Solution-phase synthesis and electrochemical hydrogen storage of ultra-long single-crystal selenium submicrotubes

Article abstract of DOI:10.1021/jp054214k

Ultra-long single-crystalline trigonal selenium submicrotubes were synthesized using a facile one-step solution-phase approach with the assistance of nonionic surfactant Polyoxyethylene(20)sorbitan monolaurate (Tween-20), which turned out to be significant for the formation of ultra-long Se submicrotubes. XRD, Raman, SEM, and TEM were adopted to characterize the morphology, structure and phase composition of the as-prepared Se products. It was found that the length of the obtained Se submicrotubes was over 100 μm. By variation of the experimental parameters, the t-Se spheres, nanowires, and broken microtubes can be prepared. The possible growth mechanism of the ultra-long selenium submicrotubes was explained. In addition, we have also demonstrated that the synthesized ultra-long t-Se submicrotubes using the Tween-20-assisted approach can electrochemically charge and discharge with the high capacity of 265 mAh/g (corresponding to 0.97 wt % hydrogen in SWNTs) under normal atmosphere at room temperature. Cyclic voltammetry was adopted to investigate the adsorption-oxidation behavior of ultra-long selenium submicrotubes. It was observed that the morphology of the synthesized selenium products had a remarkable influence on their capacity of electrochemical hydrogen storage. These differences in hydrogen storage capacity are likely due to the size and density of tubes as well as the microcosmic morphology of different Se samples. The as-obtained ultra-long Se submicrotubes are expected to find wide applications in hydrogen storage, high-energy batteries, and optoelectronic, biologic, and catalytic fields as well as in the studies of structure-property relationships. This simple Tween-assisted approach might be extended to the preparations of one-dimensional nanostructures of tellurium and other anisotropic materials.

Full text of DOI:10.1021/jp054214k

22830
J. Phys. Chem. B 2005, 109, 22830-22835
Solution-Phase Synthesis and Electrochemical Hydrogen Storage of Ultra-Long
Single-Crystal Selenium Submicrotubes
Bin Zhang, Wei Dai, Xingchen Ye, Weiyi Hou, and Yi Xie*
Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at
Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China
ReceiVed: July 29, 2005; In Final Form: October 15, 2005
Ultra-long single-crystalline trigonal selenium submicrotubes were synthesized using a facile one-step solution-
phase approach with the assistance of nonionic surfactant Polyoxyethylene(20)sorbitan monolaurate (Tween-
20), which turned out to be significant for the formation of ultra-long Se submicrotubes. XRD, Raman, SEM,
and TEM were adopted to characterize the morphology, structure and phase composition of the as-prepared
Se products. It was found that the length of the obtained Se submicrotubes was over 100 µm. By variation
of the experimental parameters, the t-Se spheres, nanowires, and broken microtubes can be prepared. The
possible growth mechanism of the ultra-long selenium submicrotubes was explained. In addition, we have
also demonstrated that the synthesized ultra-long t-Se submicrotubes using the Tween-20-assisted approach
can electrochemically charge and discharge with the high capacity of 265 mAh/g (corresponding to 0.97 wt
% hydrogen in SWNTs) under normal atmosphere at room temperature. Cyclic voltammetry was adopted to
investigate the adsorption-oxidation behavior of ultra-long selenium submicrotubes. It was observed that the
morphology of the synthesized selenium products had a remarkable influence on their capacity of
electrochemical hydrogen storage. These differences in hydrogen storage capacity are likely due to the size
and density of tubes as well as the microcosmic morphology of different Se samples. The as-obtained ultra-
long Se submicrotubes are expected to find wide applications in hydrogen storage, high-energy batteries, and
optoelectronic, biologic, and catalytic fields as well as in the studies of structure-property relationships.
This simple Tween-assisted approach might be extended to the preparations of one-dimensional nanostructures
of tellurium and other anisotropic materials.
Introduction
intriguing 1D Se materials have been synthesized.12-24 Trigonal
selenium nanorods of high purity have been fabricated using
laser ablation of selenium powders12 as well as the reduction
of selenious acid under ultrasonic irradiation.13 The t-selenium
nanowires are also created by sonochemical approach14 and the
solution-mediated transformation from Se powders or semicon-
ductor nanoparticles.15 CVD was utilized to prepare hexagonal
selenium nanowires with the assistance of silicon as the
catalyst.16 Interestingly, Xie et al. found that selenium microballs
with monoclinic structure could form t-Se nanorods in ethanol
solutions.17 In a recent report, we also fabricated t-Se nano-
ribbons and nanowires network using the CVD method.18
Emphatically, Xia’s group took advantage of t-Se nanowires
as the templates to successfully prepare CdSe and Pt nano-
tubes,19-20 and this thought-provoking work may offer an
important method to synthesize other materials with hollow
structure and will broaden the application of Se materials.
