Initial Oxidation Process Research Articles

An Accurate Growth Mechanism and Photocatalytic Degradation Rhodamine B of Crystalline Nb2O5 Nanotube Arrays

To effectively improve photocatalytic activity, the morphology and crystallinity of semiconductor photocatalysts must be precisely controlled during the formation process. Self-aligned Nb2O5 nanotube arrays have been successfully fabricated using the electrochemical anodization method. A novel growth mechanism of Nb2O5 nanotubes has been proposed. Starting from the initial oxidation process, the “multi-point” corrosion of fluoride ions is a key factor in the formation of nanotube arrays. The inner diameter and wall thickness of the nanotubes present a gradually increasing trend with increased dissociative fluorine ion concentration and water content in the electrolyte. With dehydroxylation and lattice recombination, the increased crystallinity of Nb2O5 represents a reduction of lattice defects, which effectively facilitates the separation and suppresses the recombination of photo-generated carriers to enhance their catalytic degradation activity.

Initial oxidation and surface stability diagram of Ge(100) as a function of the temperature and oxygen partial pressure through ab initio thermodynamics

Density functional theory calculations in conjunction with thermodynamic modeling were performed to examine the oxygen adsorption on a Ge(100) c(4 × 2) surface and the subsequent initial oxidation. For several possible adsorption sites, the adsorption energy of atomic oxygen as well as the atomic configuration and electronic properties of the adsorbed structure were examined. Then the effects of the surface coverage of oxygen from 1/64 to 1/4 monolayers on the adsorption energy were considered. Through the surface Gibbs free energy as a function of the temperature (T) and oxygen partial pressure (), the (T, ) surface stability diagram was predicted for the O/Ge(100) c(4 × 2) surface. The theoretical prediction well reproduced previous experimental observations and provides an insight to control the initial oxidation process of Ge surface with tuned T

Initial oxidation processes of ultrathin hafnium film and hafnium disilicide islands on Si(100)-2 × 1 surfaces studied using core-level X-ray photoelectron spectroscopy

We investigated the initial oxidation of ultrathin hafnium (Hf) film on Si(100)-2 × 1 [Hf/Si(100)] using high resolution Hf 4f5/2, 7/2, Si 2p1/2, 3/2, and O 1s core-level photoelectron spectroscopy. Ultrathin Hf/Si(100) film was prepared using electron-beam evaporation in an ultrahigh vacuum chamber below 1.5 × 10−8 Pa. Our results revealed the formation of metallic Hf layers containing a few Si atoms on the Si(100)-2 × 1 substrate. A Hf monosilicide (HfSi) component was also formed in the vicinity of the interface region. Metallic Hf rapidly oxidized, transforming into hafnium dioxide (HfO2) and its suboxides, after exposure to O2 molecules at <4.1 Langmuir. In accordance with the oxidation of metallic Hf, Si atoms in metallic Hf layers were also oxidized into typical Si (sub)oxides and Hf silicate. The other HfSi component at the interface was nearly unreactive with O2 molecules. These facts suggest that the metallic Hf component plays a vital role in the initial oxidation of the ultrathin Hf/Si(100) film. After annealing from 873 K to 973 K, the Hf suboxides in low ionic valences progressed into fully oxidized HfO2. Once the annealing temperature reached ~1073 K, oxygen atoms were entirely removed from the ultrathin HfO2/Si(100) film containing SiO2 at the interface. Simultaneously, ultrathin HfO2 layers changed into islands of Hf disilicide (i-HfSi2) on a bare Si(100)-2 × 1 surface. The i-HfSi2 component showed slight reactivity with O2 molecules at 298 K. In contrast to the initial oxidation of clean Si(100)-2 × 1 surface, the dangling bonds on bare Si(100)-2 × 1 surface among i-HfSi2 oxidized preferentially. This was due to some back-bonds of the Si dimers on bare Si(100)-2 × 1 that were occupied by Hf atoms in the form of HfSi2.

Reversible Oxidation of Blue Phosphorus Monolayer on Au(111)

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.

