Synthesis of ultra-long hollow chalcogenide nanofibers

Article abstract of DOI:10.1039/c1cc12312b

Electrospinning and galvanic displacement reaction are combined to fabricate ultra-long hollow chalcogen and chalcogenide nanofibers in a cost-effective and high throughput manner. This procedure exploits electrospinning to fabricate ultra-long sacrificial nanofibers with controlled dimensions and morphology, thereby imparting control over the composition and shape of the nanostructures evolved during galvanic displacement reaction. It is believed to be a general route to form various ultra-long hollow semiconducting nanofibers.

Full text of DOI:10.1039/c1cc12312b

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COMMUNICATION
Synthesis of ultra-long hollow chalcogenide nanofibersw
Kun-Jae Lee,ab Hanbok Song, Young-In Lee, Hyunsung Jung, Miluo Zhang,
a
a
b
b
a
b
Yong-Ho Choa* and Nosang V. Myung*
Received 20th April 2011, Accepted 19th June 2011
DOI: 10.1039/c1cc12312b
Electrospinning and galvanic displacement reaction are combined
to fabricate ultra-long hollow chalcogen and chalcogenide
nanofibers in a cost-effective and high throughput manner. This
procedure exploits electrospinning to fabricate ultra-long sacrificial
nanofibers with controlled dimensions and morphology, thereby
imparting control over the composition and shape of the nano-
structures evolved during galvanic displacement reaction. It is
believed to be a general route to form various ultra-long hollow
semiconducting nanofibers.
by the difference in redox potentials between the solid material
to be displaced and the ions in the electrolyte and is well suited
4
to high-throughput processing under nearly ambient conditions.
It has been previously utilized to create metal nanostructures
with hollow interiors exhibiting a spectrum of geometries and
5
multi-walled metal nanoshells and to coat Si with metal films/
6
nanoparticles.
In this work, we combined electrospinning and galvanic
displacement reaction to demonstrate cost-effective high
throughput fabrication of ultra-long hollow chalcogen and
chalcogenide nanofibers. This procedure exploits electrospinning
to fabricate ultra-long sacrificial nanofibers with controlled
dimensions, morphology, and crystal structures, providing a
large material database to tune electrode potentials, thereby
imparting control over the composition and shape of the
nanostructures that evolved during galvanic displacement
reaction. Tellurium and bismuth telluride were selected as
proof of concept materials because they possess unique
1
Nanoengineered materials with advanced architectures are
critical building blocks to modulate conventional material
properties or amplify interface behavior for enhanced device
performance. While several techniques exist for creating
one-dimensional heterostructures, electrospinning has emerged
as a versatile, scalable, and cost-effective method to synthesize
ultra-long nanofibers with controlled diameter (a few nano-
metres to several micrometres) and composition. In addition,
different morphologies (e.g., nano-webs, beaded or smooth
cylindrical fibers, and nanoribbons) and structures (e.g.,
core–shell, hollow, branched, helical and porous structures)
can be readily obtained by controlling different processing
7
electrical, optical, and chemical properties which lead to
many potential applications including sensors, thermoelectrics,
non-volatile phase change memory, solar cells, and infrared
detectors. For example, elemental tellurium (Te) is a p-type
2
parameters. Although various nanofibers including polymers,
semiconductor with electrical properties that depend on crystal
8
orientation. Bi
carbon, ceramics and metals have been synthesized using
direct electrospinning or through post-spinning processes,
limited works were reported on the compound semiconducting
nanofibers because of incompatibility of precursors. A few
works demonstrated the synthesis of compound semiconducting
nanofibers (e.g., SiC, GaAs) by vapor phase reaction of
electrospun cation precursors with anions, which exhibited
limited scalability due to extreme operating conditions and
Te
2
3
and its alloys are currently the most
widely used thermoelectric materials for room temperature
9
power generators or coolers. Theoretical calculation predicts
that hollow nanostructures can enhance thermoelectric efficiency
by reducing the thermal conductivity because of a stronger
1
0
phonon-surface scattering. In addition, ultra-long nano-
structures allow one to harvest more energy because of the
ability to cover greater temperature gradients.
3
high operating cost due to expensive equipment.
Te and Bi
x
Te1Àx hollow nanofibers were synthesized by
Galvanic displacement reaction (GDR) is a simple and
versatile yet powerful technique for selectively changing the
composition and/or morphology of nanostructures. GDR
takes place via a spontaneous electrochemical reaction driven
galvanic displacement reaction of electrospun nickel nanofibers
at room temperature. Nickel nanofibers were synthesized by
calcinating Ni acetate/PVP nanowires at 500 1C for 3 hours in
air to form NiO nanofibers, followed by thermal reduction at
2 2
400 1C for 3 hours in reducing environment (5% H + 95% N )
to form Ni nanofibers. Fig. 1 shows the morphologies of
synthesized Ni acetate/PVP (a), NiO (b), and Ni (c) nanofibers.
XRD patterns (Fig. S1, ESIw) and HR-TEM images with
SAED pattern (Fig. S2, ESIw) confirmed the formation of
polycrystalline nickel nanofibers. The diameter and size
distribution of nickel nanofibers were tuned by electrospinning
parameters such as nickel acetate concentration and applied
voltage at fixed PVP concentration (Table S1, ESIw).
a
Department of Chemical and Environmental Engineering,
University of California-Riverside, Riverside, CA 92521, USA.
E-mail: myung@engr.ucr.edu; Fax: +1 951 8277710
Department of Fine Chemical Engineering/Bionano Technology,
b
Hanyang University, Ansan 426-791, Korea.
E-mail: choa15@hanyang.ac.kr; Fax: +82 31 4186490
w Electronic supplementary information (ESI) available: Experimental
and supplementary characterization details. See DOI: 10.1039/
c1cc12312b
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9107–9109 9107

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Fig. 2 (a, b) TEM and (c) HR-TEM (inset: FFT) images of
Fig. 1 FE-SEM images of (a) electrospun PVP/Ni acetate nanofibers,
b) NiO nanofibers after thermal oxidation at 500 1C in air, (c) Ni
nanofibers after thermal reduction at 400 1C in 5% H /N , and
d) Bi Te hollow nanofibers after galvanic displacement.
synthesized Bi
e) EDS analysis confirmed the formation of polycrystalline hollow
Bi Te nanofibers after galvanic displacement reaction. The electrolyte
x
Te1Àx hollow nanofibers. (d) SAED pattern and
(
(
2
2
x
y
(
x
y
+
3
+
consisted of 20 mM Bi + 1 mM HTeO
2 3
+ 1 M HNO at room
The driving force for galvanic displacement reactions is the
temperature.
difference in redox potentials, a fundamental electrochemical
process which is the basis of battery technology. When nickel
FFT and SAED patterns (inset of Fig. 2(c) and Fig. 2(d))
corroborate XRD results with lattice spacing corresponding
nanofibers are immersed into an acidic nitric solution containing
+
only HTeO
2
+
ions, nickel nanofibers are galvanically displaced
ions, due to the difference in the reduction
2 3
to rhombohedral Bi Te . EDS line scans show composition
by HTeO
2
homogeneity (Fig. 2(e)). SEM images of synthesized hollow Te
nanofibers are shown in Fig. S5 (ESIw) with the corresponding
EDS profile.
2+
0
O
+
potential of Ni /Ni (E = À0.257 V vs. SHE) and HTeO
2
/
0
Te (E = 0.551 V vs. SCE) as described in eqn (1).
O
+
2
+
0
0
2+
HTeO (aq) + 3H (aq) + 2Ni (s) - Te (s) + 2H O + 2Ni
The mechanism for creating these hollow nanostructures by
galvanic displacement reactions has been described previously
5
by Xia’s group. The generalized scheme starts with particle
2
(
1)
+
In the electrolyte containing both Bi and HTeO
3
+
2
ions, Te is
nucleation and growth of the more noble material on the
surface of the sacrificial metal nanostructure, forming a thin,
porous sheath. As the shell fills in, diffusion across the casing
allows for continued oxidation/dissolution of the sacrificial
metal. The end result is a hollow nanostructure with an
interior roughly resembling the exterior of the sacrificial metal
galvanically displaced by nickel, following spontaneous under-
potential deposition of Bi on Te to form Bi Te due to the
formation (i.e., DG
2
3
0
negative Gibbs free energy of Bi
2
Te
3
f
=
À1
À899.088 kJ mol ). The following reaction describes the
1
1
formation of Bi Te (eqn (2)) :
2
3
3
Bi (aq) + 3HTeO
+
+
0
+
(
Fig. 2(a) and (b)).
2
2
(aq) + 9Ni (s) + 9H (aq) -
2+
Bi Te (s) + 9Ni (aq) + 6H O(aq)
(2)
2
3
2
1
2
Based on thin film results from our prior works, the deposited
Bi content in hollow Bi
x
Te1Àx nanofibers was varied by
2
] ratio. The deposited Bi content,
3
+
adjusting the [Bi ]/[HTeO
+
measured by energy dispersive X-ray spectroscopy (EDS)
