Remarkably Corrosion Resistant Graphene Coating on Steel Enabled Through Metallurgical Tailoring.

Graphene coatings developed by chemical vapor deposition (CVD) that possess extraordinary/unique characteristics as barrier against aggressive environment can improve the corrosion resistance of Ni and Cu by up to two orders of magnitude. However, because of some compelling technical reasons, it has thus far been a nontrivial challenge to develop graphene coatings on the most commonly used engineering alloy, mild steel (MS). To circumvent the challenge simply by first electroplating MS with a Ni layer is attempted, and then developing CVD graphene over the Ni layer. However, this approach proved too simplistic and does not work. This necessitated an innovative surface modification of MS (based on basic metallurgical principles) that enabled successful CVD of graphene coating on MS. The graphene coating thus developed is demonstrated to improve the corrosion resistance of mild steel by two orders of magnitude in an aggressive chloride solution, through electrochemical testing. This improvement was not only sustained for the entire test duration of >1000 h; but there is a clear trend for the resistance to be possibly everlasting. The optimized surface modification that enabled development of CVD graphene coating on mild steel is generic in nature, and it should enable graphene coating on other alloy systems, which would otherwise not be possible.

Graphene-Coated Ni-Cu Alloys for Durable Degradation Resistance of Bi-Polar Plates for Proton Exchange Membrane Fuel Cells: Remarkable Role of Alloy Composition.

Bipolar plates, a critical component of proton exchange membrane fuel cell (PEMFC), are constructed out of alloys of Ti, Pt, Cr, or graphitic materials that have limitations. Electrical conductivity, cost, and corrosion resistance are among the critical considerations for bi-polar plate material. Graphene, which possesses impressive conductivity and toughness, is an attractive option as coating on metallic substrates of PEMFC bipolar plates. This study investigates corrosion resistance and its durability due to graphene developed by chemical vapor deposition on a pure Ni-Cu alloy and a commercial Ni-Cu alloy in 0.5 m H2 SO4 environment, with a view to exploring use of graphene coated Ni-Cu alloys for the construction of PEMFC bipolar plates. The graphene coating on the pure alloy shows remarkably superior corrosion resistance than the commercial alloy that is attributed to the former's ability to develop considerably defect-free graphene.

Formation of a two-dimensional oxide via oxidation of a layered material.

We investigate the oxidation mechanism of the layered model system GeAs. In situ X-ray photoelectron spectroscopy experiments performed by irradiating an individual flake with synchrotron radiation in the presence of oxygen show that while As leaves the GeAs surface upon oxidation, a Ge-rich ultrathin oxide film is being formed in the topmost layer of the flake. We develop a theoretical model that supports the layer-by-layer oxidation of GeAs, with a logarithmic kinetics. Finally, assuming that the activation energy for the oxidation process changes linearly with coverage, we estimate that the activation energy for As oxidation is almost twice that for Ge at room temperature.

Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon.

Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.

Operando Surface Characterization of InP Nanowire p-n Junctions.

We present an in-depth analysis of the surface band alignment and local potential distribution of InP nanowires containing a p-n junction using scanning probe and photoelectron microscopy techniques. The depletion region is localized to a 15 nm thin surface region by scanning tunneling spectroscopy and an electronic shift of up to 0.5 eV between the n- and p-doped nanowire segments was observed and confirmed by Kelvin probe force microscopy. Scanning photoelectron microscopy then allowed us to measure the intrinsic chemical shift of the In 3d, In 4d, and P 2p core level spectra along the nanowire and the effect of operating the nanowire diode in forward and reverse bias on these shifts. Thanks to the high-resolution techniques utilized, we observe fluctuations in the potential and chemical energy of the surface along the nanowire in great detail, exposing the sensitive nature of nanodevices to small scale structural variations.

Graphene Coating Obtained in a Cold-Wall CVD Process on the Co-Cr Alloy (L-605) for Medical Applications.

Graphene coating on the cobalt-chromium alloy was optimized and successfully carried out by a cold-wall chemical vapor deposition (CW-CVD) method. A uniform layer of graphene for a large area of the Co-Cr alloy (discs of 10 mm diameter) was confirmed by Raman mapping coated area and analyzing specific G and 2D bands; in particular, the intensity ratio and the number of layers were calculated. The effect of the CW-CVD process on the microstructure and the morphology of the Co-Cr surface was investigated by scanning X-ray photoelectron microscope (SPEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Nanoindentation and scratch tests were performed to determine mechanical properties of Co-Cr disks. The results of microbiological tests indicate that the studied Co-Cr alloys covered with a graphene layer did not show a pro-coagulant effect. The obtained results confirm the possibility of using the developed coating method in medical applications, in particular in the field of cardiovascular diseases.

