Article pubs.acs.org/Langmuir
Self-Assembly of Polytetrafluoroethylene Nanoparticle Films Using Repulsive Electrostatic Interactions Chuan Du, Jiadao Wang,* and Darong Chen State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P.R. China ABSTRACT: An approach for manufacturing polytetrafluoroethylene nanoparticle films using repulsive electrostatic interactions was developed. This approach used the strong repulsive force between colloidal nanoparticles and a substrate surface to cause the colloidal nanoparticles to suspend and selfassemble at a near-wall equilibrium position. A suspended monolayer was formed and was subsequently deposited on the substrate surface. A relatively large-area (3 × 3 cm2), closepacked unordered monolayer of polytetrafluoroethylene nanoparticles was observed. Multilayer nanoparticle films were also generated by increasing the particle concentration and deposition time. This work confirms the feasibility of nanoparticle self-assembly under repulsive electrostatic interactions and provides new routes for the large-area fabrication of monolayer and multilayer close-packed nanoparticle films.
1. INTRODUCTION Polytetrafluoroethylene (PTFE) is a well-known high molecular weight material with excellent properties that include an ultralow coefficient of friction,1 chemical stability,2 a low dielectric constant,3 and hydrophobicity.4 PTFE films have received an increasing amount of attention from researchers. Methods used for fabricating PTFE films include ion plating,5 vapor-phase e-beam-assisted deposition,6 pulsed laser deposition,7 and superfine powder spraying.8 However, these methods are very complicated and have high costs.9 Self-assembly techniques have received considerable attention because of their simplicity and high efficiency and have been widely applied in sensors,10 biochips,11 photonic crystals,12 and surface-enhanced Raman scattering.13 However, reports of the fabrication of PTFE films using self-assembly are rare. To date, a number of self-assembly methods have been investigated, including dip-coating,14,15 spin-coating,16,17 the Langmuir−Blodgett technique,18,19 interfacial self-assembly,20,21 and electrostatic self-assembly.22,23 Dip-coating, spincoating, and the Langmuir−Blodgett technique are hard to control and are subject to significant environmental constraints.24 It is also difficult to fabricate large-area films using these techniques. Interfacial self-assembly, including at liquid/ air interfaces and oil/water interfaces, is often used to fabricate monolayers of nanoparticles (NPs) with good uniformity and few defects. This method is much simpler, but the transfer of the monolayer from the interface to the substrate surface is a difficult and delicate process.25 Electrostatic self-assembly provides a more versatile, tunable method, because the electrostatic interactions between particles and substrates can be either attractive or repulsive, and the magnitude and range of the interactions can be controlled.26 © XXXX American Chemical Society
To date, the overwhelming majority of studies have used attractive interactions in the electrostatic self-assembly process and have obtained excellent results. For example, Grzybowski’s group used the electrostatic attraction between positively charged Au NPs and negatively charged Ag NPs to fabricate binary NP crystals with a diamondlike lattice.27 Huskens et al. used electrostatic attractions to direct the deposition of NPs to produce colloidal patterns.28 Richard A. Vaia et al.29 and Kostiainen et al.30 applied electrostatic self-assembly to bioparticles; the former successfully organized viral particles into photonic crystals and the latter fabricated binary NP superlattices using protein cages. Repulsive electrostatic interactions are typically unfavorable and are used less frequently; when they are used, it is typically in combination with the hydrophobic31 or magnetic32 attractive interactions of Janus particles or with the strong van der Waals attractions of lower symmetries such as nanotriangles.33 However, it is difficult to predict the precise nature of the attractive or repulsive electrostatic interactions that might exist between various kinds of materials. To obtain attractive electrostatic interactions or make Janus particles, a complicated surface modification process is always required to change the surface charges or other chemical properties; this increases the difficulty, decreases the efficiency, and limits the material options for the overall process. In addition, a shortcoming of attractive electrostatic self-assembly is that it is difficult to fabricate NP films with good uniformity and high density because the colloidal NPs will be confined by the strong electrostatic attraction once they contact the substrate surReceived: September 12, 2013 Revised: January 7, 2014
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face.34 This random sequential adsorption process was simulated by Adamczyk et al. using a Monte Carlo simulation,35 which predicted the presence of a short-range-ordered but long-range-disordered colloid monolayer. It is fundamentally difficult to overcome this disadvantageous long-range disorder, as has been found by many researchers.36−38 We therefore focused on the repulsive electrostatic interactions, with the aim of finding something useful for the self-assembly of NPs. In this paper, we report a strategy for the fabrication of largearea, close-packed unordered NP films based on electrostatic self-assembly under the influence of repulsive interactions. It is typically difficult for colloidal particles to adsorb on a substrate in the presence of repulsive interactions, let alone form a film;39,40 these properties clearly act against self-assembly. Reports of the fabrication of NP films using repulsive electrostatic interactions are therefore rare. The approach presented in this paper used the strong repulsive forces between PTFE colloidal particles and an aluminum alloy substrate to cause the colloidal particles to suspend and selfassemble at a near-wall equilibrium position. A suspended monolayer was formed and deposited on the substrate surface; the integrity of the monolayer’s structure was preserved during and after deposition. Using this method, a 3 × 3 cm2, closepacked unordered monolayer of PTFE particles was fabricated on an aluminum alloy substrate. In addition, by increasing the particle concentration and deposition time, multilayer closepacked unordered PTFE NP films could also be generated.
the 2D schematic illustration (Figure 1) have been simplified to spheres for clarity. Because the density of the PTFE particles (2.1 g/cm3) was higher than that of the solution (∼1 g/cm3), the colloidal NPs sedimented under the action of gravity. When the particles moved into the zone of influence of the electric field produced by the electrical double layer (EDL) associated with the substrate, they were stopped at a certain equilibrium position in the EDL by the strongly repulsive force. If the diameter of the PTFE particles was approximately equal to the thickness of the substrate’s EDL, only the lowest layer of colloidal particles was influenced. When more NPs sedimented downward, the lowest layer became more concentrated. The ion concentration in the EDL of substrate increased, and the charge on the colloidal particle surface was redistributed; both of these factors decreased the distance between the particles. Finally, the particles aggregated in the lowest layer. The suspended monolayer was formed gradually in this manner and eventually fell onto the substrate under the influence of its own increasing weight and the push from the colloidal particles above; the integrity of the monolayer’s structure was preserved during and after deposition. Finally, the monolayer was sintered onto the surface by increasing the temperature, and the redundant colloidal solution was pumped out. Large amounts of deionized water were used to wash the sample, to remove any residual particles.
