Morphological control in solvothermal synthesis of copper sulphides on copper foil

Article abstract of DOI:10.1016/j.materresbull.2006.05.030

Copper sulphides Cu₉S₅ and polymorphs of Cu₂S were synthesized apparently for the first time on copper foil through its solvothermal reaction with sulphur (the former being used both as substrate and reactant), with 75% fill of the autoclave with water, ethanol or ethylenediamine as solvents. Water as solvent produced micron-sized hemispheres of Cu₉S₅ (composed of much smaller particles of size ～5-6 nm), truncated by forming interfaces and exhibiting relatively large dihedral angles (80-120°). Ethanol produced fibre-like or thread-like (～40 nm diameter) morphological form of Cu₉S₅, generally woven into network-structures. Ethylenediamine, which produced Cu₂S (monoclinic), as also β-Cu₂S (hexagonal), assisted formation of hexagonal micron-sized single crystal rods with a hexagonal end face in conjunction with β-Cu₂S.

Full text of DOI:10.1016/j.materresbull.2006.05.030

Materials Research Bulletin 42 (2007) 345–353
www.elsevier.com/locate/matresbu
Morphological control in solvothermal synthesis of
copper sulphides on copper foil
*
S. Gorai, D. Ganguli, S. Chaudhuri
Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India
Received 6 December 2005; received in revised form 17 May 2006; accepted 29 May 2006
Available online 7 July 2006
Abstract
Copper sulphides Cu
9 5 2
S and polymorphs of Cu S were synthesized apparently for the first time on copper foil through its
solvothermal reaction with sulphur (the former being used both as substrate and reactant), with 75% fill of the autoclave with water,
ethanol or ethylenediamine as solvents. Water as solvent produced micron-sized hemispheres of Cu (composed of much smaller
9 5
S
particles of size ꢀ5–6 nm), truncated by forming interfaces and exhibiting relatively large dihedral angles (80–1208). Ethanol
produced fibre-like or thread-like (ꢀ40 nm diameter) morphological form of Cu S , generally woven into network-structures.
9
5
2 2
Ethylenediamine, which produced Cu S (monoclinic), as also b-Cu S (hexagonal), assisted formation of hexagonal micron-sized
single crystal rods with a hexagonal end face in conjunction with b-Cu
#
2
S.
2006 Elsevier Ltd. All rights reserved.
Keywords: A. Chalcogenides; A. Nanostructures; B. Chemical synthesis; D. Optical properties
1
. Introduction
Extensive attention has been paid to the synthesis and characterization of sulphides, due to their interesting
properties and potential applications.
Copper sulphides in different stoichiometries are widely used as p-type semiconductors in solar cells, as super ionic
materials and as optical filters [1–2]. They are also widely applied in thin films and composite materials for their
unique optical electrical properties [3–6]. Copper sulphide is often used for ammonia gas sensing at room temperature
and these gas sensitive parameters have been found to be dependent upon chemical composition and the morphology
of the Cu S materials [7,8].
x
Various wet chemical techniques have been employed for obtaining a variety of shapes in nanoparticles or
microparticles of Cu S [1,6,9–14]. Previously, some researchers [12–14] including ourselves [15] reported about the
x
synthesis of Cu S crystals by solvothermal/hydrothermal procedure where salts of copper (such as copper nitrate,
x
copper chloride, copper sulphate) were used as the precursor of copper.
In the present work, as compared to the above, we used an unexplored combination for synthesis where instead of
copper salt, copper metal foils were employed as the source of Cu as also as the substrate for growth and different
solvents were used for gathering an idea about their roles in shape control of Cu S (x ꢀ 2). The results point towards
x
unique and significant shape tailorabilities as a function of the selection of a solvent; the range of shapes was from
*
Corresponding author. Tel.: +91 33 24734971x201; fax: +91 33 24732805.
E-mail address: mssc2@iacs.res.in (S. Chaudhuri).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.05.030

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S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
nanoparticles and their agglomerates (solvent: water) to thread-like and net-like forms (solvent: ethanol) to well-
developed rodlike single crystals (solvent: ethylenediamine). In this paper, we also report about the differences in
optical properties of the synthesized copper sulphide samples as a function of composition/structure and morphology.
2
. Experimental
About 0.49 g of Cu foil and 0.19 g of sulphur powder were put into a Teflon-lined stainless steel autoclave. This was
filled up to 75% of the total volume of the Teflon cylinder with different solvents, namely: (1) water, (2) ethanol and (3)
ethylenediamine (en). The autoclave was maintained at 130 8C for 12 h and subsequently cooled at room temperature.
The obtained black product was washed carefully with water and ethanol. The products prepared by using water,
ethanol and ethylenediamine were named as sample-I, II and III, respectively.
The samples were characterized by X-ray powder diffraction (XRD), using a Seifert (3000 P) unit with
monochromatic Cu Ka radiation (Ni filter). To determine the morphology and particle size, both scanning electron
microscope (SEM, Hitachi Model S-2300) and transmission electron microscope (TEM, JEOL-2010) were used.
Absorption spectra of the samples dispersed in spectroscopic grade alcohol were recorded on Hitachi U-3410
spectrophotometer.
3
. Results and discussion
3
.1. Crystalline product identification by XRD
The X-ray diffraction pattern (XRD) of Cu foil (which is not shown here) showed it to match well with that of the
JCPDS (04-0836) pattern. The pattern indicates the sample to be apparently free from any preferred orientation but
with a strong development of the (1 1 1) plane, conforming to the JCPDS standard pattern.
Water and ethanol as solvents produced well-crystallized phases, the XRD patterns (Fig. 1a and b) of which looked
similar, and matched very well with that of Cu S (JCPDS 26-0476). When ethylenediamine was used as the solvent,
the product(s) identified by XRD seemed to be Cu S phase(s). One of them, Cu S (JCPDS 33-0490), crystallizes in the
9
5
2
2
˚
monoclinic system with the following cell dimensions (A): a = 15.235, b = 11.885, c = 13.496. This phase, though not
prefixed with the term ‘‘a’’, has a companion polymorphic phase called b-Cu S. The b-polymorph crystallizes in the
2
Fig. 1. XRD patterns of the reaction products obtained by using (a) water and (b) ethanol.

S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
347
Fig. 2. (a) A typical XRD pattern of the reaction product in ethylenediamine system, (b) standard JCPDS pattern of b-Cu S, and (c) standard JCPDS
2
2
pattern of Cu S.
˚
hexagonal system with the following cell dimensions in A (JCPDS 26-1116): a = 3.961 and c = 6.722. Fig. 2 shows a
typical XRD pattern obtained in this work (Fig. 2a) and the two standard patterns for b-Cu S (Fig. 2b) and Cu S
2
2
(
Fig. 2c). Results of slow scanning of the obtained XRD pattern in the region 2u = 36.58–38.58 and 458–498 (Cu Ka)
are shown in Fig. 3a and b. Within the broad region of 2u = 358–408, several peaks are observed at 378–388: three peaks
of Cu S and one of b-Cu S are known to be present in this region according to the JCPDS standard patterns. It is
therefore difficult to assign any of these to the b-form, though the possibility of formation of both cannot be ruled out.
2
2
In comparison, in case of slow scan in the range 2u = 458–498, there is a strong indication of the presence of b-Cu S in
2
the pattern, especially in the region around 2u = 46 as Cu S has only one strong peak (I/I = 70) at 45.921, and b-Cu S
has its strongest peak (I/I = 100) at 45.776. Deconvolution of the two peaks in the region 458–46.58 2u (Fig. 4) clearly
2
0
2
0
show the presence of two peaks at 45.788 and 45.998. The conclusion reached is that the former peak is due to b-Cu2S
and the latter, due to Cu S.
2
3
.2. Optical property studies
UV–vis spectrophotometry was used to characterize the absorption properties of the obtained samples. Fig. 5a
presents the UV–vis spectrum of sample-I, which shows the absorption edge to be about ꢀ690 nm; the absorption edge
of sample-II is at ꢀ655 nm, as shown in Fig. 5b. The absorption spectrum of sample-III (Fig. 5c) represents an
absorption edge at ꢀ1035 nm, which is comparable with the absorption onset of bulk Cu S (1022 nm) [6].
2
3
.3. Morphological variations
It is clear from the SEM/TEM pictures shown below (Figs. 6, 8 and 9) that the three solvents yielded distinctly
different morphologies of the products. These morphologies will be described here with possible routes of
formation.
3
.3.1. Water as solvent
Water as the solvent yielded nanoparticles (ꢀ5–6 nm) that agglomerated into large, micrometer-sized, i.e. up to
ꢀ
10 mm particles with rounded to spherical shape, as in Fig. 6a–c. Fig. 6d indicates the high resolution

