Surface Chemistry

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
Molecular/Atomic Interactions
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 lt 20 kJ/mol
• gt 1 layer adsorbed
• Not Specific
• Kinetic Fast - since it is a non-activated
  process
• Chemisorption
• Exothermic
• lDHc l gt 100 kJ/mol
• Only 1 layer adsorbed
• Specific
• Kinetic Depends of the activation energy

4
Covalent/Ionic

• Directional
• Partial Exchange of electrons
• Formation of Molecular orbitals

• 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,...

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

Induced dipole
Permanent 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
  energy ?
• 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) gt fcc (100) gt fcc (110)

(100) face 8 neighbors
12 neighbors
(110) face 7 neighbors
(111) face 9 neighbors
Determination of crystals shapes
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

Unrelaxed surface
Relaxed surface (d1-2 lt 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
• TTerrace
• LLedge
• KKink

• 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