Preparation And Photocatalytic Properties Of BiOX

The indirect semiconductor bismuth oxyhalides?BiOX?consist of[Bi₂O₂]²⁺layers sandwiched between two slabs of halogen ions,which has attracted widespread attention due to its unique layered structure.BiOF has the largest band gap?3.6 eV?and only responds to ultraviolet light.BiOI has the smallest band gap?1.78 eV?and can respond to higher wavelength visible light.It is well known that the photocatalytic process for semiconductor materials include the light photoexcitation,the succedent separation and transfer of photogenerated charge carriers,and the ultimate redox reaction on the semiconductor surface.BiOX's layered structure provides enough space to polarize the corresponding atoms and orbits to form internal electrostatic field,which is essential for carrier separation.Therefore,it is still an efficient strategy by tuning the IEF for BiOX photocatalysts with layered structure.In this paper,BiOX?X=F,Cl,Br,I?was synthesized by hydrothermal method and chemical precipitation method.Bi?NO₃?₃.5H₂O and NaF/KCl/NaBr/KI were used as Bi source and X source,respectively.The structure and morphology of BiOX catalysts were determined by X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,ultraviolet-visible diffuse reflection absorption spectrum?UV-Vis DRS?,X-ray photoelectron spectroscopy?XPS?,Raman and specific surface area?BET?.The types of vacancies were determined by electron spin paramagnetic resonance spectroscopy?ESR?and positron annihilation spectroscopy?PAL?.The efficiency of carrier separation is characterized by the transient photocurrent curve,electrochemical impedance spectroscopy,photoluminescence spectrum?PL?combined with first-principles calculations.The photoreduction efficiency of CO₂ under simulated sunlight was used to evaluate the catalytic ability of BiOX.The activity of the isomorphous fluorine-substituted BiOBr_x I_1-x catalyst was evaluated by degrading the dyes methyl orange?MO?and rhodamine B?RhB?.Experimental results prove that:1.The BiOBr sample was observed to exhibite the best CO₂ photoreduction performance under the simulated sunlight.The CO and CH₄ evolution rates are 21.6?mol g^-1 h^-1 and 1.2?mol g^-11 h^-1,respectively.The yields of CO and CH₄ under different light intensities indicate that high light intensity is conducive to improving the selectivity of photocatalytic CO₂ reduction products and is beneficial to the production of CO.XPS and PAL results revealed the presence of oxygen vacancies?OVs?in BiOX,which is beneficial to the adsorption and activation of CO₂.The results of ESR experiments show that the characteristic signal peaks of OVs weaken when BiOX photocatalytically reduces CO₂ under light conditions,and the characteristic signal peaks of OVs gradually increase again under continued light conditions,proving that CO₂ preferentially adsorbs on the surface of OVs and light irradiation can generate OVs.The CO₂ photocatalytic reduction isotope tracer experiments performed in H₂¹⁸O show that H₂¹⁸O participates in the process of photocatalytic CO₂ reduction.2.Density functional theory was used to calculate the band structure,electron density and state density of BiOX in Materials Studio 2017 CASTEP?Cambridge Sequential Total Energy Package?.The influence of OVs on the structure of BiOX was further analyzed.In the band structure diagram,the top of the valence band and the bottom of the conduction band of BiOX are located at different positions in K space,which proves that BiOX is an indirect semiconductor.The band gap of BiOX decreases as the number of X atoms increases.The valence band of BiOX-OVs is more gentle than that of pure BiOX,which proves that photo-generated carriers on BiOX-OVs have better separation efficiency.The density of state?DOS?distributions of BiOX and BiOX-OVs show that the bands of O-2p and X-np mainly constitute the valence band,and the band of Bi-6p mainly constitutes the conduction band.BiOX-OVs has a higher DOS than BiOX,which proves that the introduction of OVs can generate more photo-generated carriers.As the atomic number of X increases,the electronegativity gradually decreases,thus reducing the electron transfer energy.However,as the ionic radius increases,the electron transfer energy increases.Therefore,the electron density of O in BiOBr is the highest in BiOX?X=F,Cl,Br,I?,and shows the best CO₂ photoreduction activity in the BiOX family.3.The fluorine isomorphously substituted BiOBr_xI_1-x solid solution with different OVs concentrations was synthesized by the facile chemical precipitation method at room temperature.The sample of BiOF_0.4Br_0.5I_0.1 showed the highest photocatalytic activities toward the degradation of MO and RhB under visible light irradiation in the family.It was found that BiOF_0.4Br_0.5I_0.1 demonstrated a degradation rate of 6.42 h^-1,which is 10.7,14.6,11.7 times higher than that of pristine BiOBr,BiOI and BiOBr_0.9I_0.1,respectively.The maximum photocurrent density and the lowest photoluminescence?PL?intensity were observed for BiOF_0.4Br_0.5I_0.1,suggesting that the enhanced separation efficiency of photoproduced e^--h⁺pairs is the main reason for the boost in photocatalytic performance.The PAL and ESR exploration testified that^·O₂^-and h⁺play essential roles for the degradation of pollutants.It is assumed that the excellent photocatalytic performance is due to the synergistic effect of IEF and OVs by fluorine substitution,which could change the charge distribution and promote the separation of photogenerated carriers.