photocatalysis

The earliest mention came in 1911, when German chemist Dr. Alexander Eibner integrated the concept in his research of the illumination of zinc oxide (ZnO) on the bleaching of the dark blue pigment, Prussian blue.{{Cite journal|last=Eibner|first=Alexander|date=1911|title=Action of Light on Pigments I|journal=Chem-ZTG|volume=35|pages=753–755}}{{Cite book|title=Design of Advanced Photocatalytic Materials for Energy and Environmental Applications|url=https://archive.org/details/designadvancedph00coro|url-access=limited|last1=Coronado|first1=Juan M.|last2=Fresno|first2=Fernando|last3=Hernández-Alonso|first3=María D.|last4=Portela|first4=Racquel|publisher=Springer|year=2013|isbn=978-1-4471-5061-9|location=London|pages=[https://archive.org/details/designadvancedph00coro/page/n10 1]–5|doi=10.1007/978-1-4471-5061-9|hdl=10261/162776}} Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination,{{Cite journal|last1=Bruner|first1=L.|last2=Kozak|first2=J.|date=1911|title=Information on the Photocatalysis I The Light Reaction in Uranium Salt Plus Oxalic Acid Mixtures|journal=Elktrochem Agnew P|volume=17|pages=354–360}} while in 1913, Landau published an article explaining the phenomenon of photocatalysis. Their contributions led to the development of actinometric measurements, measurements that provide the basis of determining photon flux in photochemical reactions.{{Cite journal|last=Landau|first=M.|date=1913|title=Le Phénomène de la Photocatalyse|journal=Compt. Rend.|volume=156|pages=1894–1896}} After a hiatus, in 1921, Baly et al. used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under visible light.{{Cite journal|last1=Baly|first1=E.C.C.|last2=Helilbron|first2=I.M.|last3=Barker|first3=W.F.|date=1921|title=Photocatalysis. Part I. The Synthesis of Formaldehyde and Carbohydrates from Carbon Dioxide and Water.|url=https://zenodo.org/record/1503399|journal=J Chem Soc|volume=119|pages=1025–1035|doi=10.1039/CT9211901025}}

In 1938 Doodeve and Kitchener discovered that {{chem|TiO|2}} , a highly-stable and non-toxic oxide, in the presence of oxygen could act as a photosensitizer for bleaching dyes, as ultraviolet light absorbed by {{chem|TiO|2}} led to the production of active oxygen species on its surface, resulting in the blotching of organic chemicals via photooxidation. This was the first observation of the fundamental characteristics of heterogeneous photocatalysis.{{Cite journal|last1=Goodeve|first1=C.F.|last2=Kitchener|first2=J.A.|date=1938|title=The Mechanism of Photosensitization by Solids|journal= Transactions of the Faraday Society|volume=34|pages=902–912|doi=10.1039/tf9383400902}}

Research in photocatalysis again paused until 1964, when V.N. Filimonov investigated isopropanol photooxidation from ZnO and {{chem|TiO|2}} ;{{Cite journal|last=Filimonov|first=V.N.|date=1964|title=Photocatalytic Oxidation of Gaseous Isopropanol on ZnO + {{chem|TiO|2}} |journal=Dokl. Akad. Nauk SSSR|volume=154|issue=4|pages=922–925}} while in 1965 Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) explored oxidation/photooxidation of {{Chem|CO|2}} and organic solvents from ZnO radiance.{{Cite journal|last1=Ikekawa|first1=A.|last2=Kamiya|first2=M.|last3=Fujita|first3=Y.|last4=Kwan|first4=T.|date=1965|title= On the Competition of Homogeneous and Heterogeneous Chain Terminations in Heterogeneous Photooxidation Catalysis by Zinc Oxide|journal= Bulletin of the Chemical Society of Japan|volume=38|pages=32–36|doi=10.1246/bcsj.38.32}}{{Cite journal|last1=Doerffler|first1=W.|last2=Hauffe|first2=K.|date=1964|title=Heterogeneous Photocatalysis I. Influence of Oxidizing and Reducing Gases on the Electrical Conductivity of Dark and Illuminated Zinc Oxide Surfaces|journal=J Catal|volume=3|issue=2|pages=156–170|doi=10.1016/0021-9517(64)90123-X}}{{Cite journal|last1=Kato|first1=S.|last2=Mashio|first2=F.|date=1964|title= Titanium Dioxide-Photocatalyzed Liquid Phase Oxidation of Tetralin|journal= The Journal of the Society of Chemical Industry, Japan|volume=67|issue=8|pages=1136–1140|doi=10.1246/nikkashi1898.67.8_1136|doi-access=free}} In 1970, Formenti et al. and Tanaka and Blyholde observed the oxidation of various alkenes and the photocatalytic decay of N₂O, respectively.{{Cite journal|last1=Formenti|first1=M.|last2=Julliet F.|first2=F.|last3=Teichner SJ|first3=S.J.|date=1970|title=Controlled Photooxidation of Paraffins and Olefins over Anatase at Room Temperature|journal=Comptes Rendus de l'Académie des Sciences, Série C|volume=270C|pages=138–141}}{{Cite journal|last1=Tanaka|first1=K.I.|last2=Blyholde|first2=G.|date=1970|title=Photocatalytic and Thermal Catalytic Decomposition of Nitrous Oxide on Zinc Oxide.|journal=J. Chem. Soc. D|volume=18|issue=18|pages=1130|doi=10.1039/c29700001130}}

