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Ag-doped CaF2 microcubes
Journal of Luminescence 166 (2015) 361–365
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fabrication and luminescence properties of Tb3 þ and Tb3 þ /Ag-doped CaF2 microcubes Lin Sun a, Enzhou Liu a, Jun Fan a,n, Xiaoyun Hu b, Jun Wan a, Juan Li a, Hua Li a, Yang Hu a a b
School of Chemical Engineering, Northwest University, Xi'an 710069, PR China Department of Physics, Northwest University, Xi'an 710069, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 January 2015 Received in revised form 25 April 2015 Accepted 28 April 2015 Available online 8 May 2015
CaF2:Tb3 þ and CaF2:Tb3 þ /Ag microcubes were synthesized by the microwave–hydrothermal method. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), and UV–vis absorption spectroscopy (UV–vis). SEM results reveal that monodisperse and uniform microcube with smooth surfaces can be prepared by this approach. The PL spectra indicate that the 5D4–7F5 (544 nm) transition is the most intensive when excited at 213 nm. It is shown that CaF2: Tb3 þ with 4 mol% Tb3 þ concentration exhibited the highest emission intensity. The experimental results indicate that Ag nanoparticles can greatly enhance the light emission from Tb3 þ in CaF2:Tb3 þ sample, which was attributed to the local field enhancement and the increased radiative decay rates of Tb3 þ caused by Ag nanoparticles. Additionally, the comparison of emission for CaF2:Tb3 þ /Ag and CaF2:Tb3 þ (1 mol%) particles reveal that the emission intensity of the former is about 5.7 times as strong as that of the latter. & 2015 Elsevier B.V. All rights reserved.
Keywords: CaF2 microcubes Luminescence Surface plasmon resonance Radiative decay rates
1. Introduction Fluoride compounds have been widely applied in optics and electronics due to their transparency in a wide range from the VUV to IR, low phonon energy and refractive index. [1–5]. Among the fluoride compounds, the calcium fluoride (CaF2) is a kind of typical alkaline-earth fluorides, a particular feature of which is the typical fluorite structure consisting of a simple cubic array of fluoride ions with every alternate cube occupied by a divalent cation Ca [6]. This fluorite structure enables high doping concentration of foreign ions. The rare earth (RE) ions doped CaF2 could be used as laser material and fluorescent labeling material in biological applications [7,8]. As a result, considerable attention has been focused on this field recently. Up to now, An appreciable amount of reports have been published on the photoluminescence of CaF2 doped different lanthanide ions, such as Yb3 þ , Eu3 þ , Er3 þ and Ce3 þ [9–12]. In addition to these RE ions mentioned above, Terbium (Tb3 þ ) element is one important and particular RE ion with intense intra-4f transitions, and a long life time for excited states [13–16]. Recently, various methods such as Li þ codoping, energy transfer and core–shell design have been used to
n
Corresponding author. Tel.: þ 86 29 88002223; fax: þ 86 29 88305252. E-mail address: [email protected] (J. Fan).
http://dx.doi.org/10.1016/j.jlumin.2015.04.048 0022-2313/& 2015 Elsevier B.V. All rights reserved.
improve luminescence property of RE ions [17–20]. As one of the efficient methods, noble-metal nanoparticles (NPs) co-doping, have been widely developed to enhance the luminescence intensity of RE ions based on their surface plasmon resonance (SPR) effect [21–26]. The noble metal NPs nearby fluorescent centers at suitable separations can magnify the fluorescence intensity hundreds times. Qi reported that Ag NPs could effectively increase the fluorescence intensity of Er3þ in glass [27]. Results show that the intensity of Er3 þ increases by 7.2 times after doping of Ag NPs, and the above optical enhancement is attributed to the localized field enhancement near Er3þ. However, so far, the influence of Ag nanoparticles on luminescent performance of CaF2:Tb3 þ microcubes have not been systematically investigated. There have been several reports on synthesis of CaF2 particles by various techniques. Lin et al. synthesized CaF2 hollow spheres with tunable particle size (300–930 nm) by a facile hydrothermal process [28]. Yan and co-workers Had prepared cubic CaF2 nanoplates from the thermolysis of alkaline earth metal trifluoroacetate (M(CF3COO)2) in hot surfactant solutions [29]. Li et al. prepared sub-10 nm monodispersed CaF2:Yb3 þ /Er3 þ nanocrystals according to liquid–solid solution (LSS) strategy [30]. Sun and Li synthesized RE:CaF2 nanopowders with diameter of 100–300 nm by 10–20 h of hydrothermal treatment [31]. In this article, we present a facile microwave–hydrothermal method to obtain CaF2:Tb3 þ microcubes with excellent luminescent properties. The advantages of this method compared with other
362
L. Sun et al. / Journal of Luminescence 166 (2015) 361–365
techniques include low process temperature, short reaction time and uniform heating, which leading to enhanced structure and morphological properties for the materials. The Tb3 þ /Ag-doped CaF2 microcubes with various doping concentrations were also synthesized. The dependence of luminescence intensity on Tb3þ concentrations (0.1– 4.5 mol%) was studied. The influence of Ag NPs on luminescence performance of CaF2:Tb3þ was investigated in detail, and the mechanism of fluorescence enhancement was discussed as well.
2. Experiments 2.1. Preparation of samples All the reagents used in this study were analytical grade without further purification. A series of CaF2:Tb3 þ particles were prepared according to the procedure as follows. Typically, a total of 3 mmol of Ca(NO3)2 and Tb(NO3)3 was dissolved in 20 mL of deionized water to form a transparent solution (Ca2 þ :Tb3 þ ¼100 x:x, x represents the molar percentage of Tb3 þ ; it varied from 0.1 to 4.5 in our experiment). Then 5 mmol of Ethylene Diamine Tetraacetic Acid (EDTA) was added into above solution, followed by addition of several drops of ammonia (25 wt%) under vigorously stirring to form a clear solution with the pH around 7.0. Afterwards, 20 mL of 0.4 M NH4F was slowly added into the above solution under vigorous stirring. Subsequently, above mixture solution was directly exposed to a microwave irradiation at 120 1C for 15 min. The final precipitates were centrifuged and washed repeatedly with water and ethanol. Finally, CaF2:Tb3 þ particles were obtained after drying at 60 1C for 12 h. Besides, CaF2:Tb3 þ (1 mol%)/y Ag particles were synthesis by adding Tb(NO3)3 and AgNO3 together, here, y represents the millimolar (mM) concentration of Ag þ in the reaction solution, it varied from 0.3 to 2.1 in intervals of 0.3 in our experiment. EDTA was employed as the complexing reagent. 2.2. Characterization XRD was performed on a Shimadzu XRD-6000 powder diffractometer with monochromatized CuKα radiation (λ ¼0.1540562 nm) to analysis the phase structure of the samples. Morphology of the particles was observed by scanning electron microscope (SEM) with energy-dispersive X-ray (EDS) analysis. X-ray photoelectron spectroscopy (XPS) measurements were obtained with a Kratos AXIS NOVA spectrometer. Photoluminescence spectra were measured by a fluorescence spectrometer (Hitachi, F-7000). Besides, the absorption property of samples was recorded using a UV–vis spectrophotometer (Shimadzu, UV-3600).
3. Results and discussion 3.1. Microstructure analysis The crystal structure of the samples was characterized by XRD. Fig. 1 shows the XRD of the as-synthesized CaF2:Tb3 þ and CaF2: Tb3 þ /Ag microcubes. All diffraction peaks of the particles agree well with the date of cubic CaF2 phase. The result indicates that the structure integrity of CaF2 is not altered by the doping. Although the diffraction peaks of these samples are in the similar position, the peak intensity is different with the introduction of different additives. And the diffraction peaks of Ag are not observed, which may be attributed to the fact that the content of Ag is so low that it could not be detected by XRD diffractometer. The existence of Ag is proved by the EDS, XPS and UV–vis analysis below. Fig. 2 depicts the SEM images of the Tb3 þ and the Tb3 þ /Agdoped CaF2 products. It can be clearly seen that the microcubes
Fig. 1. XRD patterns of (a) CaF2:Tb3 þ microcubes, (b) CaF2:Tb3 þ /Ag microcubes.
with sharp corners, edges, and smooth surfaces have been fabricated in large quantities, and their main diameters range from 400 to 500 nm. It can be seen clearly that the CaF2:Tb3 þ cubes maintain the original morphology after loading of Ag NPs. Further investigation shows that EDTA plays a critical role in controlling the morphology of the samples. As shown in Fig. 2(e), CaF2:Tb3 þ prepared without EDTA was composed of irregular particles. This is because the reaction between Ca2 þ and F produced by hydrolysis of NH4F is so fast that nucleation occurs at an outburst speed without EDTA. When Ca2 þ was treated with EDTA before microwave irradiation, the stable Ca2 þ -EDTA complex could be formed. Under microwave irradiation conditions, Ca2 þ ions were continuously supplied at a convenient rate by gradual dissociation of the Ca2 þ -EDTA complex. This kind of Ca2 þ feeding mode might lead to uniform CaF2 microcubes. The EDS spectrum is used to analyze the elements in the selected area, and the result is shown in Fig. 3. Ca, Tb, Ag, and F are clearly observed in the spectrum. It suggests that microwave–hydrothermal method is an appropriate way to prepare Tb3 þ /Ag-doped CaF2 microcubes. The Ag 3d XPS spectra of the sample CaF2:Tb3 þ /Ag with 1.5 Ag loading are presented in Fig. 4. As is known, the Ag 3d5/2 peak of Ag0, AgO and Ag2O is at 368.2 eV, 367.0 eV and 367.7 eV, respectively. Fig. 4 shows the binding energy of Ag 3d5/2 at 368.3 eV and Ag 3d3/2 at 374.4 eV, and the energy spinning of two lines was about 6ev, suggesting that Ag species is in the metallic state. 3.2. Luminescence properties Fig. 5 represents the emission spectra of the CaF2 doped with different Tb3 þ concentrations under excitation at 213 nm. The spectra consist of four emission bands located at about 480, 544, 580 and 623 nm, which are originating from the 5D4-7F6, 5 D4-7F5, 5D4-7F4 and 5D4-7F3 transitions of Tb3 þ , respectively, with the green emission transition 5D4-7F5 at 544 nm as the most prominent group. The intensity of the emission at 544 nm increases with the increase of Tb3 þ concentration. However, this increase is no longer significant when the concentration of Tb3 þ is more than 4 mol%. It may be related with the concentration quenching phenomenon, which is due to the cross-relaxation between neighboring Tb3 þ ions as per the following mechanism: Tb3 þ (5D3) þTb3 þ (7F6)-Tb3 þ (5D4) þTb3 þ (7F0) Fig. 6 shows the excitation spectra of CaF2 doped with different Tb3 þ concentrations by monitoring the green emission at 544 nm. It exhibits characteristic absorptions of Tb3 þ . The strongest
L. Sun et al. / Journal of Luminescence 166 (2015) 361–365
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Fig. 2. SEM images of (a) CaF2:Tb3 þ microcubes, (b) the magnification of image, (c) CaF2:Tb3 þ /Ag microcubes and (d) CaF2:Tb3 þ samples prepared without EDTA.
Fig. 3. EDS pattern of CaF2:Tb3 þ /Ag sample.
