Revealing the Role of the Intermediates during the Synthesis of BaTi5O11

Article abstract of DOI:10.1021/acs.inorgchem.9b00865

BaTi₅O₁₁ has been extensively studied because of their microwave dielectrics properties. Traditionally, it is difficult to achieve this material as single-phase. Here, we report an effective method to obtain BaTi₅O₁₁ powder with nanometer-scale crystals, by solid-state reaction at moderate temperatures and using as precursors nanostructured particles consisting of BaTiO₃ and TiO₂. The main advantage is the intimate contact between the BaTiO₃ and TiO₂ that ensure, when the solid-state reaction takes place, the formation of complex solid compounds from three or more constituents. The formation mechanism of BaTi₅O₁₁ has been studied as a function of both the thermal treatment and the time reaction. The reaction was monitored by Raman spectroscopy combined with Confocal microscopy, the aim of this characterization technique is to provide the description of the general strategy and design principles to obtain BaTi₅O₁₁ powder. Consequently, this work is a challenging task for the compositional and structural study of complex inorganic nanoparticles.

Full text of DOI:10.1021/acs.inorgchem.9b00865

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Revealing the Role of the Intermediates during the Synthesis of
BaTi5O11
,†
†
‡
Carmen María Alvarez-Docio,* Julian Jimenez Reinosa, Giovanna Canu, María Teresa Buscaglia,‡
́
́
́
‡
†
́
́
Vincenzo Buscaglia, and Jose Francisco Fernandez
†Ceramics for Smart Systems Group, Ceramic and Glasses Institute (ICV-CSIC), C/Kelsen 5, Madrid, Spain
‡Synthesis of Advanced Functional Oxides Group, Institute of Condensed Matter Chemistry and Technologies for Energy
(IENI-CNR), Via de Marini 6, Genoa, Italy
S
* Supporting Information
ABSTRACT: BaTi5O11 has been extensively studied because of
their microwave dielectrics properties. Traditionally, it is difficult
to achieve this material as single-phase. Here, we report an
effective method to obtain BaTi5O11 powder with nanometer-scale
crystals, by solid-state reaction at moderate temperatures and using
as precursors nanostructured particles consisting of BaTiO3 and
TiO2. The main advantage is the intimate contact between the
BaTiO3 and TiO2 that ensure, when the solid-state reaction takes
place, the formation of complex solid compounds from three or
more constituents. The formation mechanism of BaTi5O11 has
been studied as a function of both the thermal treatment and the
time reaction. The reaction was monitored by Raman spectroscopy
combined with Confocal microscopy, the aim of this characterization technique is to provide the description of the general
strategy and design principles to obtain BaTi5O11 powder. Consequently, this work is a challenging task for the compositional
and structural study of complex inorganic nanoparticles.
involved during the reaction, and the final morphology of
BaTi5O11 material, because of the thermal processing, are still
not clearly defined.10
INTRODUCTION
■
The different crystalline phases present in the binary system of
BaTiO3−TiO2 exhibit excellent microwave properties.1 Specif-
ically, BaTi5O11 ceramic has demonstrated potent functional
dielectric characteristics, resulting in an interesting advanced
material for microwave communications.2−4 Owing to its high
interest, BaTi5O11 powder as a single-phase has been the
subject of research, and a great number of researchers have
tried to produce this material in a variety of ways. O’Bryan and
Thomson5 synthesized BaTi5O11 by the traditional ceramic
method, such as the solid-state reaction of BaCO3 and TiO2 at
high temperatures (1100−1400 °C); however, this method
could not produce single-phase material. Ritter et al.6 produced
the BaTi5O11 compound by a controlled hydrolysis. They
relied on O’Bryan’s study to predict the excellent dielectric
properties of the BaTi5O11 material. Other chemical processes
were effectively used to obtain the fine powder of BaTi5O11 as
a single-phase. Fukui et al.7 reported the effects of the thermal
treatment on the sintering of BaTi5O11 ceramics from an
alkoxide-derived powder. Employing a liquid-mix method,
Javadpour and Eror8 obtained BaTi5O11 by calcining the
mixture obtained after the reaction of tetraisopropyl titanate,
barium carbonate, and an ethylene glycolcitric acid solution.
Lu et al.9 and Tangjuank et al.10 obtained BaTi5O11 powder
using a sol−gel process; in both cases, the precursors were
sintered at temperatures from 700 to 1200 °C. In addition, the
evolution in the structure of intermediates, the mechanism
Chemical reactions may be optimized by reaction monitor-
ing because it provides information about the chemical
development at every stage of the reaction system.11
Transmission Electron Microscopy (TEM) and X-ray
Diffraction (XRD)12 are currently used to structurally
determine the reaction phases. However, reaction in nano-
structured materials still represents a challenge because the
structural analysis of nanoregions is limited in multiple phases
systems. Confocal Raman microscopy is a powerful tool for
research, which may introduce significant enhancements over
other common analytical techniques. It is a nondestructive,
noninvasive, and contact- and label-free technique and
discloses evidence about the stress and strain states, crystal
structure, electronic properties, and lattice vibrations. Con-
sequently, it permits detecting the different phases present in
the materials.13 Moreover, the combination of confocal Raman
spectroscopy and confocal microscopy, allows us to system-
atically study the structure−microstructure of a wide variety of
materials and composites, as well as to resolve the spatial
distribution of the compounds, owing to the high spectral and
spatial resolution of this technique.14−16 In this sense, Raman
Received: March 27, 2019
© XXXX American Chemical Society
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Figure 1. Diffraction patterns of (a) as-obtained BaTiO3 particles, (b) fresh BaTiO3/TiO2 composite particles, and (c) 10 days aged BaTiO3/TiO2
composite particles (the arrow indicates the peaks of amorphous titania). Raman spectrum obtained for (d) as-obtained BaTiO3 particles and (e)
10 days aged BaTiO3/TiO2 composite particles (peaks signaled as blue triangles, green squares, and red stars correspond to BaCO3, BaCl2, and
internal defects, respectively). FESEM micrographs of (f) as obtained BaTiO3 particles and (g) 10 days aged BaTiO3/TiO2 composite particles.
Particle size distribution of (h) as-obtained BaTiO3 particles and (i) 10 days aged BaTiO3/TiO2 composite particles.
calculated to have an overall stoichiometry corresponding to Ti/Ba =
5. The suspension was maintained at 95 °C for 1 h. The resulting
yellow powder, was then filtrated, washed with water until it was free
from chloride ions, and finally dried.
Finally, the resulting BaTiO3/TiO2 powders were introduced in a
cylindrical steel body of 1 cm in diameter, and they were compacted
by isostatic cold pressing. Then, sintering was carried out, in a
conventional muffle furnace, at different temperatures between 400 to
1000 °C with a heating rate of 5 °C/min. The hold time at the
maximum temperature was 1 min in each treatment.
Characterization. The crystalline structure of the samples was
studied by XRD in a D8 Bruker diffractometer with Cu Kα radiation
and a Lynx Eye detector.
Differential thermal (DTA) analysis joined to thermogravimetric
(TG) analysis was carried out to study the thermal behavior of the
materials by using a Netzch STA 409 Thermo-Analyzer. The assay
conditions correspond to the synthesis process: 20−1400 °C with
heating rate of 5 °C/min.
spectroscopy is an effective technique for the characterization
of the different phases obtained during a solid-state reaction.
In conclusion, this work will contribute to study the thermal
evolution of core−shell BaTiO3/TiO2 nanoparticles to obtain
single-phase BaTi5O11 particles. BaTiO3 nanoparticles were
synthesized through a hydrothermal method and consequently
coated accordingly with a TiO2 shell. Therefore, the samples
were characterized by Raman spectroscopy, X-ray Photo-
electron Spectroscopy (XPS) and XRD; in order to evaluate
the behavior of the reagents used for the preparation of the
core−shell particles and its effects in the solid-state reaction
during the thermal treatments.
EXPERIMENTAL PROCEDURE
■
Synthesis. The synthesis of barium titanate particles was
accomplished following this chemical reaction:17
BaCl2(aq) + TiOCl2(aq) + 4NaOH(aq)
A XPS study determined the surface composition of the samples in
a K-Alpha, Thermo Scientific equipment applying pass energy of 200
eV for survey scans and 40 eV for high resolution scans, and shifting
the C 1s peak to 285 eV.
The morphology of the particles was observed by using a Field
Emission Scanning Electronic Microscope, FESEM, S-4700 from
Hitachi. The morphological parameters were obtained by processing
FESEM micrographs with Leica Qwin Image software. The obtained
results consisted of a statistical study of values from more than 150
measured particles.
→ BaTiO3(s) + 4NaCl(aq) + 2H2O
(l)
where (aq) designates an aqueous solution.
The reaction was performed in a continuous Segmented Flow
Tubular Reactor (SFTR), similar to that used by Jongen et al.18 The
reaction was carried out at 95 °C, and the residence time in the
tubular reactor (length, 12 m; inner diameter, 4 mm) was 10 min. The
BaTiO3 precipitate obtained after reaction was centrifuged, it should
be washed with water until it was free from chloride ions, and finally
dried. Additional details about the preparation and properties of the
barium titanate particles used as templates can be found at the
Supporting Information.19,20
The previous obtained BaTiO3 particles were introduced in a
peroxotitanium(IV) solution (prepared as described in the Supporting
Information) under continuous slow stirring at room temperature.
The amount of Ti(IV) solution spent for the coating process was
A confocal micro-Raman Witec α 300R equipment equipped with
532 nm excitation laser was using to achieve a Raman study of the
samples by applying an incident laser power of 40 mW. The results
treatment was carried out with a Witec Control Plus Software.
B
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Figure 2. XPS spectra of as-prepared BaTiO3 (pink line) and aged BaTiO3/TiO2 (green line) composite particles: (a) C1s, (b) O1s, (c) Ti2p, and
(d) Ba3d elements.
1
(Figure 1d). This polymorph is based on cubic Pm3m (Oh )
symmetry and the phonon modes are represented by 3F1u
RESULTS AND DISCUSSION
■
+
The crystalline structure of the precursors is determined by
XRD (Figure 1). Figure 1a shows that the as-precipitated
BaTiO3 particles possess the typical pseudocubic structure of
hydrothermal particles (JCPDS 01-075-0214). In addition, it is
possible to distinguish the presence of BaCO3 (JCPDS 00-001-
0506), despite the precautions taken during the synthesis.
Figure 1b,c shows the XRD patterns of the BaTiO3/TiO2
particles, both as prepared and after 10 days aging. The
diffractogram of the freshly prepared sample are characterized
by the superposition of narrow diffraction peaks corresponding
to the cubic BaTiO3 and a broad hump at 25−35° 2θ that
could be associated with the amorphous titania phase.21,22
After 10 days of aging, the diffraction peaks of BaTiO3 almost
disappear, leaving only the characteristic diffraction spectrum
of amorphous hydrated titania, which does not reveal any
crystal characteristics; it only shows two wide bands, one
centered at ∼32° 2θ and the other at ∼50° 2θ.23 According to
the previous studies,24−27 the amorphous phase of peroxo-
titanium hydrate with a unknown structure is generated when
the spontaneous hydrolysis at room temperature of the peroxo-
titanium solution occurs.23 In the temperature range of 80−
100 °C, the peroxo-titanium hydrate evolves toward the
formation of an amorphous hydrated titania.25−27 Conse-
quently, we suggest that the amorphous layer achieved in this
work consists mainly of an amorphous titania comprising
hydration water.23,28
F2u.29 Although no Raman-active mode is predicted for the
cubic phase, it has been reported that it typically shows two
wide Raman bands at around 250 and 520 cm−1.29−32 The as-
precipitated BaTiO3 presents Raman bands centered at about
299 and 521 cm−1, which indicates local disorder of titanium
atoms in a nominally pseudocubic phase. Raman bands at 667
and 1059 cm−1 are also detected in the samples and are
assigned to the BaCO3 phase,30,33,34 while the Raman band at
181 cm−1 is attributed to the BaCl2 phase35−37 (a complete
assignment of the BaCl2 spectrum is given in the Supporting
Information, Figure S1). The Raman band at 807 cm−1 should
have been caused by the deformation of hydroxyl lattice
groups, they are located in the BaTiO3 structure as defects in
oxygen sites.38,39
The Raman spectrum of the 10 days aged precipitate of
BaTiO3/TiO2 sample shows several broad Raman features
located at about 280, 380, 450, 670, 810, and 870 cm−1 (Figure
1e). These Raman bands correspond to the vibration modes of
the Ti−O−Ti bonds.23,27,40 It is also worth highlighting that
the strong Raman band at 525 cm−1, which is characteristic of
the Ti−O−O groups in gels composed of peroxy-titanium, is
not observed in the 10 days aged precipitate.23,27 Thus, it could
be excluded the existence of an important quantity of residual
peroxy groups in the 10 days aged precipitated particles.
