Physical chemistry of solid surfaces

Lecture 4

郭修伯

Surface

• A large fraction of surface atoms per unit volume– 1 cm3 cube of iron -> surface atom 10-5%

– 1000 nm3 cube of iron -> surface atom 10%

Fig 2.1

Table 2.1

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

Surface energy

• Definition– the energy required to create a unit area of

“new” surface

2

1

,,ab

PTn

NA

G

i

surface area half bond length

surface atomic density

number of broken bonds

when brake into two pieces

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

– surface restructuring• through combining surface dangling bonds into

strained new chemical bonds

Fig 2.5

– 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

– composition segregation or impurity enrichment on the surface

• enrichment of surfactants on the surface of a liquid

• through solid-state diffusion

Fig 2.7

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

Sintering & Ostwald ripening

Fig 2.9

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

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

Fig 2.14

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

Electrostatic repulsion stabilization

Fig 2.18

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

Polymeric stabilization

Fig 2.21