| Abstract |
The zinc oxide (ZnO) is one of the most widely studied metal oxide materials, which in recent years has become a very popular material because of the great potential in various applications such as optoelectronic devices, optical waveguides gas sensors etc.
In this thesis, thin films of pure (neat) ZnO as well as ZnO thin films doped with copper (ZnO: Cu) are prepared, using a zinc nitrate precursor solution on a glass (amorphous) substrate by spray pyrolysis method. The deposition was done by varying some deposition conditions as the concentration of the precursor solution, the duration of deposition and the level of impurities Cu, while maintaining constant the number of technical parameters (such as substrate temperature, pressure of the carrier gas, etc.). The main objective of this study was to investigate the effects of the change of deposition conditions on structural, optical and electrical properties of ZnO and ZnO: Cu films, as well as their surface morphology. In addition the effect of varying the deposition conditions and properties of the films on the surface wettability and photocatalytic activity have been investigated.
Structural characterization shows that ZnO and ZnO: Cu films are polycrystalline structure with hexagonal wurtzite, mainly oriented (most of them) along the axis c, perpendicular to the substrate surface. It is found by the XRD data of neat films of ZnO that the increasing of the concentration of the precursor solution, as well as in the two deposition times, yields to the growing of the average grain size along the c-axis and to the reducing of the structural disturbances (i.e., the crystalline quality of films gradually improving), but while there are more oriented crystallites in various other directions compounding the overall crystallinity of the films. The study of the structure of ZnO: Cu, films, in the contrast, shows that as the dopant concentration of copper in both deposition time increases, the grain size is reduced, while increasing the structural disturbances and changing the preferred orientation.
According the images SEM, net of ZnO films exhibited a relatively dense and compact continuous surface morphology without cracks but with projections and/or holes with different configurations and form stamp. This morphology appears to be significantly modified by increasing the concentration of the precursor solution and the deposition time, although in the higher concentration samples, the surface appears to be more rugged. In the SEM images also observed that the surfaces consist of small nearly spherical particles of different sizes on the order of nanometers (50-400 nm, approximately), which are distributed to non-uniformly across to the surface, while in many cases appear to be a dense and compact assemblage in the area of micro-scale. The surface morphology of the ZnO: Cu membranes, occurrence of variations in relation to that of films of pure ZnO. The surfaces of the ZnO: Cu, films were quite smooth and uniform, whereas the spherical particles and the agglomerates are reduced in size by increasing the dopant concentration of copper in both deposition times.
The study of the optical properties of pure ZnO films showed a drastic reduction in permeability and an increasing in absorption in the visible region of the E/M spectrum, by varying the deposition conditions. A similar behavior is found in the ZnO: Cu, films during the increasing of dopant concentration of copper in both deposition times. The optical absorption spectra of all films, also exhibited an abrupt end at 380-400 nm, which corresponds to the energy gap (Eg). The energy gap of pure ZnO films show a relatively small decrease in the variation of deposition conditions, and the same happens in ZnO: Cu, films with increasing dopant concentration of copper in both deposition times. In all cases, the energy gap of the films exhibit less than the stoichiometric value of bulk ZnO (≈ 3,34 eV).
Due to the high resistance that exhibited in thin films, in this thesis it could not have be carried out Hall measurement in order to determine the conductivity type (n-type or p-type) of the films. However, measurements of the surface currents in two cases: 1) in the dark, and 2) under the influence of UV radiation have been performed. The results show a trend of reduction of the surface current in pure ZnO films in both cases measurements by varying the deposition conditions, and the same happens in ZnO: Cu, films with increasing dopant concentration of copper in both deposition times.
Net of ZnO films show generally a hydrophobic behavior, which is decreased with the change of deposition conditions. The observed effect of wettability depends on the characteristic size of the surface structures (films containing both the micro- and nanoscale surface features) of the semiconductive properties of ZnO, as well as by the increasing of influence of surface chemistry. The effects of impurities in copper wettability of ZnO: Cu films decrease slightly the hydrophobic nature. The nature of ZnO and ZnO: Cu films are also found to change from hydrophobic to hydrophilic by the action of ultraviolet (UV) radiation, although it is reversible.
Finally, photocatalysis experiments of ZnO and ZnO: Cu films show that the films have almost no photocatalytic activity in the visible spectrum, while under the influence of ultraviolet (UV) radiation the films show a low photocatalytic activity, which however could not be determined by varying the concentration of the precursor solution and deposition time, due to some experimental imperfections.
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Αυτοκαθαριζόμενες και φωτοκαταλυτικές ιδιότητες επιφανειών με ενώσεις οξειδίου του Zn (ZnO)
