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Novel hexagonal WO3 nanopowder with metal decorated carbon nanotubes as NO2 gas sensor
Available online at www.sciencedirect.com
Sensors and Actuators B 133 (2008) 151–155
Novel hexagonal WO3 nanopowder with metal decorated carbon nanotubes as NO2 gas sensor Csaba Bal´azsi a,∗ , Katar´ına Sedl´ackov´a a , Eduard Llobet b , Radu Ionescu b a
Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, Konkoly-Thege M. u´ t 29-33, 1121 Budapest, Hungary b MINOS, 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-WO3 matrix. 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 NO2 hazardous gas. In comparison, the sensitivity of hex-WO3 to NO2 was in the temperature range between 150 ◦ C and 250 ◦ C, while the hex-WO3 /MWCNTs composites were sensitive to NO2 gas 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. NO2 which results from combustion and automotive emissions [1], is very important for the environmental protection and human health. The adsorption of gases basically occurs at the surface level of a sensing film, and an increase in the active surface area of the semiconductor oxide would enhance the properties of the materials used for gas sensors. The mechanism of the electrical conductivity change of the oxide under gas exposure is understood in terms of adsorption–desorption reactions involving surface oxygen 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 WO3 powder [5,6], by radio-frequency-sputtering from metallic W [7] or WO3 targets [8] in an Ar/O2 atmosphere, by chemical vapor deposition [9] and by wet chemistry such as the sol–gel process [10].
∗
Corresponding author. E-mail address: [email protected] (C. Bal´azsi).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.02.006
In the present work, acidic precipitation is carried out by a nanocrystalline processing route; the preparation of nanocrystalline hexagonal WO3 is demonstrated. Multi-walled carbon nanotubes (MWCNTs) were added to hexagonal WO3 nanopowder with the aim to further lower the operating temperature of sensors [11]. The carbon nanotubes were decorated with metallic nanoclusters (Au and Ag) in order to obtain an improved sensitivity to NO2 at room temperature [12]. The gas sensing properties of the hex-WO3 and hybrid hex-WO3 /MWCNTs composites were tested in the presence of very low concentrations of NO2 (nitrogen dioxide). 2. Experimental 2.1. Preparation of hexagonal WO3 nanopowder Tungstic acid samples were prepared by acidic precipitation from sodium tungstate solution according to Zocher method [13]. 10.5 g Na2 WO4 ·2H2 O was dissolved in water and a hydrochloric acid solution 18% in excess of equimolar reaction was added to this at a reaction temperature not higher than 5 ◦ C. The resulted centrifuged H2 WO4 ·H2 O precipitates were dispersed in water again and were passed to high-temperature
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Fig. 1. TEM images of WO3 nanopowder. (a) Orthorhombic WO3 1/3H2 O “mother” phase, and (b) hexagonal WO3 phase.
treatment in an autoclave at 125 ± 5 ◦ C. The resulted powder was dried in a desiccator, and after that was heat treated (annealing 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 The MWCNTs were obtained from Mercorp [16]. They were prepared by arc discharge without use of catalysts. The MWCNTs 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, respectively [17].
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 dm3 test 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 injections, 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.
2.3. Sensors fabrication 3. Results and discussion A drop coating method was employed for depositing the sensing 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 nanoclusters were dissolved in glycerol (700 mg in 1 ml), and then a precise amount of hex-WO3 nanopowder was added in order 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 micromachined 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 carried out in air, and it ensured a good adherence of the deposited materials to the sensor substrates.
The TEM analysis of tungsten oxide powders prepared from the Zocher type tungstic acid gel confirmed the change in the phases. In Fig. 1a is shown the structure of WO3 1/3 H2 O “mother” phase as obtained after hydrothermal preparation. The selected area electron diffraction (SAED) of nanopowder shows the orthorhombic phase of WO3 1/3 H2 O crystallites. The average size of WO3 1/3 H2 O crystallites is ∼80–100 nm. A dehydration process (WO3 1/3 H2 O to WO3 ) was taking place during calcination of powders (at 330 ◦ C, 90 min, air). This dehydration was accompanied by a structural change; from orthorhombic WO3 1/3H2 O hex-WO3 was obtained. The electron 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-WO3 crystallites is ∼50–100 nm.
