Large scale preparation of zinc nanosheets by thermochemical reduction of ZnS powders

Article abstract of DOI:10.1016/S0009-2614(03)00472-X

Zn nanosheets have been synthesized by a simple thermochemical reaction process at a N₂ atmosphere with CO and H₂ gases which were produced in situ. The synthesized Zn nanosheets have dimensions of tens of microns and an average thickness of about 20 nm. These nanosheets were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and a high-resolution transmission electron microscope (HRTEM) equipped with an X-ray energy dispersive spectrometer. The nanosheets are single crystals with a hexagonal structure, which grow along both [1 0 0] and [0 1 0] directions within the (0 0 1) planes.

Full text of DOI:10.1016/S0009-2614(03)00472-X

Chemical Physics Letters 372 (2003) 640–644
www.elsevier.com/locate/cplett
Large scale preparation of zinc nanosheets by
thermochemical reduction of ZnS powders
*
Ying-Chun Zhu , Yoshio Bando
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
Received 11 January 2003; in final form 26 February 2003
Abstract
Zn nanosheets have been synthesized by a simple thermochemical reaction process at a N
2
atmosphere with CO and
H
2
gases which were produced in situ. The synthesized Zn nanosheets have dimensions of tens of microns and an
average thickness of about 20 nm. These nanosheets were studied by X-ray diffraction (XRD), scanning electron mi-
croscopy (SEM), and a high-resolution transmission electron microscope (HRTEM) equipped with an X-ray energy
dispersive spectrometer. The nanosheets are single crystals with a hexagonal structure, which grow along both [1 0 0]
and [0 1 0] directions within the (0 0 1) planes.
Ó 2003 Elsevier Science B.V. All rights reserved.
1
. Introduction
chargeable alkaline Zn-air battery was recently
assembled by using a porous Zn/ZnO electrode [7].
Zinc metal is also an important precursor for the
preparation of ZnO, ZnS, ZnSe, and ZnTe etc.
semiconductors, which have extensive applications
in electronics, photoelectronics and photonics.
Although zinc metal is an important material but
there are only a few reports on nanostructured zinc
metal [8–13]. Wang prepared Zn nanobelts by re-
ducing ZnS with carbon at 1000 °C below the
sublimation point of ZnS [12], in which a gas–solid
reaction process is involved. We herein report a
new and simple method to prepare single crystal-
line zinc nanosheets in a large scale production, in
which a gas–gas reaction process is involved. The
gas–gas process may take place more effectively.
The formation process of zinc nanosheets involves
the thermal evaporation of ZnS powders, decom-
position and reduction of ZnS vapors, and solidi-
Great interests in nanostructured materials
have been focused on controlling the shapes of
materials and finding novel properties. Many
materials including metals, oxides, sulfides, nit-
rides and carbides etc. have been shaped into
one-dimensional structures, such as nanotubes,
nanowires, nanobelts etc. [1–3]. Two-dimensional
nanostructure such as nanosheets is also an im-
portant category of nanostructured materials to
which increasing attention has been paid recently
[4–6]. As is well known, zinc metal is an important
material for the fabrication of batteries. Re-
*
Corresponding author. Fax: +81-298-51-6280.
E-mail addresses: yczhu2010@yahoo.com.cn, yingchun.zhu
nims.go.ip (Y.-C. Zhu).
@
0
009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00472-X

