Phase evolution of Cu-S system in ethylene glycol solution: The effect of anion and PVP on the transformation of thiourea

Article abstract of DOI:10.1002/cjoc.201300200

The transformation mechanisms of thiourea in ethylene glycol solution was systematically investigated in this report, which shows the transformation process is influenced by the anion (NO^3-, Cl^-, Br ^-) and polyvinylpyrrolidone (PVP). Thiourea (tu) isomerizes into ammonium thiocyanate when NO^3- is present, regardless of the existence of PVP. For Cl^-, thiourea coordinates with copper anion to form [Cu(tu)]Cl·1/2H₂O complex whether PVP is present. When it comes to Br^-, thiourea hydrolyzes in the cooperation of PVP or coordinates with copper anion to form [Cu(tu)Br]·1/2H₂O complex without PVP. The different transformation routes will lead to different phase evolution of the Cu-S system. This work may provide a new understanding of the transformation of thiourea in ethylene glycol solution. The optical properties of the as-prepared copper sulfides exhibit signi?cant stoichiometry-dependent features which may have potential applications in semiconductor photovoltaic devices. The effect of anions and PVP on the transition of thiourea in ethylene glycol solution was studied in detail. Copyright

Full text of DOI:10.1002/cjoc.201300200

FULL PAPER
DOI: 10.1002/cjoc.201300200
Phase Evolution of Cu－S System in Ethylene Glycol
Solution: the Effect of Anion and PVP on the
Transformation of Thiourea
Zhen Fang,* Chenyan Wang, Fan Fan, Shenghua Hao, Liuyang Long,
Yixuan Song, and Tingting Qiang
Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science,
Anhui Normal University, Wuhu, Anhui 241000, China
The transformation mechanisms of thiourea in ethylene glycol solution was systematically investigated in this
report, which shows the transformation process is influenced by the anion (NO3−, Cl−, Br−) and polyvinylpyrroli-
done (PVP). Thiourea (tu) isomerizes into ammonium thiocyanate when NO3− is present, regardless of the existence
of PVP. For Cl−, thiourea coordinates with copper anion to form [Cu(tu)]Cl•1/2H2O complex whether PVP is pre-
sent. When it comes to Br−, thiourea hydrolyzes in the cooperation of PVP or coordinates with copper anion to form
[Cu(tu)Br]•1/2H2O complex without PVP. The different transformation routes will lead to different phase evolution
of the Cu－S system. This work may provide a new understanding of the transformation of thiourea in ethylene
glycol solution. The optical properties of the as-prepared copper sulfides exhibit significant stoichiometry-
dependent features which may have potential applications in semiconductor photovoltaic devices.
Keywords anion, semiconductors, thiourea, transformation
polyhedron[7] and so on, the simultaneous precise stoi-
chiometry and morphology control of copper sulfides is
still difficult.
Introduction
As an important member of semiconductors, transi-
tion-metal chalcogenides have attracted a lot of attention
because of their excellent properties and potential ap-
plications.[1] Among these transition-metal chalco-
genides, copper sulfides are the most intensively studied
materials and exhibit various interesting applications in
solar cell devices, optical filters, cathode materials in
lithium batteries, nonenzymatic sensors, photothermal
therapy agents, photocatalysis, peroxidase-like activity,
supercapacitors, and thermoelectric materials.[2] The
physical properties of copper sulfides largely depend on
the morphology, stoichiometry and valence state. Ac-
cording to the previous reports, copper sulfides exist in
at least nine different CuxS phases possessing different
compositions and crystal structures,[3] including villa-
maninite (CuS2), covellite (CuS), yarrowite (Cu1.12S),
spionkopite (Cu1.39S), geerite (Cu1.6S), anilite (Cu1.75S),
digenite (Cu1.8S), djurleite (Cu1.96S) and chalcocite
(Cu2S). However, the multiformity of copper sulfides in
composition made it a great challenge for the control of
stoichiometry and morphology at the same time. To date,
although various morphologies of CuxS have been fab-
ricated, such as dendrites,[4] rattle-type structures,[5]
ball-flowers,[2i] nanotubes,[2e] hollow architectures,[2c,2j,6]
Thiourea (H2NCSNH2) is a cheap and commonly
used sulfur source for the synthesis of metal sulfides,
which has been employed for the preparation of copper
sulfide micro/nanocrystals with various morphologies
and superior properties.[2e,2i,8] The thiourea has a strong
tendency to coordinate with metallic cations, forming
semiorganic materials such as bisthiourea cadmium
chloride, zinc thiourea sulphate, zinc thiourea chloride
and tris-thiourea copper(I) chloride.[9] The thiourea
molecule may take effect mainly in two ways. For the
first way, thiourea decomposes mildly and continuously
to release S2− under hydrothermal/solvothermal circum-
stances or even in common ambient conditions. The
resulting S2− ions then associate effectively with the
metal cations to generate metal sulfide materials.[8d,10] In
the second way, thiourea could isomerize into ammo-
nium thiocyanate under solvothermal condition, em-
ploying metal thiocyanate as the intermediate prod-
uct.[5,11]
Kumar et al.[3a,3b] reported that different copper-
thiourea complexes with different counter anions influ-
enced on the final copper sulfide stoichiometry in bulk.
