Harrick PlasmaApplicationsAlter Surface ChemistyPhotocatalysis

Photocatalysis is a process where light energy, usually from ultraviolet (UV) sources, is used to accelerate a chemical reaction through the activation of a catalyst. The ability to degrade harmful substances and improve material properties under light exposure makes photocatalysis a key technology in advancing sustainable solutions to industrial problems.

Plasma treatment serves multiple purposes in photocatalysis. It can clean material surfaces to create a pristine base, modify surface chemistry to enhance coating adhesion, and physically alter material properties through etching.


TiO2 Photocatalysis

One of the most widely studied and utilized photocatalysts is titanium dioxide (TiO2), known for its chemical stability, low cost, non-toxic nature, and commercial availability. When TiO2 is exposed to UV light, it generates reactive oxygen species (ROS) that can break down organic pollutants, bacteria, and other contaminants. TiO2 is frequently applied as a colloidal solution to coat substrates with TiO2 nanoparticles (NPs).

To improve adhesion between the nanoparticles and the substrate, scientists frequently plasma treat the substrates before deposition. Mejia et. al. plasma treated nylon fabric before coating the textile with TiO2 NPs. Upon staining the fabric with red wine, the nanoparticles photocatalytically reacted with the wine to reduce stain discoloration.

A similar self-cleaning effect of TiO2 was demonstrated by Krogman et. al, who studied photocatalytic degradation of toxic volatile organic compounds (VOCs). Rapid degradation of VOCs is essential for military protective clothing. Krogman plasma treated plastic sheeting before coating it with PDADMAC / TiO2 via layer-by-layer (LbL) deposition. The coated sheets photocatalytically degraded the chemical warfare agent chloroethyl ethyl sulfate (CEES).

Besides destroying harmful chemicals and unwanted stains, TiO has also been used on antibacterial surfaces. Vierira et. al. created a microstructured polydimethylsiloxane (PDMS)-based surface to trap bacteria. After functionalizing the PDMS via plasma treatment, Vierira incubated it in APTES and polysodium4-styrenesulfonate (PSS). The resulting negatively charged surfaces were coated with TiO2 nanoparticles. These nanoparticles photocatalytically reacted with the trapped bacteria to kill them.

 During photocatalysis, changes have been observed not only in the unwanted chemicals/bacteria, but also in the TiO2 itself. Panchanathan et. al. studied the evolution of TiO2 wettability (hydrophilicity) as a function of UV light exposure time. Panchanathan first plasma treated glass slides and stainless-steel mesh before depositing a polymer binder (PAH) and a colloidal TiO­2 nanoparticle solution via layer-by-layer (LbL) deposition. The resulting samples contained 30 PAH/TiO2 bilayers which formed a nanoporous film. After calcination to remove the PAH, the samples were submerged in oil and illuminated with UV light. During photocatalysis, the TiO2 layer returned to its naturally hydrophilic state as the hydrophobic oil contamination degraded (Figure 1). Panchanathan used these findings to create a kinetic model linking the photocatalytic removal of oil to the evolution of TiO­2 wettability.


TiO2 Photocatalysis

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.

TiO2 Photocatalysis Articles

Krogman, K. C., Zacharia, N. S., Grillo, D. M., & Hammond, P. T. (2008). “Photocatalytic layer-by-layer coatings for degradation of acutely toxic agents”. Chemistry of Materials, 20(5), 1924–1930. https://doi.org/10.1021/cm703096w

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. https://doi.org/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). https://doi.org/10.1002/admi.201700462

Vieira, A., Rodríguez-Lorenzo, L., Leonor, I. B., Reis, R. L., Espiña, B., & dos Santos, M. B. (2023). “Innovative Antibacterial, Photocatalytic, Titanium Dioxide Microstructured Surfaces Based on Bacterial Adhesion Enhancement”. ACS Applied Bio Materials, 6(2), 754–764. https://doi.org/10.1021/acsabm.2c00956


Effect of TiO­2 Morphology on Photocatalysis

Some researchers have found that TiO2 morphology may affect photocatalytic activity. Brockenstedt et. al. compared water disinfection rates using TiO2 ­nanoparticles (TiO2 NPs) and porous TiO2 nanowires (TiO2 NW) exposed to UV light. The nanoparticles reduced E. coli bacteria levels faster than the porous nanoparticles did. Brockenstedt proposed that the nanowires’ porosity reduced the number of sites to which E. coli could attach as well as the number of sites which the UV light could reach. Together, these limitations caused smaller photocatalytic activity for water disinfection.

Brockenstedt also studied the water disinfection capabilities of reused photocatalyst. After autoclaving, decanting, and drying the aqueous E. coli / photocatalyst solutions, the resulting TiO2 NP or TiO2 NW photocatalyst solids were divided into two portions. One portion of each photocatalyst was decarburized using air plasma, while the other portion was not. Both portions were then added to E. coli contaminated water to study their disinfection rates. After 2 hours in the contaminated water, the plasma treated TiO2 nanowires removed twice as much E. coli as the non-plasma treated nanowires did. This effect was even more pronounced for the TiO2 nanoparticles: after just 1 hour in the contaminated water, the plasma treated TiO2 NPs removed four times as much E. coli as the non-plasma treated nanoparticles did.

In addition to nanoparticles and nanowires, TiO2 can also take the form of nanosheets. These nanosheets can be doped to enhance their photocatalytic activity when exposed to non-UV light. Kong et. al. used argon plasma on boron-doped TiO2 nanosheets to etch organic contamination and create a porous structure with higher surface area. This plasma treatment exposed additional active sites without altering the TiO­2 nanostructure. Plasma treatment also induced the formation of oxygen vacancies and Ti3+ defects on the surface. The O vacancies facilitated the adsorption of the reactant molecules to increase reaction efficiency. In addition, this defect generation effectively narrowed the energy band gap of TiO2 from >3 eV (UV range) to within the energy range of visible light. Thus, these TiO2 nanosheets could absorb light from the visible spectrum following plasma treatment.

Using hydrogen generation and photochemical measurements, Kong found that plasma-treated TiO2 had 4 times the photoresponse current density of untreated TiO2 under full spectrum illumination. This improved photocatalytic performance may ultimately enhance the efficiency of water splitting reactions for hydrogen generation.

Besides boron, TiO2 has also been doped with other elements in the hopes of improving photocatalytic behavior. One such element is platinum (Pt). Gayle et. al. found that platinum-doped TiO2 (Pt-TiO2) degraded Methylene Blue dye quicker than pure TiO2 did. Using this information, Gayle constructed a Pt-TiO2 self-cleaning layer. Gayle plasma cleaned glass slides prior to spin-coating with a self-cleaning sol-gel solution containing Pt-TiO2 nanoparticles. This self-cleaning layer was then used to degrade a stearic acid (SA) contaminant layer.


Effect of TiO¬2 Morphology on Photocatalysis Articles

Bockenstedt, J., Vidwans, N. A., Gentry, T., & Vaddiraju, S. (2021). “Catalyst recovery, regeneration and reuse during large-scale disinfection of water using photocatalysis”. Water (Switzerland), 13(19). https://doi.org/10.3390/w13192623

Gayle, A. J., Lenef, J. D., Huff, P. A., Wang, J., Fu, F., Dadheech, G., & Dasgupta, N. P. (2022). „Visible-Light-Driven Photocatalysts for Self-Cleaning Transparent Surfaces”. Langmuir, 38(38), 11641–11649. https://doi.org/10.1021/acs.langmuir.2c01455

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. https://doi.org/10.1016/j.apcatb.2018.02.019

Toxicity Studies of TiO­2 Photocatalysts

Although TiO2 is generally regarded as nontoxic, recent studies suggest it may cause oxidative stress in human cells.As reported by Parra-Ortiz et. al, oxidative stress can contribute to inflammation, infection, and sepsis. This is concerning for workers in TiO2 manufacturing facilities, as well as people who are exposed to TiO2 from sunscreen, paint, or even food products. Jayaram et. al. proposed that reactive oxygen species (ROS) generated by TiO2 nanoparticles (TiO2 NPs) oxidize proteins which bind to the nanoparticles. Jayaram discovered that these ROS originate from surface defects (oxygen vacancies) in the TiO2 NPs.

After using plasma treatment to increase the number of surface defects on the TiO2 NPs, Jayaram observed an increase in reactive oxygen species and a corresponding increase in cell oxidative stress.


Toxicity Studies of TiO¬2 Photocatalysts Articles

Jayaram, D. T., Runa, S., Kemp, M. L., & Payne, C. K. (2017). “Nanoparticle-induced oxidation of corona proteins initiates an oxidative stress response in cells.” Nanoscale, 9(22), 7595–7601. https://doi.org/10.1039/c6nr09500c

Parra-Ortiz, E., Malekkhaiat Häffner, S., Saerbeck, T., Skoda, M. W. A., Browning, K. L., & Malmsten, M. (2020). “Oxidation of Polyunsaturated Lipid Membranes by Photocatalytic Titanium Dioxide Nanoparticles: Role of pH and Salinity”. ACS Applied Materials and Interfaces, 12(29), 32446–32460. https://doi.org/10.1021/acsami.0c08642

Quartz Crystal Microbalance (QCM) Studies of TiO2 Photocatalysts

Oxidative stress caused by TiO2 has also been studied using quartz crystal microbalance (QCM). Parra-Ortiz et. al. deposited polyunsaturated fatty acids onto silicon dioxide (SiO) substrates to form lipid bilayers. The samples were then plasma cleaned and placed in a QCM cell. After adding a TiO2 nanoparticle solution, Parra-Ortiz noted that vesicle attachment to the nanoparticles (and thus oxidative stress) was pH and salinity-dependent. This was likely due to the varying aggregation of TiO2 nanoparticles under different pH and salinity conditions. The authors determined that -OH radicals from the TiO­2 are responsible for the observed vesicle attachment and oxidation.

Another use of quartz crystal microbalance to study TiO2 photocatalysis was reported by Lim et. al. . Lim incorporated TiO­2 into immunoassays for detecting biomolecules via photocatalytic silver staining. After plasma treating TiO2 nanoparticles and modifying them in 3-GPTMS, Lim incubated the nanoparticles in a cardiac troponin (cTnI) detection antibody. A quartz crystal microbalance (QCM) sensor chip was incubated in the resulting solution. After adding a silver nitrate (AgNO3) solution to the chip, the chip was exposed to UV light, causing photocatalytic silver staining. This process increased the TiO2 nanoparticle size and mass. As a result, the QCM resonance frequency increased by up to 2 kHz compared to sensors without the TiO/silver staining addition. This signal amplification increased the cTnI detection sensitivity by 17-fold.


Quartz Crystal Microbalance (QCM) Studies of TiO2 Photocatalysts Articles

Lim, J. Y., & Lee, S. S. (2021). „Quartz crystal microbalance cardiac Troponin I immunosensors employing signal amplification with TiO2 nanoparticle photocatalyst”. Talanta, 228. https://doi.org/10.1016/j.talanta.2021.122233

Parra-Ortiz, E., Malekkhaiat Häffner, S., Saerbeck, T., Skoda, M. W. A., Browning, K. L., & Malmsten, M. (2020). “Oxidation of Polyunsaturated Lipid Membranes by Photocatalytic Titanium Dioxide Nanoparticles: Role of pH and Salinity”. ACS Applied Materials and Interfaces, 12(29), 32446–32460. https://doi.org/10.1021/acsami.0c08642

Graphitic carbon nitride (C3N4) Photocatalysts

Graphitic carbon nitride (C3N) is a photocatalyst which can be used in water purification. Adding polyethylenimine (PEI) increases the number of reactive oxygen species (ROS) generated by the photocatalyst and improves adhesion of the bacteria to the photocatalyst. Once the bacteria adhere to the photocatalyst, they are destroyed by the reactive oxygen species. Zeng observed the adhesion of E. coli bacteria to the PEI-modified photocatalyst using atomic force spectroscopy. Before applying PEI/C3Nto the AFM cantilevers, Zeng plasma cleaned the cantilevers to remove contaminants.


Graphitic carbon nitride (C3N4) Photocatalysts Articles

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. https://doi.org/10.1016/j.apcatb.2020.119095

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