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shell heterostructures
Journal of Luminescence 188 (2017) 96–100
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Synthesis and up-conversion luminescence properties of BaFBr-Er3+@SiO2 core/shell heterostructures
MARK
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Corina Elisabeta Secu , Mihail Secu, Marin Cernea National Institute of Materials Physics, Bucharest, Magurele 077125, Romania
A R T I C L E I N F O
A B S T R A C T
Keywords: Core-shell Up-conversion luminescence BaFBr:Er3+ phosphor
BaFBr:Er3+@SiO2 core-shell composites were synthesized starting from BaFBr:Er3+ phosphor core grains (BaFBr doped with 1 at% Er3+) prepared by coprecipitation method. A sol-gel process was used to obtain the outer SiO2 layer shell from silica sol precursor. The polycrystalline BaFBr:Er3+@SiO2 heterostructures were formed following calcination at 700 °C, in air. The resulting material was characterized in terms of microstructure, photoluminescence and up-conversion luminescence properties. The results indicate that the as-obtained composite has a core-shell structure and shows good luminescence properties. The BaFBr:Er3+@SiO2 heterostructure is made of BaFBr:Er3+ square sheets encased in a SiO2 shell layer with thickness values between 100 and 400 nm. Under 810 nm laser light pumping, the core-shell heterostructure exhibits Er3+ green upconversion luminescence bands ((2H11/2, 4S3/2) → 4I15/2) at 525 and 545 nm, ascribed to a two-photon process.
1. Introduction There has been an increased interest towards the development of new, highly fluorescent labels for biological applications, such as imaging and biodetection assays in both in-vitro and in-vivo ([1] and references therein) and novel antimicrobial and antibiofilm agents [2]. Compared to conventional biological labels (e.g. organic dye markers), rare earths (RE3+) doped materials exhibiting efficient upconversion effects (i.e. near-infrared (NIR) conversion into the visible spectral range) have several advantages, such as weak background fluorescence, high detection sensitivity (and signal to noise ratio), and high lightpenetration depth in tissues [3–6]. In addition, biological cells and tissues have very weak absorption in the NIR region and as such increasing the CW laser power does not cause any significant heat damage or photodamage to living organisms. However, their use in biological applications poses issues related to long-term stability, toxicity due to exposure to the body and emission of light almost exclusively in the visible region. One of the simplest and most popular strategies that have been proposed to overcome these shortcomings has been to coat the RE-doped nanocrystals with a silica shell, which is highly biocompatible and has a well-documented surface chemistry in terms of biological interactions [7]. Therefore, a core-shell heterostructure is created that has new and improved physical properties [8–10]. The silica coating was shown to improve the luminescence and up-conversion efficiency by enhancing the energy transfer from the sensitizer to the activator [11,12]. Silica can be easily tailored to
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Corresponding author. E-mail address: cesecu@infim.ro (C.E. Secu).
http://dx.doi.org/10.1016/j.jlumin.2017.04.015 Received 26 October 2016; Received in revised form 31 March 2017; Accepted 9 April 2017 Available online 12 April 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
assume a spherical morphology [7], with size values in the nano to micro interval [13], and core-shell phosphor materials with various morphologies have been obtained. The size of the core-shell particles can be controlled by the silica layer. In addition, the silica shell acts as protective coating around a sensitive core (improving the chemical stability) and reduces the growth rate of the core particles during the fabricating process [14]. On one hand, in the core-shell composites, it is expected that the SiO2 shell and the nanocrystalline core will interact with each other, which affects PL properties of the core-shell heterostructure [15]. So far, various RE3+-doped materials (LaF3 [16] Y2O3 [17], CeO2 [18] BaCl2 [19]) and core-shell heterostructures [20–22] showing efficient up-conversion effects have been reported. Apart from these, it was shown that Eu2+-doped BaFBr is also suitable for several technological applications such as X-ray storage phosphors [23], biological imaging [24] micro-dosimeter [25] and core-shell heterostructures [26]. Moreover, owing to its low phonon energy (about 300 cm−1 [27]) that assures efficient radiative emission, high chemical stability and non-hydroscopic behavior, this material can also be considered as host for UC applications. Herein, we report about the synthesis of BaFBr:Er3+@SiO2 coreshell phosphor heterostructures. Structural and morphological characterizations have been carried out, along with photoluminescence and UC luminescence measurements of this new heterostructure.
Journal of Luminescence 188 (2017) 96–100
C.E. Secu et al.
2. Experimental procedures 2.1. Samples preparation For the synthesis of Ba(1−x)Er2x/3FBr (x=0.01) powder we used the coprecipitation method [28] with ethanol/water mixture as solvents [26,29], starting from barium bromide dihydrate BaBr2·2H2O (99.3%, Alpha Aesar), erbium bromide nonahydrate ErBr3·9H2O (99.99%, Alpha Aesar) and ammonium hydrogen difluoride (NH4F·HF) (≥98.5%, Fluka) as fluorination agent. Barium bromide and erbium bromide were mixed together and then dissolved in a mixture of water and ethanol (volume ratio H2O:ethanol =1:3). Separately, the amount of ammonium hydrogen difluoride required to obtain BaFBr was dissolved in a mixture of H2O and ethanol (volume ratio H2O:ethanol =1:4). The two solutions of bromides and ammonium hydrogen difluoride were mixed, under vigorous stirring, at room temperature, for 1 h. The resulting BaFBr:Er3+ precipitate was separated from the solvent by centrifugation and washed with ethanol and methanol alternatively for several times, until the F- and Br- ions in excess were completely eliminated. Finally, the precipitate dried at 60 °C, 5 h was calcined at 700 °C, 1 h in air and BaFBr:Er3+ powder was obtained. The composite core–shell BaFBr:Er3+@SiO2 was prepared by coating the BaFBr:Er3+ powder grains with a continuous layer of SiO2 gel (using the Stöber method [13]). The BaFBr:Er3+ powder was first dispersed into a solution of 2-propanol containing 2 M distilled water and disaggregated by sonication during 0.5 h. Knowing that the precipitation of silica by decomposition of tetraethyl orthosilicate [Si(OC2H5)4] depends on the concentrations of ammonia and water [30,31], the pH of the 2-propanol solution was adjusted to 9 by adding aqueous ammonia. Then, the suspension containing BaFBr:Er3+ grains was heated at 40 °C and tetraethyl orthosilicate (TEOS) (Aldrich, 99.99%) was added under magnetic stirring and dissolved to form the silica layer. We used a weight ratio BaFBr:Er3+:SiO2 of 4:1, similar to the one reported in our previous works [26], in order to obtain a SiO2 shell having a thickness of several hundreds of nanometers. The as-prepared dispersion was kept under magnetic stirring for 4 h in order to polymerize the silica gel onto the surface of the BaFBr:Er3+ core grains. The thickness of silica gel shell depends on the suspension viscosity, temperature and times of the gelling process of silica sol precursor. The formation mechanism of the BaFBr:Er3+ core-shell heterostructure is the following: first, the silicate groups (SiO44-) are adsorbed on the surface of BaFBr:Er3+ particles, via the oxide groups (oxygen bridges -O-). Then, the slow polymerization of the silicate groups promotes the formation of a thin and homogeneous silica layer coating the BaFBr:Er3+ grains [32–34]. The resulting coated material was isolated by centrifugation at 4000 rpm and then dried at 80 °C for 24 h. In order to crystallize the BaFBr:Er3+@SiO2 core-shell heterostructure, the dried silica precursor gel was calcined at 700 °C, 1 h in air.
Fig. 1. XRD patterns of the BaFBr:Er3+ precipitate calcined at 700 °C (a) and BaFBr:Er3+@SiO2 core-shell powder calcined at 700 °C (b).
3. Results and discussion 3.1. X-ray diffraction Fig. 1 presents the XRD patterns of the BaFBr:Er3+ precipitate and BaFBr:Er3+@SiO2 core-shell powder, calcined at 700 °C. All diffraction peaks of BaFBr:Er3+ and BaFBr:Er3+@SiO2 are attributed to the tetragonal BaFBr phase, P4/nmm space group (JCPDS 24–0090). In order to investigate the phase composition and crystallographic characteristics in a more quantitative way, the XRD patterns of the BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders were analyzed by full pattern fitting (Pawley method) using TOPAS 3 [35] (Table 1). It shows that BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders exhibit small variations of the crystallographic characteristics by incorporation of 1 at% Er3+ in the BaFBr crystalline lattice and by coating of BaFBr:Er3+ grains with a SiO2 layer. This can be explained by doping of BaFBr with a small amount of Er3+(1 at% Er3+) and by the compression effect due to the densification of the SiO2 layer-gel, during the calcination of the core-shell composite. 3.2. Scanning electron microscopy analysis The SEM images of the core BaFBr:Er3+ and composite core-shell BaFBr:Er3+@SiO2 powders are depicted in Fig. 2. TEM investigations showing borders, phases and lattice fringes in the structure have been presented in our previous paper (Secu et al. [26]). The EDX spectrum of Er3+-doped BaFBr, presented in Fig. 2(c), shows the peaks corresponding to Ba, F, Br and Er. The quantitative EDX analysis of BaFBr:Er3+ powder (Table 2) indicates an atomic ratio Ba:F:Br:Er close to the stoichiometric ratio. It can be seen that the grains of the Er3+-doped BaFBr powder are square sheets with a relatively wide size distribution. The average thickness of the plates is about 80 nm and the average length about 500 nm; several aggregates can be observed (Fig. 2(a, b)). The thickness of the SiO2 layer shell is not uniform (from 100 to
2.2. Samples characterization The morphological and microstructural characterizations of the BaFBr:Er3+@SiO2 core-shell heterostructures were carried out by scanning electron microscope (SEM, FEI Quanta Inspect F), equipped with an energy-dispersive X-ray spectroscopy (EDX) module. The chemical elements, phase components, and crystalline structure were detected by EDX and powder X-ray diffraction (XRD, Bruker-AXS type D8 ADVANCE X-ray diffractometer, Cu-Kɑ source at λ=0.15406 nm). Photoluminescence, UC luminescence spectra and luminescence decays measurements were recorded at room temperature by using a FluoroMax 4P spectrophotometer; spectra were corrected for the spectral sensitivity of the instrument. For the power dependence of the UC luminescence intensity we used a laser diode centred at 810 nm, with a maximum power of 200 mW focused to a spot size of about 3 mm in diameter.
