Preparation Of Perovskite Potassium Niobate-based Nanofibers And Their Photocatalytic Performance For Hydrogen Evolution

Hydrogen energy is a potential renewable clean energy source to solve the current energy and environmental crisis facing the world,and the photocatalytic hydrogen production is a highly promising hydrogen production technology in the future.KNbO₃,a typical ferroelectric oxide,exhibits unique activity advantages in photocatalytic hydrogen production reactions by its spontaneous polarization.Due to its wide band gap,KNbO₃can only absorb the ultraviolet region in sunlight,which limits its application in the field of photocatalysis.Although there are many reports on the photocatalytic modification of KNbO₃,the photocatalytic hydrogen evolution performance of KNbO₃obtained by modification is not satisfactory at present.Therefore,in this thesis,the photocatalytic hydrogen evolution performance of KNbO₃nanofibers was optimized by ion doping and composite heterojunction modification,and their photocatalytic hydrogen evolution mechanism was investigated.The main research contents and conclusions are as follows:（1）The[KNbO₃]_1-x-[Bi(Ni_1/2Ti_1/2)O₃]_x（x=0-0.05）nanofibers doped with Bi³⁺ions at the A-site and Ni²⁺and Ti⁴⁺ions at the B-site were prepared by electrospinning.It is found that with the increase of Bi³⁺,Ni²⁺and Ti⁴⁺ion doping,the crystal structure of the samples change from a single orthorhombic phase to a mixed phase with the coexistence of orthorhombic and tetragonal phases,and the content of tetragonal phase gradually increases,while the ferroelectricity is continuously weakened.Meanwhile,Bi³⁺,Ni²⁺and Ti⁴⁺ion doping can effectively reduce the optical band gap and enhance the visible hydrogen evolution activity,but reduce the rate of hydrogen evolution under simulated sunlight.The enhancement of visible hydrogen evolution activity is due to the Ni 3d gap state caused by Ni²⁺ion doping can make the photogenerated electrons d-d jump between transition metals.When x=0.01 content,the sample has the best visible hydrogen evolution activity,and its hydrogen evolution rate is 28.5μmol·g^-1·h^-1,after corona polarization,its visible hydrogen evolution rate reaches 55μmol·g^-1·h^-1,which is nearly twice as high as that of unpolarized ones,indicating that increasing the built-in electric field of ferroelectrics can improve the separation efficiency of photogenerated electrons and holes,thus enhancing the photocatalytic activity of the samples.（2）K_1-xNa_xNbO₃（x=0-1）nanofibers doped with Na⁺ions at the A-site were prepared by electrospinning.The results show that the crystal structure of the samples first change from orthorhombic phase to monoclinic phase and then to orthorhombic phase with the increase of Na⁺doping.The doping of an appropriate amount of Na⁺enhances the photocatalytic hydrogen evolution performance of the samples,and the best photocatalytic hydrogen evolution activity is achieved at x=0.5 with the hydrogen evolution rates of 11506 and 33μmol·g^-1·h^-1under simulated sunlight and visible light,respectively.The negative shifts of the conduction band and valence band edges and the contraction distortion of NbO₆octahedra caused by Na⁺doping are the main reasons for the enhanced photocatalytic activity under simulated sunlight and visible light,respectively.（3）K_0.5Na_0.5NbO₃/g-C₃N₄heterojunctions were synthesized by electrospinning and calcination methods,and their photocatalytic hydrogen evolution performance was investigated in comparison with that of the mechanically mixed samples.It is found that the heterojunctions can effectively reduce the band gap,promote the visible light absorption,and significantly improve the photocatalytic hydrogen evolution activity under visible light compared with the mechanically mixed samples.The enhanced activity is attributed to the ability of photogenerated electrons and holes to transfer between the conduction and valence bands of K_0.5Na_0.5NbO₃and g-C₃N₄,thus improving the separation efficiency of electrons and holes.Further research shows that when the content of g-C₃N₄is 20wt%,the heterojunction has the best separation of photogenerated electrons and holes,and its rate of photocatalytic hydrogen evolution is 1.7 times higher than that of the simple mechanical mixed sample under visible light.This is caused by the largest negative shift difference value of the conduction and valence band edges between the heterojunction and the mechanical mixed sample.（4）A novel K_0.5Na_0.5NbO₃/Sr Nb_0.03Ti_0.94Cr_0.03O₃composite nanofiber with visible light response and high stability was prepared by coaxial electrospinning.The research shows that there is a heterostructure effect between K_0.5Na_0.5NbO₃and Sr Nb_0.03Ti_0.94Cr_0.03O₃,which makes the composite nanofiber show better visible hydrogen evolution performance than the individual component,and the best hydrogen evolution activity is exhibited at 10wt%of K_0.5Na_0.5NbO₃with a hydrogen evolution rate of250μmol·g^-1·h^-1,which is 7.6 and 1.8 times higher than that of K_0.5Na_0.5NbO₃and Sr Nb_0.03Ti_0.94Cr_0.03O₃.It is mainly related to the energy band bending caused by the spontaneous polarization of K_0.5Na_0.5NbO₃.