Room-temperature formation of hollow Cu(2)O nanoparticles.

2010 WILEY-VCH Verlag Gmb The synthesis of colloidal nanoparticles (NPs) possessing hollow or core/shell structures has attracted a great deal of attention because these materials exhibit unique physical and chemical properties that allow them to be used in catalysis and drug delivery. These particles are typically produced using sacrificial templates, such as polystyrene or silica. Nevertheless, this templating strategy often limits the core size to within a few hundred nanometers. In 2004, Yin et al. demonstrated the first preparation of hollow CoO NPs through the oxidation of Co NPs, using the concept of the nanoscale Kirkendall effect. Since then, many hollow and core/shell nanocrystals of oxides and chalcogenides have been synthesized this way. Colloidal synthesis of metal NPs accompanied by the Kirkendall process often produces high yields of monodisperse hollow nanoparticles. The synthesis of hollow metal oxide nanocrystals usually involves two distinct processes: surface oxidation (resulting in the formation of core/shell nanostructures) and vacancy coalescence induced by outward diffusion of the metal atoms (resulting in the formation of hollow structures). Metals such as Fe, Cu, Al, and Zn undergo surface oxidation when they are exposed to ambient atmosphere at room temperature. Further oxidation is prohibited by the surface oxide layer, the thickness of which is usually on the order of several nanometers. The formation of hollow nanocrystals is often performed in solution phase at elevated temperatures to accelerate the outward diffusion of metal ions from the core. Only a few examples of hollow metal oxide nanospheres formed at low temperature have been reported. The oxidation of Cu NPs is interesting to study because Cu possesses multiple oxidation states and usually forms the stable oxides, Cu2O and CuO. The Cu/Cu2O/CuO system has been applied to facilitate oxidation reactions in the bulk; as a result, it might be useful as an alternative for noble metals in various catalytic systems. Cu2O is an environmentally friendly p-type semiconductor having a band gap of 2 eV and a high optical absorption coefficient, which make it an excellent candidate for solar-energy-conversion applications. The shape-controlled synthesis of Cu2O microand nanocrystals has been explored to conduct the fabrication process in the potential application. With high surface area and lowmaterial density, hollow NPs are one important class of those nanocrystals. Several hollow Cu2O nanocrystals with defined interior architectures have been prepared previously through the dry oxidation of Cu NPs or the reduction of self-assembled CuO particles. Furthermore, the high diffusion rate of Cu in copper oxides makes it an ideal material for studying the Kirkendall effect. In this work, we describe the solution-phase synthesis of highly monodisperse Cu NPs and, by controlling the oxidation process, the formation of Cu@Cu2O core/shell structures, hollow Cu2O nanospheres, and solid Cu2O nanospheres. We synthesized the Cu NPs through the thermal decomposition of copper(I) acetate (CuOAc) in trioctylamine (TOA) in the presence of tetradecylphosphonic acid (TDPA), forming a purplish-red colloidal solution. The dark-red NPs were precipitated by adding ethanol and were collected through centrifugation. The NPs were readily dispersed in organic solvents, including hexane and chloroform. Analysis using transmission electron microscopy (TEM) indicated that the presence of TDPA, a strongly binding capping agent, led to the formation of monodisperse Cu nanospheres (Fig. 1a and 1b). The average size of the Cu nanoparticles was 8.4 ( 0.8) nm. The narrow size distribution was demonstrated from the formation of a large-area closely packed array of Cu NPs on the TEM grid after evaporation of the solvent (Fig. 1a). No size-selection steps were applied before the array formation. Changing the molar ratio of TDPA to the Cu precursor allowed us to tune the size of the Cu nanoparticles. For example, we obtained larger Cu NPs (average size: 14.7 nm) when the TDPA/CuOAc ratio was increased to 1 (Fig. 1c). To study the oxidation of the nanostructures, we dispersed the NPs in hexane and chloroform under ambient conditions. Both of these NP solutions appeared reddish green in color immediately after dispersing the particles. The color of each solution gradually changed to forest green. The green color of the hexane solution persisted for several months, whereas the chloroform solution turned yellow after a few hours under ambient conditions. We used TEM and X-ray diffraction (XRD) to examine the nanostructural transformations of the NPs in both solutions during the oxidation process. TEM images of the oxidized products prepared from hexane (Fig. 1d) and chloroform (Fig. 1e) revealed core/shell and hollow spherical structures, respectively. They also indicated that the monodispersity of the nanoparticles was maintained during the oxidation. High-resolution TEM (HRTEM) images revealed that the as-prepared nanoparticles had five-fold symmetry and possessed a very thin surface layer that featured a different spacing from that of the core (Fig. 1f). The