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Layer-by-Layer Growth of Attractive Binary Colloidal Particles Kwan Wee Tan,‡ Guang Li,‡ Yaw Koon Koh,†,‡ Qingfeng Yan,†,‡ and C. C. Wong*,†,‡ Singapore-MIT Alliance, N3.2-01-36, 65 Nanyang DriVe, Singapore 637460, and School of Materials Science and Engineering, Nanyang Technological UniVersity, N4.1 Nanyang AVenue, Singapore 639798 ReceiVed March 24, 2008. ReVised Manuscript ReceiVed May 30, 2008 We investigate the two-dimensional (2D) colloidal structures formed by oppositely charged polystyrene monolayers grown layer-by-layer, where the electrostatic forces are recruited to assist in the packing of the layers. Our results show a transition through several 2D-superlattices to more close-packed structures with increasing ionic strength. The observed geometrical packing constraints of the 2D-superlattice structures agree well with the estimated Debye screening length of the electric double layer. By tuning interaction forces between charged colloids, electrostatic interactions could enhance the template-directed self-assembly process to achieve more complex and diverse structures.
1. Introduction Colloidal self-assembly offers a bottom-up approach to form ordered two-dimensional (2D) and three-dimensional (3D) crystalline structures. To date, the majority of the studies on colloidal self-assembly have been focused on colloidal particles with purely repulsive electrostatic interactions due the required stability of the colloidal suspensions. In such cases, the particles can be approximated as weakly interacting hard sphere particles and the structures formed in monodispersed colloidal systems are mostly limited to close-packed lattices, such as the facecentered cubic structure.1–3 Only three equilibrium binary phases, LS, LS2, and LS13, of large (L) and small (S) hard spheres have been located by computer simulations and experiments.4–15 However, more complex and diverse structures have been predicted by introducing weak reversible attractions between different colloidal species.16,17 Such ionic colloidal crystals (ICCs) are stabilized by attractive electrostatic interactions, analogous to those in atomic ionic materials.18,19 With well-defined symmetries of defect-tolerable photonic band gaps, binary ICCs * To whom correspondence should be addressed. E-mail: wongcc@ ntu.edu.sg. † Singapore-MIT Alliance. ‡ Nanyang Technological University.
(1) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (2) Koh, Y. K.; Wong, C. C. Langmuir 2006, 22, 897. (3) Teh, L. K.; Tan, N. K.; Wong, C. C.; Li, S. Appl. Phys. A: Mater. Sci. Process. 2005, 81 (7), 1399. (4) Hachisu, S.; Yoshimura, S. Nature 1980, 283, 188. (5) Yoshimura, S.; Hachisu, S. Prog. Colloid Polym. Sci. 1983, 68, 59. (6) Bartlett, P.; Ottewill, R. H.; Pusey, P. N. J. Chem. Phys. 1990, 93, 1299. (7) Bartlett, P.; Ottewill, R. H.; Pusey, P. N. Phys. ReV. Lett. 1992, 68, 3801. (8) Hunt, N.; Jardine, R.; Bartlett, P. Phys. ReV. E 2000, 62, 900. (9) Eldridge, M. D.; Madden, P. A.; Frenkel, D. Mol. Phys. 1993, 80, 987. (10) Eldridge, M. D.; Madden, P. A.; Frenkel, D. Mol. Phys. 1993, 79, 105. (11) Eldridge, M. D.; Madden, P. A.; Pusey, P. N.; Bartlett, P. Mol. Phys. 1995, 84, 395. (12) Underwood, S. M.; van Megen, W.; Pusey, P. N. Physica A 1995, 221, 438. (13) Schofield, A. B. Phys. ReV. E 2001, 64, 051403. (14) Kranendonk, W. G. T.; Frenkel, D. J. Phys.: Condens. Matter 1989, 1, 7735. (15) Radin, C.; Sadun, L. Phys. ReV. Lett. 2005, 94, 015502. (16) Tkachenko, A. V. Phys. ReV. Lett. 2002, 89, 148303. (17) Bartlett, P.; Campbell, A. I. Phys. ReV. Lett. 2005, 95, 128302. (18) Leunissen, M. E.; Christova, C. G.; Hynninen, A.-P.; Royall, C. P.; Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A. Nature 2005, 437, 235. (19) Maskaly, G. R.; Garcia, R. E.; Carter, W. C.; Chiang, Y.-M. Phys. ReV. E 2006, 73, 011402. (20) Hynninen, A.-P.; Thijssen, J. H. J.; Vermolen, E. C. M.; Dijkstra, M.; van Blaaderen, A. Nat. Mater. 2007, 6, 202.
