveals that the nanorod isa single crystal in nature The distance between the adja-cent fringes was examined to be 0270 nm correspondingto that of the (110) planes of CuO Based on SAED andHRTEM analyses it can be demonstrated that the CuOnanorods grow along the [010] direction The direction isthe same with the previous reports of CuO nanoleaves andnanoplatelets2326

The structure and crystal phase of the resultant CuOnanobundles were investigated by XRD pattern as shownin Figure 2(f) All the observed peaks were indexed tothe monoclinic phase CuO (JCPDS 05-0661 a= 4684 Aringb = 3425 Aring c = 5129 Aring and = 9947 space groupC2c No other characteristic peaks for impurities suchas Cu(OH)2 or Cu2O were detected indicating the highpurity of CuO nanobundles obtained via our current syn-thetic route The XRD results are well consistent with theabove HRTEM and SAED analyses The samples of CuOnanobundles were further examined by Raman spectraCuO belongs to the c62h space group with two moleculesper primitive cell There are 12 zone-center optical phononmodes including six infrared active modes (3Au + 3Bu)three acoustic modes (Au+2Bu) and three Raman activemodes (Ag + 2Bg) Figure 2(g) shows the Raman spectraof the as-synthesized CuO nanobundles It can be seen that

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

there are three Raman peaks at 273 320 and 608 cmminus1

corresponding to the Ag Bg and Bg modes of CuO nano-structures respectively The results are in good agree-ment with the reported values19 and demonstrate that CuOnanobundles are single crystal with a monoclinic structureIn order to investigate the growth mechanism of the

as-prepared CuO nanobundles systematic time-dependentexperiments were carried out at 130 C Figures 3(andashd)shows the SEM images of CuO samples synthesized at130 C with a reaction time of 100 min 2 10 and18 h illustrating the morphological evolution of the CuOnanobundles A large number of flake-like CuO wereobtained when the hydrothermal reaction proceeded for100 min and the typical SEM image was shown inFigure 3(a) The surface of some CuO nanoflakes was

(a)

(c) (d)

(e)(f)

(b)

Fig 3 SEM images of CuO products prepared with SDBS at 130 C with a hydrothermal reaction time of (a) 100 min (b) 2 h (c) 10 h and (d) 18 h(e) SEM image of the sample prepared without SDBS for 24 h and (f) XRD pattern of products prepared with SDBS at 130 C for various time

rough and some flakes connected to each other At theinitial stage the orthorhombic Cu(OH)2 precursor precip-itated as small nanorods or nanoflakes due to the connec-tion of (010) planes through H-bonds1840 In the followinghydrothermal period Cu(OH)2 lost H2O molecules bybreaking the interplanar H-bonds resulting in the forma-tion of monoclinic flake-like CuO nanostructures It shouldbe noted that 100 min was the appropriate time to com-plete the reaction as investigated by the XRD patternshown in Figure 3(f) Some blue precursors coexisted withthe black CuO products when the reaction time was lessthan 100 min which suggested the transformation fromthe Cu(OH)2 precursor to the final resultant CuO wasnot fully complete When the reaction was prolonged for2 h the edges of most CuO nanoflakes partially split into

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

many irregular short nanostrips due to the splitting pro-cess By careful observation some tips of the short stripswere curved while the center part of the flakes was stillintegrated (Fig 3(b)) This process is similar to the forma-tion of CuO nanoribbons and nanorings33 With the assis-tance of SDBS in the low temperature region the CuOnanoflakes split easily into small nanoribbons possiblybecause the intense Brownian movement of the surfac-tant molecules might destroy the integrity of CuO crys-tal facets As the reaction time was increased further thepartially split parts of these CuO nanoflakes might growthicker and larger to produce rod-like structures As theSEM image shown in Figure 3(c) many irregular rod-likestructures with a variety of lengths were produced whenthe hydrothermal reaction time was 10 h One ends ofthese nanorods were connected together in the central partof flakes which was preserved well as those in the pre-vious stage However the nanorods were not straight andthe diameter varied according to the position It was obvi-ous that the CuO rod-like structures were formed in situfrom the split parts of nanostrips and the various lengthsmight result from the different extent of partially split-ting When the hydrothermal treatment was progressedfor 18 h the CuO nanobundles were formed to someextent (Fig 3(d)) Compared to the CuO products obtainedfrom 24 h hydrothermal treatment process the nanobun-dles produced at this stage were not perfect Howevermany straight nanorods existed with one ends connectingtogether to form a whole as center of the bundle Thewhole center was the one preserved from the nanoflake atthe initial reaction stage With reaction time prolongingthe CuO nanobundles will continue to grow into perfect