excitation band is around 213 nm, which is associated with the transition from the ground state 7F6 to 5D3 of Tb3 þ , while the weak band located at 258 nm is assigned to the transition from the ground state 7F6 to 5D4 state. In addition, the intensity of the excitation band increases with increasing concentration of Tb3 þ up to 4 mol%, then decreases. This is consistent with the emission spectra results. A comparison of the luminescence for the CaF2: Tb3 þ (1 mol%) and CaF2: Tb3 þ (1 mol%)/Ag is shown in Fig. 7. It can be found that the luminescence intensities from Tb3 þ depend on Ag concentration. The fluorescence enhancement is observed after Ag NPs doping, and the maximum enhancement is obtained in CaF2: Tb3 þ (1 mol%)/Ag (1.5), which is 5.7 times higher than that of samples without Ag NPs. With the further increase of Ag-doping
concentration, the fluorescence intensity decreases dramatically but it is still higher than the undoped one. In order to verify the luminescence enhancement mechanisms of Tb3 þ in the CaF2:Tb3 þ /Ag samples, the luminescence lifetimes of CaF2:Tb3 þ /Ag samples with different concentrations of Ag were characterized, as shown in Fig. 8. The samples are excited at 213 nm and monitored at 544 nm under Time Scan mode, and the lifetime values are calculated by the software of the spectrophotometer (FL Solutions for F-7000). It can be seen that the fluorescence lifetime values of CaF2:Tb3 þ (1 mol%)/y Ag samples are 3.406, 3.256, 3.118, 2.716, 2.582, 2.390, 3.308 and 3.352 ms corresponding to the Ag concentration 0.0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.1, respectively. It is known that the fluorescence life time decreases with quantum yield enhancing due to the radiative rate
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Fig. 4. XPS spectrum of Ag 3d binding energy of CaF2:Tb3 þ /1.5Ag composite.
Fig. 7. Emission spectra of CaF2:Tb3 þ (1 mol%)/Ag materials with different concentrations of Ag excited at 213 nm.
Fig. 5. Emission spectra of CaF2:Tb3 þ samples with different concentrations of Tb3 þ under excitation at 213 nm.
Fig. 8. The fluorescence lifetime of CaF2:Tb3 þ (1 mol%)/Ag samples with different concentrations of Ag.
Fig. 6. Excitation spectra of CaF2 doped with different concentrations of Tb3 þ , monitored at 544 nm.
Fig. 9. Absorption spectra of CaF2:Tb3 þ (1 mol%)/Ag samples.
decays increasing. Therefore, it can be concluded from above results that the lifetime of CaF2:Tb3 þ /Ag decrease obviously with the increasing of Ag concentration from 0 to 1.5, suggested that luminescence enhancement of Tb3 þ in the samples is attributed to the increasing of radiative decays rate. The luminescence intensity at 544 nm of CaF2:Tb3 þ /Ag was decreased when the concentration of Ag is about 1.8 in contrast to other samples with lower concentration of Ag, as shown in Fig. 7, which may be contributed to the decreasing of radiative decays’ rate. It is noted that the
increasing of radiative decays’ rate in the CaF2:Tb3 þ /Ag microcube is responsible for their luminescence enhancement. 3.3. Absorption spectrum of CaF2: Tb3 þ /Ag Fig. 9 displays UV–vis absorption spectra of the samples. For the as-prepared Tb3 þ -doped and Tb3 þ /Ag-doped samples, the strong absorption in the range below 220 nm may be attributed to host absorptions. Compared with CaF2:Tb3 þ , CaF2:Tb3 þ /Ag shows
L. Sun et al. / Journal of Luminescence 166 (2015) 361–365
a sharp absorption around 410 nm, which is attributed to the surface plasmon absorption of Ag NPs, this gives the valid evidence that Ag NPs exist in the samples. The fluorescence enhancement of Tb3 þ may be attributed to local field enhancement of metallic NPs. The local field enhancement may enhance absorption, emission, and energy transfer from CaF2 host to Tb3 þ ions. Further observation indicates that the intensity of surface plasmon absorption is proportional to increasing Ag amount. It can be concluded from above results that the fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag NPs. 4. Conclusions In summary, Tb3 þ and Tb3 þ /Ag-doped CaF2 microcubes were prepared by a microwave–hydrothermal method. The strong characteristic emission of Tb3 þ is observed, and the highest fluorescence intensity is obtained when the doping concentration of Tb3 þ is 4 mol%. Besides, the emission intensity of Tb3 þ at 544 nm is greatly enhanced by Ag NPs, fluorescence life time reduces with increasing Ag NPs doping concentration. The luminescence enhancement of Tb3 þ in the CaF2:Tb3 þ /Ag materials is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag NPs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21306150, 21176199, 21476183 and 51372201), the Shaanxi Provincial Research Foundation for Basic Research, China (No. 2013JQ2003), the Scientific Research and Industrialization Cultivation Foundations of the Education Department of Shaanxi Provincial Government, China (No. 2013JK0693), the Scientific Research Foundation of North-west University (No 12NW19), and the Scientific Research Foundation of Northwest University (No PR12216).
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fabrication and luminescence properties of Tb3 þ and Tb3 þ /Ag-doped CaF2 microcubes Lin Sun a, Enzhou Liu a, Jun Fan a,n, Xiaoyun Hu b, Jun Wan a, Juan Li a, Hua Li a, Yang Hu a a b
School of Chemical Engineering, Northwest University, Xi'an 710069, PR China Department of Physics, Northwest University, Xi'an 710069, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 January 2015 Received in revised form 25 April 2015 Accepted 28 April 2015 Available online 8 May 2015
CaF2:Tb3 þ and CaF2:Tb3 þ /Ag microcubes were synthesized by the microwave–hydrothermal method. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), and UV–vis absorption spectroscopy (UV–vis). SEM results reveal that monodisperse and uniform microcube with smooth surfaces can be prepared by this approach. The PL spectra indicate that the 5D4–7F5 (544 nm) transition is the most intensive when excited at 213 nm. It is shown that CaF2: Tb3 þ with 4 mol% Tb3 þ concentration exhibited the highest emission intensity. The experimental results indicate that Ag nanoparticles can greatly enhance the light emission from Tb3 þ in CaF2:Tb3 þ sample, which was attributed to the local field enhancement and the increased radiative decay rates of Tb3 þ caused by Ag nanoparticles. Additionally, the comparison of emission for CaF2:Tb3 þ /Ag and CaF2:Tb3 þ (1 mol%) particles reveal that the emission intensity of the former is about 5.7 times as strong as that of the latter. & 2015 Elsevier B.V. All rights reserved.
Keywords: CaF2 microcubes Luminescence Surface plasmon resonance Radiative decay rates
1. Introduction Fluoride compounds have been widely applied in optics and electronics due to their transparency in a wide range from the VUV to IR, low phonon energy and refractive index. [1–5]. Among the fluoride compounds, the calcium fluoride (CaF2) is a kind of typical alkaline-earth fluorides, a particular feature of which is the typical fluorite structure consisting of a simple cubic array of fluoride ions with every alternate cube occupied by a divalent cation Ca [6]. This fluorite structure enables high doping concentration of foreign ions. The rare earth (RE) ions doped CaF2 could be used as laser material and fluorescent labeling material in biological applications [7,8]. As a result, considerable attention has been focused on this field recently. Up to now, An appreciable amount of reports have been published on the photoluminescence of CaF2 doped different lanthanide ions, such as Yb3 þ , Eu3 þ , Er3 þ and Ce3 þ [9–12]. In addition to these RE ions mentioned above, Terbium (Tb3 þ ) element is one important and particular RE ion with intense intra-4f transitions, and a long life time for excited states [13–16]. Recently, various methods such as Li þ codoping, energy transfer and core–shell design have been used to
n
Corresponding author. Tel.: þ 86 29 88002223; fax: þ 86 29 88305252. E-mail address: [email protected] (J. Fan).
http://dx.doi.org/10.1016/j.jlumin.2015.04.048 0022-2313/& 2015 Elsevier B.V. All rights reserved.
improve luminescence property of RE ions [17–20]. As one of the efficient methods, noble-metal nanoparticles (NPs) co-doping, have been widely developed to enhance the luminescence intensity of RE ions based on their surface plasmon resonance (SPR) effect [21–26]. The noble metal NPs nearby fluorescent centers at suitable separations can magnify the fluorescence intensity hundreds times. Qi reported that Ag NPs could effectively increase the fluorescence intensity of Er3þ in glass [27]. Results show that the intensity of Er3 þ increases by 7.2 times after doping of Ag NPs, and the above optical enhancement is attributed to the localized field enhancement near Er3þ. However, so far, the influence of Ag nanoparticles on luminescent performance of CaF2:Tb3 þ microcubes have not been systematically investigated. There have been several reports on synthesis of CaF2 particles by various techniques. Lin et al. synthesized CaF2 hollow spheres with tunable particle size (300–930 nm) by a facile hydrothermal process [28]. Yan and co-workers Had prepared cubic CaF2 nanoplates from the thermolysis of alkaline earth metal trifluoroacetate (M(CF3COO)2) in hot surfactant solutions [29]. Li et al. prepared sub-10 nm monodispersed CaF2:Yb3 þ /Er3 þ nanocrystals according to liquid–solid solution (LSS) strategy [30]. Sun and Li synthesized RE:CaF2 nanopowders with diameter of 100–300 nm by 10–20 h of hydrothermal treatment [31]. In this article, we present a facile microwave–hydrothermal method to obtain CaF2:Tb3 þ microcubes with excellent luminescent properties. The advantages of this method compared with other
362
L. Sun et al. / Journal of Luminescence 166 (2015) 361–365
techniques include low process temperature, short reaction time and uniform heating, which leading to enhanced structure and morphological properties for the materials. The Tb3 þ /Ag-doped CaF2 microcubes with various doping concentrations were also synthesized. The dependence of luminescence intensity on Tb3þ concentrations (0.1– 4.5 mol%) was studied. The influence of Ag NPs on luminescence performance of CaF2:Tb3þ was investigated in detail, and the mechanism of fluorescence enhancement was discussed as well.
2. Experiments 2.1. Preparation of samples All the reagents used in this study were analytical grade without further purification. A series of CaF2:Tb3 þ particles were prepared according to the procedure as follows. Typically, a total of 3 mmol of Ca(NO3)2 and Tb(NO3)3 was dissolved in 20 mL of deionized water to form a transparent solution (Ca2 þ :Tb3 þ ¼100 x:x, x represents the molar percentage of Tb3 þ ; it varied from 0.1 to 4.5 in our experiment). Then 5 mmol of Ethylene Diamine Tetraacetic Acid (EDTA) was added into above solution, followed by addition of several drops of ammonia (25 wt%) under vigorously stirring to form a clear solution with the pH around 7.0. Afterwards, 20 mL of 0.4 M NH4F was slowly added into the above solution under vigorous stirring. Subsequently, above mixture solution was directly exposed to a microwave irradiation at 120 1C for 15 min. The final precipitates were centrifuged and washed repeatedly with water and ethanol. Finally, CaF2:Tb3 þ particles were obtained after drying at 60 1C for 12 h. Besides, CaF2:Tb3 þ (1 mol%)/y Ag particles were synthesis by adding Tb(NO3)3 and AgNO3 together, here, y represents the millimolar (mM) concentration of Ag þ in the reaction solution, it varied from 0.3 to 2.1 in intervals of 0.3 in our experiment. EDTA was employed as the complexing reagent. 2.2. Characterization XRD was performed on a Shimadzu XRD-6000 powder diffractometer with monochromatized CuKα radiation (λ ¼0.1540562 nm) to analysis the phase structure of the samples. Morphology of the particles was observed by scanning electron microscope (SEM) with energy-dispersive X-ray (EDS) analysis. X-ray photoelectron spectroscopy (XPS) measurements were obtained with a Kratos AXIS NOVA spectrometer. Photoluminescence spectra were measured by a fluorescence spectrometer (Hitachi, F-7000). Besides, the absorption property of samples was recorded using a UV–vis spectrophotometer (Shimadzu, UV-3600).
3. Results and discussion 3.1. Microstructure analysis The crystal structure of the samples was characterized by XRD. Fig. 1 shows the XRD of the as-synthesized CaF2:Tb3 þ and CaF2: Tb3 þ /Ag microcubes. All diffraction peaks of the particles agree well with the date of cubic CaF2 phase. The result indicates that the structure integrity of CaF2 is not altered by the doping. Although the diffraction peaks of these samples are in the similar position, the peak intensity is different with the introduction of different additives. And the diffraction peaks of Ag are not observed, which may be attributed to the fact that the content of Ag is so low that it could not be detected by XRD diffractometer. The existence of Ag is proved by the EDS, XPS and UV–vis analysis below. Fig. 2 depicts the SEM images of the Tb3 þ and the Tb3 þ /Agdoped CaF2 products. It can be clearly seen that the microcubes
Fig. 1. XRD patterns of (a) CaF2:Tb3 þ microcubes, (b) CaF2:Tb3 þ /Ag microcubes.
with sharp corners, edges, and smooth surfaces have been fabricated in large quantities, and their main diameters range from 400 to 500 nm. It can be seen clearly that the CaF2:Tb3 þ cubes maintain the original morphology after loading of Ag NPs. Further investigation shows that EDTA plays a critical role in controlling the morphology of the samples. As shown in Fig. 2(e), CaF2:Tb3 þ prepared without EDTA was composed of irregular particles. This is because the reaction between Ca2 þ and F produced by hydrolysis of NH4F is so fast that nucleation occurs at an outburst speed without EDTA. When Ca2 þ was treated with EDTA before microwave irradiation, the stable Ca2 þ -EDTA complex could be formed. Under microwave irradiation conditions, Ca2 þ ions were continuously supplied at a convenient rate by gradual dissociation of the Ca2 þ -EDTA complex. This kind of Ca2 þ feeding mode might lead to uniform CaF2 microcubes. The EDS spectrum is used to analyze the elements in the selected area, and the result is shown in Fig. 3. Ca, Tb, Ag, and F are clearly observed in the spectrum. It suggests that microwave–hydrothermal method is an appropriate way to prepare Tb3 þ /Ag-doped CaF2 microcubes. The Ag 3d XPS spectra of the sample CaF2:Tb3 þ /Ag with 1.5 Ag loading are presented in Fig. 4. As is known, the Ag 3d5/2 peak of Ag0, AgO and Ag2O is at 368.2 eV, 367.0 eV and 367.7 eV, respectively. Fig. 4 shows the binding energy of Ag 3d5/2 at 368.3 eV and Ag 3d3/2 at 374.4 eV, and the energy spinning of two lines was about 6ev, suggesting that Ag species is in the metallic state. 3.2. Luminescence properties Fig. 5 represents the emission spectra of the CaF2 doped with different Tb3 þ concentrations under excitation at 213 nm. The spectra consist of four emission bands located at about 480, 544, 580 and 623 nm, which are originating from the 5D4-7F6, 5 D4-7F5, 5D4-7F4 and 5D4-7F3 transitions of Tb3 þ , respectively, with the green emission transition 5D4-7F5 at 544 nm as the most prominent group. The intensity of the emission at 544 nm increases with the increase of Tb3 þ concentration. However, this increase is no longer significant when the concentration of Tb3 þ is more than 4 mol%. It may be related with the concentration quenching phenomenon, which is due to the cross-relaxation between neighboring Tb3 þ ions as per the following mechanism: Tb3 þ (5D3) þTb3 þ (7F6)-Tb3 þ (5D4) þTb3 þ (7F0) Fig. 6 shows the excitation spectra of CaF2 doped with different Tb3 þ concentrations by monitoring the green emission at 544 nm. It exhibits characteristic absorptions of Tb3 þ . The strongest
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Fig. 2. SEM images of (a) CaF2:Tb3 þ microcubes, (b) the magnification of image, (c) CaF2:Tb3 þ /Ag microcubes and (d) CaF2:Tb3 þ samples prepared without EDTA.
Fig. 3. EDS pattern of CaF2:Tb3 þ /Ag sample.
excitation band is around 213 nm, which is associated with the transition from the ground state 7F6 to 5D3 of Tb3 þ , while the weak band located at 258 nm is assigned to the transition from the ground state 7F6 to 5D4 state. In addition, the intensity of the excitation band increases with increasing concentration of Tb3 þ up to 4 mol%, then decreases. This is consistent with the emission spectra results. A comparison of the luminescence for the CaF2: Tb3 þ (1 mol%) and CaF2: Tb3 þ (1 mol%)/Ag is shown in Fig. 7. It can be found that the luminescence intensities from Tb3 þ depend on Ag concentration. The fluorescence enhancement is observed after Ag NPs doping, and the maximum enhancement is obtained in CaF2: Tb3 þ (1 mol%)/Ag (1.5), which is 5.7 times higher than that of samples without Ag NPs. With the further increase of Ag-doping
concentration, the fluorescence intensity decreases dramatically but it is still higher than the undoped one. In order to verify the luminescence enhancement mechanisms of Tb3 þ in the CaF2:Tb3 þ /Ag samples, the luminescence lifetimes of CaF2:Tb3 þ /Ag samples with different concentrations of Ag were characterized, as shown in Fig. 8. The samples are excited at 213 nm and monitored at 544 nm under Time Scan mode, and the lifetime values are calculated by the software of the spectrophotometer (FL Solutions for F-7000). It can be seen that the fluorescence lifetime values of CaF2:Tb3 þ (1 mol%)/y Ag samples are 3.406, 3.256, 3.118, 2.716, 2.582, 2.390, 3.308 and 3.352 ms corresponding to the Ag concentration 0.0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.1, respectively. It is known that the fluorescence life time decreases with quantum yield enhancing due to the radiative rate
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Fig. 4. XPS spectrum of Ag 3d binding energy of CaF2:Tb3 þ /1.5Ag composite.
