Transmission electron microscopy observation of oxide layer growth on Cu nanoparticles and formation process of hollow o
- PDF / 603,999 Bytes
- 6 Pages / 585 x 783 pts Page_size
- 3 Downloads / 177 Views
J.-G. Lee Korea Institute of Machinery and Materials, Changwon-City 641-831, Korea
H. Mori Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047, Japan (Received 1 April 2007; accepted 6 July 2007)
The growth of a Cu2O layer on Cu nanoparticles at 323–373 K was investigated by transmission electron microscopy to elucidate the influence of voids formed at the Cu/Cu2O interface on the oxidation rate. The thickness of the Cu2O formed on Cu nanoparticles with an initial diameter of 10 to ∼35 nm was measured as a function of oxidation time. During the initial oxidation stage until the oxide film is about 2.5 nm thick, the oxide film on nanoparticles of 10 to ∼35 nm in diameter grows rapidly at an almost consistent rate. After that, however, the growth rate of smaller nanoparticles decreases drastically compared with that of larger ones, suggesting that the voids formed near the Cu/Cu2O interface prevent Cu atoms from diffusing outward, because the volume ratio of voids to inner Cu in the case of smaller nanoparticles is considerably higher than that for larger ones at the same oxidation time.
I. INTRODUCTION
In the current research on nanotechnologies, controlling the shape of nanoparticles is regarded as one of the most important objectives because changes in shape influence several physical and chemical properties.1 In particular, there is an increasing interest in methods to fabricate hollow nanostructures since their unique shape suggests they can be used as delivery vehicle systems, as well as for fillers and catalysts, and could also bring about changes in chemical, optical, and catalytic properties.2,3 Chemical reaction processes are the most common of the methods that have been developed to synthesize hollow nanospheres of various materials, including carbons,4 polymers,5 metals,6 and inorganic materials.7 Recently, it has been reported by Yin et al. that when isolated nanocrystals of cobalt are exposed to sulfur and oxygen at relatively low temperatures near 400 K, the initially solid nanocrystals turn to hollow spheres of cobalt sulfides and oxides.8,9 Thereafter, the formation of hollow oxide as a result of oxidation of Fe,10 Zn,11 and Cu and Al12 nanoparticles was confirmed. Up to now, it
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0362 2930 J. Mater. Res., Vol. 22, No. 10, Oct 2007 http://journals.cambridge.org Downloaded: 16 Mar 2015
has been commonly recognized that the rapid outward diffusion of metal ions from the metal core to the outer oxide shell during oxidation or sulfidation is accompanied by the generation of vacancies, which then cluster, and finally a nanohole is formed in the oxide particles. In the course of the oxidation of such nanoparticles, voids are produced at the interface between core metals and the outer oxide shells, and then the remaining core is connected with the shell by bridges. Wang et al.10 have mentioned that voids at the interface may lead to a reduced transp
