Na2so4 monocrystal nanowires - aspect ratio control and electron beam radiolysis

Article abstract of DOI:10.1021/ic100874r

This paper presents the synthesis of water-dissolvable Na ₂SO₄ nanowires and nanorods by a simple chemical reaction between CuSO₄ and NaBH₄ In ethylene glycol. By adjusting the pH and the monomer concentration, the aspect ratio and size of the Na ₂SO₄ nanowires could be tuned. Na₂SO ₄ nanorods, nanowhiskers, nanowires, and submicrorods were obtained. Optimal chemical potential is believed to be the dominant driving force for the growth of Na₂SO₄ nanowires during the synthesis. We also demonstrated the Na₂SO₄ nanotubes obtained by the electron beam radiolysis of Na₂SO₄ nanowires. The mechanism of selective radiolysis is also investigated.

Full text of DOI:10.1021/ic100874r

6
748 Inorg. Chem. 2010, 49, 6748–6754
DOI: 10.1021/ic100874r
Na SO Monocrystal Nanowires;Aspect Ratio Control and
2
4
Electron Beam Radiolysis
†
,‡
†,§
‡
||
‡
||
||
Ruiting Zheng, Jinwei Gao, Tengfei Yang, Yucheng Lan, Guoan Cheng, Dezhi Wang, Zhifeng Ren,
,†
and Gang Chen*
†
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts
‡
0
2139 , Key Laboratory of Radiation Beam Technology and Materials Modification of Ministry of Education,
§
College of Nuclear Science and Technology, Beijing Normal University; Beijing, 100875, China, School of
Physics and Telecommunication Engineering, South China Normal University, Guangzhou, 510631, China, and
||
Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467
Received May 3, 2010
This paper presents the synthesis of water-dissolvable Na SO nanowires and nanorods by a simple chemical reaction
4
2
between CuSO and NaBH in ethylene glycol. By adjusting the pH and the monomer concentration, the aspect ratio
4
4
and size of the Na SO nanowires could be tuned. Na SO nanorods, nanowhiskers, nanowires, and submicrorods
2
4
2
4
were obtained. Optimal chemical potential is believed to be the dominant driving force for the growth of Na SO4
2
nanowires during the synthesis. We also demonstrated the Na SO nanotubes obtained by the electron beam
2
4
radiolysis of Na SO nanowires. The mechanism of selective radiolysis is also investigated.
4
2
1
. Introduction
Crystallization and physical properties of inorganic salt
In this paper, we investigate the growth mechanism of
Na SO nanowires and report a simple technique to control
2
4
the aspect ratio and diameter of Na SO nanowires at room
2
4
crystals have been well studied. Compared with bulk crystals,
however, less research has been carried out on inorganic salt
nanocrystals. Recently, NaCl, Na SO , and Na CO in-
organic salt nanocrystal arrays have been obtained by micro-
engineering the shape of nanocrystals in a microchannal or
temperature. We also report Na SO nanotubes obtained by
2
4
the electron beam (EB) radiolysis of the Na SO nanowires.
2
4
2
4
2
3
Na SO is an environmentally friendly and a water-dissol-
2
4
1
vable inorganic salt that can be easily removed by water.
Hence, Na SO nanorods, nanowhiskers, nanowires, sub-
2
3
2
4
a microwell. Water-dissolvable Na SO nanowires and
2
4
4
micrometer rods, and nanotubes reported inthis paper can be
ideal candidate templates for the synthesis of hollow struc-
tures for many other applications.
(NH ) PtCl microcrystals have been adopted as templates
4 2 6
to synthesize hollow micro/nano structures. Such salt-based
templates are advantageous compared to using other water-
insoluble nanowires and nanoparticle as templates that
5
require selective etching or calcinating in the subsequent tem-
plate removal, in terms of simplicity, environmental impacts,
and costs. However, the growth mechanism and shape
control of inorganic salt nanocrystals still await further
investigation.
2. Experimental Section
2
.1. Reagent. Copper sulfate pentahydrate (CuSO
4
5H
2
O,
, 98%), hydrochloric acid
HCl, 10% V/V aqueous solution), sodium chloride (NaCl,
9.0%), sodium hydroxide (NaOH, 50% w/w aqueous solu-
3
9
9%), sodium borohydride (NaBH
4
(
9
tion), and ethanol (anhydrous, 94-96%) were purchased from
Alfa Aesar and used without further purification. Polyvinylpyr-
rolidone (PVP) (K30, molecular weight of 40 000) and ethylene
glycol (EG) (anhydrous, 99.8%) were purchased from Sigma-
Aldrich. Deionized water was purchased from Mallinckrodt Baker.
*
Corresponding author. Telephone: 1-617-253-0006. Fax: 1-617-258-
802. E-mail: gchen2@mit.edu.
1) Suh, K. Y.; Khademhosseini, A.; Eng, G.; Langer, R. Langmuir 2004,
5
(
2
0, 6080–6084.
(
(
2) Sato, Y.; Koide, Y. Chem. Lett. 2009, 38, 674–675.
2 4 2 4
2.2. Growth of Na SO Nanowires. Na SO nanowires were
3) Pu, Y. C.; Hwu, J. R.; Su, W. C.; Shieh, D. B.; Tzeng, Y.; Yeh, C. S.
prepared by the reaction of CuSO4 with NaBH4 at room
temperature. Typically, 0.25 g CuSO4 5H O and 1 mmol PVP
J. Am. Chem. Soc. 2006, 128, 11606–11611.
4) Lou, X. W.; Yuan, C.; Zhang, Q.; Archer, L. A. Angew. Chem., Int. Ed.
006, 45, 3825–3829.
5) (a) Tenne, R.; Rao, C. N. R. Philos. Trans. R. Soc., A 2004, 362, 2099–
125. (b) Bommel, K. J. C.; Friggeri, A.; Shinkai, S.; Bommel, K. J. C; Friggeri,
3
2
(
(
homogeneous solution. Then 0.15 g NaBH is added into the
in repeating unit,) are dissolved in 50 mL EG to form a
2
(
4
solution, under vigorous stirring. The color of the solution
changed from bluish to brown in 2 min. After 30 min, there
were no bubbles generated in the solution, indicating that the
2
A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (c) Lou, X. W.;
Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987–4019.
pubs.acs.org/IC
Published on Web 06/15/2010
r 2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6749
Figure 1. Microstructure characterization of as-prepared nanowires.
(a) XRD pattern of the as-prepared nanowires. (b) SEM image of the
Na SO nanowires. (c) TEM image and SAED pattern of a nanorod
grown along the [001] direction.
2
4
reaction was complete. The aspect ratio of Na SO nanowires
2
4
Figure 2. TEM images of Na
ratios. (a) Na SO nanorods with an average aspect ratio of 12,
b) Na SO nanowires with an average aspect ratio of 26, and (c) Na SO
nanowires with an average aspect ratio exceeding 100.
2 4
SO nanowhiskers with different aspect
was tuned by the adjustment of reactant concentration and
pH of the CuSO solution in the range of 2-7 with HCl. The
as-prepared products were centrifuged from the solution at
000 rpm for 15 min by a Marathon21000 centrifuger (Fisher
2
4
4
(
2
4
2
4
4
Scientific). The derived products were washed with ethanol three
times to get the white reaction products and were preserved in
ethanol as a suspension. Transmission electron microscopy (TEM)
samples were prepared by dispersing a drop of the above suspension
on carbon films supported by copper grids. The pH in solution
is measured by an Exstik pH meter (Exstik pH100, EXTECH
Instrument Corporation). All the synthesis conditions and the
products are listed in the Supporting Information, Table S1.

6
750 Inorganic Chemistry, Vol. 49, No. 14, 2010
Zheng et al.
nanowires were observed by a field emission scanning electron
microscopy (SEM) (JEOL 6320, Japan) at 5 kV. Crystal struc-
ture and growth orientation of the Na SO nanowires were
2 4
characterized by TEM (JEOL JEM 200CX, Japan) at 200 kV
and high-resolution transmission electron microscopy HRTEM
(JEOL 2010 and JEOL 2011, Japan) at 200 kV. The EB radio-
lysis of Na SO nanowires was carried out by both TEM and
HRTEM.
2
4
3. Results and Discussions
Figure 1 illustrates the microstructures of typical Na SO4
2
nanowires synthesized at pH = 3. Figure 1a is the XRD
pattern of a reaction product in the 2θ range of 10-80°. The
pattern could be distinctly indexed to an orthorhombic phase
˚
with lattice constants a = 5.607, b = 8.955, and c = 6.967 A
for Na SO (Joint Committee on Powder Diffraction Stan-
2
4
dards (JCPDS) no. 83-1570). A little residual cubic phase
˚
Cu O with lattice constant a=4.26 A (JCPDS no. 71-4310)
2
was also observed, whichcould be removed by a gradient cen-
trifugation technique. Different from both prism and needle
Na SO4 crystals grown from aqueous solution at room
2
6
temperature, the precipitates obtained in our experiment
were all Na SO nanowires. SEM observation of the pre-
2
4
cipitates is illustrated in Figure 1b. The precipitates exhibit
typical wire morphology with diameters of 100-300 nm and
with aspect ratios exceeding 100. Selected area electron
diffraction (SAED) patterns of Na SO nanowires, as shown
2
4
in Figure 1c, reveal the single crystalline nature of the nano-
wires. The preferred growth direction of these Na SO nano-
2
4
wires was determined to be along the [001] direction, i.e.,
c axis of orthorhombic phase. The space group of our
Na SO nanowires is Cmcm, (001) has a zero XRD intensity
2
4
because of systematic extinction, (002) has a nonzero struc-
ture factor, and can be detected.
The aspect ratio of Na SO nanowires could be tuned by
2
4
adjusting the reactant concentration and pH of the CuSO4
solution. Figure 2a shows the TEM image of reaction pro-
ducts prepared by the reaction of 0.125 g CuSO 5H O and
4
3
2
0
.075 g NaBH in EG at pH = 7. Na SO nanorods with a
4 2 4
diameter of 20-50 nm and an aspect ratio of 9-15 were
observed. Figure 2 b illustrates the TEM image of reaction
products synthesized by the reaction of 0.25 g CuSO 5H O
4
3
2
and 0.15 g NaBH in EG at pH = 7. Na SO nanowhiskers
4
2
4
with diameters of 70-120 nm and aspect ratios of 19-33
were produced. If the reaction of 0.25 g CuSO 5H O and
4
3
2
0
.15 g NaBH was performed in EG at pH = 3, then Na SO
4
2
4
nanowires with diameters of 100-300 nm and average aspect
ratios exceeding 100 could be obtained (Figure 2c).
We speculate that Na SO tends to form nanowires due to
2
4
7
an orientational crystallization mechanism. With the help of
H O molecules that existed in CuSO 5H O crystal, CuSO
2
4
3
2
4
could be dissolved in EG and form a uniform ionic solution.
When the NaBH is added in the solution, the bluish Cu is
2þ
4
reduced into yellowish Cu O nanoparticles, and H is gene-
2
2
Figure 3. TEM images of Na
different processes. (a) Na SO
CuSO and NaBH with the addition of PVP and two drops of 0.1 M
NaCl solution; (b) Na SO nanorods obtained by the reaction of H SO and
NaOH in EG with the existence of PVP; and (c) Na SO nanorods obtained
by the reaction of CuSO and NaBH in EG without the help of PVP.
2
SO
4
nanowires and nanorods prepared by
rated. At the same time, Na SO nanorods will homoge-
2
4
nanowires obtained by the reaction of
2
4
4
4
neously nucleate from the solution, because the solvability8
of Na SO in EG is extremely low at room temperature.
2
4
2
4
2
4
2
4
4
4
(
6) Amirthalingam, V.; Karkhanavala, M. D.; Rao, U. R. K. Acta
Crystallogr. 1977, A33, 522–532.
7) (a) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389–1395.
b) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343–3353. (c) Wang,
2.3. Microstructure Characterization and EB Radiolysis of
Na SO Nanowires. Crystal structures of the Na SO nanowires
(
2
4
2
4
(
were identified by a powder X-ray diffractometer (XRD) (Rigaku
˚
X.; Li, Y. Angew. Chem., Int. Ed. 2002, 41, 4790–4793.
(8) (a) Vener, R. E.; Thompson, A. R. Ind. Eng. Chem. 1949, 41, 2242–
2247. (b) Vener, R. E.; Thompson, A. R. Ind. Eng. Chem 1950, 42, 171–174.
RU300, Japan), employing Cu KR radiation (λ= 1.5418 A) at
5
0 kV and 300 mA. The sizes and morphologies of the Na SO
2 4

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6751
Figure 4. TEM images of Na
is the SAED pattern of this submicrorod, and the white arrows show the thickness of the outer layer is about 10 nm; (c) growth edges of the Na
layer indicated by arrows; and (d) the transmission image of a broken Na SO submicrorod.
2
SO
4
submicrorods. (a) Morphology of Na
2
SO
4
submicrorods; (b) a typical end structure of a Na
2
SO
4
submicrorod, the inset
SO outer
2 4
2
4
9
The reactions can be expressed as
Na SO monomer concentrations, Na SO nanorods grow
2 4 2 4
longer and form nanowhiskers with average aspect ratio of
26, as shown in Figure 2b. However, it is hard to increase
the aspect ratio of Na SO nanowhiskers by further increas-
þ
-
∼
NaBH
4
þ 2C
2
H
6
O
2
f Na þ BðC
2
H
4
O
2
Þ2 þ 4H
2
v
2
4
ing the mass of NaBH , because the dissolution rate of
4
2
þ
þ
-
NaBH in EG is too slow to keep up with the growth rate
2
Cu þ 2NaBH
4
þ 5H
2
O f Cu
2
OV þ 2Na þ 2BO
2
4
11
of Na SO nanowhiskers. According to a previous study,
2
4
þ 7H
2
v þ 4Hþ
dilute HCl could accelerate the dissolution rate of NaBH4.
We also found that the dissolving period of 0.075 g NaBH in
4
5
0 mL EG decreases from 320 to 150 s, while the pH value
of EG solution changes from 7 to 3 by adding two drops of
.1 M HCl. In the acidic environment, the Na SO monomer,
þ
42 -
f Na
2
Na þ SO
2 4
SO V
0
2
4
Like other orthorhombic one-dimensional nanostruc-
tures, Na SO tends to form nanorods when it precipitates
from the solution, because of the highly anisotropic nature of
the Na SO orthorhombic structure. The first-precipitated
Na SO nanorods could serve as seeds. When subsequent
Na SO monomers precipitatefromthe solution, they tend to
aggregate along the c axis of the seeds and grow up. At low
Na SO monomer concentrations, only nanorods can be
obtained, as indicated in Figure 2a, since the diffusion of
Na SO monomers is not fast enough. With the increase of
1
0
which is consumed by the growth of Na
the dissolution of NaBH , is in balance, so the Na SO
4
nanowires with average aspect ratio beyond 100 could be
obtained (Figure 2c). The above discussion suggests that the
Na SO monomer, not the H concentration, holds the key
for synthesizing Na SO nanowires with high aspect ratios. A
higher chemical potential generated by a higher monomer
concentration is preferable for the growth of nanowires. This
can be further confirmed by another experiment: with the
SO and provided by
2 4
2
4
4
2
2
4
2
4
þ
2
4
2
4
2
4
2
4
2
4
addition of two drops of 0.1 M NaCl solution, Na SO4
2
nanowires with aspect ratios beyond 100 could also be
obtained by the reaction of 0.25 g CuSO 5H O and 0.15 g
(
9) (a) Dasgupta, M.; Mahanti, M. K. Transit. Meta. Chem. 1986, 11,
86–288. (b) Chen, Y. Catal. Today 1998, 44, 3–16.
10) (a) Li, X. L.; Liu, J. F.; Li, Y. D. Appl. Phys. Lett. 2002, 81, 4832–
834. (b) Zhang, Y. W.; Yan, Z. G.; You, L. P.; Si, R.; Yan, C. H. Eur. J. Inorg.
4
3
2
2
4
NaBH , as shown in Figure 3a. However, an extremely high
4
(
Chem. 2003, 10, 4099–4104. (c) Yu, Y.; Jin, C. H.; Wang, R. H.; Chen, Q.; Peng,
L. M. J. Phys. Chem. B 2005, 109, 18772–18776.
(11) Zhang, J.; Fisher, T. S.; Gore, J. P.; Hazra, D.; Ramachandran, V. P.
Int. J. Hydrogen Energy 2006, 31, 2292–2298.

6
752 Inorganic Chemistry, Vol. 49, No. 14, 2010
Zheng et al.
2 4
Figure 5. The structure evolutions of Na SO nanowires by the electron beam irradiation of: (a) 30 s, (b) 2 min, (c) 6 min, (d) 8 min, (e) 10 min,
and (f ) 15 min.
chemical potential is disadvantageous to synthesize Na SO4
of Na SO nanowires. The principal role played by PVP is to
2 4
2
nanowires. If 0.01 M H SO reacts with 0.02 M NaOH, a
suppress nucleation, thereby increasing the competitive nat-
ure of the growth. Indeed, only Na SO nanorods can be
2
4
high density of Na SO nanorods with aspect ratios less than
2
4
2
4
1
0 is obtained, as indicated in Figure 3b. Since the neutraliza-
synthesized without the help of surfactant (Figure 3c).
Figure 4 displays Na SO submicrorods prepared by the
tion reaction between H SO and NaOH is very quick, the
2
4
2
4
concentration of Na SO monomers is highly supersatu-
reaction of 0.5 g CuSO 5H O with 0.3 g NaBH and 4 mmol
4 2 4
2
4
3
rated. A large amount of seeds form immediately, and a high
nucleation rate suppresses the growth of Na SO nanowires.
PVP in EG. Figure 4a clearly indicates that nanowhiskers
grow coarser rather than longer with higher monomer con-
centration. Many Na SO submicrorods with a diameter
2
4
According to above analysis, an optimal chemical potential is
the key factor for synthesizing Na SO nanowires with high
2
4
larger than 200 nm are synthesized. Figure 4b reveals that
the obtained submicrorods have a core-shell structure; the
thickness of each layer of the shell is about 10 nm. The core
2
4
aspect ratios. Besides the pH value and the monomer con-
centrations, the surfactant (PVP) also influences the growth

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6753
Na SO coated with amorphous materials (see Supporting
2
4
Information, Figure S1). At the ends of nanowire, it is
observed that the inner Na SO nanowires are wrapped by
2
4
outer Na SO layers with identical crystal orientation. In the
2
4
area indicated by arrows in Figure 4b and c, some growth
edges can be clearly found. And there are notable characters
of the growth edges (indicated by arrows.) of Na SO whose
2
4
outer layer can be found in Figure 4c. Figure 4d shows the
transmission image of a broken Na SO nanowire. Na SO
4
2
4
2
near the cross-section is radiolysised and left two big and
several small cavities behind. There is no evidence of
any liquid existing in the core of nanowire, because the liquid
-
8
will vaporize immediately at the vacuum of 10 Torr in
TEM.
These multilayer shell structures of Na SO submicrorods
2
4
are different from nanotubes that grow up under a “rolling
12
mechanism”. We did not find this shell structure in the
Na SO nanowires, whose diameters are less than 50 nm. We
2
4
conjectured that the rolling growth of Na SO submicrorods
2
4
starts from those nanowhiskers with small diameters. At the
beginning of the reaction, the concentration of Na SO4
2
monomers is higher; a higher chemical potential makes Na SO4
2
nanorods grow into Na SO nanowhiskers, as indicated by
2
4
Figure 2b. However, due to the quick growth of Na SO4
2
nanowhiskers, the concentration of Na SO monomers in
2
4
solution decreases quickly. Lower chemical potentials cannot
provide enough driving force for the growth of Na SO4
2
nanowhiskers, so they stop growing. With the prolonging
of reaction time, more and more NaBH is alcoholyzed. The
4
concentration of Na SO monomers increases again, but
2
4
nanowhiskers cannot grow along their growth axes anymore
because these growth surfaces are shielded by the surfactant.
Comparing with the nucleation of new particles, Na SO4
2
tends to deposit and epitaxially grow around the surface of
existing Na SO nanowires (Figure 4b and c). Such a growth
2
4
process will quickly decrease the chemical potential and can
also minimize the total surface energy. Finally, Na SO4
2
submicrorods are formed, as shown in Figure 4a.
Na SO4 nanowires are unstable under electron beam
2
radiation. A series of interesting structure evolutions can be
observed in a continuous electron beam irradiation of
Na SO nanowires in TEM. Figure 5 shows the structure
2
4
change of Na SO nanowires. A 200 kV electron beam with a
2
4
2
beam intensity of 10 pA/cm is targeted on a sample surface
for different times. Initially the electron irradiation (for
about 30 s) develops lots of small bubbles in each individual
nanowire (Figure 5a). These bubbles expand, merge, and
grow up under a continuous uniform irradiation from 2 to
6
min. In this course, a molten material flows in the ‘core’ of
nanowires (Figure 5b and c). After 8 min of irradiation, some
regions of the outer ‘shell’ were broken by the inner pressure,
and the gas and molten phases in the nanowires began to
vaporize (Figure 5d and e). Finally, nanotubes form after
15 min of irradiation (Figure 5f ).
It is well-known that radiolysis phenomena exists in
inorganic compounds under electron beam irradiation by
electronic excitation. Usually radiolysis happens homoge-
neously, however. The observed selective radiolysis is beyond
Figure 6. TEM images of Na
a) A typical Na SO nanowire after 2 min EB irradiation (200KV,
0pA). The insets are SAED pattern and the HRTEM image of this
nanowire separately. (b) The irradiated Na SO nanotubes. The inset is
the SAED pattern of these nanotubes. (c) Annealed Na SO nanowires
after 60 min irradiation (200KV, 130pA).
2 4
SO nanowires under EB irradiation.
13
(
1
2
4
2
4
2
4
(12) (a) Li, Y. D.; Wang, J. W.; Deng, Z. X.; WU, Y. Y.; Yang, P. D. J. Am.
Chem. Soc. 2001, 123, 9904–9905. (b) Sun, X. M.;Chen, X.; Li, Y. D. Inorg. Chem.
and shell have the same crystal structure, which is confirmed
by the inset diffraction pattern. The shell is crystalline
2
002, 41, 4996–4498. (c) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am.
Chem. Soc. 2002, 124, 1411–1416.

6
754 Inorganic Chemistry, Vol. 49, No. 14, 2010
Zheng et al.
our expectation. According to the patterns of XRD and
SAED, there is no other phase in these irradiated nanowires.
So we deduce that the formation of Na SO nanotubes has a
phenomena nor nanotube formation can be observed in the
course. The electron dose in a unit area is proportional to the
irradiation time multiplying beam density. So the irradiation
2
4
close relation with their original nanowire structures. When
the Na SO nanorods/nanowires quickly grow along the
resistance of Na SO4 nanowires is increased more than
2
100 times after the annealing treatment. This result indicates
that the defect density is obviously reduced by annealing.
Finally we would like to mention that the simple synthesis
technology and diversity of morphology of Na SO nano-
2
4
þ
axial direction, some H or other ion-related defects or
impurities may be kept in inner part of the nanowires. The
presence of defects and impurities makes Na SO nanorods
2
4
2
4
or nanowires sensitive to the electron beam because those
defects and impurities provide preferred sites for localized
excitations and locally decreased lattice binding energies
materials will bring some opportunities for chemistry, mate-
rials, and biology. These Na SO nanorods, nanowhiskers,
2
4
nanowires, submicrorods, and nanotubes are candidate tem-
plates for nanotubes, nanocapsule, and microcapsules by
coating them with designed materials or their precursors and
by removing the Na SO with water. In addition, the simple,
[13a].
Figure 6 is some further observations of Na SO nano-
2
4
wires under electron beam irradiation. When the samples are
irradiated by electron beam, small bubbles first appear in the
inner part of Na SO nanowires (the gas in the bubbles may
2
4
facile, and cheap synthetic approach developed in this paper
can be extended to the preparation of other salt crystalline
nanomaterials with highly anisotropic structures. And these
size-controllable, environmentally friendly salt nanomater-
ials enrich the family of nanomaterials.
2
4
be O , SO , or H O) because of the higher defects and the
2
2
2
impurities density in this region. At the same time, electron
beams will result in local heating. Degradation of surfactants
attached on the surfaces of nanowires will take out part of the
energy and the cool from the surface of nanowires. So the
inner part of Na SO nanowires will be melted first by the EB
4. Conclusions
2
4
We have demonstrated the synthesis of Na SO crystalline
2
4
heating and form a solid Na SO ‘shell’ with molten Na SO
4
2
4
2
nanowires in bulk quantities by a simple and clean solution-
based wet chemistry method. The size and aspect ratios of as-
synthesized Na SO nanowires are finely tuned by the ad-
‘
core’. Figure 6a shows TEM images of a typical Na SO4
2
nanowire with [001] growth direction (the insets are the
SAED pattern and the HRTEM image of this nanowire).
After 2 min of EB irradiation (200KV, 10pA), bubbles are
formed in the nanotubes, but the surface of nanotubes still
keep the prefect crystal structure. We can observe the lattice
fringes from the inset HRTEM image of the nanotube sur-
face. The interplanar spacing is about 0.703 nm, which
corresponds to the (001) plane of orthorhombic Na SO .
2
4
justment of pH and monomer concentrations. Mechanisms
for the growth processes are proposed that can explain the
observations consistently. It is expected that the synthetic
method employed in this work can be extended to the
preparation of other crystalline one-dimensional nanomate-
rials with highly anisotropic structures. Our experiments also
demonstrated that Na SO nanotubes can be obtained by the
2
4
2
4
After the expansion of the bubbles, when the pressure of gas
and molten Na SO exceed the strength limit of Na SO
selective radiolysis of Na SO nanowires. This approach also
2
4
2
4
2
4
could be used in preparing other hollow micronanostruc-
tures. Further research in preparing hollow structures with
different particle size for biomedical applications is in pro-
gress.
‘
shell’, the ‘shell’ will be broken, the inner gas and molten
-
8
phase leak and volatilize immediately at the vacuum of 10
Torr, forming hollow nanotubes. These tubes are still
Na SO , as confirmed by SAED pattern in Figure 6b. In
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4
order to verify our assumption, we put the same sample on a
hot plate and annealed at 300 °C for 3 h. When the annealed
samples were observed in the TEM again, their irradiation
Acknowledgment. This work is supported by the Na-
tional Science Foundation Nanotechnology and Inter-
disciplinary Research Team (NSF NIRT) (CBET-
2
resistance is improved. Under a beam density of 130 pA/cm ,
0506830) and the NSF grant (CBET-0830098). R.T.Z.
the cavities appear after 3 min of irradiation. They gradually
grow up and become stable after 20 min of observation. A
few structure changes can be observed for another 40 min
of irradiation, as indicated in Figure 6c. Neither flowing
also gratefully acknowledges partial financial support
from the China Scholarship Council, Beijing Nova pro-
ject (2006A32) and the National Basic Research Program
of China (No: 2010CB832905). We wish to thank Dr.
Scott Speakman and Dr. Y. Zhang for their helpful
discussion during this work.
(
13) (a) Hobbs, L. W.Introduction to Analytical Electron Microscopy. In
Radiation effects in analysis by TEM; Hren, J. J., Goldstein, J. I., Joy, D. C.,
Eds.; Plenum: New York, 1987; pp 399-445. (b) Hobbs, L. W. Scanning
Microsc., Suppl. 1990, 4, 171–183. (c) Egerton, R. F.; Crozier, P. A.; Rice, P.
Ultramicroscopy 1987, 23, 305–312. (d) Egerton, R. F.; Li, P.; Malac, M. Micron
Supporting Information Available: All the synthesis condi-
tions and the products are listed. This material is available free
of charge via the Internet at http://pubs.acs.org.
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004, 35, 399–409.