Preparation And Performance Study Of Surface Acoustic Wave Ammonia Sensors

When preparing high performance surface acoustic wave ammonia sensors,the three couplings are critical.The coupling between the gas molecules to be measured and the sensor sensitive layer is the first coupling.The second coupling is the coupling between the sensor element and the test system.The last coupling relies on signal analysis to convert the experimental data into gas concentration.In order to get a better coupling effect,some possible ways are proposed.The prepared sensitive layer should have good selective adsorption to the target gas molecules.At the same time,good compatibility of the sensitive film with the surface acoustic wave device is critical?for example,good adhesion to the piezoelectric substrate and no damage to the SAW during the preparation process?.Finally,the preparation of sensitive membranes should be easy and their physical parameters?such as thickness,roughness,uniformity,etc.?should be easily controlled.In this paper,we choose zinc oxide and graphene oxide as the sensitive layer.The former is representative of metal semiconductor oxides,and the latter is one of the most sensitive materials in recent years.Surface acoustic wave ammonia sensors based on zinc oxide nanotube?NT?array and graphene oxide nanofilm were successfully prepared.ZnO NT array sensitive membranes were prepared by a strategy of deposition of long nanorods?NRs?and then conversion of NRs into hollow tubes.The ZnO seed layer was firstly grown on the ST-cut quartz by sol-gel method and spin coating method,then the ZnO NRs array was grown by chemical bath method,and finally the ZnO NRs were etched into ZnO NTs by neutral green KCl solution.In this article,a wurtzite ZnO NT array was successfully grown on ST-cut SAW devices,and the size of the NTs was small,as well as good C-axis characterized by XRD.However,this SAW sensor has a large Q and a large insertion loss.Graphene oxide was prepared by the modified Hummers method,and then the graphene oxide sensitive layer was prepared on the ST-cut surface acoustic wave device by spin coating method.The rich oxygen-containing functional groups on the surface of graphene oxide have great significance for enhancing the sensitivity and selectivity of the sensor.The presence of oxygen-containing functional groups on the GO film makes it very sensitive to ammonia at room temperature?e.g.,with a positive frequency shift of 620 Hz for 500 ppb ammonia gas?,and the SAW device shows an excellent selectivity?as compared to other gases such as H₂,H₂S,CO and NO₂?and a good reproducibility.In addition,we systematically studied the sensing mechanism of the sensor and found that the elastic loading is the main reason for its positive frequency shift.