1. 7316 Chem. Commun., 2013, 49, 7316--7318 This journal is c The Royal Society of Chemistry 2013
Cite this: Chem. Commun., 2013,
49, 7316
Precursor driven one pot synthesis of wurtzite and
chalcopyrite CuFeS2†
Prashant Kumar, Sitharaman Uma and Rajamani Nagarajan*
A facile precursor dependent single step, one pot solution based
synthesis of wurtzite and chalcopyrite polymorphs of CuFeS2 has been
developed by reacting a Cu(I) thiourea complex with Fe2(SO4)3 and
FeCl3 separately in ethylene glycol.The phases have been characterized
by structural refinements, SEM-EDX, TEM-SAED, Raman, UV-Visible
spectroscopy and TGA measurements.
Polymorphism and the structure–property relationship of solids are
quite synonymous in chemical research and play a pivotal role in
determining their eventual use for any particular application.1
Cu–S
system represents a gallery of stoichiometric and non-stoichiometric
compositions exhibiting polymorphism.2
Complex crystal structure,
the highly mobile nature of Cu and the existence of mixed valence
states in copper sulfides2,3
offer chemical and structural freedom for
the easy diffusion of other metal ion/ions in them resulting in
ternary and quaternary sulfides. With the advent of nanoscience and
nanotechnology, stabilization of high energy phases, wurtzite and
zinc blende, which are the structural variants of generic chalcopyrite
structures, by solution based methods has been achieved for the
I–III–VI2 type compounds, where group I and III elements belong to
coinage metals and p-block elements of the periodic table, respec-
tively, e.g. CuInS2, CuGaS2.4,5
Generation of such polymorphs has
been quite beneficial to tune the Fermi energy of these photovoltaic
materials over a wide range during their fabrication.6
To the best of
our knowledge, there exists no report on the existence of wurtzite
structure with a group III element belonging to the transition metal
series. Presence of transition metal ions in wurtzite arrangement
may evolve interesting properties (as magnetic semiconductors for
spintronics) due to the introduction of d orbitals with unpaired
electrons in the band structure.6
Chalcopyrite (CH–CuFeS2) is the
extensively studied system among the ternary compositions contain-
ing transition metal ions and its structure consists of Cu1+
and Fe3+
at alternate tetrahedral sites of a cubic close packed sulfur network
in tetrahedral symmetry (space group I%42d).7
In this communication,
we report a rapid, one pot, single step solution based synthesis of
CuFeS2 in wurtzite (WZ) structure for the first time. Additionally,
CuFeS2 in the well known chalcopyrite structure has been synthe-
sized by varying the precursor. In the present set of reactions, the
Cu(I) complex, ([Cu4(tu)9](NO3)4Á4H2O (tu = thiourea)), was refluxed
with iron(III) salts, Fe2(SO4)3, FeCl3 separately in ethylene glycol.
Thiourea, present in the complex, acted as the sulfur source.
The powder X-ray diffraction (PXRD) pattern of the product
obtained from the reaction of [Cu4(tu)9](NO3)4Á4H2O with Fe2(SO4)3
is reproduced in Fig. S1 (ESI†). The search-match procedure for the
observed peak positions with the known compositions in the ICSD
database using the High score plus software8a
did not provide any
solution resembling the fingerprints of binary and ternary sulfides
containing copper and/or iron. Initially, the sample was subjected
to elemental color mapping of its Field Emission Scanning Electron
Microscopy (FE-SEM) images to determine the elements present,
their homogeneity, purity, phase formation, and the results are
reproduced in Fig. 1(a). From the mapping, the product has been
found to contain Cu, Fe and S elements that are homogeneously
distributed within the particles. The ratio of Cu:Fe:S obtained
from the EDX analysis is 1.16:1:1.80 (Fig. S2, ESI†). By wet
chemical analysis and atomic absorption spectroscopy measure-
ments, a similar ratio of the three constituents has been realized
(details are provided in ESI†). Preliminary indexing of the PXRD
pattern using the TREOR8b
program indicated that all the reflec-
tions could be indexed in a hexagonal system. Additionally, it was
recognized that the intensity pattern of the reflections matched
closely with the observed PXRD pattern of WZ–CuInS2.4
The crystal
structure analysis of the product was therefore carried out by
considering the structural model of wurtzite CuInS2,9
wherein the
In3+
site is replaced by Fe3+
-ions and positional refinement for
copper and iron was carried out using the FULLPROF10
program.
The diffraction pattern fitted well in the hexagonal symmetry
(space group P63mc (186)) with lattice constants of a = 3.726 (3) Å
Materials Chemistry Group, Department of Chemistry, University of Delhi,
Delhi 110007, India. E-mail: rnagarajan@chemistry.du.ac.in;
Fax: +91-11-2766 6605; Tel: +91-11-2766 2650
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of [Cu4(tu)9](NO3)4Á4H2O and CuFeS2, PXRD patterns and EDX analysis of
WZ–CuFeS2 and CH–CuFeS2, Rietveld refinement plots of WZ- and CH–CuFeS2,
crystallographic data of WZ- and CH–CuFeS2, TEM images of WZ- and CH–
CuFeS2, TGA traces of WZ- and CH–CuFeS2 in a nitrogen atmosphere and the PL
spectrum of CH–CuFeS2 obtained upon exciting at l = 500 nm. See DOI: 10.1039/
c3cc43456g
Received 9th May 2013,
Accepted 19th June 2013
DOI: 10.1039/c3cc43456g
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2. This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7316--7318 7317
and c = 6.132 (4) Å (Fig. 1(b)) after automatic background correction.
The structural positional parameters after the final cycle of refine-
ment are summarized in ESI† (Fig. S3 and Tables S1 and S2). The
estimated average crystallite size of this sample by Scherrer analysis
is 17 nm. Crystal structure of the synthesized WZ–CuFeS2, generated
using the DIAMOND8c
software using the refined parameters, is
presented in Fig. 1(c). The Selected Area Electron Diffraction (SAED)
pattern of this sample is shown in Fig. 1(d); the diffraction spots
corresponding to the (002), (110), (012)/(102), (011)/(101) planes
match well with its PXRD pattern. The TEM image of the sample
is shown in Fig. S4 (ESI†).
WZ–CuFeS2 nanocrystals show micro flower-like morphology
with an average diameter of 0.8–1 mm (Fig. 2(a)). It shows four
bands at 287, 351, 470 and 564 cmÀ1
in the room temperature
Raman spectrum (Fig. 2(b)). While the positions of the bands match
well with the ones reported for CH–CuFeS2,11
the intensity of the
band at 470 cmÀ1
is slightly lower than that of the band at 287 cmÀ1
.
Following the assignment of these modes for CH–CuFeS2,11
the
observed bands in the present case have been assigned to A1, B2 and
E phonon modes of the lattice. The band at 564 cmÀ1
may be the
first overtone of 287 cmÀ1
. The observed intensity changes in the A1
and E modes may arise from internal changes in the dipole
moments in the hexagonal lattice consisting of Cu1+
and Fe3+
ions.
Fig. 2(c) presents the UV-Visible absorption spectrum of the synthe-
sized WZ–CuFeS2, dispersed in n-hexane. CuFeS2 is known to show a
ligand to metal charge transfer spectrum due to the presence of
empty d orbitals in Fe3+
. Prominent absorption occurring in the
range of 500–780 nm confirms the charge transfer in this system.12
The optical band gap of WZ–CuFeS2 has been estimated to be 0.7 eV
(inset in Fig. 2(c)). A strong emission centered at 750 nm is observed
in the photoluminescence (PL) spectrum upon exciting the sample
at l = 500 nm (Fig. 2(d)). Although this kind of emission has been
attributed to the luminescence from donor to acceptor defect levels
in WZ–CuInS2 and WZ–CuGaS2,13
intraband optical transitions from
Cu1+
present in CuFeS2 cannot be ruled out.11
Upon reacting FeCl3 with the same Cu–tu precursor, for-
mation of tetragonal CuFeS2 (JCPDS File no. 83-0983) is evident
from its PXRD pattern (Fig. 3(a)). Lattice dimensions obtained
from the Rietveld refinement in space group I%42d (122) are
a = 5.386 (5) Å and c = 10.391 (1) Å (Fig. S5 and Tables S3 and S4,
ESI†). The average crystallite size has been estimated to be
10 nm by the Scherrer analysis. Similar to the wurtzite phase, a
flower-like morphology comprising of plate shaped petals has
been noticed for CH–CuFeS2 in its FESEM images (Fig. 3(b)).
EDX analysis confirmed the presence of Cu, Fe and S yielding a
ratio of 1.25 : 1.08 : 1.70 (Fig. S6, ESI†). Rietveld refinements of
CuFeS2 in WZ and CH structures have been compared in Fig. S7
(ESI†). The SAED-TEM pattern of this sample (shown in the
inset of Fig. 3(b) and Fig. S8, ESI†) endorses the tetragonal
symmetry by the presence of spots corresponding to (112) and
(220) planes. The room temperature Raman spectrum presented
in Fig. 3(c) also confirms the chalcopyrite structure exhibiting
bands at 287, 351 and 470 cmÀ1
corresponding to the A1, B2 and
E phonon modes respectively.11
The UV-Visible spectrum of the
CH–CuFeS2 sample is shown in Fig. 3(d) and the calculated
band gap of 0.6 eV (inset in Fig. 3(d)) is quite comparable to the
reported range of 0.53–0.7 eV.12
The observed band gap of
the CH–CuFeS2 is marginally smaller than that of its WZ
counterpart and such trends have been reported earlier for
ternary and quaternary sulfides based on ZnS structure.14,15
The temperature dependent weight loss pattern of WZ–CuFeS2
differs slightly from that of CH–CuFeS2 as shown by their
thermograms in a nitrogen atmosphere (Fig. S9, ESI†). A PL
emission spectrum similar to the one observed for WZ–CuFeS2
has been obtained for CH–CuFeS2 samples upon excitation at
l = 500 nm (Fig. S10, ESI†).
Solution based synthesis of CH–CuFeS2 was first reported by
Roberts16
who reacted CuS and FeS precipitates under mild
conditions. Earlier studies on the solution based synthesis of
I–III–VI2 including CuFeS2 have been centred mainly on the
generation of different shapes and sizes of the ternary sulfides
in which the change in metal ion precursors in a single solvent
Fig. 1 (a) Elemental distribution of S, Fe and Cu in the samples, (b) Rietveld fit of
the powder X-ray diffraction (PXRD) pattern (observed, calculated (profile matching),
and difference profiles are given as red, blue, and green lines, respectively) of the
product obtained from the reaction of [Cu4(tu)9](NO3)4Á4H2O with Fe2(SO4)3ÁxH2O in
ethylene glycol for 1.5 h under refluxing conditions, (c) structure of wurtzite form of
CuFeS2, and (d) SAED pattern of wurtzite CuFeS2.
Fig. 2 (a) FE-SEM images, (b) room temperature Raman spectrum, (c) UV-Visible
absorption spectrum with the inset of the energy (eV) vs. (Ahn)2
plot of the
optical absorption data, and (d) photoluminescence spectrum of WZ–CuFeS2
obtained by exciting at l = 500 nm.
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3. 7318 Chem. Commun., 2013, 49, 7316--7318 This journal is c The Royal Society of Chemistry 2013
system did not result in any symmetry variation of the final
product.17
Our study is the first report in which the change in
symmetry of the ternary sulfide has been achieved under
identical conditions just by changing the Fe3+
salt using a
single solvent system, viz., ethylene glycol. It is believed that
the less coordinating ability of ethylene glycol promotes the
homogenous reaction between the reactants effectively paving
the way for the rapid inclusion of Fe3+
in the Cu–S lattice.
Among the plausible mechanisms responsible for the formation
of CuFeS2 in hexagonal and tetrahedral symmetries, simultaneous
nucleation of hexagonal CuxS and hexagonal FexS crystals and their
interdiffusion may occur during the dissociation of the Cu–tu
precursor in the presence of Fe2(SO4)3. Such a proposition would
require minimum energy for the rapid lattice formation due to
symmetry match. Along the similar lines, reaction of tetragonal
CuxS nuclei with tetragonal FexS nuclei resulting in tetragonal
CuFeS2 might proceed when FeCl3 is employed. It is relevant to
point out that incorporation of 0.05% and 5% Fe3+
has stabilized
the monoclinic and tetragonal symmetries of Cu2S.18
Nevertheless
the dramatic change in the symmetry of CuFeS2 by just changing the
anion of the iron salt justifies the need to understand the intriguing
chemistry in these systems.
In summary, a simple non-injection synthetic approach to
generate CuFeS2 in WZ and CH structures by reacting different
Fe3+
salts with the same Cu–tu precursor in ethylene glycol has
been developed. This process is viable and scalable as quantitative
yields (average yield of 80%) of nano sized powders that are readily
redispersable in non-polar solvents are obtained. The discovery of a
new phase (WZ–CuFeS2) will be of immediate significance to tune
the optoelectronic and magnetic properties.
The authors thank DST (Nanomission) and DST (SR/S1/PC-
07/2011(G)), New Delhi, Govt of India, for the financial support
to carry out this work.
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Fig. 3 (a) Rietveld fit of the PXRD pattern along with the JCPDS file of CH–CuFeS2
(observed, calculated (profile matching), and difference profiles are given as red,
blue, and green lines, respectively), (b) FE-SEM image with the SAED pattern in the
inset, (c) room temperature Raman spectrum, (d) UV-Visible absorption spectrum
of the product obtained from the reaction of [Cu4(tu)9](NO3)4 with FeCl3 in
ethylene glycol for 1.5 h under refluxing conditions. The inset in (d) shows the
plot of energy (eV) vs. (Ahn)2
of the optical absorption data.
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