large - scale synthesis and gas sensing application of vertically

Scanning electron microscopy and transmission electron microscopy analyses reveal an interesting three-order hierarchical nanostructure from small, single-crystalline nanorods via nanoro

Contents lists available atScienceDirect Sensors and Actuators B: Chemical

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 / s n b

Large-scale synthesis and gas sensing application of vertically aligned and

double-sided tungsten oxide nanorod arrays

Xiaoping Shena,b, Guoxiu Wanga,∗, David Wexlera

a Institute for Superconducting and Electronics Materials, School of Mechanical, Materials and Mechatronics Engineering,

University of Wollongong, Wollongong, New South Wales 2522, Australia

b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212000, China

a r t i c l e i n f o

Article history:

Received 25 June 2009

Received in revised form 17 August 2009

Accepted 6 September 2009

Available online 17 September 2009

Keywords:

Tungsten oxide

Nanorod arrays

Hydrothermal synthesis

Gas sensing

Sensor

a b s t r a c t

Large-scale vertically aligned and double-sided Co-doped hexagonal tungsten oxide nanorod arrays have been successfully synthesized by a facile hydrothermal method without using any template, catalyst,

or substrate Scanning electron microscopy and transmission electron microscopy analyses reveal an interesting three-order hierarchical nanostructure from small, single-crystalline nanorods via nanorod bundles to double-sided nanorod arrays The optical absorption properties of the Co-doped WO3 sam-ples were investigated by ultraviolet–visible spectroscopy, and the results indicate that the Co-doped

WO3nanostructures are semiconducting with direct band gaps of 2.26 eV and 2.77 eV The gas sens-ing performance of the as-prepared Co-doped WO3double-sided nanorod arrays was tested towards a series of typical organic solvents and fuels The sample shows excellent gas sensing performance towards 1-butanol vapor, with rapid response and high sensitivity We propose that the double-sided nanorod arrays are formed from urchin-like microspheres via a self-assembly and fusion process This new syn-thesis strategy could be extended to prepare other well-aligned nanorod arrays for many functional applications

© 2009 Elsevier B.V All rights reserved

1 Introduction

Enormous efforts have been devoted to the development of

syn-thetic strategies for highly organized hierarchical nanostructures

consisting of nanoscale building blocks[1–4], because assembling

the synthesized nanoscale building blocks into advanced

struc-tures is a necessary approach for applications in integrated devices

Highly ordered arrays of nanorods or nanowires in particular have

aroused continuous interest due to their diverse properties and

potential applications in data storage, catalysis, sensing, field

elec-tron emission, and optoelecelec-tronic devices [5–8] Two versatile

strategies based on templates and patterned catalysts, respectively,

have been developed to produce nanorods in the form of

large-area arrays[9–11] However, although the template-based method

can provide good control over the uniformity and dimensions

of nanorods, removal of the template through a post-synthesis

process may cause damage to the nanorod arrays In addition,

most nanorods synthesized using the template-based method are

polycrystalline in structure, which may limit their use in device

fab-rication and fundamental studies The method involving the use of

a patterned catalyst is able to generate nanorods with controllable

∗ Corresponding author Fax: +61 2 42215731.

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

sizes and highly crystalline structures However, the catalyst may cause contamination of the resultant nanorod arrays, which is often

a great disadvantage to their application As a template-free and catalyst-free method, epitaxial growth of free-standing nanorod arrays has recently been attained, in which a substrate with an excellent lattice match to the overlying materials is vital to guide the assembly of one-dimensional (1D) nanoarrays [12,13] As a result, this method has largely been limited to particular materials, notably zinc oxide Therefore, the development of novel and more effective strategies for preparing highly ordered nanorod arrays is

of great significance for their practical applications in nanotechnol-ogy

Tungsten oxide (WO3), as an important n-type semiconductor, has received wide attention owing to its promising application in gas sensors, heterogeneous catalysts, chromogenic devices, solar-energy devices, and field electron emission[14–16] The syntheses

of various WO3 nanostructures have been reported, including nanorods, nanowires, nanotubes, nanobelts, urchin-like super-structures, and nanostructured thin films [17–19] Moreover, it has been found that metal-doped tungsten oxide may exhibit improved optical and electrical properties For example, Na-doped tungsten oxide has proved to be a high temperature supercon-ductor [20], and Cr-doped WO3 nanocrystals showed excellent gas sensing performance towards acetone[21] Herein, we report

a novel strategy for preparing large-area Co-doped WO3 double-0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.

326 X Shen et al / Sensors and Actuators B 143 (2009) 325–332

sided nanorod arrays without using any template, catalyst, or

substrate The resulting products were characterized by X-ray

diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field

emission scanning electron microscopy (FE-SEM), high-resolution

transmission electron microscopy (HRTEM), selected area electron

diffraction (SAED), Raman spectroscopy, and ultraviolet–visible

(UV–vis) spectroscopy A mechanism is proposed to explain the

growth of the double-sided nanorod arrays Moreover, the gas

sensing performance of the as-synthesised Co-doped WO3

nanos-tructures was investigated towards a series of flammable and toxic

organic solvents We found that the as-synthesised Co-doped WO3

nanorod arrays have a fast, highly sensitive, and fairly selective

response to 1-butanol gas

2 Experimental details

All chemicals are ACS reagent and were used directly as

purchased from Sigma–Aldrich In a typical experiment for

synthe-sizing Co-doped hexagonal tungsten oxide double-sided nanorod

arrays, 0.815 g of Na2WO4·2H2O was dissolved in 10 mL of distilled

water The solution was acidified to a pH range of 1–1.2 using HCl

solution (3 mol L−1) Then, 0.63 g of H2C2O4was added to the

mix-ture and the mixmix-ture was diluted to 25 mL After that, a stable

WO3sol was formed 16 mL of the WO3sol was transferred into

a 20 mL Teflon-lined autoclave, and then 0.80 g Na2SO4and 5.0 mg

Co(NO3)2·6H2O were added to the solution The autoclave was

sealed and maintained at 180◦C for 10 h After the autoclave was

naturally cooled to room temperature, the resulting solid products

were separated by centrifugation, washed three times with

dis-tilled water and ethanol, respectively, and dried at 60◦C overnight

To further improve the crystallinity and phase purity, the products

were annealed at 400◦C for 4 h

The as-prepared samples were characterized by X-ray

diffrac-tion (XRD, Cu K␣ radiation, Philips 1730), scanning electron

microscopy (SEM, JSM-6460 and JSM-6700F), transmission

elec-tron microscopy (TEM), and high-resolution TEM (HRTEM, JEOL

2011) Samples for TEM were prepared by dropping the products

on a carbon-coated copper grid after strong ultrasonic dispersion

in absolute ethanol X-ray photoelectron spectroscopy (XPS)

mea-surements were carried out with an ESCALab220i-XL spectrometer

by using a twin-anode Al K␣ (1486.6 eV) X-ray source All the

spectra were calibrated according to the binding energy of the

adventitious C1s peak at 284.8 eV The band gap of the Co-doped

WO3 was determined by UV–vis spectroscopy (Shimadzu 1700)

Raman spectroscopy (HR 800) was performed at room temperature

with an excitation wavelength of 632.8 nm

The gas sensing properties were measured using a WS-30A gas

sensor measurement system The gas sensor was fabricated as

fol-lows: the Co-doped WO3sample was mixed with polyvinyl acetate

(PVA) binder (1 wt.%) to form a slurry, and then pasted onto a

ceramic tube (2 mm in diameter) by a doctor blade to form a thin

film between two Au electrodes, which had been previously printed

on the ceramic tube and were connected with four platinum wires

As a comparison, gas sensing properties of commercial WO3

pow-der (Fluka) were also measured The commercial WO3powder is a

well crystalline powder with a crystal size in the range of a few

hun-dreds nanometers.Fig S-1shows the FE-SEM image of commercial

WO3powders (Supplementary data) Given amounts of test gases

were injected into the testing chamber by a microsyringe Gas

sens-ing measurements were carried out at a worksens-ing temperature of

200◦C with 30–40% relative humidity.Fig S-2shows a schematic

diagram of the sensor system (Supplementary data) The gas sensor

was fabricated according to the method described in Section2 The

working temperature of the sensor can be controlled by adjusting

the heating voltage (V ) across a resistor inside the ceramic

tube A reference resistor is put in series with the sensor to form

a complete measurement circuit In the test process, a working voltage (Vworking) was applied By monitoring the voltage (Voutput) across the reference resistor, the response of the sensor in air or in

a test gas could be measured The gas sensing response is defined

as the ratio of the stationary electrical resistance of the sensor in air (Rair) to the resistance in the test gas (Rgas), i.e., R = Rair/Rgas

