Adsorbate-induced Changes Research Articles

Simulating Structural Dynamics of Metal Catalysts under Operative Conditions.

Structural reconstructions of metal catalysts have been recognized as common phenomena during catalytic reactions, which play a key role in their activities in heterogeneous catalysis. Precisely identifying the structures under the operative conditions becomes a prerequisite to establish a reliable structure-activity relationship and further rationalize the design of metal catalysts. However, real-time capture of the structural variations of catalysts at the atomic level with high-temporal resolution is a grand challenge for present in situ characterizations. During the past decade, significant progress has been made in theory to couple the structures with the reaction conditions to reproduce the experimental observations and predict the adsorbate-induced changes of catalysts in composition, morphology, size, etc. Modeling the dynamic correlation between the structure and activity of the metal catalysts brings us advanced knowledge of heterogeneous catalysis and becomes indispensable for accurate evaluation of the performance of metal catalysts.

In Situ STM Investigations of Cu(100) and Bimetallic Ag/Cu(100) Model Catalysts for CO2 Electroreduction in Bicarbonate Solution

The electrochemical conversion of carbon dioxide (CO2RR) to valuable chemicals and fuels plays a crucial role in the sustainable energy economy as it represents a promising potential for the renewable energy storage as well as closing the carbon loop.[1,2] Thus, the design and development of a highly efficient and selective electrocatalyst for CO2RR has become a topic of great importance in recent years. Among elemental metals, copper exhibits unique catalytic behaviour to reduce CO2 towards multi-carbon products with reasonable Faradaic efficiency due to its moderate CO-binding energy.[3,4] However, the selectivity and activity of pure copper is still far away for a practical application. The integration of a secondary metal species provides a promising strategy to improve the catalytical performance of the copper catalyst. Combination with silver is particularly interesting for such bimetallic catalysts, since Ag is a highly efficient catalyst for CO2RR to CO, probably the most important intermediate in the CO2RR, and has been shown to promote the formation of multi-carbon products.[5]Here, we present in situ scanning tunnelling microscopy studies of well-defined Cu and AgCu electrodes in CO2-satured 0.1 M KHCO3, which is one of the most common electrolytes for CO2 electroreduction. For Cu(100) in the double layer potential range, coexistence of two ordered (bi)carbonate adlayer phases was observed (Fig. 1a,b, published in [6]). These adlayer phases have a highly complex structure, exhibit dynamic fluctuations, and undergo a potential dependent order-disorder transition at about 0 V vs. the reversible hydrogen electrode. Detailed density functional calculations reveal that the observed layer is composed of carbonate and water that coadsorb on the electrode surface, underlining the key role of water in the stabilization of the (bi)carbonate adlayers.[6]Upon decreasing the potential to the onset of CO2RR, the formation of Cu nanoclusters is observed on the Cu(100) surfaces by in situ STM. This surface restructuring can be assigned to the influence of CO intermediates formed in the CO2RR. Upon subsequent increase of the potential back into the double layer range, the clusters disperse, resulting in a highly disordered interface structure that contains Cu adatoms, (bi)carbonates, and further molecular adsorbates. This irreversible change of the molecular-scale electrode surface is supported by spectroscopic results.AgCu bimetallic model catalysts were prepared by electrodeposition of submonolayer Ag coverage on Cu(100) in 0.1 M H2SO4. Silver forms islands on Cu(100) with a hexagonal quasi-Ag(111) atomic lattice. After the preparation and characterization of the Ag-decorated Cu(100), the electrolyte was exchanged under potential control to 0.1 M KHCO3. The electrolyte exchange results in the distinct restructuring on the surface in the double layer potential range, in which the islands develop a strongly anisotropic shape. In addition, appearance of new islands and a reduced surface mobility are observed. Furthermore, in contrast to the Ag-free Cu(100), high-resolution STM images show a disordered molecular adlayer.Our results demonstrate the pronounced role of adsorbed (bi)carbonate at the catalyst/electrolyte interface and its impact on the surface morphology and dynamics already at potentials in the double layer regime. These adsorbate-induced structural changes need to be considered for a fundamental understanding and rational design of electrocatalysts for CO2RR.The Authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via SPP 2080, project no. 327886311.

