Microfluidic synthesis of colloidal nanomaterials

Abstract

This thesis focuses on microfluidics based approaches for synthesis and surface-engineering of colloidal particles. Bottom-up assembly through colloidal nucleation and growth is a popular route to the controlled synthesis of nanomaterials. Standard bench-scale synthetic chemistry techniques often involve non-uniform spatial and temporal distributions of concentration and temperature, and are not readily scalable. Photolithography-based microfabrication enables the application of classical techniques of chemical reaction engineering to design chemical reactors that cannot be realized easily at the macroscale, and that closely approach theoretical 'idealized' reactor configurations. In addition, the microfluidic format allows precisely controlled reaction conditions such as rapid mixing, and concentration and temperature uniformity. The goal of this thesis was to design microfluidic reactors for synthesis of core-shell colloidal particles with tunable sizes. Microscale segmented gas-liquid flows overcome the large axial dispersion effects associated with single-phase laminar flows. Microchannel devices that yielded uniform, stable gas-liquid segmented flows over three orders of magnitude in flow velocity were first developed.

(cont.) Extensive experimental studies of the transport, dynamics and stability of such flows were then conducted with pulsed-laser fluorescent microscopy, optical stereomicroscopy and micro particle image velocimetry (-PIV). Flow segmentation not only reduces axial dispersion, but also allows rapid micromixing of miscible liquids through internal recirculations in the liquid phase. This added functionality is especially useful in syntheses involving colloidal particles that, due to inherently low diffusivity, cannot be rapidly mixed by laminar diffusive techniques. Continuous segmented flow reactors were then developed for the synthesis of colloidal silica and titania particles by sol-gel chemistry. Particle sizes could be tuned by varying the rates of flow of reactants, or by varying the chip temperature. Particle size distributions comparable to or narrower than the corresponding stirred-flask synthesis, with little agglomeration or shape distortion were obtained. Coating of colloidal particles with one or more layers of different materials is used to modify their optical, chemical or surface properties. Core-shell particles are often prepared by controlled precipitation of inorganic precursors onto core particles.

(cont.) Synthesis of such structures requires precise control over process parameters to prevent precipitation of secondary particles of shell material and agglomeration of primary particles. Particles coated with titania are exceptionally difficult to synthesize due to the high reactivity of the titania precursors, which makes controlled precipitation difficult. A novel continuous flow microfluidic reactor with sequential multi-point precursor addition was developed for colloidal overcoating processes. Silica particles were coated with uniform titania layers of tunable thickness by the controlled hydrolysis of titanium ethoxide, with no secondary particle formation or agglomeration. An integrated reactor for continuous silica synthesis and in-situ series overcoating with titania was then developed using a two-level stacked reactor fabrication process. Finally, multi-step nanomaterials synthesis and surface coating with incompatible chemistries requires the development of microfluidic 'unit operations' equivalent to particle filtration. In this context, rapid, continuous microfluidic particle separation was demonstrated using transverse free-flow electrophoresis.