Investigation Of Preparation And Performance Of SAW Gas Sensor Based On SnO₂ Composite Film

When a surface acoustic wave?SAW?gas sensor is placed in the gas environment,the gas molecules will contact with the sensitive layer of the sensor.When the gas molecules enter the sensitive layer,the following three changes may occur:the effective mass of the film?mass loading?,the elastic modulus/density/viscosity?elastic loading?or the conductivity?electrical loading?,all of which can change the resonant frequency of SAW devices.In this paper,SiO₂,SnO₂ and CuO,which are common metal semiconductor oxides,were prepared by precursor colloids,and composite films were prepared as gas sensing materials.The SAW ammonia sensor based on SiO₂-SnO₂ composite film and the SAW hydrogen sulfide sensor based on SnO₂-CuO composite film were successfully prepared.?1?Firstly,SiO₂ sol was prepared by the St?ber method,then SnO₂ precursor colloid was prepared,and then mixed sol was prepared by volume ratio of 1:1.The prepared mixed sol was spin coated on ST-cut SAW quartz device,and annealed in the air at 450°C for 2 hours to obtain SiO₂-SnO₂ composite film sensitive layer.In ammonia environment,the frequency shifts of SnO₂ sensitive layer and SiO₂-SnO₂ composite sensitive layer were positive,while that of SiO₂ sensitive layer was negative.The positive frequency shift is related to the dehydration and condensation of hydroxyl group,which makes the film harder and lighter.The negative frequency shift is mainly due to the increase of mass load caused by the adsorption of ammonia molecules by water molecules on the film surface.Compared with carbon monoxide,hydrogen,nitrogen dioxide and methane,the gas sensor based on the SiO₂-SnO₂ sensitive layer has a positive frequency shift of 631 Hz in 3 ppm ammonia environment.It has a good selectivity for ammonia gas,as well as a good repeatability and stability.?2?Firstly,SnO₂ and CuO precursor colloids were prepared by the sol-gel method.With volume ratio of 1:1,the mixed sol was spin coated on the ST-cut SAW device.The CuO-SnO₂ sensitive layer was obtained after annealed for 2 hours in the air at 400°C.SnO₂ and CuO are n-type semiconductors and p-type semiconductors,respectively,which form p-n junction.When CuO reacts with H₂S,p-n junction disappears,so that the change of conductivity is of great significance to enhance the sensitivity and selectivity of the sensor.The detection limit of SAW hydrogen sulfide sensor based on CuO-SnO₂ sensitive layer in indoor environment can reach 100 ppb,and the frequency shift is-978 Hz.Compared with some common gases such as ammonia,nitrogen dioxide,carbon monoxide and hydrogen,it has better selectivity for hydrogen sulfide.In addition,after systematically studying the sensing mechanism of the sensor based on CuO-SnO₂ sensitive layer,it is found that conductivity and mass load are the main reasons for the negative frequency shift.