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Title: The surface chemistry and bulk electronic structure of bismuth based pyrochlore oxides
Author: Walker, Robert
ISNI:       0000 0004 6347 1475
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Metal oxides of bismuth show applications from photocatalysts to dielectric materials often due to the influence of the so called “lone pair” electrons that give rise to structural distortions and modified electronic structure. These effects upon which fundamental properties are derived are still not yet fully understood. Polycrystalline Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 materials - the latter in both pyrochlore (p) and monoclinic (m) phases - were prepared via co-precipitation and, except for Bi 2 Hf 2 O 7 (m), thin films. The bulk, surface, and electronic properties were characterised using a combination of X-ray diffraction and spectroscopic techniques. The structural determination, completed in Chapter 4, confirmed the formation of a directional lone pair by O 2p-assisted Bi 6s-6p hybridisation in all except Bi 2 Zr 2 O 7 . X-ray photoelectron spectroscopy quantified a Bi surface excess that in Chapter 5 was probed further by low-energy ion spectroscopy achieving greater surface sensitivity. The top atomic layer was determined to be almost exclusively BiO x with surface relaxation of the directional lone pair suggested as the driving force behind this surface reconstruction. By in-situ diffuse reflectance infrared Fourier-transformed spectroscopy, CO 2 was found to strongly chemisorb onto the surface of Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 (p) indicating a basic surface. Both the surface reconstruction and surface basicity were not observed in Y 2 Ti 2 O 7 demonstrating the clear influence of the Bi cation. In Chapter 6, the electronic structure of the films was investigated by XPS and polarisation-dependent HAXPES and indicated a strong Bi contribution to the valence band either by O 2p-assisted Bi 6s-6p hybridisation or, as in Bi 2 Zr 2 O 7 , anti-bonding interactions between O 2p and Bi 6s. The Bi 6s was shown to shift the valence band towards the Fermi edge, in addition, a valence band with a high metal character is beneficial for the formation of holes and mobility while Bi 6p contributions to the conduction band improve the stability of excited electrons from the valence band with high O 2p character.
Supervisor: Payne, David ; Ryan, Mary Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral