Flame synthesis of nanostructured TiO2 for energy and environmental applications

• Main Author: Wu, Shuyang
• Other Authors: Xu Rong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Nanostructured materials; Engineering::Nanotechnology
• Online Access: https://hdl.handle.net/10356/146558
• Institution: Nanyang Technological University

Energy shortage, global warming and air pollution have become the main energy and environmental issues over the past few decades. Numerous strategies have been explored to solve these problems, such as generation of energy from green renewable resources to replace traditional fossil fuels, conversion of CO2 to chemical fuels and degradation of air pollutants to benign products. Among these methods, photocatalytic water splitting, CO2 reduction and volatile organic compound (VOC) degradation have been widely studied as promising ways to convert solar energy to chemical energy and eliminate the air pollution because sunlight, water, and CO2 are abundant on earth and the reactions take place in mild conditions. For these photocatalytic applications, semiconductor materials are widely used as the photocatalysts due to the unique band structure with separated electrons and holes. Among the various semiconductors, TiO2 has been a benchmark material for the reactions because of many advantages such as excellent photocatalytic activity, high abundance, good stability and low toxicity. However, the applications of TiO2 are largely obstructed by the problems of high recombination of the charges and poor absorption of visible light. Hence, various strategies have been used to modulate the properties of TiO2 to improve the charge separation efficiency and enhance the visible light utilization such as fabrication of the hetero-phase junctions and disordered structure with defects. The overall goal of this thesis is to apply flame synthesis method to fabricate TiO2 nanoparticles with controllable phase composition and defect states, which can be utilized for efficient photocatalytic applications. Firstly, this research work is focused on the fabrication of mixed-phase TiO2 using flame synthesis method for photocatalytic H2 generation. The TiO2 nanoparticles composed of anatase, rutile and TiO2-II phases are prepared via the one-step flame stabilized on a rotating surface (FSRS) method. The phase composition can be directly tuned by adjusting the flame conditions. The optimized sample with rutile/anatase/ TiO2-II mixed phases exhibits excellent photocatalytic H2 evolution rate of 21.9 mmol h-1 g-1 with extra-low 0.1 wt% Pt loaded. The apparent quantum efficiency (AQE) can reach as high as 39.4% at 360 nm. The less studied TiO2-II phase is studied for photocatalytic H2 evolution for the first time. It is found that a small amount of TiO2-II in the mixed-phase TiO2 could efficiently promote the charge separation and transport. The flame-made TiO2 with surface defects could efficiently anchor the ultra-small Pt clusters of around 0.63 nm, even with single atom Pt identified. The percentage of zero valence Pt is largely enhanced on the optimized sample compared with P25 TiO2, which improves the utilization of noble metals. Another strategy to improve the charge separation and the visible light absorption is to fabricate the non-stoichiometric TiO2-x. Thus, in the second part of this work, we for the first time apply the facile FSRS method for fast fabrication of TiO2-x with controllable defect quantity and position. The tuning of the defects can be easily achieved by varying the deposition time. Besides, temperature programmed oxidation (TPO) analysis was applied for the first time and found to be an excellent technique in both differentiating and quantifying oxygen vacancies (OVs). Photocurrent density and photocatalytic H2 evolution results consistently indicate that a moderate level of OVs can greatly enhance the charge transfer. Importantly, the OVs locked at GBs can facilitate the anchoring and reduction of Pt species. Moreover, the presence of Ti3+ and defects has a significant impact on the conversion of CO2 to CO with the highest photocatalytic activity of 44.7 mol h-1 g-1 on TiO2-6 min. As revealed by temperature programmed desorption (TPD), the defective surface of TiO2-x can promote the adsorption and the bindings of CO2. This work demonstrates that flame synthesis can be used as a facile and scalable process to obtain TiO2-x with controllable defects. Next, the performance of TiO2-x is evaluated by isopropyl alcohol (IPA) photo-oxidation. The optimized sample TiO2-x-20 min shows the highest IPA mineralization yield of 22.7% under visible light in 6 hours while 98.8% mineralization yield is achieved within 70 min under UV-Vis irradiation. The role of O2•-, h+ and •OH radicals in the steps of IPA degradation was systematically investigated. The result indicates h+ and O2•- radicals play an essential role in the overall degradation process while the •OH radicals mainly accelerate the degradation of acetone to CO2. Lastly, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method is applied to investigate the reaction mechanisms of CO2 photoreduction and IPA photo-oxidation on flame-made TiO2-x samples. The in situ DRIFTS study on CO2 photoreduction reveals that the surface OVs facilitate the chemical activation of CO2 molecules through one-electron injection process. This results in the formation of the key intermediate CO2- species which facilitates the CO2 photoreduction. The in situ DRIFTS study of the IPA photo-oxidation reveals that the dissociative chemisorption of IPA is greatly improved by surface OVs, forming the isopropoxide species. Formate and acetate species are identified as the key reaction intermediates and their production is largely promoted by TiO2-x with optimum OVs. Therefore, this part of work provides an in-depth understanding of the photoreaction mechanisms for CO2 reduction and IPA oxidation on defective TiO2-x. It provides a correlation between the surface chemistry and the photocatalytic activities which further benefits the photocatalyst design. In summary, the research works in this thesis mainly focus on the flame synthesis of ultrafine TiO2 nanoparticles with tunable properties (i.e., phase composition, surface chemistry, oxygen vacancy defect content and distribution) and exploring their photocatalytic applications in water splitting, CO2 reduction and IPA oxidation. In particular, the fabrication of multiple-phase junctions and a moderate level of defects in TiO2 is proved an effective approach to improve the charge separation and transport. The presence of surface defects significantly enhances the reactant species adsorption, chemical activation and subsequent conversion. It also improves the noble metal utilization by promoting the deposition and reduction process. It is believed that the research findings in this work would provide valuable insights in controlling the properties of metal oxides by flame synthesis conditions and development of such materials for a variety of energy conversion and environmental applications. In overall, this thesis contributes to bridging the gap between the flame synthesis of nanostructured materials and the applications of these materials in the field of catalysis.