Photoluminescence of barium-calcium titanates obtained by the microwave-assisted hydrothermal method (MAH)

Article abstract of DOI:10.1016/j.cplett.2010.01.065

Ba_1-xCa_xTiO₃ (x = 0, 0.25, 0.50, 0.75, and 1.0) samples report the association of three types of clusters synthesized by the microwave-assisted hydrothermal at 140 °C for 40 min. In order to evaluate influence of structura

Full text of DOI:10.1016/j.cplett.2010.01.065

Chemical Physics Letters 488 (2010) 54–56
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Photoluminescence of barium–calcium titanates obtained by the
microwave-assisted hydrothermal method (MAH)
a,*
a
a
b
c
a
c
A.E. Souza , R.A. Silva , G.T.A. Santos , M.L. Moreira , D.P. Volanti , S.R. Teixeira , E. Longo
a
DFQB, UNESP, Univ. Estadual Paulista, 19060-080 Presidente Prudente, SP, Brazil
LIEC, Depto. de Química, UFSCar, Univ. Federal de São Carlos, São Carlos, SP, Brazil
LIEC, IQ, UNESP, Univ. Estadual Paulista, Araraquara, SP, Brazil
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Àx x 3
Ba1 Ca TiO (x = 0, 0.25, 0.50, 0.75, and 1.0) samples report the association of three types of clusters syn-
Received 28 November 2009
In final form 26 January 2010
Available online 1 February 2010
thesized by the microwave-assisted hydrothermal at 140 °C for 40 min. In order to evaluate influence of
structural order–disorder degree among them, photoluminescence (PL) emission, X-ray diffraction, FT-
Raman spectroscopy and Ultraviolet–Visible absorption were used. The PL emission of the crystalline
phases grows up with the calcium concentration and reaches the highest PL emission to 0.75 Ca concen-
tration, which is supported by the symmetry break caused by the phase transition. This PL emission is
3
higher than that of the CaTiO phase.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Potential applications enhance the importance of ATiO
friendly conditions, producing ordered and disordered PL materials
with visible broad-band centered at green region.
3
(A = Ca,
Ba, Sr, and Pb) compound as ferroelectric, semiconductor and cat-
alytic materials [1–3]. Although, the ceramic properties of the bar-
2
. Experimental
3
ium titanate (BaTiO ) have been widely investigated, more recently
Barium–calcium titanates (BCT) were prepared according to
there is an interest in studying nanoscale particles with tetragonal
structure, due to the properties which depend strongly on the crys-
talline structure and grain size.
procedure: (0.01 À x) mol BaCl
2
Á2H
2
O and (x) mol CaCl
2
Á2H
2
O
(
x = 0, 0.0025, 0.0050, 0.0075, and 0.01) were dissolved in 20 mL
Ò
of deionized water within the Teflon cup of the MAH reaction
chamber. With constant stirring and under nitrogen gas bubbling,
3
BaTiO (BT) nanoparticles can be prepared by several methods
[
4–8]; among the chemical methods, the hydrothermal is one of
0
4
.01 mol of C12H28TiO was added to the solution, and after stirring
the promising routes for production of oxides with extremely fine
particles, with different morphology and homogeneous distribu-
tion of grain sizes [3,8–13]. In recent publication [14] it was ob-
served that CaTiO powders synthesized using the MAH method
3
show that intermediate energy states within the band gap are
mainly responsible for photoluminescence (PL) emission. Also,
for 10 min at room temperature, 50 ml of KOH (6 mol) was quickly
added to act as a mineralization agent. Afterward the cup was put
within the reaction chamber which was sealed and installed inside
the microwave oven. The synthesis process was carried out at
1
40 °C for 40 min (at a heating rate of 140 °C/min) and maximum
pressure of 4 bar. The system was cooled to room temperature and
the powder ceramic was washed in deionized water and centri-
fuged several times until neutral pH (pH 7). The crystalline powder
was dried at 110 °C for 12 h. The as-synthesized powders were
characterized by X-ray diffraction (Rigaku, model D/Max-2500/
3 3
SrZrO and SrTiO [15,16] powders and multilayer thin films, pre-
pared using other methods, present PL emission at different colors
which can be associated to numerous states within the forbidden
band gap.
