Green photocatalysts are photocatalysts derived from environmentally friendly sources. They are synthesized from natural, renewable, and biological resources, such as plant extracts, biomass, or microorganisms, minimizing the use of toxic chemicals and reducing the environmental impact associated with conventional photocatalyst production.
A photocatalyst is a material that absorbs light energy to initiate or accelerate a chemical reaction without being consumed in the process. They are semiconducting materials which generate electron-hole pairs upon light irradiation. These photogenerated charge carriers then migrate to the surface of the photocatalyst and interact with adsorbed species, triggering redox reactions. They are promising candidates for a wide range of applications, including the degradation of organic pollutants in wastewater, the reduction of harmful gases, and the production of hydrogen or solar fuels. Many methods exist to produce photocatalysts via both conventional and more green approaches including hydrothermal synthesis or sol-gel, the difference being in the material sources used.
Green precursor materials for photocatalysts
Green sources A green source for photocatalyst synthesis refers to a material that is renewable, biodegradable, and has minimal environmental impact during its extraction and processing. This approach aligns with the principles of green chemistry, which aim to reduce or eliminate the use and generation of hazardous substances in chemical processes. Green sources are abundant, readily available, and often considered as waste materials, thus offering a sustainable and cost-effective alternative to conventional photocatalyst precursors.
Plant-based precursors Plant extracts and agricultural waste products have emerged as promising green sources for photocatalyst production, offering attractive alternatives to conventional precursors due to their abundance, biodegradability, and cost-effectiveness. Extracts from various plant parts, such as leaves, roots, and fruits, contain phyto-chemicals that can act as reducing and stabilizing agents in nanoparticle synthesis, contributing to the formation of desired photocatalyst morphologies. Meanwhile, waste materials from agricultural processes, such as rice husks and sugarcane bagasse, are rich in cellulose and lignin. These components can be used as precursors for carbon-based photocatalyst or as templates for the synthesis of porous nano-materials.
+Plant-Based Nanoparticle/Nanocatalysts: Synthesis, Size, and ShapePlantCommon/Popular NameNPs synthesized and producedSize of NPs (nm)Shape of NPsReferenceCitrus limettaSweet Lime/MosambiCdO54Quasi-sphericalDillenia indicaElephant AppleCuO15SphericalMikania micranthaMile-a-minute Weed/American RopeCuO15SphericalJackfruitJackfruitLa2O330Needle-shapedSansevieria trifasciataSnake Plant/Mother-in-Law's TongueZnFe2O45–20SphericalCommelina benghalensisBenghal Dayflower/Tropical SpiderwortAg–ZnO–CSs20-100SphericalCommelina benghalensisBenghal Dayflower/Tropical SpiderwortAu–ZnO–CSs50-400SphericalSenna siameaSiamese Cassia/Kassod TreeZnO37.39SphericalEpipremnum aureumPothos/Devil's Ivy/Money PlantZnO29SphericalChinese MahoganyChinese MahoganyLO22.56Long rod-like particlesCitrullus colocynthisColocynth/Bitter AppleCu17 ± 4.2SphericalAegle marmelosBael/Bengal QuinceFeO18.78SphericalCouroupita guianensisCannonball TreeCaO25.2Clusters with irregular forms Notes:
NPs: Nanoparticles CSS: Core-Shell Structure The table summarizes various plant-based nanoparticles and nanocatalysts, including their synthesis methods, particle sizes, shapes, and corresponding references.
Bio-waste precursors Utilizing bio-waste, such as food waste and animal waste, for green photocatalyst synthesis offers a dual benefit of waste management and material production. These waste streams are rich in organic matter, which can be converted into valuable carbon-based photocatalyst through various thermochemical processes. +Bio-Waste/Agro-Waste Derived Nanomaterials: A Summary of Synthesis, Size, and ShapeBio-wasteNPs synthesized and producedSize of NPs (nm)Shape of NPsReferenceWaste oyster shellsnHAp/ZnO/GO9–22SphericalRice huskTiO26.2–7.6Irregular sharp cylinder-like particlesWaste of chicken eggshellCaO@NiO15-20Rod-like shapePapaya (Carica papaya L.) peel biowasteCuO85–140Agglomerated sphericalDragon fruit (Hylocereus polyrhizus) peel biowasteZnO56SphericalLongan seeds biowasteZnO10–100Irregular and hexagonalBanana pseudo stemTiO29.98–24.56PolyhedralAgro-waste durva grassZrO215-35SphericalAgricultural waste Hibiscus cannabinusγ-Fe2O3/Si48.3SphericalCitrus reticulata Blanco (C. reticulata) wasteZnO9HexagonalRooibos tea wasteFe2O3–SnO2 -Tone-like structures, tiny rod-like structures, and well-dispersedSugarcane bagasseCu2O38.02Irregular Notes/Explanations:
NPs: Nanoparticles nHAp/ZnO/GO: Nano-hydroxyapatite/Zinc Oxide/Graphene Oxide composite CaO@NiO: Calcium Oxide coated with Nickel Oxide y-Fe2O3/Si: Gamma-Iron(III) Oxide supported on Silicon Fe2O3-SnO2: Iron Oxide-Tin Oxide composite
Marine macroalgae/seaweed precursors Seaweed is a highly promising green source for photocatalyst synthesis due to its rapid growth rates and minimal environmental requirements. It does not require freshwater or fertilizers for cultivation, making it a sustainable and environmentally friendly option. Various seaweed species have been explored for their ability to produce nanoparticles and to act as templates for the synthesis of photocatalytic materials. +Bio-Fabrication of Nanoparticles Using Marine Macroalgae ExtractsSpecies of MacroalgalBioactive SubstancesPhytochemical ActivitiesNPs synthesized and producedSize of NPs (nm)Shape of NPsReferenceSargassum vulgarePolyphenols, polysaccharides, phytohormones, carotenoids, vitamins, unsaturated fatty acids and free amino acids.Reducing and capping agentsZn50-150SphericalSargassum myriocystumPhenolReducing and capping agentsAg20 ± 2.2Well dispersed hexagonalSargassum coreanumPolysaccharides, polyphenols, lignansReducing and stabilizing agentAg19Distorted spherical shapeSargassum spp.Phenolics compoundsCapping agentAg2-35SphericalPadina tetrastromaticaFavonoids, steroids, saponins, tannins, phenols and proteinsReducing and stabilizing agentAu11.4Nearly sphericalSargassum spp.Ase terpenoids, flavones, and polysaccharidesCapping and stabilization agentFe3O423.60 ± 0.62Agglomerated sphericalSargassum tenerrimumPolyphenol and proteinsReducing, capping, and stabilizing agentsAg22.5SphericalSargassum duplicatumProteins containing amide and carboxyl groups and carbohydratesReducing and stabilizing agentAg20-50SphericalCaulerpa sertularioidesAlkaloids, phenols, flavonoids, tannins, terpenoids, carbohydrates, glycosides, amino acids, and proteinsReducing and capping agentAg24-57SphericalGalaxaura elongata, Turbinaria ornata, and Enteromorpha flexuosaAlkaloids, flavonoids, phenolic compounds, proteins, and sugarsReducing and capping agentAg20-25SphericalLobophora variegataPolyphenol, bromophenols, lobophorones, and sulphated polysaccharideReduction, capping and stabilizing agentAg6.5-10Ovalred marine algae (Bushehr province, Iran)Amino acids, polysaccharides, carbohydratesReducing and coating agentNiO32.64Spherical Notes/Explanations:
NPs: Nanoparticles
Dispersion and stability of green sources +Marine Macroalgae as Green Stabilizing Agents for Nanoparticle Synthesis: Dispersion and StabilityReferenceMarine MacroalgaeBiogenic Capping AgentsNPs synthesized and producedZeta PotentialStabilityPDIDispersionPotential ApplicationsSargassum spp.PolyphenolsAg−22.6 mVHigh stability0.246MonodispersityPollutant detection in environmentalPolycladia crinitaPrimary and tertiary amines, polysaccharides, amino acidsSe− 13.9 mVHigh stability -PolydispersedDrug deliveryCystoseira tamariscifoliaPolyphenols and polysaccharidesAu−24.6 ± 1.5 mVHigh stability - -BiomedicalPolysiphonia urceolataPhenols (bromophenols), terpenes, steroids, carbohydrates, and polypeptidesCeO2NPs, NiONPs and CeO2/NiO NCS -High stability -PolydispersedToxic ofloxacin remediation and antibacterial (green surfactant)Padina boergeseniiPhenolic compounds, aromatic amine groups, nitro compounds, and aliphatic aminesSe-ZnO−16.4 mVHigh stability0.262PolydispersedBiomedicine (anti-cancer)Ulva lactucaPolyphenols, flavonoids, terpenoids, polysaccharides, and proteinsAg−59.0 mVHigh stability1.092MonodispersedAzo-dyes Photodegradation and biomedical usageEnteromorpha proliferaAlcohol, thiol, carbon dioxide, and ketanine, alkene, carboxylic acid and amine and alkene compoundAg− 30.8 mVHigh stability0.277PolydispersedBiomedical fieldSargassum wightiiPolyphenolsZnO− 49.39 mVHigh stability0.150PolydispersedBiomedical fieldTurbinaria ornataFlavonoid and phenolicAg–63.3 mVHigh stability0.313MonodispersedBiomedical fieldSargassum angustifoliumPolyphenolsAg− 27 mVHigh stability0.15MonodispersedBiomedicine (anti-bactrerial)Gracilaria birdiaePolysaccharidesAg−28.7 ± 0.7 mV - −31.7 ± 0.4 mVHigh stability0.35 -0.68MonodispersedBiomedicine Notes/Explanations:
NPs: Nanoparticles Zeta Potential: A measure of the surface charge of nanoparticles, which influences their stability and dispersion. PDI: Polydispersity Index, a measure of the size distribution of nanoparticles.
Common green precursor materials for photocatalysts +Green Synthesis of Nano-materials Using Plant and Bio-Waste ExtractsMaterialGreen Source(s)Advantages of SourceReferenceTiO2Plant extracts (e.g., Aloe vera)Abundant, biocompatibleZnOAgricultural waste (e.g., rice husks)Renewable, low cost, high surface area in derived materialsCuOPlant extracts (e.g., Hibiscus sabdariffa L.)Biocompatible, non-toxic, can act as reducing and capping agentsCeO2Plant extracts (e.g., Azadirachta indica)Abundant, eco-friendlyCarbon quantum dotsBio-waste (e.g., food waste)Waste management, cost-effective, tunable propertiesGraphene quantum dotsBio-waste (e.g., Spent tea leaves)Waste management, cost-effective, tunable properties
Photocatalyst synthesis methods
Hydrothermal synthesis
Hydrothermal synthesis is a green method that utilizes water under high pressure and temperature to facilitate chemical reactions. It often avoids the need for organic solvents and offers control over crystal size and morphology, making it a versatile approach for producing various photocatalyst materials.
Microwave-assisted synthesis
Microwave-assisted synthesis employs microwaves to provide rapid and uniform heating, leading to faster reaction rates and potential for significant energy savings compared to conventional heating methods. This technique is increasingly favored in green synthesis due to its reduced energy consumption and potential for shorter reaction times.
Sol-gel method
The sol-gel method involves the formation of a gel from a solution, followed by its conversion into a solid material through controlled drying and calcination. It is a versatile technique widely used in the production of various photocatalyst materials, offering advantages in terms of controlling material composition and morphology.
Comparing photocatalyst synthesis methods The table below provides a comparison of the advantages, potential limitations, and suitability of different green synthesis methods: