Sol-Gel Process

The sol-gel process belongs to the class of wet chemical synthesis. The methodology of producing small particles is called the sol-gel process and it is an important method to synthesize nanomaterials. This method is mostly employed in synthesizing metal oxides.1761

The sol-gel process can be defined as a chemical reaction where an ion or molecular compound forms a three-dimensional network via the formation of oxygen bond between the ions. The reaction is also accompanied by the loss of water or other smaller molecules. In general, the sol-gel process is a polycondensation reaction which gives a three-dimensional network.1771

In the first step of the sol-gel process, a monomer or the starting material is converted into a sol (colloidal solution) which acts as the precursor for further gel formation. The gel so formed consists of polymers or discrete particles. The sol-gel method being a low temperature process gives control over the product composition and is economically feasible, making it a preferred method for nanomaterial synthesis. Chlorides and metal alkoxides are used commonly as the precursor, which is then hydrolyzed and polycondensed to produce colloids. Materials synthesized by this method find application in numerous fields such as in energy, optics, sensors, space, and separation technology like chromatography and in medicine.1761

Gel can be formed in several ways in the sol-gel process. In certain situations, a small change in the reaction condition results in products with different structures, though the precursor used is similar in both cases.1781

There are basically three steps namely, hydrolysis, condensation, and polymerization in the preparation of nanoparticles through the sol-gel process. Metal organic compounds are generally used as the precursor for this method.1791 The presence of organic ligands (such as -OCH, or OC2H?) which bind onto the metal or metalloid atom makes metal alkoxides a preferred precursor for the sol-gel process. These groups present on them can react with water easily.1801

In the first step, a hydrolysis reaction occurs in which a species (M-OH) which is unstable is formed and it undergoes further reaction with other species.1771 The factors which affect the hydrolysis reaction are nature of solvent, nature of alkyl group, water-to-alkoxide molar ratio temperature, and the presence of a catalyst (base or acid).1811In the step that follows, the unstable M-OH group reacts with another molecule of M-OH or M-OR group to form a condensation product (M-O-M) along with the elimination of water or alcohol. This results in the formation of a three-dimensional network. The solid particles hence formed will be suspended in the liquid forming the sol. They further cross-link with each other to form the gel.1771

The preparation of mono-sized nanoparticles of silica is an important application of the sol-gel process. Particles with varying size can be determined by controlling the process of hydrolysis and polymerization.1721

In a sol-gel process, at a precipitation preventing pH colloidal particles are mixed with water to form a colloidal solution. The metal alkoxide precursor M(OR)4 undergoes hydrolysis and polycondensation reaction on coming in contact with water. This results in the formation of colloidal dispersion of small particles which is then converted into a 3-D network of corresponding inorganic oxide. With further progression, 3-D networks start to form from the colloidal particles. The property of the so-formed gel is determined by the size of the particles and condensed silica.1821 The viscosity will increase as the gelation process proceeds. During the gelation process, fibers are spun together with a controlled change in viscosity. Gelation will result in agglomeration with colloidal particles. The agglomeration is due to electrical interactions among the components.1831

Excess solvent is removed from the complex network through the process of drying. Cracks may be produced on the gels if capillary stress develops (if pores are small) during drying. To avoid this problem, small pores are eliminated by hypercritical evaporation or by addition of surfactants. Larger pore size and a stronger network will reduce the stress, which will help in reducing cracking. The gel formed is then heated at an elevated temperature in order to cause densification of gel.1761

Once the formation of gel is complete, the solvent is removed. Depending on the method used in removing the solvent, different types of gels, namely cryogels, xerogels, or aerogels can be obtained. Methods of drying employed include ambient drying, freeze drying, or supercritical drying. Xerogels are formed when the process of drying causes shrinking in the gel and if no shrinking is observed, then an aerogel is formed. Xerogels are mostly obtained by employing ambient drying, whereas supercritical drying results in aerogels.181-841 Certain advantages of using the sol-gel process are listed below:

  • 1. The method is versatile.
  • 2. This method allows preparation of highly purified materials.
  • 3. It provides the provision of an easy way for the introduction of trace elements.
  • 4. Allows for the synthesis of special materials.
  • 5. It helps in saving energy as the process employs a low processing temperature1681

The method of the sol-gel process offers molecular level mixing. It is also capable of improving the chemical homogeneity of the composite formed here. Since a low temperature is used, materials that are liable to degradation at high temperature such as growth factors, biomolecules, and proteins can be included.1851

Laser Pyrolysis

Laser pyrolysis belongs to a category of gas phase synthesis.1861 The basis of laser pyrolysis process is the interaction between laser photon, gaseous reactant, and a sensitizer. The role of the sensitizer is to function as an energy transfer agent which transfers energy onto the reactants by collision. The precursor used for the reaction can be either in gaseous or liquid form. If a liquid precursor is being used, then it must be atomized in the synthesis region so as to improve its interaction with a laser beam.1871

In laser pyrolysis, the compound will undergo excitation by absorbing the energy of laser radiation. This energy is transmitted to the reaction medium thus increasing the temperature of the reaction medium rapidly. The reactants undergo decomposition on being exposed to high temperature. These precursors on dissociation form nanoparticles which then undergo a sudden cooling effect. Growth of the particle is stopped by the sudden drop in temperature yielding nanosized particles. The physical and chemical uniformity of the particles is obtained by adjusting parameters such as flow rate, pressure, laser power, and types of precursor.1881

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