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

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

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

Hierarchical porosity design enables highly recyclable and efficient Au/TiO2 composite fibers for photodegradation of organic pollutants

Titanium dioxide (TiO2) nanomaterials are ideal for photocatalytic degradation of organic pollutants but remain infeasible for industrial and municipal wastewater treatment because they cannot simultaneously satisfy two essential criteria for practical application, i.e., high performance and good recyclability. Here, we design and create hierarchically porous TiO2 fibers by dual-polymer templating sol–gel electrospinning combined with precise control over crystallization. The produced fibers own unique interconnected macropores throughout the fiber body that enable significantly enhanced light absorption and unlimited mass transport, making them ideal hosts for anchoring plasmonic nanoparticles (NPs). The Au NP-coupled TiO2 fibers have photocatalytic efficiencies up to 6.6 times higher than plain TiO2 fibers, showing comparable ability as commercial P25 nanopowder in photodegrading methyl blue (MB) and achieving complete decomposition of methyl orange (MO) in 90 min while P25 degrades only 66% MO. Unlike P25 or anatase TiO2 nanopowders that non-reversibly disperse/aggregate in water, our composite fibers can be recollected through natural sedimentation, and their superior performance remains for at least six cycles. This work offers a practical and feasible design for high-performance recyclable photocatalysts for industrial-scale water treatment

Lipid-modulated assembly of magnetized iron-filled carbon nanotubes in millimeter-scale structures

Biomolecule-functionalized carbon nanotubes (CNTs) combine the molecular recognition properties of biomaterials with the electrical properties of nanoscale solid state transducers. Application of this hybrid material in bioelectronic devices requires the development of methods for the reproducible self-assembly of CNTs into higher-order structures in an aqueous environment. To this end, we have studied pattern formation of lipid-coated Fe-filled CNTs, with lengths in the 1–5 µm range, by controlled evaporation of aqueous CNT-lipid suspensions. Novel diffusion limited aggregation structures composed of end-to-end oriented nanotubes were observed by optical and atomic force microscopy. Significantly, the lateral dimension of assemblies of magnetized Fe-filled CNTs was in the millimeter range. Control experiments in the absence of lipids and without magnetization indicated that the formation of these long linear nanotube patterns is driven by a subtle interplay between radial flow forces in the evaporating droplet, lipid-modulated van der Waals forces, and magnetic dipole–dipole interactions. Keywords

Driving fiber diameters to the limit: nanoparticle-induced diameter reductions in electrospun photoactive composite nanofibers for organic photovoltaics

Electrospun photoactive nanofibers hold significant potential for enhanced photon absorption and charge transport in organic photovoltaics. However, electrospinning conjugated polymers with fiber diameters comparable to exciton diffusion lengths for efficient dissociation, is difficult. Previously, spinning sub-100 nm poly(3-hexylthiophene) (P3HT) fibers has required the auxiliary polymer, poly(ethylene oxide) (PEO), and large antisolvent additions. Therefore, its success differs considerably across donor polymers, due to variable antisolvent addition limits before precipitation. Herein, plasmonic nanoparticle infusion into P3HT nanofibers is used to modulate viscosity and deliver a novel and unrivaled strategy to achieve reduced fiber diameters. Following PEO removal, the fibers measure 55 nm in diameter, 30% lower than any previous report – providing the shortest exciton diffusion pathways to the heterojunction upon electron acceptor infiltration. The nanoparticle-containing nanofibers present a 58% enhancement over their pristine thin-film counterparts. ~17% is ascribed to plasmonic effects, demonstrated in thin-films, and the remainder to along-fiber polymer chain alignment, introduced by electrospinning. The anisotropy of light absorbed when polarized parallel versus perpendicular to the fibers increases from 0.88 to 0.62, suggesting the diameter reduction improves the alignment, resulting in greater electrospinning-induced enhancements. Controlled by the electrospinning behavior of PEO, our platform may be adapted to contemporary donor-acceptor systems

Season of Prescribed Fire Determines Grassland Restoration Outcomes After Fire Exclusion and Overgrazing