A breakthrough occurred in 1972, when Akira Fujishima and Kenichi Honda discovered that electrochemical photolysis of water occurred when a {{chem|TiO|2}} electrode irradiated with ultraviolet light was electrically connected to a platinum electrode. As the ultraviolet light was absorbed by the {{chem|TiO|2}} electrode, electrons flowed from the anode to the platinum cathode where hydrogen gas was produced. This was one of the first instances of hydrogen production from a clean and cost-effective source, as the majority of hydrogen production comes from natural gas reforming and gasification.{{Cite journal|last1=Fujishima|first1=A.|last2=Honda|first2=K.|date=1972|title=Electrochemical Photolysis of Water at a Semiconductor Electrode|journal=Nature|volume=238|issue=5358|pages=37–38|doi=10.1038/238037a0|pmid=12635268|bibcode=1972Natur.238...37F }} Fujishima's and Honda's findings led to other advances. In 1977, Nozik discovered that the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, could increase photoactivity, and that an external potential was not required.{{Cite journal|last=Nozik|first=A.J.|date=1977|title=Photochemical Diodes|journal=Appl Phys Lett|volume=30|issue=11|pages=567–570|doi=10.1063/1.89262|bibcode=1977ApPhL..30..567N}} Wagner and Somorjai (1980) and Sakata and Kawai (1981) delineated hydrogen production on the surface of strontium titanate (SrTiO₃) via photogeneration, and the generation of hydrogen and methane from the illumination of {{chem|TiO|2}} and PtO₂ in ethanol, respectively.{{Cite journal|last1=Wagner|first1=F.T.|last2=Somorjai|first2=G.A.|date=1980|title=Photocatalytic and Photoelectrochemical Hydrogen Production on Strontium Titanate Single Crystals|url=https://escholarship.org/uc/item/72f8n0w6|journal=J Am Chem Soc|volume=102|issue=17|pages=5494–5502|doi=10.1021/ja00537a013|bibcode=1980JAChS.102.5494W }}{{Cite journal|last1=Sakata|first1=T.|last2=Kawai|first2=T.|date=1981|title=Heterogeneous Photocatalytic Production of Hydrogen and Methane from Ethanol and Water|journal=Chem Phys Lett|volume=80|issue=2|pages=341–344|doi=10.1016/0009-2614(81)80121-2|bibcode=1981CPL....80..341S}}

For many decades photocatalysis had not been developed for commercial purposes. However, in 2023 multiple patents were granted to a U.S. company, (Pure-Light Technologies, Inc.) that has developed various formulas and processes that allow for widespread commercialization for VOC reduction and germicidal action.R Young. US Patents 11,906,157; 11,680,506;17/393,065 Chu et al. (2017) assessed the future of electrochemical photolysis of water, discussing its major challenge of developing a cost-effective, energy-efficient photoelectrochemical (PEC) tandem cell, which would, “mimic natural photosynthesis".{{Cite journal|last1=Chu|first1=S.|last2=Li|first2=W.|last3=Yan|first3=Y.|last4=Hamann|first4=T.|last5=Shih|first5=I.|last6=Wang|first6=D.|last7=Mi|first7=Z.|date=2017|title=Roadmap on Solar Water Splitting: Current Status and Future Prospects|journal=Nano Futures|publisher=IOP Publishing Ltd|volume=1|issue=2|pages=022001|doi=10.1088/2399-1984/aa88a1|bibcode=2017NanoF...1b2001C }}

In heterogeneous catalysis the catalyst is in a different phase from the reactants. Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, ¹⁸O₂–¹⁶O₂ and deuterium-alkane isotopic exchange, metal deposition, water detoxification, and gaseous pollutant removal.

