Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

NH_x(x=1～3)在金属Ir表面吸附与解离的第一性原理研究

采用密度泛函理论,在slab模型下,研究了NH_x(x=1~3)在Ir(100)、Ir(111)和Ir(110)表面上的最稳定吸附位置、几何构型以及逐步脱氢分解过程,计算了相应的吸附能和活化能.计算结果表明,在Ir(100)、Ir(111)面上,NH_3是以C_3轴垂直吸附在顶位,在Ir(110)上,NH_3是以N-Ir键与表面成68.6°吸附在顶位,且吸附能依赖于表面的结构而不同,相比而言,NH_3更容易吸附在开放表面Ir(100)、Ir(110)面上,说明NH_3在这些表面的吸附具有结构敏感性.NH_(x(x=1~3))的分解,在Ir(100),NH_3的吸附与分解存在竞争,在Ir(110)面NH_3最容易分解,在Ir(111)面NH_3是分子性吸附,不能分解.NH_2、NH在三个表面均能够分解,在Ir(110)面活化能均较高.     

甲基和羟基在Ir(111)表面吸附的密度泛函理论研究

在超原胞近似和slab模型基础上,采用周期性密度泛函理论,在0.11覆盖度（ML）下,对甲基与羟基在Ir(111)表面的吸附进行了研究,得到了甲基和羟基在Ir(111)表面不同吸附位置的吸附能和吸附构型,计算了它们的振动频率,同时分析了甲基和羟基共吸附于Ir(111)表面的情况。结果表明,甲基和羟基在Ir(111)表面的最稳定吸附位置都是top位,甲基是碳端向下吸附,羟基是通过氧端向下倾斜吸附。通过频率分析发现吸附后CH3中C-H键的对称伸缩振动、反对称伸缩振动以及剪切振动频率均发生了红移,而羟基中的O-H键的振动频率发生蓝移现象。通过计算对比发现甲醇分解为甲基和羟基过程是一个放热反应,从热力学角度来说该反应是可行的。     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Structure transformation of Ir clusters on Ir surfaces

A small Ir cluster can assume either a one-dimensional linear-chain structure or a two-dimensional island-like structure. We present a study of the energetics of the 1 D to 2 D structure transformation of three-atom Ir clusters on the Ir(111) and (001) surfaces. On the (111) plane, the temperature dependence of the ratio of the probabilities of observing a three-atom cluster in the 1 D and 2 D structures exhibits a simple linear Arrhenius behavior. The 2 D island structure is found to be more stable with the cluster binding energy lower by 0.098±0.004 eV. On the (001) plane, the 1 D chain structure is more stable with the cluster binding energy lower by 0.335±0.015 eV. From these energies, the relative pair interaction at three different bond lengths can be derived. The relative pair potential is found to be non-monotonic in distance dependence. We explain the (1×5) reconstruction of the Ir(001) surface as being caused by the large difference in the pair binding energy of the first and second nearest-neighbor bonds. In addition, we find a significant deviation from the simple linear Arrhenius behavior at low temperatures for the three-atom Ir cluster on the Ir (001) plane, indicating that the entropy factor is temperature dependent.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

Adsorption of O and CO on Ir(100) from first principles

The adsorption of O and CO on Iridium (100) surface with different coverages (Θ = 1.0, 0.5, 0.25 monolayer (ML)) is studied using density functional theory (DFT). The most energetically preferred site of adsorption for O is found to be the bridge site. However, the top site is the preferred one for CO at coverages of 0.25 ML and 0.5 ML. Oxygen adsorbed on the bridge site at 0.25 ML and 0.5 ML coverages causes a row pairing. A missing row reconstruction appears in the case of 0.25 ML coverage. We find that the adsorption of O (CO) on Ir(100) surface causes disruptions of Ir–Ir bonds in the metal, which reduces (increases) the Ir–Ir bond length.     

The preparation, characterisation and oxygen adsorption on Ir/Pd(110) alloys

Ir-Pd alloys were formed on Pd(110) by evaporation of Ir and subsequent heating in vacuo. Ir overlayers were stable up to 600 K. At higher temperatures Pd diffusion to the surface occurred, which resulted in a Pd-dominated surface over a range of Ir coverages. The transformation of the surface initially dominated by Ir to one where a more homogeneous distribution of the two metals was obtained was probed with angle-resolved XPS. This showed that above 600 K a rapid loss of Ir from the surface region took place, which was followed by a slower loss at higher temperatures. Mixed surface layers could thus be formed which varied in composition depending on the initial amounts of Ir deposited. At low Ir coverages, these surfaces showed an oxygen adsorption behaviour resembling that of Pd(110) in terms of desorption temperatures, but with an increased amount of oxygen desorbing from subsurface sites.     

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects – wrinkles – are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.     

Self-assembly of one-dimensional surface structures: long-range interactions in the growth of Ir and Pd on W(110)

Self-assembly of one-dimensional surface structures is examined by tracking single Ir and Pd atoms on W(110) as they incorporate into chains of Ir and Pd, respectively. Ir adatoms move parallel to the chains, but do not come close to the chain sides; incorporation occurs only at the ends. Pd adatoms also migrate parallel to Pd chains, and incorporate at the ends. Occasionally, in the end region, they do jump to the side of a chain, and migrate there until they find an end site. Incorporation behavior for both Ir and Pd is driven by highly anisotropic, long-ranged interactions between atoms on the surface.     

