Presence Of Surface Oxide Research Articles

Kinetics of Hydrogen Evolution Reaction on Monometallic Bulk Electrodes in Various Electrolytic Solutions

The hydrogen evolution reaction (HER) holds pivotal significance in electrochemical energy conversion. In this study, we present essential HER kinetic parameters encompassing nine metals (Ag, Au, Co, Cr, Fe, Ni, Pt, W, and Zn) evaluated within seven distinct electrolytes (0.1 mol dm−3 HClO4, 0.1 mol dm−3 HCl, 0.5 mol dm−3 NaCl, 1 mol dm−3 KH2PO4, 0.1 mol dm−3 KOH, 0.1 mol dm−3 LiOH, and 1 mol dm−3 KOH). Through careful measures to restrain oxide formation, HER activity was measured on clean electrodes, while the assessment of HER activity on oxidatively treated metals was also performed. By correlating HER exchange current densities with calculated hydrogen binding energies, we show that the shape of HER volcano curves is largely preserved in studied electrolytes, at least around their apexes. Additionally, depending on the metal–electrolyte combination, the presence of surface oxide can have both positive and negative effects on HER kinetics. Finally, we collated HER kinetic data for bulk surfaces from diverse literature sources, offering a comprehensive overview of the kinetic parameters governing hydrogen evolution across distinct electrolytic environments. These insights have practical significance, guiding the development of new catalytic materials for different water electrolysis technologies, optimizing electrolyte formulations for boosting HER, and enhancing energy efficiency and catalytic performance through catalyst–electrolyte synergies.

Electrochemical Corrosion Characterization of Submicron WC-12Co Coatings Produced by CGS and HVAF Compared with Sintered Bulks

The electrochemical corrosion performance of WC-12 wt% Co in coating and bulk forms has been evaluated in a 3.56 wt% NaCl solution. The coatings were deposited by means of thermal spray techniques, i.e., cold gas spraying (CGS) and high-velocity air fuel (HVAF) spraying, while bulks with different WC sizes were manufactured by conventional pressing and sintering. Microstructural characterizations and phase composition determinations were carried out using scanning electron microscopy and X-ray diffraction. Differences in WC grain size and morphology, carbide dissolution, and cobalt binder phase transformation are discussed according to the inherent characteristics of each processing method. Together with surface roughness (polished/as-sprayed), these features have been observed to directly affect the electrochemical corrosion performance. Electrochemical measurements (open circuit potential, polarization resistance, electrochemical impedance spectroscopy, and polarization curves) showed that the as-sprayed CGS coating presented an electrochemical behavior similar to those of the bulk materials. This was attributed to the higher metallic character of this coating in comparison to that of the HVAF coating. The polished HVAF coating showed anodic activity lower than those of the bulk samples, most likely due to the presence of cobalt–tungsten carbide phases and eventually the lower amount of Co available for dissolution. Finally, the as-sprayed HVAF coating showed very high resistivity due to the presence of surface oxides generated during the deposition process.

A first-principles study of thiadiazole and dimercapto-thiadiazole adsorption on copper and silver surfaces

Copper and silver are strategic metals in industry. However, corrosion in these metals severely limits their broad engineering applications. Organic molecules, such as thiadiazole (TD) derivatives have been effectively utilized as corrosion inhibitors for Cu and Ag metals and their alloys. Therefore, a fundamental understanding of their adsorption behavior on metal surfaces is key to the understanding of their effect on corrosion inhibition. In this work, we used first-principles calculations based on the density functional theory (DFT) to systematically study the adsorption behaviors of 1,3,4-thiadiazole (TD) and 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on the low-index surfaces of pure Cu and Ag. Our results show that the average adsorption energy of DMTD is much lower than that of TD on both Cu and Ag surfaces. This is attributed to the removal of H atoms from DMTD in the DFT calculations as they easily dissociate in the aqueous solution, along with the presence of two more sulfur atoms in the DMTD structure as anchoring sites. It is also revealed that both S and N atoms in DMTD participate in the bond formation with Ag, while the three S atoms of DMTD prefer to form bonds with Cu. This is further validated from the calculated charge density difference before and after the adsorption and agrees well with the X-ray photoelectron spectroscopy measurements. Finally, the differences between the adsorption energies from DFT calculations and experimental extrapolations are discussed. They can be ascribed to the existence of H atoms in DMTD due to their plausible partial ionization in real experiments, as well as the presence of surface oxides and contact with the electrolyte in the experiments. This is further validated by simulating the adsorption of fully-, partially- and non-ionized DMTDs on the (111) surfaces of both Cu and Ag. Our work sheds light on the adsorption behaviors of TD derivatives on both Cu and Ag, which can potentially be used as a criterion to discover and determine appropriate corrosion inhibitors for metal protection.

