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Shape-controlled synthesis of Cu2O nanocrystals assisted by PVP and application as catalyst for synthesis of carbon nanofibers
ARTICLE IN PRESS
Journal of Crystal Growth 304 (2007) 206–210 www.elsevier.com/locate/jcrysgro
Shape-controlled synthesis of Cu2O nanocrystals assisted by PVP and application 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 important influence 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 in plasmon resonance [1]. Therefore, the synthesis of nanoparticles with well-controlled shape and size has become a focus for material researches [2]. Among the metal oxides, cuprous oxide (Cu2O) has received considerable attention in recent years for its intrinsic properties. Due to smaller band gap, Cu2O has potential applications in solar energy conversion [3], photocatalyst [4], lithium ion batteries [5] and gas sensors [6]. Hence, more efforts have been devoted to shapecontrolled synthesis Cu2O particles with various methods. Corresponding author. Tel./fax: +86 532 84022869
E-mail address: [email protected] (Z. Cui). 0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.01.043
Liu et al. reported that cubic and pyramidal Cu2O particles were synthesized by electrodeposition on InP(0 0 1) wafers [7]. An arc discharge method for preparing Cu2O spherical nanoparticles was also reported [8]. Kumar et al. fabricated cubic crystalline Cu2O embedded in a polyaniline matrix by the sonochemical method [9]. Nanoparticles with various shapes, such as cubic and hollow cubic [10], hollow spherical [11], octahedral [12] and triangular platelike [13], have been synthesized by solutionphase method since it is simple and effective. However, the Cu2O nanoparticles synthesized by organic solution-phase method were rarely reported. Here, we describe a new organic solution-phase method for fabricating cuprous oxide nanostructures with different shapes and narrow size distribution. These nanostructures were obtained by reducing Cu(NO3)2 3H2O with ethylene glycol in the presence of PVP at 160 1C. The catalytic property of the as-prepared Cu2O nanoparticles was tested in the synthesis of carbon nanofibers (CNFs) by polymerization of acetylene at a lower temperature.
ARTICLE IN PRESS H. Zhang et al. / Journal of Crystal Growth 304 (2007) 206–210
2. Experimental section 2.1. Preparation of Cu2O nanostructures All of the chemical reagents used in the experiment were analytical grade. Cu2O nanostructures were synthesized by the following procedures. First, 5 ml of anhydrous ethylene glycol was put into a beaker, and heated at 160 1C for 1 h. Next, 3 ml of ethylene glycol solution of Cu(NO3)2 3H2O (0.03–0.15 M) and 3 ml of ethylene glycol solution of PVP (0.30–0.75 M in repeating unites, molecular weight 5000–700,000) were added to above solution. The mixture of reaction solution was stirred about 50 min and continued to be heated at 160 1C for 2 h. Finally, the yellow deposition was separated from the solution by centrifugation and washed with ethanol for several times. After removing the supernatant, the deposition containing particles was collected and redispersed in ethanol for further characterization.
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particle catalyst was put in a ceramic boat, which was placed in the center of the reactor. Acetylene was used as the gas resource of the reaction. When the temperature of the reactor was heated to 250 1C, the acetylene gas was put in the reactor and CNFs were grown on these Cu2O particles. 2.3. Characterization of the Cu2O nanostructures The as-prepared Cu2O nanostructures were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and transmission electron microscope (TEM). XRD pattern was obtained using a D/MAX-500 X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 A˚), and the operation voltage and current were maintained at 40 kV and 70 mA, respectively. The morphologies of Cu2O were examined by a FE-SEM (JEOL JSM-6700) operated at 5.0 kV and a TEM (JEM-2000EX) operated at an acceleration voltage of 160 kV.
2.2. CNFs preparation 3. Results and discussion The preparation of CNFs has been described in other literature [14]. In brief, the main apparatus was a fixed-bed quartz glass tube reactor (6 cm inside diameter 90 cm), which was electrically heated from the outside. The Cu2O
When the ratio of PVP/Cu(NO3)2 3H2O was in the range of 10–15, cubic particles were obtained. Fig. 1 shows the results of TEM, SEM and XRD at the ratio of
Fig. 1. (a, b) TEM and SEM images of cuprous oxide particles synthesized at PVP/Cu(NO3)2 3H2O ¼ 12.5 (Cu(NO3)2 3H2O 0.06 M, PVP 0.75 M) at 160 1C for 2 h. The inset shows the selected-area electron diffraction (SAED) pattern obtained by focusing the electron beam on two cuprous oxide particles. (c) An XRD pattern of the as-prepared nanocubes.