(a)

(e) (d)

(b)

(c)

Fig 4 Schematic diagram of a possible growth mechanism for CuO nanobundles The entire procedure includes five main steps (a) blue Cu(OH)2precursor precipitates as very small nanoflakes due to the H-bonds in its (010) planes (b) Irregular CuO nanoflakes with a rough surface form bythe dehydration of Cu(OH)2 molecules (C) The edges of CuO nanoflakes split partially as a result of an interaction between SDBS and the CuOcrystal surface while the central area is still integral (d) Irregular nanorods are formed in situ at the previous partially split positions and (e) Straightnanorods and perfect CuO nanobundles are formed finally

ones as what are displayed in Figures 1(a and b) In con-trast the sample prepared at 130 C for 24 h without sur-factant of SDBS only included some small nanoplateletsand some layered CuO nanostructures due to the directdecomposition of Cu(OH)2 precursor (Fig 3(e))A possible growth mechanism for the formation of

bundle-like CuO nanostructures is proposed based on theabove controlled experimental results The growth of CuOnanobundles in our case consisted of a five-step process asschemed in Figure 4(1) The Cu(OH)2 precursor precipitates as very smallnanoflakes due to the H-bonds existing and connecting its(010) planes (step a)1840

(2) Flake-like CuO nanostructures with some rough sur-faces form through both the decomposition of Cu(OH)2molecules and the breakage of H-bonds (step b)(3) The edges of the CuO nanoflakes split partially withthe assistance of the surfactant SDBS while the centralpart is still integrated (step c)(4) Irregular CuO nanorods with different lengths areformed in situ at the previous partially split positions Therods are connected together with one end at the centralpart of CuO nanoflakes (step d)(5) The irregular CuO nanorods transform gradually intostraight ones and perfect bundle-like CuO nanostructuresare produced (step e) finally

It is obvious that SDBS is very crucial during the entireCuO growth process In the beginning SDBS interactsdirectly with the CuO facets to produce partially split CuOnanoflakes and subsequently it acts as a soft template forthe growth of CuO nanorods and the formation of the finalbundles

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

5126 J Nanosci Nanotechnol 10 5121ndash5128 2010

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

J Nanosci Nanotechnol 10 5121ndash5128 2010 5127

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

there are three Raman peaks at 273 320 and 608 cmminus1

corresponding to the Ag Bg and Bg modes of CuO nano-structures respectively The results are in good agree-ment with the reported values19 and demonstrate that CuOnanobundles are single crystal with a monoclinic structureIn order to investigate the growth mechanism of the

as-prepared CuO nanobundles systematic time-dependentexperiments were carried out at 130 C Figures 3(andashd)shows the SEM images of CuO samples synthesized at130 C with a reaction time of 100 min 2 10 and18 h illustrating the morphological evolution of the CuOnanobundles A large number of flake-like CuO wereobtained when the hydrothermal reaction proceeded for100 min and the typical SEM image was shown inFigure 3(a) The surface of some CuO nanoflakes was

(a)

(c) (d)

(e)(f)

(b)

