Room temperature synthesis of In2S3 micro- and nanorod textured thin films

Article abstract of DOI:10.1016/j.materresbull.2007.04.026

Thin films of indium sulfide (In₂S₃) micro- and nanorods were successfully prepared by sulfurization of electrodeposited metal indium layers. The films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV-vis spectroscopy. From XRD and TEM observations it was concluded that the In₂S₃ nanorods and microrods have ～50 nm and ～0.5 μm diameter, respectively. A plausible top-growth mechanism was proposed for the formation of the nanorods in which the hydroxide layer was found to play an important role. The micro- and nanorods showed optical bandgap of ～2.2 and ～2.54 eV, respectively. This facile and cost effective method may be extended to fabricate other metal chalcogenide nanostructures on solid substrates.

Full text of DOI:10.1016/j.materresbull.2007.04.026

Materials Research Bulletin 43 (2008) 983–989
www.elsevier.com/locate/matresbu
Room temperature synthesis of In S micro- and
2
3
nanorod textured thin films
Anuja Datta, Subhendu K. Panda, Soma Gorai, Dibyendu Ganguli,
*
Subhadra Chaudhuri
Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India
Received 1 February 2007; received in revised form 5 April 2007; accepted 19 April 2007
Available online 24 April 2007
Abstract
Thin films of indium sulfide (In
2 3
S ) micro- and nanorods were successfully prepared by sulfurization of electrodeposited metal
indium layers. The films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission
electron microscope (TEM) and UV–vis spectroscopy. From XRD and TEM observations it was concluded that the In S nanorods
2
3
and microrods have ꢀ50 nm and ꢀ0.5 mm diameter, respectively. A plausible top-growth mechanism was proposed for the
formation of the nanorods in which the hydroxide layer was found to play an important role. The micro- and nanorods showed
optical bandgap of ꢀ2.2 and ꢀ2.54 eV, respectively. This facile and cost effective method may be extended to fabricate other metal
chalcogenide nanostructures on solid substrates.
#
2007 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; C. X-ray diffraction; D. Microstructure; D. Optical properties
1
. Introduction
One-dimensional (1D) nanostructured materials have attracted significant attention due to their fascinating
optical, electrical, catalytic and magnetic properties compared to their bulk counterparts. These new classes of
materials have potential applications in all branches of sciences and technologies. Till now many physical and
chemical methods such as vapor–liquid–solid (VLS) growth [1], chemical vapor deposition (CVD) [1], solvothermal,
etc. [1] have been developed to prepare 1D structured semiconductor materials which are complex and costly and
therefore it is still a challenging task to develop low temperature simple methods to prepare 1D nanostructures in
practically usable forms. So forth, many soft chemical methods were developed to meet the requirement, in most of
which the products are in powder form that again limits their use in direct fabrication of nanodevices. Hence, growth
of those structures directly on the solid substrates is a definitive step particularly when the growth is achieved at room
temperature.
In2S3 is an important member of III–VI group mid-gap semiconducting sulfides, particularly useful for
optoelectronic, solar cells and photoelectric device applications [2–5]. In S with diverse morphologies, both in thin
film and powder form have been synthesized by different techniques [6–8]. However, reports on the growth of In S 1D
2
3
2
3
*
Corresponding author. Tel.: +91 33 24734971x201; fax: +91 33 24732805.
E-mail address: mssc2@iacs.res.in (S. Chaudhuri).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.04.026

