Insights from in situ and environmental TEM on the oriented attachment of α-Fe2O3 nanoparticles during α-Fe2O3 nanorod formation

Abstract

Acicular α-Fe₂O₃ nanorods (NRs), at an intermediate stage of development, were isolated using a snapshot valve-assisted hydrothermal synthesis (HS) technique, for the purpose of complementary in situ transmission electron microscopy (iTEM) and environmental TEM (ETEM) investigations of the effect of local environment on the oriented attachment (OA) of α-Fe₂O₃ nanoparticles (NPs) during α-Fe₂O₃ NR growth. Observations of static snapshot HS samples suggested that α-Fe₂O₃ NPs undergo reorientation following initial attachment, consistent with an intermediate OA stage, prior to ‘envelopment’ with the developing NR to adopt a perfect single crystal. Conversely, the heating of partially developed α-Fe₂O₃ NRs up to 250 °C, under vacuum, during iTEM, demonstrated the progressive coalescence of loosely packed α-Fe₂O₃ NPs and the coarsening of α-Fe₂O₃ NRs, without any direct evidence for an intermediate OA stage. Direct evidence was obtained for the action of an OA mechanism prior to the consumption of α-Fe₂O₃ NPs at the tips of developing α-Fe₂O₃ NRs during ETEM investigation, under an He pressure of 5 mbar at 500 °C. However, α-Fe₂O₃ NPs more strongly attached to the side-walls of developing α-Fe₂O₃ NRs were more likely to be consumed through a local NP destabilisation and reordering process, in the absence of an OA mechanism. Hence, the emerging ETEM evidence suggests a competition between OA and diffusion processes at the α-Fe₂O₃ NP coalescence stage of acicular α-Fe₂O₃ NR crystal development, depending on whether the localised growth conditions facilitate freedom of NP movement.