Materials Science and Engineering B 179 (2014) 48–51
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Europium fluoride based luminescent materials: From hydrogels to porous cryogels, and crystalline NaEuF4 and EuF3 micro/nanostructures Hongkang Wang a , Yu Wang a , Jie Zhang a , Nikolai Gaponik b , Andrey L. Rogach a,∗ a b
Department of Physics and Materials Science & Centre for Functional Photonics, City University of Hong Kong, Hong Kong Special Administrative Region Physical Chemistry/Electrochemistry TU Dresden, Bergstr. 66b, 01062 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 27 May 2013 Received in revised form 7 October 2013 Accepted 14 October 2013 Available online xxx Keywords: Europium fluoride complex Luminescent materials Hydrogels Cryogels NaEuF4 and EuF3 nanostructures
a b s t r a c t Europium fluoride based hydrogels, porous cryogels, and crystalline NaEuF4 and EuF3 nanostructures were prepared by wet chemical route at room temperature by using NaF or NH4 F as the reactants and gelation agents, in the absence of any other additives. The phase, morphology and thus resultant photoluminescence properties of luminescent products are demonstrated on the basis of the formation and evolution of europium fluoride complexes (EuF6 3− ). The characteristic emission of Eu3+ ions is strongly dependent on their existing states and also the adopted fluorides (both cations and anions of NaF or NH4 F). The amorphous NaF- and NH4 F-mediated cryogels with highly porous structure and uniform distribution of optically active Eu3+ ions show much (one or two order) stronger luminescence intensity than their corresponding hydrogels and crystalline NaEuF4 and EuF3 nanostructures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photoluminescence properties of lanthanide ions have attracted much attention, owing to several unique features such as sharp emission bands, long lifetimes and high PL quantum yields, which render them highly attractive for applications in optoelectronics and bio-imaging [1–4]. Due to the low molar extinction coefficient of the forbidden lanthanide f–f transitions [5–7], direct excitation of the lanthanide ions is inefficient, and it has been a common synthetic strategy to incorporate them into host matrices [5,8–12]. A uniform distribution of the optically active lanthanide ions in the host matrix is an important pre-requisite to achieve efficient luminescence [13]. Fluoride compounds with general formulae of ALnF4 or LnF3 (A: alkaline metal; Ln: lanthanide element) offer multiple advantages such as high transparency, large band gap, chemical stability, and low phonon energy minimizing the multiphonon relaxation and improving the luminescence efficiencies [13–15]. Synthesis of Eu-based fluoride compounds, such as NaEuF4 and EuF3 structures with different phases, shapes and sizes can be achieved by means of various synthetic approaches such as hydrothermal [11,16,17], sonochemical [18], and wet chemical routes [19,20], as well as sol–gel techniques [5,21,22]. At the same
∗ Corresponding author. Tel.: +852 34429532; fax: +852 34420538. E-mail address:
[email protected] (A.L. Rogach). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.10.008
time, there have been just a few publications exploring hydrogels, aerogels or cryogels including lanthanide ions [5,21,22]. The latter are highly porous inorganic structures of extremely low density. Recently, aerogels composed of light emitting semiconductor quantum dots have been explored [23,24], which open opportunities for the development of optical sensors and LEDs [25]. Fabrication of light-emitting lanthanide based materials in form of gels and aerogels may in its own turn open the way to novel type of optical sensors and detectors based on up-conversion phenomena. In this work, we employ NaF and NH4 F as both gelation agents and reactants to prepare luminescent cryogels as well as NaEuF4 and EuF3 nanostructures from Eu-fluoride based hydrogels.
