Graphene hybrid nanomaterials : investigation of charge transport and electrochemical properties

• Main Author: Verawati
• Other Authors: School of Materials Science & Engineering
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/51864
• Institution: Nanyang Technological University

With exceptional electrical, thermal and physical properties , graphene has garnered much interest from the scientific as well as technological point of view. Over the past few years, various preparation methods of graphene have been developed and unique characteristics uncovered that have resulted in graphene being considered for applications ranging from catalysis, opto-electronics, sensors to energy storage amongst others. Importantly, the properties of graphene can be tuned and enhanced by creating hybrid graphene - nanomaterials systems, that leverage the best properties of graphene, e.g. 2-D electrical and thermal conductivity, with semiconducting and catalytic properties of transition metal oxide and noble material nanoparticles. Here, graphene oxide was synthesized through chemical oxidation method and further modified to support noble metal nanoparticles (Au, Ag and Pt), porphyrin molecules, or sulfur decoration. We found that wet chemical synthesis can be utilized for reducing the insulating GO into conducting reduced graphene oxide (rGO) as well as decorating GO with nanostructures by reacting GO sheet with nanostructure precursors in optimized proportion. Interaction between rGO and the nanostructure was investigated in terms of electron transfer interaction, electrochemical dynamic and gas molecule detection. Removal of oxygen functionality on GO is the key factor in achieving conducting rGO, which can be done by providing necessary electron for the reduction. By utilizing GO’s ability to mediate charge, two routes have been demonstrated in this thesis. The first involves photoexcitation of porphyrin molecule to generate electron-hole pairs, followed by electron injection to GO, thus reducing it. The level of reduction could be adjusted by modifying the duration of illumination. The second route involves utilization of catalysts such as platinum to generate alcohol radical, which is used to remove the oxygen functionalities on the sheet. In addition, to the novel techniques for the synthesis of rGO, the electronic properties of hybrid rGO materials were also studied. Decoration by metal nanoparticles induces p-doping to rGO via fermi energy alignment, which results in the formation of potential step at the metal and rGO interface under equilibrium condition. The potential step is important to drive charge transfer between foreign species, such as gas molecules, and rGO sheet, which subsequently result in enhanced sensing capability. Furthermore, decoration by metal nanoparticles successfully imparted chemical sensitivity towards H2S gas molecules. Decoration of GO by high catalytic materials such as platinum or sulfur provides the graphene with specific catalytic properties favorable for utilization as counter-electrode in dye sensitized solar cell (DSC). High catalytic properties towards electrolyte (I-/I3 -) redox couple are evident in DSC shown by higher redox current density in cyclic voltammetry (CV) measurements of Pt decorated GO (Pt- GO) and sulfur decorated GO (GO-S) as compared to pure rGO. Decorated GO counter electrodes give better photovoltaic performance as compared to undecorated GO counter electrode.