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

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

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 40 - 800 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

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

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

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

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,...)

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

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

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

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 )

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)

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

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