Most heterogeneous photocatalysts are transition metal oxides and semiconductors. Unlike metals, which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The difference in energy between the filled valence band and the empty conduction band in the MO diagram of a semiconductor is the band gap.{{cite journal|doi=10.1021/cr00035a013|title=Photocatalysis on {{chem|TiO|2}} Surfaces: Principles, Mechanisms, and Selected Results|year=1995|last1=Linsebigler|first1=Amy L.|last2=Lu|first2=Guangquan.|last3=Yates|first3=John T.|journal=Chemical Reviews|volume=95|issue=3|pages=735–758 }} When the semiconductor absorbs a photon with energy equal to or greater than the material's band gap, an electron excites from the valence band to the conduction band, generating an electron hole in the valence band. This electron-hole pair is an exciton. The excited electron and hole can recombine and release the energy gained from the excitation of the electron as heat. Such exciton recombination is undesirable and higher levels cost efficiency. Efforts to develop functional photocatalysts often emphasize extending exciton lifetime, improving electron-hole separation using diverse approaches that may rely on structural features such as phase hetero-junctions (e.g. anatase-rutile interfaces), noble-metal nanoparticles, silicon nanowires and substitutional cation doping.{{Cite journal|first1=Saila|last1=Karvinena|first2=Pipsa|last2=Hirvab|first3=Tapani A|last3=Pakkanen|date=2003|title=Ab initio quantum chemical studies of cluster models for doped anatase and rutile {{chem|TiO|2}} |journal= Journal of Molecular Structure: Theochem|volume=626|issue=1–3|pages=271–277|doi=10.1016/S0166-1280(03)00108-8}} The ultimate goal of photocatalyst design is to facilitate reactions of the excited electrons with oxidants to produce reduced products, and/or reactions of the generated holes with reductants to produce oxidized products. Due to the generation of positive holes (h⁺) and excited electrons (e⁻), oxidation-reduction reactions take place at the surface of semiconductors irradiated with light.

In one mechanism of the oxidative reaction, holes react with the moisture present on the surface and produce a hydroxyl radical. The reaction starts by photo-induced exciton generation in the metal oxide (MO) surface by photon (hv) absorption:

:MO + hν → MO (h⁺ + e⁻)

Oxidative reactions due to photocatalytic effect:

:h⁺ + H₂O → H⁺ + •OH

:2 h⁺ + 2 H₂O → 2 H⁺ + H₂O₂

:H₂O₂→ 2 •OH

Reductive reactions due to photocatalytic effect:

:e⁻ + O₂ → •O₂⁻

:•O₂⁻ + HO₂• + H⁺ → H₂O₂ + O₂

:H₂O₂ → 2 •OH

Ultimately, both reactions generate hydroxyl radicals. These radicals are oxidative in nature and nonselective with a redox potential of E₀ = +3.06 V.{{cite journal|last1=Daneshvar|first1=N|last2=Salari|first2=D|last3=Khataee|first3=A.R|year=2004|title=Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to {{chem|TiO|2}} |journal=Journal of Photochemistry and Photobiology A: Chemistry|volume=162|issue=2–3|pages=317–322|doi=10.1016/S1010-6030(03)00378-2}} This is significantly greater than many common organic compounds, which typically are not greater than E₀ = +2.00 V.{{Citation |title=Appendix B: Tables of Physical Data |date=2014-10-18 |work=Fundamentals and Applications of Organic Electrochemistry |pages=217–222 |editor-last=Fuchigami |editor-first=Toshio |place=Chichester, United Kingdom |publisher=John Wiley & Sons Ltd |language=en |doi=10.1002/9781118670750.app2 |isbn=978-1-118-67075-0 |editor2-last=Inagi |editor2-first=Shinsuke |editor3-last=Atobe |editor3-first=Mahito|doi-access=free }} This results in the non-selective oxidative behavior of these radicals.

Titanium dioxide, a wide band-gap semiconductor, is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made {{chem|TiO|2}} an important material in many practical applications. Investigation of TiO₂ in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases is common. The absorption of photons with energy equal to or greater than the band gap of the semiconductor initiates photocatalytic reactions. This produces electron-hole (e⁻ /h⁺) pairs:{{Cite journal|last1=Ibhadon|first1=Alex|last2=Fitzpatrick|first2=Paul|date=2013-03-01|title=Heterogeneous Photocatalysis: Recent Advances and Applications|journal=Catalysts|volume=3|issue=1|pages=189–218|doi=10.3390/catal3010189 |doi-access=free}} {{Creative Commons text attribution notice|cc=bysa3|from this source=yes}}

:TiO2 ->[{hv}][{}] e- (TiO2) + h+(TiO2)

Where the electron is in the conduction band and the hole is in the valence band. The irradiated {{chem|TiO|2}} particle can behave as an electron donor or acceptor for molecules in contact with the semiconductor. It can participate in redox reactions with adsorbed species, as the valence band hole is strongly oxidizing while the conduction band electron is strongly reducing.

In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase. The process by which the atmosphere self-cleans and removes large organic compounds is a gas phase homogenous photocatalysis reaction.{{Cite journal |last1=He |first1=Fei |last2=Jeon |first2=Woojung |last3=Choi |first3=Wonyong |date=2021-05-05 |title=Photocatalytic air purification mimicking the self-cleaning process of the atmosphere |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=2528 |doi=10.1038/s41467-021-22839-0 |pmc=8100154 |pmid=33953206|bibcode=2021NatCo..12.2528H }} The ozone process is often referenced when developing many photocatalysts:

:2 O3(g) + H2O(g) ->[{hv}] O2(g) + 2OH.

