Electrophoretic deposition

Charged surfaces electrostaticallyattract oppositely charged species(counter-ions) in the solvent orsolution. A combination ofelectrostatic forces, Brownian motionand osmotic forces results in theformation of the so-called doublelayer structure.

The electrophoretic deposition technique has been widely explored,particularly in film deposition of ceramic and organoceramicmaterials on cathode from colloidal dispersions.The main aspects of electrophoretic method are:

a) the material for electrophoretic deposition method need not beelectrically conductive.

b) nanosized particles in colloidal dispersions are typically stabilizedby electrostatic or electrosteric mechanisms.

If an external electric field is applied to a colloidal system or a sol, theconstituent charged particles are set in motion. This type of motion isreferred to as electrophoresis. When a charged particle is in motion, someof the solvent or solution surrounding the particle will move with it, sincepart of the solvent or solution is tightly bound to the particle. The planeseparating the tightly bound liquid layer from the rest of the liquid is calledthe slip plane. The electric potential at the slip plane is known as thezeta potential. Zeta potential is an important parameter in determining thestability of a colloidal dispersion; a zeta potential larger than about 25-30mV is typically required to stabilize a system.

The term electrodeposition is often used unclearly, referring either to electroplating or to electrophoreticdeposition (EPD). The electroplating process is based on a solution of ionic species, usually in water, whileEPD occurs in a suspension of particles. In electroplating, there is a charge transfer during the deposition toproduce the metal or oxide layer in the electrode, while in EPD the deposition occurs without any reactioninvolved. In fact, the principal driving force for EPD is the charge and the electrophoretic mobility of theparticles in the solvent under the influence of an applied electric field, with the drawback that the solventshould be organic in order to avoid water electrolysis.

L. Santos et al.  Electrodeposition of WO3 Nanoparticles for Sensing Applications in Nanotechnology and Nanomaterials » "Electroplating of Nanostructures", book edited by M. Aliofkhazraei, ISBN 978‐953‐51‐2213‐5, 2015

Zeta potential is determined by the particle surface chargedensity, the concentration of counter ions in the solution, solventpolarity and temperature. The zeta potential, around a sphericalparticle can be described as:

with

where Q is the charge on the particle, a is the radius of the particleout to the shear plane, r is the relative dielectric constant of themedium, and ni and zi are the bulk concentration and valence of thei-th ion in the system, respectively.A positively charged surface results in a positive zeta potentialin a dilute system. A high concentration of counter ions, however,can result in a zeta potential of the opposite sign.The mobility µ of a nanoparticle in a colloidal dispersion or a solis dependent on the dielectric constant of the liquid medium r,the zeta potential of the nanoparticle and the viscosity of thefluid . Several forms for this relationship have been proposed, suchas the Huckel equation:

IHP Inner Helmotz planeOHP Outer Helmotz plane

Electrophoretic deposition exploits the motion of charged particlesto grow films or monoliths by enriching the solid particles from acolloidal dispersion onto the surface of an electrode. If particles arepositively charged (having a positive zeta potential), then thedeposition of solid particles will occur at the cathode. Otherwise,deposition will be at the anode.

The electrostatic double layers collapse upon deposition on thegrowth surface, and the particles coagulate. Strong attractive forces(formation of chemical bonds) between two particles, develop once theparticles coagulate. The process can be viewed as a compaction ofnanosized particles. Such films or monoliths are porous (presence ofvoids inside). Typical packing densities, defined as the fraction of solid,are less than 74%, which is the highest packing density for uniformlysized spherical particles.The process depends on the concentration ofparticles in the colloidal dispersions, zetapotential, applied electric field and reactionkinetics between particle surfaces. Slowreaction and slow arrival of nanoparticles ontothe surface allow sufficient particle relaxationon the deposition surface, so that a highpacking density is expected.

In many cases, films or monolithsgrown by electrophoreticdeposition are electric insulators.They are porous and the surface ofthe pores can be electricallycharged just like the nanoparticlesurfaces. Furthermore, the poresare filled with solvent or solutionthat contain counter ions.Using radiation tracked-etchedpolycarbonate membranes with anelectric field, Limmer et al. grewnanowires with diameters 40-175nm and length of 10 mcorresponding to the thickness ofthe membrane. The materialsinclude anatase TiO2, amorphousSiO2, perovskite structured BaTiO3and Pb(Ti,Zr)O3. Nanorods grownby sol electrophoretic depositionare polycrystalline or amorphous.

