Role of Surfactant Adsorption in Controlling Morphology of Single

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Role of Surfactant Adsorption in Controlling Morphology of SingleWalled Carbon Nanotubes/Polythiophene Nanohybrid Hui Zhang and Hua Wu* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland ABSTRACT: We have prepared nanohybrids of short single-walled carbon nanotubes (SWNTs) covered by a polythiophene (PT) layer. The SWNTs have an average length of 1 μm and are well dispersed in water in the presence of sodium dodecyl benzene sulfonate (SDBS). The PT layer on the short SWNTs has been produced by catalytic oxidative polymerization in aqueous solution. It is found that the morphology of the PT layer is oriented by the morphology of the adsorbed SBDS on the SWNTs. It follows that, depending on the amount of SDBS added in the system, the PT on the SWNTs can be not only in the form of layer but also in the form of several spherical-like particles strung along each SWNT (i.e., in the beaded form). This result confirms the proposal (Vaisman et al. Adv. Colloid Interface Sci. 2006, 128−130, 37) that the SDBS adsorption on the SWNT sidewall is in the form of condensed assemblies. It is this condensed assembly that templates the beaded form of PT on the SWNTs.

1. INTRODUCTION Carbon nanotubes, especially single-walled carbon nanotubes (SWNTs), have stimulated a great amount of new applications in various fields in the last two decades, such as in drug delivery, electronic devices and sensors, reinforcement for materials, etc.1−7 In most of these applications, to achieve the desired performance and economic optimum, the SWNTs have to be well dispersed in a solvent and their sidewall needs to be properly modified.8−10 Since the SWNTs after synthesis are normally in the form of bundles and are almost insoluble in any solvent, it is required to have proper techniques to produce unbundled dispersible SWNTs. The general methodology is to split the bundles using energy in the presence of stabilizing agents, which prevent reagglomeration of the SWNTs. Different classes of stabilizing agents have been used such as surfactants in aqueous solutions,11−13 polymers (wrapping around the SWNTs) in organic solutions,14−18 and ionic liquids.18 In the case of dispersing the SWNTs in water, the most commonly used surfactant is sodium dodecyl benzene sulfonate (SDBS),11,19,20 whose benzene ring, due to its structure similarity to that of SWNTs, would improve dispersion efficiency. It is proposed in the literature19 that the SDBS adsorption on the tube sidewall would produce specific self-organization. Three possible states have been proposed: (a) the SWNT encapsulated in the center of a cylindrical surfactant micelle, (b) random adsorption of surfactant molecules on the SWNT sidewall, and (c) surfactant molecules in the form of condensed assemblies on the SWNT sidewall. However, few experimental evidence can be found in the literature indicating clearly the adsorption state. Another challenge in the applications of SWNTs is how to develop low-cost and industrially feasible approaches to modify or functionalize their sidewall. There are two classes of approaches for the modification, covalent,13−17 and noncovalent.20,21 The covalent functionalization of SWNTs would result in changes in carbon hybridization from sp2 to sp3, leading to possible partial loss of conjugation, thus loss of electron-acceptor and electron-donor properties. In the case of © 2013 American Chemical Society

noncovalent modifications, the SWNT sidewall is functionalized by employing π−π stacking or hydrophobic interactions (e.g., aromatic compounds, polymers, etc.). In this case, the properties of SWNTs can be preserved. Polymers, especially conjugated polymers, have been shown to be excellent wrapping materials for the noncovalent functionalization of SWNTs. Among them, polythiophene (PT) and its derivatives, as one class of the most important intrinsically conducting polymers, have been shown to have great application potential in the field of organic electronics, especially photovoltaic devices. The noncovalent association of PT with SWNTs can form novel electron donor−acceptor nanohybrids, which may be directly used in photovoltaics.22 In this work, we investigate noncovalent modification of short SWNTs by PT through direct emulsion polymerization in aqueous phase. The short SWNTs were obtained by breaking the original bundled ones under intense shear flow and well dispersed in water in the presence of SDBS. The main goal of this work is first to investigate the adsorption behavior of SDBS on the short SWNTs and then to explore how the state of the SDBS adsorption affects the formed PT state on the SWNT sidewall. As will be seen, our results confirm that there is strong correlation between the state of the SDBS adsorption and the final PT form on the SWNT sidewall.

