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Unadded and 0.5 mol% Pd-added ZnO bulk and thin films were prepared by sintering and sputtering, respectively, and their CO gas sensing properties were investigated. The effects of Pd addition, sensing temperature (100–500 °C), and humidity on the CO gas response were discussed. In the bulk sensors, Pd-addition lowered the temperature for the maximum CO gas response (sensitivity) from 400 to 300 °C, whereas the thin film sensors (unadded and Pd-added) exhibited maximum gas response at 200 °C. The Pd-addition enhanced the CO gas response in thin film sensors, and it was also effective for reducing the interference from humidity in both bulk and thin film sensors.

Two types of H2Ti3O7 powders were prepared by ion exchange and hydrothermal methods, and the H2 sensing properties of the H2Ti3O7-derived TiO2 sensors were examined. In the ion exchange method, Na2Ti3O7 was first synthesized via a solid-state reaction, and H2Ti3O7 was obtained from Na+/H+ exchange on Na2Ti3O7. H2Ti3O7 was also prepared via a hydrothermal reaction of TiO2 powder in a NaOH solution. The morphology, size, and phase evolution of H2Ti3O7 were found to be strongly dependent on the preparation methods. The TiO2 sensors fabricated by the H2Ti3O7 powders calcined at 700 °C (ion exchanged) exhibited an excellent gas response (S = 30,000) to 1.0% H2/N2 at 500 °C, which was three orders of magnitude higher than that of the hydrothermally synthesized powder and commercial anatase powder even though its specific surface area was smaller. The higher H2 gas response in the TiO2 sensor derived from the ion exchanged H2Ti3O7 is discussed in terms of the metastable β-TiO2 and anatase phases.

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.