3 Results and discussion

The phase of the obtained products was determined by XRD

As shown inFig 1, all the diffraction peaks of the annealed product can be readily indexed to hexagonal structure WO3with lattice con-stants a = 7.3223 Å and c = 7.6574 Å, which are slightly smaller than the standard values for bulk hexagonal WO3(JCPDS No 85-2460,

a = 7.3242 Å and c = 7.6624 Å) Before annealing, the sample showed several impurity peaks, suggesting that the annealing is necessary

to obtain pure phase WO3 In addition, the relative strength of the (0 0 2) peak was significantly increased by the annealing treatment, suggesting that annealing also improves the crystallinity of the product It is well known that hexagonal (h) WO3is a metastable phase and can transform into monoclinic (m) WO3 at high tem-perature Recently, Szilágyi et al pointed out that the structure

of hexagonal WO3cannot be maintained without some stabilizing ions or molecules in the hexagonal channels, and thus the exis-tence of strictly stoichiometric hexagonal WO3is questionable[22] X-ray photoelectron spectroscopy (XPS) (Fig 2) was employed to determine the chemical composition and oxidation states of the elements in the present sample It was revealed that the product contains the elements tungsten, oxygen, sodium and cobalt with

a W:O:Na:Co molar ratio of about 1:2.92:0.40:0.02 The presence

of such a high Na+content of the product strongly supports Szilá-gyi’s viewpoint This may explain why alkaline (Na+, K+, Cs+, etc.)

or NH4 ions were needed to prepare h-WO3 Alkaline (Na+, K+, Cs+, etc.) or NH4 ions can stabilize the hexagonal structure in such a way that they are located in the hexagonal channels of crystallites and block the thermodynamically favored hexagonal–monoclinic transformation

As shown inFig 2, the O1s peak is located at 530.7 eV, which

is ascribed to the W–O peak The W4f peaks located at 36.4 eV and 38.4 eV can be attributed to W4f7/2and W4f5/2, respectively, which are in good agreement with the reported values[23] These two peaks are well separated without any shoulder, which indicates that almost all W atoms are in the +6 oxidization state[24] The

Fig 1 X-ray diffraction patterns of Co-doped WO3 nanorod arrays: (a) before annealing and (b) after annealing.

Fig 2 XPS spectra of Co-doped WO3nanorod arrays.

Na1s peak at 1070.9 eV is consistent with the +1 oxidation state

of sodium[25] A Co2p signal located at 780.3 eV was detected,

showing that Co exists in the +2 oxidation state in the product[26]

However, the Co2p signal is weak due to the low Co content in

the product Therefore, the above results confirmed the successful

preparation of the Co-doped hexagonal tungsten oxide

Fig 3 shows FE-SEM images of the Co-doped WO3 sample

Fig 3(a) is a general view of a double-sided nanorod array It

can be seen that the double-sided nanorod array has a uniform

thickness of about 10␮m, with the nanorods growing from the

center in two opposite directions.Fig 3(b) and (c) are top views

of the nanorod array at different magnifications It can be seen

that the nanorods, with a diameter of about 200 nm, are densely

arranged in a well ordered way to form a large-area ordered array

It was found that the area of a single ordered array can reach

1.0× 104␮m2 Furthermore, the high-magnification FE-SEM image

(Fig 3(d)) reveals that the nanorods are actually composed of

a number of smaller nanorods with a diameter of about 20 nm

Therefore, they will be referred to as nanorod bundles hereafter

We also performed FE-SEM observation on a cross-section of the

double-sided nanorod array (as shown in Fig S-3,

supplemen-tary data) Elemental energy dispersive X-ray (EDX) mapping of

Co was obtained on the same area We found that the

distribu-tion of the dopant element Co is uniform in the cross-secdistribu-tion The

above results unambiguously demonstrate that a novel Co-doped

WO3 nanostructure with a three-order hierarchical architecture

from small nanorods via nanorod bundles to double-sided nanorod

arrays has been achieved

A further investigation of the Co-doped WO3 nanorod arrays

was conducted by TEM and HRTEM analysis The low-magnification

TEM image (Fig 4(a)) shows some straight nanorod bundles with

a diameter of about 200 nm and a length up to about 5␮m A

high-magnification TEM image (Fig 4(b)) reveals that the nanorod

bundles consist of smaller nanorods with a diameter of about

20 nm These results are consistent with the FE-SEM observations Selected area electron diffraction (SAED) pattern (inset inFig 4(b)) taken from this nanorod bundle shows regular diffraction spots, which can be indexed as hexagonal WO3 single crystal recorded along the [1 1 0] zone axis and demonstrates that the WO3nanorods grow along the [0 0 1] direction The SAED pattern also reveals that the nanorods in the bundles consist of a perfectly oriented assembly, which is further confirmed by the HRTEM analysis We have performed extensive TEM observations on different individual bundles and found that all of them consist of small nanorods and grow along [0 0 1] crystal direction (Fig S-4, supplementary data)