3. EXPERIMENTAL SECTION
2. METHODOLOGY The approach can be divided into four steps: first, the colloidal NPs sedimented under the action of gravity (Figure 1A);
Three different materials, namely 6061 aluminum alloy (AA), copper (Cu, 99.99%), and 304 stainless steel (SS), were used as substrates. All of the substrates were cut to a size of 3 × 3 cm2 and polished. They were then cleaned using an ultrasonic cleaner, in acetone, ethanol, and deionized water, each in turn for 30 min, and then dried in nitrogen. The monodisperse PTFE NP colloidal solution (3F New Material Co. Ltd., Shanghai, China) was diluted using deionized water to give five different mass fractions: 0.6, 1, 1.5, 2 , and 3 wt %. The substrates were placed horizontally into the diluted PTFE colloidal solution and maintained at 25 °C for a certain deposition time in a constant temperature chamber. The temperature was then increased to 60 °C, and this temperature was maintained for 15 min, for sintering. The cleaning procedure was carried out after the temperature was allowed to decrease naturally to 25 °C. SEM micrographs were taken using an FEI Quanta 200 FEG field emission scanning electron microscope with an accelerating voltage of 15 kV. The particle size and zeta potential of the diluted monodisperse PTFE NP colloidal solution (1.5 wt %, pH = 7.6) were measured by a Malvern Zetasizer Nano ZS Instrument at 25 °C. The surface potential of the substrates was measured using a conventional three-electrode cell in an electrochemical working station (EG&G, model 273A). The counter and reference electrodes were platinum mesh and a saturated calomel electrode (SCE), respectively. The working electrode was scanned using a rate of 2 mV/s. Small angle X-ray diffraction patterns were recorded on an X-ray diffractometer (Rigaku D/MAX 2500, Tokyo, Japan) by using Cu Kα radiation (40 kV, 200 mA). The samples for SAXS were cut into 1.5 × 1.5 cm2 and mounted on a glass sample holder. The step width was 0.01° and the scan speed was 1°/s. The analytical parameters were DS = 1/6°, SS = 1/6°, and RS = 0.15 mm.
Figure 1. 2D schematic illustration of the self-assembly approach using repulsive electrostatic self-assembly: (A) colloidal NPs sedimenting, (B) formation of a suspended PTFE NP monolayer, (C) deposition of the integral suspended monolayer, and (D) sintering and cleaning.
second, a near-wall suspended monolayer was formed (Figure 1B); third, the integral suspended monolayer was deposited on the substrate surface (Figure 1C); fourth, the deposited monolayer was sintered onto the substrate and cleaned (Figure 1D). There were two fundamental conditions that had to be met for this method to work: the first was that the material density of the colloidal particles had to be higher than that of the solution; the other was that the force between the colloidal particles and the substrate had to be strongly repulsive. NPs in
4. RESULTS AND DISCUSSION In the electrostatic self-assembly process, the interaction between the particles and the substrate surface, the particle concentration, and the deposition time may affect the ordering, compactness, and number of layers in the resulting selfassembled structures. The surface potential is one of the most B
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Figure 2. (a) Particle size distribution and (b) correlation coefficient of the PTFE colloidal NPs.
important parameters and has a strong impact on the interactions between the colloidal particles and the substrate surface. In the experiments presented here, three different materials with different surface potentials were chosen as substrates, and the particle concentration and deposition time were varied. 4.1. Effect of Surface Potential. The size of PTFE colloidal NPs was measured by Dynamic Light Scattering and only one distinct peak was found in the size distribution. The average size was 193.8 nm and the polydispersity index (PDI) was 0.047 (less than 0.05), which indicated a highly monodisperse colloidal solution.41 The particle size distribution and correlation coefficient are shown in Figure 2. The PTFE colloidal particle is anisotropic and has a shape that is actually like a capsule. The length of the capsulelike NP is 280 ± 43 nm, while the gyration radius is 60 ± 14 nm. With the use of repeated measurements, the average zeta potential of the PTFE NPs after dilution (1.5 wt %, pH = 7.6) was determined to be −31.4 mV. The surface potentials of the aluminum alloy (AA), stainless steel (SS), and copper (Cu) substrates were −1.33, −0.20, and −0.13 V, respectively, in the same solution without particles (Figure 3). The self-assembly results for the three substrates with different surface potentials are shown in Figure 4 for the 1.5 wt % diluted PTFE colloidal solution after a deposition time of 60
Figure 4. SEM images of the PTFE monolayers on the (a and b) AA, (c and d) SS, and (e and f) Cu substrates, with a 1.5 wt % diluted PTFE colloidal solution and a deposition time of 60 min.
