pubs.acs.org/Langmuir © 2010 American Chemical Society
Copper-Ion-Assisted Self-Assembly of Silicate Clays in Rod- and Disklike Morphologies Wei-Cheng Tsai‡ and Jiang-Jen Lin*,†,‡ †
‡
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, and Department of Materials Science and Technology, National Chung Hsing University, Taichung 40227, Taiwan Received January 22, 2010. Revised Manuscript Received March 5, 2010
Self-assembled rodlike (0.3-2.5 μm in diameter and 5.3-31 μm in length) and disklike microstructures (1.8-10.6 μm in width and 0.1-1.0 μm in thickness) are uniquely present in amorphous clay aggregates. Clay units were prepared by intercalation of Naþ-montmorillonite (Naþ-MMT) with copper ions (Cu2þ) and poly(oxypropylene)-amine salt (POP) in simultaneous or stepwise ionic exchange reactions. Differences in process control during incorporation of Cu2þ and hydrophobic POP greatly affected the layer structure of the clay units (d spacing of 12-53 A˚) and consequently their amphiphilic dispersion properties. By controlling the dispersion in water and drying at 80 °C, highly ordered selfassembly structures were obtained, presumably as a result of self-piling of clay units in competing vertical and horizontal directions. In general, association with Cu2þ yielded units with a disklike microstructure, in contrast to the rod-like structure obtained for POP-intercalated clay. The self-assembled structures were characterized using X-ray diffraction, UV adsorption, thermal gravimetric analysis, zeta potential, scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques. Control of the clay self-piling process provides a new synthetic route for the fabrication of bottom-up microstructures that are potentially useful for templates, sensors, and electronic devices.
Introduction Processes for the fabrication of ordered self-assembled structures from nanometer to micrometer scale are fundamental for the bottom-up method used in nanotechnology applications.1-3 Diverse geometric shapes and structures such as helical fibrils,4 wormlike,5 fibrous,6 bowl-shaped,7 ringlike,8 and bundle arrays9 have been widely reported. Polymer-inorganic composites can give rise to tubular,10 fibrous,11 rod,12 lamellar,13 triangular,14 *Corresponding author. Tel.: þ886-2-3366-5312. Fax: þ886-2-8369-1384. E-mail:
[email protected]. (1) Baker, W. O. Science 1981, 211, 359. (2) Usuki, A.; Hasegawa, N.; Kadoura, H.; Okamoto, T. Nano Lett. 2000, 290, 1536. (3) Glotzer, S. C. Science 2004, 306, 419. (4) (a) Wang, M.; Yang, Y. L.; Deng, K.; Wang, C. J. Phys. Chem. C 2007, 111, 6194. (b) Nagarkar, R. P.; Hule, R. A.; Pochan, D. J.; Schneider, J. P. J. Am. Chem. Soc. 2008, 130, 4466. (5) (a) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, Y. P. Science 2004, 306, 98. (b) Ma, J. W.; Li, X.; Tang, P.; Yang, Y. J. Phys. Chem. B 2007, 111, 1552. (6) (a) Ma, M.; Krikorian, V.; Yu, J. H.; Thomas, E. L.; Rutledge, G. C. Nano Lett. 2006, 6, 2969. (b) Vandermeulen, G. W. M.; Kim, K. T.; Wang, Z.; Manners, I. Biomacromolecules 2006, 7, 1005. (7) (a) Schenning, A. P. H. J.; Escuder, B.; Nunen, J. L. M.; Bruin, B.; L€owik, D. W. P. M.; Rowan, A. E.; Gaast, S. J.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 2001, 66, 1538. (b) Liu, X.; Kim, J.-S.; Wu, J.; Eisenberg, A. Macromolecules 2005, 38, 6749. (8) (a) Chen, Z.; Cui, H.; Hales, K.; Li, Z.; Qi, K.; Pochan, D. J.; Wooley, K. L. J. Am. Chem. Soc. 2005, 127, 8592. (b) Jeremic, A.; Quinn, A. S.; Cho, W. J.; Taatjes, D. J.; Jena, B. P. J. Am. Chem. Soc. 2006, 128, 26. (9) Lin, J. J.; Tsai, W. C.; Wang, C. H. Langmuir 2007, 23, 4108. (10) (a) Yang, Y.; Suzuki, M.; Owa, S.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2007, 129, 581. (b) Zhou, Y.; Shimizu, T. Chem. Mater. 2008, 60, 325. (11) (a) Lee, G. S.; Lee, Y. J.; Choi, S. Y.; Park, Y. S.; Yoon, K. B. J. Am. Chem. Soc. 2000, 122, 12151. (b) Wang, T.; Reinecke, A.; C€olfen, H. Langmuir 2006, 22, 8986. (12) (a) Rajh, T.; Thurnauer, M. C.; Thiyagarajan, P.; Tiede, D. M. J. Phys. Chem. B 1999, 103, 2172. (b) Yang, Z.; Xia, Y.; Zhu, Y.; Mokaya, R. Chem. Mater. 2007, 19, 6317. (13) (a) Hussein, M. Z.; Long, C. W. Mater. Chem. Phys. 2004, 85, 427. (b) Zhong, Y. W.; Matsuo, Y.; Nakamura, E. J. Am. Chem. Soc. 2007, 129, 3052. (14) (a) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (b) Kewalramani, S.; Kmetko, J.; Dommett, G.; Kim, K.; Evmenenko, G.; Mo, H.; Dutta, P. Thin Solid Films 2007, 515, 5627.
Langmuir 2010, 26(12), 10177–10182
and micellar15 particles. However, few examples of the selfassembly of sheetlike clays have been reported in the literature, perhaps because of handling difficulties attributable to their polydisperse geometric shapes and size inhomogeneity. Common silicate clays are composed of multiple layers of thin platelets (ca. 1 nm) with the lateral dimension ranging from ca. 50 nm to several micrometers.16,17 Beause of the ionic charges on the plate surface, which has a high aspect ratio, silicate clays can form ordered orientations and exhibit liquid crystal behavior.18 Besides the platelike geometric shape, the surface electric charges are crucial for noncovalent bonding and ionic interactions. We previously reported that ionic exchange reactions of poly(oxyalkylene)-diamine hydrogen chloride (POP) with montmorillonite expanded the basal spacing and altered the hydrophobic properties.19-21 Incorporation of hydrophobic POP rendered the clay strong tendency for forming ordered self-assemblies.22-24 The forming process may involve two different noncovalent bonding forces including the POP hydrophobic aggregation and the silicate ionic interaction in separate directions. The presence of ionic charges on the clay surface means that charge attraction is probably an important factor, besides the high aspect ratio, that contributes to the formation of rodlike particles.25,26 Experiments (15) (a) Frankamp, B. L.; Uzun, O.; IIhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892. (b) Kim, B.-S.; Qiu, J.-M.; Wang, J.-P.; Taton, T. A. Nano Lett. 2005, 5, 1987. (16) Murray, H. H. Appl. Clay Sci. 2000, 17, 207. (17) Cadene, A.; Durand-Vidal, S.; Turqa, P.; Brendle, J. J. Colloid Interface Sci. 2005, 285, 719. (18) Gabriel, J. P.; Davidson, P. Adv. Mater. 2000, 12, 9. (19) Lin, J. J.; Chen, Y. M. Langmuir 2004, 20, 4261. (20) Lin, J. J.; Chen, I. J.; Chou, C. C. Macromol. Rapid Commun. 2003, 24, 492. (21) Chou, C. C.; Shieu, F. S.; Lin, J. J. Macromolecules 2003, 36, 2187. (22) Lin, J. J.; Chou, C. C.; Lin, J. L. Macromol. Rapid Commun. 2004, 25, 1109. (23) Lin, J. J.; Chen, Y. M.; Tsai, W. C.; Chiu, C. W. J. Phys. Chem. C 2008, 112, 9637. (24) Chiu, C. W.; Chu, C. C.; Dai, S. A.; Lin, J. J. J. Phys. Chem. C 2008, 112, 17940. (25) Lin, J. J.; Chu, C. C.; Chou, C. C.; Shieu, F. S. Adv. Mater. 2005, 3, 301. (26) Lin, J. J.; Chu, C. C.; Chiang, M. L.; Tsai, W. C. Adv. Mater. 2006, 18, 3248.
Published on Web 03/17/2010
DOI: 10.1021/la100313j
10177
Article
Tsai and Lin
Figure 1. Chemical structure of hydrophobic POP-amine used as intercalating agent.
revealing that the self-assemblies are transformable under electron beam bombardment25 and modifiable for the creation of Lotus surfaces26 demonstrated that noncovalent bonding interactions are involved. In the present study, we investigated the effect of Cu2þ ions on clay self-assembly. When Cu2þ competes with hydrophobic POP for incorporation into the layered galleries, the balance between hydrophobic and charge interactions dictates the self-piling direction and hence the morphology. The formation of rodand disklike microstructures is discussed and a mechanism for clay self-piling is proposed.
Experimental Section Materials. Sodium montmorillonite (Naþ-MMT), which is an Naþ type of aluminosilicate clay with a cation exchange capacity (CEC) of 140 mequiv/100 g, was supplied by Nanocor Inc. MMT is a 2:1 layered smectite clay comprising an edge-shared octahedral sheet sandwiched between two tetrahedral sheets, with an average polydisperse dimension of 100 100 1 nm3 for each sheet and approximately 8-10 platelets for one sheet. Copper sulfate (CuSO4, 98%) was obtained from Sigma. Poly(oxypropylene)-diamine with an average molecular weight of 2000 g/mol was obtained from Huntsman; its chemical structure is shown in Figure 1. Preparation of Cu2þ/POP-Intercalated Naþ-MMT.
Three methods for the preparation of Cu2þ/POP-MMT clays were developed. The experimental procedures involved MMT intercalation with Cu2þ alone and simultaneous or stepwise ionic exchange with Cu2þ and POP. In a typical procedure, Naþ-MMT (3.0 g, 4.2 meq) was placed in a 1-L beaker and deionized water (300 mL) was added. The slurry was stirred vigorously using a mechanical stirrer and heated at 80 °C for 3 h. For intercalation of MMT with Cu2þ alone (method 1), CuSO4 (0.07-0.7 g, 0.2-2 CEC) was added to the Naþ-MMT slurry and heated at 80 °C for 3 h. For simultaneous addition of Cu2þ and POP (method 2), POP (8.4 g, 1.0 CEC) acidified with HCl (37% in water, 4.2 mmol) at a ratio of Hþ/NH2 of 1:2 was mixed with CuSO4 under stirring for 1 h, then poured into the swollen Naþ-MMT slurry. For stepwise addition of Cu2þ and POP (method 3), CuSO4 was added to the vessel containing the swollen Naþ-MMT slurry and heated at 80 °C for 3 h under stirring and then POP was added and heated at 80 °C for a further 3 h. After being cooled to ambient temperature, the precipitate was collected and washed thoroughly with deionized water and then EtOH and dried in an oven at 80 °C. Samples were dispersed in water at a concentration of ca. 1 wt % for selfassembly experiments and other analyses. Samples were generally prepared by spreading the 1 wt % solution on a glass plate and drying it in an oven at 80 °C for 4 h. The samples had been prepared at least twice and shown the reproducible morphology observation. Analyses. The Cu2þ concentration in water was determined by measuring the UV absorbance of samples at 796 nm on a Jasco V-530 UV-visible spectrophotometer with comparison to a standard curve. The amount of Cu2þ adsorbed per gram of MMT was calculated from the difference in Cu2þ concentration in water before and after the adsorption process. X-ray diffraction (XRD) patterns were obtained on a powder diffractometer (Shimadzu SD-D1 using a Cu target at 35 kV, 30 mA). The d spacing was calculated according to Bragg’s equation (nλ = 2dsin θ) by fitting a series of θ values to determine the value of d for n = 1. 10178 DOI: 10.1021/la100313j
Thermogravimetric analysis (TGA) patterns were obtained on a Perkin-Elmer Pyris 1 analyzer by heating samples from 100 to 800 at 10 °C/min in air. Scanning electron microscopy (SEM) observations were carried out on a Topcon ABT-150a instrument at an acceleration voltage of 20 kV. A ZetaPlus zetameter (Brookhaven Instruments Corp., NJ) was used to characterize the ionic properties of Naþ-MMT, Cu2þ-MMT, and Cu2þ/POP-MMT. The zeta potential of the samples was measured for aqueous suspensions at a concentration of 0.01 wt %.