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A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
forms are not profuse. In this work, In S micro- and nanorods have been synthesized by a novel two-step approach at
2
3
room temperature. Electrodeposition method has been utilized to deposit indium metal layers on conducting
molybdenum coated glass substrates. Electrodeposition is a widely used technique that allows preparing large area thin
films at low-temperature with fair control over the film thickness [9–11]. In the next step, gas–solid route was chosen to
sulfurize the metal films, which is an acceptable simple and reliable approach to prepare nanostructures at room
temperature [12–15]. The results of the work indicated that the combined technique is very facile and can be
successfully applied to synthesize uniform 1D nano- and microstructured chalcogenides on solid substrates.
2
. Experimental details
In S micro- and nanorods were synthesized by a two-step route, viz. electrodeposition followed by gas–solid
2 3
reaction. Molybdenum coated (MC) glasses were used as the conducting substrates for all the experiments in this
work. Firstly, thin layers of metallic indium (In) were electrodeposited on the substrates at a chosen voltage across
the electrodes. The thickness of the electrodeposited In layer was observed to be directly proportional to the
voltage applied in our system. For electrodeposition, the substrates were first cleaned by soap solution and then
ultrasonicated in ethanol for about 0.5 h. Finally the substrates were degreased in 2-propanol vapor for 1 h to
remove any organic impurities. The electrolytic solution was prepared by adding 0.9 mmol InCl in 30 ml of water.
3
The pH of the solution was adjusted to ꢀ3 by adding 0.1N HCl. The substrate (2 cm ꢁ 1 cm) was used as the
cathode and In foil (2 cm ꢁ 1 cm) was used as the anode. Deposition of In metal film was carried out for 20 min
then removed from the solution and dried in air. A smooth In layer covering the substrate surface was obtained. In
the second step, the films were kept in a reactor and the system was deaerated with N to remove oxygen. Next,
2
sulfurization was performed through gas–solid reaction at room temperature in static H S atmosphere for certain
2
period at a constant pressure of ꢀ1.1 atm in the closed reactor. The H S gas was prepared by the reaction of Na S
2
2
and HCl. After ꢀ10 min the color of the shiny In layer changed to reddish orange indicating the transformation of
In to In S . The samples obtained after 12 h sulfurization of In layer deposited at 1 and 2 V will be termed as S1
2
3
and S2 in the text. Confirmatory experiments were also carried out by varying sulfurization time as well the
electrodeposition voltage. Contrastive experiment was conducted by heating the metal film prior to sulfurization to
understand the effect of annealing.
˚
X-ray diffractometry (Seifert-3000P) was used to identify the phases using Cu Ka radiation with a 1.54056 A
wavelength. Morphology of the samples was studied using scanning electron microscopy (SEM, Hitachi S3200) and
transmission electron microscopy (TEM, JEOL 2010). For TEM studies the indium sulfide layer was scraped from the
substrate surface carefully and the material thus obtained was dispersed in spectrochemical grade 2-propanol. A few
drops of the dispersed solution were then deposited on carbon coated Cu grids. Optical reflectance measurements were
carried out by a UV–vis–NIR spectrophotometer (Hitachi U-3410).
3
. Results and discussions
Fig. 1a shows the X-ray patterns of the electrodeposited metal layers at 1 and 2 V prior to sulfurization, indicating
sharp peaks of metallic In along with the presence of In(OH) phase, originating as a result of hydrolysis reaction
3
(
discussed later on). It can be observed that the peak corresponding to In(OH)3 is more intense at lower
elctrodeposition voltage, indicating that the hydrolysis reaction is more vigorous at lower voltage. Fig. 1b shows the X-
ray diffraction spectra of the samples after sulfurization, confirming the formation of phase pure In S (JCPDS card
no. 32-0456). There was no signature of the In(OH) phase after sulfurization which indicated that the hydroxide phase
2
3
3
that formed during the elctrodeposition stage participated completely during sulfurization. An intense peak indexing
Mo (1 1 0) (JCPDS card no. 42-1120) also appeared for all the samples, contributed by the highly crystalline MC
substrate.
Fig. 2a and b shows the nature of the electrodeposited metal surfaces prior to sulfurization recorded by the SEM
images. The images revealed that the surfaces of the initially electrodeposited In layers were devoid of any significant
morphology. The surface of the metal layers was porous at lower voltage at 1 V whereas, the porosity decreased and a
densely packed surface was obtained as the electrodeposition voltage was increased to 2 V. Fig. 2c is the SEM image of
S1, which shows the formation of In S nanorods uniformly over the entire surface. The rods were observed to be of
2
3
diameter 50 nm and length ꢀ0.5 mm and are grown nearly vertical to the substrate surface. A transition from nanorod

A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
985
Fig. 1. (a) XRD patterns of the In metal layers electrodeposited on molybdenum substrates and (b) XRD patterns of the In
microstructured films.
2
S
3
nano- and
to microrod was observed in S2 when the initial electrodeposition voltage was increased to 2.0 V. Fig. 2d shows the
formation of microrods of ꢀ0.43 mm diameter and length 2 mm in sample S2.
To reveal the effect of elctrodeposition voltage on the morphology, experiments were conducted by varying the
initial electrodeposition voltage. When the elctrodeposition voltage was lowered to 0.6 V and sulfurization was
Fig. 2. SEM imagesof(a) porousindiummetalfilmdepositedat1.0 V, (b) compactanduniformindium metal film depositedat2.0 V, (c) In
2 3
S nanorods
2 3
in S1 formed on indium metal film deposited at 1.0 V and (d) microrods of In S nanorods in S2 formed on indium metal film deposited at 2.0 V.

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A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
Fig. 3. SEM images of (a) In
2
S
3
2 3
microparticles formed after sulfurization of In layer deposited at 0.6 V and (b) In S nanoparticles formed after
sulfurization of In layer deposited at 2.5 V.
carried out, microparticles were formed as shown in Fig. 3a. On the other hand, when the initial electrodeposition
voltage was increased to 2.5 V, nanoparticles of ꢀ50 nm appeared after sulfurization as shown in Fig. 3b. This
indicates that initial electrodeposition voltage has an important role to play in controlling the final morphology of
the products.
A detailed microstructural study of the products was carried out by TEM. Fig. 4a shows a cluster of nanorods
originating from a globular base. The lengths were calculated to be ꢀ0.5 mm and diameters were ꢀ50 nm near their
bases, which gradually decreased towards the top to ꢀ40 nm. The high-resolution image of a single rod as shown in
Fig. 4b revealed distinct lattice spacings in two directions. The interplanar spacing of 0.35 nm corresponds to the
(
with interplanar spacing of 0.32 nm corresponds to the (3 1 1) plane of In S . The planes were equispaced and showed
3 0 0) plane of In S and it was observed that the length of the nanorods is parallel to this plane. The other set of planes
2 3
2
3
continuity over the entire rod-length, indicating single crystalline nature. The inset of Fig. 4b shows the FFT of the
lattice fringes, which also reveals the single crystalline nature of the nanorods.
To reveal the growth process of the nanorods experiment was carried out for shorter sulfurization period. X-ray
analysis of the product after 8 h of sulfurization (figure not shown) revealed the presence of minor In(OH) phase along
3
with the In S phase indicating that the hydroxide phase was partially consumed after 8 h. The HRTEM study of the
2
3
partially grown nanorods indicated the presence of rough and poorly crystalline thin outer surface layer of ꢀ2 nm
around the nanorods (Fig. 4c). The interplanar spacing of this layer was calculated to be 0.23 nm, which matches well
with the (2 2 2) plane of In(OH) . The inset of Fig. 4c shows the SAED pattern with two prominent central rings
3
consisting of spots, which correspond to the diffraction from (4 0 0) and (6 0 0) planes of In S . Besides, there are
2
3

A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
987
Fig. 4. (a) Low resolution TEM image of the nanorods, (b) HRTEM image of In
2 3
S nanorod. Inset shows the FFT, (c) HRTEM image of a partially
grown In nanorod showing a thin layer of In(OH) shell. Inset shows the SAED pattern.
3
2
S
3
some diffuse spots present in the diffraction pattern for which the d-spacing of the spots correspond to the (6 4 0) and
8 0 0) planes of In(OH) . We assume that the In S rods have actually grown from this In(OH) layer.
3 2 3 3
(
3
The formation of In(OH) was assumed to be promoted by the hydrolysis of In in the aqueous deposition bath
+
3
[
16], which took place accordingly as:
In þ 3OHꢂ ! InðOHÞ3
3
þ
(1)

