Conference Papers in Science

Volume 2014 (2014), Article ID 683219, 5 pages

http://dx.doi.org/10.1155/2014/683219

Conference Paper

NO₂ Sensing Properties of WO₃ Thin Films Deposited by Rf-Magnetron Sputtering

Savita Sharma,1,2 Monika Tomar,³ Nitin K. Puri,² and Vinay Gupta¹

¹Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India

²Department of Applied Physics, Delhi Technological University, Delhi 110042, India

³Physics Department, Miranda House, University of Delhi, Delhi 110007, India

Received 8 February 2014; Accepted 11 March 2014; Published 10 April 2014

Academic Editors: P. Mandal, R. K. Shivpuri, and G. N. Tiwari

This Conference Paper is based on a presentation given by Savita Sharma at “National Conference on Advances in Materials Science for Energy Applications” at “National Conference on Advances in Materials Science for Energy Applications” held from 9 January 2014 to 10 January 2014 in Dehradun, India.

Copyright © 2014 Savita Sharma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Tungsten trioxide (WO₃) thin films were deposited by Rf-magnetron sputtering onto Pt interdigital electrodes fabricated on corning glass substrates. NO₂ gas sensing properties of the prepared WO₃ thin films were investigated by incorporation of catalysts (Sn, Zn, and Pt) in the form of nanoclusters. The structural and optical properties of the deposited WO₃ thin films have been studied by X-ray diffraction (XRD) and UV-Visible spectroscopy, respectively. The gas sensing characteristics of all the prepared sensor structures were studied towards 5 ppm of NO₂ gas. The maximum sensing response of about 238 was observed for WO₃ film having Sn catalyst at a comparatively lower operating temperature of 200°C. The possible sensing mechanism has been highlighted to support the obtained results.

1. Introduction

Nitrogen dioxide (NO₂) is one of the most harmful gases to the ecosystem and provides a major contribution to air pollution [1]. The detection of NO₂ is crucial for monitoring environmental pollution resulting from combustion processes, particularly industrial emissions or vehicle exhaust [2]. Occupational Safety and Health Administration (OSHA, United States Department of Labour) declares the Permissible Exposure Limit (PEL) of NO₂ gas as 5 ppm for general industries and 20 ppm as Immediately Dangerous to Life or Health Concentrations (IDLHs) [3]. NO₂ gas is the main precursor for ozone layer depletion in lower atmosphere and also produces acid rain which is slowly damaging the ecosystem. Commercially, many NO₂ gas sensors are available in the market but they have poor sensitivity, high operating temperature, bulky size and are very costly. Thus, there is an urgent requirement of cheap, highly sensitive and selective NO₂ gas sensors which could be operated at lower operating temperature.

Gas sensors based on metal oxide semiconductors are used in a wide variety of applications including gas monitoring and detection applications [4–6]. Considerable research has been carried out on the development of chemical sensors based on semiconductor metal oxides such as SnO₂, ZnO, and TiO₂ because of their high sensitivity towards many reducing as well as oxidizing gases [7–9]. Tungsten trioxide has attractive electrical properties and reactivity to oxidizing gases making it one of the best candidates for gas sensing applications [10–12]. The tungsten trioxide (WO₃) thin films and nanostructures are seen to be an excellent candidate for NO₂ gas detection [13, 14] because the W transition metal is found to be with different oxidation states (W⁵⁺, W⁶⁺) enhancing the oxidizing power of NO₂ gas molecules onto the surface of WO₃ metal oxide. Thus, in the present work, WO₃ thin film based gas sensors have been exploited for the trace level (5 ppm) detection of NO₂ gas. Effect of different catalysts (Sn, Zn, and Pt) incorporated on the surface of SnO₂ thin film in the form of nanoclusters has also been studied for NO₂ gas detection.

2. Experimental Details

WO₃ thin film has been deposited by Rf-magnetron sputtering technique using a 2-inch W metal target (99.999% pure) in a reactive gas ambient of Ar and O₂ gas mixture. WO₃ thin films were fabricated onto the corning glass substrates patterned with Pt interdigital electrodes (IDEs). The description of fabrication of Pt IDEs has been mentioned elsewhere [9]. The typical growth conditions used for the deposition of WO₃ thin film are summarized in Table 1. WO₃ thin films were deposited under the growth pressure of 10 mTorr with argon to oxygen ratio of 60 : 40 by applying a Rf power of 50 W.

Table 1: Deposition parameters for WO₃ thin film using Rf magnetron sputtering.

Target
W metal target (99.999% pure)

Substrate
IDE/corning glass; corning glass

Target to substrate distance
6 cm

Sputtering pressure
10 mT

RF power
50 Watt

Gas composition (Ar : O₂)
60 : 40

For the formation of nanocrystalline WO₃ thin film, in situ annealing was carried out at a substrate temperature of 400°C for 1 hour in 60% Ar: 40% O₂ environment. In order to get the enhanced sensing response characteristics, Sn, Zn, and Pt metal catalysts were incorporated onto the surface of WO₃ thin film in the form of nanoclusters on separate sensors. Sn, Zn, and Pt were also deposited using sputtering technique using their respective metal targets under the sputtering pressure of 10 mT and in 100% argon gas ambient. Nanoclusters of metal catalysts were deposited using a shadow mask of uniformly distributed pores of 600 μm diameter onto the surface of WO₃ thin film. X-ray diffraction (XRD) study was carried out using Bruker D-8 X-ray diffractometer. Optical properties have been studied via UV-visible spectrophotometer (Lambda 35). WO₃ film thickness was measured using surface thickness profiler (DEKTAK 150). Gas sensing characteristics of the prepared WO₃ thin film based sensors were studied in a specially designed “Gas Sensor Measurement and Calibration System (GSMCS).” Changes in the sensor resistance were recorded after every second using a digital multimeter (Keithley 2700) interfaced with computer. The sensing response of prepared WO₃ thin film based structures was calculated using the relation: , where is the resistance of sensor in the presence of target NO₂ gas and is the resistance of the sensor in the absence of target NO₂ gas.

3. Results and Discussion

Figure 1 shows the X-ray diffraction (XRD) spectra of WO₃ thin films deposited on corning glass substrate at a substrate temperature of 400°C. The XRD spectra show the formation of orthorhombic structure (JCPDS Card no. 35-0270) of WO₃ and do not show the presence of any other secondary phase indicating the growth of single phase and polycrystalline WO₃ thin films. It is observed from Figure 1 that reflection corresponding to (002) plane is dominant and is relatively intense indicating preferential orientation of the deposited WO₃ thin film. The crystallite size calculated from the Debye Scherrer formula is found to be 17.73 nm which is relatively small leading to increase in grain boundaries suitable for gas sensing applications.

Figure 1: XRD patterns of the WO₃ thin film.

UV-visible transmission spectra of annealed WO₃ thin film deposited over corning glass substrate were measured in the wavelength range of 190 to 1100 nm and are shown in Figure 2(a). It can be seen that the film is highly transparent (88%) in the visible region showing good optical quality and low absorption losses. The presence of fringes at higher wavelength confirms that the prepared WO₃ thin film is free from any inhomogeneity. The onset of sharp fundamental absorption edge at about 310 nm was observed in the deposited WO₃ thin films (Figure 2(a)).

(a)

(b)

Figure 2: (a) UV-visible transmittance spectra of the WO₃ thin film prepared on corning glass substrate. (b) Tauc Plot of WO₃ thin film.

The Tauc plot of versus photon energy () (where is absorption coefficient, is planks constant, and is frequency of the incident radiation) of the WO₃ thin film deposited at 10 mT sputtering pressure is shown in Figure 2(b). Optical bandgap of the WO₃ thin film was calculated from the linear portion of the Tauc plot. Linear region is clearly seen in Figure 2(b) at > 3.5 eV for WO₃ thin film which is in accordance with the well-known absorption law being characteristic for direct optical transitions. Estimated value of bandgap of the WO₃ thin film is found to be 3.90 eV and is close to the reported value [15].

