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CePO4:Tb nanophosphors and their photoluminescence properties
Applied Surface Science 266 (2013) 22–26
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrasonic-assisted synthesis of core–shell structure CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb nanophosphors and their photoluminescence properties Yao-yao Fan a , Zong-chao Hu a,∗ , Jian Yang a , Chao Zhang a , Ling Zhu b a b
College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China College of Chemistry and Biological Engineering, Changsha University of Science & Technology, Changsha 410076, China
a r t i c l e
i n f o
Article history: Received 9 August 2012 Received in revised form 8 November 2012 Accepted 12 November 2012 Available online 23 November 2012 Keywords: Ultrasonic Core–shell structure Photoluminescence Nanophosphors
a b s t r a c t CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb and (Ce, Tb, Gd)PO4 (4–8) nm × (35–73) nm sized nanobars with the hexagonal crystal system have been obtained by ultrasonic-assisted synthesis and characterized by X-ray diffraction (XRD), FT-IR spectrum, transmission electron microscopy (TEM), photoluminescence (PL). The shell thickness of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core/shell structure is 1.04 nm and 1.10 nm respectively. Under ultraviolet excitation, these nanophosphors show Tb3+ characteristic emission, 5 D4 –7 FJ (J = {6, 5, 4, 3}) and the fluorescence of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb increases superficially compared with CePO4 :Tb and the co-precipitated (Ce, Tb, Gd)PO4 . The photoluminescence intensity of CePO4 :Tb/GdPO4 is 33 times, 7 times, 2 times as high as that of CePO4:Tb, GdPO4/CePO4:Tb and (Ce, Tb, Gd)PO4 , respectively. It is worth mentioning that the increasing amount of intensity of CePO4 :Tb/GdPO4 is double than that of GdPO4 /CePO4 :Tb. A possible formation mechanism for the fluorescent efficiency enhancement has been proposed. The results are helpful in developing effective phosphors and have potential applications in field emission display (FED) and plasma display panels (PDP). © 2012 Elsevier B.V. All rights reserved.
1. Introduction The utilization of beneficial and non-renewable rare earth metal resources has become higher and higher, in many different areas. Rare-earth [1] metal not only can play a crucial role in optoelectronic, petroleum chemical industry, machinery, energy and environmental protection but also can be used for data storage, biochemical and chemical sensor materials. Rare earth hydrides have been extensively applied in the electric light source, permanent high-performance magnets, hydrogen storage, catalysis [2] and other functional materials. One of the widest applications of rare earth metals is the synthesis of fluorescent materials whose functions depend strongly on their composition and structure [3–5]. Reasonable and efficient use of rare earth metals is an important approach to solve the problem of resource shortage. Research on fluorescent materials has attracted significant interests [6,7] due to potential applications. With the requirements of photoluminescence intensity and quantum efficiency, rare-earth phosphates as a composite matrix of nanophosphor have also attracted considerable attention. Rambabu et al. [8] have studied photoluminescence materials with different rare-earth ions doped into rare-earth phosphates, for example, LnPO4 :Tb3+ (Ln = Y, La, Gd) and LnPO4 :Eu3+ , which
showed that (Gd0.25 , La0.75 )PO4 :Tb3+ and (Gd0.5 , Y0.5 )PO4 :Tb3+ have appeared a bright green light as two kinds of green promising phosphors. Yu et al. [9] have prepared LaPO4 :(Ce3+ , Tb3+ ) luminescent thin film by Pechini type sol–gel method and compared with larger particles and Tb3+ doped LaPO4 in nanoscale colloid, and the photoluminescence properties have not changed. Optical properties of nanowires and nanoparticles of LaPO4 have been compared by Song et al. [10]. It was found that the great changes of photoluminescence properties depended on the altered morphology of powder. Meyssamy et al. [11,12] have studied Eu3+ and Tb3+ doped LaPO4 nanofiber, and found that nanofiber photoluminescence materials appeared strong light. The photoluminescence quantum yield of CePO4 :Tb/LaPO4 core–shell nanoparticles can be improved [13,14]. Herein we report the synthesis of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core–shell [15] structure phosphors by ultrasonic method and the investigation of their structural characteristics, photoluminescence properties and the mechanism of the fluorescent efficiency change in core to shell conversion, so as to provide a theoretical foundation for the development of efficient photoluminescence materials. 2. Experimental 2.1. Sample preparations
∗ Corresponding author. Tel.: +86 13985113898; fax: +86 08513625867. E-mail address: [email protected] (Z.-c. Hu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.050
All chemicals used in the experiments were of analytical purity, bought from Sinopharm Chemical Reagent Co. Ltd. The precursor
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solutions were dissolved with rare-earth oxides. (1) In a typical procedure for synthesizing CePO4 :Tb phosphors [13], 20 mL Ce(NO3 )3 (2.07 mmol) and Tb(NO3 )3 (0.23 mmol) were added into 30 mL H3 PO4 (11.5 mol) aqueous solution to produce the mixed solution, then used 1:1 ammonia solution to adjust to pH = 1. After that, the mixed procedure was irradiated at 80 W for 2 h with ultrasonic to produce a viscous gel. The obtained gel was centrifuged and washed repeatedly with distilled water and ethanol, filtered to the ultimate white CePO4 :Tb phosphors, which was kept for 24 h at the temperature of 60 ◦ C. (2) CePO4 :Tb/GdPO4 was synthesized via taking half of CePO4 :Tb phosphors dispersed in 10 mL distilled water, then the solution was added 10 mL Gd(NO3 )3 (2.07 mmol) irradiated for 30 min by ultrasonic to produce the transparent and viscous gel. The obtained gel was mixed in 30 mL H3 PO4 (11.5 mmol) aqueous solutions and adjusted to pH = 1 using 1:1 ammonia solution. Subsequently the remaining steps were the same as CePO4 :Tb phosphors’. (3)The method of GdPO4 /CePO4 :Tb phosphors synthesis is the same as CePO4 :Tb/GdPO4 . At first, the core of GdPO4 was synthesized with 20 mL Gd(NO3 )3 (2.3 mmol) solutions and 30 mL H3 PO4 (11.5 mmol) aqueous solutions irradiated for 2 h in ultrasonic. Fewer steps are consistent with CePO4 :Tb phosphors. The coating of GdPO4 /CePO4 :Tb phosphors was gotten by using the synthesis as CePO4 :Tb/GdPO4 without adding Tb(NO3 )3 . (4) (Ce, Tb, Gd)PO4 phosphors synthesized via all solutions with the same content of CePO4 Tb/GdPO4 were mixed together, then irradiated for 2 h in ultrasonic. The remaining steps the same as CePO4 :Tb phosphors’. 2.2. Characterization Photoluminescence excitation and emission spectra of the nanoparticles were acquired by a Hitachi F-4500 photoluminescence spectrometer (PL, HITACHI, JPN). FT-IR spectra were obtained by using the KBr technique with Fourier transforms infrared spectrometer (FT-IR, FT-IR Presting-21, JPN). The morphology of the samples was characterized on a TECNAI G2 20 transmission electron microscope (TEM, FEI Ltd., USA), with an accelerating voltage of 200 KV. The crystal phase of the as-prepared products was identified by X’Pert Pro diffractometer (Panalytical Co., Holland) at a scan ˚ The samrate of 10◦ min−1 using Cu K␣1 radiation ( = 1.5406 A). ples were synthesized by ultrasonic cell disruption system with 80 W (JY92-IIDN, CHN). 3. Results and discussions 3.1. Structure and morphology 3.1.1. Phase identification The XRD patterns of as-synthesized products are shown in Fig. 1(a). It is clear that CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb and (Ce, Tb, Gd)PO4 can be readily indexed to phase-pure hexagonal morphology cerium phosphate hydrate and gadolinium phosphate hydrate, corresponding to (JCPDS-751880) (JCPDS-210337) (JCPDS-350614) (JCPDS-350614), respectively. All of the obtained rare-earth phosphate phosphors remain a solitary phase after doping Tb3+ , which indicates Tb3+ is effectively doped into crystal lattices of cerium phosphate hydrate and gadolinium phosphate hydrate. Because rare-earth ions have comparable radius, coordination structure and physical–chemical properties as a result when one rare earth ion is replaced by another rare earth ion, the structures are alike extremely [16] and the positions of the products are not sizable. In addition, narrow and high peaks indicated their good crystallinity at room temperature. It can be observed from Fig. 1(a) that the main peaks are [1 0 2] and [2 0 0], revealing that the [1 0 2] and [2 0 0] of crystals prefer to choose to grow. According to Bravais law [17], the main reason is
Fig. 1. (a) XRD pattern of samples, (b) the [2 0 0] lattice plane in the CePO4 crystal cell and (c) the [1 0 2] lattice plane in the CePO4 crystal cell.
superior surface energy. Fig. 1(b and c) displays the [1 0 2] which is asymmetrical and only one phosphorus atom is occupied, and the [2 0 0] which is symmetric and is occupied with four oxygen atoms, five cerium atoms, three phosphorus atoms, in the CePO4 crystal cell. 3.1.2. Infrared spectroscopy (IR) characterization The FT-IR spectra presented in Fig. 2 were obtained using the KBr technique with FT-IR Presting-21.
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Fig. 2. (a) The FT-IR spectra of samples and (b) the FT-IR spectra of CePO4 :Tb/GdPO4 .
There are few differences for all of infrared spectra of the asprepared samples in Fig. 2(a). However, it is worth mentioning that there is a remarkable difference, which O–P tetrahedron dissymmetry stretching vibration in CePO4 :Tb/GdPO4 and (Ce, Tb, Gd)PO4 appear blue shift [18] in the range of 1050–1070 cm−1 , because the interatomic distance of Gd O is shorter than Ce O’s. In addition, for example, Fig. 2(b) is the infrared spectroscopy of CePO4 :Tb/GdPO4. It is characteristic bands that appear at 549 cm−1 , 625 cm−1 and 1068 cm−1 for PO4 3− . Bands at 549 cm−1 , 625 cm−1 are attributed to the O P O bending vibration [18]. Band at 1068 cm−1 is assigned to the O P tetrahedron dissymmetry stretching vibration. There are two peaks from 500 cm−1 to 600 cm−1 , indicating the samples are the hexagonal morphology of rare earth orthophosphate with eight ligands. The consequence is cooperated with XRD analysis. Bands observed from 3000 cm−1 to 3500 cm−1 and 1623 cm−1 are ascribed respectively to H2 O stretching vibration and bending vibration [18,19]. Traces of 2855 cm−1 and 2926 cm−1 are attributed to the dissymmetry and symmetry stretching vibration of CH2 [19], and the absorption bands observed at 1450 cm−1 are assigned to the CH2 scissor bending vibration, which can be caused by residual ethanol.
3.1.3. TEM analysis TEM images of the products are shown in Fig. 3(a)–(d). It can be seen that all of the synthesized phosphors are regular sticks.
It is indicated that the size of CePO4 :Tb is 4.08 nm × 34.65 nm, CePO4 :Tb/GdPO4 ’s is 6.16 nm × 51.21 nm, GdPO4 /CePO4 :Tb’s is 6.28 nm × 66.04 nm, and (Ce, Tb, Gd)PO4 ’s is 7.86 nm × 73.17 nm. The shell thickness of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core/shell structure is 1.04 nm and 1.10 nm respectively. The shell thickness of CePO4 :Tb/GdPO4 is smaller than GdPO4 /CePO4 owing to the radius of Gd3+ less than Ce3+ ’s. In addition, it is observed that the crystal grows longitudinally corresponding to the result of XRD characterization.
3.2. Photoluminescence characterization Fig. 4(a) and (b) is obtained excitation (monitored with 545 nm) and emission (monitored with 314 nm) spectra of the sample at room-temperature. Photoluminescence photographs of GdPO4 :Tb, CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb, (Gd, Ce, Tb)PO4 are shown in Fig. 4(c) on the UV lamp at 254 nm. The characteristic emission peaks 5 DJ (J = {3, 4})–7 FJ (J = {6, 5, 4, 3}) [20] are observed in Fig. 4(a), corresponding 5 D4 –7 F6 (491 nm), 5 D4 –7 F5 (545 nm), 5 D4 –7 F4 (587 nm), 5 D4 –7 F3 (621 nm) [18]. The obvious differences of photoluminescence intensity were observed from the spectra, and the sequence was ICePO4 :Tb < I(Gd,Ce,Tb)PO4 < IGdPO4 /CePO4 :Tb < ICePO4 :Tb/GdPO4 . The photoluminescence intensity of CePO4 :Tb/GdPO4 was 33 times, 7times, 2 times compared with CePO4 :Tb, (Gd, Ce, Tb)PO4 , GdPO4 /CePO4 :Tb
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Fig. 3. TEM images of samples: (a) CePO4 :Tb, (b) CePO4 :Tb/GdPO4 , (c) GdPO4 /CePO4 :Tb and (d) (Gd, Ce, Tb)PO4 .
