Microwave technology has emerged as one of the useful methods for nanoparticle synthesis. Despite many studies on this area, its underlying mechanism has not been clearly understood. In this study, in-situ observation of nanoparticle growth was carried out and the profiles and behaviors of produced bubbles and particles under microwave effect were reported. Sizes of bubble and nanoparticle particle were measured during and after microwave irradiation using a DLS apparatus and behavior of particle growth and superheating effect were observed. From the experimental data, it is apparent that the maximum bubble sizes were greatly influenced by the irradiation power and solute concentration. Particle number density, which is related to the initial solute concentration, is also an important factor for the bubble size produced during the irradiation. Finally, through in-situ observation of superheating effect, the behavior was frequently caused by the irradiation at higher power. To prevent superheating effect, influencing factors such as the irradiation power and number density should be controlled to ensure a stable operation of particle formation process.

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Microwave is advantageous for nanoparticle synthesis because nanostructured crystals and mono-dispersed nanoparticle size can be obtained due to the rapid growth induced by quick thermal response. In previous studies, it has been confirmed that an individual application of two-stage irradiation and anti-solvent addition is effective in promoting stable nucleation and smaller particle generation. Nevertheless, to date no studies have been reported in applying the two methods above simultaneously and examining their synergistic effect. This study, therefore, aims to investigate the possibility of producing finer particles using a stable operation without any superheating and abrupt bubble growth. From the experimental data, it is evident that the beneficial combined effect of two-stage irradiation and addition of anti-solvent on the stable finer nanoparticle formation could be observed and confirmed. It is expected that the findings and recommendations from this work may be useful in optimizing two-stage microwave-assisted nanoparticle synthesis with an addition of anti-solvent.

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