One-step hydrothermal formation of Bi2O3 nanourchins with radially ultrathin nanotubes

Article abstract of DOI:10.1143/JPSJ.79.094802

A novel Bi₂O₃ urchin-like microarchitecture with high-density radially oriented Bi₂O₃ nanotubes (NTs) was fabricated by a one-step hydrothermal approach without any template or surfactant at a relative low temperature of 90 °C. The radial Bi ₂O₃ NTs on the surface of nanourchins (NUs) were 4-7nm in diameter and ～ 1.7 μm in length. The mean wall thickness of the NTs was as thin as 1.5 nm. Based on the time-dependent evolutions of morphology examined by scanning electron microscope (SEM), a rolling-up mechanism was demonstrated to explain the formation of the ultrathin Bi₂O ₃ NTs. The ultrathin and long NTs bring on the high surface-to-volume ratios of the Bi₂O₃ urchin-like microarchitectures, which makes them great potential applications in gas sensors, photocatalysis, environmental purification, and solar energy conversion.

Full text of DOI:10.1143/JPSJ.79.094802

Journal of the Physical Society of Japan
Vol. 79, No. 9, September, 2010, 094802
#2010 The Physical Society of Japan
One-Step Hydrothermal Formation of Bi O Nanourchins
2
3
with Radially Ultrathin Nanotubes
Hongbing LU1 , Shimin WANG , Binghai DONG , Zuxun XU ,
;2
2
2
2
2
1
ꢀ
Li ZHAO , and Jinchai LI
1Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology,
Wuhan University, Wuhan 430072, China
2
Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials,
Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China
(
Received April 8, 2010; accepted July 12, 2010; published September 10, 2010)
A novel Bi2O3 urchin-like microarchitecture with high-density radially oriented Bi2O3 nanotubes
NTs) was fabricated by a one-step hydrothermal approach without any template or surfactant at a
(
relative low temperature of 90 C. The radial Bi2O3 NTs on the surface of nanourchins (NUs) were 4–
ꢁ
7
nm in diameter and ꢂ1:7 mm in length. The mean wall thickness of the NTs was as thin as 1.5 nm.
Based on the time-dependent evolutions of morphology examined by scanning electron microscope
SEM), a ‘‘rolling-up mechanism’’ was demonstrated to explain the formation of the ultrathin Bi2O3 NTs.
(
The ultrathin and long NTs bring on the high surface-to-volume ratios of the Bi2O3 urchin-like
microarchitectures, which makes them great potential applications in gas sensors, photocatalysis,
environmental purification, and solar energy conversion.
KEYWORDS: nanostructures, crystal growth, chemical techniques, optical properties
DOI: 10.1143/JPSJ.79.094802
routes to synthesize NTs of artificial lamellar/layered
structure materials.
Bismuth oxide (Bi2O3) is an interesting material and very
1
.
Introduction
Effectively controlling the sizes, morphologies, and
structures of nanomaterials is important for the development important in modern solid-state technology. It has five
of nanoscience and technology, especially for their applica- polymorphic forms that are labeled as ꢀ-Bi2O3 (monoclinic),
tions with morphology-dependent properties.1,2) Since the ꢁ-Bi2O3 (tetragonal), ꢂ-Bi2O3 (body centred cubic), ꢃ-Bi2O3
3
)
18)
carbon nanotubes (NTs) were discovered in 1991, consid- (cubic), and !-Bi2O3 (triclinic). Among them, the low-
erable effort has been placed on the design and synthesis temperature ꢀ-phase and the high-temperature ꢃ-phases are
of different kinds of nanotubular materials. Compared with stable, but the others are high-temperature metastable
19)
other structured morphologies, hollow tubular nanostruc- phases. The tetragonal ꢁ-Bi O , which has a distorted
2
3
tures exhibit enhanced or novel functionalities due to their defect-fluorite structure, is a metastable phase at ambient
higher surface area and the capability of forming composite conditions. Bi2O3 has been widely used in gas sensors,
structures by embedding specific particles in the interiors, optical coatings, photovoltaic cells, and so on, owing to their
and they thus may find potential applications in a wide range peculiar properties, such as large band gap, high refractive
of areas, including catalysis, drug delivery, storage and index and dielectric permittivity, as well as marked photo-
2
0,21)
release systems, bioencapsulation, nanoreactors, and tem- conductivity.
–8)
The tungsten-stabilized Bi O solid elec-
2 3
4
plates for functional architectural composite materials. trolyte has been studied as a sensitive material for CO gas
2
It is well known that many natural lamellar solids can be sensors. It has been proposed that the Bi O is a highly
2
2)
2
3
rolled up into tubular structures under the appropriate selective oxide for NO detection in front of the NO2 as
conditions. This is reasonable because the layered structures interfering species.
23)
It has also been used for smoke
tend to bend to eliminate dangling bonds by the intralayer detection.20,24) For these applications, particle size, porosity,
linkage. As a result, the syntheses of many inorganic tubular and specific surface area are of major importance. Many
9)
10)
11)
12)
structures such as WS2, BN,
MoS2,
NTs have been achieved. However, these nanotube nanostructures with high surface area. However, to the best
synthesis methods require either extreme conditions such of our knowledge, the production of hollow Bi O nano-
TiO2,
and efforts were therefore exerted to tailor hollow Bi2O3
13)
V O
2
5
2
3
as high temperatures or preformed templates. Moreover, materials has lingered far behind. To date, only two papers
naturally existing lamellar/layered materials are limited, and on the synthesis of hollow Bi2O3 nanostructures have been
25)
some of them cannot be transferred into nanosheets because published. Yang et al. prepared Bi2O3 NTs by annealing
the strong interaction between adjacent layers holds the the precursor metallic bismuth NT template in air at 200–
layers tightly.1 Only some protonic layered materials 220 C for 14–18 h. Li et al. also synthesized Bi2O3 NTs
containing weak hydrogen bonds in the interlayer are able by annealing the precursor single-crystalline Bi nanowires in
to be exfoliated by interacting organic molecules, mainly anodic alumina membrane (AAM) via a multistep oxidation
4)
ꢁ
26)
15–17)
ꢁ
long-chain molecule amines, and some still are hard to process at 500–750 C for about 14 h. However, compli-
roll into NTs because of their rigid, crystalline features.
Therefore, it remains a challenge to find simple and mild as two-step or multistep annealing processes, high temper-
1
5,16)
cated procedures were involved in the above methods, such
ature and long reaction time (above 14 h). Herein, we
demonstrate a one-step hydrothermal approach for the
ꢀ
E-mail: jcli@acc-lab.whu.edu.cn
094802-1

J. Phys. Soc. Jpn., Vol. 79, No. 9
H. LU et al.
synthesis of spherical microstructures with a high-density,
radially oriented array of ultrathin Bi2O3 NTs that we have
denoted ‘‘nanourchins (NUs)’’. The average diameter of
the Bi2O3 NTs is as thin as 5.5 nm, which contributes to the
high surface-to-volume ratio of the NUs. The synthesizing
processes are simple, low-cost and free of template or
surfactant, and capable of low-temperature and large-scale
production.
2
.
Experimental Methods
.1 Preparation of samples
In a typical synthesis, Bi(NO3)3 6H2O and hexamethyl-
2
.
enetetramine (HMT) with equal molar ratio were dissolved
into 50 mL of distilled water to form a 0.003 M solution.
Si(100) p-type substrates were etched by hydrofluoric acid
and cleaned by the action of deionized water and ethanol.
The cleaned Si substrates were placed at the bottom of a
sealed bottle containing the above solution. Then, the bottle
ꢁ
was put into a water tank and maintained at 90 C for 3 h.
Finally, the products on the silicon substrates were washed
with deionized water and ethanol several times.
2
.2 Characterization
The morphology and crystalline structure of the samples
were characterized by a Sirion FEG SEM equipped for
energy-dispersive x-ray spectroscopy (EDX), D8 advanced
x-ray diffraction (XRD), JEOL JEM 2010 transmission
electron microscopy (TEM), and high-resolution transmis-
sion electron microscopy (HRTEM). The Raman spectra
were recorded on a Renishaw (RMRe1000) Raman micro-
Fig. 1. Low-magnification SEM image (a) and high-magnification SEM
image (b) of typical Bi O nanostructures with urchin-like shapes.
2
3
þ
spectroscope with a 514.5 nm Ar laser as an excitation
source. Optical absorption spectra were recorded using an
ultraviolet–visible–near infrared (UV–vis–NIR) dual-beam
spectrophotometer (Varian Cary5000) with wavelengths
from 800 to 200 nm.
3
.
Results and Discussion
The product morphology was examined by SEM, as
shown in Fig. 1. From the low-magnification SEM image
Fig. 1(a)], it is clearly observed that the products consist
[
of a large quantity of spherical NUs of micrometer-sized
dimensions. The NUs have typical diameters of 2.5–5 mm.
Figure 1(b) presents the higher magnification SEM image
of a single nanourchin. It is evident that the nanourchin
is composed of many Bi2O3 wirelike nanostructures. The
Bi2O3 wirelike nanostructures radiate from the center of the
crystals forming a spherical urchin shaped structure. Further
investigation shows that the wirelike nanostructures are in
fact Bi O NTs that grow radially on the nanourchin. This
2
3
will be confirmed by TEM (Fig. 3). The lengths of the NTs
are up to several micrometers.
The crystal structure and phase composition of the
products were characterized by XRD and EDX analysis.
As shown in Fig. 2(a), most of the diffraction peaks in the
spectrum can be indexed to tetragonal ꢁ-Bi2O3 structure
Fig. 2. (a) XRD pattern of the Bi2O3 NUs; (b) EDX spectrum of the
Bi2O3 NUs.
˚
(
indicated by squares) with lattice parameters of a ¼ 7:742 A
˚
and c ¼ 5:631 A (JCPDS Card No. 27-0050). The lattice
parameters and indexing are in good agreement with most of
2
7,28)
˚
˚
˚
the reported results.
spectrum also shows two weak peaks corresponding to the (JCPDS Card No. 14-0699). Therefore, the XRD results
On the other hand, in Fig. 2(a), the tice constants a ¼ 5:848 A, b ¼ 8:166 A, and c ¼ 7:510 A
monoclinic ꢀ-Bi2O3 structure (indicated by stars) with lat- indicate that the product is mainly a tetragonal ꢁ-Bi2O3
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J. Phys. Soc. Jpn., Vol. 79, No. 9
H. LU et al.
Fig. 3. (a) Typical bright-field TEM image of several Bi2O3 NUs, exhibiting urchin-like structures. (b,c) High-magnification TEM images of the radially
oriented one-dimensional nanostructures grown on the surfaces of the NUs, showing their tubular structures.
Fig. 4. SEM images of the products recorded at different growth stages: (a) 0.5, (b) 1, and (c) 2 h.
structure, with a trace of monoclinic ꢀ-Bi2O3. In order to stage (0.5 h), the primary nanocrystals tend to aggregate
identify the chemical composition of the product, the EDX together to form solid sphere, as presented in Fig. 4(a). The
method was also utilized [Fig. 2(b)]. Besides the Si signal surface of the spherical particle is rough and accidented.
coming from the Si substrate, only Bi and O signals have Compared with the complete NUs (see Fig. 1), the primary
been detected, suggesting that the as-prepared samples are spherical particle shows a reduced diameter of about 0.5 mm,
indeed made of Bi and O.
which is approximate 12.5% of the diameter of the NUs. As
To further investigate the product morphology and the reaction proceeds (1 h), some thin nanosheets are found
structure, TEM was employed. The sample for TEM to cover with the surface of the spherical particle, as shown
observations was obtained by scraping the Bi2O3 NUs on in Fig. 4(b). When the reaction time increases to 3 h, a
the silicon substrate and then dispersing them in ethanol and significant change occurs in the morphology of the Bi O3
2
finally dropping a droplet of the mixed solution on a TEM microcrystals. Thin NTs and half-tube nanostructures in-
microgrid. Figure 3(a) shows a typical bright field TEM stead of nanosheets are observed on the surface of the
image of several Bi2O3 NUs, clearly demonstrating their urchin-like sphere, as shown in Fig. 4(c). Some of the NTs
urchin-like structures. Figures 3(b) and 3(c) exhibit the are closed, while most of them are unclosed, showing the
high-magnification TEM images of a part of an individual half-tube structures [presented in the inset of Fig. 4(c)].
nanourchin. It is apparent that the radially oriented one- This gives strong evidences that the half-tube structures and
dimensional nanostructures covering with the NUs are NTs are formed by rolling the nanosheets. With the further
indeed hollow tubular structures, where the contrast between prolonged reaction time, the half-tube structures will be
the sidewall and the inside hollow region can be clearly rolled up completely to form integrated NTs, and finally a
identified [Fig. 3(c)]. The NTs have typical outer diameters complete Bi O nanourchin is formed [see Fig. 1(b)].
2
3
of 4–7 nm, inner diameters of usually 2–4 nm, and lengths
On the basis of the previous analysis, we proposed a
of up to several micrometers. The mean wall thickness is reasonable mechanism for the formation of Bi2O3 NUs.
only 1.5 nm. The average diameter and length of the NTs are First, in the initial stage, Bi2O3 nanocrystals are produced
5
of ca. 309.
.5 nm and 1.7 mm, respectively, yielding a large aspect ratio and aggregated to microspheres through the reaction
.
between Bi(NO3)3 6H2O and HMT. In the second stage,
In this synthesis method, neither templates nor surfactants the rough surfaces of the microspheres may offer energeti-
were used. For a complete view of the formation process cally favored sites for the adsorption of reaction molecules.
of Bi O NUs and their growth mechanism, detailed time- Thus, numerous thin nanosheets grow radially on the rough
2
3
dependent evolutions of morphology were examined by surfaces of the microspheres, forming urchin-like structures.
SEM at various growth stages (0.5, 1, 2 h) of the hydro- In the third stage, the thin nanosheets grow further and curl
thermal process, which are shown in Fig. 4. In the initial up themselves, resulting in the half-tube structures. Where
094802-3

J. Phys. Soc. Jpn., Vol. 79, No. 9
H. LU et al.
Fig. 6. Optical absorption spectrum of the Bi2O3 NUs, the inset shows the
2
plot of ðꢀhꢄÞ versus hꢄ for the Bi2O3 NUs.
Fig. 5. Typical Raman spectrum of the Bi2O3 NUs.
ꢃ1
does the driving force for rolling of the nanosheets come 150 cm are defined by the displacements of both Bi and
from? The hydrothermal treatment should be a suitable O atoms. The molecular structure of the bismuth oxide
35)
explanation and there are some evidences. The bending and polyhedra in ꢁ-Bi2O3 is proposed to be BiO6 octahedra with
˚
roll-up of a thin layer to form tubular nanostructures is a all Bi–O bonds equivalent at 2.40 A. The bismuth oxide
thermally driven process. A layer with dangling bonds at polyhedron in ꢁ-Bi2O3 has a molecular structure similar to
36)
its periphery is unstable. The decrease in the number of that found in ꢀ-Bi2O3. Hence, the five Raman bands of the
ꢃ1
dangling bonds and reduction in the area of the active solid– Bi O NUs observed at 129, 143, 156, 250, and 449 cm
3
2
vapor or solid–liquid interface that occur upon bending are expected to be due to five unique Bi–O stretches
compensate the energy barrier associated with the strain of representing (A , B ) modes of Bi O .
35)
g
g
2
3
bending.2
9,30)
Thus, the formation of seamless NTs can be
The UV–vis absorption spectrum of the as-grown Bi2O3
energetically favorable. The rolling of the lamellar structure NUs is shown in Fig. 6. It is known that an estimate of
may be initiated by a stress of either a structural or an the optical band gap Eg for direct interband transitions can
electrical nature caused by the asymmetry of the layer, and be made by using the following equation for a semi-
also affected by the chemical environment, such as the ionic conductor:
strength, pH and temperature.15,31) Zhang et al.32) have
demonstrated that ultrathin Si and Ge nanofilms may self-
2
ðꢀhꢄÞ ¼ Bðhꢄ ꢃ E Þ;
g
bend without external stress load, under its own intrinsic where hꢄ is the photon energy, ꢀ is the absorption
surface-stress imbalance arising from surface reconstruction. coefficient, B is the parameter that relates to the effective
The curling and distortion of the ultrathin Bi2O3 nanosheets masses associated with the valence and conduction bands.
and NTs were also observed under electron beam irradiation Therefore, the optical band gap for the absorption edge can
during the HRTEM measurement. Therefore, it is reasonable be obtained by extrapolating the linear portion of the plot
2
that the rolling of the ultrathin Bi2O3 nanosheets into the ðꢀhꢄÞ versus hꢄ to ꢀ ¼ 0. The inset of Fig. 6 shows the plot
2
NTs takes place under the current hydrothermal treatment of ðꢀhꢄÞ versus hꢄ calculated from Fig. 6. From the result,
or other proper conditions. The emergence of the half-tube the direct band gaps are determined to be 3.72 eV for
structure intermediates holds strong evidence for the ‘‘roll- the Bi2O3 NUs. It has been reported that depending on
ing-up mechanism’’ of the Bi2O3 NTs, which is similar preparation technology, the band gap of Bi2O3 may change
33)
34)
21,25,26,37)
to that reported by Wang et al. and Guo et al. where from 2 to 3.96 eV.
.
WO3 H2O and [Ni(NH3)6]Cl2 NTs were produced by rolling
4
.
Conclusions
In summary, using a simple one-step hydrothermal
reaction for more than 3 h, and a fully developed nanourchin method, we have synthesized the novel Bi O urchin-
.
of the WO3 H2O and Ni complex nanosheets, respectively.
Finally, half-tubes grow into intact NTs after hydrothermal
2
3
containing large numbers of radially oriented Bi O NTs is like microarchitecture consisting of high-density radially
3
2
obtained.
oriented arrays of ultrathin Bi2O3 NTs. The radial Bi2O3
The optical property of the Bi2O3 NUs was examined by NTs on the surface of NUs are 4–7 nm in diameter and
micro-Raman and UV–vis absorption spectrum measure- ꢂ1:7 mm in length. The mean wall thickness of the NTs is as
ments. Typical Raman spectrum of the Bi2O3 NUs is thin as 1.5 nm, contributing to the high surface-to-volume
displayed in Fig. 5. The spectrum shows two weak bands at ratios of the urchin-like microarchitectures. The formation
ꢃ1
ꢃ1
2
and 200 cm (indicated by the rectangle). This dominant based on the time-dependent evolutions of morphology
50 cm and 449 cm , and a dominant peak between 110 procedure is demonstrated to be a ‘‘rolling-up mechanism’’
ꢃ1
peak is asymmetric and can be well Gaussian fitted to three during the growth process. These types of special three-
ꢃ1
bands, which are located at about 129, 143, and 156 cm dimensional microarchitectures with high surface-to-volume
shown in the inset of Fig. 5). In general, the vibration ratios may find potential applications in gas sensors, photo-
(
modes of Bi2O3 above 150 cm are due to the displace- catalysis, environmental purification, and solar energy
ments of the O atoms, while the modes in the range of 120– conversion.
ꢃ1
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J. Phys. Soc. Jpn., Vol. 79, No. 9
H. LU et al.
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Acknowledgments
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This work was supported by the National Natural
Science Foundation of China (Grant Nos. 10975109 and
1
(
1
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