2.4. Experimental techniques
3.2. Structural properties of WO3 /MWCNTs
The structural characterizations of the samples were investigated 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 TEM images recorded on the novel hex-WO3 /metal decorated MWCNTs composites (Figs. 2 and 3, for MWCNTs decorated with Ag and Au nanoparticles, respectively) show a fair good dispersion of both materials. This achieve-
3.1. Structural properties of WO3 nanopowder
C. Bal´azsi et al. / Sensors and Actuators B 133 (2008) 151–155
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Fig. 2. TEM images of hexagonal WO3 with Ag decorated MWCNTs. (a) TEM image of hexagonal WO3 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-WO3 were investigated. Fig. 4a shows the response of a hex-WO3 sensor to 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 conductivity change drastically decreased (more than a factor of four at 150 ◦ C, while at room temperature it lose completely its property to sense NO2 in the concentration range up to 1 ppm). In the next step, the sensing characteristics of the novel composite materials were also studied. Fig. 4b shows the response of a hex-WO3 /Au-decorated MWCNTs sensor to increasing concentration of NO2 . This response, recorded for the sensor
operated at room temperature, demonstrates that the addition of a small amount of MWCNTs to the original hex-WO3 matrix can improve the sensing potential in terms of room temperature NO2 detection. 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 hex-WO3 matrix plays a crucial role, and it depends on 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 Agdecorated MWCNTs (not shown) and 1:500 for Au-decorated MWCNTs (Fig. 4b), respectively. Other weight ratios made the hybrid materials to loose their property to sense NO2 at room temperature. The addition of carbon nanotubes to hex-WO3 modifies furthermore the semiconducting characteristics of the active layer of sensors. While hex-WO3 is an n-type semiconductor, the hybrid hex-WO3 /MWCNTs behaves as a p-type material (similarly to the carbon nanotubes), as its resistance decreases under NO2 (i.e., oxidizing gas) exposure (see Fig. 4b).
Fig. 3. TEM images of hexagonal WO3 with Au decorated MWCNTs. (a) TEM image of hexagonal WO3 nanograins and MWCNTs, and (b) detail of Au decorated carbon nanotubes.
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Fig. 4. Resistance change experienced by the gas sensors exposed to increasing concentrations of NO2 . (a) h-WO3 film operated at 250 ◦ C, and (b) Au-MWCNTs/hexWO3 (1/500 wt%) film operated at room temperature.
4. Conclusion Hexagonal tungsten oxide nanopowders were successfully prepared by acidic precipitation from a sodium tungstate solution. TEM analysis of nanopowders showed that the average size of hexagonal nanoparticles was 50–100 nm. Chemical gas sensors based on hex-WO3 operating at 250 ◦ C showed a good potential to detect very low amounts of NO2 , but they significantly lose this sensing characteristic at lower operating temperatures, while at room temperature they could not detect NO2 at all. The elaboration of a new gas sensitive composite materials technology was afterwards reported. A new approach was introduced 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-WO3 matrix. 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 Universitaires Notre Dame de la Paix, Namur, Belgium, for providing us the metal decorated carbon nanotubes. References [1] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors–a comprehensive review, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111–188.
