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

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