min. It can be seen that the PTFE NPs were deposited on each of the substrates. On the AA substrate (Figure 4, a and b), the PTFE NPs were close-packed, and covered the whole surface. The image showing the edge of the NP film (Figure 4a) clearly indicated that this close-packed NP film was a monolayer. Small-angle X-ray diffraction patterns (Figure 5) show only one distinct diffraction peak, indicating the good short-range regularity of their structures. Owing to the nonsphere shape
Figure 3. Dynamic potential curves for the aluminum alloy (AA, red), copper (Cu, blue), and stainless steel (SS, black) substrates in the 1.5 wt % PTFE colloidal solution without particles. C
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PTFE colloidal solution was 1.61 × 10−5 mol/L. λD is 75.76 nm, according to eq 3.42 n = n+ + n− = n0 exp[−zeψ (D)/KT ] + n0exp[zeψ (D)/KT ]
(1)
tanh[zeψ (D)/4KT ] = tanh(zeψs/4KT ) exp( −D/λD)
(2)
λD = 0.304/ n0 nm
(3)
The ion concentration and electric potential could therefore be calculated; Figure 6 shows the results of these calculations. The ion concentration increased exponentially as the distance between the particle and substrate surface decreased (Figure 6a). The increase in the ion concentration made the ion atmosphere of the particles thinner.43 Moreover, the electric field from the substrate surface redistributed the surface charges of the colloidal particles; that is, the negative charges on the surface of particles move in the opposite direction to the electric field (Figure 6b). As a result, the negative charges on the surface of the particles gathered at the far end of the particles, away from the substrate. Cations were in the majority in the ion atmosphere of the particles. They were not only influenced by the electric field of the substrate but also were attracted by the moving negative charges on the particle surface. Under the influence of these two competing factors, the positive ions became distributed on the two sides of the particles perpendicular to the substrate. The ion atmosphere on the two sides of the particles parallel to the substrate surface thus thinned further (Figure 6b). As shown in Figure 4 (panels a and b), all the NPs contact the substrate with the cylindrical surface, which is a result of energy minimization and indicates a similar behavior to NPs in the suspended layer. Because twice the gyration radius of the PTFE NPs was bigger than the thickness of the substrate’s EDL, only the lowest layer (the suspended layer) of the particles was influenced. As the suspended layer sedimented, these two effects became much stronger because the ion concentration and the electric potential in the substrate’s EDL increased more rapidly. This gradually bridged the distance between particles in the suspended layer, and aggregation then occurred by Brownian motion, which is the essence of the self-assembly processes. Finally, the suspended monolayer was completed and deposited
Figure 5. Small angle X-ray diffraction patterns of the PTFE monolayer on the AA substrate.
and big size differences of the NPs, the long-range regularity of the NP film is not good, which means the film is unordered and noncrystalline. Nonclose-packed particle films were formed on the SS and Cu substrates. On the SS substrate (Figure 4, c and d), the PTFE NPs self-assembled to form chain structures. On the Cu substrate (Figure 4, panels e and f), the NPs were randomly distributed, and it appeared that no self-assembly occurred. It is believed that these differences were caused by the variations in the surface potential. Because the surface potential of the AA substrate was much larger than that of the SS and Cu substrates, the AA substrate had the largest repulsive barrier. When the PTFE NPs entered the influence of the field from the substrate’s EDL, they would have been stopped by the large repulsive barrier and a suspended layer of particles would have formed gradually at a certain position in the EDL as more particles sedimented. In accordance with the DLVO theory, the ion concentration and electric potential in the EDL can be described by eqs 1 and 2 for a 1:1 electrolyte, respectively.42 ψ (D) is the potential, ψs is the surface potential of the substrate, D is the distance between particle and substrate, λD is the Debye length, n is the ion concentration, and n0 is the ion concentration in the bulk solution. Here, n0 of the 1.5 wt %
Figure 6. Schematic diagram of changes in the colloidal NPs’ ion atmosphere arising from the different increases of (a) ion concentration and (b) electric potential in the EDL of different substrates (AA, black; SS, red; Cu, blue). (The ion concentration in the bulk solution was 1.6 × 10−5 mol/L, electrolyte type 1:1.) D
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Figure 7. SEM images of the PTFE NP films formed on the AA substrate using colloidal solutions of different mass fractions: (a) 0.6, (b) 1, (c) 2, and (d) 3. The inset images shown in (c and d) show the edges of the NP films.
on the substrate. In contrast with traditional electrostatic selfassembly, the process of self-assembly in this approach occurred in a near-wall suspended layer, which gave the particles more degrees of freedom and made the self-assembly more effective. The above analysis was based on the condition that the surface potential was sufficiently large to make the interaction between NPs and substrate strongly repulsive, such as the AA substrate. However, if the surface potential was not large enough, two outcomes were possible. The first case can be illustrated using the Cu substrate, whose surface potential was very small. When the particles were sedimenting, they could cross the repulsive barrier easily under the action of gravity and the pressure from the particles above. The increase of ion concentration near the Cu substrate (Figure 6a) was not sufficient to thin the EDL of NPs to the degree necessary for aggregation. In this case, the particles were deposited on the substrate surface randomly and were unable to form a suspended layer. The second case can be illustrated using the SS substrate, whose surface potential was intermediate, not large enough but also not very small. As the particles were sedimenting, they initially stopped at a position in the substrate’s EDL. As time went on, more particles became trapped in the suspended layer. Before the suspended layer was completed, however, the repulsive barrier was overcome by the
combined influence of gravity on the particles in the suspended layer and the pressure from the particles above. The incomplete suspended layer fell to the substrate surface, and the particles self-assembled during the descent process because of the rapid increase in the ion concentration and the electric potential (Figure 6, panels a and b). This was why a partially selfassembled monolayer was observed on the surface of the SS substrate. 4.2. Effects of Particle Concentration and Deposition Time. As well as a sufficient surface potential, the formation of a suspended layer also requires a sufficient number of particles deposited to the equilibrium position in a certain deposition time. The particle concentration and deposition time were therefore also critical factors influencing the self-assembly results. By increasing the particle concentration, the suspended layer could be made to form, complete, and descend more quickly. After the suspended layer descended to the substrate, the EDL of the substrate changed because the PTFE NP monolayer effectively became the new substrate. Because the surface potential of PTFE was much lower than that of the AA substrate, the strong, repulsive barrier could not exist. The particles above could therefore pass through the weakly repulsive barrier to become deposited on the monolayer. The E
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deposited monolayer therefore acted effectively as a template; the particles in the solution above sedimented downward and self-assembled on this template,44 resulting in a multilayered, close-packed NP film. In contrast, when the particle concentration was decreased, the NPs that gathered at the equilibrium position during the deposition were not packed densely enough to form a suspended monolayer and thus could not descend onto the substrate. Figure 7 shows that at mass fractions of 0.6 and 1 wt %, few particles were deposited on the AA surface; the lower the mass fraction, the fewer particles were deposited. At mass fractions of 2 and 3 wt %, two layers and four layers were formed, respectively. The inset images in Figure 7 (panels c and d) show the edges of the NP films, from which the number of layers could be counted. Changing the deposition time produced similar results (Figure 8). With short deposition times of 20 (Figure 8a)
Figure 9. Small angle X-ray diffraction patterns of the PTFE NP films with different layers on the AA substrate.
