New Strategy: Molten Salt‐Assisted Synthesis to Enhance Lanthanide Upconversion Luminescence

Abstract

Lanthanide‐doped upconversion luminescent materials (LUCMs) have attracted much attention in diverse practical applications because of their superior features. However, the relatively weak luminescence intensity and low efficiency of LUCMs are the bottleneck problems that seriously limit their development. Unfortunately, most of the current major strategies of luminescence enhancement have some inherent shortcomings in their implementation. Here, a new and simple strategy of molten salt‐assisted synthesis is proposed to enhance lanthanide upconversion luminescence for the first time. As a proof‐of‐concept, a series of rare earth oxides with obvious luminescence enhancement are prepared by a one‐step method, utilizing molten NaCl as the high‐temperature reaction media and rare earth chlorides as the precursors. The enhancement factors at different reaction temperatures are systematically investigated by taking Yb3+/Er3+ co‐doped Y2O3 as an example, which can be enhanced up to more than six times. In addition, the molten salts are extended to all alkali chlorides, indicating that it is a universal strategy. Finally, the potential application of obtained UCL materials is demonstrated in near‐infrared excited upconversion white light‐emitting diodes (WLEDs) and other monochromatic LEDs.

A low-temperature self-flux route to prepare hexagonal rare earth fluorides and manipulation of upconversion luminescence in rodlike NaYbF4:Tm3+/Fe3+ crystals

“…Low doping can promote the nucleation and growth of particles to form larger particles whereas heavy doping can inhibit the nucleation and growth of particles and make the particle size smaller. 12,30–32 The high-resolution transmission electron microscope (HRTEM) image of the NaYbF 4 : 1 mol% Tm 3+ /10 mol% Fe 3+ (89Yb1Tm10Yb) sample (inset in Fig. 4h) reveals the single-crystalline attribute, as corroborated by the observed clear lattice fringes with a measured d -spacing of 0.296 nm, in good agreement with the lattice spacing in the (110) planes of h-NaYbF 4 (JCPDS no.…”

“…A great array of techniques have been developed to prepare various fluorides. 29–32 Unfortunately, the high-temperature solvothermal, hydrothermal and thermal decomposition methods broadly suffer from certain inherent handicaps of rigid synthesis conditions, exorbitant crude materials and the generation of substantial environmental loads, hindering their further large-scale industrial application, 29–32 which has stimulated continuous efforts towards finding facile large-scale techniques with simplicity, versatility, low cost, and environmental friendliness. Encouragingly, the low-temperature molten-salt (flux) technique has been shown to be an effective and convenient method for synthesizing fluoride micro-/nano-materials, including tetragonal (t-)LiYF 4 , 33 hexagonal (h)-NaYF 4 (ref.…”

“…34 and 35) and h-NaBiF 4 , 36 in a molten NH 4 NO 3 system at relatively low temperature owing to the above advantages. 30–32 Rationally manoeuvring their size and morphology requires extensive stringent control over numerous experimental variables, including the usage amounts of crude materials NaNO 3 , NH 4 F, RE(NO 3 ) 3 · x H 2 O (RE = rare earth) and NH 4 NO 3 , the reaction temperature and the reaction duration. 34,35 Moreover, the molten NH 4 NO 3 flux has a drastic influence upon the shape and size of the final fluoride products.…”

Molten Salt Synthesis and Upconversion of Cubic ZrO2 Submicron Phosphors for Temperature Sensor and Anticounterfeit Applications

“…25,26 More importantly, the latest research demonstrated that the MSS method can enhance lanthanide upconversion luminescence in the Y 2 O 3 :Yb/Er system. 27 Unfortunately, there are few works about cubic ZrO phosphors synthesized by the MSS method and their upconversion (UC) luminescence. 28 In this work, a simple MSS method was proposed to synthesize rare earth-doped cubic ZrO 2 crystals using NaCl and KCl as the MS.…”

“…In general, owing to a very high melting point of 2715 °C, stabilized ZrO 2 is typically prepared through solid-state reaction at very high temperatures (>1400 °C). − Meanwhile, it is widely known that the molten salt synthesis (MSS) method can be used to grow crystals at relatively low temperatures (much lower than those of the conventional solid-state reaction method) by using molten salts (MS) with a low melting point. , More importantly, the latest research demonstrated that the MSS method can enhance lanthanide upconversion luminescence in the Y 2 O 3 :Yb/Er system . Unfortunately, there are few works about cubic ZrO 2 phosphors synthesized by the MSS method and their upconversion (UC) luminescence .…”