Cent. Eur. J. Chem. • 8(6) • 2010 • 1281-1287
DOI: 10.2478/s11532-010-0104-1
Central European Journal of Chemistry
Formation and characterization of CuxS layers
on polyethylene film using the solutions
of sulfur in carbon disulfide
Research Article
Ingrida Ancutienė , Vitalijus Janickis
*
Department of Inorganic Chemistry, Faculty of Chemical Technology,
Kaunas University of Technology, LT–50254 Kaunas, Lithuania
Received 20 May 2010; Accepted 23 August 2010
Abstract: Results of the formation of copper sulfide layers using the solutions of elemental sulfur in carbon disulfide as precursor for sulfurization
are presented. Low density polyethylene film can be effectively sulfurized in the solutions of rhombic (α) sulfur in carbon disulfide. The
concentration of sulfur in polyethylene increases with the increase of the temperature and concentration of sulfur solution in carbon
disulfide and it little depends on the duration of sulfurization. Electrically conductive copper sulfide layers on polyethylene film were
formed when sulfurized polyethylene was treated with the solution of copper (II/I) salts. CuxS layer with the lowest sheet resistance
(11.2 Ω cm–2) was formed when sulfurized polyethylene was treated with copper salts solution at 80oC. All samples with formed CuxS
layers were characterized by X-ray photoelectron spectroscopy. XPS analysis of obtained layers showed that on the layer’s surface
and in the etched surface various compounds of copper, sulfur and oxygen are present: Cu2S, CuS, CuO, S8, CuSO4, Cu(OH)2 and
water. The biggest amounts of CuSO4 and Cu(OH)2 are present on the layer’s surface. Significantly more copper sulfides are found in
the etched layers.
Keywords: Polyethylene film • Rhombic (α) sulfur • Carbon disulfide • Sulfurization• Copper sulfide layers
© Versita Sp. z o.o.
1. Introduction
Copper sulfides are a particularly interesting class of
metal sulfides due to their ability to form with various
stoichiometries. The Cu–S binary system consists of the
stoichiometric end members Cu2S (chalcocite) and CuS
(covellite) with several stable and metastable phases of
varying stoichiometry between those two compositions.
Their structures and valence state results in some
unique physical and chemical properties. Their optical
and electrical properties have been the topic of many
studies [1-4].
Copper sulfide is an interesting material for its metallike electrical conductivity, chemical-sensing capability
and ideal characteristics for solar absorption [5]. Covellite,
which has a CuS composition, attracts special interest
because it shows a metal-like electrical conductivity
down to liquid helium temperature [1]. Copper sulfides
thin films have shown promise as gas sensor material,
which can operate at room temperatures [6-8]. Copper
sulfide coatings would find notable applications as solar
radiation absorbers [9], as p-type semiconductors in
solar cells [10] and as solar radiation control coatings
[11]. They are widely used in thin films and composite
materials [12-14], etc.
The properties of copper sulfide are affected by
the accurate stoichiometry, which depends on the
preparative conditions used for the thin film deposition
[3]. Copper sulfides, usually denoted as CuxS, have
been fabricated in thin film forms by different techniques
such as successive ionic layer adsorption and reaction,
chemical bath deposition, electrodeposition, electroless
deposition, anodization, spray pyrolysis, etc. [15].
Thiourea, thioacetamide, thiosulfate, sodium sulfide
and hydrogen sulfide are generally used as precursors
for various materials sulfurization. The aqueous
solutions of sodium polysulfides, Na2Sn (n≈4.8) [16] and
of polythionic acids, H2SnO6 (n = 4÷45) [17-19] are also
proposed for the sulfurization of dielectrics. However,
solutions of sodium polysulfides are highly alkaline
and solutions of higher polythionic acids are acidic and
complicated to prepare.
The formation of sulfide films by sorption–diffusion
* E-mail: [email protected]
1281
Formation and characterization of CuxS layers
on polyethylene film using the solutions
of sulfur in carbon disulfide
method has attracted much attention because of its
simplicity, low cost, low temperatures of formation
processes (20–80oC) and availability of starting
materials. Hydrophobic polymer (polyethylene) adsorbs
elemental sulfur from Na2Sn [20] or H2SnO6 [21] solutions.
