In the experiment above, photons from a light source (out of frame on the right hand side) are absorbed by the surface of the titanium dioxide disc, exciting electrons within the material. These then react with the water molecules, splitting it into its constituents of hydrogen and oxygen. In this experiment, chemicals dissolved in the water prevent the formation of oxygen, which would otherwise recombine with the hydrogen.

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst.[1] In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide (TiO2).

History

Early mentions of photocatalysis (1911–1938)

The earliest mention of photocatalysis dates back to 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.[2][3] Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination,[3][4] 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.[3][5] After a brief stint of lack of research on photocatalysis, in 1921, Baly et al. used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under light in the visible spectrum.[3][6] However, it wasn't until 1938 that Doodeve and Kitchener discovered that TiO2, 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 TiO2 led to the production of active oxygen species on its surface, resulting in the blotching of organic chemicals via photooxidation. This would actually mark the first observation of the fundamental characteristics of heterogeneous photocatalysis.[3][7]

Advances in photocatalysis research (1964–1981, present)

Research in photocatalysis subsided for over 25 years due to lack of interest and absence of practical applications. However, in 1964, V.N. Filimonov investigated isopropanol photooxidation from ZnO and TiO2;[3][8] at around the same time, Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) explored oxidation/photooxidation of carbon dioxide and organic solvents from ZnO radiance.[3][9][10][11] A few years later, in 1970, Formenti et al. and Tanaka and Blyholde observed the oxidation of various alkenes and the photocatalytic decay of nitrous oxide (N2O), respectively.[3][12][13]

A breakthrough in photocatalysis research occurred in 1972, when Akira Fujishima and Kenichi Honda discovered that electrochemical photolysis of water occurred when a TiO2 electrode irradiated with ultraviolet light was electrically connected to a platinum electrode. As the ultraviolet light was absorbed by the TiO2 electrode, electrons flowed from it (the anode; site of the oxidation reaction) to the platinum electrode (the cathode; site of the reduction reaction) 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 back then came – and still today comes – from natural gas reforming and gasification.[3][14] Fujishima's and Honda's findings have led to other advances in photocatalysis; for instance, 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.[3][15] Further research conducted by Wagner and Somorjai (1980) and Sakata and Kawai (1981) delineated the production of hydrogen on the surface of strontium titanate (SrTiO3) via photogeneration, and the generation of hydrogen and methane from the illumination of TiO2 and PtO2 in ethanol, respectively.[3][16][17]

Research and development in photocatalysis, especially in electrochemical photolysis of water, continues today, but so far the process has not been developed for commercial purposes. In 2017, Chu et al. 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.”[3][18]

Types of photocatalysis

Homogeneous photocatalysis

In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase. The most commonly used homogeneous photocatalysts include ozone and photo-Fenton systems (Fe+ and Fe+/H2O2). The reactive species is the •OH radical, which is used for various purposes. The mechanism of hydroxyl radical production by ozone can follow two paths:[19]

O3 + hν → O2 + O(1D)
O(1D) + H2O → •OH + •OH
O(1D) + H2O → H2O2
H2O2 + hν → •OH + •OH

Similarly, the Fenton system produces hydroxyl radicals by the following mechanism:[20]

Fe2+ + H2O2→ HO• + Fe3+ + OH
Fe3+ + H2O2→ Fe2+ + HO•2 + H+
Fe2+ + HO• → Fe3+ + OH

In photo-Fenton type processes, additional sources of OH radicals should be considered, such as photolysis of H2O2 and reduction of Fe3+ ions under UV light:

H2O2 + hν → HO• + HO•
Fe3+ + H2O + hν → Fe2+ + HO• + H+

The efficiency of Fenton type processes is influenced by several operating parameters like the concentration of hydrogen peroxide, pH and intensity of UV. The main advantage of this process is the ability of using sunlight with light sensitivity up to 450 nm, thus avoiding the high costs of UV lamps and electrical energy. These reactions have been proven more efficient than other examples of photocatalysis but the disadvantages of the process are the low pH values, which are required since iron precipitates at higher pH values and the fact that iron has to be removed after treatment.