X-ray Photoelectron Spectroscopy Studies of Nanoparticles Dispersed in Static Liquid.

For nanoparticles active for chemical and energy transformations in static liquid environment, chemistries of surface or near-surface regions of these catalyst nanoparticles in liquid are crucial for fundamentally understanding their catalytic performances at a molecular level. Compared to catalysis at a solid-gas interface, there is very limited information on the surface of these catalyst nanoparticles under a working condition or during catalysis in liquid. Photoelectron spectroscopy is a surface-sensitive technique; however, it is challenging to study the surfaces of catalyst nanoparticles dispersed in static liquid because of the short inelastic mean free path of photoelectrons traveling in liquid. Here, we report a method for tracking the surface of nanoparticles dispersed in static liquid by employing graphene layers as an electron-transparent membrane to separate the static liquid containing a solvent, catalyst nanoparticles, and reactants from the high-vacuum environment of photoelectron spectrometers. The surfaces of Ag nanoparticles dispersed in static liquid sealed in such a graphene membrane liquid cell were successfully characterized using a photoelectron spectrometer equipped with a high vacuum energy analyzer. With this method, the surface of catalyst nanoparticles dispersed in liquid during catalysis at a relatively high temperature up to 150 °C can be tracked with photoelectron spectroscopy.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. II. Photoelectron spectroscopy and microscopy.

Chemical waves in the H2 + O2 reaction on a Rh(111) surface alloyed with Ni [ΘNi < 1.5 monolayers (ML)] have been investigated in the 10-7 and 10-6 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior. The coincidence of a high oxygen signal with a low Ni 2p intensity, which seemingly contradicts the chemical attraction between O and Ni, has been explained with a phase separation of the oxygen covered Rh(111)/Ni surface into a 3D-Ni oxide and into a Ni poor metallic phase. Macroscopic NiO islands (≈1 µm size) formed under reaction conditions have been identified as 2D-Ni oxide. Titration experiments of the oxygen covered Rh(111)/Ni surface with H2 demonstrated that the reactivity of oxygen is decreased by an order of magnitude through the addition of 0.6 ML Ni. An excitation mechanism is proposed in which the periodic formation and reduction of NiO modulate the catalytic activity.

Fabrication of Ti substrate grain dependent C/TiO2 composites through carbothermal treatment of anodic TiO2.

Composite materials of titania and graphitic carbon, and their optimized synthesis are highly interesting for application in sustainable energy conversion and storage. We report on planar C/TiO2 composite films that are prepared on a polycrystalline titanium substrate by carbothermal treatment of compact anodic TiO2 with acetylene. This thin film material allows for the study of functional properties of C/TiO2 as a function of chemical composition and structure. The chemical and structural properties of the composite on top of individual Ti substrate grains are examined by scanning photoelectron microscopy and micro-Raman spectroscopy. Through comparison of these data with electron backscatter diffraction, it is found that the amount of generated carbon and the grade of anodic film crystallinity correlate with the crystallographic orientation of the Ti substrate grains. On top of Ti grains with ∼(0001) orientations the anodic TiO2 exhibits the highest grade of crystallinity, and the composite contains the highest fraction of graphitic carbon compared to Ti grains with other orientations. This indirect effect of the Ti substrate grain orientation yields new insights into the activity of TiO2 towards the decomposition of carbon precursors.

Preparation of a membrane-sealed cell for studying catalyst nanoparticles in flowing gas with high vacuum x-ray photoelectron spectrometer.

Here a sealing-style x-ray photoelectron spectroscopy study of the surface of a 1.0 wt. %Ni/TiO2 nanoparticle catalyst in a flowing mixture of CO and O2 at 1 bar was performed with a graphene membrane-sealed Si3N4 window-based miniature cell. We report the details on how a commercial Si3N4 window is modified before assembling a graphene membrane, how single-layer graphene membranes are transferred from their metal supports to the modified Si3N4 window, how a modified Si3N4 window covered with a double-layer graphene membrane is assembled onto a blank cell cap, how a nanoparticle catalyst is introduced to the cell cap and then the cell cap is installed onto a cell body to form a complete reaction cell, and how a complete cell is interfaced with a high vacuum chamber of an XPS system before an XPS study of 1.0 wt. %Ni/TiO2 catalyst surface in a flowing mixture for 0.2 bar CO and 0.8 bar O2 is performed. How the characterization of a catalyst using this type of graphene membrane-sealed Si3N4 window-based miniature cell is relevant to the finding of the actual surface chemistry of a catalyst during catalysis is discussed.

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction.

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO2) in H2 oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO2) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO2, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO2 support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Effect of Carbon Nanotubes on the Na+ Intercalation Capacity of Binder Free Mn2V2O7-CNTs Electrode: A Structural Investigation.

Improvements in sodium intercalation in sodium cathodes have been debated in recent years. In the present work, we delineate the significant effect of the carbon nanotubes (CNTs) and their weight percent in the intercalation capacity of the binder-free manganese vanadium oxide (MVO)-CNTs composite electrodes. The performance modification of the electrode is discussed taking into account the cathode electrolyte interphase (CEI) layer under optimal performance. We observe an intermittent distribution of the chemical phases on the CEI, formed on these electrodes after several cycles. The bulk and superficial structure of pristine and Na+ cycled electrodes were identified via micro-Raman scattering and Scanning X-ray Photoelectron Microscopy. We show that the inhomogeneous CEI layer distribution strongly depends on the CNTs weight percentage ratio in an electrode nano-composite. The capacity fading of MVO-CNTs appears to be associated with the dissolution of the Mn2O3 phase, leading to electrode deterioration. This effect is particularly observed in electrodes with low weight percentage of the CNTs in which the tubular topology of the CNTs are distorted due to the MVO decoration. These results can deepen the understanding of the CNTs role on the intercalation mechanism and capacity of the electrode, where there are variations in the mass ratio of CNTs and the active material.

Soft X-ray imaging and spectromicroscopy: new insights in chemical state and morphology of the key components in operating fuel-cells.

Fuel cells are one of the most appealing environmentally friendly devices for the effective conversion of chemical energy into electricity and heat, but still there are key barriers to their broad commercialization. In addition to efficiency, a major challenge of fuel-cell technology is the durability of the key components (interconnects, electrodes, and electrolytes) that can be subject to corrosion or undesired morphology and chemical changes occurring under operating conditions. The complementary capabilities of synchrotron-based soft X-ray microscopes in terms of imaging, spectroscopy, spatial and time resolution, and variable probing depths are opening unique opportunities to shed light on the multiple processes occurring in these complex systems at microscopic length scales. This type of information is prerequisite for understanding and controlling the performance and durability of such devices. This paper reviews the most recent efforts in the implementation of these methods for exploring the evolving structure and chemical composition of some key fuel cell components. Recent achievements are illustrated by selected results obtained with simplified versions of proton-exchange fuel-cells (PEFC) and solid-oxide fuel-cells (SOFC), which allow in situ monitoring of the redox reactions resulting in: 1) undesired deposits at interconnects and electrodes (PEFC); 2) material interactions at the electrode-electrolyte interface (PEFC); 3) release of corrosion products to the electrolyte phase (PEFC, and 4) mass-transport processes and structural changes occurring at the high operation temperatures of SOFC and promoted by the polarization.

In situ study of electrochemical activation and surface segregation of the SOFC electrode material La0.75Sr0.25Cr0.5Mn0.5O(3±Î´).

Mixed-conducting perovskite-type electrodes which are used as cathodes in solid oxide fuel cells (SOFCs) exhibit pronounced performance improvement after cathodic polarization. The current in situ study addresses the mechanism of this activation process which is still unknown. We chose the new perovskite-type material La(0.75)Sr(0.25)Cr(0.5)Mn(0.5)O(3±Î´) which is a potential candidate for use in symmetrical solid oxide fuel cells (SFCs). We prepared La(0.75)Sr(0.25)Cr(0.5)Mn(0.5)O(3±Î´) thin film model electrodes on YSZ (111) single crystals by pulsed laser deposition (PLD). Impedance spectroscopy (EIS) measurements show that the kinetics of these electrodes can be drastically improved by applying a cathodic potential. To understand the origin of the enhanced electrocatalytic activity the surfaces of operating LSCrM electrodes were studied in situ (at low pressure) with spatially resolving X-ray photoelectron spectroscopy (µ-ESCA, SPEM) and quasi static secondary ion mass spectrometry (ToF-SIMS) after applying different electrical potentials in the SIMS chamber. We observed that the electrode surfaces which were annealed at 600 °C are enriched significantly in strontium. Subsequent cathodic polarization decreases the strontium surface concentration while anodic polarization increases the strontium accumulation at the electrode surface. We propose a mechanism based on the reversible incorporation of a passivating SrO surface phase into the LSCrM lattice to explain the observed activation/deactivation process.

