Self-Assembled Growth of AgIn(MoO4)2 Submicroplates into Hierarchical Structures and Their Near-Infrared Luminescent Properties Shuyan Song,†,‡ Yu Zhang,† Jing Feng,†,‡ Yan Xing,† Yongqian Lei,†,‡ Weiqiang Fan,†,‡ and Hongjie Zhang*,†
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 848–852
State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Changchun 130022, Chinese Academy of Sciences, Beijing 100864, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed April 30, 2008; ReVised Manuscript ReceiVed October 9, 2008
ABSTRACT: Layer-controlled hierarchical flowerlike AgIn(MoO4)2 microstructures with “clean” surfaces using submicroplates as building blocks without introducing any template have been fabricated through a low-cost hydrothermal method. The nearinfrared luminescence of lanthanide ion (Nd, Er, and Yb) doped AgIn(MoO4)2 microstructures, in the 1300-1600 nm region, was discussed and is of particular interest for telecommunication applications. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, electron diffraction, and photoluminescence spectra were used to characterize these materials. Introduction The fabrication and self-organization of inorganic materials on different scales with a special size and well-defined shapes are attracting increasing interest, the resulting materials possessing substantially enhanced properties and the potential to design new materials and devices in various fields.1-11 Hierarchical micro/nanostructures, assemblies using nanoparticles, nanorods, nanoplates, and nanobelts as building blocks with specific morphologies and higher order, have received great interest in materials chemistry and nanotechnology owing to their important role in the systematic study of structure-property relationships and their improved physical and chemical properties over their single component.12-19 Exploration of reasonable synthetic methods for controlled construction of complex 3D architectures of other functional materials via a chemical selfassembly route is currently an intensive and hot research topic. The hydrothermal method, a solution-phase chemical method, is one of the most promising routes in these reports, due to its low cost and potential advantage for large-scale production.20-22 Recently, luminescent lanthanide complexes have attracted much attention because of their fundamental scientific importance and potential utility in a wide variety of photonic applications, such as planar waveguide amplifiers, plastic lasers, light-emitting diodes, and luminescent probes.23-25 There is a growing interest to use this luminescence in materials with nanosize, because of their peculiar properties in comparison with bulk phases.26,27 Also, lanthanide-doped materials, such as molybdates, oxides, phosphates, fluorides, and vanadates, have great utilizations, including phosphors, display monitors, X-ray imaging, scintillators, lasers, and amplifiers for fiber-optic communications based on the electronic, optical, and chemical characteristics arising from the 4f electrons.28,29 The luminescence efficiency of these materials is often limited by the dynamics of the lanthanide ion, which depends on the interaction between lanthanide ions and the host. A great number of nanostructure materials have been reported that are doped with lanthanide ions such as Eu3+ and Tb3+, which emit visible * To whom correspondence should be addressed. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry. ‡ Chinese Academy of Sciences and Graduate School of the Chinese Academy of Sciences.
Scheme 1. Growth Mechanism of AgIn(MoO4)2 Hierarchical Microstructures
light.30,31 In addition, processable nanostructures doped with near-infrared (NIR) emitting lanthanide as an active dopant for many different inorganic lattices, producing white-light emission by adjusting the ratio of blue, green, and red emissions, would be of particular interest as the active materials in telecommunication components, lasers, and LEDs.32-34 Metal molybdates represent an important class of inorganic materials with wide applications in various fields. Their intriguing photoluminescence behavior, structural properties, and photocatalytic properties make them good potential candidates in heterogeneous catalysis and building of humidity sensors, optical fibers, and scintillators.35-37 In comparison with the many studies on the metal tungstates, fewer investigations have been explored on metal molybdates.38,39 Also, there are only a few reports on the synthesis of nanosized double tungstates and molybdates.40,41 As a ternary metal molybdate, AgIn(MoO4)2 has a tetragonal structure with strong anisotropy. It can serve as a useful host lattice for rare earth ions to produce phosphors emitting a variety of colors, because their strong anisotropy (which can generate large optical cross sections) and large lanthanide impurity acceptance make these crystals very efficient laser hosts and laser Raman shifters compared to the wide range of other available hosts. Thus, special efforts to synthesize AgIn(MoO4)2 hierarchical structures with smaller sizes but a higher order are warranted to tailor their electronic and optical
10.1021/cg800445v CCC: $40.75 2009 American Chemical Society Published on Web 12/03/2008
AgIn(MoO4)2 Submicroplate Self-Assembled Growth
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Figure 1. FE-SEM images (a, b), TEM image (c), and XRD patterns (d) of the AgIn(MoO4)2 hierarchical microstructures.
