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Dy3+ microrods: Formation mechanism, energy transfer and luminescence properties
Accepted Manuscript 3+ 3+ 3+ Hexagonal Crown-Capped NaYF4:Ce /Gd /Dy Microrods: Formation Mechanism, Energy Transfer and Luminescence Properties Mingye Ding, Yuting Li, Daqin Chen, Hongwei Lu, Junhua Xi, Zhenguo Ji PII:
S0925-8388(15)31529-2
DOI:
10.1016/j.jallcom.2015.10.273
Reference:
JALCOM 35828
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2015 Revised Date:
28 October 2015
Accepted Date: 29 October 2015
Please cite this article as: M. Ding, Y. Li, D. Chen, H. Lu, J. Xi, Z. Ji, Hexagonal Crown-Capped 3+ 3+ 3+ NaYF4:Ce /Gd /Dy Microrods: Formation Mechanism, Energy Transfer and Luminescence Properties, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.273. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Hexagonal Crown-Capped NaYF4:Ce3+/Gd3+/Dy3+ Microrods: Formation Mechanism, Energy Transfer and Luminescence Properties
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Mingye Ding a, Yuting Li a, Daqin Chen a,*, Hongwei Lu a, Junhua Xi a, Zhenguo Ji a College of Materials and Environmental Engineering, Hangzhou Dianzi University,
Hangzhou, 310018 P. R. China Abstract
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In this work, hexagonal crown-capped β-NaYF4:Ce3+/Gd3+/Dy3+ microrods with remarkably uniform morphology and size have been successfully synthesized for the
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first time via a facile hydrothermal route. The phase and morphology evolution of β-NaYF4 crystals are carefully tracked during hydrothermal growth by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron diffraction (TEM), respectively. The possible growth mechanism of hexagonal crown-capped NaYF4 microrods has been proposed. Furthermore,
the
luminescence
properties
of
β-NaYF4:Ce3+/Gd3+/Dy3+
are
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systemically investigated by the photoluminescence excitation spectra, emission spectra and decay curves. Impressively, upon the allowed Ce3+ 4f-5d electronic transition excitation, the efficient energy transfer from sensitizer (Ce3+) to activator (Dy3+) occurs via energy migration process Ce3+ → (Gd3+)n → Dy3+, in which Gd3+
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ions act as bridge centers to induce intense yellow luminescence from Dy3+ activators.
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Keywords: Crown-capped microrods, hexagonal NaYF4, crystal growth, energy transfer, Dy3+ luminescence
*
Corresponding author: Tel.: +86 0571 87713537 Fax. : +86 0571 87713537
E-mail address: [email protected] (D. Chen) 1
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1. Introduction Recently, lanthanide (Ln3+) ions doped luminescent materials have received
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increasing research interests because of their widely tunable emission colors, sharp
4
emission peaks, large Stokes shifts, high photostability and low toxicity.[1-5] Until
5
now, large numbers of rare earth (RE) inorganic compounds, including oxides,[6]
6
oxysulfides,[7]
7
vanadates[14] and tungstates,[15] have been extensively investigated for their
8
potential application in the fields of white light-emitting diodes,[16] sensors,[17]
9
anti-counterfeiting,[18] flat-panel displays,[19] solid-state lasers,[20] solar cells[21]
10
and biological labels.[22] Among the reported host materials, RE-based fluorides with
11
general formula NaREF4 have been considered as excellent host lattices for various
12
optically active Ln3+ ions, because they possess low phonon energy (< 400 cm-1),
13
good optical transparency over a wide wavelength range and adequate thermal and
14
chemical stability. As we known, NaREF4 exists in two polymorphic forms, i.e., cubic
15
(α) and hexagonal (β) phase, depending on the synthesis conditions and methods.
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Previous studies have proven that the hexagonal NaREF4 series are more efficient
17
than their cubic counterparts because of their unique crystal structure.[23] Specifically,
18
hexagonal NaYF4 (β-NaYF4) has been most commonly chosen as excellent host
19
lattice for achieving efficient downconversion (DC) and upconversion (UC)
20
luminescence of lanthanide ions for decades. Up to now, β-NaYF4 nano/microcrystals
21
with various morphologies and sizes, including nano/microspheres, nano/microtubes,
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nano/microprisms, nano/microrods, nanoarrays, micro-bipyramids, microplates, have
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sulfides,[10]
fluorides,[11-13]
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phosphates,[9]
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oxyfluorides,[8]
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been successfully synthesized by several chemical methods such as solid-state reaction, molten salt, solvothermal, co-precipitation and hydrothermal methods.[24-28] However, fabrication of hexagonal NaYF4 microrods with crown-capped shape have been rarely reported, which may force the advancement of these rare earth fluorides
27
with better luminescence properties, finally accelerating the spread of new
28
applications.
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As we known, the luminescence of Ln3+-activated materials originates basically
30
from electronic transitions within the 4fn configuration (4f → 4f) of the lanthanide 2
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dopants. The luminescence properties of the lanthanide-doped materials can be
2
precisely controlled through changing the lanthanide activators doped in the host
3
lattice. The trivalent lanthanide ions, such as Eu3+, Tb3+, Sm3+ and Dy3+ have attracted
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wide attention due to their unique optical properties.[29, 30] For example, Tb3+ and
5
Eu3+ ions are well-known as green-emitting and red-emitting activators because of the
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5
7
transitions of Eu3+, respectively.[31, 32] Sm3+ ions can also act as useful activator for
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their orange luminescence, originating from the 4G5/2 → 6HJ/2 (J = 5, 7 and 9)
9
transitions of Sm3+.[33] Particularly, Dy3+ ions, possessing two dominant transitions
10
in visible region: 4F9/2 → 6H13/2 transition (yellow emission) and 4F9/2 → 6H15/2
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transition (blue emission), have been paid growing attention for their potential
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applications in white light-emitting diodes, plasma display planes, field emission
13
displays and mercury-free lamps.[34, 35] In spite of the single-doping model,
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activators co-doping with sensitizer ions have a higher absorption coefficient, leading
15
to more efficient luminescence than single-doped system by taking advantage of the
16
effective energy transfer from sensitizers to activators.[36, 37] Ce3+ ions are usually
17
used as sensitizer in order to improve the luminescence intensity of Ln3+ ions (e. g. Eu,
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Tb), because the emission band of Ce3+ ions matches well with several f-f absorption
19
bands of Ln3+ ions. Simultaneously, Gd3+ ions could act as intermediate sublattices to
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facilitate the energy transfer from Ce3+ ions to the Ln3+ ions via Ce3+ → (Gd3+)n →
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Ln3+ energy hopping, resulting in the promotion of the photon conversion from
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ultraviolet radiation to visible emissions. Despite these above developments, the
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D4 → 7FJ (J = 1, 2, 3 and 4) transitions of Tb3+ and 5D0 → 7FJ (J = 6, 5, 4 and 3)
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energy transfer Ce3+ → Gd3+ → Dy3+ in β-NaYF4 host lattice and the luminescence properties of β-NaYF4:Ce3+/Gd3+/Dy3+ have been rarely reported and still needed further investigation.
In the present work, we demonstrate a facile and effective hydrothermal strategy
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for the synthesis of monodisperse β-NaYF4:Ce3+/Gd3+/Dy3+ microrods with hexagonal
28
crown-capped shape. Based on the time-dependent experimental results, the possible
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formation mechanism for the phase and morphology evolution process has been
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proposed. The luminescence behaviors of Ce3+, Gd3+ and Dy3+ co-doped β-NaYF4 3
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products are systemically investigated. In addition, the energy transfer mechanism of
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β-NaYF4:Ce3+/Gd3+/Dy3+ samples is discussed in detail.
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2. Experimental section
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2.1. Reagents.
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Yttrium chloride hydrate (YCl3.6H2O, 99.99%), gadolinium chloride hexahydrate
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(GdCl3.6H2O, 99.99%), cerium chloride hydrate (CeCl3.7H2O, 99.99%) and
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dysprosium nitrate hydrate (Dy(NO3)3.6H2O, 99.99%) were purchased from Beijing
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Founde Star Science & Technology Co., Ltd (China). Sodium fluoride (NaF, 98%)
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and sodium citrate (C6H5Na3O7.2H2O, 99%) were supplied by Sinopharm Chemical
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Reagent Co., Ltd (China). Deionized water was used throughout. All chemicals were
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of analytical grade reagents and used directly without further purification.
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2.2 Synthesis.
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In a typical procedure for the synthesis of β-NaYF4:Ce3+/Gd3+/Dy3+ microrods, 2
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mmol of RECl3.6H2O (RE = Y, Ce, Gd and Dy) were firstly dissolved in 10 mL
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deionized water with magnetic stirring to form rare earth chloride solution. Then 20
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mL of trisodium citrate (Na3Cit, 0.1 mol/L) was added into the resulting solution to
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form the metal-citrate complex. After vigorous stirring for 30 min, 30 mL of NaF
18
aqueous solution (0.4 mol/L) were dropped into the above mixture under thoroughly
19
stirring for 15 min. Subsequently, the as-obtained mixing solution was transferred to a
20
100 mL Teflon-lined autoclave and heated at 220 oC for 24 h. After the autoclave was
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cooled down to room temperature naturally, the precipitates were collected by
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centrifugation and washed with deionized water and ethanol in sequence. The
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collected samples were dired in air at 60 oC for 12 h. 2.3 Characterization
Powder X-ray diffraction (XRD) measurements were performed on a ARL X’TRA
diffraction meter at a scanning rate of 10 o/min in the 2θ range from 10 o to 80 o with
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Cu Kα radiation (λ = 0.15046 nm). Scanning electron microscopy (SEM) micrographs
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were obtained using a field emission scanning electron microscope (FE-SEM,
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SU8010, Hitachi). An energy dispersive spectroscopy (EDS) facility attached to the
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JEOL JSM-6460 SEM was employed to analyze the chemical composition of the 4
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as-synthesized products. Transmission electron microscopy (TEM) and selected area
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electron diffraction (SAED) pattern were recorded on a JEM-200CX with a field
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emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple
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CCD spectrometer. The excitation and emission spectra, luminescence decay curves
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were recorded with an Edinburgh Instruments FS5 fluorescence spectrophotometer.
