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Journal of
Nanoscience and Nanotechnology
Vol. 10, 8322–8327, 2010
One-Dimensional Single-Crystalline Bismuth Oxide
Micro/Nanoribbons: Morphology-Controlled
Synthesis and Luminescent Properties
B. Ling1 , X. W. Sun1 ∗ , J. L. Zhao1 , Y. Q. Shen2 , Z. L. Dong2 ∗ ,
L. D. Sun3 , S. F. Li4 ∗ , and S. Zhang3
1
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
3
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798
4
College of Engineering, China Agricultural University, Beijing 100083, China
2
RESEARCH ARTICLE
Delivered by Ingenta to:
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Based on a facile vapor-phase transport method
without
any catalyst
and template, one-dimensional
: 202.6.242.69were fabricated on silicon substrates in
micro/nanoribbons
single-crystalline bismuth oxide (Bi2 O3 IP
Tue, 28 Dec
2010 18:39:15
large quantities and morphology-controlled
fabrication
of Bi2 O3 was achieved from a single precursor. The widths of Bi2 O3 ribbons varied from 0.2 to 20 m depending on the deposition temperatures. The thickness was in the range of 0.1–2 m and the length reached several hundred
micrometers and even millimeter range. The detailed composition and structural analysis confirmed
the single-crystalline nature of -Bi2 O3 micro/nanoribbons with monoclinic structure. The photoluminescence spectrum of a single Bi2 O3 ribbon showed a broadband emission from 450 to 750 nm
in the visible region, consisting two peaks located at 589 and 697 nm which were primarily originated from the impurity ions and crystal defects. A self-catalyzed vapor–solid model was proposed
to account for the growth mechanism of Bi2 O3 ribbons with different morphologies.
Keywords: Bi2 O3 Micro/Nanoribbons, Vapor Phase Transport, Photoluminescence.
1. INTRODUCTION
Research in one-dimensional (1D) metal oxide nanostructures with different morphologies has attracted considerable attention owing to their dimension-dependent
physical properties and unique applications in nanoscale
electronic and optoelectronic devices in recent years.1 2
Bismuth oxide (Bi2 O3 is an interesting material and very
important in modern solid-state technology owing to its
unique structure and physical properties, such as a phasedependent energy band gap (2.85 eV for -Bi2 O3 and
2.58 eV for -Bi2 O3 , high refractive index, high dielectric permittivity (r = 190), high oxygen ion conductivity
(1.0 S cm−1 , and good photoconductivity and photoluminescence (PL).3–6
Although bulk Bi2 O3 has been widely used in various
technological fields such as the third-order nonlinear optical glass, oxide-conductors, solar cells, and gas sensors,
the synthesis of 1D Bi2 O3 has not been widely investigated
∗
Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 12
till now. However, controlled fabrication of Bi2 O3 nanostructures is of great importance for both physical research
and potential applications in photocatalyst, sensors and
optoelectronic devices. Among the five polymorphs of
Bi2 O3 that are -Bi2 O3 (monoclinic), -Bi2 O3 (tetragonal), -Bi2 O3 (body centered cubic), -Bi2 O3 (cubic),
and -Bi2 O3 (triclinic), -Bi2 O3 exhibits p-type electronic
conductivity (dominated by hole charge carriers) at room
temperature.4 7 However, reports on controlled fabrication of pure single -crystalline Bi2 O3 nanostructures is
very limited and the reported morphologies are nanoparticle, nanosquaresheet and nanorod.8–11 Thus, both from
fundamental and application viewpoints, it is meaningful to develop a simple and effective method to achieve
dimensionally-controlled high-quality -Bi2 O3 .
Herein, we report a simple and effective approach of
fabricating single crystalline -Bi2 O3 micro/nanoribbons
with length up to several hundred micrometers with
controllable morphologies. Single crystalline -Bi2 O3
micro/nanoribbons were synthesized simultaneously at two
temperature regions of 550–650 C and 650–850 C,
respectively.
1533-4880/2010/10/8322/006
doi:10.1166/jnn.2010.3051
Ling et al.