The morphology of as precipitates BaTiO3 nanoparticles is
shown in the Figure 1f. The stoichiometric particles are small
in size (≈100 nm) and they have a nearly spherical shape.41
The Raman spectrum for the as-precipitated BaTiO3 sample
shows the typical spectrum for BaTiO3 in a pseudocubic phase
C
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Moreover, into Figure S2 it could be observed smaller primary
particles (≈100 nm), that are aggregated to build up the so
obtained BaTiO3 particles.
whereas the peaks detected at 464.4 and 458.6 eV in the XPS
spectrum of Ti2p for the aged BaTiO3/TiO2 particles is
assigned to the Ti4+ in TiO2.51 Finally, the Ba3d spectra in both
samples BaTiO3 and BaTiO3/TiO2 look slightly different,
Figure 2d. In the case of the BaTiO3 particles, there are two
signals due to the Ba3d species (Ba3d3/2 and Ba3d5/2) that can be
deconvoluted into two peaks at around 793.8 and 795.2 eV,
which is consistent to barium in titanate and in carbonate
According to similar previous studies,42 the amorphous
titania coating generated onto the surface of barium titanate
nanoparticles is owing to the hydrolysis of the peroxotitanium
solution. The formation of the coating is promoted by the
differences in surface potential between the BaTiO3 particles
and the amorphous titania when the pH of the suspension was
adjusted to 9. Thus, the amorphous titania self-assemble
spontaneous at the barium titanate nanoparticle surface.23
Furthermore, the morphology of the 10 days aged BaTiO3/
TiO2 composite particles evidence the rearrangement of the
primary particle of BaTiO3 during the formation of the titania
coating, Figure 1g. It may be noted that the size of the
BaTiO3/TiO2 composite particles is slightly smaller than the
BaTiO3 nanoparticles (insets of Figure 1f,g), this fact confirms
that the amorphous titania attacks the BaTiO3 aggregates and
therefore produces rearrangement. Moreover, the evolution of
the crystallinity with the aging indicates that this attack also
advance far from the disaggregation and promotes the
reduction of crystallinity of the BaTiO3 nanoparticles. This
kind of chemical attack only could be justified if some reactant
remains in the so obtained powder.
5/2
species, respectively. Moreover, for the Ba3d the two peaks
located about 778.5 and 779.9 eV, also correspond to barium
in titanate and in carbonate species, respectively. However, in
the case of the aged BaTiO3/TiO2 particles the Ba3d peak can
3/2
5/2
be resolved into Ba3d at 795.7 eV and Ba3d at 780.3 eV
that it is assigned to barium in BaTiO3 without presence of
BaCO3. It is worth noting that for the BaTiO3 sample there
was no detected chlorine from BaCl2, in contrast with Raman
results, where the Cl2p for the BaCl2 should present a peak with
a binding energy of 202 eV52 that was not present in the
samples. Furthermore, the BaTiO3 precipitate was carefully
washed with water. Accordingly, with the silver nitride test
method,53 the supernatant is chloride ion free. These two ideas
may suggest that the BaCl2 could be found between the
BaTiO3 aggregates.
Based on the XPS analysis, the weight percentages of Ti, Ba,
O, and C were also calculated and listed in Table 1. The
A systematic surface characterization study of the particles
was carried out by XPS, which is an effective technique to
understand the chemistry produced at the surface of materials,
because it provides evidence about the local bonding
environment of the samples.43,44 Moreover, it is a powerful
tool for characterizing the chemical structure of nanoparticles
at the surface and bulk.45,46 A survey scan from 1300 to 0 eV
(shown in Supporting Information, Figure S3) indicated the
existence of barium, titanium, oxygen, and carbon in both the
as obtained BaTiO3 and 10 days aged BaTiO3/TiO2 particles.
Additional high-resolution scans (Figure 2a−d) were con-
ducted in the selected binding energy ranges for C1s, O1s, Ti2p,
and Ba3d, respectively, in order to quantify the elemental
composition of the samples. The C1s peak can be deconvoluted
into two principal bands (see Figure 2a) with a peak centered
at 285 eV, corresponding to the C−H/C−C species and the
other one centered at 286.4 eV, due to CO groups. The
amount of the carbon atoms bonded as C−C/C−H and as
CO is 75% and 15%, respectively. These signals of carbon,
owing to the environmental contamination, are assigned as
absorbed carbon and absorbed CO2. However, the C1s peak
displays a third component at 288.9 eV for the as prepared
BaTiO3 particles and at 289.2 eV for the 10 days aged BaTiO3/
TiO2 particles having a ∼10% contribution and it has been
Table 1. Element Contents (%) of Core BaTiO3 and Aged
BaTiO3/TiO2 Composite Particles Based on XPS Analysis
sample
At % Ti At % Ba At % O At % C Ba/Ti
BaTiO3
BaTiO3/TiO2 (10 days)
13.70
16.71
14.34
4.54
48.85
47.38
23.11
31.37
1:1
1:4
relative atomic percentages indicate that the Ba:Ti proportion
of BaTiO3 particles is 1:1, whereas for the BaTiO3/TiO2
composite particles, the Ba:Ti ratio is 1:4, which corresponds
with the BaTi4O9 compound. These data evidence that the
BaTiO3/TiO2 composite sample presents Ba2+ cations at the
surface that could be related to two aspects: (a) diffusion effect
during the synthesis process or (b) incomplete covering of
nanoparticles by the amorphous titania shell.
Figure 3 shows the thermal evolution study of the different
samples (DTA/TG). The TG curve of both samples shown a
first weight loss in the 40−200 °C temperature range, related
to the evaporation of absorbed water. These mass losses
continue gradually in the range 200−600 °C related to the
chemisorbed OH groups in the nanoparticle crystal lattice.54
Besides, the BaTiO3 nanoparticles shows two distinct mass
losses at higher temperatures signaled by two DTG peaks. The
first DTG peak, centered at ∼816 °C, is associated with the
thermal decomposition of BaCO3 and CO2 elimination, in
agreement of the endothermic peak observed at this temper-
47,48
2−
assigned to CO3 ions.
The O1s signal displays also three
components, as seen in Figure 2b. For the BaTiO3 particles,
there is an intense band near 529.2 eV assigned to oxygen
atoms in the Ba−O−Ti structure.49,50 In the case of the aged
BaTiO3/TiO2 particles, this peak appears at 530.1 eV and it is
attributed to the oxygen atoms in Ti−O−Ti structure.49,50 The
peak at 530.5 eV (BaTiO3 particles) or at 531.3 eV (aged
BaTiO3/TiO2 particles) corresponds to the hydroxyl group on
the surface of nanoparticles.47 The O1s emission line at 531.7
eV (BaTiO3 particles) or at 531.9 eV (aged BaTiO3/TiO2
particles) is assigned to the absorbed oxygen as O−C of CO2.
ature.55 At higher temperature, the second DTG peak (∼911
−
̅
°C) could be associated with the Cl removal, in spite of the
fact that the melting point of BaCl2 occurs at a higher
temperature and it is an endothermic phenomena.12 In the
case of aged BaTiO3/TiO2 composite, the thermogravimetric
curves show the loss of adsorption water at temperatures <200
°C and a weak exothermic peak at ∼750 °C without relevant
mass variation, that could correspond to the phases
crystallization. In order to determine the structural nature of
such thermally activated reactions, a XRD and Raman
characterization is afforded.
The fitted Ti2p spectrum for the core BaTiO3 nanoparticles,
3/2
Figure 2c shows the main peaks assigned to Ti2p1/2 and Ti2p
,
centered at 463.8 and 458.0 eV, respectively, which are in good
agreement with the bonding energies of Ti4+ in BaTiO3,48
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Figure 3. DTA/TG analysis at a 5 °C/min heating rate for the samples: (a) BaTiO3 and (b) BaTiO3/TiO2 composite particles.
Figure 4. XRD patterns of 1000 °C thermally treated (a) BaTiO3 and (b) BaTiO3/TiO2. Raman spectrum of 1000 °C thermally treated (c)
BaTiO3 and (d) BaTiO3/TiO2. FESEM micrographs of 1000 °C thermally treated (e) BaTiO3 and (f) BaTiO3/TiO2.
A XRD study of the samples after a thermal treatment at
1000 °C is shown in Figure 4a,b. After thermal treatment at
1000 °C, the BaTiO3 sample presents a main tetragonal
structure (JCPDS 01-081-2205), with a minor secondary phase
corresponding to the barium orthotitanate species, Ba2TiO4
(JCPDS 00-038-1481). The removal of barium carbonate
observed in the precursor is in the origin of the appearance of
such a secondary phase. The as-prepared coated powder
(BaTiO3/TiO2 composite) was also calcined in air at 1000 °C,
and the diffractogram is shown in Figure 4b. The diffraction
pattern for the BaTiO3/TiO2 composite heat-treated at 1000
°C shows mostly the peaks of the BaTi5O11 phase, in spite of
their low resolution (JCPDS 00-035-0805). No evidence of
other titanate phases are detected within the XRD resolution
limit (of the order of 1 wt %). Hence, it can be asserted that
the amorphous titania reacts with BaTiO3 nanoparticles, and
the system evolves toward the formation of nanocrystalline
BaTi5O11.
centered at about 186 and 521 cm−1 correspond to the
transversal component of the mode with the A1 symmetry (A1
(TO)),38 whereas the peak at about 719 cm−1 can be assigned
to the longitudinal mode of A1 symmetry (A1 (LO)).39 The
Raman mode at about 307 cm−1 is related to the B1 mode,
indicating asymmetry within the TiO6 octahedral.56 Finally, the
Raman characterization for the 1000 °C thermally treated
BaTiO3/TiO2 sample indicates the formation of BaTi5O11 is
complete (Figure 4d).8,9,57 In addition, it is observed that a
material densification in which grain boundaries and sintering
necks appear in a nanostructured material having a grain size
ranging from 40 to 200 nm (Figure 4e,f).
Despite obtaining the desired phase, the role of intermediate
compounds and the evolution of phase composition with
temperature is not adequately resolved by the use of XRD. To
clarify this point, both precursors are isothermally treated at
400, 500, 600, 700, 800, 900, and 1000 °C for 1 h and
Confocal Raman microscopy (CRM) has been used to
determine the nature of intermediate chemical species and
The Raman spectrum of the 1000 °C thermally treated
BaTiO3 sample is shown in Figure 4c. The Raman modes
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Figure 5. (a) Average Raman spectra for BaTiO3 thermally treated at selected temperatures. The main Raman modes of the different phases are
signaled by color-coding: BaTiO3 in red; BaCO3 in blue; and BaCl2 in green. Raman images of the different phases for (b−e) untreated BaTiO3
sample and (f−i) 1000 °C thermally treated BaTiO3 sample.
the evolution with the thermal treatment of main crystalline
phases.
The evolution of average Raman spectra obtained for
BaTiO3/TiO2 composite sample treated at various temper-
atures is shown in Figure 6a. The Raman image of the aged
and untreated BaTiO3/TiO2, Figure 6b, only shows a broad
Raman band associated with Ti−O−Ti bonds, signaled by a
magenta box. We study the Raman spectra as a function of the
temperature treatment, between 400 and 700 °C (for the sake
of brevity, this spectra is shown in the Supporting Information,
Figure S4 and it shows Raman peaks at 116, 140, 228, 264,
271, 362, 424, 630, and 845 cm−1). As it was reported before,
these Raman peaks correlate with the orthorhombic structure
of BaTi4O9.57−59 In the Raman image (Figure 6e) it can be
observed that the amount of the amorphous titanium decreases
as the BaTi4O9 crystallizes, signaled as yellow. In addition, the
increase of the thermal treatment temperature favors the
appearance of BaTi5O11 (signaled in cyan regions, Figure 6f).
Taking into account the results of the Raman spectra, the
system seems to have almost entirely transformed into
BaTi5O11, since this phase is observed from 800 °C and is
correlated with the crystallization, which is denoted by an
exothermic peak at ∼750 °C in the DTA analysis (Figure 3b).
The existence of crystalline BaTi5O11 is also in agreement with
the XRD pattern of the 1000 °C thermally treated sample
(Figure 4b).
The CRM study of the BaTiO3 precursor at different
temperatures has been carried out by mapping the surface of
samples pressed into disks, Figure 5. Raman images of the
untreated BaTiO3 and 1000 °C thermally treated BaTiO3
samples are depicted to show the distribution of the main
phases, Figure 5b−e and f−i, respectively. The Raman images
of the untreated BaTiO3 sample, presented in Figure 5b−e,
show three distinguishable phases: BaTiO3 as the main phase
(signaled by red regions, Figure 5b); BaCO3 as disperse in
agglomerates with a particle size of ∼2 μm (signaled by blue
region, Figure 5c); and unreacted BaCl2 is distributed in the
whole area where BaTiO3 is present (signaled by green, Figure
5d). This fact, together with the convergence between BaTiO3
and BaCO3 in Figure 5b,c, indicates that the BaCO3 could be
at the surface of the agglomerates, while the existence of
unreacted BaCl2 protects the BaTiO3 against the carbonation.
Figure 5a shows the average Raman spectrum of the core
BaTiO3 samples treated at different temperatures. The BaCO3
disappears at temperatures >700 °C (see Figure 5a), as
signaled by the lack of the Raman signal at 1059 cm−1 (in
concordance with DTA-Tg study). It is worth noting that the
unreacted BaCl2 coexists with BaTiO3 for the whole range of
temperatures.
From the above discussion, by combining the results of the
thermal analysis with the XRD, XPS, and CRM analyses, the
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Figure 6. (a) Study of the Raman spectra evolution as a function of thermal treatment for the BaTiO3/TiO2 composite sample. The main Raman
modes of the different phases are signaled by color-coding: amorphous Ti in magenta, BaTi4O9 in yellow, and BaTi5O11 in cyan. Raman images of
the phase distribution for (b) an untreated sample of BaTiO3/TiO2 sample, (c−e) 400 °C thermally treated BaTiO3/TiO2 sample, and (f) 1000 °C
thermally treated BaTiO3/TiO2 sample.
reaction mechanism of BaTi5O11 formation through solid-state
reaction from core−shell BaTiO3@TiO2 nanoparticles can be
schematically illustrated as shown in Figure 7. The first stage
portrays the core BaTiO3 raw materials at room temperature
obtained by a hydrothermal synthesis in a Segmented Tubular
Reactor. BaTiO3 particles possess spherical shape morphology
and they form aggregates (corresponding FE-SEM micro-
graphs are included in Supporting Information, Figure S2).
Raman images show that some BaTiO3 nanoparticles are
carbonated forming BaCO3 at room temperature. In addition,
the BaCl2 precursor remains in contact with BaTiO3 particles.
Even when the precipitated was carefully washed with water
and chloride ions are absent in the supernatant by the silver
nitride test method, this compound is found in the BaTiO3
aggregates. The second stage in the scheme illustrates the
reaction when an amorphous TiO2 is generated by coating the
core BaTiO3 from an aqueous solution of TiOCl2. The
presence of the amorphous titania phase produces three main
effects: first, the elimination of BaCO3; second, through the
removal of BaCl2 a rearrangement of the agglomerates; and,
finally, the amorphization of BaTiO3. XRD and Raman studies
expose that BaTiO3 is almost completely decomposed in the
presence of amorphous TiO2, which penetrates through the
structure of BaTiO3 by the BaCl2 phase. According to the XPS
data, it is also evident that the BaTiO3/TiO2 composite sample
presents Ba2+ cations at the surface that could be related to the
migration of the Ba2+ cations during the synthesis process.
Thus, an amorphous TiO2 is generated having a nucleus of
BaTiO3. At the third stage, evidence of the crystallization of
BaTi4O9 phase appears in the presence of amorphous titania.
Finally, at 800 °C, the excess of amorphous TiO2 continues to
penetrate the structure and form nanoparticles of the BaTi5O11
phase.
Figure 7. Scheme representing the main stages of the BaTi5O11
formation. Up-left scheme illustrates the former BaTiO3 nanoparticle
agglomerateas. The next up-right scheme represents the initial
reaction between the core-BaTiO3 aggregates and the peroxotitanium-
(IV) solution, the final room temperature reaction of the core−shell
BaTiO3/TiO2 to produce an amorphous titania with BaTiO3 rests.
The down-right scheme denotes the beginning of the formation of the
BaTi4O9 particles surrounded by the amorphous Titania phase.
Finally, the down-left scheme depicts the formation of the BaTi5O11
particles.
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DOI: 10.1021/acs.inorgchem.9b00865
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Inorganic Chemistry
Article
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CONCLUSIONS
■
In the attempt of obtaining a core−shell structure of TiO2@
BaTiO3, both the phases evolution and the role of the
intermediate compounds during the synthesis of nano-
structured BaTi5O11 have been studied. Core BaTiO3 particles
in the form of aggregates present precursor rests as BaCl2 and
local carbonation. The process of shell formation by an
amorphous titania onto core BaTiO3 provokes both the
carbonates and the chlorides removal. The progressing of the
reaction tooks 10 days and, as the chloride is completely
removed, the BaTiO3 particles are chemically attacked by the
amorphous titania phase. The thermal treatment of the so
obtained materials produces a single phase of BaTi5O11. The
intermediate phases are identified by a combination of
characterization methodologies as Raman confocal, XRD and
XPS. This example shows a strategy to synthesize complex
single-phase composition by controlling of the intermediate
phase and, in particular, to the residues of the precursors that
contributed to the reaction mechanism during the thermal
treatment. Finally, the development of BaTi5O11 powders is
attractive because this material exhibits excellent microwave
properties that allow its use in monolithic microwave
integrated circuits with reduced particle dimensions.
(11) Wiss, J.; Zilian, A. Online Spectroscopic Investigations (FTIR/
Raman) of Industrial Reactions: Synthesis of Tributyltin Azide and
Hydrogenation of Chloronitrobenzene. Org. Process Res. Dev. 2003, 7
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00865.
■
S
(12) Rojas-Hernandez, R. E.; Rubio-Marcos, F.; Gonca̧ lves, R. H.;
́
́
Rodriguez, M. A.; Veron, E.; Allix, M.; Bessada, C.; Fernandez, J. F.
Original Synthetic Route to Obtain a SrAl2O4 Phosphor by the
Molten Salt Method: Insights into the Reaction Mechanism and
Enhancement of the Persistent Luminescence. Inorg. Chem. 2015, 54
(20), 9896−9907.
Raman spectrum of as-obtained core BaTiO3 particles,
FESEM micrographs of core BaTiO3 particles, survey
XPS spectrum, and Raman spectrum of BaTiO3/TiO2 at
400 °C, showing the BaTi4O9 phase (PDF)
́
́
(13) Alvarez-Docio, C. M.; Reinosa, J. J.; Del Campo, A.; Fernandez,
J. F. Investigation of Thermal Stability of 2D and 3D CoAl2O4
Particles in Core-Shell Nanostructures by Raman Spectroscopy. J.
Alloys Compd. 2019, 779, 244−254.
AUTHOR INFORMATION
Corresponding Author
*E-mail: carmenma.docio@icv.csic.es.
■
(14) Torres-Carrasco, M.; de la Rubia, M. A.; Moragues, A.;
́
Fernandez, J. F.; Reyes, E.; del Campo, A. New Insights in Weathering
Analysis of Anhydrous Cements by Using High Spectral and Spatial
Resolution Confocal Raman Microscopy. Cem. Concr. Res. 2017, 100
(June), 119−128.
ORCID
́
Carmen María Alvarez-Docio: 0000-0002-3984-3078
́
(15) Reinosa, J. J.; del Campo, A.; Fernandez, J. F. Indirect
Giovanna Canu: 0000-0003-0819-9352
Measurement of Stress Distribution in Quartz Particles Embedded in
a Glass Matrix by Using Confocal Raman Microscopy. Ceram. Int.
2015, 41 (10), 13598−13606.
́
́
Jose Francisco Fernandez: 0000-0001-5894-9866
Notes
The authors declare no competing financial interest.
(16) Enríquez, E.; De la Rubia, M. A.; Del Campo, A.; Rubio-
Marcos, F.; Fernandez, J. F. Characterization of Carbon Nanoparticles
in Thin-Film Nanocomposites by Confocal Raman Microscopy. J.
Phys. Chem. C 2014, 118 (19), 10488−10494.
ACKNOWLEDGMENTS
■
The authors express their thanks to the projects NANOMIND
CSIC201560E068 and MAT2017-86450-C4-1-R for their
financial support. C.M.A.-D. is also indebted to MINECO
for a Predoctoral Grant (BES-2014-069779), which is
cofinanced with FEDER funds. C.M.A.-D. is grateful to
MINECO for a short stay scholarship (EEBB-I-16-11587).
(17) Bowen, P.; Donnet, M.; Testino, A.; Viviani, M.; Buscaglia, M.
T.; Buscaglia, V.; Nanni, P. Synthesis of Barium Titanate Powders by
Low-Temperature Aqueous Synthesis Using a New Segmented Flow
Tubular Reactor. Key Eng. Mater. 2001, 206−213, 21−24.
(18) Jongen, N.; Donnet, M.; Bowen, P.; Lemaître, J.; Hofmann, H.;
Schenk, R.; Hofmann, C.; Aoun-Habbache, M.; Guillemet-Fritsch, S.;
Sarrias, J.; et al. Development of a Continuous Segmented Flow
Tubular Reactor and the “Scale-out” Concept − In Search of Perfect
Powders. Chem. Eng. Technol. 2003, 26 (3), 303−305.
(19) Testino, A.; Buscaglia, M. T.; Buscaglia, V.; Viviani, M.;
Bottino, C.; Nanni, P. Kinetics and Mechanism of Aqueous Chemical
Synthesis of BaTiO3 Particles. Chem. Mater. 2004, 16 (8), 1536−
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