pubs.acs.org/Langmuir © 2010 American Chemical Society
Layer-by-Layer Assembly of Charged Particles in Nonpolar Media Kwadwo E. Tettey,† Michael Q. Yee,‡ and Daeyeon Lee*,† †
Department of Chemical and Biomolecular Engineering and ‡Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received January 6, 2010. Revised Manuscript Received March 10, 2010
Layer-by-layer (LbL) assembly of charged species such as nanoparticles and polymers has been widely used to generate functional thin films with unique wetting, optical, catalytic, and biological properties. Although LbL assembly is a versatile tool for creating functional thin films on a variety of substrates, it is generally restricted to aqueous media, in which electrolytes ionize readily due to the large dielectric constant of water. LbL assembly of non-water-soluble materials would expand the range of film properties and functionalities that are attainable. In this study, we have successfully performed LbL deposition of charged particles in a nonpolar solvent, toluene. In toluene, carbon black (CB) and alumina acquired negative and positive surface charge, respectively, in the presence of a charge-inducing agent, Aerosol OT (AOT). The dependence of particle surface charge on the concentration of AOT in toluene was probed by electrophoretic mobility analysis. The two oppositely charged particles were sequentially deposited onto glass slides to form CB/Al2O3 nanocomposite thin films. UV-vis spectroscopy, optical profilometry, and thermogravimetric analysis (TGA) were used to investigate the effect of assembly conditions (i.e., the concentration of AOT in each suspension) on the composition and growth behavior of CB/Al2O3 nanocomposite films. Our results demonstrate that LbL assembly can indeed be performed using charged particles in nonpolar media. Such possibility will widen the library of materials that can be incorporated into thin films based on the LbL technique, which can ultimately lead to the generation of multifunctional nanocomposite thin films.
Introduction Layer-by-layer (LbL) assembly involves the sequential deposition of oppositely charged species to create conformal nanocomposite thin films. Over the past two decades, the LbL technique has been used as a versatile tool for creating functional thin films with a wide range of applications1-4 on various substrates including planar supports,5 colloids,6-9 and porous membranes.2 The properties and structures of LbL films can be precisely tuned by varying assembly conditions such as pH and ionic strength. By incorporating a variety of materials including nanoparticles,3,10,11 polymers,5,12 and biomolecules,13-15 functional thin films with *Corresponding author. E-mail:
[email protected]. (1) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2003. (2) Dotzauer, D. M.; Dai, J. H.; Sun, L.; Bruening, M. L. Nano Lett. 2006, 6, 2268–2272. (3) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 2305–2312. (4) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413–418. (5) Decher, G. Science 1997, 277, 1232–1237. (6) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111–1114. (7) Caruso, F.; Donath, E.; Mohwald, H. J. Phys. Chem. B 1998, 102, 2011– 2016. (8) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202–2205. (9) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mohwald, H. Colloids Surf., A 1998, 137, 253–266. (10) Hattori, H. Adv. Mater. 2001, 13, 51–54. (11) Huang, S.; Cen, X.; Peng, H.; Guo, S.; Wang, W.; Liu, T. J. Phys. Chem. B 2009, 113, 15225–15230. (12) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481–486. (13) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405–409. (14) Lang, J.; Lin, M. H. J. Phys. Chem. B 1999, 103, 11393–11397. (15) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117–6123. (16) Dawidczyk, T. J.; Walton, M. D.; Jang, W.-S.; Grunlan, J. C. Langmuir 2008, 24, 8314–8318. (17) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59–63. (18) Mertz, D.; Vogt, C.; Hemmerle, J.; Mutterer, J.; Ball, V.; Voegel, J. C.; Schaaf, P.; Lavalle, P. Nat. Mater. 2009, 8, 731–735.
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unique mechanical,4 optical,16,17 wetting,3 biological,18 and catalytic2,19 properties have been generated. Layer-by-layer assembled thin films have traditionally been assembled in aqueous solutions or, in some instances, in relatively polar media such as chloroform,20 alcohol,21 and formamides.22,23 Materials dispersed in nonpolar solvents are typically not suitable candidates for LbL assembly due to their poor solubility and stability in aqueous media. To incorporate these materials into thin films based on LbL assembly, the materials have to be modified to make them water-soluble, which, in many cases, is not trivial to accomplish. This limitation can be an obstacle to generating functional LbL thin films since a large number of unique nanomaterials such as quantum dots, magnetic nanoparticles, and polymers are indeed synthesized and dispersed in nonpolar solvents.24-27 Furthermore, the choice of water as a solvent for LbL assembly sometimes limits the use of materials that are sensitive to water. These materials include substrates that dissolve in water and polymers and nanoparticles that lose their unique properties in the presence of moisture.20,22 Layer-by-layer assembly in nonpolar solvents (i.e., ε ∼ 2-5)28 is challenging because materials dispersed in nonpolar solvents typically do not acquire charge. This fundamental limitation has (19) Krogman, K. C.; Lowery, J. L.; Zacharia, N. S.; Rutledge, G. C.; Hammond, P. T. Nat. Mater. 2009, 8, 512–518. (20) Hirsjarvi, S.; Peltonen, L.; Hirvonen, J. Colloids Surf., B 2006, 49, 93–99. (21) Beyer, S.; Mak, W. C.; Trau, D. Langmuir 2007, 23, 8827–8832. (22) Kamineni, V. K.; Lvov, Y. M.; Dobbins, T. A. Langmuir 2007, 23, 7423– 7427. (23) Tuo, X. L.; Chen, D.; Cheng, H.; Wang, X. G. Polym. Bull. 2005, 54, 427– 433. (24) Hyeon, T. Chem. Commun. 2003, 927–934. (25) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (26) Roncali, J. Chem. Rev. 1992, 92, 711–738. (27) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (28) Roberts, G. S.; Sanchez, R.; Kemp, R.; Wood, T.; Bartlett, P. Langmuir 2008, 24, 6530–6541.
Published on Web 04/14/2010
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most likely inhibited attempts to perform LbL assembly in nonpolar solvents. The difficulty of charging materials in nonpolar media can be understood by estimating the Bjerrum length in a medium. The Bjerrum length (λB = e2/4πε0εkBT) describes a characteristic ion-ion separation length at which the thermal energy of the system (kBT) becomes comparable to the electrostatic energy between the ions. While the Bjerrum length is only 0.7 nm in water (ε ∼ 80) at room temperature, it is 24 nm in toluene (ε ∼ 2.4), indicating that a 12 nm solvation layer has to form spontaneously around each ion for complete ionization to occur. Thus, it is energetically unfavorable to generate charged species in nonpolar media. Although materials dispersed in nonpolar solvents generally do not acquire charge, recent studies have shown that the addition of an amphiphilic surfactant such as Aerosol OT (AOT) can impart charge on particles in nonpolar solvents.28-31 The stabilization of colloidal particles in nonpolar solvents has been shown to occur through electrostatic repulsion, and repulsive forces between particles have been directly measured,32,33 indicating that electrostatic effects are significant under appropriate conditions. It is believed that the adsorption of added surfactants onto particles and the presence of surfactant micelles in solution play a crucial role in inducing surface charge on particles.28 The aim of this study is to demonstrate that LbL assembly of charged particles can be achieved in a nonpolar solvent. We use commonly available particles, namely carbon black (CB) and alumina, as two species to be incorporated into thin films via LbL assembly from a common nonpolar solvent, toluene. AOT is added to suspensions of CB and alumina in toluene to impart surface charge onto these particles. We show that nanocomposite thin films of CB and alumina can be assembled on glass slides based on LbL assembly. Furthermore, we show that the composition and growth behavior of the CB/alumina films can be varied by independently controlling the concentrations of AOT in each particle suspension. Our results demonstrate that LbL assembly of charged species in nonpolar media can lead to generation of nanocomposite thin films.
Experimental Section Electrophoretic Mobility Measurements. 200 mM Aerosol OT (AOT) (Sigma-Aldrich) in toluene (Fisher) was prepared and diluted to 100, 20, 10, 2, and 1 mM AOT/toluene solutions. 0.1 wt % Al2O3 (Cabot SpectrAl 100) and CB (Columbian Chemicals Conductex 7055 Ultra) suspensions were prepared in pure toluene and sonicated for 1 h to obtain fine dispersions. The alumina suspension was vigorously shaken for ∼30 s to disperse the particles, and then 3 mL was immediately transferred and mixed with 3 mL of each AOT/toluene solution to yield 0.05 wt % alumina in AOT/toluene solutions. The alumina suspensions in AOT/toluene were subsequently sonicated for 1 h. The same procedure was used for CB. Electrophoretic mobility measurements were performed with a Beckman Coulter Delsa Nano-C. Alumina suspensions were allowed to settle overnight to obtain a homogeneous top layer for use in electrophoretic mobility measurements. CB suspensions for electrophoretic mobility measurements were centrifuged at 5000 rpm for 5 min and then filtered with a 5 μm PTFE syringe filter to obtain a homogeneous distribution of particles for an appropriate intensity signal. Three measurements (29) Hsu, M. F.; Dufresne, E. R.; Weitz, D. A. Langmuir 2005, 21, 4881–4887. (30) Keir, R. I.; Suparno; Thomas, J. C. Langmuir 2002, 18, 1463–1465. (31) Smith, P. G.; Patel, M. N.; Kim, J.; Milner, T. E.; Johnston, K. P. J. Phys. Chem. C 2007, 111, 840–848. (32) Sainis, S. K.; Germain, V.; Mejean, C. O.; Dufresne, E. R. Langmuir 2008, 24, 1160–1164. (33) Sainis, S. K.; Merrill, J. W.; Dufresne, E. R. Langmuir 2008, 24, 13334– 13347.
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were made for each suspension at an electric field of 85.2 V/cm. The nitrogen surface area (NSA) of CB and alumina are 55 and 95 m2/g, respectively, as provided by the manufacturers. Alumina and CB particles were used as received.
Layer-by-Layer Assembly of CB and Alumina in Toluene. AOT/toluene solutions were prepared by making 60 mL of 200 mM AOT/toluene solution by adding AOT in pure toluene and then sonicating for 20 min to ensure that AOT was completely dissolved. The 200 mM AOT/toluene solution was subsequently diluted to 20 and 2 mM solutions. 0.1 wt % suspensions of CB and alumina each in vials of 60 mL of pure toluene were sonicated for 20 min. The particle suspension in pure toluene was shaken vigorously for 30 s, and then 30 mL of each particle suspension was mixed with AOT/toluene solution in a 1:1 ratio to yield 60 mL of 0.05 wt % particle in 100, 10, and 1 mM AOT/toluene. The particles in AOT/toluene solutions were subsequently sonicated for 20 min. Particle suspensions were used for LbL assembly 5 min after sonication. LbL assembly was performed on glass slides (Fisherbrand plain microscope slides), which were cleaned by sonicating in 1.0 M NaOH for 20 min, thorough rinsing in deionized (DI) water (18.2 MΩ cm), and drying with compressed air. The first LbL rinse bath consisted of 60 mL of AOT/toluene at the same AOT concentration as the particle suspensions. The remaining rinse baths consisted of 60 mL of pure toluene. Altogether, the LbL assembly consisted of baths of 0.05 wt % CB in AOT/toluene, AOT/toluene rinse, toluene, and toluene followed by 0.05 wt % alumina in AOT/toluene, AOT/toluene rinse, toluene, and toluene. A StratoSequencer VI (NanoStrata Inc.) was programmed to expose the substrates to each particle suspension for 10 min followed by 2, 1, and 1 min of rinse steps. Control samples were generated by substituting CB for alumina so that only CB would be sequentially deposited on glass slides. While samples assembled from particle suspensions with 1, 10, and 100 mM AOT yielded uniform films, those assembled from particle suspensions with 0.5 mM AOT were not highly uniform. Film Characterization. UV-vis absorbance measurements were performed using a Cary 5000 (Varian Inc.) UV-vis-NIR spectrophotometer. The absorbance at 500 nm was used for all data analysis. Scanning electron microscopy (SEM) images were taken using an FEI 600 Quanta FEG ESEM at 5 kV and at a working distance of 10 mm. Atomic force microscopy (AFM) images were taken using an Agilent Technologies/Molecular Imaging PicoPlus AFM. Film thickness measurements were obtained using a Zygo NewView 6K series optical profilometer. A small scratch was made on the film in order to use the bare glass substrate as a reference height (Figure S1a). The height profile on either side of the scratch (Figure S1b) was integrated and normalized with the profile length to get an average film thickness (eq S1 in Supporting Information). This procedure was repeated for three random line segments on each film. The film composition was determined by using thermogravimetric analysis (TA Instruments model SDT 2960). Thick films (60 bilayers) were assembled on glass slides and then scraped off into a platinum TGA pan. The temperature was increased at 10 °C/min to 110 °C and then held for 20 min to remove moisture. The temperature was subsequently ramped at 10 °C/min to 1000 °C. Film conductivity measurements were performed with a Cascade Microtech C4S 4-Point probe head coupled with an Agilent DC power supply unit and Keithley 2000 multimeters. Current-voltage measurements were performed on four random positions on 30 bilayer films in order to obtain the sheet resistance. The sheet resistance of a sample and its respective film thickness were used to determine the resistivity of the film. The inverse of the film resistivity was calculated to find the film conductivity.
Results and Discussion Charging of Alumina and Carbon Black in Toluene. One of the essential properties required for a material to be incorporated into thin films using layer-by-layer (LbL) assembly is that it is DOI: 10.1021/la1000655
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Figure 1. (a) Alumina and (b) carbon black dispersed in toluene containing different concentrations of AOT ([AOT]). Electrophoretic mobility of dispersed (c) alumina and (d) carbon black as a function of [AOT] in toluene. Error bars indicate standard deviations from three measurements.
charged in its medium. Alumina and carbon black (CB) do not acquire charge in pure toluene and precipitate due to poor colloidal stability as shown by the leftmost solutions in parts a and b of Figure 1, respectively. We impart surface charge on CB and alumina particles in toluene by adding a surfactant, Aerosol OT (AOT). CB and alumina particles became well dispersed and colloidally stable in toluene upon adding AOT in a wide range of concentrations as shown in Figure 1a,b. The particle suspensions were stable for several weeks, although sedimentation gradually occurred with time. Sedimented particles, however, could be easily redispersed with gentle agitation. The effect of [AOT] on the particle surface charge in toluene was further studied by taking electrophoretic mobility measurements. The magnitude of the electrophoretic mobility is indicative of the surface charge of particles. The most important result is that CB and alumina acquired opposite charge in solution as shown in Figure 1c,d; CB is negatively charged, whereas alumina becomes positively charged in AOT/toluene solutions. We believe that the difference in the polarity of CB and alumina surfaces led to the acquisition of opposite surface charge by these two particles. A previous study using TiO2 nanoparticles demonstrated that the surface charge of TiO2 particles in AOT/toluene depends on the surface polarity (or hydrophilicity) of the particles rather than its bulk composition.31 Indeed, while alumina could be suspended to form homogeneous dispersion in water, CB precipitated in water, which confirms that alumina has a hydrophilic surface, whereas CB is hydrophobic. Our electrophoretic mobility measurements are also consistent with a previous study that demonstrated the acquisition of negative surface charge by CB in several nonpolar solutions with AOT.34,35 Our results (Figure 1c,d) show that the electrophoretic mobility of alumina and CB depends on the concentration of AOT. The electrophoretic mobility of alumina has a maximum value around 10 mM AOT and gradually decreases as the AOT concentration is (34) Morrison, I. D. Colloids Surf., A 1993, 71, 1–37. (35) Kitahara, A.; Karasawa, S.; Yamada, H. J. Colloid Interface Sci. 1967, 25, 490–495.
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Figure 2. Heteroaggregation of charged CB and alumina in toluene at (a) time = 0 h and (b) time = 16 h. The concentration of AOT in each mixture is 100 mM.
increased above 10 mM. The magnitude of the electrophoretic mobility of CB decreases gradually as the concentration of AOT is increased above 0.5 mM. For CB, no peak in the magnitude of electrophoretic mobility is detected within the concentration range of our investigation; however, we believe that a maximum value (in magnitude) lies between 0 and 0.5 mM AOT since CB in pure toluene has little charge, as evidenced by its poor colloidal stability in pure toluene (Figure 1b). Previous studies have shown similar trends for both hydrophilic and hydrophobic particles.30,31,35 We believe that, at low AOT concentration, the adsorption of AOT molecules on the surface of the particles increases the surface charge of the particles and, hence, their electrophoretic mobility.34,36 The gradual decrease of electrophoretic mobility beyond a maximum value is likely due to attraction of counterions to the particle surface, which in turn leads to neutralization of the surface charge.30,31 (36) Kemp, R.; Sanchez, R.; Mutch, K. J.; Bartlett, P. Langmuir (DOI: 10.1021/ la904207x).
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Figure 3. AFM images of glass slides after deposition of (a) alumina and (b) CB in 10 mM AOT/toluene. Each side of the AFM image corresponds to 20 μm.
Heteroaggregation of Carbon Black and Alumina in Toluene. Attractive interactions between a pair of materials are necessary to form LbL assembled thin films. A simple method to test for the presence of attractive forces in a medium is to mix suspensions of oppositely charged species. Heteroaggregation of oppositely charged colloidal particles in aqueous media has been investigated extensively,37 but similar phenomena involving two oppositely charged particles in nonpolar solvents has not been well documented.38,39 To test for the existence of attractive interactions between oppositely charged particles, CB and alumina in AOT/toluene solution were mixed together in a 1:1 ratio. The mixture became clear overnight, indicating that the two particles aggregated and precipitated from the solution as shown in Figure 2. The heteroaggregation of oppositely charged CB and alumina strongly suggests the existence of attractive forces in this nonpolar medium. Layer-by-Layer Deposition of Charged Carbon Black and Alumina in Toluene. The acquisition of opposite surface charge by CB and alumina and the heteroaggregation of the two particles in AOT/toluene solutions strongly suggest that LbL assembly of CB and alumina is feasible. Before performing LbL assembly of the two particles, we determined the first particle to be deposited on glass slides by exposing cleaned glass slides to suspensions of CB and alumina in 10 mM AOT/toluene solutions. AFM images (Figure 3) of the glass slides showed that the particle density of CB is much higher than that of alumina after rinse steps. From these results, it can be inferred that the glass surface acquires a positive charge in AOT/toluene solutions in contrast to the negative charge that glass acquires in aqueous solutions. Indeed, a previous study has shown that silica particles acquire positive charge in AOT/decane solution, which is consistent with our observation.30 Layer-by-layer assembly of charged CB and alumina in toluene was performed to generate CB/alumina nanocomposite thin films. The assembled films were observed to become darker with increasing number of deposited bilayers as shown in Figure 4a. To quantify the film growth, UV-vis absorbance at 500 nm was measured as a function of the number of deposited bilayers (Figure 4b). These results show that the absorbance increases linearly with the number of deposited bilayers, suggesting that the (37) Islam, A. M.; Chowdhry, B. Z.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 62, 109–136. (38) Damerell, V. R.; Mattson, R. J. Phys. Chem. 1944, 48, 134–141. (39) van Ewijk, G. A.; Philipse, A. P. Langmuir 2001, 17, 7204–7209.
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Figure 4. (a) Photograph of LbL assembled CB/alumina films on glass slides. The text on glass slides represents the number of deposited bilayers (e.g., 3BL = 3 bilayers). (b) Absorbance (at 500 nm) of CB/alumina LbL films on glass slides as a function of the number of deposited bilayers. Absorbance measurements were taken for 1 (b), 10 (2), and 100 mM (1) AOT in CB and alumina. Absorbance for control samples are indicated by 9.
films grow linearly; this is a hallmark of LbL assembly of oppositely charged species. A control experiment was performed to test the possible contribution of nonspecific adsorption of CB particles and/or evaporation-induced particle deposition40 on film growth. With repeated exposure of glass slides to CB suspensions with rinse steps, the slides did not become significantly darker after the formation of the first layer of CB particles (Figure S2). Measured absorbance on the control slides (Figure 4b) confirms that the absorbance of control films is much lower than LbL films and that the increase in the darkness of control samples with increasing number of deposited layers is small. These results indicate that the alternate deposition of oppositely charged alumina and CB leads to the buildup of LbL nanocomposite thin films. (40) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303–1311.
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Figure 5. SEM images of CB/alumina nanocomposite films after deposition of (a) 3 bilayers, (b) 6 bilayers, and (c) 30 bilayers. (d) A crosssectional SEM image of 30 bilayer CB/alumina nanocomposite thin film on a Si wafer. The CB/alumina films were assembled using 10 mM AOT in particle suspensions.
