Growth mechanism and electrochemical properties of Transition metal oxide nanostructures Mn3O4 and α-Fe2O3

Abstract

This thesis describes the growth and properties of epitaxially grown manganese oxide (Mn304) and iron oxide (Fe203) nanocrystals. A 2-step phase separation approach has been employed to achieve nanostructures with controlled surface orientation. AFM, SEM and TEM have all been carried out to determine the topography, as well as interfacial relationship between substrate and sample. It provides a thorough investigation of the structural formation mechanism and how they correlate to the overall chemical and physical properties. ( 10 1) oriented M0304 is used as an example to demonstrate terrace formation on the surface of individual island as a result of impurities. Terraces spirals in both right and left handed direction have been observed, with the upward climb distance approximately equal to half of the c-axis. The effect of increasing deposition frequency was also investigated. Mn304 deposited at high frequency exhibited much larger and flatter structures. This can be explained by adatom arrangement, where at higher deposition frequency adatoms have less time to achieve 'vertical climb' hence resulting in flatter nano-islands. Having an understanding ofnanostructure formation also allows us to explore the electrochemical properties of(OOI) (101) and (I 12) orientations of Mn304. 1 0,000 cyclic voltammetry cycles have been run and repeated for each sample. Results suggest that the ( 112) orientation had the highest reactivity. Simulated atomic model of all three orientations confirms that the electrochemical response increase corresponds to higher induced dipole moment. This is because increased number of dipole moments can cause surface instability that is more prone to electrochemical reactions. The study into the interplay between surface morphology and property has been extended to iron oxide nanocrystals (a-F20 3). We found resistive switching behaviors when measured under conductive C-AFM, under applied voltage between -5V to +5V on nanocrystals between height range 25nm to 45nm. We observe hysteresis loop under reverse bias which exhibit rectifier diode behaviour, which also increases with nanocrystal height. This phenomenon is attributed to the alternation of Fe between the 2+ and 3+ valence states which is enhanced in larger (taller) nanocrystals.