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis.

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Critical Roles of Impurities and Imperfections in Various Phases of Materials.

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain-relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.

Electronic structure factors and the importance of adsorbate effects in chemisorption on surface alloys

The chemisorption energy is an integral aspect of surface chemistry, central to numerous fields such as catalysis, corrosion, and nanotechnology. Electronic-structure-based methods such as the Newns-Anderson model are therefore of great importance in guiding the engineering of material surfaces with optimal properties. However, existing methods are inadequate for interpreting complex, multi-metallic systems. Herein, we introduce a physics-based chemisorption model for alloyed transition metal surfaces employing primarily metal d-band properties that accounts for perturbations in both the substrate and adsorbate electronic states upon interaction. Importantly, we show that adsorbate-induced changes in the adsorption site interact with its chemical environment leading to a second-order response in chemisorption energy with the d-filling of the neighboring atoms. We demonstrate the robustness of the model on a wide range of transition metal alloys with O, N, CH, and Li adsorbates yielding a mean absolute error of 0.13 eV versus density functional theory reference chemisorption energies.

Combined Deep Learning and Classical Potential Approach for Modeling Diffusion in UiO-66.

Modeling of diffusion of adsorbates through porous materials with atomistic molecular dynamics (MD) can be a challenging task if the flexibility of the adsorbent needs to be included. This is because potentials need to be developed that accurately account for the motion of the adsorbent in response to the presence of adsorbate molecules. In this work, we show that it is possible to use accurate machine learning atomistic potentials for metal-organic frameworks in concert with classical potentials for adsorbates to accurately compute diffusivities though a hybrid potential approach. As a proof-of-concept, we have developed an accurate deep learning potential (DP) for UiO-66, a metal-organic framework, and used this DP to perform hybrid potential simulations, modeling diffusion of neon and xenon through the crystal. The adsorbate-adsorbate interactions were modeled with Lennard-Jones (LJ) potentials, the adsorbent-adsorbent interactions were described by the DP, and the adsorbent-adsorbate interactions used LJ cross-interactions. Thus, our hybrid potential allows for adsorbent-adsorbate interactions with classical potentials but models the response of the adsorbent to the presence of the adsorbate through near-DFT accuracy DPs. This hybrid approach does not require refitting the DP for new adsorbates. We calculated self-diffusion coefficients for Ne in UiO-66 from DFT-MD, our hybrid DP/LJ approach, and from two different classical potentials for UiO-66. Our DP/LJ results are in excellent agreement with DFT-MD. We modeled diffusion of Xe in UiO-66 with DP/LJ and a classical potential. Diffusion of Xe in UiO-66 is about a factor of 30 slower than that of Ne, so it is not computationally feasible to compute Xe diffusion with DFT-MD. Our hybrid DP-classical potential approach can be applied to other MOFs and other adsorbates, making it possible to use an accurate DP generated from DFT simulations of an empty adsorbent in concert with existing classical potentials for adsorbates to model adsorption and diffusion within the porous material, including adsorbate-induced changes to the framework.

Deprotonation and cation adsorption on the NiOOH/water interface: A grand-canonical first-principles investigation

Nickel-based oxides are highly active, cost-effective materials for the oxygen evolution reaction in alkaline conditions. Recent experimental studies have revealed the importance of surface deprotonation and alkali metal cation adsorption on the activity of Ni oxide surfaces, in contact with aqueous alkaline electrolyte. As a first step to elucidate the role of the alkali adsorption for the activity, we performed first-principles electronic structure calculations to address the stable surface structures of β-NiOOH(0001) as a function of the operating conditions in an electrochemical environment. We present a grand-canonical approach to compute the surface Pourbaix diagram of the β-NiOOH/water interface for the processes of deprotonation and alkali metal cation adsorption. The results of this study emphasize the importance of double-layer effects, including the adsorbate-induced change of surface dipole moments and the rearrangement of water molecules due to their strong interaction with the adsorbed species, for the most stable interface structure.

