Host composition dependent tuneable morphology and luminescent property of the CaXSrYBa1?X?YWO4:RE3+(RE=Pr, Ho, and Er) phosphors

Article abstract of DOI:10.1016/j.jallcom.2017.01.255

Novel Pr³⁺, Ho³⁺, and Er³⁺single-doped Ca_XSr_YBa_1?X?YWO₄phosphors were successfully prepared via a facile hydrothermal method. The hydrothermal process was conducted in aqueous condition without the use of any organic solvent, surfactant, or catalyst. The effects of doping-host composition and RE³⁺doping concentration on the emission intensity were investigated to optimize the luminescent properties of Ca_XSr_YBa_1?X?YWO₄:RE³⁺phosphors. Experimental results demonstrate that the morphologies of the products vary gradually and regularly with the change of the host composition, in which the anisotropic growth played a key role. Moreover, the down-conversion emissions of Pr³⁺, Ho³⁺, and Er³⁺in Ca_XSr_YBa_1?X?YWO₄host were successfully realized. After optimizing the luminescent properties, Ca_0.4Sr_0.6WO₄:0.01Pr³⁺, Ca_0.8Sr_0.2WO₄:0.01Ho³⁺, and Ca_0.6Sr_0.4WO₄:0.005Er³⁺exhibited optimal luminescent property, with orange, yellowish-green, and green emissions, respectively.

Full text of DOI:10.1016/j.jallcom.2017.01.255

Accepted Manuscript
Host composition dependent tuneable morphology and luminescent property of the
3+
Ca Sr Ba
WO :RE (RE=Pr, Ho, and Er) phosphors
4
X
Y
1-X-Y
Linlin Li, Hong-Yue Wu
PII:
S0925-8388(17)30289-X
DOI:
10.1016/j.jallcom.2017.01.255
JALCOM 40622
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 November 2016
Revised Date: 17 January 2017
Accepted Date: 22 January 2017
Please cite this article as: L. Li, H.-Y. Wu, Host composition dependent tuneable morphology and
3+
luminescent property of the Ca Sr Ba
Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.01.255.
WO :RE (RE=Pr, Ho, and Er) phosphors, Journal of
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X
Y
1-X-Y
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Host composition dependent tuneable morphology and luminescent property of the
CaXSrYBa1-X-YWO4:RE3+ (RE=Pr, Ho, and Er) phosphors
Linlin Lia*, Hong-Yue Wub
aDepartment of Chemistry, Tonghua Normal University, Tonghua 134002, PR China
bCollege of Chemistry, Jilin University, Changchun 130026, PR China
*Corresponding author. Tel.: +86 0435 3208077.
E-mail address: mumuandchacha@163.com
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Abstract
Novel Pr3+, Ho3+, and Er3+ single-doped CaXSrYBa1-X-YWO4 phosphors were successfully
prepared via a facile hydrothermal method. The hydrothermal process was conducted in aqueous
condition without the use of any organic solvent, surfactant, or catalyst. The effects of doping-host
composition and RE3+ doping concentration on the emission intensity were investigated to
optimize the luminescent properties of CaXSrYBa1-X-YWO4:RE3+ phosphors. Experimental results
demonstrate that the morphologies of the products vary gradually and regularly with the change of
the host composition, in which the anisotropic growth played a key role. Moreover, the
down-conversion emissions of Pr3+, Ho3+, and Er3+ in CaXSrYBa1-X-YWO4 host were successfully
realized.
After
optimizing
the
luminescent
properties,
Ca0.4Sr0.6WO4:0.01Pr3+,
Ca0.8Sr0.2WO4:0.01Ho3+, and Ca0.6Sr0.4WO4:0.005Er3+ exhibited optimal luminescent property,
with orange, yellowish-green, and green emissions, respectively.
Keywords: Hydrothermal method; Doping-host combination; Tungstate; Phosphor
1. Introduction
Rare-earth (RE) ions have received enormous attention based on their unique electronic and
optical characteristics developed from the different arrangement of their 4f electrons [1].
Therefore, most rare-earth elements are often doped into many hosts and used as light-emitting
materials and laser materials. Luminescence of rare-earth ions can be categorized as
down-conversion and up-conversion emission processes, based on the mechanism of luminescence
[2]. Among them, the down-conversion process is the conversion of higher energy photons into
lower energy photons, which obeys Stokes' law [3]. The up-conversion process can emit a higher
energy photon. In this process, the sequential absorption of two or more photons leads to the
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emission of light, which is well known as an anti-Stokes emission process under near-infrared
(NIR) or infrared light (IR) (low energy) excitation [4,5]. Accordingly, phosphors can be divided
into down-conversion phosphors and up-conversion phosphors, which are usually distinguished by
the doped rare-earth ions. The commonly used down-conversion activators include Eu3+, Tb3+,
Sm3+, and Dy3+, which have abundant emission peaks in the whole visible region under UV light
excitation [6-8]. Pr3+, Er3+, Tm3+, and Ho3+ ions are the most common activators that generate
up-conversion luminescence because of the ladder-like arrangement of their energy levels [9,10].
Among them, Er3+ shows the highest up-conversion efficiency, which could be due to the similar
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energy gaps from I15/2 to I11/2 and I11/2 to F7/2 [11]. However, in up-conversion process, some
additional sensitizers must be added to donate their absorbed energy to the above activators, and
Yb3+ ion is a particularly suitable sensitizer for these activator ions [12,13]. Pr3+, Ho3+, and Er3+
ions also can be excited by UV, near-UV, and blue light and then emit their nature light in the
visible region. However, reports on this subject are still limited. As far as we know Tamrakar et al.
[14] and Upadhyay et al. [15] prepared a Gd2O3:Er3+ phosphor via the combustion synthesis
method. Xin et al. [16] used solid-state method to synthesize a versatile phosphor
Sr10[(PO4)5.5(BO4)0.5](BO2):Pr3+. Zou et al. [17] developed a novel red emitting long persistent
luminescent phosphor Ca2Ga2GeO7:Pr3+ by mean of solid state reaction. All the above works have
studied the down conversion properties of Er3+ and Pr3+.
Metal tungstates have emerged as an important family of luminescent materials owing to their
excellent spectroscopic properties, chemical stabilities [18], and potential applications as a catalyst
[19], as a scintillator [20], for production of laser and phosphors and optical fibers [21-23]. MWO4
(M=Ca, Sr, and Ba) possess the tetragonal scheelite structure with the space group I41/a. In these
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2-
compounds, W6+ is coordinated by four O2- at a tetrahedral site, which makes WO4 relatively
stable. M2+ ions are coordinated by eight O2- from near four WO42- with S4 symmetry [24]. As well
known, WO42− group has strong absorption in the UV region, so the energy transfer process from
WO42− group to rare-earth ions can easily occur, which can greatly enhance the external quantum
efficiency of rare-earth ions doped materials [25]. Therefore, the MWO4 could be a suitable host
for Pr3+-, Ho3+-, and Er3+-activated phosphors. For instance, the emission spectrum of pure
CaWO4 shows an intense broadband ranging from 300 to 600 nm with a maximum at
approximately 420 nm, and the blue emission band of CaWO4 is due to its intrinsic emission. A
number of studies concerning alkaline earth metal ions replace with each other (entirely or partly)
in the same host, and some results show that the optical property of RE ions can been changed
[24]. A variety of methods, such as sol-gel process, co-precipitation, hydrothermal and
sonochemical methods, have been developed to prepare tungstates. Particularly, the hydrothermal
method has been proven to be an efficient and convenient technique for the large-scale production
of inorganic materials with diverse morphologies and regular particle sizes under practicable
environments [26]. As a typical solution-based approach, this route is based on solution-phase
colloidal chemistry, in which the reactants can be homogeneously mixed at molecular or ion level
in solutions, thereby allowing the products to be precisely tuned in terms of composition, shape,
and size [3]. Alkaline earth metal ions can replace each other in the same host due to the similar
chemical properties and small ionic radius difference. Thus, CaXSrYBa1-X-YWO4 materials can be
prepared by hydrothermal method.
The emission intensity of RE3+-doped phosphors could be manipulated by some conventional
factors, such as doping-host composition, particle size, shape, phase, electric-field, doping
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concentration, and so on. According to the reports, the rare-earth ions are embedded into the host
lattice in low concentration (on the whole ≤ 5 mol %) to reduce the concentration quenching of
luminescence, and not changing the crystal structure of the host material [27,28]. Generally, an
optimal doping concentration exists, at which the emission intensity has the maximum value.
Many studies also showed that the emission intensities of RE3+-doped phosphors are dependent on
their size. Larger phosphors will show stronger total luminescence emission and a sharp increase
in quantum yield. Less activator ions are found on the surface that suffer from the ligand induced
or surface defect induced quenching of luminescence, as larger phosphors have smaller surface
area, eventually resulting in less non-radiative relaxations of the surface activator ions [3,29,30].
As for the host composition, which can form solid solutions by adjusting the cations or anions of
the host compound [31], the sub-lattice structures around the luminescent center ions (RE3+) will
be expected to be somewhat diverse. Therefore, the luminescent intensity of rare-earth ions should
be enhanced. Unfortunately, reports about the effect of doping-host composition on the
luminescent property are scarce and unsystematic. Because the host composition also can affect
other factors, such as particle size, shape, phase, and electric-field, and these effects are
complicated and hard to control.
Based on the above investigations and motivated by the attempt to develop novel efficient
phosphors, series of Pr3+, Ho3+, and Er3+ single-doped CaXSrYBa1-X-YWO4 phosphors were
prepared via a facile hydrothermal method. The morphologies of the products vary gradually and
regularly with the change of the host composition. The down-conversion emissions of Pr3+, Ho3+,
and Er3+ in CaXSrYBa1-X-YWO4 host were successfully realized in our work. Additionally, after
optimizing their luminescent properties, Ca0.4Sr0.6WO4:0.01Pr3+, Ca0.8Sr0.2WO4:0.01Ho3+, and
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Ca0.6Sr0.4WO4:0.005Er3+ exhibited the optimal luminescent property.
2. Experimental
2.1. Sample preparation
CaXSrYBa1-X-YWO4:RE3+ (RE=Pr, Ho, and Er) phosphors were synthesized via a facile
hydrothermal method. Na2WO4·2H2O (A.R.), Ca(NO3)2·4H2O (A.R.), Sr(NO3)2 (A.R.), Ba(NO3)2
(A.R.), Pr6O11 (99.99 %), Ho2O3 (99.99 %), and Er2O3 (99.99 %) were used as starting materials.
Rare-earth nitrate aqueous solutions were produced by dissolving the corresponding rare-earth
oxides in nitric acid solution under stirring. The resulting solutions were evaporated with heating,
and then the residues were again dissolved in distilled water, transferred to a volumetric flask, and
diluted. All the doping ratios of Pr3+, Ho3+, and Er3+ were molar in our experiments. In a typical
synthesis process, the stoichiometric weights of M(NO3)2·nH2O (M=Ca, Sr, and Ba) were
dissolved into 20 mL deionized water, and then the required volume of RE(NO3)3 solution was
added into the above solution, resulting in the formation of the mixed colorless solution of
RE(NO3)3 and M(NO3)2 (labeled as M(NO3)2:RE3+). Meanwhile, 3.0 mmol of Na2WO4·2H2O was
dissolved into 15 mL deionized water. Then, under vigorous stirring, the above M(NO3)2:RE3+
mixed solution was slowly dropped into Na2WO4 solution. White precipitate was immediately
observed in the glass beaker. The vigorous stirring was maintained for 30 min. Finally, the
precursor solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was
subsequently sealed and maintained at 120 °C for 12 h. After that, the autoclave was allowed to
cool down to room temperature naturally. The resulted precursor was separated by centrifugation,
washed by deionized water and absolute ethanol for several times, and then dried in vacuum at
70 °C. At last, the final sample was obtained.
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Series of phosphors with different proportion of Ca, Sr, and Ba were synthesized to optimize the
luminescent properties of CaXSrYBa1-X-YWO4:RE3+. These phosphors are marked as No. 1-21, and
showed in table 1. Total ionic radius of alkaline earth ions is X r(Ca2+) + Y r(Sr2+) + (1-X-Y)
r(Ba2+) for CaXSrYBa1-X-YWO4 host, and all the total ionic radii were listed in table 1.
2.2. Characterization
The samples were examined by X-ray diffraction (XRD) measurements performed on a Rigaku
D/max-II B X-ray diffractometer with monochromatic Cu Kα radiation. Morphology and size of
the as-synthesized products were characterised by field-emission scanning electron microscopy
(FE-SEM, S-4800, Hitachi), and employed the accelerating voltage of 5 kV. Constituent elements
of the as-prepared phosphors were detected by energy dispersive spectroscopy (EDS) using an
X-ray detector attached to the FE-SEM instrument. Detailed morphological and structural analyses
were observed by using a Hitachi 8100 transmission electron microscope (TEM, Hitachi, Tokyo,
Japan). Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi
F-7000 fluorescence spectrophotometer equipped with a 150 W Xe lamp as the excitation source.
Photoluminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital
Oscilloscope (1 GHz) using a tuneable laser as the excitation. All the measurements were
performed at room temperature.
3. Results and discussion
3.1. Phase structure and composition
Measurements on the powder X-ray diffraction for all the samples were performed to verify the
phase purity, and all the diffraction peaks can be well indexed to the standard data of pure
single-phase scheelite structure CaWO4, SrWO4, or BaWO4. The XRD patterns of all samples are
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almost the same as each other, hence only the typical XRD patterns of Er3+-doped samples No.
7-11 are shown in Fig. 1a, as well as the standard data of SrWO4 (JCPDS No. 08-0490). It can be
seen that the diffraction patterns of CaXSrYBa1-X-YWO4:0.005Er3+ (X=0.2; Y=0.8, 0.6, 0.4, 0.2, 0)
can be indexed with the pure tetragonal phase (space group: I41/a), and they are mostly consistent
with the standard data of scheelite structured SrWO4, except that there is a discernible shift in the
position of the diffraction peaks, which can be explained by the difference in the total ionic radii
of alkaline earth ions. The ionic sizes (CN=8) of these alkaline earth ions follow the trend: r(Ca2+)
(1.12 Å) < r(Sr2+) (1.26 Å) < r(Ba2+) (1.42 Å) [32]. With increasing Ba2+ ions concentration, the
total ionic radii increased as shown in table 1, and the diffraction peaks of
CaXSrYBa1-X-YWO4:0.005Er3+ shift to a lower degree, in accordance with the Bragg equation:
λ=2dsinθ. From the schematic illustration of the tetragonal phase structure Ca/Sr/BaWO4 (Fig. 1b),
it can be seen that the central W6+ is coordinated by four equivalent O2- at a tetrahedral site, while
M2+ (M=Ca, Sr, or Ba) ions share corners with eight adjacent O2- from near four WO42- without an
inversion centre. Considering the smaller ionic radius difference between M2+ and Er3+, the doped
Er3+ ions are expected to randomly occupy the Ca/Sr/Ba sites in the host lattice, thus no peaks
corresponding to any other phases or impurities can be detected, which confirmed by the XRD
results. Moreover, the above results also confirm that these products are pure substance, rather
than a mixture of CaWO4, SrWO4, and BaWO4.
EDS analysis of some scientifically selected products was performed to verify the presence of
RE3+ and determine the chemical composition of the as-prepared phosphors. However, this
analysis cannot be performed on the samples which contain both Sr and W, because the Lα signal
of Sr overlaps with the Mα signal of W in the energy scale of about 1.9 keV. So only the EDS
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spectra of Ca0.8Ba0.2WO4:0.01Ho3+, Ca0.6Ba0.4WO4:0.01Pr3+, Ca0.4Ba0.6WO4:0.005Er3+, and
Ca0.2Ba0.8WO4:0.005Er3+ phosphors are presented in Fig. 2. These EDS data confirm the presence
of calcium (Ca), barium (Ba), tungsten (W), oxygen (O), and the corresponding rare-earth element
(Ho, Pr, and Er). With increasing Ba2+ concentration, it also can be seen clearly that the intensity
of Ca peak decreased while that of Ba peak increased. The approximate molar ratios of Ca and Ba
extracted from the EDS data are about 3.8, 1.5, 0.66, and 0.24 respectively, which is considerably
closer to the theoretical value of 4.0, 1.5, 0.67, and 0.25. The results reveal that the obtained
phosphors are consistent with the originally designed composition, and the phosphors with
different proportion of Ca, Sr, and Ba can be successfully prepared by the hydrothermal method.
3.2. Morphological analysis
The morphology and size of a series of 0.5% Er3+-doped CaXSrYBa1-X-YWO4 were investigated
by FE-SEM. The SEM images of Er3+-doped single-component CaWO4, SrWO4, and BaWO4 are
presented in Fig. S1 (in the Supporting Information). In order to write concisely, only the chemical
formulas of the host are employed in this section. From Fig. S1a, it can be seen clearly that the
as-prepared CaWO4 shows the microsphere-like shape with an average diameter of about 5 ꢀm.
SEM image of SrWO4 shown in Fig. S1b reveals that the product is composed of abundant
hamburger-like microstructures, and the diameter of these micro-hamburgers is about 6 ꢀm.
Moreover, each micro-hamburger has a joint boundary dividing the particle into two hemispheres
or four quarter spheres like SrWO4. This phenomenon can be assigned to crystal twinning and
splitting [33]. Fig. S1c displays the morphology of the BaWO4. It can be seen clearly that the
product is composed of uniform micro-cylinders, and there are some sawtooth at both ends of the
cylinder. These cylinders are about 2 ꢀm in length and 1 ꢀm in diameter.
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SEM images of SrWO4:0.0025Er3+, SrWO4:0.005Er3+, SrWO4:0.01Er3+, and SrWO4:0.015Er3+
were displayed in Fig. S2. It can be observed from Fig. S2 that with the increase of Er3+ ions
doping concentration the shape and size of these samples are almost the same. It is because that
the Er3+ ions have been embedded into the SrWO4 lattices and the doping concentrations of Er3+
ions were so few that can't change the crystal structure and morphology of the sample.
SEM images of Er3+-doped double-component CaXSr1-XWO4 (X=0.8, 0.6, 0.4, and 0.2) and
CaWO4, SrWO4 for comparison are shown in Fig. 3. The images show that the morphologies of
these samples change from microsphere-like to hamburger-like shape gradually with increasing
Sr2+ ions concentration. As shown in Fig. 3b and c, both the Ca0.8Sr0.2WO4 and Ca0.6Sr0.4WO4
present a microsphere-like shape similar to that of CaWO4, the average diameters of them are
about 3 ꢀm and 6 ꢀm, respectively. With further increase of Sr2+ ions concentration, the joint
boundary begins to appear and the joint boundary becomes more and more obvious as shown in
Fig. 3d and e.
Fig. 4 shows the SEM images of Er3+-doped double-component CaXBa1-XWO4 (X=0.8, 0.6, 0.4,
and 0.2), as well as CaWO4, BaWO4 for comparison. It can be observed in Fig. 4 that the
morphologies of these particles change from microsphere-like to microcylinder-like shape
regularly with increasing Ba2+ ions concentration. Ca0.8Ba0.2WO4 reveals a near microsphere-like
shape with inhomogenous size ranging from 2 ꢀm to 3 ꢀm, as shown in Fig. 4b. SEM image of
Ca0.6Ba0.4WO4 in Fig. 4c shows that the middle parts of these microspheres begin to crack. With
further increasing Ba2+ ions concentration, the micro-cylinders with about 3 ꢀm in length and 2
ꢀm in diameter appeared, as shown in Fig. 4d. It can be clearly observed that the Ca0.2Ba0.8WO4
sample is also composed of micro-cylinders (Fig. 4e) and closer to the shape of BaWO4. The
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average length of these micro-cylinders is about 3 ꢀm, and the average diameter is about 2 ꢀm.
Moreover, it can be obviously seen that the sawtooth at the ends of the micro-cylinder becomes
more and more obvious with increasing Ba2+ ions concentration.
Fig. 5 depicts the SEM images of Er3+-doped double-component SrXBa1-XWO4 (X=0.8, 0.6, 0.4,
and 0.2) and SrWO4, BaWO4 for comparison. By comparing the morphologies, it can be seen that
the shapes of these particles change from hamburger-like to cylinder-like gradually with
increasing Ba2+ ions concentration. Similar to the shape of SrWO4, Sr0.8Ba0.2WO4 shows a
hamburger-like shape with an average diameter of about 3 ꢀm (Fig. 5b). As shown in Fig. 5c, the
morphology of Sr0.6Ba0.4WO4 has a great change, and shows a dumbbell-like microstructure. The
length of such Sr0.6Ba0.4WO4 dumbbell is about 4 ꢀm, and the width of the two terminals is about
3 ꢀm, while the middle parts come near to 2 ꢀm. It can be seen clearly from Fig. 5d that the
as-prepared Sr0.4Ba0.6WO4 shows the micro-cylinder shape with some sawtooth at both ends of the
cylinder, and the average length of these cylinders is about 4 ꢀm. With increasing Ba2+ ions
concentration, the morphologies are more and more close to the micro-cylinder. The SEM image
in Fig. 5e illustrates that the Sr0.2Ba0.8WO4 sample is composed of a uniform cylinder-like
structure with a length of about 3 ꢀm.
Fig. 6 displays the SEM images of Er3+-doped three-component Ca0.6Sr0.2Ba0.2WO4,
Ca0.4Sr0.4Ba0.2WO4,
Ca0.4Sr0.2Ba0.4WO4,
Ca0.2Sr0.6Ba0.2WO4,
Ca0.2Sr0.4Ba0.4WO4,
and
Ca0.2Sr0.2Ba0.6WO4 samples, respectively. These images clearly indicate that the final products
tend to form the similar morphology to the single component CaWO4, SrWO4, or BaWO4,
depending on their main component. Fig. 6a gives the SEM image of Ca0.6Sr0.2Ba0.2WO4, it can be
seen that the as-prepared product consists of microspheres with an average diameter of about 2.5
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ꢀm. In the case of Ca0.4Sr0.4Ba0.2WO4 sample (Fig. 6b), the morphology is also the
microspheres-like shape with about 3 ꢀm in diameter. From Fig. 6c, it can be seen that with
increasing the Ba2+ ions concentration, the intermediate position of the Ca0.4Sr0.2Ba0.4WO4
microspheres has an obvious crack. Similar to the morphology of SrWO4, the microstructure of
Ca0.2Sr0.6Ba0.2WO4 is near the hamburger-like shape, and each micro-hamburger has a joint
boundary dividing the particles into two hemispheres, as shown in Fig. 6d. The morphology of
Ca0.2Sr0.4Ba0.4WO4 is converted to uniform dumbbell-like microstructure. The length of such
Ca0.2Sr0.4Ba0.4WO4 dumbbell is about 3.5 ꢀm, and the width of the two terminals is about 3 ꢀm,
while the middle parts come near to 2 ꢀm. From Fig. 6f, it can be seen that the Ca0.2Sr0.2Ba0.6WO4
product exhibits cylinders-like shape with some sawtooth at both ends of the cylinder. The low
magnification SEM images of typical three-component phosphors are also indicated in Fig. S3. It
can be seen that these samples are consists of almost mono-dispersed and homogeneous particles
with narrow size distribution.
TEM and selected area electron diffraction (SAED) analysis of the representative Er3+-doped
three-component products was performed to obtain the detailed morphology and crystal structure
of the above deferent shapes. The TEM images in Fig. 7 clearly illustrate that both the
morphologies and sizes of these six samples are consistent with that observed from the SEM
images which shown in Fig. 6. All the shapes of SAED patterns (inserted in TEM images) are
fully concentric rings, implying that the products are polycrystalline. The interplanar spaces were
calculated from the diameters of the rings, and compared with those in the JCPDS standard.
Moreover, all SAED patterns show the same (1 1 2), (2 0 4), (1 1 6), and (3 1 6) planes which in
good accordance with the XRD results.
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According to the above results, it can be seen that each sample have only one shape, also
proving that the products are pure substance, rather than a mixture of CaWO4, SrWO4, and
BaWO4. Moreover, in our experiments, the morphologies of the products vary gradually and
regularly with the change of the host composition when other conditions are kept constant, which
suggests that the composition plays a crucial role in the formation of CaXSrYBa1-X-YWO4 with
different morphologies. It is believed that the morphologies of the final products mainly relate to
the intrinsic structure of the sample, the growth kinetics during the reaction, and the surface charge
distribution of the crystal nucleus in the growth process [34-36]. Based on the experimental
phenomena and the above results, we can carefully conclude that the formation of the above
morphologies can be explained through three consecutive stages. Firstly, fast nucleation: at the
initial stage, direct mixing of two solutions which containing Ca/Sr/Ba(NO3)2 and Na2WO4, white
precipitate was immediately observed, due to the simple precipitation reaction. The formed tiny
nuclei of Ca/Sr/BaWO4 can serve as precursors of the crystal. Secondly, dissolution and
re-crystallization: when the precipitate was transferred into the autoclave, under the hydrothermal
2-
condition, the original nuclei dissolved and released Ca2+, Sr2+, Ba2+, and WO4 gradually. This
step made the next reaction of Ca2+, Sr2+, Ba2+ and WO42- slow down, and results in the formation
of different shape Ca/Sr/BaWO4 particles. Because the crystal faces of the samples with different
proportion of Ca, Sr, and Ba have different growth rate. Even in on sample, the growth rates of
different faces are different, which is called anisotropic growth. Eventually, further growth: each
particle aggregated during the reaction process, which driven by the electrostatic interaction and
the minimization of the total energy of the system, and finally result in the formation of the above
different morphologies. Anisotropic growth is believed to be an important process to form diverse
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morphologies [37], and the different proportion of Ca, Sr and Ba is a sensitive factor in deciding
the anisotropic growth, so the morphologies change with the host composition.
3.3. Luminescence properties of Pr3+, Ho3+ and Er3+ single-doped SrWO4 phosphors
Fig. 8a illustrates the excitation (EX) and emission (EM) spectra of SrWO4:0.01Pr3+ phosphor.
Monitored at 644 nm, the excitation spectrum consists of an intense broadband and three sharp
excitation peaks. The wide band ranging from 250 to 320 nm is responsible for the charge transfer
band (CTB) of O2−→W6+ within the tungstate group. The strong CTB is favourable for the
effective energy transfer and luminescence of Pr3+. The sharp excitation peaks which located at
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448, 472, and 485 nm are corresponded to H4→3P2, H4→3P1, and H4→3P0 transitions of Pr3+,
respectively [38]. It can be seen that the SrWO4:Pr3+ phosphor can be effectively excited by blue
light. Under the excitation of 448 nm light, the corresponding emission spectrum exhibits a series
of emission peaks belonging to the characteristic transitions of Pr3+. Seven major emission peaks
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at about 528, 553, 600, 616, 644, 682, and 736 nm are assigned to the P1→3H5, P0→3H5,
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3
3
3
1D2→3H4, P0→3H6, P0→3F2, P0→3F3, and P0→3F4 transitions, respectively [39]. The strong
sharp red emission peak at 644 nm should be of value in improving the colour rendering index.
The measured excitation and emission spectra of SrWO4:0.01Ho3+ phosphor are presented in
Fig. 8b. The excitation spectrum was obtained by monitoring 657 nm emission which
corresponding to the 5F5→5I8 transition of Ho3+. Its excitation spectrum also consists of two parts,
one is an intense broadband ranging from 250 to 320 nm attributed to the CTB, the other one is
composed of some sharp lines from 350 to 500 nm, which is ascribed to the intra f-f transitions of
Ho3+ ions. These obvious excitation peaks at 360, 400, 417, 450, and 482 nm are attributed to the
5
5
5
5
electronic transitions from the I8 ground state to the G2, G4, 5G5, 5F1, and F2 excited states of
14

ACCEPTED MANUSCRIPT
Ho3+ ions, respectively. The remarkable excitation bands locate in the blue region indicate that the
SrWO4:0.01Ho3+ can act as a potential blue light exciting phosphor. From the corresponding
emission spectrum excited at 450 nm, we can see that the emission spectrum consists of three
5
prominent peaks centered at 541, 614, and 657 nm. These peaks are assigned to the F4/5S2→5I8,
5F2→5I7, and 5F5→5I8 transitions of Ho3+ ions, respectively [40]. Moreover, the strongest emission
at 657 nm is observed as two sub-peaks, due to the Stark energy splitting, which is the (2J+1)
Stark components of J-degeneracy splitting. It is influenced by the crystal field around Ho3+ ions
in the host lattice.
Fig. 8c presents the excitation and emission spectra of the SrWO4:0.005Er3+ phosphor.
Monitored with the characteristic 4S3/2→4I15/2 emission of Er3+ at 550 nm, the excitation spectrum
contains the absorption of the host and the general f-f transitions of Er3+ ions. The strongest
excitation peak centered at 378 nm is due to the transition of Er3+ ions from the ground state 4I15/2
4
to the excited state G11/2. Upon the excitation of 378 nm light, the emission spectrum of
SrWO4:0.005Er3+ phosphor should be divided into three parts: the sharp peaks in the 513-536 nm
2
4
and 536-569 nm green region are assigned to the H11/2→4I15/2 and S3/2→4I15/2 transitions
4
respectively, while the weak peaks in the 644-678 nm red region are assigned to F9/2→4I15/2
transition [41]. Moreover, each emission is found to be two sub-peaks just like that of Ho3+ ions.
In order to clearly show the transition process of these rare-earth ions, the energy level schemes of
Pr3+, Ho3+, and Er3+ are shown in Fig. 8d. According to the above results, it can be seen that the
down-conversion emission of Pr3+, Ho3+, and Er3+ in SrWO4 host have been successfully realized
in our work.
The photoluminescence decay curves of SrWO4:0.01Pr3+, SrWO4:0.01Ho3+, and
15

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SrWO4:0.005Er3+ phosphors were monitored at their strongest emission peaks and shown in Fig. 9.
It indicates that all the decay curves can be well fitted into a single exponential function as I=I0
exp (-t/τ), in which I is the emission intensity at time t, τ is the decay lifetime. The fitting results
were 0.38499, 1.5078, and 0.52749 ms for SrWO4:0.01Pr3+, SrWO4:0.01Ho3+, and
SrWO4:0.005Er3+ phosphors, respectively.
3.4. Influence of the doping concentrations on the luminescent properties of Pr3+, Ho3+, and
Er3+ doped SrWO4 phosphors
The emission intensity of RE3+-doped phosphors could be manipulated by doping-host
combination, particle size, shape, phase, electric-field, doping concentration, and so on, as
mentioned above. Therefore the influences of doping concentrations and host compositions on the
luminescent properties of Pr3+, Ho3+, and Er3+ single-doped CaXSrYBa1-X-YWO4 phosphors are
systematically studied in this section.
The emission spectra of SrWO4 phosphors with different RE3+ concentrations excited at their
strongest excitation wavelength were presented in Fig. 10a-c, and the variation trend of relative
emission intensity with different RE3+ concentrations was shown in Fig. 10d. It can be observed
from the emission spectra of SrWO4:xPr3+ (Fig. 10a) that the peak positions and their regular
pattern of every curve are very similar except for the intensity. With increase of Pr3+ ions
concentration, the emission intensities increase firstly and reach a maximum at x=0.01, then
decrease with the further increase of Pr3+ ions concentration, excessive doping leads to
concentration quenching of the Pr3+ emission. The emission spectra of SrWO4:yHo3+ reveal the
similar variation trend as SrWO4:xPr3+, as shown in Fig. 10b. When Ho3+ ions are doped into
SrWO4 host, an initial increasing of emission intensity is observed with increase of Ho3+ ions
16

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doping concentration, which can be summarized as the increase of luminescence centres. Then the
emission intensity reaches the maximum value at y=0.01 and finally decreases beyond 0.01 due to
concentration quenching, which results from energy transfer between adjacent luminescence
centers. It can be seen clearly from Fig. 10c that the intensities of emission transitions of
SrWO4:zEr3+ phosphors enhance with the increase of Er3+ ions concentration and reach a
maximum at z=0.005. Then they reduce inversely because of concentration quenching. In
summary, the above results suggest that the optimum doped concentrations of Pr3+, Ho3+, and Er3+
in SrWO4 host are 0.01, 0.01, and 0.005, respectively.
With the increase of RE3+ ions concentration, the average distance between RE3+ ions will
reduce. The excitation energy may transfer between the close RE3+ ions [42]. For this reason, it is
necessary to obtain the critical distance (RC), which is the critical separation between donors
(activators) and acceptors (quenching site). The critical distance RC of the energy transfer between
the same activators Pr3+, Ho3+, or Er3+ in the SrWO4 host can be estimated according to the
following equation:
1
3


3V



RC = 2
(1)

4πxc N


Where χc is the critical concentration, N is formula units in the SrWO4 unit cell and V is the
volume of the unit cell. By taking the experimental and analytic values of χc, N, and V (0.01, 4,
349.80 Å3 for Pr3+; 0.01, 4, 348.48 Å3 for Ho3+; 0.005, 4, 350.66 Å3 for Er3+, respectively) the
critical transfer distances of Pr3+, Ho3+, and Er3+ in the SrWO4 host are found to be about 25.57,
25.53, and 32.24 Å, respectively.
In addition, concentration quenching usually is the result of non-radiative energy transfer
among luminescent centers. Non-radiative energy transfer from one RE3+ to another RE3+ may
17

ACCEPTED MANUSCRIPT
take place via exchange interaction, radiation reabsorption, or electric multipolar interaction [43].
The exchange interaction needs a large direct or indirect overlap between donor and acceptor,
which is responsible for the energy transfer for forbidden transitions and shorter critical distances
of less than 5 Å [44]. In single-doped SrWO4 phosphors the RC are much larger than 5 Å, hence,
the mechanism of exchange intercalation is ineffective. The mechanism of radiation reabsorption
comes into force only when the emission and the excitation spectra have broad overlap. Thus, the
small spectra overlap for the SrWO4:Pr3+/Ho3+/Er3+ phosphors indicates that the radiation of
reabsorption can be ignored in this case. As a result, the energy transfer process of RE3+ ions in
SrWO4 would be due to electric multipolar intercalation. There are three electric multipolar
interactions: dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole, respectively.
According to the theory of Dexter, The emission intensity I is expressed by the formula given in
the following [45]:
/ 3
I / x = k[1+
β
(x)θ ]−1
(2)
Where x is the activator concentration, k and β are constants under the same excitation condition
for a certain system, θ represents the interaction mechanism between rare-earth ions, and θ = 6, 8,
or 10 for electric dipole-dipole, dipole-quadrupole, or quadrupole-quadrupole interactions,
respectively. Fig. 11 shows that the relationship of log(I/x) versus logx is linear and the slopes are
-3.60, -2.11, and -2.54 for Pr3+, Ho3+, and Er3+ respectively. By using the above formula (2), the
values of θ are found to be 10.80, 6.33, and 7.62, which are approximately equal to 10, 6, and 8.
Therefore, the dominant concentration quenching mechanisms for Pr3+, Ho3+, and Er3+ in SrWO4
host are electric quadrupole-quadrupole, dipole-dipole, and dipole-quadrupole interaction,
respectively.
18

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3.5. Influence of the host compositions on the luminescent properties of Pr3+, Ho3+, and Er3+
doped CaXSrYBa1-X-YWO4 phosphors
Series of phosphors with different proportion of Ca, Sr, and Ba were synthesized to further
optimize the luminescent properties of CaXSrYBa1-X-YWO4:RE3+. The excitation and emission
spectra of all these phosphors were examined. There are so many spectra, so only that of the
typical Pr3+-doped three-component samples were presented in Fig. 12. It can be clearly observed
that the peak positions and their regular pattern of both excitation and emission spectra are very
similar with each other except for the intensity. All the strongest excitation peaks are located at
448 nm, and the strongest emission peaks are located in a very narrow range (644-648 nm, show
in the insert of Fig. 12b). These transitions are assigned to the 4f-4f intra-configurational
transitions, which are shielded by neighbouring 5s25p6 shells, so these transitions have low
susceptibility to crystal-field and environment.
Fig. 13 a, b, and c show the intensity variation trend of CaXSrYBa1-X-YWO4:0.01Pr3+ at 644 nm,
CaXSrYBa1-X-YWO4:0.01Ho3+ at 657 nm, and CaXSrYBa1-X-YWO4:0.005Er3+ at 550 nm,
respectively. In order to ensure the repeatability and accuracy of these data, three repeated tests
have been conducted under the same reaction conditions.
It can be seen from Fig. 13a that the luminescent intensity of the phosphors prepared three times
may change slightly, but they have the same variation trend in the three tests (in addition to the
second point in the test 3). The optimum doping-host composition is Ca0.4Sr0.6WO4 for Pr3+ ions.
3
The emission at 644 nm is ascribed to the P0→3F2 hypersensitive transition of Pr3+
(
∆
S=0,
∆
∆
L=2) [46], which is corresponding to the forced electric dipole transition, and strongly
J=2,
varies with the chemical environment surrounding of Pr3+. The f-f hypersensitivity transition of
19

ACCEPTED MANUSCRIPT
rare-earth ions is related to numerous factors. Moreover, the factors that affect the transitions are
very complicated [47]. The ionic radius, morphology, and crystallinity of the prepared phosphors
in our experiment all varied with the host composition, which leads to the emission intensities
reveal a non-monotonic decreasing (or increasing) variation trend. On one hand, it is possible that
increasing the total ionic radius of alkaline earth metal ions can distort the hosts’ structures. In
other words, it can lead to the larger degree of disorder and lower local symmetry of Pr3+ ions.
3
Eventually, it can improve the P0→3F2 transition of Pr3+. On the other hand, with the increasing
of the total ionic radius, the bond covalency of CaXSrYBa1-X-Y-O will decrease and the length of
the Pr-O bond becomes longer. Depending on the theory of B. R. Judd, the higher covalency has
the greater transition intensity [46], and the long Pr3+-O2- distance decreases the exchange
interaction probability for Pr3+ ions to transfer energy. So the lower covalency and longer distance
3
of Pr-O bond may result in the decrease of P0→3F2 transition intensity. And the results of the
emission intensity led by the above two aspects are contradictory. Furthermore, the morphology
and size of the synthetic phosphors also have great influence on the emission intensity of
RE3+-doped phosphors. The phosphors with nearly spherical shape and big size will have stronger
emission intensity. Because larger crystals imply a smaller surface area and less RE3+ ions near to
the surface to suffer from the ligand-induced and surface-defect quenching of luminescence,
resulting in less nonradiative decay losses of the surface luminescence ions [3]. The improved
emission intensity in the spherical phosphors can be basically attributed to the high packing
densities and low scattering of light [48]. Eventually, as a comprehensive result of the above
factors, the emission intensities reveal a non-monotonic decreasing (or increasing) variation trend,
as shown in Fig. 13a.
20

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The emission intensity variation trend of CaXSrYBa1-X-YWO4:0.01Ho3+ at 657 nm shows that the
emission intensities of the phosphors prepared three times have the same variation trend (Fig. 13b).
The optimum host composition is Ca0.8Sr0.2WO4 for Ho3+ ions. The intensity variation trend of
CaXSrYBa1-X-YWO4:0.005Er3+ at 550 nm is also the same (except the fifth point in the test 1), as
shown in Fig. 13c. The optimum host composition is Ca0.6Sr0.4WO4 for Er3+ ions. Like
CaXSrYBa1-X-YWO4:0.01Pr3+, the emission intensities of CaXSrYBa1-X-YWO4:0.01Ho3+ and
CaXSrYBa1-X-YWO4:0.005Er3+ phosphors also reveal a non-monotonic decreasing (or increasing)
variation trend due to the multiple factors. Unfortunately, the present work did not obtain a precise
rule, because these effects are complicated and hard to control, needing further investigation. As
can be seen from the present results, the optimum host compositions only contain Ca and Sr. So in
our later work, we will research the influence of different proportion of Ca and Sr on the
luminescent properties of CaXSr1-XWO4:RE3+ phosphors.
Chromaticity coordinates is one of the important factors in evaluating the performance of the
prepared phosphors. So the chromaticity coordinates of Ca0.4Sr0.6WO4:0.01Pr3+,
Ca0.8Sr0.2WO4:0.01Ho3+, and Ca0.6Sr0.4WO4:0.005Er3+ phosphors have been calculated based on
the corresponding emission spectrum and using the CIE 1931 colour matching functions. The
chromaticity coordinates are (0.572, 0.420), (0.393, 0.591), and (0.290, 0.686) respectively, and
they have been represented in Fig. 13d. The corresponding correlated colour temperatures are
1865, 4617, and 6224 K respectively. It can be seen that the Ca0.4Sr0.6W04:0.01Pr3+ phosphor will
emit orange light, Ca0.8Sr0.2W04:0.01Ho3+ will emit yellowish green light, and
Ca0.6Sr0.4W04:0.005Er3+ will emit green light.
4. Conclusions
21

ACCEPTED MANUSCRIPT
In summary, CaXSrYBa1-X-YWO4:RE3+ (RE=Pr, Ho, and Er) phosphors have been successfully
prepared via a facile hydrothermal method. The morphologies of the products vary gradually and
regularly with the change of the host composition, in which the anisotropic growth has played a
key role. Down-conversion emissions of Pr3+, Ho3+, and Er3+ in CaXSrYBa1-X-YWO4 host have
been successfully realized. The emission intensities of CaXSrYBa1-X-YWO4:RE3+ phosphors reveal
a non-monotonic decreasing (or increasing) variation trend due to the multiple factors.
Additionally, among these phosphors, Ca0.4Sr0.6WO4:0.01Pr3+, Ca0.8Sr0.2WO4:0.01Ho3+, and
Ca0.6Sr0.4WO4:0.005Er3+ have the optimal luminescent property, and these phosphors will emit
orange, yellowish green, and green light, respectively.
Acknowledgment
This present work was financially supported by Department of Chemistry, Tonghua Normal
University and College of Chemistry, Jilin University.
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