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Aiming at a better fundamental understanding of the chemistry of bimetallic PtAg/Pt(111) surfaces, we have investigated the stability, electronic properties and CO adsorption properties of bimetallic PtAg surfaces, including pseudomorphic Ag film covered Pt(111) surfaces and PtxAg1-x/Pt(111) monolayer surface alloys, using periodic density functional theory calculations. The data provide detailed insights into the relative stabilities of different surface configurations, as indicated by their formation enthalpies and surface energies, and changes in their electronic properties, i.e., in the projected local densities of states and shifts in the d-band center. The adsorption properties of different Ptn ensembles were systematically tested using CO as a probe molecule. In addition to electronic ligand and strain effects, we were particularly interested in the role of different adsorption sites and of the local COad coverage, given by the number of CO molecules per Pt surface atom in the Ptn ensemble. Different from PdAg surfaces, variations in the adsorption energy with adsorption sites and with increasing local coverage are small up to one COad per Pt surface atom. Finally, formation of multicarbonyl species with more than one COad per Pt surface atom was tested for separated Pt1 monomers and can be excluded at finite temperatures. General trends and aspects are derived by comparison with comparable data for PdAg bimetallic surfaces. Fundamental insights relevant for applications of bimetallic Pt catalysts, specifically PtAg catalysts, are briefly discussed.

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Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported by a UiO-66 metal-organic framework (Cu/UiO-66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density-functional-theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2 , by reaction of O2,ad with COad , leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as the rate limiting step. This is removed in a second activated step.

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The development of synthetic particles that emulate real viruses in size, shape, and chemical composition is vital to the development of imprinted polymer-based sorbent materials (molecularly imprinted polymers, MIPs). In this study, we address surrogates for adenovirus type 5 (Adv 5) via the synthesis and subsequent modification of icosahedral gold nanoparticles (iAuNPs) decorated with the most abundant protein of the Adv 5 (i.e., hexon protein) at the surface. CTAB-capped iAuNPs with dimensions in the range of 40-90 nm were synthesized, and then CTAB was replaced by a variety of polyethylene glycols (PEGs) in order to introduce suitable functionalities serving as anchoring points for the attachment of the hexon protein. The latter was achieved by non-covalent linking of the protein to the iAuNP surface using a PEG without reactive termination (i.e., methoxy PEG thiol, mPEG-SH, Mn=800). Alternatively, covalent anchoring points were generated by modifying the iAuNPs with a bifunctional PEG (i.e., thiol PEG amine, SH-PEG-NH2) followed by the addition of glutaraldehyde. X-ray photoelectron spectroscopy (XPS) confirmed the formation of the anchoring points at the iAuNP surface. Next, the amino groups present in the amino acids of the hexon protein interacted with the glutaraldehyde. iAuNPs before and after PEGylation were characterized using dynamic light scattering (DLS), XPS, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and UV-Vis spectroscopy, confirming the CTAB-PEG exchange. Finally, the distinct red shift obtained in the UV-Vis spectra of the pegylated iAuNPs in the presence of the hexon protein, the increase in the hydrodynamic diameter, the change in the zeta potential, and the selective binding of the hexon-modified iAuNPs towards a hexon-imprinted polymer (HIP) confirmed success in both the covalent and non-covalent attachment at the iAuNP surface.

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Near-infrared (NIR) light-activated photosensitization represents an encouraging therapeutic method in photodynamic therapy, especially for deep tissue penetration. In this context, two-photon activation, i.e., utilization of photons with relatively low energy but high photon flux for populating a virtual intermediate state leading to an excited state, is attractive. This concept would be highly advantageous in photodynamic therapy due to its minimal side effects. Herein, we propose that the combination of plasma protein serum albumin (HSA) containing several Ru complexes and NIR two-photon excitable carbon nanodots (Cdots), termed HSA-Ru-Cdots, provides several attractive features for enhancing singlet oxygen formation within the mitochondria of cancer cells stimulated by two-photon excitation in the NIR region. HSA-Ru-Cdot features biocompatibility, water solubility, and photostability as well as uptake into cancer cells with an endosomal release, which is an essential feature for subcellular targeting of mitochondria. The NIR two-photon excitation induced visible emission of the Cdots allows fluorescence resonance energy transfer (FRET) to excite the metal-to-ligand charge transfer of the Ru moiety, and fluorescence-lifetime imaging microscopy (FLIM) has been applied to demonstrate FRET within the cells. The NIR two-photon excitation is indirectly transferred to the Ru complexes, which leads to the production of singlet oxygen within the mitochondria of cancer cells. Consequently, we observe the destruction of filamentous mitochondrial structures into spheroid aggregates within various cancer cell lines. Cell death is induced by the long-wavelength NIR light irradiation at 810 nm with a low power density (7 mW/cm2), which could be attractive for phototherapy applications where deeper tissue penetration is crucial.

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Supported gold nanoparticles are widely studied catalysts and are among the most active known for the low-temperature water-gas shift reaction, which is essential in fuel and energy applications, but their practical application has been limited by their poor thermal stability. The catalysts deactivate on-stream via the growth of small Au nanoparticles. Using operando X-ray absorption and in situ scanning transmission electron microscopy, we report direct evidence that this process can be reversed by carrying out a facile oxidative treatment, which redisperses the gold nanoparticles and restores catalytic activity. The use of in situ methods reveals the complex dynamics of supported gold nanoparticles under reaction conditions and demonstrates that gold catalysts can be easily regenerated, expanding their scope for practical application.

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Vanadate compounds, such as V3O7·H2O, are of high interest due to their versatile applications as electrode material for metal-ion batteries. In particular, V3O7·H2O can insert different ions such as Li+, Na+, K+, Mg2+ and Zn2+. In that case, well resolved crystal structure data, such as crystal unit-cell parameters and atom positions, are needed in order to determine the structural information of the inserted ions in the V3O7·H2O structure. In this work, fundamental crystallographic parameters, i.e. atomic displacement parameters, are determined for the atoms in the V3O7·H2O structure. Furthermore, vanadium ions were substituted by molybdenum in the V3O7·H2O structure [(V2.85Mo0.15)O7·H2O] and the crystallographic positions of the molybdenum ions and their oxidation state are elucidated.

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Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc2 ) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while "bulk-like water" regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc2 electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g-1 (based on the AC weight) at a current density of 5 A g-1 . They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%).

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Li-rich layered oxides (LRLO) with specific energies beyond 900 Wh kg-1 are one promising class of high-energy cathode materials. Their high Mn-content allows reducing both costs and the environmental footprint. In this work, Co-free Li1.2 Mn0.6 Ni0.2 O2 was investigated. A simple water and acid treatment step followed by a thermal treatment was applied to the LRLO to reduce surface impurities and to establish an artificial cathode electrolyte interface. Samples treated at 300 °C show an improved cycling behavior with specific first cycle capacities of up to 272 mAh g-1 , whereas powders treated at 900 °C were electrochemically deactivated due to major structural changes of the active compounds. Surface sensitive analytical methods were used to characterize the structural and chemical changes compared to the bulk material. Online DEMS measurements were conducted to get a deeper understanding of the effect of the treatment strategy on O2 and CO2 evolution during electrochemical cycling.

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As part of an extensive study of the interaction between Zn and Au in Zn/Au(111) model systems, we have systematically investigated the low-temperature (LT) nucleation and growth behavior of Zn on the Au(111) surface as well as the thermal stability of the resulting structures toward sintering, intermixing, and dissolution by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). Zn deposition at LT, at 105 K (STM) or 80 K (XPS), leads to nucleation and two-dimensional growth of Zn islands mainly at the elbows of the Au(111) herringbone reconstruction, with a slight preference for island formation at pinched-in (pi) rather than bulged-out (bu) elbows. Local surface intermixing during LT Zn deposition leads to local perturbations of the Au(111) herringbone reconstruction, which results in the formation of additional nucleation sites (edge sites). At higher coverages (>0.11 ML), island coalescence sets in. Testing the thermal stability by annealing experiments, we find the structures to be stable up to about 200 K, while at higher temperatures, 2D sintering, intermixing, and dissolution set in, with subtle differences between pi- and bu-elbow islands. This indicates largely comparable activation barriers for the underlying (surface-)diffusion and exchange processes. Upon annealing to 330 K, all island structures dissolved. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and can be considered to be typical of metal deposition systems with comparable barriers for 2D Zn detachment/sintering and intermixing/bulk diffusion.

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Hydrogen production from electrocatalytic water splitting in electrolyzers is a key process to store excess electric energy produced from intermittent renewable energy sources. For proton exchange membrane (PEM) electrolyzers, carbon supported platinum particles exhibit the highest rates for the hydrogen evolution reaction (HER); however, high Pt costs limit the wide spread use of this technology. By employing a graphene layer grown on a Ru(0001) single crystal as a template for Pt nanocluster (NC) growth, we studied the dependence of the HER activity on the NC size using NCs of different sizes. We provide clear quantitative experimental evidence for a volcano-like relationship between the HER activity and the NC size which has been missing so far. For Pt NCs with very low sizes below 2 nm, we found stunningly improved exchange HER current densities. The highest exchange current density was observed for Pt NCs with an average size of ca. 38 atoms. These Pt38 NCs do not only surpass the Pt-mass-specific activity of commercial Pt electrode materials by well above three orders of magnitude, also their exchange current density is located close to the maximum exchange current density for the HER predicted theoretically for transition metal surfaces. The present work provides a strong stimulus for future research towards technically feasible Pt NC catalysts with cluster sizes in the range of few tens of Pt atoms.

