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Thin films from functionalized carbon nanotubes using the layer-by-layer technique
Thin Solid Films 551 (2014) 68–73
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Thin films from functionalized carbon nanotubes using the layer-by-layer technique Timo Bohnenberger a,⁎, Lidija D. Rafailovic b, Christian Weilach c, David Hubmayr a, Ulrich Schmid a a b c
Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7, A-1040 Vienna, Austria CEST Centre of Electrochemical Surface Technology, Viktor Kaplan Strasse 2, 2700 Wiener Neustadt, Austria Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 21 November 2013 Accepted 27 November 2013 Available online 3 December 2013
a b s t r a c t In this study the surface of carbon nanotubes is functionalized to generate thin films with a highly porous network structure, as it is desired for electrode materials in electrochemical energy storages. To do so, the nanotubes are carefully treated in acids and analyzed to determine optimum deposition parameters. © 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotubes Layer-by-layer technique Purification
1. Introduction In the recent years, many fundamental as well as application oriented research activities have been done in the field of CNTs. Apart from having superior mechanical properties CNTs are especially attractive as a high surface area material when they are targeted via a porous network structure. These specific properties can be most beneficially used in a wide range of applications such as in filtration [1–3] and in electrochemical devices [4–7]. Especially in the latter case, electrodes can be formed with superior properties for an efficient device operation compared to activated carbon mostly used nowadays. In detail, CNT networks promise to provide enhanced electrical conductivity and to be tunable in pore size distribution. This specific property is of utmost importance in supercapacitor technology, as mesoporous structures with a pore size around 7 nm are most efficient in providing surface area for double layer capacitance [8–11]. In general, there are two standard approaches reported in literature to form thin films of carbon nanotubes. In many studies various vapor deposition techniques were applied to form forests of vertically aligned nanotubes on metallic substrates [12–14] which have in fact good ion accessibility to the network, but there proved to be a limit of the number of nanotubes per square restricting the density of such structures. Secondly, electrophoretic Abbreviations: CNT, carbon nanotubes; MWNT, multiwalled carbon nanotubes; LbL, layer-by-layer; SDS, sodiumdodecylsulfate; CTAB, cetyl trimethylammonium bromide; DI, deionized; EDC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; Sulfo-NHS, Nhydroxysulfosuccinimide; XPS, X-ray photoelectron spectroscopy; PAH, poly(allylamine hydrochloride); PSS, poly-styrenesulfonic acid; PAA, polyacrylic acid; AFS, atomic sensitivity factor; SEM, scanning electron microscopy; BE, binding energy. ⁎ Corresponding author. Fax: +43 1 58801 36698. E-mail address: [email protected] (T. Bohnenberger). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.107
deposition [15,16] has fast kinetics and allows good control over the generated CNT films, but it requires substrates with good electrical conductivity. In contrast, the LbL assembly can be performed in principal on every substrate which provides sufficient surface charges. This charge can be measured as Zeta (ζ)-potential and should reside in the range of ± (40–60) mV for stable film morphology [17]. Mainly nanocomposites of CNTs and weak polyelectrolytes were realized by LbL technique and evaluated so far [18–20]. To ensure, however, the enhanced electrical conductivity provided by the pristine CNTs, the focus of this study is on the realization of networks made only of pure nanotubes [21] instead of composite structures. To reach this goal it is essential to investigate in detail the characteristics of the pH dependant surface charges. CNT-based films are made by alternately dipping a cellulose nitrate substrate into dispersions of complementary functionalized CNTs. These dispersions have to be carefully prepared to adjust their ζ-potential at a level high enough to result in good film stability. In addition, the preparation procedure includes an efficient purification by a treatment in concentrated acids. Also, the ζ-potential of functionalized CNTs is compared to common surfactants such as SDS and CTAB, which are often used to implement charges on colloids. In case of the CNTs, the negative surface charges are provided by carboxylic acid groups [22] attached to the tubes' side walls and positive charges that originate from amine groups [23]. 2. Experimental details MWNTs were purchased from Nanocyl. They were functionalized and sonicated with a concentration of 0.1 mg·ml− 1 either in pure DI-water or as pristine MWNTs in the presence of a surfactant. For this purpose, a Hielscher Sonicator UP400S with external cooling
T. Bohnenberger et al. / Thin Solid Films 551 (2014) 68–73
circuit was used. The cooling prevented an excessive formation of gas bubbles due to the increase in temperature during operation. Otherwise, this would lead to a substantial drop of ultrasonic energy transfer from the sonotrode into the dispersion. For the preparation of a stable dispersion without the addition of any surfactants, the \COOH groups were created by refluxing the CNTs at 75 °C in a mixture of concentrated H2SO4/HNO3 (98%/70%) with a ratio 3:1 for 2 h and 4 h, respectively. The CNTs were cleaned thoroughly by filtering them through a membrane and excessive rinsing in deionized water. Afterwards, they were dried under nitrogen flow at 50 °C. To create amine functionalized nanotubes, the originally oxidized ones were treated in a bath of SOCl2 at 75 °C for 6 h to chlorinate the \CO groups. Next, toluene was added to the bath and the temperature was increased to 120 °C so that the toluene evaporated and drove the acid out of the distilling flask. To free the tubes from any acidic residues, they were cleaned again as described. This step was followed by refluxing them for 24 h in a solution of ethylenediamine in dehydrated toluene (ratio: 1:1), which replaces the Cl-atoms by a molecule with a NH2-tail. Finally, the nanotubes were cleaned and dried again. Additional dispersions were made, using pristine MWNTs in DI-water, but with addition of 0.01 mg·ml − 1 SDS or 0.03 mg·ml− 1 CTAB, respectively. Also, a combination of both approaches was made, dispersing MWNT-COOH with addition of SDS and MWNT-NH2 with CTAB.
