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carbon nanotubes hybrid material
Current Applied Physics 12 (2012) 1575e1579
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Microwave-induced fabrication of copper nanoparticle/carbon nanotubes hybrid material Nattaporn Leelaviwat a, Siriporn Monchayapisut a, Chantamanee Poonjarernsilp b, Kajornsak Faungnawakij c, Kyo-Seon Kim d, Tawatchai Charinpanitkul a, * a
Center of Excellence in Particle Technology, Faculty of Engineering, Chulalongkorn University, Payathai, Pathumwan, Bangkok 10330, Thailand Faculty of Engineering, Rajamangala University of Technology Krungthep, Sathorn, Bangkok, Thailand National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Pathumthani, Thailand d Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwon-Do, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 19 April 2012 Accepted 2 May 2012 Available online 21 May 2012
Copper nanoparticle/multi-walled carbon nanotube (MWCNT) hybrid material could be fabricated by microwave irradiation to a suspension of MWCNTs dispersed in copper precursor with the presence of ethylene glycol. Microscopic and spectroscopic analyses could confirm the uniform dispersion of copper oxide nanoparticles hybridized on the outer surface of MWCNTs. Reduction of Cu2þ ions to Cu2O and Cu nanoparticles was ascribed to functional groups which were generated after the precursor was irradiated by microwave. Sufficient irradiation time of 5 min or longer played an important role to induce the agglomeration of copper oxide nanoparticles on the MWCNT surface. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Microwave irradiation Hybrid material Carbon nanotube Copper oxide
1. Introduction Carbon nanotubes (CNTs) have been heralded as a promising material regarding to their excellent mechanical and electrical properties [1]. Meanwhile, metal-CNTs hybrid materials have recently been recognized as a new class of modified CNTs having novel and excellent properties compared to conventional CNTs. Among various metals, Cu appears to be suitable for functionalizing CNTs owing to its good thermal and electrical conductivities with economical cost. In addition, both CNTs and Cu nanoparticles have been broadly employed in various micro- or nano-electronic devices because of their excellent stability and conductivity [2,3]. There are some previous works reporting that metal-loaded CNTs, e.g. Au/CNTs, Pt/CNTs, and Cu/CNTs, could be prepared by some techniques, including metal-vapor deposition, chemical-reduction deposition in aqueous phase, arc discharge, electroless deposition, and microwave irradiation treatment [4e9]. In comparison with others methods, microwave irradiation treatment has attracted attention of many researchers as a promising technique for the preparation of size-controlled metallic nanostructures due to its
* Corresponding author. Tel./fax: þ66 2 2186480. E-mail address: [email protected] (T. Charinpanitkul). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.05.002
rapid and controllable heating which is essential for hybridization of precursors. Therefore, an investigation on the preparation of Cu-nanoparticle/multi-walled carbon nanotube (MWCNT) hybrid materials using a microwave irradiation has been carried out and discussed in this work. A household microwave oven has been employed for inducing hybridization of MWCNTs dispersed in a solution of copper precursor. As a source of reducing agent, ethylene glycol (EG) was introduced to MWCNT suspension subject to microwave treatment because of its high dielectric constant [10]. Commercial MWCNTs were pretreated with acid treatment in prior to hybridizing with Cu nanoparticles Effect of microwave irradiation time on morphology of the Cu/MWCNTs hybrid materials was examined and plausible mechanism of their formation was also discussed. 2. Experimental Commercial MWCNTs was purchased from Bayer Materials Science Co. Ltd. Analytical grade nitric acid (HNO3; 69e70%, J.T. Baker, USA), sulfuric acid (H2SO4; 95e97%, QReC, New Zealand), ethylene glycol (EG; 99.5%, QReC, New Zealand), copper (II) sulfate pentahydrate (CuSO4$5H2O; Asia Pacific Specialty Chemicals, APS) and sodium hydroxide (NaOH; Ajax Finechem) were used as received without any purification.
