NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film
Introduction
As the air pollution becoming more and more serious, requirements to gas sensitive devices increase. In these years, there has been increasing interest to detect nitrogen oxide gases since they are main source of acid rain and photochemical smog [1]. The air quality standard for NO2, suggest by Italian legislation for ambient air (“long-term exposure”), is 100 ppb (attention level) [2]. American standards, concentration of NO and NO2 should not exceed 3 and 25 ppm respectively while in Japan, these fall in ppb range [3]. Thus, there is a strong demand for cheap, reliable, sensitive gas sensors targeting NOX.
Metal oxide gas sensors are promising for use in detection of low concentrations of NOX due to their low cost, compatibility with microfabrication technologies, high sensitivity and availability of a large variety of metal oxides with different gas sensing characteristics [4]. Transition metal oxides, such as TiO2, SnO2, ln2O3, WO3, ZnO, Nb2O5 or MoO3, and several combinations of these have been widely investigated so far [5], [6], [7], [8]. Among them, tungsten trioxide (WO3), which shows promising properties especially if possesses a large surface area [9], is considered as one of the most interesting materials in the field of gas sensors based on metals oxides semiconductors [10]. An evident positive correlation relationship was found between the surface areas of gas sensors and their sensor response [11]. Accordingly, the performance of WO3 gas sensor can be significantly enhanced by increasing their surface areas, which provides more surface adsorption sites for the reaction of NO2 gas. At the same time, the deposition process affects the sensor performance largely since it affects the morphology and structure of the sensing material (and thus, its properties) [3], [12].
Porous WO3 materials, which provide high surface areas have been prepared by sol–gel process [13], anodization of tungsten foils [14], [15] or sputtered tungsten thin films [16], [17]. Porous WO3 materials obtained by anodization of tungsten thin films deposited on conductive substrates are not suitable for the fabrication of resistive-type gas sensor for their substrate conductivity. In this article, anodization of sputtered tungsten thin film deposited on insulating substrate was studied for the application of gas sensor. Metallic tungsten films were deposited on alumina substrates, which were attached with a pair of interdigitated Pt electrodes, through a DC magnetron sputtering process. Porous WO3 films were then prepared by anodic oxidation of the W films. We studied the gas sensing properties of porous WO3 towards NO2 gas ranging from 100 ppb to 5 ppm at operating temperature of 100–200 °C. The selectivity to NO2 at operating temperature of 100–250 °C was investigated as well.
Section snippets
Experiment
The alumina substrates were ultrasonically cleaned first with deionized water, acetone, ethanol consecutively for 15 min each, in order to remove grease and other possible pollutants from the substrate surface, and dried at room temperature in ambient air. After cleaned, a pair of interdigitated platinum (Pt) electrodes with a thickness of 100 nm was deposited on the alumina substrate using RF magnetron sputtering method by using an interdigitated shadow mask.
A DPS-III ultra-high vacuum facing
Microstructure characterizations
To investigate the effect of anodization on the morphology of the film, the top view SEM images of different films were taken (Fig. 3). It can be seen from Fig. 3(a), the sputtered W layer exhibits a compact stratified structure. The size of W particles is about several microns. After anodization, many cracks appear on the surface of W film, which makes it porous (inset in Fig. 3(b)). Compared with the porous morphology before annealing, it can be seen that the thermal annealing results in the
Conclusions
In this work, a novel porous WO3 gas sensor was fabricated by applying electrochemical anodization treatment to metallic tungsten film deposited on alumina substrates, which had been attached with a pair of interdigitated Pt electrodes. SEM showed a porous coral-like structure was formed after annealing. The results of XRD confirmed the formation of crystalline WO3 with a grain size of about 9 nm. Compared with sputtered WO3 sensor, the porous WO3 sensor exhibited markedly higher response value,
Acknowledgments
Project supported by the National Natural Science Foundation of China (grant nos. 60771019 and 60801018), Tianjin Key Research Program of Application Foundation and Advanced Technology, China (grant no. 11JCZDJC15300), Tianjin Natural Science Foundation, China (grant no. 09JCYBJC01100), and the New Teacher Foundation of Ministry of Education, China (grant no. 200800561109)
Jing Zeng received his bachelor degree in Electronic Science and Technology from Tianjin University in 2009. He is now a graduate student at Tianjin University. His current research is focused on the metal oxide based gas-sensing materials and gas-sensing mechanism.
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Jing Zeng received his bachelor degree in Electronic Science and Technology from Tianjin University in 2009. He is now a graduate student at Tianjin University. His current research is focused on the metal oxide based gas-sensing materials and gas-sensing mechanism.
Ming Hu received a M.S. in microelectronics and solid-state electronics from Tianjin University in 1991. She is now a full professor in department of electronics science and technology in Tianjin University. Her research interests include MEMS, gas sensor, and sensitive materials and functional thin film devices.
Weidan Wang received her bachelor degree in Electronic Science and Technology from North University of China in 2008. She is now a graduate student at Tianjin University. Her current research is focused on the tungsten oxide based gas sensor and material adsorption properties simulation.
Huiqing Chen received his bachelor degree in Applied Physics from Hebei Normal University in 2009. He is now a graduate student at Tianjin University. His current research is focused on the porous silicon based gas-sensing materials and gas-sensing mechanism.
Yuxiang Qin received a Ph.D. in microelectronics and solid-state electronics from Tianjin University in 2007. She is currently an associate professor in department of electronics science and technology in Tianjin University. Her research interest is in the areas of oxide semiconductor gas sensor, field emission materials and devices.