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All the measurements were performed at room temperature.
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3. Results and Discussion
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3.1 Crystal Structure and Morphology
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The composition and structure of the samples were first characterized by X-Ray
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Diffraction (XRD) technique. Fig. 1 shows the XRD patterns of the as-synthesized
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NaYF4 products co-doping with Ce3+, Gd3+ and/or Dy3+ ions as well as the standard
12
data of pure hexagonal NaYF4. As shown, strong and sharp diffraction peaks can be
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well indexed to the pure hexagonal NaYF4 phase, which is consistent with the
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standard card (JCPDS No. 16-0334). The crystal structure of β-NaYF4 is determined
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with lattice parameters of a = 0.596 nm and c = 0.353 nm, space group P63/mmc.[38]
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No obvious extra diffraction peaks are observed in all the XRD patterns, revealing
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that Y3+ can be replaced by Ce3+, Gd3+ and Dy3+ with no effect on the crystallinity or
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phase change because of their almost ionic radii and identical valence.
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The typical morphology of the as-synthesized β-NaYF4:10%Ce3+, 30%Gd3+,
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1%Dy3+ sample is presented in Fig. 2. As can be seen from SEM images (Fig. 2a and
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2b), the as-prepared sample is composed of a large quantity of hexagonal
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crown-capped microrods with good uniformity, monodispersity and well-defined
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crystal facets. Analysis of a number of microrods reveals that these microparticles have an average size of 5.3 µm in length and 3.3 µm in diameter, indicating an average of aspect ratio of about 1.6 for each rod. The magnified SEM image shows a close-up view of one isolated microrod (Fig. 2c), which clearly displays the hexagonal
27
prism structure of the sample. From the magnified SEM image in Fig. 2d, it can be
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clearly seen that the top end of the hexagonal rods is not flatness but like a crown. As
29
an analogy, six triangle walls incline outward a bit on the six edges of the hexagonal
30
column, which construct a short prismatic cup erecting on top/bottom side, resulting 5
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in the formation of hexagonal crown-capped NaYF4 microrod. The length and
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thickness of these triangle walls are about 0.8-1.2 µm and 0.15-0.49 µm, respectively.
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Fig.
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β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ microrods, confirming the existence of Na, Y,
5
Gd, F, Ce and Dy elements. The morphology sketch (Inset in Fig. 2e) clearly indicates
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the preferred growth direction. Because hexagonal NaYF4 has high anisotropic
7
structure, the growth direction is confined to the [0001] direction and it preferentially
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grows into prismatic shape.[39] To the best of our knowledge, this kind of β-NaYF4
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microrods with crown-like top ends has been synthesized and observed for the first time.
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3.2 Growth mechanism
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2e shows the Energy Dispersive Spectrometer (EDS) spectrum of
The time-dependent experiments were carried out to explore the formation
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process of crown-capped β-NaYF4 microrods. Fig. 3 shows the XRD patterns of the
14
intermediates obtained at different reaction time intervals as well as the standard data
15
of cubic NaYF4 (α-NaYF4: JCPDS No. 77-2042) and hexagonal NaYF4 (β-NaYF4:
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JCPDS No. 16-0334) phases for comparison. Obviously, the samples exhibit
17
distinguishingly different XRD patterns at different reaction periods. At a short
18
reaction time of 3 h, only cubic NaYF4 (α-NaYF4) can be obtained (Fig. 3a). When
19
the reaction time is prolonged to 4 h, a new hexagonal phase, namely, β-NaYF4,
20
emerges in addition to the α-phase NaYF4, resulting in the mixture of predominantly
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α-NaYF4 and minor β-NaYF4 (Fig. 3b). This result indicates that the sample
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transforms gradually from cubic to hexagonal phase with the further reaction. With
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the reaction proceeding for 12 h, the cubic NaYF4 disappears thoroughly and pure hexagonal phase of NaYF4 exists, as shown in Fig. 3c. At a reaction time of 24 h, the phase of the sample remains unchanged, but the intensities of diffraction peaks increase remarkably (Fig. 3d). This implies that the crystallinity of the microcrystals
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increases with the increasing of the reaction time. Form the above analysis, it can be
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drawn conclusion that the crystal evolves from α-phase to mixed phase and finally to
29
β-phase with prolongation of reaction time.
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At the meantime, the morphologies of the intermediate products were carefully 6
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investigated at different reaction times. Fig. 4 shows the typical TEM and SEM
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images of NaYF4 samples obtained at different reaction periods. Remarkably, the
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intermediates exhibit distinctively different morphologies in the process of crystal
4
growth. At t = 3 h, the as-obtained α-NaYF4 sample consists of spherical nanoparticles
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with an average diameter of 31 nm, as presented in Fig. 4a and Fig. S1. With the
6
reaction proceeding only 4 h, relatively uniform microrods with well-defined
7
hexagonal cross-section begin to appear. The intermediate sample consists of
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hexagonal microrods (1.82 µm in length and 0.70 µm in diameter) and some irregular
9
nanoparticles, which correspond to β-NaYF4 and α-NaYF4, respectively (Fig. 4b).
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Noticeably, the top face of the hexagonal rods is extremely flat without any other
11
trace, as shown in the inset image of Fig. 4b. At a reaction time of 12 h, pure β-NaYF4
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with rod-like morphology can be obtained, as shown in Fig. 4c. Meanwhile, the
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α-NaYF4 nanoparticles disappear completely, indicating that the phase transition (α →
14
β) significantly results in the dramatic change in the final morphology of NaYF4
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crystals. The mean size of these microrods is estimated to be 0.98 µm in diameter and
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2.08 µm in length. It is noteworthy that convex triangle walls on the edges of the
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hexagonal column begin to appear and grow higher than the central part with the
18
extension of reaction time (Inset in Fig. 4c). Quite interestingly, on aging for a longer
19
period up to 24 h, the outer triangle walls become increasingly sharp and a flat plane
20
appears in the end of the inner structure, leading to the final hexagonal crown-capped
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NaYF4 rods (Fig. 4d). Further extending the reaction time to 24 h also results that the
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size of the as-obtained samples continues to augment significantly (5.3 µm in length
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and 3.3 µm in diameter). The above morphology evolution process provides us some clue for understanding the formation mechanism of hexagonal crown-capped β-NaYF4 microrods.
Based on the time-dependent experiments, a possible phase and morphology
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evolution mechanism is presented in Scheme 1 and described as follows. At the
28
beginning, the aqueous solution containing rare earth cations (Y3+) and sodium citrate
29
(Na3Cit) is directly mixed to form a precursor. Through strong coordination
30
interaction, the citrate anions (Cit3-) introduced into the reaction system can form 7
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Y3+-Cit3- complexes with Y3+ ions. Simultaneously, NaF dissolves in the aqueous
2
solution to form Na+ and F- ions. Y3+ + Na3Cit → Na+ + Y3+-Cit3- (complex)
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(1)
NaF → Na+ + F-
(2)
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Under the hydrothermal conditions of high temperature and pressure, the
6
chelating ability of Y3+-Cit3- complexes could be weakened by slow degrees, resulting
7
that Y3+ ions would be released gradually into the reaction solution. As a result, the
8
concentration of free Y3+ ions in the solution increases gradually, which thus helps to
9
regulate the nucleation and subsequent growth of the crystals. After the addition of the
10
aqueous solution containing Na+ and F- ions, Y3+ ions in the reaction system react
11
with Na+ and F- ions quickly to generate small nuclei. At a short reaction time, these
12
nuclei quickly congregate together and grow into α-NaYF4 nanoparticles. The cubic
13
NaYF4 seeds possess isotropic unit cell structures, which commonly induce an
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isotropic growth of particles and therefore spherical particles are clearly observed in
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order to minimize the surface energy of various crystal facets.
