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Quantitative study on the interaction of Ag+ and Pd2 + with CNT-graft-PCA (polycitric acid) in aqueous solution
Journal of Molecular Liquids 180 (2013) 39–44
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Quantitative study on the interaction of Ag + and Pd 2 + with CNT-graft-PCA (polycitric acid) in aqueous solution N. Sarlak a, b,⁎, M. Adeli a, M. Karimi a, M. Bordbare c, M.A. Farahmandnejad b a b c
Department of chemistry, Faculty of science, University of Lorestan, Khorram Abad, Iran Engineering Research Institute, Tehran, Iran Department of chemistry, Faculty of Science, The University of Qom, P. O. Box: 37185-359, Qom, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 12 November 2012 Accepted 27 December 2012 Available online 9 January 2013 Keywords: Quantitative study Silver nanoparticle Palladium nanoparticle Polycitric acid Carbon nanotube Complexation
a b s t r a c t The reactions between multi walled carbon nanotube graft polycitric acid (MWCNT-graft-PCA) and Ag+ and Pd2+ ions at 25 °C are investigated spectrophotometrically. Encapsulation of Ag (I) and Pd (II) on the surface of (MWCNT-graft-PCA) as a function of pH, sonication time and reaction time was studied. The results indicated that the absorbance increased until pH=6 and pH=8 for Ag and Pd respectively and then decreased. The effects of sonication time on the reactions were studied and 20 min was selected for both of reactions. Also the time of reaction between MWCNT-graft-PCA and (Ag, Pd) vs. absorbance was studied and the stirring time of 160 and 100 min was selected for these reactions respectively. Also knowing the number of light-absorbing species is a critical step for subsequent quantitative and qualitative solution equilibrium studies. Behind the number of various complexes formed the stability constants for the combination of MWCNT-graft-PCA (ligand L) with Ag+ and Pd2+ ions at 25 °C and variant pH, and various mole ratios are estimated by the SQUAD program. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, nanotechnology has become an important field in science and technology, allowing one to manipulate matter at the nanometer scale and to incorporate nanostructures and nano-processes into working technological innovations [1–5]. Considerable interest has been focused on the possibility of the construction of new assemblies from nanoparticles (NPs) and other components to yield superstructure nanomaterials with unique and useful optical, electronic, and magnetic properties [1–4]. Carbon nanotubes (CNTs) are potentially excellent one-dimensional nanoscale materials because of their excellent physical properties [6] and morphology that can be carefully functionalize [7,8]. The chemical modification and the covalent functionalization of carbon nanotubes with organic species like long chain alcohols and amines, dendrimers and polymers have been reported recently [9–11]. Also there are few reports about the functionalization of carbon nanotubes that open the area of metallo-organic chemistry to nanotubes such as the synthesis of nanotubes covalently complexed to molecular coordination compounds [12], interconnecting carbon nanotubes with an inorganic metal complex [13], sidewall oxidation and complexation of carbon nanotubes by base-catalyzed cycloaddition of transition metal oxide [14]. Recently a method has been developed for the attaching of ⁎ Corresponding author at: Department of chemistry, Faculty of science, University of Lorestan, Khorram Abad, Iran. E-mail address: [email protected] (N. Sarlak). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.12.034
self-assembled gold nanoparticles (Au-NPs) onto the surface of sidewalls and ends of thiol-terminated multi-walled carbon nanotubes (MWNTs) functionalized with orthomercaptoaniline which acts as a bridging agent [15]. Among them, one of the most intriguing applications of CNTs is the polymer/CNT nanocomposites. In the past decade, there has been an increasing interest in the studies of polymer/CNT nanocomposites due to the unique combination of promising properties and construction of multifunctional structures of each component [16]. Lately biodegradable nanocomposites containing multi walled carbon nanotubes (MWCNT) and polycitric acid (PCA) were successfully synthesized which hereby metal nanoparticles could be trapped through the conjugating of polymers to CNTs [17]. CNT grafted polymer (CNT-g-polymers) hybrid materials are good candidates to support metal nanoparticles such as platinum group metals (silver, palladium and so on). The unique properties of palladium, silver and other platinum group metals account for their widespread use. In this work, MWCNTs were opened and functionalized by acid (MWCNT-COOH) and citric acid was polymerized on their surface. Trapping of Ag+ and Pd 2+ ions by MWCNT-g-PCA hybrid materials was led to encapsulated silver and palladium nanoparticles onto the surface of MWCNTs via complexation reactions. The parameters for complexation reactions of (Ag +, Pd 2+) ions with MWCNT-g-PCA were optimized using UV–Visible spectrophotometry and then the complex-forming equilibriums and the stability constants of MWCNTg-PCA-Ag and MWCNT-g-PCA-Pd were calculated using SQUAD
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N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
according to the following procedure [17]. Functionalized MWCNTCOOH (0.05 g) was added to a polymerization ampoule equipped with magnetic stirrer and vacuum inlet then monohydrate citric acid (2.5 g) added to ampoule and it was sealed under vacuum. The mixture was heated up to 120 °C and stirred in this temperature for 30 min. After removing the water by vacuum inlet, reaction temperature was raised to 140 °C and stirred at this temperature for 1 h. Water as a byproduct of reaction was removed by vacuum inlet and reaction temperature was raised to 160 °C. Polymerization continued in this temperature under dynamic vacuum for 1.5 h. The mixture was cooled and dissolved in THF and product was precipitated in cyclohexane. Purified product was obtained as a viscous coffee-brown compound in %85 yields.
program. It is one of the most widespread programs and algorithms for determining the stability constant from absorbance data. Conceptually, the novelty of this derivatization is that nanotube can be considered as a primary ligand with respect to the metal atom. To the best of our knowledge, this is the first report of optimizing the parameters of reaction between MWCNT-g-PCA and platinum group metals (Ag and Pd) and it is the first report of applying SQUAD program for studying MWCNT complexation reactions with metal ions. 2. Experimental 2.1. Chemicals and solutions All the chemicals were of analytical reagent grade. Carbon nanotubes were prepared by Cheap Tubes Inc. The outer diameter of CNT was between 20 and 40 nm. Monohydrate citric acid, AgNO3 and PdCl2 were purchased from Merck. Two stock solutions of metal ions which were obtained from their various salts (AgNO3 and PdCl2) were prepared in 100 ml volumetric flasks and the solutions were used for the preparation of the various mole ratio mixtures of MWCNT-g-PCA. Buffer solutions were prepared using (Na2HPO4, NaH2PO4) 0.2 M and the concentrated (HNO3, NH3) solutions for adjusting pH of reaction solutions of hybrid materials with PdCl2 and AgNO3 respectively which were purchased from Merck. All experiments were carried out at 25 °C, and all titrations were repeated at least three times.
2.3.2. Preparation of MWCNT-g-PCA nanocomposites containing silver nanoparticles Preparation of MWCNT-g-PCA containing Ag nanoparticles was reported recently [17], but we prepared this nanocomposite in optimum reaction conditions such as pH, sonication time and the stirring time of reaction between hybrid materials and Ag + ions. Dilute solution of MWCNT-g-PCA was mixed with aqueous solution of silver nitrate containing 1 M (AgNO3) at pH 6. After 20 min dispersing in an ultrasonic bath (22 kHz) the solution was stirred at room temperature for 160 min. Water was evaporated by vacuum and residue was dissolved in THF and precipitated in cyclohexane. With optimization reaction conditions, the time needed for maximum loading of Ag nanoparticles in polymeric shell decreased from 8 h [17] to 160 min.
2.2. Apparatus and software The pH values were measured by a model Jenway 3015 pH meter using a Metrohm combined glass electrode. A Pharmacia model LKB UV–visible Ultraspect (III) single beam spectrophotometer that connected to a Pentium II computer with 1-cm quartz cells was used for recording the absorbance measurements. An ultrasonic bath (30 kHz, manufactured in United Kingdom) was used to well disperse metal ions in the polymeric shell of hybrid material. The calculations were made with a Pentium 4 computer and the SQUAD Program for determination of stability constants was used. Also third dimension diagrams were drawn with MATLAB R2007b software.
2.3.3. Preparation of MWCNT-g-PCA nanocomposites containing palladium nanoparticles MWCNT-g-PCA-Pd nanocomposite was prepared recently [19] but we prepared it in optimum reaction conditions (pH, sonication time, stirring time). Water solutions of PdCl2 (0.26 g in 3 ml) and MWCNT-g-PCA (0.2 g in 10 ml) were mixed at pH 8 and placed in an ultrasonic bath (22 kHz) for 20 min to well disperse metal ions in the polymeric shell of hybrid material. Then it was stirred at room temperature for 100 min. Water was evaporated by vacuum oven and residue was dissolved in THF and precipitated in cyclohexane. Product was dried by vacuum oven at 60 °C for 2 h. With optimization reaction conditions, the time needed for maximum loading of Pd nanoparticles in polymeric shell decreased from 8 h [19] to 100 min.
2.3. Procedure 2.3.1. Preparation of MWCNT-g-PCA hybrid materials MWCNTs were opened according to reported procedures in the literatures [18]. MWCNT-g-PCA hybrid materials were prepared
0.33 0.31 0.29
Abs.
0.27 0.25 0.23 0.21 0.19 0.17 0.15 1
2
3
4
5
6
7
8
9
10
pH + Fig. 1. Effect of pH on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in CAg = 1.2 × 10−3 M, CL = 4.1 × 10−4 M.
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
1
0.9
Abs.
0.8
0.7
0.6
0.5
0.4
1
3
5
7
9
11
pH Fig. 2. Effect of pH on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in 2+ = 2.5 × 10−4 M, CL = 4.9 × 10−4 M. CPd
41
oxygen-containing functional groups (\COOH and \OH) play an important role in anchoring metal nanoparticles on the walls of CNTs. These surface functional groups provide active sites for interaction with metal ions. Citric acid is a cheap and available compound which has been used to produce, disperse and stabilize metal nanoparticles abundantly. Conjugation of citric acid on the surface of CNTs not only causes solubility of CNTs in aqueous and organic solvents but also anchoring metal nanoparticles on the walls of CNTs. In addition an aqueous solution of Ag+ or Pd 2+ to MWCNT-g-PCA hybrid material and sonication the mixture lead to the dispersion of these ions in hyper branched polycitric acid shell. Carboxylic groups of implanted polycitric acid shell act as donor agents and metal ions act as acceptors. As a result, metal nanoparticles were encapsulated on the surface of MWCNT-g-PCA. The absorption peak of ligand to metal charge transfer (LMCT) is shown in the UV spectra of complexation reaction of Ag and Pd nanoparticles with MWCNT-poly (citric acid) during the time [17,19]. Enhancement of the intensity of absorption bands during the time demonstrates a gradual increase in surface plasmon absorption. This behavior indicates the increase of encapsulated metal nanoparticles in the polymer host.
3. Results and discussion 3.2. Optimization of the reaction parameters 3.1. Preliminary investigation The novel functionalized MWCNT structures were found to have remarkable catalytic effects when used as support. It is found that the
The effects of parameters (such as pH, ultrasonic time and stirring time) on the reactions of Pd (II) and Ag (I) with MWCNT-graft-PCA hybrid materials were followed by UV–vis experiments. The effects of
0.35 0.34
Abs.
0.33 0.32 0.31 0.3 0.29 0.28 0
5
10
15
20
25
30
35
Sonication time (min) Fig. 3. Effect of sonication time on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in C
+ −3 Ag = 3 × 10
M, CL = 1 M.
1.1 1.05
Abs.
1 0.95 0.9 0.85 0.8 0.75 0
5
10
15
20
25
30
35
Sonication time (min) 2+ Fig. 4. Effect of sonication time on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in CPd = 3 × 10−3 M, CL = 0.03 M.
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N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
0.4
0.35
Abs.
0.3
0.25
0.2
0.15 50
100
150
200
250
Time (min) Fig. 5. Effect of stirring time on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in C
+ −3 Ag = 3 × 10
M, CL = 1 M.
1.05 0.95
Abs.
0.85 0.75 0.65 0.55 0.45 0.35 0
20
40
60
80
100
120
140
160
180
Time (min) 2+ Fig. 6. Effect of stirring time on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in CPd = 3 × 10−3 M, CL = 0.03 M.
pH on the reactions are shown in Figs. 1, 2. As can be seen, the pH of solutions plays an important role on the reactions. In Figs. 1, 2 the absorbance vs. pH has an increasing pattern until pH 6 and pH 8 for Ag (I) and Pd (II) respectively and then decreases with increasing pH which shows the decomposition of the metal complexes. It is due to the formation of the metal hydroxide species in high pH value. The sonication time effect on the (Ag+, Pd 2+) ion reactions with hybrid materials is
shown in Figs. 3, 4 respectively. As shown in Figs. 3, 4 the absorbance increase with increasing sonication time up to 20 min and then it remained constant. Therefore the time of 20 min was selected for further studies. Ultrasonication imparts a high energy density in the solution. Cavitation can break the aggregated nanotubes apart, which will improve the dispersion of nanotubes and improve the surface property of the nanocomposite. The effects of temperature on the reactions 2.5
3 2.5
2
Abs.
Abs.
2 1.5
1.5
1 1 0.5 0 500
450
400
350
300
250
200 2
3
5
4
6
7
8
pH
Wavelength (nm) Fig. 7. Absorbance spectra of the mixture of MWCNT-g-PCA with Ag+ in a mole ratio of CM/CL = 1 and different pH values.
0.5 500
450
400
350
300
Wavelength (nm)
250
200
4
5
6
7
8
9
pH
Fig. 8. Absorbance spectra of the mixture of MWCNT-g-PCA with Pd2+ in a mole ratio of CM/CL = 1 and different pH values.
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
7.00E-04
Table 1 Concentrations of Ag+ and Pd2+ at different CM/CL.
L
6.00E-04
LH2Ag
Species
5.00E-04
c (mol/L)
43
CM/CL 4
+
Ag Pd2+
4.00E-04
3 −3
1.320 × 10 5.915 × 10−4
2 −3
1.237 × 10 5.546 × 10−4
1 −3
1.100 × 10 4.929 × 10−4
0.5 −4
8.250 × 10 3.697 × 10−4
5.500 × 10−4 2.465 × 10−4
LH3
3.00E-04 2.00E-04
L(Ag)3
1.00E-04
Ag LH(Ag)2
0.00E+00 2
3
4
5
-1.00E-04
6
7
8
9
pH
Fig. 9. Distribution diagrams of the relative concentration of all complex species in equilibrium between MWCNT-g-PCA and Ag+ for CM/CL = 1.
were studied and it was found that its effect is negligible. Therefore all experiments were carried out in room temperature. The effects of stirring time on the reactions vs. absorbance were studied (Figs. 5, 6). As can be seen in Figs. 5, 6 the absorbance has increased with increasing stirring time up to 160 min and 100 min for hybrid materials containing Ag and Pd nanoparticles respectively and then decreased. In these times, the loading of metal nanoparticles on the polymeric shell of hybrid materials is at maximum and with increasing stirring time, the complexes of hybrid materials and metal ions decomposed and as a result the absorbance of nanocomposites decreased. 3.3. Absorption spectra The absorption spectra at 200–500 nm at five mole ratios of CM/CL = 0.5, 1, 2, 3 and 4 of the metal cations and MWCNT-g-PCA at the pH range 3–9 were recorded. A sample absorption spectrum of the pH titration of MWCNT-g-PCA with Ag+ and Pd 2+ is shown in Figs. 7, 8. 3.4. Determination of stability constants In the first step, the number of absorbing species and the stability constants of the resulting complexes were calculated from absorption data matrices at each metal to ligand ratio. This needs specification of the exact formula of the individual species involved. The overall stability constants of all resulting complexes between MWCNT-gPCA and metal ions in the studied pH range were calculated by the SQUAD program. The outputs of the SQUAD, besides the overall
5.00E-04
LH2Pd
stability constants, contain standard deviations of the estimated constants and distribution diagrams of all species. When calculating the equilibrium constants, care must be taken to consider only the feasible region for each constant. The best region for determining equilibrium constant agrees to the pH where the concentrations of the equilibrium species are nearly equal. This accrues near the intersections of the profiles shown in Figs. 9, 10. The distribution curves, results of SQUAD, showed that MWCNT-g-PCA formed different protonated complex species. Absorbance mole ratio data matrices by applying the procedure of efficient experimentation and computational strategy at CM/CL = 0.5, 1, 2, 3 and 4 were obtained. The concentrations of Ag + and Pd 2+ were presented at ratios 0.5, 1, 2, 3 and 4 in Table 1. The complexation reaction of the metal ions with MWCNT-g-PCA as a ligand was supported by visually detected spectral changes, which related to the gradual variation of the solution from a light yellow color of the primary solution to the deep yellow and increasing in the darkness of the solution. The chemical model concerns the number of light-absorbing species coexisting in the equilibrium mixture, their stoichiometries, and their stability constants which are estimated by regression analysis and, at the same time, the curves of molar absorption coefficients in their dependence on wavelength. For a set of current values of βpqr, the free concentrations of the metal [M], the ligand [L], and [H] (known from pH measurements) for each solution are calculated, and then the concentrations of all the complexes in the equilibrium mixture, [MpLqHr]j; j = 1, …, nc, forming for n solutions the matrix C is found. As well as the fit achieved, it is also necessary to examine the physicochemical sense of the model parameter estimates, such as positive values in the concentration matrix, positive values of molar absorptivities and the concentration fraction of the complex species in the mixture. If a complex species is present in a fraction lower than 5–10% (minor species), evaluating such an equilibrium can fail; i.e., from the spectral view, it acts as “noise” only, insufficient for an evaluation of its own equilibrium and complicating to evaluate other equilibriums. This problem can be solved by augmenting the set of spectra with a single spectrum of the molar absorptivities of the unknown species. Calculated stability constants of the resulting complexes by given programs are listed in Table 2.
L
4.50E-04 4.00E-04 LH3
c (mol/L)
3.50E-04
Table 2 Log βpqr and standard deviation (s(Log βpqr)) of the most probable chemical model in the MWCNT-g-PCA complexforming system with Pd2+ and Ag+ ions and estimated by SQUAD program in CM/CL =1 for(M=Ag+) CM/CL =0.5 for (M=Pd2+).
3.00E-04 2.50E-04 2.00E-04 Pd
LH(Pd)2
1.50E-04 1.00E-04
L(Pd)3
5.00E-05 0.00E+00
2
3
4
5
6
7
8
9
pH Fig. 10. Distribution diagrams of the relative concentration of all complex species in equilibrium between MWCNT-g-PCA and Pd2+ for CM/CL = 0.5.
Species
Log βpqr
LH3 LH2Pd LH(Pd)2 L(Pd)3 LH3 LH2Ag LH(Ag)2 L(Ag)3
14.7903 ± 0.8044 12.2715 ± 0.5061 6.86 ± 0.4458 10.5553 ± 0.8282 14.7903 ± 0.8044 17.9927 ± 0.2953 14.5943 ± 0.6267 11.9250 ± 0.8688
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N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
4. Conclusion The reaction conditions of Ag and Pd with MWCNT-graft-PCA were optimized and we could to obtain these nanocomposites in a short time in comparison to the previously reported research. The stability constants of the complexes of MWCNT-g-PCA with Ag + and Pd 2+ in various mole ratios (CM/CL = 0.5, 1, 2, 3 and 4) were estimated using spectrophotometric titration. Tentative methods which all are implemented in the computer program SQUAD are applied for determining the light-absorbing species. References [1] [2] [3] [4] [5]
K. Keren, R.S. Berman, E. Buchstab, U. Sivan, E. Braun, Science 302 (2003) 1380–1382. H. Yan, S.H. Park, G. Finkelstein, J.H. Reif, T.H. LaBean, Science 301 (2003) 1882–1884. M. Zheng, A. Jagota, M.S. Strano, et al., Science 302 (2003) 1545–1548. D.J. Pochan, Z. Chen, H. Cui, K. Hales, K. Qi, K.L. Wooley, Science 306 (2004) 94–97. H. Ago, K. Petritsch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Advanced Materials 11 (1999) 1281–1285. [6] P. He, M. Bayachou, Langmuir 21 (2005) 6086–6092.
[7] G. Lu, P. Maragakis, E. Kaxiras, Nano Letters 5 (2005) 897–900. [8] S. Li, P. He, J. Dong, Z. Guo, L. Dai, Journal of the American Chemical Society 127 (2005) 14–15. [9] S. Niyogi, M.A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen, M.E. Itkis, R.C. Haddon, Accounts of Chemical Research 35 (2002) 1105–1113. [10] Y. Lin, A.M. Rao, B. Sadanadan, E.A. Kenik, Y.-P. Sun, The Journal of Physical Chemistry. B 106 (2002) 1294–1298. [11] Y.P. Sun, W. Huang, Y. Lin, K. Fu, A. Kitaygorodskiy, L.A. Riddle, Y.J. Yu, D.L. Carroll, Chemistry of Materials 13 (2001) 2864–2869. [12] S. Banerjee, S.S. Wong, Nano Letters 2 (2002) 49–53. [13] F. Frehill, J.G. Vos, S. Benrezzak, A.A. Koόs, Z. Kόnya, M.G. Rüther, W.J. Blau, A. Fonseca, J.B. Nagy, L.P. Birό, A.I. Minett, M. in het Panhuis, Journal of the American Chemical Society 124 (2002) 13694–13695. [14] X. Lu, F. Tian, Y. Feng, X. Xu, N. Wang, Q. Zhang, Nano Letters 2 (2002) 1325–1327. [15] J.-M. Yeh, K.-Y. Huang, S.-Y. Lin, Y.-Y. Wu, C.-C. Huang, S.-J. Liou, Journal of Nanotechnology 2009 (2009) 1–7. [16] P.M. Ajayan, Chemical Reviews 99 (1999) 1787–1799. [17] M. Adeli, A. Bahari, H. Hekmatara, NANO: Brief Reports and Reviews 3 (2008) 37–44. [18] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159–162. [19] M. Adeli, E. Mehdipour, M. Bavadi, Journal of Applied Polymer Science 116 (2010) 2188–2196.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Quantitative study on the interaction of Ag + and Pd 2 + with CNT-graft-PCA (polycitric acid) in aqueous solution N. Sarlak a, b,⁎, M. Adeli a, M. Karimi a, M. Bordbare c, M.A. Farahmandnejad b a b c
Department of chemistry, Faculty of science, University of Lorestan, Khorram Abad, Iran Engineering Research Institute, Tehran, Iran Department of chemistry, Faculty of Science, The University of Qom, P. O. Box: 37185-359, Qom, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 12 November 2012 Accepted 27 December 2012 Available online 9 January 2013 Keywords: Quantitative study Silver nanoparticle Palladium nanoparticle Polycitric acid Carbon nanotube Complexation
a b s t r a c t The reactions between multi walled carbon nanotube graft polycitric acid (MWCNT-graft-PCA) and Ag+ and Pd2+ ions at 25 °C are investigated spectrophotometrically. Encapsulation of Ag (I) and Pd (II) on the surface of (MWCNT-graft-PCA) as a function of pH, sonication time and reaction time was studied. The results indicated that the absorbance increased until pH=6 and pH=8 for Ag and Pd respectively and then decreased. The effects of sonication time on the reactions were studied and 20 min was selected for both of reactions. Also the time of reaction between MWCNT-graft-PCA and (Ag, Pd) vs. absorbance was studied and the stirring time of 160 and 100 min was selected for these reactions respectively. Also knowing the number of light-absorbing species is a critical step for subsequent quantitative and qualitative solution equilibrium studies. Behind the number of various complexes formed the stability constants for the combination of MWCNT-graft-PCA (ligand L) with Ag+ and Pd2+ ions at 25 °C and variant pH, and various mole ratios are estimated by the SQUAD program. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, nanotechnology has become an important field in science and technology, allowing one to manipulate matter at the nanometer scale and to incorporate nanostructures and nano-processes into working technological innovations [1–5]. Considerable interest has been focused on the possibility of the construction of new assemblies from nanoparticles (NPs) and other components to yield superstructure nanomaterials with unique and useful optical, electronic, and magnetic properties [1–4]. Carbon nanotubes (CNTs) are potentially excellent one-dimensional nanoscale materials because of their excellent physical properties [6] and morphology that can be carefully functionalize [7,8]. The chemical modification and the covalent functionalization of carbon nanotubes with organic species like long chain alcohols and amines, dendrimers and polymers have been reported recently [9–11]. Also there are few reports about the functionalization of carbon nanotubes that open the area of metallo-organic chemistry to nanotubes such as the synthesis of nanotubes covalently complexed to molecular coordination compounds [12], interconnecting carbon nanotubes with an inorganic metal complex [13], sidewall oxidation and complexation of carbon nanotubes by base-catalyzed cycloaddition of transition metal oxide [14]. Recently a method has been developed for the attaching of ⁎ Corresponding author at: Department of chemistry, Faculty of science, University of Lorestan, Khorram Abad, Iran. E-mail address: [email protected] (N. Sarlak). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.12.034
self-assembled gold nanoparticles (Au-NPs) onto the surface of sidewalls and ends of thiol-terminated multi-walled carbon nanotubes (MWNTs) functionalized with orthomercaptoaniline which acts as a bridging agent [15]. Among them, one of the most intriguing applications of CNTs is the polymer/CNT nanocomposites. In the past decade, there has been an increasing interest in the studies of polymer/CNT nanocomposites due to the unique combination of promising properties and construction of multifunctional structures of each component [16]. Lately biodegradable nanocomposites containing multi walled carbon nanotubes (MWCNT) and polycitric acid (PCA) were successfully synthesized which hereby metal nanoparticles could be trapped through the conjugating of polymers to CNTs [17]. CNT grafted polymer (CNT-g-polymers) hybrid materials are good candidates to support metal nanoparticles such as platinum group metals (silver, palladium and so on). The unique properties of palladium, silver and other platinum group metals account for their widespread use. In this work, MWCNTs were opened and functionalized by acid (MWCNT-COOH) and citric acid was polymerized on their surface. Trapping of Ag+ and Pd 2+ ions by MWCNT-g-PCA hybrid materials was led to encapsulated silver and palladium nanoparticles onto the surface of MWCNTs via complexation reactions. The parameters for complexation reactions of (Ag +, Pd 2+) ions with MWCNT-g-PCA were optimized using UV–Visible spectrophotometry and then the complex-forming equilibriums and the stability constants of MWCNTg-PCA-Ag and MWCNT-g-PCA-Pd were calculated using SQUAD
40
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
according to the following procedure [17]. Functionalized MWCNTCOOH (0.05 g) was added to a polymerization ampoule equipped with magnetic stirrer and vacuum inlet then monohydrate citric acid (2.5 g) added to ampoule and it was sealed under vacuum. The mixture was heated up to 120 °C and stirred in this temperature for 30 min. After removing the water by vacuum inlet, reaction temperature was raised to 140 °C and stirred at this temperature for 1 h. Water as a byproduct of reaction was removed by vacuum inlet and reaction temperature was raised to 160 °C. Polymerization continued in this temperature under dynamic vacuum for 1.5 h. The mixture was cooled and dissolved in THF and product was precipitated in cyclohexane. Purified product was obtained as a viscous coffee-brown compound in %85 yields.
program. It is one of the most widespread programs and algorithms for determining the stability constant from absorbance data. Conceptually, the novelty of this derivatization is that nanotube can be considered as a primary ligand with respect to the metal atom. To the best of our knowledge, this is the first report of optimizing the parameters of reaction between MWCNT-g-PCA and platinum group metals (Ag and Pd) and it is the first report of applying SQUAD program for studying MWCNT complexation reactions with metal ions. 2. Experimental 2.1. Chemicals and solutions All the chemicals were of analytical reagent grade. Carbon nanotubes were prepared by Cheap Tubes Inc. The outer diameter of CNT was between 20 and 40 nm. Monohydrate citric acid, AgNO3 and PdCl2 were purchased from Merck. Two stock solutions of metal ions which were obtained from their various salts (AgNO3 and PdCl2) were prepared in 100 ml volumetric flasks and the solutions were used for the preparation of the various mole ratio mixtures of MWCNT-g-PCA. Buffer solutions were prepared using (Na2HPO4, NaH2PO4) 0.2 M and the concentrated (HNO3, NH3) solutions for adjusting pH of reaction solutions of hybrid materials with PdCl2 and AgNO3 respectively which were purchased from Merck. All experiments were carried out at 25 °C, and all titrations were repeated at least three times.
2.3.2. Preparation of MWCNT-g-PCA nanocomposites containing silver nanoparticles Preparation of MWCNT-g-PCA containing Ag nanoparticles was reported recently [17], but we prepared this nanocomposite in optimum reaction conditions such as pH, sonication time and the stirring time of reaction between hybrid materials and Ag + ions. Dilute solution of MWCNT-g-PCA was mixed with aqueous solution of silver nitrate containing 1 M (AgNO3) at pH 6. After 20 min dispersing in an ultrasonic bath (22 kHz) the solution was stirred at room temperature for 160 min. Water was evaporated by vacuum and residue was dissolved in THF and precipitated in cyclohexane. With optimization reaction conditions, the time needed for maximum loading of Ag nanoparticles in polymeric shell decreased from 8 h [17] to 160 min.
2.2. Apparatus and software The pH values were measured by a model Jenway 3015 pH meter using a Metrohm combined glass electrode. A Pharmacia model LKB UV–visible Ultraspect (III) single beam spectrophotometer that connected to a Pentium II computer with 1-cm quartz cells was used for recording the absorbance measurements. An ultrasonic bath (30 kHz, manufactured in United Kingdom) was used to well disperse metal ions in the polymeric shell of hybrid material. The calculations were made with a Pentium 4 computer and the SQUAD Program for determination of stability constants was used. Also third dimension diagrams were drawn with MATLAB R2007b software.
2.3.3. Preparation of MWCNT-g-PCA nanocomposites containing palladium nanoparticles MWCNT-g-PCA-Pd nanocomposite was prepared recently [19] but we prepared it in optimum reaction conditions (pH, sonication time, stirring time). Water solutions of PdCl2 (0.26 g in 3 ml) and MWCNT-g-PCA (0.2 g in 10 ml) were mixed at pH 8 and placed in an ultrasonic bath (22 kHz) for 20 min to well disperse metal ions in the polymeric shell of hybrid material. Then it was stirred at room temperature for 100 min. Water was evaporated by vacuum oven and residue was dissolved in THF and precipitated in cyclohexane. Product was dried by vacuum oven at 60 °C for 2 h. With optimization reaction conditions, the time needed for maximum loading of Pd nanoparticles in polymeric shell decreased from 8 h [19] to 100 min.
2.3. Procedure 2.3.1. Preparation of MWCNT-g-PCA hybrid materials MWCNTs were opened according to reported procedures in the literatures [18]. MWCNT-g-PCA hybrid materials were prepared
0.33 0.31 0.29
Abs.
0.27 0.25 0.23 0.21 0.19 0.17 0.15 1
2
3
4
5
6
7
8
9
10
pH + Fig. 1. Effect of pH on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in CAg = 1.2 × 10−3 M, CL = 4.1 × 10−4 M.
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
1
0.9
Abs.
0.8
0.7
0.6
0.5
0.4
1
3
5
7
9
11
pH Fig. 2. Effect of pH on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in 2+ = 2.5 × 10−4 M, CL = 4.9 × 10−4 M. CPd
41
oxygen-containing functional groups (\COOH and \OH) play an important role in anchoring metal nanoparticles on the walls of CNTs. These surface functional groups provide active sites for interaction with metal ions. Citric acid is a cheap and available compound which has been used to produce, disperse and stabilize metal nanoparticles abundantly. Conjugation of citric acid on the surface of CNTs not only causes solubility of CNTs in aqueous and organic solvents but also anchoring metal nanoparticles on the walls of CNTs. In addition an aqueous solution of Ag+ or Pd 2+ to MWCNT-g-PCA hybrid material and sonication the mixture lead to the dispersion of these ions in hyper branched polycitric acid shell. Carboxylic groups of implanted polycitric acid shell act as donor agents and metal ions act as acceptors. As a result, metal nanoparticles were encapsulated on the surface of MWCNT-g-PCA. The absorption peak of ligand to metal charge transfer (LMCT) is shown in the UV spectra of complexation reaction of Ag and Pd nanoparticles with MWCNT-poly (citric acid) during the time [17,19]. Enhancement of the intensity of absorption bands during the time demonstrates a gradual increase in surface plasmon absorption. This behavior indicates the increase of encapsulated metal nanoparticles in the polymer host.
3. Results and discussion 3.2. Optimization of the reaction parameters 3.1. Preliminary investigation The novel functionalized MWCNT structures were found to have remarkable catalytic effects when used as support. It is found that the
The effects of parameters (such as pH, ultrasonic time and stirring time) on the reactions of Pd (II) and Ag (I) with MWCNT-graft-PCA hybrid materials were followed by UV–vis experiments. The effects of
0.35 0.34
Abs.
0.33 0.32 0.31 0.3 0.29 0.28 0
5
10
15
20
25
30
35
Sonication time (min) Fig. 3. Effect of sonication time on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in C
+ −3 Ag = 3 × 10
M, CL = 1 M.
1.1 1.05
Abs.
1 0.95 0.9 0.85 0.8 0.75 0
5
10
15
20
25
30
35
Sonication time (min) 2+ Fig. 4. Effect of sonication time on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in CPd = 3 × 10−3 M, CL = 0.03 M.
42
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
0.4
0.35
Abs.
0.3
0.25
0.2
0.15 50
100
150
200
250
Time (min) Fig. 5. Effect of stirring time on the interaction of MWCNT-g-PCA with Ag+ at λ = 350 nm in C
+ −3 Ag = 3 × 10
M, CL = 1 M.
1.05 0.95
Abs.
0.85 0.75 0.65 0.55 0.45 0.35 0
20
40
60
80
100
120
140
160
180
Time (min) 2+ Fig. 6. Effect of stirring time on the interaction of MWCNT-g-PCA with Pd2+ at λ = 360 nm in CPd = 3 × 10−3 M, CL = 0.03 M.
pH on the reactions are shown in Figs. 1, 2. As can be seen, the pH of solutions plays an important role on the reactions. In Figs. 1, 2 the absorbance vs. pH has an increasing pattern until pH 6 and pH 8 for Ag (I) and Pd (II) respectively and then decreases with increasing pH which shows the decomposition of the metal complexes. It is due to the formation of the metal hydroxide species in high pH value. The sonication time effect on the (Ag+, Pd 2+) ion reactions with hybrid materials is
shown in Figs. 3, 4 respectively. As shown in Figs. 3, 4 the absorbance increase with increasing sonication time up to 20 min and then it remained constant. Therefore the time of 20 min was selected for further studies. Ultrasonication imparts a high energy density in the solution. Cavitation can break the aggregated nanotubes apart, which will improve the dispersion of nanotubes and improve the surface property of the nanocomposite. The effects of temperature on the reactions 2.5
3 2.5
2
Abs.
Abs.
2 1.5
1.5
1 1 0.5 0 500
450
400
350
300
250
200 2
3
5
4
6
7
8
pH
Wavelength (nm) Fig. 7. Absorbance spectra of the mixture of MWCNT-g-PCA with Ag+ in a mole ratio of CM/CL = 1 and different pH values.
0.5 500
450
400
350
300
Wavelength (nm)
250
200
4
5
6
7
8
9
pH
Fig. 8. Absorbance spectra of the mixture of MWCNT-g-PCA with Pd2+ in a mole ratio of CM/CL = 1 and different pH values.
N. Sarlak et al. / Journal of Molecular Liquids 180 (2013) 39–44
7.00E-04
Table 1 Concentrations of Ag+ and Pd2+ at different CM/CL.
L
6.00E-04
LH2Ag
Species
5.00E-04
c (mol/L)
43
CM/CL 4
+
Ag Pd2+
4.00E-04
3 −3
1.320 × 10 5.915 × 10−4
2 −3
1.237 × 10 5.546 × 10−4
1 −3
1.100 × 10 4.929 × 10−4
0.5 −4
8.250 × 10 3.697 × 10−4
5.500 × 10−4 2.465 × 10−4
LH3
3.00E-04 2.00E-04
L(Ag)3
1.00E-04
Ag LH(Ag)2
0.00E+00 2
3
4
5
-1.00E-04
6
7
8
9
pH
Fig. 9. Distribution diagrams of the relative concentration of all complex species in equilibrium between MWCNT-g-PCA and Ag+ for CM/CL = 1.
were studied and it was found that its effect is negligible. Therefore all experiments were carried out in room temperature. The effects of stirring time on the reactions vs. absorbance were studied (Figs. 5, 6). As can be seen in Figs. 5, 6 the absorbance has increased with increasing stirring time up to 160 min and 100 min for hybrid materials containing Ag and Pd nanoparticles respectively and then decreased. In these times, the loading of metal nanoparticles on the polymeric shell of hybrid materials is at maximum and with increasing stirring time, the complexes of hybrid materials and metal ions decomposed and as a result the absorbance of nanocomposites decreased. 3.3. Absorption spectra The absorption spectra at 200–500 nm at five mole ratios of CM/CL = 0.5, 1, 2, 3 and 4 of the metal cations and MWCNT-g-PCA at the pH range 3–9 were recorded. A sample absorption spectrum of the pH titration of MWCNT-g-PCA with Ag+ and Pd 2+ is shown in Figs. 7, 8. 3.4. Determination of stability constants In the first step, the number of absorbing species and the stability constants of the resulting complexes were calculated from absorption data matrices at each metal to ligand ratio. This needs specification of the exact formula of the individual species involved. The overall stability constants of all resulting complexes between MWCNT-gPCA and metal ions in the studied pH range were calculated by the SQUAD program. The outputs of the SQUAD, besides the overall
5.00E-04
LH2Pd
stability constants, contain standard deviations of the estimated constants and distribution diagrams of all species. When calculating the equilibrium constants, care must be taken to consider only the feasible region for each constant. The best region for determining equilibrium constant agrees to the pH where the concentrations of the equilibrium species are nearly equal. This accrues near the intersections of the profiles shown in Figs. 9, 10. The distribution curves, results of SQUAD, showed that MWCNT-g-PCA formed different protonated complex species. Absorbance mole ratio data matrices by applying the procedure of efficient experimentation and computational strategy at CM/CL = 0.5, 1, 2, 3 and 4 were obtained. The concentrations of Ag + and Pd 2+ were presented at ratios 0.5, 1, 2, 3 and 4 in Table 1. The complexation reaction of the metal ions with MWCNT-g-PCA as a ligand was supported by visually detected spectral changes, which related to the gradual variation of the solution from a light yellow color of the primary solution to the deep yellow and increasing in the darkness of the solution. The chemical model concerns the number of light-absorbing species coexisting in the equilibrium mixture, their stoichiometries, and their stability constants which are estimated by regression analysis and, at the same time, the curves of molar absorption coefficients in their dependence on wavelength. For a set of current values of βpqr, the free concentrations of the metal [M], the ligand [L], and [H] (known from pH measurements) for each solution are calculated, and then the concentrations of all the complexes in the equilibrium mixture, [MpLqHr]j; j = 1, …, nc, forming for n solutions the matrix C is found. As well as the fit achieved, it is also necessary to examine the physicochemical sense of the model parameter estimates, such as positive values in the concentration matrix, positive values of molar absorptivities and the concentration fraction of the complex species in the mixture. If a complex species is present in a fraction lower than 5–10% (minor species), evaluating such an equilibrium can fail; i.e., from the spectral view, it acts as “noise” only, insufficient for an evaluation of its own equilibrium and complicating to evaluate other equilibriums. This problem can be solved by augmenting the set of spectra with a single spectrum of the molar absorptivities of the unknown species. Calculated stability constants of the resulting complexes by given programs are listed in Table 2.
L
4.50E-04 4.00E-04 LH3
c (mol/L)
3.50E-04
Table 2 Log βpqr and standard deviation (s(Log βpqr)) of the most probable chemical model in the MWCNT-g-PCA complexforming system with Pd2+ and Ag+ ions and estimated by SQUAD program in CM/CL =1 for(M=Ag+) CM/CL =0.5 for (M=Pd2+).
3.00E-04 2.50E-04 2.00E-04 Pd
LH(Pd)2
1.50E-04 1.00E-04
L(Pd)3
5.00E-05 0.00E+00
2
3
4
5
6
7
8
9
pH Fig. 10. Distribution diagrams of the relative concentration of all complex species in equilibrium between MWCNT-g-PCA and Pd2+ for CM/CL = 0.5.
Species
Log βpqr
LH3 LH2Pd LH(Pd)2 L(Pd)3 LH3 LH2Ag LH(Ag)2 L(Ag)3
14.7903 ± 0.8044 12.2715 ± 0.5061 6.86 ± 0.4458 10.5553 ± 0.8282 14.7903 ± 0.8044 17.9927 ± 0.2953 14.5943 ± 0.6267 11.9250 ± 0.8688
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4. Conclusion The reaction conditions of Ag and Pd with MWCNT-graft-PCA were optimized and we could to obtain these nanocomposites in a short time in comparison to the previously reported research. The stability constants of the complexes of MWCNT-g-PCA with Ag + and Pd 2+ in various mole ratios (CM/CL = 0.5, 1, 2, 3 and 4) were estimated using spectrophotometric titration. Tentative methods which all are implemented in the computer program SQUAD are applied for determining the light-absorbing species. References [1] [2] [3] [4] [5]
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