Titanium dioxide
Names | |
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IUPAC names
Titanium dioxide
Titanium(IV) oxide | |
Other names | |
Identifiers | |
3D model (
JSmol ) |
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ChEBI | |
ChEMBL | |
ChemSpider | |
ECHA InfoCard
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100.033.327 |
E number | E171 (colours) |
KEGG | |
PubChem CID
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RTECS number
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UNII | |
CompTox Dashboard (EPA)
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Properties | |
TiO 2 | |
Molar mass | 79.866 g/mol |
Appearance | White solid |
Odor | Odorless |
Density |
|
Melting point | 1,843 °C (3,349 °F; 2,116 K) |
Boiling point | 2,972 °C (5,382 °F; 3,245 K) |
Insoluble | |
Band gap | 3.05 eV (rutile)[1] |
+5.9·10−6 cm3/mol | |
Refractive index (nD)
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Thermochemistry | |
Std molar
entropy (S⦵298) |
50 J·mol−1·K−1[2] |
Std enthalpy of (ΔfH⦵298)formation |
−945 kJ·mol−1[2] |
Hazards | |
NFPA 704 (fire diamond) | |
Flash point | not flammable |
NIOSH (US health exposure limits): | |
PEL (Permissible)
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TWA 15 mg/m3[3] |
REL (Recommended)
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Ca[3] |
IDLH (Immediate danger) |
Ca [5000 mg/m3][3] |
Safety data sheet (SDS) | ICSC 0338 |
Related compounds | |
Other cations
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Hafnium dioxide
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Titanium(II) oxide Titanium(III) oxide Titanium(III,IV) oxide | |
Related compounds
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Titanic acid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Titanium dioxide, also known as titanium(IV) oxide or titania /taɪˈteɪniə/, is the inorganic compound with the chemical formula TiO
2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891.[4] It is a white solid that is insoluble in water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tonnes.[5][6][7] It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion.[8]
Structure
In all three of its main dioxides,
Molten titanium dioxide has a local structure in which each Ti is coordinated to, on average, about 5 oxygen atoms.[9] This is distinct from the crystalline forms in which Ti coordinates to 6 oxygen atoms.
Synthetic and geologic occurrence
Synthetic TiO2 is mainly produced from the mineral
Mineralogy and uncommon polymorphs
Titanium dioxide occurs in nature as the minerals rutile and anatase. Additionally two high-pressure forms are known minerals: a monoclinic baddeleyite-like form known as akaogiite, and the other has a slight monoclinic distortion of the orthorhombic α-PbO2 structure and is known as riesite. Both of which can be found at the Ries crater in Bavaria.[11][12][13] It is mainly sourced from ilmenite, which is the most widespread titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The metastable anatase and brookite phases convert irreversibly to the equilibrium rutile phase upon heating above temperatures in the range 600–800 °C (1,110–1,470 °F).[14]
Titanium dioxide has twelve known polymorphs – in addition to rutile, anatase, brookite, akaogiite and riesite, three metastable phases can be produced synthetically (monoclinic, tetragonal, and orthorhombic ramsdellite-like), and four high-pressure forms (α-PbO2-like, cotunnite-like, orthorhombic OI, and cubic phases) also exist:
Form | Crystal system | Synthesis |
---|---|---|
Rutile | Tetragonal | |
Anatase | Tetragonal | |
Brookite | Orthorhombic | |
TiO2(B)[15] | Monoclinic | Hydrolysis of K2Ti4O9 followed by heating |
TiO2(H), hollandite-like form[16] | Tetragonal | Oxidation of the related potassium titanate bronze, K0.25TiO2 |
TiO2(R), ramsdellite-like form[17] | Orthorhombic | Oxidation of the related lithium titanate bronze Li0.5TiO2 |
TiO2(II)-(α-PbO2-like form)[18] | Orthorhombic | |
Akaogiite (baddeleyite-like form, 7 coordinated Ti)[19] | Monoclinic | |
TiO2 -OI[20] | Orthorhombic | |
Cubic form[21] | Cubic | P > 40 GPa, T > 1600 °C |
TiO2 -OII, cotunnite(PbCl2)-like[22] | Orthorhombic | P > 40 GPa, T > 700 °C |
The
Titanium dioxide (B) is found as a mineral in magmatic rocks and hydrothermal veins, as well as weathering rims on perovskite. TiO2 also forms lamellae in other minerals.[26]
Production
The largest TiO
2 pigment processors are
2 pigment demand for 2010 was 5.3 Mt with annual growth expected to be about 3–4%.[30]
The production method depends on the feedstock. In addition to ores, other feedstocks include upgraded
Chloride process
In chloride process, the ore is treated with chlorine and carbon to give titanium tetrachloride, a volatile liquid that is further purified by distillation. The TiCl4 is treated with oxygen to regenerate chlorine and produce the titanium dioxide.
