| Author: | Cai, Haozhao |
| Title: | Study of ferroelectric nanosheets for water purification applications |
| Advisors: | Kwok, K. W. (AP) |
| Degree: | M.Phil. |
| Year: | 2024 |
| Subject: | Water -- Purification Photocatalysis Nanostructured materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xiii, 95 pages : color illustrations |
| Language: | English |
| Abstract: | The photocatalytic oxidation process has been widely adopted for the decomposition and degradation of organic pollutants, as well as for the generation of hydrogen through the splitting of water molecules. Due to its environmentally friendly characteristics and economic advantages, this process is gaining increasing significance in the treatment of various wastewater types, particularly those originating from the textile, pharmaceutical, and agricultural industries. However, the practical application effectiveness of semiconductors has been restricted by their inherent limitations, including a wide bandgap, low light absorption capability, and a high electron-hole recombination rate. These factors collectively impede their photocatalytic efficiency. Ferroelectric materials, characterized by a polar unit cell and the ability to display reversible spontaneous polarization upon the application of an external electric field, have garnered significant attention. On the other hand, there is a mounting interest in exploring the impact of ferroelectric polarization on adsorption properties, encompassing both physical adsorption and chemical adsorption. In this study, our objective is to explore the potential of ferroelectric materials for water purification through adsorption and photocatalysis. In this work, bismuth layer-structured ferroelectric Na0.5Bi4.5Ti4O15 (NBiT) has been chosen as the based material due to its layer structure with strongly anisotropic ferroelectric properties along the c-axis. Also, it is suggested that the reduced the optical band gap and particle size can be promoted through acceptor doping. Fe3+ was chosen as the dopant for replacing Ti4+ in the NBiT nanosheets because they have comparable radii. 0, 1, 2 mol of Fe3+ and 0, 0.5, 1, 2 mol of excess Fe3+ are doped in NBiT nanosheets to fabricate Na0.5Bi4.5FexTi4-xO15 + yFe (NBiFeT x-y, x=0,1,2; y=0,0.5,1,2) nanosheets. To investigate the appropriate mole ratio of the salt mixture and product in molten salt synthesis technique, NBiFeT 0-0 is synthesized using mole ratios of 1:15:15, 1:45:45, and 1:60:60. Based on a comprehensive scanning electron microscope (SEM) analysis, the optimal salt-to-product ratio as 1:60:60. As substantiated by X-ray diffraction (XRD) patterns, all the NBiFeT nanosheets have the standard crystal structure of Na0.5Bi4.5Ti4O15. Furthermore, an expansion of the lattice spacing of NBiFeT nanosheets is resulted after the Fe (exact) doping, resulting in a shift of the XRD pattern towards lower 2θ values. Moreover, the doping induces a reduction in the particle size. Interestingly, it has been found that an excessive amount of Fe leads to the creation of BiFeO3 (BFO), which can improve the adsorption capability. The TEM analysis shows that the exposed plane of the NBiFeT 0-0 nanosheets is primarily {001} facet. The ferroelectric property has also been investigated by the dielectric properties and P-E loop measurements, in the form of ceramic disk. As the temperature increases from room temperature to 100°C, observed remnant polarization (Pr) increase from 0.3 µC/cm² to 1.39 µC/cm², while the observed saturated polarization (Ps) from 1.4 µC/cm² to 3.3 µC/cm². When Fe or an excess amount of Fe is doped into the NBiFeT 0-0 nanosheets, the band gap decreases significantly. Undoped and Fe-doped NBiFeT nanosheets have been studied for their photocatalytic performances. The results demonstrate a significant degradation rate of 98% for RhB after 180 minutes of irradiation with NBiFeT 0-0 nanosheets. A hypochromic shift is observed in the absorption maximum of the RhB solution during the photocatalytic degradation process. The de-ethylation mechanism of RhB serves as a reliable indicator for the photocatalytic degradation process. Also, the investigation indicates that the inherent spontaneous polarization within the NBiFeT x-y nanosheets contributes to the enhancement of their photocatalytic performance. Moreover, an investigation into the effectiveness of Fe-doped NBiFeT 0-0 nanosheets for photocatalytic degradation. The degradation efficiency of RhB for NBiFeT 1-0 and NBiFeT 2-0 are 92% and 67%, respectively. About the effect of excessive Fe doping on photocatalytic degradation, the degradation efficiencies of RhB are measured as 98%, 96%, and 90% for NBiFeT 0-0, NBiFeT 0-0.5 and NBiFeT 0-2, respectively. The results highlight a decrease in photocatalytic performance as the increased level of excess Fe doping. Moreover, the results of trapping experiment of active species suggested that superoxide radicals (•O2-) is the main species in RhB photocatalytic degradation. Then, the effect of pH values on photocatalytic performance is examined. The surface activity of NBiFeT 0-0 nanosheets exhibit optimal behavior at a pH of 7.1. Conversely, under alkaline conditions with a pH of 11, the abundance of hydroxyl groups (OH−) hinders the occurrence of the adsorption effect. The adsorption capability of Fe-doped NBiFeT 1-0 nanosheets is investigated. Among all the nanosheets, the NBiFeT 1-1 nanosheets exhibit the highest adsorption capability, which can adsorb 92% of RhB. The adsorption capability increases after Fe doping, while the adsorption efficiency of NBiFeT 0-0 is 7% and NBiFeT 1-0 is 67%. On the other hand, the adsorption capability of NBiFeT 2-0 is measured as 22%. The decrease in adsorption capability observed in NBiFeT 2-0 nanosheets should be attributed to the retardation caused by dye molecules within the pore channels. To study the effect of excess Fe doping on adsorption, NBiFeT 1-y (y=0,0.5,1,2) nanosheets have been used. The result demonstrates that by increasing the level of excess Fe doping, the adsorption capability increased. This should be attributed to the high concentration of Fe, which has the potential to react with Bi during the synthesis process, leading to the formation of BiFeO3. It has been reported that BiFeO3-coated material can enhance the adsorption capability. Additionally, the pH value of the RhB solution also exerts an influence on the adsorption. In an alkaline system, almost no dark adsorption occurs. Conversely, the adsorption capability experiences an enhancement in an acidic environment attributed to the attractive force between nanosheets and RhB dye. Finally, Ni and Eu are doping into the NBiFeT 1-1 nanosheets to enhance their magnetic properties. The Ms value increases from the initial 0.12 emu/g to 1.22 emu/g after the Ni, Eu doping. However, it is important to note that the adsorption capability of NBiFeT 1-1 decreases from 92% to 81% after the magnetism enhancement. |
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| Access: | open access |
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