Native Oxide Surface Layer Research Articles

Current Collector Primers with Improved Physical and Electrochemical Properties

Lithium-ion batteries are the dominant technology for powering the growing markets of electric vehicles, consumer electronics, and grid storage. However, to meet the increasing demands for higher energy density, faster charging, lower cost, and longer cycle life, continued research of new materials and designs are required. One of the key components that affects the performance and durability of lithium-ion batteries is the current collector, which provides a conductive substrate for active materials. The interfacial conductivity, adhesion, and corrosion resistance of these foils is largely dictated by their native oxide surface layer. To improve any of these features, specialized primers may be applied prior to electrode manufacturing, presenting a unique electrode/active material interface. For instance, carbon-coatings may be applied to aluminum to improve the conductivity and adhesion of lithium iron phosphate (LFP) formulations. These carbon coatings are limited in their cost, thickness, and performance, particularly in next generation cell designs and application methods. PPG is exploring novel current collector coating technologies addressing the physical, electrochemical, and stability requirements of these chemistries and manufacturing process. In this presentation, we will elaborate on coatings development and the effect that novel primer layers have on mechanical properties and single-layer pouch cell cycle life.

Plasmon assisted synthesis of TiN-supported single-atom nickel catalysts

We report the deposition of single atom nickel catalyst on refractory plasmonic titanium nitride (TiN) nanomaterials supports using the wet synthesis method under visible light irradiation. TiN nanoparticles efficiently absorb visible light to generate photoexcited electrons and holes. Photoexcited electrons reduce nickel precursor to deposit Ni atoms on TiN nanoparticles’ surface. The generated hot holes are scavenged by the methanol. We studied the Ni deposition on TiN nanoparticles by varying light intensity, light exposure time, and metal precursor concentration. These studies confirmed the photodeposition method is driven by hot electrons and helped us to find optimum synthesis conditions for single atoms deposition. We characterized the nanocatalysts using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). We used density functional theory (DFT) calculations to predict favorable deposition sites and aggregation energy of Ni atoms on TiN. Surface defect sites of TiN are most favorable for single nickel atoms depositions. Interestingly, the oxygen sites on native surface oxide layer of TiN also exhibit strong binding with the single Ni atoms. Plasmon enhanced synthesis method can facilitate photodeposition of single atom catalysts on a wide class of metallic supports with plasmonic properties.

On the high-temperature oxidation of ZnSb for thermoelectric applications

Thermoelectric ZnSb has a native surface oxide layer of Sb2O5 +ZnO, while HT oxidation in air resulted in selective oxidation to ZnO, with no detectable Sb by STEM-EDS. The ZnO consisted of an inner nano-granular and a more columnar outer layer. Thermogravimetry showed parabolic oxidation kinetics at 200–325 °C with an activation energy of 117 kJ/mol, comparing well with literature for oxidation of Zn. Two-stage 16O2 +18O2 oxidation followed by SIMS suggests growth by mixed Zn and O diffusion through a protective ZnO layer that still cannot prevent considerable high temperature oxidation in air, with formation of insulating electrical contacts, unless metallised.

Nitrogen plasma passivated niobium resonators for superconducting quantum circuits

Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal–air interface, which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling-factor-adjusted two-level-system loss tangent FδTLS of (2.9±0.5)·10−7 for resonators with a 3 μm center strip and (1.0±0.3)·10−7 for a 20 μm center strip, exceeding the performance of unpassivated samples by a factor of four.

Effects of native oxidation on Ti/TiO2 nanodot arrays and their plasmonic properties compared to Au nanodot arrays

Unlike for noble metals, localized surface plasmon resonance (LSPR) of metallic nanodot array (NDA), which forms a surface oxide layer, has not been fully explored. We have investigated the surface oxide layers, LSPR, and surface-enhanced Raman scattering (SERS) of Au and Ti/TiO2 NDAs with similar shapes. Au NDA and Ti/TiO2 NDA with 80 ± 5 nm diameters were fabricated on indium tin oxide glass substrates using anodic aluminum oxide masks. The native oxide surface layer of Au NDA was very thin, whereas the Ti/TiO2 NDA’s surface was identified as TiO2 due to its inherent oxide layer. XPS depth profiling confirmed that the Ti metal’s surface transformed into a native oxide layer of TiO2 on Ti metal. The SERS signal of the Au NDA was approximately 104 times higher than that of the Ti/TiO2 NDA. The Au NDA’s LSPR had a strong peak centered at 659 nm, while the Ti/TiO2 NDA’s LSPR had a broad peak centered at ~420 nm. The Ti/TiO2 NDA’s LSPR was weak because the native surface layer dampened the LSPR but was clearly observed in the visible light region. These results suggested that Ti/TiO2 NDA has the potential for use as a visible light-responsive photocatalyst in solar energy systems.

Improved methodological concepts for processing liquid Mg at high temperature

Abstract In this paper, new improvements of methodological concepts upon examining wettability of high vapor pressure liquid metal systems (e.g. Mg-based alloys) in contact with refractory materials, are presented and discussed. In this regard, high-temperature experiments on molten magnesium (Mg) in contact with graphite as a refractory substrate, were performed by utilizing a newly developed testing device and by applying a suitable experimental procedure. The wetting experiments were carried out by the sessile drop method and under identical testing conditions (700 °C/10 min under a protective gas atmosphere). Two different procedures were applied: the classical contact heating (CH) or a newly introduced capillary purification (CP) one. The contact angle behaviors observed under the same conditions were strongly influenced by the applied procedure. Specifically, in the case of using the CH procedure, a presence of native surface oxide layer on the metal surface hinders the observations of melting process, making not possible to experimentally determine the wetting kinetics curve θ=f(t). Contrarily, during the wetting test performed on the Mg/graphite couple by applying the CP procedure, the native surface oxide layer was mechanically removed during the squeezing of the molten Mg through the hole of a capillary. Indeed, an oxide-free squeezed Mg-drop with regular and spherical shape was successfully obtained and dispensed on the graphite substrate. Consequently, the reliable contact angle value around θ=150° for the Mg/graphite system, was measured within the wetting test.

Native Oxide Seeded Spontaneous Integration of Dielectrics on Exfoliated Black Phosphorus.

Two-dimensional (2D) semiconductors have been a central focus for next-generation electronics and optoelectronics owing to their great potential to extend the scaling limits in a silicon transistor. However, due to the lack of surface dangling bonds in most 2D semiconductors, such as graphene and transition metal dichalcogenides (TMDs), the direct growth of the high-κ film on these 2D materials via an atomic layer deposition (ALD) technique often produces dielectrics with poor quality, which hinders their integration in the modern semiconductor industry. Here, we comprehensively investigate the ALD growth of the Al2O3 layer on 2D exfoliated black phosphorus (BP). Intriguingly, we found that the 2D BP with "silicon-like" characteristics possesses a native surface oxide layer PxOy after air exposure. The PxOy-induced surface dangling bonds enable the spontaneous integration of the high-quality Al2O3 layer on the BP flake without any pretreatments to functionalize the surface. Additionally, the Al2O3 layer could effectively passivate BP to prevent its degradation in ambient conditions, which addresses the most serious problem of the BP material. Moreover, the Al2O3-encapsulated BP field-effect transistor (FET) exhibits good electrical transport performance, with a high hole mobility of ∼420 cm2 V-1 s-1 and electron mobility of ∼80 cm2 V-1 s-1. Moreover, the high-quality Al2O3 layer can also be integrated into the top-gated BP transistor and inverter. Our findings reveal the silicon-like characteristics of BP for the high-κ ALD dielectric growth technology, which promises the seamless integration of 2D BP in the modern semiconductor industry.

Intermediate Structures of Pt-Ni Nanoparticles during Selective Chemical and Electrochemical Etching.

Both chemical and electrochemical etching are effective methods for tailoring the surface composition of Pt-based catalytic bimetallic nanoparticles (NPs). However, the detailed nanoscale etching mechanisms, which are needed for achieving fine control over the etch processes, are still not understood. Here, we study selective chemical and electrochemical Ni etching of Pt-Ni rhombic dodecahedron NPs using in situ liquid-phase transmission electron microscopy. Our real-time observations show that the intermediate NP structures evolve differently in the two cases. Chemical etching of Ni starts from localized pits on the NP surface, in contrast to the uniform dissolution of Ni during the electrochemical etching. Our study reveals how oxidative etching participates in the removal of a non-noble metal and the subsequent formation of noble-metal-rich NPs. The mechanistic insights reported here highlight the role of a native surface oxide layer on the etching behavior, which is important for the design of NPs with specific surface composition for applications in electrocatalysis.

