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Materials Chemistry and Physics 126 (2011) 128–132

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ree-like SnO2 nanowires and optical properties

ao Taoa,∗, Qi-yuan Chena, Hui-ping Hua, Ying Chenb

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, ChinaARC Centre of Excellence for Functional Nanomaterials, Institute for Technology Research and Innovation, Deakin University, Waurn Ponds, Victoria 3217, Australia

r t i c l e i n f o

rticle history:eceived 11 June 2010eceived in revised form

a b s t r a c t

Tree-like SnO2 nanowires have been grown by a vapor–solid process using a milled SnO2 powder asthe evaporation source. Phase, structural evolution and chemical composition were investigated usingX-ray diffraction (XRD), X-ray spectrometry (EDS), and scanning electron microscopy (SEM). The process

2 November 2010ccepted 29 November 2010

eywords:in dioxideanowires

yields a large proportion of ultra-long rutile nanowires of 50–150 nm diameter and lengths up to severaltens of micrometers. High-resolution transmission electron microscopy (HRTEM) shows that the SnO2

nanowires are single crystals in the (1 0 1) growth direction with scattered smaller crystals or nanowiresas the tree branches. The SnO2 nanostructures were also examined using Fourier transform infra-red (FT-IR) and photoluminescence (PL) spectroscopy. A strong emission band centered at 548 nm dominated the

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. Introduction

Tin dioxide (SnO2) materials with a wide band gap of 3.6 eV havettracted considerable attention due to their potential applications,ncluding in lithium-ion rechargeable batteries [1], solar cells [2],hotoconductors [3], and gas sensors [4]. The development of opto-lectronic devices using SnO2 nanomaterials has also been pursuedxtensively in recent years. However, bulk tin oxide has a largexciton binding energy of 130 meV which limits its application inptoelectronic devices because of its forbidden dipole transition5]. Much effort has been spent trying to change the structure ofin dioxide materials in order to overcome this problem to achievefficient luminescence. Fortunately, one dimensional (1D) SnO2anostructures with a much larger surface to volume ratio, such asanoribbons, nanowires, nanotubes, nanorods [6–10], have been

ound to be more suitable for optoelectronic applications becausef quantum confinement in the reduced dimensionality whichan destroy the wave function symmetry and avoid the forbid-en dipole, allowing direct gap luminescence in 1D nanostructures11–13]. Furthermore, the size and morphology of nanostructuresery strongly affect their optical properties [14–19]. For example,

he photoluminescence (PL) spectrum of beak-like SnO2 nanorodsxhibited strong visible light emission with a peak at 602 nm [17]nd a PL emission peak at 593 nm has been observed from verticallyligned SnO2 nanowires [18]. The PL spectrum of SnO2 fishbone-

∗ Corresponding author at: Institute of Metallurgical and Applied Physical Chem-stry, School of Chemistry and Chemical Engineering, Central South University,hangsha 410083, Hunan, China. Tel.: +86 731 88877364; fax: +86 731 88879602.

E-mail address: yunqing5252@sina.com (T. Tao).

254-0584/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rioi:10.1016/j.matchemphys.2010.11.052

nowires.Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

like nanoribbons showed a strong green emission about 500 nm[19]. The promise that 1D nanostructures of a particular size ornovel morphology may dramatically improve the optical propertiesfor optoelectronic applications has stimulated great enthusiasm.

A variety of methods have been proposed to obtain 1D SnO2nanostructures, including sol–gel [20], thermal chemical vapordeposition (CVD) [21], thermal evaporation [6] and pulsed laserablation [22]. Generally, 1D SnO2 nanostructures can be easily syn-thesized by thermal evaporation. However, in its current form, thismethod has several notable drawbacks such as low production, thatoften a high temperature is required for evaporation, and somedifficulties in the elimination of metal catalysts that could act asimpurities or defects that affect optical properties.

Recently, a self-catalysis growth method, which uses ball-milledSnO2 powder or a ball-milled mixture of SnO and Sn powderas an evaporation source, has been successfully used to produce1D SnO2 nanostructures with high purity by conventional ther-mal evaporation at lower temperature [23,24]. Compared withunmilled powders, the improvement in the evaporation behav-ior for the milled powders can be attributed to a high density oflattice defects and structural changes induced by the ball milling[25–27]. For example, the evaporation of milled and unmilled ZnOsamples have been investigated under the same conditions, andthe milled sample lost 17 wt%, whereas the unmilled sample lostonly 5 wt% [25]. However, the formation mechanism of the 1D SnO2nanowires produced by evaporation of the milled tin oxides has

never been discussed in detail. Improved nanowire performancerequires the development of 1D SnO2 nanostructures of novelsize and shape. This paper reports the synthesis of high qualitytree-like SnO2 nanowires using milled SnO2 powder as the evapo-ration source. The tree-like SnO2 nanowires form via a controlled

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apor–solid process during annealing. The effect of shape of thenO2 nanostructures on the infrared vibration and room tempera-ure photoluminescence (PL) characteristics has been investigated.he PL spectrum of the treelike SnO2 nanowires exhibits a strongmission peak at 548 nm.

