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Controlled modification of multi-walled carbon nanotubes with CuO, Cu2O and Cu nanoparticles
Solid State Sciences 11 (2009) 655–659
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Controlled modification of multi-walled carbon nanotubes with CuO, Cu2O and Cu nanoparticles Xiuying Wang, Feng Zhang, Baiying Xia, Xingfu Zhu, Jiesheng Chen, Shilun Qiu, Ping Zhang, Jixue Li* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2008 Received in revised form 27 September 2008 Accepted 15 October 2008 Available online 30 October 2008
Multi-walled carbon nanotubes (MWNTs) have been successfully modified with CuO, Cu2O and Cu nanoparticles via a simple method, and the calcination temperature, the amount of NH3$H2O and soaking time play critical roles in controlling the final products. The modified MWNTs have been characterized by X-ray diffraction, infrared spectrum, scanning electron and transmission electron microscopies. Optical absorption of the obtained products has also been investigated, and the quantum confinement effect was illustrated in the absorption spectra. Ó 2008 Published by Elsevier Masson SAS.
Keywords: Carbon nanotubes Controlled modification Nanoparticle Heterojunction
1. Introduction Modification of carbon nanotubes (CNTs) has been paid a great deal of interest as a fascinating model system for fundamental scientific research with the potential technological applications. In particular, modification of CNTs with metal oxides is of significance for the development of CNT-based devices. Up to now, various metal oxides such as TiO2, SnO2, ZnO, Fe2O3, MnO2 and RuO2 have been reported to modify CNTs [1–10]. However, to search for a simple and low-cost approach to modify CNTs with the metal oxides is still an active research focus today. Recently, Cu and their oxides have attracted a great deal of attention because of their various applications. For instance, CuO has been widely used as the basis of high temperature superconductors, lithium ion electrode materials and giant magneto resistance materials [11–13]. Cu2O, a p-type semiconductor (Eg ¼ 2.17 eV) with particular optical and magnetic properties, has also been applied in a series of fields, for example solar energy conversion, electronics, magnetic storage, catalysis, and gas sensors [14–19]. Copper, one of the most normal conductors, is usually found in both fundamental researches and technical applications. Therefore, the modification of CNTs with Cu or their oxides is anticipated to extend their applications in many fields such as nano-electronic devices and novel catalysts due to the unique mechanical, electrical and thermal properties of the CNTs. It has * Corresponding author. Tel.: þ86 431 85168581; fax: þ86 431 85168624. E-mail address: [email protected] (J. Li). 1293-2558/$ – see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.10.009
been reported that Cu2O nanoparticles could be deposited on the surface of multi-walled carbon nanotubes (MWNTs) via a polyol process or anodic oxidation of copper electrode in an alkaline solution [20,21], and Cu/CNT composites could be fabricated by the electrochemical co-deposition method [22]. Nevertheless, all those methods are complicated processes, and the costs are usually high. Herein we report a simple and cost-effective approach to modify the MWNTs with CuO, Cu2O and Cu nanoparticles via copper– ammonium complex (Cu(NH3)2þ 4 ) covalently attached to the MWNTs. The components and morphologies of the products can be well controlled by adjusting the calcination temperature, the amount of NH3$H2O and soaking time. 2. Experimental section 2.1. Treatment of MWNTs The commercial MWNTs were treated with nitric acid (80 wt%) at 80 C for 4 h, washed with deionized water several times and dried at 100 C. 2.2. Preparation of CuO/MWNT and Cu/MWNT composites In our experiment, Cu(CH3COO)2$H2O was used as copper source. At first, 16 mg Cu(CH3COO)2$H2O were dissolved in 5 mL deionized water. Then 0.10 mL NH3$H2O (25 wt%) solution was slowly added into the above solution under continuous stirring to form the precursor solution and 10 mg MWNTs were soaked in the
X. Wang et al. / Solid State Sciences 11 (2009) 655–659
solution for 17 h. Subsequently, the MWNTs were centrifugally separated from the solution, dried at 70 C in vacuum, giving the intermediate products. Finally, the intermediate products were calcined at 300 C and 500 C in the presence of N2 for 2 h, respectively, and the CuO/MWNT and Cu/MWNT composites were obtained.
