Electronic structures of CO adsorbed Pt-skin surface on Pt–Co and Pt–Ni alloys

By using angle resolved photoemission spectroscopy, we investigate the electronic structures of Pt-skin layer of Pt–Co and Pt–Ni alloys with CO molecules on the surface. Measured Fermi surface maps and band dispersions reflect the signatures of chemical bonding between Pt-skin layer and CO molecules. Furthermore, the degree of chemical bonding strength of CO molecules, estimated from the energy shift of the participating bands, is found to be reduced on both Pt bimetallic alloys. Our results show how the surface band structure of Pt bimetallic alloys is modified with molecular orbitals of CO molecules on the surface, revealing the important role of the electronic structure in the determination of chemical properties of bimetallic alloys.     

低能Pt原子团簇沉积过程的分子动力学模拟

利用分子动力学模拟系统研究了低能Pt38,Pt141和Pt266原子团簇与Pt(001)表面的相互作用过程,详细分析了初始原子平均动能为0.1,1.0和10eV的原子团簇的沉积演化过程及其对基体表面形貌的影响.研究表明,初始原子平均动能是描述低能原子团簇的重要参量.当团簇的平均原子动能较低时,团簇对基体表层原子点阵损伤较小,基本属于沉积团簇;随着入射团簇的原子平均动能的增加,团簇对表层原子点阵结构的破坏能力增强,当团簇的原子平均动能增加到10eV时,团簇已经显现出注入特征.低能原子团簇对基体表面形貌的影响 关键词： 分子动力学模拟 低能原子团簇 载能沉积     

Atomistic simulation of Pt trimer on Pt(1 1 1) surface

The diffusion of Platinum trimer on Pt(1 1 1) is studied at different temperatures by molecular dynamics (MD) simulation. The structure stability is studied by cluster binding energy. The interaction between adatoms and surface atoms is discussed based on the calculated phonon density of state of Pt trimer. The diffusion coefficients of Pt trimer are derived from mean square displacement of cluster’s mass-center, which is obtained by long simulation times (?0.2 s) and tracing of interstitial atoms on surface. Then the diffusion prefactor and migration energy are deduced from Arrhenius relation. The calculated results are in reasonable agreement with experiment. In addition, using the diffusion prefactor and migration energy, the efficiency of Pt trimer as a critical nucleus for three-dimensional growth of thin films is discussed.     

递归法研究Pt对Ti合金腐蚀影响

采用递归法计算了Ti及Ti合金的电子态密度、环境敏感镶嵌能、费米能级和格位能等电子结构参量.计算发现Pt在晶体中环境敏感镶嵌能和格位能高于表面,从电子层面证实Pt易在 Ti合金表面偏聚.偏聚在表面的Pt有序能为正值,故Pt以有序相（Pt与Ti的化合物）形式分布在合金表面.晶体表面Pt 与Ti的化合物电极电位较低,它与Ti形成微电池.在腐蚀介质的作用下,Pt与Ti的化合物分解,Pt沉淀到晶体表面造成Pt在合金表面形成凹凸不平的Pt电催化层.Pt电催化层加强Ti钝化作用,从而提高了Ti合金的抗腐蚀能力. 关键词： 电子结构 Ti合金表面 钝化     

Defects mediated diffusion in Pt/Co/Pt multilayers induced by dense electronic excitations

Present study compares the effects of 200 MeV Ag¹⁵⁺ and 100 MeV O⁷⁺ ion irradiations on the structural, interfacial mixing and magnetic properties of annealed Pt/Co/Pt layers fabricated by DC magnetron sputtering. X-ray diffraction analysis shows that ion irradiations coupled with post annealing results in the formation of the face centred tetragonal L1₀ CoPt phase. Irradiation using 200 MeV Ag¹⁵⁺ ions having higher ionizing energy transfer to the film was found to be more efficient in causing structural phase transition as compared with that using 100 MeV energy O⁷⁺ ions having lower ionizing energy transfer at similar fluence. Rutherford back scattering analysis reveals the role of defect mediated inter-atomic diffusion in tailoring the alloy composition of the film irradiated by different energetic ions. A broad magnetic switching field distribution for O⁷⁺ ion irradiated films compared to Ag¹⁵⁺ ion irradiation was evident from the magnetic measurements. The contribution of alloy composition to switching field distribution has been discussed in details. Above results showed that the electronic energy loss and fluence dependent defects, generated by irradiation, played an important role in tuning the structural, atomic diffusion and magnetic reversal properties of Pt/Co/Pt.     

Phonon dispersion of quasi-freestanding graphene on Pt(111)

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).     