Most homogeneous photocatalytic reactions are aqueous phase, with a transition-metal complex photocatalyst. The wide use of transition-metal complexes as photocatalysts is in large part due to the large band gap and high stability of the species.{{cite book |doi=10.1016/B978-0-12-821192-2.00005-X |chapter=Nanomaterials for photocatalysis |title=Nanotechnology and Photocatalysis for Environmental Applications |date=2020 |last1=Tahir |first1=Muhammad Bilal |last2=Iqbal |first2=Tahir |last3=Rafique |first3=Muhammad |last4=Rafique |first4=Muhammad Shahid |last5=Nawaz |first5=Tasmia |last6=Sagir |first6=M. |pages=65–76 |isbn=978-0-12-821192-2 }} Homogeneous photocatalysts are common in the production of clean hydrogen fuel production, with the notable use of cobalt and iron complexes.

Iron complex hydroxy-radical formation using the ozone process is common in the production of hydrogen fuel (similar to Fenton's reagent process done in low pH conditions without photoexcitation):

:Fe^2+ +H2O2 ->[{hv}] Fe^3+ +OH- + HO.

:Fe^3+ +H2O2 ->[{hv}] Fe^2+ H+ + HO2.

:Fe^2+ +HO. -> Fe^3+ +OH-

Complex-based photocatalysts are semiconductors, and operate under the same electronic properties as heterogeneous catalysts.{{cite book |doi=10.1016/B978-0-12-823007-7.00003-1 |chapter=Nanostructured photocatalysts: Introduction to photocatalytic mechanism and nanomaterials for energy and environmental applications |title=Nanostructured Photocatalysts |date=2021 |last1=Nguyen |first1=Chinh Chien |last2=Nguyen |first2=Dang Le Tri |last3=Nguyen |first3=Dinh Minh Tuan |last4=Nguyen |first4=Van-Huy |last5=Nanda |first5=Sonil |last6=Vo |first6=Dai-Viet N. |last7=Shokouhimehr |first7=Mohammadreza |last8=Do |first8=Ha Huu |last9=Kim |first9=Soo Young |last10=Van Le |first10=Quyet |pages=3–33 |isbn=978-0-12-823007-7 }}

A plasmonic antenna-reactor photocatalyst is a photocatalyst that combines a catalyst with attached antenna that increases the catalyst's ability to absorb light, thereby increasing its efficiency.

A Silicon dioxide catalyst combined with an Au light absorber accelerated hydrogen sulfide-to-hydrogen reactions. The process is an alternative to the conventional Claus process that operates at {{Convert|800-1000|C|abbr=on}}.{{Cite web |last=Blain |first=Loz |date=2022-11-02 |title=Light-powered catalyst makes profitable hydrogen from stinky waste gas |url=https://newatlas.com/energy/hydrogen-sulfide-remediation/ |access-date=2022-11-30 |website=New Atlas |language=en-US}}

A Fe catalyst combined with a Cu light absorber can produce hydrogen from ammonia ({{Chem|NH|3}}) at ambient temperature using visible light. Conventional Cu-Ru production operates at {{Convert|650-1000|C|abbr=on}}.{{Cite web |last=Blain |first=Loz |date=2022-11-29 |title=Revolutionary photocatalyst is huge news for green hydrogen and ammonia |url=https://newatlas.com/energy/light-catalyst-ammonia-rice/ |access-date=2022-11-30 |website=New Atlas |language=en-US}}

File:ZnO paper.tif (dark fibers) and tetrapodal zinc oxide micro particles (white and spiky) in paper.]]Photoactive catalysts have been introduced over the last decade, such as {{chem|TiO|2}} and ZnO nanorods. Most suffer from the fact that they can only perform under UV irradiation due to their band structure. Other photocatalysts, including a graphene-ZnO nanocompound counter this problem.{{cite journal |last1=Rouzafzay |first1=F. |last2=Shidpour |first2=R. |year=2020 |title=Graphene@ZnO nanocompound for short-time water treatment under sun-simulated irradiation: Effect of shear exfoliation of graphene using kitchen blender on photocatalytic degradation |journal=Alloys and Compounds |volume=829 |pages=154614 |doi=10.1016/J.JALLCOM.2020.154614 }} For several decades, there have been numerous attempts to develop active photocatalysts with broad light absorption capabilities. High-entropy photocatalysts, first introduced in 2020,{{cite journal |last1=Edalati |first1=P. |last2=Wang |first2=Q. |last3=Razavi-khosroshahi |first3=H. |last4=Fuji |first4=M. |last5=Ishihara |first5=T. |last6=Edalati |first6=K. |title=Photocatalytic hydrogen evolution on a high-entropy oxide |journal=Journal of Materials Chemistry A |date=February 2020 |volume=8 |issue=7 |pages=3814–3821 |doi= 10.1039/C9TA12846H }} are the result of one such effort. They have been utilized for hydrogen production, oxygen production, carbon dioxide conversion, and plastic waste conversion.{{cite journal |last1=Hai |first1=H.T.N. |last2=Nguyen |first2=T.T. |last3=Nishibori |first3=M. |last4=Ishihara |first4=T. |last5=Edalati |first5=K. |title=Photoreforming of plastic waste into valuable products and hydrogen using a high-entropy oxynitride with distorted atomic-scale structure |journal=Applied Catalysis B |date=May 2025 |volume=365 |pages=124968 |doi= 10.1016/j.apcatb.2024.124968 |arxiv=2412.18722 |bibcode=2025AppCB.36524968H }}

Micro-sized ZnO tetrapodal particles added to pilot paper production.{{Cite journal|last1=Sandberg|first1=Mats|last2=Håkansson|first2=Karl|last3=Granberg|first3=Hjalmar|date=2020-10-01|title=Paper machine manufactured photocatalysts - Lateral variations|journal=Journal of Environmental Chemical Engineering|language=en|volume=8|issue=5|pages=104075|doi=10.1016/j.jece.2020.104075 |doi-access=free}} The most common are one-dimensional nanostructures, such as nanorods, nanotubes, nanofibers, nanowires, but also nanoplates, nanosheets, nanospheres, tetrapods. ZnO is strongly oxidative, chemically stable, with enhanced photocatalytic activity, and has a large free-exciton binding energy. It is non-toxic, abundant, biocompatible, biodegradable, environmentally friendly, low cost, and compatible with simple chemical synthesis. ZnO faces limits to its widespread use in photocatalysis under solar radiation. Several approaches have been suggested to overcome this limitation, including doping for reducing the band gap and improving charge carrier separation.{{Cite journal|last1=Nunes|first1=Daniela|last2=Pimentel|first2=Ana|last3=Branquinho|first3=Rita|last4=Fortunato|first4=Elvira|last5=Martins|first5=Rodrigo|date=2021-04-16|title=Metal Oxide-Based Photocatalytic Paper: A Green Alternative for Environmental Remediation|journal=Catalysts|volume=11|issue=4|pages=504|doi=10.3390/catal11040504 |doi-access=free|hdl=10362/124574|hdl-access=free}}

{{Main|Photocatalytic water splitting}}

Photocatalytic water splitting separates water into hydrogen and oxygen:{{cite journal |title=Strategies for the Development of Visible-light-driven Photocatalysts for Water Splitting |author1=Kudo, Akihiko |author2=Kato, Hideki |author3=Tsuji, Issei |journal=Chemistry Letters |volume=33 |year=2004 |issue=12 |page=1534|doi=10.1002/chin.200513248}}

:{{chem2|2 H2O -> 2 H2 + O2}}

The most prevalently investigated material, {{chem|TiO|2}} , is inefficient. Mixtures of {{chem|TiO|2}} and nickel oxide (NiO) are more active. NiO allows a significant explоitation of the visible spectrum.{{Cite journal|last1=Banić|first1=Nemanja|last2=Krstić|first2=Jugoslav|last3=Stojadinović|first3=Stevan|last4=Brnović|first4=Anđela|last5=Djordjevic|first5=Aleksandar|last6=Abramović|first6=Biljana|date=September 2020|title=Commercial TiO 2 loaded with NiO for improving photocatalytic hydrоgen prоduction in the presence оf simulated solar radiation|journal=International Journal of Energy Research|language=en|volume=44|issue=11|pages=8951–8963|doi=10.1002/er.5604 |doi-access=free}} One efficient photocatalyst in the UV range is based on sodium tantalite (NaTaO₃) doped with lanthanum and loaded with a nickel oxide cocatalyst. The surface is grooved with nanosteps from doping with lanthanum (3–15 nm range, see nanotechnology). The NiO particles are present on the edges, with the oxygen evolving from the grooves.

