Search

Fumed silica with a specific area of 295 m2/g was carbonized by successive phenyltrimethoxysilane treatments followed by annealing in inert atmosphere up to 650 °C. Emission, excitation, kinetics, and photo-induced bleaching effects were investigated by steady state and time-resolved photoluminescence spectroscopies. The local chemistry was also studied by infrared transmission spectroscopy. Strong ultraviolet and visible photoluminescence was observed in the samples after the chemical treatments/modifications and thermal annealing. It has been shown that ultraviolet photoluminescence in chemically modified fumed silica is associated with phenyl groups, while near ultraviolet and visible emission in annealed samples originated from inorganic pyrolytic carbon precipitates dispersed in the silica host matrix. Two types of emission bands were identified as a function of the annealing temperature: one is in the near UV and the other is in the visible range. Based on the emission/excitation analysis of these two bands, as well as on correlations with the synthesis conditions, a structural-energy concept of light-emitting centers has been proposed. According to this model, the light-emitting centers are associated with carbon clusters that can be bonded or adsorbed on the silica surface. This has been validated by a detailed (S)TEM-electron energy-loss spectroscopy study, confirming the inhomogeneous distribution of nanoscale carbon precipitates at the surface of the silica nanoparticles. These carbon precipitates are mostly amorphous although they possess some degree of graphitization and local order. Finally, the fraction of sp2 carbon in these nanoclusters has been estimated to be close to 80%. [ABSTRACT FROM AUTHOR]

We report an initial magnetic study of highly crystalline nanoparticles of the layered-kagome compound BaCo3(VO4)2(OH)2. Quasi-spherical nanoparticles with size in the range of 9–25 nm were elaborated for the first time via a new synthetic route at ambient pressure. Rietveld refinement of X-ray diffraction data, vibrational spectroscopies and high-resolution scanning transmission electron microscopy indicates that the rhombohedral crystal structure of BaCo3(VO4)2(OH)2 is not modified by nanostructuring. Magnetisation measurements are consistent with high-spin Co2+ ions for which unquenched orbital angular momentum is present. Temperature-dependent magnetic susceptibility shows no apparent magnetic ordering or spin freezing down to 2 K and suggests finite-size or surface effects at low temperatures. [ABSTRACT FROM AUTHOR]

In this work, SiO2/GaN MOS structures have been fabricated using Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) for deposition of silicon dioxide (SiO2) at low temperature (300 °C) on N-type, NID and P-type GaN epitaxial layers. Surface preparation involving chemical, UV-Ozone oxidation and oxygen plasma oxidation have been investigated by XPS analysis of the GaN surfaces prior to SiO2 deposition. The association of UV ozone and plasma oxidation allows a complete removal of carbon contamination and has a huge beneficial effect on the quality of the SiO2/GaN interface. Electrical C-V characterizations put into evidence the improved quality of the SiO2/GaN interface with a low interface trap density of 1010 cm-2 eV-1. The advantage of this soft interface treatment is thus specially observed for the N-type samples without annealing step, whereas improvements are still needed in the case of NID and P-type samples. [ABSTRACT FROM AUTHOR]

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80–330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm−2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1–5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii–Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm−1 Oe−1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films. [ABSTRACT FROM AUTHOR]

Dielectric properties of α-MoO3 are investigated by a combination of valence electron-energy-loss spectroscopy and ab initio calculation at the random-phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO3 has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-N2,3 edges as well as the low-energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of α-MoO3 is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak inε2, but only along the [010] direction. Thanks to a detailed analysis at specific k points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity. [ABSTRACT FROM AUTHOR]

Abstract: Recent analyses from the Curiosity rover at Yellowknife Bay (Gale crater, Mars) show sedimentary rocks deposited in a lacustrine environment and containing smectite clays thought to derive from the alteration of olivine. However, little is known about the weathering processes of olivine under early martian conditions, and about the stability of smectite clays in particular. Here, we present a 3-month experiment investigating the weathering of forsteritic olivine powders (Fo90) under a dense CO2 atmosphere, and under present-day terrestrial conditions for comparison. The experiment also evaluates the potential effects of hydrogen peroxide (H2O2), as a representation of the highly oxidizing compounds produced by photochemical reactions throughout martian history. The weathered samples were characterized by means of near-infrared spectroscopy (NIR), X-ray diffraction (XRD), transmission electron microscopy with energy dispersive X-ray spectrometry (TEM-EDX), Mössbauer spectroscopy and thermogravimetry. The results show that a Mg-rich smectite phase formed from the weathering of olivine under CO2 conditions, although in lower abundance than under terrestrial conditions. The main secondary phase formed under CO2 turns out to be a silica-rich phase (possibly acting as a “passivating” layer) with a non-diagnostic near-infrared spectral signature. The use of H2O2 highlights the critical importance of both the redox conditions and Fe content of the initial olivine on the nature of the secondary phases. [Copyright &y& Elsevier]

MXenes, a family of two-dimensional (2D) transition metal carbides, have been discovered as exciting candidates for various energy storage and conversion applications, including green hydrogen production by water splitting. Today, these materials mostly remain interesting objects for in-depth fundamental studies and scientific curiosity due to issues related to their preparation and environmental stability, limiting potential industrial applications. This work proposes a simple and inexpensive concept of composite electrodes composed of molybdenum- and titanium-containing MAX phases and MXene as functional materials. The concept is based on the modification of the initial MAX phase by the addition of metallic Ni, tuning Al- and carbon content and synthesis conditions, followed by fluoride-free etching under alkaline conditions. The proposed methodology allows producing a composite electrode with a well-developed 3D porous MAX phase-based structure acting as a support for electrocatalytic species, including MXene, and possessing good mechanical integrity. Electrochemical tests have shown a high electrochemical activity of such electrodes towards the hydrogen evolution reaction (HER), combined with a relatively high areal capacitance (up to 10 F cm -2 )., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A two-dimensional (2D) silicene-germanene alloy, siligene (Si x Ge y ), a single-phase material, has attracted increased attention due to its two-elemental low-buckled composition and unique physics and chemistry. This 2D material has the potential to address the challenges caused by low electrical conductivity and the environmental instability of corresponding monolayers. Yet, the siligene structure was studied in theory, demonstrating the material's great electrochemical potential for energy storage applications. The synthesis of free-standing siligene remains challenging and therefore hinders the research and its application. Herein we demonstrate nonaqueous electrochemical exfoliation of a few-layer siligene from a Ca 1.0 Si 1.0 Ge 1.0 Zintl phase precursor. The procedure was conducted in an oxygen-free environment applying a -3.8 V potential. The obtained siligene exhibits a high quality, high uniformity, and excellent crystallinity; the individual flake is within the micrometer lateral size. The 2D Si x Ge y was further explored as an anode material for lithium-ion storage. Two types of anode have been fabricated and integrated into lithium-ion battery cells, namely, (1) siligene-graphene oxide sponges and (2) siligene-multiwalled carbon nanotubes. The as-fabricated batteries both with/without siligene exhibit similar behavior; however there is an increase in the electrochemical characteristics of SiGe-integrated batteries by 10%. The corresponding batteries exhibit a 1145.0 mAh·g -1 specific capacity at 0.1 A·g -1 . The SiGe-integrated batteries demonstrate a very low polarization, confirmed by their good stability after 50 working cycles and a decrease in the solid electrolyte interphase level that occurs after the first discharge/charge cycle. We anticipate the growing potential of emerging two-component 2D materials and their great promise for energy storage and beyond.

The "gold dust defect" (GDD) appears at the surface of ferritic stainless steels (FSS) and degrades their appearance. Previous research showed that this defect might be related to intergranular corrosion and that the addition of aluminium improves surface quality. However, the nature and origin of this defect are not properly understood yet. In this study, we performed detailed electron backscatter diffraction analyses and advanced monochromated electron energy-loss spectroscopy experiments combined with machine-learning analyses in order to extract a wealth of information on the GDD. Our results show that the GDD leads to strong textural, chemical, and microstructural heterogeneities. In particular, the surface of affected samples presents an α-fibre texture which is characteristic of poorly recrystallised FSS. It is associated with a specific microstructure in which elongated grains are separated from the matrix by cracks. The edges of the cracks are rich in chromium oxides and MnCr 2 O 4 spinel. In addition, the surface of the affected samples presents a heterogeneous passive layer, in contrast with the surface of unaffected samples, which shows a thicker and continuous passive layer. The quality of the passive layer is improved with the addition of aluminium, explaining the better resistance to the GDD.

The conversion of CO 2 into desirable multicarbon products via the electrochemical reduction reaction holds promise to achieve a circular carbon economy. Here, we report a strategy in which we modify the surface of bimetallic silver-copper catalyst with aromatic heterocycles such as thiadiazole and triazole derivatives to increase the conversion of CO 2 into hydrocarbon molecules. By combining operando Raman and X-ray absorption spectroscopy with electrocatalytic measurements and analysis of the reaction products, we identified that the electron withdrawing nature of functional groups orients the reaction pathway towards the production of C 2+ species (ethanol and ethylene) and enhances the reaction rate on the surface of the catalyst by adjusting the electronic state of surface copper atoms. As a result, we achieve a high Faradaic efficiency for the C 2+ formation of ≈80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm -2 for C 2+ products., (© 2021. The Author(s).)

In this study, substochiometric hole-selective molybdenum oxide (MoO x ) contacts in crystalline silicon (c-Si) solar cells were investigated by a combination of transmission electron microscopy (TEM) and spatially resolved electron energy-loss spectroscopy (SR-EELS). It was observed that a ≈ 4 nm SiO x interlayer grows at the MoO x /c-Si interface during the evaporation of MoO x over a c-Si substrate. SR-EELS analyses revealed the presence of a 1.5 nm diffused MoO x /indium tin oxide (ITO) interface in both as-deposited and annealed samples. Moreover, the presence of a 1 nm thin layer with a lower oxidation state of Mo was detected at the SiO x /MoO x interface in an as-deposited state, which disappears upon annealing. Overall, it was evident that no hole-blocking interlayer is formed at the MoO x /ITO interface during annealing and homogenization of the MoO x layer takes place during the annealing process. Furthermore, device simulations revealed that efficient hole collection is dependent on MoO x work function and that reduction in the work function of MoO x results in loss of band bending and negatively impacts hole selectivity.

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

As nanomaterials are dominating 21st century's scene, multiple functionality in a single (nano)structure is becoming very appealing. Inspired by the Land of the Rising Sun, we designed a bifunctional (gas-sensor/photochromic) nanomaterial, made with TiO 2 whose surface was simultaneously decorated with copper and silver (the Cu/Ag molar ratio being 3:1). This nanomaterial outperformed previous state-of-the-art TiO 2 -based sensors for the detection of acetone, as well as the Cu-TiO 2 -based photochromic material. It indeed possessed splendid sensitivity toward acetone (detection limit of 100 ppb, 5 times lower than previous state-of-the-art TiO 2 -based acetone sensors), as well as reduced response/recovery times at very low working temperature, 150 °C, for acetone sensing. Still, the same material showed itself to be able to (reversibly) change in color when stimulated by both UV-A and, most remarkably, visible light. Indeed, the visible-light photochromic performance was almost 3 times faster compared to the standard Cu-TiO 2 photochromic material-that is, 4.0 min versus 10.8 min, respectively. It was eventually proposed that the photochromic behavior was triggered by different mechanisms, depending on the light source used., Competing Interests: The authors declare no competing financial interest.

We have synthesized quaternary chalcogenide-based misfit nanotubes LnS(Se)-TaS 2 (Se) (Ln = La, Ce, Nd, and Ho). None of the compounds described here were reported in the literature as a bulk compound. The characterization of these nanotubes, at the atomic level, has been developed via different transmission electron microscopy techniques, including high-resolution scanning transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy. In particular, quantification at sub-nanometer scale was achieved by acquiring high-quality electron energy-loss spectra at high energy (∼between 1000 and 2500 eV). Remarkably, the sulfur was found to reside primarily in the distorted rocksalt LnS lattice, while the Se is associated with the hexagonal TaSe 2 site. Consequently, these quaternary misfit layered compounds in the form of nanostructures possess a double superstructure of La/Ta and S/Se with the same periodicity. In addition, the interlayer spacing between the layers and the interatomic distances within the layer vary systematically in the nanotubes, showing clear reduction when going from the lightest (La atom) to the heaviest (Ho) atom. Amorphous layers, of different nature, were observed at the surface of the nanotubes. For La-based NTs, the thin external amorphous layer (inferior to 10 nm) can be ascribed to a Se deficiency. Contrarily, for Ho-based NTs, the thick amorphous layer (between 10 and 20 nm) is clearly ascribed to oxidation. All of these findings helped us to understand the atomic structure of these new compounds and nanotubes thereof.

We propose an innovative, easy-to-implement approach to synthesize aligned large-area single-crystalline graphene flakes by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.

We present for the first time a nonaqueous sol-gel route to produce ultrasmall (<2 nm) magnetic bimetallic CoPt 3 nanoparticles (NPs). The one-pot procedure is carried out at low temperature (180 °C) using benzyl alcohol, acting as both reducing agent and solvent. The highly monodisperse CoPt 3 NPs were investigated with innovative advanced X-ray methods (whole powder pattern modeling), HR-STEM, XPS, and SQUID magnetometry. XPS showed Co was mostly in metallic form, but with a very small amount of CoO on the NP surface. The spherical NPs had an ultrasmall diameter of 1.6 nm and could self-assemble in aligned linear chains, or nanobelts, of single NPs. They are superparamagnetic, with blocking temperature of ∼20 K and coercivity at 10 K of 27.9 kA m -1 (∼350 Oe). However, there is evidence of a second magnetic phase (probably CoO) in the ZFC magnetization curve, which enhances their magnetization values, without significantly affecting their superparamagnetism.

Misfit layered compounds (MLCs) have generated significant interest in recent years as potential thermoelectric materials. MLC nanotubes could reveal behavior that is entirely different from the bulk material. Recently, new chemical strategies were exploited for the synthesis of nanotubular forms of chalcogenide-based MLCs, which are promising candidates for thermoelectric materials. However, analogous synthesis of oxide-based MLC nanotubes has not been demonstrated until now. Here, we report a chemical strategy for synthesis of cobalt-oxide-based misfit nanotubes. A combination of high-resolution (scanning) transmission electron microscopy (including image simulations), spatially resolved electron energy-loss spectroscopy, electron diffraction, and density functional theory (DFT) calculations is used to discover the formation of a phase within these nanotubes that differs significantly from bulk calcium cobaltite MLCs. Furthermore, DFT calculations show that this phase is semiconducting with a band gap in excess of 1 eV, unlike bulk calcium cobaltite MLCs, which are known to be metallic. Through systematic experiments, we propose a formation mechanism for these nanotubes that could also apply more generally to realizing other oxide-based MLC nanotubes.

The electron energy-loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface, and a number of new tools. The database is accessible at https://eelsdb.eu/ and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions and the new design gives greater emphasis on contributors' original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities. In accordance with the ongoing drive toward open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra.

Ultrathin Ag2Mo3O10·2H2O nanowires (NWs) were synthesized by soft chemistry under atmospheric pressure from a hybrid organic-inorganic polyoxometalate (CH3NH3)2[Mo7O22] and characterized by powder X-ray diffraction, DSC/TGA analyses, FT-IR and FT-Raman spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Their diameters are a few tens of nanometers and hence much thinner than that found for silver molybdates commonly obtained under hydrothermal conditions. The optical properties of Ag2Mo3O10·2H2O NWs before and after UV irradiation were investigated by UV-vis-NIR diffuse reflectance spectroscopy revealing, in addition to photoreduction of Mo(6+) to Mo(5+) cations, in situ photogeneration of well-dispersed silver Ag(0) nanoparticles on the surface of the NWs. The resulting Ag@Ag2Mo3O10·2H2O heterostructure was confirmed by electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and Auger spectroscopy. Concomitant reduction of Mo(6+) and Ag(+) cations under UV excitation was discussed on the basis of electronic band structure calculations. The Ag@Ag2Mo3O10·2H2O nanocomposite is an efficient visible-light-driven plasmonic photocatalyst for degradation of Rhodamine B dye in aqueous solution.