Table 1 Crystallographic parameters calculated for BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders calcinated at 700 °C. Crystal parameters
Lattice constants:
BaFBr undoped powder a (Å) c (Å)
Cell volume, V(Å3) Crystallographic phase
97
BaFBr:Er3+ doped with 1 at % Er3+
BaFBr:Er3+@ SiO2 core-shell
4.5109(3) 4.5108(8) 4.5106(3) 7.4359(6) 7.4438(3) 7.4384(4) 151.311 151.467 151.341 tetragonal BaFBr, P4/nmm space group
Journal of Luminescence 188 (2017) 96–100
C.E. Secu et al.
Fig. 2. SEM images Er3+-doped BaFBr powder “as-prepared” recorded with various magnifications ×20 000 (a), ×50 000 (b) and the corresponding EDX spectrum (c); SEM images of BaFBr:Er3+@SiO2 core shell heterostructures calcined at 700 °C recorded with various magnifications ×20 000 (d) and ×40 000 (e).
400 nm) (Fig. 2(d, e)). The SiO2 shell layer covers one or more grains of Er3+-doped BaFBr because it is difficult to separate core grains from the aggregates.
Table 2 EDX quantification of the Er3+-doped BaFBr and core-shell BaFBr:Er3+@SiO2 powders. Element / EDX line
wt%
at%
F/K Ba / L Er / L Br/ K
7.57 57.69 1.64 33.10
32.06 33.81 0.79 33.34
3.3. Photoluminescence and up-conversion luminescence Photoluminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures, “as-prepared” and calcined, respectively, are depicted in Fig. 3. The spectra show two green luminescence bands at around 525 and 545 nm assigned to Er3+ ion transitions (2H11/2, 4S3/ 4 2)→ I15/2, accompanied by a much smaller band at about 665 nm due 98
Journal of Luminescence 188 (2017) 96–100
C.E. Secu et al.
Fig. 3. Photoluminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures, “as-prepared” and calcined at 700 °C; the inset shows the luminescence decay profiles of the 550 nm emission band for both samples.
Fig. 4. Up-conversion luminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures; the inset shows the double logarithmic plot of luminescence intensity vs. incident laser power for the green ((2H11/2, 4S3/2) → 4I15/2) emission.
to 4F9/2→4I15/2 transition (Fig. 3); the intensity of the emission is much smaller in the “as-prepared” core shell sample. In the “as-prepared” core-shell samples the luminescence signal is weak, but after calcination the luminescence signal increases by about one order of magnitude (Fig. 3) due to the non-radiative relaxation processes which are revealed by the PL decay measurements. The time decay profile of the 545 nm luminescence band ((2H11/2, 4S3/2) → 4I15/ 2) recorded on “as-prepared” core-shell sample is not exponential and we determined an excited state lifetime of 0.34 ± 0.01 ms. For the calcined core-shell sample the decay curve has been well fitted with single exponential decay with the characteristic times of about 0.62 ± 0.01 ms. The measured luminescence lifetime (τm) is given by the radiative transition rate (1/τR) and the non-radiative relaxation rate (WNR):
luminescence intensity is proportional with the nth power of the incident pump power, where n is the number of the pumping photons required to excite RE (rare earth) ions from the ground state to the emitting excited state [39]. Laser pump power dependence of the green UC luminescence intensity showed a quadratic laser power dependence indicating a two-photon process (Fig. 4 – inset). In Fig. 5, the energy level diagram of Er3+ ions is depicted along with possible up-conversion processes occurring. We assume that energy transfer between spatially separated Er3+ ions is most likely the dominant process involved in the up-conversion mechanism [39]. Accordingly, the energy transfer between two adjacent Er3+ ions excited to the 4I9/2 state by the laser light will bring about one ion to the 2H9/2 excited state: Er3+- Er3+[4I9/2 (Er3+) + 4I9/2 (Er3+) → 2H9/2 (Er3+) + 4I15/2 (Er3+)], followed by the rapid multiphonon transition to the lower lying levels 4S3/2 and 4F9/2, followed by the emission of green ((2H11/2, 4S3/2) → 4I15/2) luminescence [40]. The red luminescence (4F9/2 → 4I15/2) cannot be observed because the number of phonons required to bridge the non-radiative relaxation between 4S3/2 and 2F9/2 (about 2800 cm−1 energy gap [41]) is high, due to the small energy of the phonons in the BaFBr lattice (about 300 cm−1) [27], and therefore the multiphonon relaxation rate is very low.
1/ τm = 1/ τR + WNR Assuming similar environment around the Er3+- dopant in the “asprepared” and calcinated core-shell samples, it result that the radiative decay rate is the same. This means that the non-radiative relaxation rate decreases in the calcined core-shell sample, and hence the luminescence lifetime increases, as was observed. We suppose that during the calcination the core crystallinity is improved accompanied by reducing of the non-radiative relaxation rate related to “growth” related defects. In order to reveal the role played by the silica layer we have performed the calcination of the BaFBr/Er sample (in air, to keep the same conditions as for the core-shells). The incipient aggregation of primary nano-crystallites results in the formation of polycrystalline secondary particles, which are clearly observed in the SEM micrographs (not shown). The increase of the PL signal is about one order of magnitude but the PL lifetime remains short of about 0.3 ms. The PL signal increase after calcination is not as significant as in the case of Eu2+-doped BaFBr nanocrystalline powders [38] (annealed in controlled (reducing) atmosphere) because of the hydroxyl groups and oxygen incorporation during the annealing. Therefore the SiO2 coating prevents such contamination and the aggregation of primary nanocrystallites, thus retaining the advantage of performing the annealing in air. Apart from this, a weak influence of surface defects (related to the silanol groups and hydroxyl) at the interface between nanocrystal and shell might be present [36]. It was shown that surface quenching is effective up to a depth of about 7 nm from the surface [37]. Under 810 nm laser light pumping the BaFBr:Er3+@SiO2 core shell heterostructure showed green luminescence due to the (2H11/2, 4S3/2) → 4I15/2 transition of Er3+ ions (Fig. 4). In order to investigate the UC (up-conversion) mechanism, the pump power dependencies of the green (2H11/2, 4S3/2) → 4I15/2 emission were measured (Fig. 4). It is known that if the laser power is well below the saturation level, the UC
Fig. 5. Energy level schemes of Er3+ and the up-conversion luminescence yielded by the 810 nm laser light pumping of BaFBr:Er3+@SiO2 core shell heterostructure.
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4. Conclusions BaFBr:Er3+@SiO2 core-shell heterostructures have been prepared in a two-step process from BaFBr:Er3+ core grains synthesized by coprecipitation method, followed by the deposition of a silica gel layer shell from sol precursor of SiO2. Scanning electron microscopy (SEM) investigations of BaFBr:Er3+@SiO2 demonstrated a core-shell structure composed from BaFBr:Er3+ core grains with square sheets shape and a SiO2 shell layer. Under 810 nm laser light pumping the BaFBr:Er3+@SiO2 core-shell heterostructures show green ((2H11/2, 4S3/2) → 4I15/2) up-conversion luminescence ascribed to a two-photon processes. The favourable optical properties of the BaFBr:Er3+@SiO2 coreshell heterostructures, synthesized using our low-cost approach, demonstrate that they are promising candidates for biological applications, such as imaging and biodetection. It is also expected that the material performances (i.e. the UC efficiency) can be improved by adjusting Er3+-ions concentration or by Yb3+ co-doping, whereas UC luminescence wavelength can be tuned by using other dopants such as Ho3+ or Tm3+. Acknowledgements The authors gratefully acknowledge the Romanian Research Ministry (“Core Program 2016–2017” project no. P1) for financial support. The authors acknowledge Dr. Iuliana Pasuk for help and useful discussions on XRD results. References [1] J.C.G. Bünzli, Lanthanide luminescence for biomedical analyses and imaging, Chem. Rev. 110 (2010) 2729–2755. [2] J. Lellouche, A. Friedman, J.P. Lellouche, A. Gedanken, E. Banin, Improved antibacterial and antibiofilm activity of magnesium fluoride nanoparticles obtained by water-based ultrasound chemistry, Nanomed. Nanotechnol. Biol. Med. 8 (2012) 702–771. [3] H.S. Mader, P. Kele, S.M. Saleh, O.S. Wolfbeis, Upconverting luminescent nanoparticles for use in bioconjugation and bioimaging, Curr. Opin. Chem. Biol. 14 (2010) 582–596. [4] M. Haase, H. Schäfer, Upconverting nanoparticles, Angew. Chem. Int. Ed. Engl. 50 (2011) 5808–5829. [5] M.V. DaCosta, S. Doughan, Y. Han, U.J. Krull, Lanthanide upconversion nanoparticles and applications in bioassays and bioimaging: a review, Anal. Chim. Acta 832 (2014) 1–33. [6] G. Chen, J. Shen, T.Y. Ohulchanskyy, N.J. Patel, A. Kutikov, Z. Li, J. Song, R.K. Pandey, H. Agren, P.N. Prasad, G. Han, (α-NaYbF4:Tm3+)/CaF2 core/shell nanoparticles with efficient near-infrared to near-infrared upconversion for highcontrast deep tissue bioimaging, ACS Nano 6 (2012) 8280–8287. [7] C. Barbé, J. Bartlett, L. Kong, K. Finnie, H.Q. Lin, M. Larkin, S. Calleja, A. Bush, G. Calleja, Silica particles: a novel drug-delivery system, Adv. Mater. 16 (2004) 1959–1966. [8] M. Cernea, B.S. Vasile, A. Boni, A. Iuga, Synthesis, structural characterization and dielectric properties of Nb doped BaTiO3/SiO2 core-shell heterostructure, J. Alloy. Compd. 587 (2014) 553–559. [9] M. Ohmori, E. Matijević, Preparation and properties of uniform coated inorganic colloidal particles: 8. Silica on iron, J. Colloid Interface Sci. 160 (1993) 288–292. [10] K. Kömpe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Müller, M. Haase, Greenemitting CePO4:Tb/LaPO4 core-shell nanoparticles with 70% photoluminescence quantum yield, Angew. Chem. Int. Ed. 42 (2003) 5513–5516. [11] F. Vetrone, R. Naccache, V. Mahalingam, C.G. Morgan, J.A. Capobianco, The activecore/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles, Adv. Funct. Mater. 19 (2009) 2924–2929. [12] J.W. Stouwdam, F.C.J.M. van Veggel, Improvement in the luminescence properties and processability of LaF3/Ln and LaPO4/Ln nanoparticles by surface modification, Langmuir 20 (2004) 11763–11771. [13] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and up-conversion luminescence properties of BaFBr-Er3+@SiO2 core/shell heterostructures
MARK
⁎
Corina Elisabeta Secu , Mihail Secu, Marin Cernea National Institute of Materials Physics, Bucharest, Magurele 077125, Romania
A R T I C L E I N F O
A B S T R A C T
Keywords: Core-shell Up-conversion luminescence BaFBr:Er3+ phosphor
BaFBr:Er3+@SiO2 core-shell composites were synthesized starting from BaFBr:Er3+ phosphor core grains (BaFBr doped with 1 at% Er3+) prepared by coprecipitation method. A sol-gel process was used to obtain the outer SiO2 layer shell from silica sol precursor. The polycrystalline BaFBr:Er3+@SiO2 heterostructures were formed following calcination at 700 °C, in air. The resulting material was characterized in terms of microstructure, photoluminescence and up-conversion luminescence properties. The results indicate that the as-obtained composite has a core-shell structure and shows good luminescence properties. The BaFBr:Er3+@SiO2 heterostructure is made of BaFBr:Er3+ square sheets encased in a SiO2 shell layer with thickness values between 100 and 400 nm. Under 810 nm laser light pumping, the core-shell heterostructure exhibits Er3+ green upconversion luminescence bands ((2H11/2, 4S3/2) → 4I15/2) at 525 and 545 nm, ascribed to a two-photon process.
1. Introduction There has been an increased interest towards the development of new, highly fluorescent labels for biological applications, such as imaging and biodetection assays in both in-vitro and in-vivo ([1] and references therein) and novel antimicrobial and antibiofilm agents [2]. Compared to conventional biological labels (e.g. organic dye markers), rare earths (RE3+) doped materials exhibiting efficient upconversion effects (i.e. near-infrared (NIR) conversion into the visible spectral range) have several advantages, such as weak background fluorescence, high detection sensitivity (and signal to noise ratio), and high lightpenetration depth in tissues [3–6]. In addition, biological cells and tissues have very weak absorption in the NIR region and as such increasing the CW laser power does not cause any significant heat damage or photodamage to living organisms. However, their use in biological applications poses issues related to long-term stability, toxicity due to exposure to the body and emission of light almost exclusively in the visible region. One of the simplest and most popular strategies that have been proposed to overcome these shortcomings has been to coat the RE-doped nanocrystals with a silica shell, which is highly biocompatible and has a well-documented surface chemistry in terms of biological interactions [7]. Therefore, a core-shell heterostructure is created that has new and improved physical properties [8–10]. The silica coating was shown to improve the luminescence and up-conversion efficiency by enhancing the energy transfer from the sensitizer to the activator [11,12]. Silica can be easily tailored to
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Corresponding author. E-mail address: cesecu@infim.ro (C.E. Secu).
http://dx.doi.org/10.1016/j.jlumin.2017.04.015 Received 26 October 2016; Received in revised form 31 March 2017; Accepted 9 April 2017 Available online 12 April 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
assume a spherical morphology [7], with size values in the nano to micro interval [13], and core-shell phosphor materials with various morphologies have been obtained. The size of the core-shell particles can be controlled by the silica layer. In addition, the silica shell acts as protective coating around a sensitive core (improving the chemical stability) and reduces the growth rate of the core particles during the fabricating process [14]. On one hand, in the core-shell composites, it is expected that the SiO2 shell and the nanocrystalline core will interact with each other, which affects PL properties of the core-shell heterostructure [15]. So far, various RE3+-doped materials (LaF3 [16] Y2O3 [17], CeO2 [18] BaCl2 [19]) and core-shell heterostructures [20–22] showing efficient up-conversion effects have been reported. Apart from these, it was shown that Eu2+-doped BaFBr is also suitable for several technological applications such as X-ray storage phosphors [23], biological imaging [24] micro-dosimeter [25] and core-shell heterostructures [26]. Moreover, owing to its low phonon energy (about 300 cm−1 [27]) that assures efficient radiative emission, high chemical stability and non-hydroscopic behavior, this material can also be considered as host for UC applications. Herein, we report about the synthesis of BaFBr:Er3+@SiO2 coreshell phosphor heterostructures. Structural and morphological characterizations have been carried out, along with photoluminescence and UC luminescence measurements of this new heterostructure.
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2. Experimental procedures 2.1. Samples preparation For the synthesis of Ba(1−x)Er2x/3FBr (x=0.01) powder we used the coprecipitation method [28] with ethanol/water mixture as solvents [26,29], starting from barium bromide dihydrate BaBr2·2H2O (99.3%, Alpha Aesar), erbium bromide nonahydrate ErBr3·9H2O (99.99%, Alpha Aesar) and ammonium hydrogen difluoride (NH4F·HF) (≥98.5%, Fluka) as fluorination agent. Barium bromide and erbium bromide were mixed together and then dissolved in a mixture of water and ethanol (volume ratio H2O:ethanol =1:3). Separately, the amount of ammonium hydrogen difluoride required to obtain BaFBr was dissolved in a mixture of H2O and ethanol (volume ratio H2O:ethanol =1:4). The two solutions of bromides and ammonium hydrogen difluoride were mixed, under vigorous stirring, at room temperature, for 1 h. The resulting BaFBr:Er3+ precipitate was separated from the solvent by centrifugation and washed with ethanol and methanol alternatively for several times, until the F- and Br- ions in excess were completely eliminated. Finally, the precipitate dried at 60 °C, 5 h was calcined at 700 °C, 1 h in air and BaFBr:Er3+ powder was obtained. The composite core–shell BaFBr:Er3+@SiO2 was prepared by coating the BaFBr:Er3+ powder grains with a continuous layer of SiO2 gel (using the Stöber method [13]). The BaFBr:Er3+ powder was first dispersed into a solution of 2-propanol containing 2 M distilled water and disaggregated by sonication during 0.5 h. Knowing that the precipitation of silica by decomposition of tetraethyl orthosilicate [Si(OC2H5)4] depends on the concentrations of ammonia and water [30,31], the pH of the 2-propanol solution was adjusted to 9 by adding aqueous ammonia. Then, the suspension containing BaFBr:Er3+ grains was heated at 40 °C and tetraethyl orthosilicate (TEOS) (Aldrich, 99.99%) was added under magnetic stirring and dissolved to form the silica layer. We used a weight ratio BaFBr:Er3+:SiO2 of 4:1, similar to the one reported in our previous works [26], in order to obtain a SiO2 shell having a thickness of several hundreds of nanometers. The as-prepared dispersion was kept under magnetic stirring for 4 h in order to polymerize the silica gel onto the surface of the BaFBr:Er3+ core grains. The thickness of silica gel shell depends on the suspension viscosity, temperature and times of the gelling process of silica sol precursor. The formation mechanism of the BaFBr:Er3+ core-shell heterostructure is the following: first, the silicate groups (SiO44-) are adsorbed on the surface of BaFBr:Er3+ particles, via the oxide groups (oxygen bridges -O-). Then, the slow polymerization of the silicate groups promotes the formation of a thin and homogeneous silica layer coating the BaFBr:Er3+ grains [32–34]. The resulting coated material was isolated by centrifugation at 4000 rpm and then dried at 80 °C for 24 h. In order to crystallize the BaFBr:Er3+@SiO2 core-shell heterostructure, the dried silica precursor gel was calcined at 700 °C, 1 h in air.
Fig. 1. XRD patterns of the BaFBr:Er3+ precipitate calcined at 700 °C (a) and BaFBr:Er3+@SiO2 core-shell powder calcined at 700 °C (b).
3. Results and discussion 3.1. X-ray diffraction Fig. 1 presents the XRD patterns of the BaFBr:Er3+ precipitate and BaFBr:Er3+@SiO2 core-shell powder, calcined at 700 °C. All diffraction peaks of BaFBr:Er3+ and BaFBr:Er3+@SiO2 are attributed to the tetragonal BaFBr phase, P4/nmm space group (JCPDS 24–0090). In order to investigate the phase composition and crystallographic characteristics in a more quantitative way, the XRD patterns of the BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders were analyzed by full pattern fitting (Pawley method) using TOPAS 3 [35] (Table 1). It shows that BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders exhibit small variations of the crystallographic characteristics by incorporation of 1 at% Er3+ in the BaFBr crystalline lattice and by coating of BaFBr:Er3+ grains with a SiO2 layer. This can be explained by doping of BaFBr with a small amount of Er3+(1 at% Er3+) and by the compression effect due to the densification of the SiO2 layer-gel, during the calcination of the core-shell composite. 3.2. Scanning electron microscopy analysis The SEM images of the core BaFBr:Er3+ and composite core-shell BaFBr:Er3+@SiO2 powders are depicted in Fig. 2. TEM investigations showing borders, phases and lattice fringes in the structure have been presented in our previous paper (Secu et al. [26]). The EDX spectrum of Er3+-doped BaFBr, presented in Fig. 2(c), shows the peaks corresponding to Ba, F, Br and Er. The quantitative EDX analysis of BaFBr:Er3+ powder (Table 2) indicates an atomic ratio Ba:F:Br:Er close to the stoichiometric ratio. It can be seen that the grains of the Er3+-doped BaFBr powder are square sheets with a relatively wide size distribution. The average thickness of the plates is about 80 nm and the average length about 500 nm; several aggregates can be observed (Fig. 2(a, b)). The thickness of the SiO2 layer shell is not uniform (from 100 to
2.2. Samples characterization The morphological and microstructural characterizations of the BaFBr:Er3+@SiO2 core-shell heterostructures were carried out by scanning electron microscope (SEM, FEI Quanta Inspect F), equipped with an energy-dispersive X-ray spectroscopy (EDX) module. The chemical elements, phase components, and crystalline structure were detected by EDX and powder X-ray diffraction (XRD, Bruker-AXS type D8 ADVANCE X-ray diffractometer, Cu-Kɑ source at λ=0.15406 nm). Photoluminescence, UC luminescence spectra and luminescence decays measurements were recorded at room temperature by using a FluoroMax 4P spectrophotometer; spectra were corrected for the spectral sensitivity of the instrument. For the power dependence of the UC luminescence intensity we used a laser diode centred at 810 nm, with a maximum power of 200 mW focused to a spot size of about 3 mm in diameter.
Table 1 Crystallographic parameters calculated for BaFBr, BaFBr:Er3+ and BaFBr:Er3+@SiO2 powders calcinated at 700 °C. Crystal parameters
Lattice constants:
BaFBr undoped powder a (Å) c (Å)
Cell volume, V(Å3) Crystallographic phase
97
BaFBr:Er3+ doped with 1 at % Er3+
BaFBr:Er3+@ SiO2 core-shell
4.5109(3) 4.5108(8) 4.5106(3) 7.4359(6) 7.4438(3) 7.4384(4) 151.311 151.467 151.341 tetragonal BaFBr, P4/nmm space group
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Fig. 2. SEM images Er3+-doped BaFBr powder “as-prepared” recorded with various magnifications ×20 000 (a), ×50 000 (b) and the corresponding EDX spectrum (c); SEM images of BaFBr:Er3+@SiO2 core shell heterostructures calcined at 700 °C recorded with various magnifications ×20 000 (d) and ×40 000 (e).
400 nm) (Fig. 2(d, e)). The SiO2 shell layer covers one or more grains of Er3+-doped BaFBr because it is difficult to separate core grains from the aggregates.
Table 2 EDX quantification of the Er3+-doped BaFBr and core-shell BaFBr:Er3+@SiO2 powders. Element / EDX line
wt%
at%
F/K Ba / L Er / L Br/ K
7.57 57.69 1.64 33.10
32.06 33.81 0.79 33.34
3.3. Photoluminescence and up-conversion luminescence Photoluminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures, “as-prepared” and calcined, respectively, are depicted in Fig. 3. The spectra show two green luminescence bands at around 525 and 545 nm assigned to Er3+ ion transitions (2H11/2, 4S3/ 4 2)→ I15/2, accompanied by a much smaller band at about 665 nm due 98
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Fig. 3. Photoluminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures, “as-prepared” and calcined at 700 °C; the inset shows the luminescence decay profiles of the 550 nm emission band for both samples.
Fig. 4. Up-conversion luminescence spectra recorded on BaFBr:Er3+@SiO2 core shell heterostructures; the inset shows the double logarithmic plot of luminescence intensity vs. incident laser power for the green ((2H11/2, 4S3/2) → 4I15/2) emission.
to 4F9/2→4I15/2 transition (Fig. 3); the intensity of the emission is much smaller in the “as-prepared” core shell sample. In the “as-prepared” core-shell samples the luminescence signal is weak, but after calcination the luminescence signal increases by about one order of magnitude (Fig. 3) due to the non-radiative relaxation processes which are revealed by the PL decay measurements. The time decay profile of the 545 nm luminescence band ((2H11/2, 4S3/2) → 4I15/ 2) recorded on “as-prepared” core-shell sample is not exponential and we determined an excited state lifetime of 0.34 ± 0.01 ms. For the calcined core-shell sample the decay curve has been well fitted with single exponential decay with the characteristic times of about 0.62 ± 0.01 ms. The measured luminescence lifetime (τm) is given by the radiative transition rate (1/τR) and the non-radiative relaxation rate (WNR):
luminescence intensity is proportional with the nth power of the incident pump power, where n is the number of the pumping photons required to excite RE (rare earth) ions from the ground state to the emitting excited state [39]. Laser pump power dependence of the green UC luminescence intensity showed a quadratic laser power dependence indicating a two-photon process (Fig. 4 – inset). In Fig. 5, the energy level diagram of Er3+ ions is depicted along with possible up-conversion processes occurring. We assume that energy transfer between spatially separated Er3+ ions is most likely the dominant process involved in the up-conversion mechanism [39]. Accordingly, the energy transfer between two adjacent Er3+ ions excited to the 4I9/2 state by the laser light will bring about one ion to the 2H9/2 excited state: Er3+- Er3+[4I9/2 (Er3+) + 4I9/2 (Er3+) → 2H9/2 (Er3+) + 4I15/2 (Er3+)], followed by the rapid multiphonon transition to the lower lying levels 4S3/2 and 4F9/2, followed by the emission of green ((2H11/2, 4S3/2) → 4I15/2) luminescence [40]. The red luminescence (4F9/2 → 4I15/2) cannot be observed because the number of phonons required to bridge the non-radiative relaxation between 4S3/2 and 2F9/2 (about 2800 cm−1 energy gap [41]) is high, due to the small energy of the phonons in the BaFBr lattice (about 300 cm−1) [27], and therefore the multiphonon relaxation rate is very low.