Homogeneous photocatalysis can also be conducted by Cu(II)/Cu(I) complexes.The photoredox behavior of Cu(II) complexes, similar to Fe(III) complexes, is derived mostly from the reactive decay of their LMCT states. Excitation to LMCT states can be achieved by direct sunlight when the ionization energy of the ligands coordinated to Cu(II) is not very high. In consequence of the reactive decay of the LMCT excited state by inner-sphere electron transfer, the Cu(II) central atom is reduced to Cu(I), whereas the ligand is oxidized to its radical and leaves the coordination sphere:[21]

The photoredox behaviour is demonstrated by the simple Cu(II) complexes with halogens. After excitation of [CuClx] 2−x the metal centre is reduced and Cl• and Cl2 radicals are formed:[22]

[23]

[23]

The Cl2 radicals are strong oxidation and chlorination agents. For instance they are able to oxidize phenol and its derivatives to para-benzochinone and CO2.

Heterogeneous photocatalysis

Heterogeneous catalysis has the catalyst 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, 18O216O2 and deuterium-alkane isotopic exchange, metal deposition, water detoxification, gaseous pollutant removal, etc.

Most common heterogeneous photocatalysts are transition metal oxides and semiconductors, which have unique characteristics. 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 void region of energy, which extends from the top of the filled valence band to the bottom of the vacant conduction band, is called the band gap.[24] When a photon with energy equal to or greater than the material's band gap is absorbed by the semiconductor, an electron is excited from the valence band to the conduction band, generating a positive hole in the valence band. Such a photogenerated electron-hole pair is termed 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 lead to an inefficient photocatalyst. For this reason efforts to develop functional photocatalysts often emphasize extending exciton lifetime, improving electron-hole separation using diverse approaches that often rely on structural features such as phase hetero-junctions (e.g. anatase-rutile interfaces), noble-metal nanoparticles, silicon nanowires and substitutional cation doping.[25] 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 and excited electrons, oxidation-reduction reactions take place at the surface of semiconductors irradiated with light.

In one mechanism of the oxidative reaction, the positive 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 surface (MO stands for metal oxide):

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 + H2O + H+ → H2O2 + O2
H2O2 → 2 •OH

Ultimately, hydroxyl radicals are generated in both the reactions. These hydroxyl radicals are very oxidative in nature and nonselective with a redox potential of E0 = +3.06 V.[26]

TiO2 is a good and common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made TiO2 an important material in many practical applications. TiO2, a wide band-gap semiconductor, is commonly investigated in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases. Photocatalytic reactions are initiated by the absorption of illumination with energy equal to or greater than the band gap of the semiconductor. This produces electron-hole (e /h+ ) pairs:[27]

where the electron is in the conduction band and the hole is in the valence band. The irradiated TiO2 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.[27]

Applications

SEM image of wood pulp (dark fibers) and tetrapodal zinc oxide micro particles (white and spiky) in paper.[28]
  • Paper production: Large scale photocatalysis by micro-sized ZnO tetrapodal particles added to pilot paper production.[28] The most common are one-dimensional nanostructures, such as nanorods, nanotubes, nanofibers, nanowires, but also nanoplates, nanosheets, nanospheres, tetrapods. ZnO has a strong oxidation ability, chemical stability, enhanced photocatalytic activity, and a large free-exciton binding energy. Also, it is non-toxic, earth abundant, biocompatible, biodegradable, environmentally friendly, low cost, and compatible with simple chemical synthesis. ZnO has also restrictions to its widespread use in photocatalysis under solar radiation as previously mentioned. Thus, several approaches have been suggested to overcome this limitation, including nonmetal and metal doping for reducing the band gap and improving the charge carrier separation.[29]
  • Separating water into hydrogen and oxygen by photocatalytic water splitting.[30] Combustion of fossil fuels is causing a huge amount of air pollutants, such as nitrogen oxides, sulfur oxides,[31][1] and carbon oxides.[32] Using sunlight as a renewable energy source is therefore becoming increasingly interesting.[33] In order to continue exploring photocatalytic hydrogen production efficiency, the most prevalently investigated titanium dioxide (TiO2), which has limited photocatalytic hydrogen production efficiency, was further loaded with different amounts of nickel oxide (NiO). From the results obtained, it can be cоncluded that the addition of NiO leads tо a significant explоitation of the visible part оf the spectrum.[34] An efficient photocatalyst in the UV range is based on sodium tantalite (NaTaO3) doped with lanthanum and loaded with a cocatalyst nickel oxide. The surface of the sodium tantalite crystals is grooved with nanosteps that are a result of doping with lanthanum (3–15 nm range, see nanotechnology). The NiO particles which facilitate hydrogen gas evolution are present on the edges, with the oxygen gas evolving from the grooves.
  • Use of titanium dioxide in self-cleaning glass. Free radicals[35][36] generated from TiO2 oxidize organic matter.[37][38] The rough wedge-like TiO2 surface was subsequently modified with a hydrophobic monolayer of octadecylphosphonic acid (ODP). TiO2 surfaces that were plasma etched for 10 second and subsequent surface modification 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 TiO2 photocatalysis. Due to the wide band gap of TiO2, light absorption by the semiconductor material and resulting superhydrophilic conversion of undoped TiO2 requires ultraviolet radiation (wavelength <390 nm) and thereby restricts the practical use of self-cleaning phenomenon to outdoor applications.[39]
  • Disinfection of water by supported titanium dioxide photocatalysts,[40] a form of solar water disinfection (SODIS).[41][42]
  • Use of titanium dioxide in self-sterilizing photocatalytic coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact).[43]
  • Oxidation of organic contaminants using magnetic particles that are coated with titanium dioxide nanoparticles and agitated using a magnetic field while being exposed to UV light.[44]
  • Conversion of carbon dioxide into gaseous hydrocarbons using titanium dioxide in the presence of water.[45] As an efficient absorber in the UV range, titanium dioxide nanoparticles in the anatase and rutile phases are able to generate excitons by promoting electrons across the band gap. The electrons and holes react with the surrounding water vapor to produce hydroxyl radicals and protons. At present, proposed reaction mechanisms usually suggest the creation of a highly reactive carbon radical from carbon monoxide and carbon dioxide which then reacts with the photogenerated protons to ultimately form methane. Although the efficiencies of present titanium dioxide based photocatalysts are low, the incorporation of carbon based nanostructures such as carbon nanotubes[46] and metallic nanoparticles[47] have been shown to enhance the efficiency of these photocatalysts.
  • Sterilization of surgical instruments and removal of unwanted fingerprints from sensitive electrical and optical components.[48]
  • A less-toxic alternative to tin and copper-based antifouling marine paints, ePaint, generates hydrogen peroxide by photocatalysis.
  • Antifouling coatings for filtration membranes,[49] which can also act as separation layer[50] and photocatalyst for degradation of contaminants of emerging concern.[51] or Cr(VI) removal.[52]