1D Single-Crystalline Bi2 O3 Micro/Nanoribbons: Morphology-Controlled Synthesis and Luminescent Properties
2. EXPERIMENTAL DETAILS
(b) region II: 650–750 C, (c) region III: 750–850 C,
respectively. Figures 1(d–f) are the corresponding enlarged
SEM images of Figures 1(a–c), respectively. In the low
temperature region I, large quantities of nanoribbons were
observed with thickness of ∼100 nm, width in the range of
0.2–1 m, and length up to several hundred micrometers
and even several millimeters. In the medium temperature
region II, microribbons with thickness of 0.4–2 m, width
of 2–10 m and length up to several hundred micrometers were observed. In high temperature region III, the
width of some microribbons further increase to more than
20 m while the thickness and the length did not show
much change compared to region II.
J. Nanosci. Nanotechnol. 10, 8322–8327, 2010
8323
RESEARCH ARTICLE
The synthesis was carried out in a horizontal tube furnace
by vapor-phase transport (VPT) method without any template or catalyst. The VPT system used in our experiment
can be found elsewhere.12 The source material used was
Bi2 O3 powder (99.999%, Strem Chemicals, Inc.) mixed
thoroughly with graphite powder (weight ratio 1:1), which
was placed at one end of a small quartz tube. Si(100)
wafers serving as the substrate were ultrasonically cleaned
with acetone, isopropyl alcohol, and deionized water in
sequence, each for 15 minutes, and was dried in pure nitrogen gas flow before being placed at the other end of the
small quartz tube. The central temperature of the furnace
was increased to 1000 C at a rate of 10 C min−1 and kept
3.2. Composition and Structural Analysis
for 60 min under a constant Ar flow of 250 sccm (standard
cubic centimeters per minute) and O2 flow of 2 sccm. The
Figure 2(a) shows a typical XRD pattern of the products.
substrates were located at a temperature region of 550–
All the diffraction peaks can be indexed as monoclinic 850 C according to the predetermined furnace temperaBi2 O3 with lattice constants of a = 5
849 Å, b = 8
169 Å,
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ture profile. After 60 min evaporation, the small
quartzby Ingenta
c = 7
512 Å and = 112
98 (JCPDS 41-1449), indicatA*STAR,
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tube was drawn out of the furnace and cooled down to
ing single phase of our products. Figure 2(b) shows the
IP
:
202.6.242.69
room temperature. A wool-like layer of light yellow color
typical Raman scattering spectra recorded from the 1D
Tue, 28 Dec 2010
18:39:15
was found on the substrates.
Bi2 O3 micro/nanostructures with an excitation wavelength
The morphology and crystal structure were characterof 514.5 nm, for a low frequency region ranging from
ized by scanning electron microscopy (SEM, JEOL JSM50 to 600 cm−1 . The peak positions of the Raman scatter5600LV, operated at 10 kV), X-ray diffraction (XRD,
ing throughout the whole deposition temperature regions
Rigaku, operated at 40 kV) with Cu K1 radiation ( =
are similar. All 14 peaks agree well with the literatures,13
1
5418 Å), and transmission electron microscopy (TEM,
which further confirms that the nanostructure is composed
JEM-2010, operated at 200 kV). The chemical composiof monoclinic -Bi2 O3 . The Raman bands observed in the
tion was analyzed using X-ray photoelectron spectroscopy
region below 120 cm−1 are mainly attributed to the dis(XPS, KRATOS, AXIS ULTRA) using a monochromaplacements of Bi atoms while those above 150 cm−1 are
tized Al K X-ray source (1486.6 eV). All XPS spectra
mainly due to the displacements of the O atoms. Raman
were calibrated by C 1s peak (284.9 eV) to compensate the
bands in the range of 120–150 cm−1 are defined by the
charging effect. Raman scattering spectra of the samples
displacement of both Bi and O atoms.14 XPS survey scan
were measured by a Renishaw Ramanscope with an argon
of the Bi2 O3 micro/nanoribbons (Fig. 2(c)) shows the preslaser operating at 514.5 nm as the excitation source. The
ence of Bi, O and C. Figures 2(d and e) show the high
PL spectrum of a single Bi2 O3 microribbon was recorded
resolution XPS spectra of O 1s and Bi 4f in the Bi2 O3
by a Renishaw inVia Raman microscope with 325 nm laser
micro/nanoribbons, respectively. The quantification analyexcitation.