offer great technological potential for photonic applications.20,21 These unique colloidal crystals may also be used in biological, microelectronic, and catalytic applications.21 The layer-by-layer (LbL) growth process of crystallization has been explored recently to fabricate binary colloidal crystals.22–26 In this process, a monolayer of L-colloids is first grown to guide the S-particles which deposit subsequently over the L-colloidal template. These steps can then be repeated to grow a 3D structure of the desired thickness. The deviation from close packing is due to the template effect combined with size difference between alternate layers. By controlling the drying rate, van Blaaderen et al. fabricated LS, LS2, and LS3 structures consisting of 203 nm and 101 or 110 nm silica spheres by LbL vertical deposition.23 The resulting packing structure is a function of the templating effect of the underlying monolayer, size ratio, and concentration of colloidal suspension. In an alternate method termed “confined convective assembly”, two glass substrates are pressed together with a small gap to produce a confined thin film of colloidal suspension and one of the glass substrates is subsequently lifted up at a constant rate.25 A monolayer of colloidal particles is deposited, and LbL growth is done with multiple passes. With this method, it is possible to form more binary superlattices such as LS2, LS3, LS4, and LS5 structures. In the above-mentioned cases and recent experiments,27–29 repulsive particles are used, which means that the driving force for equilibrium packing is the entropic force. To our knowledge, LbL crystallization of oppositely charged colloids has not been systematically studied. It is possible to introduce an additional level of control by making use of attractive electrostatic forces in binary particles systems. In the LbL case, the underlying template can provide an ordered potential landscape which can aid in improving the order of the next layer.22 However, a problem with using attractive electrostatic forces is the possibility of irreversible aggregation, resulting in disordered structures. Thus, the repulsive forces between the like-charged particles (21) Nelson, E. C.; Braun, P. V. Science 2007, 318, 924. (22) Wang, D.; Mo¨hwald, H. J. Mater. Chem. 2004, 14, 459. (23) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; van Blaaderen, A. Science 2002, 296, 106. (24) Wang, D.; Mo¨hwald, H. AdV. Mater. 2004, 16, 244. (25) Kim, M. H.; Im, S. H.; Park, O. O. AdV. Mater. 2005, 17, 2501. (26) Zhou, Z.; Yan, Q.; Li, Q.; Zhao, X. S. Langmuir 2007, 23, 1473. (27) Rugge, A.; Tolbert, S. H. Langmuir 2002, 18, 7057. (28) Kitaev, V.; Ozin, G. A. AdV. Mater. 2003, 15, 75. (29) Mukhopadhyay, R.; Al-Hanbali, O.; Pillai, S.; Hemmersam, A. G.; Meyer, R. L.; Hunter, A. C.; Rutt, K. J.; Besenbacher, F.; Moghimi, S. M.; Kingshott, P. J. Am. Chem. Soc. 2007, 129, 13390.
10.1021/la8009089 CCC: $40.75 2008 American Chemical Society Published on Web 07/24/2008
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Figure 1. SEM micrographs of S-colloids deposited on the L-colloidal monolayer template by FCVD with (a) Igepal CO-720 and (b) DTAB. Both specimens are suspended in deionized water solvent with no addition of KCl electrolyte. Igepal CO-720 does not dissociate into free ions and is more suitable for LbL growth of attractive binary colloids. Table 1. Experimental Parameters of LbL Binary Colloidal Films Fabrication bottom layer S/N 1 2 3 4 5 6 7 8
top layer
d of PS (nm)
volume fraction (φ, %)
surfactant (conc/µM)
d of PS (nm)
volume fraction (φ, %)
550
0.50
Igepal CO-720 (70.3)
250
0.50
surfactant conc (µM)
KCl conc (µM)
Igepal CO-720 (70.3) 10 1000 0.05 DTAB (324) 10 1000
become extremely important in stabilizing the system before the desired deposition. This force is affected significantly by the ionic strength of the suspension. In our study, we grow bilayer structures with a positively charged S-polystyrene (PS) on top of a negatively charged L-PS monolayer. The effect of ionic strength is examined, and we observe a difference in the final order as a function of the ionic strength which will be discussed in detail.
2. Experimental Section Polystyrene Colloids. Polystyrene (PS) colloidal spheres with diameters 550 and 250 nm were synthesized using the emulsifierfree emulsion polymerization method.30 In order to obtain particles with both positive and negative surface charges, amindine- and sulfate-type initiators were used, respectively. The zeta potential of the particles was measured with a Brookhaven ZetaPlus zeta potential analyzer. The negatively charged 550 nm L-PS colloids have a zeta potential ζL ) -41.87 mV, and the 250 nm S-PS are positively charged with zeta potential ζS ) +80.98 mV. The PS spheres were then dispersed in deionized water to form suspensions of specific volume fractions detailed in Table 1. Fabrication of Negatively Charged L-Monolayer. The first monolayer of negatively charged L-colloids was grown on top of borosilicate glass microslides (Menzel-Glazer) at 35 °C using the flow-controlled vertical deposition (FCVD) method as described elsewhere.31 The glass microslides were cleaned using acetone and piranha solution and then rinsed profusely with deionized water before use. Nonionic surfactant, polyoxyethylene nonylphenol (Igepal CO-720, Sigma-Aldrich), was added to reduce the surface tension of the PS suspension.32–34 The pumping flow velocity was set at (30) Shim, S.-E.; Cha, Y.-J.; Byun, J.-M.; Choe, S. J. Appl. Polym. Sci. 1999, 71, 2259. (31) Zhou, Z.; Zhao, X. S. Langmuir 2004, 20, 1524. (32) Marquez, M.; Grady, B. P. Langmuir 2004, 20, 10998. (33) Zhou, Z.; Li, Q.; Zhao, X. S. Langmuir 2006, 22, 3692.
0.5794 µm/s for the L-PS suspension with volume fraction φ ) 0.5%. Fabrication of Binary PS Colloidal Film. For samples 1-4, the S-PS colloids were diluted with deionized water to volume fraction φ ) 0.5%. However, in samples 5-8, the volume fraction of the 250 nm PS suspension was φ ) 0.05% to achieve a 1:1 number ratio with respect to the 550 nm colloids.19 Dodecyltrimethylammonium bromide (DTAB, Acros Organics) and Igepal CO-720 were added to reduce the surface tension of the suspension.32–34 Potassium chloride (KCl, Fisher Scientifiic) was added as the electrolyte to adjust the ionic strength in the suspension. The second layer of 250 nm S-PS colloids was grown using the same technique on top of the previously formed L-monolayer to form a bilayer of oppositely charged colloidal films. For all samples, the pumping flow velocity was manipulated to obtain a monolayer. The final morphologies of the colloidal films were imaged with a JEOL JSM 6340 field-emission scanning electron microscope (SEM).
3. Results and Discussion Effect of Surfactant. Surfactants play a role in the ordering of LbL structures. The surfactant is needed to reduce the water surface tension and improve the confinement effect during the assembly process to achieve monolayer deposition. As shown in Figure 1, we observe some indication of ordered LS2 structures with the use of nonionic Igepal CO-720 [(C2H4O)n · C15H24O, n ) 10.5-12]. However, aggregation and disorder dominate the specimens with the addition of cationic surfactant DTAB [CH3(CH2)11N(CH3)3Br]. DTAB dissociates into long chains of DTA+ and Br- ions in water. Br- ions are attracted to the positively charged S-particles and act as counterions, thus making electrostatic repulsion between the like-charged particles more short-ranged. This results (34) Yan, Q.; Teh, L. K.; Shao, Q.; Wong, C. C.; Chiang, Y.-M. Langmuir 2008, 24, 1796.
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Figure 2. Positive-charged S-PS monolayer pattern is grown on negative-charged L-monolayer in (a) deionized, (b) low ionic, and (d) high ionic conditions with Igepal CO-720 and (f) high ionic conditions with DTAB. Panels (c) and (e) show the magnified views of the LS2 and more close-packed LSn structures, respectively. In general, order is vastly improved with the addition of small amounts of KCl.
in irreversible aggregation and disordered structures. In contrast, the Igepal CO-720 surfactant is nonionic and does not affect the electrostatic repulsion between the particles. The positive particles stay dispersed and can deposit according to the underlying particle template. Ordered structures as observed in Figure 1a are more likely to be formed in this way. We measured the zeta potentials of negatively charged colloidal specimens added with Igepal CO-720 and validated that the nonionic surfactant molecules that could be attached on the colloid surfaces do not affect the charge properties (see the Supporting Information). This highlights that the choice of surfactant is important to achieve ordered structure by the LbL method between oppositely charged particles. Ideally, the surfactant added should affect only the surface tension of water to improve wettability but not the interaction between the particles.32–34 It should also be noted that the addition of the surfactant to lower the surface
tension is important to prevent film peeling and retain the integrity of the L-monolayer template as shown in our previous work.34 Effect of Capillary Forces. Capillary forces are 2D forces acting in the plane of the meniscus boundary, meaning that they are highly directional as compared to the interaction forces between the particles. Past literature reported that colloidal selfassembly at liquid menisci is driven solely by capillary forces using vertical deposition techniques.31,35 Kralchevsky and Nagayama36 studied the capillary force acting between colloidal particles and found that, for floating particles in a wetting fluid, lateral capillary forces are negligible for particles smaller than (35) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (36) Kralchevsky, P. A.; Nagayama, K. Particles at fluids interfaces and membranes: attachment of colloid particles and proteins to interfaces and formation of two-dimensional arrays; Elsevier: Amsterdam, 2001.
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10 µm. As observed in our recent work,37 particles with a Peclet number Pe < 1 remain suspended in the solvent and capillary forces cannot overcome the repulsive potential between similarly charged particles, meaning that interaction forces provide the primary driving force for self-assembly. For the 250 nm positive particles used in the experiments, the Peclet number is ∼10-3. Thus, in the considerations of novel structures such as ionic colloidal crystals, the effect of capillary forces is normally ignored. Effect of Ionic Strength. In an electrostatically repulsive system of like-charged colloids, the amount of surface charge and the ionic strength of the suspension will affect the equilibrium colloidal structure suspended in the fluid phase.27,38 In general, body-centered cubic crystals are favored due to long-range interactions in dilute suspensions and face-centered cubic crystals are more predominant in concentrated suspensions. When an electrolyte such as KCl is added into the suspension, the dissociated K+ and Cl- will attach onto the highly charged colloid surfaces as counterions and reduce the effective charges. Thus, the interparticle interactions become more short-ranged and favor the formation of close-packed structures. The structural evolution of monodispersed colloidal crystals as a function of increasing ionic strength has been reported by Bevan et al.39 The effect of ionic strength in the LbL growth of binary ionic colloidal structures is investigated by adding KCl (1:1 electrolyte) to