. Experiments

.1. Ball milling

Three grams of commercial SnO2 powder (Sigma–Aldrich,9.9% purity, sublimation point: 1800–1900 ◦C and melting point:630 ◦C) was milled in a steel magneto-ball mill for 100 h at roomemperature under argon atmosphere of 100 kPa to prepare a pre-ursor for evaporation. The magnet was located near the bottom ofhe mill at 45◦ to the vertical to create strong milling impacts usingrotation speed of 160 rpm [28].

.2. Annealing process

The SnO2 nanostructures were prepared by thermal evapora-ion of the ball milled SnO2 powder as follows: 0.5 g of the millednO2 powder was loaded into an alumina combustion boat, whichas covered by another alumina boat to inhibit the escape of vaporhases. The spaces between the two alumina boats allow air to dif-use inside during cooling. The system was placed into the centerf a conventional horizontal tube furnace. The growth of productsas investigated as a function of the annealing temperature and

ime. In the first case, the samples were heated from room temper-ture to either 900, 1000, or 1100 ◦C and isothermally kept for 1 h.n the second case, the milled samples were isothermally heated at100 ◦C for different times (between 10 min and 2 h). All the heat-

ng processes were performed in argon flow (50 ml/min). Followinghe heating, both ends of the furnace tube were opened and theamples were allowed to cool naturally to room temperature withccess of air.

.3. Material characterization

The phase of the SnO2 nanostructures was characterized using-ray diffraction (XRD) (Philips 3020, Co target, 40 kV, 30 mA,

ig. 1. Typical images of the tree-like nanowires: (a) low magnification SEM; (b and c) highattern; (f) HRTEM of the nanowires shown in (d).

d Physics 126 (2011) 128–132 129

� = 0.1789 nm). Scanning electron microscopy (FESEM, Zeiss Ultra-Plus) and transmission electron microscopy (TEM, Philips CM300)were used to examine the morphology and structure of thesamples, and energy-dispersive X-ray spectrometry (EDS) was con-ducted in the scanning electron microscope to determine chemicalcomposition. Fourier transform infra-red spectra (FT-IR) of sam-ples as powder-pressed KBr pellets were obtained in the wavenumber range from 4000 to 400 cm−1 using a Bruker VERTEX-70 spectrometer at room temperature. The room-temperaturephotoluminescence (PL) spectrum was recorded with a SPEX®

FluoroLog®-3 fluorescence spectrophotometer.

3. Results and discussion

The morphology of the tree-like SnO2 nanowires produced byheating the as-milled SnO2 powder at 1100 ◦C for 1 h in Ar flowfollowed by cooling in air was examined by SEM. A typical low-magnification SEM image (Fig. 1a) indicates that a large quantityof ultra-long nanowires of 50–150 nm diameter and lengths upto several tens of micrometers is produced. Interestingly, somenanocrystals of regular shape and about 50–300 nm width areattached to the sides and tips of nanowires, or passed through bynanowires, as can be seen in the high-magnification SEM images(Fig. 1b and c). The chemical composition of the crystals and wiresis determined to be close to SnO2 by EDS. Fig. 1d shows the bright-field image, further confirming the tree-like morphology of the1D SnO2 nanostructures. Fig. 1e shows the corresponding elec-tron diffraction pattern. The pattern matches the diffraction patternof a single SnO2 crystal taken along the [1 0 2̄] axis. Fig. 1f is theHRTEM image of the same 1D nanostructures in which the lat-tice spacing of 0.47 nm (corresponding to the (0 2 0) lattice planes)is resolved. The TEM data establishes that the growth directionis parallel to the [1 0 1] crystal axis and the side surfaces are±(0 1 0) and ±(1 0 1̄) facets. The results indicate that the crystallineorientation of the nanowires is the same as reported previously[7].

Fig. 2 shows an XRD pattern of the tree-like SnO2 nanowires.All of the observed diffraction peaks can be ascribed to a tetragonalrutile SnO2 structure with lattice constants of a = 0.4735 nm andc = 0.3185 nm, consistent with the standard data file (JCPDS No. 41-1445).

-magnification SEM; (d) bright-field TEM; (e) the corresponding electron diffraction

130 T. Tao et al. / Materials Chemistry an

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Fig. 2. XRD pattern of the as-prepared tree-like SnO2 nanowires.

