Green photocatalyst

Green photocatalysts are photocatalysts derived from environmentally friendly sources.^[1][2] 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.^[3][4]

A photocatalyst is a material that absorbs light energy to initiate or accelerate a chemical reaction without being consumed in the process.^[5] They are semiconducting materials which generate electron-hole pairs upon light irradiation. These photogenerated charge carriers^[6] then migrate to the surface of the photocatalyst and interact with adsorbed species, triggering redox reactions.^[7] 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.^[8] 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.

Trend of Scopus-indexed publications on green photocatalysts, including bio-waste, macroalgae, and plant-based materials, from 2000 to 2024

VOSviewer analysis (© 2024 Centre for Science and Technology Studies, Leiden University) of 5,375 Scopus documents (1999–2026) retrieved using the search query "TITLE-ABS-KEY(green AND photocatalyst) AND PUBYEAR > 1999 AND PUBYEAR < 2026" reveals key trends in green photocatalyst research, including a focus on environmentally friendly synthesis methods and applications in environmental remediation and energy production

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.^[3][4] 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.^[3][4] Green sources are abundant, readily available, and often considered as waste materials, thus offering a sustainable and cost-effective alternative to conventional photocatalyst precursors.^[9]

Different synthesis approaches available for the preparation of metal nanoparticles for various application including as Green Photocatalyst

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.^[10] 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,^[10][11] 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.^[12] These components can be used as precursors for carbon-based photocatalyst or as templates for the synthesis of porous nano-materials.^[13][14]

Phenolic compounds role in the M. oleifera NPs synthesis

Plant-Based Nanoparticle/Nanocatalysts: Synthesis, Size, and Shape

Plant	Common/Popular Name	NPs synthesized and produced	Size of NPs (nm)	Shape of NPs	Reference
Citrus limetta	Sweet Lime/Mosambi	CdO	54	Quasi-spherical	[15]
Dillenia indica	Elephant Apple	CuO	15	Spherical	[16]
Mikania micrantha	Mile-a-minute Weed/American Rope	CuO	15	Spherical	[16]
Jackfruit	Jackfruit	La2O3	30	Needle-shaped	[17]
Sansevieria trifasciata	Snake Plant/Mother-in-Law's Tongue	ZnFe2O4	5–20	Spherical	[18]
Commelina benghalensis	Benghal Dayflower/Tropical Spiderwort	Ag–ZnO–CSs	20-100	Spherical	[19]
Commelina benghalensis	Benghal Dayflower/Tropical Spiderwort	Au–ZnO–CSs	50-400	Spherical	[19]
Senna siamea	Siamese Cassia/Kassod Tree	ZnO	37.39	Spherical	[20]
Acacia nilotica	Gum Arabic Tree	Ag	5.72 ± 0.16	Spherical	[21]
Epipremnum aureum	Pothos/Devil's Ivy/Money Plant	ZnO	29	Spherical	[22]
Chinese Mahogany	Chinese Mahogany	LO	22.56	Long rod-like particles	[23]
Citrullus colocynthis	Colocynth/Bitter Apple	Cu	17 ± 4.2	Spherical	[24]
Aegle marmelos	Bael/Bengal Quince	FeO	18.78	Spherical	[25]
Couroupita guianensis	Cannonball Tree	CaO	25.2	Clusters with irregular forms	[26]

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.

Electron microscope images of ZnO nanoparticles synthesized by chemical and green methods using beetroot, cedar, and pomegranate extracts at different resolutions

SEM images of ZnO nanoparticles synthesized by chemical and green methods using beetroot, cedar, and pomegranate extracts at different resolutions

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.^[27] These waste streams are rich in organic matter, which can be converted into valuable carbon-based photocatalyst through various thermochemical processes.^[28][29]

Bio-Waste/Agro-Waste Derived Nanomaterials: A Summary of Synthesis, Size, and Shape

Bio-waste	NPs synthesized and produced	Size of NPs (nm)	Shape of NPs	Reference
Waste oyster shells	nHAp/ZnO/GO	9–22	Spherical	[30]
Rice husk	TiO2	6.2–7.6	Irregular sharp cylinder-like particles	[31]
Waste of chicken eggshell	CaO@NiO	15-20	Rod-like shape	[32]
Papaya (Carica papaya L.) peel biowaste	CuO	85–140	Agglomerated spherical	[33]
Dragon fruit (Hylocereus polyrhizus) peel biowaste	ZnO	56	Spherical	[34]
Longan seeds biowaste	ZnO	10–100	Irregular and hexagonal	[35]
Banana pseudo stem	TiO2	9.98–24.56	Polyhedral	[36]
Agro-waste durva grass	ZrO2	15-35	Spherical	[37]
Agricultural waste Hibiscus cannabinus	γ-Fe2O3/Si	48.3	Spherical	[38]
Citrus reticulata Blanco (C. reticulata) waste	ZnO	9	Hexagonal	[39]
Rooibos tea waste	Fe2O3–SnO2	-	Tone-like structures, tiny rod-like structures, and well-dispersed	[40]
Sugarcane bagasse	Cu2O	38.02	Irregular	[41]

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

Green synthesis of ZnO nanoparticles using extracts from three marine macroalgae: (A) Ulva lactuca, (B) Ulva intestinalis, and (C) Sargassum muticum

Marine macroalgae/seaweed precursors

Seaweed is a highly promising green source for photocatalyst synthesis due to its rapid growth rates and minimal environmental requirements.^[42] It does not require freshwater or fertilizers for cultivation, making it a sustainable and environmentally friendly option.^[43][44] Various seaweed species have been explored for their ability to produce nanoparticles and to act as templates for the synthesis of photocatalytic materials.^[45][46][47]

Bio-Fabrication of Nanoparticles Using Marine Macroalgae Extracts

Species of Macroalgal	Bioactive Substances	Phytochemical Activities	NPs synthesized and produced	Size of NPs (nm)	Shape of NPs	Reference
Sargassum vulgare	Polyphenols, polysaccharides, phytohormones, carotenoids, vitamins, unsaturated fatty acids and free amino acids.	Reducing and capping agents	Zn	50-150	Spherical	[48]
Sargassum myriocystum	Phenol	Reducing and capping agents	Ag	20 ± 2.2	Well dispersed hexagonal	[49]
Sargassum coreanum	Polysaccharides, polyphenols, lignans	Reducing and stabilizing agent	Ag	19	Distorted spherical shape	[50]
Sargassum spp.	Phenolics compounds	Capping agent	Ag	2-35	Spherical	[51]
Padina tetrastromatica	Favonoids, steroids, saponins, tannins, phenols and proteins	Reducing and stabilizing agent	Au	11.4	Nearly spherical	[52]
Sargassum spp.	Ase terpenoids, flavones, and polysaccharides	Capping and stabilization agent	Fe3O4	23.60 ± 0.62	Agglomerated spherical	[53]
Sargassum tenerrimum	Polyphenol and proteins	Reducing, capping, and stabilizing agents	Ag	22.5	Spherical	[54]
Sargassum duplicatum	Proteins containing amide and carboxyl groups and carbohydrates	Reducing and stabilizing agent	Ag	20-50	Spherical	[55]
Caulerpa sertularioides	Alkaloids, phenols, flavonoids, tannins, terpenoids, carbohydrates, glycosides, amino acids, and proteins	Reducing and capping agent	Ag	24-57	Spherical	[56]
Galaxaura elongata, Turbinaria ornata, and Enteromorpha flexuosa	Alkaloids, flavonoids, phenolic compounds, proteins, and sugars	Reducing and capping agent	Ag	20-25	Spherical	[57]
Lobophora variegata	Polyphenol, bromophenols, lobophorones, and sulphated polysaccharide	Reduction, capping and stabilizing agent	Ag	6.5-10	Oval	[58]
red marine algae (Bushehr province, Iran)	Amino acids, polysaccharides, carbohydrates	Reducing and coating agent	NiO	32.64	Spherical	[59]

Notes/Explanations:

• NPs: Nanoparticles

Dispersion and stability of green sources

Marine Macroalgae as Green Stabilizing Agents for Nanoparticle Synthesis: Dispersion and Stability

