Efficient synthesis of lithium rare-earth tetrafluoride nanocrystals via a continuous flow method
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BSTRACT As an important upconversion illuminant material, LiREF4 nanocrystals were efficiently synthesized in a continuous reactor with the assistant of a new precursor solution. The employed trioctylamine solvent in the solution had a strong interaction with HF, and helped to avoid the generation of unnecessary solid components as LiF and NH4REF4 during the reaction. A silicon carbide reactor was developed to carry out the synthesis reaction, where LiYF4:Yb,Er/Ho/Tm quickly nucleated in 5 min at 300 °C. The reaction time to successfully prepare 15 nm sized nanocrystals was less than 30 min, and the space-time yield of the flow synthesis method was 14.8 times that of a control group batch reaction. The prepared nanocrystals had a strong illuminant ability, which could find its use in the area of security mark printing.
KEYWORDS lithium rare earth tetrafluoride, upconversion illuminant nanocyrstal, silicon carbide reactor, flow chemistry
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Introduction
Lanthanide-doped upconversion luminescent nanocrystals are anti-stokes fluorescent materials with many advantages such as long lifetime, high light intensity, and good quantum yields [1]. They have a wide range of applications in fields of solar energy utilization, criminal investigations, anti-counterfeiting identification, and biological detection [2–7]. NaREF4 and LiREF4 (RE represents the rare-earth elements) are attractive upconversion materials for their low phonon energy and non-radiative relaxation frequency [8]. In the application of upconversion nanocrystals, there are two types of NaREF4 crystals, cubic and hexagon, which are named to α-NaREF4 and β-NaREF4, respectively [9]. β-NaREF4 is more preferable between them for its higher illuminant intensity. Therefore, α-NaREF4 is usually converted to β-NaREF4 via an Oswaldripening progress at high temperature [10]. Different from NaREF4, there is only one tetragonal phase for LiREF4, which is better at quantum yield than β-NaREF4 for the reductions of self-quenching and ligand non-radiative relaxation effects [11]. The rare-earth elements are generally composed of three parts as the luminous core in NaREF4 or LiREF4: host matrix, sensitizer, and activator [12]. The relatively optimized combinations of those elements in NaREF4 and LiREF4 are about 80% Y (atom ratio) as the host matrix, 18% Yb as the sensitizer and 2% Er/Ho/Tm as the activator, launching green and blue lights under the irritation of 980 nm near-infrared (NIR) light, respectively [13–15]. For the syntheses of NaREF4 and LiREF4 nanocrystals, many methods employing hydrothermal [16, 17], thermal decomposition [18], sol-gel [19] and solvothermal co-precipitation [20] technologies have been reported in the past 20 years. Among them, the solvothermal co-precipitation method accomplished Address correspondence to [email protected]
with a mixture of oleic acid (HOA) and 1-octadecene (ODE) as the ligand and solvent is more preferable for the low toxicity of the reaction system [21–25]. NaREF4 is commonly synthesized by this method, but few approach
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