Pad’sos Catalans 26, 43007 Tarragona, Spain Received 9 November 2007; received in revised form 4 February 2008; accepted 5 February 2008 Available online 12 February 2008 Abstract In thi
Trang 1Csaba Bal´azsi , Katar´ına Sedl´ackov´a , Eduard Llobet , Radu Ionescu
aResearch Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, Konkoly-Thege M ´ut 29-33,
1121 Budapest, Hungary
bMINOS, Department d’Enginyeria Electronica, Universitat Rovira i Virgili, Av Pad’sos Catalans 26, 43007 Tarragona, Spain
Received 9 November 2007; received in revised form 4 February 2008; accepted 5 February 2008
Available online 12 February 2008
Abstract
In this work, hexagonal tungsten oxide (hex-WO3) nanopowders were prepared by acidic precipitation from a sodium tungstate solution TEM analysis of nanopowders showed that the average size of the hexagonal nanoparticles was 50–100 nm Novel hybrid composites were fabricated
by embedding a low amount of carbon nanotubes into the hex-WO3matrix Metallic nanoclusters (Ag, Au) were added to the carbon nanotubes for improving the gas sensing properties of the films The addition of MWCNTs lowered the temperature range of sensitivity of the hex-WO3
nanocomposites to NO2hazardous gas In comparison, the sensitivity of hex-WO3to NO2was in the temperature range between 150◦C and 250◦C, while the hex-WO3/MWCNTs composites were sensitive to NO2gas at room temperature
© 2008 Elsevier B.V All rights reserved
Keywords: Hexagonal WO3 ; Carbon nanotube; Sensing properties; TEM
1 Introduction
Detection of hazardous gases, e.g NO2which results from
combustion and automotive emissions[1], is very important for
the environmental protection and human health The
adsorp-tion of gases basically occurs at the surface level of a sensing
film, and an increase in the active surface area of the
semi-conductor oxide would enhance the properties of the materials
used for gas sensors The mechanism of the electrical
conduc-tivity change of the oxide under gas exposure is understood in
terms of adsorption–desorption reactions involving surface
oxy-gen vacancies[2] Among of other candidates, tungsten oxides
have been commonly applied as sensing layers for hazardous
gas detection[3,4] Various crystalline forms of tungsten oxides
can be prepared by thermal evaporation of WO3powder[5,6],
by radio-frequency-sputtering from metallic W[7]or WO3
tar-gets[8]in an Ar/O2atmosphere, by chemical vapor deposition
[9]and by wet chemistry such as the sol–gel process[10]
∗Corresponding author.
E-mail address:balazsi@mfa.kfki.hu (C Bal´azsi).
In the present work, acidic precipitation is carried out by a nanocrystalline processing route; the preparation of
nanopow-der with the aim to further lower the operating temperature of sensors[11] The carbon nanotubes were decorated with metal-lic nanoclusters (Au and Ag) in order to obtain an improved
composites were tested in the presence of very low concentra-tions of NO2(nitrogen dioxide)
2 Experimental
2.1 Preparation of hexagonal WO 3 nanopowder
Tungstic acid samples were prepared by acidic precipitation from sodium tungstate solution according to Zocher method
hydrochloric acid solution 18% in excess of equimolar reac-tion was added to this at a reacreac-tion temperature not higher than
5◦C The resulted centrifuged H
2WO4·H2O precipitates were dispersed in water again and were passed to high-temperature
0925-4005/$ – see front matter © 2008 Elsevier B.V All rights reserved.
doi: 10.1016/j.snb.2008.02.006
Trang 2Fig 1 TEM images of WO 3 nanopowder (a) Orthorhombic WO 3 1/3H 2 O “mother” phase, and (b) hexagonal WO 3 phase.
