Structural studies of electroactive transition metal oxide films
thesisposted on 15.12.2014, 10:40 by Nicola Rita St. Jude. Farley
This thesis is concerned with measuring the short-range structural changes that are induced when electrochromic transition metal oxide films are electrochemically oxidised and reduced. Three systems are studied: electrochemically-deposited nickel hydroxide, sol-gel-derived niobium oxide and oxide films anodically-grown on tungsten. Structural changes are measured using extended X-ray absorption fine structure (EXAFS), a technique that is sensitive to atomic structure up to about 5A, providing interatomic distances, and the number and identity of neighbouring atoms. Standard and quick transmission, energy dispersive, fluorescence and reflection modes are utilised. Additionally, the crystal impedance technique is used to study rheological changes during the sol-gel conversion of niobium oxide, and the X-ray reflectivity of the tungsten oxide films is measured at several stages of growth.;For the nickel hydroxide system, a distorted first Ni-O shell is observed in the anodically charged electrode, which reverts to a regular octahedral form as charge density decreases. An antagonistic movement is observed in the first Ni-Ni shell. For the niobium oxide system, the structures of both uncharged and cathodically charged electrodes are determined. The initially distorted first Nb-O shell of the uncharged Nb2O5 film is replaced by a regular octahedral Nb-O shell when charged. Again, a reversed effect is displayed by the first Nb-Nb shell. Also, the rheological development taking place during the sol-gel transformation is characterised. For the tungsten oxide films, a separation of W-O and W-W shells initially occurs during formation, reaching a maximum of 1.3A, then diminishes with a further increase of electrode charge density. Shell separation coincides with cation insertion during the formation of HWO3; the subsequent shell convergence results from cation expulsion when WO3 is being formed.