Preparation,Properties,and Applications Of Copper Nanowire-Graphene Composites For High-Performance Flexible Transparent Electrodes For Optoelectronic Devices

One dimensional（1D）copper nanowires（Cu NWs）have the potential for a wide range of applications in constructing modern electronic devices due to its low cost,high-conductivity,and mechanical flexibility.Therefore,it has drawn more and more attention and has become one of the hot research fields.However,Cu NWs are easily oxidized even in the normal environment,thereby increasing the electrical resistance of the conductive network.This shortcoming of Cu NWs impedes its applications greatly.Therefore,preventing oxidation of Cu NWs is an urgent issue that needs to be solved.Shielding Cu NWs from moisture and oxygen is a good strategy.Such as coating Cu NWs using inert metals or polymers.However,the conductivity of Cu NWs would be sacrificed or impaired using the existed methods.Also,these methods either required complex and expensive processing steps or the protective layers had weak bonding with the Cu NWs or porous structure,which could not completely avoid atmospheric gases and moisture from contacting the Cu NWs.Therefore,it is a still challenging task for the preparation of the oxidation stable Cu NWs without compromising its conductivity,and promoting its application.Graphene is a well-known two-dimensional material,which has wonderful characters of high-conductivity,large surface area,high in-plane stiffness,and impermeability against O₂ and H₂O.Thus,it is a good potential material to protect the Cu NWs from oxidation by forming a protecting layer on the Cu NWs without impairing the conductivity.However,the reported methods of using the graphene as a protecting material until now have numerous drawbacks,such as short-term protection effect,complex processing,high-cost,and environmental pollution,etc.In this thesis,a new strategy of preventing Cu NWs from oxidation while keeping excellent conductivity is proposed,in which the composites of Cu NWs with pristine graphene nanosheets（GNs）and with the hydroxylated graphene nanosheets（GNs-OH）were synthesized using hydrothermal and solvothermal synthesis methods.The structure,morphology,and properties of the obtained composites were characterized and analyzed using varied modern tools.The mechanisms of forming the composites and their oxidation stability were explored.The applicability and superiority of the composites in flexible transparent electrodes and transparent heaters were studied.Also,the coating of Cu NWs with Ag was carried out via an electroless-plating method by using a newly developed Ag precursor,and its structure,properties,and application were studied.The main results and specific conclusions are as follows:A modified liquid-phase exfoliation method assisted with ultrasound was developed to preparethe few-layer（<5 layers）GNs from graphite.The curcumin with richπ-πbonds was used as a dispersant and ethanol was used as an exfoliation media.The exfoliation was conducted with the help of high-power horn ultrasound equipment.The influences of the dispersant concentration,initial graphite amount,ultrasound power,and exfoliation time were schematically investigated.It was found that at the optimum parameters(0.5 mg·m L^-1concentration of dispersant,5 mg·m L^-1 of graphite amount,1.08 k W of ultrasound power,and 8 h of exfoliation time),the yield of the GNs in ethanol achieved to 34.2%.The structure and morphology of the GNs were further analyzed using the high-resolution transmission electron microscope（HR-TEM）,Raman spectrum,atomic force microscope（AFM）,and X-ray diffraction,which confirmed that the obtained GNs were highly crystalline and several hundreds of nanometers in lateral size with average layers number less than 5.Also,the as-exfoliated GNs suspension in ethanol was stable for a long time.The higher yield,long-term dispersion stability,and better structural integrity of the exfoliated GNs were mainly attributed to the complexing between the exfoliated GNs and the curcumin through theπ-πas well as hydrophobic interaction,which prevented the re-stacking of the exfoliated GNs and kept suspended in the ethanol.One-pot hydrothermal synthesis approach was developed to produce copper nanowires-graphene nanosheets（Cu NWs-GNs）composite.The synthesis of Cu NWs in the composite was conducted in the presence of GNs and ethanol,which could affect the capping effect of oleylamine（OAm）and the growth of the Cu NWs.Therefore,the influences of GNs amount,OAm amount,and ethanol volume on the growth of the Cu NWs,and the structure of the Cu