Fig 3 SEM images of CuO products prepared with SDBS at 130 C with a hydrothermal reaction time of (a) 100 min (b) 2 h (c) 10 h and (d) 18 h(e) SEM image of the sample prepared without SDBS for 24 h and (f) XRD pattern of products prepared with SDBS at 130 C for various time

rough and some flakes connected to each other At theinitial stage the orthorhombic Cu(OH)2 precursor precip-itated as small nanorods or nanoflakes due to the connec-tion of (010) planes through H-bonds1840 In the followinghydrothermal period Cu(OH)2 lost H2O molecules bybreaking the interplanar H-bonds resulting in the forma-tion of monoclinic flake-like CuO nanostructures It shouldbe noted that 100 min was the appropriate time to com-plete the reaction as investigated by the XRD patternshown in Figure 3(f) Some blue precursors coexisted withthe black CuO products when the reaction time was lessthan 100 min which suggested the transformation fromthe Cu(OH)2 precursor to the final resultant CuO wasnot fully complete When the reaction was prolonged for2 h the edges of most CuO nanoflakes partially split into

5124 J Nanosci Nanotechnol 10 5121ndash5128 2010

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

many irregular short nanostrips due to the splitting pro-cess By careful observation some tips of the short stripswere curved while the center part of the flakes was stillintegrated (Fig 3(b)) This process is similar to the forma-tion of CuO nanoribbons and nanorings33 With the assis-tance of SDBS in the low temperature region the CuOnanoflakes split easily into small nanoribbons possiblybecause the intense Brownian movement of the surfac-tant molecules might destroy the integrity of CuO crys-tal facets As the reaction time was increased further thepartially split parts of these CuO nanoflakes might growthicker and larger to produce rod-like structures As theSEM image shown in Figure 3(c) many irregular rod-likestructures with a variety of lengths were produced whenthe hydrothermal reaction time was 10 h One ends ofthese nanorods were connected together in the central partof flakes which was preserved well as those in the pre-vious stage However the nanorods were not straight andthe diameter varied according to the position It was obvi-ous that the CuO rod-like structures were formed in situfrom the split parts of nanostrips and the various lengthsmight result from the different extent of partially split-ting When the hydrothermal treatment was progressedfor 18 h the CuO nanobundles were formed to someextent (Fig 3(d)) Compared to the CuO products obtainedfrom 24 h hydrothermal treatment process the nanobun-dles produced at this stage were not perfect Howevermany straight nanorods existed with one ends connectingtogether to form a whole as center of the bundle Thewhole center was the one preserved from the nanoflake atthe initial reaction stage With reaction time prolongingthe CuO nanobundles will continue to grow into perfect

(a)

(e) (d)

(b)

(c)

Fig 4 Schematic diagram of a possible growth mechanism for CuO nanobundles The entire procedure includes five main steps (a) blue Cu(OH)2precursor precipitates as very small nanoflakes due to the H-bonds in its (010) planes (b) Irregular CuO nanoflakes with a rough surface form bythe dehydration of Cu(OH)2 molecules (C) The edges of CuO nanoflakes split partially as a result of an interaction between SDBS and the CuOcrystal surface while the central area is still integral (d) Irregular nanorods are formed in situ at the previous partially split positions and (e) Straightnanorods and perfect CuO nanobundles are formed finally

ones as what are displayed in Figures 1(a and b) In con-trast the sample prepared at 130 C for 24 h without sur-factant of SDBS only included some small nanoplateletsand some layered CuO nanostructures due to the directdecomposition of Cu(OH)2 precursor (Fig 3(e))A possible growth mechanism for the formation of

bundle-like CuO nanostructures is proposed based on theabove controlled experimental results The growth of CuOnanobundles in our case consisted of a five-step process asschemed in Figure 4(1) The Cu(OH)2 precursor precipitates as very smallnanoflakes due to the H-bonds existing and connecting its(010) planes (step a)1840

(2) Flake-like CuO nanostructures with some rough sur-faces form through both the decomposition of Cu(OH)2molecules and the breakage of H-bonds (step b)(3) The edges of the CuO nanoflakes split partially withthe assistance of the surfactant SDBS while the centralpart is still integrated (step c)(4) Irregular CuO nanorods with different lengths areformed in situ at the previous partially split positions Therods are connected together with one end at the centralpart of CuO nanoflakes (step d)(5) The irregular CuO nanorods transform gradually intostraight ones and perfect bundle-like CuO nanostructuresare produced (step e) finally

