Synthesis of Nonstoichiometric Cu2ZnSnS4 from ZnS by Cation Exchange

Article abstract of DOI:10.1002/ejic.201700137

A cation exchange reaction of wide-band-gap crystalline ZnS with CuCl₂ and SnCl₄ in triethylene glycol was developed to synthesize nonstoichiometric Cu₂ZnSnS₄ with crystallite size more than 100 nm. The materials were characterized by powder X-ray diffraction, electron microscopy, Raman, and energy dispersive X-ray elemental mapping. The method allows the formation of Zn-rich phases and good control of the crystal size and shape. No narrow-band-gap highly conductive Cu_xS impurities were observed.

Full text of DOI:10.1002/ejic.201700137

A Journal of
Accepted Article
Title: Synthesis of Nonstoichiometric Cu2ZnSnS4 by cation exchange
of ZnS
Authors: Yi Jin and George Chumanov
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700137
Link to VoR: http://dx.doi.org/10.1002/ejic.201700137

European Journal of Inorganic Chemistry
10.1002/ejic.201700137
Synthesis of Nonstoichiometric Cu ZnSnS by cation exchange of
2
4
ZnS
Yi Jin, George Chumanov*
Corresponding Authors: *E-mail: gchumak@clemson.edu
http://www.clemson.edu/science/departments/chemistry/people/faculty/chumanov.html
Department of Chemistry, Center for Optical Materials Science and Engineering
Technologies (COMSET), Clemson University, Clemson, South Carolina 29634, United
States
Key words: Cu
2
ZnSnS , Zn-rich, solar cells, Raman spectrum, nanocrystals
4
Abstract
A cation exchange reaction of wide band gap crystalline ZnS with CuCl
2
and SnCl
4
in triethylene glycol was developed to synthesize nonstoichiometric Cu ZnSnS
2
4
with
crystallite size more than 100 nm. The materials were characterized by powder X-ray
diffraction, electron microscopy, Raman, and energy dispersive X-ray elemental mapping.
The method allows the formation of Zn-rich phases and good control of the crystal size and
shape. No narrow bandgap high conductive Cu S impurities were observed.
x
Introduction
Cu
2
ZnSnS (CZTS) is a p-type semiconductor with a band gap of 1.41.6 eV and
4
4
1 [1]
a high light absorption coefficient of over 10 cm . It is made of earth-abundant and
nontoxic elements and has good stability in the ambient environment. CZTS is investigated
[
2]
[3]
as a light absorption material in solar cells, photocatalyst and photocathode for water
splitting[1,4]. A range of techniques is used to synthesize CZTS, including organic solvent
assistant direct synthesis,[4] hydro/solvent thermal synthesis, chemical vapor deposition,
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
pulse laser deposition, sputtering, evaporation, electrochemical deposition, sol-gel coating,
[
1]
spray pyrolysis, and successive ionic layer absorption and reaction. Sulfurization by H S
2
or S at around 500 C is an essential step in all these methods to obtain highly crystallized
CZTS. The high-temperature sulfurization of CZTS also results in sulfurization of metal
oxide semiconductors such as ZnO, TiO
2
, thereby limiting in situ fabrication of highly
efficient solar cells. As synthesized, CZTS is also often contaminated with impurities such
[5]
as highly conductive or narrow bandgap Cu
x
S, Sn S, and Cu-Sn-S. These impurities trap
x
carriers and act as recombination centers. As a result, electrons and holes cannot reach the
surface of the semiconductor leading to low conversion efficiency of photocatalysts and
low open circuit voltage, current density and fill factor in solar cells.
