Three-Dimensional Graphene Materials:Functionalization,Micromorphology Control And Property Study

Multi-dimensional graphene materials can be applied in energy storage,energy transformation,stimulus response and other related fields due to their unique physical and chemical properties.In recent years,multi-dimensional macroscopic graphene materials have attracted wide attention.In this article,we have prepared different three-dimensional(3D)graphene based materials by using hydrothermal method and freeze-casting process,including carbon quantum dots doped carbon aerogel,doping-free multi-defective graphene and air-bubbles filled 3D graphene materials.(1)Carbon aerogel decorated with carbon quantum dots——Three-dimensional multichannel internal structure:Carbon quantum dots(CQDs)can be embedded into the polymer skeleton with covalent bonds through a simple hydrothermal polymerization process.The CQD aerogel were obtained after lyophilization(use ice crystals as temple to build the internal channel structure)and pyrolysis(1050 ^oC under argon atmosphere)procedures.The CQD aerogel shows very high specific surface area(1027 m² g^-1)and rich mesoporous structures,which can be used as electrode materials for supercapacitors.The electrochemical tests inidcate that the specific capacitance of CQD aerogel based supercapactior is 294.7 F g^-1at 0.5 A g^-1discharge current density.Meanwhile,the CQD aerogel based supercapacitor exhibits high stable capacitive performance,which shows no obvious capacitance decrement(reduced about 6%)even after 1000 charge and discharge cycles.(2)Doping-free and multi-defective graphene:We prepared doping-free and multi-defective graphene(DMG)using a high temperature hydrothermal method.In a sealed reaction system,water molecules are turned into gas state under 600 ^oC and the theoretical pressure is up to 4500 MPa.The graphene layers can be etched by steam under such high pressure,leading to the muti-defective sheet structure and increased specific surface area with more active sites.Thus,the DMG presents excellent catalytic performance of oxygen reduction with a high half peak potential of 0.746 V and a low Tafel slope of 50.3 m V dec^-1,which is comparable to those heteroatoms doped graphene based materials.Meanwhile,the DMG can be used as electrode materials for supercapacitor due to its excellent electro-conductivity and high specific surface area.The specific capacitance of DMG is 203.8 F g^-1at 1 A g^-1.A small capacitance loss of 7.3%can be observed at 10 A g^-1 after 3000 cycling.(3)Air-bubbles filled 3D graphene materials:Graphene oxide(GO)dispersion and the surfactant solution are mixed under stirring,while the air bubbles can be introduced into the mixture.Here,we successfully prepared homogeneous air bubbles filled 3D graphene materials by using the air bubbles as soft temple and a simple freeze-casting process.It shows excellent compression performances.This experiment is mainly divided into two parts:(i)Solidified graphene foams(SGF),ultraelastic and strong air-bubbled graphene foamUnder the compound effect of two surfactants,we prepared this ultraelastic graphene foam.Sodium dodecyl sulphate(SDS)was used as the bubbling agent to create homogeneous air bubbles inside the GO dispersion;Nonidet P-40(NP-40,octylphenoxypolyethoxyethanol)was used as the solidifying agent,which solidified the graphene foam during annealing.The solidified degree can be controlled by the amount of NP-40.As a result,SGF shows excellent compression performance even after high temperature annealing(1000 ^oC),which can also fully recover to its original state at 99%compressive strain and the corresponding compressive stress is up to 5.4 MPa.Furthermore,SGF shows a good electroconductibility(55.21 S m^-1)and a small resistance variation during compression(only 60%resistance variation under 80%compressive strain).Thus it can be used as a conductive compressive pressure switch.(ii)Ultralight sparkling graphene block(SGB),ultrasensitive pressure sensorThe ultralight sparkling graphene block with large bubbles(100?300?m)can be obtained through a vigorous stirring process by using polyethylene glycol sorbitan monooleate(Tween 80)as the sparkling agent.Due to its connected internal network,SGB exhibits excellent compression performance and relative low compressive stress.It can fully recover to its original state at 95%compressive strain and the corresponding compressive stress is 2 k Pa.During compression,SGB shows ultrahigh current variation(under 0.1 V)and the pressure sensitivity is 229.8 k Pa^-1,which is much higher than other graphene based piezoresistive pressure sensor.Meanwhile,due to its extraordinary elasticity,the SGB could detect other kinds of forces,like bending and twisting,which shows great potential for multifunctional sensing applications.