pH induced size tuning of Gd2Hf2O7:Eu3+ nanoparticles and its effect on their UV and X-ray excited luminescence
pH induced size tuning of Gd2Hf2O7:Eu3+ nanoparticles and its effect on their UV and X-ray excited luminescence
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DOI：
10.1016/j.jlumin.2020.117605
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发表时间：
2020-12-01
影响因子：
3.6
通讯作者：
Mao, Yuanbing
中科院分区：
文献类型：
--
作者：
Gupta, Santosh K.;Garcia, Mitzy A. Penilla;Mao, Yuanbing
粒径可调纳米颗粒在光电子学、药物传递、磁学等领域发挥着重要作用。此外，设计尺寸可调的NPs而不将其暴露于高温和长时间热处理是制造无团聚和无缺陷NPs的迫切需要。在这项工作中，我们使用熔融盐合成（MSS）方法设计了Gd2Hf2O7:Eu3+ (GHOE) NPs，其前驱体由不同的沉淀剂浓度（相应的沉淀溶液的pH）制成。所得NPs的粒径随着沉淀溶液pH的升高而减小，形成缺陷萤石结构，其中大部分Eu3+离子位于GdO8扭曲的scalenhedra上，一小部分位于Hf4+位置，并且在Eu-Hf'附近存在氧空位。由于表面缺陷和颗粒聚类的最佳平衡，以5.0% NH4OH为沉淀剂制备的GHOE NPs具有最大的光致发光和辐射致发光输出以及内部量子产率。Judd-Ofelt分析证实，这些GHOE NPs具有最低的非辐射跃迁概率、最亮的红色发射、最大的分支比和最高的辐射跃迁率。随着沉淀溶液pH值的增加，GHOE主体中Eu3+离子的基团对称性从D-4 -> C-4v -> C-3v依次下降，然后趋于饱和，符合pH依赖性不对称比值。欧米茄(2)/欧米茄(4)比值单调增加证实了极化程度增强，Eu-O键的共价性增强。我们在这些光学材料上的工作将有助于科学界在低合成温度下制造出尺寸可调的纳米颗粒，用于高效发光器件。
Size tunable nanoparticles (NPs) have played an important role in areas of optoelectmnics, drug delivery, magnetism and many others. Moreover, designing size-tunable NPs without exposing them to high temperature and long time thermal treatment is highly desirable to make agglomerate- and defect-free NPs. In this work, we have designed Gd2Hf2O7:Eu3+ (GHOE) NPs using the molten-salt synthesis (MSS) method with the precursors made from varying precipitant concentrations (correspondingly the pH of the precipitating solution). The obtained NPs have a decreasing size as the pH of the precipitating solution rises and a defect fluorite structure with a large fraction of Eu3+ ions localized at GdO8 distorted scalenohedra, a small fraction residing at Hf4+ site, and the presence of oxygen vacacnies in the vicinity of Eu-Hf'. Maximum photoluminescence and radioluminescence outputs and internal quantum yield have been observed from the GHOE NPs made from 5.0% NH4OH as precipitant due to optimum balance of surface defects and particle clustering. Judd-Ofelt analysis has confirmed that these GHOE NPs have lowest non-radiative transition probability, brightest red emission, largest branching ratio, and highest radiative transition rate. With increasing pH of the precipitating solution, the group symmetry of Eu3+ ions in the GHOE host decreases systematically from D-4 -> C-4v -> C-3v and then saturates, consistent with the pH dependent asymmetry ratio value. Also, the extent of polarizability enhances and the Eu-O bond becomes more covalent as confirmed by the monotonic increase of Omega(2)/Omega(4) ratio. Our work on these optical materials will assist the scientific community to make size-tunable nanoparticles at low synthesis temperature for efficient luminescent devices.