Synthesis, Characterization And Photocatalytic Activity Of BiOX (X=Cl, Br And I) Photocatalysts

In past decades, various advanced oxidation processes (AOPs) have been developed for environmental remediation. Among them, heterogeneous photocatalysis has attracted considerable attention. It is regarded as a potential solution to the recent severe problems of energy shortages and environment crises. Titanium dioxide (TiO2) has many advantageous properties including highly oxidative, chemically stable, inexpensive, and non-toxic nature and it is widely investigated for antimicrobial, deodorization, and air and water purifications. It has undoubtedly been proven to be the most excellent photocatalyst for the oxidative decomposition of organic compounds. However, it is only active under UV excitation (λ<380nm) because of its large band gap energy of 3.2 eV. This limits its further application in the visible-light region(k>400nm). Moreover, the rapid recombination of photoinduced electrons and holes greatly lowers the quantum efficiency. Therefore, it is of great interest to improve the generation and separation of photoinduced electron-hole pairs in TiO2 for further applications.Modification of TiO2, such as doping with nonmetal or metal atoms, coupling with other narrow band-gap semiconductor and surface dye-sensization, has received a lot of attention to improve its dissatisfactory quantum efficiency. Besides the focused work on TiO2 modification, many efforts were concentrated on other novel photocatalysts such as sulfides, oxysulfides, nitrides, oxynitrides, and oxyhalides. Recently, Bi-based multiple compounds have drawn much attention for their potential application in photocatalysis. Bismuth oxyhalides, BiOX (X=Cl, Br, I), are of great importance because of their optical properties and promising industrial applications. All BiOX compounds crystallize in the tetragonal matlockite structure, a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms. Bismuth oxychloride is one of the simplest members of the Sillen family expressed by [M2O2][Clm]or[M3O4+n][Clm](m=1-3) where bismuth oxide based fluorite-like layers, [M2O2] or [M3O4+n], are intergrown with double chlorine layers. These oxyhalide compounds can be applied as catalysts, ferro-electric materials, and pigments. They can also be used as excellent photocatalysts to decompose organic compounds into inorganic substances for purifying textile dye polluted wastewater.This disssertation first introduced the fundamental concepts, classification, properties and characteristic technologies of advanced oxidation processes (AOPs), and then reviewed the research and development of the nano-materials as ptotocatalysts. The synthetic strategies of hierarchical BiOX (X=Cl, Br, I) microspheres consisting of nanoplates, a heterostructured BiOI/TiO2 photocatalyst with different Bi to Ti molar ratios and iodine self-doped bismuth oxyiodide (BiOIx, x= 1,1.5,2,2.5 and 3) powders were devoleped. These resulting materials were characterized by XRD,XPS,SEN,TEM,SPS and photocatalytic activity test.The detailed works were shown as the following:1.A general one-pot solvothermal process was explored to prepare BiOX (X=Cl, Br, I) powders by employing ethylene glycol as the solvent. The as-prepared BiOX powders were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, and nitrogen sorption. The resulting BiOX samples were phase-pure and of hierarchical microspheres consisting of nanoplates. Ethylene glycol was found to not only act as the reaction medium but also play an important role in the formation of hierarchical BiOX nanoplate microspheres. On the basis of characterization results, we proposed a possible process for the growth of hierarchical BiOX nanoplate microspheres. The band gaps of the as-prepared powders were estimated to about 3.22,2.64, and 1.77 eV for BiOCl, BiOBr, and BiOI, respectively. Moreover, we evaluated their photocatalytic activities on the degradation of methyl orange and compared them with TiO2 (Degussa, P25) under UV-vis light irradiation and C-doped TiO2 under visible light (λ,>420nm) irradiation, respectively. It was found that all the BiOX samples were photocatalytically active and BiOI exhibited excellent activity under both UV-vis and visible light irradiation because of its suitable band gap. The resulting hierarchical BiOX nanoplate microspheres are very promising photocatalysts for degrading organic pollutants and other applications.2. BiOI/TiO2 heterostructures with different Bi to Ti molar ratios were synthesized through a simple soft-chemical method at a temperature as low as 80℃. The as-prepared powders were characterized by X-ray powder diffraction, electron microscopy, UV-vis diffuse reflectance spectroscopy, nitrogen sorption, and X-ray photoelectron spectroscopy. The photocatalytic activities of these BiOI/TiO2 heterostructures were evaluated on the degradation of methyl orange under visible-light irradiation(λ>420 nm). The results revealed that the BiOI/TiO2 heterostructures exhibited much higher photocatalytic activities than pure BiOI and TiO2, respectively, and 50%BiOI/TiO2 showed the best activity among all these heterostructured photocatalysts. The presence of BiOI could extend the spectral response of TiO2 from the UV to visible region, enabling the heterostructures to effectively degrade the organic dye pollutant under visible-light irradiation. Surface photovoltage spectroscopy and transient photovoltage measurements were used to confirm the formation of heterojunction and probe charge transfer between BiOI and TiO2. The visible-light photocatalytic activity enhancement of BiOI/TiO2 heterostructures could be attributed to its strong absorption in the visible region and low recombination rate of the electron hole pairs because of the heterojunction formed between BiOI and TiO2. The study suggests the BiOI/TiO2 heterostructures are new types of visible-light-driven photocatalysts for envi ronmental applications.3.Iodine self-doped bismuth oxyiodide (BiOIx, x=1,1.5,2,2.5 and 3) powders were synthesized through a simple soft-chemical method. The as-prepared powders were characterized by X-ray powder diffraction, electron microscopy, UV-vis diffuse reflectance spectroscopy, nitrogen sorption and X-ray photoelectron spectroscopy. The resulting BiOIx powders were of hierarchical microspheres consisting of nanoplates. The photoactivities of BiOIx were evaluated on the degradation of methyl orange (MO) and removal of NO, both under visible light irradiation(k>420 nm). It was found that BiOIx (x=1.5) showed the best photoactivity among them on the degradation of MO and removal of NO. To reveal the effects of iodine self-doping on BiOI, we discussed the features of band structures of BiOIx on the basis of density functional theory (DFT) calculations. The calculation results reveal that the highly active photocatalytic performance is closely related to tuning of electronic and band structures of iodine self-doped BiOI. This could change the band sructures and optical absorption properties, and thus leading to the effective separation of photo-induced electron/hole pairs. Surface photovoltage spectroscopy and transient photovoltage measurements were further used to probe charge transfer process in BiOIx. Moreover, the correlation among band structure, optical properties and photocatalytic activities of BiOIx was discussed in detail.These results show that the iodine self-doped BiOI is more beneficial to the optical absorption properties and fast separation of photo-induced e/h pairs because of its modified electronic and band structures. This successful example of self-doping for valence band control method may inspire studies to develop other self-doped ternary semiconductor photocatalysts driven by visible light irradiation.