  • Decomposition of crude oil with TiO2 nanoparticles: by using titanium dioxide photocatalysts and UV-A radiation from the sun, the hydrocarbons found in crude oil can be turned into H2O and CO2. Higher amounts of oxygen and UV radiation increased the degradation of the model organics. These particles can be placed on floating substrates, making it easier to recover and catalyze the reaction. This is relevant since oil slicks float on top of the ocean and photons from the sun target the surface more than the inner depth of the ocean. By covering floating substrates like woodchips with epoxy adhesives, water logging can be prevented and TiO2 particles can stick to the substrates. With more research, this method should be applicable to other organics.
  • Decontamination of water with photocatalysis and adsorption: the removal and destruction of organic contaminants in groundwater can be addressed through the impregnation of adsorbents with photoactive catalysts. These adsorbents attract contaminating organic atoms/molecules like tetrachloroethylene to them. The photoactive catalysts impregnated inside speed up the degradation of the organics. Adsorbents are placed in packed beds for 18 hours, which would attract and degrade the organic compounds. The spent adsorbents would then be placed in regeneration fluid, essentially taking away all organics still attached by passing hot water counter-current to the flow of water during the adsorption process to speed up the reaction. The regeneration fluid then gets passed through the fixed beds of silica gel photocatalysts to remove and decompose the rest of the organics left. Through the use of fixed bed reactors, the regeneration of adsorbents can help increase the efficiency.
  • Some photoactive catalysts have been introduced over the last decade, such as TiO2 and ZnO nano rodes. Most of them suffer from the fact that they can only perform under UV irradiation due to their band structure. Therefore, some other photocatalysts, including a graphene-ZnO nanocompound, have been introduced over the last few years to counter this problem.[53]
  • Decomposition of polyaromatic hydrocarbons (PAHs). Triethylamine (TEA) was utilized to solvate and extract the polyaromatic hydrocarbons (PAHs) found in crude oil. By solvating these PAHs, TEA can attract the PAHs to itself. Once removed, TiO2 slurries and UV light can photocatalytically degrade the PAHs. The figure shows the high success rate of this experiment. With high yielding of recoveries of 93–99% of these contaminants, this process has become an innovative idea that can be finalized for actual environmental usage. This procedure demonstrates the ability to develop photocatalysts that would be performed at ambient pressure, ambient temperature, and at a cheaper cost.
  • Photocatalysis of organic reactions by polypyridyl complexes,[54] porphyrins[55] or other dyes[56] is extensively used by organic chemists to produce materials inaccessible by classical approaches. Most of the photocatalytic dye degradation studies reported have been with titanium dioxide as a photocatalyst. However, it is a major disadvantage of TiO2 that it absorbs only in the UV region since it has a band gap of around 3.2 eV. Among the different phases of TiO2, the anatase form of TiO2 is mostly employed due to its higher photon absorption characteristics. It is clear that the phase composition of TiO2 has a role to play in degradation of dyes.[57]
  • Light2CAT, a research and innovation project funded by the European Commission from 2012 to 2015, aimed to develop a modified TiO2 that is able to absorb light in the visible spectrum (commercial TiO2 can only be activated under ultraviolet light, which is hard to achieve) and include this modified TiO2 into concrete for the construction of concrete structures, to mitigate air quality in European cities. The absorption of visible light activates this modified TiO2, which degrades harmful pollutants via photocatalysis, such as NOx (a combination NO and NO2, in which the latter is harmful to human health), into  harmless components, such as NO3-. The modified TiO2 was utilized in concrete in three separate European cities: Copenhagen and Holbæk, Denmark, and Valencia, Spain. The installation of this “self-cleaning” concrete lead to a 5-20% reduction in NOx over the course of a year.[58][59]
  • Photocatalytic bioethanol production, research by Professor Linda Lawton, Robert Gordon University and her collaborators under CyanoSol funded by BBSRC.[60]

Quantification of Photocatalytic Activity

ISO 22197-1:2007 specifies a test method for the determination of the nitric oxide removal performance of materials that contain a photocatalyst or have superficial photocatalytic films.[61]

Specific FTIR systems are used to characterize photocatalytic activity or passivity, especially with respect to volatile organic compounds (VOCs), and representative matrices of the binders applied.[62]

Recent studies show that mass spectrometry can be a powerful tool to determine photocatalytic activity of certain materials by following the decomposition of gaseous pollutants such as nitrogen oxides or carbon dioxide [63]

See also

References

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