First-principles investigations of iridium low index surfaces

We report density functional theory (DFT) calculations for the surface energy, work function, and interlayer spacings for Ir(1 1 1), Ir(1 0 0), and Ir(1 1 0) surfaces using pseudopotential method and plane waves basis set. We investigate the convergence of the surface energy as a function of the number of layers in the slab for the Ir(1 0 0) surface. The results show that the surface energies calculated using the bulk total energies obtained by a fit to a series of slab total energies converge within 0.01 J/m². We also investigate the convergence of the work function and interlayer spacings as a function of the number of layers in the slab, for the Ir(1 0 0) surface. Employing the local-density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation functional, we obtained a very good agreement of the calculated surface energies and work functions with experimental results. For the studied surfaces, the calculations give interlayer relaxations that are in an excellent agreement with available low-energy electron diffraction (LEED) analysis. Furthermore, we discuss the performance of the LDA and GGA for the exchange correlation functional in describing the various surface properties. The results show that calculations using GGA give results that are in a better agreement with experiment than the LDA.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: II. Structural determination by LEED intensity analysis

The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.     

Step edge diffusion and the structure of nanometer-size Ir islands on the Ir(111) surface

We report an FIM study of the structure of nanometer-size Ir islands on the Ir(111) surface. In this experiment, the number of atoms in an island is carefully controlled by field evaporation and vapor deposition. When this number can be fitted to a hexagonal atomic arrangement, the stable structure is found to be a perfect hexagon. In other cases, an addition of one ledge atom can reverse the symmetry of a small island or change its shape. We also compare diffusion of adatoms on the Ir(311) and (331) surfaces to that of ledge-atoms along the A- and B-type steps of the (111) layer, and the relative binding energies of a ledge atom at these steps.     

Structural properties of a monolayer graphite film on the (111)Ir surface

The structural properties of a monolayer graphite film prepared on the (111)Ir surface through thermal decomposition of benzene molecules were studied. The study was carried out in ultrahigh vacuum using scanning tunneling microscopy, which allowed observation of the atomic structure of the film. It is shown that, on extended smooth regions of the Ir surface, a continuous graphite film with a regular arrangement of carbon atoms in a planar hexagonal lattice is formed. The orientation of zigzag carbon atom chains coincides with the 〈110〉 direction on the Ir surface. Structural defects of the (5, 7) configuration were revealed in the film. A comparison of the topographies of the film and the (111)Ir surface shows that the graphite layer smoothly (without discontinuities) flows over subnanometer topographical features existing on the Ir surface and that the distance between the graphite film and the metal surface in this case can reach 1 nm.     

Interaction of silver atoms with iridium and with a two-dimensional graphite film on iridium: Adsorption,desorption, and dissolution

The initial stages in the interaction of silver with the (111)Ir surface and with a two-dimensional graphite film (2D GF) on (111)Ir were studied by high-resolution electron Auger spectroscopy in ultrahigh vacuum. The growth mechanisms of silver films and the desorption fluxes of Ag atoms were determined, and their desorption energies estimated. It was found that the Ag desorption fluxes from a 2D GF on Ir and from a thick silver film on the pure metal are similar and considerably (an order of magnitude) smaller than the sublimation fluxes from bulk silver at the same temperatures. The activation energy for desorption from a submonolayer film varies from 3.2 eV for coverage θ=1 to 3.7 eV at θ ～ 0. It was shown that silver atoms do not penetrate into the substrate bulk throughout the temperature range covered (300–1800 K).     

Adsorption of hydrogen on rhodium; Comparison with hydrogen adsorption on platinum and iridium

The adsorption of hydrogen on Rh has been studied (i) on a single crystal tip using field electron microscopy, and (ii) on a filament carrying this tip, using thermal desorption spectroscopy. The results are compared to those of other Group VIII metals. An isosteric heat of adsorption of 19 kcal/mole was found at low coverage, decreasing slightly with increasing coverage. This heat is substantially lower than that on Ru and Ir, determined by the same method. The work function increases by 0.4 eV, a value comparable to data reported for Ni and Ru, but significantly larger than those of Ir and Pt. An electropositive state of hydrogen as observed for Pt and Ir was not found for Rh. A small fraction of the adsorbed hydrogen is not desorbed at temperatures where other transition metal surfaces are completely denuded. This β₂-hydrogen which is desorbed only at 600–800 K, is tentatively assigned to a subsurface species.     

First-principles study of molecular NO dissociation on Ir(100) surface

The dissociation of NO on Ir(100) surface is investigated using density functional theory (DFT). The pathway and transition state (TS) of the dissociation of NO molecule are determined using climbing image nudge elastic band (CI-NEB). The prerequisite state of NO dissociation is determining the most stable sites of the reactant and products. We found that the most energetically stable sites are the hollow for N atom and the bridge for NO molecule as well as O atom. We found that the bending of NO is the first step of the dissociation reaction due to the increase of the back-donation from the d-band of Ir to 2π ^? orbital of NO, which causes the weakening of NO bond. The dissociation energy barrier of NO molecule on Ir(100) surface is 0.49 eV.     

Spin-liquid state in the S=1/2 hyperkagome antiferromagnet Na4Ir3O8

A spinel related oxide, Na(4)Ir(3)O(8), was found to have a three dimensional network of corner shared Ir(4+) (t(2g)(5)) triangles. This gives rise to an antiferromagnetically coupled S = 1/2 spin system formed on a geometrically frustrated hyperkagome lattice. Magnetization M and magnetic specific heat C(m) data showed the absence of long range magnetic ordering at least down to 2 K. The large C(m) at low temperatures is independent of applied magnetic field up to 12 T, in striking parallel to the behavior seen in triangular and kagome antiferromagnets reported to have a spin-liquid ground state. These results strongly suggest that the ground state of Na(4)Ir(3)O(8) is a three dimensional manifestation of a spin liquid.