Strain Regulating and Kinetics Accelerating of Micro-Sized Silicon Anodes via Dual-Size Hollow Graphitic Carbons Conductive Additives.

Micro-sized silicon (µSi) anode features fewer interfacial side reactions and lower costs compared to nanosized silicon, and has higher commercial value when applied as a lithium-ion battery (LIB) anode. However, the high localized stress generated during (de)lithiation causes electrode breakdown and performance deterioration of the µSi anode. In this work, hollow graphitic carbons with tailored dual sizes are employed as conductive additives for the µSi anode to overcome electrode failure. The dual-size hollow graphitic carbons (HGC) additives consist of particles with micrometer size similar to the µSi particles; these additives are used for strain regulation. Additionally, nanometer-size particles similar to commercial carbon black Spheron (SP) are used mainly for kinetics acceleration. In addition to building an efficient conductive network, the dual-size hollow graphitic carbon conductive additive prevents the fracture of the electrode by reducing local stress and alleviating volume expansion. The µSi anode with dual-size hollow graphitic carbons as conductive additives achieves an impressive capacity of 651.4 mAh g-1 after 500 cycles at a high current density of 2 A g-1 . These findings suggest that dual-size hollow graphitic carbons are expected to be superior conductive additives for micro-sized alloy anodes similar to µSi.

Hybrid tungsten-carbon 2D nanostructures viain situgasification of carbon substrates driven by ebeam irradiation of WO2.9microparticles.

Since the transmission electron microscope (TEM) has the capacity to observe the atomic structure of materials,in situTEM synthesis methods are uniquely suited to advance our fundamental understanding of the bottom-up dynamics that drive the formation of nanostructures. E-beam induced fragmentation (potentially identified as a manifestation of Coulomb explosion) and electron stimulated desorption are phenomena that have received attention because they trigger chemical and physical reactions that can lead to the production of various nanostructures. Here we report a simple TEM protocol implemented on WO2.9microparticles supported on thin amorphous carbon substrates. The method produces various nanostructures such as WC nanoparticles, WC supported films and others. Nevertheless, we focus on the gradual graphitization and gasification of the C substrate as it interacts with the material expelled from the WO2.9microparticles. The progressive gasification transforms the substrate from amorphous C down to hybrid graphitic nanoribbons incorporating W nanoparticles. We think these observations open interesting possibilities for the synthesis of 2D nanomaterials in the TEM.

An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene.

Three-dimensional (3D) graphene with a high specific surface area and excellent electrical conductivity holds extraordinary potential for molecular gas sensing. Gas molecules adsorbed onto graphene serve as electron donors, leading to an increase in conductivity. However, several challenges remain for 3D graphene-based gas sensors, such as slow response and long recovery time. Therefore, research interest remains in the promotion of the sensitivity of molecular gas detection. In this study, we fabricate oxygen plasma-treated 3D graphene for the high-performance gas sensing of formaldehyde. We synthesize large-area, high-quality, 3D graphene over Ni foam by chemical vapor deposition and obtain freestanding 3D graphene foam after Ni etching. We compare three types of strategies-non-treatment, oxygen plasma, and etching in HNO3solution-for the posttreatment of 3D graphene. Eventually, the strategy for oxygen plasma-treated 3D graphene exceeds expectations, which may highlight the general gas sensing based on chemiresistors.

Study of Active Janus Particles in the Presence of an Engineered Oil-Water Interface.

We present a systematic study of motion of Pt@SiO2 Janus particles at a liquid-liquid interface. A special microfluidic trap is used for creating such an interface. The increased surface energy of the large surface results in partial wetting of the substrate, leaving patches of oil on the glass surface. This allows us to directly compare the motion at the two interfaces, i.e., oil-water and solid-water interface within the same setting, guaranteeing identical conditions in terms of additional parameters. The propulsion behavior of Janus particles is found to be quantitatively similar at both surfaces. The interplay of reaction product absorption by oil, slip locking by surfactant, microscale friction, lubrication efficiency, and potential Marangoni effect controls the resemblance of motion characteristics at the two interfaces. Additionally, we also observed guidance effect on the Janus particles by the pinning line of oil patches, similar to solid side walls.

Tailoring the stoichiometry of C3N4 nanosheets under electron beam irradiation.

Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4 under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4 occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4 (C-N-C, [double bond, length as m-dash]NH or -NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, the d-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4.

In Situ Formation of Free-Standing Single-Atom-Thick Antiferromagnetic Chromium Membranes.

