AmericanChemicalSociety.com

The synthesis of nanomaterials has many potential applications in areas of photovoltaics, energy, and bio-chemical sensing. Current methods to reproducibly synthesize nanomaterials are largely limited by batch procedures which suffer from irreproducibility, large size distribution and batch to batch quality. It can also be very challenging to scale-up these methods. Microfluidic devices, and specifically microreactors, can integrate heaters and fluidic control elements to help rectify some of the problems with batch reactors as well as provide some additional advantages. For example, microfluidic channels are characterized by their high surface area to volume ratio which provides enhanced mass and heat transfer. The small reaction volumes combined with the high heat and mass transfer rates enable reactions to be performed under more aggressive conditions with higher yields than can typically be achieved with conventional reactors. Moreover, new reaction pathways deemed too difficult to control in conventional macroscopic equipment can be conducted safely because of the high heat transfer and ease of confining small volumes. This ability to work at elevated temperatures and pressures while confining potentially toxic, highly reactive starting materials is particularly important for the synthesis of novel nanostructured materials. High pressure allows for wider range of chemistry, since at sufficiently high pressure, virtually any common solvent, precursor, and ligand will remain either liquid or become supercritical at the temperatures required for nanomaterials synthesis.

We have made significant progress in designing and characterizing both capillary and microfluidic based reactors. Several of these reactors have been characterized for the ability to generate both suspended droplets in oil and also wall spanning slugs and plugs in either air or oil, respectively. The devices that were developed are extremely robust in the hands of undergraduate students, which greatly facilitates their characterization and application to chemical systems. For example, one capillary based Tee device, was utilized by an undergraduate student for an entire semester without fouling. The device itself was capable of generating wall spanning slugs with a periodicity ranging from 0.1 Ð 10.5 Hz, with reproducibilities ranging from 1.5 to 0.3%RSD, respectively. The volume range for the corresponding segmented flows ranged from 0.5- 50 nL. In addition to a simple Tee junction chemical reagents were mixed and segmented within the nanoliter droplets. These confined nanoliter droplets served as stirred nanoliter reaction vessels as the droplets were hydrodynamically pumped across the capillary. The reaction time of the droplets were tuned by changing either the carrier oil or gas phase, and the length of the reaction capillary. The combination of tuning the reaction time via either method provided a means of accurately controlling the reaction time of each plug with reproducibilities < 2.5% for reaction times up to 30 min. The ability to control both reaction time and hence the residence time distribution greatly improved the monodispersity of colloids synthesized using such methods.

We have also developed an optimized recipie for the synthesis of silica microspheres that range in diameter from 1.2 Ð 500.9 nm (1.1-1.3% RSD, respectively) by varying reaction times between 1-30 min. respecitively. The recipie that was used for this synthesis involved a solution of 13.0 M H2O and 1.0 M NH3 in ethanol which was loaded in one syringe and a solution of 0.1 M TEOS in ethanol was loaded in the second and they were injected into the microreactor at identical flow rates, mimicking the 1:1 ratio of reagent solutions used in batch synthesis. The resulting particles that were synthesized using the microreactor had a greater monodispersity by a factor of 2 fold as compared to the batch reactor. The results which indicated that the reduced residence time distribution was smaller in the microreactor largely contributed to the narrow size distributions, as compared with the batch reactions.