Improvement Of Gas Adsorption Performances Of Functionalized Monolayer MoS₂ Surfaces:A First-principles Study

Effective monitoring gas molecules,especially for common toxic gases,with high sensitivity,selectivity,simplicity and low cost has always been an urgent problem to be solved on account of the ubiquitous pollution problems.The gas sensors with high efficiency can also applied to control the fuel burning and exhaust gas emission.Due to their large surface-to-volume ratio and high mobility,the two-dimensional materials have been greatly developed in gas sensor in recent years.As a typical and promising two-dimensional material,the“Sandwich”MoS₂ not only has larger surface-to-volume ratio,compared with the graphene,but also possesses sizable band gaps?1.3-1.8 eV?,which can be directly used as field-effect transistor?FET?and sensors.As well as graphene,the MoS₂-based sensors performance can be further optimized and improved by introducing defects,metals doping and functionalization.In this paper,the behaviors of several common small gases on monolayer Mo S₂ surface were investigated via first-principle theory.The thesis is divided into two parts:Section one:The adsorption behaviors of several representative common gases?NO₂,NH₃,H₂S,SO₂,CO and HCHO?on the Ag-functionalized monolayer Mo S₂ surface are investigated with aim to explore the improvement of the single-layer MoS₂ gas sensing by the Ag-functionalization.All the adsorption configurations,charge transfer and electron properties of gas molecules on three different surfaces of Ag-Mo S₂ were systematically calculated and discussed.The results indicate the gas adsorption activities are improved significantly on Ag-P and Ag-V_S substrates compared with pristine MoS₂ and Ag-V_Mo surface.Specially,the sensitivity of surfaces to NO₂ are higher than other gases.The semiconductor type of gas/Ag-Mo S₂ systems are more determined by the metal-functionalization,however,the charge transfer processes of gases show distinct differences between each other.The different directs and amounts of charge transfer can cause the variations of the carrier densities of systems and further influence the systems'resistance.Definitely,this is benefit for detection and distinction of gases.Our further studies show that the charge transfer process is the result of coordinated action of the gain-lost electron ability of gases and the metallicity of noble metal.Section two:the adsorption and dissociation process of H₂O₂ on defect-functionalized monolayer MoS₂were studied and the mechanism of adsorption thermodynamics and decomposition of H₂O₂ on different surfaces were elucidated in this section.By calculating the adsorption energy and optical properties of H₂O₂and H₂O₂ fragments,the reactive energy of H₂O₂ dissociation processes on different surfaces,it is found that:?1?The decomposition of H₂O₂ on MoS₂ surfaces is mainly via the cleavage of O-O bond and surfaces with sulfur and molybdenum vacancy are more beneficial to H₂O₂ decomposing to HO radical;?2?The generation of HOO radical from the reaction of H₂O₂ and HO radical was inclined to proceed on perfect MoS₂ surface;?3?On perfect and sulfur vacancy surfaces,the HO and HOO radicals can be easily reacted to generate O₂and H₂O;?4?Comparing with the perfect surface,the defect surface has greater optical sensitivity for H₂O₂and its fragments;?5?The adsorption spectra of species with different adsorption concentrations on surfaces display different characteristics,which are helpful to the detection and identification of different species.