NWs-GNs composite were schematically studied.The experimental results suggested that the 1.5 mg·m L^-1concentration of GNs,17 m L of OAm amount,6.25 m L of ethanol volume,3.75 g of Cu Cl₂,4.5g of glucose amount,120 ^oC of reaction temperature and reaction time for 5 h were the optimum parameters to synthesize the Cu NWs-GNs composite consisting of high-aspect-ratio（AR=～1778）Cu NWs with coating effect of GNs.The structure and morphology of the as-synthesized Cu NWs-GNs composites were examined with the help of scanning electron microscope（SEM）,HR-TEM,selected-area-diffraction pattern（SAED）,and Raman spectrum.The results confirmed that the linear Cu NWs were grown between the GNs interlayers,thus obtaining a multilayer protective layer of GNs without the stacking of GNs.During synthesis,the glucose and OAm molecules played key roles such as a reducing agent,capping agent for the growth of Cu NWs,and dispersant for the GNs.The glucose molecules reduced the Cu²⁺ions to low-valence metallic Cu⁰ atoms and OAm molecules directed the growth of the Cu NWs between the GNs interlayers,obtaining a multilayer coating effect to address the oxidation issue of nanowires.Therefore,the relative resistance values of the composite increased to only 1.27 and 1.92 after 46 of the aging test at 25in 50±5%of relative humidity（RH）and 60 ^oC in 40±5%of RH,while the pure Cu NWs was more than 20 and 1000 after 35 and 15 days in the similar conditions.Also,the morphology,phases,and chemical composition of the nanowires in the composite remained unchanged during the stability test,which was mainly due to the barrier effect of GNs for atmospheric gases and moisture.In addition,the membranes fabricated using Cu NWs-GNs composite possessed high electrical conductivity of 2.12×10⁶ S·m^-1 at a sheet resistance（R_s）of 0.037?·sq^-1 and loading amount of 4 mg·cm^-2.The high electrical conductivity was mainly due to the high-aspect-ratio Cu NWs and high-crystalline GNs,which formed a better conductive network in the membranes.A new facile method of preparing the strongly combined copper nanowires-hydroxylated graphene nanosheets（Cu NWs-GNs-OH）composite based on a solvothermal approach was developed.In this novel synthesis method,the hydroxylated-graphene nanosheets（GNs-OH）were directly used as both a reducing agent and a protector for the Cu NWs.Firstly,the GNs-OH was obtained by treating the exfoliated GNs with O₂ plasma for 30 min followed by hydrolysis using2 M concentration of KOH at 180 ^oC for 2 h.Then,the Cu NWs-GNs-OH composite was synthesized by heating a homogenous mixture consisting of 50 mg of Cu Cl_2,25 g of OAm,and 75mg of GNs-OH at 180 ^oC for 12 h.The morphology of the as-synthesized composites was examined with the help of SEM,HR-TEM,and Fourier transform infrared spectroscopy（FTIR）,which suggested that the high-aspect-ratio（～2771）Cu NWs were grown within the graphene interlayers,thereby forming dense protective layers of GNs-OH.The oxidation stability of the composite was evaluated by testing the relative resistance values of the composite-based membrane in different conditions.The relative resistance ratio of the composite increased to only1.33,1.27,1.76,and 1.94 after the aging test at 25 ^oC in 40±5%relative humidity（RH）for 60days,at 80℃in 20±5%RH,25℃in 70±5%RH,and 80℃in 80±5%RH for 7 days respectively,while the pure relative resistance ratio of pure Cu NWs increased to 356,1462,1745,1462 after 60 days,2 days,1day,and 1 day in the similar conditions,respectively.These results indicated that the Cu NWs had excellent oxidation stability.Also,HR-TEM,X-ray diffraction,and Raman spectrum indicated that the morphology,phases,and chemical composition of the nanowires did not change.It might be attributed to the strong attachment between GNs-OH and Cu NWs because of the-OH groups via oxygen mediated carbon bonding with copper（C-O-Cu）.Therefore,graphene and copper nanowires are more closely combined and stronger.Thus,moisture and gases could not diffuse inside the composite and oxidize the nanowires.Also,this novel structure empowered excellent electrical conductivity of 2.77×10⁶ S/m at an R_s value of0.028?·sq^-1 and loading amount of 4 mg·cm^-2.The improved electrical conductivity was mainly due to the high-aspect-ratio Cu NWs high-crystalline GNs-OH and formation of close contact between the GNs-OH which allowed a better pathway for the transport of electrons in the conductive membranes.The high-performance transparent flexible electrodes（TFEs）attracted and increased interest in the scientific community to fabricate the next-generation organic light-emitting diodes,photovoltaics,flexible displays,and transparent heaters.Therefore,the applicability of the as-synthesized Cu NWs-GNs-OH composite in fabricating transparent flexible transparent electrodes（TFEs）for opto-electronic devices and transparent heaters were determined.The TFEs and transparent heaters were fabricated using Cu NWs-GNs-OH via facile spray coating method and polyethylene terephthalate（PET）as a transparent flexible substrate.The performance of the composite in the TFEs and transparent heaters was investigated in terms of R_s,transmittance,and flexibility of and heating rate,and the highest saturation temperature.The results indicated that the as-fabricated TFEs had a great transmittance of 89%at an R_s value of 21?/sq and the loading amount of 0.17 mg·cm².Also,the relative resistance was increased to about 1.1 upon compressive bending and 1.047 upon tensile bending for 1000 cycles.The relative resistance increased to 1.28upon compressive bending and about 1.35 during tensile bending upon 1 mm of radius,confirming that the TFEs had excellent mechanical flexibility,which was mainly attributed to the stable nanostructured composite of Cu NWs and GNs-OH.Besides,the fabricated transparent heaters too presented an excellent performance in terms of rapid heating,a uniform temperature distribution,and a high saturation temperature.For example,the temperature of the transparent heater rapidly increased from 30 to 33 ^oC upon applying 1 V of input bias.While the temperature quickly increased from 30 to 119 ^oC as the 8V of the input bias was applied.Therefore,it was confirmed the as-synthesized Cu NWs-GNs-OH applied to construct the high-rank electrode used in the curved devices and next-generation opto-electronics devices and transparent heaters.The Cu NWs coated with metallic silver（Ag⁰）as another protecting way were also investigated.A rapid and efficient electroless-plating way was developed to synthesize the core-shell nanowires of copper@silver（Cu@Ag）.The synthesis of Cu NWs was conducted through the hydrothermal method using 1 g of Cu Cl₂,2.5 g ofα-glucose,and 9 g of hexadecyl amine at 105℃.The electroless plating of the as-synthesized Cu NWs was conducted by using a novel Ag-amino precursor,which was prepared by mixing the silver nitrate（Ag NO₃）and 3-dimethylamino-1,2propanediol in a 1:3 ratio.The Ag-aminoalcohol precursor was mixed with the Cu NWs suspension at 200 rpm for 15 min.Then,the Cu NWs-Ag-amine complex was transferred to the 70%aqueous solution of isopropyl alcohol,which reduced the adsorbed Ag-amine complex on the Cu NWs and produced Cu@Ag core-shell nanowires.The TFEs for opto-electronics devices were fabricated through the spray deposition method followed by the nano-welding using a 5%aqueous solution of H₂O₂ to fuse the nanowires at their junction.The structure and composition of the as-prepared Cu@Ag core-shell nanowires were examined in detail using SEM,TEM,SAED,and energy dispersive spectroscopy（EDS）.The SEM micrographs of the core-shell nanowires revealed that a nano-scale thin Ag layer was coated on the Cu NWs after electroless plating.The thickness of the Ag shell on the Cu NWs was about 13nm.Furthermore,the SAED pattern of the core-shell nanowires presented a ring pattern,which was corresponded to the face-centered cubic Cu and Ag coexistence in the same nanostructure.Also,atomic mapping displayed that the Ag atoms were uniformly distributed on the edges,while Cu atoms were mainly located at the center,confirming Cu@Ag nanowires had a core-shell structure.The results of testing oxidation stability indicated that the relative resistance ratio of Cu@Ag core-shell nanowire-based membrane increased only to 1.86 and 12.54 after they had been exposed at 80℃and in 25±5%and 80±5%RH for 500h.The excellent oxidation stability of the nanowires was due to the dense coating of metallic Ag⁰on Cu NWs formed via electroless plating method.The TFEs fabricated using Cu@Ag core-shell nanowires had the highest transmittance of 90%at an R_s value of 52?/sq.Its relative resistance increased only to 1.16 and 1.08 against tensile and compressive bending for 1000 cycles.Besides,the relative resistance increased to 1.28 upon compressive bending and about 1.35 during tensile bending upon 1 mm of radius,which confirmed that the TFEs had excellent mechanical flexibility suitable for the flexible devices.The enhanced performance was mainly attributed to the strongly interconnected network of Cu@Ag core-shell nanowires.The overall method is simple and provides a choice and experimental data for the preparation of copper nanowires with high oxidation stability.