Nucleation studies of ZTC doped with l-arginine in supersaturated aqueous solutions

Article abstract of DOI:10.1016/j.physb.2009.02.034

The metastable zonewidth studies are carried out for various temperatures for supersaturated aqueous solutions of zinc thiourea chloride added with 1 mole % of l-arginine. The metastable zonewidth is increased with the addition of l-arginine. The inductio

Full text of DOI:10.1016/j.physb.2009.02.034

ARTICLE IN PRESS
Physica B 404 (2009) 1813–1818
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Nucleation studies of ZTC doped with -arginine in supersaturated
L
aqueous solutions
a
b,
Ã
c
T. Balu , T.R. Rajasekaran , P. Murugakoothan
a
Department of Physics, Aditanar College of Arts and Science, Tiruchendur 628 216, India
Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627 012, India
Post Graduate and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The metastable zonewidth studies are carried out for various temperatures for supersaturated aqueous
Received 21 January 2009
Accepted 24 February 2009
solutions of zinc thiourea chloride added with 1 mole % of
L-arginine. The metastable zonewidth is
increased with the addition of -arginine. The induction period is experimentally determined and
L
various critical nucleation parameters such as radius of critical nucleus, number of molecules in the
critical nucleus, critical free energy of nucleus and interfacial tension are also calculated based on the
classical theory for homogeneous crystal nucleation. The induction period is increased with the increase
PACS:
8
4
8
1.10.Aj
2.65.Ky
1.10.ꢀh
of L-arginine addition. The critical nucleation parameters vary with increase in doping concentration. It
is also observed that the nucleation rate increases with the increase of supersaturation. The second
harmonic generation (SHG) efficiency measurements are carried out with different doping concentra-
Keywords:
tion of
L-arginine reveal that nonlinear optical (NLO) property is enhanced by
L-arginine dopant.
Metastable zonewidth
Induction period
&
2009 Published by Elsevier B.V.
Nucleation parameters
Interfacial tension
SHG efficiency
Nonlinear optical materials
1.
Introduction
Nonlinear optical materials posses wide range of applications
kinetics of ZTC doped with
solutions. In the present study, the nucleation parameters of pure
ZTC crystals and the effect of amino acid -arginine as an added
L-arginine in supersaturated aqueous
L
in the field of telecommunication and optical information storage
devices. The search for new and efficient NLO materials has
resulted in the development of a new class of materials called
semiorganics. These materials have large nonlinearity, high
resistance, too large induced damage, low angular sensitivity
and good mechanical hardness [1–3]. Recently metal complexes of
thiourea have been explored. Some of the potential thiourea
complexes are zinc thiourea sulphate (ZTS), zinc thiourea chloride
additive (additive concentration is 1–5 mole%) on the nucleation
of these crystals are investigated.
In this report, in order to optimize the growth parameters for
the growth of bulk crystals with improved growth rates, the
solubility, metastable zonewidth and induction periods have been
determined experimentally. Metastable zonewidth is an essential
parameter for the growth of good crystals from solution, since it is
the direct measurement of the stability of the solution in its
supersaturated region [12]. The control of crystallization habit is
clearly linked to the additives added to the system and the
internal parameters like pH. Various critical nucleation para-
(ZTC), cadmium thiourea chloride (BTCC), copper thiourea
chloride (CTC), bis thiourea zinc acetate (BTZA) and cadmium
thiourea acetate (BTCA). These crystals have better nonlinearity
optical property than KDP [4–8]. ZTC is a potential semiorganic
NLO material and crystallizes in the noncentrosymmetric orthor-
meters of pure and L-arginine doped ZTC crystals for the effect of
supersaturation and on different doping concentration are
calculated.
˚
hombic space group Pnma with lattice parameters a ¼ 13.012 A,
3
˚
˚
˚
b ¼ 12.768 A, c ¼ 5.890 A and V ¼ 978.64 A [9]. Growth of ZTC
single crystals using slow cooling technique has already been
attempted [10]. Influence of solution pH on the growth and
characteristics of ZTC has also been reported by Rajasekar et al.
