An elegant room temperature procedure for the precise control of composition in the Cu-S system

Article abstract of DOI:10.1021/ic201133a

A general methodology for the precise control of the final product composition in the copper sulfide system has been established for the first time. Cu₂S, prepared by the dissociation of [Cu(tu)₃]Cl in ethylenediamine, was subjected to controlled chemical oxidation by iodine to obtain Cu_2-x (x = 0.2, 0.25, 0.88, 1.0) at room temperature. The compositions were characterized by powder X-ray diffraction, ICP spectroscopy, Raman spectroscopy, and magnetization measurements.

Full text of DOI:10.1021/ic201133a

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pubs.acs.org/IC
An Elegant Room Temperature Procedure for the Precise Control of
Composition in the CuꢀS System
Prashant Kumar and R. Nagarajan*
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India
S Supporting Information
b
system are the major strategies followed in the solution-based
synthesis to achieve the various compositions.14ꢀ18 Also, the use
ABSTRACT: A general methodology for the precise con-
trol of the final product composition in the copper sulfide
system has been established for the first time. Cu2S,
prepared by the dissociation of [Cu(tu)3]Cl in ethylenedia-
mine, was subjected to controlled chemical oxidation
by iodine to obtain Cu2ꢀx (x = 0.2, 0.25, 0.88, 1.0) at room
temperature. The compositions were characterized by
powder X-ray diffraction, ICP spectroscopy, Raman spec-
troscopy, and magnetization measurements.
of stabilizing agents and reducing agents to form a selective set of
copper sulfides is known.19,20 For example, copper(I) thiobenzoate is
decomposed in the presence of dodecane, trioctyl phosphine, and
tributyl phosphine as stabilizing agents to yield Cu1.75S and Cu2S.19
Jiang et al.20 synthesized Cu9S8 first and transformed it to CuS and
Cu7S4 (Cu1.75S) with the aid of KBH4 and SnCl4, respectively. It is
important to note that these were achieved under solvothermal
conditions in an ammoniacal medium at 60 °C.
One of the ideal approaches to achieving the various composi-
tions in the copper sulfides is to obtain the end member of the
copper-rich sulfide, Cu2S first, followed by the systematic
introduction of holes through the controlled removal of copper
from this system. Since all of the copper ions are present in the +I
oxidation state in Cu2S, either the oxidation of copper ions or
their removal must preferably be carried out at room temperature
without involving high temperatures, wherein the control will not
be very precise. Additionally, the use of higher temperatures may
introduce contaminants such as oxides at the grain boundaries.
Our recent research on the dissociation of the single source
he research interest in the copperꢀsulfur system originated
T
with mineralogists and geologists and expanded to chemists,
physicists, and material scientists due to the discovery of useful
technological applications.1 Various interesting properties ex-
hibited by the copper sulfides, such as semiconductivity, electrodes
in solar cells, fast ionic conductivity, thermo- and photoelectric
transformers, high temperature thermistors, and super-
conductivity, fueled further interest in understanding the struc-
tureꢀproperty correlation in this structurally complex CuꢀS
sytem.2ꢀ6 Copper sulfides exist in a wide range of stable and
metastable compositions that include Cu2S, Cu31S16 (Cu1.96S),
Cu9S5 (Cu1.8S), Cu7S4 (Cu1.75S), Cu9S8 (Cu1.12S), CuS, and
CuS2.7,8 It has been a great challenge for crystallographers to
understand the structures of Cu2ꢀxS phases due to the positions
of the copper atoms, within the close packed sublattice of S
atoms, which are not well-defined. They depend on the tem-
perature, and at high temperatures they are unusually mobile,
thus making these compositions partially ionic conductors. Due
to this special status, crystallographers and theorists, considered-
ing statistical distribution of copper atoms, with Cu2S to be the
ideal starting crystal structural model for obtaining other
compositions.9 Stoichiometric Cu2S exhibits a monoclinic sym-
metry below 103.5 ( 1 °C and hexagonal symmetry above that
temperature. The hexagonal phase is stable up to 436 ( 10 °C,
above which it embraces the cubic symmetry. The complex phase
diagram of the copperꢀsulfur system is dominated by Cu2ꢀxS at
higher temperatures and a broad homogeneity range containing
phases Cu1.75S, Cu1.95S, and monoclinic and hexagonal forms of
Cu2S.10 The phase relations are complicated due to the tendency
to form metastable phases.
precursors [Cu(tu)3]Cl, [Cu4(tu)9](NO3)4 4H2O, and [Cu2-
3
(tu)6]SO4 4H2O in ethylene glycol solvent, showed conclu-
3
sively that the final product stoichiometry was greatly influenced
by the anions of the starting precursor.21 Also, the use of other
copper(I)-containing single source precursors for the easy
synthesis of Cu2S described in the literature motivated us to choose
[Cu(tu)3]Cl for the present study.22ꢀ24 The greater reducing power
of ethylenediamine (as compared to ethylene glycol) and its wide-
spread use for the successful synthesis of copper sulfides, even at
room temperature from their elements, prompted us to select
ethylenediamine to be the solvent medium for preparing Cu2S.25
It was resolved to apply room temperature for the synthesis due
to the stringent requirement to make stoichiometric Cu2S.
The synthesis and characterization results of the precursor
[Cu(tu)3]Cl26 are provided in the Supporting Information. The
addition of ethylenediamine to the solid precursor results
immediately in a black colored product with the release of heat
as indicated by the temperature of the reaction flask. Vigorous
stirring, over a magnetic plate, was continued for 12 h, and the
product is obtained by filtration under vacuum and dried over a
desiccant. The powder X-ray diffraction pattern of the reaction
product is shown in Figure 1a. As Cu2S is known to exist in
monoclinic, hexagonal, and cubic symmetries, the positions of
the reflections observed in the powder X-ray diffraction pattern
Despite the existence of numerous generalized synthetic
methods available for the fabrication of metal chalcogenides in
different sizes and shapes,11ꢀ13 enough understanding toward
achieving the composition control of the final products is lacking.
This is of the utmost importance in the case of copper sulfides for
the reasons mentioned above. Variation of the copper to sulfur
ratio, tuning the pH of the medium, and use of a mixed solvent
Received: May 27, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
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reflections. Generally, noisy powder X-ray diffraction patterns for
Cu2S are reported in the literature.27,28 The reflections in the 2θ
range of 20ꢀ40° are well-defined in the product after the solvother-
mal treatment at 130 °C for 12 h. The LeBail fitting procedure of the
powder X-ray diffraction pattern (Figure 1b) converged well in
monoclinic symmetry with lattice constants of a = 15.131(2), b =
11.843(1), and c = 13.502(2) Å and β = 115.90 (6)°. The details of
the fitting procedure are provided in the Supporting Information. Also,
the Cu/S ratio, as analyzed by the inductively coupled plasma (ICP)
spectroscopy, came out to be 2:1.04.
It is important to recognize that copper sulfide formation is
extremely difficult, with CuCl and thiourea as the reactants, in an
ethylenediamine medium, due to the formation of a quite stable
complex between CuCl and ethylenediamine. This complex has
been observed to undergo dissociation under hydrothermal
conditions at 170 °C.29 In our study, this situation does not
arise, since a single source precursor, in which the copper to
sulfur bond existed before the dissociation, is used. This facili-
tated the copper sulfide formation at room temperature itself.
The formation of crystalline Cu2S, by the dissociation of the
precursor, is believed to be assisted by the mechanical energy
supplied in the form of vigorous stirring. This explanation is
further supported by the fact that mechanochemical syntheses,
from the elements, exist as an alternate synthetic strategy to make
crystalline metal chalcogenides due to their soft character.30 The
reaction between the precursor and ethylenediamine is repre-
sented as follows:
Figure 1. Powder X-ray diffraction pattern of the product obtained after the
dissociation of the complex [Cu(tu)3]Cl in ethylenediamine (a) at room
temperature and (b) under solvothermal conditions at 130 °C for 12 h. The
line representations of JCPDS file no. 83-1462 for the monoclinic Cu2S are
shown for easy identification. LeBail fitting with the difference pattern is
shown for the Cu2S obtained under solvothermal conditions.
The standard chemical oxidation procedure using the iodine
solutions in acetonitrile has been applied to control the composition
of the final product.31 A total of 0.0710 g (0.2 mol of I) of freshly
sublimed iodine in acetonitrile is added to 0.2 g of highly crystalline
Cu2S, and the reaction is carried out under constant stirring in
flowing argon. A slight excess of iodine is used, as the reaction is
performed for 24ꢀ48 h. Highly colored iodine becomes colorless
after the reaction. The product, after the reaction, is Cu1.8S, as
evidenced from its powder X-ray diffraction pattern (Figure 2a) in
which the positions and intensities of the peaks match closely with
the hexagonal Cu9S5 (JCPDS file no. 47-1748). When 0.2 g of Cu2S
was treated with 0.080 g (0.25 mol of I) for 48 h, orthorhombic
anilite, Cu7S4, is obtained, as validated by its powder X-ray diffraction
pattern (Figure 2b). Along similar lines, Cu9S8 and CuS are obtained
after reacting Cu2S with 0.3124 g (0.88 mol of I) and 0.3550 g (1 mol
of I) of iodine in acetonitrile for 48 h respectively (Figure 2c,d). The
oxidation reaction of Cu2S with iodine in acetonitrile can be
expressed by the following equation:
Figure 2. Powder X-ray diffraction pattern of (a) Cu9S5, (b) Cu7S4, (c)
Cu9S8, and (d) CuS obtained from the reaction of Cu2S with 0.20, 0.25, 0.88,
and 1 mol of iodine solution in acetonitrile for 2 days at 25 °C.
Cu2S þ x=2I2 f Cu2ꢀxS þ xCuI
CuI, formed in the reaction, gets dissolved in acetonitrile and
repeated washing is done to make sure it is not present with the
copper sulfide.
The Raman spectrum of the prepared copper sulfides en-
dorsed the results from the powder X-ray diffraction studies,
showing the shifting of the bands to higher wave numbers, as one
moves from Cu2S to CuS, confirming earlier reports (Figure 3).32
matched closely with the monoclinic Cu2S (see JCPDS file no.
83-1462). But, the intensities of the reflections were not match-
ing, as reported in the JCPDS file, probably due to the partially
disordered nature of the Cu2S obtained by this procedure. As the
monoclinic is a lower symmetry, compared to hexagonal, the
presence of these two might be masking the intensities of the
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’ ACKNOWLEDGMENT
The authors thank DST (Nanomission) Govt of India for
funding this research.
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Figure 3. Raman spectra of Cu2S, Cu9S5, Cu9S8, and CuS.
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Supporting Information. Magnetization measurements
b
at room temperature of the prepared copper sulfides and the
LeBail fitting details of the powder X-ray diffraction pattern of the
monoclinic Cu2S. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: rnagarajan@chemistry.du.ac.in.
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Notes
Scope of the reactionThough this study has been concentrated to
copper sulfides, a similar approach can effectively be applied for
composition control in other metal sulfides.
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