Since the discovery of the photocatalytic splitting of water on TiO2 electrode by Fujishima and Honda in 1972, enormous effort has been spent on the study of TiO2 under light illumination, due to its various potential applications, such as photovoltaics and photocatalysis. The optical properties, in particular the absorption, of TiO2 are essential to its photon-driven applications. Typically, TiO2 absorbs in the UV regime, which is only a small fraction of the sun’s energy (< 10%). The performance of TiO2 can be enhanced by shifting the onset of its absorption from the UV to the visible region. Metals have been employed to tune the electronic structure of TiO2-based material. The photocatalytic reactivity of metal-doped TiO2 depends on many factors, and metal doping can result in thermal instability and increased carrier trapping. The desired visible-light absorption of TiO2 can be also achieved by using main group dopants.
In this dissertation, different non-metal elements, C, N and S, are incorporated to the lattice of TiO2 to induce the absorption in the visible-light regime. Both bottom-up and top-down methods are used to synthesize these doped TiO2 nanoparticles. The optical, physical, electronic and photocatalytic properties of these doped TiO2 nanoparticles are explored with different techniques. The relationship between the optical, electronic and photocatalytic properties are elucidated. The photocatalytic performance of the doped TiO2 nanoparticles is applied not only to the model photodegradation of methylene blue, but also on other industrial dyes under natural sun-light illumination. The non-metal-doped TiO2 nanoparticles demonstrated improved photocatalytic performance over the non-doped TiO2 nanoparticles, i.e. in the visible-light regime.
On the other hand, as the size of nanoparticles decreases, the surface-to-volume ratio increases dramatically (~ 1/r), so does the surface area (1/r2). The high surface area brought by the small size of nanoparticles becomes more important for the optical and electronic properties of nanomaterials, compared to the bulk materials. Besides the well-know quantum-confinement effect, the surface effect should be taken into account for small nanoparticles. Thus in this dissertation, the synthesis and properties of II-VI (CdSe, CdSe/CdS) semiconductor nanoparticles are investigated to elucidate the surface effect on the properties of nanoparticles, which helps to understand the photocatalytic property of TiO2 nanomaterials, since the main catalytic reactions occur on the surface. The gradual crystallization of small nanoparticles, as well as its effect on the optical properties is elucidated. The interface strain/stress in the CdSe/CdS core/shell system is explored on their optical properties, as well as the hot carrier relaxation dynamics in CdSe nanoparticles.