Reversible Self‐Assembly of Metal Chalcogenide/Metal Oxide Nanostructures Based on Pearson Hardness

Abstract

Nanotechnology has reached a stage of development where not individual nanoparticles but rather systems of greater complexity are the focus of concern. These complex structures incorporate two or more types of materials, an example of which is the formation of metal–semiconductor hybrids, which effectively combine the properties of both materials. The assembly of multicomponent nanoparticles from constituents with different optical, electrical, magnetic, and chemical properties can lead to novel functionalities that are independent of the individual components and may be tailored to fit a specific application. These applications include such far-reaching challenges as solar energy conversion, biological sensors, mechanical and optical devices, and potential methods for drug delivery and medical diagnostics. A specific challenge is to assemble nanoparticles into a hierarchical structure. Nanotubes (NT-MQ2) [7] and fullerenes (IF-MQ2) [8] of layered metal chalcogenides are the purely inorganic analogues of carbon fullerenes and nanotubes, and exhibit analogous mechanical and electronic properties. They consist of metal atoms sandwiched between two inert chalcogenide layers. Their physical properties are related to their crystal structures, which contain MQ2 slabs with metal atoms sandwiched between two inert chalcogen layers. These MQ2 layers are stacked with only van der Waals contacts between them. The steric shielding of the metal atoms by the chalcogen surface layers from nucleophilic attack by oxygen or organic ligands makes chalcogenide nanoparticles highly inert and notoriously difficult to functionalize. Some progress has been made by employing chalcophilic transition metals in combination with multidentate surface ligands: The 3d metals “wet” the sulfur surface of the chalcogenide nanoparticles whilst the multidentate surface ligands partially block one hemisphere of the metal coordination environment. This steric shielding prevents an aggregation of the chalcogenide nanoparticles through interparticle cross-linking. The assembly of aggregates from different types of nanoparticles typically relies on chemical modifications of the nanoparticle surface to achieve a specific linkage. A bifunctional organic linker molecule having specific anchor groups for each type of nanoparticle is bound with one of its anchor groups to the first type of the pre-synthesized nanoparticles. In a subsequent step, the second anchor group is used for the attachment of the second type of nanoparticles. The goal is to attach a controlled number of target molecules while avoiding aggregation through nonspecific interactions with surfaces and other particles in solution. To achieve that goal, the nanoparticles have to be stabilized with a protecting layer containing some chemical anchor points for further modification. This covalent chemical attachment offers high stability in different solvents and ionic environments. Therefore, current strategies for the functionalization of nanoparticles rely on either 1) non-covalent physisorption of linker molecules to the surface of the nanoparticles, 2) electrostatic anchoring of an additional polymeric layer, or 3) the use of short bifunctional cross-linkers. These processes lead to low yields or low surface coverage. An alternative strategy is to grow nanoparticles directly on the nanotubes by using colloidal nanoparticle synthesis methods. Colloidal nanoparticles may have an affinity based on their acid–base properties, functional groups, or Pearson hardness for nanotube surfaces that allows their attachment without the aid of linkers. Herein we present a novel synthetic strategy based on Pearson s HSAB (hard/soft acid–base) principle. that allows the formation of a hierarchical assembly of metal chalcogenide/metal oxide nanostructures. The metal oxide particles can be functionalized in a subsequent reaction step at room temperature to tailor the chalcogenide surfaces or to reversibly detach them from the chalcogenide surfaces with excess surface ligand (Scheme 1). The recycled chalcogenide [*] J. K. Sahoo, Dr. M. N. Tahir, Dr. A. Yella, T. D. Schladt, Prof. Dr. W. Tremel Institut f r Anorganische Chemie und Analytische Chemie der Johannes Gutenberg Universit t Duesbergweg 10–14, 55099 Mainz (Germany) Fax: (+49)6131-39-25605 E-mail: tremel@uni-mainz.de