Medium-high frequency sonication dominates spherical-SiO2 nanoparticle size

Abstract

Spherical SiO₂ nanoparticles (SSNs) have been inventively synthesized using the Stöber method with sonication at medium-high frequencies (80, 120, and 500 kHz), aiming to control SSN size and shorten reaction time. Compared to the conventional method, such sonication allowed the Stöber reaction complete in 20-60 min with a low molar ratio of NH₄OH/tetraethyl orthosilicate (0.84). The hydrodynamic diameters of 63-117 nm of SSNs were obtained under sonication with 80, 120, and 500 kHz of ultrasonic frequencies. Moreover, the SSNs obtained were smaller at 120 kHz than at 80 kHz in a multi-frequencies ultrasonic reactor, and the SSN size decreased with increasing ultrasonic power at 20 °C, designating the sonochemical unique character, namely, the SSN-size control is associated with the number of microbubbles originated by sonication. With another 500 kHz ultrasonic bath, the optimal system temperature for producing smaller SSNs was proven to be 20 °C. Also, the SSN size decreased with increasing ultrasonic power. The smallest SSNs (63 nm, hydrodynamic diameter by QELS, or 21 nm by FESEM) were obtained by sonication at 207 W for 20 min at 20 °C. Furthermore, the SSN size increased slightly with increasing sonication time and volume, favoring the scale-up of SSNs preparation. The mechanisms of controlling the SSN size were further discussed by the radical's role and effects of ammonia and ethanol concentration.

Keywords: Medium-high frequencies; Number and size of cavitation bubbles; Sonication; Spherical SiO(2) nanoparticles; Stöber method.

Graphical abstract

Scheme 1

Stöber preparation of monodisperse SSNs by hydrolysis and condensation of TEOS .

Fig. 1

Setups for sonication with ultrasonic generators and baths (A) 40, 80, and 120 kHz; (B) 500 kHz.

Fig. 2

Dependence of SSN hydrodynamic diameter size by QELS (A) and yield (B) on ultrasonic power at 80 and 120 kHz. Conditions: ca. 10 mL of mixture (99.8 % ethanol: 6.00 mL; 8.1 % NH₄OH: 0.40 mL; 98.0 % TEOS: 0.25 mL; H₂O: 3.00 mL) was sonicated for 20 min at 20 °C in a 3 L water bath.

Fig. 3

Dependence of SSN hydrodynamic diameter size by QELS and yield on ultrasonic power at 500 kHz. Conditions: ca. 10 mL of mixture (99.8 % ethanol: 6.00 mL; 8.1 % NH₄OH: 0.40 mL; 98.0 % TEOS: 0.25 mL; H₂O: 3.00 mL) was sonicated for 20 min at 20 °C in a 3 L water bath.

Fig. 4

Influence of reaction volume on SSN size (QELS) and yield. Conditions: various volumes of mixture (99.8 % ethanol/8.1 % NH₄OH/98.0 % TEOS/H₂O = 24.0:1.6:1.0:12.0 (volume)) were sonicated at 500 kHz, 207 W for 60 min at 20 °C in a 3 L water bath.

Fig. 5

Dependence of SSN size (QELS) and the yield on the reaction temperature. Conditions: ca. 50 mL of mixture (99.8 % ethanol: 30.00 mL; 8.1 % NH₄OH: 2.00 mL; 98.0 % TEOS: 1.25 mL; H₂O: 15.00 mL) was sonicated at 500 kHz, 207 W for 60 min in a 3 L water bath.

Fig. 6

Influence of sonication time on SSN size (QELS) and yield. Conditions: ca. 50 mL of mixture (99.8 % ethanol: 30.00 mL; 8.1 % NH₄OH: 2.00 mL; 98.0 % TEOS: 1.25 mL; H₂O: 15.00 mL) was sonicated at 500 kHz, 207 W at 20 °C in a 3 L water bath.

Fig. 7

FESEM images of the smallest SSNs obtained with sonication at (A1) 80 kHz, 63 W; (B1) 120 kHz, 78 W; and (C1) 500 kHz 207 W, and the particles size distribution of SSNs obtained with sonication at (A2) 80 kHz, 63 W; (B2) 120 kHz, 78 W; and (C2) 500 kHz, 207 W. Other conditions: ca. 10 mL of mixture (99.8 % ethanol: 6.00 mL, 8.1 % NH₄OH: 0.40 mL, 98.0 % TEOS: 0.25 mL, H₂O: 3.00 mL) was sonicated for 20 min at 20 °C in 3 L of water bath.