Recently, much attention has been devoted to the synthesis of
tubular selenium.21-24 In our group, Se microtubes with ca.
1-1.5 mm in length, 15 µm in diameter, and 5 µm in thickness
were fabricated by dissolving Se powders in different organic
solvents at mild temperatures.21 Se nanotubes of good quality
were also prepared by reducing the as-prepared precursor of
Na2SeSO3 in the micelles of C12E23 under acidic condition,
assisted by additional ultrasonic bath.22 Selenium salts are used
as Se source to fabricate the irregular or broken tubular
structure.23 In a very recent report, Se tubes are also synthesized
via a three-step method.24 However, to the best of our
During the past few decades, much attention has been focused
on one-dimensional (1D) nanostructures, such as nanowires,
nanorods, nanobelts, and nanotubes, owing to their fundamental
significance in investigating the dependence of various physical
properties on dimensionality and size reduction, as well as their
potential applications in many fields.1-3 Since the discovery of
carbon nanotubes in 1991,4 intense interest has been initiated
to design novel methods to synthesize inorganic tubes. However,
most investigations have been concentrated on the compounds
with layered or quasi-layered structure.5-7 Emphatically, there
are few reports on the synthesis of single-crystalline semicon-
ductor tubular materials. For the preparation of nanotubes/
submicrotubes with nonlayered structures, template-assisted
route is still considered as a potential and effective method.8,9
Among various semiconductor materials, trigonal selenium
(t-Se) is well-known as a valuable material with good photo-
electric, catalytic, semiconducting properties, and attractive
commercial use in photocopiers, rectifiers and xerography.
Moreover, trigonal selenium attracts wide research interest
because the extended spherical chains of Se atoms in the trigonal
phase can be a natural template to guide the growth preferen-
tially along the c-axis.10,11 Consequently, it is logical to expect
that the novel t-Se structure may be important both in theoretical
studies and in technological applications, especially in construct-
ing the semiconductor-based devices. At present, a wealth of
* To whom correspondence should be addressed. Telephone and Fax:
86-551-3603987. E-mail: yxie@ustc.edu.cn.
10.1021/jp054214k CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/12/2005

Selenium Submicrotubes
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22831
knowledge, there have been few reports on the regular single-
crystalline selenium submicrotubes, especially these whose
diameter is longer than 10 µm, through simple one-step synthesis
route.
As for various materials, one of the challenging and intriguing
problems is to find/study their potential properties and thus to
achieve materials’ applications in life-relating fields. Energy
crisis are considered to be one of the most troublesome
embarrassments in current world. Hydrogen-based energy is
well-known as an effective alternative for the fossil-fuel-
dominated energy, but the crux is how to synthesize materials
with high hydrogen uptake.25 Electrochemical storage of
hydrogen is found to be a good method because of the relative
low temperature and high stability. It has been observed that
layered-like materials26-30 (eg. CNT, BN nanotubes, MoS2
nanotubes, TiS2 nanotubes) have attracting hydrogen storage
capacity. However, the electrochemical hydrogen storage in-
vestigations of materials with chainlike or quasi-layered structure
are still vacant. As mentioned above, selenium atoms in infinite
spiral chains parallel to the c-axis in the trigonal phase are bound
with covalent bonds, whereas the chains are packed into a
hexagonal lattice through van der Waals’ interactions.18 We
preliminarily assume that the interstitial sites among Se chains
and the pores of Se submicrotubes may potentially store
hydrogen, which function similar to those of layered nanotubes.
Therefore, the electrochemical hydrogen storage behavior of
ultra-long Se submicrotubes deserves our special research
efforts.
