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Facile synthesis of visible light excited La1-xGdxF3: Eu3+ micro-meter sphere using polyvinylpyrrolidone as a template
JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 285
Facile synthesis of visible light excited La1–xGdxF3: Eu3+ micro-meter sphere using polyvinylpyrrolidone as a template ZHENG Yuhui (䚥⥝ᚴ), LIU Diqing (߬Ᏹ⏙), WANG Qianming (⥟ࠡᯢ) (School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China) Received 10 June 2010; revised 16 October 2010
Abstract: Lanthanide fluorides exhibited unique luminescent properties in terms of their low phonon energy can restrict the luminescence quenching and extend luminescent lifetimes. Here, a room-temperature co-precipitation method was used to synthesize europium(III) activated La1–xGdxF3 solid phosphors. X-ray diffraction (XRD) data confirmed the crystalline phases of synthesized sample belongs to orthorhombic system. All the as-derived materials exhibited red luminescence (5D0ĺ7F1) under the excitation at longer wavelengths (394 and 466 nm). The powder with the most intense emission was achieved in terms of 10 mol.% doping concentration (Eu content, La/Gd=1/9) and sample sintered at 700 ºC. Scanning electron microscopy (SEM) investigated the morphology and crystalline of the samples, showing that many regular and large balls (5–10 ȝmol/L) were dispersed within the micro-meter scale composites. We proved that the above crystal growth structures were controllable and predicable based on the surface functionalization by polyvinylpyrrolidone ligand. Keywords: europium; fluoride; luminescent; visible light; rare earths
Europium activated fluorides as a bulk material have been paid much attention for basic science field and could be used in displays or red emitting phosphors in lamps. They have been proved to be an excellent host for lanthanide ions in terms of its low phonon energy that can restrict the luminescence quenching and extend luminescent lifetimes[1–6]. Current access to lanthanide containing luminescent particles is through hydrothermal, micro-emulsion or precipitation in ethanol. In our research, firstly, polyvinyl-pyrrolidone (PVP) as an amphiphilic chelating and stabilizing ligand was chosen to render the aggregated samples miscible with aqueous solution[7]. Moreover, its pyrrolidone moiety can provide coordination sites to bind lanthanide ions and shield it from the influence by hydroxyl vibrations. Secondly, PVP as an efficient surfactant coating ligand could control particles growth and modify their morphology. In this paper, we report the synthesis and photo-luminescence properties of PVP modified La1–xGdxF3:Eu3+ phosphors that can exhibit strong red emission. To our knowledge, very limited publications have described the effects of PVP modification agents on lanthanide fluoride luminescent particles.
1 Experimental All the starting materials were obtained from commercial suppliers and used as received. Fluorescence spectra were measured on Edinburgh FLS920 spectrometer, respectively. Dynamic light scattering was measured at BI-200SM. Scanning electronic microscope (SEM) was measured with JSM-6360LV. X-ray powder diffraction were investigated at
Y-2000 of Dandong Aolong company, P.R.China. Surface area were calculated by Brunauer-Emmett-Teller (BET) method. The preparation of sintered La1–xGdxF3:Eu: For the synthesis of La1–xGdxF3: Eu (10%Tb), Gd(NO3)3 (0.01 mol/L), La(NO3)3(0.05 mol/L) and Eu(NO3)3 (0.005 mol/L) were mixed together, then NaF and polyvinyl-pyrrolidone (0.3 g) was added. At this stage the clear solution became precipitated. The whole liquid under intense stirring was heated to 75 ºC and the temperature was maintained for 2 h. The solid samples were collected and washed with acetone and water (4/1). If the sintered sample is needed, the above resulting mixture was poured into crucible and then calcinated at different temperature. After 4 h the powder was obtained. The other products with various La/Gd or Eu3+ ratio were obtained in the same way.
2
Results and discussion
The co-precipitation approach was used for the preparation of La1–xGdxF3:Eu3+ composites, it can be indexed that the orthorhombic crystal phase for La0.1Gd0.9F3:Eu3+ is formed sintered at 600 ºC when the content of Gd3+ is the prevalent (Fig. 1). Ref. [8] proved the hexagonal phase of LnF3 corresponds to light rare earth elements (La-Sm) and we also found the diffraction peak changes in sample La0.9Gd0.1F3:Eu3+ (Figs. not shown). The peak positions at (020), (111), (210) are all similar and agree well with the JCPDS card (12-0788). These results show the crystalline phase could be
Foundation item: Project supported by the Natural Science Foundation of China (21002035) Corresponding author: WANG Qianming (E-mail: [email protected]; Tel.: +86-20-39310258) DOI: 10.1016/S1002-0721(10)60344-4
286
Fig. 1 XRD pattern of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%) powder
achieved in terms of the facile synthetic procedure. The effects of the ratio of La/Gd, content of Eu3+, thermal treatment temperature and polyvinyl-pyrrolidone ligand: Fig. 2 shows the excitation spectrum of synthesized lowtemperature La1–xGdxF3:Eu3+ which was monitored at 592 nm emission of europium ions. The sharp and narrow bands located at 365, 397 and 466 nm are assigned to be the characteristic absorption peaks of transitions from 7F0 to 5D4, 5L6 and 5D2 energy levels[8]. Especially the 7F0ĺ5D2 transition is favorable for longer wavelength sensitization. The emission spectra were measured under the excitation of 397 nm shows the bands at 555, 592 and 615 nm respectively, attributed to the transition of 5D1ĺ7F2, 5D0ĺ7F1 and 5D0ĺ7F2 (Fig. 3).
JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010
Interestingly, the 5D0ĺ7F1 optical transition is prevalent and orange-red luminescence could be observed by naked eyes. Results show that the ideal ratio between La and Gd should be 1:9, the 100% Gd3+ content will also decrease the emission intensities. To clarify the best dopant concentration, 1–15 mol.% amount of europium were added into La0.1Gd0.9F3 matrices. As provided in Fig. 4, the luminescence intensities of green peaks have varied considerably in terms of different amounts of Eu3+. Data suggested that 10 mol.% dopants gave the strongest emission band and concentration quenching effect was observed in the case of 15% Eu3+ addition. We also investigated the influence of sintering temperature from 450 to 700 ºC (Fig. 5). According to the emission spectra, the thermal treatment at 700 ºC brought to the most intense red luminescence. It is estimated that the higher temperature calcination will benefit for the formation of more rigid host lattice which will enhance the luminescence efficiency. More interestingly, the electric-dipole transition of 5 D0ĺ7F2 became dominating after the sintering process, whereas magnetic-dipole transition 5D0ĺ7F1 was the strongest based on the co-precipitation method. It shows that the inversion symmetry of europium ions disappeared in the crystal lattice during the thermal treatment and 5D0ĺ7F2 is hypersensitive to the environment changes. As is known to us, the emission of lanthanide ions was
Fig. 2 Excitation spectrum (Em=592 nm) of synthesized low-temperature La0.1Gd0.9F3:Eu3+(10 mol.%)
Fig. 4 Emission spectra of synthesized low-temperature La0.1Gd0.9F3: Eu3+ (1–15 mol.%)
Fig. 3 Emission spectra of synthesized low-temperature La1–xGdxF3: Eu3+ (10 mol.%)
Fig. 5 Emission spectra of La0.1Gd0.9F3:Eu3+ (10 mol.%) sintered at different temperature
ZHENG Yuhui et al., Facile synthesis of visible light excited La1–xGdxF3: Eu3+ micro-meter sphere using polyvinylpyrrolidone … 287