2. ADSORPTION OF SDBS ON SWNTS 2.1. Preparation of SWNT Dispersion. The SDBS was purchased from Tokyo Chemical Industry (Germany) and the SWNTs with an average length of about 30 μm from Chengdu Organic Chemicals (China). After desired amounts of the SDBS and SWNTs were added into deionized water, the Special Issue: Massimo Morbidelli Festschrift Received: Revised: Accepted: Published: 9088

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mixture was sonicated using a Transsonic 460/H instrument (εlma, Germany) for 6 h. The operation was carried out in an ice−water bath in order to avoid thermal damage of the tube sidewall. Then, the obtained SWNT suspension was forced to pass through a Microfluidizer processor (M-110Y, Microfluidics, US), equipped with a commercial z-shape microchannel of rectangular cross-section, at an extremely high shear rate of 1.94 × 106 1/s at least 10 times. Details of the experimental setup may be found in our previous work.23 The dispersed broken SWNTs were characterized by both SEM imaging and a light scattering technique. Figures 1a and b

Figure 2. Zimm plot of the static light scattering data to estimate the average radius of gyration (Rg) of the SWNTs after breakage.

particular, a set of SWNT dispersions are prepared at the same SWNT weight fraction, φ = 0.0004, but at different SDBS concentrations, and allowed to equilibrate under sonication for at least 8 h. Then, the SWNTs in all the dispersions are removed by centrifugation (Multifuge-3-S-R, Heraeus) at 4000 rpm for 2 h. After the SWNTs were removed, the surface tension of the mother liquor of each sample is measured, using the instrument, DCAT 21 (dataphysics, Germany), and its value allows us to evaluate the concentration of SDBS in water at the adsorption equilibrium based on the measured relation (calibration curve) between the surface tension and the SDBS concentration in water. Thus, the amount of SDBS adsorbed on the SWNTs equals the total amount of SDBS added in the system minus that present in water. Figure 3 shows the obtained adsorption isotherm in the form of the SDBS density on the SWNT surface (Γ) as a function of

Figure 1. SEM pictures of SWNTs (a) before breakage and (b) after breakage.

show the SEM pictures of the SWNTs before and after breakage in the z-MC, respectively. It is seen that after breakage, the SWNTs are short and well dispersed, with an average length around 1 μm. The Zimm plot of the static light scattering data for the SWNTs after breakage is shown in Figure 2, from which the average radius of gyration (Rg) is obtained and equal to 244 nm. If we consider the SWNTs to be rigid, the relation between Rg and length (L) is given by Rg2 = L2/12 + (ae2 + ai2)/2, where ae and ai are external and internal radii of the tube. Then, from the Rg value, we may estimate the average length of the SWNTs after breakage, which is about L ≈ 840 nm, very consistent with the result from the SEM picture. 2.2. Adsorption Isotherm of SDBS on SWNTs. The adsorption isotherm of SDBS on the SWNTs has been investigated using the standard surface tension technique. In

Figure 3. Adsorption isotherm of the SDBS on the SWNTs, shown in the form of the SDBS surface density (Γ) as a function of the SDBS concentration in water at equilibrium. The simulation curve is given by eq 1.

the SDBS concentration in water at equilibrium (Cs,eq). It is seen that the isotherm exhibits an S-shape curve, similar to those observed in the literature for SDBS on the poly(vinyl chloride) particles24 and on the polystyrene particles.25 The Sshape isotherm indicates that the SDBS adsorption on the 9089

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SWNTs follows the two-step adsorption model.26 On the basis of this model, at low surfactant concentrations, the first step is dominant, where the surfactant molecules adsorb on the surface individually and adopt an extended conformation aligned with the surface in order to maximize the hydrophobic attraction with the surface. Then, when most of the surface has been covered by the individual surfactant molecules, leading to a monolayer of gaseous-like (G) surfactant molecules, the second step prevails, where n surfactant molecules start to interact among themselves via van der Waals forces and are anchored together forming condensed assemblies (K), as proposed in the literature.19 This second step leads to substantial increase in the surfactant surface density. Figure 4a and b schematically

assemblies. It should be mentioned that the two-step adsorption mechanism can take place only in the surfactant concentration below the critical micellar concentration (CMC), because if it occurs above the CMC, the surfactant would first form micelles in water, instead of forming the condensed assemblies on the SWNT surface. In fact, for SDBS, the measured CMC is 2.1 × 10−3 mol L−1, which is also consistent with the value (∼1.9 × 10−3 mol L−1) reported in the literature,24 and the entire adsorption isotherm in Figure 3 is within Cs,eq < CMC = 2.1 × 10−3 mol L−1. From the saturation SDBS density on the SWNT surface, Γ∞, if we know the specific surface area of our SWNTs, At, we may estimate the surface area occupied by each SDBS molecule on the SWNT surface, Am. The At values reported in the literature27 vary substantially, in the range of At ∈ [320, 1600] m2 g−1. Then, from these we obtain Am ∈ [7.9, 39.6] Å2. The Am value for SDBS has been estimated in the literature for different surfaces. On a polystyrene particle surface, Paxton28 found that the isotherm is of Langmuir type with Am = 62 Å2, while Jodar-Reyes25 obtained an S-shape isotherm with Am = 43 Å2. On a PVC particle surface, Vale and McKenna24 observed an S-shape isotherm with Am = 52 Å2, and on quartz surface, instead, Shen et al.29 obtained a Langmuir-type adsorption with Am = 33.2 Å2. Our Am (= 39.6 Å2) value estimated at the largest At (= 1600 m2 g−1) value is very close to those reported in the literature. It should be noted however that the Am value estimated in this way is only the projection area of the adsorbed surfactant molecule on the tube surface, and the real Am value should account for the curvature of the condensed assemblies and can be significantly larger than the estimated value.

Figure 4. Schematic representation of the two-step surfactant adsorption on the SWNTs: (a) step 1, in the gaseous-like (G) form, and (b) step 2, formation of condensed (K) assemblies.

represents adsorption steps 1 and 2, respectively, assuming a beaded form for the condensed assemblies. Note that a beaded form is possible in the case of SWNTs because the diameter of the SWNTs and the length of an SDBS molecule are on the same order of magnitude. The mathematical form of this twostep adsorption isotherm can be written as follows:26 Γ=

3. MODIFICATION OF SWNTS WITH PT 3.1. Polymerization Process. Typical layered structures in organic (polymer) electronics require that the conducting polymers should be in the form of thin films, which are usually generated from polymer solutions. It is known that PT conventionally made from chemical and electrochemical polymerization cannot be dissolved in common solvents. To solve this insolubility problem, some kinds of substituents are often incorporated on the PT backbone such as alkyl, alkoxyl, perfluoroalkyl, amine, carboxyl groups, etc., making PT soluble or dispersible in common organic solvents, water, or supercritical fluids. On the other hand, the introduced side chains may be undesired in terms of light harvesting and charge transport. They may also soften the materials, allowing small molecules and constituents to diffuse inside. Recently, Wang et al.30 have shown that soluble PT, without introducing any side chains, can be synthesized by heterogeneous oxidative polymerization in aqueous solution. This polymerization technique has been used in this work to generate PT for noncovalent functionalization of SWNTs. A 50 mL portion of short SWNT dispersion (φ = 0.001) with a defined SDBS surface density and 1 g of thiophene monomer (99.5%) were mixed and sonicated for 30 min. A 250-mL flask equipped with a mechanical stirrer was used to perform the polymerization. The reaction started at 50 °C after adding 9 g of hydrogen peroxide solution, with the H2O2 concentration of 30 wt %, and 5 mL of CuSO4 solution, with the Cu2+ concentration of 40 mM. The polymerization proceeded for 7 h, at which the conversion of thiophene can reach about 90%.30 Then, the standard SEM imaging was used to characterize the morphology of the obtained SWNT/PT nanohybrids.

Γ∞k1Cs,eq(1/n + k 2Cs,eq n − 1) 1 + k1Cs,eq(1 + k 2Cs,eq n − 1)

(1)

where k1 and k2 are adsorption constants for adsorption steps 1 and 2, respectively, and Γ and Γ∞ are the equilibrium and the saturation densities of SDBS on the SWNT surface, respectively. After fitting the experimental adsorption isotherm with eq 1, we obtain the model parameters, Γ∞ = 6.62 × 10−3 mol g−1, k1 = 1.3 × 105 L mol−1, k2 = 4.0 × 109 (L mol−1)n−1, and n = 4.1. The obtained n value is small but still within the range reported in the literature (n ∈ [4, 15]).26 Note that for the SDBS adsorption on the poly(vinyl chloride) particles, Vale and McKenna24 obtained an extremely small value of n = 1.33. This arises mostly due to their limited data of the adsorption isotherm at low SDBS concentrations. In fact, all their data are in the range of Cs,eq > 1.0 × 10−4 mol L−1, while, based on our data in Figure 3, the first-step adsorption occurs for Cs,eq < 4.0 × 10−4 mol L−1, dominantly when Cs,eq < 1.0 × 10−4 mol L−1. This means that their data cannot define well the first-step adsorption; thus, it is difficult to obtain a reliable two-step adsorption isotherm. In the regime of the first-step adsorption, the SWNT surface is covered by individual SDBS molecules. Then, as the SDBS concentration further increases, condensed assemblies start to form on the surface and their number increases progressively until the SWNT surface is completely covered by the 9090

