synthesis and high gas sensitivity of tin oxide nanotubes

Goua,b aSchool of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia bInstitute for Superconducting & Electronic Materials, University of Wo

Sensors and Actuators B 131 (2008) 313–317

Synthesis and high gas sensitivity

of tin oxide nanotubes G.X Wanga,b,∗, J.S Parkb, M.S Parkb, X.L Goua,b

aSchool of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia

bInstitute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia

Received 24 August 2007; received in revised form 14 November 2007; accepted 14 November 2007

Available online 24 November 2007

Abstract

Semiconductor tin oxide (SnO2) nanotubes have been synthesised in bulk quantities using a sol–gel template (AAO membrane) synthetic technique The morphology and crystal structure of SnO2nanotubes were characterised by a field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM) The as-prepared SnO2nanotubes are polycrystalline with an outer diameter of 200 nm, an inner diameter of about 150 nm and a length extending to tens of micrometers SnO2nanotube sensors exhibited high sensitivity towards ethanol gas

© 2007 Elsevier B.V All rights reserved

Keywords: Tin oxides; Nanotubes; Sol–gel; Gas-sensors; Nanocrystallites

1 Introduction

One-dimensional (1D) nanostructures including nanotubes,

nanowires, and nanoribbons have attracted both intensive and

extensive research, which can be mainly attributed to their

unique chemical and physical properties, and their intriguing

technological applications [1,2] In particular, 1D

semicon-ductor nanostructures provide building-blocks for fabricating

functional nanoscale electronic, optoelectronic, photonic,

chem-ical and biomedchem-ical devices based on the bottom-up paradigm

[3–7]

Among all the potential applications, nanoscale chemical and

biological sensors are generally considered as one of the

impor-tant areas for nanotechnology to enter into practical applications

[8] The high surface-to-volume ratio of 1D nanostructures

induces extremely high sensitivity to adsorbed chemical or

bio-logical species on the surface of nanosensors Lieber et al have

developed silicon nanowire sensors and implemented them as

the real-time sensors for detecting pH and biological species

∗Corresponding author at: School of Mechanical, Materials and

Mecha-tronic Engineering, University of Wollongong, Northfield Avenue, NSW 2522,

Australia Fax: +61 2 42215731.

E-mail address:gwang@uow.edu.au (G.X Wang).

[9] The principle of the Si nanowire sensors is based on the conductance (surface charge) change caused by protonation and deprotonation associated with the adsorbed molecular species Single and multiple In2O3 nanowire sensors have shown high sensitivity to NO2 and NH3 gas [10,11] SnO2 is a wide-bandgap (3.6 eV) semiconductor The electronic conductivity

of SnO2 is significantly influenced by the effects on its sur-face states of molecular adsorption It has been widely explored

as an effective gas sensor, traditionally in the forms of thin or thick films with low sensitivity and long response time [12] Recently, SnO2 nanobelts have been tested for their sensitiv-ity to environmental pollutants such as CO and NO2 [13] Photochemical SnO2nanoribbon sensors have been fabricated for detecting low concentration of NO2 at room temperature under UV light [14] Polycrystalline SnO2 nanowire sensors were also developed for sensing ethanol, CO and H2gas[15] SnO2 nanohole array sensors exhibited reversible response to

H2[16] Herein, we describe the synthesis of polycrystalline SnO2

nanotubes using the sol–gel template method, and the fabrica-tion of SnO2 nanotube sensors Due to their one dimensional and tubular structure, SnO2 nanotube sensors exhibited high sensitivity and quick response time for detecting ethanol and ammonia gas

0925-4005/$ – see front matter © 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2007.11.032

200 nm pore, 60␮m in thickness, and 47 mm in diameter)

were used as the template for preparing SnO2nanotubes The

chemicals used were tin(II) chloride dehydrate (SnCl2·2H2O,

Aldrich, A.C.S reagent), sodium hydroxide (Aldrich, 98%) and

hydrochloric acid (36%, Merck) SnO2nanotubes were

synthe-sised via a sol–gel and sintering process following these steps:

(i) 3.38 g SnCl2, 4.7 ml ethanol and 0.3 ml HCl were mixed

together and aged for 24 h, during which time the colour of the

solution changed from white to pale yellow and finally

form-ing a transparent and highly viscous gel Then, 0.3 ml deionised

water was added to the as-prepared gel to form a solution; (ii)

