Synthesis of polyoxometalates-based materials for energy, environmental and biological applications

• Main Author: Ong, Boon Chong
• Other Authors: Dong Zhili
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Material testing and characterization; Science::Chemistry::Inorganic chemistry::Synthesis
• Online Access: https://hdl.handle.net/10356/155589
• Institution: Nanyang Technological University

Today, energy crisis and environmental pollution are among the most concerning global issues. The overdependence of fossil fuels for energy supply creates a problem because they are not sustainable for long-term usage. Thus, it is crucial to search for alternative energy sources. In addition, the burning of fossil fuels emits tons of harmful pollutants causing the increase in pollution level. Besides fuel combustion, the rapid industrial development in the recent decades also poses deleterious effects towards environment by the release of harmful industrial effluent. Direct contact with these toxic materials may harm our health and even cause cell mutation. Polyoxometalates (POMs), molecular oxide aggregates with diversified structure and chemical versatility, have potential to be applied in various application owing to their unique structures and properties. In the first part of the study, the potential of POMs in the energy sector was investigated, specifically the photoelectrochemical water oxidation, the coupled reaction in the production of hydrogen fuel. Cobalt-based POMs with cobalt and phosphorus as heteroatoms were immobilized on titanium dioxide (TiO2) by layer-by-layer method. At the theoretical water oxidation potential of 1.23 V vs. RHE, a high photocurrent density of 0.79 mA/cm2 was attained by POMs coated TiO2 photoelectrode, almost double that of TiO2 with a recorded value of 0.45 mA/cm2. Besides that, a negatively shifted onset potential and an improvement in the photocurrent efficiency from 0.24 % to 0.50 % was noticed for POMs modified TiO2 photoelectrode in comparison to pristine TiO2 photoelectrode. The optimum POMs thickness was determined to be 6 layers, in which the water oxidation performance started to decline with a further increase in POMs coating thickness due to the limitation of light penetration and inhibition of charge carriers mobility. Upon an in-depth investigation, the improvement in the overall reaction rate for POMs immobilized TiO2 photoelectrode could be elucidated by the ability of POMs to function as active water oxidation catalyst as well as effective hole extraction layer. In addition, the modified photoanode also manifested high stability over operational time of more than 10000 s, proving the rational design of TiO2/POMs composite film towards photoelectrochemical water oxidation application under a neutral environment. On top of the immobilization on TiO2, the as-synthesized cobalt-based POMs were coupled with bismuth vanadate (BiVO4) to catalyze the water oxidation reaction. In term of light capturing ability, BiVO4 is sensitive towards visible light irradiation, resulting in a broader range of light wavelength absorption that is crucial in the light stimulated reaction. The nascent BiVO4 was a mixed phase compound, consisting of both monoclinic and tetragonal structures, with an approximate dimension of 2.0 µm x 0.8 µm. The amount of oxygen evolved from BiVO4/POMs coupling system within 1 h reaction time was 189.7 µmol, comparable to the system without POMs but with a much longer reaction time of 5 h. The introduction of POMs into the reaction system had shorten the time in obtaining a similar product quantity, illustrating a much higher reaction rate was attained by the POMs-containing system. At the end of 5 h reaction, 372 µmol oxygen was collected from the BiVO4/POMs coupling system, which was roughly twofold that of BiVO4 system. The enhancement in the oxygen evolution reaction was owing to the positive synergic effect between the coupling materials. POMs as cocatalyst could extract the photogenerated holes from BiVO4 prior charge recombination and improve the charge transfer mechanism. In the next study, novel mixed-addenda POMs were synthesized by the addition of dopants into POMs. At first, cobalt-based POMs with cobalt and vanadium as heteroatoms were synthesized by a simple crystallization-driven self-assembly method. It was followed by the introduction of molybdenum as secondary addenda atoms into POMs framework. In the electrochemical water oxidation study, at the same applied voltage of 1.7 V vs. RHE, modified mixed-addenda POMs showed a current density of 2.35 mA/cm2, which was almost 3 times higher than pristine unmodified POMs of 0.79 mA/cm2. Furthermore, a lower onset potential of 1.62 V vs. RHE was attained by mixed-addenda POMs with a difference of 130 mV in comparison to original POMs with the recorded value of 1.75 V vs. RHE. All these positive results proved the ability of POMs to oxidize water effectively. Additionally, mixed-addenda POMs displayed a remarkable enhancement in the electrochemical water oxidation activity which could be elucidated by the presence of positive synergic effect between molybdenum and tungsten addenda atoms in POMs framework. The alteration of addenda atoms in POMs will modify their properties. Last but not least, experimental investigation was conducted to scrutinize the ability of POMs to behave as bifunctional materials, particularly in the environmental and biological sector. In the dye degradation test, the as-synthesized POMs exhibited excellent catalytic activity towards decolorization of methylene blue (MB) and rhodamine B (RB) dyes solution with the presence of peroxymonosulphate (PMS). The positive synergic effect between POMs and PMS was substantiated by a relatively meager degradation of less than 10 % in the system without POMs, in which POMs involved actively in the activation of PMS towards free radicals generation for dye degradation. MB and RB dyes were entirely decolorized within 60 min with the presence of 40 mg/L POMs and 150 mg/L PMS. The POMs/PMS system was pH dependance with a decrease in dye degradation efficiency at elevated pH. The effect of pH was more prominent in RB dye with a plummeted degradation efficiency from 93.3 % to 41.12 % with an increase in solution pH from 7 to 11. In the biological study, POMs showed non-toxic behavior towards normal cells that minimized the safety risk for the large-scale wastewater treatment application. Remarkably, POMs exhibited prominent anticancer property with more than 35% of A549 lung cancer cells and MDA-MB-231 breast cancer cells were killed with 250 mg/L POMs. In a comparative study, the killing efficacy of POMs at 250 mg/L is higher against breast cancer cells compared to lung cancer cells, highlighting the cell-type dependence of the cytotoxic activity. The selective lethality of POMs towards the cancer cells was inspected to be caused by differential extents of cellular apoptosis. In conclusion, this thesis presents the synthesis of POMs-based materials and the investigation of their prospect in diverse application. The findings from this study contribute to the advancement of POMs-based materials in the energy, environmental and biological sectors for future practical application.