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Photoluminescence in nanocrystalline MMoO4 (M = Ca, Ba) synthesized by a polymerized complex method
Materials Science and Engineering B 127 (2006) 154–158
Photoluminescence in nanocrystalline MMoO4 (M = Ca, Ba) synthesized by a polymerized complex method Jong-Won Yoon, Jeong Ho Ryu ∗ , Kwang Bo Shim Department of Ceramic Engineering, Ceramic Processing Research Center (CPRC), Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea Received 11 July 2005; received in revised form 29 August 2005; accepted 8 October 2005
Abstract Nanocrystalline MMoO4 (M = Ca, Ba) were successfully synthesized at low temperatures via a polymerized complex route and room temperature photoluminescent properties were investigated in detail. Prepared nanocrystalline MMoO4 (M = Ca, Ba) showed primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The photoluminescent spectra were decomposed to several individual Gaussian components in green and blue wavelength range. These photoluminescent features could be interpreted by Jahn–Teller splitting effect on [MoO4 2− ] tetrahedron of the nanocrystalline MMoO4 (M = Ca, Ba). © 2005 Elsevier B.V. All rights reserved. Keywords: MMoO4 (M = Ca; Ba); Polymerized complex method; Photoluminescence; Jahn–Teller splitting effect
1. Introduction Metal molybdates are important inorganic materials that have a high application potential in various fields [1–3], such as photoluminescence and electro-optic applications. Metal molybdates of relatively large bivalent cations (MMoO4 , ionic ˚ M = Ca, Sr, Ba, Pb) exist in the so-called scheelradius > 0.99 A, ite structure (scheelite = CaWO4 ), where the tungsten atom adopts tetrahedral coordination [4]. In general, metal molybdate powders are synthesized by conventional solid-state reaction method [4]. However, MMoO4 (M = Ca, Ba) powders prepared by the solid-state reaction are relatively large with irregular morphology, and inhomogeneous compounds might be easily formed because MoO3 has a tendency to vaporize at high temperatures [5]. These problems can be avoided by applying advanced wet chemical route as a modified Pechini process [6]. This method has been already used successfully to prepare highly pure powders of various complex oxides [7] and even various superconductors [8] with multiple cationic compositions. The metal molybdates exhibit the blue or green luminescent spectra, which is based on the radiative transition within tetrahedral [MoO4 2− ] group [1–3,9]. Mainly, the detailed studies for
∗
Corresponding author. Tel.: +82 2 2220 0543; fax: +82 2 2291 7395. E-mail address: [email protected] (J.H. Ryu).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.10.015
the luminescent properties have been focused on single crystals [1–3,9,10]. Moreover, nano-sized inorganic materials have attracted much interest to scientists in recent years, due to their wide range of optical and electrical properties. However, only few studies on the synthesis [11] and luminescent properties of the nanocrystalline MMoO4 (M = Ca, Ba) powders prepared via wet chemical route have been reported. In this study, we successfully synthesized nanocrystalline MMoO4 (M = Ca, Ba) by a polymerized complex method. The precursors and powders were evaluated for the crystallization process, thermal decomposition and particle morphology. Furthermore, room-temperature photoluminescent properties of the synthesized nanocrystalline MMoO4 (M = Ca, Ba) were investigated in detail. 2. Experimental Metal nitrate [Ca(NO3 )2 ·4H2 O and Ba(NO3 )2 , purity 99.99%, Junsei Chemical Co. Ltd., Japan] and ammonium para-molybdate [(NH4 )6 Mo7 O24 ·4H2 O, purity 99.99%, Wako Chemical Co. Ltd., Japan] were used as the metallic cations. Ethylene glycol (HOCH2 CH2 OH, EG, purity 99.9%, High Purity Chemical Co. Ltd., Korea) and citric acid [HOC(CO2 H)(CH2 CO2 H)2 , CA, purity 99.9%, Yukiri Pure Chemical Co. Ltd., Japan] were used as the solvent and chealating agent for the process. The citrate solutions were prepared
J.-W. Yoon et al. / Materials Science and Engineering B 127 (2006) 154–158
