Electronic properties and surface chemistry of vanadium oxide systems: New insights from synchrotron X-ray techniques

Abstract

Vanadium oxides are considered exemplary materials within both fundamental condensed matter physics and heterogeneous catalysis research. The metal-insulator transition (MIT) inVO2 has been at the centre of attempts to understand the role of electron correlations in transition metal oxides since its discovery more than 50 years ago, and it has been investigated extensive lyas a prototype in the development of Mott insulator transistors, switches and memory devices, which may allow further miniaturisation of computer chips. Meanwhile, vanadium oxide based materials with oxidation state V3+ - V5+ catalyse reactions which produce many of the important organic molecules most chemists take for granted. The work presented in this thesis examines VO2 films on oxide substrates with these two perspectives in mind, using complementary synchrotron X-ray spectroscopy techniques to investigate their chemistry inside and out. With an aim to expand the limited range of experimental density of states measurements on doped VO2, V L2,3 and O K edge X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) studies were performed on VO2/Al2O3 films, doped with varied levels of Cr or Nb which respectively increased and decreased MIT temperature. XAS was carried out at two different X-ray incidence angles and at temperatures above and below the MIT temperature for each sample. XES was compared for pure, Cr and Nb doped samples above and below the MIT temperature. The results suggest that Cr-doping stabilises VO2 in an insulating phase and alters its structural symmetry, which is consistent with formation of the M2 phase. Changes induced by Nb were more subtle and may be related to an increased lattice parameter and reduced octahedral distortion without a change in symmetry. Cr and Nb doped samples showed opposite trends in the intensity of t2g states relative to eg states, which suggests the degree of hybridisation between O 2p orbitals and V 3d orbitals of different symmetry plays a key role in the influence of both Cr and Nb doping on the MIT in VO2. These studies provide empirical data against which theoretical models can be compared. This work has also revealed variation between samples and measurements as a result of other factors such as orientation, surface phases and sample degradation. To fill a deficit of in situ studies looking at the first layer of adsorbed molecules on simple vanadium oxide surfaces, O 1s/V 2p, C 1s/K 2p and valence band X-ray photoelectron spectroscopy studies were performed on VO2 films with single-crystal metal oxide substrates in the presence of water and methanol vapour. In a series of isotherm and isobar experiments on VO2 films with a variety of different preparation methods, contamination levels, thicknesses and orientations, molecular water adsorption generally followed a consistent pattern: hydroxide coverage exceeded molecular water coverage at the minimum relative humidity, a molecular water adsorption onset occurred at 0.01-0.1 % relative humidity, and coverage of both species was higher in the experiments where relative humidity is decreased. These findings are consistent with the general trends seen on oxide surfaces and support previous studies pointing at hydrogen bonding between surface species as an important contributor to water adsorption behaviour. Methanol isotherm and isobar experiments on VO2/TiO2 (100) films were consistent with previous studies which have shown that dissociative adsorption to methoxy and hydroxy is a key reaction on VOx surfaces. Observation of an adsorption onset at a consistent temperature, rather than relative pressure indicated that, in contrast to water adsorption, temperature-dependent surface reactions play a role in methanol adsorption. For both water and methanol adsorption, differences in reactivity between films could be related to the availability of reduced surface sites. Various levels of potassium contamination were observed on the films in these experiments. Results suggested it plays an important role in determining the vanadium oxidation state, with clear consequences for the reactivity of the films that should be considered for catalytic systems, and electronic devices. Together, the investigations of bulk and surface properties presented in this thesis generally indicate that VO2 films are quite sensitive to changes in the both the bulk composition and the surrounding environment. Contamination and surface reactions may be widely overlooked sources of variability between studies on the MIT in VO2, which could contribute to the lack of consensus over the exact sequence of electronic and structural changes that occurs as the transition proceeds. Dopants such as Cr or K could affect the reactivity and reducibility of vanadium oxide surfaces, with consequences both in catalytic applications and for the lifetime of electronic devices. More broadly, observations in these studies highlight issues which are relevant to a wide range of materials science studies, including sample stability, experimental reproducibility, and the possible widespread effects of contaminants like potassium on results.