Initial Oxide Growth Research Articles

Temperature Dependent Operando High Energy Surface X-Ray Diffraction Studies of Pt(111) Oxidation

Platinum electrocatalysts are still widely used as cathode material in proton-exchange membrane fuel cells (PEMFC) because of their high ORR activity. However, their long-term stability is limited due to degradation processes. This degradation is partly caused by Pt oxidation and oxide reduction, which lead to irreversible surface restructuring and subsequently Pt dissolution. To increase the lifetime of PEMFCs it is mandatory to understand the mechanisms behind the degradation on an atomistic level. This requires studies by structure sensitive in situ methods, in particular, also studies at the elevated temperatures typically employed in PEMFCs operation.Previous studies revealed that in the initial stage of oxide formation Pt atoms are extracted out of the surface and form an ultra-thin oxide layer directly above the surface. This extraction is long known as place exchange [1], but the atomic-scale details of the surface oxide structure, the extraction mechanism, and especially the kinetics of the initial oxide growth were only recently clarified [2-7]. It was found that the structural arrangement of the Pt atoms within the surface oxide on the Pt(111) and the Pt(100) surface is distinctly different, leading to differences in the oxidation/reduction kinetics, the reversibility of the surface oxidation/reduction process, and the amount of Pt dissolution into the electrolyte.Recently, we performed fast operando high-energy surface X-ray diffraction (HESXRD) studies, in which we investigated the initial extraction of Pt atoms on Pt(111) during potential cycles. Surprisingly, it was found that the Pt extraction during the anodic sweep is largely independent of the potential sweep rate, although the peak cyclic voltammograms (CVs) associated with the charge transfer during surface oxidation showed a pronounced shift [7-9]. This suggests that the Pt extraction is a fast process, rather driven by potential and not directly coupled to the slow formation of surface oxygen species. However, the Pt extraction during the anodic sweep and the re-insertion during the cathodic sweep still exhibit significant hysteresis, indicating a complex multistep oxidation kinetics.Previous electrochemical studies of Pt(111) electrodes revealed that the described influence of oxidation/reduction kinetics and the reversibility of the charge transfer processes increase at elevated temperature [10]. However, the temperature dependence of the Pt extraction behavior is currently unknown, because all previous structural studies of oxide growth and reduction have been performed exclusively at room temperature.For the experiments at elevated temperatures we developed a new setup for HESXRD studies of single crystals, which combines a hanging meniscus electrochemical cell for operando X-ray diffraction measurements with an inductive heating device in a joint Ar-filled compartment. This setup allows direct (re-)preparation of the aligned Pt single crystals on the diffractometer. By this, the time between preparation and measurement is reduced down to a few minutes and sample transfer through air is rigorously avoided.Temperature-dependent HESXRD measurements at temperatures up to 343 K were performed at beamline ID31 of the European Synchrotron Radiation Facility. The simultaneously measured electrochemical current and X-ray intensity indicate kinetic shifts in the charge transfer but a rather temperature independent Pt extraction process.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1.

Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film.

Etching of molybdenum via a combination of low-temperature ozone oxidation and wet-chemical oxide dissolution

Etching of molybdenum was demonstrated in two steps. Mo was first oxidized in an ozone gas ambient to form molybdenum oxide. It is shown that comparable oxide thicknesses can be obtained in ozone and oxygen but at lower temperatures for the former. Initial oxide growth is fast but then considerably slows down due to its diffusion-limited character. The metal-oxide thickness can be controlled by temperature and defines the amount of metal etch per cycle (EPC). XPS analysis showed that the thermally grown oxide is MoO3. In the second, wet-chemical step, MoO3 was dissolved selectively toward the Mo metal using an aqueous solution. The dissolution rate of amorphous MoO3 formed in O3 at temperatures below ∼230 °C is fast, but the dissolution of MoO3 formed at Tox > 230 °C was shown to be incomplete. Cross-section TEM showed a matrix of amorphous oxide with crystallized MoO3 islands, the latter more difficult to dissolve. However, the crystalline phase could be completely and selectively removed using a more concentrated NH4OH solution at an elevated temperature (70 °C). The EPC was determined for temperatures between 150 and 290 °C. The etch rates increased with temperature from 1–2 nm/cycle at 150 °C to 5–6 nm/cycle at 290 °C. This hybrid thermal-wet etching sequence is well suited for vertical and lateral recess etching as it shows a controlled and isotropic dissolution of polycrystalline Mo at the nanoscale. Furthermore, the process shows a progressive surface smoothening upon increasing the number of etching cycles.

