Synthesis of shape-controlled b-In2S3nanotubes through oriented
attachment of nanoparticles
Yu Hee Kim,aJong Hak Lee,bDong-Wook Shin,bSung Min Park,bJin Soo Moon,c
Jung Gyu Namcand Ji-Beom Yoo*ab
Received (in Cambridge, UK) 26th October 2009, Accepted 7th January 2010
First published as an Advance Article on the web 22nd January 2010
b-In2S3 nanotubes were synthesized using an organic solution
pyrolysis route. The shape of the b-In2S3 nanotubes was
controlled from hexagonal nanoplates to nanotubes simply by
changing the reaction time. The growth mechanism of the nano-
tubes was explained by oriented attachment. The b-In2S3nano-
tubes had a diameter, wall thickness and length of 5.0 nm, 0.79 nm
and >10 lm, respectively. The diameter of the b-In2S3nanotubes
was found to be dependent on the sulfur concentration.
One-dimensional (1D) nanocrystalline semiconducting mate-
rials, such as nanosheets, nanotubes, nanowires and nanobelts
are attractive building blocks for high-performance nano-scale
devices as both device elements and interconnects.1–8In
particular, 1D nanocrystalline materials have several advan-
tages in terms of high surface-to-volume ratio, facile strain
relaxation on flexible substrates and efficient 1D electron
transport, so that they act as good channels and are easier
to handle than nanoparticles in device fabrication. The struc-
ture and properties of InS have been reported to be similar to
those of GaS in group III–VI; the mechanical properties of
GaS nanotubes are related to the tube diameter and wall
thickness.9,10Especially, III–VI materials has been recently
used as programmable materials in phase-change random
access memory (PRAM) due to their higher electrical resistivity
than Ge2Sb2Te5, a widely used phase-change material for
PRAM.11,12Highly resistive phase-change materials help to
reduce the programming current in PRAM switching activity.
In2S3is a III–VI group semiconductor with three different
structures: a defective cubic structure, a-In2S3; a defective
spinel structure, b-In2S3, which shows a stable state with a
tetragonal structure below 1027 K; and a layered hexagonal
structure, g-In2S3.13,14Especially, b-In2S3 is an important
n-type semiconductor with a band gap of 2.0–2.3 eV, which
seems to be an ideal candidate to substitute toxic CdS as the
buffer layer widely used in CuInSe2 based solar cells and
medical applications as bioconjugates for cancer diagnosis.15,16
The properties of nanostructured b-In2S3materials were found
to depend on their chemical composition, shape (morphology
and uniformity) and dimensions.14
Several groups have reported a range of techniques for
fabricating b-In2S3nanocrystals with different morphologies,
such as urchinlike microspheres, dendrites, nanofibers, half
shells and hollow microspheres consisting of nanoflakes and
nanobelts. These methods include laser-induced synthesis,17
H2S or (NH4)2S gas treatment,18,19sonochemical methods,20
hydrothermal,21solvent-reduction routes,22and single-source
precursor approaches.23However, there are few reports on the
synthesis of shape-controlled of b-In2S3nanotubes.
This study examined the formation of shape-controlled
b-In2S3 nanotubes as well as their growth mechanism and
optical properties. This paper reports the synthesis of b-In2S3
nanotubes using an organic solution pyrolysis route. This
method is a simple and stable process operating at high
temperatures that can allow easy control of the shape of
b-In2S3nanocrystals. The shape of b-In2S3nanocrystals has
a clear dependence on the process parameters and growth
mechanism with reaction time, which can be used to control
the shape of the b-In2S3nanocrystals.
Sulfur powder (0.014 g, 0.45 mmol) was dissolved in
oleylamine (9 ml) to form a sulfur–oleylamine complex as
the sulfur source. Indium chloride (0.1 g, 0.45 mmol) was then
added to the mixture solution followed by purging at 50 1C for
30 min under an argon atmosphere. The reaction mixture was
heated and maintained at the reflux temperature of 220 1C. As
the thermal reduction proceeded, the color of the mixture
solution changed to yellow indicating the formation of b-In2S3
nuclei. To investigate the nanotube formation, the mixture
solution was heated for 0.5–5 h. The diameter of nanotubes
was controlled by varying the amount of sulfur (0.014 g, 0.028 g,
0.042 g). After centrifugation and repeated washing, the
particles were dispersed readily in a range of hydrocarbon
solvents, such as toluene, hexane and chlorobenzene.
The size and morphology of the nanocrystals were examined
by transmission electron microscopy (TEM, JEOL JEM3010)
operated at a 300 kV accelerating voltage. The composition of
the nanocrystals was analyzed by energy dispersive X-ray
spectroscopy (EDX) installed in a field emission transmission
electron microscope (JSM6700F). The phase and crystallo-
graphic structure of the nanocrystals were characterized by a
12 kW X-ray diffraction source (12 kW-XRD, RIGAKU) with
Cu-Ka radiation (l = 0.15418 nm). The UV/Vis absorption
spectrum of b-In2S3 was recorded using a SHIMADZU
The size and morphology of the as-synthesized b-In2S3
nanocrystals were characterized by TEM. Fig. 1(a) shows
TEM images of a bundle of b-In2S3nanotubes. The inset in
Fig. 1(a) shows a high resolution transmission electron micro-
scopy (HR-TEM) image of a b-In2S3nanotube. The b-In2S3
aSchool of Advanced Materials Science and Engineering,
Sungkyunkwan University, Suwon, 440-746, Korea.
E-mail: firstname.lastname@example.org; Fax: 82-31-290-7410; Tel: 82-31-290-7413
bSKKU Advanced Instituted of Nanotechnology (SAINT),
Sungkyunkwan University, Suwon, 440-746, Korea
cSamsung Advanced Institute of Technology, Yongin, 446-712, Korea
2292 | Chem. Commun., 2010, 46, 2292–2294 This journal is ? c The Royal Society of Chemistry 2010
COMMUNICATIONwww.rsc.org/chemcomm | ChemComm
nanotubes were coated with oleylamine. The image confirms
that the nanocrystals were nanotubes. The nanotubes had a
diameter, wall thickness and length of 5.0 nm, 0.79 nm and
>10 mm, respectively. EDS (Fig. 1(b)) confirmed that the
nanotubes were indeed b-In2S3with an In:S atomic ratio of
1:1.5. Fig. 1(c) shows the selected area electron diffraction
(ED) of the b-In2S3nanocrystallites. The ED pattern revealed
d spacings of 0.32 and 0.18 nm, corresponding to the (311)
and (440) planes, respectively. The high-resolution TEM
(HR-TEM) image of a nanotube confirmed a d-spacing of
0.32 nm in the tube (Fig. 1(d)).
Fig. 2 shows a TEM image of the nanocrystals with different
shapes depending on the reaction time. Almost hexagonal
b-In2S3nanoplates were obtained at a reaction time of 30 min
(Fig. 2(a)) and the diameter of the b-In2S3nanoparticles was
approximately 13 nm. When the reaction time was extended to
1 h (Fig. 2(b)), the particle size increased and the building
blocks of the b-In2S3nanoplates with a size and thickness of
approximately 47 nm and 0.79 nm were stacked with a parallel
alignment (inset of Fig. 2(b)) though the surfaces of the
nanoplates were not entirely flat.
On futher increasing the reaction time the nanoparticles
began to change shape. Fig. 2(c) shows a TEM image of the
reordered nanoparticles when the reaction time was increased
to 1.5 h. The b-In2S3appeared to grow in the longitudinal
direction and a mixture of nanotubes and stacked nano-
particles was observed. After 2.5 h, the stacked nanoparticles
changed further to nanotubes and nanosheets (Fig. 2(d)), and
after 5 h only nanotubes were observed (Fig. 2(e)). Thus the
shape changed from hexagonal nanoplates to b-In2S3nano-
tubes at an In:S ratio of 1:1.5 with increasing reaction time.
The XRD pattern in Fig. 2(f) was indexed to b-In2S3according
to the JCPDS card (32–0456). The XRD patterns of the
nanocrystals for a reaction time of 30 min and 5 h showed
peaks for the (311) and (440) planes, respectively.
At the initial stages, the crystalline phase of the hexagonal
nanoparticles is critical to the formation of the nanotube
shape. According to Penn and Li,7,24when structurally similar
surfaces of particles approach, there is a driving force to form
chemical bonds between the atoms of the opposing surfaces to
achieve full coordination. Bonding between the particles reduces
the overall energy by removing the surface energy associated
with dangling bonds and oriented attachment involves the
spontaneous self-organization of adjacent particles so that
they share a common crystallographic orientation, followed
by joining of these particles at a planar interface. Additionally,
oleylamine is a surfactant which has a long hydrocarbon chain
that facilitates the growth of nanoparticles and also plays an
important role in determining the morphology of the b-In2S3
nanocrystals.25,26In our experiments, oleylamine is deposited
preferentially on the (111) surface with low binding energy
which can facilitate the directed nanoparticle collision and
due to the supply of thermal energy, oriented attachment
proceeded through the alignment of adjacent nanoparticles
in a parallel direction, i.e. 1D attachment (Fig. 3), which shows
a schematic diagram of the growth mechanism. The nuclei
provided the seed for further growth to form larger structures.
The nanotubes were formed by a rearrangement of the nano-
plates aligned in the parallel direction and grew in a single
direction in 1D growth through self-assembly. Therefore,
oriented attachment of b-In2S3nanocrystals along the h110i
axis was observed.
The diameter of the nanotubes was affected by the sulfur
concentration in solution. Nanotubes with various diameters
were confirmed by TEM (Fig. 4). Oleylamine-capped b-In2S3
nanotubes with various diameters were observed. The nanotube
a HRTEM image of a nanotube with a wall thickness of 0.79 nm.
(b) EDS data for the b-In2S3nanotubes synthesized showing a 1:1.5 In:S
ratio. (c) ED pattern of the b-In2S3nanotubes. (d) HRTEM image of a
b-In2S3nanotube showing a d-spacing of 0.32 nm.
(a) TEM images of the b-In2S3nanotubes. The inset shows
nanoparticles to nanotubes according to the reaction time: (a) 13 nm
sized nanoparticles formed at 30 min; (b) the nanoplates formed at 1 h
(inset shows a side view of the nanoplates); (c) transformed selected
area of the nanoplates at 1.5 h; (d) b-In2S3nanotube and nanosheets at
2.5 h, and (e) b-In2S3nanotubes at 5 h. (f) XRD patterns of the b-In2S3
nanocrystals at different reaction times.
TEM images of b-In2S3 showing the shape changes from
oriented attachment self-assembly.
Schematic diagram showing the formation of nanotubes by
This journal is ? c The Royal Society of Chemistry 2010Chem. Commun., 2010, 46, 2292–2294 | 2293
diameter could be controlled by changing the sulfur con-
centration. Nanotubes with a 5.0 nm diameter were obtained
with a 1:1.5 molar ratio (In 0.1 g, S 0.014 g) (Fig. 4(a)). As the
sulfur concentration increased from 0.028 to 0.042 g, the
diameter decreased from 2.3 to 1.8 nm (Fig. 4(b) and (c)).
The diameter of the nanotubes could thus be controlled by
changing the sulfur concentration and suggests that as sulfur
concentration is increased, ligand molecules interact more
strongly due to an increase in the density of sulfur atoms
and the passivation of facets, resulting in thinner nanotubes.
Fig. 4(d) shows the UV-vis spectra of the b-In2S3nanotubes
with different diameters resulting from a change in sulfur
concentration. The absorption peak was slightly red-shifted
at a larger nanotube diameter and the absorption intensity of
the nanotubes with a thinner diameter showed a clear increase.
The nanotube samples (5.0, 2.3, 1.8 nm) had optical band gaps
of 1.94, 2.15 and 2.22 eV, which is in good agreement with the
reported value of 2.0–2.3 eV for bulk b-In2S3. The thinner
tubes had a larger band gap. Therefore, a binary chalcogenide
semiconductor shows a quantum size effect due to their size-
dependent properties, which is believed to be responsible for
the variety of band gaps of b-In2S3nanotubes compared to
bulk b-In2S3. Fig. 4(e) and (f) show examples of nanoparticles
grown at a reaction time of 10, 20 min with 0.042 g of sulfur.
In particular, nanoplates as building blocks were dispersed
and aggregated. However, when the reaction time is increased,
the steady growth of the nanoparticles was re-ordered into a
1D structure similar to the previous result. Comparing to
the previous result, particle formation was much faster than
that at low concentrations. Both the nanotube diameter and
reaction rate could be controlled by the sulfur concentration.
In summary, indium sulfide nanocrystals were synthesized
with various sizes and shapes according to the reaction time.
The oleylamine-capped nanoparticles helped control the shape
because oleylamine acts as a surfactant with a capping ligand.
The process allows uniform-size b-In2S3nanocrystals to be
obtained without a further size-selection process. Various
b-In2S3nanostructures with a range of shapes from hexagonal
nanoplates to nanotubes were obtained simply by changing
the reaction time, and the diameter of the nanotubes was
controlled by the sulfur concentration.
This study was partially funded by the BK21 Project of the
School of Advanced Materials Science & Engineering,
Basic Science Research Program through National Research
Foundation of Korea (NRF) funded by the Ministry of
Education. Science and Technology (2009–0083540), Samsung
Advanced Institute of Technology.
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nanotubes; the nanotube diameter decreases with increasing sulfur
concentration. (d) Optical absorption of b-In2S3 nanotubes with
different diameters. Shape evolution of the nanoparticles using
0.042 g of sulfur; 1.8 nm for (e) 10 min, (f) 20 min.
TEM images of (a) 5.0 nm, (b) 2.3 nm (c) 1.8 nm b-In2S3
2294 | Chem. Commun., 2010, 46, 2292–2294 This journal is ? c The Royal Society of Chemistry 2010