First-principles Nonadiabatic Quantum Dynamics Of Photocatalytic Water Splitting

Conventional ground state density functional theory molecular dynamics has revealed the fundamental physical picture in many materials,engaging in various applications in both physical and chemical regions.Considering the tremendous time dependent nonadiabatic phenomena,the newly time-dependent density functional theory（TDDFT）,are intensively required.Based on the TDDFT,we developed the time dependent ab initio package employing numerical atomic orbital basis and the time dependent ab initio plane wave code employing plane wave basis,to treat the excited state molecular dynamics（MD）with nonadiabatic nature.To deal with the electron-electron correlation,we implement the effective Hubbard U for on-site Coulomb repulsion in TDDFT,aiming at further strongly correlated materials.Adopting the TDDFT-MD simulation methods,we investigated nonadiabatic dynamics for the photocatalytic water splitting on the water/semiconductor interface,where motions of electrons and nuclei are strongly coupled.Although many photocatalytic applications have been achieved,the underlying mechanisms and atomic dynamics are far from known.In this thesis,we show the main results as follows:1.We firstly presented the nonadiabatic quantum dynamics at the water interface involving the non-metal photocatalyst--graphitic carbon nitride（g-C₃N₄）,identifying explicitly the charge transfer channel and the relations to bond breaking/forming.The proposed three-step mechanism clarifies the hole-driven hydrogen transfer process as the important roles in water splitting.The split two hydrogen atoms can be bonded together to form hydrogen gas,and the remanent OH radicals may produce intermediate products（e.g.H₂O₂）.2.We investigated photocatalytic water splitting process on the most important metal oxide,Ti O₂.We found two pathways,electron dominated water splitting involving surface oxygen atom and hole-induced water splitting helped by photoexcited polaron,on rutile Ti O₂（110）surface.The entangled photoexcited 3d electronic dynamics and nuclear motions of polaron,including Jahn-Teller distortion and recovery process,triggers the hole transfer from polaron to water molecules.This result is beyond traditional Born-Oppenheimer dynamics.This polaron-assisted water splitting dynamics also suggest the key mechanism in the perovskite-type materials,towards new perspectives for enhancing photocatalytic activity and material design.Representing an advance in studying quantum dynamics of photocatalytic water splitting,our results bring about a comprehensive understanding about the nonadiabatic electronic-nuclear motions at the microscopic level and critical new insights for the characterization and further development of efficient water-splitting photocatalysts from a dynamic perspective.Towards the full quantum dynamics,we present an advance in the combination of real-time TDDFT with ring polymer molecular dynamics（RPMD）,to include the nuclear quantum effects（NQE）.Thus,we can treat both molecular（ozone）and periodic systems（graphene）.When NQE are taken into account,we found the wave packet splitting of ozone upon photoexcitation,and ultrafast exponential decay of carrier dynamics effected by the quantum motions in graphene.Such quantum nature cannot be observed in the mixed quantum-classical dynamics,such as TDDFT-MD.Our full quantum simulation method can be potentially applied to many other materials to study the quantum effect of both excited state dynamics and nuclear motions.We summarize the recent progresses in studying photocatalytic water splitting on the g-C₃N₄ and rutile Ti O₂（110）surface,as well as our newly developed full quantum simulation method in this thesis.These results will provide deep understanding for photoinduced phenomena and quantum motions.