Chemistry Department, Rutgers University , Newark, New Jersey 07102, United States.
ACS Nano. 2013 Sep 24;7(9):8147-57. doi: 10.1021/nn403429v. Epub 2013 Sep 3.
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.
Hummers 法通常用于从石墨颗粒制备氧化石墨烯 (GO)。氧化过程还会导致石墨烯片被切割成小碎片。从热力学的角度来看，剧烈的、有些随机的氧化切割过程不太可能直接得到没有破坏石墨烯固有π共轭结构和相关奇异性质的石墨烯纳米片。在 Hummers 法中，KMnO4 和浓 H2SO4 溶液中的 NO2+(硝酰离子)通过不同的氧化机制作为氧化剂。从实验观察和理论计算来看，似乎 KMnO4 在观察到的氧化切割和开环过程中起主要作用。我们发现 KMnO4 还限制了硝酰离子对石墨烯基面的氧化刻蚀，从而减缓了用于纳米片制备的石墨烯断裂过程。通过故意排除 KMnO4 并利用纯硝酰离子氧化，辅以微波加热引起的独特热和动力学效应，我们发现石墨颗粒可以转化为具有其π共轭芳香结构和性质基本保留的石墨烯纳米片。不需要任何后还原过程来去除 Hummers GO 形成过程中产生的高浓度含氧基团，所制备的石墨烯纳米片表现出强烈的吸收，在可见光和近红外 (NIR) 区域几乎与波长无关，这是本征石墨烯片的典型光学性质。我们首次证明，这些石墨烯纳米片可以在 NIR 激发下产生强烈的光声信号。光声转换对 NIR 激发波长的依赖性较弱，这与以前报道的所有其他近红外光声对比剂都不同。