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Ru/Al2 O3 is a highly stable, but less active catalyst for methanation reactions. Herein we report an effective approach to significantly improve its performance in the methanation of CO2 /H2 mixtures. Highly active and stable Ru/γ-Al2 O3 catalysts were prepared by high-temperature treatment in the reductive reaction gas. Operando/in situ spectroscopy and STEM imaging reveals that the strongly improved activity, by factors of 5 and 14 for CO and CO2 methanation, is accompanied by a flattening of the Ru nanoparticles and the formation of highly basic hydroxylated alumina sites. We propose a modification of the metal-support interactions (MSIs) as the origin of the increased activity, caused by modification of the Al2 O3 surface in the reductive atmosphere and an increased thermal mobility of the Ru nanoparticles, allowing their transfer to modified surface sites.

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The development of sensitive biosensors, such as gallium nitride (GaN)-based quantum wells, transistors, etc., often makes it necessary to functionalize GaN surfaces with small molecules or even biomolecules, such as proteins. As a first step in surface functionalization, we have investigated silane adsorption, as well as the formation of very thin silane layers. In the next step, the immobilization of the tetrameric protein streptavidin (as well as the attachment of chemically modified iron transport protein ferritin (ferritin-biotin-rhodamine complex)) was realized on these films. The degree of functionalization of the GaN surfaces was determined by fluorescence measurements with fluorescent-labeled proteins; silane film thickness and surface roughness were estimated, and also other surface sensitive techniques were applied. The formation of a monolayer consisting of adsorbed organosilanes was accomplished on Mg-doped GaN surfaces, and also functionalization with proteins was achieved. We found that very high Mg doping reduced the amount of surface functionalized proteins. Most likely, this finding was a consequence of the lower concentration of ionizable Mg atoms in highly Mg-doped layers as a consequence of self-compensation effects. In summary, we could demonstrate the necessity of Mg doping for achieving reasonable bio-functionalization of GaN surfaces.

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As part of an extensive effort to explore the function of Au/ZnO catalysts in the synthesis of methanol from CO2 and H2, we have systematically investigated the temperature dependent growth, structure formation, and surface intermixing of Zn on the herringbone reconstructed Au(111) surface and the thermal stability of the resulting surfaces by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). After Zn deposition at low temperatures, at about 105 K (STM) or below (XPS), we observed nucleation and two-dimensional growth of Zn islands mainly at the elbow sites of the Au(111) herringbone reconstruction. This results in local perturbations of the reconstruction pattern of the Au(111) substrate, which can create additional nucleation sites. XPS data indicate that Zn dissolution into deeper layers is kinetically hindered under these conditions, while local exchange with the Au surface layer, in particular at the elbow sites during nucleation, cannot be excluded. Zn deposition at room temperature, in contrast, results in near-surface alloy formation with a strongly distorted pattern of the herringbone reconstruction and condensation of the Zn and exchanged Au adatoms at ascending steps, together with some loss of Zn into deeper layers. Upon annealing, Zn atoms diffuse to lower layers and eventually to the Au bulk, and the surface successively regains its original Au(111) herringbone structure, which is almost reached after 500 K annealing. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and near-surface alloy formation.

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The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3 O4 (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3 O4 (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O2 ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3 O4 (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 VRHE (RHE: reversible hydrogen electrode) at 0.1 mA cm-2 was almost unchanged.

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The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar+ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.

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Amorphous and graphitized nitrogen-doped (N-doped) carbon spheres are investigated as structurally well-defined model systems to gain a deeper understanding of the relationship between synthesis, structure, and their activity in the oxygen reduction reaction (ORR). N-doped carbon spheres were synthesized by hydrothermal treatment of a glucose solution yielding carbon spheres with sizes of 330 ± 50 nm, followed by nitrogen doping via heat treatment in ammonia atmosphere. The influence of a) varying the nitrogen doping temperature (550-1000 °C) and b) of a catalytic graphitization prior to nitrogen doping on the carbon sphere morphology, structure, elemental composition, N bonding configuration as well as porosity is investigated in detail. For the N-doped carbon spheres, the maximum nitrogen content was found at a doping temperature of 700 °C, with a decrease of the N content for higher temperatures. The overall nitrogen content of the graphitized N-doped carbon spheres is lower than that of the amorphous carbon spheres, however, also the microporosity decreases strongly with graphitization. Comparison with the electrocatalytic behavior in the ORR shows that in addition to the N-doping, the microporosity of the materials is critical for an efficient ORR.

