The morphology evolution, tunable down-conversion luminescence, and energy transfer of [CaY]F2crystals doped with Li+/Ce3+/Tb3+

Article abstract of DOI:10.1039/d0dt02955f

Octahedral [CaY]F2 crystals with an average particle size of 1 μm were synthesized via a mild one-step hydrothermal route without employing any surfactants. Various morphologies, including cubes, truncated cubes, truncated octahedrons, and spheres, were achieved via manipulating the amount of EDTA used, and a possible growth mechanism was proposed based on the surface energies of different crystal planes and the influence of the surfactant. XRD, SEM, EDS, TEM, HRTEM, and PL analysis were used to characterize the products. The effects of the morphologies and Li+ doping concentrations on the luminescence intensities of the [CaY]F2:Ce3+/Tb3+ phosphors were explored, and the strongest luminescence intensity is obtained when the sample is cubic with (100) crystal faces and the doping concentration of Li+ is 0.25 mol%. Additionally, multicolor emission (blue → aquamarine blue → green) was obtained from [CaY]F2:Ce3+/Tb3+ phosphors via adjusting the doping concentration of Tb3+, which resulted from the Ce3+ → Tb3+ energy transfer behavior; the energy transfer here happened through a dipole-dipole mechanism. This work may result in the as-synthesized phosphors having great application potential in many optoelectronic device fields, such as in displays and multicolor lighting.

Full text of DOI:10.1039/d0dt02955f

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The morphology evolution, tunable down-
conversion luminescence, and energy transfer of
Cite this: Dalton Trans., 2020, 49,
15433
[CaY]F2 crystals doped with Li+/Ce3+/Tb3+†
Yini Mao, Zhiyi Wang, Rui Ye, Li Jiang, Shanshan Hu* and Jun Yang
*
Octahedral [CaY]F2 crystals with an average particle size of 1 μm were synthesized via a mild one-step
hydrothermal route without employing any surfactants. Various morphologies, including cubes, truncated
cubes, truncated octahedrons, and spheres, were achieved via manipulating the amount of EDTA used,
and a possible growth mechanism was proposed based on the surface energies of different crystal planes
and the influence of the surfactant. XRD, SEM, EDS, TEM, HRTEM, and PL analysis were used to character-
ize the products. The effects of the morphologies and Li+ doping concentrations on the luminescence
intensities of the [CaY]F2:Ce3+/Tb3+ phosphors were explored, and the strongest luminescence intensity is
obtained when the sample is cubic with (100) crystal faces and the doping concentration of Li+ is
0.25 mol%. Additionally, multicolor emission (blue → aquamarine blue → green) was obtained from [CaY]
F2:Ce3+/Tb3+ phosphors via adjusting the doping concentration of Tb3+, which resulted from the Ce3+
→
Received 23rd August 2020,
Accepted 22nd October 2020
Tb3+ energy transfer behavior; the energy transfer here happened through a dipole–dipole mechanism.
This work may result in the as-synthesized phosphors having great application potential in many opto-
electronic device fields, such as in displays and multicolor lighting.
DOI: 10.1039/d0dt02955f
rsc.li/dalton
approaches.13–17 It must be additionally emphasized here that
among these synthetic methods, hydrothermal methods are
1. Introduction
Rare-earth elements have gained the reputation of being favored by researchers due to their simple operation, low
“strategic elements of the 21st century” due to their excellent energy consumption, and strong controllability, mainly in
physical and chemical characteristics.1,2 Rare-earth functional terms of morphology and size.18 The aspects mentioned above
materials (with properties such as magnetism, hydrogen are the reasons why rare-earth luminescent materials play an
storage and catalytic abilities, luminescence, etc.) based on indispensable role in the fields of lighting, displays, biological
rare-earth elements also show unique properties, therefore, imaging, fluorescent labeling and medical detection.19
they are listed as one of the nine key materials needed for Furthermore, fluoride-based rare-earth luminescent materials
implementing a strong national manufacturing strategy.3 As a have become the focus of most researchers due to their lower
representative class of rare-earth functional materials, and as a phonon energies, higher refractive indices, better chemical
type of inorganic material,4 rare-earth luminescent materials stabilities, larger Stokes shifts, and lower crystallization temp-
of numerous types, including oxides, fluorides, phosphates, eratures.20 In the past five years, Dennis T. Klier et al. prepared
molybdates, tungstates, aluminates, etc.,5–10 exhibit excellent NaYF4:Yb,Er up-conversion nanoparticles via hydrothermal
optical performance, such as strong light absorption, high synthesis and characterized their temperature-dependent
conversion efficiencies, pure light colors, long fluorescent life- up-conversion luminescence with different amounts of Gd3+ as
times, and so on.11,12 So far, a variety of synthetic methods an additional dopant.14 M. Heise et al. obtained Eu3+-doped
have been proposed to prepare rare-earth luminescent metal fluorides (CaF2, SrF2, BaF2, and PbF2) with fluorite struc-
materials, including high-temperature solid-phase, hydro- tures via a solvent-free mechanochemical synthesis route
thermal,
solvothermal,
sol–gel,
and
co-precipitation (high-energy ball milling) and measured the luminescence life-
times of the as-prepared nanoparticles.20 Zhao and co-workers
synthesized Sr2ScF7:Ln3+ (Ln = Ce, Tb, Eu, Sm, Dy, Er, Tm, Ho,
School of Chemistry and Chemical Engineering, Southwest University,
No. 2 Tiansheng Road, Beibei District, Chongqing 400715, China.
E-mail: hushan3@swu.edu.cn, jyang@swu.edu.cn
and Yb) nanocrystals through a one-step hydrothermal method
and studied the growth mechanism of the synthesized nano-
crystals and their tunable up-/down-conversion luminescence
properties, including white-light emission.21
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt02955f
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It is worth noting that [CaY]F2 is also a member of the flu- Company (China). Ethylene diamine tetraacetic acid (EDTA)
oride material family, and it possesses the excellent properties was acquired from Aladdin Industrial Corporation (China). All
of these materials, but existing research relating to it is rela- of the chemical reagents listed above were used directly in
tively sparse, focusing only on two aspects: the improvement experiments. More particularly, rare-earth chlorides (LnCl3)
of synthesis methods and the up-conversion luminescence were self-prepared via dissolving the corresponding rare-earth
properties.22–24 For example, Liu and co-workers prepared rare- oxides in hot dilute HCl and then evaporating excess HCl; it
earth-doped [CaY]F2 nanoparticles based on the thermal was necessary to continuously add H2O2 solution during the
decomposition method reported by Capobianco et al., and dissolution of CeO2.
studied the up-conversion luminescence performance of [CaY]
F2:Yb3+,Er3+ phosphors at different Yb3+,Er3+ doping concen-
2.2. Synthesis
trations and the emission mechanism.22 Hu and co-workers
In the typical synthesis of the [CaY]F2 host, certain proportions
of CaCl2 and YCl3 were dissolved in 35 mL of deionized water
synthesized Tm3+–Yb3+ co-doped [CaY]F2 nanocrystals via uti-
lizing a hydrothermal method, and the effects of the calcina-
tion temperature, hydrothermal reaction temperature, and
rare-earth-ion doping concentration on the up-conversion
luminescence properties were investigated.23 Wang et al. pre-
pared a novel photocatalyst composed of N-doped TiO2 and
[CaY]F2:Yb3+,Tm3+ using a combination of dealloying and a
hydrothermal method.24 And our group reported the synthesis,
luminescence properties, and temperature-sensing capabilities
of [CaY]F2:Yb3+,Er3+ phosphor products in a previous research
work.25 However, not only has no one reported on the various
morphologies of [CaY]F2 crystals and their possible evolution
mechanism, but no research group has studied the intrinsic
reasons for the down-conversion luminescence performance of
[CaY]F2:Ce3+/Tb3+ phosphors doped with Li+ ions,22–26 which
means that it will be a novel and challenging task to under-
stand the crystal morphology evolution process and lumine-
scence mechanism in detail.
to form a homogeneous solution upon stirring at room temp-
erature. Afterwards, 0.7685 g of NaBF4 was added to the mixed
solution and then the mixture was fully stirred for half an
hour. Finally, the obtained white suspended solution was
transferred into a 50 mL stainless-steel Teflon-lined autoclave,
sealed, and heated at 200 °C for 24 h. After the reaction was
completed and the container was naturally cooled to room
temperature, the product was collected and further purified
using ethanol and deionized water several times, and then
dried at 60 °C for 12 h. The preparation processes for the other
products in the present work are basically the same, except the
material ratios of the rare-earth elements are adjusted. If an
additive was involved, it was added first, and then the other
raw materials were added according to the above process after
stirring for 30 minutes. It is worth noting that during Li+-
doping, the added ratio of Y3+ should be reduced accordingly.
2.3. Characterization
In the present work, [CaY]F2 crystals with regular octahedral
shape were first synthesized via a mild hydrothermal method,
and then the original evolution laws of crystal growth were
systematically analyzed based on the surface energy of each
crystal plane. Moreover, [CaY]F2 crystals with rich mor-
phologies, including cubes, truncated cubes, truncated octa-
hedrons, and spheres, were synthesized via adding different
amounts of EDTA. At the same time, the effects of EDTA on
the crystal growth process were explored in detail to reveal the
possible evolution mechanism of the different morphologies.
In addition, the effects of the doped amount of Li+ ions on the
luminescence performance of [CaY]F2:Ce3+/Tb3+ phosphors
were explored on the basis of lattice defects caused by charge
imbalance. Finally, the Ce3+ → Tb3+ energy transfer process
and mechanism in the [CaY]F2 host were investigated, with
rich luminescent colors arising at different Tb3+ doping
concentrations.
To identify the phases of the products, X-ray diffraction (XRD)
measurements were made using Purkinje General Instrument
MSAL-XD3 apparatus with Cu Kα radiation (λ = 0.15406 nm)
at a scanning rate of 8° min−1 in the 2θ range of 10°–90°.
Morphology analysis and energy dispersive spectrometry (EDS)
characterization were carried out using a field-emission scan-
ning electron microscope (FE-SEM, Hitachi, S-4800), and trans-
mission electron microscopy (TEM), high-resolution trans-
mission electron microscopy (HRTEM) and selected-area elec-
tron diffraction (SAED) pattern measurements were carried out
using a JEM-2100F microscope. PL excitation and emission
spectra were obtained using an F-7000 spectrophotometer
(Hitachi, Japan) equipped with a 150 W xenon lamp as the
excitation source.
3. Results and discussion
3.1. Phases and morphologies
2. Experimental section
2.1. Chemicals
Fig. 1 shows the XRD pattern, the EDS spectrum, and an SEM
image of pure [CaY]F2 crystals. [CaY]F2 was found to be cubic
ˉ
The rare earth oxides Y2O3, CeO2, and Tb4O7, with purity of phase with the space group Fm3m (225) (a = b = c = 5.5 Å and Z
99.99%, were purchased from Ganzhou Guangli High-tech = 4). As shown in Fig. 1a, the diffraction peaks of the sample
Materials Company (China). Calcium chloride (CaCl2), lithium match perfectly with the standard card (JCPDS No. 31-0293),
chloride (LiCl), sodium fluoroborate (NaBF4), and hydrochloric which means that the product has high purity.27 It can also be
acid (HCl) were obtained from Chuandong Chemical Reagents observed that the half widths of the diffraction peaks in the
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ticles are regular octahedrons with an average particle size of
about 1 μm. Such an appearance and size fully indicate that
the product has high crystallinity.28,29
To further explore the micro-morphology of the [CaY]F2
crystals, TEM and HRTEM analysis was performed on the pre-
pared products. Fig. 2a shows a TEM image of the pure [CaY]
F2 crystals, Fig. 2b shows a HRTEM image of a selected area
(marked with a red frame in Fig. 2a), and Fig. 2c indicates
images of an octahedron projected in different directions. It
can be seen from Fig. 2c that an octahedron can be projected
in different directions to obtain four different shapes, namely
a diamond (S1), a regular hexagon (S2), a square (S3), and a rec-
tangle (S4), which are projected in the direction of an edge, a
surface, a vertex, and the direction parallel to the surface,
respectively.30 The above-mentioned four shapes can be
observed in Fig. 2a, which proves once again that the [CaY]F2
crystals appear as regular octahedron. Also, the HRTEM image
Fig. 1 Pure [CaY]F2 crystals: (a) the XRD pattern, (b) the EDS spectrum,
and (c) an SEM image.
XRD pattern are narrow (Fig. 1a), which reflects the high crys-
tallinity of the prepared crystals.28 Moreover, we also calculated
the unit cell parameters of the sample using the measured
XRD data, and the result is a = b = c = 5.512 Å; this is basically
consistent with the theoretical value and can be obtained only
under conditions of extremely high purity.29 At the same time,
Fig. 1b also provides support for the conclusion obtained from
Fig. 1a that a high-purity product was synthesized. As shown
in Fig. 1b, the product consists of only three elements, Ca, Y,
and F, with the molar ratio of (Ca + Y) : F = 1 : 1.98, which is
approximately equal to the theoretical value of 1 : 2. Fig. 1c
Fig. 2 (a) A TEM image of pure [CaY]F2 crystals, (b) a HRTEM image
from the selected area, and (c) images of an octahedron projected in
shows an SEM image of the product, in which the crystal par- different directions.
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taken inside the red frame shows lattice spacings of 0.200 nm illustration of the morphology evolution of [CaY]F2 crystals
and 0.317 nm, which correspond to the interplanar spacings with varying amounts of EDTA. During the process of increas-
of the (220) and (111) crystal planes of cubic [CaY]F2, ing the amount of EDTA from 0.35 g to 0.5 g, the crystals show
respectively.
four different morphologies, which are reflected intuitively in
XRD patterns of products synthesized under different EDTA Fig. 3f. At the beginning, the amount of EDTA is equal to
conditions are shown in Fig. 3a. The diffraction peaks of the 0.35 g, and the morphology of the product is surprisingly cube
four samples all belong to cubic [CaY]F2 and there are no shaped. More interestingly, when the amount of EDTA is
other peaks, indicating that the purities of the four samples changed to 0.4 g, a morphology is obtained that is formed via
obtained are high.27 In addition, it can be found that the diffr- cutting off the eight corners of the cubes. Upon continuing to
action peaks corresponding to the (111) and (220) crystal increase the amount of EDTA to 0.45 g, the appearance of trun-
planes are strong and sharp, reflecting that samples with high cated octahedrons occurs. Finally, the product is approxi-
crystallinity can be obtained when the amount of EDTA added mately uniformly spherical under the conditions of 0.5 g of
during synthesis is within a certain range.31
EDTA. Although the appearance of the product changes as
Fig. 3b–e shows SEM images of [CaY]F2 crystals synthesized described above, the size is always on the micro-scale (Fig. 3b–
with different amounts of EDTA, and Fig. 3f shows a schematic e), which is completely consistent with the results obtained
from the XRD patterns (Fig. 3a).
Regular octahedron crystals were obtained without using
any additives (Fig. 1c), and another series of different mor-
phologies was obtained after adding EDTA during synthesis
(Fig. 3b–e). According to Gibbs–Wulff theory, the surface
energy of each crystal plane is different and has the rough
order of {hkl} ≫ {110} > {100} > {111}, where {hkl} represents
the high-index crystal plane.32 The growth rates of the crystal
planes also follow the above order, because the growth rate of
each crystal plane is positively related to its surface energy.33
Similarly, the final morphology of the crystal is also closely
related to the above-mentioned relationship; there is such a
regular pattern in the process of crystal growth that the crystal
planes with faster growth rates tend to disappear, while the
crystal planes with relatively slower growth rates, such as {110},
{100}, and {111}, tend to be retained.34,35 This is the rule
explaining surface crystal plane elimination, which is why the
geometric appearances of the crystals that we produced are
mostly composed of {110}, {100}, and {111} crystal planes.33–35
Fig. 4a shows the evolution of the crystal shape from a cube
(No. 1) to an octahedron (No. 4) due to the varying growth
rates of the (100) and (111) crystal planes. For polyhedral crys-
tals with regular shapes, the growth rate ratio R(100)/(111)
between the (100) and (111) crystal planes is usually used to
explain the evolution of crystal morphology. In our experi-
ments, we obtained cube (0.35 g of EDTA), truncated cube
(0.4 g of EDTA), truncated octahedron (0.45 g of EDTA), and
regular octahedron (without EDTA) shaped crystal particles,
which correspond to R = 0.58, R = 0.70, R = 1.15, and R = 1.73,
respectively.36 Fig. 4b shows a schematic diagram of how the
(100) and (111) crystal planes change from shape 1 to shape 4.
According to the direction of the arrow, the evolution process
of the (100) crystal plane from existence to absence and of the
(111) crystal plane from nothing to existence can be seen intui-
36
tively with the change of R(100)/(111)
.
Fig. 4c shows an illus-
tration of the evolution of crystal growth along the (100) and
(111) crystal plane directions. The law of conservation of the
crystal plane angle states that the angle (dihedral angle)
between any two crystal planes remains unchanged during the
growth of a crystal.37 As shown in Fig. 4c, the growth rate of
the (100) crystal plane is faster than that of the (111) plane,
Fig. 3 XRD patterns of [CaY]F2 crystals prepared with varying amounts
of EDTA (a), SEM images of [CaY]F2 crystals prepared with different
amounts of EDTA: (b) 0.35 g, (c) 0.4 g, (d) 0.45 g, and (e) 0.5 g, and a
schematic illustration of the morphology evolution of [CaY]F2 crystals
with varying amounts of EDTA (f).
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kinds of surfactants will cause preferential adsorption effects
on some crystal planes, resulting in a change in the order of
surface energies (growth rates).40,41 Here, the existence of
EDTA swapped the surface energy order of the (111) and (100)
crystal planes. When EDTA is absent, the composition of the
crystal surface is dominated by the (111) crystal plane with
the slowest growth rate, and there is no (100) crystal plane
(Fig. 4d). After adding EDTA during the synthesis process, it
will selectively adsorb on the (100) crystal plane of the [CaY]F2
crystals via electrostatic interactions, making the binding
energy of the (100) crystal plane smaller and correspondingly
reducing the growth rate.41,42 This will result in the exposure
of the (100) surface, and will contribute to the generation of
the final geometric appearance of the crystal.35 The proportion
changed depending on the amount of EDTA added (Fig. 4d).
When the amount of EDTA is equal to 0.35 g, the (100) crystal
plane is the main one (Fig. 4d). Then, the proportion of the
(100) crystal plane gradually decreases as the amount of EDTA
increases (Fig. 4d). Therefore, a product with desired mor-
phology can be obtained via adding an appropriate amount of
surfactant during the synthesis of the crystals.43
3.2. Down-conversion luminescence
3.2.1. Morphology-dependent luminescence properties.
Based on the morphologies of the samples, [CaY]F2:0.1%Tb3+
phosphors synthesized with 0 g of EDTA (octahedrons), 0.35 g
of EDTA (cubes), 0.4 g of EDTA (truncated cubes), 0.45 g of
EDTA (truncated octahedrons), and 0.5 g of EDTA (spheres)
were used as examples to explore the effects of morphology on
luminescence properties.
Fig. 5 shows the emission spectra of [CaY]F2:0.1%Tb3+
phosphors synthesized with different EDTA content levels, and
an illustration showing the corresponding morphology of each
sample. The morphologies of the products are almost the
same as those without Tb3+ doping, which indicates that
doping a relatively small amount of rare-earth ions will not
Fig. 4 (a) The evolution of the crystal shape from a cube (1) to an octa-
hedron (4) with the varying growth rates of the (100) and (111) crystal
planes, (b) a schematic diagram of the (100) and (111) crystal planes
changing from shape 1 to shape 4, (c) an illustration showing the evol-
ution of crystal growth along different directions, and (d) a schematic
diagram of the effects of EDTA on crystal morphology.
and the angle between the two is a constant value. The proportion
of the (100) crystal plane on the crystal surface (red) gradually
decreases and finally disappears, and the proportion of the (111)
plane on the crystal surface (black) gradually increases.37,38 This
trend can be clearly seen in the direction of the arrow, which
again points out that the final morphology of the crystal depends
on the relative growth rates of the crystal planes, and the pro-
portion of the crystal plane with a slower growth rate on the
surface of the crystal product is relatively large.33
However, it is worth noting that we did not directly obtain
the crystal morphology evolution pattern seen in Fig. 4a
during the preparation process, but instead obtained a variety
of products with different morphologies in the order seen in
Fig. 4d, which is due to the influence of the surfactant.39
Fig. 5 The emission spectra of [CaY]F2:0.1%Tb3+ phosphors syn-
thesized with different EDTA content levels; the corresponding mor-
During the process of crystal growth, the addition of some phologies of the samples are shown in the illustration.
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cause changes in the morphologies of the products.44 In explored. Fig. 7 shows the emission spectra of [CaY]F2:10%
addition, Y3+ and Tb3+ belong to the heavy rare-earth group, Ce3+ (Fig. 7a) and [CaY]F2:0.7%Tb3+ (Fig. 7b) phosphors as a
which is also an auxiliary factor in the maintenance of mor- function of Li+-doping concentration, and the insets show
phology.45 It can be seen from Fig. 5 that the cube-shaped charts of emission peak intensity as a function of Li+-doping
sample whose surface structure is dominated by the (100) concentration. It can be seen that the luminescence intensity
crystal plane has the largest luminescence intensity, and then of the phosphor is greatly affected by the Li+-doping concen-
the luminescence intensity gradually decreases with a decrease tration. As the doping concentration of Li+ increases, the
in the proportion of the (100) crystal plane on each crystal luminescence intensity first increases and then decreases, and
surface, indicating that the doped rare-earth light-emitting ions it is surprising to find that the luminescence intensities of the
(Tb3+) were mainly concentrated on the (100) crystal plane.46 In two phosphors both reach maximum values when the Li+-
other words, changes in the morphologies (the proportions of doping concentration is 0.25 mmol, that is, when Li+ replaces
crystal planes) of the samples caused by EDTA are the root just half of the total amount (0.50 mmol) of Ln3+ (Ln = Y, Ce,
cause of the different luminescence intensities of each sample.
Tb). This can be reasonably explained using Fig. 6c. In theory,
In order to further prove the above conclusions that Tb3+ when Li+ ions and Y3+/Ce3+/Tb3+ ions enter Ca2+ lattice sites at
was mainly doped into the (100) crystal plane, the interplanar a ratio of 1 : 1, the charge will be completely balanced to mini-
spacing values before and after doping are calculated and mize lattice defects.49 And, naturally, the luminescence inten-
shown in Table 1. We know that the doping of rare-earth ions sity at this time is the largest. Too much or too little Li+-ion
into the crystal lattice of the host will cause changes in the doping will reduce the luminescence intensity due to corres-
interplanar spacing21 (here, the change in the interplanar ponding lattice defects (Fig. 6a and d), so the luminescence
spacing after doping is represented by Δd), which can reflect intensity shows a quadratic function trend of rising and then
the ion-doping situation to a certain extent. The larger the Δd falling (the insets of Fig. 7 ).49
value, the greater the possibility of doping into the corres-
ponding crystal plane.46 According to the variation of Δd, it
3.3. Energy transfer behavior
can be seen that Tb3+ ions are indeed doped mainly into the
Fig. 8 shows the excitation and emission spectra of [CaY]
F2:10%Ce3+, [CaY]F2:0.7%Tb3+, and [CaY]F2:10%Ce3+,0.7%Tb3+
(200) crystal plane of [CaY]F2, which is also the (100) crystal
plane. Therefore, the results in Table 1 provide reasonable
phosphors. It can be seen from Fig. 8a that the emission spec-
support for the conclusions made from Fig. 5. For further
proof, we analyzed the quantum yields of samples with
different morphologies (Tables S1 and S2 in the ESI†), and the
results obtained are consistent with the experimental results
shown in Fig. 5.
3.2.2. Influence of the Li+-doping concentration. In [CaY]
F2, Ca2+ and Y3+ ions actually occupy the same lattice sites,
with the same structure as CaF2; however, Y3+ occupying Ca2+
lattice sites will cause an imbalance of positive and negative
charges, which will then cause lattice defects.47 Lattice defects
usually act as luminescence quenchers, and positive mono-
valent alkali metal cations are often required to act as charge
compensators in this case.47 Li+ not only has similar electro-
negativity as Y3+, but it also has a radius that is not much
different from Y3+, so charge compensation can be carried out
through doping with Li+ ions.48 As shown in Fig. 6a, cation
vacancies are introduced in order to maintain charge balance
in the case without Li+-ion doping, and charge compensation
can be performed after doping with Li+ ions (Fig. 6b).47,48
Based on the above analysis, the effects of the Li+-doping
concentration on down-conversion luminescence were
Table 1 [CaY]F2:0.1%Tb3+ interplanar spacing of the four main crystal
planes
(111)
(200)
(220)
(311)
Fig. 6 A schematic representation of charge compensation without Li+
(a) and with Li+ co-doping in [CaY]F2 (b), and with Li+ co-doping in [CaY]
F2:Ce3+/Tb3+ phosphors (c), and a schematic diagram of the introduc-
tion of A-vacancies (d).
[CaY]F2
3.170
3.169
0.001
2.751
2.744
0.007
1.943
1.941
0.002
1.658
1.657
0.001
[CaY]F2:0.1%Tb3+
Δd
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wavelength of 364 nm, the excitation spectrum is also com-
posed of a broad peak, which is generated during the tran-
sition of electrons from the ground state (2F5/2) to the excited
state (5d).50 As shown in Fig. 8b, upon excitation at 215 nm,
the emission spectrum of [CaY]F2:0.7%Tb3+ is composed of
multiple emission peaks in the range of 380–600 nm, with a
maximum at 546 nm (5D4
→
7F5). The remaining emission
7
7
peaks are located at 380 nm (5D3 → F6), 417 nm (5D3 → F5),
7
7
7
438 nm (5D3 → F4), 491 nm (5D4 → F6), 546 nm (5D4 → F5),
and 587 nm (5D4 7F4). At a monitoring wavelength of
→
546 nm, it is observed that the excitation spectrum mainly con-
sists of a strong excitation peak at 215 nm and some weak
peaks in the range of 230–380 nm, which are attributed to a
7
4f8-4f75d1 spin-allowed transition (7F6 → DJ) and spin-forbid-
den transition (7F6 → 9DJ), respectively, in Tb3+.51,52
By comparing Fig. 8(a) and (b), it can be found that the
emission spectrum of Ce3+ overlaps with the excitation spec-
trum of Tb3+ to a certain extent, from which it can be prelimi-
narily inferred that Ce3+ → Tb3+ energy transfer may occur in
the [CaY]F2 host.53 It can be seen from Fig. 8c that not only the
emission peak from Ce3+ but also the emission peak from Tb3+
are obtained at an excitation wavelength of 257 nm, revealing
that the Tb3+ ions are essentially excited through the Ce3+ ions.
More importantly, the emission peak from Tb3+ obtained
under these conditions is much stronger than that obtained
under excitation at 215 nm, which indicates that energy
has been successfully transferred from Ce3+ to Tb3+.53 It can
also be seen from Fig. 8c that the excitation spectrum of
[CaY]F2:10%Ce3+,0.7%Tb3+ monitored at 546 nm (Tb3+ emis-
sion) is similar to that monitored at 364 nm (Ce3+ emission),
with differences only in relative intensity; also, the excitation
spectrum of [CaY]F2:10%Ce3+,0.7%Tb3+ monitored at 364 nm
(Ce3+ emission) seen in Fig. 8c is the same as the excitation
spectrum of [CaY]F2:10%Ce3+ monitored at 364 nm (Ce3+ emis-
sion) in Fig. 8a, and there is no characteristic excitation
peak from Tb3+ ions, which proves that there is no energy
migration from the 4f-5d excited state of Tb3+ to the 4f-5d
excited state of Ce3+. Since then, we explored the luminescence
properties of a series of Ce3+ and Tb3+ co-doped phosphors.
Fig. 9a shows the emission spectra of [CaY]F2:10%Ce3+,x%Tb3+
(x = 0, 0.3, 0.5, 0.7, 0.9, 1.3, and 1.5) phosphors. The doping
concentration of Ce3+ is fixed at 10%. With an increase in
Tb3+, the intensity of the characteristic peak at 546 nm (Tb3+,
Fig. 7 Emission spectra of [CaY]F2:10%Ce3+ (a) and [CaY]F2:0.7%Tb3+ (b)
phosphors as a function of Li+ doping concentration; the insets show
charts of emission peak intensity as a function of Li+-doping concentration.
5D4
→
7F5) gradually increases and reaches a maximum
at x = 0.7, then gradually weakens due to concentration
quenching effects.54 Correspondingly, the intensity of the
characteristic peak at 364 nm (Ce3+, 5d → 4f) continued to
decrease due to the occurrence of Ce3+ → Tb3+ energy
transfer.55 Fig. 9b shows the CIE chromaticity coordinates
of the phosphors mentioned above. When the Tb3+ concen-
tration was changed from 0% to 1.5%, the chromaticity co-
ordinates methodically changed from blue (0.1589, 0.0496)
to aquamarine blue (0.2268, 0.3349), and finally to green
Fig. 8 The excitation and emission spectra of [CaY]F2:10%Ce3+ (a),
[CaY]F2:0.7%Tb3+ (b), and [CaY]F2:10%Ce3+,0.7%Tb3+ (c), respectively.
trum of Ce3+ consists of a broad and strong peak at 364 nm (0.2516, 0.5126), which indicates that the regulation of lumi-
when 257 nm was used as the excitation wavelength, which is nescent color can be achieved during the energy transfer
derived from a Ce3+ transition from 5d to 4f. At the monitoring process.
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Fig. 10 The relative emission intensities of Ce3+ at 364 nm and Tb3+ at
546 nm, and the ET efficiency of Ce3+ → Tb3+ in [CaY]F2:10%Ce3+,x%
Tb3+ phosphors.
1.2, and 1.5) were measured to further understand the excited
state dynamics of the energy transfer process (Fig. S1†). The
decay time of Ce3+ decreases monotonically with an increase
in Tb3+ concentration, which proves that efficient energy trans-
fer from Ce3+ to Tb3+ occurred.
Then, the Ce3+ → Tb3+ ET mechanism was explored.
According to the literature, the ET mechanism is closely
related to the critical distance (RC) between the sensitizer and
activator ions. When RC ≥ 4 Å, ET mainly occurs through elec-
tric multipolar interactions, and when RC < 4 Å, exchange
interactions are taken to be dominant.57 The following
formula can be used to calculate the critical distance RC:57
Fig. 9 (a) A series of emission spectra from [CaY]F2:10%Ce3+,x%Tb3+
and (b) the corresponding CIE chromaticity coordinates of [CaY]F2:10%
Ce3+,x%Tb3+ phosphors (x = 0, 0.3, 0.5, 0.7, 0.9, 1.3, and 1.5).
ꢀ
ꢁ
1=3
3V
4πχcN
RC ꢁ 2
For a deeper understanding of the process of Ce3+ → Tb3+
energy transfer, we calculated the energy transfer efficiencies
of [CaY]F2:10%Ce3+,x%Tb3+ (x = 0, 0.3, 0.5, 0.7, 0.9, 1.3, and
1.5) phosphors via the following formula:56
where π is a constant, χc represents the total concentration of
Ce3+ and Tb3+ when the luminescence intensity of Ce3+ is half
that in the absence of Tb3+ (it is estimated that the concen-
trations of Ce3+ and Tb3+ here are 10% and 0.7%, respectively),
V represents the volume of the unit cell, and N is the number
of molecules in a unit cell. For [CaY]F2, substituting V = 166.4,
N = 4, and χc = 0.107 into the formula results in RC = 9.06 Å,
which is >4 Å, so Ce3+ → Tb3+ energy transfer here occurs
mainly through electric multipolar interactions. Electric multi-
polar interactions can be further divided into three types:
dipole–dipole (D–D), dipole–quadrupole (D–Q), and quadru-
pole–quadrupole (Q–Q) interaction.58 The following relation-
ship can be used to make an accurate judgment:59
IS
ηT ¼ 1 ꢀ
IS0
where IS and IS0 in the formula indicate the emission intensity
of Ce3+ in the presence and absence of Tb3+, respectively, and
ηT is the energy transfer efficiency. According to the calculation
results, it can be concluded that the energy transfer efficiency
can reach 87.1% in the experimental range. Fig. 10 reveals the
relative emission intensity trends of Ce3+ (364 nm) and Tb3+
(546 nm), and the energy transfer efficiency change with the
Tb3+ concentration, which can more intuitively reflect the con-
clusions obtained in Fig. 9a. Before the emission of Tb3+ at
ηS0
ηS
n
3
/ C
546 nm reached its strongest value, the energy transfer where ηS0/ηS can be approximated by IS0/IS, which is the ratio
efficiency increased rapidly from 0% to 60.2%, and then the of the Ce3+ emission intensity in the absence and presence of
growth rate gradually decreased, and the efficiency slowly Tb3+, C represents the total concentration of Ce3+ and Tb3+
reached 87.1%. The luminescence decay curves of Ce3+ emis- ions, and n = 6, 8, 10 correspond to the above-mentioned D–D,
sion from [CaY]F2:10%Ce3+,x%Tb3+ (x = 0, 0.3, 0.5, 0.7, 0.9, D–Q, and Q–Q interactions, respectively. The linear relation-
15440 | Dalton Trans., 2020, 49, 15433–15442
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This project is financially sponsored by the Natural Science
Foundation of Chongqing, China (cstc2020jcyj-msxmX0332).
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