Oxide Nucleation Research Articles

Transformation of sluggish higher valent molybdenum into electrocatalytically active amorphous carbon doped MoO2/MoO3-x nanostructures using phyllanthus reticulatus fruit extract as natural reducing agent in supercritical fluid processing

Electrochemical hydrogen evolution by molybdenum dioxide and molybdenum sub-oxides has gathered much attention owing to their excellent stability and high catalytic activity. However, the reduction of higher valent molybdenum oxides to lower valent molybdenum oxides using conventional chemical process is a challenging issue. Here, supercritical fluid (SCF) and hydrothermal assisted direct synthesis of carbon doped lower valent molybdenum oxides i.e., MoO2/MoO3-x using ammonium heptamolybdate tetrahydrate and phyllanthus reticulatus fruit extract is demonstrated. The phyllanthus reticulatus fruit extract was used as natural reducing agent and it facilitated the reduction, nucleation and surface capping of lower valent molybdenum oxides. XRD and XPS analysis confirmed the phase pure monoclinic MoO2 nanostructures formation with mildly oxidized molybdenum suboxides (MoO3-x) thin layer on the surface. The partial conversion of phyllanthus reticulatus fruit extract to carbon nanostructures that were doped in MoO2/MoO3-x nanostructures was further confirmed using Raman spectroscopic analysis. The carbon doped MoO2/MoO3-x nanostructures prepared by SCF and hydrothermal method exhibited superior electrochemical hydrogen evolution reaction with the low overpotential of 160 and 207 mV at a current density of 10 mA cm−2, respectively.

Oxidation behavior of the dilution zone of a dissimilar metal weld between low alloy steel and Alloy 82 in high temperature water

The investigation on oxidation behavior of the dilution zone of a dissimilar metal weld (DMW) between low alloy steel (LAS) A508-III and Alloy 82 weld metal (WM) in high temperature water was carried out. The results indicated that there were two types of oxide films in the dilution zone of the weld metal depending on the types of the neighboring fusion boundaries (FBs). One of the oxide films has a single crystal grain with spinel structure as internal layer which formed in the dilution zone of weld metal close to the martensite FB (area B). The other oxide film has a compact internal layer composed of nano-sized Cr-rich oxides found in the dilution zone of WM close to the sharp FB or other areas in the dilution zone (area A). Both the two types of oxide films exhibited double layer structure，regardless of the thickness of the oxide film. The single crystal grain of the internal layer of the oxide film in area B was attributed to the low Cr content of the substrate matrix, which resulted in a small number of nucleation of oxides at early stage of oxidation and the low-oxidation-rate was because of the stability of Fe-rich oxide and the protection of martensite due to galvanic corrosion.

Atomic Layer Deposition Nucleation on Isolated Self-Assembled Monolayer Functional Groups: A Combined DFT and Experimental Study

We computationally design and experimentally corroborate the atomic layer deposition (ALD) of metal oxides with tailorable nucleation on selected self-assembled monolayers (SAMs) decorated with terminal functional groups. The low-temperature ALD processes for Al2O3, MnO, and especially ZnO are inhibited on nonpolar alkyl-terminated SAMs with high efficacy, in agreement with the thermodynamics calculated for reactivity with the alkyl-terminated SAMs as well as for potentially physisorbed precursors. Functional-SAM surfaces are also considered for the uniform nucleation of ALD oxides, while mixed alkyl-/functional-group-terminated SAMs are considered for the site-directed synthesis of metal oxide clusters. Long alkyl-chain SAMs with functional group termination including OH, COOH, and SH are predicted to react exergonically over a variable energy barrier which depends strongly on ALD precursor chemistry as well as functional group density. Experiments confirm that more efficient nucleation may be catalyzed via a single ALD precursor cycle of a more reactive precursor (e.g., trimethylaluminum) followed by growth of targeted metal oxides (ZnOx and MnOx). The computational strategy and experimental approach may be applicable to the selective deposition of technologically important hybrid organic–inorganic barrier films as well as catalytically relevant metal oxide and mixed-metal oxide precision few-atom clusters.

Exploring the synthesis conditions to control the morphology of gold-iron oxide heterostructures

Gold–iron oxide nano-heterostructures with a clear and well-defined morphology were prepared via a seed-assisted method. The synthesis process and the events of heterogeneous nucleation during the decomposition of the iron precursor were carefully studied in order to understand the mechanism of the reaction and to tailor the architecture of the fabricated heterostructures. When employing Au seeds of 3 and 5 nm, nanoparticles with a dimer-like morphology were produced due to the occurrence of a single iron oxide nucleation event. Otherwise, multi-nucleation events could be favored by two mechanisms: (i) by the incorporation of a reducing agent and the slowing down of the heating protocol, leading to a core–shell system; (ii) by the increase of the Au seed size to 8 nm, leading to a flower-like system. Further increase of the Au seed size to 12 nm using similar synthesis conditions promotes the homogeneous nucleation and growth of the iron oxide phase, without formation of heterostructures. An in-depth study was performed on the gold–iron oxide heterostructures to confirm the epitaxial growth of the oxide domain over the Au seed and to analyze the elemental distribution of the components within the heterostructures. Finally, it was found that the modification of the plasmonic properties of the Au nanoparticles are strongly influenced by the architecture of the heterostructure, with a more pronounced damping effect for the systems produced after multi-nucleation events.

Data-assisted physical modeling of oxygen precipitation in silicon wafers

A quantitative continuum model for oxide precipitation in silicon is presented that accounts for vacancy absorption and shape change as mechanisms of precipitate stress relief. All model parameters except one, the Si/SiO2 interface free energy, are fixed at values established in prior studies of microdefect formation. The interface free energy is described by an 8-parameter function, whose functional form and dependencies were based on an analysis of electronic structure calculations of small oxide cluster thermodynamics. The interface energy function parameters are regressed, using global optimization, to an experimental benchmark consisting of 13 wafer thermal anneals, with different temperature-time histories and resulting in widely varying measured final oxide precipitate densities. We demonstrate that the model is able to capture the benchmark features well with multiple parameter combinations and that additional constraints are required to fully specify a unique solution. We also show that a simple, single-parameter, constant interface free energy model cannot fully capture the diverse experimental benchmark, highlighting the complexity of oxide precipitation. The precipitation model is used to analyze the mechanisms responsible for several features of oxide nucleation and growth during wafer annealing.

Atomistic Origin of the Complex Morphological Evolution of Aluminum Nanoparticles during Oxidation: A Chain-like Oxide Nucleation and Growth Mechanism.

Metal nanoparticles usually show different oxidation dynamics from bulk metals, which results in various oxide nanostructures because of their size-related surface effects. In this work, we have found and investigated the chain-like nucleation and growth of oxides on the aluminum nanoparticle (ANP) surface, using molecular dynamics simulations with the reactive force-field (ReaxFF). After nucleation, the chain-like oxide nuclei could stay on the ANP surface and continue growing into an oxide shell, extend outward from the surface to form longer oxide chains, or detach from the ANP to generate independent oxide clusters, which is highly dependent on the oxygen content, temperature, and nanoparticle size. Our results emphasize the complicated interplay between the surface structure of nanoparticles and the environmental conditions in determining the formation of oxides, which provides insights into the atomic-scale oxidation mechanism of metal nanoparticles.

Non-equilibrium crystallization pathways of manganese oxides in aqueous solution

Aqueous precipitation of transition metal oxides often proceeds through non-equilibrium phases, whose appearance cannot be anticipated from traditional phase diagrams. Without a precise understanding of which metastable phases form, or their lifetimes, targeted synthesis of specific metal oxides can become a trial-and-error process. Here, we construct a theoretical framework to reveal the nanoscale and metastable energy landscapes of Pourbaix (E-pH) diagrams, providing quantitative insights into the size–dependent thermodynamics of metastable oxide nucleation and growth in water. By combining this framework with classical nucleation theory, we interrogate how solution conditions influence the multistage oxidation pathways of manganese oxides. We calculate that even within the same stability region of a Pourbaix diagram, subtle variations in pH and redox potential can redirect a non-equilibrium crystallization pathway through different metastable intermediates. Our theoretical framework offers a predictive platform to navigate through the thermodynamic and kinetic energy landscape towards the rational synthesis of target materials.