Y.-C. Zhu, Y. Bando / Chemical Physics Letters 372 (2003) 640–644
641
fication of zinc vapors. This method can produce
single crystalline zinc nanosheets in a large scale.
The as-prepared nanosheets may be used as pre-
cursors to synthesize ZnO and other zinc-related
compounds nanostructures [10,11].
was measured by a SEM as shown in Fig. 1a. Most
of the as-prepared products have a sheet-shaped
morphology with a dimension of tens of microns.
The thickness of the as-prepared nanosheets is
about 20 nm as confirmed in Fig. 1b. The EDS
analysis reveals that the composition of the
nanosheets is zinc metal as shown in the inset of
Fig. 1b. The carbon and copper in inset of Fig. 1b
should be the contribution of the copper grid and
the carbon film coated on the grid. The X-ray
diffraction pattern of zinc nanosheets is shown in
Fig. 2. The sample shows reflections at 2.47, 2.30,
2
. Experimental
Commercial ZnS powders were used as pre-
cursors. A RF induction furnace was used for the
thermal reaction process. The ZnS powders were
put into a graphite crucible, and graphite powders
and fibers were put into the other one. The two
crucibles were enclosed into a graphite susceptor
with the crucible containing ZnS powders located
above the other crucible. The susceptor was put in
the center of the RF induction furnace with a
ꢀ
2.09, 1.77, 1.68, and 1.34 A, which matches with
those assigned to the hexagonal zinc phase
(JCPDS Card: 04-0831). The adornments on the
surface of the nanosheets are Zn nanobelts, small
2
flowing N atmosphere (1.5 l/min). The positions
of the two crucibles were adjusted so that the
graphite powders and ZnS powders were heated at
1
N =H O stream (obtained from bubbling N
500–1600 and 1200–1250 °C, respectively. The
2
2
2
through distilled water) was managed to pass
through the graphite powders, where CO and H
gases were generated from the reaction of graphite
and water. Then the N stream together with the
CO and H gases passed through zinc sulfide
2
2
2
powders. ZnS powders were evaporated, decom-
posed, and reduced into Zn vapors in the graphite
susceptor, zinc vapors were then solidified on the
quartz wall of the furnace. The products were
collected on the quartz wall of the furnace at a
temperature of 200–400 °C. The as-prepared
product was characterized by X-ray diffractometer
and scanning electron microscopy. The sample was
also characterized using a JEM-3000F (JEOL)
high-resolution transmission field emission elec-
tron microscopy (HRTEM) operated at 300 kV,
which was attached with an X-ray energy disper-
sive spectrometer.
3
. Results and discussion
Fig. 1. SEM morphology and composition analyses of Zn
nanosheets obtained from the thermochemical reduction of ZnS
powders: (a) SEM image of zinc nanosheets; (b) cross-section of
Zn nanosheets. Inset: EDS spectrum of zinc nanosheets.
Products with a silver-gray color were obtained
from the thermochemical reduction of ZnS pow-
ders. The morphology of the as-prepared products

642
Y.-C. Zhu, Y. Bando / Chemical Physics Letters 372 (2003) 640–644
indicated by arrows. Fig. 4b shows a small piece of
zinc nanosheets, of which the angles of adjacent
edges are also 120°. The surface of the nanosheets
should be the (0 0 1) plane of the hexagonal zinc
phase. The angles of 120° should be those of the
(1 0 0) and (0 1 0) planes, and the edges should
correspond to the (1 0 0) and (0 1 0) planes, re-
spectively. The proposal is confirmed by HRTEM
and ED measurements as shown in Fig. 5.
Fig. 5a is the HRTEM morphology of a zinc
nanosheet, which was taken with the electron
beam perpendicular to the surface of the nano-
sheet. The lattice fringes show hexagonal patterns
and the d-spacing indicated by parallel lines is 0.23
nm which corresponds to the distance between the
(1 0 0) planes. The corresponding ED patterns of
Fig. 5a are shown in Fig. 5b, which can be indexed
as a hexagonal Zn with lattice constants of
a ¼ 0:26 nm and c ¼ 0:49 nm recorded from
[0 0 1] zone axis, which is consistent well with the
ideal values of the hexagonal Zn phase (JCPDS
Card: 04-0831). The ED pattern corresponds to
the XRD results as shown in Fig. 2. The foregoing
evidence reveals that the as-prepared Zn nano-
sheets grow along [1 0 0] and [0 1 0] direction within
the (0 0 1) plane. It is difficult to measure the pre-
cise thickness of the Zn nanosheets by TEM be-
cause they are very soft and have a large width to
thickness ratio. When the sample is collected with
copper grids, all the nanosheets lay on the carbon
film.
Fig. 2. XRD pattern of zinc nanosheets.
nanosheets, and other nanostructures which have
dimensions from tens of nanometers to several
microns as shown in Fig. 3.
Fig. 3 shows some special structures discovered
in the products. When the nanosheets were ex-
posed to the electron beam during the TEM
measurement, some of them may shrink into or-
derly striated patterns as shown in Fig. 3a. Zn
nanobelts are also formed during the thermo-re-
duction process as can be seen in Fig. 3b. The zinc
nanobelts have a zig-zag morphology with a width
around 200 nm and a length of several microme-
ters. Fig. 3c shows a grille-shaped zinc pattern
which consists of parallel zinc nanobelts.
The morphology of the zinc nanosheets was
also analyzed by TEM as shown in Fig. 4. Fig. 4a
reveals a part of a large zinc nanosheet with zig-
zag edges. The angles of adjacent edges are 120° as
The formation mechanism of ZnS–Zn nanoc-
ables is proposed as following. At high tempera-
Fig. 3. TEM images of special zinc nanostructures found in the as-prepared products: (a) zinc nanosheet with ordered stria patterns
formed by the electron beam irradiation; (b) zinc nanobelts; (c) grille-shaped zinc pattern consisting of parallel zinc nanobelts.

Y.-C. Zhu, Y. Bando / Chemical Physics Letters 372 (2003) 640–644
643
Fig. 4. TEM images of zinc nanosheets: (a) TEM image of a part of a large zinc sheet. The angles of the adjacent edges are 120° as
indicated by the crossed arrows; (b) TEM image of a small piece of zinc nanosheets.
ture regions, carbon and H
and H which were carried by the N gas. In the
2
O react to produce CO
2
2
reductive atmosphere, ZnS powders may evaporate
into ZnS vapors, and then ZnS vapors decompose
into Zn and S
and CO gases resulting in the formation H
COS gases. At low temperature regions, Zn vapors
were solidified to form Zn nanosheet, H S and
COS gases were carried away by the N -flow. The
2
vapors. S
2
vapors may react with H
2
2
S and
2
2
involved reactions during the formation process of
Zn nanosheets are listed as follows. The Gibbs free
energy values of these reactions were estimated on
the base of the thermodynamic properties [14,15].
It can be seen that all the reactions are thermody-
namically spontaneous process since the Gibbs free
energies are negative.
C
ðsÞ
þ H O ! COðgÞ þ H2ðgÞ
2
ðgÞ
ðDG°1873 K ¼ ꢀ125 kJ=molÞ
ð1Þ
ð2Þ
ZnSðsÞ ! ZnSðgÞðDG°1500 K ¼ ꢀ72 kJ=molÞ
1
ZnSðgÞ ! ZnðgÞ þ S2ðgÞðDG°1500 K ¼ ꢀ50 kJ=molÞ
2
ð3Þ
Fig. 5. HRTEM image of zinc nanosheets: (a) HRTEM mor-
phology of the zinc nanosheet which was taken with the electron
beam perpendicular to the surface of the nanosheet (b) ED pat-
tern of zinc nanosheet corresponding to the HRTEM image (a).
1
H
2
þ S2ðgÞ ! H
2
SðDG°1500 K ¼ ꢀ16 kJ=molÞ
2
ð4Þ

644
Y.-C. Zhu, Y. Bando / Chemical Physics Letters 372 (2003) 640–644
1
nanosheets may form orderly striated patterns
under the electron beam irradiation, which may
lead to special applications in electronics.
CO þ S2ðgÞ ! COSðgÞðDG°1500 K ¼ ꢀ217 kJ=molÞ
2
ð5Þ
ZnðgÞ ! ZnðsÞðDG°573 K ¼ ꢀ67 kJ=molÞ
ð6Þ
Acknowledgements
The formation process of zinc nanosheets in-
volves a vapor–solid mechanism. As the tempera-
ture of the ZnS precursor was controlled above
the sublimation point of ZnS (ca. 1180 °C) [16],
ZnS vapors may be generated rapidly, which were
decomposed and reduced into Zn vapors. When
the Zn vapors were cooled down quickly at low
temperatures of 200–400 °C, large quantities of Zn
nucleuses were generated and the Zn nucleuses
grow along [1 0 0] and [0 1 0] direction forming Zn
nanosheets. Wang prepared Zn nanobelts by re-
ducing ZnS with carbon at 1000 °C under a tem-
perature below the sublimation point of ZnS [12].
The different shapes of the products may be as-
cribed to the differences in the reaction process.
The reduction of ZnS in reference [12] is a gas–
solid process, while the reduction of ZnS in the
present work is a gas–gas process which may take
place more quickly and completely, and thus
generates more Zn vapors. Large quantities of Zn
nanosheets were found accumulating quickly on
the quartz wall of the furnace.
This work was supported by the Japan Science
Promotion Society (JSPS) fellowship at the Na-
tional Institute for Materials Science, Tsukuba,
Japan.
References
[
[
[
1] S. Iijima, T. Ichihashi, Nature 36 (1993) 603.
2] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
3] Y.C. Zhu, H.L. Li, Y. Koltypin, Y.R. Hacohen, A.
Gedanken, Chem. Commun. 24 (2001) 2616.
[4] M. Shirai, K. Igeta, M. Arai, J. Phy. Chem. 105 (2001)
211.
7
5] N. Miyamoto, H. Yamamoto, R. Kaito, K. Kuroda,
Chem. Commun. (2002) 2378.
[
[
[
6] T. Sasaki, M. Watanabe, J. Phys. Chem. 101 (1997) 10159.
7] E. Deiss, F. Holzer, O. Hass, Electrochim. Acta 47 (2002)
3995.
[
[
8] J. Narayan, R.K. Venkatesan, A. Kvit, J. Nanopart. Res. 4
(
2002) 265.
9] Y. Zhu, H. Wang, P.P. Ong, J. Phys.: condens. Mater. 13
2001) 787.
(
[
[
[
[
[
[
[
10] R. Wu, C.S. Xie, H. Xia, A.H. Wang, Scripta Mater. 43
(2000) 841.
11] S.S. Chang, S.O. Yoon, H.J. Park, A. Sakai, Mater. Lett.
In summary, Zn nanosheets, nanobelts and
other nanostructures were synthesized in a large
quantity by the thermochemical reduction of ZnS
5
3 (2002) 432.
12] Y. Wang, L. Zhang, G. Meng, C. Liang, G. Wang, S. Sun,
Chem. Commun. (2001) 2632.
2 2
powders in the N atmosphere with CO and H
gases which were produced in situ. The as-pre-
pared Zn nanosheets are the hexagonal phase
growing in both [1 0 0] and [0 1 0] axis directions.
The formation of these nanosheets is a thermo-
dynamically spontaneous growth process via the
nucleation of Zn metal and the growth in [1 0 0]
and [0 1 0] directions. It is also found that the Zn
13] Y.C. Zhu, H.L. Li, Y. Koltypin, A. Gedanken, J. Mater.
Chem. 12 (2002) 729.
14] I. Barin, Thermochemical Data of Pure Substances, VCH
Publishers, New York, 1989.
15] R.C. Weast, Handbook of Chemistry and Physics, Chem-
ical Rubber Publishing Company, Ohio, 1974.
16] F.D. Rossini, D.D. Wagman, W.H. Evans, S. Levine,
I. Jaffe, Nat. Bur. Std. (U.S.) Circ. (1952) 500.