Physical chemistry of solid surfaces

1
Physical chemistry of solid surfaces

• Lecture 4
• ???

2
Surface

• A large fraction of surface atoms per unit volume
• 1 cm3 cube of iron -gt surface atom 10-5
• 1000 nm3 cube of iron -gt surface atom 10

Fig 2.1
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Table 2.1
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Surface energy

• Origin
• Atoms or molecules on a solid surface posses
  fewer nearest neighbors or coordination numbers,
  thus have unsatisfied bonds exposed to the
  surface
• Huge surface energy for nanomaterials
• Thermodynamically unstable/metastable
• tend to growth to reduce the surface energy

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Surface energy

• Definition
• the energy required to create a unit area of
  new surface

number of broken bonds
surface atomic density
when brake into two pieces
surface area
half bond length
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Surface energy

• For a given surface with a fixed surface area,
  the surface energy can be reduced through
• surface relaxation
• the surface atoms or ions shift inwardly

Fig 2.4
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• surface restructuring
• through combining surface dangling bonds into
  strained new chemical bonds

Fig 2.5
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• surface adsorption
• through chemical or physical adsorption of
  terminal chemical species onto the surface by
  forming chemical bonds or weak attraction forces
  such as electrostatic or van der Waals forces

Fig 2.6
chemical adsorption
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• composition segregation or impurity enrichment on
  the surface
• enrichment of surfactants on the surface of a
  liquid
• through solid-state diffusion

Fig 2.7
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Reduction of overall surface energy at the
overall system level

• Combining individual nanostructure together to
  form large structures so as to reduce the overall
  surface area
• sintering high temp (70 melting pt.)
• Ostwald ripening wide range temp solvent
  (large grow and small eliminate)
• agglomeration of individual nanostructures
  without altering the individual nanostructures

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Sintering Ostwald ripening
Fig 2.9
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Electrostatic stabilization

• a solid emerges in a polar solvent or an
  electrolyte solution
• surface charge develops by
• preferential adsorption of ions
• dissociation of surface charged species
• isomorphic substitution of ions
• accumulation or depletion of electrons at the
  surface
• physical adsorption of charged species onto the
  surface

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Surface charge distribution

• The distributions of ions and counter ions are
  controlled by
• Coulomic force or electrostatic force
• Entropic force or dispersion
• Brownian motion
• Inhomogenous distribution
• double layer structure
• separated by the Helmholtz plane

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Fig 2.14
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Van der Waals attraction potential

• The sum of the molecular interaction for all
  pairs of molecules
• weak force and becomes significant only at a very
  short distance
• agglomeration of nanoparticles the combination
  of van der Waals force and Brownian motion
• Prevent agglomeration electrostatic repulsion
  and steric exclusion

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Electrostatic repulsion stabilization
Fig 2.18
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Steric exclusion stabilization

• Also polymeric stabilization
• Widely used in stabilization of colloidal
  dispersions
• thermodynamic stabilization particles are always
  redispersible
• high concentration can be accommodated
• not electrolyte sensitive
• suitable to multiple phase systems

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Polymeric stabilization
Fig 2.21