METHODS

METHODS

1. AQUEOUS

   • SOL-GEL
     • Used to prepare ceramic materials such as metal oxides, nitrides, and carbides. transformation of a sol into a gel, which is then usually thermally treated to obtain the final material. Sols are solutions of the precursor compounds the precursors react to form a network of bonds, resulting in a solid skeleton of metal−oxygen bonds within a continuous liquid phase. Sol−gel processes usually result in materials that are amorphous or poorly crystalline because they are produced at or near room temperature. Therefore, a thermal treatment is generally needed to transform the dried gel into the final desired material.
   • HYDROTHERMAL
     • Typically, bases such as ammonia or alkaline hydroxides are employed to form a titanium hydroxide intermediate, which is then dehydrated to titania under the applied hydrothermal reaction conditions at relatively high temperatures (usually between 150 and 250 °C). Reaction time is also important in determining the final morphology of the particles, as the crystals can proceed through intermediate, metastable states before reaching the final equilibrium morphology.
   • Is formed in 2 steps: hydrolysis and condensation.
     • The hydrolysis step results in the formation of a metal complex with water in the form of aquo, hydroxo, or oxo complexes depending on the metal cation an de reaction condition.
     • Condensation is the reaction between two or more of the formed complexes to give the Ti−O−Ti bonds, which are the precursors of the final oxide network. After condensation, oxolation can also occur with the formation of additional oxo bridges between metal fragments through nucleophilic addition. Usually this step is accompanied by water elimination, so that the final Ti−O−Ti bond is formed.
     • Condensation rates are much faster than hydrolysi

2. NONAQUEOUS

   • In nonaqueous methods, water is not used as the primary reaction solvent, such that titania can form through mechanisms other than hydrolysis and condensation. One proposed advantage of the nonaqueous approaches is better control over reaction rates than is possible in sol−gel chemistry, where hydrolysis typically occurs very quickly and is difficult to modulate.
     • When water is not present or produced in situ, hydrolysis and condensation can generally be better controlled because these reactions occur much more slowly.
   • SOLVOTHERMAL
     • A larger variety of surfactants or structuredirecting agents can be employed in solvothermal methods to drive the shape and morphology of the crystallites. The choice of the solvent has also been found to dramatically influence the shape and size of the formed crystallites. Under solvothermal conditions, titania formation can occur even in the absence of any additional compounds other than an oxygenated titanium precursor such as a titanium alkoxide. This is because some titanium alkoxides can thermally decompose into titania and an unsaturated (alkene) organic compound.
       • The“benzyl alcohol” It consists of the reaction between TiCl4 and benzyl alcohol to form an intermediate alkoxide that is then hydrolyzed to form thefinal titania crystallites.
   • NONHYDROLYTIC
     • Is based on reactions that produce the required Ti−O−Ti bonds in the presence of surfactants that modulate the growth of titania. One of the advantages of nonaqueous surfactant-assisted synthesis is that the reactivity of the titanium precursors can be further modulated by the choice of the surfactant molecules that act as protecting agents.