Simultaneous amperometric and gasochromic measurements of α-MoO3 gas sensors optimized with controlled content of Pd sensitizer and device configuration

MoO3 is a promising material for sensing reducing gases due to its high oxidation state (Mo6+) and relatively low melting point, making it easy to reduce to lower oxidation states (Mo5+ and Mo4+). In this study, we explore the importance of Pd (palladium) sensitizer deposition time (14, 21, and 28 s) using RF-magnetron sputtering (RFMS) on H2 gas sensing performances of ultra-thin alpha-MoO3 obtained by thermal oxidation of sputtered MoS2 at 380 degrees C. We also examine the effect of the position of contact material on H2 sensing properties of ultra-thin alpha-MoO3 was studied. Our results demonstrate that ultra-thin MoO3 sensor, with bottom contact and Pd deposition time of 21 s has shown an exceptionally high response of 3.3 x 108 at operating temperature of 75 degrees C with relatively rapid response and recovery times of 52 and 55 s, respectively. The detection limit of 10 ppm with a response of 90 % is also achieved at 75 degrees C. Following optimizitation of Pd sensitizer deposition time, these conditions were applied to thicker MoO3 nano-wall structures. This allowed for the utilization of gasochromism in addition to chemiresistive sensing, with both measurements conducted simultaneously. Thicker nano-wall structured sensors have shown a faster response and recovery time of 23 and 22 s with a response of 9.8 x 105 at an operating temperature of 150 degrees C. The transmittance variation is noticeable from Delta T% of 8.3 at 700 nm wavelength for 100 ppm H2 to 22 % for 1 % H2 concentration at 100 degrees C with an optical response in 19 s. Also, the hydrogenation mechanism of MoO3 is discussed in depth by using double-injection model, as applied in electrochromic devices.