deposition time, monolayer and multilayer close-packed NP films could be fabricated successfully. In addition, the particle concentration and deposition time were relevant parameters; the combination of these two factors was the key to controlling the number of layers. The self-assembly mechanism of this approach to form a monolayer or multilayer is hard to be analyzed quantitatively as the colloidal system is very complex. The high surface potentials of substrates and low ion concentration of solution make the DLVO theory invalid to provide an analytical solution for this system.45,46 The so-called Derjaguin approximations and linear-superposition approximations (LSA) become less accurate at small ka and kh (k is the inverse Debye length, a is the NP radius, and h is the distance between NPs), where the EDL thickness, particle size, and particle separation are all of similar magnitude, which is especially relevant to charged NPs.26 Therefore, it is very hard to calculate the exact repulsive forces between NPs themselves and between a NP and the substrate. The above qualitative analysis on the self-assembly mechanism and the experiment results are mainly based on EDL theory. In addition, there are also some other theories helpful to explain the results. For example, in the studies of nonequilibrium phenomena, exclusion zones47,48 were found near various substrate surfaces under colloidal solutions owing to long-range repulsion. Particles were gathered at the edge of the exclusion zone, which was similar to the formation of the suspended layer. It is also believed that in systems of likecharged colloidal particles, pairwise interactions are all repulsive. However, recent work46,49,50 has shown that threebody interactions between like-charged particles can be attractive in systems where ka ∼1, which is conducive to explain the self-assembly of NPs in the suspended layer. The theories of nonequilibrium phenomena and three-body interactions significantly supplement the EDL theory and can explain the self-assembly results and mechanism to a certain extent, but both of them are also imperfect and difficult to understand. In the future study, these two theories will be considered and combined with the EDL theory in order to give more precise and quantitative analysis of the self-assembly method proposed in this paper.
Figure 8. SEM images of the PTFE NP films formed on the AA substrate using a 1.5 wt % colloidal solution and different deposition times of (a) 20, (b) 30, (c) 120, and (d) 240 min. The inset images in (c and d) show the edges of the NP films.
and 30 (Figure 8b) min, few particles were deposited on the AA surface. This was because the decreased deposition time did not allow sufficient time for the suspended layer to form. When the deposition time was increased to 120 (Figure 8c) and 240 (Figure 8d) min, three layers were formed, which can be seen from the inset images in Figure 8 (panels c and d). Since a monolayer could be formed at a particle concentration of 1.5 wt % and within 60 min, at deposition times longer than 60 min the particles in the solution sedimented and self-assembled based on the newly formed template (the first NP layer), which resulted in multilayered, close-packed NP films. Small-angle Xray diffraction patterns of the PTFE NP films with different layers (Figure 9) show similar short-range regularity and longrange irregularity. In accordance with the results shown in Figures 7 and 8, it can be concluded that if the repulsive barrier was large enough, and if the NP concentration was high enough to form a monolayer at the equilibrium position during a certain
5. CONCLUSIONS In summary, we have demonstrated an electrostatic selfassembly approach for the manufacture of PTFE NP films using F
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repulsive interactions. Large-area, close-packed, unordered monolayers and multilayers of PTFE NPs were obtained on an aluminum alloy substrate. Experimental parameters, including the surface potential, particle concentration, and deposition time were varied, and it was shown that these factors had a great effect on the quality and number of layers of the NP films. The feasibility of this NP self-assembly technique using repulsive electrostatic interactions expands the material options and range of applications for traditional electrostatic selfassembly and also provides new routes for the large-area fabrication of monolayer and multilayer close-packed NP films. This study enriches the NP self-assembly research field and has potential for industrial applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Tel: 8610-62796458. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 51375253, 51075228, and 51021064)
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REFERENCES
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based on electrostatic self-assembly. Langmuir 2010, 26, 17936− 17942. (35) Adamczyk, Z.; Zembala, M.; Siwek, B.; Warszyński, P. Structure and ordering in localized adsorption of particles. J. Colloid Interface Sci. 1990, 140, 123−137. (36) Zheng, H.; Rubner, M. F.; Hammond, P. T. Particle assembly on patterned “plus/minus” polyelectrolyte surfaces via polymer-onpolymer stamping. Langmuir 2002, 18, 4505−4510. (37) Fang, Y.; Guo, S.; Zhu, C.; Zhai, Y.; Wang, E. Self-assembly of cationic polyelectrolyte-functionalized graphene nanosheets and gold nanoparticles: a two-dimensional heterostructure for hydrogen peroxide sensing. Langmuir 2010, 26, 11277−11282. (38) Himmelhaus, M.; Takei, H. Self-assembly of polystyrene nano particles into patterns of random-close-packed monolayers via chemically induced adsorption. Phys. Chem. Chem. Phys. 2002, 4, 496−506. (39) Li, X.; Scheibe, T. D.; Johnson, W. P. Apparent decreases in colloid deposition rate coefficients with distance of transport under unfavorable deposition conditions: A general phenomenon. Environ. Sci. Technol. 2004, 38, 5616−5625. (40) Shen, C.; Li, B.; Huang, Y.; Jin, Y. Kinetics of coupled primaryand secondary-minimum deposition of colloids under unfavorable chemical conditions. Environ. Sci. Technol. 2007, 41, 6976−6982. (41) Ouyang, Y.; Shi, H.; Fu, R.; Wu, D. Highly monodisperse microporous polymeric and carbonaceous nanospheres with multifunctional properties. Sci. Rep. 2013, 31430. (42) Israelachvili, J. N. Intermolecular and surface forces, revised 3rd ed.; Academic Press: Waltham, MA, 2011. (43) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Direct measurement of colloidal forces using an atomic force microscope. Nature 1991, 353, 239−241. (44) Wang, D.; Möhwald, H. Template-directed colloidal selfassembly, the route to “top-down” nanochemical engineering. J. Mater. Chem. 2004, 14, 459−468. (45) Bostrom, M.; Williams, D. R. M.; Ninham, B. W. Specific ion effects: Why DLVO theory fails for biology and colloid systems. Phys. Rev. Lett. 2001, 87, 168103−168103. (46) Dobnikar, J.; Chen, Y.; Rzehak, R.; von Grünberg, H. H. Manybody interactions in colloidal suspensions. J. Phys.: Condens. Matter 2003, 15, 263−268. (47) Zheng, J. M.; Pollack, G. H. Long-range forces extending from polymer-gel surfaces. Phys. Rev. E 2003, 68, 031408. (48) Schurr, J. M.; Fujimoto, B. S.; Huynh, L.; Chiu, D. T. A theory of macromolecular chemotaxis. J. Phys. Chem. B 2013, 117, 7626− 7652. (49) Merrill, J. W.; Sainis, S. K.; Dufresne, E. R. Many-body electrostatic forces between colloidal particles at vanishing ionic strength. Phys. Rev. Lett. 2009, 103, 138301. (50) Hynninen, A. P.; Dijkstra, M.; van Roij, R. H. H. G. Effect of three-body interactions on the phase behavior of charge-stabilized colloidal suspensions. Phys. Rev. E 2004, 69, 061407.
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Self-Assembly of Polytetrafluoroethylene Nanoparticle Films Using Repulsive Electrostatic Interactions Chuan Du, Jiadao Wang,* and Darong Chen State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P.R. China ABSTRACT: An approach for manufacturing polytetrafluoroethylene nanoparticle films using repulsive electrostatic interactions was developed. This approach used the strong repulsive force between colloidal nanoparticles and a substrate surface to cause the colloidal nanoparticles to suspend and selfassemble at a near-wall equilibrium position. A suspended monolayer was formed and was subsequently deposited on the substrate surface. A relatively large-area (3 × 3 cm2), closepacked unordered monolayer of polytetrafluoroethylene nanoparticles was observed. Multilayer nanoparticle films were also generated by increasing the particle concentration and deposition time. This work confirms the feasibility of nanoparticle self-assembly under repulsive electrostatic interactions and provides new routes for the large-area fabrication of monolayer and multilayer close-packed nanoparticle films.
1. INTRODUCTION Polytetrafluoroethylene (PTFE) is a well-known high molecular weight material with excellent properties that include an ultralow coefficient of friction,1 chemical stability,2 a low dielectric constant,3 and hydrophobicity.4 PTFE films have received an increasing amount of attention from researchers. Methods used for fabricating PTFE films include ion plating,5 vapor-phase e-beam-assisted deposition,6 pulsed laser deposition,7 and superfine powder spraying.8 However, these methods are very complicated and have high costs.9 Self-assembly techniques have received considerable attention because of their simplicity and high efficiency and have been widely applied in sensors,10 biochips,11 photonic crystals,12 and surface-enhanced Raman scattering.13 However, reports of the fabrication of PTFE films using self-assembly are rare. To date, a number of self-assembly methods have been investigated, including dip-coating,14,15 spin-coating,16,17 the Langmuir−Blodgett technique,18,19 interfacial self-assembly,20,21 and electrostatic self-assembly.22,23 Dip-coating, spincoating, and the Langmuir−Blodgett technique are hard to control and are subject to significant environmental constraints.24 It is also difficult to fabricate large-area films using these techniques. Interfacial self-assembly, including at liquid/ air interfaces and oil/water interfaces, is often used to fabricate monolayers of nanoparticles (NPs) with good uniformity and few defects. This method is much simpler, but the transfer of the monolayer from the interface to the substrate surface is a difficult and delicate process.25 Electrostatic self-assembly provides a more versatile, tunable method, because the electrostatic interactions between particles and substrates can be either attractive or repulsive, and the magnitude and range of the interactions can be controlled.26 © XXXX American Chemical Society
To date, the overwhelming majority of studies have used attractive interactions in the electrostatic self-assembly process and have obtained excellent results. For example, Grzybowski’s group used the electrostatic attraction between positively charged Au NPs and negatively charged Ag NPs to fabricate binary NP crystals with a diamondlike lattice.27 Huskens et al. used electrostatic attractions to direct the deposition of NPs to produce colloidal patterns.28 Richard A. Vaia et al.29 and Kostiainen et al.30 applied electrostatic self-assembly to bioparticles; the former successfully organized viral particles into photonic crystals and the latter fabricated binary NP superlattices using protein cages. Repulsive electrostatic interactions are typically unfavorable and are used less frequently; when they are used, it is typically in combination with the hydrophobic31 or magnetic32 attractive interactions of Janus particles or with the strong van der Waals attractions of lower symmetries such as nanotriangles.33 However, it is difficult to predict the precise nature of the attractive or repulsive electrostatic interactions that might exist between various kinds of materials. To obtain attractive electrostatic interactions or make Janus particles, a complicated surface modification process is always required to change the surface charges or other chemical properties; this increases the difficulty, decreases the efficiency, and limits the material options for the overall process. In addition, a shortcoming of attractive electrostatic self-assembly is that it is difficult to fabricate NP films with good uniformity and high density because the colloidal NPs will be confined by the strong electrostatic attraction once they contact the substrate surReceived: September 12, 2013 Revised: January 7, 2014
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face.34 This random sequential adsorption process was simulated by Adamczyk et al. using a Monte Carlo simulation,35 which predicted the presence of a short-range-ordered but long-range-disordered colloid monolayer. It is fundamentally difficult to overcome this disadvantageous long-range disorder, as has been found by many researchers.36−38 We therefore focused on the repulsive electrostatic interactions, with the aim of finding something useful for the self-assembly of NPs. In this paper, we report a strategy for the fabrication of largearea, close-packed unordered NP films based on electrostatic self-assembly under the influence of repulsive interactions. It is typically difficult for colloidal particles to adsorb on a substrate in the presence of repulsive interactions, let alone form a film;39,40 these properties clearly act against self-assembly. Reports of the fabrication of NP films using repulsive electrostatic interactions are therefore rare. The approach presented in this paper used the strong repulsive forces between PTFE colloidal particles and an aluminum alloy substrate to cause the colloidal particles to suspend and selfassemble at a near-wall equilibrium position. A suspended monolayer was formed and deposited on the substrate surface; the integrity of the monolayer’s structure was preserved during and after deposition. Using this method, a 3 × 3 cm2, closepacked unordered monolayer of PTFE particles was fabricated on an aluminum alloy substrate. In addition, by increasing the particle concentration and deposition time, multilayer closepacked unordered PTFE NP films could also be generated.
