Titanium dioxide

Titanium dioxide, also known as titanium(IV) oxide or titania, is the naturally occurring oxide of titanium, chemical formula TiO₂. When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. It is noteworthy for its wide range of applications, from paint to sunscreen to food colouring.

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Natural occurrence

Titanium dioxide occurs in four forms:

• rutile, a tetragonal mineral usually of prismatic habit, often twinned;
• anatase or octahedrite, a tetragonal mineral of dipyramidal habit;
• brookite, an orthorhombic mineral. Both anatase and brookite are relatively rare minerals;
• Titanium dioxide (B) or TiO₂(B), a monoclinic mineral.

Titanium dioxide occurrences in nature are never pure; it is found with contaminant metals such as iron. The oxides can be mined and serve as a source for commercial titanium. The metal can also be mined from other minerals such as ilmenite or leucoxene ores, or one of the purest forms, rutile beach sand. Star sapphires and rubies get their asterism from rutile impurities present in them.^[1]

Production

Crude titanium dioxide is purified via titanium tetrachloride in the chloride process. In this process, the crude ore (containing at least 90% TiO₂) is reduced with carbon, oxidized with chlorine to give titanium tetrachloride. This titanium tetrachloride is distilled, and re-oxidized with oxygen to give pure titanium dioxide.^[2]

Another widely used process utilizes ilmenite as the titanium dioxide source, which is digested in sulfuric acid. The by-product iron(II) sulfate is crystallized and filtered-off to yield only the titanium salt in the digestion solution, which is processed further to give pure titanium dioxide.

Applications

Titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index (n=2.7), in which it is surpassed only by a few other materials. Approximately 4 million tonnes of pigmentary TiO₂ are consumed annually worldwide. When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors and some gemstones, for example "mystic fire topaz". TiO₂ is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets) as well as most toothpastes. Used as a white food colouring, it has E number E171. In cosmetic and skin care products, titanium dioxide is used both as a pigment and a thickener. It is also used as a tattoo pigment and styptic pencils.

This pigment is used extensively in plastics and other applications for its UV resistant properties where it acts as a UV absorber, efficiently transforming destructive UV light energy into heat.

In ceramic glazes titanium dioxide acts as an opacifier and seeds crystal formation. In almost every sunscreen with a physical blocker, titanium dioxide is found because of its high refractive index, its strong UV light absorbing capabilities and its resistance to discolouration under ultraviolet light. This advantage enhances its stability and ability to protect the skin from ultraviolet light. Sunscreens designed for infants or people with sensitive skin are often based on titanium dioxide and/or zinc oxide, as these mineral UV blockers are less likely to cause skin irritation than chemical UV absorber ingredients, such as avobenzone.

Titanium oxide is also used as a semiconductor.^[3]

As a photocatalyst

Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet light. Recently it has been found that titanium dioxide, when spiked with nitrogen ions, is also a photocatalyst under visible light. The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals. It can also oxidize oxygen or organic materials directly. Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for sterilizing, deodorizing and anti-fouling properties and is also used as a hydrolysis catalyst. It is also used in the Graetzel cell, a type of chemical solar cell.

Titanium dioxide has potential for use in energy production: as a photocatalyst, it can

1. carry out hydrolysis; i.e., break water into hydrogen and oxygen. Were the hydrogen collected, it could be used as a fuel. The efficiency of this process can be greatly improved by doping the oxide with carbon, as described in "Carbon-doped titanium dioxide is an effective photocatalyst". ^[4]
2. produce electricity when in nanoparticle form. Research suggests that by using these nanoparticles to form the pixels of a screen, they generate electricity when transparent and under the influence of light. If subjected to electricity on the other hand, the nanoparticles blacken, forming the basic characteristics of a LCD screen. According to creator Zoran Radivojevic, Nokia has already built a functional 200-by-200-pixel monochromatic screen which is energetically self-sufficient.

As TiO₂ is exposed to UV light, it becomes increasingly hydrophilic; thus, it can be used for anti-fogging coatings or self-cleaning windows. TiO₂ incorporated into outdoor building materials, such as paving stones in noxer blocks, can substantially reduce concentrations of airborne pollutants such as volatile organic compounds and nitrogen oxides.

For wastewater remediation

TiO₂ offers great potential as an industrial technology for detoxification or remediation of wastewater due to several factors.

1. The process occurs under ambient conditions very slowly, direct UV light exposure increases the rate of reaction.
2. The formation of photocyclized intermediate products, unlike direct photolysis techniques, is avoided.
3. Oxidation of the substrates to CO₂ is complete.
4. The photocatalyst is inexpensive and has a high turnover.
5. TiO₂ can be supported on suitable reactor substrates.

Other applications

It is also used in resistance-type lambda probes (a type of oxygen sensor).

Titanium dioxide is what allows osseointegration between an artificial medical implant and bone.

Titanium dioxide in solution or suspension can be used to cleave protein that contains the amino acid proline at the site where proline is present. This breakthrough in cost-effective protein splitting took place at ASU in 2006.^[5]

Titanium dioxide on silica is being developed as a form of odor control in cat litter. The purchased photocatalyst is vastly cheaper than the purchased silica beads, per usage, and prolongs their effective odor-eliminating life substantially.

The Pilkington Activ glass has a special nano-scale, extremely thin hydrophilic coating of microcrystalline titanium oxide which catalyses the break-down of organic surface contamination by ultraviolet light from the sun. ^[6]

Historical uses

The Vinland map, the map of America ("Vinland") that was supposedly drawn during mid-15th century based on data from the Viking Age, has been declared a forgery on the basis that the ink on it contains traces of the TiO₂-form anatase; TiO₂ was not synthetically produced before the 1920s. Recently (1992) a counter-claim has been made that the compound can be formed from ancient ink.^{[citation needed]}

Titanium dioxide white paint was used to paint the Saturn V rocket, which is so far the only rocket that has sent astronauts to the moon. In 2002, a spectral analysis of J002E3, a celestial object, showed that it had titanium dioxide on it, giving evidence it may be a Saturn V S-IVB.

See also

• Noxer, a building material incorporating TiO₂.

References