Tin oxide nanowires, nanoribbons, and nanotubes

Article abstract of DOI:10.1021/jp013214r

Nanowires, sandwiched nanoribbons, and nanotubes of SnO₂ are synthesized using elevated temperature synthesis techniques, and their structures are characterized in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition to the normal rutile structured SnO₂, it has been possible to form an orthorhombic superlattice-like structure in the present study. The orthorhombic structure can form in a thin nanowire, coexist with the normal rutile structured SnO₂ in a sandwiched nanoribbon, or occur in the form of nanotubes. This result is distinct from that for bulk SnO₂ where pressures in excess of 150 kbar are required to form the orthorhombic form. The orientation relationship between the orthorhombic SnO₂ and the rutile structured SnO₂ is determined to be [001]₀ a?￥ [102ì?]_t and (100)₀ a?￥ (010)_t for the nanowires and sandwiched nanoribbons, and [001]₀ a?￥ [317]_t and (110)₀ a?￥ (451)_t for the nanotubes. Although the growth direction of the rutile structured SnO₂ nanowires is along [101]_t, two growth directions are found to occur in the nanostructures having the orthorhombic SnO₂ structure. They are [010]₀ for nanowires and [1ì?10]₀ for the sandwiched nanoribbons and nanotubes. The results in this study and the observation of orthorhombic SnO₂ may result from the formation of the products in an oxygen deficient environment.

Full text of DOI:10.1021/jp013214r

1274
J. Phys. Chem. B 2002, 106, 1274-1279
Tin Oxide Nanowires, Nanoribbons, and Nanotubes
Z. R. Dai,† J. L. Gole,‡ J. D. Stout,‡ and Z. L. Wang*,†
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430
ReceiVed: August 18, 2001; In Final Form: October 26, 2001
Nanowires, sandwiched nanoribbons, and nanotubes of SnO2 are synthesized using elevated temperature
synthesis techniques, and their structures are characterized in detail by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). In addition to the normal rutile structured SnO2, it has been
possible to form an orthorhombic superlattice-like structure in the present study. The orthorhombic structure
can form in a thin nanowire, coexist with the normal rutile structured SnO2 in a sandwiched nanoribbon, or
occur in the form of nanotubes. This result is distinct from that for bulk SnO2 where pressures in excess of
150 kbar are required to form the orthorhombic form. The orientation relationship between the orthorhombic
SnO2 and the rutile structured SnO2 is determined to be [001]o || [102h]t and (100)o || (010)t for the nanowires
and sandwiched nanoribbons, and [001]o || [317]t and (110)o || (451)t for the nanotubes. Although the growth
direction of the rutile structured SnO2 nanowires is along [101]t, two growth directions are found to occur in
the nanostructures having the orthorhombic SnO2 structure. They are [010]o for nanowires and [1h10]o for the
sandwiched nanoribbons and nanotubes. The results in this study and the observation of orthorhombic SnO2
may result from the formation of the products in an oxygen deficient environment.
1. Introduction
layered mixtures of Sn foil and SnO powder or simply SnO
powder as the source material under more stringent vacuum
conditions. A detailed microstructure study of the synthesized
SnO2 nanowires, nanoribbons, and nanotubes have been carried
out, and their growth directions and morphology have been
determined. Besides the rutile structure, an orthorhombic
structured SnO2 has also been observed in the nanostructures.
This is in contrast to its observation only in the high-pressure
phase for bulk tin oxide.
Searching for interconnects is a challenge to future nano-
electronics. With the reduction in feature size of devices, it is
essential to find one-dimensional nanomaterials that can inte-
grate these devices and interconnects. Semiconductor nano-
wires1,2 and the recently discovered semiconducting oxide nano-
belts3,4 are potential candidates for fabrication of nanoscale
devices using the integrity of the individual nanowires and belts.
However, there is considerable additional potential for the oxides
as they represent the most diverse class of materials, with
properties covering almost all aspects of material science and
physics in areas including superconductivity, ferroelectricity,
magnetism, and more. Oxides are fundamental to smart and
functional materials due to two unique characteristics: variation
in valence state and oxygen vacancies. The synthesis techniques
and the products we discuss here reflect especially the latter
property.