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Thin films of carbon nanotubes were made by alternately dipping a membrane of cellulose nitrate in dispersions with positive and negative ζ-potential for at least 15 min. Between the single steps, the sample was washed in deionized water three times for 3 min each to get rid of any weakly attached nanotubes. To finish the pure nanotube thin film, it was dipped for 1 min into a crosslinking agent so that the single tubes become covalently bonded to make the film more durable. As agent 0.1 mol of EDC in water was used. To improve the efficiency of the reaction, 0.1 mol Sulfo-NHS was added to stabilize the carbodiimide in an aqueous solution. The characterization of the functionalized CNTs was done by XPS with a non-monochromatic Al/Mg dual anode and a PHOIBOS EA 150 hemispherical analyzer. To determine the condition of highest Zeta (ζ)-potential of the dispersions an Acoustic Spectrometer DT-1200 was used, scanning the amount of surface charges depending on the pH value. The membranes were weighed before and after the dipping process in a Sartorius analytical balance to monitor the progress of nanotube adsorption per cycle. The surface characteristics of the CNT-based thin films after the dipping process were analyzed using a Hitachi SU8030 scanning electron microscope. Electrical measurements have been done by van-der-Pauw method.
a)
a)
b) b)
Fig. 1. ζ-Potential measurements for a) pristine and functionalized CNTs dispersed with SDS or CTAB and b) dispersions of modified CNTs without any surfactants.
Fig. 2. XPS characteristics representing the different species in the C1s region after a) 2 h and b) 4 h treatment with acids by plotting measured intensity vs. binding energy. It is evident that the amount of carboxylic groups is increased by acid treatment while this is also a good method to remove any carbide.
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Table 1 Results of XPS measurements showing the content of different C-species. Functional group
Content [%] 2 h treated sample (COOH)
Content [%] 4 h treated sample (COOH)
Content [%] functionalized sample (NH2)
C\C/C_C C_O(C_N) COOH π–π*
69.3 10.8 12.1 7.9
69.7 7.8 20.4 2
81.3 8.4 6.3 4
Table 2 Prepared systems for the LbL process.
System 1 System 2 System 3
Positive dispersion
Negative dispersion
MWNT-NH2 MWNT-NH2 MWNT + CTAB
MWNT-COOH (2 h treated) MWNT-COOH (4 h treated) MWNT + SDS
Another possibility to achieve a high amount of surface charges is to functionalize the nanotubes directly. Having this goal in mind, the best suited functional groups are COOH and NH2 [29] which are also the key elements in PAA and PAH, an alternative combination of polyelectrolytes for LbL-assembly. Considering carboxylic groups, they get deprotonated with increasing pH value, which leads to a saturation behavior at pH values higher than five, as shown in Fig. 1b. As can be seen, there is a difference in surface charge depending on the duration of acid treatment. The longer the nanotubes were treated, the higher the ζ-potential. However, it is known from literature, that excess acid treatment will increase the defect density in the shells of the CNTs and therefore reduce their electric conductivity [29,30]. Since the ζ-potential is high enough for both, dispersion and LbL-process, the duration of treatment was not extended. A close look to the trace of the 2 h treated sample reveals that the surface charge density slightly decreases at pH above eight. A possible explanation is the
3. Results and discussion As the LbL technique is well established for films synthesized from two oppositely charged weak polyelectrolytes like PAH and PSS as the most prominent combination [24,25], the realization of a thin film of carbon nanotubes requires some modifications since their intrinsic surface charge is rather low. There are several approaches reported, to increase the amount of electrostatic charges to the surface of the tubes. A typical approach to prepare colloidal dispersions is to add some surface active agents (surfactants) like SDS and CTAB [26–28]. As can be seen in the comparative measurements of the ζ-potential in Fig. 1a, both of them provide the necessary electrostatic forces required for the LbL-technique. SDS and CTAB are ionic detergents which provide a high amount of electrostatic charges if their specific molecules are attached to a tube. While SDS has an anionic sulfate group as head which leads to a highly negative ζ-potential for pH values larger than 4, CTAB is a cationic detergent with also a high ζ-potential over a large pH range. It is important to notice, that once a surfactant is added, the functionalization of the nanotube is of minor importance with respect to the ζ-potential. This indicates a mechanism, where the monomers of the surfactant attach well to the like charged nanotube and screen the functional groups from the bulk medium due to their higher chain length.
