Heat-induced reshaping and coarsening of metal nanoparticle-graphene oxide hybrids.

摘要

Glutathione-capped gold nanoparticles of size 1, 3, and 10 nm, CTAB-stabilized gold nanorods, as well as ro-carboxylate-functionalized palladium nanoparticles were synthesized and self-assembled onto graphene oxide to study their coarsening or reshaping behaviors upon heating at different temperatures ranging from 50 °C to 300 °C. These engineered nanoparticle- or nanorod-graphene oxide hybrid materials were studied by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) spectroscopy, and UV-Vis spectroscopy.;The spherical nanoparticles would undergo coalescence to become larger particles and the nanorods would undergo reshaping to spherical particles. UV-Vis results show that the plasmonic band of gold nanoparticles at 520 nm would shift to higher wavelength indicating the coarsening into larger particles upon heating. Transmission electron microscopy results were generally in good agreements with the UV-Vis results and would be used as a direct tool to observe the structural changes of gold nanoparticles upon heat treatments.;Without the presence of graphene oxide, the nanoparticle coalescence began at the temperature between 150 and 200 °C for all three nanoparticles with different core sizes. But with the presence of graphene oxide, nanoparticles start to coalesce at the temperature below 150 °C. The gold nanorods have two plasmonic bands at ∼780 and ∼520 nm. The bands at ∼780 nm for gold nanorods would disappear when the gold nanorods-graphene oxide is heated at 50 °C indicating the complete reshaping of nanorods even at such a low temperature. Gold nanorods themselves are more stable and do not undergo the reshaping completely until the sample is heated above 150 °C. Since graphene oxide is an excellent thermal conductor, we propose that graphene oxide could transfer heat to the nanoparticles and nanorods efficiently, disrupt the interaction of stabilizing ligands, and make them to either coalesce or undergo reshaping at a lower temperature.;Nanoparticle- and nanorod-graphene oxide hybrid materials were also used to study the effect of covalent and non-covalent interactions between gold nanoparticles or nanorods and graphene oxide during coarsening or reshaping, respectively. Non-covalent interactions were studied by directly adding graphene oxide to aqueous solutions containing water-soluble metal nanoparticles or nanorods, and covalent interactions were achieved by the self-assembly of the same nanoparticles onto thiolated graphene oxide that was prepared by coupling L-cystine using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). When nanoparticles and nanorods are attached to graphene oxide through additional -covalent bonds, they are more strongly immobilized and therefore would undergo less coalescence and slower reshaping upon heating.