Near Room Temperature Light-Activated WS2-Decorated rGO as NO2 Gas Sensor

Abstract

The NO2 response in the range of 200 ppb to 1 ppm of a chemoresistive WS2-decorated rGO sensor has been investigated at operating temperatures of 25 °C and 50 °C in dry and humid air (40% RH) under dark and Purple Blue (PB) light conditions (λ = 430 nm). Few-layers WS2, exfoliated by ball milling and sonication technique, with average dimensions of 200 nm, have been mixed with rGO flakes (average dimension 700 nm) to yield WS2-decorated rGO, deposited on Si3N4 substrates, provided with platinum (30 μm gap distance) finger-type electrodes. TEM analysis showed the formation of homogeneous and well-dispersed WS2 flakes distributed over a thin, continuous and uniform underlying layer of interconnected rGO flakes. XPS and STEM revealed a partial oxidation of WS2 flakes leading to the formation of 18% amorphous WO3 over the WS2 flakes. PB-light irradiation and mild heating of the sensor at 50 °C substantially enhanced the baseline recovery yielding improved adsorption/desorption rates, with detection limit of 400 ppb NO2 and reproducible gas responses. Cross sensitivity tests with humid air interfering vapor highlighted a negligible influence of water vapor on the NO2 response. A charge carrier mechanism between WS2 and rGO is proposed and discussed to explain the overall NO2 and H2O response of the WS2–rGO hybrids.

Vertically aligned carbon nanotubes, MoS2–rGo based optoelectronic hybrids for NO2 gas sensing

“…The photogenerated carriers release sites occupied by ambient molecules (such as O 2 and H 2 O molecules) on the flakes surface and make them active sites for the adsorption of NO 2 molecules 10 . They also provide the activation energy needed to desorb the trapped molecules during the recovery time, thereby improving the sensitivity and performance of the device 10,11,15,16 . Two type of active sites contribute in the absorption of NO 2 molecules on the MoS 2 -rGo flakes including low energy (1) www.nature.com/scientificreports/ adsorption sites (sp 2 bonded carbon) and high energy adsorption sites 17 (defects, sulfur/oxygen vacancies).…”

“…The high electron mobility of rGo derive electrons to the less conductive MoS 2 flakes. Therefore, the concentration of minority carrier (electron) significantly decreases in rGo and less electrons can reach the contacts and subsequently the device resistance decreases in the presence of NO 2 gas 10 . Under laser illumination, many electron/hole pairs are generated which provide activation energy for desorption of adsorbed O 2 , H 2 O and NO 2 molecules from both rGo and MoS 2 surfaces.…”

“…In addition to preventing sensor saturation, the photogenerated carriers provide further active sites for NO 2 molecules by removing O 2 and H 2 O molecules, leading to an increase of the device sensitivity. As a result, the recovery time of sensor can be improved and device acts as a rapid sensor even at room temperature 10 . The Eqs.…”

“…Hence, active sites can be released for the absorption of the analyte molecules. Second, the photoexcited carriers accelerate desorption process of the adsorbed molecules by releasing the occupied sites during recovery time 10 .…”

“…As a result, various optical active materials are integrated with graphene to fabricate high performance gas sensors. The most prominent candidates are the 2d materials 10,11 . Among various 2d materials 12,13 , transition open Nanoscale Physics Device Lab (NPDL), Department of Physics, University of Kurdistan, Sanandaj 66177-15175, Iran.…”