First-Principles Study Of Photocatalytic Properties Of Two-Dimensional Germanium-Based Compounds

With the rapid development of global technology and economy,energy and environmental issues have become increasingly prominent.Solving these problems,especially the development of clean and pollution-free renewable energy,has become an urgent scientific issue to be addressed.In the field of solar energy development,photocatalytic water splitting for hydrogen production is a promising technology.However,traditional photocatalysts suffer from problems such as narrow spectral response range,high carrier recombination rate,low driving force for oxidation reaction,and weak surface reaction activity,which limit their development and application.Therefore,the development of new photocatalytic materials is a critical scientific issue that urgently needs to be addressed.Compared with traditional bulk semiconductor materials,two-dimensional（2D）materials and their heterostructures have many rich physical properties,such as nanometer-thick thickness,large specific surface area,tunable optical and electronic properties,and have shown great potential for applications in the field of photocatalysis.Germanium-based 2D semiconductors,such as 2D germanene and its derivatives,have excellent electronic and optical properties and are expected to become efficient photocatalysts.However,research on germanium-based semiconductor photocatalysts is not yet in-depth.Based on the above situation,this study conducted theoretical research on the photocatalytic performance of novel germanium-based 2D semiconductors and their heterostructures.The main research results are as follows:（1）A new two-dimensional material,GeN₃,was obtained from the theoretical replacement of GeP₃ with N-equivalent electrons.Its cohesive energy,phonon dispersion spectrum,and molecular dynamics simulation were calculated to demonstrate its dynamical and thermodynamic stability.The study found that GeN₃ is a covalent compound with a highest phonon frequency of 1417 cm^-1.The electronic properties and optical absorption indicate that the single-layer GeN₃ is an indirect bandgap semiconductor with a bandgap of 1.96 e V and has good light absorption properties in the visible light region.The valence band maximum and conduction band minimum completely crossed the oxidation-reduction potential of water decomposition,meeting the basic requirements of photocatalytic water splitting.In addition,its ultra-high carrier mobility and anisotropic characteristics are favorable for the separation of photo-generated electron-hole pairs.Moreover,the bandgap of GeN₃ single-layer can be transformed from indirect to direct under external strain and electric field.（2）The electronic and photocatalytic properties of strain-tuned porous 2D GeP₂S₆semiconductor were investigated.Computational analysis revealed the material’s high stability,indicating that it can maintain its activity for a long time in catalytic reactions.The GeP₂S₆ monolayer material has a direct bandgap of 2.06 e V,which can absorb visible and ultraviolet light.Through adsorption energy and differential charge density analysis,it was found that H₂O molecules on the GeP₂S₆ monolayer are physically adsorbed,which is beneficial for reactant adsorption and product desorption.The GeP₂S₆ monolayer exhibits good robustness to strain,and under biaxial compressive strain,it can efficiently catalyze the reduction of H₂O in acidic environments,achieving a solar-to-hydrogen conversion efficiency of 12.9%.Finally,the multilayer GeP₂S₆ electronic properties are found to be arranged with edges,which is conducive to photocatalytic water resolution and oxygen reaction（3）Based on first-principles calculations,the mechanical properties,electronic structures,strain responses,optical absorption abilities,and photocatalytic properties of penta-Ge X₂（X=B,C,N）systems with five-membered ring structures were studied,and candidate materials for photocatalytic water splitting were screened.The calculation results show that these five-membered ring structures exhibit good energy,dynamic,thermodynamic,and mechanical stability,and a more in-depth structural stability analysis is obtained by calculating the bond strength.Compared to traditional ultra-thin 2D nano materials,the penta-Ge X₂（X=B,C,N）system has a relatively small Young’s modulus and a larger critical strain.Among them,the penta-GeC₂ monolayer has a band gap and edge position suitable for photocatalytic water splitting,and the Gibbs free energy of its water splitting reaction is calculated,indicating that it can spontaneously carry out photocatalytic total water splitting reaction under neutral conditions.（4）The photocatalytic water splitting performance driven by the oxidation-reduction process of the all-solid-state Z-typeβ-Ge Se/Hf S₂ heterojunction was investigated.In order to further expand the spectral absorption response range and improve the oxidation-reduction ability,the structure,electronic properties,optical absorption,carrier transport,and photocatalytic activity of theβ-Ge Se/Hf S₂ heterojunction were systematically studied based on the successfully preparedβ-Ge Se and Hf S₂ monolayers.The results show that the heterojunction exhibits high carrier mobility,withβ-Ge Se and Hf S₂ electrodes reaching 1311 cm²V^-1s^-1 and 5806 cm²V^-1s^-1,respectively.At the same time,the heterojunction maintains the excellent light absorption performance of the monolayer materials and has a direct Z-type photocatalytic mechanism in the all-solid-state.Based on the calculation of the band edge distribution and the Gibbs free energy of the catalytic reaction,it was found that under visible light irradiation,theβ-Ge Se/Hf S₂ heterojunction can effectively capture water molecules and promote spontaneous photocatalytic water splitting reaction over a wide p H range from 0 to 7.This work demonstrates that the direct Z-typeβ-Ge Se/Hf S₂ heterojunction with enhanced visible light absorption and carrier transport performance is an ideal material for photocatalytic water splitting and has the potential for experimental preparation.