Study On Doping Modification And Optical Properties Of LaCrO₃-Based Materials

Lanthanum chromate?LaCrO₃?with perovskite structure,due to its excellent electrical,magnetic,catalytic and optical properties,is widely used in solid oxide fuel cells,magnetohydrodynamic generators,thermistors,photocatalysis and coatings for photothermal conversion,etc.Currently,doping is one of the most effective method to improve the infrared performance of LaCrO₃,resulting in improvement of absorption capability by enhancing free carriers absorption,impurity absorption and lattice vibration absorption.In this study,the thermal decomposition process of La_0.8Ca_0.2CrO₃ precursors prepared by sol-gel and high-temperature solid state methods were respectively investigated via thermogravimetric and differential scanning calorimetry,X-ray diffraction and Fourier transform infrared spectroscopy analyses,and then the optimum calcination temperature can be determined.Subsequently,the phase composition,microstructure and optical properties of Ca-doping and Ca/Ni co-doping LaCrO₃ powders were characterized using X-ray diffraction,scanning electron microscope,X-ray photoelectron spectroscopy and Ultraviolet?visible?near infrared spectroscopy,etc.The main results and conclusions are as follows:The optimum calcination temperature was determined to be 800? when La_0.8Ca_0.2CrO₃powders were synthesized via sol-gel route.The La_0.8Ca_0.2CrO₃ sample possesses the highest crystallinity in all the doping samples,and a small amount of CaCrO₄ appears when the molar concentration of calcium exceeds 0.2.The as-fabricated La_1-xCa_xCrO₃ nanoperovskite powders of^?20 nm are nearly spherical,while the grain sizes decrease with increasing the doping concentration of calcium.In addition,the substitution of La³⁺by Ca²⁺can generate more oxygen vacancies and cause valance state shift from Cr³⁺to Cr⁶⁺to maintain the charge balance.The infrared absorption performance of La_0.8Ca_0.2CrO₃ sample is the optimal under the mid-infrared range of 2.5?25?m,all the samples have a wide absorption band in 200?800 nm,and the decrease degree of E_g value is more serious while increasing the doping concentration of calcium from 0.1 to 0.2,which demonstrates that the doping amount of calcium can be optimized to be0.2.When La_0.8Ca_0.2CrO₃ powders were prepared by solid state method,the calcination temperature was optimized to be 1200?,and there can be a trend of Cr³⁺to oxidize to Cr⁶⁺andthe generation of La₂CrO₆ and CaCrO₄ below 1000?.La_0.8Ca_0.2CrO₃ sample microparticles show the nearly spherical structure with a diameter of^?0.5?m and agglomerate severely.After doping,the enhancement of absorption capability is obvious in the two spectral regions of 2.5?25?m and 200?1400 nm,and the E_g value decreases from 3.04 to 2.46 eV,indicating that calcium doping improves the optical absorption performance of LaCrO₃.Three impurity phases of Ni,NiO and La₂CrO₆ generate in La_0.8Ca_0.2Cr_1-yNi_yO₃ samples synthesized by sol-gel method when the doping concentration of nickel exceeds 0.3.Moreover,the substitution of Cr³⁺by Ni²⁺can reduce the covalency of La-O and Ca-O bonds,and increase the contents of Cr⁵⁺and Cr⁶⁺and the concentration of oxygen vacancies.After nickel doping,the optical absorption performance of La_0.8Ca_0.2CrO₃ powders improves obviously due to the formation of“light traps”derived from lots of gaps.In all the co-doping samples,La_0.8Ca_0.2Cr_0.5Ni_0.5O₃ and La_0.8Ca_0.2Cr_0.9Ni_0.1O₃ possess respectively the optimal mid-infrared absorption and excellent ultraviolet?visible?near infrared absorption properties,and the decrease degree of E_g value of La_0.8Ca_0.2Cr_0.9Ni_0.1O₃ is the most obvious.