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An experimental and theoretical study of RuO2/TiO2 heterostructures as photoelectrocatalysts for hydrogen evolution reaction (HER).

Heterostructured RuO2–Fe2O3 nanocomposite can be viewed as a bi-functional electrocatalyst for HER and OER because it exploits the synergistic effects of heterostructures and active sites at its interface.

Li metal electrodes have the potential to increase the energy density of conventional lithium-ion batteries by up to 60%. However, challenges regarding the cycling efficiency and safety of these electrodes still remain. Effective surface treatments seem to be a promising approach for resolving issues such as dendrite formation or excessive reaction with the electrolyte. LiF has been reported to exhibit most of the properties required of a coating material in this context, offering high electro-/chemical stability, mechanical strength, high surface energies, etc. Nevertheless, contradicting reports on the effectiveness of LiF coatings to improve the cycling efficiency of Li metal electrodes raise doubts on its effectiveness to protect the Li metal surface. The mechanism of protection for this material is also not well understood. Thus, a comparative study was conducted, selecting three different approaches for the synthesis of a LiF coating on Li metal and following with detailed electrochemical analysis. Using the same experimental setup for all samples enabled direct comparison between the selected coatings, and subsequently, a reliable evaluation on the effectiveness of LiF as protective coating.

The design of active and selective co-catalysts constitutes one of the major challenges in developing heterogeneous photocatalysts for energy conversion applications. This work provides a comprehensive insight into thermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. We demonstrate that the volcano-type HER profile as a function of calcination temperature is independent of the type of the Cu precursor but is affected by changes in oxidation state and location of the copper species. Supported by DFT modeling, our data suggest that low temperature (200 °C) do not affect the Cu oxidation state, but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance. This work examines electronic and structural effects that are in control of the photocatalytic activity and can be transferred to other systems for further advancing photocatalysis., Calcination of Cu/TiO2 is detrimental to its photocatalytic HER performance. We relate this to heat-activated Cu diffusion into the TiO2 lattice, which decreases accessibility of the Cu sites on the surface and generates charge recombination centers.

Water electrolysis is one of the most capable processes for supplying clean fuel. Herein, two novel ionic Ru(II)-Fe(II) complexes, [Ru(tmphen)

In this study we investigate how electronic properties of buried semiconductor/ionic liquid interfaces may influence on charging effects observed by X-ray photoelectron spectroscopy (XPS). Droplets of the ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) were deposited on silicon and germanium semiconductor surfaces with different doping states. Core level spectra were recorded from the ionic liquid/ultra-high vacuum interface with and without irradiation of the droplets using low energy electrons from a flood gun. Changing electron intensity results in shifts of the binding energy of all XPS signals to lower values. Magnitude of shifts is found to directly depend on the intensity of the electron beam and to correlate with the nominal resistivity of the substrates. The observations are qualitatively explained by flood gun induced changes of the electronic properties of the [EMIM][Tf2N]/substrate interface and suggested to refer to space charge layer effects.

L-Valine-based oligopeptides with the chemical structure Z–(L-Val)3–OMe and Z–(L-Val)2–L-Cys(S-Bzl)–OMe form stable organogels in butanol. Both peptides are efficient gelators, but Z–(L-Val)2–L-Cys(S-Bzl)–OMe crystallizes more readily than Z–(L-Val)3–OMe. The two peptides can form mixed fibers, which also gel butanol. The resulting organogels are very similar to oligovaline organogels reported earlier (Mantion and Taubert, Macromol. Biosci., 2007, 7, 208) as they also form highly ordered peptide fibers with a predominant β-sheet structure and diameters of ca. 100 nm. The fibers can be mineralized with silver nanoparticles using DMF as a reducing agent. The fraction of the sulfur-containing peptide Z–(L-Val)2–L-Cys(S-Bzl)–OMe controls the shape and size of the resulting nanoparticles. At high Z–(L-Val)2–L-Cys(S-Bzl)–OMe content, small spherical particles are distributed all over the fiber. Lower contents of Z–(L-Val)2–L-Cys(S-Bzl)–OMe lead to a size increase of the particles and to more complex shapes like plate-like and raspberry-like silver particles. The interactions between peptide and silver ions or silver particles takes place via a complexation of the silver ions to the sulfur atom of the thioether moiety, and are shown to be the key interaction in controlling particle formation.

In the absence of a protecting atmosphere, the surfaces of rocky bodies in the solar system are affected by significant space weathering due to the exposure to the solar wind [1]. Fundamental knowledge of space weathering effects, such as optical changes of surfaces as well as the formation of an exosphere is essential for gaining insights into the history of planetary bodies in the solar system [2]. Primarily the exospheres of Mercury and Moon are presently of great interest and the interpretation of their formation processes relies on the understanding of all space weathering effects on mineral surfaces.Sputtering of refractory elements by solar wind ions is one of the most important release processes. We investigate solar wind sputtering by measuring and modelling the sputtering of pyroxene samples as analogues for the surfaces of Mercury and Moon [3, 4]. These measurements with thin film samples on Quartz Crystal Microbalance (QCM) substrates allow recording of sputtering yields in-situ and in real time [5]. For the simulation of kinetic sputtering from the ion-induced collision cascade we use the software SDTrimSP with adapted input parameters that consistently reproduce measured kinetic sputtering yields [4, 6].This study focuses on investigating the potential sputtering of insulating samples by multiply charged ions [7]. Changes of these sputtering yields with fluence are compared to calculations with a model based on inputs from SDTrimSP simulations. This leads to a very good agreement with steady-state sputtering yields under the assumption that only O atoms are sputtered by the potential energy of the ions. The observed decreasing sputtering yields can be explained by a partial O depletion on the surface [4]. Based on these findings expected surface composition changes and sputtering yields under realistic solar wind conditions can be calculated. Our results are in line with previous investigations (see e.g. [8, 9]), creating a consistent view on solar wind sputtering effects from experiments to established modelling efforts. References:[1] B. Hapke, J. Geophys. Res.: Planets, 106, 10039 (2001).[2] P. Wurz, et al., Icarus, 191, 486 (2007).[3] P.S. Szabo, et al., Icarus, 314, 98 (2018).[4] P.S. Szabo, et al., submitted to Astrophys. J. (2020).[5] G. Hayderer, et al., Rev. Sci. Instrum., 70, 3696 (1999).[6] A. Mutzke, et al., “SDTrimSP Version 6.00“, IPP Report, (2019).[7] F. Aumayr, H. Winter, Philos. Trans. R. Soc. A, 362, 77 (2004).[8] H. Hijazi, et al., J. Geophys. Res.: Planets, 122, 1597 (2017).[9] S.T. Alnussirat, et al., Nucl. Instrum. Methods Phys. Res. B, 420, 33 (2018).

Bodies in space without a dense atmosphere are affected by several erosive space weathering processes [1, 2]. These processes are responsible for modifications of surface properties as well as for the formation of an exosphere. During the BepiColombo mission, an on-board mass spectrometer will probe this exosphere on Mercury. Knowledge the different processes causing its formation is crucial for the interpretation of the obtained mass spectrometry data [3, 4]. Sputtering by solar wind ions is expected to be one of the key drivers of the particle release that leads to the formation of the exosphere. In addition, these ions can modify the regolith, become implanted and are released into the exosphere [5, 6].We emulate solar wind precipitating onto the surface of Mercury by irradiating analogue material with mass over charge selective ion sources. For this study in particular, a magnesium rich augite (Ca,Fe)(Mg,Fe)[Si2O6] sample was used as analogue. The material is deposited onto a Quartz Crystal Microbalance (QCM) as thin film. Chemical composition and thickness of these films was investigated by means of ion-beam analysis [7]. The QCM-technique allows for real time measurements of mass changes during the experiments. This approach enables us to determine sputter yields due to ion impact, as well as projectile implantation and rerelease. Furthermore, the target is heatable and desorbed atoms can be analyzed with a quadrupole mass spectrometer. The setup thus allows for Thermal Desorption Spectroscopy (TDS) measurements. Together with the QCM, temperature dependence of the projectile rerelease and the total mass change during heating cycles can be determined.Irradiation of the targets with He+ at solar wind energies of 4keV were performed and simulated using the program SDTrimSP [8, 9]. A significant amount of helium is implanted upon irradiation, leading to a fluence-dependent mass change rate. When an equilibrium of implantation and rerelease is reached the mass loss due to sputtering of target material is observed. This saturation happens after a fluence of about 1021 ions per m2, which corresponds to an irradiation by the solar wind of several hundred years on the surface of Mercury [3]. The study shows that helium is mobile during ion irradiation, and released thermally from the sample at about 400K. Combining TDS results and measurements of the mass change, the helium volume abundance after saturation was estimated to about 10-15%. The results of this study therefore provide a more detailed understanding of the interaction between helium from solar wind and Mercury analogues.References:[1] Hapke B.: J. Geophys. Res. Planet., 106, 10039, 2001.[2] Noble S.K., et al.: Sol. Syst. Res. 37, 31, 2003.[3] Wurz P., et al.: Planet. Space Sci., 58, 1599, 2010.[4] Benkhoff J., et al.: Planet. Space Sci. 58, 2, 2010.[5] Sasaki S., et al.: Adv. Space Res., 33, 2152, 2004.[6] Hartle R., et al.: J. Geophys. Res., 80, 3689, 1975.[7] Moro M.V., et al.: Thin Solid Films, 686, 137416, 2019.[8] Mutzke A., et al.: IPP Report, 2019.[9] Szabo P.S., et al.: Icarus, 314, 98, 2018.