It is obvious that SDBS is very crucial during the entireCuO growth process In the beginning SDBS interactsdirectly with the CuO facets to produce partially split CuOnanoflakes and subsequently it acts as a soft template forthe growth of CuO nanorods and the formation of the finalbundles

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

5126 J Nanosci Nanotechnol 10 5121ndash5128 2010

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Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

J Nanosci Nanotechnol 10 5121ndash5128 2010 5127

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

5128 J Nanosci Nanotechnol 10 5121ndash5128 2010

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

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LE

Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

many irregular short nanostrips due to the splitting pro-cess By careful observation some tips of the short stripswere curved while the center part of the flakes was stillintegrated (Fig 3(b)) This process is similar to the forma-tion of CuO nanoribbons and nanorings33 With the assis-tance of SDBS in the low temperature region the CuOnanoflakes split easily into small nanoribbons possiblybecause the intense Brownian movement of the surfac-tant molecules might destroy the integrity of CuO crys-tal facets As the reaction time was increased further thepartially split parts of these CuO nanoflakes might growthicker and larger to produce rod-like structures As theSEM image shown in Figure 3(c) many irregular rod-likestructures with a variety of lengths were produced whenthe hydrothermal reaction time was 10 h One ends ofthese nanorods were connected together in the central partof flakes which was preserved well as those in the pre-vious stage However the nanorods were not straight andthe diameter varied according to the position It was obvi-ous that the CuO rod-like structures were formed in situfrom the split parts of nanostrips and the various lengthsmight result from the different extent of partially split-ting When the hydrothermal treatment was progressedfor 18 h the CuO nanobundles were formed to someextent (Fig 3(d)) Compared to the CuO products obtainedfrom 24 h hydrothermal treatment process the nanobun-dles produced at this stage were not perfect Howevermany straight nanorods existed with one ends connectingtogether to form a whole as center of the bundle Thewhole center was the one preserved from the nanoflake atthe initial reaction stage With reaction time prolongingthe CuO nanobundles will continue to grow into perfect

(a)

(e) (d)

(b)

(c)

Fig 4 Schematic diagram of a possible growth mechanism for CuO nanobundles The entire procedure includes five main steps (a) blue Cu(OH)2precursor precipitates as very small nanoflakes due to the H-bonds in its (010) planes (b) Irregular CuO nanoflakes with a rough surface form bythe dehydration of Cu(OH)2 molecules (C) The edges of CuO nanoflakes split partially as a result of an interaction between SDBS and the CuOcrystal surface while the central area is still integral (d) Irregular nanorods are formed in situ at the previous partially split positions and (e) Straightnanorods and perfect CuO nanobundles are formed finally

ones as what are displayed in Figures 1(a and b) In con-trast the sample prepared at 130 C for 24 h without sur-factant of SDBS only included some small nanoplateletsand some layered CuO nanostructures due to the directdecomposition of Cu(OH)2 precursor (Fig 3(e))A possible growth mechanism for the formation of

bundle-like CuO nanostructures is proposed based on theabove controlled experimental results The growth of CuOnanobundles in our case consisted of a five-step process asschemed in Figure 4(1) The Cu(OH)2 precursor precipitates as very smallnanoflakes due to the H-bonds existing and connecting its(010) planes (step a)1840

(2) Flake-like CuO nanostructures with some rough sur-faces form through both the decomposition of Cu(OH)2molecules and the breakage of H-bonds (step b)(3) The edges of the CuO nanoflakes split partially withthe assistance of the surfactant SDBS while the centralpart is still integrated (step c)(4) Irregular CuO nanorods with different lengths areformed in situ at the previous partially split positions Therods are connected together with one end at the centralpart of CuO nanoflakes (step d)(5) The irregular CuO nanorods transform gradually intostraight ones and perfect bundle-like CuO nanostructuresare produced (step e) finally

It is obvious that SDBS is very crucial during the entireCuO growth process In the beginning SDBS interactsdirectly with the CuO facets to produce partially split CuOnanoflakes and subsequently it acts as a soft template forthe growth of CuO nanorods and the formation of the finalbundles

J Nanosci Nanotechnol 10 5121ndash5128 2010 5125

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ARTIC

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Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

5126 J Nanosci Nanotechnol 10 5121ndash5128 2010

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RESEARCH

ARTIC

LE

Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

J Nanosci Nanotechnol 10 5121ndash5128 2010 5127

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

5128 J Nanosci Nanotechnol 10 5121ndash5128 2010

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Temperature is also served as one of the important fac-tors which influences the final morphology of resultantCuO nanostructures On the basis of our experimentalresults temperature determines the extent of splitting pro-cess for the nanoflakes Figure 5 shows SEM images ofCuO samples prepared at different temperatures over a 24 hperiod When the synthesis was carried out at 100 Conly thick strip-like CuO was generated and many tips ofthe partially split strips were curved to form a hook-likeshape At 100 C the low temperature could not providesufficient energy for the intense movement of surfactantmolecules and the crystal facets showed less destructionAs a result only very slight splitting process occurred orig-inally and resulted in the preservation of large integratedcentral areas of nanoflakes as displayed in Figure 5(a) Itappears that 130 C is the optimal temperature for the for-mation of perfect CuO nanobundles possibly because theappropriate extent of splitting of the initial CuO nanoflakescan be obtained with the assistance of surfactant moleculesunder this synthetic temperature and the related SEMimage is also shown in Figure 5(b) If we kept increas-ing the reaction temperature to 160 C or above it wasclearly observed from Figures 5(cndashd) that CuO nanobun-dles became much less prevalent while isolated nanorodswere dominant In particular for the samples preparedat 190 C almost uniform separate CuO nanorods wereachieved In the higher temperature region in our presentwork CuO nanoflakes were split completely into nanos-trips at the initial stage of the reaction and the central partof the flakes could not be preserved any more Therefore

(a) (b)

(d)(c)

Fig 5 SEM images of CuO products prepared at (a) 100 C (b) 130 C (c) 160 C and (d) 190 C with a hydrothermal reaction time of 24 h

isolated nanorods developed from the split nanostrips couldbe finally obtained It is interesting that the length of theisolated CuO nanorods is almost comparable to that ofnanorods constituted the CuO nanobundlesThe final morphology of the CuO nanostructures is

determined by many factors instead of only the reactiontime or temperature as investigated above It is widelyaccepted that the final shape of the resultant CuO is alsorelevant to the concentration of the starting materials pHof the system solvent surfactant type and so on Forexample only flower-like CuO consisting of nanosheetscould be obtained when the same approach and syntheticparameters were employed except that the surfactant ofSDBS was substituted with cetyltrimethylammonium bro-mide (CTAB) From the FESEM and TEM images inFigures 6(andashc) we found that most nanosheets were inthe range of 05ndash2 m while the size of each CuOmicroflower was approximately 2ndash4 m A close observa-tion (Fig 6(c)) revealed that the surface and edges of thesenanosheets were not smooth The SAED patterns (inset inFig 6(c)) and typical XRD pattern shown in Figure 6(d)demonstrate that the CuO microflowers are single crys-talline in the monoclinic phase with no impuritiesGenerally the surfactant is considered to kinetically

control the growth rates of different crystallographic facetsof CuO nanostructures through preferentially adsorbingand desorbing on these facets However the mechanism ofmorphology-control using surfactants is quite complicatedsince different surfactants may play different roles for adefinite nanomaterial which might lead to the formation

5126 J Nanosci Nanotechnol 10 5121ndash5128 2010

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

J Nanosci Nanotechnol 10 5121ndash5128 2010 5127

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

5128 J Nanosci Nanotechnol 10 5121ndash5128 2010

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Chen et al Three-Dimensional CuO Nanobundles Consisted of Nanorods

