Advances in Chemical Engineering and Science, 2012, 2, 465-473 Published Online October 2012 (
Photoluminescent Properties of CoMoO4 Nanorods
Quickly Synthesized and Annealed in a Domestic
Microwave Oven
Ana P. de Moura1, Larissa H. de Oliveira2, Paula F. S. Pereira2, Ieda L. V. Rosa2*, Máximo S. Li3,
Elson Longo1, José A. Varela1
1Chemistry Institute, University of Sao Paulo State, Araraquara, Brazil
2Chemistry Department, Federal University of Sao Carlos, São Carlos, Brazil
3Physical Institute, University of Sao Paulo, São Carlos, Brazil
Email: *
Received July 3, 2012; revised August 6, 2012; accepted August 18, 2012
A simple way to prepare
- and
-CoMoO4 nanorods is reported in this paper. CoMoO4·xH2O nanorod precursors were
obtained using the microwave-assisted hydrothermal (MAH) method. By annealing the as-prepared CoMoO4·xH2O
precursor at 600˚C for 10 min in a domestic microwave oven,
- and
-CoMoO4 nanorods were prepared. These pow-
ders were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier trans-
form Raman microscopy and ultraviolet visible absorption spectroscopy (UV-vis spectra) as well as photoluminescence
(PL) measurements. Based on the results, these materials revealed nanorod morphology. PL spectra obtained at room
temperature for
- and
-CoMoO4 particles exhibited maximum components around the blue light emission. The results
show that the domestic microwave oven has been successfully employed to obtain
- and
-CoMoO4 nanoparticles.
Keywords: Photoluminescence; Nanorods; Molybdates; Microwave-Hydrothermal Method
1. Introduction
Materials belonging to the molybdate and tungstate fami-
lies have a long history of practical applications due to
their excellent optical properties in phosphors, laser ma-
terials, and scintillation detectors [1-3].
Tungstate and molybdate based materials with a
schelite-type structure, have molybdenum (or tungsten)
atoms in a tetrahedral coordination as well as large biva-
lent cations (ionic radius > 0.99 Å such as Ca, Ba, Pb and
Sr) as lattice modifiers [4]. However, when the modifier
atoms are small bivalent cations (ionic radius < 0.77 Å;
e.g., Fe, Mn, Co, Ni, Mg and Zn), the tungstate and mo-
lybdate-based materials have a wolframite-type structure
where the molybdenum (or tungsten) atom adopts an
overall six-fold coordination [5,6].
Among these materials, cobalt molybdate (CoMoO4) is
one of the most important components of industrial cata-
lysts for the partial oxidation of hydrocarbons and pre-
cursors in the synthesis of hydrodesulphurization cata-
lysts [7-9]. In addition, CoMoO4 can also be applied in
the electronics and bioscience industries due to its struc-
tural, magnetic, electronic and antibacterial properties
[9-17]. Therefore, four stoichiometric type CoMoO4 crys-
talline structures are reported by the literature. The phase
-CoMoO4 was verified at low temperatures [18]; at high
temperatures, the
-CoMoO4 phase was stable [19] while
at high pressures, the hydrated phase (CoMoO4·xH2O) is
the more stable phase [10].
In recent years, different chemical methods have been
employed to synthesize CoMoO4 powders such as solid
state reaction [18,20], co-precipitation [21], sol-gel [22]
and hydrothermal methods [14], etc. However, some of
these methods require long annealing times and result in
higher costs [23] as well as other disadvantages such as
the formation of a large amount of organic waste,
polydisperse particles sizes and an undefined morphol-
Komarneni et al. [24,25] combined microwave radia-
tion with the hydrothermal system to synthesize ceramic
powders. The use of microwave energy in a conventional
hydrothermal system promoted the development of a new
technique which facilitated rapid heating and rates of
crystallization [26]. Thus, the MAH method can be em-
ployed as an efficient synthesis route in processing mate-
rials due to rapid kinetics which accelerates the crystalli-
zation process through the enhancement of the nucleation
*Corresponding author.
opyright © 2012 SciRes. ACES
rate. In this method, chemical reaction is related to the
effect of microwave radiation during the experimental
procedure which interacts with the permanent dipole of
the solvent and induces molecular vibration and pro-
motes rapid heating as a result of molecular rotation [27].
Therefore, in this study, CoMoO4·xH2O precursors
were prepared by the MAH method and then heat treated
using microwave radiation to form the desired
- and
-CoMoO4 phases. These materials were chemically,
structurally and morphologically investigated by XRD,
FE-SEM, (UV-vis) and Raman spectroscopy; their opti-
cal properties were also investigated by PL measure-
ments at room temperature.
2. Experimental Section
2.1. Synthesis of the Precursors
In a typical procedure, 2 mmol of sodium molybdate
solution was dissolved in 50 mL of distilled water. Then,
2 mmol of cobalt nitrate hexaydrate was dissolved in 50
mL of deionized water which was slowly added to the
sodium molybdate solution under magnetic stirring
which produced a homogeneous solution (pH = 6). The
reaction mixture was transferred to a Teflon-lined stain-
less autoclave which was finally sealed and placed in the
MH system using 2.45 GHz microwave radiation with
maximum power of 800 W. MH conditions were kept at
140˚C for 1, 2, 4, 8, 16 and 32 minutes. Then, the auto-
clave was naturally cooled to room temperature. These
as-prepared purple precursor powders were labeled as P1,
P2, P4, P8 P16 and P32, respectively. Precursors P1, P2,
P4, P8, P16 and P32 were water washed several times
until a neutral pH was obtained and then dried at 60˚C
for 8 h under atmospheric air in a conventional furnace.
2.2. Synthesis of
- and
-CoMoO4 Powders
CoMoO4 powders were obtained from the thermal de-
composition of precursors P1, P2, P4, P8, P16 and P32.
These precursor powders were placed in ceramic cruci-
bles and heated in a microwave sintering furnace at
600˚C for 10 minutes. The calcinated brown-like pow-
ders were labeled D1, D2, D4, D8 D16 and D32 samples,
2.3. Characterizations
Precursors P1, P2, P4, P8 P16 and P32 and
- and
-CoMoO4 powders D1, D2, D4, D8 D16 and D32 were
characterized by powder XRD using a Rigaku-DMax
2500PC, (Japan) with Cu Kα radiation (λ = 1.540598 Å)
in the 2θ range from 5˚ to 75˚ using a scanning rate of
0.02˚/min. The morphologies of the samples were veri-
fied using FE-SEM (Jeol JSM 6330F). The particle size
of precursors and CoMoO4 powders were determined
using FE-SEM images and were calculated using the
Image J. program. UV-vis spectra were taken using Cary
5G (Varian, USA) equipment in the diffuse reflection
mode. The thermal decomposition of the precursor pow-
ders was studied by thermogravimetric analysis (TGA/
DTA) on a TGA 2050 thermal analysis device (American
TA Corporation). TGA determination was carried out in
air at a heating rate of 20˚C/min1 in the range from room
temperature to 900˚C.
PL precursor measurements were taken in a Thermal
Jarrel-Ash Monospec 27 monochromator and a Hamama-
tsu R446 photomultiplier. The 350.7 nm exciting wave-
length of a krypton ion laser (Coherent Innova) was used
with the nominal output power of the laser power kept at
200 mW. All the measurements were taken at room tem-
3. Results and Discussion
3.1. Precursor Nanorods
3.1.1. XRD Patterns of the Precursor Nanorods
Figure 1 shows XRD patterns of precursor P1 from MAH
processing at 140˚C for 1 minute. The results revealed
that MAH treatment led to the formation of a pure phase
CoMoO4·xH2O with good crystallinity (see Figure 1).
All reflection peaks of different samples prepared at
different times can be easily indexed as the Co-
MoO4·xH2O phase of the purple powders. The formation
of CoMoO4·xH2O pure powders with a monoclinic
structure and cell parameters of
= 9.501 Å, b = 8.897 Å,
c = 7.751 Å and
= 113.83˚ (Joint Committee on Pow-
der Diffraction Standards (JCPDS) N˚. 26-0477) was
verified. These observations confirm that MH processing
results in the formation of a product based on XRD pat-
terns of the as-prepared P1 powder obtained from the
MAH method at 140˚C for 1 min.
Figure 1. XRD patterns of the precursor P1 microwave-
hydrothermal assisted at 140˚C for 1 minute.
Copyright © 2012 SciRes. ACES
3.1.2. FE-SEM Images of the Precursors Nanorods
Figure 2(A) shows FEG-SEM images of the P1 powders.