treatment in an autoclave at 125± 5◦C The resulted powder was
dried in a desiccator, and after that was heat treated
(anneal-ing at ∼330◦C for 90 min in ambient air) The details of the
preparation are described in our previous papers[14,15]
2.2 Metal decoration of the carbon nanotubes
prepared by arc discharge without use of catalysts The
MWC-NTs powder presents 99% of carbon with 30–40% nanotube
content Subsequently, the nanotubes were modified with Au or
Ag nanoclusters by thermally evaporating gold or silver atoms
onto the MWCNTs surface from a gold or silver wire,
respec-tively[17]
2.3 Sensors fabrication
A drop coating method was employed for depositing the
sens-ing materials onto silicon based microhotplates Full details on
these sensor substrates can be found in paper[18] To prepare
the deposition paste, MWCNTs decorated with the metallic
nan-oclusters were dissolved in glycerol (700 mg in 1 ml), and then
to obtain the desired proportions of CNT/WO3(i.e 1/250 and
1/500 wt% for Ag- and Au-decorated CNTs, respectively) The
mass ratios selected were based on previous studies[11,19,20]
The dispersion and adequate mixture of the components were
enhanced by stirring the solution in an ultrasonic bath for 2 h at
75◦C The resulting paste was then dropped onto the
microma-chined silicon membranes by using a microinjector (JBE1113
Dispenser, I&J FISNAR Inc., USA) The deposited films were
dried at 170◦C for 1 h in order to burn out the organic vehicle,
and finally annealed at 400◦C for 2 h This process was
car-ried out in air, and it ensured a good adherence of the deposited
materials to the sensor substrates
2.4 Experimental techniques
The structural characterizations of the samples were
inves-tigated by transmission electron microscopy (TEM) TEM
analysis and selected area electron diffraction (SAED) were
carried out on a Philips CM-20 microscope operating at 200 kV
The gas sensing properties of the hybrid composite films were tested in the presence of very low concentrations of NO2 To perform the measurements, the gas sensors were placed inside
a 5.3 dm3test chamber, and the desired concentrations of NO2
(ranging from 100 ppb to 1 ppm) were introduced by the direct injection method using a gas-tight chromatographic syringe A fan was employed to provide the homogeneity of gas diffusion inside the test chamber After each series of successive injec-tions, the sensor chamber was flushed using pure dry air for
2 h, which ensured the cleaning of both the chamber and the sensor surface During this process, the sensors were heated at
250◦C in order to speed up gas desorption An Agilent 34970A
multimeter was used for continuously monitoring the electrical resistance of the sensors during the measurement process The data acquired were stored in a PC for further analysis
3 Results and discussion
3.1 Structural properties of WO 3 nanopowder
The TEM analysis of tungsten oxide powders prepared from the Zocher type tungstic acid gel confirmed the change in the
“mother” phase as obtained after hydrothermal preparation The selected area electron diffraction (SAED) of nanopowder shows the orthorhombic phase of WO31/3 H2O crystallites The aver-age size of WO31/3 H2O crystallites is∼80–100 nm
tak-ing place durtak-ing calcination of powders (at 330◦C, 90 min, air).
This dehydration was accompanied by a structural change; from
elec-tron diffraction of heat treated sample confirmed the hexagonal phase of WO3 (Fig 1b) From TEM analysis, the crystalline derivative consists of aggregates (∼500 nm) of rods and the average size of hex-WO3crystallites is∼50–100 nm
3.2 Structural properties of WO 3 /MWCNTs
MWC-NTs decorated with Ag and Au nanoparticles, respectively) show a fair good dispersion of both materials This
Trang 3achieve-Fig 2 TEM images of hexagonal WO 3 with Ag decorated MWCNTs (a) TEM image of hexagonal WO 3 nanograins and MWCNTs, and (b) detail of Ag decorated carbon nanotubes.
ment allows for envisaging the possibility of these new hybrid
material composites to combine the sensing properties of the two
components
3.3 Gas sensing properties
At first, the gas sensing properties of hex-WO3were
increasing concentration of NO2 This result was recorded for
the sensor operated at 250◦C When the operating temperature
was lowered below 250◦C (in this paper not shown), the
con-ductivity change drastically decreased (more than a factor of
four at 150◦C, while at room temperature it lose completely
1 ppm)
In the next step, the sensing characteristics of the novel
com-posite materials were also studied.Fig 4b shows the response
concentration of NO2 This response, recorded for the sensor
operated at room temperature, demonstrates that the addition of
can improve the sensing potential in terms of room temperature
NO2detection On the other hand, no response was obtained by these sensors operated at higher temperatures up to 250◦C.
