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Modified dispersion of functionalized multi-walled carbon nanotubes in acetonitrile
Chemical Physics Letters 492 (2010) 258–262
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Modified dispersion of functionalized multi-walled carbon nanotubes in acetonitrile Heng Li a,b, Jia Cai Nie a, Sándor Kunsági-Máté b,* a b
Department of Physics, Beijing Normal University, Beijing 100875, People’s Republic of China Department of General and Physical Chemistry, University of Pécs, H-7624 Pécs, Ifjúság 6, Hungary
a r t i c l e
i n f o
Article history: Received 25 March 2010 In final form 20 April 2010 Available online 22 April 2010
a b s t r a c t The dispersion of hydroxylated multi-walled carbon nanotubes was modified in non-protic acetonitrile solvent using a treatment by ethanol. The dispersion was examined by photoluminescence and Rayleigh-scattering methods. In spite of well known very low solubility of nanotubes, present results showed presence of nanotube dimers in the solution with considerable concentration. Applying a qualitative model, DH = 46.6 ± 12 kJ/mol and DS = 29.9 ± 7 J/K mol enthalpy and entropy changes were obtained during formation of nanotube dimers. This highly negative entropy term is of great importance for the deposition of carbon nanotubes by liquid phase epitaxy to enlarge the surface coverage. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a sort of fascinating nanomaterials which has attracted wide interest in many areas of science and technology since 1991 [1]. As a result of substantial van der Waals tube–tube attractions, the dispersion of CNTs is insufficient in most common solvents. This unfavorable property inhibits their investigations in solution chemistry and also implies many limitations for their practical applications. Therefore continuous efforts have been made to improve their solubility [2–4]. The complexity of the solubilization of nanotubes is highlighted by the case of aniline solvent. Aniline was reported first as a very fruitful solvent [5], later this idea was revised and neglectable dissolution of nanotubes in aniline was reported [6]. In our recent works [7–9] we have demonstrated that solubilization in aniline can be performed but only with about three orders lower concentrations than it reported first time [5]. According to the solubilization, it is well known that functionalizing with suitable groups on the sidewalls of CNTs reduces the van der Waals interactions between the tubes, enabling separation of nanotube bundles into individual, separated tubes [10]. On that way CNTs can become soluble in aqueous [11] or organic solvents [12]. Furthermore, functionalization of nanotubes with –OH or –COOH groups improves the solubility in solvents with higher permittivity, such as the primary alcohols, e.g. in ethanol (er(T = 298 K) = 26). All of the efforts mentioned above are based on the reduction of the nanotube–nanotube interaction by increasing the nanotube–solvent interactions. As a result, nanotube monomers with low concentrations will be presented in the solutions.
* Corresponding author. E-mail addresses: [email protected] (J.C. Nie), [email protected] (S. Kunsági-Máté). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.053
In our recent work [13], we showed that the high reduction of the entropy during adsorption of carbon nanotubes onto the CeO2 surface inhibits itself the high coverage of the substrate. Applying this idea to the case of liquid phase epitaxy (LPE), considerable adsorption of carbon nanotube monomers onto the surface cannot be predicted. However, due to their higher rigidity and therefore considerably reduced entropy content, adsorption of nanotube dimers onto any kind of surface could be enhanced just only because of the lower reduction of the entropy during the adsorption process. Unfortunately, the low solubility of nanotubes does not support formation of nanotube dimers. Accordingly, in this work a method for the modified dispersion of functionalized CNTs during its solubilization will be discussed. Non-protic acetonitrile was chosen to avoid the possible effect of the self-organization of the solvent on the solubilization process. Primary alcohols were applied as co-solvents to modify the dispersion of the nanotubes. The results showed presence of nanotube dimers in the solution with considerable concentration, which is of great interest for the deposition of CNTs onto some specific surfaces by using LPE. 2. Experimental methods Hydroxylated multi-walled CNTs (MWCNT-OH, purity > 95%) with 8–10 nm diameter and 5–15 lm length were purchased from Guangzhou Heji Trade Co. (China) and were then used for sample preparation, cited later as ‘untreated CNTs’. ‘treated CNTs’ was then prepared as follows: powder of ‘untreated CNTs’ was dissolved in EtOH at room-temperature using ultrasonic stirrer for 15 min. After 12 h equilibration, the clear fraction of the CNTs–EtOH saturated solution was then transported into evaporating vessels. The so-called ‘treated CNTs’ were obtained after evaporating the EtOH. Both the pristine and treated nanotubes were dissolved in acetonitrile as follows: after 5 min ultrasonic shaking the saturated
H. Li et al. / Chemical Physics Letters 492 (2010) 258–262
