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Decoration of multi-walled carbon nanotubes with silver nanoparticles and investigation on its colloid stability
Materials Chemistry and Physics 139 (2013) 113e117
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Decoration of multi-walled carbon nanotubes with silver nanoparticles and investigation on its colloid stability F. Ahmadpoor, S. Mojtaba Zebarjad*, K. Janghorban Dept. Metallurgical Engineering, Engineering Faculty, Shiraz University, Shiraz, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Oxygen containing groups on acid treated MWCNTs act as sites for deposition silver nanoparticles on MWNTs. < Ultrasonication help to decrease of Ag nanoparticles size on acid treated MWCNTs. < Stability of MWCNTs decorated using silver nanoparticles in distilled water than acid treated MWCNTs has decreased.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2012 Received in revised form 13 December 2012 Accepted 28 December 2012
In this paper, multi-walled carbon nanotubes (MWCNTs) were decorated with silver nanoparticles which have been successfully synthesized by chemical reduction of Ag cations on the acid treated MWCNTs surface by sodium hydroxide. Ag/MWCNTs were characterized by TEM and XRD. The results indicated that silver nanoparticles were homogeneously dispersed on the outer surface of MWCNTs. The dispersibility of pristine, acid treated and decorated carbon nanotubes in distilled water were investigated by zeta potential and optical image. The results revealed that the dispersion of acid treated carbon nanotubes in distilled water was better than those of pristine MWCNTs and Ag/MWCNTs. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Metals Chemical synthesis Infrared spectroscopy (IR)
1. Introduction Carbon nanotubes (CNTs) were first discovered by Iijima in 1991 [1]. This one dimensional type of carbon allotropes has attracted a great deal of interest because of its outstanding properties such as high aspect ratio, ultra-light weight, high tensile strength, excellent electrical conductivity, chemical and thermal stability [2]. The external surface of CNT serves as a specific template for the deposition of metal nanoparticles. The integration
* Corresponding author. Tel./fax: þ98 7112307293. E-mail addresses: [email protected], [email protected] (S.M. Zebarjad). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.071
of these two classes of materials (CNTs and nanoparticles) has received extensive attention because these new hybrid nanostructures show properties of two components [3]. Decoration of CNTs with noble metal nanoparticles such as Au, pd, Pt, Ag are attractive due to the unique properties of metal nanoparticles [4e 6]. Among them, decoration of CNTs with Ag nanoparticles has received considerable attention because of their potential applications as advanced nanocomposites [5], antibacterial activity [7], catalyst behavior [8] and sensor [9,10]. In literature, vapor deposition [11], thermal decomposition [4,12], gamma irradiation [13] and chemical reduction [14,15] have been employed to decorate CNTs with Ag. However in most cases, agglomeration of silver nanoparticles is a major drawback in uniform decoration of CNT surface.
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Fig. 1. FTIR spectra of pristine MWNTs.
Naturally pristine carbon nanotubes are hydrophobic and tend to aggregation in water and organic solvents due to the presence of high van der waals forces between tubes. Functionalization is an efficient technique to prevent carbon nanotubes aggregation. Covalent and noncovalent functionalization are two effective methods to improve dispersion in solvents [16]. Sheih et al. [17] reported that dispersibility of covalent functionalized carbon nanotubes in polar solvents such as water was improved because of ionization of COOH groups on the treated carbon nanotubes. The studies on the dispersibility of carbon nanotubes in water which presented in the literature limited to investigation of pristine and acid treated carbon nanotubes. In the present study, we applied a simple and effective approach to decoration of multiwall carbon nanotubes with silver nanoparticles based on the wet chemistry reaction. We chose sodium hydroxide (NaOH) as a reducing agent to prepare Ag-MWCNTs. Ultrasonication is applied to homogeneously decorate carbon
nanotubes with silver nanoparticles. As well as we studied the dispersibility of pristine, acid treated and decorated carbon nanotubes with silver nanoparticles in water. 2. Experimental procedure 2.1. Materials Multi-walled carbon nanotubes were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China) with a purity of >95%. Their diameter and length were 40e60 nm and 5e15 mm respectively. NaOH (99%) and silver nitrate (98%) were purchased from Merck. 2.2. Acid treatment of MWCNTs 0.1 g Pristine MWCNTs were suspended in 50 concentrated HNO3 and ultrasonicated for 2 h then the suspension stirred for 2 h.