Results and Discussion Intercalation of Naþ-MMT with POP and Cu2þ. Pristine Naþ-MMT swells in water, whereas the clay modified with hydrophobic POP was dispersible in toluene. As the amount of POP intercalated in the MMT galleries was increased to 1.0 equiv. of its cation exchange capacity (CEC), it caused the clay to aggregate or precipitate from water. Tailoring of the hydrophobicity was further modified by addition of Cu2þ in a complementary manner. MMT intercalated with only Cu2þ at 1.0-2.0 CEC (method 1) was water dispersible. Simultaneous (method 2) and stepwise (method 3) intercalation of Naþ-MMTs with Cu2þ and POP allowed the incorporation of different ratios of the ionic species. An increase from 0.2 to 0.6 equivalents of Cu2þ to POPMMT (at 1.0 CEC) failed to alter the hydrophobicity but the sample was still dispersible in toluene (Table 1). With a further increase in Cu2þ to 1.0-2.0 equiv. of the MMT ion capacity, the samples were dispersible in water. These results imply that Cu2þ species can exchange with POP-MMT, so that more metal ions are incorporated, which overcome the POP hydrophobicity in the silicate galleries. The approach provides a systematic method for synthesizing clay hybrids with different solvation properties. Effect of Cu2þ on Ion Exchange in MMT for Different Intercalation Methods. XRD of Cu2þ/POP-MMT. Intercalation of Cu2þ in Naþ-MMT is possible because of the large surface area and high CEC of the clay particles, both of which are essential for Cu2þ intercalation. The process is influenced by the intercalation method at different stoichiometric Cu2þ/POP CEC ratios. During intercalation, the sodium counterions in NaþMMT are replaced with Cu2þ and organic amine salts. The d spacing increased from 12.4 A˚ for Naþ-MMT to 13.3 A˚ for Cu2þ/ MMT at a CEC ratio of 2.0 for method 1 because of partial intercalation of Cu2þ and its swelling property in water (Table 1). POP-MMT had a d spacing of 53 A˚. However, on intercalation with Cu2þ/POP via methods 2 and 3, d decreased from 51 to 39 A˚ and from 52 to 42 A˚, respectively, with a gradual decrease in the Cu2þ/POP CEC ratio from 0.2 to 2.0. This result is attributed to the fact that the Cu2þ/POP component of the hybrids contains more Cu2þ and less POP. However, addition at a Cu2þ/POP CEC ratio of 2.0 yielded a d spacing of at least 39 A˚ (method 2), indicating preferential intercalation of POP at this concentration. Thermal Stability of Cu2þ/POP Confined in Silicates. Cu2þ intercalation in the clay interlayer spacing was confirmed by TGA, as shown in Table 1. The decomposition temperature for a weight loss of 5 wt % revealed that Cu2þ/POP-MMT decomposed more rapidly than POP-MMT. In the case of method 2, when the Cu2þ/POP CEC ratio was increased from 0.2 to 2.0, the decomposition temperature decreased from 257 to 201 °C (change of 56 °C); in contrast, in the case of method 3, the decomposition temperature decreased from 255 to 210 °C (change of 45 °C). The results indicate that Cu2þ ions influence the catalytic cracking temperature, resulting in thermal instability. The decomposition temperature for method 2 was lower than that for method 3 at the CEC ratio larger than 1.4, because simultaneous intercalation resulted in greater amounts of Cu2þ in the clay interlayers than Langmuir 2010, 26(12), 10177–10182
Tsai and Lin
Article Table 1. Parameters for Intercalated Cu2þ/POP in MMT Layered Silicates dispersibilitye
CuSO4 intercalation (CEC) CuSO4/POP CEC ratio in MMT
d (A˚)a
wt fractionb
T5 (°C)c
Foundd
Calc.
zeta potential (mV)
H2O
toluene
-
þ
-16 -28
þ þ
-
-23 -29 -31 -33
þ þ þ þ
þ þ -
POP/MMT 0/1
53
67:33
263
0
0
CuSO4-MMT (Method 1) 0.2/0 2/0
12.4 13.3
0.2 2.0
0.11 0.52
Simultaneous Intercalation of CuSO4 and POP (Method 2) 0.2/1 0.6/1 1/1 1.2/1 1.4/1 2/1
51 48 45 44 41 39
58:42 56:44 57:43 57:43 50:50 47:53
257 249 235 230 209 201
0.2 0.6 1.0 1.2 1.4 2.0
0.16 0.30 0.53 0.61 1.01 1.10
Stepwise Intercalation of CuSO4 and POP (Method 3) 0.2/1 52 60:40 255 0.2 0.16 þ 0.6/1 48 55:45 245 0.6 0.33 þ 1/1 45 54:46 236 1.0 0.49 -30 þ 1.2/1 45 55:45 228 1.2 0.60 -34 þ 1.4/1 44 47:53 225 1.4 0.67 -37 þ 2/1 42 46:54 210 2.0 0.72 -40 þ a XRD n = 1 basal spacing calculated according to Bragg’s equation (nλ = 2dsin θ). b Weight ratio of organic/inorganic fractions determined by TGA at 800 °C (in air). c Temperature at which 5 wt % weight loss occurred during TGA (in air). d Calculated from UV measurements. e þ, dispersible; -, aggregation (0.1 g of sample in 1.0 g of solvent).
Figure 2. Amount of Cu2þ intercalated into MMT versus Cu2þ/ POP CEC ratio used for simultaneous or stepwise intercalation.
stepwise intercalation. This result was confirmed by UV absorbance measurements. Intercalation Performance of Simultaneous and Stepwise Methods. The amount of Cu2þ intercalated using three different methods was determined by UV-visible spectrophotometry. As shown in Table 1, the largest amount of Cu2þ intercalated or adsorbed in methods 1-3 was 0.52, 1.10, and 0.72, respectively, at a Cu2þ equivalent of 2.0 CEC (Figure 2). Intercalation of Cu2þ by methods 2 and 3 was preferred over that by method 1 because of the possible aggregation of Cu2þ ions and associated formation of Cu2þ/POP colloids in the interlayer spaces. Furthermore, method 2 was better than method 3 in terms of the preparation procedure. Langmuir 2010, 26(12), 10177–10182
In method 2, Cu2þ/POP forms a specific chemical complex27 of amine/Cu2þ that has high affinity for clay galleries and can thus promote Cu2þ intercalation. Cu2þ species significantly masked the surfactant molecules, facilitating complex entry into the clay galleries, which were overloaded with complex above the average CEC. However, Cu2þ intercalation occurs to a lesser extent in method 3 because only some of the Cu2þ is intercalated (0.52 CEC) in Naþ-MMT at a CEC ratio of 2.0. As a result, some of the active sites on the Naþ-MMT surface are covered by Cu2þ, so Cu2þ/POP complexes can occupy only a few of the remaining interlayer spaces. Zeta Potential of Cu2þ/POP-MMT. To determine the ionic behavior of Cu2þ/POP-MMTs, we measured the zeta potential of various water-soluble samples. As shown in Figure 3, the zeta potential of Cu2þ-MMT at 0.01 wt % decreased from -16 to -28 mV when the Cu2þ intercalation CEC for method 1 was increased from 0.2 to 2.0. However, in the case of method 2, the zeta potential decreased from -23 to -33 mV when the Cu2þ/POP CEC ratio was increased from 1.0 to 2.0. Similarly, in the case of method 3, the zeta potential decreased from -30 to -40 mV. The dependence of zeta potential on Cu2þ can be explained by the large number of newly exposed Cu2þ ions on the surface of the Cu2þ/POP-MMT hybrids. Further, the trend of zeta potentials of method 3 is higher in negative sign than that of other two methods, indicating the Cu2þ ions association mostly around silicates rather than in the clay galleries. The explanation is confirmed by UV absorption and TGA measurements. The difference in Cu2þ exposure or location on MMT surface alters the Cu2þ/POP-MMT self-assembly properties. Surface Morphology of Self-Assembled Hybrids. Simultaneous Intercalation of Cu2þ/POP in MMT. A control (27) Kobayashi, S.; Higashimura, H. Adv. Mater. 2003, 28, 1015.
