Synthesis, Characterization And Visible-light Photocatalytic Activity Of Bismuth Oxide Iodides Hierarchical Micro/Nano-Architecture

Energy shortage and environmental pollution are two major challenges currently faced by human beings. Semiconductor photocatalytic materials show great potential for solar energy conversion and environmental protection, and have been widely applied in air and water purification, H₂ production by water splitting as well as dye-sensitized solar cells, etc., which have attracted increasing attention over the past two decades. However, the performance of most photocatalysts requires to be further improved due to their narrow light-response range as well as low quantum efficiency. Therefore, design and development of new and highly efficient visible light photocatalytic materials have become the focus of current research.Recently, bismuth oxyhalides (BiOX, X = F, Cl, Br and I) have drawn much attention because of their possessing unique structure of alternate [Bi₂O₂]²⁺ sheets with the X- slabs and the internal electric fields between positive slabs and anionic slabs, which are more effective in improving the separation of photoinduced electron-hole pairs, and therefore have demonstrated excellent photocatalytic activities and are offering a new family of promising photocatalysts. Among them, BiOI has the strongest absorption in the visible light region because it has the smallest band gap (¹.8 eV). Except for the BiOI, there are other complex bismuth oxyiodide compounds, which have a different ratio of Bi to I, including Bi₂O₄I₅, Bi₇O₉I₃, -Bi₅O₇I and?-Bi₅O₇I. Since the valence band in BiOX is mainly composed of Bi 6s, O 2p and X 5p orbits, while the conduction band is based on Bi 6s and Bi 6p orbits, the bandgap energy of bismuth oxyiodides may gradually increase by substitution of I with O in their structures, resulting in a lower bandgap than that of Bi₂O₃ (².8 eV). Consequently, all these kinds of bismuth oxyiodides are expected to demonstrate visible light photocatalytic activity and it is also interesting to obtain the different oxidation-reduction ability by tuning the ratio of Bi/I.In addition to its electronic and band structure of the semiconductor, microstructures of the materials, especially in size, three-dimensional structures, surface morphology, the ratio of crystal planes, porous and internal channels, are obviously influencing the performance, efficiency and applications of a specific photocatalyst. Generally, photocatalysts with three-dimensional (3D) micro/nano-structures based on the assembly of low-dimensional building blocks are expected to have enhanced photocatalytic performance although the synthesis of such complex superstructure remains a challenge for nanotechnology.In this dissertation, we focus on design and development of new and highly efficient bismuth oxyiodides as photocatalysts with visible light response and 3D hierarchical micro/nano-structures. The resulting materials were characterized carefully by various techniques, their structural formation mechanism was discussed in detail, and their photocatalytic activities were tested under the visible light irradiation.The main contents were shown as the following:1. Self-assembled three dimensional (3D) BiOI microspheres consisting of nanoplatelets were synthesized at low temperature using ethanol-water mixed solvent as reaction media and NH3·H2O as pH an adjustment. The as-prepared BiOI was characterized by XRD, SEM, TEM, UV-vis DRS, and BET. The possible formation mechanism for the architectures was discussed. It was found that the mixed solvents and alkali play key roles in the formation of the BiOI microspheres. The photocatalytic activity of the as-prepared sample was evaluated by degradation of phenol in water under visible light irradiation. The 3D BiOI microspheres show much higher photocatalytic activity than the random BiOI platelets. The TOC measurement after the degradation process indicated that phenol was effectively mineralized over the BiOI microspheres. In addition, the BiOI microspheres are stable during the reaction and can be used repeatedly. The high catalytic performance of the 3D BiOI microspheres comes from their narrow band gap, high surface area and high surface-to-volume ratio.2. A novel Bi₇O₉I₃ material with hierarchical nano/micro-architecture is successfully synthesized by a one-step, template and surfactant-free solution method. The as-prepared product was characterized by various techniques. XRD, XPS and TG measurements confirm that the composition of the as-fabricated sample is Bi₇O₉I₃. SEM and TEM observation reveals that the as-synthesized sample is microsized plate-like structure with dense nanosheets standing on their surfaces. The time-dependent morphology of the Bi₇O₉I₃ sample was investigated, and a possible formation mechanism of the hierarchical structure is proposed. More importantly, the Bi₇O₉I₃ exhibits an excellent photocatalytic activity than that of BiOI towards degradation of phenol under visible light irradiation. The high catalytic performance of the Bi₇O₉I₃ hierarchical structure comes from its electronic band structure, high surface area and high surface-to-volume ratio. In addition, the Bi₇O₉I₃ hierarchical architecture is stable during the reaction and can be used repeatedly. The present work not only gives insight into understanding the hierarchical growth behaviour of complex bismuth oxide iodides in a solution-phase synthetic system, but also provides a new way to improve the photocatalytic performance by designing desirable structures and morphologies.3. The hierarchical Bi₇O₉I₃ microplates, BiOI microspheres and BiOBr flowers were successfully synthesized via a facile, rapid and reliable microwave-assisted one-pot method, employing Bi(NO₃)₃·5H₂O, KI and CTAB as starting reagents without using any other additives. The as-prepared powders were characterized by XRD, SEM, and UV-vis DRS. The compositions, structures, morphologies and influential factors of the products were discussed based on the additional experiments. The photocatalytic activities of the as-prepared samples were evaluated towards the degradation of phenol solution under visible light irradiation. It was found that under the same conditions, Bi₇O₉I₃ microplates exhibited the best activity, BiOI microspheres showed the second and BiOBr flowers presented the worst. The high catalytic performance of the Bi₇O₉I₃ microplates comes from its relatively strong oxidative capacity, wider light-response range as well as hierarchical nano/micro-architecture. And its structure and activities remain unchanged after repeated four times. This work provides a facile, rapid, low-energy consumption route to prepare novel BiOX hierarchical architecture.