Properties of nanostructured oxide formed during oxidation of a zirconium wire by supercritical water

Synthesis of ZrO₂ during oxidation of a zirconium wire by supercritical water at P = 25 MPa and T = 500 or 525°C has been investigated. It is established that an inhomogeneous nanostructured ZrO₂ layer is formed as a result of oxidation. Rate of 〈Zr〉 oxidation, oxide porosity and morphology, and average size and structure of crystallites are determined. The thermal conductivity of the synthesized ZrO₂ layer in supercritical water and in nitrogen is determined by pulsed electric heating of a partially oxidized wire. The low values of thermal conductivity (about 0.2 W/(m K)) correspond to a layered structure of porous material, with layers orientated parallel to the oxidized-metal surface.     

Synthesis of Fe x O y nanoparticles during iron oxidation by supercritical water

It is established that iron is oxidized by supercritical water (SCW) with the formation of H₂ and nanoparticles of iron oxides (Fe₃O₄, FeO, and γ-Fe₂O₃). The kinetics of H₂ production and iron oxidation has been studied by SCW injection at T = 673, 723, 773, 823, and 873 K into a reactor with iron particles. Data of X-ray diffraction and transmission electron microscopy show that the phase composition and morphology of synthesized oxide nanoparticles depend on the SCW temperature.     

Pyramidal nanostructures of zinc oxide

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 degrees C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 +/- 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere.     

Continuous production of fine zinc oxide nanorods by hydrothermal synthesis in supercritical water

Continuous production of highly crystalline ZnO nanorods by supercritical hydrothermal synthesis was reported in this article. Zinc nitrate aqueous solution was pressurized to 30 MPa at room temperature and rapidly heated to 673 K by mixing with supercritical water and then fed into a tubular reactor. Residence time is about 10 s. Production of ZnO nanorod particles with uniform particle size distribution showed a strong ultraviolet light emission at room temperature. This article also reported in-situ surface modification of ZnO nanorods with organic reagents by the supercritical hydrothermal synthesis.     

Synthesis of Fe-group metal oxide nanostructures by thermal oxidation and their magnetic properties

In this paper, we review the preparation of Fe-group metal oxide nanostructures by the thermal oxidation method developed in our lab. By this method, we have prepared several kinds of nanostructures, including nanowires and nanoleaves. The magnetic properties of these nanostructures have also been studied. By carefully controlling the reacting time, temperature, and humidity, we have prepared alpha-Fe2O3, gamma-Fe2O3, Fe3O4, and Co3O4 nanowires and alpha-Fe2O3 nanoleaves by heating the substrates in proper atmosphere. The alpha-Fe2O3 and Co3O4 nanowires are produced by directly oxygenating pure metal at 550 to approximately 650 degrees C and 480-520 degrees C, separately. The gamma-Fe2O3 and Fe3O4 nanowires are produced by reducing as-prepared alpha-Fe2O3 nanowires in a mixture of N2 and H2. The nanowires are about 10-20 microm, with diameter of about 20 to approximately 100 nm. Most of the nanowire arrays are grown vertically from the surface of the substrate at a high surface density (10(8)-10(9) cm(-2)). Compared with the nanowires prepared by hydrothermal process and template method, Most of our nanowires are structurally uniform and single crystallites. The magnetic properties of these nanostructures are also studied, and demonstrate some novel properties.     

Synthesis and growth mechanism of zinc oxide nanostructures in arc discharge

Nanodimensional tubes, needles, and tetrapods of zinc oxide (ZnO) with lengths up to ～1 μm and diameters up to several hundred nanometers have been synthesized for the first time using an arc discharge method. A distinctive feature of the proposed method is the broad variety of evaporated and deposited products. The possible mechanism of evaporation, nucleation, and growth is suggested. Initially, the primary sheets are growing; these sheets turn into rolls and tubes, which eventually transform into needles. Tetrapods crystallize and grow in the gas phase and form a white snowlike deposit.     