Initial oxidation of U3Si2 studied by in-situ XPS analysis

U3Si2 attracted much attention as candidate nuclear fuel for some favorable properties such as higher uranium density and thermal conductivity than that of UO2. Until now, the oxidation and hydrolysis mechanism of U3Si2 have not been fully explained. In this paper, we fabricated U3Si2 by arc-melting method and characterized surface morphology and structure by SEM and XRD, respectively. The initial oxidation process of U3Si2 was studied by in-situ XPS analysis. The results showed that the oxidized product of U3Si2 were firstly uranium oxides and then uranium silicate. By XPS peak fitting process, the percentage of oxides were obtained and showed that the oxides percentage increased roughly obeyed the exponential law.

Synthesis, Structure, Electrochemical, and Spectroscopic Properties of Hetero-Bimetallic Ru(II)/Fe(II)-Alkynyl Organometallic Complexes.

A series of heterobimetallic wire-like organometallic complexes [( tpy-C6H4-R)(PPh3)2Ru-C≡C-Fc]+ ( tpy-C6H4-R = 4'-(aryl)-2,2':6',2''-terpyridyl, Fc = [(η5-Cp)2Fe], R = -H, -Me, -F, -NMe2 in complexes 5-8, respectively) featuring ferrocenyl and 4'-(aryl)-2,2':6',2''-terpyridyl ruthenium(II) complexes as redox active metal termini, have been synthesized. Various spectroscopic tools, such as multinuclear NMR, IR spectra, HRMS, CHN analyses, and single crystal X-ray crystallography have been utilized to characterize the heterobimetallic complexes. The electrochemical and UV-vis-NIR spectroscopic studies have been investigated to evaluate the electronic delocalization across the molecular backbones of the Ru(II)-Fe(II) heterobinuclear organometallic dyads. Electrochemical studies reveal two well-separated reversible redox waves as a result of successive oxidation of the ferrocenyl and Ru(II) redox centers. The spin density distribution analyses reveal that the initial oxidation process is associated with the Fe(II)/Fe(III) couple followed by one electron oxidation of the ruthenium(II) center. The high Kc value (0.11-1.73 × 1012) and intense NIR absorption, with molar absorption coefficient (in the order of 103 M-1 cm-1) for the RuIIFeIII mixed-valence species, signify strong electronic communication between the two metal termini. The electronic coupling constant ( Hab) has been estimated to be 492 and 444 cm-1 for the structurally characterized complexes 6 and 7, respectively. The redox and NIR absorption features indicate that the mixed-valence system of the heterobinuclear dyads belongs to a Robin and Day "class II" system.

Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding

An overlay coating material was deposited on a single crystal superalloy SRR99 by laser cladding. The microstructure and oxidation behavior of this coating was investigated through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that although the composition of the coating was chosen based on the γ' composition in René N5 superalloy, the primary solidification phase of this coating during laser cladding was γ-Ni. Furthermore, under the laser cladding condition, fine parallel dendrites grew epitaxially in the coating from the substrate, indicating the single crystal structure of the substrate was reproduced. When the single crystal MCrAlY coating was oxidized at 1000℃, both θ-Al2O3 and α-Al2O3 formed during initial oxidation process. As the oxidation time proceeded, the presence of θ-Al2O3 facilitated the formation of NiAl2O4 spinel oxide. Once the spinel was observed, it flourished and induced some porosity in the scale. When the scale thickness increased to 6–7 μm, large area spallation of the scale began.

Evolution of Surface Morphology of Oxide Formed During Initial-Stage Oxidation of γ-TiAl Alloy Using In Situ Environmental SEM

Initial-stage oxidation characteristics of a γ-TiAl alloy were investigated by using in situ environmental scanning electron microscopy for both heating and isothermal oxidation processes. Selective oxidation led to primary nucleation and growth on the aluminum-rich γ phase. Under different isothermal oxidation times, the oxide morphology changed to a linear structure between 750 °C and 774 °C and to an acicular structure at 800 °C. The initial oxidation process of γ-TiAl alloy could be divided into three stages: nucleation; growth; and morphological transformation and subsequent growth stages.

