Luminescent property of CaW[O.sub.4] powders prepared with aqueous reactions.
Scheelite calcium tungstate is well known for their interesting luminescence and structural particularities and therefore has been extensively studied during the past century. For example, Kroger wrote a monograph presenting a complete summary of the luminescence properties of these and related materials(Kroger, F.A., 1948). It was also used for 75 years in X-ray photography as screen intensifies due to its capability of absorbing X-rays and converting their energy into radiation enabling the blackening of the photographic film(Blasse, G. and B.C. Grabmaier, 1994). Nowadays the challenge is to use CaW[O.sub.4] as solid-state optoelectronic devices like lasers, optical fibers components or scintillators (Nikl, M., P. Bohacek, 2002; Nikl, M., P. Bohacek, 2000; Kobayashi, M., M. Ishii, 1993; Nikl, M., 2000). CaW[O.sub.4] is also the object of interesting structural studies because it presents a great variety of phases depending on the preparation conditions (Errandonea, D., M. Somayazulu, 2003; Errandonea, D., F.J. Manjon, 2004; Eung Soo Kim, Soon Ho Kim, 2006) D. Errandonea, F.J. Manjon, M. Somayazulu and D. Hausermann, J. Solid State Chem. Ill (2004), p. 1087. Article | PDF (507 K) | View Record in Scopus | Cited By in Scopus (30).
The nanomaterials and nanostructures are of a great interest for the modern science and technology. The properties and phenomena of these materials and structures are mainly due to the quantum confinement (QC), which is determined by the sizes of 10-20 interatomic distances, and to the surface/interface effects, which are amplified by the enormous surface/volume ratio([10.sup.8]-[10.sup.8][m.sup.-]1). Luminescent property of CaW[O.sub.4] nanocrystal may be dependent on the character of the powders. The character is found to depend on the synthesis process and processing parameters, such as, reaction temperature, and pH, et al.
The scheelite CaW[O.sub.4] nano-crystalline has been synthesized with various techniques, including combustion process(Xiaoming Lou and Donghua Chen, 2008), the hydrothermal process(Fang Leia and Bing Yan, 2008), sonochemical method (Titipun Thongtem, Anukorn Phuruangrat), pulsed laser induced synthesis(Jeong Ho Ryu,
Sin Young Bang, 2007), solids reaction method(Eung Soo Kim, Soon, 2006), a molten salt method(Yonggang Wang, Junfeng Ma, 2006). In this work, we reported (i) low cost synthesis of CaW[O.sub.4] nanocrystal with aqueous reaction-calcination processes and (ii) the excellent luminescent properties of the synthesized CaW[O.sub.4] nanocrystals and (iii) the discussion on air atmosphere effect of the luminescence property.
2. Experimental Procedure
The CaW[O.sub.4] powders were prepared by a method based on an approach previously used for fabrication of 3D metal tungstates (MW[O.sub.4], M=Mn, Co, Ni and Cu). Equimolecular [Ca(N[O.sub.3]).sub.2] and [Na.sub.2] W[O.sub.4] were respectively dissolved in distilled water. The two solutions are of concentration of 0.01M respectively. Two solutions were then slowly mixed with a constant stirring. In the process of the mixing, a white precipitation was fast formed in the precursor. By filtering and washing with distilled water, the precipitation was then dried at 100[degrees]C for 4h and calcined at different temperatures of 400-700[degrees]C for 1h and 2h respectively.
The phase indentification of the CaW[O.sub.4] powders was conducted at room temperature using X-Ray diffractometer (XRD, Cu[K.sub.[alpha]1], [lambda]=0.15406nm, Model No. D/Max-2200PC, Rigaku, Japan). Scanning electron microscopy (SEM, Model No: JXM-6700F, Japan) was used to analyze the particle morphology and the agglomeration of the powders. The luminescent properties of the CaW[O.sub.4] nanocrystals were measured on the luminescent spectrophotometer (Modal No: LS-55, PE, US).
3. Results and discussion
The CaW[O.sub.4] powders calcined at 400-500[degrees]C for 1h show grey color. With increasing calcining temperature and calcining time, all other powders appear white color. The XRD patterns of the CaW[O.sub.4] powders are shown in Fig. 1 and Fig. 2, which indicated that monoclinic scheelite structure was an only XRD-detactable phase for all powders prepared in this experiment. The intensities of the XRDe peaks of the powders calcined at 600[degrees]C and 700[degrees]C for 1h were respectively about 3000 counts. The intensity increased to 3500 counts with increasing calcining temperature to 700[degrees]C for 2h. The large intensity of The XRD peak corresponds to a large increase in crystallinity of the powders. The average crystalline sizes calculated by Scherrer's equation were 28.5-44.7nm, as summarized in table 1. The lattice constant a and c of the CaW[O.sub.4] powders were calculated from XRD data analyses, and are summarized in table 1. The CaW[O.sub.4] powders possess of a larger constant a and a less c for calcining time of 1h than for 2h.
[FIGURE 1 OMITTED]
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The SEM micrographs of the CaW[O.sub.4] powders are shown in Fig. 3, which indicated that the CaW[O.sub.4] powders were of particle size about 100-1000nm, which increased remarkably with calcining temperature and calcining time. The particle calcined at 600-700[degrees]C for 1h and calcined at 400-500[degrees]C for 2h appear nearly spherical morphology, but become quasi-tetragonal at 600-700[degrees]C for 2h.
Scherrer's equation is only available in crystalline size range of 1-100 nm, the result calculated by Scherrer's equation is also affected by widening of diffraction peak resulted from micro-strain and dislocation in crystalline particle. So that, the sizes determined from XRD should only be approximate data, the real sizes of the synthesized powder grains should be the results determined by SEM analysis.
The luminescence properties of the CaW[O.sub.4] nanocrystals were determined on the luminescent spectrophotometer and shown in Fig. 4 and Fig. 5. The wavelengths of monitor and excitation are at 425nm (2.92eV) and 247nm (5.02) respectively. The powders showed a broad excitation band and a broad deep bluegreen emission band. Two main emissions are near 406nm (~3.05eV) and 424nm (~2.92eV). Table 2 summarizes the positions and intensities of excitation and emission bands of the CaW[O.sub.4] powders. The intensities of excitation peaks and emission peaks increased with increasing calcining temperature for either time of 1h and 2h, which can be attributed to increase in the grain size of the CaW[O.sub.4] powders. The peak intensities also increased with calcining time, except for emission peaks of the powders calcined at 700oC for 2h are lower than that for 1h, however these increases were not matched with the increases of grain sizes and crystallinity as that with the calcining temperature. This phenomenon can be explained as follow. The air atmosphere changed the defect subsystem of the particle surface in longer period of the higher temperature calcination. Yakovyna et al (2008) reported that thermal treatment in different atmospheres causes change of the defect subsystem of the CaW[O.sub.4] crystal, resulting in the alteration of the spectral dependences. Thermal treatment in oxygen atmospheres at 900-1200[degrees]C resulted in a relative decrease in the intensity of the emission band (~2.9eV) which is excited at 230nm (~5.4eV). However the intensity of the emission band (~2.3eV) obviously increased when excited below the fundamental absorption edge 4.7eV (4.4eV, [lambda],=280nm) but decreased abruptly with increasing thermal treatment temperature to 1200[degrees]C. It is generally accepted that emission of tungstate with scheelite structure excited at high energy is due to radiative decay of self-trapped excitations localized at regular W[O.sub.4.sup.2] complexes while emission excited at low energy is associated with anion defective W[O.sub.3] complexes(Mikhailik, V.B., H. Kraus, 2004; Mikhailik, V.B., H. Kraus, 2005). The air atmosphere would similarly resulted in decreases of anion defective W[O.sub.3] complexes in the long calcining period of 2h. The excitation (5.02eV, 247nm) was also near the low energy (4.7eV). Thus, the powders prepared in this work do not shown emission intensities that matched with their grain sizes at calcining time of 2h. In addition, the calcining time of 2h resulted in larger anisotropy in atomic arrangements and larger crystallinity of the crystal than that of 1h. Orhan et al (2005) reported that the luminescence intensity of the CaW[O.sub.4] is much higher in the disordered films than in the crystalline one. Their experimental results strongly indicate that the luminescence of the CaW[O.sub.4] excited at 488nm is very sensitive to its structure and that relatively weak variations in the atomic arrangements can induce significant changes in the emission spectra. Their result also could be associated with the anion defective W[O.sub.3] complexes due to the films were calcined in air and oxygen atmospheres (Orhan et al 2005).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The stronger light diffusion of the powder with a less size generally resulted in less intensity of luminescence, and so larger grain size is required for higher luminescence property. However, effect of specific surface area used for the absorption and emission of the light could becomes superior with increasing grain size to enough large. This also may be a reason of the change in luminescence intensity with the grain size of the powder.
