Investigation Of ZnO Gas-Sensing Mechanism And Application Of Amplification Of The Sensor's Signals

The volatile organic compounds(VOCs)are silent killers of human health due to their potential toxicity,carcinogenicity and mutagenicity,making it very urgent to monitor the content of VOCs in the environment on a large scale.In addition,some specific VOCs can be used as biomarkers of certain diseases.But it is challenging to determine the exact content of the trace VOCs in the breath.The metal oxide(MOX)sensors have promising applications in these two aspects.In this paper,in order to improve the sensitivity and selectivity(considering the humidity)of MOX sensors,the gas-sensing sensing mechanism of ZnO is explored and the sensor's signal amplification technology is designed.Finally,new materials and new technologies are tried to apply to detect the trace concentration xylene and acetone.The content and results are as follows(1)The single ZnO nanocomb gas-sensing device is used as a model to investigate the influence of the noble metal Au on the gas-sensing performance of nanocombs.The single-temperature-zone chemical vapor deposition(CVD)method is utilized to prepare the ZnO nanocombs by using ZnO powders and graphite as the raw materials.The different sizes of Au particles are further loaded to investigate their effects on the response to the acetone gas for ZnO nanocombs.It is found that the nanocombs loaded with the 3.7±0.7 nm Au have the responses of 3 and 21 to 0.2 ppm(part per million)and 5 ppm acetone respectively,much higher than those of the pure-phase nanocombs.At the same time,the selectivity to acetone is enhanced However,if the size of the gold particles further increases to 11.7±3.0 nm,the gas-sensing performance is seriously deteriorated.(2)The single ZnO nanowire gas-sensing device is used as a model to investigate the role of donor-and acceptor-defects in gas-sensing mechanism.ZnO nanowires are synthesize by dual-temperature-zone CVD,also with the ZnO powders and graphite as raw materials.A single nanowire field-effect transistor(FET)is constructed to study the electron mobility and electron concentration versus the diameter of the nanowire,from which the surface charge layer is calculated to be 43.6±3.7 nm.The results of the acetone gas-sensing test show that when exposed to 5 ppm of acetone,the nanowires with the diameter of 110 nm exhibit a response of 42,while those with the diameter of 80 nm exhibit a response of 5,which is contradictory to the particle model.Importantly,this trend is independent of temperature in the range of 200 ?-375 ? and is hardly affected by the contact between the nanowires.The photoluminescence properties of ZnO nanowires with different diameters are probed by using the micro-photoluminescence technology to obtain the crystal defects of the single nanowire.The results show that the donor content of the 110 nm ZnO nanowire is the maximum and the content of the acceptor is the minimum.As a result,donor-and acceptor-defects play a predominant role in gas-sensing performance.Finally,a gas-sensing mechanism is proposed:the more the donors are,the fewer the acceptors are,the better the gas-sensing performance is(3)The signals of the MOX sensor are modulated(i.e.,amplified and reduced)by using different types of FETs.Firstly,the p-and n-type depletion mode FETs are employed to capture and amplify the weak signal of the MOX sensors,with amplifying the responses by 5-6 times.The limit of detection can be extended to the lower concentration but the response-recovery time of the MOX sensors are not affected.The amplifying mechanism is that the exponential increase of its resistance is induced by the slight increase of the absolute gate voltage of the FET.The amplifying technology,which is different from the lock-in amplifier and operational amplifier,is simple,easy to design,suitable for all of resistive MOX sensors.Thus,the signal amplification technology enhances the capability of the MOX sensors to detect the low-concentration gas and has great potential in the air quality monitoring and breath analysis for diseases.Secondly,the p-and n-type enhancement mode FETs can strongly reduce the signals of the MOX sensors.The reducing mechanism is that the exponential decrease caused by the increase of the absolute gate voltage of the FET.(4)The new materials and new FET amplification technologies are developed to detect the trace VOCs.On one hand,the synergetic p+n FETs allow the commercial sensors(Figaro TGS2602)to detect 10 ppb(part per billion)of xylene(response is 2.6).In the high-humidity environment,the synergetic p+n FETs make TGS2602 sensors still remain better sensitivity and selectivity to the xylene,which can be affected by the humidity a little.Therefore,whether or not the xylene concentration is above the standard in indoor air(?42 ppb)can be easily assessed by using the synergetic p+n FETs circuit.On the other hand,Mn-doped ZnO(MZO)gas-sensing materials are fabricated by the coprecipitation method and they display a high sensitivity and selectivity to acetone.To enhance the response of the MZO sensors to trace concentration acetone,the interlocking p+n FETs are designed,causing a transilient response of the MZO sensors to 2 ppm acetone under the condition of high humidity,and the MZO sensors have a better ability of anti-interference,which is greatly beneficial to the qualitative screening of the diabetic patients,saving the time and the cost greatly.(5)Zooming p+n FETs are designed to handle the problem of signal saturation of the MOX sensors in the high-concentration target gas.It can reduce the sensitivity to the low-concentration gas but increase the sensitivity to the very high-concentration gas,realizing the goal that the effective detection range of the MOX sensors is tuned to the higher concentration.The principle of the zooning p+n FETs is that the n-type enhancement mode FET works firstly to suppress the signals and the amplifying role of the p-type depletion mode FET depends completely on the concentration of the target gas.This p-type FET will be drnven to play an amplifying role only if the concentration is large enough.Now,the effective detection range of the MOX sensors can be flexibly changed by selecting a suitable type of FET according to the actual need.