3
+
+
2
initially increased linearly with the log of the [Bi ]/[HTeO ]
3
+
ratio and plateaued for the [Bi ]/[HTeO
+
2
] ratio greater than
3
0 (Fig. S3, ESIw). Although elemental hollow Te nanofibers
were formed by the galvanic displacement of nickel nanofibers
+
from the electrolyte containing only HTeO2 ions (Fig. S5,
ESIw), elemental Bi nanofibers were not formed from the
+
3
electrolyte containing only Bi
ions similar to thin film
work. X-Ray diffraction pattern of synthesized hollow Bi Te
nanofibers confirmed the formation of highly crystalline
rhombohedral intermetallic Bi Te crystals without separate
Bi and Te phases (Fig. S1(c), ESIw).
Fig. 1(d) and 2 show the SEM, TEM, and HR-TEM images
with SAED patterns of synthesized hollow Bi Te nanofibers.
The inner diameter of Bi Te nanofibers was similar to the
13
x
y
2
3
x
y
Fig. 3 (a, b) Optical and (c) SEM images of the synthesized Bi
hollow nanofiber mat from a nickel nanofiber mat. The electrolyte
x
Te1Àx
x
y
+
3
+
consisted of 20 mM Bi + 1 mM HTeO2 + 1 M HNO at room
outer diameter of sacrificial nickel nanofibers with the tube
3
thickness of approximately 13 nm (Fig. 2(b) and Fig. S4, ESIw).
temperature.
9
108 Chem. Commun., 2011, 47, 9107–9109
This journal is c The Royal Society of Chemistry 2011

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To demonstrate the scalability and manufacturability of this
process a mat of ultra-long hollow BiTe nanofiber was formed
from a nickel nanofiber mat (Fig. 3).
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as-prepared and annealed Te-rich Bi Te (i.e. Bi Te ) shows
2
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30 70
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reduction of defect density.
4
5
In conclusion, electrospinning and galvanic displacement
reaction were combined to synthesize tellurium and bismuth
telluride hollow nanofibers in a cost-effective manner. This
approach is believed to be a general route to form ultra-long
hollow semiconducting nanofibers as numerous electrospun
nanofibers can be utilized. Moreover, by exploiting the
redox potential dependent reactions of galvanic displacement,
nanofiber materials can be extended (semiconductor/metal
core–shell and branched) to new and exotic nanofibers.
This research work was supported by the Pioneer Research
Center Program through National Research Foundation of
Korea (2010-0002231) funded by the Ministry of Education,
Science and Technology (MEST), and the Fundamental R & D
Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy, Republic of Korea. K.-J.
Lee acknowledges financial support from the National Research
Foundation of Korea Grant [NRF-2009-352-D00158].
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9107–9109 9109