Conversion of chirality to twisting via sequential one-dimensional and two-dimensional growth of graphene spirals.

The properties of two-dimensional (2D) van der Waals materials can be tuned through nanostructuring or controlled layer stacking, where interlayer hybridization induces exotic electronic states and transport phenomena. Here we describe a viable approach and underlying mechanism for the assisted self-assembly of twisted layer graphene. The process, which can be implemented in standard chemical vapour deposition growth, is best described by analogy to origami and kirigami with paper. It involves the controlled induction of wrinkle formation in single-layer graphene with subsequent wrinkle folding, tearing and re-growth. Inherent to the process is the formation of intertwined graphene spirals and conversion of the chiral angle of 1D wrinkles into a 2D twist angle of a 3D superlattice. The approach can be extended to other foldable 2D materials and facilitates the production of miniaturized electronic components, including capacitors, resistors, inductors and superconductors.

Interplay of Magnetic Properties and Doping in Epitaxial Films of h-REFeO3 Multiferroic Oxides.

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO3 multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (TN ). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO3 . Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.

Ultrathin Single Crystalline MgO(111) Nanosheets*.

Synthesizing high-quality two-dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long-range single-crystallinity and clean high-energy surfaces are required. Reported here is the synthesis of single-crystalline MgO(111) nanosheets by a two-step process involving the formation of ultrathin Mg(OH)2 nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, three-coordinated O2- sites and low-coordinated Mg2+ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C2 species.

Growth and Termination Dynamics of Multiwalled Carbon Nanotubes at Near Ambient Pressure: An in Situ Transmission Electron Microscopy Study.

Understanding the growth mechanism of carbon nanotubes (CNTs) has been long pursued since its discovery. With recent integration of in situ techniques into the study of CNT growth, important insights about the growth mechanism of CNT have been generated, which have improved our understanding significantly. However, previous in situ experiments were mainly conducted at low pressures which were far from the practical conditions. Direct information about the growth dynamics under relevant conditions is still absent and thus is highly desirable. In this work, we report atomic-scale observations of multiwalled CNT (MWCNT) growth and termination at near ambient pressure by in situ transmission electron microscopy. On the basis of the real-time imaging, we are able to reveal that the working catalyst is constantly reshaping at its apex during catalyzing CNT growth, whereas at the base the catalyst remains faceted and barely shows any morphological change. The active catalyst is identified as crystalline Fe3C, based on lattice fringes that can be imaged during growth. However, the oscillatory growth behavior of the CNT and the structural dynamics of the apex area strongly indicate that the carbon concentration in the catalyst particle is fluctuating during the course of CNT growth. Extended observations further reveal that the catalyst splitting can occur: whereas the majority of the catalyst stays at the base and continues catalyzing CNT growth, a small portion of it gets trapped inside of the growing nanotube. The catalyst splitting can take place multiple times, leading to shrinkage of both, catalyst size and diameter of CNT, and finally the growth termination of CNT due to the full coverage of the catalyst by carbon layers. Additionally, in situ observations show two more scenarios for the growth termination, that is, out-migration of the catalyst from the growing nanotube induced by (i) Oswald ripening and (ii) weakened adhesion strength between the catalyst and CNT.

Structural Evolution and Dynamics of an In2O3 Catalyst for CO2 Hydrogenation to Methanol: An Operando XAS-XRD and In Situ TEM Study.

