Oxidation of a ZnS nanobelt into a ZnO nanotwin belt or double single-crystalline ZnO nanobelts

Article abstract of DOI:10.1016/j.ssc.2003.10.009

Single-crystalline wurtzite ZnS nanobelts are synthesized by the vapor phase transport (VPT) process. When oxidized, a single-crystalline ZnS nanobelt turns into a ZnO nanotwin belt containing two twinning parts with a sharp and clear edge, which is a (0001) twinning plane parallel to and running through the length direction. The two twinning parts in a ZnO nanotwin belt have the same crystalline direction, [0001], along their width, and the [011?0] and [112?0] crystalline directions along the length direction. On some ZnO nanobelts, nanovoids appear along the twinning planes and when those nanovoids connect with each other, one original ZnS nanobelt can divide into two single-crystalline ZnO nanobelts with quite clear edges. Published by Elsevier Ltd.

Full text of DOI:10.1016/j.ssc.2003.10.009

Solid State Communications 129 (2004) 233–238
www.elsevier.com/locate/ssc
Oxidation of a ZnS nanobelt into a ZnO nanotwin belt or double
single-crystalline ZnO nanobelts
Yujie Lia, Liping Youa, Ran Duana, Pengbo Shia, Guogang Qina,b,
*
aDepartment of Physics and State Key Lab for Mesoscopic Physics, Peking University, Beijing 100871, People’s Republic of China
bInternational Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic of China
Received 16 September 2003; accepted 15 October 2003 by R. Phillips
Abstract
Single-crystalline wurtzite ZnS nanobelts are synthesized by the vapor phase transport (VPT) process. When oxidized, a
single-crystalline ZnS nanobelt turns into a ZnO nanotwin belt containing two twinning parts with a sharp and clear edge, which
is a (0001) twinning plane parallel to and running through the length direction. The two twinning parts in a ZnO nanotwin belt
ꢀ
ꢀ
have the same crystalline direction, [0001], along their width, and the ½0110Š and ½1120Š crystalline directions along the length
direction. On some ZnO nanobelts, nanovoids appear along the twinning planes and when those nanovoids connect with each
other, one original ZnS nanobelt can divide into two single-crystalline ZnO nanobelts with quite clear edges.
q 2003 Published by Elsevier Ltd.
PACS: 61.82.Rx; 81.05.Ys
Keywords: A. Nanostructures; A. Semiconductors; D. Optical properties; D. Phase transitions
1. Introduction
by a chemical vapor deposition process [15]. Nanosheets of
ZnS achieved from a lamellar molecular precursor
ZnS·(NH2CH2CH2NH2)0.5 can be oxidized into ZnO
dendrites [16]. In this paper, we report a novel phenomenon
that when oxidizing, a wurtzite single-crystalline ZnS
nanobelt turns into a ZnO nanobelt containing two twinning
parts with a (0001) twinning plane parallel to and running
through the length direction. On some ZnO nanobelts,
nanovoids appear along the twinning planes and when those
nanovoids connect with each other, one original ZnS
nanobelt can divide into two single-crystalline ZnO
nanobelts with quite clear edges.
ZnO is one of the mostly interested functional oxides in
recent years for its catalytic [1], electrical [2], photochemi-
cal [3], optoelectronic [4] and especially room-temperature
ultraviolet lasing [5–7] properties. Single ZnO nanowire
based lasing has firstly been reported by Huang et al. [8].
ZnO thin films and other low dimensional structures can be
made by various approaches [8–13]. As-synthesized ZnO
nanorods [9,10], nanowires [8,11], nanotubes [12] and
nanobelts [13] are mostly of the [0001] direction along the
longer dimension. ZnO nanobelts fabricated by vapor phase
transport (VPT) method were used as templates to
synthesize ZnO/ZnS nanocables and ZnS nanotubes through
chemical reactions with H2S in water solution [14]. Most
recently, hexagonal ZnO nanowires were used as templates
for the epitaxial overgrowth of single crystal GaN nanotubes
2. Experimental
The growth of the ZnS nanobelts takes place in a
horizontal quartz tube. Commercial ZnS powders with
purity of 99% were used as the source material and were
kept at 900–950 8C for 1 h in the reactor tube. High purity
Ar gas with a flow rate of 100–200 sccm was introduced
*
Corresponding author. Address: Department of Physics and
State Key Lab for Mesoscopic Physics, Peking University, Beijing
100871, People’s Republic of China. Tel.: þ86-10-627-51618; fax:
þ86-10-627-51615.
0038-1098/$ - see front matter q 2003 Published by Elsevier Ltd.
doi:10.1016/j.ssc.2003.10.009

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Y. Li et al. / Solid State Communications 129 (2004) 233–238
into the reactor tube. The products were deposited on the Si
single-crystalline wafers coated with 2–6 nm thick Au films
and sited downstream the source. The oxidation of the ZnS
nanobelts was performed at 900–950 8C in an atmosphere
of pure O2 with the flow rate of 100 sccm for 1 h.
Transmission electron microscopy (TEM) analysis was
carried out using a Hitachi H-9000 NAR microscope
equipped with an energy-dispersive X-ray spectroscope.
Photoluminescence (PL) spectra were recorded by a Hitachi
F-4500 fluorescence spectrophotometer with a Xe-lamp as
the excitation light source at room temperature. A filter of
310 nm was used during the PL measurement processes.
X-ray photo-electron energy spectra (XPS) were recorded
by a VG ESCA LABS XPS spectrometer.
3. Results and discussions
ZnS nanobelts obtained have uniform widths along the
length directions. The lengths of the nanobelts are in the
range of 50–100 mm, while the widths 150–600 nm. A low
magnification TEM image of the as-deposited nanobelts is
given in Fig. 1(a). The energy dispersive X-ray spectroscopy
analysis proves that the nanobelts are composed of Zn and S
with the atomic ratio of 1:1 within the experimental errors.
Au tips are found to locate at the free-growing ends of the
ZnS nanobelts, as shown by the arrows in Fig. 1(a) and the
ZnS nanoblets may have been grown by the vapor-liquid-
solid mechanism with the catalyst of Au as described by
Wagner and Ellis [17]. As-synthesized ZnS nanobelts are
single crystals with the wurtzite structure. The crystalline
directions of the ZnS nanobelts along the width and the
length dimensions make angles within 0–158 to [0001] and
ꢀ
½0110Š; respectively. As an example, a ZnS nanobelt with
the width and the length directions very near to the [0001]
ꢀ
and the ½0110Š crystalline directions, respectively, is shown
in Fig. 1(b) with a high resolution TEM image shown in the
down-right inset and a corresponding selective area electron
diffraction (SAED) pattern in the upper-right inset.
Fig. 1. (a) A low magnification TEM image of ZnS nanobelts. The
arrows show the solidified Au catalyst nanoclusters at the free-
growing ends of the nanobelts. (b) A high magnification TEM
image of a ZnS nanobelt with the width and the length directions
Post-growth oxidation of the ZnS nanobelts at tempera-
tures of 900–950 8C was performed to receive ZnO
nanostructures. In most cases, the original ZnS nanoblets
turn into ZnO nanotwin belts containing two parts with a
clear and sharp edge lying along and running through the
length direction. The widths of the two parts are close to
each other. Fig. 2(a) shows a TEM image of a typical ZnO
nanotwin belt after oxidation. A SAED analysis, shown in
the inset of Fig. 2(a), recorded from the ZnO nanotwin belt
displays two sets of spotted patterns that correspond to the
ꢀ
along the [0001] and ½0110Š crystalline directions, respectively.
ꢀ
ꢀ
length dimension are ½0110Š and ½1120Š for the two parts in
all the ZnO nanotwin belts. Fig. 2(b) is a high resolution
TEM image of the places around the interface of a ZnO
nanotwin belt. The clear and sharp interface between the
ꢀ
ꢀ
two parts with the length directions of ½0110Š and ½1120Š is
the (0001) twinning plane. The formation of the nanotwins
can be regarded as that each unit cell in one side of the
(0001) twinning plane rotates around the [0001] twinning
axis by 308 relative to that in the other side. The twinning
process probably involves the big difference in the mole
volumes of ZnS and ZnO. For wurtzite ZnS and ZnO
crystals, the relative densities are 3.980 and 5.606 g/cm3
ꢀ
ꢀ ꢀ
½1100Š and ½2110Š zone axises of two wurtzite ZnO crystals,
respectively. As pointed out earlier, the width and the length
directions of the as-synthesized ZnS nanobelts make angles
ꢀ
within 0–158 to [0001] and ½0110Š; respectively. After
oxidation, we observed that the crystalline directions along
the width dimension are both [0001], while those along the