Sulfate process
Chemical manufacturing plants using the sulfate process, require ilmenite concentrate (45–60% TiO2) or pretreated feedstocks as a suitable source of titanium.[32] In the sulfate process, ilmenite is treated with sulfuric acid to extract iron(II) sulfate pentahydrate. The resulting synthetic rutile is further processed according to the specifications of the end user, i.e. pigment grade or otherwise.[33] In another method for the production of synthetic rutile from ilmenite the Becher process first oxidizes the ilmenite as a means to separate the iron component.
Specialized methods
For specialty applications, TiO2 films are prepared by various specialized chemistries.[34] Sol-gel routes involve the hydrolysis of titanium alkoxides, such as titanium ethoxide:
- Ti(OEt)4 + 2 H2O → TiO2 + 4 EtOH
This technology is suited for the preparation of films. A related approach that also relies on molecular precursors involves chemical vapor deposition. In this application, the alkoxide is volatilized and then decomposed on contact with a hot surface:
- Ti(OEt)4 → TiO2 + 2 Et2O
Applications
Pigment
First mass-produced in 1916,
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, supplements, medicines (i.e. pills and tablets), and most toothpastes; in 2019 it was present in two-thirds of toothpastes on the French market.[39] In food, it is commonly found in products like ice creams, chocolates, all types of candy, creamers, desserts, marshmallows, chewing gum, pastries, spreads, dressings, cakes, and many other foods.[40] In paint, it is often referred to offhandedly as "brilliant white", "the perfect white", "the whitest white", or other similar terms. Opacity is improved by optimal sizing of the titanium dioxide particles.
Thin films
When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors; it is also used in generating decorative thin films such as found in "mystic fire topaz".
Some grades of modified titanium based pigments as used in sparkly paints, plastics, finishes and cosmetics – these are man-made pigments whose particles have two or more layers of various oxides – often titanium dioxide,
The iridescent effect in these titanium oxide particles is unlike the opaque effect obtained with usual ground titanium oxide pigment obtained by mining, in which case only a certain diameter of the particle is considered and the effect is due only to scattering.
Sunscreen and UV blocking pigments
In
Titanium dioxide and zinc oxide are generally considered to be less harmful to coral reefs than sunscreens that include chemicals such as oxybenzone, octocrylene and octinoxate.[46]
Nanosized titanium dioxide is found in the majority of physical
The risk assessment of different titanium dioxide nanomaterials in sunscreen is currently evolving as nano-sized TiO2 is different from the well-known micronized form.
Initial studies indicated that nano-TiO2 particles could penetrate the skin causing concern over the use of nano-TiO2. These studies were later refuted, when it was discovered that the testing methodology couldn't differentiate between penetrated particles and particles simply trapped in hair follicles and that having a diseased or physically damaged dermis could be the true cause of insufficient barrier protection.[49]
SCCS research found that when nanoparticles had certain photostable coatings (eg.
