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M A TE RI A L S C HA RACT ER I ZA TI O N 65 ( 20 1 2 ) 1 0 9–1 1 4
Available online at www.sciencedirect.com
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Morphology and non-isothermal crystallization kinetics of
CuInS2 nanocrystals synthesized by solvo-thermal method
M.A. Majeed Khana,⁎, Sushil Kumarb , M.S. Alsalhia, c , Maqusood Ahameda ,
Mansour Alhoshana, d , Salman A. Alrokayana , Tansir Ahamade
a
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia
Department of Physics, Chaudhary Devi Lal University, Sirsa 125055, India
c
Department of Physics and Astronomy, King Saud University, Riyadh 11451, Saudi Arabia
d
Chemical Engineering Department, King Saud University, Riyadh 11451, Saudi Arabia
e
Department of Chemistry, King Saud University, Riyadh 11451, Saudi Arabia
b
AR TIC LE D ATA
ABSTR ACT
Article history:
Nanocrystals of copper indium disulphide (CuInS2) were synthesized by a solvo-thermal
Received 11 September 2011
method. The structure, morphology and non-isothermal crystallization kinetic behavior
Received in revised form
of samples were investigated using X-ray diffraction, field emission scanning electron mi-
20 December 2011
croscopy, field emission transmission electron microscopy, thermogravimetric analysis
Accepted 14 January 2012
and differential thermal analysis techniques. Non-isothermal measurements at different
heating rates were carried out and the crystallization kinetics of samples were analyzed
Keywords:
using the most reliable non-isothermal kinetic methods. The kinetic parameters such as
CuInS2
glass transition temperature, thermal stability, activation energy, Avrami exponent etc.
Nanocrystals
were evaluated.
Solvo-thermal
© 2012 Elsevier Inc. All rights reserved.
Crystallization kinetics
Morphology
1.
Introduction
The synthesis and properties of nanostructured I–III–VI2 chalcopyrite materials including copper indium disulphide
(CuInS2) have been actively studied because the shape and
size of these nanoscale semiconductors may exert a significant influence on their optoelectronic functions and device
performance, and sometimes induce unique physical and
chemical properties different from their bulk counterparts.
Some novel features have been found in the nanoscale semiconductor systems, such as an obvious enhancement of
solar energy conversion efficiencies of photovoltaic devices
covered with nanocrystalline-based semiconductor absorber
layers [1,2]. The chalcopyrite CuInS2 is a ternary semiconductor with a bandgap of 1.53 eV [3]. It is a suitable, environmen⁎ Corresponding author. Tel.: +966 1 467 61 88; fax: +966 1 467 06 62.
E-mail address: majeed_phys@yahoo.co.in (M.A. Majeed Khan).
1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.matchar.2012.01.009
tally safe and cost effective material for terrestrial solar cell
applications [4]. Elemental solvo-thermal reaction [5] has
been developed to synthesize this chalcopyrite material.
Different solvents such as benzyl alcohol [6] and ethylene
glycol [7] can be used as the reaction medium in the solvothermal routes. Recently, desirable morphologies of CuInS2
such as shape-controlled nanocrystals [8–10], nanorods [11],
and pyramidal crystals [12], have been reported.
Thermal analysis methods such as differential scanning
calorimetry can be used to investigate the reaction kinetics
of a broad range of materials, including metals, polymers
and glass-forming solids. The crystallization kinetic process
can proceed under either isothermal or non-isothermal conditions. From the experimental view, the non-isothermal crystallization is more readily accessible than the isothermal
110
M A TE RI A L S CH A RACT ER IZ A TI O N 65 (2 0 1 2 ) 1 0 9–1 1 4
crystallization. Several researchers studied the crystallization kinetics of chalcogenide materials [13,14]. In this work,
we report on structure, morphology and crystallization kinetics of CuInS2 nanocrystals synthesized by solvo-thermal
method using inorganic and organic compounds as starting
materials. The characterization techniques used were X-ray
diffraction, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), thermogravimetric analysis (TGA)
and differential thermal analysis (DTA).
2.
Experimental
2.1.
Synthesis of CuInS2 Nanocrystals
CuCl2 (0.1 mmol) and InCl3 (0.1 mmol) were dissolved in 20 ml
deionized water and stirred at room temperature for 30 min.