Fire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C4 grasslands and savannas. Although interest is growing in prescribed fire for grassland restoration, we have little long-term experimental evidence of the influence of burn season on the recovery of herbaceous plant communities, encroachment by trees and shrubs, and invasion by exotic grasses. We conducted a prescribed fire experiment (seven burns between 2001 and 2019) in historically fire-excluded and overgrazed grasslands of central Texas. Sites were assigned to one of four experimental treatments: summer burns (warm season, lightning season), fall burns (early cool season), winter burns (late cool season), or unburned (fire exclusion). To assess restoration outcomes of the experiment, in 2019, we identified old-growth grasslands to serve as reference sites. Herbaceous-layer plant communities in all experimental sites were compositionally and functionally distinct from old-growth grasslands, with little recovery of perennial C4 grasses and long-lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C3 grasses and forbs; and fall and winter sites were intermediate in composition to the unburned and summer sites. Despite compositional differences, all treatments had comparable plot-level plant species richness (range 89–95 species/1000 m2). At the local-scale, summer sites (23 species/m2) and old-growth grasslands (20 species/m2) supported greater richness than unburned sites (15 species/m2), but did not differ significantly from fall or winter sites. Among fire treatments, summer and winter burns most consistently produced the vegetation structure of old-growth grasslands (e.g., mean woody canopy cover of 9%). But whereas winter burns promoted the invasive grass Bothriochloa ischaemum by maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlled B. ischaemum invasion. Our results support a growing body of literature that shows that prescribed fire alone, without the introduction of plant propagules, cannot necessarily restore old-growth grassland community composition. Nonetheless, this long-term experiment demonstrates that prescribed burns implemented in the summer can benefit restoration by preventing woody encroachment while also controlling an invasive grass. We suggest that fire season deserves greater attention in grassland restoration planning and ecological research

Effect of Temperature and Acoustic Pressure During Ultrasound Liquid-Phase Processing of Graphite in Water

Copyright © 2021 The Author(s). Ultrasound-assisted liquid-phase exfoliation is a promising method for manufacturing two-dimensional materials. Understanding the effect of ultrasonication parameters such as the temperature and input power on the developed pressure field is pivotal for optimization of the process. Limited research has been carried out to determine the optimal temperature for exfoliation, with some data generating disputed results. Simply maximizing the sonication power does not necessarily produce a higher yield because of shielding. In this study, a high-temperature calibrated cavitometer was used to measure the acoustic pressure generated in different graphite solutions in deionized water at various temperatures (from 10°C to 70°C) and input power conditions (from 20% to 100%). In addition, high-speed optical imaging provided insight on the shock wave generation from transient bubble collapses under different sonication conditions. The optimal sono-exfoliation parameters were determined to be 20% input power at 10°C for graphite flake solution, and 100% input power at 40°C to 50°C for graphite powder solution.UK Engineering and Physical Sciences Research Council (EPSRC) to the project “Sustainable and industrially scalable ultrasonic liquid phase exfoliation technologies for manufacturing 2D advanced functional materials” (EcoUltra2D), with grant nos. EP/R031665/1; EP/R031401/1; EP/R031819/1; EP/R031975/1

Multiscale interactions of liquid, bubbles and solid phases in ultrasonic fields revealed by multiphysics modelling and ultrafast X-ray imaging

Data availability: Data will be made available on request. Appendix A. Supplementary data: Supplementary data to this article can be found online at: https://doi.org/10.1016/j.ultsonch.2022.106158.Copyright © 2022 The Authors. The volume of fluid (VOF) and continuous surface force (CSF) methods were used to develop a bubble dynamics model for the simulation of bubble oscillation and implosion dynamics under ultrasound. The model was calibrated and validated by the X-ray image data acquired by ultrafast synchrotron X-ray. Coupled bubble interactions with bulk graphite and freely moving particles were also simulated based on the validated model. Simulation and experiments quantified the surface instability developed along the bubble surface under the influence of ultrasound pressure fields. Once the surface instability exceeds a certain amplitude, bubble implosion occurs, creating shock waves and highly deformed, irregular gas-liquid boundaries and smaller bubble fragments. Bubble implosion can produce cyclic impulsive stresses sufficient enough to cause µs fatigue exfoliation of graphite layers. Bubble-particle interaction simulations reveal the underlying mechanisms for efficient particle dispersion or particle wrapping which are all strongly related to the oscillation dynamics of the bubbles and the particle surface properties.UK Engineering and Physical Sciences Research Council (Grant Nos. EP/R031819/1; EP/R031665/1; EP/R031401/1; EP/R031975/1); Royal Society