High pressure synthesis of amorphous TiO2 nanotubes

view Abstract Citations (157) References (75) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Density Amorphous Ice, the Frost on Interstellar Grains Jenniskens, P. ; Blake, D. F. ; Wilson, M. A. ; Pohorille, A. Abstract Most water ice in the universe is in a form which does not occur naturally on Earth and of which only minimal amounts have been made in the laboratory. We have encountered this "high-density amorphous ice" in electron diffraction experiments of low-temperature (T < 30 K) vapor-deposited water and have subsequently modeled its structure using molecular dynamics simulations. The characteristic feature of high-density amorphous ice is the presence of "interstitial" oxygen pair distances between 3 and 4 A. However, we find that the structure is best described as a collapsed lattice of the more familiar low-density amorphous form. These distortions are frozen in at temperatures below 38 K because, we propose, it requires the breaking of one hydrogen bond, on average, per molecule to relieve the strain and to restructure the lattice to that of low-density amorphous ice. Several features of astrophysical ice analogs studied in laboratory experiments are readily explained by the structural transition from high-density amorphous ice into low-density amorphous ice. Changes in the shape of the 3.07 microns water band, trapping efficiency of CO, CO loss, changes in the CO band structure, and the recombination of radicals induced by low-temperature UV photolysis all covary with structural changes that occur in the ice during this amorphous to amorphous transition. While the 3.07 microns ice band in various astronomical environments can be modeled with spectra of simple mixtures of amorphous and crystalline forms, the contribution of the high-density amorphous form nearly always dominates. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/176585 Bibcode: 1995ApJ...455..389J Keywords: INFRARED: ISM: LINES AND BANDS; ISM: DUST; EXTINCTION; ISM: MOLECULES; LINE: PROFILES; MOLECULAR DATA; METHODS: LABORATORY; Density (Mass/Volume); Amorphous Materials; Ice; Frost; Interstellar Matter; Astronomy; Astrophysics; Cosmic Dust; Crystallinity; Electron Diffraction; Hydrogen Bonds; Molecular Dynamics; Radicals; Shapes; Trapping; Universe; Vapor Deposition; Water; Astrophysics full text sources ADS |

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

(0 0 1) Facet-exposed anatase-phase TiO2 nanotube hybrid reduced graphene oxide composite: Synthesis, characterization and application in photocatalytic degradation

Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra‐high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocation substructures, including free dislocations, loops, tiltwalls, dislocation tangles and subboundaries. They were produced by high‐temperature ductile deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470 ± 6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APDs in ordered omphacites from Shima, coincident with ages given by single‐zircon U‐Pb dating (471 ± 2 Ma). HRTEM reveals C2/c and P2/n space groups in different parts of one single omphacite crystal, and no exsolution is observed in the studied samples, which is attributed to rapid cooling. It is suggested that the UHP eclogites underwent a long period of annealing at high temperatures, followed by relatively rapid cooling. These data provide valuable information for the formation and exhumation mechanism of UHP eclogites in the Dabie high‐pressure (HP) and UHP metamorphic belt.

Formation of TiO2 nanoribbons by anodization under high current density

TiO2 nanotubes are a promising system for photoelectrochemical applications due to their high surface area ratio, 1D charge transport properties, and stability in solution and under light.1 However, their photoelectrochemical performance is limited due to the presence of trap states inside the TiO2 band gap, facilitating the recombination of electron/hole pairs. We found that TiO2 nanotubes anodized in a low water content electrolyte had a lower photocurrent response compared to those anodized in a 11 vol% water electrolyte.2 A subsequent study attributed this observation to a higher density of these deep level trap state: 6.5 x 1016 cm-3 for TiO2 nanotubes anodized in a 2 vol% water electrolyte compared with 2.4 x 1016 cm-3 for nanotubes anodized in a 11 vol% water electrolyte.3 As the low water content electrolyte produces TiO2 nanotubes with the widest variety of lengths, it is desirable to improve their performance by reducing the influence of these trap states. Modification of TiO2 nanotubes by Li and H has been observed to improve the photoelectrochemical performance, with the intercalation inducing the formation of Ti3+.4 TiO2 nanotubes were synthesized by a double anodization procedure and anodized in an electrolyte containing 0.3wt% NH4F and 2 vol% H2O in ethylene glycol. Li and H modification was carried out by applying a potential of -1.55 VSCE in 1M LiClO4 and 0.5M H2SO4 respectively. Photocurrent measurements under simulated sunlight (Figure 1) in a solution containing 0.2M Na2SO4 and 0.1M NaCH3COO (pH = 7) showed a 2 fold enhancement in the photoelectrochemical performance under simulated sunlight at a potential of 1.0 VSCE for both lithium and hydrogen modified nanotubes. Faster onset transients and electrochemical impedance spectroscopy measurements showed that the intercalation of Li and H effectively passivated the trapping behavior of the TiO2 nanotubes, leading to higher photocurrents. High resolution TEM was performed to obtain atomic-scale imaging of the TiO2 nanotubes (Figure 2). At the outside walls of the Li modified TiO2 nanotubes, a darkened zone is observed in the wall, suggesting that the intercalation penetrates ~10 nm into the nanotube walls. Acknowledgments We gratefully acknowledge the support of the ARCS Foundation.

Context. Thermal inertias of atmosphereless icy planetary bodies happen to be very low.Aims. We relate the thermal inertia to the regolith properties such as porosity, grain size, ice form and heat transfer processes to understand why it is low. We interpret the dichotomy in thermal inertia of the surface of Mimas in terms of changes in regolith properties. We predict how the thermal inertia of these bodies may vary with heliocentric distance depending on these properties.Methods. We combine available models of conductivity by contact or radiation to understand what heat transfer process is predominant.Results. The magnitude of the thermal inertia of a porous icy regolith is mainly governed by the crystalline or amorphous ice forms, and the quality of contacts between grains. Beyond the orbit of Jupiter, thermal inertias as low as a few tens J/m2 / K/s1/2 are difficult to reproduce with plausible porosity and grains sizes made of crystalline ice unless contacts are loose. This is, on the contrary, straightforward for regoliths of sub-cm-sized grains made of amorphous water ice. This study points out the importance of including the temperature dependence of thermophysical properties of water ice forms and the radiative conduction in thermal models of these bodies. The relatively high thermal inertia of the leading face of Mimas can be explained by a regolith of crystalline ice grains in tight contacts, which are eventually sintered by the bombardment of high energy electrons. The low thermal inertia of its trailing face is easily reproduced by a regolith of moderate porosity with sub-mm-sized grains of amorphous ice. The characteristic decrease of thermal inertia with heliocentric distance of icy atmosphereless surfaces and the very low thermal inertia of relevant trans-Neptunian objects are easily explained if amorphous ice is present at cm depths below a thin layer of crystalline ice.

High-performance supercapacitors based on amorphous C-modified anodic TiO2 nanotubes

One of the major challenges of implantology is to design nanoscale modifications of titanium implant surfaces inducing osseointegration. The aim of this study was to investigate the behavior of rat osteoblasts cultured on anodized TiO2 nanotubes of different crystallinity (amorphous and anatase phase) up to 24 days. TiO2 nanotubes were fabricated on VT1–0 titanium foil via a two-step anodization at 20 V using NH4F as an electrolyte. Anatase-phase samples were prepared by heat treatment at 500 °C for 1 h. VT1–0 samples with flat surfaces were used as controls. Primary rat osteoblasts were seeded over experimental surfaces for several incubation times. Scanning electron microscopy (SEM) was used to analyze tested surfaces and cell morphology. Cell adhesion and proliferation were investigated by cell counting. Osteogenic differentiation of cells was evaluated by qPCR of runt-related transcription factor 2 (RUNX2), osteopontin (OPN), integrin binding sialoprotein (IBSP), alkaline phosphatase (ALP) and osteocalcin (OCN). Cell adhesion and proliferation, cell morphology and the expression of osteogenic markers were affected by TiO2 nanotube layered substrates of amorphous and anatase crystallinity. In comparison with flat titanium, along with increased cell adhesion and cell growth a large portion of osteoblasts grown on the both nanostructured surfaces exhibited an osteocyte-like morphology as early as 48 h of culture. Moreover, the expression of all tested osteogenic markers in cells cultured on amorphous and anatase TiO2 nanotubes was upregulated at least at one of the analyzed time points. To summarize, we demonstrated that amorphous and anodized TiO2 layered substrates are highly biocompatible with rat osteoblasts and that the surface modification with about 1500 nm length nanotubes of 35 ± 4 (amorphous phase) and 41 ± 8 nm (anatase phase) in diameter is sufficient to induce their osteogenic differentiation. Such results are significant to the engineering of coating strategies for orthopedic implants aimed to establish a more efficient bone to implant contact and enhance bone repair.

TiO2 nanotubes as alternative cathode in microbial fuel cells: Effect of annealing treatment on its performance

The phase transitions of one-dimensional (1D) anatase TiO2 nanowires were studied by in situ high-pressure synchrotron X-ray diffraction and Raman scattering up to 37 GPa. A direct anatase-to-baddeleyite transformation was observed at ∼9 GPa, which is clearly different from the size-dependent phase transition behaviors for nanocrystalline TiO2. We found the higher compressibility in the c-axis compared to the a-axis for anatase nanowires that may be attributed to both the crystal structural feature and the growth direction of the nanowires. The Ti–O bonds show abnormal changes during the anatase-to-baddeleyite phase transition. This phase transition of the TiO2 nanowires shows obvious morphology-tuned behaviors. Upon decompression, the baddeleyite phase transformed into α-PbO2 phase. The morphology of the TiO2 nanowires shows excellent stability and TiO2 nanowires with α-PbO2 phase were obtained at ambient conditions through a compression–decompression cycle. These results indicate that the nanoscale quas...

Introduction There are 12 crystalline forms of ice. At ordinary pressures the stable phase is termed ice I, terminology that followed Tammann's (1900) discovery of high-pressure phases. There are two closely related low-pressure variants: hexagonal ice, denoted Ih, and cubic ice, Ic. Ice Ih is termed ordinary ice whose hexagonal crystal symmetry is reflected in the shape of snow-flakes. Ice Ic is made by depositing water vapor at temperatures lower than about −130 ℃. High-pressure ices are of little interest in relation to geophysical processes on Earth, but constitute the primary materials from which many extra-terrestrial bodies are made. We describe their structure and creep properties in Chapter 8. In addition to the 12 crystalline forms, there are two amorphous forms. One is termed low-density ice (940 kg m −3 at −196 ℃ at 1 atmosphere) and the other, high-density ice (1170 kg m −3 , same conditions). The density of ice Ih is 933 kg m −3 at the same temperature and pressure (Hobbs, 1974). Amorphous ices can be made at low temperatures in five ways (see review by Mishima and Stanley, 1998): by condensing vapor below −160 ℃; by quenching liquid; by compressing ice Ih at −196 ℃; by electron irradiation; and through transformation upon warming from one amorphous state to another. Although once thought to be a nanocrystalline material, amorphous ice is now considered to be truly glassy water. Its mechanical behavior remains to be explored. In this chapter we address the structure of ice Ih.

The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii 1 km < R < 100 km was investigated. The radioisotopes considered were 26Al, 40K, 232Th, 235U, and 238U. Except for the 26Al abundance, which was varied, the other initial abundances were kept fixed, at values derived from those of chondritic meteorites and corresponding to a gas-to-dust ratio of 1. The initial models were homogeneous and isothermal (To = 10 K) amorphous ice spheres, in a circular orbit at 10(4) AU from the Sun. The main object of this study was to examine the conditions under which the transition temperature from amorphous into cubic ice (Ta = 137 K) would be reached. It was shown that the influence of the short-lived radionuclide 26Al dominates the effect of other radioactive species for bodies of radii up to approximately 50 km. Consequently, if we require comets to retain their ice in amorphous form, as suggested by observations, an upper limit of approximately 4 x 10(-9) is obtained for the initial 26Al abundance in comets, a factor of 100 lower than that of the inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of a coexistence of molten cometary cores and extended amorphous ice mantles is ruled out. Larger icy spheres (R > 100 km) reached Ta even in the absence of 26Al, due to the decay of the other radionuclides. As a result, a crystalline core formed whose relative size depended on the composition assumed. Thus the outermost icy satellites in the solar system, which might have been formed of ice in the amorphous state, have probably undergone crystallization and may have exhibited eruptive activity when the gas trapped in the amorphous ice was released (e.g., Miranda).

Interface feature characterization and Schottky interfacial layer confirmation of TiO2 nanotube array film

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics to describe supercooled liquids and glasses. Here we use the PEL formalism and computer simulations to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) at different temperatures. We employ the ST2 water model for which the LDA-HDA transformations are remarkably sharp, similar to what is observed in experiments, and reminiscent of a first-order phase transition. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that low-density liquid (LDL) configurations are located in the same megabasin as LDA, and that high-density liquid (HDL) configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid is different from the path followed by the amorphous solid. At higher pressure, we also study the liquid-to-ice-VII first-order phase transition, and find that the behavior of the PEL properties across this transition is qualitatively similar to the changes found during the LDA-HDA transformation. This similarity supports the interpretation that the LDA-HDA transformation is a first-order phase transition between out-of-equilibrium states. Finally, we compare the PEL properties explored during the LDA-HDA transformations in ST2 water with those reported previously for SPC/E water, for which the LDA-HDA transformations are rather smooth. This comparison illuminates the previous work showing that, at accessible computer times scales, a liquid-liquid phase transition occurs in the case of ST2 water, but not for SPC/E water.