Reasearch On Photo-Gas Sensitive Properties Based On ZnO Nanowalls/rGO At Room Temperature

NO₂,which is harmful to human health,is one of the sources of haze and acid rain.Therefore,detection of NO₂ is particularly important.As a wide-band gap material,ZnO nanowalls（ZnO NW）has excellent properties in photoelectric and gas sensitivity.However,the high operating temperature（200-400 ℃）limits their application in some fields.Under UV irradiation,a large number of electron-hole pairs can be generated to increase free carrier concentration,which accelerate the adsorption and desorption.Therefore,the gas-sensitivity of ZnO based gas sensor can be improved effectively by light illumination.However,photogenerated electron-hole pairs recombine seriously in ZnO.Due to porous graphene quickly fast transformation carrier.Addition of it can effectively inhibit the recombination of photogenerated electron-hole pairs.Therefore,the ZnO NW/rGO heterojunction is constructed to detect NO₂ at room temperature under UV irradiation.In this paper,porous reduction graphene（rGO）are prepared by spraying method combined with high temperature thermal reduction method,ZnO nanowalls are then grown on this rGO films by dip coating method to fabricate ZnO NW/rGO heterojunction,then its photo-gas sensing properties are researched.The conclusions are as follows:（1）ZnO nanowalls prepared at 6000 μm/s pilling rate has the better photo sensitivity（5.009）to 365 nm wavelength light at room temperature,the response-recovery times are 43s-10 s,respectively.When it is exposed to 50 ppm NO₂ under UV illumination,the gas sensitivity is 33.172,the response-recovery times are 40 s-4 s,respectively.However,when it is free of UV irradiation,the gas sensitivity to 50 ppm NO₂ is only 3.226.Therefore,the gas sensing properties of the sensor can be greatly improved by the UV illumination.The dependence of light on the gas sensing properties are explored via the surface photocurrentanalysis irradiated by 300-500 nm light.The photocurrent peak value of ZnO nanowalls surface reaches the maximum（11×10-12A）as the illuminated light wavelength at 365 nm,which means the sample has a high photoinduced charge separation efficiency.Thus,a large number of electron-hole pairs will be generated under UV irradiation,then those pairs will migrate to the sample surface under the influence of the electric field and react with NO₂.Therefore,the gas sensing properties of ZnO nanowalls to NO₂ is improved.（2）Pourous rGO prepared at spray concentration of 0.2 mg/mL has the better gas sensitivity（1.376）to 50 ppm NO₂ at room temperature,the response-recovery times are 24 s-50 s,respectively.The gas sensitivity of rGO to 50 ppm NO₂ decreases from 1.376 to 1.068 with the increase of its spraying concentration 0.2 mg/mL to 0.8 mg/mL.The integral value of GDI/I is increased with the increase of spraying concentration via Raman analysis.In addition,defects are positively correlated with I/IGD integral values,which decreases the gas sensitivity of rGO.（3）The ZnO/rGO heterojunction prepared at 6000 μm/s pilling rate and 0.2 mg/mL spray concentration has the better photo sensitivity（6.279）to 365 nm wavelength light at room tenmerature,the response-recovery times are 35 s-8 s,respectively.When it is exposed to50 ppm NO₂ under UV illumination,the gas sensitivity is 35.103,the response-recovery times are 38 s-3 s,respectively.The heterojunction has higher surface photocurrent（19×10-12A）than pure ZnO nanowalls（11×10-12 A）,thus more electron-hole pairs are generated.In addition,the photogenerated electron-hole recombinations are effectively inhibited by rGO because of its excellent electrical conductivity,which leads to more photogenerated pairs react with O₂ and NO₂.