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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 an average edge length of 130 nm. And the corresponding selected area electron diffraction (SAED) patterns (the inset in Fig. 1a) reveal that cuprous oxide particles are single crystal. Fig. 1c shows an XRD pattern of the asprepared cuprous oxide cubes. All the peaks are labeled and can be readily indexed to a crystalline cubic phase Cu2O with lattice constant a ¼ 4.260 A˚ (JCPDS 65-3288). The lattice constant was calculated by Scherrer formula to be a ¼ 4.263 A˚, which was consistent with the literature standard value. In the preparation processing, the molar ratio of PVP/ Cu(NO3)2 3H2O has a significant effect on the shape of Cu2O nanocrystals. Fig. 2 shows SEM images of Cu2O particles synthesized at various molar ratios of PVP/ Cu(NO3)2 3H2O after reaction for 2 h. As the molar ratio of PVP/Cu(NO3)2 3H2O decreases from 5 to 3, the shape of the nanocrystals changes from nanosphere to nanorod. When the ratio of PVP/Cu(NO3)2 3H2O is in the range of 5–7, all the particles appear spherical and the average size of these particles is about 260 nm. When the ratio of
PVP/Cu(NO3)2 3H2O is in the range of 2–3, the nanorods were obtained. The average diameter and length of asprepared nanorods are 70 and 700 nm, respectively. The shape and size distribution of Cu2O nanoparticles are also influenced by the reaction temperature. To study the influence of reaction temperature on the morphologies of cuprous oxide, Cu2O nanoparticles were prepared at different temperatures (120 and 140 1C). Fig. 3a shows a SEM image of Cu2O nanoparticles synthesized at 120 1C for 2 h, and the edge length of cubic Cu2O nanoparticles varies between 60 and 180 nm and these cubic particles have rough surfaces. Fig. 3b shows a SEM image of the Cu2O nanoparticles that synthesized at 140 1C for 2 h. The mean size of these particles decreases to 130 nm, although most of these nanoparticles have not been completely evolved into nanocubes. Recalling Figs. 1b, 2a, and 3b, we can know that the average size of the Cu2O nanoparticles was decreased and the size uniformity of the nanoparticles was improved with increasing temperatures. The reasons for this may be of that the increase in the reaction temperature results in a rapid diffusion and molecular dispersion, which makes it possible for Cu2O
Fig. 2. SEM images of cuprous oxide particles and rods synthesized at various PVP/Cu(NO3)2 3H2O ratios at 160 1C for 2 h. (a) PVP/ Cu(NO3)2 3H2O ¼ 6.25 (Cu(NO3)2 3H2O 0.12 M, PVP 0.75 M) and (b) PVP/Cu(NO3)2 3H2O ¼ 2 (Cu(NO3)2 3H2O 0.15 M, PVP 0.3 M).
Fig. 3. SEM images of Cu2O nanoparticles prepared at same PVP/Cu(NO3)2 3H2O with different temperature for 2 h (PVP/Cu(NO3)2 3H2O ¼ 2, PVP 0.75 M, Cu(NO3)2 3H2O 0.06 M): (a) 120 1C and (b) 140 1C.
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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.
