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 26-1
Particles suspended in a polar solvent or electrolyte are often charged -hence can be directed by an electric field due to: 1. Oxidation or reduction 2. 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 26-2
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 layer and rest of double layer Zeta potential is not measurable directly but it can be calculated using theoretical models and an experimentally-determined electrophoretic mobility or dynamic electrophoretic mobility. 26-3
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 26-4
The zeta potential (ξ) is given by: (for a spherical particle) ξ = Q/ 4πε r a(1 + κa) with : κ 2 = e 2 En i z i2 / ε r ε 0 kt Q = charge on particle; a is the radius of the particle out to the slip plane ε r is the relative dielectric constant of the medium n = concentration of ion z = charge (valence of ion) 26-5
Dielectric constant 101 If 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 = ε o (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 ε to 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. 26-6
Note: a positively charged surface results in a positive zeta potential in a 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 ε r ε 0 ξ / 3Bη where η is the viscosity of the fluid. Zeta potential will determine whether deposition is at cathode or anode 26-7
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 Fe 2 O 3 nanocrystals. Such film mixtures are potentially multifunctional nanomaterials. When Au nanocrystals are added to a hexane solvent containing either CdSe or Fe 2 O 3 nanocrystals, there is deposition of a film containing either only CdSe or Fe 2 O 3 nanocrystals, respectively, and this occurs on only one electrode. 26-8
Templates: New Topic We have met this idea before: Porous Alumina is used as a template to grow nanotubes wires grow up can be electrochemical, electroless and/or electrophoretic can produce highly organized nanostructures we now look at more aspects of template synthesis 26-9
Templates are porous: can be inorganic or inorg/organic hybrid IUPAC (International Union of Pure and Applied Chemistry) gives three classes of porous materials: 1. macroporous ; d > 50 nm 2. mesoporous ; 2 < d < 50 nm 3. microporous ; d < 2 nm http://www.iupac.org/ Important topic whole Journal devoted to this field 26-10
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 threedimensional 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. 26-11
Chemistry in confined spaces: Good example Zeolites: Microporous and Mesoporous Materials Zeolites: Microporous more next lecture 26-12