Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

Summary

Silica nanoparticles were prepared using acid-catalysis of a siloxane precursor and microwave-assisted synthetic techniques resulting in the controlled growth of nanomaterials ranging from 30-250 nm in diameter. The growth dynamics can be controlled by varying the initial silicic acid concentration, time of the reaction, and temperature of reaction.

Abstract

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle diameters ranging from 30-250 nm by varying the reaction conditions. Through the selection of a microwave compatible solvent, silicic acid precursor, catalyst, and microwave irradiation time, these microwave-assisted methods were capable of overcoming the previously reported shortcomings associated with synthesis of silica nanoparticles using microwave reactors. The siloxane precursor was hydrolyzed using the acid catalyst, HCl. Acetone, a low-tan δ solvent, mediates the condensation reactions and has minimal interaction with the electromagnetic field. Condensation reactions begin when the silicic acid precursor couples with the microwave radiation, leading to silica nanoparticle sol formation. The silica nanoparticles were characterized by dynamic light scattering data and scanning electron microscopy, which show the materials' morphology and size to be dependent on the reaction conditions. Microwave-assisted reactions produce silica nanoparticles with roughened textured surfaces that are atypical for silica sols produced by Stöber's methods, which have smooth surfaces.

Introduction

Silica nanoparticles (SiO₂ NPs) were first synthesized by Stöber¹ and through modifications^2-7 have become the preferred method for SiO₂ NPs synthesis. Typically, Stöber reactions are catalyzed by alkaline conditions where silica sols are formed. Acid-catalyzed reactions are used less frequently than alkaline-catalyzed reactions due to the greater degree of difficulty of hydrolysis of the siloxane precursor. Unlike alkaline-catalyzed reactions, acid-catalyzed reactions preferentially form silica gels.⁸

Microwave-assisted chemical reactions are an emerging tec....

Protocol

1. Preparation and Calculations

1. Prepare 1 mM HCl solution using concentrated hydrochloric acid, 37%, and water.
   Note: Caution should always be used when handling concentrated acids. Concentrated acid should always be added to water, never add water to concentrated acid.
2. Determine the desired concentration of TMOS, siloxane precursor, for the microwave reaction. A TMOS concentration of 25 mM will be used for procedural demonstrations.

2. Hydrolysis of TMOS and Preparation of Reaction Solution

1. For a 25 mM reaction solution of TMOS in acetone, obtain a 50 ml plastic conical tube, T....

Results

Temperature, pressure, and microwave power traces for a representative SiO₂ NP microwave-assisted reaction are presented in Figure 1. The microwave reaction plot is divided into three sections - ramping, reaction, and cooling. A reaction temperature of 125 °C and reaction time of 60 sec are used in this representative SiO₂ NP reaction. During the ramping portion, the power is maximized at 300 W (or near max power) so that the reaction temperature can be reached quickly without over .......

Discussion

The microwave-assisted methods described in this manuscript are advantageous over conventional heating methods because SiO₂ NPs can be synthesized accurately, precisely, and quickly. The following criteria should be followed to eliminate any potential issues associated with SiO₂ NPs formation by these microwave-associated techniques: 1) use of a catalyst such as 1 mM HCl, 2) hydrolysis of the TMOS should be completed before addition of acetone, 3) use of an aprotic solvent such as acetone, 4) use of.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Tetramethylorthosilicate	Sigma Aldrich	218472
Hydrochloric acid, 37%	Sigma Aldrich	435570
Acetone	Fisher	A949SK
Sulfuric acid	EMD Millipore	SX1244
Hydrogen peroxide, 30%	EMD Millipore	HX0635
Discover microwave reactor	CEM
10 ml Borosilicate reaction vial	CEM	908035
10 ml Snap cap	CEM	909210
3 mm Stir bar	Fisher Scientific	14-513-65
Highly polished silicon wafers	Broker	SP064483
S4800 SEM	Hitachi
Zetasizer Nano90	Malvern
Polystyrene cuvette, (10 mm x 10 mm x 45mm)	Sarstedt	67.754
5415D centrifuge	Eppendorf
Hummer 6.2 sputter system	Anatech