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Titanium dioxide

|Section2={{Chembox Properties | Formula = | MolarMass = 79.866 g/mol | Appearance = White solid | Odor = odorless | Density = 4.23 g/cm3 (Rutile) 3.78 g/cm3 (Anatase) | MeltingPtC = 1843 | BoilingPtC = 2972 | Solubility = insoluble | BandGap = 3.05 eV (rutile) | RefractIndex = 2.488 (anatase) 2.583 (brookite) 2.609 (rutile) | MagSus = +5.9·10−6 cm3/mol }} |Section4={{Chembox Thermochemistry | DeltaHf = −945 kJ·mol−1 | Entropy = 50 J·mol−1·K−1 }} |Section7={{Chembox Hazards | ExternalSDS = ICSC 0338 | EUClass = Not listed | NFPA-H = 1 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = | FlashPt = Non-flammable | IDLH = Ca mg/m3 | REL = Ca | PEL = TWA 15 mg/m3 }} |Section8={{Chembox Related | OtherAnions = | OtherCations = Zirconium dioxide Hafnium dioxide | OtherFunction = Titanium(II) oxide Titanium(III) oxide Titanium(III,IV) oxide | OtherFunction_label = titanium oxides | OtherCompounds = Titanic acid }} }} Titanium dioxide, also known as titanium(IV) oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. Generally it is sourced from ilmenite, rutile and anatase. It has a wide range of applications, from paint to sunscreen to food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million metric tons. "Titanium" in 2014 Minerals Yearbook. USGS

Occurrence

Titanium dioxide occurs in nature as the well-known minerals rutile, anatase and brookite, and additionally as two high pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form, both found recently at the Ries crater in Bavaria. One of these is known as akaogiite and should be considered as an extremely rare mineral. Akaogiite. mindat.org It is mainly sourced from ilmenite ore. This is the most widespread form of 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 . Titanium dioxide has eight modifications – in addition to rutile, anatase, and brookite, three metastable phases can be produced synthetically ( monoclinic, tetragonal and orthorombic), and five high-pressure forms (α-PbO2-like, baddeleyite-like, cotunnite-like, orthorhombic OI, and cubic phases) also exist: The cotunnite-type phase was claimed by L. Dubrovinsky and co-authors to be the hardest known oxide with the Vickers hardness of 38 GPa and the bulk modulus of 431 GPa (i.e. close to diamond's value of 446 GPa) at atmospheric pressure. However, later studies came to different conclusions with much lower values for both the hardness (7–20 GPa, which makes it softer than common oxides like corundum Al2O3 and rutile TiO2) and bulk modulus (~300 GPa). The oxides are commercially important ores of 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. 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. Spectral lines from titanium oxide are prominent in class M stars, which are cool enough to allow molecules of this chemical to form.

Production

The production method depends on the feedstock. The most common method for the production of titanium dioxide utilizes the mineral ilmenite. Ilmenite is mixed with sulfuric acid. This reacts to remove the iron oxide group in the ilmenite. The by-product iron(II) sulfate is crystallized and filtered-off to yield only the titanium salt in the digestion solution. This product is called synthetic rutile. This is further processed in a similar way to rutile to give the titanium dioxide product. Synthetic rutile and titanium slags are made especially for titanium dioxide production. The use of ilminite ore usually only produces pigment grade titanium dioxide. Another method for the production of synthetic rutile from ilmenite utilizes the Becher Process. Rutile is the second most abundant mineral sand. Rutile found in primary rock cannot be extracted hence the deposits containing rutile sand can be mined meaning a reduced availability to the high concentration ore. Crude titanium dioxide (in the form of rutile or synthetic rutile) is purified via converting to titanium tetrachloride in the chloride process. In this process, the crude ore (containing at least 70% TiO2) is reduced with carbon, oxidized with chlorine to give titanium tetrachloride; i.e., carbothermal chlorination. This titanium tetrachloride is distilled, and re-oxidized in a pure oxygen flame or plasma at 1500–2000 K to give pure titanium dioxide while also regenerating chlorine. Aluminium chloride is often added to the process as a rutile promotor; the product is mostly anatase in its absence. The preferred raw material for the chloride process is natural rutile because of its high titanium dioxide content. One method for the production of titanium dioxide with relevance to nanotechnology is solvothermal Synthesis of titanium dioxide. image]]