readily quenched by high vibration oscillators causing by surface impurities and polar solvents, the excited energy would be dissipated through strong interactions. But poly vinyl-pyrrolidone (PVP) could coordinate to lanthanide ions through its oxygen atom, the complete coordination environment will avoid the possibilities of non-irradiation. In this case, the introduction of PVP provided an effective way to improve the luminescence intensity (Fig. 6). In order to study more about the internal surface structure, the characterization of the nitrogen adsorption and desorption was measured. The sintered material possesses SBET of 352 m2/g (Fig. 7). Without the assistance of the ligand PVP, the BET surface of La1–xGdxF3:Eu3+ remained to be 50–60 m2/g (Figs not given). However, the micrometer crystals can be finely dispersed and particle aggregation was largely prohibited. Therefore, PVP plays an important role in controlling particle growth and surface functionality for the preparation of europium activated fluoride materials. The morphology of the sintered powders was investigated by scanning electron microscopy. SEM graph shows microstructure of the samples appears to be regular spherical balls with aggregation and an average particle size of 5–10 ȝmol/L (Fig. 8). We estimated that polyvinylpyrrolidone is critical to the surface modification and it helps to form these round shape structure. A close examination of dynamic light scattering (DLS) clarifies the calcinated sample (Fig. 9). Results show that the as-formed particles have average diameters from 5 to
Fig. 8 SEM graphs of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC)
Fig. 9 DLS curve of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC)
10 ȝmol/L and these aggregates with micro-meter size consist with the evidence obtained in SEM measurements.
3 Conclusions Fig. 6 Emission spectra of La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC) with or without PVP
Fig. 7 N2 adsorption-desorption isotherms of sintered La0.1Gd0.9F3: Eu3+ (10 mol.%, 700 ºC)
Fluorides have been considered as an excellent host for lanthanide ions based on its low phonon energy. XRD data showed the crystalline phase of synthesized sample is orthorhombic system. The prepared materials exhibited red luminescence under the excitation at visible range. The powder with the strongest emission was obtained using 10 mol.% doping concentration (Eu content, La/Gd=1/9) and sample sintered at 700 ºC. Scanning electron microscopy (SEM) proved that many spherical particles (5–10 ȝmol/L) were dispersed within the micro-meter scale composites. We have proved the above crystal growth structures were controlled by polyvinylpyrrolidone ligand and the latter also contributes to the emission of europium ions. Acknowledgements: Q. M. appreciates Start Funding of South China Normal University G21117 and Natural Science Foundation
288 of China (21002035).
References: [1] Stouwdam J W, van Veggel F C J M. Near-infrared emission of redispersible Er3+, Nd3+, and Ho3+ doped LaF3 nanoparticles. Nano Lett., 2002, 7: 733. [2] Grzyb T, Lis S. Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method. J. Rare Earths, 2009, 27: 588. [3] Stouwdam J W, van Veggel F C J M. Improvement in the luminescence properties and processability of LaF3/Ln and LaPO4/ Ln nanoparticles by surface modification. Langmuir, 2004, 20: 11763. [4] Zhang X Q, Fan X P, Qiao X S, Luo Q. NaGdF4:Ce3+ and (Ce,
JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010 Gd)F3 nanoparticles: Hydrothermal synthesis and luminescence properties. Mater. Chem. Phys., 2010, 121: 274. [5] Crichton W A, Bouvier P, Winkler B, Grzechnik A. The structural behaviour of LaF3 at high pressures. Dalton Trans., 2010, 39: 4302. [6] Wang Q, You Y M, Ludescher R D, Ju Y G. Syntheses of optically efficient (La1–x–yCexTby)F3 nanocrystals via a hydrothermal method. J. Lumin., 2010, 130: 1076. [7] Wang F, Liu X G. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev., 2009, 38: 976. [8] Li C X, Yang J, Yang P P, Lian H Z, Lin J. Hydrothermal Synthesis of Lanthanide Fluorides LnF3 (Ln) La to Lu) Nano-/Microcrystals with Multiform Structures and Morphologies. Chem. Mater., 2008, 20: 4317.
Facile synthesis of visible light excited La1–xGdxF3: Eu3+ micro-meter sphere using polyvinylpyrrolidone as a template ZHENG Yuhui (䚥⥝ᚴ), LIU Diqing (߬Ᏹ⏙), WANG Qianming (⥟ࠡᯢ) (School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China) Received 10 June 2010; revised 16 October 2010
Abstract: Lanthanide fluorides exhibited unique luminescent properties in terms of their low phonon energy can restrict the luminescence quenching and extend luminescent lifetimes. Here, a room-temperature co-precipitation method was used to synthesize europium(III) activated La1–xGdxF3 solid phosphors. X-ray diffraction (XRD) data confirmed the crystalline phases of synthesized sample belongs to orthorhombic system. All the as-derived materials exhibited red luminescence (5D0ĺ7F1) under the excitation at longer wavelengths (394 and 466 nm). The powder with the most intense emission was achieved in terms of 10 mol.% doping concentration (Eu content, La/Gd=1/9) and sample sintered at 700 ºC. Scanning electron microscopy (SEM) investigated the morphology and crystalline of the samples, showing that many regular and large balls (5–10 ȝmol/L) were dispersed within the micro-meter scale composites. We proved that the above crystal growth structures were controllable and predicable based on the surface functionalization by polyvinylpyrrolidone ligand. Keywords: europium; fluoride; luminescent; visible light; rare earths
Europium activated fluorides as a bulk material have been paid much attention for basic science field and could be used in displays or red emitting phosphors in lamps. They have been proved to be an excellent host for lanthanide ions in terms of its low phonon energy that can restrict the luminescence quenching and extend luminescent lifetimes[1–6]. Current access to lanthanide containing luminescent particles is through hydrothermal, micro-emulsion or precipitation in ethanol. In our research, firstly, polyvinyl-pyrrolidone (PVP) as an amphiphilic chelating and stabilizing ligand was chosen to render the aggregated samples miscible with aqueous solution[7]. Moreover, its pyrrolidone moiety can provide coordination sites to bind lanthanide ions and shield it from the influence by hydroxyl vibrations. Secondly, PVP as an efficient surfactant coating ligand could control particles growth and modify their morphology. In this paper, we report the synthesis and photo-luminescence properties of PVP modified La1–xGdxF3:Eu3+ phosphors that can exhibit strong red emission. To our knowledge, very limited publications have described the effects of PVP modification agents on lanthanide fluoride luminescent particles.
1 Experimental All the starting materials were obtained from commercial suppliers and used as received. Fluorescence spectra were measured on Edinburgh FLS920 spectrometer, respectively. Dynamic light scattering was measured at BI-200SM. Scanning electronic microscope (SEM) was measured with JSM-6360LV. X-ray powder diffraction were investigated at
Y-2000 of Dandong Aolong company, P.R.China. Surface area were calculated by Brunauer-Emmett-Teller (BET) method. The preparation of sintered La1–xGdxF3:Eu: For the synthesis of La1–xGdxF3: Eu (10%Tb), Gd(NO3)3 (0.01 mol/L), La(NO3)3(0.05 mol/L) and Eu(NO3)3 (0.005 mol/L) were mixed together, then NaF and polyvinyl-pyrrolidone (0.3 g) was added. At this stage the clear solution became precipitated. The whole liquid under intense stirring was heated to 75 ºC and the temperature was maintained for 2 h. The solid samples were collected and washed with acetone and water (4/1). If the sintered sample is needed, the above resulting mixture was poured into crucible and then calcinated at different temperature. After 4 h the powder was obtained. The other products with various La/Gd or Eu3+ ratio were obtained in the same way.
2
Results and discussion
The co-precipitation approach was used for the preparation of La1–xGdxF3:Eu3+ composites, it can be indexed that the orthorhombic crystal phase for La0.1Gd0.9F3:Eu3+ is formed sintered at 600 ºC when the content of Gd3+ is the prevalent (Fig. 1). Ref. [8] proved the hexagonal phase of LnF3 corresponds to light rare earth elements (La-Sm) and we also found the diffraction peak changes in sample La0.9Gd0.1F3:Eu3+ (Figs. not shown). The peak positions at (020), (111), (210) are all similar and agree well with the JCPDS card (12-0788). These results show the crystalline phase could be
Foundation item: Project supported by the Natural Science Foundation of China (21002035) Corresponding author: WANG Qianming (E-mail: [email protected]; Tel.: +86-20-39310258) DOI: 10.1016/S1002-0721(10)60344-4
286
Fig. 1 XRD pattern of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%) powder
achieved in terms of the facile synthetic procedure. The effects of the ratio of La/Gd, content of Eu3+, thermal treatment temperature and polyvinyl-pyrrolidone ligand: Fig. 2 shows the excitation spectrum of synthesized lowtemperature La1–xGdxF3:Eu3+ which was monitored at 592 nm emission of europium ions. The sharp and narrow bands located at 365, 397 and 466 nm are assigned to be the characteristic absorption peaks of transitions from 7F0 to 5D4, 5L6 and 5D2 energy levels[8]. Especially the 7F0ĺ5D2 transition is favorable for longer wavelength sensitization. The emission spectra were measured under the excitation of 397 nm shows the bands at 555, 592 and 615 nm respectively, attributed to the transition of 5D1ĺ7F2, 5D0ĺ7F1 and 5D0ĺ7F2 (Fig. 3).
JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010
Interestingly, the 5D0ĺ7F1 optical transition is prevalent and orange-red luminescence could be observed by naked eyes. Results show that the ideal ratio between La and Gd should be 1:9, the 100% Gd3+ content will also decrease the emission intensities. To clarify the best dopant concentration, 1–15 mol.% amount of europium were added into La0.1Gd0.9F3 matrices. As provided in Fig. 4, the luminescence intensities of green peaks have varied considerably in terms of different amounts of Eu3+. Data suggested that 10 mol.% dopants gave the strongest emission band and concentration quenching effect was observed in the case of 15% Eu3+ addition. We also investigated the influence of sintering temperature from 450 to 700 ºC (Fig. 5). According to the emission spectra, the thermal treatment at 700 ºC brought to the most intense red luminescence. It is estimated that the higher temperature calcination will benefit for the formation of more rigid host lattice which will enhance the luminescence efficiency. More interestingly, the electric-dipole transition of 5 D0ĺ7F2 became dominating after the sintering process, whereas magnetic-dipole transition 5D0ĺ7F1 was the strongest based on the co-precipitation method. It shows that the inversion symmetry of europium ions disappeared in the crystal lattice during the thermal treatment and 5D0ĺ7F2 is hypersensitive to the environment changes. As is known to us, the emission of lanthanide ions was
Fig. 2 Excitation spectrum (Em=592 nm) of synthesized low-temperature La0.1Gd0.9F3:Eu3+(10 mol.%)
Fig. 4 Emission spectra of synthesized low-temperature La0.1Gd0.9F3: Eu3+ (1–15 mol.%)
Fig. 3 Emission spectra of synthesized low-temperature La1–xGdxF3: Eu3+ (10 mol.%)
Fig. 5 Emission spectra of La0.1Gd0.9F3:Eu3+ (10 mol.%) sintered at different temperature
ZHENG Yuhui et al., Facile synthesis of visible light excited La1–xGdxF3: Eu3+ micro-meter sphere using polyvinylpyrrolidone … 287
