Preparation of Monodispersed Eu3+:CaMoO4 Nanocrystals with Single Quasihexagon Sheng Yu,†,‡ Zhoubin Lin,† Lizhen Zhang,† and Guofu Wang*,† Fujian Institute of Research on the Structure of Matter, State Key Laboratory of Structural Chemistry, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2397–2399
ReceiVed September 13, 2006; ReVised Manuscript ReceiVed September 12, 2007
ABSTRACT: This paper reports the preparation of Eu3+:CaMoO4 nanocrystals by the microemulsion-mediated hydrothermal method. The synthesized Eu3+:CaMoO4 nanocrystals were characterized by XRD, transmission electron microscopy, high-resolution transmission electron microscopy, excitation, and emission spectra. The result shown that Eu3+:CaMoO4 nanocrystals with sizes of 40–90 nm exhibited a monodispersed and single quasihexagon. Eu3+:CaMoO4 nanocrystals exhibit a strong red fluorescence at 614 nm. This result may bring opportunity for the development of the other nanocrystals. Recently, the optical properties of rare-earth-doped nanocrystals have attracted much interest because of their potential application as phosphors in lighting and display devices, such as plasma display panel (PDP) and field emission display (FED). Main research was focused on the Eu3+-doped nanocrystalline materials1–5 because Eu3+-doped nanophosphors give rise to strong red emission and play an important role in emissive display technology. Currently, much research was focused on the properties of nanodimensioned Y2O3: Eu3+ because it is the unprecedented red emitting phosphor with high luminescence quantum efficiency in fluorescence lamps and projection television tubes.6–11 However, because Y2O3-based phosphors are relatively expensive, research on non-rare-earth hosts with low cost is gaining interest. Pode et al. reported the preparation of CaWO4:(Bi3+, Eu3+) powder and its luminescence,12 which was suggested as an efficient red phosphor. Recently, the attentions were paid to the molybdate except the tungstate.13 Calcium molybdate (CaMoO4) is an important member of the metal molybdate families that have much potential application in various fields, such as in photoluminescence14 and microwave applications.15 CaMoO4 crystal belongs to the tetragonal system with space group I41/a and unit-cell parameters: a ) 5.223 Å.16 CaMoO4 powders can be prepared by several methods, such as the coprecipitation method,17 combustion method,18 and conventional solid-state reaction.19 However, using these methods, the obtained powders have relatively large size and inhomogeneous morphology and composition. As is well-known in synthesizing nanocrystals, controlling the single morphology of nanocrystals is important except the size. Therefore, in order to obtain nanocrystalline materials with homogeneous morphology and composition, we applied the microemulsion-mediated hydrothermal method to synthesize Eu3+:CaMoO4 nanocrystals. This paper reports the synthesis and photoluminescence properties of europium-doped calcium molybdate (Eu3+:CaMoO4) nanocrystals with monodispersed and novel quasihexagonal morphology. The chemicals used were cationic surfactant CTAB (99%+), calcium chloride (99%+), sodium molybdate (99%+), europium chloride (99%+), cyclohexane (99%+), 1-butanol (99%+), and ethanol (95%+). Deionized water was used as the solvent. Eu3+: CaMoO4 nanocrystallines were synthesized by the microemulsionmediated hydrothermal method. The microemulsion-mediated hydrothermal method consists of a microemulsion process and hydrothermal process. The synthetic procedures were as follows: First, 0.0027 mol CaCl2, 0.0002 mol EuCl3, and 1 mL of H2O were * Corresponding author. Tel: 86-591-3714636. Fax: 86-591-3714636. E-mail:
[email protected]. † State Key Laboratory of Structural Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
Figure 1. XRD patterns of Eu3+:CaMoO4 nanocrystals.
mixed to form an aqueous solution, and the formed solution was mixed with a mixture of CTAB (2 g)/cyclohexane (60 mL)/1butanol (3 mL) to form the microemulsion solution. Second, 0.001 mol Na2MoO4 solution was mixed with a mixture of CTAB (2 g)/ cyclohexane (60 mL)/1-butanol (3 mL) to form another microemulsion solution. After being stirred, the two optically transparent microemulsion solutions were mixed together and stirred for 20 min. Third, the obtained new microemusion solution was then transferred into 20 mL stainless Teflon-lined autoclave. The sealed tank was heated to 180 °C at a rate of 5 °C/min, held at this temperature for 10 h in an oven, and then cooled to room temperature naturally. Lastly, a centrifuge collected the precipitates with white color. After being washed several times using the deionized water and ethanol, the precipitates were dried at room temperature in atmosphere. The crystalline phase of the products was identified by X-ray powder diffraction (XRD) using a D/max-rA type diffractometer and employing CuKR radiation (λ ) 0.1541nm) at room temperature, as shown in Figure 1. The size and morphology of products were determined by a transmission electron microscopy (TEM, JEOL2010) operating at 200 kV. Structural information of the nanocrystals was measured by a high-resolution transmission electron microscopy (HRTEM). The excitation and emission spectra of the products were measured using an Edinburgh Analytical Instruments FLS920 fluorescence spectrophotometer.
10.1021/cg060611o CCC: $37.00 2007 American Chemical Society Published on Web 11/03/2007
2398 Crystal Growth & Design, Vol. 7, No. 12, 2007
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Figure 2. TEM images of quasihexagonal Eu3+:CaMoO4 nanocrystals.
Figure 3. TEM and HRTEM images of Eu3+:CaMoO4 nanocrystal. (a) Typical quasihexagonal Eu3+:CaMoO4 nanocrystal. (b) FFT pattern of Eu3+:CaMoO4 nanocrystal taken from a portion of the nanocrystal in (a).
Figure 4. Excitation spectra of Eu3+:CaMoO4 nanocrystals in the range of (a) 220–340 and (b) 340–500 nm.
According to the structure data of CaMoO4 crystal,16 all data of X-ray powder diffraction can be indexed as shown in Figure 1 The result of X-ray powder diffraction shows that the precipitates belong to CaMoO4 crystal. In Figure 1, no diffraction peaks of impurities were not detected. The strong and sharp diffraction peaks suggest that the as-prepared Er3+:CaMoO4 nanocrystals are well-crystallized.
Figure 2 shows the TEM images of Eu3+:CaMoO4 nanocrystals. The Eu3+:CaMoO4 nanocrystals exhibit a monodispersed and single quasihexagonal morphology, and the sizes of the quasihexagonal nanocrystal are 40–90 nm. A typical quasihexagonal nanocrystal is shown in Figure 3a. A high-resolution transmission electron microscopy (HRTEM) image was taken from a portion of it, as
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Crystal Growth & Design, Vol. 7, No. 12, 2007 2399 red 614 nm transition (5D0 f 7F2) is definitely stronger than the orange 592 nm transition (5D0 f 7F1), which is allowed in this case, as the europium does not occupy a center of symmetry. Eu3+:CaMoO4 nanocrystals have been successfully synthesized by the microemulsion-mediated hydrothermal method. Eu3+: CaMoO4 nanocrystals exhibit a monodispersed and single quasihexagon. Previously, although the microemulsion-mediated hydrothermal method was already successfully applied to prepared MgF2 and SrCO3 nanocrystallines with nanorod, nanoball, and nanoline morphologies,20,21 to the best of our knowledge, the monodispersed and single quasihexagonal nanocrystals have not been achieved via the microemulsion-mediated hydrothermal method. The Eu3+: CaMoO4 nanocrystals with the monodispersed and single quasihexagonal morphology exhibit a strong red fluorescence at 614 nm. This result may bring opportunities for the development of the other nanocrystals.
Acknowledgment. This project was supported by the National Natural Science Foundation of China (60378031) and the Key project of Science and Technology of Fujian Province (2001F004). Figure 5. Emission spectrum of Eu3+:CaMoO4 nanocrystals. 3+
shown in Figure 3b, which confirms that the Eu :CaMoO4 nanocrystal is a single crystal with uniform lattice structure, free of defects and dislocations. The corresponding fast Fourier transform (FFT) pattern (inset of Figure 3b) was indexed according to the tetragonal phase CaMoO4 along the [241˘ ] zone axis. The above results show that Eu3+:CaMoO4 nanocrystals with the monodispersed and single quasihexagon can be obtained by the microemulsion-mediated hydrothermal method. In this method, the microemulsion process can control the monodispersity of the products, and the hydrothermal process can improve the crystallinity of the products. Panels a and b in Figure 4 show the excitation spectra of Eu3+: CaMoO4 with excitation by the 614 nm wavelength radiation at the room temperature. The excitation bands consist of a broad excitation band in the range of 220–340 nm and some sharp excitation lines in the range of 340–500 nm. The ultraviolet excitation spectrum substantiates that the broad excitation band located at around 280 nm is excited from the ground state of the 4f shell of Eu ion to a Eu–O charge-transfer state. Figure 5 shows the emission spectrum of Eu3+:CaMoO4 with excitation by the 280 nm wavelength radiation at the room temperature. The these sharp emission transition lines correspond to the transitions from the excited state 5D0 of Eu3+ to the lower levels 7FJ (J ) 0–4), which are marked in Figure 5. It is wellknown that the probability of the 5D0 f 7F2 transition is very sensitive to the chemical surroundings of the Eu3+ ions. The intensity of the electric-dipole transition (5D0 f 7F2) is significantly affected by the degree of symmetry in the environments around Eu3+ ions. Conversely, the 5D0 f 7F1 emission is allowed by magnetic dipole considerations, being relatively not sensitive to the local symmetry. Thus, the Eu3+ ion located symmetry in crystal can be determined by the ratio of the 5D0 f 7F1 and 5D0 f 7F2 transitions. From Figure 5 it can be found that the intensity of the
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