Figure 3 shows the variation of sensing response of all the prepared WO₃ thin film based sensor structures (WO₃, WO₃/Sn, WO₃/Zn, and WO₃/Pt) as a function of temperature towards 5 ppm of NO₂ gas. It is observed that the value of resistance of the sensors in the presence of air () increases to a higher stable value () when target NO₂ gas is inserted into chamber due to the oxidising nature of NO₂. When the NO₂ gas is flushed out of the chamber, all the fabricated sensors retain their original high resistance value ().

Figure 3: Variation of sensor response with temperature for the sensor structures: WO₃/Sn, WO₃/Zn, and WO₃/Pt towards 5 ppm of NO₂ gas.

Figures 4 and 5 show the variation of sensor resistance in the presence of air () and in the presence of NO₂ gas (), respectively, with temperature for all the prepared sensors (WO₃/Pt, WO₃/Sn, and WO₃/Zn nanoclusters). It can be observed from Figures 4 and 5 that the value of and decreases with increase in temperature. When NO₂ gas interacts with the sensor surface at a particular temperature, the value of resistance increases from to . The maximum increase in the value of is observed for WO₃/Zn nanoclusters sensor structure; however, the higher value of as compared to other sensor structures restricts the further enhancement in sensing response. Moreover, the maximum sensor response has been observed for WO₃/Sn sensor structure which may be attributed to the possible spillover of NO and O⁻ species onto the uncovered surface of WO₃ thin film. It can be observed from Figure 3 that the maximum sensing response of ~238 has been obtained for the WO₃/Sn sensor at a low operating temperature of 200°C. The obtained sensor response for all the sensors with response and recovery speeds at their respective operating temperatures is summarized in Table 2. It may be seen that the WO₃/Sn sensor exhibits comparatively high sensing response with fast response and recovery speeds at low operating temperature of 200°C. WO₃/Pt and WO₃/Zn sensors show a moderate response of 118 and 173, respectively, at high operating temperatures of 270°C and 250°C, respectively. For WO₃/Sn sensors, NO₂ gas reacts with free Sn sites as well in the similar way as with W sites and captures electrons thus giving a high sensor response [14]. NO₂ gas on interaction with Sn nanoclusters gets converted into NO and O⁻ species which spill over the WO₃ surface and captured electrons from WO₃ thin film thereby increasing sensing response.

Table 2: Sensor response, response time, and recovery time at their respective operating temperatures for the fabricated sensor structures.

Sensor
Sensor response
Operating temperature (°C)
Response time
Recovery time

WO₃/Sn
238
200
32 sec
15.3 min

WO₃/Zn
173
250
5.2 min
18 min

WO₃/Pt
118
270
4.6 min
17 min

Figure 4: Variation of sensor resistance in air () with temperature for the sensor structures: WO₃/Sn, WO₃/Zn, and WO₃/Pt.

Figure 5: Variation of sensor resistance in the presence of 5 ppm NO₂ gas with temperature for sensor structures: WO₃/Sn, WO₃/Zn, and WO₃/Pt.

4. Conclusion

The enhanced sensing response of 238 towards 5 ppm of NO₂ was obtained for WO₃/Sn sensor structure at a low operating temperature of 200°C as compared to other fabricated sensors, namely, WO₃/Zn, WO₃/Pt, and WO₃. Enhanced sensing response for WO₃/Sn sensor structure is attributed to the spillover of NO₂ species over WO₃ surface thus enhancing the adsorption of NO₂ gas on the sensor surface that leads to an increased sensor response.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are thankful to the Department of Science and Technology (DST) and GAIL India Ltd. for the financial support to carry out this work. One of the authors (Savita Sharma) is thankful to the Delhi Technological University (DTU) for the teaching assistantship.

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