Small Gaseous Molecules (CH₄, CO₂, H₂S) Metal Oxide Sensing

Gas sensors play an important role in the monitoring of toxic,hazardous,flammable and explosive gases.Researchers have been working tirelessly on how to improve gas sensitivity.At present,the regulation of the morphology and crystal plane of gas-sensing materials,addition of dopants,and surface modification are main methods to improve gas sensitivity.In this work,density functional theory is applied to study the structural characteristics and gas sensing performance of h-WO₃ by using the DMol³ module in Material Studio software.We find different crystal plane exposure and the help of Au clusters on surface are two effective methods to improve h-WO₃ gas sensitivity.The research contents mainly include the following aspects:?1?Density functional theory?DFT?has been used to investigate the structural characteristics of bulk,surface and 2D layered structures of h-WO₃.And the CH₄ sensing mechanism on h-WO₃?001?and?110?surfaces was studied.The calculated results indicate that the flat?001?surface is nearly not sensitive to CH₄ gas.However,on the ridge-like?110?surface,large transfers occur on both four-fold coordinated W_4c and hole₂,the side bridge site between W_4c and O_1c,with charge transfers up to 0.286e and 0.266e,respectively.Allowing the CH₄ molecule to be as close as possible to the surface due to the open and cuspidated surface structure should be the intrinsic reason for the high activity of the?110?surface.A 4-fold improvement in sensing ability is obtained.In addition,the nature of the physical adsorption creates potential for the material to be reused.Therefore,the presence of a highly active?110?surface solves the insensitivity issue of pure WO₃ and makes it a promising material for methane detection.?2?The same method was used to study the growth of Au_n?n=1-5?on h-WO₃?001?perfect and oxygen vacancy surfaces.Calculation found that Au_n?n=1-5?clusters all tend to deposit three-dimensional 3D structure on the surface.On this basis,the sensing of CH₄ on Au_n?n=1-5?modified h-WO₃?001?surface was studied.The calculated results show that the physically adsorbed CH₄ achieves charge transfer up to 0.347 e and 0.321 e on the Au₁ modified h-WO₃?001?surface,which is much larger than that on the unmodified h-WO₃?001?surface,and realized high efficient sensing of CH₄.The active site is cationic Au⁺.The relative small adsorption energy?about 0.55 eV?guarantees the possibility of recycling this material at low temperatures.As the number of gold atoms increases,the active site has a tendency to shift to metallic Au⁰.Interestingly,the charge transfer on the larger gold clusters(Au_2-5)are small.Then,Au cluster on the h-WO₃?001?surface has the single-atom catalytic potential for CH₄ sensing.?3?The sensing ability of the Au cluster modified h-WO₃ material for CO₂ and H₂S was studied by the same method.The results show that the modification of the Au cluster greatly improves the sensing performance of h-WO₃ for these two gases.Especially for CO₂ sensing,pure h-WO₃ can not realize CO₂ sensing and the charge transfer is only0.035e.While the charge transfer is improved to 0.139e after single Au modification,indicating that the Au clusters modified h-WO₃ has great potential in relatively inert gas sensing.And the presence of Au clusters also significantly improved the H₂S adsorption and sensing abilities.Therefore,the Au clusters modified h-WO₃ is a potential gas sensing material and worthy of further commercial development.