Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces: I. Fe on TiO2(1 1 0)

The electronic states of the Fe overlayers on TiO₂(1 1 0) surfaces have been investigated using normal-emission and resonant photoelectron spectroscopy with synchrotron radiation. It was found that Fe grows in a Stranski-Krastanov mode. At low coverages, Fe deposition on TiO₂(1 1 0) is supposed to create surface Ti³⁺(3d¹) ions leading to the same in-gap emission as that is produced by surface oxygen vacancies of TiO₂. At high coverages, Fe-induced in-gap emission is evolved into a bulk Fe spectrum. However, at the beginning, a Fermi edge is not observed, indicating that the small Fe clusters of non-metallic nature are formed. A sharp Fermi edge is formed at higher coverages, indicating that the cluster becomes metallic as the size increases.     

Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces II. Cr on TiO2(0 0 1)

The electronic states of the Cr overlayers on TiO₂(0 0 1) surfaces have been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. At lower coverages, Cr deposition on TiO₂(0 0 1) creates two well separated in-gap emissions due to the formation of surface Ti³⁺ (3d¹) ions and Cr³⁺ (3d³) ions. At higher coverages, the in-gap emission is developed into the 2-peak-structure emission of Cr 3d character. The corresponding state is considered to be of metallic nature from the viewpoint of the high ability of oxygen adsorption, but has no Fermi edge, indicating a possibility of forming small Cr clusters on TiO₂(0 0 1) at this stage.     

Quantitative determination of the local structure of H2O on TiO2(1 1 0) using scanned-energy mode photoelectron diffraction

O 1s scanned-energy mode photoelectron diffraction has been used to determine the local structure of molecular water on TiO₂(1 1 0). The adsorption site is found to be atop five-fold coordinated surface Ti atoms, confirming the results of published total energy calculations and STM imaging. The Ti-O_w bondlength is found to be 2.21 ± 0.02 Å, much longer than Ti-O bondlengths in bulk TiO₂ and for the formate (HCOO-) species adsorbed on this surface. This is consistent with relatively weak bonding, and in general agreement with total energy calculations, although all of the published calculations yield bondlengths somewhat longer than the experimental value. Structural optimisation based on the photoelectron diffraction data also provides some information on the associated substrate relaxation. In particular, the bondlength of the five-fold coordinated surface Ti atom to the O atom directly below shows the same contraction (relative to the bulk) as is found for the clean surface, reinforcing the picture of rather weak bonding of the water to this same Ti surface atom.     

TiO2-rich reconstructions of SrTiO3(0 0 1): a theoretical study of structural patterns

We have recently reported structure solutions for the (2 × 1) and c(4 × 2) reconstructions of SrTiO₃(0 0 1) based on high-resolution electron microscopy, direct methods analysis of diffraction data and density functional theory. Both reconstructions were found to be TiO₂-rich and feature a single overlayer of TiO₂ stoichiometry on top of a bulk-like TiO₂ layer. Qualitatively, the two reconstruction geometries differ in the cation sub-lattice of the overlayer only, where Ti atoms occupy half of the fivefold cation sites. In the present work we use density functional theory to generate a number of variations of this structural motif in search of patterns of stability. We find a reliable predictor for the reconstruction energy in the ability of oxygen atoms to relax vertically out of the overlayer plane to minimize non-bonded oxygen-oxygen repulsions. Out-of-plane relaxation of oxygen atoms in turn is modulated by the number and relative position of coordinating Ti atoms, which yields simple empirical rules as to how cations are distributed in low energy reconstructions.     

Structural, electronic properties and stability of the (1 × 1) PbTiO3 (1 1 1) polar surfaces by first-principles calculations

Under GGA, the structural, electronic properties and stabilities of four different (1 × 1) terminations of cubic PbTiO₃ (1 1 1) surface, the directly cleaved (1 1 1)-Ti and (1 1 1)-PbO₃ terminations and the constructed (1 1 1)-TiO and (1 1 1)-PbO₂ ones, have been systematically studied by using projector-augmented wave method implemented in VASP. For (1 1 1)-Ti and (1 1 1)-PbO₃ terminations, Ti-O bonds between the outermost two layers are enhanced after relaxation, while those between the second and the third layers are weakened. In addition, a contraction of O-O distance in surface PbO₃ layer is also found for (1 1 1)-PbO₃ termination. Moreover, electronic structures of both (1 1 1)-Ti and (1 1 1)-PbO₂ terminations are significantly influenced by structure relaxations, and the effects of the surface on the DOS are dominantly on the Ti layers, especially the CB. For a constructed (1 1 1)-TiO termination, the relaxation results show both Ti-O bonds between the outermost two layers and those between the second PbO₃ layer and the third Ti layers are enhanced. For a constructed (1 1 1)-PbO₂ termination, Ti-O bonds between the outermost two layers are also enhanced as in the (1 1 1)-TiO termination, however, inequivalent Ti-O bonds between the second layer Ti atom and the third layer O atoms are found, with two bonds expanding and the other one contracting. Results of electronic structure calculations show these two constructed terminations are all insulating and changes of DOS originate dominantly from modifications of surface compositions. Furthermore, it is found that for all four different (1 1 1) terminations, the movements of the cation and/or anion on the outermost layer along the surface normal direction after relaxation all result in a reduction of the space electric field. In O and Pb external environments, it is predicted that (1 1 1)-PbO₂ termination is the most stable one in O- and Pb-rich environments, however, the (1 1 1)-Ti termination is stable one in O- and Pb-poor conditions. The (1 1 1)-TiO termination also shows a stability domain in moderate O and Pb environments.     