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The preparation processes of Cu2O/MWNT heterojunctions and Cu2O/MWNT composites are the same as that of CuO/MWNT composites, except that the amount of the NH3$H2O solution changed to 0.20 mL, and the soaking times were 17 h and 48 h.
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Two theta (degrees) Fig. 1. XRD patterns of the pure MWNTs, CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites.
The products were characterized by X-ray diffraction (XRD, Rigaku XRD spectrometer with a Cu Ka line of 1.5418 Å), scanning electron microscopy (SEM, JEOL JSM-6700F at 5 kV), and transmission electron microscopy (TEM, JEOL JEM 3010 at 300 kV)
Fig. 2. SEM, TEM and HRTEM images of (a–c) CuO/MWNT, (d–f) Cu2O/MWNT and (g–i) Cu/MWNT composites.
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equipped with energy dispersive X-ray spectroscopy (EDS). The infrared (IR) spectra were recorded on an IFS 66 V/S FTIR spectrometer using KBr pellets. UV–visible spectra were recorded on a Perkin Elmer Lambda 20 UV–visible spectrophotometer. 3. Results and discussion Fig. 1 demonstrates the XRD patterns of the pure MWNTs, CuO/ MWNT, Cu2O/MWNT and Cu/MWNT composites. It is found that the characteristic peaks of the XRD patterns of the modified MWNTs are in good agreement with those of CuO (JCPDS no. 80-1916), Cu2O (JCPDS no. 05-0667) and Cu (JCPDS no. 85-1326), suggesting that no impurities exist in all of the obtained products. Fig. 2 shows the SEM images and TEM images of the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites. It is obviously seen that CuO, Cu2O and Cu nanoparticles are homogeneously coated on the surface of MWNTs and intimately attached to the MWNTs. The particle size distributions of the CuO and Cu2O nanoparticles are in narrow ranges, mainly around 20 nm and 15 nm, whereas that of the Cu nanoparticles is mainly in a wide range of 20–100 nm. To better understand the components of the obtained products, HRTEM has also been recorded. The distances between the parallel lattice planes of the nanoparticles are 0.25 nm, 0.24 nm and 0.20 nm, respectively, which are consistent with the space between (111) planes of CuO, (111) planes of Cu2O and (111) planes of Cu. The compositions of the obtained products were also studied by EDS (Fig. 3), obtained by narrowing down and focusing the beam directly onto the selective nanoparticles on the MWNTs. The EDS demonstrates that the atomic ratio of Cu to O is about 1.0, 1.8, and 3.5 in the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites, respectively, which indicates the phase of the obtained products is CuO, Cu2O and Cu. However, it should be noted that the EDS (Fig. 3c) confirms that oxygen does exist in Cu/MWNT composites. It is most likely that the O atoms are bound to the C atoms in MWNTs but the possibility that Cu-bound O atoms are also present cannot be excluded. In addition, the signals of C and Ni present in all the EDS were originated from the MWNTs and Ni grid. On the basis of the results of XRD, HRTEM and EDS, it is believed that the assynthesis products are pure CuO/MWNT, Cu2O/MWNT and Cu/ MWNT composites, and no impurities are present. Fig. 4a–b shows the SEM images of Cu2O/MWNT heterojunctions. It is seen that Cu2O nanoparticle with the particle size of about 100 nm is attached to the tip of MWNT, in agreement with the observation through transmission electron microscope imaging (Fig. 4c). In order to study the structure of the obtained product, the XRD pattern has been conducted (Fig. 4d). The characteristic peaks of the XRD pattern are in good agreement with those of Cu2O, indicating that the phase of the obtained products are Cu2O. The composition of the obtained product was also studied by EDS. Fig. 4e demonstrates that the atomic ratio of Cu to O is about 2.4, which indicates the component of the obtained products is Cu2O. In order to investigate the growth mechanism of the CuO/ MWNT, Cu2O/MWNT and Cu/MWNT composites, IR spectra were carried out (Fig. 5). A band at 1720 cm1 associated with the stretch mode of carboxylic groups is observed in the IR spectrum of the acid-treated MWNTs, indicating that carboxylic groups are formed due to the oxidation of some carbon atoms on the surface of the MWNTs by nitric acid. In the IR spectrum of the intermediate products, new bands in the range of 3400–3100 cm1, 900– 650 cm1 attributed to the N–H stretching vibration and around 1405 cm1 assigned to C–N stretching vibration are clearly are attached to the observed, demonstrating that Cu(NH3)2þ 4 MWNTs through the reaction with the carboxylic groups. Moreover, the bands at 585 cm1, 620 cm1 and 650 cm1 relative to CuO, Cu2O and Cu are obviously detected, indicating the formation of the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites.