Semiconducting and amphoteric SnO2 is a key functional
material that has been used extensively for optoelectronic
devices5,6 and gas sensors which detect leakages of reducing
gases such as H, S, CO, and others.7-12 Recently, a series of
binary semiconducting oxide nanobelts (or nanoribbons), such
as ZnO, In2O3, Ga2O3, CdO, and PbO2 and including SnO2 have
been successfully synthesized by simply evaporating the source
compound.3-4,13 The source material used for the synthesis of
SnO2 nanobelts is primarily SnO2 but also SnO powders. Stoi-
chiometric SnO2 nanobelts with a high quality rutile crystalline
structure and uniform geometry have been synthesized.
In this paper, we report the synthesis and structures of SnO2
nanowires, nanoribbons, and nanotubes produced using primarily
2. Experimental Methods
The technique of high temperature thermal oxide synthesis14
has been adopted to synthesis procedures which, in concert with
the control of entrainment flow rates, reactant mixtures, and
annealing procedures, has allowed the synthesis of silicon/silica
based nanotubes and nanoarrays.16 With some modification these
synthesis techniques allow the formation of tin oxide (SnO2)
nanowires, nanoribbons and nanotubes through the use of SnO/
Sn and SnO based mixtures in a double concentric alumina tube
configuration heated to 1050-1150 °C in a Lindberg Blue tube
furnace configuration.
The inner tube of the furnace is vacuum sealed by two water
cooled stainless steel end pieces, attached and tightly lock-press
fit against custom viton O-rings.15 At one end of the furnace,
for the present tin-based synthesis, ultrahigh purity nitrogen gas
enters through the upstream stainless steel end piece, passes
through a matched set of zirconia insulators to the central region
of the inner tube oven, and flows over an alumina crucible
containing the source Sn/SnO or SnO mixtures, usually at a
flow rate of 100 sccm.15 The total pressure in the inner tube
can be changed from 200 to 800 Torr (attached bubbler),
controlled by a mechanical pump attached to the inner alumina
tube through a downstreamwater cooled stainless steel end piece.
This end piece is mechanically attached to a water cooled plate,15
whose temperature is adjustable, located at the fringes of the
* To whom correspondence should be addressed. E-mail: zhong.wang@
mse.gatech.edu.
† School of Materials Science and Engineering, Georgia Institute of
Technology, Atlanta.
‡ School of Physics, Georgia Institute of Technology.
10.1021/jp013214r CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/12/2002

Tin Oxide Nanowires, Nanoribbons, and Nanotubes
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1275
TABLE 1: List of the Samples and Synthesis Conditions
Used in the Present Study
source
T
pressure collecting
sample
material
(°C)
(Torr)
place
#1 (10/12/00)
Sn foil + SnO 1125 400
layered
tube wall
Figure 1(a)
N2 flow gas
#2 (10/11/00)
Sn foil + SnO 1125 200
layered
tube wall
Figure 1(b)
N2 flow gas
#3 (10/11/00)
Sn foil + SnO 1125 200
layered
N2 flow gas
cold plate
35-40 °C
Figure 1(c)
#4 (4/20/00)
Sn foil + SnO 1050 200
layered
tube wall
Figure 2
Ar flow gas
#5 (5/31/00)
Figure 3, Figure 7 +N2
Figure 8 entrainment
SnO powder
1050 250-700 cold plate
25-40 °C
#6 (Likely 10/11) Sn foil + SnO 1125 200
layered
Figure 5, Figure 6 N2 flow gas
cold plate
25-40 °C
#7 (6/21/00)
Figure 1(a)
SnO powder
N2 entrainment
1100 255-500 tube wall
Figure 1. (a) SEM image showing the morphology of the as-
synthesized products for the Sn/SnO system (deposited on inner wall
of the concentric alumina tube). (b) Enlarged SEM image of a local
area for image (a). The sample was made using a layered Sn foil/SnO
mixture. The synthesis was conducted at a furnace temperature at the
central region of 1125 °C and a chamber pressure of 400 Torr at a
flow rate of 100 sccm. The sample was collected from the walls of the
concentric tube. (c) SEM image showing disklike products formed on
a cold plate central to the upstream region of the flow system. See text
for discussion.
tube furnace hot zone. The entrance and exit regions of the
alumina tube are insulated by a machined set of zirconia
blocks.15 A light to dark gray fluffy product is collected on the
alumina tube wall near the point where the double concentric
alumina tube configuration exits the Lindberg oven and where
the temperature is in the range between 450 and 500 °C.