Structure and chemistry of surface oxides on ZnMgAl corrosion protection coatings with varying alloy composition

A principal investigation of the surface and shallow sub-surface region of corrosion protection coatings based on the ZnMgAl system was performed for three different compositions of Mg and Al, varying from 1.5 wt% up to 3.7 wt%, which are known to behave on a macroscopic scale in an interchangeable manner with respect to properties including corrosion resistance, forming behavior or weldability. Therefore, special attention was paid to the effect of the Mg and Al concentration on the chemical composition, structure and the thickness of the resulting native oxide surface layer as well as on the underlying microstructure of the coating on the nanometer scale. For this purpose the chemical composition of the naturally grown oxide surface layer was analyzed by means of X-ray photoelectron spectroscopy, whereas Auger electron spectroscopy was applied in order to obtain the spatial distribution of the surface constituents. Back-scattered electron micrographs revealed in addition the underlying coating microstructure. The thickness of the oxide surface layers was investigated by transmission electron microscopy. It was found that the resulting surface layers are rather similar, especially in terms of their chemical nature, and highly uniform, despite the high variations of Mg and Al concentrations of more than 100% for the different coatings. Only for the lowest here considered amounts of Mg and Al the thickness and composition of the oxide surface layer starts to reflect the underlying complex microstructure.

Impact-bonding with aluminum, silver, and gold microparticles: Toward understanding the role of native oxide layer

Bonding during cold spray has been phenomenologically associated with the breakup of the native surface oxide layer to produce intimate metallic contacts. Here, we resolve the instant of bonding for metals both with and without a significant native oxide, through direct in-situ observation of individual microparticles. We show that fracture of the native oxide layer is not the only physical barrier to impact adhesion; even noble gold must exhibit unstable jetting upon impact in order to attain bonding. However, surface oxide certainly presents an additional barrier to bonding and can increase the critical velocity by a large factor.

Reprint of “Modification of the GaAs native oxide surface layer into the layer of the Ga2O3 dielectric by an Ar+ ion beam”

Poor dielectric properties of GaAs oxides are the drawback of the GaAs-based electronics preventing using these oxides as dielectric layers. The elemental and chemical compositions of the GaAs native oxide layer slightly irradiated by Ar+ ions with the fluence Q ~1 ∗ 1014 ions/cm2 have been studied by the synchrotron-based photoelectron spectroscopy. The effect of selective and total decay of arsenic oxides followed by diffusive escape of arsenic atoms from the oxide layer has been revealed. The effect results in three-fold Ga enrichment of the upper layer of the native oxide and in strong domination (~90 at%) of the Ga2O3 phase which is known to be a quite good dielectric with the bandgap width as wide as 4.8 eV. A band diagram was obtained for the native oxide nanolayer on the n-GaAs wafer. It has been shown that this natural nanostructure has a character of a p-n heterojunction.

Modification of the GaAs native oxide surface layer into the layer of the Ga2O3 dielectric by an Ar+ ion beam

Poor dielectric properties of GaAs oxides are the drawback of the GaAs-based electronics preventing using these oxides as dielectric layers. The elemental and chemical compositions of the GaAs native oxide layer slightly irradiated by Ar+ ions with the fluence Q ~1 ∗ 1014 ions/cm2 have been studied by the synchrotron-based photoelectron spectroscopy. The effect of selective and total decay of arsenic oxides followed by diffusive escape of arsenic atoms from the oxide layer has been revealed. The effect results in three-fold Ga enrichment of the upper layer of the native oxide and in strong domination (~90 at%) of the Ga2O3 phase which is known to be a quite good dielectric with the bandgap width as wide as 4.8 eV. A band diagram was obtained for the native oxide nanolayer on the n-GaAs wafer. It has been shown that this natural nanostructure has a character of a p-n heterojunction.

On the Effect of Native SiO2 on Si over the SPR-mediated Photocatalytic Activities of Au and Ag Nanoparticles.

In hybrid materials containing plasmonic nanoparticles such as Au and Ag, charge-transfer processes from and to Au or Ag can affect both activities and selectivity in plasmonic catalysis. Inspired by the widespread utilization of commercial Si wafers in surface-enhanced Raman spectroscopy (SERS) studies, we investigated herein the effect of the native SiO2 layer on Si wafers over the surface plasmon resonance (SPR)-mediated activities of the Au and Ag nanoparticles (NPs). We prepared SERS-active plasmonic comprised of Au and Ag NPs deposited onto a Si wafer. Here, two kinds of Si wafers were employed: Si with a native oxide surface layer (Si/SiO2 ) and Si without a native oxide surface layer (Si). This led to Si/SiO2 /Au, Si/SiO2 /Ag, Si/Au, and Si/Ag NPs. The SPR-mediated oxidation of p-aminothiophenol (PATP) to p,p'-dimercaptoazobenzene (DMAB) was employed as a model transformation. By comparing the performances and band structures for the Si/Au and Si/Ag relative to Si/SiO2 /Au and Si/SiO2 /Ag NPs, it was found that the presence of a SiO2 layer was crucial to enable higher SPR-mediated PATP to DMAB conversions. The SiO2 layer acts to prevent the charge transfer of SPR-excited hot electrons from Au or Ag nanoparticles to the Si substrate. This enabled SPR-excited hot electrons to be transferred to adsorbed O2 molecules, which then participate in the selective oxidation of PATP to DMAB. In the absence of a SiO2 layer, SPR-excited hot electrons are preferentially transferred to Si instead of adsorbed O2 molecules, leading to much lower PATP oxidation.

A study of the nanoscale and atomic-scale wear resistance of metallic glasses

Metallic glasses are found to be applicable in micromechanical systems. When the size of the mechanical components is reduced to the micrometer and sub-micrometer level, the native surface oxide layer starts playing an important role in contact mechanical applications of metallic glasses. The nanoscale and atomic-scale scratch wear resistance of metallic glasses is studied in the present work using an atomic force microscopy at different loads. The wear rate is correlated with the properties of the metallic glasses studied and their respective surface oxides.

Effects of Titanium Layer Oxygen Scavenging on the High-k/InGaAs Interface.

One of the main challenges in the path to incorporating InGaAs based metal-oxide-semiconductor structures in nanoelectronics is the passivation of high-k/InGaAs interfaces. Here, the oxygen scavenging effect of thin Ti layers on high-k/InGaAs gate stacks was studied. Electrical measurements and synchrotron X-ray photoelectron spectroscopy measurements, with in situ metal deposition, were used. Oxygen removal from the InGaAs native oxide surface layer remotely through interposed Al2O3 and HfO2 layers observed. Synchrotron X-ray photoelectron spectroscopy has revealed a decrease in the intensity of InOx features relative to In in InGaAs after Ti deposition. The signal ratio decreases further after annealing. In addition, Ti 2p spectra clearly show oxidation of the thin Ti layer in the ultrahigh vacuum XPS environment. Using capacitance-voltage and conductance-voltage measurements, Pt/Ti/Al2O3/InGaAs and Pt/Al2O3/InGaAs capacitors were characterized both before and after annealing. It was found that the remote oxygen scavenging from the oxide/semiconductor interface using a thin Ti layer can influence the density of interface traps in the high-k/InGaAs interface.

Thermal outgassing rates of low-carbon steels

Outgassing rates of three low-carbon steels were measured using rate-of-rise and throughput methods. Outgassing rates of water vapor during pump-down were higher than those of stainless steels, probably due to the nature of native surface oxide layer. However, hydrogen outgassing rates without a high temperature pretreatment were as low as (1–4) × 10−10 Pa m3 s−1 m−2, which is much lower than that of untreated stainless steels. No dramatic reduction was observed in H2 outgassing after vacuum annealing at 850 °C for 12 h, suggesting that the low-carbon steels had been fully degassed during the steelmaking processes. This may be due to the use of the Ruhrstahl-Hausen vacuum process during steel refining instead of an older process, such as argon-oxygen decarburization. The extremely low H2 outgassing rate from low-carbon steels makes them applicable for use in ultrahigh vacuum or even extreme high vacuum applications, particularly where magnetic field shielding is needed.

The size effect of titania-supported Pt nanoparticles on the electrocatalytic activity towards methanol oxidation reaction primarily via the bifunctional mechanism

We prepared Pt nanoparticles of different particle sizes by plasma enhanced atomic layer deposition (PEALD) on the native oxide surface layer of Ti thin films, and investigated the Pt particle size effect on the electrocatalytic activity towards methanol oxidation reaction (MOR) in acidic media. The average Pt nanoparticles size ranges from 3 nm to 7 nm depending on the number of the PEALD reaction cycles. The electronic interaction between Pt nanoparticles and the TiO2 support is insignificant according to x-ray photoelectron spectroscopy analyses, suggesting that the influence of the Pt particle size on the electrocatalytic activity can be mainly described by the bifunctional mechanism. From cyclic voltammetry measurements, Pt particles of smaller size have a better CO tolerance in MOR. We proposed the reaction steps for the electrooxidation of CO adspecies on Pt nanoparticles on the basis of the bifunctional mechanism. The electrode with Pt nanoparticles of ∼5 nm in size shows the best electrocatalytic performance in terms of CO tolerance and electrochemical stability.

Toward Silicon Anodes for Next-Generation Lithium Ion Batteries: A Comparative Performance Study of Various Polymer Binders and Silicon Nanopowders

Silicon is widely regarded as one of the most promising anode materials for lithium ion and next-generation lithium batteries because of its high theoretical specific capacity. However, major issues arise from the large volume changes during alloying with lithium. In recent years, much effort has been spent on preparing nanostructured silicon and optimizing various aspects of material processing with the goal of preserving the electrode integrity upon lithiation/delithiation. The performance of silicon anodes is known to depend on a large number of parameters and, thus, the general definition of a "standard" is virtually impossible. In this work, we conduct a comparative performance study of silicon anode tapes prepared from commercially available materials while using both a well-defined electrode configuration and cycling method. Our results demonstrate that the polymer binder has a profound effect on the cell performance. Furthermore, we show that key parameters such as specific capacity, capacity retention, rate capability, and so forth can be strongly affected by the choice of silicon material, polymer binder and electrolyte system - even the formation of metastable crystalline Li15Si4 is found to depend on the electrode composition and low potential exposure time. Overall, the use of either poly(acrylic acid) with a viscosity-average molecular weight of 450.000 or poly(vinyl alcohol) Selvol 425 in combination with both silicon nanopowder containing a native oxide surface layer of ∼1 nm in diameter and with a monofluoroethylene carbonate-based electrolyte led to improved cycling stability at high loadings.

Annealing induced low coercivity, nanocrystalline Co–Fe–Si thin films exhibiting inverse cosine angular variation

Co–Fe–Si based films exhibit high magnetic moments and are highly sought after for applications like soft under layers in perpendicular recording media to magneto-electro-mechanical sensor applications. In this work the effect of annealing on structural, morphological and magnetic properties of Co–Fe–Si thin films was investigated. Compositional analysis using X-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a native oxide surface layer consisting of oxides of Co, Fe and Si on the surface. The morphology of the as deposited films shows mound like structures conforming to the Volmer–Weber growth model. Nanocrystallisation of amorphous films upon annealing was observed by glancing angle X-ray diffraction and transmission electron microscopy. The evolution of magnetic properties with annealing is explained using the Herzer model. Vibrating sample magnetometry measurements carried out at various angles from 0° to 90° to the applied magnetic field were employed to study the angular variation of coercivity. The angular variation fits the modified Kondorsky model. Interestingly, the coercivity evolution with annealing deduced from magneto-optical Kerr effect studies indicates a reverse trend compared to magetisation observed in the bulk. This can be attributed to a domain wall pinning at native oxide layer on the surface of thin films. The evolution of surface magnetic properties is correlated with morphology evolution probed using atomic force microscopy. The morphology as well as the presence of the native oxide layer dictates the surface magnetic properties and this is corroborated by the apparent difference in the bulk and surface magnetic properties.

Enhanced Lithiation of Doped 6H Silicon Carbide (0001) via High Temperature Vacuum Growth of Epitaxial Graphene

The electrochemical lithiation capacity of 6H silicon carbide (0001) is found to increase by over 1 order of magnitude following graphitization at 1350 °C in ultrahigh vacuum. Through several control experiments, this Li-ion capacity enhancement is correlated with SiC substrate doping and removal of the native oxide surface layer by thermal annealing, which renders both the bulk and surface electrically conductive. Characterization via multiple depth-resolved spectroscopies shows that lithium penetrates the activated SiC upon lithiation, the bulk lattice spacing does not appreciably change, and the surface structure remains largely intact. The electron energy-loss spectroscopy (EELS) extracted compositional ratio of Li to Si is approximately 1:1, which indicates an intrinsic bulk Li capacity in activated SiC of 670 mAh g–1. In addition, inelastic X-ray scattering spectra show changes in the Si chemical bonding configuration due to lithiation. X-ray scattering data show a decrease in the SiC Bragg peak intensity during lithiation, suggesting changes to the bulk crystallinity, whereas the emergence of a diffuse scattering feature suggests that lithiation is associated with the development of substrate defects. Overall, these results illustrate that the electrochemical capacity of a traditionally inert refractory material can be increased substantially via surface modification, thus suggesting a new strategy for improving the performance of next generation Li-ion battery electrodes.