In recent years, resource scarcity and environmental degradation have been eroding human life, and solar powered photocatalytic technology is an effective way to solve the energy crisis and environmental pollution. Photocatalysts are a general term for optoelectronic semiconductor materials with photocatalytic functions, which have been widely reported and applied in various fields such as H2O2 production, photocatalytic hydrogen production, CO2 reduction, degradation of organic pollutants, nitrogen fixation, photocatalytic antibacterial, etc. However, traditional photocatalysts such as TiO2, g-C3N4, MoS2, ZnO, WO3, bismuth based organic compounds, etc. are limited in their utilization of visible light due to issues such as wide bandgap, small specific surface area, and severe recombination of photo generated electron hole pairs. At present, methods used to improve the performance of photocatalysts include morphology control, element doping, precious metal deposition, and heterojunction construction. Usually, other chemical substances need to be introduced, which not only increases costs but also easily causes secondary pollution. In this context, plasma method has been found to have unique advantages in surface treatment of materials, such as no need to add other chemical substances, low modification cost, simple and rapid reaction process, suitable for large-scale modification of catalysts, mild modification process, no generation of secondary pollutants, and green environmental protection. Therefore, plasma modified photocatalysts have gradually been valued and extensively studied.

Plasma treatment

Plasma is an ionized gas composed of electrons, atoms, ions, or free radicals, and is considered the fourth form of matter besides solids, liquids, and gases. The surface modification of photocatalytic materials by plasma involves etching, doping, manufacturing defects, introducing functional groups, and stripping morphology through electron bombardment. In addition, plasma method can also be used to prepare photocatalysts, which can not only reduce metal ions into metal nanoparticles through the active species generated by discharge, but also fix the components with photocatalytic performance on suitable substrates through plasma deposition, sputtering, plasma electrolytic oxidation and other methods to improve the dispersion and stability of the active components.

Plasma is usually generated by gas discharge, and the application of gas-phase plasma technology in surface treatment of photocatalysts has a long history. Different plasma treatment atmospheres can result in varying modification effects on materials, such as using plasma to modify g-C3N4 in O2 or N2 atmospheres. Oxygen plasma treatment not only increases the specific surface area of g-C3N4, but also effectively reduces the recombination of photo generated electrons and holes. The activity of g-C3N4-O photocatalytic degradation of RhB after treatment is 4.2 times higher than that of the original g-C3N4, while the material treated with N2 plasma not only reduces the surface area, but also significantly weakens the photocatalytic activity. In addition, processing materials under a single atmosphere or mixed atmosphere plasma can also affect the modification effect. Common single plasma atmospheres include argon (Ar), nitrogen (N2), oxygen (O2), hydrogen (H2), ammonia (NH3), air, etc. Mixed atmospheres include Ar/O2, Ar/H2, and Ar/NH3, etc. 

Single atmosphere plasma treatment

The main function of Ar plasma is etching and stripping, which generates more defects, increases active sites, and also has certain N and O doping ability. O2 plasma mainly has the functions of oxidation and etching. H2 plasma has the functions of etching and reduction. NH3 plasma has the functions of N doping, etching, and reduction.

In summary, gas-phase plasma treatment of photocatalytic materials has the characteristics of fast, effective, and stable. The basic principle of plasma treatment modified photocatalytic materials is to use the active components generated by discharge to etch the surface of the material, which can increase the hydrophilicity of the material surface and introduce defects into the material, thereby accelerating charge separation and migration rate, improving light absorption and utilization performance.