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Abstract
It is widely accepted that one dimensional (1D)-TiO2 nanotube (TNT) presents a light scattering effect and enhanced electron charge transport, both critical properties in photoelectrochemical and photocatalysis applications. In this thesis, 1D-TNT-based photoelectrodes were synthesized using electrochemical synthetic methods and systematically optimized for enhancement in solar energy conversion applications. In the first part of this project, the geometry of TNT arrays was optimized by controlling anodization conditions in the ethylene glycol based-electrolyte including NaF. In the second part of the project, the TNT array was used as a photoelectrode for dye-sensitized solar cells (DSSCs). The well-ordered 1D-TNT photoelectrodes exhibited an enhanced electron lifetime compared to TiO2 particle-based photoelectrodes. This was attributed to the retarding of the electron recombination rate by shallow traps within the TNT structure. The porphyrin dye (GD2)-adsorbed DSSCs with TNT photoelectrodes showed significantly enhanced photovoltaic performance through the light soaking (LS) treatment leading to better charge injection. It was also demonstrated that the TNT arrays could be applied for the synthesis of highly conductive pure reduced graphene oxide (RGO) through photocatalytic reduction. Furthermore, the reduction degree and functional groups of the pure RGO were controllable by varying reduction time in the presence of formic acid as a hole scavenger or by changing the hole scavenger. The electrical conductivity and dispersibility of the pure RGO can be tuned by controlling the reduction degree and carboxyl groups bound to the edge of the sp2 domains. The last part of the project demonstrates two strategies to modify the TNT array to enhance photoelectrochemical performances as a photoelectrode: 1) extension of light absorption by coupling with visible responsive semiconductors via the square wave pulsed-electrodeposition and 2) enhancement of electron charge transport by incorporating highly conductive carbon-based materials via the combined electrophoretic deposition-anodization (CEPDA). The CuInS2 (CIS)-TNT array presented enhanced photoelectrochemical performance with a good heterojunction structure. The RGO-TNT array exhibited enhanced photocurrent density and conductance compared to the pure TNT array, attributed to the high conductivity of well-incorporated RGO sheets into the TNT array.