The sol-gel method is a versatile and widely used technique for synthesizing titanium dioxide (TiO2) nanoparticles. This process involves the transition of a liquid precursor solution into a solid gel phase through hydrolysis and condensation reactions, followed by drying and calcination to obtain crystalline TiO2 nanoparticles. The method offers precise control over particle size, morphology, and phase composition, making it suitable for applications in photocatalysis, solar cells, and coatings.

Chemical precursors play a critical role in sol-gel synthesis. Titanium alkoxides, such as titanium tetraisopropoxide (TTIP), titanium tetraethoxide (TEOT), and titanium tetramethoxide (TMOT), are commonly used due to their high reactivity with water. These alkoxides undergo hydrolysis in the presence of water, forming titanium hydroxide species. The general hydrolysis reaction can be represented as:
Ti(OR)4 + 4H2O → Ti(OH)4 + 4ROH
where R represents an alkyl group (e.g., isopropyl, ethyl). The hydrolysis rate is influenced by the alkoxide's steric hindrance, with bulkier groups slowing the reaction.

Condensation follows hydrolysis, leading to the formation of Ti-O-Ti bonds through oxolation or olation mechanisms. Oxolation involves the elimination of water, while olation results in the formation of hydroxyl bridges:
Ti-OH + HO-Ti → Ti-O-Ti + H2O (oxolation)
Ti-OH + Ti-OH → Ti-(OH)2-Ti (olation)
These reactions contribute to the growth of a three-dimensional network, forming a gel.

The pH of the reaction medium significantly impacts the sol-gel process. Acidic conditions (pH < 3) promote slower hydrolysis and condensation rates, resulting in linear or weakly branched polymers and smaller nanoparticles. In contrast, basic conditions (pH > 7) accelerate condensation, leading to denser, more highly branched structures and larger particles. The isoelectric point of TiO2 is around pH 6, where aggregation is most likely due to reduced electrostatic repulsion.

Catalysts are often employed to control reaction kinetics. Acidic catalysts (e.g., nitric acid, hydrochloric acid) protonate alkoxide groups, enhancing electrophilicity and promoting hydrolysis. Basic catalysts (e.g., ammonia, sodium hydroxide) deprotonate hydroxyl groups, increasing nucleophilicity and favoring condensation. The choice of catalyst affects particle size distribution and gel structure.

Precursor concentration and solvent selection also influence nanoparticle properties. Higher precursor concentrations increase the rate of gelation but may lead to inhomogeneous particle growth. Solvents like ethanol, isopropanol, or water-alcohol mixtures are used to dissolve alkoxides and control viscosity. Water-to-alkoxide molar ratios typically range from 2:1 to 20:1, with higher ratios favoring complete hydrolysis but potentially causing precipitation.

Temperature is another critical parameter. Lower temperatures (below 50°C) slow reaction kinetics, yielding smaller particles with narrow size distributions. Elevated temperatures accelerate gelation but may cause particle agglomeration. Aging the gel at controlled temperatures allows further polymerization and structural reorganization.

Drying the gel removes residual solvents and water, producing a xerogel. Conventional drying at ambient conditions can lead to capillary forces that collapse the porous structure. Supercritical drying with CO2 avoids this issue, producing aerogels with high surface areas. Calcination converts the amorphous TiO2 xerogel or aerogel into crystalline phases. Heating temperatures between 300°C and 600°C favor the anatase phase, while temperatures above 600°C promote rutile formation. Prolonged calcination or higher temperatures increase crystallite size due to sintering.

Characterization techniques are essential for analyzing sol-gel-derived TiO2 nanoparticles. X-ray diffraction (XRD) determines crystallinity and phase composition. The anatase phase exhibits peaks at 2θ = 25.3°, 37.8°, and 48.0°, while rutile shows peaks at 2θ = 27.4°, 36.1°, and 54.3°. The average crystallite size can be estimated using the Scherrer equation applied to peak broadening.

Transmission electron microscopy (TEM) provides direct imaging of particle size, shape, and aggregation. Sol-gel-derived TiO2 nanoparticles typically range from 5 to 50 nm, depending on synthesis conditions. High-resolution TEM (HRTEM) reveals lattice fringes corresponding to anatase (101) or rutile (110) planes, confirming crystallinity.

Nitrogen adsorption-desorption isotherms (BET analysis) measure surface area and porosity. Anatase nanoparticles often exhibit surface areas between 50 and 150 m²/g, with mesopores (2–50 nm) formed during gel drying. Pore size distribution is calculated using the Barrett-Joyner-Halenda (BJH) method.

Fourier-transform infrared spectroscopy (FTIR) identifies surface hydroxyl groups and organic residues. Bands near 3400 cm⁻¹ and 1630 cm⁻¹ correspond to adsorbed water and hydroxyl groups, while peaks below 1000 cm⁻¹ are attributed to Ti-O-Ti vibrations.

The sol-gel method allows tuning of TiO2 nanoparticle properties for specific applications. Smaller particles with high surface area enhance photocatalytic activity, while controlled phase composition (anatase/rutile mixtures) improves charge separation in solar cells. Surface modification during synthesis, such as doping or functionalization, further tailors performance.

In summary, sol-gel synthesis of TiO2 nanoparticles offers precise control over material properties through careful selection of precursors, catalysts, and processing conditions. Understanding the interplay between hydrolysis, condensation, and calcination enables the design of nanoparticles with optimized size, crystallinity, and surface characteristics for diverse technological applications.