Analysis of Initial Oxidation Process of 2205 Duplex Stainless Steel in Closed Container at High Temperature

Analysis of Initial Oxidation Process of 2205 Duplex Stainless Steel in Closed Container at High Temperature

Characterization of initial intergranular oxidation processes in alloy 600 at a sub-nanometer scale

Intergranular oxidation in Alloy 600 exposed to 480 °C hydrogenated steam was examined at the nano-scale level with analytical electron microscopy and atom probe tomography. The fundamental processes of minor element oxidation and diffusion-induced grain boundary migration (DIGM) were explored. Intergranular oxidation was observed, with Ti and Al oxidation preceding Cr oxidation. Extensive DIGM is observed, with concentrations of minor elements far exceeding bulk values. Calculations are performed which support that the mechanism of DIGM causes large-scale segregation of minor elements. Also, discrete oxide particles ahead of the oxide front provide evidence for classical intergranular internal oxidation at temperatures below 500 °C.

Influence of imidazolium-based ionic liquids on coal oxidation

In order to find the ionic liquids (ILs) which can significantly suppress coal oxidation, nine imidazolium-based ILs, [EMIm][BF4], [EMIm]Ac, [BMIm][BF4], [BMIm]Ac, [AMIm][BF4], [HOEtMIm][BF4], [HOEtMIm][NTf2], [AOEMIm][BF4] and [EOMIm][BF4], were used to treat lignite sample to reveal the inhibitory effect of ILs on coal oxidation activity. The TGA results showed that the ILs can reduce the mass loss of coal, of which the ILs [HOEtMIm][BF4] and [HOEtMIm][NTf2] showed the strongest reducing effect, while the IL [AMIm][BF4] showed the weakest reducing effect. Differential thermal analysis (DTA) results indicated that the heat release of [HOEtMIm][BF4] and [HOEtMIm][NTf2]-treated coal were very less. At the same time, the oxidation reactions of the two coals were delayed. IR results showed that ILs treatment can effectively destroy the associated hydroxyl in coal and change the content of carbonyl and ether bonds. In the process of coal oxidation, the oxidation activity of hydrogen bonding, ether bonds in [HOEtMIm][BF4] and [HOEtMIm][NTf2]-treated coal were weakened significantly at the initial oxidation process. With the oxidation processing, the content of generated carbonyl was reduced. The micro mechanism of the reduced coal oxidation activity was ascribed to the weaker oxidation activity of the ether bonds and the inhibited formation of the carbonyl groups.

Initial stage oxidation on nano-trenched Si(1 0 0) surface

As the size of an electronic element shrinks to nanoscale, trench design of Si strongly influences the performance of related semiconductor devices. By reactive force field molecular dynamics (ReaxFF MD) simulation, the initial stage oxidation on nano-trenched Si(1 0 0) angled 60°, 90°, 120°, 150° under temperatures from 300 K to 1200 K has been studied. Inhomogeneous oxidation at the convex–concave corners of the Si surface was observed. In general, the initial oxidation process on the Si surface was that, firstly, the O atoms ballistically transported into surface, then a high O concentration induced compressive stress at the surface layers, which prevented further oxidation. Compared to the concave corner, the convex one contacted a larger volume of oxygen at the very beginning stage, leading an anisotropic absorption of O atoms. Afterwards, a critical compression was produced at both the convex and concave corners to limit the oxidation. As a result, an inhomogeneous oxide film grew on nano-trenched Si. Meanwhile, due to enhanced O transport and compression relaxation by increasing temperature, the inhomogeneous oxidation was more obvious under 1200 K. These present results explained the observed experimental phenomena on the oxidation of non-planar Si and provided an aspect on the design of nano-trenched electronic components in the semiconductor field.

