Study On Room Temperature NH₃ Sensors Based On Phenanthrene Pyrazine Derivatives Organic Sensitive Materials

Resistive gas sensor has the outstanding characteristics of miniaturization,low cost and easy integration,which is the key core sensing device for the construction of Internet of Things-gas monitoring,while the low power consumption is an important development trend in the future.Ammonia（NH₃）sensors have significant applications in the fields of environmental monitoring,agricultural and animal husbandry supervision and medical diagnosis and treatment.The development of high-performance room temperature NH₃ sensors which meets the detection requirements of different scenarios is the research focus and difficulty in this field,as well as an effective strategy to achieve low-power sensors.This paper focused on the room temperature NH₃ sensors based on organic semiconductor sensing materials,starting from the design and selection of novel organic small molecules sensitive materials and the optimization of device structure.The planar room temperature NH₃ sensors based on the sensitive materials of dicyano substituted phenanthropyrazine-benzene（DPA-Ph-DBPz DCN）and discyano substituted phenanthopyrazine（TPA-DCPP）were fabricated,and the flexible self-healing room temperature NH₃ sensor based on the dichyano substituted phenanthopyrazine derivative（TPA-3DCNPZ）sensitive material with the twisted configuration and the laminated structure.By means of the density functional theory（DFT）calculation,the sum frequency generation spectroscopy and the electrochemical impedance measurement technical method,the sensitivity mechanism of the sensors was systematically studied.The main research contents were as follows:（1）The planar room temperature NH₃ sensor based on DPA-Ph-DBPz DCN was constructed via the design and selection of 3,6-bis（4-（diphenylamino）phenyl）dibenzo[a,c]pyrazine-11,12-dinitrile（DPA-Ph-DBPz DCN）organic material with the D-π-A-π-D type as sensitive material and spin-coating process.The results indicated that the DPA-Ph-DBPz DCN sensor exhibited the highest response value（72.73%）to 100 ppm NH₃ at room temperature and 98%relative humidity,the response and recovery time to20 ppm NH₃ were 48 s and 15 s,and the lowest detection limit was 2 ppm.Meanwhile,the sensor device exhibited great repeatability,selectivity and stability to NH₃.It was demonstrated by DFT calculation that the well sensitivity of the sensor was mainly attributed to the excellent NH₃ adsorption characteristics of the cyano and pyrazine groups on the sensitive material of DPA-Ph-DBPz DCN.Especially,under the high humidity condition,the pre-adsorption of H₂O further enhanced the NH₃ adsorption capacity of DPA-Ph-DBPz DCN and effectively improved the NH₃ sensitivity.（2）The planar room temperature NH₃ sensor based on 7,10-bis（4-（diphenylamino）phenyl）-2,3-dicyanopyrazino phenanthrene（TPA-DCPP）organic sensitive material with the D-π-A-π-D type was fabricated by spin-coating process.Through optimizing the connection mode of pyrazine and cyano groups of organic molecule,the adsorption characteristic of pyrazine-N was regulated to improve the NH₃sensitivity of the sensor.The results of gas sensitive detection exhibited that the baseline resistance of the TPA-DCPP sensor was 2 orders of magnitude lower at 98%relative humidity than 17%relative humidity and room temperature,and showed the highest response value（76.4%）to 100 ppm NH₃ and the lowest detection limit（500 ppb）.In addition,the sensor device also exhibited the excellent response and recovery properties,selectivity and long-term stability of 396 days to NH₃.Via the complex impedance test,the sum frequency generation spectroscopy test and DFT calculation,it was further explained that the improved NH₃ sensitivity of the TPA-DCPP sensor was attributed to the strong NH₃ adsorption characteristic of cyano and pyrazine group sites of TPA-DCPP sensitive material,which effectively improving the conductivity of the sensitive material.Furthermore,the mechanism of synergistic adsorption effect of H₂O and NH₃on sensitization of the sensor under high humidity condition was revealed.（3）The6,6’,6’’-（nitrilotris（benzene-4,1-diyl））tris（5-phenylpyrazine-2,3-dicarbonitrile）（TPA-3DCNPZ）organic sensitive material was designed and prepared successfully,that was fabricated on alumina ceramic substrate to construct the planar NH₃ sensor.The planar NH₃ sensor based on the TPA-3DCNPZ sensitive material exhibited the highest response value（86.8%）to 100 ppm NH₃ and the lowest detection limit（300 ppb）at room temperature and 98%relative humidity.In order to solve the problem which the baseline resistance of the planar sensor was too large,using TPA-3DCNPZ as the sensitive material,the room temperature NH₃ sensor with the laminated structure was fabricated on the flexible self-healing polycarbonate（PU）substrate by thermal evaporation and spin-coating process.Through the design of the laminated sensor structure,the charge transmission path in the sensitive material was shortened,the conductivity of the sensor was improved,and the baseline resistance of the device was significantly decreased.The research results indicated that the baseline resistance of the Au/TPA-3DCNPZ/Ag-PU laminated sensor device was about 3.48 MΩat 15%relative humidity,which was 5 orders of magnitude lower than that of the planar TPA-3DCNPZ sensor.Moreover,the TPA-3DCNPZ sensor based on laminated structure also exhibited the low detection limit（1 ppm）to NH₃ at room temperature and 98%relative humidity,with excellent selectivity,bending stability and self-healing properties.The complex impedance test and DFT calculation proved the sensitization mechanism of the TPA-3DCNPZ sensor to NH₃ under different humidity conditions.