Abstract
Water pollution caused by the release of organic contaminants such as dyes, pharmaceutical pollutants and pathogenic bacteria has become a huge concern to rural and urban communities. All these contaminants are considered hazardous. Therefore, it very crucial to remove them before reaching rural and urban communities for human consumption. The aim of the study was to explore the properties of piezoelectric/ferroelectric based materials and their ability to degrade or remove organic dyes, pharmaceuticals in wastewater using photocatalytic, piezocatalytic and piezo-photocatalytic methods. Furthermore, the antibacterial activity of piezoelectric/ferroelectric based materials was investigated. The materials were prepared using various methods such as photoreduction, green synthesis, hydrothermal and Dr Blade (tape casting). Characterization techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL) and UV-diffuse reflectance spectroscopy (UV-DRS) were used to study the structures, morphologies, and optical properties of the piezo-photocatalyst thin film. The surface area and thermal stability of the prepared materials were determined using Brunauer-Emmett-Teller (BET) and thermogravimetric analysis (TGA). Energy dispersive X-ray spectrometry (EDX) technique was employed to investigate elemental composition of the as-synthesised materials. The piezoelectric and piezoelectrochemical properties of the piezo-photocatalyst thin film electrodes were evaluated using electrochemical impedance spectroscopy (EIS), chronoamperometry, and Mott-Schottky plots. These studies were performed prior to piezo-photocatalytic degradation of organic pollutants and removal of bacteria.
In the first section, Silver (Ag) doped barium titanate (BaTiO3) deposited on fluorine-doped tin oxide (FTO) glass via tape casting technique was employed for piezo-photocatalytic removal of methylene blue (MB) and ciprofloxacin (CIP). The piezo-photocatalyst (BaTiO3/Ag) was prepared via the hydrothermal method followed by the photoreduction method. The UV-DRS revealed the SPR (Surface Plasmon Resonance) of Ag nanoparticles on the surface of BaTiO3 (BTO) at 505 nm. TEM images showed the average size of Ag nanoparticles on the BaTiO3 surface was 10-15 nm. Under ultrasonic
vii
irradiation, the composite thin film achieved a piezocatalytic degradation efficiency of 90 % for MB under basic condition (pH 10). The result indicated that FTO substrate was an appropriate conducting substrate for BTO towards the piezo-photocatalytic degradation process. The FTO substrate was further employed as a conducting substrate for the deposition of BTO/AgNPs composite to produce FTO/BTO/AgNPs piezo-photocatalyst thin film. Under synergistic effect (ultrasonic and light irradiation), piezo-photocatalyst thin film composite resulted in the highest degradation removal of 72 and 98 % for CIP and MB, respectively. The findings indicated that Ag doping and the synergistic effect (coupling of piezocatalysis and photocatalysis) could supress electrons and holes recombination within BTO/AgNPs, thus resulting in an increased efficiency of removal.
Subsequently, in the second section (Chapter 5), another BaTiO3-based catalyst was prepared and employed for piezo-photocatalytic degradation and antibacterial disinfection. A BaTiO3/SnO2 heterojunction composite with different SnO2 content was tape casted on FTO substrate. Chronoamperometry and electrochemical impedance spectroscopy (EIS) were employed to investigate piezo-electrochemical properties of the prepared piezo-photocatalyst thin films. The FTO/BTO@0.2%SnO2 exhibited a higher piezo-electrochemical current than pristine and other composites, suggesting strong internal piezoelectric field which separated electrons and holes from recombination, resulting in enhanced piezo-photocatalytic performance. This was further confirmed by charge transfer resistance (Rct) which was obtained from Nyquist plots. Piezo-photocatalytic removal of methyl orange (MO), methylene blue (MB) and ciprofloxacin (CIP) using FTO/BTO/SnO2 thin film was investigated. The FTO/BTO@0.2%SnO2 showed the highest piezo-photocatalytic degradation efficiencies of 94, 92 and 64 % and TOC removal of 86, 73 and 48 % for MO, MB and CIP, respectively. The values were significantly higher than those obtained from a pristine FTO/BTO thin film, which further confirmed that the formation BTO/SnO2 heterojunction composite can enhance piezo-photocatalytic removal. In addition, the BTO@0.2%SnO2 composite showed a strong antibacterial efficiency towards gram-negative E.coli than gram-positive S.aureus.
In the final results section (Chapter 6), a tape casting method was used for the deposition of a tri-component composite (Sb-ZnO/MoS2) on the surface of FTO substrate. The tri-component composite was applied for piezo-photocatalytic degradation of MB, MO and
viii
CIP, as well as for removal of pathogenic bacteria in wastewater. Due to a better charge separation, the tri-component composite (FTO:Sb-ZnO/MoS2) achieved the highest piezo-photocatalytic removal percentages of 95, 82 and 72 % for MB, MO and CIP, respectively. The degree of mineralisation percentages obtained from the TOC analyser were recorded to be 76, 70 and 52 % for MB, MO and CIP, respectively. For bacterial disinfection, it was found that the tri-component composite (Sb-ZnO/MoS2) inhibited the bacterial growth of both gram-negative and gram-positive bacteria (E.coli and S.aureus) in wastewater.
Based on the results obtained using the various materials, the study showed that piezoelectric/ferroelectric based thin films are highly effective in piezo-photocatalytic removal of various organic pollutants including dyes and pharmaceuticals in water. Furthermore, these materials could also be used for bacterial disinfection. Therefore, this makes them highly suitable for use in real-world wastewater treatment.