Effects Of Doped Transition Metal Ions On Band Structure And Visible Light Photocatalytic Performance Of BiNbO₄

In recent years,non-TiO₂ based metal oxide semiconductor have been one of the hotspots in the visible-light photocatalysts fields.Among them,the bismuth oxide semiconductor materials are considered as a kind of potential visible light catalysts because of chemical stability and high efficiency.BiNbO₄,a member of the bismuth oxide photocatalysts family,exhibits better photocatalytic activity than TiO₂ under ultraviolet radiation,but it faces problems such as large band gap and low quantum yield.In this paper,We have systematically studied the effects of doping three different transition metal elements(Ti⁴⁺?Fe³⁺?Ni²⁺)on the optical band gap,energy structure and visible-light photocatalytic activity of BiNbO₄.The results and analysis are as follows:1.BiNb_1-xTi_xO₄,BiNb_1-xFe_xO₄ and BiNb_1-xNi_xO₄ samples were prepared by the solid-state reaction method.2.Conmpared with the parent phase BiNbO₄,BiNb_1-xTi_x O₄ sample maintained the same orthorhombic structure,microscopic morphology and the average particle size.The introduction of Ti ions did not cause valence changes in other elements,but the oxygen vacancies were introduced.By Ti⁴⁺doping,the optical band gap reduction was caused by the mixing of the Ti 3d orbital with the original conduction band.The visible-light photocatalytic activity of the Ti ions doped samples were obviously enhanced,which may be due to a suitable optical band gap,excellent redox ability and surface oxygen vacancy defect.3.With the doping of Fe ions,the crystal structure and average particle size of BiNb_1-xFe_xO₄did not change.But the specific surface area of doped samples decreased sharply and then increased with the concentration of dopant.The introduction of Fe ions greatly reduced the band gap of parent BiNb O₄ and broadened the response to visible-light absorption,probably because of the separation of Fe 3d orbital energy level involved the formation of conduction bands and valance bands respectively.However,the Fe ions doped samples exhibit lower visible-light photocatalytic degradation efficiency than pure phase BiNbO₄.This may be attributed to the small specific surface areas,the weak redox ability and a new photo-generated carrier recombination center.4.For Ni doping system,the doped samples kept the same crystal structure and the average particle size unchanged,but the specific surface area decreased.Optical band gap was reduced with the increase of doping concentration.A new energy band caused by oxygen vacancies was introduced between the valence band and the conduction band,which may become a new photo-generated carrier recombination center,obstructing the migration process of photogenerated carriers.That was why the visible-light photocatalytic performance of doped sample was only slightly improved compared with pure phase BiNbO₄,even with a suitable optical band gap.