Stephen J. Pennycook

An oxygen reduction electrocatalyst based on carbon nanotube–graphene complexes

Oxygen reduction reaction catalysts based on precious metals such as platinum or its alloys are routinely used in fuel cells because of their high activity. Carbon-supported materials containing metals such as iron or cobalt as well as nitrogen impurities have been proposed to increase scalability and reduce costs, but these alternatives usually suffer from low activity and/or gradual deactivation during use. Here, we show that few-walled carbon nanotubes, following outer wall exfoliation via oxidation and high-temperature reaction with ammonia, can act as an oxygen reduction reaction electrocatalyst in both acidic and alkaline solutions. Under a unique oxidation condition, the outer walls of the few-walled carbon nanotubes are partially unzipped, creating nanoscale sheets of graphene attached to the inner tubes. The graphene sheets contain extremely small amounts of irons originated from nanotube growth seeds, and nitrogen impurities, which facilitate the formation of catalytic sites and boost the activity of the catalyst, as revealed by atomic-scale microscopy and electron energy loss spectroscopy. Whereas the graphene sheets formed from the unzipped part of the outer wall of the nanotubes are responsible for the catalytic activity, the inner walls remain intact and retain their electrical conductivity, which facilitates charge transport during electrocatalysis.

Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy

Direct imaging and chemical identification of all the atoms in a material with unknown three-dimensional structure would constitute a very powerful general analysis tool. Transmission electron microscopy should in principle be able to fulfil this role, as many scientists including Feynman realized early on. It images matter with electrons that scatter strongly from individual atoms and whose wavelengths are about 50 times smaller than an atom. Recently the technique has advanced greatly owing to the introduction of aberration-corrected optics. However, neither electron microscopy nor any other experimental technique has yet been able to resolve and identify all the atoms in a non-periodic material consisting of several atomic species. Here we show that annular dark-field imaging in an aberration-corrected scanning transmission electron microscope optimized for low voltage operation can resolve and identify the chemical type of every atom in monolayer hexagonal boron nitride that contains substitutional defects. Three types of atomic substitutions were found and identified: carbon substituting for boron, carbon substituting for nitrogen, and oxygen substituting for nitrogen. The substitutions caused in-plane distortions in the boron nitride monolayer of about 0.1 Å magnitude, which were directly resolved, and verified by density functional theory calculations. The results demonstrate that atom-by-atom structural and chemical analysis of all radiation-damage-resistant atoms present in, and on top of, ultra-thin sheets has now become possible.

High-resolution Z-contrast imaging of crystals

Abstract The use of a high-angle annular detector in a scanning transmission electron microscope is shown to provide incoherent images of crystalline materials with strong compositional sensitivity. How this occurs, even in the presence of strong dynamical diffraction of the low-angle beams, becomes very clear in a Bloch wave description of the imaging, which shows that only tightly bound s -type Bloch states contribute significantly to the image. Interference effects are therefore precluded and the image can be described as a convolution. There are no contrast reversals with thickness or defocus and no Fresnel fringe effects at interfaces. Each atomic column contributes to the image independently of its neighbors until the s -states themselves overlap. With an optimum imaging probe the nature of the convolution can be visualized intuitively to a scale well below the resolution limit. To first order, therefore, each object has only one possible image, and since the same probe is used for all objects, an unknown structure can be interpreted directly. These ideas will be illustrated with images from semiconductors, superconductors, and alloys.

Nanoscale nickel oxide/nickel heterostructures for active hydrogen evolution electrocatalysis

Active, stable and cost-effective electrocatalysts are a key to water splitting for hydrogen production through electrolysis or photoelectrochemistry. Here we report nanoscale nickel oxide/nickel heterostructures formed on carbon nanotube sidewalls as highly effective electrocatalysts for hydrogen evolution reaction with activity similar to platinum. Partially reduced nickel interfaced with nickel oxide results from thermal decomposition of nickel hydroxide precursors bonded to carbon nanotube sidewalls. The metal ion-carbon nanotube interactions impede complete reduction and Ostwald ripening of nickel species into the less hydrogen evolution reaction active pure nickel phase. A water electrolyzer that achieves ~20 mA cm(-2) at a voltage of 1.5 V, and which may be operated by a single-cell alkaline battery, is fabricated using cheap, non-precious metal-based electrocatalysts.

Dopamine as a Carbon Source: The Controlled Synthesis of Hollow Carbon Spheres and Yolk‐Structured Carbon Nanocomposites

A facile and versatile synthesis using dopamine as a carbon source gives hollow carbon spheres and yolk-shell Au{at}Carbon nanocomposites. The uniform nature of dopamine coatings and their high carbon yield endow the products with high structural integrity. The Au{at}C nanocomposites are catalytically active.

Z-contrast stem for materials science

Abstract The use of a high-angle annular detector on a scanning transmission electron microscope (STEM) for imaging crystalline materials with strong chemical sensitivity is described. The image can be used to form an elemental map with high efficiency which can be quantified directly in terms of atomic concentrations. Examples of ion-implanted silicon recrystallized by solid- or liquid-phase epitaxial growth will be shown, illustrating the usefulness of the image in studies of phase transformations and grain boundary segregation. At high resolution it is possible to resolve a crystal lattice, while preserving the strong chemical sensitivity of the image. A simply interpretable image results, with a minimum dependence on objective lens defocus and specimen thickness, and examples of high-temperature superconductors will be shown.

Atomic-resolution chemical analysis using a scanning transmission electron microscope

This corrects the article DOI: 10.1038/366143a0

Time-resolved imaging of gas phase nanoparticle synthesis by laser ablation

The dynamics of nanoparticle formation, transport, and deposition by pulsed laser ablation of c-Si into 1–10 Torr He and Ar gases are revealed by imaging laser-induced photoluminescence and Rayleigh-scattered light from gas-suspended 1–10 nm SiOx particles. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles, while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. Nanoparticles unambiguously formed in the gas phase were collected on transmission electron microscope grids for Z-contrast imaging and electron energy loss spectroscopy analysis. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described.

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Band Gap Engineering and Layer-by-Layer Mapping of Selenium-Doped Molybdenum Disulfide

Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure, and hence, control of dopant concentration within each individual layer of these compounds provides a powerful tool to efficiently modify their physical and chemical properties. The main challenge arises when quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of molybdenum disulfide substitutionally doped with a broad range of selenium concentrations, resulting in over 10% optical band gap modulations in atomic layers. Chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local optical band gaps. Furthermore, in a bilayer structure, the dopant distribution is imaged layer-by-layer. This work demonstrates that each layer in the bilayer system contains similar local Se concentrations, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.