Compared to van der Waals two-dimensional (2D) layers with lateral covalent bonds, metallic bonding systems favor close-packed structures, and thus, free-standing 2D metals have remained, for the most part, elusive. However, a number of theoretical studies suggest a number of metals can exist as 2D materials and a few early experiments support this notion. Here we demonstrate free-standing single-atom-thick crystalline chromium (Cr) suspended membranes using aberration-corrected transmission electron microscopy and image simulations. Density functional theory studies confirm the 2D Cr membranes have an antiferromagnetic ground state making them highly attractive for spintronic applications. Moreover, the work also helps consolidate the existence of a new family of 2D metal layers.

In Situ N-Doped Graphene and Mo Nanoribbon Formation from Mo2 Ti2 C3 MXene Monolayers.

Since the advent of monolayered 2D transition metal carbide and nitrides (MXenes) in 2011, the number of different monolayer systems and the study thereof have been on the rise. Mo2 Ti2 C3 is one of the least studied MXenes and new insights to this material are of value to the field. Here, the stability of Mo2 Ti2 C3 under electron irradiation is investigated. A transmission electron microscope (TEM) is used to study the structural and elemental changes in situ. It is found that Mo2 Ti2 C3 is reasonably stable for the first 2 min of irradiation. However, structural changes occur thereafter, which trigger increasingly rapid and significant rearrangement. This results in the formation of pores and two new nanomaterials, namely, N-doped graphene membranes and Mo nanoribbons. The study provides insight into the stability of Mo2 Ti2 C3 monolayers against electron irradiation, which will allow for reliable future study of the material using TEM. Furthermore, these findings will facilitate further research in the rapidly growing field of electron beam driven chemistry and engineering of nanomaterials.

Active Assembly of Spheroidal Photocatalytic BiVO4 Microswimmers.

We create single-component photocatalytic bismuth vanadate (BiVO4) microswimmers with a spheroidal shape that move individually upon irradiation without any asymmetrization step. These particles form active assemblies which we investigate combining an experimental approach with numerical simulations and analytical calculations. We systematically explore the speed and assembly of the swimmers into clusters of up to four particles and find excellent agreement between experiment and theory, which helps us to understand motion patterns and speed trends. Moreover, different batches of particles can be functionalized separately, making them ideal candidates to fulfill a multitude of tasks, such as sensing or environmental remediation. To exemplify this, we coat our swimmers with silica (SiO2) and selectively couple some of their modules to fluorophores in a way which does not inhibit self-propulsion. The present work establishes spheroidal BiVO4 microswimmers as a versatile platform to design multifunctional microswimmers.

Contribution to understand the biomineralization of bones.

INTRODUCTION: The goal is to propose a material scientific hypothesis for the atomic arrangement of calcium phosphates during the mineralization of bones. MATERIALS AND METHODS: It was reached by the analysis of bones of healthy and osteoporotic rats using analytical transmission electron microscopic methods. RESULTS: Electron diffraction patterns show hydroxyapatite (HAP) as dominant phase within the mineralized areas. In the electron energy loss spectrum, a double peak of the phosphorous L-edge seems to be a characteristic feature of the phosphorous binding in biological HAP. The hypothesis bases on periodic features on the collagen surface which agree with distances between oxygen atoms in the (200) plane of octacalcium phosphate (OCP). Bridge pillars for the HAP network consist of OCP coupled with a half unit cell on collagen by oxygen-hydrogen bridges. Possibly, the metastable OCP bridges are only a transient step, while the mineralization is starting. OCP and HAP couple by similar distances of calcium atoms in an interface close to the (100) planes of the OCP and the HAP network. To reach the perfect overlap of the equidistant Ca atoms, the HAP network has to be rotated by 22.5° around the a-axis, 11.5° around the c-axis of HAP, and 10.1° around an axis perpendicular to a and c. CONCLUSIONS: A supercell based on this idea is able to explain the dominance of HAP in the electron diffraction patterns, the arrangement of the (002) lattice planes perpendicular to the collagen fiber axis, and sections of high-resolution TEM images.

Applications of 2D MXenes in energy conversion and storage systems.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Cu@TiO2 Janus microswimmers with a versatile motion mechanism.

We report novel metal-capped TiO2 photochemically-active colloids endowed with a 'hybrid drive': directional motion is achieved in water upon UV illumination, as well as in dilute peroxide solutions upon illumination with UV or visible light. From the different behaviours of nearby particles, we infer that distinct reaction pathways affect the local composition and flow of the solution.

In Situ Electron Driven Carbon Nanopillar-Fullerene Transformation through Cr Atom Mediation.