Additional products are collected on the cold plate which is
positioned ∼6 in. into the tube furnace, fronting the upstream
zirconia insulator. This cold plate is maintained at a temperature
of 25-40 °C.
If the fluffy gray to gray-black, SnOx, product is heated slowly
to 1100 °C at Ptotal ∼200 Torr under oxidative conditions (1%
O2 seeded argon) using a crucible directed concentric nozzle
flow configuration, we observe its complete conversion to a
white powder-like product which is more dense and consistent
with bulk SnO2. Further, if this same fluffy gray product is
heated slowly under a reducing atmosphere (Ar/4% H2 @ 1100
°C) it is converted completely to metallic tin. The conditions
for the synthesis of all the product materials discussed in the
following sections are presented in Table 1.
The as-synthesized products are characterized by scanning
electron microscopy (SEM) (Hitachi S-800 FEG), transmission
electron microscopy (TEM) (Hitachi HF-2000 FEG with energy-
dispersive X-ray spectroscopy (EDS)) at 200 kV and a JEOL
4000EX high-resolution TEM (HRTEM) at 400 kV.
Figure 2. (a) Low magnification TEM image of a rutile structured
SnO2 nanowire. (b) High-resolution TEM image of the nanowire. (c)
Corresponding FFT of the image, and (d) SAD pattern from the
nanowire. See text for discussion.
3. Results and Discussion
3.1 Morphology. The as-synthesized products were first
inspected using SEM. Figure 1(a) depicts a typical distribution
in morphology for the fluffy gray products collected on the wall
of the alumina tube furnace following an SnO/Sn based
synthesis. It is evident that the products are dominated by wire-
like structures whose diameter varies over a broad range from
several tens of nanometers to a micrometer. The typical length
of the wires ranges from several tens to several hundred
micrometers. The cross sectional shape of the wires can vary
from virtually circular to the larger rectangle-like structure
indicated by a white arrowhead in the enlarged SEM image
(Figure 1(b)). The chemical composition of the wires is
determined by EDS to be close to SnO2. In addition to the wire-
like structures, Figure 1(c) demonstrates that disklike structures
characterize the sample collected at considerably lower tem-
peratures on the cold plate. The typical diameter of these disks
is 1-2 µm with a thickness of several tens of nanometers. Some
tiny nanodisks are also identified. The chemical composition
of these disks, however, corresponds to SnO and not SnO2. In
the present paper, we will focus on the study of the wire-like
nanostructures produced as the as-synthesized products of
primarily the Sn/SnO synthesis.
3.2 Tetragonal (Rutile) Structured SnO2 Nanowires. All
of the wire-like products including those several tens of
nanometers in diameter (Figure 2(a)) are found to be straight.
We have investigated the crystallography of these SnO2 nano-
wires using select area electron diffraction (SAD) (Figure 2(d))
and HRTEM imaging (Figure 2(b)) combined with a fast Fourier

1276 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Dai et al.
Figure 3. (a) TEM image of an SnO2 nanowire with twin structure,
(b) HRTEM image of the twin, where the inset is the corresponding
FFT of the image. (c) Electron diffraction pattern from the nanowire
shown in (a). The sample was made by vaporizing an SnO powder
sample at a furnace temperature of 1050 °C and a chamber pressure
(argon) of 250 Torr. The sample was collected from a cold plate placed
in the flow channel.
Figure 4. (a) Low magnification TEM image of an individual
orthorhombic SnO2 nanowire, and (b) the corresponding HRTEM
image. The inset at the upper right-hand corner is a select area electron
diffraction pattern obtained for the nanowire shown in (a) and the inset
at the bottom right-hand corner is a FFT of the HRTEM image. The
sample was made using a layered Sn foil/SnO reactant mixture. The
furnace temperature was 1050 °C and the chamber pressure 200 Torr.
The sample was collected from the cold plate after being deposited at
25 °C.
transform (FFT) analysis technique (Figure 2(c)). The results
indicate that the SnO2 nanowires have a normal rutile crystal
structure (a ) 0.470 nm, and c ) 3.188 nm).17 This crystal-
lographic configuration of the SnO2 nanowires is similar to that
of the previously observed SnO2 nanobelts.3,4 The SnO2 nano-
wires can also display a rectangular cross section enclosed by
((010) and ((101h) facet planes, and a thickness-to-width aspect
ratio ranging from 1:2 to 1:5, which is smaller than that for the
SnO2 nanobelts (1:5-1:10).3,4 The growth direction of these
SnO2 nanowires is parallel to the [101] crystal direction.
As we find that the diameter of the nanowires varies from
wire to wire over a broad range, we observe that structural
defects are introduced especially in the nanowires of large
diameter. Figure 3(a) depicts the TEM image of an SnO2
nanowire with a single twin structure. The corresponding SAD
pattern is shown in Figure 3(c). Here, the “twin” reflections
are indexed with a subscript “T” and the remaining matrix
reflection indices are marked without subscript. The zone axis
of the diffraction pattern is [010] for the rutile structured SnO2.
The twinning plane is determined to be (101h) and the twinning
direction is [101], parallel to the growth direction of the SnO2
nanowire. The twin also can be regarded as being formed by
one part of the crystal (twin) rotated 180° along the normal
direction of the (101h) crystal plane while the remaining sections
of the crystal (matrix) maintain the original orientation. Shown
in Figure 3(b) is a HRTEM image around the twin boundary
that is completely coherent, with no relative displacement. The
inset corresponds to the FFT of the HRTEM image, which is
consistent with the experimental electron diffraction pattern
given in Figure 3(c).
3.3 Orthorhombic Structured SnO2 Nanowires. In comple-
ment to the normal rutile structured SnO2 nanowires, we have
found that some of the additional SnO2 nanowires display a
crystal structure different from rutile SnO2. Depicted in Figure
4(a) is a low magnification TEM image of one of these SnO2
nanowires. Figure 4(b) is the corresponding HRTEM image.
The inset at the upper right-hand corner of Figure 4(b) is an
experimental SAD pattern. A matched pattern is reproduced by
the FFT of the HRTEM image, as inserted at the bottom right-
hand corner of Figure 4(b). The configuration of the reflections
with strong intensities in the SAD pattern (inset) is the same as
that for the [102h] crystal zone of rutile structured SnO2 (Figure
2(d)). Some extra weak reflections, however, are clearly seen
in the SAD pattern inserted in Figure 4(b). Their geometrical
configuration is commensurate to that of the strong reflections,
indicating the formation of a superlattice structure (or super-
structure) associated with the normal rutile SnO2. The super-
structure is determined to match that of orthorhombic structured
SnO2 having the lattice parameters: a ) 0.4714 nm, b ) 0.5727
nm, and c ) 0.5214 nm. This observation is somewhat
surprising as previous bulk phase studies suggest that the
orthorhombic structure is formed only under high-pressure
conditions.18 The SAD pattern corresponds to the [001]o crystal
zone of the orthorhombic SnO2 superstructure. The indexes with
a subscript “o” correspond to the ortho-rhombic SnO2 super-
structure, whereas the basic reflections indexed on the basis of
the normal rutile structured SnO2 are denoted with a subscript
“t”. The determined orientation relationship between the ortho-
rhombic SnO2 and the tetragonal (rutile) SnO2 corresponds to
[001]o || [102h]t and (100)o || (010)t. Assuming the cross-sectional
shape of the nanowire is rectangular, the orthorhombic SnO2
nanowire is enclosed by ((100)o/((010)t and ((001)o/((101h)t,
its growth direction being along the [010]o crystal direction that
is parallel to [101]t of the rutile structured SnO2.
3.4 Sandwiched SnO2 Nanoribbons. Another typical mor-
phology observed for the wire-like nanostructures is shown in
Figure 5(a). Here, the nanostructure consists of two side layers
whose thickness is about 20 nm and a core layer with a width
∼120 nm. Considering the contrast over the length of the
nanostructure and the results of chemical imaging, the wire-
like nanostructure is found to have a rectangular cross section
with a smaller aspect ratio than that observed previously for
the nanobelts.3 It is, therefore referred to as a sandwiched
nanoribbon to distinguish it from the nanobelts.3 Although small
parts of the side layers are missing, the nanoribbon appears to
have a generally uniform width over its entire length. The
composition of the sandwiched nanoribbon also approaches that
of SnO2 as determined by EDS. The SAD pattern (Figure 5(b))
indicates that the nanoribbon is not a purely rutile structured
SnO2 but likely has significant orthorhombic structure (SnO2)
because the super-reflections corresponding to orthorhombic
SnO2 are observed. To further understand the crystal structure

Tin Oxide Nanowires, Nanoribbons, and Nanotubes
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1277
Figure 6. (a) HRTEM image of the edge area of the sandwiched
nanoribbon shown in Figure 5(a). Inset is the FFT of the corresponding
image.
Figure 5. (a) TEM image of an individual sandwiched nanoribbon.
(b) Select area electron diffraction pattern from the ribbon, (c,d) Micro-
diffraction patterns taken from the center and edge of the nanoribbon,
respectively. The sample was made using a layered Sn foil/SnO reactant
mixture. The furnace temperature was 1050 °C as the chamber ranged
from 250 to 700 Torr over the course of the run. The sample was
collected from the cold plate. See text for discussion.
of the nanoribbon, electron micro-diffraction patterns have been
recorded for the core layer (Figure 5(c)) and side layer (Figure
5(d)), respectively. Evidently, the reflections corresponding to
the rutile structured SnO2 are very strong in the micro-diffraction
pattern taken of the center of the core layer (Figure 5(c))
although weak reflections corresponding to orthorhombic SnO2
can be identified. The micro-diffraction pattern from the side
layer (Figure 5(d)), however, displays a notably stronger
diffraction feature for orthorhombic SnO2.
The results imply that the nanoribbon core layer is likely to
adopt the rutile structure, whereas the side layers adopt pre-
dominantly the orthorhombic structure. The superlattice reflec-
tion observed in the pattern recorded from the center of the
nanoribbon may come from the top and bottom surface layers
that are dominated by the orthorhombic structure. The HRTEM
image recorded near the edge layer displayed in Figure 6, and
the corresponding FFT given in the inset verifies that ortho-
rhombic structured SnO2 is formed in the edge layer. The surface
of the SnO2 nanoribbon edge is not flat at the atomic level, as
indicated by its HRTEM image (Figure 6). The orthorhombic
SnO2 and the normal rutile SnO2 are coherent with orientation
relationships: [001]o || [102h]t and (100)o || (010)t. This is virtu-
ally the same relationship as that deduced for the corresponding
orthorhombic SnO2 nanowires considered in Section 3.3. The
sandwiched SnO2 nanoribbons, however, are enclosed by
((110)o/((231)t and ((001)o/((101h)t, and their growth direc-
tions are parallel to [1h10]o/[11h1]t.
Figure 7. (a) and (b), correspond to TEM images of two wide
nanoribbons, and (c) and (d) are enlarged images showing the detailed
microstructures of the nanoribbons. The sample was made using SnO
powder at a furnace temperature of 1050 °C, a chamber pressure of
250 Torr, and the sample was collected from a coldfinger placed in
the channel.
increases. Although the contrast displayed is complicated, the
entire nanoribbon is single crystalline and the SAD pattern is
identical to that given in Figure 5(b). Shown in Figure 7(c)-
(d) are two enlarged images of local areas in the nanoribbon
(Figure 7(b)). Here, dislocations can be identified, as indicated
by the open arrowheads.
In addition, a domain-like boundary can also be identified in
the nanoribbon, as marked by solid arrowheads in Figure 7(d).
Figure 8 corresponds to an HRTEM image associated with the
boundary region. This indicates that a domain boundary is
formed by the rutile structured SnO2 (t-SnO2) and the ortho-
rhombic structured SnO2 (o-SnO2). The boundary is semi-
coherent and not abrupt on the atomic scale. The dash line
marked in Figure 8 represents this boundary schematically.
These observations thus suggest that the large width nanoribbons
correspond to a mixed phase SnO2, where dislocations and
domain boundaries coexist.
The contrast over sandwiched SnO2 nanoribbons with smaller
widths, similar to that shown in Figure 6(a), is, in general,
uniform. With an increase in width, however, the contrast over
the nanoribbons becomes complex, as indicated in Figure 7,
parts a and b, where two sandwiched nanobands with width of
order 500 nm are displayed. One of the nanobands (Figure 7(b))
has only one side layer. The thickness of the surface layers does
not significantly increase as the width of the nanoribbon
Bulk orthorhombic SnO2 has been synthesized at a high
pressure of 158 kbar.18 Although its lattice parameters have been
determined, the atomic positions in the crystal have not been

1278 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Dai et al.
Figure 10. (a), (b), and (c) are TEM images of SnO2 nanotubes, and
(d) a select area electron diffraction pattern taken for the SnO2 nanotube
shown in (b). The sample was made using SnO powder at a furnace
temperature of 1100 °C, a chamber pressure of 250 Torr, and the sample
was collected from the walls of the tube furnace.
Figure 8. HRTEM image showing the domains formed by rutile-SnO2
and orthorhombic-SnO2 in the nanoribbons displayed in Figure 7.
The volume decrease is 1.8%. The volume change results in
the semi-coherent boundary between the orthorhombic and rutile
structured SnO2 (Figure 8) and might be responsible for the
missing parts of the side layers in the sandwiched SnO2
nanoribbon shown in Figure 5. The [001]o (c-axis) crystal
direction is not exactly parallel to [102h]t. Instead, there is an
included angle of 1.7° between [001]o and [102h]t, which can be
barely detected from the select area electron diffraction patterns
shown in Figures 4 and 5(b). The growth directions of the
wire-like nanostructures are also indicated as [010]o/[101]t and
[1h10]o/[11h1]t.
3.5 SnO2 Nanotubes. Figure 10(a)-(c) corresponds to TEM
images showing yet another characteristic of the wire-like
nanostructures observed in the present as-synthesized samples.
The chemical composition of these wire-like nanostructures also
corresponds to SnO2. The bright contrast for the center part of
the wire-like nanostructures is always accompanied by the dark
sides even with tilting of the sample under TEM observation.
This indicates that the wire-like nanostructures have hollow
cores, i.e., they correspond to SnO2 nanotubes. The diameter
size of the SnO2 nanotubes varies from 50 nm (Figure 10(c)-
(d)) to 350 nm (Figure 10(a)) and their typical length is several
micrometers. The hollow core is not always continuous through
the entire length of the sample, especially for those nanotubes
of smaller (diameter) size (Figure 10(c)-(d)). For the SnO2
nanotubes with large diameters, the hollow core displays a
zigzag shape (Figure 10(a)).
Electron diffraction analyses indicate that the SnO2 nanotubes
correspond to a single crystal. Shown in Figure 10(e) is an SAD
pattern taken for the SnO2 nanotube depicted in Figure 10(b).
Here, the reflections with strong intensities are indexed to belong
to the[317h]t zone axis of the rutile structured SnO2, and the weak
reflections correspond to an orthorhombic SnO2 superstructure
along the [001]o zone axis. The orientation relationship between
the orthorhombic SnO2 and the rutile SnO2 structures is
Figure 9. Schematic diagram of the crystal structures of rutile SnO2
and orthorhombic SnO2, where the at, bt, and ct represent the base
vectors of the rutile structured SnO2, and the ao, bo, and co denote the
base vectors for the orthorhombic SnO2.
reported. To understand the structural relationship between the
orthorhombic and normal rutile SnO2, a schematic diagram of
the crystal structures is presented in Figure 9. This figure is
constructed on the basis of the orientation relationships between
the two structures that we have determined in the present study.
The rectangular parallelepiped BOAC-B1D1A1C1 represents a
unit cell for the rutile structured SnO2 crystal. Here, the large
spheres denote oxygen atoms and small spheres tin atoms. Six
unit cells of rutile SnO2 are drawn in the diagram, as indicated
by the dotted lines. The rectangular parallelepiped A1OBC1-
E1O1G1F1 is a unit cell for the orthorhombic SnO2. This unit
cell of the orthorhombic SnO2 can be regarded as forming from
the parallel-piped A1OBC1-EDGF in the rutile structured SnO2
by shearing along the [101]t crystal direction, followed by a
slight compression along the c-axis of orthorhombic SnO2. The
shearing angle is 20.2°, and the compression in length is 1.6%.
The dimensions of the a-axis and b-axis of the orthorhombic
SnO2 are also adjusted very slightly (0.5% compression in the
a-axis and 0.3% expansion in the b-axis) relative to the
corresponding spacing distances in the rutile SnO2 structure.
determined to be [001]o || [317]t and ((110)o || (451)t. If we
consider only the orthorhombic SnO2, the growth direction of
the SnO2 nanotubes is parallel to [1h10]o, identical to that of the
SnO2 sandwiched nanoribbons.

Tin Oxide Nanowires, Nanoribbons, and Nanotubes
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1279
4. Summary
the observations made in the SnO-based nanobelt synthesis,3
where only a rutile SnO2 crystal structure is observed, if we
assume that the present experiments, especially those involving
Sn foil/SnO, result in an oxygen deficient product synthesis.
We suggest that the ability to produce the orthorhombic
superlattice structure, if it results from an oxygen deficient
environment, may have important implications for syntheses in
similar systems. These will be the subject of future study in
our laboratory.
In this study, we have demonstrated the synthesis of both
rutile and orthorhombic crystalline tin oxide (SnO2) nanostruc-
tures in the form of nanowires, nanoribbons, and nanotubes we
have used two starting products, primarily interlayered Sn foil/
SnO mixtures or simply SnO powders. The system used in this
study operates from a base pressure of 20 milliTorr as the
pressure ranges given in Table 1 correspond primarily to total
pressures (Ar, N2 ...) obtained with 100 sccm entrainment gas
flows under adjustable “baffled” forepumping of the system.
There are several important factors which distinguish the
present study. First and foremost is our ability to synthesize an
orthorhombic (a ) 0.4714 nm, b ) 0.5727 nm, c ) 0.5214 nm)
SnO2 “superlattice” structure at nominal pressures (primarily
N2). If we compare the results that we obtain for the SnO only
and interleaved Sn foil/SnO synthesis mixtures, then we find
that the latter combination greatly increases our ability to
produce SnO2 nanowires. The data in Figure 4, which displays
a strong SAD pattern associated with the rutile SnO2 structure
and weak reflections associated with the orthorhombic “super-
lattice” structure, in conjunction with our initial synthesis
materials suggests the possibility that the weak structure may,
in fact, be associated with an oxygen deficiency in the
synthesized SnO2. The orthorhombic and rutile structures of
SnO2 do display very similar unit cell parameters suggesting
that the orthorhombic structure might be readily converted.
It is relevant that the “fluffy” gray to gray-black material
which we synthesize can be converted through controlled heating
at ∼1100 °C under a 1% oxygen/UHP Argon flow to a
somewhat more dense white SnO2 powder (the same synthesis
product mixture under an Ar (4% H2) flow is reduced completely
to metallic tin). The present results are therefore consistent with
References and Notes
(1) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208.
(2) Yu, D. P.; Bai, Z. G.; Feng, S. Q.; Lee, C. S.; Bello, I.; Sun,
X. S.; Tang, Y. H.; Zhou, G. W.; Zhang, Z. Solid State Commun. 1998,
105, 403.
(3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L, Science 2001, 291, 1947.
(4) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Solid State Commun. 2001,
118, 351.
(5) Tatsuyama, C.; Ichimura, S. Jpn. J. Appl. Phys. 1976, 15, 843.
(6) Aoki, A.; Sasakura, H. Jpn. J. Appl. Phys. 1970, 9, 582.
(7) Watson, J. Sens. Actuators 1984, 5, 29.
(8) Yamazoe, N. Sens. Actuators B 1991, 5, 7.
(9) Sberveglieri, G. Sens. Actuators B 1992, 6, 239.
(10) Dieguez, A.; et al. Sens. Actuators B 1996, 31, 1.
(11) Ansari, S. G.; et al. Thin Solid Films 1997, 295, 271.
(12) Nayral, C.; et al. Appl. Surf. Sci. 2000, 164, 219.
(13) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Appl. Phys Lett. 2000,
submitted.
(14) Lee, S. T.; Wang, N.; Zhang, Y. F.; Tang, Y. H. MRS Bulletin
1999, 24, 36.
(15) Gole, J. L.; Stout, J. D.; Rauch, W. L.; Wang, Z. L. Appl. Phys.
Lett. 2000, 76, 2346.
(16) Wang, Z. L.; Gao, R. P.; Gole, J. L.; Stout, J. D. AdV. Mater. 2000,
12, 1938.
(17) Baou, W. H. Acta Crystallogr. 1956, 9, 515.
(18) Suito, K.; Kawai, N.; Masuda, Y. Mater. Res. Bull. 1975, 10,
677.