TiO2 nanowires by template-based electrochemicallyinduced sol-gel deposition.When an external electric fieldis applied, TiO2

+ clustersdiffused to cathode andunderwent hydrolysis andcondensation reactions,resulting in deposition ofnanorods of amorphous TiO2gel. After heat treatment at240°C (24 hrs in air),nanowires of single crystalTiO2 with anatase structurewere synthesized. Theformation of single crystal TiO2is via crystallization ofamorphous phase at anelevated temperature,whereas nanoscale crystallineTiO2 particles are believed toassemble epitaxially to formsingle crystal nanorods.

Nanorods and nanotubes of ZnO from colloidal sols.The ZnO colloidal solution was deposited into the pores of anodicalumina membranes at voltages in the range of 10-400 V. Lowervoltages led to dense, solid nanorods, while higher voltages causedthe formation of hollow tubules. The suggested mechanism is that thehigh voltages cause dielectric breakdown of the anodic alumina,causing it to become charged similarly to the cathode. Electrostaticattraction between the ZnO nanoparticles and the pore walls then leadsto tubule formation.

A similar effect can be obtained in ZnO nanowires and nanotubessynthesis by solution changing the temperature.

Evolution of the morphology of ZnO nanocrystals ranging from rods to tubes while the solution was kept at 90 °C for 3 h and then cooled down to (a) 80 °C (20 h), (b) 60 °C (20 h) and (c) 50 °C (20 h).

Ki‐Woong Chae et al. Low‐temperature solution growth of ZnO nanotube arrays Beilstein J. Nanotechnol. 2010, 1, 128–134.

Template fillingA liquid precursor or precursor mixture is used to fill the pores.The main requests for template filling are:

1) The wetability of the pore wall must be good enough to permitthe penetration and complete filling of the liquid precursor. Forfilling at low temperatures, the surface of pore walls can be easilymodified to be either hydrophilic or hydrophobic by introducing amonolayer of organic molecules.

2) The template materials have to be chemically inert.3) A careful control of shrinkage during solidification is required.

If adhesion between the pore walls and the filling material isweak or solidification starts at the center, or from one end of thepore, or uniformly, solid nanorods are most likely to form.However, if the adhesion is very strong, or the solidification startsat the interfaces and proceeds inwardly, it is most likely to formhollow nanotubes.

Formation of various oxide nanorods and nanotubes by simply fillingthe templates with colloidal dispersions. Colloidal dispersions wereprepared using appropriate sol-gel processing. The filling of thetemplate is accomplished by immersion of the template in a stablesol for various time. The capillary force is believed to drive the solinto the pores, if the surface chemistry of the pores is appropriatelymodified to have a good wetability. After the pores are filled with sol,the templates are withdrawn from the sol and dried prior to firing atelevated temperatures. The firing at elevated temperaturesproduces: the removal of template so that free standing nanorods canbe obtained and densification of the sol-gel derived nanorods.

Colloidal dispersion filling

The capillary force may ensure the complete filling of colloidaldispersion inside pores of the template. Upon drying and subsequentfiring processes, a significant amount of shrinkage is expected.However, most nanorods shrank a little only. One possiblemechanism could be the diffusion of solvent through the membrane,leading to the enrichment of solid along the internal surface oftemplate pores. This process can explain also the formation ofnanotubules.Drawbacks of the filling procedure are the difficult to ensure thecomplete filling of the template pores and that the obtainednanorods are commonly polycrystalline or amorphous.

Melt and solution fillingMetallic, semiconductor and polymeric nanowires can also besynthesized by filling a template with molten metal, acqueoussolution of the appropriate materials and monomer solution.Examples are:Bismuth nanowires by pressure injection of molten bismuth into thenanochannels of an anodic alumina template. The anodic aluminatemplate was degassed and immersed in the liquid bismuth at 325°Cand then high pressure Ar gas of about 4500 psi was applied to injectliquid Bi into the nanochannels of the template for 5 hr.Au, Ag and Pt nanowires synthesized in mesoporous silica templatesfilled with aqueous solutions of the appropriate metal salts and afterdrying and chemical treatment to convert the salts to pure metal.Semiconductor nanowires can be obtained by filling the pores of amesoporous silica template with an aqueous solution of Cd and Mnsalts, drying the sample, and react it with H2S gas to convert to(Cd,Mn)S.Polymeric fibrils have been made by filling a monomer solution,which contains the desired monomer and a polymerization reagent,into the template pores and then polymerizing the monomer solution.

