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Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine
JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009, p. 895
Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine LI Wenyu (ᴢ᭛ᅛ), LIU Yingliang (߬ᑨ҂), AI Pengfei (㡒吣亲), CHEN Xiaobo (䰜ᇣम) (Department of Chemistry, Jinan University, Guangzhou 510632, China) Received 24 December 2008; revised 27 April 2009
Abstract: Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were prepared by a solvothermal procedure. Rod-like Y(OH)3 was firstly synthesized by hydrothermal method to serve as the precursor. Y2O2S:Eu3+, Mg2+, Ti4+ powders were obtained by calcinating the precursor at CS2 atmosphere. The Y2O2S:Eu3+, Mg2+, Ti4+ phosphor with diameters of 30–50 nm and lengths up to 200–400 nm inherited the rod-like shape from the precursor after calcined at CS2 atmosphere. The Y2O2S:Eu3+, Mg2+, Ti4+ nanorods showed hexagonal pure phase, good dispersion and exhibited bright red luminescence. After irradiation by 265 or 325 nm for 5 min, the phosphor emitted red long-lasting phosphorescence, and the phosphorescence could be seen with the naked eyes in the dark clearly for more than 1 h after the irradiation source was removed. It was considered that the long-lasting phosphorescence was due to the persistent energy transfer from the traps to the Ti4+ and Mg2+ ions to generate the red-emitting long-lasting phosphorescence. Keywords: yttrium oxysulfide; rod-like structure; nanomaterials; luminescence; rare earths
During the recent half-century there have been considerable interests in the long-lasting phosphors because of their potential applications in safety indicators, fluorescent lamps, urgent illumination system, and cathode ray tubes, etc.[1–4] From the point of practical application, red is one of the three fundamental colors, and a red or orange afterglow phosphor is most suitable as illuminating light sources and appropriate for various displays. Therefore, red long-lasting phosphors with high luminescence and good chemical stability are badly needed. Yttrium oxysulfide has been known for a long time as an excellent red phosphor host material. While doped with Eu, Mg, Ti, a red long-lasting phosphor with the afterglow time of above 3 h has been synthesized[5]. But until now, the progress on the systemic research of Y2O2S:Eu3+, Mg2+, Ti4+ is very slow and the luminescent mechanism is not well disclosed. The research of the long-lasting phosphors is mainly focused on the bulk materials. However, one-dimensional rare-earth nanocrystals have recently attracted great attention because of their wide applications in fabrication of optical, electronic, biochemical and medical devices[6–9]. If the rare earth compounds were fabricated in the form of one-dimensional nanostructures, they would have some new properties as a result of both their marked shape-specific and
quantum-confinement effects. For luminescent materials, the phosphorescent properties are greatly affected by grain size, and many new properties can be obtained when the grain size reaches nanoscale. There are some methods for the preparation of fine powders in nanosize, including sol-gel method, chemical precipitation, hydrothermal synthesis, and so on. The solvothermal method which exhibits some advantages of low processing temperature, high homogeneity and purity of the products has become a promising method for the preparation of well-crystallized nanomaterials. Recently, some rare earth hydroxides with controlled morphology have been reported[10–13]. As for lanthanide oxysulfides, only La2O2S, Gd2O2S and Eu2O2S are known as nanorods[14–16], La2O2S and Nd2O2S as nanowires[17] and Y2O2S as nanotubes[11]. There is still great difficulty in developing an effective route to synthesis high-quality (single-crystalline, well-shaped and phase-pure) nanocrystals. Until now the way to produce Y2O2S:Eu3+, Mg2+, Ti4+ nanorods has rarely been reported yet. In this paper, we reported that, for the first time to our knowledge, Y2O2S:Eu3+, Mg2+, Ti4+ nanorods have been prepared by solvothermal method followed by a calcination process in CS2 atmosphere, and their photoluminescent properties were characterized at room temperature. Such Y2O2S:Eu3+, Mg2+, Ti4+ nanorods showed persistent red
Foundation item: Project supported by the National Natural Science Foundation of China (20671042, 50872045) and the Natural Science Foundations of Guangdong Province (0520055, 7005918) Corresponding author: LI Wenyu (E-mail: [email protected]; Tel.: +86-20-85221813) DOI: 10.1016/S1002-0721(08)60358-0
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JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009
emission after UV illumination, exhibiting potential in photoluminescent application.
measured on a fluorophotometer (Hitachi F-4500). All measurements were carried out at room temperature.