Nanoscale phase separation in the oxide layer at GeTe (111) surfaces.

As a semiconductor ferroelectric, GeTe has become a focus of renewed attention due to the recent discovery of giant Rashba splitting. It already has a wide range of applications, from thermoelectricity to data storage. Its stability in ambient air, as well as the structure and properties of an oxide layer, define the processing media for device production and operation. Here, we studied a reaction between the GeTe (111) surface and molecular oxygen for crystals having solely inversion domains. We evaluated the reaction kinetics both ex situ and in situ using NAP XPS. The structure of the oxide layer is extensively discussed, where, according to HAADF-STEM and STEM-EDX, nanoscale phase separation of GeO2 and Te is observed, which is unusual for semiconductors. We believe that such behaviour is closely related to the ferroelectric properties and the domain structure of GeTe.

How the Environment Encourages the Natural Formation of Hydrated V2O5.

Herein, we report the microscopic and spectroscopic signatures of the hydrated V2O5 phase, prepared from the α-V2O5 powder, which was kept in deionized water inside an airtight glass container for approximately 2.5 years. The experimental results show an evolution of the V4+ component in V 2p3/2 core energy level spectra, and a peak corresponding to σ-OH- bond appeared in the valence band spectra in the hydrated V2O5 powder sample due to the water intercalation. Vanadium metal oxide particles were found to be self-nucleated into micro/nanorods after a long period of exposure to an extremely humid environment. The distinct features in the spectra obtained with high-resolution transmission electron microscopy, micro-Raman scattering, and X-ray photoelectron spectroscopy confirmed the presence of structural water molecules for the first time in the long-aged naturally hydrated V2O5 phase.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Electrochemical activation of molecular nitrogen at the Ir/YSZ interface.

Nitrogen is often used as an inert background atmosphere in solid state studies of electrode and reaction kinetics, of solid state studies of transport phenomena, and in applications e.g. solid oxide fuel cells (SOFC), sensors and membranes. Thus, chemical and electrochemical reactions of oxides related to or with dinitrogen are not supposed and in general not considered. We demonstrate by a steady state electrochemical polarisation experiments complemented with in situ photoelectron spectroscopy (XPS) that at a temperature of 450 °C dinitrogen can be electrochemically activated at the three phase boundary between N(2), a metal microelectrode and one of the most widely used solid oxide electrolytes--yttria stabilized zirconia (YSZ)--at potentials more negative than E = -1.25 V. The process is neither related to a reduction of the electrolyte nor to an adsorption process or a purely chemical reaction but is electrochemical in nature. Only at potentials more negative than E = -2 V did new components of Zr 3d and Y 3d signals with a lower formal charge appear, thus indicating electrochemical reduction of the electrolyte matrix. Theoretical model calculations suggest the presence of anionic intermediates with delocalized electrons at the electrode/electrolyte reaction interface. The ex situ SIMS analysis confirmed that nitrogen is incorporated and migrates into the electrolyte beneath the electrode.

Crystal Orientation Dependent Oxidation Modes at the Buried Graphene-Cu Interface.

We combine spatially resolved scanning photoelectron spectroscopy with confocal Raman and optical microscopy to reveal how the oxidation of the buried graphene-Cu interface relates to the Cu crystallographic orientation. We analyze over 100 different graphene covered Cu (high and low index) orientations exposed to air for 2 years. Four general oxidation modes are observed that can be mapped as regions onto the polar plot of Cu surface orientations. These modes are (1) complete, (2) irregular, (3) inhibited, and (4) enhanced wrinkle interface oxidation. We present a comprehensive characterization of these modes, consider the underlying mechanisms, compare air and water mediated oxidation, and discuss this in the context of the diverse prior literature in this area. This understanding incorporates effects from across the wide parameter space of 2D material interface engineering, relevant to key challenges in their emerging applications, ranging from scalable transfer to electronic contacts, encapsulation, and corrosion protection.