Crystal Growth & Design - ACS Publications - American Chemical

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Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

pubs.acs.org/crystal

Reducing Grain Size and Enhancing Luminescence of NaYF4:Yb3+, Er3+ Upconversion Materials Lihua He,† Xiao Zou,† Xiang He,† Fengying Lei,† Na Jiang,† Qiaoji Zheng,† Chenggang Xu,*,† Yongfu Liu,*,‡ and Dunmin Lin*,† †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China

‡

ABSTRACT: A variety of NaYF4 :Yb/Er upconversion materials were successfully synthesized via a facile hydrothermal method by altering the volume ratios (0:1, 1:1, 2:1, 3:1, 4:1, 5:1) of ethanol to water and various alcohols (methanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, polyethylene glycol, and phemethylol), and the grain size and luminescence of the materials were effectively tailored. The addition of alcohol effectively reduces the grain size of the materials and simultaneously leads to the surprising enhancement of the UC emission. The increase in the volume ratio of EtOH/H2O causes the transformation of the crystal structure of the materials from hexagonal to cubic phase. The particle size of the material with the EtOH/H2O of 2:1 is almost 1000 times smaller than that of the material without EtOH, and the strongest luminescence is observed at the EtOH/H2O of 2:1. The type of alcohol in the solvent exhibits a significant influence on the luminescent properties of the materials. The NaYF4:Yb/Er nanocrystals obtained using different alcohols as solvent possess much a smaller grain size and stronger upconversion luminescence than that using water as solvent, and the NaYF4:Yb/Er materials with n-propyl alcohol exhibit the strongest luminescence intensity. The mechanism of the reduction of grain size and enhancement in the luminescence of the NaYF4:Yb3+, Er3+ upconversion materials has been proposed. This study may suggest a simple method to significantly reduce the grain size and improve the upconversion luminescence properties of NaYF4-based fluorescence material. grain size, and luminescence of the NaYF4 materials.19,20 Many investigations have shown that the UC luminescence intensity of the NaYF4 materials can be effectively controlled through adjusting their morphology.21−27 For example, Li’s group successfully prepared NaYF4 nanocrystals with tunable shape and size in a water/alcohol/oleic acid system;28,29 Lin et al. synthesized β-NaYF4 microcrystals with diverse morphologies by a hydrothermal method using sodium citrate as surfactant;30−32 and Zhao’s group demonstrated a facile hydrothermal method to synthesize uniform β-NaYF4.33,34 However, from the investigations on the NaYF4 materials, it can be clearly noted that the enhancement in luminescent intensity is generally accompanied by the significant increase of the grain size of the materials, as shown in Figure 1.35−38 From Figure 1, the emission intensity of NaYF4:Yb3+, Er3+ microcrystals shows a continuous improvement with grain size increasing, suggesting that increasing the size of the β-NaYF4: Yb, Er microprisms is a very effective method of maximizing the UC emission efficiency;35 the UC and DC emission intensities of βNaYF4:Yb, Er nanocrystals synthesized by a solvothermal decomposition approach can be enhanced with increasing

1. INTRODUCTION Over the last few decades, rare-earth-doped upconversion nanoparticles (UCNPs) have attracted much attention due to their superior luminescence properties (e.g, good chemical stability, low cytotoxicity, high luminescence efficiency, long lifetime, large Stokes shifts, and biological friendliness).1,2 It has been demonstrated that there are great applications of UCNPs in the fields of upconversion lasers,3 solar cells,4,5 and thermal sensors.6 Especially, one of the most important application fields is biomedical science such as biosensing, photoswitching bioimaging, and selective detecting cysteine.7−9 As is well-known, fluoride-containing host materials are attracting extensive attention over other materials because of their high photochemical stability, low toxicity, high quantum yield, and lower phonon energies which refrain from nonradiative transitions of rare earth ions.10−14 Among fluoridecontaining materials, NaYF4, as one of the most efficient hosts for upconversion emission, possesses the merits of low phonon energies (