The ionic liquid [EMIM][TFSI] was investigated by X-ray Photoelectron Spectroscopy (XPS) on different substrates, ranging from metallic to semiconductor type materials. Parallel angle-resolved XPS was performed utilising a wide-angle lens hemispherical analyser with an acceptance angle of 60°. ARXPS data show no preferential orientation/accumulation of certain species at the ionic liquid/ultra-high vacuum interface. Charging caused by the electron emission process and electron flood gun experiments was recorded. The data indicate that the binding energies of the spectra depend on the properties of the substrate material, specifically on the substrate/ionic liquid interface. Observations are discussed by means of electric double layer and other processes that may effect on the potential of the ionic liquid.

In this work, the supported ionic liquid phase (SILP) method was applied for the immobilization of a newly developed, well-defined hydride Fe(II) PNP pincer complex dissolved in ionic liquid (IL) onto polymer-based spherical activated carbon. This novel SILP catalyst was structurally characterized by electron microscopy, N2 adsorption–desorption, FTIR, and XPS measurements and was used for the hydrogenation of aldehydes to alcohols. For an optimized pore filling degree, this system showed excellent activity in the chemoselective hydrogenation of different aldehydes and proved to be reusable in at least seven consecutive reaction cycles without any loss in activity. Significantly lower reaction rates were observed, however, compared to a recent study of the same catalyst supported on silica, which was ascribed to the different pore sizes of these two support materials.

New dye functionalized polymeric carbon nitrides with increased visible light response were designed and fabricated. In this research, Rose Bengal (RB) was immobilized on g-C3N4 (CN) and carboxylated g-C3N4 (CCN) through the formation of a covalent bond. The covalent bonding of Rose Bengal to the CN or CCN resulted in the transfer of electrons from the dye to the semiconductor and the enhancement of the light harvesting performance, which promoted the photocatalytic efficiency. The photocatalytic activity of CN, CCN, and their hybrids with RB toward selective oxidation of benzyl alcohol was investigated under the visible light irradiation. The results obtained from the GC–MS analyses related to the oxidation reaction displayed the highest percentage of benzyl alcohol conversion and benzaldehyde selectivity for the hybrid structures compared to the bare RB, CN, CCN, or a mixture of RB and CN (or CCN). The effect of several parameters, including the amount of photocatalyst and promoter, irradiation time, and photocatalyst type, was studied and optimized. The recovery of RB-CCN (the most active catalyst) was surveyed in five runs, and the results verify that the activity is mostly maintained. Finally, the optimized structure, energy gap, and frontier orbitals of Rose Bengal/graphitic carbon nitride were investigated by DFT methods.

