| Rare earth luminescent materials have become the main materials in many different technological areas, including information display, illumination engineering, X-ray intensifying screen, X-ray computed tomography(X-CT), light emitting diodes, biological labeling and diagnostics, and upconversion lasers. Nearly, the hotspots in the research fields of rare earth luminescent materials mainly include exploration of new luminescent material systems and synthesis of highly dispersed ultrafine phosphors. In the present work, a new Gd2O2SO4:Eu3+ red luminescent material was developed. Gd2O2SO4:Eu3+ phosphors with various sizes were synthesized using a solid-liquid method, a homogenous precipitation method, and a co-precipitation method, and their crystal structure, electronic structure and optical properties were studied. It was found that the Gd2O2SO4:Eu3+ phosphors not only have good thermal stability, but also ideal red light emission performance with peak emission at 618nm, making them suitable candidates in the field of high resolution red light emission. Moreover, Gd2O2S:Pr3+ phosphors with various sizes were synthesized using a reduction method, a homogenous precipitation method, and a co-precipitation method, and their crystal structure, electronic structure and optical properties were studied. The Gd2O2S:Pr3+ phosphors are prospective in the fields of soft X-ray detection for "water window", X-ray microscopy and upconversion luminescence. Finally, Gd2O2S:Pr3+ scintillation ceramic was fabricated by pressureless reaction sintering method, and their luminescent properties were studied.Gd2O2SO4 phosphors in micrometer scale were synthesized by a solid-liquid method from the commercially available Gd2O3, Eu2O3,and H2SO4 starting materials. Since there are no complete data on crystal structure and electronic structure of the Gd2O2SO4 system, crystal structure of Gd2O2SO4 was calculated from the X-ray diffraction data, and electronic structure was calculated on this basis. The results show that the crystal structure of Gd2O2SO4 belongs to orthorhombic system with a space group PMNB (No.62):a=12.996A, b=8.117A, c=4.184A. The atom positions, distances and angles between different atoms of Gd2O2SO4 crystal cell were also determined. The Gd2O2SO4 is an indirect-gap insulator with the indirect band gap energy of 4.9eV, which is lower than estimated value 8.4eV. The emission spectrum of (Gd0.95,Eu0.0)2O2SO4 under 270nm UV light excitation demonstrates the strongest emission peak located at 618nm, attributing to the electric dipole 5D0â†'7F2 transition of Eu3+ions.Gd2O2SO4:Eu3+submircrometer phosphors were synthesized by a homogeneous method from the commercially available Gd2O3, Eu2O3, H2SO4 and urea starting materials. The results reveal that single phase Gd2O2SO4 phosphors can be synthesized at 900℃by controlling the molar ratio (M) of precipitant to Gd2(SO4)3. The optimal M value is found to be 10, and the produced particles are spherical in shape with a particle size of 300-500nm. Under 270nm UV light excitation, the strongest emission peak of the (Gd1-x,Eux)2O2SO4 submircrometer phosphor is located at 618nm, attributing to the electric dipole 5D0â†'7F2 transition of Eu3+ions. The quenching concentration of Eu3+ ions for the phosphor is 5mol%, and the concentration quenching mechanism is the electric dipole-dipole interactions. The decay study reveals that the 5D0â†'7F2 transition of Eu3+ ions has a single exponential decay behavior.Gd2O2SO4:Eu3+ nano-sized phosphors were synthesized by a co-precipitation method from the commercially available Gd2O3, Eu2O3; H2SO4, and NaOH starting materials. The results reveal that molar ratio (m) of NaOH to Gd2(SO4)3 has great effect on the composition of precursor. If m<4, the precursor is amorphous Gd2(OH)4SO4·nH2O, but the productivity is low. If m>4, the composition of precursor is away from Gd2(OH)4SO4·nH2O, with the existence of Gd2O3 after the later calcination. The optimum m is found to be 4, and pure phase can be obtained by calcining the precursor in air at 900℃for 2 hours. The particles are nearly spherical and well dispersed, with a particle size of 30-50nm. Under 270nm UV light excitation, the strongest emission peak of the (Gd1-x,Eux)2O2SO4 nano-sized phosphor is located at 618nm, attributing to the electric dipole 5D0â†'7F2 transition of Eu3+ ions. The quenching concentration of Eu3+ ions is 10mol%, the concentration quenching mechanism is the exchange interaction among the Eu3+ions. The decay study reveals that the 5D0â†'7F2 transition of Eu3+ ions has also a single exponential decay behavior. However, its luminescent intensity and lifetime of the 5D0â†'7F2 transition is smaller than those of the submicrometer phosphors.(Gd1-x,Prx)2O2S micrometer phosphors were synthesized by reduction method from the Gd2O2SO4:Pr micrometer phosphors synthesized by solid-liquid method. Its crystal structure, electronic structure and optical properties were studied. The results show that Gd2O2S is an indirect-gap semiconductor with the indirect band gap energy of 2.57eV, which is lower than the experimental value 4.37eV. The emission spectrum of (Gd1-x,Prx)2O2S under 303nm UV light excitation demonstrates the strongest emission peak located at 515nm, attributing to the 3P0â†'3H4 transition of Pr3+ions. The quenching concentration of Pr3+ ions for (Gd1-x,Prx)2O2S micrometer phosphors is 1mol%. The Gd2O2S:Pr3+ submircrometer phosphors and the Gd2O2S:Pr3+ nano-sized phosphors were produced from the precursors synthesized by a homogeneous and a co-precipitation method, respectively. The results show single phase Gd2O2S can be synthesized by calcining the precursors at temperature higher than 700℃for 1 hour in flowing hydrogen. The former particles are spherical and 300-500nm in size, and the latter particles are 20-40nm in size, with a near spherical shape. Under 303nm and 300nm UV light excitation, the strongest emission peaks of the (Gd1-x,Prx)2O2S submircrometer and nanometer phosphors are located at 515nm and 512nm, respectively, attributing to the 3P0â†'3H4 transition of Pr3+ ions. The 3P0â†'3H4 transition of Pr3+ ions has a single exponential decay behavior. The luminescent intensity and lifetime of 3P0â†'3H4 transition increase with increasing calcination temperature for Gd2O2S:Pr3+ submircrometer and nano-sized phosphors and the former has higher luminescent intensity and longer lifetime compared with the latter.Gd2O2S:Pr3+ scintillation ceramic was fabricated by sintering at 1750℃in a hydrogen atmosphere from the Gd2O2SO4:Pr powder synthesized by solid-liquid method. The fabricated scintillation ceramic has a relative density higher than 99%, with certain luminescent properties.In summary, the synthesized Gd2O2SO4:Eu3+ and Gd2O2S:Pr3+ luminescent materials have high luminescent performance and the preparation conditions are simple, easy to control and low cost, with good potential of practical applications. |
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