The Preparation Of Functionalized Graphene Oxide And Its Interaction With Conjugated Polymers

Graphene is a novel 2 dimensional carbon material with one-atom layer thickness and is attracting growing research attentions owning to its excellent mechanical,electrical,optical,and thermal properties.Being the most common derivative of graphene,graphene oxide has a similar structure but quite different properties compared to graphene.The structure of graphene oxide can be described by the well-known Lerf-Klinowski model,which points out that graphene oxide is composed of a graphene sheet skeleton modified by carboxyl and carbonyl groups at the edges as well as hydroxyl and epoxy groups on the basal plane.These oxygen containing groups renders graphene oxide hydrophilicity and solution processibility in aqueous media.As a result,mono-layered dispersed graphene oxide sheets in water can be easily exfoliated from graphite oxide by ultrasound sonication,which is cost effectively synthesized from bulk graphite by Hummers method and other modified chemical methods.The hydrophilicity can provide convenience for the application of graphene oxide in photocatalysis,drug delivery and bio-sensing,but unfortunately limit the dispersion of graphene oxide in organic solvents.Therefore,various modification routes,including covalent and non-covalent functionalization methods,have been proposed to acquire functionalized graphene oxide with dispersibility in organic solvents,which can effectively broaden the application of graphene oxide in organic phase.Graphene oxide as well as functionalized graphene oxide have great application potentials in the areas of nanocomposites,photocatalysis,energy storage,energy conversion,etc.As flexible 2-D materials with huge specific surface area(～2630 m2/g),they can act as skeletons or platforms for the loading of other component and provide large interfaces for reaction or charge transportation,leading to the improved performance of applications where graphene oxide incorporated.Graphene oxide is an electronically hybrid material that contains both conducting π-states from sp2 carbon sites and the σ-states of the spa-bonded carbons with large band gap.The overall band gap of graphene oxide thus can be tuned by changing the ratio of the sp2 and sp3 fractions by reduction.Therefore,reduction is able to controllably transform GO from an insulator to a semiconductor and to a graphene-like semi-metal.Nowadays,organic semiconductors including conjugated polymers have achieved great progress for the applications in organic lighting-emitting diodes,organic photovoltaics and other optoelectronic devices owing to their flexibility,solution processibility and accessibility at low cost.Among these,the developing of organic photo-detectors and solar cells based on conjugated polymer blends converting solar energy to electricity is the research hotspots.Great efforts have been took to explore the incorporation of graphene oxide into conjugated polymer blend films to alter the film structures and the charge transport properties,aiming at improving the performance of optoelectronic devices based on these polymer blend films.As the majorities of conjugated polymers can only dispersed in organic non-polar solvents,the functionalization of graphene oxide to acquire dispersibility in non-polar solvents is the key to gain good incorporation of functionalized graphene oxide into conjugated polymer blends.In order to obtain functionalized graphene oxide with dispersibility in non-polar solvents and achieve performance improvement of optoelectronic devices based on functionalized graphene oxide incorporated conjugated polymer blend films,we systematically investigated the methods for the preparation of functionalized graphene oxide as well as the interaction between functionalized graphene oxide and conjugated polymer.The main contents of these research works can be include the follows:1.The ionic functionalization of graphene oxideWe improved the preparation method of DDAB ionically functionalized graphene oxide(DDAB-GO)with which DDAB-GO can be effectively transferred from water phase to a non-polar solvent,o-dichlorobenzene(DCB),without the need for pH adjusting.It was proposed that the oxidative debris(OD)adhering on the graphene oxide(GO)can be largely eliminated during the phase transfer of DDAB-GO.A purification process was developed to extract the redundant DDAB molecules transferred to DCB,leading to a purified and stable dispersion of DDAB-GO in DCB.DDAB-GO and a conjugated polymer,P3HT,can form stable blend dispersion in DCB.In the liquid phase,DDAB-GO sheets act as 2 dimensional platforms for the assembling of P3HT molecules.The charge transfer or energy transfer processes from P3HT to DDAB-GO was verified to occur under photo-excitation which implies the possibility of DDAB-GO as an acceptor material.2.The controllable reduction of ionically functionalized graphene oxideA solvo-thermal reduction route based on a polar solvent,dimethylformamide(DMF)was proposed to reduce the DDAB ionically functionalized graphene oxide(DDAB-GO)dispersed in DCB to obtain DDAB functionalized reduced graphene oxide(DDAB-rGO)with controllable reduction level.We experimental measured the band gap alignment(LUMO/HOMO positions)of DDAB-GO and DDAB-rGO which shows that the band gap of DDAB-GO decreases after reduction and the LUMO/HOMO positions of DDAB-rGO approaches to each other as the reduction level increase.The photo-physical studies of P3HT:DDAB-GO and P3HT:DDAB-rGO blend films indicated that only electron transfers from photoexcited P3HT leading to the improved performance of photodetector while both electron and hole in P3HT can be transferred to DDAB-rGO reduced by 5 hours due to the HOMO level mismatching.In this case,the electrons and holes transferred to DDAB-rGO are mostly recombined and make no contribution to photocurrent generation.Therefore,the control and measurement of the band gap alignment of functionalized GO are essential to acquire the optimized band matching with the conjugated polymer for achieving improved performance of optoelectronic devices.3.The covalent functionalization of ionically functionalized graphene oxideIt was proved that DDAB ionically functionalized graphene oxide(DDAB-GO)dispersed in DCB can be covalently functionalized by(CI).The obtained CI-DDAB-GO possesses better compatibility with P3HT compared to DDAB-GO,which is consistence with the improved dispersibility in non-polar solvents after CI functionalization.The incorporation of CI-DDAB-GO into P3HT blend film can effectively decrease the dark current and increase the on-off ratio of photodiodes based on the blend film.This performance enhancement observed was deeply interpreted by the proposed structural model.This results suggested CI-DDAB-GO has great advantage in incorporation into polymer blends to improve their properties and device performance.4.The functionalization of graphene oxide based on π-π interactionAn effective method for preparing poly(3-hexylthiophene)(P3HT)functionalized graphene oxide(P-GO)based on the π-π interaction was developed.During the P3HT assisted phase transfer of GO from water/ethanol to DCB based on solvent evaporation,P3HT molecules can adhering to the surface of GO due to theπ-π interaction between them,which renders P-GO good dispersibility DCB and accessibility to polymer blend composites.P-GO was incorporated into P3HT:PCBM blends by solution mixing and shown to facilitate phase separation of P3HT and PCBM in P3HT:PCBM blend films to achieve a more optimum morphology for polymer photovoltaic cells.Bulk heterojunction P3HT:PCBM solar cells exhibit～18%power conversion efficiency enhancement with 2.5wt%P-GO incorporation.