J. Phys. Chem. C 2010, 114, 2573–2582
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Self-Assembled 3D Urchin-Like NaY(MoO4)2:Eu3+/Tb3+ Microarchitectures: Hydrothermal Synthesis and Tunable Emission Colors Zhenhe Xu, Chunxia Li,* Guogang Li, Ruitao Chai, Chong Peng, Dongmei Yang, and Jun Lin* State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: December 3, 2009; ReVised Manuscript ReceiVed: January 5, 2010
Novel 3D urchin-like NaY(MoO4)2 microarchitectures have been successfully synthesized by a complexingagent-assisted hydrothermal process followed by a subsequent heat treatment process. The shape and size of the double alkaline rare earth molybdates precursor microstructures can be tuned effectively by controlling the reaction conditions, such as reaction time, the amount of organic additive trisodium citrate (Cit3-), and the kinds of organic additives. The possible formation mechanism for urchin-like microarchitectures has been presented. It is found that the Cit3- organic molecule, acting as the chelating agent and shape modifier, plays a key role in fine-tuning the precursor microstructures. The as-formed precursor could transform into NaY(MoO4)2 with their original urchin-like morphology and slight shrinkage in the size after postannealing process. Under UV and low-voltage electron beam excitation, 5 mol % Eu3+ and 5 mol % Tb3+ doped NaY(MoO4)2 samples exhibit strong red and green emission, corresponding to the characteristic lines of Eu3+ and Tb3+, respectively. Moreover, the luminescence colors of the Eu3+ and Tb3+ codoped NaY(MoO4)2 samples can be tuned from red, yellow, and green-yellow to green by simply adjusting the relative doping concentrations of the activator ions under a single wavelength excitation, which might find potential applications in the fields such as light display systems and optoelectronic devices. 1. Introduction In modern chemistry and materials science, the chemical and physical properties of functional materials consisting of either inorganic compounds or inorganic/organic hybrids are fundamentally related to their size, shape, and dimensionality.1 So, rational control over these factors has become an important research issue in recent years,2 allowing us not only to observe unique properties of the materials but also to tune their chemical and physical properties as desired. Many research efforts have focused on the rational ways to control the shape, size, and dimensionality of nanomaterials. Among various methods used, self-assembly of inorganic nanobuilding blocks into onedimensional (1D), two-dimensional (2D), and three-dimensional (3D) ordered hierarchical nanostructures is fascinating because the variation of the arrangements of the building blocks provides a means to tune the property of the material.3 In particular, threedimensional (3D) nanostructured architectures have been explored for a new generation of advanced devices such as catalysis, water treatments, sensors, and energy4-10 owing to some improved properties originating from their nanobuilding blocks and the manners in which they are organized. Up to now, a wide variety of inorganic materials, including metal,11a metal oxide,11b,c hydrate,11d borate,11e molybdate,11f,g and tungstate,11h have been successfully prepared with complex 3D hierarchical shapes. However, the exploration of reasonable synthetic methods for controlled construction of complex 3D architectures of other inorganic functional materials via a chemical selfassembly route is still an intensive and hot research topic. Recently, the hydrothermal method as a typical solution-based * Authors to whom correspondence should be addressed. E-mail: cxli@ ciac.jl.cn (Dr C. X. Li) and
[email protected] (Prof. J. Lin).
approach has been proven an effective and convenient process in preparing various inorganic materials with diverse controllable morphologies and architectures in terms of cost and potential for large-scale production.12 Furthermore, during the hydrothermal process, one of the promising and popular strategies of controlling the shape and size of a targeted material is to select carefully an appropriate organic additive with functional groups that selectively adheres to a particular crystal facet and effectively slows the growth of that facet relative to others, leading to the morphological modification of the crystals.12c Among a variety of organic additives, trisodium citrate (Cit3-) is one of the most common and important organic molecules that has been used extensively as the stabilizer and structure-directing agent to control the nucleation, growth, and alignment of crystals.13,14 In our previous researches, we have studied systematically the key roles of Cit3- in tailoring the crystal phases, shapes, and sizes of NaREF4 (RE ) Y, Yb, and Lu) and REF3 nano/ microcrystals.15 As a continuation and extension of this work, here we will extend this method to the fabrication of NaY(MoO4)2 crystals to test the role of Cit3- in controlling crystal phases and shapes of the final products. Recently, the materials emitting multiple colors have become a research focus because of their important role in the field of light display systems, lasers, and optoelectronic devices.16 As the most frequently used activators in luminescent materials, rare earth ions have been playing an important role in modern lighting and display fields due to the abundant emission colors based on their 4ff4f or 5df4f transitions.17 As important solidstate materials, rare earth molybdates have attracted much interest in the past three decades because of their remarkable properties in areas such as ferroelectricity and ferroelasticity, laser hosts, phosphors, ionic conductivity of oxygen, etc.18
10.1021/jp9115029 2010 American Chemical Society Published on Web 01/22/2010
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Among them, double alkaline rare earth molybdates with the general formula of NaY(MoO4)2 form a wide variety of inorganic compounds having tetragonal and monoclinic symmetries that have attracted great interest in recent years.19 Nevertheless, previous work about the NaY(MoO4)2 family compounds was reported mostly on the single crystals to be used as laser crystal materials for a long period of time.20 Recently, stimulated by both the promising applications and the interesting properties, much attention has been directed to the controlled synthesis of rare earth molybdates with different shapes and the investigation of their size/shape-dependent properties in the past several years.21 Nevertheless, to the best our knowledge, there are no systematic accounts about the effects of Cit3- on the morphological growth process of monodisperse NaY(MoO4)2 3D complex architectures with uniform size and morphology, which provides superiority for their further application. Furthermore, for applications such as lamps and screens, where the phosphor is applied as a layer, narrow size distribution is strongly desirable to achieve homogeneous devices and lower the cost by decreasing the amount of matter deposited.21c In this paper, we report a controllable route for the production of novel self-assembled 3D urchin-like NaY(MoO4)2 microarchitectures by a complexing-agent-assisted hydrothermal process followed by further calcination treatment. We can control the size and shape of the products by simply changing multiple experimental parameters such as the kinds of organic additives, reaction time, and the amount of Cit3-. Furthermore, the formation process and luminescence properties of the assynthesized samples have been investigated in detail. 2. Experimental Section 2.1. Materials. The rare earth oxides Ln2O3 (Ln ) Y, Eu) (99.99%) and Tb4O7 (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry, and other chemicals were purchased from Beijing Chemical Company, China. All chemicals are of analytical grade reagents and used directly without further purification. 2.2. Preparation. In a typical synthesis, 0.5 mmol of Y2O3 was dissolved in dilute HCl, resulting in the formation of a colorless solution of YCl3. After evaporation followed by drying at 100 °C for 12 h in ambient atmosphere, a powder of YCl3 was obtained. Then, the YCl3 powder was added into 20 mL of aqueous solution containing 1.2 mmol of trisodium citrate (Cit3-) to form the Y3+-Cit3- complex. After vigorous stirring for 30 min, 1 mmol of Na2MoO4 · 2H2O was added into the above solution. After additional agitation for 30 min, the as-obtained mixing solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed and maintained at 180 °C for 12 h. As the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 80 °C for 12 h. The final products (NaY(MoO4)2) were retrieved through a heat treatment at 800 °C in air for 4 h. Eu3+and Tb3+-codoped double alkaline rare earth molybdates precursor microstructures were prepared in a similar manner except that Eu2O3 and Tb4O7 together with Y2O3 were the starting materials. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ ) 0.15405 nm). Fourier transform infrared spectroscopy (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the
Xu et al.
Figure 1. XRD pattern of the as-prepared precursor through a hydrothermal process at 180 °C.
KBr pellet technique. Thermogravimetric (TGA) data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 °C · min-1 in an air flow of 100 mL · min-1. The X-ray photoelectron spectra (XPS) were taken on a VG ESCALAB MK II electron energy spectrometer using Mg KR (1253.6 eV) as the X-ray excitation source. The morphology and structure of the samples were inspected using a field emission scanning electron microscope (FE-SEM, XL 30, Philips) equipped with an energydispersive X-ray (EDX) spectrometer and a transmission electron microscope. Low- and high-resolution transmission electron microscopy (TEM) was performed by using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The ultraviolet-visible photoluminescence (PL) excitation and emission spectra were recorded with 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 (