Herein, we report for the first time a facile one-step
hydrothermal method for the fabrication of ultra-long single-
crystalline Se submicrotubes using SeO2 as the selenium source
with the assistance of nonionic surfactant Polyoxyethylene(20)
sorbitan monolaurate (Tween-20). It is observed that the length
of the as-prepared submicrotubes is over 100 µm. The possible
formation mechanism of t-Se submicrotubes is presented. This
novel soft template route can be potentially extended to fabricate
submicrotubes of Te and other anisotropic materials. Other
similar nonionic surfactants (eg. Span-20, Span-40, Span-60,
Span-85, and other Tween series) are expected to serve as the
soft templates to synthesize Se tubes in our approach, and the
size and diameter of Se tubes may be modulated by varying
the surfactants and their molar ratio to selenium. In addition,
we have demonstrated that the ultra-long selenium submicro-
tubes can electrochemically charge and discharge with the high
capacity of 265 mAh/g (amounts to 0.97 wt % hydrogen in
SWNTs26-30) at room temperature. The electrochemical dis-
charge capacities of Se nanomaterials with different morphol-
ogies were also investigated. It is found that the discharge
capacity is sensitive to the morphologies and the diameters of
Se tubular materials. Furthermore, the preparation and electro-
chemical studies of Se nanomaterials will offer great opportuni-
ties to explore the dependence of novel properties of material
on its morphology and is essential to manufacturing potential
optoelectronic devices. The obtained ultra-long selenium sub-
microtubes are expected to find wide applications in hydrogen
storage, high-energy batteries, and catalytic fields.
Figure 1. (a) XRD pattern of the as-prepared Se submicrotubes. All
the diffraction peaks can be indexed according to trigonal Se, indicating
the high crystallinity and the preferential growth along [001] direction.
(b) Raman scattering spectrum of the selenium submicrotubes. The
resonance peak at 236.3 cm-1 is a characteristic stretching mode of
t-Se, revealing the nature of trigonal phase of the obtained Se
submicrotubes.
form a homogeneous solution under constant strong stir. Then
0.5 mL of hydrazine hydrate (N2H4‚H2O, 85%) was slowly
dropped into the above solution at room temperature, and the
solution became brick red quickly, indicating the formation of
amorphous selenium (a-Se) spherical particles.10-11 After react-
ing for 15 min while giving out the gas (N2) yielded in the
evolution, the resulting solution was transferred into a 50 mL
Teflon-lined autoclave. The autoclave was sealed, maintained
at 120 °C for 24 h, and cooled to room-temperature naturally.
The black precipitate was collected and washed for three times
with absolute ethanol and distilled water, respectively. Then the
sample was dried in a vacuum at 50 °C for 6 h.
Characterization with SEM, XRD, Raman, and TEM. The
X-ray diffraction patterns (XRD) of the products were recorded
with a Rigaku Dmax diffraction system using a Cu KR source
(λ ) 0.154178 nm). The scanning electron microscopy (FESEM)
images were taken with a JEOL-JSM-6700F field emitting
scanning electron microscope (FE-SEM, 15 kV). Transmission
electron microscopy (TEM) images and electron diffraction
pattern were obtained with Hitachi 800 system at 200 kV. The
specimens of TEM measurements were prepared via spreading
a droplet of ethanol suspension onto a copper grid, coated with
a thin layer of amorphous carbon film, and allowed to dry in
air.
Electrochemical Measurements. The electrochemical mea-
surements were carried out following the method reported in
ref 26 with slight modifications. Briefly, the electrode was
fabricated by directly pressing the prepared selenium samples
on a sheet of nickel foam at 50 MPa. All the experiments were
performed in a three-electrode cell in 5 M KOH at 25 °C under
ambient pressure. The Se submicrotubes products were used as
the working electrode, Ni(OH)2/NiOOH was used as the counter
electrode, and Hg/HgO was used as the reference electrode. The
obtained Se submicrotubes electrode was charged for 3 h at a
constant current density after a 2 s rest. The charging and
discharging current density was controlled at 50 mA/g except
for especial illumination. All the electrochemical hydrogen
storage experiments were carried out using the Land battery
system (CT2001A). The cyclic voltammetric (CV) measure-
ments were carried out by an electrochemical workstation (CHI
660).
Experimental Section
Chemicals. All the chemicals were of analytical grade and
were used as received. Aqueous solutions were prepared using
distilled water.
Results and Discussion
Preparation of Ultra-Long Se Submicrotubes. In a typical
procedure, 0.4 g of SeO2 and 1 mL of Tween-20 were first added
to 20 mL of distilled water, and the mixture was dispersed to
The crystal structure and phase composition of Se products
were first characterized using X-ray powder diffraction (XRD).
Figure 1a displays a representative XRD pattern of the as-

22832 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Zhang et al.
Figure 2. Typical FESEM images of the obtained t-Se submicrotubes. (a) at a low magnification, indicating that the ultra-long t-Se submicrotubes
can be fabricated on a large scale. (b) at a high magnification, revealing the tubular shape of the Se samples. (c, d) at a higher magnification, further
confirming the tubular morphology. Scale bars (c, d): 100 nm.
prepared Se samples, suggesting their high crystallinity. All the
diffraction peaks can be readily indexed to the triagonal phase
of selenium (JCPDS 06-362). The abnormal intensity of (100)
peaks clearly indicates that these t-Se products had predomi-
nately developed along the unique [001] direction, which is
parallel to the helical chains of Se atoms or the c axis.
Raman spectroscopy has been proven to be a powerful tool
to analyze the phase of Se materials due to their characteristic
signals in the spectrum.31 Typically, the resonance peak of t-Se
is located at ∼235 cm-1 whereas those of amorphous selenium
and monoclinic selenium are centered at ca. 250 and ca. 260
cm-1, respectively.31 As shown in Figure 1b, only one peak at
Figure 3. (a, b) Representative TEM image of one t-Se submicrotube
and the associated SAED pattern, showing that the thickness of tube
wall is ca. 120 nm. The [11h0] zone axis of the trigonal selenium
submicrotube was identified, indicating the preferential growth along
[001] direction. (c) Typical TEM image of one small t-Se submicrotube,
showing that the thickness of tube is about 40 nm.
236 cm-1 was observed in the frequency region between 200
and 300 cm-1, which was assigned to the characteristic mode
of t-Se. This testifies that the as-prepared Se products have a
high crystal quality, as revealed in the XRD data.
The morphology of the obtained Se products was char-
acterized using field emission scanning electron microscopy
(FESEM) and the typical FESEM images are displayed in Figure
2. From the low magnification FESEM image (Figure 2a), the
as-prepared Se submicrotubes are relatively uniform and can
be fabricated on a large scale by this simple Tween-20-assisted
solution approach. This higher-magnification FESEM image in
Figure 2b clearly manifests the product’s tubular structure.
Figure 2c shows that the wall thickness of the bigger Se tubes
is about 120 nm, whereas the wall thickness of small t-Se
nanotube is ca. 40 nm (Figure 2d). Also from Figure 2, it can
be seen that the lengths of Se tubes are over 100 µm, implying
that ultra-long selenium submicrotubes have been successfully
synthesized on a large scale by our presented method.
The morphology and structure of the t-Se submicrotubes
products are further detected by transmission electron micros-
copy (TEM) and selected area electron diffraction (SAED).
Figure 3a displays the typical TEM image of the obtained
selenium product, revealing that the obtained sample has a
tubular morphology with a uniform thickness of ca. 120 nm, in
good agreement with the FESEM image in Figure 2. Figure 3b
shows the associated SAED pattern of the Se submicrotubes,
recorded with the incident electron beam perpendicular to the
wide surface of the submicrotube. The index of the spots in the
SAED pattern indicates that the submicrotubes is a single crystal
and predominantly grows along the [001] direction (c-axis),
consistent with previous observations on Se 1D nano-
materials.10-20 Through extensive investigations on individual
submicrotubes from selenium products with SAED, we found
that this orientation was maintained in all of the products. Figure
3c also gives the TEM image of the t-Se submicrotube with
relatively small diameter, suggesting that its wall thickness is
uniform and lower (ca. 40 nm) than that of big submicrotubes.
The SAED measurements also indicate that the small selenium
submicrotubes preferentially developed along the [001] direc-
tion. The SEM, XRD, TEM, and Raman spectroscopic results
demonstrate that t-Se submicrotubes with high crystallinity and
purity can be successfully synthesized by using the Tween-
assisted hydrothermal method.
To ascertain the effect of the nonionic surfactant on the
morphology, a series of parallel experiments were performed.
In the absence of Tween-20, while other conditions remains
unchanged, it was found that Se submicrotubes cannot be
obtained, and the as-prepared products are selenium spheres
(Figure 4a), suggesting that tween-20 is necessary for the
formation of t-Se tubes. When the volume of Tween-20 was
increased to 2 mL, the obtained samples are broken Se
microtubes with ca. 5-10 µm in diameter, 20-80 µm in length
(Figure 4b). Compared with the ultra-long Se submicrotubes,
the diameter of the broken selenium microtubes become bigger
while their length being shorter. We founded that the appropriate
amount of Tween-20 for the synthesis of tubular selenium
structure is 1.5-3 mL. When the hydrothermal temperature was
controlled at 90 °C, selenium nanowires were obtained and a
small quantity of cross-like junctions could also be observed in
the final wire-like products, as shown in Figure 4c, indicating
that the reaction temperature is a critical factor for the formation
of Se submicrotubes.
Although the exact mechanism for the surfactant-modulated
formation of ultra-long t-selenium submicrotubes is still under

Selenium Submicrotubes
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22833
Figure 4. FESEM images of the obtained samples in different conditions: (a) t-Se spheres prepared from 0.4 g of SeO2, 0 mL of Tween-20, and
20 mL of H2O for 24 h at 120 °C under hydrothermal condition; (b) broken t-Se microtubes synthesized from 0.4 g of SeO2, 2 mL of Tween-20
and 20 mL of H2O for 24 at 120 °C under hydrothermal condition; (c) t-Se nanowires produced 0.4 g of SeO2, 2 mL of Tween-20 and 20 mL of
H2O for 24 h at 90 °C under hydrothermal condition. In Figure 4c, a small quantity of cross-like junctions can be observed.
partly dissolved and transformed into the series-sphere-joined
aggregates because of the Se anisotropic nature. When the
hydrothermal reaction was processed for 0.5 h, the as-prepared
products are Se submicrotubes with rough outer surface, as
displayed in Figure 5b. A higher SEM image clearly manifests
that many particles and spheres-linked aggregates coexist in the
outer surface of Se tubes along a certain direction (the inset in
Figure 5b). With the reaction time increased to 2 h, the tubular
morphology of the Se submicrotubes becomes obvious (Figure
5c), and their crystallinity also becomes better. However, there
are still a few particles at the outer wall of selenium sub-
microtubes, which are further confirmed by the TEM image
inserted in Figure 5c. In the following process, the crystallinity
of Se tube is increased with the prolonged reaction time. When
the hydrothermal reaction continues for 24 h, the outer surface
of Se submicrotubes is quite smooth (learned from the SEM
image in Figure 5d), indicating the high crystallinity. XRD and
SAED measurements show that the ultra-long selenium has a
preferential growth along the [001] direction. We believe that
Tween-20 micelles play a very important role in the formation
of Se submicrotubes. It was thought that a-Se spheres were first
formed on the outer shell of Tween-20 micelles because of the
good solubility for a-Se22 as a result of much water entrapped
Figure 5. SEM image of the as-prepared Se samples collected at
different hydrothermal time: (a) 0 h, collected quickly after hydrazine
hydrate was added to the reaction mixture; (b) 0.5 h, the inset is a
higher magnification of SEM, reflecting the particles composition of
selenium tube; (c) 2 h, the right inset showing TEM image of one tube;
(d) 24 h.
by the poly(oxyethylene) chains of Tween-20. In the following
process, the neighboring a-Se spheres at the outer shell of
Tween-20-based micelles quickly dissolved and recrystallized
along the [001] direction due to the inherently anisotropic
structure of t-Se,10-20 and then formed the tubular structure.
Under the hydrothermal condition, the dissolved a-Se particles
were diffused to the surface of Se submicrotubes, as revealed
in Figure 5c, and provided the Se source for the durative growth
to the ultra-long selenium submicrotubes. When the hydro-
thermal reaction continued, the selenium particles dissolved
in the micelles were completely consumed and thus the
crystallinity of Se submicrotubes became better. As for the
formation of broken Se tubes and Se nanowires under other
conditions, it was believed that the relative ratio of Tween-20
to the dissolved a-Se particles and the interstitial sites among
the neighboring Tween micelles were mainly responsible for
the difference in the as-prepared products’ morphologies, similar
to Qi’s discussion.22 It should be pointed out that the explanation
for the growth process of ultra-long submicrotubes is rough,
and more systematic work needs to be done to get a better
knowledge on the formation mechanism, which is now under-
way in our group.
investigation, the directing role of Tween-20 is undoubtedly
significant. In the absence of Tween-20, the synthesized products
were not the selenium tubular structure, but the spherelike Se
particles, as seen in Figure 4a. To understand the growth
mechanism of ultra-long Se submicrotubes, four samples at
different reaction stages were collected. Figure 5a shows the
SEM image of the Se samples collected quickly after 0.5 mL
of hydrazine hydrate (N2H4‚H2O) was added to the mixture of
SeO2, Tween-20 and H2O. As demonstrated in Figure 5a, the
products mainly consisted of spherical nanoparticles and series-
sphere-joined aggregates. Further Raman spectroscopic analysis
(not shown here) shows that the intermediates are composed of
the coexistence of t-Se and a-Se phase, suggesting that SeO2
was rapidly reduced to the coexistence of Se spheres and quasi-
1D nanomaterials by N2H4 in the presence of Tween-20. Similar
results of the coexistence of two kinds of Se phases have been
reported by Xia et al. in preparing Se nanowires on the basis of
TEM observations.10 Owing to the very unstable properties of
amorphous selenium spheres under current approach, it is
difficult to obtain the pure spherical Se particles, but it is
reasonable to think that the SeO2 is first reduced to form
amorphorous selenium particles (a-Se) and then these Se spheres
It was found that the ultra-long selenium submicrotubes had
the function of electrochemical hydrogen storage. Figure 6
shows the charge-discharge voltage changes of ultra-long t-Se
submicrotubes as a function of capacity at a constant current
density at room temperature. In the charge curve of ultra-long
t-Se submicrotubes (Figure 6a), one weak and perceptible

22834 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Zhang et al.
Figure 6. Charge-discharge curves of the ultra-long t-Se submicrotubes at a given constant current density: (a) 80 mA/g; (b) 50 mA/g.
storage of the ultra-long Se tubes is sensitive to the charge-
discharge current density. The value of discharging capacity is
higher than those of MoS2 and BN nanotubes.27,28 The high
capacity was considered to be pertinent to the enhanced
electrocatalytic activity of the highly porous of selenium tubular
structure and the weak van der Waals’ interactions among
infinite spiral chains. This indicates that ultra-long t-Se sub-
microtubes can be potentially applied as the material of
electrochemical hydrogen storage.
Cyclic voltammograms (CVs) were also performed to further
Figure 7. Cyclic voltammograms of the different electrodes in 5 M
investigate the electrochemical hydrogen adsorption-desorption
KOH at a sweep rate of 50 mV/s: (a) porous nickel; (b) ultra-long
behaviors of porous Ni and ultra-long selenium submicrotubes
t-Se submicrotubes supported on the porous nickel substrate.
supported on the porous nickel substrate. In the CV of the porous
nickel electrode (the inset in Figure 7a), one broad oxidation
peak of hydrogen is observed at ca. -0.55 V vs Hg/HgO, and
one desorption peak of hydrogen can be found around -0.92
voltage plateau is seen at ca. 2 mAh/g. With the increase of the
electrochemical capacity, the potential decreased quickly, but
remain unchanged when the charge capacity reaches 14 mAh/
V, which are typical CV behaviors of nickel. When the working
g. One new obvious plateau of potential is observed between
electrode was replaced by the ultra-long t-Se submicrotubes
14 and 150 mAh/g. This indicates that two different hydrogen
supported on the porous nickel substrate, the anodic oxidation
adsorption sites25 exist in the synthesized ultra-long selenium
peak of hydrogen appears at about -0.47 V (Figure 7b). As
submicrotubes, in other words, there are two different electro-
shown in Figure 7b, the hydrogen desorption peak can also be
chemical steps in the charging process. It is assumed that the
observed prior to hydrogen oxidation peak, indicating the
H was first adsorbed into the pores/inner walls of selenium
possible existence of the strong chemisorption of hydrogen,
submicrotubes and then diffused into the interstitial sites among
similar to the observations and discussion on CVs of BN
the Se chains, which is similar to the hydrogen storage
nanotubes.28 In the following cathodic process, one adsorption
mechanism of single-walled carbon nanotubes proposed on the
peak of hydrogen appears at about -0.94 V (close to the values
of the BN and MoS2 nanotubes27,28). Interestingly, one new
current peak (not observed in the previous CV investigations25-28
of CNT, MoS2 and BN nanotubes) can be clearly seen at ca.
-0.67 V, which is probably attributed to the weak adsorption
of hydrogen on the Se tubes or the reduction peaks of hydrogen.
As for the charge-discharge mechanism of selenium sub-
microtubes, a simple scheme on the adsorption and release of
hydrogen was tentatively displayed as shown in Scheme 1.
basis of the density-functional calculation.32 This two plateaus
phenomenon become more obvious (Figure 6b) when the charge
current density was decreased from 80 to 50 mA/g, suggesting
that the charge current density has an important effect on the
charge behaviors of Se submicrotubes, which may be related
to the electrochemical diffusion and adsorption kinetics. As
shown in Figure 6a, the plateau of the discharging potential is
observed at ca. -0.58 V and the discharging capacity of 200
mAh/g can be obtained, which amounts to a hydrogen storage
capacity of 0.74 wt % in SWNTs.26-30 When the constant
current density becomes 50 mA/g, the discharge voltage plateau
is not distinct and the discharge capacity was increased to 265
mAh/g (corresponds to 0.97 wt % hydrogen in SWNTs, Figure
6b), revealing that the capacity of electrochemical hydrogen
To study the morphology effect of t-Se on the capacity of
electrochemically hydrogen storage, the charge-discharge
curves of the synthesized broken selenium microtubes and
spherelike Se particles as a function of capacity at a constant
current density of 50 mA/g under normal atmosphere were
Figure 8. Charge-discharge curves of the broken Se microtubes (a, SEM image shown in Figure 4b) and spherelike Se particles (b, SEM image
displayed in Figure 4a) at a given constant current density of 50 mA/g.

Selenium Submicrotubes
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22835
SCHEME 1. Electrochemical Hydrogen Storage of
Ultra-Long t-Se Submicrotubes
nonionic surfactants (eg. Span-20, Span-40, Span-60, Span-85,
and other Tween series) might potentially serve as the soft
templates to synthesize submicrotubes/nanotubes of Se and other
anisotropic materials with high electrochemical hydrogen storage
capacity.
Acknowledgment. This work was financially supported by
the National Natural Science Foundation of China. The authors
acknowledge kind help from Mr. Linfeng Fei at USTC with
TEM measurements. B.Z. also thanks Prof. Qingsong Tong at
Fujian Normal University for valuable discussion.
measured and are shown in Figure 8. For the broken selenium
microtubes samples, the value of the discharge capacity is 132
mAh/g, whereas the discharge capacity of spherelike Se particles
is only 52 mAh/g. This manifests that the morphologies have
exerted a noticeable influence on the electrochemical hydrogen
storage ability of Se samples and also indicates that the tubular
structure of selenium products is a crucial factor for the high
hydrogen storage ability of Se tubes. Different from the ultra-
long Se submicrotubes, only one voltage plateau in the charge
curves was seen in broken selenium microtubes and Se particles.
We believe that the tubular structure and the tube’s diameter of
selenium samples mainly contribute to the differences in the
charging process. For the broken Se microtubes, the H atom
may rapidly diffuse to the inter walls in a very short time owing
to the microscale diameter of tube. It is well-known that the
electrochemical capacity is a direct proportion function of the
charging time. Therefore, the charging capacity in the process
of hydrogen adsorbed into the pores/inter walls of selenium
microtubes is rather small, and thus the corresponding potential
plateau is invisible in the charging curve. As for the spherelike
Se particles, there is only a diffusion process between layers of
selenium chains (weak van der Waals’ interactions) because of
the absence of pores or tubes.
References and Notes
(1) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32,
435.
(2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayer, B.; Gates,
B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353.
(3) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418.
(4) Iijima, S. Nature (London) 1991, 354, 56.
(5) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem.
Soc. 2002, 124, 1411.
(6) Suenaga, K.; Colliex, C.; Demoncy, N.; Loiseau, A.; Pascard, H.;
Willaime, F. Science 1997, 278, 653.
(7) Huo, K. F.; Hu, Z.; Fu, J. J.; Xu, H.; Wang, X. Z.; Lu, Y. N. J.
Phys. Chem. B 2003, 107, 11316.
(8) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Liu, Z. W.; Golberg, D.; Ringer,
S. P. AdV. Mater. 2005, 17, 975.
(9) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.;
Yang, P. Nature (London) 2003, 422, 599.
(10) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. AdV. Funct. Mater.
2002, 12, 219.
(11) Gates, B.; Yin, Y. D.; Xia, Y. N. J. Am. Chem. Soc. 2000, 122,
12582.
Conclusions
(12) Jiang, Z. Y.; Xie, Z. X.; Xie, S. Y.; Zhang, X. H.; Huang, R. B.;
Zheng, L. S. Chem. Phys. Lett. 2003, 368, 425.
(13) Wang, X. J.; Zheng, X. W.; Lu, J.; Xie, Y. Ultrason. Sonochem.
2004, 11, 307.
(14) Mayers, B. T.; Liu, K.; Sunderland, D.; Xia, Y. N. Chem. Mater.
2003, 15, 3825.
(15) (a) Cheng, B.; Samulski, E. T. Chem. Commun. 2003, 2024. (b)
Tang, Z. Y.; Wang, Y.; Sun, K.; Kotov, N. A. AdV. Mater. 2005, 17, 358.
(16) Ren, L.; Zhang, H. Z.; Tan, P. G.; Chen, Y. F.; Zhang, Z. S.; Chang,
Y. Q.; Xun, J.; Yang, F. H.; Yu, D. P. J. Phys. Chem. B 2004, 108, 4627.
(17) Xie, S. Y.; Wang, C. F.; Zhang, X. H.; Jiang, Z. Y.; Xie, Z. X.;
Tian, Z. Q.; Huang, R. B.; Zheng, L. S. J. Mater. Chem. 2003, 13, 1447.
(18) (a) Cao, X. B.; Xie, Y.; Zhang, S. Y.; Li, F. Q. AdV. Mater. 2004,
16, 649. (b) Cao, X. B.; Xie, Y.; Li, L. Y. AdV. Mater. 2003, 15, 1914.
(19) Jiang, X. C.; Mayers, B.; Herricks, T.; Xia, Y. N. AdV. Mater. 2003,
15, 1740.
(20) Mayers, B.; Jiang, X. C.; Sunderland, D.; Cattle, B.; Xia, Y. N. J.
Am. Chem. Soc. 2003, 125, 13364.
(21) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. Y. J. Mater. Chem. 2002, 12, 2755.
(22) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Cheng, H. M. AdV. Mater. 2004,
16, 1023.
(23) Chen, Y. T.; Sun, Q. Y.; Li, H. L. Chem. Lett. 2003, 32, 448.
(24) Zhang, H.; Yang, D. R.; Ji, Y. J.; Ma, X. Y.; Xu, J.; Que, D. L. J.
Phys. Chem. B 2004, 108, 1179.
(25) Dai, G. P.; Liu, M.; Chen, D. M.; Hou, P. X.; Tong, Y.; Cheng, H.
M. Electrochem. Solid-State Lett. 2002, 5, E13.
(26) Dai, G. P.; Liu, C.; Liu, M.; Wang, M. Z.; Cheng, H. M. Nano.
Lett. 2002, 2, 503.
(27) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. J.
Am. Chem. Soc. 2001, 123, 11813.
(28) Chen, X.; Gao, X. P.; Zhang, H.; Zhou, Z.; Hu, W. K.; Pan, G. L.;
Zhu, H. Y.; Yan, T. Y.; Song, D. Y. J. Phys. Chem. B 2005, 109, 11525.
(29) Tenne, R. Angew. Chem., Int. Ed. 2003, 42, 5124.
(30) Rajalakshmi, N.; Dhathathreyan, K. S.; Govindaraj, A.; Satishkumar,
We have demonstrated a simple and practical solution-phase
route to trigonal phase ultra-long selenium submicrotubes with
the assistance of Tween-20. The SEM observations show that
the final products were the t-Se tubular structure with a length
of over 100 µm. The high crystallinity and purity of the ultra-
long selenium submicrotubes were confirmed by XRD, Raman
and TEM studies. It was found that the prepared selenium
submicrotubes had a preferential growth along the [001]
direction. By changing the experimental parameters, broken Se
microtubes, Se nanowires, and spherelike particles were suc-
cessfully synthesized. Interestingly, we also found that the ultra-
long t-Se submicrotubes could store hydrogen in an electro-
chemical process. A high electrochemical discharge capacity
of 265 mAh/g (amounts to 0.97 wt % hydrogen in SWNTs)
was obtained at room temperature in the ultra-long Se sub-
microtubes electrode. For the ultra-long selenium submicrotubes,
the phenomenon of two charging plateaus was seen. We deem
that there are two steps in the process of hydrogen storage: the
H was first adsorbed into the pores/inner-walls of selenium
submicrotubes and then diffused into the interstitial sites among
the Se chains. It has been demonstrated that the morphology of
the synthesized Se products had a remarkable influence on the
ability of electrochemical hydrogen storage. These differences
in hydrogen storage capacity are likely due to the diameter and
length of tubes as well as the microcosmic morphology of
different selenium samples. The as-obtained ultra-long Se
submicrotubes are expected to find wide applications in
hydrogen storage, high-energy batteries, and optoelectronic,
biologic, and catalytic fields as well as in the studies of
structure-property relationships. In addition, ultra-long selenium
submicrotubes might be used as the templates to produce other
ultra-long one-dimensional functional nanomaterials. Impor-
tantly, by the same solution-phase approach, other similar
B. C. Electrochim. Acta 2000, 45, 4511.
(31) (a) Yannopoulos, S. N.; Andrikopoulos, K. S. J. Chem. Phys. 2004,
121, 4747. (b) Poborchii, V. V.; Kolobov, A. V.; Tanaka, K. Appl. Phys.
Lett. 1999, 74, 215.
(32) Lee, S. M.; An, K. H.; Lee, Y. H.; Seifert, G.; Frauenheim, T. J.
Am. Chem. Soc. 2001, 123, 5059.