International Scholarly Research Network
ISRN Nanomaterials
Volume 2012, Article ID 909647, 13 pages
doi:10.5402/2012/909647
Research Article
Optimization of the Synthesis of Nanostructured
Tungsten-Molybdenum Bimetallic Oxide
H. Hassan,1 T. Zaki,1 S. Mikhail,1 A. Kandil,2 and A. Farag2
1 Petroleum
Refining Division, Department of Catalysis, Egyptian Petroleum Research Institute, Cairo 11727, Egypt
of Chemistry, Faculty of Science, Helwan University, Helwan 11421, Egypt
2 Department
Correspondence should be addressed to H. Hassan, hebachem@yahoo.com
Received 16 May 2012; Accepted 10 June 2012
Academic Editors: R. Azimirad, C. Li, and F. Miao
Copyright © 2012 H. Hassan et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Mo0.5 W0.5 O3 nanoparticles were prepared through the Pechini process and were characterized using X-ray diffraction (XRD),
transmission electron microscopy (TEM), FT-IR spectrometer, and differential thermal analysis (TG-DSC) analyses. The
polyesterification reaction, as the starting step, has a profound influence on the dispersion of the resulting nanoparticles. The
molar ratios CA : TM = 2 and EG : CA = 1.5 are favorable for the preparation of Mo0.5 W0.5 O3 nanoparticles having average particles
size ranging from 2 to 9 nm. Meanwhile, the molar ratios CA : TM = 4 and EG : CA = 0.19 are favorable for the preparation
of Mo0.5 W0.5 O3 nanoparticles having an average particles size ranging from 11 to 29 nm. For the calcination step, increased
calcination time (eight hours) at 500◦ C is advantageous for allowing the monometallic phases enough time to transform into
the desired bimetallic Mo0.5 W0.5 O3 phase.
1. Introduction
Molybdenum trioxide (MoO3 ) and tungsten trioxide (WO3 )
are well-known metal oxides with similar physical and
chemical properties. They show n-type semiconducting
properties related to the presence of lattice defects, mainly
oxygen defects [1, 2], and they have been extensively studied
for their potential applicability in gas sensing devices [3, 4]
and catalysis [5, 6]. Additionally, due to the unique activity of
trioxides of W and Mo in nonstoichiometric forms, they have
been extensively studied as electrochemical materials [7].
The Mox Wl−x O3 system exhibits “displacive” or “reconstructive” phase transitions induced by hydrogen intercalation and temperature, respectively, leading to rearrangement
in the local electronic and atomic structures [8]. Polycrystalline Mox Wl−x O3 (x = 0.1, 0.2, 0.3, 0.5, 0.7, and 0.9) solid
solutions were obtained by high-temperature synthesis [9].
With the progress in nanotechnology, nanoparticles have
attracted increasing attention to their unique properties
[10]. One of the recently investigated methods for the
simple preparation of nano-oxide composites that include
molybdenum or tungsten atoms is the Pechini method
[11, 12]. However, research into the catalytic activity of
molybdenum-tungsten bimetallic oxides is rare despite their
promising gas sensing potential [13, 14].
In this work, the synthesis of molybdenum-tungsten
oxide nanoparticles is presented. The procedure includes
the formation of composites containing both molybdenum
or tungsten atoms in a 1 : 1 ratio using the polymeric
method. The influence of citric acid and ethylene glycol
concentrations, as well as total metal molar ratios, on the
properties of the final products was studied. In addition,
the effect of the sequence of the synthetic steps (i.e.,
polymerization and chelation) was explored. Furthermore,
the impacts of the temperature and time of calcination were
examined.
2. Experimental
For most of the prepared samples, aqueous ethylene glycol
(EG) solutions of different concentrations were added to
an aqueous solution of citric acid (CA) for polymerization
under constant stirring and at 60◦ C for 1 h. Second, the
polyester was heated to 80◦ C with stirring, and then 500 mL
of a TM solution (1 : 1 Mo : W metal ratio of ammonium
2
heptamolybdate and ammonium metatungstate precursors)
was added and stirred for an additional hour. Accordingly,
molybdic and tungstic acids were created, and the pH
of the medium decreased to 1.4. In the third step, the
solutions were slowly heated to 140◦ C until the water was
completely removed (∼2.5 hours). The gels obtained were
dried at 150◦ C overnight in an electrical furnace to yield
solid resins. The resulting resins were ground in an agate
mortar and subjected to a pyrolysis process at 450◦ C for 4 h
in glazed alumina crucibles. Finally, the pyrolyzed product
was subjected to calcination at 500◦ C for 4 h in the presence
of purified air. The MoWO3 samples were named MWRC RE (RC is CA : TM, and RE is EG : CA) corresponding to the
ratios used.
To understand the influence of the sequence of the
preparation steps, MW2–1.5 was also prepared in the reverse
fashion (i.e., the TM solution was added to the CA solution
at 60◦ C first, and then, the EG solution was added at 80◦ C).
This sample was named RMW2–1.5.
To study the role of the calcination step on the final characteristics of the produced bimetallic oxides, two factors were
investigated. The first factor was the calcination temperature,
and the second was the calcination duration.
The molybdenum-tungsten samples selected for thermal
treatment temperature evaluation were named MWRC RE (T) (RC is CA : TM, RE is EG : CA and T is calcination temperature) corresponding to the preparation and treatment
conditions used. At relatively high calcination temperatures,
that is, 750, 850, and 950◦ C, a physically separated needled
sample was collected in addition to the ordinary final
powder product, and the separated samples were coded
MWRC -RE (S750), MWRC -RE (S850), and MWRC -RE (S950),
respectively.
The bimetallic oxide samples subjected to different
calcination times were labeled MWRC -RE (8) and MWRC RE (12), which referred to calcination for eight and twelve
hours, respectively. The sample calcined for four hours was
named MWRC -RE .
The evolution of the samples was followed by X-ray
powder diffraction (XRD) on a Bruker AXS D8 Advance
equipped with Ni-filtered copper Kα1 radiation (λ =
1.5404 Å) in the range 2θ = 20–80◦ . Transmission electron
microscopy images were recorded on a JEOL-1400 TEM
at 120 kV. FTIR spectra were recorded using an FT-IR
spectrometer, model 960M000 g, ATI Mattson Infinity Series.
Differential thermal analysis was recorded on a Labsys TGDSC16 apparatus manufactured by Setaram Instrumentation. The sample was ground to 20 mesh, and α-alumina
was used as an inert reference material. The experiments
were carried out at a heating rate of 10◦ C/min in an argon
atmosphere.
3. Results and Discussion
3.1. The Role of Reactant Molar Ratios
3.1.1. Influence of EG Molar Ratio. Figure 1 presents the
XRD patterns of MoW oxides samples with increasing EG
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molar fractions and a constant CA : TM molar ratio. The
XRD patterns show one main phase, which is Mo0.5 W0.5 O3
(JCPD: 28–0667), and two secondary phases, monoclinic
MoO3 (JCPD: 80–0347) and cubic WO3 (JCPD: 41–0905),
in all the samples tested. TEM images (Figures 2(a) and
2(b)) reveal that the average particle size increased (from
3–10 nm to 10–16 nm) as the EG molar fraction increased
(from MW1–3 to MW1–6).
With increasing EG molar fractions, the average particle
size continued to increase up to 18 nm (Figure 2(c), MW1–
12), and aggregates started to appear. Both particle enlargement and aggregation became more obvious in the MW1–24
sample (Figure 2(d)), which was prepared using the highest
molar fraction of EG.
3.1.2. Influence of CA Molar Ratio. According to previous experimental data, the MW1–3 sample has bimetallic
nanoparticles that were prepared using the optimum EG
molar ratio, so this sample was selected. Thus, the EG molar
ratios versus TM and CA of 1 and 3, respectively, were
selected as the initial values for studying the influence of the
CA molar ratio on the properties of the produced bimetallic
nanoparticles.
Figure 3 presents the XRD patterns of MoW oxide
samples prepared using different CA molar ratios versus both
TM and EG. According to Figure 3(b), Mo0.5 W0.5 O3 was the
only phase in MW2–1.5. However, the other samples (MW1–
3, MW4–0.75, MW8–0.38, and MW16–0.19) contained
secondary phases of MoO3 and WO3 in addition to the major
phase (Mo0.5 W0.5 O3 ). The XRD patterns did not reveal any
change in the intensities of the Mo0.5 W0.5 O3 reflections with
an increasing CA molar ratio.
TEM images (Figures 4(a) and 4(b)) indicated that the
average particle size increased from 3–10 nm to 4–16 nm
as the CA molar ratio increased (from MW1–3 to MW2–
1.5). However, when the CA molar ratio increased four
and eight times, as in the case of MW8–0.38 and MW16–
0.19, respectively, the average particle size increased from
4–16 nm (Figure 4(b)) to 11–19 nm (Figure 4(c)) and 20–
36 nm (Figure 4(d)).
Despite the slightly increased size of the MW2–1.5
nanoparticles in comparison to the MW1–3 nanoparticles
(Figures 4(a) and 4(b)), the presence of the Mo0.5 W0.5 O3
phase as a mono-phase gave this sample unique properties.
3.1.3. Influence of the Sequence of Preparation Steps. Figure 5
presents XRD patterns of both MW2–1.5 and RMW2–1.5
samples. This figure shows that starting with the chelation
reaction (i.e., the reaction between TM and CA) did not
alter the nature of the final phase when Mo0.5 W0.5 O3 was the
only phase (e.g., MW2–1.5). However, the crystallinity of the
produced phase decreased with a change to the sequence of
preparation steps.
The TEM images of both samples (Figure 6) did not
reveal the average particle size shift due to the rearranged
order of the preparation steps. However, in RMW2–1.5, the
prepared Mo0.5 W0.5 O3 appeared as aggregations, while the
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(MW1–3)
(MW1–6)
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(MW1–12)
(MW1–24)
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0
0
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(c)
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2θ
(d)
Figure 1: XRD patterns of (a) MW1–3 (b) MW1–6, (c) MW1–12 and (d) MW1–24.
(a)
(c)
(b)
(d)
Figure 2: Transmission electron micrographs of nanocrystalline particles of (a) MW1–3, (b) MW1–6, (c) MW1–12, and (d) MW1–24.
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(MW1–3)
(MW2–1.5)
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Relative intensity
Relative intensity
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0
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(a)
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(b)
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(MW4–0.75)
(MW8–0.38)
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Relative intensity
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Relative intensity
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(c)
(d)
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(MW16–0.19)
Relative intensity
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60
40
20
0
20
30
40
50
2θ
60
70
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(e)
Figure 3: XRD patterns of (a) MW1–3, (b) MW2–1.5, (c) MW4–0.75, (d) MW8–0.38, and (e) MW16–0.19.
MW2–1.5 sample showed the Mo0.5 W0.5 O3 as well dispersed
nanoparticles.
3.1.4. Influence of TM Molar Ratio. To investigate the
influence of the TM molar ratio on the properties of the
produced nanoparticles, a reactant mixture with excess citric
acid was chosen because the CA plays the main role in the
chelation step. Accordingly, CA molar ratios versus TM and
EG of 16 and 0.19, respectively (MW16–0.19), were selected
for studying the influence of the TM molar fraction on the
properties of the produced bimetallic nanoparticles.
Figure 7 presents the XRD patterns of MoW oxide
samples prepared using regular increments of TM molar
ratios against CA and EG. In Figure 7(b), the XRD pattern
of MW8–0.19 indicated duplication of the intensities of
the reflections related to the Mo0.5 W0.5 O3 phase. The
same reflections were seen in the pattern of MW16–0.19
(Figure 7(a)), but the WO3 and MoO3 phases were still
present in the prepared sample.
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(a)
(b)
(c)
(d)
Figure 4: Transmission electron micrographs of nanocrystalline particles of (a) MW1–3, (b) MW2–1.5, (c) MW8–0.38, and (d) MW16–
0.19.
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(MW2–1.5)
(RMW2–1.5)
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Relative intensity
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2θ
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2θ
(a)
(b)
Figure 5: XRD patterns of (a) MW2–1.5 and (b) RMW2–1.5.
(a)
(b)
Figure 6: Transmission electron micrographs of nanocrystalline particles of (a) MW2–1.5 and (b) RMW2–1.5.
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120
(MW16–0.19)
(MW8–0.19)
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Relative intensity
Relative intensity
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(a)
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(MW4–0.19)
(MW2–0.19)
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(b)
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2θ
(c)
(d)
Figure 7: XRD patterns of (a) MW16–0.19, (b) MW8–0.19, (c) MW4–0.19, and (d) MW2–0.19.