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Y3+-Cit3- + Na+ + F- → α-NaYF4 + Cit3-
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Nevertheless, these cubic-phased NaYF4 nanoparticles are thermodynamically
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unstable and evolve ineluctably to hexagonal-phased NaYF4 seeds through
19
dissolution-renucleation process.[40] Obviously, the phase transformation (α → β)
20
leads to the prominent change in the morphology of NaYF4 crystals, which can be
21
ascribed to different characteristic unit cell structures of diverse crystallographic
22
phases. Different from α-NaYF4, β-NaYF4 seeds possess anisotropic unit cell
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structure,
which
generally
induces
the
anisotropic
growth
along
the
crystallographically active direction. With the further reaction, anisotropic growth of hexagonal-phased NaYF4 becomes dominating through rapid dissolving of cubic-phased NaYF4 and releasing of monomers into reaction solution, resulting in
27
the sudden nucleation of β-NaYF4 and subsequent growth along [0001] direction to
28
hexagonal cylinder.[41] The dissolution-reconstruction process of α-NaYF4
29
nanoparticles preferentially occurs at the circumferential edges of each hexagonal
30
microrod along crystallographically reactive direction through the Ostwald ripening 8
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process, resulting that the morphology of the sample changes from spherical
2
nanoparticles (α-NaYF4) into hexagonal microrods (β-NaYF4). α-NaYF4 → β-NaYF4
3
(4)
Subsequently, the growth rate of β-NaYF4 crystals in the crystal growth stage is
5
closely related to the amount of monomers released during the fast dissolution of
6
α-NaYF4 nanoparticles, which is considered as single-source precursors to supply
7
monomers.[42] As the reaction proceeded, the vicinal monomers is continually
8
consumed, resulting that the concentration of monomers near β-NaYF4 microrods
9
decreases gradually and the nearby α-NaYF4 nanoparticles accelerate dissolution as a
10
supplement.[25] Then the released monomers move to the hexagonal rods and meet
11
the extending top corners of initial hexagonal rod firstly, leading to the accelerated
12
growth rate of the top corners. Thus the crystal growth along [0001] direction could
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be suppressed a little and the β-NaYF4 crystals would also grow along the twelve
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21 0], [011 0], [1120], [1010], [2110], [1100], directions ([2110], [1100], [1
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[1210] , [0110] , [1120] and [1010] ), resulting in the formation of
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dodecagon-shaped rods (Fig. S2). Obviously, the top corners can absorb more
17
monomers than the interior, giving rising to the formation of the six convex triangle
18
walls on the edges of hexagonal rods. With a prolonged growth time, the β-NaYF4
19
crystal shows a hexagonal crown-capped morphology.
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3.3 Photoluminescence properties
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Generally, the direct energy transfer from Ce3+ to activator Ln3+ ions (Ln = Eu,
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Tb, Sm and Dy) is rather inefficient.[43] To make this energy transfer process more
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efficient, Gd3+ ions are usually employed to act as an intermediate sublattice for energy transfer from Ce3+ to Ln3+ through Ce3+ → (Gd3+)n → Ln3+ energy migration process. Herein, in the following part, we try to illustrate this energy transfer process by selecting Gd3+ and Dy3+ as migrator and trap center, respectively.
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Firstly, we investigate the photoluminescence properties of NaYF4:Ce3+ and
28
NaYF4:Ce3+/Gd3+ samples to validate the efficient Ce3+ → Gd3+ energy transfer
29
process. Fig. 5 shows room temperature excitation and emission spectra of
30
β-NaYF4:10%Ce3+ and β-NaYF4:10%Ce3+, 30%Gd3+ samples. It can be seen from the 9
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excitation spectrum that the maximum of the characteristic excitation band lies at
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about 259 nm, which is assigned to the 4f → 5d transition of Ce3+ (left line, Fig. 5a).
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Specifically, in β-NaYF4:10%Ce3+, 30%Gd3+ sample, the excitation spectrum for Gd3+
4
emission (311 nm) is dominated by the strong and broad absorption band peaked at
5
around 259 nm due to the Ce3+ 4f → 5d transition followed by the weak and sharp
6
Gd3+ line at about 273 nm (8S → 6I), reveals that the possibility of energy transfer
7
from Ce3+ ions to Gd3+ ions (left line, Fig. 5b). Upon the excitation into the 4f → 5d
8
transition of Ce3+ at 259 nm, the emission spectrum of β-NaYF4:10%Ce3+ shows a
9
broadband emission with a maximum at about 309 nm, which is assigned to the
10
electric-dipole-allowed 4f-5d transition of Ce3+ (right line, Fig. 5a). However, the
11
sharp emission peak corresponding to 6P7/2 → 8S7/2 of Gd3+ (311 nm) can be clearly
12
observed in β-NaYF4:10%Ce3+, 30%Gd3+ sample (right line, Fig. 5b). Interestingly,
13
there is a large spectral overlap between the Gd3+ absorption line and the Ce3+
14
emission band, which significantly satisfies the requirement of energy transfer from
15
Ce3+ to Gd3+, resulting in the efficient Ce3+ → Gd3+ energy transfer process.[44]
16
Secondly, we confirm the existence of Gd3+ → Dy3+ energy transfer by comparing the
17
luminescence properties of β-NaYF4:1%Dy3+ and β-NaYF4:30%Gd3+, 1%Dy3+
18
samples. Fig. 6 presents their excitation and emission spectra. For β-NaYF4:1%Dy3+,
19
the excitation spectrum consists of the 4f-4f characteristic excitation lines of Dy3+
20
with its 4f9 configuration from 300 to 450 nm (left line, Fig. 6a). The main excitation
21
lines originate from the 6H15/2 ground state to different excited states: 322 nm (6P3/2),
22
349 nm (6P7/2), 363 nm (6P5/2), 386 nm (4M11/2) and 423 nm (4G11/2). Under excitation
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into the 6H15/2 → 6P5/2 transition at 349 nm, there are two obvious sharp lines centered at 481 nm and 572 nm, corresponding to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions, respectively (right line, Fig. 6b).[45] By comparison, the excitation spectrum of β-NaYF4:30%Gd3+, 1%Dy3+ exhibits the Gd3+ absorption line (273 nm, 8S → 6I) and
27
f-f transition lines of Dy3+ (left line, Fig. 6b). Excitation into the strongest 8S → 6I
28
transition of Gd3+ at 273 nm yields the emission of Gd3+ at 311 nm and the
29
characteristic emissions of Dy3+ at 481 nm and 572 nm, which are assigned to the
30
Gd3+: 6P7/2 → 8S7/2 transition and Dy3+: 4F9/2 → 6HJ/2 (J = 15, 13) transitions, 10
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respectively (right line, Fig. 6b), suggesting the occurrence of energy transfer from
2
the Gd3+ sublattice to Dy3+ emission center.[46] The above results confirm that the
3
energy transfer Ce3+ → Gd3+ and Gd3+ → Dy3+ exist in NaYF4:Ce3+/Gd3+/Dy3+
4
samples. To further interpret the role of Gd3+ in the Ce3+ → Dy3+ energy transfer process,
6
the luminescence properties of NaYF4:Ce3+/Dy3+ and NaYF4:Ce3+/Gd3+/Dy3+ are
7
compared and investigated in detail. Fig. 7 shows the excitation, emission spectra and
8
decay curves of β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+,
9
1%Dy3+ samples. From the excitation spectrum (left line, Fig. 7a) and emission
10
spectrum (right line, Fig. 7a) of β-NaYF4:10%Ce3+, 1%Dy3+ sample, it can be seen
11
that broad Ce3+ emission with a maximum 309 nm along with weak Dy3+ emissions
12
(481 nm and 572 nm), exhibiting the inefficient Ce3+ → Dy3+ energy transfer process.
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For β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ in Fig. 7b, by monitoring the 572 nm
14
emission of Dy3+, the characteristic 4f-4f transition lines within the Dy3+ 4f9
15
configuration together with strong and broad band at 259 nm due to 4f-5d transition of
16
Ce3+ can be obviously observed (left line, Fig. 7b). Excitation into the Ce3+ band at
17
259 nm yields weak and sharp emission of Gd3+ at 311 nm and strong emission of
18
Dy3+ (right line, Fig. 7b), demonstrating the important role of Gd3+ sublattice as an
19
intermediary in the energy transfer process from Ce3+ to Dy3+. With the assistance of
20
Gd3+ ion, Ce3+ ion can transfer its excitation energy to Dy3+ ion through Ce3+ →
21
(Gd3+)n → Dy3+ energy migration process. The experimental decay lifetimes of Dy3+
22
for 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions also provide evidence to support this
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conclusion. The fluorescence decay curves of Dy3+ (4F9/2 → 6H15/2/6H13/2) for β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples are shown in Fig. 6c and Fig. 6d, respectively. The average lifetime can be calculated by[47]
27
∗ ∗ τave =
(5)
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where I(t) represents the emission intensity at time t. Apparently, owing to the
29
efficient energy migration process, the obtained lifetimes of Dy3+ for 4F9/2 → 6H15/2 11
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and 4F9/2 → 6H13/2 transitions increase from 1.10 ms (481 nm) and 1.06 ms (572 nm)
2
of NaYF4:Ce3+/Dy3+ to 2.50 ms (481 nm) and 2.45 ms (572 nm) of
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NaYF4:Ce3+/Gd3+/Dy3+. The above results powerfully demonstrate that Gd3+ ions play
4
an important role in facilitating the energy transfer from Ce3+ ions to Dy3+ ions.
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In the following experiments, we study on the luminescence properties of
6
NaYF4:Ce3+/Gd3+/Dy3+ microrods with different Dy3+ dopant concentration to
7
demonstrate that the emission intensity can be precisely adjusted by tuning the Dy3+
8
dopant concentration. It is noted that the crystallographic phase and general
9
morphology of the final samples have not changed by the doping small amount of
10
Dy3+ ions (Fig. S3 and Fig. S4). Fig. 8 shows the emission spectra of
11
β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ (x = 0 ~ 10) samples. Under Ce3+ excitation
12
band at 259 nm, the obtained emission spectrum exhibits three obvious lines centered
13
at 311 nm, 481 nm and 572 nm, corresponding to 6P7/2 → 8S7/2 transition of Gd3+ and
14
4
15
intensity of Gd3+ decreases gradually with the increasing dopant concentration of Dy3+,
16
which may be ascribed to the enhanced energy transfer from Gd3+ to Dy3+. However,
17
the 572 nm emission intensity of Dy3+ increases with the increment of Dy3+ dopant
18
concentration from 0 mol% to 1 mol%, and then decreases gradually when the Dy3+
19
concentration is further increased to 10 mol%. The 481 nm emission intensity of Dy3+
20
presents the same variation tendency (Fig. S5), which can be attributed to the
21
concentration quenching effect.[48] As observed by naked eyes, these samples exhibit
22
yellow emission under 254 nm ultraviolet lamp. Moreover, the emission intensity of
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F9/2 → 6HJ/2 (J = 15, 13) transitions of Dy3+. Obviously, the 311 nm emission
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β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ is the strongest, agreeing well with the results of emission spectra.