[2] M. Gillet, R. Delamare, E. Gillet, Growth, structure and electrical conduction of WO3 nanorods, Appl. Surf. Sci. 254 (2007) 270– 273. [3] M. Stankova, X. Vilanova, E. Llobet, J. Calderer, C. Bittencourt, J.J. Pireaux, X. Correig, Influence of the annealing and operating temperatures on the gas-sensing properties of rf sputtered WO3 thin-film sensors, Sens. Actuators B Chem. 105 (2005) 271–277. [4] Y.G. Choi, G. Sakai, K. Shimanoe, N. Yamazoe, Wet process-based fabrication of WO3 thin film for NO2 detection, Sens. Actuators B Chem. 101 (2004) 107–111. [5] A. Al Mohammed, M. Gillet, Phase transformations in WO3 thin films during annealing, Thin Solid Films 408 (2002) 302– 309. [6] M. Akiyama, Z. Zhang, J. Tamaki, N. Miura, N. Yamazoe, T. Harada, Tungsten oxide based semiconductor sensor for detection of nitrogen oxides in combustion exhaust, Sens. Actuators B Chem. 13–14 (1993) 619– 620. [7] Z. Xu, J.F. Vetelino, R. Lee, D.C. Parker, Electrical properties of tungsten trioxide films, J. Vac. Sci. Technol. A8 (1990) 3434– 3438. [8] H. Kaneko, S. Nishimoto, K. Miyake, N. Suedomi, Physical and electrochemichromic properties of RF sputtered tungsten oxide films, J. Appl. Phys. 59 (1986) 2526–2534. [9] S. Pal, C. Jacob, The influence of substrate temperature variation on tungsten oxide thin film growth in an HFCVD system, Appl. Surf. Sci. 253 (2007) 3317–3325. [10] J. Shieh, H.M. Feng, M.H. Hon, H.Y. Juang, WO3 and W–Ti–O thin-film gas sensors prepared by sol–gel dip-coating, Sens. Actuators B Chem. 86 (2002) 75–80. [11] E.H. Espinosa, R. Ionescu, B. Chambon, G. Bedis, E. Sotter, C. Bittencourt, A. Felten, J.-J. Pireaux, X. Correig, E. Llobet, Hybrid metal oxide and multiwall carbon nanotube films for low temperature gas sensing, Sens. Actuators B Chem. 127 (2007) 137–142. [12] E.H. Espinosa, R. Ionescu, C. Bittencourt, A. Felten, R. Erni, G. Vantendeloo, J.-J. Pireaux, E. Llobet, Metal-decorated multi-wall carbon nanotubes for low temperature gas sensing, Thin Solid Films 515 (2007) 8322– 8327. [13] M. Yoshimura, W.L. Suchanek, K. Byrappa, Soft solution processing: a strategy for one-step processing of advanced inorganic materials, MRS Bull. 25 (9) (2000) 17–25. [14] Cs. Balazsi, J. Pfeifer, Structure and morphology changes caused by wash treatment of tungstic acid precipitates, Solid State Ionics 124 (1999) 73– 81. [15] Cs. Balazsi, in: R. Kuzel, J. Fiala, Z. Weiss (Eds.), Materials Structure, Bulletin of the Czech and Slovak Crystallographic Association, vol. 6, no. 2, 1999, pp. 135– 139, Prague. [16] http://www.mercorp.com. [17] A. Felten, C. Bittencourt, J.-F. Colomer, G. Van Tendeloo, J.-J. Pireaux, Nucleation of metal clusters on plasma treated multi wall carbon nanotubes, Carbon 45 (2007) 110–116.
C. Bal´azsi et al. / Sensors and Actuators B 133 (2008) 151–155 [18] D.G. Rickerby, N. W¨atchter, M.C. Horrillo, J. Guti´errez, I. Garcia, C. Can´e, Structural and dimensional control in micromachined integrated solid state gas sensors, Sens. Actuators B 69 (2000) 314–319. [19] C. Bittencourt, A. Felten, E.H. Espinosa, R. Ionescu, E. Llobet, X. Correig, J.-J. Pireaux, WO3 films modified with functionalized multi-wall carbon nanotubes: morphological, compositional and gas response studies, Sens. Actuators B 115 (2006) 33–41. [20] R. Ionescu, E.H. Espinosa, R. Leghrib, A. Felten, J.J. Pireaux, R. Erni, G. Van Tendeloo, C. Bittencourt, N. Canellas, E. Llobet, Novel hybrid materials for gas sensing applications made of metal-decorated MWCNTs dispersed on nano-particle metal oxides, Sens. Actuators B 131 (2008) 174–182. [21] http://daq.state.nc.us/rules/rules/D0407.pdf.
Biographies Csaba Bal´azsi received his PhD in 2000 from the University Miskolc, Hungary. 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
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nanocomposites for high temperature (Si3 N4 /CNT), sensor (hex-WO3 ) and bioapplications (hydroxyapatite). Katar´ına Sedl´ackov´a graduated as material engineer from the Slovak Technical University, Bratislava in 2000. She received her PhD in 2002 from the same university. She is currently deputy head of the Thin Film Physics Department at Research Institute for Technical Physics and Materials Science, Budapest, Hungary. Her main areas of interest are the carbon based nanocomposites and their structural characterization by transmission electron microscopy. Eduard Llobet graduated in telecommunication engineering from the Universitat Polit`ecnica de Catalunya (UPC), Barcelona, Spain in 1991, and received his PhD in 1997 from the same University. He is an associate professor of electronics at the University Rovira i Virgili (Tarragona, Spain). His main research 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.
Sensors and Actuators B 133 (2008) 151–155
Novel hexagonal WO3 nanopowder with metal decorated carbon nanotubes as NO2 gas sensor Csaba Bal´azsi a,∗ , Katar´ına Sedl´ackov´a a , Eduard Llobet b , Radu Ionescu b a
Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, Konkoly-Thege M. u´ t 29-33, 1121 Budapest, Hungary b MINOS, 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-WO3 matrix. 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 NO2 hazardous gas. In comparison, the sensitivity of hex-WO3 to NO2 was in the temperature range between 150 ◦ C and 250 ◦ C, while the hex-WO3 /MWCNTs composites were sensitive to NO2 gas 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. NO2 which results from combustion and automotive emissions [1], is very important for the environmental protection and human health. The adsorption of gases basically occurs at the surface level of a sensing film, and an increase in the active surface area of the semiconductor oxide would enhance the properties of the materials used for gas sensors. The mechanism of the electrical conductivity change of the oxide under gas exposure is understood in terms of adsorption–desorption reactions involving surface oxygen 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 WO3 powder [5,6], by radio-frequency-sputtering from metallic W [7] or WO3 targets [8] in an Ar/O2 atmosphere, by chemical vapor deposition [9] and by wet chemistry such as the sol–gel process [10].
∗
Corresponding author. E-mail address: [email protected] (C. Bal´azsi).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.02.006
In the present work, acidic precipitation is carried out by a nanocrystalline processing route; the preparation of nanocrystalline hexagonal WO3 is demonstrated. Multi-walled carbon nanotubes (MWCNTs) were added to hexagonal WO3 nanopowder with the aim to further lower the operating temperature of sensors [11]. The carbon nanotubes were decorated with metallic nanoclusters (Au and Ag) in order to obtain an improved sensitivity to NO2 at room temperature [12]. The gas sensing properties of the hex-WO3 and hybrid hex-WO3 /MWCNTs composites were tested in the presence of very low concentrations of NO2 (nitrogen dioxide). 2. Experimental 2.1. Preparation of hexagonal WO3 nanopowder Tungstic acid samples were prepared by acidic precipitation from sodium tungstate solution according to Zocher method [13]. 10.5 g Na2 WO4 ·2H2 O was dissolved in water and a hydrochloric acid solution 18% in excess of equimolar reaction was added to this at a reaction temperature not higher than 5 ◦ C. The resulted centrifuged H2 WO4 ·H2 O precipitates were dispersed in water again and were passed to high-temperature
152
C. Bal´azsi et al. / Sensors and Actuators B 133 (2008) 151–155
Fig. 1. TEM images of WO3 nanopowder. (a) Orthorhombic WO3 1/3H2 O “mother” phase, and (b) hexagonal WO3 phase.
treatment in an autoclave at 125 ± 5 ◦ C. The resulted powder was dried in a desiccator, and after that was heat treated (annealing 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 The MWCNTs were obtained from Mercorp [16]. They were prepared by arc discharge without use of catalysts. The MWCNTs 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, respectively [17].
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 dm3 test 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 injections, 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.
2.3. Sensors fabrication 3. Results and discussion A drop coating method was employed for depositing the sensing 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 nanoclusters were dissolved in glycerol (700 mg in 1 ml), and then a precise amount of hex-WO3 nanopowder was added in order 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 micromachined 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 carried out in air, and it ensured a good adherence of the deposited materials to the sensor substrates.
The TEM analysis of tungsten oxide powders prepared from the Zocher type tungstic acid gel confirmed the change in the phases. In Fig. 1a is shown the structure of WO3 1/3 H2 O “mother” phase as obtained after hydrothermal preparation. The selected area electron diffraction (SAED) of nanopowder shows the orthorhombic phase of WO3 1/3 H2 O crystallites. The average size of WO3 1/3 H2 O crystallites is ∼80–100 nm. A dehydration process (WO3 1/3 H2 O to WO3 ) was taking place during calcination of powders (at 330 ◦ C, 90 min, air). This dehydration was accompanied by a structural change; from orthorhombic WO3 1/3H2 O hex-WO3 was obtained. The electron 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-WO3 crystallites is ∼50–100 nm.
