Electrospun CeO2 nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor

doi:10.1016/j.snb.2015.09.148

Sensors and Actuators B: Chemical

Volume 223, February 2016, Pages 730-737

https://doi.org/10.1016/j.snb.2015.09.148 Get rights and content

Abstract

Nowadays the working frequencies of surface acoustic wave (SAW) sensors are usually not higher than 500 MHz, typically in the range of a few dozen to hundred megahertz, while an increase of working frequency should be beneficial to the sensor performance. Thus, a high frequency SAW resonator operating at 1.56 GHz was fabricated for relative humidity (RH) detection. The CeO₂ nanoparticles (NPs)/polyvinylpyrrolidone (PVP) nanofibers were prepared by electrospinning of PVP solution with CeO₂ NPs working as the sensitive layer. The inorganic CeO₂ NPs were synthesized previously using a hydrothermal method. In contrast with SAW sensor working at lower resonant frequency (879 MHz), the resonant frequency shift of the sensor based on 1.56 GHz was about −2.5 MHz in the RH range of 11% to 95%, which was approximately 8 times of the former one. Further analysis demonstrated that the additional acoustoelectric loading effect arising from increased electrical conductivity of CeO₂/PVP nanofibers in high RH improved the frequency response compared with pure PVP nanofibers based SAW sensor. Moreover, the SAW sensor based on inorganic/organic nanohybrid also showed high stability under humid environment and negligible cross-sensitivity effects ensuring further wireless humidity detection.

Introduction

Among the humidity detection of different sensing techniques, such as resistance type, impedance type, surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [1], [2], [3], [4], SAW sensor is a prominent form with enhanced sensitivity, low cost and simple handling. In addition, SAW devices have already been widely employed in the field of signal processing such as analog and digital filters owing to the high determined resolution and frequency stability [5]. These features ensure the SAW humidity sensor working under long-distance wireless control via antenna and battery-less operation, which makes the SAW sensor as a hopeful method for harsh environmental humidity detection.

Recently, because of the increasing volume of data transmission, there have been tremendous demands for SAW devices such as SAW filters and duplexers working at above GHz [6]. Meanwhile, high frequency SAW devices also demonstrate great advantages in other wide applications. Interestingly, minimum droplet volume down to nanoliter order could be streamed in a lab-on-chip device based on GHz SAW [7]. Likewise in the temperature and pressure sensor, there were reported that the sensitivity could be significantly enlarged as the working frequency increased to GHz range [8], [9]. However, most commercially available SAW sensors and prototypical devices in research field recently are still working at the frequency ranging from a few dozen to hundred megahertz [10], [11], [12], [13], [14], [15], [16]. Considering the similar behavior of SAW humidity sensing characterization, SAW humidity sensor exploiting high working frequency, especially in GHz range, is also promising.

There were intensive investigations focusing on the hygroscopic polymer materials such as polyaniline (PANI), Nafion, and hematoporphyrin (hp) as selective coatings on SAW humidity sensors [2], [3], [11], [12]. On the other hand, in order to overcome the intrinsic drawback of polymeric material such as low mechanical and thermal stability, metal oxide (MOX) and metal-organic framework (MOF) based SAW humidity sensors also attracted massive attentions [13], [14], [15], [16]. Surface acoustic waves are sensitive to the changes in the mass loading, viscoelastic properties, and electrical conductivity of these sensing layers. Among the variations of different properties, the conductivity change is essential to MOX based SAW humidity sensor [13], [15]. Recently, CeO₂ has been widely used in resistance-type humidity sensor [1], [17]. Owing to the small ionic radius of Ce⁴⁺ ions, the strong electric field induced around the surface of CeO₂ augments ionization of water molecules which enhances the conductivity of CeO₂ nanomaterials upon high RH. As an intriguing approach, the inorganic/organic hybrid is expected to cooperate in the sensing mechanism and suppress the drawback of single component. By this means, we report a SAW resonator operating at 1.56 GHz on 128° YX LiNbO₃ substrate and coated with CeO₂ NPs/PVP nanofibers by electrospinning method. The SAW device with 1.56 GHz was measured at relative humidity ranging from 11% to 95%, and the frequency shift was approximately −2.5 MHz which was 8 times of the sensor based on 879 MHz with the same sensitive coating. Moreover, compared with pure PVP nanofiber utilizing mass and viscoelastic loading effects, the inorganic/organic hybrid based SAW sensor showed enhanced sensitivity attributed to additional acoustoelectric loading effect. In particular, the sensor also featured with good stability and showed negligible interferences from other gases.

Section snippets

Fabrication of high frequency SAW devices

The SAW devices were fabricated on 128° YX LiNbO₃ substrate (Shanghai Institute of Optics and Fine Mechanics, CAS). We have fabricated two kinds of devices based on one port resonator structure as shown in Fig. 1(a). Each device contained one SAW interdigital transducer (IDT) and several pairs of reflectors placed on each side of the IDT. The number of IDT finger pairs (N_IDT), the number of reflectors (N_re), aperture (W) and the nominal wavelength (λ) of different samples are listed in Table 1.

The morphologies and crystal structures of the samples

Fig. 3(a) shows the SEM image of the high-purity CeO₂ NPs with dimensions ranging from 400 to 480 nm. Fig. 3(b) exhibits a low-magnification TEM image of a single CeO₂ NP, and the inserted selected area electron diffraction (SAED) image reveals the single crystal structure of the CeO₂ NP. The typical XRD pattern of CeO₂ NPs is shown in Fig. 4, and the exhibited strong peaks can be corresponding to pure cubic CeO₂ with cell constants of α = 5.41 Å (JCPDS No. 65-5925). Nanostructured CeO₂ has been

Conclusions

In summary, high frequency SAW humidity sensors operating at 1.56 GHz were fabricated on 128 YX° LiNbO₃ substrate by EBL. Inorganic/organic hybrids of CeO₂/PVP nanofibers were prepared by electrospinning method. The SAW sensors were measured upon various relative humidity ranging from 11% to 95%, and the sensor of 1.56 GHz showed the maximum frequency shift of −2.5 MHz compared with other devices. Improved performance of PC1-HF was accounted to both the high resonant frequency working at above GHz

Acknowledgement

This research was partly supported by the National Natural Science Foundation of China (Grant No. 61376073).

Yuan Liu received his B.Sc. from the Department of Aeronautics, Xiamen University, in 2011. At present, he is a Ph.D. candidate in Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. His current research interests involve the development of MEMS gas sensors and surface acoustic wave devices.

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Hui Huang received his B.E. from the Department of Mechanical and Electrical Engineering, Xiamen University in 2013. At present, he is a Ph.D. candidate at the Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. His current research interests involve the development of electrospinning instrument.

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Daoping Cai received his B.Sc. and M.S. from the Department of Chemistry, Xiamen University in 2011 and 2013, respectively. At present, he is a Ph.D. student at the Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. His current research interests involve the development of materials for applications in gas sensing and energy storage.

Bin Liu received his bachelor's degree from the Department of Electronic Engineering, Xiamen University in 2011. Now he is a Ph.D. student at the Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. His current research interests focus on fabrication techniques of gas sensors and computational verb similarity based gas classification.

Dandan Wang received her B.E. from the Department of Materials, Xiamen University in 2011. At present, she is a graduate student at the Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. Her current research interests involve the development and application of gas sensing materials.

Qiuhong Li is a professor of Physics at School of Physics and Microelectronic Science. Her work mainly focuses on the nanostructured materials for the nanodevice, gas sensor and lithium batteries application.

Taihong Wang is a professor of the Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University. He became Changjiang Scholar in 2005. His research interests focus on micro-nano devices and circuits.

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Electrospun CeO2 nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor

Abstract

Introduction

Section snippets

Fabrication of high frequency SAW devices

The morphologies and crystal structures of the samples

Conclusions

Acknowledgement

Sens. Actuators B: Chem.

Sens. Actuators

Sens. Actuators A: Phys.

Sens. Actuators B: Chem.

Anal. Chim. Acta

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Sens. Actuators B: Chem.

Synthesis of Ba-doped CeO2 nanowires and their application as humidity sensors

Nanotechnology

A surface acoustic wave humidity sensor with high sensitivity based on electrospun MWCNT/Nafion nanofiber films

Nanotechnology

A highly sensitive humidity sensor based on a nanofibrous membrane coated quartz crystal microbalance

Nanotechnology

Electrospun CeO₂ nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor

Synthesis of Ba-doped CeO₂ nanowires and their application as humidity sensors