Prashant's 8th article

Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 269–275
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of zincblende CuInS2 and Fe-substituted CuInS2 by the
reaction of binary colloids
Meenakshi Gusain, Prashant Kumar, Sitharaman Uma, Rajamani Nagarajan∗
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110 007, India
h i g h l i g h t s
• Binary colloids of Cu–S and In–S
in ethyleneglycol reacted to yield
CuInS2.
• CuInS2 exhibited zincblende arrange-
ment.
• Following this success, higher con-
centrations of iron substituted for
indium.
• This soft chemical process is simple,
robust and rapid.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 18 March 2015
Received in revised form 25 May 2015
Accepted 27 May 2015
Available online 1 June 2015
Keywords:
Copper indium sulphide
Colloids
Reactivity
Polymorph
TEM
Electron microscopy
SAED pattern
a b s t r a c t
Colloidal suspension of binary sulﬁdes of copper and indium were generated by the reaction of CuCl and
In2(SO4)3 with thiourea independently in ethyleneglycol. The colloids were identiﬁed to be Cu9S5 and
a mixture of InS and In2S3 from the TEM-SAED measurements. They were reacted to yield phase pure
CuInS2 in zincblende arrangement as endorsed by the powder X-ray diffraction pattern, TEM-SAED, lattice
fringe measurements and Raman spectroscopy measurements. Optical band gap of zincblende CuInS2
was measured using UV–visible spectroscopy. In the photoluminescence spectrum, a strong emission
centred at around 750 nm was observed on exciting the sample with = 500 nm. Following this success,
quaternary composition containing Cu, In, Fe and S in zincblende arrangement has been synthesized and
characterized. Band gap of iron substituted samples was estimated to be 1.07 eV. It showed paramagnetic
behaviour at room temperature suggesting its use as opto-magnetic material. While cubic CIS degraded
methylene blue dye solution under visible light radiation, iron substituted sample did not promote this
reaction.
© 2015 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +91-11-27662650.
E-mail address: rnagarajan@chemistry.du.ac.in (R. Nagarajan).
1. Introduction
Owing to the rich heritage as ores, tuneable semiconducting
property and structural diversity, copper containing I–III–VI2 type
compounds have always been a focus of immense interest among
http://dx.doi.org/10.1016/j.colsurfa.2015.05.027
0927-7757/© 2015 Elsevier B.V. All rights reserved.

270 M. Gusain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 269–275
researchers from various disciplines. Metastable phases of these
compounds are more often found to exhibit interesting and unusual
properties as compared to their thermodynamically stable ana-
logues. Designing synthetic strategies for the stabilization of a
metastable phase of a compound consisting of metal ions belonging
to different groups of the periodic table are quite challenging and
necessitate critical reaction controls. Primary limitations include
the formation of thermodynamically stable phases and/or the gen-
eration of secondary impurity phases [1–6]. Choice of reaction
conditions becomes much more rigid if the compound exhibits
polymorphic modifications differing in their stability by a nar-
row window of energy as in the case of CuInS2 (CIS), a widely
investigated member of I–III–VI2 type compounds. Three different
polymorphs are known for CIS, viz., chalcopyrite (CH), zinc blende
(ZB) and wurtzite (WZ) [7]. Following the Grimm–Sommerfeld rule
[8], the constituent ions of CuInS2 exhibit tetrahedral coordination,
where in well ordered arrangement of cations in cubic close packed
(ccp) sulfide lattice results in CH arrangement, while their statistical
distribution yields ZB structure. Though both CH and ZB struc-
tures possess an identical ccp network, and are energetically almost
equivalent, the CH arrangement is thermodynamically stable, while
ZB is a kinetically stable structure [9]. Wurtzite, another metastable
arrangement, consists of a statistical distribution of cations in a
hexagonal close packed (hcp) anionic sub lattice. WZ also differs
slightly in energetics from ZB. These narrowly spaced polymorphs
provided challenges and opportunities for researchers to explore
synthetic strategies to stabilize them by solution based synthetic
routes. Compared to WZ and CH arrangement, reports describing
the synthesis of metastable ZB-CIS are quite limited [10–20]. The
foremost difficulty, encountered in the solution phase synthesis
of CIS, was the differing acid and basic character of Cu (I) and In
(III) towards the sulfide ligand (Cu (I) = soft acid, In (III) = hard acid,
S2− = soft ligand), thus promoting competitive reaction between
them. This has been balanced by the use of coordinating solvents or
a mixture of them [10–21]. A majority of earlier reports emphasized
the use of amine solvent system for the synthesis of ZB structure
of CIS, probably due to easy formation of reactive polyammonium
sulfide as well as the strong coordinating ability of amines [10–21].
