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The interface between an oxide substrate and a metal overlayer may crucially influence the macroscopic behavior of technological devices such as sensors, catalysts, ceramics, and semiconductor chips. Hence investigations of adsorption, nucleation and growth of ultrathin metal films on metal oxide surfaces have attracted increasing interest during recent years. Experimental results on the growth of metal overlayers on a model oxide, TiO 2 (110), are reviewed. The emphasis is on the very initial stages of overlayer growth and on extracting general trends on metal/metal-oxide interaction by comparing results from different metal overlayers. The electronic and geometric structure, growth mode, interface formation, and thermal stability of metal films are discussed. The strength of the oxidation/reduction reaction at the interface, the wetting ability and the tendency to form a disordered layer are all related to the reactivity of the overlayer metal towards oxygen. We propose that ‘reactive adsorption’ accounts for the observed trends.

A study of ultrathin Cr films on TiO2(110) surfaces is reported. The combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) enables us to study quantitatively the growth of reactive metal films on metal oxides, and the chemical interactions at the interface. Ultrathin Cr films grow initially in a quasi-two-dimensional fashion at room temperature. Changes in oxidation states of both Ti and Cr during film formation suggest that strong chemical interactions at the interface are of great importance in the growth of reactive metal films. Cr “wets” the surface more effectively at 300 K than ultrathin films of less reactive metals, Fe and Cu. This suggests that a relation exists between the metal reactivity with oxygen and the wetting of metal films on oxides: The more reactive the metal towards oxygen, the better is the wetting ability. The chemical interaction is accompanied by charge transfer at the interface, causing a reduction of the surface work function. The interfacial interaction also causes the interfacial Cr layer to behave differently from the top metallic Cr atoms upon thermal annealing. The top layers of metallic Cr show a strong clustering tendency at 500°C, while the interfacial Cr seems to have less surface mobility and slow bulk diffusion.

As part of a systematic study of metals on oxides, we report here a study of the growth of Fe films on TiO2(110) surfaces using low‐energy ion scattering and x‐ray photoelectron spectroscopy (XPS). Our studies reveal that Fe grows initially in an island growth mode, and completely covers the TiO2 substrate for thicknesses of a few monolayers. XPS shows evidence for a strong interaction between Fe films and the oxide substrate, i.e., oxidation of Fe overlayers and reduction of the oxide substrate, at submonolayer thickness. Comparing the wetting ability of Fe with that of Cu, a less reactive metal, and with that of Cr, a more reactive metal, we observe a relation between the wetting ability of the metal films and the reactivity of metal with oxygen. Complete coverage of the TiO2 substrate can be obtained for a 1 monolayer Fe film following oxidation of the film. Studies of Fe film growth on sputtered TiO2 surfaces indicate that surface roughness also plays an important role in the morphology of the metal f...

A structural study of ultrathin Cr, Fe and Cu metal films on stoichiometric and Ar+ sputtered TiO2(110) surfaces has been carried out using low energy electron diffraction (LEED), angle-resolved XPS (ARXPS) and two-dimensional medium energy backscattered electron diffraction (MEED). Although LEED results indicate only weak long range order for all three metal films (Cr, Fe and Cu) on TiO2(110), clear evidence for a locally ordered structure can be observed using ARXPS and MEED. Both Cr and Fe films grow with bcc(100) structures on stoichiometric TiO2(110) yielding pronounced forward scattering features in ARXPS and MEED data. Cu overlayers grow with fcc(111) structures exhibiting two equivalent domain orientations on TiO2(110) that yield less pronounced features in ARXPS. However, MEED measurements of Cu films with different electron surface incidence angles show clear fcc(111) patterns for different domains. Both bcc(100) and fcc(111) MEED patterns are simulated by single scattering cluster (SSC) calculations, and the results are qualitatively consistent with experimental data. A bcc(100) short range ordering is also observed for ultrathin Cr and Fe films on long range disordered Ar+ sputtered TiO2(110) surfaces.

We have studied the growth of ultrathin (< 50 A) Cu overlayers on stoichiometric TiO2(110) surfaces. Using low energy ion scattering, the growth mode of Cu was determined to be Volmer-Weber type (three-dimensional islands) at room temperature; evidence for cluster growth is observed even at 160 K. A comparison with Fe and Cr metal films shows that the spreading of the films correlates with the heat of formation of the metal oxide of the overlayer. Fe and Cr cause a reduction of the Ti4+ surface cations at the metal/metal oxide interface as observed with X-ray photoelectron spectroscopy. No reduction of the TiO2 cations takes place upon contact with a Cu overlayer.

We have studied the stoichiometric (annealed in oxygen), the slightly oxygen‐deficient (annealed in vacuum), and the highly defective (sputtered with Ar+) TiO2(110) surfaces and their reactivities to molecular oxygen, molecular water, and 50‐eV hydrogen ions using x‐ray photoelectron spectroscopy (XPS) and low‐energy ion scattering spectroscopy (LEIS). The use of isotopically labeled 18O enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by 18O2 adsorption. LEIS (1‐keV He+) is used to analyze the chemical composition of the outermost surface layer before and after 18O2 and H218O exposure. Water adsorbs on both stoichiometric and slightly O‐deficient surfaces [with oxygen vacancies ∼0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of ∼0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the...

Medium-energy backscattered electron diffraction (MEED) studies are reported for the rutile TiO 2 (110) surface. Two-dimensional diffraction images are obtained using a retarding field analyzer with a microchannel plate-intensified detector. Background removal is accomplished using digital image processing techniques. We find that the backscattered pattern for 750 eV incident electrons is predominantly due to diffraction of the outgoing electrons (subsequent to backscattering from subsurface atoms) by overlying lattice atoms. We also find effects related to the orientation of the incident beam relative to the surface. These effects are ascribed to diffraction of the incoming electrons (prior to backscattering) and possibly the focusing of electron intensity on second-layer atoms. Single-scattering cluster (SSC) calculations are used to model the outgoing diffraction process. The results are qualitatively consistent with a bulk-like termination of the TiO 2 (110) surface.