Non-Stoichiometric Oxide Surfaces and Ultra-thin Films: Characterisation of TiO2

Abstract

Reducible transition metal oxides are key components in many catalytic, sensing and device systems however relatively little quantitative information exists on the nature of non-stoichiometric phases. In this study, we show how substrates can be prepared in a controlled manner with designed levels of non-stoichiometry. These in turn will facilitate quantitative studies of the interaction of non-stoichiometric oxide materials with adsorbates, reactants and supported metallic particles. We grow stoichiometric ultrathin films of TiO 2 in the rutile phase and (110) orientation on W(100) which represent our baseline material. Films are then doped with submonolayer excess Ti and annealed to create films with quantified levels of non-stoichiometry. The study relies heavily on previous scanning tunnelling microscopy work, which shows the dominant nature of Ti interstitial defects in the surface chemistry of TiO 2 , combined with detailed and quantitative X-ray photoelectron spectroscopy measurements.

Spectroscopy of ultrathin epitaxial rutile TiO2(110) films grown on W(100)

“…In this paper we detail the growth of electronic and vibrational properties of ultra-thin film TiO 2 (110) grown on W(100). The experimental data extends our previous study which concentrated on detailing the growth characterising the geometrical structure of thicker ultra-thin films (>2nm) [11,20]. That work showed by LEED (and some ex-situ STM) that the films grew epitaxially aligned in the (110) orientation, covered the surface and had step heights of 3.2Å as per rutile TiO 2 , making this system one of the best characterised TiO x films.…”

Electronic structure of point defects in controlled self-doping of theTiO2(110) surface: Combined photoemission spectroscopy and density functional theory study

“…[19][20][21] Nevertheless, systematic studies of nonstoichiometric TiO 2 have been presented in which the stoichiometry has been changed through three routes: ͑i͒ Ar ion bombardment, 22 which preferentially sputters oxygen; ͑ii͒ oxygen desorption induced by electronic excitations ͑e.g., electron beam͒ and the Knotek-Feibelman mechanism; 23 and ͑iii͒ Ti deposition on a stoichiometric surface. 24 These studies show that reduced species appear in XPS as a clear shoulder on the notionally Ti 4+ bulk signal ͑chemical core level shift of Ti 2p states͒ 25 and in UPS as the appearance of a Ti 3d derived defect state in the band gap. 26 This state is found in the band gap, 0.9 eV below the Fermi level, 12,[14][15][16][17][18]26 and is attributed to formation of reduced Ti 3+ .…”

“…However, there has generally been no independent way of quantifying the level of nonstoichiometry. In a recent work nonstoichiometric TiO 2 was investigated by deposition of submonolayer neutral Ti atoms onto the surface, 25 enabling the measured nonstoichiometry to be directly compared to the amount of Ti added. The use of TiO 2 ultrathin films epitaxially grown on W͑100͒ enabled separation of bulk and surface effects.…”

“…These studies provide evidence of the unsuitability of many popular exchange-correlation functionals for describing reduced Ti ions. [47][48][49][50][51][52][53] Although there exists little information on the submonolayer adsorption of Ti onto TiO 2 besides that from our previous XPS experiments, 25 the literature shows a number of publications in which the growth of semiconducting metal oxides has been studied with a combination of theory and experiment, e.g., VO x in Refs. 54 and 55. With this background, our approach is to use a combination of experimental data-XPS and UPS-and an appropriate computational method-DFT+ U-to explain the nature of reduced Ti species present upon self-doping of the TiO 2 ͑110͒ surface and assign the features in the UPS spectrum.…”

The solid–solid interface: Explaining the high and unique photocatalytic reactivity of TiO2-based nanocomposite materials

“…Surface defects such as oxygen vacancies can be easily created and are important for the surface chemistry and properties of TiO 2 [99,[142][143][144][145][146]. Oxygen vacancies are found to be color centers which respond to visible light irradiation [147][148][149][150].…”