The Preparation And Photocatalytic Performances Of Bismuth Oxide Based Photocatalysts

With the rapid development of economy, the global environmental pollution and energy shortages isbecoming serious problem which will restrict the development of human society. To remediate waterpollution, scientists throughout the world have developed several methods, including biological methods(such as activated sludge process, biofilm process, biological aerated filters, membrane bioreactors),physical-chemical methods (such as ion-exchange method, reverse-osmosis), advanced oxidation processes(such as photochemical oxidation, catalytic wet air oxidation, ozone oxidizing method, electrochemicaloxidation process, Fenton oxidation process, semiconductor photocatalysis). Among these methods,semiconductor photocatalysis offers a promising solution for energy shortage and environmental pollutionsince it’s mild reaction conditions, small selectivity (completely eliminating all kinds of contaminants), andit can decompose water into hydrogen and oxygen, inactivate viruses and/or completely eliminate all kindsof contaminants under the illumination of sunlight under ambient conditions. A prerequisite for thedevelopment of photocatalysis application is to gain access to photocatalysts with high efficiency. Currently,many types of photocatalysts have been investigated and developed, and semiconductor TiO2hasundoubtedly proven to be one of the most excellent photocatalysts. Unfortunately, due to its wide band gap(3.2eV), TiO2can only be excited by ultraviolet or near-ultraviolet radiation, which accounts for only ca.4%of the solar light spectrum. In order to efficiently utilize solar light in the visible region (700> λ>400nm), many groups investigated and developed different kinds of visible-light-driven (VLD) photocatalystsfor photocatalytic degradation of organic pollutants and disinfection of microorganisms, all of thesephotocatalysts exhibit high photocatalytic performance under visiable light irradiation, such as some simple oxide (Bi2O3and WO3) and sulfide (CdS), complex oxide (Bi2WO6, BiVO4, Bi2MoO6), nitride(C3N4,Ta3N5)and so on. Among these VLD photocatalysts, p type semiconductor Bi2O3has drew extensive attention, dueto their excellent properties, such as high refractive index, high dielectric permittivity, as well as markedphotoluminescence properties, they has been widely used in gas sensors, solid oxide fuel cells, opticalcoatings, ceramic glass manufacturing etc.. Furthermore, Bi2O3has also been proved to be a goodphotocatalyst for water splitting and pollutant decomposing under visible light irradiation. However, thereare some defects which hinder the application of Bi2O3as an efficient VLD photocatalts, such as their shortphoton-generated carrier, their relatively narrow visiable light response range and so on, research should becarried out for improving their photocatalytic performance. In this dissertation, we designed andsynthesized several kinds of Bi2O3-based photocatalysts. These Bi2O3-based photocatalysts exhibitedextended visiable light response range, enhanced photocatalytic performance compared with Bi2O3.Futhermore, The influence of the reaction parameters such as the initial concentration of dye, pH value onthe photocatalytic performance of these Bi2O3-based photocatalysts has also been investigated. The majorfindings are as follows:1. We prepared Bi2O3photocatalyst by hydrothermal method. These Bi2O3exhibit rodlike morphologywith sub-micron size, they showed strong photoabsorption thus can efficiently utilize solar light. They alsoexhibit higher photocatalytic efficiency than that of TiO2under visiable light irradiation. To further improvethe photocatalytic efficiency of Bi2O3, the influence of synthetic reaction parameters such as hydrothermalreaction temperature, time and surfactant on the sizes, light absorption and photocatalytic efficiency ofthese Bi2O3have also been investigated. The experiment results show that, the size, the light absorption aswell as the photocatalytic performance increase with the synthesis temperature. Furthermore, surfactant hasa strong effect on the structure and morphology of Bi2O3. When0.6g cetyltrimethylammonium bromide(CTAB) was added into the precursor solution, the final product showed irregular polyhedron morphology;while nanorods with smaller size were obtained when0.6g Polyvinyl Pyrrolidone (PVP) was added.2. According to the energy band theory, loading metal on the surface of semiconductors (especiallyloading metal on the surface of p type semicondutores whose work function is higher than that of metal orloading metal on the surface of n type whose work function is lower than that of metal) can lead to thetransfer of the charge carrier of semiconductors to metal, which can improve the photocatalyticperformance of semiconductors. In order to further improve the photocatalytic performance of Bi2O3, wedesigned and prepared Bi-Bi2O3heterojunction by solvothermal method. These Bi-Bi2O3photocatalystshowed a three-dimensional flower-like mesoporous structure which was built from two dimensionalultra-thin nanoplates, results in a high specific area. Furthermore, there are plenty of hierarchical pores withmeso-and macro-diameter sizes between these ultra-thin nanoplates. These hierarchical pores structure canbe transport channel for the dye. Moreover, the pores between the nanoplates exhibit a similar size with thewavelength of incident illumination, which makes they can utilize solar light more efficiently by therefraction. These Bi-Bi2O3heterojunction showed excellent photocatalytic efficiency for the degradation of RhB, it can degrade90%of RhB in60minutes under the visiable light irradiation, which was almost9and4.7times higher than that of TiO2and Bi2O3respectively. The reason for their highest photocatalyticefficiency was also investigated, there might be three reasons. First of all, the band gap of Bi-Bi2O3heterojunction was wider than that of Bi2O3, which makes the photoholes and photoelectrons of Bi-Bi2O3heterojunction with much stronger oxidation reduction ability. Secondly, the three-dimensional flower-likemesoporous structure makes Bi-Bi2O3heterojunction with high specific area and pores, and thus improvedphotocatalytic performance. Furthermore, the formation of metal-semiconductor heterojunction canimprove the separation of the photon-generated carrier.3. Compared with pure Bi2O3, the photocatalytic efficiency of Bi-Bi2O3heterojunction has beensignificantly improved, however, there is only one kind of visiable light response components Bi2O3in theheterojunction, their visiable light response range were still not wide enough. To further extend the visiablelight response range, improve their utilization of sunlight and further enhance their photocatalyticperformance, we have developed a Bi2WO6surface decorated with Bi2O3nanoparticles (abbreviated asBi2WO6SS-D-Bi2O3NP) nanojunction by surface decoration of Bi2WO6superstructures with Bi2O3nanoparticles through a dip-coating-anneal method. These nanojunctions were built from two dimensionalultra-thin nanoplates as well as plenty of hierarchical pores with meso-and macro-diameter sizes. Thesehierarchical pores structure between the nanoplates can be transport channel for the dye, furthermore, thepores between the nanoplates exhibit a similar size with the wavelength of incident illumination, whichmakes they can utilize solar light more efficiently by the refraction. This Bi2WO6SS-D-Bi2O3NPnanojunction system exhibits a broad-spectrum photoabsorption from the UV to visible-light region with anedge at ca.650nm, indicating a red shift of photoabsorption range compared with that of Bi2WO6superstructures (edge at ca.450nm), nanoscale Bi2O3powder (ca.470nm) and Bi2O3-Bi2WO6compositemicrospheres (ca.440nm) prepared through a one-step hydrothermal route. Furthermore, by using Bi2WO6SS-D-Bi2O3NP as VLD photocatalyst, the photodegradation efficiency of Rhodamine B (RhB) reaches86%which is about2.7times as that (32.3%) by Bi2WO6SS and1.3times that (64.4%) by Bi2O3-Bi2WO6composite microspheres, in20min of reaction. Besides that, this nanojunction can still efficiently degradethe colorless pollutant parachlorophenol. In particular, the photocatalytic activity of the nanojunction issuperior to the sum of the activities of two individual photocatalysts with the same weight of components(Bi2WO6SS and Bi2O3NP). Based on the above results and energy band diagram, two possible reasonshave been proposed for the higher photocatalytic activity of Bi2WO6SS-D-Bi2O3NP nanojunction:(1)substantial broadening of the photoabsorption range and (2) efficient separation of photogeneratedelectron-hole pairs.