The promise of sp2 nanomaterials remains immense, and ways to strategically combine and manipulate these nanostructures will further enhance their potential as well as advance nanotechnology as a whole. The scale of these structures requires precision at the atomic scale. In this sense electron microscopes are attractive as they offer both atomic imaging and a means to structurally modify structures. Here we show how Cr atoms can be used as physical linkers to connect carbon nanotubes and fullerenes to graphene. Crucially, while under electron irradiation, the Cr atoms can drive transformations such as catalytic healing of a hole in graphene with simultaneous transformation of a single wall carbon nanotube into a fullerene. The atomic resolution of the electron microscopy along with density functional theory based total energy calculations provide insight into the dynamic transformations of Cr atom linkers. The work augments the potential of transmission electron microscopes as nanolaboratories.

Highly photocatalytic TiO2 interconnected porous powder fabricated by sponge-templated atomic layer deposition.

A titanium dioxide (TiO2) interconnected porous structure has been fabricated by means of atomic layer deposition of TiO2 onto a reticular sponge template. The obtained freestanding TiO2 with large surface area can be easily taken out of the water to solve a complex separation procedure. A compact and conformal nanocoating was evidenced by morphologic characterization. A phase transition, as well as production of oxygen vacancies with increasing annealing temperature, was detected by x-ray diffraction and x-ray photoelectron spectroscopy, respectively. The photocatalytic experimental results demonstrated that the powder with appropriate annealing treatment possessed excellent photocatalytic ability due to the co-action of high surface area, oxygen vacancies and the optimal crystal structure.

Room temperature in situ growth of B/BOx nanowires and BOx nanotubes.

Despite significant advances in the synthesis of nanostructures, our understanding of the growth mechanisms of nanowires and nanotubes grown from catalyst particles remains limited. In this study we demonstrate a straightforward route to grow coaxial amorphous B/BOx nanowires and BOx nanotubes using gold catalyst particles inside a transmission electron microscope at room temperature without the need of any specialized or expensive accessories. Exceedingly high growth rates (over 7 µm/min) are found for the coaxial nanowires, and this is attributed to the highly efficient diffusion of B species along the surface of a nanowire by electrostatic repulsion. On the other hand the O species are shown to be relevant to activate the gold catalysts, and this can occur through volatile O species. The technique could be further developed to study the growth of other nanostructures and holds promise for the room temperature growth of nanostructures as a whole.

Study of the Long-Term High-Temperature Structural Stability of RuAl Electrodes for Microelectronic Devices.

The high-temperature stability of RuAl-based electrodes for application in microelectronic devices is analyzed for long-term duration. The electrodes are prepared on Ca3TaGa3Si2O14 (CTGS) substrates using SiO2 and Al-N-O cover and barrier layers as oxidation protection. The samples are annealed at 600, 700, or 800 °C in air for 192 h. Minor degradation is observed after thermal loading at 700 °C. The annealing at 800 °C for 192 h leads to a partial oxidation of the Al in the extended contact pad and to a complete oxidation of the Al within the structured interconnect electrodes. The different degradation of the interconnect electrodes and the contact pads is caused by their different lateral dimensions. In summary, long-term high-temperature stability is demonstrated up to at least 700 °C in air. Less oxidizing atmospheres should allow the application at higher temperatures and for a significantly longer duration.

Influence of magnetic field on electron beam-induced Coulomb explosion of gold microparticles in transmission electron microscopy.

In this work we instigated the fragmentation of Au microparticles supported on a thin amorphous carbon film by irradiating them with a gradually convergent electron beam inside the Transmission Electron Microscope. This phenomenon has been generically labeled as "electron beam-induced fragmentation" or EBIF and its physical origin remains contested. On the one hand, EBIF has been primarily characterized as a consequence of beam-induced heating. On the other, EBIF has been attributed to beam-induced charging eventually leading to Coulomb explosion. To test the feasibility of the charging framework for EBIF, we instigated the fragmentation of Au particles under two different experimental conditions. First, with the magnetic objective lens of the microscope operating at full capacity, i.e. background magnetic field B=2 T, and with the magnetic objective lens switched off (Lorenz mode), i.e. B=0 T. We observe that the presence or absence of the magnetic field noticeably affects the critical current density at which EBIF occurs. This strongly suggests that magnetic field effects play a crucial role in instigating EBIF on the microparticles. The dependence of the value of the critical current density on the absence or presence of an ambient magnetic field cannot be accounted for by the beam-induced heating model. Consequently, this work presents robust experimental evidence suggesting that Coulomb explosion driven by electrostatic charging is the root cause of EBIF.

Inherent Antibacterial Properties of Biodegradable FeMnC(Cu) Alloys for Implant Application.

