High Performance Yet Flexible Macroscopic-assembled Graphene Film

Macroscopic-assembled graphene films are the ones fabricated by the wet assembly of graphene oxide or graphene sheets with subsequent chemical,thermal or other reduction treatments.They exhibit a wide range of applications in heat transfer,power transmission,battery electrode materials,wearable sensors and detectors of the photon,acoustic and gas molecules,intrinsically attributing to the excellent properties of graphene building blocks on electron mobility,transient phonon relaxation time and mechanical strength.However,the overall performance of graphene films greatly depends on various complex parameters including the size,intrinsic structure and stacking model of graphene sheets,leading to the difficulty of graphene films to full fill the excellent properties of the ideal single-layer graphene.Therefore,more efforts on the control of the specific composition and assembly structures of graphene films to greatly improve their overall performance and thus applications are still needed.In this regard,this dissertation focuses on the structure design of flexible graphene films with high-performance.The effects of the surface chemistry,morphologies,and stacking models of graphene sheets on the performance of graphene film are investigated systematically.The main contents are listed as follows:1.The surface chemistry and the physicochemical properties of graphene oxide are studied.Most of the acidity of graphene oxide comes from hydroxyl ionization which evidenced by the catalytic reaction of S→O acetyl migration for the synthesis of thiol compounds as a catalyst,further proving that graphene oxide carries many lone pair electrons stabilized by the conjugated structural units.Under the activation of the delocalized conjugated structure and the polarization of the lone pair electrons,the activity of hydroxyl groups is greatly enhanced,resulting in a comparable acidic property to that of ionized carboxyl groups.2.High fatigue resistance graphene aerogel films were achieved with a high temperature(1600℃)reduction of GO film.The as-prepared film exhibits both high elasticity(elastic extension up to 16%)and flexibility,which can endure 10,000 cycles of bending and 1000 times repeated stretching with no reduction in performance.Porous structure makes graphene films recover to their original shapes rapidly from bending deformation within 5 ms(3 mm)and 50 ms(18 mm).In addition,this film can endure a high temperature of 400℃ and low temperature of-150℃ in the air.The perfect combination of both high-temperature inertness and porous structures offers such as-prepared graphene films potential applications in pressure detection under ultimate temperature.3.The atomic crystal-folding principle was proposed to design highly thermal-conductive yet super-flexible graphene films.Debris-free and giant graphene sheets afford macroscopic assembled graphene films a high thermal conductivity of 1940 W/mK and electrical conductivity up to 1.03 MS/m.The micro-folds from the compress of micro-gasbags in graphene lead to the flexibility.Under the action of external forces,the stretched micro-folds endow graphene films high elongation(12-16%),including elastic and plastic deformation.4.The structure transition of graphene sheets in nano-thickness graphene film under high-temperature treatment was studied by tracking the composition,defects,stacking models.The high annealing temperature and long holding time have the same influence on the defects healing and structure transition of graphene sheets.The stacking structure of graphene sheets inside films was conducted,proving an internal structure composed of folds,twist,and cross-linked structures.5.A green solid-phase transfer method was developed to prepare free-standing nano-thickness macroscopic-assembled graphene films with a diameter of 4.2 cm,the thickness of 14-100 nm and density of 2.03-2.11 g/cm3.The heat-expansion and cold-contraction effect of camphor provides the driving force for the separation between the substrate and GO film.Besides,the larger porosity of AAO and the heterogeneous reduction of hydroiodic acid provide the dynamic basis for the separation of the film.After a 2800℃ annealing process,this film holds a high thermal conductivity(1840-3600 W/mK),electrical conductivity(1.9-2.1 MS/m),mechanical strength(9.2 GPa)and elongation(12-20%).By using bromine doping method,the electron density of graphene films is increased and hence an increase of the graphene films conductivity up to 18.9 MS/m.The high light absorptivity and perfect structure of this film make it a superior photo electronic responsivity(24 A/W)than that of CVD based graphene films.