High Performance Triethylamine Gas Sensors Based On One-Dimensional Nickel Oxide And Its Composites

Gas sensors based on metal oxide semiconductors（MOSs）have been widely applied in medical health,environmental monitoring,food quality assessment,aerospace and other fields because of their all-solid state,adjustable performance,simple operation and small size.How to construct gas sensors with high sensitivity,fast response and good stability has become the focus of researchers in this field.Water vapor,as a component that can’t be ignored in the actual environment,often affects the gas sensors performance of MOSs negatively,decreasing their stability and reliability.Thus,it is necessary to carry out further research on the humidity influence,and design and develop gas sensors with a high sensitivity and good moisture resistance to meet the actual demands.At present,the detection of triethylamine gas,which is widely used,low toxic and used as a biomarker,is seriously affected by humidity changes and has poor recovery characteristics,which are particularly prominent in the n-type MOS sensors.In contrast,the p-type MOS sensors exhibit high catalytic activity,rich surface defects,and good moisture resistance,although their sensor response is relatively low.Given this situation,Ni O is selected as the primary materials in this paper and are modified by uniaxial electrospinning method through heterometal ion doping,noble metal modification,and heterostructure construction.These can change the hydrophilic/hydrophobic characteristics of the surface of the material,improve the catalytic activity and active oxygen content of the sensitive material,and adjust the carrier concentration of the sensitive body.Finally,the anti-moisture performance of the fabricated gas sensors is gradually improved,and the response/recovery speed,especially the recovery speed,is obviously improved.The related works are as follows:（1）Five groups of Sn-doped Ni O nanofibers with different Sn⁴⁺ratios were synthesized firstly by electrospinning using Sn⁴⁺dopant.It can be seen that the Sn doping can inhibit the growth of Ni O crystals from the XRD characterization results;SEM and TEM characterization results showed that the Ni O sample had a hollow structure.After Sn doping,the fiber continuity was better,and the hollow structure can still be maintained.Gas sensing studies revealed that all sensors showed a same optimal working temperature of 180°C,the optimal Sn doping content was 6 at%and the gas sensor had the highest response（16.6）to 100 ppm triethylamine.Compared with pure Ni O sensor,the response of 6 at%Sn-Ni O sensor to 100 ppm triethylamine gas was increased by 9 times,and the detection limit,response and recovery characteristics were also improved.It is worth mentioning that the 6 at%Sn-Ni O sensor exhibited a stable resistance value（R_g）in the triethylamine atmosphere from 20%RH to 80%RH.Thus,it can be concluded that an appropriate ratio of Sn doping can affect the gas sensing performance of Ni O nanofibers,making them exhibit higher response and certain moisture resistance in triethylamine atmosphere.（2）Although 6 at%Sn-Ni O sensor exhibited a high response to triethylamine gas,the sensor’s response（R_g/R_a）and baseline resistance（R_a）were easily affected by humidity,and the detection limit was only 0.5 ppm.Based on this,we further improved the above shortcomings by Rh modification.In this work,6 at%Sn-Ni O nanofibers with different Rh ratio were prepared using one-step electrospinning method and the effect of Rh addition on the morphology and gas sensing performance was also investigated.The SEM and TEM characterizations showed that all nanofibers were hollow structures and the surface of nanofibers became rougher as the Rh content increased.The XPS results showed that Rh was mainly present in the form of Rh³⁺rather than Rh⁰.The results of gas sensing investigation showed that the addition of Rh can reduce the optimal working temperature to 165°C,and the gas sensor had the highest response when the Rh ratio was 6 at%.The response to 20 ppm triethylamine was 28.3,which was higher than that of 6 at%Sn-Ni O sensor（8.3）in the last work,and the detection limit was further reduced to 50 ppb.The testing results of humidity effect showed that the response value of the sensor to 20 ppm triethylamine changed to 35.7%from 35%RH to 95%RH,which was significantly lower than that of the 6 at%Sn-doped Ni O sensor（51.9%）,indicating that the modification of Rh can further improve the humidity resistance.The improvement of gas sensing performance can be attributed to the increase of catalytic activity on the surface after Rh modification,the increase of oxygen defects and the adsorption and decomposition of water by Rh₂O₃.（3）The humidity resistance of gas sensor can gradually improve through Sn doping and further Rh modification.However,the final response value of the sensor still decreased by 35.7%in the range of low RH to high RH.Therefore,rare earth yttrium（Y）with better moisture resistance was introduced to modify the Ni O.Furthermore,the effect of Y³⁺doping contents（0 at%,1 at%,3 at%and 5 at%）on the gas sensing characteristics was also studied.XRD results showed that Y³⁺ions have been doped into Ni O crystals,SEM and TEM characterizations revealed that the nanoparticles in the nanofibers were more visible after Y doping.TEM showed that a small amount of Y₂O₃ was formed at 3 at%Y doped sample.The gas sensing investigations showed that the sensor exhibited high response and excellent selectivity to triethylamine after Y doping,and the low detection limit for triethylamine was reduced to 0.25 ppm.The investigation about the humidity effect on gas sensing performance revealed that the baseline resistance and response value of the sensors in different humidity environments tended to be more stable with the increase of Y doping.When Y doping content was more than 3 at%,the sensor responses and baseline resistances were almost unaffected by humidity,demonstrating excellent humidity independence.Thus,Y-doped Ni O can be used to prepare triethylamine gas sensors with high response and humidity resistance.（4）Although the humidity resistance of the above sensors was enhanced obviously,the recovery time of these devices were more than 10 s,which was not conducive to real-time monitoring triethylamine gas.Thus,the recovery characteristics of the sensor were improved by constructing the Ni O/Ni Fe₂O₄ heterojunction.In this work,the structure and composition of the material were controlled by controlling the relative ratio of Ni/Fe,the molar ratio of Ni/Fe was 0.5,1.0,1.5 and 3.0 respectively.XRD testing results showed that the Ni O content increased with the increase of Ni/Fe ratio.SEM and TEM characterizations revealed that the nanofibers consisted of an inner core and an outer shell,with a solid nanofiber（Fiber）core and a porous nanotube（Tube）outer shell.As the Ni/Fe ratio increased,the outer diameter of the tube decreased while the internal diameter of fiber became larger.Gas sensing investigations revealed that the fabricated sensors showed the highest response to 50 ppm triethylamine when the Ni/Fe molar ratio was 1.5 and the recovery time was only 3 s,it can return back to the initial state after eight successive cycle tests,demonstrating the superior stability.Most importantly,the gas sensor showed the excellent humidity resistance with an almost constant response to triethylamine from 30%RH to 98%RH.This indicates that the construction of suitable heterostructure is helpful to improve the recovery speed and moisture resistance of the gas sensor.