Time dependent decomposition of ammonia borane for the controlled production of 2D hexagonal boron nitride.

Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexagonal boron nitride (h-BN) by chemical vapour deposition (CVD). Its non-toxic and non-flammable properties make AB particularly attractive for industry. AB decomposition under CVD conditions, however, is complex and hence has hindered tailored h-BN production and its exploitation. To overcome this challenge, we report in-depth decomposition studies of AB under industrially safe growth conditions. In situ mass spectrometry revealed a time and temperature-dependent release of a plethora of NxBy-containing species and, as a result, significant changes of the N:B ratio during h-BN synthesis. Such fluctuations strongly influence the formation and morphology of 2D h-BN. By means of in situ gas monitoring and regulating the precursor temperature over time we achieve uniform release of volatile chemical species over many hours for the first time, paving the way towards the controlled, industrially viable production of h-BN

WS22D nanosheets in 3D nanoflowers

In this work it has been established that 3D nanoflowers of WS2 synthesized by chemical vapour deposition are composed of few layer WS2 along the edges of the petals. An experimental study to understand the evolution of these nanostructures shows the nucleation and growth along with the compositional changes they undergo

Effects of temperature and ammonia flow rate on the chemical vapour deposition growth of nitrogen-doped graphene

We doped graphene in situ during synthesis from methane and ammonia on copper in a low-pressure chemical vapour deposition system, and investigated the effect of the synthesis temperature and ammonia concentration on the growth. Raman and X-ray photoelectron spectroscopy was used to investigate the quality and nitrogen content of the graphene and demonstrated that decreasing the synthesis temperature and increasing the ammonia flow rate results in an increase in the concentration of nitrogen dopants up to ca. 2.1% overall. However, concurrent scanning electron microscopy studies demonstrate that decreasing both the growth temperature from 1000 to 900 1C and increasing the N/C precursor ratio from 1/50 to 1/10 significantly decreased the growth rate by a factor of six overall. Using scanning tunnelling microscopy we show that the nitrogen was incorporated mainly in substitutional configuration, while current imaging tunnelling spectroscopy showed that the effect of the nitrogen on the density of states was visible only over a few atom distances

A Graphene Surface Force Balance

We report a method for transferring graphene, grown by chemical vapor deposition, which produces ultraflat graphene surfaces (root-mean-square roughness of 0.19 nm) free from polymer residues over macroscopic areas (>1 cm2). The critical step in preparing such surfaces involves the use of an intermediate mica template, which itself is atomically smooth. We demonstrate the compatibility of these model surfaces with the surface force balance, opening up the possibility of measuring normal and lateral forces, including friction and adhesion, between two graphene sheets either in contact or across a liquid medium. The conductivity of the graphene surfaces allows forces to be measured while controlling the surface potential. This new apparatus, the graphene surface force balance, is expected to be of importance to the future understanding of graphene in applications from lubrication to electrochemical energy storage systems

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 atomic %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.Comment: 4 pages, 4 figures; major changes to the tex

Density Functional Theory Investigation of 2D Phase Separated Graphene/Hexagonal Boron Nitride Monolayers; Band Gap, Band Edge Positions, and Photo Activity

Creating sustainable and stable semiconductors for energy conversion via catalysis, such as water splitting and carbon dioxide reduction, is a major challenge in modern materials chemistry, propelled by the limited and dwindling reserves of platinum group metals. Two-dimensional hexagonal borocarbonitride (h-BCN) is a metal-free alternative and ternary semiconductor, possessing tunable electronic properties between that of hexagonal boron nitride (h-BN) and graphene, and has attracted significant attention as a nonmetallic catalyst for a host of technologically relevant chemical reactions. Herein, we use density functional theory to investigate the stability and optoelectronic properties of phase-separated monolayer h-BCN structures, varying carbon concentration and domain size. We find that, on average, a higher C content reduces the energetic cost of carbon inclusion per atom, as an increasingly graphitized network lowers the overall energy of the structure. Using functional HSE06, we show how the electronic bandgap of h-BN can be reduced from 5.94 to 1.61 eV with significant substitution of C in the domain (C at. % ∼ 44%) adding to the weight of evidence that suggests these segregated h-BCN systems can easily be customized. We use the location of conduction and valence band edges with respect to the potentials of HER, OER and CO2 reduction to assess the catalytic suitability of these materials, identifying three structures with appropriate band edges for these catalytic reactions. Finally, the photoactivity of the structures is assessed through TD-DFT calculations, and we propose two candidates for photocatalysis based on the segregated h-BCN system