Importantly, the quantity of MWCNTs embedded into the
type of metal used to decorate the carbon nanotubes Thus, the optimal weight ratio was found to be 1:250 in the case of Ag-decorated MWCNTs (not shown) and 1:500 for Au-Ag-decorated MWCNTs (Fig 4b), respectively Other weight ratios made the hybrid materials to loose their property to sense NO2at room temperature
The addition of carbon nanotubes to hex-WO3modifies fur-thermore the semiconducting characteristics of the active layer
(simi-larly to the carbon nanotubes), as its resistance decreases under
NO2(i.e., oxidizing gas) exposure (seeFig 4b)
Fig 3 TEM images of hexagonal WO 3 with Au decorated MWCNTs (a) TEM image of hexagonal WO 3 nanograins and MWCNTs, and (b) detail of Au decorated carbon nanotubes.
Trang 4Fig 4 Resistance change experienced by the gas sensors exposed to increasing concentrations of NO 2 (a) h-WO 3 film operated at 250 ◦C, and (b)
Au-MWCNTs/hex-WO 3 (1/500 wt%) film operated at room temperature.
4 Conclusion
Hexagonal tungsten oxide nanopowders were successfully
prepared by acidic precipitation from a sodium tungstate
solu-tion TEM analysis of nanopowders showed that the average
size of hexagonal nanoparticles was 50–100 nm Chemical gas
sig-nificantly lose this sensing characteristic at lower operating
temperatures, while at room temperature they could not detect
NO2at all
The elaboration of a new gas sensitive composite materials
technology was afterwards reported A new approach was
intro-duced when room temperatures detection of hazardous gases
was desired, consisting in embedding a low amount of metal
decorated (Ag, Au) carbon nanotubes into the hex-WO3matrix
Indeed, the new fabricated hybrid material composites were able
to detect as low as 100 ppb of NO2, with no need to heat the
sensor substrates during operation
Thus, the main achievement that we report in our work is
the creation of active films sensitive to NO2 at low operating
temperatures The detected concentration level is very close to
the ambient air quality standard for nitrogen dioxide established
by the Department of Environment and Natural Resources, USA
(i.e 53 ppb[21]), which demonstrates the high potential of our
new gas sensors
Acknowledgements
The work was supported by the bilateral NSF-OTKA-MTA
co-operation, contract no MTA: 96 OTKA: 049953 R Ionescu
acknowledges a ‘Juan de la Cierva’ research fellowship funded
by the Spanish Ministry for Science and Education The authors
are grateful to A Felten and J.J Pireaux from Falcult´es
Univer-sitaires Notre Dame de la Paix, Namur, Belgium, for providing
us the metal decorated carbon nanotubes
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Trang 5Csaba Bal´azsi received his PhD in 2000 from the University Miskolc,
Hun-gary He is currently Head of the Ceramics and Nanocomposites Department
at Research Institute for Technical Physics and Materials Science, Budapest,
Hungary His work focuses on the ceramic nanocomposites, mainly on ceramic
interests are in the fabrication and modelling of semiconductor gas sensors and
in the applications of intelligent systems to complex odour analysis.
Radu Ionescu is a postdoctoral research fellow at the Department of Electronics,
Electrical and Automatic Engineering, Rovira i Virgili University, Tarragona, Spain His main research interests are in the field of chemical gas sensors, carbon nanotubes and pattern recognition.