Abstract
In this study, a high performance transparent conductive film (TCF) was prepared by graphene nanosheets (GNS), which is a thin two dimensional nanomaterial with high electrical conductivity. There are five parts in this study: (1) the preparation of GNS-TCFs by spin coating; (2) using vacuum filtration for fabricating GNS/silver nanoparticle TCFs; (3) the preparation of GNS thin films through dip coating: (4) and (5) including the preparation of high performance GNS/silver nanowire TCFs by various dip coating methods. Part one: GNS were obtained by the reduction of graphene oxide (GO), which is a well-known layer-structured compound, obtained by oxidation of nano graphite plates (NGPs). Sodium borohydride (NaBH4) was used for the reduction of GO because NaBH4 is highly effective for reduction in this part. The carboxylic acid functional group may remain in the alkaline condition by the treatment of NaBH4. The cationic surfactant Didodecyldimethylammonium bromide (DDAB) was added into the mixture, which played vital roles. DDAB serves as a spacer. The GNS solution can be “well-dispersed” in water or a polar organic solution (HA-GNS and HB-GNS) because of the remaining DDAB. In this part, the transparent and conductive film was prepared using spin coating of the GNS dispersion solution of water or THF. The optimal performances of the TCFs are then obtained (HA-GNS and HB-GNS). The surface electrical resistance of HA-GNS is 1.5×103 Ω/□ with transmittance of 82 %, which is similar to that of HB-GNS (2.1×103 Ω/□ with transmittance of 81 %). Part two: GNS was also prepared by the reduction of GO. Aggregation and restacking of graphene nanosheets (GNS) can be efficiently inhibited by decorating the silver nanoparticles on the surface of GNS to form GNS/silver (GNS-Ag) composites, which can construct high transparent and electrically conductive thin films. Silver nanoparticles act as a useful nanospacer and conductor, which not only increase the interlayer distance but also improve the electrical conductivity between layers. A two-step reduction process using NaBH4 and ethylene glycol (EG) was also demonstrated reducing graphene oxide to GNS efficiently. The GNS-Ag composite films showed a maximum sheet resistance of 93 Ω□-1, while maintaining up to 78 % light transmittance, which was two orders of magnitude lower than that of GNS (8.2×103 Ω□-1, 81 %), and the value of DC conductivity to optical conductivity ratio was 13.5 instead of 0.25 for GNS TCFs. Part three: This study has prepared graphene nanosheet (GNS)–based transparent, conductive films (TCFs) by a self-assembly method. The water-borne polyurethane (WPU, with sulfonate functional groups) film was used as substrate. Aggregation and restacking of the GNS were inhibited efficiently by attracting the octadecyl trimethyl ammonium chloride surfactant (cationic surfactant) to the surface of the GNS (GNS-O), which can in turn attracted sulfonate groups to the WPU surface. GNS-O was deposited on WPU to form TCFs. A highly transparent and electrically conductive thin film after treatment with nitric acid (GNS-OA) was obtained. The GNS-OA composite films showed a maximum sheet electrical resistance of 1.5×103 Ω□-1, with a light transmittance of up to 79 % and a ratio of DC conductivity to optical conductivity of 0.88. Part four: Silver nanowires (AgNWs), modified by cysteamine, with a high electrical conductivity can be combined with high surface area graphene nanosheets (GNS) to form AgNW-GN hybrid nanomaterials. These materials with -NH3+ functional groups in an alkaline environment can be deposited on waterborne polyurethane surfaces with the attraction of sulfonate functional groups to prepare transparent conductive films with high transmittance and low surface electrical resistance. This self-assembly method provides highly controllable transmittance and surface electrical resistance. The AgNWs can inhibit GNS from restacking and aggregation after reduction from graphene oxide, increasing the electrical conductivity between the GN interlayers. The AgNW-GN hybrid nanomaterial films show a sheet resistance of 86 Ω/sq with 80% light transmittance, and the ratio of DC conductivity to optical conductivity reaches 19.81 (in the part three, the ratio of DC conductivity to optical conductivity of the GNS film is 0.88). Part five: The polyethylene terephalate (PET) was used as the substrate. The GO thin films which can deposit on the PET surface easily by the dip-coating processes. The GO membrane was formed at the air and the solution interface can be heated at 80 ℃ without stirring. After, the PET substrate was inserted in the GO solution, and then pulled out. The GO thin film was coated on the PET surface. The GNS-based transparent conductive films (TCFs) were obtained by reduction of HI acid. Moreover, the performance of GNS-based TCF can be further increased through decorating silver nanowires (AgNWs) by a dip-coating (the GNS-AgNW TCF). AgNWs were modified by the thiophenol, and AgNWs then possess many aromatic compounds which can be attracted by GNS due to π-π interaction. The ultrahigh performance of the GNS-AgNW TCF was obtained. The surface electrical resistance of the GNS-AgNW TCF is only 71 Ω□-1 with the 85 % light transmittance.