Nanotubes

Anatase can be converted by hydrothermal synthesis to delaminated anatase inorganic nanotubes and titanate nanoribbons which are of potential interest as catalytic supports, photocatalysts, thermal and mechanical reinforcement of polymer. In the synthesis, anatase is mixed with 10  M sodium hydroxide and heated at for 72 hours. The reaction product is washed with dilute hydrochloric acid and heated at for another 15 hours. The yield of nanotubes is quantitative and the tubes have an outer diameter of 10 to 20  nm and an inner diameter of 5 to 8 nm and have a length of 1  μm. A higher reaction temperature (170 °C) and less reaction volume gives the corresponding nanowires. Another process for synthesizing nanotubes is through anodization in an electrolytic solution. When anodized in a 0.5 weight percent HF solution for 20 minutes, well-aligned titanium oxide nanotube arrays can be fabricated with an average tube diameter of 60  nm and length of 250  nm. Based on X-ray Diffraction, nanotubes grown through anodization are amorphous. As HF is highly corrosive and harmful chemical, NH4F is now being used as the etching agent in lieu of HF. In a typical synthesis process, a formamide based non aqueous electrolyte is produced containing 0.2M NH4F and 5 vol% of DI water. The anodization process is carried out under 25V at 20oC for 20 hours, in a two electrode electrochemical cell consisting of a highly pure and thoroughly cleaned titanium plate as the anode, a copper plate or platinum wire as the cathode and the aforesaid electrolyte. The as prepared sample is annealed in air at 400oC to get anatase phase. (top) and TEM (bottom) images of chiral TiO2 nanofibers.]] Hollow TiO2 nanofibers can be also prepared by coating carbon nanofibers with titanium(IV) butoxide (Ti(OCH2CH2CH2CH3)4). The product is then heated at 550 °C for several hours in air to remove the carbon core and form TiO2 nanocrystals. When chiral carbon nanofibers are used as templates, the resulting TiO2 fibers are also chiral, i.e., they respond differently to left and right-hand circularly polarized light. Such optical activity is common for organic, but not for inorganic molecules or nanostructures; the latter are preferred for optical applications because of their superior mechanical and thermal stability.

Applications

The most important application areas are paints and varnishes as well as paper and plastics, which account for about 80% of the world's titanium dioxide consumption. Other pigment applications such as printing inks, fibers, rubber, cosmetic products and foodstuffs account for another 8%. The rest is used in other applications, for instance the production of technical pure titanium, glass and glass ceramics, electrical ceramics, catalysts, electric conductors and chemical intermediates. It is also in most red-coloured candy.

Pigment

Titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index, in which it is surpassed only by a few other materials. Approximately 4.6 million tons of pigmentary TiO2 are used annually worldwide, and this number is expected to increase as utilization continues to rise. When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors and some gemstones like "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. In paint, it is often referred to offhandedly as "the perfect white", "the whitest white", or other similar terms. Opacity is improved by optimal sizing of the titanium dioxide particles. Some grades of titanium based pigments as used in sparkly paints, plastics, finishes and pearlescent cosmetics are man-made pigments whose particles have two or more layers of various oxides – often titanium dioxide, iron oxide or alumina – in order to have glittering, iridescent and or pearlescent effects similar to crushed mica or guanine-based products. In addition to these effects a limited colour change is possible in certain formulations depending on how and at which angle the finished product is illuminated and the thickness of the oxide layer in the pigment particle; one or more colours appear by reflection while the other tones appear due to interference of the transparent titanium dioxide layers. In some products, the layer of titanium dioxide is grown in conjunction with iron oxide by calcination of titanium salts (sulfates, chlorates) around 800 °C or other industrial deposition methods such as chemical vapour deposition on substrates such as mica platelets or even silicon dioxide crystal platelets of no more than 50  µm in diameter. Pearlescence with Iriodin. pearl-effect.com The iridescent effect in these titanium oxide particles (which are only partly natural) 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. In ceramic glazes titanium dioxide acts as an opacifier and seeds crystal formation. Titanium dioxide has been shown statistically to increase skimmed milk's whiteness, increasing skimmed milk's sensory acceptance score. Titanium dioxide is used to mark the white lines of some tennis courts.Les, Caren B. (November 2008) Light spells doom for bacteria. Photonics.com The exterior of the Saturn V rocket was painted with titanium dioxide; this later allowed astronomers to determine that J002E3 was the S-IVB stage from Apollo 12 and not an asteroid.

