Presence Of Structural Defects Research Articles

Tuning Zeolite Properties for a Highly Efficient Synthesis of Propylene from Methanol.

A series of nanosized ZSM-5 samples was synthesized at 170, 150, 120, and 100 °C. Experimental data show that the decrease of crystallization temperature leads to significant changes in zeolite properties. Crystals synthesized at 100 °C exhibit many framework defects with lower acid-site density, strength, and a larger external surface area. The selectivity to light olefins and the propylene-to-ethylene ratio increases as the crystallization temperature decreases. A propylene-to-ethylene ratio of above 6 with the highest selectivity to propylene of 53 % was obtained over ZSM-5 catalyst prepared at 100 °C. The stability of the nanosized zeolite in methanol to olefins (MTO) was also improved compared to the industrial sample with a similar Si/Al ratio. This catalytic performance is a result of the decrease in the acid-site density, strength, and the crystals' size, providing a shorter diffusion path and larger external surface area. The presence of structural defects and a different external surface in the crystals has been shown to play an important role in the MTO catalyst performance.

Preparation of ternary phase Li4Ti5O12/anatase/rutile nanocomposites with defects and their enhanced capability for lithium ion storage

Developing anode materials that have high safety and high performance is essential for their application in Li ion batteries. Ternary phase Li4Ti5O12/rutile/anatase (N-LTO/A/R) nanocomposites that have abundant phase boundaries and structure defects are prepared via solvothermal synthesis and following calcination under an Ar/NH3 atmosphere. With the introduction of multiple phases and defects, the as-prepared ternary phase nanocomposites display excellent lithium storage capability in a Li half-cell as well as in full-cell batteries. The nanocomposites deliver a reversible specific capacity of 149.0 mAh g−1 at a current density of 10 A g−1 and maintain about 85% of the specific capacity after 800 cycles at a higher rate of 5 A g−1 for the Li half-cell. Even in the Li ion full-cell, the as-prepared nanocomposites deliver specific capacities of 210 and 124 mAh g−1 at current densities of 0.05 and 1 A g−1, respectively. The extraordinary electrochemical performance of the N-LTO/A/R nanocomposites is attributed to the nanostructures, phase boundaries, presence of structure defects, and larger specific surface area, which favor facilitating lithium ion diffusion and electron transfer. This study shows that the ternary phase nanocomposites are very promising as a high-performance anode in lithium ion batteries.

Nanoscale imaging of charge carrier transport in water splitting photoanodes

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current–voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.

Effect of alkaline-doping on photoelectrochemical activity of electrodeposited cuprous oxide films

P-type Cu2O films with alkaline ions (Li+, Na+ and K+) of unintentional dopants on indium tin oxide coated glass substrate are successfully fabricated via a simple electrodeposition method for photoelectrochemical (PEC) hydrogen generation. The SEM and XRD analysis show the as-grown films with the pyramid-like morphology and cubic structure, and the composition of alkaline-doped Cu2O films are examined using XPS spectroscopy to demonstrate the substitution of alkaline ions in the Cu2O lattice. The optical analyses, including the absorbance and low-temperature photoluminescence spectra, confirm a bandgap of 2.3 eV and the presence of structural defects in alkaline-doped Cu2O films. The Mott-Schottky plot shows the flat band potentials of the alkaline-doped Cu2O films to be approximately −0.1 V and the hole concentrations in the order of 1017 cm−3. Significantly, the Cu2O:Li film photocathode exhibits a higher photocurrent of −2.2 mA cm−2 at a potential of −0.6 V vs Ag/AgCl which are greater than those of Cu2O:K and Cu2O:Na films due to greater preferred orientation degrees along (111) and less structural defects, because the ionic radii of Cu and Li is similar. These results demonstrate the great potential of alkaline doped Cu2O films in solar-related applications.

Terahertz Time-Domain Imaging to Guide a Conservation Intervention on a Stratified Easel Painting

Very few real easel paintings have been scanned by means of terahertz time-domain imaging (THz-TDI) up to now. In this study, four different areas of an eighteenth century easel painting have been scanned by THz-TDI with the aim of proving useful information to the conservators for its imminent restoration treatment, owing to the complexity of its structure. Despite the unevenness of the layers composing the painting (lining canvas, original canvas, patching materials, and paint layers), the recorded THz dataset has been processed so that the THz images of the single layers composing the multilayered object have been clearly obtained. Relevant information about the painting structure, the presence of structural defects, overpainting, and the existence and extension of the original remains could have been determined. Infrared photography and X-ray radiography have been used for integrating the collected THz data and comparing the different outputs.

A comparative study of dry reforming of methane over nickel catalysts supported on perovskite-type LaAlO3 and commercial α-Al2O3

A systematic and comparative study was made to determine the influence of perovskite-type LaAlO3 and commercial α-Al2O3 on the performance of nickel-based catalysts in dry reforming of methane (DRM). The perovskite-type LaAlO3 was selected due to its characteristics of solid state semiconductor with oxygen vacancies and high structural stability. The catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), N2 adsorption-desorption, temperature programmed reduction (TPR-H2), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalyst performance was evaluated based on activity tests (600–800 °C) and short- and long-term stability (10 and 20 h) at 700 °C at a GHSV (Gas Hourly Space Velocity) of 18 and 72 L g−1 h−1. The TPR-H2 profiles indicate that the oxygen vacancies on the perovskite surface exerted a strong effect on the reduction temperature and reducibility of the NiO nanoparticles, resulting in weak Ni0/support interaction. The results of the tests after 10 h under GHSV of 18 L g−1 h−1 indicate that the Ni/LaAlO3 catalyst is 7.8 and 11.5% more stable than Ni/α-Al2O3 in the conversions of CH4 and CO2, respectively. The higher stability and activity of Ni/LaAlO3 is directly ascribed to the presence of NiO (3.38 wt%) after activation, which promoted the formation of carbon nanotubes (CNT) and increased the dispersion of the metallic phase. Even under severe conditions of activation and reaction (high GHSV), as in the long-term test, the Ni/LaAlO3 catalyst showed a 37.2% higher H2 yield than the Ni/α-Al2O3. Analyses by TEM indicate that the Ni/α-Al2O3 catalyst exhibited deactivation problems associated with sintering effects. Thus, the presence of structural defects and surfaces rich in oxygen vacancies makes LaAlO3 perovskite a potential support for application in methane catalytic reforming processes.

Gaseous Nitric Acid Activated Graphite Felts as Hierarchical Metal-Free Catalyst for Selective Oxidation of H2S

In this study, we reported on the influence of gaseous HNO3 treatment on the formation of defects decorated with oxygenated functional groups on commercial graphite felts (GFs). The gaseous acid treatment also leads to a remarkable increase of the specific as well as effective surface area through the formation of a highly porous graphite structure from dense graphite filamentous. The as-synthesized catalyst was further used as a metal-free catalyst in the selective oxidation of H2S in industrial waste effluents. According to the results, the defects decorated with oxygenated groups were highly active for performing selective oxidation of H2S into elemental sulfur. The desulfurization activity was relatively high and extremely stable as a function of time on stream which indicated the high efficiency of these oxidized un-doped GFs as metal-free catalysts for the selective oxidation process. The high catalytic performance was attributed to both the presence of structural defects on the filamentous carbon wall, which acting as a dissociative adsorption center for the oxygen, and the oxygenated functional groups, which could play the role of active sites for the selective oxidation process.

Room temperature response and enhanced hydrogen sensing in size selected Pd-C core-shell nanoparticles: Role of carbon shell and Pd-C interface

In the present work, the effect of carbon shell around size selected palladium (Pd) nanoparticles on hydrogen (H2) sensing has been studied by investigating the sensing response of Pd-C core-shell nanoparticles having a fixed core size and different shell thickness. The H2 sensing response of sensors based on Pd and Pd-C nanoparticles deposited on SiO2 and graphene substrate has been measured over a temperature range of 25 °C–150 °C. It is observed that Pd-C nanoparticle sensor shows higher sensitivity with increase in shell thickness and faster response/recovery in comparison to that of Pd nanoparticle samples. Pd-C nanoparticles show room temperature H2 sensitivity in contrast to Pd nanoparticles which respond only at higher temperatures. Role of carbon shell is also understood by investigating H2 sensing properties of Pd and Pd-C nanoparticles on graphene substrates. These results show that higher catalytic activity and electronic interaction at Pd-C interface, a complete coverage and protection of Pd surface by carbon and presence of structural defects in nanoparticle core are important for room temperature and higher sensing response.

Beryllium doped graphene as an efficient anode material for lithium-ion batteries with significantly huge capacity: A DFT study

First-principles density functional theory (DFT) calculations were performed to investigate the lithium (Li) adsorption upon beryllium (Be) doped graphene. Be acts as hole doping in graphene leaving the structure as electron deficient, offering a greater tendency for Li adsorption than in pristine and boron (B) doped graphene. The introduction of Be augments the adsorption energy of Li from −1.11 to −2.53eV/Li. Furthermore, 12, and 16 Li ions can easily be captured by one Be center in the single and double vacancy case, respectively, with the adsorption energies of −1.33eV/Li (for both the cases), showing that Be doped graphene is an excellent anode material for lithium ion batteries (LIBs). Consequently, the presence of structural defects, in particular, a divacancy is found to be more efficient in terms of Li storage capacity. A huge Li storage capacity (2303.295mAh/g) is calculated for Li8BeC7 having reasonable adsorption energy (−1.47eV/Li). Our calculated capacity is 6.19 times greater than that of the graphite.

Structural Order of Clays from Gounioube’s Deposit (Ivory Coast): Study by Electron Paramagnetic Resonance Spectroscopy

Cristallinity in kaolin samples from Gounioube’s deposit is studied. For this purpose, the behavior of iron and electronic defects related to irradiation damage in these clays is examined by using Electron Paramagnetic Resonance spectroscopy measurements, which point out the presence of structural defects in these materials. A deep inspection carried out by comparison of raw and bleached samples also confirms poorly cristallized samples kaolinite.

Quantitative thermal imperfection definition using non-destructive infrared thermography on an advanced DEMO divertor concept

The future DEMO divertor is currently under conceptual design within the European Consortium. In this regard, several concepts have been proposed and mock-ups have been fabricated to investigate their thermo-mechanical behaviour. Indeed, as a key plasma facing component, the divertor will have to withstand extreme thermal loads (up to 20 MW m−2 during slow transient events) and will have to be able to exhaust a large amount of heat. The presence of structural defects in the component may significantly affect the thermal response and must therefore be considered. A non-destructive technique based on infrared thermography is proposed here to detect defects in mock-ups where graded material was used as an interlayer between the heatsink material and the armor material. Two methods to characterize the size and location of such defects are presented. It was shown that finite element analysis combined with experimental data from infrared thermography, provides accurate means to assess quantitatively the size and position of thermal imperfections.

Morphology and Electronic Properties of Electrochemically Exfoliated Graphene.

Electrochemically exfoliated graphene (EEG) possesses optical and electronic properties that are markedly different from those of the more explored graphene oxide in both its pristine and reduced forms. EEG also holds a unique advantage compared to other graphenes produced by exfoliation in liquid media: it can be obtained in large quantities in a short time. However, an in-depth understanding of the structure-properties relationship of this material is still lacking. In this work, we report physicochemical characterization of EEG combined with an investigation of the electronic properties of this material carried out both at the single flake level and on the films. Additionally, we use for the first time microwave irradiation to reduce the EEG and demonstrate that the oxygen functionalities are not the bottleneck for charge transport in EEG, which is rather hindered by the presence of structural defects within the basal plane.

A Raman spectroscopic study of graphene cathodes in high-performance aluminum ion batteries

Few layer graphene is a promising cathode material for aluminum-ion batteries that use chloroaluminate (AlCl4-) ionic liquids as the electrolyte. A fundamental understanding of interactions between the few layer graphene cathode and the ionic liquid electrolyte is key for realizing the full potential of these systems. Through in situ Raman spectroscopy and density functional theory calculations, we show that the cathode is capable of achieving stage-one intercalation within the operating voltage window, leading to improved cell performance. We also show that the presence of structural defects in few layer graphene such as pores induced via plasma exposure or nitrogen dopants can deteriorate the cell performance by either decreasing the electrical connectivity or precluding stage-one intercalation respectively. The cathodes made with highly crystalline few layer graphene display high power and energy densities (~ 200Whkg−1 at 200Wkg−1 and ~ 160Whkg−1 at 5000Wkg−1), and are stable with no loss in performance up to 1000 cycles while fully charging to 2.4V.

Implementing Structural Disorder as a Promising Direction for Improving the Stability of PtNi/C Nanoparticles

Because of their enhanced oxygen reduction reaction (ORR) kinetics (9 and 6-fold) enhancement of specific activity and mass activity for the ORR relative to those of a commercial Pt/C catalyst, respectively), hollow PtNi/C nanoparticles are attracting a growing level of interest. This catalytic enhancement arises from the synergetic combination of strain and ligand effects, the presence of structural defects, and their hollow morphology producing a convex and concave catalytic sites. However, preventing a loss of catalytic activity under practical proton exchange membrane fuel cell (PEMFC) cathode operating conditions on alloys of platinum with other transition metals (PtM alloys, M being a transition metal) or M-rich core@Pt-rich shell nanoparticles remains highly challenging. A loss of performance is usually observed because of the dissolution of Pt and M under the harsh operating conditions of a PEMFC cathode, but the question remains unanswered for nanomaterials in which catalytic activity is not sole...

Efficient photocatalytic oxygen production over Ca-modified LaTiO2N

LaTiO2N has been considered a promising candidate for efficient photocatalytic water splitting. However, its photocatalytic activity is often hampered by the presence of structural defects and poor charge mobility. In this work, we introduced Ca into the crystal structure of LaTiO2N via a simple co-precipitation method and investigated the photocatalytic and photoelectrochemical properties for the full range solid solution La1−xCaxTiO2+yN1−y (0⩽x, y⩽1). Our results show that incorporating Ca into LaTiO2N induces a strong structural shrinkage as well as a space group change from I mma to P nma around a middle doping point (x=0.5). The presence of Ca effectively boosts the photocatalytic oxygen production and aids photocurrent extraction for LaTiO2N. The best photocatalytic activity was achieved for sample La0.3Ca0.7TiO2.7N0.3 with oxygen production rate ∼66.96μmol/h under visible light illumination (λ⩾420nm), corresponding to apparent quantum efficiency ∼3.13%. XPS analysis revealed a substantial reduction in surface Ti3+ species and a removal of A-site cation segregation for samples containing Ca. Further photoelectrochemical analysis highlights the critical role of Ca in enhancing the charge mobility of LaTiO2N. The better photocatalytic performance of Ca-modified LaTiO2N can be understood as a result of defect removal, high charge mobility, and improved surface charge migration pathways that considerably prolong the lifetimes of photogenerated charges.

Primary cilia are increased in number and demonstrate structural abnormalities in human cancer

AimsPrimary cilia play an important role in the regulation of cell signalling pathways and are thought to have a role in cancer but have seldom been studied in human cancer...

Physical properties of Al-doped MgZnO/AlGaN p–n heterojunction photodetectors

We report a p–n heterojunction ultraviolet photodetector (PD) based on an Al-doped MgxZn1−xO (AMZO)/Al0.08Ga0.92N heterojunction. AMZO films with 5 wt% of Al2O3 were deposited onto p–Al0.08Ga0.92N by using radio-frequency magnetron sputtering, and they were annealed at 700, 800, and 900 °C in a nitrogen ambient for 60 s. The photoluminescence spectra of the AMZO films were measured at 17 K and show two major peaks. A broad visible band could be attributed to the presence of structural defects. The AMZO film annealed at 800 °C showed a lower emission band in the visible region because of the presence of fewer defects compared with the other AMZO films. The wavelength that corresponds with the peak responsivity of the PD annealed at 800 °C was approximately 200 nm, and the cutoff wavelength was approximately 250 nm. Furthermore, the current–voltage characteristics of the Al0.08Ga0.92N/AMZO p–n junction PD showed a high leakage current. The PD annealed at 800 °C exhibited a dark current of 1.56 μA at a bias of −3 V.

Insights on finite size effects in ab initio study of CO adsorption and dissociation on Fe 110 surface

Adsorption and dissociation of hydrocarbons on metallic surfaces represent crucial steps on the path to carburization, eventually leading to dusting corrosion. While adsorption of CO molecules on Fe surface is a barrier-less exothermic process, this is not the case for the dissociation of CO into C and O adatoms and the diffusion of C beneath the surface that are found to be associated with large energy barriers. In practice, these barriers can be affected by numerous factors that combine to favour the CO-Fe reaction such as the abundance of CO and other hydrocarbons as well as the presence of structural defects. From a numerical point of view, studying these factors is challenging and a step-by-step approach is necessary to assess, in particular, the influence of the finite box size on the reaction parameters for adsorption and dissociation of CO on metal surfaces. Here, we use density functional theory (DFT) total energy calculations with the climbing-image nudged elastic band method to estimate the adsorption energies and dissociation barriers for different CO coverages with surface supercells of different sizes. We further compute the effect of periodic boundary condition for DFT calculations and find that the contribution from van der Waals interaction in the computation of adsorption parameters is important as they contribute to correcting the finite-size error in small systems. The dissociation process involves carbon insertion into the Fe surface causing a lattice deformation that requires a larger surface system for unrestricted relaxation. We show that, in the larger surface systems associated with dilute CO-coverages, C-insertion is energetically more favourable, leading to a significant decrease in the dissociation barrier. This observation suggests that a large surface system with dilute coverage is necessary for all similar metal-hydrocarbon reactions in order to study their fundamental electronic mechanisms, as an isolated phenomenon, free from finite-size effects.

Modification of electrical properties of Au/n-type InP Schottky diode with a high-k Ba0.6Sr0.4TiO3 interlayer

Au/Ba0.6Sr0.4TiO3 (BST)/n-InP metal/insulator/semiconductor (MIS) Schottky diodes have been analyzed by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The surface morphology of the BST films on InP is fairly smooth. The Au/BST/n-InP MIS Schottky diode shows better rectification ratio and low leakage current compared to the conventional Au/n-InP metal-semiconductor (MS) Schottky diode. Higher barrier height is achieved for the MIS Schottky diode compared to the MS Schottky diode. The Norde and Cheung's methods are employed to determine the barrier height, ideality factor and series resistance. The interface state density (NSS) is determined from the forward bias I-V data for both the MS and MIS Schottky diodes. Results reveal that the NSS of the MIS Schottky diode is lower than that of the MS Schottky diode. The Poole-Frenkel emission is found dominating the reverse current in both Au/n-InP MS and Au/BST/n-InP MIS Schottky diodes, indicating the presence of structural defects and trap levels in the dielectric film.

Carbon nanotubes containing oxygenated decorating defects as metal-free catalyst for selective oxidation of H2S

In this study, we report on the influence of gaseous HNO3 treatment, with various time and temperature, on the creation of defects decorated with oxygenated functional groups on the CNTs side-wall. The gaseous acid treatment leads to a significant increase of the oxygenated functional groups and defects on the tube wall along with a specific surface area improvement. The as-synthesized catalyst was further used as a metal-free catalyst in the selective oxidation of H2S in the waste effluents. According to the results the defects decorated with oxygenated groups are extremely active for performing partial oxidation of H2S into elemental sulfur. The desulfurization activity is extremely high and stable as a function of time on stream which indicates the high efficiency of these oxidized un-doped carbon nanotubes as metal-free for the selective oxidation process. The high catalytic performance was attributed to both the presence of structural defects on the carbon nanotubes wall which acting as a dissociative adsorption center for the oxygen and the oxygenated functional groups which could play the role of active sites.