Thermal oxidation of Ru(0001) to RuO2(110) studied with ambient pressure x-ray photoelectron spectroscopy

The thermal oxidation of Ru(0001) has been extensively studied in the surface science community to determine the oxidation pathway towards ruthenium dioxide (RuO2(110)), improving the knowledge of Ru(0001) surface chemistry. Using time-lapsed ambient-pressure x-ray photoelectron spectroscopy (APXPS), we investigate the thermal oxidation of single-crystalline Ru(0001) films toward rutile RuO2(110) in situ. APXPS spectra were continuously collected while the Ru(0001) films were exposed to a fixed O2 partial pressure of 10−2 mbar and the sample temperature was increased stepwise from room temperature to 400 °C. We initially observe the removal of adventitious carbon and subsequent formation of a chemisorbed oxygen overlayer at 250 °C. Further annealing to 300 °C leads to an increase in thickness of the oxide layer and a shift in the Ru–O component of the Ru 3d spectra, indicating the presence of a metastable O–Ru–O trilayer structure. A rapid formation of the RuO2 rutile phase with an approximate thickness of at least 2.6 nm is formed about four minutes after stabilizing the temperature at 350 °C and subsequent annealing to 400 °C, signaled by a distinct binding energy shift in both the Ru 3d and O 1s spectra, as well as quantitative analysis of XPS intensities. This observed autocatalytic oxidation process agrees well with previous theoretical models and experimental studies, and the data provide the unambiguous spectral identification of one proposed metastable precursor required for full oxidation to rutile RuO2(110). Further ex situ characterization of the grown oxide with x-ray photoelectron diffraction confirms the presence of three rotated domains of rutile RuO2(110) and reveals their orientation relative to the substrate lattice

Water Adsorption at the Tetrahedral Titania Surface Layer of SrTiO3_3(110)-(4×\times1)

The interaction of water with oxide surfaces is of great interest for both fundamental science and applications. We present a combined theoretical [density functional theory (DFT)] and experimental [Scanning Tunneling Microscopy (STM), photoemission spectroscopy (PES)] study of water interaction with the two-dimensional titania overlayer that terminates the SrTiO$_3$(110)-(4$\times$1) surface and consists of TiO$_4$ tetrahedra. STM, core-level and valence band PES show that H$_2$O neither adsorbs nor dissociates on the stoichiometric surface at room temperature, while it dissociates at oxygen vacancies. This is in agreement with DFT calculations, which show that the energy barriers for water dissociation on the stoichiometric and reduced surfaces are 1.7 and 0.9 eV, respectively. We propose that water weakly adsorbs on two-dimensional, tetrahedrally coordinated overlayers

Surface electronic structure of Ni-doped Fe3O4(001)

Magnetite (Fe3O4) doped with earth-abundant metals has emerged as a promising catalyst material, with Ni-doped magnetite (Ni/Fe3O4) being a cost-effective, durable, and highly active material for photocatalytic and electrochemical water oxidation. While previous studies have investigated the incorporation of Ni atoms into Fe3O4 single-crystalline surfaces using surface science characterization methods and density functional theory calculations, an experimental study is still required to understand the impact of Ni incorporation on the electronic structure of Ni/Fe3O4 systems. To address this, we employed angle-resolved photoemission spectroscopy, analyzed within the one-step model of photoemission by a real-space multiple scattering code to investigate the electronic structure of the reconstructed magnetite surface. Moreover, the half-metal to semiconductor phase transition upon Ni incorporation is reflected in an almost complete disappearance of states near the Fermi level. Finally, we report on the systematic changes in the unoccupied states observed with the increasing amount of Ni dopant. These findings offer insights into the influence of Ni incorporation on the electronic structure of Ni/Fe3O4, which can link to an increased catalytic activity

Dynamic Equilibrium at the HCOOH-Saturated TiO2_{2}(110)-Water Interface

Carboxylic acids bind to titanium dioxide (TiO$_{2}$) dissociatively, forming surface superstructures that give rise to a (2 × 1) pattern detected by low-energy electron diffraction. Exposing this system to water, however, leads to a loss of the highly ordered surface structure. The formate-covered surface was investigated by a combination of diffraction and spectroscopy techniques, together with static and dynamic ab initio simulations, with the conclusion that a dynamic equilibrium exists between adsorbed formic acid and water molecules. This equilibrium process is an important factor for obtaining a better understanding of controlling the self-cleaning properties of TiO$_{2}$, because the formic acid monolayer is responsible for the amphiphilic character of the surface