facile route to straight sno2 nanowires and their optical properties

These nanowires with unusual [12-1] growth direction are very straight and uniform in diameter and length.. The photoluminescence PL spectrum exhibits a wide yellow emission centered at

Journal of Alloys and Compounds xxx (2008) xxx–xxx

Contents lists available atScienceDirect Journal of Alloys and Compounds

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / j a l l c o m

Facile route to straight SnO 2 nanowires and their optical properties

P.G Lia,∗, M Leia, W.H Tanga, X Guoa, X Wangb

aDepartment of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China

bDepartment of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e i n f o

Article history:

Received 19 September 2008

Received in revised form 8 October 2008

Accepted 15 October 2008

Available online xxx

Keywords:

Nanostructured materials

Gas–solid reactions

Transmission electron microscopy

Optical property

a b s t r a c t

Rutile SnO2nanowires were fabricated by a simple chemical vapor method using as-synthesized SnO2

nanoparticles as starting material These nanowires with unusual [12-1] growth direction are very straight and uniform in diameter and length Self-catalytic vapor–liquid–solid (VLS) mechanism should

be responsible for the growth of the nanowires The photoluminescence (PL) spectrum exhibits a wide yellow emission centered at 576 nm with a relatively small orange emission at 629 nm The Raman spectrum exhibits four additional modes that are not allowed by rutile-type structure in first-order Raman-scattering at the zone center The possible reasons for the unusual PL and Raman spectrum are proposed

© 2008 Elsevier B.V All rights reserved

1 Introduction

In the past decade, one-dimensional nanostructures, such as

nanotubes, nanowires and nanobelts, have attracted

consider-able attentions because of their peculiar structure characteristics

and excellent physical properties [1–3] As an important

n-type semiconductor with wide bandgap (Eg= 3.6 eV at 300 K),

SnO2 have been widely used for transparent conductors, gas

sensor solar cells, lithium-ion batteries, and electronic devices

[4–10] Recently, one-dimensional nanowire structure has attracted

increasing attentions, owing to its enhanced surface to volume

ratio and promising applications for gas sensors [11,12] and

electronic nanodevices[13]Up to now, various methods

includ-ing direct-oxidized growth[14–16], molten-salt synthesis[17,18],

hydrothermal method [19,20], laser-ablation synthesis [21],

car-bothermal reduction[22,23], and template method[24], etc have

been developed to fabricate SnO2 nanowires However, the

as-synthesized nanowires easily bend and length and diameter is

not uniform, which limits their promising applications So,

fabri-cation of straight SnO2nanowires with uniform size and smooth

surface is still a challenge up to now In this work, we

devel-oped a novel chemical vapor method to synthesize large-scale

SnO2 nanowires with uniform size using SnO2 nanoparticles as

starting materials The structure property and growth

mecha-nism were investigated in detail In addition, some interesting

∗ Corresponding author Tel.: +86 571 86843468; fax: +86 571 86843222.

E-mail address:peigangiphy@yahoo.com.cn (P.G Li).

optical features of the SnO2 nanowires were presented in the paper

2 Experimental

The starting material is SnO 2 nanoparticles synthesized by a hydrothermal method reported by literature [25] In the experiment, an alumina boat contain-ing 5 g SnO 2 nanoparticles was loaded into the center of a horizontal alumina tube and 10 mm× 10 mm-sized 6H-SiC substrate for growth of SnO 2 nanowires was placed on the downstream end of the alumina tube Direct thermal evaporation of SnO 2 nanoparticles was performed at 1550 ◦ C for 90 min with an Ar flow rate of

300 SCCM Finally, the furnace was cooled to room-temperature and white products were deposited on 6H-SiC substrate.

Powder X-ray diffraction (XRD) of the product was characterized by PaNalytical X’Pert Pro MPD X-ray diffractometer with Cu K␣ radiation The morphology of the as-synthesized product was examined by field-emission scanning electron micro-scope (FEI XL30 S-FEG) The transition electronic microscopy (TEM) images and high-resolution TEM (HRTEM) of samples were collected on the JEOL 2010F trans-mission electron microscope The X-ray photoelectron spectra (XPS) are recorded

on a VGESCALAB MKII X-ray photoelectron spectrometer, using nonmonochroma-tized Mg K␣ X-ray as the excitation source Raman measurement was performed on

a multichannel modular triple Raman system (JY-T64000) using a 532 nm laser as excitation source Photoluminescence (PL) spectrum of the nanowires was collected

at RT using the 325 nm line of a He–Cd laser as the excitation source.

3 Results and discussion

Fig 1a shows the SEM image of the SnO2 nanoparticles syn-thesized by a hydrothermal method The average size of these nanoparticles is about 5 nm, indicating the SnO2 nanoparticles can be decomposed at relatively low temperature comparing with micropowders SEM images of the product deposited on 6H-SiC substrate are shownFig 1b and c Fig 1b shows that straight 0925-8388/$ – see front matter © 2008 Elsevier B.V All rights reserved.

doi: 10.1016/j.jallcom.2008.10.130

2 P.G Li et al / Journal of Alloys and Compounds xxx (2008) xxx–xxx

Fig 1 (a) SEM image of the SnO2 nanoparticles synthesized by hydrothermal method (b and c) SEM images of the SnO 2 nanowires (d) EDS analysis of the SnO 2 nanowires. nanowires with high density are distributed over the entire

sur-face of the substrate SEM image with higher magnification (Fig 1c)

clearly indicates that these nanowires are of uniform size and

smooth surface, and the average diameter and length of these

nanowires are 80 nm and 5␮m, respectively The energy

disper-sive X-ray spectroscopy (EDS) spectrum (Fig 1d) indicates that the

sample only consists of Sn and O element The average O/Sn ratio

is 1.92:1, close to the chemical composition of SnO2.Fig 2shows

a typical XRD pattern of the nanowires deposited on 6H-SiC

sub-strate All the diffraction peaks can be well indexed as rutile SnO2

(ICDD-PDF No 41-1445) The diffraction peaks are sharp and no

other impure peaks are detected, confirming the good crystallinity

of the nanowires The composition of the Product can be further

determined by XPS spectra (Fig 3) The binding energy centered

at 530.75, 486.88, 495.38 eV for O1s, Sn3d5/2and Sn3d3/2,

respec-tively, are in good agreement with the value of the bulk SnO2

Quantification of the Sn3d and O1s peaks gives an average Sn/O

atomic ratio of 1:1.95, indicating the O-deficient formation of the

SnO2nanowires

Fig 2 XRD pattern of the SnO2 nanowires.

Fig 3 XPS spectra of the obtained SnO2 nanowires: (a) O1s region, (b) Sn3d region.

P.G Li et al / Journal of Alloys and Compounds xxx (2008) xxx–xxx 3

Fig 4 (a) TEM image of the SnO2 nanowires (b) TEM image of a single nanowire The corresponding FFT pattern is shown in the inset (c) TEM-based EDS spectrum of the single nanowire (d) HRTEM image of the nanowire.

TEM image (Fig 4a) clearly indicates that nanowires are of

smooth surface and rather uniform size along the growth direction

Fig 4b shows a typical nanowire with a clear surface

TEM-based EDS analysis of the nanowire (Fig 4c) confirms that the

nanowire mainly consists of Sn and O element, and average Sn/O

atomic ratio of 1:1.94, which exhibits the O-deficient condition of

the nanowire The corresponding FFT pattern and HRTEM image

clearly show that the nanowire is single crystalline and grows

along [12-1] direction, which is different from common [1 0 1]

and [1 1 0] growth direction[26,27] No obvious defects and

dis-locations are observed, and the interplanar space is 0.356 nm

and 0.236 nm, which corresponds to the (101) and (200) plane

of the rutile crystalline SnO2 (Fig 4d), further confirming rutile

structure of the nanowire Based on the experimental results,

con-ventional vapor–liquid–solid (VLS) cannot dominate the growth

of the nanowires due to no metal catalyst such as tin particle

attached on the tip of nanowire Vapor–solid (VS) mechanism also

cannot explain the growth process because SnO2powder

unavoid-ably decomposes at high temperature We deduce that the growth

of the nanowires follows a self-catalytic VLS process First, SnO2

nanoparticles decomposed into Sn and SnO vapor Sn and SnO vapor

subsequently are transported to low temperature zone and form

liquid droplets The liquid droplets gradually absorb oxygen and

are further oxidized into SnO2droplets The enhanced absorption

and diffusion of tin oxides occurred at SnO2liquid tip will finally

form SnO2nanowires

A typical room-temperature photoluminescence (PL) spectrum

is shown inFig 5 The spectrum is dominated by a strong yellow

emission centered at 576 nm with a small orange emission

shoul-der at 629 nm Near band edge (NBE) emission (centered at around

320 nm) is not detected, which is ascribed to strong surface effects

due to the larger surface-aspect ratio and the more surface defects

[28] In this work, the deep-level (DL) emission such as yellow and orange emission may be induced by bulk defects such as oxygen

vacancy (Vo), and tin interstitial (Sni), etc It is interesting to observe that the orange emission disappears after annealing in air at 850◦C for 3 h, whereas no change happens as annealing at Ar and N2 atmo-sphere These results indicate that the yellow emission is related to

the Vo, whereas orange emission originates from Sni[29,30] Due to

the synthesis process is in the O-deficient condition, Vois unavoid-able exist As a native defect of the n-type SnO2, The Vo cannot

be eliminated during the above annealing process Nevertheless,

Snican be oxidized and thus removed by annealing in air or oxy-gen atmosphere So, orange emission is not commonly detected in

Fig 5 Room-temperature photoluminescence spectrum of the SnO2 nanowires.

4 P.G Li et al / Journal of Alloys and Compounds xxx (2008) xxx–xxx

Fig 6 Room-temperature Raman scattering spectrum of the SnO2 nanowires.

the O-rich synthesis condition[28,31,32] However, details on these

bulk defects such as their distributing forms need to be further

investigated

Fig 6 shows the Raman spectra of the as-synthesized SnO2

nanowires Rutile SnO2belongs to the point groupD14

4hand space

group P4n/mnm According to the group theory, the active Raman

modes B1g, Eg, A1g, and B2gcan be observed in first-order spectrum

A1gand B2gmodes vibrate in the plane perpendicular to the c-axis

while the Egmode vibrates in the direction of the c-axis[33] As

shown inFig 5, three active Raman scattering peaks at 482.3, 638.3

and 779.1 cm−1can be assigned to Eg, A1gand B2gmode,

respec-tively, in good agreement with those of rutile SnO2single crystal

[34] Nevertheless, the four additional modes are also observed,

all of which are not allowed by rutile-type structure in first-order

Raman-scattering at the zone center Among them, the mode peak

at 699.7 cm−1 reported in previous article[27]is caused by the

finite-size effects of SnO2 The other additional peaks at 548.7, 591.1

and 733.8 cm−1have not been reported up to now We deduce that

these additional modes may be in line with the defect-induced

phonon modes due to the surface disorder and large amount of O

vacancies and Sn interstitial However, the exact mechanism needs

to be further investigated

4 Conclusions

Large-scale rutile SnO2nanowires were fabricated by a simple

chemical vapor method using SnO2nanoparticles as starting

mate-rials These straight nanowires are uniform in diameter and length,

and grow along the unusual [12-1] direction The PL spectrum

exhibits a wide yellow emission centered at 576 nm with a small

orange emission shoulder at 629 nm The yellow emission is related

to the oxygen vacancies (Vo), whereas orange emission originates

from tin interstitial (Sni) The Raman spectrum presents some new

features of the nanowires The additional mode at 699.7 cm−1 is

attributed to finite-size effects of SnO2 The peaks at 548.7, 591.1, and 733.8 cm−1, respectively, were related to the defect-induced phonon modes due to the surface disorder and large amount of O vacancies and Sn interstitial

Acknowledgements

This work was supported by the key project of the National Nat-ural Science Foundation of China (60571029, 50672088) and the Zhejiang Provincial Natural Science Foundation (Z605131)

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