Ultrafine CaW[O.sub.4] powders have been successfully synthesized with aqueous reaction process. The powder characters and luminescence property of powders calcined at 400-700[degrees]C for 1-2h were studied. The synthesized powders show excellent luminescence.
The crystallinity and grain size of the powders increased with increasing calcining temperature and calcining time. The powders showed a broad excitation band and broad deep blue-green emission band. The emission intensity obviously increased with increasing calcining temperature. The emission intensity do not matched with the grain size with increasing calcining time. The calcining atmosphere of air should be responsible for the luminescence change with calcining time. The CaW[O.sub.4] powder calcined at 700[degrees]C for 1h shown a largest luminescence intensity of 926 counts at excitation wavelength of 425nm (2.92eV). The excellent luminescence property, low cost and convenient synthesis technique, make the CaW[O.sub.4] powder able for many potential luminescent applications. It will be of interest to investigate the synthesis and luminescence property of the CaW[O.sub.4] powder at various calcining atmospheres under different excitation energy to improve the luminescence intensity.
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Haiyan He, Jianfeng Huang, Liyun Cao, Xianxiang Ao
College of materials science and engineering, Shaanxi university of science and technology, China(710021)
Corresponding Author: Haiyan He, College of materials science and engineering, Shaanxi university of science and technology, China(710021), Email: firstname.lastname@example.org
Table 1: The lattice parameter of the CaW[O.sub.3] powders determined with the XRD data analysis, and grain size determined with SEM micrographs calcining schedule lattice parameter particle size a(A) c(A) c/a (nm) 500[degrees]C1h 5.244 11.323 2.159 600[degrees]C1h 5.244 11.348 2.164 310 700[degrees]C1h 5.244 11.345 2.163 790 400[degrees]C2h 5.239 11.333 2.163 130 500[degrees]C2h 5.236 11.357 2.167 380 600[degrees]C2h 5.232 11.359 2.171 770 700[degrees]C2h 5.219 11.366 2.178 860 Table 2: Characters of the excitation and emission peaks of the CaW[O.sub.4] powders ([[lambda].sub.em] = 425nm, [[lambda].sub.ex] = 247nm) Calcining Schedules Excitation Position (nm) Intensity (counts) 600[degrees]C 1h 231.0 801.08 248.5 850.70 700[degrees]C 1h 231.0 845.88 248.5 859.35 400[degrees]C 2h 231.0 762.08 247.5 771.00 500[degrees]C 2h 231.5 745.12 248.0 742.20 600[degrees]C 2h 232.0 860.51 248.0 899.57 700[degrees]C 2h 231.5 883.65 247.5 871.16 Calcining Schedules Emisssion Position (nm) Intensity (counts) 600[degrees]C 1h 405.5 875.82 425.0 869.24 700[degrees]C 1h 404.5 910.13 423.5 926.24 400[degrees]C 2h 406.0 780.03 426.0 769.06 500[degrees]C 2h 408.0 727.49 422.0 778.94 600[degrees]C 2h 408.5 893.19 424.5 905.15 700[degrees]C 2h 403.5 875.35 425.0 892.20
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|Title Annotation:||Original Article|
|Author:||He, Haiyan; Huang, Jianfeng; Cao, Liyun; Ao, Xianxiang|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||May 1, 2009|
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