以超临界和亚临界水活化掺金属化合物的酚醛树脂制备球形活性炭(英文)

以掺稀土金属化合物氯化铈的酚醛树脂为原料,采用超临界和亚临界水活化制备了具有较好中孔结构和机械强度的球形活性炭。通过77K氮气吸附对所制球形活性炭进行表征,并研究不同活化方法、活化条件对其孔隙结构的影响。结果表明:氯化铈具有催化作用,在超临界水环境下促进了孔结构的发展,优势中孔分布于4nm和7nm;而活化方法和活化压力对孔结构的影响较小。     

The effect of constant electric field on the oxidation rate of massive zinc in supercritical water and the formation of zinc oxide nanocrystals

It is established that the rate of oxidation of a massive zinc plate by supercritical water (SCW) at 673 K and 23 MPa increases and the morphology of obtained ZnO nanocrystals changes when strength E of the constant electric field applied perpendicular to the plate increases from 0 to 286 kV/m. A decrease in the SCW density at the same temperature results in the formation of a more compact nanostructured ZnO layer, while the increase in E leads to loosening of structure in the inner part of the ZnO layer.     

Synthesis of ZrO<Subscript>2</Subscript> nanoparticles during zirconium oxidation by supercritical water

We have discovered that massive samples of solid zirconium (Zr)_s are completely oxidized by supercritical water (SCW, T > 647 K, P > 22.1 MPa) with the formation of zirconium oxide nanoparticles (ZrO₂) _n . The particle size distribution, morphology, and features of the nanostructure formation depend on the process conditions. The kinetics of H₂ production and zirconium oxidation has been determined using the method of SCW injection into a reactor with (Zr)_s at various temperatures. The dependence of the oxidation induction time on the SCW parameters has been studied.     

Thermal properties of semiconductor zinc oxide nanostructures

A combination of two methods — laser modulation and 3ω — has been used to determine the heat capacity, heat conductivity, and heat diffusivity of zinc oxide nanostructures. A significant difference between the thermal parameters of zinc oxide nanostructures grown by different technological methods has been revealed. It has been shown that the relatively low heat conductivity and heat diffusivity values of oxide zinc nanostructures are due to both the internal defects and the contact resistance between the film and its base — the substrate.     

Zinc oxide nanostructures: epitaxially growing from hexagonal zinc nanostructures

The epitaxial growth of ZnO nanosheets and nanoneedles from a Zn/ZnO core/shell structure is verified by an experiment in which the ZnO nanoneedles and nanosheets are synthesized in air within an ultra-low temperature range from?250 to 400?°C by thermal oxidation of Zn films made up of hexagonal nanodiscs or nanoprisms. The hexagonal Zn structures are oxidized to form a Zn/ZnO core/shell structure with an epitaxial relationship; ZnO nanoneedles and nanosheets are found to grow epitaxially from the ZnO shell, along sixfold symmetric [Formula: see text] directions, showing the same lattice orientation as the Zn core. The stability difference among different facets of hexagonal Zn crystal structures plays a key role in the formation of ZnO nanosheets, nanoneedles and the Zn/ZnO core/shell structure, as well as ZnO hollow structures. A vapor-solid mechanism is suggested to explain the epitaxial growth process of the ZnO products. Photoluminescence properties of the ZnO nanostructures are also explored.     

A study on electrodeposited zinc oxide nanostructures

Zinc oxide (ZnO) nanostructures prepared by electrochemical deposition method from aqueous zinc nitrate solution at 65 °C onto fluorine doped tin oxide coated glass substrates were investigated. Characterization of ZnO nanostructures was realized using conventional electrochemical techniques, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Cyclic voltammetry experiments were performed to elucidate the electrodic processes that occurred when potentials were applied and the optimum potential for electrodeposition were determined. From the Mott-Schottky measurements, the flat-band potential and the donor density for the ZnO nanostructure are determined. From single-step potential experiment in the potential ranges from ?1.1 to ?1.4 V, the formation of ZnO nuclei in the early deposition stages was proceeded according to the three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth rather than a progressive one. SEM images demonstrated that the morphology of ZnO nanostructures depend greatly on the potential depositions. XRD studies revealed that the deposited films were polycrystalline in nature with wurtzite phase.     

Nucleation and growth of catalyst-free zinc oxide nanostructures

This paper deals with the investigations on the nucleation and growth of Zinc Oxide (ZnO) nanostructures in a catalyst free synthesis. The ZnO nanostructures have been formed by evaporation of Zn (99.99%) in O2 and Ar atmosphere in single zone furnace under two temperature regions, region A (approximately 1173-1073 K) and region B (approximately 873-773 K). Through application of XRD and TEM techniques, it has been shown that first ZnO is formed which changes to ZnOx through creation of oxygen vacancies. The ZnOx acts as self-catalyst and leads to formation of various nanostructures. Those observed in the present investigation are nanotetrapods (1 D, diameter approximately 70-450 nm, length approximately 2-4.5 approximatelym) nanorods (1 D, diameter approximately 45-95 nm, length approximately 2.5-4.5 microm), nanoflowers(2D, central core diameter approximately 90-185 nm, length of petals/nanorod approximately 1.0-3.5 microm) and nanoparticles (3D, size approximately 0.85-2.5 microm). These nanostructures have been revealed by SEM explorations. Attempts have been made to explain the formation of the various nanostructures in terms of the creation and distribution of the ZnOx, the temperature as well as oxygenation conditions.     

Thin polycrystalline zinc oxide films obtained by oxidation of metallic zinc films

Thin polycrystalline ZnO films were obtained by thermal oxidation of metallic Zn films, thermally deposited on various substrates, such as silica, sapphire and glass, in both air and pure oxygen atmospheres. The quality of the ZnO layers was asserted by Hall effect, cathodoluminescence and atomic force microscopy measurements. Electron concentration of 7.32×10¹² cm⁻³ and mobility of 14.2 cm²/V s with root mean square roughness of 30 nm were obtained for the 900 °C annealed ZnO films in oxygen. Room temperature cathodoluminescence spectra consisted of a narrow near band edge luminescence band and a broad defect-related green band with peak positions at 380 and 500 nm, respectively. ZnO film luminescence properties improved dramatically with the increase of annealing temperature and decrease of O₂ pressure.     

Field-emission stability of hydrothermally synthesized aluminum-doped zinc oxide nanostructures

The Al-doped ZnO (AZO) nanostructures field-emission arrays (FEAs) were hydrothermally synthesized on AZO/glass substrate. The samples with Al-dosage of 3 at.% show the morphology as nanowires vertically grown on the substrates and a structure of c-axis elongated single-crystalline wurtzite. The good field-emission (i.e., the large anode current and low fluctuation of 15.9%) can be found by AZO nanostructure FEAs with well-designed Al-dosage (i.e., 3 at.%) because of the vertical nanowires with the less structural defects and superior crystallinity. Moreover, the Full width at half maximum (FWHM) of near band-edge emission (NBE) decreased as the increase of annealing temperature, representing the compensated structural defects during oxygen ambient annealing. After the oxygen annealing at 500 degrees C, the hydrothermal AZO nanostructure FEAs revealed the excellent electrical characteristics (i.e., the larger anode current and uniform distribution of induced fluorescence) and enhanced field-emission stability (i.e., the lowest current fluctuation of 5.97%).     

OPA oxidation rates in supercritical water

Supercritical water oxidation can effectively destroy a large variety of high-risk wastes resulting from munitions demilitarization and complex industrial chemical. An important design consideration in the development of supercritical water oxidation is the information on the oxidation rate. In this paper, the oxidation rate of isopropyl amine (OPA), one of high-risk wastes resulting from munitions demilitarization, was investigated under supercritical water oxidation (SCWO) conditions in an isothermal tubular reactor. H2O2 was used as the oxidant. The reaction temperatures were ranged from 684 to 891 K and the residence times varied from 9 to 18s at a fixed pressure of 25 MPa. The conversion of OPA was monitored by analyzing total organic carbon (TOC) on the liquid effluent samples. The initial TOC concentrations of OPA varied from 7.21 to 143.78 mmol/l at the conversion efficiencies from 88.94 to 99.98%. By taking into account the dependence of reaction rate on oxidant and TOC concentration, a global power-law rate expression was regressed from 38 OPA experimental data. The resulting pre-exponential factor was 2.46(+/-0.65)x10(3)l(1.37)mmol(-0.37)s(-1); the activation energy was 64.12+/-1.94 kJ/mol; and the reaction orders for OPA (based on TOC) and oxidant were 1.13+/-0.02 and 0.24+/-0.01, respectively.     

Fabrication of zinc oxide nanostructures using solvent-assisted capillary lithography

We present a simple solvent-assisted capillary molding method to fabricate zinc oxide (ZnO) nanostructures using an ultraviolet (UV) curable polyurethane acrylate (PUA) mold. A thin film of the ZnO sol-gel precursor solution in methyl alcohol was prepared by spin coating on a solid substrate and subsequently a nanopatterned PUA mold was brought into conformal contact with the substrate under slight physical pressure (～3.5?bar). After annealing at 230?°C for 4?h, well-defined ZnO nanostructures formed with feature size down to ～50?nm, aided by capillary rise and solvent evaporation. It was found that the height of capillary rise depended highly on the applied pressure. A simple experimental setup was devised to examine the effects of pressure, revealing that the optimum pressure ranged from 3.5?bar to 5?bar. Also, ZnO nanorods could be selectively grown on patterned regions using the seed layer as a pseudocatalyst when the width of the seed layer was larger than ～200?nm.     

Formation of zinc oxide nanoparticles during electric discharge in water

A method for obtaining nanodimensional structures of zinc oxide (ZnO) by means of electric discharge in water is described. Data on the phase composition and luminescent properties of the synthesized ZnO powder deposited from a colloidal solution onto single crystal silicon surface are presented. It is shown that the partial concentrations of zinc in the oxide and metallic phases in the colloidal solution depend on the supply of oxygen to the zone of the chemical reaction. Optimum conditions for the discharge synthesis of nanodimensional powder with an almost 100% ZnO content have been found.     

Dopant induced bandgap narrowing in Y-doped zinc oxide nanostructures

In this report, hydrothermal synthesis and the absorption properties of the cubic shaped zinc oxide nanostructures doped with different amount of yttrium (Y) metal cation (0 to 15 at.%) are demonstrated. The structural and optical properties of chemically synthesized pure and Y doped ZnO powders are investigated by using powder X-ray diffraction (XRD), field emission scanning electron spectroscopy (FESEM) and transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorbance, photoluminescence (PL), and Fourier transform infra-red spectroscopy (FT-IR). It is found that the dopant ions stabilize in wurtzite hexagonal phase of ZnO upto the concentration of less than 6 at.%, which is mainly due to the fact that the ZnO lattice expands and the optical bandgap energy decreases at this level. Increasing the dopant concentration to greater than 6 at.% leads to a contraction of the lattice, which in turn produces a significant structural disorder evidenced by shift in the XRD peaks due to additional interstitial incorporation of Y. The vibrational modes of the metal oxide groups have been identified from the IR transmission spectra. The optical absorption results show that the optical bandgap energy of Y:ZnO nanocrystals is much less as compared to that of the pure bulk ZnO particles. Doping ZnO with trivalent Y produces excess number of electrons in the conduction band and thus, shifts the absorption edge and narrows down to 80 meV approximately. PL spectra are used to study the dependence of doping on the deep-level emission, which show an enhanced blue emission after Y doping. The existence of near band edge (NBE) emission and blue emission, related to zinc interstitials are observed in the luminescence spectra of Zn(1-x)Y(x)O nanostructures.     

Field emission from zinc oxide nanostructures and its degradation

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environment was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre-deposited transitional ZnO film on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10 μA/cm² was achieved at the fields of 5.7 and 6.2 V/μm from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post-test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined.