Preparation And Photocatalytic Hydrogen Production Efficiency Of A/B-site Ni Substituted KNbO₃ Ferroelectric Semiconductor Ceramics

Semiconductor photocatalysts have been playing an important role in environmental degradation,pollutant treatment and hydrogen production.In recent years,the inherent electric field of ferroelectric materials can effectively separate the carriers and improve their photocatalytic performance,which has attracted a lot of attention,thus making ferroelectric materials become a hot research material in the field of photocatalysis.In this work,we use potassium niobate（KNbO₃;KN）as parent phase successfully prepared（1-x）KNbO₃-x La NiO_3-δ（abbreviated as KN-LN;x=0.00-0.04）ferroelectric semiconductors by using La/Ni elements as dopants;（1-x）KNbO₃-x Ba(Ni_0.5Nb_0.5)O_3-δ（abbreviated as KN-BNN;x=0.00-0.20）ferroelectric semiconductors by using Ba/Ni elements as dopants;And（1-x）KNbO₃-x Bi(Ni_0.5Ti_0.5)O_3-δ（KN-BNT;x=0.00-0.04）（KN-BNT;x=0.00-0.04）ferroelectric semiconductors ceramics by using Bi/Ti/Ni as dopants.All ferroelectric semiconductors are prepared by the traditional solid-sintering method.The results provide a reference for the development of novel and efficient ferroelectric photocatalytic materials and deepen our understanding of the mechanism of ferroelectric polarization.The microstructures and photocatalytic hydrogen production efficiency of KN-LN ceramics were studied in detail.The XRD,Raman spectroscopy,and dielectric measurement confirmed the coexistence of orthogonal and tetragonal phases in x=0.01-0.04 ceramics.Under simulated sunlight,the hydrogen production rates of x=0.00,0.01,0.02,0.03 and 0.04ceramics reached 473.7,1169.7,938.0,921.2 and 815.2μmol/g.h,respectively.Moreover,all the samples have been achieved hydrogen production activity under visible light.The enhanced photocatalytic activity is attributed to the successful control of the KN-LN band gap in the range of 3.25-2.20 e V.After high-field polarization,the photocatalytic hydrogen production rate of the 0.99KN-0.01LN sample is 642.0μmol/g.h under visible light irradiation,which is 2.9 times of the unpoled one.The improved photocatalytic activity in poled samples is attributed to the improved bulk charge separation efficiency,which has been confirmed by photocurrent,electron paramagnetic resonance（EPR）,and electrochemical impedance spectroscopy（EIS）measurements.These findings indicate that the doping of La/Ni elements can improve the hydrogen production rate of KN system,which can be further improved by the high-field polarization.The microstructures,electrical properties,and photocatalytic performance of poled/unpoled KN-BNN ceramics were studied in detail.The XRD,Raman,and dielectric analysis showed that KN-BNN ceramics undergo a series of phase transitions with increasing x,namely orthorhombic to tetragonal,and tetragonal to cubic.Raman spectra showed that the crystal structure of KN-BNN ceramics became symmetrical after polarization.Dielectric test indicates that the domain in the poled ceramics gradually changes to the ferroelectric ordered state.The average hydrogen production rates of the polarized ceramics with x=0.00,0.02,0.05,0.10 and 0.20 are 707.61,737.54,847.34,424.63 and 131.34μmol/g.h,respectively,which are 1.5,2.8,3.6,1.5 and 2.2 times higher than those of the unpolarized ceramics.Besides,the Optical absorption measurement indicate that the bandgaps of KN-BNN ceramics were successfully controlled in the range of 3.25-2.05 e V by Ba/Ni doping.Therefore,KN-BNN ceramics successfully achieve visible-light photocatalytic hydrogen production.Under visible-light irradiation,the average hydrogen production rates of x=0.02,0.05,0.10,and 0.20 ceramics are 363.38,415.16,224.54,and 126.32μmol/g.h,respectively,which are 14.5,17.7,11.1,and 8.1 times higher than those of the unpoled ceramics,respectively.The enhanced photocatalytic activity in the poled samples was attributed to the enhancement of the ferroelectric field,which improves charge separation efficiency.This result which was confirmed by photocurrent,EPR,and EIS measurements.The microstructures,electrical,optical properties,and photocatalytic hydrogen production performance of KN-BNT ferroelectric semiconductors were systematically characterized.All the KN-BNT ceramics show single perovskite structure without second phase.The XRD,Raman spectra,and dielectric measurement show that KN-BNT gradually transforms from orthogonal to tetragonal phase with increasing x.Under simulated sunlight irradiation,the hydrogen production rates of x=0.00,0.02,0.05,0.10 and 0.20 ceramics are473.7,1305.4,1258.3,584.1 and 579.9μmol/g.h,respectively.This result indicates that the Bi and Ni/Ti elements is beneficial to improve the photocatalytic hydrogen production rate of KN-BNT ceramics.The improvement of photocatalytic activity is attributed to the improvement of charge separation efficiency of KN-BNT ceramics.Withing one hour of visible light illumination,the average hydrogen production rates of x=0.01,0.02,0.03 and0.04 ceramics are 55.2,29.5,19.5,and 14.6μmol/g.h,respectively.The optical absorption test results show that the band gaps of x=0.00,0.01,0.02,0.03 and 0.04 ceramics are 3.25,1.30,1.28,1.26 and 1.25 e V,respectively.Therefore,the successful reduction of the band gap makes KN-BNT ceramics have good photocatalytic activity for hydrogen production.