Synthesis and High-Temperature NOx Sensing Properties of Rhodium-doped Barium Titanate Nanoparticles

Kurzfassung

High-temperature NOx-sensing is an increasing requirement for process control of combustion systems, exhaust gas monitoring and environmental protection. The sensor has to tolerate the harsh conditions of high temperature, high humidity and coexistence of multiple gases. Good gas sensing properties are difficult to achieve at high temperatures by using simple semiconducting metal oxides. Therefore high temperature gas sensor materials with excellent sensing performance towards NO2 are desired. BaTiO3 based perovskites and their doped compounds are known as highly catalytic high-temperature materials and thus are potentially interesting to be used as sensing materials in such conditions. Our previous work with Rh-doped BaTiO3 yielded some promising sensing properties as argon was employed as carrier gas. This work reports the investigations carried out with the nanoparticles of Rh-doped BaTiO3 in argon but also in air carrier gas under humid conditions. For the synthesis of nanoparticles, the oxalic co-precipitation route is adopted. The powder was characterized and the gas sensing properties were investigated for NO2 and CO between 400-800°C. While XRD and Raman spectroscopy show the presence of a single phase identified as tetragonal BaTiO3, EDX and XPS reveal the presence of Rh and oxidation state of the different elements. The SEM reveals the spherical nanoparticles (50 nm) morphology with the slightly increasing of BET surface area in the presence of Rh. The as-prepared nanoparticles are drop-coated on the IDE sensor platforms followed by a heat treatment at 900 °C under H2 for 30 min. Sensor tests yield reasonable signal at temperatures exceeding 400°C. A higher response is obtained than that achieved at the previous work by classic chemical route. Although sensor signal decreases with temperature (400°C-800 °C), decreasing response and recovery times are measured. Sensor signal maintains its dignity in air under 5%RH at 600°C despite the presence of oxidizing environmental. Hydrogen treatment at high temperature appears to activate the material for better sensing. The reason for this phenomenon is investigated and believed to be related to Rh-diffusion out of the lattice resulting in high oxygen vacancies.