Shaping Silica Rods by Tuning Hydrolysis and Condensation of Silica Precursors.

Fabian Hagemans,Johannes D Meeldijk,Ravi Kumar Pujala,Dominique M E Thies-Weesie,Arnout Imhof,Danisha S Hotie,D A Matthijs De Winter,Alfons Van Blaaderen

doi:10.1021/acs.chemmater.8b04607

Fabian Hagemans, Johannes D Meeldijk + Show 6 more

Open Access

https://doi.org/10.1021/acs.chemmater.8b04607

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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.

Highlights

Colloids are promising building blocks in materials synthesis because of the possibility to control their size, anisotropy, and surface properties.[2−4] Recent developments to exploit the particle shape have led to several interesting applications in areas ranging from biology to materials science
Investigations of such anisotropic colloids resulted in the development of novel multifunctional and advanced materials.[5−10] Anisotropic colloidal particles have attracted a lot of attention because of their widespread applications in emulsion stabilization, optical displays, imaging, drug delivery, and active materials.[11−14] Recently, great improvements have been made in the development of novel synthetic techniques to introduce anisotropy in colloids with the goal of attaining various novel complex morphologies.[15−22] The anisotropy can be geometrical, chemical, or both
We have shown that the shape of colloidal silica rods is highly dependent on the growth conditions

Summary

■ INTRODUCTION

Colloids are promising building blocks in materials synthesis because of the possibility to control their size, anisotropy, and surface properties.[2−4] Recent developments to exploit the particle shape have led to several interesting applications in areas ranging from biology to materials science. At concentrations of 75% (v/v) (1.0 mmol) of added pre-hydrolyzed TEOS (together with 1.28 mmol water and 5.7 μmol HCl), the droplet on most particles got completely encapsulated, and rod growth stopped as a result (Figure 6d). The sequence of events, partial coverage of the droplet, thin segment formation, and return to regular rod growth are comparable to the case of hydrolyzed TEOS addition at the start of a synthesis (Figure 4). When 0.18 mmol of pre-hydrolyzed TEOS is added, the rate of silica deposition from the oil phase directly onto the water interface will increase This encapsulates part of the droplets, so that the resulting rods have an initial hollow sphere at the tip of the silica rods and a matchstick-like shape. The smaller droplet, together with an increased silica growth and changed viscoelastic properties of the droplet, causes the rod growth to be unstable and is likely to be the origin of the change in the growth direction

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES