Studies of Molecular and Colloidal Semiconductors and Their Mass Transport Properties by Voltammetry, Stochastic Electrochemistry, and Electrogenerated Chemiluminescence
AuthorKhouzani, Rezvan K.
AdvisorAlpuche-Aviles, Mario A.
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This dissertation discusses stochastic photoelectrochemistry of titanium dioxide (TiO2) nanoparticles (NPs) in methanol (on fluorine-doped tin oxide (FTO) microelectrodes and in aqueous media (Pt microelectrode). In parallel with photoelectrochemical current (PEC) measurements, colloidal behavior of TiO2 NPs has been studied using dynamic light scattering (DLS) technique. Combining PEC measurements with DLS analysis of colloidal semiconductors provides a precise correlation of nanostructure−function of NPs which is the key to improve the efficiency of electron transfer in bulk semiconductors utilized in solar cell panels. In methanol, intensive current spikes observed in PEC response and also DLS results emphasized the presence of agglomerates up to a few μm in diameter. In aqueous medium, DLS data support a stable colloid under illumination and over six months in dark (size distribution centered at 22-23 nm). However, the current magnitude of observed anodic spikes was assigned to TiO2 agglomerates of a few μm in diameter. This mismatch is subjected to further studies.We also studied mass transfer of three kinetically facile systems: Ru(NH3)63+ / Ru(NH3)62+, Fe(CN)63- / Fe(CN)64-and Fc/Fc+ at high concentration of analyte (0.1 M) with (supported media) and without electrolyte (unsupported media) via voltammetric measurement. The apparent diffusion coefficients (D) of electroactive species directly determined via choronoamperometery measurements. The insight gained from these couples can be used as a model to study the energy-relevant systems such as hydrogen power stations and lithium ion batteries which operate in highly concentrated media. In an unsupported medium, migrational contribution was observed in mass transfer of charges species: Ru(NH3)63+ toward and Fe(CN)63- away from the electrode. In the case of Ru(NH3)63+ reduction reaction in an unsupported medium, a combined effect of migration and electro-convection was observed in the absence of electrolyte. In a2supported medium, the potential shift of voltammograms was attributed to the stabilizing effect of electrolyte ions via ion pairing interactions with electroactive species. In the case of ferrocene oxidation, no evidence of migration was observed due to electroneutrality of Fc species.We also performed a detailed study on electrochemical, spectroscopic and electrogenerated chemiluminescence (ECL) properties of three classes of polycyclic aromatic hydrocarbons (PAHs): peropyrenes, chiropeopyrenes, and teropyrenes. Voltammetric behavior of these compounds was studied on Pt macroelectrode. HOMO-LUMO energetics were calculated from formal redox potentials determined by digital simulation of voltammetric responses.A decrease in the electrochemical band gap (2.50 to 2.10 eV) from peropyrene to teropyrene was consistent with the trend observed in their absorption spectra.ECL emission of some selected PAHs was measured via ion annihilation reaction. The ECL mechanism was shown to be a function of generated radical cations and radical anions stability and their molecular structure. A high ECL efficiency (ΦECL > 0.6) of some of these compounds suggest their promising applications as light emitters in ECL devices.