J.L. de Segovia

The interaction of Pt with TiO2(110) surfaces: a comparative XPS, UPS, ISS, and ESD study

The interaction of platinum with (110) single crystal surfaces of titanium dioxide (TiO2) has been studied by means of X-ray and ultraviolet photoemission spectroscopies (XPS and UPS), ion backscattering spectroscopy (ISS), Auger electron spectroscopy (AES), and electron-stimulated desorption (ESD) for Pt coverages in the sub- and monolayer range. The latter have been evaporated thermally under ultra-high vacuum conditions. We found that Pt overlayers are thermodynamically stable at room temperature with respect to the formation of Ti suboxides. At submonolayer coverages, Pt atoms adsorb preferentially on top of five-fold coordinated Ti surface atoms which act as nucleation sites for the subsequent three-dimensional growth of Pt clusters. Metallic Pt clusters are formed at the surface at elevated temperatures. UPS difference spectra show additional Pt-derived electronic states below the Fermi energy. Chemical bonds between Pt and TiO2 surface atoms are not formed under these conditions. On pre-reduced TiO2(110) surfaces, in contrast, a localized electronic charge transfer is found between Ti3+ states and Pt surface atoms.

High temperature corrosion protection of austenitic AISI 304 stainless steel by Si, Mo and Ce ion implantation

Abstract The influence of implanted silicon, molybdenum and cerium on the oxidation behaviour of a 18Cr8Ni stainless steel was studied at 1173 K up to 144 h in air under isothermal conditions in order to verify the enhanced selective oxidation of chromium by these elements. The implanted surface and the corrosion products formed were characterized by means of AES, SEM, EDS and XRD. Implanted depth profiles were calculated by TRIM96 computational code. The implanted silicon and cerium markedly improved protection against oxidation of the alloy by enhanced chromium transport while molybdenum gave rise to an accelerated oxidation due to the formation of volatile MoO 3 species. Further, ion implantation was found to play a beneficial effect against decarburization of the alloy.

The structure of TiO2(110) (1 × 1) and (1 × 2) surfaces with acetic acid adsorption — a PES study

Abstract The adsorption of acetic acid on the TiO 2 (110) surface has been studied with PES, using synchrotron radiation ∼47 eV, both at the stoichiometric (1 × 1) surface and the (1 × 2)-reconstructed surface. The results indicate a similar bonding configuration for each surface, and that the saturation acetate coverage at room temperature on the (1 × 2)-reconstructed surface is approximately half that of the (1 × 1) surface. The results are interpreted in favour of the added Ti 2 O 3 -row model of the (1 × 2) reconstruction at the clean surface.

Adsorption and thermal desorption of H2O on TiO2(001) at 250 K

Abstract In the present work a study of the thermal desorption of water adsorbed on a nearly perfect TiO 2 (001) surface at 250 K is presented. Prior to the desorption experiments, the H 2 O layer is characterized by ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES). Desorbed masses were identified using a quadrupole mass spectrometer (QMS). From an analysis of the UPS spectra, evidence is given for H 2 O dissociation into OH and O Ti H hydroxyl radicals bonded to the Ti atoms, with O Ti being one of the TiO 2 oxygen atoms. The thermal desorption spectra reveals water as the only desorbed species.

Study of the interaction of SO2 with TiO2 (110) surface

Abstract The adsorption of SO2 on TiO2(110) surfaces at 100 K has been studied by means of the X-ray photoemission and ultra-violet photoemission spectroscopy, and photon stimulated desorption techniques using Synchrotron radiation. For a saturation dose at 100 K, the SO2 is molecularly adsorbed and a chemisorbed interface is formed by SO42− and SO32− species on the surface. Annealing of the adsorbed layer to room temperature leads to the formation of sulfur derived species.

Electron stimulated desorption of ions from surfaces: techniques, methodology and some recent findings with water at metals and semiconductors☆

Abstract A review is presented of the basic principles, theory and instrumentation underlying the observation of ions with the technique of electron stimulated desorption (ESD) and the results of some recent investigations with water adsorbed at metal and semiconductor surfaces are employed to illustrate the state of research in the field. The evolution of theoretical models of desorption from the pioneering work of Redhead, Menzel and Gomer to the present state of theory with considerations of hole-hole interactions are summarised, with particular emphasis on the physical actions involved in the desorption processes. Methods of study of desorbed ions using mass spectrometers, retarding field analysers and time-of-flight techniques are examined, as well as methods for determining the angular distribution of desorbed ions. The response with ESD at the interface formed with water is particularly striking in view of the variety and the range of desorption efficiency of the ions produced. Experimental factors contributing to the possible enhancement of ion production are examined and the results of measurements of desorption parameters are discussed and compared with the predictions of theory.

Reactivity of CO on a TiO2(110) defective surface studied by electron stimulated desorption

CO adsorption on a defective TiO2(110) surface at 150 K incorporating about 1.7 × 1014cm−2 vacancies produced by Ar+ sputtering has been studied by electron stimulated desorption, ESD, and with Auger electron spectroscopy, AES, as a complementary technique. When the surface was exposed to CO up to 5 × 105L the AES spectra revealed a small C signal. From the O(KLL)Ti(LMM) ratio increase a CO coverage of 0.06 ± 0.01 ML was determined. In spite of this small amount of CO the ESD technique showed dramatic changes in ion yield and energy. The characteristic dominant peak at 4.0 eV and the small structure at about 7.0 eV of the ion kinetic energy distribution curve of the clean defective surface changed to a triple peak structure with maxima at 3.4, 4.3 and 7.4 eV. The first is identified with the most probable kinetic energy of O+ ions ejected from non-dissociated CO molecules. The 4.3 and 7.4 eV maxima correspond to the most probable kinetic energy of O+ ions desorbed from O in-plane and from the bridging O-ligand of the TiO2(110) surface, respectively. The growth of the 7.4 eV peak upon adsorption is a consequence of the filling of the bridging O-ligand initial vacancies by dissociated O from the CO molecule. The ion yield curves show, in addition to the onset at 35 eV electron energy of the clean surface, a threshold at about 290 eV electron energy corresponding to excitation of the C 1s core level. The total ion yield also shows a decrease upon CO adsorption. The O+ ion yield curves are found to be related to the secondary electron yield. The chemisorption of CO is discussed in terms of a model where the CO is molecularly adsorbed on the Ti five-fold coordination sites with a part being dissociated and leading to O adsorbed on bridging O-ligand vacancies.

Anomalous mass numbers in quadrupole mass spectrometers (QMS) at very low pressures

Abstract In this work, the results of mass spectrometric studies of the residual vacuum in a uhv all metal system are presented. H + , O + and F + ions arising from surface effects are found. The abundance of F + is extremely high, as compared with previous work. Surface spectra on a tungsten sample are also presented for comparison, as well as changes in the gases in the volume and surface ions on introducing H 2 O into the system. Surface ions from the W sample show the high dissociation of H 2 O (H + and O + ions), but no OH + ions are found. Thermal treatment of the W surface shows that F + has two desorption states. Ion energy distribution maxima for surface species from the tungsten sample are 4.4 eV for H + , 4.5 eV for F + and 10.2 eV for O + .

Adsorption of ammonia on TiO2(001) at room temperature

Abstract The ammonia adsorption on the TiO 2 (001) surface has been studied using ultraviolet photoelectron spectroscopy (UPS), UV He 1 lamp, 21.2 eV, and thermal desorption spectrometry (TDS). The layer at 300 K is formed by molecularly adsorbed ammonia. characterised by UPS structures at 11.8 and 7.4 eV. Electron damage of this initial layer as well as adsorption at a temperature of 340 K reveal that the surface species are NH 2 and OH, the O being the lattice oxygen of TiO 2 . NH 3 is the main desorbed product in thermal desorption experiments at T = 338 K, with first order kinetics and 80 kJ/mol desorption energy. Some ammonia is dissociated as NH 2 and OH and after a slight temperature increase these products are completely dissociated as N and H, yielding by recombination N 2 and H 2 m the gas phase, their maximum desorption rates being at temperatures of 343 and 364 K, respectively.

Surface mechanical effects of nitrogen ion implantation on vanadium alloys

Abstract This paper describes the modifications introduced by nitrogen implantation on V5 at.% Ti alloys at high energy. Two sets of samples were treated with three successive atomic and molecular nitrogen implantations, in order to obtain flat profiles of nitrogen concentration, namely 20 and 40%. On implanted samples microhardness tests with a final load of 2 mN showed a hardness increase by a factor of 2. Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to identify the concentrations and chemical states of the elements present in the high dose implanted V5Ti surface. The obtained results indicate a correlation between the hardness increase and the nitrogen implantation. From the spectroscopic results we can distinguish three different regions: an amorphous carbon layer up to 10 nm from the surface, followed by an interface of mixed amorphous carbon and carbide (VC–TiC) of approximately 20 nm, in which nitrogen is in the form of nitride. The third region from 30 nm is characterized by a decrease in the oxygen concentration of 40%, together with slight nitrides increase and stable carbides.