2. Experimental 2.1. Materials preparation All chemicals, including sodium fluoride (Sigma–Aldrich), ammonium fluoride (Sigma–Aldrich) were used as received without any further treatment. EuCl3 ·6H2 O precursor solution was prepared by dissolving Eu2 O3 in 37% hydrochloric acid, according to our previous publication [26]. Hydrogels were prepared by dropwise adding aqueous solution of EuCl3 into fluoride aqueous solution at room temperature. Upon exposure, a translucent floccule (hydrogel) precipitated in the solution, which can be separated by centrifugation at a low speed of 1000 r/min. Cryogels
H. Wang et al. / Materials Science and Engineering B 179 (2014) 48–51
49
Fig. 1. Digital photographs of NaF- and NH4 F-mediated hydrogels (a) and cryogels (b) under white light (top) and 365 nm UV light irradiation (bottom). Frame (c) shows a SEM image of NaF-mediated cryogel.
were prepared by freezing the collected hydrogels in a refrigerator at −80 ◦ C for 30 min, followed by overnight storage in a freeze dryer (Labconco Corp.) under vacuum. NaEuF4 and EuF3 nanostructures can be prepared by treating the NaF- or NH4 F-mediated hydrogels by either washing or natural crystallization at room temperature. 2.2. Characterization The phase structures were characterized with a Philips X’pert ˚ X-ray diffractometer (XRD) using Cu K␣ radiation ( = 1.5418 A) with an operating voltage of 40 kV and a current of 30 mA. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out respectively with a Philips XL30 FEG SEM and a Philips CM 20. Brunauer–Emmett–Teller (BET) method has been applied to investigate the specific surface area and porosity of the products by using a NOVA 1200e Surface Area and Pore Size Analyzer (Quantachrome Instruments). Steady-state and timeresolved PL spectra were measured under ambient conditions on powdered samples deposited in the chamber sandwiched between two quartz plates on an Edinburgh Instrument FLS920P spectrometer. 3. Results and discussion Hydrogels were typically prepared by dropwise adding 2.85 mL 0.2 M EuCl3 aqueous solution into 30 mL of 0.45 M aqueous solution of NaF or NH4 F at room temperature, resulting in the formation of a translucent precipitate containing fluoride complexes (EuF6 3− ) according to the reaction (1) due to the excessively high fluorine concentration [27]. This precipitate, which has a typical appearance of a hydrogel as illustrated in Fig. 1a, was separated by centrifugation and served as a starting material to form Eu-fluoride based cryogels (Fig. 1b), which could be easily accomplished by freeze-drying. Both hydrogels and cryogels show characteristic red emission of Eu3+ ions when excited by UV light at 365 nm (Fig. 1a and b). Fig. 1c shows a scanning electron microscopy (SEM) image of typical cryogel, composed of highly porous irregular particles. Xray diffraction (XRD) shows that both NaF-mediated cryogels and NH4 F-mediated cryogels are poorly crystalline (Fig. 2). Bundled rod-like structures with diameter of several tens of nanometers and length of several hundreds of nanometers (Fig. 3a and b) separate from the NaF-mediated hydrogels upon their storage at room temperature for 1–3 days. Their XRD pattern (Fig. 2) indicates the formation of hexagonal NaEuF4 with high crystallinity, where all the peaks can be indexed to this particular compound according to JCPDS Card 49-1897. The formation of crystalline NaEuF4 may occur according to the reaction (2) between Na+ ions and EuF6 3− complex, proceeding through the intermediate steps of dehydration and crystallization,
with the rod-like appearance due to anisotropic crystal growth. Disk-like EuF3 micro/nanostructures with a diameter of up to one micron and thickness of 200–400 nm (Figs. 3c,d) have been obtained by washing both freshly-prepared NaF-mediated or NH4 F-mediated hydrogels with distilled water, in order to remove the excessive coordinated fluorine ions and coexisted Na+ or NH4 + ions. Their XRD pattern (Fig. 2) can be indexed to pure hexagonal structure of EuF3 (JCPDS Card 05-0564). Translucent expanded hydrogels become white dense aggregates after several circles of washing, as the negatively charged europium fluorides (EuF6 3− ) loss the charges to form neutral EuF3 nanocrystallites according to reaction (3). Eu3+ + 6F− → EuF6 3−
(1)
EuF6 3− + Na+ → NaEuF4 + 2F−
(2)
EuF6 3− → EuF3 + 3F−
(3)
BET measurements reveal that the specific surface areas were 42.7 m2 /g for NaF-mediated cryogels and 54.0 m2 /g for NH4 Fmediated cryogels with larger total pore volume (NaF: 0.2246; NH4 F: 0.3009 cm3 /g), much higher than those of related NaEuF4 (14.6 m2 /g, 0.0826 cm3 /g) and EuF3 (4.8 m2 /g, 0.0540 cm3 /g) nanostructures. These results confirm that as-prepared cryogels are highly porous and their network composed of europium
Fig. 2. XRD patterns of the NaF-mediated cryogel (green), NH4 F-mediated cryogel (black), NaEuF4 nanorods (blue) and EuF3 nanodisks (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
50
H. Wang et al. / Materials Science and Engineering B 179 (2014) 48–51
Fig. 3. SEM images of (a and b) rod-like NaEuF4 structures derived from NaF-mediated hydrogels, and (c and d) disk-like EuF3 structures derived from NH4 F-mediated hydrogels (a and c: low magnification; b and d: high magnification). Insets in (b) and (d) show the corresponding TEM images.
fluoride units would allow inclusion of optically active Eu3+ ions, as we discuss in details below. Excitation and emission spectra of NaF- and NH4 F-mediated hydrogels, their respective cryogels, as well as NaEuF4 and EuF3 nanostructures are shown in Fig. 4. As seen from excitation spectra monitored at 614 nm, they all show similar features and no peaks centered at ∼255 nm are observed, indicating there is no Eu O charge transfer band [28] in these Eu-fluoride based luminescent materials. The strongest peaks located at ∼394 nm are observed for all the samples, which correspond to 7 F0 → 5 L6 transition of Eu3+ [4,28]. Other characteristic peaks of Eu3+ ions located from 200 to 500 nm [18] are also present. The emission spectra were obtained with the excitation wavelength of 394 nm (direct excitation of Eu3+ ). All the samples show characteristic emission features of Eu3+ in the wavelength range of 550–750 nm, corresponding to the transition from excited levels of 5 D0 → 7 Fj (j = 0–4). The emission band with peaks at ∼590 nm is ascribed to the magnetic dipole 5 D → 7 F transition and peaks at ∼614 nm is due to the electric0 1 dipole 5 D0 → 7 F2 transition. The peaks at ∼690 nm are indexed to 5 D → 7 F transition. We note the very similar emission profiles 0 4 observed for hydrogels derived from either NaF or NH4 F (Fig. 4a), which indicates that Eu3+ ions in the hydrogels are parts of a similar environment coordinated by fluorine ions, i.e. europium fluoride complexes (EuF6 3− ). Cryogels show a strong increase in the luminescence intensity as compared to the respective hydrogels they have been derived from, which means that water present in the hydrogels quenches the luminescence of Eu3+ ions to a large degree [28]. Furthermore, the hypersensitive forced electronic dipole 5 D0 → 7 F2 transition (at
