Orientation dependence of gas sensing properties of TiO2 films

Highly oriented polycrystalline TiO2 films were grown on sapphire substrates with various orientations using rf magnetron sputtering, and the effects of the crystallographic orientation on the H2 and CO gas sensing performance were investigated. The orientation of the TiO2 films was strongly dependent on the substrate orientation such that (1 0 1)/(1 0 0), (1 0 1), (0 0 1), and (1 0 0) oriented TiO2 films were formed on (1 1 2¯ 0) (a-cut), (1 1¯ 0 2) (r-cut), (1 0 1¯ 0) (m-cut), and (0 0 0 1) (c-cut) Al2O3, respectively. A fine surface feature developed in the films grown on (1 1 2¯ 0) and (1 0 1¯ 0) Al2O3 and a rather rough surface was obtained in case of the (1 1¯ 0 2) Al2O3 substrate. The deposited TiO2 films showed a short response time and high gas response toward H2 and CO balanced with N2. The TiO2 film grown on (1 1 2¯ 0) Al2O3 exhibited the highest H2 gas response possibly due to the (1 0 0) orientation and high surface area. The (0 0 1) orientation of TiO2 showed the highest selectivity toward H2 against CO.