sis of the XPS peaks show that the Bi/O atomic ratio is
To explore the structural and optical properties of a
about 2:3, which is consistent with the given formula of
single Bi2 O3 nanoribbon by TEM and PL characterizaBi2 O3 and former XRD and Raman results. Figure 2(f)
tions, the nanoribbons were ultrasonically removed from
shows the high resolution TEM (HRTEM) image recorded
the growth substrate and then transferred onto a carbonnear the edge of a Bi2 O3 nanoribbon. The measured latcoated copper grid and a clean silicon wafer by dropping
tice spacing of 2.0 Å corresponds to the spacing between
an ethanol-nanoribbon suspension. All the characteriza(010) planes of -Bi2 O3 . The white arrow indicates that
tions were performed at room temperature.
the Bi2 O3 nanoribbon grows along the [010] direction. The
selected area electron diffraction (SAED) pattern shown in
Figure 2(g) is in good agreement with the monoclinic crys3. RESULTS AND DISCUSSION
tal structure of -Bi2 O3 taken along the [001] zone axis.
Additionally, an amorphous layer with width of ∼4 nm
3.1. Morphology Characterization
can be found at the edge of the nanostructures.
Large quantities of light yellow products were collected at
a wide deposition temperature region from 550 to 850 C.
3.3. Optical Properties
Figures 1(a–c) show the typical low-magnification SEM
images of Bi2 O3 micro/nanostructures grown at differFigure 3 shows the room temperature PL spectrum of a
single Bi2 O3 microribbon using a He–Cd laser (325 nm)
ent temperature regions, i.e., (a) region I: 550–650 C,
1D Single-Crystalline Bi2 O3 Micro/Nanoribbons: Morphology-Controlled Synthesis and Luminescent Properties
(a)
(b)
(c)
(d)
RESEARCH ARTICLE
(e)
Ling et al.
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Fig. 1. SEM images of Bi2 O3 ribbons with different morphologies deposited on different temperature regions. (a) Region I: 550–650 C; (b) Region
II: 650–750 C; and (c) Region III: 750–850 C.
as the excitation source. A wide broad-band emission
from 450 to 750 nm was observed, which can be well
presented by the two Gaussian-shaped peaks centered at
589 and 697 nm, respectively. The full widths at half maximum of the two bands are ∼130 and ∼80 nm, respectively.
The 589 nm emission can be attributed to different valence
states of Bi ions such as Bi3+ , Bi2+ or Bi cluster.4 15 16 The
red/near-infrared emission may be ascribed to the recombination between the defect levels in the band gap or from
the conduction band to defect levels.4
3.4. Growth Mechanism Discussion
The growth mechanism of Bi2 O3 can be attributed to the
self-catalyzed vapor–solid (VS) mechanism instead of conventional vapor–liquid–solid (VLS) process since no metal
catalyst was used in the growth process.17 To investigate
the growth mechanism, we conducted two control experiments under the same growth condition except (1) only
using Bi2 O3 powder as the source material and (2) conducting the experiment in an open-end quartz tube (without
8324
any carrier gas and open to the atmosphere). In both experiments, no Bi2 O3 products were collected on the Si substrates which can be understood by the high melting point
of Bi2 O3 and low vapor pressure of Bi2 O3 at atmosphere.
Thus, Bi or Bi suboxides played an important role for the
nucleation of Bi2 O3 micro/nanostructures. The formation
mechanism is similar to that of other metal oxides fabricated by VPT method such as ZnO.18 19
Firstly, in the high-temperature region, carbon reduced
Bi2 O3 into Bi or Bi suboxides (BiOx , x < 1
5) by the following reactions:
C + 1/2O2 → CO
(1)
2Bi2 O3 + 3C → 4Bi + 3CO2
(2)
Bi2 O3 + 3CO → 2Bi + 3CO2
(3)
Bi2 O3 + 3 − 2xCO → 2BiOx + 3 − 2xCO2
(4)
Carbon might directly react with Bi2 O3 or first react
with oxygen to form CO. Then Bi2 O3 powders were
reduced into Bi or its suboxide under a reducing atmosphere with CO presence.
J. Nanosci. Nanotechnol. 10, 8322–8327, 2010
Ling et al.
1D Single-Crystalline Bi2 O3 Micro/Nanoribbons: Morphology-Controlled Synthesis and Luminescent Properties
(b)
(a)
(c)
(f)
(d)
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(e)
(g)
Secondly, the Bi and BiOx vapor with a low melting
point would evaporate and be continuously transferred to
the low-temperature region to form nanosized droplets,
which are ideal nuclei for Bi2 O3 nanostructure growth.
