Keerthi Savaram

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.

Synergy of oxygen and a piranha solution for eco-friendly production of highly conductive graphene dispersions

A variety of strategies for the synthesis of solution processable graphene sheets has been developed so far. However, no approach has been reported to directly produce highly conductive, low-oxygen-containing graphene sheets without relying on toxic reagents and metal containing compounds and without generating toxic by-products. With an aim of developing such an eco-friendly approach, for the first time, this work studied solution phase oxidation of graphite particles and reversible graphite intercalation compounds using molecular oxygen and piranha etching solutions. We found that the synergy of the piranha etching solution and the intercalated molecular oxygen enables controlled oxidation of graphite particles assisted by microwave heating. The controlled oxidation leads to the rapid and direct generation of highly conductive, “clean”, and low oxygen containing graphene sheets without releasing toxic gases or aromatic by-products as detected by gas chromatography-mass spectrometry (GC-MS). These highly conductive graphene sheets have unique molecular structures, different from both graphene oxide and pristine graphene sheets. It is even different from chemically reduced graphene oxide, while combining many of its merits. They can be dispersed in both aqueous and common organic solvents without surfactants/stabilizers, producing “clean” solution phase graphene sheets. “Paper-like” graphene films are generated via simple filtration, resulting in films with a conductivity of 2.3 × 104 S m−1, the highest conductivity observed so far for graphene films assembled via vacuum filtration from solution processable graphene sheets. After 2 hours of low temperature annealing at 300 °C, the conductivity further increased to 7.4 × 104 S m−1. This eco-friendly and rapid approach for the production of highly conductive and “clean” solution-phase graphene sheets would enable a broad spectrum of applications at low cost.