Sputtered SnO2:NiO thin films on self-assembled Au nanoparticle arrays for MEMS compatible NO2 gas sensors
Introduction
Due to the remarkable responses, low cost, and portability, metal oxide (MOX) gas sensors are widely investigated to detect the highly toxic gases at low concentrations, such as nitrogen dioxide (NO2). It is detrimental to both environment and human health such as leading to smog, acid rain and various diseases like edema, pneumonia, and bronchoconstriction [[1], [2], [3], [4]]. Therefore, in situ detection of NO2 is of increasing importance for assessing the air quality (standard 0.12 ppm), and extensive efforts have been focused on investigating MOX sensing materials including tin dioxide (SnO2), tungsten trioxide (WO3) and nickel oxide (NiO) [3,[5], [6], [7], [8], [9], [10], [11]]. Specially, nanostructured MOX with high surface area to volume ratio has continuously enhanced the sensing performance, such as zero-dimensional (0D) nanoparticles or clusters, one-dimensional (1D) nanowires or nanorods, two-dimensional (2D) nanosheets, and three dimensional (3D) hierarchical or porous hollow structures [[6], [7], [8],[12], [13], [14], [15], [16], [17], [18], [19], [20]]. Besides, novel metals are particularly effective chemical additives for selectivity improvement towards NO2 [1,3,15,[21], [22], [23], [24], [25]]. However, these materials have to be made into slurry and then screen printed or dip coated onto a hot-plate to fabricate a sensor device. This kind of traditional gas sensor has the disadvantages of relatively large size and thus high power dissipation, and also large sensor-to-sensor variations due to the non-uniform composition or thickness of the slurry [10,22,26,27].
Nowadays, microhotplate platforms have been designed and fabricated by microelectrical mechanical system (MEMS) techniques to effectively achieve the device miniaturization, low device-to-device variation and low energy dissipation [[28], [29], [30]]. However, the slurry-based drop coating of nanomaterials on small sensing area of MEMS microhotplates is an extremely difficult task [31,32]. The low yield and large sample-to-sample deviation in sensor response hamper the MEMS sensor fabrication in a large scale, which limits their practical use. To address this issue, it is still an urgent demand and challenge to prepare highly sensitive MOX materials by the MEMS compatible sputtering processing, which has been proved a facile method to integrate the sensing materials on microhotplate platform [[33], [34], [35]]. Up to now, there have been several examples of MEMS sensors based on sputtered MOX thin films [34,[36], [37], [38]]. For example, Kang et al. have demonstrated a sputtered SnO2 thin film based micro gas sensor, which showed a response of 6–25 ppm toluene at 450 °C [34]. And Vallejos et al. reported the micro-machined WO3-based sensors which exhibited response of 7 to 1 ppm NO2 at 250 °C [36]. Despite their success in technical research, the sensitivity of most reported sputtered films is still much lower than the conventionally chemically synthesized nanostructured MOX materials, because their amorphous and highly dense structure limits the interactions between the sensitive material and the surrounding gases. Thus, many efforts should be made to stabilize and improve the poor response kinetics of sputtered films before applying them into the air quality control systems.
In this paper, to develop practically useful NO2 sensing materials, MEMS compatible Au/SnO2:NiO thin films were fabricated through self-assembly and magnetron sputtering technique. To the best of our knowledge, there is no previous report on the application of ordered 2D AuNP arrays to activate the sensing property of semiconducting MOX. The prepared AuNP monolayer/SnO2:NiO thin film composites exhibit high response to the target NO2 gas (185 to 5 ppm) and low sample-to-sample deviation (<15%). A detection limit of 0.05 ppm can be realized at a working temperature of 200 °C. The generality of our bottom-up method was verified by designing heterostructured Au/WO3 and Au/SnO2 thin films with well-organized internal order for NO2 detection, showing the effectiveness of this heterostructure synthesis strategy for large scale fabrication of MEMS gas sensors.
Section snippets
Synthesis and self-assembly of AuNPs
Gold colloidal particles with a diameter of 9.5 nm were prepared by the Turkevich method [39]. In brief, a 20 mL solution containing 4 mL 1% (w/v) trisodium citrate and 0.2 mL 1% (w/v) tannic acid was rapidly added to an 80 mL solution containing 1 mL 1% (w/v) chloroauric acid. The mixed solution was heated and kept boiling for 10 min with continuously stirring and then cooled to room temperature. All the above reagents were purchased from J&K Scientific.
The encapsulation of the AuNPs with
Results and discussion
The morphologies of Au/SnO2:NiO heterostructures (thickness of 20 nm) fabricated by the process in Fig. 1 are illustrated in Fig. 2. The top view of one typical measured gas sensor in Fig. 2a shows the circular sensing film connected by the interdigitated Au electrodes. The thin film between two adjacent electrodes is 10 μm in length and 170 μm in width. The framed region is magnified and shown in Fig. 2b, which shows the well aligned printed AuNP monolayer covered by SnO2:NiO film. The
Conclusion
In summary, we have successfully developed a MEMS compatible heterostructured Au/SnO2:NiO sensing film for NO2 detection. Our method combines the traditional sputtering technique with large-scale self-assembled Au NPs. The SAXS, XPS, UPS and NO2 gas sensing measurements have been carried out to investigate the post-annealed Au/SnO2:NiO films. The response to 5 ppm NO2 of Au/SnO2:NiO (185) is significantly higher than that of the SnO2:NiO (<2.5) and Au/SnO2 (35). The enhancement of the gas
Acknowledgements
This research was financially supported by the National Key R&D Program of China (2016YFC0207100), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05C146 and 20150317025954531), and the State Key Laboratory of Multiphase Complex Systems (MPCS-2014-C-01).
Ying Wang received the bachelor’s degree in optics from Beijing Jiaotong University in 2010 and the pH.D. degree in physical electronics from the College of Electronics, Peking University, in 2015. She is currently an Associate Professor with the Institute of Process Engineering, CAS.
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Ying Wang received the bachelor’s degree in optics from Beijing Jiaotong University in 2010 and the pH.D. degree in physical electronics from the College of Electronics, Peking University, in 2015. She is currently an Associate Professor with the Institute of Process Engineering, CAS.
Chengyao Liu received the B.S. degree and M.S. degree in chemistry from the Beijing University of Chemical Technology separately in 2014 and 2017. He is currently working in Chengdu BOE Company.
Zhou Wang received the B.S. degree in chemical engineering and technology from the Wuhan Institute of Technology. He received the M.S. degree with the Beijing University of Chemical Technology in 2018. He is currently working in Xiamen Leading Optics Co., LTD.
Zhiwei Song received the B.S. degree in physics from Zaozhuang University. He received the M.S. degree in materials science and engineering from Beijing University of Technology. He is currently an Intermediate Engineer with National Center for Nanoscience and Technology, CAS.
Xinyuan Zhou received the B.S. degree in materials chemistry from the Taiyuan University of Technology in 2010. He is currently pursuing the pH.D. degree in materials engineering with the Institute of Process Engineering, CAS.
Ning Han received the pH.D. degree in chemical engineering from the Institute of Process Engineering, CAS, in 2010. He was a Post-Doctoral Fellow with the Department of Physics and Materials Science, City University of Hong Kong, from 2010 to 2014. He has been a Professor with IPE CAS via One Hundred Talents Plan since 2014. He is works on investigations of preparation, structure, and property of semiconductors, and has developed highly sensitive and selective gas sensing materials. He has authored over 80 articles.
Yunfa Chen received the pH.D. degree in material science from the Université Louis Pasteur Strasbourg, France, in 1993. He is currently a Professor with the Graduate University of Chinese Academy of Sciences, and a Research Professor and the Vice Director of the Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are the preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials, and industrial application of nanomaterials.