In our continuing efforts to explore synthetic strategies for the
generation of stable and metastable binary/ternary copper contain-
ing sulfides in a single and less coordinating ethyleneglycol solvent
(essentially amine free) system [22–27], synthesis of ZB form of
CIS has been examined by exploiting the reactivity of nanocrystals
of binary sulfides. It is noteworthy that glycol medium is ide-
ally suited for nano fluidic studies [28,29]. In this communication,
results from the reactions of freshly generated colloidal suspen-
sions of binary sulfides of copper and indium in ethyleneglycol are
described. Success from this approach was expanded to include
fourth element, viz., iron in CIS in zinc blende arrangement. The
obtained phases were characterized by high resolution powder
X-ray diffraction (PXRD), scanning and transmission microscopy,
UV–vis, Raman, photoluminescence (PL) spectroscopy and mag-
netic measurements. While CIS degraded methylene blue (MB)
solution under visible radiation, iron substitution sample did not
show appreciable degradation of the dye.
2. Material and methods
2.1. Synthetic procedure
Colloidal suspension of copper sulfide species was generated by
refluxing 0.098 g (1 mmol) of CuCl with 0.228 g (3 mmol) of thiourea
in 50 mL of ethyleneglycol (Merck, 99%) at 197 ◦C for 1.5 h. Inde-
pendently, a colloidal suspension of indium sulfide was produced
by refluxing 0.258 g (0.5 mmol) In2(SO4)3.xH2O (Alfa aesar, 99.99%)
with 0.228 g (3 mmol) thiourea in 50 mL of ethyleneglycol for 1.5 h
at 197 ◦C. Both the suspensions were mixed under flowing nitro-
gen atmosphere using a Schlenk line and refluxed further for 1.5 h at
197 ◦C. The product was separated by vacuum filtration and washed
with double distilled water, absolute alcohol and CS2. 0.169 g was
the weight of the final product from these reactions.
For the synthesis of CIFS, colloidal suspension of copper sulfide
was generated by refluxing 0.098 g (1 mmol) of CuCl with 0.228 g
(3 mmol) of thiourea in ethyleneglycol under refluxing conditions
for 1.5 h. Colloidal suspensions of indium sulfides and iron sulfides
were prepared independently by refluxing 0.259 g (0.5 mmol) of
In2(SO4)3·xH2O with 0.114 g (1.5 mmol) of thiourea and 0.163 g
(1 mmol) of FeCl3 with 0.152 g (2 mmol) of thiourea in 50 mL of
ethyleneglycol at 197 ◦C independently in separate round bottom
flasks for 1.5 h. The colloidal suspensions were then mixed under
inert atmosphere and refluxed further for 1.5 h. Separation of the
final product was carried out by vacuum filtration and the sample
was air dried. 0.153 g of the final product was obtained.
2.2. Characterization details
Powder X-ray diffraction (PXRD) patterns were collected using
PANalytical’s Empyrean diffractometer, equipped with PIXcel3D
detector, employing Cu K␣ radiation ( = 1.5418 ˚A) with scan step
size of 0.01313◦ and 63.495 s/step. UV–visible absorption spectra
of the samples were recorded by dispersing them in n-hexane and
using a Thermo Scientific UV-visible spectrophotometer (Model
Evolution 300). Raman spectra of the samples, in compact form,
were collected using a Renishaw spectrophotometer equipped with
a microscope having a laser with wavelength of 785 nm. Trans-
mission Electron Microscopy (TEM) and Selected Area Electron
Diffraction (SAED) were carried out on an FEI Technai G2 30 electron
microscope operating at 300 kV. The morphology and composi-
tion of the final products was observed using scanning electron
microscopy (SEM) using a Zeiss EVO 50 microscope and FE-SEM
Quanta 200 FEG microscope equipped with EDS detector. PL mea-
surements were performed on solid samples using Horiba Jobin
Yvon Fluorolog 3 Spectrofluorometer at room temperature. Mag-
netic measurements were carried out at 300 K using a Vibrating
Sample Magnetometer (Micro sense EV9). Photo degradation of
methylene blue (MB) dye solution was carried out under visi-
ble irradiation using a 450 W xenon arc lamp (Oriel, Newport,
USA) along with a water filter to cut down IR radiation and glass
cut off filter (Melles Griot-03FCG057) to permit only visible light
(400 nm ≤ ≤ 800 nm) radiation using the set-up described earlier.
