Environmental transmission electron microscopy (ETEM) has proven to be a powerful method for studying nano-scaled changes in catalyst materials during exposure to reactive gasses and elevated temperatures [1-3]. In this work, we use ETEM to observe the structural dynamics of electrospun nanofibers during calcination, i.e. while elevating the temperature in the presence of O₂. The ETEM movies show nanofiber shrinkage due to polymer oxidation followed by formation, coarsening and sintering of oxide nano-crystals in the fiber. The results show that a nanofiber can be directly transformed to a nanotube or to a nano-pearl string by controlling the calcination temperature.

Electrospinning is a powerful, efficient and cost-effective method to produce one-dimensional (1D) materials [4]. Combined with sol-gel chemistry and calcination, electrospinning has been used to produce a variety of 1D metal oxide nanomaterials that bring remarkable performance enhancement in many energy devices. (e.g. lithium ion batteries, solid oxide fuel cells). The perovskite lanthanum strontium cobalt oxide (LSC), which has been intensively studied for electrochemical devices, is synthesized in this work.

A Titan 80-300 (FEI) transmission electron microscope (TEM) equipped with at differential pumping system allowing for exposure to the specimen of a few mbar of a reactive gas was used. TEM images were recorded while exposing the fibres to 2 mbar of O₂ and ramping the temperature from room temperature to 900 °C by 5 °C/min. Thermal analysis was performed at the same oxygen partial pressure balanced in N₂ and same ramping rate to link structural dynamics with polymer oxidation and decomposition of the precursers.

[1] S.B. Simonsen, I. Chorkendorff, S. Dahl, M. Skoglundh, S. Helveg, “Coarsening of Pd nanoparticles in an oxidizing atmosphere studied by in situ TEM”, Surf. Sci. Available online, doi:10.1016/j.susc.2015.11.003

[2] S.B. Simonsen, K. Agersted, K.V. Hansen, T. Jacobsen, J.B. Wagner, T.W. Hansen, L.T. Kuhn, “Environmental TEM study of the dynamic nanoscaled morphology of NiO/YSZ during reduction”, Appl. Catal. A:gen 489 (2015) 147

[3] S.B. Simonsen, S. Dahl, E. Johnson, S. Helveg, “Direct Observations of Catalytic Soot Oxidation on a Nano-Scale using Environmental Transmission Electron Microscopy”, SAE Int. J. Mater. Manuf. 1 (2009) 199

[4] W. Zhang, P.N. Pintauro. “High Performance Nanofiber Fuel Cell Electrodes”. ChemSusChem, 4 (2011) 1753