Oxide Islands Research Articles

From Seeds to Islands: Growth of Oxidized Graphene by Two-Photon Oxidation

The mechanism of two-photon induced oxidation of single-layer graphene on Si/SiO2 substrates is studied by atomic force microscopy (AFM) and Raman microspectroscopy and imaging. AFM imaging of areas oxidized by using a tightly focused femtosecond laser beam shows that oxidation is not homogeneous but oxidized and nonoxidized graphene segregate into separate domains over the whole irradiated area. The oxidation process starts from point-like “seeds” which grow into islands finally coalescing together. The size of islands before coalescence is 30–40 nm, and the density of the islands is on the order of 1011 cm–2. Raman spectroscopy reveals growth of the D/G band ratio along the oxidation. Sharpness of the D-band which persists over a large range of oxidation and the maximal value of the intensity ratio of the D- and G-bands (∼0.8) indicates that graphene oxidation proceeds by an increase of the oxidized area rather than progression of oxidized areas to fully disordered structure. A phenomenological model is developed which explains the observations. According to the model, the probability for oxidation of a site next to an already oxidized site is 5 orders of magnitude higher than oxidation of pristine graphene. Irradiation of an extended area by raster scanning leads to a formation of an irregular nanomesh of oxide islands with a narrow size distribution. The phenomenological model yields similar results as the experiment. This study forms a basis for controlled use of two-photon oxidation for tailoring properties of graphene and patterning it with submicrometer resolution.

Comparative Analysis of Cobalt Oxide Nanoisland Stability and Edge Structures on Three Related Noble Metal Surfaces: Au(111), Pt(111) and Ag(111)

Metal oxide nanostructures and thin films grown on metallic substrates have attracted strong attention as model catalysts and as interesting inverse catalyst systems in their own right. In this study, we investigate the role of metal support in the growth and stabilization of cobalt oxide nanostructures on the three related (111) surfaces of Au, Pt and Ag, as investigated by means of high-resolution scanning tunneling microscopy and DFT calculations. All three substrates promote the growth of crystalline CoOx (x = 1−2) islands under oxidative conditions, but we find several noteworthy differences in the occurrence and stabilization of four distinct cobalt oxide island phases: Co–O bilayers, O–Co–O trilayers, Co–O–Co–O double bilayers and O–Co–O–Co–O multilayers. Using atom-resolved images combined with analysis of defect lines in bilayer islands on Au and Pt, we furthermore unambiguously determine the edge structure. Interestingly, the island shape and abundances of edge types in bilayers change radically from mixed Co/O edge terminations on Au(111) to a predominance of Co terminated edges (~91 %) on Pt(111) which is especially interesting since the Co metal edges are expected to host the most active sites for water dissociation.

Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth.

Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H2TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials.

Erosion behavior of composite Al-Cr cathodes in cathodic arc plasmas in inert and reactive atmospheres

AlxCr1−x composite cathodes with Al contents of x = 0.75, 0.5, and 0.25 were exposed to cathodic arc plasmas in Ar, N2, and O2 atmospheres and their erosion behavior was studied. Cross-sectional analysis of the elemental distribution of the near-surface zone in the cathodes by scanning electron microscopy revealed the formation of a modified layer for all cathodes and atmospheres. Due to intermixing of Al and Cr in the heat-affected zone, intermetallic Al-Cr phases formed as evidenced by x-ray diffraction analysis. Cathode poisoning effects in the reactive N2 and O2 atmospheres were nonuniform as a result of the applied magnetic field configuration. With the exception of oxide islands on Al-rich cathodes, reactive layers were absent in the circular erosion zone, while nitrides and oxides formed in the less eroded center region of the cathodes.

Surface roughness of the strained polycrystalline copper during the early stage oxidation

Using the reactive molecular dynamic simulation, the surface roughness and oxide layer morphology are studied during the early oxidation of polycrystalline copper under applied tensile strains at 900K. We find that the surface roughness of oxide layer increases with the formation and growth of oxide islands. The transformation of oxide wetting layer to the island structure occurs as a result of atom diffusion triggered by the oxidation-induced growth stress. Such a transformation process is enhanced by promoting the free energy of copper atoms, especially along the grain boundary when the external tensile strain is applied. This new insight provides a fundamental interpretation of the instability of oxide layer and the origin of surface defects under applied tensile loadings.

New Generation Devices for Gas (Liquid) Sensing

We present a short review and novel approach for the construction of conductometric sensors demonstrating considerably higher sensitivities than traditional metal oxide sensors. Sensor platforms do not require film-based technology, operate at room temperature, and can be obtained without the use of time consuming self-assembly processes. A combined nanopore coated micro-porous array, is deposited with nanostructure directing acidic metal oxide island sites which vary in their Lewis acidity, decorate the micropores, and control an electron transduction process. The interaction of analytes with these island sites varies in a predictable manner and can be modified through in-situ functionalization of their Lewis acidity through formation of the oxynittrides or oxysulfides. Microporesallow rapid Fickian diffusion of the analytes to the active nanostructured island sites whose reversible interaction with the analyte dominates the sensor response. We require only that the island sites be deposited at sufficiently low concentration so as not to interact electronically with each other. Highly accurate repeat placement of the nanostructured island depositions is not required. The nanoporous walls of the microarray act as a phase match for the deposition of a diversity of nanostructures that are selected for deposition from a variety of solution-based sources and the forgiving deposition process requires a minimum of energy consumption and time. Comparisons to a variety of metal oxide systems are considered. Observed sensitivities and the sensor system reversibility can be predicted from the recently developing IHSAB model.

In situ atomic scale visualization of surface kinetics driven dynamics of oxide growth on a Ni-Cr surface.

We report the in situ atomic-scale visualization of the dynamic three-dimensional growth of NiO during the initial oxidation of Ni-10at%Cr using environmental transmission electron microscopy. A step-by-step adatom growth mechanism in 3D is observed and a change in the surface planes of growing oxide islands can be induced by local surface kinetic variations.

Microstructural modifications in powder-metallurgically produced Al0.675Cr0.275Fe0.05 targets during cathodic arc evaporation

Recently, the authors showed that metallic droplets, originating from the Al0.675Cr0.275Fe0.05 cathode surface, play an essential role in the nucleation process of hexagonal crystallites in mixed cubic and hexagonal-structured cathodic arc-evaporated (Al0.70Cr0.25Fe0.05)2O3 films. Here, the authors investigated in detail the corresponding powder-metallurgically produced Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 targets (after the arc evaporation process) and the ejected macroparticles, which were intentionally separated and collected from the plasma stream. The 15–200 μm thick melting zone (under given process conditions not entirely covering the target surface) of both target materials predominately consists of intermetallic Al80Cr20, Al9Cr4 and dominating Al8Cr5 phases. The selective melting process, induced by the cathodic arc spot size and particle size distribution of the targets, led to the formation of Al-enriched areas. Oxide islands, which form on the target surfaces, especially contain Cr- and Fe-enriched particles. The latter are only present for Al0.675Cr0.275Fe0.05 targets, where the majority of Fe is basically dissolved in the intermetallic Al-Cr phases formed at the target surface due to the cathodic arc interaction. The chemical composition of the ejected macroparticles corresponds with these cathodic arc interaction zones. Based on here presented results, the authors can conclude that Fe-containing intermetallic phases, which are also present within the droplets, as well as Cr- and Fe-enriched particles, are the influential factors for the nucleation of hexagonal phases within arc-evaporated (Al,Cr,Fe)2O3 coatings.

Microscopic Investigation of Chemoselectivity in Ag-Pt-Fe3O4 Heterotrimer Formation: Mechanistic Insights and Implications for Controlling High-Order Hybrid Nanoparticle Morphology.

Three-component hybrid nanoparticle heterotrimers, which are important multifunctional constructs that underpin diverse applications, are commonly synthesized by growing a third domain off of a two-component heterodimer seed. However, because heterodimer seeds expose two distinct surfaces that often can both support nucleation and growth, selectively targeting one particular surface is critical for exclusively accessing a desired configuration. Understanding and controlling nucleation and growth therefore enables the rational formation of high-order hybrid nanoparticles. Here, we report an in-depth microscopic investigation that probes the chemoselective addition of Ag to Pt-Fe3O4 heterodimer seeds to form Ag-Pt-Fe3O4 heterotrimers. We find that the formation of the Ag-Pt-Fe3O4 heterotrimers initiates with indiscriminate Ag nucleation onto both the Pt and Fe3O4 surfaces of Pt-Fe3O4, followed by surface diffusion and coalescence of Ag onto the Pt surface to form the Ag-Pt-Fe3O4 product. Control experiments reveal that the size of the Ag domain of Ag-Pt-Fe3O4 correlates with the overall surface area of the Pt-Fe3O4 seeds, which is consistent with the coalescence of Ag through a surface-mediated process and can also be exploited to tune the size of the Ag domain. Additionally, we observe that small iron oxide islands on the Pt surface of the Pt-Fe3O4 seeds, deposited during the formation of Pt-Fe3O4, define the morphology of the Ag domain, which in turn influences its optical properties. These results provide unprecedented microscopic insights into the pathway by which Ag-Pt-Fe3O4 heterotrimer nanoparticles form and uncover new design guidelines for the synthesis of high-order hybrid nanoparticles with precisely targeted morphologies and properties.

Development of a Fermi Energy Distribution Based Adsorption Isotherm for Response Simulation of Nanostructure Decorated Porous Silicon Substrates

A simple MEMS/NEMS platform facilitates the modeling of the interaction of nanostructure directing metal oxide island sites with several gas phase analytes. A “response matrix” created for the nanostructured metal oxides facilitates the modeling process. Sensitive conductometric sensors operating at room temperature and atmospheric pressure, are forgiving, and do not require film based technology or lithography for their construction. Their responses are simulated using a combination diffusion/adsorption–based model where a new adsorption isotherm is derived on the basis of the Fermi distribution function. We show that diffusion dominates the conductometric response and model this process with the aid of the relative sensitivities determined previously for the metal oxides. Not only the direct response but also the derivatives of this response are used to evaluate the modeling of Fickian diffusion. The first derivative, as it is found to be linear in concentration, allows a quick evaluation of a rapid sensor response. The spectral simulations are refined to include the adsorption effects of the analyte gas and evaluate subsequent non-linear interface sensitivities. A new Fermi energy distribution –based response isotherm, based on first principles, is compared to a diversity of well-known empirical isotherms and found to be superior for the modeling of the sensor response. A comprehensive model including not only this response but also its derivative and its log-log plot are demonstrated. This modeling is exemplified for the analytes NH3 and NO.

Influence of temperature-induced copper diffusion on degradation of selective chromium oxy-nitride solar absorber coatings

Temperature-induced copper diffusion process and its influences on optical degradation and long-term stability of solar absorber coatings on copper substrates were investigated at intermediate temperatures of 248–500°C. The studied absorbers were sputtered chromium oxy-nitride absorbers having tin oxide anti-reflection coatings. The absorbers were aged by means of thermal accelerated ageing studies and short-period heat treatments up to 500°C for two hours.Ageing mechanisms and degradation of the absorbers were analysed before and after the ageing studies by optical measurements (solar absorptance with a UV/Vis/NIR spectrophotometer and thermal emittance by FTIR spectrophotometry), microstructural analysis using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM) with an EDS, composition by time-of-flight elastic recoil detection analysis (TOF-ERDA) and an X-ray photoelectron spectroscope (XPS), and adhesion by tensile test. The relation between optical degradation and diffusion mechanisms was studied using optical modelling and simulation. The results clearly revealed the mechanism of outward copper diffusion: diffusion of copper substrate atoms into the coating and through the coating to the surface, formation of copper oxide islands on the surface of the coating, and formation of voids in the substrate surface. The relation between the diffusion mechanisms and increase in thermal emittance of the absorber surface was demonstrated.

Approach to Multigas Sensing and Modeling on Nanostructure Decorated Porous Silicon Substrates

An approach to multiple gas sensing on decorated porous silicon (PS) substrates is presented. The simple microelectromechanical systems/nanoelectromechanical systems platform that we have developed facilitates the modeling of the interaction of nanostructured metal oxide islands with the analytes of interest, which are exemplified by NO and NH3. These conductometric sensors operate at room temperature and atmospheric pressure and, as they are forgiving, do not require film-based technology or lithography for their construction. We show that diffusion dominates the conductometric response. The direct response and the derivatives of this response are considered. The first derivative allows a quick evaluation of sensor response and the derivative is linear with concentration. The spectral simulations have been refined to include adsorption/desorption effects of the analyte gas and assess subsequent non-linear interface sensitivities. By including the physics of adsorption/desorption, the simulated sensor response is now a non-linear function of concentration. We model the absorption/diffusion through the use of the Langmuir absorption isotherm and find substantial agreement with experiment for the mixed analyte interactions of NH3 and NO combinations on several metal oxide decorated PS interfaces.

Multi-gas interaction modeling on decorated semiconductor interfaces: A novel Fermi distribution-based response isotherm and the inverse hard/soft acid/base concept

A unique MEMS/NEMS approach is presented for the modeling of a detection platform for mixed gas interactions. Mixed gas analytes interact with nanostructured decorating metal oxide island sites supported on a microporous silicon substrate. The Inverse Hard/Soft acid/base (IHSAB) concept is used to assess a diversity of conductometric responses for mixed gas interactions as a function of these nanostructured metal oxides. The analyte conductometric responses are well represented using a combination diffusion/absorption-based model for multi-gas interactions where a newly developed response absorption isotherm, based on the Fermi distribution function is applied. A further coupling of this model with the IHSAB concept describes the considerations in modeling of multi-gas mixed analyte–interface, and analyte–analyte interactions. Taking into account the molecular electronic interaction of both the analytes with each other and an extrinsic semiconductor interface we demonstrate how the presence of one gas can enhance or diminish the reversible interaction of a second gas with the extrinsic semiconductor interface. These concepts demonstrate important considerations in the array-based formats for multi-gas sensing and its applications.

Ultrahigh precision low-cost pinpointed SiO2 patterns nanofabrication by using traditional MEMS fabrication processes

Recent years, nanoscale patterns transfer technique from DNA origami molecules to SiO2 layer has been a promising approach of silicon based nanofabrication process. However, researchers find it difficult to locate SiO2 nano patterns with origami's shapes. Here, we present a novel and low-cost approach of ultrahigh precision SiO2 patterns nanofabrication in target position. Traditional contact lithography process is used to fabricate nanoscale silicon oxide islands array in this paper, which are used to pinpoint the position of ultrahigh precision nano patterns. Precise controllability sacrificial layer etching is the key process to realize nanoscale control of islands in process. This fabrication method is a simple and cost-effective method with high yield, which will hopefully make contributions for higher precision nanofabrication technology in the future.

Transient Structures of PdO during CO Oxidation over Pd(100)

In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O2 in the gas stoichiometry, islands of bulk oxide grow epitaxial...

(Invited) Engineering the Microstructure and Atomic Arrangement of Pt-Based ORR Catalyst for High Activity and Durability

Polymer Electrolyte Membrane Fuel Cells (PEMFC) have been developed for passenger vehicle applications for several decades. The durability and activity of the Oxygen Reduction Reaction (ORR) catalysts is one of the main obstacles to be overcome to make PEMFC vehicle commercially viable. Efforts have been focused on: 1) Developing non-precious metal catalysts. 2) Reducing the Pt loading and modifying the d-band structure of Pt by forming nano-sized core-shell structures on various cores including metallic materials, oxides, carbides and borides, and/or alloying Pt with transition metals (Cr, Ni, Co, Mn, etc.). 3) Stabilizing the Pt nano-particles by growing them larger and making the particle size more uniform. 4) Atomically engineering the Pt environment by using hybrid support of conductive metal oxide and carbon. Worldwide efforts have greatly improved the durability of the ORR catalysts, extending the life time of PEM fuel cell vehicles to about 2000 hours, which is still less than half of the required commercialization target of more than 5000 hours. Pt-based ORR catalysts still dominate in PEMFC. The main requirements for an effective ORR catalyst are low Pt loading, high activity and durability. We used a PVD based method to engineer the microstructure and atomic arrangement of Pt into a 2-D connected network on worm-shaped islands of conductive metal oxides, which are deposited on graphitic carbon. A model catalyst was first developed on flat surface graphene, and it showed much improved activity and durability. Later, this desired Pt 2-D structure and morphology was replicated onto graphitic carbon powders. In this talk, we will discuss the catalyst, its development, and structural and electrochemical characterization using RDE and in-situ single cell experiments. A mechanistic understanding for the improvement in both activity and durability will be discussed.

Interface controlled oxidation states in layered cobalt oxide nanoislands on gold.

Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER; half of the catalytic "water splitting" reaction), particularly when promoted with gold. However, the surface chemistry of cobalt oxides and in particular the nature of the synergistic effect of gold contact are only understood on a rudimentary level, which at present prevents further exploration. We have synthesized a model system of flat, layered cobalt oxide nanoislands supported on a single crystal gold (111) substrate. By using a combination of atom-resolved scanning tunneling microscopy, X-ray photoelectron and absorption spectroscopies and density functional theory calculations, we provide a detailed analysis of the relationship between the atomic-scale structure of the nanoislands, Co oxidation states and substrate induced charge transfer effects in response to the synthesis oxygen pressure. We reveal that conversion from Co(2+) to Co(3+) can occur by a facile incorporation of oxygen at the interface between the nanoisland and gold, changing the islands from a Co-O bilayer to an O-Co-O trilayer. The O-Co-O trilayer islands have the structure of a single layer of β-CoOOH, proposed to be the active phase for the OER, making this system a valuable model in understanding of the active sites for OER. The Co oxides adopt related island morphologies without significant structural reorganization, and our results directly demonstrate that nanosized Co oxide islands have a much higher structural flexibility than could be predicted from bulk properties. Furthermore, it is clear that the gold/nanoparticle interface has a profound effect on the structure of the nanoislands, suggesting a possible promotion mechanism.

Nucleation and growth of oxide islands during the initial-stage oxidation of (100)Cu-Pt alloys

The initial-stage oxidation of (100) Cu-Pt alloys has been examined by in situ environmental transmission electron microscopy and ex situ atomic force microscopy (AFM). It is shown that the oxidation proceeds via the nucleation and growth of Cu2O islands that show dependence on the alloy composition and oxidation temperature. The kinetic measurements on the oxide nucleation reveal that both the nucleation density and surface coverage of Cu2O islands can be promoted by alloying more Pt in the Cu-Pt alloys. Increasing the oxidation temperature above 700 °C results in the growth of large Cu2O islands that transits to a dendritic growth morphology. The ex situ AFM studies reveal that the nucleation of oxide islands can occur on surface terraces and the subsequent oxide growth depletes local terrace Cu atoms that results in the formation of surface pits.

Thermal evolution of cobalt deposits on Co3O4(111): atomically dispersed cobalt, two-dimensional CoO islands, and metallic Co nanoparticles.

Cobalt oxide nanomaterials show high activity in several catalytic reactions thereby offering the potential to replace noble metals in some applications. We have developed a well-defined model system for partially reduced cobalt oxide materials aiming at a molecular level understanding of cobalt-oxide-based catalysis. Starting from a well-ordered Co3O4(111) film on Ir(100), we modified the surface by deposition of metallic cobalt. Growth, structure, and adsorption properties of the cobalt-modified surface were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and infrared reflection absorption spectroscopy (IRAS) using CO as a probe molecule. The deposition of a submonolayer of cobalt at 300 K leads to the formation of atomically dispersed cobalt ions distorting the surface layer of the Co3O4 film. Upon annealing to 500 K the Co ions are incorporated into the surface layer forming ordered two-dimensional CoO islands on the Co3O4 grains. At 700 K, Co ions diffuse from the CoO islands into the bulk and the ordered Co3O4(111) surface is restored. Deposition of larger amounts of Co at 300 K leads to formation of metallic Co aggregates on the dispersed cobalt phase. The metallic particles sinter at 500 K and diffuse into the bulk at 700 K. Depending on the degree of bulk reduction, extended Co3O4 grains switch to the CoO(111) structure. All above structures show characteristic CO adsorption behavior and can therefore be identified by IR spectroscopy of adsorbed CO.

Initial corrosion attack of 304L and T22 in 2 MW biomass gasifier: a microstructural investigation

The work investigates the initial corrosion attack on a low alloyed steel and a stainless steel in a 2 MW test gasifier. The gasifier environment generates homogenous deposits that consist mainly of carbon containing species, potassium sulphate, potassium chloride and zinc sulphide. The stainless steel exhibits better corrosion resistance compared to the low alloyed steel and the analysis indicates a protective thin scale covering parts of the surface after 4 h exposure. However, in some areas the oxide scale has lost its protective properties and thicker oxide scales are seen. The thick oxide islands consist of an inward growing Fe,Cr,Ni oxide and an outward growing iron oxide. The low alloyed steel shows a more homogenous and faster initial corrosion attack. The thick scales exhibit a sharp straight line in the middle of the scale that separates the bottom spinel oxide from the outer iron rich parts of the scale. It is considered that this flat interface corresponds to the original sample surface.