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Surface Chemistry The Molecular/Atomic Interactions Chemisorbtion Physisorbtion The Free Surface energy Thermodynamics Considerations Decreasing the surface energy Description of a Surface T-L-K Model. Title. Molecular/Atomic Interactions. U(r). Chemisorption. E act.

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  1. Surface Chemistry The Molecular/Atomic Interactions Chemisorbtion Physisorbtion The Free Surface energy Thermodynamics Considerations Decreasing the surface energy Description of a Surface T-L-K Model Title

  2. Molecular/Atomic Interactions U(r) Chemisorption Eact Physisorption r DHp rep DHc rec In most of the case: Physisorption before Chemisorption • Chemisorption • Formation of molecules • Short Distance • Physisorption • No molecules formation • Long Distance

  3. Types of interactions • Physisorption • Exothermic • lDHp l < 20 kJ/mol • > 1 layer adsorbed • Not Specific • Kinetic: Fast - since it is a non-activated process • Chemisorption • Exothermic • lDHc l > 100 kJ/mol • Only 1 layer adsorbed • Specific • Kinetic: Depends of the activation energy

  4. Covalent/Ionic Covalent bonding Ionic bonding • Directional • Transfer of one or more electron from one atom to the other • Directional • Partial Exchange of electrons • Formation of Molecular orbitals • Difference of Electronegativity (capacity to attract electrons) defines the type of liaison

  5. Metallic Bonding Electrons cloud Atom • In a solid, a huge number of atoms: • Many molecular orbitals together lead to the formation of bands (conduction, valence,…) • Some electrons are delocalized and form a cloud • Is the origin of the properties of the solid: conductivity, optic, magnetic properties,...

  6. Van der Waals Forces neutral charged • Interactions between dipoles • 3 parts: • London (Dispersion) Forces • Induced dipole/ Induced dipole • Debye Forces • Permanent dipole/ Induced dipole • Keesom Forces • Permanent dipole/ Permanent dipole • Induced Dipole = polarizable molecules or atoms Permanent dipole Induced dipole

  7. Coulomb Forces and Hydrogen Bridges • Columbic interaction • Interaction between permanent charged particles • Hydrogen bondings • Directional • Electrostatic interaction between hydrogen and electronegative atoms (O, Cl, F,...)

  8. Surface Free Energy • Creation of a surface • You need energy to create a surface! • You break chemical bonds • Work to create a surface define the free surface energyγ • Thermodynamically, every system want to decrease its surface energy Driving force for solids

  9. Surface Free Energy (2) • Minimizing the surface free energy: 1. By reducing the amount of surface area exposed 2. By predominantly exposing surface planes which have a low surface free energy 3. By altering the local surface atomic geometry in a way which reduces the surface free energy Aggregation of the particles Crystal Shapes Relaxation/Reconstruction

  10. Crystal Surface • Surface • Example: fcc crystal • Bulk • In vacuum the most stable surfaces are : fcc (111) > fcc (100) > fcc (110) (100) face 8 neighbors • 12 neighbors (110) face 7 neighbors (111) face 9 neighbors Determination of crystals shapes

  11. Reconstruction change in the periodicity of the surface structure and surface symmetry Relaxation/Reconstruction (1) • Relaxation • adjustments in the surface layers spacings perpendicular to the surface Unrelaxed surface Relaxed surface (d1-2 < dbulk )

  12. More realistic case (Thin films) Solid-solid interface • (a) and (b) are abrupt interfaces since there is no mixing that occurs • The non-abrupt interfaces • mixing (or interdiffusion) • reactive (forming new chemical compounds, possibly multiple phases, the stability of which are dependent on thermodynamic parameters)

  13. T-L-K Model • Describes the structure of equilibrium surfaces • Assumption: all bonds are equal in the solid • T=Terrace • L=Ledge • K=Kink • Ex: move an atom from a terrace site to a kink site • Difference: the energy of two bonds Number of atoms doing transitions

  14. Conclusion • Adsorption • Two different types of adsorption • Physisorption • Chemisorption • Surface free energy • Driving force for solids: decreasing the surface free energy • Decrease the surface area • Expose the best surface planes • Relaxation/Reconstruction

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