First-principles Study Of SiI₂ And SiMI₄（M=Ge,Sn） Monolayers For Photocatalytic Water Splitting To Produce Hydrogen

In recent years,the global energy shortage and environmental pollution have become more and more serious.Non-renewable resources such as coal mines,oil,and natural gas are already facing exhaustion.Therefore,looking for clean and environmentally friendly renewable resources is the top priority of current social development.It is generally believed that hydrogen,as clean energy with high combustion calorific value,is one of the strong candidates for replacing fossil fuels in the future.The semiconductor photocatalytic hydrolysis hydrogen production technology driven by visible light can produce hydrogen under light conditions,which provides a brand-new method for solving energy shortages.Since the discovery of photocatalytic technology,photocatalytic splitting of water to produce hydrogen production technology has attracted much attention.With the emergence of graphene,people`s research boom in two-dimensional materials has been triggered because two-dimensional materials have unique electronic properties,optical properties,and mechanical properties,and other unusual and excellent characteristics.The performance of photocatalysts is a decisive factor for photocatalytic reactions.Compared with three-dimensional materials,two-dimensional materials have many advantages in photocatalytic applications.First of all,in terms of catalytic reaction sites,due to the larger specific surface area of the two-dimensional material,it can provide more catalytic reaction sites.The second point is that the ultra-thin properties of two-dimensional materials greatly reduce the migration distance of photo-generated carriers,thereby reducing their recombination probability and further improving the photocatalytic performance.It is precise because of the unique properties of two-dimensional materials that two-dimensional materials have always been the first choice among photocatalyst candidate materials.In this paper,two-dimensional materials of SiI<sub>2</sub>and SiMI<sub>4</sub>（M=Ge,Sn）are designed,and their stability and good photocatalytic performance are confirmed by first-principles calculations,which also provide theoretical guidance for the experimental design and preparation of photocatalytic materials.Mainly for the following two aspects:1.Based on first-principles calculations,the feasibility of SiI<sub>2</sub>monolayer as a candidate material for photocatalytic water splitting hydrogen production under sunlight irradiation was discussed.Firstly,through the phonon scattering curve and the image analysis of molecular dynamics simulation,the fully relaxed SiI<sub>2</sub>monolayer is stable.The geometric structure,electronic and optical properties,carrier mobility,and strain engineering of single-layer films have been studied.The results show that the SiI<sub>2</sub>monolayer has an indirect gap of 2.33 eV（HSE06）,and the band edge and band gap are both in line with the redox potential conditions for water splitting hydrogen production.Through the study of light absorption,we found that the monolayer has obvious optical absorption in the visible and near-ultraviolet regions,and the biaxial compressive strain can significantly enhance its optical absorption.In addition,the mobility of electrons is significantly different from that of holes,so we can expect the space charge separation phenomenon of photo-generated carriers,which makes the recombination rate of photo-generated charge pairs relatively low.Through calculation and simulation,the main adsorption sites of water molecules on the surface of the material are determined.On this basis,the change of Gibbs free energy of hydrogen evolution reaction is calculated,and the change of reaction direction and energy is judged by the calculated data.And by calculating the energy difference produced by the adsorption process,the energy change in the process of water molecules splitting on the monolayer to generate H<sub>2</sub>is proved.2.Based on density functional theory,the two-dimensional structure of SiMI<sub>4</sub>（M=Ge,Sn）monolayer film was predicted,and it was clarified that two monolayer films could be candidates for high-efficiency photocatalysts for visible-light-driven hydrogen decomposition reactions.Since the two monolayers are derived from two-dimensional diiodides,they have a similar structure to the diiodide monolayer.Phonon dispersion and first-principles molecular dynamics,respectively confirmed the dynamic stability and thermal stability of the obtained monolayer film.In addition,the work function and cohesive energy results indicate that the SiMI<sub>4</sub>monolayer has a stable structure.The calculation results of electronic properties show that the indirect band gaps of the two two-dimensional structures are 2.45 eV and 2.43 eV,respectively,and the edge of the bandgap matches the hydrogen evolution reaction conditions.We also studied the optical properties of the SiMI<sub>4</sub>monolayer and its effect under strain conditions.In addition,taking into account that the carrier mobility in different directions may be different,the carrier mobility of two monolayer`s electrons and holes are calculated along with two different directions.Finally,the Gibbs free energy of the hydrogen evolution reaction and the energy change during the reaction are calculated,as well as the main adsorption sites of water molecules on the surface,and the energy of the direct decomposition reaction of water molecules on the surface is summarized and calculated.All results support that these two monolayer films are potential candidates for visible light photocatalytic water splitting to produce hydrogen.