ELECTRICAL AND THERMAL CONDUCTIVITIES OF AU NANOPARTICLE DECORATED - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS ELECTRICAL AND THERMAL CONDUCTIVITIES OF AU NANOPARTICLE DECORATED GRAPHENE NANOPLATELET ‘PAPER’ J. Xiang 1 , L. T. Drzal 1 * 1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, U.S.A, * Corresponding author (drzal@egr.msu.edu ) Keywords : graphene nanoplatelet, polyethyleneimine, microwave assisted heating 1 Introduction reduction processes. Research conducted in MSU has led to a process that can produce exfoliated Numerous research efforts are underway directed at graphene nanoplatelets with controlled thicknesses discovering the superior and unique properties of ranging from 1-10 nm and platelet diameters from single layer graphene and multilayer graphene 100 to 10000 nm [3]. Polyethyleneimine, a nanoplatelets. Typical methods to synthesize metal hydrophilic polymer with primary, secondary and nanoparticle on graphene surface involve covalent tertiary amino groups and a positive charge in the functionalization of graphene surface to induce neutral aqueous solution, adsorbs onto the highly anchoring sites for the metal precursor and hydrophobic GNP surface and stabilizes the GNP subsequent attachment of the reduced nanoparticles particles in water. In addition, PEI contains one of to the surface [1]. This approach results in the the highest densities of amino groups among all disruption of the sp 2 bonded carbon atoms in the polymers, donating electrons that help reduce metal basal plane which leads to reduced transport ions [4]. Au/GNP particles were then dispersed in properties of the graphene because of additional water and made into a ‘paper’ by vacuum assisted scattering sites. Another widely adopted technique self assembly. The electrical and thermal involves nanoparticle growth on non-covalently conductivity of the Au/GNP paper were functionalized graphene surfaces, which preserves characterized. It is believed adsorbed PEI is likely to the intrinsic properties of graphene nanosheets interfere with both electron and phonon transport thanks to the minimum chemical perturbation of the within the hybrid paper. Therefore, a thermal basal planes. Other techniques of metal nanoparticle annealing at 340 ˚ C was applied to remove the PEI decoration on graphitic nanostructure include within the paper by thermal decomposition. Since electrodeposition, evaporation, solventless bulk the GNP is highly oxidation resistant to temperatures synthesis and etc. While these methods have some greater than 500 ˚ C, chemical changes in the GNP processing advantages over solution-phase did not take place during annealing. It is also worth techniques, they are usually quite expensive and noting that the hybrid paper sample as prepared by energy intensive. self-assembly is highly porous. In order to reduce Despite the many publications on superior porosity and enhance particle alignment in the paper, electrochemical properties of nanoparticle/graphene the annealed samples were compacted in a hydraulic hybrid, none discusses the transport properties of press at room temperature. The impact of gold this hybrid material when made into a ‘paper-like’ nanoparticles on both electron and phonon transport structure where the nanoparticles are located at the in this hybrid paper under different experimental surfaces and interfaces of the graphene nanosheets. conditions (thermal annealing, cold compaction) is In this work, we reported a fast, one-pot synthesis of discussed. gold nanoparticle decorated graphene nanoplatelets (GNP) in the presence of a polyethyleneimine (PEI) matrix with microwave assisted heating [2]. GNPs 2. Results and discussions are few layer graphene nanosheets produced from At pH <10, the positively charged polymer chain microwave exfoliation of graphite intercalated also induces electrostatic repulsion that contributes compounds followed by a combination of size to good dispersion of the nanoplatelets. As the

tetrachloroauric acid dissolved in water was mixed particles. In addition, more PEI chains would adsorb - ions complex with the GNP suspensions, the AuCl 4 on to the surface of GNP acting as templates for Au with the positive functional groups of PEI adsorbed nanoparticle growth. There is no obvious on the GNP surface (chloride ligands were replaced agglomeration of Au nanoparticles on the surface of by amine groups of PEI). The adsorbed PEI chains, GNP, which is also believed to be the result of PEI in this case, serve as templates for subsequent Au encapsulation that stabilizes the nanoparticles in the nanoparticle growth on the surface of GNP. The solution electrostatically. addition of H + ions also lowers the pH of the Au/GNP paper was prepared by vacuum assisted self solution to around 6, inducing more positive charges assembly. Figure 2 shows the photos of the as-made on the chain of PEI and more electrostatic attraction papers after being removed from the filter membrane. - [5]. The reduction of Au 3+ to Au 0 was with AuCl 4 The as-made paper is very flexible and shows then carried out in a microwave oven where the certain mechanical robustness under bending. Table heating rate is fast enough to cause a rapid 1 shows the apparent densities of the samples at temperature increase in the solution due to the high different conditions. polarity of water molecules, creating a high Thermal diffusivity of the samples prepared at concentration of radicals which facilitates the electron transfer from the radicals of PEI to the different experimental conditions (as-made, annealed, annealed and cold compacted) were metal precursor. As soon as the solution is measured by Nanoflash 447 (Netzsch Instruments). supersaturated with metal atoms, Au atoms form nuclei. The critical size of the nucleus as well as Given the density of the sample, and the specific heat, which was measured by differential scanning nucleation activation energy control the nucleation calorimetry, thermal conductivity of the sample can rate and depend on the surface tension of the nuclei- be obtained: κ = α ρ C p where κ is thermal solvent interface when the radius of nuclei is small. conductivity with a unit of W/m o K, α is thermal Therefore, a higher surface tension corresponds to a diffusivity with a unit of mm 2 /s, and C p is specific larger critical nucleus size and a lower nucleation heat with a unit of J/g o K. rate, which should be avoided in synthesizing monodispersed small nanoparticles. Upon formation As shown in Figure 3a, the through-plane thermal of the nucleus, polyethyleneimine, with a lower conductivity of all the samples followed the same surface tension than Au atoms, adsorbs on the trend with a slight increase after thermal annealing surface of the metal atoms reducing the nucleation and a reduction after cold compaction. It is believed energy barrier and critical nucleus size. As a result, PEI adsorbed either on the GNP surface or on the more primary particles with low surface energy are Au nanoparticles scatter the phonons in this paper- formed leading to generation of smaller secondary like structure, contributing to a larger thermal particles whose growth and coarsening are also interface resistance which is a major impediment to affected by surface tension. Scheme 1 represents the phonon transport [6, 7]. A thermal annealing typical procedures and the interactions between the treatment removed most of the PEI in the paper, metal ions and the active sites on GNP particles and presumably reducing the interfacial resistance as subsequent formation of nanoparticles on GNP. suggested by a 20% improvement in through-plane Figure 1 shows the SEM images of Au nanoparticle diffusivity. However, upon annealing and cold decorated GNP prepared at 0.3wt% and 0.6wt% PEI compaction, thermal conductivity decreased for all in the solution. It is found that the size and loading the samples. Compaction of the sample effectively of the Au nanoparticles synthesized by this eliminated large pores within the paper created technique correlated with the concentration of PEI in during filtration and the nanoplatelets were more the solution. The size of the nanoparticles was aligned due to the compressive stress. The better reduced while the loading increased with higher orientation of the nanoplatelets reduced the through- concentration of PEI in the solution. With a higher plane phonon transport due to the intrinsically low concentration of PEI in the solution, the number of thermal conductivity of GNP with multiple layers of nuclei with a lower surface energy increased which graphene held together with weak Van der Waals leads to formation of more primary and secondary forces and the high probability of interface scattering