The local electronic properties of individual Pt atoms adsorbed on TiO2(110) studied by Kelvin probe force microscopy and first-principles simulations.

Noble metal nanostructures dispersed on metal oxide surfaces have applications in diverse areas such as catalysis, chemical sensing, and energy harvesting. Their reactivity, chemical selectivity, stability, and light absorption properties are controlled by the interactions at the metal/oxide interface. Single-atom metal adsorbates on the rutile TiO2(110)-(1 × 1) surface have become a paradigmatic model to characterize those interactions and to understand the unique electronic properties of these supported nanostructures. We combine Kelvin probe force microscopy (KPFM) experiments and density functional theory (DFT) calculations to investigate the atomic-scale variations in the contact potential difference of individual Pt atoms adsorbed on a hydroxylated (h) TiO2(110)-(1 × 1) surface. Our experiments show a significant drop in the local contact potential difference (LCPD) over Pt atoms with respect to the TiO2 surface, supporting the presence of an electron transfer from the Pt adsorbates to the substrate. We have identified two characteristic regimes by LCPD spectroscopy. At far tip-sample distances, LCPD values show a weak distance dependence and can be attributed to the intrinsic charge transfer from Pt to the oxide support. Beyond the onset of short-range chemical interactions, LCPD values exhibit a strong distance dependence that we ascribe to the local structural and charge rearrangements induced by the tip-sample interaction. These findings also apply to other electropositive adsorbates such as potassium and the hydrogen atoms forming the OH groups that are present on the h-TiO2(110) surface, promoting KPFM as a suitable tool for the understanding of electron transfer in catalytically active materials.

Graphene Tunable Transparency to Tunneling Electrons: A Direct Tool To Measure the Local Coupling.

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems.

Hydrogen activation, diffusion, and clustering on CeO2(111): a DFT+U study.

We present a comprehensive density functional theory+U study of the mechanisms underlying the dissociation of molecular hydrogen, and diffusion and clustering of the resulting atomic species on the CeO2(111) surface. Contrary to a widely held view based solely on a previous theoretical prediction, our results show conclusively that H2 dissociation is an activated process with a large energy barrier ~1.0 eV that is not significantly affected by coverage or the presence of surface oxygen vacancies. The reaction proceeds through a local energy minimum--where the molecule is located close to one of the surface oxygen atoms and the H-H bond has been substantially weaken by the interaction with the substrate--, and a transition state where one H atom is attached to a surface O atom and the other H atom sits on-top of a Ce(4+) ion. In addition, we have explored how several factors, including H coverage, the location of Ce(3+) ions as well as the U value, may affect the chemisorption energy and the relative stability of isolated OH groups versus pair and trimer structures. The trimer stability at low H coverages and the larger upward relaxation of the surface O atoms within the OH groups are consistent with the assignment of the frequent experimental observation by non-contact atomic force and scanning tunneling microscopies of bright protrusions on three neighboring surface O atoms to a triple OH group. The diffusion path of isolated H atoms on the surface goes through the adsorption on-top of an oxygen in the third atomic layer with a large energy barrier of ~1.8 eV. Overall, the large energy barriers for both, molecular dissociation and atomic diffusion, are consistent with the high activity and selectivity found recently in the partial hydrogenation of acetylene catalyzed by ceria at high H2/C2H2 ratios.

Role of surface reconstructions in (111) silicon fracture.

The (111) cleavage in crystalline silicon was investigated by hybrid quantum/classical atomistic simulations showing that its remarkable stability is largely due to asymmetric π-bonded reconstructions of the cleavage surfaces created by the advancing crack front. Further simulations show that the same reconstructions can induce an asymmetric dynamical response to added shear stress components. This explains why [211] upward steps are much more common than [211] downward steps on (111) cleavage surfaces, while "zigzag" cleavage with alternated (111) and (111) facets will still occur in crystal samples fractured under [110] uniaxial loading.

Theoretical study on hydrogen-bond effects in IR spectra of high- and low-temperature phases of nitric acid dihydrate.

The low- and high-temperature phases (alpha and beta, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H(3)O(+)) x (H(2)O), designated A and B, with different hydrogen-bonding (HB) characteristics. The theoretical study reveals that type A complexes are weakly bound and could be described as (H(3)O)(+) and H(2)O aggregates, with decoupled vibrational modes, whereas in type B structures the proton is situated close to the centre of the O...O bond and induces strong vibrational coupling. The proton-transfer mode is predicted at quite different wavenumbers in each complex, which provides an important differentiating spectral feature, together with splitting of some bands in beta-NAD. Theoretical spectra are estimated by using two GGA parameterizations, namely, PBE and BLYP. The potential-energy surface for each type of HB in NAD is also studied, as is the spectral influence of displacement of the shared H atom along the O-O bond. The results are compared to literature infrared spectra recorded by different techniques, namely, transmission and reflection-absorption, with both normal and tilted incident radiation. This work provides a thorough assignment of the observed spectra, and predictions for some spectra not yet available. The usefulness of high-level theoretical calculations as performed herein to discriminate between two phases of a solid crystal is thus evidenced.

A computational study of the adsorption of small Ag and Au nanoclusters on graphite.

The adsorption of silver and gold atoms, and M2, M6, and M13 (M=Ag or Au) clusters on the (0001) graphite surface has been investigated computationally using the density functional theory (DFT) with periodic boundary conditions and plane wave basis functions. The surface has been modeled as a single carbon sheet. The role of dispersion forces has been studied with an empirical classical model. The results show that the clusters avoid hollow sites on the graphite surface, and that the metal atoms favor atop and bond sites. Large structural changes are observed in octahedral M6 and icosahedral M13 clusters on the graphite surface when compared with gas-phase geometries. The results also indicate that if accurate results are required, the dispersion forces between metal and carbon atoms should be included in the studied systems.