The reaction container was heated to 240 °C under nitrogen
flow. When the solution color changed from turbid green to
slightly yellow, this solution was mixed homogeneously
with thiourea–formaldehyde resin. The resulting polymer
metal complex has been used as a single-source precursor to
synthesize CuInS2. The aqueous mixture of prepared polymer
metal complex was transferred into a 100 ml Teflon-lined autoclave and treated at 250 °C for 2 h. After cooling to room
temperature, the resulting precipitate was isolated by centrifugation, followed by washing with distilled water and pure
ethanol several times to remove the possible by products,
and then dried in vacuum at 80 °C for 10 h to obtain the final
product. The purified CuInS2 nanocrystals were then redissolved in hexane. All the chemicals used in the experiment
were of analytical grade and were used without further
purification.
2.2.
Characterization
The crystallographic structure of the products was determined by powder (XRD) pattern using an X-ray diffractometer
(PANalytical X'Pert) with CuKα radiation (λ = 1.5406 Å) as the
X-ray source. The morphology of the CuInS2 nanocrystals
was investigated by FESEM and FETEM. The images were
obtained with FESEM (JEOL, JSM-7600F) at an accelerating voltage of 15 kV. The fine powder of CuInS2 nanocrystals were dispersed in ethanol on a carbon-coated copper grid and the
images were obtained with ultra high resolution FETEM
(JEOL, JEM-2100F) at an accelerating voltage of 200 kV. The reaction type and weight loss were confirmed using TGA/DTA
thermal system (Shimadzu, DTG-60).
3.
Results and Discussion
3.1.
Microscopy Studies
The phase and crystallinity of the prepared samples were
investigated by X-ray diffraction pattern as shown in Fig. 1.
A most intense peak at 2θ = 27.62° is obtained along (112)
orientation. The diffraction peaks can be indexed to the
chalcopyrite tetragonal phase of CuInS2 and exhibit the
polycrystalline nature of the samples. The location of
Fig. 1 – X-ray powder diffraction pattern of CuInS2
nanocrystals and inset shows the HRTEM image for the same.
peaks is in good agreement with JCPDS reference patterns
for the corresponding bulk phases [15]. The calculated lattice
parameters a = 5.523 Ǻ and c = 11.125 Ǻ corresponding to
prominent peaks (112) and (204) are in good agreement with
the values reported in literature [JCPDS card No. 27–159].
Fig. 2 shows the scanning electron microscopy image of
CuInS2 nanocrystals. This picture reveals that most of the
nanocrystals have smooth surface with size about 20 nm.
Fig. 3(a) shows the transmission electron microscopy image
of CuInS2 nanocrystals. The average crystal size is estimated
by considering the large number of crystals and is found to be
about 22 nm. The selected area electron diffraction (SAED)
pattern (Fig. 3b) has concentric rings with sharp spots,
which is a good indication of the polycrystalline nature with
partial orientation of CuInS2 nanoparticles in the sample. The
crystallinity of the synthesized nanoparticles was also supported from the observed lattice fringes of around 0.32 nm in
HRTEM image (Inset of Fig. 1).
Fig. 2 – SEM image of CuInS2 nanocrystals.
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M A TE RI A L S C HA RACT ER I ZA TI O N 65 ( 20 1 2 ) 1 0 9–1 1 4
thermogravimetric analysis and differential thermal analysis
at a heating rate of 20 °C/min. A careful look at the TGA curve
reveals that the portion between room temperature (27 °C)
and 452 °C shows a stable composition for CuInS2 as there
is no weight loss in this temperature region. After that a
considerable and sharp weight loss occurs in between 452
and 563 °C. Again, there is no weight loss between 563 and
800 °C. The DTA curve has a large exothermic peak at
~ 533 °C which may be attributed to the fact that the volatile
sulfur generated by the dissociation of CuInS2 at this temperature (~ 533 °C) reacts with oxygen to form sulfur dioxide. The
loss of this sulfur dioxide is confirmed by weight loss observed in temperature region 452 to 563 °C in the TGA curve.
A small endothermic peak is observed at about 394 °C in the
DTA curve which may be due to the loss of weakly coordinated hydrazine species as well as the loss of excess chalcogen
species from the system. Glass characterizing temperatures
such as glass transition temperature Tg, crystallization temperature Tc, and peak temperature (Tp) were determined for
each heating rate and are given in Table 1(a).
In non-isothermal study, the stability of material can be
expressed [16] by the temperature difference ΔT = Tc − Tg. A
larger value of ΔT leads to higher thermal stability of material.
For CuInS2 nanocrystals, no significant effect of heating rate
on ΔT was observed. The crystallization enthalpy (ΔHC) is evaluated using the formula
ΔH c ¼
Fig. 3 – (a) TEM image of CuInS2 nanocrystals (b) SAED pattern
of CuInS2 nanocrystals.
