| Hydrogen is a clean and sustainable energy source with high calorific value. Photocatalytic water splitting using limitless solar energy has great practical significance for hydrogen generation. TiO2 that is abundant in nature, non-toxic and chemically stable has been extensively studied as potential photocatalysts for water splitting. However, due to the large band gap (e.g. Eg= 3.2 eV for anatase TiO2) and high electron-hole recombination rate, the application of TiO2 has been severely hindered. In view of this, based on an efficient synthetic route, named as continuous hydrothermal flow syntheses (CHFS) using supercritical water, a series of modified TiO2 had been prepared in this thesis, which were subsequently utilized as effective photocatalysts for water splitting under UV radiation. It is expected that the work conducted herein could provide experimental and theoretical basis for effective TiO2 photocatalytic application and development.Firstly, we prepared 2 mol% Ni, La, Fe and Zn ions doped nano TiO2 photocatalysts by using the supercritical water hydrothermal route, and investigated their photocatalytic activitiesfor hydrogen production via photocatalytic water splitting. Experimental results revealed that the Fe and Zn ions doping had decreased the TiO2 photocatalytic water splitting efficiency, and the La and Ni ions doping could effectively improve the photocatalytic activity, especially for Ni ion doping that promoted the most.Further study indicated that the Ni doping could introduce impurity levels in the TiO2 band gap, which had effectively expanded the range of optical response. Moreover, the Ni ion in the TiO2 lattice could act as a shallow well to trap the hole, which promoted the separation of photo-induced electron-hole pairs. The Ni at surface might exist in the form of NiO and had a strong interaction with TiO2, forming a heterojunction.With a built-in electric field force, the photo-induced electrons were aggregated on the conduction band of TiO2, where the photo-generated holes were migrated to the valence band of NiO. This had effectively inhibited the electron-hole recombination, hence greatly improving the water splitting efficiency over the TiO2. Experimental results revealed that the doping level of 1 mol% Ni would yield the best photocatalytic performance with the hydrogen production efficiency at ca.1278.4 ?mol/g/h, which was ca.18.6 folds higher than that of pure TiO2.The pure TiO2 was also subjected to a heat treatment under nitrogen atmosphere, which had effectively led to the formation of ultrafine TiO2 nano-crystallites with defects and C. It was noted that under supercritical water crystallizing environment, the TiO2 was shown with trace amounts of lactic acid groupsat surface. These species were removed after ageing in N2 atmosphere, which took away part of surficial reactive oxygen species of TiO2, leading to the formation of oxygen vacancies and elemental C at surface. These oxygen vacancies were shallow energy level defects, which could trap the photo-induced electrons and released them soon, This had effectively promoted the separation and migration of TiO2 photo electron-hole pair. On the other hand, C at surface might also contribute to the electron-hole separation promotion as the conductive C was evidenced able to transfer the electrons, thus improve the photocatalytic activity of TiO2. Experimental results revealed that when the heat treatment temperature was at 300?, the resulted sample had the highest hydrogen production capacity with the production efficiencyat ca.196.0 ?mol/g/h, which was 4 folds higher than that of pure TiO2 and 44 folds higher than that of commercial P25. |
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