Aqueous sol-gel synthesis and thermoanalytical study of the alkaline earth molybdate precursors

Article abstract of DOI:10.1007/s10973-013-3579-0

The preparation and characterization of the M-Mo-O nitrate-tartrate (M = Mg, Ca, Sr, and Ba) gels, which were produced by the simple aqueous sol-gel method and calcined at 500, 600, 700, 800, 900, and 1,000 °C temperatures are reported. The crystalline al

Full text of DOI:10.1007/s10973-013-3579-0

J Therm Anal Calorim
DOI 10.1007/s10973-013-3579-0
Aqueous sol–gel synthesis and thermoanalytical study
of the alkaline earth molybdate precursors
Gediminas Braziulis
Ruta Stankeviciute
•
Gytautas Janulevicius
Arturas Zalga
•
•
Received: 6 September 2013 / Accepted: 28 November 2013
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2013
Abstract The preparation and characterization of the
importance in a variety of applications such as phosphors
[1–3], scintillation materials [4, 5], microwave devices [6],
catalysts [7], or optoelectronic devices [8, 9]. From the
structural point, the differences between alkaline earth
0
0
M –Mo–O nitrate–tartrate (M = Mg, Ca, Sr, and Ba) gels,
which were produced by the simple aqueous sol–gel
method and calcined at 500, 600, 700, 800, 900, and
1
,000 °C temperatures are reported. The crystalline alka-
metal molybdates (AMoO , A = Mg, Ca, Sr, and Ba)
4
line earth metal molybdates (MgMoO , CaMoO , SrMoO ,
0
mainly consist in ionic radius of the A-site. If the ionic
˚
radius of the A-site ion is smaller than 1.0 A as is the case
4
4
4
and BaMoO ) and as-prepared M –Mo–O nitrate–tartrate
4
gels investigated by thermal analysis (TG/DSC), X-ray
diffraction (XRD), and scanning electron microscopy
for MgMoO ceramic, the compounds show a wolframite
4
structure with octahedral coordination. Alternatively, if the
˚
ionic radius of the A-site is larger than 1.0 A as is the case
(
SEM). TG/DSC analysis showed the possible decompo-
2
for Ca , Sr , and Ba in AMoO , the compounds show
?
2?
2?
sition mechanism of synthesized gels. XRD studies
allowed the identification of main types of crystalline
structures in the MgMoO , CaMoO , SrMoO , and
4
a scheelite structure with tetrahedral coordination [10, 11].
Moreover, as well as the crystal structure, the surface
morphology also significantly affects both the optical and
electrical properties of alkaline earth metal molybdates.
Thus, according to this the synthesis method that could be
chosen for the preparation of these ceramic materials has
significant importance.
4
4
4
BaMoO systems. Moreover, SEM analysis revealed the
4
changes of surface morphology of the final compounds
depending on annealing temperatures.
Keywords Alkaline earth metals ꢀ Sol–gel processing ꢀ
Thermal analysis
In the last several decades, many different preparation
techniques [12–17] have been used for the preparation of
AMoO (A = Mg, Ca, Sr, and Ba) ceramics. Nowadays,
4
Introduction
the results of obtained compounds, depending on the
preparation route, are important and have significant
influence to the science and technologies. Therefore, the
choice of synthesis technique usually depends on variety
factors that can shorten, facilitate, and reduce the prepa-
ration way. From this point of view, the solution-based
synthetic methods play a crucial role in the design and
production of fine ceramics and they have been successful
in overcoming many of the limitation of the traditional
solid-state, high-temperature methods. The use of solution
chemistry can eliminate major problems, such as long
diffusion paths, impurities, and agglomeration, and result
in products with improved homogeneity [18–20]. More-
over, the sintering temperature that usually strong affects
Metal molybdates of the general formula AMoO4
(
A = Mg, Ca, Sr, Ba, Cd, Zn, Pb, etc.) have been studied
extensively for decades, owing to their technological
G. Braziulis ꢀ G. Janulevicius ꢀ R. Stankeviciute ꢀ A. Zalga (&)
Department of Applied Chemistry, Faculty of Chemistry, Vilnius
University, Naugarduko Str. 24, 03225 Vilnius, Lithuania
e-mail: arturas.zalga@chf.vu.lt
A. Zalga
Department of General Physics and Spectroscopy, Faculty of
Physics, Vilnius University, Sauletekio Av. 9, Bld. III,
1
0222 Vilnius, Lithuania
123

G. Braziulis et al.
the properties of final compounds is crucial factor for the
preparation of both nano-sized materials and thin films on
different substrates.
instrument using a sample mass of about 10 mg and a
-
1
3
-1
heating rate of 20 °C min in flowing air (20 cm min
)
at ambient pressure from room temperature to 1,000 °C.
X-ray diffraction (XRD) patterns have been recorded in air
at room temperature with a powder X-ray diffractometer
In this work, the aqueous sol–gel synthesis method was
0
successfully used for the preparation of M –Mo–O nitrate–
0
tartrate (M = Mg, Ca, Sr, and Ba) gel precursors, which
Rigaku MiniFlex II using Cu Ka radiation. The spectra
1
-
1
additional at 500, 600, 700, 800, 900, and 1,000 °C tem-
peratures were calcined.
were recorded at the standard rate of 1.5 2h min . The
scanning electron microscope (SEM) Hitachi TM3000 was
used to study the surface morphology and microstructure of
the obtained ceramic samples.
Experimental
The samples MMoO (where M = Mg, Ca, Sr, and Ba)
4
Results and discussion
were prepared by an aqueous nitrate–tartrate sol–gel syn-
thesis route taking tartaric acid (TA) as a complexing
Thermal analysis
agent.
The
magnesium(II)
nitrate
hexahydrate
(
Mg(NO ) ꢀ6H O, 99.97 % AlfaAesar), calcium(II) nitrate
In order to explain the thermal decomposition behavior and
crystallization processes of the synthesized M–Mo–O
(M = Mg, Ca, Sr, and Ba) nitrate–tartrate gel precursors,
the thermal analysis (TG/DSC) was performed. TG/DSC
measurements of the alkaline earth metal molybdate
nitrate–tartrate gel precursors carried out up to 1,000 °C at
3
2
2
tetrahydrate (Ca(NO ) ꢀ4H O, 99.98 % AlfaAesar), stron-
3
2
2
tium(II) nitrate (Sr(NO ) , 99.97 % AlfaAesar), barium(II)
2
3
nitrate (Ba(NO ) , 99.95 % AlfaAesar), and molybdenum
2
3
oxide (MoO , 99.95 % Alfa Aesar) were used as starting
3
materials and weighed according to the desired stoichi-
-
1
ometric ratio. Nitric acid (HNO ), distilled water, and
3
a
heating rate of 20 °C min
in flowing air
3
-1
ammonia (NH ꢀ4H O) were used as solvents and reagents
(20 cm min ) at ambient pressure with a sample mass of
about 5 mg. The curves of analyzed samples are shown in
Fig. 1, respectively.
3
2
to regulate the pH of the solutions. TA (C H O , 99.5 %)
4 6 6
was used as a complexing agent. First, MoO was dissolved
3
in 25 ml of concentrated ammonia solution by stirring at
At the initial stage of the explanation of TG curve,
shown in Fig. 1a, should be noted that the final mass loss of
about 78 % occurred to 580 °C temperature, which gives
the conclusion about the decomposition of all volatile
organic parts in the gel precursor. In general, the decom-
position processes of the Mg–Mo–O nitrate–tartrate system
could be roughly divided into eight periods that clearly
indicate all mass losses and heat flow transformations
occurring during the sample heating in the appropriate
range of temperature. The first mass loss of about 1 % in
the TG curve goes from 40 to 140 °C temperatures and is
related with the removal of surface absorbed water from
the gel and/or water from the coordination sphere of the
metal complexes. A broad endothermic peak with heat flow
7
0–80 °C temperature. Then TA with a molar ratio of
Mo/TA = 0.25, dissolved in a small amount of distilled
water was added with a continuous stirring at the same
temperature to the reaction mixture. Next, after several
hours the stoichiometric amount of alkaline earth nitrate
was mixed with the previous solution. To prevent precip-
itation, the excess of ammonia was neutralized with con-
centrated HNO until the pH reached the value of *1.0.
3
Finally, the same amount of the aqueous solution of the
complexing agent TA was repeatedly added to the reaction
mixture to prevent crystallization of metal salts during the
gelation process. The beaker with the solution was closed
with a watch glass and left for 1 h with continuous stirring.