2
Fig. 3. Results of the gravimetric measurements of the thin films on a 6.3 cm wide area. Note the stopping of film growth at around five bi-layers for the weakly acid treated sample. The inserted dashed line serves only as a guide to the eye.
Fig. 4. Typical SEM micrographs taken from a and b) the 2 h and c) the 4 h acid-treated sample. The CNT network in the pictures a) and b) is covered by a film of amorphous carbon which does not mix with the tubes homogeneously but seems to agglomerate to islands. In contrast, the film in micrograph c) is completely free of these carbide islands, showing a uniform porous structure instead.
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Fig. 5. Comparative measurements of sheet resistance.
Table 3 Measured Br− content of the investigated sample relating to carbon bonds. The depicted values represent the average figure of five different samples, each.
Br− content
Fresh sample
After 1 day
After 7 days
7.8%
7.1%
6.7%
amount of carboxylic groups attached at carbonaceous impurities to the tubes. At basic pH values they detach and do not contribute when performing the ζ-potential measurements [31]. The NH2 amine groups change their degree of ionization with changing pH, according to the characteristics given in Fig. 1c. Their ζ-potential is highest for acidic environment and decreases rapidly under basic conditions. In accordance to the American Society for Testing and Materials [17], good film stability requires a ζ-potential of at least ±30 mV which is achieved by both types of functional groups as well as for the samples with surfactants added. Other studies have shown that the amount of surfactant per volume to disperse colloids reaches an optimum depending on their concentration [32], demonstrating that the addition of SDS and CTAB must be carefully controlled. The exact quantity was determined when monitoring the behavior of the dispersions over time with an optical microscope. Since nanotubes are only visible in the agglomerated state this technique proved to be a simple and reliable method.
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Considering the minor functionalized CNT-COOH, the positive slope at pH values larger than eight in Fig. 1b indicates the existence of carbonaceous impurities which may cause some problems during the LbL assembly. This assumption is proven by XPS measurements, yielded in Fig. 2. The height of the peak from the sp2 hybridized carbons at 284.3 eV is taken as reference value for all other investigations done in the Cs1 area. The ratio between this peak and the one for O\C_O at ~ 288 eV changes for the strongly treated nanotubes indicating a much higher degree of carboxylation at the cost of double bonds, as shown in Table 1. The purification effect of prolonged acid treatment can be extracted from the peak high at a binding energy of 283 eV in Fig. 2a. This peak is assigned to amorphous carbide and is removed for the 4 h treated sample. In this work, there were three systems prepared for LbL assembly as is given in Table 2 The dipping process generated thin CNT films with different numbers of bi-layers on a cellulose nitrate membrane. Since the membrane has a softly modulated surface topography it is difficult to measure the exact film height with techniques such as contact surface profilometry or atomic force microscopy. Therefore, the gravimetric method was chosen to estimate the amount of attached nanotubes per dipping step. The corresponding results are shown in Fig. 3 indicating a linear behavior of film growth as expected. There was a clear difference in adsorption for functionalized MWNTs treated 2 h and 4 h, respectively. This is probably due to the carbonaceous impurities which in fact are also highly carboxylated, but seem to provide only a weak bonding to the neighboring layers. Consequently, it has to be due to the substrates large excess charges that the first few layers are firmly attached. According to the measurement results shown in Fig. 3, however, the electrostatic influence of the substrate is approximately limited to the first five bi-layers. Considering the film growth for systems two and three, there seems to be no difference in growth rate. Analyzing the topography (see Fig. 4) illustrates the large difference caused by the longer functionalization. The combination of strong acids removes any carbon-containing residues from the dispersion. In case of the short treated nanotubes large areas of the surface are covered with amorphous carbon already indicated by the XPS measurements of the non-acid treated CNTS earlier in Fig. 2. This carbide deposition makes the attachment of additional layers rather difficult, explaining the results of the gravimetric measurement shown in Fig. 3. Since the goal of this study was the development of thin MWNT films that take advantage of the good electrical conductivity of the single tube, the electrical sheet resistance of the films was measured by the
Fig. 6. Schematics yielding the three possibilities during EDC crosslinking reaction: EDC form an intermediate with the \COOH groups and (at the top) reacts with an amine group and forms a stable amide bond (in the middle), decays due to hydrolysis (at the bottom) are stabilized by addition of Sulfo-NHS and may again be able to react with amine groups.
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van-der-Pauw method. On one hand, surfactants inhibit current flow by enclosing the single CNTs. On the other hand, the aggressive chemical treatment of the functionalized CNTs has increased their defect density and therefore the electrical resistivity [33]. The results of the measurements are shown in Fig. 5. As can be seen, there is only a small difference in the measured data for systems two and three. Even though the conductivities of both film types happen to be almost equal, they are influenced from different effects. CNTs in system two are treated in acids for several hours, which certainly damages their lattice structure and reduces their capability for electron transport. The residual surfactant constituent in system three also degrades the CNT conductive properties [34]. However, this implies questions relating surfactant leaching and therefore the aging of membranes made from system three. A loss of CTAB would result in incorporation of other ions, a process that might influence the electrical properties of the membranes. To verify such behavior, CTAB content was monitored by XPS for fresh samples and samples being dipped in KOH for one and seven days, respectively. KOH was chosen to simulate the intended operating environment of these membranes, potentially used as electrodes in supercapacitors. Table 3 provides the according results, showing the Br− content of the samples compared to carbon. From these results, it seems evident, that most of the leachable material has been removed by the intense water rinsing during the assembling process. The subsequent electrical characterization showed no significant difference between the samples. Overall, the film made from functionalized nanotubes has a slightly better conductivity than the film with surfactants added. Since the difference is not very high, there is one other advantage of surface modified CNTs. The functional groups can be used to bond the single layers covalently, which should result in far stronger and longer lasting films. There are several possibilities to bond carboxylic and amine groups together. As a thermal treatment or an irradiation procedure [21,35] is not applicable because of the low temperature stability of the cellulose nitrate, the crosslinking agent EDC was chosen. After connecting to the carboxylic groups it is either replaced by an amine group which is the desired route or it decays. Adding a further agent (e.g. Sulfo-NHS) stabilizes the carbodiimide and increases the robustness of the process (Fig. 6). XPS measurements of the N1s spectra were made to verify the crosslinking and to evaluate the efficiency of the process with and without the addition of Sulfo-NHS as stabilizer. As can be seen in Fig. 7b and c, there is a good crosslinking in both cases. However, when comparing the peak heights at 400 eV, the stabilized reaction has a lower probability. It seems that in case of excessive use of EDC, a stabilizer becomes disadvantageous for the efficiency of bonding.
a)
b)
c)
4. Conclusions and outlook In this work, CNT-based thin films are realized with LbL technique without the addition of polyelectrolytes. Positive and negative charges were implemented by either use of surfactants or functional groups. For a homogenous microstructure it is important to clean the nanotubes from any impurities such as particles of amorphous carbon not only to guarantee a high ζ-potential, but also to provide sufficient surface charges for the subsequentially deposited layers during the dipping procedure. If this requirement is not fulfilled, only the very first CNT layers are attached to the substrate surface whereas any additional layers are washed off. The purification was achieved by a suitable duration of acid treatment. Evaluation of sheet resistance of the films made from functionalized CNTs or surfactant wrapped CNTs revealed only a slightly better conductivity of the former ones. Since the CNT networks are primary hold together by van-der-Waals attractive forces, there is the possibility of film delamination in case of mechanical stress applied. The functionalized CNTs offer a possibility to avoid this problem. They can
Fig. 7. XPS measurements in the N1s region for a) a sample which was not crosslinked, b) a sample crosslinked in EDC solution, and c) a sample crosslinked in Sulfo-NHS stabilized EDC solution. Since the ratio of the peak heights indicates the efficiency of the crosslinking process where NH+ 3 groups are transformed to NH\C_O groups, crosslinking in diluted EDC solution was preferred for all following experiments.
be chemically crosslinked, adding covalent bonding to the van-derWaals forces. The well dispersed nanotube fibers adsorbed very homogeneously on the substrate surface and formed a dense mesoporous network.
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Based on this result, a large amount of possibilities for potential use in technical applications is offered especially for ion adsorption needed in electrodes in electrochemical energy storage and conversion devices. In the near future, further research activities have to focus on the determination of important film properties such as porosity, film thickness and electrical performance in dependence of deposition conditions. For this purpose a change in substrate will be necessary as the curving of a soft cellulose nitrate membrane prevents an exact thickness measurement. Furthermore, its own porosity makes it difficult to evaluate this parameter for the CNT network. Finally, we hope that all these promising results will pave the way for device architectures with enhanced performance, especially in the field of supercapacitors. References [1] S.G.J. Heijman, R. Hopman, Colloids Surf. A Physicochem. Eng. Asp. 151/1–2 (1999) 303. [2] V.F. Medinat, T. Webster, M. Ramaratnam, J.S. Devinny, J. Environ. Sci. Health Part A: Environ. Sci. Eng. Toxicol. 30/2 (1995) 407. [3] M.W. LeChevallier, T.S. Hassenauer, A.K.C, G.A. McFeters, Appl. Environ. Microbiol. 48/5 (1984) 918. [4] In: B.E. Conway (Ed.), Electrochemical Supercapacitors, Kluwer Academic/Plenum Publishers, New York, 1999. [5] P. Ramesh, M.E. Itkis, J.M. Tang, R.C. Haddon, J. Phys. Chem. C 112 (24) (2008) 9089. [6] V. Meille, Appl. Catal. A Gen. 315 (2006) 1. [7] L. Dai, D.W. Chang, J.-B. Baek, W. Lu, Small 8 (8) (2012) 1122. [8] Z. Niu, W. Zhou, J. Chen, G. Feng, H. Li, W. Ma, J. Li, H. Dong, Y. Ren, D. Zhao, S. Xie, Energy Environ. Sci. 4 (4) (2011) 1440. [9] C. Zheng, W. Qian, F. Wei, Mater. Sci. Eng. B 177 (13) (2012) 1138. [10] Y. Huang, J. Liang, Y. Chen, Small 8 (12) (2012) 1805. [11] K.H. An, W.S. Kim, Y.S. Park, Y.C. Choi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Adv. Mater. 13 (7) (2001) 497.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Thin films from functionalized carbon nanotubes using the layer-by-layer technique Timo Bohnenberger a,⁎, Lidija D. Rafailovic b, Christian Weilach c, David Hubmayr a, Ulrich Schmid a a b c
Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7, A-1040 Vienna, Austria CEST Centre of Electrochemical Surface Technology, Viktor Kaplan Strasse 2, 2700 Wiener Neustadt, Austria Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 21 November 2013 Accepted 27 November 2013 Available online 3 December 2013
a b s t r a c t In this study the surface of carbon nanotubes is functionalized to generate thin films with a highly porous network structure, as it is desired for electrode materials in electrochemical energy storages. To do so, the nanotubes are carefully treated in acids and analyzed to determine optimum deposition parameters. © 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotubes Layer-by-layer technique Purification
1. Introduction In the recent years, many fundamental as well as application oriented research activities have been done in the field of CNTs. Apart from having superior mechanical properties CNTs are especially attractive as a high surface area material when they are targeted via a porous network structure. These specific properties can be most beneficially used in a wide range of applications such as in filtration [1–3] and in electrochemical devices [4–7]. Especially in the latter case, electrodes can be formed with superior properties for an efficient device operation compared to activated carbon mostly used nowadays. In detail, CNT networks promise to provide enhanced electrical conductivity and to be tunable in pore size distribution. This specific property is of utmost importance in supercapacitor technology, as mesoporous structures with a pore size around 7 nm are most efficient in providing surface area for double layer capacitance [8–11]. In general, there are two standard approaches reported in literature to form thin films of carbon nanotubes. In many studies various vapor deposition techniques were applied to form forests of vertically aligned nanotubes on metallic substrates [12–14] which have in fact good ion accessibility to the network, but there proved to be a limit of the number of nanotubes per square restricting the density of such structures. Secondly, electrophoretic Abbreviations: CNT, carbon nanotubes; MWNT, multiwalled carbon nanotubes; LbL, layer-by-layer; SDS, sodiumdodecylsulfate; CTAB, cetyl trimethylammonium bromide; DI, deionized; EDC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; Sulfo-NHS, Nhydroxysulfosuccinimide; XPS, X-ray photoelectron spectroscopy; PAH, poly(allylamine hydrochloride); PSS, poly-styrenesulfonic acid; PAA, polyacrylic acid; AFS, atomic sensitivity factor; SEM, scanning electron microscopy; BE, binding energy. ⁎ Corresponding author. Fax: +43 1 58801 36698. E-mail address: [email protected] (T. Bohnenberger). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.107
