Growth of SnO2 nanowires with uniform branched structures

J.X. Wang, D.F. Liu, X.Q. Yan, H.J. Yuan, L.J. Ci, Z.P. Zhou, Y. Gao, L. Song,L.F. Liu, W.Y. Zhou, G. Wang, S.S. Xie*

Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-65#, Beijing 100080, China

Received 3 September 2003; received in revised form 5 December 2003; accepted 5 January 2004 by B. Jusserand

Abstract

SnO2 nanowires have been prepared using the active carbon reaction with the fine SnO2 powder at low temperature (700 8C).

These nanowires show rectangular cross-section, with their widths ranging from 10 to 50 nm. Branched nanowires with definite

included angle are also observed in these products. The morphology and microstructure of the single crystalline SnO2

nanowires and the branched nanowires are characterized by means of scanning electron microscopy (SEM), high-resolution

transmission electron microscopy (HRTEM), selective area electron diffraction (SAED) and Raman spectrum. In addition, the

possible growth mechanism of the SnO2 nanowires and branched nanowires is also discussed.

q 2004 Elsevier Ltd. All rights reserved.

PACS: 81.10; 81.05.Y

Keywords: A. Nanomaterials; A. Semiconducting materials; C. Crystal morphology

1. Introduction

Semiconductor metal oxide nanowires have attracted

significant attention of the researchers due to the funda-

mental importance and the wide range of their potential

applications in nanodevices [1–7]. As a wide band

semiconductor (Eg ¼ 3.6 eV at 300 K), SnO2 is a key

functional material that has been used extensively for gas

sensor, transparent conductor, and nanoelectronic device

[8–10]. Until now, considerable effort has been focused on

the synthesis and application of the SnO2 thin film or

nanoparticles [11–15]. Recently, SnO2 nanowires, nano-

belts and nanotubes are prepared by simple evaporation of

the source compound over 1000 8C [16–20]. SnO2 nano-

belts as gas sensor and field-effect transistor are also

reported [10,20]. In addition, the branched carbon nanotubes

(so call ‘Y’ shape or branched shape) materials have been

synthesized by different methods in many groups [21–23],

for example, Li et al. produced the branched carbon

nanotubes in template with branched nanochannels [23].

In this study, we prepared SnO2 nanowires using the active

carbon reaction with the fine SnO2 powder at low

temperature 700 8C. Moreover, branched SnO2 nanowires

and nanowires network structures, which have definite

included angle and high crystalline, were also observed. In

contrast with the synthesis methods previously reported

[16–19], both the straight nanowires and branched nano-

wires prepared in our method have the rectangular cross-

section. Furthermore, the growth mechanisms of these SnO2

nanostructures are also discussed.

2. Experiment

Our experiment was carried out in a high-temperature

tube furnace. Fig. 1 represents schematically the apparatus

we used. A horizontal quartz tube was mounted inside the

furnace. The mixture of active carbon powder 1.2 g and fine

SnO2 powder 5 g was placed on a quartz wafer and the

substrates for growth of nanowires were put on the place 3–

8 cm away from the quartz wafer. The substrates we used in

0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2004.01.003

Solid State Communications 130 (2004) 89–94

www.elsevier.com/locate/ssc

* Corresponding author. Tel.: þ86-10-82649081; fax: þ86-10-

82640215.

E-mail addresses: jxwang@aphy.iphy.ac.cn (J.X. Wang),

ssxie@aphy.iphy.ac.cn (S.S. Xie).

experiment were n-type silicon (2–3 V). A thin Au film was

deposited on the substrates using an ion sputter films

deposition system (Hitachi, E-1010). The thickness of Au

film was estimated to be approximately 5 nm. After the

wafer was inserted into the center of the quartz tube, the

quartz tube was first evacuated by a vacuum pump. Then

the furnace was rapidly heated up to 700 8C from the room

temperature under a N2 flow at a rate of about 100 sccm.

During the experiment, the pressure was kept at 200 Torr

and the heating last for 240 min. Subsequently, the furnace

was cooled to the room temperature naturally and the

products were collected on the substrates from different

regions I, II and III of the furnace (as shown in Fig. 1). All

these regions have the same temperature of 700 8C. It was

observed that thick white cotton-wool-like products are

deposited on the surface of the substrates. Then, these as-

prepared products were characterized by field emission

scanning electron microscopy (FESEM; Philips XL 30

FEG), transmission electron microscopy (HRTEM; JEOL

2010F at 200 kV), and energy-disperse X-ray spectrum.

Raman measurements were also carried out on a micro-

Raman spectrometer (JY T64000 France) at room tempera-

ture. The 514.5 nm emission from argon ion laser was used.

3. Result and discussion

Fig. 2(a) is a typical SEM image showing the general

view of the morphology of the as-prepared product in region

I (as shown in Fig. 1). It was found that large-scale wire-like

material is produced. High magnification SEM image (Fig.

2(b)) shows the wire-like materials have cross-rectangle

section. Unlike the SnO2 belts previously reported [4,20],

the width-to-thickness aspect ratio of these nanowires are

2:1–4:1, obviously smaller than that of nanobelts. Actually,

their width ranges from 20 to 60 nm and the length of these

nanowires varies from several tens to hundreds of

micrometers. All these nanowires are smooth and uniform

along the fiber axis.

A straight nanowire collected in region I with the

diameter almost 20 nm is displayed in TEM image (Fig.

3(a)). Inset shows a nanowire with a catalyst particle on its

tip. These catalyst particles are not easily found in the SEM

image, presumably because the nanowires are too long.

To provide the further insight of nanowires, HRTEM and

area diffraction (SAED) were used. Fig. 3(b) reveals that the

SnO2 nanowires are structurally uniform and single crystal-

line. The interplanar spacing is measured to be 0.34 nm,

which corresponds to the (110) plane of a rutile SnO2 lattice.

The inset SAED pattern of the nanowires in Fig. 3(b) was

recorded with the electron beam along [1̄13]. From SAED,

the growth direction of SnO2 nanowire is found to be [301],

different from previous reports [18].

Raman spectrum is exhibited (Fig. 4) to further

determine the characteristic of the nanowires. Rutile SnO2

belongs to the space group D4h and displays four Raman-

active modes in bulk SnO2. In Fig. 4, three fundamental

Raman scattering peaks at 472, 629 and 773 cm21can be

observed, respectively. The peak at 472 cm21 can be

assigned to the Eg, the peak at 629 cm21 can be indexed

to the A1g mode and the peak at the 773 cm21 can be

identified as the B2g mode. These peaks indicate the typical

feature of the rutile structure of the SnO2 nanowires. There

also exist two weak Raman peaks at 498 and 689 cm21.

These two peaks, which have not been detected in bulk

SnO2, were reported to be A2u mode and Eu(2) mode,

respectively [19]. The Raman activities of these two modes

Fig. 1. Schematic diagram of apparatus used in experiment.

Fig. 2. (a) SEM images of the SnO2 nanowires prepared using active carbon and SnO2 powder. (b) Enlarged SEM image which shows

rectangular cross-section.

J.X. Wang et al. / Solid State Communications 130 (2004) 89–9490

may be due to a size effect of the thin nanowires. In our case,

the peak at 540 cm21 has not been reported in the Raman

study of SnO2 nanowires. This peak is identified to be of S2

mode, which is believed to be the consequence of the

disorder activation of SnO2 nanowires.

Many models are proposed to describe the growth

mechanism of the one-dimensional materials, such as VLS

(vapor–liquid–solid) and VS (vapor–solid) mechanism

[24]. The inset image in Fig. 3(a) shows a particle on the tip

of the nanowire, which indicates the growth mechanism of

SnO2 nanowires in our experiment that can be considered to

be VLS. It is reported that the Sn droplets on the tips are

essential for the growth of the nanowires [16]. However, in

our experiment, EDX reveals that the top particles are

composed of Au, Sn and O, which indicates Au particles

also play an important role in the growth of SnO2 nanowires.

The following chemical reaction will take place during the

thermal evaporation process:

CðsÞ þ SnO2ðsÞ ¼¼ SnOðgÞ þ COðgÞ ð1Þ

COðgÞ þ SnO2ðsÞ ¼¼ SnOðgÞ þ CO2ðgÞ ð2Þ

2SnOðgÞ ¼¼ SnðlÞ þ SnO2ðsÞ ð3Þ

SnO2 powder is first reduced by active carbon powder at

700 8C, as the reaction (1) described. Then SnO2 powder

will continue to be reduced by the CO gas formed from

reaction (1). SnO vapor is the common resultant of these two

reactions. The SnO vapor can be transported to the

deposition zone by the carrying gas. As we all know SnO

is metastable, it will decompose into Sn and SnO2 above

600 8C, as shown in reaction (3). Considering the low

melting point of Tin (231.9 8C), Sn particles are still liquid

at reaction temperature. These Sn droplets fell on the

substrate and form Sn–Au alloyed droplets by reacting with

the Au particles. At the same time, these alloyed droplets

can provide the energetically favored sites for adsorption of

SnO vapor. Subsequently, the decomposition of SnO will

result in the precipitation of SnO2 and the SnO2 nanowires

are formed.

