Development of photocatalytic functional coatings via magnetron sputtering deposition and their integration into a laboratory-scale water treatment reactor

Grao, Matthieu (2022) Development of photocatalytic functional coatings via magnetron sputtering deposition and their integration into a laboratory-scale water treatment reactor. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Photocatalysis is an advanced oxidation process that only requires a photocatalyst, an appropriate light source and water to decompose a wide variety of pollutants. In recent years, it has been extensively studied for environmental wastewater treatment applications. Although promising, it has yet to be globally adopted, as it faces many challenges; namely cost, complexity and efficiency. To achieve this goal, two strategies have been employed in the A bismuth titanate composite photocatalyst was investigated as an alternative to the conventionally used titanium dioxide, to tackle the well-known drawbacks of fast chargecarrier recombination and low quantum efficiency of titania. Optimum deposition and annealing conditions were identified, to deposit bismuth titanate coatings by reactive magnetron sputtering. Bismuth titanate proved to be more efficient at decomposing methylene blue under UV light, with a pseudo first rate order constant of 1.40 and 0.52 (10-5 s-1) for bismuth titanate and titania coatings on glass substrates, respectively. Moreover, the reusability assessment of bismuth titanate thin films revealed an incremental performance increase after each consecutive test, leading to a 6-fold increase in photocatalytic activity. This behaviour was investigated by XPS and TRPL analysis and attributed to the formation of oxygen vacancies under UV irradiation. The oxygen vacancies-rich bismuth titanate was termed “black bismuth titanate” due to its dark colour, as opposed to the opaque white colour of the pristine one. present thesis. First, photocatalytic thin films were produced by pulsed DC magnetron sputtering, a method of high industrial relevance, onto substrates specifically selected to improve the performance of immobilised systems. Second, the deposition process was scaledup and the thin films were integrated into laboratory scale water treatment systems. Titanium dioxide (or titania) is, to date, the most popular photocatalyst due to its stability, low toxicity and low cost. When deposited by reactive magnetron sputtering, titania often requires substrate bias or annealing to be photocatalytic. In this present work, crystalline titanium dioxide was successfully deposited on woven stainless-steel in a one step process, using low pressure and high-power deposition conditions. Anatase and rutile polymorphs were identified by XRD analysis, whilst Raman mapping indicated excellent coating coverage of the mesh substrate. The pseudo first rate order constant was as high as 5.3 × 10-5 s-1, for methylene blue degradation, and the performance was maintained over 10 consecutives cycling experiments. Large sheets of photocatalytic titania coated mesh were produced in a one-step process and integrated in a bespoke laboratory-scale photocatalytic reactor. The reactor was built from inexpensive and readily available consumer market parts, to facilitate a widespread adoption of such water treatment methods. The influence of variable parameters was studied, using a DOE template, to maximize the degradation rate of methylene blue, leading to the removal of more than 90% of the model pollutant after 5 h. This substrate/catalyst combination proved to be effective at degrading methylene blue, with no evident performance degradation after 10 repeated cycles, equivalent to 360 h of consecutive use. Glass stirring rods were coated with bismuth titanate and integrated in the bespoke laboratory-scale photocatalytic reactor. Dye degradation tests carried out in the presence of scavengers and revealed that photocatalytic reactions were driven by superoxide and holes, when using pristine bismuth titanate and by superoxide, electrons and holes, when using black bismuth titanate. Finally, the reactor was used to successfully degrade levofloxacin, a reserve antibiotic, which was verified by UV-Vis spectroscopy and inhibition zone test in the presence of three different pathogens. Overall, this piece of work describes the use of reactive magnetron sputtering to deposit efficient photocatalytic coatings, under optimised deposition conditions. These photocatalysts were then integrated in a laboratory-scale water treatment reactor, which was optimised to maximise the photocatalytic activity of the system. The results of this work address the technical viability and economic feasibility challenges faced by photocatalytic waste treatment.