deposition [15,16] has fast kinetics and allows good control over the generated CNT films, but it requires substrates with good electrical conductivity. In contrast, the LbL assembly can be performed in principal on every substrate which provides sufficient surface charges. This charge can be measured as Zeta (ζ)-potential and should reside in the range of ± (40–60) mV for stable film morphology [17]. Mainly nanocomposites of CNTs and weak polyelectrolytes were realized by LbL technique and evaluated so far [18–20]. To ensure, however, the enhanced electrical conductivity provided by the pristine CNTs, the focus of this study is on the realization of networks made only of pure nanotubes [21] instead of composite structures. To reach this goal it is essential to investigate in detail the characteristics of the pH dependant surface charges. CNT-based films are made by alternately dipping a cellulose nitrate substrate into dispersions of complementary functionalized CNTs. These dispersions have to be carefully prepared to adjust their ζ-potential at a level high enough to result in good film stability. In addition, the preparation procedure includes an efficient purification by a treatment in concentrated acids. Also, the ζ-potential of functionalized CNTs is compared to common surfactants such as SDS and CTAB, which are often used to implement charges on colloids. In case of the CNTs, the negative surface charges are provided by carboxylic acid groups [22] attached to the tubes' side walls and positive charges that originate from amine groups [23]. 2. Experimental details MWNTs were purchased from Nanocyl. They were functionalized and sonicated with a concentration of 0.1 mg·ml− 1 either in pure DI-water or as pristine MWNTs in the presence of a surfactant. For this purpose, a Hielscher Sonicator UP400S with external cooling
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circuit was used. The cooling prevented an excessive formation of gas bubbles due to the increase in temperature during operation. Otherwise, this would lead to a substantial drop of ultrasonic energy transfer from the sonotrode into the dispersion. For the preparation of a stable dispersion without the addition of any surfactants, the \COOH groups were created by refluxing the CNTs at 75 °C in a mixture of concentrated H2SO4/HNO3 (98%/70%) with a ratio 3:1 for 2 h and 4 h, respectively. The CNTs were cleaned thoroughly by filtering them through a membrane and excessive rinsing in deionized water. Afterwards, they were dried under nitrogen flow at 50 °C. To create amine functionalized nanotubes, the originally oxidized ones were treated in a bath of SOCl2 at 75 °C for 6 h to chlorinate the \CO groups. Next, toluene was added to the bath and the temperature was increased to 120 °C so that the toluene evaporated and drove the acid out of the distilling flask. To free the tubes from any acidic residues, they were cleaned again as described. This step was followed by refluxing them for 24 h in a solution of ethylenediamine in dehydrated toluene (ratio: 1:1), which replaces the Cl-atoms by a molecule with a NH2-tail. Finally, the nanotubes were cleaned and dried again. Additional dispersions were made, using pristine MWNTs in DI-water, but with addition of 0.01 mg·ml − 1 SDS or 0.03 mg·ml− 1 CTAB, respectively. Also, a combination of both approaches was made, dispersing MWNT-COOH with addition of SDS and MWNT-NH2 with CTAB.
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Thin films of carbon nanotubes were made by alternately dipping a membrane of cellulose nitrate in dispersions with positive and negative ζ-potential for at least 15 min. Between the single steps, the sample was washed in deionized water three times for 3 min each to get rid of any weakly attached nanotubes. To finish the pure nanotube thin film, it was dipped for 1 min into a crosslinking agent so that the single tubes become covalently bonded to make the film more durable. As agent 0.1 mol of EDC in water was used. To improve the efficiency of the reaction, 0.1 mol Sulfo-NHS was added to stabilize the carbodiimide in an aqueous solution. The characterization of the functionalized CNTs was done by XPS with a non-monochromatic Al/Mg dual anode and a PHOIBOS EA 150 hemispherical analyzer. To determine the condition of highest Zeta (ζ)-potential of the dispersions an Acoustic Spectrometer DT-1200 was used, scanning the amount of surface charges depending on the pH value. The membranes were weighed before and after the dipping process in a Sartorius analytical balance to monitor the progress of nanotube adsorption per cycle. The surface characteristics of the CNT-based thin films after the dipping process were analyzed using a Hitachi SU8030 scanning electron microscope. Electrical measurements have been done by van-der-Pauw method.
a)
a)
b) b)
Fig. 1. ζ-Potential measurements for a) pristine and functionalized CNTs dispersed with SDS or CTAB and b) dispersions of modified CNTs without any surfactants.
Fig. 2. XPS characteristics representing the different species in the C1s region after a) 2 h and b) 4 h treatment with acids by plotting measured intensity vs. binding energy. It is evident that the amount of carboxylic groups is increased by acid treatment while this is also a good method to remove any carbide.
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Table 1 Results of XPS measurements showing the content of different C-species. Functional group
Content [%] 2 h treated sample (COOH)
Content [%] 4 h treated sample (COOH)
Content [%] functionalized sample (NH2)
C\C/C_C C_O(C_N) COOH π–π*
69.3 10.8 12.1 7.9
69.7 7.8 20.4 2
81.3 8.4 6.3 4
Table 2 Prepared systems for the LbL process.
