**2. Synthesis of MO-NPS and MO-NCPs**

MO-NPs can be obtained through two opposite approaches including topdown and bottom-up. The former technique consists in successive mechanical operations of divisions and fragmentations, or in the irradiation of the bulk phase with a powerful energy source including UV, X-Rays, electron beam [5]. Using the bottom-up approach, MO-NPs are formed within the reaction medium, subsequently to the clustering of single atoms. The growth in size is time-dependent, and may be facilitated by additional treatment such as calcination [6].

#### **2.1 Physical methods**

#### *2.1.1 Spray pyrolysis*

Spray pyrolysis is generally used for the preparation of thin-films or pulverulent materials. It consists in the spraying of a solution or a suspension containing the metallic precursors in an oven, followed by a high temperature treatment. This method allows the formation of spherical oxides as the shape of the oxides are strongly dependent of the drops generated at the entrance of the furnace. Given the rapid rate of nucleation at high temperature, there is a one-droplet, one-particle mechanism.

In addition to the shrinkage occurring following the formation of oxides, micrometers precursors can allow the formation of particles in the nanometer size range [7]. NCPs can also be obtained through this method by mixing several metal ions in the precursor solution. The formulation of the composites is controlled by adjusting the stoichiometric proportions of each metallic species in the solution [8, 9].

## *2.1.2 Chemical vapor deposition (CVD)*

This technique is used mostly for the preparation of 2D metallic or inorganic materials of nanometric thickness. Usually, volatiles precursors are delivered on a heated surface on which a thin layer of materials is deposited upon a chemical reaction in vapor phase. Additional physical processes such as evaporation or sputtering are usually required to complete the synthesis.

**133**

*Nanocomposite and Nanofluids: Towards a Sustainable Carbon Capture, Utilization, and Storage*

During the preparation of MOs by CVD, oxidation and hydrolysis are the primary chemical reactions taking place in the presence of oxidizing agents (oxygen

Sonochemistry is a branch of chemistry supported by the formation, growth, and collapse of bubbles in a liquid upon irradiation with high intensity ultrasound waves. The bubbles can reach a temperature and pressure as high as 5000°C and 500 mPa respectively [11]. These conditions increase the chemical reactivity of the species in the reactor. When the water is the solvent, radical •OH, H2O2 and O3 are generated, leading thereby to oxidant medium suitable for the preparation of MO. Treatment of solutions of copper (Cu), zinc (Zn) and cobalt (Co) acetates under

a high-intensity ultrasonic horn has been used to produce nanosized CuO, ZnO, and CoO3 respectively [12]. This method has also been used for the preparation of

Hydrothermal or solvothermal synthesis (if water is the solvent) is a synthesis method in which the precursors (dissolved or dispersed in water) are placed in an autoclave where the reaction takes place at high temperature and pressure [14]. Hydrothermal allows the synthesis of MO-NPs from a wide variety in shape, size, structure, and composition, provided that key-factors such as temperature, pressure, pH, concentration of reactants are well-controlled [15–20].

It involves the hydrolysis and condensation of metal alkoxides, acetates, nitrates, sulfates, and chlorides. Their hydrolysis in solution results from the dispersion of metal hydroxides, which further undergo condensation leading to the formation of 3-dimensional network namely a gel. The gel is either dried by removing the solvent or treated by chemical reaction to give the condensed MO materials [21]. The solvent used is generally water (aqueous sol-gel). In most non-aqueous synthesis, additives such as surfactant can be required to control the morphology of the particles and most importantly to prevent/reduce their aggregation [22, 23].

The co-precipitation method entails the co-precipitation of cations in an aqueous media by the addition of an alkaline solution. A short burst of nucleation occurs when the concentration of the species reaches its critical super-saturation, and then, there is a slow growth of the nuclei by diffusion of the solutes to the surface of the crystal. MO-NPs are dried further or sometimes calcined at temperatures above

The co-precipitation method can be relevant for the synthesis of both MO-NP and MO-NCP [25]. The size and morphology of the nanoparticles produced by this method can be controlled by varying the molar ratio of the two precursor ions, the base used, mixing rate, the pH, and the temperature [26]. Unfortunately, it produces wastewaters with very basic pH needs to be treated before been discarded.

500°C, in which case there is an alteration of the structure of material [24].

or ozone) and the precursors are usually metal alkoxides [10].

nanocomposite when the suitable precursors are mixed [13].

*DOI: http://dx.doi.org/10.5772/intechopen.95838*

*2.1.3 Sonochemistry*

**2.2 Chemical methods**

*2.2.2 Sol-gel synthesis*

*2.2.3 Co-precipitation*

*2.2.1 Hydrothermal synthesis*

During the preparation of MOs by CVD, oxidation and hydrolysis are the primary chemical reactions taking place in the presence of oxidizing agents (oxygen or ozone) and the precursors are usually metal alkoxides [10].