readily quenched by high vibration oscillators causing by surface impurities and polar solvents, the excited energy would be dissipated through strong interactions. But poly vinyl-pyrrolidone (PVP) could coordinate to lanthanide ions through its oxygen atom, the complete coordination environment will avoid the possibilities of non-irradiation. In this case, the introduction of PVP provided an effective way to improve the luminescence intensity (Fig. 6). In order to study more about the internal surface structure, the characterization of the nitrogen adsorption and desorption was measured. The sintered material possesses SBET of 352 m2/g (Fig. 7). Without the assistance of the ligand PVP, the BET surface of La1–xGdxF3:Eu3+ remained to be 50–60 m2/g (Figs not given). However, the micrometer crystals can be finely dispersed and particle aggregation was largely prohibited. Therefore, PVP plays an important role in controlling particle growth and surface functionality for the preparation of europium activated fluoride materials. The morphology of the sintered powders was investigated by scanning electron microscopy. SEM graph shows microstructure of the samples appears to be regular spherical balls with aggregation and an average particle size of 5–10 ȝmol/L (Fig. 8). We estimated that polyvinylpyrrolidone is critical to the surface modification and it helps to form these round shape structure. A close examination of dynamic light scattering (DLS) clarifies the calcinated sample (Fig. 9). Results show that the as-formed particles have average diameters from 5 to
Fig. 8 SEM graphs of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC)
Fig. 9 DLS curve of sintered La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC)
10 ȝmol/L and these aggregates with micro-meter size consist with the evidence obtained in SEM measurements.
3 Conclusions Fig. 6 Emission spectra of La0.1Gd0.9F3:Eu3+ (10 mol.%, 700 ºC) with or without PVP
Fig. 7 N2 adsorption-desorption isotherms of sintered La0.1Gd0.9F3: Eu3+ (10 mol.%, 700 ºC)
Fluorides have been considered as an excellent host for lanthanide ions based on its low phonon energy. XRD data showed the crystalline phase of synthesized sample is orthorhombic system. The prepared materials exhibited red luminescence under the excitation at visible range. The powder with the strongest emission was obtained using 10 mol.% doping concentration (Eu content, La/Gd=1/9) and sample sintered at 700 ºC. Scanning electron microscopy (SEM) proved that many spherical particles (5–10 ȝmol/L) were dispersed within the micro-meter scale composites. We have proved the above crystal growth structures were controlled by polyvinylpyrrolidone ligand and the latter also contributes to the emission of europium ions. Acknowledgements: Q. M. appreciates Start Funding of South China Normal University G21117 and Natural Science Foundation
288 of China (21002035).
References: [1] Stouwdam J W, van Veggel F C J M. Near-infrared emission of redispersible Er3+, Nd3+, and Ho3+ doped LaF3 nanoparticles. Nano Lett., 2002, 7: 733. [2] Grzyb T, Lis S. Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method. J. Rare Earths, 2009, 27: 588. [3] Stouwdam J W, van Veggel F C J M. Improvement in the luminescence properties and processability of LaF3/Ln and LaPO4/ Ln nanoparticles by surface modification. Langmuir, 2004, 20: 11763. [4] Zhang X Q, Fan X P, Qiao X S, Luo Q. NaGdF4:Ce3+ and (Ce,
JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010 Gd)F3 nanoparticles: Hydrothermal synthesis and luminescence properties. Mater. Chem. Phys., 2010, 121: 274. [5] Crichton W A, Bouvier P, Winkler B, Grzechnik A. The structural behaviour of LaF3 at high pressures. Dalton Trans., 2010, 39: 4302. [6] Wang Q, You Y M, Ludescher R D, Ju Y G. Syntheses of optically efficient (La1–x–yCexTby)F3 nanocrystals via a hydrothermal method. J. Lumin., 2010, 130: 1076. [7] Wang F, Liu X G. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev., 2009, 38: 976. [8] Li C X, Yang J, Yang P P, Lian H Z, Lin J. Hydrothermal Synthesis of Lanthanide Fluorides LnF3 (Ln) La to Lu) Nano-/Microcrystals with Multiform Structures and Morphologies. Chem. Mater., 2008, 20: 4317.