The obtained clear solution was concentrated by slow
evaporation at 80 °C in an open beaker. A yellow trans-
parent gel formed after nearly 90 % of the water has been
evaporated under continuous stirring. After drying in an
oven at 105 °C, fine-grained powders were obtained. The
precursor gels were calcined for 5 h at 500 °C in alumina
crucibles and reground carefully in an agate mortar. Since
-
1
of 388 mJ and DH = 69.7 J g on the DSC curve corre-
sponds to the first period of mass loss. By further
increasing of temperature from 150 to 240 °C, the mass of
Mg–Mo–O nitrate–tartrate precursor to 16.5 % has been
decreased. A sharp exothermic peak of about 1,280 mJ
-
1
(DH = -230 J g
) confirmed either by the initial
decomposition of excess of TA in the precursor gel or by
the decomposition reactions of mixed-metal nitrate and
ammonium salts that formed during the sol–gel processing.
The third mass loss (4.2 %) of precursor gel in 245–320 °C
temperature range has been observed. The endothermic
-
1
the gels are very combustible, slow heating (1 °C min ),
especially between 150 and 300 °C, was found to be
essential. After intermediate grinding, the obtained pow-
ders were repeatedly annealed for 5 h at 500, 600, 700,
-
1
800, 900, and 1,000 °C. Thermal measurements were
performed with TG–DSC, STA 6000 Perkin-Elemer
peak of about 510 mJ (DH = 92 J g ) shows both the
further decomposition of either TA or metal tartrates and
1
23

Aqueous sol–gel synthesis and thermoanalytical study
(
a) 100
attributed to the beginning of burning processes of residual
organic compounds in the precursor gel. Finally, the last
sixth mass loss of about 4 % from 470 to 580 °C temper-
ature accords with the burning of elemental carbon in the
–
–
–
0
1
2
30
20
10
TG
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
Exo
residual sample. This process by strong exothermic peak of
-1
about 4,920 mJ (DH = -882 J g
) was confirmed,
DSC
respectively. By further increasing of temperature, no mass
losses in the TG curve have observed. Meanwhile, the
results obtained from DSC measurement clearly indicated
that at least two broad endothermic peaks 525 mJ
0
0
-
1
-1
(DH = 94 J g ) and 6,603 mJ (DH = 1,185 J g ) in the
00–1,000 °C range of temperature have been related.
(b) 100
–100
6
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
Exo
Both of them came with the crystallization process of final
structural materials. Moreover, according to the stabilized
mass at temperatures higher than 800 °C, no impurity
phases either crystalline or amorphous have formed.
In conclusion, TG/DSC analysis of Mg–Mo–O nitrate–
tartrate gel precursor suggests that single-phase crystalline
–
–
–
–
80
60
40
20
TG
DSC
MgMoO compound at relative low temperature of 550 °C
4
should form.
0
–
Slightly different view of TG–DSC curves by thermal
treatment of Ca–Mo–O nitrate–tartrate gel precursor,
presented in Fig. 1b, is shown. In this case, overall mass
loss of about 74 % of analyzed specimen to 775 °C
temperature has occurred. The thermal decomposition
processes of the Ca–Mo–O nitrate–tartrate system like-
wise in previous case also into eight periods have divi-
ded. The first mass loss of about 2 % in the TG curve
takes from 60 to 130 °C temperatures and is related with
the removal of surface absorbed water from the gel and/
or water from the coordination sphere of the metal
complexes. An endothermic peak of heat flow of 403 mJ
(
c) 100
TG
40
20
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
Exo
–
0
DSC
20
(
d) 100
–
–
0
20
10
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
-
1
Exo
and DH = 40 J g on the DSC curve corresponds to the
first period of mass loss. The initial thermal dissociation
of TA in the precursor gel or the decomposition reaction
of mixed-metal nitrate and ammonium salts that were
formed during the sol–gel processing start from 130 to
10
TG
2
3
4
0
0
0
DSC
1
40 °C temperature, respectively. This mass loss of about
9 % follows the exothermic transformations with a peak
-
1
width of about 2,247 mJ (DH = -217 J g ). The third
mass loss of about 14 % from 210 °C temperature, in
this case, to the final decomposition of the excess of TA
in the sample could be attributed. The exothermic peak
50
1000
2
00
400
600
800
Temperature/°C
-
1
of about 2,238 mJ (DH = -215.9 J g ) from 240 to
50 °C temperature corresponds with the third period of
Fig. 1 Combined TG–DSC curves of the Mg–Mo–O (a), Ca–Mo–O
b), Sr–Mo–O (c), and Ba–Mo–O (d) nitrate–tartrate gel precursors in
flowing air
3
(
mass loss. Next to that, the burning of the small amount
of elemental carbon of about 1 % that has formed during
the partial decomposition and intermolecular rearrange-
ment of TA has been observed. Comparatively to the low
mass loss in this stage, a strong exothermic peak of
the intermolecular rearrangement of organic compounds.
Thereafter, on the following increasing of temperature
fourth (2.7 %) and fifth (1 %) mass loss stages have star-
ted. Both small exothermic peaks of about 195 mJ
-
1
about 1,898 mJ (DH = -183 J g ) was measured. The
fifth stage on the DCS curve from 520 to 670 °C tem-
perature with no mass changes in the analyzed sample
-
1
-1
(
DH = -35 J g ) and 22 mJ (DH = -4 J g ) are
123

G. Braziulis et al.
characterized by endothermic effect of about 1,996 mJ
-
investigation of Sr–Mo–O gel precursor, the TG curve
has remained constant. However, in the DSC curve a
broad endothermic band related with the crystallization
process near to the 1,000 °C temperature has been
observed. In conclusion, TG/DSC analysis of Sr–Mo–O
nitrate–tartrate gel precursor showed that pure single-
phase crystalline SrMoO4 compound at temperature of
600 °C should form. However, the crystallization process
1
(
DH = 192.5 J g ) to the crystallization of CaMoO4
phase is related. Finally, the last mass loss of about 12 %
from 700 to 780 °C temperature with the decomposition
of carbonates that formed during the partial oxidation of
metal tartrates in the Ca–Mo–O gel precursor have
occurred. A strong exothermic peak of about 8,225 mJ
-
1
(
DH = -793 J g ) in this period corresponds to the
decomposition process of hydrocarbons with the evapo-
rating of CO and H O gases. As seen from the DSC
of SrMoO starts under 500 °C temperature that clearly
4
the results obtained from DSC curve.
2
2
curve in the 780–880 °C temperature range, another
broad exothermic band of about 2,450 mJ (DH =
The last combined TG–DSC curves of Ba–Mo–O
nitrate–tartrate gel precursor in Fig. 1d are shown. The
character of mass loss in the TG curve is similar com-
paring with previous cases when the mass of Ba–Mo–O
nitrate–tartrate gel precursor over 600 °C temperature
remains constant. However, the general views of
TG–DSC curves of the Ba–Mo–O sample and conclu-
sions that have made from this thermal analysis are
significantly different in comparing with the three cases
presented before. The TG–DSC analysis of Ba–Mo–O
nitrate–tartrate gel precursor was divided into five mass
loss periods. The first exothermal peak of about 977 mJ
-
236.4 J g ) is observed. This negative heat effect is
1
-
closely related with the earlier exothermic process and
the further crystallization of CaMoO phase in the same
4
matter of time. In conclusion, it should be noted that the
final decomposition of Ca–Mo–O nitrate–tartrate gel
precursor occurs at the temperature of 780 °C. However,
the crystallization process, likewise as was mentioned in
previous case, also starts at relatively low temperature of
5
50 °C.