Thermal morphological evolution of platinum nano-particles in Pt–Al2O3 nano-composites

Temperature morphological evolution of nonpercolated granular nano-structures of platinum nano-particles embedded in an insulating alumina matrix was investigated by X-rays scattering in grazing angle reflection mode. In the investigated temperature range of 298–823 K, it was found that the annealing treatment tends to increase the Pt nano-particles' size and to produce a quasi-mono-disperse Pt nano-particles followed by a reduction of the barrier thickness between them. The percolation temperature is estimated to be of the order of 890 K. Using the rate constant governing the growth of the Pt nano-particles, the corresponding activation energy was determined to be about 90 kJ/mol.     

Analysis of energy and force of Pt adatom on Pt (0 0 1) surface by MAEAM

The energy and perpendicular force of a Pt adatom on Pt (0 0 1) surface have been calculated by MAEAM. With increasing the distance of the adatom from the surface, the energy and force maps can be classified into four regions: repulsive region, transformed region, strongly attractive region and weakly attractive region. In repulsive region, the maximum (minimum) values of the energy and repulsive force appear on the top (hole) of the first layer atoms of Pt (0 0 1) surface due to stronger pair-potential interaction. In other regions, the energy and force maps are more complicated than those in repulsive region due to the effects of the many body interactions and nonspherical distribution of the electrons of the atoms in crystal. The most stable position is 0.1664 nm above the hole of the first layer atoms for a Pt adatom on Pt (0 0 1) surface.     

Growth of Pt thin films on WSe2

The morphology and structure of Pt deposited on a WSe₂(0 0 0 1) van der Waals surface have been investigated by reflection high energy electron diffraction and scanning tunneling microscopy. At room temperature, the initial growth is characterized by the formation of three-dimensional fcc Pt islands with (1 1 1) orientation. In contrast, at higher temperatures of about 450 °C the formation of a novel chemically ordered Pt-Se alloy is observed. Based on the diffraction patterns, a tetragonal DO₂₂-type structure of a Pt₃Se compound is suggested. With increasing Pt thickness, this chemically ordered alloy disappears and an additional superstructure occurs, which is accompanied by the coalescence of the islands. The observed superstructure is attributed to a strong Se diffusion towards the growth surface, forming most likely a PtSe₂ alloy with the CdI₂-type layered structure on the top surface. Due to the lateral lattice mismatch between the Pt(1 1 1) layers and the PtSe₂(1 1 1) top layer, a Moiré pattern with a period of 1.1 nm is created, which might be used as a long-range atomic pattern for further nanostructure growth.     

Atomic scale characterization of the Pt/TiO2 interface

We present results from an investigation of the Pt/TiO(2) catalyst system using a combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Evidence of a strong interaction between the Pt particles and the support is found to be dependent on the Pt cluster size, being manifested either as an encapsulation of the Pt particles by the support or a distortion of the structure of the Pt particles. In the case of clusters that are only a few atoms in size, we show direct evidence of an epitaxial nucleation relationship between Pt and Titania. The results also show unexpectedly that Pt particles exhibit a preferential nucleation on rutile rather than anatase.     

Fabrication of electrode Pt nanotubes for semiconductor capacitors

Template-wetting process was used to fabricate Pt electrode nanotubes for the further application in 3D nanotube capacitors. Anodic alumina oxide (AAO) was used as a template, which can be fabricated by means of two-step anodization process. After the wetting process, released Pt nanotubes have been obtained by selective etching of the template using KOH solution. Pt nanotubes formation and tube walls morphology were investigated by FE-SEM technique. Furnace annealing (FA) effects at temperatures ranging from 200 to 600 °C with 100^oC step on nucleation and crystal growth condition of Pt nanotubes have been examined. Crystallization of the Pt nanotube inside the template has been examined by step-by-step annealing at different temperatures, and confirmed by XRD.     

Preservation of the Pt(100) surface reconstruction after growth of a continuous layer of graphene

Scanning tunneling microscopy shows that a layer of graphene can be grown on the hex-reconstructed Pt(100) surface and that the reconstruction is preserved after growth. A continuous sheet of graphene can be grown across domain boundaries and step edges without loss of periodicity or change in direction. Density functional theory calculations on a simple model system support the observation that the graphene can have different rotation angles relative to the hex-reconstructed Pt surface. The graphene sheet direction can be changed by incorporating pentagon-heptagon defects giving rise to accommodation of edge dislocations. The defect formation energy and the induced buckling of the graphene have been characterized by DFT calculations.     

Oxidation of Pt(111) under near-ambient conditions

The oxidation of Pt(111) at near-ambient O2 pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin α-PtO2 trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.     