solution was left staying for 48 h. During this time the equilibrium between the precipitated and diluted fraction of the solutions were fully separated. Clear fractions of these solutions were then used for the investigations. All the manipulations described above were carried out at room temperature (T = 288 K), except for the measurements performed to determine the temperature-dependence of the dispersion. The effect of bounding was analysed by photoluminescence (PL) spectra, while the particle size was investigated by Rayleigh-scattering methods. Both experiments were carried out by Fluorolog s3 spectrofluorometer (Jobin-Yvon/SPEX). Photon counting method with 0.1 s integration time was used, the excitation and emission bandwidths were set to 1 nm. A 2 mm thickness of the fluorescent probes with front face detection was applied to eliminate the inner filter effect. Same instrument was applied to detect the Rayleigh-scattering. Same bandwidths and layer thickness but with right angle detection rearrangement was applied for these investigations. 3. Results and discussion Fig. 1 shows the PL spectra of the ‘untreated’ and ‘treated’ CNTs taken from the clear fraction of their supersaturated solution in acetonitrile. In comparison with the untreated CNTs, considerable red shift of the emission peak of the solutions of treated CNTs can be obtained using 337 nm of excitation. This large shift can be described by the known effect of interaction of carbon rings of aromatic molecules [14,15]. Similar shifts of the emission spectra were also observed during interaction of the aniline aromatic rings with the single-walled carbon nanotube in our previous studies [7– 9]. Furthermore, the characteristic Raman peak of the acetonitrile solvent was observed at about 375 and 373 nm using 337 nm and 335 nm excitations, respectively. This Raman shift of 3010 cm1 reflects the C–H vibration of the solvent molecules. Fig. 1 also shows extremely enhanced Rayleigh-scattering in the solutions of the treated CNTs, which properties will be discussed in detail later. It is the basic property of the fluorescence of any material if there is only a single conformation in the solutions, the site of the emission peak should be independent from the excitation wavelengths due to the Kasha’s rule [16–18] in condensed phase. Any dependence of the emission peak on the excitation wavelength reflects presence of at least two different species in the solutions which has different emission properties. In the following, we
Fig. 1. PL spectra of EtOH treated and untreated MWCNT-OH in acetonitrile under the excitation at about 335 nm (T = 288 K).
259
will discuss the emission properties of the solutions of the ‘untreated’ and ‘treated’ CNTs using the above viewpoints. The site of the fluorescence peak of the ‘untreated’ CNTs solution is independent from the excitation wavelength, highlighting the presence of only one class of CNTs (probably monomers) in the solution. In this particular case, presence of nanotubes with only monomeric form is probably due to the very low solubility. In contrast, the site of the fluorescence peak of the treated CNTs solution depends on the excitation wavelength. Fig. 2a shows the typical emission spectra of ‘treated’ CNTs solution under different excitation wavelength while Fig. 2b shows the intensity of the maxima of the fluorescence peaks of the ‘treated’ CNTs solution plotted against the excitation wavelength. This curve was obtained by considering and subtracting the enhanced background (see Fig. 1) originated from the unexpectedly enhanced Rayleigh-scattering of the solutions of treated nanotubes in acetonitrile. According to previous results observed, e.g. on a family of ionic liquids [19–21], we can assume the presence of different classes of bounded nanotubes solved in the acetonitrile solutions where the shift of the PL peak proportional to the interface of the interacted nanotubes. Note here, the shape of the fluorescence spectra associated to the different excitation wavelengths is almost same. This property means that the peak intensity represents (i.e. is proportional to) the amount of materials excited at the given wavelength. It can be seen clearly in Fig. 2 that there is a maxima on the curve at about 370 nm of excitation, highlighting presence of
Fig. 2. (a) Typical emission spectra of EtOH treated MWCNT-OH in acetonitrile under different excitation wavelength, the black arrows show the emission maxima and the appropriate excitation wavelength also indicated (T = 298 K). (b) Excitation dependence of the emission intensity of EtOH treated MWCNT-OH in acetonitrile (T = 298 K).
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Fig. 3. Excitation dependence of the emission maximum of EtOH treated MWCNTOH in acetonitrile (T = 298 K).
Table 1 Temperature-dependence of the cross of lines represents the different fractions in the treated nanotube solutions. Temperature (K)
Site of the cross I (cm1)
Site of the cross II (cm1)
288 293 298
21 699 22 207 22 510
24 915 25 225 26 922
a specific structure in the solutions with highest probability. To get information about the chemical equilibrium associated to the formation of bounded nanotubes, we need to determine first the number of different classes of bounded nanotubes existed in the solutions with considerable concentration. Fig. 3 shows the site of the PL peaks of the treated CNTs solution plotted against the excitation wavenumber. These data was obtained at 298 K. Considering that the shift observed on the emission peak reflects the gradual change of conformation between different classes of CNTs (i.e., the gradation of the interface of the bounded CNTs), it is obvious to assume that the different slopes in Fig. 3 represent the different types of transitions when the emission changes from one class of the bounding to another. The linear fitting was done by the least square method onto the measured points associated to the same slope, and the crosses of the fitted lines were obtained (see Fig. 3 and Table 1). Therefore, the crosses are associated to the different classes of the bundled CNTs, i.e., the nanotube dimer and trimer (see Fig. 3). Result shows presence of three different bounding procedures in the solution at a given temperature. The excitation wavenumber of the dimer cross is about 26 922 cm1, corresponding to a wavelength of 371 nm which suggests that the specific structure in the solutions with highest probability is just the nanotube dimer (see also Fig. 2b). These measurements were repeated also at 293 K and 288 K. The crosses determined above shifted towards higher wavenumbers with increase of temperature (see Table 1), highlighting the decrease of the interface of the bounded CNTs. Note here, the monomer emission which does not change with the excitation wavenumber located at 22 472 cm1 (see also Fig. 1, the red1 peak at 445 nm). The temperature-dependent distribution of different CNTs classes suggests weak interactions which stabilize the bounded structures. Therefore the chemical equilibrium shifts to-
1 For interpretation of color in Figs. 1–4, the reader is referred to the web version of this article.
Fig. 4. (a) Excitation dependence of the Rayleigh-scattering intensity of EtOH treated MWCNT-OH in acetonitrile compared to MWCNT-OH in EtOH and untreated MWCNT-OH in acetonitrile ((T = 288 K). (b) Fitting results of temperature dependence of EtOH treated MWCNT-OH in acetonitrile under ‘small particle approximation’. Inset: fitting result of thermodynamic parameters.
wards the smaller particles at higher temperature. At last, due to the change observed in the PL properties of the treated CNTs solutions, p–p interactions of aromatic rings of nanotubes could be identified as forces to stabilize the larger nanotube structures [22,23]. To obtain information about the size of the structures formed in the acetonitrile solutions, Rayleigh-scattering measurements was performed. Fig. 4a shows the Rayleigh-scattering spectra observed on the treated and untreated nanotubes in acetonitrile solvent and on the nanotubes dissolved in EtOH. Result clearly shows unexpectedly high Rayleigh-scattering in the case of treated nanotubes. As the Rayleigh-scattering is even suppressed for the nanotube solutions dissolved in EtOH, one can hardly ascribe this significant enhancement to the increase of the solubility of the nanotubes treated by EtOH. Therefore, it is reasonable to assume that the solubility of CNTs does not change considerably under the treatment. In agreement with this assumption the saturated solutions of our functionalized nanotubes have 13 mg/dm3 concentration at 288 K independently from the treatment. Considering 1 lm as the average length of the nanotube, about 6 109 M concentration of the nanotube can be estimated in the solutions. Therefore, same mass density of the dissolved nanotubes can be considered in both
H. Li et al. / Chemical Physics Letters 492 (2010) 258–262
solutions. Then, the enhancement of the scattering can be discussed under the Rayleigh-equation using the idea that the size of some particles is increased by association of the nanotubes. When two individual nanotubes are associated, the effective diameter of the particles is increased. Mention however, that it is not exactly correct that the diameter of a dimer is two times larger than a monomer. In fact, the scattering section of dimer will show an angle dependence. As a result, supposing that the diameter of a monomer is d, the effective diameter of dimer which is responsible for the Rayleigh-scattering is not 2d but
Z
!1=6
p 2
6
ð2d cos hÞ dh
1:7763d:
ð1Þ
0
So the actual scattering of a dimer should be 1:77636 ¼ 31:416 times higher than before. Therefore, if all the nanotubes would associate into the dimer forms, the concentration of the particles would reduce to the half, while the scattering of one particle (dimer) would increase to (1.7763)6 = 31.416. Totally, 15.71 times higher intensity could be detected after association of the nanotubes. However, experiments validate only about 10 times higher intensity in case of treated nanotubes, which highlights presence of nanotube in the solutions both in monomer and dimer forms. To examine this property quantitatively, we assume negligible amount of nanotube trimers in the solution (it is reasonable also from Fig. 2), therefore the equilibrium of the dimer formation will be examined within this model. Let us use c for the total concentration of the individual nanotubes in the solution of the treated nanotubes and c1 for the concentration of the monomers, then the dimer concentration c2 can be expressed as follows: c2 = (c c1)/2, since c = c1 + 2c2. The intensity of Rayleigh-scattering in the solution of the treated nanotubes can be written as follows [24,25]:
14 c c 1 6 I ¼ I0 k c1 d þ ; ð2 dÞ6 k 2
ð2Þ
where k is a constant for a given material. Now we can use the above equation to fit the data plotted in Fig. 4b. Then, the ratio of the concentrations of the monomer and dimer (bounded) nanotubes can be calculated, therefore the stability constant (Ks) can be determined as follows:
c2 Ks ¼ 2 : c1
ð3Þ
Using the Rayleigh-scattering data measured at different temperatures (Fig. 4b, T = 288 K, 293 K and 298 K) the thermodynamic parameters of the nanotube association can be determined using the van’t Hoff equation (see similar applications in our previous works [26–28]):
ln K s ¼
DG DH DS ¼ þ ; RT RT R
ð4Þ
where DG, DH and DS are the free enthalpy, enthalpy and entropy change of the association reaction, respectively. R is the gas constant, T reflects the temperature. Using the stability constants from Eq. (3), we are able to calculate the thermodynamic parameters by plotting the ln Ks values against the reciprocal temperatures (see the inset in Fig. 4b). Results show an enthalpy change of 46.6 kJ/ mol and an entropy change of 29.9 J/K mol of the association of the nanotubes, respectively. These values validate 37.7 kJ/mol free enthalpy change which shows the considerable association of the nanotubes around room temperature. These values have relatively high uncertainty, ±12 kJ/mol and ±7 J/K mol in the enthalpy and entropy terms, respectively. This property comes either from the simplification of the model by neglecting the higher ordered association or the uncertainty of the measured Rayleigh spectra.
261
The very enhanced association results in 87% individual CNTs bounded into the dimers at room temperature. This high association of the nanotubes is probably due to the etching effect of ethanol on the nanotube wall, since the interaction of ultra-thin wires which formed during the etching can also participate in the association [29]. Furthermore, the negative entropy value reflects more ordered structure of the nanotubes after the association and the formation of rigid nanotube dimers. This property has very important consequences in many applications. For example, according to our recent theoretical work [13], examinations of depositing CNTs onto a structured CeO2 surface highlighted determinant role of the entropy during such interaction. However, if a rigid structure of nanotube dimer reaches the surface, the reduction of the freedom, therefore, reduction of the entropy becomes much lower compared to when the flexible individual nanotube adsorbs to the surface. This process itself can enhance the coverage of any surfaces during epitaxial growth of nanotube layers from solution phase. 4. Summary and conclusion Dissolution of MWCNT-OH into the non-protic acetonitrile solvent was performed and investigated by PL and Rayleigh-scattering method. Either the MWCNT-OH powder was used as it is or previously treated by EtOH. These two types of powders were diluted by acetonitrile and the solvation was followed by photoluminescence and Rayleigh-scattering methods. Results show presence of three fractions of the nanotubes treated by EtOH and highlight presence of nanotube dimers with high concentration at 298 K. Using a qualitative model, DH = 46.6 kJ/mol and DS = 29.9 J/ K mol was determined as thermodynamic parameters of the molecular association. These findings offer not only a very simple way to get markedly modified dispersion of MWCNT-OH, but it is a controllable method to obtain different species of bundled CNTs by changing the temperature. It has a very important practical consequence especially when someone would use LPE method to deposit CNTs onto some specific surfaces, i.e., the highly negative entropy term is of great importance for the LPE deposition of CNTs to enlarge the surface coverage. Acknowledgments This work was supported by the Chinese–Hungarian Intergovernmental S&T Cooperation Programme (Project No: CH-4-32/ 2008:CN-54/2007). This work was also supported partly by the National Natural Science Foundation of China (Grant Nos. 50772015 and 10974019) and the Specialized Research Fund for the Doctoral Program of High Education of China (SRFDP-200800270004). References [1] S. Iijima, Nature 354 (1991) 56. [2] M. Alvaro, P. Atienzar, P. De la Cruz, J.L. Delgado, H. Garcia, F. Langa, J. Phys. Chem. B 108 (2004) 12691. [3] G. Zhang, P. Qi, X. Wang, Y. Lu, D. Mann, X. Li, H. Dai, J. Am. Chem. Soc. 128 (2006) 6026. [4] T. Palacin et al., J. Am. Chem. Soc. 131 (2009) 15394. [5] Y. Sun, S.R. Wilson, D.I. Schuster, J. Am. Chem. Soc. 123 (2001) 5348. [6] D.F. Perepichka, F. Wudl, S.R. Wilson, Y. Sun, D.I. Schuster, J. Mater. Chem. 14 (2004) 2749. [7] B. Peles-Lemli, P. Ács, L. Kollár, S. Kunsági-Máté, Fuller. Nanotubes Carbon Nanostruct. 16 (4) (2008) 247. [8] B. Peles-Lemli, L. Kollár, S. Kunsági-Máté, Fuller. Nanotubes Carbon Nanostruct., in press, doi:10.1080/15363831003782916. [9] B. Peles-Lemli, G. Matisz, A.M. Kelterer, W.M.F. Fabian, S. Kunsági-Máté, J. Phys. Chem. C 114 (2010) 5898. [10] K.A. Fernando, Y. Lin, Y.P. Sun, Langmuir 20 (2004) 4777. [11] J. Chen et al., J. Phys. Chem. B 105 (2001) 2525. [12] M.B. Kannan Balasubramanian, Small 1 (2005) 180. [13] S. Kunsági-Máté, J.C. Nie, Surf. Sci. 604 (2010) 654.
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[14] C. Rajesh, C. Majumder, H. Mizuseki, Y. Kawazoe, J. Chem. Phys. 130 (2009) 124911. [15] M.Z. Hossain, Appl. Phys. Lett. 96 (2010) 053118. [16] M. Kasha, Photochem. Photobiol. 7 (1968) 561. [17] M. Kasha, J. Chem. Phys. 20 (1952) 71. [18] M. Kasha, Int. J. Quantum Chem. 45 (1993) 689. [19] A. Adhikari, D.K. Das, D.K. Sasmal, K. Bhattacharyya, J. Phys. Chem. A 113 (2009) 3737. [20] A. Adhikari, S. Dey, D.K. Das, U. Mandal, S. Ghosh, K. Bhattacharyya, J. Phys. Chem. B 112 (2008) 6350. [21] A. Adhikari, K. Sahu, S. Dey, S. Ghosh, U. Mandal, K. Bhattacharyya, J. Phys. Chem. B 111 (2007) 12809.
[22] Y. Iizumi et al., Chem. Commun. 46 (2010) 1293. [23] S. Gorantla et al., Nano Res. 3 (2010) 92. [24] C.F. Bohren, D. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley, New York, 1983. [25] S. Chakraborti, Am. J. Phys. 75 (2007) 824. [26] S. Kunsági-Máté, A. Kumar, P. Sharma, L. Kollar, M.P. Nikfardjam, J. Phys. Chem. B 113 (2009) 7468. [27] S. Kunsági-Máté, K. Szabo, B. Desbat, J.L. Bruneel, I. Bitter, L. Kollar, J. Phys. Chem. B 111 (2007) 7218. [28] S. Kunsági-Máté, Zs. Csók, A. Tuzi, L. Kollár, J. Phys. Chem. B 112 (2008) 11743. [29] G. Yu, J. Gong, S. Wang, D. Zhu, S. He, Z. Zhu, Carbon 44 (2006) 1218.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Modified dispersion of functionalized multi-walled carbon nanotubes in acetonitrile Heng Li a,b, Jia Cai Nie a, Sándor Kunsági-Máté b,* a b
Department of Physics, Beijing Normal University, Beijing 100875, People’s Republic of China Department of General and Physical Chemistry, University of Pécs, H-7624 Pécs, Ifjúság 6, Hungary
a r t i c l e
i n f o
Article history: Received 25 March 2010 In final form 20 April 2010 Available online 22 April 2010
a b s t r a c t The dispersion of hydroxylated multi-walled carbon nanotubes was modified in non-protic acetonitrile solvent using a treatment by ethanol. The dispersion was examined by photoluminescence and Rayleigh-scattering methods. In spite of well known very low solubility of nanotubes, present results showed presence of nanotube dimers in the solution with considerable concentration. Applying a qualitative model, DH = 46.6 ± 12 kJ/mol and DS = 29.9 ± 7 J/K mol enthalpy and entropy changes were obtained during formation of nanotube dimers. This highly negative entropy term is of great importance for the deposition of carbon nanotubes by liquid phase epitaxy to enlarge the surface coverage. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a sort of fascinating nanomaterials which has attracted wide interest in many areas of science and technology since 1991 [1]. As a result of substantial van der Waals tube–tube attractions, the dispersion of CNTs is insufficient in most common solvents. This unfavorable property inhibits their investigations in solution chemistry and also implies many limitations for their practical applications. Therefore continuous efforts have been made to improve their solubility [2–4]. The complexity of the solubilization of nanotubes is highlighted by the case of aniline solvent. Aniline was reported first as a very fruitful solvent [5], later this idea was revised and neglectable dissolution of nanotubes in aniline was reported [6]. In our recent works [7–9] we have demonstrated that solubilization in aniline can be performed but only with about three orders lower concentrations than it reported first time [5]. According to the solubilization, it is well known that functionalizing with suitable groups on the sidewalls of CNTs reduces the van der Waals interactions between the tubes, enabling separation of nanotube bundles into individual, separated tubes [10]. On that way CNTs can become soluble in aqueous [11] or organic solvents [12]. Furthermore, functionalization of nanotubes with –OH or –COOH groups improves the solubility in solvents with higher permittivity, such as the primary alcohols, e.g. in ethanol (er(T = 298 K) = 26). All of the efforts mentioned above are based on the reduction of the nanotube–nanotube interaction by increasing the nanotube–solvent interactions. As a result, nanotube monomers with low concentrations will be presented in the solutions.
* Corresponding author. E-mail addresses: [email protected] (J.C. Nie), [email protected] (S. Kunsági-Máté). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.053
In our recent work [13], we showed that the high reduction of the entropy during adsorption of carbon nanotubes onto the CeO2 surface inhibits itself the high coverage of the substrate. Applying this idea to the case of liquid phase epitaxy (LPE), considerable adsorption of carbon nanotube monomers onto the surface cannot be predicted. However, due to their higher rigidity and therefore considerably reduced entropy content, adsorption of nanotube dimers onto any kind of surface could be enhanced just only because of the lower reduction of the entropy during the adsorption process. Unfortunately, the low solubility of nanotubes does not support formation of nanotube dimers. Accordingly, in this work a method for the modified dispersion of functionalized CNTs during its solubilization will be discussed. Non-protic acetonitrile was chosen to avoid the possible effect of the self-organization of the solvent on the solubilization process. Primary alcohols were applied as co-solvents to modify the dispersion of the nanotubes. The results showed presence of nanotube dimers in the solution with considerable concentration, which is of great interest for the deposition of CNTs onto some specific surfaces by using LPE. 2. Experimental methods Hydroxylated multi-walled CNTs (MWCNT-OH, purity > 95%) with 8–10 nm diameter and 5–15 lm length were purchased from Guangzhou Heji Trade Co. (China) and were then used for sample preparation, cited later as ‘untreated CNTs’. ‘treated CNTs’ was then prepared as follows: powder of ‘untreated CNTs’ was dissolved in EtOH at room-temperature using ultrasonic stirrer for 15 min. After 12 h equilibration, the clear fraction of the CNTs–EtOH saturated solution was then transported into evaporating vessels. The so-called ‘treated CNTs’ were obtained after evaporating the EtOH. Both the pristine and treated nanotubes were dissolved in acetonitrile as follows: after 5 min ultrasonic shaking the saturated
H. Li et al. / Chemical Physics Letters 492 (2010) 258–262
solution was left staying for 48 h. During this time the equilibrium between the precipitated and diluted fraction of the solutions were fully separated. Clear fractions of these solutions were then used for the investigations. All the manipulations described above were carried out at room temperature (T = 288 K), except for the measurements performed to determine the temperature-dependence of the dispersion. The effect of bounding was analysed by photoluminescence (PL) spectra, while the particle size was investigated by Rayleigh-scattering methods. Both experiments were carried out by Fluorolog s3 spectrofluorometer (Jobin-Yvon/SPEX). Photon counting method with 0.1 s integration time was used, the excitation and emission bandwidths were set to 1 nm. A 2 mm thickness of the fluorescent probes with front face detection was applied to eliminate the inner filter effect. Same instrument was applied to detect the Rayleigh-scattering. Same bandwidths and layer thickness but with right angle detection rearrangement was applied for these investigations. 3. Results and discussion Fig. 1 shows the PL spectra of the ‘untreated’ and ‘treated’ CNTs taken from the clear fraction of their supersaturated solution in acetonitrile. In comparison with the untreated CNTs, considerable red shift of the emission peak of the solutions of treated CNTs can be obtained using 337 nm of excitation. This large shift can be described by the known effect of interaction of carbon rings of aromatic molecules [14,15]. Similar shifts of the emission spectra were also observed during interaction of the aniline aromatic rings with the single-walled carbon nanotube in our previous studies [7– 9]. Furthermore, the characteristic Raman peak of the acetonitrile solvent was observed at about 375 and 373 nm using 337 nm and 335 nm excitations, respectively. This Raman shift of 3010 cm1 reflects the C–H vibration of the solvent molecules. Fig. 1 also shows extremely enhanced Rayleigh-scattering in the solutions of the treated CNTs, which properties will be discussed in detail later. It is the basic property of the fluorescence of any material if there is only a single conformation in the solutions, the site of the emission peak should be independent from the excitation wavelengths due to the Kasha’s rule [16–18] in condensed phase. Any dependence of the emission peak on the excitation wavelength reflects presence of at least two different species in the solutions which has different emission properties. In the following, we
Fig. 1. PL spectra of EtOH treated and untreated MWCNT-OH in acetonitrile under the excitation at about 335 nm (T = 288 K).
259
will discuss the emission properties of the solutions of the ‘untreated’ and ‘treated’ CNTs using the above viewpoints. The site of the fluorescence peak of the ‘untreated’ CNTs solution is independent from the excitation wavelength, highlighting the presence of only one class of CNTs (probably monomers) in the solution. In this particular case, presence of nanotubes with only monomeric form is probably due to the very low solubility. In contrast, the site of the fluorescence peak of the treated CNTs solution depends on the excitation wavelength. Fig. 2a shows the typical emission spectra of ‘treated’ CNTs solution under different excitation wavelength while Fig. 2b shows the intensity of the maxima of the fluorescence peaks of the ‘treated’ CNTs solution plotted against the excitation wavelength. This curve was obtained by considering and subtracting the enhanced background (see Fig. 1) originated from the unexpectedly enhanced Rayleigh-scattering of the solutions of treated nanotubes in acetonitrile. According to previous results observed, e.g. on a family of ionic liquids [19–21], we can assume the presence of different classes of bounded nanotubes solved in the acetonitrile solutions where the shift of the PL peak proportional to the interface of the interacted nanotubes. Note here, the shape of the fluorescence spectra associated to the different excitation wavelengths is almost same. This property means that the peak intensity represents (i.e. is proportional to) the amount of materials excited at the given wavelength. It can be seen clearly in Fig. 2 that there is a maxima on the curve at about 370 nm of excitation, highlighting presence of
Fig. 2. (a) Typical emission spectra of EtOH treated MWCNT-OH in acetonitrile under different excitation wavelength, the black arrows show the emission maxima and the appropriate excitation wavelength also indicated (T = 298 K). (b) Excitation dependence of the emission intensity of EtOH treated MWCNT-OH in acetonitrile (T = 298 K).
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H. Li et al. / Chemical Physics Letters 492 (2010) 258–262
Fig. 3. Excitation dependence of the emission maximum of EtOH treated MWCNTOH in acetonitrile (T = 298 K).
Table 1 Temperature-dependence of the cross of lines represents the different fractions in the treated nanotube solutions. Temperature (K)
Site of the cross I (cm1)
Site of the cross II (cm1)
288 293 298
21 699 22 207 22 510
24 915 25 225 26 922
a specific structure in the solutions with highest probability. To get information about the chemical equilibrium associated to the formation of bounded nanotubes, we need to determine first the number of different classes of bounded nanotubes existed in the solutions with considerable concentration. Fig. 3 shows the site of the PL peaks of the treated CNTs solution plotted against the excitation wavenumber. These data was obtained at 298 K. Considering that the shift observed on the emission peak reflects the gradual change of conformation between different classes of CNTs (i.e., the gradation of the interface of the bounded CNTs), it is obvious to assume that the different slopes in Fig. 3 represent the different types of transitions when the emission changes from one class of the bounding to another. The linear fitting was done by the least square method onto the measured points associated to the same slope, and the crosses of the fitted lines were obtained (see Fig. 3 and Table 1). Therefore, the crosses are associated to the different classes of the bundled CNTs, i.e., the nanotube dimer and trimer (see Fig. 3). Result shows presence of three different bounding procedures in the solution at a given temperature. The excitation wavenumber of the dimer cross is about 26 922 cm1, corresponding to a wavelength of 371 nm which suggests that the specific structure in the solutions with highest probability is just the nanotube dimer (see also Fig. 2b). These measurements were repeated also at 293 K and 288 K. The crosses determined above shifted towards higher wavenumbers with increase of temperature (see Table 1), highlighting the decrease of the interface of the bounded CNTs. Note here, the monomer emission which does not change with the excitation wavenumber located at 22 472 cm1 (see also Fig. 1, the red1 peak at 445 nm). The temperature-dependent distribution of different CNTs classes suggests weak interactions which stabilize the bounded structures. Therefore the chemical equilibrium shifts to-
1 For interpretation of color in Figs. 1–4, the reader is referred to the web version of this article.
Fig. 4. (a) Excitation dependence of the Rayleigh-scattering intensity of EtOH treated MWCNT-OH in acetonitrile compared to MWCNT-OH in EtOH and untreated MWCNT-OH in acetonitrile ((T = 288 K). (b) Fitting results of temperature dependence of EtOH treated MWCNT-OH in acetonitrile under ‘small particle approximation’. Inset: fitting result of thermodynamic parameters.
wards the smaller particles at higher temperature. At last, due to the change observed in the PL properties of the treated CNTs solutions, p–p interactions of aromatic rings of nanotubes could be identified as forces to stabilize the larger nanotube structures [22,23]. To obtain information about the size of the structures formed in the acetonitrile solutions, Rayleigh-scattering measurements was performed. Fig. 4a shows the Rayleigh-scattering spectra observed on the treated and untreated nanotubes in acetonitrile solvent and on the nanotubes dissolved in EtOH. Result clearly shows unexpectedly high Rayleigh-scattering in the case of treated nanotubes. As the Rayleigh-scattering is even suppressed for the nanotube solutions dissolved in EtOH, one can hardly ascribe this significant enhancement to the increase of the solubility of the nanotubes treated by EtOH. Therefore, it is reasonable to assume that the solubility of CNTs does not change considerably under the treatment. In agreement with this assumption the saturated solutions of our functionalized nanotubes have 13 mg/dm3 concentration at 288 K independently from the treatment. Considering 1 lm as the average length of the nanotube, about 6 109 M concentration of the nanotube can be estimated in the solutions. Therefore, same mass density of the dissolved nanotubes can be considered in both
H. Li et al. / Chemical Physics Letters 492 (2010) 258–262
solutions. Then, the enhancement of the scattering can be discussed under the Rayleigh-equation using the idea that the size of some particles is increased by association of the nanotubes. When two individual nanotubes are associated, the effective diameter of the particles is increased. Mention however, that it is not exactly correct that the diameter of a dimer is two times larger than a monomer. In fact, the scattering section of dimer will show an angle dependence. As a result, supposing that the diameter of a monomer is d, the effective diameter of dimer which is responsible for the Rayleigh-scattering is not 2d but
Z
!1=6
p 2
6
ð2d cos hÞ dh
1:7763d:
ð1Þ
0
So the actual scattering of a dimer should be 1:77636 ¼ 31:416 times higher than before. Therefore, if all the nanotubes would associate into the dimer forms, the concentration of the particles would reduce to the half, while the scattering of one particle (dimer) would increase to (1.7763)6 = 31.416. Totally, 15.71 times higher intensity could be detected after association of the nanotubes. However, experiments validate only about 10 times higher intensity in case of treated nanotubes, which highlights presence of nanotube in the solutions both in monomer and dimer forms. To examine this property quantitatively, we assume negligible amount of nanotube trimers in the solution (it is reasonable also from Fig. 2), therefore the equilibrium of the dimer formation will be examined within this model. Let us use c for the total concentration of the individual nanotubes in the solution of the treated nanotubes and c1 for the concentration of the monomers, then the dimer concentration c2 can be expressed as follows: c2 = (c c1)/2, since c = c1 + 2c2. The intensity of Rayleigh-scattering in the solution of the treated nanotubes can be written as follows [24,25]:
14 c c 1 6 I ¼ I0 k c1 d þ ; ð2 dÞ6 k 2
ð2Þ
where k is a constant for a given material. Now we can use the above equation to fit the data plotted in Fig. 4b. Then, the ratio of the concentrations of the monomer and dimer (bounded) nanotubes can be calculated, therefore the stability constant (Ks) can be determined as follows:
c2 Ks ¼ 2 : c1
ð3Þ
Using the Rayleigh-scattering data measured at different temperatures (Fig. 4b, T = 288 K, 293 K and 298 K) the thermodynamic parameters of the nanotube association can be determined using the van’t Hoff equation (see similar applications in our previous works [26–28]):
ln K s ¼
DG DH DS ¼ þ ; RT RT R
ð4Þ
where DG, DH and DS are the free enthalpy, enthalpy and entropy change of the association reaction, respectively. R is the gas constant, T reflects the temperature. Using the stability constants from Eq. (3), we are able to calculate the thermodynamic parameters by plotting the ln Ks values against the reciprocal temperatures (see the inset in Fig. 4b). Results show an enthalpy change of 46.6 kJ/ mol and an entropy change of 29.9 J/K mol of the association of the nanotubes, respectively. These values validate 37.7 kJ/mol free enthalpy change which shows the considerable association of the nanotubes around room temperature. These values have relatively high uncertainty, ±12 kJ/mol and ±7 J/K mol in the enthalpy and entropy terms, respectively. This property comes either from the simplification of the model by neglecting the higher ordered association or the uncertainty of the measured Rayleigh spectra.