to nucleate more, rapidly and homogeneously. Thus, the monodisperse Cu2O nanocubes are synthesized at higher temperature. The catalytic property of the as-prepared Cu2O nanoparticles was tested also in the synthesis of CNFs by polymerization of acetylene. Fig. 4a shows the SEM image of the carbon fibers synthesized with Cu2O nanocubes as catalysts. We can see that the diameter of most of carbon fibers is about 170.2 mm and the length of these carbon fibers is up to few micrometers. Some coil carbon fibers with an average fiber diameter of 100 nm are also shown in Fig. 4a. When spherical Cu2O particles were used as catalyst, more uniform carbon fibers with an average diameter of 200 nm are synthesized (Fig. 4b). From these SEM images, we can infer that the diameter of carbon fibers was influenced by the size of Cu2O nanoparticles. Why carbon fibers with larger diameters were fabricated with small size catalyst particles? The reason may be that the small cubic Cu2O nanoparticles tend to aggregate into larger particles at higher temperatures and carbon fibers with larger diameters were synthesized on these aggregative particles. The reactions processes for synthesis of Cu2O nanostructures in this experiment can be described as follows: 2HOCH2 CH2 OH ! 2CH3 CHO þ 2H2 O; Cu2þ þ CH3 CHO þ 2OH ! Cuþ þ CH3 COOH þ H2 O; 2Cuþ þ 2OH ! 2CuOH ! Cu2 O þ H2 O: The effect of PVP on the morphology of the cuprous oxide can be explained by the selective adsorption of PVP polymer on the surfaces of the cuprous oxide [15]. It is believed that PVP kinetically controlled the growth rates of various faces of Cu2O through adsorption on these surfaces. Chemical interaction (the formation of coordination bonds between PVP and Cu2O surfaces) might also be involved because there seemed to exist in selectivity for the functional group on the capping reagent. Similar to the ‘‘poisoning’’ mechanism proposed to account for the anisotropic growth of other materials. PVP macromole-
cules could selectively interact with different faces of Cu2O nanostructures through Cu–O and Cu–N coordination bonds. As a result, the growth rates of some surfaces covered by PVP would be greatly reduced, and those of surfaces 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. The equaxial growth occurs, so spherical and cubic cuprous oxide particles were obtained. If the concentration of PVP was low, only a part of the surfaces of the cuprous oxide were covered by PVP, leading to an anisotropic growth. In this case, cuprous oxide rods were obtained. 4. Conclusion In this work, we used a new organic solution-phase method for fabricating cuprous oxide nanostructures with different shapes. When the molar ratio of PVP/ Cu(NO3)2 3H2O is high, cubic and spherical particles were synthesized; when the molar ratio of PVP/Cu (NO3)2 3H2O is low, Cu2O nanorods are fabricated. Increasing with reaction temperatures, monodisperse particles were obtained. The catalytic property of the asprepared Cu2O nanoparticles was tested in the synthesis of CNFs by polymerization of acetylene. Uniform CNFs with diameter of 200 nm were obtained by using the spherical particles. The effect of PVP on the morphology of the cuprous oxide was discussed. Acknowledgment This work has been financially supported by the Natural Science Foundation of Shan Dong Province. References [1] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, J. Chem. Phys. 116 (2002) 6755. [2] S.W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642.
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[3] R.N. Briskman, Sol. Energy Mater. Sol. Cells 27 (1992) 361. [4] M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J. Kondo, K. Domen, Chem. Commun. 3 (1998) 357. [5] D.B. Wang, D.B. Yu, M.S. Mo, X.M. Liu, Y.T. Qian, Colloid Interface Sci. 261 (2003) 565. [6] J.T. Zhang, J.F. Liu, Y.D. Li, et al., Chem. Mater. 18 (2006) 867. [7] R. Liu, F. Oba, E.W. Bohannanm, F. Ernst, J.A. Switzer, Chem. Mater. 15 (2003) 4882. [8] W.T. Yao, S.H. Yu, Y. Zhou, J. Jiang, Q.S. Wu, L. Zhang, J. Jiang, J. Phys. Chem. B 109 (2005) 14011. [9] R.V. Kumar, Y. Mastai, Y. Diamant, A. Gedanken, J. Mater. Chem. 11 (2001) 1209. [10] (a) L.F. Guo, C.J. Murphy, NanoLetters 3 (2003) 231; (b) D.B. Wang, M.S. Mo, D.B. Yu, L.Q. Xu, F.Q. Li, Y.T. Qian,
Crystal Growth Des. 5 (2003) 717; (c) J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369. [11] (a) Y. Chang, J.J. Teo, H.C. Zeng, Langmuir 21 (2005) 1074; (b) A. Muramatsu, T. Sugimoto, J. Colloid Interface. Sci. 189 (1997) 167. [12] (a) X. Zhang, Y. Xie, F. Xu, X.H. Liu, D. Xu, Inorg. Chem. Commun. 6 (2003) 1390; (b) C.H. Lu, L.M. Qi, J.H. Yang, X.Y. Wang, D.Y. Zhang, J.L. Xie, J.M. Ma, Adv. Mater. 17 (2005) 2562; (c) H.L. Xu, W.Z. Wang, W. Zhu, J. Phys. Chem. B 110 (2006) 13829. [13] C.H. Bernard Ng, W.Y. Fan, J. Phys. Chem. B 110 (2006) 20801. [14] Y. Qing, Z.K. Zhang, Z.L. Cui, Carbon 41 (2003) 3072. [15] M. Almeida, L. Alcacer, J. Crystal Growth 62 (1983) 183.