DOI: 10.1021/la100313j
10179
Article
experiment revealed that pristine Naþ-MMT, Cu2þ-MMT (method 1, 2.0 CEC Cu2þ), and POP-MMT (1.0 CEC POP, basal spacing of 53 A˚ and 67 wt % organic content) did not have a regular particle shape. Because POP has hydrophobic characteristics, it is an inherently amorphous and flexible material. The selfassembly ability of water-soluble Cu2þ/POP-MMT hybrids was investigated. In the case of method 2 at a Cu2þ/POP CEC ratio of 1.4 (basal spacing of 41 A˚ and 50 wt % organic content), short rodlike and lamellar arrays with a diameter of 0.3-2.2 μm and length of 5.3-7.2 μm were formed and a layer structure could be clearly be observed on the surface (Figure 4A, B). In comparison, at a Cu2þ/POP CEC ratio of 2.0 (basal spacing of 39 A˚ and 47 wt.% organic content), directional and ordered rodlike microarrays were formed with a diameter of 0.3-2.5 μm and length
Figure 3. Zeta potentials obtained by intercalation of MMT with only Cu2þ, simultaneous intercalation, and stepwise intercalation with Cu2þ and POP-salt versus CEC of intercalated Cu2þ.
Tsai and Lin
of 8-31 μm (Figure 4C, D). These results imply that a higher Cu2þ amount can promote self-assembly of the hybrids and at a suitable Cu2þ/POP ratio the complementary Cu2þ and hydrophobic POP tend to balance each other in the interlayer, yielding regular agglomerate. Stepwise Intercalation of Cu2þ/POP-MMT. SEM images revealed that the crystalline phases resulting from method 3 were different from those observed for method 2. At a Cu2þ/POP CEC ratio of 1.4 (basal spacing of 44 A˚ and 47 wt % organic content), small disk-like particles with a width of 1.8-2.4 μm and thickness of 0.1-0.4 μm were formed (Figure 5A, B). In comparison, at a Cu2þ/POP CEC ratio of 2 (basal spacing of 42 A˚ and 46 wt % organic content), large and directional disklike particles were formed (Figure 5C, D), for which the width and thickness ranged from 4.1 to 10.6 μm and from 0.3 to 1.0 μm, respectively. Elemental analysis of the surface morphology of the selfassembled structures was carried out by energy-dispersive X-ray spectroscopy (EDS) (see Figure S1 in the Supporting Information). In the case of method 2, clay hybrids had a high silicon content (Figure 4D) with a silicon/copper ratio of 49.69:26.99; by contrast, method 3 yielded a silicon/copper ratio of 33.41:42.01 (Figure 5D). The apparent difference in composition of the surface of the clay hybrids indicates a possible composition for dictating self-assembly morphology. Proposed Mechanism for the Formation of Different SelfAssembled Morphologies. A conceptual illustration of Cu2þ/ POP intercalation in Naþ-MMT is shown in Scheme 1. We visualize that Cu2þ deposits in the clay interlayer gallery and accompanied with the POP intercalation which is responsible for the thickness of the MMT spacing. The amount of Cu2þ between the clay layers contributes the strength of ionic attraction force for the Cu2þ/POP-MMT hybrids. The difference of the Cu2þ amount and location in the hybrid structure derived from the simultaneous or stepwise intercalation is responsible for the self-assembling behavior. For simultaneous intercalation with a high Cu2þ
