Preparation And Humidity-sensitive Properties Of Ultrafine Nano SnO₂ And TiO₂

As a kind of detecting device, humidity sensor can convert the content of water vapor in the atmosphere or a particular environment to electrical signals, and it has been applied widely for environment monitoring, food strorage and preservation of culture relics, etc. The disadvantages of the conventional semiconductor humidity sensors are narrow testing range, long response and recovery time, small sensitivity and poor stability, etc, which limits their applicabitities in the more stringent conditions. Morphology structure and grain diameter of humidity-sensitive materials and doped with other ions to the material, which can affect the humidity-sensitive properties of the materials. Therefore, the study of changing the humidity-sensitive material of semiconductor humidity sensors from the three aspects and improve humidity-sensitive properties have positive significance.In this paper, coral-like SnO₂ mesoporous material and four kinds of SnO₂ and Ti O2 solid solution materials with molar ratio of 2:1, 6:1, 8:1, 10:1, respectively, were prepared by hydrothermal method using Sn Cl4·5H2O and Ti（OC4H9）4 as raw materials. SnO₂ and SnO₂-Ti O2 materials were characterized by XRD, FE-SEM, TEM, BET, etc, and humidity sensing properties of the moisture sensors which made of SnO₂, SnO₂-Ti O2 system, Ti O2 have been investigated, the main results are as follows:1. Made two kinds of grain diameter of Ti O2（A12 and P25） from the humidity sensors to test the humidity-sensitive properties. Through the tests, the impedance values of sensors increased with the relative humidity level decreasing, and they had different best working frequency, A12 was 1 k Hz and P25 was 500 Hz. Under their respective best working frequency, the sensitivity of A12 was 790 when the relative humidity changed from 11% to 95%, which showed that the response and recovery time was 6 and 3 s, respectively. For P25 the sensitivity was 36.3 and the recovery time was 2 s, but the response time was very long. It can be seen that the grain diameter of material had a great influence on the humidity-sensitive properties, and the smaller grain diameter of humidity sensing material made from sensor showed the better properties.2. The as-prepared SnO₂ material has tetragonal rutile structure, and its average crystalline size was 10.3 nm. The surface morphology of SnO₂ material was similar to a kind of marine organism called coral. The coral-like SnO₂ was mesoporous structure, its pore size was 5.7 nm, and the specific surface area reached up to 63.3 m2/g. The higher specific surface area and mesoporous structure would be advantageous for water vapor adsorption-desorption and enhance the humidity-sensitive properties. The results of humidity sensing tests exhibited that the best working frequency of the humidity sensor based on SnO₂ was 500 Hz. Under the best working frequency the response and recovery time of this sensor was 4 and 43 s, respectively, and the sensitivity was 1008 when the relative humidity changed from 11% to 95%. Compared with the literatures it can be seen that morphology change can improve the humidity-sensitive properties of the humidity sensor based on SnO₂.3. The as-prepared SnO₂-Ti O2 solid solution materials with 4 kinds of molar ratio have tetragonal rutile structure, no Ti O2 crystallization peaks were observed, which demonstrated that the Ti4+ replaced the Sn4+ into SnO₂ crystal lattice by isomorphous way and formed the solid solution. The crystalline size of 4 kinds of SnO₂-Ti O2 powder were 5.5 nm（2:1）,6.4 nm（6:1）,8.7 nm（8:1） and 9.9 nm（10:1）, respectively. The SnO₂-Ti O2 solid solution materials have coral-like morphology and cross-linked sacking structure. Meanwhile, some pores and holes were on the surface. The SnO₂-Ti O2 solid solution materials were mesoporous material. The pore size was 14 nm, 15 nm, 17 nm and 20 nm, and the specific surface area was 64.38 m2/g, 62.78 m2/g, 63.40 m2/g, and 60.59 m2/g, respectively. During the testing, the amount of doped Ti O2 had a great influence on the humidity-sensitive properties. The results of humidity sensing tests exhibited that the best working frequency of 4 kinds of humidity sensors were 500 Hz, and the sensor based on SnO₂-Ti O2 with molar ratio was 6:1 showed the best properties. Under the best working frequency the response and recovery time of this sensor was 2.6 and 40 s, respectively, better than the humidity sensor based on SnO₂, and the sensitivity was 12080 when the relative humidity changed from 11% to 95%. Sensitivity not only the SnO₂-Ti O2 with molar ratio was 6:1, but also the other 3 ratio of SnO₂-Ti O2 was far more than the humidity sensor based on SnO₂. Obviously, Ti O2 doping improved the humidity-sensitive properties of coral-like SnO₂ sensor, and the amount of Ti O2 doping had different effect on the humidity-sensitive properties.