Nanosized CuO Particles via a Supramolecular Strategy

Abstract

Carbon monoxide, emitted from many industrial processes and transportation activities, is considered as a major component of air pollution. Catalytic oxidation is an efficient way to convert CO to CO 2 at low temperature, and noble metals (Au, Pt, Pd, and Ru) are efficient catalysts for lowtemperature CO oxidation. [1] Due to the limited availability of precious metals, attention has been given to base metals as catalysts, especially copper oxide. [2] Among the oxides of transition metals, CuO (tenorite) nanoparticles have attracted much attention because they have been shown to be an industrially important material that can be widely used in applications such as solar energy transformation, [3] magnetic phase transitions, [4] gas sensors, [5] catalysts, [6] and superconductors. [7] CuO is a p-type covalent semiconductor, having a bandgap between 1.2 and 1.5 eV. [8] Though some methods have been suggested for the preparation of CuO nanoparticles, such as chemical vapor deposition, [3] sol-gel techniques, [6] quick precipitation, [9] sonochemistry, [10] electrochemistry, [11] one-step solid-state reactions, [12] and alcohothermal reactions, [13] a technique to synthesize very small oxide nanoparticles with a narrow distribution in an organic host has been still lacking. Therefore, it was a challenge to find a convenient, mild, and simple route to produce CuO nanoparticles by using a suitable macrocyclic organic host. Very recently, we developed a process for the fabrication of CuO nanoparticles (average size: 10-20 nm) using a b-cyclodextrin-encapsulated precursor of copper acetate. [14] In this Communication, we focus on the synthesis of still smaller CuO nanoparticles (average size: 5 nm) using cucurbit[7]uril (CB[7], Scheme 1) as an encapsulating agent for the precursor (copper acetate) by utilizing a host-guest binding strategy for the first time.Cucurbituril (CB[6]), a macrocycle comprising six glycouril units, forms stable host-guest complexes with small molecules such as aliphatic and aromatic amines. [15] Based on our knowledge and results in supramolecular and nanochemistry, [16] we have reported water-soluble, stable inclusion complexes of b-cyclodextrin with guest molecules such as [60]fullerene, [17] also supramolecular polymers such as poly(azomethine), [18] and a supramolecular complex of CB[7] with [60]fullerene using the "supramolecular masking concept". [19] Inspired by these approaches developed in our laboratory and our experience with metal compounds, [20] we now adopt this concept to the preparation of inclusion complexes of CB[7] with metals for the preparation of nanoparticles. Although several adducts of cucurbituril and s-block metals (Na + , K + , Rb + , Cs + , Ca 2 + ) have already been described, [21] no isolated inclusion complexes of CB[7] with transition metals or metal acetates have been reported to date. To this end, we encapsulated copper acetate in CB [7] for the first time.The complex prepared in this work may be of interest for applications in catalysis, since a high specif...

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