Titanium Dioxide, Quantum Dot And Their Nanocomposites: Preparation And Photoelectrochemical Properties

In view of the global energy crisis and environmental issues, semiconductorphotocatalysis is regarded as one of the green technologies, which is able to splitwater to produce clean hydrogen energy, to degrade organic pollutants to purify theenvironment, and to reduce carbon dioxide into hydrocarbon fuels. Among variousphotocatalysts, TiO2has been attracting much attention due to its excellent propertiessuch as low cost, non-toxicity, chemical stability, and high activity. On the other hand,TiO2is usually used as a solar cell material (photoanode), due partially, to its goodelectronic conductivity. However, TiO2has a wide band gap energy (Eg~3.2eV) andcan only absorb UV light. From the practical point of view, TiO2is usually sensitizedby, for instances, some dyes and quantum dots so as to absorb more visible or, evennear infrared light.The photocatalytic property of a semiconductor material is generally related tothe crystal structure, electronic structure, defect, morphology, and surface area etc. Inthis context, a variety of TiO2nanostructures with different morphologies, crystalstructures were synthesized through a hydrothermal method under various conditions.In the second part of this thesis, PbS QDs and PbSe0.5S0.5QDs were prepared througha metal-organic route and then hybridized with one-dimensional TiO2nano-materialsusing a self-assembly method, forming the QDs/1-D TiO2nanocomposite.Thesynthesized materials were characterized by means of X-ray diffraction (XRD),scanning electron microscopy (SEM) and transmission electron microscopy (TEM),and evaluated by conducting the photocatalytic degradation of a model dye, methylorange (MO). The main results are as follows: TiO2nanotubes are crystallized in asmaller size with the anatase structure while the TiO2wires are crystallized in arelatively larger size with the mixed phases of brookite and anatase. In terms of unitsurface area, the TiO2nanowires show a higher activity than TiO2nanotubes forphotocatalytic degradation of MO dye under the same irradiation conditions. Theresults showed that the synthesized QDs were well crystallized in regular sphericalshape with a monodispersion in particle size, and the coverage ratio of QDs on thesurface of TiO2could be controlled by changing the ligand dosage and reaction time. UV-Vis-NIR absorption and photoluminescence (PL) measurements confirmed thatowing to the hybridized QDs, the absorbance of QDs/1-D TiO2nanocomposite wassuccessfully extended to near infrared region, exhibiting an identical characteristicabsorption edge of QDs. Compared to the pure QDs, the fluorescence intensity andthe decay time of the QDs/1-D TiO2nanocomposite decreased dramatically, implyingan apparent electron transfer from QDs to TiO2. The present study has proven thatcombining near infrared QDs with one-dimensional TiO2nano-materials might bepromising method for effectively improving the photoelectrochemical properties ofTiO2.