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A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
However, the low content of In(OH) phase at 2 V is still not clearly understood. Perhaps, the hydrogen bubbling near
3
the cathode at higher voltage reduced the pH of the electrodepositing aqueous solution and prohibited the deposition of
In(OH) on the indium surface [17].
3
The obtained films, when exposed to H S gas, converted to indium sulfide according to the reaction:
2
3
H2S þ 2InðOHÞ3 ! In2S3 þ 6H2O
(2)
ꢂ2
6.74
The solubility product of In(OH) is smaller (K = 10 ) than the solubility product of In S (K = 10 ) [16], this is a
3
2 3
probable reason why In(OH) being unstable in the H S atmosphere easily changed into In S .
3
2
2 3
From the above observations it is evident that in the morphological growth process the regulatory parameter was the
formation of In(OH) layer on the metal surface which is again controlled by the deposition voltage. To understand the
3
importance of In(OH) in regulating the growth of the 1D structures, a contrastive experiment was carried out in which
3
the metal film was heated at 100 8C in vacuum to get rid of the In(OH) layer. When the hydroxide free layer was
3
sulfurized only particles were obtained. This again proves that the hydroxide phase surely helps in the one-
dimensional growth on the metal film surface. The formation of the nanorods on the substrate surface can be explained
by the top growth mechanism [12–14,18]. The growth of the nanorods was initiated from the metal surface, which
acted both as a substrate and a precursor. In S formed by the in situ reaction with the as supplied H S gas according to
2
3
2
the reaction stated above. The In(OH) phase might have acted as a fluid transport layer on the surfaces of the nanorods
3
3
and supply In ions to the rod top. The In ions reacted with the S supplied by the H S gas and resulted the
+
3+
2ꢂ
2
unidirectional growth of the nanorods [12,14].
For the formation of particles at other conditions, the reason may be given as due to the absence/very low presence
of In(OH) that prohibited unidirectional growth. The porosity and compactness of the metal surface also has an
3
additional role. Cementation of the particles by the adsorbed water in the porous film at low electrodeposition voltage
may give rise to particle agglomerates rather than the 1D structures.
The prepared In S nanorods and microrods showed good quality of optical properties. Fig. 5 represents optical
2
3
reflectance spectra of the films. The films showed ꢀ30% reflectance value at the band edge. The differential spectra
(
dR/dl versus l) of the reflectance traces are shown in the inset. The optical bandgaps calculated from the first order
differential maximum of the reflectance spectra were ꢀ2.54 and 2.2 eV for S1 and S2, respectively. As the diameters of
the nanorods and the nanopaticles were well above the quantum confinement effect, the slight increase from the bulk
bandgap energy value (2.0 eV) [15,19] could be related to the morphology variation and to the difference in the
crystallinity.
Fig. 5. Optical reflectance spectra of the samples. Inset showing the dR/dl vs. l diagram.

A. Datta et al. / Materials Research Bulletin 43 (2008) 983–989
989
4
. Conclusions
Indium sulfide (In S ) nanorods and microrods have been successfully prepared by sulfurization of the
2
3
electrodeposited metal indium layers at room temperature. The films were characterized by X-ray diffraction,
scanning electron microscope, transmission electron microscope and UV–vis optical spectrophotometer. The
formation of the micro- and nano-structures was found to be critically dependent on the indium hydroxide phase
formed during the electrodeposition stage, which helped to carry ions to the top and facilitated one-dimensional
growth. This kind of low cost, room temperature synthesis of well patterned chalcogenide films of good optical
properties can be used for direct device applications. This method may also be extended to control many other
nanopatterned film surfaces.
Acknowledgements
Authors thank Mr. K.K. Das of IACS for recording the SEM micrographs. One of the authors (A.D.) expresses her
sincere gratitude to the Council of Scientific and Industrial Research (CSIR, Government of India) for sanction of a
research fellowship during the tenure of the work.
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