Highly Responsive Pd-Decorated MoO 3 Nanowall H 2 Gas Sensors Obtained from In-Situ-Controlled Thermal Oxidation of Sputtered MoS 2 Films.

Among transition metal oxides, MoO 3 is a promising material due to its layered structure and different oxidation states, making it suitable for different device applications. One of the methods used to grow MoO 3 is radio frequency magnetron sputtering (RFMS), which is the most compatible method in industry. However, obtaining nanostructures by RFMS for metal oxides is challenging because of compact morphology film formation. In this study, α-MoO 3 with vertical nanowalls is obtained by a two-step process; deposition of magnetron-sputtered MoS 2 vertical nanowalls and postoxidation of these structures without changing the morphology. In situ transmittance and electrical measurements are performed to control the oxidation process, which shed light on understanding the oxidation of MoS 2 nanowalls. The transition from MoS 2 to α-MoO 3 is investigated with partially oxidized MoS 2 /MoO 3 samples with different thicknesses. It is also concluded that oxidation starts from nanowalls perpendicular to the substrate and lasts with oxidation of basal planes. Four different thicknesses of α-MoO 3 nanowall samples are fabricated for H 2 gas sensors. Also, the effect of Pd deposition on the H 2 -sensing properties of sensors is deeply investigated. An outstanding response of 3.3 × 10 5 as well as the response and recovery times of 379 and 304 s, respectively, are achieved from the thinnest Pd-loaded sample. Also, the gas-sensing mechanism is explored by gasochromic measurements to investigate the sensor behaviors under the conditions of dry air and N 2 gas as the carrier gas.