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 when it is given the E number E171.

Occurrence

Titanium dioxide occurs in nature as the well-known naturally occurring minerals rutile, anatase and brookite, additionally two high pressure forms, the monoclinic baddeleyite form and the orthorhombic α-PbO₂ form have been found at the Ries crater in Bavaria. The most common form is rutile, which is also the most stable form. Anatase and brookite both convert to rutile upon heating. Rutile, anatase and brookite all contain six coordinate titanium. Additionally there are three metastable forms produced synthetically and five high pressure forms:

Form	Crystal system	Synthesis
rutile	tetragonal
anatase	tetragonal
brookite	orthorhombic
TiO2(B)	monoclinic	Hydrolysis of K2Ti4O9 followed by heating
TiO2(H), hollandite form	tetragonal	Oxidation of the related potassium titanate bronze, K0.25TiO2
TiO2(R), ramsdellite form	orthorhombic	Oxidation of the related lithium titanate bronze Li0.5TiO2
TiO2(II)-(α-PbO2 form)	orthorhombic
baddeleyite form, (7 coordinate Ti)	monoclinic
TiO2 -OI	orthorhombic
cubic form	cubic
TiO2 -OII, cotunnite, PbCl2	orthorhombic

The naturally occurring 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.

Spectral lines from titanium oxide are prominent in class M stars, which are cool enough to allow molecules of this chemical to form.

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.

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. Another method for upgrading ilmenite is called the Becher Process.

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 tons 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.

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, or doped with metal oxide like tungsten trioxide, is also a photocatalyst under visible and UV 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.

The photocatylic properties of titanium dioxide were discovered by Akira Fujishima in 1967. The process on the surface of the titanium dioxide was called the Honda-Fujishima effect.

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".
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 or paints, 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 Arizona State University in 2006.

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.

In 1995 the Research Institute of Toto Ltd. discovered the superhydrophilicity phenomenon for glass coated with titanium dioxide and exposed to sun light. Professor Fujishima and his group discovered that This resulted in the development of self-cleaning glass.

Titanium dioxide is also used as a material in the memristor, a new electronic circuit element.

Historical uses

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

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.
• Timeline of hydrogen technologies

Toxicology

Titanium dioxide dust, when inhaled, has recently been classified by the International Agency for Research on Cancer (IARC) as an IARC Group 2B carcinogen possibly carcinogenic to humans. Titanium dioxide accounts for 70% of the total production volume of pigments worldwide. It is widely used to provide whiteness and opacity to products such as paints, plastics, papers, inks, foods, and toothpastes. It is also used in cosmetic and skin care products, and it is present in almost every sunblock, where it helps protect the skin from ultraviolet light.

With such widespread use of titanium dioxide, it is important to understand that the IARC conclusions are based on very specific evidence. This evidence showed that high concentrations of pigment-grade (powdered) and ultrafine titanium dioxide dust caused respiratory tract cancer in rats exposed by inhalation and intratracheal instillation*. The series of biological events or steps that produce the rat lung cancers (e.g. particle deposition, impaired lung clearance, cell injury, fibrosis, mutations and ultimately cancer) have also been seen in people working in dusty environments. Therefore, the observations of cancer in animals were considered, by IARC, as relevant to people doing jobs with exposures to titanium dioxide dust. For example, titanium dioxide production workers may be exposed to high dust concentrations during packing, milling, site cleaning and maintenance, if there are insufficient dust control measures in place. However, it should be noted that the human studies conducted so far do not suggest an association between occupational exposure to titanium dioxide and an increased risk for cancer.

The Workplace Hazardous Materials Information System (WHMIS) is Canada's hazard communication standard. The WHMIS Controlled Products Regulations require that chemicals, listed in Group 1 or Group 2 in the IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans, be classified under WHMIS Class D2A (carcinogenic). The classification decision on titanium dioxide has been published on the IARC website and in a summary article published in The Lancet.

Representatives from Health Canada (National Office of WHMIS) recently consulted with the Quebec CSST and CCOHS (the two main agencies providing WHMIS classifications to the public) regarding the implications of the IARC decision to the WHMIS classification of titanium dioxide. It was agreed that titanium dioxide does now meet the criteria for WHMIS D2A (carcinogen) based on the information released by IARC to date, and that it is not necessary to wait for release of the full monograph.

Manufacturers and suppliers of titanium dioxide are advised to review and update their material safety data sheets and product labels based on this new information as soon as possible. Employers should review their occupational hygiene programs to ensure that exposure to titanium dioxide dust is eliminated or reduced to the minimum possible. Workers should be educated concerning this potential newly recognized risk to their health and trained in proper work procedures.

References

• Kutal, C., Serpone, N. (1993). Photosensitive Metal Organic Systems: Mechanistic Principles and Applications. American Chemical Society, Washington D.C.

External links

• International Chemical Safety Card 0338
• NIOSH Pocket Guide to Chemical Hazards
• "Fresh doubt over America map", bbc.co.uk, [[30 July] 2002]
• Titanium Dioxide Classified as Possibly Carcinogenic to Humans, 2007 (if inhaled as a powder)
• A description of TiO₂ photocatalysis
• Crystal structures of the three forms of TiO₂
• "Architecture in Italy goes green", Elisabetta Povoledo, International Herald Tribune, [[November 22], 2006]
• "A Concrete Step Toward Cleaner Air", Bruno Giussani, BusinessWeek.com, [[November 8], 2006]
• "Titanium Dioxide Classified as Possibly Carcinogenic to Humans", Canadian Centre for Occupational Health and Safety, August, 2006
• "Diflucan antifungal med has titanium dioxide "inactive ingregient" in suspension - see page 1 of linked full prescribing info from Pfizer". Is it carcinogenic like the powder breathing is?