A conversion route towards tubular SiO2 using rod-like BaSiF6 as a novel template

Article abstract of DOI:10.1016/j.jssc.2009.03.037

A simple hydrothermal reaction between Ba(NO₃)₂ and K₂SiF₆ results in the formation of 1D rod-like BaSiF₆. The BaSiF₆ rods can act as efficient precursors for production of tubular SiO

Full text of DOI:10.1016/j.jssc.2009.03.037

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Journal of Solid State Chemistry 182 (2009) 1679–1684
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
A conversion route towards tubular SiO using rod-like BaSiF as
2
6
a novel template
a
a
a
b
a,Ã
c
Hao-Xiang Zhong , Qing-Li Huang , Ying-Li Ma , Jian-Ming Hong , Xue-Tai Chen , Zi-Ling Xue
a
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures,
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
b
Analytic Center of Nanjing University, Jiangsu, Nanjing 210093, PR China
c
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
2 6
A simple hydrothermal reaction between Ba(NO3)2 and K SiF results in the formation of 1D rod-like
Received 21 January 2009
Received in revised form
6 6
BaSiF . The BaSiF rods can act as efficient precursors for production of tubular SiO2 by hydrothermal
reaction in alkaline solutions. Powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS),
transmission electron microscopy (TEM), high resolution electron microscopy (HRTEM), and field
emission scanning electron microscopy (FESEM) were used to characterize the phase and morphology of
the final product. The experiments indicated the amount of NaOH, reaction temperature, and reaction
time played important roles in the transformation process. A possible growth mechanism of tubular
silica was proposed.
2
6 March 2009
Accepted 28 March 2009
Available online 16 April 2009
Keywords:
Template synthesis
BaSiF
6
&
2009 Elsevier Inc. All rights reserved.
Tubular silica
Hydrothermal synthesis.
1.
Introduction
and morphology of the final product and contain nothing of the
chemical composition of the desired product. Herein, we report on
Recently, much research effort has been devoted to the design
the synthesis of the tubular SiO
2
in alkaline solutions by
and synthesis of tubular materials since the first discovery of
carbon nanotubes [1]. Up to now, various tubular structures such
as metals (Au [2] and Te [3]), sulfides [4], nitrides [5] and oxides
employing rod-like BaSiF as both the physical and chemical
6
templates. No other additive was used in the current synthesis.
The advantage of using such physical/chemical template is that
the final removal of the template is not required since the
template has been sacrificed. Another benefit is the ease control of
the composition of the final material.
2 2 x 2
(SiO [6], ZnO [7], TiO [8], VO [9], and CeO [10]) have been
fabricated. Amongst the oxide systems, silica has attracted
particular attention because of their potential applications
as a catalyst support material [11], as a core material for
biosensors and biomarkers [12], as storage/delivery containers
for biochemical substances [13] and in optoelectronic nanodevice
design [14,15]. Two synthetic approaches to silica nanotubes have
been reported, namely, template approach [16] and direct
chemical/physical routes [17,18]. As a representative synthetic
method, several templates have been developed for the synthesis
2
.
Experimental details
2
.1. Synthesis of BaSiF precursor
6
All chemical reagents used in this study were of analytical
grade. Ba(NO (0.1 mol/L, 10 mL) and K SiF (0.1 mol/L, 10 mL)
3
)
2
2
6
2
of tubular SiO , such as surfactant [19], organogelators [20,21],
were mixed and stirred for 10 min. Then the mixture solution was
transferred into a 30 mL stainless Teflon-lined autoclave and
heated at 120 1C for 6 h. The powder samples obtained were
centrifuged, washed with distilled water, and dried at 60 1C.
biological substrates [22], organic ammounium carboxylate
crystals [23], nanoporous membrane materials [24,25], carbon
nanotubes [26,27], fiber-like crystals of [Pt(NH
ZnS [29], V O [30] and so on. However to the best of our
ꢀ H
knowledge, most reported templates for the preparation of
3 4 3 2
) ](HCO ) [28],
3
O
7
2
tubular SiO
2
only act as a physical template to control the size
2.2. Synthesis of tubular SiO
2
The BaSiF precursor (0.54 mmol) was dispersed into distilled
6
Ã
water (25 mL). After the addition of NaOH (1.80 mmol), and stirred
for 10 min, the mixture was transferred into a 30 mL stainless
Corresponding author. Fax: +86 25 83314502.
E-mail address: xtchen@netra.nju.edu.cn (X.-T. Chen).
0
022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.03.037

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7
00
00
00
00
00
00
00
0
6
5
4
3
2
1
1
0
20
30
40
50
60
70
80
2
Theta(degree)
6
Fig. 1. (a) XRD and (b) SEM patterns of the as-prepared BaSiF .
3
3
2
2
1
1
50
00
50
00
50
00
after washing
before washing
5
0
0
1
0
20
30
40
50
60
70
2Theta(degree)
Fig. 2. (a) XRD of the product before and after washing; (b and c) SEM, and HRTEM images of the product before washing (inset, EDS); (d–g) TEM, SEM, and HRTEM images
of the product after washing (inset, EDS, ED); (h) XPS spectrum of the tube after washing.

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1681
1
00000
O1s
8
6
4
2
0000
0000
0000
0000
0
Okll
Ckll
C1s
Si2s Si2P
1
000
800
600
400
200
0
Binding Energy (eV)
Fig. 2. (Continued)
Teflon-lined autoclave and heated at 120 1C for 6 h. The samples
3. Results and discussion
were collected, washed several times with distilled water, and
dried at 60 1C for 4 h. A mixture of BaF
obtain pure SiO , an acid treatment was employed to remove BaF
The mixture of BaF and SiO was added directly into 1.5 mol/L
HNO solution, and then were stirred for about 3 h. The solid
sample obtained was washed several times with water, and dried
at 60 1C for 4 h.
2
and SiO
2
was obtained. To
3.1. Synthesis and characterizations
2
2
.
3 2 2 6
The direct reaction between Ba(NO ) and K SiF readily
2
2
generates rod-like BaSiF
the precursor BaSiF is shown in Fig. 1a. All peaks can be indexed
¯m, with lattice
6
with a high yield. The XRD pattern of
3
6
to a pure rhombohedral phase (space group: R
3
˚
˚
constant a ¼ 7.198 A, c ¼ 7.020 A; JCPDS 78-0031). No other
impurity peaks are detected in the XRD pattern. Fig. 1b shows a
typical SEM image of the product, from which many rods with
lengths up to several micrometers and diameters around
300–500 nm can be clearly found.
2.3. Characterizations
The crystalline phases of the products were analyzed by X-ray
diffraction (XRD) on
a
Shimadzu XRD-6000 powder X-ray
˚
radiation l ¼ 1.5418 A) at a scanning rate
It was expected that the rod-like BaSiF
precursor and/or template to generate the 1D nano/micro-
structures of Ba- or Si-containing material. Because BaSiF is
unstable in alkaline solutions, the precursor can be treated with
basic solution to obtain pure SiO . When BaSiF was reacted with
ca. 3 molar equivalence of NaOH via a simple hydrothermal
procedure, and then followed by the removal of the produced BaF
through acid treatment, the pure SiO tubes were obtained. Fig. 2a
6
could act as the
diffractometer (CuK
a
ꢁ1
of 0.051 s
in the 2
y
range 10–701. The X-ray photoelectron
6
spectra (XPS) were recorded on an ESCALAB MK II X-ray
photoelectron spectrometer, using Mg K-X-ray as the excitation
source. The sizes and morphologies of the products were studied
by field emission scanning electron microscopy (FESEM, HITACHI-
S4800), transmission electron microscopy (TEM, TECNAI-12), and
high resolution electron microscopy (HRTEM, JEM-2010). X-ray
fluorescence (XRF) analysis was performed on an ARL-9800
spectrometer.
2
6
2
2
shows the XRD patterns of the products prepared by the
hydrothermal process before and after the acid treatment,
respectively. Before washing with acid, it is found that the