TiO
2 is used extensively in plastics and other applications as a white pigment or an opacifier and for its UV resistant properties where the powder disperses light – unlike organic UV absorbers – and reduces UV damage, due mostly to the particle's high refractive index.[55]
Other uses of titanium dioxide
In ceramic glazes, titanium dioxide acts as an opacifier and seeds crystal formation.
It is used as a
The exterior of the Saturn V rocket was painted with titanium dioxide; this later allowed astronomers to determine that J002E3 was likely the S-IVB stage from Apollo 12 and not an asteroid.[56]
Research
Patenting activities
Between 2002 and 2022, there have been 459 patent families that describe the production of titanium dioxide from ilmenite, and this number is growing rapidly. The majority of these patents describe pre-treatment processes, such as using smelting and magnetic separation to increase titanium concentration in low-grade ores, leading to titanium concentrates or slags. Other patents describe processes to obtain titanium dioxide, either by a direct hydrometallurgical process or through two industrially exploited processes, the sulfate process and the chloride process.[57]
Acid leaching might be used either as a pre-treatment or as part of a hydrometallurgical process to directly obtain titanium dioxide or synthetic rutile (>90 percent titanium dioxide, TiO2). The sulfate process represents 40% of the world’s titanium dioxide production and is protected in 23% of patent families. The chloride process is only mentioned in 8% of patent families, although it provides 60% of the worldwide industrial production of titanium dioxide.[57]
Key contributors to patents on the production of titanium dioxide are companies from China, Australia and the United States, reflecting the major contribution of these countries to industrial production. Chinese companies Pangang and Lomon Billions Groups are the main contributors and hold diversified patent portfolios covering both pre-treatment and the processes leading to a final product.[57]
Photocatalyst
Nanosized titanium dioxide, particularly in the anatase form, exhibits
The photocatalytic properties of nanosized titanium dioxide were discovered by
In 1995 Fujishima and his group discovered the superhydrophilicity phenomenon for titanium dioxide coated glass exposed to sun light.[63] This resulted in the development of self-cleaning glass and anti-fogging coatings.
Nanosized TiO2 incorporated into outdoor building materials, such as paving stones in
Using TiO2 as a photocatalyst, attempts have been made to mineralize pollutants (to convert into CO2 and H2O) in waste water.[72][73][74] The photocatalytic destruction of organic matter could also be exploited in coatings with antimicrobial applications.[75]
Hydroxyl radical formation
Although nanosized anatase TiO2 does not absorb visible light, it does strongly absorb ultraviolet (UV) radiation (hv), leading to the formation of hydroxyl radicals.[76] This occurs when photo-induced valence bond holes (h+vb) are trapped at the surface of TiO2 leading to the formation of trapped holes (h+tr) that cannot oxidize water.[77]
- TiO2 + hv → e− + h+vb
- h+vb → h+tr
- O2 + e− → O2•−
- O2•− + O2•−+ 2 H+ → H2O2 + O2
- O2•− + h+vb → O2
- O2•− + h+tr → O2
- OH− + h+vb → HO•
- e− + h+tr → recombination
- Note: Wavelength (λ)= 387 nm[77] This reaction has been found to mineralize and decompose undesirable compounds in the environment, specifically the air and in wastewater.[77]
Nanotubes
Anatase can be converted into non-carbon nanotubes and nanowires.[78] Hollow TiO2 nanofibers can be also prepared by coating carbon nanofibers by first applying titanium butoxide.[79]
Health and safety
As of 2006, titanium dioxide has been regarded as "completely nontoxic".[4] Widely-occurring minerals and even gemstones are composed of TiO2. All natural titanium, comprising more than 0.5% of the Earth's crust, exists as oxides. Although no evidence points to acute toxicity, recurring concerns have been expressed about nanophase forms of these materials. Studies of workers with high exposure to TiO2 particles indicate that even at high exposure there is no adverse effect to human health.[80]
The European Union removed the authorization to use titanium dioxide (E 171) in foods, effective 7 February 2022, with a six months grace period.[81]
