Study On Microstructure Regulation Of Two-dimensional MoO₃ And Its Hydrogen Sulfide Gas Sensing Performance

Hydrogen sulfide（H₂S）is not only flammable and explosive,but also has strong neurotoxicity,which poses a great threat to human life safety.Therefore,it is more and more urgent to develop efficient H₂S gas detection technology.Metal oxide semiconductor gas sensor has become the research focus in the field of gas detection because of its high sensitivity and low cost.Among many metal oxides,molybdenum trioxide（MoO₃）has attracted researchers’attention because of its good H₂S gas selectivity.However,as a typical layered metal oxide,MoO₃ has a dense layer of oxygen atoms on its surface,which seriously prevents Mo active sites from participating in gas adsorption,resulting in that MoO₃ needs a high temperature of over 300℃to exert its good gas sensing performance,which is not conducive to the safe detection of flammable and explosive gases such as H₂S.To solve the above problems,MoO₃ porous two-dimensional nanosheets were synthesized based on morphological genetic strategy,and the enhancement effect of Mo active sites on gas adsorption was discussed by combining H₂S gas sensing measurements and theoretical calculations.Subsequently,the surface oxygen defect regulation and Co doping modification were carried out on the MoO₃ porous two-dimensional nanosheets to find the effective ways to remove oxygen shielding,thus further improving the H₂S gas sensing performance of the MoO₃ porous two-dimensional nanosheets and reducing their working temperature.The main research contents of this paper are as follows:Firstly,two-dimensional MoSe₂,WSe₂ and Co_0.85Se nanosheets were synthesized by a simple one-step hydrothermal method,and porous two-dimensional nanosheets of MoO₃,WO₃ and Co₃O₄ were prepared by high-temperature oxidation of two-dimensional selenide nanosheets.At the same time,the formation mechanism of pores is proposed based on the unit cell shrinkage,which proves that this morphological genetic strategy can be extended to synthesize other porous two-dimensional metal oxide nanosheets.Secondly,the H₂S gas sensing performance of MoO₃ porous two-dimensional nanosheets was systematically studied.Compared with commercial MoO₃ nanosheets,the response value of MoO₃ porous two-dimensional nanosheets to 1 ppm H₂S gas at 250℃was 7.2 times（9.44/1.31）.Density functional theory（DFT）calculations showed that the existence of pores leads to the exposure of some Mo active sites in the inner layer,and these edge Mo active sites have higher adsorption energy and stronger adsorption activity for H₂S and O₂ gas molecules.To further remove the shielding effect of outer oxygen atoms on Mo active sites and improve the response of MoO₃ porous two-dimensional nanosheets to H₂S gas,the surface oxygen atoms of MoO₃ porous two-dimensional nanosheets were knocked out by defect engineering,and defect-rich MoO₃ porous two-dimensional nanosheets were successfully prepared.It was found that when the hydrogen treatment time was 10 minutes,the working temperature of the defect-rich MoO₃ porous two-dimensional nanosheets decreased from 250℃to 200℃,and the response value to 1 ppm H₂S gas increased to12.91,which was 1.4 times（12.91/9.44）of the original MoO₃ porous two-dimensional nanosheets.DFT calculations showed that the exposure of Mo active sites caused by oxygen defects is not only beneficial to the adsorption and transformation of O₂ gas,but also shows shorter adsorption bond length,higher adsorption energy and more charge transfer in the process of H₂S gas adsorption,which is the main reason for the enhancement of sensing performance of defect-rich MoO₃ porous two-dimensional nanosheets.Finally,Co metal site was introduced into the defect-rich MoO₃ porous two-dimensional nanosheets by doping,which further improved the H₂S gas sensing performance.It was found that doping 0.50 mol%Co²⁺reduced the optimal working temperature of defect-rich MoO₃ porous two-dimensional nanosheets from 200℃to140℃,and the response value of 1 ppm H₂S gas increased to 34.12,which was 2.6 times（34.12/12.91）of that of undoped defect-rich MoO₃ porous two-dimensional nanosheets.DFT calculations showed that the introduction of Co metal site made the inert oxygen atoms around it become brand-new active adsorption sites,which significantly enhanced the adsorption reaction of gas molecules on the surface oxygen atoms,thus improving the sensing performance.