Editing Titanium dioxide (section)

===Photocatalyst===
Nanosized titanium dioxide, particularly in the anatase form, exhibits [[Photocatalysis|photocatalytic activity]] under ultraviolet (UV) irradiation. This photoactivity is reportedly most pronounced at the {001} planes of anatase,<ref>{{cite journal |author=Liang Chu |journal=Scientific Reports |volume=5 |pages=12143 |title=Anatase TiO<sub>2</sub> Nanoparticles with Exposed {001} Facets for Efficient Dye-Sensitized Solar Cells |doi=10.1038/srep12143 |pmid=26190140 |pmc=4507182 |bibcode=2015NatSR...512143C |year=2015}}</ref><ref>{{cite journal |author= Li Jianming and Dongsheng Xu |title=tetragonal faceted-nanorods of anatase TiO<sub>2</sub> single crystals with a large percentage of active {100} facets |journal= Chemical Communications |volume=46 |issue=13 |pages=2301–3 |year=2010 |doi=10.1039/b923755k |pmid=20234939}}</ref> although the {101} planes are thermodynamically more stable and thus more prominent in most synthesised and natural anatase,<ref>{{cite journal |author= M Hussein N Assadi |title= The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO 2: A theoretical study |url=https://www.researchgate.net/publication/304714130 |journal= Applied Surface Science |volume= 387 |pages=682–689|year=2016|bibcode=2016ApSS..387..682A|doi=10.1016/j.apsusc.2016.06.178 |arxiv= 1811.09157|s2cid= 99834042 }}</ref> as evident by the often observed tetragonal dipyramidal [[Crystal habit|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.<ref>{{cite journal |title=Sand Supported Mixed-Phase TiO<sub>2</sub> Photocatalysts for Water Decontamination Applications |journal= Advanced Engineering Materials |year=2014 |volume=16|issue=2 |pages=248–254|doi=10.1002/adem.201300259 |arxiv=1404.2652 |last1= Hanaor |first1= Dorian A. H. |last2= Sorrell |first2= Charles C. |bibcode= 2014arXiv1404.2652H|s2cid= 118571942 }}</ref> 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.<ref name=Visible>{{cite journal |author1=Kurtoglu M. E. |author2=Longenbach T. |author3=Gogotsi Y. |year= 2011|title= Preventing Sodium Poisoning of Photocatalytic TiO<sub>2</sub> Films on Glass by Metal Doping|journal= International Journal of Applied Glass Science|volume= 2|issue= 2|pages= 108–116|doi= 10.1111/j.2041-1294.2011.00040.x}}</ref> The strong [[Redox|oxidative potential]] of the [[Electron hole|positive holes]] oxidizes water to create [[hydroxyl radical]]s. 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 nanosized titanium dioxide were discovered by [[Akira Fujishima]] in 1967<ref name=fujishima/> and published in 1972.<ref>{{cite journal |doi=10.1038/238037a0|title=Electrochemical Photolysis of Water at a Semiconductor Electrode|year=1972|journal=Nature|volume=238|pages=37–8|pmid=12635268|issue=5358|bibcode= 1972Natur.238...37F|last1=Fujishima|first1=Akira|last2=Honda|first2=Kenichi|s2cid=4251015}}</ref> The process on the surface of the titanium dioxide was called the {{ill|Honda-Fujishima effect|ja|本多-藤嶋効果}}.<ref name=fujishima>[https://web.archive.org/web/20050608091634/http://www.nanonet.go.jp/english/mailmag/2005/044a.html "Discovery and applications of photocatalysis – Creating a comfortable future by making use of light energy"]. ''Japan Nanonet Bulletin'' Issue 44, 12 May 2005.</ref> In [[thin film]] and [[nanoparticle]] form, titanium dioxide has the potential for use in energy production: As a photocatalyst, it can 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.<ref>{{cite news |work=Advanced Ceramics Report|date=1 December 2003|url=http://www.highbeam.com/doc/1G1-110587279.html|archive-url=https://web.archive.org/web/20070204161415/http://www.highbeam.com/doc/1G1-110587279.html|url-status=dead|archive-date=4 February 2007|title=Carbon-doped titanium dioxide is an effective photocatalyst|quote=This carbon-doped titanium dioxide is highly efficient; under artificial visible light, it breaks down chlorophenol five times more efficiently than the nitrogen-doped version.}}</ref> 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.<ref>[https://www.sciencedaily.com/releases/2011/01/110128165212.htm Cheap, Clean Ways to Produce Hydrogen for Use in Fuel Cells? A Dash of Disorder Yields a Very Efficient Photocatalyst]. Sciencedaily (28 January 2011)</ref> Visible-light-active nanosized anatase and rutile has been developed for photocatalytic applications.<ref>{{cite journal |last=Karvinen|first=Saila|title=Preparation and Characterization of Mesoporous Visible-Light-Active Anatase|journal=Solid State Sciences|volume=5 2003|issue=8|pages=1159–1166|bibcode=2003SSSci...5.1159K|year=2003|doi=10.1016/S1293-2558(03)00147-X}}</ref><ref>{{Cite journal |last1=Bian |first1=Liang |last2=Song |first2=Mianxin |last3=Zhou |first3=Tianliang |last4=Zhao |first4=Xiaoyong |last5=Dai |first5=Qingqing |date=June 2009 |title=Band gap calculation and photo catalytic activity of rare earths doped rutile TiO2 |url=https://linkinghub.elsevier.com/retrieve/pii/S1002072108602707 |journal=Journal of Rare Earths |language=en |volume=27 |issue=3 |pages=461–468 |doi=10.1016/S1002-0721(08)60270-7|url-access=subscription }}</ref>

In 1995 Fujishima and his group discovered the [[superhydrophilicity]] phenomenon for titanium dioxide coated glass exposed to sun light.<ref name=fujishima/> This resulted in the development of [[self-cleaning glass]] and [[anti-fog]]ging coatings.

Nanosized TiO<sub>2</sub> incorporated into outdoor building materials, such as paving stones in [[noxer block]]s<ref>[http://www.cptechcenter.org/publications/task15/task15_vol2/track12am.pdf Advanced Concrete Pavement materials] {{Webarchive|url=https://web.archive.org/web/20130620080135/http://www.cptechcenter.org/publications/task15/task15_vol2/track12am.pdf |date=20 June 2013}}, National Concrete Pavement Technology Center, Iowa State University, p. 435.</ref> or paints, could reduce concentrations of airborne pollutants such as [[volatile organic compound]]s and [[nitrogen oxide]]s.<ref>Hogan, Jenny (4 February 2004) [https://www.newscientist.com/article/dn4636 "Smog-busting paint soaks up noxious gases"]. ''New Scientist''.</ref> A TiO<sub>2</sub>-containing cement has been produced.<ref>[https://content.time.com/time/specials/packages/0,28757,1852747,00.html TIME's Best Inventions of 2008]. (31 October 2008).</ref>

Using TiO<sub>2</sub> as a photocatalyst, attempts have been made to mineralize pollutants (to convert into CO<sub>2</sub> and H<sub>2</sub>O) in waste water.<ref name="Mineralize Pollutants">{{cite book |last=Winkler |first=Jochen |title=Titanium Dioxide |year=2003 |isbn=978-3-87870-148-4 |pages=115–116 |publisher=Vincentz Network |location=Hannover}}</ref><ref>{{cite journal |doi=10.1016/j.apcatb.2003.11.010|title=TiO<sub>2</sub>-assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetic and mechanistic investigations|year=2004|last1=Konstantinou|first1=Ioannis K|last2=Albanis|first2=Triantafyllos A|journal=Applied Catalysis B: Environmental|volume=49|issue=1 |pages=1–14|bibcode=2004AppCB..49....1K }}</ref><ref>{{cite journal |last1=Hanaor |first1=Dorian A. H. |last2=Sorrell |first2=Charles C. |title= Sand Supported Mixed-Phase TiO<sub>2</sub> Photocatalysts for Water Decontamination Applications |journal= Advanced Engineering Materials |year=2014 |volume=16 |issue=2 |pages=248–254 |doi=10.1002/adem.201300259 |arxiv=1404.2652|s2cid=118571942 }}</ref> The photocatalytic destruction of organic matter could also be exploited in coatings with antimicrobial applications.<ref>{{cite journal |last1=Ramsden|first1=Jeremy J.|title=Photocatalytic antimicrobial coatings|journal=Nanotechnology Perceptions|date=2015|volume=11|issue=3|pages=146–168|doi=10.4024/N12RA15A.ntp.15.03|doi-access=free}}</ref>

====Hydroxyl radical formation====
Although nanosized anatase TiO<sub>2</sub> does not absorb visible light, it does strongly absorb [[ultraviolet]] (UV) radiation (''hv''), leading to the formation of hydroxyl radicals.<ref name=":1">{{cite book|title=Kirk-Othmer Encyclopedia of Chemical Technology|last1=Jones|first1=Tony|last2=Egerton|first2=Terry A.|date=2000|publisher=John Wiley & Sons, Inc.|isbn=978-0-471-23896-6|language=en|chapter=Titanium Compounds, Inorganic|doi=10.1002/0471238961.0914151805070518.a01.pub3}}</ref> This occurs when photo-induced valence bond holes (h<sup>+</sup><sub>vb</sub>) are trapped at the surface of TiO<sub>2</sub> leading to the formation of trapped holes (h<sup>+</sup><sub>tr</sub>) that cannot oxidize water.<ref name=":8">{{cite journal |last1=Hirakawa|first1=Tsutomu|last2=Nosaka|first2=Yoshio|date=23 January 2002|title=Properties of O2•-and OH• formed in TiO<sub>2</sub> aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions|journal=Langmuir|volume=18|issue=8|pages=3247–3254|doi=10.1021/la015685a}}</ref>

: TiO<sub>2</sub> + ''hv'' → e<sup>&minus;</sup> + h<sup>+</sup><sub>vb</sub>
: h<sup>+</sup><sub>vb</sub> → h<sup>+</sup><sub>tr</sub>
: O<sub>2</sub> + e<sup>&minus;</sup> → O<sub>2</sub><sup>•&minus;</sup>
: O<sub>2</sub><sup>•&minus;</sup> + O<sub>2</sub><sup>•&minus;</sup>+ 2{{H+}} → H<sub>2</sub>O<sub>2</sub> + O<sub>2</sub>
: O<sub>2</sub><sup>•&minus;</sup> + h<sup>+</sup><sub>vb</sub> → O<sub>2</sub>
: O<sub>2</sub><sup>•&minus;</sup> + h<sup>+</sup><sub>tr</sub> → O<sub>2</sub>
: {{OH-}} + h<sup>+</sup><sub>vb</sub> → HO•
: e<sup>&minus;</sup> + h<sup>+</sup><sub>tr</sub> → recombination
: Note: Wavelength (λ)= 387 nm<ref name=":8"/> This reaction has been found to mineralize and decompose undesirable compounds in the environment, specifically the air and in wastewater.<ref name=":8"/>

[[File:TiO2crystals.JPG|thumb|Synthetic single crystals of TiO<sub>2</sub>, c. 2–3 mm in size, cut from a larger plate]]