Titanium dioxide dust, when inhaled, has been classified by the
The US National Institute for Occupational Safety and Health recommends two separate exposure limits. NIOSH recommends that fine TiO2 particles be set at an exposure limit of 2.4 mg/m3, while ultrafine TiO
2 be set at an exposure limit of 0.3 mg/m3, as time-weighted average concentrations up to 10 hours a day for a 40-hour work week.[84]
As of May 2023, following the European Union 2022 ban, the U.S. states California and New York were considering banning the use of titanium dioxide in foods.[85]
Environmental waste introduction
Titanium dioxide (TiO₂) is mostly introduced into the environment as nanoparticles via wastewater treatment plants.[86] Cosmetic pigments including titanium dioxide enter the wastewater when the product is washed off into sinks after cosmetic use. Once in the sewage treatment plants, pigments separate into sewage sludge which can then be released into the soil when injected into the soil or distributed on its surface. 99% of these nanoparticles wind up on land rather than in aquatic environments due to their retention in sewage sludge.[86] In the environment, titanium dioxide nanoparticles have low to negligible solubility and have been shown to be stable once particle aggregates are formed in soil and water surroundings.[86] In the process of dissolution, water-soluble ions typically dissociate from the nanoparticle into solution when thermodynamically unstable. TiO2 dissolution increases when there are higher levels of dissolved organic matter and clay in the soil. However, aggregation is promoted by pH at the isoelectric point of TiO2 (pH= 5.8) which renders it neutral and solution ion concentrations above 4.5 mM.[87][88]
National policies on food additive use
TiO2 whitener in food was banned in France from 2020, due to uncertainty about safe quantities for human consumption.[89]
In 2021, the
As of 2024, the Food and Drug Administration (FDA) in the United States permits titanium dioxide as a food additive. It is commonly used to increase whiteness and opacity in dairy products (lowfat milk, cream, ice cream, yogurt, etc), candies, frostings, fillings, and many other foods. The FDA permits the product's ingredients list to identify titanium dioxide as "color added" or "artificial colors" and does not require that titanium dioxide be explicitly named.[93][94][95]
Research as an ingestible nanomaterial
Due to the potential that long-term ingestion of titanium dioxide may be toxic, particularly to cells and functions of the gastrointestinal tract, preliminary research as of 2021 was assessing its possible role in disease development, such as inflammatory bowel disease and colorectal cancer.[96]
Culture and society
Companies such as Dunkin' Donuts dropped titanium dioxide from their merchandise in 2015 after public pressure.[97] Andrew Maynard, director of Risk Science Center at the University of Michigan, rejected the supposed danger from use of titanium dioxide in food. He says that the titanium dioxide used by Dunkin' Brands and many other food producers is not a new material, and it is not a nanomaterial either. Nanoparticles are typically smaller than 100 nanometres in diameter, yet most of the particles in food grade titanium dioxide are much larger.[98] Still, size distribution analyses showed that batches of food-grade TiO₂ always include a nano-sized fraction as inevitable byproduct of the manufacturing processes.[99]
See also
- Delustrant
- Dye-sensitized solar cell
- List of inorganic pigments
- Noxer blocks, TiO2-coated pavers that remove NOx pollutants from the air
- Suboxide
- Surface properties of transition metal oxides
- Titanium dioxide nanoparticle
Sources
This article incorporates text from a free content work. Licensed under CC-BY. Text taken from Production of titanium and titanium dioxide from ilmenite and related applications, WIPO.
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External links
- International Chemical Safety Card 0338
- NIOSH Pocket Guide to Chemical Hazards
- "Titanium Dioxide Classified as Possibly Carcinogenic to Humans", Canadian Centre for Occupational Health and Safety, August, 2006 (if inhaled as a powder)
- A description of TiO2 photocatalysis
- Titanium and titanium dioxide production data (US and World)