Preparation of nanostructured particles by sol-gel method

Introduction:

The advent of the sol-gel process occurred in the year $1921$. In the $1960s$, its development was given due to the need for new synthesis methods in the nuclear industry.

The sol-gel method is a widely used wet chemical technique to fabricate nanostructured materials. This technique is used to prepare nanoparticles of ceramics, glassy, and composite materials at relatively low temperatures based on wet chemical processing. It involves the conversion of a precursor solution (i.e. sol) into a solid three-dimension network (i.e. gel) through hydrolysis and condensation reactions of precursor compounds. There are following steps are given below to fabricate nanostructured material through the sol-gel method.

1. Precursor Compound Selection: The first step involves selecting the appropriate precursor compounds, usually metal alkoxides or inorganic salts, that will form the desired material upon hydrolysis and condensation. These precursors should be soluble in a solvent i.e. metal salt in water or metal alkoxide in an organic solvent like alcohol.

2. Hydrolysis (Formation of Sol): It involves the conversion of a homogeneous solution of the precursor into a colloidal solution (i.e. The colloidal particles of precursor stably disperse in a solvent is called a Sol or colloidal solution.). In the hydrolysis process, Alcohol like ethanol or isopropanol is used as a solvent. Water or an acidic/basic catalyst is then introduced to initiate hydrolysis of the precursor molecules. This results in the breaking of metal-oxygen bonds in the precursor, generating metal hydroxide or oxide species. The reactions are given below.

$M-OR + H_{2}O \rightarrow M-OH +R-OH$

Where

$M=OR \rightarrow$ Metal Alkoxide
$M-OH \rightarrow$ Metal Hydroxide
$M \rightarrow Si, Ti,\: Zn,\: Al,\: Sn \:\: etc $

3. Condensation (Formation of Gel): The colloidal solution is kept for aging. During aging, the various condensation chemical reaction (i.e. polymerization) between two metal hydroxyl species leads to $M-O-M$ bonds with the release of $h_{2}O / R-OH$. This condensation process continues till finally results in a gel interconnected, rigid, and porous inorganic networks covered completely with the liquid phase. This transformation is known as a sol-gel transition.

$M-OR + HO=M \rightarrow M-O-M +R-OH$

4. Gelation: As the condensation reactions continue, the sol transforms into a three-dimensional network with a continuous solid phase interspersed with a liquid phase. This semi-solid network is known as a gel. The gelation process can be controlled by adjusting factors like precursor concentration, solvent composition, pH, and temperature.

5. Drying: Once the gel has formed, the excess solvent is removed through a drying process. The drying can be done by various techniques such as evaporation, supercritical drying, or freeze-drying. Careful drying is essential to prevent cracking or collapse of the gel structure.

6. Xerogel: If the solvent is dehydrated under ambient conditions (i.e. Removal of $R-OH$ groups). Xerogel is produced.

7. Calcination: In many cases, the dried gel must undergo a thermal treatment called calcination. This involves heating the gel at elevated temperatures to remove any remaining organic components and to induce further crystallization and growth of the desired material. The final crystalline structure and properties of the nanostructured material are developed during this step.

8. Characterization: The resulting nanostructured material is then characterized using various techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and spectroscopy to analyze its composition, structure, particle size, and morphology.

Advantages of the Sol-gel process:

Versatile: Sol-gel method provides better control over the structure of particles, size of particles, and porosity. There is a possibility of incorporating nanoparticles and organic materials into sol-gel derived oxides.

Extended composition ranges: The sol-gel method allows the preparation of any oxide composition as well as also some non-oxides and the production of new hybrid organic-inorganic materials, which do not exist naturally.

Better Homogeneity: Due to mixing at the molecular level; high purity

Less energy consumption: There is no requirement for the melting temperature since the three-dimensional network structure can be achieved at relatively low temperatures.

Coating and thin films, monoliths, composites, porous membranes, powders, and fibers.

No need for special or expensive equipment.

Disadvantages of Sol-Gel:

Cost of precursors

Shrinkage of a wet gel upon drying, which often leads to fracture due to the generation of large capillary stresses and consequently, makes difficult the attainment of large monolithic pieces

Preferential precipitation of a particular oxide during the formation of colloidal solution i.e. Sol (in multicomponent glasses) due to the different reactivity of the alkoxide precursors

Difficult to avoid residual porosity and $OH$ groups.

Application of Sol-Gel:

Protective Coating

Thin films and fibers

Opto-mechanicalspan

Nanoscale powders

Medicine