(a)
(c)
(b)
(d)
Figure 8: Transmission electron micrographs of nanocrystalline particles of (a) MW16–0.19, (b) MW8–0.19, (c) MW4–0.19, and (d) MW2–
0.19.
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On the other hand, the Mo0.5 W0.5 O3 phase became the
unique phase with further increases in the molar ratio of
TM, whereas the XRD pattern of MW4–0.19 (Figure 7(c))
indicated the absence of WO3 and MoO3 reflections. Furthermore, the reflections related to the Mo0.5 W0.5 O3 phase
increased in comparison with the XRD pattern of MW8–0.19
(Figure 7(b)).
Further increasing the TM molar ratio versus CA and EG
(i.e., the CA molar ratios versus TM and EG became 2 and
0.19, resp., (MW2–0.19)), the secondary phases WO3 and
MoO3 returned to the final product along with Mo0.5 W0.5 O3
(Figure 7(d)). Consequently, the reflection intensity of the
main phase decreased in comparison with the MW4–0.19
sample (Figure 7(c)). TEM images (Figures 8(a) and 8(b))
of the same samples demonstrated that the average particle
size decreased from 20–36 nm to 15–32 nm as the TM molar
ratio increased. Upon further increases in the TM molar ratio
(MW4–0.19), the average particle size gradually decreased to
11–29 nm (Figure 8(c)).
In the highest TM molar ratio sample (MW2–0.19),
the average particle size was drastically smaller than that
of the other samples. The TEM images of the samples
illustrated nanoparticles with average sizes in the range of
4–7 nm (Figure 8(d)). In addition, nonmeasurable ultra-fine
nanoparticles with diameters smaller than 2 nm appeared as
dark gray patches distributed around the larger nanoparticles, which appeared as black spots.
3.2. The Role of the Thermal Treatment. The products of
the polymerization-chelation reactions were subjected to
two different types of thermal treatment. The first thermal
treatment was pyrolysis in a nitrogen atmosphere at 450◦ C
for four hours. The second treatment was calcination, which
varied by temperature and time. At relatively high calcination
temperatures (i.e., 750, 850, and 950◦ C), physically separated
needle samples were collected in addition to the ordinary
powdered final products, and the new separated samples
were named MW1–3(S750), MW1–3(S850), and MW1–
3(S950), respectively.
3.2.1. Influence of Calcination Temperature. The X-ray
diffraction patterns of nanoparticles produced at different
thermal treatment temperatures (Figure 9) showed that for
the lowest calcination temperature (450◦ C, MW1–3(450),
Figure 9(a)) and the sample calcined at 600◦ C (Figure 9(c)),
the intensity of the strongest reflection (monoclinic MoO3
(JCPD: 80–0347) and cubic WO3 (JCPD: 41–0905) at a
distance of 3.71 Å) increased. Meanwhile, the intensity of
the strongest reflection (Mo0.5 W0.5 O3 , JCPD: 28–0667 at
a distance of 3.783 Å) decreased with respect to the XRD
pattern of the MW1–3 sample, which was calcined at 500◦ C
(Figure 9(b)). Nevertheless, the previously mentioned phases
were the only phases created on the final product. As the
calcination temperature increased to 750◦ C (Figure 9(d)),
a thermal deformation of the bimetallic nanoparticles
occurred where the expected phase Mo0.5 W0.5 O3 disappeared
7
and was replaced by two deformed phases: W0.47 Mo0.53 O3
(JCPD: 32–1392) and W0.4 Mo0.6 O3 (JCPD: 76–1280).
The monometallic nanoparticles, which were monoclinic
MoO3 (JCPD: 80–0347) and cubic WO3 (JCPD: 41–0905),
remained in the final powdered sample. However, a new,
physically needled phase was easily separated manually from
the product after cooling. This needled sample, named
MW1–3(S750), was subjected to X-ray diffraction analysis
in the range 2θ = 4–80◦ , which indicated that this phase
consisted mainly of monoclinic MoO3 (JCPD: 47–1320), and
orthorhombic MoO3 (JCPD: 05–0508) (Figure 10(a)).
Upon increasing the calcination temperature to 850◦ C
(Figure 9(e)), the deformed bimetallic phases in MW1–
3(750) were replaced with a new phase (W0.71 Mo0.29 O3 ,
JCPD: 76–1279) for the sample MW1–3(850), in which tungsten is the major component. In addition to the bimetallic
oxide, the monometallic oxides monoclinic MoO3 (JCPD:
80–0347) and WO3 (JCPD: 01–0486) were still present in the
final powdered product (Figure 9(e)).
The crystallinity of the molybdenum oxides present in
the physically separated needled sample (MW1–3(S850),
Figure 10(b)), monoclinic MoO3 (JCPD: 47–1320), and
orthorhombic MoO3 (JCPD: 05–0508), increased sharply,
as indicated from the strong enhancement in reflection
intensities with respect to MW1–3(S750) (Figure 10(a)). The
decrease in the intensity of the noisy hump that occurred
in the range of 2θ = 4–20◦ also indicated the increased
crystallinity.
Finally, at the highest calcination temperature (950◦ C),
complete separation between the two metals in the oxide
occurred, whereas the XRD pattern of the powdered sample,
MW1–3(950), indicated that the sample contained only a
triclinic WO3 (JCPD: 32–1395) phase (Figure 9(e)). Meanwhile, the XRD pattern of the needled sample, MW1–
3(S950), indicated that this sample consisted of monoclinic
MoO3 (JCPD: 47–1320), and orthorhombic MoO3 (JCPD:
05–0508) phases accompanied with strongly increased crystallinity of the molybdenum oxides (Figure 10(c)).
The elucidation of powdered sample XRD patterns was
facilitated by using the FT-IR spectra of MW1–3 and
MW1–3(950) in Figure 11. The FT-IR spectrum of MW1–
3 identified a broad band located in the range of 475–
1050 cm−1 , which had distinguishable apexes at 633, 767,
845, and 975 cm−1 (Figure 11(a)). The band located at
975 cm−1 was assigned to (W=O) and (Mo=O) terminal
groups [15, 16], while the bands at 633 and 845 cm−1 were
attributed to the asymmetric and symmetric stretches of W–
O–W bridges, respectively [15, 17, 18].
According to the literature, vibration of the Mo–O–Mo
bond should result in a band with an apex wavenumber
of 882 cm−1 [16]. There was no clear band at this spot,
but we could identify a broadband covering surrounding
wavenumbers.
The band at 767 cm−1 was assigned to Mo–O–W stretching. This assumption was possible due to its absence in the
spectrum of MW1–3(950), while the broadband that covered
the range of 475–1050 cm−1 was split into two apexes at
633 and 845 cm−1 (Figure 11(b)). The FT-IR spectrum of
MW1–3(950) nanoparticles illustrated a narrowing in this
8
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120
120
(MW1–3(450))
(MW1–3)
100
Relative intensity
Relative intensity
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0
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2θ
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(MW1–3(650))
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(MW1–3(750))
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2θ
(c)
(d)
250
300
(MW1–3(850))
(MW1–3(950))
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Relative intensity
200
Relative intensity
60
(b)
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Relative intensity
Relative intensity
(a)
140
100
50
2θ
150
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50
200
150
100
50
0
0
20
30
40
50
2θ
60
70
80
(e)
20
30
40
50
2θ
(f)
Figure 9: XRD patterns of (a) MW1–3(450), (b) MW1–3, (c) MW1–3(650), (d) MW1–3(750), (e) MW1–3(850), and (f) MW1–3(950).
peak due to the absence of bands related to the vibration of
the Mo–O–Mo bond at 882 cm−1 . Moreover, a new shoulder
appeared at 1040 cm−1 and was attributed to the stretching
of W–O bonds at the surface [18]. The band at 1620 cm−1
was assigned to the H–O–H bending vibrations of water,
which is easily introduced into the system during the IR
measurements [18].
The FT-IR spectrum of the needled sample, MW1–
3(S950), indicated that it contained orthorhombic MoO3 ,
which has two significant bands at 975 cm−1 for a terminal
Mo=O stretching vibration and 840 cm−1 for the vibration
of the Mo–O–Mo species [17]. Bands at 492 and 533 cm−1
were most likely due to asymmetric and symmetric stretches
of the monoclinic MoO3 phase (Figure 12). As previously
mentioned, the bands at 1625 and 3430 cm−1 were assigned
to the H–O–H bending and stretching vibrations of the
hydration water [19].
It is known that molybdenum oxide (MoO3 ) melts
and sublimes at approximately 800◦ C [20, 21]. However,
according to the XRD patterns and the FT-IR spectra in the
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(MW1–3(S750))
(MW1–3(S850))
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Relative intensity
Relative intensity
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(a)
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(b)
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(MW1–3(S950))
Relative intensity
100
80
60
40
20
0
20
30
40
50
60
70
80
2θ
(c)
Figure 10: XRD patterns of (a) MW1–3(S750), (b) MW1–3(S850), and (c) MW1–3(S950).
presented case study, the decomposition of the bimetallic
nanoparticles and, consequently, the sublimation process
started early (i.e., at approximately 700◦ C). Such an assumption was confirmed via the thermal analysis of MW1–
3, where the thermal profile of the MoW nanoparticles
(Figure 13) showed a strong decline in the DSC curve
towards the endothermic direction at temperatures higher
than 650◦ C.
No significant endothermic peak was identified until
880◦ C, and then another endothermic peak appeared at
933◦ C. The thermal behavior of the MW1–3 sample may
be explained as follows. Between 700 and 800◦ C, the
decomposition and deformation of the Mo0.5 W0.5 O3 phase
occurred, as did sublimation of the free MoO3 particles.
The sublimation of the free MoO3 at such a relatively low
temperature with respect to the normal thermal behavior
of the molybdenum oxide bulk phase [20, 21] might be
due to its size. On a nano-scale, the particles become more
sensitive to thermal changes and, consequently, make the
thermal transformation process easier than that for bulky
particles.
At temperatures from 800 to 900◦ C, the W0.47 Mo0.53 O3
and W0.4 Mo0.6 O3 phases deformed into W0.71 Mo0.29 O3 .
This deformation was accompanied by sublimation of the
produced free MoO3 phase. At 933◦ C, the final form of the
MoW nanoparticles, W0.71 Mo0.29 O3 , lost its molybdenum
component and transformed into triclinic WO3 .
The TEM images of the MW1–3 and MW1–3(650)
samples, which are similar in their phase compositions
(Figures 14(a) and 4(b)), illustrated that the average particle
sizes increased from 3–10 nm to 6–16 nm as the calcination
temperature increased from 500 to 650◦ C. Despite a change
in the type of phases created after thermal treatment at high
calcination temperatures (i.e., 750 and 850◦ C), the general
feature of these powdered samples was a low average particle
size (6–11 nm for MW1–3(750) and 3–10 nm for MW1–
3(850); Figures 14(c) and 14(d), resp.).
The TEM images of the nanoparticles of the triclinic
tungsten oxide sample MW1–3(950) large agglomerates
with average particle sizes from 46 to 115 nm (Figure 14(e)).
By focusing on these agglomerates, the TEM image showed
nanoparticles with average particle sizes from 1 to 12 nm,
which appeared as black spots, in addition to nonmeasurable
ultra-fine nanoparticles with diameters smaller than
1 nm (dark gray patches distributed around the larger
nanoparticles, Figure 14(f)).
3.2.2. Influence of Calcination Time. As mentioned above,
the optimal temperature for the calcination process was
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140
4
Exo
2
Heat flow (µV)
120
Transmittance (%)
100
80
2943
60
1204
1382
1450
1620
0
−2
−4
870.8◦ C◦
880.5 C
−6
−8
933.4◦ C
−10
0
40
100
200
633
400
500
600
700
800
900 1000
Temperature (◦ C)
20
Series1
845
0
5000
4000
3000
2000
767
1000
Figure 13: Thermal analysis (DSC) of MW1–3.
0
Wavenumber (cm−1 )
b
a
Figure 11: FTIR spectra of (a) MW1–3 and (b) MW1–3(950).
140
120
100
Transmittance (%)
300
1055
1375
1625
974
80
60
40
796
840
20
0
5000
533 492
4000
3000
Wavenumber
2000
1000
0
(cm−1 )
Series1
Figure 12: FTIR spectrum of MW1–3(S950).
500◦ C. However, there is another factor that may play
an important role in the thermal treatment of bimetallic
nanoparticles: the duration of the calcination process. To
understand the importance of this factor, the influence of
three different calcination times, four, eight, and twelve
hours, on the characteristics of the final product were
investigated. The bimetallic oxide samples that underwent
the different calcination times are labeled MW1–3(8) and
MW1–3(12), referring to calcination for eight and twelve
hours, respectively. In addition, the MW1–3 sample was
tested, which was calcined for four hours at 500◦ C.
The X-ray diffraction patterns of the nanoparticles
produced after different thermal treatment times (Figure 15)
showed that increased calcination time did not significantly
change the crystallinity of the produced nanobimetallic
oxides. This assertion was supported by the unchanging
intensities of the reflections in the oxides.
The reflection that belonged to the monometallic phases
monoclinic MoO3 (JCPD: 80–0347) and cubic WO3 (JCPD:
41–0905), which could easily be identified in the XRD pattern of MW1–3 (Figure 15(a)), weakened and became indistinct after eight hours of calcination at 500◦ C (Figure 15(b)).
Such reflections disappeared in the X-ray pattern of
MW1–3(12), indicating that the produced nanoparticles of
molybdenum-tungsten were a pure bimetallic Mo0.5 W0.5 O3
phase (JCPD: 28–0667, Figure 15(c)).
The TEM image of the nanoparticles of MW1–3(8)
did not show a distinct difference in average particle size
(Figure 16(b)) when compared with the TEM image of the
nanoparticles of MW1–3 (Figure 16(a)). Both samples had
average particle sizes from 3 to 10 nm. This observation
eliminated the possibility of sintering during the long
calcination time. On the contrary, increased calcination time
was advantageous, as it gave the monoclinic MoO3 and
cubic WO3 phases enough time to transform into the desired
bimetallic Mo0.5 W0.5 O3 phase.
Upon initial inspection, the TEM image of the nanoparticles of the bimetallic Mo0.5 W0.5 O3 phase in MW1–3(12)
indicated large agglomerates with average particle sizes from
41 to 160 nm (Figure 16(c)). By zooming on these agglomerates, however, the TEM image revealed nanoparticles with
average particle sizes from 2 to 9 nm, which appeared
as black spots, in addition to nonmeasurable ultrafine
nanoparticles with diameters smaller than 2 nm (dark gray
patches Figure 16(d)).
4. Conclusions
The Pechini method can be successfully applied for the
preparation of Mo0.5 W0.5 O3 , and the ratios of the reactants
strongly affected the purity of the final products.
(i) The molar ratios CA : TM = 2 and EG : CA = 1.5
were the most favorable for the preparation of
Mo0.5 W0.5 O3 nanoparticles having average particle
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(a)
(b)
(c)
(d)
(e)
Figure 14: Transmission electron micrographs of nanocrystalline particles of (a) MW1–3, (b) MW1–3(650), (c) MW1–3(750), (d) MW1–
3(850), (e) MW1–3(950), and (e) MW1–3(950).
120
120
(MW1–3)
(MW1–3(4))
Relative intensity
100
80
60
40
80
60
40
20
20
0
0
20
30
40
50
60
70
80
20
30
40
50
2θ
2θ
(a)
(b)
120
(MW1–3(8))
100
Relative intensity
Relative intensity
100
80
60
40
20
0
20
30
40
50
60
70
80
2θ
(c)
Figure 15: XRD patterns of (a) MW1–3, (b) MW1–3(4), and (c) MW1–3(8).
60
70
80
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(a)
(b)
(c)
(d)
Figure 16: Transmission electron micrographs of nanocrystalline particles of (a) MW1–3, (b) MW1–3(8), (c) MW1–3(12), and (d) MW1–
3(12).
sizes from 2 to 9 nm. Meanwhile, the molar ratios
CA : TM = 4 and EG : CA = 0.19 were optimal for
the preparation of Mo0.5 W0.5 O3 nanoparticles having
average particle sizes from 11 to 29 nm.
(ii) The sequence of the preparation steps affects
nanoparticle dispersion, whereas the polyesterification reaction, as a starting step, also strongly
influenced the dispersion of the nanoparticles.
(iii) Increased calcination time (eight hours) at a suitable
temperature (500◦ C) was advantageous, as it gave the
monometallic phases enough time to transform into
the desired bimetallic W0.5 Mo0.5 O3 phase.
[6]
[7]
[8]
[9]
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