| Abstract |
This PhD dissertation examines the deposition of TiO2 and ZnO thin films and nanostructures using chemical techniques in order to study their photocatalytic properties in
terms of destruction of organic pollutants. The objectives of this study concerned the control of basic characteristics of the materials above, the determination of the growth technique that offered optimal photocatalytic activity and the possible extension of photocatalytic coatings in large dimensions and industrial range.
TiO2 and ZnO thin films and nanostructures are deposited on glass substrates using various chemical techniques, such as the sol‐gel / spin‐coating technique, Ultrasonic Spray
Pyrolysis (USP) and Aqueous Solution Growth (ASG) based on aqueous solutions.
Sol‐gel / spin‐coating is a widely known technique for the production of uniform thin films, with very good homogeneity and high optical quality. In this work, both TiO2 and
ZnO thin films were deposited on glass substrates, while the parameters that affect their crystalline and optical properties were studied in‐depth. TiO2 thin films using solgel
technique give intense photocatalytic activity, while Aluminum doping of ZnO thin films is particularly interesting as it leads to increased photocatalytic efficiencies.
Ultrasonic Spray Pyrolysis (USP) is an excellent technique suitable for the deposition of large surfaces using simple processes easily transported in industrial production, while the user can modify the morphology and crystalline properties of the as‐grown samples. To the best of our knowledge, USP was applied for the first time within this work for the growth of photocatalytic coatings, giving ZnO samples with promising results regarding the photocatalytic destruction of organic pollutants.
Aqueous Solution Growth (ASG) gives the possibility to grown ZnO nanowires’ arrays using aqueous solutions at low temperatures (95 oC). It is demonstrated that seeded glass substrates are necessary in order to produce highly c‐axis oriented ZnO
nanowires, whose diameter can be accurately tuned by varying the grain size of the seed layer. On the other hand, the nanowires’ length is mainly determined by growth time. An important originality of this PhD dissertation constitutes the deposition of
well aligned ZnO nanowires with controlled geometry, using ZnO seed layers also grown at 95 oC, which gives the possibility to apply the growth technique on thermal sensitive substrates, such as plastics etc.
Using the chemical techniques above, we studied the morphology, the optical and crystalline properties of the as‐grown samples, along with their surface‐to‐volume ratio.
The photocatalytic activity of our samples was determined using both methylene blue (M. Blue) and stearic acid (SA) as model compounds, since M. Blue behaves like common
wastewater from textile industries and SA provides a reasonable model compound for solid films that deposit on exterior and interior surfaces.
The most critical parameters for the photocatalytic efficiency of thin films are their crystallinity, their homogeneity and their grain size, since it determines their effective surface area. TiO2 and ZnO thin films show high photocatalytic activity as their formal quantum efficiency (FQE) values are higher than the ones mentioned in the literature for
TiO2 or ZnO samples or commercially available catalysts such as Degussa P25.
ZnO nanowires’ arrays grown on glass using ASG, demonstrate significant
photocatalytic activity, probably because of their enhanced crystallinity and high surface‐tovolume
ratio. Moreover, they exhibit a remarkable reversible transition from hydrophobic to super‐hydrophilic behavior after exposure to UV light, resulting in reversible efficient
wettability changes. Such capability may be useful for the production of coating suitable for self‐cleaning applications.
|
Ανάπτυξη νανοδομών και λεπτών υμενίων ZnO και TiO2 με χημικές τεχνικές και μελέτη της φωτοκαταλυτικής τους δράσης
