Super-hydrophobic tin oxide nanoflowers

Article abstract of DOI:10.1039/b407313d

Super-hydrophobic 3D SnO₂ flowers with nanoporous petals were produced from the 3D Sn nanoflowers using a controlled shape-preserving thermal oxidation process.

Full text of DOI:10.1039/b407313d

Super-hydrophobic tin oxide nanoflowers
Aicheng Chen,* Xinsheng Peng, Kallum Koczkur and Brad Miller
Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1.
E-mail: aicheng.chen@lakeheadu.ca
Received (in Cambridge, UK) 18th May 2004, Accepted 8th June 2004
First published as an Advance Article on the web 15th July 2004
Super-hydrophobic 3D SnO
were produced from the 3D Sn nanoflowers using a controlled
shape-preserving thermal oxidation process.
2
flowers with nanoporous petals
SnO
3D Sn nanostructures seen in Fig. 1a and 1b. However, the high-
magnification SEM images (Fig. 1c and 1f) reveal that the SnO
2
nanoflowers observed in Fig. 1d and 1e are very similar to the
2
nanopetal porosity is greatly increased when compared with the as-
synthesized nanopetals of the Sn nanoflowers.
X-ray diffraction (XRD) analysis was used to further investigate
the change of the composition and structure of the 3D nanoflowers
upon RGTO oxidation. Typical XRD patterns of the Sn nano-
Size, shape and dimensionality strongly affect the properties of
nanomaterials. A three-dimensional (3D) integrated platform of
nanostructured metallic or semiconducting materials is highly
desirable for advanced nanoscale electronic and optoelectronic
1
applications. Being a smart semiconductor, tin dioxide (SnO
2
) has
2
flowers and the 3D SnO nanostructures are shown in Fig. 2. All the
attracted much attention due to its broad application in many fields,
for instance surface coatings, selective gas sensors and lithium
peaks marked by a star are derived from the titanium substrate. Fig.
2a reveals that the as-prepared Sn nanoflowers are in the b-tin
2
–4
secondary batteries.
dimensional (1D) SnO
nanotubes, nanorods and nanowires have been fabricated by
Apart from nanoparticles, different one-
2
nanostructures such as nanoribbons,
5
,6
physical/chemical vapor deposition processes, the solution-based
method7a and an anodic oxidation technique. However, there is
7b
2
no report on 3D porous SnO nanostructures. In this communica-
tion, we report for the first time the synthesis of 3D SnO
2
nanoflowers with nanoporous petals produced from the 3D Sn
nanoflowers using a controlled structure-preserving thermal oxida-
tion process.
The 3D Sn nanoflowers were formed on a titanium substrate by
thermal-pyrolysis of a tin organometallic precursor dibutyltin
3
dilaurate (DBTDL). Briefly, titanium plates (30 3 15 3 0.8 mm )
were pretreated as follows: degreased using acetone, etched in a 30
wt% HCl solution at 80 °C for 10 min to remove the titanium oxide
layer, then dried in a vacuum oven at 40 °C. A thermal pyrolysis
procedure was performed in a horizontal tube furnace equipped
with a ceramic tube. The pretreated Ti substrate and a ceramic boat
containing the tin organometallic precursor were put into the
ceramic tube. Before heating, a constant stream of ultrapure argon
(99.999%) at a flow rate of 200 sccm was introduced into the
system for 4 h. Then the furnace was heated up to 350 °C at a rate
of 50 °C min2 . The temperature was kept at 350 °C for 1.5 h, and
then naturally cooled to room temperature. During the heating and
cooling processes, the flow rate of the ultrapure argon was
maintained at 100 sccm. The thermal oxidation process approx-
imating rheotaxial growth and thermal oxidation (RGTO)8 was
1
,9
used to convert the metallic 3D Sn nanoflowers to 3D SnO
2
Fig. 1 SEM images (a) low magnification, (b) individual flower-like
structure, (c) one piece of a porous nanopetal of the as-prepared Sn
nanoflowers; (d) low magnification, (e) typical flower-like morphology, (f)
high-magnification of the petals of SnO formed by the RGTO process. The
2
insets are the shapes of water droplets on the corresponding surfaces.
nanoflowers. Before carrying out complete oxidation of the Sn
nanoflowers at 500 °C for 6 h in air, a thin supporting tin oxide skin
was grown on the surface of the tin nanoflowers in air for 2 h at 200
°
C. This deliberate and crucial step was taken to maintain the shape
and integrity of the as-synthesized tin nanostructures during the
conversion of Sn into SnO as the melting point of tin is low (232
C).
Fig. 1 shows scanning electron microscopy (SEM) images of the
2
°
as-synthesized Sn nanostructures. A large number of Sn nano-
flowers were formed as shown in Fig. 1a. More detailed
morphologies of the flower-like nanostructures with porous
nanopetals are shown in Fig. 1b and 1c. Fig. 1b shows that several
dozens of porous nanopetals connect with each other forming 3D
nanoflowers by self-assembly. These Sn nanopetals are about 50
nm thick, tens of mm wide and Fig. 1c reveals that they are porous
with random holes having diameters between tens and several
hundreds of nanometers. After the RGTO oxidation, the as-
synthesized 3D Sn nanoflowers were completely converted into
Fig. 2 XRD patterns of (a) the as-synthesized tin nanostructures, (b) after
SnO
2
3D nanoflowers without any obvious shape change. The 3D
complete RGTO oxidation.
1