Cobalt Phosphorous Trisulfide as a High-Performance Electrocatalyst for the Oxygen Evolution Reaction.

Two-dimensional (2D) layered materials are currently one of the most explored materials for developing efficient and stable electrocatalysts in energy conversion applications. Some of the 2D metal phosphorous trichalcogenides (M2P2X6 or MPX3 in its simplified form) have been reported to be useful catalysts for water splitting, although results have been less promising for the sluggish oxygen evolution reaction (OER) due to insufficient activity or compromised stability. Herein, we report the OER catalysis of a series of M2P2X6 (M2+ = Mn, Fe, Co, Zn, Cd; X = S, Se). From the series of MPX3, CoPS3 yields the best results with an overpotential within the range of values usually obtained for IrO2 or RuO2 catalysts. The liquid-phase exfoliation of CoPS3 even improves the OER activity due to abundant active edges of the downsized sheets, accompanied by the presence of surface oxides. The influence of the OER medium and underlying substrate electrode is studied, with the exfoliated CoPS3 reaching the lowest overpotential at 234 mV at a current density of 10 mA/cm2, also able to sustain high current densities, with an overpotential of 388 mV at a current density of 100 mA/cm2, and excellent stability after multiple cycles or long-term operation. Quantum chemical models reveal that these observations are likely tied to moieties on CoPS3 edges, which are responsible for low overpotentials through a two-site mechanism. The OER performance of exfoliated CoPS3 reported herein yields competitive values compared to those reported for other Co-based and MPX3 in the literature, thus holding substantial promise for use as an efficient material for the anodic water-splitting reaction.

On the Insignificant Role of the Oxidation Process on Ultrafast High-Spatial-Frequency LIPSS Formation on Tungsten.

The presence of surface oxides on the formation of laser-induced periodic surface structures (LIPSS) is regularly advocated to favor or even trigger the formation of high-spatial-frequency LIPSS (HSFL) during ultrafast laser-induced nano-structuring. This paper reports the effect of the laser texturing environment on the resulting surface oxides and its consequence for HSFLs formation. Nanoripples are produced on tungsten samples using a Ti:sapphire femtosecond laser under atmospheres with varying oxygen contents. Specifically, ambient, 10 mbar pressure of air, nitrogen and argon, and 10−7 mbar vacuum pressure are used. In addition, removal of any native oxide layer is achieved using plasma sputtering prior to laser irradiation. The resulting HSFLs have a sub-100 nm periodicity and sub 20 nm amplitude. The experiments reveal the negligible role of oxygen during the HSFL formation and clarifies the significant role of ambient pressure in the resulting HSFLs period.

Effect of parallel deposition path and interface material flow on resulting microstructure and tensile behavior of Al-Mg-Si alloy fabricated by additive friction stir deposition

In this paper, the effect of overlapping parallel additive depositions on microstructure and mechanical properties in Additive Friction Stir Deposition (AFS-D) was examined. In particular, the AFS-D process was employed to make parallel depositions of AA6061 with a 6.35 mm overlap to effectively bond the two parallel layers. The AFS-D process draws on similar physics to friction stir welding in that frictional heat and plastic deformation is exploited to deposit metallic materials from the center of a hollow rotating tool as it traverses across the build table to produce consecutive metallurgically bonded layers. In this work, the microstructural aspects of the 6.35 mm overlapping raster interface were characterized using optical and scanning electron microscopy. Aluminum oxide particles were observed at the raster interface, which were located at layer boundaries. Additional grain refinement was also apparent as a direct result of multiple stirring passes within the overlapping deposition region. Mechanical characterization via microindentation and monotonic tensile testing observed a hardness gradient in the overlapping region, but the parallel deposition layers exhibited comparable tensile strength to a single row of deposited AA6061. The influence of existing oxides on the mechanical results was observed to have limited effect on the properties longitudinally across the raster. This study determined that parallel layers of AA6061 can be successfully deposited via the AFS-D technique. The resulting deposit exhibited a strong metallurgical bond across the parallel layers despite the presence of surface oxidation on the unprepared feedstock and substrate.

