(87a) Synthesizing Complex Nano-Colloid and Nano-Fiber Morphologies by DC and Ac Electrosprays
AIChE Annual Meeting
Monday, November 13, 2006 - 12:30pm to 12:55pm
A precise technique for depositing nano colloids or fibers onto targets is reported using a special DC/AC electrospray deposition method (ESD). Compared with other deposition methods such as casting, spinning or chemical vapor deposition, ESD possesses the ability to deposit and manipulate uniform nano-sized particles and fibers without damaging the load, since the whole procedure is conducted under room temperature and atmospheric pressure. In addition, the deposition pattern of particles or fibers can be controlled using patterned target electrodes and electrostatic forces that can also vary dynamically in time to produce additional complexity in the deposited pattern. In this work, we describe the use of two kinds of fluids as spray samples, 1) carbon nanotubes solution (CNT), 2) polymer solutions (PVP and PEO) with high viscosity. CNT coating with different geometries is the intended goal of the first solution. Polymer membranes and fibers with CNT composites are the intended patterns for the second solution. Both DC and AC electrosprays are used to effect different pattern length scales and polymerization chemistry. Depending upon the liquid properties, coating structures with different morphologies are obtained. Porous composite membranes, fabrics weaved by single fibers, fractal CNT forests and CNT linear aggregates have all been observed. Some of the CNT morphologies provide large surface area and high conductivity but with minimum CNT usage, ideal for enhancing the reactive area of a membrane electrode in micro-fuel cells and for electro-chemical biosensors. The spraying is done with an imposed electric potential bias (> 1kV) across a metal needle with a blunt tip and a glassy carbon electrode which serves as the target ground. A conductive ionomer, Nafion, acting as both surfactant and binder between the CNT and the target electrode, is added into the CNT solution at a concentration of 0.5 wt%. CNT concentration is varied to obtain optimal results. If the concentration of CNT is too low, the spray time becomes excessively long. On the other hand, if the concentration is too high, vertical fractal structures are formed. Although fractal shapes possess large surface area, they are mechanically unstable and can be partially washed away when contacted with liquids. Hence an optimal concentration range exists, at which SEM results of the coated electrode show uniform Nafion nano-colloids connected by CNT, forming a sponge-like surface. These Nafion colloids (~ 1 micron in diameter) provide mechanical binding sites for the CNT network while CNT serves as catalyst and current transducers, thus improving the strength of the CNT catalyst. The performance of the coated electrode has been investigated by cyclic voltammetry and a remarkable increase in current density for hydroxide redox reaction is observed. The amperometric response current is found to vary proportionally to the concentration of hydroxide in the analytically important millimolar concentration range. The polymer nano-fibers (PVP & PEO) fabricated by the ESD method can be used as catalytic carriers as well as high-performance membranes for biological research and medical treatment, electronic parts or high-performance batteries. However, the bottleneck in conventional DC spray/spin is the slow fiber production from the single-cone spray. Multiple jets can scale-up the throughput to provide reasonable nanofiber production rates. We effect multiple Taylor cones from a single meniscus by using a low-frequency AC field to resonantly excite the drop. This forced resonance yields a high spatial jet density at the liquid-gas interface, with a single jet ensuing from each cone. The multi-cone ejections can produce as much as an order of magnitude higher spraying rate for rapid coating. We can hence achieve high-throughput spraying by judiciously selecting the applied frequency.