Bismuth is a metallic element from the Group 15 (VA) in the periodic table and it has a valence electron configuration of 6s26p3. Hence, to form a compound bismuth is likely to lose three electrons resulting in +3 charge. A common compound of bismuth is bismuth oxide (Bi2O3) and in the presence of vanadium or iodine, two of the bismuth oxide derivatives known as bismuth vanadate (BiVO4) or bismuth oxyiodide (BixOyIz), respectively, can be formed.
In this dissertation, three bismuth oxide-based compounds, namely, Bi2O3, BiVO4, and BixOyIz, have been synthesized by an evaporation-induced crystallization method, followed by chemical etching. To introduce porosity, ordered mesoporous silicas and alumina were employed as the hard templates. Alternatively, a modified method based on the synthesis of mesoporous carbon was adopted, during which bismuth nitrite was added as the bismuth oxide precursor.
The products have been characterized by various techniques including powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, which verified their successful preparation. Morphology and porosity were studied by electron microscopy and nitrogen adsorption-desorption measurements. The synthesized Bi2O3 and BiVO4 were firstly applied for iodide adsorption. Due to the scarcity of iodine as well as its threat to the environment if in excess, iodine as a waste needs to be captured. As compared with ion-exchange resins and silver-containing materials, which are two popular iodide adsorbents, bismuth-containing compounds show some important advantages, such as high iodide capture capacity and fast kinetics. In this dissertation, the influence of the pH, iodide/adsorbent ratio, temperature, crystallite size, and competing ions was explored from the viewpoint of optimization of the capture process. Further study of the iodide adsorbed bismuth compounds confirms that the capture of iodide by BiVO4 and Bi2O3 is a chemisorption process with the formation of bismuth oxyiodide (BixOyIz). Furthermore, iodide ions are able to penetrate into the bulk of BiVO4 and Bi2O3, which is believed to be responsible for their high capture capacity.
Diffuse reflectance spectroscopy measurements of Bi2O3, BiVO4, and the iodide-adsorption product BixOyIz confirmed that these bismuth compounds have small band gaps (2.13 – 2.94 eV), which make them visible-light-responsive photocatalysts. Photocatalytic experiments have been carried out by Cr(VI) reduction under simulated sunlight irradiation. Results revealed that BixOyIz has the best photocatalytic performance toward Cr(VI) reduction among the aforementioned three bismuth-containing materials. The apparent quantum efficiency was measured as 1.48% for a BixOyIz sample, which is among the highest without the use of a hole scavenger. When bismuth oxide was prepared in the presence of carbon precursors and a surfactant soft template, Bi2O3 nanoparticles were obtained and deposited on the resultant mesoporous carbon. In addition, carbon was found to be able to reduce Bi2O3 into metallic bismuth (Bi0), resulting in Bi0/Bi2O3/C composites, which showed an improved performance toward dye (methylene blue) degradation under daylight illumination as compared with bare Bi2O3.
Overall, this dissertation presents fundamental study of the synthesis, characterization, adsorption and photocatalytic applications of some popular bismuth oxide-based materials, and may be helpful for the development of other metal oxide compounds for environmental and energy-related applications.