Photocatalysis

Harrick Plasma → Applications → Energy → Catalysis → Photocatalysis

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_2­ 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.

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 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.

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.

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_2­) 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_2­ /silver staining addition. This signal amplification increased the cTnI detection sensitivity by 17-fold.

Graphitic carbon nitride (C₃N₄) Photocatalysts

Graphitic carbon nitride (C₃N_4­) 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/C₃N_4­ to the AFM cantilevers, Zeng plasma cleaned the cantilevers to remove contaminants.