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Journal of Crystal Growth 304 (2007) 206–210

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Shape-controlled synthesis of Cu2O nanocrystals assisted by PVP andapplication as catalyst for synthesis of carbon nanofibers

Hui Zhang, Xue Ren, Zuolin Cui�

Key Laboratory of Nanomaterials, Qingdao University of Science & Technology, Zhengzhou Road 53, Qingdao 266042, PR China

Received 15 September 2006; received in revised form 22 December 2006; accepted 24 January 2007

Communicated by B.A. Korgel

Available online 7 March 2007

Abstract

In this paper, cuprous oxide (Cu2O) nanostructures with different shapes, such as spheres, cubes and rods, have been synthesized by

reducing copper nitrate trihydrate with ethylene glycol in the present of poly(vinylypyrrolidone) (PVP). The molar ratio of PVP (in the

repeating unit)/Cu(NO3)2 � 3H2O and reaction temperature have significant effects on the formation and growth of these Cu2O

nanostructures. The Cu2O nanorods were fabricated at the molar ratios of PVP/Cu(NO3)2 � 3H2O 2–3, while spherical and cubic

nanoparticles were formed at the ratios of PVP/Cu(NO3)2 � 3H2O 5–7 and 10–15. Increasing with reaction temperatures, monodisperse

particles were obtained. The as-synthesized nanoparticles and nanorods were investigated by transmission electron microscopy (TEM),

and field emission scanning electron microscopy (FE-SEM) and X-ray diffractometry (XRD). With the as-prepared nanoparticles as

catalyst, carbon nanofibers (CNFs) were synthesized by catalytic polymerization of acetylene at a lower temperature (250 1C). The effects

of the catalyst particle sizes on the morphologies of the carbon fibers were studied.

r 2007 Published by Elsevier B.V.

Keywords: A1. Nanocube; A1. Nanosphere; A1. Nanorod; B1. Carbon nanofibers

1. Introduction

The shape and size of nanoparticles have importantinfluence on material physical and chemical properties,such as electronic, optical, thermal and catalytic properties.For example, colloidal silver particles with triangle,pentagon and sphere show red, green and blue colors inplasmon resonance [1]. Therefore, the synthesis of nano-particles with well-controlled shape and size has become afocus for material researches [2].

Among the metal oxides, cuprous oxide (Cu2O) hasreceived considerable attention in recent years for itsintrinsic properties. Due to smaller band gap, Cu2O haspotential applications in solar energy conversion [3],photocatalyst [4], lithium ion batteries [5] and gas sensors[6]. Hence, more efforts have been devoted to shape-controlled synthesis Cu2O particles with various methods.

e front matter r 2007 Published by Elsevier B.V.

rysgro.2007.01.043

ing author. Tel./fax: +86 532 84022869

ess: cuizl@public.qd.sd.cn (Z. Cui).

Liu et al. reported that cubic and pyramidal Cu2O particleswere synthesized by electrodeposition on InP(0 0 1) wafers[7]. An arc discharge method for preparing Cu2O sphericalnanoparticles was also reported [8]. Kumar et al. fabricatedcubic crystalline Cu2O embedded in a polyaniline matrix bythe sonochemical method [9].Nanoparticles with various shapes, such as cubic and

hollow cubic [10], hollow spherical [11], octahedral [12] andtriangular platelike [13], have been synthesized by solution-phase method since it is simple and effective. However, theCu2O nanoparticles synthesized by organic solution-phasemethod were rarely reported.Here, we describe a new organic solution-phase method

for fabricating cuprous oxide nanostructures with differentshapes and narrow size distribution. These nanostructureswere obtained by reducing Cu(NO3)2 � 3H2O with ethyleneglycol in the presence of PVP at 160 1C. The catalyticproperty of the as-prepared Cu2O nanoparticles was testedin the synthesis of carbon nanofibers (CNFs) by polymer-ization of acetylene at a lower temperature.

ARTICLE IN PRESSH. Zhang et al. / Journal of Crystal Growth 304 (2007) 206–210 207

2. Experimental section

2.1. Preparation of Cu2O nanostructures

All of the chemical reagents used in the experiment wereanalytical grade. Cu2O nanostructures were synthesized bythe following procedures. First, 5ml of anhydrous ethyleneglycol was put into a beaker, and heated at 160 1C for 1 h.Next, 3ml of ethylene glycol solution of Cu(NO3)2 � 3H2O(0.03–0.15M) and 3ml of ethylene glycol solution ofPVP (0.30–0.75M in repeating unites, molecular weight5000–700,000) were added to above solution. The mixtureof reaction solution was stirred about 50min andcontinued to be heated at 160 1C for 2 h. Finally, theyellow deposition was separated from the solution bycentrifugation and washed with ethanol for several times.After removing the supernatant, the deposition containingparticles was collected and redispersed in ethanol forfurther characterization.