the 2D schematic illustration (Figure 1) have been simplified to spheres for clarity. Because the density of the PTFE particles (2.1 g/cm3) was higher than that of the solution (∼1 g/cm3), the colloidal NPs sedimented under the action of gravity. When the particles moved into the zone of influence of the electric field produced by the electrical double layer (EDL) associated with the substrate, they were stopped at a certain equilibrium position in the EDL by the strongly repulsive force. If the diameter of the PTFE particles was approximately equal to the thickness of the substrate’s EDL, only the lowest layer of colloidal particles was influenced. When more NPs sedimented downward, the lowest layer became more concentrated. The ion concentration in the EDL of substrate increased, and the charge on the colloidal particle surface was redistributed; both of these factors decreased the distance between the particles. Finally, the particles aggregated in the lowest layer. The suspended monolayer was formed gradually in this manner and eventually fell onto the substrate under the influence of its own increasing weight and the push from the colloidal particles above; the integrity of the monolayer’s structure was preserved during and after deposition. Finally, the monolayer was sintered onto the surface by increasing the temperature, and the redundant colloidal solution was pumped out. Large amounts of deionized water were used to wash the sample, to remove any residual particles.
3. EXPERIMENTAL SECTION
2. METHODOLOGY The approach can be divided into four steps: first, the colloidal NPs sedimented under the action of gravity (Figure 1A);
Three different materials, namely 6061 aluminum alloy (AA), copper (Cu, 99.99%), and 304 stainless steel (SS), were used as substrates. All of the substrates were cut to a size of 3 × 3 cm2 and polished. They were then cleaned using an ultrasonic cleaner, in acetone, ethanol, and deionized water, each in turn for 30 min, and then dried in nitrogen. The monodisperse PTFE NP colloidal solution (3F New Material Co. Ltd., Shanghai, China) was diluted using deionized water to give five different mass fractions: 0.6, 1, 1.5, 2 , and 3 wt %. The substrates were placed horizontally into the diluted PTFE colloidal solution and maintained at 25 °C for a certain deposition time in a constant temperature chamber. The temperature was then increased to 60 °C, and this temperature was maintained for 15 min, for sintering. The cleaning procedure was carried out after the temperature was allowed to decrease naturally to 25 °C. SEM micrographs were taken using an FEI Quanta 200 FEG field emission scanning electron microscope with an accelerating voltage of 15 kV. The particle size and zeta potential of the diluted monodisperse PTFE NP colloidal solution (1.5 wt %, pH = 7.6) were measured by a Malvern Zetasizer Nano ZS Instrument at 25 °C. The surface potential of the substrates was measured using a conventional three-electrode cell in an electrochemical working station (EG&G, model 273A). The counter and reference electrodes were platinum mesh and a saturated calomel electrode (SCE), respectively. The working electrode was scanned using a rate of 2 mV/s. Small angle X-ray diffraction patterns were recorded on an X-ray diffractometer (Rigaku D/MAX 2500, Tokyo, Japan) by using Cu Kα radiation (40 kV, 200 mA). The samples for SAXS were cut into 1.5 × 1.5 cm2 and mounted on a glass sample holder. The step width was 0.01° and the scan speed was 1°/s. The analytical parameters were DS = 1/6°, SS = 1/6°, and RS = 0.15 mm.
Figure 1. 2D schematic illustration of the self-assembly approach using repulsive electrostatic self-assembly: (A) colloidal NPs sedimenting, (B) formation of a suspended PTFE NP monolayer, (C) deposition of the integral suspended monolayer, and (D) sintering and cleaning.
second, a near-wall suspended monolayer was formed (Figure 1B); third, the integral suspended monolayer was deposited on the substrate surface (Figure 1C); fourth, the deposited monolayer was sintered onto the substrate and cleaned (Figure 1D). There were two fundamental conditions that had to be met for this method to work: the first was that the material density of the colloidal particles had to be higher than that of the solution; the other was that the force between the colloidal particles and the substrate had to be strongly repulsive. NPs in
4. RESULTS AND DISCUSSION In the electrostatic self-assembly process, the interaction between the particles and the substrate surface, the particle concentration, and the deposition time may affect the ordering, compactness, and number of layers in the resulting selfassembled structures. The surface potential is one of the most B
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Figure 2. (a) Particle size distribution and (b) correlation coefficient of the PTFE colloidal NPs.