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N. Leelaviwat et al. / Current Applied Physics 12 (2012) 1575e1579
1 g of the pristine MWCNTs were sonicated in 200 ml of a mixture of concentrated sulfuric acid (H2SO4) and nitric acid (HNO3) with a volumetric ratio of 3 to 1 at room temperature for 3 h [6]. The MWCNTs dispersed in the acid mixture were washed with de-ionized water for several times until pH value reached neutral and were dried in an oven overnight. 0.114 g of CuSO4$5H2O precursor was added into the mixed solution of EG (99 ml) and NaOH (0.55 g) in a glass beaker with stirring for 20 min at room temperature. Then 0.05 g of the acidtreated MWCNTs was added to 100 ml mixture of Cu2þ ions and sonicated at room temperature for 20 min. The MWCNTs dispersion was later placed in a household microwave oven (LG MS-2127CW, 2450 MHz, 800 W) for microwave irradiation with an input power of 360 W for designated irradiation time in a range of 1e9 min. Finally the irradiated dispersion was cooled down to room temperature in prior to being filtered, washed with de-ionized water for several times and dried in an oven overnight. Functional groups on the surface of acid-treated MWCNTs were identified using Fourier transform infrared spectrophotometer (FTIR, Spectrum I, Perkin Elmer). The morphology of acid-treated MWCNTs and Cu/MWCNT hybrid materials was analyzed using Transmission Electron Microscope (TEM, JEM-2100, JEOL). To clarify the achievement of Cu loading onto MWCNTs, solid samples were subject to X-ray diffraction analyzer (XRD, AXS-D8, Bruker) using CuKa radiation, with 40 kV and 40 mA, at 0.02 scan rate (in 2q) with step of 0.3 s per step. Zeta potential of samples was also measured by using Zetasizer (Malvern Nano-ZS). 3. Results and discussion In general, MWCNTs are a hydrophobic material, resulting in limitation of their contact to metal ions dissolved in aqueous solution. In order to deposit metals onto the surface of MWCNTs, some functional groups, such as carbonyl (>C]O), carboxyl (eCOOH) and hydroxyl (eOH) would be transferred to MWCNT surface. Concentrated acids can be used to introduce functional groups on the surfaces of carbonaceous materials, resulting in the increased hydrophilicity of the treated MWCNTs, leading to an improvement in accessibility of metal species to MWCNT surface [7]. As shown in Fig. 1, FT-IR spectra of the pristine and acid-treated MWCNTs reveal typical peaks at 1222, 1435, 1539, 1614 and 1719 cm1. It should be noted that a peak at 1719 cm1 represents C]O stretching of carboxylic acids, and another peak at 1222 cm1
is referred to CeO stretching functionalities on the surface of the acid-treated MWCNTs. A peak at 1539 cm1 is assigned to C]C stretching while another peak at 1435 cm1 represents OeH bending functional group and the last peak around 1614 cm1 corresponds to the H-bonded carbonyl groups (C]O) conjugating with C]C in the graphene wall [6]. Comparison of spectra of the pristine and acid-treated MWCNTs would reveal that the acidtreated MWCNTs contain an increased amount of functional groups on their surface. To confirm the functionalities of the acidtreated MWCNTs, the pristine MWCNTs exhibited a zeta potential of 9.02 mV while the treated MWCNTs possessed a zeta potential of 23.60 mV. Therefore, it is reasonably implied that acid treatment could lead to functionalization of the acid-treated MWCNTs. As also reported by Schierz et al., the zeta potential was decreased by the de-protonation of functional groups, which would be carboxylic groups as confirmed by FT-IR analysis [8]. As depicted in Fig. 2(a), the pristine MWCNTs exhibit tubular morphology with an average diameter of about 15e20 nm and smooth wall surface. The effect of microwave irradiation time on microstructure of Cu/MWCNT hybrid materials fabricated with the microwave irradiation power of 360 W was observed from TEM micrographs shown in Fig. 2(b)e(f). The irradiation time of 1 min could result in a scarce amount of nanoparticles attaching to the MWCNT surface (Fig. 2(b)). With the irradiation time longer that 1 min, there were increasing amount of nanoparticles anchoring on the MWCNTs surface. Microwave irradiation for 3 min would result in deposition of spherical nanoparticles with a larger average size of 8 nm on the outer surface of MWCNTs while the number of nanoparticles depositing on MWCNTs also tremendously increased with the irradiation time of 5 min as could be observed in Fig. 2(c) and (d). However, Fig. 2(e) and (f) illustrates that with longer irradiation time of 7 and 9 min, microwave would induce the agglomeration of spherical nanoparticles of which average size became larger. These results clearly suggest that microwave irradiation time would exert significant effect on the formation of spherical nanoparticles which would grow up and finally undergo an agglomeration process [7]. XRD pattern (a) in Fig. 3 illustrates two distinct diffraction peaks at 2q ¼ 25.5 and 42.5 which could confirm the presence of pristine MWCNTs [2,9]. XRD pattern (b) reveals that the product sample irradiated by microwave for 1 min would contain only few amount of Cu2O and Cu. These results suggested that the sufficient irradiation time would be required for the reduction of Cu2þ ions to yield Cu2O and Cu which would further anchor on the outer surface
Fig. 1. FT-IR spectra of (a) pristine MWCNTs and (b) acid-treated MWCNTs.
N. Leelaviwat et al. / Current Applied Physics 12 (2012) 1575e1579
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Fig. 2. TEM images of (a) pristine MWCNTs and hybrid materials with different irradiation time: (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 9 min.
of MWCNTs [3]. It should be further noted that the diffraction pattern (c) exhibits three peaks at 2q ¼ 36.4 , 42.3 and 61.0 . These peaks would confirm the presence of Cu2O on the MWCNT surface which was irradiated with microwave for 3 min [10,11]. With the irradiation time of 5 min or longer, the agglomerating Cu2O nanoparticles on MWCNTs surface could be detected as shown in Fig. 2(d)e(f). The gradual agglomeration of Cu2O nanoparticles would be attributed to the continuous nucleation of Cu2O which would also be further reduced by ethylene glycol to form Cu [x]. This postulation would be verified by the XRD patterns (d)e(f).
Such diffraction exhibits two additional peaks at 2q ¼ 43.3 and 50.5 which would confirm the presence of Cu nanoparticles on the MWCNT surface [10e13]. In short summary, these results would suggest that the irradiation time of 5 min was sufficient for the reduction of the Cu2þ ions to yield Cu2O and Cu nanoparticles. Regarding to the intensity of Cu2þ, Cuþ and Cu peaks, it should be noted that while the Cu2þ would be significantly lower, the Cu peak is almost not changed. These results would be ascribed to the limited amount of EG. First, EG would undergo the reduction of Cu2þ to Cuþ. Then the remaining EG would further reduce Cuþ to
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Fig. 3. XRD patterns of (a) pristine MWCNTs and hybrid materials with different irradiation time : (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 9 min.
Fig. 4. Schematic representation of Cu nucleation and growth on MWCNT surface.
Cu. Therefore, when the reduction reaction further proceeds to change Cu2þ to Cuþ and Cu, though the amount of Cu would significantly decrease, resulting in the lower intensity of Cu2O peak, there would be fewer EG available for further reducing Cuþ to Cu. In addition, Ahn et al. (2002) also report the microwave-induced low-temperature crystallization of Si which grain size exhibits a slightly increasing tendency by controlling the annealing temperature [5]. It would suggest that microwave irradiation could help accelerating the crystal growth. In our investigation, the synthesizing temperature would be affected by the irradiation time. Taking into account the Arrhenius correlation, the higher temperature would accelerate the reaction of Cu2þ reduction and Cu nucleation. With the higher content of Cu nuclei, the Cu crystal growth would automatically be enhanced. However, with the limited amount of reducing agent (EG) the formation of Cu nanoparticles would not significantly increase. Based on all microscopic and spectroscopic analyses, it would possible to consider that formation of Cu2O and Cu nanopartciles depositing on the surface of MWCNTs would involve with 3 consecutive steps, namely adsorption, reduction and agglomeration as proposed in Fig. 4. First, the Cu2þ precursor would be adsorbed onto the surface of acid-treated MWCNTs due to the electrostatic attraction of functional group which could be confirmed by the zeta potential measurement summarized in Table 1. While the zeta potential of acid-treated MWCNTs surface is 23.6 mV, CuSO4$5H2O precursor possesses the potential of
