TiO2-supported Au144 nanoclusters for enhanced sonocatalytic performance.

The production of reactive oxygen species (ROS), such as hydroxyl radicals, by ultrasonic activation of semiconductor nanoparticles (NPs), including TiO2, has excellent potential for use in sonodynamic therapy and for the sonocatalytic degradation of pollutants. However, TiO2 NPs have limitations including low yields of generated ROS that result from fast electron-hole recombination. In this study, we first investigated the sonocatalytic activity of TiO2-supported Au nanoclusters (NCs) (Au NCs/TiO2) by monitoring the production of hydroxyl radicals (•OH) under ultrasonication conditions. The deposition of Au144 NCs on TiO2 NPs was found to enhance sonocatalytic activity for •OH production by approximately a factor of 2. Electron-hole recombination in ultrasonically excited TiO2 NPs is suppressed by Au144 NCs acting as an electron trap; this charge separation resulted in enhanced •OH production. In contrast, the deposition of Au25 NCs on TiO2 NPs resulted in lower sonocatalytic activity due to less charge separation, which highlights the effectiveness of combining Au144 NCs with TiO2 NPs for enhancing sonocatalytic activity. The sonocatalytic action that forms electron-hole pairs on the Au144/TiO2 catalyst is due to both heat and sonoluminescence from the implosive collapse of cavitation bubbles. Consequently, the ultrasonically excited Au144 (3 wt. %)/TiO2 catalyst exhibited higher catalytic activity for the production of •OH because of less light shadowing effect, in contrast to the lower catalytic activity when irradiated with only external light.

Effect of ultrasonic frequency and surfactant addition on microcapsule destruction.

In a previous study, we found that cavitation bubbles cause the ultrasonic destruction of microcapsules containing oil in a shell made of melamine resin. The cavitation bubbles can be smaller or larger than the resonance size; smaller bubbles cause Rayleigh contraction, whereas larger bubbles are not involved in the sonochemical reaction. The activity in and around the bubble (e.g., shear stress, shock wave, microjet, sonochemical reaction, and sonoluminescence) varies substantially depending on the bubble size. In this study, we investigated the mechanism of the ultrasonic destruction of microcapsules by examining the correlations between frequency and microcapsule destruction rate and between microcapsule size and cavitation bubble size. We evaluated the bubbles using multibubble sonoluminescence and the bubble size was changed by adding a surfactant to the microcapsule suspension. The microcapsule destruction was frequency dependent. The main cause of microcapsule destruction was identified as mechanical resonance, although the relationship between bubble size and microcapsule size suggested that bubbles smaller than or equal to the microcapsule size may also destroy microcapsules by applying shear stress locally.