The luminescence decay behavior of Gd3+ and Dy3+ in the NaYF4:Ce3+/Gd3+/Dy3+
are systematically studied in order to further understand the energy transfer process.
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Fig. 9a shows the luminescence decay curves (excited at 259 nm and monitored at 311
28
nm) of Gd3+ ions as a function of Dy3+ dopant concentration. As shown, the excited
29
state lifetime of Gd3+ decreases continuously with increasing Dy3+ content (Fig. 9b
30
and Table S1). As both nonradiative and radiative processes contribute to the 12
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deactivation of an excited state, the experimental lifetime of the excited state (τ) can
2
be expressed as[49]
3
= +
(6)
Where WR is the radiative rate and WNR represents the sum of the rates of diverse
5
nonradiative processes including energy transfer. The sharp decrease of Gd3+ lifetime
6
with the adding of Dy3+ dopant concentration indicates the increasing energy transfer
7
efficiency from Gd3+ to Dy3+, which significantly increases the nonradiative rate of
8
Gd3+. The luminescence decay curves (excited at 259 nm and monitored at 572 nm) of
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Dy3+ ions in β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ (x = 0 ~ 10) samples are
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exhibited in Fig. 9c. The experimental lifetime of Dy3+ for 4F9/2 → 6H13/2 transition
11
decreases from 4.81 ms to 0.16 ms with increasing Dy3+ dopant concentration from 0
12
mol% to 10 mol% (Fig. 9d and Table S1). The Dy3+ lifetime for 4F9/2 → 6H15/2
13
transition presents the same downward trend, as shown in Fig. S6 and Table S1.
14
Interestingly, with the increment of Dy3+ dopant concentration, the rising edge of
15
decay curves change from a gentle incline into a sharp fast-rising slope, which is
16
usually used to describe the energy migration time from Ce3+ to Dy3+.[36] The
17
decrease of rising edge time reveals that the average traveling time of the excitation
18
energy via the Gd3+ subsystem decreases gradually with the adding of Dy3+ ions.
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Based on the above results, the Gd-mediated energy migration process is
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depicted in Scheme 2. Under the ultraviolet excitation (λ = 259 nm), the excitation
21
photons are absorbed by Ce3+ to populate the 5d excited state followed by fast energy
22
transfer from Ce3+ to Gd3+. Subsequently, the energy migration process among the
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Gd3+ sublattices takes place. The migration energy is finally trapped by Dy3+ ions, leading to the blue emission centered at 481 nm (4F9/2 → 6H13/2) and yellow emission at 572 nm (4F9/2 → 6H15/2) observed by naked eyes. Simultaneously, radiative decay
26
from the excited state 6P7/2 to 8S7/2 level (Gd3+) results in near ultraviolet emission at
27
311 nm. In this energy migration process, Gd3+ can act as an important intermediate
28
subsystem to significantly improve the efficiency of energy transfer from Ce3+ to Dy3+.
29
When an elevated number of Dy3+ dopants are introduced into the host lattice, the
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interatomic distance between Ce3+ and Dy3+ decreases, resulting that the average
2
traveling distance for the excitation energy could be shortened accordingly. Therefore,
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the excitation energy can reach the Dy3+ activator within a shorter time, leading to a
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sharper rising edge on the decay curves of Dy3+ ions.
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4. Conclusion
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Monodisperse and uniform hexagonal crown-capped β-NaYF4:Ce3+/Gd3+/Dy3+
7
microrods have been successfully synthesized via a facile hydrothermal method.
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Based on a series of time-dependent experiments, the phase and morphology
9
evolution as well as the formation mechanism of hexagonal NaYF4 microrods with
10
crown-like top ends are preliminarily investigated and discussed in detail. Upon
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ultraviolet light excitation at 259 nm, intense yellow emission of Dy3+ ions has been
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achieved through efficient energy migration from Ce3+ to Dy3+, in which Gd3+ plays
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an essential intermediate role. Finally, the luminescence mechanism is proposed and
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can be attributed to the strong absorption of ultraviolet irradiation by sensitizers
15
(Ce3+), followed by energy migration to Gd3+ sublattices, from which the energy is
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transferred to activators (Dy3+), resulting in the emission from Dy3+ ions. These
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results not only enrich the knowledge of fluoride chemistry but also contribute to the
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principles of phase transition and crystal growth of inorganic functional materials.
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Acknowledgements
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This project has been financially supported by Zhejiang Provincial Natural
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Science Foundation of China (LQ15E020004 and LR15E020001), the National
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Natural Science Foundation of China (61372025 and 21271170), the 151 talent’s
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projects in the second level of Zhejiang Province and the college students’ activities of science and technology innovation in Zhejiang Province (2015R407033).
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Figure Captions
4
Fig. 1 XRD patterns of the as-synthesized β-NaYF4:10%Ce3+ (a), β-NaYF4:10%Ce3+, 30%Gd3+
5
(b), β-NaYF4: 1%Dy3+ (c), β-NaYF4:30%Gd3+, 1%Dy3+ (d), β-NaYF4:10%Ce3+, 1%Dy3+ (e),
6
β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ (f) samples. The standard data of hexagonal NaYF4
7
(JCPDS No. 16-0334) is given as a reference.
8
Fig. 2 (a) Low magnification, (b) high magnification, and (c, d) typical individual SEM images, (e)
9
EDS spectrum and morphology sketch for the as-synthesized β-NaYF4:10%Ce3+, 30%Gd3+,
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1%Dy3+ microrods.
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Fig. 3 XRD patterns for NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples as a function of reaction
12
time: (a) 3 h, (b) 4 h, (c) 12 h, (d) 24 h. The standard data of cubic NaYF4 (JCPDS No. 77-2042)
13
and hexagonal NaYF4 (JCPDS No. 16-0334) is given as a reference.
14
Fig. 4 SEM images for NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples as a function of reaction
15
time: (a) 3 h, (b) 4 h, (c) 12 h, (d) 24 h.
16
Scheme 1. Schematic illustration for the possible formation process of hexagonal crown-capped
17
NaYF4 microrods.
18
Fig. 5 (a) PL excitation (λem = 309 nm) and emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+
19
sample; (b) PL excitation (λem = 311 nm) and emission spectra (λem = 259 nm) of
20
β-NaYF4:10%Ce3+, 30%Gd3+ sample.
21
Fig. 6 (a) PL excitation (λem = 572 nm) and emission spectra (λex = 349 nm) of β-NaYF4:1%Dy3+
22
sample; (b) PL excitation (λem = 572 nm) and emission spectra (λem = 273 nm) of
23
β-NaYF4:30%Gd3+, 1%Dy3+ sample.
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Fig. 7 (a) PL excitation (λem = 309 nm) and emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+,
1%Dy3+ sample; (b) PL excitation (λem = 572 nm) and emission spectra (λem = 259 nm) of β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ sample; (c) luminescence decay curves of 4F9/2 → 6H15/2
27
transition in β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples. All
28
the decay spectra were recorded by monitoring the emission at 481 nm under an excitation of 259
29
nm; (d) luminescence decay curves of 4F9/2 → 6H13/2 transition in β-NaYF4:10%Ce3+, 1%Dy3+ and
30
β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples. All the decay spectra were recorded by 17
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monitoring the emission at 572 nm under an excitation of 259 nm.
2
Fig. 8 Emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ samples with
3
different Dy3+ dopant concentrations from 0 mol% to 10 mol%. Insets are the corresponding
4
luminescence photographs of these samples under a UV lamp at room temperature (λex = 254 nm).
5
Fig. 9 (a) Luminescence decay curves of Gd3+ ions (6P7/2 → 8S7/2) in β-NaYF4:10%Ce3+, 30%Gd3+,
6
x%Dy3+ samples. All the decay spectra were recorded by monitoring the emission at 311 nm under
7
an excitation of 259 nm; (b) Dependence of the decay time of the Gd3+ ions on the Dy3+ dopant
8
concentration; (c) Luminescence decay curves of Dy3+ ions (4F9/2 → 6H13/2) in β-NaYF4:10%Ce3+,
9
30%Gd3+, x%Dy3+ samples. All the decay spectra were recorded by monitoring the emission at
10
572 nm under an excitation of 259 nm; (d) Dependence of the decay time of the Dy3+ ions on the
11
Dy3+ dopant concentration.
12
Scheme 2. Schematic energy level diagram showing the luminescence mechanism in the
13
β-NaYF4:Ce3+/Gd3+/Dy3+ samples. P1: excitation into the 4f-5d transition of Ce3+ ion; P2: transfer
14
of the excitation energy to Gd3+ ion; P3: energy migration among the Gd3+ ions. This process
15
could repeat many times, depending on the dopant concentration of Dy3+ ions; P4: trapping of the
16
migrating energy; P5: emission from the Dy3+ ion; P6: competitive emission from Gd3+ ion.
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Highlights Hexagonal crown-capped β-NaYF4 microrods has been successfully synthesized. The growth mechanism of hexagonal crown-capped NaYF4 microrods was proposed.
