Langmuir 2007, 23, 10505-10510
10505
Assembly of Highly Ordered Nanoparticle Monolayers at a Water/ Hexane Interface Yong-Kyun Park, Sang-Hoon Yoo, and Sungho Park* Department of Chemistry, BK21 School of Chemical Materials Science & SKKU AdVanced Institute of Nanotechnology, Sungkyunkwan UniVersity, Suwon 440-746, South Korea ReceiVed May 17, 2007. In Final Form: August 3, 2007 This paper reports a methodology for preparing ordering hydrophilic metal nanoparticles into close-packed 2-dimensional arrays at a hexane-water interface with alkanethiol in the hexane layer. The destabilization of metal nanoparticles by the addition of alcohol caused the nanoparticles to adsorb to an interface where the surface of entrapped Au nanoparticle was in situ coated with the long-chain alkanethiols present in a hexane layer. The adsorption of alkanethiol to the nanoparticle surface caused the conversion of the electrostatic repulsive force to a van der Waals interaction, which is a key feature in forming highly ordered close-packed nanoparticle arrays.
1. Introduction The assembly of nanoparticles into two- or three-dimensional structures is of considerable interest given their already widespread and potential applications in electronic and optical devices. In line with these attempts, a myriad of methods that utilize selfassembly concepts, such as solvent evaporation,1-3 covalent attachment,4-6 electrostatic immobilization,7 and Langmuir monolayer transfer,8-10 have been adopted. Among them, a Langmuir method has attracted a great deal of interest owing to its ability to form large two-dimensional nanoparticle arrays with a tunable interparticle distance.7-9 One requirement with this approach is to use capped nanoparticles with hydrophobic molecules in order to make them float in an air/water interface. For hydrophilic nanoparticles in an aqueous medium, there are several reports on the direct ordering process of the nanoparticles at an interface with the assistance of strongly adsorbing organic materials.11 However, this approach has often led to the inhomogeneous ordering of nanoparticles as well as the appearance of void areas between the nanoparticle aggregates. Given that there are relatively few synthetic routes for noble metal nanoparticles in organic solvents compared with ones in aqueous media, it is necessary to develop a new approach for ordering metal nanoparticles synthesized in an aqueous medium. Recently, it was reported that hydrophilic nanoparticles can be assembled into 2-dimensional arrays at liquid-liquid interfaces, which is induced by the destabilization of nanoparticles with the addition of a low-dielectric solvent to an aqueous colloidal * Corresponding author. E-mail:
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suspension.12-14 When a particle is hydrophilic, it has a contact angle
