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J. Phys. Chem. C 2010, 114, 6928–6936
Mesoporous SrF2 and SrF2:Ln3+ (Ln ) Ce, Tb, Yb, Er) Hierarchical Microspheres: Hydrothermal Synthesis, Growing Mechanism, and Luminescent Properties Cuimiao Zhang,† Zhiyao Hou,† Ruitao Chai,† Ziyong Cheng,† Zhenhe Xu,† Chunxia Li,† Ling Huang,‡ and Jun Lin*,† State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China, and School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore 639798 ReceiVed: December 13, 2009; ReVised Manuscript ReceiVed: March 5, 2010
Uniform and well-dispersed SrF2 hierarchical microspheres have been successfully synthesized by a facile and friendly hydrothermal method. The experimental results indicate that such 3D hierarchical SrF2 microspheres with size about 1 µm are assembled by numerous nanocrystals (about 100 nm). Nitrogen adsorption-desorption measurement suggests that mesopores exist in these hierarchical microarchitectures. The formation process of the hierarchical SrF2 microspheres and luminescence properties of SrF2:Ln3+ (Ln ) Ce, Tb, Yb, Er) microspheres have been investigated in detail. It is found that reaction time and trisodium citrate play a key role in forming the hierarchical microspheres. In addition, the lanthanide ions (Ln3+)-doped SrF2 hierarchical microspheres show strong green (Ln3+ ) Ce3+, Tb3+) photoluminescence with 45% quantum efficiency and cathodoluminescence centered at 542 nm corresponding to the 5D4 f 7F5 transition of Tb3+ under excitation of ultraviolet light and low-voltage electron beam, respectively. In addition, SrF2:Yb3+,Er3+ hierarchical microspheres show bright yellow upconversion emission under 980 nm laser diode excitation. The as-synthesized SrF2:Ln3+ luminescent microspheres might find some potential applications in areas of photoluminescence and field emission displays. 1. Introduction Generally, the chemical and physical properties of inorganic micro/nanostructures are fundamentally related to their chemical composition, size, phase, surface chemistry, shape, and also dimensionality.1–4 Therefore, the systematic control over these factors of inorganic materials represents a great challenge in the modern synthetic field.5,6 Nanomaterials have many interesting properties that differ from those of the bulk materials.7 However, most nanomaterials often have a natural tendency toward aggregation, which is always assumed to be the main hindrance to their practical application. Recently, extensive work has been devoted to the investigation of complex microarchitectures, especially, three-dimensional (3D) hierarchical architectures assembled by nanostructured building blocks such as nanoplates, nanoparticles, nanoribbons, nanorods, and so forth.4,8 It is very useful for practical application that this kind of material not only possesses some improved properties originating from their building blocks named nanocrystals, but also solves the problem of nanoparticle agglomeration.4 On the other hand, such materials are not only a crucial step for the realization of “bottom-up” techniques toward to future nanodevices but also offer opportunities to explore their novel collective optical, mechanical, magnetic, and electronic properties.9–12 There have been extensive studies to explore approaches to the synthesis of hierarchical materials, such as flower-like SrCO3 superarchitectures,8 hierarchical ZnO nanostructures,13 dandelionlike ZnS microspheres,14 and so on. Although great progress has been achieved on the synthesis approaches for hierarchical * To whom correspondence should be addressed. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Nanyang Technological University.
