Hierarchical Bi2Se3-xSx microarchitectures assembled from ultrathin polycrystalline nanosheets: Solvothermal synthesis and good electrochemical performance

Article abstract of DOI:10.1039/c3ta12030a

A simple solvothermal route in a binary solution of ethylene glycol and diethylenetriamine has been applied to synthesize hierarchical flower like Bi₂Se_3-xS_x. Through SEM and TEM observations, the flower like Bi₂Se_3-xS_x is assembled from poly-crystalline nanosheets with average thicknesses of 18 nm. It is found that the volume of thioglycolic acid, the reaction temperature and the volume ratio of ethylene glycol to diethylenetriamine play important roles in the formation of flower like Bi₂Se_3-xS_x. Based on the experimental results, the mechanism for the formation of the flower like Bi ₂Se_3-xS_x is discussed. Meanwhile, electrochemical measurements reveal that the as-prepared flower like Bi ₂Se_3-xS_x delivers a high discharge capacity and good cycle stability in electrochemical hydrogen storage.

Full text of DOI:10.1039/c3ta12030a

Journal of
Materials Chemistry A
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PAPER
Hierarchical Bi2Se3–xSx microarchitectures assembled
from ultrathin polycrystalline nanosheets: solvothermal
synthesis and good electrochemical performance†
Cite this: J. Mater. Chem. A, 2013, 1,
10942
a
a
a
a
Rencheng Jin, Junshen Liu, Yanbin Xu, Guihua Li, Gang Chen b and Lixia Yanga
*
*
A simple solvothermal route in a binary solution of ethylene glycol and diethylenetriamine has been
applied to synthesize hierarchical flower like Bi2Se3ꢀxSx. Through SEM and TEM observations, the
flower like Bi2Se3ꢀxSx is assembled from poly-crystalline nanosheets with average thicknesses of
18 nm. It is found that the volume of thioglycolic acid, the reaction temperature and the volume
ratio of ethylene glycol to diethylenetriamine play important roles in the formation of flower like
Bi2Se3ꢀxSx. Based on the experimental results, the mechanism for the formation of the flower like
Bi2Se3ꢀxSx is discussed. Meanwhile, electrochemical measurements reveal that the as-prepared flower
like Bi2Se3ꢀxSx delivers a high discharge capacity and good cycle stability in electrochemical hydrogen
storage.
Received 23rd May 2013
Accepted 10th July 2013
DOI: 10.1039/c3ta12030a
www.rsc.org/MaterialsA
solvothermal method.22,23 Bi2Se3 nanosheets have been ach-
ieved via the hydrothermal co-reduction method and micro-
Introduction
wave heating using an ionic liquid.24,25 Bi2Se3 hollow spheres
and microrods assembled from nanoparticles have been
fabricated via a solvothermal and a microwave-assisted tech-
In the past few decades, fast development and demand in
industry has intensively focused on various applications of
nanomaterials. Considerable efforts have been made to design
and precisely control the morphology of nanomaterials with
tuned properties. Among these nanostructures, dimensionality
is one of the most dening parameters. The same chemical
materials with different dimensionalities can exhibit drastically
different properties. Until now, 0D (quantum dots),1,2 1D
(nanowires,3 nanobelts,4,5 nanorods,6 nanotubes7), 2D (nano-
sheets,8,9 nanoplates10,11), and 3D (microspheres,12 micro-
owers,13 urchin-like morphologies14) crystals have been well
studied. However, 3D hierarchical microstructures assembled
by higher order organization of nanosheet building blocks have
been reported far less.
Bismuth triselenide, a narrow band gap semiconductor
with the tetradymite structure, has attracted much attention
due to its potential applications in thermoelectrics, optoelec-
tronic devices, etc.15–19 In addition, Bi2Se3 is a well-known
material with a lamellar structure, which makes it potentially
applicable in electrochemical hydrogen storage devices and
lithium-ion batteries.20,21 Various morphologies of Bi2Se3 have
been fabricated via diverse procedures. For instance, Bi2Se3
nanorods and nanotubes have been synthesized by the
nique.21,26
A low temperature solution route has been
employed to prepare hierarchical rose-like Bi2Se3 and nano-
structured bismuth selenide thin lms consisting of 2D
nanosheet blocks.20,27 Although many methods have been
applied for the synthesis of 3D Bi2Se3 architectures, no report
have been involved with the synthesis of 3D sulfur doped
Bi2Se3 produced by the self-assembly of 2D nanosheets with
thicknesses of less than 18 nm.
