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daidongfa

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Abstract. Er3+/Yb3+ codoped pyrochlore-type yttrium titanate (Y0.985-xEr0.015Ybx)2Ti2O7 (x=2.5 %, 5.0 %, 7.5 %, 10.0 % and 12.5 %) nanopowders were successfully prepared by the Pechini-type sol−gel process with citric acid as chelating ligands and polyethylene glycol as cross-linking agent. The preparation process was monitored by X-ray diffraction, thermal-gravimetric differential thermal analysis and Fourier transform-infrared experiments and the microstructures and morphologies of as-prepared samples were characterized by transmission electron microscopy and high resolution transmission electron microscopy images. Detailed proofs showed that the samples annealed at 800 C for 1 h exhibit good crystallinity, dispersibility and narrow size distribution with average size of 20-30 nm. For all the Er3+/Yb3+ codoped pyrochlore-type (Y0.985-xEr0.015Ybx)2Ti2O7 nanoparticles, bright visible upconversion emission for all the samples can be clearly observed with naked eyes under 980 nm excitation at room temperature. As for amorphous (Y0.91Er0.015Yb0.075)2Ti2O7 samples, upconversion spectra exhibit the two broad erbium emission bands centered at approximately 546 nm and 660 nm, which are ascribed to the 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions, respectively, and relatively weaker broad emission bands at about 524 nm, which is attributed to 2H11/2→4I15/2 transitions. As the annealing temperature increases, there exits Stark splitting effects, and the upconversion luminescence intensities increase, whih was attributed to the removal of the hydroxyl groups and to the decrease of the surface area caused by the growth of the nanoparticles. There exists the optimal Yb3+ ion concentration of 7.5 mol % doping in (Y0.985-xEr0.015Ybx)2Ti2O7 nanocrystals which can give rise to a maximum upconversion luminescence intensity. These are the efficient upconversion nanophosphors.
3.4 TEM spectra
Transmission electron microscopy (TEM) was employed to obtain direct information about the size and structure of the as-prepared Er3+ /Yb3+ cooped (Y0.91Er0.015Yb0.075)2Ti2O7 nanocrystals. The typical TEM results of (Y0.91Er0.015Yb0.075)2Ti2O7 nanocrystals prepared at 800C for1 h are shown in Fig. 5. From Fig. 5(a), it can be found that quasi-spherical nanopowders have good dispersibility, and the average size estimated from TEM is around 20-30 nm, which is quite consistent to above-mentioned XRD results. The insert in Fig. 5(a) shows the corresponding selected-area electron diffraction (SAED) patterns, which is the characteristic rings of nano-polycrystals. In Fig. 5b, the typical high-resolution transmission electron microscopy (HRTEM) images indicate the clear and regular crystal lattice distance suggesting that highly crystalline (Y0.91Er0.015Yb0.075)2Ti2O7 were formed. The lattice fringes with the interplanar distance of 0.292 nm obtained from the HRTEM images was attributed to the (222) plane of the cubic structure.
4. Conclusion
Abstract. Er3+/Yb3+ codoped pyrochlore-type yttrium titanate (Y0.985-xEr0.015Ybx)2Ti2O7 (x=2.5 %, 5.0 %, 7.5 %, 10.0 % and 12.5 %) nanopowders were successfully prepared by the Pechini-type sol−gel process with citric acid as chelating ligand and polyethylene glycol as cross-linking agent. The samples annealed at 800 C for 1 h exhibit good crystallinity, dispersibility and narrow size distribution with average size of 20-30 nm; And also, there exists the optimal Yb3+ ion concentration of 12.5 mol % doping in (Y0.985-xEr0.015Ybx)2Ti2O7 nanocrystals which can give rise to a maximum upconversion luminescence intensity. The increase of the upconversion luminescence intensities with the calcination temperature was attributed to the removal of the hydroxyl groups and to the decrease of the surface area caused by the growth of the nanoparticles.

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摘要：通过佩基尼溶胶凝胶法，柠檬酸和聚乙二醇分别作为螯合配体和交联剂成功制备出了Er3+/Yb3+共掺杂焦绿石型钛酸钇(Y0.985-xEr0.015Ybx)2Ti2O7 (x=2.5 %, 5.0 %, 7.5 %, 10.0 % 和 12.5 %)纳米粉体。制备过程通过XRD、TG-DSC和FTIR考察，合成样品的微结构和形貌通过TEM和HRTEM表征。详细的实验结果表明样品C经1h处理后显示了好的晶型、分散度已经平均粒径在20-30nm的窄的分布。对所有的Er3+/Yb3+共掺杂焦绿石型钛酸钇(Y0.985-xEr0.015Ybx)2Ti2O7纳米粒子来说，室温下样品的可见光上转换光谱发射可在980nm激发态处清楚地观察到。对于无定形的(Y0.91Er0.015Yb0.075)2Ti2O7样品来说，在大约546nm和660nm处显示了两个宽的铒上转换光谱发射带，这可以分别用电子从4S3/2→4I15/2以及4F9/2→4I15/2迁移的机理解释。随着煅烧温度的升高，会出现Stark分裂效应，上转换发光强度增大，这是由于羟基的去除以及纳米粒子的长大从而导致表面积减小引起的。Yb3+掺杂(Y0.985-xEr0.015Ybx)2Ti2O7纳米晶的最佳浓度在7.5 mol %，此浓度下上转换发光强度最大。
3.4 TEM图谱
用TEM可以直接得到制备出Er3+ /Yb3+共掺杂(Y0.91Er0.015Yb0.075)2Ti2O7纳米晶体尺寸和结构的信息。样品C经800℃、1h处理的TEM图如图5所示，可以看到近球状的纳米粉体有着好的分散度，从TEM可以看到平均粒径在20-30nm，这与XRD的结果一致。图5（a）的插图是选区分析图谱（SAED），是纳米晶的特征环。图5b中，HRTEM显示(Y0.91Er0.015Yb0.075)2Ti2O7形成了清晰、具有周期性的晶体结构，其晶格兼具也说明了晶型的形成。晶面间距离0.292nm对应着立方结构的（222）面。
4 结论