V. V. Vysotskii

Transformation of metal-oxide nanostructures in the process of afteroxidation of iron reactively evaporated in oxygen atmosphere

The methods of atomic force microscopy combined with the digital processing of images were used to study the change in the morphology of nanostructured metal-oxide coatings obtained by reactive evaporation at 10−6 torr in the atmosphere of oxygen and further low-temperature iron afteroxidation in an air atmosphere at different temperatures. It is shown that a nanostructured layer with a unimodal distribution according to the size and degree of nonsphericity of the particle is formed in the case of sputtering. Using the sample detection methods, it is shown that a unimodal distribution is transformed into a bimodal distribution at an increase in the reoxidation temperature from 50 to 200°C, after which it becomes a trimodal distribution, which proves that the building blocks of a nanostructured layer are nanoparticles and four- and 16-particle surface nanoparticle conglomerates. This structure can explain the functional properties of these coatings, which are of practical importance.

On the electrocontact oxidation of vacuum iron nanocondensates: II. Structural characteristics

Changes in the composition and morphology of thin-film conductors based on a nanostructured metal-oxide nanocomposite, which was obtained by the vacuum deposition of iron in an oxygen atmosphere at 10−5–10−3 mmHg on an amorphous glass substrate, induced by external electric current are studied using atomic force microscopy and Raman spectroscopy. The most pronounced zone of degradation is observed in the middle part of the conductor. Compared to the undistorted parts of the conductor, the degradation zone is characterized by an increased content of magnetite phase, which is formed as a result of the prevailing further oxidation of nanoparticles constituting the film. The surface morphology of the degraded part is characterized by the appearance of extended structures, some of which are nanoparticles and submicroparticles oriented in the direction of the applied electric current, which can be due to both the electric mass transfer of the metal and its electric oxidation. The other kind of extended structures are nanofibers composed of adjoining and coalesced metal-oxide nanoparticles, which can appear due to the electric oxidation in the electric contact areas between nanograins and leads to the coalescence of the neighboring nanograin chains into nanofibers. It is proposed that the electrocontact oxidation should be used as a method of creating fibrous metal-oxide nanocomposites based on vacuum deposits of iron or other metals.

The formation of urchinlike nanostructures under thermal oxidation and depassivation of iron particles

Methods of scanning electron microscopy with an X-ray probe, Raman spectroscopy, and thermogravimetry are used to study the morphology and composition of metal–oxide nanostructures obtained by thermal oxidation of spherical microparticles of iron powder with a diameter of 1–3 μm. It is shown that unidimensional hematite nanoflakes and fibers grow under atmospheric annealing at 300°C and above in the radial direction from powder microparticles, which is accompanied by depassivation and acceleration of corrosion layer growth. An increase in temperature and oxidation time results in transformation of microparticles into urchinlike multilayer nanoparticles consisting of elongated crystalline α-Fe2O3 nanowhiskers growing normally to the surface of the particle consisting of an iron core and shells of the magnetite and hematite phases. Growth of hematite nanowhiskers continues until the metallic core is completely oxidized.

Heterophase activation of the growth processes of 1D hematite nanostructures under oxidation of plastically deformed iron powders

Methods of scanning electron microscopy with an X-ray probe, Raman spectroscopy, and thermogravimetry are used to study the morphology and composition of metal-oxide nanocomposites obtained by intense plastic deformation of iron powder and the further annealing in the TGA mode. It is shown that high-temperature annealing of activated powder deformation results in the formation of “hematite wood”—a layer of “nanoflakes” and “nanoleaves” growing vertically on the composite surface. In addition to its intrinsic functional (sensor, catalytic, semiconductor, adsorption) properties, this structure possesses pronounced primer properties for further formation of protective surface layers.

Alternating-current electrocontact oxidation of Kh18N10T steel

Properties of surface oxide layers formed on extended Kh18N10T steel specimens upon oxidation at residual pressure P = 10−4–101 mmHg under typical heating in a furnace and alternating-current (50 Hz) electrocontact heating (300°C) are studied. A strong activating effect of alternating-current heating on the oxidation of steel at P = 0.01 mmHg is noticed. Compared to typical heating in a furnace, electrocontact heating promotes saturation of the surface oxide with chromium and homogenization of the chromium distribution in the depth of the oxide and the resulting formation of iron chromite.

Formation of nanocomposites of platinum with nanotubular titanium dioxide

Metal oxide nanocomposites of platinized titania nanotube arrays (Pt-TNT) have been formed via electrochemical anodization-precipitation. Initially, the vertically aligned amorphous TiO2 nanotube arrays (TNTs) were formed in accordance with potentiostatic electrochemical anodization of titanium with the use of 1 M H2SO4 + 0.3% HF electrolyte. Then, using the potentiostatic and pulse electrodeposition, precipitates of Pt on TNT were formed from 0.04 M solution of chloroplatinic acid (CPA). It has been shown with the use of SEM and Raman spectroscopy that, in order to prepare functional metal oxide Pt-TNT nanocomposites, pulse electrodeposition and subsequent annealing at 400°C are preferable, because they lead to TNTs that are homogeneously distributed along the surface and in bulk (with anatase structure) for platinized layers and conglomerates.

On electrocontact oxidation of iron nanocondensates

The transformation (under the effect of current passed) of a nanostructured metal-oxide coating produced by vacuum deposition of iron in an oxygen atmosphere at a pressure of 10−4 mm Hg on an amorphous glass substrate is studied by measuring voltammetric characteristics. In the freshly deposited film, a linear characteristic is observed that may correspond to direct quantum mechanical tunneling in thin oxide interlayers between metal nanograins. With an increase in the applied current density, when a certain threshold is reached, the conductivity of the conductor rapidly drops because of the thermo- and electrooxidation. In a degraded conductor at a prethreshold current density, a nonlinear voltammetric characteristic is observed, which may be related to the change in the mechanism of current flow restricted by the volume charge, as well as to the negative temperature resistance coefficient typical of fine-crystalline granular nanocomposites.

The effect of titanium support on the morphological properties of growth of titanium-oxide nanotubes and platinum deposit

It is shown that the thickness and structure of the layer of titanium-oxide nanotubes obtained by anodization of titanium foil in fluoride-containing solutions are determined by crystallographic orientation of surface metallic grains. In addition, this orientation appears when platinum deposits are applied onto nanooxide. Optical microimages in polarized light can be used for estimation of the quality of the crystallographic orientation of surface metallic grains, as well as for control of the thickness of oxide nanotubular layers. Shiny grains are characteristic for a nonuniformly (steplike) etched support with formation of a “humpy” light-scattering grain structure. Dull grains usually have the flat (0001) orientation; the process of nanotube growth occurs uniformly over the surface and is considerably hindered, which is probably related to the higher atomic density of the (0001) plane and protective properties of the barrier oxide. It is shown that TiO2 nanotubes are formed at a growing rate on bright grains and grains with the crystallographic orientation allowing formation of a thick oxide layer.

Synthesis and Structure–Energy Characteristics of an MOF Al-BTC Organometallic Framework Structure

An MOF Al-BTC organometallic framework is synthesized using solvothermal synthesis from aluminum salt and trimesic acid in the N′,N-dimethylformamide organic solvent. The obtained MOF Al-BTC has a bimodal porous structure containing micropores and mesopores. Results of studying the structure–energy characteristics of Al-BTC according to the equations of Dubinin–Radushkevich, Kelvin, and BET are presented. Specific surface area SBET = 1422 m2/g. Morphological and X-ray structural characteristics of MOF Al-BTC are studied.

Activation of metal oxidation over the zone of electrodiffusion

It is shown that an oxide layer saturated by chromium oxides is formed on the surface of chromium steel at a higher rate under electrocontact (104–105 A/cm2) vacuum dc annealing (10–2 Torr, 300°C) than under furnace heating. Such activation of oxidation is due to the formation of an electrodiffusion zone in the surface steel layer. At further stages, grain boundaries emerge to the metal surface that act as oxidant transportation channels from the surrounding medium into the conductor bulk, which results in accelerated oxide formation in the bulk of the surface metal layer. Apart from the uniform oxide layer, individual hematite nanoflakes and nanoleaves with the thickness of 50–40 nm and average diameter of 450 nm are formed on the positive electrode and grow vertically on the steel surface. The average surface density of nanoparticles is 108 1/cm2. Such activation of metal oxidation over the zone of electrodiffusion can provide pronounced properties for accelerated formation of protective surface layers, in addition to its intrinsic functional (sensor, catalytic, semiconductor, adsorption) properties.