Fig. 2. FTIR spectra of acid treated MWCNTs.
F. Ahmadpoor et al. / Materials Chemistry and Physics 139 (2013) 113e117
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2 h. Then, the decorated MWCNTs were separated from the solution by filtering and excess of Naþ, Agþ and NO3 ions were removed by multiple washing steps with deionized water and dried at 30 C. 2.4. Characterization X-ray diffraction (XRD) patterns were obtained by PC1800X diffractometer using Ka (l ¼ 1.5404 A). Transmission electron microscopy (TEM) was performed by CM 120 Philips at 120 kV. The TEM samples were prepared by drying a droplet of the Ag-MWCNTs suspension on a 300 mesh size copper grids coated carbon film. Fourier transform infrared (FTIR) spectra was recorded on Bruker TENSOR 27 in the 400e4000 cm1 wavenumber range. Zeta potential measurements were conducted on a Zetasizer 2000 system (Malvern, UK) with irradiation by a 658 nm HeeNe laser. 3. Results and discussion 3.1. FTIR analysis FTIR is used as a qualitative technique to characterize the samples for the identification of surface functional groups on the outer surface of MWCNTs. Fig. 1 shows FTIR spectra of pristine MWCNTs with a broad peak at 3440 cm1 which refers to eOH stretching mode of the hydroxyl group [18]. Carbonyl group of quinine type at 1640 cm1 on pristine MWCNTs can be because of oxidation of MWCNTs surfaces during purification by the manufacture. C]C stretching band at 1460 cm1, asymmetric/symmetric methylene (CH2), stretching bands at 2920 cm1 and 2850 cm1 are appeared in pristine MWCNTs spectra [19,20]. For acid treatment MWCNTs (Fig. 2), the FTIR spectrum displays a broad peak at 3450 cm1 can be attributed to the OH group [21]. The intensity of C]C (1460 cm1) is decreased with nitric acid treatment and C]O characteristic peak is visible at 1720 cm1 [22]. The peaks between 1050 and 1300 cm1 are related to CeO from ethers, alcohols, anhydrides, lactones or carboxylic acids [23]. In acid treated MWCNTs spectrum the symmetric and asymmetric CH2 bands are significantly decreased. Nitric acid initially attacks active sites on MWCNTs such as CH2 at defect sites and other functional groups are created by defect-consuming and defect generating mechanism [20].
Fig. 3. TEM images of MWCNTs decorated with Ag nanoparticles.
The resulted material was collected on filter paper and then washed with distilled water until neutral pH. Finally, the functionalized MWCNTs were dried at 90 C overnight. 2.3. Decoration of silver on MWCNTs 50 mg functionalized MWCNTs ultrasonicated in 20 ml deionized water for 2 h. Then 20 ml aqueous solution of 0.1 M AgNO3 was added dropwise to the MWCNTs suspension. A solution of 0.1 M NaOH was added to the MWCNTs-AgNO3 till the pH reaches 6. Subsequently, the solution was incubated and ultrasonicated for
Fig. 4. XRD pattern of decoration carbon nanotubes with silver nanoparticles.
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Fig. 5. Schematic diagram of formation mechanism of Ag/CNTs.
Specimen
Avg. zeta potential (mV)
electrostatic interaction to form eCOO Agþ and release Hþ ions. NaOH solution with hydroxyl ions result in situ reduction of Ag cations to Ag0 on carbon nanotubes [26].