These droplets can enhance the adsorption and diffusion
of Bi oxides at the tips of Bi2 O3 nanostructures during
growth.18
Finally, due to further oxidation, the concentration of
oxygen in the droplets increases, and then Bi2 O3 nanostructures started to form as given by:
4Bi + 3O2 → 2Bi2 O3
(5)
2BiOx + 3/2 − xO2 → Bi2 O3
(6)
Figure 4 shows the proposed growth process of
temperature-dependent Bi2 O3 micro/nanostructures. Bulk
quantities of products are found in all temperature regions
from I to III, however, the average width of the ribbons
increases gradually from region I to III. According to
Blackley and Jackson’s proposal,20 the nucleation probability can be expressed as:
PN ∝ exp− 2 /k2 T 2 ln Fig. 3.
Room temperature PL spectrum of a single Bi2 O3 microribbon.
J. Nanosci. Nanotechnol. 10, 8322–8327, 2010
(7)
where is the edge energy per atom, k is the Boltzmann
constant, T is the temperature in Kelvin, and is the
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RESEARCH ARTICLE
Fig. 2. (a) XRD pattern of the synthesized Bi2 O3 ribbons. (b) Raman spectrum of the -Bi2 O3 ribbons. (c) XPS survey spectrum for the surface of
Bi2 O3 ribbons. High-resolution XPS spectra of (d) O 1s and (e) Bi 4f in Bi2 O3 ribbons. (f) HRTEM image and (g) SAED pattern of a single Bi2 O3
nanoribbon along the [001] zone axis.
1D Single-Crystalline Bi2 O3 Micro/Nanoribbons: Morphology-Controlled Synthesis and Luminescent Properties
Ling et al.
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Fig. 4. Schematic diagram of temperature-dependent growth mechanism of single crystalline Bi2 O3 micro/nanoribbons. The scale bar in the SEM
images of seed droplets (after synthesis for 1–2 min) corresponds to 20 m.
supersaturation ratio in the vapor. Based on this equation, two-dimensional (2D) nucleation events will increase
when the temperature and supersaturation of the gas reactant are increased. The dependence of PN on T and has been verified in other metal oxide growth by VPT
method.21–24
In our case, both the temperature and supersaturation
of the gas reactant are higher in the region closer to the
source material. Thus, 2D nucleation is predominant in
higher temperature region (region II–III) by the agglomeration Bi or Bi suboxide droplets. Since the diffusion
length of the seed (Bi or Bi suboxide) droplets is larger
at higher substrate temperature, the size of the nucleation droplets are expected to be much larger at higher
temperature region III, which corresponds well with the
SEM images taken from different regions after synthesis
for 1–2 min. Since the mobility of adatom is higher at
a higher temperature region in terms of thermodynamics,
the adatom BiOx molecular species deposited on the base
and surface area of the ribbon could move quickly to feed
the continuous growth of the ribbon. Therefore, the gradi8326
ent of deposition temperature and supersaturation level of
vapor reactants led the growth of Bi2 O3 from nanoribbons
(region I) to microribbons (regions II and III).
4. CONCLUSION
In summary, 1D single crystalline Bi2 O3 micro/
nanoribbons have been synthesized for the first time
in a single process by the VPT method without the
presence of any catalyst or template. The product morphologies are temperature-dependent, thus can be collected from different temperature regions. The widths
of Bi2 O3 ribbons varied from 0.2 to 20 m and the
thickness was in the range of 0.1–2 m and the length
reached millimeter range. The composition and structural analysis by XRD, XPS, HRTEM, and Raman scattering spectra confirmed the single-crystalline nature of
-Bi2 O3 micro/nanoribbons. PL spectrum from a single
Bi2 O3 microribbon showed a defect-related broadband
emission in the visible region. The growth of Bi2 O3 ribbons is controlled by the self-catalyzed VS mechanism and
J. Nanosci. Nanotechnol. 10, 8322–8327, 2010
Ling et al.
1D Single-Crystalline Bi2 O3 Micro/Nanoribbons: Morphology-Controlled Synthesis and Luminescent Properties
different morphologies were related to the temperature and
supersaturation gradient.
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IP : 202.6.242.69
Tue, 28 Dec 2010 18:39:15
Received: 10 November 2009. Accepted: 24 February 2010.
RESEARCH ARTICLE
J. Nanosci. Nanotechnol. 10, 8322–8327, 2010
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