Electrical Properties of Ultrathin Platinum Films by Plasma-Enhanced Atomic Layer Deposition

The ability to deposit thin and conformal films has become of great importance because of downscaling of devices. However, because of nucleation difficulty, depositing an electrically stable and thin conformal platinum film on an oxide nucleation layer has proven challenging. By using plasma-enhanced atomic layer deposition (PEALD) and TiO2 as a nucleation layer, we achieved electrically continuous PEALD platinum films down to a thickness of 3.7 nm. Results show that for films as thin as 5.7 nm, the Mayadas–Shatzkes (MS) model for electrical conductivity and the Tellier–Tosser model for temperature coefficient of resistance hold. Although the experimental values start to deviate from the MS model below 5.7 nm because of incomplete Pt coverage, the films still show root mean square electrical stability better than 50 ppm over time, indicating that these films are not only electrically continuous but also sufficiently reliable for use in many practical applications.

Al2O3/Si0.7Ge0.3(001) & HfO2/Si0.7Ge0.3(001) interface trap state reduction via in-situ N2/H2 RF downstream plasma passivation

A novel method for passivating the interface between Si0.7Ge0.3 using an in-situ downstream RF plasma consisting of a nitrogen-rich mixture of H2 and N2 gases at 250 °C prior to atomic layer deposition (ALD) of Al2O3 and HfO2 was demonstrated. XPS spectra of the interface with Al2O3 indicated the presence of a nitride layer enriched in SiONx and depleted in Ge relative to the substrate. The electrical properties of this interface were characterized using I-V and variable frequency C-V measurements of MOS capacitors. The N2/H2 plasma passivation process produced a reduced density of interface trap states (Dit) and lower gate leakage compared with ex-situ HF clean and sulfur passivation for Al2O3 gate oxides. The lowered leakage current and Dit observed compared with HF(aq) or sulfur-passivated surfaces were consistent with enhanced oxide nucleation due to N2/H2 plasma passivation lowering carbon surface contamination and dangling bonds. TEM/EELS analysis of the interface was consistent with the presence of a thin interfacial nitride layer suppressing the formation of GeO bonds at the interface to form an SiOx-rich interlayer (IL).

Optimization of Chemi‐adsorption, EDLC, and Redox Capacitance Through Electro‐precipitation Synthesis of Fe3O4/NiO@rGO/h‐BN for the Development of Hybrid Supercapacitor

Abstract3D Fe3O4/NiO was grafted on to the 2D rGO/h‐BN by electro‐precipitation method. Nitrogen of h‐BN moiety and oxygen functional groups of rGO played the role of negative active site to trap the metallic cations. Electrochemical charge storage mechanism was optimized by controlling the stoichiometry and defect contents of Fe3O4/NiO@rGO/h‐BN. Stoichiometry of the electro‐precipitated samples was tailored in presence of negative active sites of rGO/h‐BN and applied D.C. bias of the electrochemical bath. In addition, the nucleation and growth of metal oxides were influenced by the stacking and vacancy defects of rGO/h‐BN sheets. High specific capacitance (1328 F g−1) of Fe3O4/NiO@rGO/h‐BN was attributed to the synergistic effect of electrochemical double layer capacitance of rGO, chemi‐adsorption of –OH ions on Lewis acid (boron of h‐BN moiety) and redox capacitance of Fe3O4/NiO in alkaline medium. In addition, the presence of pyrrolic defect at the rGO/h‐BN stacking region acted as the nucleation site and provided additional redox capacitance by shifting the Fermi level towards the valance band. An asymmetric supercapacitor (ASC) was constructed using Fe3O4/NiO@rGO/h‐BN and thermally reduced GO as positive and negative electrode, respectively. ASC showed high energy (82 W h Kg−1) and power density (5600 W Kg−1) along with low relaxation time constant (2.2 ms) and high stability (79%) after 10,000 charge discharge cycles.

Crystallization characteristics of zinc oxide under electric field and Raman spectrum analysis of polarized products

It is of great theoretical and practical significance to study the regulation of the structure, morphology and properties of nanomaterials by using high voltage electric field in the field of functional materials. Here, ZnO nanocrystalline powders are synthesized under the condition of high voltage electric field. The effect of electric field on the structure, point defect and Raman spectrum of ZnO is studied.The structure, Raman shift and defect distribution of the product are characterized by (XRD), scanning electron microscope (SEM) and Raman spectroscopy (Raman spectroscopy).The results show that the complete crystallization time and temperature of zinc oxide under high voltage electric field are longer and higher than those without electric field. The direct current electric field can significantly promote the nucleation of zinc oxide in the precursor and reduce the rate of crystallization.The morphologies of ZnO obtained under different electric field intensities are obviously different. At a lower electric field intensity, ZnO presents lamellar or stripy morphology that is formed by many 50 nm-diameter nanoparticles. At a higher electric field intensity, ZnO exhibits short conical particles. It can be inferred that the high voltage electric field inhibits the growth of zinc oxide along the c axis (the strongest polar direction).The Raman spectra of the cathode surface and the anode surface showing obvious difference after the nano-ZnO powder has been polarized in the DC electric field.The intensity of the second-order optical phonon mode <i>A</i><sub>1</sub>(LO) on the anode surface at 1050 cm<sup>–1</sup> increases significantly under the condition of obvious leakage current, and the ratio (<i>I</i><sub>1</sub>/<i>I</i><sub>2</sub>) of Raman intensity (<i>I</i><sub>1</sub> = 438 cm<sup>–1</sup> and <i>I</i><sub>2</sub> = 1050 cm<sup>–1</sup>) is linearly related to the field strength of the polarized electric field.When the positive and negative sides of the sample disc turn over, the 1050 cm<sup>–1</sup> peak increases on the anode surface and tends to disappear on cathode surface.The zinc vacancies with negative charge move toward the anode and the concentration of zinc vacancies on one side of the anode increases significantly, which makes the surface of zinc oxide nanoparticles in the local area of the anode surface exhibit obvious negative electric properties, and increases the local electric field significantly to form a double Shaw base barrier.The Raman shift of 1050 cm<sup>–1</sup> belongs to the second order optical phonon <i>A</i><sub>1</sub> (LO) vibrational mode, which is usually in inactive or silent state. When the current passes through, the grain boundary double Schottky barrier is established, which enhances the vibration of the <i>A</i><sub>1</sub> (LO) phonon and increases its Raman frequency shift.It can be concluded that the enhancement of the 1050 cm<sup>–1</sup> Raman peak on the anode surface is related to the redistribution of defects in ZnO grains and the double Schottky barrier.

Nucleation and Growth of Oxide Particles on Fe-Mn (1 Wt.%) Binary Alloy during Annealing

The nucleation and growth of oxides formed on Fe-Mn (1 wt.%) binary alloy during annealing were investigated. The recrystallized samples were heated to 800°C and kept at this temperature for 60 s in N2 / H2 (5 vol.%) with a dew point of-40°C. The heating process was also interrupted at different temperatures, namely 650, 700, 750 and 800°C. The annealed samples were then observed by scanning electron microscopy. Image analysis was performed to obtain the surface density and size of the oxides. The influence of the ferrite grain orientation on the oxides’ nucleation and growth was studied by Electron Back Scattered Diffraction. The annealed Fe-Mn surfaces were covered by oxide crystals whose shape, surface density and size strongly depended on the underlying ferrite grain orientation. The oxides’ nucleation occurred first on Fe {110} grain, their surface density was the largest on Fe {100}, and their size was the largest on Fe {110}.