Titanium dioxide takes part in self-cleaning glass. Free radicals{{cite web |url=http://www.calutech.com/photocatalytic-oxidation.htm|title=Snapcat Photo Catalytic Oxidation with {{chem|TIO|2}} (2005)|publisher=CaluTech UV Air|access-date=2006-12-05}}{{cite journal |vauthors=Kondrakov AO, Ignatev AN, Lunin VV, Frimmel FH, Braese S, Horn H |title=Roles of water and dissolved oxygen in photocatalytic generation of free OH radicals in aqueous {{chem|TiO|2}} suspensions: An isotope labeling study |journal=Applied Catalysis B: Environmental |volume=182 |pages=424–430 |year=2016 |doi=10.1016/j.apcatb.2015.09.038}} generated from {{chem|TiO|2}} oxidize organic matter.{{cite web |url=http://www.titaniumart.com/photocatalysis-ti02.html |title=Photocatalysis Applications of {{chem|TiO|2}} |work=Titanium Information |publisher=titaniumart.com}}{{cite journal |vauthors=Kondrakov AO, Ignatev AN, Frimmel FH, Braese S, Horn H, Revelsky AI |title=Formation of genotoxic quinones during bisphenol A degradation by {{chem|TiO|2}} photocatalysis and UV photolysis: A comparative study |journal=Applied Catalysis B: Environmental |volume=160 |pages=106–114 |year=2014 |doi=10.1016/j.apcatb.2014.05.007}} The rough wedge-like {{chem|TiO|2}} surface can be modified with a hydrophobic monolayer of octadecylphosphonic acid (ODP). {{chem|TiO|2}} surfaces that were plasma etched for 10 seconds and subsequent surface modifications with ODP showed a water contact angle greater than 150◦. The surface was converted into a superhydrophilic surface (water contact angle = 0◦) upon UV illumination, due to rapid decomposition of octadecylphosphonic acid coating resulting from {{chem|TiO|2}} photocatalysis. Due to {{chem|TiO|2}} 's wide band gap, light absorption by the semiconductor material and resulting superhydrophilic conversion of undoped {{chem|TiO|2}} requires ultraviolet radiation (wavelength <390 nm) and thereby restricts self-cleaning to outdoor applications.{{cite journal |last1=Banerjee |first1=Swagata |last2=Dionysiou |first2=Dionysios D. |last3=Pillai |first3=Suresh C. |title=Self-cleaning applications of TiO2 by photo-induced hydrophilicity and photocatalysis |journal=Applied Catalysis B: Environmental |date=October 2015 |volume=176-177 |pages=396–428 |doi=10.1016/j.apcatb.2015.03.058 }}

• Water disinfection/decontamination,{{cite journal |last1=Shafiq |first1=Iqrash |last2=Hussain |first2=Murid |last3=Shehzad |first3=Nasir |last4=Maafa |first4=Ibrahim M. |last5=Akhter |first5=Parveen |last6=Amjad |first6=Um-e-salma |last7=Shafique |first7=Sumeer |last8=Razzaq |first8=Abdul |last9=Yang |first9=Wenshu |last10=Tahir |first10=Muhammad |last11=Russo |first11=Nunzio |title=The effect of crystal facets and induced porosity on the performance of monoclinic BiVO4 for the enhanced visible-light driven photocatalytic abatement of methylene blue |journal=Journal of Environmental Chemical Engineering |date=August 2019 |volume=7 |issue=4 |pages=103265 |doi=10.1016/j.jece.2019.103265 }} a form of solar water disinfection (SODIS).{{cite journal |vauthors=McCullagh C, Robertson JM, Bahnemann DW, Robertson PK |title=The application of {{chem|TiO|2}} photocatalysis for disinfection of water contaminated with pathogenic micro-organisms: a review |journal=Research on Chemical Intermediates |volume=33 |issue=3–5 |pages=359–375 |year=2007 |doi=10.1163/156856707779238775 }}{{cite journal| last1=Hanaor| first1=Dorian A. H.| last2=Sorrell| first2=Charles C.| title= Sand Supported Mixed-Phase {{chem|TiO|2}} 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| bibcode=2014arXiv1404.2652H }} Adsorbents attract organics such as tetrachloroethylene. Adsorbents are placed in packed beds for 18 hours. Spent adsorbents are placed in regeneration fluid, essentially removing organics still attached by passing hot water opposite to the flow of water during adsorption. The regeneration fluid passes through fixed beds of silica gel photocatalysts to remove and decompose remaining organics.
• {{chem|TiO|2}} self-sterilizing coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact).{{cite journal|vauthors=Cushnie TP, Robertson PK, Officer S, Pollard PM, Prabhu R, McCullagh C, Robertson JM | title=Photobactericidal effects of {{chem|TiO|2}} thin films at low temperature | journal=Journal of Photochemistry and Photobiology A: Chemistry | year= 2010 | volume= 216 | issue= 2–3 | pages= 290–294 | doi=10.1016/j.jphotochem.2010.06.027 | url=https://zenodo.org/record/888756 }}
• Magnetic {{chem|TiO|2}} nanoparticle oxidation of organic contaminants agitated using a magnetic field.{{cite journal |doi=10.1021/es0508121 |pmid=16295874 |first=William L. |last=Kostedt IV |author2=Jack Drwiega |author3=David W. Mazyck |author4=Seung-Woo Lee |author5=Wolfgang Sigmund |author6=Chang-Yu Wu |author7= Paul Chadik |title=Magnetically agitated photocatalytic reactor for photocatalytic oxidation of aqueous phase organic pollutants |journal=Environmental Science & Technology |publisher=American Chemical Society |year=2005 |volume=39 |issue=20 |pages=8052–8056|bibcode=2005EnST...39.8052K }}
• Sterilization of surgical instruments and removal of fingerprints from electrical and optical components.{{Cite web |title=New visible light photocatalyst kills bacteria, even after light turned off |url=https://www.sciencedaily.com/releases/2010/01/100119121539.htm |website=ScienceDaily}}

