MECHANISMS OF ENHANCED ACTIVITY OF MODEL TiO2/CARBON and TiO2/METAL NANOCOMPOSITE PHOTOCATALYSTS

Doctoral thesis, 2013

TiO2 is the most widely used photocatalyst for energy and environmental applications because of its outstanding physical and chemical properties. However, the quantum efficiency of TiO2 is greatly diminished by fast electron-hole pairs recombination and light absorption only in the UV region. This thesis deals with two approaches aiming to improve TiO2 performance through design of model systems. Emphasis is given to understand the underlying photocatalytic mechanisms. In the first approach, a model consisting of TiO2/Graphitic carbon nanocomposites has been fabricated to address the role of carbon on charge carrier dynamics and light absorption. Composites showed a significant enhanced photocatalytic activity compared to bare TiO2 films in the UV region for the test reaction of gas phase methanol photooxidation. The enhanced activity was attributed to improved charge carrier separation at the interface between carbon and TiO2 films. Photoluminescence (PL) studies confirmed further the improved charge carrier separation in composites, showing reduced PL intensity and shorter lifetime. Another type of model TiO2/Graphene composite photocatalysts was prepared with the aim of demonstrating the role of graphene synthesizing techniques on the performance of TiO2 photocatalytic activity. Graphene prepared by different methods resulted in different morphology and electrical conductivity. These modifications strongly affected charge carrier dynamics as revealed by PL studies and in turn affected the photocatalytic activity. In the second approach, model systems consisting of TiO2 and optically active Au and Ag metal nanoparticles (NPs) in different configurations were designed to address the role of localized surface plasmon resonance (LSPR)-related effects on the photocatalytic activity of undoped TiO2 films. Sample irradiation conditions were chosen to excite TiO2 and NPs separately as well as simultaneously. A substantial enhanced photoactivity was observed for TiO2/Metal composites compared to bare TiO2 films. The model systems were evaluated in two different photoreactor systems with two different photocatalytic reactions (methanol and ethylene photooxidation). The enhanced activity was attributed to (i) Interfacial charge transfer from TiO2 to NPs, when they are in direct contact with each other and (ii) Plasmonic near-and far-field effects, when there is an overlap of the plasmonic band with the TiO2 absorption band. In addition, TiO2/Cu nanocomposite samples were prepared and tested under UV irradiation with the aim of understanding Cu NPs’ chemical stability at conditions resembling CO2 photoreduction to hydrocarbons in water. The chemical transformation of Cu was followed in situ using LSPR spectroscopy.