Ligand Exchange with Thiols: Effects on Composition and Morphology of Colloidal CoPt Nanoparticles

Abstract

Among the multitude of colloidal nanoparticles, bimetallic systems have been increasingly attracting interest due their tunable physical (e.g. magnetism) and chemical (e.g. catalytic) properties. Oxidation-resistant magnetic bimetallic particles can be self-assembled into ordered layers to become the building blocks for a future generation of data storage devices [1] or enable the retrieval and recycling of catalytically active organic molecules in a chemical reactor.[2] On the other hand, the composition of such nanoparticles can be chosen in a way that the particles themselves show an increased activity and selectivity for catalytic processes [3,4] due to their bimetallic nature. Potential applications also include the growing field of biotechnology where functionalized magnetic nanoparticles could enable the field-guided localized delivery of pharmaceutical agents [5] or the hyperthermal treatment of tumors [6] or provide means for an efficient and highly sensitive separation of biomolecules. [7,6] Many characteristics of the particles, such as solubility, biocompatibility, adhesive properties and chemical functionality, which are crucial for most of these applications, are determined by the shell of ligand molecules bound to their surface during preparation. To selectively tune the chemical properties, ligand exchange strategies have been developed in the recent years allowing the addition of new functionalities to the nanoparticles. [8][9][10] As compared to monometallic particles, ligand exchange on bimetallic systems is more challenging as complications can arise due to different binding affinities between the ligand molecules and the two metals, possibly not only resulting in changes in particle diameter and size-distribution but also in stoichiometry and shape by selective dissolution of one metal component. Previous studies report only small changes in morphology and composition when for example, exchanging nonpolar ligands to polar ligands. [11][12][13][14][15][16] Little attention, however, has been paid to a systematic investigation of all possible effects taking place during the exchange process. Focusing on thiol ligands, which are particularly versatile and widespread in colloidal chemistry, we show for cobalt-platinum nanoparticles that the thiol ligands interact predominantly with one component resulting in composition and morphology changes after long exposure times. At the same time, we can show that the original amine ligands are not completely removed.The alloy particles chosen for the study represent a wellcharacterized [17][18][19][21][22] system with a narrow selectable size distribution in which the magnetic cobalt component is passivated against oxidation through the addition of platinum, resulting in particles with potential use in magnetic and catalytic applications. In their as-prepared state, these particles are enveloped by a binary ligand system of 1-adamantanecarboxylic acid (1-ACA) and hexadecylamine (HDA). Due to their strong tendency for binding to cobalt, [23] platinum [24]...

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