*2.5.1. Sol-gel process parameters affecting properties of TiO<sup>2</sup>*

There are various parameters that influence the size and properties of the TiO<sup>2</sup> particles produced via the sol-gel process. To get TiO2 particles with desirable properties, the parameters that influence hydrolysis and condensation reactions of the sol-gel process should be controlled. It has been established that some parameters are more important than others. The parameters include pH, nature and concentration of the catalyst, water/precursor molar ratio, reaction temperature, precursor concentration, type of solvent and type of precursor [61, 62].

#### *2.5.1.1. Precursor concentration*

Particle size increases with increasing precursor concentration due to enhanced coagulation and sintering resulting from the large concentration of TiO2 nuclei generated at high TTIP precursor concentrations [63]. Increase in precursor concentration increases the crystallinity of the anatase, and enhances transformation from anatase to rutile [64].

#### *2.5.1.2. Water content*

is basically a dispersion of colloidal particles in a liquid, and condensation leads in the formation of a gel. Compared to the methods discussed above, the sol-gel process is very promising for synthesis and preparation of inorganic and organic-inorganic hybrid nanomaterials because it allows the use of low processing temperatures (<100°C) and molecular level composition homogeneity [56]. Particle size and shape and are easy to control using the sol-gel method.

The sol-gel process produces fine, spherical powders of uniform size and has been widely

nium (IV) alkoxides [57]. One of the most attractive features of the sol-gel process is the possibility to shape the resulting material into desired forms such as fiber, film and monodispersed powder. Several steps and conditions are applied in a sol-gel process to control the final mor-

Typical precursors are metal oxides and metal chlorides. A metal alkoxide consists of an M─O─R linkage where M is the metal, O is oxygen and R is an alkyl group. The polarization that takes place in the M─O bond makes it susceptible to nucleophilic attack. In the presence of water, the alkoxide undergoes a nucleophilic substitution reaction in which the alkoxy groups (OR) are replaced by the hydroxyl groups from water and this process is called hydrolysis. The metal hydroxide groups will link and generate a hydrated metal-oxide network

The chemistry, hydrolysis and poly condensation reactions are very convenient to obtain both

Ti─OH + OR─Ti → Ti─O─Ti + ROH (2)

Ti─OH + OH─Ti → Ti─O─Ti + H2 O (3)

Polycondensation turns monomers into oligomers and, lastly, polymers. As long as the number of alkoxide groups, is greater than 2, complex random branching may occur finally lead-

Metal alkoxides used for the sol-gel process are generally very reactive and thus there is need for controlling the reactivity in order to obtain sols and gels with desirable properties by using modifiers or addition of chelating ligands such as β-diketones, carboxylic acids or other complex ligands [56]. These modifiers react with alkoxides giving rise to new molecular precursors that can be used in sol-gel processing to provide better control of the hydrolysiscondensation process. These new precursors reduce reactivity and functionality, prevent condensation and lead to formation of species that are smaller. Livage et al. in 1988 investigated

Modification by modifiers reduces the number of M─OR bonds available for hydrolysis and thus hydrolytic susceptibility. If β-diketones are used, they decrease the nuclearity resulting in small particles since these ligands are surface capping reagents and polymerization lockers.

the use of acetylacetone to improve the sol-gel processing of metal alkoxides [59].

Carboxylate ligands such as acetic acid mostly act as bridging chelating ligands.

<sup>4</sup>−*<sup>n</sup>* (OH)

*<sup>n</sup>* + *n*ROH (1)

phology as suggested by Mehrotra and Singh [58] in **Figure 4**.

which eventually forms small nuclei and this process is called condensation.

materials and normally proceeds via an acid-catalyzed step of tita-

used to synthesize TiO2

160 Titanium Dioxide - Material for a Sustainable Environment

polymeric and particulate titanium sols:

ing to fractal structures.

TiOR + mH2 O → Ti (OR)

The amount of water is a crucial parameter in controlling the hydrolysis reaction. Xiaobo [31] reported that the development of Ti─O─Ti chains through alkoxylation is favored when the content of water is low, with low hydrolysis rates and excess titanium alkoxide in the reaction mixture. The amount of water should not be too low otherwise the hydrolysis of the alkoxides with water will be incomplete and condensation occurs between the monomers of (OH)x Ti(OR)4−x [65]. Other researchers reported that the ratio of water to the alkoxide required for particle formation should be greater than 2.5 as deduced from the equation R= [H2 O]/[TEOT] > 2.5 [66]. The largest R value reported was 7 which gave particles with average size of 300 nm. If the amount of water is increased, a stronger nucleophilic reaction between water and alkoxide molecules occurs resulting in more alkoxyl groups being substituted by OH groups of water. The monomers obtained then interact with each other to form a three-dimensional network structure. When R is over a critical value, the hydrolysis is more complete and more alkoxides convert to the corresponding metal hydrates, M(OH)z which then react with each other to form particle-like polymers [65]. The formation of Ti(OH)4 is favored by high hydrolysis rates caused by large amounts of water.

According to a study by Yu and Wang [65], the molar ratio of H2 O/alkoxides (Rw) used in the sol-gel process strongly affect the characteristics of the resultant oxides as shown in **Figure 5(a)**. They suggested that the reaction mechanisms for sol-gel conversion depend on Rw used and they came up with three different mechanisms as shown in **Figure 5(b)**. All mechanisms could occur at the same time but there would be a dominant one [65]. Case I: When Rw is less than four (Rw < 4), the hydrolysis between alkoxides and water is incomplete hence condensation reaction occurs between the monomers. Case II: When Rw is increased to between two and four (2 < Rw ≤ 4), stronger nucleophilic reaction between water and alkoxide molecules monomers occurs and a three-dimensional network is formed. Case III: When Rw used is over the critical value the hydrolysis will be complete. The alkoxides will convert to the corresponding metal hydrates. The monomers will react with each other to form particle-like polymers.
