Synthesis and surface chemistry of high quality wurtzite and kesterite Cu2ZnSnS4 nanocrystals using tin(II) 2-ethylhexanoate as a new tin source

Article abstract of DOI:10.1039/c5cc04151a

A novel synthesis method for the preparation of Cu₂ZnSnS₄ nanocrystals is presented using a liquid precursor of tin, namely tin(ii) 2-ethylhexanoate, which yields small and nearly monodisperse NCs either in the kesterite or in the wurtzite phase depending on the sulfur source (elemental sulfur in oleylamine vs. dodecanethiol).

Full text of DOI:10.1039/c5cc04151a

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Facile synthesis and surface chemistry of high quality wurtzite and
kesterite Cu2ZnSnS4 nanocrystals using tin(II) 2-ethylhexanoate as
a new tin source
Received 00th January 20xx,
Accepted 00th January 20xx
Grzegorz Gabka,a Piotr Bujak,a* Maciej Gryszel,a Andrzej Ostrowski,a Karolina Malinowska,b
DOI: 10.1039/x0xx00000x
Grazyna Z. Zukowska,a Fabio Agnesec,d, Adam Prona and Peter Reissc,e,f**
www.rsc.org/
A novel synthesis method for Cu2ZnSnS4 nanocrystals is presented very similar metal precursors9 and also from metal
using a liquid precursor of tin, namely tin(II) 2-ethylhexanoate, dithiocarbamate complexes,8c however, the addition of
which yields small and nearly monodisperse NCs either in the alkanethiols as a source of sulfur is necessary in this case. Both
kesterite or in the wurtzite phase depending on the sulfur source kesterite- or wurtzite-type CZTS NCs can also be prepared in a
(elemental sulfur in oleylamine vs. dodecanethiol).
controllable manner by varying the reactivity of the sulfur
precursor.10 Injection of the solution of higher reactivity
(prepared by heating S/octadecene (ODE) for 2 hrs at 155°C)
leads to NCs of wurtzite-type structure while the less reactive
sulfur precursor solution (obtained by heating the sulfur
solution 1 hr) yields kesterite NCs. The opposite is observed if
the synthesis of NCs is carried out in OLA: the use of a highly
reactive precursor solution, namely sulfur dissolved in OLA,
Cu2ZnSnS4 (CZTS) quaternary semiconductor nanocrystals
(NCs) have been drawing a significant research interest in the
past few years as materials for solar1 or thermal energy
conversion to electricity.2 Contrary to the NCs previously used
for this purpose, they do not contain toxic (cadmium, lead) or
rare (indium) elements. Their direct band gap which is close to
the optimum value (
∼1.5 eV) and their high absorption
results in kesterite NCs whereas
a less reactive 1-
coefficient (
∼
104 cm-1) make them especially adequate for the
dodecanethiol (DDT)/OLA precursor solution produces
wurtzite ones.11 These two examples unequivocally show that
the reactivity of the sulfur precursor cannot play a predictive
role as far as the crystallographic structure of the resulting
CZTS NCs is concerned. The hot-injection method provides a
versatile approach for the temporal separation of nucleation
and growth of NCs, leading to narrow size dispersion.
However, a drawback of this method is the limited availability
of metal precursors, which could easily be dissolved at room
temperature. Therefore, generally only the chalcogenide
precursors are injected. In the case of multinary NCs the ability
to inject also the metal precursors would give the opportunity
to reduce the formation of secondary phases during the
temperature increase of the reaction mixture.
use in solar cells.3 So far, for solar cells using CZTS-derived NCs,
in particular mixed Cu2ZnSnSxSe4-x NCs of optimized band gap,
the highest reported energy conversion efficiency amounted
to 8.6%.4 In the case of thin films of Cu2ZnSnSxSe4-x deposited
from hydrazine solutions of the precursors even higher
efficiencies were measured, up to 12.6%.5
CZTS NCs have been mainly reported in two different
crystallographic forms: tetragonal (kesterite) and hexagonal
(wurtzite), the former being thermodynamically more stable.6
High temperature syntheses with the use of simple metal salts
in combination with elemental sulfur in oleylamine (OLA) as
the solvent yield NCs of the kesterite structure.2,7 Alternatively,
metal salts and sulfur can be replaced by one-component
precursors, namely metal dithiocarbamate complexes.8
In this communication we report a novel synthesis of CZTS
NCs, in which we replace solid and difficult to dissolve
precursors of tin by liquid tin(II) 2-ethylhexanoate. This
compound, never previously used in nanocrystal synthesis, is a
popular catalyst for polymerization of L-lactide.12 Application
of this new tin precursor in combination with the appropriate
sulfur precursor (DDT or DDT+S/OLA) and reaction conditions
(heating up vs. hot injection) allows for a strict control of the
NCs’ structure and size. Finally, we demonstrate by means of
NMR spectroscopy that the obtained wurtzite and kesterite
CZTS nanocrytals exhibit distinct differences in the nature of
their surface capping ligands.
