The surface chemistry and bulk electronic structure of bismuth based pyrochlore oxides
File(s)
Author(s)
Walker, Robert
Type
Thesis
Abstract
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.
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.
Version
Open Access
Date Issued
2016-12
Date Awarded
2017-06
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Payne, David
Ryan, Mary
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)