Herein, we report the synthesis of sulfur doped Bi2Se3
complex architectures which are composed of numerous self-
assembled nanosheets with thicknesses of less than 18 nm
through a simple solvothermal method using ethylene glycol
and diethylenetriamine as mixed solvents and convenient
inorganic salts as precursors combined with thioglycolic acid as
a surfactant and sulfur source. Some factors affecting the
morphology of the Bi2Se3ꢀxSx crystals, such as the volume of
thioglycolic acid, the reaction temperature and the volume ratio
of ethylene glycol to diethylenetriamine are systematically
investigated. Based on the SEM observations, the formation
mechanism of such hierarchical architectures is proposed,
which is related to a thioglycolic acid assembly process and an
anisotropic growth mechanism. Moreover, the electrochemical
properties of the pure Bi2Se3 nanosheets and sulfur doped
ower like Bi2Se3 were investigated. The sulfur doped ower like
Bi2Se3 exhibits both excellent electrochemical hydrogen storage
and lithium-ion battery performances.
aSchool of Chemistry & Materials Science, Ludong University, Yantai 264025, P. R.
China. E-mail: jinrc427@126.com; Fax: +86 535 6696162; Tel: +86 535 6696162
bDepartment of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R.
China. E-mail: gchen@hit.edu.cn; Fax: +86 451 86413753; Tel: +86 451 86413753
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta12030a
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rest. Cyclic voltammogram measurements were performed on
a CHI604C electrochemical workstation at a scan rate of
Experimental
Materials synthesis
5 mV sꢀ1
.
All chemicals were purchased from the indicated suppliers
and used without any further purication. In a typical proce-
dure for synthesizing Bi2Se3ꢀxSx, 0.194 g of Bi(NO3)3$5H2O
and 0.102 g of Na2SeO3 were dissolved into 20 mL ethylene
glycol with vigorous stirring. Then 0–1.5 mL of thioglycolic
acid was added into the solution and stirred for another 10
min. The prepared homogeneous solution was transferred
into a Teon-lined stainless steel autoclave with a capacity of
40 mL. Subsequently, 8 mL of diethylenetriamine and 2 mL of
hydrazine (80%) were added into an autoclave in turn. Aer
Li-ion battery performance
The electrochemical Li intercalation property measurements
were made by using homemade button cells with lithium metal
as the counter and reference electrodes at room temperature.
The active material (Bi2Se3), conductivity agent (acetylene
black), and polymer binder (poly(vinylidene diuoride), PVDF)
with a weight ratio of 70 : 15 : 15 were dispersed into 1-methyl-
2-pyrrolidinone (NMP) to form a homogenous slurry. Then, the
slurry was coated onto copper foil substrates and dried in a
vacuum at 120 ꢁC for 12 h to ensure the evaporation of the NMP.
Subsequently, the prepared copper foil was pressed at 6 MPa
and cut into circular strips with diameters of 8 mm. The elec-
trolyte was 1 mol Lꢀ1 of LiPF6 in a mixture of ethylene carbonate
and diethyl carbonate with a volume ratio of 50 : 50. The cells
were assembled in an Ar-lled glovebox. The cells were charged
and discharged on CT2001A Land battery system at a constant
current of 50 mA gꢀ1 and a xed voltage window between 0.01 V
and 3 V.
ꢁ
that, the autoclave was sealed and held in the oven at 180 C
for 4 h, and then cooled naturally to room temperature.
The black precipitates were collected and washed with
distilled water and absolute ethanol several times, and nally
dried in
a h for further
vacuum at 80 ꢁC for 12
characterization.
Materials characterization
The crystalline structures of the samples were investigated by
X-ray diffraction (Rigaku-D/MAX-2550PC, Cu Ka radiation,
˚
l ¼ 1.54056 A). The morphology of the obtained samples was
Results and discussion
assessed on a eld emission scanning electron microscope
(FESEM, FEI Quanta 200F) and with transmission electron
microscopy (TEM, FEI Tecnai G2 S-Twin) with an operating
voltage of 300 kV. Scanning transmission electron microscopy
X-ray energy dispersive spectrometry (STEM-XEDS) was also
performed on an FEI Tecnai G2 S-Twin transmission electron
microscope, equipped for energy dispersive X-ray spectroscopy
in the STEM mode. The elemental composition was investigated
with a 1 nm probe size and 20 cm camera length. The surface
analysis was conducted using X-ray photoelectron spectroscopy
with an Al Ka chromatic X-ray source (1486.60 eV, PHI 5700
ESCA System, USA). Nitrogen adsorption–desorption measure-
ments were performed on an AUTOSORB-1-MP surface analyzer
at 77.35 K. Raman spectroscopy was performed (Horiba Jobin
Yvon Co. Ltd) using a 632.8 nm laser with an incident power of
0.5 mW. The composition of the product was determined using
inductively coupled plasma analysis (ICP, NexION 300,
America).
X-Ray diffraction (XRD) patterns were used to determine the
composition and structure of the product synthesized at 180 ꢁC
for 4 h with the assistance of thioglycolic acid. All the diffraction
peaks can be indexed to the rhombohedral structure of Bi2Se3
(JCPDS 33-0214), and some other peaks originating from Bi2O3
can be detected (Fig. 1a). In the experiment, Bi(NO3)3 can be
reduced to elemental Bi by hydrazine. However, the obtained
elemental Bi is not stable, and transforms to Bi2O3 when
exposed to air. The lattice parameters calculated from the peak
Electrochemical hydrogen storage performance
The electrochemical hydrogen storage properties were assessed
following the reported method with slight modication.28 The
electrode was fabricated by directly pressing the obtained
Bi2Se3 samples on a sheet of nickel foam at 20 MPa. The Bi2Se3
nanostructured electrode was used as the working electrode,
and Ni(OH)2/NiOOH and Hg/HgO electrodes as the counter
electrode and reference electrode, respectively. The electro-
chemical hydrogen storage experiments were performed on a
CT2001A Land battery system in a three-electrode cell in 6 mol
Lꢀ1 KOH at 25 ꢁC under a normal atmosphere. The Bi2Se3
nanostructured electrode was charged for 4 h at a current
Fig. 1 (a) XRD pattern of the prepared flower like hierarchical architectures of
the Bi2Se3ꢀxSx synthesized at 180 ꢁC with the assistance of thioglycolic acid.
FESEM images of the as-synthesized Bi2Se3ꢀxSx hierarchical architectures at
density of 50 mA gꢀ1 and then discharged to 0.1 V aer a 60 s different magnifications: (b) low magnification, (c and d) high magnification.
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˚
positions of the 015, 1010, and 110 planes are a ¼ b ¼ 4.1390 A,
˚
c ¼ 28.6218 A, which are slightly lower than those of pure phase
Bi2Se3. The morphology of the samples was characterized by
SEM. The low magnication SEM image shows that the samples
are composed of well dened microowers with diameters
ranging between 800 nm and 4 mm (Fig. 1b). High-magnica-
tion SEM images (Fig. 1c and d) reveal that the microowers are
comprised of large numbers of nanosheets with smooth
surfaces. Observed carefully, these nanosheets are about 18 nm
in thickness, and are connected to each other through the axis
to form the hierarchical ower like morphology. The composi-
tion of the product was evaluated by energy dispersive spec-
troscopy (EDS). In Fig. S1,† three elements, Bi, Se and S with an
atomic ratio of 30.82 : 41.01 : 17.87 can be found, suggesting
that S can be doped into the product. In order to determine
accurate atomic ratios of the bismuth, selenide and sulfur, ICP
measurements were carried out; the atomic ratio of bismuth,
selenide and sulfur is close to 0.90 : 1.02 : 0.33.
TEM provides further insight into the microstructural details
of the samples. Fig. 2a shows the uniform ower like Bi2Se3ꢀxSx
structures, which agrees well with the SEM observations. Fig. 2b
reveals the HRTEM image of a single nanosheet marked by a
circle. The clear interplanar spacings of 0.355 nm and 0.303 nm
are assigned to the (101) and (015) planes of rhombohedral
Bi2Se3. However, different growth directions of Bi2Se3ꢀxSx can
be observed, which indicates the poly-crystallinity of the nano-
sheets. In order to ascertain the poly-crystalline nature of a
single nanosheet, HRTEM measurements taken from different
positions on the nanosheet were recorded and the results are
presented in Fig. 2c and d. Combining these images, on can
conclude that the Bi2Se3ꢀxSx nanosheets are composed of some
smaller nanocrystals. The corresponding selected area electron
diffraction (SAED) in Fig. 2e further proves that the single
nanosheet is poly-crystalline. Meanwhile, a lateral view of the
as-obtained nanosheets along the thickness direction in Fig. 2d
shows that the thickness of the nanosheet is about 6.1 nm.
Fig. 2f and g provide further evidence for the fact that ultrathin
tiny nanosheets are obtained aer solvothermal treatment.
Scanning transmission electron microscopy X-ray energy
dispersive spectrometry (STEM-XEDS) was conducted to inves-
tigate the nanoscale elemental composition as well as the
spatial uniformity of the elemental distribution. The elemental
mapping in Fig. 2i shows that three elements, Bi, Se, and S are
distributed homogeneously throughout the nanosheet. The
result indicates that sulfur anions can be doped into the Bi2Se3
nanosheets in this case.
Fig. 2 (a) Low-magnification TEM image of the Bi2Se3ꢀxSx hierarchitectures, (b)
HRTEM image of a single nanosheet taken from the circle in (a), (c and d) HRTEM
images of a single nanosheet taken from the square and rhombus in (a),
respectively, (e) the corresponding SAED pattern of a single nanosheet, (f and g)
the magnified TEM images viewed from the frontal perspective, (h) low-magni-
fication STEM image of an individual flower like Bi2Se3ꢀxSx, (i) STEM-XEDS
elemental maps of Bi (green), Se (yellow), and S (red) created based on the XEDS
spectrum, scale bar: 10 nm.
To further investigate the compositions of the as-obtained
samples, X-ray photoelectron spectroscopy (XPS) was carried
out. The survey scan spectrum of the ower like Bi2Se3ꢀxSx
(Fig. 3a) reveals the presence of Bi and Se in the sample. The
binding energies at 158.9 and 164.2 eV in the high-resolution
XPS spectra are assigned to Bi 4f7/2 and Bi 4f7/2, respectively
In previous reports, Raman spectroscopy was used to study
(Fig. 3b).29 The peak at 55.2 eV can be attributed to the ꢀ2 the vibrational modes and electron–phonon coupling in Bi2Te3
oxidation state of Se (Fig. 3c).30 Furthermore, the weak signal at and Bi2Se3.32,33 The layered Bi2Se3 is composed of repeating
225.3 eV corresponds to the S 2s transition (Fig. 3d).29,31 This units with each consisting of ve atomic Se1–Bi–Se2–Bi–Se1
result further shows the existence of sulfur anions in the ower layers, which are weakly bound by van der Waals forces.34
like Bi2Se3ꢀxSx.
Therefore, two different chemical environments of Se atoms can
10944 | J. Mater. Chem. A, 2013, 1, 10942–10950
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be found, as shown in Fig. 4b. The Brunauer–Emmett–Teller
(BET) surface area of the ower like Bi2Se3ꢀxSx calculated from
nitrogen adsorption is 28.57 m2 gꢀ1. The larger surface area may
result in a larger contact interface between the electrode and
electrolyte and therefore improves the capacity of the electrode
material.
The intrinsic crystal properties of the hexagonal Bi2Se3
dominate the formation of the plate-structure Bi2Se3, thus,
Bi2Se3 nanosheets or nanoplates can be easily fabricated in the
absence of any templates. As for the synthesis of hierarchical
Bi2Se3ꢀxSx nanostructures, the concentration of thioglycolic
acid plays a crucial role. In the absence of thioglycolic acid,
some randomly assembled nanosheets rather than the ower
like Bi2Se3ꢀxSx were obtained (Fig. 5a). The thicknesses of these
nanosheets are about 18.6 nm (Fig. 5b). The corresponding XRD
pattern in Fig. 6 shows that the obtained sample is composed of
pure phase Bi2Se3. Compared with the ower like Bi2Se3ꢀxSx,
the full width at half maximum is narrow and the intensity
peaks are much stronger, indicating that the obtained Bi2Se3
nanosheets are highly crystalline. The lattice parameters
calculated from the peak positions of the 015, 1010, and 110
Fig. 3 XPS spectra of flower like Bi2Se3ꢀxSx: (a) survey scan, (b) Bi 4f, (c) Se 3d, (d)
S 2s.
be observed (labelled as Se1 and Se2). The chemical bonding
between Bi–Se2 is of a pure covalent nature, and the bonding
between Bi–Se1 is slightly ionic but still covalent.35 For the
Bi2Se3ꢀxSx compounds, the replacement of Se at the Se2 sites is
preferential because S is more electronegative than Se. In the
Raman spectra of Bi2Se3 (see Fig. S2†), two obvious peaks
˚
˚
planes are a ¼ b ¼ 4.1398 A, c ¼ 28.6411 A, which are bigger
than those of the ower like Bi2Se3ꢀxSx. If 0.5 mL of thioglycolic
acid was added into the reaction system, more assembly of
nanosheets can be achieved (Fig. 5c). The magnied SEM image
depicts that the surfaces of these aggregated structures are also
2
2
assigned to Eg and A1g can be observed. Meanwhile, for the
ower like Bi2Se3ꢀxSx, the two peaks shi to high frequency
positions. This may be attributed to the replacement of Se by S
at the Se2 sites. Similar phenomena can be found in other
bismuth
chalcogenides,
such
as
Bi2Te3ꢀxSex
and
Bi2Se3ꢀxTex.33,36
The porosity of the ower like Bi2Se3ꢀxSx was investigated
using nitrogen gas adsorption–desorption isotherms. As shown
in Fig. 4a, the ower like structures present a typical type IV
isotherm with an H3 type hysteresis loop at the relative pressure
(P/P0) ranging from 0.4 to 1.0, indicating the mesoporous nature
of the material. The hysteresis loop in the pressure range
between 0.4 and 0.9 can probably be attributed to the pores
formed between the primary particles. The high pressure part of
the hysteresis loop (0.9 < P/P0 < 1) is probably related to the
textural larger pores that formed between secondary particles
due to aggregation of the nanosheets. These results were further
conrmed by the corresponding pore size distribution. The
pore sizes of both mesopores and macropores up to 100 nm can
Fig. 4 (a) Typical nitrogen adsorption–desorption isotherm of the flower like
Fig. 5 FESEM images of the products obtained with different volumes of thio-
Bi2Se3ꢀxSx, (b) the corresponding pore-size distribution.
glycolic acid: (a and b) 0 mL, (c and d) 0.5 mL, (e and f) 1.5 mL.
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microspheres can be synthesized (Fig. 7a). The variation of the
morphology may be attributed to the lower reaction rate at a
lower reaction temperature. When the temperature is increased
to 180 ꢁC and 200 ꢁC, the nucleation and growth rate of the
Bi2Se3ꢀxSx accelerate. Therefore, uniform ower like Bi2Se3ꢀxSx
generates (Fig. 1 and 7b).
It is noted that the volume ratio of ethylene glycol to dieth-
ylenetriamine is also a crucial parameter. The different volume
ratios of the solvents have a much greater effect on the physical
and chemical properties of the mixed solvents, which will
inuence the reactivity, solubility, and diffusion behavior of the
regents and the intermediates.39–41 The uniform ower like
products could only be obtained with an appropriate ratio of
solvents. When no diethylenetriamine was added into the
reaction system, the product is mainly uniform nanoplates
(Fig. 8a). The magnied SEM image in Fig. 8b shows that the
obtained products are predominantly hexagonal-based nano-
Fig. 6 XRD pattern of the product synthesized in the absence of thioglycolic
acid.
constructed from many nanosheets and take on a ower like plates, and the edge length of the plates is in the range of 100–
appearance (Fig. 5d). Compared with the morphology shown in 300 nm with an average thickness of 50 nm. If 4 mL of dieth-
Fig. 1, these nanosheets are bigger and more loosely packed. ylenetriamine is used in the synthesis process, some over-
Upon increasing the volume of thioglycolic acid to 1 mL, lapping nanoplates, monodisperse nanoplates, and some
uniform ower like Bi2Se3ꢀxSx can be synthesized (Fig. 1). nanoparticles coexist (Fig. 8c and d). When the amount of the
Fig. 5e is the overall view of the sample prepared with 1.5 mL of diethylenetriamine is 8 mL, the morphology of the Bi2Se3
thioglycolic acid, demonstrating that the product contains lots changes to uniform microowers (Fig. 1). When the volume of
of microspheres with diameters of 300–800 nm. Careful diethylenetriamine reaches 14 mL or 20 mL, some micro-
inspection reveals that the microspheres are composed of spheres can be synthesized without the ower like morphology
numerous nanosheets (Fig. 5f). As in the discussion above, the (Fig. 8e and f).
formation mechanism of such hierarchical ower like
In previous reports, 3D rose-like hierarchical architectures
Bi2Se3ꢀxSx is proposed as a thioglycolic acid directed assembly of Bi2Se3 have been shown to exhibit good electrochemical
process combined with an anisotropic growth mechanism. hydrogen storage properties.20 Therefore, the electrochemical
When no or less thioglycolic acid is employed, the aggregation hydrogen storage behavior of the current sulfur doped ower
effect is reduced. That is, the morphology of the products is like Bi2Se3 was measured at a current density of 50 mA gꢀ1 at
determined by the crystal structure, thus, randomly assembled room temperature. The as-prepared ower like Bi2Se3ꢀxSx
nanosheets can be achieved. However, why can randomly electrode was charged for 4 h and discharged to ꢀ0.1 V versus
assembled nanosheets rather than monodisperse nanosheets an Hg/HgO electrode. The result is shown in Fig. 9a, which
be obtained? In our reaction system, diethylenetriamine is used shows two obvious plateaus in the charging curve with the
as a solvent. Meanwhile, it can act as surfactant and promote rst one at 20 mA h gꢀ1 and the second one at 135 mA h gꢀ1
.
the aggregation of the Bi2Se3 nanoparticles to some degree due The two plateaus in the charging curve show that two different
to the existence of an amidogen group. Upon increasing the hydrogen adsorption sites exist in the ower like Bi2Se3ꢀxSx
amount of thioglycolic acid to 1 and 1.5 mL, the aggregation nanostructures. Such a two-potential-plateau phenomenon has
effect increases. The original Bi2Se3ꢀxSx nuclei are attacked by also been observed in nanostructured materials with
a
the hydrogen bonds of the thioglycolic acid molecules. Due to lamellar structure including Bi2S3,42,43 Cu(OH)2,44 Ni3S2
the hydrogen bonding and electrostatic effects of these mole- (ref. 45) and Sb2Se3.46 This demonstrates that the hydrogen
cules, the capped nuclei will aggregate together to form spher-
ical particles with many high-energy sites. Similar to the
rhombohedral Sb2Te3 and Bi2Te3,37,38 the layered anisotropic
lattice structure of Bi2Se3 prefers to grow along the top–bottom
crystalline plane rather than along the c-axis, resulting in the
formation of nanosheets. With solvothermal treatment, the
anisotropic growth mechanism dominates. Therefore, nano-
sheets composed of lots nanocrystals are nally obtained. In
addition, thioglycolic acid can be used as a sulfur source, and
S2ꢀ anions are generated by pyrolyzing the thioglycolic acid. As
a result, the sulfur doped Bi2Se3 forms. It is found that the
reaction temperature is another important factor inuencing
the morphology of the Bi2Se3ꢀxSx. When the reaction tempera-
Fig. 7 FESEM images of the products prepared at different reaction tempera-
ture is decreased to 140 ꢁC, only nanoparticles and tures: (a) 140 ꢁC, (b) 200 ꢁC.
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surface area of the obtained ower like Bi2Se3ꢀxSx was
measured to be 28.57 m2 gꢀ1, higher than that of the
assembled Bi2Se3 nanosheets (5.65 m2 gꢀ1). In addition, other
possibilities such as the poly-crystallinity of the nanosheets
and the sulfur doping effect caused by pyrolyzing the thio-
glycolic acid should be considered. The cycle performances of
the ower like Bi2Se3ꢀxSx and assembled Bi2Se3 nanosheet
electrodes were investigated and are shown in Fig. 10. Aer
being cycled 40 times at a current density of 50 mA gꢀ1, the
discharging capacity of the ower like Bi2Se3ꢀxSx remains at
118.2 mA h gꢀ1, but the pure phase assembled Bi2Se3 nano-
sheets are only at 66.1 mA h gꢀ1. This indicates that the as-
prepared ower like Bi2Se3 has a stronger resistance against
oxidation and corrosion than that of the assembled Bi2Se3
nanosheets.42,47 The excellent capacity for the hierarchical
architectures not only implies their potential application in
hydrogen storage, but also promotes a new strategy for other
materials with enhanced properties.
Fig. 11 shows the cyclic voltammetry (CV) curves for the
obtained Bi2Se3 and Bi2Se3ꢀxSx. When the ower like Bi2Se3ꢀxSx
electrode was used as the working electrode, one cathodic
adsorption or reduction peak of hydrogen at ca. ꢀ0.682 V (versus
Hg/HgO) can be observed (Fig. 11a). During the following
anodic polarization, two anodic peaks appear at ꢀ0.476 V and
ꢀ0.309 V, which correspond to the hydrogen desorption (Hdes
)
and hydrogen oxidation (Hoxi), respectively. The occurrence of
the hydrogen desorption peak prior to the hydrogen oxidation
peak demonstrates the presence of the strong hydrogen chem-
isorption. Such a phenomenon has been observed and dis-
cussed for CVs of Bi2S3 nanoowers, nanorod networks, and BN
nanotubes. Furthermore, the CV curves of the assembled Bi2Se3
nanosheet electrode were also tested for comparison. As can be
seen in Fig. 11b, one cathodic peak appears at ꢀ0.702 V, lower
than that of the ower like Bi2Se3ꢀxSx electrode, and two anodic
peaks are at ꢀ0.477 V and ꢀ0.308 V. This result further provides
a complementary explanation for the better electrochemical
hydrogen storage properties of the ower like Bi2Se3ꢀxSx.
Bismuth selenide with a layered structure is expected to
exhibit excellent electrochemical properties in a lithium ion
Fig. 8 FESEM images of the obtained product after solvothermal treatment at
180 ꢁC in media with different volume ratio of diethylenetriamine to ethylene
glycol: (a and b) 0 : 28, (c and d) 4 : 24, (e) 14 : 14, (f) 8 : 20.
Fig. 9 Charge–discharge curves of the different morphologies of Bi2Se3, (a)
flower like Bi2Se3ꢀxSx hierarchical architectures, (b) assembled Bi2Se3 nanosheets.
Current density: 50 mA gꢀ1
.
was rst adsorbed into the interstitial sites/pores of the hier-
archical structures and then entered into the interlayers of the
crystals. As shown in Fig. 9a, the plateau of the discharging
potential appeared at ca. ꢀ0.54 V and ꢀ0.49 V, and a dis-
charging capacity of 197 mA h gꢀ1 can be obtained, which is
higher than that of hierarchical rose-like Bi2Se3.20 For
purposes of comparison, the electrode material was replaced
by the randomly assembled Bi2Se3 nanosheets. Only one dis-
charging potential at ca. ꢀ0.55 V is observed between 10 and
146 mA h gꢀ1 in Fig. 9b, while a discharging capacity of
168 mA h gꢀ1 was achieved. The lower discharging capacity
may be attributed to fewer interstitial pores in the randomly
Fig. 10 Cycle life of the synthesized flower like Bi2Se3ꢀxSx constructed from
assembled Bi2Se3 nanosheets. As discussed above, the BET ultrathin nanosheets and assembled Bi2Se3 nanosheet electrodes.
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Fig. 11 Cyclic voltammograms of Bi2Se3 electrodes with different morphologies:
(a) flower like Bi2Se3ꢀxSx hierarchitectures, (b) assembled Bi2Se3 nanosheets. Scan
rate: 5 mV sꢀ1
.
battery. Therefore, the electrochemical performance of the
obtained products with respect to Li insertion/extraction was
investigated. Fig. 12a shows the charge–discharge voltage
prole of ower like Bi2Se3ꢀxSx measured at a constant current
of 50 mA gꢀ1. The anode in the rst charge–discharge process
presents a charge capacity of 813.2 mA h gꢀ1, with a high
discharge capacity of about 920.1 mA h gꢀ1. The irreversible
capacity of 106.9 mA h gꢀ1 can be achieved in the rst cycle,
which may be ascribed to the formation of a solid electrolyte
interphase (SEI) lm onto the surface of the electrode mate-
rial.48,49 For the initial cycle, two obvious plateaus at ca. 1.52 V
and 0.61 V (vs. Li+/Li) appear in the Li intercalation process.
The rst three discharge and charge curves of the ower like
Bi2Se3ꢀxSx are 920.1, 832.4 and 707.6 mA h gꢀ1, respectively.
The values are higher than those of hierarchical Bi2Se3
microrods.21 For the pure phase assembled Bi2Se3 nanosheets,
similar discharge plateaus and irreversible capacity loss can be
observed. The rst three discharge capacities are 859.8, 719.1
and 647.2 mA h gꢀ1, respectively (Fig. 12b), which are much
lower than those of ower like Bi2Se3ꢀxSx. This greatly
enhanced storage capacity is possibly due to the ultrathin
thicknesses of the nanosheets and the poly-crystalline struc-
tures. The nanometer-sized architectures provide negligible
diffusion and probably faster phase transitions, and the inte-
rior spaces can effectively buffer drastic volume changes
during the electrochemical reactions.50,51 In addition, the
porous structure of the Bi2Se3ꢀxSx should be considered. The
Fig. 12 Charge–discharge voltage profiles of the (a) flower like Bi2Se3ꢀxSx and
(b) assembled Bi2Se3 nanosheets for the first, second, and third cycles at a current
rate of 50 mA gꢀ1, (c) cycling performance comparison of flower like Bi2Se3ꢀxSx
and assembled Bi2Se3 nanosheets at a current density of 50 mA gꢀ1 with a
voltage window of 0.01 V–3 V.
porous structure of the product can enhance the Li ion storage ower like Bi2Se3ꢀxSx is about 16.32 S cmꢀ1 at room temper-
and accommodate the volume change, resulting in a high ature, while for the pure phase assembled Bi2Se3 nanosheets it
storage capacity and good stability of the electrode.52 From is 4.06 S cmꢀ1. As a result, the ower like Bi2Se3ꢀxSx exhibits
another point of view, the BET surface area and the electrical better electrochemical properties. Fig. 12c shows the cycle life
conductivity may have some effect on the electrochemical of the products with different morphologies. The discharge
performance. The higher surface area and the higher electrical capacity of ower like Bi2Se3ꢀxSx before the 15th cycle is always
conductivity will promote the electrochemical reactions higher than the theoretical capacity of commercial graphite
between Bi2Se3ꢀxSx and Li+ in the electrolyte. In previous (372 mA h gꢀ1), and the discharge capacity remains at 235.1
reports, doping sulfur anions has been shown to increase the mA h gꢀ1 aer the 30th cycle. Compared with the ower like
electrical conductivity of Sb2Se3 nanowires.53 In order to Bi2Se3ꢀxSx, the cycle performance of the assembled Bi2Se3
ascertain the conductivity of the product, a four-probe method nanosheets is much poor and the discharge capacity remains
was carried out.54 The electrical conductivity for sulfur doped at only 123.4 mA h gꢀ1 aer the 30th cycle.
10948 | J. Mater. Chem. A, 2013, 1, 10942–10950
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Journal of Materials Chemistry A
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In summary, sulfur doped ower like Bi2Se3 has been synthe-
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presence of thioglycolic acid. The obtained ower like archi-
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concluded that the thioglycolic acid directed assembly process
and anisotropic growth mechanism are responsible for the
formation of the ower like structures. The as-synthesized
ower like Bi2Se3ꢀxSx displays a high discharge capacity, and
good cycle performance in electrochemical hydrogen storage
and lithium ion storage, demonstrating that it may be a
promising material as a candidate for hydrogen storage devices
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Acknowledgements
We are grateful for the nancial support from the National
Natural Science Foundation of China (project no. 51102128 and
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