Fig. 3. EDS spectra of (a) CuO/MWNT, (b) Cu2O/MWNT and (c) Cu/MWNT composites.
The process of the modification of MWNTs is multistep, in which various chemical reactions occur. It is impossible to pinpoint the exact chemical reactions, but the likely formation route can be inferred on the basis of XRD patterns and IR spectra. Firstly, acid treatment introduces the carboxylic groups (–COOH) into the MWNTs and NH3$H2O is added into Cu2þ solution to form 2þ Cu(NH3)2þ 4 solution. Afterward, Cu(NH3)4 diffuses to the MWNTs by the electrostatic attraction as the acid-treated MWNTs are soaked into the precursor solution. Subsequently, the MWNTs are separated from the solution centrifugally and dried at 70 C in vacuum, and the intermediate products MWNT–CONH–Cu(NH3)2þ 3 are obtained due to the reaction of the amide and carboxyl [5,23– 25]. Finally, the intermediate products are converted to the final products via the calcination treatment. It is worth noting that the calcination temperature, the amount of NH3$H2O and soaking time play critical roles in controlling the final products. When the precursor solution prepared with 0.10 mL
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Fig. 4. (a and b) SEM images, (c) TEM image, (d) XRD pattern and (e) EDS spectrum of the Cu2O/MWNT heterojunctions.
diffusion of Cu(NH3)2þ to some extent. So when the precursor 4 solution prepared with 0.1 mL NH3$H2O, in which less NHþ 4 were were dispersed homogeneously present, was used, Cu(NH3)2þ 4 around the MWNTs after 17 h and the CuO and Cu nanoparticles
a b Transmittance
NH3$H2O was used, CuO/MWNT and Cu/MWNT composites were obtained after calcination treatment at 300 C and 500 C for 2 h in N2 flows, respectively. It indicates that the decomposition of CONH–Cu(NH3)2þ 3 groups gave birth to CuO and NH3 at 300 C, and the CuO/MWNT composites were obtained, whereas when the calcination temperature was elevated to 500 C, the oxidation– reduction reaction between CuO and NH3 would take place [26,27], resulting in the formation of Cu/MWNT composites. It is also found that further increasing the amount of NH3$H2O and calcination at 300 C can result in the formation of Cu2O. When the amount of NH3$H2O exceeds 0.20 mL, the final products completely consist of Cu2O. It is most likely that COONH4 groups besides CONH– groups exist in the intermediate products, and the Cu(NH3)2þ 3 amount increases with the increasing of NH3$H2O. Therefore, more NH3 were present due to the decomposition of more COONH4 groups and reduced CuO to Cu2O [28]. In addition, it should be noted that the soaking time had the important influence on the morphologies of Cu2O/MWNT complexes. When the soaking time was retained at 17 h, Cu2O nanoparticle attached to the tip of MWNT was obtained and a Cu2O/MWNT heterojunction was formed. But when the soaking time elongated to 48 h, uniform Cu2O nanoparticles with the particle size of about 15 nm were obtained and homogeneously coated on the surface of MWNTs as that of CuO/MWNT and Cu/MWNT composites. It can be explained that NHþ 4 present in the precursor solution could inhibit the
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Wave numbers (cm-1) Fig. 5. IR spectra of the (a) acid-treated MWNTs, (b) intermediate products using the precursor solution prepared with 0.10 mL NH3$H2O, (c) CuO/MWNT, (d) Cu2O/MWNT and (e) Cu/MWNT composites.
X. Wang et al. / Solid State Sciences 11 (2009) 655–659
The amount of NH3$H2O, calcination temperature and soaking time play critical roles in controlling the components and morphologies of the final products. The quantum confinement effect was illustrated in the absorption spectra of the products with the different morphologies. The successful synthesis of CuO/MWNT, Cu2O/ MWNT and Cu/MWNT composites and Cu2O/MWNT heterojunctions opens new possibility for exploitation of controlled modification of MWNTs.
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Acknowledgement This work was supported by National Science Foundation of China under Grant No. 10674054. 500
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Wavelength (nm) Fig. 6. UV–visible absorption spectra of the (a) Cu2O/MWNT composites and (b) Cu2O/ MWNT heterojunctions.
were homogeneously formed around the MWNTs. Oppositely, diffuse a longer soaking time was needed to make Cu(NH3)2þ 4 homogeneously around the MWNTs when the precursor solution with more NHþ 4 was used. As a result, the Cu2O/MWNT composites were obtained as the soaking time was 48 h. But when the acidtreated MWNTs were soaked in the precursor solution with more 2þ NHþ 4 for 17 h, the concentration of Cu(NH3)4 around the tips of the MWNTs was rather high due to the presence of a large number of carboxylic groups on the tips of the MWNTs. Therefore the amount of CONH–Cu(NH3)2þ 3 groups around the tips of MWNTs was large and resulted in formation of large Cu2O nanoparticles after calcination treatment. UV–visible absorption spectra of Cu2O/MWNT composites and Cu2O/MWNT heterojunctions are shown in Fig. 6. Two absorption bands at 552 and 567 nm, attributed to band-to-band transition in the Cu2O nanoparticles, are observed, respectively, since the MWNTs exhibit no absorption bands in the region between 300 and 1000 nm [29]. The band gaps of Cu2O calculated from the UV– visible spectra are about 2.23 and 2.17 eV. It is clear that the absorption band shifts from 567 nm to 552 nm upon decreasing the particle size of the Cu2O attached to the MWNTs, such a blue-shift with the decrease of particle size is attributed to the quantum confinement effect [30]. Since both the Cu2O/MWNT composites and Cu2O/MWNT heterojunctions have narrow band gaps, they may be potential photocatalysts for water splitting and organic contamination removal by visible light. 4. Conclusion In summary, a simple and cost-effective approach has been developed to modify MWNTs with CuO, Cu2O and Cu nanoparticles.
References [1] C.S. Kuo, Y.H. Tseng, H.Y. Lin, C.H. Huang, C.Y. Shen, Y.Y. Li, S.I. Shah, C.P. Huang, Nanotechnology 18 (2007) 465607. [2] S. Banerjee, S.S. Wong, Nano Lett. 2 (2002) 195. [3] W.Q. Han, A. Zettl, Nano Lett. 3 (2003) 681. [4] G.M. An, N. Na, X.R. Zhang, Z.J. Miao, S.D. Miao, K.L. Ding, Z.M. Liu, Nanotechnology 18 (2007) 435707. [5] X.Y. Wang, B.Y. Xia, X.F. Zhu, J.S. Chen, S.L. Qiu, J.X. Li, J. Solid State Chem. 181 (2008) 822. [6] J.W. Liu, X.J. Li, L.M. Dai, Adv. Mater. 18 (2006) 1740. [7] X.B. Fan, F.Y. Tan, G.L. Zhang, F.B. Zhang, Mater. Sci. Eng. A 454–455 (2007) 37. [8] Yan S.C., Wang H.T., Qu P., Zhang Y., Xiao Z.D., Synth. Met., 2008, doi:10.1016/j. synthmet.2008.07.024. [9] Z. Fan, J.H. Chen, B. Zhang, B. Liu, X.X. Zhong, Y.F. Kuang, Diamond Relat. Mater. 