# Green photocatalyst

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 Shape :
| Plant | Common/Popular Name | NPs synthesized and produced | Size of NPs (nm) | Shape of NPs |
| Citrus limetta | Sweet Lime/Mosambi | CdO | 54 | Quasi-spherical |
| Dillenia indica | Elephant Apple | CuO | 15 | Spherical |
| Mikania micrantha | Mile-a-minute Weed/American Rope | CuO | 15 | Spherical |
| Jackfruit | Jackfruit | La$_2$O$_3$ | 30 | Needle-shaped |
| Sansevieria trifasciata | Snake Plant/Mother-in-Law's Tongue | ZnFe$_2$O$_4$ | 5–20 | Spherical |
| Commelina benghalensis | Benghal Dayflower/Tropical Spiderwort | Ag–ZnO–CSs | 20-100 | Spherical |
| Commelina benghalensis | Benghal Dayflower/Tropical Spiderwort | Au–ZnO–CSs | 50-400 | Spherical |
| Senna siamea | Siamese Cassia/Kassod Tree | ZnO | 37.39 | Spherical |
| Acacia nilotica | Gum Arabic Tree | Ag | 5.72 ± 0.16 | Spherical |
| Epipremnum aureum | Pothos/Devil's Ivy/Money Plant | ZnO | 29 | Spherical |
| Chinese Mahogany | Chinese Mahogany | LO | 22.56 | Long rod-like particles |
| Citrullus colocynthis | Colocynth/Bitter Apple | Cu | 17 ± 4.2 | Spherical |
| Aegle marmelos | Bael/Bengal Quince | FeO | 18.78 | Spherical |
| Couroupita guianensis | Cannonball Tree | CaO | 25.2 | Clusters 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 Shape :
| Bio-waste | NPs synthesized and produced | Size of NPs (nm) | Shape of NPs |
| Waste oyster shells | nHAp/ZnO/GO | 9–22 | Spherical |
| Rice husk | TiO$_2$ | 6.2–7.6 | Irregular sharp cylinder-like particles |
| Waste of chicken eggshell | CaO@NiO | 15-20 | Rod-like shape |
| Papaya (Carica papaya L.) peel biowaste | CuO | 85–140 | Agglomerated spherical |
| Dragon fruit (Hylocereus polyrhizus) peel biowaste | ZnO | 56 | Spherical |
| Longan seeds biowaste | ZnO | 10–100 | Irregular and hexagonal |
| Banana pseudo stem | TiO$_2$ | 9.98–24.56 | Polyhedral |
| Agro-waste durva grass | ZrO$_2$ | 15-35 | Spherical |
| Agricultural waste Hibiscus cannabinus | γ-Fe$_2$O$_3$/Si | 48.3 | Spherical |
| Citrus reticulata Blanco (C. reticulata) waste | ZnO | 9 | Hexagonal |
| Rooibos tea waste | Fe$_2$O$_3$–SnO$_2$ | - | Tone-like structures, tiny rod-like structures, and well-dispersed |
| Sugarcane bagasse | Cu$_2$O | 38.02 | Irregular |
Notes/Explanations:
* NPs: Nanoparticles
* nHAp/ZnO/GO: Nano-hydroxyapatite/Zinc Oxide/Graphene Oxide composite
* CaO@NiO: Calcium Oxide coated with Nickel Oxide
* y-Fe$_2$O$_3$/Si: Gamma-Iron(III) Oxide supported on Silicon
* Fe$_2$O$_3$-SnO$_2$: 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 Extracts :
| Species of Macroalgal | Bioactive Substances | Phytochemical Activities | NPs synthesized and produced | Size of NPs (nm) | Shape of NPs |
| Sargassum vulgare | Polyphenols, polysaccharides, phytohormones, carotenoids, vitamins, unsaturated fatty acids and free amino acids. | Reducing and capping agents | Zn | 50-150 | Spherical |
| Sargassum myriocystum | Phenol | Reducing and capping agents | Ag | 20 ± 2.2 | Well dispersed hexagonal |
| Sargassum coreanum | Polysaccharides, polyphenols, lignans | Reducing and stabilizing agent | Ag | 19 | Distorted spherical shape |
| Sargassum spp. | Phenolics compounds | Capping agent | Ag | 2-35 | Spherical |
| Padina tetrastromatica | Favonoids, steroids, saponins, tannins, phenols and proteins | Reducing and stabilizing agent | Au | 11.4 | Nearly spherical |
| Sargassum spp. | Ase terpenoids, flavones, and polysaccharides | Capping and stabilization agent | Fe$_3$O$_4$ | 23.60 ± 0.62 | Agglomerated spherical |
| Sargassum tenerrimum | Polyphenol and proteins | Reducing, capping, and stabilizing agents | Ag | 22.5 | Spherical |
| Sargassum duplicatum | Proteins containing amide and carboxyl groups and carbohydrates | Reducing and stabilizing agent | Ag | 20-50 | Spherical |
| Caulerpa sertularioides | Alkaloids, phenols, flavonoids, tannins, terpenoids, carbohydrates, glycosides, amino acids, and proteins | Reducing and capping agent | Ag | 24-57 | Spherical |
| Galaxaura elongata, Turbinaria ornata, and Enteromorpha flexuosa | Alkaloids, flavonoids, phenolic compounds, proteins, and sugars | Reducing and capping agent | Ag | 20-25 | Spherical |
| Lobophora variegata | Polyphenol, bromophenols, lobophorones, and sulphated polysaccharide | Reduction, capping and stabilizing agent | Ag | 6.5-10 | Oval |
| red marine algae (Bushehr province, Iran) | Amino acids, polysaccharides, carbohydrates | Reducing and coating agent | NiO | 32.64 | Spherical |
Notes/Explanations:
* NPs: Nanoparticles

### Dispersion and stability of green sources

Marine Macroalgae as Green Stabilizing Agents for Nanoparticle Synthesis: Dispersion and Stability :
| Marine Macroalgae | Biogenic Capping Agents | NPs synthesized and produced | Zeta Potential | Stability | PDI | Dispersion | Potential Applications |
| Sargassum spp. | Polyphenols | Ag | −22.6 mV | High stability | 0.246 | Monodispersity | Pollutant detection in environmental |
| Polycladia crinita | Primary and tertiary amines, polysaccharides, amino acids | Se | − 13.9 mV | High stability | - | Polydispersed | Drug delivery |
| Cystoseira tamariscifolia | Polyphenols and polysaccharides | Au | −24.6 ± 1.5 mV | High stability | - | - | Biomedical |
| Polysiphonia urceolata | Phenols (bromophenols), terpenes, steroids, carbohydrates, and polypeptides | CeO$_2$NPs, NiONPs and CeO$_2$/NiO NCS | - | High stability | - | Polydispersed | Toxic ofloxacin remediation and antibacterial (green surfactant) |
| 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) |
| Ulva lactuca | Polyphenols, flavonoids, terpenoids, polysaccharides, and proteins | Ag | −59.0 mV | High stability | 1.092 | Monodispersed | Azo-dyes Photodegradation and biomedical usage |
| 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 |
| Sargassum wightii | Polyphenols | ZnO | − 49.39 mV | High stability | 0.150 | Polydispersed | Biomedical field |
| Turbinaria ornata | Flavonoid and phenolic | Ag | –63.3 mV | High stability | 0.313 | Monodispersed | Biomedical field |
| Sargassum angustifolium | Polyphenols | Ag | − 27 mV | High stability | 0.15 | Monodispersed | Biomedicine (anti-bactrerial) |
| 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 |
| TiO$_2$ | Plant extracts (e.g., Aloe vera) | Abundant, biocompatible |
| ZnO | Agricultural waste (e.g., rice husks) | Renewable, low cost, high surface area in derived materials |
| CuO | Plant extracts (e.g., Hibiscus sabdariffa L.) | Biocompatible, non-toxic, can act as reducing and capping agents |
| CeO$_2$ | Plant extracts (e.g., Azadirachta indica) | Abundant, eco-friendly |
| Carbon quantum dots | Bio-waste (e.g., food waste) | Waste management, cost-effective, tunable properties |
| Graphene quantum dots | Bio-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:
Comparison of Common Green Nanomaterials Synthesis Methods :
| Method | Description | Advantages | Potential Limitations | Suitable for... |
| 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 |
| 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 |
| 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 |