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3.2. Morphology of the SWNT/PT Nanohybrids. The first type of the SWNT/PT nanohybrids was synthesized at the initial SDBS density on the SWNT surface, Γ 5.8 × 10−3 mol g−1, smaller than Γ∞(= 6.62 × 10−3 mol g−1). This Γ value from Figure 3 corresponds to the situation in Figure 4b, where the SDBS adsorption on the SWNT surface is dominated by the condensed assemblies (K), i.e., in the beaded form. In this case, we were initially expecting that if the polymerization occurs within the condensed assemblies, we would obtain a beaded form of PT along the SWNTs. However, the SEM pictures shown in Figure 5 are out of our expectation, with nonbeaded

along the polymerization. Then, even though initially there are condensed SDBS assemblies on the tube surface, they are progressively consumed by covering the increased total surface from the polymer layer. The thickness of the PT layer from the SEM pictures in Figure 5 is not very uniform, and the thicker part is probably the location of an initial condensed SDBS assembly. There are also some SWNTs that are bound together after polymerization. This indicates that along the polymerization, as the total surface area increases, the SDBS surface density decreases too much, leading to decrease in the SWNT stability. In fact, we also tried to perform the polymerization at an even lower Γ value but failed due to aggregation of the SWNTs during the polymerization. The occurrence of the aggregation is related to the dynamic adsorption of SDBS on the SWNTs. In particular, along the polymerization the SDBS surface density decreases progressively due to the increase in the total surface area. Then, when the SDBS surface density decreases to a certain level, the repulsion from the surface charges may become insufficient to stabilize the SWNTs. On the basis of the above results, to obtain a beaded form of PT along the SWNTs, we should have the condensed SDBS assemblies on the tube surface not only before but also during the polymerization. To realize this, we should have a reservoir of SDBS that can continuously supply SDBS to cover the increased total surface during the polymerization. Thus, we decided to add substantially large amount of SDBS to the system such that not only the adsorption of SDBS on the SWNTs is initially saturated (Γ = Γ∞ = 6.62 × 10−3 mol g−1) but also the SDBS concentration in water (Cs,eq 1.01 × 10−2 mol L−1) is 5 times larger than the CMC. In this way, a large amount of micelles are formed in water, which serve as the SDBS reservoir. Figure 6 shows the SEM pictures of the SWNTs after polymerization under such conditions. It is seen that the morphology of the SWNTs is substantially different from that in Figure 5. Now, not only are the SWNTs covered by PT but also several spherical-like PT particles are strung together along each SWNT, i.e., the expected beaded form. From the upper picture of Figure 6, we can also observe many individual PT particles that are not connected to the SWNTs. These particles arise obviously because of unconsumed individual micelles presented in water.

4. CONCLUSIONS We have prepared the single-walled carbon nanotubes (SWNTs)/polythiophene (PT) nanohybrids through heterogeneous oxidative polymerization, starting from short SWNTs dispersed in water and stabilized by ionic surfactant, sodium dodecyl benzene sulfonate (SDBS). From the measured S-shape adsorption isotherm, the SDBS adsorption on the SWNTs follows the two-step adsorption. At low SDBS concentrations, the SDBS molecules adsorb on the SWNT surface individually (the first step), and then, when most of the surface has been covered by the individual SDBS molecules, the second step prevails, where the SDBS molecules start to interact among themselves forming condensed assemblies. The SEM pictures reveal that the morphology of the obtained SWNT/PT nanohybrids depends substantially on the adsorption state of SDBS on the SWNT surface and can be either in the form of wrapped PT layer or in the form of spherical-like PT particles strung together along each SWNT, i.e., in the beaded form. The prerequisite for the latter is that not only the adsorbed SDBS molecules on the SWNT surface

Figure 5. SEM pictures of the PT particles and SWNTs after polymerization with the initial SDBS surface density, Γ = 5.8 × 10−3 mol g−1, corresponding to the situation in Figure 4b, where the SDBS adsorption is dominated by the condensed assemblies (K).

form of PT along the SWNTs after polymerization, even though there are few spherical particles around. It should be noted however that, comparing to that in Figure 1b, the diameter of the SWNTs in Figure 5 has increased substantially. This confirms that the polymerization does occur on the SWNT surface. Thus, now the SWNTs are surrounded by PT. To explain the unexpected nonbeaded form of PT in Figure 5 from the initial condensed assemblies of the SDBS adsorption on the SWNT surface, we should consider the fact that the total external surface area of the SWNTs increases substantially 9091

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Figure 6. SEM pictures of the SWNTs after polymerization with the initial SDBS surface density at the saturation, Γ = Γ∞ = 6.62 × 10−3 mol g−1, and the equilibrium SDBS concentration in water, Cs,eq = 1.01 × 10−2 mol L−1 ≫ CMC (= 2.1 × 10−3 mol L−1), thus in the presence of micelles.

must be in the form of condensed assemblies but also there are significant amount of SDBS micelles in water as reservoir for SDBS consumption as the total surface area increases along the polymerization. The beaded morphology of PT along the SWNTs have also confirmed that the adsorbed SDBS molecules on the SWNTs can be indeed in the condensed assembly state.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swiss National Science Foundation (Grant No. 200020_147137/1) is gratefully acknowledged. REFERENCES

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