The formation of hollow metal oxide nanoparticles through the oxidation process at low temperatures from 295 to 423 K has been studied by transmission electron microscopy for Cu, Al, and Pb. For Cu and Al, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. On the other hand, Pb nanoparticles turn to solid PbO because the diffusivity difference D Pb Ͻ D O in PbO does not lend itself to the formation of vacancy clusters. The oxide growth behavior of Cu and Al nanoparticles of a larger size at 423 K are summarized as follows: ͑i͒ for Al, the rapidly forming oxide layer on its surface stops growing once it reaches a critical thickness of about 1.5 nm, ͑ii͒ the growth of Cu 2 O continues until hollow Cu 2 O of a certain thickness is formed. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides: the rapid outward diffusion of metal ions based on the CabreraMott theory plays an important role in the formation of hollow Al-oxides, whereas the Kirkendall effect at the Cu/ Cu 2 O interface, where Cu diffuses much faster than oxygen, brings about the formation of hollow Cu 2 O.

The oxidation behaviour of Ni nanoparticles at temperatures from 573 to 673 K and the formation process of hollow oxide particles were studied by transmission electron microscopy.In the course of oxidation, a single large void was observed at one site of the interface between inner Ni and outer NiO layer due to the vacancy clustering which occurs during the oxidation process resulting from the rapid outward diffusion of Ni ions through the NiO layer. This suggests that supersaturated vacancies generated at the interface migrate to the site over a long-range distance and aggregate at the site. Ni nanoparticles were fully oxidized to become hollow NiO, in which nano-holes in the form of vacancy clusters were located at the off-centred positions. The de-centring of the voids in hollow NiO is probably due to the large mobility of vacancies inside Ni during oxidation.

No abstract

No abstract

The structural stability of hollow Cu 2 O and NiO nanoparticles associated with reduction and oxidation reactions at high temperatures was studied by transmission electron microscopy (TEM). Hollow Cu2O and NiO in annealing under 5.0×10-5 Pa was observed to have shrunk at 473 and 623 K, respectively, where the reduction reactions from oxides to metals started. As a result of shrinking associated with reduction, hollow oxides turned into solid metal nanoparticles after annealing at higher temperatures for a long time. In addition, hollow oxides shrunk and collapsed through high-temperature oxidation. It was found that shrinking of hollow oxides during oxidation occurs at temperature where the diffusion coefficients of slower diffusing species reach around 10 -22 m 2 s -1 . Annealing at high temperatures both in a vacuum and in air leads to atomic movement that results in the annihilation of nano-holes inside hollow nanoparticles, and a consequent reduction in e the extra inner-surface energy.2

SummaryHollow nanostructures are ranked among the top materials for applications in various modern technological areas including energy storage devices, catalyst, optics and sensors. The last years have witnessed increasing interest in the Kirkendall effect as a versatile route to fabricate hollow nanostructures with different shapes, compositions and functionalities. Although the conversion chemistry of nanostructures from solid to hollow has reached a very advanced maturity, there is still much to be discovered and learned on this effect. Here, the recent progress on the use of the Kirkendall effect to synthesize hollow nanospheres and nanotubes is reviewed with a special emphasis on the fundamental mechanisms occurring during such a conversion process. The discussion includes the oxidation of metal nanostructures (i.e., nanospheres and nanowires), which is an important process involving the Kirkendall effect. For nanospheres, the symmetrical and the asymmetrical mechanisms are both reviewed and compared on the basis of recent reports in the literature. For nanotubes, in addition to a summary of the conversion processes, the unusual effects observed in some particular cases (e.g., formation of segmented or bamboo-like nanotubes) are summarized and discussed. Finally, we conclude with a summary, where the prospective future direction of this research field is discussed.

The growth of a Cu 2 O layer on Cu nanoparticles at 323-373 K was investigated by transmission electron microscopy to elucidate the influence of voids formed at the Cu/Cu 2 O interface on the oxidation rate. The thickness of the Cu 2 O 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/Cu 2 O 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.

The electron-irradiation-induced crystallization of amorphous Al 2 O 3 (a-Al 2 O 3 ) was investigated by in-situ transmission electron microscopy under the wide electron-energy region of 25-300 keV. The formation of c-Al 2 O 3 nanocrystallites was induced by irradiating the a-Al 2 O 3 thin film along with the formation of nanovoids in the crystalline grains regardless of the acceleration voltage. The crystallization became more pronounced with decreasing the electron energy, indicating that electronic excitation processes play a dominant role in the formation of c-Al 2 O 3 . Radial distribution analyses suggested that a-Al 2 O 3 transforms to c-phase via the "excited" ("stimulated") amorphous state, in which the breaking and rearrangement of unstable short-range Al-O bonds, i.e., fivefold-coordinated Al-O (AlO 5 ) basic units, occur.