Study Of Pr³⁺-doped CaTiO₃ And Its Photocatalytic Performance

As a typical representative of the perovskite oxide,CaTiO₃ is widely used because of its good ferroelectric,dielectric,pyroelectric effect.Preparation of the CaTiO₃:Pr³⁺red long afterglow materials are the most popular.High temperature solid state reaction is the conventional method for preparing CaTiO₃:Pr³⁺which requires high calcination temperature,but induces sintering and aggregation of particles.It has been observed that the preparation of the powders by sol-gel,chemical precipitation and hydrothermal synthesis method can lead to significant reinforcement of the red photoluminescence intensity.The reason why they are not widely used in the industrialized production is that the processing routines are complicated and the duration is long.In recent years,many scholars have studied the photocatalytic performance of CaTiO₃,they have found out that CaTiO₃ has higher photocatalytic activity under UV visible light irradiation,but the most common degradation object was aqueous solution,and the research on the organics degradation was less.In this work,the perovskite red phosphors activated by Pr³⁺were prepared via the combustion method,using citric acid as a fuel and complex agent.The samples were characterized by X-ray powder diffraction?XRD?,scanning electron microscope?SEM?and fluorescence spectrometer respectively.The CaTiO₃:Pr³⁺can be obtained with high purity and good crystallinity,which has perovskite structure?PDF#22-0153?.The diffraction peak intensities enhance with the increasing of calcination temperature and holding time,also the grain size increases.Instead,the amount of citric acid(CA/(Ca²⁺+Ti⁴⁺))and Pr³⁺have little influence on the phase of samples.However,with the increasing of calcination temperature and holding time,the sample particles will be sintered and aggregated.The CaTiO₃:Pr³⁺can exhibit maximum photoluminescence intensity prepared at900oC for 1 h,the optimal CA/(Ca²⁺+Ti⁴⁺)and Pr³⁺concentration are 1.4 and 0.1mol%,respectively.The excitation spectra(?_em=616 nm)mainly locates at 274 nm and 323 nm,which is the result of O?2p??Ti?3d?e_g and O?2p??Ti?3d?t_2g transition.The emission(?_em=274 nm)peak is centred at 616 nm,which is according to the special ¹D₂?³H₄ transition of Pr³⁺.CaTiO₃:0.1 mol%Pr³⁺,0.1 mol%R⁺?R=Li,Na,K,Ag?were sucessfully synthesized by the combustion method,where R is used as charge compensators.The doped monovalent R⁺can replace a part of Ca²⁺and form negative charged detects in order to compensate the positive defects due to the replacement of Ca²⁺by Pr³⁺,which is useful to improve the luminescent properties of samples.In addition,the phosphors,Ca_1-xSr_xTiO₃:Pr³⁺with x=0,0.1,0.2,0.3,0.4,0.5,and 0.6,were synthesized by using the combustion method at 900oC for 1 h.The intensity of diffraction peak at about 2?=23.5odecreases and increases at about 2?=40.9owith the concentration of doped Sr²⁺ion chaging from 0 to 0.6.It indicates that the crystal structure of the samples has changed from perovskite to calcium strontium titanate.When x=0.4,the obtained samples exhibits the maximum emission intensity.The photocatalytic properties of CaTiO₃:0.1 mol%Pr³⁺on the degradation of methylene blue?MB?solution were also studied.The photocatalytic rate is nearly51.66%after irradiation for 6 h under UV light,and the optimized initial concentration of methylene blue solution and amount of photocatalyst are 5 mg/L,0.1g respectively.What's more,the luminescent materials,Ca_xTiO₃:0.1 mol%Pr³⁺?x=0.7,0.8,0.85,0.9,1?were prepared via the combustion method.With the Ca deficiency increases,it has found some anatase and rutile in the diffraction phases of the samples,which may due to the Ti excess.The enhancement of luminescence maybe result from Ca vacancy compensation for Ca deficiency and the replacement of Pr³⁺-Ti⁴⁺,forming[Pr_Ti]?to compensate for the defect of[Pr_Ca]?.Ca deficiency can improve the photocatalytic activity on the methylene blue solution,and the degradation rate is 93.6%under UV irradiation for 7 h when x=0.85.However,the degradation rate decreases with Ca deficiency increases,which is probabilily resulted from the decreasing luminescence intensity.