Journal of Crystal Growth 304 (2007) 206–210 www.elsevier.com/locate/jcrysgro
Shape-controlled synthesis of Cu2O nanocrystals assisted by PVP and application 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 important influence 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 in plasmon resonance [1]. Therefore, the synthesis of nanoparticles with well-controlled shape and size has become a focus for material researches [2]. Among the metal oxides, cuprous oxide (Cu2O) has received considerable attention in recent years for its intrinsic properties. Due to smaller band gap, Cu2O has potential applications in solar energy conversion [3], photocatalyst [4], lithium ion batteries [5] and gas sensors [6]. Hence, more efforts have been devoted to shapecontrolled synthesis Cu2O particles with various methods. Corresponding author. Tel./fax: +86 532 84022869
E-mail address: [email protected] (Z. Cui). 0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.01.043
Liu et al. reported that cubic and pyramidal Cu2O particles were synthesized by electrodeposition on InP(0 0 1) wafers [7]. An arc discharge method for preparing Cu2O spherical nanoparticles was also reported [8]. Kumar et al. fabricated cubic crystalline Cu2O embedded in a polyaniline matrix by the sonochemical method [9]. Nanoparticles with various shapes, such as cubic and hollow cubic [10], hollow spherical [11], octahedral [12] and triangular platelike [13], have been synthesized by solutionphase method since it is simple and effective. However, the Cu2O nanoparticles synthesized by organic solution-phase method were rarely reported. Here, we describe a new organic solution-phase method for fabricating cuprous oxide nanostructures with different shapes and narrow size distribution. These nanostructures were obtained by reducing Cu(NO3)2 3H2O with ethylene glycol in the presence of PVP at 160 1C. The catalytic property of the as-prepared Cu2O nanoparticles was tested in the synthesis of carbon nanofibers (CNFs) by polymerization of acetylene at a lower temperature.
ARTICLE IN PRESS H. Zhang et al. / Journal of Crystal Growth 304 (2007) 206–210
2. Experimental section 2.1. Preparation of Cu2O nanostructures All of the chemical reagents used in the experiment were analytical grade. Cu2O nanostructures were synthesized by the following procedures. First, 5 ml of anhydrous ethylene glycol was put into a beaker, and heated at 160 1C for 1 h. Next, 3 ml of ethylene glycol solution of Cu(NO3)2 3H2O (0.03–0.15 M) and 3 ml of ethylene glycol solution of PVP (0.30–0.75 M in repeating unites, molecular weight 5000–700,000) were added to above solution. The mixture of reaction solution was stirred about 50 min and continued to be heated at 160 1C for 2 h. Finally, the yellow deposition was separated from the solution by centrifugation and washed with ethanol for several times. After removing the supernatant, the deposition containing particles was collected and redispersed in ethanol for further characterization.
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particle catalyst was put in a ceramic boat, which was placed in the center of the reactor. Acetylene was used as the gas resource of the reaction. When the temperature of the reactor was heated to 250 1C, the acetylene gas was put in the reactor and CNFs were grown on these Cu2O particles. 2.3. Characterization of the Cu2O nanostructures The as-prepared Cu2O nanostructures were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and transmission electron microscope (TEM). XRD pattern was obtained using a D/MAX-500 X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 A˚), and the operation voltage and current were maintained at 40 kV and 70 mA, respectively. The morphologies of Cu2O were examined by a FE-SEM (JEOL JSM-6700) operated at 5.0 kV and a TEM (JEM-2000EX) operated at an acceleration voltage of 160 kV.
