(398b) New Applications of Electrophoretic Deposition

Electrophoretic deposition (EPD) is a technique in which charged particles dispersed in a liquid are deposited onto a substrate under the force of an applied electric field. EPD has many advantages besides its benign processing conditions, such as a high uniformity, the ability to produce deposits fast and continuously, and a low level of contamination. For the past 20 years, my research group has investigated EPD of a variety of micron and, more recently, nanosized powders, including phosphors for high-resolution information displays, zeolites for fabrication of modified electrodes and supported membranes, and single-walled carbon nanotube structures. Recently, we have use EPD to deposit nano-sized powders, specifically phosphors for solid state lighting and electrocatalysts for a thermochemical hydrogen production process. The typical EPD process used a bath consisting of the micron-sized particles of interest suspended in isopropanol (IPA) with dissolved nitrate salt. The fundamentals of the EPD process were systematically investigated in our work with micron-sized phosphors, including the dissociation of nitrate salts in IPA and the zeta potential of charged particles, the formation of the adhesive agents, and the factors that affect the adhesion strength of deposits. Then, the EPD process was combined with photolithography to deposit triads of phosphor stripes for producing high-resolution color displays.

Recently, we have used EPD for depositing micron-sized and ~100 nm phosphors for a “remote phosphor” configuration in a near UV-LED-based light source for improved white light extraction efficiency. It was demonstrated that EPD could be used to deposit red, green, blue, yellow and orange phosphors to generate white light using a near UV-emitting LED by either depositing a phosphor blend or layer-by-layer in a sequential method.  Individual and phosphor blend coatings were prepared by EPD of red, green, blue, yellow and orange emitting phosphors. The deposition rates of the individual phosphor films were ~1-5 mm/min. The blend depositions composed of both three and four phosphor compositions emit white light located on or near the black body locus on the CIE chromaticity diagram. Phosphor films were also prepared by sequential deposition of red/orange and green/blue compositions, to generate white light. When the layered films were flipped over and illuminated in this orientation, they showed approximately the same luminescence characteristics. No change in the reabsorption ratio of green/blue emission by the red/orange phosphor was found regardless of the deposited order of the layered films. These applications of EPD of phosphor for white solid state lighting are promising and effective due to easy tuning of emissive color by varying the phosphor blend compositions.

Although nanoparticles of a variety of materials have been coated by EPD, there have been few direct comparisons of EPD of nano- and micron-sized particles of the same material. EPD of nano-, nano core/SiO₂ shell and micron-sized (Ba_0.97Eu_0.03)₂SiO₄ phosphor particles for application in a near-UV LED-based light source was studied. EPD from an amyl alcohol bath was able to produce uniform films for allparticle sizes, whereas uniform films were produced only of micron-sized particles in an isopropyl alcohol bath. A new equation was developed for predicting the deposited mass, considering the change in concentration of particles in the bath from both settling and deposition, showed good agreement with the experimental values.

The solar sulfur ammonia (SA) thermochemical cycle is a potential process for splitting water to produce hydrogen.  The hydrogen production sub-cycle in the SA cycle consists of the electrolytic oxidation of ammonium sulfite to ammonium sulfate. The anodic reaction is kinetically slow and nanoparticle catalysts are being investigated to improve this reaction rate.  For this purpose, nanoparticles were coated onto graphite substrates by EPD.  Synthesized nanoparticles of cobalt ferrite (20 nm) and purchased platinum cobalt on graphite (50 nm) were mixed into either 100% ethanol or 90% water/10% isopropanol with hexadecyltrimethylammonium bromide (CTAB).  Then the nanoparticles were deposited onto either graphite paper or felt using EPD.  Linear sweep voltammetry in 2 M ammonium sulfite was performed to test electrocatalytic activity of the deposits compared to blank graphite substrates.  The morphology of deposits was examined by scanning electron microscopy.