4.85 mV. Such difference in the surface potential would help induce Cu2þ ions to attach with the charged surface of acid-treated MWCNTs [14]. In the 2nd step those Cu2þ ions would be further reduced by EG to form Cuþ and Cu nuclei which would further grow up on the surface of the acid-treated MWCNTs as long as the Cu2þ ions are still available. It should be noted that the zeta potential of Cu/MWCNT hybrid material prepared by the microwave treatment is 10.8 mV. The increasing zeta potential value would be ascribed to the contribution of Cu species which possess positive zeta potential [12]. Finally, the excessive Cu2þ ions available in the solution would further be reduced by EG, resulting in the continuous growth of agglomerating Cu2O and Cu nanoparticles on the outer surface of MWCNTs [13]. It should be summarized that the irradiation time of 5 min would be essential for inducing the formation of copper nanoparticles hybridized with MWCNTs with the presence of EG and NaOH. The longer irradiation time would further enhance the agglomeration of copper nanoparticles as long as there is excessive Cu2þ ions in the system. Table 1 Zeta potential of each sample. Zeta potential (mV) Pristine MWCNTs Acid-treated MWCNTs CuSO4$5H2O precursor Cu/MWCNTs hybrid material
9.02 23.60 4.85 10.80
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4. Conclusion
References
The microwave irradiation can be used to induce Cu2þ ions to deposit on the negatively charged surface of the acid-treated MWCNTs, resulting in the presence of Cu and Cu2O nanoparticles depositing on the outer surface of MWCNTs. Based on all analytical results, it would be implied that specific functional groups generated from EG with the presence of NaOH would play an important role as a reducing agent to transform Cu2þ ions to Cu2O and Cu nanoparticles. The presence of Cu2O and Cu nanoparticles on the MWCNT surface would undergo three consecutive steps of adsorption, reduction and agglomeration. The microwave irradiation time of 5 min or longer would be essential for the formation of uniform dispersion of copper nanopraticles on the MWCNT surface.
[1] X.Y. Tao, X.B. Zhang, J.P. Cheng, F. Liu, Mater. Chem. Phys. 104 (2007) 210e214. [2] T. Charinpanitkul, A. Soottitantawat, N. Tonanon, W. Tanthapanichakoon, Mater. Chem. Phys. 116 (2009) 125e128. [3] F. Wang, S. Arai, M. Endo, Electrochem. Commun. 6 (2004) 1042e1044. [4] W.X. Chen, J. Zhao, J.Y. Lee, Z. Liu, Mater. Chem. Phys. 91 (2005) 124e129. [5] J.H. Ahn, J.N. Lee, Y.C. Kim, B.T. Ahn, Curr. Appl. Phys. 2 (2002) 135e139. [6] E. Tamburri, S. Orlanducci, M.L. Terranova, F. Valentini, G. Palleschi, A. Curulli, F. Brunetti, D. Passeri, A. Alippi, M. Rossi, Carbon 43 (2005) 1213e1221. [7] J.H. Lee, B.E. Park, Y.M. Lee, S.H. Hwang, W.B. Ko, Curr. Appl. Phys. 9 (2009) e152ee156. [8] A. Schierz, H. Zänker, Environ. Poll. 157 (2009) 1088e1094. [9] J. Klanwan, N. Akrapattangkul, V. Pavarajarn, T. Seto, Y.Otani, T. Charinpanitkul, Mater. Lett. 64 (2010) 80e82. [10] P. Martis, B.R. Venugopal, J.F. Seffer, J. Delhalle, Z. Mekhalif, Acta Materialia 59 (2011) 5040e5047. [11] Y. Zhao, J.-J. Zhu, J.-M. Hong, N. Bian, H.-Y. Chen, Euro. J. Inor. Chem. 20 (2004) 4072e4080. [12] M. Blosi, S. Albonetti, M. Dondi, C. Martelli, G. Baldi, J. Nanoparticle Res. 13 (2011) 127e138. [13] H.T. Zhu, C.Y. Zhang, Y.S. Yin, J. Crystal Growth 270 (2004) 722e728. [14] L. Zhao, L. Gao, Carbon 42 (8e9) (2004) 1858e1861.
Acknowledgments This research work is partially supported by the Centenary Fund and Graduate School of Chulalongkorn University to Center of Excellence in Particle Technology.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Microwave-induced fabrication of copper nanoparticle/carbon nanotubes hybrid material Nattaporn Leelaviwat a, Siriporn Monchayapisut a, Chantamanee Poonjarernsilp b, Kajornsak Faungnawakij c, Kyo-Seon Kim d, Tawatchai Charinpanitkul a, * a
Center of Excellence in Particle Technology, Faculty of Engineering, Chulalongkorn University, Payathai, Pathumwan, Bangkok 10330, Thailand Faculty of Engineering, Rajamangala University of Technology Krungthep, Sathorn, Bangkok, Thailand National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Pathumthani, Thailand d Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwon-Do, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 19 April 2012 Accepted 2 May 2012 Available online 21 May 2012
Copper nanoparticle/multi-walled carbon nanotube (MWCNT) hybrid material could be fabricated by microwave irradiation to a suspension of MWCNTs dispersed in copper precursor with the presence of ethylene glycol. Microscopic and spectroscopic analyses could confirm the uniform dispersion of copper oxide nanoparticles hybridized on the outer surface of MWCNTs. Reduction of Cu2þ ions to Cu2O and Cu nanoparticles was ascribed to functional groups which were generated after the precursor was irradiated by microwave. Sufficient irradiation time of 5 min or longer played an important role to induce the agglomeration of copper oxide nanoparticles on the MWCNT surface. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Microwave irradiation Hybrid material Carbon nanotube Copper oxide
1. Introduction Carbon nanotubes (CNTs) have been heralded as a promising material regarding to their excellent mechanical and electrical properties [1]. Meanwhile, metal-CNTs hybrid materials have recently been recognized as a new class of modified CNTs having novel and excellent properties compared to conventional CNTs. Among various metals, Cu appears to be suitable for functionalizing CNTs owing to its good thermal and electrical conductivities with economical cost. In addition, both CNTs and Cu nanoparticles have been broadly employed in various micro- or nano-electronic devices because of their excellent stability and conductivity [2,3]. There are some previous works reporting that metal-loaded CNTs, e.g. Au/CNTs, Pt/CNTs, and Cu/CNTs, could be prepared by some techniques, including metal-vapor deposition, chemical-reduction deposition in aqueous phase, arc discharge, electroless deposition, and microwave irradiation treatment [4e9]. In comparison with others methods, microwave irradiation treatment has attracted attention of many researchers as a promising technique for the preparation of size-controlled metallic nanostructures due to its
* Corresponding author. Tel./fax: þ66 2 2186480. E-mail address: [email protected] (T. Charinpanitkul). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.05.002
rapid and controllable heating which is essential for hybridization of precursors. Therefore, an investigation on the preparation of Cu-nanoparticle/multi-walled carbon nanotube (MWCNT) hybrid materials using a microwave irradiation has been carried out and discussed in this work. A household microwave oven has been employed for inducing hybridization of MWCNTs dispersed in a solution of copper precursor. As a source of reducing agent, ethylene glycol (EG) was introduced to MWCNT suspension subject to microwave treatment because of its high dielectric constant [10]. Commercial MWCNTs were pretreated with acid treatment in prior to hybridizing with Cu nanoparticles Effect of microwave irradiation time on morphology of the Cu/MWCNTs hybrid materials was examined and plausible mechanism of their formation was also discussed. 2. Experimental Commercial MWCNTs was purchased from Bayer Materials Science Co. Ltd. Analytical grade nitric acid (HNO3; 69e70%, J.T. Baker, USA), sulfuric acid (H2SO4; 95e97%, QReC, New Zealand), ethylene glycol (EG; 99.5%, QReC, New Zealand), copper (II) sulfate pentahydrate (CuSO4$5H2O; Asia Pacific Specialty Chemicals, APS) and sodium hydroxide (NaOH; Ajax Finechem) were used as received without any purification.
