Understanding the mechanisms for photocatalytic efficiency enhancement : a zinc oxide perspective

• Κύριος συγγραφέας: Δημητρόπουλος, Μαρίνος
• Άλλοι συγγραφείς: Γιαννόπουλος, Σπύρος
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2019
• Θέματα: Zinc oxide; Mechanisms; Defects; Photocorrosion; Photocatalysis; Nanomaterials; Οξείδιο του ψευδαργύρου; Μηχανισμοί; Ατέλειες; Φωτοδιάβρωση; Φωτοκατάλυση; Νανοϋλικά; 541.395

An increasing fraction of the current world population has no access to safe water, which conveys severe consequences since inappropriate sanitary conditions are responsible for disease propagation and a growing number of deaths. Heavy metals, microorganisms, pharmaceuticals, pesticides, dyes, etc. are among the most frequent pollutants of surface and subsurface water. Undoubtedly, clean water scarcity is becoming progressively one of the most prominent problems on the planet with unpredictable economic and social risks. Among the methods that have been developed for water disinfection, advanced oxidation processes such as heterogeneous photocatalysis appear as an emerging technology for the decomposition of most of the organic pollutants. Since the early development of photocatalysis in the 1970s, TiO2 was established as the archetype photocatalyst due to its relatively high efficiency, low cost and availability. However, during the last decades a substantial number of new photocatalytic materials have been developed as potential substitutes of TiO2. In recent years, ZnO has attracted a lot of attention due to its extraordinary characteristics such as large free-exciton binding energy, very high electron mobility and biosafety. But in order to further improve the immigration of photo-induced charge carriers to the surface during excitation state, considerable efforts have to be exerted to further improve heterogeneous photocatalysis under various types of illumination. The modification of existing semiconductors has been an attractive research subject in the recent years along with the exploration of novel semiconducting photocatalysts. The criteria for an ideal photocatalyst such as high surface area, appropriate bandgap, efficient visible light absorption, and high carrier mobility should be taken into consideration to overcome the limitations of prototype semiconductor photocatalysts. Thus, fabricating and engineering the appropriate materials for each unique application is a challenging task since the key parameters that need to be considered simultaneously are vast, and require a deep understanding on the various mechanisms that partake in the photocatalytic process. The purpose of this study is to shed some light on how the photocatalytic efficiency is affected by altering key material characteristics, such as native defects, and give insights on the mechanisms that govern the nature of defects and photocorrosion of ZnO in photocatalysis. For assessment reasons, most experiments are compared to the world standard photocatalyst, TiO2. For the evaluation of the role of defects in photocatalysis, mechanical activation (ball-milling) was em-ployed as it provides a means to introduce lattice defects in a controlled way. The activated nanopowders were fully explored using techniques probing structure and optical properties. Notably, the defect density exhibits non-monotonic behavior against milling conditions, which can be interpreted by considering the penetration depth of each technique. For TiO2 milled powders phase-transformation effects on the crystal lattice are observed (phase transition and partial amorphization). On the other hand, PC activity kinetics are in good correlation with defect density for both materials. Results on ZnO photocorrosion dictate that UV pre-irradiation of pristine powder deteriorates systematically the PC efficiency up until a saturation point, where prolonged irradiation times seem to regenerate the material close to its initial condition. Additionally, the recyclability of ZnO was tested with repetitive pollutant (MB) degradation cycles. Apart from tuning intrinsic defects and ZnO photocorrosion, morphology engineering is one of the main research hotspots for photocatalyst design. For that reason, separately from ZnO and TiO2, alternative materials with interesting properties and perks that could have a significant impact on the ecosphere of photocatalysis, are fabricated and investigated briefly.