Synergistic Effect of Mo plus Cu Codoping on the Photocatalytic Behavior of Metastable TiO2 Solid Solutions

Codoping with Cu and Mo is shown to have a synergistic effect on the photocatalytic activity of TiO2. The enhancement in activity is observed only if the synthesis route results in TiO2 in which (Cu, Mo) codopants are forced into the TiO2 lattice. Using X-ray photoelectron spectroscopy, Cu and Mo are shown to be present in the +2 and +6 oxidation states, respectively. A systematic study of the ternary system shows that TiO2 containing 6 mol % CuO and 1.5 mol % MoO3 is the most active ternary composition. Ab initio calculations show that codoping of TiO2 using (Mo, Cu) introduces levels above the valence band, and below the conduction band, resulting in a significant reduction in the band gap (similar to 0.8 eV). However, codoping also introduces deep defect states, which can have a deleterious impact on photoactivity. This helps rationalize the narrow compositional window over which the enhancement in photocatalytic activity is observed.

Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150 degrees C and then to a stable hexagonal structure at high temperatures (>= 250 degrees C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)(1-x)Se-x thin films (0 <= x <= 0.50) crystallize directly to the stable hexagonal structure for x >= 0.10, when annealed at temperatures >= 150 degrees C. The intermediate NaCl structure has been observed only for x, 0.10. Chemically ordered network of GST is largely modified for x >= 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)(1-x)Se-x films are better candidates for phase change memory applications.

Metastable monoclinic and orthorhombic phases and electric field induced irreversible phase transformation at room temperature in the lead-free classical ferroelectric BaTiO3

For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family, namely, BaTiO3, KNbO3, PbTiO3, and Na1/2Bi1/2TiO3, the dielectric and piezoelectric properties of BaTiO3 are considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of BaTiO3 exhibit anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in BaTiO3 remained unexplained so far from the structural standpoint. In this work, we have used high-resolution synchrotron x-ray powder diffraction, atomic resolution aberration-corrected transmission electron microscopy, in conjunction with a powder poling technique, to reveal that at 300 K (i) the equilibrium state of BaTiO3 is characterized by coexistence of metastable monoclinic Pm and orthorhombic (Amm2) phases along with the tetragonal phase, and (ii) strong electric field switches the polarization direction from the 001] direction towards the 101] direction. These results suggest that BaTiO3 at room temperature is within an instability regime, and that this instability is the fundamental factor responsible for the anomalous dielectric and piezoelectric properties of BaTiO3 as compared to the other homologous ferroelectric perovskite compounds at room temperature. Pure BaTiO3 at room temperature is therefore more akin to lead-based ferroelectric alloys close to the morphotropic phase boundary where polarization rotation and field induced ferroelectric-ferroelectric phase transformations play a fundamental role in influencing the dielectric and piezoelectric behavior.

Hierarchical sampling for metastable conformers determines biomolecular recognition: the case of malectin and diglucosylated N-glycan interactions

Structural information over the entire course of binding interactions based on the analyses of energy landscapes is described, which provides a framework to understand the events involved during biomolecular recognition. Conformational dynamics of malectin's exquisite selectivity for diglucosylated N-glycan (Dig-N-glycan), a highly flexible oligosaccharide comprising of numerous dihedral torsion angles, are described as an example. For this purpose, a novel approach based on hierarchical sampling for acquiring metastable molecular conformations constituting low-energy minima for understanding the structural features involved in a biologic recognition is proposed. For this purpose, four variants of principal component analysis were employed recursively in both Cartesian space and dihedral angles space that are characterized by free energy landscapes to select the most stable conformational substates. Subsequently, k-means clustering algorithm was implemented for geometric separation of the major native state to acquire a final ensemble of metastable conformers. A comparison of malectin complexes was then performed to characterize their conformational properties. Analyses of stereochemical metrics and other concerted binding events revealed surface complementarity, cooperative and bidentate hydrogen bonds, water-mediated hydrogen bonds, carbohydrate-aromatic interactions including CH-pi and stacking interactions involved in this recognition. Additionally, a striking structural transition from loop to beta-strands in malectin CRD upon specific binding to Dig-N-glycan is observed. The interplay of the above-mentioned binding events in malectin and Dig-N-glycan supports an extended conformational selection model as the underlying binding mechanism.

Experimental studies on the influence of intermediate principal stress and inclination on the mechanical behaviour of angular sands

A comprehensive experimental study has been made on angular sand to investigate various aspects of mechanical behavior. A hollow cylinder torsion testing apparatus is used in this program to apply a range of stress conditions on this angular quartzitic fine sand under monotonic drained shear. The effect of the magnitude and inclination of the principal stresses on an element of sand is studied through these experiments. This magnitude and inclination of the principal stresses are presented as an ``ensemble measure of fabric in sands''. This ensemble measure of fabric in the sands evolves through the shearing process, and reaches the final state, which indeed has a unique fabric. The sand shows significant variation in strength with changing inclination of the principal stresses. The locus of the final stress state in principal stress space is also mapped from these series of experiments. Additional aspects of non-coaxiality, a benchmarking exercise with a few constitutive models is presented here. This experimental approach albeit indirect shows that a unique state which is dependent on the fabric, density and confining stress exists. This suite of experiments provides a well-controlled data set for a clear understanding on the mechanical behavior of sands.

Mode coupling theory analysis of electrolyte solutions: Time dependent diffusion, intermediate scattering function, and ion solvation dynamics

A self-consistent mode coupling theory (MCT) with microscopic inputs of equilibrium pair correlation functions is developed to analyze electrolyte dynamics. We apply the theory to calculate concentration dependence of (i) time dependent ion diffusion, (ii) intermediate scattering function of the constituent ions, and (iii) ion solvation dynamics in electrolyte solution. Brownian dynamics with implicit water molecules and molecular dynamics method with explicit water are used to check the theoretical predictions. The time dependence of ionic self-diffusion coefficient and the corresponding intermediate scattering function evaluated from our MCT approach show quantitative agreement with early experimental and present Brownian dynamic simulation results. With increasing concentration, the dispersion of electrolyte friction is found to occur at increasingly higher frequency, due to the faster relaxation of the ion atmosphere. The wave number dependence of intermediate scattering function, F(k, t), exhibits markedly different relaxation dynamics at different length scales. At small wave numbers, we find the emergence of a step-like relaxation, indicating the presence of both fast and slow time scales in the system. Such behavior allows an intriguing analogy with temperature dependent relaxation dynamics of supercooled liquids. We find that solvation dynamics of a tagged ion exhibits a power law decay at long times-the decay can also be fitted to a stretched exponential form. The emergence of the power law in solvation dynamics has been tested by carrying out long Brownian dynamics simulations with varying ionic concentrations. The solvation time correlation and ion-ion intermediate scattering function indeed exhibit highly interesting, non-trivial dynamical behavior at intermediate to longer times that require further experimental and theoretical studies. (c) 2015 AIP Publishing LLC.

Dissecting Ubiquitin Folding Using the Self-Organized Polymer Model

Folding of Ubiquitin (Ub), a functionally important protein found in eukaryotic organisms, is investigated at low and neutral pH at different temperatures using simulations of the coarse-grained self-organized-polymer model with side chains (SOP-SC). The melting temperatures (T-m's), identified with the peaks in the heat capacity curves, decrease as pH decreases, in qualitative agreement with experiments. The calculated radius of gyration, showing dramatic variations with pH, is in excellent agreement with scattering experiments. At T-m Ub folds in a two-state manner at low and neutral pH. Clustering analysis of the conformations sampled in equilibrium folding trajectories at T-m with multiple transitions between the folded and unfolded states, shows a network of metastable states connecting the native and unfolded states. At low and neutral pH, Ub folds with high probability through a preferred set of conformations resulting in a pH-dependent dominant folding pathway. Folding kinetics reveal that Ub assembly at low pH occurs by multiple pathways involving a combination of nucleation-collapse and diffusion collision mechanism. The mechanism by which Ub folds is dictated by the stability of the key secondary structural elements responsible for establishing long-range contacts and collapse of Ub. Nucleation collapse mechanism holds if the stability of these elements are marginal, as would be the case at elevated temperatures. If the lifetimes associated with these structured microdomains are on the order of hundreds of microseconds, then Ub folding follows the diffusion collision mechanism with intermediates, many of which coincide with those found in equilibrium. Folding at neutral pH is a sequential process with a populated intermediate resembling that sampled at equilibrium. The transition state structures, obtained using a P-fold analysis, are homogeneous and globular with most of the secondary and tertiary structures being native-like. Many of our findings for both the thermodynamics and kinetics of folding are not only in agreement with experiments but also provide missing details not resolvable in standard experiments. The key prediction that folding mechanism varies dramatically with pH is amenable to experimental tests.

Light Induced Metastable State of Silver Nitroprusside Probed by Raman Spectroscopy

Low temperature Raman spectroscopic measurements on silver nitroprusside (AgNP), Ag-2Fe(CN)(5)NO] powders display reversible features of a partially converted metastable state. The results are compared with similarly observed metastable state in case of sodium nitroprusside (NaNP) and the differences have been discussed in terms of possible resistance to metastable state formation offered by silver atoms on the basis of hard soft acid base (HSAB) theory.

Polyketide Quinones Are Alternate Intermediate Electron Carriers during Mycobacterial Respiration in Oxygen-Deficient Niches

Mycobacterium tuberculosis (Mtb) adaptation to hypoxia is considered crucial to its prolonged latent persistence in humans. Mtb lesions are known to contain physiologically heterogeneous microenvironments that bring about differential responses from bacteria. Here we exploit metabolic variability within biofilm cells to identify alternate respiratory polyketide quinones (PkQs) from both Mycobacterium smegmatis (Msmeg) and Mtb. PkQs are specifically expressed in biofilms and other oxygen-deficient niches to maintain cellular bioenergetics. Under such conditions, these metabolites function as mobile electron carriers in the respiratory electron transport chain. In the absence of PkQs, mycobacteria escape from the hypoxic core of biofilms and prefer oxygenrich conditions. Unlike the ubiquitous isoprenoid pathway for the biosynthesis of respiratory quinones, PkQs are produced by type III polyketide synthases using fatty acyl-CoA precursors. The biosynthetic pathway is conserved in several other bacterial genomes, and our study reveals a redox-balancing chemicocellular process in microbial physiology.

On variant distribution and coarsening behavior of the a phase in a metastable beta titanium alloy

The stereology, variant distribution and coarsening behavior of semicoherent alpha(hcp) precipitates in a beta(bcc) matrix of a Ti5553 alloy has been analyzed, and a dominant 3-variant cluster has been observed in which the variants are related to each other by an axis-angle pair <<11(2)over bar> 0 >/60 degrees. Shape and spatial distribution independent elastic self and interaction energies for all pairwise and triplet combinations of a have been calculated and it is found that the 3-cluster combination that is experimentally observed most frequently has the lowest energy for the semicoherent state. The coarsening behavior of the delta distribution follows LSW kinetics after an initial transient, and has been modeled by phase field methods. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Structural Evolution and Property Changes in Nd_{60}Al_{10}Fe_{20}Co_{10} Bulk Metallic Glass During Crystallization

The structural evolution and property changes in Nd60Al10Fe20Co10 bulk metallic glass (BMG) upon crystallization are investigated by the ultrasonic method, x-ray diffraction, density measurement, and differential scanning calorimetry. The elastic constants and Debye temperature of the BMG are obtained as a function of annealing temperature. Anomalous changes in ultrasonic velocities, elastic constants, and density are observed between 600–750 K, corresponding to the formation of metastable phases as an intermediate product in the crystallization process. The changes in acoustic velocities, elastic constants, density, and Debye temperature of the BMG relative to its fully crystallized state are much smaller, compared with those of other known BMGs, the differences being attributed to the microstructural feature of the BMG.

Formation of metastable liquid catalyst during subeutectic growth of germanium nanowires.

Lattice-resolved, video-rate environmental transmission electron microscopy shows the formation of a liquid Au-Ge layer on sub-30-nm Au catalyst crystals and the transition of this two-phase Au-Ge/Au coexistence to a completely liquid Au-Ge droplet during isothermal digermane exposure at temperatures far below the bulk Au-Ge eutectic temperature. Upon Ge crystal nucleation and subsequent Ge nanowire growth, the catalyst either recrystallizes or remains liquid, apparently stabilized by the Ge supersaturation. We argue that there is a large energy barrier to nucleate diamond-cubic Ge, but not to nucleate the Au-Ge liquid. As a result, the system follows the more kinetically accessible path, forming a liquid even at 240 degrees C, although there is no liquid along the most thermodynamically favorable path below 360 degrees C.

G-jitter effects on half floating-zone convection in intermediate-frequency range

The g-jitter influence on thermocapillary convection and critical Marangoni number in a liquid bridge of half-floating rone was discussed in the low frequency range of 0.4 to 1.5 Hz in a previous paper. This paper extended the experiments to the intermediate frequency range of 2 to 18 Hz, which htrs often been recorded as vibration environment of spacecrafts. The experiment was completed on the deck of a vibration machine, which gave a periodical applied acceleration to simulate the effects of g-jitter. The experimental results in the intermediate frequency range are different from that in the low frequency range. The velocity field and the shape of the free surface have periodical fluctuations in response to g-jitter. The amplitude of the periodical varying part of the temperature response decreases obviously with increasing frequency of g-jitter and vanishes almost when the frequency of g-jitter is high enough. The critical Marangoni number is defined to describe the transition from a periodical convection in response to g-jitter to an oscillatory convection due to internal instability, and will increase with increasing g-jitter frequency. According to the spectral analysis, it can be found that the oscillatory part of temperature is a superposition of two harmonic waves if the Marangoni number is larger than a critical value.