Synthesis And Studies Of Photovoltaic Properties Of Titanium Dioxide Nanomaterials With Different Morphologies

As a common wide-band gap semiconductor, Titanium dioxide (TiO₂) materials have been applied in many aspects, such as solar energy conversion, photodegradation of organic pollutants, and fabricating photovaltaic devices, et al. In the past few years, more and more new physical or chemical properties and applications of TiO₂ nanomaterials were observed and developed, as the continuous breakthroughs made in its preparation of different sizes and shapes. In addition to the well-known size confinement effect, TiO₂ nanomaterials were also found to have size, morphology and structure dependent optical and photoelectronic properties, meanwhile, we found that for all the applications of TiO₂ nanomaterials, such as photocatalyst, solar energy conversion and photovoltaic devices, their operating principles are all in fact correlated with the charge transfer properties in TiO₂ nanomaterials after its absorption of light. Thus, it is necessary to further understand the photo-induced charge transfer properties of TiO₂ nanomaterials of different morphologies, which is significantly important for its applications.Based on above analysis, in this thesis, we developed a new relatively environmentally friendly reaction system, in which the ethylene glycol (EG) was used to manipulate the hydrolysis rates of the titanium alkoxides, to synthesize TiO₂ based nanomaterials with different of morphologies, and further studied the photovoltaic properties of the as-synthesized TiO₂ nanomaterials through the Surface Photovoltage (SPV) and Surface Photocurrent (SPC) techniques. The information providing in this thesis should be beneficial for understanding the photocatalyst process and fabricating photovoltaic devices.The main contents are as follows:1. Synthesized TiO₂ one-dimensional nanowires with different types of crystalline phases via sol-gel method: Anatase, Anatase/Rutile mixed phase, and Rutile. Systematically studied the photovoltaic properties of these samples through the SPC, SPV and TPV techniques. The conclusions are as follows: 1. The surface of the anatase TiO₂ nanowires has more active sites, and the amount of the active sites gradually decrease as the change of the crystalline phase from anatase to rutile. 2. The mobility of the photo-induced electrons is significantly different for different types of TiO₂ nanowires, and found that the electrons move faster in anatase TiO₂ nanowires than that of rutile sample. The results should be beneficial for fabricating TiO₂ nanowires based photovoltaic devices.2. Developed a facile method to synthesis high-crystallinity anatase TiO₂ nanocrystals under mild conditions. Found that EG played the role to both control the hydrolysis and condensation rates of titanium isopropoxide, and H2O is the key reagent for the anatase formation, and the amount of H2O has been demonstrated to be an important parameter for the fast anatase phase formation. The simplicity (only three reagents are involved) and reproducibility of this method makes this route be possibly large-scaled. Besides, found that the surfactant used in the reaction system has significant effect on the photovoltaic properties of the nanocrystals. Meanwhile, it has been found that the nanocrystals exhibit the fast response to oxygen under the UV illumination at room temperature.3. Developed a facile method to synthesize monodisperse gold-doped titania spheres under high concentration of titanium precursor simply by introducing trace amount of CA into the reaction system, and the size of the spheres can be easily tuned between 392 and 587 nm through the changing of the amount of CA added. The detailed growth mechanism has been discussed where it was found that CA acts as the stabilizing agent in the reaction. Anatase titania spheres with gold nanodots (-7 nm) on its surface has been synthesized by the heating treatment. SPC and SPV investigations show that the gold nanodots have dual roles in the gold-doped anatase titania spheres: one is as the electron acceptor in the UV region, while the other is as the electron donor when it was illuminated by the visible light. TPV measurement indicates that the decay time of the injected plasmon-induced electrons is in the millisecond timescales and gradually increased as the increasing of the amount of gold doped. The information providing here should be beneficial for further understanding the charge transfer properties in the Au-TiO₂ system under the illumination of different wavelengths of light, synthesizing high efficient visible light catalysts and fabricating photovoltaic devices with higher performance.