Theoretical Study On The Thermoelectrical Properties Of Two-Dimensional Double Transition Metal MXenes Materials

With the rapid development of the social economy and global population growth,fossil energy is widely tapped and used,and non-renewable resources will gradually be in shortage,so how to effectively develop new renewable energy sources and seek new technologies for energy conversion has become the focus of people to explore energy.Thermoelectric materials are a class of functional materials that can realize heat and electricity conversion through the movement of electrons and phonons without the help of external media and have the advantages of no mechanical movement and stable equipment,which have certain advantages in energy utilization.Graphene and other two-dimensional materials have been heavily investigated theoretically and experimentally for their excellent physical properties in recent years.Two-dimensional materials have more boundary scattering,which makes them have lower lattice thermal conductivity and have the potential to be superior thermoelectric materials.Two-dimensional semiconductor-type MXenes materials were found to have higher electrical and lower thermal conductivities,which are promising for research in thermoelectric development.In 2014,researchers discovered two-dimensional dual-transition metal MXenes materials,and the presence of two transition metals provides superior flexibility in property tuning compared to a single transition metal in conventional MXenes materials.Therefore,the two-dimensional double-transition metal materials ScYCT₂（T=F,OH）and ZrTiCO₂are used as the subjects of this paper;the effects of surface functionalization and biaxial tensile strain on the thermoelectric properties of the materials are investigated.The stability and electronic structure of the materials are studied by using the first principle calculation method based on density functional theory;the electrical transport properties and electronic thermal conductivity of the materials are studied by using Boltzmann transport theory;the lattice thermal conductivity of the materials is studied by using Slack model,and the thermal transport properties of the materials are obtained by combining the electronic thermal conductivity;the electrical and thermal transport properties analyze the thermoelectric properties of the materials.The VASP calculation software and Boltz Tra P2 program package are mainly applied.The main results and conclusions of this paper are as follows:I.The thermoelectric properties of ScYC,a two-dimensional double transition metal MXenes material functionalized with F and OH,are analyzed to explore the effect of surface functionalization on the thermoelectric properties.First-principles calculations based on density generalization theory show that the F and OH functionalized ScYC is mechanically and dynamically stable and can be an independent two-dimensional planar structure without a substrate.In addition,the band gaps of monolayer ScYCF₂and monolayer ScYC（OH）₂are1.09 e V and 0.55 e V,respectively,indicating that monolayer ScYCT₂（T=F,OH）is a semiconductor.Boltzmann transport theory calculations found that p-type ScYCF₂has the largest Seebeck coefficient of 862μV/K at room temperature;the power factor of n-type ScYC（OH）₂is the largest under the same conditions,which means it may have better thermoelectric properties;the electronic thermal conductivity increases rapidly at carrier concentrations more fabulous than 10¹³cm^-2,so better thermoelectric materials with better performance are expected to be obtained at carrier concentrations up to 10¹³cm^-2.Calculations of the Slack model show that the OH-functionalized ScYC has a lower lattice thermal conductivity.The performance merit of the thermoelectric materials is evaluated comprehensively by the thermoelectric superiority value ZT,and we predict that the maximum ZT value of two-dimensional double transition metal ScYCT₂material is above 1.In particular,the ZT value of n-type ScYCF₂can reach 3.18 at 900 K.The ZT value of n-type ScYCT₂is larger than that of the corresponding p-type ScYCT₂,indicating that n-type ScYCT₂has better thermoelectric properties,which may be related to the more excellent electrical conductivity of the n-type material.For F-functionalized ScYC,p-type and n-type ScYCF₂are high-temperature thermoelectric materials,and for OH-functionalized ScYC,p-type ScYC（OH）₂is a high-temperature thermoelectric material,while n-type ScYC（OH）₂is a medium-temperature thermoelectric material.These findings provide new ideas for the future experimental synthesis of thermoelectric materials.II.The thermoelectric properties of the two-dimensional biaxial transition metal MXenes material ZrTiCO₂are investigated to analyze the effect of 0%to 2%biaxial tensile strain on its thermoelectric properties.The calculated results of stability show that the monolayer ZrTiCO₂after applying different biaxial tensile strains is still mechanically and dynamically stable and can be an independent two-dimensional planar structure without a substrate.Calculations of the electronic structure show that the positions of the valence band top and conduction band bottom of monolayer ZrTiCO₂change as the biaxial tensile strain increases from 0%to 2%,but this does not affect its semiconductor properties.The calculated results of the electron transport properties find that the power factor of n-type ZrTiCO₂without strain at 300 K temperature is 7.83×10^-4W/m K²,and the power factor of n-type ZrTiCO₂after applying 2%biaxial tensile strain is 1.19×10^-3W/m K²,which is about 51.98%increase,indicating that the biaxial tensile strain can improve the electron transport properties of the material.And the power factor of p-type ZrTiCO₂is 3.32×10^-4W/m K²,which is smaller than the power factor of n-type ZrTiCO₂,which means that n-type ZrTiCO₂has stronger electron transport ability than p-type ZrTiCO₂.Calculations of the thermal transport properties show that the phonon scattering intensity increases as the biaxial tensile strain increases from 0%to 2%,resulting in a substantial decrease in the lattice thermal conductivity of monolayer ZrTiCO₂.The ZT value calculation revealed that the ZT value of n-type ZrTiCO₂at 900 K is 1.49 in the absence of strain and increased to 2.87 under 2%biaxial tensile strain,which implies that biaxial tensile strain significantly increased the ZT value of monolayer ZrTiCO₂.The two-dimensional double transition metal MXenes material ZrTiCO₂has excellent thermoelectric properties and can be applied as a high-temperature thermoelectric material,and biaxial tensile strain can significantly improve its thermoelectric properties.The results show that monolayer ScYCT₂（T=F,OH）and monolayer ZrTiCO₂are semiconductors,both of which have high thermoelectric properties and can be applied as thermoelectric materials.Among them,F-functionalized ScY has the highest ZT value,and biaxial tensile strain can improve the thermoelectric properties of monolayer ZrTiCO₂.