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Nanoscience and Nanotechnology: An International Journal
Universal Research Publications. All rights reserved
ISSN: 2278-1374
Original Article
Chemical Synthesis of Copper Sulphide nanoparticles embedded in PVA matrix
S.K. Nath1 & P.K. Kalita2
1
2
Department of Physics, B.H. College, Howly, Assam, India
Nano-Science Research Laboratory, Guwahati College Guwahati -781021, Assam, India
Email: [email protected], [email protected]
Received 28 June 2012; accepted 14 July 2012
Abstract
Thin films of copper sulphide nanoparticles were deposited on glass substrates at room temperature 303K to 363K through
chemical route.Equivolume and equimolar 0.01M solution of copper acetate and thiourea was used for the synthesis in an
alkali medium. The X-ray diffraction showed both the covellite and the chalcocite phase of copper sulphides with
hexagonal & orthorhombic crystal structure respectively. The formation of nanoparticles was confirmed by UV-visible
absorption and PL measurements which showed an enhancement of band gap. The HRTEM images exhibited the spherical
size of nanoparticles varied from 20nm at room temperature to200nm at high temperature growth. The PL spectra showed
the identical blue-green emission irrespective of growth temperature.
© 2011 Universal Research Publications. All rights reserved
Key words. Chemical synthesis, CuS/Cu2S, PVA, Quantum confinement
1. Introduction
Nanostructures materials have attracted a great deal of
attention in the last few years for their unique
characteristics that cannot be obtained from conventional
macroscopic materials. Owing to quantum size effects and
surface effects [1-4], nanoparticles can display novel
optical, electrical, magnetic, chemical and structural
properties that might find important technological
applications. Copper sulphide has fairly complex crystal
chemistry owing to its ability to form sub- stochiometric
compounds CuxS (2≥x≥1) [5]. CuS phase exists in two
forms, the amorphous brown chalcocite CuS and green
crystalline covellite [6]. CuxS has many forms in bulk at
room temperature. They are chalcocite (orthorhombic
Cu2S), djurleite (mono-clinic-prismatic Cu1.95S) etc. [7, 8].
In the chalcocite group, Cu2-xS (0≤x≤0.6), eight compounds
exist. They are γ-chalcocite Cu2S (low, orthorhombic), βchalcocite Cu2S (high, hexagonal), djurleite Cu 1.97S,
digenite
Cu1.80S, roxbyite Cu 1.78S, anilite Cu 1.75S,
geerite Cu 1.60S and spionkopite Cu 1.60S. While γchalcocite, djurleite and anilite form low temperature
phases, the tetragonal
Cu 1.96S phase, low digenite, Cu
S
and
roxbite,
Cu
S
form metastable phases [9]. As
1.8
1.75
CuS is a P-type material, it has a wide scope in
photovoltaic device application such as solar cells. Its band
gap is 1.27eV. So in this nano-region CuS may exhibit its
potential in the visible region [5, 10, 11]. Keeping above
8
aspects, an experimental work on the chemical synthesis of
copper sulphide nanoparticle has been undertaken and
presented in this paper.
2. Experimental
2.1 Synthesis
Thin films of CuS nanopaarticles were deposited on glass
substrates by reacting copper acetate monohydrate (Cu (ac)
2) with thiourea (H2NCSNH2) in the presence of polyvinyl
alcohol (PVA) as capping agents. A 3% PVA solution was
mixed with copper acetate solution to act as capping agent.
Equivolume and eqimolar (.01M) solution of copper acetate
and thiourea were used for the synthesis. Ammonia was
added to the PVA mixed copper acetate solution to make
copper ion complex where thiourea solution was added
drop wise for the formation of final copper sulphide matrix
solution. The synthesized copper sulfide solution became
greenish in colour.The reaction mechanism can be given as
followsCu2+ +4NH3 ↔ Cu (NH3)42+
NH3 + H2O ↔ NH4+ + OH(NH3)2CS +2OH- ↔ S2- + 2H2O + H2CN2
Cu2+ + S2- → CuS(s)
However copper (2) ions can readily be reduced to copper
(1) ions by thiourea. The PVA in the reaction solution is
expected to stabilize copper sulphide particles. The as
formed copper (1) ions, then have the probability to
Nanoscience and Nanotechnology: An International Journal 2012; 2(1): 8-12
combine with sulphide ions to form chalcocite particles
(Cu2S) as follows2 Cu2+ + 2 (NH2)2CS ↔ 2Cu+ + (NH2 )2 (NH)2 C2S2 +2H+
2Cu+ + S2-→ Cu2S
The glass substrates were cleaned and these were dipped to
cast thin films. The films were taken for XRD (X-ray
diffraction) and the filtrate solution for other optical
measurements. In the present study, the growth temperature
was increased from 303K (Sample1:S1) to 363K (Sample2:
S2).
2.2 Characterizion
The structural characterizion was done by X-ray diffraction
technique and transmission electron microscope (TEM).
The X-ray diffractograms were recorded using copper Kα
radiation. The tube was operated at 20kV 30mA. The 2θ
varied from 15o to 70o with scanning rate (0.05o). The TEM
morphology was recorded using JEM 2100 (Jeol Electronic
Microscope) at accelerating voltage 200kV and beam
current 102μA. The optical spectra of nanoparticle were
measured using USB-2000 UV-visible spectrometer over
the range 300nm to 700nm.The PL spectra were also
recorded in the wavelength range 350nm to 700nm. The
atomic absorption was recorded by Atomic absorption
spectrometer (Perkin Elemer 3110)
3. Results and discussion
3.1 Structural characterizion
Copper sulphide nanoparticles were prepared through the
reaction of copper acetate and thiourea as discussed in
experimental details. Different sizes of nanoparticles were
obtained with respect to the growth of temperature. The
XRD pattern of deposited copper sulphide exhibited weak
diffraction peaks superimposed on PVA matrix (Fig.1). The
diffraction peaks at 19.50 and 240 corresponding to (101)
and (200) planes were corresponding to PVA. This type of
weak diffraction peaks were reported by other workers
particularly when the sample is embedded in a polymer
matrix like PVA and PNIPAM (poly N-isopropy
lacrylamide) [12, 13, 22]. The XRD showed prominent
(102), (103) and (110) planes of CuS covellite phase
corresponding to 2θ at 44o, 57o, 53.85o. The other prime
peaks at 23.9o, 26.5o, 30.65o and 32.95o were corresponding
to (016), (046), (400) and (412) planes of chalcocite
(Cu2S). This was confirmed from standard JCPDS data
no.2-1272 for covellite (hexagonal) and chalcocite copper
sulphide with orthorhombic phase respectively [14, 15].
Cu2S had tendency to form chalcocite orthorhombic type
crystal at room temperature as reported by other workers
[14]. The natures of diffraction pattern of PVA capped
copper sulphide at room temperature303K and at elevated
temperature (363K) were quite identical. However, the
prominence of Cu2S peaks at high temperature indicated an
increase in overall particle size. The formation of Cu2S was
also supported by atomic absorption measurements where
the concentration of copper increased at high temperature
(Table1).The HRTEM showed the shape and size of
particles formed in the samples which were of mainly
spherical type (Fig.2). Most of the other workers also found
spherical and hexagonal copper sulphide particles. The
average individual particle size deposited at room
9
temperature was around 10-20nm whereas it got
agglomerism to form overall size up to 200nm at high
temperature. Similar type of TEM morphology was also
found in chitosan capped copper sulphide [10]. TEM
morphology clearly indicated that PVA had acted as a good
stabilizer where the particles were nicely embedded
irrespective of growth temperature.
