P-Doped Porous Carbon as Metal Free Catalysts for Selective Aerobic Oxidation with an Unexpected Mechanism.

An extremely simple and rapid (seconds) approach is reported to directly synthesize gram quantities of P-doped graphitic porous carbon materials with controlled P bond configuration. For the first time, it is demonstrated that the P-doped carbon materials can be used as a selective metal free catalyst for aerobic oxidation reactions. The work function of P-doped carbon materials, its connectivity to the P bond configuration, and the correlation with its catalytic efficiency are studied and established. In direct contrast to N-doped graphene, the P-doped carbon materials with higher work function show high activity in catalytic aerobic oxidation. The selectivity trend for the electron donating and withdrawing properties of the functional groups attached to the aromatic ring of benzyl alcohols is also different from other metal free carbon based catalysts. A unique catalytic mechanism is demonstrated, which differs from both GO and N-doped graphene obtained by high temperature nitrification. The unique and unexpected catalytic pathway endows the P-doped materials with not only good catalytic efficiency but also recyclability. This, combined with a rapid, energy saving approach that permits fabrication on a large scale, suggests that the P-doped porous materials are promising materials for "green catalysis" due to their higher theoretical surface area, sustainability, environmental friendliness, and low cost.

Direct production of graphene nanosheets for near infrared photoacoustic imaging.

Hummers method is commonly used for the fabrication of graphene oxide (GO) from graphite particles. The oxidation process also leads to the cutting of graphene sheets into small pieces. From a thermodynamic perspective, it seems improbable that the aggressive, somewhat random oxidative cutting process could directly result in graphene nanosheets without destroying the intrinsic π-conjugated structures and the associated exotic properties of graphene. In Hummers method, both KMnO4 and NO2(+) (nitronium ions) in concentrated H2SO4 solutions act as oxidants via different oxidation mechanisms. From both experimental observations and theoretical calculations, it appears that KMnO4 plays a major role in the observed oxidative cutting and unzipping processes. We find that KMnO4 also limits nitronium oxidative etching of graphene basal planes, therefore slowing down graphene fracturing processes for nanosheet fabrication. By intentionally excluding KMnO4 and exploiting pure nitronium ion oxidation, aided by the unique thermal and kinetic effects induced by microwave heating, we find that graphite particles can be converted into graphene nanosheets with their π-conjugated aromatic structures and properties largely retained. Without the need of any postreduction processes to remove the high concentration of oxygenated groups that results from Hummers GO formation, the graphene nanosheets as-fabricated exhibit strong absorption, which is nearly wavelength-independent in the visible and near-infrared (NIR) regions, an optical property typical for intrinsic graphene sheets. For the first time, we demonstrate that strong photoacoustic signals can be generated from these graphene nanosheets with NIR excitation. The photo-to-acoustic conversion is weakly dependent on the wavelength of the NIR excitation, which is different from all other NIR photoacoustic contrast agents previously reported.