Facile development of high performance QCM humidity sensor based on protonated polyethylenimine-graphene oxide nanocomposite thin film

https://doi.org/10.1016/j.snb.2016.01.105Get rights and content

Highlights

  • A breakthrough was achieved for preparing P-PEI-GO thin film under mild conditions.

  • A high performance humidity QCM sensor was developed based on P-PEI-GO thin film.

  • The humidity hysteresis, stability and durability of sensors were greatly enhanced.

  • Schematic models of adsorption sites and sensing mechanism were established.

Abstract

In this paper, a protonated polyethylenimine (P-PEI)-graphene oxide (GO) nanocomposite thin film, referred as P-PEI-GO, was deposited on the quartz crystal microbalance (QCM) by a facile dipping process for the development of high performance humidity sensors. The morphological and structural characteristics of P-PEI-GO thin film were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscope. The humidity-sensing properties, such as response and recovery, sensitivity, repeatability, selectivity, humidity hysteresis and long-term stability, were investigated in a wide range of working humidity (11–97% RH). The resultant P-PEI-GO nanocomposite thin film possesses much larger specific surface area and richer hydrophilic groups than pure P-PEI film, leading to higher sensitivities and shorter response/recovery times of the sensor. Especially, the sensor preserves excellent durability, small hysteresis, good selectivity and stability compared with individual P-PEI or GO film sensor, which could be ascribed to the high mechanical modulus of GO and weak interaction between P-PEI and GO. Finally, the humidity sensing mechanism of proposed sensor was discussed in details. This research provides a feasible and effective approach to construct high performance humidity sensor operated at room temperature based on GO nanocomposites.

Introduction

Humidity sensor has attracted tremendous attention due to its wide and essential applications in both industrial and domestic circumstances such as indoor air quality monitoring, heating ventilating air conditioning, manufacturing process control, agriculture and electric devices/foodstuffs storage [1], [2]. Therefore, there is a great demand to develop sensitive, reliable and low-cost humidity sensors ensuring humidity monitoring within a wide operating range. A key factor that could determine the humidity sensor's performance is humidity-sensing material. Up to now, different kinds of sensitive materials have been employed in humidity sensors, such as ceramics [3], [4], polymers [5], [6], electrolytes [7], meal oxides [8], [9], [10], carbon nanotubes (CNTs) [11] and graphene [12], [13], [14] etc. However, pure humidity-sensing materials suffer from their own shortcomings and limited performances. For example, the major drawbacks of single polymeric materials including polyvinylpyrrolidone (PVP) and polyethylenimine (PEI) are their confining sensing range, poor durability against water and drift in baseline with time. Furthermore, the polymer layer is liable to swell, stretch or peel off from the substrate when exposed to the high humid atmosphere, leading to the damage of sensor's functionality [5], [15]. D. Bin's group has shown that the flat PEI film coated quartz crystal microbalance (QCM) humidity sensor could not measure the signal once the relative humidity (RH) exceeded 80% [16]. On the other hand, novel graphene oxide (GO) has aroused much interest for humidity sensing due to its decorated oxygen functional groups (hydroxyl, epoxy and carboxylic acid groups) with high hydrophilicity [12], [14], [17]. Typically, the GO film-QCM sensor with excellent humidity sensing performances has been obtained by Yao et al., whereas a slight lag of water molecules desorption was observed in the high RH range, which might be ascribed to variations in the interlayer stress of GO thin film derived from swelling effect [17]. Meanwhile, long-term use would deteriorate sensor's performances due to the physical coating of GO film on the substrate [15], which restricts further application of these sensors.

Recent research has shown that the humidity-sensing properties of pure material could be tuned by modification or compositing technique via noncovalent or covalent method, which provides a new route for developing high performance humidity-sensing materials. For instance, metal oxide hybrids [8], functionalized multiwalled carbon nanotubes (MWCNTs) [18] or GO [14], MWCNTs/GO composite [19], polyamide modified PEI [16] and electrolytes/MWNTs [20] or electrolytes/GO [21], have been successfully used for improving humidity-sensing characteristics. Several possible factors might be responsible for the enhanced humidity properties. In terms of structure, the positive effect of some nanomaterials (such as MWCNTs or graphene) in the composite film is to provide a large specific surface area and thus hydrophilic active sites could be increased [20]. In addition, the composite might provide effective channels for the diffusion of water molecules [19], [20]. Moreover, synergistic effect [8] also contributes to the improved humidity sensing properties.

The working operation principles of above mentioned humidity sensors are mainly based on the change in either electrical (resistive, capacitive and impedance) or mass (bulk acoustic wave (BAW) and surface acoustic wave (SAW)) properties due to the adsorption of water molecules. Among them, QCM (a typical BAW device) is a sensitive and versatile tool for measuring adsorption of a variety of compounds on surfaces. In principle, QCM is very attractive for humidity sensing application as it employs the most basic physical effect, which occurs when water molecules adsorption takes place on its electrodes. Therefore, QCM devices combined with different sensitive materials have been frequently employed for humidity detection by measuring the shift of its resonant frequency according to the Sauerbrey equation [3], [6], [10], [16].

As a versatile building block for the construction of adsorbents, PEI possesses high amine density and accessible primary amine sites on chain ends [22]. Therefore, PEI can easily interact with the oxygenated groups of GO sheet, which is profitless for the development of humidity sensitive thin films. To our knowledge, PEI-GO composite humidity-sensing film has not been explored yet. This work is anticipated to open a possibility in developing PEI-GO nanocomposite thin film for humidity sensing application, which retains abundant amine and oxygenated functional groups of PEI and GO, respectively. A great breakthrough was achieved by the preparation of protonated PEI (noted as P-PEI) under mild conditions, and a high-performance QCM humidity sensor composed of P-PEI-GO nanocomposite thin film was demonstrated in this paper. We systematically investigated humidity-sensing characteristics (response and recovery, humidity hysteresis, reproducibility, selectivity and stability) of resultant QCM sensor, and the plausible humidity sensing mechanism was also discussed for illuminating how the surface morphology or functional groups of P-PEI-GO film and humidity sensing properties are correlated.

Section snippets

Preparation of P-PEI-GO composite solution

Ultrapure water with a resistance of 15.0 MΩ was used throughout the preparation of all solutions. GO aqueous dispersion at pH 4 (2 mg/ml, Sinocarbon Materials Technology Co., Ltd.) was sonicated for 60 min prior to use. Branched PEI aqueous solution (50% (w/v), Sigma–Aldrich Co., LLC.) was diluted to 2% (w/v) with the original pH value of ca. 9, which was adjusted to ca. 4 by adding appropriate amounts of hydrochloric acid (HCl) dropwise, i.e., total 8.8 ml 0.1 M HCl was injected into 2.0 ml PEI

Characterization of sensitive thin films

Fig. 5 shows the FESEM images of (a) pure P-PEI, (b) pure GO and (c) P-PEI-GO thin film, respectively. It reveals that the pure P-PEI film exhibits a uniform and smooth surface morphology whereas the single GO sheets are not perfectly flat and display obvious wrinkles. One interesting observation here is the formation of slimsy ripple-like surface of P-PEI-GO thin film. The specific surface area of sensitive film was obtained from N2 adsorption isotherm using Brunauer–Emmett–Teller (BET)

Conclusion

In summary, a highly sensitive, stable and low cost humidity sensor based on P-PEI-GO nanocomposite thin film coated QCM has been developed by a very facile dipping approach. Compared with the pure P-PEI and GO thin film under the same process, the nanocomposite thin film took full advantage of large specific area and high mechanical modulus of GO, superimposed hydrophilic groups, and weak electrostatic interaction between GO and P-PEI. Therefore, the resultant sensor exhibited remarkably

Acknowledgements

This work is partially supported by the National Science Funds for Creative Research Groups of China (Grant No. 61421002), Program for New Century Excellent Talents in University (Grant No. NCET-13-0096) and the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201413).

Huiling Tai received her B.Sc. degree in the field of electronic materials and components in 2003 and got her Ph.D. degree in optical engineering from University of Electronic Science and Technology of China (UESTC), Chengdu, China, in 2009. Currently she is an associate professor of School of Optoelectronic Information at UESTC. Her major interests are sensitive materials and gas/humidity sensors.

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Huiling Tai received her B.Sc. degree in the field of electronic materials and components in 2003 and got her Ph.D. degree in optical engineering from University of Electronic Science and Technology of China (UESTC), Chengdu, China, in 2009. Currently she is an associate professor of School of Optoelectronic Information at UESTC. Her major interests are sensitive materials and gas/humidity sensors.

Zhen Yuan received his B.Sc. degree from School of Optoelectronic Information from University of Electronic Science and Technology of China (UESTC), Chengdu, China in 2013. He is taking successive postgraduate and doctoral programs of study for doctoral degree in School of Optoelectronic Information at UESTC since 2013. His major is optical engineering. His current research interests include gas sensor and humidity sensor based on polymer, graphene and their derivatives.

Chunhua Liu received her B.Sc. degree in information display and photoelectric technology from University of Electronic Science and Technology (UESTC), Chengdu, China in 2013. She is now a doctoral student in UESTC. Her current research interest is focused on the gas sensor and MEMS.

Zongbiao Ye received his B.Sc. degree from University of Electronic Science and Technology of China (UESTC), Chengdu, China in 2012. Since Sep. 2012, he has been a full time Ph.D. student at School of Optoelectronic Information in UESTC. His current research interests include gas sensing mechanism and gas sensors based on graphene composites.

Guangzhong Xie got his B.Sc. and M.Sc. degrees in the field of material physics from Sichuan University. He got his Ph.D. degree from University of Electronic Science and Technology of China (UESTC), Chengdu, China in 2007. Now he is a professor of School of Optoelectronic Information at UESTC. His research interests are sensitive materials and sensors.

Xiaosong Du received his B.Sc. degree in the field of material science and engineer from Tsinghua University, Beijing, China, in 1992. Then he got his M.Sc. degrees in the field of material physics from Sichuan University, Chengdu, China, in 1995. And he got his Ph.D. degree in microelectronics and solid state electronics from University of Electronic Science and Technology of China (UESTC), Chengdu, China, in 2002. Now he is a professor of School of Optoelectronic Information at UESTC. His research interests are sensitive materials and QCM sensors.

Yadong Jiang graduated from Department of Material Science & Engineering at University of Electronic Science and Technology of China (UESTC) with a B.Sc. degree in 1986. Then he got his M.Sc. and Ph.D. degrees in Materials Physics and Chemistry in 1989 and 2001, respectively, He is a professor and dean of School of Optoelectronic Information at UESTC. His major research interests include optoelectronic material and devices, sensitive materials and sensors.

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