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In Situ FTIR Study of the Photocatalytic Properties of TiO2 and Conductivity of Polyaniline

Lohrasbi, Mehdi
http://rave.ohiolink.edu/etdc/view?acc_num=akron1416220154

Abstract Details

2014, Doctor of Philosophy, University of Akron, Chemical Engineering.
The surface of semiconductors, such as TiO2, has long been shown to have the capability of activating adsorbed molecules and converting the photon energy to chemical energy. The activation of adsorbed molecules occurs when electron-hole pairs are generated upon exposure to light of appropriate energy and their charges are transferred to the adsorbed molecules. This dissertation presents a fundamental study on four photocatalytic reactions on TiO2, focusing on the charge transfer processes and their subsequent influence on the reaction mechanisms. For this purpose, thermal reduction of TiO2 in H2, water dissociation on UV-irradiated TiO2, and photocatalytic degradation of benzene and ethanol were studies, using an in-situ IR spectroscopic approach. In-situ IR spectroscopy allows monitoring the interaction between the adsorbed molecules and the catalyst surface and the formation of reaction products. In addition, in-situ IR approach is capable of providing evidence for accumulation of photogenerated electrons in the conduction band of TiO2, observed as a structureless IR adsorption (i.e., background shift) in the 1000 to 3000 cm-1 region. As TiO2 was reduced in H2, an IR absorbance band was observed around 1015 cm-1. Since the similar band was not observed in Ar and O2, the formation of this band was attributed to the removal of lattice oxygen atoms and formation of Ti3+ sites on the reduced TiO2. UV photoexcited TiO2 under H2 environment caused the rise of IR background indicating the accumulation of electrons in conduction band. The accumulation of photogenerated electrons in the conduction band was attributed to the promotion effect of photogenerated holes in accepting electrons from H-H bond breaking. The injection of electrons from H2 oxidation into the photogenerated holes was found to hinder the dissociation of H2O to OH on TiO2. On the surface of TiO2, five IR bands at 3727, 3692, 3681, 3664, and 3632 cm-1 were distinguished corresponding to stretching vibration of OH groups. The function of these OH groups was investigated by adsorption and photocatalytic oxidation of benzene. Benzene adsorption showed that the OH groups at 3692, 3681, and 3664 have stronger OH•••p-electron interaction with the benzene ring compare to the OH groups at 3727 cm-1. These OH groups were adsorbed and desorbed benzene with the same rate and were assigned to bridged hydroxyls. The OH groups at 3727 cm-1 were assigned to the terminal hydroxyls, since they had the weakest interaction with the benzene ring. The subsequent photocatalytic oxidation of benzene on TiO2 revealed the function of terminal OH groups as adsorption sites and the function of bridged OH groups as providers of OH radicals. The reaction of adsorbed hydrogen, benzene and ethanol with photogenerated holes on TiO2 has was followed by the rise of IR background, attributed to the accumulation of photogenerated electrons in TiO2 conduction band. Measurements of the electrical conductivity of TiO2, while an adsorbed molecule such as ethanol was reacting with holes showed the increase in the electrical conductivity. The conductivity of TiO2 film, as an inorganic semiconductor and polyaniline (PANI), as an organic semiconductor was found to be proportional to the extent of IR background absorbance. The integration of fundamental catalysis research and technology-based devices motivated the study of long term stability of dye-sensitized solar cells (DSSCs) based on N719 sensitized and TiO2 working electrode. The photovoltaic performance of DSSCs showed an 88% reduction in the short circuit current and 89% reduction in efficiency after aging period of 4500 h in ambient conditions. The contribution of electrolyte degradation in the efficiency loss was determined by injecting freshly prepared electrolyte into the aged cells and re-evaluating their photovoltaic performance. It was observed that only 4.5% of the efficiency loss was due to the electrolyte degradation. Physical explanations for the degradation in the performance of as fabricated and aged DSSCs were concluded by extraction of sensitized-electrons lifetime and charge transfer resistance parameters, describing the DSSC performance at TiO2-dye/electrolyte interface. Molecular-level explanations for degradation of N719 dye were investigated by FTIR and UV-vis measurements on as fabricated and aged DSSCs. The thiocyanate ligand (-N=C=S) was found to be the most susceptible part of N719 dye regarding degradation under the ambient conditions. In the aged DSSCs, H2O was found to substitute the -N=C=S ligand and the typical consequence was the increase in the internal resistance and deterioration of the fill factor of the solar cell.
Steven Chuang, Dr. (Advisor)
David Perry, Dr. (Committee Member)
Edward Evans, Dr. (Committee Member)
George Chase, Dr. (Committee Member)
Hamid Bahrami, Dr. (Committee Member)
259 p.
Chemical Engineering; Chemistry
In-situ FTIR spectroscopy, TiO2, Photocatalysis, Conductivity

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Citations

  • Lohrasbi, M. (2014). In Situ FTIR Study of the Photocatalytic Properties of TiO2 and Conductivity of Polyaniline [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1416220154

    APA Style (7th edition)

  • Lohrasbi, Mehdi. In Situ FTIR Study of the Photocatalytic Properties of TiO2 and Conductivity of Polyaniline . 2014. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1416220154.

    MLA Style (8th edition)

  • Lohrasbi, Mehdi. "In Situ FTIR Study of the Photocatalytic Properties of TiO2 and Conductivity of Polyaniline ." Doctoral dissertation, University of Akron, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1416220154

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Akron and OhioLINK.