The photocatalysis process uses light energy, typically from ultraviolet (UV) sources, to increase the rate of a chemical reaction by activating a catalyst. It can degrade dangerous substances and improve material properties under light exposure, making it an essential technology in advancing sustainable solutions to industrial challenges.
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Plasma treatment has several purposes in photocatalysis. It aids in cleaning material surfaces to create a pristine base, modifies the surface chemistry to improve coating adhesion, and physically alters the material properties through etching.
TiO2 Photocatalysis
One of the most commonly studied and used photocatalysts is titanium dioxide (TiO2), as it is known for its low cost, chemical stability, non-toxic nature, and commercial availability. When exposed to UV light, TiO2 creates reactive oxygen species (ROS) capable of breaking down bacteria, organic pollutants, and other contaminants. TiO2 is often applied as a colloidal solution to coat substrates with TiO2 nanoparticles (NPs).
Adhesion between the substrate and the nanoparticles can be improved if scientists treat the substrates with plasma before deposition. Mejia et al. plasma-treated nylon fabric before coating it with TiO2 NPs. When the fabric was stained with red wine, the nanoparticles photocatalytically reacted with the wine and reduced the stain's discoloration.
Krogman et al., who researched the photocatalytic degradation of toxic volatile organic compounds (VOCs), showed a similar self-cleaning effect of TiO2. Quick degradation of VOCs is crucial in protective clothing for the military. Krogman plasma-treated plastic sheeting before coating it with PDADMAC / TiO2 via layer-by-layer (LbL) deposition. The coated sheets were able to photocatalytically degraded chloroethyl ethyl sulfate (CEES), a chemical warfare agent.
Aside from destroying harmful chemicals and undesirable stains, TiO2 has also been utilized on antibacterial surfaces. Vieira et al. designed a microstructured polydimethylsiloxane (PDMS)-based surface to contain bacteria. Following the functionalizing of the PDMS through plasma treatment, Vieira incubated it in APTES and polysodium4-styrenesulfonate (PSS). The negatively charged surfaces were then coated with TiO2 nanoparticles. The nanoparticles photocatalytically reacted with the contained bacteria to eliminate them.
Throughout photocatalysis, alterations have been seen in the undesired chemicals/bacteria and in the TiO2 itself.
Panchanathan et al. studied the evolution of TiO2 wettability (hydrophilicity) as a function of UV light exposure time. They plasma-treated stainless-steel mesh and glass slides before depositing a polymer binder (PAH) and a colloidal TiO2 nanoparticle solution via layer-by-layer (LbL) deposition.
These samples contained 30 PAH/TiO2 bilayers, which formed a nanoporous film. Following calcination to eliminate the PAH, the samples were immersed in oil and exposed to UV light. During photocatalysis, the TiO2 layer recovered its naturally hydrophilic state while the hydrophobic oil contamination degraded (Figure 1). Panchanathan used these findings to develop a kinetic model that links the photocatalytic removal of oil to the evolution of TiO2 wettability.
Figure 1. Evolution of water contact angle (WCA) of a TiO2 -coated surface as a function of UV illumination time and UV light intensity (I). The TiO2-coated surface was submerged in oil during the sessile droplet tests. As UV illumination time increases, the TiO2 layer eventually returns to its naturally hydrophilic state (WCA = 10°) due to photocatalytic degradation of the surrounding oil. Data reproduced from Panchanathan et. al.
Effect of TiO2 Morphology on Photocatalysis
Some researchers have found that TiO2 morphology can affect photocatalytic activity. Brockenstedt et al. compared water disinfection rates utilizing TiO2 nanoparticles (TiO2 NPs) and porous TiO2 nanowires (TiO2 NW) exposed to UV light.
The nanoparticles caused a quicker reduction in E. coli bacteria levels than the porous nanowires. Brockenstedt surmised that the porosity of the nanowires reduced the number of sites that the E. coli could attach to and the number of sites that the UV light could reach. In combination, these limitations resulted in smaller photocatalytic activity for water disinfection.
Brockenstedt also studied the capabilities of reused photocatalysts in water disinfection. Following autoclaving, decanting, and drying of the aqueous E. coli / photocatalyst solutions, the TiO2 NP or TiO2 NW photocatalyst solids were separated into two portions.
A single portion of each photocatalyst was decarburized through air plasma, while the other was not. Both portions were subsequently added to water contaminated with E. coli to study their disinfection rates. After two hours in the contaminated water, the TiO2 nanowires that were plasma-treated removed twice the amount of E. coli as the non-plasma-treated nanowires. This effect was more apparent in the TiO2 nanoparticles. After only one hour in the contaminated water, the plasma-treated TiO2 NPs eliminated four times the amount of E. coli as the untreated nanoparticles.
Not limited to nanoparticles and nanowires, TiO2 can also take the form of nanosheets. These nanosheets can be doped to improve their photocatalytic activity upon exposure to non-UV light. Kong et al. used argon plasma on boron-doped TiO2 nanosheets to etch organic contamination and produce a porous structure with greater surface area.
This plasma treatment exposed additional active sites without altering the TiO2 nanostructure. It also created oxygen vacancies and Ti3+ surface defects. The O vacancies facilitated reactant molecule adsorption to enhance reaction efficiency. This generation of defects effectively narrowed the energy band gap of TiO2 from >3 eV (UV range) to within the visible light energy range. This meant these TiO2 nanosheets could absorb light from the visible spectrum after plasma treatment.
Using hydrogen generation and photochemical measurements, Kong discovered that plasma-treated TiO2 had four times the photoresponse current density of TiO2 that was not plasma-treated, under full spectrum illumination. This enhanced photocatalytic performance could ultimately improve the water-splitting reaction efficiency for hydrogen generation.
Besides boron, TiO2 has been doped with various elements to improve photocatalytic behavior. An example would be platinum (Pt). Gayle et al. discovered that platinum-doped TiO2 (Pt-TiO2) degraded Methylene Blue dye faster than pure TiO2. With this information, Gayle developed a Pt-TiO2 self-cleaning layer. They plasma-cleaned glass slides before spin-coating with a self-cleaning sol-gel solution that contained Pt-TiO2 nanoparticles. This layer was subsequently used to degrade a stearic acid (SA) contaminant layer.
Toxicity Studies of TiO2 Photocatalysts
Even though TiO2 is commonly considered non-toxic, recent studies have suggested it may cause oxidative stress in human cells. Parra-Ortiz et al. reported that oxidative stress can contribute to infection, inflammation, and sepsis. This affects those working in TiO2 manufacturing facilities and those who could be exposed to TiO2 via paint, sunscreen, or even food products.
Jayaram et al. suggested that reactive oxygen species (ROS) produced by TiO2 nanoparticles (TiO2 NPs) oxidize proteins which then bind to the nanoparticles. Jayaram has shown that these ROS arise from surface defects (oxygen vacancies) in the TiO2 NPs.
Following the use of plasma treatment to raise the number of surface defects on the TiO2 NPs, Jayaram saw an increase in reactive oxygen species and a rise in cell oxidative stress that corresponds.
Quartz Crystal Microbalance (QCM) Studies of TiO2 Photocatalysts
Oxidative stress due to TiO2 has been studied utilizing quartz crystal microbalance (QCM). Parra-Ortiz et al. placed polyunsaturated fatty acids onto silicon dioxide (SiO2) substrates to create lipid bilayers. The samples were plasma-cleaned and put into a QCM cell. After adding a TiO2 nanoparticle solution, Parra-Ortiz et al. observed that vesicle attachment to the nanoparticles (and the resulting oxidative stress) was pH and salinity-dependent. This was likely due to the different aggregations of TiO2 nanoparticles under varying pH and salinity conditions. The authors concluded that -OH radicals from the TiO2 cause the vesicle attachment and oxidation witnessed.
A further use of quartz crystal microbalance in the study of TiO2 photocatalysis was investigated by Lim et al. Lim incorporated TiO2 into immunoassays to detect biomolecules through photocatalytic silver staining.
Following plasma treatment of TiO2 nanoparticles and their modification in 3-GPTMS, Lim et al. incubated the nanoparticles in a cardiac troponin (cTnI) detection antibody. The quartz crystal microbalance (QCM) sensor chip was incubated in the resulting solution.
Following the addition of a silver nitrate (AgNO3) solution to the chip, it was exposed to UV light, which caused photocatalytic silver staining. Through this process, the TiO2 nanoparticles increased in mass and size. The resulting QCM resonance frequency increased by up to 2 kHz in comparison to the sensors that did not have the TiO2 /silver staining addition. The signal amplification raised the cTnI detection sensitivity by 17-fold.
Graphitic Carbon Nitride (C3N4) Photocatalysts
Graphitic carbon nitride (C3N4) is a valuable photocatalyst for purifying water. The addition of polyethyleneimine (PEI) increases the amount of reactive oxygen species (ROS) created by the photocatalyst and enhances the bacteria's adhesion to it.
As the bacteria adhere to the photocatalyst, they are eliminated by the reactive oxygen species. Zeng et al. saw E. coli bacteria adhesion on the PEI-modified photocatalyst through atomic force spectroscopy. Prior to the application of PEI/C3N4 to the AFM cantilevers, Zeng et al. plasma cleaned the cantilevers to eliminate contaminants.
References
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Mejía, M. I., Marín, J. M., Restrepo, G., Pulgarín, C., Mielczarski, E., Mielczarski, J., Stolitchnov, I., & Kiwi, J. (2009). “Innovative UVC light (185 nm) and radio-frequency-plasma pretreatment of nylon surfaces at atmospheric pressure and their implications in photocatalytic processes”. ACS Applied Materials and Interfaces, 1(10), 2190–2198. 10.1021/am900348u
Panchanathan, D., Kwon, G., Qahtan, T. F., Gondal, M. A., Varanasi, K. K., & McKinley, G. H. (2017). “Kinetics of Photoinduced Wettability Switching on Nanoporous Titania Surfaces under Oil”. Advanced Materials Interfaces, 4(21). 10.1002/admi.201700462
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Kong, X., Xu, Y., Cui, Z., Li, Z., Liang, Y., Gao, Z., Zhu, S., & Yang, X. (2018). “Defect enhances photocatalytic activity of ultrathin TiO2 (B) nanosheets for hydrogen production by plasma engraving method”. Applied Catalysis B: Environmental, 230, 11–17. 10.1016/j.apcatb.2018.02.019
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Zeng, X., Liu, Y., Xia, Y., Uddin, M. H., Xia, D., McCarthy, D. T., Deletic, A., Yu, J., & Zhang, X. (2020). “Cooperatively modulating reactive oxygen species generation and bacteria-photocatalyst contact over graphitic carbon nitride by polyethylenimine for rapid water disinfection”. Applied Catalysis B: Environmental, 274. 10.1016/j.apcatb.2020.119095
This information has been sourced, reviewed, and adapted from materials provided by Harrick Plasma.
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