change the ionic strength. Binary films were grown under three main conditions which we refer to as deionized (0 µM KCl), low ionic strength, (10 µM KCl) and high ionic strength (1000 µM KCl). As mentioned in the earlier section, we observe some resemblance of LS2 2D-superlattice structure ordering on top of the L-monolayer template under deionized conditions as shown in Figure 2a. This phenomenon could be attributed to the effects of the size ratio between the two different types of particles and template effect as observed in many binary systems.22–29 When KCl is added to increase the ionic strength in the suspension, ordering is vastly improved. Figure 2b shows the dominance of large spatial areas of high quality LS2 2D-superlattice structures. This provides the first direct evidence of the effect of ionic strength in attractive binary colloidal films. When ionic strength is further increased by 2 orders of magnitude (high ionic strength), more closely packed structures such LS3 and LS4 are preferentially formed as depicted in Figure 2d. The effect of ionic strength becomes clearer when the Debye screening length and Yukawa interactions19 are calculated for the suspensions. In polar solvents such as water, charged colloidal particles are surrounded by an electric diffuse double layer of counterions. From Gouy-Chapman theory,40 the thickness of the diffuse layer can be approximated by the Debye screening length, κ-1, which is given as
κ-1 )
(
2
e ε0εrkBT
)
∑ zi2ci/
-1⁄2
It is a key parameter that measures how the combination of valency z, ion concentration c/i , and solvent dielectric constant εr contribute to the screening of interactions between charges in solution at temperature T. The elementary electron charge and Boltzman’s constant are denoted as e and kB, respectively. The (37) Koh, Y. K.; Yip, C. H.; Chiang, Y.-M.; Wong, C. C. Langmuir 2008, 24, 5245. (38) Sirota, E. B.; Ou-Yang, H. D.; Sinha, S. K.; Chaikin, P. M.; Axe, J. D.; Fujii, Y. Phys. ReV. Lett. 1989, 62, 1524. (39) Bevan, M. A.; Lewis, J. A.; Braun, P. V.; Wiltzius, P. Langmuir 2004, 20, 7045. (40) Hunter, R. J.; White, L. R. Foundations of colloid science; Clarendon Press, Oxford University Press: Oxford [Oxfordshire], New York, 1987.
Figure 3. (a) Yukawa energy calculations for interparticle interactions for repulsive S-S and attractive L-S interactions. The (i and iv) green lines represent deionized conditions; (ii and v) blue lines represent low ionic conditions; and (iii and vi) red lines represent high ionic conditions. The interparticle separation distance, r, is measured from the surface of each particle. (b) Schematic illustration of the range of L-S attraction and S-S repulsion at (i) low ionic strength and (ii) high ionic strength. The Debye length and range of electrostatic interactions decreases with increasing ionic strength in the suspension.
equation shows that, at low ionic strength, the thickness of the double layer is larger. Also, the Debye screening length of the oppositely charged L- and S-PS colloids is independent of particle size and thus identical for the same ionic strength condition. At 35 °C, the Debye length is 865.584 nm for deionized water19 and 98.197 and 9.869 nm for low ionic and high ionic strength, respectively. In oppositely charged binary systems, the two different particles will experience strong electrostatic attraction and result in heterocoagulation or rapid formation of randomly shaped conglomerates of macroscopic size which sediment quickly as shown in the Supporting Information. Heterocoagulation is a problem in realizing ICCs, as it typically prevents the particles from having enough mobility to rearrange themselves to form the equilibrium ICC structure. In LbL growth, heterocoagulation is prevented by the sequential growth of alternate layers of
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Figure 4. (a) SEM micrograph of positive S-colloid monolayer on negative glass in deionized condition and (b) the high magnified image.