{{chem|TiO|2}} conversion of carbon dioxide into gaseous hydrocarbons.{{cite journal |first=S. S. |last=Tan |author2=L. Zou |author3=E. Hu |title=Photocatalytic reduction of carbon dioxide into gaseous hydrocarbon using {{chem|TiO|2}} pellets|doi=10.1016/j.cattod.2006.02.057 |journal=Catalysis Today |volume=115 |issue=1–4 |year=2006 |pages=269–273}} The proposed reaction mechanisms involve the creation of a highly reactive carbon radical from carbon monoxide and carbon dioxide which then reacts with photogenerated protons to ultimately form methane. Efficiencies of {{chem|TiO|2}} -based photocatalysts are low, although nanostructures such as carbon nanotubes{{cite journal |first1=Y. Yao |author2=G. Li |author3=S. Ciston |author4=R. M. Lueptow |author5=K. Gray |title=Photoreactive {{chem|TiO|2}} /Carbon Nanotube Composites: Synthesis and Reactivity |journal=Environmental Science & Technology |publisher=American Chemical Society|doi=10.1021/es800191n |pmid=18678032 |volume=42 |year=2008 |pages=4952–4957 |last1=Yao |issue=13|bibcode=2008EnST...42.4952Y }} and metallic nanoparticles{{cite journal |first=A. L. |last=Linsebigler |author2=G. Lu |author3=J.T. Yates |title=Photocatalysis on {{chem|TiO|2}} Surfaces: Principles, Mechanisms, and Selected Results|doi=10.1021/cr00035a013|journal=Chemical Reviews |volume=95 |year=1995 |pages=735–758 |issue=3 }} help.

ePaint is a less-toxic alternative to conventional antifouling marine paints that generates hydrogen peroxide.

Photocatalysis of organic reactions by polypyridyl complexes,{{cite book |doi=10.1002/9783527674145 |title=Visible Light Photocatalysis in Organic Chemistry |date=2018 |last1=Stephenson |first1=Corey |last2=Yoon |first2=Tehshik |last3=MacMillan |first3=David W. C. |isbn=978-3-527-33560-2 }}{{pn|date=February 2025}} porphyrins,{{Cite journal |last1=Barona-Castaño |first1=Juan C. |last2=Carmona-Vargas |first2=Christian C. |last3=Brocksom |first3=Timothy J. |last4=De Oliveira |first4=Kleber T. |date=March 2016 |title=Porphyrins as Catalysts in Scalable Organic Reactions |journal=Molecules |language=en |volume=21 |issue=3 |pages=310 |doi=10.3390/molecules21030310 |pmc=6273917 |pmid=27005601 |doi-access=free}} or other dyes{{Cite journal |last1=Sirbu |first1=Dumitru |last2=Woodford |first2=Owen J. |last3=Benniston |first3=Andrew C. |last4=Harriman |first4=Anthony |date=2018-06-13 |title=Photocatalysis and self-catalyzed photobleaching with covalently-linked chromophore-quencher conjugates built around BOPHY |journal=Photochemical & Photobiological Sciences |language=en |volume=17 |issue=6 |pages=750–762 |doi=10.1039/C8PP00162F |pmid=29717745 |doi-access=free|bibcode=2018PhPhS..17..750S }} can produce materials inaccessible by classical approaches. Most photocatalytic dye degradation studies have employed {{chem|TiO|2}}. The anatase form of {{chem|TiO|2}} has higher photon absorption characteristics.{{Cite journal |last=Viswanathan |first=Balasubramanian |date=December 2017 |title=Photocatalytic Degradation of Dyes: An Overview |journal=Current Catalysis, 2018, 7, 000-000 |pages=3}}

Photocatalyst radical generation species allow for the degradation of organic pollutants into non-toxic compounds at a high efficiency. Use of CuO nanosheets to breakdown azo bonds in food dyes is one such example, with 96.99% degradation after only 6 minutes.{{Cite journal |last1=Nazim |first1=Mohammed |last2=Khan |first2=Aftab Aslam Parwaz |last3=Asiri |first3=Abdullah M. |last4=Kim |first4=Jae Hyun |date=2021-02-02 |title=Exploring Rapid Photocatalytic Degradation of Organic Pollutants with Porous CuO Nanosheets: Synthesis, Dye Removal, and Kinetic Studies at Room Temperature |journal=ACS Omega |language=en |volume=6 |issue=4 |pages=2601–2612 |doi=10.1021/acsomega.0c04747 |pmc=7859952 |pmid=33553878 }} Degradation of organic matter is a highly applicable property, particularly in waste processing.

