Development and characterisation of photocatalytic surfaces that are active under fluorescent light

Ostovarpour, Soheyla (2015) Development and characterisation of photocatalytic surfaces that are active under fluorescent light. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Hygiene and cleaning are of paramount importance to the food and beverage industry, but they also cause considerable costs to the industry in terms of production (energy, water and chemicals) and downtime of working hours for cleaning equipment. At the same time, environmental demands urge for reduction in water consumption and the use of more environmentally friendly chemicals. Photocatalytically active metal oxides, of which TiO2 is most often used, generate strong oxidizing conditions when illuminated with UVA light (400- 315 nm). In addition, they create hydrophilic surfaces, which can be cleaned by “sheeting” water that carries the dirt away. TiO2 coatings therefore have the potential to reduce the attachment and viability of microbial cells. TiO2 requires UV light in order to become activated, but doping the coatings with different transition metals can improve the visible light activity. In this project the physical vapour deposition technique of magnetron sputtering was used to produce well characterised photocatalytic coatings, to see whether the addition of the transition metal dopants W (tungsten), Nb (niobium), Ta (tantalum) and Mo (molybdenum) to TiO2 coatings enhanced photocatalytic activity and antimicrobial properties under fluorescent light. The optimum photocatalytic activity, assessed by methylene blue degradation was shown by TiO2-Mo (7 at. %) after annealing at a temperature of 600 °C presenting a mixed phase anatase/rutile crystal structure. These materials also exhibited an antibacterial effect under fluorescent light against Escherichia coli. However, the TiO2-Mo and TiO2-W (10 at.%) coatings inactivated E. coli under fluorescent light over a shorter incubation time (24 hours) in comparison to TiO2-Nb and TiO2-W (3.80 at.%) (48 hours). The TiO2-Nb film did not show any bactericidal effect in the dark, but the Mo and higher concentration of W doped sample inactivated the bacteria in the dark as well as in the light after 24 hours. In this study, photocatalytic and bactericidal activity of TiO2-Mo 7 at.% , TiO2-Nb 0.25 at.%, TiO2-W 3.8 at.% and TiO2-W 10 at.% are demonstrated, when irradiated with fluorescent light with some surfaces also being innately antimicrobial in the dark. The chemical and mechanical stability of TiO2-Mo and TiO2 surfaces were assessed for their effectiveness in situ in bottle filling lines in breweries prior to and after placement. The results from the brewery trials demonstrated some durability with slight loss of Mo (0.5 at.%), 50% loss of photocatalytic activity, and some loss of adhesion to the substratum. In conclusion, the bactericidal properties of TiO2 doped coatings can be exploited in environments where surface contamination is an issue. The difference between innate (TiO2- Mo) and photo-activated (TiO2-Nb) antimicrobial properties can also be exploited in appropriate environments. Some additional work on batch variability and coating stability is required.