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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

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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

rec

Hp

rep

Eact

Chemisorption

Physisorption

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Types of interactions

Type of interactions Nature of interactionsCovalent Bonding Chemisorption Electrons ExchangeIonic Bonding Chemisorption Electrons ExchangeMetallic Bonding Chemisorption Electrons ExchangeCoulomb Forces Physisorption Charge interactionsVan der Waals Forces Physisorption Charge interactionsHydrogen Bonding Physisorption Charge interactions

■ Physisorption Exothermic lHp l < 20 kJ/mol > 1 layer adsorbed Not Specific Kinetic: Fast - since it is a non-activated process

■ Chemisorption Exothermic lHc l > 100 kJ/mol Only 1 layer adsorbed Specific Kinetic: Depends of the activation energy

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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

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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

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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

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Coulomb Forces and Hydrogen Bridges

■ Hydrogen bondings Directional Electrostatic interaction

between hydrogen and electronegative atoms (O, Cl, F,...)

24 r

QQF

o

ba

■ Columbic interaction Interaction between

permanent charged particles

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Surface Free Energy

Creation of a surfaceYou 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

dAdW

T=∞

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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

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Crystal Surface

■ Example: fcc crystal Bulk

➔ In vacuum the most stable surfaces are :

fcc (111) > fcc (100) > fcc (110)

Surface

(100) face8 neighbors

(110) face7 neighbors

(111) face9 neighbors

Determination of crystals shapes

12 neighbors

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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 (d1-2 < dbulk

)

Unrelaxed surface

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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)

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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

terracekink WWG kTGNen

Number of atoms doing transitions

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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