Stability and Electrochemical Applications of Some Metal Clusters and Stepped Surfaces

Author:Bo Yi Fan

Supervisor:zhao ming


Degree Year:2019





In recent years,with the development of technology,metal clusters have achieved precise synthesis at the atomic level and breakthroughs in the determination of crystal structure.Unlike the continuous band structure of plasma metal nanoparticles,these clusters,such as thiolate-protected gold clusters(Aum(SR)n)and pure Cu clusters,have quantized electronic structures in ultra-small sizes,and they are also widely used in fields such as catalysis and optics.However,what determines the geometric and electronic properties of Aum(SR)n,namely,the structure--property relationship,is up for debate,despite decades of work on this topic.Copper is a potential electrocatalyst for CO2 electroreduction.The roles of the size and symmetry of the Cu clusters as well as the temperature in the CO2-reduction process remain elusive,which hinders the development of advanced catalysts.Moreover,the CO2 reduction reactions always occur on metal surface,where the water-solid interfaces have an important influence on the reaction.The accurate configuration of water-solid interface contributes to the development of experimental and theoretical studies.Over the past decade,the atomistic structures of water molecules on metal surfaces also have been extensively investigated.However,these studies have been exclusively performed on atomically flat surfaces(close-packed),such as Pt(111),Cu(110),Cu(111),and Ru(0001),which are far from realistic metal surfaces that consist of steps and terraces.This drawback substantially prohibits our understanding of the geometry and evolution of water/solid interface.In view of the above problems,the main content of this paper is divided into the following three parts:(1)Through DFT calculations,including nonlocal many-body dispersion(MBD)interactions,the geometric and electronic properties of Aum(SR)n clusters are investigated.Calculations demonstrate that the MBD interactions are essential for correctly describing the geometry and energy of the clusters.Greater anisotropic polarization and more atoms distributed in the shell of the clusters lead to more pronounced MBD interactions and higher stability of the clusters.Furthermore,the HOMO--LUMO gap of the clusters strongly depends on the gold core.These results provide critical clues for understanding and designing Aum(SR)n clusters.(2)We investigate the roles of the size and symmetry of the Cu clusters as well as the temperature in the CO2-reduction process.We find that the decrease of icosahedron Cu clusters’size but the increase of truncated octahedron Cu clusters’size contributes to the selectivity of CO2 reduction.In addition,the symmetry of Cu clusters modulates the selectivity of CO2reduction at room temperature.We also note that the increase of temperature is favorable to CO production.Our findings not only contribute to a thorough understanding of CO2 reduction on Cu clusters,but also provide clues for designing catalysts in future experiments.(3)We use a genetic algorithm method on top of density functional theory to determine water structures on Pt(221)and(553)surfaces.By including screened van der Waals(vdW)forces,we uncover a series of novel 1D and 2D water structures,which are essentially determined by the atomic geometry of Pt surfaces.We find that with increasing water coverage,water-metal vdW interactions,water-metal electrostatic interactions,and water-water interactions in turn dictate the evolution of water structures.In particular,the step feature provides the templating effects for the formation of 1D water chains by modulating water-metal interactions,whereas the terrace is crucial to the formation of 2D water networks by altering H-bonds.These findings rationalize several key experimental observations and provide critical clues for understanding water/solid interfaces.