Hoffman, Amy Jo (1993) Photocatalytic reactions on quantum-sized semiconductor colloids : photoinitiated polymerization of vinylic monomers, formation of hydrogen peroxide and organic peroxides, oxidation of carboxylic acids, and synthesis of humic-like material. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-09272005-134829
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Ultrasmall semiconductor particles (diameters 1-10 nm) of CdS, ZnO, and TiO2 have been synthesized. These particles exhibit the quantum-size effect, which is characterized by a shift of the absorption onset (i.e., bandgap energy) from the visible region for the bulk material to the UV for ultrasmall diameters. The ability of these colloids to photocatalyze various chemical transformations was investigated.
Semiconductors were found to be efficient photoinitiators of polymerization of vinylic monomers. Q-sized colloids demonstrated significantly higher rates of photopolymerization than their bulk-sized counterparts. The semiconductor photoinitiation efficiencies were correlated to the reduction potentials of the conduction band electrons. The rates of polymerization were found to depend upon the solvent nature, the monomer and initiator concentrations, and the incident light intensity. The polymerization reaction proceeds via a mechanism involving anionic initiation by conduction band electrons. The valence band holes, which are formed upon illumination, are scavenged by the solvent. The observed rate of polymerization increased proportionally to the square root of the incident light intensity; this functional dependence arises from the second-order termination reactions of the free-radical intermediates.
Aqueous, oxygenated suspensions of quantum-sized ZnO particles with added hole scavengers produced steady-state H2O2 concentrations as high as 2 mM upon bandgap illumination. Isotopic labeling experiments demonstrated that H2O2 was produced via the reduction of adsorbed oxygen by conduction band electrons. The quantum yields followed an inverse square-root dependency on absorbed light intensity. Quantum yields as high as 30% were obtained. The quantum yield of H2O2 production increased as the Q-sized particle diameter decreased.
Q-sized ZnO particles also photocatalyzed the oxidation of acetate, formate, and glyoxylate. The observed product distributions were discussed regarding direct oxidation by the valence band holes or indirect oxidation by surface-bound hydroxyl radical intermediates. In illuminated semiconductor suspensions, radical intermediates react with the semiconductor surface, forming trapped species which are further oxidized either by semiconductor holes or by self-injection of a second electron into the conduction band.
In the final investigation of semiconductor photocatalyzed reactions, humic-like substances were formed in illuminated aqueous Q-sized suspensions containing catechol or gallic acid. Their formation was attributed to polymerization-type reactions initiated by [...] radicals.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||2 April 1993|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||28 Sep 2005|
|Last Modified:||26 Dec 2012 03:03|
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