Figure 4. SEM micrographs of simultaneously intercalated Cu2þ/POP-MMT hybrids at CEC ratios of (A) 1.4/1/1 (with short rodlike and
lamellar arrays having diameters of 0.3-2.2 μm and lengths of 5.3-7.2 μm) and (C) 2/1/1 (with ordered rodlike microarrays having diameters of 0.3-2.5 μm and lengths of 8.0-31 μm). (B) Magnified image of A; (D) magnified image of C.
10180 DOI: 10.1021/la100313j
Langmuir 2010, 26(12), 10177–10182
Tsai and Lin
Article
Figure 5. SEM micrographs of Cu2þ/POP-MMT hybrids intercalated stepwise (first Cu2þ and then POP) at CEC ratios of (A) 1.4/1/1 (with small disklike morphologies having widths of 1.8-2.4 μm and thicknesses of 0.1-0.4 μm) and (C) 2/1/1 (with large, directional disklike morphologies having widths of 4.1-10.6 μm and thicknesses of 0.3-1.0 μm). (B) Magnified image of A; (D) magnified image of C. Scheme 1. Conceptual Illustration of Simultaneous and Stepwise Intercalation of Cu2þ/POP into Naþ-MMT in Resulting Their Vertical and Horizontal Growth into Rod- And Disklike Morphologies, Respectively
Langmuir 2010, 26(12), 10177–10182
DOI: 10.1021/la100313j
10181
Article
content in the clay interlayer, there is a strong tendency for formation of lengthy rodlike microarrays. By contrast, in the case of stepwise intercalation, the adsorption of Cu2þ mostly on the MMT surface leads to a strong tendency for horizontal selfassembly into disklike morphology. It appears that two main driving forces, the POP hydrophobic and Cu2þ ionic attraction, control the self-assemblies in a complementary manner. By comparison, the hydrophobic POP favors the rodlike formation via vertical growth while the presence of Cu2þ ions around the MMT surface may largely alter the self-assembling route into disklike morphologies.
Conclusion The MMT clays intercalated with hydrophobic POP and CuSO4 in three different methods were prepared. The resultant Cu2þ/POP-MMT hybrids in different compositions can be dispersed in water and evaporated into varied morphologies of the self-assemblies. The modified MMT samples tended to self-assemble into rod and disklike microstructures mainly
10182 DOI: 10.1021/la100313j
Tsai and Lin
depending on the Cu2þ presence. Ionic interactions and hydrophobic phase separation were the predominant noncovalent bonding forces directing the self-assembly. Stepwise and simultaneous Cu2þ/POP intercalation methods affected the location of the Cu2þ ions, particularly MMT surface association and interlayer space encapsulation, and thus the self-assembly behavior. An understanding of the mechanism underlying platelike clay self-assembly could help in the fabrication of highly ordered microstructures from clay for many potential material applications. Acknowledgment. We acknowledge financial support from the Ministry of Economic Affairs and the National Science Council (NSC) of Taiwan. Supporting Information Available: Elemental analysis of the Cu2þ/POP/MMT hybrids surfaces by energy-dispersive X-ray spectrometry (EDS) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(12), 10177–10182