The effects of annealing temperature and time on the millednO2 powder were investigated to determine the formation mech-nism of the tree-like SnO2 nanowires. The transformation of theilled SnO2 powder as a function of the annealing temperature

s shown in Fig. 3. Fig. 3a shows the typical agglomeration of theilled SnO2 powder that consists of particles of various size and

rregular shape. After annealing at 900 ◦ C the morphology of theample (Fig. 3b) is obviously changed with the average size of par-icles increasing and more aggregation occurring. With an increasen temperature to 1000 ◦ C (Fig. 3c), there is a complete transforma-ion of the material from nanoparticles to nanocrystals of regularhape. At a temperature of 1100 ◦ C, the material shows an almostomplete transformation from nanocrystals to tree-like nanowiresFig. 3d).

The transformations in the shape of the milled SnO2 powder as aunction of the annealing time, at constant temperature (1100 ◦ C),re shown in Fig. 4. The morphology of the SnO2 milled powderfter 60 min of annealing has been shown in Fig. 3d. The morphol-gy after 10, 30 and 120 min of annealing are shown in Fig. 4a–c,espectively. Statistical analysis of these SEM images indicates that

he quantity of SnO2 particles decreases while the amount of wiresncreases with increasing annealing time over the examined rangef 0–120 min. The elongated shape with a rectangular cross-sections preserved with longer annealing resulting in an increased aspectatio and general nanowire enlargement.

ig. 3. SEM images of (a) the as-milled powder and the material after heating in Ar flowir.

d Physics 126 (2011) 128–132

On the basis of the above SEM observations, we believe thatheating temperature and time are the key determinants of themorphology of the SnO2 nanostructures. The growth of the SnO2nanowires from the milled SnO2 powder can be understood as fol-lows. At temperatures higher than 1000 ◦C, SnO2 decomposes toSnO (vapor) and O2 (vapor) [29]. It is generally agreed that SnOvapor is metastable and will decompose to SnO2 (vapor) and Sn(vapor) [30], which may be oxidized if enough oxygen is availablein the environment. The vapors are inhibited from escaping fromthe interior of the boats by the assembly in which two alumina boatsare placed one on top of another. This arrangement is important forcontrolling the location of the products during the cooling in air. Thevapor concentration increases during extended annealing. Vaporsform at an early stage of annealing and increase gradually duringannealing. It is believed that the degree of supersaturation of thevapors plays a key role in the formation of one-dimensional nanos-tructures [31]. No catalyst has been added in these experiments andno nanoparticles were found at the tips of the nanowires. Thus, thegrowth of these SnO2 nanowires appears to be via a vapor–solid(VS) process [32,33].

Although some models have been proposed to describe theVS (vapor–solid) growth mechanism of one-dimensional materi-als [34–36], we propose a possible mechanism (Fig. 5) to explainthe growth of a uniform tree-like structure. It is proposed that ini-tially the vapors produced by heating the milled powder are usedmainly for forming smaller SnO2 crystals. These crystals wouldself-agglomerate into clusters and subsequently each cluster in theagglomerate would easily grow in one-dimension, favoring the for-mation of nanowires during the cooling in air, if there is sufficientvapor. So a low vapor concentration produced by insufficient heat-ing time could lead to the formation, during cooling, of irregularSnO2 nanowires covered with many small crystals of SnO2, as inFig. 4b. This is due to the low vapor concentration which couldnot provide an adequate amount of smaller SnO2 crystals to sup-port the continuing growth of nanowires. When the heating timewas extended to 1 h, a higher vapor concentration was achieved

which transformed into smaller crystals of SnO2. These crystalswould accumulate and form some bigger SnO2 crystals, easily lead-ing to the formation of nanowires (Fig. 3d). When the heating timereached 2 h, the concentration of vapors could have be supersatu-rated, which benefits the continuing growth of regular nanowires

for 1 h at different temperatures: (b) 900 ◦C; (c) 1000 ◦C; (d) 1100 ◦C and cooling in

T. Tao et al. / Materials Chemistry and Physics 126 (2011) 128–132 131

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and nanowires. An excitation wavelength of 252 nm was used. ThePL spectra of SnO2 nanocrystals and pure nanowires (Fig. 7) havea dominant emission peak at 402 (3.1 eV) and 602 nm (2.1 eV),respectively. Similar results have been previously observed forSnO2 nanoparticles [40] and nanobelts [41]. The PL spectra of the

Fig. 4. SEM images of as-milled powders after annealing in Ar flow at 110

Fig. 4c). The expectation is that the size and morphology of thes-prepared SnO2 nanostructures should be closely related to theapor concentration during annealing of the milled SnO2 powder.higher degree of supersaturation of the vapors can easily lead to

he formation of nanowires during cooling in air. A moderate con-entration of vapors is required for the growth of the tree-like SnO2anowires. A low concentration of the vapors favors the formationf crystals.

