Titanium dioxide nanoparticles, also called ultrafine titanium dioxide or nanocrystalline titanium dioxide or microcrystalline titanium dioxide, are particles of titanium dioxide (TiO2) with diameters less than 100 nm. Ultrafine TiO2 is used in sunscreens due to its ability to block ultraviolet radiation while remaining transparent on the skin. It is in rutile crystal structure and coated with silica or/and alumina to prevent photocatalytic phenomena. The health risks of ultrafine TiO2 from dermal exposure on intact skin are considered extremely low, and it is considered safer than other substances used for ultraviolet protection. Nanosized particles of titanium dioxide tend to form in the metastable anatase phase, due to the lower surface energy of this phase, relative to the equilibrium rutile phase. Surfaces of ultrafine titanium dioxide in the anatase structure have photocatalytic sterilizing properties, which make it useful as an additive in construction materials, for example in antifogging coatings and self-cleaning windows. In the context of TiO2 production workers, inhalation exposure potentially presents a lung cancer risk, and standard hazard controls for nanomaterials are relevant for TiO2 nanoparticles.
Properties Of the three common TiO2 polymorphs (crystal forms), TiO2 nanoparticles are produced in the rutile and anatase forms. Unlike larger TiO2 particles, TiO2 nanoparticles are transparent rather than white. Ultraviolet absorption characteristics are dependent on the crystal size of titanium dioxide, and ultrafine particles have strong absorption against both ultraviolet-A (320-400 nm) and ultraviolet-B (280-320 nm) radiation. Light absorption in the ultraviolet range occurs because of the presence of strongly bound excitons. The wavefunction of these excitons has a two-dimensional character and extends on the {001} plane. TiO2 nanoparticles have photocatalytic activity It is n-type semiconductor and its band gap between the valence and the conductivity bands is wider than of many other substances. The photocatalysis of TiO2 is a complex function of the physical characteristics of the particles. Doping TiO2 with certain atoms its photocatalytic activity could be enhanced. In contrast, pigment-grade TiO2 usually has a median particle size in the 200–300 nm range. Because TiO2 powders contain a range of sizes, they may have a fraction of nanoscale particles even if the average particle size is larger. In turn ultafine particles usually form agglomerates and particle size could be much larger than crystal size.
Synthesis Most manufactured nanoscale titanium dioxide is synthesized by the sulfate process, the chloride process or the sol-gel process. In the sulfate process, anatase or rutile TiO2 is produced by digesting ilmenite (FeTiO3) or titanium slag with sulfuric acid. Ultrafine anatase form is precipitated from sulfate solution and ultrafine rutile from chloride solution. In the chloride process, natural or synthetic rutile is chlorinated at temperatures of 850–1000 °C, and the titanium tetrachloride is converted to the ultrafine anatase form by vapor-phase oxidation. It is not possible to convert pigmentary TiO2 to ultrafine TiO2 by grinding. Ultrafine titanium dioxide could be obtained by different kind of processes as precipitation method, gas-phase reaktion, sol-gel method, and atomic layer deposition method.
Uses Ultrafine TiO2 is believed to be one of the three most produced nanomaterials, along with silicon dioxide nanoparticles and zinc oxide nanoparticles. It is the second most advertised nanomaterial in consumer products, behind silver nanoparticles. Due to its long use as a commodity chemical, TiO2 can be considered a "legacy nanomaterial." Ultrafine TiO2 is used in sunscreens due to its ability to block ultraviolet radiation while remaining transparent on the skin. TiO2 particles used in sunscreens typically have sizes in the range 5–50 nm. Ultrafine TiO2 is used in housing and construction as an additive to paints, plastics, cements, windows, tiles, and other products for its ultraviolet absorption and photocatalytic sterilizing properties, for example, in antifogging coatings and self-cleaning windows. Engineered TiO2 nanoparticles are also used in light-emitting diodes and solar cells. In addition, the photocatalytic activity of TiO2 can be used to decompose organic compounds in wastewater. TiO2 nanoparticle products are sometimes coated with silica or alumina, or doped with another metal for specific applications.
Health and safety
Consumer For sunscreens, health risks from dermal exposure on intact skin are considered extremely low and are outweighed by the risk of ultraviolet radiation damage, including cancer from not wearing sunscreen. TiO2 nanoparticles are considered safer than other substances used for ultraviolet protection. However, there is concern that skin abrasions or rashes, or accidental ingestion of small amounts of sunscreen, are possible exposure pathways. Cosmetics containing nanomaterials are not required to be labeled in the United States, although they are in the European Union.
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