The combined TG–DSC curves of Sr–Mo–O nitrate–
-
1
tartrate gel precursor in Fig. 1c are displayed. The
decomposition of all volatile parts in Sr–Mo–O sample to
(DH = -219 J g ) from 60 to 165 °C temperature
indicates that the first mass loss of about 5 % in the
sample is closely related with the thermal decomposition
of ammonium and nitrate ions to the N and NO gas-
6
00 °C temperature has occurred. The general behavior
of TG–DSC curves of Sr–Mo–O gel in six periods has
divided. In the first period, the evaporation of adsorbed
water molecules with a mass loss of about 0.5 % and an
2
x
eous products. This exothermal process overcomes the
endothermic peak of the evaporation of water molecules
from the gel that was not well expressed in the previous
cases. By the further increase of temperature from 170 to
300 °C, the typical decomposition process of TA with
mass loss of about 60 % has been observed. A strong
exothermic behavior of about 8,383 mJ (DH =
-
1
endothermic peak of about 426 mJ (DH = 72 J g ) to
40 °C temperature was occurred. After that in the sec-
ond period with a mass loss of about 14 % from 150 to
40 °C temperature, the decomposition of TA and either
1
2
ammonium or nitrate ions was started. The decomposi-
tion of abovementioned compounds to CO2 and NOx
gases by the exothermic peak of about 551 mJ (DH =
-
1
-1,878 J g ) in the DSC curve was expressed. After
that in the range of 300–500 °C temperature further
decomposition and the rearrangement of organic parts in
the gel precursor with mass loss of about 10 % has
occurred. Finally, from the 500 °C temperature the
combustion of residual carbon with mass loss of about
10 % in the sample has started. In addition, it is inter-
esting to note that this burning process in at least to
mass losses of about 2.5 and 1 % has been divided. In
the DSC curve two exothermic peaks of about 2,486 mJ
-
1
-
94 J g ) was confirmed. Next to that in the third and
fourth periods from 245 to 500 °C temperatures the
further decomposition of metal tartrates with mass loss of
about 9 % overlaps with the intermolecular rearrange-
ment of residual products of TA. This conclusion by
well-defined exothermic peak of about 166 mJ (DH =
-
1
-
28 J g ) from 255 to 270 °C temperatures and by
overlapped both exothermic and endothermic processes
in the range of 280–450 temperature has suggested.
Further increasing of temperature from 450 to 505 °C
leads to the start of crystallization of the final SrMoO4
phase that confirmed a broad endothermic peak of about
-
1
-1
(DH = -557 J g ) and 233 mJ (DH = -52 J g ) have
been shown. By the further increase of temperature, the
mass in the TG curve remains constant. The endothermic
character of the DSC curve lets us conclude that in this
-
1
1
75 mJ (DH = 30 J g ) in the DSC curve. Finally, the
range of temperature the crystallization of BaMoO has
4
mass loss of about 7 % from 520 to 600 °C temperature
with an exothermic peak of about 8,038 mJ (DH =
dominated.
In conclusion, the thermal decomposition of above-
described fourth gel precursors occurred in different ways
depending on the alkali earth metal in the starting
materials.
-
1,366.5 J g ) by combusting of residual organic
1
-
compounds and as-formed inorganic carbon has been
characterized. In the last six period of thermal
1
23

Aqueous sol–gel synthesis and thermoanalytical study
X-ray diffraction
mass loss and suggested possible crystallization point of
final material with initial composition of MgMoO phase.
4
The XRD data were collected at room temperature with Cu
Figure 3 exhibits powder XRD patterns of the CaMoO4
samples obtained by sintering the dried Ca–Mo–O nitrate–
tartrate gel precursor for 5 h at 500, 700, and 1,000 °C
temperatures in air atmosphere at ambient pressure.
Despite the fact that according to the TG–DSC analysis
overall mass loss of about 74 % of analyzed Ca–Mo–O gel
precursor to 775 °C temperature has occurred, the crys-
tallization of Powelite-structured CaMoO4 phase only
above 500 °C in XRD pattern (Fig. 3) has been identified.
By increasing of temperature from 500 to 700 and
Ka radiation and a scan range from 10° to 70° for 2h.
1
XRD patterns of Mg–Mo–O nitrate–tartrate gel annealed at
different temperatures shown in the top three panels in
Fig. 2, are matched with that of standard ICSD card of
MgMoO shown in the bottom panel. In addition, XRD
4
results indicated that even at 500 °C crystalline monoclinic
MgMoO phase has formed. By increase of temperature to
4
1
,000 °C, the characteristic peaks of investigated samples
became sharper and no new crystal phases have appeared.
