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1 Title Surface Chemistry The Molecular/Atomic Interactions ➔ Chemisorbtion ➔ Physisorbtion The Free Surface energy ➔ Thermodynamics Considerations ➔ Decreasing.

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Presentation on theme: "1 Title Surface Chemistry The Molecular/Atomic Interactions ➔ Chemisorbtion ➔ Physisorbtion The Free Surface energy ➔ Thermodynamics Considerations ➔ Decreasing."— Presentation transcript:

1 1 Title 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

2 2 Molecular/Atomic Interactions ■ Chemisorption  Formation of molecules  Short Distance ■ Physisorption  No molecules formation  Long Distance In most of the case: Physisorption before Chemisorption U(r) r  Hc r ec  Hp r ep E act Chemisorption Physisorption

3 3 Types of interactions ■ Physisorption  Exothermic  l  H p l < 20 kJ/mol  > 1 layer adsorbed  Not Specific  Kinetic: Fast - since it is a non-activated process ■ Chemisorption  Exothermic  l  H c l > 100 kJ/mol  Only 1 layer adsorbed  Specific  Kinetic: Depends of the activation energy

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

5 5 Metallic Bonding ■ 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,... Electrons cloud Atom

6 6 Van der Waals Forces ■ 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 chargedneutral Induced dipolePermanent dipole

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

8 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 energy γ  Thermodynamically, every system want to decrease its surface energy Driving force for solids

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

11 11 Relaxation/Reconstruction (1) ■ Relaxation adjustments in the surface layers spacings perpendicular to the surface ■ Reconstruction change in the periodicity of the surface structure and surface symmetry Relaxed surface (d 1-2 < d bulk ) Unrelaxed surface

12 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 13 T-L-K Model ■ Ex: move an atom from a terrace site to a kink site ➔ Difference: the energy of two bonds ■ Describes the structure of equilibrium surfaces ■ Assumption: all bonds are equal in the solid ■ T=Terrace ■ L=Ledge ■ K=Kink Number of atoms doing transitions

14 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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