Except the single-crystalline nanowires, however, differ-

ent morphologies are found in the products collected from

different regions of the furnace. In region II (region II in Fig.

1), the products also display wire-like features similar to the

nanowires mentioned above, but among these nanowires,

many of them have the branched structures, which are

marked by arrow A–H in SEM image (Fig. 5(a)). Unlike the

Y-shape junction previously reported [21–23], these

branched nanowires are just formed by one stem nanowire

and one branch nanowire. Some of the nanowires were

connected by several junctions and resulted in a network of

the nanowires, (as the arrow A–H marked). Similar with the

Fig. 3. (a) A typical TEM image of SnO2 nanowires with diameter about 10 nm, inset is a nanowire with a catalyst particle on the tip. (b)

HRTEM image of the individual straight SnO2 nanowire, inset is the SAED pattern. The growth direction of the nanowire is [301].

Fig. 4. Raman spectrum of the SnO2 nanowires.

J.X. Wang et al. / Solid State Communications 130 (2004) 89–94 91

straight nanowires, both the stem and branch of the branched

nanowires have the rectangle cross-section, as shown in

enlarge SEM image (Fig. 5(b)). In addition, in the nearest

region from the reactant (region III in Fig. 1), the product

shows a sheet-like feature that is similar with the previously

reported [4], which we will described in another paper.

TEM images of branched nanostructures are shown in

Fig. 6. Fig. 6(a) is a typical TEM picture of the branched

nanowires. Both the stem and branch of the branched

nanowires are straight and uniform, and with the width of

about 20 nm. The induced angle between the arms is

measured to be 68 8C. In our TEM study, we have observed

more than 10 nanowires and found that almost all of the

branched nanowires have the constant included angles.

Moreover, some interesting structures are also found in the

as-prepared product. Fig. 6(b) shows a network of

nanowires in which the nanowires are parallel to each

other. Three angles among the three nanowires are also 688,

which are marked by the arrow a, b, and c in the image. A

structure with parallelogram-shape is displayed in Fig. 6(c).

Two parallelograms in different panels are formed by the

connection of the nanowires. The angle between two

nanowires (marked by E and F) is also found to have the

same value. These structures provide the strong evidence

that the branched SnO2 nanowires tend to form the junction

in a definite included angle.

Comparing with the straight nanowire, HRTEM image

of the branched nanowire in Fig. 7 shows that a thin

amorphous layer exists at the surface of the junction. On the

other hand, the clear lattice fringes indicate the single-

crystalline of the junction. The lattice fringe distances are

0.26 nm, which is coincidence with the d space (101) plane

of the rutile SnO2. The SAED pattern taken from the

junction of branched nanowires (inset in the Fig. 7) can be

indexed to be [010] zone axis of the rutile structured SnO2

crystal. Further analysis indicated that the growth direction

of the stem and branch are [1̄01] and [101] crystalline

orientations, respectively. This result can be confirmed by

the fact that the included angle is 688 between [1̄01] and

[101] crystalline orientation.

On the basis of the experimental data, a schematic

diagram of the crystal structures is used to describe the

structural relationship between the branch and stem

nanowire. As shown in Fig. 8, the rectangular parallelepiped

ABCO–DGFE represents unit cell for the rutile structured

SnO2 crystal and the spheres denote the Sn atoms (O atoms

are not drawn). The stem of branched nanowire is

represented by the hexahedron ABA1B1–LNPM and the

branch of branched nanowire is represented by the

hexahedron ABFE–JKIH; the growth orientations of these

hexahedrons are [1̄01] and [101], respectively. The plane

ABLN is indexed to be (101) plane while the OCGD is

indexed to be (1̄01) plane. These planes correspond to the

growing plane of the stem and branch nanowire, respect-

ively. In addition, because the nanowires shows rectangular

cross-section, the enclosed facet planes of the branched

nanowires can be determined in this diagram. The enclosed

facet planes of stem nanowire are ^ (010) and ^(101) while

those of branch nanowire are ^ (010) and ^ (ı̄01),

respectively.