Defects at the TiO2(1 0 0) surface probed by resonant photoelectron diffraction

We report photoelectron diffraction (PED) experiments of weakly sub-stoichiometric TiO₂(1 0 0) rutile surfaces. Apart from standard core-level PED from the Ti-2p3/2 line, we have studied valence band PED from the defect induced Ti-3d states in the insulating band gap. For maximum yield, the latter were resonantly excited at the Ti-2p absorption edge. The PED patterns have been analyzed within the forward scattering approximation as well as by comparison with simulated PED patterns obtained in multiple scattering calculations. The analysis shows that the defect induced Ti-3d charge is mainly located on the second layer Ti atoms.     

First principle study of SrTiO3 (0 0 1) surface and adsorption of NO on SrTiO3 (0 0 1)

The results of first-principles calculations about the two possible terminations of (0 0 1) surface of SrTiO₃ perovskite and adsorption of NO on SrTiO₃ (0 0 1) surface were presented. Both surface parameters (atomic structures and electronic configurations) and adsorption parameters (bond, energy and charge) of NO on SrTiO₃ (0 0 1) surface, which have never been investigated before as far as we know were investigated using density functional theory calculations with the local-density approximation (DFT-LDA). It was found that the two possible terminations of SrTiO₃ (0 0 1) surface have large surface relaxation, which leads to surface polarization and exhibits different reactivity toward the dissociative adsorption of NO. The electronic states of TiO₂-terminated surface have larger difference than that of bulk, so it is more favorable for adsorption of NO than SrO-terminated surface.     

Theoretical study of Ti and Fe surface alloys on Al(0 0 1) substrate

Accurate density-functional calculations are performed to investigate the formation of Ti and Fe ultrathin alloys on Al(0 0 1) surface. It is demonstrated that a deposition of Ti monolayer on Al(0 0 1) substrate leads to the formation of Al₃Ti surface alloy with Ti atoms arranged according to the L1₂ stacking, distinct from the D0₂₂ structure characteristic of a bulk Al₃Ti compound. A quest for the reason of this distinct atomic arrangement led us to the study of the surface structure of Al₃Ti(0 0 1) compound. It is concluded that even the Al₃Ti(0 0 1) surface is terminated with three layers assuming a L1₂ stacking and hence this stacking fault can be classified as a surface-induced stacking fault. Several possibilities of Fe atoms distributed in the surface region of Al(0 0 1) have been examined. The most stable configuration is the one with the compact Fe monolayer on Al(0 0 1) and covered by one Al monolayer. Lastly, our calculations show that there is no barrier for the penetration of Fe adatoms below the Al(0 0 1) surface; however, such a barrier is present for a Ti-alloyed Al(0 0 1) surface.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

Driving force for the WO3(0 0 1) surface relaxation

The optimized structure of the WO₃(0 0 1) surface with various types of termination ((1 × 1)O, (1 × 1)WO₂, and c(2 × 2)O) has been simulated using density functional theory with the Perdew-Wang 91 gradient corrected exchange-correlation functional. While the energy of bulk WO₃ depends weakly on the distortions and tilting of the WO₆ octahedra, relaxation of the (0 0 1) surface results in a significant decrease of surface energy (from 10.2 × 10⁻² eV/Ų for the cubic ReO₃-like, c(2 × 2)O-terminated surface to 2.2 × 10⁻² eV/Ų for the relaxed surface). This feature illustrates a potential role of surface relaxation in formation of crystalline nano-size clusters of WO₃. The surface relaxation is accompanied by a dramatic redistribution of the density of states near the Fermi level, in particular a transformation of surface electronic states. This redistribution is responsible for the decrease of electronic energy and therefore is suggested to be the driving force for surface relaxation of the WO₃(0 0 1) surface and, presumably, similar surfaces of other transition metal oxides.     

A DFT + U description of oxygen vacancies at the TiO2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO₂ rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti³⁺ sites. We consider the electronic structure and geometry of the oxygen deficient TiO₂ rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the +U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (?4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (?4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.     

First-principles study of CaTiO3 oxygen-vacancies (0 0 1) surface

The structural and electronic properties of fully relaxed CaTiO₃ oxygen-vacancies (0 0 1) surface with CaO and TiO₂ terminations are investigated by first-principles plane waves ultrasoft pseudopotential method based on local density approximation. The present results show that the direction and the magnitudes of the atomic relaxations for oxygen-vacancies surface are different from that of the perfect surface. Compared with the TiO₂-terminated oxygen-vacancy surface, the CaO-terminated oxygen-vacancy surface are likely to be observed in oxygen environment conditions. Much different from the perfect surface, the oxygen-vacancy surface becomes metallic caused by some states in the conduction band are lowered and pulled down in the band-gap region.     

A surface X-ray diffraction study of TiO2(1 1 0)(3 × 1)-S

Surface X-ray diffraction has been used to investigate the structure of TiO₂(1 1 0)(3 × 1)-S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)-S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO2(1 1 0) substrates

Growth modes and electronic properties were analyzed for Au nano-particles grown on stoichiometric and reduced TiO₂(1 1 0) substrates by medium energy ion scattering (MEIS) and photoelectron spectroscopy(PES) using synchrotron-radiation-light. Initially, two-dimensional islands (2D) with a height of one and two atomic layers grow and higher coverage increases the islands height to form three-dimensional (3D) islands for the stoichiometric TiO₂(1 1 0) substrate. In contrast, 3D islands start to grow from initial stage with a small Au coverage (?0.1 ML, 1 ML = 1.39 × 10¹⁵  atoms/cm²: Au(1 1 1)) probably due to O-vacancies acting as a nucleation site. Above 0.7 ML, all the islands become 3D ones taking a shape of a partial sphere and the Au clusters change to metal for both substrates. We observed the Au 4f and Ti 3p core level shifts together with the valence band spectra. The Ti 3p peak for the O-deficient surface shifts to higher binding energy by 0.25 ± 0.05 eV compared to that for the stoichiometric surface, indicating downward band bending by an electron charge transfer from an O-vacancy induced surface state band to n-type TiO₂ substrate. Higher binding energy shifts of Au 4f peaks observed for both substrates reveal an electron charge transfer from Au to TiO₂ substrates. The work functions of Au nano-particles supported on the stoichiometric and reduced TiO₂ substrates were also determined as a function of Au coverage and explained clearly by the above surface and interface dipoles.     

Surface energy of M2AC(0 0 0 1) determined by density functional theory (M = Ti, V, Cr; A = Al, Ga, Ge)

We have studied the correlation between the valence electron configuration and the electronic structure of M₂AC(0 0 0 1) surfaces (M = Ti, V, Cr; A = Al, Ga, Ge) by density functional theory. The A surface termination is the most stable configuration for all systems studied according to our surface energy data. As the M valence electron population is increased, the surface energy increases by 22% and 12% for A = Al and Ga, respectively, while it decreases by 29% for A = Ge. This can be understood by evaluating the valence electron concentration induced changes in the surface density of states. Antibonding surface Md-Ap states are present as Ti is substituted by Cr in M₂AC(0 0 0 1) for A = Al and Ga, while antibonding surface Md-Ap states are not present as Ti is substituted by Cr in M₂GeC(0 0 0 1).     

First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

Surface properties of rutile TiO2(1 1 0) from molecular dynamics and lattice dynamics at 300 K: Variable-charge model results

Geometric structure, atomic vibrations and atomic charges and their thermally induced fluctuations have been calculated as a function of depth in, and thickness of, rutile TiO₂(1 1 0) slabs, within the framework of the variable-charge potential of Swamy and Gale [V. Swamy, J.D. Gale, Phys. Rev. B 62 (2000) 5406] at 300 K. Molecular dynamics simulations and lattice dynamics calculations were performed with a 2D periodic slab model for slab thicknesses between 3 and 11 triple layers (approximately 9-35 Å). Odd-even oscillations with respect to the number of slab layers are found for the surface relaxation for very thin slabs, and for the (slowly converging) rumpling in the middle of the slab. The Ti and O atomic charges in the outermost three atomic layers differ from the rest of the slab (they are less ionic); the thermal vibrations do not alter this picture. The atomic mean-square amplitudes are some 50% larger (more for O, less for Ti) at the surface than in the middle of the slab and decay rather slowly to the bulk values. Comparisons with the results of a rigid-ion potential for titania [M. Matsui, M. Akaogi, Mol. Simul. 6 (1991) 238] are presented for non-electronic properties.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

Interdiffusion of adsorbed Pb and Bi on Cu(1 0 0)

The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(1 0 0) has been studied, at T = 513 K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consist of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features lead to the conclusion that Pb atoms are more strongly bound at surface alloyed sites in Cu(1 0 0) than Bi atoms. Once the total coverage (Pb + Bi) on the substrate reaches about one monolayer, Pb and Bi are dealloyed from the substrate, and the interdiffusion profiles become essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of ∼10⁻¹³ m²/s. This is considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion is different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction, leading to the conclusion that binding of Bi atoms to the Cu(1 0 0) substrate is stronger than that of Pb atoms in the highest-coverage surface dealloyed layers.