Hydrogen Sensing Mechanism of WS2 Gas Sensors Analyzed with DFT and NAP-XPS

Abstract

Nanostructured tungsten disulfide (WS2) is one of the most promising candidates for being used as active nanomaterial in chemiresistive gas sensors, as it responds to hydrogen gas at room temperature. This study analyzes the hydrogen sensing mechanism of a nanostructured WS2 layer using near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT). The W 4f and S 2p NAP-XPS spectra suggest that hydrogen makes physisorption on the WS2 active surface at room temperature and chemisorption on tungsten atoms at temperatures above 150 °C. DFT calculations show that a hydrogen molecule physically adsorbs on the defect-free WS2 monolayer, while it splits and makes chemical bonds with the nearest tungsten atoms on the sulfur point defect. The hydrogen adsorption on the sulfur defect causes a large charge transfer from the WS2 monolayer to the adsorbed hydrogen. In addition, it decreases the intensity of the in-gap state, which is generated by the sulfur point defect. Furthermore, the calculations explain the increase in the resistance of the gas sensor when hydrogen interacts with the WS2 active layer.

Operando Investigation of WS2 Gas Sensors: Simultaneous Ambient Pressure X-ray Photoelectron Spectroscopy and Electrical Characterization in Unveiling Sensing Mechanisms during Toxic Gas Exposure

“…Therefore, in agreement with the literature, we can assign them to hexagonal WS 2 and WO 3 , respectively. 14,15,39 The W 4f 5/2 peak of WO 3 overlaps with the W 5p 3/2 peak of the WS 2 .…”

“…In another work, Minekazi et al 39 used APXPS and DFT to investigate the hydrogen sensing mechanism of chemiresistive WS 2 gas sensors on a silicon substrate. APXPS measurements were performed stepwise, exposing the sample to 1000 and 5000 ppm of H 2 , starting from UHV, at RT and 150 °C.…”

“…There are recent reports of ambient pressure X-ray photoelectron spectroscopy (APXPS) being used to study the sensing mechanism of metal oxide-based sensing devices, ,− while very few have used it to investigate TMDs. , However, most of these previous studies have encountered challenges in concurrently measuring the electrical response and electronic properties, thus limiting the potential of operando dynamic APXPS to in situ static measurements. We summarize the most significant reports in the following paragraphs.…”