Pristine CNTs Acid treated CNTs Ag/CNTs
13.97 26.7 24.69
3.3. Colloid stability
Table 1 Zeta potential of pristine, acid treated and Ag/CNTs.
3.2. Morphology and structural observation of Ag-MWCNTs TEM is a powerful technique to characterize the morphology and microstructure of Ag/CNTs nanocomposites. Typical TEM images of decoration carbon nanotubes with silver nanoparticles are shown in Fig. 3. It can be seen clearly from the TEM that spherical Ag nanoparticles with the average diameter around 5 nm are well deposited on the outside wall of MWCNTs and there is no evidence of isolated Ag nanoparticles. Moreover, the silver nanoparticles are distributed on the surface of MWCNTs without agglomeration. It is observed that silver nanoparticles have been homogeneously attached to the outer surface of MWCNTs which corroborated the effect of oxygen containing group on the formation of MWCNTs/Ag nanocomposites. The structure and chemical composition of Ag/MWCNTs was verified by XRD and result is shown in Fig. 4. The graphitized nature of MWCNTs is confirmed by crystalline planes C (002) and C (101) reflection at 2q values 26 and 44 , respectively. Reflection peaks at 2q values 38 , 45 , 64 , 77 and 82 which can be indexed as (111), (200), (220) and (311) respectively that corresponding to fcc Ag nanoparticles [24]. No characteristic peaks of silver nitrate and silver oxide were observed in XRD spectrum. TEM and XRD characterization demonstrate that Ag nanoparticles successfully have been decorated on acid treated MWCNTs. Fig. 5 shows the schematic diagram of decorated carbon nanotubes with silver nanoparticles. After acid treatment of MWNTs with HNO3, the oxygen containing groups on carbon nanotubes can used as a nucleation sites for deposition of Ag nanoparticles [25]. Ag cations in aqueous solution of silver nitrate can interact with carboxyl groups on carbon nanotubes through
Zeta potential is the electric potential difference between charged carbon nanotubes and a liquid medium. The value of zeta potential can be related to stability of aqueous carbon nanotubes suspension. The zeta potential represents the repulsion force between carbon nanotubes in dispersed medium. Colloids with high zeta potential (negative or positive) above 15 mV are stable while those with low zeta potential tend to coagulate [27]. Table 1 shows the zeta potential of aqueous dispersion of pristine, acid treated MWCNTs and also Ag/MWCNTs. The higher zeta potential of acid treated MWCNTs are should be attributed to carboxylic acid groups on the surface of carbon nanotubes. Ionization these groups in water produce carboxylate anions that expel each other and resulting improvement the dispersion of acid treated carbon nanotubes in water. Zeta potential of Ag/MWCNTs is less negative than acid treated carbon nanotubes because of consuming carboxylic acid by silver nanoparticles to decoration carbon nanotubes. The reduction of zeta potential in Ag/MWCNTs is a point of positive effect of carboxylic group for decoration carbon nanotubes with silver. Fig. 6(a) and (b) shows dispersibility behavior of pristine, acid treated and Ag decorated MWCNTs in water after 0.5 h and 168 h respectively. Pristine CNTs have less COOH content groups as described in FTIR and zeta potential analysis, therefore exhibit poor dispersion and stability in water after 0.5 and 168 h. Acid treated CNTs have more COOH groups. Ionization these groups form Hþ and COO that repulsion forces increase and provide good stability after 168 h. Some of the COOH groups on acid treated MWNTs sacrificed after decoration by Ag nanoparticles; therefore Ag/CNTs have less COOH groups than acid treated CNTs. Nevertheless the stability of an aqueous solution of Ag/CNTs is much better than pristine CNTs because of residual COOH groups after decoration. The result of zeta
Fig. 6. Dispersibility behavior of nanofluids (05 mg ml1), (a) 0.5 h after sonication, (b) 168 h after sonication. (Pristine CNT (left), acid treated CNT (middle) and Ag/CNT (right)).
F. Ahmadpoor et al. / Materials Chemistry and Physics 139 (2013) 113e117
potential can corroborate that higher zeta potential leads to higher stability. 4. Conclusions In summary, we have successfully prepared Ag decorated MWCNTs by a simple method. The TEM and XRD results confirmed that silver nanoparticles loaded on the surface of MWNTs homogeneously. The results of zeta potential and sediment photograph show that the dispersion of carbon nanotubes in distilled water with acid treatment increased but decoration carbon nanotubes with silver nanoparticles decreased. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56e58. [2] Xiang Wang, Tian Xia, Susana Addo Ntim, Zhaoxia Ji, Saji George, Huan Meng, Haiyuan Zhang, Vincent Castranova, Somenath Mitra, André E. Nel, Quantitative techniques for assessing and controlling the dispersion and biological effects of multiwalled carbon nanotubes in mammalian tissue culture cells, ACS Nano 4 (2010) 7241e7252. [3] Vasilios Georgakilas, Dimitrios Gournis, Vasilios Tzitzios, Lucia Pasquato, Dirk M. Guldie, Maurizio Prato, Decorating carbon nanotubes with metal or semiconductor nanoparticles, Journal of Materials Chemistry 17 (2007) 2679e2694. [4] B. Xue, P. Chen, Q. Hong, J. Lin, K.L. Tan, Growth of Pd, Pt, Ag and Au nanoparticles on carbon nanotubes, Journal Materials Chemistry 11 (2001) 2378e2381. [5] Fei Xin, Lin Li, Decoration of carbon nanotubes with silver nanoparticles for advanced CNT/polymer nanocomposites, Composites: Part A 42 (2011) 961e967. [6] Suresh Kumar, Inderpreet Kaur, Keya Dharamvir, Lalit M. Bharadwaj, Controlling the density and site of attachment of gold nanoparticles onto the surface of carbon nanotubes, Journal of Colloid and Interface Science 369 (2012) 23e27. [7] Raja Mohan, A.M. Shanmugharaj, Ryu Sung Hun, An efficient growth of silver and copper nanoparticles on multiwalled carbon nanotube with enhanced antimicrobial activity, Journal of Biomedical of Materials Research B 96B (2011) 119e126. [8] D.J. Guo, H.L. Li, Highly dispersed Ag nanoparticles on functional MWNT surfaces for methanol oxidation in alkaline solution, Carbon 43 (2005) 1259e 1264. [9] D.W.H. Fam, A.I.Y. Tok, A.L. Palaniappan, P. Nopphawan, Anup Lohani, S.G. Mhaisalkar, Selective sensing of hydrogen sulphide using silver nanoparticle decorated carbon nanotubes, Sensors and Actuators B 150 (2009) 189e192. [10] Minh-Phuong Ngoc Bui, Xuan-Hung Pham, Kwi Nam Han, Cheng Ai Li, Yong Shin Kim, Gi Hun Seong, Electrocatalytic reduction of hydrogen peroxide by silver particles patterned on single-walled carbon nanotubes, Sensorsr and Actuators B 150 (2010) 436e441.
