David A. Cullen

Bulk Production of a New Form of sp2 Carbon : Crystalline Graphene Nanoribbons

We report the use of chemical vapor deposition (CVD) for the bulk production (grams per day) of long, thin, and highly crystalline graphene ribbons (<20-30 microm in length) exhibiting widths of 20-300 nm and small thicknesses (2-40 layers). These layers usually exhibit perfect ABAB... stacking as in graphite crystals. The structure of the ribbons has been carefully characterized by several techniques and the electronic transport and gas adsorption properties have been measured. With this material available to researchers, it should be possible to develop new applications and physicochemical phenomena associated with layered graphene.

Ex-MWNTs: Graphene Sheets and Ribbons Produced by Lithium Intercalation and Exfoliation of Carbon Nanotubes

We found that multiwalled carbon nanotubes (MWNTs) can be opened longitudinally by intercalation of lithium and ammonia followed by exfoliation. Intercalation of open-ended tubes and exfoliation with acid treatment and abrupt heating provided the best results. The resulting material consists of: (i) multilayered flat graphitic structures (nanoribbons), (ii) partially open MWNTs, and (iii) graphene flakes. We called the completely unwrapped nanotubes ex-MWNTs, and their large number of edge atoms makes them attractive for many applications.

Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions

The establishment of covalent junctions between carbon nanotubes (CNTs) and the modification of their straight tubular morphology are two strategies needed to successfully synthesize nanotube-based three-dimensional (3D) frameworks exhibiting superior material properties. Engineering such 3D structures in scalable synthetic processes still remains a challenge. This work pioneers the bulk synthesis of 3D macroscale nanotube elastic solids directly via a boron-doping strategy during chemical vapour deposition, which influences the formation of atomic-scale “elbow” junctions and nanotube covalent interconnections. Detailed elemental analysis revealed that the “elbow” junctions are preferred sites for excess boron atoms, indicating the role of boron and curvature in the junction formation mechanism, in agreement with our first principle theoretical calculations. Exploiting this material’s ultra-light weight, super-hydrophobicity, high porosity, thermal stability, and mechanical flexibility, the strongly oleophilic sponge-like solids are demonstrated as unique reusable sorbent scaffolds able to efficiently remove oil from contaminated seawater even after repeated use.

Nitrogen-mediated carbon nanotube growth: diameter reduction, metallicity, bundle dispersability, and bamboo-like structure formation.

Carbon nanotube growth in the presence of nitrogen has been the subject of much experimental scrutiny, sparking intense debate about the role of nitrogen in the formation of diverse structural features, including shortened length, reduced diameters, and bamboo-like multilayered nanotubules. In this paper, the origin of these features is elucidated using a combination of experimental and theoretical techniques, showing that N acts as a surfactant during growth. N doping enhances the formation of smaller diameter tubes. It can also promote tube closure which includes a relatively large amount of N atoms into the tube lattice, leading to bamboo-like structures. Our findings demonstrate that the mechanism is independent of the tube chirality and suggest a simple procedure for controlling the growth of bamboo-like nanotube morphologies.

Heterodoped nanotubes: theory, synthesis, and characterization of phosphorus-nitrogen doped multiwalled carbon nanotubes.

Arrays of multiwalled carbon nanotubes doped with phosphorus (P) and nitrogen (N) are synthesized using a solution of ferrocene, triphenyl-phosphine, and benzylamine in conjunction with spray pyrolysis. We demonstrate that iron phosphide (Fe(3)P) nanoparticles act as catalysts during nanotube growth, leading to the formation of novel PN-doped multiwalled carbon nanotubes. The samples were examined by high resolution electron microscopy and microanalysis techniques, and their chemical stability was explored by means of thermogravimetric analysis in the presence of oxygen. The PN-doped structures reveal important morphology and chemical changes when compared to N-doped nanotubes. These types of heterodoped nanotubes are predicted to offer many new opportunities in the fabrication of fast-response chemical sensors.

Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

Replacing platinum in air-fed fuel cells Replacing expensive and scarce platinum catalysts in polymer electrolyte membrane fuel cells for the oxygen reduction reaction (ORR) with ones based on non-noble metals would speed up the adoption of hydrogen fuel vehicles. Most of the candidate replacement catalysts that have shown high performance do so only when running on pure oxygen. Chung et al. developed an iron-nitrogen-carbon catalyst from two nitrogen precursors that forms a high-porosity structure and exhibits high ORR performance when running on air. The proposed catalytically active site is FeN4. Science, this issue p. 479 A hierarchically structured iron-nitrogen-carbon catalyst for the oxygen reduction reaction is highly active in air. Platinum group metal–free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H2)–air fuel cell. Herein, we demonstrate H2-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN4), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

Chemical Vapor Deposition Synthesis of N-, P-, and Si-Doped Single-Walled Carbon Nanotubes

Here we report the synthesis of single-walled carbon nanotube bundles by chemical vapor deposition in the presence of electron donor elements (N, P, and Si). In order to introduce each dopant into the graphitic carbon lattice, different precursors containing the doping elements (benzylamine, pyrazine, triphenylphosphine, and methoxytrimethylsilane) were added at various concentrations into ethanol/ferrocene solutions. The synthesized nanotubes and byproduct were characterized by electron microscopy and Raman spectroscopy. Our results reveal intrinsic structural and electronic differences for the N-, P-, and Si- doped nanotubes. These tubes can now be tested for the fabrication of electronic nanodevices, and their performance can be observed.

An Atomistic Branching Mechanism for Carbon Nanotubes: Sulfur as the Triggering Agent†

A combination of theoretical techniques, high-resolution microscopy, and energy-dispersive X-ray spectroscopy shows the role sulfur plays in branching phenomena during carbon nanotube (CNT) network growth. A model is proposed in which small amounts of sulfur are enough to trigger the growth of a bud in a CNT, leading to kink formation and subsequent branch growth.

Ultrasensitive gas detection of large-area boron-doped graphene

Significance The gas-sensing performance of graphene could be remarkably enhanced by incorporating dopants into its lattice based on theoretical calculations. However, to date, experimental progress on boron-doped graphene (BG) is still very scarce. Here, we achieved the controlled growth of large-area, high-crystallinity BG sheets and shed light on their electronic features associated with boron dopants at the atomic scale. As a proof-of-concept, it is demonstrated that boron doping in graphene could lead to a much enhanced sensitivity when detecting toxic gases (e.g. NO2). Our results will open up new avenues for developing high-performance sensors able to detect trace amount of molecules. In addition, other new fascinating properties can be exploited based on as-synthesized large-area BG sheets. Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Super-Stable, Highly Monodisperse Plasmonic Faradaurate-500 Nanocrystals with 500 Gold Atoms: Au∼500(SR)∼120

Determining the composition of plasmonic nanoparticles is challenging due to a lack of tools to accurately quantify the number of atoms within the particle. Mass spectrometry plays a significant role in determining the nanoparticle composition at the atomic level. Significant progress has been made in understanding ultrasmall gold nanoparticles such as Au25(SR)18 and Au38(SR)24, which have Au core diameters of 0.97 and 1.3 nm, respectively. However, progress in 2-5 nm-diameter small plasmonic nanoparticles is currently impeded, partially because of the challenges in synthesizing monodisperse nanoparticles. Here, we report a plasmonic nanocrystal that is highly monodisperse, with unprecedentedly small size variability. The composition of the superstable plasmonic nanocrystals at 115 kDa was determined as Au(500±10)SR(120±3). The Au(~500) system, named Faradaurate-500, is the largest system to be characterized using high resolution electrospray (ESI) mass spectrometry. Atomic pair distribution function (PDF) data indicate that the local atomic structure is consistent with a face-centered cubic (fcc) or Marks decahedral arrangement. High resolution scanning transmission electron microscopy (STEM) images show that the diameter is 2.4 ± 0.1 nm. The size and the shape of the molecular envelope measured by small-angle X-ray scattering (SAXS) confirms the STEM and PDF analysis.