Titanium Dioxide

Titanium dioxide (chemical formula TiO₂), commonly referred to as titania, is one of the most widely used inorganic compounds in the modern world. Distinguished by its brilliant whiteness, chemical stability, and versatile optical properties, it serves as a crucial material across industries including paints, cosmetics, plastics, and environmental technologies. Its significance has further increased with advances in nanotechnology, where titanium dioxide nanoparticles have found new applications and raised fresh scientific and regulatory interest.

Structure, Polymorphs and Physical Properties

Titanium dioxide naturally occurs in several crystal structures, known as polymorphs. The three primary forms are rutile, anatase, and brookite.

• Rutile is the most stable and dense form at room temperature. It has a high refractive index and superior opacity, making it ideal for pigment production.
• Anatase, although metastable, is highly valued for its photocatalytic properties, especially when synthesised as nanoparticles. It exhibits a larger surface area and greater activity under ultraviolet light.
• Brookite is comparatively rare and less significant industrially, though of mineralogical interest.

Other high-pressure phases can form under extreme geological conditions but are not commercially relevant.
Titanium dioxide is a white, odourless, and non-toxic powder with a molar mass of 79.87 g/mol. It is insoluble in water and organic solvents, though it can react with strong acids at high temperatures. One of its most important characteristics is its high refractive index, which is greater than that of diamond. This property allows it to scatter light efficiently, producing an intense whiteness and opacity. Additionally, TiO₂ functions as a semiconductor with a band gap of around 3.0–3.2 eV, making it photoactive under ultraviolet radiation.

Industrial Production

Titanium dioxide is primarily manufactured through two industrial processes: the sulfate process and the chloride process.

1. Sulfate ProcessThis traditional method begins with ilmenite ore (FeTiO₃) or titanium slag. The raw material is digested in concentrated sulfuric acid to yield soluble titanium salts. After purification and precipitation, titanium hydroxide is calcined to produce either rutile or anatase pigment. This process is relatively low-cost but produces acidic waste that requires treatment.
2. Chloride ProcessIn this method, titanium-bearing ores are chlorinated to form titanium tetrachloride (TiCl₄), which is then oxidised at high temperature in the presence of oxygen to form TiO₂. The chloride process yields high-purity pigment and generates less waste, making it more suitable for large-scale, environmentally regulated production.

Control over particle size, surface coatings, and crystal phase is crucial to achieve specific functional properties for varied applications.

Applications

Pigments and CoatingsTitanium dioxide is best known as a white pigment used in paints, coatings, plastics, papers, and printing inks. It provides brightness, whiteness, and excellent opacity, requiring smaller quantities than most other white pigments. In ceramics and glazes, TiO₂ functions as an opacifier, enhancing the visual appearance and durability of products.
Cosmetics and SunscreensMicronised or nano-sized TiO₂ is widely used in sunscreens and personal care products as a physical UV filter. It reflects, scatters, and absorbs ultraviolet radiation, providing effective protection against UVA and UVB rays. Surface-treated forms of TiO₂ are often used to minimise the generation of reactive radicals and to improve dispersion in creams and lotions.
Food and PharmaceuticalsTitanium dioxide has long been used as a whitening and opacifying agent in foods, confectionery, and pharmaceuticals, where it was designated as food additive E171. It enhanced the appearance of sweets, sauces, and tablet coatings. However, recent health assessments have led to regulatory bans in some regions due to potential concerns about nanoparticle safety and genotoxicity.
Photocatalysis and Environmental UsesOne of the most remarkable properties of TiO₂, particularly in the anatase form, is its photocatalytic activity under ultraviolet light. This allows it to decompose organic pollutants, bacteria, and odours, making it invaluable in environmental purification technologies. TiO₂ is used in:

• Self-cleaning glass and coatings that break down grime under sunlight.
• Air purification systems that remove volatile organic compounds.
• Water treatment and wastewater remediation.

In renewable energy, titanium dioxide serves as a semiconductor layer in dye-sensitised solar cells and is also being investigated for hydrogen generation through photocatalytic water splitting.
Other ApplicationsTitanium dioxide’s high refractive index and dielectric constant make it useful in optical coatings, capacitors, and electronic components. It is also incorporated into rubber, adhesives, and plastics to improve mechanical strength, UV resistance, and visual appeal. Furthermore, it acts as an essential intermediate in titanium metal production.

Safety, Toxicology and Environmental Aspects

While bulk titanium dioxide has long been regarded as inert and safe, its nanoparticulate forms have sparked significant debate due to potential biological and environmental impacts.
Health ConsiderationsInhalation of fine TiO₂ dust can irritate the respiratory tract, and chronic exposure at high levels has been associated with lung inflammation in experimental animals. Consequently, inhalable titanium dioxide dust is classified by some health authorities as “possibly carcinogenic to humans” (Group 2B).
Oral ingestion of food-grade titanium dioxide has been reconsidered in recent years due to uncertainties surrounding genotoxicity, or DNA damage potential. Although no direct human harm has been conclusively proven, regulatory agencies in certain regions have chosen to err on the side of caution by prohibiting its use in food products.
For dermal use in cosmetics and sunscreens, titanium dioxide is generally considered safe, as it does not penetrate healthy skin and remains on the surface, functioning as a physical barrier.
Environmental ImpactNano-sized titanium dioxide can enter water systems through wastewater discharge, primarily from personal care products and coatings. In aquatic environments, it may cause oxidative stress in microorganisms and algae when exposed to sunlight. Its persistence and potential bioaccumulation are still being studied, with efforts directed towards developing safer, surface-modified particles that minimise environmental impact.

Advantages and Limitations

Advantages

• Exceptional whiteness and brightness, enabling efficient pigmentation.
• Excellent chemical and thermal stability, maintaining colour and structure under harsh conditions.
• Non-toxic and biocompatible in bulk form, suitable for numerous consumer applications.
• Photocatalytic and UV-resistant properties extend its functionality beyond traditional pigment use.
• Adaptability through surface treatment, doping, and particle size control allows tailoring for advanced uses.

Limitations

• Nanoparticle toxicity and inhalation risks necessitate strict industrial hygiene controls.
• Photocatalytic activity can sometimes cause undesirable degradation of organic binders in paints or plastics if not adequately coated.
• Regulatory restrictions in certain sectors, especially food, may limit market scope.
• Production processes, particularly the sulfate method, can generate environmentally burdensome waste.

Market and Future Perspectives

Titanium dioxide remains one of the most important and high-volume materials in the global chemical industry. The pigment market continues to dominate, with demand driven by the construction, automotive, and consumer goods sectors.
At the same time, research is expanding towards functional nanomaterials and sustainable technologies. Innovations include doped TiO₂ that absorbs visible light for improved photocatalysis, self-cleaning architectural materials, and next-generation solar energy devices. Efforts are also directed at developing eco-friendly production methods and non-toxic alternatives that maintain whiteness while reducing potential health concerns.