the AAO templates were impregnated by vacuum suction The

solution was forced to pass through the pores of the template

and adhere on the pore walls; (iii) the impregnated template was

dried at 100◦C and then sintered at 500◦C for 3 h to convert the

tin hydroxide to tin oxide; (iv) after sintering, the AAO

mem-brane was dissolved in 6 M NaOH solution The undissolved

SnO2 nanotubes were collected and washed through a

filter-ing process to remove Na+and Al3+ The crystal structures and

morphologies of the SnO2nanotubes were characterised using

X-ray diffraction (XRD, Philips 1730), field emission scanning

electron microscopy (FE-SEM, JEOL JSM-6700F) and

trans-mission electron microscopy (TEM, JEOL 2011) The specific

surface area was measured by the Brunauer–Emmett–Teller

(BET) method at 77 K using a NOVA 1000 high-speed gas

sorption analyzer (Quantachrome Corporation, USA) The gas

sensing properties of the as-prepared SnO2nanotubes and SnO2

nanopowders (61 nm in average particle size (APS),

Nanostruc-tured & Amorphous Materials Inc., USA) were measured using

a WS-30A gas sensor measurement system SnO2nanotubes and

nanopowders were mixed with polyvinyl acetate (PVA) binder

to form a slurry, and then pasted on to ceramic tubes (2 mm in

diameter) between Au electrodes, which were connected with

four platinum wires The fabricated sensors were fitted into the

gas-sensing measurement apparatus Given amounts of ethanol

and ammonia gas were injected into the testing chamber by a

micro-syringe injector The gas sensing response was defined as

the ratio Rair/Rgas, where Rair and Rgas are the electrical

resis-tance of the sensors in air and in gas, respectively The gas

sensing measurement was carried out at a working temperature

of 200◦C.

3 Results and discussion

Fig 1shows the X-ray diffraction patterns of SnO2

nanopow-ders and SnO2nanotubes All diffraction lines can be indexed

to the tetragonal rutile phase (JCPDS #41-1445) It should be

noted that SnO2nanotubes have much broader diffraction peaks

and lower diffraction intensities than that of SnO2nanopowders,

indicating a much small crystal size for the nanotubes The

aver-age crystal size of SnO2nanotubes was calculated to be about

15 nm using the Scherrer equation d = κλ/β cos θ The general

morphology of SnO2nanotubes was observed by FE-SEM and is

shown inFig 2 The as-prepared SnO2nanotubes have lengths of

Fig 1 X-ray diffraction patterns of SnO 2 nanotubes and nanopowders.

a few micrometers The SnO2nanotubes were partially broken, which could have been induced during the sintering process or the subsequent filtering process The inset inFig 2is a top view

of the SnO2nanotube bundle, from which we can clearly see the hollow and tubular structure with an outer diameter of 200 nm

We measured the BET surface areas of commercial nanosize SnO2 powders and as-prepared SnO2 nanotubes SnO2 nano-size powders have a BET surface area of 15.2 m2/g, while SnO2

nanotubes have a surface area of 45.6 m2/g The crystal structure

of the SnO2nanotubes was further analysed by TEM and high resolution TEM (HRTEM) A general TEM image of a SnO2

nanotube is shown inFig 3(a) The SnO2nanotubes are poly-crystalline, with the small nanosize crystals bonded together through the sintering process Selected area electron diffrac-tion (SAED) was performed on the individual SnO2nanotubes (the inset inFig 3(a)) The indexed ring patterns confirmed the tetragonal crystal structure of the SnO2nanocrystals that form the nanotube.Fig 3(b) shows a high resolution TEM image of

a SnO2 nanotube, in which the individual crystal sizes are in

Fig 2 FESEM image of SnO 2 nanotubes The inset is a top view of SnO 2

nanotube bundle.

Fig 3 (a) TEM image of a single SnO 2 nanotube Inset: selected area

electron-diffraction pattern (b) HRTEM image of a portion of a SnO 2 nanotube.

the range of 10–20 nm The lattice spacing was measured to be

0.47 nm

SnO2nanotubes were tested as chemical sensors of ethanol

and ammonia gas As a comparison, the sensing properties of

SnO2nanopodwers (APS: 61 nm) were also tested The gas

sen-sitivities were measured in air at 25◦C under a relative humidity

(RH = 40–50%) Through pre-testing, we first determined that

the optimised sensor working temperature was 200◦C, at which

both SnO2nanotubes and SnO2nanopowders exhibited an

opti-mal performance Subsequently, all sensing measurements were

conducted at this working temperature Fig 4(a) shows the

real-time gas sensing response towards ethanol vapor for SnO2

nanotube and nanopowder sensors The ethanol vapor

concen-trations were varied Initially, the SnO2nanotube sensor showed

similar response to the SnO2 nanopowders at the very low

concentration (10 ppm) However, as the ethanol vapor

concen-tration increased, the SnO2nanotube sensor demonstrated larger

response In general, on increasing the gas concentrations, the

response increase proportionally.Fig 4(b) shows the gas

sens-ing response versus the ethanol concentrations in the range of

10–1000 ppm It should be noted that SnO2nanotubes have more

than 1.5 times larger response than the corresponding

nanopow-ders This result is comparable to the previously reported ethanol

Fig 4 (a) Real-time sensing response to ethanol gas in air Inset: equivalent electrical circuit for SnO 2 nanopowder sensor and SnO 2 nanotube sensor (b) Sensing response vs ethanol vapor concentration.

gas sensing performance using nanocrystalline SnO2 powders with an average crystallite size of 8 nm[17]

By analysing the transient response characteristics of SnO2

nanotube and nanopowder sensors, we found that the response time to gas on and recovery time to gas off take less than 5 s When examining the shape of the response curves inFig 4(a), we can see that the SnO2nanotube sensor required more response time to reach its maximum value at all concentrations when the gas was on; similarly, there was also a delay before recovery when the gas was off This retard response behavior of SnO2

nanotube sensor is typically related to the small crystal size and 1D structure of the nanotubes It can be explained by using the equivalent electric circuit models shown in the inset inFig 4(a) SnO2 nanopowders could be considered as a simple resistor because individual crystals are loosely agglomerated There-fore, the SnO2 nanopowder sensor shows straight lines in the response profiles On the other hand, the SnO2nanotubes can be modeled as a capacitor connected in parallel with a resistor and then serially connected with another resistor The capacitance behavior mainly comes from the grain boundaries between the tiny nanosize crystals that form the nanotubes[18] This model

Fig 5 Real-time sensing response to ammonia gas in air.

can satisfactorily explain the retarded response behavior of the

SnO2nanotube sensor

The responses towards ammonia are shown inFig 5 When

attempting to detect ammonia gas, the SnO2nanopowder sensor

showed no response at low concentration, and a slight change

in the resistance at high concentrations, but the response was

unstable and had serious fluctuations In contrast, the SnO2

nan-otube sensor was active even at 10 ppm Its response towards

ammonia increased proportionally with the increasing gas

con-centration However, the overall sensing response performance

towards ammonia gas is much lower than that to ethanol gas for

both SnO2nanosize powders and nanotubes

The response curves in Figs 4 and 5 clearly indicated a

sensing mechanism that could be described as gas surface

chemisorption and electron acceptance, resulting in a decrease

in the sensor resistance SnO2 is an n-type wide band gap

semiconductor Its electronic conduction originates from point

defects, which either are oxygen vacancies or foreign atoms that

act as donors or acceptors In the ambient environment, SnO2

nanocrystals are expected to adsorb both oxygen and

mois-ture, in which moisture may be adsorbed as hydroxyl groups

The adsorbed O2− and OH− groups trap electrons from the

conduction band of SnO2nanocrystals, inducing the formation

of a depletion layer on the surface of the SnO2 nanocrystals

[19] When exposed to ethanol vapour, CH3CH2OH molecules

are chemisorbed at the active sites on the surface of the SnO2

nanocrystals These ethanol molecules will be oxidised by the

adsorbed oxygen and lattice oxygen (O2−) of SnO

2at the sensor working temperature During this oxidation process, electrons

will transfer to the surface of the SnO2 nanocrystals to lower

the number of trapped electrons, inducing a decrease in the

resistance A similar mechanism should be ascribed to the

detec-tion of NH3gas because NH3is commonly considered to work

as a reducing agent and to donate electrons [20] Therefore,

when exposed to NH3molecules, a SnO2sensor responds with

the increased conductivity SnO2 nanotubes consist of small

nanocrystals joined together into 1D tubular structure, resulting

in many more active sites for gas chemisorption In addition,

both the inner and outer walls of SnO2nanotubes can adsorb a

large number of gas molecules Consequently, SnO nanotubes

4 Conclusions

In summary, polycrystalline SnO2nanotubes have been pre-pared via the sol–gel template method FE-SEM observation shows the tubular 1D nanostructure TEM and HRTEM analy-sis confirmed the polycrystalline nature and tetragonal crystal structure of the SnO2nanotubes The SnO2nanotubes exhibited

an enhanced sensitivity to ethanol gas

Acknowledgements

This work was supported by the Australian Research Council (ARC) through ARC Discovery project “Synthesis of nanowires and their application as nanosensors for chemical and biological detection” (DP0559891)

Appendix A Supplementary data

Supplementary data associated with this article can be found,

in the online version, atdoi:10.1016/j.snb.2007.11.032

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Biographies

G.X Wang received his PhD degree in Materials Science and Engineering in

2001 from University of Wollongong, Australia He is currently working as a senior lecturer at School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong His major research interests include nanostructured functional materials, materials chemistry in energy storage and conversion, and development of chemical and biological sensors.

J.S Park received his Master degree in Materials Engineering in 2005 from

Andong National University, Korea Currently, he is a PhD candidate at Insti-tute for Superconducting and Electronic Materials, University of Wollongong, Australia.

M.S Park received his Master degree in Materials Science and Engineering

in 2005 from Korea Advanced Institute of Science and Technology, Korea He

is a currently PhD candidate at Institute for Superconducting and Electronic Materials, University of Wollongong, Australia.

X.L Gou received his PhD degree in Chemistry in 2006 from Nankai University,

China He is a research fellow at Institute for Superconducting and Electronic Materials, University of Wollongong, Australia His research interests include chemical synthesis of functional nanosize inorganic materials and development

of gas sensors.