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Semiconductor photocatalysis has been intensively studied in recent decades for a wide variety of application such as water treatment and hydrogen and oxygen production from water splitting. Bare photocatalysts are having the problem of electron hole recombination and photocorrosion; to overcome this problem this thesis investigates various types of UV and visible light active photocatalysts including TiO2, CdS and WO3 for water splitting. Nanocomposites of carbons and platinium metal with the UV photocatalst TiO2 were synthesized and compared the effects of individual dopants on the resulting physicochemical properties and corresponding photocatalytic activities with respect to the catalysis of several reactions under UV-visible-light irradiation. TiO2, carbons and Pt with CdS were synthesized and composites of Co-Pi, Ni-Bi, Ag with WO3 were synthesized, each modification uniquely effects photocatalytic process and photoelectrochemical behavior and shows implication for photocatalytic hydrogen, oxygen and photocurrent generation.
1. TiO2 composites with seven different carbon materials (activated carbons, graphite, carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene oxides, and reduced graphene oxides) that are virgin or treated with nitric acid are prepared through evaporation and drying method. The photocatalytic activities of the as-prepared samples are evaluated in terms of H2 production from aqueous methanol (1 M) solution (photocatalytic reduction: PCR) and degradation of aqueous pollutants (phenol, methylene blue, and rhodamine B) (photocatalytic oxidation: PCO) under AM 1.5-light irradiation. Despite varying effects depending on the kinds of carbon materials and their surface treatment, composites typically show enhanced PCR activity with maximum 50 times higher H2 production as compared to bare TiO2. Conversely, the carbon-induced synergy effects on PCO activities are insignificant for all three substrates. Colorimetric quantification of hydroxyl radicals supports the absence of carbon effects. However, platinum deposition on the binary composites displays the enhanced effect on both PCR and PCO reactions. These differing effects of carbon materials on PCR and PCO reactions of TiO2 are discussed in terms of physicochemical properties of carbon materials, coupling states of TiO2/carbon composites, interfacial charge transfers. Various surface characterizations of composites (UV-vis diffuse reflectance, SEM, FTIR, surface area, electrical conductivity, and photoluminescence) are performed to gain insight on their photocatalytic redox behaviors.
2. Multi-walled carbon nanotubes (CNTs) effectively catalyzes photocatalytic hydrogen production in heterojunction suspensions under solar visible light (AM 1.5G; > 420 nm). Due to the high catalytic activity of CNTs, use of Pt can be significantly reduced. For this, quaternary composites (CdS/TiO2/Pt/CNTs) are prepared by the creation of CdS on platinized TiO2 (TiO2/Pt) subsequently to which chemically oxidized CNTs are loaded. A binary (CdS/TiO2) and two ternaries (CdS/TiO2/Pt and CdS/TiO2/CNTs) are also prepared for comparison. A TEM analysis for the quaternary sample shows that TiO2 is a central component that holds Pt nanoparticles, CNTs, and CdS clusters, while the last is spatially away from the catalysts. Photoluminescence (PL) emission bands of the binary excited at 325 and 410 nm are reduced by loading either Pt or CNTs, and further by co-loading of both catalysts. This suggests that the recombination of photogenerated charges under UV or visible light is inhibited due to cascaded charge transfer between TiO2 and CdS, which is further decreased by Pt and/or CNTs. Photolysis confirms that either Pt or CNTs catalyzes effectively photocatalytic H2 production in aqueous CdS/TiO2 suspensions with sulfide/sulfite electron donor under visible light. Such activity is significantly enhanced by over 50% by co-loading of Pt and CNTs. It is found that the Pt amount can be reduced to approximately five- or one-tenth by additional loading of CNTs under an optimal condition. The maximized performance of the quaternary is also found in the significantly enhanced photocurrent generation compared to the two ternaries. The detailed mechanism and implications are discussed.
3. The photo-oxidation of water is studied in the presence of light source AM 1.5 using tungsten oxide (WO3, micro Size) semiconductors in the presence of electron acceptors AgNO3 (SEA) both pre deposited on the surface of WO3 and in aqueous solutions (in- situ) and also with cocatalysts cobalt phosphate (Co- Pi) and nickel borate (Ni-Bi) photodeposited on the surface of WO3 to help mediate the oxidation of water, in all the cases seen the Ag+ in aqueous solution is more effective for water oxidation, no oxygen evolution is enhanced when Ag+ is pre deposited on the surface of WO3, WO3/Co-Pi, WO3/Ni-Bi and in situ WO3/CoCl2 also enhanced O2 prodction. SEA and SHA is also studied to know the optimum deposition time of the cocatalysts. The results suggest that Co−Pi, Nickel borate played the role of cocatalyst, which can trap photogenerated holes and catalysis oxygen from water, while the Ag+ sucked the electron leading to the enhancement of electron and hole separation efficiency.
Thesis Advisor: Prof. Hyunwoong Park