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Photocatalysis
Photocatalysis
Photocatalysis
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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 (TiO
2) 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 photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions."[1] In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts.[2] The use of each catalysts depends on the preferred application and required catalysis reaction.

History

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Early mentions (1911–1938)

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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.[3][4] Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination,[4][5] 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.[4][6] 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.[4][7]

In 1938 Doodeve and Kitchener discovered that 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 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.[4][8]

1964–2024

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Research in photocatalysis again paused until 1964, when V.N. Filimonov investigated isopropanol photooxidation from ZnO and TiO
2 ;[4][9] while in 1965 Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) explored oxidation/photooxidation of CO
2 and organic solvents from ZnO radiance.[4][10][11][12] In 1970, Formenti et al. and Tanaka and Blyholde observed the oxidation of various alkenes and the photocatalytic decay of N2O, respectively.[4][13][14]

A breakthrough occurred in 1972, when Akira Fujishima and Kenichi Honda discovered that electrochemical photolysis of water occurred when a TiO
2 electrode irradiated with ultraviolet light was electrically connected to a platinum electrode. As the ultraviolet light was absorbed by the 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.[4][15] 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.[4][16] 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 TiO
2 and PtO2 in ethanol, respectively.[4][17][18]

For many decades photocatalysis had not been developed for commercial purposes. (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".[4][19]

Types of photocatalysis

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Heterogeneous photocatalysis

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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.[20] 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.[20] 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.[21] 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.[22] 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.[23] This is significantly greater than many common organic compounds, which typically are not greater than E0 = +2.00 V.[24] This results in the non-selective oxidative behavior of these radicals.

TiO
2, a wide band-gap semiconductor, is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made 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.[21] 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:[21]

Where the electron is in the conduction band and the hole is in the valence band. The irradiated 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.[21]

Homogeneous photocatalysis

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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.[25] The ozone process is often referenced when developing many photocatalysts:

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.[26] Homogeneous photocatalysts are common in the production of clean hydrogen fuel production, with the notable use of cobalt and iron complexes.[26]

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):[26]

Complex-based photocatalysts are semiconductors, and operate under the same electronic properties as heterogeneous catalysts.[27]

Plasmonic antenna-reactor photocatalysis

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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 SiO
2 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 800–1,000 °C (1,470–1,830 °F).[28]

A Fe catalyst combined with a Cu light absorber can produce hydrogen from ammonia (NH
3) at ambient temperature using visible light. Conventional Cu-Ru production operates at 650–1,000 °C (1,202–1,832 °F).[29]

Applications

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SEM image of wood pulp (dark fibers) and tetrapodal zinc oxide micro particles (white and spiky) in paper.[30]

Photoactive catalysts have been introduced over the last decade, such as 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.[31] For several decades, there have been numerous attempts to develop active photocatalysts with broad light absorption capabilities. High-entropy photocatalysts, first introduced in 2020,[32] are the result of one such effort. They have been utilized for hydrogen production, oxygen production, carbon dioxide conversion, and plastic waste conversion.[33]

Paper

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Micro-sized ZnO tetrapodal particles added to pilot paper production.[30] 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.[34]

Water splitting

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Main article: Photocatalytic water splitting

Photocatalytic water splitting separates water into hydrogen and oxygen:[35]

2 H2O → 2 H2 + O2

The most prevalently investigated material, TiO
2 , is inefficient. Mixtures of TiO
2 and nickel oxide (NiO) are more active. NiO allows a significant explоitation of the visible spectrum.[36] 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.

Self-cleaning glass

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Titanium dioxide takes part in self-cleaning glass. Free radicals[37][38] generated from TiO
2 oxidize organic matter.[39][40] The rough wedge-like TiO
2 surface can be modified with a hydrophobic monolayer of octadecylphosphonic acid (ODP). 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 TiO
2 photocatalysis. Due to TiO
2 's wide band gap, light absorption by the semiconductor material and resulting superhydrophilic conversion of undoped TiO
2 requires ultraviolet radiation (wavelength <390 nm) and thereby restricts self-cleaning to outdoor applications.[41]

Disinfection and cleaning

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  • Water disinfection/decontamination,[42] a form of solar water disinfection (SODIS).[43][44] 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.
  • TiO
    2     self-sterilizing coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact).[45]
  • Magnetic TiO
    2     nanoparticle oxidation of organic contaminants agitated using a magnetic field.[46]
  • Sterilization of surgical instruments and removal of fingerprints from electrical and optical components.[47]

Hydrocarbon production from CO
2

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TiO
2 conversion of CO
2 into gaseous hydrocarbons.[48] 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 TiO
2 -based photocatalysts are low, although nanostructures such as carbon nanotubes[49] and metallic nanoparticles[50] help.

Paints

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

Photocatalysis of organic reactions by polypyridyl complexes,[51] porphyrins,[52] or other dyes[53] can produce materials inaccessible by classical approaches. Most photocatalytic dye degradation studies have employed TiO
2. The anatase form of TiO
2 has higher photon absorption characteristics.[54]

Filtration membranes

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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.[55] 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.[56] 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.[57]

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.[58]

Air Filtration

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Light2CAT was a project funded by the European Commission from 2012 to 2015. It aimed to develop a modified TiO
2 that can absorb visible light and include this modified TiO
2 into construction concrete. The 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.[59][60]

Quantification

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ISO 22197-1:2007 specifies a test method for the measurement of NO
2 removal for 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, and representative binder matrices.[62]

Mass spectrometry allows measurement of photocatalytic activity by tracking the decomposition of gaseous pollutants such as nitrogen NOx or CO
2.[63]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photocatalysis

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from Grokipedia
Photocatalysis is a catalytic process that accelerates a photoreaction in the presence of light and a photocatalyst, which absorbs photons to generate reactive species while regenerating itself unchanged during the reaction. Typically involving semiconductors such as (TiO₂), photocatalysis harnesses light energy—often or visible—to drive reactions that would otherwise be thermodynamically unfavorable under ambient conditions. This phenomenon enables applications in and production by mimicking natural . The fundamental mechanism of photocatalysis begins with the absorption of a with energy equal to or greater than the photocatalyst's bandgap, exciting an from the valence band to the conduction band and leaving a positively charged hole. These charge carriers then migrate to the catalyst's surface, where electrons can reduce like oxygen or protons to form reactive intermediates (e.g., superoxide radicals), while holes oxidize or organic pollutants to generate hydroxyl radicals. Efficient charge separation is crucial to minimize recombination, often enhanced by doping, heterojunctions, or co-catalysts like noble metals (e.g., Pt or Au). Common photocatalysts include metal oxides (TiO₂, ZnO), sulfides (CdS), and non-metal materials like (g-C₃N₄), selected for their stability, low cost, and tunable bandgaps. Research in photocatalysis traces back to early 20th-century observations of light effects on pigments, but modern developments accelerated with the 1972 discovery of the Honda-Fujishima effect, demonstrating UV-induced on TiO₂ electrodes to produce and oxygen. This milestone, published in Nature, sparked widespread interest in heterogeneous photocatalysis for environmental applications, leading to over 10,000 publications annually by the . Subsequent advances include visible-light-responsive materials and nanostructured designs, addressing limitations like TiO₂'s wide bandgap (3.2 eV), which restricts it to UV light comprising only ~5% of solar energy. Key applications of photocatalysis span environmental purification, where it degrades organic pollutants, dyes, and volatile organic compounds (VOCs) in water and air via advanced oxidation processes, achieving efficiencies over 87% for certain antibiotics under UV/ozone conditions. In energy conversion, it facilitates hydrogen production through water splitting (e.g., rates up to 3223.9 μmol g⁻¹ h⁻¹ for oxygen evolution) and CO₂ reduction to fuels, supporting carbon-neutral technologies. Emerging uses include self-cleaning surfaces, plastic deconstruction, and sustainable organic synthesis, positioning photocatalysis as a cornerstone of green chemistry amid challenges like scalability and mechanistic reproducibility.

Fundamentals

Definition and Principles

Photocatalysis is defined as the acceleration of a photoreaction by the presence of a that is activated through the absorption of , with the remaining unchanged and not consumed during the process. This differs from general , which typically relies on to lower activation barriers without involving absorption to excite the . In photocatalysis, the light-driven activation enables reactions that would otherwise proceed slowly or not at all under ambient conditions, often facilitating processes through the generation of reactive intermediates. The fundamental principles of photocatalysis center on the absorption of by , leading to the formation of electronic states. When a with sufficient strikes —typically a or molecular —it promotes an from the to an , creating charge-separated such as electron-hole pairs in semiconductors. These are highly reactive and can participate in oxidation or reduction reactions with substrates adsorbed on the catalyst surface. A key prerequisite is that the must match or exceed the (bandgap) of , ensuring effective excitation; this is governed by the relation E=hνE = h\nu, where EE is the , hh is Planck's constant, and ν\nu is the of the . Common light sources for photocatalysis include ultraviolet (UV) radiation, visible , and solar irradiation, with the choice depending on the catalyst's absorption spectrum and the desired application . For instance, UV often provides higher photons suitable for wide-bandgap catalysts, while visible or solar enables more sustainable, broader-spectrum utilization. This photon-catalyst interaction underpins the versatility of photocatalysis in driving environmentally benign transformations, such as pollutant degradation or conversion.

Photocatalytic Mechanism

Photocatalysis in semiconductors begins with the excitation step, where photons with equal to or greater than the material's bandgap are absorbed, promoting electrons from the valence band (VB) to the conduction band (CB) and leaving behind positively charged holes in the VB. This process generates electron-hole pairs essential for subsequent reactions; for instance, in (TiO₂), a widely studied semiconductor with a bandgap (E_g) of approximately 3.2 eV, ultraviolet light (wavelength < 387 nm) drives this excitation. The key describing this is: TiO2+hν (hν≥Eg) → e−(CB)+h+(VB)\text{TiO}_2 + h\nu \ (h\nu \geq E_g) \ \rightarrow \ e^-(\text{CB}) + h^+(\text{VB}) This bandgap value ensures TiO₂ responds primarily to UV light, limiting its solar efficiency but highlighting the need for bandgap engineering in practical applications. Following excitation, charge separation and migration occur as electrons move toward the CB and holes toward the VB, driven by the built-in electric field or surface states. However, rapid recombination of these carriers—often within picoseconds—releases energy as heat or light, significantly reducing photocatalytic efficiency by limiting the availability of reactive charges at the surface. To mitigate recombination, charge carriers must migrate to the catalyst surface before recombining, a process influenced by factors such as particle size, defect density, and morphology. Prevention strategies include doping or coupling with co-catalysts, which extend carrier lifetimes and enhance quantum yields. For example, in TiO₂-carbon composites such as those incorporating biochar or activated carbon, the carbon adsorbs pollutants to concentrate them near the TiO₂ surface; upon light absorption by TiO₂, electron-hole pairs are generated, producing hydroxyl (•OH) and superoxide (•O₂⁻) radicals that oxidize the adsorbed substances, while the carbon facilitates electron transfer to minimize recombination and enhance efficiency. The separated charges then generate reactive species through interfacial reactions. Holes in the VB act as strong oxidants, typically reacting with adsorbed or hydroxide ions to produce hydroxyl radicals (•OH), potent oxidizers for degrading organics: H2O+h+ → ⋅OH+H+\text{H}_2\text{O} + h^+ \ \rightarrow \ \cdot\text{OH} + \text{H}^+ Meanwhile, electrons in the CB serve as reductants, reducing molecular oxygen to radicals (O₂•⁻): e−+O2 → ⋅O2−e^- + \text{O}_2 \ \rightarrow \ \cdot\text{O}_2^- These species, along with others like peroxides, drive oxidation-reduction cycles. Surface reactions proceed via adsorption of reactants (e.g., pollutants or substrates) onto active sites, where from the bulk solution to the surface governs overall kinetics; poor adsorption or limitations can bottleneck the process, emphasizing the role of high surface area in efficient catalysts. To further enhance efficiency, advanced mechanisms like type-II heterojunctions and Z-schemes address recombination and limitations in single semiconductors. In type-II heterojunctions, two semiconductors with staggered band alignments facilitate charge transfer: photoexcited electrons from the CB of one (higher energy) migrate to the CB of the other (lower energy), while holes transfer oppositely from VB to VB, achieving spatial separation without severely weakening potentials. This configuration, as seen in systems like WO₃/BiVO₄, prolongs carrier lifetimes and boosts activity under visible light. Z-scheme mechanisms, inspired by natural , involve two semiconductors linked by an electron mediator (e.g., IO₃⁻/I⁻) or direct interface, where electrons from the CB of one recombine with holes from the VB of the other, effectively "resetting" low-potential carriers while preserving high-potential electrons and holes for strong reduction and oxidation, respectively. This preserves superior abilities compared to type-II systems and enhances charge separation, as demonstrated in early conceptual work and modern direct Z-schemes like g-C₃N₄/TiO₂. Such designs significantly improve quantum efficiency, particularly for solar-driven processes.

History

Early Developments (1911–1960)

The concept of photocatalysis emerged in the early 20th century, with the term first appearing in scientific literature in 1911 through the work of German chemist Alexander Eibner. Eibner observed that zinc oxide (ZnO) accelerated the light-induced bleaching of the dark blue pigment Prussian blue (ferric ferrocyanide) in an aqueous suspension, attributing the process to a catalytic action enhanced by illumination. This discovery highlighted the role of semiconductors in facilitating photochemical reactions without being consumed, laying foundational groundwork for heterogeneous photocatalysis, though the attribution of the term's origin has been debated with some crediting earlier mentions around 1910 by Russian researcher Plotnikow. In the 1920s and 1930s, research expanded on dye sensitization and photochemical reductions, often exploring and pigment stability. British chemist Edward C. C. Baly and colleagues applied the term "photocatalysis" to describe light-driven reduction to and carbohydrates using inorganic catalysts like salts, mimicking photosynthetic processes. Concurrently, studies on dye-sensitized reactions investigated how organic dyes extended the photoresponse of semiconductors to visible light, as seen in experiments with ZnO and for pigment degradation. A notable advancement came in 1938 when C. F. Goodeve and J. A. Kitchener demonstrated the photosensitization of (TiO₂), showing that dye-sensitized TiO₂ particles enhanced oxygen adsorption under visible light, which accelerated the oxidation of adsorbed dyes—a key early insight into semiconductor-mediated photocatalysis. This period also saw the first patent for a photocatalytic oxidation process in 1934, issued to Frans Nosicka for a process for the photochemical oxidation of organic and inorganic compounds using light irradiation and catalysts such as derivatives, with auxiliary catalysts like metal salts. The 1940s and 1950s marked a slowdown in photocatalysis research due to disruptions, with efforts shifting toward practical applications like . Silver halide emulsions, such as AgBr and AgCl, had long been central to photographic processes since the , but post-war studies elucidated their photocatalytic nature: light absorption by these generated electron-hole pairs that reduced silver ions to metallic clusters, forming the in a catalytic amplification during development. Investigations into semiconductor photoeffects resumed, focusing on ZnO and TiO₂ for surface reactions, including early explorations of photooxidation in air purification and pigment chalking in paints, though progress remained sporadic without the systematic frameworks that would emerge later.

Modern Advances (1960–2025)

The modern era of photocatalysis began in the 1960s with exploratory work on electrodes, but a pivotal breakthrough occurred in 1972 when Akira Fujishima and Kenichi Honda reported the electrochemical photolysis of water using a (TiO₂) under irradiation, producing and oxygen without external bias—a phenomenon now known as the Honda-Fujishima effect. This discovery demonstrated TiO₂'s potential for solar-driven , sparking widespread interest in photocatalytic energy conversion and establishing Fujishima and Honda as foundational figures in the field. During the 1980s and 1990s, research expanded significantly into environmental applications, with TiO₂ photocatalysts showing efficacy in degrading organic pollutants in water and air, such as through photo-oxidation processes that mineralize contaminants into harmless byproducts. Concurrently, Arthur Nozik's studies on quantum dots highlighted size-dependent quantum confinement effects in semiconductors, enabling enhanced dynamics and multiple generation for improved photocatalytic efficiency, laying groundwork for nanostructured systems. The 2000s marked a surge in efforts to extend photocatalysis beyond UV light, with the development of visible-light-responsive catalysts through anion doping, exemplified by nitrogen-doped TiO₂, which narrowed the bandgap to enable activity under solar-spectrum illumination for applications like pollutant degradation. This period also saw the rise of nanocomposites, integrating semiconductors with materials like or metals to improve charge separation and surface area, boosting overall quantum yields in processes such as degradation and hydrogen evolution. In the and early , plasmonic enhancements emerged as a key innovation, where nanoparticles like silver or gold on supports exploited resonance to amplify light absorption and generate hot electrons, enhancing reaction rates for and CO₂ reduction by up to several orders of magnitude in select systems. Metal-organic frameworks (MOFs) gained traction as photocatalysts around 2010, offering tunable and band structures for selective production, with early reports demonstrating evolution from using zirconium-based MOFs. materials, particularly halide perovskites, advanced rapidly in this decade for , leveraging their adjustable bandgaps and high absorption coefficients to achieve significant rates under visible light in composite forms. Michael Grätzel's contributions, including the 1991 invention of dye-sensitized solar cells using mesoporous TiO₂, profoundly influenced these developments by pioneering mesoscopic architectures that improved charge transfer in photocatalytic systems. Recent years have integrated computational tools, with 2024 advances employing machine learning to optimize doping strategies in photocatalysts like perovskites and g-C₃N₄/TiO₂ heterojunctions, predicting compositions that enhance CO₂ reduction selectivity toward fuels such as methanol. As of 2024, research on graphitic carbon nitride (g-C₃N₄) has advanced mechanistic understanding of its role in solar-to-fuel conversion through water splitting. In 2025, artificial intelligence has been increasingly applied to accelerate photocatalyst discovery for hydrogen production, while new organic photoredox systems enable super-reducing transformations.

Types of Photocatalysis

Heterogeneous Photocatalysis

Heterogeneous photocatalysis involves the acceleration of photo-induced chemical reactions at interfaces between a solid catalyst and fluid phases (liquid or gas), where the catalyst, typically a semiconductor material, exists in a different phase from the reactants. This process leverages the photocatalytic properties of materials like titanium dioxide (TiO₂) to drive redox reactions, such as the oxidation of pollutants or reduction of species, under light irradiation. Unlike solution-based systems, the heterogeneous setup relies on immobilized or suspended solid catalysts that facilitate charge separation and transfer at the solid-fluid boundary. Key advantages of heterogeneous photocatalysis include the straightforward separation and recovery of from the reaction mixture, enabling reusability over multiple cycles without significant loss of activity. Additionally, these systems offer high surface-to-volume ratios when using nanostructured or powdered forms, enhancing reactant adsorption and reaction efficiency, while being cost-effective and environmentally compatible due to the stability and non-toxicity of common semiconductors like TiO₂. These features make heterogeneous approaches particularly suitable for large-scale applications where catalyst is essential. The fundamental processes in heterogeneous photocatalysis begin with absorption in the bulk of the , exciting electrons from the valence band to the conduction band and generating electron-hole pairs. These charge carriers must then diffuse to the catalyst surface without recombining, where the solid-fluid interfaces enable efficient transfer of holes to oxidize adsorbed substrates or electrons to reduce acceptors, such as oxygen in aqueous media. The role of these interfaces is critical, as they promote selective adsorption and minimize bulk recombination, thereby dictating overall quantum efficiency. Representative examples include TiO₂ suspensions in aqueous environments for degrading organic pollutants like dyes or pesticides, where fine particles maximize interfacial contact and achieve near-complete mineralization under UV light. In practical setups, fixed-bed reactors with TiO₂-coated substrates allow continuous operation for gas-phase purification, such as removing volatile organic compounds from air streams, by maintaining stable catalyst immobilization. The kinetics of these systems often follow a rate law expressed as
r=k[substrate][h+]r = k [\text{substrate}] [h^+]
where rr is the reaction rate, kk is the rate constant, [substrate] is the reactant concentration, and [h⁺] denotes the surface concentration of photogenerated holes, highlighting the dependence on both substrate availability and hole flux.

Homogeneous Photocatalysis

Homogeneous photocatalysis involves chemical reactions accelerated by in systems where the photocatalyst and reactants share the same phase, most commonly a liquid solution. This contrasts with heterogeneous systems by eliminating phase boundaries, allowing for seamless molecular interactions. Molecular catalysts, including complexes and organic dyes, serve as the active , absorbing photons to reach excited states that facilitate processes under mild conditions. A key advantage of homogeneous photocatalysis is the uniform mixing of catalyst and reactants, which promotes efficient and high reaction rates without limitations across interfaces. Additionally, the potentials of these molecular catalysts can be precisely tuned through design or modifications, enabling selective activation of specific substrates and optimization for targeted transformations. This tunability, combined with strong visible-light absorption, often results in superior activity and selectivity compared to other catalytic regimes. The core process begins with photoexcitation, frequently involving metal-to-ligand charge transfer (MLCT) in complexes, where incident light promotes an from the metal's d-orbitals to the ligand's π* orbitals, generating a charge-separated state with enhanced capabilities. This can then donate or accept electrons to/from substrates or sacrificial agents—such as (S₂O₈²⁻) for oxidation or triethylamine for reduction—to propagate the and regenerate the ground-state catalyst. The in the shifts dramatically, approximated by the relation E∗=E−ΔEE^* = E - \Delta E where E∗E^* is the excited-state potential, EE is the ground-state potential, and ΔE\Delta E represents the excitation energy (often the 0-0 transition energy). This shift enables thermodynamically unfavorable ground-state reactions to proceed under light. Representative examples illustrate the versatility of these systems. Tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)₃]²⁺, functions as a robust photosensitizer for homogeneous water oxidation, absorbing blue light (λ ≈ 450 nm) to form the oxidative [Ru(bpy)₃]³⁺ species, which interacts with water oxidation catalysts and sacrificial electron acceptors like Na₂S₂O₈, achieving turnover frequencies up to 0.13 s⁻¹ at neutral pH while highlighting stability challenges from ligand dissociation. Organic dyes offer metal-free alternatives; eosin Y, for instance, drives dye-sensitized reactions such as reductive α-dehalogenation of aryl bromides or enantioselective α-alkylation of aldehydes with alkyl halides under green LED irradiation, yielding up to 92% enantiomeric excess in continuous-flow setups due to its strong visible-light absorption and long-lived triplet state.

Plasmonic and Advanced Variants

Plasmonic photocatalysis represents an advanced hybrid approach that integrates metal nanostructures with semiconductors to harness resonance (LSPR) for enhanced absorption and generation. In these systems, nanoparticles, such as or silver, exhibit LSPR when excited by visible , leading to the oscillation of electrons that generates hot electrons—high-energy carriers capable of injection into the adjacent semiconductor's conduction band. This process overcomes the limitations of traditional wide-bandgap semiconductors like TiO₂, which primarily absorb , by extending photocatalytic activity into the visible spectrum and improving quantum efficiency through reduced electron-hole recombination. The efficiency of hot electron generation and utilization in plasmonic systems is governed by the plasmon decay pathways, where the total decay rate γ\gamma decomposes into radiative (γrad\gamma_\text{rad}) and non-radiative (γnon-rad\gamma_\text{non-rad}) components: γ=γrad+γnon-rad\gamma = \gamma_\text{rad} + \gamma_\text{non-rad} The non-radiative pathway dominates in metals, producing hot s via , while the radiative pathway contributes to light scattering; optimizing the balance through nanostructure design, such as particle size and , maximizes hot electron injection yields up to 40% in select configurations. A key innovation in plasmonic photocatalysis is the antenna-reactor concept, which decouples harvesting from the catalytic reaction site to enhance selectivity and efficiency. In this design, a plasmonic "antenna" (e.g., aluminum nanocrystals) absorbs and generates hot carriers, which are then transferred to a separate "reactor" particle (e.g., nanoparticles) where occurs, minimizing thermal losses and enabling precise control over reaction pathways. This modular approach has demonstrated enhancements in photocatalytic reactions, including CO₂ reduction. Beyond plasmonics, advanced variants include photocatalytic fuel cells (PFCs), which combine photocatalysis with electrochemical conversion to simultaneously degrade pollutants and generate . In PFCs, a photoanode (e.g., TiO₂-based) oxidizes organic substrates under light illumination, producing electrons that flow to a for oxygen reduction, while achieving mineralization of dyes like . This dual-functionality addresses in , with recent designs incorporating bifunctional catalysts to boost short-circuit current densities by 2-3 times. Bio-photocatalysis integrates enzymatic biocatalysts with photocatalytic materials to enable selective, mild-condition transformations inspired by natural . In these hybrid systems, photocatalysts like generate reactive species (e.g., reduced flavins) that drive enzyme-catalyzed reactions, such as C-H bond activation in non-natural substrates, under visible light. This synergy leverages enzymes' specificity and photocatalysts' light-driven capabilities, as seen in flavin-dependent monooxygenases paired with nanoparticles for asymmetric synthesis. Representative examples illustrate these variants' impact. Gold-decorated TiO₂ (Au-TiO₂) hybrids enhance visible-light-driven reactions through LSPR-induced hot , with formaldehyde oxidation rates increasing up to 5-fold at moderate . Recent advancements in perovskite-plasmonic tandems achieve improved charge separation for CO₂ reduction by leveraging perovskites' narrow bandgaps alongside plasmonic field enhancement. Emerging variants also include upconversion-assisted systems and 2D material hybrids, which further extend light utilization and efficiency in photocatalysis as of 2025.

Materials

Common Photocatalysts

Titanium dioxide (TiO₂) is one of the most widely used photocatalysts due to its high chemical stability, non-toxicity, and suitable electronic properties. It exists in several polymorphs, with and being the most common for photocatalytic applications. The phase has a bandgap of approximately 3.2 eV, while has a slightly narrower bandgap of about 3.0 eV, limiting both to UV light absorption. The conduction band edge of TiO₂ is positioned at around -0.29 V vs. NHE (at pH 0), and the valence band edge at +2.91 V vs. NHE, enabling effective oxidation and reduction processes. TiO₂ is highly stable under photocatalytic conditions and poses low risks, making it suitable for environmental uses. A common synthesis method for TiO₂ is the sol-gel process, involving of titanium precursors like followed by to form the desired phase. A well-known commercial example is Degussa P25 TiO₂, a of ~80% and 20% , valued for its enhanced charge separation due to the phase junction. Zinc oxide (ZnO) is another prominent photocatalyst, with a direct bandgap of 3.37 eV, also restricting it primarily to UV activation. Its conduction band edge is approximately -0.5 V vs. NHE, and valence band edge at +2.87 V vs. NHE (at 0), providing strong oxidative potential. ZnO exhibits good in neutral and alkaline environments but can dissolve in acidic conditions, and it has moderate concerns related to zinc ion release. Synthesis often employs sol-gel methods using precursors, followed by annealing, or hydrothermal routes for controlled morphology. ZnO is frequently used in powder form for its high surface area and reactivity. Cadmium sulfide (CdS) serves as a visible-light-responsive photocatalyst with a narrower bandgap of about 2.4 eV, allowing absorption up to ~520 nm. The conduction band is positioned at -0.52 V vs. NHE, and the valence band at +1.88 V vs. NHE (at 0), though its less positive valence band limits some oxidation reactions. CdS suffers from photocorrosion instability under prolonged illumination and high toxicity due to leaching, restricting its practical deployment. Common synthesis involves chemical precipitation or hydrothermal methods using and salts, often requiring stabilizers to mitigate degradation. Despite challenges, CdS is valued for its tunable optoelectronic properties in composite systems. For visible-light activity, (WO₃) is employed, featuring a bandgap of 2.6–2.8 eV and band edges at +0.4 V (conduction) and +3.0 V (valence) vs. NHE (at 0), offering strong oxidation capability but limited hydrogen potential. WO₃ demonstrates excellent across a wide range and low , synthesized typically via sol-gel or hydrothermal processes from precursors. (α-Fe₂O₃), a naturally abundant , has a bandgap of ~2.2 eV with conduction band at ~+0.2 V and valence band at ~+2.4 V vs. NHE (at 0), enabling visible-light response; it is highly stable, non-toxic, and often derived from natural minerals or synthesized by precipitation and calcination. (g-C₃N₄), a metal-free polymeric , possesses a bandgap of 2.7 eV, with conduction band at -1.3 V and valence band at +1.4 V vs. NHE (at 0), providing good reduction ability and moderate oxidation. It is thermally and chemically stable up to 500°C, non-toxic, and easily synthesized by thermal polycondensation of or at 500–550°C. The following table summarizes key electronic properties of these materials:
MaterialBandgap (eV)Conduction Band (V vs. NHE, pH 0)Valence Band (V vs. NHE, pH 0)StabilityToxicity
TiO₂ ()3.2-0.29+2.91HighLow
ZnO3.37-0.5+2.87Good (pH-dependent)Moderate
CdS2.4-0.52+1.88Low (photocorrosion)High
WO₃2.6–2.8+0.4+3.0HighLow
α-Fe₂O₃2.2+0.2+2.4HighLow
g-C₃N₄2.7-1.3+1.4HighLow
These base materials form the foundation for many photocatalytic systems, with their intrinsic properties dictating suitability for specific light spectra and reaction types.

Modifications and Nanostructures

Modifications to photocatalysts, such as doping and the formation of heterostructures, are essential strategies to enhance separation, extend light absorption into the visible and regions, and improve overall efficiency. These approaches address inherent limitations in wide-bandgap semiconductors like TiO₂, which primarily respond to ultraviolet light. By introducing dopants or engineering interfaces at the nanoscale, photocatalysts can achieve bandgap narrowing and reduced recombination rates, leading to higher quantum yields in applications like and pollutant degradation. Doping involves incorporating metal or non-metal elements into the photocatalyst lattice to modify electronic properties. Metal doping, such as (Pt) on TiO₂, acts as a co-catalyst to facilitate electron trapping and promote hydrogen evolution in , with Pt nanoparticles enhancing the rate by several folds compared to undoped TiO₂. For instance, Pt-deposited TiO₂ nanosheets have demonstrated hydrogen production rates exceeding 1000 μmol·g⁻¹·h⁻¹ under UV due to improved charge transfer at the metal- interface. The at this interface, which hinders back- transfer, is quantified by the barrier height φ_B = φ_m - χ_s, where φ_m is the metal and χ_s is the ; this barrier typically ranges from 0.5 to 1.0 eV for Pt-TiO₂, promoting efficient charge separation. Non-metal doping, particularly nitrogen (N) incorporation into TiO₂, narrows the bandgap from ~3.2 eV to ~2.5 eV, enabling visible-light response by mixing N 2p states with O 2p orbitals in the valence band. This modification, first demonstrated in substitutional N-doped TiO₂, results in photocatalytic activity for degradation under visible light, with degradation efficiencies up to 80% higher than pristine TiO₂. Bandgap narrowing through non-metal doping generally shifts absorption edges by 0.5–1.0 eV, depending on dopant concentration, while maintaining sufficient conduction band position for reactions. Heterostructures combine two or more semiconductors to spatially separate photogenerated charges, mitigating recombination. In Type-II heterojunctions, such as CdS/TiO₂, the drives electrons from the higher-bandgap material to the lower one, prolonging carrier lifetimes and enhancing photocatalytic rates by up to 10 times compared to single components. Z-scheme heterostructures, mimicking natural , involve an electron mediator (e.g., Au nanoparticles) that recombines less energetic carriers while preserving high potentials; for example, TiO₂/g-C₃N₄ Z-schemes achieve CO₂ reduction to CH₄ with quantum efficiencies exceeding 1% under visible light. These configurations improve charge separation efficiency to over 90% in optimized systems. Nanoscale engineering further optimizes photocatalyst performance by increasing surface area and exploiting quantum effects. Nanoparticles (e.g., TiO₂ spheres <10 nm) provide high specific surface areas up to 200 m²/g, accelerating reactant adsorption and reaction kinetics in dye degradation. Nanotubes, such as TiO₂ nanotubes, offer one-dimensional pathways for charge transport, reducing recombination and boosting by 2–5 times relative to bulk forms. Thin films enable scalable applications like self-cleaning surfaces, with thicknesses of 100–500 nm balancing light penetration and quantum efficiency. Quantum confinement in these nanostructures raises the bandgap by 0.2–0.5 eV for particles below 5 nm, enhancing driving forces but requiring careful size control to avoid excessive blue-shifts that limit visible absorption. Recent advances in 2024–2025 have introduced metal-organic framework (MOF)-derived catalysts, where of MOFs like ZIF-8 yields porous carbon-supported metal oxides with hierarchical structures, achieving hydrogen evolution rates over 5000 μmol·g⁻¹·h⁻¹ due to enhanced light harvesting and charge transfer. Similarly, black TiO₂, featuring oxygen vacancies and Ti³⁺ states, extends absorption into the region (up to 1000 nm), with recent defect-engineered variants showing 3–4 times higher photocatalytic activity for under simulated compared to white TiO₂. These developments underscore the role of defect engineering in bridging UV-visible-IR response gaps.

Applications

Environmental Remediation

Photocatalysis plays a pivotal role in by harnessing light to drive reactions that degrade pollutants and inactivate pathogens, primarily through the generation of (ROS) such as hydroxyl radicals (•OH) and anions (•O₂⁻). These processes offer a sustainable approach to addressing , air, and contamination without secondary , as photocatalysts like TiO₂ can be reused and operate under ambient conditions. Seminal work on TiO₂ photocatalysis, dating back to the 1970s, has established its efficacy for pollutant mineralization into CO₂ and H₂O. In , photocatalysis effectively degrades organic dyes and pesticides while reducing . For dyes such as , TiO₂ under UV light achieves near-complete degradation (up to 95%) via •OH-mediated N-deethylation and ring cleavage, serving as a model for textile wastewater remediation. Pesticides like and are mineralized using ZnO/TiO₂ composites, with efficiencies exceeding 99% in 120 minutes under UV irradiation, through ROS-induced of P-O and C-S bonds. Heavy metal reduction, exemplified by Cr(VI) to Cr(III) conversion, occurs via photoexcited electrons on MOF/TiO₂ hybrids, reducing in batch systems, as demonstrated in high-impact studies on charge transfer mechanisms. Air purification leverages photocatalysis for volatile organic compound (VOC) oxidation and removal. Formaldehyde, a common indoor VOC, is oxidized to CO₂ and H₂O on mesoporous TiO₂ surfaces with up to 95.8% efficiency under UV light (30 ppm initial concentration), involving stepwise radical attacks on the C-H bond. abatement uses TiO₂ coatings to convert NO to nitrates, achieving 67% removal in low-concentration flows (ISO 22197-1 standard), enhanced by dopants like MnOₓ that improve charge separation. Disinfection via photocatalysis inactivates bacteria like E. coli through ROS-induced and protein damage on TiO₂ films. Under UV-A exposure, TiO₂ achieves 99.9% inactivation in aqueous suspensions within 60-120 minutes, with mechanisms confirmed by EPR showing •OH dominance. Representative examples include photocatalytic membranes that integrate TiO₂ into matrices for simultaneous and degradation, reducing dye permeation by 90% while maintaining flux, offering advantages in continuous . As of 2025, advances in microplastic breakdown using TiO₂-based systems have shown up to 50% weight loss for under , via chain scission, addressing emerging aquatic pollutants. Processes typically employ batch reactors for lab-scale optimization, achieving high degradation rates but limited throughput, whereas continuous flow reactors with immobilized catalysts enable scalability for industrial use, though challenges persist in catalyst stability and light penetration. Scalability issues, including high energy costs for UV sources and photocatalyst recovery, hinder widespread adoption, with ongoing research focusing on visible-light-responsive materials to leverage solar energy.

Energy Production

Photocatalysis plays a pivotal role in production by harnessing to drive thermodynamically uphill reactions, such as and reduction, thereby generating storable solar fuels like and hydrocarbons. These processes mimic aspects of natural , converting abundant solar photons into without emitting greenhouse gases during operation. Key challenges include overcoming energy barriers and achieving high under visible , which constitutes the majority of the solar spectrum. Recent advances as of 2025 have achieved solar-to- (STH) efficiencies exceeding 1% in unbiased particulate systems, with half-cell efficiencies up to 9.91%. Photocatalytic water splitting involves the decomposition of water into and using photocatalysts under light irradiation. The overall reaction is 2H2O→2H2+O22H_2O \rightarrow 2H_2 + O_2, with a standard free energy change of ΔG∘=237.2\Delta G^\circ = 237.2 kJ/mol, corresponding to a theoretical minimum potential of 1.23 V. This process comprises two half-reactions: the (HER), 2H++2e−→H22H^+ + 2e^- \rightarrow H_2 (0 V vs. NHE), and the reaction (OER), 2H2O→O2+4H++4e−2H_2O \rightarrow O_2 + 4H^+ + 4e^- (1.23 V vs. NHE at pH 0). However, significant overpotentials—typically 0.4–0.6 V for HER and over 1 V for OER—arise due to sluggish kinetics, charge recombination, and the multi-electron nature of OER, necessitating cocatalysts like to lower these barriers. A representative example is the Pt/TiO2_2 system, where nanoparticles deposited on TiO2_2 enhance HER rates by providing active sites for proton reduction, achieving evolution rates exceeding 100 μmol/h/g under UV irradiation in sacrificial donor solutions. Recent Z-scheme photocatalyst systems, which combine two semiconductors with an electron mediator to preserve potentials, have demonstrated stable overall with solar-to- (STH) efficiencies up to 0.22% over 100 hours of visible-light operation. In parallel, photocatalytic CO2_2 reduction converts greenhouse gas into value-added fuels, addressing both energy and climate challenges. The process involves multi-electron transfers to form products like carbon monoxide (CO) via CO2+2H++2e−→CO+H2OCO_2 + 2H^+ + 2e^- \rightarrow CO + H_2O or methane (CH4_4) via CO2+8H++8e−→CH4+2H2OCO_2 + 8H^+ + 8e^- \rightarrow CH_4 + 2H_2O, but selectivity remains a major hurdle due to competing hydrogen evolution and the formation of intermediates like formate. Copper(I) oxide (Cu2_2O) has emerged as a promising p-type semiconductor for this reaction, particularly for methanol production, where the (110) facet of Cu2_2O particles selectively catalyzes CO2_2 reduction to CH3_3OH with an internal quantum yield of ~70% under visible light, attributed to facet-specific adsorption of CO2_2. These solar fuel generation approaches draw from artificial photosynthesis concepts, where photocatalysts emulate photosystems to couple light absorption with catalytic cycles for sustainable H2_2 or hydrocarbon synthesis. Efficiency in these systems is often quantified using the apparent quantum yield (Φ\Phi), defined as Φ=moles of product formedmoles of photons absorbed×100%\Phi = \frac{\text{moles of product formed}}{\text{moles of photons absorbed}} \times 100\%, which accounts for the fraction of incident photons driving the reaction. High Φ\Phi values, such as those exceeding 10% in optimized Z-scheme configurations for partial reactions, underscore progress toward practical production, though overall STH efficiencies remain below 1% for unbiased systems due to material and kinetic limitations.

Self-Cleaning and Antimicrobial Surfaces

Photocatalytic self-cleaning surfaces primarily rely on (TiO₂) films that exhibit hydrophilic properties under (UV) light exposure, enabling the degradation of organic contaminants and facilitating their removal by water rinsing. When illuminated by UV light, TiO₂ generates , such as hydroxyl radicals, which oxidize and break down organic dirt adhering to the surface into harmless compounds like and water. This photocatalytic process, combined with the superhydrophilic nature of the irradiated TiO₂ surface—where the water contact angle drops below 5°—allows rain or moisture to spread into a thin sheet, effectively washing away the decomposed residues without leaving streaks. A seminal commercial example is Activ™ , which features a 15 nm-thick nanocrystalline TiO₂ coating applied via an online process during manufacturing, demonstrating sustained self-cleaning performance on building facades exposed to natural . In parallel, photocatalytic TiO₂ coatings provide effects by producing the same that damage bacterial cell walls, viral envelopes, and proteins, leading to inactivation without the need for chemical additives. For bacteria such as and Staphylococcus aureus, TiO₂ surfaces achieve over 99% reduction in viable cells under UV or even visible light when doped, through mechanisms like and DNA disruption. Against viruses, including , TiO₂-based inactivate up to 99.9% of the within 1-8 hours under indoor lighting conditions, as demonstrated by coatings that disrupt the and viral . These properties make TiO₂ coatings suitable for hospital applications, such as wall and equipment surfaces, where they reduce microbial contamination in high-touch areas, contributing to infection control in clinical environments. Practical implementations extend to photocatalytic paints and textiles, which integrate TiO₂ nanoparticles for dual self-cleaning and functionality in everyday settings. Photocatalytic paints, often water-based and containing TiO₂, are applied to interior walls to degrade organic stains and inhibit bacterial growth, with one formulation showing 90% removal of indoor pollutants like volatile organic compounds under ambient light. For textiles, TiO₂ coatings on fabrics enable superhydrophilic self-cleaning of oil-based stains via UV-induced photocatalysis, while also providing antibacterial efficacy against E. coli with log reductions exceeding 5 under sunlight exposure. Emerging in 2025, advanced nano-coatings for incorporate TiO₂ with dopants like silver or , applied to ventilation surfaces to achieve 91% bacterial reduction using standard indoor lighting, enhancing in enclosed spaces without active energy input. The durability of these TiO₂ coatings is critical for long-term performance, with adhesion and resistance influenced by substrate preparation and coating morphology. Strong interfacial bonding, achieved through techniques like sol-gel deposition or plasma treatment, prevents , maintaining photocatalytic activity after simulated cycles equivalent to years of outdoor exposure. tests reveal that dense, crack-free TiO₂ films retain over 80% of their hydrophilicity and efficacy following 1000 hours of UV and humidity cycling, though nanoparticle leaching can occur if adhesion is poor, necessitating silica interlayers for enhanced stability on porous substrates like . In real-world applications, such as exterior paints, TiO₂ coatings demonstrate adhesion strengths above 5 MPa and resist chalking from and pollutants, ensuring sustained functionality over 5-10 years.

Industrial Processes

Photocatalysis has emerged as a promising in industrial , particularly for producing hydrocarbons from (CO₂) or through reduction processes that mimic natural . In these applications, semiconductor photocatalysts like modified (g-C₃N₄) enable the conversion of CO₂ into C1–C5 hydrocarbons via C–C coupling, achieving 100% selectivity without sacrificial agents under visible light . Recent advancements include a nanoparticle-based system that hydrogenates CO₂ to with 99% selectivity, offering a pathway for scalable production of key chemical feedstocks essential for plastics and fuels . Similarly, photocatalytic reduction of syngas-derived intermediates supports the synthesis of longer-chain hydrocarbons, with multi-carbon products highlighted in ongoing efforts to address industrial carbon utilization challenges. Selective oxidation represents another cornerstone of photocatalytic , accounting for approximately 30% of modern chemical production and enabling the transformation of alcohols, arenes, and other substrates into valuable aldehydes, ketones, and acids. Heterogeneous photocatalysts, such as rhodium-modified materials, facilitate the visible-light-driven oxidation of alcohols to carbonyl compounds with high activity and selectivity, bypassing traditional high-temperature or stoichiometric oxidants. In the of —a precursor for polyesters—photocatalytic oxidation of achieves superior yields compared to conventional methods, leveraging TiO₂-based systems to minimize energy input and waste. These processes are particularly advantageous in fine chemical manufacturing, where precise control over ensures and reduces byproduct formation. In the coatings industry, UV-curable photocatalytic additives, primarily TiO₂ nanoparticles, are integrated into formulations to enhance durability and functionality during manufacturing. These additives polymerize rapidly under ultraviolet light, enabling low-VOC paints that incorporate self-regulating photocatalytic properties for industrial applications like automotive and architectural finishes. Acrylic-based photocatalytic coatings, for instance, maintain structural integrity under low-intensity UV-A irradiation (1–10 W/m²), supporting efficient large-scale production while providing long-term surface activation. Photocatalysis also plays a role in and , where it aids bleaching and anti-odor treatments to meet quality standards without harsh chemicals. For production, photocatalytic generation of (H₂O₂) serves as an eco-friendly bleaching agent, decomposing under mild conditions to achieve whiteness comparable to traditional methods while reducing toxicity. In textiles, TiO₂-based photocatalytic coatings decompose odor-causing volatile organic compounds upon exposure, with fabrics containing 80–100% photocatalyst fibers demonstrating sustained deodorizing efficacy after multiple washes. Representative examples illustrate the transition to industrial scales, including photocatalytic filtration membranes that integrate TiO₂ for in-situ degradation during processes like recycling in chemical plants. These hybrid /photocatalytic systems reduce fouling and enhance separation efficiency, with ceramic TiO₂ membranes showing promise for high-throughput industrial . In pharmaceutical synthesis, a 2024 scale-up of metallaphotoredox-catalyzed C–O coupling for intermediates like ethers achieved continuous flow production at multigram scales, demonstrating viability for with minimal catalyst loading. Despite these advances, economic barriers hinder widespread , including high initial costs for photocatalyst synthesis and reactor scaling, which can exceed traditional methods by 2–5 times due to delivery inefficiencies. Efforts like Toyota's systems highlight progress, where photocatalytic elements contribute to on-site H₂ generation for industrial fueling, though full integration remains limited by energy efficiency below 10% in pilot setups. Addressing these requires innovations in low-cost and photoreactor design to achieve cost parity with conventional .

Quantification and Characterization

Measurement Techniques

Photocatalytic activity is typically evaluated using experimental setups that simulate relevant conditions while allowing precise control over reaction parameters. Batch reactors, such as stirred suspensions in vessels, are widely employed for their simplicity in assessing initial activity, where the photocatalyst is dispersed in a solution and irradiated under controlled conditions. Continuous flow systems, including fixed-bed or fluidized-bed reactors, are preferred for scalability studies and real-world simulations, enabling steady-state operation and higher throughput by circulating reactants over immobilized catalysts. Light sources in these setups commonly include xenon arc lamps, which provide a broad spectrum approximating with high intensity in the UV and visible regions, or solar simulators calibrated to AM 1.5G standards for outdoor relevance. Degradation of organic pollutants serves as a primary indicator of photocatalytic performance and is routinely monitored through UV-Vis spectroscopy, which tracks the temporal decrease in absorbance of chromophoric species like dyes or aromatic compounds at specific wavelengths. This technique offers real-time, non-destructive analysis, often integrated into reactor setups for in-line monitoring, and is particularly effective for model pollutants such as or phenol due to their distinct spectral signatures. To identify intermediate and final products, gas chromatography-mass spectrometry (GC-MS) is utilized post-reaction, separating volatile and semi-volatile compounds for structural elucidation via mass fragmentation patterns, ensuring complete mineralization pathways are verified. The involvement of reactive species is probed using (EPR) spectroscopy, which detects short-lived radicals such as hydroxyl (•OH) or (O₂⁻•) by employing spin-trapping agents like 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to stabilize them for spectral analysis. This method provides direct evidence of radical generation under illumination, distinguishing between surface-bound and solution-phase species through hyperfine splitting patterns. For deeper mechanistic insights, in-situ transient (TAS) examines dynamics, capturing ultrafast processes like electron-hole separation and recombination on picosecond to microsecond timescales using pump-probe configurations with femtosecond pulses. Standardization efforts enhance comparability across studies, with ISO protocols such as ISO 10678:2024 specifying procedures for measuring TiO₂ photocatalytic activity in aqueous media via decolorization under UV-A irradiation, including details on catalyst loading, flux, and sampling. Similarly, ISO 22197-1:2016 outlines NO gas-phase removal tests for air-purifying materials, defining reactor configurations and irradiance levels to benchmark TiO₂-based systems. These measurements yield raw data from which performance indicators are subsequently calculated.

Efficiency Metrics

Efficiency metrics in photocatalysis provide standardized quantitative measures to evaluate and compare the of photocatalytic systems, focusing on utilization, reaction rates, and conversion. These metrics account for factors such as absorption, dynamics, and product formation, enabling researchers to assess scalability and practical viability without relying on raw experimental data alone. Key parameters include apparent quantum efficiency, rate constants derived from kinetic models, solar-to-hydrogen efficiency, , and incident photon-to-current efficiency, each tailored to specific aspects of photocatalytic processes. Apparent quantum efficiency (AQE), also known as photonic efficiency, quantifies the ratio of the number of reaction events (e.g., molecules degraded or molecules produced) to the number of incident photons at a given or over a range. Unlike true , which uses absorbed photons, AQE employs incident photons to reflect practical performance under real illumination conditions, making it suitable for heterogeneous systems where absorption is incomplete. For instance, in photocatalytic evolution, AQE values exceeding 10% at visible s (e.g., 450 nm) have been reported for optimized TiO₂-based catalysts, highlighting improvements in visible-light utilization. The metric is calculated as: AQE=Number of eventsNumber of incident photons×100%\text{AQE} = \frac{\text{Number of events}}{\text{Number of incident photons}} \times 100\% This approach facilitates direct comparisons across studies, though variations arise from illumination sources and reactor geometries. Rate constants in photocatalysis are often derived from the Langmuir-Hinshelwood (LH) model, which describes the kinetics of surface-mediated reactions assuming adsorption of reactants onto the catalyst surface prior to photodegradation. The model posits that the reaction rate rr follows r=kθr = k \theta, where kk is the apparent rate constant and θ\theta is the fractional surface coverage of the pollutant, typically expressed as θ=KC1+KC\theta = \frac{K C}{1 + K C} with KK as the adsorption equilibrium constant and CC as the pollutant concentration. In the low-concentration limit, this simplifies to a pseudo-first-order form with r=k′Cr = k' C, where k′=kKk' = k K. Seminal work on organic contaminant degradation using TiO₂ demonstrated that LH-derived constants vary with hydroxyl radical attack mechanisms, yielding kk values on the order of 10⁻³ to 10⁻¹ min⁻¹ for pollutants like phenol under UV irradiation. These constants enable prediction of degradation efficiency and optimization of catalyst loading. For energy production applications, particularly water splitting, solar-to-hydrogen (STH) efficiency measures the fraction of incident solar energy converted into chemical energy stored in hydrogen. Defined as the ratio of the energy content of produced hydrogen (based on its higher heating value) to the total solar input, STH is given by: STH=n˙H2×ΔH×tPsun×A×t×100%\text{STH} = \frac{\dot{n}_{\text{H}_2} \times \Delta H \times t}{P_{\text{sun}} \times A \times t} \times 100\% where n˙H2\dot{n}_{\text{H}_2} is the hydrogen production rate, ΔH\Delta H is the enthalpy of combustion (typically 286 kJ/mol), PsunP_{\text{sun}} is the solar irradiance (e.g., 100 mW/cm² under AM 1.5G), AA is the illuminated area, and tt is time. Benchmark STH values for particulate photocatalysts like modified GaN reach up to 9.2% under concentrated sunlight, underscoring the metric's role in evaluating overall system performance for scalable solar fuel production. Turnover number (TON) assesses catalyst durability and site-specific activity by representing the total number of reaction cycles (e.g., substrate molecules converted) per active catalytic site over the reaction duration. In photocatalysis, TON is dimensionless and calculated as the moles of product formed divided by the moles of active sites, though estimating site density in heterogeneous systems often relies on surface area measurements like BET analysis. High TON values, such as exceeding 10⁴ for CO₂ reduction on metal-loaded TiO₂, indicate robust catalysts resistant to deactivation, with the metric emphasizing long-term stability over initial rates. Incident photon-to-current efficiency (IPCE) is crucial for photoelectrochemical photocatalysis, quantifying the of incident photons at a specific that generate collectible electrons in an external circuit. Expressed as IPCE = (1240 × J_ph) / (λ × P_in) × 100%, where J_ph is the (mA/cm²), λ is the (nm), and P_in is the incident (mW/cm²), it deconvolutes contributions from harvesting, charge separation, and transfer. For example, IPCE peaks above 80% at 400 nm have been achieved in BiVO₄ photoanodes for water oxidation, providing insights into wavelength-dependent efficiencies without full solar simulation. This metric bridges photocatalytic and photovoltaic evaluations, particularly for hybrid systems.

Challenges and Future Directions

Current Limitations

One of the primary efficiency gaps in photocatalysis stems from the limited response to visible light in many semiconductor materials, such as TiO₂ and ZnO, which primarily absorb in the (UV) region, comprising only about 5% of the solar . This restriction confines practical applications to UV sources, reducing overall solar utilization and quantum yields. Additionally, rapid charge recombination—where photogenerated electron-hole pairs recombine before participating in redox reactions—further diminishes efficiency, with studies indicating that less than 10% of photoredox events lead to productive reactive in some systems. Stability issues pose significant barriers to long-term photocatalyst performance, particularly photocorrosion, where materials degrade under illumination due to anodic or cathodic dissolution. For instance, (CdS) exhibits notable photocorrosion, limiting its durability in aqueous environments despite its favorable for visible-light absorption. remains challenging, as transitioning from laboratory-scale setups to industrial reactors often results in reduced due to limitations and difficulties in catalyst immobilization without performance loss. Cost factors hinder widespread adoption, including the reliance on rare and expensive metal dopants like and in molecular photocatalysts, which elevate material expenses. Furthermore, UV-dependent systems require energy-intensive UV lamps with short lifespans and high operational costs, making them economically unviable for large-scale solar-driven processes. Environmental concerns arise from the toxicity of certain photocatalysts, such as Cd-based materials, which can leach into treated water, posing risks to ecosystems and human health. Incomplete pollutant mineralization may also generate toxic byproducts or intermediates, complicating and potentially exacerbating rather than resolving it. One prominent emerging trend in photocatalysis involves extending light absorption into the visible and near-infrared (NIR) spectrum to harness a larger portion of , addressing the limitations of UV-dependent systems. Upconversion nanoparticles (UCNPs), such as lanthanide-doped NaYF₄:Yb³⁺,Er³⁺, convert NIR photons into higher-energy visible or UV light through sequential absorption and energy transfer processes, enhancing generation in wide-bandgap semiconductors like TiO₂ or ZnO. For instance, UCNPs integrated with Bi₂WO₆ have demonstrated improved photocatalytic evolution rates of up to 41.3 mmol g⁻¹ h⁻¹ under simulated solar irradiation by boosting electron-hole separation and reducing recombination. Tandem systems further amplify this extension by coupling multiple catalytic steps, such as S-scheme heterojunctions (e.g., CeO₂/Bi₂S₃) for CO₂ reduction to CO followed by Pd-catalyzed to amides, achieving 14.05 mmol g⁻¹ CO yield with 98% selectivity under visible/NIR light due to the narrow 1.29 eV bandgap of Bi₂S₃. These approaches enable efficient solar-driven reactions like and pollutant degradation, with core-shell nanostructures optimizing interfacial charge transfer. Sustainability efforts are driving the development of metal-free and bio-derived photocatalysts to minimize resource scarcity and environmental impact associated with rare-earth or transition-metal dependencies. Metal-free organic photocatalysts, such as push-pull heterocycles with D–π–A–π–D frameworks, exhibit strong visible-light absorption and high H₂ evolution rates exceeding 10,000 μmol g⁻¹ h⁻¹ without sacrificial agents, owing to their tunable potentials and low toxicity. Bio-derived photocatalysts, particularly biochar-based composites like Fe₃O₄/BiOBr/ from , promote principles by achieving 95.51% degradation of under visible light through enhanced adsorption and charge separation via biochar's porous structure. These materials also facilitate biomass valorization, converting waste like to H₂ at rates of 202 μmol h⁻¹ g⁻¹, aligning with for conversion. Integration of photocatalysis with (AI) and flow chemistry is accelerating catalyst discovery and process scalability. AI-driven frameworks, including models for predicting optimal heterostructures, have optimized nanomaterial synthesis for CO₂ reduction, yielding up to 20-fold gains by simulating bandgap and defect sites via integration. In flow chemistry, continuous microreactors enable homogeneous photocatalysis for reactions like decarboxylative coupling, achieving space-time yields 430 times higher than batch methods due to uniform light distribution and rapid mixing, as demonstrated in C(sp²)–C(sp³) bond formations with 90% conversion in 40 minutes. These hybrid systems support real-time optimization, reducing energy consumption and enabling industrial-scale synthesis of pharmaceuticals and fine chemicals. Looking toward 2025 and beyond, (QD) enhancements and global commercialization initiatives are poised to propel photocatalysis into practical deployment. (CQDs), with their upconversion properties and defect-engineered surfaces, boost photocurrent by 8-fold in composites like CQDs/ZnFe₂O₄, enabling 99.5% degradation and promising scalable H₂O₂ production under visible light. These advancements align with EU Green Deal targets, which emphasize photocatalytic systems for net-zero emissions by 2050, including funding for waste-to-fuel devices and CO₂ utilization technologies to achieve 55% reductions by 2030 through industrialized photocatalysts. Such prospects underscore a shift toward AI-QD hybrids in flow reactors for commercial and energy production.

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