The synthesis and characterization of Pb5S2I6 whiskers and tubules

Article abstract of DOI:10.1016/S1387-7003(03)00084-4

Pb₅S₂I₆ whiskers and tubules were synthesized from the reaction among lead chloride, thiourea, and excess sodium iodide under hydrothermal conditions at 200 °C for 20-40 h. XRD, SEM, XPS, ICP-AES, and TEM characterized the final products. Most products are whiskers with structure of 3-4 mm in length, 0.5-2.0 μm in diameter for a singular one. Meanwhile, about 10% tubules are produced in the process. The tubules are 3-6 mm in length, 8-20 μm in diameter, and 1-3 μm in thickness. Nanowhiskers were also produced in the route at 180-200 °C for 8-10 h. Raman spectra show that the Pb₅S₂I₆ crystals have complex vibrational modes of PbS and PbI₂.

Full text of DOI:10.1016/S1387-7003(03)00084-4

Inorganic Chemistry Communications 6 (2003) 670–674
www.elsevier.com/locate/inoche
The synthesis and characterization of Pb S I whiskers and tubules
5 2 6
*
Qing Yang, Kaibin Tang , Chunrui Wang, Jian Zuo, Yitai Qian
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China
Received 18 December 2002; accepted 4 March 2003
Abstract
Pb5S2I6 whiskers and tubules were synthesized from the reaction among lead chloride, thiourea, and excess sodium iodide under
hydrothermal conditions at 200 °C for 20–40 h. XRD, SEM, XPS, ICP-AES, and TEM characterized the final products. Most
products are whiskers with structure of 3–4 mm in length, 0.5–2.0 lm in diameter for a singular one. Meanwhile, about 10% tubules
are produced in the process. The tubules are 3–6 mm in length, 8–20 lm in diameter, and 1–3 lm in thickness. Nanowhiskers were
also produced in the route at 180–200 °C for 8–10 h. Raman spectra show that the Pb5S2I6 crystals have complex vibrational modes
of PbS and PbI2.
Ó 2003 Elsevier Science B.V. All rights reserved.
PACS: 61.10.N; 61.66.F; 61.72.
Keywords: Chalcogenide halides; Crystals; Hydrothermal reaction; X-ray photoelectron spectroscopy; Raman spectroscopy
1
. Introduction
were also studied [17,18]. More recently, iodide-doped
lead sulfide crystals have been developed as semicon-
ducting sensors for determining nitrogen oxides (NO,
NO2) [19].
Chalcogenide halides are a group of important solid-
state materials having many interesting properties such
as high conductivity, photoconductivity, ferroelectricity,
and piezoelectricity [1–4]. Owing to their novel prop-
erties having possible potential applications and pos-
sessing technological importance, chalcogenide halides
have excited growing interest in the scientific world
during the past several decades [5–10]. Pb S I as an
important IV–V–VI chalcogenide halide compound was
first synthesized by heating the mixture of PbS and PbI2
at 350 °C in 1968 [11]. The phase diagram of the
PbS–PbI2 was investigated by X-ray diffraction and
DTA methods [12]. Thereafter, the crystallization con-
ditions of Pb5S2I6 in the system Pb–S–I–R–H2O (R
being a solvent) have been investigated by Popolitov
and co-workers [13–16]. Meanwhile, the dielectric and
photosemiconducting properties of the Pb5S2I6 crystals
Currently, both one-dimensional (1D) materials and
three-dimensional (3D) mesoscale tubular ones have
attracted great attention because of their potential
application in microscopic research and the develop-
ment of devices. Meanwhile, hydrothermal synthesis as
a versatile synthetic technique has been intensively
employed to prepare inorganic materials including
single crystals and nanocrystals. With this method, we
have successfully synthesized several different chalco-
genide (including tubular) crystals [20,21]. Based on
our recent researches, we first obtained the tubular
Pb5S2I6 crystals from a hydrothermal reaction of io-
dine disproportionation [22]. Compared with the re-
5
2 6
action among Pb, S, I , and HI at above 300 °C [14],
2
the reaction we reported is moderate. In order to
extend the synthesis of tubular crystals and to inves-
tigate the properties, we reported the preparation of
the Pb5S2I6 whiskers and tubular crystals under hy-
drothermal reaction from iodide I-source at 200 °C
and measure the crystals by Raman spectroscopy in
the present paper.
*
Corresponding author. Tel.: +86-551-3601600; fax: +86-551-
3
601600.
E-mail addresses: qyoung@ustc.edu.cn (Q. Yang), kbtang@ustc.
edu.cn (K. Tang).
1
387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00084-4

Q. Yang et al. / Inorganic Chemistry Communications 6 (2003) 670–674
671
2
. Experimental
change among lead chloride, thiourea, and excess so-
dium iodide at a moderate temperature. The formation
of the products may be proposed as follows:
2
.1. Synthesis
ðNH
2
Þ2CS þ 2H
2
O ! H
2
S þ CO
2
þ 2NH
3
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
Pb5S2I6 crystals were synthesized in a Teflon-lined
stainless steel autoclave through the hydrothermal syn-
thetic route. In a typical experiment, an appropriate
amount of analytically pure PbCl (1.0 g), ðNH Þ CS
PbCl þ H S ! PbS þ 2HCl
2
2
PbCl
2
þ 2NaI ! PbI þ 2NaCl
2
2
2 2
and NaI (with a molar ratio of 5:2:6–12) was added into
the Teflon-lined autoclave, which was filled up to 90% of
the total volume (50 ml) with distilled water. Then, the
autoclave was maintained at 200 °C for 20–40 h and
cooled to room temperature, naturally. The as-prepared
precipitates were filtrated, washed with carbon disulfide,
distilled water, and absolute ethanol several times to
remove byproducts. After drying in a vacuum at 60–70
2PbS þ 3PbI
2
! Pb
5 2 6
S I
þ
À
HCl þ NH ! NH þ Cl
In these procedures, PbS was first formed, which could
be detected in a short reactive period (2 h). The early-
formed PbS provides a nucleus for subsequent combi-
nation with PbI
crystals. Reaction (5) is the one for by-products.
Fig. 1 shows the X-ray diffraction (XRD) patterns of
the polycrystalline powder ground from the products.
The XRD spectra could be indexed to the monoclinic
Pb5S2I6 phase with lattice parameters a ¼ 14:333,
b ¼ 4:432, c ¼ 14:529, and b ¼ 98:0°, which are in
agreement with the reported data for Pb5S2I6 [12]. No
other characteristic peaks of impurities such as PbS or S
were observed. The peaks of by-products of NaCl or
3
4
2 5 2 6
in (4) for the formation of the Pb S I
°C for 2 h, the samples in orange red color were ob-
tained.
2
.2. Characterization of products
The products were determined by X-ray diffraction
XRD) operated on a Chain Dandong rotating anode
(
X-ray diffractometer with graphite monochromatized
ꢀ
Cu Ka radiation (k ¼ 1:54178 A). The scanning rate of
NH Cl were not obviously observed in the spectra.
4
0
.060°/s was applied to record the patterns in the 2h
The scanning electron microscopy (SEM) images of
as-grown Pb5S2I6 crystals are shown in Fig. 2. It can be
seen that the morphologies of Pb5S2I6 crystals are rod-
like whiskers (Fig. 2(a)). Typically, the structure is 3–4
mm in length, 0.5–2.0 lm in diameter for a singular
crystal. With further observation, some larger crystals
have hollow structure. Electron microscope images of
the samples reveal that the tubules are about 10% in the
total yield. The tubules are 3–6 mm in length, 8–20 lm
in diameter, and 1–3 lm in thickness (Figs. 2(b–c)). Fig.
range of 10–60°. The morphology of the crystals was
observed by scanning electron microscopy (SEM),
which was performed on a Hitachi X-650 scanning
electron microanalyzer. The verification of product
formation was obtained by X-ray photoelectron spectra
(
XPS), which was collected on an ESCALAB MK II X-
rays photoelectron spectrometer by using nonmono-
chromatized Mg Ka X-ray as the excitation source.
Meanwhile, elemental analysis was done by ICP-AES,
which was carried on an Atomscan Advantage (Thermo
Jarrell Ash Corporation). Transmission electron mi-
croscopy (TEM) photographs were taken with a Hitachi
Model H-800 transmission electron microscope, using
an accelerating voltage of 200 kV. The samples for these
measurements were dispersed in absolute ethanol with
arc ultrasonic generator and then the solutions were
dropped onto copper grids coated with amorphous
carbon films. Raman spectra of the samples were re-
corded on a Spex 1403 Raman spectrometer with 514.5
nm excitation lines, which were recorded at ambient
2(b) shows a tubule with thin wall, whereas Fig. 2(c)
shows one with thick wall. Comparing with our previous
synthesis [22], we obtained similar tubular crystals from
temperature. The spectra were obtained in the range 40–
À1
8
1
step.
00 cm
at a laser power of 150 mW with slit of
À1
40 lm (ꢀ1 cm ) and an integration time of 0.5 s per
3
. Results and discussion
In our hydrothermal synthetic route, the Pb5S2I6
whiskers and tubules were achieved from the reaction
Fig. 1. Powder X-ray diffraction (XRD) patterns of Pb5S2I6 products.

672
Q. Yang et al. / Inorganic Chemistry Communications 6 (2003) 670–674
Conditions of the reaction system were investigated
in the experiments. The pH value of reaction system
mainly affects the formation and the morphology of the
Pb5S2I6 crystals. The acidic condition is favorable for
the growth of the Pb5S2I6 crystals. On one hand, acid
2
þ
condition is favorable for the transition of Pb from
PbS to Pb S I , and on the other hand, the acidic con-
5
2 6
dition promotes the formation of the tubular crystals
because the soluble effect of acid could form nucleus
with hollow structure. Otherwise, PbS is predominated
in the products in basic conditions from the experi-
ments. In our previous synthesis [22], the acidic condi-
tions were obtained by the I2 disproportionation under
hydrothermal conditions. In the present route, it may be
adjusted by dropping 2–5 ml of 5% HCl in solution if it
is necessary. When we did not drop acid into the system,
the yield of the tubules was not more than 10%. When
we dropped HCl into the system down to pH less than 3,
the yield of the tubules had about a half time increase. It
is noted that over acidic condition (pH less than 1) will
make the crystals become irregular. According to phase
Fig. 2. SEM images of the as-produced Pb5S2I6 crystals. (a) The low-
magnification image of the crystals. (b–c) The typical images of the
tubular crystals. (d) The end surface of a crystal with some minute
hollows.
diagram of Pb–S–I–R–H O, reaction temperature is also
2
a critical factor to the formation of Pb S I . When
5
2 6
temperature was less than 160 °C, the products could
not be well crystallized. Other factors such as the
reagents and the feedstock may affect the products and
the yield in the experiments. S-sources of Na2S and
the different reagents. We could conclude that the
morphology of the whiskers is mainly caused by the
anisotropies and the different growth speeds of crystal
plane. In the route, the tubules may be formed from
some hollow nucleus. Fig. 2(d) is a SEM image of an end
surface of a crystal. It is observed that the surface is not
even and it contains some hollow structures, which
could serve as the hollow nucleus to form tubules in the
epitaxial growth along axial direction.
ðNH4Þ S were also used to synthesize the target crystals
2
in the route. These basic S-sources are unfriendly for the
formation of Pb5S2I6 crystals unless an acidic condition
is adjusted (by adding HCl into the reaction system).
Based on the investigation from the experiments, it
could be concluded that the anisotropic 1D crystal
growth is the intrinsic growth nature of the monoclinic
Pb5S2I6, while the hollow structure of the crystals could
be obtained by altering the growth conditions. In the
route, Pb5S2I6 nanowhiskers with 50–300 nm in diam-
eter and more than 10 lm in length were also synthe-
To provide evidence for the formation of the Pb5S2I6
crystals, XPS was employed to analyze the obtained
products. Fig. 3(a) is the survey spectra of samples ob-
tained in the process. From the spectra, the XPS results
show that the presence of Pb, S, and I from the binding
energies, which are consistent with the formation of
Pb S I . The close-up spectrum of Pb was shown in Fig.
sized among PbCl2, ðNH2Þ CS and NaI at 180–200 °C
2
for 8–10 h (Fig. 4).
The Raman spectra of the samples obtained in the
process are shown in Fig. 5. Although the spectra of
Pb5S2I6 have not been reported before in the literature,
Pb5S2I6 could be considered as a complex system as
PbS–PbI2 generally. That is, they could be shown as the
complex behaviors of the two vibrational modes of PbS
and PbI2 in Raman spectroscopy. The intense peak lo-
5
2 6
3
4
(b) and the obtained value of the binding energy for Pb
f7=2 is 139.45 eV, which is near to the binding energy of
Pb 4f7=2 in PbSO4 [23]. Fig. 3(c) is the spectra for S 2p
(
(
161.50 eV) core, which shows that no elemental sulphur
164.00 eV) [24] was observed in the as-produced Pb5S2I6
À1
crystals. The broadened peak of S 2p may show the two
cating at 133 cm (Fig. 5(a)) could be signed as the
lattice mode vibration for PbS [26]. The broad peak
2
À
coordination states of S in the crystals. Elemental io-
dine was also analyzed by XPS and the close-up spectra
show the binding energy for I 3d5=2 is 620.20 eV (shown
in Fig. 3(d)). The value is close to the reported value for I
À1
observed at 270 cm could be attributed to two-pho-
non process in PbS [27]. The weak broad band observed
À1
at about 435 cm could be assigned to the first over-
tones of the LO phonon mode for PbS according to the
literature [28]. Meanwhile, the acoustic modes of PbS
3
d5=2 (620.50 eV) in Rb3Sb2I9 [25]. Elemental analysis
was employed to analyze the composition of as-prepared
products by ICP-AES and the results revealed that the
obtained Pb S I crystals are close to stoichiometry.
À1
have been observed at 46 and 62 cm [27b]. However,
owing to the differences between PbS and Pb5S2I6, the
5
2 6

Q. Yang et al. / Inorganic Chemistry Communications 6 (2003) 670–674
673
Fig. 3. XPS analysis of the as-produced Pb5S2I6 crystals: (a) The survey spectra of the products. (b) The close-up Pb 4f. (c) The close-up S 2p. (d) The
close-up I 3d.
À1
relative intensities of the two peaks have changed and
À1
peak at 125 cm , not been assigned, may only attribute
to the Pb5S2I6 crystals. Fig. 5(b) shows the spectra for
the nanowhiskers prepared in the route. The overall
spectra have no obvious difference from the crystals
(Fig. 5(a)). We may conclude that the nanowhiskers
the peak at 62 cm is shifted from its normal position
À1
(68 cm ). With further investigation, we could identify
the PbI2 vibrations from the spectra. The peak at
À1
7
PbI [29]. The peaks at 97–118 cm could be assigned
7 cm is attributed to the Raman active mode E of
À1
g
2
to the optical phonon modes [30]. It is noted that the
À1
intense peak of A1g at 97 cm in PbI2 becomes ex-
tremely weak, which may also be caused by the combi-
nation of PbS and PbI2 in Pb5S2I6 crystals. The shoulder
Fig. 5. Raman spectra of the samples. (a) The spectra for the as-
prepared Pb5S2I6 whiskers and tubules. The inset is the close-up
À1
spectra of the crystals from 40 to 200 cm . (b) The spectra for the
Fig. 4. TEM image of the Pb5S2I6 nanowhiskers produced in the route.
Pb5S2I6 nanowhiskers.

674
Q. Yang et al. / Inorganic Chemistry Communications 6 (2003) 670–674
have no size confinement because no peak shift was
observed in the spectra.
[7] J. Weissenstein, J. Horak, Czech. J. Phys. 24 (1974) 235.
[8] M. Sergent, O. Fischer, M. Decroux, C. Perrin, R. Chevrel, J.
Solid State Chem. 22 (1977) 87.
[9] (a) J.D. Corbett, R.K. McMullan, D.J. Prince, Inorg. Chem. 10
(1971) 1749;
(b) A.H. Cowley, S.T. Cohen, Inorg. Chem. 3 (1964) 780.
4
. Summary
[
[
[
10] A. Ibanez, J.C. Jumas, J.O. Fourcade, E. Philippot, M. Maurin, J.
Solid State Chem. 48 (1983) 272.
11] A. Rabenan, H. Rau, G. Rosenstein, Naturwissenschaften 55
In summary, Pb S2I6 whiskers and tubules were
5
achieved from the moderate reaction among lead chlo-
ride, thiourea, and excess sodium iodide under hydro-
thermal conditions at 200 °C for 20–40 h. Meanwhile,
the nanowhiskers with 50–300 nm in diameter and more
than 10 lm in length were also synthesized in the route
at 180–200 °C for 8–10 h. The possible mechanism and
affects on the formation of the Pb5S2I6 crystals with
different size and morphologies are discussed. The Ra-
man spectra show that the Pb S I crystals have com-
(
1968) 82.
12] A. Rabenan, H. Rau, Z. Anorg. Allg. Chem. 369 (1969)
295.
[13] Yu.M. Mininzon, V.I. Popolitov, A.N. Lobachev, Sov. Phys.
Crystallogr. 22 (1977) 717.
[
14] V.I. Popolitov, A.N. Lobachev, V.F. Peskin, Yu.M. Mininzon,
Ferroelectrics 21 (1978) 421.
[15] V.I. Popolitov, G.F. Plakhov, Neorg. Mater. 25 (1989) 726.
[16] V.I. Popolitov, G.F. Plakhov, Neorg. Mater. 27 (1991) 2618.
[17] V.I. Popolitov, PisÕma Zh. Tekh. Fiz. 22 (1996) 78.
5
2 6
[
18] K. Klaus-Kurasiak, W. Wojciak, J. Signalaufzeichnungsmater 10
1982) 105.
plex vibrational modes of PbS and PbI . The spectra
2
(
also show that the prepared nanowhiskers have the same
structure as the bulk crystals.
[
[
19] V.F. Markov, L.N. Maskaeva, J. Anal. Chem. 56 (2001) 754.
20] (a) J.Q. Hu, Q.Y. Lu, B. Deng, K.B. Tang, Y.T. Qian, G.E.
Zhou, X.M. Liu, Inorg. Chem. Commun. (1999) 569;
(b) J. Hu, B. Deng, Q. Lu, K. Tang, R. Jiang, Y. Qian, G. Zhou,
H. Cheng, Chem. Commun. (2000) 715.
Acknowledgements
[
[
21] (a) Q. Yang, K.B. Tang, C.R. Wang, Y.T. Qian, W.C. Yu, G.E.
Zhou, F.Q. Li, J. Mater. Chem. 11 (2001) 257;
The project supported by Anhui Provincial Natural
Science Foundation (Grant no. 3044901) and the Na-
tional Natural Science Foundation of China are grate-
fully acknowledged. We are indebted to Prof. G.E.
Zhou, C.Y. Xu, F.Q. Li, J.X. Wu, and Dr. X.M. Liu for
their help in obtaining the XRD, SEM, XPS, and TEM
results and the valuable comments.
(
b) Q. Yang, K. Tang, C. Wang, F. Li, B. Hai, G. Shen, C. An,
W. Yu, Y. Qian, J. Cryst. Growth 233 (2001) 287.
22] C.R. Wang, K.B. Tang, Q. Yang, S.H. Lu, G.E. Zhou, F.Q. Li,
W.C. Yu, Y.T. Qian, J. Cryst. Growth 226 (2001) 175.
[23] V.I. Nefedov, Y.V. Salyn, X. Keller, Zh. Neorg. Khim. 24 (1979)
2564.
[
24] N.N. Sergushin, V.I. Nefedov, I.A. Rozanov, Zh. Neorg. Khim.
1 (1976) 856.
2
[
[
25] M.J. Tricker, Inorg. Chem. 13 (1974) 743.
26] B. Minceva-Sukarova, M. Najdoski, I. Grozdanov, C.J. Chun-
nilall, J. Mol. Struct. 410 (1997) 267.
References
[
27] (a) K.K. Nanda, S.N. Sahu, R.K. Soni, S. Tripathy, Phys. Rev. B
58 (1998) 15405;
(b) D.T. Krauss, F.W. Wise, Phys. Rev. B 55 (1997) 9860.
[
[
[
[
1] B. Reuter, K. Hardel, Angew. Chim. 72 (1960) 138.
2] J. Fenner, A. Rabenau, Z. Anorg. Allg. Chem. 426 (1976) 7.
3] R. Nitsche, W.J. Merz, J. Phys. Chem. Solids 13 (1960) 154.
4] E. Fatuzzo, G. Harbeke, W.J. Merz, R. Nitsche, H. Roetschi, W.
Ruppel, Phys. Rev. 127 (1962) 2034.
[28] Y. Batonneau, C. Br ꢁe mard, J. Laureyns, J.C. Merlin, J. Raman
Spectrosc. 31 (2000) 1113.
[29] A. Saitoh, T. Komatsu, T. Karasawa, Phys. Rev. B 62 (2000)
11398.
[
[
5] T.M. Leonova, V.V. Sviridov, Inorg. Mater. 5 (1969) 1016.
6] K. Takei, H. Hagiwara, H. Tanaka, Bull. Chem. Soc. Jpn. 50
[30] R. Mu, Y.S. Tung, A. Ueda, D.O. Henderson, J. Phys. Chem. 100
(1996) 19927.
(1977) 1341.