These results are in good agreement with the well-known
tendency that at higher annealing temperatures, the crys-
tallinity of mixed-metal oxides increases. These obtained
results are consistent with the conclusions made from
TG–DSC analysis of Mg–Mo–O nitrate–tartrate gel pre-
cursor that showed well-expressed ending temperature of
1,000 °C, the characteristic peaks of CaMoO crystal phase
4
became sharper and better indexed that denote the growing
of crystallite size in the final material. In addition,
according to the XRD and TG–DSC results of investigated
Ca–Mo–O precursor and sintered CaMoO4 compound
could be confirmed the conclusion that the mass loss above
1
0
20
30
40
50
60
70
1
0
20
30
40
50
60
70
o
o
CaMoO4 sample annealed at 1000 C temperature
MgMoO4 sample annealed at 1000 C temperature
o
o
CaMoO4 sample annealed at 700 C temperature
MgMoO4 sample annealed at 700 C temperature
o
o
MgMoO4 sample annealed at 500 C temperature
CaMoO
4
sample annealed at 500 C temperature
MgMoO4 ICSD 20418
CaMoO ICSD 23699
4
1
0
20
30
40
50
60
70
10
20
30
40
50
60
70
2θ
/
°
2θ/
°
Fig. 2 Standard ICSD card of MgMoO
Mg–Mo–O nitrate–tartrate gel precursor annealed at 500, 700, and
,000 °C temperatures
4
and XRD patterns of the
4
Fig. 3 Standard ICSD card of CaMoO and XRD patterns of the
Ca–Mo–O nitrate–tartrate gel precursor annealed at 500, 700, and
1,000 °C temperatures
1
123

G. Braziulis et al.
7
00 °C temperature because of the combustion of either
18.92°, 24.27°, 26.89°, 28.74°, and 30.94° appeared, which
hydrocarbons and/or elemental carbon has occurred.
Moreover, according to the consistent results of XRD
patterns of both investigated systems to explain the marked
differences between TG–DSC curves an additional inves-
tigation is required.
belongs to monoclinic BaMo O crystal phase (ICSD
3
10
50274) and compose 13.75 % as side phase in tetragonal
crystal structure (ICSD 56109) displayed in a bottom panel
(Fig. 5). By further increasing of annealing temperature to
700 °C, the impurity peaks of monoclinic BaMoO crystal
4
XRD patterns of Sr–Mo–O nitrate–tartrate gel precursor
annealed at 500, 700, and 1,000 °C temperatures in Fig. 4
are shown. In this case, all used annealing temperatures
are suitable for preparation of single-phase crystalline
phase disappeared and only characteristic plane-structured
reflections of tetragonal phase have dominated. Finally, the
XRD pattern, displayed in next-to-last panel, shows the
possible rearrangement of characteristic planes in tetrago-
SrMoO compound. Moreover, these XRD data of sintered
4
nal BaMoO crystal phase by increasing of temperature to
4
Sr–Mo–O nitrate–tartrate system are in a good agreement
with the TG–DSC results presented in Fig. 1c.
1,000 °C. This conclusion has confirmed by the strong
expressed endothermic signal in the DSC curve above
700 °C, displayed in Fig. 1d.
Slightly different results compared with previous cases
after annealing of Ba–Mo–O nitrate–tartrate gel precursor
at 500, 700, and 1,000 °C temperatures have obtained.
Corresponding XRD patterns of this system in Fig. 5 are
displayed. For Ba–Mo–O nitrate–tartrate sample annealed
at 500 °C temperature, an extra XRD peaks at about
In summary, the XRD data of all investigated samples
have revealed that using aqueous sol–gel processing at
relatively low temperature of 500 °C is possible to syn-
thesize pure MgMoO , CaMoO , and SrMoO crystal
4
4
4
phases. Meanwhile, XRD patterns of the Ba–Mo–O
1
0
20
30
40
50
60
70
10
20
30
40
50
60
70
o
SrMoO sample annealed at 1000 C temperature
BaMo O ICSD 50274
3 10
4
o
BaMoO4 sample annealed at 1000 C temperature
o
SrMoO sample annealed at 700 C temperature
4
o
BaMoO4 sample annealed at 700 C temperature
o
SrMoO sample annealed at 500 C temperature
4
o
BaMoO4 sample annealed at 500 C temperature
*
*
*
*
*
SrMoO ICSD 173120
4
BaMoO ICSD 56109
4
1
0
20
30
40
50
60
70
10
20
30
40
50
60
70
°
°
2θ/
2θ
/
Fig. 4 Standard ICSD card of SrMoO
Sr–Mo–O nitrate–tartrate gel precursor annealed at 500, 700, and
,000 °C temperatures
4
and XRD patterns of the
4
Fig. 5 Standard ICSD cards of BaMoO and XRD patterns of the
Ba–Mo–O nitrate–tartrate gel precursor annealed at 500, 700, and
1,000 °C temperatures
1
1
23

Aqueous sol–gel synthesis and thermoanalytical study
Fig. 6 SEM micrographs of Mg–Mo–O (a 700 °C, b 1,000 °C), Ca–Mo–O (c 700 °C, d 1,000 °C), Sr–Mo–O (e 700 °C, f 1,000 °C), and
Ba–Mo–O (g 700 °C, h 1,000 °C) nitrate–tartrate gel precursors annealed at different temperatures
123

