Chapter 5
Langmuir—Blodgett Monolayers as Templates for the Self-Assembly of Zirconium Organophosphonate Films Houston Byrd, John Κ. Pike, Margaret L. Showalter, Scott Whipps, and Daniel R. Talham
Downloaded by FUDAN UNIV on April 27, 2017 | http://pubs.acs.org Publication Date: July 16, 1994 | doi: 10.1021/bk-1994-0561.ch005
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Department of Chemistry, University of Florida, Gainesville, FL 32611 Organized molecular assemblies are being explored for potential uses ranging from electronics and photonics applications to the active components in chemical sensors. These molecular assemblies are sometimes organized layer-by-layer, or even molecule-by-molecule. For one-layer-at-a-time depositions, the order of the active surface or "template" layer plays a critical role in organizing the assembly. In this article, we show how a Langmuir-Blodgett monolayer of octadecylphosphonic acid can be used as the template layer for monolayer and multilayer depositions of zirconium phosphonates. The highly organized and well characterized Langmuir-Blodgett template allows subsequent assembly steps to be quantified. The organization of molecules in the "self-assembled" layers reflects the order in the original template layer. The ability to tailor the chemical and structural characteristics of surfaces at the molecular level is the key to many strategies for developing new electronic and optoelectronic devices, solar energy conversion media, functional coatings, and of course, chemical sensors. Many methodologies exist for chemically derivitizing surfaces, but procedures for controlling surface functionalities at the molecular level fall into two general categories, Langmuir-Blodgett (LB) and so-called "self-assembly" (SA) methods (7-3). In the LB technique (7-3), molecules (or polymers) are mechanically manipulated into an organized array on a water surface before transfer to a solid support. SA methods (3,4) rely on the affinity of chemical functionalities for specific chemical surfaces in order to bind and orient molecules. The SA of organic thiols on gold surfaces is a topical example (3). Other SA methods take advantage of specific chemical interactions, but are not limited to a specific surface. An example here is the formation of Si-0 linkages between chlorosilanes and surface hydroxyl functionalities (5). The OH group can be on an oxide surface, or it can be an alcohol from an organic surface-derivitizing agent Several schemes for multilayer build-up take advantage of this Si-0 linkage (5-7). Another approach to SA multilayers takes advantage of the strong affinity of phosphonate and phosphate groups for the 2x* ion. Mallouk and co-workers (8-12) +
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Corresponding author 0097-6156/94/0561-0049$08.00/0 © 1994 American Chemical Society
Mallouk and Harrison; Interfacial Design and Chemical Sensing ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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INTERFACIAL DESIGN AND CHEMICAL SENSING
demonstrated that multilayers of α,ω-diphosphonic acids can be built-up one layer at a time by alternately adsorbing the α,ω-diphosphonic acid and Zr * ions from solution (Scheme 1). Our interest in organic thin films is in using organized organic assemblies as templates for developing single layers of inorganic extended lattice systems (13,14). Inorganic layers formed in this way should provide an opportunity to study chemical, electronic, and magnetic interactions in the limits of two-dimensions. We have investigated routes for preparing single-layer analogs of the transition metal phosphonates (13). In the solid-state, these are mixed organic/inorganic layered solids where binding within layers is strong, and interlayer interactions are van der Waals in nature (15). The critical step in preparing the metal phosphonate layers should be the organization of the surface or "template" layer. A lessonfrominvestigations of thiol adsorption onto gold is that the organization of the surface plays an important role in organizing the adsorbate (3,16-18). If inorganic layers are formed at an organic surface then the extent of structural coherence in the inorganic layer is expected to be limited by the size of organized domains in the organic template layer. Several methods have been explored for preparing a phosphorylated surface for use in multilayer depositions (8,9,1920). While most of these provide a high density of phosphonate binding sites, none are expected to provide the ordered array required to form an inorganic extended lattice. In an effort to produce an ordered array, we developed a Langmuir-Blodgett route to a phosphonic acid template layer (13). In this article, we compare this surface to a phosphonic acid template layer prepared by selfassembling long alkyl chain molecules. The LB template layer provides a more ordered surface, and we show that zirconium phosphonate bilayers can be produced at the LB template. In addition, the LB template can be used for the build-up of multilayers, using the method of Mallouk (8-12), and the organization of the LB template layer is reflected in the multilayer films that are produced. The results show that when developing functional assemblies the organization of the template layer plays an important role in controlling the architecture of the film.
Downloaded by FUDAN UNIV on April 27, 2017 | http://pubs.acs.org Publication Date: July 16, 1994 | doi: 10.1021/bk-1994-0561.ch005
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The Template Layer. Our approach to a "self-assembling" template layer is outlined in Scheme 2. In the first step, ω-hexadecenylbromide (21) is hydrosilylated with trichlorosilane and selfassembled to a silicon oxide surface to form a bromide terminated monolayer. Conversion of the bromide to the phosphonate, by reaction with triethylphosphite, is carried out on the assembled monolayer and followed by acid hydrolysis to the phosphonic acid. All of the conversions performed on the surface are followed by ATR-FTIR and XPS, and go to completion to the extent that can be determined by XPS (22). To form the zirconium phosphonate bilayer, the phosphorylated substrate is immersed into an aqueous solution of ZrOCh to bind Zi* at the phosphonic acid sites. To form the complete bilayer, the zirconated surface is thenrinsedwith water and placed into a solution of octadecylphosphonic acid in order to self-assemble the capping layer according to Scheme 1. The Langmuir-Blodgett template layer is prepared according to Scheme 3. A Langmuir monolayer of octadecylphosphonic acid on pure water is transferred to an OTS covered surface by dipping down though the air/water interface into a vial. The vial is removedfromthe trough, and Z1OCI2 is added to the vial. After 30 minutes, the zirconated surface isrinsedwith water and is then ready for binding the capping layer. If instead, zirconium ion is added directly to the LB subphase, zirconium binds to the +
Mallouk and Harrison; Interfacial Design and Chemical Sensing ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
BYRD ET AL.
Langmuir-Blodgett Monolayers as Tempfotes
A) Single Layer ^—P0 Zr< £—PO^Zr^ 2—P0 Zr
