Photocatalysts absorb light to generate electron-hole pairs and facilitate reduction-oxidation reactions, all without altering the catalyst material. In the presence of ambient water and oxygen, photocatalysts can assist with water splitting reactions for hydrogen generation as well as produce reactive oxygen species (ROS), namely hydroxyl and oxygen radicals, which may be used for cleaning and disinfecting applications [1].

TiO2, a semiconducting metal oxide, has been heavily considered for its photocatalytic capabilities because of its chemical inertness, long-term stability under various conditions, low cost, and availability for commercial use. Oftentimes, TiO2 must be fabricated into nanostructures with high surface area to provide an abundance of active sites for maximal photocatalytic performance [1].

With relatively low applied powers, our plasma cleaners are well-suited for treating such nanostructures and limiting surface modification to the nanoscale level without affecting the crystal structure or structural integrity. Here, we explore plasma treatment of TiO2 nanostructures for two different applications.

MODIFY TiO2 NANOSHEETS TO ENHANCE HYDROGEN GENERATION

Kong et al. demonstrated the use of plasma treatment on TiO2 (B) nanosheets to introduce surface defects, thereby enhancing the photocatalytic activity. Argon plasma etched organic contamination and created a porous structure with higher surface area, thereby exposing more active sites without altering the TiO2 crystal structure.

Plasma also induced the formation of oxygen vacancies and Ti3+ defects on the surface. The O vacancies facilitated the adsorption of the reactant molecules to increased 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, extending light absorption into the visible spectrum.

From performing hydrogen generation and photochemical measurements, the authors found that the photoresponse current density from plasma-treated TiO2 was 4 times that of pristine (untreated) TiO2 under full spectrum illumination. This improved photocatalytic performance may ultimately enhance the efficiency of water splitting reactions for hydrogen generation.

MODIFY TiO2 NANOPARTICLES TO ENHANCE BACTERIAL DISINFECTION

Yemmireddy and Hung developed an antimicrobial coating of TiO2 nanoparticles (NPs) on high-density polyethylene (HDPE) cutting boards and investigated the effect of NP loading and O2 plasma treatment on the TiO2 bactericidal efficacy. In testing the disinfection efficacy on E. coli bacteria, the authors found that increasing NP loading increased the log reduction (a measure of the extent of microbe removal or destruction). In addition, plasma treatment further improved the log reduction compared to untreated coatings.

Plasma cleaning removed the residual polymer layer that partially masked the NP surface — a side effect from the pressurized heat treatment step that was performed to promote NP adhesion to the cutting board surface. Plasma etching exposed more of the NP surface, increased active sites, and facilitated greater ROS generation. Short plasma exposures (1 min) did not significantly improve the log reduction, possibly because of incomplete removal of the polymer overlayer. However, longer times (15 min) did not further improve the log reduction and even resulted in reduced bactericidal efficacy, suggesting that prolonged plasma exposure may lead to structural damage and loss of TiO2 NPs.

The highest disinfection level was obtained with 0.0625 mg/cm2 NP loading and 5 min plasma treatment. Furthermore, the TiO2 coating maintained consistent bactericidal activity even after up to 5 wash/clean cycles, demonstrating the mechanical durability of the coating. The authors conclude that plasma treatment can enhance the disinfection properties of TiO2 NPs without compromising their structural stability (adhesion) on the cutting board surface.

Relevant Articles from Harrick Plasma Users

  • Kong X, Xu Y, Cui Z, Li Z, Liang Y, Gao Z, Zhu S and Yang X. “Defect enhances photocatalytic activity of ultrathin TiO2 (B) nanosheets for hydrogen production by plasma engraving method”. Appl. Catal. B (2018) 230: 11-17.

  • Yemmireddy VK and Hung Y-C. “Photocatalytic TiO2 coating of plastic cutting board to prevent microbial cross-contamination”. Food Control (2017) 77: 88-95.

Supplemental References (Do not report using Harrick Plasma instruments)

[1] Hashimoto K, Irie H and Fujishima A. “TiO2 photocatalysis: a historical overview and future prospects”. J. J. Appl. Phys. (2005) 44: 8269-8295.

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