Experimental And First-principles Study On Gas-sensing Properties Of 2D SnS/SnS₂ In-plane Heterostructures

Methanol is a kind of flammable,volatile and toxic organic vapor,the high-sensitive and safe detection of trace methanol at room temperature is of great significance for environmental safety and human health protection.As a typical two-dimensional material,tin sulfide（SnS）has received extensive attention in the field of gas sensor owe to its unique honeycomb fold structure,super specific surface area and excellent inoxidizability.At room temperature,SnS can show certain sensitive response to methanol vapor,and is a potential room temperature methanol sensitive material.However,the ability of intrinsic SnS to detect trace methanol is still insufficient.To further effectively regulate the sensing performance of 2D SnS,the SnS/SnS₂ in-plane heterostructures were constructed in this paper,and the sensitivity to methanol was studied by the method of experimental study and theoretical calculation.And the inherently sensitive mechanism of response enhancement is also explored.The stacked flower-like structure of 2D SnS nanosheets were prepared by solvothermal method,and the gas-sensitive properties of different VOCs molecules were tested.The results show that the pure-phase SnS is weakly sensitive to trace methanol.In addition,the adsorption selectivity of the intrinsic SnS structure for methanol molecules was also simulated based on density functional theory,and it is found that methanol formed physical adsorption on the surface of SnS.Further,two-dimensional SnS₂ nanosheets were prepared by hydrothermal method,and SnS/SnS₂ in-plane heterostructures were synthesized by Ar atmosphere controllable annealing process to induce the partial phase transformation of two-dimensional SnS₂,and the effects of different annealing temperature on phase composition,surface morphology and gas sensitivity of the heterostructure system were investigated.It is found that the SnS/SnS₂ in-plane heterostructure with three-dimensional sparse porous morphology can be obtained by annealing at 450℃,and it shows high sensitivity to methanol at room temperature.The lowest detection limit for methanol at room temperature is as low as 100 ppb,and the response value to 25 ppm methanol can reach 14.85,which is about 7.1 times higher than that of pure phase SnS.Finally,based on the structural characterization of the experimental samples,single-layer SnS/SnS₂ in-plane heterostructure model with local lattice reconstruction at interface was constructed,and the sensitization mechanism of excellent sensitivity response of SnS/SnS₂ lateral heterostructure to trace methanol was analyzed by density functional theory calculation method.The calculation results show that the introduction of the in-plane heterostructure leads to local symmetry breaking;the system changes from indirect band gap to direct band gap with a sharp decrease in band gap value,and the electrical conductivity increases accordingly;the built-in electric field formed at the interface guides the reconstruction of local carrier distribution effectively,and realizes the strong adsorption of methanol molecules at fixed point.The combination of experimental study and adsorption characteristics of methanol molecules at different sites on the surface of SnS/SnS₂ in-plane heterostructure concludes that the local irregularity caused by strong lateral covalent bonding can not only increase the number of adsorption sites（vacancies）,but also trigger strong adsorption on methanol molecules,thus realizing the physical/chemical synergistic adsorption of methanol molecules on the surface of SnS/SnS₂ lateral heterostructures.