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S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
Fig. 3. Slow scan XRD patterns of the reaction product in ethylenediamine system in the region: (a) 36.5–38.58 2u and (b) 45–498 2u. * indicates the
presence of b-Cu S.
2
transmission electron microscopic (HRTEM) images of the nanoparticles. The interplanar spacing was calculated
and was found to be ꢀ0.33 nm, which corresponds to the (1 0 1) plane of Cu S . The most important and
9
5
interesting point, however, is the nature of assemblage and specific joining of these relatively large agglomerates
with formation of interfaces reminiscent of sintered products obtained through fired compacts of spherical solid
particles [16]. The other parallelism is related to soap bubbles in random cellular networks [17], and their
similarity with grain growth in metals [18]. Both the forms are expressions of their tendency to minimize the
available surface area or energy. Fig. 7 shows a general picture of some of the aspects of such cases. In the present
case, each micron-sized particle is composed of a large number of nanosized particles which do not show
localized agglomeration or growth; this indicates the initial generation of a large number of nuclei and their
limited growth because of necessarily restricted material supply [19]. It is observed from Fig. 6 that the dihedral
angles are quite large [20], going from about 808 to 1208. In terms of sintering, this can indicate at least an
intermediate stage of developing contact between two (or more) spherical or spheroidal particles through material
migration, as during the initial stage [16], there should generally be a neck formation and the dihedral angle
should be small (Fig. 7).

S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
349
Fig. 4. Deconvoluted XRD spectrum of the reaction product in ethylenediamine system in the region 46.0–46.68 2u.
The similarity of the present joining of agglomerated particles on Cu foil surface with joining via sintering
practically ends here: (a) firstly, the basic starting particles in sintering are generally single crystals, polycrystals or
glass or other amorphous materials, in spherical to highly irregular shapes, compacted into green bodies and (b)
secondly, material exchange between two such particles in point contact (leading to neck formation [16,21]) and
eventually to contact via an interface requires (generally) high thermal energy [22]. In the present case, the maximum
temperature resorted to was 130 8C and neck formation at the contact region was not noticed; further, no compaction
was used, as the observed microstructure developed inside the reaction vessel.
The phenomena described here can be analyzed in the following way. The first step was formation of nuclei of
copper sulphide (Cu S ) followed by growth on the copper foil, covering the available surface. A concomitant or
9
5
subsequent phenomenon is continuing growth of the nuclei, but in a restricted way, because with the limited source
materials, all the individual particles could not grow freely to submicron- to micron-levels [20]. As a result, further
formation of nuclei and their growth could take place only on the previously deposited layer that showed preferential
growth at a limited number of locations on the Cu foil. This could, among others, be guided by the polycrystalline
Fig. 5. UV–vis absorption spectra of copper sulphide samples prepared by solvothermal technique.

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S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
Fig. 6. Electron microscopy images of copper sulphide powders synthesized by using water as solvent: (a) low resolution SEM image, (b) higher
resolution SEM image of the same sample, (c) TEM image of the same, and (d) HRTEM image of nanoparticles.
nature of the Cu foil with a number of grains and grain boundaries, and the expectedly rough surfaces of the initial
spatially restricted deposits on it. These growing deposits formed hemispherical mounds, constrained to touch one
another and form interfaces at a very low (1308) temperature.
3
.3.2. Ethanol as solvent
The products derived from ethanol (Fig. 8) differ distinctly from those derived from water in that they are composed of
porous and discontinuous networks (Fig. 8) woven of thin (ꢀ40 nm) fibres. Thread-like or fibre-like forms, sometimes
leading to net-like forms are known in case of ethanol-mediated synthesis [23,24]. Relative solubility characteristics (e.g.
polarity) of different solvents are considered to be important in determining final solid product morphologies like
complex compositesof one-dimensional forms [25]. Itisknownfrom availableliterature [26] thatsulphur, a hydrophobic
substance, is insoluble in water (e = 80.10), slightly soluble in ethanol (e = 25.3), and highly soluble in CS (e = 2.63) and
2
benzene (e = 2.28) [27]. This is likely to be an indication that in the present case (with water and ethanol as solvents),
ethanol may lead to the formation of complex one-dimensional forms, while water is not expected to do so.
3
.3.3. Ethylenediamine as solvent
It has been already indicated above that the crystals formed through solvothermal treatment involving
ethylenediamine could be both monoclinic Cu S and b-Cu S, which crystallized in the hexagonal system.
2
2
A literature scan shows that products of hydrothermal/solvothermal synthesis need not necessarily be only one
compound. Examples are co-synthesis of Ni S and NiS [28], both rhombohedral and hexagonal polymorphs of NiS
3
2
[
29] and polymorphs of zirconia [30].
Fig. 9a shows predominant growth of microrods of 5–10 mm length from the copper foil surface; the end faces of
most of these microrods are hexagonal. The hexagons are not always regular, with two opposite edges sometimes
differing in length from the other four. The single crystalline nature of these rods was confirmed through selected area
electron diffraction analysis (Fig. 9b).
The typical morphological pattern discussed above should be related to the commonly observed templating ability
of the chelating agent ethylenediamine to assist one-dimensional growth of notably oxides and sulphides [31–34].
Ethylenediamine is known to assist dissolution of sulphur [11], which has also been clearly observed in the present

S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
351
Fig. 7. Some general trends in interactions of particles: (a) particles in free movement; (b) particles in point contact, without material exchange; (c)
particles initially in point contact, but forming a neck as a result of material exchange, through, e.g. evaporation–condensation, as in ceramics. Note a
small dihedral angle; (d) particles forming interfaces at an advanced stage of interaction. Note a large dihedral angle; (e) a final, ideal form of
connectivity, as in ceramics or foam due to aggregation of soap bubbles in a cellular network.
Fig. 8. Electron microscopic images of copper sulphide powders synthesized by using ethanol as solvent: (a) low resolution SEM image. Inset of the
figure shows the higher resolution SEM image of it; (b) TEM image of the same sample.

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S. Gorai et al. / Materials Research Bulletin 42 (2007) 345–353
Fig. 9. (a) SEM images of copper sulphide powders synthesized by using ethylenediamine as solvent. Inset of the figure shows the hexagonal end
face of a rod. (b) HRTEM image of a single rod with the corresponding SAED pattern in the inset.
work. The Cu foil reacted with the dissolved sulphur and formed Cu S under ionization equilibrium. The formation of
2
+
Cu (as against Cu ) was revealed by the formation of Cu S, identified as pure sulphide phase by XRD analysis:
2+
2
Cu þ S ! Cu2S , 2Cu þ S2ꢁ
The equilibrium was disturbed in en solution and Cu ion co-ordinated with en molecule to form stable [Cu(en) ]
þ
2
+
+
n
10.8
complex (k = 10 ). The complex ions with bidentate ligands can be linked by the hydrogen bonds and self-assemble
to form chains, which facilitates the oriented growth of Cu S:
2
þ
þ
Cu þ n en , ½CuðenÞnꢂ
þ
2ꢁ
½
CuðenÞ ꢂ þ S ! Cu Sꢃm en þ ðn ꢁ mÞ en
n
2
Cu2Sꢃm en ! Cu2S þ m en
At elevated temperature and pressure the stability of the above copper complex was decreased and finally at our
reaction temperature (130 8C) Cu S was obtained.
2
4
. Conclusions
Compositions and morphological characteristics of some copper sulphides synthesized through solvothermal
reactions of sulphur with a copper foil were found to be controlled basically by the solvent, when other parameters
were constant. The results were:
(
(
(
1) Water as solvent led to the crystallization of Cu S agglomerated grains composed of nanoparticles; the grains
9 5
formed interfaces with one another with large dihedral angles, indicating an advanced stage of connectivity.
2) Ethanol as solvent also yielded crystalline Cu S , but with a network morphology composed of nanofibre or
9
5
nanowire nets.
3) Ethylenediamine produced monoclinic Cu S, as also hexagonal b-Cu S in the form of single crystal rods with
commonly hexagonal (regular and distorted) end faces. The hexagonal symmetry of b-Cu S apparently helped
2
2
2
ethylenediamine to form the typical one-dimensional morphology with hexagonal structure.
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