Hydrothermal synthesis of TiO₂ nanostructures for photocatalitic and photovoltaic applications

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2017
Erdoğan, Nursev
Titanium dioxide (TiO2) nanostructures with different crystal structures and various morphologies were synthesized by hydrothermal process to utilize them in photocatalytic and photovoltaic applications. The investigations were conducted in three different sets of systematic experimental studies. The first set of experiments was based on the synthesis of TiO2 nanostructures in the presence of strong sodium hydroxide (NaOH) catalyzer. Temperature and molarity of NaOH were kept constant while hydrothermal reaction duration and precursor materials were changed during syntheses. The nanostructures obtained were in Na-titanate crystal structure, which were protonated for H-titanate and then calcined to get a TiO2 powder. The synthesized products were characterized using X-ray Diffraction (XRD), Field Emission Electron Microscopy (FESEM), High Resolution Electron Microscopy (HRTEM), Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) techniques. Photocatalytic properties of nanostructures were investigated using of methylene blue degradation in aqueous solution by a UV-Vis spectrophotometer. Best photoactivity was recognized in the products obtained when amorphous TiO2 powders were used as precursor. Two of the powders with different morphologies were mixed to practice this mixture in the fabrication of dye-sensitized solar cell (DSSC). The second set of experiments was performed using nitric acid (HNO3) catalyzer to synthesize TiO2 powder in one step. Various hydrothermal synthesis temperatures, durations, and acid concentrations were employed to obtain powders with different morphologies and crystal structures. Synthesized powders were characterized using FESEM, HRTEM, and XRD. An evolution mechanism was proposed for the resultant TiO2 powders. Four different powders in terms of crystal structure and morphology were selected for the fabrication of DSSCs. The third set of experiments was conducted to enhance the photoactivity of powders synthesized by HNO3 catalyzed hydrothermal process via doping by iron (Fe) and/or molybdenum (Mo). It was realized that nitrogen (N) also doped into the TiO2 structure as a third dopant during co-doping of Fe and Mo. Mono-, co-, and triple- doped TiO2 powders were characterized using FESEM, XRD, HRTEM, X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Brauner Emmett Teller (BET) surface area analyses. Doping caused to evolve rutile structure instead of anatase. Results revealed that hotoactivity of rutile structure was enhanced by triple-doping. Selected powders were used in the preparation of a paste for the photoanode component of DSSC. Photoanodes were characterized using UV-Vis spectrophotometer, surface profilometer, FESEM, XRD and Electrochemical Impedance Spectroscopy (EIS). Photovoltaic measurements revealed that best efficiency (2.86 %) among single layer photoanodes could be harvested by the photoanode composed of anatase and a small amount of rutile crystals. By titanium chloride (TiCl4) treatment efficiency increased to 4.86 %. The highest efficiency harvested was 4.26 % by bi-layer photoanodes built using anatase and rutile electrode as down layer and another anatase and rutile electrode with different ratio as top layer.

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Citation Formats
N. Erdoğan, “Hydrothermal synthesis of TiO₂ nanostructures for photocatalitic and photovoltaic applications,” Ph.D. - Doctoral Program, Middle East Technical University, 2017.