Abstract
The water crisis has been an issue in South Africa and the whole world. The increase in population limits the supply of freshwater leading to a lack of clean water. Most of the available water is contaminated with various pollutants such as organics, inorganics, and microbial. Inorganic pollutants, in particular, are of interest because they are toxic and non-biodegradable. Anions and metals are inorganic pollutants of main environmental concern. Specifically, in this work, the presence of the two toxic selenium oxidation states, i.e. Se(IV) and Se(VI) in water is the main concern. Amongst the methods investigated for the removal of these anions from wastewater advanced oxidative processes (AOPs), specifically photocatalysis is the most attractive technique. In this work, the challenge is designing a visible light active, efficient, and reusable photocatalyst for the reduction of toxic Se(IV) and Se(VI). Metal-organic frameworks (MOFs) have attracted worldwide attention in photocatalysis application due to their outstanding properties, more specifically UiO- 66 because of its stability. However, UiO-66 suffers from low separation efficiency of photogenerated charge carriers and can only absorb UV light. UiO-66-based heterostructure have become a solution to enhance photocatalytic properties of UiO-66. In this study a series of FL-BP/UiO-66 heterostructures with various contents of UiO-66 were prepared by sonication method. Using X-ray diffraction (XRD) and selected area electron diffraction (SAED), the synthesized UiO-66 was found to be amorphous whereas Few Layers of Black Phosphorus (FL-BP) was crystalline. The XRD patterns of the heterostructures had characteristic peaks of both starting materials indicating that the heterostructures were formed. Laser Raman spectroscopy and Fourier transform-infrared (FTIR) spectroscopy revealed functional groups typical of both UiO-66 and FL-BP whereas the elements attributable to these materials were observed from X-ray photoelectron spectroscopy (XPS) and electron dispersive spectroscopy (EDS). Transmission electron microscopy (TEM), scanning electron microscopy (SEM) showed that the heterostructure composed of both UiO-66 and BP, with the nanocubes of UiO-66 embedded on the nanosheet of BP. A redshift in the position of absorption peaks of the heterostructures relative to that of UiO-66 was observed indicating that the v intended consequence of compositing UiO-66 with FL-BP was achieved. On the other hand, photoluminescence (PL) and electrochemical (EIS) studies showed that the heterostructures had lower electron-hole recombination rates compared to virgin UiO-66 and FL-BP. The Mott-Schottky plots confirmed p-n type heterojunction. Formation of the heterostructure promoted the separation of photogenerated charge carriers and enhanced visible light absorption. The conditions (solution pH, the mass of photocatalyst, pollutant initial concentration, and reductant concentration) in which the activities of the heterostructures towards the photoreduction of Se(IV) and Se(VI) were assessed were optimized using virgin UiO-66. Using these conditions, it was found that the heterostructures had higher photocatalytic activities compared to pristine UiO-66, FL-BP, and bulk BP. Heterostructure FL/U(40) showed the highest photocatalytic activity of all the materials tested. The reduction of Se(IV) and Se(VI) followed the first-order reaction. Also, FL/U(40) was shown to perform consistently high over 5 photocatalytic cycles with a total loss in activity of less than 10 %. The increased photocatalytic activity exhibited by the composites was attributed to a combination of phenomena that occurred upon the excitation of the heterostructures. These include the ability of the materials to absorb visible light and the cross-flow of the electrons and holes at the interfaces of the two materials.
M.Sc. (Chemistry)