Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
J Am Chem Soc. 2011 Aug 17;133(32):12810-23. doi: 10.1021/ja2048013. Epub 2011 Jul 22.
The production of graphene with open band gaps for the manufacturing of graphene-based electronic and optical devices requires synthesis methods to either control the number of layers to enrich AB-stacked bilayer or trilayer graphene or control the extent of functionalization of monolayer graphene. Solution-phase dispersion of graphene is promising for both methods to create printable electronics and nanocomposites. However, both methods face common challenges, including controlling the surface morphology, reducing the turbostratic layering, and enhancing the dispersion stability. To address these challenges at the molecular level, we successfully combined molecular simulations, theoretical modeling, and experimental measurements. First, we probed the surface structure and electrostatic potential of monolayer graphene dispersed in a sodium cholate (SC) surfactant aqueous solution, which exhibits 2D sheets partially covered with a monolayer of negatively charged cholate ions. Similar to the case of carbon nanotube functionalization, one may regulate the binding affinity of charged reactants for graphene functionalization by manipulating the surface morphology. Subsequently, we quantified the interactions between two graphene-surfactant assemblies by calculating the potential of mean force (PMF) between two surfactant-covered graphene sheets, which confirmed the existence of a metastable bilayer graphene structure due to the steric hindrance of the confined surfactant molecules. The traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was found to be adequate to explain the long-range electrostatic repulsions between the ionic surfactant-covered graphene sheets but was unable to account for the dominant, short-range steric hindrance imparted by the confined surfactant molecules. Interestingly, one faces a dilemma when using surfactants to disperse and stabilize graphene in aqueous solution: on the one hand, surfactants can stabilize graphene aqueous dispersions, but on the other hand, they prevent the formation of new AB-stacked bilayer and trilayer graphene resulting from the reaggregation process. Finally, the lifetime and time-dependent distribution of various graphene layer types were predicted using a kinetic model of colloid aggregation, and each graphene layer type was further decomposed into subtypes, including the AB-stacked species and various turbostratic species. The kinetic model of colloid aggregation developed here can serve as a useful tool to evaluate the quality of graphene dispersions for subsequent substrate-transferring or functionalization processes.
为了制造基于石墨烯的电子和光电器件，需要具有开带隙的石墨烯的生产方法，该方法要么控制层数以富集 AB 堆叠的双层或三层石墨烯，要么控制单层石墨烯的功能化程度。石墨烯的溶液分散体对于这两种方法来说都是很有前景的，可以用来制造可印刷的电子产品和纳米复合材料。然而，这两种方法都面临着共同的挑战，包括控制表面形态、减少乱层化程度和增强分散稳定性。为了解决这些分子水平上的挑战，我们成功地结合了分子模拟、理论建模和实验测量。首先，我们探测了单层石墨烯在十二烷基硫酸钠（SC）表面活性剂水溶液中的表面结构和静电势，该溶液显示出部分被带负电荷的胆酸钠离子单层覆盖的二维片层。与碳纳米管功能化的情况类似，通过操纵表面形态，可以调节带电反应物与石墨烯的结合亲和力，从而调节石墨烯的功能化。随后，我们通过计算两个覆盖有表面活性剂的石墨烯片之间的平均力势（PMF）来量化两个石墨烯-表面活性剂组装体之间的相互作用，这证实了由于受限表面活性剂分子的空间位阻，存在亚稳态双层石墨烯结构。传统的德加古林-兰德维厄-奥弗贝克（DLVO）理论足以解释离子型表面活性剂覆盖的石墨烯片之间的长程静电排斥，但无法解释受限表面活性剂分子赋予的主要短程空间位阻。有趣的是，在使用表面活性剂在水溶液中分散和稳定石墨烯时，人们面临着两难的困境：一方面，表面活性剂可以稳定石墨烯的水溶液分散体，但另一方面，它们会阻止由于再聚集过程而形成新的 AB 堆叠的双层和三层石墨烯。最后，使用胶体聚集的动态度量模型预测了各种石墨烯层类型的寿命和时间依赖性分布，并且进一步将每个石墨烯层类型分解为亚类型，包括 AB 堆叠的物种和各种乱层化的物种。这里开发的胶体聚集动态度量模型可以作为评估石墨烯分散体质量的有用工具，以用于后续的基底转移或功能化过程。
