Article pubs.acs.org/JPCC
Effect of Functional Group Topology of Carbon Nanotubes on Electrophoretic Alignment and Properties of Deposited Layer Mohammad Mostafa and Soumik Banerjee* School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920, United States ABSTRACT: Thin films of aligned carbon nanotubes (CNTs) have several interesting properties including the ability to transport ions, electrons, and thermal energy. The current study employed molecular dynamics (MD) simulations to determine the effect of varying functionalization topologies of CNTs on their deposition characteristics under applied electric fields of varying strength. The results indicate that the dynamics of CNT alignment along the direction of applied electric field is relatively faster and smoother in case of pristine CNTs compared to that of functionalized CNTs. Considering CNTs of identical length, pristine CNTs are aligned the closest to the direction of the electric field followed by sidefunctionalized and end-functionalized CNTs with the total alignment time being roughly similar. With increase in the strength of electric field, E, total alignment time decreases and is inversely proportional to E2. The final alignment angle (θ∞) and extent of oscillatory response in the case of side- and end-functionalized CNTs are diminished. In contrast with the alignment dynamics, the migration dynamics of pristine CNTs, which tend to agglomerate, is slower and shows some discontinuity compared to the functionalized CNTs. Analysis of the final structure of the deposited CNTs indicate that side-functionalized CNTs produce the most uniformly aligned deposit at relatively weaker electric fields followed by end-functionalized, and pristine CNTs, due partly to their greater extent of solvation, and are therefore a better choice for deposition of uniform CNT films on substrates.
1. INTRODUCTION Graphitic carbon nanostructures such as CNTs have been the focus of scientific research owing to their interesting physicochemical properties, such as high mechanical strength, electrochemical energy storage capacity, and thermal and electrical conductivity.1−10 Because of their excellent electronic properties, uniform deposits of CNTs are widely used in numerous electronic devices.1 CNTs are being employed for fabricating electrode materials with extremely high electrochemically accessible specific surface area and high conductivity.2 To leverage their high mechanical strength, CNTs are also utilized to fabricate high strength polymer−composite materials.2,3 Other applications of aligned arrays of CNTs include field emitters,4 transistors,5 and photoactive layers.6,7 Solution processed deposition techniques provide cost-effective means to deposit highly pure thin films of CNTs with uniform alignment, shape, and density and are widely employed in various applications.8−16 In particular, electrophoretic deposition (EPD),14,17−22 which is based on the motion of charged particles in a spatially uniform electric field, has shown very interesting results in terms of controlled alignment and deposition of thin films of graphene19,20 carbon nanohorns (CNHs)17 and CNTs on substrates.10,11,15 EPD generated CNT coatings find a wide range of applications including field emission devices, biomedical scaffolds, catalyst supports, structural composites, and coatings as well as electrodes with high specific surface area for fuel cells, capacitors, and gas sensors. In comparison with © 2014 American Chemical Society
other solution-based deposition techniques, EPD offers various advantages, including straightforward sample preparation, control over film thickness, excellent homogeneity, and high packing density.23 While the EPD technique is primarily employed in ceramic coatings and composite materials processing industries,24 potential applications of EPD for fabrication of CNT films are being currently explored.25 CNTs in suspensions accrue negative charges and therefore migrate toward the anode by means of electrophoresis when exposed to external dc electric field.26 The major advantage of thin film deposition of CNTs via EPD over traditional techniques is that EPD also intrinsically ensures aligned deposition of CNTs on substrates. Because of their dielectric properties, CNTs are polarized in the presence of electric field, resulting in the generation of a dipole moment which causes them to align along the electric field. The interactions between the solvent molecules, CNTs and the substrate play a vital role in the alignment and deposition process.12,27 The most common solvents used in EPD include distilled water, mixtures of acetone and ethanol, and pure organic solvents such as ethanol, isopropyl alcohol, n-pentanol, tetrahydrofuran, dimethylformamide, and deionized water with pyrrole.15 The complex intermolecular interactions depend on the specific molecular architecture of the CNTs, including the nature of Received: December 21, 2013 Revised: April 29, 2014 Published: May 6, 2014 11417
dx.doi.org/10.1021/jp412537d | J. Phys. Chem. C 2014, 118, 11417−11425
The Journal of Physical Chemistry C
Article
Figure 1. (a) A simulation snapshot showing the initial configuration of the 3-dimensional simulation domain, in the case of end-functionalized nanotubes, is presented. Some of the nanotubes at the bottom and top boundaries along the y-direction appear broken due to periodicity. (b) The three distinct CNT structures, based on their nature of functionalization, are shown. The top, middle, and bottom images represent sidefunctionalized, pristine, and end-functionalized nanotubes, respectively. Cyan: C atoms; white: H atoms; red: O atoms; pink: Au atoms.
al.15 In order to simulate electrophoretic migration, a net charge of one electron (1e) was assigned to each CNT. Figure 1a illustrates the initial configuration of the simulated system for end-functionalized CNTs, and Figure 1b shows the molecular structure of the three types of CNTs that were considered. We used the (100) face of pure gold crystal, shown as X-direction in Figure 1, as a model conducting surface on which the deposition of CNTs was examined. Each substrate comprises layers of thickness equivalent to two unit cells and mimics an electrode (anode or cathode) in an actual EPD process. The nine CNTs are initially randomly scattered in ethanol. In order to make the total system overall charge neutral, we introduced nine positively charged sodium (Na+) counterions. The system is periodic in three dimensions and consists of a thin 0.6 nm buffer zone in each extremity along the X-direction. The purpose of this buffer zone is to keep the two electrodes separate and eliminate physical contact. The overall system dimension is 11.81 × 8.97 × 6.12 nm3 as shown in the figure. Finally, in an effort to evaluate the effect of electric field, these three systems were exposed to electric fields of strength 1−5 V/ nm, in steps of 1 V/nm.
functional groups present on them. Therefore, it is very important to identify functional groups that enhance interactions and lead to proper dispersion of CNTs in solvent media to ensure homogeneous and ordered deposition. However, the effect of various topologies of functional groups on the orientation, deposition, and agglomeration of CNTs subjected to an external electric field is not well understood. In this study, we employed MD simulations to determine the effect of the topology of functional groups on CNTs on their alignment in a polar solvent and subsequent deposition on a substrate in the presence of applied electric field. We compared preferential alignment and deposition characteristics of sidefunctionalized, end-functionalized, and pristine CNTs in ethanol as the model solvent. We considered −COOH as a representative functional group, which commonly results from sonication of CNTs in mildly acidic solvent. The reorientation of CNTs was analyzed by calculating the ensemble average of the angle between the axes of CNTs and the direction of applied electric field as a function of simulation time. In order to determine the relative orientation of the nanotubes, we obtained the distribution of angles between the axes of distinct CNTs. The electrophoretic migration characteristics were analyzed by tracking the position of the center of mass of the CNT ensemble with time. We also characterized the structure of the deposited film by evaluating the density distribution of the CNTs along the direction normal to the substrate. To evaluate the association of CNTs in the final deposit, the radial distribution functions (RDFs) between the CNTs with respect to their centers of mass were obtained. Overall, results from the MD simulations provide valuable insight into the variation of CNT alignment and the properties of the deposited film for varied nature of functionalization of CNTs.
3. COMPUTATIONAL METHOD The MD simulations in the present study were carried out using Gromacs 4.5.5 MD simulation package.28 We used the optimized potentials for liquid simulations-all atom (OPLSAA)29 model as the force field for modeling the CNT and ethanol molecules. The equations of motion were integrated with a time step of 1 fs. The cutoff distances for van der Waals and Coulombic interactions were taken as 1.5 nm. Long-range electrostatic interactions were computed using the particlemesh Ewald method.30 We employed a stochastic velocity rescaling thermostat31 for temperature control with relaxation time of 4 ps. Each Au atom was tethered with a harmonic potential describing their vibration about a mean position. All simulations were performed at 300 K. In order to obtain accurate density of the simulation domain between the substrates (i.e., solvent), we performed several NVT runs during equilibration and solvated the system repeatedly. Once density of the system reached a converged value, we equilibrated the system for at least an additional 10 ns until the total energy of each system were minimized. The final equilibrated systems with end-functionalized, side-functionalized, and pristine CNTs comprised 57 498, 58 785, and 58
2. SIMULATION CONFIGURATION In an effort to evaluate the effect of electric field on the reorientation and deposition of CNTs with varying functional topologies, we simulated 15 different systems comprising CNTs, solvent molecules, and substrates. Each simulation domain comprises nine CNTs of chirality (6,6) and length 2 nm, without considering functional groups. Each simulated CNT(6,6) has 216 carbon atoms without considering the atoms in the functional groups. We considered six carboxylic (−COOH) functional groups on the sides or at the end of the CNTs, based on realistic systems characterized by Boccaccini et 11418
dx.doi.org/10.1021/jp412537d | J. Phys. Chem. C 2014, 118, 11417−11425
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Figure 2. Snapshots from MD simulations at 40 ns illustrating the final configuration for three distinct CNT structures, based on nature of functionalization, are shown. The strength of the applied electric field in each case is 1 V/nm. The (a) profile and (b) top views of end-functionalized nanotubes, (c) profile and (d) top views of side-functionalized nanotubes, and (e) profile and (f) top views of pristine nanotubes are provided. The circled region in (f) shows an agglomerate of pristine nanotubes aligned normal to the substrate.
Figure 3. Time evolution of ensemble-averaged angle between the axis of the nanotubes and the direction of applied electric field of strength (a) 1 V/nm and (b) 2 V/nm are presented.
induced dipole in terms of equivalent partial charges with the assumption that the partial charges are invariant with orientation for moderate initial angles between nanotube axes and the applied electric field. The polarizabilities were obtained from ab initio calculations performed on neutral molecules. However, in our case each CNT has 1e charge. The 1e charge was distributed on the upper half of the nanotube such as to create an appropriate residual dipole independent of the applied electric field. For all other atoms in the simulated systems, partial charges were adopted based on OPLS-AA force field. The force field parameters for Au atoms were based on previous study of Iori et al.40 Changes in the partial atomic charges of the atoms of ethanol due to external electric field were assumed to be negligible for the range of electric field applied in this study. In order to demonstrate effect of different electric field on electrophoretic rotation (i.e., alignment) and deposition of CNT, we considered five different systems and applied 1−5 V/nm dc electric field. The duration of the corresponding NVT production runs were varied depending on the strength of electric field and ran up to 40 ns for the lowest field strength of 1 V/nm.
155 atoms respectively with 5562, 5705, and 5659 ethanol molecules. Upon application of electric field, CNTs are polarized, resulting in induced dipoles. The accurate evaluation of polarizability of CNT, which has been of considerable interest to the scientific community,32−35 is important in order to simulate the CNT alignment process. Ab initio calculations36 presented in the literature indicate that the longitudinal probability is roughly proportional to the length of the CNT and that the presence of functional groups on CNTs does not alter the polarization characteristics significantly. We used restricted HF method with 6-31+G(d,p) basis set to calculate the longitudinal polarizability (αl) of CNT(6,6) using GAMESS version 1 MAY 2013 R1.37 The basis sets used in the present study are appropriate since recent studies reported in the literature calculated electronic properties of nanotubes accurately using lower basis sets.38,39 Nanotubes of varying length and comprising 72, 96, 120, 144, and 168 atoms were considered for calculating the longitudinal polarizability. Our results indicate that the longitudinal polarizabilities of CNTs vary in a roughly linear form with changes in length. On the basis of this observation, we calculated the polarizability of nanotubes (5951.4 bohr3) by linear regression and used it as an input parameter for the MD simulations. Finally, the axial dipole moment was calculated based on the relationship p = αlE, where E is the strength of the electric field. We introduced
4. RESULTS AND DISCUSSION In an effort to present a qualitative overview of the final configuration of functionalized CNTs with varying molecular architecture, we present representative snapshots of the 11419
dx.doi.org/10.1021/jp412537d | J. Phys. Chem. C 2014, 118, 11417−11425
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Table 1 provides θ∞ for the three different architectures based on CNT functionalization and also for varying electric
deposited CNTs on substrate in Figures 2a−f. Figures 2a,c,e show side views of the deposited film with end-functionalized, side-functionalized, and pristine CNTs, respectively, demonstrating their relative alignment with respect to the direction of electric field. While the side-functionalized and pristine CNTs are almost completely aligned along the direction of the applied electric field, the end-functionalized CNTs are oriented at a finite angle with the direction of the electric field. Figures 2b,d,f show top views of the deposited CNT film with distinct molecular architectures. In case of end-functionalized CNTs, shown in Figure 2b, the relative alignment between the CNTs displays a somewhat broad distribution. While some of these CNTs are nearly parallel, others are oriented at an angle. However, considering their alignment with respect to the direction of the electric field, they tend to be oriented at a specific angle. The side-functionalized and pristine CNTs, shown in Figures 2d and 2f, respectively, are aligned in nearparallel configuration. The region in Figure 2f marked by a circle shows that the pristine CNTs form relatively large clusters. The observed aggregation of the pristine carbon nanostructures in polar solvents prior to deposition is in qualitative agreement with a recent experimental study conducted by Oakes et al.17 On the basis of the characterization of the deposition of CNHs, Oakes et al. concluded that these nanostructures deposit on the substrate through an aggregation−flocculation mechanism. In order to quantify the observations presented in Figure 2, we analyzed the molecular trajectories and calculated ensemble averages of the angles of the axes of CNTs with respect to the direction of the applied electric field (θavg). Figures 3a and 3b present time evolutions of θavg for electric fields of strength 1 and 2 V/nm, respectively. Comparison of the dynamics of CNT alignment along the applied electric field for different topologies of functional groups, shown in Figure 3a, demonstrates that the response of pristine CNTs is relatively faster and smoother than the other two cases. In case of the functionalized CNTs, the alignment process is oscillatory following some degree of initial alignment. This oscillatory behavior is caused by the net dipole induced in the −COOH functional group, which makes an angle θ∞ with the axis of the CNT for both side- and end-functionalized CNTs. Therefore, the presence of functional groups causes CNTs to be aligned at a finite angle with applied electric field instead of perfect alignment. Even for pristine CNTs, we observed some departure from perfect alignment. Such slight offset (
