The Effect Of Heat Treatment On The Photocatalytic Properties Of Low-temperature Derived Titania Thin Films With Nanostructures

Semiconductor photocatalysis has attracted much attention over the past 3 decades. Among them, titania (titanium dioxide, TiO₂) has been widely recognized as an ideal n-type semiconductor serving as photocatalysis, due to its excellent physical and chemical properties. Till now, many research groups have successfully applied TiO₂ photocatalysis to dispose organic compounds in wastewater. However, the scale-up application is still limited, because of its limitations such as poor utilization rate of solar energy and low photocatalytic activity, and so on.In this thesis, two kinds of crystalline titania thin films have been fabricated through either directly reacting metallic Ti with 30wt% H₂O₂ at 80℃ for various durations, or direct hydrolysis of TiF₄ aqueous solution at 60 ℃ for various durations. Related mechanisms on the low-temperature growth of the crystalline titania thin films have been discussed. The effects of subsequent thermal treatment of the films on the photodegradation of Rhodamine B (RB), a laser dye, in water were also studied. And finally, efforts were made to disclose the possible causing of a so-called "natural ageing" behaviors we recently found on the titania thin films, which defines the gradually increased ability of the films to assist photodegradation of RB in water through preservation under an ambient conditions for various durations.Titania thin films with either porous nanostructure or well-aligned nanorods were synthesized through direct oxidation of Ti with H₂O₂ for various durations, at a low temperature of 80℃. Field emission scanning electron microscope (FE-SEM) observations revealed that the surface morphology of the titania thin films evolved from a condensed thin film, to porous film, and then to well-aligned nanorod film after 10 min, 1 h and 36 h reactions, respectively. The X-ray diffraction (XRD) results show that mainly amorphous titania was obtained before 48 h of reaction;only after 60 h and 72 h can one achieve crystallized structure, that is, a mixture of anatase and rutile, before heat treatment. After a subsequent thermal treatment in air at 450 ℃ for 1 h, the amorphous titania crystallized to pure anatase whereas the crystallized titania remained unchanged. The Ti(IV) ions concentrations in the solution were measured using inductively coupled plasma (ICP)atomic-emission spectroscopy. The results show that the Ti(IV) concentration increased quickly at the beginning stage and reached a peak value after 1 h reaction, after which it began to drop and then remained stable till 12 h reaction. Between 12 h and 24 h reaction, the Ti(IV) concentration decreased sharply, and almost no Ti(IV) could be detected in the solution after 24 h reaction. The H2O2 concentration in solution was tested according to National Standard GB 1616-79. The results illustrate that H2O2 concentration decreased monotonously with increasing reaction time, and there was hardly any H2O2 molecules exiting in residual solution after 24 h reaction. Based on the above results, we concluded that the well-aligned titania nanorods film derived from directly reacting Ti with 30 wt% H2O2 at 80 °C for 72 h was formed through an "oriented attachment" procedure. The possible low-temperature crystallization mechanism includes dissolution precipitation and in-situ crystallization.Crystallized titania thin films were also prepared by hydrolysis of TiF4 aqueous solution at 60 °C for up to 72 h. XRD, FE-SEM, HRTEM, UV-Vis and Raman spectroscopic analysis demonstrate that the obtained titania film consisted of well-crystallized anatase nanoparticles and some distributed agglomerates on the condensed film. The film thickness increased with increasing soaking time in the solution. The low-temperature growth of the anatase thin film was contributed to the F atoms, which inhibited the deposition rate and hence favored the crystallization procedure.The effects of a subsequent heat treatment at various temperatures on the PA of the titania thin films, derived from direct oxidation of Ti plates, were studied. The results show that the changes on PA of the titania film can be divided into three stages with increasing heat treatment temperatures. At the first stage (=340 °C), the PA decreased sharply as the heating temperature increased, which can be ascribed to the loss of surface hydroxyl group and some other superoxide radicals;at the second stage (340⁴50 °C), for the significantly improved crystallinity, the PA slightly increased with further increasing temperatures;at the third stage (500⁷00 °C), the increased crystal sizes resulted from heating led to a decreased PA.The effects of a subsequent heat treatment on the PA of the low-temperature prepared anatase thin films, derived from hydrolysis of TiF4 aqueous solution at 60 °C for 72 h, were also studied. The results show that the as-deposited film, together with the two films subjected to calcination at 200 °C and 250 °C, exhibited quite poor PA;when heated at 300 °C,comparing with 250 °C, there was an significant increase in the PA;when heated at 300, 400 and 500 °C, the PA of the film increased with the increasing calcination temperatures;and then the PA decreased when the film was heated at temperatures beyond 500 °C. Further studies revealed that the reason for the obvious change of PA between heated at 250 and 300 °C is the evaporation of F" ions at 283 °C;and the following increase of PA between heated at 300 and 500 °C is ascribed to the improvement of crystallinity of TiO2 films;when heated beyond 500 °C, the remarkably increasing crystal size resulted in the reduced PA. It is found that the PA increased with the increasing film thickness.The last section of this essay focused on the effects of an ambient preservation of the titania thin films on their abilities to assist photodegradation of RB in water. It is found that the PA increased to a certain value with the prolonged preservation time, which we called "natural aging". This phenomenon, which was irrelevant to the surface morphologies, preparation methods and crystal structures of the films, is ubiquitous. Electron spin resonance (ESR) test indicated that large amounts of hydroxyl groups were detected from the titania nanorod films derived from direct oxidation of Ti plate in H2O2 solution at 80 °C for 72 h. After the subsequent heat treatment at 450 °C for 1 h, nearly all the hydroxyl groups lost. However, increased amounts of the hydroxyl groups could be observed after natural ageing the as-heat treated film at ambient conditions for 10 days. Further studies demonstrate that the increasing amounts of surface hydroxyl groups with increasing durations of the natural ageing procedure contributed to the enhanced PA. The recovery rate of surface hydroxyl groups after heat treatment depended on the preservation conditions. Abundant oxygen and water molecules favored the recovery of surface hydroxyl groups, and the UV irradiation also accelerated the recovery procedure of surface hydroxyl groups.