In Situ Observation of Hematite Nanoparticle Aggregates Using Liquid Cell Transmission Electron Microscopy

Juan Liu, Zhiwei Wang, Anxu Sheng, Feng Liu, Fuyu Qin, and Zhong Lin Wang, 2016

Image courtesy of Environmental Science & Technology

Abstract

Aggregation of nanoparticles impacts their reactivity, stability, transport, and fate in aqueous environments, but limited methods are available to characterize structural features and movement of aggregates in liquid. Here, liquid cell transmission electron microscopy (LCTEM) was utilized to directly observe the size, morphology, and motion of aggregates that were composed of 9 and 36 nm hematite nanoparticles, respectively, in water or NaCl solution. When mass concentrations were same, the aggregates of 9 nm nanoparticles were statistically more compact and slightly larger than those of 36 nm nanoparticles. Aggregates in both samples were typically nonspherical. Increasing ionic strength resulted in larger aggregates, and also enhanced the stability of aggregates under electron-beam irradiation. In water, small aggregates moved randomly and approached repeatedly to large aggregates before final attachment. In NaCl solution, small aggregates moved directly toward large aggregates and attached to the latter quickly. This observation provided a direct confirmation of the DLVO theory that the energy barrier to aggregation is higher in water than in salt solutions. This study not only presented the influences of particle size and ionic strength on aggregation state, but also demonstrated that LCTEM is a promising method to link aggregation state to dynamic processes of nanoparticles.

Impact Statement

The aggregation dynamics and behavior or hematite nanoparticles was studied using liquid cell transmission electron microscopy. The impact of nanoparticle size and ionic strength of the liquid as a function of aggregation was quantified. Salt solutions, such as NaCl were found to drive aggregation as the ionic strength was increased. and resulted in more stable aggregates over time.
Keywords: Nanoparticles; Interactions