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The very enhanced association results in 87% individual CNTs bounded into the dimers at room temperature. This high association of the nanotubes is probably due to the etching effect of ethanol on the nanotube wall, since the interaction of ultra-thin wires which formed during the etching can also participate in the association [29]. Furthermore, the negative entropy value reflects more ordered structure of the nanotubes after the association and the formation of rigid nanotube dimers. This property has very important consequences in many applications. For example, according to our recent theoretical work [13], examinations of depositing CNTs onto a structured CeO2 surface highlighted determinant role of the entropy during such interaction. However, if a rigid structure of nanotube dimer reaches the surface, the reduction of the freedom, therefore, reduction of the entropy becomes much lower compared to when the flexible individual nanotube adsorbs to the surface. This process itself can enhance the coverage of any surfaces during epitaxial growth of nanotube layers from solution phase. 4. Summary and conclusion Dissolution of MWCNT-OH into the non-protic acetonitrile solvent was performed and investigated by PL and Rayleigh-scattering method. Either the MWCNT-OH powder was used as it is or previously treated by EtOH. These two types of powders were diluted by acetonitrile and the solvation was followed by photoluminescence and Rayleigh-scattering methods. Results show presence of three fractions of the nanotubes treated by EtOH and highlight presence of nanotube dimers with high concentration at 298 K. Using a qualitative model, DH = 46.6 kJ/mol and DS = 29.9 J/ K mol was determined as thermodynamic parameters of the molecular association. These findings offer not only a very simple way to get markedly modified dispersion of MWCNT-OH, but it is a controllable method to obtain different species of bundled CNTs by changing the temperature. It has a very important practical consequence especially when someone would use LPE method to deposit CNTs onto some specific surfaces, i.e., the highly negative entropy term is of great importance for the LPE deposition of CNTs to enlarge the surface coverage. Acknowledgments This work was supported by the Chinese–Hungarian Intergovernmental S&T Cooperation Programme (Project No: CH-4-32/ 2008:CN-54/2007). This work was also supported partly by the National Natural Science Foundation of China (Grant Nos. 50772015 and 10974019) and the Specialized Research Fund for the Doctoral Program of High Education of China (SRFDP-200800270004). References [1] S. Iijima, Nature 354 (1991) 56. [2] M. Alvaro, P. Atienzar, P. De la Cruz, J.L. Delgado, H. Garcia, F. Langa, J. Phys. Chem. B 108 (2004) 12691. [3] G. Zhang, P. Qi, X. Wang, Y. Lu, D. Mann, X. Li, H. Dai, J. Am. Chem. Soc. 128 (2006) 6026. [4] T. Palacin et al., J. Am. Chem. Soc. 131 (2009) 15394. [5] Y. Sun, S.R. Wilson, D.I. Schuster, J. Am. Chem. Soc. 123 (2001) 5348. [6] D.F. Perepichka, F. Wudl, S.R. Wilson, Y. Sun, D.I. Schuster, J. Mater. Chem. 14 (2004) 2749. [7] B. Peles-Lemli, P. Ács, L. Kollár, S. Kunsági-Máté, Fuller. Nanotubes Carbon Nanostruct. 16 (4) (2008) 247. [8] B. Peles-Lemli, L. Kollár, S. Kunsági-Máté, Fuller. Nanotubes Carbon Nanostruct., in press, doi:10.1080/15363831003782916. [9] B. Peles-Lemli, G. Matisz, A.M. Kelterer, W.M.F. Fabian, S. Kunsági-Máté, J. Phys. Chem. C 114 (2010) 5898. [10] K.A. Fernando, Y. Lin, Y.P. Sun, Langmuir 20 (2004) 4777. [11] J. Chen et al., J. Phys. Chem. B 105 (2001) 2525. [12] M.B. Kannan Balasubramanian, Small 1 (2005) 180. [13] S. Kunsági-Máté, J.C. Nie, Surf. Sci. 604 (2010) 654.
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