A mixture of 0.025 g of catalyst and 100 mL of the dye solution (pre-
pared using double distilled water) was loaded into a reactor and
stirred in the dark at room temperature for 30 min to allow the
complete equilibration of the adsorption/desorption of substrate on
the catalyst. 5 mL aliquots from the reaction, at various time inter-
vals, were sampled and the UV–visible absorption spectra of the
supernatant solutions were recorded after centrifuging the catalyst.
3. Results and discussion
An attempt has been made to identify the binary sulfide species
generated in colloidal form with the aid of PXRD and TEM measure-
ments. PXRD patterns of black coloured and dirty white coloured
products from the reaction of CuCl with thiourea and In2(SO4)3 with
thiourea are reproduced in Fig. 1. Both the patterns were noisy,
suggesting the poor crystalline nature of them. Although the pat-
terns were noisy, reflections at 2Â positions close to Cu9S5 could
be located for the species formed from the reaction of CuCl with
thiourea. Such a distinct resemblance with the PXRD pattern of
either InS or In2S3 was not perceived for the product from the
reaction of In2(SO4)3 with thiourea. Taking cue from these results,
SAED pattern of these two colloids were obtained from HR-TEM

M. Gusain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 269–275 271
Fig. 1. PXRD pattern of the product from the refluxing reaction of (a) In2(SO4)3
with thiourea in ethyleneglycol, (b) CuCl with thiourea in ethyleneglycol and (c) the
product obtained by mixing colloidal suspensions containing In–S and Cu–S species.
Along with digital photograph of In–S and Cu–S species.
experiments (Fig. 2). Both the SAED patterns showed bright spots
indicative of crystallinity at the microscopy level. While the SAED
pattern of Cu–S species could be indexed to [1 0 1], [0 0 1 5], [1 0 7],
[0 0 1 4], [1 1 0], [0 0 2 7], [1 1 1 5] planes of Cu9S5 (JCPDS file no 47-
1748), presence of [1 1 3], [1 1 2] and [2 2 0]/[0 0 1 2], [1 0 9]/[2 1 3],
[0 1 1] planes due to InS (orthorhombic, JCPDS file no 86-0639) and
In2S3 (tetragonal, JCPDS file no 73-1366) were identifiable in the
SAED pattern of the species generated in the case of indium.
The product obtained from the reaction of these two colloids
showed sharp reflections in its PXRD pattern (shown in Fig. 1(c)).
The observed peak positions and the intensity profile matched very
closely with the PXRD pattern reported for the ZB-CIS in the liter-
ature [16]. All the reflections could satisfactorily be indexed in a
cubic unit cell with a = 5.540 (19) ˚A. The Rietveld refinement of the
PXRD pattern was therefore carried out by TOPAS software in F-
43m space group [30]. As the difference between the experimental
and the theoretically simulated data was minimum, the product
was confirmed to possess zinc blende structure (Fig. 3(a), Table S1
and S2). Elemental mapping using EDX technique also confirmed
the presence of Cu, In and S in the ratio of 1:1:2 (Inset of Fig. 3(a)).
Well defined spots, were observed in the TEM-SAED pattern of the
sample. They were indexed to [0 0 2], [0 2 2] and [3 1 1] hkl planes
of CIS (Fig. 3(b)). An HR-TEM result shows the lattice spacing of
3.21 ˚A and 2.77 ˚A corresponding to the [1 1 1] and [2 0 0] h k l planes
(Fig. 3(c)). Hexagonal morphology of the CIS was quite evident in
both the FESEM and TEM images (Fig. 3(d)).
Vibration modes at 255, 295, 307 and 340 cm−1 were located in
the Raman spectrum of CIS (Fig. 4(a)). Band located at 295 cm−1
originated from the symmetric vibration of the sulfur sublat-
tice (A1 mode). Other bands at 255 cm−1, and 340 cm−1 were
assigned to E1
LO/B1
2LO and E3
LO modes, respectively. The band at
307 cm−1 appeared at the higher side of the CH A1 mode and was
attributed to the A1 mode of CuAu structural arrangement usually
observed in CIS preparations [31–33]. Closer examination of the
spectrum revealed critical differences in the positions and inten-
sities observed for ZB and WZ [26]. Strong absorption, over the
entire range of visible light with absorption edge near 800 nm was
exhibited by CIS in the UV–visible absorption spectrum (Fig. 4(b)).
The direct band gap estimated by extrapolation of the straight line
of (˛h )2 versus h plot was 1.28 eV (Fig. 4(c)). When the sample
was excited with = 500 nm, a strong emission centred at around
750 nm, arising probably the defect donor-acceptor levels, was
observed (Fig. 4(d)) [26].
The applicability of this approach to introduce the fourth ele-
ment in CIS, especially from the d-block of the periodic table, was
examined. PXRD pattern of the sample from the reaction of the
freshly generated Cu–S, In–S and Fe–S species in ethyleneglycol
is shown in Fig. 5(a). From the successful Rietveld refinement fit
(a = 5.510 (36) ˚A) of the PXRD pattern of the product in F-43m space
group, its ZB structure was confirmed (Fig. 5(a) and Table S3, S4).
SAED pattern of the sample also supported the PXRD results in
which the [0 0 2], [0 2 2], [3 1 1] planes were noticed (Fig. 5(b)). EDX
analysis revealed the presence of copper, indium, iron and sulfur
in the ratio of 1:0.6:0.4:1.8 (Fig. S1). A marginal decrease in the
cubic lattice constant for Fe-substituted samples from CIS could be
justified from the ionic sizes of Cu+ (0.46 ˚A), In3+ (0.62 ˚A) and Fe3+
(0.49 ˚A, high spin) in fourfold coordination as well from the ran-
dom occupation of these ions within the available crystallographic
sites in the ZB structure [34,35]. As the ZB structure is a cation dis-
ordered polymorph of I–III–VI2, increased concentrations of iron
can be substituted for indium. To the best of our knowledge, this
is the highest concentration of iron incorporated in the CIS lattice.
TEM and SEM images of the iron substituted samples are presented
in Fig. 5(c) and (d), respectively. Preservation of hexagonal mor-
phology as observed in CIS occurred in iron substituted samples as
well.
The deconvoluted Raman spectrum of CIFS is presented in
Fig. 6(a) in which four bands at 256, 286, 297, and 334 cm−1 are
evident. As compared to CIS, the bands at 307 and 340 cm−1 were
antistoke shifted with reduced intensity. Also, the change in the
location of the band at 297 cm−1 confirmed the change in vibration
mode caused by the introduction of iron in the CIS lattice [31–33].
Band gap of CIFS from the UV–visible spectroscopy measurements
Fig. 2. SAED pattern of the product from the refluxing reaction of (a) CuCl with thiourea in ethyleneglycol (h k l planes representing Cu9S5 (hexagonal)) and (b) In2(SO4)3
with thiourea in ethyleneglycol ([*] denotes h k l planes from InS (orthorhombic) and others represent the spots due to In2S3 (tetragonal)).

Fig. 3. (a) Rietveld reﬁnement of the PXRD pattern (b) SAED pattern (c) HR-TEM and (d) SEM and TEM image of CIS. EDX analysis of CIS sample is provided as inset in (a).
Fig. 4. (a) Raman spectrum (b) UV–vis absorption spectrum (c) Tauc plot and (d) Photoluminescence emission spectrum of CIS at = 500 nm.

Fig. 5. (a) Rietveld refinement of the PXRD pattern (b) SAED pattern (c) TEM image (d) SEM image of CIFS.
was 1.07 eV, lower than CIS (Fig. 6(b) and its inset). Incorporation of
iron in CIS lattice resulted in higher concentration of defect states as
evident from the increased intensity of the emission band in the PL
spectrum of the sample (Fig. 6(c)). The paramagnetic character of
the sample exhibited by the magnetization measurements at room
temperature confirmed the introduction of magnetically active iron
species in CIS lattice with g = 5.642 × 10−6 emu/g (Fig. 6(d)). While
in the case of Fe doped Si QDs, the magnetic moment of iron is
quenched, the paramagnetic character of iron has been preserved
in the present case [36].
Generally, it is believed that this metastable phase can only be
realized using high boiling solvents (boiling point above 300 ◦C)
[10–21]. However, in the present case, use of relatively low boil-
ing ethyleneglycol has yielded this high temperature polymorph. It
may be due to many reasons. Firstly, high viscosity of ethylene-
glycol might have promoted the intermixing of binary colloids.
Generation of defect spinel, In2S3 and Cu9S5 having many vacant
cation positions could be providing symmetry compatibility and
interdiffusability to produce cubic CIS. The colloids of individual
binary lattices possess high surface energy making them reactive
species.
The following mechanism may be operative leading to the for-
mation of ZB-CIS
CuCl + thiourea Cu9S5 (Cu1.8S)
Ethyleneglycol
In2(SO4)3 + thiourea In2S3
Ethyleneglycol
Cu1.8S + In2S3 Cu1.8In2S4 ≈ CuInS2
Ethyleneglycol
Disordered cubic zincblende structure might be the result of
non-equilibrium conditions followed in terms of generating inde-
pendently the binary sulfide colloids under hot conditions and
mixing them. While using amines, size controlled nucleations are
possible, but this kind of non equilibrium (independent mixing)
synthetic condition/protocol if followed in amine solvent systems
may lead to phase segregation due to the difference in reactivity of
the metal ions.
To demonstrate the potential applicability of CIS as photocata-
lyst [37,38], degradation of Methylene blue in aqueous solution has
been performed under visible radiation (400 nm ≤ ≤ 800 nm). The
photocatalytic degradation of MB dye by CIS was evaluated by mon-
itoring the successive decrease in absorption intensity of MB dye as
a function of exposure time in the presence of catalyst. Fig. 7 shows
temporal changes in the concentration of MB (Initial concentration:
1.0 × 10−5 M, 100 mL) measured as maximal absorption in UV–vis
spectra during the course of experiments in the presence of 0.025 g
of catalyst. The absorbance maxima at 664 nm decreased period-
ically upon increasing irradiation times. The plot of C/Co versus
irradiation time (inset of Fig. 7) showed that 90% of degradation
occurred within 80 min. CIFS did not promote the degradation of
MB dye molecules under similar experimental conditions.

Fig. 6. (a) Raman spectrum (b) UV–vis absorption spectrum (c) Photoluminescence emission spectrum of CIFS in comparison to CIS ( exc = 500 nm) and (d) Plot of magnetization
versus applied magnetic field of CIFS at room temperature. Tauc plot of CIFS is shown in the inset of (b).
Fig. 7. Temporal changes in absorbance spectra of 10 ␮M methylene blue (MB) dye
solution in the presence of CIS under visible irradiation. Inset shows the concentra-
tion of the dye solution in the dark and under visible irradiation in the presence of
CIS.
4. Conclusion
To summarize, metastable zincblende polymorphic modifica-
tion of CuInS2 in pure form was synthesized in ethyleneglycol,
without using air, moisture sensitive precursors, longer durations
of reactions and higher dissociation temperatures. As the process
produce quantitative yields (up to 70%), it can facilitate the study of
hitherto unexplored properties and rigorous relationships between
the structure and property of this system. These results also validate
the hypothetical mechanism proposed earlier for the formation
of copper containing ternary sulfides by solution phase synthesis
[9,16]. Following this methodology, higher amounts of iron could
be substituted for indium retaining the zincblende arrangement.
While CuInS2 showed effective degradation of non-biodegradable
methylene blue dye solution, iron substitution for indium did not
promote the degradation. A new scientific philosophy can be envis-
aged in which this entire system can mimic the solid state reactions
in solution where the diffusion of the metal ions is greatly facilitated
by their smaller size and the viscosity of the solvent.
Acknowledgements
Authors wish to record their sincere thanks to DST (Nanomis-
sion) and DST (SB/S1/PC-08/2012) for funding this research. MG and
PK thanks CSIR, New Delhi and DST for their fellowship. Thanks are
due to University of Delhi, Delhi for the usage of facilities of USIC,
M.Tech (NSNT) programme.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.colsurfa.2015.05.
027
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