The preparation of an evaporation source by ball milling is cru-ial for the achievement of sufficient evaporation at a relativelyow temperature. An un-milled SnO2 powder was heated under theame conditions and no significant changes were observed. Possi-le reasons for the low evaporation temperature are not only thehange in surface area but also the structural changes in the milledaterials, as has been suggested previously [25,29].The infrared (IR) spectra of the SnO2 samples presented in Fig. 6

how that the positions and even the number of observed bandsiffer between samples. Fig. 6a shows the spectra of the originalnO2 powder in which two peaks 616 and 671 cm−1 are observed,hich can be attributed to Sn–O–Sn antisymmetric vibrations [37].

he infrared pattern of the SnO2 nanowires is shown in Fig. 6b,he peaks at 630 and 702 cm−1 can be assigned to Eu TO and A2u

O infrared vibration modes of rutile SnO2 structures, respectively38]. The most important IR feature is the peak at 561 cm−1 sim-lar to the surface vibration mode of the peaks at 564 cm−1 [38]r 561 cm−1 [31] seen in SnO2 nanobelts resulting from nanosize

Fig. 5. A possible growth process for the formation of the SnO2 nanowires.

r different time: (a) 10 min; (b) 30 min; (c) 2 h followed by cooling in air.

effects. Three peaks at 562, 644, and 694 cm−1 can be observed inthe infrared pattern of tree-like SnO2 nanowires (Fig. 6c). The dis-tinct differences between the nanowires without attached crystalsand the tree-like nanowires is the middle peak of the IR spectra(Fig. 6b and c) is one strong peak at 630 cm−1 for the nanowires andone weak peak at 644 cm−1 for the tree-like nanowires, as expectedfrom their different surface structure shown in Figs. 3d and 4c. Theinfrared spectrum of the SnO2 nanocrystals (Fig. 6d) shows threeweak peaks at 578, 635, and 701 cm−1. These results suggest thatthe peaks in the infrared spectra vary in number and frequencybecause of the different shapes and therefore the proportions ofthe crystalline faces present at the surface. The differences for dif-ferent faces can be attributed to polarization phenomena inducedon the surface of the materials by the electromagnetic field of theinfrared radiation [39].

Fig. 7 shows the room temperature photoluminescence spec-tra of the as synthesized SnO2 nanocrystals, tree-like nanowires

Fig. 6. FT-IR spectra for the as-synthesized products: (a) the original SnO2 powder;(b) the pure nanowires; (c) the tree-like nanowires; (d) the nanocrystals.

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ig. 7. The photoluminescence spectra of as-prepared SnO2 products: nanocrystalsblue); tree-like nanowires (red); pure nanowires (black). (For interpretation of theeferences to color in this figure caption, the reader is referred to the web versionf the article.)

ree-like nanowires, in comparison with the SnO2 nanocrystals andure nanowires, has a strong PL peak centered at 548 nm (2.3 eV).he band gaps of the reported 1D SnO2 nanostructures such asanowires and nanobelts are between 3.74 eV and 3.81 eV [42], andhe band gap of SnO2 nanocrystals is about 4.1 eV [40]. All of thesere larger than the value of 3.62 eV for bulk SnO2 due to quantumize effects [43]. Therefore, the observed luminescence bands can-ot be assigned to the direct recombination of a conduction electron

n the Sn 4d band with a hole in the O 2p valence band. These broadmissions can be attributed to different luminescent centers such asarious defects and tin interstitials or dangling bonds in the nanos-ructures [44]. The intrinsic defects such as oxygen vacancies whichommonly act as luminescent centers can form defect levels locatedigh in the gap, trapping electrons from the valence band to makecontribution to the luminescence [45]. On the other hand, the

trong PL peak of the tree-like SnO2 nanowires can be understoodo result from surface defects [46] since the volume fraction of theanowires is different and the defect structure of the nanocrystalsttached to the surface of wires and the main wires may also vary.urther investigation is required to clarify this mechanism.

. Conclusions

SnO2 nanostructures have been grown by heating ball-millednO2 powder using different annealing conditions. Tree-likeanowires were formed under appropriate growth conditions, andhe growth process was found to be dominated by a VS mecha-ism. High-resolution transmission electron microscopy (HRTEM)howed that the SnO2 nanowires are single crystals in the (1 0 1)rowth direction. FT-IR and PL measurements have shown that theree-like nanostructures lead to superb physical properties. The PLpectrum of such tree-like nanowires exhibits a strong PL peak at48 nm. This clearly shows that the interface and nanostructure are

mportant determinants of the optical properties.

cknowledgment

The authors acknowledge financial support from the Nationalasic Research Program of China (No: 2007CB613601).

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d Physics 126 (2011) 128–132

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