Metastable wurtzite-type CZTS NCs can be obtained from
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Table 1 Reaction parameters and characteristics of the synthesized CZTS nanoparticles.§
Sample
Type
Solvent
Sulfur source
Size (nm)
Composition
Structure
CZTS-1
CZTS-2
CZTS-3
CZTS-4
HU
HI
ODE
ODE
ODE
DCB
DDT
6.0 +/-2.0
3.4 +/-0.7
5-18
Cu2.04Zn1.00Sn1.37S4.76
Cu1.94Zn1.00Sn0.92S3.81
Cu1.41Zn1.00Sn0.72S3.14
Cu2.10Zn1.00Sn0.92S3.89
Wurtzite
Kesterite
Kesterite
Kesterite
DDT+ S/OLA
S/OLA
HI
HI
DDT+ S/OLA
3.0 +/-0.9
§ HU: heating-up; HI: hot-injection; ODE: 1-octadecene; DCB: 1,2-dichlorobenzene; DDT: 1-dodecanethiol; OLA: oleylamine.
In all syntheses the new precursor was used in
combination with copper(II) oleate and zinc acetate as
precursors of metals. Commercially available tin(II) 2-
ethylhexanoate is a liquid stable in air, which readily mixes
with commonly used solvents like ODE or OLA. This new set
of metal precursors was tested in the two most popular
CZTS NCs synthesis methods, namely heating-up and hot-
injection. Table 1 gives an overview of the synthetic
parameters and characteristics of the obtained samples.
composition of the obtained NCs was Cu2.04Zn1.00Sn1.37S4.76
as determined by EDS (see Fig. S4 ESI† for the
corresponding spectrum). Their small size should be
pointed out, 6.0 +/-2.0 nm (see Fig.2 and Fig. S2 ESI†).
Along with nanoparticles described in Ref. 9, they are the
smallest wurtzite-type CZTS NCs reported to date.
Fig. 2 TEM images of the different samples at identical magnification and
HRTEM images of wurtzite-CZTS-1 and kesterite-CZTS-2 NCs.
The same set of precursors was also tested in the hot-
injection method of the CZTS NCs preparation. In this
procedure
a mixed solution of sulfur and tin(II) 2-
ethylhexanoate in OLA were injected into a mixture of
copper(II) oleate, zinc acetate and DDT in ODE heated to
Fig. 1 X-ray diffractograms of the different samples. CZTS-1: red bars indicate
the diffraction pattern of wurtzite CZTS9a,11; CZTS-2/3/4: the peak positions
of bulk kesterite CZTS are indicated (JCPDS 26-0575).
170
kept at this temperature for additional 30 minutes. The
resulting NCs CZTS-2 of the almost “ideal” 2:1:1:4
°C. Then the temperature was raised to 230 °C and
(
)
composition Cu1.94Zn1.0Sn0.92S3.81 and 3.4 +/-0.7 nm size
were essentially monodisperse (see Fig. 2) and exhibited
the kesterite structure (JCPDS No. 26-0575) (Fig.1). In this
synthesis two sulfur sources were used, namely DDT and
elemental sulfur in OLA. To identify the role of both
precursors we performed an additional synthesis without
DDT. This procedure led to NCs in the kesterite structure
The heating-up method (230 °C, 3h) with the use of the
above described set of metal precursors and DDT in ODE as
the sulfur precursor led to hexagonal (wurtzite) CZTS NCs
(denoted as CZTS-1), as evidenced by the diffractogram
shown in Fig. 1 9a,11
Raman spectroscopy is a very sensitive
.
tool for investigation of the phase purity of CZTS NCs. In
particular, even small amounts of ZnS, undetectable in
powder diffractograms, give rise to Raman bands at higher
wavenumbers as compared to the characteristic diagnostic
band of kesterite at 331 cm-1 (see Fig. S1 in ESI†).9b The
(
CZTS-3
,
Fig.
1
)
having
a
composition
of
Cu1.41Zn1.00Sn0.72S3.14. They were much bigger and more
polydisperse than those of the CZTS-2 batch, showing a
2 | Chem. Commun., 2015, 00, 1-3
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diameter in a size range of 5 - 18 nm (Fig.2). CZTS-4 was
prepared using the same parameters as for CZTS-2 but with
1,2-dichlorobenzene instead of ODE as the solvent (vide
infra).
To the contrary, 1H NMR spectra of the ligands
recovered through the inorganic core dissolution of CZTS-2
indicated ODE as essentially the sole organic molecule. No
lines attributable to DDT and oleic acid, clearly observed in
the spectra of CZTS-1, could be detected in this case. To
verify whether the recovered ODE originated from the
solvent or alternatively from the chemical transformation
of other molecules present in the reaction mixture, we
carried out an additional synthesis of CZTS NCs, in which
ODE was replaced by 1,2-dichlorobenzene as the solvent. In
this case a solution of sulfur and tin(II) 2-ethylhexanoate in
OLA was injected into a solution of copper(II) oleate and
zinc acetate in dichlorobenzene, previously heated to 150
To summarize this synthetic part of the paper, NCs of
wurtzite structure (CZTS-1) are obtained with DDT as the
only source of sulfur in a heating-up synthesis. The reaction
likely proceeds via binary and ternary wurtzite-type Cu-S
and Cu-Zn-S phases, favoring this crystal structure in the
final product.10 In the case of the kesterite-type NCs CZTS-2
and CZTS-4, DDT serves for the complexation of the Cu and
Zn precursors in ODE, while the actual predominant sulfur
source in this case is S/OLA. This can be judged from the
observed color changes. With only DDT temperatures of at
least 180-200°C are required for observing a gradual color
change from orange to black indicative of NC formation.
When injecting S/OLA together with the tin precursor at
°
C. The temperature was then raised to 180
°C and kept
constant for additional 1 hour. Since 180 C is the boiling
°
point of dichlorobenzene, the heat exchange is very
effective with no necessity of applying a strict temperature
regime. In these conditions essentially monodisperse
kesterite NCs of small size (3.0 +/-0.9 nm) formed, showing
a composition of Cu2.10Zn1.00Sn0.92S3.89 (see CZTS-4 in Fig. 1
170°C (CZTS-2
turns black. However, also in this type of reaction DDT plays
crucial role as it intervenes in the formation of
, CZTS-4), the reaction mixture immediately
a
intermediate precursor complexes and the control of the
reaction kinetics. Its absence in the reaction mixture results
in the formation of larger NCs of higher polydispersity
and 2). Note that in this case a small admixture of the
anilite (Cu7S4) phase is observed in addition to the kesterite
phase. It is evidenced by two reflections of very low
(
CZTS-3). More detailed studies of the reaction mechanism
intensity at 46.4° and 32,3° unequivocally ascribed to
are currently undertaken.
anilite (JCPDS 22-0250).14 Both the size and composition are
almost identical with the values obtained from the
synthesis in ODE (CZTS-2), indicating at a first glance that
the choice of the solvent did not significantly influence the
reaction mechanism. Raman spectroscopy (see Fig. S1 in
ESI†) revealed the presence of the pure kesterite structure
with a peak at 333 cm-1, unequivocally excluding the
presence of a mixture of phases such as Cu2SnS3 and ZnS.15
In our recent paper we have described an efficient
method for the identification of NCs’ surface ligands based
on NMR studies of organic molecules recovered after the
dissolution of the inorganic cores.13 This procedure
provides more precise information concerning the organic
shell than direct studies of organic ligands capped NCs since
their NMR spectra suffer from line broadening caused by
the limited mobility of the surface-bound molecules.
Detailed description of the applied ligands recovery
1
In Fig. 3 the H NMR spectrum of the ligands recovered
from CZTS-4 is compared with the corresponding spectrum
1
of the CZTS-1 ligands. To facilitate the attribution H NMR
1
procedure can be found in ESI†. As seen from the H NMR
spectrum shown in Fig. 3, for CZTS-1 (wurtzite) DDT
molecules were recovered, playing a dual role as the sulfur
precursor and capping ligand. Additionally, lines
attributable to oleic acid (OA) (the second expected ligand)
and to ODE (solvent) could be distinguished.
spectra of free ODE, DDT, OLA and oleic acid are collected
in Fig. S5 (ESI†). Surprisingly, distinct signals of ODE were
present in the 1H NMR of the ligands recovered from CZTS-4
NCs, despite the fact that no ODE was used at any step of
their preparation. The presence of ODE in the recovered
organic part of the NCs was additionally confirmed by GC-
MS whose spectrum unequivocally indicated the presence
of ODE (Fig. S6, ESI†). No evidence of DDT and oleic acid
could be found, similarly as in the case of the other two
kesterite-type NCs studied (CZTS-2 and CZTS-3). It is
therefore clear that ODE had to be formed from OLA or
oleic acid either in the course of NCs’ synthesis or during
the process of the inorganic core dissolution and ligands
recovery. The formation of 1-alkene from OLA must
proceed via an elimination reaction accompanied by
hydrogenation. This type of elimination reaction has been
reported when heating fcc Co nanoparticles with OLA to >
200 °C.16 In our case
a possible reaction mechanism
comprises i) bond formation between a surface metal atom
and a carbon atom of the α-CH2 group of OLA; ii) cleavage
of the C-N bond in this complex under simultaneous
Fig. 3 1H NMR spectra of the NCs’ surface ligands recovered after dissolution
of the inorganic cores. Top: CZTS-1 (wurtzite); bottom: CZTS-4 (kesterite).
formation of 1-alkene via
β-H elimination, affording ODE
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and eventually other side-products like octadecadiene,
heptadecene and hepatdecadiene.
images were obtained using the equipment purchased
within CePT Project No. POIG.02.02.00-14-024/08-00.
The optical band gap of CZTS-4 (kesterite), was
determined on the basis of UV-vis absorption spectroscopy
as 1.65 eV (see Fig. S7, ESI†). The slightly increased value as
compared to the values calculated theoretically (1.56 eV)
and measured experimentally (1.4-1.6 eV) for bulk
kesterite17 could be indicative of quantum confinement
effects. The Bohr exciton radius of kesterite, calculated on
the basis of available literature data,16 is rather small (in the
range of 2.2 to 3.4 nm, depending on assumptions used for
the calculations), hence NCs of 3.0 +/-0.9 nm size,
described here should reveal quantum confinement, as
observed experimentally.
Notes and references
1
(a) K. Ramasamy, M. A. Malik, P. O’Brien, Chem.
Commun., 2012, 48, 5703; (b) S. Chen, A. Walsh, X.-G.
Gong, S.-H. Wei, Adv. Mater., 2013, 25, 1522; (c) U.
Ghorpade, M. Suryawanshi, S. W. Shin, K. Gurav, P. Patil,
S. Pawar, C. W. Hong, J. H. Kim, S. Kolekar, Chem.
Commun., 2014, 50, 11258.
2
3
H. Yang, L. A. Jauregui, G. Zhang, Y. P. Chen, Y. Wu, Nano
Lett., 2012, 12, 540.
(a) H. Katagiri, K. Jimbo, W. S. Maw, K. Oishi, M. Yamazaki,
H. Araki, A. Takeuchi, Thin Solid Films, 2009, 517, 2455;
(b) S. Siebentritt, S. Schorr, Prog. Photovolt. Res. Appl.,
2012, 20, 512.
In the UV-vis-NIR spectrum of the wurtzite-type NCs
CZTS-1 (Fig. S7, ESI†) no broad absorption band in the NIR
4
5
W.-C. Hsu, H. Zhou, S. Luo, T.-B. Song, Y.-T. Hsieh, H.-S.
Duan, S. Ye, W. Yang, C.-J. Hsu, C. Jiang, B. Bob, Y. Yang,
ACS Nano, 2014, 8, 9164.
W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K.
Todorov, Y. Zhu, D. B. Mitzi, Adv. Energy Mater., 2014, 4,
spectral range was observed. This can be taken as an
additional proof of phase purity since the NIR band has
been attributed to the heterophase system Cu1.92S-CZTS.9d
We have determined the optical band gap to be 1.75 eV, i.e.
significantly higher than the value reported for wurtzite-
type CZTS NCs (1.4-1.5 eV).9 It should be however noted
that the literature values were measured for nanoparticles
exceeding 20 nm in size, whereas the NCs reported here are
much smaller (6.0 +/-2.0 nm). Furthermore, band gap
variation arising from deviation from the 2:1:1:4
stoichiometry is not taken into account.
1301465.
D. Aldakov, A. Lefrançois, P. Reiss, J. Mater. Chem. C,
2013, , 3756.
6
7
1
(a) T. Kameyama, T. Osaki, K.-ichi. Okazaki, T. Shibayama,
A. Kudo, S. Kuwabata, T. Torimoto, J. Mater. Chem., 2010,
20, 5319; (b) T. Rath, W. Haas, A. Pein, R. Saf, E. Maier, B.
Kunert, F. Hofer, R. Resel, G. Trimmel, Sol. Energy Mater.
Sol. Cells, 2012, 101, 87; (c) A. Shavel, D. Cadavid, M.
Ibáñez, A. Carrete, A. Cabot, J. Am. Chem. Soc., 2012, 134
1438.
,
8
9
(a) C. Zou, L. Zhang, D. Lin, Y. Yang, Q. Li, X. Xu, X. Chen, S.
Huang, CrystEngComm, 2011, 13, 3310; (b) A. Khare, A.
W. Wills, L. M. Ammerman, D. J. Norris, E. S. Aydil, Chem.
Commun., 2011, 47, 11721; (c) M. D. Regulacio, C. Ye, S.
H. Lim, M. Bosman, E. Ye, S. Chen, Q.-H. Xu, M.-Y. Han,
Chem. Eur. J., 2012, 18, 3127.
(a) X. Lu, Z. Zhuang, Q. Peng, Y. Li, Chem. Commun., 2011,
47, 3141; (b) A. Singh, H. Geaney, F. Laffir, K. M. Ryan, J.
Am. Chem. Soc., 2012, 134, 2910; (c) M. Li, W.-H. Zhou, J.
Guo, Y.-L. Zhou, Z.-L. Hou, J. Jiao, Z.-J. Zhou, Z.-L. Du, S.-X.
Wu, J. Phys. Chem. C, 2012, 116, 26507; (d) H.-C. Liao, M.-
H. Jao, J.-J. Shyue, Y.-F. Chen, W.-F. Su, J. Mater. Chem. A,
Conclusions
To summarize, we have presented a new method for the
phase-controlled synthesis of small sized CZTS NCs of
narrow size distribution through the introduction of the
new tin precursor tin(II) 2-ethylhexanoate. Used in a
heating-up method with DDT as the sulfur source, it leads
to wurtzite-type NCs, while a hot-injection method using
the same set of precursors but S/OLA as the sulfur source
yields NCs in the kesterite phase. The two types of CZTS NCs
exhibit distinct differences in surface chemistry involving
ligand transformation in the case of the kesterite NCs. An
increased optical band gap with respect to literature data
for bulk kesterite CZTS is observed, indicative of quantum
confinement effects. The fine-control of the composition,
crystal structure and band gap allow for the study of the
influence of these parameters on the properties of
optoelectronic devices using CZTS NCs, enabling
optimization of their performance.
2013, 1, 337; (e) J. Chang, E. R. Waclawik, CrystEngComm,
2013, 15, 5612.
10 Y. Zou, X. Su, J. Jiang, J. Am. Chem. Soc., 2013, 135, 18377.
11 Z. Li, A. L. K. Lui, K. H. Lam, L. Xi, Y. M. Lam, Inorg. Chem.,
2014, 53, 10874.
12 R. E. Drumright, P. R. Gruber, D. E. Henton, Adv. Mater.,
2000, 12, 1841.
13 G. Gabka, P. Bujak, M. Gryszel, K. Kotwica, A. Pron, J.
Phys. Chem. C, 2015, 119, 9656.
14 V. T. Tiong, Y. Zhang, J. Bell, H. Wang, CrystEngComm.,
2014, 16, 4306.
15 P. A. Fernandes, P. M. P. Salomé, A. F. da Cunha, J. Alloys
Compd., 2011, 509, 7600-7606.
16 (a) K. M. Nam, J. H. Shim, H. Ki, S.-I. Choi, G. Lee, J. K.
Jang, Y. Jo, M.-H. Jung, H. Song, J. T. Park, Angew. Chem.
Int. Ed., 2008, 47, 9504; (b) S. Mourdikoudis, L. M. Liz-
Marzán, Chem. Mater., 2013, 25, 1465.
This research was carried out in the framework of the
project entitled “New solution processable organic and
hybrid (organic/inorganic) functional materials for
electronics, optoelectronics and spintronics” (Contract No.
TEAM/2011-8/6), which is operated within the Foundation
for the Polish Science Team Programme cofinanced by the
EU European Regional Development Fund. One of the
authors (P.B.) wants to acknowledge a partial financial
support of the Warsaw University of Technology. The TEM
17 C. Persson, J. Appl. Phys., 2010, 107, 053710.
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