Photoluminescent BaMoO4 nanopowders prepared by complex polymerization method (CPM)

Article abstract of DOI:10.1016/j.jssc.2005.11.020

The BaMoO₄ nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO₄ powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL) and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO₄ at 300 °C was disordered. At 400 °C and higher temperature, BaMoO₄ crystalline scheelite-type phases could be identified, without the presence of additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around 40 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40-50 nm. The sample that presented the highest intensity of the red emission band was the one heat treated at 400 °C for 2 h, and the sample that displayed the highest intensity of the green emission band was the one heat treated at 700 °C for 2 h. The CPM was shown to be a low cost route for the production of BaMoO ₄ nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The optical properties observed for BaMoO₄ nanopowders suggested that this material is a highly promising candidate for photoluminescent applications.

Full text of DOI:10.1016/j.jssc.2005.11.020

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Journal of Solid State Chemistry 179 (2006) 671–678
www.elsevier.com/locate/jssc
Photoluminescent BaMoO nanopowders prepared by complex
4
polymerization method (CPM)
a,
Ana Paula de Azevedo Marques , Dulce M.A. de Melo , Carlos A. Paskocimas ,
Ã
a
b
c
Paulo S. Pizani , Miryam R. Joya , Edson R. Leite , Elson Longo
c
d
e
a
Laborat o´ rio de An a´ lise T e´ rmica e Materiais, Departamento de Qu ı´ mica, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil
b
Departamento de Engenharia Mec aˆ nica, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil
Laborat o´ rio de Semicondutores, Departamento de F ı´ sica, Universidade Federal de S a˜ o Carlos, 13565-905 S a˜ o Carlos, SP, Brazil
Laborat o´ rio Interdisciplinar de Eletroqu ı´ mica e Cer aˆ mica, CMDMC, Departamento de Qu ı´ mica, Universidade Federal de S a˜ o Carlos 13565-905,
c
d
S a˜ o Carlos, SP, Brazil
CMDMC, LIEC, Instituto de Qu ı´ mica, Universidade Estadual Paulista, 14801-907 Araraquara, SP, Brazil
e
Received 27 August 2005; received in revised form 7 November 2005; accepted 18 November 2005
Available online 28 December 2005
Abstract
The BaMoO nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO
4
4
powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL)
and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO at 300 1C was
4
disordered. At 400 1C and higher temperature, BaMoO crystalline scheelite-type phases could be identified, without the presence of
4
additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around
4
0 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40–50 nm. The
sample that presented the highest intensity of the red emission band was the one heat treated at 400 1C for 2 h, and the sample that
displayed the highest intensity of the green emission band was the one heat treated at 700 1C for 2 h. The CPM was shown to be a low
4
cost route for the production of BaMoO nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The
optical properties observed for BaMoO nanopowders suggested that this material is a highly promising candidate for photoluminescent
4
applications.
r 2005 Elsevier Inc. All rights reserved.
4
Keywords: BaMoO ; Nanopowders; Complex polymerization method; Photoluminescence; Amorphous
1
. Introduction
BaMoO can have both structures, tetragonal distorted
luminescent properties of these type materials, numerous
investigations have been carried out [3]. These materials
have been synthesized by a variety of techniques, such as
the Czochralski technique [1], spontaneous crystallization
method [4] and calcinations method [5,6]. However,
4
scheelite (b) and cubic (a), depending of temperature
applied [1]. BaMoO4 with a scheelite structure is an
important material that presents a high application
potential in various fields. Recently, crystal structures
based on the scheelite type have attracted much interest
because of their applications as a scintillating medium and
in other electro-optical devices (i.e., solid-state laser and
optical fibers) [2]. Aiming at a deeper understanding of the
BaMoO powders are relatively large and lack homoge-
4
neous morphology and composition. Such lack of homo-
geneity is due to the MoO tendency to vaporize at high
3
temperatures [7]. In the Complex Polymerization Method
(CPM) this problem is reduced because the synthesis occurs
at a low temperature and the immobilization of the metal
complexes in such rigid organic polymeric networks can
reduce the metal segregation, thus ensuring the composi-
tional homogeneity at the molecular scale. This is of vital
Ã
Corresponding author.