2.2. CNFs preparation 3. Results and discussion The preparation of CNFs has been described in other literature [14]. In brief, the main apparatus was a fixed-bed quartz glass tube reactor (6 cm inside diameter 90 cm), which was electrically heated from the outside. The Cu2O
When the ratio of PVP/Cu(NO3)2 3H2O was in the range of 10–15, cubic particles were obtained. Fig. 1 shows the results of TEM, SEM and XRD at the ratio of
Fig. 1. (a, b) TEM and SEM images of cuprous oxide particles synthesized at PVP/Cu(NO3)2 3H2O ¼ 12.5 (Cu(NO3)2 3H2O 0.06 M, PVP 0.75 M) at 160 1C for 2 h. The inset shows the selected-area electron diffraction (SAED) pattern obtained by focusing the electron beam on two cuprous oxide particles. (c) An XRD pattern of the as-prepared nanocubes.
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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 an average edge length of 130 nm. And the corresponding selected area electron diffraction (SAED) patterns (the inset in Fig. 1a) reveal that cuprous oxide particles are single crystal. Fig. 1c shows an XRD pattern of the asprepared cuprous oxide cubes. All the peaks are labeled and can be readily indexed to a crystalline cubic phase Cu2O with lattice constant a ¼ 4.260 A˚ (JCPDS 65-3288). The lattice constant was calculated by Scherrer formula to be a ¼ 4.263 A˚, which was consistent with the literature standard value. In the preparation processing, the molar ratio of PVP/ Cu(NO3)2 3H2O has a significant effect on the shape of Cu2O nanocrystals. Fig. 2 shows SEM images of Cu2O particles synthesized at various molar ratios of PVP/ Cu(NO3)2 3H2O after reaction for 2 h. As the molar ratio of PVP/Cu(NO3)2 3H2O decreases from 5 to 3, the shape of the nanocrystals changes from nanosphere to nanorod. When the ratio of PVP/Cu(NO3)2 3H2O is in the range of 5–7, all the particles appear spherical and the average size of these particles is about 260 nm. When the ratio of
PVP/Cu(NO3)2 3H2O is in the range of 2–3, the nanorods were obtained. The average diameter and length of asprepared nanorods are 70 and 700 nm, respectively. The shape and size distribution of Cu2O nanoparticles are also influenced by the reaction temperature. To study the influence of reaction temperature on the morphologies of cuprous oxide, Cu2O nanoparticles were prepared at different temperatures (120 and 140 1C). Fig. 3a shows a SEM image of Cu2O nanoparticles synthesized at 120 1C for 2 h, and the edge length of cubic Cu2O nanoparticles varies between 60 and 180 nm and these cubic particles have rough surfaces. Fig. 3b shows a SEM image of the Cu2O nanoparticles that synthesized at 140 1C for 2 h. The mean size of these particles decreases to 130 nm, although most of these nanoparticles have not been completely evolved into nanocubes. Recalling Figs. 1b, 2a, and 3b, we can know that the average size of the Cu2O nanoparticles was decreased and the size uniformity of the nanoparticles was improved with increasing temperatures. The reasons for this may be of that the increase in the reaction temperature results in a rapid diffusion and molecular dispersion, which makes it possible for Cu2O
Fig. 2. SEM images of cuprous oxide particles and rods synthesized at various PVP/Cu(NO3)2 3H2O ratios at 160 1C for 2 h. (a) PVP/ Cu(NO3)2 3H2O ¼ 6.25 (Cu(NO3)2 3H2O 0.12 M, PVP 0.75 M) and (b) PVP/Cu(NO3)2 3H2O ¼ 2 (Cu(NO3)2 3H2O 0.15 M, PVP 0.3 M).
Fig. 3. SEM images of Cu2O nanoparticles prepared at same PVP/Cu(NO3)2 3H2O with different temperature for 2 h (PVP/Cu(NO3)2 3H2O ¼ 2, PVP 0.75 M, Cu(NO3)2 3H2O 0.06 M): (a) 120 1C and (b) 140 1C.
