(41e) Engineered Microfluidic Mixing for Green Nanocrystal Fabrication

Metal oxide nanocrystals are finding increasing application as photocatalysts, sensor elements, piezoelectric transducers, and pigments. Traditional methods for fabricating nanocrystals rely on costly, difficult-to-scale, high-temperature batch methods. Solution-phase techniques for fabricating these crystals, while promising in terms of scalability, require toxic, flammable, and unrecoverable organic solvents. We are developing a route to the high-yield production of zinc oxide (ZnO) and titanium dioxide (TiO2) nanocrystals based on microfluidic reactions in environmentally friendly ionic liquid solvents. These techniques take advantage of the unique mixing properties of microfluidic reactors, and are scalable by simple microfluidic parallelization.

Ionic liquids (ILs) are nonvolatile, polar, organic salts that are liquid at room temperature. Their lack of volatility, non-flammability, and capacity for recycling have made them attractive solvents for green chemistry applications. They are especially attractive for nanocrystal fabrication reactions since they can act simultaneously as solvents and surface passivation agents. They are ideal in this role because they have low interfacial tensions, leading to high nucleation rates and small, uniform particles. While ILs have been successfully utilized in the batch syntheses of ZnO and TiO2 nanocrystals, their use has not been translated to a microfluidic platform until now. By transferring IL-based fabrication technologies to a microfluidic system, we are able to leverage the advantages of microfluidic mixing in an environmentally friendly reaction that is easily optimized to form small, homogeneous nanocrystals.

Nanoparticle formation is a nucleation-and-growth process, with nucleation occurring at a higher concentration of nanoparticle precursor (solute) than growth. Since the growth rate is constant, this kind of system can only produce monodisperse particles when the nucleation occurs during a short-lived burst. That is, since particles nucleated at different times will grow to different final sizes, monodispersity can only be obtained if nucleation ends very quickly after beginning. In bulk reaction systems, this is the case only if nucleation can be treated as a critical process, with a sudden concentration-dependant transition between nucleating and non-nucleating states and the process of nucleation itself rapidly consuming free solute. In bulk, therefore, the propensity of a given system to form monodisperse particles is entirely a function of its thermo¬dynamics. Microfluidic systems, however, allow for precise manipulation of the conditions under which reagents come into contact and mix, thereby allowing for control over the nucleation and growth phases of particle formation.

We are developing two mixing schemes for IL-based nanofabrication reactions in microfluidic channels, each with its distinct advantages. In laminar flow mixing, reactants flow through the microchannel side-by-side, with mixing occurring by diffusion between the streams. In droplet mixing, reactants are co-encapsulated in moving droplets in the microchannel, with mixing occurring by convection within each droplet.

A laminar flow configuration is the typical flow profile formed at a junction of two converging continuous microfluidic flow streams. Since the Reynolds number in a microchannel is low, there is no turbulent mixing between the two streams. Rather, they flow side by side and mixing occurs via diffusion between the two streams. This configuration allows for slow stream mixing and confinement of reactions to the interface between the streams. Diffusional mixing in laminar flow is a potentially ideal configuration for nanoparticle fabrication reactions with a thermodynamically constrained nucleation burst. Nucleation occurs at the interface as the reactant streams begin to interdiffuse. If the nucleation burst is rapid, the concentration of reactants at the interface region drops rapidly to below the level necessary for further nucleation. Particle growth is fueled and limited by the further diffusion of reactants into the zone surrounding the interface. Particles have relatively low diffusivities and remain in the interface region. Final particle size can be controlled by total residence time in the device. Since the interface region is small, with a very high surface area-to-volume ratio, heat flow is very fast and temperature disturbances do not persist.

There are a number of disadvantages to laminar flows, however. Chief among these is axial dispersion; the parabolic flow profile leads to mixing in the direction of the flow. This means that rather than having a single residence time, the fluid in the device is described by a residence time distribution. This limits the uniformity of particles that can be obtained in laminar systems. Laminar flow also limits the ability to rapidly quench a reaction (either the nucleation or growth phase of particle fabrication), since mixing between reactants and quenching agents will necessarily be slow.

Microfluidic systems also allow for flow in a droplet, or plug, profile. We are developing a system in which droplets of IL-solvated nanocrystal reactants flow down a microchannel separated by an inert fluorocarbon carrier phase. No-slip boundary conditions at channel walls induce convective flows within individual droplets. This convection leads to fast mixing within the droplet. Since droplet contents are quickly homogenized, they are potentially excellent environments for nanoparticle fabrication: nucleation begins at a single time point throughout the droplet, and solute concentration is the same throughout the droplet as growth proceeds. Droplet flow configurations also allow for rapid quenching of reactions by droplet merger, since upon merger the contents of a droplet containing a quenching agent will mix convectively with the reactants. Controlled quenching of nucleation soon after initial droplet mixing has the potential to improve particle monodispersity.

In this work, laminar and droplet-based mixing are compared for the synthesis of ZnO and TiO2 nanocrystals. Several laminar schemes are explored. These include approaches in which nanocrystal precursor reagents are introduced as solvents in IL streams and approaches in which these reagents are introduced either neat or in water solution in laminar flow adjacent to an IL stream. The effect of IL viscosity on the flow configuration is explored. Nanocrystal products are analyzed by transmission electron microscopy and X-ray diffraction.