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Employing Pt(111) supported 2D Pt-core Au-shell model catalysts, we demonstrate that 2D core-shell surfaces prepared under ultrahigh vacuum (UHV) conditions constitute excellent model systems to determine the activity of step sites in electrocatalysis, especially because UHV-scanning tunneling microscopy (STM) enables control of the quality of narrow step modifications with high accuracy on such systems. As verified with STM, cyclic voltammetry (CV), and temperature-programmed desorption (TPD) measurements, this approach allows us (i) to increase the step density by homoepitaxial growth of monolayer high islands on the respective single crystal and (ii) to modify the step sites for adsorption of reactants by selective deposition of a guest metal. Herein, STM imaging in combination with electrochemical characterization provides a direct control to ascertain a selective modification of the entire steps. Comparing the electrocatalytic activity of 2D core-shell systems with and without the shell enables us to identify the activity of step sites for electrocatalytic reactions, as demonstrated for the bulk CO electro-oxidation.

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Utilization of high-capacity alloying anodes is a promising yet extremely challenging strategy in building high energy density alkali-ion batteries (AIBs). Excitingly, it was very recently found that the (de-)sodiation of tin (Sn) can be a highly reversible process in specific glyme electrolytes, enabling high specific capacities close to the theoretical value of 847 mA h g-1. The unique solid electrolyte interphase (SEI) formed on Sn electrodes, which allows highly reversible sodiation regardless of the huge volume expansion, is herein demonstrated according to a series of in situ and ex situ characterization techniques. The SEI formation process mainly involves NaPF6 decomposition and the polymerization/oligomerization of the glyme solvent, which is induced by the catalytic effect of tin, specifically. This work provides a paradigm showing how solvent, salt, and electrode materials synergistically mediate the SEI formation process and obtains new insights into the unique interfacial chemistry between Na-alloying electrodes and glyme electrolytes, which is highly enlightening in building high energy density AIBs.

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Aiming at a better understanding of correlations between the activity and selectivity of Au electrodes in the oxygen reduction reaction (ORR) under controlled transport conditions, we have investigated this reaction by combined electrochemical and in situ FTIR measurements, performed in a flow cell set-up in an attenuated total reflection (ATR) configuration in acid and alkaline electrolytes. The formation of incomplete reduction products (hydrogen peroxyde/peroxyls) was detected by a collector electrode, the onset of OHad formation was probed by bulk CO oxidation. Using an electroless-deposited, annealed Au film on a Si prism as working electrode and three different electrolytes for comparison (sulfuric acid, perchloric acid, sodium hydroxide solution), we could derive detailed information on the anion adsorption behavior, and could correlate this with the ORR characteristics. The data reveal pronounced effects of the anions and the pH on the ORR characteristics, indicated e. g., by a grossly different activity and selectivity for the 4-electron pathway to water/hydroxyls, with the onset ranging from ca. 1.0 V in alkaline electrolyte to 0.6 V in sulfuric acid electrolyte, and the selectivity for the 4-electron pathway ranging from 100 % (alkaline electrolyte, low overpotentials) to 40 % (acidic electrolytes, alkaline electrolyte at high overpotentials). In contrast, the effect of the ORR on the anion adsorption characteristics is small. Anion effects as well as correlations between anion adsorption and ORR are discussed.

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Aiming at a detailed molecular understanding of the initial stage of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, we have investigated the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) (solvent/electrolyte) and Li (Li+ ion shuttle) on well-defined Li-poor Li4Ti5O12(111) and Li-rich Li4.3Ti5O12(111) surfaces/electrodes in a combined surface science and electrochemical model study. X-ray photoelectron spectroscopy (XPS) measurements reveal that postdeposition of Li0 under ultrahigh vacuum (UHV) conditions on a Li-poor Li4Ti5O12(111) surface precovered with a molecularly adsorbed [BMP][TFSI] adlayer leads to little IL decomposition at 80 and 300 K. We assume that most of the Li diffuses through the IL adlayer and rapidly inserts into the Li4Ti5O12(111) bulk. More pronounced IL decomposition was obtained upon IL deposition on a Li-rich Li4.3Ti5O12 phase at 80 K and subsequent heating to 300 K. Cyclic voltammograms (CVs) recorded on the Li4Ti5O12(111) electrodes in Li-TFSI/[BMP][TFSI] indicate an almost reversible Li (de-)insertion, with a slight decay of the amount of (de-)inserted Li with increasing cycle number. XPS measurements performed on the electrode after potential cycling show low intensity signals of IL decomposition products, in addition to dominant signals from residual IL electrolyte, which are related to reaction of the adsorbed IL with Li inserted into/extracted from Li4Ti5O12 during the CV. The results indicate a close similarity between IL decomposition products formed under UHV and under electrochemical conditions, underlining the validity of this experimental approach and the potential of such kind of model studies for obtaining detailed understanding of the SEI formation.