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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.
to nucleate more, rapidly and homogeneously. Thus, the monodisperse Cu2O nanocubes are synthesized at higher temperature. The catalytic property of the as-prepared Cu2O nanoparticles was tested also in the synthesis of CNFs by polymerization of acetylene. Fig. 4a shows the SEM image of the carbon fibers synthesized with Cu2O nanocubes as catalysts. We can see that the diameter of most of carbon fibers is about 170.2 mm and the length of these carbon fibers is up to few micrometers. Some coil carbon fibers with an average fiber diameter of 100 nm are also shown in Fig. 4a. When spherical Cu2O particles were used as catalyst, more uniform carbon fibers with an average diameter of 200 nm are synthesized (Fig. 4b). From these SEM images, we can infer that the diameter of carbon fibers was influenced by the size of Cu2O nanoparticles. Why carbon fibers with larger diameters were fabricated with small size catalyst particles? The reason may be that the small cubic Cu2O nanoparticles tend to aggregate into larger particles at higher temperatures and carbon fibers with larger diameters were synthesized on these aggregative particles. The reactions processes for synthesis of Cu2O nanostructures in this experiment can be described as follows: 2HOCH2 CH2 OH ! 2CH3 CHO þ 2H2 O; Cu2þ þ CH3 CHO þ 2OH ! Cuþ þ CH3 COOH þ H2 O; 2Cuþ þ 2OH ! 2CuOH ! Cu2 O þ H2 O: The effect of PVP on the morphology of the cuprous oxide can be explained by the selective adsorption of PVP polymer on the surfaces of the cuprous oxide [15]. It is believed that PVP kinetically controlled the growth rates of various faces of Cu2O through adsorption on these surfaces. Chemical interaction (the formation of coordination bonds between PVP and Cu2O surfaces) might also be involved because there seemed to exist in selectivity for the functional group on the capping reagent. Similar to the ‘‘poisoning’’ mechanism proposed to account for the anisotropic growth of other materials. PVP macromole-
cules could selectively interact with different faces of Cu2O nanostructures through Cu–O and Cu–N coordination bonds. As a result, the growth rates of some surfaces covered by PVP would be greatly reduced, and those of surfaces 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. The equaxial growth occurs, so spherical and cubic cuprous oxide particles were obtained. If the concentration of PVP was low, only a part of the surfaces of the cuprous oxide were covered by PVP, leading to an anisotropic growth. In this case, cuprous oxide rods were obtained. 4. Conclusion In this work, we used a new organic solution-phase method for fabricating cuprous oxide nanostructures with different shapes. When the molar ratio of PVP/ Cu(NO3)2 3H2O is high, cubic and spherical particles were synthesized; when the molar ratio of PVP/Cu (NO3)2 3H2O is low, Cu2O nanorods are fabricated. Increasing with reaction temperatures, monodisperse particles were obtained. The catalytic property of the asprepared Cu2O nanoparticles was tested in the synthesis of CNFs by polymerization of acetylene. Uniform CNFs with diameter of 200 nm were obtained by using the spherical particles. The effect of PVP on the morphology of the cuprous oxide was discussed. Acknowledgment This work has been financially supported by the Natural Science Foundation of Shan Dong Province. References [1] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, J. Chem. Phys. 116 (2002) 6755. [2] S.W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642.
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[3] R.N. Briskman, Sol. Energy Mater. Sol. Cells 27 (1992) 361. [4] M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J. Kondo, K. Domen, Chem. Commun. 3 (1998) 357. [5] D.B. Wang, D.B. Yu, M.S. Mo, X.M. Liu, Y.T. Qian, Colloid Interface Sci. 261 (2003) 565. [6] J.T. Zhang, J.F. Liu, Y.D. Li, et al., Chem. Mater. 18 (2006) 867. [7] R. Liu, F. Oba, E.W. Bohannanm, F. Ernst, J.A. Switzer, Chem. Mater. 15 (2003) 4882. [8] W.T. Yao, S.H. Yu, Y. Zhou, J. Jiang, Q.S. Wu, L. Zhang, J. Jiang, J. Phys. Chem. B 109 (2005) 14011. [9] R.V. Kumar, Y. Mastai, Y. Diamant, A. Gedanken, J. Mater. Chem. 11 (2001) 1209. [10] (a) L.F. Guo, C.J. Murphy, NanoLetters 3 (2003) 231; (b) D.B. Wang, M.S. Mo, D.B. Yu, L.Q. Xu, F.Q. Li, Y.T. Qian,
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