Consequently, in the work presented, copper sulfide
layers were obtained using a solution of elemental
sulfur in a solvent. One of the best inorganic solvents for
rhombic (α) sulfur is the carbon disulfide, CS2 [22,23].
At the temperature of 20°C it dissolves 50,4, at 30°C –
61.3, at 40°C – 100.0 and at 50°C – 143.9 g of sulfur in
100 g of solvent [23].
The goal of ourpresent work was to study the
sulfurization of the hydrophobic polymeric material –
polyethylene film (PE) in the solutions of cycle – octa
sulfur in a non-aqueous solvent – carbon disulfide also
the formation of copper sulfide layers on the surface
of PE and the characterization of formed layers. The
chemical and phase composition of the CuxS layers, their
electrical conductance were studied by the methods of
atomic absorption spectroscopy, X–ray photoelectron
spectroscopy (XPS) and by the measurements of sheet
resistance.
2. Experimental Procedures
Layers of CuxS were formed on a low density PE film of
0.14 ± 0.01 mm thickness (GOST 10354-82), made in
the Vilnius plastics factory “Plasta”. Rectangular samples
of 15×60 mm were used. The density of polyethylene
was measured by a flotation method using a series of
distilled water/acetone mixtures at 20°C. Samples of PE
were immersed in liquids with different density (0.910–
0.920 g cm–3) seeking one which would neither sink
nor float. Before sulfurization, PE was cleaned in a
4 % solution of a non-ionic surfactant (Disponil AAP43),
washed thoroughly with distilled water and dried.
PE samples were sulfurized in a thermostatic vessel
using a continually stirred 0.06÷22.2 mol L–1 solution
of elemental rhombic sulfur in CS2 at the temperature
of 20÷45°C for the period of time from 0.5 to 120 min.
Removed samples were dried over CaCl2 for 24 h, and
then used in analysis and further experiments.
The concentration of sulfur in sulfurization solution
and in PE was determined by a spectrometric
cyanide method [17,24] using a photometer KФK-3
(λ = 450 nm).
For the formation of CuxS layers the samples of
sulfurized PE were treated with unstirred 0.4 mol L–1
water solution of CuSO4 with a 0.1 mol L–1 addition of
reductor (hydroquinone). The composition of this solution
1282
as a mixture of univalent and divalent copper salts
was established earlier [20]: it contains 0.34 mol L–1 of
Cu (II) salt and 0.06 mol L–1 of Cu (I) salt. Sulfurized PE
samples were treated with a copper (II/I) salts solution
at 80, 60 and 40°C for a period of time ranging from
0.25 to 30 min. Then samples with the CuxS layers were
washed with distilled water, dried over CaCl2 and used
in consequent experiments.
The CuxS layer in the PE surface was dissolved in
concentrated nitric acid, and the copper amount was
determined by the atomic absorption spectrometer
“Perkin−Elmer 503“ [25].
The conductivity of CuxS layers at a constant current
was measured using an E7-8 numerical measuring
instrument with special electrodes. The PE film was
placed under a special electrode made from two nickelplated copper plates with a dielectric material between
them one cm in length. We measured sheet resistance
(R) of the CuxS layers on the PE. The concept of sheet
resistance is used to characterize thin deposited layers.
The surface of the CuxS layers on the PE film were
analyzed by using a scanning electronic microscope
JEOL SM-IC25S (Japan), with a resolution of 10 nm.
The X-ray photoelectron spectra of CuxS layers
were recorded on a spectrometer „ESCALAB MKII”
(VG Scientific, England, radiation Mg Kα – 1253.6 eV,
output 300 W). The vacuum in the analysing chamber
was kept at a level of 1.33 10–8 Pa. The distribution
of elements in depth of copper sulfide layers formed
was determined by sputtering with an Ar+ gun with ion
energy of about 1.0 keV. The samples were etched in a
preparation chamber in a vacuum of 9.3×10–3 Pa and
a current of 100 μA cm–2; the duration of etching was
15 s. Scan photoelectron spectra were recorded for Cu
2p3/2, S 2p and O 1s. The photoelectron peaks were
calibrated against the carbon line C 1s with a binding
energy equal to 285 eV. Empirical sensitivity factors for
these elements were taken from the literature [26] and
the spectra obtained were compared with the standart
ones [27].
3. Results and Discussion
Studying the influence of the non-polar solvent CS2 on a
hydrophobic polymer [28], it was important to determine
the solvent effects of low density polyethylene. By
measuring the density of PE samples before and after
treatment with CS2, it was determined that the density of
samples after treatment in the CS2 solution for intervals
between 5 to 120 min, increased on average from the
initial density of 0.916 g cm–3 to 0.919 g cm–3. Low density
I. Ancutienė, V. Janickis
polyethylene is a semi-crystalline polymer with an
amorphous phase and a crystalline phase. The increase
in density is attributed to the increase in a crystallinity
of the samples [29]. In this case the density of the PE
samples may be increased due to the intercalation of
40
7
6
30
.
CS , mg cm
-3
5
20
4
2
10
3
1
0
0
10
20
30
40
50
60
Sulfurization time, min
Figure 1. Changes of sulfur concentration in PE with time during its
treatment with solutions of sulfurization agents at different
temperatures and concentrations: 1 – H2S33O6, 60oC,
2 mmol L–1; 2 – Na2Sn (n= 4.8), 77oC, 1.85 mol L–1; 3 –
sulfur solution in CS2, 20oC, 0.6 mol L–1; 4 – sulfur solution
in CS2, 20oC, 5.7 mol L–1; 5 – sulfur solution in CS2, 40oC,
5.7 mol L–1; 6 – sulfur solution in CS2, 40oC, 12.4 mol L–1;
7 – sulfur solution in CS2, 40oC, 22.2 mol L–1
3
0,4
2
m Cu , mg . cm -2
0,3
2xCu+ + 1/8S8 → CuxS + xCu2+.
1
0,2
0,1
0
0
5
10
15
20
25
carbon disulfide into polyethylene.
Initially, PE samples were sulfurized in constantly
stirred sulfur solutions in CS 2. The saturated
concentration of sulphur (cS) in PE (Fig. 1) was reached
very rapidly – after a few minutes, using a solution of
Na2Sn as the sulfurization agent [20]. However, by using
CS2,the obtained cS values were 2 – 4 times higher
than those reached by the Na2Sn solution, though its
temperature was higher (77°C). By using solutions of
polythionic acids [21], the saturated concentration of
sulfur in PE was reached only after 2 hours, and the
H2SnO6 solution (at 80°C) cS was ~2.5 times lower than
during sulfurization with sulfur solutions in CS2. The
maximum concentration of sulfur in PE even during
the fifth minute of sulfurization reached 33 g cm–3 at a
temperature of 40°C (Fig. 1).
The results of our study show that the solutions of
sulfur in CS2 are suitable for sulfurization of PE and
are considerably more effective compared with earlier
used agents for sulfurization – solutions of polysulfides
and high polythionic acids. The results obtained by
sulfurization of the PE in a solution of sulfur in CS2
(large cS values and quick saturation of PE by sulfur)
enabled the duration of sulfurization and the lowering
of the temperature of the sulfurization process to be
shortened.
When the sulfurized hydrophobic polymers are
treated with copper (II/I) salts solution, the layers of
copper sulfides in a matrix of polymers are obtained.
Depending on the conditions of PE sulfurization in the
sulfur solutions in CS 2 and then by treatment with copper
salts solution, bright brown or even black electrically
conductive layers of CuxS were obtained.
CuxS forms during the heterogeneous reaction
between elemental sulfur in PE and Cu (I) ions and which
are, presented in copper salts solution as follows:
30
Time, min
Figure 2. Changes of the amount of copper in CuxS layers on PE
over time.Sulfurized PE (cS = 12.8 mg cm–3) was treated
with copper (II/I) salts solution at different temperatures.
Temperature, oC: 1 – 40; 2 – 60; 3 – 80
(1)
After measuring the amount of copper in the sulfide
layer, it was determined, that it depends on the initial
concentration of sulfur in PE and that it increases when
the temperature of the copper (II/I) salts solution and the
time of treatment in also increases (Fig. 2).
A microscopic analysis established that copper
sulfide will not form on the PE surface gradually, but by
forming particular islands and thus developing a rough
coating and a noble surface. The rough surface increases
the concern of its interaction with the atmosphere which
may cause difficulties when measuring the electrical
conductivity and which may have an irregular phase
composition.
1283
Formation and characterization of CuxS layers
on polyethylene film using the solutions
of sulfur in carbon disulfide
Measurements of sheet resistance (R) show
(Fig. 3), that the curves of R dependence on the duration
of treatment with the Cu (II/I) salts solution have various
minimums. When cS in PE is approximately 12.8 mg cm–3,
at 40°C, the minimum of the R-t curve is at a 8-10 min
time interval , at 60°C – 6−8 min time interval, at 80°C –
1−2 min time interval. At 40°C the CuxS layers with low
resistance (43.1 – 81.3 Ω cm–2) were obtained only after
a 7-30 min time interval. At a higher temperature (80°C)
even after a 1–5 min time interval, the resistance was
lower (11.2 – 49.5 Ω cm–2).
The minimums in the curves can be explained
similarly to previous studies [16]. When initially a very
thin CuxS layer formed on the surface of PE, the value
x equals 1.12. This sulfide is an “abnormal”, electro
conductive semiconductor. Further, due to the diffusion of
the copper ions, the thickness of the layer increases and
the resistance decreases. In this process not only the
thickness of the layer varies, but also the stoichiometric
composition. Therefore, the value of x increases. Then
an “abnormality” of the sulfide layer on the polymer
decreases and the resistance increases. When x > 1.98,
the conductivity of the copper sulfide decreases and the
chemical composition approximates that of the Cu2S;
the sheet resistance reaches the maximum value; the
process of the layer formation comes is completed.
The composition of the copper sulfide layers samples
was studied through XPS analysis. The XPS spectra
of sulfur and copper present on the surface of copper
sulfide layer and in depth of this layer (after etching by
Ar+ ions) are shown in Fig. 4. The sulfur 2p spectrum
(Fig. 4a) shows two peaks. The peak at approximately
162 eV corresponds to the binding energy of sulfur in
the sulfide form and the peak at approximately 169 eV
corresponds to the binding energy of sulfur in sulfate
form [27,30]. The spectrum (Fig. 4b) of the argon ion
etched sample shows the dominance of the sulfide
phases.
Figure 3. Changes
of the sheet resistance of CuxS layer on PE
over time. Sulfurized PE (cS = 12.8 mg cm–3) was treated
with copper (II/I) salts solution at different temperatures.
Temperature, oC: 1 – 40; 2 – 60; 3 – 80
Table 1. XPS data of copper sulfide layers obtained on PE
Etching conditions
Element
Amount, at.%
Binding energy, eV
Possible
composition
Sample No.1 (cS = 12.8 mg cm–3, treated 3 min with Cu (II/I) salts solution at 80oC)
Surface
S
Cu
O
23.0
10.1
66.9
162.5; 169.2
932.5; 934.5
532.0; 533.8
Cu2S, CuS, CuSO4,
H2O
Etched for 15 s
S
Cu
O
41.5
51.5
7.0
162.3; 168.8
932.5
532.0; 533.6
Cu2S, CuSO4, H2O
Sample No.2 (cS = 16 mg cm–3, treated 8 min with Cu (II/I) salts solution at 60oC)
Surface
S
Cu
O
28.7
6.2
65.1
162.5; 168.7
932.6; 933.6
532.0
Cu2S, CuO, CuSO4
Etched for 15 s
S
Cu
O
53.5
32.5
14.0
162.3; 164.3
932.3
531.4
Cu2S, S8,
Cu(OH)2
Sample No.3 (cS = 16 mg cm–3, treated 8 min with Cu (II/I) salts solution at 40oC)
1284
Surface
S
Cu
O
31.0
3.5
65.5
164.3; 170.5
933.1; 934.5
532.6
S8;
CuS, CuSO4,
Etched for 15 s
S
Cu
O
56.3
41.0
2.7
162.3
932.4
531.8
Cu2S, Cu(OH)2
I. Ancutienė, V. Janickis
the peak position shifts toward lower binding energy.
This means that the concentration of Cu+ increases with
Ar+ sputtering of the surface.
The binding energy values of Cu 2p3/2, S 2p and O
1s peaks in the spectra of obtained copper sulfide layers
and analysis of the data are depicted in the following
Table (samples 1–3).
The binding energies of O 1s peaks ranging from
531.4 to 533.8 eV in the XPS spectra is well fitted by
three components that have different energies. The
binding energies have been indentified as HO– in
Cu(OH)2 at 531-532 eV [26,32], as SO42– at 532 eV [33]
and as adsorbed water at 533 – 534 eV [26,32].
The peak corresponding to the binding energy
value of 164.3 eV (samples No. 2 and 3) indicates the
presence of elemental sulfur [34]. The binding energy
at 933.6 eV measured in sample No. 2 agree with the
reported values of CuO in a range of 933.4 – 933.9 eV
[27,35].
Analysis of the data presented in the Table and a
comparison of the distribution of copper, sulfur and
oxygen in the surface of the samples 1–3 have shown
that oxygen takes the biggest part on the surface of all
samples. A large oxygen amount may be explained by
copper sulfate and water absorption on the layer surface,
as well as by insoluble copper (II) hydroxide formation
on the surface; Cu(OH)2 may be formed while washing
coatings with water. The metal sulfides forming layers
in the polymer surface matrix take the form of dendrites
[16], therefore, the copper sulfate, copper hydroxide and
water may remain in-between them.
When the layer’s surface is etched by Ar+ ions,
the content of oxygen significantly decreases and the
amounts of the atomic % of copper and sulfur increases.
According to these data, the obtained layers consist
predominantly of copper sulfides.
Data in the Table show that on the layer’s surface
and in the etched surface various compounds of copper,
sulfur and oxygen are present. The following compounds
have been indentified: Cu2S, CuS, CuO, S8, CuSO4,
Cu(OH)2 and water.
4. Conclusions
Figure 4. S2p and Cu2p3/2 XPS spectra of sample No. 1 with copper
sulfide layer: a and c1 – as-obtained surface; b and c2 –
Ar+ etched layer
XPS spectra of Cu2p3/2 for the sample No 1 are shown
in Fig. 4c; the peaks measured at 932–934.5 eV. The
binding energies of approximately 932 eV and 934.5 eV
show that copper is present in the +1 and +2 oxidation
states, respectively [31]. The relative intensity of weak
satellite structure at 934.5 eV (Fig. 4c) decreases and
Low density polyethylene film can be effectively
sulfurized in the solutions of rhombic (α) sulfur S8 in
carbon disulfide. The concentration of sulfur in the
samples increases with the increase of temperature
and concentration of sulfur solution. The use of these
solutions enables to decrease the temperature of the
sulfurization stage to 20oC and to shorten its duration
to about 10 min.
1285
Formation and characterization of CuxS layers
on polyethylene film using the solutions
of sulfur in carbon disulfide
Electrically conductive copper sulfide layers are
formed when the samples of sulfurized polyethylene
are treated with the solution of copper(II/I) salts. The
amount of copper in the obtained copper sulfide layer
increases with the increase of sulfur concentration in
the polymer and the time of treatment with copper salts
solution. The copper sulfide layers of the lowest sheet
resistance (11.2 – 49.5 Ω cm–2) are formed when the
sulfurized polyethylene film is treated 1–5 min with a
copper salts solution at 80oC.
XPS analysis of obtained layers showed that on
the layer’s surface and in the etched surface, various
compounds of copper, sulfur and oxygen are present:
Cu2S, CuS, CuO, S8, CuSO4, Cu(OH)2 and water. The
biggest amounts of CuSO4 and Cu(OH)2 are present on
the layer’s surface. Significantly more copper sulfides
are found in the etched layers.
The regularities determined an enabled formation
of copper sulfide layers of desirable composition
and electrical conductivity using the elemental sulfur
solutions in carbon disulfide as a sulfurization precursor
to polyethylene film.
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