important parameters and has a strong impact on the interactions between the colloidal particles and the substrate surface. In the experiments presented here, three different materials with different surface potentials were chosen as substrates, and the particle concentration and deposition time were varied. 4.1. Effect of Surface Potential. The size of PTFE colloidal NPs was measured by Dynamic Light Scattering and only one distinct peak was found in the size distribution. The average size was 193.8 nm and the polydispersity index (PDI) was 0.047 (less than 0.05), which indicated a highly monodisperse colloidal solution.41 The particle size distribution and correlation coefficient are shown in Figure 2. The PTFE colloidal particle is anisotropic and has a shape that is actually like a capsule. The length of the capsulelike NP is 280 ± 43 nm, while the gyration radius is 60 ± 14 nm. With the use of repeated measurements, the average zeta potential of the PTFE NPs after dilution (1.5 wt %, pH = 7.6) was determined to be −31.4 mV. The surface potentials of the aluminum alloy (AA), stainless steel (SS), and copper (Cu) substrates were −1.33, −0.20, and −0.13 V, respectively, in the same solution without particles (Figure 3). The self-assembly results for the three substrates with different surface potentials are shown in Figure 4 for the 1.5 wt % diluted PTFE colloidal solution after a deposition time of 60
Figure 4. SEM images of the PTFE monolayers on the (a and b) AA, (c and d) SS, and (e and f) Cu substrates, with a 1.5 wt % diluted PTFE colloidal solution and a deposition time of 60 min.
min. It can be seen that the PTFE NPs were deposited on each of the substrates. On the AA substrate (Figure 4, a and b), the PTFE NPs were close-packed, and covered the whole surface. The image showing the edge of the NP film (Figure 4a) clearly indicated that this close-packed NP film was a monolayer. Small-angle X-ray diffraction patterns (Figure 5) show only one distinct diffraction peak, indicating the good short-range regularity of their structures. Owing to the nonsphere shape
Figure 3. Dynamic potential curves for the aluminum alloy (AA, red), copper (Cu, blue), and stainless steel (SS, black) substrates in the 1.5 wt % PTFE colloidal solution without particles. C
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PTFE colloidal solution was 1.61 × 10−5 mol/L. λD is 75.76 nm, according to eq 3.42 n = n+ + n− = n0 exp[−zeψ (D)/KT ] + n0exp[zeψ (D)/KT ]
(1)
tanh[zeψ (D)/4KT ] = tanh(zeψs/4KT ) exp( −D/λD)
(2)
λD = 0.304/ n0 nm
(3)
The ion concentration and electric potential could therefore be calculated; Figure 6 shows the results of these calculations. The ion concentration increased exponentially as the distance between the particle and substrate surface decreased (Figure 6a). The increase in the ion concentration made the ion atmosphere of the particles thinner.43 Moreover, the electric field from the substrate surface redistributed the surface charges of the colloidal particles; that is, the negative charges on the surface of particles move in the opposite direction to the electric field (Figure 6b). As a result, the negative charges on the surface of the particles gathered at the far end of the particles, away from the substrate. Cations were in the majority in the ion atmosphere of the particles. They were not only influenced by the electric field of the substrate but also were attracted by the moving negative charges on the particle surface. Under the influence of these two competing factors, the positive ions became distributed on the two sides of the particles perpendicular to the substrate. The ion atmosphere on the two sides of the particles parallel to the substrate surface thus thinned further (Figure 6b). As shown in Figure 4 (panels a and b), all the NPs contact the substrate with the cylindrical surface, which is a result of energy minimization and indicates a similar behavior to NPs in the suspended layer. Because twice the gyration radius of the PTFE NPs was bigger than the thickness of the substrate’s EDL, only the lowest layer (the suspended layer) of the particles was influenced. As the suspended layer sedimented, these two effects became much stronger because the ion concentration and the electric potential in the substrate’s EDL increased more rapidly. This gradually bridged the distance between particles in the suspended layer, and aggregation then occurred by Brownian motion, which is the essence of the self-assembly processes. Finally, the suspended monolayer was completed and deposited
Figure 5. Small angle X-ray diffraction patterns of the PTFE monolayer on the AA substrate.
and big size differences of the NPs, the long-range regularity of the NP film is not good, which means the film is unordered and noncrystalline. Nonclose-packed particle films were formed on the SS and Cu substrates. On the SS substrate (Figure 4, c and d), the PTFE NPs self-assembled to form chain structures. On the Cu substrate (Figure 4, panels e and f), the NPs were randomly distributed, and it appeared that no self-assembly occurred. It is believed that these differences were caused by the variations in the surface potential. Because the surface potential of the AA substrate was much larger than that of the SS and Cu substrates, the AA substrate had the largest repulsive barrier. When the PTFE NPs entered the influence of the field from the substrate’s EDL, they would have been stopped by the large repulsive barrier and a suspended layer of particles would have formed gradually at a certain position in the EDL as more particles sedimented. In accordance with the DLVO theory, the ion concentration and electric potential in the EDL can be described by eqs 1 and 2 for a 1:1 electrolyte, respectively.42 ψ (D) is the potential, ψs is the surface potential of the substrate, D is the distance between particle and substrate, λD is the Debye length, n is the ion concentration, and n0 is the ion concentration in the bulk solution. Here, n0 of the 1.5 wt %
Figure 6. Schematic diagram of changes in the colloidal NPs’ ion atmosphere arising from the different increases of (a) ion concentration and (b) electric potential in the EDL of different substrates (AA, black; SS, red; Cu, blue). (The ion concentration in the bulk solution was 1.6 × 10−5 mol/L, electrolyte type 1:1.) D
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Figure 7. SEM images of the PTFE NP films formed on the AA substrate using colloidal solutions of different mass fractions: (a) 0.6, (b) 1, (c) 2, and (d) 3. The inset images shown in (c and d) show the edges of the NP films.