Y. Li et al. / Solid State Communications 129 (2004) 233–238
235
Fig. 2. (a) A TEM image of a typical ZnO nanotwin belt including two twinning parts along the length direction and with a clear and sharp edge
ꢀ ꢀ
ꢀ
in between. The SAED pattern is shown in the inset. It contains two sets of spotted patterns that corresponds to the ½2110Š and ½1100Š zone
a
b
axises of two wurtzite ZnO crystals respectively. (b) A high resolution TEM image for the nanotwin belt around the boundary. (c) ZnO
nanotwins with a nanovoid located at the twinning plane. (d) One original ZnS nanobelt was divided into two parallel nanobelts both with clear
edges.
[18], respectively. Thus the mole volumes are 24.48 and
14.51 cm3/mol for ZnS and ZnO, respectively. It means that
when a ZnS crystal changes into a ZnO one with the same
wurtzite structure, the volume may shrink to about a half of
the original one. Such great shrinkage in the volume may
produce large stress in the lattices and lead to the formation
of twins. Twin structures with the (0001) twinning planes
perpendicular to the length directions of ZnO microrods

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Y. Li et al. / Solid State Communications 129 (2004) 233–238
obtained under hydrothermal conditions have been studied
by Wang et al. [19]. However, in this paper, the (0001)
twinning planes are parallel to and running through the
length direction of the nanobelts. This kind of twin
structures maybe useful in developing new nano-devices.
Some post-oxidation ZnO nanostructures are nanotwins
with nanovoids along the twinning plane as shown in
Fig. 2(c). On some ZnO twin nanobelts, chains of nanovoids
exist. If a plenty of nanovoids are presented and connected
with each other, one ZnS nanobelt can be divided into two
nanobelts parallel in the length direction after oxidation as
shown for example in Fig. 2(d) (for better illustrating, two
separate nanobelts with only very small part still connected
with each other is chosen). It is noteworthy that ZnO
nanowires or nanoblets synthesized directly by the VPT
method are mostly of [0001] direction along the length
dimension according to the experimental results published
earlier [8–13] and to our own unpublished results. In this
paper, the single-crystalline ZnO nanobelts separated from
the original ZnO nanostructures are of [0001] along the
width direction instead of the length direction.
The XPS spectrum are measured for the post-oxidized
samples. Small amount of S is detected and the S 2p
spectra is shown in Fig. 3(a). The binding energy value
for S 2p is 167.6 eV. This indicates that the small
amount of S exists as SO422. A possible explanation for
the forming of nanovoids in the central part along the
length direction of the post-oxidation nanostructures is
presented as follows. In the presence of O2, chemical
reaction equilibriums among ZnS, ZnO, and ZnSO4 must
be taken into account,
2ZnS þ 3O2ðgÞ ! 2ZnO þ 2SO2ðgÞ
2ZnO þ 2SO2ðgÞ þ O2ðgÞ ! 2ZnSO4
ZnSO4 ! ZnS þ 2O2ðgÞ
ð1Þ
ð2Þ
ð3Þ
Fig. 3. (a) A S 2p XPS spectrum for the as-oxidized products on the
Si substrates. (b) The equilibrium partial pressure of SO2 vs. that of
O2 for stable states of the ZnO–ZnS–ZnSO4 system at 927 8C
(calculated under the ideal gas assumption).
Fig. 3(b) is the equilibrium partial pressure of SO2 vs.
that of O2 for the stable state of the ZnO–ZnS–ZnSO4
system calculated at 927 8C under the ideal gas
assumption (see supporting materials). In our exper-
iments, at the beginning stage of the oxidation process,
ZnS turns into ZnO with the twinning process through
reaction (1), which takes place at high O2 and low SO2
partial pressures according to Fig. 3(b). The concen-
tration of SO2, which is produced by reaction (1), may
be rich locally. According to Fig. 3(b), in the atmosphere
with both high SO2 and O2 partial pressure, ZnSO4 tends
to be the stable phase instead of ZnO. Thus reaction (2)
may happen locally. The ZnO (0001) twinning plane has
relatively large interface free energy and consequently
the absorbing of SO2 along the ZnO (0001) twinning
plane to form ZnSO4 by reaction (2) is preferred. The
crystalline structure and the lattice constants of ZnSO4
and ZnO are very different. The phase transition from
ZnO to ZnSO4 may cause large stress in the lattice and
result in the forming of voids along the ZnO (0001)
twinning plane. Further understanding of the dividing
process of one nanobelt into two nanobelts may be a new
way to control the shape and dimension of the
nanostructures. Studies on this topic is undergoing now.
The photoluminescence (PL) spectra excited by a
Xe-lamp of the original as-synthesized ZnS nanobelts and
the converted ZnO nanotwin belts are measured at the room
temperature. The PL spectra obtained with the excitation
light of 290 nm are depicted in Fig. 4. For as-synthesized
ZnS nanobelts, only a week PL peak around 344 nm is
detected which can be connected to the near-band edge
emission of ZnS with a bandgap of 3.8 eV. After oxidation,
a strong light emission at 380 nm dominates the PL spectra.
It corresponds to the 3.17 eV bandgap transition of ZnO.
Besides, relatively much weaker emissions around 450–
475 nm due to the deep levels are also detected for the ZnO
nanotwin belts. The full width at half maximum of the peak
at 380 nm is about 20 nm, and the PL intensity ratio of the