Deposition by centrigugationTemplate filling of nanoclusters assisted with centrifugation forceis another inexpensive method for production of nanorod arrays. Thesamples are attached to silica glass and fired at high temperature.Nanorod arrays of many oxides including silica and titania can begrown with this method. Main advantage regards its applicability toany colloidal dispersion.Main requirement: centrifugation force must be larger than therepulsion force between nanoparticles or nanoclusters.

Nanorods or nanowires can also be synthesized using consumabletemplates. Nanowires of compounds can be synthesized using atemplate-directed reaction. First nanowires of constituent element areprepared, and then reacted with chemicals containing desired elementto form final products. For example the conversion of single crystallineselenium nanowires into single crystalline nanowires of Ag2Se byreacting with aqueous AgN03 solutions at room temperature.

Nanorods can also besynthesized by reactingvolatile metal halide oroxide species with formerlyobtained carbon nanotubesto form solid carbidenanorods. Carbon nanotubeswere used as removabletemplate in the synthesis ofsilicon and boron nitridenanorods.

Converting through chemical reaction

Silica-coated goldnanoparticles were assembledon carbon nanotubes bypolymer wrapping and layer-by-layer assembly. Theprocedure was applied toordered arrays of verticallyaligned nanotubes, resultingin parallel nanotubes ofcompact nanoparticlemonolayers for potential usein photonics.

(a) SEM images of CNT forest, (b)InGaN NSs grown onCNT/SiO2/Si template usingFe/Al catalysts. (c) Highlymagnified SEM image of InGaNNSs and (d) bottom images of CNTsurrounded by GaN crystallites.

DNA is an excellent choice as a template to fabricate nanowires becauseits diameter is about 2 nm and its length and sequence can be preciselycontrolled.

DNA as template

Jan Richter et al. Construction of highly conductive nanowires on a DNA template Appl. Phys. Lett. 78, 536 (2001)

Electrospinning

The morphology of the fibersdepends on the processparameters, including solutionconcentration, appliedelectric field strength, and thefeeding rate of the precursorsolution.

Electrospun nanofibers havewide applications in optics,microelectronics, protectiveclothing, tissue engineeringand drug release systems.

Electrospinning uses electrical forces to produce polymer fibers withnanometer-scale diameters. Electrospinning occurs when the electricalforces at the surface of a polymer solution overcome the surfacetension and cause an electrically charged jet to be ejected. When the jetdries or solidifies, an electrically charged fiber remains. This chargedfiber can be directed or accelerated by electrical forces and thencollected in sheets or other useful geometrical forms.

Electrospinning has also beenexplored for the synthesis ofultrathin organic-inorganichybrid fibers. For example,porous anatase titaniananofibers was made byejecting an ethanol solutioncontaining both poly(viny1pyrrolidone) (PVP) andtitanium tetraisopropoxidethrough a needle under astrong electric field, resultingin the formation ofamorphous TiO2/PVPcomposite nanofibers.Upon pyrolysis of PVP at500°C in air, porous TiO2fibers with diameter rangingfrom 20 to 200nm, dependingon the processing parametersare obtained.

Electrospinning is aconvenient procedureto incorporate insidenanofibers othernanomaterials suchas nanoparticles.One example is theincorporation of Aunanocrystals inelectrospun fibers ofpoly(ethylene oxide)(PEO).

LithographyLithography represents another route tothe synthesis of nanowires.Main techniques are: electron beamlithography, Ion beam lithography,STM lithography, X-ray lithographyand near-field photolithography.Nanowires with diameters less than 10nm and an aspect ratio of 100 can bereadily prepared.

Nanopatterns are defined in a thinfilm of photoresist by exposing it to aUV light source through a phase shiftmask (poly(dimethysiloxane)-(PDMS)).Array of nulls in the intensity of ligth areformed at the edges of the reliefstructures (near-field opticallithography). The patterns aretransferred into the underlyingsubstrate using a reactive ion etchingprocess or a wet etching.

Fabrication of singlecrystal silicon nanowires.Silicon nanostructureswere separated fromunderlying substrate byslight over-etching.