One-pot synthesis and characterization of chalcopyrite CuInS2 nanoparticles

Article abstract of DOI:10.1039/c4ta01062k

We synthesized tetragonal chalcopyrite CuInS₂ (CIS) nanoparticles from molecular single source precursors, (Ph₃P) ₂Cu-(μ-SEt)₂In(SEt)₂, by a one-pot reaction in the presence of 3-mercaptopropionic acid at reaction times of 3 hours or less with high yields. In our approach, NaCl as a by-product was used as a heat transfer agent via a conventional convective heating method. We tuned the sizes of nanoparticles through manipulation of the reaction temperature, reaction time, precursor, and thiol concentration. The sizes of nanoparticles from 1.8 nm to 5.2 nm were obtained as reaction temperatures were increased from 150 °C to 190 °C. The method developed in this study is scalable to achieve production of ultra-large quantities of tetragonal chalcopyrite CIS nanoparticles. The resulting nanoparticles were analyzed by UV-vis spectroscopy, X-ray diffraction, EDAX, and HRTEM.

Full text of DOI:10.1039/c4ta01062k

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One-pot synthesis and characterization of
chalcopyrite CuInS2 nanoparticles
Cite this: DOI: 10.1039/c4ta01062k
a
a
b
a
c
a
*
Chivin Sun, Zehra Cevher, Jin Zhang, Bo Gao, Kai Shum and Yuhang Ren
We synthesized tetragonal chalcopyrite CuInS2 (CIS) nanoparticles from molecular single source
precursors, (Ph3P)2Cu-(m-SEt)2In(SEt)2, by a one-pot reaction in the presence of 3-mercaptopropionic
acid at reaction times of 3 hours or less with high yields. In our approach, NaCl as a by-product was
used as a heat transfer agent via a conventional convective heating method. We tuned the sizes of
nanoparticles through manipulation of the reaction temperature, reaction time, precursor, and thiol
concentration. The sizes of nanoparticles from 1.8 nm to 5.2 nm were obtained as reaction temperatures
were increased from 150 ꢀC to 190 ꢀC. The method developed in this study is scalable to achieve
production of ultra-large quantities of tetragonal chalcopyrite CIS nanoparticles. The resulting
nanoparticles were analyzed by UV-vis spectroscopy, X-ray diffraction, EDAX, and HRTEM.
Received 2nd March 2014
Accepted 4th May 2014
DOI: 10.1039/c4ta01062k
www.rsc.org/MaterialsA
decomposition of SSPs using thermolysis,14–19 photolysis,20 and
microwave irradiation.21–24 Each SSP molecule contains the
Introduction
requisite elements to form CuInS2 semiconductor materials,
offering a unique level of control over product stoichiometry
during the decomposition process. However, many of these
procedures require a combination of long reaction times of 10 to
24 hours (excluding the reaction time for preparation of SSPs)
and high reaction temperatures oen exceeding 200 ꢀC. Pak et al.
recently developed an effective approach that affords (Ph3P)2Cu-
(m-SEt)2In(SEt)2 in 94% yield on a 500 g scale.25 Pak et al. also
synthesized high yield chalcopyrite CuInS2 nanoparticles by
decomposing SSPs via microwave irradiation in the presence of
1,2-ethanedithiol at reaction temperatures as low as 100 ꢀC with
reaction times as short as 30 minutes.21–24 However, this proce-
dure requires ltration or separation of the by-product (NaCl),
evaporation of solvents, and re-dissolving of SSPs for further
reactions. The SSPs are not only exposed to air but also time
consuming by these multi-steps.
The growth of nanomaterials is dependent on the thermody-
namic and kinetic barriers in the reaction as dened by the reac-
tion trajectory and inuenced by vacancies, defects, and surface
reconstruction events. For the most part, the synthetic methods
utilize conventional convective heating due to the need for high-
temperature initiated nucleation followed by controlled precursor
addition to the reaction. Conventional thermal techniques rely on
conduction of blackbody radiation to drive the reaction. The
reaction vessel acts as an intermediary for energy transfer from the
heating mantle to the solvent and nally to the reactant molecules.
This can cause sharp thermal gradients throughout the bulk
solution and inefficient, non-uniform reaction conditions. This is
a common problem encountered in chemical scale-up and is made
more problematic in nanomaterials where uniform nucleation and
growth rates are critical to material quality.
Quantum dot (QD) based solar cells have attracted much
attention due to their potential to replace thin lm devices.1–3
One of the major advantages of employing QDs is simply
changing the particle size to absorb specic wavelengths
ranging from visible to infrared wavelengths.4 By incorporating
varying sizes of nanoparticles in multiple layers, one can ach-
ieve increased solar energy absorption in photovoltaic
devices.5,6 In order to facilitate QD based multilayer devices,
synthetic strategies that can deliver QDs in high yields with
precise size control are essential. Previous studies involved
costly molecular beam epitaxy processes, but alternative inex-
pensive fabrication methods have recently been developed. One
of the strategies to prepare QDs is to prepare nanoparticles from
molecular single source precursors (SSPs).
Various I–III–VI2 (i.e. CuInxGa1ꢁxSySe2ꢁy, where 0 # x and y #
1) semiconductor materials have been identied as promising
photovoltaic materials, since devices containing CuInxGa1ꢁxSe2
thin lms, also known as CIGS, have already achieved superior
efficiency.7–12 Other applications such as light-emitting devices
and bioimaging techniques by tuning of luminescence properties
of I–III–VI nanocrystals have been discussed.13 For CuInS2 QDs,
the Wannier–Mott bulk exciton radius is approximately 8 nm
with a bandgap of 1.45 eV and QDs with radii smaller than 8 nm
exhibit bandgaps greater than 1.45 eV.4 There have been several
reports on the formation of CuInS2 nanoparticles through the
aPhysics and Astronomy Department, The City University of New York, Hunter College,
695 Park Avenue, New York, NY 10065, USA. E-mail: yre@hunter.cuny.edu
bSun Harmonics Ltd., Hangzhou, Zhejiang, China 311121
cPhysics Department, The City University of New York, Brooklyn College, 2900 Bedford
Avenue, Brooklyn, NY 11210, USA
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Herein, we report the one-pot synthesis of chalcopyrite CuInS2 overnight under N2. Precipitated sodium ethanethiolate was
nanoparticles by decomposition of SSPs in the presence of 3- ltered and washed twice with fresh diethyl ether to remove any
mecaptopropanic acid via a conventional convective heating excess of ethanethiol. The product was then dried in vacuo to
method. The advantage of this reaction can be seen in the provide white powder and stored under a nitrogen atmosphere.
simplicity of the straightforward reaction and low-cost prepara-
tion of CIS nanoparticles. As we looked to overcome the slightly
air sensitive SSPs and time consumed for separation of the by-
product (NaCl), evaporation of solvents, and re-dissolving of SSPs
for further reactions, we discovered that reaction of SSPs con-
taining NaCl, as the by-product, can provide desired products
without sacricing the yield, processing time, and exposure of
SSPs to air. We also discovered that NaCl acts as an intermediary
for energy transfer from the heating mantle to the solvent and
nally to the reactant molecules.
Synthesis of molecular single source precursors (SSPs)
A single source precursor was synthesized according to improved
procedures as shown in Fig. 1. Benzyl acetate was used as a
solvent without further purication or drying. The synthesis of
CIS SSPs includes three simple steps: (1) 2 molar equivalents
(10.00 g, 38.13 mmol) of Ph3P (triphenylphosphine) and 1 molar
equivalent (1.885 g, 19.06 mmol) of anhydrous Cu(I)Cl (copper(I)
chloride) were mixed with 100 mL of benzyl acetate in a round-
ꢀ
bottom ask (either one or two neck) and stirred at 80 C for 4
hours under a nitrogen atmosphere. (We rst dissolved Ph3P in
benzyl acetate and then we added Cu(I)Cl.) White suspensions
including the intermediate product (Ph3P)2Cu–Cl were formed in
the rst step. (2) 1 molar equivalent (4.22 g, 19.06 mmol) of
Experimental section
Materials
ꢀ
anhydrous InCl3 (indium(III) chloride) was added at 80 C and
Triphenylphosphine (Ph3P, 99+%), indium(III) chloride (InCl3,
anhydrous 99.999%, metals basis), copper(I) chloride (CuCl,
anhydrous, 99.999%, metals basis), and benzyl acetate
(C6H5CH2CO2CH3, 99%) were purchased from Alfa Aesar. Etha-
nethiol (CH3CH2SH, 99+%) and 3-mercaptopropionic acid
(HSCH2CH2CO2H, 99+%) were purchased from Acros Organics.
Ethyl ether ((CH3CH2)2O, anhydrous, BHT stabilized/certied
ACS) was purchased from Fisher Scientic. All other reagents were
obtained from commercial sources and used without further
purication. Unless stated otherwise, all reactions were per-
formed in a fume hood, under inert conditions (in a dry nitrogen
kept stirring for another 1 hour under N2. The solution turned
essentially clear with a light yellow color. We obtained the Cu–In
coordination compound. (3) NaSEt (7.000 g, 83.21 mmol, 4.366
equiv.) was added to this solution and stirred for an additional
night at 80 ꢀC. This series of steps ends with a crude suspension
of the CIS SSPs in sodium chloride. At this point, there was no
ltration or separation of sodium chloride as the by-product and
there was no evaporation of the solvent.
Synthesis of CuInS2 (CIS) nanocrystals
atmosphere) obtained by implementation of standard Schlenk To the crude suspension of CSI SSPs prepared above, 3-mer-
techniques. Sodium ethanethiolate was prepared by reuxing captopropionic acid (7.5 mL) was added as neat. The reaction
ethanethiol (215.2 mL) over sodium metal (66.8 g, cut into small mixture was heated achieving reaction temperatures from
pieces, size ꢂ0.5–1 cm3) in a suspension of diethyl ether (800 mL) 150 C to 190 C as desired for about 3 hours under N2. Upon
ꢀ
ꢀ
Fig. 1 A general illustration for steps involved in the formation of CuInS2 nanoparticles. The inset shows CuInS2 nanoparticles prepared from
SSPs in the presence of 3-mercaptopropionic acid in benzyl acetate at 150, 170, and 190 ꢀC respectively from left to right.
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completion, the reaction was cooled to room temperature to experiments, a high boiling point solvent (i.e. benzyl acetate)
yield precipitation of CuInS2 nanoparticles. The resulting was applied without further purication or drying. For SSP
nanoparticles were isolated from the benzyl acetate solution by synthesis, there are three simple step reactions as shown in
centrifugation, collected, and washed one time with CH3OH, Fig. 1. (1) Ph3P (2 equiv.), benzyl acetate and CuCl (1 equiv.)
ꢀ
two times with 75 : 25 (v/v) mixtures of CH3OH and H2O, and were heated for 4 hours at 80 C to form a white suspension
two more time with CH3OH. The product was then dried including the intermediate product (Ph3P)2Cu–Cl. (We rst
in vacuo to provide yellow to dark-orange powder. This method dissolved Ph3P in benzyl acetate and then we added Cu(I)Cl.)
has been successfully adapted to prepare up to 5 g of nano- (2) Anhydrous InCl3 (1 equiv.) was added and stirred for about
particles in a single 250 mL reaction vessel.
60 min at 80 ꢀC to form the Cu–In coordination compound. (3)
NaSEt (4.366 equiv.) was added and stirred for an additional
night at 80 ꢀC to obtain CIS SSPs. A similar mechanism of
chemical reactions was discussed in our previous work.25 The
resulting starting materials, single source precursors (SSPs),
are slightly air-sensitive and are stored in an argon lled inert
atmosphere drybox (Vac Atmospheres) to prevent decompo-
sition. Accordingly, we eliminated steps involving ltering,
evaporating, and drying of the SSP. This enables the forma-
tion of SSP complexes without exposure to air. We have effi-
ciently synthesized CuInS2 (CIS) nanocrystals by one-pot
reaction, rst mixing and forming a clear solution mixture of
the SSP at 80 ꢀC and then heating up the mixture to a
temperature of 150–190 ꢀC in 3-mercatopropanic acid for
product formation (CIS nanocrystals) via a conventional
heating technique.
We have determined that the optimal temperature for
formation of CIS nanocrystals is around 150 ꢀC, which is lower
than the widely reported reaction temperatures via a conven-
tional heating technique. Hepp et al.14 synthesized CIS nano-
crystals at temperatures above 200 ꢀC. At 200 ꢀC, the CIS was not
formed, and only an intermediate precursor comprising a
mixture of CuxSy and InxSy as hypothesized was formed. These
results suggest that in order to avoid formation of the inter-
mediate precursor the optimal temperature for the synthesis of
CIS nanocrystals should be above 200 ꢀC. Our strategy for
avoiding the formation of the intermediate precursor was to
Characterization
Transmission electron microscopy (TEM) images were captured
with a JEOL JEM 2100 transmission electron microscope using a
LaB6-Cathode and an acceleration voltage of 200 kV. Energy
dispersive X-ray (EDX) analysis was also conducted on a JEOL
JEM 2100 transmission electron microscope equipped with a
sapphire Si(Li) detecting unit from EDAX, using an acceleration
voltage of 200 kV.
Powder X-ray diffraction (XRD) patterns were acquired with a
PANalytical X'Pert Pro X-ray powder diffraction system using
incident beam hardware: a ceramic tube (Cu anode), soller slit
0.04 radians, divergence slit 1/2, mask 10 mm and diffracted
beam hardware: anti-scattering slit 1/2, Ni-lter, receiving slit 1/4,
soller slit 0.04 radians, and a proportional detector (Xe-lled).
Scans were collected for 25 min employing a 0.06ꢀ step width at a
rate of 2 s per step resulting in a 2q scan range from 20–65ꢀ.
Absorption spectra of nanoparticles were obtained from the
UV-vis data recorded on a Perkin Elmer Lambda 950 spectro-
photometer. A plane glass was measured as the background
transmission at room temperature.
Results and discussion
We improved our previous procedures (as described in ref. 25)
to synthesize the single source precursors. In our
Fig. 2 Electron diffraction and HRTEM structure analysis of CuInS2 nanoparticles prepared from SSPs in the presence of 3-mercaptopropionic
acid in benzyl acetate at 170 ꢀC.
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rst form the SSP complexes without ltration or separation of
sodium chloride. We hypothesized that sodium chloride could
be the heat agent, which could transfer heat from the reaction
ask to the SSP more effectively.
Hepp et al. observed formation of CIS nanoparticles in
5 hours using traditional heating methods. The total reaction
time and work up for synthesis of CIS nanocrystals using our
procedure is much shorter than that reported for converting
the SSP into CuInS2 nanocrystals.14–24 Sodium chloride is
soluble in both water (359 g Lꢁ1) and methanol (14.9 g Lꢁ1
)
26
and the solubility of sodium chloride in water, methanol,
ethanol, and their mixed solvents had been studied by Pinho
and Macedo.27 First nanoparticles were isolated from the
benzyl acetate solution by using 100% methanol to remove
most of the solvent and other impurities. Then a mixture of
CH3OH and H2O (75 : 25 by volume) was used to remove most
of the sodium chloride. To ensure that all of sodium chloride
had been removed, the mixture of CH3OH and H2O (75 : 25 by
volume) had been used to wash nanoparticles. Finally,
nanoparticles were washed two more times with 100%
methanol.
Fig. 3 XRD data of CuInS2 nanoparticles prepared at 170 ꢀC.
Fig. 1 displays a standard route for forming CuInS2 nano-
particles. The process includes adding 2 molar equivalents of
Ph3P (triphenylphosphine) to 100 mL of benzyl acetate to
form a solution; adding 1 molar equivalent of anhydrous CuCl
and anhydrous InCl3 to the stirring solution; adding 4 molar
equivalents of precursor forming material NaSEt; and adding
3-mercaptopropionic acid to form a reaction mixture. The
reaction mixture was then exposed to heat and maintained at
temperatures in the range of 150 ꢀC to 190 ꢀC for about 2
hours to form CuInS2 particles in the chalcopyrite phase (i.e.,
CuInS2 chalcopyrite particles). As previously indicated, the
reaction temperature, reaction time and concentration of
single source precursors and thiol concentration were
controlled to form various sizes of the CuInS2 chalcopyrite
particles. The inset shows the resulting color change of the
nanoparticles that settled at the bottom of sample vessels.
Progressively darker colors represent higher reaction
temperatures representing respective sizes of CuInS2 nano-
particles from small to large.
Fig. 4 UV-vis absorption spectra of typical CuInS2 nanoparticles
prepared from 150 to 190 ꢀC, respectively.
pattern 85-1575 (JCPDS-03-065-2732). This is consistent with
our electron diffraction analysis. Furthermore, a careful
evaluation of the gradual sharpening of the peaks in the XRD
spectra is indicative of the increasing particle sizes with
increasing reaction temperatures. The absorption behaviors
of the nanoparticles showed the expected blue-shi with
decreasing sizes which represent small to large bandgaps
(Fig. 4).
Fig. 2 shows the electron diffraction and HRTEM structure
images of CuInS2 nanoparticles prepared from SSPs in the
presence of 3-mercaptopropionic acid in benzyl acetate at
ꢀ
170 C. From our best observation, the nanoparticles appear
We also further observed that the size was controlled based
on the concentration of 3-mercaptopropionic acid. For example,
in the absence of thiol, chalcopyrite nanoparticles were not
produced at below 200 ꢀC.15 It was determined that at least
7.5 mL of 3-mercaptopropionic acid was required to produce
chalcopyrite nanoparticles. When 25 mL of 3-mercaptopro-
pionic acid was used as the reaction solvent at 170 ꢀC, the
largest chalcopyrite nanoparticles were collected. The absorp-
tion behaviors of the nanoparticles showed the expected
blue-shi with decreasing sizes on decreasing the concentra-
tion of 3-mercaptopropionic acid which represent small to large
band gaps as shown in Fig. 5.
to have diameters of about 4 nm with a narrow size distri-
bution. According to the EDAX data, all particles formed in
our studies consist of Cu, In, and S. The nanoparticle sizes
from 1.8 nm to 5.2 nm were conrmed by evaluation of
HRTEM images as reaction temperatures were varied from
150 ꢀC to 190 ꢀC. The nanoparticle sizes from yellow to dark-
orange were conrmed by evaluation of the XRD data (Fig. 3).
We determined volume-weighted crystal diameters (Scherrer
equation with a shape factor of 0.9).28 In particular, the XRD
patterns show that the CuInS2 nanoparticles are crystalline
with the chalcopyrite phase with major peaks at 2q ¼ 28, 46,
and 55ꢀ attributing to (112), (204) and (116) phases. The
peaks are consistent with the tetragonal CuInS2 reference
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Fig. 5 UV-vis absorption spectra of typical CuInS2 nanoparticles
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Conclusion
Potentially, these nanocrystals can be incorporated into next-
generation quantum-dot-based solar cells. We have shown by
exploiting the conventional convective heating method, one-pot
decomposition of single source precursors to afford CuInS2
nanoparticles in the presence of 3-mercaptopropionic acid and
NaCl as the by product. We can prepare CuInS2 nanoparticles
with diameters ranging from 1.8–5.2 nm with a very high-yield
of chalcopyrite CuInS2 nanoparticles. Short reaction times of 3
hours or less are required for the preparation of these nano-
particles. The reaction temperature and 3-mercaptopropionic
acid concentration are all critical for ne control of the nano-
particle size. Gram quantities of CuInS2 nanoparticles can be
obtained by the method developed in this study, demonstrating
its potential in providing ultra-large quantities of tunable I–III–
VI2 compound nanoparticles with a small amount of solvent in
applications for low-cost non-vacuum-based CIS solar cells.
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Conflict of interest
21 J. S. Gardner, E. Shurdha, C. Wang, L. D. Lau,
R. G. Rodriguez and J. J. Pak, J. Nanopart.Res., 2008, 10, 633.
22 C. Sun, R. D. Westover, G. Long, C. Bajracharya, J. D. Harris,
A. Punnoose, R. G. Rodriguez and J. J. Pak, Int. J. Chem. Eng.,
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The authors declare no competing nancial interest.
Acknowledgements
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26 Wikipedia, the free encyclopedia, http://en.wikipedia.org/
wiki/Sodium_chloride.
This work was partially sponsored by Sun Harmonics Ltd. and
by NYSTAR through the Photonics Center for Applied Tech-
nology at the City University of New York. The authors also
thank Dr Alexey Bykov and Prof. Hiroshi Matsui for instrument
assistance.
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