The Room Temperature Gas Sensor Based on Polyaniline/SrGe4O9 Nanocomposites for Ppt-Level NH3 Detection
Authors: Zhang Yajie , Zhang Junxin , Jiang Yadong , Tai Huiling 
Published: May 2020

Introduction

It is an urgent requirement to development of ammonia sensors with low detection limits for environmental monitoring and disease detection [1]. In recent years, much effort has been devoted to developing high performance gas sensors with low detection limit and room temperature detection. However, the development of gas sensors with ppt-level detection limit still faces enormous challenges.
PANI-based material has a good selective NH3 response at room temperature. However, the applications of PANI-based sensors have been limited by their slow response speed and high detection limit [2]. To address such limitations, Sensors based on the PANI-based nanocomposite materials have been demonstrated to be effective method for low detection limit to NH3 at room temperature. SrGe4O9 is one of the wide-bandgap metal oxide semiconductors and have the best sensing performance to NH3 among alkaline earth metal germanate. In addition, the morphology of SrGe4O9 can be regulated by various synthetic means. Hence, a flexible NH3 sensor based on PANI/SrGe4O9 nanocomposites thin film was proposed for ultra-low level NH3 detection.

Method

SrGe4O9 nanowires were synthesized by a facile one-step hydrothermal synthesis, particular steps are described as fellows: 4mmol Sr(CH3COO)2 and 16mmol CeO2 dispersed in 60mL deionized water, then transferred to a 100 mL Teflon-lined stainless autoclave. The solution was stirred for 40 min and heated at 180℃ for 24h, the reaction product washed 3 times alternately with deionized water and dried at 50 ° C for 24 h. To fabricate the PANI-SrGe4O9 flexible gas sensor, the interdigital electrode was first soaked in PDDA solution (1%) and PSS solution (2 mg/ml, PH≈1, adjusted by HCl). After that, the thin sensor was fabricated by in-situ self-assembly method at ~10℃. Specifically ,100 μL Aniline (C6H5NH2) and 5 mg SrGe4O9 nanowires was dissolved in 20ml HCl (2 M). Subsequently, 10 ml APS ((NH4)2S2O8) was gradually added to the mixture within 120 s. The flexible interdigital electrode was immersed into the mixture when the color of solution turn to pale blue. After 15 min the substrate with PANI-SrGe4O9 nanocomposite thin film was removed and washed with HCl (2 M). Finally, the sensor was dried at room temperature for one day. For comparison, pure PANI thin film was also fabricated by the same method.

Results and Conclusions

In this work, a flexible and ultra-highly sensitive NH3 sensor based on polyaniline/SrGe4O9 nanowires nanocomposites was successfully fabricated via a combined route of facile hydrothermal method and in situ chemical oxidation polymerization method to load the gas sensing material on flexible polyimide (PI) substrate. XRD pattern revealed that polyaniline and SrGe4O9 phases coexisted in the as-prepared nanocomposites. Morphology results indicated polyaniline nanoparticles anchored on the SrGe4O9 nanowires with an average diameter of 100 nm and a length of 2–5 mm. In addition, The NH3 sensing properties of the as-fabricated sensors were evaluated at room temperature (∼25℃) and results indicated that the PANI/SrGe4O9 sensor exhibited higher response value of 344% to 50 ppm NH3, which was greatly superior to the pure PANI. Furthermore, the present sensor also showed fast response speed (37.3s), satisfactory long-term stability, good bending resistance and excellent selectivity towards 50 ppm NH3 at room temperature. The response of the PANI/SrGe4O9 sensor increased with the increasing relative humidity, which can further expand the practical application of the PANI/SrGe4O9 sensor. What's more, the sensor demonstrated an ultra-low detection limit of 50 ppt NH3 at room temperature in 60%RH, which was reported for the first time. Such dramatically enhanced gas-sensing properties of PANI/SrGe4O9 nanofilm might be owing to the synergism effects, the formation of p-n heterojunction at the interface between the n-type SrGe4O9 and p-type PANI, and high surface area of PANI/SrGe4O9 nanocomposite. This research offers new strategies for developing supersensitive gas NH3 sensor and the PANI/SrGe4O9 sensor points its promising applications in ultrasensitive environmental detection and human disease detection.

References

[1] S.Q.Li, P.F.Lin, L.P. Zhao, et al., The room temperature gas sensor based on Polyaniline@flower-likeWO3 nanocomposites and flexible PET substrate for NH3 detection, Sens. Actuators B. 259 (2018) 505–513. doi:10.1016/j.snb.2017.11.081.
[2] N.R.Tanguy, M.Thompson, N.Yan, A review on advances in application of polyaniline for ammonia detection, Sens. Actuators B. 257 (2018) 1044–1064. doi:10.1016/j.snb.2017.11.008.
[3] C.H. Liu, H.L.Tai, P.Zhang, et al., A high-performance flexible gas sensor based on self-assembled PANI-CeO2 nanocomposite thin film for trace-level NH3 detection at room temperature, Sens. Actuators B. 261 (2018) 587–597. doi:10.1016/j.snb.2017.12.022.

Figure 1