1/ τm = 1/ τR + WNR Assuming similar environment around the Er3+- dopant in the “asprepared” and calcinated core-shell samples, it result that the radiative decay rate is the same. This means that the non-radiative relaxation rate decreases in the calcined core-shell sample, and hence the luminescence lifetime increases, as was observed. We suppose that during the calcination the core crystallinity is improved accompanied by reducing of the non-radiative relaxation rate related to “growth” related defects. In order to reveal the role played by the silica layer we have performed the calcination of the BaFBr/Er sample (in air, to keep the same conditions as for the core-shells). The incipient aggregation of primary nano-crystallites results in the formation of polycrystalline secondary particles, which are clearly observed in the SEM micrographs (not shown). The increase of the PL signal is about one order of magnitude but the PL lifetime remains short of about 0.3 ms. The PL signal increase after calcination is not as significant as in the case of Eu2+-doped BaFBr nanocrystalline powders [38] (annealed in controlled (reducing) atmosphere) because of the hydroxyl groups and oxygen incorporation during the annealing. Therefore the SiO2 coating prevents such contamination and the aggregation of primary nanocrystallites, thus retaining the advantage of performing the annealing in air. Apart from this, a weak influence of surface defects (related to the silanol groups and hydroxyl) at the interface between nanocrystal and shell might be present [36]. It was shown that surface quenching is effective up to a depth of about 7 nm from the surface [37]. Under 810 nm laser light pumping the BaFBr:Er3+@SiO2 core shell heterostructure showed green luminescence due to the (2H11/2, 4S3/2) → 4I15/2 transition of Er3+ ions (Fig. 4). In order to investigate the UC (up-conversion) mechanism, the pump power dependencies of the green (2H11/2, 4S3/2) → 4I15/2 emission were measured (Fig. 4). It is known that if the laser power is well below the saturation level, the UC
Fig. 5. Energy level schemes of Er3+ and the up-conversion luminescence yielded by the 810 nm laser light pumping of BaFBr:Er3+@SiO2 core shell heterostructure.
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4. Conclusions BaFBr:Er3+@SiO2 core-shell heterostructures have been prepared in a two-step process from BaFBr:Er3+ core grains synthesized by coprecipitation method, followed by the deposition of a silica gel layer shell from sol precursor of SiO2. Scanning electron microscopy (SEM) investigations of BaFBr:Er3+@SiO2 demonstrated a core-shell structure composed from BaFBr:Er3+ core grains with square sheets shape and a SiO2 shell layer. Under 810 nm laser light pumping the BaFBr:Er3+@SiO2 core-shell heterostructures show green ((2H11/2, 4S3/2) → 4I15/2) up-conversion luminescence ascribed to a two-photon processes. The favourable optical properties of the BaFBr:Er3+@SiO2 coreshell heterostructures, synthesized using our low-cost approach, demonstrate that they are promising candidates for biological applications, such as imaging and biodetection. It is also expected that the material performances (i.e. the UC efficiency) can be improved by adjusting Er3+-ions concentration or by Yb3+ co-doping, whereas UC luminescence wavelength can be tuned by using other dopants such as Ho3+ or Tm3+. Acknowledgements The authors gratefully acknowledge the Romanian Research Ministry (“Core Program 2016–2017” project no. P1) for financial support. The authors acknowledge Dr. Iuliana Pasuk for help and useful discussions on XRD results. References [1] J.C.G. Bünzli, Lanthanide luminescence for biomedical analyses and imaging, Chem. Rev. 110 (2010) 2729–2755. [2] J. Lellouche, A. Friedman, J.P. Lellouche, A. Gedanken, E. Banin, Improved antibacterial and antibiofilm activity of magnesium fluoride nanoparticles obtained by water-based ultrasound chemistry, Nanomed. Nanotechnol. Biol. Med. 8 (2012) 702–771. [3] H.S. Mader, P. Kele, S.M. Saleh, O.S. Wolfbeis, Upconverting luminescent nanoparticles for use in bioconjugation and bioimaging, Curr. Opin. Chem. Biol. 14 (2010) 582–596. [4] M. Haase, H. Schäfer, Upconverting nanoparticles, Angew. Chem. Int. Ed. Engl. 50 (2011) 5808–5829. [5] M.V. DaCosta, S. Doughan, Y. Han, U.J. Krull, Lanthanide upconversion nanoparticles and applications in bioassays and bioimaging: a review, Anal. Chim. Acta 832 (2014) 1–33. [6] G. Chen, J. Shen, T.Y. Ohulchanskyy, N.J. Patel, A. Kutikov, Z. Li, J. Song, R.K. Pandey, H. Agren, P.N. Prasad, G. Han, (α-NaYbF4:Tm3+)/CaF2 core/shell nanoparticles with efficient near-infrared to near-infrared upconversion for highcontrast deep tissue bioimaging, ACS Nano 6 (2012) 8280–8287. [7] C. Barbé, J. Bartlett, L. Kong, K. Finnie, H.Q. Lin, M. Larkin, S. Calleja, A. Bush, G. Calleja, Silica particles: a novel drug-delivery system, Adv. Mater. 16 (2004) 1959–1966. [8] M. Cernea, B.S. Vasile, A. Boni, A. Iuga, Synthesis, structural characterization and dielectric properties of Nb doped BaTiO3/SiO2 core-shell heterostructure, J. Alloy. Compd. 587 (2014) 553–559. [9] M. Ohmori, E. Matijević, Preparation and properties of uniform coated inorganic colloidal particles: 8. Silica on iron, J. Colloid Interface Sci. 160 (1993) 288–292. [10] K. Kömpe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Müller, M. Haase, Greenemitting CePO4:Tb/LaPO4 core-shell nanoparticles with 70% photoluminescence quantum yield, Angew. Chem. Int. Ed. 42 (2003) 5513–5516. [11] F. Vetrone, R. Naccache, V. Mahalingam, C.G. Morgan, J.A. Capobianco, The activecore/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles, Adv. Funct. Mater. 19 (2009) 2924–2929. [12] J.W. Stouwdam, F.C.J.M. van Veggel, Improvement in the luminescence properties and processability of LaF3/Ln and LaPO4/Ln nanoparticles by surface modification, Langmuir 20 (2004) 11763–11771. [13] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69.
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