614 nm) shows reversed emission intensity for the NaF-mediated hydrogel and cryogel (Fig. 4a and b). The 5 D0 → 7 F1 transition at 590 nm is largely independent on the local symmetry of the Eu3+ ions [29], so that the intensity ratio between 5 D0 → 7 F2 and 5 D → 7 F can be used as an index of the asymmetry of Eu3+ site 0 1 [30] – the higher the ratio the lower the symmetry of Eu3+ ions [31]. It increases from 0.87 for NaF-mediated hydrogel to 1.73 for the respective cryogel and to 2.02 for NaEuF4 nanorods, implying the non-centrosymmetric cation environment of Eu3+ ions [29,32]. On the contrary, the ratio for the NH4 F-mediated hydrogel and the respective cryogel remains the same (0.71), despite of the strongly enhanced luminescence intensity of the latter. This points out on the strong influence of cations (Na+ and NH4 + ) on the microenvironment of Eu3+ ions in NaF- and NH4 F-mediated hydrogels and cryogels. For the EuF3 nanodisks, the Stark splitting of the 5 D0 → 7 Fj (j = 1, 2, 4) transition is observed (Fig. 4c) indicating the diverse sites of Eu3+ ions. The PL dynamics of Eu3+ in the NaF- and NH4 F-mediated hydrogels, their respective cryogels, and NaEuF4 and EuF3 nanostructures was studied by measuring PL decay curves monitored within 5 D → 7 F transition under direct excitation of Eu3+ at 394 nm. PL 0 2 lifetimes derived from mono-exponential decays are very similar (0.96 ms and 1.00 ms) for NaF- and NH4 F-mediated hydrogels, which further confirms their similar composition. NaF-mediated cryogel has the highest lifetime of 3.10 ms as compared to that of NH4 F-mediated cryogel (1.60 ms), NaEuF4 (1.60 ms), and EuF3 (2.00 ms), which may relate to the homogeneous distribution of the optically active Eu3+ ions in that particularly porous cryogel matrix [13]. Besides, the cations (Na+ vs NH4 + ) may also contribute
H. Wang et al. / Materials Science and Engineering B 179 (2014) 48–51
51
at room temperature by using NaF or NH4 F as the reactants and gelation agents, in the absence of any other additives. Their formation mechanism and characteristic emission properties are discussed in terms of formation and further evolution of europium fluoride complexes (EuF6 3− ). Among all the products with different state, phase and morphology, NaF-mediated cryogels showed the strongest luminescence intensity and the longest lifetime comparing with hydrogels, NaEuF4 and EuF3 nanostructures, which is attributed to their porous structure and uniform distribution of optically active Eu3+ ions. The synthesis introduced here is easy to carry out and up-scalable, and it diversifies reported fabrication routes toward highly efficient luminescent materials based on lanthanide ions. Acknowledgement This work was financially supported by a grant G HK008/10 from the Germany/Hong Kong Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the German Academic Exchange Service (DAAD). References
Fig. 4. Emission (main frames, excitation wavelength 394 nm) and excitation (insets, detection wavelength 614 nm) spectra of NaF- and NH4 F-mediated hydrogels (a), their respective cryogels (b), and NaEuF4 and EuF3 nanostructures (c).
to the different PL properties of NaF- and NH4 F-mediated cryogels. Molecular dynamics simulations showed that the adsorption energy of Na+ cations on the SnO2 crystal faces is larger than that of NH4 + , resulting in the preferential anisotropic one-dimensional growth in the presence of Na+ ions [33]. Similarly, the possible high adsorption energy of Na+ cations upon the europium fluoride complex may dilute the optically active ions in the cryogel host matrix, as usually a high concentration of optically active ions leads to a nonradiative decay (concentration quenching) [13]. 4. Conclusions In summary, we demonstrated the preparation of Eu-fluoride based hydrogels, cryogels, and NaEuF4 nanorods or EuF3 nanodisks