architectures (such as chemical vapor deposition methods or solution-phase chemical routes), they usually require catalysts, expensive and even toxic templates or surfactants, high temperature, and a series of complicated procedures.8 Therefore, it is still a big challenge to develop simple and reliable synthetic methods for hierarchical architectures with designed chemical components and controlled morphologies, which strongly affect the properties of nano/micromaterials. The solid fluoride materials have attracted much attention due to their unique properties, such as low-energy phonons, high ionicity, electron-acceptor behavior, high resistivity, and anionic conductivity.15–17 These properties lead to a wide range of potential optical applications in optics, biological labels, and lenses,18,19 as well as components of insulators, gate dielectrics, wide-gap insulating overlayers, and buffer layers in semiconductor-on-insulator structures.20 As an important kind of alkaline earth metal fluoride, strontium fluoride (SrF2) is dielectric and thus has great applications in microelectric and optoelectric devices.20,22 However, to our knowledge, there are few reports on the controlled synthesis of well-controlled SrF2 microstructures.22 The major reason might be that the rapid precipitation reaction between soluble strontium salts and NaF/NH4F in aqueous solutions makes it difficult to achieve a controlled nucleation and growth process, which is prerequisite to obtain uniform and well-defined nano/microstructures.21,23 On the other hand, SrF2 used as an attractive host for phosphors and activated with lanthanide ions (Ln3+) have also been reported to display unique up/down-conversion luminescence properties arising from their 4f electron configuration.24,25 It is reasonable to expect that uniform fluorides will play an important role in technological applications, including high-density optical storage devices, color displays, and so on.18
10.1021/jp911775z 2010 American Chemical Society Published on Web 03/31/2010
Mesoporous SrF2 and SrF2:Ln3+ Hierarchical Microspheres Herein, we demonstrate a simple and one-step strategy for the synthesis of mesoporous SrF2 and SrF2:Ln3+ (Ln ) Ce, Tb, Yb, Er) hierarchical microspheres via a hydrothermal process. The external surface of as-obtained SrF2 microspheres, which consists of numerous randomly aggregated nanoparticles of about 100 nm, is extensively rough and porous. The effects of various reaction conditions (reaction time and organic additive) on the morphology of the SrF2 samples have been discussed in detail. On the other hand, the Ce3+ and Tb3+ codoped SrF2 microspheres show an intense green photoluminescence (PL) and cathodoluminescence (CL) under UV light irradiation and low-voltage electron beam, respectively. And the SrF2:Yb3+,Er3+ microspheres exhibit bight upconversion (UC) luminescence through single laser excitation at 980 nm. The power dependence of UC emission intensities and the upconversion mechanisms of SrF2:Yb3+,Er3+ sample under 980 nm laser excitation have also been investigated. 2. Experimental Section 2.1. Chemicals and Materials. The initial chemicals, including Tb4O7, Yb2O3, Er2O3, and Ce(NO3)3, with purity of higher than 99.99% were purchased from Changchun Applied Chemistry Science and Technology Limited, China, Tb4O7, Yb2O3, and Er2O3 were dissolved in dilute HNO3 solution and then removed the residual HNO3 by heating and evaporation, resulting in the formation of clear solutions of Tb(NO3)3, Yb(NO3)3, and Er(NO3)3, respectively. Other chemicals were analytical grade reagents purchased from Beijing Chemical Corporation. All the initial chemicals in this work were used without further purification. 2.2. Preparation. Mesoporous SrF2 microspheres were prepared by a hydrothermal process. In a typical experiment, 2 mmol of Sr(NO3)2 and 4 mmoL of trisodium citrate (labeled as Cit3-) were dissolved in deionized water to form a 20 mL solution of 1. Then, 4 mmol of NaBF4 was added into 15 mL of H2O to form solution 2. After vigorous stirring for 15 min, solution 2 was introduced into solution 1. The white precipitation appeared immediately. After additional agitation for 20 min, the as-obtained suspension was transferred into a Teflon bottle (50 mL) that was held in a stainless steel autoclave, sealed, and maintained at 180 °C for 24 h. As the autoclave cooled to room temperature naturally, the precipitate was separated by centrifugation, then washed with deionized water and ethanol in sequence. Finally, the precipitate was dried in air at 70 °C for 24 h to obtain the final SrF2 microspheres. Additionally, different hydrothermal treatments in the absence of trisodium citrate and different reaction time at 180 °C were selected to investigate the morphological evolution of the SrF2 microspheres. The luminescent SrF2:Ce3+,Tb3+ or SrF2:Yb3+,Er3+ microspheres were prepared by the same procedure as above, except for adding a stoichiometric amount of Tb(NO3)3 (2 mol %), Ce(NO3)3 (2 mol %), Yb(NO3)3 (3 mol %), and Er(NO3)3 (1 mol %) aqueous solutions into solution 1 at the initial stage. 2.3. Characterization. The X-ray diffraction (XRD) patterns of the samples were carried out on a D8 Focus diffractometer (Bruker) with use of Cu KR radiation (λ ) 0.15405 nm). FTIR spectra were performed on a Perkin-Elmer 580B infrared spectrophotometer, using the KBr pellet technique. The morphology of the samples was inspected with use of a scanning electron microscope (SEM; S-4800, Hitachi). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were obtained from a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Nitrogen adsorption/
J. Phys. Chem. C, Vol. 114, No. 15, 2010 6929
Figure 1. XRD patterns of as-prepared SrF2 (a), SrF2:Ce3+,Tb3+ (b), and SrF2:Yb3+,Er3+ (c) samples by hydrothermal process at 180 °C for 24 h, and the standard data of SrF2 (JCPDS No. 06-0262) as a reference.
desorption analysis was measured with a Micromeritics ASAP 2020 M apparatus. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method. The photoluminescence (PL) measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were carried out in an ultrahigh vacuum chamber (