1 Experimental
2 Results and discussion
Firstly, 0.091 mol Y2O3 was dissolved in concentrated HNO3. Then appropriate amount of ammonium aqueous solution was added dropwise. The co-precipitated powders were centrifugally separated, washed with distilled water and butanol three times, and then mixed with 40 ml butonal. The solution was transferred to a teflon-lined stainless autoclave and maintained at 260 ºC for 5 h and then cooled down to room temperature. The desired white hydroxide nanorods were filtered, washed with distilled water and acetone three times, and finally dried at 80 ºC for 6 h. In the second place, sulfur powder was put in a sealed graphite crucible, and heated to 800 ºC for 4 h. Then the dried precursor together with the mixture of 0.005 mol Eu2O3, 0.001 mol Mg(OH)2.4MgCO3.6H2O, and 0.001 mol TiO2 were placed into the graphite crucible and calcined at 1100 ºC for 4 h. Both the as-prepared Y(OH)3 precursor and the final Y2O2S: Eu3+, Mg2+, Ti4+ phosphorescent products were characterized. The structures of the products were determined by powder X-ray diffraction (Bruker D8 Focus). The morphologies of the powders were observed by employing scanning electron microscopy (SEM, Philips XL-30), transmission electron microscopy (TEM, Philips TECNAI 10) and high resolution transmission electron microscopy (HRTEM, Fei TECNAI G2 F20). The photoluminescence spectra and intensity were
2.1 Crystal structure of the products Fig. 1 shows pure phases of Y(OH)3 precursor generated by a solvothermal process and Y2O2S after calcined in CS2 environment. It can be seen from the XRD patterns that both Y(OH)3 and Y2O2S possess hexagonal structures. No impurity peaks are observed. As for Y2O2S, lattice parameters are as a=0.375 nm, c=0.656 nm by calculation, which are very close to the standard lattice parameters provided by the powder diffraction file, PDF #24-1424. Co-doped Eu3+, Mg2+ and Ti4+ occupy the lattice sites in Y2O2S structure to form a uniform solid solution, with a nominal chemical composition of Y2O2S:Eu3+, Mg2+, Ti4+. 2.2 Morphology of the Y(OH)3 precursor As shown in Fig. 2, Y(OH)3 nanorods with uniform size and good distribution are obtained by the solvothermal method, showing the advantage of this method in preparing the particles with uniform size. A TEM micrograph is presented in Fig. 2(c), indicating obviously that the surface of the nanorods is smooth. 2.3 Morphology of the Y2O2S:Eu3+, Mg2+, Ti4+ nanorods The calcination of the Y(OH)3 in CS2 atmosphere leads to
Fig. 1 XRD patterns of the obtained Y(OH)3 and Y2O2S:Eu3+, Mg2+, Ti4+ nanorods
Fig. 2 Morphology of Y(OH)3 precursor (a) SEM image; (b) and (c) TEM micrographs
LI Wenyu et al., Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine
the formation of Y2O2S. As shown in Fig. 3, all the crystals are of rod-like shape with the width diameters of 30–50 nm and the lengths ranging from 200 to 400 nm. For oxysulfide products, their 1D linear morphology and diameters are nearly identical to those of the initial Y(OH)3 nanorods, implying that the rod-like shape is kept after the high temperature calcination. Although the exact mechanism is not clear now, it should be mentioned that the atmosphere in which the precursor is calcined plays an important part in keeping the rod-like shape. In our previous experiments[18], when the mixture was calcined in a flowing N2, air or H2S environment, only Y2O2S:Eu3+, Mg2+, Ti4+ particles were obtained. In this solid-gas reaction under such high temperature, we believe that the atmosphere is a key factor of a close morphological retention between the starting Y(OH)3 and the final products. The detailed research of the mechanism is under way. 2.4 HRTEM examination of the Y2O2S:Eu3+, Mg2+, Ti4+ nanorods The Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were also examined by using high resolution transmission electron microscopy. From Fig. 4(a) we can see individual nanorods with the diameter of about 50 nm. A HRTEM micrograph of the nanorod is shown in Fig. 4(b), from which the single crystalline
897
nature of the nanorod is confirmed. The spacing between the two adjacent lattice planes is 0.365 nm, which is just in good agreement with the interplanar crystal spacing of (1 0 1) of hexagonal phase Y2O2S:Eu3+, Mg2+, Ti4+. As is revealed in Fig. 4(c), the corresponding selected area electron diffraction pattern obtained is quite consistent with the target Y2O2S phase. These results further confirm that the final products are pure phased and single crystalline nanorods. 2.5 Luminescence property of the synthesized red phosphor For the sample calcined in CS2 atmosphere under 1100 ºC, the excitation spectrum, shown in Fig. 5(a), consists mainly of a wide band with two peaks at about 260 and 325 nm corresponding to Eu–O CTB (charge transfer band) and Eu–S CTB. While some weak and narrow peaks are attributed to the f-f transition of Eu3+ ions. The emission spectrum (Fig. 5(b)) excited by 325 nm indicates typical emission of Eu3+ ion. The strong red-emission lines at 615 and 625 nm are due to transition from 5D0 to 7F2 level of Eu3+ ion. Either Ti4+ or Mg2+ ion does not change the shape of excitation and emission spectra dramatically. 2.6 Afterglow decay curves of the phosphors From the decay curve in Fig. 6, it can be seen that doping both Mg2+ and Ti4+ ions can result in a long afterglow of the
Fig. 3 Morphology of Y2O2S:Eu3+, Mg2+, Ti4+ (a) SEM image; (b) and (c) TEM micrographs
Fig. 4 TEM observation of Y2O2S:Eu3+, Mg2+, Ti4+ nanorod (a) TEM micrograph for a single nanorod; (b) HRTEM image of the nanorod, showing single crystalline nature; (c) Corresponding SAED pattern
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JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009
Fig. 5 Excitation and emission spectra of the Y2O2S:Eu3+, Mg2+, Ti4+ phosphor (a) Excitation spectrum monitored at 625 nm; (b) Emission spectrum excited by 325 nm
with Mg2+ and Ti4+ ions, the phosphorescence lasted for 1 h in the light perception of the naked human eye. The introduction of Mg2+ and Ti4+ ions produced the complex hole and electron traps and resulted in long-lasting phenomenon.
References:
Fig. 6 Afterglow decay curves of the phosphors (1) Y2O2S:Eu3+ nanorods; (2) Y2O2S:Eu3+, Mg2+, Ti4+ nanorods
Y2O2S:Eu3+ phosphor. The single Eu3+ doped Y2O2S:Eu3+ phosphor shows very weak afterglow while red afterglow color can be clearly seen in the dark room for codoped Y2O2S:Eu3+, Mg2+, Ti4+. Moreover, the afterglow time of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods can last up to 1 h. We propose that introduction of the Mg2+ and Ti4+ ions to the Y2O2S compound causes the formation of new electronic donating and accepting levels between the host lattice band gap. One of the two kinds of ions absorbs energy and thermally transfers the excited electrons to the other kind of ions which serves as trap centers. The trapping of excited electrons and thermally released processes results in the afterglow.
3 Conclusions Single crystalline Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were prepared by solvothermal method. Results showed that the final nanorods with uniform size and smooth surface inherited the rod-like shape from the precursor even after calcined at CS2 atmosphere at high temperatures. While co-doped
[1] Lin Y H, Tang Z L, Zhang Z T, Nan C W. Anomalous luminescence in Sr4Al14O25:Eu, Dy phosphors. Applied Physics Letter, 2002, 81: 996. [2] Jia D, Wang X, Jia W, Yen W M. Persistent energy transfer in CaAl2O4:Tb3+, Ce3+. Journal of Applied Physics, 2003, 93: 148. [3] Kamada M, Murakami J, Ohno N. Excitation spectra of a longpersistent phosphor SrAl2O4:Eu,Dy in vacuum ultraviolet region. Journal of Luminescence, 2000, 87-89: 1042. [4] Wang X X, Zhang Z T, Tang Z L, Lin Y H. Characterization and properties of a red and orange Y2O2S-based long afterglow phosphor. Materials Chemistry and Physics, 2003, 80: 1. [5] Wang Y H, Wang Z L. Characterization of Y2O2S:Eu3+, Mg2+, Ti4+ long-lasting phosphor synthesized by flux method. Journal of Rare Earths, 2006, 24(1): 25. [6] Wang X, Zhang J, Peng Q, Li Y D. A general strategy for nanocrystal synthesis. Nature, 2005, 437: 121. [7] Xia Y N, Yang P D. Chemistry and Physics of Nanowires. Advanced Materials, 2003, 15: 351. [8] Bockrath M, Liang W J, Bozovic D, Hafner J H, Lieber C M, Tinkham M, Park H K. Resonant electron scattering by defects in single-walled carbon nanotube. Science, 2001, 291: 283. [9] Hu J T, Odom T W, Lieber C M. Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Accounts of Chemical Research, 1999, 32: 435. [10] Wang X, Li Y D. Synthesis and characterization of lanthanide hydroxide single-crystal nanowires. Angewandte Chemie International Edition, 2002, 41: 4790. [11] Wang X, Sun M, Yu D P. Rare earth compound nanotubes.
LI Wenyu et al., Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine Advanced Materials, 2003, 15: 1442. [12] Yang X F, Ning G L, Lin Y. Preparation of Eu(OH)3 and Eu2O3 Nanorods through a Simple Method. Chemistry Letters, 2007, 36: 468. [13] Mao Y B, Huang J Y, Ostroumov R, Wang K L, Chang J P. Synthesis and luminescence properties of erbium-doped Y2O3 nanotubes. Journal of Physical Chemistry C, 2008, 112: 2278. [14] Jiang Y, Wu Y, Xie Y, Qian Y T. Synthesis and characterization of nanocrystalline lanthanide oxysulfide via a La(OH)3 gel solvothermal route. Journal of the American Ceramic Society, 2000, 83: 2628. [15] Mao S P, Liu Q, Gu M, Mao D L, Chang C K. Long lasting phosphorescence of Gd2O2S:Eu,Ti,Mg nanorods via a hydro-
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thermal routine. Journal of Alloys and Compounds, 2008, 468: 367. [16] Zhao F, Yuan M, Zhang W, Gao S. Monodisperse lanthanide oxysulfide nanocrystals. Journal of American Society, 2006, 128: 11758. [17] Huang Y Z, Chen L, Wu L M. Crystalline nanowires of Ln2O2S, Ln2O2S2, LnS2 (Ln=La, Nd), and La2O2S:Eu3+ conversions via the boron-sulfur method that preserve shape. Crystal Growth & Design, 2008, 8: 739. [18] Li W Y, Liu Y L, Ai P F. Synthesis of nanocrystalline Y2O2S: Eu3+, Mg, Ti long-lasting phosphorescent materials by hydrothermal-microwave method. Chinese Journal of Inorganic Chemistry, 2008, 24: 772.
Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine LI Wenyu (ᴢ᭛ᅛ), LIU Yingliang (߬ᑨ҂), AI Pengfei (㡒吣亲), CHEN Xiaobo (䰜ᇣम) (Department of Chemistry, Jinan University, Guangzhou 510632, China) Received 24 December 2008; revised 27 April 2009
Abstract: Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were prepared by a solvothermal procedure. Rod-like Y(OH)3 was firstly synthesized by hydrothermal method to serve as the precursor. Y2O2S:Eu3+, Mg2+, Ti4+ powders were obtained by calcinating the precursor at CS2 atmosphere. The Y2O2S:Eu3+, Mg2+, Ti4+ phosphor with diameters of 30–50 nm and lengths up to 200–400 nm inherited the rod-like shape from the precursor after calcined at CS2 atmosphere. The Y2O2S:Eu3+, Mg2+, Ti4+ nanorods showed hexagonal pure phase, good dispersion and exhibited bright red luminescence. After irradiation by 265 or 325 nm for 5 min, the phosphor emitted red long-lasting phosphorescence, and the phosphorescence could be seen with the naked eyes in the dark clearly for more than 1 h after the irradiation source was removed. It was considered that the long-lasting phosphorescence was due to the persistent energy transfer from the traps to the Ti4+ and Mg2+ ions to generate the red-emitting long-lasting phosphorescence. Keywords: yttrium oxysulfide; rod-like structure; nanomaterials; luminescence; rare earths
During the recent half-century there have been considerable interests in the long-lasting phosphors because of their potential applications in safety indicators, fluorescent lamps, urgent illumination system, and cathode ray tubes, etc.[1–4] From the point of practical application, red is one of the three fundamental colors, and a red or orange afterglow phosphor is most suitable as illuminating light sources and appropriate for various displays. Therefore, red long-lasting phosphors with high luminescence and good chemical stability are badly needed. Yttrium oxysulfide has been known for a long time as an excellent red phosphor host material. While doped with Eu, Mg, Ti, a red long-lasting phosphor with the afterglow time of above 3 h has been synthesized[5]. But until now, the progress on the systemic research of Y2O2S:Eu3+, Mg2+, Ti4+ is very slow and the luminescent mechanism is not well disclosed. The research of the long-lasting phosphors is mainly focused on the bulk materials. However, one-dimensional rare-earth nanocrystals have recently attracted great attention because of their wide applications in fabrication of optical, electronic, biochemical and medical devices[6–9]. If the rare earth compounds were fabricated in the form of one-dimensional nanostructures, they would have some new properties as a result of both their marked shape-specific and
quantum-confinement effects. For luminescent materials, the phosphorescent properties are greatly affected by grain size, and many new properties can be obtained when the grain size reaches nanoscale. There are some methods for the preparation of fine powders in nanosize, including sol-gel method, chemical precipitation, hydrothermal synthesis, and so on. The solvothermal method which exhibits some advantages of low processing temperature, high homogeneity and purity of the products has become a promising method for the preparation of well-crystallized nanomaterials. Recently, some rare earth hydroxides with controlled morphology have been reported[10–13]. As for lanthanide oxysulfides, only La2O2S, Gd2O2S and Eu2O2S are known as nanorods[14–16], La2O2S and Nd2O2S as nanowires[17] and Y2O2S as nanotubes[11]. There is still great difficulty in developing an effective route to synthesis high-quality (single-crystalline, well-shaped and phase-pure) nanocrystals. Until now the way to produce Y2O2S:Eu3+, Mg2+, Ti4+ nanorods has rarely been reported yet. In this paper, we reported that, for the first time to our knowledge, Y2O2S:Eu3+, Mg2+, Ti4+ nanorods have been prepared by solvothermal method followed by a calcination process in CS2 atmosphere, and their photoluminescent properties were characterized at room temperature. Such Y2O2S:Eu3+, Mg2+, Ti4+ nanorods showed persistent red
Foundation item: Project supported by the National Natural Science Foundation of China (20671042, 50872045) and the Natural Science Foundations of Guangdong Province (0520055, 7005918) Corresponding author: LI Wenyu (E-mail: [email protected]; Tel.: +86-20-85221813) DOI: 10.1016/S1002-0721(08)60358-0
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JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009
emission after UV illumination, exhibiting potential in photoluminescent application.
measured on a fluorophotometer (Hitachi F-4500). All measurements were carried out at room temperature.
1 Experimental
2 Results and discussion
Firstly, 0.091 mol Y2O3 was dissolved in concentrated HNO3. Then appropriate amount of ammonium aqueous solution was added dropwise. The co-precipitated powders were centrifugally separated, washed with distilled water and butanol three times, and then mixed with 40 ml butonal. The solution was transferred to a teflon-lined stainless autoclave and maintained at 260 ºC for 5 h and then cooled down to room temperature. The desired white hydroxide nanorods were filtered, washed with distilled water and acetone three times, and finally dried at 80 ºC for 6 h. In the second place, sulfur powder was put in a sealed graphite crucible, and heated to 800 ºC for 4 h. Then the dried precursor together with the mixture of 0.005 mol Eu2O3, 0.001 mol Mg(OH)2.4MgCO3.6H2O, and 0.001 mol TiO2 were placed into the graphite crucible and calcined at 1100 ºC for 4 h. Both the as-prepared Y(OH)3 precursor and the final Y2O2S: Eu3+, Mg2+, Ti4+ phosphorescent products were characterized. The structures of the products were determined by powder X-ray diffraction (Bruker D8 Focus). The morphologies of the powders were observed by employing scanning electron microscopy (SEM, Philips XL-30), transmission electron microscopy (TEM, Philips TECNAI 10) and high resolution transmission electron microscopy (HRTEM, Fei TECNAI G2 F20). The photoluminescence spectra and intensity were
2.1 Crystal structure of the products Fig. 1 shows pure phases of Y(OH)3 precursor generated by a solvothermal process and Y2O2S after calcined in CS2 environment. It can be seen from the XRD patterns that both Y(OH)3 and Y2O2S possess hexagonal structures. No impurity peaks are observed. As for Y2O2S, lattice parameters are as a=0.375 nm, c=0.656 nm by calculation, which are very close to the standard lattice parameters provided by the powder diffraction file, PDF #24-1424. Co-doped Eu3+, Mg2+ and Ti4+ occupy the lattice sites in Y2O2S structure to form a uniform solid solution, with a nominal chemical composition of Y2O2S:Eu3+, Mg2+, Ti4+. 2.2 Morphology of the Y(OH)3 precursor As shown in Fig. 2, Y(OH)3 nanorods with uniform size and good distribution are obtained by the solvothermal method, showing the advantage of this method in preparing the particles with uniform size. A TEM micrograph is presented in Fig. 2(c), indicating obviously that the surface of the nanorods is smooth. 2.3 Morphology of the Y2O2S:Eu3+, Mg2+, Ti4+ nanorods The calcination of the Y(OH)3 in CS2 atmosphere leads to
Fig. 1 XRD patterns of the obtained Y(OH)3 and Y2O2S:Eu3+, Mg2+, Ti4+ nanorods
Fig. 2 Morphology of Y(OH)3 precursor (a) SEM image; (b) and (c) TEM micrographs
LI Wenyu et al., Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine
the formation of Y2O2S. As shown in Fig. 3, all the crystals are of rod-like shape with the width diameters of 30–50 nm and the lengths ranging from 200 to 400 nm. For oxysulfide products, their 1D linear morphology and diameters are nearly identical to those of the initial Y(OH)3 nanorods, implying that the rod-like shape is kept after the high temperature calcination. Although the exact mechanism is not clear now, it should be mentioned that the atmosphere in which the precursor is calcined plays an important part in keeping the rod-like shape. In our previous experiments[18], when the mixture was calcined in a flowing N2, air or H2S environment, only Y2O2S:Eu3+, Mg2+, Ti4+ particles were obtained. In this solid-gas reaction under such high temperature, we believe that the atmosphere is a key factor of a close morphological retention between the starting Y(OH)3 and the final products. The detailed research of the mechanism is under way. 2.4 HRTEM examination of the Y2O2S:Eu3+, Mg2+, Ti4+ nanorods The Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were also examined by using high resolution transmission electron microscopy. From Fig. 4(a) we can see individual nanorods with the diameter of about 50 nm. A HRTEM micrograph of the nanorod is shown in Fig. 4(b), from which the single crystalline
897
nature of the nanorod is confirmed. The spacing between the two adjacent lattice planes is 0.365 nm, which is just in good agreement with the interplanar crystal spacing of (1 0 1) of hexagonal phase Y2O2S:Eu3+, Mg2+, Ti4+. As is revealed in Fig. 4(c), the corresponding selected area electron diffraction pattern obtained is quite consistent with the target Y2O2S phase. These results further confirm that the final products are pure phased and single crystalline nanorods. 2.5 Luminescence property of the synthesized red phosphor For the sample calcined in CS2 atmosphere under 1100 ºC, the excitation spectrum, shown in Fig. 5(a), consists mainly of a wide band with two peaks at about 260 and 325 nm corresponding to Eu–O CTB (charge transfer band) and Eu–S CTB. While some weak and narrow peaks are attributed to the f-f transition of Eu3+ ions. The emission spectrum (Fig. 5(b)) excited by 325 nm indicates typical emission of Eu3+ ion. The strong red-emission lines at 615 and 625 nm are due to transition from 5D0 to 7F2 level of Eu3+ ion. Either Ti4+ or Mg2+ ion does not change the shape of excitation and emission spectra dramatically. 2.6 Afterglow decay curves of the phosphors From the decay curve in Fig. 6, it can be seen that doping both Mg2+ and Ti4+ ions can result in a long afterglow of the
Fig. 3 Morphology of Y2O2S:Eu3+, Mg2+, Ti4+ (a) SEM image; (b) and (c) TEM micrographs
Fig. 4 TEM observation of Y2O2S:Eu3+, Mg2+, Ti4+ nanorod (a) TEM micrograph for a single nanorod; (b) HRTEM image of the nanorod, showing single crystalline nature; (c) Corresponding SAED pattern
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JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009
Fig. 5 Excitation and emission spectra of the Y2O2S:Eu3+, Mg2+, Ti4+ phosphor (a) Excitation spectrum monitored at 625 nm; (b) Emission spectrum excited by 325 nm
with Mg2+ and Ti4+ ions, the phosphorescence lasted for 1 h in the light perception of the naked human eye. The introduction of Mg2+ and Ti4+ ions produced the complex hole and electron traps and resulted in long-lasting phenomenon.
References:
Fig. 6 Afterglow decay curves of the phosphors (1) Y2O2S:Eu3+ nanorods; (2) Y2O2S:Eu3+, Mg2+, Ti4+ nanorods
Y2O2S:Eu3+ phosphor. The single Eu3+ doped Y2O2S:Eu3+ phosphor shows very weak afterglow while red afterglow color can be clearly seen in the dark room for codoped Y2O2S:Eu3+, Mg2+, Ti4+. Moreover, the afterglow time of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods can last up to 1 h. We propose that introduction of the Mg2+ and Ti4+ ions to the Y2O2S compound causes the formation of new electronic donating and accepting levels between the host lattice band gap. One of the two kinds of ions absorbs energy and thermally transfers the excited electrons to the other kind of ions which serves as trap centers. The trapping of excited electrons and thermally released processes results in the afterglow.
3 Conclusions Single crystalline Y2O2S:Eu3+, Mg2+, Ti4+ nanorods were prepared by solvothermal method. Results showed that the final nanorods with uniform size and smooth surface inherited the rod-like shape from the precursor even after calcined at CS2 atmosphere at high temperatures. While co-doped
[1] Lin Y H, Tang Z L, Zhang Z T, Nan C W. Anomalous luminescence in Sr4Al14O25:Eu, Dy phosphors. Applied Physics Letter, 2002, 81: 996. [2] Jia D, Wang X, Jia W, Yen W M. Persistent energy transfer in CaAl2O4:Tb3+, Ce3+. Journal of Applied Physics, 2003, 93: 148. [3] Kamada M, Murakami J, Ohno N. Excitation spectra of a longpersistent phosphor SrAl2O4:Eu,Dy in vacuum ultraviolet region. Journal of Luminescence, 2000, 87-89: 1042. [4] Wang X X, Zhang Z T, Tang Z L, Lin Y H. Characterization and properties of a red and orange Y2O2S-based long afterglow phosphor. Materials Chemistry and Physics, 2003, 80: 1. [5] Wang Y H, Wang Z L. Characterization of Y2O2S:Eu3+, Mg2+, Ti4+ long-lasting phosphor synthesized by flux method. Journal of Rare Earths, 2006, 24(1): 25. [6] Wang X, Zhang J, Peng Q, Li Y D. A general strategy for nanocrystal synthesis. Nature, 2005, 437: 121. [7] Xia Y N, Yang P D. Chemistry and Physics of Nanowires. Advanced Materials, 2003, 15: 351. [8] Bockrath M, Liang W J, Bozovic D, Hafner J H, Lieber C M, Tinkham M, Park H K. Resonant electron scattering by defects in single-walled carbon nanotube. Science, 2001, 291: 283. [9] Hu J T, Odom T W, Lieber C M. Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Accounts of Chemical Research, 1999, 32: 435. [10] Wang X, Li Y D. Synthesis and characterization of lanthanide hydroxide single-crystal nanowires. Angewandte Chemie International Edition, 2002, 41: 4790. [11] Wang X, Sun M, Yu D P. Rare earth compound nanotubes.
LI Wenyu et al., Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ nanorods via a solvothermal routine Advanced Materials, 2003, 15: 1442. [12] Yang X F, Ning G L, Lin Y. Preparation of Eu(OH)3 and Eu2O3 Nanorods through a Simple Method. Chemistry Letters, 2007, 36: 468. [13] Mao Y B, Huang J Y, Ostroumov R, Wang K L, Chang J P. Synthesis and luminescence properties of erbium-doped Y2O3 nanotubes. Journal of Physical Chemistry C, 2008, 112: 2278. [14] Jiang Y, Wu Y, Xie Y, Qian Y T. Synthesis and characterization of nanocrystalline lanthanide oxysulfide via a La(OH)3 gel solvothermal route. Journal of the American Ceramic Society, 2000, 83: 2628. [15] Mao S P, Liu Q, Gu M, Mao D L, Chang C K. Long lasting phosphorescence of Gd2O2S:Eu,Ti,Mg nanorods via a hydro-
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