oppositely charged particles; that is, opposite charges are never in suspension together. Zeta potential measurements have affirmed that the dried L-monolayer template retains similar (if not identical) negative charge conditions upon rehydration in the fluid medium for the sequential growth of the positive PS colloid layer. Rehydration restores the charges of the SO4- ligands remaining on the polystyrene sphere surface, enabling the monolayer to be negatively charged. During the deposition of the positively charged S-colloids over the negative L-monolayer, two different pair electrostatic interactions are present in the suspension: S-S repulsion and L-S attraction. Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations based on Yukawa-type potential19 in Figure 3a show that repulsion between positive S-colloids decreases significantly with every 10-fold decrease of the Debye screening length as the ionic strength in the suspension increases. Hence, the like-charged S-PS colloids will be able to approach one another more closely. On the other hand, ionic strength affects the proximity at which a positive S-colloid begins to feel the attractive forces of the negative L-template (a closer distance for a higher KCl concentration). Electrostatic forces also play a key role in granting mobility to the colloids in the assembly phase.19,28 We postulate the range of the electrostatic interactions is the controlling factor that determines the final 2D-superlattice structures. Under deionized conditions, the Debye screening length of the oppositely charged L- and S-colloids is at its maximum. Positive S-colloids are electrostatically attracted to the interstitial sites of the negative L-template which exert the strongest pull on the oppositely charged particles and could explain for the appearance of the LS2 structure over certain spatial areas of the template. Due to the long-range nature of the electrostatic interactions, the S-particles are pulled onto the template before ordering among the S-particles can take place. This undermines the opportunity of the S-particles to reorder themselves above the template. This is most evidently shown in Figure 4 where the uniform potential landscape of the negative borosilicate glass pulls the positive S-colloids down indiscriminately to form a disordered layer. The lateral mobility of the adsorbed particles is relatively low due to the strong adhesion to the substrate. Thus the L-template imposes a screening effect over the negatively charged glass substrate and induces electrostatic order for oppositely charged colloidal systems. We found that the optimum condition to form a well-ordered LS2-superlattice structure is when the ionic strength of the suspension is low (10 µM KCl). As shown in Figure 3a, the range of the L-S and S-S electrostatic interactions is suppressed. The in-plane positive S-particles are able to approach one another
closer and self-organize into an ordered arrangement. The shorterrange nature of the L-S attraction also implies the S-colloids have additional time to arrange before settling onto the template. The attractive pull on the positive S-colloids is lower as compared to the deionized condition and provides the particles extra kinetic mobility to configure into a stable in-plane colloidal phase. Finally, when the well-ordered S-particles gets closer, the attractive interaction forces increase and pull the particles into the interstitial sites of the template. The observed geometrical packing constraints of the 2D-superlattice structures agree well with the estimated Debye screening length of the electric double layer. Thus, the formation of a LS2superlattice is effectively enhanced by the Debye screening factor in low ionic strength conditions. When the electrolyte concentration is further increased to 1000 µM, the electrostatic interaction range of the positive S-particles is further reduced and more close-packed structures of LS3- and LS4-superlattices are formed (Figure 2). However, we found it is harder to control the formation of LSn structures in high ionic strength conditions. This is consistent with what has been reported for structures of like-charged colloidal crystal monolayers as a function of electrolyte concentration. The size of the ordered spatial areas decreases exponentially with increasing ionic strength.41 At extremely high ionic strength, colloid particles can become unstable and agglomerate into small aggregates. These larger colloidal aggregates lose mobility to rearrange and settle onto the attractive template as a colloidal glassy layer. Thus, low ionic strength is preferred in ICC binary systems to keep the screening ratio19 small and allow sufficient mobility between the oppositely charged colloids to reconfigure. Stronger repulsive interactions between like-charged colloids improve the mobility of the particles, and a lower energy is needed for coordinated reconfiguration to achieve the equilibrium structures.
4. Conclusion We have demonstrated the role of ionic strength in LbL growth of attractive binary colloids to fabricate large spatial areas of LS2 for an optimum KCl concentration of 10 µM. We have also found that colloids possess maximum mobility to configure into ordered 2D-superlattices in low ionic conditions compared to deionized conditions. With a further increase of electrolyte concentration, mobility falls and more closed-packed structures and even possible disorder are expected. (41) Ro¨dner, S. C.; Wedin, P.; Bergstro¨m, L. Langmuir 2002, 18, 9327.
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Both lower- and higher-density binary structures are present in the low and high ionic strength conditions. We attribute these observations to particle mobility due to the local effects of temperature, ionic strength variation, and concentration gradient in certain regions, and further optimization is needed to enhance the ordering of attractive binary colloidal structures. Nonetheless, this is one key step in understanding the influence of electrostatic attractions in templated self-assembly and could prove beneficial in designing new alternative lattices and complex structures.
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Acknowledgment. The authors thank the Singapore-MIT Alliance for financial support. Supporting Information Available: Zeta potential measurements looking into the effects of nonionic surfactant on the charge conditions of the L-PS colloids and images of heterocoagulation of oppositely charged binary colloids. This material is available free of charge via the Internet at http://pubs.acs.org. LA8009089
