Growth of Supported Gold Nanoparticles in Aqueous Phase Studied by in Situ Transmission Electron Microscopy.

Nanoparticle growth has long been a significant challenge in nanotechnology and catalysis, but the lack of knowledge on the fundamental nanoscale aspects of this process has made its understanding and prediction difficult, especially in a liquid phase. In this work, we successfully used liquid-phase transmission electron microscopy (LP-TEM) to image this process in real time at the nanometer scale, using an Au/TiO catalyst in the presence of NaCl as a case study. In situ LP-TEM clearly showed that the growth of Au nanoparticles occurred through a form of Ostwald ripening, whereby particles grew or disappeared, probably via monomer transfer, without clear correlation to particle size in contrast to predictions of classical Ostwald ripening models. In addition, the existence of a significant fraction of inert particles that neither grew nor shrank was observed. Furthermore, in situ transmission electron microscopy (TEM) showed that particle shrinkage was sudden and seemed a stochastic process, while particle growth by monomer attachment was slow and likely the rate-determining step for sintering in this system. Identification and understanding of these individual nanoparticle events are critical for extending the accuracy and predictive power of Ostwald ripening models for nanomaterials.