This work focuses on the wettability transition of nodular cast iron (NCI) modified using a femtosecond laser-produced micro/nano-structure. Laser parameters such as laser fluences (0.10 to 3.18 J/cm2), and scan speed (0.2 to 1.0 mm/s) were varied to realize hierarchical or dual-phase structures. Raman spectroscopy was performed on samples treated at 0.10 to 0.63 J/cm2 to investigate the mechanism involved in the disappearance of the graphite nodule with increasing laser fluence. Just-after the laser treatment, surface was superhydrophilic in nature and progressively became superhydrophobic after cleaning and subsequent application of vacuum pressure for 8 h. In the present investigation, a static contact angle (SCA) of 164 ± 4° was measured. The temperature-dependent durability of the superhydrophobic surface was analyzed by heating at a temperature range from 80 °C to 240 °C. The results showed a stable superhydrophobic characteristic up to 120 °C even after five cycles of repetition. Scanning electron microscope (SEM) analysis was conducted to capture the micrograph of structures. Moreover, the chemisorption mechanism during the vacuum process and over the period was analyzed using X-ray photoelectron spectroscopy (XPS) that indicated a higher concentration of non-polar functional groups on the surface could effectively increase the SCA.

We investigated the effect of different electrolyte additives such as vinyl carbonate (VC), succinic anhydride (SA), and lithium bis(oxalato)borate (LiBOB) on the chemical composition of Solid Electrolyte Interphase (SEI) layer and the electrochemical performance of Si/MC electrodes. The electrochemical behavior of the Si/MC electrodes was evaluated by galvanostatic charge/discharge and rate capability tests. It was found that various electrolyte compositions result in the formation and the resulting properties of the SEI of the electrodes with different compounds. From X-ray Photoelectron Spectroscopy (XPS) analysis, the electrodes cycled in SA- and LiBOB-containing electrolytes show a higher amount of lithium carbonates and a thicker SEI layer than those in standard and VC-containing electrolyte. The electrode with VC 5w% shows an excellent reversible capacity of ∼1100 mAh g −1 up to 100 charge-discharge cycles.

Although N-heterocyclic carbenes (NHCs) have been demonstrated as suitable ligands for the stabilisation of gold nanoparticles (AuNPs) through a variety of methods, the manner in which such AuNPs form is yet to be fully elucidated. We report a simple and fast one-step synthesis of uniform chiral (L/D)-histidin-2-ylidene stabilised gold nanoparticles using the organometallic Au(I) complex as a well defined starting material. The resulting nanoparticles have an average size of 2.35 ± 0.43 nm for the L analog whereas an average size of 2.25 ± 0.39 nm was found for the D analog. X-ray photoelectron spectroscopy analyses reveal the presence of Au(I) and Au(0) in all NHC stabilised AuNPs. In contrast, measured X-ray photoelectron spectra of dodecanethiol protected gold nanoparticles showed only the presence of a Au(0) species. This observation leads us to postulate that AuNPs synthesised from organometallic NHC–Au(I) complexes exhibit a monolayer of Au(I) surrounding a Au(0) core. This work highlights the importance of synthetic method choice for NHC-stabilized AuNPs, as this could determine Au oxidation states and resulting AuNP properties such as catalytic activities and stabilities.

Correction for ‘Elucidating the formation and active state of Cu co-catalysts for photocatalytic hydrogen evolution’ by Jasmin S. Schubert et al., J. Mater. Chem. A, 2021, DOI: 10.1039/d1ta05561e.

Silicon/mesoporous carbon (Si/MC) composites with optimum Si content, in which the volumetric energy density would be maximized, while volume changes would be minimized, have been developed. The composites were prepared by dispersing Si nanoparticles in a phenolic resin as a carbon source, subsequent carbonization, and etching with hydrofluoric acid (HF). Special attention was paid to understanding the role of HF etching as post-treatment to provide additional void spaces in the composites. The etching process was shown to reduce the SiO2 native layer on the Si nanoparticles, resulting in increased porosity in comparison to the non-etched composite material. For cell optimization, vinylene carbonate (VC) was employed as an electrolyte additive to build a stable solid electrolyte interphase (SEI) layer on the electrode. The composition of the SEI layer on Si/MC electrodes, cycled with and without VC-containing electrolytes for several cycles, was then comprehensively investigated by using ex-situ XPS. The SEI layers on the electrodes working with VC-containing electrolyte were more stable than those in configurations without VC, this explains why our sample with VC exhibits lower irreversible capacity losses after several cycles. The optimized Si/MC composites exhibit a reversible capacity of ~800 mAhg&minus, 1 with an average coulombic efficiency of ~99 % over 400 cycles at C/10.

