Microfluidic Synthesis of Titania Shells on Colloidal Silica

Abstract

Core-shell colloidal materials with tailored structural, electronic, photonic and chemical properties have a wide range of applications including coatings, pigments, electronics, catalysis, separations and diagnostics.[1] Typical particle cores are polymeric (e.g., polystyrene), [2] inorganic [3] (e.g., silica) or metallic [4] (e.g., gold) in size ranges between 2 nm to 10 lm. Titania-silica core-shell particles in the sub-micron size range are of particular interest for several applications, including catalysis, [5] pigments (as whiteners) [6] and imaging materials. [7] Microfluidic synthesis schemes, when combined with segmented flow for fast mixing and well defined residence time distribution RTD), produce cores with precisely controlled and narrow size distribution. [8][9][10][11][12] Here we extend this approach to a multi-step addition microfluidic system for growth of well defined shell coatings without complications of secondary nucleation. Photolithography-based microfabrication enables microfluidic designs that provide uniform addition through periodically spaced side inlets along the length of synthesis channel. This procedure is functionally equivalent to slow dropwise addition in conventional batch synthesis with the added advantages of control and continuous processing inherent to microfluidic synthesis. We specifically demonstrate coating colloidal silica core particles with titania layers of tunable thickness in a one-step, continuous flow process through controlled hydrolysis of titanium tetraethoxide (TEOT). Several methods have been reported for titania-coated-silica synthesis, but many of these batch processes suffer from difficulties in controlling overcoat thickness, avoiding secondary nucleation and aggregation, and maintaining a narrow particle size distribution. Silica particles were coated with titania layers of varying thicknesses by aging titanyl sulfate (TiOSO 4 ) in the presence of silica particles. [6,13,14] Thick, irregular layers were obtained via a multi-step coating process that involved repeated filtration and redispersion cycles. Titanium n-butoxide hydrolysis in tetrahydrofuran yielded monolayer coatings on silica particles.[5] Thicker coatings (< 7 nm) were obtained by the hydrolysis of titanium n-butoxide in ethanol. [15,16] Titanium tetraethoxide (TEOT) was also used as a precursor to obtain thick (> 50 nm) coatings. [17,18] Other methods to synthesize titania-silica particles include the use of polyelectrolyte-coated silica spheres as templates for sol-gel reactions, [19] and the deposition of alternating coatings of cationic polyelectrolytes and anionic titania nanosheets on the surface of silica.[20]The titanium alkoxide precursors used in sol-gel coating are highly sensitive to the presence of water, readily hydrolyzing and condensing, [21] which makes it difficult to control thickness and uniformity of the coating process as well as to avoid nucleation of secondary titania particles and aggregation of cores. In order to circumvent these complications, low alkoxide and...

Liquid–liquid two-phase flow and mass transfer characteristics in packed microchannels

“…Matsuyama et al (2007) demonstrated experimentally that the two miscible liquids slug droplets could be formed by introducing the immiscible carrier phase, and showed this slug-based microfluidics could offer rapid mixing and the reactant arrangements by internal circulation in slug. Reactions in droplets in microfluidic channels have also attracted many researchers due to the elimination of Taylor dispersion by internal circulation, such as the measurement of fast reaction kinetics parameters , protein crystallization (Zheng et al, 2004) synthesis of nanoparticles (Khan and Jensen, 2007), etc. However, only under very low flow velocity or extremely large volumetric flux ratio of immiscible liquid-liquid two phases, the droplets flow can be obtained in microchannels.…”

Ideal micromixing performance in packed microchannels

“…The mixing of multiple reagents was isolated in the liquid slugs by introducing the immiscible carrier phase in the winding microchannels, and the chaotic advection appeared in slugs by internal circulations, which could stretch and fold the fluids striation, eventually the intensification of the mixing in droplets could be obtained (Bringer et al, 2004). Reactions in slugs in microchannels have attracted many researchers due to the elimination of Taylor dispersion and the enhanced mixing performance by internal circulation, such as measurement of fast reaction kinetics parameters (Song and Ismagilov, 2003), protein crystallization (Zheng et al, 2004), synthesis of nanoparticles (Khan and Jensen, 2007), etc. Nevertheless, the multiphase flow approach involve the separation of the gas-liquid two phases or the liquid-liquid two phases after the outlet of the microchannel.…”

Nanoparticle synthesis in microreactors

“…The first one is passive mixing, as described above, which depends on the complexity of the channel structures. The other is active mixing, which utilizes segmentation to improve mixing (Khan and Jensen, 2007). Either liquid-liquid or gas-liquid segmented flow reactor can be used for this purpose Gunther and Jensen, 2006).…”