3.2 Optical characterizion
The absorption spectra of sample (S1) and sample (S2)
synthesized at 303K and 363K were shown in Fig. 3a and
Fig. 3b respectively. The UV-visible measurements
exhibited an absorption peak at 364nm and a shoulder
around 629nm for copper sulphide synthesized at room
temperature Fig.3a. This broad absorption suggested
covellite phase [10, 16] of copper sulphide which is blue
shifted because of quantum confinement from its bulk in IR
region. The short wavelength side peak corresponds to that
of chalcocite phase [5].The existence of chalcocite Cu2S in
chemically bath deposited samples was also revealed by
other workers [5, 11]. Haram, et al [17] found absorption at
475nm corresponding to chalcocite phase. F Li, et al [11]
found absorption at 314nm and 601nm. CuxS had mainly
stable phases with varied stoichiometry because of effect of
3d electrons. On increasing the growth temperature, shorter
wavelength peak red shifted to 381nm whereas the other is
blue shifted to 601nm. This clearly indicated the presence
of both CuS and Cu2S in the prepared samples. Similar
simultaneous red and blue shifts on temperature refluxing
in short and long wavelength region were reported by Boey
et al [10]. They found broad absorbance at near IR region
at 1025nm, characteristic of covellite CuS and that of
absorbance at 475nm due to chalcocite Cu2S in chitosan
capped copper sulphide. Hence it could have been been
inferred that like chitosan the PVA could act as a reducing
agent besides being a stabilizer or protecting agent. Some
noise absorption was also observed around 364nm in the
short wavelength side for room temperature grown copper
sulphide. This might be due to the surface oxidation of
chalcocite under aqueous and ambient atmosphere.
Chalcocite reacted with air and developed CuSO 4 surface
phase in addition to cuprite (Cu2O) and tenorite (CuO) [18].
Fig.1. XRD patterns of Copper Sulphides nanoparticles
Nanoscience and Nanotechnology: An International Journal 2012; 2(1): 8-12
Fig.2. TEM images of Copper Sulphides (S1: a, b) & (S2: c, d)
Fig.3. UV-visible absorption spectrum of CuS nanoparticles (S1: a) & (S2: b)
Fig.4. PL spectra for Copper Sulphides nanoparticles for samples S1 & S2
10
Nanoscience and Nanotechnology: An International Journal 2012; 2(1): 8-12
Table 1: Band gaps and copper concentration
Sample
Temperature
(K)
S1
S2
303
363
Abs. edge(nm)
Short
Long
364
381
629
601
Cu
con.(ppm)
3.79
5.41
3.3 PL spectroscopy
Fluorescence spectroscopy is a kind of electromagnetic
spectroscopy which analyses fluorescence from a sample. It
involves using a beam of light, usually UV light that
excites the electrons in the molecules of certain compounds
and causes them to emit light of a lower energy, typically
but not necessarily visible light. Photoluminescence of
those two CuS samples at room temperature and high
temperature are shown in Fig.4a and 4b.The PL spectra was
taken using excitation wavelength 325nm. The figures
show inhomogeneous broadening of emission. In Sample
(S2), PL peaks occur at 360nm, 396nm, 468nm and a broad
shoulder at extreme blue region. A close observation
indicates the presence of some weak blue-green emission at
451nm, 482nm, and 493nm. Similar results are found for
Sample (1) at 362nm, 396nm, 468nm and 600nm.Weak
peaks are found at 432nm and 474nm.The PL intensity
increases with the rise of deposition temperature. Similar
PL observations were also reported by other workers [18,
19, 20]. Blue emission was owing to the spherical copper
sulphides [11, 21].
4. Conclusion
The copper sulfide nanoparticles can be prepared through
simple chemical route. PVA was found to play a key role in
the confinement as well as in stabilization process. The
XRD confirms the formation of copper sulphides
embedded in PVA by showing the diffraction peaks of the
corresponding materials. Copper sulphides exhibit both
covellite and orthorhombic chalcocite phase irrespective of
growth temperature. This is supported by UV- visible
measurements which show chalcocite Cu2S absorbance at
364nm and covellite CuS at 629nm.In increasing the
growth temperature, the short wavelength absorption is red
shifted whereas the long wavelength absorption is blue
shifted. This is in well agreement with the reported results.
The PL measurements show blue-green emission for both
the copper sulphide samples which confirms the spherical
shape of CuS particles.
5. Acknowledgement
The authors thank the Chemistry Department, Gauhati
University, Guwahati, SAIF, NEHU, Shillong and IIT,
Guwahati for carrying out this research work.
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Source of support: Nil; Conflict of interest: None declared
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Nanoscience and Nanotechnology: An International Journal 2012; 2(1): 8-12