| Formaldehyde is one of the main indoor pollution gases, mainly from the indoor decoration materials, building materials and textiles. In recent years, it has been shown that formaldehyde can make people suffer from more than10kinds of cancer. Therefore, more and more attentions have been paid to the detection of formaldehyde. Semiconductor oxide sensors are widely caught the attention of researchers, because of simple working principle, large response, simple production process and low cost. But these sensors usually exhibit obvious sensitivity of ppm level to formaldehyde when operating temperature is around200-400℃. Recent efforts have focused on utilization of nanosized particles or porous nanostructural architectures and/or resorting to UV-vis illumination during the sensor operation to lower operation temperatures of the sensors.Nanoporous TiO2powders were prepared following a two-step method in this work. The highly ordered uniform TiO2nanotube arrays were first grown by the electrochemical anodization and subsequently, mechanically milled to obtain the porous TiO2. The fabricated TiO2were characterized by XRD, SEM, TEM, BET and XPS. A resistive-type HCHO sensor based on prepared nanotube powders was fabricated on Al2O3ceramic substrate. The sensitivity, selectivity, stability and response&recovery time of sensors were tested in a static test system, and meanwhile the HCHO response mechanism of sensors was explored. The influence of relative humidity and O2on gas sensing response was studied by a dynamic gas-sensing test system. Through analyzing the impedance spectra under different formaldehyde concentration, an equivalent circuit of sensors based on CPE and Warburg elements was eatablished, and the function of particles was discussed in the gas response process. The change in the DC resistance and AC impedance was monitored respectively to be used as the sensor signals and the response of the sensor to different formaldehyde concentrations from lOppm to50ppm at room temperature was measured. Comparatively, the impedance-type signals measured by AC method were more stable than those DC resistance signals. This has been preliminarily proposed that relative to the DC measurements, the polarization potential barrier due to the building-up of the protons (H+) dissociated from the water molecular adsorbed on the oxide surface could be effectively suppressed under AC measurements, thus leading to the better signal stability. In addition, a pair of wireless sensor nodes were developed by using perpared sensors, which can detect formaldehyde concentration in real-time, and their reliable communication distance is40-100m. ZigBee was used as the communication protocol between the two nodes. Slave node is responsible for data acquisition and sending of formaldehyde concentration, temperature and humidity, and the master node is mainly responsible for state control of slave node as well as receiving and showing of sensor datas. The received data can be displayed in the LCD of the master node,and also can be sent to the PC through the USB for system analysis. Therefore, as-developed sensor nodes can be used for gas sensing testing, or worked as portable indoor formaldehyde detector. |
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