Available online at www.sciencedirect.com
Procedia Engineering 47 (2012) 116 – 119
Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
Visible light activated tungsten oxide sensors for NO2 detection at room temperature Chao Zhanga,*, Abdelhamid Boudibaa, Carla Bittencourtb, Rony Snydersb,c, Marie-Georges Oliviera,c, Marc Debliquya a
Service de Science des Matériaux, Faculté Polytechnique, University of Mons, Mons, Belgium b Chimie des Interactions Plasma-Surfaces ,CIRMAP, University of Mons, Mons, Belgium c Materia Nova Research Center, Mons, Belgium
Abstract The sensing characteristics of tungsten oxide (WO3) gas sensors under visible light illumination have been investigated at room temperature. Sensing measurements were performed on sputtered (thin-film) and screen-printed (thick film) WO3 layers towards sub-ppm NO2. Light sources with different wavelength were employed to illuminate the WO3 sensors. The results showed that the light illumination effectively improved the NO2 sensing properties and the 475 nm light (blue colour) was the most effective for the WO3 sensors. © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense © 2012 Sp. z.o.o. Published by Elsevier Ltd.
Keywords: tungsten oxide; room temperature; NO2 sensors, light illumination
1. Introduction Semiconductor metal oxide gas sensors are often operated at elevated temperatures (100~400°C) in order to accelerate chemical reactions between the metal oxide surface and target gas molecules. Working at a high temperature implies expensive sensor architectures, difficulties in maintaining stable sensor response and high-power consumption. In recent years, there have been reports of gas sensors based on the photo-activation of metal oxide semiconductors [1, 2]. Illuminating these sensors with ultra-violet (UV) light is a feasible alternative to activate chemical reactions at oxide surface without the necessity of heating. Nevertheless, UV-sources are power-hungry and are expensive. Compared to UV-sources,
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1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. doi:10.1016/j.proeng.2012.09.098
Chao Zhang et al. / Procedia Engineering 47 (2012) 116 – 119
visible-sources are inexpensive and energy saving. Therefore, visible light is a potential candidate to substitute UV light to activate semiconductor metal oxide gas sensors. Among metal-oxides, tungsten oxide (WO3) is a good candidate for low concentration NO2 sensors [3]. The WO3 sensing layers usually work at high temperature to get rapid response. As reported in a previous work [3], the WO3 sensors prepared by atmospheric plasma spray need to be heated at about 200°C in order to get an acceptable response and recovery time. In this work, sensors are activated by visible light illumination. We show that the sensing characteristics of the WO3 sensors under blue light at room temperature are comparable to the ones operated at 200°C. The effect of illumination using different wavelength lights on the sensing of NO2 will be discussed. 2. Experimental method Commercial WO3 powder was used for producing screen-printed thick films (20 μm). Thin WO3 films (0.5 μm) were deposited by radio-frequency sputtering using pressed WO3 powder at the target to deposit thin films. Both the thin and thick films were prepared on Al2O3 substrates equipped with Au interdigital electrodes. After the deposition, the sensors were heat-treated at 700°C for 1 hour to get a stable film structure. The surface morphologies of the films were inspected using scanning electron microscopy. The phase constitutions of the films were measured by X-ray diffraction using a CuKα radiation. 2θ scanning rates of 1°·min-1 for the range from 10 to 90° while 0.02°·min-1 for the range from 22.5 to 22.5° were used during test. Light illumination and gas sensing test were performed inside a sealed plastic chamber as shown in Fig. 1. Different wavelength (purple, blue, green and orange as well as ultra violet) bulbs which are made up of 20 LEDs are used. The distance between the bulb and WO3 films is fixed at 45mm. The sensors were connected to a tailor-made system to measure electrical resistance. The NO2 gas with a concentration of 160/320 ppb was used as a target gas, which was calibrated by NOx gas analyser at the outlet of the chamber. The moisture level was controlled to be 50% at 25°C (by bubbling in deionized water at 20°C). The gas flow rate is always 1 l/min. The sensors were operated at ambient temperature (at about 20±5°C).
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Fig. 1. Scheme of the test chamber and WO3 sensor structure
3. Results and discussion The XRD results show that, after the heat-treatment at 700°C for 1 h, the films presented wellcrystallized WO3 phase structure (The XRD patterns were not listed here). Fig. 2 and 3 show the surface
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morphologies of WO3 films. In the screen-printed thick films showed high porosity, and this is an advantage for metal oxide gas sensors.
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Fig. 2. SEM images of screen-printed WO3 film (a) low resolution; (b) high resolution.
Fig. 3 presents the surface morphologies of Al2O3 substrate and sputtered WO3 thin film on Al2O3 substrate. As the thickness of film is only 400 nm, the surface morphology was determined by the substrate structure. However, it is easy to observe WO3 grains from Fig. 3(b).
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Fig. 3. SEM images of r.f. sputtered WO3 film on Al2O3 substrate (a) low resolution; (b) high resolution.
Fig. 4 shows the variation of film electrical resistances versus 160 and 320 ppb NO2 under different illumination (orange, green, blue and purple bulbs). For comparison, a UV bulb was also utilised. The resistances of the films decreased for decreasing the light wavelength. The light illumination creates lightgenerated oxygen ions O2− (hν) weakly bound to WO3 inducing the formation of a small depletion region [4]. When the rate of oxygen adsorption and desorption reaches an equilibrium, the thickness of the depletion layer and the conductivity of WO3 gradually achieve a stable state. Afterwards, the film resistance is again stable in air under light illumination. The sensors present different response and response time towards NO 2 depending on the wavelength of the used illumination light. The green and purple light produce the highest response while the shortest response time was observed for the blue light. The WO3 sensor illuminated by blue light showed the same response characteristics of the WO3 when heated at 200°C. Thus, we can suggest that illumination with blue light is promising method to activate WO3 sensors at room temperature.
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Fig. 4: Electrical resistance of (a) screen-printed and (b) r.f. sputtered WO3 films versus 160 and 320 ppb NO2
4. Conclusions We showed that WO3 films prepared by screen-printing and sputtering had their response characteristics to sub-ppm NO2 at room temperature improved when they were illuminated with visible light. The baseline resistance of the WO3 sensors was decreased for decreasing light wavelength. The best response and recovery time to NO2 was obtained when the blue light is used. Further work will be performed with varying the fluence of the light and gas humidity. The effect of the illumination on surface reactions will be considered. Acknowledgements This work was carried out in the framework of the “Cold Plasma” ARC project of the University of Mons, financially supported by the Communauté Française de Belgique and also in the framework of the Opti2mat “Programme d’Excellence” supported by the Walloon Region of Belgium. CB and RS thank the support of MP901 (NanoTP). References [1] Comini E, Faglia G, Sberveglieri G. UV light activation of tin oxide thin films for NO2 sensing at low temperatures. Sens. Actuators B, 2001;78:73–7. [2] Saura J. Gas-sensing properties of SnO2 pyrolytic films subjected to UV radiation, Sens. Actuators B, 1994;17:211–4. [3] Zhang C, Debliquy M, Boudiba A, Liao H, Coddet C. Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection.. Sens Actuators B, 2010;144:280-8. [4] Fan S.W, Srivastava AK, Dravidc VP. UV-activated room-temperature gas sensing mechanism of polycrystalline ZnO. Appl Phys Lett, 2009;95:106–142.