(a)

(c)

(d)

(b)

Fig 6 (a) SEM image and (b) TEM image of CuO microflowers with a panoramic view (c) TEM image and SAED pattern (in the inset) of thepetals of the CuO microflowers (d) XRD pattern of the obtained CuO microflowers

of different final shapes The molecule of SDBS has ahydrophilic group of DBSminus that can coordinate with Cu2+

cations to produce Cu(DBS)2 (Eq (1)) With increasingtemperature and the presence of NaOH the compoundCu(DBS)2 can be transformed gradually into Cu(OH)2precursor and then this blue precursor can precipitate(Eq (2)) When the temperature is high enough CuOcan be produced by the dehydration of Cu(OH)2 and thesimultaneous release of DBSminus anions (Eq (3)) which candetermine the extent of splitting of the initially gener-ated CuO nanoflakes by interacting directly with its sur-faces as well as act as a template for the following crystalgrowth The detailed chemical reactions occurring duringthe hydrothermal process can be expressed as follows

Cu2++2DBSminus minusrarr CuDBS2 (1)

CuDBS2+2OHminus minusrarr CuOH2+2DBSminus (2)

CuOH2 minusrarr CuO+H2O (3)

Although SDBS was found from the above analysis toplay important roles as both ligand and soft template thepractical amounts of starting materials for SDBS and cop-per salts (0021 M0037 M) in the experiments was lessthan the stoichiometric ratio based on the reaction equa-tions which probably make the SDBS act more as template

to large extent SDBS was closely associated with alkaliduring the entire reaction process which made its rolemore complex than that of CTAB in the synthesis of CuOmicroflowers The detailed reasons for SDBS- and CTAB-assisted formation of different final CuO nanostructuresneeds to be studied further and the related work is cur-rently under way

4 CONCLUSIONS

In summary we have employed a simple hydrother-mal method to prepare CuO nanobundles with size of08ndash1 m in the presence of SDBS These nanobundleswere assembled by many nanorods with one ends grow-ing together to form a center and another ends radiatinglaterally from it The length and the diameter of theseCuO nanorods are in the range of 200ndash300 nm and about20ndash30 nm respectively It was found that reaction timetemperature and surfactants played important roles in theformation of such novel CuO nanostructures Isolated CuOnanorods were obtained when the temperature increased to190 C and CuO microflowers composed of nearly rhom-bic nanosheets were achieved at 130 C when CTAB wasemployed instead of SDBS

J Nanosci Nanotechnol 10 5121ndash5128 2010 5127

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

1 Y N Xia P D Yang Y G Sun Y Y Wu B Mayers B GatesY D Yin F Kim and Y Q Yan Adv Mater 15 353 (2003)

2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

7 1350 (2007)7 X S Fang C H Ye L D Zhang J X Zhang J W Zhao and

P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

(1995)9 F Gao Q Y Lu S H Xie and D Y Zhao Adv Mater 14 1537

(2002)10 Y D Yin and G Y Hong J Nanopart Res 8 755 (2006)11 G X Cao X Y Song H Y Yu C H Fan Z L Yin and S X

Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

P K Patanjali Appl Phys Lett 84 1180 (2004)15 J A Switzer H M Kothari P Poizot S Nakanishi and E W

Bohannan Nature 425 490 (2003)16 G H Du and G Van Tendeloo Chem Phys Lett 393 64 (2004)17 M H Cao C W Hu Y H Wang Y H Guo C X Guo and E B

Wang Chem Commun 1884 (2003)18 Z H Yang J Xu W X Zhang A P Liu and S P Tang J Solid

State Chem 180 1390 (2007)19 W Wang Z Liu Y Liu C Xu C Zheng and G Wang Appl

Phys A 76 417 (2003)20 Y Chang and H C Zeng Cryst Growth Des 4 397 (2004)

21 H M Xiao S Y Fu L P Zhu Y Q Li and G Yang Eur J InorgChem 2007 1966 (2007)

22 Q Liu Y Y Liang H J Liu J M Hong and Z Xu Mater ChemPhys 98 519 (2006)

23 C H Lu L M Qi J H Yang D Y Zhang N Z Wu and J MMa J Phys Chem B 108 17825 (2004)

24 H L Xu W Z Wang W Zhu L Zhou and M L Ruan CrystGrowth Des 7 2720 (2007)

25 Q Liu H J Liu Y Y Liang Z Xu and Y Gui Mater Res Bull41 697 (2006)

26 G F Zou H Li D W Zhang K Xiong C Dong and Y T QianJ Phys Chem B 110 1632 (2006)

27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

5128 J Nanosci Nanotechnol 10 5121ndash5128 2010

Delivered by Ingenta toSung Kyun Kwan University

IP 115145200148Mon 03 May 2010 030739

RESEARCH

ARTIC

LE

Three-Dimensional CuO Nanobundles Consisted of Nanorods Chen et al

Acknowledgments The authors are grateful for thefinancial support from the BK21 Project through Schoolof Advanced Materials Science and Engineering

References and Notes

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2 M A EI-Sayed Acc Chem Res 34 257 (2001)3 A M Cao J S Hu H P Liang and L J Wan Angew Chem Int

Ed 44 4391 (2005)4 L Fan and R Guo Cryst Growth Des 8 2150 (2008)5 Y L Hou H Kondoh and T Ohta Chem Mater 17 3994

(2005)6 Y Y Li J P Liu X T Huang and G Y Li Cryst Growth Des

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P Yan Small 1 422 (2005)8 C B Murray C R Kagan and M G Bawendi Science 270 1335

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Sun Mater Res Bull 41 232 (2006)12 H Takeda and K Yoshino Phys Rev B 67 5109 (2003)13 A L Sauvet and J Fouletier J Power Sources 101 259 (2001)14 A Chowdhuri V Gupta K Sreenivas R Kumar S Mozumdar and

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27 R A Zarate F Hevia S Fuentes V M Fuenzalida and A ZuacutentildeigaJ Solid State Chem 180 1464 (2007)

28 K B Zhou R P Wang B Q Xu and Y D Li Nanotechnology17 3939 (2006)

29 H Y Chen S M Park J H Lee X H Meng D W Shin andJ B Yoo Electron Mater Lett 4 161 (2008)

30 Z H Liang and Y J Zhu Chem Lett 34 214 (2005)31 L K Zheng and X J Liu Mater Lett 61 2222 (2007)32 H W Hou Y Xie and Q Li Cryst Growth Des 5 201 (2005)33 X Q Wang G C Xi S L Xiong Y K Liu B J Xi W C Yu

and Y T Qian Cryst Growth Des 7 930 (2007)34 X Y Song H Y Yu and S X Sun J Colloid Interface Sci 289

588 (2005)35 S Z Li H Zhang Y J Ji and D R Yang Nanotechnology 15

1428 (2004)36 B Liu and H C Zeng J Am Chem Soc 126 8124 (2004)37 M M Titirici M Antonietti and A Thomas Chem Mater 18

3808 (2006)38 M Vaseem A Umar S H Kim and Y B Hahn J Phys Chem C

112 5729 (2008)39 M Vaseem A Umar Y B Hahn D H Kim K S Lee J S Jang

and J S Lee Catalysis Commun 10 11 (2008)40 W X Zhang X G Wen and S H Yang Inorg Chem 42 5005

(2003)41 L X Yang Y J Zhu H Tong L Li and L Zhang Mater Chem

Phys 112 442 (2008)42 C Q Chen Y H Zheng Y Y Zhan X Y Lin Q Zheng and

K M Wei Cryst Growth Des 8 3549 (2008)

Received 19 May 2009 Accepted 4 August 2009

5128 J Nanosci Nanotechnol 10 5121ndash5128 2010