Connecting Oxide Nucleation and Growth to Oxygen Diffusion Energetics on Stepped Cu(011) Surfaces: An Experimental and Theoretical Study

Current fundamental understanding of the reaction mechanisms controlling Cu oxidation encompasses early-stage chemisorption and O surface diffusion, as well as later-stage Cu oxide nano-island nucl...

Carbothermal reduction of oxidized tin and recovery of deep supercool

Deep supercooling of tin is a characteristic sign of a high-purity sample having fewer nucleation sites. However, due to the risk of oxidation during the cell fabrication (especially during the initial sample filling process), oxidized tin can be present in the sample, which manifests itself as a nucleation agent and significantly affects the realization of the freezing temperature of tin. In this work, the effect of the oxidation of high-purity tin (nominally 99.9999% pure) and an effective means of reducing the oxidized sample were investigated. We fabricated an open-type tin freezing point cell using 1.15 kg of high purity tin shot, which was poured into the crucible in air, giving rise to a high risk of oxidation. The oxidized tin sample showed abnormally small supercools of about 0.1 °C which were caused by additional nucleation sites introduced by the oxidation. The oxidized sample also exhibited a melting temperature inversion over the entire range of the melting with a temperature depression of about 1.9 mK. The oxidized sample was carbothermally reduced by melting the sample in its graphite crucible and holding at a temperature of 750 °C for a total duration of 365 h. The carbothermal reduction of the oxidized sample caused the satisfactory recovery of the deep supercool, and the recovered supercool was about 10.6 °C after about 100 h of reduction, and 10.7 °C after 365 h of reduction. At the end of the reduction process, the melting curve inversion was significantly alleviated, having 0.1 mK temperature depression over less than 10% of the melting range. Importantly, the carbothermal reduction at 750 °C was found to be an effective means of removing the oxide nucleation sites and of recovering the characteristic deep supercool of tin.

UV-Ozone Functionalization of 2D Materials

Integrating two-dimensional (2D) materials into the current nanoelectronic process requires control over the deposition of gate oxides onto these materials. Atomic layer deposition (ALD) relies on surface dangling bonds that are scarce for 2D materials. This review summarizes the advances made in understanding and controlling the nucleation of ALD oxides on these materials. As an example, we focus on ozone-based processes including UV-ozone pretreatments, which we have found to effectively functionalize the surface of molybdenum disulfide. Furthermore, we discuss the advantages and limitations of various functionalization or seeding techniques, such as limits in scalability or damage to the 2D materials.

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.

Improve the Stability of Hybrid Halide Perovskite via Atomic Layer Deposition on Activated Phenyl-C61 Butyric Acid Methyl Ester.

Atomic layer deposition (ALD) of oxide film on [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) shows a great promise to dramatically improve the ambient stability of hybrid halide perovskite. The nucleation of an ALD oxide on PCBM is critical to reliably apply this strategy. In this paper, we present the first study of the nucleation behavior of ALD oxides, including Al2O3 and ZnO, on PCBM. We find that PCBM film acts a gas diffusion barrier blocking the ALD reactants (diethyl zinc) from etching the underlying CH3NH3PbI3. However, ZnO is not able to nucleate on PCBM. We further identify that trimethyl aluminum, a strongly Lewis acid, reacts readily with C═O on PCBM to generate a seeding layer for nucleating ZnO ALD. This new chemical route is highly reliable and can be used to synthesize ALD ZnO coatings over PCBM. The synthesized PCBM/Al2O3-ZnO dramatically improves the stability of CH3NH3PbI3 against the ambience and even against liquid water. The result signifies the importance of understanding of nucleation of ALD in enabling reliable barrier coatings for hybrid halide perovskites.

The formation of (Al2O3)n clusters as a probable mechanism of aluminum oxide nucleation during the combustion of aluminized fuels: Numerical analysis

The model of formation and growth of stoichiometric (Al2O3)n clusters during the combustion of aluminized fuels has been developed. In this model, the thermodynamic properties of large clusters (n = 5–75) have been determined by matching the thermodynamic properties of small clusters n = 2–4, calculated earlier by quantum-chemical methods, with similar characteristics of liquid droplets. The developed model was used for a numerical simulation of the formation of (Al2O3)n clusters during the combustion a single aluminum particle with a diameter of 200 μm in O2/Ar atmosphere. It has been shown that, during the first 12 ms after the aluminum particle ignition, the rapid growth of clusters occurs. The mass of clusters in the combustion zone is comparable to the mass of aluminum oxide. The modeling results indicate that the growth of (Al2O3)n clusters can be the most probable mechanism of the formation of condensation nuclei of alumina.

High resolution characterization of sulfur-assisted degradation in alloy 800

Environmentally assisted cracking (EAC) initiation tests were carried out by subjecting Alloy 800 tensile specimens to 0.55 mol/kg SO42− solution, pH280°C 3, at 280 °C using slow rise-time cyclic loading and in-situ crack detection. EAC, intergranular corrosion (IGC), and pitting were observed. Transmission electron microscopy analysis revealed Ti- and Cr-rich oxides in cracks, and sulfur incorporated in oxide(s) or as sulfide compound(s). This oxide/sulfide film is likely impaired, producing a slip dissolution-type EAC mechanism. For pitting, only a nano-scale sulfur layer was identified at pit-metal interfaces. This high surface coverage of adsorbed sulfur limited oxide nucleation and accelerated metal dissolution.

New Insights into Thermal Chromia Growth on Fe18Cr(10Ni) Model Alloys at 900°C: Scaling Kinetics and Microstructures

Specimen of Fe-18Cr(-10Ni) (wt.-%) were isothermally oxidized in dry and wet high p(O2) gas at 900°C. The morphology and microstructural evolution of the chromia scales formed were analysed by SEM-EBSD. At high pO2 test gas containing O2 and H2O, the pO2 is independent of the H2O content of the test gas. However, both types of oxygen species, i.e. O2 as well as H2O contribute to the chromia scaling reaction. The effect of specimen thickness on chromia scaling allows the change of the concentration of the intrinsic defects and their diffusivities in thermally grown chromia scales independently from the test gas applied. Chromia scales are under pressure during scale growth. For thermodynamic reasons the concentration and diffusivity of the intrinsic, native defects in oxides under hydro-static pressure varies with the amount of pressure applied. Ion beam polished oxide cross-sections were analysed in terms of oxide grain growth and nucleation rates of new oxide at the oxide-gas interface. Chromia scales formed on the various substrates at high p(O2) test gas grew at different rates, and microstructures formed on Fe18Cr(-10Ni) differ in grain shape and size. The chromia growth kinetic was found to be depending on the thickness of the specimens used, the H2O content of the gas, and its Ni content. The scale growth mechanism consists of two interlinked levels. At the meta-level oxide nucleation and oxide grain growth are affected by the cation flux in the oxide scale formed and the nature of the oxygen species in the gas. This fixes the number, size and shape of the oxide grains as well as the number of oxide grain boundaries available as short-circuit diffusion paths. The lattice level denotes the atomic level. Hereby the cation flux in the single grain by lattice diffusion and cation vacancy condensation are linked. The defect structure in the oxide lattice is important. The factors affecting it are the Cr diffusion in the substrate and the presence of Ni in the alloy, local p(O2), H-defects and oxide growth stress. The experimental findings will be discussed with strong focus on the oxide growth mechanism, the chromia microstructure of the scales formed under the various conditions and the concentration and mobility of the native defects in the chromia scales and its interactions with H-containing species originating from water vapour in the test gas. Furthermore, the role of Fe as base and Ni as alloying element in the metal substrate will be discussed. This is done by considering the differences between the Ni-Cr-O and Fe-Cr-O phase diagrams and the consequence of the existence of the Fe3+ ion on the miscibility of the oxides formed by the various cations.