G. Braziulis et al.
nitrate–tartrate gel precursor annealed at 500 °C tempera-
ture the characteristic peaks that attributed to the mono-
clinic and tetragonal crystal phases of BaMoO4 are
observed. By increase of the annealing temperature to
In conclusion, it is clear that the surface morphology of
investigated samples has significantly affected by both
sintering temperature and nature of alkaline earth element.
Summarizing results obtained from TG–DSC, XRD, and
SEM measurements can be concluded that aqueous sol–gel
synthesis method is a suitable technique for the preparation
of single-phase MgMoO4, CaMoO4, SrMoO4, and
BaMoO4 compounds at relatively low temperature of
7
00 °C only tetragonal crystal structure of BaMoO has
4
found, as presented in Fig. 5.
SEM micrographs
5
00 °C. Moreover, according to the SEM results the for-
The surface morphology of MgMoO , CaMoO , SrMoO ,
4
mation of spherical crystals with well-expressed planes and
4
4
and BaMoO4 compounds prepared by aqueous sol–gel
synthesis method and annealed at 700 and 1,000 °C tem-
peratures by scanning electron microscopy (SEM) was
investigated. The representative SEM micrographs of
edges have started up to 700 °C in MgMoO , CaMoO , and
4
4
SrMoO cases and below 700 °C in BaMoO case that are
4
4
closely related with the relative low melting point of bar-
ium molybdate crystal phase.
MgMoO sample are shown in Fig. 6a, b. The surface of
4
the Mg–Mo–O nitrate–tartrate gel precursor annealed at
7
00 °C temperature consists of the porous irregular parti-
Conclusions
cles of size from 500 nm to 3 lm. With further increasing
of annealing temperature to 1,000 °C, the spherical crys-
talline particles in size of about 1–5 lm have estimated.
These obtained results are in a good agreement with the
broad endothermic band of about 6,603 mJ in the stage of
temperature from 800 to 1,000 C presented in DSC curve
that characterizes the crystallization and agglomeration
processes of final crystalline material.
Summarizing results obtained from TG–DSC, XRD, and
SEM measurements can be concluded that aqueous sol–gel
synthesis method is a suitable technique for the preparation
of single-phase MgMoO4, CaMoO4, SrMoO4, and
BaMoO4 compounds at relatively low temperature of
500 °C. Moreover, according to the SEM results the for-
mation of spherical crystals with well-expressed planes and
edges have started up to 700 °C in MgMoO , CaMoO , and
Slightly different results from SEM micrographs, pre-
sented in Fig. 6c, d, by combustion of Ca–Mo–O nitrate–
tartrate gel precursor were obtained. Figure 6c shows SEM
image of Ca–Mo–O sample sintered at 700 °C. In this case,
the growing of homogenous spherical grains with an
average diameter of about 200–300 nm is observed.
Thereafter, the annealing temperature of 1,000 C leads to
the formation of the crystals with well-expressed spherical
shapes and edges that size varies from 1 to 5 lm.
4
4
SrMoO cases and below 700 °C in BaMoO case that are
4
4
closely related with the relative low melting point of bar-
ium molybdate crystal phase.
Acknowledgements The study was funded from the European
Community’s social foundation under Grant Agreement No. VP1-3.1-
ˇ
SMM-08-K-01-004/KS-120000-1756.
Interesting results by combustion of Sr–Mo–O nitrate–
tartrate gel precursor from SEM micrographs in Fig. 6e, f
were observed. The homogeneous spherical grains of
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