Abstract
This work investigated the application of bismuth ferrite perovskite, its composites and heterojunction as photoanodes in removal of organic pollutants from water using photoelectrochemical process. The various materials used were synthesised via hydrothermal method, electrodeposition method and solid-state method. Their structural, optical and morphological properties were studied using x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Visible diffuse reflectance spectroscopy (UV-VIS DRS), field emission scanning electron microscope with energy dispersive x-ray spectrometry (FESEM/EDX). The photoanodes were electrochemically and photoelectrochemically characterised using electrochemical impedance spectroscopy (EIS), Mott Schottky (MS) analysis, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry experiment. The extent of degradation, mineralisation and degradation products were measured with UV/vis spectroscopy, total organic carbon (TOC) and ultra-high performance liquid chromatography-mass spectroscopy (UPLC-MS).
Bismuth ferrite perovskite (BFO, BiFeO3) doped with lanthanum on to fluorine doped tin oxide as substrate (La-BFO) was successfully synthesised via hydrothermal method with low calcination temperature. The result shows that all La doped BFO exhibited greater absorption in the visible light region, and the optimum photoanode (10% La-BFO) showed greater photocurrent response about three times higher than the pristine BFO. The BFO and 10% La-BFO photoanodes were employed in degradation of 5 mg/L orange II dye and the removal rate were 55.3% and 84.2% respectively within 120 min with applied potential of 2 V. Also 10% La-BFO was used for the degradation of 5 mg/L Congo red, methylene blue, sulfamethoxazole and acetaminophen in aqueous solution and the removal rate showed 76, 93.7, 36.8 and 54% respectively. These results reveal that the composite formation of La-BFO increased the absorption of visible light owing to reduced bandgap by the formation of impurity level which brings about a shift in the Fermi level thus suppressing the recombination of photogenerated electron-hole pair and improving the charge transfer resistance thereby increasing the photoelectrocatalytic (PEC) efficiency. Another milestone involved the decorating the BFO with graphite nanoparticles (GNPs) to improve its properties. With high current mobility and high thermal conductivity exhibited by GNPs, incorporating GNPs into BFO would enhance the photocurrent response and suppression of recombination of photogenerated charge carrier exhibited by BFO by providing trapping sites to encourage excellent charge carrier separation. The results show marked enhancement in the photocurrent density and low charge transfer resistance, thanks to the excellent electrically conductivity of GNPs. The degradation rate of 48, 50 and 78% with TOC removal of 30, 38 and 56% was observed in removal of 5 mg/L ciprofloxacin in aqueous solution using bias potential of 1.5 V for 4 h. These results showed the synergistic effects of doping BFO with GNPs and the performance of PEC compared photocatalysis (PC) and electrochemical (EC) processes. It also highlights the importance of incorporating GNPs into a semiconductor to harness its excellent properties. The ciprofloxacin aliquots were analysed using UPLC-MS to obtain degradation product. The intermediate obtained followed direct oxidation of piperazine ring. The reusability study and effects of bias potential applied proved the stability of the photoanode prepared.
Furthermore, BFO was coupled with bismuth vanadate (BVO) to form a p-n heterojunction which was prepared using hydrothermal and solid-state method. The BFO/BVO heterojunction photoanode showed greater photocurrent response than the individual semiconductors. The Mott-Schottky plot was used to calculate the charge carrier density and we observed that the heterojunction exhibited greater charge concentration than pristine semiconductors. These improvements were attributed to an excellent band alignment which aid the facile transfer of electrons to the cathode thereby suppressing electron-hole pair recombination and providing ample lifetime for the electron to participate in reduction of pollutants. The removal rate of ciprofloxacin for BFO, BVO and BFO/BVO was 56%, 49% and 80.3% respectively using bias potential of 2V for 4 h.
Furthermore, to improve on catalyst adhesion on the substrate and to minimize the leaching of the catalyst into the solution, a chemical bath electrodeposition method was employed owing to its facile, homogeneity and wide area coverage. Thin films of BFO were successfully electrodeposited using citric acid as chelating agent. The photoanodes labelled 300s, 600 s, 1200 s, 1800 s and 3600 s were prepared and characterised. The FESEM cross sectional area was used to measure the thickness of the film. Results showed that the bandgap were increasing as the electrodeposition time increases. Result showed that photoanodes with shorter electrodeposition time exhibited better absorption of visible light, greater photocurrent response and improved charge transfer and as a result suppressed recombination of photogenerated electron-hole pair more than 1800 s and 3600 s photoanodes due to agglomeration. The electrodeposition approach improved electrode stability in the presence of high voltage while mitigating the problem of leaching of catalyst during degradation process.
The findings in this study reveal that bismuth ferrite composites and heterojunction photoanode are excellent photoanodes in photoelectrochemical degradation of organic dyes and recalcitrant pollutants such as pharmaceuticals. This thesis shows that these materials have potential for real life application in wastewater remediation through photoelectrochemical process.