Thermodynamic Cyclic Voltammograms Based onAb InitioCalculations: Ag(111) in Halide-Containing Solutions

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

Electrosorption at metal surfaces from first principles

Electrosorption of solvated species at metal electrodes is a most fundamental class of processes in interfacial electrochemistry. Here, we use its sensitive dependence on the electric double layer to assess the performance of ab initio thermodynamics approaches increasingly used for the first-principles description of electrocatalysis. We show analytically that computational hydrogen electrode calculations at zero net-charge can be understood as a first-order approximation to a fully grand canonical approach. Notably, higher-order terms in the applied potential caused by the charging of the double layer include contributions from adsorbate-induced changes in the work function and in the interfacial capacitance. These contributions are essential to yield prominent electrochemical phenomena such as non-Nernstian shifts of electrosorption peaks and non-integer electrosorption valencies. We illustrate this by calculating peak shifts for H on Pt electrodes and electrosorption valencies of halide ions on Ag electrodes, obtaining qualitative agreement with experimental data already when considering only second order terms. The results demonstrate the agreement between classical electrochemistry concepts and a first-principles fully grand canonical description of electrified interfaces and shed new light on the widespread computational hydrogen electrode approach.

Van der Waals Interaction Activated Strong Electronic Coupling at the Interface between Chloro Boron-Subphthalocyanine and Cu(111).

In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby “pulling” the molecule toward the surface. This reveals two adsorption regimes with significantly different adsorption distances, molecular conformations, and adsorbate-induced changes of the work function. Notably, the above-mentioned massive hybridization of electronic states, also observed in photoelectron spectroscopy, is obtained solely for one of the regimes. We show that this regime is accessible only as a consequence of the planarization of the molecular backbone resulting from the van der Waals attraction between the molecule and the surface. The results of this study indicate that for certain metal–molecule combinations unusually strong interfacial electronic interactions can be triggered by van der Waals forces creating a situation that differs from the usually described cases of physisorptive and chemisorptive interactions.

Catalytic CO Oxidation on Nanocatalysts

CO oxidation, as one of the simplest catalytic reactions, has been widely studied in heterogeneous catalysis. Since CO can be easily adsorbed on active metals like Au, Pt, Pd, Rh, and Ru, many researchers have attempted to observe surface phenomena, including adsorption, surface restructuring, and oxidation kinetics on diverse metal surfaces ranging from single crystals to nanoparticles (NPs). The rapid advances in nanochemistry have further stimulated studies of catalytic reactions, and in-depth understanding of CO oxidation is possible based on analyses of how the size, shape, and composition affect the activity and determine the most effective active site of metal NPs. Recent research has demonstrated that the CO oxidation activity varies with the size of metallic NPs. The oxidation state of the NPs varies as a function of the size, and the surface oxide layers formed on NPs have been found to be important in enhancing or suppressing CO oxidation. Surface segregation of bimetallic NPs also influences the CO oxidation rate. The NP surfaces undergo significant adsorbate-induced structural changes, as confirmed by various in situ characterization techniques under catalytically relevant CO oxidation conditions. Based on the knowledge of support-induced catalytic properties, various metal-support interactions have been investigated for enhancing CO oxidation. By changing the reaction environments to either CO- or O-rich atmospheres, the synergetic effect at the interfaces of NPs and support oxides have been largely clarified. Versatile nanostructures with confined shells of small metal NPs have also been designed as core@shell-type catalysts. Using in situ characterization techniques combined with well-defined NPs, it is possible to study CO oxidation on the molecular level in order to gain vital mechanistic insights under catalytic working conditions.

Adsorbate-Induced Changes in Magnetic Interactions in Fe2(dobdc) with Adsorbed Hydrocarbon Molecules

Metal–organic frameworks, which are a special case of coordination polymers, form a class of materials with numerous applications due to their high porosities and large internal surface areas and in some cases also due to paramagnetic metal ions. A family of such materials of general formula M2(dobdc) (where M is a divalent metal ion, and dobdc4– is 2,5-dioxido-1,4-benzenedicarboxylate) has attracted considerable attention for gas separation, catalysis, and magnetism. In this work, we explore the magnetic properties of a member of this family, Fe2(dobdc), both in the activated form (bare MOF after solvent removal) and when hydrocarbons are bound to the open coordination sites of the metal. We report quantum mechanical electronic structure calculations using both cluster models and periodic models, and we compare our results to previously reported theoretical studies. We find that hydrocarbon adsorption only mildly affects the isotropic couplings but that the isotropic magnetic couplings obtained with hybrid exchange–correlation functionals for the cluster models were found to be in good agreement with the experimental values, and the ones obtained using local exchange–correlation functionals with empirical Coulomb and exchange integrals for the periodic models were also found to agree well with experiments. Furthermore, local density functionals with empirical Coulomb and exchange integrals for periodic models are found to give good agreement with the experimental result that the adsorption of ethylene changes the magnetic ordering. We also used second-order n-electron valence state perturbation theory and contracted spin–orbit configuration interaction to study the role of adsorption on the crystal-field splitting of the quintet manifold and on the single-ion anisotropy of the iron center.

Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces.

Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green's functions for finite geometries and electron plane wave expansion for periodic systems. The "particle in a box" picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

Towards an Atomic-Scale Understanding of the Adsorbate-Driven Formation of High-Index Faces

Preferential adsorption takes place at the step ledge between adjacent crystal faces, which usually serve as active sites for breaking chemical bonds. In this paper, we present a structural model to interpret the habit modification of single crystals in terms of the step geometries relationship between crystal faces. A new series of high index faces parallel to the ledge between adjacent facets can be explicitly determined from the presence of the symmetry operators in the space group. The relative stability of these new faces undergoes a faceting transition, driven by the adsorbate-induced changes of the step configuration. Combined with the chemical bond-geometric approach, our predictions accurately reproduce the tapering evolutions of KDP crystals in the present of metallic ions. The current work provides a new insight on how changes affecting elementary steps on one face are translated into the emergence of a new crystallographic face.

Quantifying the origin of inter-adsorbate interactions on reactive surfaces for catalyst screening and design.

The adsorption energy of reactant molecules and reaction intermediates is one of the key descriptors of catalytic activity of surfaces and is commonly used as a metric in screening materials for design of heterogeneous catalysts. The efficacy of such screening schemes depends on the accuracy of calculated adsorption energies under reaction conditions. These adsorption energies can depend strongly on interactions between adsorbed molecules in the adlayer. However, these interactions are typically not accounted for in screening procedures that use DFT-based zero-coverage adsorption energies. Identifying the physical mechanisms behind these interactions is essential to model realistic catalyst surfaces under reaction conditions and to understand the dependence of adsorption energies on reaction parameters like surface strain and composition. This article describes a method to quantitatively resolve the observed inter-adsorbate interactions into various direct adsorbate-adsorbate interactions (i.e. Coulombic and steric) and surface-mediated interactions (i.e. adsorbate-induced surface relaxation and change in electronic structure) by combining density functional theory and cluster-expansion calculations of coverage-dependent adsorption energies. The approach is implemented on a model catalyst surface of FeS2(100) reacting with H2S molecules. We find that the adsorption energy of H2S molecules can be affected by over 0.55 eV by the repulsive inter-adsorbate interactions caused primarily by the adsorbate-induced changes to the electronic structure of the FeS2 surface. These interactions also show a strong monotonic dependence on surface strain, being three times stronger on compressively strained surfaces than on surfaces under tensile strain. The large magnitude of inter-adsorbate interactions as well as their strong dependence on lattice strain demonstrate the need for using coverage-dependent adsorption energies for more accurate screening, for example for strained catalytic systems like core-shell and overlayer structures.

Adsorbate-induced structural changes in 1-3 nm platinum nanoparticles.

We investigated changes in the Pt-Pt bond distance, particle size, crystallinity, and coordination of Pt nanoparticles as a function of particle size (1-3 nm) and adsorbate (H2, CO) using synchrotron radiation pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) measurements. The ∼1 nm Pt nanoparticles showed a Pt-Pt bond distance contraction of ∼1.4%. The adsorption of H2 and CO at room temperature relaxed the Pt-Pt bond distance contraction to a value close to that of bulk fcc Pt. The adsorption of H2 improved the crystallinity of the small Pt nanoparticles. However, CO adsorption generated a more disordered fcc structure for the 1-3 nm Pt nanoparticles compared to the H2 adsorption Pt nanoparticles. In situ XANES measurements revealed that this disorder results from the electron back-donation of the Pt nanoparticles to CO, leading to a higher degree of rehybridization of the metal orbitals in the Pt-adsorbate system.

Interactive Surface Chemistry of CO2 and NO2 on Metal Oxide Surfaces: Competition for Catalytic Adsorption Sites and Reactivity

Interactive surface chemistry of CO2 and NO2 on BaOx/Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbate-induced morphological changes. After NO2 adsorption, nitrated BaOx/Pt(111) surfaces do not reveal available adsorption sites for CO2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO2 adsorption on thick BaOx(>10MLE)/Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaOx/Pt(111) overlayers does not prevent NO2 uptake. Carbonated BaOx(1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO2 adsorption can also further interact with NO2, forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO2 and CO2, carbonate species formed on Lewis base (O2–) sites enable the formation of nitrites on Lewis acid (Ba2+) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaOx aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaOx domains, nitration and carbonation results in redispersion and spreading of the BaOx domains on the Pt(111) substrate.

Nucleation, Growth, and Adsorbate-Induced Changes in Composition for Co–Au Bimetallic Clusters on TiO2

The nucleation, growth, and CO-induced changes in composition for Co–Au bimetallic clusters deposited on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO2(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of clusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of ∼3–5 Å in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80–100% Au for bulk Au fractions greater than 50%, but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni–Au and Pt–Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO-induced diffusion is less extensive on Co–Au than on the Ni–Au and Pt–Au surfaces.

Giant Ambipolar Rashba Effect in the Semiconductor BiTeI

We observe a giant spin-orbit splitting in the bulk and surface states of the noncentrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases, it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics-a large, robust spin splitting and ambipolar conduction-are present in this material.

Adsorbate-Induced Changes in the Surface Composition of Bimetallic Clusters: Pt−Au on TiO2(110)

The growth, surface composition, and chemical activity of bimetallic Pt−Au clusters on TiO2(110) have been investigated. Scanning tunneling microscopy (STM) experiments demonstrate that the deposition of Au on Pt clusters results in the formation of bimetallic Pt−Au clusters due to the seeding of the mobile Au atoms at existing Pt nuclei. The composition of the top surface layer of the clusters was studied by low energy ion scattering (LEIS) for bulk compositions ranging from 25%−87.5% Pt with total metal coverages of 0.25 and 0.50 ML. For both coverages, the cluster surfaces consisted of nearly pure Au at Pt compositions of 50% and lower; however, a mix of Au and Pt atoms were found at the cluster surfaces at higher fractions of deposited Pt. These results are consistent with bulk thermodynamics, which predicts a Pt core−Au shell structure based on the lower surface free energy of Au compared to Pt and the large bulk miscibility gap for the two metals. The adsorption of CO on the Pt−Au clusters at room temperature promotes the diffusion of Pt to the surface of the clusters, and this phenomena is most pronounced for the clusters that are initially pure Au at the surface. Density functional theory calculations demonstrate that it is thermodynamically favorable for Pt to diffuse to the cluster surface in order to bind to CO. In contrast, the extent of CO2 production via sequential adsorption of O2 and CO on the Pt−Au clusters reflects the surface Pt content before adsorption. For CO oxidation, the first step in the reaction is the dissociation of O2 at Pt sites. Since this process requires more than one contiguous Pt site, it is not surprising that O2 dissociation cannot occur on the Pt−Au clusters that are ∼100% Au at the surface before CO exposure, given the low probability for ensembles of Pt sites to form at the surface.