TiO₂ — Titanium Dioxide

Titanium dioxide (TiO₂), also known as titania, is a naturally occurring oxide of titanium. It is one of the most important white pigments in the world, known for its high brightness, opacity, and ability to scatter visible light. Found naturally as minerals such as rutile, anatase, and brookite, titanium dioxide has extensive applications in paints, coatings, plastics, paper, food coloring, and sunscreens.

Because of its non-toxic nature, high refractive index, and chemical inertness, titanium dioxide is widely used in both industrial and consumer products. In addition, it exhibits photocatalytic properties, enabling it to decompose organic pollutants under ultraviolet light. This property makes it a key material for environmental purification, solar cells, and self-cleaning surfaces.

Titanium dioxide is composed of titanium (Ti⁴⁺) and oxygen (O²⁻) ions in a 1:2 ratio. The compound exists in three main crystalline forms:

• 1. Rutile: The most stable and dense form of TiO₂, used primarily in pigments and coatings. It has a tetragonal crystal structure and exhibits a high refractive index (~2.7).
• 2. Anatase: A metastable form with a slightly lower density, often used in photocatalysts and solar energy devices because of its high surface activity and light absorption capacity.
• 3. Brookite: The rarest polymorph of TiO₂, also tetragonal but with a more complex crystal arrangement, mostly found in natural mineral deposits.

In all these structures, titanium atoms are surrounded by six oxygen atoms forming octahedral geometry, while oxygen atoms are bonded to three titanium atoms. The strong Ti–O bonds give titanium dioxide its exceptional stability and hardness.

Each crystalline form has different electronic and optical properties, influencing its industrial use. For instance, rutile TiO₂ is preferred for pigments, while anatase TiO₂ is preferred for photocatalysis and UV protection.

Commercial titanium dioxide is primarily manufactured through two processes — the sulfate process and the chloride process. Both methods start from titanium-rich ores such as ilmenite (FeTiO₃) or rutile (TiO₂).

• 1. Sulfate Process: This method involves dissolving ilmenite or titanium slag in concentrated sulfuric acid, forming titanyl sulfate. The solution is then hydrolyzed and calcined to produce pure TiO₂.

• 2. Chloride Process: In this modern process, rutile or synthetic titanium feedstocks are reacted with chlorine gas at high temperatures in the presence of carbon, forming titanium tetrachloride (TiCl₄). The TiCl₄ vapor is then oxidized to produce TiO₂.

The chloride process yields higher purity and finer particles, making it the preferred industrial method for producing pigment-grade titanium dioxide.

Physical Properties:

• White crystalline powder with high opacity and brightness.
• Insoluble in water and organic solvents but dissolves slowly in hot, concentrated acids like H₂SO₄ and HF.
• Exhibits high refractive index and low absorption, making it ideal for optical applications.
• Stable under UV radiation, high temperatures, and most chemical environments.

Chemical Properties:

• 1. Amphoteric Nature: Titanium dioxide reacts with both acids and bases to form salts and titanates.

\( TiO_2 + 2NaOH + H_2O \rightarrow Na_2TiO_3 + 2H_2O \)

• 2. Reduction: When heated with carbon, TiO₂ can be reduced to metallic titanium or lower oxides.

\( TiO_2 + 2C \xrightarrow{heat} Ti + 2CO \uparrow \)

• 3. Photocatalysis: Under UV light, TiO₂ generates electron-hole pairs that can oxidize organic compounds or reduce water into hydrogen and oxygen.

\( TiO_2 + hv \rightarrow e^- + h^+ \)

This photocatalytic activity makes TiO₂ valuable for environmental purification and solar energy conversion.

Titanium dioxide is one of the most versatile and commercially valuable inorganic compounds in modern industry. Some major applications include:

• 1. Pigments and Paints: TiO₂ is the primary white pigment used in paints, coatings, plastics, inks, and papers due to its excellent light scattering and UV resistance.
• 2. Cosmetics and Sunscreens: Acts as a physical UV filter in sunscreens, protecting skin from harmful UVA and UVB radiation without penetrating the skin.
• 3. Photocatalysts: Anatase TiO₂ is widely used in self-cleaning surfaces, antibacterial coatings, and air and water purification systems due to its photocatalytic efficiency.
• 4. Food and Pharmaceuticals: Used as a whitening agent (food additive E171) in candies, tablets, and toothpaste, though its use is being reviewed for safety in some countries.
• 5. Ceramics and Glass: Enhances opacity and refractive index in high-quality ceramics and optical glass.
• 6. Solar Cells: Utilized in dye-sensitized solar cells (DSSCs) for improving light absorption and electron transport.

Overall, titanium dioxide’s combination of brightness, stability, and reactivity makes it indispensable across diverse industries.

Although titanium dioxide is considered chemically inert and non-toxic, concerns have been raised regarding its nanoparticle form, which may pose inhalation risks. The compound is classified as a possible carcinogen (Group 2B) by the International Agency for Research on Cancer (IARC) when inhaled as dust.

Safety Measures:

• Handle powdered TiO₂ in well-ventilated areas and wear protective masks.
• Avoid generating dust or aerosol forms during processing.
• Use gloves and safety goggles when handling bulk material.
• Dispose of TiO₂ waste according to environmental regulations.

In general, titanium dioxide used in consumer products such as sunscreens, food, and paints is safe due to its low bioavailability and inert nature.