by dissolving appropriate molar ratio of citric acid in ethylene glycol (CA:EG molar ratio = 1:4). After complete homogenization of the citrate solution, metal nitrate and ammonium paramolybdate were dissolved in the molar ratio of total chelate metal cations (TO) and citric acid (TO:CA molar ratio = 1:5). By keeping the solution at a temperature of 100 ◦ C for 4 h under constant stirring, the solution became viscous pale yellow solution. In order to promote polymerization and remove excess of solvents, the solution was slowly heated up to 180 ◦ C, and maintained at this temperature for 12 h. The solution became more viscous and changed its color from pale yellow to brown. No visible precipitation was observed along such heating process. Finally, the viscous solution was heated up to 250 ◦ C for 12 h, and a solidified dark brown glassy resin was obtained. The resin was converted into powder by grinding with a Teflon bar. Heat-treatment of the precursor was performed at various temperatures from 300 to 700 ◦ C for 3 h in air. The crystallization process of the polymeric precursors was examined by thermogravimetry-differential thermal analysis (TG-DTA, SETRAM, France), using a sample weight of about 10 mg and a heating rate of 5 ◦ C/min. The phase after the heat treatment were identified by ordinary X-ray diffraction (XRD, Cu K␣, 40 kV, 30 mA, Rigaku, Japan) with a scan rate of 3 ◦ C/min. The microstructure and surface morphology of the nanocrystalline MMoO4 (M = Ca, Ba) were studied by transmission electron microscopy (TEM, JEM 2010, JEOL). The
Fig. 1. TG-DTA curves of the (a) CaMoO4 and (b) BaMoO4 precursors in flowing air.
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room-temperature photoluminescent (PL) spectra were recorded using a Japan Hitachi 850 spectrophotometer. 3. Results and discussion 3.1. Synthesis of nanocrystalline MMoO4 (M = Ca, Ba) The crystallization process of the MMoO4 (M = Ca, Ba) was analyzed by using TG-DTA (Fig. 1) and XRD (Fig. 2). In both cases of MMoO4 (M = Ca, Ba), with the increase of temperature, the weight loss of the precursors occurs in the TG curve up to 500 ◦ C. Thereafter the weight remains constant, indicating that the decomposition of all organic materials completed in the precursor, and crystallization of the MMoO4 (M = Ca, Ba) occurred below 500 ◦ C. No significant plateau, corresponding to welldefined intermediate products, appeared in the heating process. The DTA curves of the preparation of CaMoO4 and BaMoO4 show one sharp exothermic peak at 475 and 450 ◦ C respectively, which indicate that the MMoO4 (M = Ca, Ba) precursors begin to decompose from near 350 ◦ C and phase formation proceeds subsequently to 500 ◦ C.
Fig. 2. XRD patterns of the (a) CaMoO4 and (b) BaMoO4 nanocrystallites heattreated from 300 to 700 ◦ C for 3 h.
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3.2. PL of the synthesized MMoO4 (M = Ca, Ba) nanocrysallites Fig. 4 exhibits emission spectra of the nanocrystalline MMoO4 (M = Ca, Ba) heat-treated at (a) 400 ◦ C, (b) 500 ◦ C and (c) 600 ◦ C. It is generally known that the emission spectrum of the metal molybdates is mainly due to the charge-transfer transitions within the [MoO4 2− ] complex [1–3,9]. With the excitation at 240 nm, the nanocrystalline MMoO4 (M = Ca, Ba) exhibited PL emission in green and blue wavelength range. Despite of the somewhat different shapes of the spectra due to the different heat-treatment temperatures, common spectral features can be found. All spectra show broad peaks on which is superimposed considerable several fine structures. This band structure of the nanocrystalline MMoO4 (M = Ca, Ba) significantly resemble to the reported spectra of the metal tungstates [12]. These emission structures invoke possible presence of Gaussian components [13]. The decomposition of the band into individual Gaussian components resulted in three Gaussians for CaMoO4 and BaMoO4 (the fourth Gaussian component around 480 nm of BaMoO4 is considered to be produced by the Frenkel defect
Fig. 3. TEM and EDP photographs of the nanocrystalline (a) CaMoO4 and (b) BaMoO4 heat-treated at 600 ◦ C for 3 h.
Fig. 2 represents the XRD patterns of the MMoO4 (M = Ca, Ba) materials heat-treated for 3 h at 300–700 ◦ C. In Fig. 2, the MMoO4 (M = Ca, Ba) powders heat-treated at 300 ◦ C were amorphous without any crystallized phases. At 400 ◦ C, the characteristic MMoO4 (M = Ca, Ba) phase appeared and all of the prominent peaks corresponding to the MMoO4 (M = Ca, Ba) phase were clearly observed above 500 ◦ C, without any peaks assigned to either CaO, BaO, BaCO3 , or MoO3 phases. The TG-DTA and XRD results demonstrate that the crystallization of the MMoO4 (M = Ca, Ba) precursors is entirely completed at a temperature of 500 ◦ C, which is much lower than that prepared by a conventional solid-state reaction (about 900–1000 ◦ C) [4]. Fig. 3 shows the morphology of the nanocrystalline MMoO4 (M = Ca, Ba) heat-treated at 600 ◦ C for 3 h, respectively. The particles show primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The corresponding electron diffraction patterns (EDP) of the powders showed distinct rings, characteristics of an assembly of nanocrystallites.
Fig. 4. The room-temperature emission spectra of the nanocrystalline CaMoO4 and BaMoO4 heat-treated at (a) 400 ◦ C, (b) 500 ◦ C and (c) 600 ◦ C for 3 h. The decomposed Gaussian components are given with luminescent spectra.
J.-W. Yoon et al. / Materials Science and Engineering B 127 (2006) 154–158
Fig. 5. Schematic diagram of the crystal-field splitting and hybridization of the molecular orbitals of a tetragonal [MoO4 2− ] complex. The numbers in parentheses indicate the degeneracy of the [MoO4 2− ] complex, with the ‘*’ indicating anti-bonding (unoccupied) states. The shaded boxes are included to emphasize the fact that the discrete states were broadened by neighboring cluster interactions in the solid.
structure) to achieve a good agreement with the experimental data, which are given in Fig. 4. An explanation for the emission shapes in Fig. 4 might be given considering Jahn–Teller splitting effect [14,15] on excited states of tetrahedral [MoO4 2− ] anion in the MMoO4 (M = Ca, Ba). A schematic diagram for the crystal-field splitting and hybridization of the [MoO4 2− ] complex is shown in Fig. 5, similar to that given by Zhang et al. [16]. The ground state of the system corresponds to filling all one-electron states below the band gap, resulting in a many-electron state of 1 A1 symmetry. The lowest excited states involve one hole in the t1 (primarily O 2p) states and one electron in the e (primarily Mo 4d) states, corresponding to the many-electron states 1 T1 , 3 T1 , 1 T2 and 3 T2 . Of these states only the transition 1 A1 ↔ 1 T2 is a dipoleallowed transition. However, it is the lower 3 T1 or 3 T2 states, which were shown to account for the intrinsic luminescence by a spin–forbidden transition to the ground 1 A1 state in optically detected electron paramagnetic resonance experiments on CaMoO4 [17]. Both 1 A1 → 1 T1 , 1 T2 and 1 A1 → 3 T1 , 3 T2 transitions, causing at room temperature the radiating transition from triple levels 3 T1 , 3 T2 → 1 A1 at the lowest transition energy, are clearly detected in the luminescent spectra [17]. Resulting theoretical emission bands have superimposed broad shape [12,13]. Such a spectral feature is similar to the emission structure measured in this study and we can assume that the Jahn–Teller splitting effect essentially determines the emission shape of the nanocrystalline MMoO4 (M = Ca, Ba). The theoretical curve does not fit the experimental data perfectly. The discordance might be due to perturbation of local band structure [18], which can cause some variation in the experimental emission spectra. In addition, weak green and red emission bands were observed in BaMoO4 . These additional emission bands can be interpreted by the existence of Frenkel defects structure (oxygen ion shifted
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to the inter-site position with simultaneous creation of vacancy) in the surface layers of nanocrystallites [19] similar to what is observed in CaWO4 [20]. The PL spectra of the MMoO4 (M = Ca, Ba) prepared from 400 to 600 ◦ C had same peak positions. However, the luminescent intensity of sample prepared at 600 ◦ C was much stronger than that of sample prepared at 400 and 500 ◦ C. The PL intensity of sample prepared at 700 ◦ C was almost same as that of the sample prepared at 600 ◦ C. These results indicate that the PL intensity depends strongly on crystallinity of the nanocrystalline MMoO4 (M = Ca, Ba). Generally, it is noted that synthesizing process for obtaining particles with high crystallinity plays an important role in the improvement of luminescent efficiency [21] and the difference of the crystallinity of phosphor powders causes variations in luminescence efficiency. Therefore, it can be considered that the enhancement of PL intensity with the heat-treatment temperature up to 700 is due to the increment of crystallinity. 4. Conclusions Nanocrystalline MMoO4 (M = Ca, Ba) were successfully synthesized via the polymerized complex route and room temperature photoluminescent properties were investigated. Crystallization of the MMoO4 (M = Ca, Ba) entirely completed at 500 ◦ C. The prepared MMoO4 (M = Ca, Ba) nanocrystallites showed primarily dispersed and homogeneous morphology with a size of 20–40 nm. The nanocrystalline MMoO4 (M = Ca, Ba) prepared from 400 to 600 ◦ C exhibited broad peaks on which is superimposed considerable several fine structures in green and blue wavelength range, which could be explained by the Jahn–Teller splitting effect on the [MoO4 2− ] tetrahedron. The enhancement of PL intensity with the heat-treatment temperature up to 700 was due to the increment of crystallinity of MMoO4 (M = Ca, Ba) nanocrystallites. References [1] R. Grasser, E. Pitt, A. Scharmann, G. Zimmerer, Phys. Status Solid. B 69 (1975) 359–368. [2] S.B. Mikhrin, A.N. Mishin, A.S. Potapov, P.A. Rodnyi, A.S. Voloshinovskii, Nucl. Inst. Meth. A 486 (2002) 295–297. [3] D.A. Spassky, S.N. Ivanov, V.N. Kolobanov, V.V. Mikhailin, V.N. Zemskov, B.I. Zadneprovski, L.I. Potkin, Rad. Meas. 38 (2004) 607– 610. [4] W. Sleight, Acta Crystallogr. B 28 (1972) 2899–2902. [5] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, J. Am. Ceram. Soc. 80 (1997) 765–769. [6] M.P. Pechini, US Patent No. 3330697, 11 July 1967. [7] M. Yoshimura, J. Ma, M. Kakihana, J. Am. Ceram. Soc. 81 (1998) 2721–2724. [8] M. Kakihara, M. Yoshimura, H. Mazaki, H. Yosuoka, L. Borjesson, J. Appl. Phys. 71 (1992) 3904–3910. [9] B.K. Chandrasekhar, W.B. White, Mater. Res. Bull. 25 (1990) 1513–1518. [10] L.B. Barbosa, D.R. Ardila, C. Cusatis, J.P. Andreeta, J. Cryst. Growth 235 (2002) 327–332. [11] V. Thangadurai, C. Knittlmayer, W. Weppner, Mater. Sci. Eng. B 106 (2004) 228–233.
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[12] M. Nikl, P. Bohacek, E. Mihokova, M. Kobayashi, M. Ishii, Y. Usuki, V. Babin, A. Stolovich, S. Zazubovich, M. Bacci, J. Lumin. 87–89 (2000) 1136–1139. [13] K. Polak, M. Nikl, K. Nitsch, M. Kobayashi, M. Ishii, Y. Usuki, O. Jarolimek, J. Lumin. 72–74 (1997) 781–783. [14] Y. Toyozawa, M. Inoue, J. Phys. Soc. Jpn. 21 (1966) 1663–1679. [15] E.G. Reut, Izv. Akad. Nauk SSSR, Ser. Fiz. 43 (1979) 1186–1193. [16] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Phys. Rev. B 57 (1998) 12738–12750.
[17] J.V. Tol, Van der Waals, Mol. Phys. 88 (3) (1996) 803–820. [18] N.V. Klassen, Proceedings of the SCINT 95 on Inorganic Scintillators and Their Applications, Delft, 28 August–1 September 1995, Delft University Press, Delft, 1997, p. 475. [19] V.B. Mikhailik, H. Kraus, D. Wahl, M.S. Mykhaylyk, Phys. Status. Solid. B 242 (2005) R17–R19. [20] R. Grasser, A. Scharmann, K.-R. Strack, J. Lumin. 27 (1982) 263– 272. [21] S.-H. Wu, H.-C. Cheng, J. Electrochem. Soc. 151 (2004) H159–H163.
Photoluminescence in nanocrystalline MMoO4 (M = Ca, Ba) synthesized by a polymerized complex method Jong-Won Yoon, Jeong Ho Ryu ∗ , Kwang Bo Shim Department of Ceramic Engineering, Ceramic Processing Research Center (CPRC), Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea Received 11 July 2005; received in revised form 29 August 2005; accepted 8 October 2005
Abstract Nanocrystalline MMoO4 (M = Ca, Ba) were successfully synthesized at low temperatures via a polymerized complex route and room temperature photoluminescent properties were investigated in detail. Prepared nanocrystalline MMoO4 (M = Ca, Ba) showed primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The photoluminescent spectra were decomposed to several individual Gaussian components in green and blue wavelength range. These photoluminescent features could be interpreted by Jahn–Teller splitting effect on [MoO4 2− ] tetrahedron of the nanocrystalline MMoO4 (M = Ca, Ba). © 2005 Elsevier B.V. All rights reserved. Keywords: MMoO4 (M = Ca; Ba); Polymerized complex method; Photoluminescence; Jahn–Teller splitting effect
1. Introduction Metal molybdates are important inorganic materials that have a high application potential in various fields [1–3], such as photoluminescence and electro-optic applications. Metal molybdates of relatively large bivalent cations (MMoO4 , ionic ˚ M = Ca, Sr, Ba, Pb) exist in the so-called scheelradius > 0.99 A, ite structure (scheelite = CaWO4 ), where the tungsten atom adopts tetrahedral coordination [4]. In general, metal molybdate powders are synthesized by conventional solid-state reaction method [4]. However, MMoO4 (M = Ca, Ba) powders prepared by the solid-state reaction are relatively large with irregular morphology, and inhomogeneous compounds might be easily formed because MoO3 has a tendency to vaporize at high temperatures [5]. These problems can be avoided by applying advanced wet chemical route as a modified Pechini process [6]. This method has been already used successfully to prepare highly pure powders of various complex oxides [7] and even various superconductors [8] with multiple cationic compositions. The metal molybdates exhibit the blue or green luminescent spectra, which is based on the radiative transition within tetrahedral [MoO4 2− ] group [1–3,9]. Mainly, the detailed studies for
∗
Corresponding author. Tel.: +82 2 2220 0543; fax: +82 2 2291 7395. E-mail address: [email protected] (J.H. Ryu).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.10.015
the luminescent properties have been focused on single crystals [1–3,9,10]. Moreover, nano-sized inorganic materials have attracted much interest to scientists in recent years, due to their wide range of optical and electrical properties. However, only few studies on the synthesis [11] and luminescent properties of the nanocrystalline MMoO4 (M = Ca, Ba) powders prepared via wet chemical route have been reported. In this study, we successfully synthesized nanocrystalline MMoO4 (M = Ca, Ba) by a polymerized complex method. The precursors and powders were evaluated for the crystallization process, thermal decomposition and particle morphology. Furthermore, room-temperature photoluminescent properties of the synthesized nanocrystalline MMoO4 (M = Ca, Ba) were investigated in detail. 2. Experimental Metal nitrate [Ca(NO3 )2 ·4H2 O and Ba(NO3 )2 , purity 99.99%, Junsei Chemical Co. Ltd., Japan] and ammonium para-molybdate [(NH4 )6 Mo7 O24 ·4H2 O, purity 99.99%, Wako Chemical Co. Ltd., Japan] were used as the metallic cations. Ethylene glycol (HOCH2 CH2 OH, EG, purity 99.9%, High Purity Chemical Co. Ltd., Korea) and citric acid [HOC(CO2 H)(CH2 CO2 H)2 , CA, purity 99.9%, Yukiri Pure Chemical Co. Ltd., Japan] were used as the solvent and chealating agent for the process. The citrate solutions were prepared
J.-W. Yoon et al. / Materials Science and Engineering B 127 (2006) 154–158
by dissolving appropriate molar ratio of citric acid in ethylene glycol (CA:EG molar ratio = 1:4). After complete homogenization of the citrate solution, metal nitrate and ammonium paramolybdate were dissolved in the molar ratio of total chelate metal cations (TO) and citric acid (TO:CA molar ratio = 1:5). By keeping the solution at a temperature of 100 ◦ C for 4 h under constant stirring, the solution became viscous pale yellow solution. In order to promote polymerization and remove excess of solvents, the solution was slowly heated up to 180 ◦ C, and maintained at this temperature for 12 h. The solution became more viscous and changed its color from pale yellow to brown. No visible precipitation was observed along such heating process. Finally, the viscous solution was heated up to 250 ◦ C for 12 h, and a solidified dark brown glassy resin was obtained. The resin was converted into powder by grinding with a Teflon bar. Heat-treatment of the precursor was performed at various temperatures from 300 to 700 ◦ C for 3 h in air. The crystallization process of the polymeric precursors was examined by thermogravimetry-differential thermal analysis (TG-DTA, SETRAM, France), using a sample weight of about 10 mg and a heating rate of 5 ◦ C/min. The phase after the heat treatment were identified by ordinary X-ray diffraction (XRD, Cu K␣, 40 kV, 30 mA, Rigaku, Japan) with a scan rate of 3 ◦ C/min. The microstructure and surface morphology of the nanocrystalline MMoO4 (M = Ca, Ba) were studied by transmission electron microscopy (TEM, JEM 2010, JEOL). The
Fig. 1. TG-DTA curves of the (a) CaMoO4 and (b) BaMoO4 precursors in flowing air.
155
room-temperature photoluminescent (PL) spectra were recorded using a Japan Hitachi 850 spectrophotometer. 3. Results and discussion 3.1. Synthesis of nanocrystalline MMoO4 (M = Ca, Ba) The crystallization process of the MMoO4 (M = Ca, Ba) was analyzed by using TG-DTA (Fig. 1) and XRD (Fig. 2). In both cases of MMoO4 (M = Ca, Ba), with the increase of temperature, the weight loss of the precursors occurs in the TG curve up to 500 ◦ C. Thereafter the weight remains constant, indicating that the decomposition of all organic materials completed in the precursor, and crystallization of the MMoO4 (M = Ca, Ba) occurred below 500 ◦ C. No significant plateau, corresponding to welldefined intermediate products, appeared in the heating process. The DTA curves of the preparation of CaMoO4 and BaMoO4 show one sharp exothermic peak at 475 and 450 ◦ C respectively, which indicate that the MMoO4 (M = Ca, Ba) precursors begin to decompose from near 350 ◦ C and phase formation proceeds subsequently to 500 ◦ C.
Fig. 2. XRD patterns of the (a) CaMoO4 and (b) BaMoO4 nanocrystallites heattreated from 300 to 700 ◦ C for 3 h.
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J.-W. Yoon et al. / Materials Science and Engineering B 127 (2006) 154–158
3.2. PL of the synthesized MMoO4 (M = Ca, Ba) nanocrysallites Fig. 4 exhibits emission spectra of the nanocrystalline MMoO4 (M = Ca, Ba) heat-treated at (a) 400 ◦ C, (b) 500 ◦ C and (c) 600 ◦ C. It is generally known that the emission spectrum of the metal molybdates is mainly due to the charge-transfer transitions within the [MoO4 2− ] complex [1–3,9]. With the excitation at 240 nm, the nanocrystalline MMoO4 (M = Ca, Ba) exhibited PL emission in green and blue wavelength range. Despite of the somewhat different shapes of the spectra due to the different heat-treatment temperatures, common spectral features can be found. All spectra show broad peaks on which is superimposed considerable several fine structures. This band structure of the nanocrystalline MMoO4 (M = Ca, Ba) significantly resemble to the reported spectra of the metal tungstates [12]. These emission structures invoke possible presence of Gaussian components [13]. The decomposition of the band into individual Gaussian components resulted in three Gaussians for CaMoO4 and BaMoO4 (the fourth Gaussian component around 480 nm of BaMoO4 is considered to be produced by the Frenkel defect
Fig. 3. TEM and EDP photographs of the nanocrystalline (a) CaMoO4 and (b) BaMoO4 heat-treated at 600 ◦ C for 3 h.
Fig. 2 represents the XRD patterns of the MMoO4 (M = Ca, Ba) materials heat-treated for 3 h at 300–700 ◦ C. In Fig. 2, the MMoO4 (M = Ca, Ba) powders heat-treated at 300 ◦ C were amorphous without any crystallized phases. At 400 ◦ C, the characteristic MMoO4 (M = Ca, Ba) phase appeared and all of the prominent peaks corresponding to the MMoO4 (M = Ca, Ba) phase were clearly observed above 500 ◦ C, without any peaks assigned to either CaO, BaO, BaCO3 , or MoO3 phases. The TG-DTA and XRD results demonstrate that the crystallization of the MMoO4 (M = Ca, Ba) precursors is entirely completed at a temperature of 500 ◦ C, which is much lower than that prepared by a conventional solid-state reaction (about 900–1000 ◦ C) [4]. Fig. 3 shows the morphology of the nanocrystalline MMoO4 (M = Ca, Ba) heat-treated at 600 ◦ C for 3 h, respectively. The particles show primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The corresponding electron diffraction patterns (EDP) of the powders showed distinct rings, characteristics of an assembly of nanocrystallites.
Fig. 4. The room-temperature emission spectra of the nanocrystalline CaMoO4 and BaMoO4 heat-treated at (a) 400 ◦ C, (b) 500 ◦ C and (c) 600 ◦ C for 3 h. The decomposed Gaussian components are given with luminescent spectra.
J.-W. Yoon et al. / Materials Science and Engineering B 127 (2006) 154–158
Fig. 5. Schematic diagram of the crystal-field splitting and hybridization of the molecular orbitals of a tetragonal [MoO4 2− ] complex. The numbers in parentheses indicate the degeneracy of the [MoO4 2− ] complex, with the ‘*’ indicating anti-bonding (unoccupied) states. The shaded boxes are included to emphasize the fact that the discrete states were broadened by neighboring cluster interactions in the solid.
structure) to achieve a good agreement with the experimental data, which are given in Fig. 4. An explanation for the emission shapes in Fig. 4 might be given considering Jahn–Teller splitting effect [14,15] on excited states of tetrahedral [MoO4 2− ] anion in the MMoO4 (M = Ca, Ba). A schematic diagram for the crystal-field splitting and hybridization of the [MoO4 2− ] complex is shown in Fig. 5, similar to that given by Zhang et al. [16]. The ground state of the system corresponds to filling all one-electron states below the band gap, resulting in a many-electron state of 1 A1 symmetry. The lowest excited states involve one hole in the t1 (primarily O 2p) states and one electron in the e (primarily Mo 4d) states, corresponding to the many-electron states 1 T1 , 3 T1 , 1 T2 and 3 T2 . Of these states only the transition 1 A1 ↔ 1 T2 is a dipoleallowed transition. However, it is the lower 3 T1 or 3 T2 states, which were shown to account for the intrinsic luminescence by a spin–forbidden transition to the ground 1 A1 state in optically detected electron paramagnetic resonance experiments on CaMoO4 [17]. Both 1 A1 → 1 T1 , 1 T2 and 1 A1 → 3 T1 , 3 T2 transitions, causing at room temperature the radiating transition from triple levels 3 T1 , 3 T2 → 1 A1 at the lowest transition energy, are clearly detected in the luminescent spectra [17]. Resulting theoretical emission bands have superimposed broad shape [12,13]. Such a spectral feature is similar to the emission structure measured in this study and we can assume that the Jahn–Teller splitting effect essentially determines the emission shape of the nanocrystalline MMoO4 (M = Ca, Ba). The theoretical curve does not fit the experimental data perfectly. The discordance might be due to perturbation of local band structure [18], which can cause some variation in the experimental emission spectra. In addition, weak green and red emission bands were observed in BaMoO4 . These additional emission bands can be interpreted by the existence of Frenkel defects structure (oxygen ion shifted
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to the inter-site position with simultaneous creation of vacancy) in the surface layers of nanocrystallites [19] similar to what is observed in CaWO4 [20]. The PL spectra of the MMoO4 (M = Ca, Ba) prepared from 400 to 600 ◦ C had same peak positions. However, the luminescent intensity of sample prepared at 600 ◦ C was much stronger than that of sample prepared at 400 and 500 ◦ C. The PL intensity of sample prepared at 700 ◦ C was almost same as that of the sample prepared at 600 ◦ C. These results indicate that the PL intensity depends strongly on crystallinity of the nanocrystalline MMoO4 (M = Ca, Ba). Generally, it is noted that synthesizing process for obtaining particles with high crystallinity plays an important role in the improvement of luminescent efficiency [21] and the difference of the crystallinity of phosphor powders causes variations in luminescence efficiency. Therefore, it can be considered that the enhancement of PL intensity with the heat-treatment temperature up to 700 is due to the increment of crystallinity. 4. Conclusions Nanocrystalline MMoO4 (M = Ca, Ba) were successfully synthesized via the polymerized complex route and room temperature photoluminescent properties were investigated. Crystallization of the MMoO4 (M = Ca, Ba) entirely completed at 500 ◦ C. The prepared MMoO4 (M = Ca, Ba) nanocrystallites showed primarily dispersed and homogeneous morphology with a size of 20–40 nm. The nanocrystalline MMoO4 (M = Ca, Ba) prepared from 400 to 600 ◦ C exhibited broad peaks on which is superimposed considerable several fine structures in green and blue wavelength range, which could be explained by the Jahn–Teller splitting effect on the [MoO4 2− ] tetrahedron. The enhancement of PL intensity with the heat-treatment temperature up to 700 was due to the increment of crystallinity of MMoO4 (M = Ca, Ba) nanocrystallites. References [1] R. Grasser, E. Pitt, A. Scharmann, G. Zimmerer, Phys. Status Solid. B 69 (1975) 359–368. [2] S.B. Mikhrin, A.N. Mishin, A.S. Potapov, P.A. Rodnyi, A.S. Voloshinovskii, Nucl. Inst. Meth. A 486 (2002) 295–297. [3] D.A. Spassky, S.N. Ivanov, V.N. Kolobanov, V.V. Mikhailin, V.N. Zemskov, B.I. Zadneprovski, L.I. Potkin, Rad. Meas. 38 (2004) 607– 610. [4] W. Sleight, Acta Crystallogr. B 28 (1972) 2899–2902. [5] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, J. Am. Ceram. Soc. 80 (1997) 765–769. [6] M.P. Pechini, US Patent No. 3330697, 11 July 1967. [7] M. Yoshimura, J. Ma, M. Kakihana, J. Am. Ceram. Soc. 81 (1998) 2721–2724. [8] M. Kakihara, M. Yoshimura, H. Mazaki, H. Yosuoka, L. Borjesson, J. Appl. Phys. 71 (1992) 3904–3910. [9] B.K. Chandrasekhar, W.B. White, Mater. Res. Bull. 25 (1990) 1513–1518. [10] L.B. Barbosa, D.R. Ardila, C. Cusatis, J.P. Andreeta, J. Cryst. Growth 235 (2002) 327–332. [11] V. Thangadurai, C. Knittlmayer, W. Weppner, Mater. Sci. Eng. B 106 (2004) 228–233.
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