2
. Experimental
[
11]. Hitherto no attempt has been made to study the nucleation
2.1. Synthesis and solubility of ZTC
Ã
Zinc thiourea chloride (ZTC) was synthesized using analar
Corresponding author. Tel.: +91462333887; fax: +914622322973.
E-mail address: trrajasekaran@yahoo.com (T.R. Rajasekaran).
grade zinc chloride and thiourea in a stoichiometric ratio 1:2.
0921-4526/$ - see front matter & 2009 Published by Elsevier B.V.
doi:10.1016/j.physb.2009.02.034

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T. Balu et al. / Physica B 404 (2009) 1813–1818
The required quantity of zinc chloride and thiourea were esti-
mated from the following reaction:
100
ZTC with L-Arginine
Pure ZTC
ZnCl
2
þ 2½CSðNH
2
Þ
2
ꢁ ! Zn½CSðNH
2
Þ
2 2 2
ꢁ Cl
Solubility curve
The required quantities of the component salts were very well
dissolved in double distilled water and thoroughly mixed using a
magnetic stirrer and the mixture was heated at 50 1C till a white
crystalline salt of ZTC was obtained. The synthesized salt was
purified by repeated recrystallization process.
The solubility of ZTC at a pH value of 3.4 has been determined
for four different temperatures, i.e., 30, 35, 40 and 45 1C. The
solubility was determined by dissolving the solute in water in air
tight container maintained at a constant temperature with
continuous stirring. After attaining the saturation, the equilibrium
concentration of the solute was analysed gravimetrically [13]. The
solubility of ZTC as a function of temperature is shown in Fig. 1.
80
60
40
280
290
300
Temperature (K)
310
320
2.2. Metastable zonewidth studies
Two hundred milliliters of ZTC solution saturated at 30 1C was
prepared in accordance with the solubility diagram. Then the
solution was taken in two beakers each containing 100 ml. In one
Fig. 2. Metastable zonewidth of pure and
L-arginine added ZTC.
beaker, 1 mole% of L-arginine was added. The solutions were
filtered using a whatman filter paper. These beakers were loaded
in a constant temperature bath and controlled to an accuracy of
zonewidth for different saturation temperatures for pure and
-arginine added ZTC solutions are shown in Fig. 2. There was no
remarkable change in pH with the addition of -arginine
0
.01 1C provided with a cryostat for cooling below room
L
temperature. The solution was preheated to 5 1C above the
saturation temperature for 1 h before cooling. It was continuously
stirred using a motorized stirrer to ensure homogeneous con-
centration and temperature throughout the entire volume of the
L
(
1–5 mole%).
2.3. Induction period measurements and interfacial tension
solution. Metastable zone width of pure and
L-arginine doped ZTC
was measured by polythermal method [14]. In this method, the
equilibrium saturated solution was cooled at the rate of 4 1C h
ꢀ1
Induction periods of pure and -arginine doped ZTC solutions
L
were measured by means of the isothermal method [15].
A solution with supersaturation ratio 1.25 was prepared and
placed in a constant temperature bath (accuracy 70.01 1C) at
from the overheated temperature until the first visible crystal
nucleus was observed. Since the time taken for the formation of
first visible nucleus after attainment of critical nucleus is very
small, the first nucleus observed may be taken as critical nucleus.
The direct vision method is adopted for studying the nucleation
temperature. The difference between the saturated temperature
and the nucleation temperature is taken to be the metastable zone
width of the system. The experiment was repeated for solutions
saturated at the temperatures 30, 35, 40 and 45 1C. The metastable
3
4 1C. The time required for the growth of critical nucleus to a
detectable size is negligibly small compared with the time needed
between the attainment of supersaturation and the appearance of
a crystal of detectable size and hence this can be taken as
induction period. The appearance of first speck of the nucleus was
seen at the bottom of the container. Induction periods were
recorded for different supersaturation ratios 1.25, 1.3, 1.35, 1.4 and
1.45. Repeated trials were performed to ascertain the correctness
of the observed results. Results are presented in Fig. 3. The plot of
100
2
ln
t against 1/(ln S) is shown in Fig. 4. The induction period is
written as
ꢀ
ꢂꢁ
D
kT
G
t
/ exp
,
(1)
80
60
40
where
DG* is the critical free energy of the nucleus, k is the
Boltzmann constant and T is the constant temperature of the
solution.
ꢂ
D
kT
G
ln
t
¼ ln B þ
,
(2)
3
16pg
3DGv
ꢂ
D
G ¼
,
(3)
2
where B is a constant,
bulk energy change per unit volume and is given by
g
is the interfacial tension and
DG
v
is the
D
G
v
¼ ꢀ =V;
where
¼ kT ln S:
D
m
(4)
(5)
300
305
310
315
320
Temperature (K)
Dm
Fig. 1. Solubility of ZTC at pH of 3.4.

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T. Balu et al. / Physica B 404 (2009) 1813–1818
and
1815
1
1
1
:0 mole % L-Arginine
:1 mole % L-Arginine
:3 mole % L-Arginine
5000
4000
3000
2000
1000
0
k ¼ R=N
where N
A
,
(8)
A
is the Avogadro’s number, R is the gas constant and t is
1:5 mole % L-Arginine
the induction period at temperature T and supersaturation ratio S.
According to Eq. (7) the plot of ln
line with a slope given by
2
t
against 1/ (ln S) is a straight
3
2
3
A
16
pg V N
3
m ¼
.
(9)
3
3
R T
Since ln B weakly depends on the temperature, the interfacial
tension of the solid relative to the solution has been calculated
from the slope of the line as
g
3
3
3
R T m
3
g
¼
.
3
(10)
2
16pV N
A
1.25
1.30
1.35
Supersaturation, S
1.40
1.45
2.4. Evaluation of nucleation parameters
The change in Gibbs free energy (
Fig. 3. Plot of S versus t.
DG) between the crystalline
phase and the surrounding mother liquor results in a driving
force, which stimulates crystallization. For a rapid crystallization,
1
:0 mole % L-Arginine
DGo0, the free energy required to form a spherical nucleus is
8.5
8.0
7.5
7.0
6.5
6.0
5.5
1:1 mole % L-Arginine
given by [16]
1
:3 mole % L-Arginine
:5 mole % L-Arginine
1
1
3
r3
r2
DG ¼
p
DG
v
þ 4p
g,
(11)
where
DG is the energy change per unit volume and r is the
V
radius of the nucleus. The first term expresses the formation of the
new surface and the second term represents the difference in
the chemical potential between the crystalline phase (
surrounding mother liquor ( ). At the critical state, the free
energy formation obeys the condition d(
G)/dr ¼ 0. Hence the
radius of the critical nucleus [17,18] has been computed as
m) and the
m
0
D
rꢂ ¼ ꢀ2
g=DG
.
(12)
v
The nucleation rate J which is the number of critical nuclei formed
per unit time and per unit volume has been calculated using the
equation
ꢀꢀ
ꢂꢁ
D
kT
G
J ¼ A exp
,
(13)
8
12
16
20
3
0
where A is the pre-exponential factor (ꢄ10 for solution) [19].
1
/ (ln S)2
The number of molecules in the critical nucleus [20] is expressed
as
2
Fig. 4. Plot of 1/(ln S) versus ln(
t
).
ꢂ
3
iꢂ ¼ 4
p
3
ðr Þ
.
(14)
In the above expression, V is the molar volume of the crystal and is
V
obtained from the expression
Using the interfacial tension value
g
, the nucleation parameters
such as r*, G* and i* were calculated.
D
Molecular weight
V ¼
,
(6)
density ꢃ N
A
2.5. Second harmonic generation efficiency measurements
and the supersaturation ratio S ¼ C/C*, C is the actual concentra-
tion of the solution and C* is the equilibrium concentration of the
solution.
