First-principles Study Of The Photocatalytic Properties Of Double-layer Mo Chalcogenide

In an era of increasing environmental pollution and energy crisis,the search for new and efficient photocatalytic materials has become a popular area of research.Two-dimensional materials exhibit excellent photocatalytic properties due to their large surface area and unique electronic structure.Two-dimensional heterojunctions composed of two different two-dimensional materials have the advantages of adjustable band structure and effective separation of photogenerated carriers,overcoming the disadvantages of unsuitable band alignment and high carrier compliance of single two-dimensional materials,making two-dimensional heterojunctions outstanding in photocatalysis.In this paper,a first-principles approach has been adopted to study the bilayer molybdenum-based dichalcogenides.The crystal structure,band structure,density of states,adsorption energy and other performance parameters of the photocatalytic material MoSe2/MoSeTe van der Waals heterojunction have been systematically investigated,and its application prospects in the field of photocatalytic water splitting have been analysed.The main contents of this thesis are as follows:(1)We have carried out calculations on the band structure etc.of bilayer molybdenum-based dichalcogenides.The results show that only four materials,bilayer MoSe2,bilayer MoTe2,sulphur-linked bilayer MoSSe and MoSe2/MoSeTe,meet the requirements for photocatalytic water splitting,of which MoSe2/MoSeTe is a heterojunction material.We also calculated the crystal structure and band structure of monolayer MoSe2 and MoSeTe with different crystalline phases,and the results show that triangular phase MoSe2 and MoSeTe are not suitable for photocatalysis,and only hexagonal phase MoSe2,MoSeTe and their heterojunctions can be used for photocatalysis.(2)The influence of the stacking structure of the MoSe2/MoSeTe heterojunction on its properties is investigated in detail.Binding energy calculations show that the molybdenum-based dichalcogenide is a material with weak interlayer interactions,with a maximum layer-to-layer binding energy of235.42 meV.Phonon spectroscopy shows that the staggered MoSe2/MoSeTe heterojunction has high stability,while the small imaginary frequencies in the phonon spectra indicate that the directly stacked MoSe2/MoSeTe heterojunction is less stable.Both have the characteristics of type II heterojunctions.The band structure study shows that the top of the valence band is higher for MoSeTe and the bottom of the conduction band is lower for MoSe2 in both stacks,with a gap of 1.64 eV.The frontline orbital study also supports this conclusion,with the highest occupied orbitals in the MoSeTe layer and the lowest unoccupied orbitals in the MoSe2 layer.(3)Compared to the direct stacking type,the staggered stacking type MoSe2/MoSeTe heterojunction produces a significant built-in electric field between the layers,which can facilitate the separation of photogenerated carriers into the two material layers.Adsorption calculations show that both stacking types have similar adsorption properties,with each intermediate of the oxygen evolution reaction tending to adsorb on the Te atom,in agreement with previous analyses.For the staggered stacked MoSe2/MoSeTe heterojunction,we also observed layer misalignment during adsorption due to the low interlayer binding energy of the molybdenum-based dichalcogenides.We also applied strain to the material and found that the applied strain can effectively modulate the band structure,with compressive strain increasing the band gap and further improving the redox capability of the photogenerated carriers.The Gibbs free energy study of the oxygen evolution reaction shows that an overpotential of 0.91 V is required to complete the oxygen evolution reaction,while the oxidation capacity of the photogenerated hole is insufficient and thus a co-catalyst or sacrificial agent must be added to complete the photocatalytic water splitting.