Probing the catalytic activity of porous graphene oxide and the origin of this behaviour.

Graphene oxide, a two-dimensional aromatic scaffold decorated by oxygen-containing functional groups, possesses rich chemical properties and may present a green alternative to precious metal catalysts. Graphene oxide-based carbocatalysis has recently been demonstrated for aerobic oxidative reactions. However, its widespread application is hindered by the need for high catalyst loadings. Here we report a simple chemical treatment that can create and enlarge the defects in graphene oxide and impart on it enhanced catalytic activities for the oxidative coupling of amines to imines (up to 98% yield at 5 wt% catalyst loading, under solvent-free, open-air conditions). This study examines the origin of the enhanced catalytic activity, which can be linked to the synergistic effect of carboxylic acid groups and unpaired electrons at the edge defects. The discovery of a simple chemical processing step to synthesize highly active graphene oxide allows the premise of industrial-scale carbocatalysis to be explored.

Oriented graphene nanoribbon yarn and sheet from aligned multi-walled carbon nanotube sheets.

Highly oriented graphene nanoribbons sheets and yarns are produced by chemical unzipping of self-standing multiwalled carbon nanotube (MWNT) sheets. The as-produced yarns - after being chemically and thermally reduced - exhibit a good mechanical, electrical, and electrochemical performance.

Room-temperature metastability of multilayer graphene oxide films.

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Field emission from atomically thin edges of reduced graphene oxide.

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. The development and implementation of tip emitters have been hampered by the need to position them sufficiently apart to achieve field enhancement, limiting the number of emission sites and therefore the overall current. Here we report low threshold field (< 0.1 V/µm) emission of multiple electron beams from atomically thin edges of reduced graphene oxide (rGO). Field emission microscopy measurements show evidence for interference from emission sites that are separated by a few nanometers, suggesting that the emitted electron beams are coherent. On the basis of our high-resolution transmission electron microscopy, infrared spectroscopy, and simulation results, field emission from the rGO edge is attributed to a stable and unique aggregation of oxygen groups in the form of cyclic edge ethers. Such closely spaced electron beams from rGO offer prospects for novel applications and understanding the physics of linear electron sources.

The role of intercalated water in multilayered graphene oxide.

A detailed in situ infrared spectroscopy analysis of single layer and multilayered graphene oxide (GO) thin films reveals that the normalized infrared absorption in the carbonyl region is substantially higher in multilayered GO upon mild annealing. These results highlight the fact that the reduction chemistry of multilayered GO is dramatically different from the single layer GO due to the presence of water molecules confined in the ∼1 nm spacing between sheets. IR spectroscopy, XPS analysis, and DFT calculations all confirm that the water molecules play a significant role interacting with basal plane etch holes through passivation, via evolution of CO(2) leading to the formation of ketone and ester carbonyl groups. Displacement of water from intersheet spacing with alcohol significantly changes the chemistry of carbonyl formation with temperature.

Structural evolution during the reduction of chemically derived graphene oxide.

The excellent electrical, optical and mechanical properties of graphene have driven the search to find methods for its large-scale production, but established procedures (such as mechanical exfoliation or chemical vapour deposition) are not ideal for the manufacture of processable graphene sheets. An alternative method is the reduction of graphene oxide, a material that shares the same atomically thin structural framework as graphene, but bears oxygen-containing functional groups. Here we use molecular dynamics simulations to study the atomistic structure of progressively reduced graphene oxide. The chemical changes of oxygen-containing functional groups on the annealing of graphene oxide are elucidated and the simulations reveal the formation of highly stable carbonyl and ether groups that hinder its complete reduction to graphene. The calculations are supported by infrared and X-ray photoelectron spectroscopy measurements. Finally, more effective reduction treatments to improve the reduction of graphene oxide are proposed.