Sol-gel synthesis and photoluminescence of K2NiF4-type structure phosphors CaxSr1-xGdyY1-yAlO 4:zEu3+ with hybrid precursors

Article abstract of DOI:10.1016/j.jallcom.2006.09.085

Ca_xSr_1-xGd_yY_1-yAlO ₄:zEu³⁺ (x = 0.2-1.0, y = 0-1.0, z = 0.01-0.07) was synthesized by a hybrid precursor assembly sol-gel technology. We got sol solutions by stoichiometric rare earth nitrate, Sr(NO₃)₂, Ca(NO₃)₂, Al(NO₃)₃, at the same time investigated the relationship between the sol formed and experiment variable including Ph, temperature, concentration, and so on. Through drying and calcining precursors, we got luminescent materials powder. The particle size of luminescent materials is about 40 nm characterized by XRD and having thick three-dimension grains as SEM shown. Not only co-doping Ca²⁺ and Sr²⁺ but also changing the ratio of Gd³⁺ and Y³⁺ cannot change the crystalline structure in Ca_xSr_1-xGd_yY_1-yAlO ₄, it also formed the crystalline structure as that of pure-phase CaGdAlO₄. All these photoluminescence materials show good emission spectra and their luminescent intensity depend on the concentrating of Eu³⁺: in all these luminescent materials, there exist emission come from Eu³⁺ activator' transitions of ⁵D₀-⁷F_J (J = 0-3) and their emission intensity increase as adding to the concentration of Eu³⁺.

Full text of DOI:10.1016/j.jallcom.2006.09.085

Journal of Alloys and Compounds 441 (2007) 214–218
Sol–gel synthesis and photoluminescence of K2NiF4-type structure
phosphors CaxSr1−xGdyY1−yAlO4:zEu3+ with hybrid precursors
Junjie Wu, Bing Yan∗
Department of Chemistry, Tongji University, Shanghai 200092, China
Received 7 August 2006; received in revised form 22 September 2006; accepted 23 September 2006
Available online 7 November 2006
Abstract
CaxSr1−xGdyY1−yAlO4:zEu3+ (x = 0.2–1.0, y = 0–1.0, z = 0.01–0.07) was synthesized by a hybrid precursor assembly sol–gel technology. We
got sol solutions by stoichiometric rare earth nitrate, Sr(NO3)2, Ca(NO3)2, Al(NO3)3, at the same time investigated the relationship between
the sol formed and experiment variable including Ph, temperature, concentration, and so on. Through drying and calcining precursors, we got
luminescent materials powder. The particle size of luminescent materials is about 40 nm characterized by XRD and having thick three-dimension
grains as SEM shown. Not only co-doping Ca2+ and Sr2+ but also changing the ratio of Gd3+ and Y3+ cannot change the crystalline struc-
ture in CaxSr1−xGdyY1−yAlO4, it also formed the crystalline structure as that of pure-phase CaGdAlO4. All these photoluminescence materials
show good emission spectra and their luminescent intensity depend on the concentrating of Eu3+: in all these luminescent materials, there exist
5
emission come from Eu3+ activator’ transitions of D0–7FJ (J = 0–3) and their emission intensity increase as adding to the concentration of
Eu3+.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Phosphors; Sol–gel synthesis; Luminescence
1. Introduction
phosphors. And thereby, they were expected to act as potential
kind of luminescent materials. A kind of material with the
general formula ABCO4 where A site is occupied by alkaline
earth ions and B site by a rare earth cation while C site is other
element such as Al, N, Ga and so on attracts more attention
of sciences because these type materials with the structure
of K2NiF4 are splendid host matrix [9,10]. CaGdAlO4:Eu3+
is one of the best red emitting luminescent materials for its
steady chemical and physical characters. On the other hand, it
is well known that much more attention has been directed to the
sol–gel approach in luminescent materials [11–13] because the
direct advantages of this technology include obtaining novel
chemical compositions with unique properties, excellent purity
and more convenient preparation routes. Through a sol–gel
process, it is possible to synthesize phosphors with small size.
It is easy to control the composition and homogeneity [14–20].
The active precursors result in low calcination temperature
and minimize the potential for cross contamination. Many
researchers have done work to synthesize phosphors by sol–gel
processing.
The researchers who are concerned about the development
of luminescent materials [1,2], in general speaking have pay
more attention to the efficient productions during the past
decades. Their original particular interest were inorganic
luminescent materials, practical applications in almost and
device involving the artificial production of light. Cathode ray
tubes lamps, X-ray detectors field emission display (FED) and
plasma displays (PDP) are well known examples [3–6] and all
these industrial appliances have increased the demand for these
materials with increasingly better characteristics in term of
stability, brightness and industrial processing ability [7,8]. To
acquire more efficient luminescent materials, great efforts have
been devoted to the rare-earth activated phosphors because
of their good luminescent characteristics, stability in high
vacuum, and absence of corrosive gas emission under electron
bombardment compared with currently used sulfide-based
∗
In this work, europium, yttrium and gadolinium salicylate
coordination polymers were selected as the precursor of lumi-
Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.
E-mail address: byan@tongji.edu.cn (B. Yan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.09.085

J. Wu, B. Yan / Journal of Alloys and Compounds 441 (2007) 214–218
215
nescent species, polyacrylamide (PAM) as dispersing medium
procedure can be shown in the following:
template, Al(NO3)3, Ca(NO3)2 and Sr(NO3)2 providing the
elements matrix need, and urea for a fuel. These precursors
were in situ sol–gel composed and the luminescent materials
CaxSr1−xGdyY1−yAlO4:zEu3+ was achieved by the further
thermolysis of the hybrid precursors at 1000 ◦C, respectively.
The luminescent performance, structure and grain sizes were
systematically examined.
yGd(NO3)3 + (1 − y)Y(NO3)3 + (0.01–0.07) Eu(NO3)3 + 3.2Hsal
+ NH3·H2O → [YxGd1−xEu(Sal)3]n + Al(NO3)3 + xCa(NO3)2
+ (1 − x) Sr(NO3)2 + 5.0PAM + 5.0CO(NH2)2
+ NH3·H2O → CaxSr1−xGdyY1−yEuAlO4 + CO2↑ + NH3↑
+ NO2↑ + H2O ↑
2. Experimental
2.2. Physical measurements
2.1. Chemicals and procedures
The particle size was characterized by means of X-ray diffraction (XRD,
Bruke, D8-Advance, 40 Kv and 20 mA, Cu K␣). The morphology and
microstructure were characterized with scanning electronic microscope (SEM,
Philips XL-30). Excitation and emission spectra at room temperature were deter-
minedwithPerkin-ElmerLS-55modelfluorophotometer(scanrate = 1500 nm/s,
excitation slit width = 10 nm, emission width = 5 nm).
The rare earth oxide Eu2O3, Y2O3 and Gd2O3 were dissolved
in diluted HNO3 to make their rare earth nitrates. The synthesis of
CaxSr1−xGdyY1−yAlO4:zEu3+ by rare earth salicylate coordination polymer-
PAM precursors were described as the following process: excess salicylic acid
(5.0 mmol) was dissolved into 95% ethanol and its Ph was adjusted to be
about 7.0 with ammonia solution. Then mixed solutions of rare earth nitrates
were added and mixed homogenously. Stoichiometric Sr(NO3)2, Ca(NO3)2,
Al(NO3)3 and solutions of urea (5.0 mmol), PAM (5.0 mmol) were added into
the above solution. Heating and stirring for about 4 h at 80 ◦C, light yellow gel
were obtained, dried and the precursors were achieved. The hybrid precursors
were calcined in a resistance stove for 4 h at 1000 ◦C. The typical synthesis
3. Results and discussion
The XRD patterns for these phosphors were mea-
sured and showed the similar crystalline structure. Fig. 1
Fig. 1. The XRD patterns of the CaGdAlO4:Eu3+ (A), Ca0.4Sr0.6GdAlO4:Eu3+ (B) and CaGd0.8Y0.2AlO4:Eu3+ (C) crystalline powders.

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J. Wu, B. Yan / Journal of Alloys and Compounds 441 (2007) 214–218
presents the selected XRD patterns of the CaGdAlO4:Eu3+,
Ca0.4Sr0.6GdAlO4:Eu3+ and CaGd0.8Y0.2AlO4:Eu3+ crystalline
powders calcined at 1000 ◦C for 4 h, respectively. The diffrac-
tion patterns for all these materials were found to be exactly the
sameasthatofCaGdAlO4 reportedinJCPDSCard. Itmeansthat
there exist pure phases CaGdAlO4 no matter changing the ratio
of Y3+ and Gd3+ ions or co-doping alkaline-earth metal ions-
Ca2+ and Sr2+ which can not change the structure of matrix,
they maintain the crystalline structure of CaGdAlO4. The struc-
ture of CaGdAlO4 can explain this phenomenon. To Gd3+ and
Y3+ ions in this material, they are in similar ionic radii, it means
the two ions can replace their sites in the crystalline structures.
When it takes place, CaGdxY1−xAlO4 maintains the same struc-
ture as that in CaGdAlO4. On the other hand, co-doped Sr2+ and
Ca2+ ions are at the same situation. It is well known that Sr2+ and
Ca2+ belonging to alkaline-earth metal have the similar physi-
cal and chemical character, so the crystal structure will remain
constant when Sr2+ ions substitute for Ca2+ ions.
We further used the scanning electron microscope (SEM)
to measure the CaGdAlO4:Eu3+, Ca0.4Sr0.6GdAlO4:Eu3+ and
CaGd0.8Y0.2AlO4:Eu3+ samples. Fig. 2 shows the representa-
tive SEM micrograph for them, respectively. It can be seen that
all products mainly consist of solid micron crystalline struc-
tures, which exhibit well-proportional grain structures for the
high temperature 1000 ◦C of thermal decomposition. The typical
particle is estimated to be about 30–60 nm in dimension which
is coincident with that calculated by Scherrer formula (accord-
ing to work out by Scherrer formula, we could see that the radii
of these grains concentrate on about 40 nm). Besides this, the
three-dimensional sizes of crystalline grain are very thick as
they shown in pictures from SEM to afford high power, it needs
to refer that the crystalline powder and micrometer dimension
for these powders with high strength would be very useful for
the application to obtain high efficient phosphors because these
micro-crystalline materials can result in the high luminescent
intensities. All these excellent three-dimensional sizes of crys-
talline owed to the sol–gel synthesis technology which we adopt
to preparing phosphors.
All the excitation spectra of CaxSr1−xGdyY1−yAlO4:zEu3+
show the similar feathers as it exhibited in Fig. 3. We take repre-
sentative excitation spectrum of CaGdAlO4:Eu3+ which is taken
at an emission wavelength of 613 nm, annealed at 1000 ◦C, for
example. The excitation spectrum consists of bands (201, 208,
240, 250, 256, 275 nm) between 200 and 280 nm. The wide band
centered at 240 nm originate in transitions toward the charge
transfer state (CTS) corresponding to Eu3+–O2− interactions.
The peaks at 201 and 208 nm are attributed to the host absorption
4
band (HAB). The f–f transitions within the Eu3+, F6 electron
configuration cannot be checked in long wavelength.
As it shown in Fig. 4, the emission spectra of
CaGdAlO4:zEu3+ (z = 0.01, 0.02, 0.03, 0.05, 0.07 mmol) verifies
thefactthat theluminescencepropertiesof Eu3+ dependstrongly
Fig. 2. SEM of CaGdAlO4:Eu3+ (A), Ca0.4Sr0.6GdAlO4:Eu3+ (B) and CaGd0.8Y0.2AlO4:Eu3+ (C).

J. Wu, B. Yan / Journal of Alloys and Compounds 441 (2007) 214–218
217
Fig. 5. Emission spectrum of CaYAlO4:zEu3+ (z = 0.02, 0.03, 0.05, 0.07 mmol).
Fig. 3. Excitation spectra of CaGdAlO4:Eu3+
.
on their doping concentration in the host lattices. From this pic-
ture we can see that the emission spectra consist of four peaks sit-
uated at 562, 583, 610, 648 nm which agree with the mechanism
of Eu3+ activator. As we known, the emission lines of Eu in many
different doping concentrations. Of course, all these situations
take place in YGdAlO4:zEu3+ (z = 0.02, 0.03, 0.05, 0.07 mmol)
shown in Fig. 5.
Fig. 6 compared the luminescent intensities with different
composition of host matrix in CaxSr1−xGdAlO4 x = 0.2, 0.4,
0.6, 0.8 (doping the same ratio of Tb3+ (5 mol%)). The emission
compounds originate mainly from the transitions of 5D1 → FJ
7
5
7
(J = 0–4) and D0 → FJ (former lying in the period between
450 and 550 nm, later in 560 and 650 nm). According to other
researcher reported, in low concentrating of Eu3+, there exist
7
spectra are composed of 5D0 → FJ (J = 0–3) emission lines of
Eu3+. It can be found from Fig. 6, there existed emission inten-
sity alternated with changing the concentration ratio of Ca2+
and Sr2+ ions while their emission spectral shapes have no differ-
ence. As x = 0.4, the production exhibited the strongest emission
intensity. The reason for this phenomenon, we believe lies in the
factors as following: the Ca2+ and Sr2+ are very similar in their
ionic radii which makes them show almost the same character in
compounds, so in CaxSr1−xGdAlO4:Eu3+ phosphors, Ca2+ and
Sr2+ ions share with the same sites in crystalline lattice. These
could be verified by the XRD patterns. Because there is unsta-
ble crystalline structure in CaGdAlO4, when co-doping Sr2+
7
emissions come form 5D1 → FJ, while increasing the Eu3+ ions
the luminescence from 5D1 level becomes less probable and that
from 5D0 level is enhanced. It can be seen in CaGdAlO4:zEu3+
the fluorescence spectra of the as-prepared phosphors are domi-
nated by the 5D0 emission. The dominant peak is located 610 nm
7
attributed to electron transition 5D0 → F2. On the other hand,
the emission intensity increases consecutively. When the con-
centration of Eu3+ increases up to 0.07 (7 mol%), the corre-
sponding phosphor exhibits the strongest emission, although the
doping concentration has only little influence on the correspond-
ing intensities and the distinction of luminescence intensities for
Fig. 4. Emission spectrum of CaGdAlO4:zEu3+ (z = 0.01, 0.02, 0.03, 0.05,
0.07 mmol).
Fig. 6. Luminescence intensities of CaxSr1−xGdAlO4:Eu3+ (x = 0.2, 0.4, 0.6,
0.8 mmol).

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J. Wu, B. Yan / Journal of Alloys and Compounds 441 (2007) 214–218
ent compositions have only little influence on the luminescent
intensities.
4. Conclusion
In summary, CaxSr1−xGdyY1−yAlO4:zEu3+ (x = 0.2–1.0,
y = 0–1.0, z = 0.01–0.07) were synthesized by hybrid precursor
assembly sol–gel technology. From XRD, we can see that there
existed pure phase CaGdAlO4 no matter co-doping alkaline
earth ions-Ca2+, Sr2+ or changing the ratio of Y3+ and Gd3+ ions,
both XRD and SEM results indicate that these phosphors exhibit
well-proportional grain structures with micrometer dimension.
All these photoluminescence materials show splendid emission
spectra and their luminescent intensity depend on the concen-
trating of Eu3+.
Acknowledgements
Fig. 7. luminescence spectrum of CaGdyY1−yAlO4:Eu3+ (y = 0.2, 0.4, 0.6,
0.8 mmol).
The work was supported by the Science of Shanghai Univer-
sity for Excellent Youth Scientists and National Natural Science
Foundation of China (20671072).
and Ca2+ in matrix, it can likely offset this defeat. As x = 0.4,
Ca0.4Sr0.6GdAlO4 is most steady, so it shows most strongly
emission intensity.
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local site(withoutinversion center), thehypersensitivetransition
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5D0 → F2 is often dominated in their emission spectra whose
7
peaks situated at 610 nm. This is in agreement with the mecha-
nism known to be responsible for the luminescence in the bulk
material. The absorption of UV photons by the CaGdyY1−yAlO4
groups inside the host matrix is followed by a nonradiative
transfer to the europium ions. These latter come back to the
ground state through a radiative transition. In CaGdyY1−yAlO4
matrix, because the Gd3+ and Eu3+ are very similar in their
ionic radii and there exist the same crystalline structure as that
of CaGdAlO4 in CaGdyY1−yAlO4, the doped Eu3+ occupied
the Gd3+ sites resulting in the hypersensitive red emission tran-
7
sition 5D0 → F2 of Eu3+ being the most prominent group in its
emission. Due to the weaker interaction between Eu3+ and O2−
ions in CaGdyY1−yAlO4 doped with Eu3+, no little level split
can be caused by the crystal field and no obvious split of Eu3+
emission bands has been observed. Besides this, it also can be
observed that the luminescent intensity reaches the strongest at
the host composition of CaGd0.8Y0.2AlO4 although the differ-