In our previous work,[5] we have reported the synthesis
*
E-mail: fzfscn@mail.ahnu.edu.cn; Tel.: 0086-0553-3937135; Fax: 0086-0553-3869302
Received March 18, 2013; accepted June 28, 2013; published online July 19, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300200 or from the author.
Chin. J. Chem. 2013, 31, 1015—1021
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of rattle-like Cu1.8S nanostructures via the isomerization
of thiourea in ethylene glycol (Figures S1 and S2).
Herein, our comparative experiments showed that the
different anions and PVP existing in the solution do
have significant effect on the transition process of thio-
urea during the reaction. The effect of anions and PVP
on the transition of thiourea in ethylene glycol (EG) so-
lution was further studied in this report. The transforma-
tion of thiourea can be classified into three types, in-
cluding isomerization into ammonium thiocyanate, for-
mation of complex and hydrolyzation when different
anions were used. This work provides a systematic and
comprehensive understanding of the transformation of
thiourea in EG solution and offers an effective and sig-
nificant chemical mechanism in the synthesis of copper
sulfides.
lyzer (PerkinElmer). Raman spectra of the samples were
measured at room temperature using a Jobin Yvon/
Labram HR800 spectrometer with laser wavelength of
514 nm.
Results and Discussion
Figure 1A shows the XRD pattern of the sample ob-
tained when employing CuCl2•2H2O as the copper
source in the presence of PVP. The XRD pattern shows
that the white precursor is well crystallized and all of the
diffraction peaks in this pattern can be indexed to be the
monoclinic phase of [Cu(tu)]Cl•1/2H2O (JCPDS NO.
53-0121). No diffraction peaks of other impurities could
be seen, which indicate the as-prepared sample was pure.
The EDS result (Figure 1B and Figure S3) clearly
confirms the crystal consists of Cu, S, N, and Cl, and the
molar ratio of the element Cu to S to Cl is about 1∶
1.01∶1.04, which matches well with the formula of
[Cu(tu)]Cl•1/2H2O. The C, H, N and S elemental analy-
sis (Table S1) of the white precursor from experiment is
also in agreement with the calculated ratio of
[Cu(tu)]Cl•1/2H2O. Figure 1C shows a representative
SEM image of the as-obtained rod-like [Cu(tu)]Cl•
1/2H2O precursor. These nanorods, with length of sev-
eral micrometers and width of about 300－400 nm, have
smooth surfaces along its entire length. Thermogravi-
metric analysis (TGA) and differential thermal analysis
(DTA) were used to study the thermal decomposition
behavior of white precursor in order to have a better
understanding of the [Cu(tu)]Cl•1/2H2O complex, as
shown in Figure 1D . The mass loss is about 7.2% in the
temperature range 18－220 ℃ corresponding to the
following dehydration process:
Experimental
Chemicals and synthesis
All reagents used in this work were of analytical
grade and were purchased from the commercial market
without any further purification. In a typical synthesis, 2
mmol copper source (Cu(NO3)2•3H2O, CuCl2•2H2O,
CuCl or CuBr2 ) and 2 mmol thiourea (Tu) were dis-
solved in 20 mL EG, respectively. The resulting solu-
tions were stirred for a certain time to obtain a clear so-
lution. In the next step, 2.0 g PVP (K30, Mw≈58000)
was added into the copper-EG solution under stirring.