The morphology of CB/alumina LbL films was characterized by using scanning electron microscopy (SEM) as shown in Figure 5. SEM images (Figure 5a-c) show that the surface coverage increases with the number of deposited bilayers. The cross-sectional SEM image (Figure 5d) of a 30 BL sample illustrates that the nanocomposite film uniformly covers the surface. For samples with small number of bilayers as shown in Figure 5a, the particles are seen to cluster into isolated regions on the surface. These islands grow laterally and eventually merge to form a vertically growing, uniform thin film at large numbers of bilayers, as seen in Figure 5c. Similar film morphology transformations have been observed in LbL assembled films of a charged nanoparticle and an oppositely charged polyelectrolyte in aqueous solutions.41 This study concluded that the lateral expansion mode (i.e., lateral growth of isolated domains) is a result of particles adhering to existing islands rather than the bare surface. This phenomenon was attributed to a compensation effect that reduces the number of adsorbed particles as the area of the film expands by (i) partial desorption of previously adsorbed particles during adsorption of the next layer or (ii) increased electrostatic repulsion between charged components during film growth. (41) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101–1110.
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Effect of Assembly Conditions on the Physicochemical Properties of CB/Alumina Nanocomposite Thin Films. The versatility of LbL assembly in aqueous solutions is in the possibility of generating thin films with controlled structure and properties. This can be readily achieved by varying assembly conditions such as pH or ionic strength of the aqueous solution.1,42,43 Since the concentration of AOT was shown to influence the surface charge of CB and alumina in toluene, we examined the effect of AOT concentration on the composition and growth behavior of CB/ alumina films. The effect of these assembly conditions on the absorbance per bilayer and the thickness of 15 bilayer samples are shown in parts a and b of Figure 6, respectively. These results show that the concentration of AOT in either suspension plays a critical role in changing the physical properties of the film. A similar trend is seen for the absorbance and thickness measurements; that is, the lowest values occur when the AOT concentration of CB suspension is 10 mM, whereas the highest values are obtained for 1 mM AOT in CB suspension. The concentration of AOT in CB suspensions shows a more pronounced effect compared to its concentration in alumina suspensions. (42) Lee, D.; Omolade, D.; Cohen, R. E.; Rubner, M. F. Chem. Mater. 2007, 19, 1427–1433. (43) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213–4219.
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Article Table 1. Composition and Conductivity of CB/Alumina Films Determined from TGA and Four-Point Probe Measurement, Respectivelya [AOT] (mM)
mCB
mAl2O3
mAOT
mCB/ (mCB þ mAl2O3)
Conductivity (S/m)
1 10 100
57 43 44
37 47 44
6 10 12
61 48 50
28.49 ( 0.44 8.54 ( 0.01 21.31 ( 0.02
a
Figure 6. Histograms showing (a) the absorbance per bilayer (arbitrary units) and (b) the thickness (nm) of the 15 bilayer films as a function of the concentration of AOT (mM) in alumina and CB suspensions.
Figure 7. Change in mass of CB/alumina films as a function of temperature using thermogravimetric analysis (TGA). Black, red, and blue lines represent thermograms of CB/alumina films assembled in 1, 10, and 100 mM AOT/toluene solutions, respectively. The dashed line represents the derivative of 1 mM thermogram.
The compositions of CB/alumina films were further analyzed using thermogravimetric analysis (TGA).44 Figure 7 shows the changes in mass with respect to temperature for CB/alumina nanocomposite thin films generated from suspensions with 1, 10, and 100 mM AOT. Each TGA thermogram for different CB/ alumina films shows two distinct decomposition regimes. The first degradation regime between 200 and 300 °C arises from the decomposition of residual AOT in the nanocomposite thin films. Figure 7 shows that in this regime the drop in % mass increases with the concentration of AOT in the particle suspensions ([AOT]), which indicates that the amount of residual AOT in the films increases with [AOT]. The second regime is from the decomposition of CB. Interestingly, the onset of CB decomposition is seen to occur at lower temperatures as [AOT] is increased. Changes in the decomposition temperature of a carbon-based (44) Nguyen, P. M.; Zacharia, N. S.; Verploegen, E.; Hammond, P. T. Chem. Mater. 2007, 19, 5524–5530.
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mi indicates wt % of component i.
material has been observed in carbon nanofiber-polymer composites.45 In that study, the changes in decomposition temperature were attributed to polymer-nanofiber interactions that modulate the thermal stability of the carbon nanofiber. We believe that the decomposition of residual AOT in the film, possibly on the surface of CB, facilitates the oxidation of CB at low temperatures. The compositions of CB/alumina nanocomposite thin films as determined from TGA are summarized in Table 1. The relative mass fraction of CB in the nanocomposite thin films agrees well with the results shown in Figure 6; the CB/ alumina film assembled at 1 mM AOT contains the largest amount of CB. Our results clearly demonstrate that modifying the surface charge of CB and alumina particles by changing [AOT] provides a versatile way of controlling the structural properties of CB/alumina nanocomposite films. Electrical Properties of CB/Alumina Nanocomposite Thin Films. Carbon black is often used as a filler to improve the electrical conductivity of insulators. Electrically conductive ceramics can be used in a broad range of applications such as static dissipation and protection.46 To determine the electrical properties of our CB/alumina films, we measured the conductivity of CB/alumina films generated from 1, 10, and 100 mM AOT solutions (Table 1). For all three samples, the films became conductive due to the percolation of CB within the films. However, the measurements show that the conductivity of films depends on the assembly condition (i.e., [AOT]). We also found that a positive correlation between the relative mass fraction of CB and the film conductivity exists, which is consistent since CB is the conductive component in the films.
Conclusion and Outlook In conclusion, we have demonstrated that layer-by-layer (LbL) assembly of oppositely charged materials can be achieved in a nonpolar solvent, toluene. A surfactant, AOT, was used to induce negative and positive surface charge on carbon black (CB) and alumina, respectively. While each particle was stable in AOT/ toluene solution via charge stabilization, a mixture of the two particles resulted in heteroaggregation, suggesting the existence of an attractive force. We showed that LbL assembly of CB/alumina films can be performed on glass slides and that the concentration of the charge inducing agent plays a crucial role in controlling the properties of the films such as composition and thickness. The exact relationship between the surface charge of individual particles and the growth behavior of LbL films in nonpolar media warrants further study.42,47 The simplicity of this procedure is advantageous for creating nanocomposite thin films of ceramics and CB in nonpolar (45) Chipara, M.; Lozano, K.; Hernandez, A.; Chipara, M. Polym. Degrad. Stab. 2008, 93, 871–876. (46) Menchavez, R. L.; Fuji, M.; Takahashi, M. Adv. Mater. 2008, 20, 2345– 2351. (47) Lee, D.; Gemici, Z.; Rubner, M. F.; Cohen, R. E. Langmuir 2007, 23, 8833– 8837.
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solvents. Composite structures composed of carbon and metal oxides have been investigated for various applications as catalysts,48 adsorbents,49 and conductive nanopowders.50 Furthermore, other functional materials such as magnetic nanoparticles, quantum dots, and conjugated polymers synthesized in nonpolar solvents could potentially be charged and incorporated into nanocomposite thin films based on LbL deposition. Such possibility will undoubtedly expand the versatility of LbL assembly as a general film fabrication technique and the use of these (48) Lopez-Salinas, E.; Espinosa, J. G.; Hernandez-Cortez, J. G.; Sanchez-Valente, J.; Nagira, J. Catal. Today 2005, 109, 69–75. (49) Villieras, F.; Leboda, R.; Charmas, B.; Bardot, F.; Gerard, G.; Rudzinski, W. Carbon 1998, 36, 1501–1510. (50) Inam, F.; Yan, H. X.; Jayaseelan, D. D.; Peijs, T.; Reece, M. J. Eur. Ceram. Soc. 2010, 30, 153–157. (51) Michel, M.; Taylor, A.; Sekol, R.; Podsiadlo, P.; Ho, P.; Kotov, N.; Thompson, L. Adv. Mater. 2007, 19, 3859–3864. (52) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nat. Mater. 2005, 4, 366–377. (53) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem., Int. Ed. 2008, 47, 2930–2946.
9980 DOI: 10.1021/la1000655
Tettey et al.
conducting films as electrodes in power generation and storage devices.51-53 Acknowledgment. This work was supported by the PENN MRSEC DMR-0520020 and the Nano/Bio Interface Center through the National Science Foundation NSEC DMR0425780. We thank Professor Russell Composto (University of Pennsylvania) for the use of the UV-vis spectrophotometer and the AFM. We also thank Professor Robert Carpick (University of Pennsylvania) for the use of the optical profilometer. We thank Dr. Renliang Xu and Vamshi Akknuru (Beckman Coulter, Inc.) for their help with electrophoretic mobility analysis. We also thank Columbian Chemicals and Cabot Corp. for their generous provision of carbon black and alumina particles, respectively. Supporting Information Available: Photograph of control experiment samples and film profiles from optical profilometry. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(12), 9974–9980
