A “Mix and Match” Ionic-Covalent Strategy for Self-Assembly of Inorganic Multilayer Films

Multilayer thin films consisting of anionic R-zirconium phosphate (R-ZrP) sheets, tetrameric zirconium hydroxide cations [Zr4(OH)8(H2O)16] (Zr4), and alkanediylbis(phosphonic acid) (CnBPA) have been grown on silicon and gold surfaces by sequential adsorption reactions. The thin films were characterized by ellipsometry, X-ray diffraction, reflectance infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Alternately dipping cationic substrates into exfoliated R-ZrP-containing suspensions, aqueous zirconium oxychloride, and ethanolic C16BPA solutions generates a mixed ionic/covalent multilayer structure. The tetrameric Zr4 cation adsorbs onto the R-ZrP surface, providing a covalent anchoring point for the growth of the C16BPA layer. Adsorbing a second layer of zirconium ions onto the C16BPA layer allows one to continue the layer growth sequence using either covalent (metal/phosphonate) or ionic (R-ZrP/polycation) interlayer connections. A multilayer film with a repeating R-ZrP/Zr4/C16BPA/Zr4 sequence is sufficiently well-ordered in the stacking direction to give a Bragg peak in the diffraction pattern. The intensities of infrared absorbances in the symmetric and asymmetric C-H stretching regions, which arise from C16BPA, are linear with the C16BPA layer number. This “mix and match” approach provides a versatile means of assembling multilayer heterostructures from both ionic and covalent building blocks, with essentially any desired sequence of layers. Molecular self-assembly is one of the simplest and most effective methods for preparing thin films.1 Compared with other film-growth techniques, such as spin coating, laser ablation, sol-gel processing, electroplating, and chemical vapor deposition, self-assembly provides better control of structure at the molecular level. Self-assembled thin films are therefore useful vehicles for studying a variety of surface chemical phenomena on both flat and curved surfaces. They are of interest as interfacial models and in potential applications in many surface-related technologies, including catalysis, chemical sensing, microelectronics, corrosion inhibition, adhesion, and tribology.2 While the self-assembly of organic monolayers is now a wellunderstood process,1c the preparation of stable and ordered multilayer films remains a challenge, and relatively few approaches to it have been demonstrated. Most of these involve linking of sequentially grown self-assembled monolayers through covalent3-5 or noncovalent6,7 interactions. Inorganic materials are particularly interesting in this regard, because a full range of interlayer bonding schemes (covalent,4,8 coordinate covalent,5 and ionic9-12) has been demonstrated, and many different kinds of materials (metal phosphates, oxides, colloidal metal particles, coordination networks, intercalation compounds) have been grown as ultrathin films. 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Chem. Lett. 1996, 257. (e) Isayama, M.; Nomiyama, K.; Kunitake, T. AdV. Mater. 1996, 8(8), 641. 12184 J. Am. Chem. Soc. 1997, 119, 12184-12191 S0002-7863(97)02569-9 CCC: $14.00 © 1997 American Chemical Society voids for chemical sensing,14 and ordered multilayers that act as templates for their own replication.15 The technique does have some important drawbacks, most notably the requirement for specific functional groups and limitations on the lateral area occupied by a given molecule. On the other hand, multilayers joined by ionic contacts are more easily prepared, because specific functional groups or metal-ligand interactions are not required. Ionic multilayers are also very forgiving with respect to the size of molecules that can be included in the layer growth process. A drawback of the ionic method, at least with organic polyelectrolytes, is that there is significant interpenetration of sequentially grown layers. This problem can be eliminated by using certain inorganic sheets as polyanions;16 still, it is not easy to control molecular orientation or intermolecular distances to the extent possible with covalent multilayer films. In this paper we describe a “mix and match” strategy that allows one to switch between ionic and covalent interlayer connections within a multilayer adsorption sequence. The technique employs well-characterized covalent and ionic components (zirconium phosphonates and R-Zr(HPO4)2‚H2O/poly(allylamine hydrochloride) (R-ZrP/PAH)), which are themselves compatible with many other covalent and ionic materials. Metal-phosphonate sequential adsorption has previously been used to prepare covalent multilayers with different sequences of divalent, trivalent, or tetravalent metal ions and phosphonates; likewise, R-ZrP as a lamellar colloid can be combined with a very wide variety of organic and inorganic polyelectrolytes.17 Hence we can anticipate that the “mix and match” method will enable the preparation of self-assembled films from numerous ionic and covalent components. In this paper we demonstrate this idea, and use surface-sensitive techniques (ellipsometry, surface infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy) to characterize some representative mixed ionic/covalent multilayer films in detail. Experimental Section Materials. Zirconium oxychloride octahydrate (ZrOCl2•8H2O, 99.99%), hafnium oxychloride octahydrate (HfOCl2‚8H2O, 99.99+%), and tetrabutylammonium hydroxide (TBA+OH-, 40 wt % solution in water) were obtained from Aldrich and used as received. (4Aminobutyl)dimethylmethoxysilane (4-ABDMMS), from United Chemical Technologies, Inc. and 2-mercaptoethylamine hydrochloride (2MEA) from Sigma Chemical Co. were also used as received. All other chemicals were reagent grade and were obtained from commercial