Reference	Marine Macroalgae	Biogenic Capping Agents	NPs synthesized and produced	Zeta Potential	Stability	PDI	Dispersion	Potential Applications
[51]	Sargassum spp.	Polyphenols	Ag	−22.6 mV	High stability	0.246	Monodispersity	Pollutant detection in environmental
[60]	Polycladia crinita	Primary and tertiary amines, polysaccharides, amino acids	Se	− 13.9 mV	High stability	-	Polydispersed	Drug delivery
[61]	Cystoseira tamariscifolia	Polyphenols and polysaccharides	Au	−24.6 ± 1.5 mV	High stability	-	-	Biomedical
[62]	Polysiphonia urceolata	Phenols (bromophenols), terpenes, steroids, carbohydrates, and polypeptides	CeO2NPs, NiONPs and CeO2/NiO NCS	-	High stability	-	Polydispersed	Toxic ofloxacin remediation and antibacterial (green surfactant)
[63]	Padina boergesenii	Phenolic compounds, aromatic amine groups, nitro compounds, and aliphatic amines	Se-ZnO	−16.4 mV	High stability	0.262	Polydispersed	Biomedicine (anti-cancer)
[64]	Ulva lactuca	Polyphenols, flavonoids, terpenoids, polysaccharides, and proteins	Ag	−59.0 mV	High stability	1.092	Monodispersed	Azo-dyes Photodegradation and biomedical usage
[65]	Enteromorpha prolifera	Alcohol, thiol, carbon dioxide, and ketanine, alkene, carboxylic acid and amine and alkene compound	Ag	− 30.8 mV	High stability	0.277	Polydispersed	Biomedical field
[66]	Sargassum wightii	Polyphenols	ZnO	− 49.39 mV	High stability	0.150	Polydispersed	Biomedical field
[67]	Turbinaria ornata	Flavonoid and phenolic	Ag	–63.3 mV	High stability	0.313	Monodispersed	Biomedical field
[68]	Sargassum angustifolium	Polyphenols	Ag	− 27 mV	High stability	0.15	Monodispersed	Biomedicine (anti-bactrerial)
[69]	Gracilaria birdiae	Polysaccharides	Ag	−28.7 ± 0.7 mV - −31.7 ± 0.4 mV	High stability	0.35 -0.68	Monodispersed	Biomedicine

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 Extracts

Material	Green Source(s)	Advantages of Source	Reference
TiO2	Plant extracts (e.g., Aloe vera)	Abundant, biocompatible	[70]
ZnO	Agricultural waste (e.g., rice husks)	Renewable, low cost, high surface area in derived materials	[71]
CuO	Plant extracts (e.g., Hibiscus sabdariffa L.)	Biocompatible, non-toxic, can act as reducing and capping agents	[72]
CeO2	Plant extracts (e.g., Azadirachta indica)	Abundant, eco-friendly	[73]
Carbon quantum dots	Bio-waste (e.g., food waste)	Waste management, cost-effective, tunable properties	[74]
Graphene quantum dots	Bio-waste (e.g., Spent tea leaves)	Waste management, cost-effective, tunable properties	[75]

Photocatalyst synthesis methods

Schematic representation of the preparation of Lemon Peel, LP-ZnO NPs by hydrothermal method

Hydrothermal synthesis

Main article: Hydrothermal synthesis

Hydrothermal synthesis is a green method that utilizes water under high pressure and temperature to facilitate chemical reactions.^[76] 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.^[76]

Microwave-assisted synthesis

Main article: Microwave chemistry

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.^[77] This technique is increasingly favored in green synthesis due to its reduced energy consumption and potential for shorter reaction times.^[77]

Sol-gel method

Main article: Sol-gel

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.^[78] It is a versatile technique widely used in the production of various photocatalyst materials, offering advantages in terms of controlling material composition and morphology.^[78]

The schematic representation of the sol-gel synthesis of ZnO NPs using different types of chitosan sources and their application in antibacterial and photocatalytic degradation of MB dye

Comparing photocatalyst synthesis methods

The table below provides a comparison of the advantages, potential limitations, and suitability of different green synthesis methods:

Comparison of Common Green Nanomaterials Synthesis Methods

Method	Description	Advantages	Potential Limitations	Suitable for...	Reference
Hydrothermal Synthesis	Water under high pressure & temperature facilitate chemical reactions	Avoids organic solvents, controls crystal size & morphology	Longer reaction times, specialized equipment needed	Producing various photocatalytic materials	[79]
Microwave-Assisted Synthesis	Microwaves provide rapid & uniform heating	Faster reaction rates, energy efficient	Limited scalability, potential for uneven heating	Synthesis of nanomaterials with controlled size & morphology	[80]
Sol-Gel Method	Gel from a solution is converted into a solid material	Versatile in producing various materials, controls composition & morphology	Requires careful control of parameters, can be time-consuming	Metal oxide nanoparticles, thin films, and coatings	[81]

Applications of photocatalysts

Wastewater treatment

Photocatalytic degradation mechanism of Safranin O dye pollutant using Centaurea behen leaf-AgNP composites under sunlight irradiation

Degradation of organic pollutants

Green photocatalyst effectively break down organic contaminants in wastewater into less harmful products through a process known as photocatalytic oxidation.^[82] Upon light irradiation, the photocatalyst generates reactive oxygen species (ROS), such as hydroxyl radicals (•OH) and superoxide radicals (O₂•-), which attack and decompose organic pollutants.^[83] Green photocatalyst synthesized from plant extracts or agricultural waste have shown promising results in degrading various dye molecules, including methylene blue, rhodamine B, and methyl orange.^[84] Green photocatalyst have demonstrated the ability to remove pharmaceutical contaminants such as carbamazepine,^[85] ibuprofen,^[86] tetracycline^[87][88] from wastewater. Additionally, green photocatalyst have been successfully employed in the degradation of pesticides such as alachlor.^[89]

Green synthesis of magnetic nanocomposites using Eucalyptus globulus leaf extract and sugarcane bagasse biochar for the photocatalytic degradation of ciprofloxacin and amoxicillin

Plant-Based Synthesis of Nanoparticles for Environmental Remediation (Organic Compounds Degradation)

Plant	Bioactive substances	NPs synthesized and produced	Size of NPs (nm)	Shape of NPs	Applications	Ref
Froriepia subpinnata	Flavonoids and phenolic	Ag	18	Hemispherical and hexagonal	Antimicrobial and adsorption of the Azo dye Acid-Red 58	[90]
Rhododendron arboreum	Steroids, terpenoids, alkaloids, saponins, phenols, flavonoids, tannins, glycosides and polyphenolic	ZnO	29.424	Spherical	Dye photodegradation	[91]
Elettaria cardamomum	Phenolic	CoFe2O4	20–50	Spherical	Phenol red dye photodegradation	[92]
Zingiber officinale	Phenolic	CoFe2O4	20–50	Spherical	Phenol red dye photodegradation	[92]
Tillandsia recurvata	Tannins, reducing sugars, and carbohydrates	ZnO	12–61	Spherical	Methylene blue (MB) photodegradation	[93]
Ajuga iva	Carbohydrates, phenol groups, acidic fractions	Ag	100-300	Polygonal poly–dispersed	Methylene blue (MB) photodegradation	[94]
Macleaya cordata	Phenolic	CuO	80	rectangular and square with irregular rod	Methylene blue (MB) photodegradation and antibacterial	[95]
Coleus scutellariodes	Phenolic	NiO	23	Rod shape	Antibiotic (rufloxacin) photodegradation	[96]
Eupatorium adenophorum	Sesquiterpenoids, triterpenes, flavonoids, phenolics, coumarins, steroids, polyphenols, and phenylpropanols	Ag	30–400	Spherical	Rhodamin B photodegradation	[97]

Notes/Explanations:

• NPs: Nanoparticles
• CoFe₂O₄: Cobalt Ferrite

Magnetic separation of green synthesized of magnetic nanocomposites using Eucalyptus globulus leaf extract and sugarcane bagasse biochar for the photocatalytic degradation of antibiotics, ciprofloxacin and amoxicillin

Removal of heavy metals

In addition to degrading organic pollutants, green photocatalyst can also contribute to the removal of toxic heavy metals from wastewater. The large surface area and functional groups present on green photocatalyst, particularly those derived from carbon-based sources like bio-waste, can effectively adsorb heavy metal ions from the water.^[98] Furthermore, photogenerated electrons^[99] from the green photocatalyst can reduce heavy metal ions to their less toxic elemental forms, which can then be more easily removed from the wastewater.^[98]

Antibacterial mechanism of Cb-AgNPs: disruption of cell membrane, generation of reactive oxygen species (ROS), and damage to cellular components

Antibacterial activity

Mechanisms of action

Green photocatalyst exhibit potent antibacterial properties due to their ability to generate ROS upon light irradiation.^[100] These ROS, including hydroxyl radicals and superoxide radicals, can damage bacterial cell walls and membranes, leading to cell death.^[101]

Antibacterial activity of Ligustrum vulgare berry extracts derived silver nanoparticles (LV-AgNPs) against P. aeruginosa and E. coli at various concentrations

Examples and applications

Several green photocatalyst have shown promising antibacterial activity. ZnO nanoparticles synthesized using plant extracts have demonstrated strong antibacterial activity against a wide range of bacteria, including E. coli and Staphylococcus aureus.^[102] TiO₂-based photocatalyst, particularly those doped with silver or copper, exhibit enhanced antibacterial properties under visible light irradiation, making them suitable for disinfection applications.^[103] Potential applications of these materials include water disinfection and the creation of antibacterial surfaces. Green photocatalyst can be used to disinfect water by killing harmful bacteria, offering a sustainable alternative to conventional disinfection methods.^[103] Incorporating them into coatings or surfaces can create self-sterilizing materials, reducing the risk of bacterial contamination in healthcare settings and other environments.^[103]

Plant-Based Synthesis of Nanoparticles for Biomedical Applications (Antimicrobial)

Plant	Bioactive substances	NPs synthesized and produced	Size of NPs (nm)	Shape of NPs	Applications	Ref
Piper guineense (Uziza)	Phenolics and flavonoids	ZnO	7.39	Spherical and well-dispersed	Antibacterial	[104]
Olea Europaea	Protein, carbonyl, carboxyl, amide, and phenols	Ag/Ag2O	45	Spherical	Antimicrobial	[105]
Froriepia subpinnata	Flavonoids and phenolic	Ag	18	Hemispherical and hexagonal	Antimicrobial and adsorption of the Azo dye Acid-Red 58	[90]
Vitex negundo	Flavonoids	ZnO	40-50	Spherical	Antibacterial and Anticancer	[106]

Notes/Explanations:

• NPs: Nanoparticles
• Ag/Ag₂O: Silver/Silver Oxide Composite

Toxicity assessments

Importance of toxicity evaluation

Cytotoxic effect of shilajit-derived ZnO nanoparticles on HeLa cancer cells compared to cisplatin and normal Vero cells

Despite their sustainable origins, a thorough evaluation of the potential toxicity of green photocatalyst is essential to ensure their safe and responsible application in various settings. Even though they are synthesized from environmentally benign materials, their unique properties and nanoscale dimensions can potentially pose risks to human health and the environment.^[107] It is crucial to assess the potential for adverse effects before widespread implementation of these materials in water treatment, air purification, or biomedical applications.

Methods for toxicity assessment

Various methods are employed to assess the potential toxicity of green photocatalyst. Eco-toxicity tests expose organisms such as algae, daphnia, or fish to varying concentrations of the photocatalyst to evaluate their effects on growth, reproduction, or mortality.^[108] These tests provide valuable insights into the potential impact of green photocatalyst on aquatic ecosystems. Cytotoxicity assays are conducted in laboratory settings using human cell lines to evaluate the potential toxicity of green photocatalysts to human cells.^[109][110] These assays help determine the potential for adverse effects on human health upon exposure to these materials.

Toxicity Assessment of Marine Macroalgae-Derived Nanoparticles

Reference	Macroalgal–NPs	Animal/Organism Model	Toxicity Test	Exposure Duration	Concentration/Dose	Toxicity
[111]	Ericaria amentacea–AgNPs	Artemia salina	Brine shrimp test	24 h	17.08 μg/mL	Low
[112]	Sargassum polycystum–AgNPs	Artemia salina	Brine shrimp test	24 h and 48 h	20 to 100 ppm	Low
[113]	Polycladia myrica–GZ	Amphibalanus amphitrite	Barnacle larvae cytotoxicity	24 h	0.031 mg mL−1	Low
[114]	Kappaphycus alvarezii–ZnONPs	3T3	MTT assay	24 h and 48 h	5, 10, 20, 25, 50 and 100 μg/mL	Low
[114]	Kappaphycus alvarezii–ZnONPs	MCF 7	MTT assay	48 h	75 μg/mL	High

Notes/Explanations:

• NPs: Nanoparticles
• MTT Assay: 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, a colorimetric method to assess cell viability.

See also

• Catalysis – Process of increasing the rate of a chemical reaction
• Heterogeneous catalysis – Type of catalysis involving reactants & catalysts in different phases of matter
• Industrial catalysts – Catalysts used in industry
• Light harvesting materials – Materials that convert light to energy
• Nanoparticle – Particle with size less than 100 nm
• Photocatalysis – Acceleration of a photoreaction in the presence of a catalyst