As shown in Fig 4(c) and (d), the parallel lattice fringes among the different primary nanorods and grain boundaries clearly show the oriented aggregation and high crystallinity of the primary nanorods The degree of fusion of the primary nanorods in the bundles shows a gradual increase from the top to the base of the bundles.Fig 4(e) shows a HRTEM image of the middle section of

a primary nanorod, from which the (1 0 0) lattice planes with a d-spacing of 0.64 nm can be clearly distinguished It was found that preferential growth occurred parallel to the (1 0 0) lattice plane, which is in consistent with the [0 0 1] growth direction determined

by SAED.Fig 4(f) shows a lattice resolved HRTEM image of the tip

of a primary nanorod The corresponding SAED pattern is presented

as the inset inFig 4(f), confirmed the single-crystalline nature of the primary nanorod

Based on the experimental observations, we propose that the formation process of the Co-doped WO3 double-sided nanorod arrays can be divided into three steps In the initial stage, the nanoparticles are quickly grown and spontaneously aggregate into large spheres to minimize their surface area In the second step, the as-formed microspheres serve as substrates for epitaxial growth of c-axis oriented WO3 nanorods with the assistance of SO4 −ions

At the same time, because of their high surface energy, the adja-cent WO nanorods tend towards oriented attachment and then

328 X Shen et al / Sensors and Actuators B 143 (2009) 325–332

Fig 3 FE-SEM images of Co-doped WO3nanorod arrays: (a) a general side view of double-sided nanorod arrays, (b) and (c) top views of the nanorod arrays at different magnifications, (d) high-magnification FEG-SEM image of the Co-doped WO3 nanorod arrays, with the inset showing a high-resolution view of a single nanorod bundle. partly fuse to form bunch-like structures [27–29] As observed

by TEM analysis, the bunch-like structures become more smooth

and regular from the tip to the base of the bundles We believe

that Ostwald-ripening works simultaneously with the oriented

attachment to remedy the defects, leading to a smooth and

regu-lar surface of the base parts Thus, urchin-like architectures with

WO3 nanorod bundles on the surfaces of the microspheres are

formed This is supported by the evidence that several

hemispher-ical regions (Fig 5(a)) and sphere-like cores with nanorod bunches

on their surfaces (Fig 5(b)) have been found in the Co-doped WO3

products This process is similar to what happens in the synthesis

of WO3 like microspheres Finally, the as-formed

urchin-like microspheres further self-assemble and fuse into well-aligned

double-sided nanorod arrays with the help of dopant Co2+and an

accompanying Ostwald-ripening process To the best of our

knowl-edge, this is the first time that the observation of large-area uniform

double-sided nanorod arrays formed by self-assembly and fusion of

urchin-like nanostructures has been reported.Scheme 1shows the

schematics of the formation process of the Co-doped WO3

double-sided nanorod arrays, elucidating the growth mechanism of the

nanorod arrays

Raman scattering is very sensitive to the microstructure of

nanocrystalline materials, so it was employed to determine the

nanostructure of the Co-doped WO3nanorod arrays As shown in

Fig 6(a), the Raman spectrum of the Co-doped WO3

nanostruc-tures shows five obvious Raman peaks located at around 270 cm−1,

327 cm−1, 713 cm−1, 808 cm−1, and 927 cm−1 The Raman shifts are

consistent with the fundamental modes of crystalline h-WO3 The

bands at 713 cm−1and 808 cm−1 can be assigned to the O–W–O

stretching modes, while the bands at 270 cm−1and 327 cm−1

cor-respond to the O–W–O bending modes of the bridging oxygen The

weak Raman peak at 927 cm−1 may be attributed to a stretching

mode of the terminal W O Although this latter band is

charac-teristic of tungsten oxide hydrates, it can appear in WO3via the

adsorption of water molecules[30] The Raman spectrum provides

clear evidence for the high structural quality and phase-pure nature

of Co-doped WO3nanorod arrays The optical absorption properties

of the as-prepared Co-doped WO3 nanostructures were

investi-gated at room temperature by UV–visible spectroscopy As shown

in Fig 6(b), the spectrum shows one absorption peak at about

281 nm WO3is an n-type semiconductor[31], and its optical band gap can be estimated using the following formula:

where ˛ is the absorption coefficient, h is the photon energy, B

is a constant relative to the material, Egis the band gap, and n

is either 1/2 for an indirect transition or 2 for a direct transition The (˛h)2versus h curve for the product is shown in the inset

inFig 4(b) The value of h extrapolated to ˛ = 0 gives the absorp-tion band gap energy Two regions with a linear relaabsorp-tionship are observed in the ranges of 3.5–4.2 eV and 5.0–6.0 eV, respectively, giving two Egvalues of 2.77 eV and 2.26 eV The band gap of 2.77 eV can be attributed to the transition between the 2p valence band formed by oxygen and the 5d conduction band of tungsten, while the 2.26 eV bandgap may be associated with the O−II→ CoIIcharge transfer process (with the CoIIlevel located below the conduction band) However, the band gap of 2.77 eV is much lower than the reported direct band gaps of WO3 The band edge position for amor-phous WO3in contact with an aqueous electrolyte at a pH of∼1 is about 3.2 eV[32] Two distinct direct interband transition energies

of 3.52 eV and 3.74 eV for WO3were also observed by Koffyberg

et al.[33] The lower band gap of the Co-doped WO3may reflect doping effects It is well known that in doped compound semi-conductors, in contrast with un-doped ones, the impurity states play a special role in the electronic energy structures and transi-tion probabilities In additransi-tion, it is found that the best fit of Eq.(1)

to the absorption spectrum of the product gives n = 2, which sug-gests that the as-obtained Co-doped WO3is semiconducting with direct transitions at these energies

Chemical sensors play an important role in the areas of emis-sions control, environmental protection, public safety, and human health Much more public concern over serious environmental issues is further promoting the development of sensors with both high sensitivity and rapid response It has been well documented that the ultra-high surface-to-volume ratios of nanostructured materials make their electrical conductivities extremely sensitive

to surface-adsorbed species and make them excellent candidates for gas sensing applications[34,35] The gas sensing performance

Fig 4 TEM and HRTEM images of Co-doped WO3nanorod arrays: (a) low TEM images of separated nanorod bundles, (b) high-magnification TEM images showing the bundle-like nanostructures, (c) and (d) HRTEM images showing the lattice structures of the nanorod bundles, (e) and (f) lattice resolved HRTEM images of the primary nanorods The inset in (b) and (f) are the corresponding SAED patterns.

of the Co-doped WO3 nanorod arrays was investigated for

sev-eral ordinary organic solvents and fuels, including acetone, ethanol,

propanol, butanol, toluene, heptane, acetic acid, and gasoline Some

of these chemicals are very important industrial raw materials,

and the others are arousing more and more attention because of

the possibility of their use as automotive fuels or gasoline

compo-nents Therefore, highly sensitive gas sensors are important to the

practical applications of these flammable gases

The real-time sensing responses towards 1-butanol of sensors based on the Co-doped WO3 nanostructures and on commercial

WO3 powder are displayed inFig 7(a) It can be seen that the sensing response (Rair/Rgas) of the sensor increased abruptly on the injection of 1-butanol, then decreased rapidly, and recovered to its initial value after the test gas was purged The magnitude of the response of the sensor based on the Co-doped WO3 nanorod arrays improved dramatically with increasing concentration of the

Fig 5 (a) FE-SEM image shows a hemispherical region (enclosed by the circle) in a nanorod array (b) Cross-sectional FE-SEM image showing the sphere-like cores (as

indicated by the circles) in the Co-doped WO3 double-sides nanorod arrays.

330 X Shen et al / Sensors and Actuators B 143 (2009) 325–332

Scheme 1 Schematic diagram of the proposed growth mechanism of the Co-doped WO3double-sided nanorod arrays.

test gas and was much higher than that of the commercial

pow-der This means that the Co-doped WO3nanorod arrays are much

more sensitive to 1-butanol than the commercial powder After

many cycles between the test gas and fresh air, the resistance of

the sensor was still able to recover its initial state, which

indi-cates that the sensor has an excellent reversibility The response

time and recovery time (defined as the time required to reach 90%

Fig 6 (a) Raman spectrum of the Co-doped WO3double-sided nanorod arrays (b)

UV–vis spectrum of the Co-doped WO3 double-sided nanorod arrays The inset is

2

of the final equilibrium value) of the sensor were only 1–2 s and 2–4 s, respectively The response characteristic curves of the sen-sors towards other gases are similar to that for 1-butanol and are not shown here The major charge carriers are electrons for n-type semiconductors Upon exposure to a reducing gas, the density of n-type charge carriers (electrons) would increase due to surface

Fig 7 (a) Real-time sensing responses towards 1-butanol of the sensor made from

the Co-doped WO3 double-sided nanorod arrays and commercial WO3 powders (b) Sensing responses vs gas concentrations of various gases, including 1-butanol,

adsorption and chemical reaction between the gas and the

oxy-gen adsorbates (electron acceptors), resulting in a decrease in the

sensor resistance

The sensing responses as a function of vapor concentration from

5 ppm to 1000 ppm are shown inFig 7(b) It can be seen that the

responses took on an exponential rate of increase at first (below

200 ppm), which then changed to a linear increase in the range

of 200–1000 ppm As a whole, the sensing responses decreased

in the sequence of 1-butanol, acetone, toluene, 2-propanol, acetic

acid, ethanol, gasoline, and heptane The Co-doped WO3

nanostruc-tures exhibited a high sensing response to 1-butanol vapor, and

the Rair/Rgasvalue was 8.5 at the very low concentration of 5 ppm,

but reached 232 at 1000 ppm The sensing responses to 2-propanol,

ethanol and gasoline are 71, 50, and 37, respectively, at the

concen-tration of 1000 ppm As shown inFig 5(b), the sensing responses

of the Co-doped WO3 nanorod arrays towards acetone, toluene,

acetic acid, and heptane are 122, 95, 66, and 31, respectively These

results indicate that the Co-doped WO3nanostructure-based

sen-sor is highly sensitive to these organic gases It should be noted that

the relatively low operation temperature helps to decrease the

con-sumption of energy and can improve the suitability of the sensor

in some particular situations It is well known that WO3, as an

n-type semiconductor, is a good candidate for detecting the inorganic

gases O3, NOx, and H2S[36], but is less sensitive to hydrocarbons

Nevertheless, our investigations illustrate that doping, and control

of the morphology and size of the nanorod arrays has endowed

WO3with better sensing performance towards hydrocarbon gases

at the relatively low operation temperature of 200◦C

4 Conclusions

In summary, well-aligned Co-doped WO3double-sided nanorod

arrays have been synthesized by a facile hydrothermal method

without using any template, catalyst, or substrate An

interest-ing three-order hierarchical nanostructure, from small nanorods

via nanorod bundles to double-sided nanorod arrays, has been

observed by FE-SEM and TEM It was discovered that the

double-sided nanorod arrays are formed from urchin-like microspheres

via a self-assembly and fusion process This may provide a new

strategy for large-scale preparation of well-aligned nanorod arrays,

with the advantages of simplicity, low cost, and no introduced alien

species The Co-doped WO3 nanorod arrays are semiconducting,

with direct band gaps of 2.26 eV and 2.77 eV, and show good

sens-ing performance towards 1-butanol vapor, with rapid response and

high sensitivity The results highlight the potential applications

of the Co-doped WO3double-sided nanorod arrays in monitoring

flammable and toxic organic gases

Acknowledgements

This work was financially supported by the Australian Research

Council (ARC) through an ARC Discovery project (DP0559891)

Appendix A Supplementary data

Supplementary data associated with this article can be found, in

the online version, atdoi:10.1016/j.snb.2009.09.015

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Biographies

X.P Shen received his PhD degree in inorganic chemistry in 2005 from Nanjing

University, China He is a professor at School of Chemistry and Chemical

Engineer-ing, Jiangsu University since 2006 He is currently working as a visiting Professor at

School of Mechanical, Materials and Mechatronic Engineering, University of

Wol-longong, Australia His major research interests include nanostructured materials

and molecule-based magnetic materials.

G.X Wang received his PhD degree in Materials Science and Engineering in 2001

from University of Wollongong, Australia He currently is working as an Associate Professor at School of Mechanical, Materials and Mechatronic Engineering, Univer-sity of Wollongong His major research interests include nanostructured functional materials, materials chemistry in energy storage and conversion, and development

of chemical and biological sensors.

D Wexler received his PhD degree in Materials Science and Engineering in 1991

from Monash University, Australia He currently is working as a senior research fellow at School of Mechanical, Materials and Mechatronic Engineering, University

of Wollongong His major research interests include nanomaterials synthesis and TEM and HRTEM characterization of materials.