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[11] K.C. Chin, A. Gohel, W.Z. Chen, H.I. Elim, W. Ji, G.L. Show Chong, et al., Gold and silver coated carbon nanotubes: an improved broad-band optical limiter, Chemical Physics Letters 409 (2005) 85e88. [12] Li Zhenzi, Lijun Fan, Tao Zhang, Kang Li, Facile synthesis of Ag nanoparticles supported on MWCNTs with favorable stability and their bactericidal properties, Journal of Hazardous Materials (2011). [13] S.D. Oh, B.K. So, S.H. Choi, A. Gopalan, K.P. Lee, K.R. Yoon, Dispersing of Ag, Pd, and PteRu alloy nanoparticles on single-walled carbon nanotubes by c-irradiation, Materials Letters 59 (2005) 1121e1124. [14] Putian Wang, Jinbao Guo, Huihui Wang, Yan Zhang, Jie Wei, Functionalized multi-walled carbon nanotubes filled ultraviolet curable resin nanocomposites and their applications for nanoimprint lithography, Journal of Physical Chemistry C 113 (2009) 8118e8123. [15] Yuyang Liu, Jing Tang, Xianqiong Chen, Wei Chen, G.K.H. Pang, J.H. Xin, A wetchemical route for the decoration of CNTs with silver nanoparticles, Carbon 44 (2006) 381e392. [16] V.N. Khabashesku, Covalent functionalization of carbon nanotubes: synthesis, properties and applications of fluorinated derivatives, Russian Chemical Reviews 80 (2011) 705e725. [17] Y.T. Sheih, et al., The effect of pH and ionic strength on the dispersion of carbon nanotubes in poly (acrylic acid) solutions, Polymer International 61 (2011) 554e559. [18] X. Jiang, J. Gu, X. Bai, L. Lin, Y. Zhang, The influence of acid treatment on multiwalled carbon nanotubes, Pigment & Resin Technology (2009) 165e173. [19] Blazej Scheibe, Ewa Borowiak-Palen, Ryszard J. Kalenczuk, Oxidation and reduction of multiwalled carbon nanotubes preparation and characterization, Materials Characterization 61 (2010) 185e191. [20] Yu-Lin Hsin, Jyun-Yi Lai, Kuo Chu Hwang, Shen-Chuan Lo, Fu-Rong Chen, J.J. Kai, Rapid surface functionalization of iron-filled multi-walled carbon nanotubes, Carbon 44 (2006) 3328e3335. [21] Babu P. Ramesh, W.J. Blau, P.K. Tyagi, D.S. Misra, N. Ali, J. Gracio, G. Cabral, E. Titus, Thermogravimetric analysis of cobalt-filled carbon nanotubes deposited by chemical vapour deposition, Thin Solid Films 494 (2006) 128e132. [22] Nosrat Izadi, Ali Morad Rashidi, Samira Golzardi, Zeinab Talaei, Ali Reza Mahjoub, Mir Hassan Aghili, Hydrogen sulfide sensing properties of multi walled carbon nanotubes, Ceramics International 38 (2012) 65e75. [23] Paulina Caenete-Rosales, Valeria Ortega, Alejandro Álvarez-Lueje, Soledad Bolloa, Mónica González, Alejandro Ansón, María Teresa Martínez, Influence of size and oxidative treatments of multi-walled carbon nanotubes on their electrocatalytic properties, Electrochimica Acta 62 (2012) 163e171. [24] S. Ramaprabhu, Neetu Jha, Thermal conductivity studies of metal dispersed multiwalled carbon nanotubes in water and ethylene glycol based nanofluids, Journal of Applied Physics 106 (2009) 084317. [25] T.W. Ebbesen, H. Hiura, M.E. Bisher, M.M.J. Treacy, J.L. Shreeve-Keyer, R.C. Haushalter, Decoration of carbon nanotubes, Advanced Materials 8 (1996) 155e157. [26] G. Wee, Wai F. Mak, N. Phonthammachai, A. Kiebele, M.V. Reddy, B.V.R. Chowdari, G. Gruner, M. Srinivasan, S.G. Mhaisalkara, Particle size effect of silver nanoparticles decorated single walled carbon nanotube electrode for supercapacitors, Journal of The Electrochemical Society 157 (2010) A179eA184. [27] Wenming Zhao, Mei Li, Zhongyi Zhang, Hua-Xin Peng, EMI shielding effectiveness of silvernanoparticle-decorated multi-walled carbon nanotube sheets, International Journal of Smart and Nano Materials 1 (2010) 249e260.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Decoration of multi-walled carbon nanotubes with silver nanoparticles and investigation on its colloid stability F. Ahmadpoor, S. Mojtaba Zebarjad*, K. Janghorban Dept. Metallurgical Engineering, Engineering Faculty, Shiraz University, Shiraz, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Oxygen containing groups on acid treated MWCNTs act as sites for deposition silver nanoparticles on MWNTs. < Ultrasonication help to decrease of Ag nanoparticles size on acid treated MWCNTs. < Stability of MWCNTs decorated using silver nanoparticles in distilled water than acid treated MWCNTs has decreased.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2012 Received in revised form 13 December 2012 Accepted 28 December 2012
In this paper, multi-walled carbon nanotubes (MWCNTs) were decorated with silver nanoparticles which have been successfully synthesized by chemical reduction of Ag cations on the acid treated MWCNTs surface by sodium hydroxide. Ag/MWCNTs were characterized by TEM and XRD. The results indicated that silver nanoparticles were homogeneously dispersed on the outer surface of MWCNTs. The dispersibility of pristine, acid treated and decorated carbon nanotubes in distilled water were investigated by zeta potential and optical image. The results revealed that the dispersion of acid treated carbon nanotubes in distilled water was better than those of pristine MWCNTs and Ag/MWCNTs. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Metals Chemical synthesis Infrared spectroscopy (IR)
1. Introduction Carbon nanotubes (CNTs) were first discovered by Iijima in 1991 [1]. This one dimensional type of carbon allotropes has attracted a great deal of interest because of its outstanding properties such as high aspect ratio, ultra-light weight, high tensile strength, excellent electrical conductivity, chemical and thermal stability [2]. The external surface of CNT serves as a specific template for the deposition of metal nanoparticles. The integration
* Corresponding author. Tel./fax: þ98 7112307293. E-mail addresses: [email protected], [email protected] (S.M. Zebarjad). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.071
of these two classes of materials (CNTs and nanoparticles) has received extensive attention because these new hybrid nanostructures show properties of two components [3]. Decoration of CNTs with noble metal nanoparticles such as Au, pd, Pt, Ag are attractive due to the unique properties of metal nanoparticles [4e 6]. Among them, decoration of CNTs with Ag nanoparticles has received considerable attention because of their potential applications as advanced nanocomposites [5], antibacterial activity [7], catalyst behavior [8] and sensor [9,10]. In literature, vapor deposition [11], thermal decomposition [4,12], gamma irradiation [13] and chemical reduction [14,15] have been employed to decorate CNTs with Ag. However in most cases, agglomeration of silver nanoparticles is a major drawback in uniform decoration of CNT surface.
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F. Ahmadpoor et al. / Materials Chemistry and Physics 139 (2013) 113e117
Fig. 1. FTIR spectra of pristine MWNTs.
Naturally pristine carbon nanotubes are hydrophobic and tend to aggregation in water and organic solvents due to the presence of high van der waals forces between tubes. Functionalization is an efficient technique to prevent carbon nanotubes aggregation. Covalent and noncovalent functionalization are two effective methods to improve dispersion in solvents [16]. Sheih et al. [17] reported that dispersibility of covalent functionalized carbon nanotubes in polar solvents such as water was improved because of ionization of COOH groups on the treated carbon nanotubes. The studies on the dispersibility of carbon nanotubes in water which presented in the literature limited to investigation of pristine and acid treated carbon nanotubes. In the present study, we applied a simple and effective approach to decoration of multiwall carbon nanotubes with silver nanoparticles based on the wet chemistry reaction. We chose sodium hydroxide (NaOH) as a reducing agent to prepare Ag-MWCNTs. Ultrasonication is applied to homogeneously decorate carbon
nanotubes with silver nanoparticles. As well as we studied the dispersibility of pristine, acid treated and decorated carbon nanotubes with silver nanoparticles in water. 2. Experimental procedure 2.1. Materials Multi-walled carbon nanotubes were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China) with a purity of >95%. Their diameter and length were 40e60 nm and 5e15 mm respectively. NaOH (99%) and silver nitrate (98%) were purchased from Merck. 2.2. Acid treatment of MWCNTs 0.1 g Pristine MWCNTs were suspended in 50 concentrated HNO3 and ultrasonicated for 2 h then the suspension stirred for 2 h.