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1 g of the pristine MWCNTs were sonicated in 200 ml of a mixture of concentrated sulfuric acid (H2SO4) and nitric acid (HNO3) with a volumetric ratio of 3 to 1 at room temperature for 3 h [6]. The MWCNTs dispersed in the acid mixture were washed with de-ionized water for several times until pH value reached neutral and were dried in an oven overnight. 0.114 g of CuSO4$5H2O precursor was added into the mixed solution of EG (99 ml) and NaOH (0.55 g) in a glass beaker with stirring for 20 min at room temperature. Then 0.05 g of the acidtreated MWCNTs was added to 100 ml mixture of Cu2þ ions and sonicated at room temperature for 20 min. The MWCNTs dispersion was later placed in a household microwave oven (LG MS-2127CW, 2450 MHz, 800 W) for microwave irradiation with an input power of 360 W for designated irradiation time in a range of 1e9 min. Finally the irradiated dispersion was cooled down to room temperature in prior to being filtered, washed with de-ionized water for several times and dried in an oven overnight. Functional groups on the surface of acid-treated MWCNTs were identified using Fourier transform infrared spectrophotometer (FTIR, Spectrum I, Perkin Elmer). The morphology of acid-treated MWCNTs and Cu/MWCNT hybrid materials was analyzed using Transmission Electron Microscope (TEM, JEM-2100, JEOL). To clarify the achievement of Cu loading onto MWCNTs, solid samples were subject to X-ray diffraction analyzer (XRD, AXS-D8, Bruker) using CuKa radiation, with 40 kV and 40 mA, at 0.02 scan rate (in 2q) with step of 0.3 s per step. Zeta potential of samples was also measured by using Zetasizer (Malvern Nano-ZS). 3. Results and discussion In general, MWCNTs are a hydrophobic material, resulting in limitation of their contact to metal ions dissolved in aqueous solution. In order to deposit metals onto the surface of MWCNTs, some functional groups, such as carbonyl (>C]O), carboxyl (eCOOH) and hydroxyl (eOH) would be transferred to MWCNT surface. Concentrated acids can be used to introduce functional groups on the surfaces of carbonaceous materials, resulting in the increased hydrophilicity of the treated MWCNTs, leading to an improvement in accessibility of metal species to MWCNT surface [7]. As shown in Fig. 1, FT-IR spectra of the pristine and acid-treated MWCNTs reveal typical peaks at 1222, 1435, 1539, 1614 and 1719 cm1. It should be noted that a peak at 1719 cm1 represents C]O stretching of carboxylic acids, and another peak at 1222 cm1
is referred to CeO stretching functionalities on the surface of the acid-treated MWCNTs. A peak at 1539 cm1 is assigned to C]C stretching while another peak at 1435 cm1 represents OeH bending functional group and the last peak around 1614 cm1 corresponds to the H-bonded carbonyl groups (C]O) conjugating with C]C in the graphene wall [6]. Comparison of spectra of the pristine and acid-treated MWCNTs would reveal that the acidtreated MWCNTs contain an increased amount of functional groups on their surface. To confirm the functionalities of the acidtreated MWCNTs, the pristine MWCNTs exhibited a zeta potential of 9.02 mV while the treated MWCNTs possessed a zeta potential of 23.60 mV. Therefore, it is reasonably implied that acid treatment could lead to functionalization of the acid-treated MWCNTs. As also reported by Schierz et al., the zeta potential was decreased by the de-protonation of functional groups, which would be carboxylic groups as confirmed by FT-IR analysis [8]. As depicted in Fig. 2(a), the pristine MWCNTs exhibit tubular morphology with an average diameter of about 15e20 nm and smooth wall surface. The effect of microwave irradiation time on microstructure of Cu/MWCNT hybrid materials fabricated with the microwave irradiation power of 360 W was observed from TEM micrographs shown in Fig. 2(b)e(f). The irradiation time of 1 min could result in a scarce amount of nanoparticles attaching to the MWCNT surface (Fig. 2(b)). With the irradiation time longer that 1 min, there were increasing amount of nanoparticles anchoring on the MWCNTs surface. Microwave irradiation for 3 min would result in deposition of spherical nanoparticles with a larger average size of 8 nm on the outer surface of MWCNTs while the number of nanoparticles depositing on MWCNTs also tremendously increased with the irradiation time of 5 min as could be observed in Fig. 2(c) and (d). However, Fig. 2(e) and (f) illustrates that with longer irradiation time of 7 and 9 min, microwave would induce the agglomeration of spherical nanoparticles of which average size became larger. These results clearly suggest that microwave irradiation time would exert significant effect on the formation of spherical nanoparticles which would grow up and finally undergo an agglomeration process [7]. XRD pattern (a) in Fig. 3 illustrates two distinct diffraction peaks at 2q ¼ 25.5 and 42.5 which could confirm the presence of pristine MWCNTs [2,9]. XRD pattern (b) reveals that the product sample irradiated by microwave for 1 min would contain only few amount of Cu2O and Cu. These results suggested that the sufficient irradiation time would be required for the reduction of Cu2þ ions to yield Cu2O and Cu which would further anchor on the outer surface
Fig. 1. FT-IR spectra of (a) pristine MWCNTs and (b) acid-treated MWCNTs.