Polycrystalline and amorphous materials have also much more crystal defects and
dangling bonds than single crystal materials and these disorder states increase the carrier
scattering thereby decreasing their lifetime, mobility, and diffusion length. The
semiconductor in the single crystal state shows significantly better carrier mobility and
diffusion length than in the polycrystalline and amorphous states. However, there is no
effective method currently available for low temperature synthesis of large crystals of pure
CZTS. Furthermore, many studies have shown that Zn-rich CZTS exhibits much better
[
2]
photovoltaic effect than Cu-rich CZTS. Therefore, it is essential to develop an effective
method for the synthesis of highly crystalline CZTS with the full control of Zn
concentration in the product. Cation exchange reactions is an effective method to
synthesize complex compounds with certain morphologies.[6] The first cation exchange
synthesis of CZTS was the reaction of Cu
2
SnS with zinc diethyldithiocarbamate in a
3
mixture of 1-dodecanethiol and trioctylamine at 250 C resulting in ~15 nm CZTS
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
nanoparticles.[7] However, the synthesis of large crystals of the reactant Cu
SnS was
2
3
shown to be difficult. CZTS thin plates with a thickness of about 12 nm were also
synthesized by the cation exchange of CuS with zinc 2,4-pentanedionate monohydrate and
tin(IV)chloride bis(2,4-pentanedionate) in a mixture of oleylamine, sulfur and 1-
[8]
dodecanethiol at 260 C. In a slightly modified method, in which CuS with
Zn(CH COO) 2H O was reacted with SnCl 2H O in triethylene glycol at 205 C, CZTS
nanoplates with the thickness of about 35 nm were obtained. All these methods start from
relatively thermodynamically stable Cu SnS and CuS. Residuals of these phases are
3
2
2
2
2
[
9]
2
3
detrimental to the performance of the solar cells because of their narrower band gap and
high conductivity.[10]
Powder X-ray diffraction (PXRD) and Raman spectroscopy are widely accepted
methods for characterizing CZTS and its impurity phases.[11] PXRD is used to determine
the crystal structure of materials but cannot distinguish tetragonal Cu
2
ZnSnS
4
from cubic
ZnS and Cu SnS due to the similarity of the diffraction patterns from these phases (Figure
2
3
S1). Contrary, Raman spectroscopy yields distinctly different spectra of these phases and
as such is a complementary technique to PXRD (Table S1). X-ray absorption near-edge
structure[12] and neutron diffraction[13] are also helpful to distinguish these phases but are
seldom used due to their scarce availability.
Here, we present the synthesis of Cu
exchange reaction starting with wide bandgap (3.5 eV) and low conductive cubic ZnS and
CuCl and SnCl in triethylene glycol. This new method is compared to a previously
published similar cation exchange reaction, in which CuS was used instead of ZnS as a
x
Zn
y
Sn2-0.25x-0.5y
S
4
(NS-CZTS) by a cation
2
4
[
9]
starting material. It is demonstrated that, using ZnS instead of CuS results in a superior
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
CZTS product, at the same time permitting control of Zn composition. The composition of
NS-CZTS was controlled by the feeding ratio of the reactants enabling zinc rich products.
The new cation exchange method is expected to promote the development of highly
efficient CZTS devices.
Results and discussion
Cu
2
ZnSnS nanosheets were previously synthesized from narrow band CuS using
4
[
9]
a cation exchange reaction. We started with this method in an attempt to synthesize high
quality CZTS material to be used in efficient solar cells. Hexagonal CuS with 80 nm
average crystallite size was synthesized from CuCl and thiourea via the hydrothermal
2

1
reaction (Figure S2). The product exhibited a characteristic band at 474 cm in Raman
spectra (Figure S3). The cation exchange reaction was then carried out at 220 C for 96 h
[
9]
in a PTFE reactor as described in the literature . PXRD revealed that the product contained
Cu
2
S and Cu
ZnSnS (Figure S4). However, its Raman spectrum showed only a weak peak at 338
ZnSnS and a high fluorescent background (Figure S5). The
cation exchange reaction was further modified by increasing the reaction temperature from
20 C to the boiling point of triethylene glycol (285 C) and refluxing the mixture for 22
2
ZnSnS
4
or possibly other phases with a similar diffraction pattern as that of
Cu
2
4

1
cm often assigned to Cu
2
4
2
h. The product was NS-CZTS with the average crystallite size of 52 nm but PXRD also
contained two additional peaks at 46.6 and 48.8 that could not be reliably assigned and
were assumed to be due to impurities (Figure S6). The product was further soaked in either
2
.0 M HCl or 1.0 M NH at room temperature for 30 min followed by thorough water
3
washing in the attempt of removing the impurities. The HCl treatment did not change the
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
PXRD diffraction pattern of the product as the peaks from the impurities were not removed
(
Figure S7). In contrast, the impurity peak at 48.8 disappeared after treating with NH
Raman spectra of the as-synthesized product showed two peaks at 337 and 288 cm-
.[14] No observable
3
.
1
that are often assigned to non-polar A-symmetry modes of Cu
2
ZnSnS
4
-
1
peak was found around 476 cm indicating no detectable Cu
After 30 min treatment with HCl or NH , Raman spectra of the product did not change, but
longer reaction times resulted in the formation of Cu S as was evident from the appearance
of a peak at 476 cm . A similar behavior was also observed when using SnCl  5H
instead of SnCl (Figure S4 & S5).
x
S in the product (Figure S7).
3
x
-1
4
2
O
2
Further studies also showed that this cation exchange reaction was poorly
reproducible and the final product was strongly affected by the synthetic history of the CuS.
For example, CuS synthesized from 5.0 equivalent thiourea exhibited lower reactivity to
2
+
2+
Zn and Sn than that synthesized from 1.0 equivalent thiourea. As a result, the product
contained unreacted Cu S as was confirmed by the presence of characteristic diffraction
peaks at 28.0 (111), 32.4 (200), 46.3 (220), 54.8 (311) in PXRD pattern and a Raman
2
-1
peak at 472 cm (Figure S4 & S5). Also, as is shown in the reaction (1), the copper-rich
environment promotes the formation the copper-rich product. The copper-rich product is
often contained narrow bandgap defects that trap the carriers, thereby reducing the
efficiency of photovoltaic devices.[2] For these reasons, CuS was not considered as a
suitable reactant for a cation exchange reaction to produce high quality CZTS.
4CuS + Zn(CH
3
COO)
2
+ SnCl
2
→ Cu
2
ZnSnS
4
+ CuCl
2
3
+ Cu(CH COO)
2
(1)
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
It was rationalized that if the cation exchange reaction is carried out with ZnS as a
starting material, Zn-rich CZTS will result. The following possible reactions were
identified.
4
ZnS + 2CuCl
OOP* (2)
ZnS + CuCl
ZnS + 2CuCl → Cu
CuS + Triethylene glycol → Cu
CuCl + Triethylene glycol → CuCl + HCl + OOP* (6)
CuS +ZnCl
OOP* (7)
CuS +2ZnS + SnCl
8)
Cu
2
+ SnCl
4
+ Triethylene glycol → Cu
2
ZnSnS
4
+ 3ZnCl + 2HCl +
2
2
→ CuS + ZnCl
2
(3)
2
S + ZnCl
2
(4)
S + H
2
2
2
S + OOP* (5)
2
4
2
+ SnCl
4
+ Triethylene glycol → Cu
2
ZnSnS
4
+ 2CuCl + 2HCl +
2
2
4
+ Triethylene glycol → Cu
2
ZnSnS
(9)
4
+ ZnCl + 2HCl + OOP*
2
(
2
S + 3ZnS + SnCl
4
→ Cu
2
ZnSnS
4
+ 2ZnCl
2
Note: OOP*  organic oxidation products from triethylene glycol, which are complicated
mixtures and will not be discussed here.[15]
[
16]
The reaction of ZnS (pKsp  26.1) with CuCl
2
and SnCl
4
is more thermodynamically
favorable than the reaction of CuS (pKsp  40.3) with Zn(CH COO) and SnCl (reaction
2)). The formation of CuS and Cu S from ZnS appears also to be thermodynamically
feasible and kinetically more favorable than the formation of Cu ZnSnS (reactions (3) &
4)) because CuS and Cu S have been observed as intermediates (Figure 12, S10S11).
CuS can be further reduced to Cu S (reaction (5)) and CuCl is produced from CuCl
reaction (6)) by triethylene glycol acting as a reducing agent at high temperature. It is
[16]
3
2
2
(
2
2
4
(
2
2
2
(
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
reasonable to assume that reactions (7), (8), and (9) can proceed because they are similar
to reaction (1). According to our results, the reactions (5) proceeded fast when using ZnS
with the crystallite size of 5 nm. After adding CuCl into the suspension of 5 nm ZnS in
2
triethylene glycol at room temperature, the white suspension changed to black CuS in 30
seconds. In contrast, ZnS with a crystallite size more than 130 nm reacted significantly
slower with CuCl
30 nm) and CuCl
temperature for 5 h indicating no noticeable CuS. This implies that the formation of CuS
is a kinetically controlled reaction that is favorable at low temperatures because k T is in
the denominator in Arrhenius equation (k is the Boltzmann constant and T is the absolute
2
under the same conditions. Specifically, the reaction mixture of ZnS (
1
2
in triethylene glycol still had white color after stirring at room
B
B
temperature). Therefore, raising the temperature of the cation exchange reaction and using
large ZnS crystals favored the formation of CZTS product at the same time minimizing
trapping of CuS kinetic product due to increasing rates of reactions (2), (7), (8) and (9).
Figure 1. PXRD patterns of NS-CZTS synthesized from ZnS, CuCl
the reflux conditions for different reaction times. The simulated PXRD pattern is of
tetragonal Cu ZnSnS . Diffraction peak at 46.4 is assigned to (220) plane of cubic Cu
2
, and SnCl
4
 5H
2
O at
2
4
2
S
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
(PDF card #: 00-053-0522). Crystallite sizes of NS-CZTS were 123, 112, 101, 120, and
1
29 nm for the reaction time of 0.5, 1, 2, 3, and 24 h, respectively.
Figure 2. Raman spectra of NS-CZTS synthesized from ZnS, CuCl
2
, and SnCl
4
 5H
2
O at
-1
the reflux conditions for different reaction times. Raman peaks at 288 and 335 cm are
-1
-1
assigned to NS-CZTS; peak at 349 cm is assigned to ZnS, and peak at 472 cm is assigned
to Cu S.
Highly crystallized ZnS with the crystallite size more than 130 nm was obtained
from the hydrothermal reaction of Zn(CH COO) with thiourea (Figure S8). The ZnS
exhibited a characteristic Raman peak at 351 cm (Figure S9). First, the cation exchange
reaction was carried out with tin (II) (SnCl ), ZnS and CuCl and the product contained
NS-CZTS together with unassignable impurities as shown in the PXRD (Figure S10). Only
2
3
2

1
2
2

1
one peak at 337 cm of CZTS was detected in the Raman spectrum (Figure S11). The
cation exchange reaction was then carried out using tin (IV) (SnCl  5H O) at 200 C for
h. The product contained a mixture of hexagonal CuS and NS-CZTS as confirmed by
4
2
2
-1
PXRD (Figure S10) and Raman showing two peaks at 334 cm from NS-CZTS and at 471
-1
cm from CuS (Figure S11). The reaction temperature was further increased to the boiling
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
point of triethylene glycol (285 C). The product contained only NS-CZTS (Figure 1 & 2).
The effect of the reaction time on the formation of NS-CZTS was further studied by
carrying the reaction at 0.5 h, 1.0 h, 2.0 h, 3.0 h, and 24 h in the mixture of ZnS, CuCl2,
and SnCl
4
 5H
2
O at the reflux reaction condition. Cu S was detected as a by-product after
2
the reaction time of 0.5 h and 1.0 h. together with the unreacted ZnS after 0.5 h as indicated
-1
by a characteristic Raman peak at 349 cm (Figure 1 & 2). The longest studied reaction
time, 24 h, generated also impurities detectable by PXRD but not by Raman spectroscopy.
The crystallite size of NS-CZTS did not change when increasing reaction time from 0.5 h
to 24 h and was determined by the size of the initial ZnS material because of the nature of
cation exchange reactions.
Figure 3. PXRD patterns of NS-CZTS synthesized from ZnS, CuCl
with different molar feed ratios at the reflux condition. The simulated PXRD pattern is of
tetragonal Cu ZnSnS . Crystallite sizes of NS-CZTS were 47, 83, 124, and 97 nm for the
2
, and SnCl
4
 5H O
2
2
4
molar feed ratio of 1 : 0.5 : 4, 1.6 : 0.8 : 4, 1.9 : 0.95 : 4, and 1.6 : 0.91 : 4, respectively.
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
Figure 4. Raman spectra of NS-CZTS synthesized from ZnS, CuCl
2
, and SnCl
4
 5H O
2
with different molar feed ratios at the reflux conditions. Raman peaks at 288, 335 and 370
-1
cm are assigned to Cu
The effect of the reactant feeding ratios was studied at the reflux condition for 2 h
reactions. Only NS-CZTS was formed for the molar feed ratio of CuCl : SnCl : ZnS from
: 0.5 : 4 to 1.9 : 0.95 : 4 and the crystallite size increased from 47 nm to 124 nm due to
the nonstoichiometric ratio of the reactants (Figure 3 & Table S2). According to literature,
the zinc-rich Cu ZnSnS with an atom ratio of Cu : Sn :Zn  1.6 : 0.91 : 1 exhibited high
performance in the solar cells. The molar feeding ratio of CuCl : SnCl : ZnS = 1.6 : 0.91 :
was also used to synthesize NS-CZTS with a crystallite size of 97 nm. The NS-CZTS
2
ZnSnS
4
.
2
4
1
2
4
2
4
4
-1
-1
material exhibited down shift of the Raman peak from 335 cm to 330 cm when compared
to the other materials synthesized from ZnS in this study (Figure 4 & Table S2).
Scanning electron microscopy revealed that ZnS crystallized into the shape of
irregular plates and polyhedrons when the reaction was carried out at 220 C (Figure 5).
After the cation exchange reaction with CuCl
2
and SnCl
4
 5H O at the reflux conditions
2
for 2 h, the shape did not change but plates and polyhedrons exhibited rough edges. This
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
[
6]
means that the original shape of the ZnS was preserved in the NS-CZTS product. It was
concluded that a desired shape of NS-CZTS can be obtained by controlling the shape of
initial ZnS, which is easier to achieve due to the simpler reaction. As such, the described
approach provides better control for the preparation of predefined Cu
2
ZnSnS nanocrystals.
4
The synthesized NS-CZTS nanocrystals exhibited a uniform distribution of all
elements in any individual particles; however, small particles contained more copper and
tin than the large particles, as shown in the EDX elemental maps (Figure 6 & S12S17).
No detectable diffusion edges rich in copper or tin neither ZnS core was found in the
elemental maps. Quantitative analysis revealed nonstoichiometric atom ratios that
increased from [Cu]/([Zn]  [Sn])  0.12 & [Zn]/[Sn]  31.10 to [Cu]/([Zn]  [Sn])  0.48
&
[Zn]/[Sn]  6.68 with increasing of the molar feed ratio of CuCl
2
: SnCl : ZnS from 1 :
4
0
4
.5 : 4 to 1.6 : 0.8 : 4. The further increase of the molar feed ratio to stoichiometric 2 : 1 :
yielded the similar atom ratio of [Cu]/([Zn]  [Sn])  0.48 & [Zn]/[Sn]  6.96. This
2


4
similarity indicates that the cation exchange of Zn by Cu and Sn is leveling off while
approaching the stoichiometric ratio in the product. The cation exchange reaction should
also be controlled by other factors, such as crystal defects in ZnS, which decreased the
[
6]
possibility of the formation of ZnS/NS-CZTS core-shell structure. The elemental maps
of either single large NS-CZTS particle measured using relatively low resolution SEM
(Figure S16) or high resolution STEM maps of a single small particle (Figure S17) show
no evidence of core-shell structures.
As mentioned in the introduction part, a combination of PXRD and Raman
spectroscopy is widely accepted as a standard way for characterizing CZTS materials and
their impurities.[11] It is important to emphasize that the nonstoichiometric NS-CZTS
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
phases exhibit characteristic PXRD patterns and Raman spectra similar to that of
Cu ZnSnS for a broad range of [Cu]/([Zn]  [Sn]) of NS-CZTS atom ratios between 0.12
and 0.28. Therefore, it is difficult to distinguish stoichiometric Cu ZnSnS from
2
4
2
4
nonstoichiometric NS-CZTS using the combination of these techniques. The other
characterization techniques, such as elemental mapping, are necessary to fully characterize
the chemical composition of CZTS materials.
Figure 5. SEM images of (a) ZnS synthesized by the reaction of Zn(CH
3
COO)
2
with
thiourea at 220 C (b) NS-CZTS synthesized from the molar feed ratio of CuCl
2
: SnCl :
4
ZnS = 2 : 1 : 4. at reflux condition for 2 h.
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
Figure 6. SEM image and EDX elemental maps of NS-CZTS synthesized from the molar
feed ratio of CuCl
2
: SnCl : ZnS = 2 : 1 : 4. at reflux condition for 2 h.
4
Conclusion
A new method based on a cation exchange reaction starting with ZnS crystals for
the synthesis of nonstoichiometric NS-CZTS with the crystallite size more than 100 nm
was developed. The method does not result in the contamination with low bandgap and
high conductive Cu S impurities that are often detrimental to the applications of CZTS
x
materials. Contrary, the method permits the synthesis of beneficial Zn-rich phases. The
synthesized NS-CZTS exhibited a uniform distribution of all elements throughout the
individual particles. High similarities of PXRD patterns and Raman spectra of
nonstoichiometric NS-CZTS and stoichiometric Cu
2
ZnSnS indicates the importance of
4
using other techniques to fully characterize the chemical composition of the materials. This
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
cation exchange method is expected to further facilitate the development of Cu
2
ZnSnS
4
based devices.
Experimental section
Materials
Anhydrous tin(II) chloride (Alfa Aesar, 99%), zinc acetate dihydrate (J. T. Baker,
Baker analyzed reagent), anhydrous copper (II) chloride, (Acros Organics, 99%), tin (IV)
chloride pentahydrate (Fisher Science Education, lab grade), triethylene glycol (TCI,
9
9.0%) and thiourea (Alfa Aesar, 99%) were used as received.
Instrumentation
Powder X-ray diffraction data was collected using a Rigaku Ultima IV X-ray
diffractometer with Cu K radiation ( = 1.5418 Å) at 25 C. PXRD pattern simulation
was performed with Mercury 3.5.1 software by using cubic ZnS (a = 5.40 Å),[17] hexagonal
CuS (a = b = 3.729 Å c = 16.11 Å),[18] cubic Cu
SnS
(a = b = c = 5.43 Å)[19] and tetragonal
2
3
Cu
2
ZnSnS
4
(a = b = 5.434 Å, c = 10.850 Å)[20] single crystal structures. X-ray powder
diffraction analysis was performed on Rigaku PDXL 1.8.1.0 software. Crystallite sizes
were calculated from PXRD using Scherrer equation after correcting the instrumental line
broadening using the peak width of the width standard data obtained using LaB external
6
standard reference. Electron microscope images were obtained with Hitachi S-4800 field
emission scanning electron microscopes (SEM) and HD2000 field emission scanning
transmission electron microscope (STEM) equipped with the capability of energy-
dispersive X-ray spectroscopy. Particles were drop-casted on polished silicon wafers
(Wafer Works Corp., dopant, As, resistivity  0.006   cm, ) for SEM analysis and on
copper grids with carbon type-B support films for STEM analysis. Raman spectra were
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
+
collected using 514.5 nm excitation light from an Ar ion laser (Innova 200, Coherent, 500
mW). Scattered light was collected by a f/1.2 camera lens in a backscattering geometry and
analyzed by a triple spectrometer (Triplemate 1877, Spex) equipped with a CCD detector
(iDUS 420, Andor). Raman spectra of a mixture of chloroform and bromoform were used
for the spectral calibration.
Synthesis of CuS
CuCl
L ). Thiourea (0.335 g, 4.4 mmol) was dissolved in H
was then added to the CuCl solution drop by drop with stirring at room temperature over
2
(0.538 g, 4.00 mmol) was dissolved in hydrochloric acid (4.0 mL, 1.0 mol

1
2
O (4.0 mL). The thiourea solution
2
a period of 10 min. The reaction mixture was further stirred at room temperature for
additional 10 min and then sealed in 23 mL PTFE lined digestion vessel. The reaction
mixture was then heated in an oven to a set temperature of 220 C and kept for 24 h. After
cooling to room temperature, the colorless supernatant was discarded and the black
precipitate was washed with H
5 h.
2
O three time and then dried in a vacuum oven at 60 C for
1
Synthesis of NS-CZTS from CuS
Zn(CH COO)  2H O (28 mg, 0.13 mmol) and SnCl (25 mg, 0.13 mmol) were
3
2
2
2
dissolved in triethylene glycol (5 mL). CuS (50 mg, 0.52 mmol) was then added to the
solution followed by stirring at room temperature for 10 min. The reaction mixture was
transferred to 23 mL PTFE lined digestion vessel and kept in an oven at a set temperature
of 220 C for 96 h. Alternatively, the reaction mixture was transferred to a 25 mL round
bottom flask and refluxed with stirring for 22 h. After cooling to room temperature, the
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
reaction mixture was centrifuged and the black precipitate was washed twice with ethanol
and twice with water.
Synthesis of ZnS
Zn(CH
dissolved in H
digestion vessel and kept in a 220 C oven for 24 h. After cooling to room temperature, the
3
COO)
2
 2H
2
O (2.63 g, 12.0 mmol) and thiourea (0.913 g, 12.0 mmol) were
2
O (8 mL). The solution was then transferred to a 23 mL PTFE lined
colorless supernatant was discarded and the white precipitate was washed with H
time and then dried in a vacuum oven at 60 C for 15 h.
Synthesis of NS-CZTS from ZnS
2
O three
CuCl
2
(54 mg, 0.40 mmol) and SnCl
4
• 5H O (70 mg, 0.20 mmol) were dissolved
2
in triethylene glycol (5 mL). ZnS (78 mg, 0.80 mmol) was then added to the solution
followed by refluxing for 3 h. After cooling to room temperature, the reaction mixture was
centrifuged and separated into a slightly yellow-brown supernatant and a black precipitate.
The black precipitate was washed twice with ethanol and twice with water.
Supporting information
Supporting information and ORCID(s) from the author(s) for this article are available on
the WWW under http://dx.doi.org/ .
Acknowledgments
We gratefully acknowledge the support of this work from Clemson University Center for
Optical Materials Science and Engineering Technologies.
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European Journal of Inorganic Chemistry
10.1002/ejic.201700137
Table-of-Contents
A cation exchange reaction of ZnS with CuCl
2
and SnCl was developed to synthesize
4
nonstoichiometric Cu
2
ZnSnS with crystallite size more than 100 nm. The method allows
4
the formation of Zn-rich phases and good control of the crystal size and shape.
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