Figure 2. FE-SEM images of AgIn(MoO4)2 submicrocrystals obtained at different concentrations of AgNO3: (a) 0.025 mmol, (b) 0.05 mmol, (c) 0.1 mmol, and (d) 0.2 mmol.
properties. Herein, we presente a template-free hydrothermal method for the fabrication of layer-controlled hierarchical flowerlike AgIn(MoO4)2 microstructures. The formation mechanism was discussed in detail together with their crystal structure. At the same time, near-infrared luminescent properties of lanthanide ion doped AgIn(MoO4)2 microstructures were investigated. Experimental Section All chemical reagents were of analytical grade and used as received without further purification. In a typical procedure, 5.0 mL of aqueous solution containing dissolved 0.0340 g of AgNO3 (0.20 mmol) and 0.0764 g of In(NO3)3 · 4.5H2O (0.20 mmol) was mixed with 5 mL of Na2MoO4 · 2H2O (0.0968 g, 0.40 mmol) aqueous solution. The suspension containing white precipitates was stirred for a further 40 min. Then the resulting suspension was transferred into a stainless steel autoclave with a Teflon liner of 23 mL capacity and heated in an oven at 180 °C
for 24 h. The average pressure produced in the autoclaves was about 1.0 MPa. After the autoclave was air-cooled to room temperature naturally, the resulting precipitates were filtered, washed with distilled water and absolute ethanol, and finally dried under vacuum at 60 °C for 4 h. Ln3+-doped AgIn(MoO4)2 microstructures were prepared by the same hydrothermal treatment as the undoped sample except that 0.01 mmol of Ln(NO3)3 · 6H2O (Ln ) Er, Nd, and Yb) and 0.19 mmol of In(NO3)3 · 4.5H2O were used instead of 0.20 mmol of In(NO3)3 · 4.5H2O. Ln3+-doped AgIn(MoO4)2 bulk materials were prepared by a solidstate reaction according to the literature.42 High-purity Ag2O, In2O3, Ln2O3, and MoO3 were mixed stoichiometrically and calcined at 823 K for 4 h and then at 1073 K for 24 h. X-ray power diffraction (XRD) patterns of the samples were collected on a Rigaku Dmax 2000 X-ray diffractometer with Cu KR radiation (λ ) 0.154178 nm) and 2θ ranging from 10° to 70°. Field-emission scanning electron microscopy (FE-SEM) images were obtained with an XL30 ESEM FEG microscope. Transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy
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Figure 3. Crystal structure of AgIn(MoO4)2: view from the c axis (a) and a axis (b).
Figure 4. SEM images of the samples obtained at different times: (a) 6 h and (b) 12 h. (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope with a LaB6 filament and an accelerating voltage of 200 kV. Room-temperature photoluminescence (PL) spectra were recorded on a Fluorolog-3 with a 400 W xenon lamp as the excitation source at room temperature. The luminescence decay curve was excited by a 320 nm laser (frequency doubled of a 640 nm dye laser narrow scan, Ridiant Dyes Laser Co., pumped by a continuum PPII 8100 Nd:YAG).
Results and Discussion The XRD pattern of the as-prepared product is shown in Figure 1d. All peaks appearing could be indexed to the pure phase of monoclinic AgIn(MoO4)2 (JCPDS card no. 36-0312), and no impurities are discernible. Figure 1a is the typical SEM image of the as-grown products, from which a number of uniform hierarchical flowerlike AgIn(MoO4)2 microstructures of diameter 4 µm are clearly observed. No other morphologies can be detected, indicating a high yield of these 3D microstructures. Higher magnification SEM images shown in Figure 1b demonstrate the detailed structural information of the sample. The observed spheres are constructed by many 2D sheets, which are densely packed and form a multilayered structure. Careful examination shows that some microstructures actually contain a concave in the middle section. In Figure 1b, we can further find that each layer/sheet is made up of numerous square submicroplates with an average side length of 200 nm. The tiny plates are attached side by side into integrated sheets, which is similar to the process of forming CuO43 and Bi2WO614 nanosheets from small nanoblocks. On the basis of the above results, the as-prepared microstructures can be generally classified as hierarchical structures. More details about the structure of the microstructures were investigated using TEM and SAED. The SAED images (Figure 1c) show that the obtained microstructures are building by many single-crystal submicroplates. It is worth mentioning that these hierarchical microstructures are sufficiently stable that they cannot be destroyed into dispersed submicroplates even after long periods of ultrasonication.
Figure 5. Emission spectrum for AgIn0.95Ln0.05(MoO4)2 (Ln ) Nd, Er, Yb) 3D hierarchical microstructures: (a) AgIn(MoO4)2:Er (λex ) 343 nm, green line); (b) AgIn(MoO4)2:Nd (λex ) 315 nm, red line); (c) AgIn (MoO4)2:Yb (λex ) 350 nm, blue line).
Figure 2 shows the SEM images of AgIn(MoO4)2 submicrocrystals obtained at different concentrations. On the basis of the SEM and TEM studies and experimental process, along with the crystal structure, we believe that the formation of such intricate microstructures is achieved via a hierarchical assembly process in terms of the change of concentration (Scheme 1). At first, AgIn(MoO4)2 tiny submicroplates were formed at low concentration, which was a typical Ostwald ripening process. At the beginning of the reaction, the crystalline nucleus supersaturated solution occurred, and then the larger particles grew at the cost of the small ones because there were different solubilities between relatively larger and smaller particles according to the Gibbs-Thomson law.44 The unique structural feature determined the morphology of AgIn(MoO4)2 submicrocrystals. From the structural images of AgIn(MoO4)2 (Figure 3), it is clear that the (100) and (010) facets bear more dangling bonds (which determine the chemical potential according to the
AgIn(MoO4)2 Submicroplate Self-Assembled Growth
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Figure 6. Luminescence decays for AgIn0.95Ln0.05(MoO4)2: (a) AgIn(MoO4)2:Er; (b) AgIn(MoO4)2:Nd; (c) AgIn(MoO4)2:Yb.
Gibbs-Thomson law) than any other facets and that growth along the [100] and [010] directions can release more energy, thus making the (100) and (010) facets of higher chemical potential than the other facets. Therefore, AgIn(MoO4)2 square submicroplates were obtained. Upon increasing the concentration of the reactant [AgNO3, 0.05 mmol (the concentrations of the other reactants were used stoichiometrically)], the monomer concentration in the solution is higher, which makes the chemical potential of these tiny submicroplates significantly higher than in the first stage. Driven by minimization, the chemical potential of the tiny submicroplates, and the total energy of the system, the small primary submicroplates begin to aggregate together to form submicroscaled microsheets. Further increasing the concentration (AgNO3, 0.2 mmol), the degree of the assembly process is enlarged, and finally the sheets are arranged into 3D hierarchical microstructures. The reaction time also influences the morphology of AgIn(MoO4)2. For example, when the crystallization duration is shorter than 12 h (AgNO3, 0.2 mmol), the yield of the desired mircrospheres is low and some amorphous products are observed as shown in Figure 4. Since AgLn(MoO4)2 and AgIn(MoO4)2 belong to the same crystal system and space group, AgIn(MoO4)2 can easily serve as a useful host lattice for rare earth ions to produce phosphors emitting a variety of colors. The luminescent properties of AgIn0.95Ln0.05(MoO4)2 (Ln ) Nd, Er, Yb) 3D hierarchical microstructures have been systematically investigated. The emission spectra of the AgIn(MoO4)2:Er 3D hierarchical microstructures was obtained by exciting the sample at 343 nm. The broad emission band extending from 1450 to 1653 nm and centered at 1535 nm is shown in Figure 5 and is attributed to the transition from the first excited state (4I13/2) to the ground state (4I15/2) of the partially filled 4f shell of Er3+. It is interesting to notice that the satellite peaks at 1490, 1547, 1562, and 1575 nm represent the transitions among the multiplet manifolds. This energy-level splitting caused by the crystal field of the matrix indicates that the active Er3+ centers are located within a welldefined crystalline environment. The transition around 1535 nm is in the right position for telecommunication applications. The full width at half-maximum (fwhm) of the emission spectrum
is 70 nm, which is very wide and has the potential of opticalamplification applications. The NIR emission spectra of AgIn(MoO4)2:Nd and AgIn(MoO4)2:Yb are depicted in Figure 5, which were collected by excitation at 315 and 350 nm, respectively. The NIR spectrum of AgIn(MoO4)2:Nd consists of three bands centered at λ ) 883, 1067, and 1344 nm, which are attributed to the f-f transitions of 4F3/2 (emitting level) f 4 I9/2, 4F3/2 f 4I11/2, and 4F3/2 f 4I13/2, respectively. The relative intensity sequence of these three transitions is I(4F3/2 f 4I11/2) > I(4F3/2 f 4I9/2) > I(4F3/2 f 4I13/2). Among the three bands of the emission spectra, the intensity of the transition at 1067 nm is the strongest, and for a long time this center has been found to have potential application in laser systems. In the NIR spectrum of AgIn(MoO4)2:Yb, the prominent 980 nm emission band can be observed, which is assigned to the 2F5/2 f 2F7/2 transition of the Yb3+ ion. It should also be noted that the Yb3+ ion emission band is not a single sharp band, but an envelope of bands arising at the lower energy side than the primary 980 nm band. Similar splitting has been reported previously.45,46 This was the result of splitting of the energy level of the Yb3+ ion as a consequence of crystal field effects. By using these lanthanide ions, the spectral region from 1300 to 1600 nm, which is of particular interest for telecommunication applications, can be covered completely. The emission spectra of the AgIn0.95Ln0.05(MoO4)2 hierarchical structures are almost the same as those of the bulk samples (Figures S2-S4, Supporting Information). Unlike other nanocrystals showing unique absorption and fluorescence characteristics such as CdSe and ZnO due to quantum size effects, since the luminescence of both lanthanide-doped AgIn(MoO4)2 hierarchical structures and bulk samples originates from the f-f transitions of 4f shells which are well-shielded by the 5s and 5p shells of Ln3+ ions and the transitions of the f electrons are mainly affected by the local symmetry of the crystal site, size effects on the luminescence of lanthanide-doped AgIn(MoO4)2 hierarchical structures are expected to be weak. Time-resolved measurements were carried out on these NIRluminescent compounds upon excitation at 397 nm and monitored around the most intense emission lines (at 1540 nm for AgIn(MoO4)2:Er, 1067 nm for AgIn(MoO4)2:Nd, and 980 nm
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for AgIn(MoO4)2:Yb). The luminescence decays for AgIn0.95Ln0.05(MoO4)2 (Ln ) Er, Nd, Yb) 3D hierarchical microstructures (Figure 6) are double-exponential functions of time, with lifetimes of 30 µs (51.9%) and 10 µs (48.1%) for AgIn(MoO4)2: Er, 3.7 µs (74.6%) and 7.4 µs (25.4%) for AgIn (MoO4)2:Nd, and 10 µs (36.6%) and 4.0 µs (63.4%) for AgIn(MoO4)2:Yb, respectively. The reason the lifetimes form double-exponential functions may be caused by lanthanide ions replacing two kinds of crystallographically unique indium ions in the structure. Conclusions In this work, layer-controlled hierarchical flowerlike AgIn(MoO4)2 microstructures using single-crystal submicroplates as building blocks were synthesized by the hydrothermal method without introducing any template or surfactant. The good nearinfrared luminescent properties of lanthanide ion doped AgIn(MoO4)2 microstructures, together with their interesting structure, open the field for telecommunication applications. Acknowledgment. We are grateful for financial aid from the National Natural Science Foundation of China (Grant Nos. 20631040 and 20610102007) and MOST of China (Grant Nos. 2006CB601103 and 2006DFA42610). Supporting Information Available: XRD patterns of the AgIn(MoO4)2 submicrocrystals obtained at low concentration (Figure S1) and emission spectra of the bulk AgIn0.95Ln0.05(MoO4)2 samples (Figures S2-S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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