Implant-related infections or inflammation are one of the main reasons for implant failure. Therefore, different concepts for prevention are needed, which strongly promote the development and validation of improved material designs. Besides modifying the implant surface by, for example, antibacterial coatings (also implying drugs) for deterring or eliminating harmful bacteria, it is a highly promising strategy to prevent such implant infections by antibacterial substrate materials. In this work, the inherent antibacterial behavior of the as-cast biodegradable Fe69Mn30C1 (FeMnC) alloy against Gram-negative Pseudomonas aeruginosa and Escherichia coli as well as Gram-positive Staphylococcus aureus is presented for the first time in comparison to the clinically applied, corrosion-resistant AISI 316L stainless steel. In the second step, 3.5 wt % Cu was added to the FeMnC reference alloy, and the microbial corrosion as well as the proliferation of the investigated bacterial strains is further strongly influenced. This leads for instance to enhanced antibacterial activity of the Cu-modified FeMnC-based alloy against the very aggressive, wild-type bacteria P. aeruginosa. For clarification of the bacterial test results, additional analyses were applied regarding the microstructure and elemental distribution as well as the initial corrosion behavior of the alloys. This was electrochemically investigated by a potentiodynamic polarization test. The initial degraded surface after immersion were analyzed by glow discharge optical emission spectrometry and transmission electron microscopy combined with energy-dispersive X-ray analysis, revealing an increase of degradation due to Cu alloying. Due to their antibacterial behavior, both investigated FeMnC-based alloys in this study are attractive as a temporary implant material.

Surface Modified ß-Ti-18Mo-6Nb-5Ta (wt%) Alloy for Bone Implant Applications: Composite Characterization and Cytocompatibility Assessment.

Commercially available titanium alloys such as Ti-6Al-4V are established in clinical use as load-bearing bone implant materials. However, concerns about the toxic effects of vanadium and aluminum have prompted the development of Al- and V-free ß-Ti alloys. Herein, a new alloy composed of non-toxic elements, namely Ti-18Mo-6Nb-5Ta (wt%), has been fabricated by arc melting. The resulting single ß-phase alloy shows improved mechanical properties (Young's modulus and hardness) and similar corrosion behavior in simulated body fluid when compared with commercial Ti-6Al-4V. To increase the cell proliferation capability of the new biomaterial, the surface of Ti-18Mo-6Nb-5Ta was modified by electrodepositing calcium phosphate (CaP) ceramic layers. Coatings with a Ca/P ratio of 1.47 were obtained at pulse current densities, -jc, of 1.8-8.2 mA/cm2, followed by 48 h of NaOH post-treatment. The thickness of the coatings has been measured by scanning electron microscopy from an ion beam cut, resulting in an average thickness of about 5 µm. Finally, cytocompatibility and cell adhesion have been evaluated using the osteosarcoma cell line Saos-2, demonstrating good biocompatibility and enhanced cell proliferation on the CaP-modified Ti-18Mo-6Nb-5Ta material compared with the bare alloy, even outperforming their CaP-modified Ti-6-Al-4V counterparts.

Fast synthesis of large-area bilayer graphene film on Cu.

Bilayer graphene (BLG) is intriguing for its unique properties and potential applications in electronics, photonics, and mechanics. However, the chemical vapor deposition synthesis of large-area high-quality bilayer graphene on Cu is suffering from a low growth rate and limited bilayer coverage. Herein, we demonstrate the fast synthesis of meter-sized bilayer graphene film on commercial polycrystalline Cu foils by introducing trace CO2 during high-temperature growth. Continuous bilayer graphene with a high ratio of AB-stacking structure can be obtained within 20 min, which exhibits enhanced mechanical strength, uniform transmittance, and low sheet resistance in large area. Moreover, 96 and 100% AB-stacking structures were achieved in bilayer graphene grown on single-crystal Cu(111) foil and ultraflat single-crystal Cu(111)/sapphire substrates, respectively. The AB-stacking bilayer graphene exhibits tunable bandgap and performs well in photodetection. This work provides important insights into the growth mechanism and the mass production of large-area high-quality BLG on Cu.

Understanding the growth of amorphous SiO2 nanofibers and crystalline binary nanoparticles produced by laser ablation.

The pulsed-laser evaporation synthesis of silica nanofibers and crystalline binary nanoparticles is investigated in detail. By careful adjustment of the synthesis parameters one can tailor the product to form high yield nanofibers or binary nanoparticles. Some control on their diameters is also possible through the synthesis parameters. Oxidation of the nanofibers occurs upon exposure to air after the reaction.