Langmuir-Blodgett Assembly of Soft Carbon Sheets
- PDF / 359,132 Bytes
- 7 Pages / 432 x 648 pts Page_size
- 76 Downloads / 150 Views
Langmuir-Blodgett Assembly of Soft Carbon Sheets Laura J. Cote, Jaemyung Kim and Jiaxing Huang Department of Materials Science and Engineering, Northwestern University, Evanston, USA Abstract Graphene oxide sheets have recently gained immense interest as a building block for graphene based materials and devices. Rapid developments have been made in the chemistry and applications of GO. However, assembly, too, plays a critical role in the final properties of bulk graphene based materials as it determines the microstructures of the 2D sheets. There is thus a pressing need for controllable assembly strategies. Based on the recent identification of the pH dependent surfactant-like behavior of GO sheets, we are now able to control the tiling morphologies of such sheets to produce thin films with either wrinkled or overlapped types of microstructures. This allows for the deconvolution of the effects of these two basic morphological features in the electrical and optical properties of the resulting thin films, providing a well-defined example of the processing-microstructure-properties relationship for this unique soft material building block. Introduction Graphite oxide sheets, now known as graphene oxide (GO), are the product of the liquidphase oxidation-exfoliation reaction of graphite powder with strong oxidants such as a mixture of sulfuric acid and potassium permanganate.1,2 The resultant GO consists of exfoliated single atomic layers of sp2-hybridized carbon atoms derivatized by phenol hydroxyl and epoxide groups across the basal plane and ionizable carboxylic acid groups along the edges,3,4 as shown schematically in Fig. 1a. Charge repulsion of the ionized acid groups allows GO to form stable single layer aqueous dispersions.5 In recent years, rapid progress has been made in the chemistry6-8 and applications of GO as a solution-processable precursor for graphene-based materials and devices.9-12 However, there remains a pressing need for rational assembly strategies of these two-dimensional (2D) sheets since assembly plays a critical role in controlling microstructures and thus properties of the final materials. The combination of the hydrophilic hydroxyl and carboxyl acid groups on the edges with the unoxidized hydrophobic polyaromatic nanographene domains remaining on the basal plane (Fig. 1a) also makes GO a unique tethered 2D surfactant sheet. Indeed, GO has been found to be enriched at interfaces without being modified by surfactants13 and capable of lowering interfacial energies.14 Additionally, the amphiphilicity of GO can be tuned by pH since it affects the degree of ionization (Fig. 1b) or number of charges on the sheets, as confirmed by zeta potential measurements (Fig. 1c). The surfactant-like properties of GO suggest that interfacial assembly of the sheets could potentially be controlled by tuning their amphiphilicity. Indeed, pH can be used as a tuning parameter to guide the assembly of GO sheets at the air-water interface. Interacting sheets can be coaxed into a continuous film with two distinct types
Data Loading...