Besides the several works about binary titanates, no one re-
search has been reported on the preparation of ternary titanates
using MAH method. In this work, the microwave radiation associ-
ated with hydrothermal method was employed to obtain ternary
PC), Cu Ka radiation, with a scan speed of 4°/min and a step width
of 0.02°. Raman spectra were obtained using a micro-Raman
Renishaw spectrograph model inVia equipped with
a Leica
microscope (50Â objectives with ꢀ1
l
m2 spatial resolution) and
x 3
Ba1ÀxCa TiO (x = 0, 0.25, 0.50, 0.75, and 1.0) compounds under
À1
CCD detector. Scan ranges of 100–1400 cm , using the 633 nm
wavelength of a He–Ne laser. Photoluminescence (PL) spectra were
collected with a Thermal Jarrel-Ash Monospec 27 monochromator
and a Hamamatsu R446 photomultiplier. The 350.7 nm exciting
wavelength of a krypton ion laser (Coherent Innova) was used;
the nominal output power of the laser was kept at 200 mW.
*
Corresponding author. Address: Rua Roberto Simonsen 305, 19060-080
Presidente Prudente, SP, Brazil. Fax: +55 18 3221 5682.
E-mail addresses: agda@fct.unesp.br, agda_pb@ig.com.br (A.E. Souza).
0
009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.01.065

A.E. Souza et al. / Chemical Physics Letters 488 (2010) 54–56
55
Ultraviolet–Visible (UV–Vis) spectroscopy for the spectral depen-
dence of optical absorbance of all samples was taken using a Cary
G UV–Vis NIR Spectrophotomer in the total reflection mode. Two
5
reference standards Labsphere Certified Reflectance (SRS 94-010
and SRD 02-010) were used. All measurements were taken at room
temperature.
(e)
(d)
3
. Results and discussion
X-ray diffraction patterns (Fig. 1) show tetragonal perovskite
structure (PDF 83-1878) and a small amount of barium carbonates
PDF 01-0506) for the composition Ba1ÀxCa TiO with x < 1. The
(c)
(
x
3
tetragonal phase can be denoted by the peak nearby 2h = 45°. In
this case, the splitting of the cubic (2 0 0) into tetragonal (2 0 0)
and (0 0 2) reflections was observed, indicating the high tetrago-
nality of this sample. In addition, for concentrations x = 0.50 and
(
b)
(a)
0
0
.25, orthorhombic barium/calcium carbonate BaCa(CO
3-0322) was identified in these samples. XRD data show the pres-
(CT) orthorhombic perovskite phase (PDF 82-
229), to x = 1 (CT) which was also observed by Marques et al. [17]
3 2
) (PDF
2
00
400
600
800
1000
1200
-
1
ence of a pure CaTiO
0
using polymeric precursor method and Moreira et al. using the
MAH method [3]. For x = 0.75 concentration, the diffraction peaks
3
Raman Shift (cm )
Fig. 2. Raman spectra of the Ba1ÀxCa
d) x = 0.75; (e) x = 1.
x 3
TiO sample: (a) x = 0; (b) x = 0.25; (c) x = 0.50;
(
3
show only the calcium carbonate formation CaCO (PDF 01-0837)
and, the halo near 2h = 30° in the X-ray pattern indicates the for-
mation of titanate with short-range order. This result was con-
firmed in two new samples which were prepared, one at the
same previous conditions and other with a shorter treatment time
from x = 0 to 0.50 are characteristic of the barium titanate. Sample
with x = 0.75 shows a Raman spectra different of the other samples,
characterizing the new phase not identified by XRD data.
The nine CT Raman modes observed in the range from 145 to
(
10 min). The results show that x = 0.75 is the worse concentration
À1
8
15 cm
are attributed orthorhombic structures which are in
for the obtaining of pure BCT phase.
agreement with the literature [18]. The Raman mode from 177 to
Fig. 2 shows Raman spectra of the BCT to different composi-
À1
À1
3
37 cm , is attributed to the O–Ti–O bending modes. The 462
tions. The peak at 305 cm in the samples with Ba indicates the
À1
À1
and 537 cm are ascribed to the torsional mode, and 664 cm
tetragonal BT phase formation is the dominate phase [3]. The mode
À1
is assigned to the Ti–O symmetric stretching vibration. As shown
by Cavalcante et al. [18], the positions of the peaks in Raman spec-
trum have a small shift according to the method used to prepare
the compounds. Also, the stoichiometric composition can change
the peak positions and the intensities.
at 716 cm is related to the highest frequency longitudinal optical
mode. The higher relative intensity of the band in comparison with
2
+
the other tetragonal bands for nanoparticles can be related to Ba
À1
defects in the BT lattice [3]. The peaks at 515, 262, and 180 cm
are assigned to the fundamental TO mode of A symmetry which
1
According to Fig. 1, the XRD patterns of the three intermediary
compositions (x = 0.25, 0.50, and 0.75) show a band around
comprise the main difference in Raman spectra among tetragonal
and orthorhombic phase of BT. The asymmetry in the peak at
À1
2
h = 30° indicating that these samples still present a small struc-
tural disorder. The XRD pattern of the Ba0.25Ca0.75TiO sample
shows only the Ca(CO) crystalline phase and the band which char-
5
15 cm suggests the existence of coupling of TO modes associ-
ated with the tetragonal phase. The presence of these peaks in
the Raman spectrum indicates a tetragonal structure to the BT
3
3
acterizes a short-range order structure. XRD data serve to explain
PL results. The PL property is affected by the relationship between
short and long-range structural lattice order–disorder [19]. Consid-
ering that calcium carbonate does not have a significant influence
on the PL emission, the very high PL intensity of this sample (Fig. 3)
can be associated with the existence of two cubic–octahedral clus-
ters. These clusters have a quite different charge distribution for
BaO12 and CaO12, which are presents in singular ratios for each
sample, and they are associated under different conditions with
and is in agreement with XRD data [3]. The short peak at
À1
1
060 cm confirms the presence of a small amount BaCO
3
in all
samples. This peak disappears when the calcium concentration is
raised as confirmed by XRD and Raman data. The Raman modes
6
an octahedral TiO cluster. Also, it is observed that the PL spectrum
maximum shifts to a higher k (nm), and its intensity increases with
the calcium concentration, indicating that the CaO12 cluster intro-
duces a break of local symmetry in the tetragonal lattice and
produces new orthorhombic structures. As the PL provides infor-
mation about ions in relation to their neighborhood (i.e., about
their clusters associations), the higher PL intensity in the Ba0.25-
(
e)
(d)
(c)
(b)
Ca0.75TiO
by structural changes and the amount of the disordered phase
short and medium-range disorder) present in this material. All
3
composition must be related to the disorder introduced
(
100)
20
(
101)
(111)
(202)
(
102)
(
a)
(
the samples present the maximum component emission around
the green light range (between 495 and 545 nm).
Three types of additional energy levels exist in the forbidden
band of titanates: free-exciton levels, STE levels and defect or
impurity levels; all energy levels are near to the conduction band
10
30
40
50
60
70
2
θ (degree)
Fig. 1. XRD patterns of the Ba1ÀxCa
d) x = 0.75; (e) x = 1.
x
3
TiO sample: (a) x = 0; (b) x = 0.25; (c) x = 0.50;
(

5
6
A.E. Souza et al. / Chemical Physics Letters 488 (2010) 54–56
x = 0.25, the band gap to BCT phase is smaller than to BT (x = 0)
phase, suggesting that the BCT phase has a higher degree of or-
der–disorder than the BT phase. This occurs because the BT tetrag-
onal structure is disordered by the CaO12 clusters making available
additional electronic levels in the forbidden band gap. For x = 0.50,
the high structural distortion can result in a higher band gap value
than in the prior phases. For x = 0.75, the band gap has the highest
calculated value which could be associated with the structural
threshold among tetragonal and orthorhombic phases. For the
intermediate phases (0.25–0.75), it is observed that the band gap
energy rises with the Ca concentration.
(d)
(e)
(
c)
b)
a)
4. Conclusions
(
The results reported here illustrate the use of the MAH method
in the synthesis of barium–calcium titanates. They show that the
tetragonal BCT phase can be obtained under calcium concentra-
tions of 0.25 and 0.50. For 0.75, there is no formation of titanate
with long-range order due to be the threshold of tetragonal–ortho-
rhombic phase transition. The samples present PL emission in the
visible green region. The PL emission of the BCT phase rises with
the calcium concentration and has the highest value for 0.75 which
is higher than the calcium titanate value. The higher PL emission is
supported by the symmetry break caused by the phase transition
that occurs with this concentration of calcium.
(
400
500
600
700
800
Wavelenght (nm)
Fig. 3. Photoluminescence spectra of the Ba1ÀxCa
x = 0.25; (c) x = 0.50; (d) x = 0.75; (e) x = 1.
x
TiO
3
sample: (a) x = 0; (b)
Table 1
Average grain size of the Ba1ÀxCa
x
TiO
3
samples.
Samples
Band gap
Crystallite size
(nm) [20]
Acknowledgements
(
eV)
BaTiO
3
3.36
3.24
3.44
3.78
3.51
39
34
17
N.I.
45
The authors gratefully acknowledge Danilo Cardoso Ferreira
and José Leopoldo Costantino for Raman data and FAPESP, CAPES
and, CNPq for financial support.
