Shaping Silica Rods by Tuning Hydrolysis and Condensation of Silica Precursors

Abstract

We present the synthesis of colloidal silica particles with new shapes by manipulating the growth conditions of rods that are growing from polyvinylpyrrolidone-loaded water-rich droplets containing ammonia and ethanol. The silica rods grow by ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS). The lengthwise growth of these silica rods gives us the opportunity to change the conditions at any time during the reaction. In this work, we vary the availability of hydrolyzed monomers as a function of time and study how the change in balance between the hydrolysis and condensation reactions affects a typical synthesis (as described in more detail by our group earlier1). First, we show that in a “standard” synthesis, there are two silica growth processes occurring; one in the oil phase and one in the droplet. The growth process in the water droplet causes the lengthwise growth of the rods. The growth process in the oil phase produces a thin silica layer around the rods, but also causes the nucleation of 70 nm silica spheres. During a typical rod growth, silica formation mainly takes place in the droplet. The addition of partially hydrolyzed TEOS or tetramethoxysilane (TMOS) to the growth mixture results in a change in balance between the hydrolysis and condensation reaction. As a result, the growth also starts to take place on the surface of the water droplet and thus from the oil phase, not only from inside the droplet onto a silica rod sticking out of the droplet. Carefully tuning the growth from the droplet and the growth from the oil phase allowed us to create nanospheres, hollow silica rods, hollow sphere rod systems (colloidal matchsticks), and bent silica rods.

Low‐dose liquid cell electron microscopy investigation of the complex etching mechanism of rod‐shaped silica colloids

“…[ 51 ] Finally we used the information obtained to explore the chemical composition of newly developed, even more complexly structured silica rod‐shaped particles known as crooked silica rods. Such crooked rods have been developed recently in our group [ 52 ] and that of others [ 53 ] as such particles can form interesting new colloidal liquid crystal phases. [ 54 ]…”

Seeded-Growth of Silica Rods from Silica-Coated Particles

“…Moreover, with this method, the silica rods can even be shaped by modifying conditions during growth 3,19 or via post-treatment to create even more complex particles. 20,21 Subsequent research then found that when certain colloidal particles (e.g., TiO 2 , FeO x , and MnO x ) are also present in the synthesis, the silica rods can grow from the water droplets attached to (or Pickering stabilized by) the colloidal “seed” particles, resulting in anisotropic composite silica rods. 14,15,19,22−24 This is known as “seeded-growth”, and the resulting particles are known as “matchsticks”.…”

Chemistry and interactions of silica based particles studied by liquid cell electron microscopy

“…Moreover, it has been shown that by changing the reaction conditions (e.g. tuning hydrolysis and condensation of silica precursors or changing the reaction temperature) during the growth of rods, various shapes of silica particles with inhomogeneous chemical structures can be synthesized [85,86].…”

“…As a result, the rods grew into two segments connected at an angle. It is assumed that the addition of pre-hydrolyzed TEOS initiates an interval of more rapid condensation and therefore was also seen [85] at the surface of the emulsion droplet and thereby the droplet was seen to shrink [274], and growth of the normal rod was interrupted. Most likely, the reaction temperature also enhanced the shrinking of the droplet by the increased dissolution of water in the pentanol phase and boosted the hydrolysis and condensation of the pre-TEOS.…”

“…Finally, we used the information obtained to explore the chemical composition of newly developed, even more complexly structured silica rodshaped particles known as crooked silica rods. Such crooked rods have been developed recently in our group [85] and that of others [270] as such particles can form interesting new colloid based liquid crystal phases [271].…”