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product can be indexed to a cubic BaF
Fm3m), which is consistent with the values in the standard cards
JCPDS Card no. 04–0452). But after the acid treatment, only a
broad peak at 2
indicated the amorphous nature of the product. The above
experimental results imply that the product before washing is
2
phase (space group:
2
observed for the tubular SiO , indicating it is in the amorphous
state. The corresponding selected-area electron diffraction
(SAED) pattern (inset, Fig. 2g) shows only diffuse rings without
diffraction spots, which further confirm its amorphous nature. The
corresponding elemental composition is confirmed by EDS (inset,
Fig. 2d) to be Si and O with an approximate atomic ratio of 1:2
(the Cu signal comes from the copper grids.), indicating that these
(
y
¼ 20–301 was found in the XRD pattern, which
the mixture of BaF
peaks of BaF in the mixed product are very strong, the halo peak
of amorphous SiO could not be seen in the diffraction pattern of
the mixed product.
2
and the amorphous SiO2. Since the diffraction
2
2
tubes are amorphous SiO . No obvious peak of other element such
2
as Ba is detected. XRF data of the product indicated that only
0.3 atm% Ba existed, which demonstrated that nearly all of the
Fig. 2b shows the typical SEM images of the mixture product
before treating with acid. There are many irregular particles or
BaF
HNO
2
produced along with SiO
2
was washed away by treating with
was
3
for 3 h. The surface composition of the amorphous SiO
2
aggregates along with SiO
particles confirmed that main chemical composition is BaF
Fig. 2b). The HRTEM image of a region of an irregular particle
shows it is structurally uniform with interplanar spacing of about
2
tubes. The EDS of these irregular
(inset,
further probed by XPS (Fig. 2h). The relatively strong peaks at
154.5, 103.0, and 532.5 eV can be attributed to the bonding
energies of Si2s, Si2p, and O1s, respectively. These results are in
agreement with those reported [15,31]. No obvious peaks for Ba or
F were observed besides C peaks from the XPS chamber.
2
0
(
.36 nm, which corresponds to the (111) lattice spacing of BaF
Fig. 2c). Therefore, the identity of these irregular particles was
demonstrated to be BaF by HRTEM and EDS. The images of the
products after treating with acid were shown in Figs. 2d–g. The
flower-like SiO microstructures consisted of SiO tubes could be
clearly found from the images in Figs. 2d and e. A tip of a cracked
SiO tube clearly shows a hollow interior of tubular structure with
2
The above observations showed that the rod-like BaSiF
be transformed into a mixture of BaF and SiO . After treating
with acid, tubular SiO was prepared. XRD and TEM were carried
6
could
2
2
2
2
2
2
out on the products isolated at different reaction stages to
investigate the factors involved in the transformation process
2
diameters in the range of 300–400 nm, and shell thickness less
than 60 nm (Fig. 2f). These are further confirmed by the HRTEM
2
image of a single SiO tube (Fig. 2g). No lattice fringe could be
1
1
1
1
1
800
600
400
200
000
2
4 h
2 h
1
6
h
2
1
h
8
6
4
2
00
00
00
00
0
h
0
.5 h
1
0
20
30
40
50
60
70
80
2Theta (degree)
Fig. 3. (a) TEM images of the products correspond to reaction stages at 2.0 h (inset,
EDS) and (b) XRD patterns of the products obtained at reaction stages of 0.5, 1.0,
Fig. 4. (a and b) SEM images of the product obtained with 2.3 mmol NaOH at
120 1C for 6 h after washing with acid.
2
.0, 6.0, 12.0, and 24.0 h, respectively.

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1683
and the formation of the tubular SiO
was shortened to 0.5 h, only random rods with a diameter around
00–500 nm were observed. The X-ray diffraction pattern of the
product showed that it was pure BaSiF with the rhombohedral
2
. When the reaction time
into BaF
2
and SiO
2
(Fig. 2a). Because SiO
2
is amorphous in nature,
only those diffraction peaks of BaF
reaction was further prolonged (12, 24 h), the product still
preserved the tubular morphology. It can be deduced that the
2
were observed. When the
3
6
structure (Fig. 3b). Increasing reaction time up to 1 h, the rods and
a small quantity of tubes coexisted. The X-ray diffraction pattern
formation of tubular SiO
precursors BaSiF nanorods formed. Second, the precursor is
reacted with NaOH to yield a mixture of SiO and BaF . Finally,
BaF is removed by acid treatment, which results in the final
hollow morphology.
The following reactions occur in the formation of tubular SiO
2
involves three steps. First, the
6
of the product is mainly the rhombohedral BaSiF
6
(Fig. 3b). When
2
2
the reaction time reaches at 2.0 h, the images in Fig. 3a showed
that the partially filled rods or partial tubes were found. It appears
that the tubular structures were formed from the erosion of the
template starting at the tips of the rod. EDS (inset, Fig. 3a) of
the partial tube shows the Ba, Si, F, and O elements coexisted, and
the amount of Si is much more than that of Ba. The X-ray
diffraction pattern of the product obtained at this stage showed
2
2
:
2
þ
2ꢁ
Ba þ SiF6 ! BaSiF
6
#
(1)
(2)
BaSiF
6
þ 4OHꢁ ! BaF
2
# þSiO
2
2
þ 4F þ 2H O
2 6
the mixed phases of the cubic BaF and BaSiF . When the reaction
It has been found that the amount of alkaline have great influence
on the products. When the amount of NaOH was less than
time was further prolonged to 6 h (Fig. 2b), the mixed
morphologies of tubes and irregular particles coexisted. The X-
ray diffraction pattern of the product showed that the only cubic
1
.5 mmol, the BaSiF
mixture of BaF and SiO
increased to 1.8 mmol, a mixture of BaF
and the tubular SiO was obtained after acid washing. Further
increasing the amount of NaOH to 2.3 mmol, the SiO tubes
6
rods did not completely transform into the
. When the amount of NaOH was
and SiO was produced,
2
2
2 6
BaF existed, indicating that BaSiF was completely transformed
2
2
2
2
with the diameter of 50 nm were produced after washing with
acid (Fig. 4a). It should be noted that the size of this tubular
product is much smaller than that of the precursor BaSiF
6
. Only
irregular small sphere were obtained with 5 mmol NaOH.
The reaction temperature was also found to affect the
transformation. At a low temperature such as 80 1C, the product
was still BaSiF rods. When temperature was raised to 120 1C, SiO
6 2
tubes were produced after washing with acid (Figs. 2e–h). Further
increasing the temperature to 160 1C, the bamboo-like tubes of
SiO
2
were obtained after washing the mixture (Figs. 5a and b).
with the
When the temperature was further raised to 180 1C, SiO
2
long bamboo-like tubes were still kept after washing the mixture
with acid.
3.2. Growth mechanism
On the basis of all the above observations, a template-based
growth mechanism of the straight silica tube is proposed. The
above findings suggest that the three-stage processes are
responsible for the formation of the tubular silica. The possible
2 6
formation mechanism of SiO was illustrated in Scheme 1. BaSiF
rods were formed first, which might be due to the anisotropical
nature of crystal structure and intrinsic growth characteristics.
The TEM image of the product obtained after reaction for 2 h
shown in Fig. 3a clearly suggest that the erosion started on both
ends. Therefore it is reasonable to assume that the reactivity rates
of the different crystalline planes were significantly varied due to
the different surface energies when BaSiF
NaOH. Two ends of BaSiF rod reacted with NaOH much faster
than the side surfaces. Therefore the erosion of the BaSiF first
occurred on the two tips, yielding the primary SiO and BaF
particles. At the same time, these produced SiO particles could
coat on the exterior surface of BaSiF while the produced BaF
remained in the hole or the solution. With the increasing of the
6
was treated with
6
6
2
2
2
6
2
Fig. 5. TEM images of the products obtained at different temperature (a and b)
60 1C, 6 h, after washing.
1
Scheme 1. Schematic illustration of the tubular silica formation.

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reaction time, the interior was gradually hollow and the silica
sheath was developed. When the BaSiF was completely
consumed, the mixture of SiO tubes and BaF particles were
formed. The final removal of BaF led to the pure tubular SiO
Therefore, in this formation process of silica tubes, BaSiF rods
both served as the precursor and the template. The different
reactivity of crystalline planes of BaSiF rods with NaOH was
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2
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2
In summary, we have successfully fabricated tubular SiO and
rod-like BaF
hydrothermal treatment of BaSiF
at a mild temperature (120 1C) produces SiO
2
by using BaSiF
6
nanorods as precursors. The
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[
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This work was supported by National Basic Research Program
of China (nos. 2006CB806104 and 2007CB925102 to XC), Natural
Science Grant of China (no. 50572037 to XC), and US National
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[
[
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