System 1 System 2 System 3
Positive dispersion
Negative dispersion
MWNT-NH2 MWNT-NH2 MWNT + CTAB
MWNT-COOH (2 h treated) MWNT-COOH (4 h treated) MWNT + SDS
Another possibility to achieve a high amount of surface charges is to functionalize the nanotubes directly. Having this goal in mind, the best suited functional groups are COOH and NH2 [29] which are also the key elements in PAA and PAH, an alternative combination of polyelectrolytes for LbL-assembly. Considering carboxylic groups, they get deprotonated with increasing pH value, which leads to a saturation behavior at pH values higher than five, as shown in Fig. 1b. As can be seen, there is a difference in surface charge depending on the duration of acid treatment. The longer the nanotubes were treated, the higher the ζ-potential. However, it is known from literature, that excess acid treatment will increase the defect density in the shells of the CNTs and therefore reduce their electric conductivity [29,30]. Since the ζ-potential is high enough for both, dispersion and LbL-process, the duration of treatment was not extended. A close look to the trace of the 2 h treated sample reveals that the surface charge density slightly decreases at pH above eight. A possible explanation is the
3. Results and discussion As the LbL technique is well established for films synthesized from two oppositely charged weak polyelectrolytes like PAH and PSS as the most prominent combination [24,25], the realization of a thin film of carbon nanotubes requires some modifications since their intrinsic surface charge is rather low. There are several approaches reported, to increase the amount of electrostatic charges to the surface of the tubes. A typical approach to prepare colloidal dispersions is to add some surface active agents (surfactants) like SDS and CTAB [26–28]. As can be seen in the comparative measurements of the ζ-potential in Fig. 1a, both of them provide the necessary electrostatic forces required for the LbL-technique. SDS and CTAB are ionic detergents which provide a high amount of electrostatic charges if their specific molecules are attached to a tube. While SDS has an anionic sulfate group as head which leads to a highly negative ζ-potential for pH values larger than 4, CTAB is a cationic detergent with also a high ζ-potential over a large pH range. It is important to notice, that once a surfactant is added, the functionalization of the nanotube is of minor importance with respect to the ζ-potential. This indicates a mechanism, where the monomers of the surfactant attach well to the like charged nanotube and screen the functional groups from the bulk medium due to their higher chain length.
2
Fig. 3. Results of the gravimetric measurements of the thin films on a 6.3 cm wide area. Note the stopping of film growth at around five bi-layers for the weakly acid treated sample. The inserted dashed line serves only as a guide to the eye.
Fig. 4. Typical SEM micrographs taken from a and b) the 2 h and c) the 4 h acid-treated sample. The CNT network in the pictures a) and b) is covered by a film of amorphous carbon which does not mix with the tubes homogeneously but seems to agglomerate to islands. In contrast, the film in micrograph c) is completely free of these carbide islands, showing a uniform porous structure instead.
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Fig. 5. Comparative measurements of sheet resistance.
Table 3 Measured Br− content of the investigated sample relating to carbon bonds. The depicted values represent the average figure of five different samples, each.
Br− content
Fresh sample
After 1 day
After 7 days
7.8%
7.1%
6.7%
amount of carboxylic groups attached at carbonaceous impurities to the tubes. At basic pH values they detach and do not contribute when performing the ζ-potential measurements [31]. The NH2 amine groups change their degree of ionization with changing pH, according to the characteristics given in Fig. 1c. Their ζ-potential is highest for acidic environment and decreases rapidly under basic conditions. In accordance to the American Society for Testing and Materials [17], good film stability requires a ζ-potential of at least ±30 mV which is achieved by both types of functional groups as well as for the samples with surfactants added. Other studies have shown that the amount of surfactant per volume to disperse colloids reaches an optimum depending on their concentration [32], demonstrating that the addition of SDS and CTAB must be carefully controlled. The exact quantity was determined when monitoring the behavior of the dispersions over time with an optical microscope. Since nanotubes are only visible in the agglomerated state this technique proved to be a simple and reliable method.
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Considering the minor functionalized CNT-COOH, the positive slope at pH values larger than eight in Fig. 1b indicates the existence of carbonaceous impurities which may cause some problems during the LbL assembly. This assumption is proven by XPS measurements, yielded in Fig. 2. The height of the peak from the sp2 hybridized carbons at 284.3 eV is taken as reference value for all other investigations done in the Cs1 area. The ratio between this peak and the one for O\C_O at ~ 288 eV changes for the strongly treated nanotubes indicating a much higher degree of carboxylation at the cost of double bonds, as shown in Table 1. The purification effect of prolonged acid treatment can be extracted from the peak high at a binding energy of 283 eV in Fig. 2a. This peak is assigned to amorphous carbide and is removed for the 4 h treated sample. In this work, there were three systems prepared for LbL assembly as is given in Table 2 The dipping process generated thin CNT films with different numbers of bi-layers on a cellulose nitrate membrane. Since the membrane has a softly modulated surface topography it is difficult to measure the exact film height with techniques such as contact surface profilometry or atomic force microscopy. Therefore, the gravimetric method was chosen to estimate the amount of attached nanotubes per dipping step. The corresponding results are shown in Fig. 3 indicating a linear behavior of film growth as expected. There was a clear difference in adsorption for functionalized MWNTs treated 2 h and 4 h, respectively. This is probably due to the carbonaceous impurities which in fact are also highly carboxylated, but seem to provide only a weak bonding to the neighboring layers. Consequently, it has to be due to the substrates large excess charges that the first few layers are firmly attached. According to the measurement results shown in Fig. 3, however, the electrostatic influence of the substrate is approximately limited to the first five bi-layers. Considering the film growth for systems two and three, there seems to be no difference in growth rate. Analyzing the topography (see Fig. 4) illustrates the large difference caused by the longer functionalization. The combination of strong acids removes any carbon-containing residues from the dispersion. In case of the short treated nanotubes large areas of the surface are covered with amorphous carbon already indicated by the XPS measurements of the non-acid treated CNTS earlier in Fig. 2. This carbide deposition makes the attachment of additional layers rather difficult, explaining the results of the gravimetric measurement shown in Fig. 3. Since the goal of this study was the development of thin MWNT films that take advantage of the good electrical conductivity of the single tube, the electrical sheet resistance of the films was measured by the
Fig. 6. Schematics yielding the three possibilities during EDC crosslinking reaction: EDC form an intermediate with the \COOH groups and (at the top) reacts with an amine group and forms a stable amide bond (in the middle), decays due to hydrolysis (at the bottom) are stabilized by addition of Sulfo-NHS and may again be able to react with amine groups.
