Antimicrobial effects of silver nanoparticle-microspots on the mechanical properties of single bacteria.

Silver nanoparticles (AgNPs) conjugated with polymers are well-known for their powerful and effective antimicrobial properties. In particular, the incorporation of AgNPs in biocompatible catecholamine-based polymers, such as polydopamine (PDA), has recently shown promising antimicrobial activity, due to the synergistic effects of the AgNPs, silver(I) ions released and PDA. In this study, we generated AgNPs-PDA-patterned surfaces by localised electrochemical depositions, using a double potentiostatic method via scanning electrochemical cell microscopy (SECCM). This technique enabled the assessment of a wide parameter space in a high-throughput manner. The optimised electrodeposition process resulted in stable and homogeneously distributed AgNP-microspots, and their antimicrobial activity against Escherichia coli was assessed using atomic force microscopy (AFM)-based force spectroscopy, in terms of bacterial adhesion and cell elasticity. We observed that the bacterial outer membrane underwent significant structural changes, when in close proximity to the AgNPs, namely increased hydrophilicity and stiffness loss. The spatially varied antimicrobial effect found experimentally was rationalised by numerical simulations of silver(I) concentration profiles.

Icosahedral gold nanoparticles decorated with hexon protein: a surrogate for adenovirus serotype 5.

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.

Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal-Organic Framework.

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.

On-Chip Direct Laser Writing of PAN-Based Carbon Supercapacitor Electrodes.

The carbonization of polyacrylonitrile (PAN) by direct laser writing to produce microsupercapacitors directly on-chip is reported. The process is demonstrated by producing interdigitated carbon finger electrodes directly on a printed circuit board (PCB), which is then employed to characterize the supercapacitor electrodes. By varying the laser power, the process can be tuned from carbonization to material ablation. This allows to not only convert pristine PAN films into carbon electrodes, but also to pattern and cut away non-carbonized material to produce completely freestanding carbon electrodes. While the carbon electrodes adhere well to the printed circuit board, non-carbonized PAN is peeled off the substrate. Specific capacities as high as 260 µF cm⁻2 are achieved in a supercapacitor with 16 fingers.

CO Oxidation on Planar Au/TiO2 Model Catalysts under Realistic Conditions: A Combined Kinetic and IR Study.

The oxidation of CO on planar Au/TiO2 model catalysts was investigated under pressure and temperature conditions similar to those for experiments with more realistic Au/TiO2 powder catalysts. The effects of a change of temperature, pressure, and gold coverage on the CO oxidation activity were studied. Additionally, the reasons for the deactivation of the catalysts were examined in long-term experiments. From kinetic measurements, the activation energy and the reaction order for the CO oxidation reaction were derived and a close correspondence with results of powder catalysts was found, although the overall turnover frequency (TOF) measured in our experiments was around one order of magnitude lower compared to results of powder catalysts under similar conditions. Furthermore, long-term experiments at 80 °C showed a decrease of the activity of the model catalysts after some hours. Simultaneous in-situ IR experiments revealed a decrease of the signal intensity of the CO vibration band, while the tendency for the build-up of side products (e. g. carbonates, carboxylates) of the CO oxidation reaction on the surface of the planar model catalysts was rather low.

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies.

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO2 proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO2 reduction. Here, by employing findings from operando X-ray absorption spectroscopy at the near and extended Au and Ce LIII energy edges, we focus on the fundamental aspects of highly active Au/CeO2 catalysts, mainly in the CO oxidation for understanding their complex structure-reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O2 activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO2 support, and the dynamics of O2 activation.

Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co3 O4 (111) Thin Films.

In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), Li and a Co3 O4 (111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li-ion batteries (LIBs). We employed mostly X-ray photoelectron spectroscopy (XPS) in combination with dispersion-corrected density functional theory calculations (DFT-D3). If the surface is pre-covered by BMP-TFSI species (model electrolyte), post-deposition of Li (Li+ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li2 NSO2 CF3 , LiF, Li2 S, Li2 O2 , Li2 O, but also kinetic products like Li2 NCH3 C4 H9 or LiNCH3 C4 H9 of BMP. Simultaneously, Li adsorption and/or lithiation of Co3 O4 (111) to Lin Co3 O4 takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co0 could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Low-temperature nucleation and growth of Zn on Au(111) and thermal stability toward (surface) alloy formation.

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.

Adlayer growth vs spontaneous (near-) surface alloy formation: Zn growth on Au(111).

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.

Raising the COx Methanation Activity of a Ru/γ-Al2 O3 Catalyst by Activated Modification of Metal-Support Interactions.

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.

Morphology-Engineered Highly Active and Stable Ru/TiO2 Catalysts for Selective CO Methanation.

Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2 - and H2 -rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non-deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2 -{100} and Ru/TiO2 -{101} are very stable, while Ru/TiO2 -{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal-support interactions (MSIs). The stronger MSIs on the defect-rich TiO2 -{100} and TiO2 -{101} supports stabilize flat Ru nanoparticles, while on TiO2 -{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2 -{001}.

Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode | electrolyte interface in Li-ion batteries.

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO2(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li3N, Li2S, LixSOy, and Li2O. In the absence of a [BMP]+[TFSI]- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO2) above a critical temperature, forming LiOx and Ti3+ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li+ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

Direct observation of magnetic metastability in individual iron nanoparticles.

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.

Switchable Polyacrylonitrile-Copolymer for Melt-Processing and Thermal Carbonization-3D Printing of Carbon Supercapacitor Electrodes with High Capacitance.

Polyacrylonitrile (PAN) represents the most widely used precursor for carbon fibers and carbon materials. Carbon materials stand out with their high mechanical performance, but they also show excellent electrical conductivity and high surface area. These properties render carbon materials suitable as electrode material for fuel cells, batteries, and supercapacitors. However, PAN has to be processed from solution before being thermally converted to carbon, limiting its final format to fibers, films, and non-wovens. Here, a PAN-copolymer with an intrinsic plasticizer is presented to reduce the melting temperature and avoid undesired entering of the thermal carbonization regime. This plasticizer enables melt extrusion-based additive manufacturing (EAM). The plasticizer in the PAN-copolymer can be switched to increase the melting temperature after processing, allowing the 3D-melt-printed workpiece to be thermally carbonized after EAM. Melt-processing of the PAN copolymer extends the freedom-in-design of carbon materials to mold-free rapid prototyping, in the absence of solvents, which enables more economic and sustainable manufacturing processes. As an example for the capability of this material system, open meshed carbon electrodes are printed for supercapacitors that are metal- and binder-free with an optimized thickness of 1.5 mm and a capacitance of up to 387 mF cm-2 .

The adsorption of oxygen and coadsorption of CO and oxygen on structurally well-defined PdAg surface alloys.

The dissociative interaction of oxygen with structurally well-defined monolayer Pd(x)Ag(1-x)/Pd(111) surface alloys of different compositions, with well-known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012, 14, 10754-10761), and the coadsorption of/reaction with CO on oxygen pre-covered surfaces were studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd(67)Ag(33)(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold-hollow sites on Pd(3) sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.

Reversible Growth of Gold Nanoparticles in the Low-Temperature Water-Gas Shift Reaction.

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.

Enhanced Electrochemical Capacity of Spherical Co-Free Li1.2 Mn0.6 Ni0.2 O2 Particles after a Water and Acid Treatment and its Influence on the Initial Gas Evolution Behavior.

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.

Coadsorption of hydrogen and CO on hydrogen pre-covered PtRu/Ru(0001) surface alloys.

The influence of pre-adsorbed hydrogen on the adsorption of CO on well-defined PtRu/Ru(0001) surface alloys and the modification of the adsorption properties of both adsorbate species due to coadsorption was studied by a combination of temperature-programmed desorption and infrared spectroscopic measurements. The hydrogen binding strength is weakened both by surface alloy formation and by CO coadsorption. In the densely packed coadsorbate layers, which can be formed after CO post-adsorption, the adsorption properties of the adsorbed CO molecules are also affected significantly. Depending on the Pt surface concentration, the CO adsorption process on hydrogen pre-covered surfaces is blocked to a different extent with a decreasing blocking tendency for Pt-rich alloys.

From adlayer islands to surface alloy: structural and chemical changes on bimetallic PtRu/Ru(0001) surfaces.

The correlation between structural and chemical properties of bimetallic PtRu/Ru(0001) model catalysts and their modification upon stepwise annealing of a submonolayer Pt-covered Ru(0001) surface up to the formation of an equilibrated Pt(x)Ru(1-x)/Ru(0001) monolayer surface alloy was investigated by scanning tunneling microscopy and by the adsorption of CO and D(2) probe molecules. Both temperature-programmed desorption and IR measurements demonstrate the influence of the surface structure on the adsorption properties of the bimetallic surface, which can be explained by changes of the composition of the adsorption ensembles (ensemble effects) for D adsorption and by changes in the electronic interaction (ligand effects, strain effects) of the metallic constituents for CO and D adsorption upon alloy formation.

Planar Au/TiO2 model catalysts: fabrication, characterization and catalytic activity.

Different types of planar Au/TiO(2) model catalysts are produced on TiO(2)(110) single-crystal substrates and thin TiO(2) films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO(2)(110) model catalysts with a stoichiometric TiO(2)(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO(2) model catalysts with thin TiO(2) films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO(2)(110) substrates on the CO oxidation activity is discussed.