Titanium dioxide (TiO2) has been extensively studied due to its remarkable properties, including photoelectricity, high dielectric constant, catalytic conversion, humidity treatment of polluted air and water, oxygen sensitivities, and its ability to act as a photocatalyst, facilitating environmentally beneficial reactions such as water splitting to generate hydrogen.
Titanium dioxide (TiO2), or Titania, primarily exists in three natural phases: anatase, brookite, and rutile. These phases have different crystalline structures; brookite is orthorhombic, anatase is tetragonal, and rutile is also tetragonal. Anatase and brookite are metastable phases, while rutile is the most stable and can transform from other phases when heated to higher temperatures. Rutile, with its higher refractive index and ultraviolet absorptivity, is commonly used in pigments, paints, and ultraviolet absorbents. The phase, composition, and surface states of titanium dioxide strongly influence its charge properties and electronic structure, affecting its photocatalytic activity.
Titanium dioxide also exhibits excellent chemical stability and enhances the brightness of colored pigments. Titanium white, known for its high refractive index and optical properties of UV light absorption and scattering, is widely used in sunscreen applications. Proper surface treatment of titanium dioxide particles is crucial to prevent the degradation of organic substrates by the filler particles. Silica coating on titanium dioxide has been extensively studied for its ability to enhance powder durability, although it may lead to opacity loss due to particle agglomeration during liquid-phase manufacturing. Titanium dioxide (TiO2) is a wide bandgap semiconductor, absorbing only a small fraction of solar light. Efforts have been made to extend titanium dioxide’s absorption to the visible region through doping with transition metals, nonmetal atoms, and organic materials. Controlling the morphology and structure of nanoscale titanium dioxide is essential for its various applications. Anatase-phase titanium dioxide nanocrystals, known for their high photocatalytic activity, have garnered attention due to their distinct properties compared to bulk materials. The annual global production of titanium dioxide exceeds 4 million tons, with significant usage in paints, plastics, paper, and various other applications.
Crystal Structure of Titanium Dioxide (TiO2)
Titanium dioxide (TiO2) exists in three natural phases: brookite, anatase, and rutile. These phases have different crystalline structures; brookite is orthorhombic, while anatase and rutile are tetragonal. Both crystals have each Ti atom surrounded by six adjacent O atoms in approximately regular but slightly different octahedral arrangements. Rutile has a more compact structure than anatase, accounting for its higher refractive index. Rutile’s crystal structure consists of corner-shared TiO6 octahedra in a tetragonal cell, while anatase’s structure consists of edge-shared TiO6 octahedra. Rutile is more stable than anatase and is the most abundant form of titanium dioxide.
Properties of Titanium Dioxide (TiO2)
Chemical Properties
Pure titanium dioxide is a colorless, crystalline solid with high stability, non-volatility, and insolubility. It is amphoteric, with more acidic than basic characteristics. Titanium dioxide exists in three fundamental crystal forms: anatase, rutile, and brookite, with rutile being the most stable. Rutile exhibits a higher refractive index, specific gravity, and chemical stability compared to anatase. The high refractive index of titanium dioxide contributes to its hiding power in paints and pigments.
Optical Properties
Titanium dioxide exhibits brightness and whiteness, with reflectance properties similar to a perfect reflecting diffuser. Controlling color during manufacturing is crucial for achieving the desired brightness and whiteness. Flocculation, the formation of loose clumps of titanium dioxide particles in a fluid system, can affect opacity and tinting strength. Titanium dioxide pigments are highly effective in imparting whiteness and brightness to various materials, including paints, plastics, papers, fibers, and enamels. Surface treatments are used to optimize dispersion in different applications.
Applications of Titanium Dioxide
Titanium dioxide finds applications in dye-sensitized solar cells (DSSCs), organic solar cells, photocatalysis, chemical sensors, microelectronics, and electrochemistry. Its ability to scatter visible light and absorb UV light energy enhances the weatherability and durability of polymer products. Titanium dioxide is widely used in paints, plastics, paper, fibers, and enamels due to its high refractive index, brightness, and opacity.