Synthesis And Properties And Sensing Applications Of Graphene Functional Structures

Graphene is a two–dimensional(2D) monolayer of sp2–hybridized carbon atoms tightly packed into a honeycomb lattice structure. The intriguing optical, electrical, mechanic and absorptive properties mainly arise from its truly atomic thickness and strictly 2D structure. It is of great importance to translate the excellent properties of 2D graphene sheet into the macroscopic, functional structure of graphene for improving the properties and developing the applications. However, it is difficult to realize these goals due to the lack of effective synthetic method. Herein, effective ways are developed to prepare and build graphene functional structure with special morphologies to make breakthroughs in optical, electrical performances and sensor applications.Different from traditional carbon source such as CH4(decomposition temperature >1000 oC), large–scale graphene film has been fabricated using liquid ethanol as carbon source in lower temperature(800 oC). Compared with CH4, ethanol has a lower decomposition temperature. It is discussed that the effect of different experimental conditions on the quality and layer numbers of the graphene, including catalyst metal, concentration of carbon source, growth temperature and time. The graphene films prepared in these conditions exhibit high transparency(88.1%–97.5%) and low sheet resistance(127 Ω/sq–384 Ω/sq).It is demonstrated the massive fabrication of 1D graphene/PVA core–sheath fibers with both excellent mechanical and superior electrical properties from ultralong CVD graphene, and their application in flexible strain sensors. Different from naked graphene fiber in other references, graphene core–sheath fiber can be applied in safe conductor and sensing devices. Compared with the typical rGO fibers, graphene core–sheath fibers fabricated from CVD obtain the improvement on electrical conductivity(9.6×103 S/m) and mechanical stress(590 Mpa). Strain sensors based on as–prepared fibers have high sensitive electrical response toward various bending radius from 4 mm to 9 mm and strain from 0 to 6.39%. Our fiber sensors exhibit outstanding durability and stability after 200 times bending and stretching cycles. The gauge factor of graphene core–sheath fiber sensor is calculated to be ~5.02 under 0–6.3% strain. The intrinsic dynamic fracture procedure and mechanism of the core–sheath nanofibers were investigated.It is demonstrated the fabrication of 2D graphene meshes with infrared transparent conductivity and electromagnetic(EMI) shielding properties. Graphene meshes have been fabricated by CVD using copper meshes with different geometric construction as sacrificial substrates. The structure parameter τ of graphene meshes affect the IR transmittance(2500–6500 nm), good electrical conductivity, and EMI shielding efficiency of as–fabricated materials. The highest IR light transmittance is 87.85% with EMI shielding efficiency and conductivity of 4 d B and 66.7 S/cm when the parameter τ is 0.7. The highest EMI shielding efficiency is 12.86 d B with transmittance and conductivity of 70.85% and 127 S/cm when the parameter τ is 0.54. Based on the experimental and theoretical analysis, the transmittance mechanism of graphene meshes is explained. And the EMI shielding efficiency is prominently dependent on microwave absorption.Graphene/PDA film with 3D functional structure has been prepared to detect environmentally hazardous volatile organic compounds(VOCs). It is beneficial to assemble PDA on graphene substrate due to atomic plane of graphene and π–π interaction between graphene and polymer. PDA molecular assemble characteristics is studied by scanning tunneling microscopy(STM) microscope. This colorimetric sensor shows sensitive response with vapor range from 0.01% to 20%. The colorimetric responses of sensor to THF and chloroform are more sensitive compared to methanol and DMF vapors. The color change caused by relatively high concentration VOCs can even be perceived by the naked eyes. An important discovery is the logarithmic relationship between chromatic response(CR) and the VOC concentration(N) in the range of 0.01%–10%, CR=Cln N+D, where D and D are constants, depending on the type of VOCs. It is possible for graphene/PDA sensor to give quantitative values for practical applications. The structural conformation changes of PDA molecule, caused by its interaction with VOCs, are directly observed by STM, which revealed the intrinsic mechanism of chromatic variety of PDA at the molecular level.The research works in this thesis would provide some theoretical and technical support for synthesis of graphene functional structure materials. It makes graphene functional structure materials can be applied in more fields due to the improvement of the optical, electrical, mechanical properties.