Pyroxenes ((Ca, Mg, Fe, Mn)2Si2O6) belong to the most abundant rock forming minerals that make up the surface of rocky planets and moons. Therefore, sputtering of pyroxenes by solar wind ions has to be considered as a very important process for modifying the surface of planetary bodies. This is increased due to potential sputtering by multiply charged ions; to quantify this effect, sputtering of wollastonite (CaSiO3) by He2+ ions was investigated. Thin films of CaSiO3 deposited on a quartz crystal microbalance were irradiated, allowing precise, in situ, real time sputtering yield measurements. Experimental results were compared with SDTrimSP simulations, which were improved by adapting the used input parameters. On freshly prepared surfaces, He2+ ions show a significant increase in sputtering, as compared to equally fast He+ ions. However, the yield decreases exponentially with fluence, reaching a lower steady state after sputtering of the first few monolayers. Experiments using Ar8+ ions show a similar behavior, which is qualitatively explained by a preferential depletion of surface oxygen due to potential sputtering. A corresponding quantitative model is applied, and the observed potential sputtering behaviors of both He and Ar are reproduced very well. The results of these calculations support the assumption that mainly O atoms are affected by potential sputtering. Based on our findings, we discuss the importance of potential sputtering for the solar wind eroding the lunar surface. Estimated concentration changes and sputtering yields are both in line with previous modeling for other materials, allowing a consistent perspective on the effects of solar wind potential sputtering.

The influence of surface morphology modifications on the sputtering yield of thin Fe films by monoenergetic Ar ions is studied by using a highly sensitive quartz crystal microbalance (QCM) technique. The morphology changes are induced by prolonged sputtering up to a total Ar fluence of 8 × 1021 m−2. Atomic force microscopy (AFM) measurements are performed to analyse the sample topography before and after irradiation and to determine surface roughness parameters. Numerical modelling with the codes SDTrimSP and SDTrimSP-2D are performed for comparison. Our investigations show that by using the local distribution of projectile impact angles, as derived from AFM measurements, as well as the elemental composition of the samples as an input to the codes SDTrimSP and SDTrimSP-2D the agreement between experiment and simulations is substantially improved.

Application of X-ray photoelectron spectroscopy (XPS) in electrochemistry research is introduced by means of selected examples ranging from fundamental to more applied studies. Recent developments and early works in that field are reviewed. Different ways of electrode preparation and transfer from ambient pressure into ultrahigh vacuum are described. Examples refer to ex situ, quasi in situ, and in situ electrochemical XPS investigations in aqueous, nonaqueous, and ionic liquid electrolytes.

Synopsis Sputtering by solar wind ions is investigated on thin films of the mineral wollastonite (CaSiO3) as an analogue material for the lunar surface. Sputtering yields are measured with a Quartz Crystal Microbalance (QCM) using H and He ions as well as multiply charged Ar ions at different incidence angles. The results of these irradiations show that kinetic sputtering can be very well reproduced using SDTrimSP, while potential sputtering significantly increases the expected erosion of airless bodies in the solar system.

Polymer electrolyte fuel cells (PEFCs) have been the subject of extensive investigations in the last decade as they represent an alternative power source to conventional engines and secondary batteries. At present, one of the main drawbacks hindering PEFCs widespread commercialization is the slow kinetics of the oxygen reduction reaction (ORR) at the cathode and its corrosion stability. Most of the PEFC cathode catalysts are based on Pt or Pt-alloy nanoparticles supported on high-surface area carbons. Despite the good performance as cathode catalyst, Pt-supported on carbon suffers from corrosion stability, being fuel cell lifetime determining. Indeed, in real fuel cell operation, the cathode can reach potentials as high as 1.5 V, which can lead to severe oxidation of the carbon support. Carbon corrosion results in loss of carbon surface area and detachment of the Pt-nanoparticles, and, thus, in loss of the electrode integrity with rapid failure of the fuel cell.[1] Therefore, a growing interest is raising towards alternative, more stable support materials to carbon, such as carbides and oxides. The most research on carbides has been oriented towards tungsten carbide, due to its high electronic conductivity and thermal stability. However, the corrosion stability of, e.g., WC is still controversial, since oxidation of tungsten carbide at high potentials has been recently reported. [2] On the other hand, the use of metal oxides in their highest oxidation state should lead to more stable supports, and based on this idea several investigations on oxide-based catalysts have been reported in the literature in the last years. Thermochemically calculated pHpotential diagrams have shown that only few oxides are stable at pH=0, 80 °C, and with an applied potential of 1.0 V vs. standard hydrogen electrode. [3] In this contribution, theoretical ab-initio DFT calculations have been also performed to calculate the stability potential windows, theoretical Pourbaix diagrams and dynamic modeling of the metal oxide surface changes upon application of different potentials. Among the stable oxides, we have selected tin-based compounds as a novel, durable support for PEMFC catalysts since, besides their stability, they can also achieve high conductivity and they are relatively low-cost materials. Model thin-film electrodes of doped-tin oxide have been fabricated by reactive magnetron sputtering on glassy carbon substrates. A systematical investigation using X-ray photoelectron spectroscopy (XPS) has been carried out to correlate the discharge power and the oxygen content in the plasma to the oxidation state of the tin-based films, which is essential to achieve a stable oxide support in its highest oxidation state. The model thin film electrodes have been also characterized by X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). The electrochemical characterization of the model electrodes was carried out both on pure oxide films and on Pt-supported on the oxides. Cyclic voltammetry and rotating disc electrode measurements were performed in liquid electrolyte to evaluate the electrochemical stability and activity of the oxide-based model electrodes. In addition to thin film model electrodes, doped tin oxide nanopowders have been synthesized by a modified sol gel method. Figure 1 shows the XRD pattern of 5 at% Bi-doped tin oxide powder prepared from aqueous solution containing tin citrate and bismuth nitrate. Chelation of cations is achieved by adding citric acid to the solution and using ethylene glycol as polymerization agent. Once the gel was achieved, it was dried at 150 °C overnight and then calcined at 600 °C for 2 h to obtain single phase oxides. Figure 2 shows a typical SEM micrograph of the calcined Bi-doped tin oxide nanopowder. The oxide powder electrodes were also investigated by XPS in order to correlate the powder processing to the dopant distribution (surface segregation). Finally cyclic voltammetry and rotating disc electrode measurements were performed in liquid electrolyte for both pure oxides and Pt/oxides catalysts.

Precise control of tensile stress and intrinsic damping is crucial for the optimal design of nanomechanical systems for sensor applications and quantum optomechanics in particular. In this letter we study the in uence of oxygen plasma on the tensile stress and intrinsic damping of nanomechanical silicon nitride resonators. Oxygen plasma treatments are common steps in micro and nanofabrication. We show that oxygen plasma of only a few minutes oxidizes the silicon nitride surface, creating several nanometer thick silicon dioxide layers with a compressive stress of 1.30(16)GPa. Such oxide layers can cause a reduction of the e ective tensile stress of a 50 nm thick stoichiometric silicon nitride membrane by almost 50%. Additionally, intrinsic damping linearly increases with the silicon dioxide lm thickness. An oxide layer of 1.5nm grown in just 10s in a 50W oxygen plasma almost doubled the intrinsic damping. The oxide surface layer can be e ciently removed in bu ered HF.

A cyclic approach to silicon nitride dry-etching is presented, which differs in concept from most established high aspect ratio etching processes. Alternating steps of CHF3 etching and oxygen plasma treatment are applied to form vertical sidewalls. During the CHF3 etching step, an etch-inhibiting fluorocarbon film gradually forms on silicon nitride surfaces, whereas the oxygen plasma step removes the fluorocarbon layer and restores the bare nitride surface. By adjusting the timing between the two steps, the etch-inhibition by the fluorocarbon film can be controlled to yield vertical sidewalls. Using x-ray photoelectron spectroscopy, the formation and removal of the fluorocarbon film are confirmed, and its chemical composition is analyzed. The authors show the influence of cycle step duration on etched sidewall angles and present the results of an optimized set of etching parameters for smooth and vertical sidewalls. By feeding only one gas at a time to the plasma reactor, they avoid having to control the ...