2.4. Experimental techniques
3.2. Structural properties of WO3 /MWCNTs
The structural characterizations of the samples were investigated 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 TEM images recorded on the novel hex-WO3 /metal decorated MWCNTs composites (Figs. 2 and 3, for MWCNTs decorated with Ag and Au nanoparticles, respectively) show a fair good dispersion of both materials. This achieve-
3.1. Structural properties of WO3 nanopowder
C. Bal´azsi et al. / Sensors and Actuators B 133 (2008) 151–155
153
Fig. 2. TEM images of hexagonal WO3 with Ag decorated MWCNTs. (a) TEM image of hexagonal WO3 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-WO3 were investigated. Fig. 4a shows the response of a hex-WO3 sensor to 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 conductivity change drastically decreased (more than a factor of four at 150 ◦ C, while at room temperature it lose completely its property to sense NO2 in the concentration range up to 1 ppm). In the next step, the sensing characteristics of the novel composite materials were also studied. Fig. 4b shows the response of a hex-WO3 /Au-decorated MWCNTs sensor to increasing concentration of NO2 . This response, recorded for the sensor
operated at room temperature, demonstrates that the addition of a small amount of MWCNTs to the original hex-WO3 matrix can improve the sensing potential in terms of room temperature NO2 detection. 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 hex-WO3 matrix plays a crucial role, and it depends on 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 Agdecorated MWCNTs (not shown) and 1:500 for Au-decorated MWCNTs (Fig. 4b), respectively. Other weight ratios made the hybrid materials to loose their property to sense NO2 at room temperature. The addition of carbon nanotubes to hex-WO3 modifies furthermore the semiconducting characteristics of the active layer of sensors. While hex-WO3 is an n-type semiconductor, the hybrid hex-WO3 /MWCNTs behaves as a p-type material (similarly to the carbon nanotubes), as its resistance decreases under NO2 (i.e., oxidizing gas) exposure (see Fig. 4b).
Fig. 3. TEM images of hexagonal WO3 with Au decorated MWCNTs. (a) TEM image of hexagonal WO3 nanograins and MWCNTs, and (b) detail of Au decorated carbon nanotubes.
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Fig. 4. Resistance change experienced by the gas sensors exposed to increasing concentrations of NO2 . (a) h-WO3 film operated at 250 ◦ C, and (b) Au-MWCNTs/hexWO3 (1/500 wt%) film operated at room temperature.
4. Conclusion Hexagonal tungsten oxide nanopowders were successfully prepared by acidic precipitation from a sodium tungstate solution. TEM analysis of nanopowders showed that the average size of hexagonal nanoparticles was 50–100 nm. Chemical gas sensors based on hex-WO3 operating at 250 ◦ C showed a good potential to detect very low amounts of NO2 , but they significantly lose this sensing characteristic at lower operating temperatures, while at room temperature they could not detect NO2 at all. The elaboration of a new gas sensitive composite materials technology was afterwards reported. A new approach was introduced 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-WO3 matrix. 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 Universitaires Notre Dame de la Paix, Namur, Belgium, for providing us the metal decorated carbon nanotubes. References [1] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors–a comprehensive review, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111–188.
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C. Bal´azsi et al. / Sensors and Actuators B 133 (2008) 151–155 [18] D.G. Rickerby, N. W¨atchter, M.C. Horrillo, J. Guti´errez, I. Garcia, C. Can´e, Structural and dimensional control in micromachined integrated solid state gas sensors, Sens. Actuators B 69 (2000) 314–319. [19] C. Bittencourt, A. Felten, E.H. Espinosa, R. Ionescu, E. Llobet, X. Correig, J.-J. Pireaux, WO3 films modified with functionalized multi-wall carbon nanotubes: morphological, compositional and gas response studies, Sens. Actuators B 115 (2006) 33–41. [20] R. Ionescu, E.H. Espinosa, R. Leghrib, A. Felten, J.J. Pireaux, R. Erni, G. Van Tendeloo, C. Bittencourt, N. Canellas, E. Llobet, Novel hybrid materials for gas sensing applications made of metal-decorated MWCNTs dispersed on nano-particle metal oxides, Sens. Actuators B 131 (2008) 174–182. [21] http://daq.state.nc.us/rules/rules/D0407.pdf.
Biographies Csaba Bal´azsi received his PhD in 2000 from the University Miskolc, Hungary. 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
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nanocomposites for high temperature (Si3 N4 /CNT), sensor (hex-WO3 ) and bioapplications (hydroxyapatite). Katar´ına Sedl´ackov´a graduated as material engineer from the Slovak Technical University, Bratislava in 2000. She received her PhD in 2002 from the same university. She is currently deputy head of the Thin Film Physics Department at Research Institute for Technical Physics and Materials Science, Budapest, Hungary. Her main areas of interest are the carbon based nanocomposites and their structural characterization by transmission electron microscopy. Eduard Llobet graduated in telecommunication engineering from the Universitat Polit`ecnica de Catalunya (UPC), Barcelona, Spain in 1991, and received his PhD in 1997 from the same University. He is an associate professor of electronics at the University Rovira i Virgili (Tarragona, Spain). His main research 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.