Construction Of Bismuth-based Photocatalysts With Micro-nano Structure And Their Photocatalytic Performance In Degradation Of Aromatic VOCs

Due to the energy saving,high efficiency and simple operation etc,photocatalytic oxidation method driven by solar energy is considered to be one of the most promising processing technology for removing VOCs from ambient environment.In terms of practical application,how to increase photocatalytic decomposition efficiency towards VOCs is a crucial issue in environmental engineering.Compared with the traditional semiconductor photocatalysts,Bi-based photocatalysts gradually become hot materials due to the special electronic and crystal structure.Most of Bi-based semiconductors possess narrow band gap and are responsive to visible light.The three-dimensional layered structure,composed of the layer of[Bi2O2]2+ interleaved by double slabs of halogen atoms,is beneficial to the separation of photo-generated electrons and holes.Besides,based on the previous studies for micro-nano structure tunement of semiconductor catalysts(for example,hierarchical micro-nano structure,surface-interface structre and electronic structure,etc.)and their catalytic performance improvement,the decomposition efficiency of typical benzene series of VOCs can be enhanced if the Bi-based photocatalysts with special micro-nano structure are constructed on molecular level.Therefore,in this paper,some kinds of efficient Bi-based photocatalysts were constructed by modulating the micromorphology and electronic structure.The relationships between microstructures and charge dynamic behaviors,as well as catalytic activities were systematically investigated.The surface-interface charges transfer and catalytic reaction mechanisms were deeply elucidated by kinds of physical chemistry techniques such as transient surface photovoltage(TPV),electron spin resonance(ESR)spin-trap and in situ FTIR technique.The obtained research results are as follows:(1)The novel BiVO4 quantum tubes(Q-BiVO4)were synthesized by a low-temperature hydrothermal method,and then combined with the hierarchical architectured TiO2 micro flowers to form the Q-BiV04/TiO2 heterostructured composite of which the absorption edge was extended to 520 nm.BiVO4 quantum tubes have a uniform hollow structure with an ultrathin wall thickness of ca.1 nm and an ultranarrow diameter of ca.3 nm,the tiny structure of which was favorable for rapid transfer of photoinduced charge carriers from bulk to the surface.The constructed heterojuction further reduced the recombination rate of photoexcited electrons and holes.Besides,the introduction of BiVO4 quantum tubes extended photo-response to the visible region.The photocatalytic activities of the materials were evaluated by the decomposition of gaseous toluene.Compared to the individual BiVO4 quantum tube,BiVO4 nanoparticle and nano-BiVO4/TiO2,the Q-BiVO4/TiO2 composite exhibited higher photo activities.Under the optimum conditions(T = 298 K,P = 101.3 KPa,?>400 nm,treaction = 6 h),the conversion ratios were 89%toward toluene.Electron spin resonance(ESR)examinations confirmed that OH radicals were the predominant reactive oxygen specie involved in the degradation of gaseous toluene over Q-BiVO4/TiO2 composites.(2)A novel(3-Bi2O3/BiVO4 nanocomposite with p-n heterojuction structure,which is about 24 nm and assembled by interconnected quantum dots,has been successfully constructed through a facile hydrothermal method.Such mutli-heterostructure of?-Bi2O3/BiVO4 nanocomposite with an effective built-in electric field not only improved the mobility and separation of charge carriers but also prolonged their lifetime,eventually making the charges diffuse to the surface for the photocatalytic reaction.ESR detection results confirmed that O2-radicals were the dominant reactive oxygen species.Photocatalytic activities of the samples were evaluated by degradation of gaseous ortho-dichlorobenzene(o-DCB).The conversion ratio of o-DCB over the ?-Bi2O3/BiVO4 nanocomposite attained 70%under the optimum conditions(T = 298 K,P = 101.3 KPa,?>400 nm,treaction = 6 h),which was obviously higher than the pure BiV04 catalysts with other microstructures.The degradation intermediates of o-DCB in photocatalytic process over the BiVO4-based catalysts are identified by in situ FTIR and a possible decomposition pathway of o-DCB was proposed:Reactive oxygen species attacked the adsorbed o-DCB and then the Cl atoms were substituted by the nucleophilic effect,leading to the formation of the chlorophenols intermediates which could be further oxidized to the o-benzoquinone-type species.The ring of o-benzoquinone-type species can cleave through electrophilic substitution with bond-breaking,partially oxidized into some carboxylates that can be ultimately mineralized into CO2 and H2O,partially.(3)Three BiOBr catalysts with different morphologies and structures were prepared via solvothermal method by adjusting reaction conditions.Among them,the hierarchical architectured BiOBr microflowers were assembled by regular ultrathin nanosheets.A series of modern physical techniques confirmed that the dominant exposed facet of BiOBr catalysts was {001} facet,and the percentage of {001} facets achieved was as high as 98%for BiOBr microflowers.The result of low-temperature ESR detection displayed that in comparison with BiOBr microspheres and nanosheets,more oxygen vacancies existed in BiOBr microflowers,which not only could narrow the band gap,but also serve as electron traps as well as adsorption sites where the charge transfer to adsorbed species can prevent the electron-hole recombination.DRS result indicated that the band gap of BiOBr microflower was narrower(2.39 eV)which was attributed to the more content of oxygen vacancies.Under the optimum conditions(T = 298 K,P = 101.3 KPa,?>400 nm,treaction = 6 h),the photocatalytic activity of BiOBr microflowers for the degradation of gaseous o-DCB was superior and the conversion ratio was 68%,which was ascribed to more oxygen vacancies and high {001}facets exposure percentage,resulting in the effective separation of charge carriers under the built-in field in the[110]direction and more generation of the reactive oxygen species for photocatalytic reaction.(4)The CeO2/BiOBr composite with surface dispersive p-n heterojunction structure was fabricated by a homogeneous precipitation-solvothermal method.Steady-state and transient-state PL spectra corfirmed that the photo-excited charge carriers in CeO2/BiOBr composite display higher separation and much longer lifetime.Surface photovoltaic technique demonstrated that more surficial charges existed in the CeO2/BiOBr composite,resulting in that the value of surface photovoltage was as high as 175 ?V.In comparison with other prepared heterojunction system or pure BiOBr microflower and CeO2 nanopartical,the CeO2/BiOBr composite exhibited the highest photo activity towards the degradation of o-DCB(78%)under the optimum conditions(T = 298 K,P = 101.3 KPa,?>400 nm,treaction=6 h).The band gap analyzation and ESR experiment results confirmed that the interfacial electric field in the fabricated effective heterojunction structure was favorable to the separation and transfer of spacial photo-induced charges and extension of charge lifetime,which made more electrons reacts with adsorbed surface oxygen species to generate O2-radicals that subsequently can derive OH radicals.Ultimately,the gaseouse o-DCB was degraded efficiently under the synergistic effect of photo-induced holes and reactive oxygen species.