3.2.
Thermal Studies
The thermal properties of CuInS2 nanocrystals were determined by thermogravimetric analysis and differential thermal analysis. Samples were heated in aluminum pans from
room temperature to 800 °C at different heating rates (β) 5,
10, 15 and 20 °C/min in open air. Fig. 4 shows the plots of
KA
M
where K is the constant of instrument used, A is the area of
crystallization peak and M is the mass of sample. The enthalpy released is related to the metastability of glass and the
least stable glass with minimum (Tc − Tg) value is supposed
to have maximum ΔHC. The enthalpy of CuInS2 nanocrystals
at different heating rates is given in Table 1(a).
The volume fraction crystallized (α) at any temperature T is
given as α = ST / S, where S is the total area of exotherm between the temperature Ti (where the crystallization is just
started) and the temperature Tf (where the crystallization is
completed), and ST is the area between the initial temperature
and the generic temperature, T, ranging between Ti and Tf
(Fig. 5). The plots of α versus T for different heating rates are
Table 1 – Thermal parameters of CuInS2 nanocrystals.
(a)
Heating rate
(°C/min)
5
10
15
20
Tg
(°C)
Tc
(°C)
Tp
(°C)
Tc − Tg
(°C)
ΔHc
(J/g)
337.8
409.92
383.92
391.13
418.42
451.93
427.04
462.97
467.64
480.79
464.45
507.31
80.62
42.01
43.12
71.84
37.28
258.29
269.93
438.99
(b)
ΔEC
(kJ/mol)
Sample
Fig. 4 – TGA/DTA plot as function of temperature for CuInS2
nanocrystals.
CuInS2
n
FO
KAS
Vazquez
Ozawa
52.13 ± 5.3
59.61 ± 2.8
1.60 ± 0.18
1.66 ± 0.26
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M A TE RI A L S CH A RACT ER IZ A TI O N 65 (2 0 1 2 ) 1 0 9–1 1 4
Fig. 5 – Variation of heat flow with temperature for CuInS2
nanocrystals.
shown in Fig. 6(a). The stage ‘a’ represents nucleation which
occurs at various points in the bulk of the sample and bulk
crystallization becomes dominant. The stage ‘b’ shows the
growth of nuclei with increased rate of reaction as the surface
area of nucleation increases. The decay stage ‘c’ shows the
decrease in surface area as a result of nuclei coalescing. In
the present study, the maximum temperature of crystallization is seen to increase with increase in heating rate. The
ratio between the ordinates of differential thermal analysis
curve and the total area of exothermic peak gives the corresponding crystallization rate, which makes it possible to
build the curves of exothermic peaks represented in Fig. 6(b).
The non-isothermal decomposition process of CuInS2 was
analyzed by Kissinger–Akahira–Sunose and Flyan Ozawa isoconversional methods. The crystallization mechanism, under
non-isothermal condition, is generally understood in terms
of the activation energy of crystallization (Ec) and the degree
of conversion (n). These parameters have widely been calculated using several theoretical models reported in literature
[14,17,18]. From the heating rate dependence of the peak temperature of crystallization Tp, Kissinger [19] has developed a
model, which is perhaps the most commonly used to calculate Ec, and is given as
ln
β
T p2
¼
ΔEC
þ Const
RT p
where R is the universal gas constant (= 8.314 J K − 1 mol− 1).
The value of ΔEc for CuInS2 nanocrystals, as evaluated from
the slope of the plot of ln (β / T2p) against 1000 / Tp as shown
in Fig. 7(a), is listed in Table 1(b).
The activation energy Ec of crystallization can also be calculated from the variation of the onset crystallization temperature Tc with the heating rate using Ozawa [20] equation
lnβ ¼ −
Ec
þ Constant
RT c
The value of ΔEc, as evaluated from the slope of the plot of ln
β against 1000 / Tc as shown in Fig. 7(b), is listed in Table 1(b).
The activation energies of the sample calculated by means
of different theoretical models are summarized in Table 1(b).
The difference in the activation energy as calculated with
Fig. 6 – (a): Plot of α vs. T, and (b) heat flow vs. temperature
for CuInS2 nanocrystals.
the different models, even for single and same sample, may
be attributed to different approximations used in the models.
The Kissinger equation was basically developed for studying
the variation of peak crystallization temperature with heating
rate. According to Kissinger's method, the transformation
under non-isothermal condition is represented by first-order
(i.e. n = 1) reaction. Similar is the case with Ozawa model.