The two solutions were then mixed and transferred into
a 60 mL Teflon-lined autoclave, sealed and heated at
100－210 ℃ for 12 h and then cooled down to room
temperature naturally. The final solid product was col-
lected by centrifugation and washed several times with
alcohol and distilled water to remove surfactant and or-
ganic impurities. The as-obtained products were dried
under vacuum for 2 h at 60 ℃ for further characteriza-
tion. In the comparative experiments, no PVP was used
while other conditions were kept the same as the typical
process.
[Cu(tu)]Cl•1/2H O(s)→[Cu(tu)]Cl(s) 1/2H O(g)
＋
2
2
The maximal three mass losses (total 41.6%) take
place at approximately 232 ℃ (endothermic), 260 ℃
(endothermic), and 286 ℃ (exothermic) in the tem-
perature range 200－358 ℃ (from DTA peaks). The
main reaction is as below:
Characterization method
The products were characterized with X-ray powder
diffraction (XRD) using a Shimadzu X-ray diffractome-
ter (XRD-6000) with Cu Kα irradiation (λ＝l.54187 Å).
Scanning electron microscopy (SEM) and X-ray energy
dispersive spectroscopy (EDS) analysis of these samples
were investigated by a Hitachi S-4800 scanning electron
microscope. For UV-vis-NIR absorption spectra, the
samples were first dispersed in ethanol through ultra-
sonication, and then the supernatants were taken after
centrifugation. The absorption spectra were recorded on
a Shimadzu UV-2450 UV-vis-NIR spectrophotometer.
Thermogravimetric (TG) analysis was carried out on a
thermoelectron simultaneous DTA-TGA thermal ana-
lyzer (SDT Q600). The samples were heated at the rate
of 10 ℃/min in air atmosphere. C, H, N, and S elemen-
tal analysis was collected using CHNS Elemental Ana-
[Cu(tu)]Cl(s) →CuCl(s) SC(NH )
(l)
＋
2 2
Tu decomposed to produce complex gases at this
time. The mass increased by about 6.72% accompanied
with high exotherm at 365 ℃, which is possibly due to
the oxidation reaction.
2CuCl(s) 1/2O (g)→Cu OCl (s)
＋
2
2
2
After that there is a mass decreasing again. Finally,
CuO is the main products. The total mass loss is about
58.4%, which is in agreement with the mass loss from
[Cu(tu)]Cl•1/2H2O to CuO (56.8%). And this result is
quite similar to the previous report.[12]
The as-formed [Cu(tu)]Cl•1/2H2O, a rod-like com-
plex can further transform into Cu1.8S as the reaction
temperature increased, as shown in Figures 2 and 3.
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Chin. J. Chem. 2013, 31, 1015—1021

Phase Evolution of Cu－S System in Ethylene Glycol Solution: the Effect of Anion and PVP
Figure 1 (A) XRD pattern, (B) EDS spectrum, (C) SEM image, and (D) TG and DTA curves (the sample mass is 3.64 mg) of the
[Cu(tu)]Cl•1/2H2O precursor prepared from CuCl2•2H2O with PVP at room temperature (r.t.) with stirring for 20 min.
Figure 2
XRD patterns of the products prepared from
CuCl2•2H2O with PVP at different temperature, (a) 120 ℃, (b)
150 ℃, (c) 180 ℃, and (d) 210 ℃.
Figure
3
SEM images of the products prepared from
CuCl2•2H2O with PVP at different temperature: (A) 120 ℃, (B)
150 ℃, (C) 180 ℃, and (D) 210 ℃.
Figure 2a shows the XRD pattern of the offwhite prod-
ucts obtained at 120 ℃, which indicates that it is the
mixed phases of monoclinic [Cu(tu)]Cl•1/2H2O and
hexagonal CuS. The corresponding SEM image (Figure
3A) indicates that the decomposition of [Cu(tu)]Cl•
1/2H2O complexes starts to take place and the sample is
comprised of rod-like and flower-like structures. If the
reaction is carried out at 150 ℃, only hexagonal phase
CuS can be obtained (a＝3.788 Å, c＝16.33 Å, JCPDS
No. 79-2321), as shown in Figure 2b. And from SEM
image (Figure 3B), it can be seen that the morphology
of the products is flower-like, and the rod-like was not
detected, which indicates that all those [Cu(tu)]Cl•
1/2H2O complex precursors have completely converted
into CuS products. Upon increasing the reaction tem-
perature to 180 ℃, hexagonal CuS and cubic Cu1.8S
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coexist in the sample according to the XRD result (Fig-
ure 2c). With the temperature further increasing to 210
℃, the hexagonal phase CuS was completely trans-
formed into cubic phase Cu1.8S (Figure 2d) with lattice
parameters of a＝5.577 Å (JCPDS No. 72-1996). From
SEM observations (Figures 3C, 3D), it is clearly seen
that the flower-like architectures were collapsed with the
temperature elevating and phase changing. This result
confirmed that temperature was the primary parameters
influencing the stoichiometry of copper sulfides and it is
favorable to generate cubic phase Cu1.8S in our reacting
system as the reaction temperature increased.
Figure S4 and Figure S5 present the XRD patterns
and SEM images of the as-synthesized sample in the
absence of PVP while keeping other reaction parameters
constant. Same process from rod-like [Cu(tu)]Cl•
1/2H2O complex to Cu1.8S is observed. However, the
transformation temperature is higher than that when
PVP is used. In other words, the sample obtained at 210
℃ is still a mixture phase of hexagonal CuS and cubic
Cu1.8S. This phenomena may result from the weak re-
ducing ability of PVP.[13] SEM images (Figure S5) show
that PVP can also reduce the size of products. If CuCl
was used as the copper source, no matter with or with-
out PVP, white rod-like [Cu(tu)]Cl•1/2H2O complex
was persistently obtained at room temperature, which
was decomposed as the temperature increased, as shown
in Figures S6－S9. From the above results, it is clear
that the formation of [Cu(tu)]Cl•1/2H2O complex can
not be avoided in the presence of chloride anion in EG
solvent.
When CuBr2 was used as the copper source, Tu dis-
plays a double-face character determined by the pres-
ence or absence of PVP. Figures 4 and 5 show the XRD
patterns and SEM images of the products as-obtained at
different temperature, using CuBr2 as the copper source
in the presence of PVP. Figure 4a is the XRD pattern of
the grayish black product obtained by reacting at 120
℃ for 12 h (no reaction took place below 120 ℃), on
which all the peaks can be indexed to be hexagonal
phase Cu8S5 (a＝3.863 Å, c＝46.1 Å,
Figure 5 SEM images of the products prepared from CuBr2
with PVP at different temperature, (A) 120 ℃, (B) 150 ℃, (C)
180 ℃ and (D) 210 ℃.
JCPDS No. 33-0491). In Figure 5A, the product is
composed of nest-like structures with hollow cores with
sizes of about 800－1200 nm, and no other structures,
such as rod-like or wire-like, are observed any more. No
precursor was observed this time. As the reaction tem-
perature increased from 120 to 210 ℃, form XRD pat-
terns in Figure 4, it is observed a phase transition from
hexagonal phase Cu8S5 to cubic phase Cu7.2S4. The
completely transformation of Cu8S5 into cubic phase
Cu7.2S4 takes place at 210 ℃ (Figure 4d). SEM images
in Figure 5A－5D are in good agreement with XRD
patterns. At 120 and 150 ℃, Cu8S5 kept nest-like ar-
chitectures consisting of very thin flakes. When the tem-
perature increased to 180 or 210 ℃, it also kept the
nest-like architectures, but the flakes on the nest-like
architectures were destroyed. This result was caused by
the phase transition.
It is surprising for us to observe that white precipita-
tion emerged immediately when the CuBr2 solution and
Tu solution were mixed at room temperature. Figure 6A
shows the XRD pattern of the white precipitation. The
diffraction peaks possess the same profile as
[Cu(tu)]Cl•1/2H2O complex, except for the diffraction
peak shift to the low angle. This kind of shift can be
attributed to the enlargement of the crystal lattice con-
stants resulting from the substitution of Cl with Br. The
formula of the white precursor could be written as
[Cu(tu)]Br•1/2H2O. Corresponding EDS spectrum (Fig-
ure 6B and Figure S10) of the precursor demonstrates
that the crystal consists of Cu, S, N, C, and Br, and the
molar ratio of Cu to S to Br is about 1.17∶1∶1.17,
which is close to the stoichiometry of [Cu(tu)]Br•
1/2H2O.
Furthermore, the C, H, N, and S elemental analysis
(Table S2) of the white precursor from experiment is
according with that of calculated one. The morphology
of the sample was investigated by SEM. Figure 6C is
the SEM image of [Cu(tu)]Br•1/2H2O nanowires, which
Figure 4 XRD patterns of the products prepared from CuBr2
with PVP at different temperature: (a) 120 ℃, (b) 150 ℃, (c)
180 ℃ and (d) 210 ℃.
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Phase Evolution of Cu－S System in Ethylene Glycol Solution: the Effect of Anion and PVP
Figure 6 (A) XRD pattern, (B) EDS spectrum, (C) SEM image, and (D) TG and DTA curves (the sample mass is 5.81 mg) of the
[Cu(tu)]Br•1/2H2O precursor prepared from CuBr2 without PVP at r.t. with stirring for 20 min.
220 to 600 ℃ accompanying with complex endother-
mic-exothermic processes (from DTA peaks). These
indicate that the decomposition steps between 220 and
600 ℃ are complex multistep processes. From TG-
DTA curves, we find the total mass loss is larger than
the calculated one, which is based on the hypothesis that
all copper content is converted into CuO. This phe-
nomenon also occurred in the other copper thiourea
bromide complex, which may come from the excrescent
water molecule in the complex or possible nonstoi-
chiometry of the product.[14]
Interestingly, the white nanowires complex con-
verts into slab-like complex (Figure 7a and Figure S11A)
at a higher reaction temperature (120 ℃). From the
EDS spectrum (Figure S12), it is obvious to see that the
slab-like product is also [Cu(tu)]Br•1/2H2O complex.
This process is probably attributed to the anisotropic
growth of wire-like complex form slab-like morphology
at this condition. While at 150 ℃, hexagonal phase
flower-like CuS (Figure 7b and Figure S11B) was ob-
tained. Finally, the hexagonal phase CuS will transfer
into cubic phase Cu1.8S (Figure 7d) as the temperature
increased.
From above results, it is conclusive that the anion
and PVP have an important effect on the Cu－S system
evolution based on the Tu transition (as show in Scheme
1) and the results of the experiment are summarized in
Figure 7 XRD patterns of the products prepared from CuBr2
without PVP at different temperature: (a) 120 ℃, (b) 150 ℃, (c)
180 ℃, and (d) 210 ℃.
clearly shows that the lengths of these nanowires are 10
－20 μm. The thermal decomposition of [Cu(tu)]Br•
1/2H2O complex was also studied using TG-DTA
analysis (Figure 6D). The first mass loss (7.4%) in re-
gion of 22－220 ℃ can be assigned to the elimination
of water (crystallization water and adsorption water).
[Cu(tu)]Br•1/2H O(s)→[Cu(tu)]Br(s) 1/2H O(g)
＋
2
2
Then the maximal mass loss (71.7%) started from
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Scheme 1 Schematic illustration of the anions influence on the
is that it can act as a reducing agent.
