TiO2-based nanocomposites for photocatalytic hydrogen generation applications: materials synthesis, applications and mechanism studies

摘要

Solar to chemical energy conversion through photocatalysis has attracted great research interest since it offers potential solutions for human beings to mitigate their dependence on non-renewable energy source. Among the developed photocatalysts, TiO2 has been extensively investigated for solar fuel production. In the first part of this project, gold (Au) embedded boron (B)-doped TiO2 photocatalysts were synthesized which showed superior activity compared to solely B-doped or Au embedded TiO2. Isotopic tracer studies indicated that the produced hydrogen originated predominantly from water rather than methanol. With the aid of high-performance liquid chromatography (HPLC) analyses of the liquid intermediate products, a possible reaction pathway for the photocatalytic hydrogen production from the water/methanol mixture was proposed. In the second part, Au, platinum (Pt) and alloyed Au-Pt deposited TiO2 were prepared. Compared to bare TiO2, Au-Pt/ TiO2 showed a remarkable photocatalytic hydrogen evolution enhancement of up to 10 times. In situ 13C and 1H nuclear magnetic resonance (NMR) spectroscopic studies showed that the surface-adsorbed methanol was first oxidized to formaldehyde, followed by spontaneous hydrolysis and methanolysis to methanediol and methoxymethanol. The in situ monitoring also revealed that deposition of metal nanoparticles (NPs) would not alter the reaction pathways. The third part of this project highlighted that doping of Sn4+ ions into TiO2 facilitated the phase transformation from anatase to rutile at a lower calcination temperature. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. The mixed-phase catalysts prepared by doping Sn4+ ions into TiO2 exhibited superior activity for photocatalytic hydrogen generation, relative to their counterparts prepared by conventional annealing. In the last part, a wet-chemical refluxing approach was employed for the fabrication of In2S3/Pt-TiO2 heterogeneous catalysts for hydrogen generation under visible light irradiation. When the mass ratio between Pt-TiO2 and cubic-phased In2S3 was 2, the composite catalyst showed the highest hydrogen production, which exhibited an 82-fold enhancement over that of Pt-In2S3. Optical pump-terahertz probe (OPTP) spectroscopic studies showed that the migration of photo-induced electrons from the conduction band of In2S3 to that of TiO2 and subsequently into Pt nanoparticles was found to occur within 5 picoseconds.