Study On Photoelectric Conversion Of Perovskite-phase Oxide/graphene Composites And Graphene-like MXene Materials

With dwindling fossil-fuel reserves,there is an urgent need to find alternative energy resources to meet the growing demand.Alternate energy resources must be efficient,cost-effective and ecologically friendly,the harnessing of solar energy,in this context,becomes a very attractive proposition.Photoelectric conversion technology is one of the main technologies to realize solar energy conversion.Dye-sensitized solar cells and semiconductor photocatalysis are two important ways of photoelectric conversion.In the first part of this thesis,two types of perovskite phase oxide/graphene composites were prepared experimentally and used as the counter electrode for dye-sensitized solar cells.Their electrocatalytic properties in the I-/I3-system were studied.In the second part,the semiconductor properties of a new type of graphene-like material,SC2C MXene,are calculated theoretically using density functional theory and the possibility of its application in visible-light photocatalysis is analyzed.The main contents of this thesis are as follows:(1)The nanocomposites of the perovskite-phase La0.65Sr0.35MnO3 nanoparticles with reduced graphene oxide(LSMO@RGO)have been successfully prepared via the solution combustion method.LSMO@RGO nanocomposites exhibit excellent electrocatalytic performance for reduction of triiodide,owing to the positive synergetic effect between LSMO nanoparticles and RGO sheets.As a consequence,the dye-sensitized solar cell(DSSC)with the LSMO@RGO counter electrode(CE)yielded a power conversion efficiency(PCE)of 6.57%,which is observably higher than that of using pristine LSMO CE(5.35%)or RGO CE(4.93%)alone,and up to 92%of the Pt CE(7.13%).(2)The perovskite-phase La0.5Sr0.5CoO2.91(LSCO)nanoparticles were synthesized through a sol-gel method and used as the CE for DSSC.The cell showed a power conversion efficiency of 3.24%.To further improve the PCE of CE of the LSCO nanoparticles,the nanocomposites of the LSCO nanoparticles with reduced graphene oxide(LSCO@RGO)have been prepared via a simply physical mixing method and served as the efficient CE for DSSC.The LSCO@RGO nanocomposites exhibit excellent electrocatalytic performance for reduction of triiodide,owing to the positive synergetic effect between LSCO nanoparticles and RGO sheets.As a consequence,the DSSC with the LSCO@RGO nanocomposites CE yielded a PCE of 6.32%,which is observably higher than that of using pristine LSCO CE(3.24%)or RGO CE(4.54%)alone,and up to 88%of the Pt CE(7.18%).(3)In view of the diverse functional groups left on the MXene during the etching process,we systematically computed the influence of surface-group types on the structural,electronic and optical properties of SC2CT2(T=-O,-OH,-F)MXenes.For all geometries of the SC2CT2 MXenes,the geometry I of SC2CT2,which has the functional groups locating above the opposite-side Sc atoms,are lowest-energy structure.Accordingly,the energetically favorable Sc2CO2-I,SC2CF2-I and Sc2C(OH)2-I were selected for further evaluation of the photocatalytic activities.We found that the SC2CO2-I is metallic,while SC2CF2-I and Sc2C(OH)2 are semiconductors with visible-light absorptions and promising carrier mobilities.Compared with the Sc2C(OH)2-I,the Sc2CF2-I has not only more suitable band gap(1.91 eV),but also the higher redox capability of photo-activated carriers,which should have better photocatalytic performance.