Lecture 7 - Surface stabilization

Lecture 7 Surface - MSE 803 NANOMATERIALS AND NANOTECHNOLOGY Class 7 Surface Stablization Professor Xudong Wang 213 MSE Building Spring 2010

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MSE 803: N ANOMATERIALS AND N ANOTECHNOLOGY Professor Xudong Wang 213 MSE Building Spring 2010 Class 7: Surface Stablization
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Nanostructure Stabilization Two major stabilization mechanisms: Electrostatic stabilization Separating nanostructures by surface charge Kinetically stable Steric stabilization Separating nanostructures by surface coating polymers thermodynamically stable
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Electrostatic Stabilization When a solid emerges (precipitates) from a polar solvent a surface charge will be developed by one, or more, of the following mechanisms: For a given solid surface in a given liquid medium, a fixed surface charge density, or electrode potential, E, will be established. Given by the Nernst Eqn: Where E o is standard potential, n i valence state of ions, α i activity of ions Surface potential varies with concentration of ions in surrounding solution Surface charge in oxides from preferential dissolution/deposition of ions i i g o F n T R E E ln
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Electrostatic Stabilization Ions absorbed on surface determine surface charge - charge determining ions Concentration of charge determining ions affects the surface potential Point of zero charge (p.z.c.) Concentration of charge determining ions corresponding to a neutral (zero-charged) surface Some p.z.c values for oxides in water: WO 3 - 0.5, V 2 O 5 - 1-2, SiO 2 - 2.5, Sn 2 O 4.5, TiO 2 - 6, ZrO 2 - 7, ZnO - 8, Al 2 O 3 - 9, MgO 12 Surface potential can then be written: ] .) . . [( 06 . 0 pH c z p E At room temp: F pH c z p T R E g ] .) . . [( 303 . 2
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Electrostatic Stabilization: Double Layer Model In solution there exists both determining ions and counter ions which have opposite charge to the surface determining ion Concentration of counter ions highest near surface & falls off with distance (Opposite profile for determining ions in solution) Double Layer: Stern Layer: Net surface charge on surface Gouy Layer: A cloud of excess counter-ions in adjacent solution, where ions are distributed
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Distribution of both determining Coulombic force or electrostatic charge Entropic force or dispersion Brownian motion In Gouy layer electric field decreases as: where h>>H, & 1/k is Debye-
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This note was uploaded on 03/23/2010 for the course MATERIAL S 803 taught by Professor Samuel during the Spring '10 term at Wisconsin.

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Lecture 7 Surface - MSE 803 NANOMATERIALS AND NANOTECHNOLOGY Class 7 Surface Stablization Professor Xudong Wang 213 MSE Building Spring 2010

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