Atomic Scale Mechanisms of Multimode Oxide Growth on Nickel-Chromium Alloy: Direct In Situ Observation of the Initial Oxide Nucleation and Growth.

The initial growth mode of oxide on alloy plays a decisive role in the development of protective oxide scales on metals and alloys, which is critical for their functionality for high temperature applications. However, the atomistic mechanisms dictating that the oxide growth remain elusive due to the lack of direct in situ observation of the initial oxide nucleation and growth at atomic-scale. Herein, we employed environmental transmission electron microscopy and the first-principles calculations to elucidate the initial atomic process of nickel-chromium (Ni-Cr) alloy oxidation. We directly revealed three different oxide growth modes of initial NiO islands on Ni-Cr alloy upon oxidation by O2, which result in distinct crystallography and morphology. The multimode oxide growth leads to irregular-shaped oxides, which is shown to be sensitive to the local mass transport. This localization of oxide growth mode is also demonstrated by the identified vigorous competence in oxide growth and thus shown to be kinetically controlled. The concept exemplified here provides insights into the oxide formation and has significant implications in other material and chemical processes involving oxygen gas, such as corrosion, heterogeneous catalysis, and ionic conduction.

Machine Learning Applied to a Variable Charge Atomistic Model for Cu/Hf Binary Alloy Oxide Heterostructures

Alloy oxide heterostructures are essential for a wide variety of energy applications, including anticorrosion coatings, bifunctional catalysis, energy storage, as well as emerging platforms for multilevel nonvolatile memory and neuromorphic devices. The key role played by oxide composition, density, and stoichiometry in governing electrochemical reactions, interface structure, and ionic transport in alloy oxides remains largely unknown. This is primarily due to the lack of variable charge interatomic potential models that can adequately describe the various atomic/ionic interactions in alloy oxides within a unified framework. Here, we introduce a charge transfer ionic potential (CTIP) model for Cu/Hf/O alloy system and demonstrate its ability to accurately capture the complex potential energy surface owing to dynamically varying interactions, including metallic (Cu/Hf), ionic (Cu/Hf/Ox), mixed environment (interfaces). We leverage supervised machine learning methods powered by genetic algorithms coupled with local simplex optimization to efficiently navigate through a high-dimensional parameter space and identify an optimum set of an independent set of CTIP parameters. We train our model against an extensive first-principles based data set that includes lattice constants, cohesive energies, equations of state, and elastic constants of various experimentally observed polymorphs of hafnia, copper oxide, hafnium, as well as copper. Our machine-learned CTIP model captures the structure, elastic properties, thermodynamics, and energetic ordering of various polymorphs. It also accurately predicts the surface properties of both oxides and their metals. To demonstrate the suitability of our CTIP model for investigating dynamic processes, we employed it to identify the atomistic scale mechanisms associated with the initial nanoscale oxidation and surface oxide growth on Cu-doped hafnia surfaces. Our machine-learned CTIP model can be used to probe the dynamic response of Cu/Hf/O alloy interfaces subjected to external stimuli (e.g., electric field, pressure, temperature, strain, etc.) and a variety of atomistic phenomena including the dynamics of switching in emerging neuromorphic platforms.

In situ XPS study of low temperature atomic layer deposition of B2O3 films on Si using BCl3 and H2O precursors

In this study, atomic layer deposition (ALD) of nanoscale boron oxide (B2O3) films on Si using BCl3/H2O precursors at room temperature was investigated using in situ x-ray photoelectron spectroscopy (XPS). B2O3 thin films are of considerable interest in ultra-shallow Si doping applications, including high aspect ratio FinFET structures, where film conformality and precise thickness control are crucial. ALD is therefore of particular interest for such applications. XPS data demonstrate that initial BCl3 exposures on Si at room temperature are self-limiting and are accompanied by partial B-Cl dissociation and Cl-Si formation. H2O exposures >1.7 × 108 Langmuir at room temperature removed Cl from B sites, but failed to remove Cl from the Cl-Si species. ALD-type growth of B2O3 was observed with an average growth rate of ∼2.5 Å/cycle and without further increase in the Cl content. The initial Cl contamination was due to Cl-Si bond formation at the interface, without Cl incorporation into the oxide film. The role of Cl in the inhibition of oxide film growth was further investigated by the reactions of BCl3/O2 on SiO2 at room temperature and 650 K. BCl3/O2 precursors exhibit negligible B2O3 growth at room temperature. At 650 K, B2O3 growth was observed with a decreasing growth rate per BCl3/O2 cycle, corresponding to an increase in the Cl:B atomic ratio. These data indicate that room temperature ALD using BCl3/H2O precursors is a potential route toward the formation of uniform B2O3 films for shallow Si doping applications, but that Cl-Si formation significantly impacts initial oxide nucleation and growth.

Cluster-size dependent phase transition of Co oxides on Au(111)

The surface structure of Co oxide nanoparticles supported on Au(111) was studied using scanning tunneling microscopy (STM) and ultra-violet photoelectron spectroscopy (UPS). Initial Co oxide growth at oxygen pressures orders of magnitude above those required to form bulk Co3O4 resulted solely in small 2-D CoO islands, exhibiting a moiré surface characteristic of the rocksalt (111) surface; continued growth led to a transition to fully oxidized 3-D Co3O4 spinel clusters. The CoO–Co3O4 transition is reversible by reduction in vacuum. It is shown that the stabilization of the reduced phase at low coverages can be explained by the higher surface energy of the support and the Co3O4 spinel structure; since a bilayer is the smallest repeat unit of this structure, oxidation increases the exposed area of the high surface energy substrate. In addition, the 2D to 3D transition required to form Co3O4 increases the occupancy of high energy edge sites, thus further stabilizing small CoO clusters against oxidation. The results show how support interactions and particle size can be used to tune the stability of oxide phases.

Model Calculations of SiC Oxide Growth Rates at Sub-Atmospheric Pressures Using the Si and C Emission Model

We found that the ‘Si and C emission model’ that we proposed as an oxidation model of SiC could not reproduce the initial oxide growth rates of SiC at sub-atmospheric pressures. The comparison between calculated and observed growth rates suggests that the oxide growth on the oxide surface is enhanced in the initial oxidation stage and thus our oxidation model is inaccurate in the description on the surface oxidation. Accordingly, we reconsidered the parameters on surface oxidation and, as a result, found that a much enlarged oxygen concentration on the oxide surface is necessary for solving the discrepancy between calculated and observed growth rates.

Strong Temperature Dependence of the Initial Oxide Growth on the Si(111)7 × 7 Surface

Strong Temperature Dependence of the Initial Oxide Growth on the Si(111)7 × 7 Surface

<i>In Situ</i> Spectroscopic Ellipsometry Study of SiC Oxidation at Low Oxygen-Partial-Pressures

We have investigated the oxidation process of SiC (000-1) C-face at low oxygen partial pressures using an in-situ spectroscopic ellipsometry. The oxide growth rate decreased steeply at the early stage of oxidation and then slowly decreased with increasing oxide thickness. The initial oxide growth rate was almost proportional to the oxygen partial pressure for both the polar directions. This result suggests that the initial interfacial reaction rate is constant regardless of the concentration of oxidants reaching the interface.

MBE grown high-quality Gd 2O 3/Si(1 1 1) hetero-structure

A nearly lattice-matched Gd 2O 3/Si(1 1 1) hetero-epitaxy was demonstrated using molecular beam epitaxy (MBE). Detailed structural studies find that the nano thick Gd 2O 3 films have a cubic phase with a very uniform thickness, an excellent crystallinity and atomically sharp interfaces. These features are characterized by the bright, streaky reconstructed reflection high-energy electron diffraction (RHEED) patterns at the initial oxide growth, the pronounced interference fringes in the X-ray reflectivity curve as well as in the crystal truncation rod around the substrate diffraction peaks using the high-resolution X-ray diffraction. The (1 1 1) axis of the thin oxide is oriented parallel to the substrate (1 1 1) normal with a 60° in-plane symmetry rotation.

Growth and decay of the Pd(1 1 1)–Pd 5O 4 surface oxide: Pressure-dependent kinetics and structural aspects

Growth and decomposition of the Pd 5O 4 surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O 2 gas pressures between 10 −7 and 6 × 10 −5 mbar, by means of an effusive O 2 beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd 5O 4 surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10 −6 mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O 2 pressure pulse and then following further growth kinetics in the lower (10 −6 mbar) pressure range. Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd 5O 4 layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a ( 67 × 67 ) R 12.2 ° superstructure. Isothermal decay of the Pd 5O 4 oxide layer at 693 K involves at first a rearrangement into the ( 67 × 67 ) R 12.2 ° structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.

Oxidation of hydrogenated crystalline silicon as an alternative approach for ultrathin SiO2 growth

This article is devoted to the growth of thin thermal SiO2 layers on hydrogenated Si substrates as a possibility to obtain improved properties of oxide-Si structure for contemporary MOS devices. Spectral ellipsometry and atomic force microscopy have been applied for characterization of the SiO2/Si interface and surface morphology. The results show that the oxidation kinetics obeys longer linear time dependence in thin-film regime. The smaller activation energies and higher initial oxide growth rates indicate facilitation of oxide growth in the early oxidation stage. Formation of a reactive less dense Si surface layer is suggested to be responsible for the higher kinetics rates and oxide thicknesses in the very initial stage.

Bias-assisted photoelectrochemical oxidation of n-GaN in H2O

Growth of gallium oxide on n-GaN was realized in H2O by bias-assisted photoelectrochemical (PEC) oxidation using Al as a counterelectrode instead of a Pt commonly used in the PEC process. Although the growth of the oxide was not observed at below 2 V, the initial oxide growth rate of 8.7 nm/min was shown at a bias of 15 V and ultraviolet light intensity of 300 mW/cm2. However, the growth rate lowered and oxide thickness was saturated to 340 nm. The saturated oxide thickness and initial growth rate were increased with the applied bias. The homogeneous oxide growth and near stoichiometric composition of Ga2O3 were observed in Auger electron spectroscopy analysis results.

Silicon Orientation Effects in the Initial Regime of Wet Oxidation

Atmospheric pressure wet oxidations at ∼785°C were performed on (100), (110), and (111) oriented silicon wafers. The experimental data revealed that the (110) plane experiences rapid initial oxide growth of the same order of magnitude as that which is known to occur during dry oxidation of all orientations of Si. The data also indicated that a crossover in the oxidation rates of the (110) and (111) surfaces occurs similar to that which has been reported for oxidation in dry ambients. Initially, the (110) orientation oxidizes fastest yielding the oxidation rate order As the growth proceeds, however, the oxidation rate of the (111) plane surpasses that of the (110) plane resulting in the order These two anomalous characteristics of silicon oxidation have previously been believed to be limited to dry oxidation. The discovery that rapid initial oxide growth also occurs during wet oxidation of (110) oriented Si reveals that the understanding of these anomalous oxidation kinetics must encompass both wet and dry oxidants and the strong dependence of the phenomena on the structure of the Si crystal surface. © 2002 The Electrochemical Society. All rights reserved.

Spectroscopic ellipsometry of oxides and interfaces thermally formed on (100)Si and (111)Si

Thin oxides and their interface, formed by dry oxidation of hydrogen-enriched (100)Si and (111)Si substrates via r.f. hydrogen plasma exposure at 850°C, are studied by multiple-angle spectroscopic ellipsometry in the range 280–632.8 nm. Comparison with the standard oxidation of RCA cleaned Si shows that on plasma treated Si the initial oxide growth rate is higher and a thicker interface is formed with a different composition. Various processes involving hydrogen are suggested to be responsible for changes in interface properties.

A study on the oxidation kinetics of silicon in inductively coupled oxygen plasma

The oxidation kinetics of silicon in inductively coupled oxygen plasma (ICP) was studied at temperatures ranging from 350 to 450 °C. The oxide growth kinetics was described by a linear-parabolic growth law, with a rapid initial growth and a negative linear-rate constant. Under oxygen pressure of 10 mTorr, the initial oxide growth at 350 °C (thickness below 25 nm) was faster than at 400 °C. An analysis of transverse-optical mode frequencies and etch rates indicated that the density of the surface oxide was lower than that of the bulk oxide. The oxidation kinetics was explained qualitatively by assuming that the ICP oxide consisted of a surface layer with a larger diffusion coefficient and a bulk layer with a smaller diffusion coefficient. On the other hand, the ICP oxidation of silicon with a thin chemical oxide showed a positive linear-rate constant and no surface layer effect, supporting the fact that the oxide grown by the ICP oxidation has a low-density surface layer with a larger diffusion coefficient.

Critical oxide cluster size on Si(111)

The initial stage of oxide growth and subsequent oxide decomposition on Si(111)-7×7 at temperatures between 350 and 720°C are studied with the optical second harmonic generation for O2 pressures (Pox) between 5×10−9 and 4×10−6Torr. The obtained pressure dependencies of the initial oxide growth rate (Rgr) and the subsequent oxide decomposition rate are associated with the cluster-forming nature of the oxidation process. For the model of oxide cluster nucleation and growth, a scaling relationship is derived among the critical oxide cluster size, i, and the experimentally measurable values of Rgr and Pox. The critical oxide cluster size, i, thus obtained from the kinetic data increases with temperature. This correlates with an increase of desorption channels and their rates in that the competition between growth and decomposition requires more stable oxide clusters, i.e. clusters with a larger critical size, for oxide to grow at higher temperatures. The increase of i with decreasing Pox is related with a decrease of Rgr: a decreased Rgr requires critical clusters with a longer lifetime, i.e. clusters with a larger size.

Role of alloying elements in steels on metal dusting

Metal dusting of Fe-Ni-Cr alloys has been observed in industrial processes in strongly carburizing atmospheres at temperatures from 450 °C to 800 °C. At temperatures below 650 °C the alloys are generally not able to form dense, well adherent oxide layers in spite of relatively high Cr-contents, therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidate the mechanism and to compare the resistance against metal dusting for high alloy steels [1] The intention of this study was to obtain additional information concerning the role of alloying elements and the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. The susceptibility to metal dusting was determined by measuring the metal loss under metal dusting conditions of Fe-20%Cr-32%Ni alloys modified with additions of different carbide formers (W, Mo, Nb) or oxide formers (Si, Al). The samples were exposed at 600°C in a CO-H 2 -H 2 O-gas mixture for repeated periods up to 500-1500 h. The attack by the oxidizing and carburizing atmosphere leads to the precipitation of internal carbides and metal dusting and more or less to formation of an oxide layer. In comparison to the undoped material, the addition of carbide formers retards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dusting under the given laboratory conditions. When carbides are present at the metal surface, they affect the initial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effect of Ce, this rare earth element is generally known to favour Cr-oxide formation and to improve the adherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting and metal wastage.

The Structure Change of Liquid Phase Deposited Silicon Oxide by Water Dilution

The silicon oxyhydrogen (Si‒OH) bonds were found in large amounts in conventional liquid‐phase‐deposited silicon dioxide using silica powder or gel as a saturation reagent in hydrofluorosilicic acid . However, a more dense structure of LPD‐ devoid of Si‒OH bonds was found if the reagent is changed to silicic acid and the concentration of is kept above 2 M. At 30°C, the water dilution of from 2.5 to 1 M tends to enhance the deposition rate of silicon oxide and to introduce substantial amount of Si‒OH bonds into the film when concentration drops to below 1.5 M. This is attributed to the fact that water dilution tends to dilute the concentration of all components but the decay of etching rate of by HF is much faster than the growth rate of , because the removal of one silicon needs more HF molecules (from one to four HF) and the growth of one silicon needs only one intermediate species. The growth rate of silicon oxide decreases when concentration is further lowered to 0.5 M. The delay time during initial oxide growth follows the growth rate, it is concluded that the delay time is the time that the solution needs to modify part of the Si‒OH or Si‒H bonds on the growth surface of to Si‒F bonds by hydrofluoric acid (HF).