**2. Preparation methods**

Hollow materials, in general, can be prepared using the Kirkendall effect and Ostwald ripening based on events, as well as the templating method (hard, soft, or

**47**

**Figure 3.**

*process in air at 300°C [12].*

*Preparation of Hollow Nanostructures via Various Methods and Their Applications*

one-pot/self-templating and free) based on the use of templates. In more detail, it

Kirkendall effect, a vacuum ordering occurs due to a change in the rate of diffusion between two or more components diffusing simultaneously. The process of different diffusion movements was proven experimentally by Smigelkas and Kirkendall [10] in 1947 that atomic diffusion occurs through the exchange of vacancies rather than by the direct replace of atoms. One example of this method is the preparation of metal oxides that can change the morphology of nanowires to nanotubes [11]. The example of nanowire formation based on Kirkendall effect is

The mechanism explaining the formation of a cavity or hollow material in the inner direction could be described as follows: cations will flow rapidly outward through the oxide layer and flow inward from the void as a counterweight to the metal oxide interfacial void. Then, the direction of flow of the material is equalized by the direction of flow of the void through condensation into the pore or eliminating the crystalline defects. The direction of material flow can also result from the phenomena of diffusion and reaction pairs at the gas/solid or liquid/solid interfaces, the formation of deformations and vacancies, or both during the growth of metal oxide or sulfide layers [13, 14]. It should be remembered that the hollows produced in the metal-metal diffusion pair or near the metal oxide interfaces of an oxide growth do not produce mono-spheres in regular directions but form a very hetero-

Ostwald Ripening is a phenomenon that is observed in solid solutions or liquid soles and explains changes in the structure of inhomogeneity with time, for example, small crystals or sol particles dissolving and being deposited back into crystals or larger sol particles. This phenomenon was first described by Wilhelm Ostwald in 1896 [15, 16] and is commonly found in oil-in-water [17] emulsions

*The schematic formation of Hollow Cu nanowires based on Kirkendall effect during the thermal oxidation* 

*DOI: http://dx.doi.org/10.5772/intechopen.95272*

described below:

**2.1 Kirkendall effect**

shown in **Figure 3**.

geneous molecular collection.

**2.2 Ostwald ripening**

*Preparation of Hollow Nanostructures via Various Methods and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.95272*

one-pot/self-templating and free) based on the use of templates. In more detail, it described below:
