For the past few decades, a frenzy of attention has been given towards the development of an assortment of photocatalysts as a solution for various environmental problems. TiO2 is the most widely used photocatalyst. TiO2 is biocompatible, chemically and thermally stable but TiO2 and a vast majority other photocatalysts have large band gaps, and hence they find applicability only in the UV region of the solar spectrum. These large band gap photocatalysts suffer a severe limitation with regard to their overall process efficiency as the UV region contributes to about 3 to 4 % of the solar spectrum in terms of energy.
This thesis concentrates on the progress towards the generation of visible light active photocatalysts. Lanthanum transition metal perovskites were synthesized to incorporate B site doping in the following fashion; LaCrxFe1-xO3, LaMnxFe1-xO3 and LaCrxMn1-xO3 (x= {1, 0.25, 0.5, 0.75}). These perovskites configurations were selected as LaCrO3 has a conduction band edge suitable to activate most photocatalytic reactions, but LaCrO3’s large band gap energetically hinders the photocatalysis. Doping LaCrO3 with Fe and Mn allowed for tuning their band gaps and made various photocatalytic reactions feasible, namely CO2 reduction and photoassisted dye degradation.
Validation of the perovskite's crystal structure was established through the study of their XRD patterns. The perovskite exhibited crystallinity throughout all doping concentrations. At some doping concentrations, due to low or high degree of tolerance factor, the presence of hexagonal and rhombohedral crystal phases was seen.
Analysis of the electronic structure of these perovskites was conducted through diffuse reflectance spectroscopy measurements and electrochemical impedance spectroscopy. Doping transition metals in B site of the perovskite led to the narrowing of band gap energy with the increase in the concentration of the higher atomic number transition metal. About 38% reduction in band gap was achieved in LaCrxFe1-xO3. The band gap constituted of Mott- Hubbard gap and charge transfer gap.
For the species LaCrxFe1-xO3, interband states exist with an energy gap as large as 1.3 eV for X=1 and 0.75. These states manifested as Urbach tails and are clearly documented in the absorption spectrum data. At x=0.5 and below, evidence of mixing is seen in LaCrxFe1-xO3, leading to the diminution of these interband states, although not to full extent, their energy was reduced by about 0.5 eV. In LaCrxMn1-xO3 and LaMnxFe1-XO3, the absence of Urbach tail and absorption edge is observed.
The band edge positions of most of these perovskites provided a large enough over potential to cause the reduction of CO2. Future efforts on the photocatalytic activity study of these perovskites through dye degradation and CO2 reduction are in progress. Preliminary results of photoassisted dye degradation are shared in this thesis.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-6977 |
| Date | 16 September 2015 |
| Creators | Suresh, Divya |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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