The Preparation And Investigation Of Formaldehyde Quartz Crystal Microbalance Gas Sensor

With the development of the national economy, the people pay more and more attention to the living condition security. Formaldehyde(HCHO) is one of major indoor air pollutants, so it is very significant to develop the sensor for low concentration formaldehyde detection with low-cost, room-temperature operation and rapid response rates. Quartz crystal microbalance(QCM) gas sensors have advantages such as high sensitivity, fast response, low noise, low-cost, ease of fabrication and good stability, et al., therefore, QCM device was chosen as the sensing structure, and formaldehyde sensors were developed based on organic polymer/multi-walled carbon nanotubes(MWNTs) composite thin films. The sensing properties of sensors were researched when exposed to low-concentration formaldehyde at room temperature. The main research contents include the following two aspects:The mixed polyethyleneimine(PEI)/multiwalled carbon nanotubes(MWNTs) composite thin films were deposited on the QCM device using spraying process. The effects of film thickness and composite ratio on sensing characteristics of QCM sensors were researched at room temperature, and the process parameters were optimized. The sensing properties of PEI/MWNTs-QCM sensor with optimal parameters were tested and analyzed when exposed to low-concentration formaldehyde. The resluts showed that the sensor exhibited the good response characteristcis within 0.6-6 ppm with regression coefficient(R) of 0.97064 and sensitivity of 0.4 Hz/ppm. The detection limit was 0.6ppm. Meanshile, the sensor showed the good reproducibility and selectivity. The stability measurement results indicated that the response values of prepared sensors changes little, but the response and recovery times prolonged. The formaldehyde-sensing properties were influenced greatly by the humidity and temperature. Combined with the X-ray photoelectron spectrometer(XPS) and UV-Vis absorption spectra characterizations, it was believed that the hydrogen-bonding interaction existed between PEI and MWMWNTs, and the sensing mechanism of composite thin films was mainly ascribed to the reversible nucleophilic addition reaction between formaldehyde gas molecules and amine functional groups on PEI structure.Based on the PEI/MWNTs composite thin films, polyvinylpyrrolidone(PVP)/ MWNTs-QCM sensors were fabricated by the sparying process, and the sensing properties of sensors were measured when exposed to different concentration formaldehyde. The results showed that the sensor performed good linear response characteristics and rapid response rates within 0~4 ppm, which also had good reproducibility, selectivity and stability. It was found that the adsorption saturation of sensors was achieved when the formaldehyde concentration was over 6ppm, and it was supposed that the high formaldehyde concentration was poisoning the sensing film. The surface morphology of PVP and PVP/MWNTs composte thin films was cpmpared and analyzed. It was believed that surface area of composite thin film was increased by mixing with MWNTs, thus the sensing properties of sensors were improved.