The critical parameters involved in between the growing
crystals and the surrounding mother liquor is the interfacial
tension. In the present study, in order to estimate the critical
The first and the most widely used technique for confirming
the SHG from prospective second order NLO materials is the Kurtz
powder technique [21] to identify the materials with noncen-
trosymmetric crystal structures. The SHG conversion efficiency of
-arginine doped ZTC crystals grown at an optimized pH of 3.4 was
studied using a 1064 nm Nd:YAG laser. The schematic of the
experimental set- up is shown in Fig. 5. The crystalline samples
L
nucleation parameters, the interfacial tension
g has been
calculated using experimentally determined induction period
values. According to the classical theory for homogeneous
formation of spherical nuclei,
were powdered into particle sizes in the range 125–150 mm. To
make relevant comparisons with known SHG materials, KDP was
also ground and sieved into the same particle size range. The
powdered samples were filled air-tight in separate micro-capillary
tubes of uniform bore of about 1.5 mm diameter. A Q-switched,
mode-locked Nd:YAG laser was used to generate about
3
2
3
A
2
1
6
3
pg V N
In
t
¼ In B þ
,
(7)
3
3
R T ðln SÞ

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T. Balu et al. / Physica B 404 (2009) 1813–1818
Iris
IR reflector
was very low compared with the pure system. This is because
-arginine suppresses the activities of heterogeneous nucleation.
Nd:YAG
L
2
Plot of ln
t against 1/(ln S) is shown in Fig. 4. In order to reduce
the effect of heterogeneous nucleation on the nucleation para-
meters, the results were obtained using the slopes determined by
2
Glass plates
a linear fit for ln
t
against 1/(ln S) . It can be noticed that the
induction period increases with increase in additive concentra-
Sample
holder
tion. The interfacial tension has been calculated at constant
IR filter
Power meter
temperature (T ¼ 307 K) for pure ZTC and
L-arginine doped ZTC
2
crystals from the graph drawn between ln
t
and 1/(ln S) . The
Prism
2
measured interfacial tension varies from 1.845 to 2.165 mJ/m .
Using the interfacial tension value, the nucleation parameters
such as the radius of critical nucleus (r*), the free energy for the
Monochromator
Detector (PMT)
formation of the critical nucleus (DG*) and the number of
molecules in the critical nucleus (i*) have been calculated for
the controlled nucleation condition. From the plot made between
the calculated critical radius r* and supersaturation (Fig. 6), we
observe that the value of r* decreases with the increase of
Fig. 5. Schematic arrangement of the experimental setup used for measuring the
SHG efficiency.
5
.7 mJ/pulse at 1064 nm as fundamental radiation. This laser
device can be operated in two different-modes. In the single-shot
mode, the laser emits an 8 ns pulse. While in the multi-shot mode,
the laser produces a continuous train of 8 ns pulse at a repetition
rate of 10 Hz. In the present study, a multi-shot mode of 8 ns laser
pulse with a spot radius of 1 mm was used. This experimental set-
1
1
0
9
8
7
6
5
1:0 mole % L-Arginine
1
:1 mole % L-Arginine
:3 mole % L-Arginine
1
1
1:5 mole % L-Arginine
5
0
up was used as a mirror and beam splitter to generate a beam
5
0
with pulse energies of about 5.7 mJ. The input laser beam was
allowed to pass though an IR reflector and then directed on the
micro-crystalline powdered samples packed in a capillary tube.
The photodiode detector and oscilloscope arrangements were
measured the light emitted in the sample. The SHG radiations of
5
32 nm (green light) emitted were collected by a photomultiplier
tube (PMT-Hamamatsu-model R 2059). The optical signal incident
on the PMT was converted into voltage output at the CRO
(Tektronix-TDS 305213).
3.
Results and discussion
1.25
1.30
1.35
1.40
1.45
Supersaturation, S
The solubility of ZTC at a pH value of 3.4 as a function of
Ã
temperature is shown in Fig. 1. From the graph it is found that the
solubility increases with temperature. Results of metastable
zonewidth measurement and induction period measurement are
presented in Figs. 2 and 3. It is found that the addition of
Fig. 6. Plot of S versus r .
6
0
5
L
-arginine slightly enhances the metastable zonewidth. It is also
1
1
1
:0 mole % L-Arginine
:1 mole % L-Arginine
:3 mole % L-Arginine
found from the graph that the zonewidth decreases with increase
in temperature. The kinetics of nucleation depends on the
thermodynamic driving force, which in turn depends on the
supersaturation, temperature and impurities present in
the system. The induction period, a measure of nucleation rate,
is determined experimentally for ZTC solution with and without
1
:5 mole % L-Arginine
4
the presence of
L-arginine at different supersaturations. The
induction period decreases exponentially with supersaturation
which suggests that the nucleation rate increases exponentially.
Therefore, the number of critical nuclei formed will be increased
which lead to spurious nucleation. Similar results have been
obtained by us for ADP admixtured TGS crystals [22] and other
30
15
researchers for their systems such as deuterated
L-arginine
phosphate (dLAP), potassium fluoride mixed dLAP and sodium
azide mixed dLAP single crystals [23], m-Nitroaniline crystals [24]
and L-histidine tetrafluoro borate (L-HFB) single crystals [25]. The
study of induction period against supersaturation gives an idea of
optimized induction period in order to control the nucleation rate
and to grow good quality of single crystals. It is observed that the
number of tiny crystals formed by the spontaneous nucleation
1.25
1.30
1.35
1.40
1.45
Supersaturation, S
Fig. 7. Plot of S versus J.

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1817
Table 1
Summary of nucleation data for pure and L-arginine doped ZTC systems.
G* ꢃ 10ꢀ
21
J
28
J ꢃ 10 nuclei/s/vol
r* ꢃ 10ꢀ10
System
S ¼ C/C*
D
m
i*
Pure ZTC
1.25
11.03
7.98
6.10
7.40
15.20
23.69
31.83
39.09
11.024
9.376
8.197
7.311
6.621
23.298
14.334
9.578
6.796
5.047
1
1
1
1
.3
.35
.4
4.85
3.98
.45
ZTC+1 mole%
ZTC+3 mole%
ZTC+5 mole%
L
L
L
-arginine
-arginine
-arginine
1.25
8.46
6.12
4.68
3.72
3.05
13.58
23.57
33.14
41.56
48.68
10.097
8.587
7.507
6.696
6.064
17.901
11.011
7.357
5.221
3.878
1
1
1
1
.3
.35
.4
.45
1.25
7.25
5.24
4.01
3.19
2.61
18.07
29.03
38.81
47.09
54.01
9.588
8.1543
7.129
6.358
5.758
15.328
9.429
6.301
4.469
3.319
1
1
1
1
.3
.35
.4
.45
1.25
6.83
4.94
3.77
3.00
2.46
19.95
31.16
41.07
49.26
55.93
9.398
7.993
6.988
6.232
5.644
14.435
8.880
5.934
4.209
3.127
1
1
1
1
.3
.35
.4
.45
Table 2
different supersaturation and different doping concentration. The
growth rate is found to be increase with supersaturation ratio for
SHG output of pure and
L-arginine doped ZTC.
pure and L-arginine doped ZTC. Also, the growth rate of L-arginine
doped ZTC (1–5 mole%) is found to be greater than the pure ZTC
for same supersaturation ratio. Improvement of SHG efficiency
due to L-arginine doping was observed and is comparable with
that of KDP.
Samples
SH signal output (mV)
Pure KDP
Pure ZTC
194
121.2
126.7
138.2
146.8
ZTC+1 mole%
ZTC+3 mole%
ZTC+5 mole%
L
L
L
-arg
-arg
-arg
Acknowledgements
Input power ¼ 5.7 mJ/pulse.
One of the authors (T.B.) is grateful to the University Grant
Commission (UGC), Government of India, for the award of
research fellowship under the Faculty Improvement Program.
This work is partially supported by FIST, Department of Science
and Technology, India.
supersaturation and consequently the nucleation rate (J) increases
with increase of supersaturation (Fig. 7). It is noted from Table 1
that the value of i* and
DG* decreases with the increase of
supersaturation and also varies with different doping concentra-
tions.
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SHG gives relative values of NLO efficiency of the material. From
the table, one can see that the second harmonic signal strength
and the second harmonic generation efficiency increases slightly
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[
[
[
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