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phase. The weak peaks (110), (200) and (101) of SnO
the Sn flowers are partially oxidized in air. This is consistent with
the observation that tin oxide is formed on the surface of Sn
nanowires due to oxidation in air at room temperature. As seen in
Fig. 2b, all peaks arising from tin disappear after RGTO oxidation,
2
indicate that
deposits on the Ti substrate, nucleates and self-assembles into
networks forming the 3D nanoflowers. In the initial stages, the
interplay between the dynamic wetting behavior and thermally-
enhanced surface diffusion of tin adatoms on the substrate may
facilitate the formation of the network patterns, rather than forming
isolated droplets or a continuous film, at the elevated temperature.
Although further investigation is necessary to elucidate the
mechanism of the growth of the 3D tin nanoflowers, we believe that
the formation of the tin nanoflowers is initiated along the grain
boundaries as these are the most thermodynamically active sites for
the precipitation of tin atoms, similar to the grain boundary
formation of ZnO nanowalls.1
9
which indicates that the tin is completely oxidized into SnO
2
. The
XRD patterns further show that the formed SnO has the cassiterite
2
structure. Please note that tin suboxide is not observed in the
formed tin oxide nanoflowers due to the 6 h long oxidation at 500
°
C in air.
Fig. 3 shows transmission electron microscopy (TEM) images
and corresponding high resolution TEM images of the nanopetals
of the nanoflowers. The as-prepared nanopetals of the Sn flowers
We further investigated the wetting properties of the Sn and
(
(
Fig. 3a) and the post-annealed nanopetals of the SnO
Fig. 3d) are porous. The shapes and the sizes of the pores are
2
nanoflowers
SnO
nanoflowers (Fig. 1a) and 155° for the SnO
contrast, we found the water contact angle is around 60° for a
smooth Sn surface and about 90° for a flat SnO surface. These
2
nanoflowers. The water contact angle is 90° for the Sn
2
flowers (Fig. 1b). In
irregular. Comparison of Fig. 3a and 3d reveals that the average
pore size of the Sn nanopetals is smaller than that of the SnO
2
2
nanopetals, in good agreement with the SEM observations. In
addition, Fig. 3a shows core–shell structures caused by a thin oxide
layer covering the tin nanopetal. Their structure and composition
are further characterized using HRTEM and energy dispersive X-
ray spectroscopy analysis (EDS). Fig. 3b indicates that the
interlayer spacing is about 0.28 nm, close to the d value (0.2793
nm) of the (101) plane of b-tin. Fig. 3c shows that these nanopetals
are composed of 97.76 at% Sn and 2.24 at% oxygen. Fig. 3e and 3f
results show that the water contact angle of the nanoflower surfaces
is much larger than that of the corresponding smooth surfaces,
suggesting that the significant increase of hydrophobicity results
from the dramatic change of the surface structure and roughness.
This observation is consistent with Cassie and Baxter’s model,
which describes the contact angle at a heterogeneous surface
composed of different materials, in our case, the trapped air in the
1
0
hollows and Sn or SnO
SnO nanoflowers originates from the contribution of the air
trapped in the interspaces of rough surfaces similar to that of well-
aligned nanorod TiO
arrays.11
In summary, 3D Sn nanoflowers and 3D SnO
2
nanoflowers. The super-hydrophobicity of
indicate that SnO
attributed to the (110) plane of cassiterite SnO
composition is 33.48 at% Sn and 66.52 at% O, close to the ratio of
: 2 in the bulk SnO showing that Sn is completely oxidized into
SnO
2
is formed with an interlayer space of 0.35 nm,
2
2
and that the element
2
1
2
2
nanoflowers were
2
.
successfully synthesized in this study; Ti is favorable for the
formation of Sn nanoflowers compared with other substrates such
as ceramic Al O and Sn. The dimensions of the nanopetals depend
2 3
on the applied temperature and the flow rate of argon. The
preservation of the shape of individual nanopetals as well as the
morphology of an entire 3D nanoflower upon oxidation suggests a
promising route for fabricating other 3D oxide nanonatures from
The above results suggest that the growth of Sn flower
nanopetals is probably controlled by a vapor–solid (VS) process.6
The tin vapor is formed from the pyrolysis of DBTDL first and then
carried by the Ar stream to the low temperature zone, where it
their 3D metal nanomaterials. The super-hydrophobic 3D SnO
2
nanoflowers with porous nanopetals possess a very high surface-to-
volume ratio, which is especially desirable for sensor design in
moisture environments.
This work was supported by the Natural Sciences and Engineer-
ing Research Council of Canada (NSERC). A. Chen acknowledges
the Ontario Premier’s Research Excellence Award.
Notes and references
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Fig. 3 (a) Low magnification TEM image, (b) and (c) are the corresponding
HRTEM and EDS of the as-prepared tin nanostructures, respectively. The
peak of copper was derived from Cu grids. (d) A typical TEM image, (e) and
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(f) are the corresponding HRTEM and EDS of the completely oxidized
products.
C h e m . C o m m u n . , 2 0 0 4 , 1 9 6 4 – 1 9 6 5
1965