Porous silicon and alumina as chemically reactive templates for the synthesis of tubes and wires of SnSe, Sn, and SnO2

Article abstract of DOI:10.1002/anie.200502665

Three in one: Ordered nanostructures of SnSe, SnO₂, and elemental tin (see SEM image) were prepared by template-assisted synthesis in porous alumina and macroporous silicon. The use of the templates as chemical reactants allowed the preparation

Full text of DOI:10.1002/anie.200502665

Angewandte
Chemie
Inorganic Nanotubes
DOI: 10.1002/anie.200502665
Porous Silicon and Alumina as Chemically
Reactive Templates for the Synthesis of Tubes and
Wires of SnSe, Sn, and SnO **
2
Lili Zhao, Maekele Yosef, Martin Steinhart,
Petra Göring, Herbert Hofmeister, Ulrich Gösele, and
Sabine Schlecht*
Highly ordered porous templates such as alumina mem-
[
1]
[2]
branes and macroporous silicon are valuable and versatile
tools for the fabrication of arrays of one-dimensional nano-
structures with high aspect ratios and narrow diameter
[
3]
distributions. With templates of macroporous silicon and
porous alumina, pore diameters from 25nm up to 1000 nm
are accessible. Here, we report on a new synthetic approach
where these templates are not simply inert shape-defining
molds but also chemical reagents for the oxidation or
reduction of tin(ii) selenide. For a given template system,
the reduction or oxidation can be initiated easily by increasing
the temperature to ꢀ 6508C. Starting from tin(ii) selenide
nanotubes with tin in a medium oxidation state, single-
crystalline nanotubes of elemental tin were produced in a
high-temperature reaction with the macroporous silicon
template as a reducing environment. In an analogous
manner single-crystalline nanowires of tin(iv) oxide (SnO2)
were obtained using the high-temperature oxidizing proper-
[
*] Dr. M. Yosef, Prof. Dr. S. Schlecht
Institut für Chemie und Biochemie
Freie Universität Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
Fax: (+49)30-838-53310
E-mail: schlecht@chemie.uni-marburg.de
L. Zhao, Dr. M. Steinhart, Dr. P. Göring, H. Hofmeister,
Prof. Dr. U. Gösele
Max-Planck-Institut für Mikrostrukturphysik
Halle (Germany)
[
**] This research was supported financially by the Deutsche For-
schungsgemeinschaft (SPP1165 “Nanowires and Nanotubes”,
STE1127/2-1 and SCHL529/2-1) and the Fonds der Chemischen
Industrie. We thank T. Geppert, K. Sklarek, and S. Grimm for
preparing the templates.
Angew. Chem. Int. Ed. 2006, 45, 311 –315
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311

