Elsevier

Sensors and Actuators B: Chemical

Volume 270, 1 October 2018, Pages 140-148
Sensors and Actuators B: Chemical

Sensitivity enhanced, stability improved ethanol gas sensor based on multi-wall carbon nanotubes functionalized with Pt-Pd nanoparticles

https://doi.org/10.1016/j.snb.2018.04.170Get rights and content

Highlights

  • An ethanol gas sensor with enhanced sensitivity of 0.938 mV/ppm was fabricated by utilizing multi-wall carbon nanotubes functionalized with Pt-Pd nanoparticles as catalyst.

  • The sensor exhibited low LOD of 3 ppm and a large dynamic response range from 3–400 ppm.

  • The performance degradation of the sensor over 7 days was <2.7%, indicating its improved stability.

Abstract

This paper reports a kind of ethanol gas sensor with enhanced sensitivity and improved stability. It utilizes multi-wall carbon nanotubes (MWCNTs) functionalized with Pt-Pd nanoparticles as catalyst. The MWCNTs with large specific surface area were used as carbon supports for the bimetallic catalyst (Pt-Pd) to enhance the electrochemical activity. The results demonstrate that the highest sensitivity (0.938 mV/ppm, which is ∼1.47 times higher than existing sensors) is realized by utilizing the catalyst with 80 wt% Pt-Pd. On the other hand, the highest value (2.877 mV/mg·ppm, ∼1.64 times higher than the reported ones) of the mass-normalized sensitivity is obtained with the 20 wt% Pt-Pd catalyst. Repeated detection of ethanol gas demonstrated a low limit of detection (LOD) of 3 ppm and a large dynamic response range from 3 to 400 ppm. The linear correlation coefficient (R2) of the signal was ≥0.998 from 0 to 400 ppm, and the performance degradation over 7 days was <2.7%, while that for existing sensors was 15%. These data illustrated that this sensor has potential applications for ethanol gas detection with high sensitivity.

Introduction

In the fields of chemical engineering, traffic safety, medical applications, and the food industry, highly sensitive, rapid, low-cost, and portable ethanol gas sensors are needed to detect and quantify low concentrations of ethanol gas [1]. However, most of the present technical solutions could not detect ethanol vapor below 10 ppm [2], [45], [47]. Furthermore, commercial ethanol gas sensors cannot fully meet the requirements for effective applications in many fields: the infrared type has high costs [3], [4], the semiconductor type requires high working temperature [5], [6], [7], and the gas chromatography devices are very complex [8], [9], [10]. For portable detection of gases at low working temperature (room temperature), the proton exchange membrane fuel cell (PEMFC)-type electrochemical sensor has received significant attention. In most PEMFCs, Nafion membrane was mainly used both as the proton exchange membrane to support the catalyst layers, and as the solid electrolyte to deliver protons. The use of a solid electrolyte could fundamentally solve the liquid leakage problem, compared with electrochemical sensors that use acid solution as electrolyte.

Meanwhile, platinum (Pt) is recognized as the most active metal to catalyze ethanol oxidation. However, there remain a few problems with it: (a) the adsorption of CO molecules on the surface of Pt catalyst, which results in poisoning and inactivating of the sensor [11], [12], [62]; (b) the aggregation and dissolution of Pt nanoparticles (NPs) reduce the active area of the electrodes [13], [14], resulting in unsatisfactory sensitivity and stability for practical devices; (c) the use of expensive Pt catalyst hinders the widespread adoption of these sensors. Thus, new catalysts should be investigated with further step in order to build better ethanol gas sensors.

Bimetallic catalysts, including Pt-Ag [15], [16], Pt-Ru [17], [18], [19], and Pt-Ir [20], [21], [22], have been extensively investigated to improve the electrochemical performance of catalysts. Among them, the Pt-Pd NPs have shown good electrochemical properties and high stability, and Pd is also cheaper than Pt [23], [24], [25]. Moreover, the amount of catalyst could be greatly reduced by dispersing it over a carbon support with high specific surface area [26], [27], [28]. Nano carbon materials including carbon nanotubes have attracted much attention in this regard, due to their excellent conductivity, high stability, and good physical and chemical properties [29], [30], [31].

Considering the fabrication process, there have been many methods to combine the catalyst layer with the Nafion film to prepare membrane electrode assembly (MEA) used in the PEMFC-type sensors, such as decal [32], [33], screen-printing [34], [35], and the blade process [36], [37]. However, these methods suffer from incomplete adhesion between the proton membrane and the catalyst layers, heavy catalyst aggregation, and lack of uniformity for the catalyst layers. In contrast, the ultrasonic spraying method could be applied to prepare noble metal catalyst layers with a highly uniform dispersion, thanks to the high-frequency vibrations from the ultrasound [38], [39], [40], [41], [42].

