| This paper is based on density functional theory using first-principles ultrasoft-pseudopotential method in the CASTEP modules of Material Studio software.Bi2O3 and Bi2O2CO3 as the research object, the research of different polymorphs Bi2O3 has different photocatalytic activity, the effect of N doping on optical and electrical properties of Bi2O3 and Bi2O2CO3, the influence of oxygen defect on bulk Bi2O2CO3 in photocatalytic activity, and lattice conversion relationship between Bi2O3 and Bi2O2CO3 were talked. By constructing appropriate models, optimizing and analysising the crystal structure, charge density difference map, the total charge density map, the formation energy, effective mass, Mulliken population and optical properties to reveal the internal mechanism of different models of Bi2O3 and Bi2O2CO3 with enhanced photocatalytic activity. The conclusions are listedas follow:(1) In order to reveal the reason that ?, ?and ? and ?-Bi2O3 show different photocatalytic activity, the models of different phase Bi2O3 were constructed; The results show that ?-Bi2O3 displaying semi-metallic characteristics is due to the existence of oxygen defect, which results in conduction band hybridization with valence band, ?,?, and ?-Bi2O3 all have a smaller effective mass and this is conducive to the separation of electrons and holes. Band structures shows the intermediate level appears in band gap of ?-Bi2O3 band gap, which reduces the electronic transition energies and is conducive to photo-generated electrons transition. UV absorption spectra indicates that ?, ?, and ?-Bi2O3 are responsive to visible light and the photocatalytic activity of the order is ?> ?> ?, this is consistent with the experimental results.(2) Constructing N-substituted-doping ?-Bi2O3 and ?-Bi2O3 models to study the effect of N doping on the crystal structure and optoelectronic properties of ?-Bi2O3 and ?-Bi2O3. The results show that the N atom and the surrounding Bi atoms to form stable N-Bi bond, and the introduction of N atom promotes charge transfer from N atom B atoms. Comparing with ?-Bi2O3, N more easily substituted ?-Bi2O3. Energy band structure shows the introduction of N atoms is formed by the new N 2p orbital configuration level in the valence band. The emergence of new energy level promotes the transition of secondary electrons and reduces the electron transition energy. Optical properties show N doping enhances response to visible light.(3) The crystal structures models of N doping bulk and the (001) facet of Bi2O2CO3 were constructed. And the mechanism of N doping Bi2O2CO3 with enhancing performance was studied. Calculation results show that the doping mechanism is consistent between N bulk doping and N surface doping. N dopant is more likely to form and exist in BiaO2CO3 in the form of interstice N comparing to substitution doping. For substitution doping, N and Bi readily form stable N-Bi bond, and this is proved by the hybridization between the N 2p orbital and Bi 6p orbital appearing near the top of the valence band. However, in N interstitial model, the interstice N and O atoms of CO32- form NO bond, and bond with the entire CO32- to form electrons delocalized group via ? bonding, which is in favor of the charge transfer between atoms. The top of valence band is formed by new level N 2p orbital configuration. Optical properties show that N doping shifts the absorption adge to the red region and enhance absorption of visible light.(4) The model with oxygen defect is to remove an oxygen atom in the the Bi2O22+ layer and CO32- layer of bulk Bi2O2CO3. The results show that oxygen defects is easily formed in the (Bi2O2)2+ layer with respect to CO32- layer. And oxygen defect as an electron capture centers is in favor of charge transfer. Energy band structure shows that the introduction of oxygen defects will form a new level in the band gap. For (Bi2O2)2+ layer, a new level is comprised from the Bi 6p orbital, while for CO32- layer, the new level is made up of Bi 6p, O 2p and N 2p orbital. The emergence of new level reduces the electron transition energy and enhances the absorption of visible light. Therefore, the surface activity of Bi2O2CO3 stems from oxygen vacancies on the Bi2O22+ layer, which is consistent with experiments results.(5) The sample of Bi2O2CO3 with oxygen defect was synthesized by UV irradiation, and the XRD, solid-phase ultraviolet spectrometry and degradation NO activity tests were charactered. XRD results show that the introduction of oxygen defects has not changed Bi2O2CO3 crystal structure, but increased the crystallinity degree of Bi2O2CO3, which is consistent with theoretical calculation results. Solids-phase ultraviolet spectrometry results showed that the formation of oxygen vacancies promote absorption of visible light for Bi2O2CO3, which corresponds with the analysis of adsorption spectrum. NO activity tests indicated that the introduction of oxygen defect or not has no effect on the degradation of NO activity.(6) It is found that a facile route to prepare ?-Bi2O3 nanosheets with active{001} facets through annealing Bi2O2CO3 nanosheet precursor Theoretical calculations show that atomic arrangement of Bi2O2CO3 and ?-Bi2O3, is similar and the lattice matching rate is high along (001) plane. It has also been shown by experiment and that ?-Bi2O3 and Bi2O2CO3 can be transformed into each other along the (001) plane, which is consistent with theoretical value. The prepared ?-Bi2O3 nanosheets exhibited enhanced photocatalytic activity toward to NO oxidation both under UV-visible light and visible light irradiation... |
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