Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy.

We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemsák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces.

The study of surface Li-Sb alloy formation and electronic structure using photoelectron spectroscopy.

The formation of surface Li-Sb alloy and its electronic structures are investigated with x-ray photoelectron spectroscopy (XPS). From the information on core-level shift of Sb 3d and Li 1s XPS spectra, the presence of charge transfer from Li to Sb is confirmed, while the chemical composition of formed alloy is estimated as Li(3)Sb. The density of states from valence band XPS shows a significant nonrigid part of band structures, suggesting strong covalent bonding between Li and Sb.

Oxygen reduction on silver low-index single-crystal surfaces in alkaline solution: rotating ring disk(Ag(hkl)) studies.

The rotating ring disk method is applied to investigate the oxygen reduction reaction on Ag single-crystal surfaces in alkaline solution over the temperature range 293-333 K. At all temperatures, the oxygen reduction reaction proceeds through the 4e- reaction pathway with a very small (approximately 0.5-2%) peroxide formation. At the same temperature and overpotentials, the kinetics of the ORR always increases in the order (100) < or = (111) < (110), indicating that the reaction is structure-sensitive. We present an interpretation of the structure sensitivity based on the supposition that it arises from (i) a potential-dependent adsorption of spectator hydroxyl ions and (ii) variations in activation energies, which are determined by the O2 adsorption on Ag single-crystal surfaces covered by adsorbed oxygenated species.

Electronic structure of phospho-olivines Li(x)FePO4 (x = 0, 1) from soft-x-ray-absorption and -emission spectroscopies.

The electronic structure of the phospho-olivine Li(x)FePO4 was studied using soft-x-ray-absorption (XAS) and emission spectroscopies. Characteristic changes in the valence and conduction bands are observed upon delithation of LiFePO4 into FePO4. In LiFePO4, the Fe-3d states are localized with little overlap with the O-2p states. Delithiation of LiFePO4 gives stronger hybridization between Fe-3d states and O-2p states leading to delocalization of the O-2p states. The Fe L-edge absorption spectra yield "fingerprints" of the different valence states of Fe in LiFePO4 and FePO4. Resonant soft-x-ray-emission spectroscopy at the Fe L edge shows strong contributions from resonant inelastic soft x-ray scattering (RIXS), which is described using an ionic picture of the Fe-3d states. Together the Fe L-edge XAS and RIXS study reveals a bonding character of the Fe 3d-O2p orbitals in FePO4 in contrast to a nonbonding character in LiFePO4.

Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles.

We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using modified embedded atom method potentials and the Monte Carlo method. The nanoparticles are constructed with disordered fcc configurations at two fixed overall concentrations (50 at. % Pt and 75 at. % Pt). We use octahedral and cubo-octahedral nanoparticles terminated by {111} and {100} facets to examine the extent of the Pt segregation to the nanoparticle surfaces at T=600 K. The model particles contain between 586 and 4033 atoms (particle size ranging from 2.5 to 5 nm). Our results imply that a complete {100}-facet reconstruction could make the cubo-octahendral Pt-Ni nanoparticles most energetically favorable. We predict that at 600 K due to segregation the equilibrium cubo-octahedral Pt50Ni50 nanoparticles with fewer than 1289 atoms and Pt75Ni25 nanoparticles with fewer than 4033 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that, due to an order-disorder transition, the Pt50Ni50 cubo-octahedral nanoparticles containing more than 2406 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogenous core.

Conformation dependence of electronic structures of poly(ethylene oxide).

The electronic structure of pure poly(ethylene oxide) (PEO) for four different polymeric chain conformations has been studied by Hartree-Fock (HF) and density functional theory (DFT) through the analysis of their valence band photoelectron spectroscopy (VB-PES), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). It is shown that the valence band of PEO presents specific conformation dependence, which can be used as a fingerprint of the polymeric structures. The calculated spectra have been compared with experimental results for PEO powder.

Surface structures of cubo-octahedral Pt-Mo catalyst nanoparticles from Monte Carlo simulations.

The surface structures of cubo-octahedral Pt-Mo nanoparticles have been investigated using the Monte Carlo method and modified embedded atom method potentials that we developed for Pt-Mo alloys. The cubo-octahedral Pt-Mo nanoparticles are constructed with disordered fcc configurations, with sizes from 2.5 to 5.0 nm, and with Pt concentrations from 60 to 90 atom %. The equilibrium Pt-Mo nanoparticle configurations were generated through Monte Carlo simulations allowing both atomic displacements and element exchanges at 600 K. We predict that the Pt atoms weakly segregate to the surfaces of such nanoparticles. The Pt concentrations in the surface are calculated to be 5-14 atom % higher than the Pt concentrations of the nanoparticles. Moreover, the Pt atoms preferentially segregate to the facet sites of the surface, while the Pt and Mo atoms tend to alternate along the edges and vertexes of these nanoparticles. We found that decreasing the size or increasing the Pt concentration leads to higher Pt concentrations but fewer Pt-Mo pairs in the Pt-Mo nanoparticle surfaces.

