**2.1 Kirkendall effect**

*Novel Nanomaterials*

**Figure 1.**

**Figure 2.**

or separation of pollutant particles from plastic contaminated water with nano-

Based on the study of specific surface area, load capacity, material transfer as well as storage, the size of the cavity makes hollow materials have extraordinary advantages in their characteristics. Having driven by these unique characteristics, the research groups eager to explore the more possible applications such as catalysis, photocatalysis, drug delivery, solar cells, supercapacitors, lithium-ion batteries, electromagnetic wave absorption, and sensors. The challenge faced in producing hollow materials at this time is to synthesize nano hollow materials which have a series of controlled structures in terms of composition and geometric configuration so that their applicative development is still constrained. However, the progress regarding the ability to manipulate both structure and morphology of nano hollow scale solid materials will have greater control over the local chemical environment [6–9]. Furthermore, the simple method used in the manufacture of nano hollow materials emphasizes the preparation process, economic review, and environmental friendliness for each of the chemicals used. This simple method is possible to produce nano hallow materials of various shapes such as nano hollow spheres (NHS), nano hollow cubes (NHC), nano hollow squared tubes (NHST), and related fibers. The applications described are the catalytic utilization of carbon dioxide into alcohol compounds, degradation of dyes, and the conversion of nano-cellulose to

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

microns or microbes was able to be done [5].

*A flowchart and the example of Zeolite nano hollow formation [3, 4].*

*The structure area of regular and hollow cubic shapes.*

alcoholic sugars by photocatalysis.

**2. Preparation methods**

**46**

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 shown in **Figure 3**.

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 heterogeneous molecular collection.

#### **2.2 Ostwald ripening**

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

#### **Figure 3.**

*The schematic formation of Hollow Cu nanowires based on Kirkendall effect during the thermal oxidation process in air at 300°C [12].*

**Figure 4.**

*Schematic of both w/o and o/w emulsion and hollow particles formation (a) using oleyamine micelles [19], and the growth of solid carbon sphere (b) based on Ostwald repining mechanism [23].*

when flocculation is found in water-in-oil [18] emulsions. Schematically the w/o and o/w emulsions are presented below in **Figure 4a**.

Ostwald ripening mechanism is well-known through several growth methods, such as island formation [20], layer by layer formation [21], and the mixed layers and islands formation [22] as illustrated in a solidified growth of carbon sphere in **Figure 4b**.

The emulsion produced in the w/o or o/w system is affected by various factors such as pressure (Laplace and osmotic), the concentration of the dispersed phase, the concentration of surfactants, and the additives used. Furthermore, the emulsifiers or surfactants used are generally biopolymers such as various proteins (whey protein isolate (WPI), β-lactoglobulin, casein, soy protein isolate (SPI), and pea protein [24], polysaccharides such as xanthan, Arabic gum, modified starch, carrageenan, pectin, and modified celluloses frequently utilized to stabilize emulsions, especially O / W and W/O/W double emulsions [25].

#### **2.3 The Smoluchowski process**

The Smoluchowski process is a process to produce nano hollow complex materials in an "integrative" nature from colloidal particles. An example of this preparation was the manufacture of titanium oxide, TiO2, and the yield observed by a high-resolution TEM [26]. The HRTEM TiO2 micrograph showed that the tiny nanocrystallites stuck to each other in the aggregated end product while keeping the overall orientation unchanged. An example of the formation of particles based on the Smoluchowski mechanism is presented in **Figure 5** below.

#### **Figure 5.**

*An example of a particle formation mechanism based on the Smoluchowski process with an emphasis on agglomeration and aggregation [27].*

**49**

**Figure 6.**

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

preparation are insensitive, easy to operate, and practice.

that a template used is moved/separated or modified.

*2.4.2 The soft templating or the endotemplate method*

surfactants, leading to mesopores up to 30 nm [31, 32].

*Schematic pathways of Au doped CeO using hard template method [30].*

These methods can effectively control the morphology, particle size, and structure during the nanomaterial manufacturing process. In general, these methods consist of two types/categories, namely: hard methods and soft (or one-pot or self) templates according to different structures. The methods of templates in their

In principle, this method is for the preparation of one-dimensional hollow materials. Materials used as hard-templates are polymer microspheres, porous membranes, plastic foam, ion exchange resins, carbon fiber, and anodic aluminum oxide (AAO) [28, 29]. Because the templates and the resulting target products have a unique structure and influence the particle size range, they play an important role in many areas of application. Furthermore, after the desired target is obtained so

One example of using the hard template method is making the ordered mesoporous CeO2 prepared via a hard-template method using SBA-15 as a structuredirecting agent. Leaching with NaOH and thermal treatment at 500°C enabled the removal of the inorganic template, thus resulting in the formation of long-range ordered CeO2. Nevertheless, small amounts of silica were present in the final oxides. The resulting CeO2 samples were used as supports for Au nanoparticles as shown in

The soft templating or the endotemplate method refers to supramolecular entities like self-assembled arrangements of structure-directing molecules such as

In the soft template method as shown schematically in **Figure 7**, compounds that function as templates are organic compounds whose molecules form aggregates through inter-molecular or intra-molecular interactions such as hydrogen bonds, chemical bonds, and electrostatic forces. The metal cations as the target as the hollow material are deposited on the surface or in the inside of the aggregate. The process of placing metal cations in the aggregate carried out using electrochemical methods, precipitation, and other synthesis/preparation methods to form metal oxide or composite materials of various shapes and sizes. Organic compounds that commonly function as templates are surfactants, polymers, biopolymers, supramolecules,

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

**2.4 Template methods**

*2.4.1 Hard-template method*

**Figure 6** below.
