Research On The Electronic Structure And Optical Properties Of Carbon Nanotubes/Semiconductor Heterojunctions

Low-dimensional carbon nanomaterials such as fullerenes,carbon nanotubes（CNTs）and graphene have brought the dawn for figuring out the energy crisis and environmental problems.Abundant experimental researches confirm that the carbon nanomaterial/semiconductorheterojunctionshaveexcellentphotocatalytic performance,but the exact mechanisms of enhanced photocatalytic performance for the carbon nanomaterials/semiconductor heterojunctions are far from being fully understood.With nanometer-scale dimensions,the properties of carbon nanomaterials are strongly dependent on their electronic structures and interactions with other materials.In the thesis,in order to reveal the photocatalytic mechanism of low-dimensional carbon nanomaterials/semiconductor heterojunctions from the electronic level,the electronic and optical properties of CNT/semiconductor（narrow band gap semiconductor Ag₃PO₄,wide band gap semiconductor Sn O₂,the new two-dimensional semiconductor materials Mo S₂ and phosphorene）heterojunctions were systematically studied by the first-principles for clarifying the photocatalytic mechanisms on the basis of the experimental results.The main innovations are gained as follows:1.An excellent visible light photocatalyst,the narrow band semiconductor Ag₃PO₄ blocks are selected to couple with CNTs.Both covalent and noncovalent interactions simultaneously exist in CNT/Ag₃PO₄ heterojunctions.The interfacial interaction in CNT/Ag₃PO₄ heterojunctions depends largely on the nature of CNTs and the relative orientations of them.The C-Ag bond in covalently bonded hybrid and type-II,staggered,band alignment in noncovalent heterojunction lead to a robust separation of photoexcited charge carriers between two constituents,thus enhanced photocatalytic activity.The type-II,staggered,band alignment or the C-Ag bond formed at the interface could effectively block the reduction of Ag+to Ag atom at the Ag₃PO₄（100）surface,thus improving its photostability.The interfacial interaction of the CNT/Ag₃PO₄ heterojunctions results into their samll band gap.The small band gap makes the CNT/Ag₃PO₄ heterojunctions absorb sunlight from ultraviolet to infrared region.Moreover,the CNTs are not only an effective sensitizer,but also a highly active co-catalyst in heterojunctions.These results have revealed the mechanism of enhanced photocatalytic performance of CNT/Ag₃PO₄ heterojunctions observed in experiments,and help for developing highly efficient carbon nanomaterial-based nanophotocatalysts.2.An ultraviolet light photocatalyst,the wide band gap semiconductor Sn O₂blocks are selected to couple with CNT,and the mechanism of their enhanced photocatalytic performance is elucidated by studying systematically the electronic and optical properties.It is found that CNT is interacted non-covalently with monolayer Sn O₂ forming van der Waals heterostructures.The type-II band alignment in CNT/Sn O₂ hybrids can facilitate the separation of photoexcited electrons and holes,improving the photocatalytic activity.The small band gap and electronic structure of CNT/Sn O₂ heterojunctions makes these hybrids having a strong optical absorption in visible light region.The CNTs have served as an effective sensitizer and highly active co-catalyst in hybrids.These results have revealed the mechanism of enhanced photocatalytic performance of CNT/Sn O₂ heterojunctions observed in experiments.3.Two-dimensional（2D）materials with three-atom thickness,Mo S₂ are selected to couple with CNT.The interfacial interaction has systematically been studied in single-wall CNT/monolayer Mo S₂ heterojunctions and its effects on electronic and optical properties by first-principles calculations.It is found that CNT is interacted non-covalently with monolayer Mo S₂ forming van der Waals heterostructures,and their interfacial interaction is closely related to tube diameter.In these heterojunctions,interestingly,Mo S₂ gaining or losing electrons depends also on tube diameter.The type-II,staggered,band alignment in CNT/Mo S₂ heterojunctions can facilitate the separation of photoexcited electrons and holes,improving the photocatalytic activity.Some charged C atoms in CNTs,initially catalytically inert,will turn out to be active sites due to charge transfer,making the CNTs to be a highly active co-catalyst in these heterojunctions.In addition,the Mo atom becomes active species in the photocatalytic reaction due to gaining electrons from CNTs.The combined effects of the above factors can result in enhanced vis-light photocatalytic performance of the CNT/Mo S₂ heterojunctions.4.Two-dimensional（2D）materials with one-atom thickness,phosphorene are selected to couple with CNT.The interfacial interactions,the electronic structure and optical properties of CNT/phosphorene heterojunctions were investigated.The interfacial interactions between CNT and phosphorene can enhance the optical absorption and the stability of the phosphorene.The type-II band alignment in CNT/phosphorene heterojunctions can facilitate the separation of photoexcited electrons and holes,improving the photocatalytic activity.Moreover,the CNTs are an active co-catalyst in hybrids.These results have shown that the CNT/phosphorene heterojunctions can be used as a high activity nanophotocatalysts.