The impact of geometric and surface electronic properties of pt-catalysts on the particle size effect in electrocatalysis.

The particle size effect on the formation of OH adlayer, the CO bulk oxidation, and the oxygen reduction reaction (ORR) have been studied on Pt nanoparticles in perchloric acid electrolyte. From measurements of the CO displacement charge at controlled potential, the corresponding surface charge density versus potential curves yielded the potentials of total zero charge (pztc), which shifts approximately 35 mV negative by decreasing the particle size from 30 nm down to 1 nm. As a consequence, the energy of adsorption of OH is more enhanced, that is, at the same potential the surface coverage with OH increases by decreasing the particle size, which in turn affects the catalytic reactions thereon. The impact of the electronically induced potential shift in the OH adsorption is demonstrated at the CO bulk oxidation, in which adsorbed OH is an educt species and promotes the reaction, and the ORR, where it can act as a surface site blocking species and inhibits the reaction.

Vibrational properties of CO at the Pt(111)-solution interface: the anomalous Stark-tuning slope.

Vibrational properties of CO have been studied on Pt(111) in acid and alkaline electrolytes by synchronous measurements of CO oxidation current (0.5 mV/s) and IRAS spectra (one spectrum for every 1 mV). We found that in acid solutions the frequency-tuning rate (dnu(CO)/dE) as well as the potential-dependent bandwidth (dDeltanu1/2/dE) deviates from expected linear relationships. This unusual potential-dependent behavior is interpreted in terms of compression/dissipation of CO islands during the CO oxidation, engendered by competitive adsorption between inactive anions from a supporting electrolyte and the reactive OH species.

Monte Carlo simulations of segregation in Pt-Re catalyst nanoparticles.

We have investigated the segregation of Pt atoms to the surfaces of Pt-Re nanoparticles using the Monte Carlo method and modified embedded-atom method potentials that we have developed for Pt-Re alloys. The Pt(75)Re(25) nanoparticles (containing from 586 to 4,033 atoms) are assumed to have disordered fcc configurations and cubo-octahedral shapes (terminated by [111] and [100] facets), while the Pt(50)Re(50) and Pt(25)Re(75) nanoparticles (containing from 587 to 4,061 atoms) are assumed to have disordered hcp configurations and truncated hexagonal bipyramidal shapes (terminated by [0001] and [1011] facets). We predict that due to the segregation process the equilibrium Pt-Re nanoparticles would achieve a core-shell structure, with a Pt-enriched shell surrounding a Pt-deficient core. For fcc cubo-octahedral Pt(75)Re(25) nanoparticles, the shells consist of almost 100 at. % of Pt atoms. Even in the shells of hcp truncated hexagonal bipyramidal Pt(50)Re(50) nanoparticles, the concentrations of Pt atoms exceed 85 at. % (35 at. % higher than the overall concentration of Pt atoms in these nanoparticles). Most prominently, all Pt atoms will segregate to the surfaces in the hcp truncated hexagonal bipyramidal Pt(25)Re(75) nanoparticles containing less than 1000 atoms. We also find that the Pt atoms segregate preferentially to the vertex sites, less to edge sites, and least to facet sites on the shell of Pt-Re nanoparticles.

Potential-dependent vibrational spectroscopy of solvent molecules at the Pt(111) electrode in a water/acetonitrile mixture studied by sum frequency generation.

Sum frequency generation (SFG) vibrational spectra of D(2)O and/or acetonitrile (CH(3)CN) on a Pt(111) single-crystal electrode were obtained as a function of applied potential in a 5 mol % water/acetonitrile mixed solvent with different 0.1 molar MSO(3)CF(3) salts (M = H(+), Li(+), Na(+), K(+), and Cs(+)). The results provide a very specific model for the composition of the inner Helmholtz layer as a function of potential and surface charge. Acetonitrile dominates the inner layer with the CN group directed toward the metal at potentials where the metal has a positive charge. As the surface becomes negatively charged, the acetonitrile orientation flips 180 degrees, with the CH(3) group pointing toward the surface. At even more negative surface charge, D(2)O displaces acetonitrile from the inner layer and is the predominant molecule on the surface. Here water is present as an oriented molecule with the oxygen end pointing toward the metal. The potential (and surface charge) where water is the dominant molecule in the inner Helmholtz layer is determined by the solvation energy of the cation.