Abstract
According to the United Nation Environment Program, the global demand in clean water is expected to be increased by thirty percent by 2050 and already two billions of people are lacking safe water on a daily basis [1]. Air pollution is another global challenge to be faced as nine out of ten people now breathe pollute air, which leads to death from stroke, lung cancer and heart disease [2]. In order to tackle these challenges, the unique physical and chemical properties of nano-materials have been explored by academia and industry. Since pioneering discovery of photo- and electrochemical water splitting in 1972, titanium dioxide (TiO2) has been extensively studied and used as a photocatalyst to overcome environmental and energy problems. However, TiO2 has a wide band gap of 3.2 eV and can be excited only by using UV light. Considering the abundance of solar energy, the development of visible-light-sensitive photocatalysts would extend applications of nano-materials to water and air treatment, self-cleaning surfaces and semiconductor industries. There are several methods to prepare visible-light-induced photocatalysts. The first strategy extends light absorption of TiO2 to visible range either through O site doping (with C and N) or through metal site doping, where Ag, Au, Pd, Pt, etc. introduced. Depending on the type of dopant, the additional energy levels reduce the band gap and make electron easier to be transferred from the valence band (VB) to the conduction band (CB). The second strategy involves the direct utilization of catalysts with visible light absorption properties. In order to be suitable for solar energy and environmental applications, the photocatalytic material should have the band gap below 3.0 eV and be stable in water and under visible light. Other methods are determined by synthesis conditions and include morphology control, surface modification, construction of p-n hetero- and homojunctions. Ideally, the combination of all available methods provides the most efficient photocatalytic materials for water treatment and solar energy applications. Among the promising candidates is the family of Bi-based oxides. Their electronic structure includes VB consisting of hybridized O 2p and Bi 6s levels (and contributing to narrow band gap), while TiO2 has only O 2p orbitals. It was shown, that dispersed Bi 6s orbitals decrease the band gap, favoring charge carrier mobility and oxidation reactions. As a result, the vast majority of Bi-based catalysts has the band gap below 3.0 eV. However, the photo-performance of bulk Bi-based catalysts is not as high as those of nanosize Bi-based oxides, because the photoelectrons and photo-holes of these materials are not easy to utilize. Current thesis systematically studies nanosize bismuth vanadate (BiVO4), bismuth niobate (BiNbO4), bismuth oxide (a-Bi2O3 and ß-Bi2O3), and their modifications towards degradation of pollutants under visible light. Research and development of visible-light-driven catalysts still remains the great challenge as the majority of lab-scale trials fails at the experimental stage. Thus, it is crucially important to develop a semi-empirical tool to predict the efficiency of water splitting and photocatalytic degradation of pollutants even before designing the catalyst with desired properties. Such approach would help to (1) reduce the development expenses and (2) minimize the timespan between the idea and final product. There is no doubt that research investigations in this field are of a great interest and very desired. There are two main objectives of this work. The first objective is to improve the photocatalytic properties of selected visible-light-driven nanosize Bi-based materials, such as BiVO4, BiNbO4, and Bi2O3. It was achieved through the catalyst-centric approach, such as cationic or anionic site doping and control of synthesis mechanisms, including morphology control, surface modification and construction of p-n hetero- and homojunctions. The photocatalytic system-centric approach covers optimization of catalyst’s amount and dye’s concentration, pH level of the system, light intensity/irradiation time and reaction kinetics. The second objective is to introduce and validate the dye degradation predicative tool based on the universal ability of chemical species to acquire electrons and, consequently be reduced. In such approach of “rational design” the reduction potentials of catalysts, organic dyes (pollutants) and reactive oxygen species (ROS) are mapped out versus normal hydrogen electrode (NHE). Energy bands of catalysts consist of VB and CB values, while organic dyes consist of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels. The efficiency of photo-dye degradation is proposed based on the relative positions of ROS, pollutants’ and catalysts’ energy bands. Furthermore, the strategy of “rational design” assists in explaining the mechanism of dye degradation (photocatalytic or photo-oxidative pathway), emphasizing on the role of electrons, holes, and ROS and their interaction with HOMO/LUMO levels of dyes. The strategy of “rational design” was successfully tested and verified on three different catalysts towards degradation of dyes under visible light.