NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film

doi:10.1016/j.snb.2011.10.059

Sensors and Actuators B: Chemical

Volume 161, Issue 1, 3 January 2012, Pages 447-452

https://doi.org/10.1016/j.snb.2011.10.059 Get rights and content

Abstract

In this paper, a novel porous WO₃ sensor was prepared by anodic oxidation of DC magnetron sputtered metallic tungsten (W) film deposited on alumina substrate. The structural and morphological properties of these films are investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Coral-like porous crystalline WO₃ film with a grain size of about 9.3 nm was obtained after annealing of the anodized W film. The porous WO₃ sensor achieved its maximum response value to NO₂ at a low operating temperature of 150 °C. In comparison to sputtered WO₃ sensor, the porous WO₃ sensor showed markedly higher responses, much better response–recovery characteristics and lower optimal operating temperature to different concentration of NO₂ gas due to its high specific surface area and small grain size.

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 NO_X.

Metal oxide gas sensors are promising for use in detection of low concentrations of NO_X 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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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.

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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.

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NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film

Abstract

Introduction

Section snippets

Experiment

Microstructure characterizations

Conclusions

Acknowledgments

Sens. Actuators B: Chem.

Sens. Actuators. B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Acta Mater.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Mater. Sci. Eng. R

Sens. Actuators B: Chem.

Electrochem. Commun.

Sens. Actuators B: Chem.

Electrochem. Commun.

Electrochim. Acta

NO₂-sensing properties of porous WO₃ gas sensor based on anodized sputtered tungsten thin film