The Gas Sensing Properties of Nanocrystalline WO3-Based Hybrid Systems

• Main Authors: Wu, Chuan-Yi, 吳泉毅
• Other Authors: Lin, Hong-Ming
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/71188645308768204825

博士 === 大同大學 === 材料工程研究所 === 91 === Tungsten oxide nanoparticles are prepared by evaporating and oxidizing the tungsten boat in helium and oxygen atmosphere and then quenched to the liquid nitrogen temperature. The as-prepared tungsten oxide nanoparticles are porous-free with uniform size. The morphology and particle size distribution of the as prepared and after sinter treatments tungsten oxide nanoparticles are revealed by TEM and AFM. The long-range order of these nanoparticles can be examined by X-ray diffraction technique. The as-prepared nanoparticles exhibit a mixture structure of monoclinic and hexagonal crystals. Preliminary X-ray diffraction results indicate that the hexagonal structure is transformed to monoclinic structure after annealing to above 600℃. In order to better distinguish the structural properties of the tungsten oxide (WO3-x) nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or tungsten can be determined. According the XAS result, the shape of the W L3-edge undergoes no considerable changes. This infers that structural transformation of tungsten oxide nanoparticle may be caused by the migration of oxygen after sintering. From the O K-edge of absorption spectrum, it suggests that a mixture phase structure is obtained when sintered below 300℃. And this result indicates that heat treatment to approximately 600℃ produces a stable structure of a monoclinic crystal of WO3. The porous nano-WO3 sensor surface adsorbs water vapor easily through the pores. The sensor resistance decreases while the sensor capacitance increases with increasing humidity. The gas sensing properties can be obtained from the changes of the sensor’s resistance as a function of time during different CO concentrations range from 120ppm to 600ppm and NO2 concentrations range from 10ppm to 350ppm. In NO2 gas detection, pure nano-WO3 gas sensor has the best sensing properties, but cannot be measured in high concentration of NO2 gas. We can find the sensitivity of hybrid CNTs/WO3 gas sensor (p-type) is obviously lower than pure nano-WO3 (n-type) gas sensor with inverse sensing properties. One of the most fussing is the memory effect of detected gas. It means the resistance of gas sensor cannot return to background after desorption. The resistance of gas sensor will continuously increase till the gas sensor loses recovery. This is caused by either an irreversible reaction or the change of sensor structure of gas adsorption during the thermal operation of sensors. For measurement at a lower temperature such as 100℃, it can enhance the stability of gas sensor and increase the life time of the heater. If the dopant CNTs/WO3 gas sensor cannot recovery after gas detecting, we can introduce a heating process to higher temperature to help it recover. This interesting phenomenon is observed dopant carbon nanotubes on the surface of WO3 nanoparticles. The sensitivity tends to increase gradually with increasing CO concentration. The WO3-based gas sensors are very sensitive for detecting CO gas. The carbon nanotubes dopant can help the nano-WO3 gas sensor to adsorb the detected gas fast and stable, but it also slows down desorption of the detected gas. The gas-sensing property of hybrid CNTs/WO3 gas sensor to O2 gas detection is similar to CO gas detection. Because carbon nanotubes is a kind of p-type semiconductor, the gas sensitivity is realized through the charge transfer in which the gas molecule to be sensed acts as an electron acceptor forming a redox couple, and the positive charge produced is delocalized over the carbon nanotubes. We can sure what kind of interaction between gaseous molecule and sensor’s surface. This problem may be resolved by using in situ measurement of FTIR spectrum and near-edge X-ray absorption fine structural spectrum to explore the gas sensing mechanisms of hybrid materials. The temperature of adsorption of NO2 can have an influence upon the respective proportion of each nitrate species, i.e. free nitrate or N-bounded nitrate. This properties can be examined by the desorption behaviors of NO2 and CO gases at varying temperatures that observe the N―O and C―O changes of infrared bands. For adsorbed CO, it can be argued that the shake-up intensity should relate to the population of the molecular 2π* orbital in the ground state. This orbital is populated with approximately one electron in the core-ionized molecule, because of charge transfer screening from the hybrid system. The relative change between the initial and final states, and thus the satellite intensity, is larger if the ground-state population is low. This indicates that the ground state CO-2π* population in the 2π* state is lower for CO on hybrid CNTs/WO3, which can be expected because of smaller interaction strengths.