Fig. 2. FTIR spectra of acid treated MWCNTs.
F. Ahmadpoor et al. / Materials Chemistry and Physics 139 (2013) 113e117
115
2 h. Then, the decorated MWCNTs were separated from the solution by filtering and excess of Naþ, Agþ and NO3 ions were removed by multiple washing steps with deionized water and dried at 30 C. 2.4. Characterization X-ray diffraction (XRD) patterns were obtained by PC1800X diffractometer using Ka (l ¼ 1.5404 A). Transmission electron microscopy (TEM) was performed by CM 120 Philips at 120 kV. The TEM samples were prepared by drying a droplet of the Ag-MWCNTs suspension on a 300 mesh size copper grids coated carbon film. Fourier transform infrared (FTIR) spectra was recorded on Bruker TENSOR 27 in the 400e4000 cm1 wavenumber range. Zeta potential measurements were conducted on a Zetasizer 2000 system (Malvern, UK) with irradiation by a 658 nm HeeNe laser. 3. Results and discussion 3.1. FTIR analysis FTIR is used as a qualitative technique to characterize the samples for the identification of surface functional groups on the outer surface of MWCNTs. Fig. 1 shows FTIR spectra of pristine MWCNTs with a broad peak at 3440 cm1 which refers to eOH stretching mode of the hydroxyl group [18]. Carbonyl group of quinine type at 1640 cm1 on pristine MWCNTs can be because of oxidation of MWCNTs surfaces during purification by the manufacture. C]C stretching band at 1460 cm1, asymmetric/symmetric methylene (CH2), stretching bands at 2920 cm1 and 2850 cm1 are appeared in pristine MWCNTs spectra [19,20]. For acid treatment MWCNTs (Fig. 2), the FTIR spectrum displays a broad peak at 3450 cm1 can be attributed to the OH group [21]. The intensity of C]C (1460 cm1) is decreased with nitric acid treatment and C]O characteristic peak is visible at 1720 cm1 [22]. The peaks between 1050 and 1300 cm1 are related to CeO from ethers, alcohols, anhydrides, lactones or carboxylic acids [23]. In acid treated MWCNTs spectrum the symmetric and asymmetric CH2 bands are significantly decreased. Nitric acid initially attacks active sites on MWCNTs such as CH2 at defect sites and other functional groups are created by defect-consuming and defect generating mechanism [20].
Fig. 3. TEM images of MWCNTs decorated with Ag nanoparticles.
The resulted material was collected on filter paper and then washed with distilled water until neutral pH. Finally, the functionalized MWCNTs were dried at 90 C overnight. 2.3. Decoration of silver on MWCNTs 50 mg functionalized MWCNTs ultrasonicated in 20 ml deionized water for 2 h. Then 20 ml aqueous solution of 0.1 M AgNO3 was added dropwise to the MWCNTs suspension. A solution of 0.1 M NaOH was added to the MWCNTs-AgNO3 till the pH reaches 6. Subsequently, the solution was incubated and ultrasonicated for
Fig. 4. XRD pattern of decoration carbon nanotubes with silver nanoparticles.
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Fig. 5. Schematic diagram of formation mechanism of Ag/CNTs.
Specimen
Avg. zeta potential (mV)
electrostatic interaction to form eCOO Agþ and release Hþ ions. NaOH solution with hydroxyl ions result in situ reduction of Ag cations to Ag0 on carbon nanotubes [26].
Pristine CNTs Acid treated CNTs Ag/CNTs
13.97 26.7 24.69
3.3. Colloid stability
Table 1 Zeta potential of pristine, acid treated and Ag/CNTs.
