Microwave-assisted ionic-liquid-based synthesis of highly crystalline CaMoO4:RE3+ (RE = Tb, Sm, Eu) and Y2Mo4O15:Eu3+ nanoparticles
Graphical abstract
Introduction
Red emitting phosphor nanomaterials that can be excited with near-UV to blue light (thus, UV- or blue-light LED as excitation sources with λexcitation > 350 nm) instead of high-energy UV-excitation (e.g. mercury discharge excitation with λexcitation < 330 nm) are in great demand for fluorescent thin-films (e.g., marking, advertisement, thin-film illumination or displays [1], [1]a), [1]b)) as well as for medical application (e.g., optical imaging, theranostics [2]). Besides red emission and nanoscaled size, additional aspects such as quantum yield, physical/chemical stability, reproducible synthesis, etc. are of major relevance. Transferring well-known red-emitting bulk-phosphors (e.g. Y2O3:Eu, YBO3:Eu, YVO4:Eu [3]) to the nanoscale is typically restricted to low quantum yields (<20%) and UV excitability (<330 nm). The recently up-coming nitride-based LED phosphors − although highly efficient as bulk-materials [4] − are not available for experimental reasons as nanoparticles by now. Thus, there is still a strong need for inorganic nanomaterials showing efficient emission of red light.
Nanoscaled molybdates have attracted considerable interest in view of various features, mainly related to catalysis and color pigments but as well including fluorescent materials [5], [5]a), [5]b). In particular, CaMoO4 is known for its fluorescence ranging from blue to green emission, depending on specific doping, particle size and crystallinity. Various high-temperature techniques, e.g., Czochralski method [6], co-precipitations [7], combustion methods [8], and solid–state reactions [9] have been used to prepare CaMoO4 powder samples that exhibit relatively large particle sizes and/or irregular morphology [6], [7], [8], [9], [10].
Molybdates MMoO4 (M = metal) with powellite structure, in general, are known as excellent phosphor host lattices as they allow an excitation via the O → Mo ligand-to-metal charge transfer (LMCT) at high absorption strength. Upon doping with rare-earth ions, intra configurational f → f transitions can result in green (Tb3+), orange (Sm3+) and red (Eu3+) emission [2], [11]. Thus, CaMoO4:RE3+ (RE = rare-earth metal) nanoparticles were synthesized by different methods, such as hydrothermal processes, urea-driven hydrolysis, co-precipitation methods, microwave-assisted synthesis, sol–gel processes or polyol syntheses [12], [12]a), [12]b), [12]c). Particularly, CaMoO4:Eu is an attractive material as it shows the characteristic Eu3+-related red emission, which is normally obtained upon high-energy UV-excitation (<330 nm). Many reports address an improvement of the fluorescent properties of CaMoO4:Eu by charge compensation (e.g., co-doping with Li+, K+, Na+) or co-doping with Bi3+ or Zr4+/Si4+ ions [13]a), [13]b), [13]c), [13]d), [13] as well as a surface passivation [14]a), [14]b), [14]. Nevertheless, the highest quantum yields reported for CaMoO4:Eu nanoparticles remain at 10–40% only [15], [15]a), [15]b), [15]c).
In this work, we use a microwave-assisted ionic-liquid-based synthesis for preparing CaMoO4:RE3+ nanoparticles (RE = Tb, Sm, Eu). Fast and direct microwave heating in temperature-resistant and high-boiling ionic liquids (ILs) guarantees for optimal particle nucleation and a high particle crystallinity. As a result, CaMoO4:RE3+ nanoparticles in the 50–70 nm range, showing intense emission and quantum yields up to 82% in the case of CaMoO4:Eu, are obtained. By replacing the divalent Ca2+ against trivalent Y3+, moreover, we could prepare nanoparticles of the new phase Y2Mo4O15:Eu. Y2Mo4O15:Eu has a particle diameter of 25–30 nm and shows intense red emission with a quantum yield of 67%. In contrast to CaMoO4:Eu, the excitation band of Y2Mo4O15:Eu is red-shifted by more than 50 nm to 400 nm, so that Y2Mo4O15:Eu can be excited with a UV-LED as a powerful, low-cost excitation source.
Section snippets
Microwave-assisted ionic-liquid-based synthesis
To obtain phase-pure and highly crystalline CaMoO4:RE3+ (RE = Tb, Sm, Eu) and Y2Mo4O15:Eu nanoparticles, a microwave-assisted synthesis in ionic liquids (ILs) was used. We already established such concept of synthesis for obtaining fluorescent LaPO4:Ce,Tb and YVO4:Eu nanoparticles [16]a), [16]b), [16] or nanoscaled In2O3:Sn (ITO = indium tin oxide) as a transparent oxide conductor [17]. For optimal conditions of particle nucleation, the starting materials (e.g. CaCl2, EuCl3 × 6H2O, (NH4)6Mo7O24)
Conclusion
The microwave-assisted synthesis in ionic liquids successfully leads to readily crystalline CaMoO4:RE3+ (RE = Tb, Sm, Eu) and Y2Mo4O15:Eu nanoparticles. The as-prepared nanoparticles exhibit a high crystallinity (without any post-sintering of powder samples) and excellent quantum yields of 52% for CaMoO4:Tb, 82% for CaMoO4:Eu, and 67% for Y2Mo4O15:Eu. The nanoparticles exhibit particle diameters of 50–70 nm in the case of CaMoO4:RE3+ and 25–30 nm in the case of Y2Mo4O15:Eu. Subsequent to
Materials
General aspects: All chemicals were used as received from the supplier. This includes the starting materials CaCl2 (VWR, 99.99%), (NH4)6Mo7O24 × 4H2O (VWR, 99%), Na2MoO4 (VWR, 99%), and the rare-earth chlorides EuCl3 × 6H2O (Aldrich, 99.99%), TbCl3 × 6H2O (Aldrich, 99.9%), SmCl3 × 6H2O (Aldrich, 99.99%), and YCl3 × 6H2O (Aldrich, 99.9%). The synthesis of [MeBu3N][N(SO2CF3)2] as the ionic liquid (IL) was performed with tributylmethylammonium chloride ([N(CH3)(C4H9)3]Cl) and lithium
Acknowledgments
The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support as well as to the DFG graduate school Karlsruhe School of Optics and photonics (KSOP) at the KIT.
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