Synthesis of Tin Oxide Branching Nanostructures by Carbothermal Reduction Process
Rosaura Salinas
University of Missouri-Saint Louis
Jingyue (Jimmy) Liu, PhD
Abstract
Tin oxide nanostructures were grown by carbothermal reduction of different
materials including tin (IV) oxide (SnO2), tin (II) oxide (SnO) and a combination of
tin (IV) oxide with zinc oxide (ZnO) powders. The source material underwent a
physical vapor deposition process in a horizontal tube furnace. The resulting
condensation occurred in the lower temperature region, either on the tube or on
substrates when these were used. When using mica as the substrate, the use of SnO2
by itself and in combination with ZnO as the source material resulted in the growth of
star-like clusters of tin oxide nanowires. These main nanowires, or stems, had
multiple branching nanowires growing from them. A single sphere was found on the
tips of each nanowire, stems and branches; and some nanowires presented the spheres
on their surface suggesting these particles act as the catalyst for the growth. This
structure was most commonly found in the region 18 to 25 cm downstream from the
furnace. The morphology of the as-synthesized nanostructures was examined using a
high resolution scanning electron microscope.
Introduction
The increased interest in the synthesis of one dimensional nanostructures such as nanowires,
belts, tubes, and particles arises from their potential applications in devices like gas sensing,
energy harvesting, catalysis, and electronics (1). SnO2 is an n-type semiconductor with a wide
band gap of 3.6 eV. It is of interest because it is a transparent conductor which exhibits
unique electrical and optical properties that are of use in technologies such as solar cells and
catalysis (2), (3). Several studies have been reported on the synthesis of SnO2 nanowires by
different methods including carbothermal reduction; however, these methods require
temperatures in excess of 1100° C, the use of precursors or vacuum pumps (4). In this paper,
we investigate the synthesis of tin oxide branching nanostructures by varying the source
material and substrates used in a physical-vapor deposition process. The effects of using tin
(II) oxide and tin (IV) oxide powders in combination with zinc oxide powders as the source
material were investigated.
Experiment
Tin oxide branching nanowires were grown by using different source materials including a
mixture of carbon with SnO, SnO2 or a combination of SnO2 with ZnO powders as source
material. An alumina boat containing the source material was placed inside the 1 meter long
quartz tube and centered with respect to the horizontal tube furnace used. In some of the
experiments, alumina and mica substrates were used for collecting the deposited materials.
These substrates were placed on a separate boat at various distances from the source material.
The source material was generally heated to about 1080° C for 1 hour or longer. Argon was
used as a carrier gas to assist the movement of the evaporated molecules to the lower
temperature regions where the substrates were positioned. A small amount of oxygen gas was
usually used to facilitate the oxidation of the evaporated Sn molecules. A schematic of the
apparatus setup is shown in figure 1. When the furnace was turned off, the tube was allowed
to cool down to room temperature. Materials which deposited on the quartz tube, the mica or
the alumina substrates were examined in a SEM to obtain information on their nanostructure
and general morphology.
Fig. 1 Schematic of tube furnace used for the synthesis of tin oxide nanostructures
Results
The evaporated materials were generally accumulated on the surfaces of the quartz tube as
well as the mica and alumina substrates. The deposited materials were usually a thin layer of
coating with a visual color ranging from light to a deep dark gray and back again to a lighter
tone as the distance from the source material increased. When the source material was SnO2
with or without ZnO, star-like clusters of nanowires were successfully synthesized. These
main, or stem, nanowires showed smaller nanowires, or branches, growing from them. Each
nanowire, stem or branch, has a spherical particle located on its tip. These spheres were
sometimes also observed on the surfaces of some of the nanowires. SEM images of these
stems, branches and spheres are shown in figure 2.
(a)
(b)
(c)
Fig. 2 SEM images of (a) SnOx star-like
nanowire cluster, (b) branch nanowires with
spheres on tips and on surface and (c) stem
nanowire with branches
These hierarchical structures are of interest because their growth seems to be guided by the
presence of the spherical particles which are presumably rich in Sn. The possibility of being
able to grow branched nanostructures will have potential applications in developing highsensitivity sensors, high efficiency solar panels and many other practical applications.
Research Efforts in Progress
The goal of our research was to determine the effect of the mixture of source materials, the
parameters that control the reaction and the use of different substrates on the final
morphology of the synthesized tin oxide nanostructures.
Having optimized these
experimental conditions, our next step is to characterize the nanostructures and investigate
their growth mechanisms.
From the observations made so far, our working hypothesis is that the Sn spheres act as the
catalyst for the growth of the SnO2 nanowires, and we propose the growth process as shown
in the schematic of figure 3.
Fig. 3 Proposed growth mechanism of the stem and branch nanowires
Acknowledgements
The author wishes to acknowledge Hongyang Liu, PhD for his assistance in obtaining the
SEM images, the Center for Nanoscience and the Department of Physics and Astronomy at
the University of Missouri-Saint Louis and the NASA-Missouri Space Grant Consortium for
their funding of this research.
Biography
Rosaura Salinas was born in McAllen, TX and has lived in the Saint Louis area since 2006.
She is currently a senior at the University of Missouri-Saint Louis pursuing a degree in
Engineering Physics. Her interests include material science and photovoltaics.
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