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The energy transfer mechanism of β-NaYF4:Ce3+/Gd3+/Dy3+ was discussed in
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S0925-8388(15)31529-2
DOI:
10.1016/j.jallcom.2015.10.273
Reference:
JALCOM 35828
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2015 Revised Date:
28 October 2015
Accepted Date: 29 October 2015
Please cite this article as: M. Ding, Y. Li, D. Chen, H. Lu, J. Xi, Z. Ji, Hexagonal Crown-Capped 3+ 3+ 3+ NaYF4:Ce /Gd /Dy Microrods: Formation Mechanism, Energy Transfer and Luminescence Properties, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.273. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Hexagonal Crown-Capped NaYF4:Ce3+/Gd3+/Dy3+ Microrods: Formation Mechanism, Energy Transfer and Luminescence Properties
a
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Mingye Ding a, Yuting Li a, Daqin Chen a,*, Hongwei Lu a, Junhua Xi a, Zhenguo Ji a College of Materials and Environmental Engineering, Hangzhou Dianzi University,
Hangzhou, 310018 P. R. China Abstract
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In this work, hexagonal crown-capped β-NaYF4:Ce3+/Gd3+/Dy3+ microrods with remarkably uniform morphology and size have been successfully synthesized for the
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first time via a facile hydrothermal route. The phase and morphology evolution of β-NaYF4 crystals are carefully tracked during hydrothermal growth by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron diffraction (TEM), respectively. The possible growth mechanism of hexagonal crown-capped NaYF4 microrods has been proposed. Furthermore,
the
luminescence
properties
of
β-NaYF4:Ce3+/Gd3+/Dy3+
are
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systemically investigated by the photoluminescence excitation spectra, emission spectra and decay curves. Impressively, upon the allowed Ce3+ 4f-5d electronic transition excitation, the efficient energy transfer from sensitizer (Ce3+) to activator (Dy3+) occurs via energy migration process Ce3+ → (Gd3+)n → Dy3+, in which Gd3+
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ions act as bridge centers to induce intense yellow luminescence from Dy3+ activators.
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Keywords: Crown-capped microrods, hexagonal NaYF4, crystal growth, energy transfer, Dy3+ luminescence
*
Corresponding author: Tel.: +86 0571 87713537 Fax. : +86 0571 87713537
E-mail address: [email protected] (D. Chen) 1
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1. Introduction Recently, lanthanide (Ln3+) ions doped luminescent materials have received
3
increasing research interests because of their widely tunable emission colors, sharp
4
emission peaks, large Stokes shifts, high photostability and low toxicity.[1-5] Until
5
now, large numbers of rare earth (RE) inorganic compounds, including oxides,[6]
6
oxysulfides,[7]
7
vanadates[14] and tungstates,[15] have been extensively investigated for their
8
potential application in the fields of white light-emitting diodes,[16] sensors,[17]
9
anti-counterfeiting,[18] flat-panel displays,[19] solid-state lasers,[20] solar cells[21]
10
and biological labels.[22] Among the reported host materials, RE-based fluorides with
11
general formula NaREF4 have been considered as excellent host lattices for various
12
optically active Ln3+ ions, because they possess low phonon energy (< 400 cm-1),
13
good optical transparency over a wide wavelength range and adequate thermal and
14
chemical stability. As we known, NaREF4 exists in two polymorphic forms, i.e., cubic
15
(α) and hexagonal (β) phase, depending on the synthesis conditions and methods.
16
Previous studies have proven that the hexagonal NaREF4 series are more efficient
17
than their cubic counterparts because of their unique crystal structure.[23] Specifically,
18
hexagonal NaYF4 (β-NaYF4) has been most commonly chosen as excellent host
19
lattice for achieving efficient downconversion (DC) and upconversion (UC)
20
luminescence of lanthanide ions for decades. Up to now, β-NaYF4 nano/microcrystals
21
with various morphologies and sizes, including nano/microspheres, nano/microtubes,
22
nano/microprisms, nano/microrods, nanoarrays, micro-bipyramids, microplates, have
24 25 26
sulfides,[10]
fluorides,[11-13]
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phosphates,[9]
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oxyfluorides,[8]
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been successfully synthesized by several chemical methods such as solid-state reaction, molten salt, solvothermal, co-precipitation and hydrothermal methods.[24-28] However, fabrication of hexagonal NaYF4 microrods with crown-capped shape have been rarely reported, which may force the advancement of these rare earth fluorides
27
with better luminescence properties, finally accelerating the spread of new
28
applications.
29
As we known, the luminescence of Ln3+-activated materials originates basically
30
from electronic transitions within the 4fn configuration (4f → 4f) of the lanthanide 2
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dopants. The luminescence properties of the lanthanide-doped materials can be
2
precisely controlled through changing the lanthanide activators doped in the host
3
lattice. The trivalent lanthanide ions, such as Eu3+, Tb3+, Sm3+ and Dy3+ have attracted
4
wide attention due to their unique optical properties.[29, 30] For example, Tb3+ and
5
Eu3+ ions are well-known as green-emitting and red-emitting activators because of the
6
5
7
transitions of Eu3+, respectively.[31, 32] Sm3+ ions can also act as useful activator for
8
their orange luminescence, originating from the 4G5/2 → 6HJ/2 (J = 5, 7 and 9)
9
transitions of Sm3+.[33] Particularly, Dy3+ ions, possessing two dominant transitions
10
in visible region: 4F9/2 → 6H13/2 transition (yellow emission) and 4F9/2 → 6H15/2
11
transition (blue emission), have been paid growing attention for their potential
12
applications in white light-emitting diodes, plasma display planes, field emission
13
displays and mercury-free lamps.[34, 35] In spite of the single-doping model,
14
activators co-doping with sensitizer ions have a higher absorption coefficient, leading
15
to more efficient luminescence than single-doped system by taking advantage of the
16
effective energy transfer from sensitizers to activators.[36, 37] Ce3+ ions are usually
17
used as sensitizer in order to improve the luminescence intensity of Ln3+ ions (e. g. Eu,
18
Tb), because the emission band of Ce3+ ions matches well with several f-f absorption
19
bands of Ln3+ ions. Simultaneously, Gd3+ ions could act as intermediate sublattices to
20
facilitate the energy transfer from Ce3+ ions to the Ln3+ ions via Ce3+ → (Gd3+)n →
21
Ln3+ energy hopping, resulting in the promotion of the photon conversion from
22
ultraviolet radiation to visible emissions. Despite these above developments, the
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D4 → 7FJ (J = 1, 2, 3 and 4) transitions of Tb3+ and 5D0 → 7FJ (J = 6, 5, 4 and 3)
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energy transfer Ce3+ → Gd3+ → Dy3+ in β-NaYF4 host lattice and the luminescence properties of β-NaYF4:Ce3+/Gd3+/Dy3+ have been rarely reported and still needed further investigation.
In the present work, we demonstrate a facile and effective hydrothermal strategy
27
for the synthesis of monodisperse β-NaYF4:Ce3+/Gd3+/Dy3+ microrods with hexagonal
28
crown-capped shape. Based on the time-dependent experimental results, the possible
29
formation mechanism for the phase and morphology evolution process has been
30
proposed. The luminescence behaviors of Ce3+, Gd3+ and Dy3+ co-doped β-NaYF4 3
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products are systemically investigated. In addition, the energy transfer mechanism of
2
β-NaYF4:Ce3+/Gd3+/Dy3+ samples is discussed in detail.
3
2. Experimental section
4
2.1. Reagents.
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Yttrium chloride hydrate (YCl3.6H2O, 99.99%), gadolinium chloride hexahydrate
6
(GdCl3.6H2O, 99.99%), cerium chloride hydrate (CeCl3.7H2O, 99.99%) and
7
dysprosium nitrate hydrate (Dy(NO3)3.6H2O, 99.99%) were purchased from Beijing
8
Founde Star Science & Technology Co., Ltd (China). Sodium fluoride (NaF, 98%)
9
and sodium citrate (C6H5Na3O7.2H2O, 99%) were supplied by Sinopharm Chemical
10
Reagent Co., Ltd (China). Deionized water was used throughout. All chemicals were
11
of analytical grade reagents and used directly without further purification.
12
2.2 Synthesis.
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In a typical procedure for the synthesis of β-NaYF4:Ce3+/Gd3+/Dy3+ microrods, 2
14
mmol of RECl3.6H2O (RE = Y, Ce, Gd and Dy) were firstly dissolved in 10 mL
15
deionized water with magnetic stirring to form rare earth chloride solution. Then 20
16
mL of trisodium citrate (Na3Cit, 0.1 mol/L) was added into the resulting solution to
17
form the metal-citrate complex. After vigorous stirring for 30 min, 30 mL of NaF
18
aqueous solution (0.4 mol/L) were dropped into the above mixture under thoroughly
19
stirring for 15 min. Subsequently, the as-obtained mixing solution was transferred to a
20
100 mL Teflon-lined autoclave and heated at 220 oC for 24 h. After the autoclave was
21
cooled down to room temperature naturally, the precipitates were collected by
22
centrifugation and washed with deionized water and ethanol in sequence. The
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collected samples were dired in air at 60 oC for 12 h. 2.3 Characterization
Powder X-ray diffraction (XRD) measurements were performed on a ARL X’TRA
diffraction meter at a scanning rate of 10 o/min in the 2θ range from 10 o to 80 o with
27
Cu Kα radiation (λ = 0.15046 nm). Scanning electron microscopy (SEM) micrographs
28
were obtained using a field emission scanning electron microscope (FE-SEM,
29
SU8010, Hitachi). An energy dispersive spectroscopy (EDS) facility attached to the
30
JEOL JSM-6460 SEM was employed to analyze the chemical composition of the 4
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as-synthesized products. Transmission electron microscopy (TEM) and selected area
2
electron diffraction (SAED) pattern were recorded on a JEM-200CX with a field
3
emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple
4
CCD spectrometer. The excitation and emission spectra, luminescence decay curves
5
were recorded with an Edinburgh Instruments FS5 fluorescence spectrophotometer.
6
All the measurements were performed at room temperature.
7
3. Results and Discussion
8
3.1 Crystal Structure and Morphology
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The composition and structure of the samples were first characterized by X-Ray
10
Diffraction (XRD) technique. Fig. 1 shows the XRD patterns of the as-synthesized
11
NaYF4 products co-doping with Ce3+, Gd3+ and/or Dy3+ ions as well as the standard
12
data of pure hexagonal NaYF4. As shown, strong and sharp diffraction peaks can be
13
well indexed to the pure hexagonal NaYF4 phase, which is consistent with the
14
standard card (JCPDS No. 16-0334). The crystal structure of β-NaYF4 is determined
15
with lattice parameters of a = 0.596 nm and c = 0.353 nm, space group P63/mmc.[38]
16
No obvious extra diffraction peaks are observed in all the XRD patterns, revealing
17
that Y3+ can be replaced by Ce3+, Gd3+ and Dy3+ with no effect on the crystallinity or
18
phase change because of their almost ionic radii and identical valence.
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The typical morphology of the as-synthesized β-NaYF4:10%Ce3+, 30%Gd3+,
20
1%Dy3+ sample is presented in Fig. 2. As can be seen from SEM images (Fig. 2a and
21
2b), the as-prepared sample is composed of a large quantity of hexagonal
22
crown-capped microrods with good uniformity, monodispersity and well-defined
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crystal facets. Analysis of a number of microrods reveals that these microparticles have an average size of 5.3 µm in length and 3.3 µm in diameter, indicating an average of aspect ratio of about 1.6 for each rod. The magnified SEM image shows a close-up view of one isolated microrod (Fig. 2c), which clearly displays the hexagonal
27
prism structure of the sample. From the magnified SEM image in Fig. 2d, it can be
28
clearly seen that the top end of the hexagonal rods is not flatness but like a crown. As
29
an analogy, six triangle walls incline outward a bit on the six edges of the hexagonal
30
column, which construct a short prismatic cup erecting on top/bottom side, resulting 5
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in the formation of hexagonal crown-capped NaYF4 microrod. The length and
2
thickness of these triangle walls are about 0.8-1.2 µm and 0.15-0.49 µm, respectively.
3
Fig.
4
β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ microrods, confirming the existence of Na, Y,
5
Gd, F, Ce and Dy elements. The morphology sketch (Inset in Fig. 2e) clearly indicates
6
the preferred growth direction. Because hexagonal NaYF4 has high anisotropic
7
structure, the growth direction is confined to the [0001] direction and it preferentially
8
grows into prismatic shape.[39] To the best of our knowledge, this kind of β-NaYF4
9
microrods with crown-like top ends has been synthesized and observed for the first time.
11
3.2 Growth mechanism
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2e shows the Energy Dispersive Spectrometer (EDS) spectrum of
The time-dependent experiments were carried out to explore the formation
13
process of crown-capped β-NaYF4 microrods. Fig. 3 shows the XRD patterns of the
14
intermediates obtained at different reaction time intervals as well as the standard data
15
of cubic NaYF4 (α-NaYF4: JCPDS No. 77-2042) and hexagonal NaYF4 (β-NaYF4:
16
JCPDS No. 16-0334) phases for comparison. Obviously, the samples exhibit
17
distinguishingly different XRD patterns at different reaction periods. At a short
18
reaction time of 3 h, only cubic NaYF4 (α-NaYF4) can be obtained (Fig. 3a). When
19
the reaction time is prolonged to 4 h, a new hexagonal phase, namely, β-NaYF4,
20
emerges in addition to the α-phase NaYF4, resulting in the mixture of predominantly
21
α-NaYF4 and minor β-NaYF4 (Fig. 3b). This result indicates that the sample
22
transforms gradually from cubic to hexagonal phase with the further reaction. With
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the reaction proceeding for 12 h, the cubic NaYF4 disappears thoroughly and pure hexagonal phase of NaYF4 exists, as shown in Fig. 3c. At a reaction time of 24 h, the phase of the sample remains unchanged, but the intensities of diffraction peaks increase remarkably (Fig. 3d). This implies that the crystallinity of the microcrystals
27
increases with the increasing of the reaction time. Form the above analysis, it can be
28
drawn conclusion that the crystal evolves from α-phase to mixed phase and finally to
29
β-phase with prolongation of reaction time.
30
At the meantime, the morphologies of the intermediate products were carefully 6
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investigated at different reaction times. Fig. 4 shows the typical TEM and SEM
2
images of NaYF4 samples obtained at different reaction periods. Remarkably, the
3
intermediates exhibit distinctively different morphologies in the process of crystal
4
growth. At t = 3 h, the as-obtained α-NaYF4 sample consists of spherical nanoparticles
5
with an average diameter of 31 nm, as presented in Fig. 4a and Fig. S1. With the
6
reaction proceeding only 4 h, relatively uniform microrods with well-defined
7
hexagonal cross-section begin to appear. The intermediate sample consists of
8
hexagonal microrods (1.82 µm in length and 0.70 µm in diameter) and some irregular
9
nanoparticles, which correspond to β-NaYF4 and α-NaYF4, respectively (Fig. 4b).
10
Noticeably, the top face of the hexagonal rods is extremely flat without any other
11
trace, as shown in the inset image of Fig. 4b. At a reaction time of 12 h, pure β-NaYF4
12
with rod-like morphology can be obtained, as shown in Fig. 4c. Meanwhile, the
13
α-NaYF4 nanoparticles disappear completely, indicating that the phase transition (α →
14
β) significantly results in the dramatic change in the final morphology of NaYF4
15
crystals. The mean size of these microrods is estimated to be 0.98 µm in diameter and
16
2.08 µm in length. It is noteworthy that convex triangle walls on the edges of the
17
hexagonal column begin to appear and grow higher than the central part with the
18
extension of reaction time (Inset in Fig. 4c). Quite interestingly, on aging for a longer
19
period up to 24 h, the outer triangle walls become increasingly sharp and a flat plane
20
appears in the end of the inner structure, leading to the final hexagonal crown-capped
21
NaYF4 rods (Fig. 4d). Further extending the reaction time to 24 h also results that the
22
size of the as-obtained samples continues to augment significantly (5.3 µm in length
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and 3.3 µm in diameter). The above morphology evolution process provides us some clue for understanding the formation mechanism of hexagonal crown-capped β-NaYF4 microrods.
Based on the time-dependent experiments, a possible phase and morphology
27
evolution mechanism is presented in Scheme 1 and described as follows. At the
28
beginning, the aqueous solution containing rare earth cations (Y3+) and sodium citrate
29
(Na3Cit) is directly mixed to form a precursor. Through strong coordination
30
interaction, the citrate anions (Cit3-) introduced into the reaction system can form 7
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Y3+-Cit3- complexes with Y3+ ions. Simultaneously, NaF dissolves in the aqueous
2
solution to form Na+ and F- ions. Y3+ + Na3Cit → Na+ + Y3+-Cit3- (complex)
3
(1)
NaF → Na+ + F-
(2)
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Under the hydrothermal conditions of high temperature and pressure, the
6
chelating ability of Y3+-Cit3- complexes could be weakened by slow degrees, resulting
7
that Y3+ ions would be released gradually into the reaction solution. As a result, the
8
concentration of free Y3+ ions in the solution increases gradually, which thus helps to
9
regulate the nucleation and subsequent growth of the crystals. After the addition of the
10
aqueous solution containing Na+ and F- ions, Y3+ ions in the reaction system react
11
with Na+ and F- ions quickly to generate small nuclei. At a short reaction time, these
12
nuclei quickly congregate together and grow into α-NaYF4 nanoparticles. The cubic
13
NaYF4 seeds possess isotropic unit cell structures, which commonly induce an
14
isotropic growth of particles and therefore spherical particles are clearly observed in
15
order to minimize the surface energy of various crystal facets.
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Y3+-Cit3- + Na+ + F- → α-NaYF4 + Cit3-
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Nevertheless, these cubic-phased NaYF4 nanoparticles are thermodynamically
18
unstable and evolve ineluctably to hexagonal-phased NaYF4 seeds through
19
dissolution-renucleation process.[40] Obviously, the phase transformation (α → β)
20
leads to the prominent change in the morphology of NaYF4 crystals, which can be
21
ascribed to different characteristic unit cell structures of diverse crystallographic
22
phases. Different from α-NaYF4, β-NaYF4 seeds possess anisotropic unit cell
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structure,
which
generally
induces
the
anisotropic
growth
along
the
crystallographically active direction. With the further reaction, anisotropic growth of hexagonal-phased NaYF4 becomes dominating through rapid dissolving of cubic-phased NaYF4 and releasing of monomers into reaction solution, resulting in
27
the sudden nucleation of β-NaYF4 and subsequent growth along [0001] direction to
28
hexagonal cylinder.[41] The dissolution-reconstruction process of α-NaYF4
29
nanoparticles preferentially occurs at the circumferential edges of each hexagonal
30
microrod along crystallographically reactive direction through the Ostwald ripening 8
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process, resulting that the morphology of the sample changes from spherical
2
nanoparticles (α-NaYF4) into hexagonal microrods (β-NaYF4). α-NaYF4 → β-NaYF4
3
(4)
Subsequently, the growth rate of β-NaYF4 crystals in the crystal growth stage is
5
closely related to the amount of monomers released during the fast dissolution of
6
α-NaYF4 nanoparticles, which is considered as single-source precursors to supply
7
monomers.[42] As the reaction proceeded, the vicinal monomers is continually
8
consumed, resulting that the concentration of monomers near β-NaYF4 microrods
9
decreases gradually and the nearby α-NaYF4 nanoparticles accelerate dissolution as a
10
supplement.[25] Then the released monomers move to the hexagonal rods and meet
11
the extending top corners of initial hexagonal rod firstly, leading to the accelerated
12
growth rate of the top corners. Thus the crystal growth along [0001] direction could
13
be suppressed a little and the β-NaYF4 crystals would also grow along the twelve
14
21 0], [011 0], [1120], [1010], [2110], [1100], directions ([2110], [1100], [1
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[1210] , [0110] , [1120] and [1010] ), resulting in the formation of
16
dodecagon-shaped rods (Fig. S2). Obviously, the top corners can absorb more
17
monomers than the interior, giving rising to the formation of the six convex triangle
18
walls on the edges of hexagonal rods. With a prolonged growth time, the β-NaYF4
19
crystal shows a hexagonal crown-capped morphology.
20
3.3 Photoluminescence properties
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Generally, the direct energy transfer from Ce3+ to activator Ln3+ ions (Ln = Eu,
22
Tb, Sm and Dy) is rather inefficient.[43] To make this energy transfer process more
23 24 25 26
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efficient, Gd3+ ions are usually employed to act as an intermediate sublattice for energy transfer from Ce3+ to Ln3+ through Ce3+ → (Gd3+)n → Ln3+ energy migration process. Herein, in the following part, we try to illustrate this energy transfer process by selecting Gd3+ and Dy3+ as migrator and trap center, respectively.
27
Firstly, we investigate the photoluminescence properties of NaYF4:Ce3+ and
28
NaYF4:Ce3+/Gd3+ samples to validate the efficient Ce3+ → Gd3+ energy transfer
29
process. Fig. 5 shows room temperature excitation and emission spectra of
30
β-NaYF4:10%Ce3+ and β-NaYF4:10%Ce3+, 30%Gd3+ samples. It can be seen from the 9
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excitation spectrum that the maximum of the characteristic excitation band lies at
2
about 259 nm, which is assigned to the 4f → 5d transition of Ce3+ (left line, Fig. 5a).
3
Specifically, in β-NaYF4:10%Ce3+, 30%Gd3+ sample, the excitation spectrum for Gd3+
4
emission (311 nm) is dominated by the strong and broad absorption band peaked at
5
around 259 nm due to the Ce3+ 4f → 5d transition followed by the weak and sharp
6
Gd3+ line at about 273 nm (8S → 6I), reveals that the possibility of energy transfer
7
from Ce3+ ions to Gd3+ ions (left line, Fig. 5b). Upon the excitation into the 4f → 5d
8
transition of Ce3+ at 259 nm, the emission spectrum of β-NaYF4:10%Ce3+ shows a
9
broadband emission with a maximum at about 309 nm, which is assigned to the
10
electric-dipole-allowed 4f-5d transition of Ce3+ (right line, Fig. 5a). However, the
11
sharp emission peak corresponding to 6P7/2 → 8S7/2 of Gd3+ (311 nm) can be clearly
12
observed in β-NaYF4:10%Ce3+, 30%Gd3+ sample (right line, Fig. 5b). Interestingly,
13
there is a large spectral overlap between the Gd3+ absorption line and the Ce3+
14
emission band, which significantly satisfies the requirement of energy transfer from
15
Ce3+ to Gd3+, resulting in the efficient Ce3+ → Gd3+ energy transfer process.[44]
16
Secondly, we confirm the existence of Gd3+ → Dy3+ energy transfer by comparing the
17
luminescence properties of β-NaYF4:1%Dy3+ and β-NaYF4:30%Gd3+, 1%Dy3+
18
samples. Fig. 6 presents their excitation and emission spectra. For β-NaYF4:1%Dy3+,
19
the excitation spectrum consists of the 4f-4f characteristic excitation lines of Dy3+
20
with its 4f9 configuration from 300 to 450 nm (left line, Fig. 6a). The main excitation
21
lines originate from the 6H15/2 ground state to different excited states: 322 nm (6P3/2),
22
349 nm (6P7/2), 363 nm (6P5/2), 386 nm (4M11/2) and 423 nm (4G11/2). Under excitation
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into the 6H15/2 → 6P5/2 transition at 349 nm, there are two obvious sharp lines centered at 481 nm and 572 nm, corresponding to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions, respectively (right line, Fig. 6b).[45] By comparison, the excitation spectrum of β-NaYF4:30%Gd3+, 1%Dy3+ exhibits the Gd3+ absorption line (273 nm, 8S → 6I) and
27
f-f transition lines of Dy3+ (left line, Fig. 6b). Excitation into the strongest 8S → 6I
28
transition of Gd3+ at 273 nm yields the emission of Gd3+ at 311 nm and the
29
characteristic emissions of Dy3+ at 481 nm and 572 nm, which are assigned to the
30
Gd3+: 6P7/2 → 8S7/2 transition and Dy3+: 4F9/2 → 6HJ/2 (J = 15, 13) transitions, 10
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1
respectively (right line, Fig. 6b), suggesting the occurrence of energy transfer from
2
the Gd3+ sublattice to Dy3+ emission center.[46] The above results confirm that the
3
energy transfer Ce3+ → Gd3+ and Gd3+ → Dy3+ exist in NaYF4:Ce3+/Gd3+/Dy3+
4
samples. To further interpret the role of Gd3+ in the Ce3+ → Dy3+ energy transfer process,
6
the luminescence properties of NaYF4:Ce3+/Dy3+ and NaYF4:Ce3+/Gd3+/Dy3+ are
7
compared and investigated in detail. Fig. 7 shows the excitation, emission spectra and
8
decay curves of β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+,
9
1%Dy3+ samples. From the excitation spectrum (left line, Fig. 7a) and emission
10
spectrum (right line, Fig. 7a) of β-NaYF4:10%Ce3+, 1%Dy3+ sample, it can be seen
11
that broad Ce3+ emission with a maximum 309 nm along with weak Dy3+ emissions
12
(481 nm and 572 nm), exhibiting the inefficient Ce3+ → Dy3+ energy transfer process.
13
For β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ in Fig. 7b, by monitoring the 572 nm
14
emission of Dy3+, the characteristic 4f-4f transition lines within the Dy3+ 4f9
15
configuration together with strong and broad band at 259 nm due to 4f-5d transition of
16
Ce3+ can be obviously observed (left line, Fig. 7b). Excitation into the Ce3+ band at
17
259 nm yields weak and sharp emission of Gd3+ at 311 nm and strong emission of
18
Dy3+ (right line, Fig. 7b), demonstrating the important role of Gd3+ sublattice as an
19
intermediary in the energy transfer process from Ce3+ to Dy3+. With the assistance of
20
Gd3+ ion, Ce3+ ion can transfer its excitation energy to Dy3+ ion through Ce3+ →
21
(Gd3+)n → Dy3+ energy migration process. The experimental decay lifetimes of Dy3+
22
for 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions also provide evidence to support this
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conclusion. The fluorescence decay curves of Dy3+ (4F9/2 → 6H15/2/6H13/2) for β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples are shown in Fig. 6c and Fig. 6d, respectively. The average lifetime can be calculated by[47]
27
∗ ∗ τave =
(5)
28
where I(t) represents the emission intensity at time t. Apparently, owing to the
29
efficient energy migration process, the obtained lifetimes of Dy3+ for 4F9/2 → 6H15/2 11
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and 4F9/2 → 6H13/2 transitions increase from 1.10 ms (481 nm) and 1.06 ms (572 nm)
2
of NaYF4:Ce3+/Dy3+ to 2.50 ms (481 nm) and 2.45 ms (572 nm) of
3
NaYF4:Ce3+/Gd3+/Dy3+. The above results powerfully demonstrate that Gd3+ ions play
4
an important role in facilitating the energy transfer from Ce3+ ions to Dy3+ ions.
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In the following experiments, we study on the luminescence properties of
6
NaYF4:Ce3+/Gd3+/Dy3+ microrods with different Dy3+ dopant concentration to
7
demonstrate that the emission intensity can be precisely adjusted by tuning the Dy3+
8
dopant concentration. It is noted that the crystallographic phase and general
9
morphology of the final samples have not changed by the doping small amount of
10
Dy3+ ions (Fig. S3 and Fig. S4). Fig. 8 shows the emission spectra of
11
β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ (x = 0 ~ 10) samples. Under Ce3+ excitation
12
band at 259 nm, the obtained emission spectrum exhibits three obvious lines centered
13
at 311 nm, 481 nm and 572 nm, corresponding to 6P7/2 → 8S7/2 transition of Gd3+ and
14
4
15
intensity of Gd3+ decreases gradually with the increasing dopant concentration of Dy3+,
16
which may be ascribed to the enhanced energy transfer from Gd3+ to Dy3+. However,
17
the 572 nm emission intensity of Dy3+ increases with the increment of Dy3+ dopant
18
concentration from 0 mol% to 1 mol%, and then decreases gradually when the Dy3+
19
concentration is further increased to 10 mol%. The 481 nm emission intensity of Dy3+
20
presents the same variation tendency (Fig. S5), which can be attributed to the
21
concentration quenching effect.[48] As observed by naked eyes, these samples exhibit
22
yellow emission under 254 nm ultraviolet lamp. Moreover, the emission intensity of
24 25 26
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F9/2 → 6HJ/2 (J = 15, 13) transitions of Dy3+. Obviously, the 311 nm emission
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β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ is the strongest, agreeing well with the results of emission spectra.
The luminescence decay behavior of Gd3+ and Dy3+ in the NaYF4:Ce3+/Gd3+/Dy3+
are systematically studied in order to further understand the energy transfer process.
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Fig. 9a shows the luminescence decay curves (excited at 259 nm and monitored at 311
28
nm) of Gd3+ ions as a function of Dy3+ dopant concentration. As shown, the excited
29
state lifetime of Gd3+ decreases continuously with increasing Dy3+ content (Fig. 9b
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and Table S1). As both nonradiative and radiative processes contribute to the 12
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deactivation of an excited state, the experimental lifetime of the excited state (τ) can
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be expressed as[49]
3
= +
(6)
Where WR is the radiative rate and WNR represents the sum of the rates of diverse
5
nonradiative processes including energy transfer. The sharp decrease of Gd3+ lifetime
6
with the adding of Dy3+ dopant concentration indicates the increasing energy transfer
7
efficiency from Gd3+ to Dy3+, which significantly increases the nonradiative rate of
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Gd3+. The luminescence decay curves (excited at 259 nm and monitored at 572 nm) of
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Dy3+ ions in β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ (x = 0 ~ 10) samples are
10
exhibited in Fig. 9c. The experimental lifetime of Dy3+ for 4F9/2 → 6H13/2 transition
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decreases from 4.81 ms to 0.16 ms with increasing Dy3+ dopant concentration from 0
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mol% to 10 mol% (Fig. 9d and Table S1). The Dy3+ lifetime for 4F9/2 → 6H15/2
13
transition presents the same downward trend, as shown in Fig. S6 and Table S1.
14
Interestingly, with the increment of Dy3+ dopant concentration, the rising edge of
15
decay curves change from a gentle incline into a sharp fast-rising slope, which is
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usually used to describe the energy migration time from Ce3+ to Dy3+.[36] The
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decrease of rising edge time reveals that the average traveling time of the excitation
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energy via the Gd3+ subsystem decreases gradually with the adding of Dy3+ ions.
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Based on the above results, the Gd-mediated energy migration process is
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depicted in Scheme 2. Under the ultraviolet excitation (λ = 259 nm), the excitation
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photons are absorbed by Ce3+ to populate the 5d excited state followed by fast energy
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transfer from Ce3+ to Gd3+. Subsequently, the energy migration process among the
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Gd3+ sublattices takes place. The migration energy is finally trapped by Dy3+ ions, leading to the blue emission centered at 481 nm (4F9/2 → 6H13/2) and yellow emission at 572 nm (4F9/2 → 6H15/2) observed by naked eyes. Simultaneously, radiative decay
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from the excited state 6P7/2 to 8S7/2 level (Gd3+) results in near ultraviolet emission at
27
311 nm. In this energy migration process, Gd3+ can act as an important intermediate
28
subsystem to significantly improve the efficiency of energy transfer from Ce3+ to Dy3+.
29
When an elevated number of Dy3+ dopants are introduced into the host lattice, the
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interatomic distance between Ce3+ and Dy3+ decreases, resulting that the average
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traveling distance for the excitation energy could be shortened accordingly. Therefore,
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the excitation energy can reach the Dy3+ activator within a shorter time, leading to a
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sharper rising edge on the decay curves of Dy3+ ions.
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4. Conclusion
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Monodisperse and uniform hexagonal crown-capped β-NaYF4:Ce3+/Gd3+/Dy3+
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microrods have been successfully synthesized via a facile hydrothermal method.
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Based on a series of time-dependent experiments, the phase and morphology
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evolution as well as the formation mechanism of hexagonal NaYF4 microrods with
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crown-like top ends are preliminarily investigated and discussed in detail. Upon
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ultraviolet light excitation at 259 nm, intense yellow emission of Dy3+ ions has been
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achieved through efficient energy migration from Ce3+ to Dy3+, in which Gd3+ plays
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an essential intermediate role. Finally, the luminescence mechanism is proposed and
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can be attributed to the strong absorption of ultraviolet irradiation by sensitizers
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(Ce3+), followed by energy migration to Gd3+ sublattices, from which the energy is
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transferred to activators (Dy3+), resulting in the emission from Dy3+ ions. These
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results not only enrich the knowledge of fluoride chemistry but also contribute to the
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principles of phase transition and crystal growth of inorganic functional materials.
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Acknowledgements
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This project has been financially supported by Zhejiang Provincial Natural
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Science Foundation of China (LQ15E020004 and LR15E020001), the National
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Natural Science Foundation of China (61372025 and 21271170), the 151 talent’s
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projects in the second level of Zhejiang Province and the college students’ activities of science and technology innovation in Zhejiang Province (2015R407033).
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Figure Captions
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Fig. 1 XRD patterns of the as-synthesized β-NaYF4:10%Ce3+ (a), β-NaYF4:10%Ce3+, 30%Gd3+
5
(b), β-NaYF4: 1%Dy3+ (c), β-NaYF4:30%Gd3+, 1%Dy3+ (d), β-NaYF4:10%Ce3+, 1%Dy3+ (e),
6
β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ (f) samples. The standard data of hexagonal NaYF4
7
(JCPDS No. 16-0334) is given as a reference.
8
Fig. 2 (a) Low magnification, (b) high magnification, and (c, d) typical individual SEM images, (e)
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EDS spectrum and morphology sketch for the as-synthesized β-NaYF4:10%Ce3+, 30%Gd3+,
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1%Dy3+ microrods.
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Fig. 3 XRD patterns for NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples as a function of reaction
12
time: (a) 3 h, (b) 4 h, (c) 12 h, (d) 24 h. The standard data of cubic NaYF4 (JCPDS No. 77-2042)
13
and hexagonal NaYF4 (JCPDS No. 16-0334) is given as a reference.
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Fig. 4 SEM images for NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples as a function of reaction
15
time: (a) 3 h, (b) 4 h, (c) 12 h, (d) 24 h.
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Scheme 1. Schematic illustration for the possible formation process of hexagonal crown-capped
17
NaYF4 microrods.
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Fig. 5 (a) PL excitation (λem = 309 nm) and emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+
19
sample; (b) PL excitation (λem = 311 nm) and emission spectra (λem = 259 nm) of
20
β-NaYF4:10%Ce3+, 30%Gd3+ sample.
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Fig. 6 (a) PL excitation (λem = 572 nm) and emission spectra (λex = 349 nm) of β-NaYF4:1%Dy3+
22
sample; (b) PL excitation (λem = 572 nm) and emission spectra (λem = 273 nm) of
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β-NaYF4:30%Gd3+, 1%Dy3+ sample.
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Fig. 7 (a) PL excitation (λem = 309 nm) and emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+,
1%Dy3+ sample; (b) PL excitation (λem = 572 nm) and emission spectra (λem = 259 nm) of β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ sample; (c) luminescence decay curves of 4F9/2 → 6H15/2
27
transition in β-NaYF4:10%Ce3+, 1%Dy3+ and β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples. All
28
the decay spectra were recorded by monitoring the emission at 481 nm under an excitation of 259
29
nm; (d) luminescence decay curves of 4F9/2 → 6H13/2 transition in β-NaYF4:10%Ce3+, 1%Dy3+ and
30
β-NaYF4:10%Ce3+, 30%Gd3+, 1%Dy3+ samples. All the decay spectra were recorded by 17
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monitoring the emission at 572 nm under an excitation of 259 nm.
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Fig. 8 Emission spectra (λex = 259 nm) of β-NaYF4:10%Ce3+, 30%Gd3+, x%Dy3+ samples with
3
different Dy3+ dopant concentrations from 0 mol% to 10 mol%. Insets are the corresponding
4
luminescence photographs of these samples under a UV lamp at room temperature (λex = 254 nm).
5
Fig. 9 (a) Luminescence decay curves of Gd3+ ions (6P7/2 → 8S7/2) in β-NaYF4:10%Ce3+, 30%Gd3+,
6
x%Dy3+ samples. All the decay spectra were recorded by monitoring the emission at 311 nm under
7
an excitation of 259 nm; (b) Dependence of the decay time of the Gd3+ ions on the Dy3+ dopant
8
concentration; (c) Luminescence decay curves of Dy3+ ions (4F9/2 → 6H13/2) in β-NaYF4:10%Ce3+,
9
30%Gd3+, x%Dy3+ samples. All the decay spectra were recorded by monitoring the emission at
10
572 nm under an excitation of 259 nm; (d) Dependence of the decay time of the Dy3+ ions on the
11
Dy3+ dopant concentration.
12
Scheme 2. Schematic energy level diagram showing the luminescence mechanism in the
13
β-NaYF4:Ce3+/Gd3+/Dy3+ samples. P1: excitation into the 4f-5d transition of Ce3+ ion; P2: transfer
14
of the excitation energy to Gd3+ ion; P3: energy migration among the Gd3+ ions. This process
15
could repeat many times, depending on the dopant concentration of Dy3+ ions; P4: trapping of the
16
migrating energy; P5: emission from the Dy3+ ion; P6: competitive emission from Gd3+ ion.
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Highlights Hexagonal crown-capped β-NaYF4 microrods has been successfully synthesized. The growth mechanism of hexagonal crown-capped NaYF4 microrods was proposed.
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The energy transfer mechanism of β-NaYF4:Ce3+/Gd3+/Dy3+ was discussed in
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