17 (2008) 1943. [10] Z.Y. Wang, G. Chen, D.G. Xia, J. Power Sources 184 (2008) 432. [11] X.G. Zheng, C.N. Xu, Y. Tomokiyo, E. Tanaka, H. Yamada, Y. Soejima, Phys. Rev. Lett. 85 (2000) 5170. [12] X.P. Gao, J.L. Bao, G.L. Pan, H.Y. Zhu, P.X. Huang, F. Wu, D.Y. Song, J. Phys. Chem. B 108 (2004) 5547. [13] K. Borgohain, S. Mahamuni, J. Mater. Res. 17 (2002) 1220. [14] W. Wang, G. Wang, X. Wang, Y. Zhan, Y. Liu, C. Zheng, Adv. Mater. 14 (2002) 67. [15] L. Gou, C.J. Murphy, Nano Lett. 3 (2003) 231. [16] R. Liu, F. Oba, E.W. Bohannan, F. Ernst, J.A. Switzer, Chem. Mater. 15 (2003) 4882. [17] C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie, J. Ma, Adv. Mater. 17 (2005) 2562. [18] J.T. Zhang, J.F. Liu, Q. Peng, X. Wang, Y.D. Li, Chem. Mater. 18 (2006) 867. [19] L. Huang, F. Peng, H. Yu, H. Wang, Solid State Sci. 11 (2009) 129. [20] Y. Yu, L.L. Ma, W.Y. Huang, J.L. Li, P.K. Wong, J.C. Yu, J. Solid State Chem. 178 (2005) 1488. [21] L.S. Zhang, J.L. Li, Z.G. Chen, Y.W. Tang, Y. Yu, Appl. Catal. A: Gen. 299 (2006) 292. [22] G. Chai, Y. Sun, J. Sun, Q. Chen, J. Micromech. Microeng. 18 (2008) 035013. [23] S.S. Wong, A.T. Woolley, E. Joselevich, C.L. Cheung, C.M. Lieber, J. Am. Chem. Soc. 120 (1998) 8557. [24] S. Ravindran, S. Chaudhary, B. Colburn, M. Ozkan, C.S. Ozkan, Nano Lett. 3 (2003) 447. [25] J.M. Haremza, M.A. Hahn, T.D. Krauss, Nano Lett. 2 (2002) 1253. [26] G.Y. Xie, Z.Y. Liu, Z.P. Zhu, Q.Y. Liu, J.R. Ma, Appl. Catal. 45 (2003) 213. [27] C.C. Liu, H. Teng, Appl. Catal. B 58 (2005) 69. [28] S. Sua´rez, J.A. Martı´n, M. Yates, P. Avila, J. Blanco, J. Catal. 229 (2005) 227. [29] K.R. Reddy, B.C. Sin, C.H. Yoo, W. Park, K.S. Ryu, J. Lee, D. Sohn, Y. Lee, Scr. Mater. 58 (2008) 1010. [30] P. He, X.H. Shen, H.C. Gao, J. Colloid Interface Sci. 284 (2005) 510.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Controlled modification of multi-walled carbon nanotubes with CuO, Cu2O and Cu nanoparticles Xiuying Wang, Feng Zhang, Baiying Xia, Xingfu Zhu, Jiesheng Chen, Shilun Qiu, Ping Zhang, Jixue Li* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2008 Received in revised form 27 September 2008 Accepted 15 October 2008 Available online 30 October 2008
Multi-walled carbon nanotubes (MWNTs) have been successfully modified with CuO, Cu2O and Cu nanoparticles via a simple method, and the calcination temperature, the amount of NH3$H2O and soaking time play critical roles in controlling the final products. The modified MWNTs have been characterized by X-ray diffraction, infrared spectrum, scanning electron and transmission electron microscopies. Optical absorption of the obtained products has also been investigated, and the quantum confinement effect was illustrated in the absorption spectra. Ó 2008 Published by Elsevier Masson SAS.
Keywords: Carbon nanotubes Controlled modification Nanoparticle Heterojunction
1. Introduction Modification of carbon nanotubes (CNTs) has been paid a great deal of interest as a fascinating model system for fundamental scientific research with the potential technological applications. In particular, modification of CNTs with metal oxides is of significance for the development of CNT-based devices. Up to now, various metal oxides such as TiO2, SnO2, ZnO, Fe2O3, MnO2 and RuO2 have been reported to modify CNTs [1–10]. However, to search for a simple and low-cost approach to modify CNTs with the metal oxides is still an active research focus today. Recently, Cu and their oxides have attracted a great deal of attention because of their various applications. For instance, CuO has been widely used as the basis of high temperature superconductors, lithium ion electrode materials and giant magneto resistance materials [11–13]. Cu2O, a p-type semiconductor (Eg ¼ 2.17 eV) with particular optical and magnetic properties, has also been applied in a series of fields, for example solar energy conversion, electronics, magnetic storage, catalysis, and gas sensors [14–19]. Copper, one of the most normal conductors, is usually found in both fundamental researches and technical applications. Therefore, the modification of CNTs with Cu or their oxides is anticipated to extend their applications in many fields such as nano-electronic devices and novel catalysts due to the unique mechanical, electrical and thermal properties of the CNTs. It has * Corresponding author. Tel.: þ86 431 85168581; fax: þ86 431 85168624. E-mail address: [email protected] (J. Li). 1293-2558/$ – see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.10.009
been reported that Cu2O nanoparticles could be deposited on the surface of multi-walled carbon nanotubes (MWNTs) via a polyol process or anodic oxidation of copper electrode in an alkaline solution [20,21], and Cu/CNT composites could be fabricated by the electrochemical co-deposition method [22]. Nevertheless, all those methods are complicated processes, and the costs are usually high. Herein we report a simple and cost-effective approach to modify the MWNTs with CuO, Cu2O and Cu nanoparticles via copper– ammonium complex (Cu(NH3)2þ 4 ) covalently attached to the MWNTs. The components and morphologies of the products can be well controlled by adjusting the calcination temperature, the amount of NH3$H2O and soaking time. 2. Experimental section 2.1. Treatment of MWNTs The commercial MWNTs were treated with nitric acid (80 wt%) at 80 C for 4 h, washed with deionized water several times and dried at 100 C. 2.2. Preparation of CuO/MWNT and Cu/MWNT composites In our experiment, Cu(CH3COO)2$H2O was used as copper source. At first, 16 mg Cu(CH3COO)2$H2O were dissolved in 5 mL deionized water. Then 0.10 mL NH3$H2O (25 wt%) solution was slowly added into the above solution under continuous stirring to form the precursor solution and 10 mg MWNTs were soaked in the
X. Wang et al. / Solid State Sciences 11 (2009) 655–659
solution for 17 h. Subsequently, the MWNTs were centrifugally separated from the solution, dried at 70 C in vacuum, giving the intermediate products. Finally, the intermediate products were calcined at 300 C and 500 C in the presence of N2 for 2 h, respectively, and the CuO/MWNT and Cu/MWNT composites were obtained.
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The preparation processes of Cu2O/MWNT heterojunctions and Cu2O/MWNT composites are the same as that of CuO/MWNT composites, except that the amount of the NH3$H2O solution changed to 0.20 mL, and the soaking times were 17 h and 48 h.
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Two theta (degrees) Fig. 1. XRD patterns of the pure MWNTs, CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites.
The products were characterized by X-ray diffraction (XRD, Rigaku XRD spectrometer with a Cu Ka line of 1.5418 Å), scanning electron microscopy (SEM, JEOL JSM-6700F at 5 kV), and transmission electron microscopy (TEM, JEOL JEM 3010 at 300 kV)
Fig. 2. SEM, TEM and HRTEM images of (a–c) CuO/MWNT, (d–f) Cu2O/MWNT and (g–i) Cu/MWNT composites.
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equipped with energy dispersive X-ray spectroscopy (EDS). The infrared (IR) spectra were recorded on an IFS 66 V/S FTIR spectrometer using KBr pellets. UV–visible spectra were recorded on a Perkin Elmer Lambda 20 UV–visible spectrophotometer. 3. Results and discussion Fig. 1 demonstrates the XRD patterns of the pure MWNTs, CuO/ MWNT, Cu2O/MWNT and Cu/MWNT composites. It is found that the characteristic peaks of the XRD patterns of the modified MWNTs are in good agreement with those of CuO (JCPDS no. 80-1916), Cu2O (JCPDS no. 05-0667) and Cu (JCPDS no. 85-1326), suggesting that no impurities exist in all of the obtained products. Fig. 2 shows the SEM images and TEM images of the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites. It is obviously seen that CuO, Cu2O and Cu nanoparticles are homogeneously coated on the surface of MWNTs and intimately attached to the MWNTs. The particle size distributions of the CuO and Cu2O nanoparticles are in narrow ranges, mainly around 20 nm and 15 nm, whereas that of the Cu nanoparticles is mainly in a wide range of 20–100 nm. To better understand the components of the obtained products, HRTEM has also been recorded. The distances between the parallel lattice planes of the nanoparticles are 0.25 nm, 0.24 nm and 0.20 nm, respectively, which are consistent with the space between (111) planes of CuO, (111) planes of Cu2O and (111) planes of Cu. The compositions of the obtained products were also studied by EDS (Fig. 3), obtained by narrowing down and focusing the beam directly onto the selective nanoparticles on the MWNTs. The EDS demonstrates that the atomic ratio of Cu to O is about 1.0, 1.8, and 3.5 in the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites, respectively, which indicates the phase of the obtained products is CuO, Cu2O and Cu. However, it should be noted that the EDS (Fig. 3c) confirms that oxygen does exist in Cu/MWNT composites. It is most likely that the O atoms are bound to the C atoms in MWNTs but the possibility that Cu-bound O atoms are also present cannot be excluded. In addition, the signals of C and Ni present in all the EDS were originated from the MWNTs and Ni grid. On the basis of the results of XRD, HRTEM and EDS, it is believed that the assynthesis products are pure CuO/MWNT, Cu2O/MWNT and Cu/ MWNT composites, and no impurities are present. Fig. 4a–b shows the SEM images of Cu2O/MWNT heterojunctions. It is seen that Cu2O nanoparticle with the particle size of about 100 nm is attached to the tip of MWNT, in agreement with the observation through transmission electron microscope imaging (Fig. 4c). In order to study the structure of the obtained product, the XRD pattern has been conducted (Fig. 4d). The characteristic peaks of the XRD pattern are in good agreement with those of Cu2O, indicating that the phase of the obtained products are Cu2O. The composition of the obtained product was also studied by EDS. Fig. 4e demonstrates that the atomic ratio of Cu to O is about 2.4, which indicates the component of the obtained products is Cu2O. In order to investigate the growth mechanism of the CuO/ MWNT, Cu2O/MWNT and Cu/MWNT composites, IR spectra were carried out (Fig. 5). A band at 1720 cm1 associated with the stretch mode of carboxylic groups is observed in the IR spectrum of the acid-treated MWNTs, indicating that carboxylic groups are formed due to the oxidation of some carbon atoms on the surface of the MWNTs by nitric acid. In the IR spectrum of the intermediate products, new bands in the range of 3400–3100 cm1, 900– 650 cm1 attributed to the N–H stretching vibration and around 1405 cm1 assigned to C–N stretching vibration are clearly are attached to the observed, demonstrating that Cu(NH3)2þ 4 MWNTs through the reaction with the carboxylic groups. Moreover, the bands at 585 cm1, 620 cm1 and 650 cm1 relative to CuO, Cu2O and Cu are obviously detected, indicating the formation of the CuO/MWNT, Cu2O/MWNT and Cu/MWNT composites.
Fig. 3. EDS spectra of (a) CuO/MWNT, (b) Cu2O/MWNT and (c) Cu/MWNT composites.
The process of the modification of MWNTs is multistep, in which various chemical reactions occur. It is impossible to pinpoint the exact chemical reactions, but the likely formation route can be inferred on the basis of XRD patterns and IR spectra. Firstly, acid treatment introduces the carboxylic groups (–COOH) into the MWNTs and NH3$H2O is added into Cu2þ solution to form 2þ Cu(NH3)2þ 4 solution. Afterward, Cu(NH3)4 diffuses to the MWNTs by the electrostatic attraction as the acid-treated MWNTs are soaked into the precursor solution. Subsequently, the MWNTs are separated from the solution centrifugally and dried at 70 C in vacuum, and the intermediate products MWNT–CONH–Cu(NH3)2þ 3 are obtained due to the reaction of the amide and carboxyl [5,23– 25]. Finally, the intermediate products are converted to the final products via the calcination treatment. It is worth noting that the calcination temperature, the amount of NH3$H2O and soaking time play critical roles in controlling the final products. When the precursor solution prepared with 0.10 mL
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Fig. 4. (a and b) SEM images, (c) TEM image, (d) XRD pattern and (e) EDS spectrum of the Cu2O/MWNT heterojunctions.
diffusion of Cu(NH3)2þ to some extent. So when the precursor 4 solution prepared with 0.1 mL NH3$H2O, in which less NHþ 4 were were dispersed homogeneously present, was used, Cu(NH3)2þ 4 around the MWNTs after 17 h and the CuO and Cu nanoparticles
a b Transmittance
NH3$H2O was used, CuO/MWNT and Cu/MWNT composites were obtained after calcination treatment at 300 C and 500 C for 2 h in N2 flows, respectively. It indicates that the decomposition of CONH–Cu(NH3)2þ 3 groups gave birth to CuO and NH3 at 300 C, and the CuO/MWNT composites were obtained, whereas when the calcination temperature was elevated to 500 C, the oxidation– reduction reaction between CuO and NH3 would take place [26,27], resulting in the formation of Cu/MWNT composites. It is also found that further increasing the amount of NH3$H2O and calcination at 300 C can result in the formation of Cu2O. When the amount of NH3$H2O exceeds 0.20 mL, the final products completely consist of Cu2O. It is most likely that COONH4 groups besides CONH– groups exist in the intermediate products, and the Cu(NH3)2þ 3 amount increases with the increasing of NH3$H2O. Therefore, more NH3 were present due to the decomposition of more COONH4 groups and reduced CuO to Cu2O [28]. In addition, it should be noted that the soaking time had the important influence on the morphologies of Cu2O/MWNT complexes. When the soaking time was retained at 17 h, Cu2O nanoparticle attached to the tip of MWNT was obtained and a Cu2O/MWNT heterojunction was formed. But when the soaking time elongated to 48 h, uniform Cu2O nanoparticles with the particle size of about 15 nm were obtained and homogeneously coated on the surface of MWNTs as that of CuO/MWNT and Cu/MWNT composites. It can be explained that NHþ 4 present in the precursor solution could inhibit the
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Wave numbers (cm-1) Fig. 5. IR spectra of the (a) acid-treated MWNTs, (b) intermediate products using the precursor solution prepared with 0.10 mL NH3$H2O, (c) CuO/MWNT, (d) Cu2O/MWNT and (e) Cu/MWNT composites.
X. Wang et al. / Solid State Sciences 11 (2009) 655–659
The amount of NH3$H2O, calcination temperature and soaking time play critical roles in controlling the components and morphologies of the final products. The quantum confinement effect was illustrated in the absorption spectra of the products with the different morphologies. The successful synthesis of CuO/MWNT, Cu2O/ MWNT and Cu/MWNT composites and Cu2O/MWNT heterojunctions opens new possibility for exploitation of controlled modification of MWNTs.
b Absorbance
659
a
Acknowledgement This work was supported by National Science Foundation of China under Grant No. 10674054. 500
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Wavelength (nm) Fig. 6. UV–visible absorption spectra of the (a) Cu2O/MWNT composites and (b) Cu2O/ MWNT heterojunctions.
were homogeneously formed around the MWNTs. Oppositely, diffuse a longer soaking time was needed to make Cu(NH3)2þ 4 homogeneously around the MWNTs when the precursor solution with more NHþ 4 was used. As a result, the Cu2O/MWNT composites were obtained as the soaking time was 48 h. But when the acidtreated MWNTs were soaked in the precursor solution with more 2þ NHþ 4 for 17 h, the concentration of Cu(NH3)4 around the tips of the MWNTs was rather high due to the presence of a large number of carboxylic groups on the tips of the MWNTs. Therefore the amount of CONH–Cu(NH3)2þ 3 groups around the tips of MWNTs was large and resulted in formation of large Cu2O nanoparticles after calcination treatment. UV–visible absorption spectra of Cu2O/MWNT composites and Cu2O/MWNT heterojunctions are shown in Fig. 6. Two absorption bands at 552 and 567 nm, attributed to band-to-band transition in the Cu2O nanoparticles, are observed, respectively, since the MWNTs exhibit no absorption bands in the region between 300 and 1000 nm [29]. The band gaps of Cu2O calculated from the UV– visible spectra are about 2.23 and 2.17 eV. It is clear that the absorption band shifts from 567 nm to 552 nm upon decreasing the particle size of the Cu2O attached to the MWNTs, such a blue-shift with the decrease of particle size is attributed to the quantum confinement effect [30]. Since both the Cu2O/MWNT composites and Cu2O/MWNT heterojunctions have narrow band gaps, they may be potential photocatalysts for water splitting and organic contamination removal by visible light. 4. Conclusion In summary, a simple and cost-effective approach has been developed to modify MWNTs with CuO, Cu2O and Cu nanoparticles.
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