This sample consists of large-scale and homogeneous
CoMoO4·xH2O particles with morphology-like nanorods.
This morphology was observed for all P1, P2, P4, P8 P16
and P32 precursors. Figure 2(B) is a histogram with the
calculated average thickness of these nanorods evaluated
as 100 - 300 nm. Nanorods lengths were determined as
ca. 1 - 3 m. All other samples prepared at different
times showed similar morphology.
3.1.3. TGA Curves of Precursor Nanorods
The thermal behavior of the P1 precursor nanostructure
was examined by TGA measurements (see Figure 3).
The TGA curve shows that the precursor decomposi-
tion occurs in two steps: 1) in a temperature range of
84˚C and 165˚C; and 2) between 168˚C and 450˚C. A
total weight loss of 5.75% was obtained which, accord-
ing to Ding and co-workers [6], is largely ascribed to
water loss corresponding to the water content in the hy-
drated product. These researchers also observed these
two steps where the desired CoMoO4 crystalline structure
occurred at 45˚C. Thus, the 600˚C temperature was cho-
sen to heat treat precursor powders to obtain the pure
Figure 2. (A) FEG-SEM micrographies images and (B)
average thickness distribution of the particles (nm) of the
CoMoO4·xH2O powder obtained by the microwave-hydro-
thermal assisted method (MH) at 140˚C for 1 min (P1).
Figure 3. TGA curves of the as-prepared CoMoO4·xH2O
precursor powder obtained from by the microwave-hydro-
thermal assisted method (MH) at 140˚C by 1 min.
- and
3.2.1. XRD Patterns of
- and
-CoMoO4 Powders
XRD patterns of
- and
-CoMoO4 phases obtained by
thermal decomposition of the precursor (CoMoO4·xH2O)
are shown in Figure 4.
XRD patterns reveal that all calcined samples have
well defined narrow diffraction peaks of two phases that
are ordered at long range.
-CoMoO4 has a monoclinic
structure with a space group of C2/m (Joint Committee
on Powder Diffraction Standards) (JCPDS N˚. 25.1434),
-CoMoO4 has a monoclinic structure with a space
group of C2/m (JCPDS No. 21-0868), respectively.
3.2.2. R aman Spectra of
- and
-CoMoO4 Powders
Figure 5 shows Raman spectra of D1, D2, D4, D8 D16
and D32 samples, respectively. Raman spectra collected
at room temperature showed that both
have similar vibrational modes related to the CoMoO4
with a space group of C2/m. These vibrational modes are
observed at 330, 369, 698, 815, 879, and 941 cm–1. The
Raman mode at 941 cm–1 is associated with the symmet-
ric stretching mode of Mo-O. The bands located at 879
cm–1 and 815 cm–1 correspond to asymmetric stretching
modes of oxygen in binding O-Mo-O. The bands ob-
served at 330 cm–1 and 369 cm–1 are related to asymmet-
ric and symmetric bending modes of the O-Mo-O, respec-
tively. The band located at 698 cm–1 can be attributed to
the symmetric stretching of the Co-O-Mo bond (see Ta-
ble 1). Figure 5 illustrates that all Raman spectra have the
same features. The results obtained are in agreement with
the literature [28], and the small variations in the positions
of the vibrational modes can be associated with the me-
thod of preparation, crystal size, morphology and strength
of interaction between the ions and the degree of struc-
tural order-disorder of these materials.
Copyright © 2012 SciRes. ACES
Figure 4. XRD patterns of the (a) D1, (b) D2, (c) D4, (d) D8,
(e) D16 and (f) D32 powders prepared by thermal decompo-
sition of the CoMoO4·xH2O precursors at 600˚C for 10 min-
Figure 5. Raman spectra of the (a) D 1, (b) D 2, (c) D 4, (d) D
8, (e) D 16 and (f) D 32 samples.
Table 1. Relative positions of the Raman active modes for
the samples of
- and
Symbol Position Vibrational Mode
P6 941 cm–1 Symmetric stretch O-Mo-O
P5, P4 879 cm–1 e 815 cm–1 Assimmetric stretch O-Mo-O
P3 698 cm–1 Symmetric stretch Co-Mo-O
P2 330 cm–1 Assimmetric torsional
mode O-Mo-O
P1 369 cm–1 Symmetric torsional
mode O-Mo-O
3.2.3. FE-SEM An al ysi s of
- and
-CoMoO4 Powders
Crystal structures of
- and
-CoMoO4 rods annealed at
600˚C were characterized by FE-SEM.
Figure 6 shows FE-SEM images of (a) D1, (b) D2, (c)
D4, (d) D8, (e) D16 and (f) D32 powders, respectively.
The nanorod-like morphology of
- and
-CoMoO4 was
similar to the precursors (Figures 2 and 6) which
indicate that this method is a good approach to prepare
powders with controlled morphology.
Figure 7 shows the average thickness distribution of
particles for samples (A) D1, (B) D2, (C) D4, (D) D8, (E)
D16 and (F) D32. According to these graphics all parti-
cles have the same thickness being which is evaluated as
ca. 250 and 350 nm. Particle lengths were determined to
be between 2 to 4 μm.
3.3.4. Ultr a vio l e t-Vi si bl e Absorption Spect roscopy
The optical energy gap is associated with the degree of
structural order and disorder of the materials in a medium
range. The order/disorder ratio leads to different defect
densities in the material which results in different distri-
butions of intermediate levels of energy between the va-
lence (VB) and conduction band (CB). Most crystalline
materials exhibit a greater degree of organization when
compared to the less crystalline materials with more
structural defects. Therefore, these materials exhibit a
high optical energy gap value because they have small
intermediate energy levels between the VB and CB [29].
The optical band gap energy (Eg) of
- and
nanorods was estimated by the method proposed by
Wood and Tauc [30]. According to these authors the Eg
is associated with absorbance and photon energy by the
following equation:
 
 , (1)
is the absorbance, h is Planck’s constant,
the frequency, Eg is the optical band gap and m is a con-
stant associated with the different types of electronic
transitions (m = 1/2, 2, 3/2 or 3 for direct allowed, indi-
rect allowed, direct forbidden and indirect forbidden
transitions, respectively). In this case, Eg values of the
powders were evaluated by extrapolating the linear por-
tion of the curve or tail. The m value suggested in our
work is 2 which indicate an indirect allowed transition.
The Uv-vis absorbance spectra of the samples as well
as the gap values are presented in Figure 8. The value of
the Eg was quantified from spectra in Figure 8. Samples
D1, D2, D4, D8, D16 and D32 have respectively gap
values of 2, 1.8, 2.1, 2.1, 2.4, and 2.0 eV. The results
obtained in this study are in accordance with the value of
1.8 eV already published [31]. The small difference in
our results is probably due to quantum size effects as
well as reaction conditions of the medium. Both effects
promote different structural defects in the materials. The
preparation time for precursors with microwave irradia-
tion results changes Eg values. A pronounced difference
between samples D2 min and D16 min is evident. Ac-
cording to another report [1], three different charge states
Copyright © 2012 SciRes. ACES
3.3.5. PL Measurements occurring in oxygen vacancies in molybdenum are
[MoO3·], [MoO3·O], and [MoO3·O] complex
states. The [MoO3·O], complex state has two paired
electrons ↑↓ and is a neutral relative to the lattice. The
single ionized [MoO3·O
V], complex state has one un-
paired electron , and the [MoO3·O
V] complex state did
not trap any electrons and was doubly positively charged
with respect to the lattice. The authors speculate that
these oxygen vacancies induced new energy in the band
gap and were attributable to the molybdenum-oxygen
complex vacancy centers. In this case, the interaction of
these clusters with microwave irradiation results in
distortions in the angles and lengths of bonds which
generates different degrees of defects and different
distributions of intermediate energy levels between VB
and CB.
VFigure 9 illustrates PL spectra of D1, D2, D4, D8 D16
and D32 powders at room temperature using excitation
of a krypton laser source at 350.7 nm. These PL spectra
exhibited a broad band in the range of 300 - 600 nm
which is ascribed to a multiphoton process where many
intermediate levels of energy are involved in the PL
process where relaxation occurs by several paths. PL
spectra observed for all powders show a maximum emis-
sion located at 450 nm (blue region of the electromag-
netic spectrum). According to Wu et al. [32,33], the
emission band shape might be explained by considering
the Jahn-Teller active vibration modes of T2 symmetry
that influence the [MoO4]2 complex anion of slightly
distorted tetrahedral symmetry which leads to a struc-
ured absorption band for the A1-T1(2) transition. t
Figure 6. FE-SEM micrographies
- and
-CoMoO4 samples: (A) D 1, (B) D 2, (C) D 4, (D) D 8, (E) D 16 and (F) D 32.
Copyright © 2012 SciRes.
Figure 7. Average thickne ss distri bution of the partic les (nm) for the samples: (A) D1, (B) D2, (C) D4, (D) D8, (E) D16 and (F)
Figure 8. UV-vis absorbance spectra of the samples: (A) D1, (B) D2, (C) D4, (D) D8, (E) D16 and (F) D32.
Copyright © 2012 SciRes. ACES
Copyright © 2012 SciRes. ACES
of energy within the band gap. These energy levels are
basically composed of oxygen 2p states (near the valence
band) and Mo 4d states (below the conduction band). In
these cases, polarizations induce a symmetry breaking
and localized energy levels that favor the trapping of
electrons. For CoMoO4 powders synthesized in this study,
the ratio of order and disorder can result from the inter-
action of microwave energy with [MoO3] and [MoO4]
clusters. The interaction of these clusters with microwave
radiation produces the formation of defects and/or distor-
tions in these materials.
- and
-CoMoO4 nanorods have a broad band
emission typical of systems where relaxation processes
occur by different paths which involve intermediary lev-
els in the “band gap”. Thus, the decomposition of these
broad bands (see Figure 10) was used to obtain informa-
tion on the PL response influenced by electronic transi-
tions. The decomposition was performed using the Peak-
Fit Program (version 4.05), and the Gaussian function
was used successfully to fit the PL peaks and tuning pa-
rameters, including peak positions and corresponding
areas (see Table 2 ). In this study, PL curves can be com-
posed of three components: 1) the blue component (with
a maximum at around 440 nm); 2) the blue-green com-
ponent (with a maximum at around 475 nm); and the
green component (with a maximum at around 525 nm).
n analysis of the evolution of the PL emission of D1,
Figure 9. PL spectra of the samples
- and
-CoMoO4: (A)
D1, (B) D2, (C) D4, (D) D8, (E) D16 and (F) D32.
Figure 9 also shows that the precursor processing time
influenced PL behavior due to the verified differences in
emission intensities and a slight shift in the maximum
intensities positions.
The PL maximum emission intensity is related to the
effects of structural order and disorder in the material
which result in different electron transfer processes due
to different distributions of intermediate levels of energy
between the VB and CB. Studies of molybdates PL [1]
suggest that distorted clusters of [MoO3] and [MoO4] in
the network lead to the formation of intermediate levels A
Figure 10. Deconvolution of the PL spectra of for the samples
- and
-CoMoO4: (A) D1, (B) D2, (C) D4, (D) D8, (E) D16 and
(F) D32.
Table 2. Data obtained by the decomposition of PL bands of
- and
-CoMoO4 samples.
Peak 1 Peak 2 Peak 3
Sample PL emission maximus (nm)
(nm) Área (%)
(nm) Área (%)
(nm) Área (%)
D 1 455 445 54.7 483 30.0 529 15.2
D 2 453 442 52.5 478 35.8 526 11.5
D 4 453 442 51.5 478 34.3 525 14.1
D 8 453 438 46.2 475 39.2 521 14.5
D 16 451 440 49.6 477 36.5 525 13.8
D 32 455 442 51.2 478 36.3 526 12.3
D2, D4, D8 D16 and D32 powders (see Table 2) reveals
that all samples exhibit a PL emission maximum at ca.
455 nm (blue emission) which indicates that the charge
transference process as well as the trapping of electrons
occurs because of a greater contribution of shallow holes
rather than deep holes [33].
4. Conclusion
A synthesis process utilizing the MAH method for a
short time was successful in obtaining precursors of both
- and
-CoMoO4 Nanorods. XRD patterns and Raman
spectra at room temperature showed that these powders
have both
phases related to the CoMoO4 with a
space group of C2/m. These materials contain nanorod
morphologies, and all samples exhibit a PL emission
maximum at ca. 455 nm (in the blue emission) which
indicates that the charge transference process as well as
the trapping of electrons occurs because of a greater con-
tribution of shallow holes rather than deep holes.
5. Acknowledgements
The authors are grateful for the financial support of Bra-
zilian agencies CNPq, FAPESP and CAPES.
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