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A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates
Journal of Non-Crystalline Solids 355 (2009) 903–907
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Rapid Communication
A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates Juan Wang a,b, Yunhua Xu b,*, Mirabbos Hojamberdiev b, Jianhong Peng b, Gangqiang Zhu b a b
School of Science, Xi’an University of Architecture and Technology, Xi’an 710055, PR China Shaanxi Key Laboratory of Nano-materials and technology, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
a r t i c l e
i n f o
Article history: Received 7 January 2009 Received in revised form 26 March 2009
PACS: 68.37.Lp 68.37.Og 78.67.Bf 81.07.Bc 81.16.Be
a b s t r a c t Porous nanoplates made of yttrium orthovanadate doped with europeum (YVO4:Eu3+) have been successfully synthesized via a chemical co-precipitation method using NH4VO3, Y2O3, Eu2O3 and ethylene glycol. To investigate the effect of temperature on the pore size and morphology of the final product, the as-synthesized YVO4:Eu3+ nanoplates were subjected to heat treatment at 300 and 600 °C for 2 h. The obtained samples were characterized by X-ray diffractometion, Fourier-transform infrared spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectroscopy. The experimental results showed that the luminescent properties were significantly enhanced with increasing pore size of the YVO4:Eu3+ porous nanoplates. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Nanocrystals Optical spectroscopy Nanoparticles Luminescence
1. Introduction Due to their potential quantum-confinement effect and low dimensionality, inorganic nanocrystals exhibit fascinating sizeand shape-dependent properties [1,2]. In general, the intrinsic properties of nanoscale materials are determined by their composition, structure, crystallinity, size and morphology [3]. Over the past few years, the synthesis of inorganic nanoscale materials with specific morphologies has been the focus of extensive studies in materials science [4,5]. In particular, the development of nanostructured optical materials has also made a very positive contribution to systematic fundamental studies of crystal growth and possible new applications. Yttrium orthovanadate (YVO4) is an essential optical material having a zirconia structure. Its beneficial characteristics make it play an important role in many devices involving the artificial production of light and display fields [6,7]. YVO4 was modified by doping with Eu3+ has been used as a red phosphor in color televisions and cathode ray tubes (CRTs) owing to its high luminescent efficiency upon electron-beam excitation [8,9]. In recent years, many efforts have been made to fabricate materials with porous nanostructures that have interesting properties different from those of their solid counter-parts [10–12]. Such * Corresponding author. Tel.: +86 29 82202531; fax: +86 29 82202886. E-mail address: [email protected] (Y. Xu). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.04.013
materials can be used as gas and heavy-metal-ion adsorbents, selective separators and light fillers. If the porous nanostructure and luminescent properties could be combined, porous nanomaterials would be ideal candidates for the photoluminescent applications. To avoid a high preparation temperature and the formation of impurity phases, several soft chemical methods, e.g., hydrothermal synthesis [13,14], hydrolyzed colloidal reaction [15] and induced precipitation [16], have been employed to date to produce nanostructured YVO4. However, all the reported methods produced solid nanoparticles without pores. To the best of our knowledge, no report on the synthesis and luminescent properties of YVO4:Eu3+ porous nanoplates has been published. Due to the specific characteristics and promising applications of this material, the development of a method that can regulate both the pore size and nanoparticle morphology of YVO4 is a challenging topic. We therefore report here a simple approach for the synthesizing YVO4:Eu3+ porous nanoplates.
2. Experimental In the present work, nanosized YVO4:Eu3+ was synthesized by means of chemical co-precipitation. Stoichiometric amounts of Y2O3 and Eu2O3 were dissolved in 20 ml HNO3 (6 mol/l) to obtain a transparent solution. Five microliters ethylene glycol (EG) was
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introduced into the solution as a complexing agent, followed by the addition of a stoichiometric amount of NH4VO3. The mixture was maintained under continuous stirring for 20 min. Subsequently, ammonia (NH3) solution was slowly dropped into the homogeneous solution under vigorous stirring to adjust its pH to 9–10. The resulting solution was preheated at 80 °C for 6 h in a water bath. Then, the as-synthesized precipitate was collected, washed with distilled water and absolute ethanol several times, and dried in vacuum at 80 °C for 6 h. Finally, the as-synthesized powders were heated up to 300 and 600 °C at a heating rate of 10 °C/min for 2 h in a laboratory furnace. The crystal structures of the samples were determined by powder X-ray diffraction (XRD) using a0 D/MAX2550 apparatus with A) at 40 kV and 50 mA. Foumonochromated CuKa (k = 1.5406 Å rier-transformed infrared (FTIR) adsorption spectra were measured using a JASCO FTIR 480 plus spectrophotometer. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selective-area electron diffraction (SAED) images were taken with a JEM-2000 Ex transmission electron microscope with an accelerating voltage of 200 kV. The excitation and emission photoluminescence (PL) spectra were measured using a Perkin–Elmer LS55 fluorescence spectrometer. 3. Results
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The XRD patterns of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are shown in Fig. 1. All the peaks can be indexed to the tetragonal phase (JCPDS Card, No.17-0341) with a zirconia structure. The results show that pure-phase YVO4:Eu3+ can be obtained by the chemical co-precipitation method at as low a temperature as 80 °C for 6 h. As the annealing temperature increases, the diffraction peaks of the products gradually become narrower and sharper, which indicates that the crystallinity of the as-annealed products has increased, and the grain size increases with the annealing temperature. The FTIR spectra of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are illustrated in Fig. 2. The large isolated absorption peak around 820 cm1 rewith a strong IR veals the typical spectral characteristic of VO3 4 absorption band in the range of 780–920 cm1. The isolated peaks prove that the crystal structures of the products are coincident with a tetragonal phase, agreeing well with the XRD results. The absorption peaks appear at around 1385 cm1, and can be ascribed to the –CH2 bending vibration. At the same time, a band due to hy-
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2θ (deg.) Fig. 1. XRD patterns of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h.
Fig. 2. FTIR spectra of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h.
droxyl stretching of the ethylene glycol is observed at 3100– 3400 cm1. It can be seen from Fig. 2 that the amount of ethylene glycol decreases as the heat-treatment temperature increases. Other weak frequencies in the absorption peaks that appear in the range of 1600–1650 cm1 are assigned to O–H. This is attributed to crystal or absorbed water in the as-synthesized YVO4:Eu3+. Fig. 3(a–c) compares the TEM images of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 °C and 600 °C for 2 h, whereas the SAED patterns and HRTEM images of the corresponding samples are represented in Fig. 3(d–h), respectively. Clearly, the sample preheated at 80 °C for 6 h consists of roughly plate-like nanoparticles 10–20 nm in size. The plate-like nanoparticles themselves seem to be composed of many small nanoparticles. After heat treatment of the as-synthesized samples at 300 °C, the plate-like nanoparticles become larger (ca. 30 nm) and some porous structure appears on their surface. A further increase in the heating temperature to 600 °C results in the formation of nanoplates with a relatively narrow particle-size distribution and a larger size (ca. 40 nm). The sample heated at 600 °C for 2 h appears to have a more porous structure than that of the sample heated at 300 °C for 2 h. The SAED pattern recorded for the as-synthesized sample (Fig. 3(d)) is composed of continuous rings, whereas the samples heated at 300 and 600 °C for 2 h show some discrete spots (Fig. 3e–f). This demonstrates that all samples are polycrystalline and that the mean particle and crystallinity increase because of the heat treatment, as expected. Fig. 3(g and h) shows the HRTEM of YVO4:Eu3+ nanoplates heated at 300 and 600 °C for 2 h. The lattice spacings of the nanoplates obtained at 300 and 600 °C are about 0.36 nm, which corresponds to the (2 0 0) plane of YVO4:Eu3+. In contrast, the porous nanoplates of YVO4:Eu3+ are different from the nanoparticles synthesized without ethylene glycol [17]. It was found that the environment of the ethylene glycol solution influenced the shape of YVO4:Eu3+ crystals that formed. The increase in pore diameter of the sample is thought to be due not only to crystal growth at the higher temperatures at 300 and 600 °C, but also to the output of CO2 from the C2H6O2 matrix. This is also an important factor that causes the increase of pore diameter in the YVO4 powders, because of the residual organic compounds. The excitation spectra and emission spectra of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are shown in Fig. 4. Evidently, the excitation spectra monitored at 616 nm have almost the same shape for all samples. Fig. 4 shows the typical room-temperature photoluminescence
J. Wang et al. / Journal of Non-Crystalline Solids 355 (2009) 903–907
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Fig. 3. TEM image of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h. SAED patterns of the as-synthesized (d) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (e) and 600 °C (f) for 2 h. HRTEM images of the YVO4:Eu3+ samples heated at 300 °C (g) and 600 °C (h) for 2 h.
emission spectra of nanoparticles at 284 nm excitation. The spectra composed of sharp lines ranging from 400 to 700 nm are associated with transitions from the excited 5D0 level to the 7FJ (J = 1, 2, 3, 4) levels of Eu3+ activators. The emission bands around 584 and 591 nm correspond to the 5D0 ? 7F1 magnetic-dipole transitions; the weak peak at 537 nm is related to the transition from 5 D1 to 7F1. The emission peaks located at about 612 and 616 nm are thought to be due mainly to transitions from 5D0 to two sublevels of the 7F2 energy level. Although the peak positions in all the emission spectra are identical, the intensities of all the peaks increased gradually with increasing pore size. The increase of the peak at 616 nm is much faster than that of the peak at 612 nm, so the predominant peak changes from 612 to 616 nm. This may be caused by the change of symmetry of the crystal fields around the Eu3+ ions. However, the luminescent properties of normal nanoplates synthesized without ethylene glycol are independent of the heating temperature and all show the same peak positions and intensities [17]. The different luminescent properties of normal nanoplates and porous nanoplates of YVO4:Eu3+ are possible because of the distinct porous structures of the crystals. Therefore,
YVO4:Eu3+ shows unusual luminescent properties when it is in the form of porous nanoplates.
4. Discussion Recently, Lagendijk and co-workers [18–20] found that a macroporous gallium phosphide semiconductor prepared by electrochemical etching shows strong scattering without optical absorption in the red part of the visible spectrum. They also demonstrated that the scattering strength of this macroporous GaP can be tuned in a wide range, depending on the density and size of pores. Li et al. [21] reported that a small pore size leads to a decrease of the photoluminescence intensity of anodic alumina membranes. In this paper, it can be observed that the luminescence intensity of all the peaks increases gradually with increasing treatment temperature and pore size. It is well known that the 5 D0 ? 7F2 transition of Eu3+ ion is almost completely determined by the crystal structure and site symmetry around the Eu3+ ions, and that the low symmetry of the rare-earth site increases the
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J. Wang et al. / Journal of Non-Crystalline Solids 355 (2009) 903–907 M
2C2 H6 O2 þ SO2 ! 4CO2 þ 6H2 O:
λex =284nm
λex =616nm
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wavelength(nm) Fig. 4. The excitation spectra of the as-synthesized (a1) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b1) and 600 °C (c1) for 2 h. Photoluminescence emission spectra of the as-synthesized (a2) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b2) and 600 °C (c2) for 2 h.
probability of radiative transitions (within the 4f electron shell) through the destruction of parity forbiddenness [22]. Chen et al. [23] reported that a higher luminescence intensity means a lower symmetry of Eu3+ sites in an annealed nanobundle-like sample. The sites offered for Eu3+ have a reduced symmetry, which is able to over-ride the parity selection rule and results in the enhanced luminescent properties [24]. Here, our porous YVO4:Eu3+ nanostructures possess higher surface energy with increasing treatment temperature and pore size, which results in a higher degree of disorder and a corresponding lower symmetry of the crystal fields around the Eu3+ ions. So it can be concluded that the low symmetry of the Eu3+ sites increases the probability of radiative transitions within the 4f electron shell. So we can further confirm that Eu3+ occupies the host lattice and replaces Y3+. Therefore, the Eu3+ dopant in porous nanoplates leads to a noticeable improvement of the luminescent properties. Another reason for these changes may be the improvement in crystallinity and the increase of particle size, as small particles do not a have high luminescent efficiency because of grain-boundary effects [25]. It is found that the improvement of crystallinity and crystalline size with increasing annealing temperature is a common phenomenon [9]. So it is understood that luminescent properties of porous nanostructures are associated with the pore size and crystallinity. By changing the pore size and crystallinity via controlled post-heat treatment of our samples, we can effectively adjust their luminescent properties. In order to form porous structured YVO4:Eu3+ products, crystal growth at the higher temperatures of 300 and 600 °C is necessary. However, here, we should also attach importance to the output of CO2 from the C2H6O2 matrix. This is also an important factor in the increase of the pore diameter in YVO4 powders, because of the decomposition of residual organic compounds. The porous nanoplates of YVO4:Eu3+ are different from the nanoparticles synthesized without ethylene glycol [17]. It was found that the environment of the ethylene glycol solution influenced the shape of the porous YVO4:Eu3+ crystals that formed. When the sample was heated at 80 °C for 6 h, no pores could be found on the surface of the nanoplates. After heat treatment of the as-synthesized samples at 300 °C, some porous structure appeared on the surface of the plate-like nanoparticles. The heat treatment promoted the decomposition of residual organic compounds, by the following reaction:
This is due to output of CO2 from the interior of the nanoplates. Consequently, pores were easily formed due to the release of these gases. As the annealing temperature increased to 600 °C, the C2H6O2 reacted sharply and more gases were produced in a short time. So the pores would become bigger owing to the improvement of the structure at the high annealing temperature. From the PL spectra, it can be seen that the intensity of all the peaks increases gradually with increasing heat-treatment temperature and pore size. Generally speaking, the luminescence not only reflects changes caused by thermal annealing, which can modify intrinsic defects and the amount of Eu3+ ions, but also is potentially a non-destructive monitor of the growth conditions, as well as the state and size of the YVO4:Eu3+ nanoparticles. An important reason for these alterations may be the improvement in crystallinity and the increase of particle size, as small particles do not have high luminescent efficiency due to grain-boundary effects [10]. In sum, this is an indication that certain properties of the as-synthesized powders, such as the improvement of crystallinity and low symmetry of the local environment surrounding the activator ions result from the increasing pore size and influence the PL spectra. The heat treatment is important for extracting the maximum luminescent efficiency. 5. Conclusions To summarize we have demonstrated an easy way to synthesize YVO4:Eu3+ porous nanoplates using NH4VO3, Y2O3, Eu2O3 and ethylene glycol as the reacting reagents. The influence of the heattreatment on the pore size and morphology of the as-synthesized product was investigated. The photoluminescence measurements showed that the luminescent intensity of the YVO4:Eu3+ porous nanoplates was enhanced with the increasing pore size of the nanocrystals. The synthesis of porous YVO4:Eu3+ nanocrystals under the current conditions will provide a new route for the development of other lanthanonvanadate materials with a porous structure at low temperature. Acknowledgements The authors are grateful to Science Foundation of Shaanxi Provincial Education Department for the financial support (Nos. 08JK346 and 08JZ38). References [1] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893. [2] M. Fernández-García, A. Martínez-Arias, J.C. Hanson, J.A. Rodriguez, Chem. Rev. 104 (2004) 4063. [3] Y. Sun, Y. Xia, Science 298 (2002) 2176. [4] J.H. Fendler, F.C. Meldrum, Adv. Mater. 7 (1995) 607. [5] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544. [6] M. Bass, IEEE J. Quantum Electron 11 (1975) 938. [7] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [8] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118. [9] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [10] S.-W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642. [11] Z.H. Kang, E.B. Wang, B.D. Mao, Z.M. Su, L. Chen, L. Xu, Nanotechnology 16 (2005) 1192. [12] Y. Sun, B. Mayers, Y. Xia, Adv. Mater. 15 (2003) 641. [13] Y.H. Wang, Y.Y. Zuo, H. Gao, Mater. Res. Bull. 41 (2006) 2147. [14] X.C. Wu, Y.R. Tao, C.Y. Song, C.J. Mao, L. Dong, J.J. Zhu, J. Phys. Chem. B 110 (2006) 15791. [15] S. Erdei, N.M. Rodriguez, F.W. Ainger, W.B. White, D. Ravichandran, L.E. Cross, J. Mater. Chem. 8 (1998) 99. [16] S. Takeshita, T. Isobe, S. Niikura, J. Lumin. 128 (2008) 1515. [17] Y.H. Li, G.G. Hong, J. Solid State Chem. 178 (2005) 645. [18] F.J.P. Schuurmans, D. Vanmaekelbergh, J. van de Lagemaat, A. Lagendijk, Science 284 (1999) 141.
J. Wang et al. / Journal of Non-Crystalline Solids 355 (2009) 903–907 [19] F.J.P. Schuurmans, M. Megens, D. Vanmaekelbergh, A. Lagendijk, Phys. Rev. Lett. 83 (1999) 2183. [20] J.G. Rivas, A. Lagendijk, R.W. Tjerkstra, D. Vanmaekelbergh, J.J. Kelly, Appl. Phys. Lett. 80 (2002) 4498. [21] G.H. Li, Y. Zhang, Y.C. Wu, L.D. Zhang, Appl. Phys. A 81 (2005) 627.
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R.D. Peacock, Struct. Bond. 22 (1975) 83. L. Chen, Y. Liu, K. Huang, Mater. Res. Bull. 41 (2006) 158. L.D. Sun, C. Qian, C.S. Liao, X.L. Wang, Solid State Commun. 119 (2001) 393. J.S. Yoo, J.D. Lee, J. Appl. Phys. 81 (1997) 2810.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Rapid Communication
A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates Juan Wang a,b, Yunhua Xu b,*, Mirabbos Hojamberdiev b, Jianhong Peng b, Gangqiang Zhu b a b
School of Science, Xi’an University of Architecture and Technology, Xi’an 710055, PR China Shaanxi Key Laboratory of Nano-materials and technology, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
a r t i c l e
i n f o
Article history: Received 7 January 2009 Received in revised form 26 March 2009
PACS: 68.37.Lp 68.37.Og 78.67.Bf 81.07.Bc 81.16.Be
a b s t r a c t Porous nanoplates made of yttrium orthovanadate doped with europeum (YVO4:Eu3+) have been successfully synthesized via a chemical co-precipitation method using NH4VO3, Y2O3, Eu2O3 and ethylene glycol. To investigate the effect of temperature on the pore size and morphology of the final product, the as-synthesized YVO4:Eu3+ nanoplates were subjected to heat treatment at 300 and 600 °C for 2 h. The obtained samples were characterized by X-ray diffractometion, Fourier-transform infrared spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectroscopy. The experimental results showed that the luminescent properties were significantly enhanced with increasing pore size of the YVO4:Eu3+ porous nanoplates. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Nanocrystals Optical spectroscopy Nanoparticles Luminescence
1. Introduction Due to their potential quantum-confinement effect and low dimensionality, inorganic nanocrystals exhibit fascinating sizeand shape-dependent properties [1,2]. In general, the intrinsic properties of nanoscale materials are determined by their composition, structure, crystallinity, size and morphology [3]. Over the past few years, the synthesis of inorganic nanoscale materials with specific morphologies has been the focus of extensive studies in materials science [4,5]. In particular, the development of nanostructured optical materials has also made a very positive contribution to systematic fundamental studies of crystal growth and possible new applications. Yttrium orthovanadate (YVO4) is an essential optical material having a zirconia structure. Its beneficial characteristics make it play an important role in many devices involving the artificial production of light and display fields [6,7]. YVO4 was modified by doping with Eu3+ has been used as a red phosphor in color televisions and cathode ray tubes (CRTs) owing to its high luminescent efficiency upon electron-beam excitation [8,9]. In recent years, many efforts have been made to fabricate materials with porous nanostructures that have interesting properties different from those of their solid counter-parts [10–12]. Such * Corresponding author. Tel.: +86 29 82202531; fax: +86 29 82202886. E-mail address: [email protected] (Y. Xu). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.04.013
materials can be used as gas and heavy-metal-ion adsorbents, selective separators and light fillers. If the porous nanostructure and luminescent properties could be combined, porous nanomaterials would be ideal candidates for the photoluminescent applications. To avoid a high preparation temperature and the formation of impurity phases, several soft chemical methods, e.g., hydrothermal synthesis [13,14], hydrolyzed colloidal reaction [15] and induced precipitation [16], have been employed to date to produce nanostructured YVO4. However, all the reported methods produced solid nanoparticles without pores. To the best of our knowledge, no report on the synthesis and luminescent properties of YVO4:Eu3+ porous nanoplates has been published. Due to the specific characteristics and promising applications of this material, the development of a method that can regulate both the pore size and nanoparticle morphology of YVO4 is a challenging topic. We therefore report here a simple approach for the synthesizing YVO4:Eu3+ porous nanoplates.
2. Experimental In the present work, nanosized YVO4:Eu3+ was synthesized by means of chemical co-precipitation. Stoichiometric amounts of Y2O3 and Eu2O3 were dissolved in 20 ml HNO3 (6 mol/l) to obtain a transparent solution. Five microliters ethylene glycol (EG) was
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introduced into the solution as a complexing agent, followed by the addition of a stoichiometric amount of NH4VO3. The mixture was maintained under continuous stirring for 20 min. Subsequently, ammonia (NH3) solution was slowly dropped into the homogeneous solution under vigorous stirring to adjust its pH to 9–10. The resulting solution was preheated at 80 °C for 6 h in a water bath. Then, the as-synthesized precipitate was collected, washed with distilled water and absolute ethanol several times, and dried in vacuum at 80 °C for 6 h. Finally, the as-synthesized powders were heated up to 300 and 600 °C at a heating rate of 10 °C/min for 2 h in a laboratory furnace. The crystal structures of the samples were determined by powder X-ray diffraction (XRD) using a0 D/MAX2550 apparatus with A) at 40 kV and 50 mA. Foumonochromated CuKa (k = 1.5406 Å rier-transformed infrared (FTIR) adsorption spectra were measured using a JASCO FTIR 480 plus spectrophotometer. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selective-area electron diffraction (SAED) images were taken with a JEM-2000 Ex transmission electron microscope with an accelerating voltage of 200 kV. The excitation and emission photoluminescence (PL) spectra were measured using a Perkin–Elmer LS55 fluorescence spectrometer. 3. Results
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The XRD patterns of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are shown in Fig. 1. All the peaks can be indexed to the tetragonal phase (JCPDS Card, No.17-0341) with a zirconia structure. The results show that pure-phase YVO4:Eu3+ can be obtained by the chemical co-precipitation method at as low a temperature as 80 °C for 6 h. As the annealing temperature increases, the diffraction peaks of the products gradually become narrower and sharper, which indicates that the crystallinity of the as-annealed products has increased, and the grain size increases with the annealing temperature. The FTIR spectra of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are illustrated in Fig. 2. The large isolated absorption peak around 820 cm1 rewith a strong IR veals the typical spectral characteristic of VO3 4 absorption band in the range of 780–920 cm1. The isolated peaks prove that the crystal structures of the products are coincident with a tetragonal phase, agreeing well with the XRD results. The absorption peaks appear at around 1385 cm1, and can be ascribed to the –CH2 bending vibration. At the same time, a band due to hy-
(b) (a) 10
20
30
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50
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2θ (deg.) Fig. 1. XRD patterns of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h.
Fig. 2. FTIR spectra of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h.
droxyl stretching of the ethylene glycol is observed at 3100– 3400 cm1. It can be seen from Fig. 2 that the amount of ethylene glycol decreases as the heat-treatment temperature increases. Other weak frequencies in the absorption peaks that appear in the range of 1600–1650 cm1 are assigned to O–H. This is attributed to crystal or absorbed water in the as-synthesized YVO4:Eu3+. Fig. 3(a–c) compares the TEM images of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 °C and 600 °C for 2 h, whereas the SAED patterns and HRTEM images of the corresponding samples are represented in Fig. 3(d–h), respectively. Clearly, the sample preheated at 80 °C for 6 h consists of roughly plate-like nanoparticles 10–20 nm in size. The plate-like nanoparticles themselves seem to be composed of many small nanoparticles. After heat treatment of the as-synthesized samples at 300 °C, the plate-like nanoparticles become larger (ca. 30 nm) and some porous structure appears on their surface. A further increase in the heating temperature to 600 °C results in the formation of nanoplates with a relatively narrow particle-size distribution and a larger size (ca. 40 nm). The sample heated at 600 °C for 2 h appears to have a more porous structure than that of the sample heated at 300 °C for 2 h. The SAED pattern recorded for the as-synthesized sample (Fig. 3(d)) is composed of continuous rings, whereas the samples heated at 300 and 600 °C for 2 h show some discrete spots (Fig. 3e–f). This demonstrates that all samples are polycrystalline and that the mean particle and crystallinity increase because of the heat treatment, as expected. Fig. 3(g and h) shows the HRTEM of YVO4:Eu3+ nanoplates heated at 300 and 600 °C for 2 h. The lattice spacings of the nanoplates obtained at 300 and 600 °C are about 0.36 nm, which corresponds to the (2 0 0) plane of YVO4:Eu3+. In contrast, the porous nanoplates of YVO4:Eu3+ are different from the nanoparticles synthesized without ethylene glycol [17]. It was found that the environment of the ethylene glycol solution influenced the shape of YVO4:Eu3+ crystals that formed. The increase in pore diameter of the sample is thought to be due not only to crystal growth at the higher temperatures at 300 and 600 °C, but also to the output of CO2 from the C2H6O2 matrix. This is also an important factor that causes the increase of pore diameter in the YVO4 powders, because of the residual organic compounds. The excitation spectra and emission spectra of the as-synthesized and nanocrystalline YVO4:Eu3+ samples heated at 300 and 600 °C for 2 h are shown in Fig. 4. Evidently, the excitation spectra monitored at 616 nm have almost the same shape for all samples. Fig. 4 shows the typical room-temperature photoluminescence
J. Wang et al. / Journal of Non-Crystalline Solids 355 (2009) 903–907
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Fig. 3. TEM image of the as-synthesized (a) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b) and 600 °C (c) for 2 h. SAED patterns of the as-synthesized (d) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (e) and 600 °C (f) for 2 h. HRTEM images of the YVO4:Eu3+ samples heated at 300 °C (g) and 600 °C (h) for 2 h.
emission spectra of nanoparticles at 284 nm excitation. The spectra composed of sharp lines ranging from 400 to 700 nm are associated with transitions from the excited 5D0 level to the 7FJ (J = 1, 2, 3, 4) levels of Eu3+ activators. The emission bands around 584 and 591 nm correspond to the 5D0 ? 7F1 magnetic-dipole transitions; the weak peak at 537 nm is related to the transition from 5 D1 to 7F1. The emission peaks located at about 612 and 616 nm are thought to be due mainly to transitions from 5D0 to two sublevels of the 7F2 energy level. Although the peak positions in all the emission spectra are identical, the intensities of all the peaks increased gradually with increasing pore size. The increase of the peak at 616 nm is much faster than that of the peak at 612 nm, so the predominant peak changes from 612 to 616 nm. This may be caused by the change of symmetry of the crystal fields around the Eu3+ ions. However, the luminescent properties of normal nanoplates synthesized without ethylene glycol are independent of the heating temperature and all show the same peak positions and intensities [17]. The different luminescent properties of normal nanoplates and porous nanoplates of YVO4:Eu3+ are possible because of the distinct porous structures of the crystals. Therefore,
YVO4:Eu3+ shows unusual luminescent properties when it is in the form of porous nanoplates.
4. Discussion Recently, Lagendijk and co-workers [18–20] found that a macroporous gallium phosphide semiconductor prepared by electrochemical etching shows strong scattering without optical absorption in the red part of the visible spectrum. They also demonstrated that the scattering strength of this macroporous GaP can be tuned in a wide range, depending on the density and size of pores. Li et al. [21] reported that a small pore size leads to a decrease of the photoluminescence intensity of anodic alumina membranes. In this paper, it can be observed that the luminescence intensity of all the peaks increases gradually with increasing treatment temperature and pore size. It is well known that the 5 D0 ? 7F2 transition of Eu3+ ion is almost completely determined by the crystal structure and site symmetry around the Eu3+ ions, and that the low symmetry of the rare-earth site increases the
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J. Wang et al. / Journal of Non-Crystalline Solids 355 (2009) 903–907 M
2C2 H6 O2 þ SO2 ! 4CO2 þ 6H2 O:
λex =284nm
λex =616nm
616nm 612nm
PL Intensity(a.u.)
284nm
312nm
(c1)
(c2)
(b1) (b2)
(a1)
(a2) 200
300
500
600
700
wavelength(nm) Fig. 4. The excitation spectra of the as-synthesized (a1) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b1) and 600 °C (c1) for 2 h. Photoluminescence emission spectra of the as-synthesized (a2) and nanocrystalline YVO4:Eu3+ samples heated at 300 °C (b2) and 600 °C (c2) for 2 h.
probability of radiative transitions (within the 4f electron shell) through the destruction of parity forbiddenness [22]. Chen et al. [23] reported that a higher luminescence intensity means a lower symmetry of Eu3+ sites in an annealed nanobundle-like sample. The sites offered for Eu3+ have a reduced symmetry, which is able to over-ride the parity selection rule and results in the enhanced luminescent properties [24]. Here, our porous YVO4:Eu3+ nanostructures possess higher surface energy with increasing treatment temperature and pore size, which results in a higher degree of disorder and a corresponding lower symmetry of the crystal fields around the Eu3+ ions. So it can be concluded that the low symmetry of the Eu3+ sites increases the probability of radiative transitions within the 4f electron shell. So we can further confirm that Eu3+ occupies the host lattice and replaces Y3+. Therefore, the Eu3+ dopant in porous nanoplates leads to a noticeable improvement of the luminescent properties. Another reason for these changes may be the improvement in crystallinity and the increase of particle size, as small particles do not a have high luminescent efficiency because of grain-boundary effects [25]. It is found that the improvement of crystallinity and crystalline size with increasing annealing temperature is a common phenomenon [9]. So it is understood that luminescent properties of porous nanostructures are associated with the pore size and crystallinity. By changing the pore size and crystallinity via controlled post-heat treatment of our samples, we can effectively adjust their luminescent properties. In order to form porous structured YVO4:Eu3+ products, crystal growth at the higher temperatures of 300 and 600 °C is necessary. However, here, we should also attach importance to the output of CO2 from the C2H6O2 matrix. This is also an important factor in the increase of the pore diameter in YVO4 powders, because of the decomposition of residual organic compounds. The porous nanoplates of YVO4:Eu3+ are different from the nanoparticles synthesized without ethylene glycol [17]. It was found that the environment of the ethylene glycol solution influenced the shape of the porous YVO4:Eu3+ crystals that formed. When the sample was heated at 80 °C for 6 h, no pores could be found on the surface of the nanoplates. After heat treatment of the as-synthesized samples at 300 °C, some porous structure appeared on the surface of the plate-like nanoparticles. The heat treatment promoted the decomposition of residual organic compounds, by the following reaction:
This is due to output of CO2 from the interior of the nanoplates. Consequently, pores were easily formed due to the release of these gases. As the annealing temperature increased to 600 °C, the C2H6O2 reacted sharply and more gases were produced in a short time. So the pores would become bigger owing to the improvement of the structure at the high annealing temperature. From the PL spectra, it can be seen that the intensity of all the peaks increases gradually with increasing heat-treatment temperature and pore size. Generally speaking, the luminescence not only reflects changes caused by thermal annealing, which can modify intrinsic defects and the amount of Eu3+ ions, but also is potentially a non-destructive monitor of the growth conditions, as well as the state and size of the YVO4:Eu3+ nanoparticles. An important reason for these alterations may be the improvement in crystallinity and the increase of particle size, as small particles do not have high luminescent efficiency due to grain-boundary effects [10]. In sum, this is an indication that certain properties of the as-synthesized powders, such as the improvement of crystallinity and low symmetry of the local environment surrounding the activator ions result from the increasing pore size and influence the PL spectra. The heat treatment is important for extracting the maximum luminescent efficiency. 5. Conclusions To summarize we have demonstrated an easy way to synthesize YVO4:Eu3+ porous nanoplates using NH4VO3, Y2O3, Eu2O3 and ethylene glycol as the reacting reagents. The influence of the heattreatment on the pore size and morphology of the as-synthesized product was investigated. The photoluminescence measurements showed that the luminescent intensity of the YVO4:Eu3+ porous nanoplates was enhanced with the increasing pore size of the nanocrystals. The synthesis of porous YVO4:Eu3+ nanocrystals under the current conditions will provide a new route for the development of other lanthanonvanadate materials with a porous structure at low temperature. Acknowledgements The authors are grateful to Science Foundation of Shaanxi Provincial Education Department for the financial support (Nos. 08JK346 and 08JZ38). References [1] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893. [2] M. Fernández-García, A. Martínez-Arias, J.C. Hanson, J.A. Rodriguez, Chem. Rev. 104 (2004) 4063. [3] Y. Sun, Y. Xia, Science 298 (2002) 2176. [4] J.H. Fendler, F.C. Meldrum, Adv. Mater. 7 (1995) 607. [5] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544. [6] M. Bass, IEEE J. Quantum Electron 11 (1975) 938. [7] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [8] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118. [9] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [10] S.-W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642. [11] Z.H. Kang, E.B. Wang, B.D. Mao, Z.M. Su, L. Chen, L. Xu, Nanotechnology 16 (2005) 1192. [12] Y. Sun, B. Mayers, Y. Xia, Adv. Mater. 15 (2003) 641. [13] Y.H. Wang, Y.Y. Zuo, H. Gao, Mater. Res. Bull. 41 (2006) 2147. [14] X.C. Wu, Y.R. Tao, C.Y. Song, C.J. Mao, L. Dong, J.J. Zhu, J. Phys. Chem. B 110 (2006) 15791. [15] S. Erdei, N.M. Rodriguez, F.W. Ainger, W.B. White, D. Ravichandran, L.E. Cross, J. Mater. Chem. 8 (1998) 99. [16] S. Takeshita, T. Isobe, S. Niikura, J. Lumin. 128 (2008) 1515. [17] Y.H. Li, G.G. Hong, J. Solid State Chem. 178 (2005) 645. [18] F.J.P. Schuurmans, D. Vanmaekelbergh, J. van de Lagemaat, A. Lagendijk, Science 284 (1999) 141.
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