The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5 V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.

Simple tripeptides are scaffolds for the synthesis and further assembly of peptide/silver nanoparticle composites. Herein, we further explore peptide-controlled silver nanoparticle assembly processes. Silver nanoparticles with a pH-responsive peptide coating have been synthesized by using a one-step precipitation/coating route. The nature of the peptide/silver interaction and the effect of the peptide on the formation of the silver particles have been studied via UV/Vis, X-ray photoelectron, and surface-enhanced Raman spectroscopies as well as through electron microscopy, small angle X-ray scattering and powder X-ray diffraction with Rietveld refinement. The particles reversibly form aggregates of different sizes in aqueous solution. The state of aggregation can be controlled by the solution pH value. At low pH values, individual particles are present. At neutral pH values, small clusters form and at high pH values, large precipitates are observed.

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et4NBF4 in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G+ bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G+ bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C–C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.

Highly oriented pyrolytic graphite (HOPG) in a solution of (C2H5)4NBF4 in propylene carbonate (PC) was investigated as a model system for the electrochemical double-layer capacitor (EDLC) negative electrode. For the first time the intercalation of (C2H5)4N+ into HOPG could be monitored by means of in situ atomic force microscopy (AFM). In analogy to the behavior in Li+ containing solution the formation of a surface film on the negative electrode could be detected. The question is addressed if this film might serve as a solid electrolyte interphase preventing sustained solvent decomposition. Keywords: Electrochemical in situ atomic force microscopy (AFM), Intercalation, Film formation, Tetraalkylammonium salt, Propylene carbonate decomposition, Electrochemical double layer capacitor (EDLC)

Electrochemically pretreated Bi 2 Pt 2-y Ir y O 7 pyrochlores exhibit oxygen reduction activity and methanol tolerance and therefore are potential candidates for catalysts in fuel cells. In order to investigate the catalytically active surface for oxygen reduction, in this study, XPS measurements obtained from Bi 2 Pt 2-y Ir y O 7 pyrochlores in synthesized state and after electrochemical treatment in 0.5 M H 2 SO 4 solution are presented. The XP-spectra of the Bi 2 Pt 2-y Ir y O 7 powders reveal some interesting features concerning core-level shifts that are attributed to the electronic properties of these materials. The catalytically active surface for oxygen reduction consists of a mixture of Pt 0 /Pt 4+ , Bi 0 /Bi 3+ and Ir 4+ . Pt is enriched at the surface during activation. Sputter depth profiling provides evidence that metallic Pt°, which is supposed to serve the catalytic active sites, is present in the bulk material.

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH−–Co2+– OH− trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)2 layer is found to be not completed which is confirmed by XPS results.

Passive layers on cobalt were prepared in 0.1 N NaOH (pH 13) under potentiostatic control with systematic variation of the relevant parameters, such as time and potential, and examined by x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. Evaluation of the XPS signals occurs on the basis of well-characterized standards. The results are compared with those obtained in borate buffer (pH 9.3), which have been published previously in this journal. Depending on the potential, cobalt forms two different passive layers. In 0.1 N NaOH (pH 13) the potential range of primary passivity goes from -0.4 to 0.3 V SHE . The passive layer consists of a pure Co(OH) 2 film that grows from 10 nm after 5 min of passivation up to 37 nm within 90 min. The layer forms most probably according to a dissolution precipitation mechanism. For E > 0.0 V SHE a thinner layer of only 4 nm (E = 0.2 V SHE for 5 min) is formed. In the range of secondary passivity at E = 0.3-0.8 V SHE Co(III) dominates the layer composition with contributions of Co 3 O 4 , CoOOH and Co 2 O 3 . After 5 min of passivation the layer thickness in this potential range increases slightly with increasing potential from 4 up to 7 nm. After 90 min the thickness reaches 9 nm, independent of the potential.

The authors present an experimental approach to investigate the effect of the target poisoning level on the microstructure, preferred orientation, and composition of aluminum nitride (AlN) and titanium nitride (TiN) films grown on natively oxidized silicon (Si) wafers. Cylindrical aluminum (Al) and titanium targets are reactively sputtered in the poisoning mode using nitrogen as the reactive gas during direct current reactive magnetron sputtering. For each target, the nitride films are deposited during the first 10 min of poisoning and after 60 min of poisoning for 10 min each. Higher substrate temperatures as well as changes in the surface morphology are observed for both AlN and TiN samples prepared after long-term poisoning. After long-term poisoning of Al, the nitrogen content is increased and the preferred crystallographic orientation is changed from [101] to [103]. In the case of TiN, the deposited film is transformed into titanium rich coatings with higher compressive stress and a preferred orienta...

Room temperature ionic liquids (RTILs) are an emerging class of electrolytes enabling high cell voltages and, in return, high energy density of advanced supercapacitors. Yet, the low temperature behavior, including freezing and thawing, is little understood when ions are confined in the narrow space of nanopores. This study shows that RTILs may show a tremendously different thermal behavior when comparing bulk with nanoconfined properties as a result of the increased surface energy of carbon pore walls. In particular, a continuous increase in viscosity is accompanied by slowed-down charge–discharge kinetics as seen with in situ electrochemical characterization. Freezing reversibly collapses the energy storage ability and thawing fully restores the initial energy density of the material. For the first time, a different thermal behavior in positively and negatively polarized electrodes is demonstrated. This leads to different freezing and melting points in the two electrodes. Compared to bulk, RTILs in the confinement of electrically charged nanopores show a high affinity for supercooling; that is, the electrode may freeze during heating.

Passive layers on cobalt were prepared under potentiostatic control in borate buffer pH 9.3 and examined with x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. The XPS signals were evaluated quantitatively on the basis of standard spectra, which yields the composition of the passive layer, i.e. the contribution of the cobalt and oxygen species Co(0), Co(II), Co(III), OH−, O2− and H2O. Depending on the potential, cobalt forms two different passive layers. The primary passive film consists of Co(II) oxide and Co(II) hydroxide and is formed at low potentials. At high potentials a secondary passive layer is formed consisting of Co(II)/Co(III) mixed oxides. The oxidation to Co(III) is related to a significant increase of the layer thickness from 1 to 4 nm. Time-resolved XPS investigations show that the primary passive film grows within

The electronic properties of citrate stabilised Ag nanoparticles with sizes ranging from 4 to 35 nm were investigated by the Kelvin probe method and high resolution XPS. Two and three dimensional assemblies of the particles were prepared by electrostatic adsorption from aqueous solution onto poly-l-lysine modified surfaces. The work function of the Ag particles increased from 5.29 +/- 0.05 to 5.53 +/- 0.05 eV as the particle size decreased. These values are approximately 0.8 eV higher than for clean polycrystalline Ag surfaces. The origin of these remarkable high work functions cannot be explained in terms of either citrate induced changes in the surface dipole or image forces in the confined metallic domains. High resolution XPS spectra of the Ag 3d(5/2) core level were characterised by broad bands and a 0.4 eV shift towards lower binding energies for the smallest particles. Comparisons with reported studies on extended Ag surfaces indicate that as-grown particles exhibit partially oxidised surfaces. The behaviour of the work function further suggests that the strength of the Ag-O bonding increases with decreasing particle sizes. These findings are highly relevant to the interpretation of the catalytic properties of Ag nanoparticles.

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Hydrous ruthenium oxide, RuO 2 ·H 2 O, was prepared according to a sol-gel process and annealed at different temperatures. The importance of high electronic conductivity for high capacity in aqueous 3 M H 2 SO 4 was revealed through two approaches. The electronic resistivity of RuO 2 ·H 2 O measured in situ as a function of the electrode potential shows a marked increase toward low potentials. This trend is more pronounced for the low-temperature annealed oxide (T ≤ 150°C) where it results in a limitation of the capacitance at E < 0.4 V vs reversible hydrogen electrode. This finding is in line with the steep rise of the electrochemical impedance in the same potential region. A possible way to overcome this limitation is to mix two differently heat treated oxides, one with high conductivity (T = 300°C, Z300), the other with optimum capacity (T = 150°C, Z150). The observed specific capacity increase of hydrous RuO 2 in the mixture from 738 to 982 F/g is attributed to an improvement of the electronic pathway along the particles of high-temperature-treated RuO 2 (Z300) toward the high-capacity Z150 particles.