We report an operando examination of a model nanocrystalline In2O3 catalyst for methanol synthesis via CO2 hydrogenation (300 °C, 20 bar) by combining X-ray absorption spectroscopy (XAS), X-ray powder diffraction (XRD), and in situ transmission electron microscopy (TEM). Three distinct catalytic regimes are identified during CO2 hydrogenation: activation, stable performance, and deactivation. The structural evolution of In2O3 nanoparticles (NPs) with time on stream (TOS) followed by XANES-EXAFS-XRD associates the activation stage with a minor decrease of the In-O coordination number and a partial reduction of In2O3 due to the formation of oxygen vacancy sites (i.e., In2O3-x). As the reaction proceeds, a reductive amorphization of In2O3 NPs takes place, characterized by decreasing In-O and In-In coordination numbers and intensities of the In2O3 Bragg peaks. A multivariate analysis of the XANES data confirms the formation of In2O3-x and, with TOS, metallic In. Notably, the appearance of molten In0 coincides with the onset of catalyst deactivation. This phase transition is also visualized by in situ TEM, acquired under reactive conditions at 800 mbar pressure. In situ TEM revealed an electron beam assisted transformation of In2O3 nanoparticles into a dynamic structure in which crystalline and amorphous phases coexist and continuously interconvert. The regeneration of the deactivated In0/In2O3-x catalyst by reoxidation was critically assessed revealing that the spent catalyst can be reoxidized only partially in a CO2 atmosphere or air yielding an average crystallite size of the resultant In2O3 that is approximately an order of magnitude larger than the initial one.

Atomic-Scale Observation of the Metal-Promoter Interaction in Rh-Based Syngas-Upgrading Catalysts.

The direct conversion of syngas to ethanol, typically using promoted Rh catalysts, is a cornerstone reaction in CO2 utilization and hydrogen storage technologies. A rational catalyst development requires a detailed structural understanding of the activated catalyst and the role of promoters in driving chemoselectivity. Herein, we report a comprehensive atomic-scale study of metal-promoter interactions in silica-supported Rh, Rh-Mn, and Rh-Mn-Fe catalysts by aberration-corrected (AC) TEM. While the catalytic reaction leads to the formation of a Rh carbide phase in the Rh-Mn/SiO2 catalyst, the addition of Fe results in the formation of bimetallic Rh-Fe alloys, which further improves the selectivity and prevents the carbide formation. In all promoted catalysts, Mn is present as an oxide decorating the metal particles. Based on the atomic insight obtained, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided.

What Drives Metal-Surface Step Bunching in Graphene Chemical Vapor Deposition?

Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.

Identifying Key Structural Features of IrOx Water Splitting Catalysts.

Hydrogen production by electrocatalytic water splitting will play a key role in the realization of a sustainable energy supply. Owing to their relatively high stability and activity, iridium (hydr)oxides have been identified as the most promising catalysts for the oxidation of water. Comprehensive spectroscopic and theoretical studies on the basis of rutile IrO2 have provided insight about the electronic structure of the active X-ray amorphous phase. However, due to the absence of long-range order and missing information about the local arrangement of structural units, our present understanding of the active phase is very unsatisfying. In this work, using a combination of real-space atomic scale imaging with atomic pair distribution function analysis and local measurements of the electronic structure, we identify key structural motifs that are associated with high water splitting activity. Comparison of two X-ray amorphous phases with distinctively different electrocatalytic performance reveals that high activity is linked to the ratio between corner- and edge-sharing IrO6 octahedra. We show that the active and stable phase consists of single unit cell sized hollandite-like structural domains that are cross-linked through undercoordinated oxygen/iridium atoms. In the less active phase, the presence of the rutile phase structural motif results in a faster structural collapse and deactivation. The presented results provide insight into the structure-activity relationship and promote a rational synthesis of X-ray amorphous IrOx hydroxides that contain a favorable arrangement of structural units for improved performance in catalytic water splitting.

Engineered Au Core@Prussian Blue Analogous Shell Nanoheterostructures: Their Magnetic and Optical Properties.

We report a new approach for the synthesis of multifunctional Au core@Prussian Blue analogous (PBA) shell nanoheterostructures that involves PBA shell growth on the surface of cyanide-stabilized gold nanoparticles. It permits the assembly of Au@KNiII [FeII (CN)6 ] core@shell and Au@KNiII [FeII (CN)6 ]@KNiII [CrIII (CN)6 ] core@shell@shell heterostructures with well-defined and size-controlled gold cores and PBA shells. These heterostructures exhibit tunable size- and shape-dependent magnetic and optical properties: (i) the surface plasmon resonance band position and intensity mainly depend on the PBA shell thickness, and (ii) the magnetic properties (the transition temperature, the coercivity, and magnetic regime) depend in a complex manner on the thickness as well as the particular morphology of the magnetic shell.

