Sol-gel is a common method that is used mostly to synthesize small metal oxide particles and mixed oxide-based composites. The sol-gel process is inexpensive, has a low reaction temperature process, and can control the amount of product obtained. The sol-gel method involves mainly three steps: hydrolysis, condensation, and drying process to produce metal oxide nanostructures. In this method, the starting materials are converted into a colloidal solution (sol), which is used as precursor material in further reactions to form a gel. The precursor materials are dissolved and reacted with water and then polycondensed to form three-dimensional (3D) gels. The obtained gel is then dried and converted to Xerogel or Aerogel based on the drying process. Based on the nature of the solvents, the sol-gel process is classified into two parts, i.e. aqueous and nonaqueous sol-gel processes . In the aqueous sol-gel process, water is the reaction media, whereas organic solvent is used as reaction media in the nonaqueous sol-gel method. Figure 11.3 illustrates the schematic diagram of the sol-gel process . In this method, the parameters, such as molar concentration of the reactants (e.g., precursors and additives), pH of the solution, heat treatment, and the type of solvent used, can be controlled to vary the morphology and size of the nanostructures . The size and shape of the nanostructures formed in this process are uniform and, therefore, the electrical, optical, and magnetic properties are enhanced. The materials obtained from the sol-gel method possess a large surface to volume ratio, therefore, these can be used in catalytic applications. Moreover, the low reaction temperature causes fewer defects and disorders in the synthesized system.
Conventional heating processes has some major limitations, such as high thermal gradient, inefficient and slow reaction kinetics, lack of homogeneity, low crystallization, and unwanted reaction conditions that have tremendous negative impact on the nucleation and size distribution of the nanostructures . Microwave-assisted synthesis is considered as a promising and eco-friendly process to prepare the metal nanostructures and metal oxides of different sizes and shapes. This process has several benefits, such as cost effectiveness, high yield of product, fast reaction, and
FIGURE 11.3 Schematic illustration of sol-gel process. Adapted and reproduced with permission from reference . Copyright 2017 Elsevier.
homogeneous heat treatment in the reaction solution. From this perspective, the microwave-assisted synthesis process is introduced as an alternative to the conventional heating system to remove the drawbacks related to conventional heating processes. Microwaves use electromagnetic radiation with a frequency range within 300 MHz-300 GHz . During the reaction, the microwaves interact with materials via two mechanisms: interaction of dipole and conduction of ions. These mechanisms are valid only when the target material and the electric field of microwave irradiation interact with each other. Figure 11.4 illustrates the generation of thermal energy due to microwave irradiation on water molecules . In this system, the water molecules try to align along the electric field, while the polar ends try to realign along the alternating electric field, causing a loss of energy in the form of heat. The rate of change of polarity of the alternating electric field is faster than the alignment of the water molecules around the dipole, causing a shift in phase and absorption of energy from the electric field .
The template-guided method is used to synthesize high-grade nanostructures with controlled morphology using an appropriate template. The important parameters of nanostructures, such as composition of phase, size of the pore, grain morphology, and shape of the nanomaterial can be tuned with the use of a suitable template
FIGURE 11.4 The effect of microwave irradiation on water molecules. Adapted and reproduced with permission from reference . Copyright 2009 Elsevier.
. In this process, the resultant solid material is formed over the template through the reaction between solid and liquid interfaces . The template provides desired morphology to the nanomaterial, which is similar to that of template morphology. In addition, the template-directed process is beneficial because it does not require external energy sources and it has a reduced reaction time, low reaction temperature, and low toxicity of the precursors . Nanostructures are formed within the template and completely removed by certain routes, which include chemical etching and calcinations. Based on the structure, the template-directed synthesis process can be classified into two routes: hard- and soft-template methods. Hard templates include porous anodic alumina, porous carbon, porous silica, carbon NPs, metal oxides, polystyrene beads, and block polymers, whereas the soft templates are polymer vesicles, droplets, bubbles, amphiphilic surfactants, and micelles. However, the template method has a major limitation that complete removal of the template might affect the structure and purity of the synthesized nanostructures.