Taking consideration of the growth mechanism of

branched nanowires, we suggest the growth of the stem of

branched nanowires may follow the same growth mechan-

ism as that of the straight nanowires with the rectangular

cross-section. When the stem SnO2 nanowires grows along

the (101) crystalline orientation, the outside faces of (101)

plane might consists of oxygen atoms with many dangling

bonds. As the grown nanowire is nearer to the reactants, the

dangling bonds of oxygen on the outside surface have more

chance to catch Sn atoms. Thus, some of the small Sn

droplets decomposed from SnO vapor can be attached on its

surface and formed the new growth sites. This point was

also supported by our SEM study. Some small droplets

attached in the surface of nanowires have been observed in

SEM observation. As we all know, because of the tetragonal

structure of the SnO2 crystal, (101) and (1̄01) plane are

kinetically identical in the growth of crystal, the same as

Fig. 5. (a) SEM image of branched nanowires, the junctions are

marked by arrow A–H. (b) High magnification SEM image of the

SnO2 nanowires also show the rectangle cross section.

J.X. Wang et al. / Solid State Communications 130 (2004) 89–9492

[101] and [1̄01] orientation. Moreover, because the (101)

plane and [101] orientation are, respectively, the close-

packed plane and close-packed orientation in rutile SnO2

crystal, these orientations are prior directions for growth of

crystal. Once the Sn droplets as the nucleation sites were

attached on the surface (101) plane of the stem nanowire, the

epitaxial growths of the branch nanowires will take place.

These branch nanowires grow along the definite directions

and then result in uniform branched nanowires. Here, the

growth process of these branch nanowires is similar to the

formation of the individual straight nanowire, but Sn

droplets replaced the Sn–Au alloy droplets as the catalyst

for the formation of the branch nanowire. If these growths

take place in several sites of the stem nanowire, the parallel

network is produced. Similarly, as the growths take place on

the top and end of the nanowires, the parallelograms

structure is formed.

In addition, the SnO2 fishbone nanostructures were

reported recently. In these structures the branches grow

perpendicularly from the main stem [25]; however, the

vertical structures were not observed in our specimen. The

k101l direction for growth of branched nanowires is

Fig. 6. TEM image of branched nanostructure (a) branched nanowire, (b) a parallel network of SnO2 nanowires. The angle marked by arrow a, b

and c are all 688 (c) a parallelogram structure formed by the connection of nanowires.

J.X. Wang et al. / Solid State Communications 130 (2004) 89–94 93

dominant under our experiment condition. This difference

may due to the using of Au catalyst particles and relatively

low growth temperature during the growth process.

Presumably different favored growth directions for SnO2

nanowire growth depend on different experiment con-

ditions. By controlling the experimental conditions in

order to favor different growth orientations, branched

nanowires with different included angles might be obtained.

These will be the subjects of further study in our laboratory.

4. Conclusion

In summary, we have demonstrated the synthesis of

single crystalline rutile SnO2 nanostructures in the forms of

nanowires and branched structures using the active carbon

and SnO2 powder at low temperature 700 8C. The

characteristics of these nanostructures were analyzed by

means of SEM, TEM, HRTEM, and Raman spectrum. The

branched nanowires, which were constructed by two

nanowires in the definite included angle, were observed in

the experiment. Both the individual straight nanowires and

the branched nanowires have the rectangle cross-section. In

addition, the growth mechanism of the SnO2 nanowires and

other nanostructures are also discussed in detail. Epitaxial

growth of branch nanowire from the kinetic equivalent

crystal planes and orientation may be the possible reasons

for the formation of the branched nanowires. These obtained

SnO2 nanostructures may have important application in

fabrication of the nano-devices.

Acknowledgements

The authors gratefully thank X.A. Yang and J.L. Jing for

the assistance in SEM and HRTEM work. This work is

supported in part by the national Natural Science Foun-

dation of China.

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Fig. 7. HRTEM image of the branched SnO2 nanowire, inset is the

SAED pattern taken along [010] axis. The growth directions of stem

and branch nanowire are [1̄01] and [101], respectively.

Fig. 8. A schematic diagram of the branched nanowire crystal

structure. Circles denote Sn atoms while O atoms are not drawn.

J.X. Wang et al. / Solid State Communications 130 (2004) 89–9494