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.

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.

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.

Notes/Explanations:

 NPs: Nanoparticles

Dispersion and stability of green sources 

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

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:

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 (O2•-), 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.

Notes/Explanations:

 NPs: Nanoparticles
 CoFe2O4: 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. TiO2-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.

Notes/Explanations:

 NPs: Nanoparticles
 Ag/Ag2O: 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.

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

References 

Catalysis
Sustainable technologies
Photochemistry