Article pubs.acs.org/JPCC
Low-Temperature Fluorination Route to Lanthanide-Doped Monoclinic ScOF Host Material for Tunable and Nearly Single Band Up-Conversion Luminescence Yonggang Wang,†,‡ Ting Wen,*,† Huina Zhang,† Jing Sun,† Miao Zhang,† Yanzhen Guo,† Wenjiao Luo,§ Mingjun Xia,∥ Yingxia Wang,*,‡ and Baocheng Yang*,† †
Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou, Henan 450006, China State Key Laboratory for Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China ∥ Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡
S Supporting Information *
ABSTRACT: Lanthanide upconversion (UC) materials that convert near-infrared excitations into visible emissions are of extensive current interest owing to their potential applications in biosensing, 3D displays, and solar cells. A wise choice of the host lattice is crucial for high-quality UC luminescence with desired emission wavelengths. From the viewpoint of structural chemistry, here we propose monoclinic scandium oxyfluoride (M-ScOF) as a promising UC host material for the following reasons: (1) the shortest Sc3+−Sc3+ distance (3.234 Å, versus 3.584 Å of Y3+−Y3+ in hexagonal NaYF4); (2) the unique crystallographic site of Sc in the structure; (3) specific coordination environment of Sc with 4O + 3F in C1 symmetry. Lanthanide doping in an individual host with such structural features is highly expected to achieve single band emission and fast energy migration for high-efficiency UC process. Experimentally, we employ a low temperature fluorination method to synthesize pure and lanthanides doped M-ScOF samples successfully by using polytetrafluoroethylene as the fluridizer. The Yb3+/Ho3+-codoped M-ScOF nanoparticles exhibit tunable UC emissions with various red/green ratios under excitation of λex = 980 nm. Nearly single-band red (∼660 nm) and near-infrared (∼805 nm) UC luminescence have been achieved in Yb3+/Er3+- and Yb3+/Tm3+incorporated samples, respectively. We believe that more attention to M-ScOF and the search for other advanced host materials based on structural chemistry perspective will greatly boost the development of high-efficiency UC phosphors in various applications such as bioprobes and chromatic displays.
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INTRODUCTION
efficiency and desired emission wavelength), it is already known that host matrix effects are essential for UC transitions of lanthanide ions and thus responsible for controllable emission bands and high efficiency.12−16 Among various fluorides and oxide host materials, hexagonal NaYF4 is currently considered to be the best choice, and its Ln3+ codoped nanocrystals are
3+
Trivalent lanthanide ions (Ln ) doped upconverting (UC) materials utilizing near-infrared (NIR) excitations and emitting visible photons offer an attractive technique in various applications such as solar cell, biosensing and bioimaging.1−6 Compared to the conventional used organic fluorophores and quantum dots, they hold intrinsic advantages of good biocompatibility, narrow emission bandwidths, weak background autofluorescence, and high penetration depth.7−11 In the search for practically available UC materials (with high© 2014 American Chemical Society
Received: February 26, 2014 Revised: April 23, 2014 Published: April 28, 2014 10314
dx.doi.org/10.1021/jp5020274 | J. Phys. Chem. C 2014, 118, 10314−10320
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Table 1. Chemical Environments of Representative UC Host Lattices for Ln3+ Doping
a
host lattice
CN (Ln3+)
ligands
site symmetry
shortest Mn+−Mn+ distance (Å)
disorder
ref
CaF2 cubic NaYF4 hexagonal NaYF4 NaScF4 Na3ScF6 M-ScOF
8 8 9 7 6 7
F F F F F 4O + 3F
Oh Oh C1 C1 Oh C1
3.869 3.868 3.584 3.471 3.243a, 5.484b 3.234
none Na/Y Na/Y none none none
12, 34 35 36 31 37 this work
The shortest Na+-Sc3+ distance. bThe shortest Sc3+-Sc3+ distance.
polytetrafluoroethylene (PTEF) as fluridizer. In a typical synthesis procedure of M-ScOF, 0.2758 g of Sc2O3 (0.002 mol; >99.5% purity) and 0.15 g of PTEF powder (0.003 mol CF2; >99% purity) were weighed and ground together with ethanol for several minutes. The resulting fine powder was placed in an alumina crucible and then sealed partially by an alumina cover. The sample was slowly heated to 400 °C at a heating rate of 2 °C/min and then to 600 °C at a heating rate of 10 °C/min. After the sample was held at the highest reacting temperature for 3 h, the furnace was allowed to cool to room temperature naturally. Experimental attempts with different Sc2O3/CF2 ratios or at various heating processes were conducted following similar procedures. Ln3+-doped M-ScOF powders were fabricated by a modified sol−gel method following with the fluorination process. First, proportional Ln2O3 (Ln = Eu, Tb, Ho, Er, Tm, Yb) was mixed with Sc2O3, and dissolved in hot nitric acid to form a clear solution. Then excessive citric acid was added under stirring. After that, the solution was heated at 80, 120, 160, 200, 240, 280, and 320 °C for 2 h at each step and finally heated at 800 °C for 5 h. The resulting powders were mixed with PTEF and followed a similar procedure for as undoped M-ScOF to generate Ln3+-doped M-ScOF samples. For a direct and visualized comparison, Ln-doped cubic and hexagonal NaYF4 samples were obtained by a facile hydrothermal method referred to in the previous literature.38 Characterization. Powder X-ray diffraction (PXRD) data of all the samples were collected at room temperature (25 °C) on a Bruker D8 Advance diffractometer using a germanium monochromatic (Cu Kα). The data in the 2θ range of 10−90° were collected in a step of 0.02° with the remaining time 1 s per step under the tube conditions 40 kV and 40 mV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED) images were performed on a transmission electron microscope (Tecnai G2 20, S-TWIN) with an accelerating voltage of 200 kV. Elemental analysis was also performed by energy dispersive X-ray spectroscopy (EDS) on the same instrument (Figure S1, Supporting Information). The differential scanning calorimetric (DSC) measurements and thermogravimetric analysis (TG) were carried out on a Q600SDT thermogravimetric analyzer under both Air and N2 atmosphere with a heating rate of 10 °C/min from 50 to 1000 °C. The up-conversion luminescence spectra were recorded at room temperature on a modified Hitachi F-4500 spectrophotometer with a 100 mW 980 nm diode (∼3 W/cm2) and a tunable 2 W 980 nm laser diode (Beijing) as the excitation source.
served as most promising candidate in terms of biomedical applications.17−24 Lanthanide ion based upconversion is a complex nonlinear optical process involving the absorption of two or more photons and subsequent shorter-wavelength emissions via intermediate long-lived energy states.25 The most efficient mechanism known currently is a two-step excitation and energy-transfer process (ETU), in which the overall UC efficiency dependent strongly on the distances between the neighboring sensitizer and activator pairs.26 In other words, fast energy migration requires short interatomic distances, and this relies greatly on the possible substitution sites within the host lattice. Sc is the smallest rare-earth element (ionic radius, 74.5 pm). Thus, Sc-based materials may represent shorter interatomic distances than those other bigger rare-earth elements involved materials with the same crystal structure. Recent studies on Sc3+-based host materials for up-conversion luminescence all make special mention of the possibility of high energy transfer efficiency benefit from shorter Ln3+−Ln3+ distance.27−33 Nevertheless, the smallest ionic radius does not consequentially lead to the shortest interatomic distance, which may mostly be determined by the local crystal structure (e.g., Ln−L bond length and Ln−L−Ln bond angle). In this work, we propose monoclinic scandium oxyfluoride (M-ScOF) as a novel host material that is promising for singleband UC generation and possible high-efficiency profiting from the shortest energy migration distance. In order to gain a direct estimation from the viewpoint of structural chemistry, we list the chemical environments of representative UC materials with high performance and also recently emerged Sc-based host materials in Table 1. First, in most of the currently studied fluorite-related host materials, such as the cubic NaYF4, the metal sites often exhibit high symmetry Oh with coordination number 8 and the same ligands, which are considered consequentially to suffer the unfavorable parity-forbidden rule of an f−f transition. Second, we may notice that the most efficient hexagonal NaYF4 possesses the shortest Y3+−Y3+ distance among the nonscandium host compounds. In addition, there are often disorders or multitype crystallographic sites within the formerly studied host structures, so it is not surprising when they display multipeak emission profiles. Compared to them, M-ScOF represents a rare example showing not only a specific 4O + 3F coordination of Sc atoms in the structure with the lowest C1 symmetry but also an exclusive Sc3+ site with the shortest Sc3+−Sc3+ distance. Accordingly, novel UC performance is expected in Ln3+codoped M-ScOF phosphors.
