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Electrophoretic Deposition

- process in which particles, suspended in a liquid medium, migrate in an electric field and deposit on an electrode

no redox

differs from electrolytic in several ways

• deposit need not be electrically conductive• preformed nanoparticles can be used

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Particles suspended in a polar solvent or electrolyte are often charged-hence can be directed by an electric field

due to:1. Oxidation or reduction2. Adsorption of charged species (e.g., polymers)3. Dissolution (remember naked ions in CHEM 2060)

The Electric Double Layer Development of a net charge at the

particle surface affects the distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counter ions (ions of opposite charge to that of the particle) close to the surface Thus an electrical double layer exists

round each particle

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Potential decreases linearly in Stern Layer

Exponential decrease in Diffuse layer

Potential at slip plane : between Stern and Diffuse layer is called the ZETA potential

Slip plane divides tightly bound layerand rest of double layer

Zeta potential is not measurable directly but it can be calculated using theoretical models and an experimentally-determined electrophoreticmobility or dynamic electrophoretic mobility.

Zeta Potential Ranges The zeta potential is the overall charge a particle acquires in a specific medium.

The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system

If all the particles have a large negative or positive zeta potential they will repel each other and there is dispersion stability

If the particles have low zeta potential values then there is no force to prevent the particles coming together and there is dispersion instability

A dividing line between stable and unstable aqueous dispersions is generally taken at either +30 or -30mV

Particles with zeta potentials more positive than +30mV are normally considered stable Particles with zeta potentials more negative than -30mV are normally considered stable

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ξ = Q/ 4πεra(1 + κa)

The zeta potential (ξ) is given by: (for a spherical particle)

with : κ2 = e2 Eni zi2 / εrε0 kT

Q = charge on particle; a is the radius of the particle out to the slip planeεr is the relative dielectric constant of the mediumn = concentration of ionz = charge (valence of ion)

Dielectric constant 101If a voltage V is applied across a capacitor of capacitance C, then the

charge Q that it can hold is directly proportional to the applied voltage V, with the capacitance C as the proportionality constant. Thus, Q =

CV, or C = Q/V. The unit of measurement for capacitance is the farad (coulomb per volt).

The capacitance of a capacitor depends on the permittivity ε of the dielectric layer, as well as the area A of the capacitor and the

separation distance d between the two conductive plates. Permittivity and capacitance are mathematically related as follows: C = ε (A/d).

When the dielectric used is vacuum, then the capacitance Co = εo (A/d), where εo is the permittivity of vacuum

(8.85 x 10-12 F/m).

The dielectric constant (k) of a material is the ratio of its permittivity εto the permittivity of vacuum εo, so k = ε/εo. The dielectric constant is therefore also known as the relative permittivity of the material. Since

the dielectric constant is just a ratio of two similar quantities, it is dimensionless. 31-6

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Note: a positively charged surface results in a positive zeta potential ina dilute system.

However, a high concentration of counter ions can result in a zeta potential of the opposite sign

The mobility (μ) of a nanoparticle depends on the dielectric constant:

μ = 2 εrε0ξ / 3Bη

where η is the viscosity of the fluid.

Zeta potential will determine whether deposition is at cathode or anode

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In some cases, electrophoretic deposition using mixtures of nanocrystals produces films composed of mixtures of the nanocrystals. This is shown below schematically for mixtures of CdSe and Fe2O3 nanocrystals. Such film mixtures are potentially multifunctional nanomaterials. When Au nanocrystals are added to a hexane solvent containing either CdSe or Fe2O3 nanocrystals, there is deposition of a film containing either only CdSe or Fe2O3 nanocrystals, respectively, and this occurs on only one electrode.

Templates: New Topic

We have met this idea before: Porous Alumina is used as a template togrow nanotubes

wires grow up can be electrochemical, electrolessand/or electrophoretic

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we now look at more aspects oftemplate synthesis

can producehighly organizednanostructures

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Templates are porous: can be inorganic or inorg/organic hybrid

IUPAC (International Union of Pure and Applied Chemistry) http://www.iupac.org/gives three classes of porous materials:1. macroporous ; d > 50 nm2. mesoporous ; 2 < d < 50 nm3. microporous ; d < 2 nm

Important topic

whole Journal devoted tothis field

Various kinds of porous materials are displayed. Top left: the “ink bottle” pore cavity is quite common and serves as an excellent nanosized beaker within which studies of chemical confinement are conducted. A structure is considered to be a pore if its width is less than its depth. Top middle: interstitial spaces are formed with hard spheres. The spaces are exploited in templating processes such as nanosphere lithography. Top right: interstitial spaces are found between cylinders formed by a sol–gel process. Such spaces are also found in single-walled carbon nanotube bundles. The interspatial cavity is defined by the van der Waals gap between and among bundles. Bottom left: enclosed cavities are not useful in the templating process due to their inaccessibility. These kinds of cavities, however, are able to affect the physical properties (e.g., thermal conductivity) of the material. Bottom middle: zeolites exhibit highly ordered structures that come in the form of channels or three-dimensional interconnected porous networks. Bottom right: the pore channels of porous alumina can be made to transverse the entire thickness of the membrane or be left capped at one end.

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Chemistry in confined spaces: Good example Zeolites:

Microporous and Mesoporous Materials

Zeolites: Microporous

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