Communications
ties of the pore walls of alumina templates etched with
sulfuric acid.
It is known that SnSe forms as the primary thermolytic
In the wetting procedure applied here, the single-source
precursor Sn(SePh) is infiltrated into the pores. In a low-
4
temperature thermolysis at about 3508C SnSe forms as the
initial thermolytic product and no chemical reaction involving
the template and the precursor or the template and the
produced SnSe takes place. Nanowires consisting of ortho-
[
4]
product of the precursor Sn(SePh)4 at temperatures above
008C. SnSe is a narrow-band-gap semiconductor with an
3
optical band gap of ꢀ 1 eV. In particular, nanoscale thin films
[
14]
of the tin(ii) chalcogenides SnS and SnSe are promising
rhombic SnSe (space group Pbnm)
with a diameter of
[
5,6]
materials for photoelectric conversion.
Polycrystalline
25nm, corresponding to the diameter of the template pores,
were obtained when the wetted samples were annealed at
3508C for 36 h. A transmission electron microscopic (TEM)
image of a bundle of released polycrystalline SnSe nanowires
is shown in Figure 1a, and the corresponding selected area
electron diffraction pattern (SAED) is shown in Figure 1b.
An X-ray diffraction pattern of the SnSe nanowires, which
indicates the formation of single-phase SnSe, is represented in
Figure 1c.
[
6]
films of SnSe can be used in energy-conversion devices
and in memory switching. Ordered structures of tin nano-
tubes can be used for the preparation of nanostructures of
catalytically active binary intermetallics such as Pt–Sn and
Au–Sn. Tin nanowires also exhibit superconducting proper-
[
7]
[
8]
[
9]
[
10]
[11]
ties. Nanoribbons, nanowires, and nanoboxes of tin(iv)
oxide were produced by template-assisted routes or by
addition of surface-capping additives and showed excellent
sensing properties. The different types of templates we used
for the preparation of nanowires and nanotubes of SnSe, Sn,
and SnO are listed in Table 1.
2
Table 1: Redox properties of different template types at elevated temper-
atures.
Template type
Pore diameter
Redox
properties
alumina (etched with sulfuric acid)
25 nm, 30 nm
oxidizing
alumina (etched with phosphoric acid) 180 nm, 400 nm mostly inert
silicon 1000 nm reducing
Self-ordered porous alumina is currently accessible with
pore diameters ranging from 25nm to 400 nm and pore
depths up to 100 mm. The pore walls are amorphous and also
contain tetrahedrally coordinated aluminum atoms, defects,
[
12,13]
and electrolyte anions.
Commonly used electrolytes are
[
1c]
sulfuric acid, yielding pores with a diameter of 25nm, and
[
1d]
Figure 1. a) TEM image of a bundle of released SnSe nanowires with a
diameter of 25 nm; b) electron diffraction pattern of a bundle of SnSe
nanowires; c) XRD pattern of SnSe nanowires in the template (reflec-
tions are calculated as a line pattern and indexed according to JCPDS
card no. 88-0287).
phosphoric acid, yielding pores with a diameter of 180 nm.
The latter pores can be widened to 400 nm by an isotropic
wet-etching step. The properties of the pore walls depend to a
great extent on the electrolyte, and the more acidic medium of
sulfuric acid together with a higher defect concentration in
thin pore walls should enhance the reactivity of the templates
with small pore diameters (Table 1). Macroporous silicon
templates cover the diameter range from 370 nm up to few
microns. Their pore depths are limited only by the thickness
of the wafers used as substrates and can reach several
hundreds of microns, thus leading to excellent aspect ratios
of the pores.
Independent of the produced target material, solid wires
formed in narrow pores with diameters of 25nm, whereas
tubes were obtained when pores with diameters of 180, 400, or
Nanotubes of SnSe with polycrystalline walls formed
when porous alumina with a pore diameter of 180 nm was
used as a template. The scanning electron micrographic
(SEM) images in Figure 2a and b show an overview and a
magnified image of an array of aligned SnSe nanotubes
released from the template. In Figure 2b it can be seen that
the nanotubes have terminating caps at their ends, resulting
from the bottom dimple of the template pores. This detail
nicely shows that the pores were filled in their full depth and
that the excellent aspect ratio of the pores can be transferred
quantitatively to the one-dimensional nanostructures. The
grain structure of the tube walls is apparent in Figure 2c,
where an individual polycrystalline SnSe tube released from a
template with a pore diameter of 400 nm is depicted. The
crystallites have diameters on the order of 10–20 nm. The
thickness of the tube walls is also in the range of 20 nm. The
SAED pattern depicted in Figure 2d shows the characteristics
[
2]
1
000 nm were used. All products resulting from a thermolysis
at 3508C showed a granular, polycrystalline morphology.
When higher temperatures of 6008C to 65 08C were applied,
the formation of single-crystalline wires and tubes was
achieved. The transition from wires to tubular structures
depending on the pore diameter of the templates also occurs
in the case of the template-assisted thermolysis of hydrogen
[
3i]
tetrachloroaurate leading to gold nanostructures.
3
12
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Angew. Chem. Int. Ed. 2006, 45, 311 –315

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Chemie
than 6008C (Table 2). The reduction can be performed as a
quantitative conversion either starting directly from molec-
ular Sn(SePh) or from independently produced nanoparti-
4
cles of SnSe with a particle size of ꢀ 50 nm. In both reactions
elemental tin was formed under the reaction conditions and
crystallized from its liquid state under cooling. Owing to the
large surface area of the pore walls of macroporous silicon
templates and a relevant number of defects and distortions
that form in the structure of the pore walls of macroporous
[
3h]
silicon during annealing, the reaction of Sn(SePh) with the
4
template pores should be even more effective than its reaction
with microcrystalline silicon powder. Accordingly, single-
crystalline microtubes consisting of elemental Sn with a
diameter of 1 mm were obtained by infiltrating macroporous
silicon molds with Sn(SePh) , annealing the wetted templates
4
at temperatures in the range of 600–6508C for 5h, and
Figure 2. SEM images of an ordered array of SnSe nanotubes with a
diameter of 180 nm: a) overview; b) magnified image showing granular
wall morphology and terminating caps; c) TEM image of a single SnSe
nanotube with a diameter of 400 nm and polycrystalline walls;
d) electron diffraction pattern of the tube section seen in (c).
subsequently cooling the samples to room temperature at a
À1
rate of 2 Kmin . Figure 3a shows an array of aligned Sn
of polycrystalline tubes of single-phase orthorhombic SnSe
(
space group Pbnm).
The intermediate oxidation state of Sn in SnSe offers the
II
opportunity of either a reduction or an oxidation of the SnSe
nanostructures by appropriate pore walls with reducing or
oxidizing properties. Such selective redox chemistry within
the pores requires that the pore walls of the alumina
templates have sufficient oxidizing power and that the pores
of the macroporous silicon have good reducing ability, as
these two template materials are the most readily accessible
ones with complementary redox properties.
To model the redox chemistry of Sn(SePh) and SnSe with
4
macroporous silicon and with the pore walls of porous
alumina, the two tin compounds were reacted with commer-
cial silicon powder under different conditions (Table 2), and
Figure 3. a) SEM image of a laterally extended array of Sn nanotubes
with a diameter of 1000 nm, protruding from the macroporous silicon
matrix; b) XRD pattern of aligned, partially released tin tubes. All
reflections are indexed on the basis of JCPDS card no. 04-673; c) tin
nanotube after removal of the silicon template; d) indexed SAED
pattern of a single tin nanotube (zone axis [123]), indicating its single-
crystalline character.
Table 2: Overview of reactions of Sn(SePh)4 and SnSe with silicon
(
reaction time: 15 h).
Tin
compound
Silicon
source
Reaction
temperature
Product
tubes protruding from a partially removed template. They are
uniform in size and have a wall thickness of approximately
Sn(SePh)4
Sn(SePh)4
Sn(SePh)4
SnSe (nanopart.)
SnSe (microcryst.)
Si template
Si powder
Si powder
Si powder
Si powder
6508C
500 or 6008C
6508C
6508C
6508C
Sn
SnSe
Sn
Sn
SnSe,
very little Sn
1
50 nm. All reflections in the X-ray diffraction pattern
[
16]
(Figure 3b) can be indexed for tetragonal b-Sn
without
ambiguity, indicating that the reduction of tin(ii) selenide is
quantitative. Figure 3c shows an individual tin tube (d =
1
000 nm) after removal of the silicon template, and an
indexed SAED pattern is depicted in Figure 3d.
the molecular precursor Sn(SePh) was heated in boiling wet
When Sn(SePh) was infiltrated into porous alumina and
4
4
diethylene glycol solvent, a system with hydroxy groups and a
small water content just like the amorphous wall structure of
annealed at 6508C a big difference in the oxidizing power of
the walls of small pores etched with sulfuric acid and the walls
of larger pores etched with phosphoric acid became apparent.
Whereas the thicker, less acidic walls of the larger pores did
not lead to the oxidation of SnSe, single-crystalline SnO2
nanowires with a high aspect ratio and diameters of 25nm
were obtained from porous alumina templates etched with
sulfuric acid. This is in accordance with the higher defect
[
12,13,15]
the pores in ordered alumina membranes.
Under these
conditions Sn(SePh) did not thermolyze into SnSe but was
4
converted into nanoparticles of SnO with a diameter of about
2
1
0 nm, as could be shown by X-ray powder diffraction. In a
series of experiments with silicon powder the formation of
elemental tin was observed at reaction temperatures higher
Angew. Chem. Int. Ed. 2006, 45, 311 –315
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313

Communications
concentration, the increased acidity, and the presence of
electrolyte anions and residual water in the pore walls of
templates etched with sulfuric acid. Figure 4a shows a TEM
Sample preparation: The templates were prepared according to
[
1,2]
procedures described elsewhere.
For the SnSe and SnO wires, as
2
[
12]
well as for the Sn tubes shown in Figure 1 and Figures 3–5, the
Sn(SePh) was melted on the template surface at a temperature of
4
image of SnO nanowires at low magnification. They typically
2
1
108C. For the SnSe tubes shown in Figure 2, a solution of 5wt%
Sn(SePh)4 in chloroform was applied dropwise onto the template
surface. Subsequently, thermolysis was performed at 3508C, and or
alternatively, SnSe was converted into SnO2 or Sn at 6508C. All
thermolytic reactions and annealing procedures were performed in
corundum crucibles under argon. The templates were selectively
removed either partially or completely by etching with a 20 wt%
aqueous solution of potassium hydroxide at 708C. The resulting
suspension was washed with deionized water several times until it was
neutral.
X-ray diffraction measurements: XRD measurements were
performed with a Philips Xꢀpert MRD diffractometer with a CuKa
radiation source, cradle, and secondary monochromator in a q/2q
geometry. The wires and tubes were aligned within the pores of the
templates. Their long axes were parallel, and the template surface was
perpendicular to the plane defined by the incident beam and the
detector.
Electron microscopy: SEM images of both the completely
released tubes, which were deposited on conductive substrates
(highly doped silicon wafers), and the partially liberated tubes
standing in the matrix of the partially etched templates were obtained
using a field-emission scanning electron microscope JEOL JSM
Figure 4. a) TEM image of single-crystalline SnO nanowires with a
diameter of 25 nm; b) indexed SAED pattern (zone axis [122]) of the
2
marked section of the SnO nanowire seen in (a).
2
have aspect ratios (length over diameter) greater than 100.
The nonuniform contrast of these nanowires is due to bending
[
17]
contours, thickness fringes, and planar defects. An indexed
SAED pattern of part of a section of a single-crystalline SnO2
nanowire (Figure 4a) is depicted in Figure 4b.
6300F at an accelerating voltage of 5kV. For TEM imaging and
A high-resolution TEM micrograph of a selected area of
electron diffraction recording, aqueous suspensions of the nano- and
micro-objects were applied dropwise onto copper grids coated with a
holey carbon film. The samples were investigated in a JEM 1010
microscope operated at 100 kV. High-resolution TEM images shown
in Figure 5were recorded with a high-resolution TEM (JEM 4010
operating at 400 kV).
an individual nanowire of tetragonal SnO (Figure 5) again
2
shows the single-crystalline nature of the wires. The inter-
planar spacing of d = 3.34 Š (Figure 5c) could be assigned to
{
110} planes of the SnO lattice described in space group P4 /
2 2
[
18]
mnm.
Received: July 29, 2005
Published online: November 30, 2005
Keywords: nanostructures · selenides · template synthesis · tin
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[
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