Preparation and application of plasmon metal enhanced titanium dioxide photocatalyst for the removal of organics in water.

Abstract

Advanced oxidation processes are capable of removing organic compounds that cannot be removed by conventional water treatment methods. Among the oxidation processes, photo-catalysis using titanium dioxide (TiO2) is a promising method but suffers from rapid electron-hole recombination rates and only absorbs UV light which is a small percentage (5 %) of the total solar radiation. Therefore there is a need to reduce the recombination rates and also extend the absorption of the photo-catalyst into the visible region which constitutes 55 % of the total solar radiation. The major aims of this study were to prepare plasmon metal decorated and doped TiO2 photo-catalysts immobilized on quartz substrates and test their photo-catalytic and antimicrobial activities. The effect of film thickness (loading) and use of different shapes of plasmon metal nanostructures was investigated. TiO2 thin films were prepared by a sputter coating technique while plasmon metal (Au & Ag)/carbon co-doped TiO2 by a simple sol gel process and plasmon metal films were prepared by the thermal evaporation technique. Different plasmon metal nanostructures (nanorods, dendrites, nanowires and spherical nanoparticles) were prepared using a wet chemical technique using sodium borohydride as the reducing agent. Nanocomposites of co-doped TiO2 photo-catalyst and plasmon elements of different proportions were also prepared. The prepared photo-catalysts were coated onto etched and MPTMS (3-Mercaptopropyl trimethoxysliane) treated quartz glass substrate which is a stable support favouring easy recovery. The prepared materials were characterized by XRD, HRTEM, TEM, HRSEM, FT-IR, SEM, PIXE and TGA while the doped TiO2 was characterized by XPS, BET, CHNS and Raman Spectroscopy. The effect of pH of solution, presence of other contaminants and salts in solution, initial concentration of the model pollutant and type of the plasmonic elements on the photocatalytic activity of TiO2 towards 4-(4-sulfophenylazo)-N,N-dimethyl aniline (methyl orange) were also investigated. The selected TiO2 photo-catalyst films were tested for antimicrobial properties. The effect of different types of plasmon elements on the antimicrobial activity of TiO2 against E. coli ATCC 3695 was evaluated under both sunlight and weak UV light. Under UV light, Ag showed the highest enhancement in photo-catalytic activity of TiO2 than Au and Cu. The photo-catalytic activity of TiO2 increased with an increase in Ag content to an optimum loading and then started to decrease with a further increase in loading. For Cu and Au, photo-activity activity increased with an increase in plasmon metal content. Under sunlight, Cu showed the highest enhancement of TiO2 photocatalytic compared to Ag and Au. The change in order of deposition showed that Au films enhanced the photo-activity better when they were deposited underneath rather than on top of TiO2 on quartz supports but Ag films performed better in enhancing photo-activity when they were deposited on top of TiO2. The use of bimetallic layers and three layer systems of different plasmon elements enhanced photo-catalytic activity better than the use of a monometallic layer. The presence of other organic contaminants and salts in solutions was found to reduce the photo-degradation of methyl orange due to preferential adsorption of other contaminants. When the pH was increased, the photocatalytic activity of TiO2 towards methyl orange was reduced. In antimicrobial studies, it was found that the plasmon elements greatly improved the antibacterial action of TiO2 against Escherichia coli ATCC 3695 in water and the best antibacterial action was observed with silver/carbon co-doped TiO2 photo-catalyst under sunlight. The doped samples consisted of polydisperse nanoparticles which were found to be beneficial for photo-catalytic activity enhancement under sunlight. No general trend was observed on the band gap reduction with an increase in plasmon metal content. Among the Ag doped photo-catalysts, the highest photo-degradation rate constant of 2.45 × 10-3 min-1 was achieved by TiO2 with a silver content of 0.5 % because it had the lowest band gap of 1.95 eV, and lowest rate constant of 1.74 × 10-3 min-1 was achieved with 0.7 % Ag loaded TiO2 towards methyl orange. For the Au doped samples, the highest photodegradation was achieved with a sample loaded with 1.0 % Au which had a photodegradation rate constant of 3.22 × 10-3 min-1. Doping with Au and Ag was found to promote anatase to rutile phase transformation, which allowed utilization of visible light but reduced the surface area of the photo-catalyst. There was no change in band gap observed in the composite systems. The use of more than one plasmon element to prepare composites was found to be more effective in enhancing the photo-catalytic activity of TiO2 than the use of one plasmon element only. The highest photo-degradation of 66.56 % after 5 hours under visible light irradiation was achieved by co-doped-TiO2/Au-Ag-Cu composite system. Durability tests showed that the selected photo-catalysts were durable and could be used for at least four times without appreciable loss of photo-catalytic activities. For instance TiO2 on a three layer system (TiO2/Au-Ag-Cu) showed 95.88 % degradation of bromocresol purple after five hours and after the fourth application/cycle it decreases by only 14.9 % to 80.98 %. Leaching of the plasmon elements increased with an increase in the number of cycles and was minimal for Au and Cu. The extent of leaching was also found to depend on the position of the plasmon element relative to the photo-catalyst. It was shown that low band gap does not always guarantee high photo-catalytic activity as some samples with high band gaps were found to have higher photo-catalytic activities than low band gap photo-catalysts. Silver was found to favour anatase to rutile phase transformation more than gold. Some secondary photo-degradation by-products of methyl orange were found. The mode in which the LC-MS is operated has an impact on the photo-degradation products that can be detected by the instrument. The study achieved its goals of enhancing the photo-catalytic activity of TiO2 both under UV and visible light irradiation. It was proved that under UV light the plasmon elements act as electron reservoirs thereby reducing electron recombination rates and under visible light irradiation they increase TiO2 photo-catalytic activity by both creating localized surface plasmons and reducing recombination rates.