Fig. 7. Emission spectra of CaF2:Tb3 þ (1 mol%)/Ag materials with different concentrations of Ag excited at 213 nm.
Fig. 5. Emission spectra of CaF2:Tb3 þ samples with different concentrations of Tb3 þ under excitation at 213 nm.
Fig. 8. The fluorescence lifetime of CaF2:Tb3 þ (1 mol%)/Ag samples with different concentrations of Ag.
Fig. 6. Excitation spectra of CaF2 doped with different concentrations of Tb3 þ , monitored at 544 nm.
Fig. 9. Absorption spectra of CaF2:Tb3 þ (1 mol%)/Ag samples.
decays increasing. Therefore, it can be concluded from above results that the lifetime of CaF2:Tb3 þ /Ag decrease obviously with the increasing of Ag concentration from 0 to 1.5, suggested that luminescence enhancement of Tb3 þ in the samples is attributed to the increasing of radiative decays rate. The luminescence intensity at 544 nm of CaF2:Tb3 þ /Ag was decreased when the concentration of Ag is about 1.8 in contrast to other samples with lower concentration of Ag, as shown in Fig. 7, which may be contributed to the decreasing of radiative decays’ rate. It is noted that the
increasing of radiative decays’ rate in the CaF2:Tb3 þ /Ag microcube is responsible for their luminescence enhancement. 3.3. Absorption spectrum of CaF2: Tb3 þ /Ag Fig. 9 displays UV–vis absorption spectra of the samples. For the as-prepared Tb3 þ -doped and Tb3 þ /Ag-doped samples, the strong absorption in the range below 220 nm may be attributed to host absorptions. Compared with CaF2:Tb3 þ , CaF2:Tb3 þ /Ag shows
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a sharp absorption around 410 nm, which is attributed to the surface plasmon absorption of Ag NPs, this gives the valid evidence that Ag NPs exist in the samples. The fluorescence enhancement of Tb3 þ may be attributed to local field enhancement of metallic NPs. The local field enhancement may enhance absorption, emission, and energy transfer from CaF2 host to Tb3 þ ions. Further observation indicates that the intensity of surface plasmon absorption is proportional to increasing Ag amount. It can be concluded from above results that the fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag NPs. 4. Conclusions In summary, Tb3 þ and Tb3 þ /Ag-doped CaF2 microcubes were prepared by a microwave–hydrothermal method. The strong characteristic emission of Tb3 þ is observed, and the highest fluorescence intensity is obtained when the doping concentration of Tb3 þ is 4 mol%. Besides, the emission intensity of Tb3 þ at 544 nm is greatly enhanced by Ag NPs, fluorescence life time reduces with increasing Ag NPs doping concentration. The luminescence enhancement of Tb3 þ in the CaF2:Tb3 þ /Ag materials is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag NPs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21306150, 21176199, 21476183 and 51372201), the Shaanxi Provincial Research Foundation for Basic Research, China (No. 2013JQ2003), the Scientific Research and Industrialization Cultivation Foundations of the Education Department of Shaanxi Provincial Government, China (No. 2013JK0693), the Scientific Research Foundation of North-west University (No 12NW19), and the Scientific Research Foundation of Northwest University (No PR12216).
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