Study of La and Cu influence on the growth inhibition and phase transformation of nano-TiO2 used for gas sensors

Lanthanum and copper have been incorporated to titanium dioxide as additives. Structural modifications on the TiO2 support induced by the foreign atoms have been observed by XRD. The bare TiO2 totally exhibits the rutile form at a temperature of 700 °C. When La is added, the anatase to rutile transition is delayed and the grain growth inhibited. Almost pure anatase phase is maintained until 800 °C with an average grain size of 7 nm for the sample modified with 10 at.% of La and 11 nm for the 5 at.% La added one. On the contrary, the inclusion of Cu to the La-containing samples in a concentration of 2 at.% promoted the phase transformation yielding an increase of the grain size. In this case, the crystallographic form observed at 700 °C is still pure anatase, but at 800 °C the samples already showed essentially rutile phase with a minor amount of anatase. Segregation of the La4Ti9O24 solid solution is observed for all the doped samples when the calcination temperature reaches the value in which anatase is converted to rutile. Electrical measurements of thick films prepared with these materials showed that La increased the base resistance of TiO2 and enhanced the sensing performances of TiO2 to CO. The effects perceived due to Cu were an improvement of the selectivity of titania in the detection of CO.