Y. Li et al. / Solid State Communications 129 (2004) 233–238
237
Acknowledgements
This work is supported by the National Nature
Foundation of China.
Appendix A
The standard mole free energies for each of the products
and reactors can be looked up in chemistry hand books [20].
Then the changes of the standard free energies and the
equilibrium constants for all the reactions (1) to (3) can be
calculated and the partial pressures of SO2 and O2 needed to
maintain the equilibrium can be derived from the equili-
brium constants. For example, at 927 8C, the mole free
energies for ZnSO4, ZnS (wurtzite) and O2(g) are
21207.02, 2311.61 and 2270.15 kJ/mol, respectively.
The changes of the standard free energies for reaction (3) is
Fig. 4. The room temperature PL spectra excited by 290 nm light
from a Xe-lamp of (a) the as-synthesized ZnS nanobelts and (b) the
converted ZnO nanotwin belts.
DGo ¼ 2311:61 þ ð2270:15Þ 2 ð21207:02Þ
near-band edge emission at 380 nm to the deep level green
emission is larger than 12:1.
¼ 355:11 kJ=mol
The equilibrium constant of this reaction is
ð1Þ
ð2Þ
ln ko ¼ 2DGo=RT
4. Conclusions
here R is the gas constant, 8.314 J/K/mol, and T is the
reaction temperature in Kelvin. Calculation leads to that
Single-crystalline ZnS nanobelts of the wurtzite structure
are synthesized by the VPT process in the presence of Au
catalyst. Oxidation of the as-synthesized ZnS nanobelts
results in a novel phenomenon that a single-crystalline ZnS
nanobelt turns into a ZnO nanotwin belt with the (0001) as
the twinning plane parallel to and running through the length
direction. The nanotwin belt has the [0001] crystalline
ln ko ¼ 235:6
On the other hand,
ð3Þ
2
ln ko ¼ ðPðO2Þ=PoÞ
ð4Þ
Here Po stands for the standard vapor pressure,
1.013 £ 105 Pa, and PðO2Þ for the partial pressure of O2 in
the reaction system. Eq. (3) determines the ZnS/ZnSO4
boundary which locates around the O2 partial pressure of
0.01 ppm in Fig. 3(b).
ꢀ
direction along the width dimension but the ½0110Š and
ꢀ
½1120Š directions along their length dimension. During the
oxidation process, nanovoids may appear along the twinning
plane and, when those nanovoids connect with each other,
one original ZnS nanobelt can be divided into two single-
crystalline ZnO nanobelts with well defined edges. Whether
the original ZnS nanobelt turns into nanotwin belts or
separates into individual single-crystalline nanobelts may be
controlled by the gas-flow conditions during the oxidation.
The crystalline direction along the width dimension of the
ZnO nanobelts obtained this way is [0001] while the ZnO
nanobelts/nanowires directly synthesized by the VPT
process have the [0001] along the length dimension. The
dividing of one nanobelt into two nanobelts may be a new
way to control the shape, dimension and crystalline
direction of the nanostructures. ZnO nanotwin belts with
the (0001) twinning planes parallel to and running through
the length directions and ZnO single-crystalline nanobelts
with [0001] along the width instead of the length direction
may have useful properties in developing new nano-devices.
The near-band edge emission of 344 and 380 nm for ZnS
and ZnO are detected, respectively.
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