Structural, electrical and hydrogen sensing properties evaluation of fabricated PdO thin films by thermal oxidation.

Palladium oxide (PdO) thin films (~ 60 nm thick) were grown on quartz substrates using a two-step process involving DC magnetron sputtering of Pd films followed by thermal oxidation in dry air at 450-750 °C for 4 h. In this work, a systematic temperature-dependent oxidation strategy is employed to directly correlate phase evolution, microstructural transformation, thickness effects, and hydrogen sensing properties of PdO thin films, an aspect that has not been extensively addressed in the existing literature. Structural, morphological, and optical characterizations confirmed full Pd-to-PdO conversion above 550 °C, with crystallite sizes increasing up to 30 nm. AFM and FESEM showed porous, domain-like morphologies, and UV-Vis analysis indicated direct band gaps in the range of 2.06-2.42 eV. XPS verified the presence of stoichiometric PdO with Pd²⁺ oxidation states. Electrical measurements revealed semiconducting behavior with thermally activated conduction, yielding activation energies between 0.045 and 0.092 eV depending on annealing temperature. The sensor response showed no significant dependence on the operating temperature, and the film annealed at 750 °C exhibited the highest sensing response at 400 ppm H₂. The Pt-contacted device detected 1000 ppm H₂ at room temperature, demonstrating stable operation without elevated temperatures. Room-temperature hydrogen sensing tests showed that PdO exhibits two distinct sensing mechanisms: a reversible p-type surface reaction at low H₂ concentrations (≤ 1000 ppm) and a reduction-driven response at high concentrations (2%), where PdO is chemically transformed into metallic Pd, leading to a sharp resistance drop and metallic-like conduction. Beyond temperature variation, PdO thin films with thicknesses of 5, 10, and 20 nm were also fabricated and annealed at 650 °C to examine the influence of film thickness on hydrogen sensing performance.