Full length articleTuning the optical and photoluminescence properties of high efficient Eu3+-doped KY3F10 phosphors by different synthetic approaches
Graphical abstract
Introduction
Over the last few decades, materials doped with luminescent lanthanide ions (Ln3+) have been widely studied because of their application in emitting diodes, solar cells, lasers, and catalysts, among others [1], [2], [3]. Fluorides are better host candidates than their respective oxides for optical applications due to the high ionicity of the metal–fluorine bonds, which provides low phonon energy of the crystal lattice (associated with atomic vibrations) [4]. As a result, the quantum efficiencies of the photoluminescent processes are higher and the lifetimes of Ln3+ are longer, varying from a few microseconds to several milliseconds [5].
In addition, fluorides have also been used as upconversion (UC) phosphors [6], [7], [8]. Indeed, lanthanide-doped phosphors are of great interest for UC because their electronic configuration makes them more suitable for the absorption of a second photon [9]. For instance, upconverting fluorides have been reported to be promising materials for bioanalytical and biomedical applications, such as bio-imaging [10], [11]. Novel multifunctional materials have also been designed combining the luminescence properties of Ln3+-doped fluorides with the plasmonic activity of metallic nanoparticles (NPs) [12], [13], [14]. Therefore, spectroscopic techniques based on plasmonic interactions, such as the well-known Surface Enhanced Raman Spectroscopy (SERS), allow us to detect and quantify different analytes with great sensitivity [15].
On the other hand, white light-emitting diodes (w-LEDs) have lately received attention because of their good stability in their physical and chemical properties. One of the most common strategies to generate these devices is the combination of orange-yellow-emitting phosphors (such as the well-known yellow-emitting Y3Al5O12:Ce3+) with blue InGaN chips [16]. Nevertheless, the main drawback of this combination is the modest color-rendering index. Additionally, it is also possible to combine near-ultraviolet LED chips with tricolor (red, green, blue) phosphors [17]. Thus, it is still important to develop and optimize the luminescence properties of new orangish-yellow-emitting phosphors. For this reason, many studies have used phosphors based on oxides, phosphates or nitrides matrices [18], [19], [20], [21], although fluorides have also been proved recently to be good candidates for w-LEDs [22]. Indeed, the high quantum efficiencies and long lifetimes of fluoride-based structures (due to the low phonon interaction of the host lattice) open up new strategies to the design of materials with interesting properties for their applications in such devices.
Fluoride crystals supporting trivalent lattice cations (such as Y3+) are especially attractive for luminescence because they allow their isovalent substitution for a lanthanide ion [23]. The potassium-yttrium-fluorine system has been studied and has yielded many phases: KYF4, KY2F7, KY3F10, KY7F22, K2YF5 and K3YF6 [24], [25], [26], [27], [28], [29]. Of these, KY3F10 has attracted a lot of attention because of its spectroscopic properties over a wide range of temperatures [4], [30]. Moreover, KY3F10 crystal melts congruently and is a suitable host for Ln3+ ions, which can easily substitute Y3+ ions in a non-center-symmetrical site (C4v symmetry) [31], [32].
Under ambient conditions, KY3F10 crystallizes in a face-centered cubic structure (fluorite-type structure) belonging to the space group, with lattice parameter a = 11.536 Å, cell volume V = 1535.20 Å3, and 8 formula units per unit cell (Z = 8) [33]. The crystallographic parameters are detailed in Table 1.
The structure can be described as square antiprisms composed of YF8 units. A central cation of yttrium is linked to two squares of non-equivalent fluorine anions. One square contains four anions in 32f sites (F1), while the other is composed of four anions in 48i sites (F2) [34], Fig. 1(a). The union of six antiprisms generates a cluster-type assemblage, whose edges are shared, leaving an empty cuboctahedron in the center formed by ions in position 48i, Fig. 1(b). The empty cuboctahedra are situated at the center of the unit cell and in the middle of its edges, Fig. 1(c). The potassium cations are distributed along channels running parallel to the a, b and c crystallographic axes [35]. Structures were drawn with VESTA software [36].
KY3F10 compounds have been synthesized via different methods, including the hydrothermal method, coprecipitation or sonochemical processes, giving rise to nano/micron-sized materials with different morphologies (spherical, semi-spherical, rodlike, etc.) [12], [13], [32], [37], [38], [39], [40]. However, these preparation methods are frequently problematic to use because of the complexity and difficulties involved in the reproducibility of the product [38].
The structural and optical properties of KY3F10 have also been reported in the literature. Nevertheless, to the best of our knowledge, no accurate attempts have been made to provide a full analysis of different synthetic procedures that studies their influence on the luminescence features of these materials. Among all the lanthanides, we have chosen Eu3+ as a dopant ion in view of its adequacy as a site-sensitive structural probe [41], [42], [43], which can be useful to discuss the optical response of the materials. In addition, the ionic radius for Y3+ (coordination number, CN = 8) is 1.019 Å and for Eu3+ (CN = 8), it is 1.066 Å [44]. Therefore, these similar values ensure a good incorporation of the Eu3+ ions in KY3F10.
The luminescence properties of the materials are highly dependent on their size, shape, and structure. A well-designed and controlled synthesis of Ln3+-doped fluorides can provide a full range of opportunities to explore new phenomena [45], [46]. In view of this, the present paper is focused on the study of the structural, morphological, and luminescence properties of Eu3+-doped KY3F10 materials. This study enriches the literature and also reveals the effects of post-hydrothermal treatments on the as-synthesized materials. In addition, we show that quantum cutting processes or similar energy transfers between Eu3+ ions allow yielding quantum efficiencies higher than 100%. For all of this, the materials investigated (with highly tunable properties) offer different possibilities depending on the synthesis process used, which can be a keystone for future optical, bioanalytical or biomedical applications. Indeed, the color-tunable emissions of the phosphors could be very interesting for their applications in w-LEDs through their combination with blue chips.
Section snippets
Materials
Reagents used were yttrium(III) nitrate hexahydrate [Y(NO3)3·6H2O 99.9%, Alfa Aesar], europium(III) nitrate hexahydrate [Eu(NO3)3·6H2O 99.9%, Strem Chemicals], potassium hydroxide [KOH 85%, Labkerm], and potassium tetrafluoroborate [KBF4 96%, Sigma-Aldrich].
Synthesis of Eu3+-doped KY3F10 compounds
Different synthetic routes were addressed to prepare KY3F10. On the one hand, the synthesis of KY3F10 was studied by means of the following methods: sonochemical, coprecipitation, and a continuous stirring process. On the other hand, these
Structural characterization
Fig. 4 shows the XRD patterns of the undoped KY3F10 (0% Eu3+) obtained by the different synthetic routes. Fig. 4(a) corresponds to the samples of routes with an ultrasonication common step (Routes 1–3). The XRD patterns show all the peaks corresponding to the cubic phase of KY3F10 (JCPDS-ICDD card 409643). Samples 0%-R1 and 0%-R2 have broad, low-intensity peaks, meaning that the materials may not be so much crystalline or are nano-sized. However, the peaks of sample 0%-R3 are very well defined
Conclusions
The structure, morphologies and luminescence properties of Eu3+-doped KY3F10 materials have been investigated in detail. The compounds with the nominal formula K(Y3-xEux)F10 were prepared according to different synthetic routes. The synthesis was performed via the sonochemical, coprecipitation and continuous stirring methods. Moreover, a combination of these methods with a successive hydrothermal treatment was also addressed. The results obtained allow us to draw the following conclusions:
- •
The
Authors contribution
All authors contributed equally to perform the required experiments, analyse the data and write the paper. All authors reviewed and edited the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
P.S-G, H.B-M, and E.C. thank the Universitat Jaume I (Project UJI-B2019-41) and Ministerio de Economía y Competitividad (Project MAT2016-80410-P) for financial support. P. S.-G. also thanks the Spanish Ministerio de Ciencia, Innovación y Universidades for an FPU predoctoral contract.
References (69)
- et al.
Dual functions of YF3:Eu3+ for improving photovoltaic performance of dye-sensitized solar cells
Sci. Rep.
(2013) - et al.
Synthesis, spectroscopic and structural studies on YOF, LaOF and GdOF nanocrystals doped with Eu3+, synthesized via stearic acid method
Opt. Mater. (Amst)
(2013) - et al.
High yield syntheses of reactive fluoride K1–x(Y, Ln)xF1+2x nanoparticles
Opt. Mater. (Amst)
(2009) - et al.
Synthesis of Yb3+/Er3+ co-doped Y2O3, YOF and YF3 UC phosphors and their application in solar cell for sub-bandgap photon harvesting
J. Lumin.
(2018) Color-tunable up-conversion emission of luminescent-plasmonic, core/shell nanomaterials - KY3F10:Yb3+, Tm3+/SiO2-NH2/Au
J. Lumin.
(2017)- et al.
Luminescent-plasmonic, lanthanide-doped core/shell nanomaterials modified with Au nanorods – up-conversion luminescence tuning and morphology transformation after NIR laser irradiation
J. Alloys Compd.
(2018) - et al.
SERS: Materials, applications, and the future
Mater. Today.
(2012) - et al.
Eu3+-doped Bi4Si3O12 red phosphor for solid state lighting: Microwave synthesis, characterization, photoluminescence properties and thermal quenching mechanisms
Sci. Rep.
(2017) - et al.
Vibrational spectrum and lattice dynamics of KY3F10 single crystals
Vib. Spectrosc.
(2005) - et al.
Infra-red excitation of visible luminescence in up-converters rare earth materials
J. Lumin.
(1979)