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Organic Chemistry Thesis
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CHAPTER-2
EXPERIMENTAL METHODS FOR SYNTHESIS
Experimental Methods for Synthesis
45 | P a g e
CHAPTER-II
2.0 EXPERIMENTAL METHODS FOR SYNTHESIS
In this chapter an attempt is made to give a systematic survey of the possible
preparation methods for nano-sized Barium Titanate. Part of these preparation
methods have been used as starting points for the work, described elsewhere in
this thesis. The preparation of Barium Titanate is mainly performed in two ways:
(1) Mixed oxide (solid state) preparation. (2) Wet-chemical preparation method,
so called because of the use of a solvent.
The solid state synthesis is not in use as mentioned above due to several
limitations. Moreover, as mentioned above there are different wet chemical
methods designed and are in practice to obtain crystalline or amorphous ceramic
powders.
The general description of some processes used widely for the preparation of
nano Barium Titanate mentioned below.
2.1 Co-precipitation Method
The basic principle of this procedure is the simultaneous precipitation from
solution of different ions in the form of insoluble highly dispersed Hydroxides or
Carbonates which are converted to final crystalline particles by thermal
treatment. This method has been applied in the preparation of Barium Titanate.
Synthesis routes based on the coprecipitation of complex metal salts remain one
of the most widely used commercial routes for the synthesis of BaTiO3.
Experimental Methods for Synthesis
46 | P a g e
The precipitation is performed mostly through the hydrolysis at low temperatures
(25-100 C) of metal-alkoxide solutions, such as Barium Isopropoxide and
Titanium amyloxide. The Clabaugh process is a wet chemical technique where
solutions of BaCl2, TiCl4 and oxalic acid are mixed and particles of barium titanyl
oxalate (BTO) precipitated [2.1]. After synthesis the particles are calcined to
drive off the oxalate and form BaTiO3. An advantage of BTO is that there is little
change in particle size during the conversion from BTO to BaTiO3 if the
calcination step is properly controlled. In addition, because the Clabaugh
process is primarily a solution-based synthesis route, good mixing and near
atomic scale homogeneity are possible. However, there are two critical issues
associated with the Clabaugh process: (1) Oswald ripening during synthesis and
(2) agglomeration and crystallite growth during calcination. To overcome the two
issues different researchers have used different approaches to correct the
problems during synthesis process.
A modified Clabaugh process has been studied by Kimel et al. [1.25] and
Szepesi [2.2]. In the modified process, a small-volume high-shear mixing
chamber is used to create turbulent fluid flow which permits particle nucleation
while limiting particle growth. After precipitation the particles are directly injected
into a quenching solution which coats the particle surface to inhibit Oswald
ripening. This method has produced BTO particles as small as 10 nm.
Experimental Methods for Synthesis
47 | P a g e
Yamaura et al. [2.3] and Park et al. [2.4] used alcohol based oxalic acid solution
during synthesis of BTO. The BTO exhibits a lower solubility in alcoholic solution
compared to aqueous solution and therefore growth by Oswald ripening is
limited. No notable growth was observed for BTO particles prepared in alcoholic
environments. However, since the solubility of Ba and Ti in alcohol solution is
incongruent and it was difficult to precipitate homogenous BTO powders.
To understand particle evolution during calcination it is important to understand
the decomposition reactions of BTO. Under isothermal conditions during
calcination BTO is believed to decompose by the following reactions [2.5-2.6]
BaTiO ( C2O4 )24 2 Ba2Ti2O2 ( C2O4 ) CO33 2 CO( g )+
2 Ba2Ti2O2 ( C2O4 ) CO33 BaCO3 TiO2 BaTiO3 CO2 CO( g )+++ 33 +
..(2.1)
On contrary the chemical reaction mentioned below can also take place
simultaneously during synthesis.
BaTiO ( C2O4 )2 BaCO3 TiO2 CO( g )+ 2+ 1/2 O2 ( g ) ++ CO2( g )
..(2.2)
Independent of the decomposition reaction, BaCO3 and TiO2 must react to form
BaTiO3
.
+ CO2( g )BaCO3 TiO2+ BaTiO3 .. (2.3)
Experimental Methods for Synthesis
48 | P a g e
The reaction can also lead to agglomeration and crystallite growth during
calcination. Wada et al. [1.47, 2.7] developed a two step calcination of BTO.
Powders with particle size ranging from 17 to 100 nm were reported.
Another group of researchers found that precise control of the heating rate is
necessary to control the final particle size of the BaTiO3 [2.4-2.8]. Using an
intermediate heating rate it yields the proper control of nucleation rate with limited
growth. At low heating rates the nucleation rate is low and the duration of the
reaction is long enough for substantial growth to occur whereas, high
temperature promotes growth of particles [2.9]. Under optimum conditions
BaTiO3 powder with a particle size ranging from 20-40 nm can be synthesized
Thermal Decomposition of Double Metal Salts
Other synthesis methods based on the thermal decomposition of double metal
salts have been presented, but the most common is the Pechini method, or the
citrate method [2.10-2.11]. This method is similar to the Clabaugh process
except that citric acid is used instead of oxalic acid to form a complex double
metal salt. The decomposition reactions involved in the Pechini method are
more complex than that of the Clabaugh process. Since both the Clabaugh
process and Pechini method are based on carboxylic acids, the formation of
BaCO3
during thermal decomposition is unavoidable [1.35].
Experimental Methods for Synthesis
49 | P a g e
2.2 SOL-GEL Process
Sol-gel is similar to that of co-precipitation. The main difference, however, is the
formation of a polymeric precipitate instead of the fine-grained powders obtained
by coprecipitation. The precursors used in sol-gel processing are metal-organic
compounds, mostly metal alkoxides. Several research groups have used a sol-
gel method for the preparation of nanoscale BaTiO3 powders [1.23-1.25, 1.27,
1.29 and 2.12-2.13]. Most sol-gel routes begin with the formation of non-aqueous
sols using high purity Ba and Ti reagents, commonly organo-metallics.
Hydrolysis of metal alkoxides at high pH causes the nucleation of Hydroxide or
oxide powders directly from solution, whereas hydrolysis at low pH produces a
gel. The nonaqueous solution in which the alkoxides are dissolved undergoes
gelation by the addition of an excess of water at temperature below 100 C, and
the resulting polymeric gel has to be converted to the final powder by annealing.
The properties of the resulting gel depend on many factors, including water
content, pH, and temperature. After gelation, the gels are dried and calcined at
high temperatures to remove the chemically bound water and crystallize the
amorphous gel. The calcination temperature is lower than that of solid-state
routes, and therefore agglomerates formed are weaker and easier to reduce
during milling. The main disadvantage of sol-gel routes is that the processes are
costly with low yields. To further reduce the particle size and tailor the particle
size distribution, Hempelmann and co-workers [2.14-2.15] performed sol-gel
synthesis in a microemulsion system. By using such a system the nucleation and
growth of the particles was confined to the aqueous phase of a water-in-oil
Experimental Methods for Synthesis
50 | P a g e
microemulsion and controls the particle size. Using different surfactant systems
and varying the experimental conditions, narrow particle size distributions with
mean sizes ranging from 3 to 16 nm were synthesized. The general reaction map
of sol-gel process is shown in Figure 2.1.
Figure 2.1 General reaction scheme of sol-gel process
Vapor phase synthesis
During exhaustive literature survey it is also found that efforts have been focused
on vapor phase synthesis routes for nanocrystalline BaTiO3 [2.16-2.18]. The
synthesis uses vapor phase Ba and Ti sources such as liquid precursors that are
Experimental Methods for Synthesis
51 | P a g e
either boiled or have inert gas bubble through them, the vapors are then mixed at
elevated temperatures and quenched. Because of the high quenching rates
growth of the particle after nucleation is severely limited. It is also possible to
use electron beam evaporation or sputtering of solid precursors to generate the
vapor [2.16]. Particle sizes less than 20 nm have been reported. One of the
major issues during synthesis is controlling the mixing of the vapors and the
chemical stoichiometry (i.e. Ba: Ti ratio) of the particles. The formation of BaCO3
is also a problem if the atmosphere is not properly controlled [2.1].
During the literature search the direct wet-chemical synthesis routes based on
precipitation have been presented. The techniques are named as low
temperature aqueous synthesis (LTAS), low temperature direct synthesis
(LTDS), solvent refluxing, or hydrothermal synthesis, all these syntheses consist
of basic synthesis steps. Aqueous solutions of Ba and Ti sources are injected
into a high pH solution, and then aged as needed. The Ba and Ti sources, pH
solution, and temperature range in different techniques leading to powders with a
variety of physical properties. Work has been carried out to precipitate BaTiO3
The high pH solution during synthesis also leads to the incorporation of large
amounts of hydroxide defects into the lattice, and since the reaction is open to
directly in an aqueous environment at or near room temperature under ambient
pressure [1.27, 1.30 and 2.19-2.22]. The resultant solutions contain large
amounts of Na and Cl it is necessary to thoroughly wash the particles after
synthesis.
Experimental Methods for Synthesis
52 | P a g e
the ambient atmosphere the presence of BaCO3 is difficult to eliminate. By
adjusting the synthesis variables (i.e. solution concentration, temperature, etc.)
particle size can be varied from 20-900 nm.
2.3 Hydrothermal Synthesis
Hydrothermal techniques are not new and were largely applied in the last century
for the synthesis of minerals of geological interest. In recent years, these
methods have attracted increasing attention for synthesis of dielectric and
piezoelectric ceramic powders [1.28, 1.31-1.32, and 2.23-2.29]. Under the
optimized synthesis conditions, powders with low defect concentrations and
controlled stoichiometry that require no further processing can be synthesized,
making hydrothermal synthesis an excellent choice for the commercial synthesis
of BaTiO3 [1.34]. In hydrothermal synthesis an aqueous solutions of barium and
titanium are mixed and sealed in a high temperature-pressure reaction vessel
and heated. Osseo-Asare et al. [2.30] and Lencka and Riman [2.31] studied the
thermodynamics of the hydrothermal formation of BaTiO3 and found that a basic
environment is necessary for precipitation of BaTiO3
The hydrothermal synthesis of BaTiO
and pH was dependent on
the Ba concentration in the starting solution.
3 is extensively commercialized and
protected by a variety of patents [2.32-2.36]. The methods invented by Abe et
al.[2.32] and Menashi et al.[2.35] are two of the primary methods used for the
commercial synthesis of hydrothermal BaTiO3. The synthesis steps in each
method are similar with differences arising in the post-synthesis treatments.
Experimental Methods for Synthesis
53 | P a g e
They used hydroxides of both Ba and Ti as the source material, which are mixed
in an aqueous solution and heated. After synthesis the powder is washed with
an acetic acid solution to remove BaCO3. However, the acid wash leads to Ba
dissolution from the particle and a Ba deficient surface. Stoichiometry is
controlled by a post-washing treatment with an insoluble Ba metal salt to adjust
to the desired Ba: Ti stoichiometry.
Menashi et al. [2.35] used an amorphous hydrous Ti-gel, Tiy(OH)x, as the Ti
precursor with Ba(OH)2 as the Ba precursor. After synthesis, the particles were
washed with a 0.005 to 0.02 M Ba(OH)2 solutions. The use of a Ba-rich wash
solution limits Ba dissolution and eliminates the need to adjust the stoichiometry
with a second treatment. Regardless of the method used to the synthesis, the
general reaction for the formation of BaTiO3
+ ++ BaTiO3Ba 2+
( g )TiO2 OH-2 H2O
during hydrothermal treatment is
shown in (equation 2.4).
.. (2.4)
Two rate-limiting mechanisms have been observed for the hydrothermal
synthesis of BaTiO3
The difference in formation mechanism is generally dependent on the phase of
the TiO
:
(1) Phase boundary and diffusion limited [2.26-2.28, 2.37]
(2) Nucleation and growth [1.28, 1.32, 2.24-2.25 and 2.29-2.38].
2 source. If TiO2 is crystalline or of large size, then the TiO2 particles
Experimental Methods for Synthesis
54 | P a g e
have a low solubility and growth occurs by the reaction of Ba2+ at the surface of
the TiO2 followed by diffusion of Ba2+ into the lattice, eventually leading to the
conversion of the TiO2 to BaTiO3. A secondary effect of this growth mechanism
is that size and morphology are limited by the size and morphology of the starting
TiO2 particles [2.39]. Hertl [2.28] studied the kinetics of hydrothermal synthesis
using a crystalline TiO2 source. At low Ba concentrations diffusion of Ba into the
lattice of the TiO2 is the rate limiting step. In contrast, at higher Ba
concentrations, the reaction of the Ba with the surface of TiO2 particles is the rate
limiting step in BaTiO3 growth.
When a highly soluble TiO2 source is used, for example, a Ti-organometallic or
sol-gel derived Ti-hydrous-oxide gel, both the Ba and Ti exhibit high solubility at
elevated temperatures and synthesis proceeds by nucleation and growth. To
fully investigate hydrothermal growth under such conditions Kershner et al. [2.29]
used TEM to image particles synthesized using a TiCl4-based gel as the TiO2
source. At all stages of growth homogenous single crystal BaTiO3 particles were
observed. If a surface reaction/diffusion mechanism was responsible for growth,
then at the early stages of growth, inhomogeneous particles with a TiO2 core and
a shell of BaTiO3 are expected; however, this was not observed. This lack of
evidence for a surface reaction/diffusion mechanism was later confirmed with
kinetic studies from Moon et al. [1.28], which led to the conclusion that a
nucleation and growth mechanism controls the growth of hydrothermal BaTiO3
when a high solubility TiO2 source is used.
Experimental Methods for Synthesis
55 | P a g e
The low temperature hydrothermal synthesis of BaTiO3 is of interest because of
the savings of time and energy. At low temperature the interface-diffusion growth
mechanism is kinetically limited. However, the mixing of the Ti and Ba is a
problem when using a Ti-gel precursor. When titanium isopropoxide is mixed
with water at high pH, a TiOy(OH)x gel readily forms. The local structure of the
gel is comprised of Ti-O-Ti bonds. It is necessary to break the Ti-O bonds for
complete mixing of the Ti and Ba [2.39]. Moon et al. [1.28] modified titanium
isopropoxide with acetylacetone which inhibits the hydrolysis of Ti and the
formation of TiOy(OH)x network [2.40-2.42]. This results in the Ti precursor
having greater water solubility and permits better mixing of the Ti and Ba. Using
a modified Ti precursor the BaTiO3 was synthesized at temperatures at 50 C
with particle sizes ranging from 50 to 350 nm.
Although high pH is necessary for synthesis it also leads to the greatest issue
with hydrothermal powders: hydroxide defects. During synthesis hydroxyl groups
are incorporated into the lattice of the particles [2.43]. After synthesis, heat
treatment of the powders is needed to remove the hydroxyl groups from the
lattice. The hydroxyl groups are compensated by the generation of oxygen
vacancies in the lattice [2.43]. If a large concentration of hydroxide defects is
present, during heat treatment the oxygen vacancies coalesce to form large
pores, which degrade electrical permittivity and physical properties, crystallinity
and density, of the bulk materials.
Experimental Methods for Synthesis
56 | P a g e
In the synthesis of BaTiO3 the quality and physical properties of the powder must
meet high standards. Defects, contamination, and incorrect stoichiometry are all
problems which will affect the densification and sintering of bulk materials. Large
intragranular pores, exaggerated grain growth and secondary phase are all
possible if the physical properties of the powder are not well-controlled [2.44].
An advantage of hydrothermal synthesis is the ability to control particle
morphology. A variety of shapes have been reported, including tubes [2.45],
hexapods [1.33], and platelets [2.46] all in the nanoscale by hydrothermal
synthesis. By limiting growth in a specific direction an anisotropic morphology is
achieved. Crystal chemistry and the presence of specific adsorbates affect the
crystal growth. Bagwell [2.47] found the stable crystal habit in hydrothermally-
derived BaTiO3 changed from the {111} plane to the {100}, {110}, and {211}
planes with the addition of polymeric additives. Since the ferroelectric properties
of BaTiO3
Barium Titanate hydrothermal syntheses are based upon the reaction between
TiO
are strongly dependent on the crystallographic orientation of the
materials, these developments in morphology control could possibly lead to an
enhancement in the electrical properties of bulk samples prepared from these
powders.
2 or Titanium Hydroxide and Ba(OH)2 at a high pH value in an aqueous
solution of NaOH. The reaction is carried out in a closed vessel at T >100C, so
that its pressure corresponds to the equilibrium vapor partial pressure at that
temperature. The temperature and concentration of reactants determine the time
Experimental Methods for Synthesis
57 | P a g e
needed for the reaction. The final product is a very fine BaTiO3 powder (100
nm) with a cubic or pseudocubic structure at room temperature, always
presenting a certain amount of OH-
The solvothermal method provides a means of using solvents as temperatures
well above their boiling points, by carrying out the reaction in a sealed vessel.
The pressure generated in the vessel due to the solvent vapor elevates the
boiling point of the solvent. Typically solvothermal methods make use of solvents
such as ethanol, toluene and water, and are widely used to synthesize zeolites,
inorganic open frame structures and other solid materials. In the past few years,
solvothermal synthesis has emerged to become the chosen method to
synthesize nanocrystals of inorganic materials. Hydrothermal processes involve
using water at elevated temperatures and pressures in a closed system, often in
the vicinity of its critical point. A more general term, solvothermal, refers to a
similar reaction in which a non aqueous solvent (organic or inorganic) is used.
Under solvo (hydro) thermal conditions, certain properties of the solvent, such as
groups trapped in the crystal structure, which
are released after thermal treatment at moderate temperature (~500 C)
necessary to obtain the thermodynamically stable tetragonal structure. A
beneficial effect of hydrothermal synthesis is the improved chemical purity of the
final product because of the dissolution-recrystallization process occurring during
hydrothermal aging. In particular, the amount of iron in the final product is
strongly reduced. This procedure nowadays has important industrial applications
2.4 Solvothermal Method
Experimental Methods for Synthesis
58 | P a g e
density, viscosity and diffusion coefficient, change dramatically and the solvent
behaves much differently from what is expected under ambient conditions. A
novel solvothermal route has been developed to synthesize highly dispersed
nanocrystalline Barium Titanate (BaTiO3), using a mixture of ethylenediamine
and ethanolamine as a solvent. The BaTiO3 nanoparticles obtained were highly
dispersed and crystalline with a cubic perovskite structure. The particle size
derived from the TEM ranged from 5 to 20 nm [2.48].
Table 2.1 represents a list of the most common synthesis routes used to produce
nanoscale BaTiO3 powders and their characteristics.
Experimental Methods for Synthesis
59 | P a g e
Table 2.1 List of common techniques and their characteristics used in synthesis of nanoscale Barium Titanate [1.34]
Method Particle Size Impurities Advantages Disadvantages Mixed Oxide 400 nm to
100s m Large quantities of Impurities due to starting materials and milling method
Easy process to perform on large scale Relatively cheap starting materials
High Impurities level High Calcination temperatures Large amount of aggregation leading to large particle sizes Milling usually required Poor stoichiometric control from particle to particles
Coprecipitation 10nm 10s m
Chloride and other impurities present from starting materials. Contamination of milling is required
Low Impurity levels. Low reaction temperatures. Stoichiometric mixing approaches atomic level.
Usually requires a milling treatment to obtain desired particle size. More time consuming than mixed oxide method. Tedious washing required to remove chloride ions.
Sol-Gel 5- 100 nm Minimal contaminants from organic precursors. Small amounts of Si contamination from glass wares.
Very low impurity levels. Stoichiometric on atomic level. Low processing temperature 20-650 C
Relatively expensive starting materials. Low temperature methods are generally time consuming with low product yields.
Vapor phase 20 nm- micron level
Small levels of contamination from starting materials
Low processing temperature from 100- (-800) C. Easy to produce nanosized particles
Some precursor materials are costly. Collection without aggregation is difficult. Stoichiometric control can be difficult
Hydrothermal 3nm micron level
Small levels of contamination from starting materials and reaction vessel. Hydrothermal (OH) defects due to aqueous synthesis.
Low processing temperatures 60-500 C. Particles are formed in solution giving potential control over agglomeration. High purity and atomic scale stoichiometry Particle morphology easily controlled.
Some precursor materials are costly. Recovery from suspension without agglomeration. Re-dispersion of agglomerates.
Experimental Methods for Synthesis
60 | P a g e
On going through the literature search one of the main disadvantages of several
routes is high-temperature calcination step which leads to the formation of hard
agglomerates.
Experimental Methods for Synthesis
61 | P a g e
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