In this work, a kind of ethanol gas sensor was developed by utilizing multi-wall carbon nanotubes (MWCNTs) functionalized with Pt-Pd NPs as catalyst, in conjunction with the ultrasonic spraying method. The resulting MEA containing Pt-Pd/MWCNTs catalyst layers was assembled into an ethanol gas sensor, and the influence of the metal content on the sensor performance was discussed. The sensors prepared with large specific surface area MWCNTs and high electro-catalytic activity Pt-Pd NPs catalyst were shown experimentally to have improved performance, high sensitivity, and long lifetimes.

Section snippets

Methods

The schematic diagram of the MEA, which is the key component of PEMFC type gas sensor, is illustrated in Fig. 1(a). The ethanol oxidation-reduction reaction (ORR) only occurs at where is simultaneously accessible to the catalyst sites, gas-phase reactants and electrolyte (also called the triple phase boundary (TPB) or the ‘three-phase reaction zone’). In addition, the electrochemical reactions could occur at room temperature without external energy source. The detailed reactions during ethanol

Characterization of the Pt-Pd/MWCNTs catalyst

To analyze their composition, different catalyst samples were characterized using X-ray techniques. As shown in Fig. 3(a), the characteristic diffraction peaks located at 26.15° could be the (002) plane of graphite in the MWCNTs. The intensity of this peak was negatively correlated with the metal content in the Pt-Pd/MWCNTs samples (20 wt% > 50 wt% > 80 wt%). Five other peaks could be seen in the Pt/MWCNTs and Pd/MWCNTs catalyst samples, which are consistent with the Pt (JCPDS Card No. 04-0802)

Conclusion

In conclusion, a kind of ethanol gas sensor with a low LOD of 3 ppm and a large dynamic range from 3 to 400 ppm was realized. The highest sensitivity (0.938 mV/ppm) is 1.47 times higher than the previous reported sensors. The performance degradation of these sensors was <2.7% over 7 days, indicating improved stability for long-term use. Moreover, the sensors showed much higher responses for ethanol than for other gases (ammonia, acetone, and hydrogen chloride). These results, taken together,

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51675517, No. 61505241), Natural Science Fund of Jiangsu Province of China (No. BK20140380, No. BK20160057), Youth Innovation Promotion Association CAS (No. 2014280, No. 2018360), Ministry of Science and Technology focused on special international cooperation projects (2016YFE0107700), Jiangsu Planned Projects for Postdoctoral Research Funds of China (No.1701010A), Major Technology Innovation Projects of Jiangsu

Qi Nie received the B.S. degree in School of Electronic and Information Engineering from Soochow University in 2015. She is currently studying for her MS degree in Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, China. Her research focuses on the development of highly sensitive VOCs gas sensors and electrochemical sensing mechanisms.

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    Qi Nie received the B.S. degree in School of Electronic and Information Engineering from Soochow University in 2015. She is currently studying for her MS degree in Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, China. Her research focuses on the development of highly sensitive VOCs gas sensors and electrochemical sensing mechanisms.

    Wei Zhang received the B.S. degree and PhD degree in Biomedical Engineering from Zhejiang University, Hangzhou, China in 2006 and 2011 respectively. She is now an associate professor in Suzhou Institute of Biomedical Engineering and Technology of Chinese Academy of Sciences. Her research is mainly focused on biointerface immobilization and biocompatible design of biosensor.

    Lirong Wang received her Ph.D. degree in the Electronics Science and Engineering Mechanical Department, China in 2006. She is now a professor in the School of Electronic and Information Engineering in Soochow University, China. Her current areas of interest include image analysis and processing. He published more than 20 articles in international journal and co-author of 10 patents.

    Zhen Guo received the PhD degree both in Graduate School of the Chinese Academy of Sciences and the University of Orléans (France) in 2011. He is currently an associate professor working in Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences. He published more than 40 articles in international journal and co-author of 5 patents. His research interest focuses on the development of sensor chips, micro/nano material assembly and processing.

    Chuanyu Li received the B.S. degree and MS degree in School of Mechanical Engineering from Shandong University in 2008 and 2011, respectively. He is currently studying for his PhD degree in Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, China. His research focuses on the development of highly sensitive piezoelectric materials, sensors and systems.

    Jia Yao received his MS degree from Nanjing University of Aeronautics and Astronautics, major in mechanical engineering. His research now focuses on signal detection and biomedical applications of piezoelectric sensors.

    Min Li received the Ph.D. degree in Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, China in 2010. She is now an Associate Professor working in Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, China. Her research now focuses on MEMS sensors.

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