Synthesis And Photocatalytic Properties Of Metal Ion-doped BiFeO₃ Nanoparticles

Energy crisis and environmental pollution have become two urgent important problems to be solved in the word,and semiconductor photocatalytic technology is expected to be one of the most effective ways to solve these two problems.In recent years,the design and development of new highly efficient visible light responsive semiconductor photocatalyst is becoming one of the research hotspots in the field of photocatalysis.Bismuth ferrite(BiFeO3,noted as BFO)has been considered as a potential visible light photocatalyst,because of its suitable bandgap,good visible light response and photoelectric properties as well as chemical stability,and many literatures has demonstrated the good photocatalytic performance of BFO.However,BFO itself has lower carrier mobility,easy recombination of photogenerated carriers,and lower light utilization rate,which seriously restrict its practicability in the field of photocatalysis.In order to further improve the photocatalytic performance of BFO,this paper attempts to modify BFO by metal ions doping,and then explore the effect of different metal ions on the morphology and photocatalytic properties of BFO,as well as uncover the photocatalytic mechanism of metal-ion doped BFO samples.The main contents in this work are as follows.1.Synthesis and photocatalytic performance of BiFeO3 nanoparticlesBFO nanoparticles were prepared by sol-gel method,using metal nitrates as precursors and tartaric acid as a chelating agent respectively.Different techniques(such as XRD,TEM,SEM,and UV-vis spectra)were employed to characterize the morphology and structure of the products,and the photocatalytic performance of BFO was evaluated by photodegradation of rhodamine B(RhB)under visible light irradiation.The experimental results show that the prepared BFO was rhombohedral perovskite phase with an average particle size of about 150 nm,and were well responsive to the visible light spectrum with a narrow bandgap of about 2.2 eV.Under visible light irradiation for 4.5 h,the RhB photodegradation rate by the prepared BFO was only 24%,suggesting a lower photocatalytic efficiency.2.Synthesis and photocatalytic performance of Gd doped BiFeO3 photocatalystsGd-doped BFO photocatalysts with different Gd3+ doping concentrations,marked as Gdl%-BFO,Gd3%-BFO and Gd5%-BFO,were prepared by adding different molar concentrations of Gd(NO3)3 to the precursors of BFO through a sol-gel method.It was found that the crystal structure of BFO was nearly unchanged by the doping of Gd3+ ions,but the magnified diffraction peaks of(104)and(110)had an obvious tendency to shift toward higher 20 value,indicating that Gd3+ ions were successfully incorporated into the crystal lattice of BFO.The particle size of the Gd-doped BFO samples was?80 nm,which was smaller than that of BFO.Compared to BFO,all the Gd-doped BFO samples exhibited much higher visible light absorption ability and photocatalytic performance.Among all the photocatalysts,the Gd3%-BFO sample exhibited the highest photocatalytic activity for RhB degradation,which was about 2.55 times that of BFO.From the trapping experiments and photoelectrochemical measurement,the mechanism for enhanced photocatalytic performance of Gd-doped BFO was also elucidated.It was proposed that the doping Gd3+ ions in BFO could form a shallow trapping level,which could effectively capture the photogenerated electrons and inhibit the recombination of electron-hole pairs,thus leading to the enhanced utilization rate of photocarriers and the higher photocatalytic activity of Gd-doped BFO.3.Synthesis and photocatalytic performance of Zr doped BiFeO3 photocatalystsZr-dopeu BFO photocatalysts with different Zr4+ doping concentrations,marked as Zrl%-BFO,Zr2%-BFO and Zr3%-BFO,were prepared by adding different molar concentrations of Zr(NO3)4 to the precursors of BFO through a sol-gel method.It was revealed that the the XRD patterns of all the prepared Zr-doped BFO samples were similar to that of pure BFO,and the(104)and(110)diffraction peaks had an obvious tendency to shift toward lower 2d value arising from the substitution of Zr4+ions.The particle size of all the Zr-doped BFO samples was relatively smaller than pure BFO,and Zr element was uniformly distributed in the BFO host.The bandgap of BFO was slightly enlarged by Zr doping,which could slightly decrease the visible light absorption but could facilitate the carrier mobility.The doping element of Zr was tetravalent in Zr-doped BFO,accompanying with surface oxygen vacancies.Among all the prepared samples,the Zr2%-BFO sample exhibited the highest photocatalytic performance under visible light irradiation(e.g.,the photodegradation efficiency for methyl orange was about 3 times that of BFO,the photocatalytic efficiency for Cr(VI)reduction was?2 times that of pure BFO).On the basis of the trapping experiments and photoelectrochemical measurements,the mechanism for the enhanced photocatalytic performance of Zr-doped BFO was further explored.It was proposed that the doping of Zr4+ ions could induce some surface oxygen vacancies in the BFO host,which could effectively capture the photogenerated electrons to facilitate the separation and migration of photogenerated charge carriers as well as react with the adsorbed oxygen for the formation of superoxide radicals,thus leading to the enhanced photocatalytic activity.