Insights into the role of grain refinement on high-temperature initial oxidation phase transformation and oxides evolution in high aluminium Fe-Mn-Al-C duplex lightweight steel

Influence of grain size on internal oxidation behaviour at 1273K in dry air of Fe-20Mn-8Al-0.45C (wt.%) has been investigated. The measured weight gains are in parabolic relation to oxidation time and parabolic rate constant degrades with the reduction of grain size. Grain size affects compositions of oxides in the initial oxidation process and diffusion abilities of cations. Fine-grained alloy becomes less permeable to oxygen than coarse-grained alloy. Grain refinement accelerates phase transformation and promotes the rapid formation of pure Al2O3 layer, contributing to the slightly improved oxidation resistance. A brief schematic model is proposed to explain oxides evolution.

Micromechanism of oxygen transport during initial stage oxidation in Si(100) surface: A ReaxFF molecular dynamics simulation study

The early stage oxidation in Si(100) surface has been investigated in this work by a reactive force field molecular dynamics (ReaxFF MD) simulation, manifesting that the oxygen transport acted as a dominant issue for initial oxidation process. Due to the oxidation, a compressive stress was generated in the oxide layer which blocked the oxygen transport perpendicular to the Si(100) surface and further prevented oxidation in the deeper layer. In contrast, thermal actuation was beneficial to the oxygen transport into deeper layer as temperature increases. Therefore, a competition mechanism was found for the oxygen transport during early stage oxidation in Si(100) surface. At room temperature, the oxygen transport was governed by the blocking effect of compressive stress, so a better quality oxide film with more uniform interface and more stoichiometric oxide structure was obtained. Indeed, the mechanism presented in this work is also applicable for other self-limiting oxidation (e.g. metal oxidation) and is helpful for the design of high-performance electronic devices.

Removal of Phenols from Aqueous Solutions by Graphene Oxide Nanosheet Suspensions

The application of aqueous graphene oxide suspension, produced directly from flake graphite by a facile oxidized chemical exfoliation approach, in removal of phenols from aqueous solutions was investigated. Chemical oxidation and exfoliation of flake graphite was based on an initial acute oxidation process using H2SO4-H3PO4 and potassium permanganate, followed by an exfoliation process. The concentration of the as-prepared aqueous graphene oxide suspension was determined as 2.45 mg.mL(-1). Furthermore, as-prepared pale yellow graphene oxide suspension was directly used to remove p-nitrophenol from aqueous solutions, and experimental conditions including contact time, temperature, and initial pH were optimized. The results indicated that the optimized conditions for removal of p-nitrophenol were found as contact time of 40 min, temperature of 40 degrees C, and pH of 6. The adsorption kinetics, adsorption thermodynamic, and adsorption isotherms for removal of p-nitrophenol were investigated. Kinetics data and adsorption isotherm were better fitted by the pseudo-second-order model and by Freundlich isotherm, respectively. The maximum adsorption capacity of the aqueous graphene oxide suspension for p-nitrophenol was 1020.848 mg.g(-1) under the optimized conditions. The maximum adsorption capacities for other three phenols including o-nitrophenol, m-nitrophenol, and 1-naphthol were 947.294 mg.g(-1), 1049.643 mg.g(-1), and 1036.739 mg.g(-1), respectively. Because of the outstanding performances such as tremendous adsorption capacity, removal efficiency, facile process, convenient operation, and low cost, aqueous graphene oxide suspensions might find their wide potential application in adsorbing and removing organic contaminants from wastewater.

Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide complexes as efficient and durable water oxidation catalysts

A series of phenylene-linked iridium bis-pyridine-2-sulfonamide (bpsa) complexes substituted with electron-donating and electron-withdrawing groups and a new bpsa complex containing naphthalene linkage were synthesized and investigated as efficient and robust water oxidation catalysts. It was found that the oxidation potentials of these substituted complexes are within a broad range, which reflects their effectively tuned electronic structures. Under various water oxidation conditions, diverse kinetics behaviors are observed. The reaction mechanism strongly depends on the structure of the catalyst, and the nature of the oxidant as well as their ratios and concentrations. As evidenced by cyclic voltammetry and DFT calculations, the addition of electron-donating substituents greatly destabilizes the HOMO of the catalyst and thus facilitates the initial oxidation process. On the other hand, the limited driving force provided by the oxidized state of an electron-rich catalyst also impedes efficient water oxidation reactions. The robustness of a water oxidation catalyst is critical, and long-term reactions have exhibited turnover numbers (TONs) of up to 16,200.

The initial oxidation behaviors of uranium nitride UNx (x = 0, 0.23, 0.68, 1.66) films

The initial oxidation behaviors of uranium nitride films with different N/U ratios have been focused in the present work. Uranium nitride films with different nitrogen content, UNx (x = 0, 0.23, 0.68, 1.66), have been prepared on the Si substrate by radio frequency magnetron sputtering method. The experimental results showed that the UNx (x = 0, 0.23, 0.68, 1.66) films were fine and dense. The initial oxidation processes of uranium nitride films were investigated in an ultra-high vacuum chamber of Auger electron spectroscopy. After 105 L oxygen exposure, the oxide layer of UO2 were formed on the surface of U, and UN0.23, UN0.68 films formed UO2 with little UNxOy, while the UN1.66 film generated UNxOy oxide layer. UN1.66 film exhibited the thinnest oxide layer and provided the best considerable protection against oxygen attack. Also, the AES transition lines of UNxOy were identified for the first time.

First-principles study of initial oxidation process of Ge(100) surfaces

Stable structures of oxygen atoms inserted into Ge(100) surfaces are investigated by first-principles calculations based on the density functional theory. Comparing the total energies of several models, the most stable structure is realized when oxygen atoms are inserted into the backbond of a lower dimer atom and the next bond along the (100) direction. We calculate the electronic density of states to reveal the origin of the stability. The structure is stable because a dangling bond of the lower dimer atom disappeared to form a four-coordinated structure. We also reveal that the dangling bond disappears from equal-amplitude plots of wave functions. These results are due to the strong electronegativity of the oxygen atom.

A study of the diffusion and pre-oxidation treatment on the formation of Al2O3 ceramic scale on NiCrAlY bond-coat during initial oxidation process

In this study, the formation of thermally growth ceramic oxides during initial oxidation on treated NiCrAlY bond-coat was investigated. The oxidation resistance of NiCrAlY bond-coat was enhanced by two approaches: (i) an inter-diffusion treatment in a tube furnace with flowing argon at 600°C for 1h, and (ii) a pre-oxidation treatment in static air at atmospheric pressure at 1000°C for 1h. Experimental results showed that, after diffusion treatment, the content of Al in the bond-coat increased. This led to the growth of θ-Al2O3 during the pre-oxidation process, which was then helped in the formation of a dense and continuous α-Al2O3/Cr2O3 multilayer on the surface of the bond-coat. Results on the cyclic oxidation at 1200°C (for time≤204h) showed that the treated NiCrAlY bond-coat exhibited better oxidation resistance than the NiCrAlY bond-coat samples without diffusion treatment and the samples pre-oxidized at 800°C for 1h. The formation of NiO, Cr2O3 and NiCr2O4 spinel oxides were suppressed due to the formation of α-Al2O3 scale during the initial oxidation on the surface of pre-treated NiCrAlY bond-coat.

The initial oxidation of Zircaloy-4 in lithiated water under autoclave conditions as examined by TEM

TEM thin foil specimens prepared using Zircaloy-4 coarse-grained samples were corroded by autoclaving at 250°C and 280°C in lithated water for a short time exposure, and the initial oxidation process before the formation of ZrO2 was investigated by TEM. The results show a variation of the crystal structure along with the increase of oxygen contents at the initial oxidation stage. When the Zr/O atomic ratio reached 6–7, 3 and 1, a commensurable long period super-lattice 9a0-2H and sub-oxides with hcp and fcc ordered structure were formed. When the Zr/O atomic ratio was 0.8, monoclinic ZrO2 was detected.