Fabrication And Application Of ZnO-SAW Sensor In Formaldehyde Detection

Chronic respiratory diseases,nasopharyngeal carcinoma,brain tumors and other diseases can be caused by long-term exposure to low dose formaldehyde.Formaldehyde is also the main cause of morbid building syndrome.Furthermore,formaldehyde has been identified as a major cause of sick building syndrome(SBS).In 2004,International Agency for Research on Cancer(IARC)determined that formaldehyde was carcinogenic to humans.In recent years,a number of analytical methods to detect formaldehyde have been reported,such as spectrophotometry,gas chromatography,high-performance liquid chromatography,ion chromatography,polarography,as well as fluorescence.However,these approaches required rigorous conditions and bulky instrumentation,and are unable to detect formaldehyde information on a real-time basis.In this paper,three kinds of ZnO-SAW sensors were prepared,the formaldehyde dry gas and formaldehyde wet gas were detected by a self-built formaldehyde gas detection platform,and the adsorption properties of formaldehyde molecules on the surface of ZnO were calculated by density functional theory(DFT).1)The binding distance of ZnO to formaldehyde was 2.081?,binding energy was 0.496 eV,charge transfer amount was 0.089 e.Adsorption of formaldehyde molecule would be favorable to electron excitation from the lower valence band to the upper valence band and conduction band,thus changing the conductivity of ZnO and producing formaldehyde sensitive signal.Each Zn atom on the surface of ZnO could adsorb one formaldehyde molecule,and all the Zn atoms on the surface could adsorb formaldehyde molecule at the same time.When there were many formaldehyde molecules in the space,some formaldehyde molecules were adsorbed on the surface of ZnO,and some formaldehyde molecules were connected with formaldehyde molecules adsorbed on the ZnO surface weakly.2)SAW chips were fabricated by photolithography,and ZnO sensitive films were prepared on the surface of SAW sensors by magnetron sputtering technique,and were characterized by XRD,SEM and AFM.The roughness of the ZnO films prepared by 50 W,100W and 150 W sputtering power were 0.648 nm,1.35 nm and 2.26 nm,respectively.The quality sensitivity of the fabricated SAW chip was1705 times of that of QCM,and the ZnO-SAW sensor had good temperature stability.3)After absorption of formaldehyde by ZnO-SAW sensor under power of 100 W at a flow rate of 0.3L/min and response time of 1min,the resonant frequency of the sensor was reduced most,frequency shift was maximum,and the average frequency shift in 5min was 5967.2Hz.Using 1ppm,5ppm,10 ppm,25ppm and 50 ppm formaldehyde respectively,it was found that the ZnO-SAW sensor prepared under power of 100 W had higher sensitivity and better linearity to different concentrations of formaldehyde,and the sensitivity to 1-50 ppm formaldehyde was 114.99Hz/ppm after fitting.After the formaldehyde was adsorbed,the resonance frequency of the sensor could be restored to the initial state by pure nitrogen,and the adsorption of the sensor to the formaldehyde dry gas was completely reversible.4)At a flow rate of 0.3L/min and a response time of 3min,the sensitivity of the ZnO-SAW sensor prepared under power of 100 W to formaldehyde at 20%RH?40%RH?60%RH were 136.34Hz/ppm?141.01Hz/ppm?152.51Hz/ppm,the higher the humidity,the higher the sensitivity of the ZnO-SAW sensor to formaldehyde.After adsorption of formaldehyde moisture,the resonant frequency of the sensor did not return to the initial state by advocating pure nitrogen under the same experimental conditions,and the adsorption of formaldehyde moisture was partially reversible.5)The adsorption ability of water molecules on the surface of ZnO was equivalent to that of formaldehyde molecules.Water molecules could not only be adsorbed on the surface of ZnO,but also form hydrogen bonds between them.When there were more water molecules,the amount of water adsorbed on the surface of ZnO increased layer by layer.When the water molecules have been adsorbed on the surface of ZnO,formaldehyde molecules have broken the hydrogen bond between the raw water molecules,forming hydrogen bonds with the water molecules,and one formaldehyde molecule could connect two water molecules.The addition of formaldehyde molecule shorten the binding distance between water molecule and ZnO surface and increased the average binding energy of the whole system.