Antiferromagnetic proximity coupling between semiconductor quantum emitters in WSe2 and van der Waals ferromagnets.

van der Waals ferromagnets have gained significant interest due to their unique ability to provide magnetic response even at the level of a few monolayers. Particularly in combination with 2D semiconductors, such as the transition metal dichalcogenide WSe2, one can create heterostructures that feature unique magneto-optical response in the exciton emission through the magnetic proximity effect. Here we use 0D quantum emitters in WSe2 to probe for the ferromagnetic response in heterostructures with Fe3GT and Fe5GT ferromagnets through an all-optical read-out technique that does not require electrodes. The spectrally narrow spin-doublet of the WSe2 quantum emitters allowed to fully resolve the hysteretic magneto-response in the exciton emission, revealing the characteristic signature of both ferro- and antiferromagnetic proximity coupling that originates from the interplay among Fe3GT or Fe5GT, a thin surface oxide, and the spin doublets of the quantum emitters. Our work highlights the utility of 0D quantum emitters for probing interface magnetic dipoles in vdW heterostructures with high precision. The observed hysteretic magneto response in the exciton emission of quantum emitters adds further new degrees of freedom for spin and g-factor manipulation of quantum states.

A Statistical Analysis of Powder Flowability in Metal Additive Manufacturing

The quality of parts fabricated with additive manufacturing is influenced by the flowability of the feedstock particles, which is the result of many factors, including chemistry (e.g., true density, the presence of surface oxides, and impurities), morphology (e.g., particle shape and the presence of satellites), and particle size distribution. This work investigates the relationship between powder characteristics and flow behavior of different powders using three flowability testing methods. Six powders of two compositions (316L stainless steel and AlSi10Mg), made using two different methods (gas‐ and water‐atomization), are investigated to rationalize the effect of powder chemistry and morphology on flow behavior. The results show that the true density of the powders can influence several flowability metrics. In addition, aspect ratio strongly influences the initiation of flow from a static condition, whereas average particle size strongly dictates the ease of maintaining that flow.

Surface analysis of fine water‐atomized iron powder and sintered material

Press and sinter is a core technology in powder metallurgy in which a metal powder is mixed with a lubricant and other additives and subsequently compacted at large mechanical pressures to create the desired shape. The component is then delubricated and finally sintered to strengthen the material. The end result of the sintering depends on both the physical properties of the powder such as particle size, morphology, and size distribution, as well as chemical properties like surface chemical composition and presence of surface oxides. In this study, the surface characteristics of 3 fractions of water‐atomized iron powder, sieved to −20, −45, and −75 μm, were investigated. This was done by using X‐ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy equipped with energy‐dispersive X‐ray spectroscopy. The powder was sintered while performing thermogravimetric analysis at 1350°C for 30 minutes in pure hydrogen. Results showed that powder of all 3 size fractions were mainly covered by a thin iron oxide layer but with some presence of submicron‐sized oxide particulates. X‐ray photoelectron spectroscopy and thermogravimetric analysis showed reasonable agreement on the oxide layer thickness, while a size dependence was found with an enrichment of oxide particulates on the finer powder fractions. Fracture surfaces of the sintered material were analyzed with X‐ray photoelectron spectroscopy and scanning electron microscopy + energy‐dispersive X‐ray spectroscopy, which indicated that full reduction of all oxides was achieved. The sintering conditions were further related to the surface properties of the powder and its size fraction.

Accelerating the oxidation rate of AlN substrate through the addition of water vapor

To apply a pre-oxidation treatment on aluminum nitride (AlN) substrate is a common practice before its metallization. In the present study, the microstructure of AlN after oxidation either in dry air or in wet air is characterized. The resulting thermal conductivity is measured. With or without the presence of water vapor, the oxidation of AlN is a reaction-dominating process. The addition of water vapor speeds up the oxidation rate by one order of magnitude. The surface oxide layer is full of nano-sized pores. The presence of surface oxide reduces the thermal conductivity by ∼15% when the thickness of oxide layer is only 3 μm.

Reaction of aqueous ammonium sulfide on SiGe 25%

SiGe 25% substrates were treated with aqueous solutions of ammonium sulfide with and without added acid to understand the adsorption of sulfur on the surface. X-ray photoelectron spectroscopy showed no sulfide layer was deposited from aqueous (NH4)2S alone and instead both Si and Ge oxides formed during immersion in the sulfur solution. The addition of hydrofluoric and hydrochloric acids dropped the pH from 10 to 8 and deposited sulfides, yet increased the oxide coverage on the surface and preferentially formed Ge oxides. The sulfur coverage grew with increasing concentrations of acid in the aqueous (NH4)2S. The simultaneous deposition of O and S is suspected to be the result of oxidized sulfur species in solution. Metal-insulator-semiconductor capacitor (MISCAP) devices were fabricated to test the electrical consequences of aqueous ammonium sulfide wet chemistries on SiGe. MISCAPs treated with acidic ammonium sulfide solutions contained fewer interface defects in the valence band region. The defect density (Dit) was on the order of 10+12 cm–2 eV−1. The flat band voltage shift was lower after the acidic ammonium sulfide treatment, despite the presence of surface oxides. Adsorption of S and potentially O improved the stability of the surface and made it less electrically active.

Effects of oxidation on tensile deformation of iron nanowires: Insights from reactive molecular dynamics simulations

The influence of oxidation on the mechanical properties of nanostructured metals is rarely explored and remains poorly understood. To address this knowledge gap, in this work, we systematically investigate the mechanical properties and changes in the metallic iron (Fe) nanowires (NWs) under various atmospheric conditions of ambient dry O2 and in a vacuum. More specifically, we focus on the effect of oxide shell layer thickness over Fe NW surfaces at room temperature. We use molecular dynamics (MD) simulations with the variable charge ReaxFF force field potential model that dynamically handles charge variation among atoms as well as breaking and forming of the chemical bonds associated with the oxidation reaction. The ReaxFF potential model allows us to study large length scale mechanical atomistic deformation processes under the tensile strain deformation process, coupled with quantum mechanically accurate descriptions of chemical reactions. To study the influence of an oxide layer, three oxide shell layer thicknesses of ∼4.81 Å, ∼5.33 Å, and ∼6.57 Å are formed on the pure Fe NW free surfaces. It is observed that the increase in the oxide layer thickness on the Fe NW surface reduces both the yield stress and the critical strain. We further note that the tensile mechanical deformation behaviors of Fe NWs are dependent on the presence of surface oxidation, which lowers the onset of plastic deformation. Our MD simulations show that twinning is of significant importance in the mechanical behavior of the pure and oxide-coated Fe NWs; however, twin nucleation occurs at a lower strain level when Fe NWs are coated with thicker oxide layers. The increase in the oxide shell layer thickness also reduces the external stress required to initiate plastic deformation.

Extensive Characterization of Oxide-Coated Colloidal Gold Nanoparticles Synthesized by Laser Ablation in Liquid.

Colloidal gold nanoparticles are a widespread nanomaterial with many potential applications, but their aggregation in suspension is a critical issue which is usually prevented by organic surfactants. This solution has some drawbacks, such as material contamination and modifications of its functional properties. The gold nanoparticles presented in this work have been synthesized by ultra-fast laser ablation in liquid, which addresses the above issues by overcoating the metal nanoparticles with an oxide layer. The main focus of the work is in the characterization of the oxidized gold nanoparticles, which were made first in solution by means of dynamic light scattering and optical spectroscopy, and then in dried form by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and finally by surface potential measurements with atomic force microscopy. The light scattering assessed the nanoscale size of the formed particles and provided insight in their stability. The nanoparticles’ size was confirmed by direct imaging in transmission electron microscopy, and their crystalline nature was disclosed by X-ray diffraction. The X-ray photoelectron spectroscopy showed measurements compatible with the presence of surface oxide, which was confirmed by the surface potential measurements, which are the novel point of the present work. In conclusion, the method of laser ablation in liquid for the synthesis of gold nanoparticles has been presented, and the advantage of this physical approach, consisting of coating the nanoparticles in situ with gold oxide which provides the required morphological and chemical stability without organic surfactants, has been confirmed by using scanning Kelvin probe microscopy for the first time.

Formation of Silicon Local Environments upon Annealing for Silicon Anodes: A 29Si Solid State NMR Study

29Si magic angle spinning nuclear magnetic resonance (MAS NMR) has been used to characterize the binding interactions and interfacial phases formed between silicon and copper in a series of all-inorganic negative electrodes. The phases identified were correlated with electrochemical performance of the electrode. The electrode materials were prepared by copper deposition on silicon particles with varying Cu:Si ratios followed by heat-treatment at varying temperatures. Initial studies of the electrochemical performance of the anode materials indicated that samples with a 1:4 copper to silicon ratios and annealed at temperatures below 700°C had the best cycling performance. Utilizing 29Si MAS NMR we have been able to show that the best electrodes possess a single silicon local environment (from elemental silicon) and very little Cu3Si intermetallic formation. Electrodes with the compositions Cu:6Si and Cu:8Si annealed at certain conditions showed additional silicon resonances at −75, −92, −107 and 750 ppm and poor electrochemical performance. Combination of 29Si Single Pulse and 1H/29Si Cross Polarization techniques were performed to study the nature of these silicon local environments. Preliminary assignments indicate that the presence of surface oxidation and hydration (from annealing process) are key variables that limit the electrochemical activity of the electrode.

Fabrication of Graphene-Based Electrochemical Capacitors

In this study, we investigate the electrochemical properties of graphene-based capacitors using cyclic voltammetry (CV), constant charge/discharge cycling, and electrochemical impedance spectroscopy combined with an equivalent circuit. An easy route incorporated with modified Hummers' method and microwave-assisted reduction is capable of preparing graphene nanosheets (GNs). Two types of capacitors fabricated with GN and graphene oxide (GO) powders are examined in 1 M H2SO4 within a potential of 0 and 0.8 V vs Ag/AgCl. The GO-based capacitor not only presents a better stable capacitance (ca. 211.7 F/g after 1000 cycles), but also a lower equivalent series resistance (ca. 7.6 Ω after 1000 cycles), when compared with the GN-based capacitor. The presence of surface oxides, attached to the edges or defects of the basal planes, imparts hydrophilic coverage for the formation of a double layer (double-layer capacitance) and active sites for reversible redox reaction (pseudocapacitance). When incorporated with structural (nanosheets) and chemical (surface oxides) factors, the GO powder serves as a promising electrode material for electrochemical capacitors or other energy-storage devices.

Fabrication of Graphene-Based Electrochemical Capacitors

In this study, we investigate the electrochemical properties of graphene-based capacitors using cyclic voltammetry (CV), constant charge/discharge cycling, and electrochemical impedance spectroscopy combined with an equivalent circuit. An easy route incorporated with modified Hummers' method and microwave-assisted reduction is capable of preparing graphene nanosheets (GNs). Two types of capacitors fabricated with GN and graphene oxide (GO) powders are examined in 1 M H2SO4 within a potential of 0 and 0.8 V vs Ag/AgCl. The GO-based capacitor not only presents a better stable capacitance (ca. 211.7 F/g after 1000 cycles), but also a lower equivalent series resistance (ca. 7.6 Ω after 1000 cycles), when compared with the GN-based capacitor. The presence of surface oxides, attached to the edges or defects of the basal planes, imparts hydrophilic coverage for the formation of a double layer (double-layer capacitance) and active sites for reversible redox reaction (pseudocapacitance). When incorporated with structural (nanosheets) and chemical (surface oxides) factors, the GO powder serves as a promising electrode material for electrochemical capacitors or other energy-storage devices.

Towards Passivation of Ge(100) Surfaces by Sulfur Adsorption from a (NH4)2S Solution: A Combined NEXAFS, STM and LEED Study

Using x-ray absorption spectroscopy, scanning tunneling microscopy and low-energy electron diffraction we have studied the surface chemistry and atomic structure of the sulfur passivation layer formed on Ge(100) surfaces upon treatment in an aqueous (NH4)2S solution at room temperature. This treatment was shown to yield incomplete sulfur coverage (<1 monolayer) and residual Ge oxides regardless of the time the substrates are immersed in the solution. Scanning tunneling microscopy images of the surface structure of the passivation layer reveal the coexistence of locally ordered and large disordered areas, attributed to S-Ge and O-Ge bonds, respectively. The passivated surfaces exhibit a pronounced (1 × 1) electron diffraction pattern. The formation of the passivation layer appears to be dependent on the state of the Ge surface prior to sulfidation, i.e. on the presence of surface oxides, which hamper the formation of a long-range ordered sulfur monolayer.

Efficient Generation of Cube-on-Face Crystallographic Texture in Iron and its Alloys

A novel method has been discovered for controlling the crystallographic orientation of body-centred cubic crystals of iron. The process allows the cube faces of the crystals to align with the major surface of the polycrystalline sample in sheet form. Such a texture is advantageous in terms of its magnetic properties and has been a long-standing goal for research on electrical steels. The mechanism controlling the favourable orientation obtained is associated with the fact that the cube faces are elastically compliant so that the texture can develop in a manner consistent with minimization of strain energy. For the same reason, the presence of surface oxide which modifies the elastic modulus of metal surface stifles the development of the correct texture. The new process is demonstrated to lead to a dramatic increase in the magnetic flux density and a reduction in iron losses. It also involves a remarkably shorter processing time than conventional texture control in electrical steels.

Synthesis, characterization, and electrochemical capacitance of amino-functionalized carbon nanotube/carbon paper electrodes

Amino-functionalization of carbon nanotubes (CNTs) attached to carbon paper (CP) has been achieved using one synthesis protocol: (i) chemical oxidation, (ii) acyl chlorination, and (iii) amidation. The amidation reaction of the carboxylic groups in oxidized the CNT/CP hybrids enables the formation of terminal amino groups on the CNT sidewalls. The functionalized CNTs were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transformed infrared spectroscopy, thermal programmed desorption, and N 2 adsorption. The cyclic voltammetry curves of an amino-modified CNT-based capacitor showed a continuum of double-layer and pseudocapacitive behavior. The presence of surface oxides and amides on CNTs imparts not only hydrophilic coverage for the formation of double-layer (double-layer capacitance) but also active sites for the redox reaction (pseudocapacitance). Based on the result of the charge/discharge cycling test, the N-modified CNT/CP capacitor exhibits an enhanced capacitance, high-rate capability, and capacitance stability with high coulombic efficiency.

The effect of thermal oxidation on laser-induced photoelectron emission during tensile deformation of polycrystalline aluminum

Many metals emit electrons when exposed to UV radiation (photon energies 4 to 8 eV). Deformation can significantly affect the intensity of these emissions. In the case of reactive metals, these emissions are also altered by the presence of surface oxides. We have characterized the effect of thermal oxides on laser-induced photoelectron emission from commercially pure polycrystalline aluminum with a view toward using these emissions as a probe of deformation processes. The thickness of oxides produced by a range of annealing treatments in air was determined by x-ray photoelectron spectroscopy. Time-of-flight measurements on photoelectrons from these surfaces under 248 nm irradiation (5 eV photons) show two peaks: a fast peak which we attributed to electrons from metallic aluminum, and a slower peak, which may be due to electrons from interface states. Surface oxide films of sufficient thickness attenuate both peaks. We show that the sensitivity of the photoelectron signals to deformation varies with thermal oxidation pretreatments and oxide film thickness and that with the appropriate oxide thickness the total photoelectron intensity becomes a sensitive probe of deformation-related processes during tensile testing.