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van-der-Pauw method. On one hand, surfactants inhibit current flow by enclosing the single CNTs. On the other hand, the aggressive chemical treatment of the functionalized CNTs has increased their defect density and therefore the electrical resistivity [33]. The results of the measurements are shown in Fig. 5. As can be seen, there is only a small difference in the measured data for systems two and three. Even though the conductivities of both film types happen to be almost equal, they are influenced from different effects. CNTs in system two are treated in acids for several hours, which certainly damages their lattice structure and reduces their capability for electron transport. The residual surfactant constituent in system three also degrades the CNT conductive properties [34]. However, this implies questions relating surfactant leaching and therefore the aging of membranes made from system three. A loss of CTAB would result in incorporation of other ions, a process that might influence the electrical properties of the membranes. To verify such behavior, CTAB content was monitored by XPS for fresh samples and samples being dipped in KOH for one and seven days, respectively. KOH was chosen to simulate the intended operating environment of these membranes, potentially used as electrodes in supercapacitors. Table 3 provides the according results, showing the Br− content of the samples compared to carbon. From these results, it seems evident, that most of the leachable material has been removed by the intense water rinsing during the assembling process. The subsequent electrical characterization showed no significant difference between the samples. Overall, the film made from functionalized nanotubes has a slightly better conductivity than the film with surfactants added. Since the difference is not very high, there is one other advantage of surface modified CNTs. The functional groups can be used to bond the single layers covalently, which should result in far stronger and longer lasting films. There are several possibilities to bond carboxylic and amine groups together. As a thermal treatment or an irradiation procedure [21,35] is not applicable because of the low temperature stability of the cellulose nitrate, the crosslinking agent EDC was chosen. After connecting to the carboxylic groups it is either replaced by an amine group which is the desired route or it decays. Adding a further agent (e.g. Sulfo-NHS) stabilizes the carbodiimide and increases the robustness of the process (Fig. 6). XPS measurements of the N1s spectra were made to verify the crosslinking and to evaluate the efficiency of the process with and without the addition of Sulfo-NHS as stabilizer. As can be seen in Fig. 7b and c, there is a good crosslinking in both cases. However, when comparing the peak heights at 400 eV, the stabilized reaction has a lower probability. It seems that in case of excessive use of EDC, a stabilizer becomes disadvantageous for the efficiency of bonding.
a)
b)
c)
4. Conclusions and outlook In this work, CNT-based thin films are realized with LbL technique without the addition of polyelectrolytes. Positive and negative charges were implemented by either use of surfactants or functional groups. For a homogenous microstructure it is important to clean the nanotubes from any impurities such as particles of amorphous carbon not only to guarantee a high ζ-potential, but also to provide sufficient surface charges for the subsequentially deposited layers during the dipping procedure. If this requirement is not fulfilled, only the very first CNT layers are attached to the substrate surface whereas any additional layers are washed off. The purification was achieved by a suitable duration of acid treatment. Evaluation of sheet resistance of the films made from functionalized CNTs or surfactant wrapped CNTs revealed only a slightly better conductivity of the former ones. Since the CNT networks are primary hold together by van-der-Waals attractive forces, there is the possibility of film delamination in case of mechanical stress applied. The functionalized CNTs offer a possibility to avoid this problem. They can
Fig. 7. XPS measurements in the N1s region for a) a sample which was not crosslinked, b) a sample crosslinked in EDC solution, and c) a sample crosslinked in Sulfo-NHS stabilized EDC solution. Since the ratio of the peak heights indicates the efficiency of the crosslinking process where NH+ 3 groups are transformed to NH\C_O groups, crosslinking in diluted EDC solution was preferred for all following experiments.
be chemically crosslinked, adding covalent bonding to the van-derWaals forces. The well dispersed nanotube fibers adsorbed very homogeneously on the substrate surface and formed a dense mesoporous network.
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