The Room-Temperature Planar Type Gas Sensor Based on DPA-Ph-Dbpzdcn/TPA-Dcpp for NH3 Monitoring Application
Authors: He Junming , Liu Fangmeng , Lu Geyu 
Published: July 2021

Introduction
Ammonia (NH3) gas sensor is the indispensable element in safety, air quality monitoring and respiratory analysis fields. The chemoresistor sensors are mainly focused in the investigation of NH3 sensors attributing to the high stability, simple sensing structure, low cost and portable device. However, solving the high working temperature issue and achieving reproducible and stable gas sensing properties by designing novel sensitive material is highly desirable for widespread development of ammonia sensor [1]. Here, the planar room-temperature NH3 sensors are fabricated through spin-coating DPA-Ph-DBPzDCN/TPA-DCPP organic semiconductor materials onto the Al2O3 substrate with Au interdigital electrodes. The two sensors exhibit favorable sensing response as 72.73%/76.4% to 100 ppm NH3 under 98% RH, respectively, also demonstrates the low detection limits, good reproducibility, selectivity and long-term stability. The DFT calculation results further demonstrate that the excellent NH3 sensing characteristics are attributed to the enhanced absorption between NH3 and the N groups of organic molecules, which formed the H2O pre-adsorption under the high RH. This work is expected it can offer some guidance and inspiration for the rational design of sensing materials and further development of room temperature gas sensor.

Results and Conclusions

The synthesis process of DPA-Ph-DBPzDCN/TPA-DCPP were detailed reported previously [2, 3]. The sensor devices were successful fabricated via spin-coated sensitive materials onto the Al2O3 substrate with Au interdigital electrodes and illustrated in Figure 1. It was illustrated from scanning electron microscopy (SEM) image that the organic sensing films were formed uniformly and tightly. Then, the response transients of sensors in different NH3 concentration under 98% and 25% RH were monitored (Figure 2). The response value improved alone with the increasing of NH3 concentration and performed the highest response value as 72.73%/76.4% to 100 ppm NH3 under the 98% RH, respectively. Furthermore, there had a decreasing trend of the lowest detectable NH3 concentration along with the increasing RH, especially, the TPA-DCPP sensor exhibited the lowest detection limit as 500 ppb under 98% RH.
Accordingly, the density functional theory (DFT) calculations were performed to gain further insight into the interaction between gas molecules and the sensitive materials, also the enhanced ammonia sensing properties under the high RH. The three most likely ammonia adsorbing sites (A: trihpenylamine-ortho C on the benzene ring; B: pyrazine-N; C: cyano-N) were selected due to the negative charge was mainly located. The configuration and the charge density difference after NH3 adsorption (Figure 3) demonstrated that the charge transfer occurred through the NH3 given electrons to organic compounds, thereby the conductivity of organic semiconductors increased until to an equilibrium state. Simultaneously, the NH3, H2O, NH3-H2O adsorption energies of the corresponding adsorbing sites were given in Table 1. It was shown that the NH3 adsorbing to organic compounds after H2O pre-adsorption were more capable compared with the NH3 adsorption only. Meanwhile, the NH3 adsorption energy before and after H2O pre-adsorption of the pyrazine-N on TPA-DCPP (-0.1809 eV, -0.4011 eV) was obviously higher than the DPA-Ph-DBPZdCN molecule (-0.14362 eV, -0.15679 eV), resulting in the response value of TPA-DCPP sensor was apparently improved compared to the DPA-Ph-DBPZdCN sensor under the low/high RH, that were attributed to the increasing of the negative charge on pyrazine-N owing to the directly link between and the pyrazine the cyano group, the change of molecular structure leading to a significantly stronger coulombic effect between pyrazine-N with NH3. Taken together, the excellent NH3 sensing characteristics were attributed to the enhanced absorption between NH3 and H2O pre-adsorption organic molecules under the high RH [4]. The DPA-Ph-DBPZdCN/TPA-DCPP could be the promising NH3 organic sensitive materials for the room temperature sensor, meanwhile, the discussion of sensing mechanism bring the further understanding of the organic gas sensors, providing the new concept of organic materials which has potential application value in gas monitoring of production and daily life at room temperature.

References

[1] Y. Zang, F. Zhang, D. Huang, C.-a. Di, Q. Meng, X. Gao and D. Zhu, Specific and Reproducible Gas Sensors Utilizing Gas-Phase Chemical Reaction on Organic Transistors, Advanced Materials. 26 (2014) 2862-2867. doi: 10.1002/adma.201305011.
[2] S. P. Wang, Y. Miao, X. J. Yan, K. Q. Ye and Y. Wang, A dibenzo a,c phenazine-11,12-dicarbonitrile (DBPzDCN) acceptor based thermally activated delayed fluorescent compound for efficient near-infrared electroluminescent devices, Journal of Materials Chemistry C. 6 (2018) 6698-6704. doi: 10.1039/c8tc01746h.
[3] S. Wang, X. Yan, Z. Cheng, H. Zhang, Y. Liu and Y. Wang, Highly Efficient Near-Infrared Delayed Fluorescence Organic Light Emitting Diodes Using a Phenanthrene-Based Charge-Transfer Compound, Angewandte Chemie International Edition. 54 (2015) 13068-13072. doi: 10.1002/anie.201506687.
[4] J. He, X. Yan, A. Liu, R. You, F. Liu, S. Li, J. Wang, C. Wang, P. Sun, X. Yan, B. Kang, J. He, Y. Wang and G. Lu, A rapid-response room-temperature planar type gas sensor based on DPA-Ph-DBPzDCN for the sensitive detection of NH3, Journal of Materials Chemistry A. 7 (2019) 4744-4750. doi: 10.1039/C8TA10840D.

Figure 1