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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 Molecular/Atomic Interactions U(r) In most of the case: Physisorption before Chemisorption Chemisorption Eact Physisorption r DHp rep DHc ■ Chemisorption ■ rec Formation of molecules Short Distance Physisorption No molecules formation Long Distance Types of interactions Type of interactions Covalent Bonding Ionic Bonding Metallic Bonding Coulomb Forces Van der Waals Forces Hydrogen Bonding ■ Physisorption ■ Chemisorption Chemisorption Chemisorption Physisorption Physisorption 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 Nature of interactions Electrons Exchange Electrons Exchange Electrons Exchange Charge interactions Charge interactions Charge interactions Covalent/Ionic Covalent bonding ■ ■ ■ Directional Partial Exchange of electrons Formation of Molecular orbitals Ionic bonding ■ ■ Directional Transfer of one or more electron from one atom to the other Difference of Electronegativity (capacity to attract electrons) defines the type of liaison Metallic Bonding ■ In a solid, a huge number of atoms: ➔ Electrons cloud ■ Atom ■ 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,... Van der Waals Forces ■ ■ Interactions between dipoles 3 parts: London (Dispersion) Forces ➔ Induced dipole/ Induced dipole neutral charged Debye Forces ➔ Permanent dipole/ Induced dipole Keesom Forces ➔ ■ Permanent dipole/ Permanent dipole Induced Dipole = polarizable molecules or atoms Permanent dipole Induced dipole Coulomb Forces and Hydrogen Bridges ■ Columbic interaction Interaction between permanent charged particles Qa Qb F 4or 2 ■ Hydrogen bondings Directional Electrostatic interaction between hydrogen and electronegative atoms (O, Cl, F,...) 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 dW dA T= ∞ 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 Crystal Surface ■ Example: fcc crystal Surface Bulk (100) face 8 neighbors 12 neighbors (110) face 7 neighbors ➔ In vacuum the most stable surfaces are : fcc (111) > fcc (100) > fcc (110) Determination of crystals shapes (111) face 9 neighbors Relaxation/Reconstruction (1) ■ Relaxation adjustments in the surface layers spacings perpendicular to the surface Unrelaxed surface Relaxed surface (d1-2 < dbulk ) ■ Reconstruction change in the periodicity of the surface structure and surface symmetry 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) 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 DG Wkink Wterrace Number of atoms doing transitions n Ne DG kT 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 ➔