Tu transition
Stimulated by the rich optical properties of CuxS (1
≤x≤2) system, we investigated the UV-vis-NIR ab-
sorption spectra and Raman spectra behaviors of the
as-obtained CuS, Cu1.8S＋CuS and Cu1.8S (obtained
when CuCl2•2H2O was as the copper source with PVP
as the surfactant). The typical UV-vis-NIR spectra are
shown in Figure S13a and the band gap (Eg) of CuS,
Cu1.8S＋CuS and Cu1.8S, estimated by extrapolating the
curve in Figure S13b, is about 1.86, 1.74 and 1.64 eV,
respectively. The UV-vis-NIR absorption spectra infor-
mation clearly indicates that the band gap decreased as
the present copper content increased, which is good in
agreement with the reported data.[16]
Table S3. The effect of various anions and PVP may be
－
explained as follows. The NO3 ion is not only a hard
counterion but also a better electron scavenger than the
other ions and can capture electrons released by the re-
ducing agent.[15] The existence of the nitrate anion could
accept electrons from Tu and induce the isomerization
of thiourea into ammonium thiocyanate. In the next step,
CuSCN formed and then transformed into Cu1.8S, re-
gardless of the existence of PVP. In the above-men-
tioned processes, PVP does not influence the phase
transformation but has an important effect on the mor-
phology. On the other hand, chloride anion is a soft
counterion and it could not receive electrons from
Tu.[15a] Under this condition, thiourea will combine with
the reduced Cu(I) to form a [Cu(tu)]Cl•1/2H2O complex,
then CuS and Cu1.8S were obtained at different tem-
perature regardless of the existence of PVP. And the
results show that the existence of PVP could not only
accelerate the transformation of [Cu(tu)]Cl•1/2H2O into
Cu1.8S, but also reduce the size distribution of the final
product. The changes in size could be attributed to the
increase of viscosity by adding PVP in the reaction sys-
tem. Bromine anion is also a soft counterion and has an
analogous character as chloride anion, so it undergoes a
same process of [Cu(tu)]Br•1/2H2O complex to Cu1.8S
Figure 8 Raman spectra of the copper sulfides prepared from
CuCl2•2H2O with PVP at different temperature: (a) 150 ℃, (b)
180 ℃ and (c) 210 ℃.
On the other hand, we know Raman spectrum is
widely used to provide information relating to the
structure and properties of semiconductors and super-
conductors. In our experiment, it is employed to charac-
terize Cu1.8S and CuS. Figure 8 shows the Raman spec-
tra of the as-obtained products. The Raman spectrum of
each one shows only one intense peak at around 473
－
1
cm , which can be assigned to the S－S stretching
in the absence of PVP. However, Tu decomposed di-
mode of S2 ions at 4e sites. The Raman shift of Cu1.8S is
－
rectly to release S2 when PVP was added, producing
－
－
1
1
observed at 470 cm , and for CuS it is at 475 cm ,
hexagonal phase Cu1.6S. The as-formed Cu1.6S will then
transform to cubic phase Cu1.8S upon the temperature
increasing. This unexpected result may be explained by
the fact that the complex formed by CuBr2 and Tu is
unstable in an alkaline environment in the presence of
PVP. This explanation could be further confirmed by
the experiment that the [Cu(tu)]Br•1/2H2O complex
prepared at room temperature could dissolve in the EG
solution of PVP, while the [Cu(tu)]Cl•1/2H2O complex
could not dissolve. As a result, when PVP was added
into the solution, [Cu(tu)]Br•1/2H2O complex does not
form. On the other hand, it must be mentioned that the
EG also plays important roles in the Cu－S system evo-
lution. One character is that it can serve as a good sol-
vent to offer a polyol process because of its high dielec-
tric constants and boiling points, and the other character
while the observed shift of Cu1.8S＋CuS is centered at
－
1
473 cm , which is between those of the CuS and Cu1.8S.
This is in accordance with the previous report that the
shifting of the bands will slightly move to higher wave
numbers with the decrease of the copper component in
most reports since it is the average sulfur valence that is
changing in these systems.[3a,3b,17]
Conclusions
In conclusion, we revealed that anion and PVP in-
fluence on the transformation of thiourea in EG solution
and this different transformation leads to different phase
evolution of Cu－S system. These results may be attrib-
uted to the different properties of various anions, re-
ducibility and alkaline of PVP and the reducing ability
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Chin. J. Chem. 2013, 31, 1015—1021

Phase Evolution of Cu－S System in Ethylene Glycol Solution: the Effect of Anion and PVP
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source with PVP as the surfactant) demonstrate an ob-
vious stoichiometry-dependent features. The result of
this study not only provides a new and systemic under-
standing of the transformation of thiourea in EG solu-
tion but also offers a better understanding of the optical
behaviors of copper sulfides.
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Acknowledgement
This work was financially supported by the National
Natural Science Foundation of China (Nos. 21171007,
20701002 and 21001006).
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Chin. J. Chem. 2013, 31, 1015—1021
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