Deformation properties of the neutron-deficient odd-A Pt and Hg nuclei

Nuclear and atomic spectroscopy measurements have provided a great number of data on the neutron-deficient Pt and Hg nuclei. The odd-A Pt and Hg with A<186 have a prolate shape, and the even-even isotopes have a triaxial shape, while the nuclear shape of the odd-A Pt and Hg with A>186 is still an open question. The energy of the low-lying levels and the nuclear moments have been calculated in the framework of a semimicroscopic “axial-rotor + 1 quasiparticle” coupling model. The predictions are compared with the experimental data and discussed. The results strongly suggest a prolate shape for the negative-parity low-lying states of the odd-A ^187–191Pt and ^187–193Hg isotopes.     

Carbon on Pt(111): Characterization and influence on the chemisorptive properties

Fully dehydrogenated carbon deposits in the monolayer range have been characterized on the Pt(111) face by LEED, Auger and Δφ observations. These techniques pointed to the graphitic character of such deposits at least beyond about 0.5 monolayer of carbon. The gradual deactivation of the Pt surface towards CO or C₆H₆ adsorption with increasing carbon coverage has been explained by the nucleation and the growth of numerous inactive graphitic islands. The loss of chemisorptive properties only occurs for a complete carbon monolayer which may be viewed as a passive film masking the metal surface. Some weakening in the binding energy of CO, C₆H₆ and H₂ has been evidenced on a surface partially covered with carbon. This additional effect of carbon has been discussed mainly in relation with Δφ measurements.     

Cyclic voltammograms for H on Pt(111) and Pt(100) from first principles

Cyclic voltammetry is a fundamental experimental method for characterizing electrochemical surfaces. Despite its wide use, a way to quantitatively and directly relate cyclic voltammetry to ab initio calculations has been lacking. We derive the cyclic voltammogram for H on Pt(111) and Pt(100), based solely on density functional theory calculations and standard molecular tables. By relating the gas phase adsorption energy to the electrochemical electrode potential, we provide a direct link between surface science and electrochemistry.     

Graphene domain boundaries on Pt(111) as nucleation sites for Pt nanocluster formation

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600 K.     

S在Pt皮肤Pt3Ni(111)面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理方法,计算并分析了S原子在 Pt皮肤Pt3Ni(111)面不同位置的吸附特性.结果表明：S原子在Pt皮肤Pt3Ni的fcc位吸附最强,吸附能为5. 49 eV;与S原子在纯净Pt(111)表面的吸附相比,S原子在Pt皮肤Pt3Ni表面相应吸附位置的吸附能变小、与近邻的Pt原子形成的S-Pt键变长,表明掺杂的Ni会减小相应位点S原子的吸附能,降低体系对S原子的吸附能力,进而减弱S吸附对体系催化能力的影响;态密度分析发现, S原子的吸附使得Pt基催化剂的催化活性降低,主要是S的2p电子引起的;这些结果将为后续研究Pt基合金电极抗S中毒效果以及探究S原子吸附后Pt3Ni的活性位提供依据.     

Electron scattering on centrosymmetric molecular dianions Pt(CN)4(2-) and Pt(CN)6(2-)

Electron scattering on stored Pt(CN)2-4 and Pt(CN)2-6 centrosymmetric molecular dianions has been performed at the electrostatic storage ring ELISA. The thresholds for production of neutral particles by electron bombardment were found to be 17.2 and 18.7 eV, respectively. The relatively high thresholds reflect the strong Coulomb repulsion in the incoming channel as well as a high energetic stability of the target electrons. A trianion resonance was identified with a positive energy of 17.0 eV for the Pt(CN)2-4 square-planar complex, while three trianion resonances were identified for the Pt(CN)2-6 octahedral complex with positive energies of 15.3, 18.1, and 20.1 eV.     

Influence of acceptor and donor adsorbates (CO,K, NH3) on Pt surface core-level shifts

4f core-level shifts have been measured for clean surfaces of Pt(111), Pt(331), and Pt(557). Surface peaks due to terrace sites are shifted toward lower binding energy (0.32 ± 0.05 eV) from the bulk peak, whereas peaks from step atoms are shifted by 0.58 ± 0.05 eV also to lower binding energy. The intensity ratios for the two sites differ considerably between the stepped Pt surfaces. Chemisorption of carbon monoxide on the Pt(331) surface is preferential to step sites, with a Pt 4f binding energy shift of ～ 1.29 eV toward higher binding energy. Chemisorption of potassium and ammonia also produces Pt 4f surface shifts which are at higher binding energy than the bulk peak. These experiments do not support the concept of electron donation by these adsorbates into metal d orbitals. The results are discussed in view of, and supported by, tight-binding LCAOMO calculations of potassium and ammonia interacting with a Pt(111) thin film.