Mineral phase transformation is a common process in materials synthesis as well as in natural environmental settings. Besides being ubiquitous in soils and sediments, iron oxides and oxyhydroxides are widespread in technological applications such as energy devices, catalysis, coatings, pigments, and sorbents. Understanding the transformation mechanisms of such phases is critical to designing novel materials and for predicting impacts of iron cycling in nature. To probe atomic-scale pathways that underlie phase transformations and to quantify the kinetic parameters controlling rates, we investigated transformation of β-ferric oxyhydroxide (β-FeOOH) nanowires, a common iron oxyhydroxide in chloride-containing environments, to anyhydrous iron oxide polymorphs via in situ temperature-controlled transmission electron microscopy (TEM). We observed that the crystal structure of β-FeOOH first undergoes disordering upon heating to 200 oC, and then the morphology evolved from solid nanowires to highly nanoporous nanowires composed of maghemite, the grain size of which is inversely related to density of the pores. Furthermore, we found that phase outcomes strongly depend on the initial size of the β-FeOOH nanowires. For diameters smaller than 20 nm, the final product is maghemite; transformation of maghemite to hematite was not observed even upon heating to 500 oC for 12 h, which only eliminated the nanoporosity. In this case the final morphology of maghemite is solid nanowires, and high resolution TEM and STEM indicated these were oriented single crystals. For initial β-FeOOH nanowires with diameters larger than 20 nm, the final product depends on the heating temperature, time, and pressure. The transformation of maghemite to hematite appeared to be by direct topotactic atomic rearrangement in the solid state. Imaging was complemented by in situ XRD, XPS and TGA-mass spectrometry measurements. In situ XRD showed amorphization of the β-FeOOH nanowires quickly upon approaching 200 oC and then maghemite was observed, consistent with the in situ TEM observation. However, the recalcitrance of maghemite conversion to hematite when starting from smaller β-FeOOH nanowires did not manifest in the XRD measurements upon increasing the heating temperature to 500 oC. In this case ex situ TEM indicated the smaller β-FeOOH nanowires merged into larger particles. Both in situ XPS and TGA-MS measurements showed the loss of water and HCl upon heating.

Investigating solid transformation of β-FeOOH to iron oxides by in situ transmission electron microscopy

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.