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Langmuir 2004, 20, 9796-9806
Preparation of Nanostructured Materials by HeterocoagulationsInteraction of Montmorillonite with Synthetic Hematite Particles Yan-Qin Ji,†,‡ Leon Black,†,§ Peter G. Weidler,† and Maria´n Janek*,† Institute for Technical Chemistry, Water, and Geotechnology, Karlsruhe Research Centre GmbH, P.O. Box 3640, D-76021 Karlsruhe, Germany, Radiochemistry Department, China Institute of Atomic Energy, P.O. Box 275(88), 102413 Beijing, China, and Materials & Engineering Research Institute, Sheffield Hallam University, Howard Street, Sheffield, S1 1WB, United Kingdom Received February 20, 2004. In Final Form: July 23, 2004 A nanostructured, porous material was prepared by heterocoagulation of negatively charged montmorillonite with positively charged synthetic spherical hematite particles. The process of heterocoagulation of such particles was monitored by turbidimetric titrations over the pH range 2.5-7.5. On the basis of the results of turbidimetric measurements, a series of solid materials were prepared for further characterization using ESEM, BET, XRD, and FTIR techniques. Environmental scanning electron microscopy detected isolated hematite particles or small hematite aggregates on montmorillonite surfaces (mass ratios 8:1 and 4:1). At a mass ratio of 1:1, exfoliated montmorillonite layers, covering the hematite particles as semitransparent blankets were seen. A low mass ratio led to compact hematite particle aggregates covering the montmorillonite surfaces. Nitrogen-gas-adsorption isotherms revealed the sorption properties to be gradually dependent upon mass ratios. Pore volume distributions showed that mesopores with diameter of about 10-20 nm were produced in the heterocoagulates with mass ratios of 4:1, 1:1, and 1:8. The sample prepared with a 4:1 mass ratio showed the greatest BET surface area, which decreased slightly upon sample calcination at 500 °C. X-ray diffraction measurements were used to investigate layer stacking, by comparing the integral intensities of d001 reflection. For this purpose, samples with 4:1 mass ratios, prepared both by heterocoagulation and mechanical grinding, were used. It was found that heterocoagulation effectively diminished the stacking of the layers to about 85%; hence, a significant amount of fundamental, 1 nm thick montmorillonite layers was achieved in this sample.
Introduction The preparation and utilization of nanostructured materials based on natural or synthetic minerals has applications in fields such as pharmaceutical and drug delivery systems, organo-inorganic nanocomposites, catalysts or catalyst carrier, pigments, adsorbents, etc.1-7 We can consider nanostructured materials from their dimensional dependency or consider changes in the reactivity of chemically bound atoms and their quantum state in the substance as its dimension is scaled down to nanometer sizes. For instance, environmental particles, such as clays, have broad industrial applications but their use as functional nanostructured materials requires their modification. Oft-cited modifications of layered clay minerals include intercalation or pillaring, where either organic or inorganic cations are introduced between the 1 nm thick layers of * Author to whom the correspondence should be addressed. E-mail:
[email protected]. Phone: +49 7247 82-4102. Fax: +49 7247 82-6639. † Current address: Karlsruhe Research Centre. ‡ Permanent address: China Institute of Atomic Energy. § Permanent address: Sheffield Hallam University. (1) Lagaly, G.; Mecking, O.; Penner, D. Colloid Polym. Sci. 2001, 279, 1090-1096. (2) Lagaly, G. Appl. Clay Sci. 1999, 15, 1-9. (3) Catalysis today: Pillared clays; Burch, R., Ed.; Elsevier: Amsterdam, 1988; Vol. 2, p 367. (4) Skoutelas, A. P.; Karakassides M. A.; Petridis D. Chem. Mater. 1999, 11, 2754-2759. (5) Matijevic´, E. Annu. Rev. Mater. Sci. 1985, 15, 483-516. (6) Matijevic´, E. Faraday Discuss. 1991, 92, 229-239. (7) Bo¨nnemann H.; Richards R. M. Eur. J. Inorg. Chem. 2001, 10, 2455-2480.
layered silicates, e.g., montmorillonite.8-12 Recent work has used pillaring with silica or titanium sol particles to produce microporous solids13-17 or functionalized membrane films.18,19 The advantage in using sol particles is that they can be made from commonly available elements forming (hydr)oxide particles, and the sizes of these particles can readily be scaled to the desired dimensions, e.g., 20-200 nm.5,20,21 Preparation of novel nanomaterials from dispersions includes syntheses derived from colloid chemistry. The (8) Meier, L. P.; Nueesch, R.; Madsen F. T. J. Colloid Interface Sci. 2001, 238, 24-32. (9) Lee, T. W.; Park, O. O.; Yoon J.; Kim, J. J. Synth. Met. 2001, 121, 1737-1738. (10) Lee, T. W.; Park, O. O.; Hong, J. M.; Kim, D. Y.; Kim, Y. Ch. Thin Solid Films 2001, 393, 347-351. (11) CMS Workshop Lectures, Vol. 6: Layer charge characteristics of 2: 1 silicate clay minerals; Mermut, A. R., Ed.; The Clay Minerals Society: Boulder, CO, 1994; p 122. (12) Balci, S.; Go¨kc¸ ay E. Mater. Chem. Phys. 2002, 76, 46-51. (13) Han, Y.-S.; Matsumoto, H.; Yamanaka, S. Chem. Mater. 1997, 9, 2013-2018. (14) Han, Y.-S.; Yamanaka, S.; Choy, J. H. Appl. Catal. A 1998, 174, 83-90. (15) Choy, J. H.; Jung, H.; Yoon, J. B. J. Synchrotron Rad. 2001, 8, 599-601. (16) Ding, Z.; Zhu, H. Y.; Greenfield, P. F.; Lu, G. Q. J. Colloid Interface Sci. 2001, 238, 267-272. (17) Lee, H.-Ch.; Jung, H.; Oh, J.-M.; Choy, J. H. Bull. Kor. Chem. Soc. 2002, 23, 477-480. (18) Vercauteren, S.; Keizer, K.; Vansant, E. F.; Luyten, J.; Leysen, R. J. Porous Mat. 1998, 5, 241-258. (19) Ariga, K.; Lvov, Y.; Ichinose, I.; Kunitake, T. Appl. Clay Sci. 1999, 15, 137-152. (20) Penners, N. H. G.; Koopal, L. K. Colloids Surf. 1986, 19, 337349. (21) Matijevic´, E. Chem. Mater. 1993, 5, 412-426.
10.1021/la0495579 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004
Nanostructured Materials and Heterocoagulation
Figure 1. Schematic preparation of delaminated montmorillonite particles (d) for (a) the preparation of organic-inorganic composites, (b) heterocoagulation with sol particles, and (c) solidstabilized emulsions; (particle charges, exchangeable cations, and solvent molecules are omitted).
physical-chemical properties of dispersions containing different particles have often already been investigated, their mutual coagulation (heterocoagulation) having attracted attention from either a theoretical or practical point of view.22-25 Selective preparation of colloidal particle aggregates may lead to specific arrangements. For instance, early studies focused on the effect of the polyelectrolyte on particle adsorption kinetics and framboidal aggregates of small polystyrene particles adsorbed onto a large particle of opposing charge were prepared.26-29 In these studies, the kinetic data of adsorption, adsorption isotherms, were determined, while simulation experiments were performed by Puertas et al.30 Similarly, adsorption of positively charged akagane´ite (β-FeOOH) particles onto negatively charged montmorillonite particles was made by Ferreiro et al.31 The formation of fractal aggregates of magnetite (Fe3O4) particles with montmorillonite was investigated by Tomba´cz et al.32 It was found that the montmorillonite particles appear to bridge between akagane´ite or magnetite particles with the ratios depending on the pH, ionic strength, and particle concentrations. We propose a general scheme for the production of functional nanostructured materials based on delaminated layered silicate particles, such as montmorillonite (Figure 1). It can be considered that, upon delamination of montmorillonite particles of thickness 1 nm, (reaction d) and of a given size distribution, their surface can be modified to form the required composite material using “particles” of tunable lateral size. For example, if the lateral size ratio is ∼1:(1/1000), the heterocoagulation (22) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638-1651. (23) Gregory, J. J. Colloid Interface Sci. 1975, 51, 44-51. (24) Hansen, F. K.; Matijevic´, E. J. Chem. Soc., Faraday Trans I 1980, 76, 1240-1262. (25) Matijevic´, E. Pure Appl. Chem. 1981, 76, 2167-2179. (26) Vincent, B.; Young C. A.; Tadros, Th. F. J. Chem. Soc., Faraday Trans. I 1980, 76, 665-673. (27) Vincent, B.; Jafelicci, M.; Luckham, P. F.; Tadros, Th. F. J. Chem. Soc., Faraday Trans. I 1980, 76, 674-682. (28) Luckham, P.; Vincent, B.; Hart, C. A.; Tadros, Th. F. Colloids Surf. 1980, 1, 281-293. (29) Harley, S.; Thompson, D. W.; Vincent, B. Colloids Surf. 1992, 62, 163-176. (30) Puertas, A. M.; Maroto, J. A.; Ferna´ndez-Barbero, A.; De Las Nieves, F. J. Colloids Surf. 1999, 151, 473-481. (31) Ferreiro, E. A.; Helmy, A. K.; Bussetti, S. G. Clay Miner. 1995, 30, 195-200. (32) Tomba´cz, E.; Csanaky, C.; Ille´s, E. Colloid Polym. Sci. 2001, 279, 484-492.
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scheme in Figure 1sreaction a occurs corresponding to the preparation of the precursor typical for pillared clays using polycations.3,12 If we increase the lateral size of the composite material so that the size ratio is ∼1:(1/100), the heterocoagulation scheme shown in reaction b arises, corresponding to the application of sol particles.13-17,31,32 Finally, if the size of the composite material can be extended to obtain ratios of ∼1:(1/10) or higher, we obtain the heterocoagulation scheme in reaction c, corresponding to the preparation of solid stabilized emulsions (e.g., Pickering emulsions).33-36 De´ka´ny et al.37 proposed another concept to produce materials as shown in Figure 1b, namely the “in situ” growing of nanoparticles in the interlamellar space of layered clay minerals using so-called “nanoreactors”. For instance, intercalates of CdS, SiO2, ZnO, and TiO2 nanoparticles have been prepared and identified by XRD, SAXS, TEM, and AFM techniques.38-42 An understanding of the colloidal behavior of the dispersed systems of different components, with regard to particle type, their chemical composition, and morphology, is required for the design of nanostructured materials based on natural particles, such as montmorillonite. Therefore, in this study, almost homodispersed spherical hematite particles (R-Fe2O3) were synthesized and their interactions with platy particles of exfoliated sodium montmorillonite in a heterocoagulation reaction was investigated. In a second step, the physicochemical properties of heterocoagulates prepared as solid powders were determined. Experimental Section Materials. Montmorillonite. M40 bentonite (Greenbond, Wyoming, USA) received from Su¨d Chemie AG (Germany) was fractionated according to the method of Stul and van Leemput,43 later modified by Tributh and Lagaly,44,45 to obtain a sodium form of the fine fraction (