## Applications of photocatalysts
### Wastewater treatment
#### Degradation of organic pollutants

Green photocatalyst effectively break down organic contaminants in wastewater into less harmful products through a process known as photocatalytic oxidation. Upon light irradiation, the photocatalyst generates reactive oxygen species (ROS), such as hydroxyl radicals (•OH) and superoxide radicals (O$_2$•-), which attack and decompose organic pollutants. 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. Green photocatalyst have demonstrated the ability to remove pharmaceutical contaminants such as carbamazepine, ibuprofen, tetracycline from wastewater. Additionally, green photocatalyst have been successfully employed in the degradation of pesticides such as alachlor.
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 |
| Froriepia subpinnata | Flavonoids and phenolic | Ag | 18 | Hemispherical and hexagonal | Antimicrobial and adsorption of the Azo dye Acid-Red 58 |
| Rhododendron arboreum | Steroids, terpenoids, alkaloids, saponins, phenols, flavonoids, tannins, glycosides and polyphenolic | ZnO | 29.424 | Spherical | Dye photodegradation |
| Elettaria cardamomum | Phenolic | CoFe$_2$O$_4$ | 20–50 | Spherical | Phenol red dye photodegradation |
| Zingiber officinale | Phenolic | CoFe$_2$O$_4$ | 20–50 | Spherical | Phenol red dye photodegradation |
| Tillandsia recurvata | Tannins, reducing sugars, and carbohydrates | ZnO | 12–61 | Spherical | Methylene blue (MB) photodegradation |
| Ajuga iva | Carbohydrates, phenol groups, acidic fractions | Ag | 100-300 | Polygonal poly–dispersed | Methylene blue (MB) photodegradation |
| Macleaya cordata | Phenolic | CuO | 80 | rectangular and square with irregular rod | Methylene blue (MB) photodegradation and antibacterial |
| Coleus scutellariodes | Phenolic | NiO | 23 | Rod shape | Antibiotic (rufloxacin) photodegradation |
| Eupatorium adenophorum | Sesquiterpenoids, triterpenes, flavonoids, phenolics, coumarins, steroids, polyphenols, and phenylpropanols | Ag | 30–400 | Spherical | Rhodamin B photodegradation |
Notes/Explanations:
* NPs: Nanoparticles
* CoFe$_2$O$_4$: Cobalt Ferrite

#### 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. Furthermore, photogenerated electrons 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.

### Antibacterial activity
#### Mechanisms of action

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

#### 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. TiO$_2$-based photocatalyst, particularly those doped with silver or copper, exhibit enhanced antibacterial properties under visible light irradiation, making them suitable for disinfection applications. 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. Incorporating them into coatings or surfaces can create self-sterilizing materials, reducing the risk of bacterial contamination in healthcare settings and other environments.
Plant-Based Synthesis of Nanoparticles for Biomedical Applications (Antimicrobial) :
| Plant | Bioactive substances | NPs synthesized and produced | Size of NPs (nm) | Shape of NPs | Applications |
| Piper guineense (Uziza) | Phenolics and flavonoids | ZnO | 7.39 | Spherical and well-dispersed | Antibacterial |
| Olea Europaea | Protein, carbonyl, carboxyl, amide, and phenols | Ag/Ag$_2$O | 45 | Spherical | Antimicrobial |
| Froriepia subpinnata | Flavonoids and phenolic | Ag | 18 | Hemispherical and hexagonal | Antimicrobial and adsorption of the Azo dye Acid-Red 58 |
| Vitex negundo | Flavonoids | ZnO | 40-50 | Spherical | Antibacterial and Anticancer |
Notes/Explanations:
* NPs: Nanoparticles
* Ag/Ag$_2$O: Silver/Silver Oxide Composite

## Toxicity assessments
### Importance of toxicity evaluation

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. 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. 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. These assays help determine the potential for adverse effects on human health upon exposure to these materials.
Toxicity Assessment of Marine Macroalgae-Derived Nanoparticles :
| Macroalgal–NPs | Animal/Organism Model | Toxicity Test | Exposure Duration | Concentration/Dose | Toxicity |
| Ericaria amentacea–AgNPs | Artemia salina | Brine shrimp test | 24 h | 17.08 μg/mL | Low |
| Sargassum polycystum–AgNPs | Artemia salina | Brine shrimp test | 24 h and 48 h | 20 to 100 ppm | Low |
| Polycladia myrica–GZ | Amphibalanus amphitrite | Barnacle larvae cytotoxicity | 24 h | 0.031 mg mL$^{−1}$ | Low |
| Kappaphycus alvarezii–ZnONPs | 3T3 | MTT assay | 24 h and 48 h | 5, 10, 20, 25, 50 and 100 μg/mL | Low |
| 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.