Improvement and mechanism for the fast response of a Pt/TiO2 gas sensor
Introduction
Detection of flammable and exhaust gases is a subject of growing importance both in energy-saving and environmental protection endeavors. Among a large group of existing chemical sensors, TiO2 has excellent sensing properties for various gases, such as CO, NOx, CH3CH2OH, H2, and O2, among others [1], [2], [3], [4], [5], [6], [7]. Efforts have been made to improve selectivity and sensitivity. One approach to enhancing the gas properties is the use of metals or additives. Many materials, such as metals [2], [7], [8], [9], [10], [11] and function polymers [12], [13], have been added to enhance sensitivity and selectivity towards different target gases. At the same time, newly developed fabrication techniques and novel device structures have also been widely used to improve gas properties. In recent decades, anodically oxidized films [4], nanostructures [14], mesoporous oxides [3], [15], and field effect transistor-type devices [16], [17] have been actively studied due to the need to develop high performance devices.
The sensing properties of semiconductive metal oxides have been further enhanced with noble metals [18], [19]. Recent studies used platinum, which has been shown to considerably improve not only sensitivity and selectivity but also the response speed of TiO2-based gas sensors. Platinum on TiO2 can be used as a model system to understand the influence of chemical and electrical effects. Trimboli and Dutta [20] developed a novel Pt–zeolite filter–TiO2 sensor that exhibited a relatively faster recovery at 7.5 min compared to pure TiO2 with a recovery of over 30 min. Moreover, platinum changes the TiO2 oxidation state by chemical sensitization and increases the gas properties as it increases the rate of the chemical process, leasing to a decrease in the concentration of negatively charged adsorbed oxygen. Comini et al. [21] doped Pt and Nb with TiO2 and obtained a response time ranging from 20 to 300 s and a recovery time of 60–70 s. Francioso et al. [22] doped Pt in TiO2 film at a Pt/Ti atomic ratio of 0.05 by sol–gel and heat-treated the film at 500 °C in a tubular oven. The microfabricated sensor showed a response time ranging from 1.5 ± 0.2 to 2.0 ± 0.2 s at 630 °C. The above-mentioned studies have tremendously improved the response properties of the sensors.
The response time of a sensor exposed to target gases is another important parameter. Improving the rate of response is a significant goal in developing TiO2-based gas sensors. Gas sensors for safety monitoring and breath-by-breath measurement or as input devices for control circuits in combustion processes are required to exhibit a short response time [23]. A fast-responding sensor can help in the analysis and control of the air-to-fuel (A/F) ratio in an internal combustion engine, which is an important factor that affects fuel efficiency and pollutant gas emissions of motor vehicles [24]. This is a very potent and widely used method to improve fuel efficiency and reduce pollution.
The aim of this work is to improve the rate of response. The thick-film technique, which is simple and cost-effective, is used to fabricate the sensors. Platinum is also used to modify the surface of the thick-film TiO2 sensors by dipping these in an H2PtCl6-containing solution and treating them at different temperature/preservation time conditions. The results showed that an appropriately treated sensor exhibited a response time of about 40 ms when exposed to H2 and 20 ms when exposed to O2 at 500–800 °C. The investigation on the sensing properties is interpreted with respect to the activation energy (E) of the sensors and the platinum particles dispersed on the TiO2 surface.
Section snippets
Film fabrication
The TiO2 thick film was fabricated on an alumina substrate (35 mm × 6 mm × 1 mm) equipped with comb-type platinum microelectrodes. The platinum electrodes were fabricated on the alumina substrate by a screen-printing method using a commercial platinum paste (Northwest Institute for Non-ferrous Metal Research, China). The substrates were then calcined at 1300 °C.
TiO2 nanopowders (TaiShan Chemical Factory Co., Ltd., China) were dispersed in ethyl cellulose by ultrasonic bath. The resulting paste was left
Surface state
The XRD spectra of H2PtCl6-modified TiO2 treated at different temperatures are shown in Fig. 1(a). All TiO2 samples had a rutile phase, but there were two peaks at 39.6° and 46.1° (2θ), which are typical of platinum. No phase transformation was observed at different treatment temperatures. The presence of platinum was firmly demonstrated by EDS, shown in Fig. 1(b), which confirmed that the loading amount of platinum on the surface of TiO2 sensors is about 4.8 at%. SEM analysis was performed to
Conclusions
TiO2 thick-film sensors were surface-modified in an H2PtCl6-containing solution by the dipping method and treated under different conditions. The treatment condition markedly influenced the distribution of platinum particles and activation energy (E). The response properties showed a conspicuous difference under various treatment conditions. In particular, the sensor treated at 900 °C for 2 h exhibited a response time of about 40 ms when exposed to H2 and 20 ms when exposed to O2 at 500–800 °C.
Acknowledgments
This work was partially supported by the Northwest Institute for Non-ferrous Metal Research of China. The author wishes to thank Mr. Z. Miao (NIN, China) and Mr. Z. Z. Zhao for their invaluable assistance in carrying out some experiments.
Maolin Zhang received his MS and BS degree from the Xi’an JiaoTong University in the Department of Electronic Science and Technology. He is currently a PhD student working in the area of sensing materials and devices.
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Maolin Zhang received his MS and BS degree from the Xi’an JiaoTong University in the Department of Electronic Science and Technology. He is currently a PhD student working in the area of sensing materials and devices.
Zhanheng Yuan graduated from the Xi’an JiaoTong University in 1976. He is currently a professor of the Xi’an JiaoTong University. His work focuses on the preparation, modification, processing of functional composite materials for gas-sensitive, chemical sensitive sensors, and anodic oxidation thin films as well as nano-technology.
Jianping Song received his PhD degree from the Xi’an JiaoTong University in 1990 and then worked in the Technical University of Denmark. He is currently a professor of the Xi’an JiaoTong University and the vice-chair of the Institute of Physical Electronics& Optoelectronics. His research interests are in the area of multifunctional sensing materials, nonlinear optics and the scanning tunneling microscopy.
Cheng Zheng received his BS degree in the Department of Electronic Science and Technology at the Xi’an JiaoTong University in 1998. Since 2002 he has been studying at the Speciality of Microelectronic & Solidelectronic Engineering of Xi’an JiaoTong University. His current fields of interest focus on the applications of titania and stannic oxide to sensors.