respectively. The photoluminescence intensity of GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 increased observably than CePO4 :Tb’s, and combined with TEM analysis, which the size of GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 are bigger than CePO4:Tb, revealing that GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 are the core–shell structure [14,21]. Although Gd3+ ion has a weak absorption under UV, the emission peak (at 310 nm) of Gd3+ ion is overlapping Ce3+ ion in a specified range. The Gd3+ emission peak originates from 6 P –8 S J 7/2 (275 nm) with quite narrow stokes shift (35 nm), which lead to no radiation relaxation easily, so the direct energy transfer from Gd3+ ion to the activator Tb3+ ions is not applicable. It is a consequence of GdPO4 :Tb with no luminance (shown in the first quartz cuvette in Fig. 3(c)). The photoemission process starts with the excitation of Ce3+ via the allowed Ce3+ 4f–5d transition and the excitation energy are subsequently transferred to the Gd3+ and migrates over the Gd3+ sublattice in Gd Ce Tb system [20,22,23] owing to CePO4 strong absorption under UV. As 5d energy states of Ce3+ ion are high, which can be transferred to Gd3+ ion efficiently [18], the emission intensity of (Gd, Ce, Tb)PO4 increase compared to CePO4 :Tb. The main reason that I(Gd,Ce,Tb)PO4 < IGdPO4 /CePO4 :Tb may be caused by the concentration of CePO4 is higher in the sample of GdPO4 /CePO4 :Tb than (Gd, Ce, Tb)PO4 , and most of CePO4 are exposed outside, which are good for energy absorption in GdPO4 /CePO4 :Tb core–shell structure. The CePO4 :Tb/GdPO4 nanobar showed the
most intense emission among the tested samples based on the following arguments: (1) Due to GdPO4 shell’s shielding effect on the surface of CePO4 :Tb nanobars, which lead to the energy cannot be transferred, a lot of the radiation recombination center greatly reduces [14,24] and the distance of lanthanum ions fluorescent center and surface quenches center increases, then the non-radiative recombination paths decrease in energy transfer process and energy quenches are inhibited [25], so the luminous intensity of CePO4 :Tb/GdPO4 is higher than CePO4 :Tb. (2) It is the same crystal for GdPO4 and CePO4 , then the Gd3+ ion gets into the CePO4 lattice easily, which increased the opportunity of Ce3+ → Gd3+ and Gd3+ → Tb3+ energy transmission. In the process for forming the precipitation of (Gd, Ce, Tb)PO4 , Gd3+ ion and Ce3+ ion competed with each other, resulting the content of GdPO4 covered on the surface of (Gd, Ce, Tb)PO4 is less than CePO4 :Tb/GdPO4 and leading to the energy loss more easily, so the fluorescence intensity of (Gd, Ce, Tb)PO4 is weaker than CePO4 PO4 :Tb/GdPO4 . (3) Although the process of energy transmission for Gd3+ → Tb3+ exists when CePO4 :Tb as the GdPO4 /CePO4 :Tb shell, the role of shielding effect cannot fair play by GdPO4 . Therefore, photoluminescence intensity of GdPO4 /CePO4 :Tb is less than CePO4 :Tb/GdPO4 but higher than CePO4 :Tb. The result shows that photoluminescence intensity of CePO4 :Tb/GdPO4 is the best. It is more suitable for developing the efficient photoluminescence materials.
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GdPO4 /CePO4 :Tb and (Gd, Ce, Tb)PO4 : nanobars utilizing rare earth oxides and phosphoric acid as raw materials. Structure and photoluminescence properties were investigated. The result shows that the samples are hexagonal rare earth orthophosphate crystals of (4–8) nm × (35–73) nm. The shell thickness of CePO4 :Tb/GdPO4 core–shell structure is 1.04 nm and GdPO4 /CePO4 :Tb 1.10 nm. [1 0 2] and [2 0 0] of crystals have priority to grow. The photoluminescence properties of core–shell structures are better than those of co-precipitated ones. Conversion from core to shell has resulted in a profound change in photoluminescence intensity. The CePO4 :Tb/GdPO4 nanobar showed the most intense emission among the tested samples, which is the result of the easiest energy transfer from Ce3+ → Gd3+ → Tb3+ . The characteristic Tb3+5 D4 –7 FJ (J = {6, 5, 4, 3}) emission lines were observed. Acknowledgments We thank Tao Yang for help with TEM. This work was supported by the Innovation Foundation by Guizhou University. References
Fig. 4. (a) Room-temperature emission spectra of samples, (b) room-temperature excitation spectra of the samples and (c) the picture under UV lamp excitation at 254 nm.
4. Conclusions In summary, a mild ultrasonic-assisted technique was successfully established to synthesize CePO4 :Tb, CePO4 :Tb/GdPO4 ,
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrasonic-assisted synthesis of core–shell structure CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb nanophosphors and their photoluminescence properties Yao-yao Fan a , Zong-chao Hu a,∗ , Jian Yang a , Chao Zhang a , Ling Zhu b a b
College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China College of Chemistry and Biological Engineering, Changsha University of Science & Technology, Changsha 410076, China
a r t i c l e
i n f o
Article history: Received 9 August 2012 Received in revised form 8 November 2012 Accepted 12 November 2012 Available online 23 November 2012 Keywords: Ultrasonic Core–shell structure Photoluminescence Nanophosphors
a b s t r a c t CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb and (Ce, Tb, Gd)PO4 (4–8) nm × (35–73) nm sized nanobars with the hexagonal crystal system have been obtained by ultrasonic-assisted synthesis and characterized by X-ray diffraction (XRD), FT-IR spectrum, transmission electron microscopy (TEM), photoluminescence (PL). The shell thickness of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core/shell structure is 1.04 nm and 1.10 nm respectively. Under ultraviolet excitation, these nanophosphors show Tb3+ characteristic emission, 5 D4 –7 FJ (J = {6, 5, 4, 3}) and the fluorescence of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb increases superficially compared with CePO4 :Tb and the co-precipitated (Ce, Tb, Gd)PO4 . The photoluminescence intensity of CePO4 :Tb/GdPO4 is 33 times, 7 times, 2 times as high as that of CePO4:Tb, GdPO4/CePO4:Tb and (Ce, Tb, Gd)PO4 , respectively. It is worth mentioning that the increasing amount of intensity of CePO4 :Tb/GdPO4 is double than that of GdPO4 /CePO4 :Tb. A possible formation mechanism for the fluorescent efficiency enhancement has been proposed. The results are helpful in developing effective phosphors and have potential applications in field emission display (FED) and plasma display panels (PDP). © 2012 Elsevier B.V. All rights reserved.
1. Introduction The utilization of beneficial and non-renewable rare earth metal resources has become higher and higher, in many different areas. Rare-earth [1] metal not only can play a crucial role in optoelectronic, petroleum chemical industry, machinery, energy and environmental protection but also can be used for data storage, biochemical and chemical sensor materials. Rare earth hydrides have been extensively applied in the electric light source, permanent high-performance magnets, hydrogen storage, catalysis [2] and other functional materials. One of the widest applications of rare earth metals is the synthesis of fluorescent materials whose functions depend strongly on their composition and structure [3–5]. Reasonable and efficient use of rare earth metals is an important approach to solve the problem of resource shortage. Research on fluorescent materials has attracted significant interests [6,7] due to potential applications. With the requirements of photoluminescence intensity and quantum efficiency, rare-earth phosphates as a composite matrix of nanophosphor have also attracted considerable attention. Rambabu et al. [8] have studied photoluminescence materials with different rare-earth ions doped into rare-earth phosphates, for example, LnPO4 :Tb3+ (Ln = Y, La, Gd) and LnPO4 :Eu3+ , which
showed that (Gd0.25 , La0.75 )PO4 :Tb3+ and (Gd0.5 , Y0.5 )PO4 :Tb3+ have appeared a bright green light as two kinds of green promising phosphors. Yu et al. [9] have prepared LaPO4 :(Ce3+ , Tb3+ ) luminescent thin film by Pechini type sol–gel method and compared with larger particles and Tb3+ doped LaPO4 in nanoscale colloid, and the photoluminescence properties have not changed. Optical properties of nanowires and nanoparticles of LaPO4 have been compared by Song et al. [10]. It was found that the great changes of photoluminescence properties depended on the altered morphology of powder. Meyssamy et al. [11,12] have studied Eu3+ and Tb3+ doped LaPO4 nanofiber, and found that nanofiber photoluminescence materials appeared strong light. The photoluminescence quantum yield of CePO4 :Tb/LaPO4 core–shell nanoparticles can be improved [13,14]. Herein we report the synthesis of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core–shell [15] structure phosphors by ultrasonic method and the investigation of their structural characteristics, photoluminescence properties and the mechanism of the fluorescent efficiency change in core to shell conversion, so as to provide a theoretical foundation for the development of efficient photoluminescence materials. 2. Experimental 2.1. Sample preparations
∗ Corresponding author. Tel.: +86 13985113898; fax: +86 08513625867. E-mail address: [email protected] (Z.-c. Hu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.050
All chemicals used in the experiments were of analytical purity, bought from Sinopharm Chemical Reagent Co. Ltd. The precursor
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solutions were dissolved with rare-earth oxides. (1) In a typical procedure for synthesizing CePO4 :Tb phosphors [13], 20 mL Ce(NO3 )3 (2.07 mmol) and Tb(NO3 )3 (0.23 mmol) were added into 30 mL H3 PO4 (11.5 mol) aqueous solution to produce the mixed solution, then used 1:1 ammonia solution to adjust to pH = 1. After that, the mixed procedure was irradiated at 80 W for 2 h with ultrasonic to produce a viscous gel. The obtained gel was centrifuged and washed repeatedly with distilled water and ethanol, filtered to the ultimate white CePO4 :Tb phosphors, which was kept for 24 h at the temperature of 60 ◦ C. (2) CePO4 :Tb/GdPO4 was synthesized via taking half of CePO4 :Tb phosphors dispersed in 10 mL distilled water, then the solution was added 10 mL Gd(NO3 )3 (2.07 mmol) irradiated for 30 min by ultrasonic to produce the transparent and viscous gel. The obtained gel was mixed in 30 mL H3 PO4 (11.5 mmol) aqueous solutions and adjusted to pH = 1 using 1:1 ammonia solution. Subsequently the remaining steps were the same as CePO4 :Tb phosphors’. (3)The method of GdPO4 /CePO4 :Tb phosphors synthesis is the same as CePO4 :Tb/GdPO4 . At first, the core of GdPO4 was synthesized with 20 mL Gd(NO3 )3 (2.3 mmol) solutions and 30 mL H3 PO4 (11.5 mmol) aqueous solutions irradiated for 2 h in ultrasonic. Fewer steps are consistent with CePO4 :Tb phosphors. The coating of GdPO4 /CePO4 :Tb phosphors was gotten by using the synthesis as CePO4 :Tb/GdPO4 without adding Tb(NO3 )3 . (4) (Ce, Tb, Gd)PO4 phosphors synthesized via all solutions with the same content of CePO4 Tb/GdPO4 were mixed together, then irradiated for 2 h in ultrasonic. The remaining steps the same as CePO4 :Tb phosphors’. 2.2. Characterization Photoluminescence excitation and emission spectra of the nanoparticles were acquired by a Hitachi F-4500 photoluminescence spectrometer (PL, HITACHI, JPN). FT-IR spectra were obtained by using the KBr technique with Fourier transforms infrared spectrometer (FT-IR, FT-IR Presting-21, JPN). The morphology of the samples was characterized on a TECNAI G2 20 transmission electron microscope (TEM, FEI Ltd., USA), with an accelerating voltage of 200 KV. The crystal phase of the as-prepared products was identified by X’Pert Pro diffractometer (Panalytical Co., Holland) at a scan ˚ The samrate of 10◦ min−1 using Cu K␣1 radiation ( = 1.5406 A). ples were synthesized by ultrasonic cell disruption system with 80 W (JY92-IIDN, CHN). 3. Results and discussions 3.1. Structure and morphology 3.1.1. Phase identification The XRD patterns of as-synthesized products are shown in Fig. 1(a). It is clear that CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb and (Ce, Tb, Gd)PO4 can be readily indexed to phase-pure hexagonal morphology cerium phosphate hydrate and gadolinium phosphate hydrate, corresponding to (JCPDS-751880) (JCPDS-210337) (JCPDS-350614) (JCPDS-350614), respectively. All of the obtained rare-earth phosphate phosphors remain a solitary phase after doping Tb3+ , which indicates Tb3+ is effectively doped into crystal lattices of cerium phosphate hydrate and gadolinium phosphate hydrate. Because rare-earth ions have comparable radius, coordination structure and physical–chemical properties as a result when one rare earth ion is replaced by another rare earth ion, the structures are alike extremely [16] and the positions of the products are not sizable. In addition, narrow and high peaks indicated their good crystallinity at room temperature. It can be observed from Fig. 1(a) that the main peaks are [1 0 2] and [2 0 0], revealing that the [1 0 2] and [2 0 0] of crystals prefer to choose to grow. According to Bravais law [17], the main reason is
Fig. 1. (a) XRD pattern of samples, (b) the [2 0 0] lattice plane in the CePO4 crystal cell and (c) the [1 0 2] lattice plane in the CePO4 crystal cell.
superior surface energy. Fig. 1(b and c) displays the [1 0 2] which is asymmetrical and only one phosphorus atom is occupied, and the [2 0 0] which is symmetric and is occupied with four oxygen atoms, five cerium atoms, three phosphorus atoms, in the CePO4 crystal cell. 3.1.2. Infrared spectroscopy (IR) characterization The FT-IR spectra presented in Fig. 2 were obtained using the KBr technique with FT-IR Presting-21.
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Fig. 2. (a) The FT-IR spectra of samples and (b) the FT-IR spectra of CePO4 :Tb/GdPO4 .
There are few differences for all of infrared spectra of the asprepared samples in Fig. 2(a). However, it is worth mentioning that there is a remarkable difference, which O–P tetrahedron dissymmetry stretching vibration in CePO4 :Tb/GdPO4 and (Ce, Tb, Gd)PO4 appear blue shift [18] in the range of 1050–1070 cm−1 , because the interatomic distance of Gd O is shorter than Ce O’s. In addition, for example, Fig. 2(b) is the infrared spectroscopy of CePO4 :Tb/GdPO4. It is characteristic bands that appear at 549 cm−1 , 625 cm−1 and 1068 cm−1 for PO4 3− . Bands at 549 cm−1 , 625 cm−1 are attributed to the O P O bending vibration [18]. Band at 1068 cm−1 is assigned to the O P tetrahedron dissymmetry stretching vibration. There are two peaks from 500 cm−1 to 600 cm−1 , indicating the samples are the hexagonal morphology of rare earth orthophosphate with eight ligands. The consequence is cooperated with XRD analysis. Bands observed from 3000 cm−1 to 3500 cm−1 and 1623 cm−1 are ascribed respectively to H2 O stretching vibration and bending vibration [18,19]. Traces of 2855 cm−1 and 2926 cm−1 are attributed to the dissymmetry and symmetry stretching vibration of CH2 [19], and the absorption bands observed at 1450 cm−1 are assigned to the CH2 scissor bending vibration, which can be caused by residual ethanol.
3.1.3. TEM analysis TEM images of the products are shown in Fig. 3(a)–(d). It can be seen that all of the synthesized phosphors are regular sticks.
It is indicated that the size of CePO4 :Tb is 4.08 nm × 34.65 nm, CePO4 :Tb/GdPO4 ’s is 6.16 nm × 51.21 nm, GdPO4 /CePO4 :Tb’s is 6.28 nm × 66.04 nm, and (Ce, Tb, Gd)PO4 ’s is 7.86 nm × 73.17 nm. The shell thickness of CePO4 :Tb/GdPO4 and GdPO4 /CePO4 :Tb core/shell structure is 1.04 nm and 1.10 nm respectively. The shell thickness of CePO4 :Tb/GdPO4 is smaller than GdPO4 /CePO4 owing to the radius of Gd3+ less than Ce3+ ’s. In addition, it is observed that the crystal grows longitudinally corresponding to the result of XRD characterization.
3.2. Photoluminescence characterization Fig. 4(a) and (b) is obtained excitation (monitored with 545 nm) and emission (monitored with 314 nm) spectra of the sample at room-temperature. Photoluminescence photographs of GdPO4 :Tb, CePO4 :Tb, CePO4 :Tb/GdPO4 , GdPO4 /CePO4 :Tb, (Gd, Ce, Tb)PO4 are shown in Fig. 4(c) on the UV lamp at 254 nm. The characteristic emission peaks 5 DJ (J = {3, 4})–7 FJ (J = {6, 5, 4, 3}) [20] are observed in Fig. 4(a), corresponding 5 D4 –7 F6 (491 nm), 5 D4 –7 F5 (545 nm), 5 D4 –7 F4 (587 nm), 5 D4 –7 F3 (621 nm) [18]. The obvious differences of photoluminescence intensity were observed from the spectra, and the sequence was ICePO4 :Tb < I(Gd,Ce,Tb)PO4 < IGdPO4 /CePO4 :Tb < ICePO4 :Tb/GdPO4 . The photoluminescence intensity of CePO4 :Tb/GdPO4 was 33 times, 7times, 2 times compared with CePO4 :Tb, (Gd, Ce, Tb)PO4 , GdPO4 /CePO4 :Tb
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Fig. 3. TEM images of samples: (a) CePO4 :Tb, (b) CePO4 :Tb/GdPO4 , (c) GdPO4 /CePO4 :Tb and (d) (Gd, Ce, Tb)PO4 .
respectively. The photoluminescence intensity of GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 increased observably than CePO4 :Tb’s, and combined with TEM analysis, which the size of GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 are bigger than CePO4:Tb, revealing that GdPO4 /CePO4 :Tb and CePO4 :Tb/GdPO4 are the core–shell structure [14,21]. Although Gd3+ ion has a weak absorption under UV, the emission peak (at 310 nm) of Gd3+ ion is overlapping Ce3+ ion in a specified range. The Gd3+ emission peak originates from 6 P –8 S J 7/2 (275 nm) with quite narrow stokes shift (35 nm), which lead to no radiation relaxation easily, so the direct energy transfer from Gd3+ ion to the activator Tb3+ ions is not applicable. It is a consequence of GdPO4 :Tb with no luminance (shown in the first quartz cuvette in Fig. 3(c)). The photoemission process starts with the excitation of Ce3+ via the allowed Ce3+ 4f–5d transition and the excitation energy are subsequently transferred to the Gd3+ and migrates over the Gd3+ sublattice in Gd Ce Tb system [20,22,23] owing to CePO4 strong absorption under UV. As 5d energy states of Ce3+ ion are high, which can be transferred to Gd3+ ion efficiently [18], the emission intensity of (Gd, Ce, Tb)PO4 increase compared to CePO4 :Tb. The main reason that I(Gd,Ce,Tb)PO4 < IGdPO4 /CePO4 :Tb may be caused by the concentration of CePO4 is higher in the sample of GdPO4 /CePO4 :Tb than (Gd, Ce, Tb)PO4 , and most of CePO4 are exposed outside, which are good for energy absorption in GdPO4 /CePO4 :Tb core–shell structure. The CePO4 :Tb/GdPO4 nanobar showed the
most intense emission among the tested samples based on the following arguments: (1) Due to GdPO4 shell’s shielding effect on the surface of CePO4 :Tb nanobars, which lead to the energy cannot be transferred, a lot of the radiation recombination center greatly reduces [14,24] and the distance of lanthanum ions fluorescent center and surface quenches center increases, then the non-radiative recombination paths decrease in energy transfer process and energy quenches are inhibited [25], so the luminous intensity of CePO4 :Tb/GdPO4 is higher than CePO4 :Tb. (2) It is the same crystal for GdPO4 and CePO4 , then the Gd3+ ion gets into the CePO4 lattice easily, which increased the opportunity of Ce3+ → Gd3+ and Gd3+ → Tb3+ energy transmission. In the process for forming the precipitation of (Gd, Ce, Tb)PO4 , Gd3+ ion and Ce3+ ion competed with each other, resulting the content of GdPO4 covered on the surface of (Gd, Ce, Tb)PO4 is less than CePO4 :Tb/GdPO4 and leading to the energy loss more easily, so the fluorescence intensity of (Gd, Ce, Tb)PO4 is weaker than CePO4 PO4 :Tb/GdPO4 . (3) Although the process of energy transmission for Gd3+ → Tb3+ exists when CePO4 :Tb as the GdPO4 /CePO4 :Tb shell, the role of shielding effect cannot fair play by GdPO4 . Therefore, photoluminescence intensity of GdPO4 /CePO4 :Tb is less than CePO4 :Tb/GdPO4 but higher than CePO4 :Tb. The result shows that photoluminescence intensity of CePO4 :Tb/GdPO4 is the best. It is more suitable for developing the efficient photoluminescence materials.
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GdPO4 /CePO4 :Tb and (Gd, Ce, Tb)PO4 : nanobars utilizing rare earth oxides and phosphoric acid as raw materials. Structure and photoluminescence properties were investigated. The result shows that the samples are hexagonal rare earth orthophosphate crystals of (4–8) nm × (35–73) nm. The shell thickness of CePO4 :Tb/GdPO4 core–shell structure is 1.04 nm and GdPO4 /CePO4 :Tb 1.10 nm. [1 0 2] and [2 0 0] of crystals have priority to grow. The photoluminescence properties of core–shell structures are better than those of co-precipitated ones. Conversion from core to shell has resulted in a profound change in photoluminescence intensity. The CePO4 :Tb/GdPO4 nanobar showed the most intense emission among the tested samples, which is the result of the easiest energy transfer from Ce3+ → Gd3+ → Tb3+ . The characteristic Tb3+5 D4 –7 FJ (J = {6, 5, 4, 3}) emission lines were observed. Acknowledgments We thank Tao Yang for help with TEM. This work was supported by the Innovation Foundation by Guizhou University. References
Fig. 4. (a) Room-temperature emission spectra of samples, (b) room-temperature excitation spectra of the samples and (c) the picture under UV lamp excitation at 254 nm.
4. Conclusions In summary, a mild ultrasonic-assisted technique was successfully established to synthesize CePO4 :Tb, CePO4 :Tb/GdPO4 ,
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