E-mail address: apamarques@liec.ufscar.br (A.P. de Azevedo Marques).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.11.020

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672
importance for the synthesis of multicomponent oxides with
complicated compositions, since the chemical homogeneity,
with respect to the cation distribution throughout the entire
gel system, often determines the compositional homogene-
ity of the final multicomponent oxides [8]. Preparation of
BaMoO4 nanopowders by soft chemical process and
displaying a high chemical homogeneity is very limited.
Besides the successful synthesis of nanopowders by the
CPM, this work also reports on the characterization and
The powders were annealed at 300, 350, 400, 450, 500, 550,
600, 700, 800 and 900 1C for 2 h in static atmosphere at a
heating rate of 5 1C/min.
BaMoO4 nanopowders were characterized by X-ray
diffraction (XRD) using Cu Ka radiation in order to
determine the structural evolution and the unit cell volume
of the nanopowders. The average crystallite diameter (Dcrys)
of the heat-treated powders was determined by XRD, using
from the (112) diffraction peak of the BaMoO phase (2y at
4
photoluminescent properties of BaMoO nanopowders.
4
around 26.51), according to the Scherrer equation (Eq.(1)),
as described by Suryanarayana [9],
2
. Experimental
B ¼ kl=ðDcrys cos yÞ,
(1)
BaMoO4 was obtained by the CPM. A flow chart
representing the synthesis of BaMoO used in this study is
where B is the full width at half maximum (FWHM), y the
Bragg angle, k is a constant and l is the wavelength of the
Cu Ka radiation. The microstructure and surface morphol-
4
outlined in Fig. 1. Molybdenum citrate was formed by the
dissolution of MoO , molybdenum trioxide, in an aqueous
ogy of the nanocrystalline powders of the BaMoO heat
3
4
solution of citric acid under constant stirring at 60–80 1C to
homogenize the solution.
treated at 700 1C were observed by high-resolution scanning
electron microscopy (HR-SEM) using an Field Emission –
Supra35 Gemini-Zeiss equipment.
After the homogenization, barium carbonate was
dissolved and the proper stoichiometric amount was added
to the molybdenum citrate solution. The complex was well
stirred for several hours at 60–80 1C to produce a clear,
homogeneous solution. After the solution was homoge-
nized, ethylene glycol was added to promote the poly-
esterification. With continued heating at 80–90 1C, the
viscosity of the solution increased, albeit devoid or any
visible phase separation. The molar ratio between barium
and molybdenum cations was 1:1, and the ratio between
citric acid and the sum of metals was 6:1. The citric acid/
ethylene glycol mass ratio was set at 60:40.
After partial evaporation of the water, the resin was heat
treated at 300 1C for 2 h, in a static atmosphere, leading to
the partial decomposition of the polymeric gel, forming an
expanded resin, constituted of partially pyrolyzed material.
The product was removed from the beaker and grinded.
The BaMoO FTIR transmittance spectra were deter-
4
ꢀ
1
mined at the 400–2100 cm
frequency range at room
temperature, using an Equinox/55 (Bruker) spectrometer.
The Raman spectroscopy data were obtained at room
temperature by a RFS/100/S Bruker FT-Raman equipment
ꢀ
1
with spectral resolution of 4 cm attached to a Nd:YAG
laser promoting an excitation light of 1064 nm, in the
ꢀ
1
frequency range of 100–1000 cm
.
The photoluminescence (PL) spectra of the BaMoO4
nanopowders were collected with a U1000 Jobin-Yvon
double monochromator coupled to cooled GaAs photo-
multiplier and a conventional photon counting system.
The 488.0 nm exciting wavelength of an argon ion laser
was used. The maximum output power of the laser was
20 mW. All measurements were taken at room tempera-
ture.
Citric Acid
+
Water
Thermal treatment
(pyrolysis)
Molybdic Acid
60-80°C
(
MoO3)
Molybdenum
citrate
De-agglomeration
Heat treatment
Barium
carbonate
60-80°C
Barium
Molybdenum
citrate in water
Ethylene glycol
BaMoO4 nanopowders
Formation of Complex
Polymeric Precursor
solution (BaMoO ) (90°C)
4
4
Fig. 1. Flowchart representing the procedure for the synthesis of BaMoO nanopowders.

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673
3
. Results and discussion
but high reaction times are required. In the present work,
the temperature and reaction time used are low, respec-
tively, around 500 1C and 2 h, and furthermore, the
reaction products do not present any additional phase.
Therefore, CPM was shown to be a low cost route for the
BaMoO has a scheelite crystal structure with tetragonal
4
6
4
symmetry C at room temperature. Its phase transition
h
from tetragonal scheelite (b) to cubic structure (a) takes
place at 1873 1C [1]. Experiments on heating and cooling
production of BaMoO , at a lower temperature and
4
BaMoO crystals revealed that the above-mentioned b2a
reaction time.
Fig. 2 shows the XRD patterns of the BaMoO4
nanopowders heat treated at different temperatures. In
4
phase transition was reversible. In the experimental
methods which require high temperature reactions, the
presence of this phase transition results in difficulties with
Fig. 2a, the BaMoO at 300 1C for 2 h was amorphous and
4
growing BaMoO single crystals from the melt.
4
no crystalline phases were observed. At 400 1C and higher
temperatures (Fig. 2b–e), the nanopowders could be
Methods for the synthesis of BaMoO are given in Table 1.
4
In the Czochralski [1] method, the spontaneous crystal-
lization method [4] and in the calcinations method reported
by Hitoki [5], high temperatures and long reaction times
are necessary for the production of binary oxide. The
calcination method described by Hasan [6] utilizes lower
temperature, around 450 1C, but the reaction products
contain an additional phase, BaO ꢁ 8H O. In the method
identified as crystalline BaMoO displaying single schee-
4
lite-type phases [3,10–13]. All diffraction peaks were
indexed according to the JCPDS database [13] and pointed
to the tetragonal symmetry.
The CPM was shown to be efficient for the synthesis of
BaMoO nanopowders. From the peak positions displayed
4
in Fig. 2, the lattice parameters were calculated using the
least square refinement from the REDE93 program. The
lattice parameters a and c for these BaMoO powders are
2
2
used by Nassif [10] the temperature is of around 600 1C,
4
also close to the values reported for bulk material
˚
Table 1
Barium molybdate synthesis methods
˚
(
a ¼ 5:5802 A and c ¼ 12:821 A) [13].
Table 2 presents the lattice parameters a and c and the
mean crystallite sizes for the BaMoO nanopowders heat
treated at different temperatures, the values are near to
Method
Temperature (1C) Time (h) Synthesis products
4
Czochralski [1]
Spontaneous
1100–1200
950
30–40
72
BaMoO
BaMoO
4
40 nm. The calculated mean crystallite sizes showed the
tendency to increase with the heat treatment temperature.
4
crystallization [4]
Calcination [5]
Calcination [6]
900
450
24
3
BaMoO
BaMoO
4
Fig. 3 presents a HR-SEM micrograph of BaMoO4
4
,
powders heat treated at 700 1C. This figure shows an
agglomerate of individual BaMoO4 particles, whose
crystallite sizes are of around of 40–50 nm.
The spontaneous Raman spectra with the assignments
of the Raman-active vibration modes of the BaMoO4
BaO
2
ꢁ 8H
2
O
BaCl
2
a
+Na MoO
2
4
[10] 600
400–500
72
2
BaMoO
BaMoO
4
CPM
4
a
The present work.
(e)
(
d)
c)
(
(
b)
a)
(
20
30
40
50
60
70
2θ (°)
Fig. 2. X-ray diffraction patterns of BaMoO
4
nanopowders heat treated at (a) 300 1C, (b) 400 1C, (c) 500 1C, (d) 600 1C and (e) 700 1C.

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674
Table 2
Composition, crystallite sizes and comparison of tetragonal lattice
The scheelite primitive cell presents 26 different vibration
modes: GTd ¼ 3Ag þ 5Au þ 5Bg þ 3Bu þ 5Eg þ 5Eu, but
only (A , B and E ) are Raman-active, while the odd
a
4
constants of tetragonal structure of BaMoO nanopowders prepared by
g
g
g
the CPM method and heat treated at different temperatures
modes (4A and 4E ) can be registered only in the infrared
u
u
b
˚
Lattice constants (A)
spectra. The tree B vibrations are silent modes; one A and
u u
Sample
Crystallite sizes
nm)
(
one E modes are acoustic vibrations. The Raman spectra
u
a
c
showed the well-resolved sharp peaks for the BaMoO4
nanopowders treated at 500–700 1C, indicating that the
synthesized powders were highly crystallized.
BaMoO
BaMoO
BaMoO
BaMoO
4
–400 1C
4
–500 1C
4
–600 1C
4
–700 1C
3571.8
4072.1
4472.3
4772.4
5.5725(5)
5.5739(6)
5.5770(7)
5.579(1)
12.833(2)
12.83(2)
12.813(3)
12.811(4)
In the Raman spectra of BaMoO4 nanopowders
annealed at 300 1C it is clear that undecomposed organic
ligands are still present. The Raman spectra of BaMoO4
heat treated at 400 1C does not present such well-resolved
sharp peaks, but its crystallization was observed by XRD.
Probably, during the Raman experiment, the PL of the
material overlapped with the peaks of the Raman spectra.
Fig. 5 shows the FTIR spectra of the BaMoO4
nanopowders. Measurements were carried out in the
transmittance mode. The representation for tetrahedral
symmetry is GTd ¼ A1ðn1Þ þ Eðn2Þ þ F2ðn3Þ þ F2ðn4Þ, but
only the F (n , n ) modes are IR active. The F (n )
a
˚
˚
a ¼ 5:5802 A and c ¼ 12:821 A [13].
b
Calculated using the (112), 100% diffraction peak.
2
3
4
2
3
vibrations are the antisymmetric stretches, and the F (n )
2
4
vibrations are bending modes. From the spectrum of
BaMoO4 annealed at 300 1C (Fig. 5a), it is clear that
undecomposed organic ligands are still present in the
powders. This spectra present several bands ascribe to
carbonates groups, second Nakamoto [14], the u(CO)
ꢀ
1
stretching occur at 2375–1100 cm
1
and the u(OCO)
stretching occur at 1100–550 cm . The spectra of the
ꢀ
BaMoO heat treated at 400–700 1C (Fig. 5b–e) display a
4
ꢀ
1
very broad absorption band from 800 to 870 cm . These
bands for the BaMoO4 nanopowders heat-treated at
4
00–700 1C are assigned to F (n ) antisymmetric stretch
2
3
vibrations, which were reported to be ascribed to the
ꢀ
2
Mo–O stretching vibration in MoO tetrahedra [14]. After
being submitted to the heat treatments at 400, 500, 600 and
4
700 1C (Fig. 5b–e), the BaMoO4 samples present well-
defined bands attributed to the Mo–O bonds. With the
increase of the heat treatment temperature, the crystalline
structure is increasingly ordered and the bands attributed
to the Mo–O bonds showed a better definition, a charac-
teristic confirmed by XRD and Raman spectroscopy.
Fig. 6 illustrates the PL spectra recorded at room
4
Fig. 3. HR-SEM micrograph of BaMoO nanopowders heat treated at
700 1C for 2 h.
nanopowders are presented in Fig. 4 and detailed in
Table 3. According to Basiev [1], the primitive cell of
temperature for the BaMoO nanopowders heat treated at
4
different temperatures, 300–900 1C, for 2 h. The samples
were excited by the 488 nm line of an argon ion laser. The
PL emission characteristics were broad and intense bands.
2
BaMoO includes two formula units. The [MoO ] ionic
ꢀ
4
4
group, with strong covalent Mo–O bonds, is peculiar to the
BaMoO4 scheelite structure. Due to weak coupling
The BaMoO emission bands peaked at a wavelength of
4
2
between the [MoO4] ionic group and the Ba
ꢀ
2+
cations,
the vibrational modes observed in the spontaneous Raman
about 2.15 eV for the amorphous nanopowders and around
2.26 eV for the crystalline nanopowders. The PL emission
spectra could be decomposed into two Gaussian-shaped
curves, ascribed to red and green emission bands.
The sample that presented the most intense red emission
band was the one heat treated at 400 1C for 2 h (Fig. 6I, II).
As for the most intense green emission band, it corre-
sponded to the sample calcined at 700 1C for 2 h (Fig. 6III,
IV). The intensity of the PL emission significantly increased
spectra of BaMoO scheelite crystals can be divided into
4
two groups, internal and external modes.
The internal vibrons correspond to the vibrations within
2
ꢀ
the [MoO4] group, with an immovable mass center. The
external or lattice phonons correspond to the motion of the
2
+
2ꢀ
Ba cations and the rigid molecular unit. [MoO4]
tetrahedral in free space have T -symmetry.
d

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675
ν1
ν2
ν3
ν4
νext
νf.r.
(
(
(
e)
d)
c)
(
b)
(
a)
0
200
400
600
800
1000
-1
Raman frequency (cm )
4
Fig. 4. Spontaneous Raman spectra of BaMoO nanopowders heat treated at (a) 300 1C, (b) 400 1C, (c) 500 1C, (d) 600 1C and (e) 700 1C.
Table 3
Raman mode frequencies in BaMoO
4
nanopowders heat treated at different temperatures
Lattice mode
6
symmetry C4h
BaMoO
00–700 1C
4
Assignments
a
b
c
5
1100–1200 1C
Room temperature
A
g
891
838
791
359
346
326
325
190
141
107
78
892
837
791
358
345
324
324
188
137
110
76
888
835
789
361
348
323
323
187
133
107
—
n
n
1
1
(A )
B
g
E
g
E
g
B
g
B
g
3
(F )
2
n
4
2
(F )
2
n
(E)
A
g
E
g
B
g
E
g
B
g
E
g
nf.r. (F
1
) free rotation
2
4
ꢀ
2+
and Ba motions
next.—external modes MoO
76
74
—
a
The present work.
Czocharalski method [1].
b
c
4
Electrochemical preparation of BaMoO solid solution films at room temperature [11].
(
4
about 18 times) comparing the powders heat treated at
00 1C for 2 h with the sample heat treated at 600 1C. The
PL intensity of the nanopowders heat treated at 400 1C for
h was 1.6, 1.8, 11.9, 9.1, 10.2, 18.3, 3.1, 4.3 and 3.9 times
higher than the intensity of the samples heat treated at 300,
50, 450, 500, 550, 600, 700, 800 and 900 1C for 2 h,
700 1C, respectively. The highest red emission PL was
obtained from the structure that was neither completely
amorphous (300 1C) nor fully ordered (600 1C). The most
crystalline sample (700 1C) presented the highest green
emission PL.
2
3
In this work, it was observed that the BaMoO samples
4
respectively.
with the most intense PL in the red region possess the
biggest red area (0.65) and the samples with the highest PL
intensity in the green region display the biggest green area
(0.74). These results are in agreement with the work of
Marques et al. [15], in which the BaMoO thin film with the
4
highest PL intensity possessed the biggest red area. The
The PL intensity and the band emission wavelength
depend on the thermal treatment history of the powders,
and therefore, to their structural disorder or order. Fig. 6
showed that the most intense PL intensity for the red and
green emission correspond to the samples heat 400 and

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676
(
e)
(
(
d)
c)
821
(
b)
(
a)
1400
1200
1000
800
600
400
-
1
Wavenumber (cm
)
Fig. 5. FTIR absorption spectra of BaMoO
4
nanopowders heat treated at (a) 300 1C, (b) 400 1C, (c) 500 1C, (d) 600 1C and (e) 700 1C.
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
0
400 °C
800
4
00 °C
Red Area
700
600
500
400
300
200
100
0
Green Area
3
4
50 °C
50 °C
1.6
1.8
2.0
2.2
2.4
2.6
1.6
1.8
2.0
2.2
2.4
2.6
(
I)
Photon energy (eV)
(II)
Photon energy (eV)
3
00
8
7
6
00
00
00
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
0
7
00 °C
2
2
1
50
00
50
Red Area
Green Area
500
400
300
200
100
0
100
5
0
0
700 °C
1.6
1.8
2.0
2.2
2.4
2.6
Photon energy (eV)
9
4
00 °C
50 °C
1.6
1.8
2.0
2.2
2.4
2.6
1.6
1.8
2.0
2.2
2.4
2.6
(
III)
Photon energy (eV)
(IV)
Photon energy (eV)
4
Fig. 6. Room-temperature PL spectra of BaMoO nanopowders excited with the 488 nm line of an argon ion laser: (I) heat treated at 400 1C for 2 h,
(II) heat treated at 300, 400 and 450 1C for 2 h, (III) heat treated at 700 1C for 2 h, (IV) heat-treated at 450, 700 and 900 1C for 2 h.

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677
nanopowder sample in this work that presented the
smallest PL intensity was the one heat treated at 600 1C
for 2 h. Its PL emission area distributed with 49% in the
red region and 51% in the green region, practically half for
each emission, indicating that different emissions compete
for the PL intensity.
PL in the red region practically vanishes and the PL in the
green region appears. It is agreed that the MoO clusters
4
are responsible for the green luminescence of molybdates.
The PL spectra show that the transition between the
completely amorphous structure and the completely
ordered structure is suitable for a good PL emission in
the red region, and, that a complete order is suitable for a
good PL emission in the green region.
Marques et al. [15] showed that the emission bands from
BaMoO thin films can be decomposed into two compo-
4
nents, two broad bands with their maxima at around
2
(
(
.07 eV (600 nm) (orange-red), peak 1, and 2.34 eV
530 nm) (green), peak 2. In this work, the red band
peak 1) represents, respectively, 59%, 65%, 54%, 49%,
4
. Conclusions
A simple method to prepare scheelite BaMoO nano-
4
2
nanopowders heat treated at 300, 400, 500, 600, 700, 800
and 900 1C for 2 h, respectively (see Table 4).
6%, 47% and 43% of the total PL band area for
powders is presented. The XRD, IR and Raman spectro-
scopy showed that the BaMoO at 300 and 350 1C were
disordered. At 400 1C and higher temperatures, crystalline
4
The decomposition of the overall PL emission spectra
indicates that the fraction of overall PL emission ascribed
to the red emission band increases with the degree of
structural disorder. In the most crystalline structure, the
fraction of the green emission band is bigger that the
fraction of the red emission band. Both observations agree
with the works of Marques et al. [15] and Orhan et al. [16].
The nanopowder sample heat treated at 400 1C possesses
the biggest red area because it is a transition between a
completely amorphous structure and a completely ordered
structure, similarly to BaMoO4 thin film reported by
BaMoO scheelite-type phases could be identified, without
4
the presence of additional phases. The lattice parameters
values are also close to values reported for the bulk
material. The calculated mean crystallite sizes was around
40 nm. The Raman spectra showed that the synthesized
powders were highly crystalline for the BaMoO nano-
4
powders treated at 500–700 1C, whereas at 400 1C the main
Raman peaks are only barely seen. The FTIR spectra of
BaMoO annealed at 400–700 1C presented a very broad
4
ꢀ
1
absorption band from 800 to 870 cm , related to the
2
4
ꢀ
Mo–O stretching vibration in MoO tetrahedra. Higher
Marques et al. [15] and the SrWO reported by Orhan et al.
4
heat treatment temperature of the materials showed more
order in the FTIR spectra, confirmed by XRD and Raman
spectroscopy. Intense visible PL at room temperature was
observed in amorphous and crystalline BaMoO4 nano-
[
16]. The nanopowders heat treated at 700 1C display the
biggest green area because it is a completely ordered
structure [15,16].
The components of the PL emission bands are probably
linked to specific atomic arrangements. In the polyester-
ification stage of the citrate solution containing the
molybdenum and barium ions occurs the structural
transformations from disordered to ordered phases [15].
Molybdenum, the lattice former, ideally has the tendency
to bond with four oxygen atoms, but before it reaches this
ideal configuration there are various coordination numbers
for Mo in the structure. Before the crystallization, the
powder samples. The BaMoO nanopowder sample that
4
presented the most intense red emission band was the one
heat treated at 400 1C for 2 h. The sample that presented
the most intense green emission band was the one heat
treated at 700 1C for 2 h. The optical properties observed
for BaMoO nanopowders suggested that this material is a
4
highly promising candidate for photoluminescent applica-
tions. The CPM method was shown to be a low cost route
for the production of BaMoO , at a lower temperature and
4
structure is a mixture of MoO clusters (x ¼ mostly 3 and
x
at a smaller time and a reduced cost.
4) intercalated by Ba atoms. This mixture of the clusters is
responsible for the PL in the red region. When the
Acknowledgments
crystallization is reached, only MoO clusters exist, the
4
The authors gratefully acknowledge the financial sup-
port of the Brazilian research financing institutions
FAPERN, CEPID, CNPq and PRONEX.
Table 4
Evolution of the red PL fraction with the structural order
Samples
Peak 1 (eV)
Peak 2 (eV)
RA/FA
References
BaMoO
BaMoO
BaMoO
BaMoO
BaMoO
BaMoO
BaMoO
4
–300 1C/2 h
–400 1C/2 h
–500 1C/2 h
–600 1C/2 h
–700 1C/2 h
–800 1C/2 h
–900 1C/2 h
1.93
1.99
1.92
1.90
2.01
2.09
2.02
2.21
2.32
2.34
2.44
2.32
2.33
2.38
0.59
0.65
0.54
0.49
0.26
0.47
0.43
[
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