Classification of carbon nanostructure families occurring in a chemically activated arc discharge reaction

Controlling the generation of empty cages, endohedral metallofullerenes and carbon nanotubes is an important challenge for the tailored synthesis of functional materials and their scaled up production. However, the reaction yields for fullerenes are low and their formation mechanism is far from being elucidated thus hampering their targeted, scaled up production and potential applications. We present a systematic study on the effect of the addition of copper nitrate as a doping agent during an arc discharge vaporization of Gd and Nd doped rods for the production of a series of fullerenes and carbon nanotubes. The incorporation of copper nitrate at a Cu/M molar ratio in the range of 6 to 7 leads to a higher yield for the high molecular weight fullerenes and endohedral fullerenes compared to small empty cages. We distinguished three different families of nanomaterials: (1) small empty cage fullerenes, (2) endohedral metallofullerenes and empty cage fullerenes with more than 88 atoms, and (3) multi-wall carbon nanotubes which were deposited on the cathode and their yield appeared to be influenced by the different reaction conditions

3D Electrospinning of Al2O3/ZrO2 fibrous aerogels for multipurpose thermal insulation

Ceramic aerogels are excellent ultralight-weight thermal insulators yet impractical due to their tendency towards structural degradation at elevated temperatures, under mechanical disturbances, or in humid environments. Here, we present flexible and durable alumina/zirconia fibrous aerogels (AZFA) fabricated using 3D sol–gel electrospinning — a technique enabling in situ formation of 3D fiber assemblies with significantly reduced time consumption and low processing cost compared to most existing methods. Our AZFAs exhibit ultralow density (> 3.4 mg cm−3), low thermal conductivity (> 21.6 mW m−1 K−1), excellent fire resistance, while remaining mechanically elastic and flexible at 1300 °C, and thermally stable at 1500 °C. We investigate the underlying structure-thermal conductivity relationships, demonstrating that the macroscopic fiber arrangement dictates the solid-phase thermal conduction, and the mesopores in the fiber effectively trap air thereby decreasing the gas conduction. We show experimentally and theoretically that directional heat transport, i.e., anisotropic thermal conductivity, can be achieved through compressing the fiber network. We further solve the moisture sensitivity problem of common fibrous aerogels through fluorination coating. The resulting material possesses excellent hydrophobicity and self-cleaning properties, which can provide reliable thermal insulation under various conditions, including but not limited to high-temperature conditions in vehicles and aircraft, humid conditions in buildings, and underwater environments for oil pipelines. Graphical Abstract: [Figure not available: see fulltext.

Nacre-like alumina with unique high strain rate capabilities

Nacre-like alumina manufactured using spark plasma sintering shows a strikingly different mechanical behaviour compared to conventional alumina. A range of sintering conditions were applied to micron-sized alumina platelet powders to form alumina with different nacre-like microstructures, density, grain size and flexural strength. We show that a microstructure of aligned sintered platelets not only mitigates the typical issue of brittleness, but also has extraordinary energy absorption capabilities. It can withstand an impact with up to three times the kinetic energy required to break monolithic alumina while maintaining structural integrity. The high-rate compressive strength is shown to be more than 50% higher than that of monolithic alumina and we show energy absorption mechanisms such as crack deflection and branching to be present. Our approach provides a fast and effective way of manufacturing aligned nacre-like ceramic microstructures that maintain structural integrity through energy dissipation and interlocking mechanisms