3.2. Morphology and structural observation of Ag-MWCNTs TEM is a powerful technique to characterize the morphology and microstructure of Ag/CNTs nanocomposites. Typical TEM images of decoration carbon nanotubes with silver nanoparticles are shown in Fig. 3. It can be seen clearly from the TEM that spherical Ag nanoparticles with the average diameter around 5 nm are well deposited on the outside wall of MWCNTs and there is no evidence of isolated Ag nanoparticles. Moreover, the silver nanoparticles are distributed on the surface of MWCNTs without agglomeration. It is observed that silver nanoparticles have been homogeneously attached to the outer surface of MWCNTs which corroborated the effect of oxygen containing group on the formation of MWCNTs/Ag nanocomposites. The structure and chemical composition of Ag/MWCNTs was verified by XRD and result is shown in Fig. 4. The graphitized nature of MWCNTs is confirmed by crystalline planes C (002) and C (101) reflection at 2q values 26 and 44 , respectively. Reflection peaks at 2q values 38 , 45 , 64 , 77 and 82 which can be indexed as (111), (200), (220) and (311) respectively that corresponding to fcc Ag nanoparticles [24]. No characteristic peaks of silver nitrate and silver oxide were observed in XRD spectrum. TEM and XRD characterization demonstrate that Ag nanoparticles successfully have been decorated on acid treated MWCNTs. Fig. 5 shows the schematic diagram of decorated carbon nanotubes with silver nanoparticles. After acid treatment of MWNTs with HNO3, the oxygen containing groups on carbon nanotubes can used as a nucleation sites for deposition of Ag nanoparticles [25]. Ag cations in aqueous solution of silver nitrate can interact with carboxyl groups on carbon nanotubes through
Zeta potential is the electric potential difference between charged carbon nanotubes and a liquid medium. The value of zeta potential can be related to stability of aqueous carbon nanotubes suspension. The zeta potential represents the repulsion force between carbon nanotubes in dispersed medium. Colloids with high zeta potential (negative or positive) above 15 mV are stable while those with low zeta potential tend to coagulate [27]. Table 1 shows the zeta potential of aqueous dispersion of pristine, acid treated MWCNTs and also Ag/MWCNTs. The higher zeta potential of acid treated MWCNTs are should be attributed to carboxylic acid groups on the surface of carbon nanotubes. Ionization these groups in water produce carboxylate anions that expel each other and resulting improvement the dispersion of acid treated carbon nanotubes in water. Zeta potential of Ag/MWCNTs is less negative than acid treated carbon nanotubes because of consuming carboxylic acid by silver nanoparticles to decoration carbon nanotubes. The reduction of zeta potential in Ag/MWCNTs is a point of positive effect of carboxylic group for decoration carbon nanotubes with silver. Fig. 6(a) and (b) shows dispersibility behavior of pristine, acid treated and Ag decorated MWCNTs in water after 0.5 h and 168 h respectively. Pristine CNTs have less COOH content groups as described in FTIR and zeta potential analysis, therefore exhibit poor dispersion and stability in water after 0.5 and 168 h. Acid treated CNTs have more COOH groups. Ionization these groups form Hþ and COO that repulsion forces increase and provide good stability after 168 h. Some of the COOH groups on acid treated MWNTs sacrificed after decoration by Ag nanoparticles; therefore Ag/CNTs have less COOH groups than acid treated CNTs. Nevertheless the stability of an aqueous solution of Ag/CNTs is much better than pristine CNTs because of residual COOH groups after decoration. The result of zeta
Fig. 6. Dispersibility behavior of nanofluids (05 mg ml1), (a) 0.5 h after sonication, (b) 168 h after sonication. (Pristine CNT (left), acid treated CNT (middle) and Ag/CNT (right)).
F. Ahmadpoor et al. / Materials Chemistry and Physics 139 (2013) 113e117
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