2.2. CNFs preparation

The preparation of CNFs has been described in otherliterature [14]. In brief, the main apparatus was a fixed-bedquartz glass tube reactor (6 cm inside diameter� 90 cm),which was electrically heated from the outside. The Cu2O

Fig. 1. (a, b) TEM and SEM images of cuprous oxide particles synthesized at

160 1C for 2 h. The inset shows the selected-area electron diffraction (SAED

particles. (c) An XRD pattern of the as-prepared nanocubes.

particle catalyst was put in a ceramic boat, which wasplaced in the center of the reactor. Acetylene was used asthe gas resource of the reaction. When the temperature ofthe reactor was heated to �250 1C, the acetylene gas wasput in the reactor and CNFs were grown on these Cu2Oparticles.

2.3. Characterization of the Cu2O nanostructures

The as-prepared Cu2O nanostructures were character-ized by powder X-ray diffraction (XRD), field-emissionscanning electron microscope (FE-SEM), and transmissionelectron microscope (TEM). XRD pattern was obtainedusing a D/MAX-500 X-ray powder diffractometer with CuKa radiation (l ¼ 1.5418 A), and the operation voltageand current were maintained at 40 kV and 70mA,respectively. The morphologies of Cu2O were examinedby a FE-SEM (JEOL JSM-6700) operated at 5.0 kV and aTEM (JEM-2000EX) operated at an acceleration voltageof 160 kV.

3. Results and discussion

When the ratio of PVP/Cu(NO3)2 � 3H2O was inthe range of 10–15, cubic particles were obtained. Fig. 1shows the results of TEM, SEM and XRD at the ratio of

PVP/Cu(NO3)2 � 3H2O ¼ 12.5 (Cu(NO3)2 � 3H2O 0.06M, PVP 0.75M) at

) pattern obtained by focusing the electron beam on two cuprous oxide

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PVP/Cu(NO3)2 � 3H2O ¼ 12.5. As shown in Fig. 1a and b,all of particles display a cubic-like morphology and have anaverage edge length of 130 nm. And the correspondingselected area electron diffraction (SAED) patterns (theinset in Fig. 1a) reveal that cuprous oxide particles aresingle crystal. Fig. 1c shows an XRD pattern of the as-prepared cuprous oxide cubes. All the peaks are labeledand can be readily indexed to a crystalline cubic phaseCu2O with lattice constant a ¼ 4.260 A (JCPDS 65-3288).The lattice constant was calculated by Scherrer formula tobe a ¼ 4.263 A, which was consistent with the literaturestandard value.

In the preparation processing, the molar ratio of PVP/Cu(NO3)2 � 3H2O has a significant effect on the shape ofCu2O nanocrystals. Fig. 2 shows SEM images of Cu2Oparticles synthesized at various molar ratios of PVP/Cu(NO3)2 � 3H2O after reaction for 2 h. As the molar ratioof PVP/Cu(NO3)2 � 3H2O decreases from 5 to 3, the shapeof the nanocrystals changes from nanosphere to nanorod.When the ratio of PVP/Cu(NO3)2 � 3H2O is in the range of5–7, all the particles appear spherical and the averagesize of these particles is about 260 nm. When the ratio of

Fig. 2. SEM images of cuprous oxide particles and rods synthesized a

Cu(NO3)2 � 3H2O ¼ 6.25 (Cu(NO3)2 � 3H2O 0.12M, PVP 0.75M) and (b) PVP

Fig. 3. SEM images of Cu2O nanoparticles prepared at same PVP/Cu(NO3)2 �

0.75M, Cu(NO3)2 � 3H2O 0.06M): (a) 120 1C and (b) 140 1C.

PVP/Cu(NO3)2 � 3H2O is in the range of 2–3, the nanorodswere obtained. The average diameter and length of as-prepared nanorods are 70 and 700 nm, respectively.The shape and size distribution of Cu2O nanoparticles

are also influenced by the reaction temperature. To studythe influence of reaction temperature on the morphologiesof cuprous oxide, Cu2O nanoparticles were prepared atdifferent temperatures (120 and 140 1C). Fig. 3a shows aSEM image of Cu2O nanoparticles synthesized at 120 1Cfor 2 h, and the edge length of cubic Cu2O nanoparticlesvaries between 60 and 180 nm and these cubic particleshave rough surfaces. Fig. 3b shows a SEM image of theCu2O nanoparticles that synthesized at 140 1C for 2 h. Themean size of these particles decreases to 130 nm, althoughmost of these nanoparticles have not been completelyevolved into nanocubes. Recalling Figs. 1b, 2a, and 3b, wecan know that the average size of the Cu2O nanoparticleswas decreased and the size uniformity of the nanoparticleswas improved with increasing temperatures.The reasons for this may be of that the increase in the

