Synthesis and characterization of CaWO4 phosphor powders in glycerol solvent by solvothermal method

Article abstract of DOI:10.14233/ajchem.2015.17846

A solvothermal method had been applied to synthesize CaWO₄ powders. We used glycerol as the solvent and designed different reaction conditions to explore the effect of different factors (such as pH, temperature, time and surfactant). All products had been characterized by X-ray diffraction, X-ray spectrometry, scanning electron microscopy and fluorescence. It is observed that the temperature played an important role in crystallinity and fluorescence. The concentration of surfactant also had an effect on its morphology. On the basis of a series of experiments, we found that the sample prepared at the condition of pH = 10, T = 160 °C, t = 12 h had well-crystalline, welldispersed and strong fluorescence.

Full text of DOI:10.14233/ajchem.2015.17846

Asian Journal of Chemistry; Vol. 27, No. 7 (2015), 2386-2388
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.17846
Synthesis and Characterization of CaWO Phosphor
4
Powders in Glycerol Solvent by Solvothermal Method
*
J. ZHANG, S.Q. FANG, Y. TANG , W.J. HOU and C.C. QIU
Chongqing Key Laboratory of Environmental Materials & Remediation Technologies, Chongqing University ofArts and Sciences, Chongqing
4
02160, P.R. China
*
Corresponding author: E-mail: tangying2282@163.com, jiangzhu415@163.com
Received: 18 April 2014;
Accepted: 23 June 2014;
Published online: 30 March 2015;
AJC-17039
A solvothermal method had been applied to synthesize CaWO powders. We used glycerol as the solvent and designed different reaction
4
conditions to explore the effect of different factors (such as pH, temperature, time and surfactant). All products had been characterized by
X-ray diffraction, X-ray spectrometry, scanning electron microscopy and fluorescence. It is observed that the temperature played an
important role in crystallinity and fluorescence. The concentration of surfactant also had an effect on its morphology. On the basis of a
series of experiments, we found that the sample prepared at the condition of pH = 10, T = 160 °C, t = 12 h had well-crystalline, well-
dispersed and strong fluorescence.
Keywords: CaWO
4
, Solvothermal, Characterization, Glycerol.
cetyl trimethyl ammonium bromide (CTAB) in the autoclave.
In a typical procedure, 20 mL of glycerol was mixed with
INTRODUCTION
In recent years, there is an increasing interest in CaWO
due to its widely applications in the field of electrical enginee-
ring , laser optics and scitillators in medical devices . It can
be used as luminescent materials which have more potential
advantages than traditional materials . The chemical
properties of CaWO are stable and it can be excited blue light
by UV-rays, X-rays and electron beam. It can also change the
fluorescence color, fluorescence intensity and luminescence
4
1
0 mL CaCl
2
(1 mol/L) in 100 mL conical flask with vigorous
WO (1 mol/L) and certain concen-
1
2-5
stirring and then 10 mL Na
2
4
tration of CTAB were added into the solution, simultaneously.
The pH of the solution was adjusted by 1 mol/L NaOH. After
stirred for 0.5 h, the total solution was added into a Teflon-
lined stainless steel autoclave with 50 mL capacity. The auto-
clave was maintained at a set reaction temperature in drying
oven for enough time and then cooled to room temperature
naturally. The resulting white precipitate was collected by
centrifugation at the speed of 1500 r/s for 10 min and then
washed three times with distilled water and alcohol. The final
products were dried in a drying oven at 80 °C for 2 h. The
reaction conditions were summarized in Table-1.
6
,7
4
8
time by doping with different rare elements, which makes it
have a wide application in the field of photo electricity.Various
9
10
methods, such as solid state reaction , Pechini method ,
1
1,12
13
combustion method , co-precipitation method , hydro-
1
4
15
thermal method , micro-emulsion method , microwave
16
17
method and solvothermal method , have been reported to
prepare CaWO crystals, powders and films. To the best of
our knowledge, there is no report about the synthesis of CaWO
with glycerol as solvent. In this paper, glycerol was used as
solvent to synthesize CaWO crystals by solvothermal method.
We controlled reaction conditions (such as pH, temperature,
time, surfactant) to synthesize CaWO powders with different
The dried samples were characterized by X-ray powder
4
diffraction (XRD) with graphite mono-chromatized CuK
radiation (tube voltage = 30 KV, tube current = 20 mA, λ =
.5418 nm, TD-3500, Shimadzu Co.). The morphologies and
α
4
1
4
sizes of the samples were characterized by scanning electron
microscopy (SEM, Quanta 250) operated at an accelerating
voltage of 200 kV. The luminescent spectra were performed
on the PL spectrometer (SLM INSTRU-MENTS). All of the
measurements were performed at room temperature.
4
morphologies and sizes and finally discussed the influence of
different conditions on luminescence properties.
EXPERIMENTAL
RESULTS AND DISCUSSION
CaWO powders were prepared by the reaction of calcium
4
chloride and hydrated sodium tungstate at different pH,
temperature, time and different concentration of surfactant
The Scheelite-type CaWO
4
had a tetragonal structure
with space group (I4 / a (88)). The calcium ions were 8-fold
1

Vol. 27, No. 7 (2015)
Synthesis and Characterization of CaWO
4
Phosphor Powders in Glycerol Solvent 2387
TABLE-1
TABLE-2
SUMMARY OF THE REACTION CONDITIONS
AND THE MORPHOLOGY OF THE PRODUCTS
ATOM RATIO OF S3
Element
Wt (%)
23.28
15.75
60.97
At (%)
66.76
18.03
15.22
Sample Temp.
No. (°C)
S1 80
Time
(h)
12
CTAB
(mol L )
O
Ca
W
pH
10
10
10
10
10
10
5
-1
Morphology
-
-
-
-
-
-
-
-
Sphere
Sphere
Sphere
Sphere
Sphere
Sphere
Sphere
Sphere
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
120
160
120
120
120
120
120
120
120
120
12
12
6
18
24
12
12
12
12
12
7
.5
W
6.0
7
10
10
10
0.15
0.5
0.85
Sphere and rod
Sphere and rod
Sphere and rod
4.5
3.0
Ca
coordinated with the oxygen atoms surrounding tungstate
1
8
groups . The XRD patterns of all of the CaWO samples
4
W
1.5
O
showed obvious diffraction peaks which were consistent
with the JCPDS No. 41-1431 and could be easily indexed to a
scheelite crystal structure. The lattice constants calculated from
XRD patterns were a = b = 5.243 Å and c = 11.373 Å. Typically,
0
0
1
2
3
4
5
6
7
8
9
Energy (keV)
the XRD spectra of CaWO labeled S1, S3, S6, S7 and S11
were shown in Fig. 1. The sharpness of the diffraction peaks
indicated that the products were well-crystalline.
4
Fig. 2. EDS spectrum of S3
about 1:1:4, corresponding to the chemical formula of CaWO
No other impurities were detected in the product.
4
.
Without the addition of CTAB, the morphology of the
products was sphere and had a little difference on size which
was from 100 to 1000 nm, while some samples were non-
uniform, or atom reunited. Only the sample of S3 had the
uniform morphology and well-dispersed. The size of which
was from 800 nm to 900 nm (Fig. 3). On the contrary, with the
addition of CTAB, the morphology of the samples had a big
difference (Fig. 4). Compared with those samples without
CTAB, it is observed that with the increasing of CTAB, the
1
2
3
4
thinner rod grew on the surface of CaWO sphere and reunited.
4
5
It can be inferred that the addition of CTAB may produce
virgate or floriform CaWO4.
20
30
40
50
60
70
2θ (°)
The fluorescence spectra of the typical samples of S1,
S3, S6, S7 and S11 were shown in Figs. 5 and 6. S11 was
excited at 237 nm. They exhibited only peak at 421 nm. By
using the light with the wavelength at 242 nm, S6 was excited
and had a broad emission peaks at 417 nm. When excited at
Fig. 1. XRD spectra of S1, S3, S6, S7 and S11. 1: S1 (pH = 10, T = 80 °C,
t = 12 h); 2: S3 (pH = 10, T = 160 °C, t = 12 h); 3: S6 ( pH = 10, T
=
(
120 °C, t = 24 h); 4: S7 (pH = 5, T = 120 °C, t = 12 h); 5: S11
CTAB = 0.85 mol/L, pH = 10, T = 120 °C, t = 12 h)
2
30 nm, S7 were emitted light at 415 nm. S1 and S3 (excited
It was obvious that the diffraction peaks of S3 and S6 were
sharper than the others in Fig. 1. Among all of the samples,
the diffraction peaks of S3 were sharpest. On the basis of above
experiments, it can be inferred that the pH value and the
concentration of CTAB had little influence on the intensity of
diffraction peaks. But the temperature and time played impor-
at the same wavelength of 240 nm) both had emission peaks
at 415 nm. Although the reaction conditions were different. It
could be inferred that the emission spectra of all samples were
mainly due to the intrinsic electronic transfer of WO
self-trapped centers with excition energies located within the
band gap . Fig. 6 showed that the widely excited wavelength
range from 230 to 240 nm could excite luminescence of
CaWO . From the emission image (Fig. 6), it is observed that
19
2-
4
or the
17
tant roles in the crystallization of CaWO
of temperature and time, the diffraction peak became much
sharper and the crystalline of CaWO became much higher.
4
. With the increase
4
4
the intensity of the fluorescence emissions in S3 was much
stronger than those of S1, S6, S7 and S11, which was connected
with its uniform morphology and well-dispersed. Besides, it
was obvious that the intensity of S7 was the weakest, which
indicated that the reaction temperature made tremendous
contribution to the emission intensity. Reaction time and the
amount of surfactant also improved the emission intensity
Moreover, the effect of the temperature was dominating.
Although the pH value had no obvious effect on the crystalline,
we could discover that when the pH value was below 7, fewer
products were obtained.
The EDS image and the atom ratio of S3 (Table-2) are
showed in Figs. 2. It can be seen that only Ca, W and O singles
were found in the spectrum. The atom ratio of Ca:W:O is

2
388 Zhang et al.
Asian J. Chem.
to some degree, while the pH value could lead to less products
and decrease emission intensity.
Conclusion
Spherical and rod like crystals of CaWO were prepared
4
by solvothermal method with the solvent of glycerol. Reaction
temperature and the amount of surfactant played an important
roles on the morphology, size and fluorescence of the resulting
products. When the higher reaction temperature and the more
surfactant was performed, the fluorescence of the product was
stronger. Moreover, the temperature made the most contri-
bution to the formation of uniform morphology and well-
dispersed. The method we used to synthesize CaWO had a
4
great advantages, such as the simpler equipment, more conve-
nience operation, more uniform morphology and stronger
luminosity than the method reported by other literatures. There-
fore, it can be expanded to the synthesis of other tungstates.
Fig. 3. SEM pattern of S3
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