Preparation Of The In₂O₃-Based Nanomaterials And Used As The Ammonia Sensor

In₂O₃ is an n-type semiconductor and it has been well studied as gas sensor material for detecting ethanol, gasoline, butane, carbon monoxide, methane and ozone, the study of using this material as the ammonia sensor is recognized by workers. So far, the nano-powers is only doped with noble metal for improving its sensitivity and selectivity of detecting Ammonia, more work should be done in this field. In this paper, the results of a systematic investigation of the synthesis and characterization of the In₂O₃ based nano-powers have been given, some correlation between the synthesis process, structure of powers and property of sensors were analyzed preliminarily, and the mechanism for NH₃,CO,H₂ selectivity is tried to be discussed. The main results are as follows: 1,Using indium chloride (InCl₃·4H₂O) as starting materials, the preparation conditions of In₂O₃ nano-powers synthesized by ammonia coprecipitation and the Sol-Gel methods, the crystallite size of In₂O₃ with ammonia coprecipitation is bigger comparatively. Preparation by Sol-Gel and the sol was stabilized with nitric acid, the effects of pH value on dispersal of Sol, the crystallite size of powers and the gas-sensitivity of sensors were studied. The results shows that controlling pH=5 can obtain smaller particles and ESEM shows that the obtained particles have the average size. And the higher or lower pH value the crystallite size is produced bigger. With increasing pH value from 5 to 9, the optimal operating current of sensors changes from 160mA to 180mA. 2.As dealed with the In(OH)₃ Sol as pro-treat process by the sintering method and the supercritical drying technology, the supercritical drying technology can prevent the particle from reuniting and control the grain-size within 100nm, but the crystallite size is bigger than the size by the sintering method. The better sensitivity to NH₃ was achieved for the particle by sintering method. 3,Structure of powers and property of sensors were effected by calcined temperature. TG-DSC and XRD show that the cubic-In₂O₃ have been formed at 350℃, with rising the calcined temperature the crystallite size of In₂O₃ increases and becomes larger. Our measurement results show that the sensitivity of In₂O₃ sensor calcined at 350℃,one hour, for detecting CO and H₂ was higher, and it is better for detecting NH₃ at particular calcined temperature(550℃). 4,In order to improve the gas sensitivity and selectivity of the pure In₂O₃ sensor, the powers doped with different valences transition metallic ions was prepared. It is found that our ammonia sensors after doped with the higher valence ions shows more obvious enhancement of selectivity than the other two reducing gases, H₂ and CO. Data of IR spectra of pure In2O3 and doped In₂O₃ shows that, as for the In₂O₃ doped with the higher valence ions, the obvious change of In-O bonds takes place at the wavenumber near 420cm^-1. The absorption bands are shifted to higher wavenumber and tended to split into several apices. Among all of our doped ions, Ti(IV)-In₂O₃ system was studied detailedly and the XRD, BET, magnetic susceptibility, FT-IR, ESEM, O₂-TPD and gas sensitivity measurement were characterized. With the doped content of Ti(IV), the particles doped with 1mol% Ti(IV) are reunited badly, but the sensor can obtain good selectivity for detecting ammonia. The sensor made by In₂O₃ doped 5mol% Mo(VI) have high sensitivity and selectivity to NH₃ at operating current (180mA). 5,According to the comprehensive analysis of the different reacting process of In₂O₃ based nano-powers to NH₃, H₂, CO, the existence of the adsorbed oxygen is testified on the surface of doped materials by O2-TPD. As the species and concentration of defects changed after doping the higher valence ions, the concentration of oxygen vacancies is decreased and the other defects are occurred. The selectivity of our doping materials is achieved for NH3 gas. Additionally, with increasing concentration of doping ions, the ions can exist on the surface of particles, and these affects the interaction between the materials and the gas molecules. And then the sensitivity to different gases and the optimal working current of sensors are changed. The different electronic configuration leads to the different adsorption and activation behaviors to NH3, H2 and CO. The adsorption sites are decreased in In2O3 to H2 and CO with doping ions. The mechanism is that the higher selectivity for NH3 sensor is due to a lone pair of electrons of ammonia molecules...