[1] K. Binnemans, Chemical Reviews 109 (2009) 4283–4374. [2] X. Zeng, J. Yuan, Z. Wang, L. Zhang, Advanced Materials 19 (2007) 4510–4514. [3] Z. Fu, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, Crystal Growth and Design 9 (2008) 616–621. [4] S. Rodriguez-Liviano, F.J. Aparicio, T.C. Rojas, A.B. Hungría, L.E. Chinchilla, M. ˜ Crystal Growth and Design 12 (2011) 635–645. Ocana, [5] S. Bhowmik, S. Banerjee, U. Maitra, Chemical Communications 46 (2010) 8642–8644. [6] J.-C.G. Bunzli, C. Piguet, Chemical Society Reviews 34 (2005) 1048–1077. [7] J.-C.G. Bünzli, Accounts of Chemical Research 39 (2005) 53–61. [8] B.M. Tissue, Chemistry of Materials 10 (1998) 2837–2845. [9] T. Tachikawa, T. Ishigaki, J.-G. Li, M. Fujitsuka, T. Majima, Angewandte Chemie 120 (2008) 5428–5432. [10] P. Ptacek, H. Schäfer, K. Kömpe, M. Haase, Advanced Functional Materials 17 (2007) 3843–3848. [11] Z. Chen, Z. Geng, D. Shao, Y. Mei, Z. Wang, Analytical Chemistry 81 (2009) 7625–7631. [12] X. Xun, S. Feng, J. Wang, R. Xu, Chemistry of Materials 9 (1997) 2966–2968. ´ [13] C. Lorbeer, J. Cybinska, A.-V. Mudring, Crystal Growth and Design 11 (2011) 1040–1048. [14] Z.-L. Wang, J.H. Hao, H.L.W. Chan, CrystEngComm 12 (2010) 1373–1376. [15] D. Zakaria, R. Mahiou, D. Avignant, M. Zahir, Journal of Alloys and Compounds 257 (1997) 65–68. [16] X. Wang, J. Zhuang, Q. Peng, Y. Li, Inorganic Chemistry 45 (2006) 6661–6665. [17] Z.-J. Wang, F. Tao, W.-L. Cai, L.-Z. Yao, X.-G. Li, Bulletin of Materials Science 34 (2011) 1371–1374. [18] L. Zhu, X. Liu, J. Meng, X. Cao, Crystal Growth and Design 7 (2007) 2505–2511. [19] M. Wang, Q.-L. Huang, J.-M. Hong, X.-T. Chen, Z.-L. Xue, Crystal Growth and Design 6 (2006) 1972–1974. [20] M. Wang, Q.-L. Huang, J.-M. Hong, X.-T. Chen, Z.-L. Xue, Crystal Growth and Design 6 (2006) 2169–2173. [21] Y. Qiao, Y. Lin, S. Zhang, J. Huang, Chemistry – A European Journal 17 (2011) 5180–5187. [22] Y. Qiao, Y. Lin, Z. Yang, H. Chen, S. Zhang, Y. Yan, J. Huang, Journal of Physical Chemistry B 114 (2010) 11725–11730. [23] N. Gaponik, A. Wolf, R. Marx, V. Lesnyak, K. Schilling, A. Eychmüller, Advanced Materials 20 (2008) 4257–4262. [24] J.L. Mohanan, I.U. Arachchige, S.L. Brock, Science 307 (2005) 397–400. [25] N. Gaponik, A.-K. Herrmann, A. Eychmüller, Journal of Physical Chemistry Letters 3 (2011) 8–17. [26] H. Wang, Y. Wang, S.V. Kershaw, T.F. Hung, J. Xu, A.L. Rogach, Particle and Particle Systems Characterization 30 (2013) 332–337. [27] S.A. Kuznetsov, M. Gaune-Escard, Journal of Nuclear Materials 414 (2011) 126–131. [28] M.N. Luwang, R.S. Ningthoujam, Jagannath, S.K. Srivastava, R.K. Vatsa, Journal of the American Chemical Society 132 (2010) 2759–2768. [29] H. Zhang, H. Li, Journal of Materials Chemistry 21 (2011) 13576–13580. [30] V. Sudarsan, F.C.J.M. van Veggel, R.A. Herring, M. Raudsepp, Journal of Materials Chemistry 15 (2005) 1332–1342. [31] A. Kar, A. Patra, Journal of Physical Chemistry C 113 (2009) 4375–4380. [32] H. Li, H. Zhang, L. Wang, D. Mu, S. Qi, X. Hu, L. Zhang, J. Yuan, Journal of Materials Chemistry 22 (2012) 9338–9342. [33] A. Birkel, N. Loges, E. Mugnaioli, R. Branscheid, D. Koll, S. Frank, M. Panthofer, W. Tremel, Langmuir 26 (2010) 3590–3595.
