1. Surface Chemistry Title 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. Molecular/Atomic Interactions
U(r) r
DHc
rec
DHp
rep
Eact
Chemisorption
Physisorption
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 Chemisorption
Exothermic
lDHp l < 20 kJ/mol
> 1 layer adsorbed
Not Specific
Kinetic: Fast - since it is a non-activated process
Chemisorption
lDHc l > 100 kJ/mol
Only 1 layer adsorbed
Specific
Kinetic: Depends of the activation energy
Chemisorption Physisorption
Temperature Range
(over which adsorption occurs) Virtually unlimited
(but a given molecule may effectively adsorb only over a small range) Near or below the condensation point of the gas
(e.g. Xe < 100 K, CO2 < 200 K)
Adsorption Enthalpy Wide range (related to the chemical bond strength) - typically kJ mol-1 Related to factors like molecular mass and polarity but typically 5-40 kJ mol-1 (i.e. ~ heat of liquefaction)
Crystallographic Specificity
(variation between different surface planes of the same crystal) Marked variation between crystal planes Virtually independent of surface atomic geometry
Nature of Adsorption Often dissociative
May be irreversible Non-dissociative
Reversible
Saturation Uptake Limited to one monolayer Multilayer uptake possible
Kinetics of Adsorption Very variable - often an activated process Fast - since it is a non-activated process

4. Covalent/Ionic Directional Partial Exchange of electrons
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

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. Van der Waals Forces Interactions between dipoles 3 parts:
charged
neutral
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 Driving force for solids
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. 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
All surfaces are energetically unfavourable in that they have a positive free energy of formation. A simple rationalisation for why this must be the case comes from considering the formation of new surfaces by cleavage of a solid and recognizing that bonds have to be broken between atoms on either side of the cleavage plane in order to split the solid and create the surfaces. Breaking bonds requires work to be done on the system, so the surface free energy (surface tension) contribution to the total free energy of a system must therefore be positive.
The unfavourable contribution to the total free energy may, however, be minimised in several ways :
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
y of a system must therefore be positive.The unfavourable contribution to the total free energy may, however, be minimised

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

11. Relaxation/Reconstruction (1)
adjustments in the surface layers spacings perpendicular to the surface
Reconstruction
change in the periodicity of the surface structure and surface symmetry
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. Number of atoms doing transitions
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 Surface free energy
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