N. Leelaviwat et al. / Current Applied Physics 12 (2012) 1575e1579
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Fig. 2. TEM images of (a) pristine MWCNTs and hybrid materials with different irradiation time: (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 9 min.
of MWCNTs [3]. It should be further noted that the diffraction pattern (c) exhibits three peaks at 2q ¼ 36.4 , 42.3 and 61.0 . These peaks would confirm the presence of Cu2O on the MWCNT surface which was irradiated with microwave for 3 min [10,11]. With the irradiation time of 5 min or longer, the agglomerating Cu2O nanoparticles on MWCNTs surface could be detected as shown in Fig. 2(d)e(f). The gradual agglomeration of Cu2O nanoparticles would be attributed to the continuous nucleation of Cu2O which would also be further reduced by ethylene glycol to form Cu [x]. This postulation would be verified by the XRD patterns (d)e(f).
Such diffraction exhibits two additional peaks at 2q ¼ 43.3 and 50.5 which would confirm the presence of Cu nanoparticles on the MWCNT surface [10e13]. In short summary, these results would suggest that the irradiation time of 5 min was sufficient for the reduction of the Cu2þ ions to yield Cu2O and Cu nanoparticles. Regarding to the intensity of Cu2þ, Cuþ and Cu peaks, it should be noted that while the Cu2þ would be significantly lower, the Cu peak is almost not changed. These results would be ascribed to the limited amount of EG. First, EG would undergo the reduction of Cu2þ to Cuþ. Then the remaining EG would further reduce Cuþ to
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Fig. 3. XRD patterns of (a) pristine MWCNTs and hybrid materials with different irradiation time : (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 9 min.
Fig. 4. Schematic representation of Cu nucleation and growth on MWCNT surface.
Cu. Therefore, when the reduction reaction further proceeds to change Cu2þ to Cuþ and Cu, though the amount of Cu would significantly decrease, resulting in the lower intensity of Cu2O peak, there would be fewer EG available for further reducing Cuþ to Cu. In addition, Ahn et al. (2002) also report the microwave-induced low-temperature crystallization of Si which grain size exhibits a slightly increasing tendency by controlling the annealing temperature [5]. It would suggest that microwave irradiation could help accelerating the crystal growth. In our investigation, the synthesizing temperature would be affected by the irradiation time. Taking into account the Arrhenius correlation, the higher temperature would accelerate the reaction of Cu2þ reduction and Cu nucleation. With the higher content of Cu nuclei, the Cu crystal growth would automatically be enhanced. However, with the limited amount of reducing agent (EG) the formation of Cu nanoparticles would not significantly increase. Based on all microscopic and spectroscopic analyses, it would possible to consider that formation of Cu2O and Cu nanopartciles depositing on the surface of MWCNTs would involve with 3 consecutive steps, namely adsorption, reduction and agglomeration as proposed in Fig. 4. First, the Cu2þ precursor would be adsorbed onto the surface of acid-treated MWCNTs due to the electrostatic attraction of functional group which could be confirmed by the zeta potential measurement summarized in Table 1. While the zeta potential of acid-treated MWCNTs surface is 23.6 mV, CuSO4$5H2O precursor possesses the potential of
4.85 mV. Such difference in the surface potential would help induce Cu2þ ions to attach with the charged surface of acid-treated MWCNTs [14]. In the 2nd step those Cu2þ ions would be further reduced by EG to form Cuþ and Cu nuclei which would further grow up on the surface of the acid-treated MWCNTs as long as the Cu2þ ions are still available. It should be noted that the zeta potential of Cu/MWCNT hybrid material prepared by the microwave treatment is 10.8 mV. The increasing zeta potential value would be ascribed to the contribution of Cu species which possess positive zeta potential [12]. Finally, the excessive Cu2þ ions available in the solution would further be reduced by EG, resulting in the continuous growth of agglomerating Cu2O and Cu nanoparticles on the outer surface of MWCNTs [13]. It should be summarized that the irradiation time of 5 min would be essential for inducing the formation of copper nanoparticles hybridized with MWCNTs with the presence of EG and NaOH. The longer irradiation time would further enhance the agglomeration of copper nanoparticles as long as there is excessive Cu2þ ions in the system. Table 1 Zeta potential of each sample. Zeta potential (mV) Pristine MWCNTs Acid-treated MWCNTs CuSO4$5H2O precursor Cu/MWCNTs hybrid material
9.02 23.60 4.85 10.80
N. Leelaviwat et al. / Current Applied Physics 12 (2012) 1575e1579
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4. Conclusion
References
The microwave irradiation can be used to induce Cu2þ ions to deposit on the negatively charged surface of the acid-treated MWCNTs, resulting in the presence of Cu and Cu2O nanoparticles depositing on the outer surface of MWCNTs. Based on all analytical results, it would be implied that specific functional groups generated from EG with the presence of NaOH would play an important role as a reducing agent to transform Cu2þ ions to Cu2O and Cu nanoparticles. The presence of Cu2O and Cu nanoparticles on the MWCNT surface would undergo three consecutive steps of adsorption, reduction and agglomeration. The microwave irradiation time of 5 min or longer would be essential for the formation of uniform dispersion of copper nanopraticles on the MWCNT surface.
[1] X.Y. Tao, X.B. Zhang, J.P. Cheng, F. Liu, Mater. Chem. Phys. 104 (2007) 210e214. [2] T. Charinpanitkul, A. Soottitantawat, N. Tonanon, W. Tanthapanichakoon, Mater. Chem. Phys. 116 (2009) 125e128. [3] F. Wang, S. Arai, M. Endo, Electrochem. Commun. 6 (2004) 1042e1044. [4] W.X. Chen, J. Zhao, J.Y. Lee, Z. Liu, Mater. Chem. Phys. 91 (2005) 124e129. [5] J.H. Ahn, J.N. Lee, Y.C. Kim, B.T. Ahn, Curr. Appl. Phys. 2 (2002) 135e139. [6] E. Tamburri, S. Orlanducci, M.L. Terranova, F. Valentini, G. Palleschi, A. Curulli, F. Brunetti, D. Passeri, A. Alippi, M. Rossi, Carbon 43 (2005) 1213e1221. [7] J.H. Lee, B.E. Park, Y.M. Lee, S.H. Hwang, W.B. Ko, Curr. Appl. Phys. 9 (2009) e152ee156. [8] A. Schierz, H. Zänker, Environ. Poll. 157 (2009) 1088e1094. [9] J. Klanwan, N. Akrapattangkul, V. Pavarajarn, T. Seto, Y.Otani, T. Charinpanitkul, Mater. Lett. 64 (2010) 80e82. [10] P. Martis, B.R. Venugopal, J.F. Seffer, J. Delhalle, Z. Mekhalif, Acta Materialia 59 (2011) 5040e5047. [11] Y. Zhao, J.-J. Zhu, J.-M. Hong, N. Bian, H.-Y. Chen, Euro. J. Inor. Chem. 20 (2004) 4072e4080. [12] M. Blosi, S. Albonetti, M. Dondi, C. Martelli, G. Baldi, J. Nanoparticle Res. 13 (2011) 127e138. [13] H.T. Zhu, C.Y. Zhang, Y.S. Yin, J. Crystal Growth 270 (2004) 722e728. [14] L. Zhao, L. Gao, Carbon 42 (8e9) (2004) 1858e1861.
Acknowledgments This research work is partially supported by the Centenary Fund and Graduate School of Chulalongkorn University to Center of Excellence in Particle Technology.