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EXPERIMENTAL SECTION Material Syntheses. Undoped M-ScOF samples were synthesized via a low-temperature fluorination route by adopting lanthanide oxides as the raw materials and
RESULTS AND DISCUSSION Syntheses and Characterization of Undoped and Ln3+-Doped M-ScOF. M-ScOF is an anion-ordered oxy10315
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Figure 1. Powder X-ray diffraction pattern of as-synthesized M-ScOF with various Sc2O3/PTEF ratios. The middle pattern with Sc2O3/CF2 = 1:1.5 is indexed to be M-ScOF (space group P21/c, a = 5.167 Å, b = 5.147 Å, c = 5.247 Å, β = 99.7°), and the red and pink bars indicate the theoretical Bragg peaks of ScF3 (PDF 46-1243) and Sc2O3 (PDF 05-0629), respectively.
Figure 2. PXRD pattern of Ln3+-doped M-ScOF samples synthesized via low-temperature fluoridation from the raw materials of oxides by using PTEF as fluridizer. Asterisks indicate a small quantity of Sc2O3 impurity in some samples. The short vertical bars represent the main theoretical Bragg reflection positions of M-ScOF (P21/c, ICSD no. 100564).
fluoride that is unstable at elevated temperature in both air and moisture.39 According to our experiments, it suffers a thermal decomposition in air or N2 atmosphere when temperature exceeds 600 °C (Figure S2, Supporting Information). The only two recorded methods for the fabrication of M-ScOF are the high-temperature reaction of Sc2O3 and ScF3 in vacancy and hydrolization of ScF3 at 800 °C.39,40 However, neither of them is capable of providing samples with high purity or suitable for well-proportioned element doping. Thus, the physical properties, especially the optical properties of Ln3+-doped M-ScOF, has never been studied probably due to the synthetic difficulties. In the present work, we employ a facile fluorination method to synthesize phase-pure M-ScOF powders by adopting Sc2O3 as the raw materials and polytetrafluoroethylene (PTEF) as fluridizer in controlled atmosphere at relatively low temperature. It is found that the formation of pure M-ScOF
samples relies on the heating procedure, the highest sintering temperature, the total time, and particularly the Sc/F ratio. Figure 1 shows the PXRD patterns of the products from proportional Sc2O3 and PTEF in a partially sealed crucible at 600 °C for 6 h. When 20% excess PTEF was used (Sc/F = 1:1.2), a small quantity of unreacted Sc2O3 could be found in the products. While in the 80% excess PTEF case, Sc2O3 was overfluoridated to ScF3. A Sc/F ratio around 1:1.5 was found appropriate to produce phase-pure M-ScOF (ICSD-100564)39 in our specific experimental conditions. For Ln3+ (Ln = Ho, Er, Tm, Yb) doped samples, lanthanides mixed Sc2O3 are prefabricated by a modified sol−gel method at 800 °C first, followed by the same fluorination process as mentioned above. Powder XRD patterns (Figure 2) show peak positions and intensities that can be well indexed in accordance with MScOF. It is worth noting that, due to the small ionic radius 10316
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Figure 3. (a) Typical TEM and HRTEM images of the as-synthesized Ln3+-doped M-ScOF nanoparticles (M-ScOF: 8% Yb, 1% Er). (b) Electrondiffraction rings indexed in monoclinic space group P21/c. (c) Crystal structure of M-ScOF showing Sc3+ ions in a 7-fold coordination environment (4O + 3F). (d) Schematic representation of Sc3+−Sc3+ distances within M-ScOF host lattice.
difference for Sc3+ and Ln3+ (Ln = Yb, Ho, Er, Tm), the highest doping concentration is restricted to a certain extent (about 10% for Yb). Trace Sc2O3 (
