Monolayer Intermixed Oxide Surfaces: Fe, Ni, Cr, and V Oxides on Rutile TiO₂(011)

Abstract

The oxide/oxide interface for monolayer oxides on rutile TiO 2 (011) substrates is investigated for several transition metal (M) oxides, with M= Fe, Ni, Cr, and V. The samples are prepared using ultra-high vacuum sample preparation procedures and are characterized with a combination of photoemission spectroscopy and scanning tunneling microscopy (STM). Furthermore, stable intermixed monolayer oxides are determined by density functional theory (DFT) based simulations and compared to the experimental results. Experimentally we find that for specific oxidation conditions a monolayer intermixed oxide is formed for all M. Although different structures as well as oxides in different oxidation states can be obtained depending on the preparation conditions, one common structure has been identified for all M. This intermixed oxide appears to have a defined composition of MTi 2 O 5 . For very small amounts of M the surface segregates into a pure TiO 2 (011)-2x1 surface and into domains of the MTi 2 O 5 phase, indicating that this intermixed oxide monolayer is a low-energy line phase in a compositional surface phase diagram. The gas-phase oxygen pressure that is required to form the intermixed MTi 2 O 5 surface increases in the order of V<Cr<Fe<Ni in accordance of the enthalpy of oxide formation for these transition metals. For V, deposition and annealing in UHV results already in the formation of the intermixed oxide without the presence of any gas phase oxygen. Exposure to background oxygen, on the other hand, results in the formation of a V-oxide adlayer with higher (3+ and 4+) oxidation states. Stable MTi 2 O 5 monolayer phases are also obtained in DFT simulations and comparison of simulated STM with experiment allows us to identify the most likely structural model.

Surface Structure of TiO2 Rutile (011) Exposed to Liquid Water

“…A pseudohexagonal structure that somehow resembles the one in Figure a has been observed in ref . Such a structure was attributed to Ca impurities segregating from crystal bulk or intentionally prepared as mixed oxide monolayer by depositing metals (Fe, Cr, Ni, V) in oxidizing atmosphere . In order to address the possible presence of such impurities, we complemented the XPS measurements with LEISa method that provides an extremely high sensitivity to the composition of the topmost atomic layer.…”

“…Such a structure was attributed to Ca impurities segregating from crystal bulk 30 or intentionally prepared as mixed oxide monolayer by depositing metals (Fe, Cr, Ni, V) in oxidizing atmosphere. 31 In order to address the possible presence of such impurities, we complemented the XPS measurements with LEIS—a method that provides an extremely high sensitivity to the composition of the topmost atomic layer. On the UHV-prepared surface only the O (mass 16) and Ti (mass 48) peaks were detected (see Figure 5 ).…”

The Activation Mechanism of Bi3+ Ions to Rutile Flotation in a Strong Acidic Environment

“…The detailed scan spectrum of Ti 2p is presented in Figure 10c and composed of two components. The characteristic of peaks 5 (464.24 eV) and 4 (458.19 eV) was obviously attributed to the XPS spectrum of Ti 2p in rutile bulk [8,31,32]. Combined with the previous analysis, peak 4 (458.84 eV) and peak 7 (461.19 eV) were tentatively assigned to the Ti 4+ states in Ti-OH and Ti-O-Bi 2+ on the surface of rutile.…”

An Ordered Mixed Oxide Monolayer Formed by Iron Segregation on Rutile-TiO2(011): Structural Determination by X-ray Photoelectron Diffraction

“…Under controlled UHV preparation conditions these metal atoms intermix with the atoms in the surface of the substrate and form mixed oxide monolayers. 8 Interestingly, we observed that the transition metals selected in our study formed an universal structure on the TiO 2 (011) surface that based on DFT simulations was identified to exhibit a MTi 2 O 5 composition for all M investigated, i.e., M = Fe, V, Ni, and Cr. There are two main aspects that motivate the study of monolayer oxides on bulk oxide substrates in the field of chemical surface properties: (i) All technological materials possess some bulk impurities (sometimes bulk dopants, including iron, are added on purpose), and these impurities/ dopants may under certain thermodynamic conditions segregate to the surface.…”