Rh-Doped Pt-Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability.

Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt(-1) combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30,000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt-Ni nanoparticles.

In Situ Graphene Growth Dynamics on Polycrystalline Catalyst Foils.

The dynamics of graphene growth on polycrystalline Pt foils during chemical vapor deposition (CVD) are investigated using in situ scanning electron microscopy and complementary structural characterization of the catalyst with electron backscatter diffraction. A general growth model is outlined that considers precursor dissociation, mass transport, and attachment to the edge of a growing domain. We thereby analyze graphene growth dynamics at different length scales and reveal that the rate-limiting step varies throughout the process and across different regions of the catalyst surface, including different facets of an individual graphene domain. The facets that define the domain shapes lie normal to slow growth directions, which are determined by the interfacial mobility when attachment to domain edges is rate-limiting, as well as anisotropy in surface diffusion as diffusion becomes rate-limiting. Our observations and analysis thus reveal that the structure of CVD graphene films is intimately linked to that of the underlying polycrystalline catalyst, with both interfacial mobility and diffusional anisotropy depending on the presence of step edges and grain boundaries. The growth model developed serves as a general framework for understanding and optimizing the growth of 2D materials on polycrystalline catalysts.

Synthesis and Assembly of Dipolar Heterostructured Tetrapods: Colloidal Polymers with "Giant tert-butyl" Groups.

We report on the first synthesis of a heterostructured semiconductor tetrapod from CdSe@CdS that carries a single dipolar nanoparticle tip from a core-shell colloid of Au@Co. A four-step colloidal total synthesis was developed, where the key step in the synthesis was the selective deposition of a single AuNP tip onto a CdSe@CdS tetrapod under UV-irradiation. Synthetic accessibility to this dipolar heterostructured tetrapod enabled the use of these as colloidal monomers to form colloidal polymers that carry the semiconductor tetrapod as a side chain group attached to the CoNP colloidal polymer main chain. The current report details a number of novel discoveries on the selective synthesis of an asymmetric heterostructured tetrapod that is capable of 1D dipolar assembly into colloidal polymers that carry tetrapods as side chain groups that mimic "giant tert-butyl groups".

GaN and GaxIn1-xN Nanoparticles with Tunable Indium Content: Synthesis and Characterization.

Semiconducting GaN and GaxIn1-xN nanoparticles (4-10 nm in diameter, depending on the metal ratio) with tunable indium content are prepared through a chemical synthesis (the urea-glass route). The bandgap of the ternary system depends on its composition, and therefore, the color of the final material can be turned from bright yellow (the color of pure GaN) to blue (the color of pure InN). Transmission electron microscopy (TEM and HRTEM) and scanning electron microscopy (SEM) images confirm the nanoparticle character and homogeneity of the as-prepared samples. X-ray diffraction (XRD), electron diffraction (EDX), elemental mapping, and UV/Vis, IR, and Raman spectroscopy investigations are used to confirm the incorporation of indium into the crystal structure of GaN. These nanoparticles, possessing adjusted optical properties, are expected to have potential applications in the fabrication of novel optoelectronic devices.

Gas sensing properties and p-type response of ALD TiO2 coated carbon nanotubes.

Amorphous titanium dioxide-coated carbon nanotubes (CNTs) were prepared by atomic layer deposition (ALD) and investigated as sensing layers for resistive NO2 and O2 gas sensors. By varying ALD process conditions and CNT structure, heterostructures with different metal oxide grain size, morphology and coating thickness were synthesized. Higher responses were observed with homogeneous and continuous 5.5 nm thick films onto CNTs at an operating temperature of 150 °C, while CNTs decorated with either discontinuous film or TiO2 nanoparticles showed a weak response close to the one of device made of bare CNTs. An unexpected p-type behavior in presence of the target gas was also noticed, independently of the metal oxide morphology and thickness. Based on previous works, hypotheses were made in order to explain the p-type behavior of TiO2/CNT sensors.

Oxide-supported IrNiO(x) core-shell particles as efficient, cost-effective, and stable catalysts for electrochemical water splitting.

Active and highly stable oxide-supported IrNiO(x) core-shell catalysts for electrochemical water splitting are presented. IrNi(x)@IrO(x) nanoparticles supported on high-surface-area mesoporous antimony-doped tin oxide (IrNiO(x)/Meso-ATO) were synthesized from bimetallic IrNi(x) precursor alloys (PA-IrNi(x) /Meso-ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA-IrNi(x)/Meso-ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core-shell particles. The core-shell IrNiO(x)/Meso-ATO catalyst displayed high water-splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.

Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting.

The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future.

The influence of intercalated oxygen on the properties of graphene on polycrystalline Cu under various environmental conditions.

Intercalation of oxygen at the interface of graphene grown by chemical vapour deposition and its polycrystalline copper catalyst can have a strong impact on the electronic, chemical and structural properties of both the graphene and the Cu. This can affect the oxidation resistance of the metal as well as subsequent graphene transfer. Here, we show, using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption near edge spectroscopy (XANES), energy dispersive X-ray spectroscopy (EDX) and (environmental) scanning electron microscopy (ESEM) that both the oxygen intercalation and de-intercalation are kinetically driven and can be clearly distinguished from carbon etching. The obtained results reveal that a charge transfer between as grown graphene and Cu can be annulled by intercalating oxygen creating quasi-free-standing graphene. This effect is found to be reversible on vacuum annealing proceeding via graphene grain boundaries and defects within the graphene but not without loss of graphene by oxidative etching for repeated (de-)intercalation cycles.

Observing graphene grow: catalyst-graphene interactions during scalable graphene growth on polycrystalline copper.

Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration.

Nanosized heterostructures of Au@Prussian blue analogues: towards multifunctionality at the nanoscale.

Access to multifunctionality at the nanoscale requires the development of hybrid nanostructures that combine materials of different natures. In this line of thought, current research on coordination polymers is not only focusing on their synthesis at the nanoscale, but also on combining these polymers with other materials. According to a novel and rational approach, single-layer Au@Prussian blue analogue (PBA) and double-layer Au@PBA@PBA' core-shell nanoparticles (NPs) may be obtained through the growth of a cyano-bridged coordination network on the gold surface. The nanosized heterostructures combine the plasmonic optical properties of the gold core and the magnetic properties of the PBA shell. Whereas the single-layer nanoparticles are paramagnetic, the double-layer nanostructures display ferromagnetism; therefore, the overall structural motif may be considered as multifunctional. The developed synthetic concept also includes an easy access to hollow PBA NPs.

Direct In- and Out-of-Plane Writing of Metals on Insulators by Electron-Beam-Enabled, Confined Electrodeposition with Submicrometer Feature Size.

Additive microfabrication processes based on localized electroplating enable the one-step deposition of micro-scale metal structures with outstanding performance, e.g., high electrical conductivity and mechanical strength. They are therefore evaluated as an exciting and enabling addition to the existing repertoire of microfabrication technologies. Yet, electrochemical processes are generally restricted to conductive or semiconductive substrates, precluding their application in the manufacturing of functional electric devices where direct deposition onto insulators is often required. Here, the direct, localized electrodeposition of copper on a variety of insulating substrates, namely Al2O3, glass and flexible polyethylene, is demonstrated, enabled by electron-beam-induced reduction in a highly confined liquid electrolyte reservoir. The nanometer-size of the electrolyte reservoir, fed by electrohydrodynamic ejection, enables a minimal feature size on the order of 200 nm. The fact that the transient reservoir is established and stabilized by electrohydrodynamic ejection rather than specialized liquid cells can offer greater flexibility toward deposition on arbitrary substrate geometries and materials. Installed in a low-vacuum scanning electron microscope, the setup further allows for operando, nanoscale observation and analysis of the manufacturing process.