reaction temperature results in a rapid diffusion andmolecular dispersion, which makes it possible for Cu2O

t various PVP/Cu(NO3)2 � 3H2O ratios at 160 1C for 2 h. (a) PVP/

/Cu(NO3)2 � 3H2O ¼ 2 (Cu(NO3)2 � 3H2O 0.15M, PVP 0.3M).

3H2O with different temperature for 2 h (PVP/Cu(NO3)2 � 3H2O ¼ 2, PVP

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Fig. 4. SEM images of carbon fibers prepared by using cuprous oxide as catalyst under atmosphere pressure at 250 1C: (a) cubic particles and (b) spherical

particles.

H. Zhang et al. / Journal of Crystal Growth 304 (2007) 206–210 209

to nucleate more, rapidly and homogeneously. Thus, themonodisperse Cu2O nanocubes are synthesized at highertemperature.

The catalytic property of the as-prepared Cu2O nano-particles was tested also in the synthesis of CNFs bypolymerization of acetylene. Fig. 4a shows the SEM imageof the carbon fibers synthesized with Cu2O nanocubes ascatalysts. We can see that the diameter of most of carbonfibers is about 170.2 mm and the length of these carbonfibers is up to few micrometers. Some coil carbon fiberswith an average fiber diameter of 100 nm are also shown inFig. 4a.

When spherical Cu2O particles were used as catalyst,more uniform carbon fibers with an average diameter of200 nm are synthesized (Fig. 4b). From these SEM images,we can infer that the diameter of carbon fibers wasinfluenced by the size of Cu2O nanoparticles. Why carbonfibers with larger diameters were fabricated with small sizecatalyst particles? The reason may be that the small cubicCu2O nanoparticles tend to aggregate into larger particlesat higher temperatures and carbon fibers with largerdiameters were synthesized on these aggregative particles.

The reactions processes for synthesis of Cu2O nanos-tructures in this experiment can be described as follows:

2HOCH2CH2OH! 2CH3CHOþ 2H2O;

Cu2þ þ CH3CHOþ 2OH� ! Cuþ þ CH3COOHþH2O;

2Cuþ þ 2OH� ! 2CuOH! Cu2OþH2O:

The effect of PVP on the morphology of the cuprousoxide can be explained by the selective adsorption of PVPpolymer on the surfaces of the cuprous oxide [15]. It isbelieved that PVP kinetically controlled the growth rates ofvarious faces of Cu2O through adsorption on thesesurfaces. Chemical interaction (the formation of coordina-tion bonds between PVP and Cu2O surfaces) might also beinvolved because there seemed to exist in selectivity forthe functional group on the capping reagent. Similar to the‘‘poisoning’’ mechanism proposed to account for theanisotropic growth of other materials. PVP macromole-

cules could selectively interact with different faces of Cu2Onanostructures through Cu–O and Cu–N coordinationbonds. As a result, the growth rates of some surfacescovered by PVP would be greatly reduced, and those ofsurfaces uncovered by PVP kept normal growth rates,leading to a highly anisotropic growth for cuprous oxide.If the concentration of PVP was high, all the surfaces of

the cuprous oxide were almost covered by PVP. Theequaxial growth occurs, so spherical and cubic cuprousoxide particles were obtained. If the concentration of PVPwas low, only a part of the surfaces of the cuprous oxidewere covered by PVP, leading to an anisotropic growth. Inthis case, cuprous oxide rods were obtained.

4. Conclusion

In this work, we used a new organic solution-phasemethod for fabricating cuprous oxide nanostructureswith different shapes. When the molar ratio of PVP/Cu(NO3)2 � 3H2O is high, cubic and spherical particleswere synthesized; when the molar ratio of PVP/Cu(NO3)2 � 3H2O is low, Cu2O nanorods are fabricated.Increasing with reaction temperatures, monodisperse par-ticles were obtained. The catalytic property of the as-prepared Cu2O nanoparticles was tested in the synthesis ofCNFs by polymerization of acetylene. Uniform CNFs withdiameter of 200 nm were obtained by using the sphericalparticles. The effect of PVP on the morphology of thecuprous oxide was discussed.

Acknowledgment

This work has been financially supported by the NaturalScience Foundation of Shan Dong Province.

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