Study On The Preparation And Gas Sensing Properties Of LaFeO₃/SnO₂ Nanocomposites

As is well known, CO₂ is one of the pollution gases. Large amounts of carbon dioxide emissions cause the greenhouse effect which will finally result in the global climate changes and natural disaster. Thus, detection and control of CO 2 become an urgent problem to solve. Compared with gases such as O₂, H2, C l2, CO, Or NOx which can easily be monitored by means of gas-solid interactions based on redox reactions, CO₂ is rather inert. Thus the development of CO₂ sensors with high performance based on simple metal oxide is difficult and challenging.P-type LaFeO₃ has attracted much attention for its special stratified structure and excellent catalytic properties. LaFeO₃, a narrow-band oxide semiconductor and exhibiting the character of mixed conductivity, shows high catalytic properties and working stability at high temperature. As well known, tin dioxide（SnO₂） is one of the most important intrinsic n-type with a wide band gap（Eg =3.6 eV）,which can perfectly match with the energy band of LaFeO₃ to form p-n junction. Thus SnO₂ has been used to make composites with LaFeO₃. The sensors based on LaFeO₃ / SnO₂ composites exhibit high gas sensitivity, fast response and low working temperature for CO₂ detection. Generally, p-type nanocomposites show an increased conductivity when interact with oxidizing gas（p-type character）. However, during the experiment, an anomalous behavior has been observed. Nanocomposites 50%SnO₂/50%LaFeO₃ and 80%SnO₂/20%LaFeO₃ showed a decreased（n-type character）rather than an increased conductivity under temperature lower than 300°C during the interaction with NO₂, and we call this a dual conductance behavior.To study the gas sensing properties and reveal the fundamental mechanism involved in the phenomenon, a series of experiments has been done. So-gel method and template method have been used to synthesize LaFeO₃ and SnO₂, respectively. Four kinds of composites with the molar ratio of SnO₂: 0; 20%; 50%; 80%, have been made to study the sensing properties towards CO₂. The results show that the as-formed p-n junction between LaFeO₃ and SnO₂ play a key role in improving the gas sensing properties.What’s more, a special phenomenon that the composites with quite high molar ratio of SnO₂ still exhibit the sensing character of p-type has attracted our attention during the test. Thus, to obtain the conversion point where the sensing character of oxide turns to n-type, we added another three composites with even higher molar ratio of SnO 2 : 85%; 90%; 95% and studied their gas sensing properties with NO₂. The effect of humidity, the type of the target gases（including NO₂ and O₂） and the concentration of the target gases on the conductive behavior of the composites have been proved to have no effect on the conductance type of the composites. It’s the temperature who decides the dual conductance behavior of the composites.