Ba0.75Ca0.25TiO
Ba0.50Ca0.50TiO
Ba0.25Ca0.75TiO
3
3
3
*
CaTiO
3
*
References
N.I.: phase not identified by XRD.
[
[
1] F.V. Motta, A.P.A. Marques, J.W.M. Espinosa, O.S. Pizani, E. Longo, J.A. Varela,
Curr. Appl. Phys. 10 (2010) 16.
2] N. Baskaran, H. Chang, Mater. Chem. Phys. 77 (2002) 889.
[
20]. The values of the gap energies can be obtained by the Wood
and Tauc method [21]. The band gap determined from UV–Vis
absorption for all samples is between 3.24 and 3.78 eV (Table 1).
The PL excitation energy used in the present work (3.52 eV corre-
sponding to 350 nm) is of the same order as these band gaps, so
it is difficult for an electron in the valence band to be directly ex-
cited to the conduction band, mainly due to thermal effects. There-
fore, it is likely that the electrons were excited first to the localized
levels within the forbidden gap and, then the observed broad-
bands in PL emissions do not result from the free-exciton recombi-
nation. In this work, we observed poly- and nanocrystalline oxide
where oxygen vacancies are known to be the most common de-
fects which usually act as radiative centers in luminescence pro-
cesses [22–24].
The broad luminescent band usually observed at low tempera-
tures in perovskite-type crystals is associated with the presence of
imperfections or defects and is typical of multiphonon and multi-
level process. The literature includes several papers explaining
favorable conditions for PL emission in materials presenting a de-
gree of order–disorder. The authors attributed the radiative decay
process to distorted octahedral, self-trapped excitons, oxygen
vacancies, surfaces states and a charge transfer via intrinsic defects
inside an oxygen octahedron [20].
[3] M.L. Moreira et al., Chem. Mater. 20 (2008) 5381.
[
[
4] E. Orhan et al., Phys. Rev. 71 (2005) 0851131.
5] Z. Xinhua, W. Junyi, Z. Zhenghai, Z. Jianmin, Z. Shunhua, L. Zhiguo, M. Naiben, J.
Am. Ceram. Soc. 91 (2008) 1.
[6] W.S. Cho, J. Phys. Chem. Solids 59 (1998) 659.
[
7] P. Durán, D. Gutierrez, J. Tartaj, M. Bañares, C. Moure, J. Eur. Ceram. Soc. 22
2002) 797.
8] K. Sridhar, Curr. Sci. 85 (2003) 1730.
(
[
[9] F.I. Pires, E. Joanni, R. Savu, M.A. Zaghete, E. Longo, J.A. Varela, Mater. Lett. 62
2008) 239.
10] Z. Chen, W. Li, W. Zeng, M. Li, J. Xiang, Z. Zhou, J. Huang, Mater. Lett. 62 (2008)
343.
(
[
4
[11] M.L. Santos, R.C. Lima, C.S. Riccardi, R.L. Tranquilin, P.R. Bueno, J.A. Varela, E.
Longo, Mater. Lett. 62 (2008) 4509.
12] D.P. Volanti et al., J. Alloys Compd. 459 (2008) 537.
13] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, O.S. Pizani, E.
Longo, J. Alloys Compd. 474 (2009) 195.
[
[
[14] M.L. Moreira et al., Acta Mater. 57 (2009) 5174.
[
[
15] V.M. Longo et al., Theor. Chem. Acc. 124 (2009) 385.
16] M.L. Moreira, J. Andrés, V.M. Longo, M.S. Li, J.A. Varela, E. Longo, Chem. Phys.
Lett. 473 (2009) 293.
[17] V. Marques et al., Solid State Sci. 10 (2008) 1056.
[
[
[
18] L.S. Cavalcante et al., Chem. Eng. J. 143 (2008) 299.
19] F.V. Motta et al., J. Lumin. 129 (2009) 686.
20] W.F. Zhang, Z. Yin, M.S. Zhang, Z.L. Du, W.C. Chen, J. Phys. Condens. Mater. 11
(1999) 5655.
21] D.L. Wood, J. Tauc. Phys. Rev. 5 (1972) 3144.
22] K. Vanheusden, W.L. Warren, J.A. Voigt, C.H. Seager, D.R. Tallant, Appl. Phys.
Lett. 67 (1995) 1280.
[
[
Table 1 shows band gap energies obtained from UV–Vis spectra
of the BCT with different compositions. These values show that for
[23] L. Foress, M. Schubnell, Appl. Phys. B 56 (1993) 363.
24] Y.C. Zhu, C.X. Ding, G.H. Ma, Z.L. Du, J. Solid State Chem. 139 (1998) 124.
[