on the substrate. In contrast with traditional electrostatic selfassembly, the process of self-assembly in this approach occurred in a near-wall suspended layer, which gave the particles more degrees of freedom and made the self-assembly more effective. The above analysis was based on the condition that the surface potential was sufficiently large to make the interaction between NPs and substrate strongly repulsive, such as the AA substrate. However, if the surface potential was not large enough, two outcomes were possible. The first case can be illustrated using the Cu substrate, whose surface potential was very small. When the particles were sedimenting, they could cross the repulsive barrier easily under the action of gravity and the pressure from the particles above. The increase of ion concentration near the Cu substrate (Figure 6a) was not sufficient to thin the EDL of NPs to the degree necessary for aggregation. In this case, the particles were deposited on the substrate surface randomly and were unable to form a suspended layer. The second case can be illustrated using the SS substrate, whose surface potential was intermediate, not large enough but also not very small. As the particles were sedimenting, they initially stopped at a position in the substrate’s EDL. As time went on, more particles became trapped in the suspended layer. Before the suspended layer was completed, however, the repulsive barrier was overcome by the
combined influence of gravity on the particles in the suspended layer and the pressure from the particles above. The incomplete suspended layer fell to the substrate surface, and the particles self-assembled during the descent process because of the rapid increase in the ion concentration and the electric potential (Figure 6, panels a and b). This was why a partially selfassembled monolayer was observed on the surface of the SS substrate. 4.2. Effects of Particle Concentration and Deposition Time. As well as a sufficient surface potential, the formation of a suspended layer also requires a sufficient number of particles deposited to the equilibrium position in a certain deposition time. The particle concentration and deposition time were therefore also critical factors influencing the self-assembly results. By increasing the particle concentration, the suspended layer could be made to form, complete, and descend more quickly. After the suspended layer descended to the substrate, the EDL of the substrate changed because the PTFE NP monolayer effectively became the new substrate. Because the surface potential of PTFE was much lower than that of the AA substrate, the strong, repulsive barrier could not exist. The particles above could therefore pass through the weakly repulsive barrier to become deposited on the monolayer. The E
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deposited monolayer therefore acted effectively as a template; the particles in the solution above sedimented downward and self-assembled on this template,44 resulting in a multilayered, close-packed NP film. In contrast, when the particle concentration was decreased, the NPs that gathered at the equilibrium position during the deposition were not packed densely enough to form a suspended monolayer and thus could not descend onto the substrate. Figure 7 shows that at mass fractions of 0.6 and 1 wt %, few particles were deposited on the AA surface; the lower the mass fraction, the fewer particles were deposited. At mass fractions of 2 and 3 wt %, two layers and four layers were formed, respectively. The inset images in Figure 7 (panels c and d) show the edges of the NP films, from which the number of layers could be counted. Changing the deposition time produced similar results (Figure 8). With short deposition times of 20 (Figure 8a)
Figure 9. Small angle X-ray diffraction patterns of the PTFE NP films with different layers on the AA substrate.
deposition time, monolayer and multilayer close-packed NP films could be fabricated successfully. In addition, the particle concentration and deposition time were relevant parameters; the combination of these two factors was the key to controlling the number of layers. The self-assembly mechanism of this approach to form a monolayer or multilayer is hard to be analyzed quantitatively as the colloidal system is very complex. The high surface potentials of substrates and low ion concentration of solution make the DLVO theory invalid to provide an analytical solution for this system.45,46 The so-called Derjaguin approximations and linear-superposition approximations (LSA) become less accurate at small ka and kh (k is the inverse Debye length, a is the NP radius, and h is the distance between NPs), where the EDL thickness, particle size, and particle separation are all of similar magnitude, which is especially relevant to charged NPs.26 Therefore, it is very hard to calculate the exact repulsive forces between NPs themselves and between a NP and the substrate. The above qualitative analysis on the self-assembly mechanism and the experiment results are mainly based on EDL theory. In addition, there are also some other theories helpful to explain the results. For example, in the studies of nonequilibrium phenomena, exclusion zones47,48 were found near various substrate surfaces under colloidal solutions owing to long-range repulsion. Particles were gathered at the edge of the exclusion zone, which was similar to the formation of the suspended layer. It is also believed that in systems of likecharged colloidal particles, pairwise interactions are all repulsive. However, recent work46,49,50 has shown that threebody interactions between like-charged particles can be attractive in systems where ka ∼1, which is conducive to explain the self-assembly of NPs in the suspended layer. The theories of nonequilibrium phenomena and three-body interactions significantly supplement the EDL theory and can explain the self-assembly results and mechanism to a certain extent, but both of them are also imperfect and difficult to understand. In the future study, these two theories will be considered and combined with the EDL theory in order to give more precise and quantitative analysis of the self-assembly method proposed in this paper.
Figure 8. SEM images of the PTFE NP films formed on the AA substrate using a 1.5 wt % colloidal solution and different deposition times of (a) 20, (b) 30, (c) 120, and (d) 240 min. The inset images in (c and d) show the edges of the NP films.
and 30 (Figure 8b) min, few particles were deposited on the AA surface. This was because the decreased deposition time did not allow sufficient time for the suspended layer to form. When the deposition time was increased to 120 (Figure 8c) and 240 (Figure 8d) min, three layers were formed, which can be seen from the inset images in Figure 8 (panels c and d). Since a monolayer could be formed at a particle concentration of 1.5 wt % and within 60 min, at deposition times longer than 60 min the particles in the solution sedimented and self-assembled based on the newly formed template (the first NP layer), which resulted in multilayered, close-packed NP films. Small-angle Xray diffraction patterns of the PTFE NP films with different layers (Figure 9) show similar short-range regularity and longrange irregularity. In accordance with the results shown in Figures 7 and 8, it can be concluded that if the repulsive barrier was large enough, and if the NP concentration was high enough to form a monolayer at the equilibrium position during a certain
5. CONCLUSIONS In summary, we have demonstrated an electrostatic selfassembly approach for the manufacture of PTFE NP films using F
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repulsive interactions. Large-area, close-packed, unordered monolayers and multilayers of PTFE NPs were obtained on an aluminum alloy substrate. Experimental parameters, including the surface potential, particle concentration, and deposition time were varied, and it was shown that these factors had a great effect on the quality and number of layers of the NP films. The feasibility of this NP self-assembly technique using repulsive electrostatic interactions expands the material options and range of applications for traditional electrostatic selfassembly and also provides new routes for the large-area fabrication of monolayer and multilayer close-packed NP films. This study enriches the NP self-assembly research field and has potential for industrial applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Tel: 8610-62796458. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 51375253, 51075228, and 51021064)
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