Sunscreen and UV blocking pigments in the industry

In cosmetic and skin care products, titanium dioxide is used as a pigment, sunscreen and a thickener. It is also used as a tattoo pigment and in styptic pencils. Titanium dioxide is produced in varying particle sizes, oil and water dispersible, and in certain grades for the cosmetic industry. Titanium dioxide is found in the majority of physical sunscreens 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. Nano-scaled (particle size of 30–40 nm)Dan, Yongbo et al. Measurement of Titanium Dioxide Nanoparticles in Sunscreen using Single Particle ICP-MS. perkinelmer.com titanium dioxide particles are primarily used in sun screen lotion because they scatter visible light less than titanium dioxide pigments while still providing UV protection. 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 believed to cause less skin irritation than other UV absorbing chemicals. This pigment is used extensively in plastics and other applications not only as a white pigment or an opacifier but also for its UV resistant properties where the powder disperses the light – unlike organic UV absorbers – and reduces UV damage, due mostly to the extremely high refractive index of the particles. Polymers, Light and the Science of TiO2, DuPont, pp. 1–2 Certain polymers used in coatings for concrete Fibre Cement Coating. dowconstructionchemicals.com or those used to impregnate concrete as a reinforcement are sometimes charged with titanium white pigment for UV shielding in the construction industry, but it only delays the oxidative photodegradation of the polymer in question, which is said to "chalk" as it flakes off due to lowered impact strength and may crumble after years of exposure in direct sunlight if UV stabilizers have not been included.

Photocatalyst

Titanium dioxide, particularly in the anatase form, exhibits photocatalytic activity under ultraviolet (UV) irradiation. This photoactivity is reportedly most pronounced at the {001} planes of anatase, although the {101} planes are thermodynamically more stable and thus more prominent in most synthesised and natural anatase, as evident by the often observed tetragonal dipyramidal growth habit. Interfaces between rutile and anatase are further considered to improve photocatalytic activity by facilitating charge carrier separation and as a result, biphasic titanium dioxide is often considered to possess enhanced functionality as a photocatalyst. It has been reported that titanium dioxide, when doped with nitrogen ions or doped with metal oxide like tungsten trioxide, exhibits excitation also 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. Hence, in addition to its use as a pigment, titanium dioxide can be added to paints, cements, windows, tiles, or other products for its sterilizing, deodorizing and anti-fouling properties and is used as a hydrolysis catalyst. It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also known as a Graetzel cell). The photocatalytic properties of titanium dioxide were discovered by Akira Fujishima in 1967 and published in 1972. The process on the surface of the titanium dioxide was called the Honda-Fujishima effect (). "Discovery and applications of photocatalysis — Creating a comfortable future by making use of light energy". Japan Nanonet Bulletin Issue 44, 12 May 2005. Titanium dioxide, in thin film and nanoparticle form has potential for use in energy production: as a photocatalyst, it can carry out hydrolysis; i.e., break water into hydrogen and oxygen. With 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. Further efficiency and durability has been obtained by introducing disorder to the lattice structure of the surface layer of titanium dioxide nanocrystals, permitting infrared absorption. Cheap, Clean Ways to Produce Hydrogen for Use in Fuel Cells? A Dash of Disorder Yields a Very Efficient Photocatalyst. Sciencedaily (28 January 2011) In 1995 Fujishima and his group discovered the superhydrophilicity phenomenon for titanium dioxide coated glass exposed to sun light. This resulted in the development of self-cleaning glass and anti-fogging coatings. TiO2 incorporated into outdoor building materials, such as paving stones in noxer blocks Advanced Concrete Pavement materials, National Concrete Pavement Technology Center, Iowa State University, p. 435. or paints, can substantially reduce concentrations of airborne pollutants such as volatile organic compounds and nitrogen oxides.Hogan, Jenny (4 February 2004) "Smog-busting paint soaks up noxious gases". New Scientist. A photocatalytic cement that uses titanium dioxide as a primary component, produced by Italcementi Group, was included in Time's Top 50 Inventions of 2008. TIME's Best Inventions of 2008. (31 October 2008). Attempts have been made to photocatalytically mineralize pollutants (to convert into CO2 and H2O) in waste water. TiO2 offers great potential as an industrial technology for detoxification or remediation of wastewater due to several factors:
  1. The process uses natural oxygen and sunlight and thus occurs under ambient conditions; it is wavelength selective and is accelerated by UV light.
  2. The photocatalyst is inexpensive, readily available, non-toxic, chemically and mechanically stable, and has a high turnover.
  3. The formation of photocyclized intermediate products, unlike direct photolysis techniques, is avoided.
  4. Oxidation of the substrates to CO2 is complete.
  5. TiO2 can be supported as thin films on suitable reactor substrates, which can be readily separated from treated water.
The photocatalytic destruction of organic matter is also exploited in photocatalytic antimicrobial coatings, which are typically thin films applied to furniture in hospitals and other surfaces susceptible to be contaminated with bacteria, fungi and viruses.

Other applications

Health and safety

Titanium dioxide is incompatible with strong reducing agents and strong acids.{{cite news|title = Hazardline|author = Occupational Health Services, Inc. | format = Electronic Bulletin|publisher = Occupational Health Services, Inc. | location = New York|date = 31 May 1988}} Violent or incandescent reactions occur with molten metals that are very electropositive, e.g. aluminium, calcium, magnesium, potassium, sodium, zinc and lithium.{{cite book|title= Dangerous Properties of Industrial Materials |author1=Sax, N.I. |author2=Lewis, Richard J., Sr. | year = 2000|publisher = Van Nostrand Reinhold | location = New York|isbn = 978-0-471-35407-9 | page = 3279|edition= 10th|volume = III}} 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. Many sunscreens use nanoparticle titanium dioxide (along with nanoparticle zinc oxide) which, despite reports of potential health risks, is not actually absorbed through the skin. Other effects of titanium dioxide nanoparticles on human health are not well understood. Nevertheless, allergy to topical application has been confirmed. Titanium dioxide dust, when inhaled, has been classified by the International Agency for Research on Cancer (IARC) as an IARC Group 2B carcinogen, meaning it is possibly carcinogenic to humans. The findings of the IARC are based on the discovery 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, the human studies conducted so far do not suggest an association between occupational exposure to titanium dioxide and an increased risk for cancer. The safety of the use of nano-particle sized titanium dioxide, which can penetrate the body and reach internal organs, has been criticized. Studies have also found that titanium dioxide nanoparticles cause inflammatory response and genetic damage in mice. The mechanism by which may cause cancer is unclear. Molecular research suggests that cell cytotoxicity due to results from the interaction between nanoparticles and the lysosomal compartment, independently of the known apoptotic signalling pathways. The body of research regarding the carcinogenicity of different particle sizes of titanium dioxide has led the US National Institute for Occupational Safety and Health to recommend two separate exposure limits. NIOSH recommends that fine particles be set at an exposure limit of 2.4 mg/m3, while ultrafine 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. These recommendations reflect the findings in the research literature that show smaller titanium dioxide particles are more likely to pose carcinogenic risk than the larger titanium dioxide particles. There is some evidence the rare disease yellow nail syndrome may be caused by titanium, either implanted for medical reasons or through eating various foods containing titanium dioxide. Dunkin' Donuts in the United States is dropping titanium dioxide from its powdered sugar donuts after public pressure.{{cite news|title=Dunkin' to stop using whitening agent | url=https://www.usatoday.com/story/money/2015/03/06/dunkin-donuts-fast-food-restaurant-food-safety/24524875/|publisher=USA TODAY | date=March 2015}} However, Andrew Maynard, director of Risk Science Center at the University of Michigan downplayed 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. Dunkin' Donuts ditches titanium dioxide – but is it actually harmful? The Conversation. March 12, 2015

See also

References

External links

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Titanium_dioxide, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Titanium_dioxide&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany