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67
CHAPTER-3 (Section-A)
Bioinspired Synthesis of Morphologically Controlled SrCO3
Superstructures by Natural Gum Acacia
3A.1 Introduction
The field of biomineralization and its synthetic counterpart, biomimetic
mineralization, has been very active in recent years [1]. Development of bioinspired
strategies for the synthesis of inorganic crystals or hybrid inorganic–organic materials
with specific size, shape, orientation, organization, complex form, and related unique
properties, due to the potential to design new materials and devices in various fields
[2]. The main theme of biomineralisation is that nucleation, growth and controlled
patterning of inorganic materials takes place by the interactions between metal and
ligand. The controlled growth experiments of carbonate crystals are carried out in
aqueous solution, and some carbonates crystallize in the air-water interface/liquid-
liquid interface [3-4]. Proteins, glycoproteins and polysaccharides play vital role in the
precipitation of carbonates and act as nucleators, growth modifiers and anchoring
units in the mineral formation. Many investigations reveal that materials obtained
from nanosized particles have unknown properties/enhanced characteristics when
compared to common materials. It is very significant not only for biomineralisation
research but also for the synthesis of nanosized inorganic materials. Biomimetic
strategies have been developed for the synthesis of organized inorganic based
structures.
Strontium carbonate although itself is not an important biomineral, is
interesting since its crystallization yields insights into the formation of the
isostructural CaCO3 phase, aragonite of which pearls and nacre are largely composed.
SrCO3 has only one polymorph, so it is suitable for the study of biomineralisation.
Strontium carbonate (SrCO3), has wide applications as an additive in the production
of glass for color television tubes, chief constituent of ferrite magnets [5-6] and
nanometer sized SrCO3 is used in chemiluminescence sensors [7]. Various kinds of
68
SrCO3 crystals with morphologies such as spheroidal, needle-like, rod-like or ribbon-
like have been prepared with the aid of urease enzyme-catalyzed reaction [8], on
centered rectangular self-assembled monolayer substrates [9], within thermally
evaporated sodium bis-2 ethylhexylsulfosuccinate thin films [10], using the fungus
Fusarium oxysporum [11], or in a simple cationic microemulsion system under solvo-
thermal conditions [12]. Shape and size controlled growth of inorganic materials using
reverse micelles or microemulsions received considerable interest in the recent past
owing to its diverse application potential in areas such as catalysis, medicine,
electronics, ceramics, pigments, cosmetics, and separation technology[13 -15].
The use of natural materials as crystal growth modifiers has been extensively
studied in many crystal systems, including calcium carbonate [16-21], silica [22-25],
calcium phosphate [26], barium sulfate [27-30], barium carbonate [31-35] and strontium
carbonate [36-38]. The creation of superstructures resembling naturally existing
biominerals [39-41] with their unusual shapes and complexity, is meanwhile already an
important branch in the broad area of biomimetics [42-46]. The biological systems are
very effective at controlling crystal growth, especially polymers have been
successfully developed to control crystallization of inorganic particles in aqueous
solutions [47- 49]. Though there are many synthesizing procedures, still the preparation
methods using polymers as crystal growth modifiers are complicated and not so easily
available. Hence, we introduced an environmentally friendly route to generate
inorganic materials with controlled morphologies by using natural biopolymer - Gum
Acacia as crystal growth modifier for the crystallization of SrCO3 superstructures.
Gum Acacia is a natural gum made of hardened sap derived from Acacia
Senegal and Acacia Seyel. GA consists of mainly three fractions (1) The major one is
a highly branched polysaccharide consisting of β-(1-3) galactose backbone with
linked branches of arabinose and rhamnose, which terminate in glucoronic acid. (2) A
smaller fraction (~10 wt % of the total) arabinogalactan–protein complex (GAGP–GA
glycoprotein) in which arabinogalactan chains is covalently linked to a protein chain
through serine and hydroxyproline groups. The attached arabinogalactan in the
complex contains ~13% (by mole) glucoronic acid. (3) The smallest fraction (~1% of
the total) having the highest protein content (~50 wt %) is a glycoprotein which
69
differs in its amino acids composition from that of the GAGP complex. Here the
functional group (-OH) present in Arabinose and Rhamnose and (-COOH) of
glucoronic acids play a crucial role in the growth and formation metal carbonates
whereas the proteinaceous core with amino acids stabilize the formed metal
carbonates [50]. It not only acts as a stabilizer [51], but also acts as surfactant and
templating agent for which the functional group moieties (–OH, COOH & -NH2) have
been found to play a key role in mimicking the biomineralization process. The
crystallization involves the formation of different hierarchical structures like rice
grain, doughnut shaped, flower shaped, hexagonal rods and cross shaped which have
never been seen before in natural biominerals. Proteins and polysaccharides with
complicated patterns of various functional groups in GA selectively adsorb on to the
metal ion thereby hindering the crystal growth, followed by the mesoscale self-
assembly of nanometer-scale building block into hierarchial superstructures [52-57]. The
key reaction of CO2 with Sr2+ ions entrapped within GA polymer leads to the growth
of beautiful structures of strontianite nanocrystalline, such an aggregated morphology
not normally observed using other surfaces as templates. The objective of the present
work is to examine the process of biomineralisation utilize natural gums for the
synthesis of SrCO3 and its effect on the morphology. The study is very significant not
only for biomineralisation research, but also for the synthesis of functional inorganic
materials.
3A.2 Experimental Section
3A.2.1 Materials
Analytical grade chemicals of SrCl2, LaCl2, TbCl2 and Gum acacia, Sodium
bicarbonate purchased from Merck, India were used without further purification.
Double distilled water was used in all experiments.
3A.2.2 Preparation of Strontium Carbonate superstructures
In a typical experiment, at room temperature, 0.2662 g (1mM) of SrCl2 was taken
along with different proportions of homogenized GA (0.5 % and 1.0%) in different 25
70
ml glass beakers. They were dissolved in 20 ml distilled water and the mixed solution
was stirred thoroughly with the help of magnetic stirrer. Then NaHCO3 (2mM; 2 ml)
solution is added by continuous stirring and kept for 24 h at room temperature. After
24 h, the crystals are filtered washed several times with distilled water and dried at
room temperature. In the case of mixed metal carbonates, 0.2662 g (1mM) of SrCl2
and 0.1083 g (0.25mM) of LaCl2 or 0.0933 g (0.25mM) of TbCl2 were used.
Hydrothermal reactions were carried out in parallel using Teflon lined autoclaves with
internal volume of 10 ml at temperatures 60 and 90 °C under autogenous pressure.
After 24 h reaction time the autoclaves were allowed to cool to 50 °C and maintained
at that temperature for about 15 h before being allowed to cool slowly to ambient
temperature over 3 to 4 h. The sizes and morphologies of the products were examined
by XRD, SEM, EDAX, TEM, TGA-MS and FT-IR.
3A.2.3 Flow chart
Table 3A.1 shows the flow chart representation of experimental conditions
with GA.
Table 3A.1: Flow chart
71
3A.2.4 Characterization Methods
X-ray diffraction measurements of the Strontium carbonate hierarchial
structures were recorded using a Rigaku diffractometer (Cu Kα radiation, λ = 0.1546
nm) running at 40 kV and 40 mA (Tokyo, Japan). FT-IR spectra of SrCO3 structures
were recorded with a Thermo Nicolet Nexus (Washington, USA) 670
spectrophotometer. Thermogravimetric Analysis (TGA) coupled to Balzer Mass (MS)
was carried out on a TGA/SDTA Mettler Toledo 851e system using open alumina
crucibles containing samples weighing about 8–10 mg with a linear heating rate of
10°C min-1. Nitrogen was used as purge gas for all these measurements. TEM images
were observed on TECNAI FE12 TEM instrument operating at 120 kV using SIS
imaging software. The particles were dispersed in methanol and a drop of it was
placed on formvar-coated copper grid followed by air drying. Scanning electron
microscopy (FEI Quanta 200 FEG with EDS) was used for morphology assessment of
SrCO3 crystals. The crystals were collected on a round cover glass (1.2 cm), washed
with deionized water and dried in a desiccator at room temperature. The cover glass
was then mounted on a SEM stub and coated with gold for SEM analysis.
3A.3 Results and Discussion
3A.3.1 Structural characterization of SrCO3 superstructures
The phase composition and structure of as obtained samples was examined by
X-ray powder diffraction (XRD). Since all the different shapes have same
composition, we have shown only the XRD pattern of the synthesized material (1%
GA) at room temperature. As can be seen in Figure 3A.1, All the observed peaks can
be perfectly indexed to a pure orthorhombic phase and no other impurities have been
detected in the synthesized products. The observed diffraction peaks (2θ [°]): can be
correlated to the (hkl) indices (110), (111), (002), (012), (200), (130), (220), (221),
(132), and (113) respectively, of pure orthorhombic strontianite (JCPDS card number:
05-418). It may also be seen that the peak of (111) is the strongest, suggesting that
SrCO3 crystals obtained in gum acacia aqueous solution grow mainly along with
72
(111) face. Along with other several strong diffraction peaks, XRD pattern suggests
that the crystallinity of SrCO3 nanocrystallites obtained is excellent.
Figure 3A.1: XRD pattern of SrCO3 superstructures prepared with 1% GA obtained
at room temperature.
3A.3.2 Morphology control of SrCO3 superstructures
The morphologies of the as-synthesized products were examined by SEM-EDAX and
TEM. Figure 3A.2a shows the typical SEM image of SrCO3 crystals obtained in the
absence of GA prepared at room temperature. As can be seen, dendrimeric crystals
with sizes ranging from 2-4 µm length and 1-3 µm diameter are formed. Remarkable
changes were observed when GA was used as crystal growth modifier. Different
clusters of rice grain cross like, dumbbell like, hexagonal rod like, dough nut shaped
and flower shaped SrCO3 superstructures were observed for 0.5 % and 1.0 % GA
concentration depending on the reaction conditions. These hierarchical clusters
consist of nanocrystallites ranging from 30-150 nm {ambient at both concentrations
Fig. 3A.2b, c} and 20-500 nm {hydrothermal at both concentrations Fig. 3A.2d, e}.
73
At room temperature, with lower molar ratio (0.5 % of GA), rice grain shaped
(1µm – 1.4µm diameter), cross shaped structures (0.9 µm diameter) and two different
forms of dumbbell shaped (1µm - 1.4µm diameter) are obtained with nanocrystallites
in the range 30 – 150 nm. The SEM micrographs of each phase at higher
magnification are shown clearly (Fig. 3A.3a, b, c). Increase in concentration (1% of
GA) led to cross like (1.5 µm length, 750 nm diameter), dumbbell shape (1.5µm
length, 300-500 nm diameter) and flower shape (2.5 µm length, 2.1 µm diameter)
structures of nanocrystallites (20-100 nm). The higher magnification images of all the
phases are separately shown (Fig. 3A.3d, e & f). As can be seen TEM images of
these phases are similar to that observed in SEM micrographs (Fig. 3A.3j, k, l,) and
SAED pattern (Fig. 3A.3m) also shown that the synthesized strontium carbonate
crystals are crystalline. Cross like and dumbbell shaped morphology was observed
commonly in both the concentrations except variation in the size. With the increase in
concentration of GA the size of the cluster increases but the nanocrystallite size
decreases. This behavior can be attributed due to the effective passivation of the
surfaces and suppression of the growth of the nanoparticles through strong
interactions with the particles via there functional molecular groups of acacia namely,
hydroxyl groups of arabinose and rhamnose , galactose and carboxylic groups of
glucoronic acid moieties. Further crystal growth of SrCO3 was monitored under
hydrothermal conditions at 60 °C and 90 °C. At 90 °C stacks of hexagonal rods (1.8
µm length, 2.1 µm diameter) were observed at lower acacia concentration
(0.5%).These hexagonal rods are arranged in a stack like manner as seen [58] whereas
at higher concentration (1 %), doughnut shaped (4.3-4.6 µm length 5.6µm diameter)
and rice grain shaped (1.6-2 µm length, 0.6-0.8 µm diameter) clusters of 15-70 nm
crystallites are identified. No such variation was seen at 60 °C except in size
variation. Remarkable changes in both size and shape were observed in the
superstructures formed at ambient and hydrothermal (90°C) conditions for higher GA
concentration (1%). The higher magnification images of all the phases are separately
shown (Fig. 3A.3g, h, i). As can be seen, the crystallites size decreases from ambient
to hydrothermal condition. No other morphologies existed except for bigger structures
when continuously increasing the temperature conditions. On increasing the
concentration of GA at room temperature, the rice grain shaped structures aggregate
and produce flower shaped structures (Fig. 3A.2c). Siamilarly at hydrothermal
reaction, stack of hexagonal rods unite to form doughnut shaped structures (Fig.
74
3A.2e). The schematic composition of SrCO3 crystal morphology is shown in the
table 3A.2.
Table 3A.2: Schematic composition and morphology of SrCO3 at different
concentrations of gumacacia (GA).
Condition Without GA 0.5%wt GA 1.0%wt GA
Ambient
Dendrimeric SrCO3 crystals with 2-4µm length
Rice grain like, Cross like structures
Crosslike,dumbell shape and flower shaped super structures
Hydrothermal
(600)
Stack of hexagonal rods
Dough nut shaped and Rice grain shaped structures
Hydrothermal
(900)
Stack of hexagonal rods
Dough nut shaped and Rice grain shaped structures
75
Figure 3A.2: SEM images of SrCO3 superstructures. a) Dendrimeric structures in the
absence of additive. b) & c) Room temp reaction process at 0.5% & 1.0% GA. d) & e)
hydrothermal (900) reaction process at 0.5% & 1.0% GA.
76
Figure 3A.3: SEM images of SrCO3 superstructures at lower magnification using different
reaction conditions. (a, b, c)0.5% GA at room temperature. (d, e, f)1% GA at room temperature. g) 0.5% GA at hydrothermal 90 °C. (h, i)1% GA at hydrothermal 90 °C room temperature.(j, k, l)TEM images of SrCO3 superstructures of 1% GA at room temperature. m) SAED pattern SrCO3 superstructures.
77
3A.3.3 TEM & EDAX
Figure 3A.4b shows that TEM images of SrCO3 obtained at room temperature
and Figure 3A.4d shows TEM image of calcined product at 700 °C. SrCO3
superstructure was observed under TEM which also shows the nanocrystallites size in
nanometer range clearly. The images are in supporting with SEM images. Figure 3A.5
EDAX elemental analysis reveals that more carbon content is seen in normal room
temperature material than calcined product. Due to calcinations low carbon content is
noticed.
Figure 3A.4: SEM & TEM images (SAED inserted). a) & b) SrCO3 with 1%GA. c) &
d) Calcined product at 700οC.
78
Figure 3A.5: EDAX elemental analysis of SrCO3 a) with GA and b) Calcined product.
79
3A.3.4 Sr-LaCO3 system and Sr-TbCO3 system
Other than strontium carbonate, mixed metal carbonates such as Sr-LaCO3 and Sr-
TbCO3 were synthesized at room temperature using higher GA (1.0 %). Figure 3A.6a,
c show the SEM images of La doped SrCO3 and Tb doped SrCO3, respectively. As
can be seen, there is a clear morphological difference between SrCO3 structures
synthesized with and without the addition of rare earths. Rod-like crystals aggregate
in the form of bunches for La whereas spheroid shape resulted for Tb. The spheroid
structures have uniform morphology and size distribution with diameters ranging
between 400 – 900 nm. Inset shows the TEM image which also shows similar
morphological features as observed by SEM. It would be instructive to understand the
chemical composition of the different features observed for both Sr-LaCO3 and Sr-
TbCO3 nanostructures. This is conveniently done by spot-profile EDAX. In addition
to the expected Sr, C and O signals, strong signals of La and Tb are seen for La doped
SrCO3 and Tb doped SrCO3, respectively as shown in Figure 3A.6b, d. Table 3A.3
illustrates the morphology of as obtained products.
Table 3A.3: Schematic composition and morphology of Sr-LaCO3 and Sr-TbCO3 at
1.0% gum acacia (GA).
Condition
Morphology of Sr-LaCO3
(1.0% wt GA)
Morphology of Sr-TbCO3
(1.0% wt GA)
Ambient
Pack of rods
Spheroid clusters with
nanocrystallites
80
Figure 3A.6: SEM images of mixed metal carbonates. a) Pack of rod like clusters of
Sr- LaCO3 (TEM inserted). b) EDAX data of Sr-LaCO3. c) Spheroid shaped clusters
of Sr-TbCO3 (TEM inserted). d) EDAX data of Sr-TbCO3.
81
3A.3.5 TGA-MS analysis
Thermogravimetric analysis coupled to mass helps us to understand the
decomposition steps more precisely as we can know the evolved gas fragments as a
function of temperature or time. As representative systems, the TG/DTG-MS
thermograms of pure GA, SrCO3 synthesized without GA, as synthesized SrCO3
using GA and SrCO3 with GA calcined at 700 °C are shown (Figure 3A.7 a, b, c, d).
TGA-MS thermogram of pure GA (Fig. 3A.7a) shows a major decomposition
step (64.5% wt. loss) in the temperature range 260 - 400 °C and as can be seen it is
during this step the evolution of the gas with mass fragment 44 a.m.u characteristic of
CO2 from the decomposition of –COOH functional groups in GA is observed. TGA
profile of SrCO3 structures synthesized without gum acacia (Fig. 3A.7b) show single
step decomposition in the range 850 – 1050 °C. The mass fragment 44 a.m.u observed
in this range clearly suggests that the SrCO3 dendrimers decomposes into SrO and
CO2. The corresponding mass loss was quite similar to the theoretical value of the
mass loss of the above decomposition (29.81%) and almost the same with that
occurred for the thermal decomposition of the high pure SrCO3 phase between 900 –
1150°C [59]. The relatively low decomposition temperature of the present SrCO3
nanocrystallites might be ascribed to the size effect of the nanoparticles existed within
the nanocrystallites. Further, SrCO3 nanocrystallites synthesized using GA (Fig.
3A.7c) showed three step decomposition pattern. The first two steps in the
temperature range 200- 300 °C and 580 – 720 °C are due to the evolution of CO2
(mass fragment 44 a.m.u) from GA component in the inorganic and organic hybrid
SrCO3 nanocrystallites and the third step in the temperature range 800-1000 °C can be
attributed mainly due to the decomposition of SrCO3 nanocrystallites into SrO and
CO2. The corresponding mass loss was found to be 24.89%. The absence of the first
two decomposition steps in calcined (at 700°C) SrCO3 synthesized with gum acacia
(Fig. 3A.7d) clearly suggests that the as synthesized SrCO3 nanocrystalites are
inorganic and organic hybrid composite. The mass loss observed in this step of
28.41% is in consonance with the weight loss observed for SrCO3 dendrimers
synthesized without gum acacia.
82
Figure 3A.7: TGA-DTG-MS thermograms of a) pure gum acacia. b) SrCO3
synthesized without GA. c) as synthesized SrCO3 using GA. d) as synthesized SrCO3
with GA calcined at 700 °C. e) - - - - TGA, -.-.-.-.- DTG and ---------- MS.
83
3A.3.6 FT-IR spectra
FT-IR spectra of SrCO3 have been studied to determine the effect of GA on the
microstructure of nanocrystallites. The IR bands at 1454.48, 1460.65 and 1451.03cm-1
corresponds to the asymmetric stretching mode of C-O bond (Fig. 3A.8a, b, c),
respectively. The sharp peaks at 857.17 and 703.0 (Fig. 3A.8a), 857.69 and 703.0
(Fig. 3A.8b) and 855.77 and 697.62 (Fig. 3A.8c) are in plane and out plane bending
CO32-. In comparison with Figure 3A.8 the C-O stretching vibration peak around 1454
cm-1 in Figure 3A.8b shifts to higher frequency by 6 cm-1 (1460 cm-1), suggesting that
GA have an influence on the superstructure of strontium carbonate. This is probably
due to the GA molecules adsorb onto the surfaces of SrCO3 nuclei and influence the
mode of crystal growth with a little change of superstructure.
Figure 3A.8: FT-IR of SrCO3 particles nucleated a) in the absence of GA. b)
presence of GA. c) calcined product at 700 οC.
84
The exact growth mechanism of the micro and nano structures is not fully
understood. We predict that the various functional groups in gum acacia molecule
bind to alkali metal cations to form complexes. Hence, the anisotropic growth
phenomenon is due to the selective adsorption of gum acacia molecules on the
specified planes of growing crystal at different reaction conditions. Based on the
above analysis a possible growth mechanism for the formation of SrCO3
superstructures at the air/solution interface is schematically shown in Figure 3A.9.
Initially the functional moieties in GA inhibit the crystal growth by the encapsulation
of Sr2+ ions which in the presence of sodium bicarbonate forms SrCO3 nanoparticles
that act as building units in sequence with a brick by brick formation mechanism
resulting in superstructure crystals. These particles are formed homogeneously in the
solution and the crystallization is based on these SrCO3 superstructures that are made
up of nanocrystallites and are built up from individual nanocrystals and are aligned in
a common crystallographic pattern. In solution, rod like particles without any
boundaries aggregate together into hierarchical flower-like superstructure at the
air/solution interface as is schematically shown in figure. During the transformation of
rods to flower like structures under the control of GA (the functional groups
influences the nucleation, nanocrystal growth, alignment or self assembly and
aggregation into a superstructure), intermediate structures are formed such as rice-
like, cross-like, and dumbbell-like. The schematic representation of growth
mechanism of SrCO3 superstructures can be seen in Figure 3A.9.
85
Figure 3A.9: Schematic representation of growth mechanism of SrCO3
superstructures.
86
3A.4 Conclusion
Organic functional groups of GA have interaction with strontium and carbonate ions
which control the morphology of strontium carbonate via non classical crystallization
process. The use of natural material, GA in the synthesis of higher ordered
Strontionite superstructures have remarkable effect in control on the nucleation,
growth and alignment of SrCO3 particles in the reaction process. The utilization of
natural materials is one of the ways in which to synthesize particles with controlled
morphologies and this method may be potentially useful to other systems also
employed to produce materials with novel morphologies. This method would allow us
pragmatically realize the kind of morphological control of biomineralisation and
should be useful for synthesizing different superstructures which might find use in
catalysis, medicine, electronics, ceramics, pigments, cosmetics, and colour television
tubes.
87
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90
SECTION-B
Shape Controlled synthesis of Barium Carbonate microclusters and
nanocrystallites using Natural polysachharide– Gum acacia.
3B.1 Introduction
The study of mineral formation in biological systems, biomineralization, provides
inspiration for novel approaches to the synthesis of new materials. Biomineralization
relies on extensive organic-inorganic interactions to induce and control the synthesis
of inorganic materials. Bio-inspired morphosynthesis of crystals have been explored
with hierarchal forms which mimic natural biominerals in the presence of organic
template with complex functionalizing patterns has been developed widely in recent
years [1-6]. Among a variety of construction methodologies of functional materials,
patterned crystal arrays of organic [7], inorganic [8,9], and their hybrid crystals [10,11],
have received considerable attention in recent years for their diverse application
potential in areas such as catalysis, medicine, pigments, cosmetics, separation
technology [12,13], nano-devices [14] and find diverse applications in nanotechnology [15,16].
Nanostructural materials have become attractive because of their unique
characteristics that can hardly be obtained from conventional bulk materials owing to
their quantum size and surface effects. So, there has been considerable interest in
fabrication of low-dimensional nanosized materials such as nanowires, nanorods and
nanotubes. Several processes have been explored in the literature for the synthesis of
nanomaterials. These processes involve both physical and chemical methods [17-
20].Various artificial complexations have been investigated as models of
biomineralization using modifier agents , e.g., urease [21], nitrilo triacetic acid, citric
acid [22], poly(acrylic acid) (PAA) [23], poly(methacrylic acid) PMMA, poly(ethylene
glycol) PEG [24], poly(allylamine hydrochloride) PAH, poly (sodium 4-styrene
sulfonate) PSS [25], (cetyl trimethyl ammonium bromide) CTAB [26]. These modifying
agents have also been thought to control the polymorph of the barium carbonate
clusters. However, the characterization of these native and modifier agents on the
crystal surface is still unclear.
91
BaCO3 has attracted a lot of recent research [27-31] due to its close relationship
with aragonite, a prevalent and important biomineral, with many important
applications in the ceramic and glass industries as well as its use as a precursor for
magnetic ferrites and/or ferroelectric materials [32]. Barium carbonate (BaCO3) is also
used as a precursor for producing superconductor and ceramic materials [33] and other
important applications in optical glass and electric condensers [34]. Therefore, in the
present study, we report the synthesis and characterization of BaCO3 nanocrystallite
using natural polymer, gum acacia.
Polymer-mediated mineralization of inorganic materials has been the subject
of intense research because polymers have been found to dramatically influence the
characteristics of an inorganic precipitate. The ability to influence the morphology
and phase of an inorganic precipitate has important technological implications [35],
because some physical properties of crystalline materials such as the brilliance of
color pigments or the dielectric function of electroceramics depend on crystal habit,
grain size, grain size distribution, impurities, or content of polymorphous
modifications. Control of nucleation, crystal growth, and organization of crystals to a
superstructure (“texture”) make these physical properties tunable and are thus
important for technical application [36].
The present investigation deals with the influence of Gum Acacia a natural
polysaccaharide as templating species for the formation of BaCO3 nanocrystallites
forming microclusters. Gum Acacia is a natural gum made of hardened sap derived
from Acacia Senegal and Acacia Seyel. GA consists of mainly three fractions (1) The
major one is a highly branched polysaccharide consisting of β-(1-3) galactose
backbone with linked branches of D-galactose (38%), L-arabinose (45%), L-
rhamnose (4%), which terminate in D-glucuronic acid (7%), and 4-O-methyl-D-
glucuronic acid (6%) . (2) A smaller fraction (~10 wt % of the total) arabinogalactan–
protein complex (GAGP–GA glycoprotein) in which arabinogalactan chains is
covalently linked to a protein chain through serine and hydroxyproline groups. The
attached arabinogalactan in the complex contains ~13% (by mole) glucoronic acid. (3)
The smallest fraction (~1% of the total) having the highest protein content (~50 wt %)
92
is a glycoprotein which differs in its amino acids composition from that of the GAGP
complex. Here the functional group (-OH) present in arabinose and rhamnose and (-
COOH) of glucoronic acids play a crucial role in the growth and formation metal
carbonates whereas the proteinaceous core with amino acids stabilize the formed
metal carbonates [37]. It not only acts as a stabilizer [38], but also acts as surfactant and
templating agent for which the functional group moieties (–OH, COOH & -NH2) have
been found to play a key role in mimicking the biomineralization process. The
crystallization involves the formation of different hierarchical structures like rods
through dumbbell to flower shaped which have never been seen before in natural
biominerals. Proteins and polysaccharides with complicated patterns of various
functional groups in GA selectively adsorb on to the metal ion thereby hindering the
crystal growth, followed by the mesoscale self-assembly of nanometer-scale building
block into hierarchial superstructures [39-44]. The key reaction of CO2 with Ba2+ ions
entrapped within GA polymer leads to the growth of beautiful structures of whitherite
nanocrystalline, such an aggregated morphology not normally observed using other
surfaces as templates. This templating species composed of many anionic moieties,
which interacts strongly with ions, crystal surfaces. Functional, water-soluble
polymers with the ability to bind ions and crystals play a major role as scale inhibitors
crystal faces and thus, promotes the crystal growth. This species also strongly modify
the morphology of growing crystals in an interesting manner. Indeed, in the presence
of such templating species, the nucleated crystals may adopt a variety of shapes. The
interacting part of the GA (a variety of functional groups) can be selected or designed
in such a way that it specifically adsorbs to a certain crystal face. The proteins and
inorganic ions regulate the phase of the deposited mineral [45, 46].
The aim of the study was to determine the effects of this GA on the
morphological and structural characteristics of the resulting different morphological
structures, with special emphasis on different phases in the growth process. Our
results demonstrate that the integration of gum acacia (GA) taking advantage of the
reaction conditions, will extend the possibilities for controlling the shape, size, and
microclusters of the inorganic crystals by means of a simple mineralization process
93
3B.2 Experimental Section
3B.2.1 Materials
Analytical grade chemicals of BaCl2, LaCl2, TbCl2, Gum acacia, Sodium
bicarbonate were purchased from Merck, India and used as such without further
purification. Double distilled water was used in all experiments.
3B.2.2 Preparation of Barium Carbonate microclusters and nanocrystallites
In a typical procedure, at room temperature, 0.24422 g (1mM) of BaCl2 was
taken along with different proportions of homogenized GA (0.5 % and 1.0%) in
different 25 ml glass beakers. They were dissolved in 20 ml distilled water and the
mixed solution was stirred thoroughly with the help of magnetic stirrer. Then
NaHCO3 (2mM; 2 ml) solution is added by continuous stirring and kept for 24 h at
room temperature. After 24 h, the crystals are filtered and washed several times with
distilled water and dried at room temperature. In the case of mixed metal carbonates,
0.24422 g (1mM) of BaCl2 and 0.1083 g (0.25mM) of LaCl2 or 0.0933 g (0.25mM) of
TbCl2 were used. Hydrothermal reactions were carried out in parallel using Teflon
lined autoclaves with internal volume of 10 ml at temperatures 60 and 90 °C under
autogenous pressure. After 24 h reaction time the autoclaves were allowed to cool to
50 °C and maintained at that temperature for about 15 h before being allowed to cool
slowly to ambient temperature over 3 to 4 h. The sizes and morphologies of the
products were examined by XRD, SEM-EDAX, TEM, and FT-IR.
94
3B.2.3 Flow chart
Table 3B.1 shows the flow chart representation of experimental conditions
with.
Table 3B.1: Flow chart.
3B.2.4 Characterization Methods
X-ray diffraction measurements of Barium carbonate clusters were recorded
using a Rigaku diffractometer (Cu Kα radiation, λ = 0.1546 nm) running at 40 kV and
40mA (Tokyo, Japan). FT-IR spectra of BaCO3 structures were recorded with a
Thermo Nicolet Nexus (Washington, USA) 670 spectrophotometer. TEM images
were observed on TECNAI FE12 TEM instrument operating at 120 kV using SIS
imaging software. The particles were dispersed in methanol and a drop of it was
placed on Formvar-coated copper grid followed by air drying. Scanning electron
microscopy (FEI Quanta 200 FEG with EDS) was used for morphology assessment of
SrCO3 crystals. The crystals were collected on a round cover glass (1.2 cm), washed
with deionized water and dried in a desiccator at room temperature. The cover glass
was then mounted on a SEM stub and coated with gold for SEM analysis.
95
3B.3 Results and Discussion
3B.3.1 Structural characterization of BaCO3 microclusters
The phase composition and structure of as obtained samples was examined by
X-ray powder diffraction (XRD). Since all the different shapes have same
composition, we have shown only the XRD pattern of BaCO3 synthesized without GA
and with GA (1%) at room temperature. The XRD pattern of BaCO3 crystals obtained
both in the presence as well as in the absence of GA was pure orthorhombic witherite
crystals (Fig. 3B.1). All the observed peaks can be perfectly indexed to a pure
orthorhombic witherite phase and no other impurities have been detected in the
synthesized products. The observed diffraction peaks (2θ [°]): can be correlated to the
(hkl) indices (111), (002), (012), (130), (221), (132) and (113), respectively, of pure
orthorhombic witherite (JCPDS card number: 71-2394).It may also be seen that the
peak of (111) is the strongest, suggesting that BaCO3 crystals obtained in gum acacia
aqueous solution grow mainly along with (111) face. Along with other several strong
diffraction peaks, XRD pattern suggests that the crystallinity of BaCO3
nanocrystallites obtained is excellent, that can be correlated from TEM (SAED)
micrograph. It can be concluded that GA has major influence on the growth and size
morphology of BaCO3 crystals formed. However, the addition of additive has no
effect on the crystal structure of the resulting BaCO3 crystals.
96
Figure 3B.1: XRD pattern of BaCO3. a) absence of GA. b) presence of GA.
97
3B.3.2 Effect of gum acacia on the morphology of BaCO3
Morphologies of the formed crystal aggregates were investigated using SEM-
EDAX and TEM. Under ambient conditions in the absence of any templating species
only rod-shaped crystals of aragonite type BaCO3 (Fig. 3B.2a) with different sizes
(700 nm to 20µm length, 200 nm to 2µm dia) were formed as expected. Remarkable
changes have been observed in the morphology of the products obtained in the
presence of the templating species – GA. Bunch of rods, dumbbell, double-dumbell
and flower shaped BaCO3 clusters were observed for 0.5 % and 1.0 % GA
concentration depending on the reaction conditions. These clusters are of sizes in
several micrometers to several nanometers in both ambient (Fig. 3B.2b, c) and
hydrothermal crystallization (Fig. 3B.2d, e). SEM images of the products formed after
24h of reaction at various concentrations of GA (0.5 % and 1.0 %) in both the reaction
conditions are shown in Figure 3B.2.
At ambient conditions, when the GA concentration was 0.5%, bunch of rods in
the form of clusters are seen as shown in Figure 3B.2b.The length of the rods present
in the clusters is in the size range from 100 nm to 400 nm. The enlarged image clearly
shows that the rods aggregate in the form of bunch like clusters (Fig. 3B.3a). When
the amount of GA added is increased to 1.0%, shorter assemblies of BaCO3
aggregates in the form of dumbbell, double dumbell and flower like clusters are
observed, as depicted in Figure 3b.2c.These clusters constitute nanosized subunits
with size around 30nm. The enlarged SEM images of both the concentrations are
separately shown in (Fig. 3B.3b).Close similar morphology was observed commonly
in both the concentrations except variation in the size of the cluster and alignment of
rods. With the increase in concentration of GA the size of the cluster increases but the
nanocrystallite size decreases. This behavior can be attributed due to the effective
passivation of the surfaces and suppression of the growth of the nanoparticles through
strong interactions with the particles via there functional molecular groups of acacia
namely, hydroxyl groups of arabinose, rhamnose and galactose and carboxylic groups
of glucoronic acid moieties. Progressive changes in the assembled BaCO3 clusters
indicated that the arrangement continued to grow principally in width rather than in
length when the crystals interlinked. Although the individual rods observed in the
absence of templating species are often disordered, they were structurally intact,
98
suggesting that there are strong interparticle interactions between adjacent rods.
Significantly, the HR-SEM images show that the BaCO3 crystals appear to be higher-
order superstructures, exhibiting close morphological alignment of the rod shaped
crystals as well as aligned growth steps. Further crystal growth of BaCO3 was
monitored under hydrothermal crystallization at 60 °C and 90 °C. Since there is no
much variation in the morphology observed, the results obtained at 90 °C are shown
in Figure 3B.2d, e. At 90 °C stacks of rods are arranged randomly and appear like
flowers at lower gum acacia concentration (0.5%), whereas at higher concentration (1
%), more number of flower like clusters are seen but there is no change in the
morphology as observed in Figure 3B.2d,e. From this it is evident that during
hydrothermal crystallization, at two different concentrations common morphology
i.e.; flower shaped clusters are seen except their number increased at higher ratio of
metal/ligand concentration, and the size of the clusters is also almost similar. This
attributes that increasing concentration does not have much effect on the crystal
morphology during hydrothermal crystallization. Moreover, temperature variation
also does not have any impact in the growth morphology except variation in size. On
the other hand, synthesis of BaCO3 nanostructures at ambient condition show
influence of concentration on the morphology and arrangement of the clusters formed.
The schematic illustration of BaCO3 crystal morphology is shown in the Table 3B.2.
Table 3B.2: Schematic composition and morphology of BaCO3 at different
concentration of gum acacia (GA).
Condition Morphology of BaCO3 without
GA
Morphology of BaCO3 (0.5%wt GA)
Morphology of BaCO3 (1.0% wt GA)
Ambient Rod like aragonite crystals
Bunch of rods in the form of cluster
Dumbell,doubledumbell and flower shaped clusters
Hydrothermal (60ο)
Rod like aragonite crystals
Flower like clusters
( less number)
Flower like clusters (more number)
Hydrothermal (90ο)
Rod like aragonite crystals
Flower like clusters
( less number)
Flower like clusters
( less number)
99
Figure 3B.2: SEM images of BaCO3 clusters. a) Rod shaped aragonite crystals in the
absence of additive. b) & c) Room temp reaction process at 0.5% & 1.0% GA. d) & e)
hydrothermal (900) reaction process at 0.5% & 1.0% GA.
100
Figure 3B.3: Enlarged SEM images of BaCO3 clusters at different reaction
conditions. a) 0.5% GA at room temperature. b) 1% GA at room temperature. c)
0.5% GA at hydrothermal 90°C. d) 1% GA at hydrothermal 90 °C.
101
3B.3.3 TEM & EDAX
Figure (3B.4) shows that TEM images of BaCO3 clusters obtained in aqueous
solution (a) before and (b) after calcinations at 700 °C. As can be seen, the
morphology of the particles obtained in the presence of GA at the air/solution
interface is dumbbell, double dumbell and flower like clusters(Fig. 3B.4a) However,
after calcination at 700 °C the structure appears to be deformed resulting in the
formation of agglomerates of smaller particles of size around 20 nm (Fig. 3B.4c) .
Figure 3B.4b, d shows the corresponding EDAX spectrum and selected area electron
diffraction (SAED) pattern of the BaCO3 clusters. Selected area electron diffraction
(SAED) pattern obtained for BaCO3 show a number of spots arranged in circular
manner which confirms the nanocrystalline nature of grown nanoparticles.
Meanwhile, the corresponding SAED pattern (Fig. 3B.4b) exerted some regularly
aligned bright spots and also blurr diffraction rings containing relatively bright spots
could be indexed as the planes of (111), (002), (012), (130), (221), (132) and (113),
which were thus in agreement with the previous XRD results .The BaCO3 clusters
were believed to be self assembled by the related crystalline nanoparticles in presence
of appropriate additives rather than the random agglomeration. From Figure 3B.4b,d,
the EDAX elemental analysis reveals that carbon content is seen more in the as-
prepared BaCO3 microclusters when compared to the calcined material, which can be
attributed to the formation of organic-inorganic hybrid material, the organic
component mainly from GA.
102
Figure 3B.4: SEM, TEM images (SAED inserted) and EDAX of BaCO3. (a, b) In the
presence of 1% GA. (c, d) Calcined product at 700 οC.
103
3B.3.4 Ba-LaCO3 System and Ba-TbCO3 System
Other than Barium carbonate, mixed metal carbonates such as Ba-LaCO3 and
Ba-TbCO3 were synthesized at room temperature using higher GA (1.0 %)
concentration. Figure 3B.5a, c shows the SEM images of La doped BaCO3 and Tb
doped BaCO3, respectively. As can be seen, there is a clear morphological difference
between BaCO3 structures synthesized with and without the addition of rare earths.
Ba-LaCO3 clusters are in form of rods whereas Ba-TbCO3 appear to be dendritic
clusters. Inset shows the TEM image in which similar morphological features are
observed and is in consonance with the observations by SEM. It would be instructive
to understand the chemical composition of the different features observed for both Ba-
LaCO3 and Ba-TbCO3 microclusters. This is conveniently done by spot-profile
EDAX. In addition to the expected Ba, C and O signals, strong signals of La and Tb
are seen for La doped BaCO3 and Tb doped BaCO3, respectively as shown in Fig.
3B.5b, d. The composition and morphology is represented in the table 3B.3.
Table 3B.3: Schematic composition and morphology of Ba-LaCO3 and Ba-TbCO3 at
1.0% gum acacia (GA).
Condition
Morphology of Ba-LaCO3
(1.0% wt GA)
Morphology of Ba-TbCO3
(1.0% wt GA)
Ambient
Short rods
Dendritic clusters
104
Figure 3B.5: SEM images of mixed metal carbonates. a) Rod like clusters of Ba-
LaCO3. b) EDAX data of Ba-LaCO3. c) Dendritic clusters of Ba-TbCO3. d) EDAX
data of Ba-TbCO3.
105
3B.3.5 FT-IR spectra
FT-IR spectra of BaCO3 have been studied to determine the effect of GA on the
microstructure of nanocrystallites. The IR bands at 1445.83 and 1426.95 cm-1
corresponds to the asymmetric stretching mode of C-O bond (Fig. 3B.6a, b),
respectively. The sharp peaks at 856.74 and 693.45 (Fig. 3B.6a), 856.44 and 689.80
(Fig. 3B.6b) are in plane and out plane bending CO32-. In comparison with Figure
3B.6a the C-O stretching vibration peak around 1426 cm-1 in Figure 3b.6b shifts to
lower frequency by 19 cm-1 (1460 cm-1), suggesting that GA have an influence on the
microstructure of barium carbonate. This is probably due to the GA molecules adsorb
onto the surfaces of BaCO3 nuclei and influence the mode of crystal growth with a
little change of superstructure.
Figure 3B.6: FT-IR of BaCO3 clusters. a) Nucleated in the presence of GA. b)
Calcined product at 700 ο C.
106
Based on the above analysis, a possible growth mechanism for the formation of
flower like BaCO3 clusters at air/solution interface are schematically shown in Figure
3B.7. First the BaCO3 rods grow under the control of GA through frequently observed
crystallization. The functional moieties in GA are selectively adsorbed on the back
bone of rods. Then these rods aggregate and self assemble to form clusters. These
clusters gives an intermediate dumbbell like arrangement and then finally to flower
shaped cluster. As we know that the rod-dumbbell-sphere morphogenesis mechanism
appears to be rather universal, however, it has to be pointed out that the exact growth
mechanism is still unknown, although some explanation was given in the literature
based on the role of intrinsic electric fields, which direct the growth of dipole crystals.
The schematic representation of growth mechanism of BaCO3 clusters are shown in
Figure 3B.7.
Figure 3B.7: Schematic representation of growth mechanism of BaCO3 clusters.
107
3B.4 Conclusions
The biomineral phase, shape and function in natural systems are often
controlled by rather complicated chemical species. Using natural gums, a variety of
crystal morphology could be produced when ambient and hydrothermal conditions
were employed. In summary, we demonstrated that simple GA can be used as crystal
growth modifiers to template unusual complex morphologies of BaCO3 crystals. We
have demonstrated the formation of micro clusters of Barium carbonate through the
ambient and hydrothermal method. Micro clusters of Barium carbonate are bunch-
like, dumbbell, double dumbbell and flowerlike arrangement, which is confirmed by
TEM micrographs. The crystalline nature of Barium carbonate is confirmed by XRD
spectra whereas FT-IR spectra confirm the structural features of Barium carbonate.
Using these simple Gums, research is being further extended for the morphogenesis of
other minerals with complex superstructures and nanostructures. The obtained BaCO3
assemblies could find applications in industrial field.
108
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