Carbon-doped Tio2: synthesis, characterization and visible light photocatalytic activities for environmental remediation

Since the discovery of photocatalytic water splitting on TiO2 electrodes in 1972, titanium dioxide (TiO2) has been studied extensively as a promising photocatalyst in air and water purification for environmental remediation. Despite its various advantages such as high reactivity, chemical stability, robustness against photocorrosion, low toxicity and low cost, two major drawbacks have hindered its usage in practical applications: the requirement for UV light excitation due to the large bandgap of TiO2 has limited its effective use of solar energy or indoor light; the recombination of photogenerated electron and hole pairs has reduced the overall quantum efficiency of TiO2 significantly. Therefore, it is desirable to improve the photocatalytic efficiency of TiO2 by extending its activities into visible light range and enhancing the charge carrier separation for more useful applications. The first part of this thesis has been devoted to the development of a simple and cost-effective process for mass-production of visible light-active carbon-doped titanium dioxide (C-TiO2) photocatalyst. C-TiO2 nanoparticles with mixed anatase/rutile phases were successfully synthesized by conventional mild oxidation of TiC precursor at 350 ºC for 2-50 hours and more aggressive oxidation at higher temperature of 400-600 ºC for 2 hours in air. With prolonged oxidation time or increase in oxidation temperature, initial decrease in crystallite size was unveiled due to cracking of TiC grains, renucleation of TiO2 and diffusion of carbon atoms. The doped carbon in the TiO2 matrix was found to exist as graphite-like carbon, interstitial carbon in the form of carbonate species and substitutional carbon in the oxygen sites. The former two carbon states serve as photosensitizers while the substitutional carbon introduces additional electronic states just above the valence band of TiO2, that collectively contribute to the visible light photocatalytic activities. The band structure and three possible pathways were thus proposed for enhanced photocatalytic oxidation processes in C-TiO2 nanoparticles. The potential of utilizing the synthesized C-TiO2 nanoparticles for photocatalytic environmental remediation was demonstrated by the removal of volatile organic compound (VOC) and disinfection of microorganisms. The C-TiO2 nanoparticles in coated form were first evaluated for the destruction of gaseous toluene as a probe VOC in the custom-built test system under both static and dynamic flow conditions with tunable light sources/intensity, feed pollutant concentration, humidity level, balancing gas and flow rates. The photocatalytic disinfection of Gram-negative Escherichia coli (E. coli) and Gram-positive Enterococcus faecalis (E. faecalis) with different cell wall structures in aqueous suspension were then investigated under different light irradiation conditions. It was found that C-TiO2 synthesized at higher oxidation temperature with shorter duration of 2 hours demonstrated better photocatalytic performance than those prepared by the conventional time-consuming mild oxidation process. The performance was comparable to the commercially available C-TiO2 Kronos vlp 7000 and superior to N-TiO2 produced by a similar recipe. Thus, the synthesis process for visible light-active C-TiO2 can be drastically simplified with a much shortened processing time, suitable for large-scale production. Further improvement of the synthesized C-TiO2 was achieved by co-modification with novel metals to enhance the charge carrier separation in the second part of this thesis. Pt/C-TiO2 and Ag/C-TiO2 were successfully prepared by a novel synthesis approach through thermal oxidation of H2PtCl6•6H2O or AgNO3-impregnated TiC precursor at 500 ºC for 2 hours in air for simultaneous TiO2 formation, C doping and Pt/Ag modification, all in a single step. The synthesized nanoparticles consisted of 70% anatase and 30% rutile, resembling the phase structure of P25 TiO2 with high photoactivities. With increasing Pt or Ag content, the anatase/rutile grain growth was retarded. The loaded Pt or Ag nanoparticles in their metallic states further enhanced the visible light absorption and charge carrier separation by serving as electron scavengers. The visible light photocatalytic oxidation mechanism with four pathways was proposed and the enhanced photocatalytic performance in comparison to C-TiO2 was successfully demonstrated with the destruction of gaseous toluene in air and decomposition of Rhodamine B in aqueous suspension. These could be attributed to the synergistic effect of C doping and Pt/Ag modification, increasing its potential of being utilized in both air and water purification with higher photocatalytic efficiency.DOCTOR OF PHILOSOPHY (EEE