The use of photocatalyst TiO₂ as a support system for filtration membranes shows promise in improving membrane bioreactors in the treatment of wastewater.{{cite journal |last1=Bae |first1=Tae-Hyun |last2=Tak |first2=Tae-Moon |title=Effect of TiO2 nanoparticles on fouling mitigation of ultrafiltration membranes for activated sludge filtration |journal=Journal of Membrane Science |date=March 2005 |volume=249 |issue=1–2 |pages=1–8 |doi=10.1016/j.memsci.2004.09.008 }} Polymer-based membranes have shown reduced fouling and self-cleaning properties in both blended and coated TiO₂ membranes. Photocatalyst-coated membranes show the most promise, as the increased surface exposure of the photocatalyst increases its organic degradation activity.{{cite journal |last1=Nascimbén Santos |first1=Érika |last2=László |first2=Zsuzsanna |last3=Hodúr |first3=Cecilia |last4=Arthanareeswaran |first4=Gangasalam |last5=Veréb |first5=Gábor |title=Photocatalytic membrane filtration and its advantages over conventional approaches in the treatment of oily wastewater: A review |journal=Asia-Pacific Journal of Chemical Engineering |date=September 2020 |volume=15 |issue=5 |doi=10.1002/apj.2533 |doi-access=free }}

Photocatalysts are also highly effective reducers of toxic heavy metals like hexavalent chromium from water systems. Under visible light the reduction of Cr(VI) by a Ce-ZrO₂ sol-gel on a silicon carbide was 97% effective at reducing the heavy metal to trivalent chromium.{{Cite journal |last1=Bortot Coelho |first1=Fabrício Eduardo |last2=Candelario |first2=Victor M. |last3=Araújo |first3=Estêvão Magno Rodrigues |last4=Miranda |first4=Tânia Lúcia Santos |last5=Magnacca |first5=Giuliana |date=2020-04-18 |title=Photocatalytic Reduction of Cr(VI) in the Presence of Humic Acid Using Immobilized Ce–ZrO2 under Visible Light |journal=Nanomaterials |language=en |volume=10 |issue=4 |pages=779 |doi=10.3390/nano10040779 |pmc=7221772 |pmid=32325680 |doi-access=free }}

Light2CAT was a project funded by the European Commission from 2012 to 2015. It aimed to develop a modified {{chem|TiO|2}} that can absorb visible light and include this modified {{chem|TiO|2}} into construction concrete. The {{chem|TiO|2}} degrades harmful pollutants such as NOx into NO₃⁻. The modified TiO₂ is in use in Copenhagen and Holbæk, Denmark, and Valencia, Spain. This “self-cleaning” concrete led to a 5-20% reduction in NOx over the course of a year.{{dead link|date=September 2023}}{{Cite web|url=https://cordis.europa.eu/project/rcn/102057/factsheet/en|title=Final Report Summary - LIGHT2CAT (Visible LIGHT Active PhotoCATalytic Concretes for Air pollution Treatment)|last=Mathiesen|first=D.|date=2012|website=European Commission}}{{Cite web|url=https://www.youtube.com/watch?v=YCkxRVSi2cU |archive-url=https://ghostarchive.org/varchive/youtube/20211213/YCkxRVSi2cU |archive-date=2021-12-13 |url-status=live|title=Light2CAT Visible Light Active PhotoCATalytic Concretes for Air Pollution Treatment [YouTube Video]|last=Light2CAT|date=2015|website=YouTube}}{{cbignore}}

ISO 22197-1:2007 specifies a test method for the measurement of {{Chem|NO|2}} removal for materials that contain a photocatalyst or have superficial photocatalytic films.{{Cite web|url=https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/04/07/40761.html|title=ISO 22197-1:2007|website=ISO}}

Specific FTIR systems are used to characterize photocatalytic activity or passivity, especially with respect to volatile organic compounds, and representative binder matrices.{{Cite web|url=http://www.coatingspromag.com/industry-news/2015/02/unique-gas-analyser-helps-to-characterize-photoactive-pigments|title=Unique Gas Analyser Helps to Characterize Photoactive Pigments | CoatingsPro Magazine|website=www.coatingspromag.com}}

Mass spectrometry allows measurement of photocatalytic activity by tracking the decomposition of gaseous pollutants such as nitrogen NOx or {{Chem|CO|2}}.{{cite journal |last1=Nuño |first1=Manuel |last2=Ball |first2=Richard J. |last3=Bowen |first3=Chris R. |title=Study of solid/gas phase photocatalytic reactions by electron ionization mass spectrometry |journal=Journal of Mass Spectrometry |date=August 2014 |volume=49 |issue=8 |pages=716–726 |doi=10.1002/jms.3396 |pmid=25044899 |bibcode=2014JMSp...49..716N |url=http://opus.bath.ac.uk/40446/1/2014_Nuno_J_Mass_Spec.pdf }}

• Light harvesting materials
• Photoelectrochemical cell
• Photolysis
• Photocatalyst activity indicator ink
• Photocatalytic water splitting
• Photoredox catalysis
• Photoelectrochemical oxidation
• Photosensitizer
• Green photocatalyst

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