Preparation And Field Emission Properties Of Graphene And Graphene-based Composite Cathode

Graphene is a two-dimensional carbon material, which has attracted a great deal of attention in the recent years. In particular, graphene has been a promising candidate for field emission source due to its atomic thickness, the high surface area, superior electrical and thermal conductivity, good chemical stability and excellent mechanical properties. Compared to other fabricating methods of graphene cathode, electrophoretic deposition has many advantages, such as low cost, versatile process and simplicity of scaling up.Firstly, graphene was deposited on Ag electrode by direct current electrophoretic deposition. The effects of graphene dispersion concentration, electrophoretic deposition current on the morphology and the field emission properties of graphene cathode were studied, respectively. It is found that when the graphene cathode was electrophoretic deposited with graphene concentration of 0.05g/L, deposited current of 6mA and electrophoretic time of 5 min, it showed the best field emission properties in our studies, with the low turn-on field of 0.65V/μm, low threshold field of 1.15V/μm and high field enhancement factor of 14827.Pulsed electrophoretic deposition was also employed for the preparation of graphene cathode on Ag electrode. The effects of reverse pulse voltage, reverse pulse working time and duty ratios on the surface morphologies and field emission properties were systematically studied. The results showed that excellent graphene cathode prepared by pulsed electrophoretic deposition with the turn-on field of 0.52V/um can be obtained at the following optimized parameters:positive pulse voltage of +30V, positive pulse working time of 500ms, reverse pulse voltage of-15V, reverse pulse working time of 5ms and the duty ratio of 0.25. This result may be attributed to the alignment of the graphene, the graphene cathodes prepared using pulsed electrophoretic possess demonstrate better alignment in the direction perpendicular to the Ag electrode than that synthesized using direct current electrophoretic deposition.To further improve the field emission properties of graphene cathode, graphene/carbon nanotubes composite cathodes were fabricated by electrophoretic deposition. Firstly, composite cathodes with different ratios of graphene and carbon nanotubes were fabricated using direct current electrophoretic deposition by controlling the weight ratios of graphene and carbon nanotubes in solution. The results indicated that the performance of graphene/carbon nanotubes composite cathodes had been improved significantly when adding carbon nanotube into graphene as the emission material. The optimized field emission performance of graphene/carbon nanotubes with the turn-on field of 0.54 V/μm and the field enhancement factor of 15810 was obtained when the weight ratio between graphene and carbon nanotubes were 6:4. The graphene/carbon nanotubes composite cathode was also prepared by pulsed electrophoretic deposition. The results exhibited that the field emission performance of graphene/carbon nanotubes composite cathode prepared by pulsed electrophoretic deposition had been improved with a lower turn-on field of 0.46 V/μm, a lower threshold field of 0.80V/μm and a higher field enhancement factor of 17316, compared with cathode prepared using direct current electrophoretic deposition.