단결정의 Brookite TiO2 나노구조 합성방법 개발 및 에너지 • 환경분야로의 응용 연구

Abstract

The population of world has increased rapidly since the industrial revolution caused advances in technology and medical science. The most important issue facing humanity that need to be solved is to supply sufficient energy to whole mankind. The fossil fuels, which have been the main fuel source, could not be the ultimate solution of energy crisis, because of limited amount of them and the environmental pollution which is caused by combustion of fuels. To replace the fossil energy, numerous studies have been investigated to find an alternative energy such as wind, hydroelectric, geothermal, hydrogen, biomass, and solar energy. The hydrogen is promising candidate for future energy, because it is the most abundant elements in the Universe, efficient fuel which produces three times as much energy per gram as gasoline, and environmentally friendly fuel that makes water after combustion with oxygen. Nowadays, 96% of hydrogen production is produced from fossil fuels by steam reformation or gasification due to economic reasons. However, the hydrogen energy produced by fossil fuels is not renewable energy in the true sense of the word because the greenhouse gas emissions occur through the process and the fossil fuels will be exhausted. An environmental pollution is another most important issue facing humanity. The release of industrial waste and toxic chemicals into the air and water threaten human health and ecosystem. Several methods including physical, biological and chemical approaches have been used for air and water purification, but they have some disadvantages such as low efficiency, secondary pollution, and economic infeasibility. The photocatalyst which uses solar energy and doesn’t produce secondary pollutant can be alternative solution of existing methods. TiO2 has been considered a promising material for various energy and environmental applications including solar cells, photoelectrochemical cells, and environmental photocatalyst due to its low cost, non-toxicity, appropriate bandgap, high photocatalytic activity, and chemical durability. TiO2 has three major crystalline polymorphs: rutile (tetragonal, space group: P42/mnm), anatase (tetragonal, space group: I41/amd), and brookite (orthorhombic, space group: Pbca). Rutile is a stable phase, whereas anatase and brookite are metastable phases that can be transformed to rutile when annealed. Most previous studies have focused on the anatase and rutile phases, while brookite TiO2 has scarcely been studied because the brookite phase rarely exists in nature and is difficult to synthesize. Because of the difficulty of obtaining pure brookite, growth mechanism and diverse properties including photocatalytic activity of it are still controversial. The aim of the present thesis is to examine undiscovered properties of the brookite and utilize it to resolve energy and environmental pollution challenges. In chapter 1, the motivation of this thesis and the theoretical background are introduced. The most urgent problems in the world which are energy and environmental crises are the motiviation of this research. Fundamental background knowledge for comprehensive understanding semiconductor photoelectrochemical cells and phocatalyst is discussed. In addition, a novel semiconductor material which is brookite TiO2 for energy and environmental applications is suggested. In chapter 2, a facile strategy to synthesize various TiO2 nanostructures including brookite phase is suggested. Different morphologies (nanowire, nanotube, nanosheet, and nanobullet) and crystal structures (anatase and brookite) of vertically aligned TiO2 nanostructures on titanium foil were obtained by a benign hydrothermal process. Especially vertically aligned brookite nanostructures were synthesized for the first time. They exhibit unique morphologies such as bullet with sharpened tip, and have an excellent crystallinity. Based on the experimental results with different concentration of NaOH, reaction time and temperature, a growth mechanism of various TiO2 nanostructures is proposed. In chapter 3, the possibility of vertically aligned brookite nanostructures as a photoanode is investigated. The photoelectrochemical (PEC) cells with two TiO2 nanostructures (anatase nanowire and brookite nanobullet) were fabricated, and their PEC properties such as photocurrent density, stability, and open circuit voltage were measured. Despite the shorter length and the smaller aspect ratio of the brookite nanoarrays, they generated a higher photocurrent density with good stability than the anatase nanoarrays due to their high conductivity and high-quality crystallinity which resulted in suppressed recombination and increased electron lifetime. In chapter 4, hydrogen doping of brookite nanostructures is attempted to enhance PEC properties. The research is designed to determine the effect of hydrogen doping in the brookite via the combined analysis of experiments and DFT theory. Hydrogen doped brookite (H:brookite) nanobullet arrays were synthesized via a well-designed solution reaction, and they shows highly improved PEC properties with excellent stability, enhanced photocurrent, and significantly high Faradaic efficiency for overall solar water splitting. To support the experimental data, ab initio density functional theory calculations were also conducted. At the interstitial doping site that has minimum formation energy, the hydrogen atoms act as shallow donors and exist as H+, which has the minimum formation energy among three states of hydrogen (H+, H0, and H-). The calculated density of states of H:brookite shows a narrowed bandgap and an increased electron density compared to the pristine brookite. In chapter 5, the unrevealed photocatalytic properties of brookite are studied. The photocatalytic degradation of RhB, TMA, and 4-CP on UV-illuminated pure brookite were investigated and compared with those of anatase and rutile. The present research explores the generation of OH radical as a main oxidant on brookite. In addition TMA, as a mobile OH radical indicator, was degraded with both pure anatase and brookite phases while it is not at all for rutile. The brookite showed the superior photoactivity among TiO2 polymorphs in spite of its smaller surface area compared to anatase. This result may be ascribed to the following properties of this brookite TiO2 film: (1) the higher driving force with more negative flat-band potential, (2) the efficient charge transfer kinetics with low resistance, and the generation of more hydroxyl radicals including mobile •OHf. The brookite nanoarrays facilitate collection and recycling of the photocatalyst with excellent stability, and exhibit accelerated decomposition rate with applied external bias. In summary, this thesis was undertaken to design the facile strategy to vertically aligned brookite nanostructures, suggest the growth mechanism of them, and explore the unrevealed properties of them. Furthermore, the brookite nanostructures were utilized to photoelectrochemical cell for overall water splitting, and environmental photocatalyst for decomposition of the pollutants. This thesis provides frameworks for the exploration of the PEC and photocatalytic properties of brookite and extend our knowledge regarding the undiscovered properties of it.