Characterization and Application of Reactor and Surface Plasmon Resonance on Photocatalysis

Wastewater treatment based on solar energy-effected photocatalytic reaction is a green process that utilizes renewable energy resources and minimizes secondary pollution. Low conversion efficiency is one of the key issues to overcome for realizing its practical application. This study aims at significantly raising the process efficiency from the viewpoints of photocatalytic reactor and visible light utilization, respectively.

First, a rotating disk reactor (RDR) has been evaluated for the application of photocatalytic decomposition of dye pollutants in water. In this process, photocatalyst (TiO2) particles are immobilized onto a disk, and dye (methyl orange)-containing solution is allowed to flow in radial direction along the surface of the disk, which is rotating and illuminated with UV light. The correlations between the fundamental characteristics of the reactor, including residence time and film thickness, and its operating variables, including volumetric flow rate and disk rotating speed, have been established by the combination of fluid dynamic and kinetic models. The results indicate that the reactor can be operating beyond mass-transfer limitation by reducing the liquid film thickness, which is a complex function of both flow rate and disk rotating speed, below certain critical value. Even under such a condition, the overall reaction rate remains strongly affected by the liquid film thickness due to the intensity attenuation of incidence light through the liquid film before reaching the TiO2 surface. With selected operation conditions, conversions greater than 50% have been achieved within only a few seconds of residence time. A reactor design equation has been derived, indicating promising scale-up potential of the process.

Second, a fascinating surface plasmon resonance (SPR) phenomenon of Ag nanostructures within visible light wavelength region has been investigated for the potential application to TiO2 photocatalysis, and emphasis has been placed on quantifying the Schottky-barrier effect (SB-effect) and the SPR effect and on looking for energy-specificity and morphology-dependence of the SPR effect. Two types of Ag/TiO¬2 composite photocatalysts, including Ag nanoparticle-on-TiO2 nanoparticle (Ag-NP/TiO2-NP) and Ag nanoring-on-TiO2 nanotube (Ag-NR/TiO2-NT), that generate different SPR modes, have been evaluated for photocatalytic bleaching of methyl orange (MO). Irradiation from ultraviolet light (UVL) and visible-light (VL) emitting diodes (LEDs) of four different wavelengths are applied to excite excitons in TiO2 and surface plasmon of Ag, respectively. Under various combinations of the UVL and VL LED, the SB- and SPR-effects due to the Ag nanostructures have been clearly distinguished. The photocatalytic activity exhibits complex dependence on the morphology of TiO2 and Ag, suggesting strong interaction between the excited electron-hole pairs and SPR effect. In particular, SPR-enabled enhancement in photocatalytic activity has been confirmed for both catalysts, and the frequency-specificity of such enhancement has for the first time been demonstrated: the extent of the enhancement decreases with increasing departure of the VL photon energy from the resonance peak energy, irrespective of the absolute value of the resonance energy. Ag-NR/TiO2-NT nanostructure in general exhibits greater SPR-enhanced enhancement than Ag-NP/TiO2-NP one.

Finally, we further delicate effort to material and reactor design for the purpose of enhancing SPR effects of noble metal on TiO2 photocatalysis. We successfully synthesize a large-scale Au NPs-embedded TiO2 thin film photocatalyst with applying a series of fabricating process involving a novel “nanospheres template method” followed by spin-coating of TiO2 thin film. The Au NPs concentration and diameter in the TiO2 matrix can be easily controlled by the sputtering time of Au target. A customized “micro-reactor” with dual-light sources is designed to evaluate the photocatalytic activities and the activity data showed clearly the SB- and SPR-enabled activity enhancement in TiO2 photocatalysis in the presence of embedded Au NPs.