Sol-gel processed TiO2-based nano-sized powders for use in thick-film gas sensors for atmospheric pollutant monitoring

Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dop e TiO2 nano-sized powders. The thermal decomposition behaviour of the precu rsors was studied using simultaneous thermogravimetric and differential the rmal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the po wders heated to 400 degreesC were crystalline in the anatase TiO2 structure . The pure TiO2 powder heated to 850 degreesC showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformat ion up to 950-1050 degreesC. Ta was soluble in the titania lattice up to th e concentration of 10 at%, while the solubility of-Nb was 5 at%. Thick film s were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650-1050 degreesC range. Sc anning electron microscopy (SEM) observations showed that the anatase-to-ru tile phase transformation induces a grain growth of about one order of magn itude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO 2 grain size at a nanometric level even at 950 degreesC, though grain growt h was observed with increasing temperature. The gas-sensitive electrical re sponse of the thick films were tested in laboratory, in environments with C O in dry and wet air. Conductance measurements showed a good gas response o nly for the nanostructured titania-based films. For field tests, the protot ype sensors were placed beside a conventional station for atmospheric pollu tant monitoring. The electrical response of the thick films was compared wi th the results of the analytical instruments. The same trend was observed f or both systems, demonstrating the use of gas sensors for this aim.