Moreover, the concept of nucleation and growth has not
been included in both Kissinger and Ozawa equations.
For CuInS2 nanocrystals under study we applied two models,
namely Ozawa [21] and Vazquez model [22], to calculate the
order of reactions occurring during linear heating. The Ozawa
method is the most commonly used method in the literature
while Vazquez model is useful to monitor the heating rate
dependence of n which can be observed as a deviation from
linear fitting in Ozawa model. Ozawa [21] extended the Avrami
equation to the non-isothermal case as follows:
ln½− lnð1−α T Þ ¼ lnK ðT Þ nlnβ
M A TE RI A L S C HA RACT ER I ZA TI O N 65 ( 20 1 2 ) 1 0 9–1 1 4
a
where K (T) is the function of heating rate, β is the heating rate
and n is the Ozawa exponent which characterizes the dimensionality of growth during transformation. According to above
equation, a plot of ln (−ln (1− α)) vs. lnβ yield a straight line
with slope equal to n (order parameter). Fig. 8 shows the variation of ln (−ln (1− α)) vs. lnβ for CuInS2 nanocrystals. The value
of n for CuInS2 nanocrystals is listed in Table 1(b).
According to Vazquez [22] it is possible to calculate the kinetic parameters using the following relations.
10.8
ln (Tp2/ β)
10.4
10.0
dα
RT p 2 ð0:37 ϕEÞ−1
n¼
dt
2 p
Tp
E
¼
ln
− lnq
RT p
ϕ
9.6
9.2
1.96
2.00
2.04
2.08
2.12
2.16
1000/Tp (K-1)
b
3.5
ln β
3.0
2.5
2.0
1.5
2.15
2.20
2.25
2.30
2.35
2.40
1000/Tc (K-1)
Fig. 7 – (a): Plot of ln (T2p / β) vs. 1000 / Tp, and (b) lnβ vs. 1000 /
Tc for CuInS2 nanocrystals.
0.0
ln[-ln(1-α)]
From the slope of this relation, one can deduce the order of
crystallization mechanism (or Avrami exponent), n. Allowing
for experimental error, the value of n is close to 2 and is
given in Table 1(b). In case of non-isothermal experiments, it
has been found that n strongly depends on the heating rate.
Only if extreme cases of nucleation occur, pure continuous
nucleation or pure site saturation, the value of n is independent of the heating rate and is constant (n = 3 or 4) [23]. The
value of n depends on the mechanism of transformation reaction. If the rate of nucleation is a function of time, so is n; n is
higher for a constant nucleation rate than when the nucleation rate increases with time and lies between those for constant and zero nucleation rate when the nucleation rate
decreases with time. When surface crystallization dominates
n ~ 1, and when bulk crystallization dominates n ≥ 3. When
both surface and bulk crystallization occur, n has a value between 1 and 3. The results represented in Table 1(b) shows
that the calculated values of the exponent n using Ozawa
and Vazquez models are in good agreement. Moreover, both
models show the same trend for CuInS2 nanocrystals.
4.
Conclusions
In summary, CuInS2 nanocrystals have been synthesized by
employing solvo-thermal method. X-ray diffraction analysis
exhibits the chalcopyrite tetragonal phase of CuInS2 nanocrystals. Morphological analysis through field emission scanning electron microscopy and field emission transmission
electron microscopy show that the particle size of the CuInS2
nanocrystals is in the range of 20–25 nm. Crystallization kinetic study through thermogravimetric analysis and differential
thermal analysis showed that the activation energy is ranging
from 52 to 59 kJ/mol. The Avrami exponent has been evaluated on the basis of Ozawa and Vazquez models and is close to 2
which corresponds to one-dimensional growth. The development of CuInS2 nanocrystals and the crystallization kinetic
study might find some usefulness in the fabrication of solar
cell devices.
0.2
-0.2
-0.4
-0.6
-0.8
1.2
113
1.6
2.0
2.4
2.8
3.2
Acknowledgments
ln β
Fig. 8 – Plot of ln [−ln (1 − α)] vs. lnβ for CuInS2 nanocrystals.
The authors gratefully acknowledged National Plan for Science and Technology (NPST), King Abdul Aziz City for Science
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M A TE RI A L S CH A RACT ER IZ A TI O N 65 (2 0 1 2 ) 1 0 9–1 1 4
and Technology (KACST), Riyadh, Saudi Arabia for financial
support under Grant no. 10-NAN1001-02.
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